Advances In Microwave And Rf Processing

Advances in Microwave and Radio Frequency Processing

Monika Willert-Porada (Ed.)

Advances in Microwave and Radio Frequency Processing Report from the 8th International Conference on Microwave and High Frequency Heating held in Bayreuth, Germany, September 3 – 7, 2001 With 469 Figures

*

P

E

*

* M

R

*

E * A

*

Professor Dr. M. Willert-Porada Chair of Materials Processing University of Bayreuth Universitätsstraße 30 D-95447 Bayreuth Germany

Library of Congress Control Number: 2005934302 ISBN-10 ISBN-13

3-540-43252-3 Springer-Verlag Berlin Heidelberg New York 978-3-540-43252-4 Springer-Verlag Berlin Heidelberg New York

This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilm or in any other way, and storage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer-Verlag. Violations are liable for prosecution under the German Copyright Law. Springer-Verlag is a part of Springer Science+Business Media springer.com © Springer-Verlag Berlin Heidelberg 2006 Printed in the Netherlands The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use.

Production: SPI Publisher Services Typesetting: SPI Publisher Services Cover Design: E. Kirchner, Heidelberg Printed on add free paper

SPIN 10850172

30/3100/SPI - 5 4 3 2 1 0

Preface Prometheus brought fire to mankind Arthur R. von Hippel “Dielectrics and Waves”, 1954 Our contribution? There are only few areas of research and development of a comparable scientific and technological extension as microwave and high frequency processing. “Processing” means not only application of radiation of 300 MHz to 300 GHz frequency to synthesis, heating or ionisation of matter but also generation, transmission and detection of microwave and radio frequency radiation. Microwave and high frequency sources positioned in the orbit are the foundation of modern satellite telecommunication systems, gyrotron tubes being presently developed in different countries all over the world will most probably be the major devices to open up a new era of energy supply to mankind be means of fusion plasma. Although initiated by military purposes during the Second World War (RADAR, Radio Detection and Ranging), microwave and high frequency utilisation has spread over almost every important aspect of normal day life since than, from individual mobile phones and kitchen microwave ovens to industrial food processing, production of composites as sustainable building materials, green chemistry, medical applications and finally infrastructure installations like GPS and Galileo, to name only few examples. These different areas of microwave and high frequency radiation application can not be unified within one group of scientists and technologists. There are several distinguished communities active e.g., in the area of telecommunication systems, strong microwaves for fusion plasma or plasma based materials processing. Research to improve fundamental knowledge leading to new non-military applications of high frequency technology, to support necessary regulations and to provide long term development of commodity and industrial applications as well as to improve the knowledge about these new technologies within the society is less well covered by scientific or professional organizations. In order to close this gap and provide a forum for fruitful discussions a group of researchers from academia and industry started to organize Microwave and High Frequency Heating Conferences in 1986, which take place every 2 years in a different European country. In 1993 AMPERE, Association for Microwave Power in Europe for Research and Education (www.ampereeeurope.org) was established and the conferences were organized on behalf of AMPERE since than. In addition to the regular conference schedule Microwaves in Chemistry meetings were added 1998 and 2000. Conference activities in the field of microwave and RF-applications show a remarkable growth: up to mid-80 of the 20th century IMPI (International Microwave Power Institute, USA) almost exclusively covered the organized activities in the field. The widespread availability of kitchen microwave ovens as well as the development of powerful microwave sources within national fusion programmes fa-

VI

Preface

cilitated use of this “cold” radiation for chemical syntheses and materials processing, with often quite unexpected results. Therefore professional organizations like e.g., the American Ceramic Society and the Materials Research Society established Microwave Symposia within their regular conferences in the period 1988-1996. Numerous Symposia Proceedings volumes came out of theses conferences (e.g., MRS Proc. Vol. 124, 189, 269, 347, 430 and Ceram. Trans. Vol. 21, 36, 59, 80), including publications from the 1st and 2nd World Congress on Microwave and Radio Frequency Processing. New professional associations enter the scene, like e.g., the Microwave Working Group in the USA, and different Societies in Japan and China. The overview and technical papers contained in this book reflect the major areas of activity not only of AMPERE members but also of a representative group of other researchers worldwide. The topics were selected from contributions of the 8th International Conference on Microwave and High Frequency Heating, organised by AMPERE and held in September 2001 in Bayreuth, Germany. The papers were referred by major specialists in the respective field. The book is intended to provide non-specialists an overview of the State of the Art in the field of microwave and high frequency hardware, measurement and modelling as well as to give the specialists insight into the most advanced R&D topics of microwave and high frequency radiation application in different disciplines. Many experts and colleagues contributed to this book. I am particularly indebted to (in alphabetical order): J.P. Bernard, France; J. Binner, UK; J. Booske, USA; S. Bradshaw, South Africa; A. Breccia, Italy; M. Brito, Japan; J.M. Catalá -Civera, Spain; T. Gerdes, Germany; J. Gerling, USA; W. Jansen, Netherlands; W. Van Loock, Belgium; R. Metaxas, UK; A. Mavretic, USA; T. Ohlsson, Sweden; P. Püschner, Germany; E. de los Reyes, Spain; A. Rosin, Germany; G. Roussy, France; A. Schmidt, Germany; V. Semenov, Russia; M. Thumm, Germany; N. Tran, Australia. My deep thanks go to the authors for their patience and effort to collect excellent papers; to colleagues for valuable suggestions and to my co-workers for many hours of work to fit the individual contributions into a book. Hopefully this book will facilitate further development of the fascinating field of microwave and high frequency processing, in a synergetic effort of many groups all over the world.

Monika Willert-Porada, Editor Bayreuth, first half of the first decade of the 21st century

Contents PART I: HARDWARE UNDERSTANDING MICROWAVE HEATING SYSTEMS: A PERSPECTIVE ON STATE-OF-THE-ART .................................................................................................3 H. C. Reader MILLIMETER-WAVE-SOURCES DEVELOPMENT: PRESENT AND FUTURE.................15 Manfred Thumm and Lambert Feher 3.5 KW 24 GHZ COMPACT GYROTRON SYSTEM FOR MICROWAVE PROCESSING OF MATERIALS ..................................................................................24 Yu. Bykov, G. Denisov, A. Eremeev, M. Glyavin, V. Holoptsev, I. Plotnikov, V. Pavlov DESIGN GUIDELINES FOR APPLICATORS USED IN THE MICROWAVE HEATING OF HIGH LOSSES MATERIALS..................................................................31 Juan V. Balbastre, E. de los Reyes, M. C. Nuño and P. Plaza DESIGN PARAMETERS OF MULTIPLE REACTIVE CHOKES FOR OPEN PORTS IN MICROWAVE HEATING SYSTEMS ............................................................39 J. M. Catalá-Civera, P. Soto, V.E. Boria, J. V. Balbastre and E. de los Reyes MICROWAVE HIGH-POWER FOUR POST AUTO-MATCHING SYSTEM ......................48 Pedro Plaza, Antoni J. Canós, Felipe L. Penaranda-Foix and Elias de los Reyes DESIGN OF AN APPLICATOR FOR PROCESSING OF NANOSCALE ZEOLITE/POLYMER COMPOSITES WITH SUPERPOSED STATIC MAGNETIC FIELD .....................................................................................................................56 Ralph Schertlen, Stefan Bossmann, Werner Wiesbeck

PART II: MEASUREMENT TECHNIQUES AND REGULATIONS MEASUREMENT TECHNIQUES FOR MICROWAVE AND RF PROCESSING ..................65 Georges Roussy DIELECTRIC CHARACTERISATION OF HIGH LOSS AND LOW LOSS MATERIALS AT 2450 MHZ .....................................................................................77 Andrew Y.J Lee and V. Nguyen Tran EUROPEAN REGULATIONS, SAFETY ISSUES IN RF AND MICROWAVE POWER ...................................................................................................................85 Walter Van Loock

VIII

Contents

FUTURE PROSPERITY OF INDUSTRIAL, SCIENTIFIC AND MEDICAL (ISM) APPLICATIONS OF MICROWAVES ...........................................................................92 David Sánchez-Hernández and José M. Catalá-Civera ELECTRIC FIELD MEASUREMENTS FOR COMMERCIALLY-AVAILABLE MOBILE PHONES ..................................................................................................103 Antonio Martínez-González, Ángel Fernández-Pascual and David Sánchez-Hernández USE OF THE DIELECTRIC PROPERTIES TO DETECT PROTEIN DENATURATION ...................................................................................................107 S. A. Barringer and C. Bircan SANDALWOOD MICROWAVE CHARACTERISATION AND OIL EXTRACTION ...........119 V. Nguyen Tran DIELECTRIC SPECTROSCOPY AND PRINCIPAL COMPONENT ANALYSIS AS A METHOD FOR OIL FRACTION DETERMINATION IN OIL-IN-WATEREMULSIONS WITH VARYING SALT CONTENT........................................................129 M. Regier, X. Yu, S. Ghio, T. Danner, H. Schubert MICROWAVE NON-DESTRUCTIVE EVALUATION OF MOISTURE CONTENT IN LIQUID COMPOSITES IN A CYLINDRICAL CAVITY AT A SINGLE FREQUENCY .........................................................................................................138 J. M. Catalá-Civera, A. J. Canós, F. Peñaranda-Foix and E. de los Reyes MILLIMETER WAVE SPECTROSCOPY OF ALUMINA-ZIRCONIA SYSTEM ................149 Saburo Sano, Akihiro Tsuzuki, Kiichi Oda, Toshiyuki Ueno, Yukio Makino and Shoji Miyake A MODIFIED CAVITY PERTURBATION TECHNIQUE FOR MEASUREMENT OF THE DIELECTRIC CONSTANT OF HIGH PERMITTIVITY MATERIALS. .................155 Sheila Oree PART III: MODELLING FINITE ELEMENTS IN THE SIMULATION OF DIELECTRIC HEATING SYSTEMS ..............................................................................................................167 G.E Georghiou, R.A Ehlers, A. Hallac, H. Malan, A.P. Papadakis and A.C. Metaxas EXAMINATION OF CONTEMPORARY ELECTROMAGNETIC SOFTWARE CAPABLE OF MODELING PROBLEMS OF MICROWAVE HEATING ...........................178 Vadim V. Yakovlev A HYBRID APPROACH FOR RESOLVING THE ELECTROMAGNETIC FIELDS INSIDE A WAVEGUIDE LOADED WITH A LOSSY MEDIUM .....................................191 Viktor Vegh, Ian W. Turner

Contents

IX

A NOVEL FDTD SYSTEM FOR MICROWAVE HEATING AND THAWING ANALYSIS WITH AUTOMATIC TIME-VARIATION OF ENTHALPYDEPENDENT MEDIA PARAMETERS .......................................................................199 Malgorzata Celuch-Marcysiak, Wojciech K.Gwarek, Macie Sypniewski SIMULATION OF MICROWAVE SINTERING WITH ADVANCED SINTERING MODELS ...............................................................................................................210 Hermann Riedel, Jiri Svoboda FINITE ELEMENT MODELLING OF THIN METALLIC FILMS FOR MICROWAVE HEATING .........................................................................................217 R.A. Ehlers and A.C. Metaxas ANALYSIS OF COUPLED ELECTROMAGNETIC AND THERMAL MODELING OF PRESSURE-AIDED MICROWAVE CURING PROCESSES ......................................226 J. M. Catalá-Civera, J. Monzó-Cabrera, A. J. Canós, F. L. Peñaranda-Foix SELECTIVE HEATING AND MOISTURE LEVELLING IN MICROWAVEASSISTED DRYING OF LAMINAR MATERIALS: AN EXPLICIT MODEL ....................234 J. Monzó-Cabrera, A. Díaz-Morcillo, J. M. Catalá-Civera, E. de los Reyes MICROWAVE HEATING OF READY MEALS – FDTD SIMULATION TOOLS FOR IMPROVING THE HEATING UNIFORMITY ........................................................243 B. Wäppling-Raaholt, P. O. Risman and T. Ohlsson PART IV: FOOD PROCESSING AND ENVIRONMENTAL ENGINEERING APPLICATIONS NOVEL AND TRADITIONAL MICROWAVE APPLICATIONS IN THE FOOD INDUSTRY ............................................................................................................259 H. Schubert and M. Regie MICROWAVE DRYING: PROCESS ENGINEERING ASPECTS ....................................271 SM Bradshaw QUALITY OF MICROWAVE HEATED MULTICOMPONENT PREPARED FOODS ..................................................................................................................282 Suvi Ryynänen SENSORY EVALUATION OF DRIED BANANAS OBTAINED FROM AIR DEHYDRATION ASSISTED BY MICROWAVES.........................................................289 Sousa, W.A.; Pitombo, R.N.M.; Da Silva, M.A.A.P.; Marsaioli, Jr., A. MICROWAVE METHOD FOR INCREASING THE PERMEABILITY OF WOOD AND ITS APPLICATIONS ........................................................................................303 G. Torgovnikov and P. Vinden

X

Contents

SELECTIVE HEATING OF DIFFERENT GRAIN PARTS OF WHEAT BY MICROWAVE ENERGY ..........................................................................................312 E. Pallai-Varsányi; M.Neményi; A.J.Kovács; E.Szijjártó MICROWAVE IN SITU REMEDIATION OF SOILS POLLUTED BY VOLATILE HYDROCARBONS ..................................................................................................321 D.Acierno, A.A.Barba, M.d'Amore ,V.Fiumara, I.M.Pinto, A.Scaglione BIO-DIELECTRIC SOIL DECONTAMINATION ..........................................................329 J.P.M. Janssen-Mommen, W.J.L. Jansen WASTE TREATMENT UNDER MICROWAVE IRRADIATION .....................................341 A. Corradi, L. Lusvarghi, M. R. Rivasi, C. Siligardi, P. Veronesi, G. Marucci, M. Annibali, G. Ragazzo ENVIRONMENTAL ASPECTS OF MICROWAVE HEATING IN POLYELECTROLYTE SYNTHESIS ...........................................................................349 E. Mateescu, G. Craciun, D. Martin, D. Ighigeanu, M. Radoiu, I. Calinescu and H. Iovu PART V: MICROWAVE APPLICATIONS IN CHEMISTRY ROLE OF MICROWAVE RADIATION ON RADIOPHARMACEUTICALS PREPARATIONS.....................................................................................................359 Enrico Gattavecchia, Elida Ferri, Biagio Esposito, Alberto Breccia FAST SYNTHESIS OF BIODIESEL FROM TRIGLYCERIDES IN PRESENCE OF MICROWAVES ......................................................................................................370 C. Mazzocchia, A. Kaddouri, G. Modica, R. Nannicini ALTERATION OF ESTERIFICATION KINETICS UNDER MICROWAVE IRRADIATION........................................................................................................377 L. A. Jermolovicius, B. Schneiderman and J. T. Senise MULTISTEP MICROWAVE-ASSISTED SOLVENT-FREE ORGANIC REACTIONS: SYNTHESIS OF 1,6-DISUBSTITUTED-4-OXO-1,4-DIHYDROPYRIDINE-3-CARBOXYLIC ACID BENZYL ESTERS ................................................386 Mauro Panunzio, Maria Antonietta Lentini, Eileen Campana, Giorgio Martelli, Paola Vicennati RECENT APPLICATIONS OF MICROWAVE POWER FOR APPLIED ORGANIC CHEMISTRY ..........................................................................................................390 Bernd Ondruschka and Matthias Nüchter LIQUID PHASE CATALYTIC HYDRODECHLORINATION OF CHLOROBENZENE UNDER MICROWAVE IRRADIATION .........................................398 Marilena T. Radoiu, Ioan Calinescu, Diana I. Martin, Rodica Calinescu

Contents

XI

CONVENTIONAL AND NEW SOLVENT SYSTEMS FOR MICROWAVE CHEMISTRY ..........................................................................................................405 Jens Hoffmann, Antje Tied, Matthias Nüchter and Bernd Ondruschka PART VI: INDUSTRIAL MICROWAVE APPLICATIONS STATE OF THE ART OF MICROWAVE APPLICATIONS IN THE FOOD INDUSTRY IN THE USA.........................................................................................417 Robert F. Schiffmann MICROWAVE VACUUM DRYING IN THE FOOD PROCESSING INDUSTRY ................426 G. Ahrens, H. Kriszio, G. Langer DEVELOPMENT OF AN INDUSTRIAL SOLID PHASE POLYMERIZATION PROCESS USING FIFTY-OHM RADIO FREQUENCY TECHNOLOGY..........................436 Joseph W. Cresko, L. Myles Phipps, Anton Mavretic RF WORLD TOUR .................................................................................................445 Jean-Paul Bernard PART VII: FUNDAMENTALS OF MICROWAVE APPLICATION TO MATERIALS PROCESSING HOW THE COUPLING OF MICROWAVE AND RF ENERGY IN MATERIALS CAN AFFECT SOLID STATE CHARGE AND MASS TRANSPORT AND RESULT IN UNIQUE PROCESSING EFFECTS .........................................................................461 John H. Booske and Reid F. Cooper ENHANCED MASS AND CHARGE TRANSFER IN SOLIDS EXPOSED TO MICROWAVE FIELDS ............................................................................................472 V.E. Semenov, K.I. Rybakov THERMAL RUNAWAY AND HOT SPOTS UNDER CONTROLLED MICROWAVE HEATING .........................................................................................482 V.E. Semenov, N.A. Zharova DENSIFICATION AND DIFFUSION PROCESSES IN THE BA,SR-TITANATE SYSTEM UNDER MICROWAVE SINTERING ............................................................491 O.I. Getman, V.V. Panichkina, V.V. Skorokhod, E.A. Shevchenko, V.V. Holoptsev OBSERVATION OF THE MICROWAVE EFFECT ON THE DIFFUSION BEHAVIOR IN 28 GHZ MILLIMETER-WAVE SINTERED ALUMINA .........................498 Toshiyuki UENO, Yukio MAKINO and Shoji MIYAKE, Saburo SANO DILATOMETER MEASUREMENTS IN A MM-WAVE OVEN .......................................506 G. Link, S. Rhee, M. Thumm

XII

Contents

IN SITU DETERMINATION OF SHRINKAGE UNDER MICROWAVE CONDITIONS .........................................................................................................514 J. Bossert, C. Ludwig, J.R. Opfermann MULTISTABLE BEHAVIOUR IN MICROWAVE HEATING OF CERAMICS ..................521 J. R. Thomas, Jr., Xiaofeng Wu, W. A. Davis PART VIII: MICROWAVE SINTERING OF CERAMICS AND METALS MICROWAVE SINTERING OF SILICON NITRIDE CERAMICS ....................................533 Kiyoshi Hirao, Mark I. Jones, Manuel E. Brito and M. Toriyama NOVEL MATERIALS PROCESSING BY MILLIMETER-WAVE RADIATION PRESENT AND FUTURE .........................................................................................541 Shoji Miyake CORRELATION BETWEEN DENSIFICATION AND E - PHASE FORMATION AT MICROWAVE SINTERING OF SI3N4 CERAMICS ......................................................553 O. I. Getman, V. V. Panichkina, V. V. Skorokhod, I. V. Plotnikov, V. V. Holoptsev SINTERING BEHAVIOUR AND MECHANICAL PROPERTIES OF MICROWAVE SINTERED SILICON NITRIDE .................................................................................562 Mark I Jones, Maria-Cecilia Valecillos, Kiyoshi Hirao, Manuel E. Brito, Motohiro Toriyama MILLIMETER-WAVE SINTERING OF HIGH PURE ALUMINA – MICROSTRUCTURE AND MECHANICAL PROPERTIES .............................................570 Yukio Makino, Shoji Miyake, Saburo Sano, Hidenori Saito, Bunkei Kyoh, Hideki Kuwahara and Akinobu Yoshikawa MICROWAVE SINTERING OF LARGE-SIZE CERAMIC WORKPIECES .......................577 S. V. Egorov, N. A. Zharova, Yu. V. Bykov, V. E. Semenov MICROWAVE ASSISTED SINTERING OF AL2O3 ......................................................583 S. Leparoux, G. Walter; Th. Lampke, B. Wielage ABSORPTION OF MILLIMETER WAVES IN COMPOSITE METAL-CERAMIC MATERIALS ..........................................................................................................591 A. G. Eremeev, I. V. Plotnikov, V. V. Holoptsev, K. I. Rybakov, A. I. Rachkovskii MICROWAVE SINTERING OF PM STEELS ..............................................................598 F. Petzoldt, B. Scholz, H. S. Park, M. Willert-Porada FORMATION OF FUNCTIONALLY GRADED CEMENTED CARBIDES BY MICROWAVE ASSISTED SINTERING IN REACTIVE ATMOSPHERES ........................609 R. Tap, M. Willert-Porada, K. Rödiger, R. Klupsch

Contents

XIII

PART IX: SYNTHESIS AND MICROWAVE PROCESSING OF POWDERS MICROWAVE PLASMA SYNTHESIS OF CERAMIC POWDERS ...................................619 Dieter Vollath, D. Vinga Szabó MICROWAVE AND CONVENTIONAL HYDROTHERMAL SYNTHESIS OF ZIRCONIA DOPED POWDERS .................................................................................627 F. Bondioli, C. Leonelli, C. Siligardi, G.C. Pellacani, S. Komarneni MICROWAVE DECOMPOSITION OF METAL ALKOXIDES TO NANOPOROUS METAL OXIDES – A MECHANISTIC STUDY ..........................................................633 F. Bauer, T. Schubert, M. Willert-Porada CHARACTERIZATION OF SIC PRODUCED BY MICROWAVES ..................................645 Juan Aguilar, Zarel Valdez, Ubaldo Ortiz, Javier Rodríguez MICROWAVE ASSISTED SYNTHESIS OF CATALYST MATERIALS FOR PEM FUEL CELLS .........................................................................................................651 T. Schubert, M. Willert-Porada EXCITATION OF SODIUM IN POWDERLIKE SILICATES BY MICROWAVE HEATING ..............................................................................................................661 M. Hasznos-Nezdei, E. Pallai-Varsányi, L. P. Szabó and S Szabó PART X: NEW APPLICATIONS AND PROCESSES RELATED TO MICROWAVE AND RF HEATING RF AND MICROWAVE RAPID MAGNETIC INDUCTION HEATING OF SILICON WAFERS .................................................................................................673 Keith Thompson, John Booske, Yogesh Gianchandani, Reid Cooper INDUSTRIAL HIGHER FREQUENCY MICROWAVE PROCESSING OF COMPOSITE MATERIALS.......................................................................................681 Lambert Feher and Manfred Thumm DRILLING INTO HARD NON-CONDUCTIVE MATERIALS BY LOCALIZED MICROWAVE RADIATION .....................................................................................687 E. Jerby and V. Dikhtyar DESIGN OF AVIONIC MICROWAVE DE-/ANTI-ICING SYSTEMS .............................695 Lambert Feher and Manfred Thumm APPLICATION OF MICROWAVE TO GLAZE AND CERAMIC INDUSTRY ....................703 C. Leonelli, C. Siligardi, P. Veronesi, A. Corradi MICROWAVE ASSISTED BINDER BURNOUT ..........................................................710 J.Grosse-Berg, M. Willert-Porada, L. Eusterbrock, G.Ziegler

XIV

Contents

CVD-PROCESSES IN MICROWAVE HEATED FLUIDIZED BED REACTORS .............720 T. Gerdes , R. Tap, P. Bahke , M. Willert-Porada PROCESSING OF CARBON-FIBER REINFORCED COMPOSITE (CFRP) MATERIALS WITH INNOVATIVE MILLIMETER-WAVE TECHNOLOGY ....................735 Christian Hunyar, Lambert Feher and Manfred Thumm BASIC RESEARCH AND INDUSTRIAL PRODUCTION USING THE SPARK PLASMA SYSTEM (SPS) .......................................................................................745 Mamoru Omori COMBINED PROCESSES: LASER ASSISTED MICROWAVE PROCESSING AND SINTERCOATING ...................................................................................................755 M. Willert-Porada, T. Gerdes, Ch. Gerk, H.S. Park

AUTHOR INDEX .............................................................................................769 SUBJECT INDEX .............................................................................................773

Understanding Microwave Heating Systems: A Perspective on State-of-the-Art H. C. Reader Department of Electrical and Electronic Engineering, University of Stellenbosch, Private Bag X1, 7602, South Africa

Introduction An interesting picture emerges when published literature on state-of-the-art in microwave heating is studied over the last five-year period. A search using engines such as ScienceDirect and ISI Web of Science finds only 4 such papers bold enough to make the claim. The two papers relevant to the present discussion, [1, 2], reveal opposite extremes. [1] reviews research on high power microwave sources, some of which can be regarded as exotic. [2] examines microwave heating of milk, using a domestic oven, where concerns focus on milk proteins, enzymes, vitamins, micro-organisms and hazardous over-heating. Interestingly, researchers seem reluctant to make dramatic claims in terms of applicator design, computational methods, control methods and combinational sources of energy. If the search is broadened to “state-of-the-art in heating”, 61 papers are found of which 3 are relevant. It would seem that workers in our discipline become bolder if “state-of-the-art” is softened to novel. 66 papers can be found containing the words novel and microwave and heating. What are the reasons for this? One simple answer is that perspectives on how people regard state-of-the-art vary considerably. Another is that manufacturers, who for obvious commercial reasons need to emphasise high-tech features, do not publish freely in academic literature. A more detailed analysis is likely to reveal that general researchers making use of microwave heating are uncertain as to what “state-of-the-art” might be. Novel, on the other hand, suggests incremental improvements with particular features coming to the fore. It may also be found that after the initial introduction of commercial microwave ovens in the late 1940's [3], and after our understanding of applicator and source physics fundamentals reached a certain level, little room exists for significant developments. A more specific technical survey can be obtained from members’ millennium statements appearing in the Ampere Newsletters of January and April 2000. A sample of hopes and predictions include:

4

Reader

x using RF 50 Ohm technology (no adjustments necessary for different load conditions) x converting long-standing laboratory research into industrial applications x developing on-line process validation tools with non-invasive sensors x using microwave developments to improve the quality of our environment x developing numerical and optimization methods with attention to the userinterface x accepting that microwave applicators have reached maturity - use them as they are! Further insights on the matter are derived from feedback in the Ampere April 2000 letter on the 7th International Conference on Microwave and High Frequency Heating where delegates ask for more on food and industrial applications, EMC/EMI and health, medical applications, modelling relevance, measurement techniques, optimisation and control tasks, and microwave sources. A direct question on “state-of-the-art” to a wide variety of active workers [4] in the field elicits the following responses: x Bernard Krieger declares: “in my opinion the best applicator is the simplest applicator and the best overall system is a hybrid system that uses the minimum amount of microwave energy and the maximum amount of conventional heating”. Krieger adds further that customers are often subtly presented with the weaknesses of microwave units (this applicator DOES deal with nonuniformity!) and not their strengths. x R F (Bob) Schiffmann, after 40 years of experience, finds that in most cases applicators have been multimode cavities - big boxes. The reason? For successful processes the load has to be quite lossy; in such cases the multimode cavities are cheap, easy to build, forgiving to variations in the load and provide good enough coupling efficiency. He argues, for a variety of reasons, that where loads have low loss the likelihood of commercial success is small. Schiffmann [5] reviews the spectacularly poor predictions on the success of domestic microwave ovens and criteria for successful microwave systems (eg. product cannot be produced any other way). He also considers barriers, one of which is the lack of understanding of microwave heating and unrealistic expectations of what can be done. x A C (Ricky) Metaxas suggests that the specific application one is considering determines whether RF or microwave, and which type of applicator, is selected. If it is large, planar, broadloom material requiring drying, then a balanced or stray-field applicator at RF is the answer. If, on the other hand, it is a small liquid one is processing, then a TM010 applicator, etc. x R V (Bob) Decareau applied lateral thought to the question and raised the consideration of harnessing power gathered in space (solar space power, or SPS), international space stations, space agriculture and microwave freeze drying in space. J M (John) Osepchuk touched upon this theme in a JMPEE guest editorial in 1998 (Vol 33 (3)) entitled “Predictions and Breakthroughs”, where he stated that there has not been a major breakthrough in the field of microwave power since the microwave oven of the 1960s. The editorial offers “candidate

Understanding Microwave Heating Systems

5

breakthroughs” which include microwave lighting, SPS (it appears that Japan has committed to developing an SPS before 2040 [4]), a new microwave power source (solid-state [4]), and a microwave clothes dryer. With all the preceding material as quite contrasting background, what criteria could be applied reasonably to define a high frequency heating system to be stateof-the-art? To explore this question, some general characteristics should be sketched.

EM Properties of High Frequency Heating Systems This paper is concerned with microwave heating systems, but it is most important to emphasise that these frequencies are not always the sensible choice for this class of heating. Consideration must be given to frequencies in other ISM (industrial, scientific and medical) allocated bands, which can be found between 6.78 MHz and 245 GHz [6]. Some of these bands are narrow and others relatively broad. Metaxas and Meredith [7] define the microwave band between 300 MHz and 30 GHz (representing wavelengths of 1 m and 0.01 m respectively). Osepchuk [4] suggests an alternative where microwaves for heating refer to any source wavelength with similar dimensions to the object of interest. The latter highlights the distinction between quasi-static or dynamic system properties. Roughly speaking, if an applicator is a tenth of a wavelength or smaller than the source wavelength, quasi-static conditions hold. Electric and magnetic fields can then be thought of separately. If the applicator is larger, electromagnetic variations will become evident. For practical purposes the definition in [7] is used here. In the microwave band where an applicator has a dimension of the order of 0.5 m, field, current and heating nonuniformity must be considered. The two most commonly used microwave heating frequencies are around 915 MHz and 2.45 GHz. Excellent accounts of these microwave heating frequencies, with historical background, are given by Osepchuk [3] and Thuéry [8]. The lower end of the ISM bands, which includes frequencies of 6.78 MHz, 13.56 MHz, 27.120 MHz and 40.68 MHz (wavelengths of 44.2 m down to 7.37 m), is spoken of as the RF band. Apart from dimensional aspects, power absorption, arcing and energy penetration into an object are pertinent issues. In chapter 2 of [9] and elsewhere it is shown that the power absorbed per unit volume (power loss density) is proportional to the frequency and to the square of the electric field (E-field). This remark must be qualified. Assumptions are that: 1) load dielectric properties do not change with frequency and processing conditions; 2) frequencies are above those where ionic conduction dominates. Water’s dielectric properties, for example, change significantly with frequency. At microwave frequencies, where the assumptions are reasonable for illustrative purposes, to achieve the same absorbed power, a twenty-five-fold increase in the source frequency would lead to a five-fold decrease in the E-field strength. As breakdown in air is seen at a value of 30 kV/cm, at standard temperature and pressure (STP), any lowering in field strength reduces arcing problems. The breakdown level is af-

6

Reader

fected by load dielectric conditions and would also occur at a lower E-field values if pressure is reduced; vacuum is sometimes preferred for various processes. From this perspective, higher microwave frequencies may be better. Higher frequencies also promote heating uniformity in multimode applicators. Energy penetration into a product, however, is inversely proportion to frequency. The same twenty-fivefold increase frequency would lead to a five-fold decrease in penetration depth. Before proceeding with microwave applicators alone, the discussion should be balanced with a consideration of RF and microwave frequency, or high frequency (HF), heating systems generally. Material displacement currents (dipolar polarisation) and conductive currents occur; conductive currents usually dominate at RF and displacement currents at microwave frequencies. RF heating is loosely associated with drying (textiles, fibre-glass bales, paper and board), heating and welding; microwave heating with food processing (including tempering and defrosting), curing and drying [10]. The advantages of HF heating over purely conventional approaches are widely described. Thuéry [8] and Roussy and Pearce [11] cite rapid heating, volumetric deposition of energy, economy of energy due to specific heat deposition, space and human resource savings, reduced pollution, ease of application, instantaneous on and off operation, adaptation to existing operations, possible product quality improvement, and automation possibilities. In the field of drying, Metaxas [10], discusses mechanisms and makes the point that purely HF drying applications are unlikely. This is best illustrated in Figure 1. The dimensions of the object or volume to be treated, and load thermal and electrical properties, are significant factors in applicator design. The frequency choice is influenced by the total required power. Microwaves are commonly employed at low powers (1 - 20 kW). Higher powers (> 200 kW) usually make use of RF. In the food industry, Schiffmann [4] indicates that several hundred microwave installations exist for tempering and bacon cooking that reach up to 500 kW. Finances must also be considered; however RF and microwave are remarkably similar when considered en toto on a per watt basis [11]. In terms of RF generators, there are two classes [10, 11]: 1) Crystal controlled frequency oscillators with power amplifiers. RF transmitter technology can be used to generate large powers for industrial heating applications. This is channelled via a matching network (in coaxial cable of 50 Ohm) to the applicator or load; 2) Power oscillators (class C) in which the load is a part of the tank resonant circuit - variable matching is an implicit part of the design. The load greatly influences the operating frequency and high harmonic generation requires filters which must be designed to allow for frequency drift. In regions, such as the US, where the operating RF frequency may vary, the power oscillator is preferred as it is much cheaper - it also achieves higher efficiencies. In Europe with electromagnetic compatibility (EMC) requirements it is usually necessary to use the first option. In very broad terms, basic RF applicators include the through-field (eg. parallel plate), the stray-field or fringe-field and variations thereof. Because of the wavelengths involved, these are very much quasi-static systems. Matching circuits are required. The parallel plate suffers from E-fields normal to the upper workpiece surface. In high loss materials, the air gap E-field must be high to obtain heating;

Understanding Microwave Heating Systems

7

Mean Moisture Content

arcing is then a consideration, requiring careful electrode design. Thin objects are also difficult to heat. Stray-field and staggered though-field rods can be used both resulting in fields that are nearly parallel to load upper surface [11].

Constant drying rate

Falling drying rate 1. Conventional 2. HF only

2

Mc 3

4

3. Simultaneous 4. Sequential combination 1 Time

Fig. 1. Moisture removal through conventional, HF and combinational systems (after [10])

Main Classes of Microwave Applicators and Sources The focus now falls on microwave systems where there are three main classes of applicator: travelling wave, near field (often lightly-resonant) and resonant. Choice depends on the nature and dimensions of the material to be heated, subject to constraints of matching, field uniformity, safety aspects, industrial factors, etc. For high volume materials, the applicator is usually a multimode cavity whose dimensions are large compared to those of the material and wavelength. For small volume loads, energy coupling to an oversized cavity is particularly inefficient single mode cavities are used instead [8]. Travelling wave applicators make use of a matched waveguide which sustains a propagating wave in a well-defined fundamental mode. Material is then judiciously inserted into sections of the guide so as to intercept the propagating energy. Sheet and filamentary materials are commonly heated in this way. Near-field applicators can be open-ended waveguides, a variety of antennas, or slotted waveguide feeds. They are found in the processing of rock ores at very high power, through the heating of wood, foods etc., at medium power, right down to the treatment of cancers by heat (hyperthermia, see [12], for example). Resonant applicators, single or multimode, usually exist in rectangular and cylindrical form. Various polygonal shapes have been attempted, mainly with efforts made to affect mode distributions. Single mode cavities are often required for

8

Reader

higher value, smaller volume products, where processing speed is not the main variable. In general, for the same power applied, single mode resonant heaters will establish much higher electric field strengths and thus power densities (107 kW/m3) [7]. Under resonance, heating is very rapid. The multimode cavity is used for bulkier items in batch or continuous processes - they are the “big boxes” associated with Schiffmann earlier [4]. Whatever the intended purpose, the microwave heater has two essential components, an energy source and an applicator and then other system components, as depicted in Figure 2. The principal concern of a batch multimode applicator is heating uniformity. Computational studies and measurement can be helpful in this regard. Given the theoretical possibility of the number of modes that can exist within a cavity, it would be natural to expect more modes to exist within increasing dimension applicators. The theoretical possibility of modes existing and the excitation of those modes should not be confused. Mode stirrers and randomises improve heating non-uniformity in batch ovens, while in continuous or conveyorbelt multimode ovens, product movement accomplishes the same purpose.

Applicator (e.g., waveguide, antenna, multimode, single mode or slotted feed cavities) Conventional heating (e.g., hot air, infrared)

Control

Circulator/ isolator/ matching

Microwave source and power supply

Fig. 2. Generic microwave heating system blocks - not all may be present (after [9]).

In a single-mode applicator, concerns of heating uniformity are replaced with interest in whether enough material can be processed. Analytical design is more likely with this cavity. In single-mode circular waveguide geometries, the TM01p (TM010 typically) are the most popular because of the coaxially placed load. TM11n can deal with larger workloads because it has two eccentric heating locations. When dealing with real loads, hybrid modes are established, and this should be considered if careful design is attempted. Impedance matching (iris, automatic stub tuners, etc) in single mode cavities is important. The most common rectangular equivalent form of this applicator is the TE01n. There are some interesting variations reported: a spherical cavity for water droplet treatment; different waveguide input angles for matching into cylindrical applicators; grooved rectangular cavity TE113 for fusing alumina or glass rods 30 mm diameter in 3 minutes; cylindrical waveguide TM01 mode and internally divided into cavities by metal disks (each disk is simply an iris) [8]. Near-field applicators, such as slotted waveguide feed, are often constructed for conveyor-belt applications. They are designed to be weakly frequency dependent and are thus fairly insensitive to slight changes in dielectric properties of the materials. This lowered dependency gives more freedom in the parameters associated

Understanding Microwave Heating Systems

9

with the load, but in units such as slotted feeds, this comes at a price - that of design complexity. In the slotted waveguide feed, the slot interrupts the normal current flow on the waveguide walls in a specific manner, yielding a chosen radiation profile. The interslot spacing is forced by the wavelength of the source, but precise slot position, with respect to an imaginary central line drawn along the wall, determines the radiation and ultimately load’s heating contours [9]. Understanding the field distribution and modal formations within applicator classes can assist in their proper choice and usage. Examples can be found in [9] and some of these will be discussed at the conference presentation.

Measurement and Design Methods Power meters, diode detectors, multimeters, oscilloscopes, spectrum analyzers (SAs), automatic network analyzers (ANAs), S-parameters, voltage standing wave ratio (VSWR), field measurement and probing methods, material dielectric property determination (a significant field in its own right) and temperature measurement are all part of a basic toolbox available to people involved in microwave heating. Neophytou and Metaxas [13] refer to the ANA as the single most important advance in recent years in the design of HF processing systems. Roussy and Pearce [11] discuss microwave measurements broadly. Power measurement is the most fundamental measurement and is accomplished by bolometers (thermally sensitive), diodes (power sensitive) or calorimetric methods. Forward and reverse power, a sign of load energy absorption, is determined by a directional coupler. Frequency measurements are made with frequency counters and spectrum analysers. Field measurements are made with electric and magnetic antennas (also can use optical devices). Several common and ingenious temperature measurement schemes are available and will not be pursued here. The usage of SA’s, ANA’s, S-parameters (relatable to VSWR and impedance parameters) and field evaluation is extensively examined in [9]. Application of this knowledge, and practical, analytical and computational methods, forms the basis of design. Attention will be given to this during the conference presentation with pictorial support. Due to space restrictions, these illustrations will only be available from the author on request.

Computational Tools and the Role They Should Play In purely design terms, a widely published view is that simulation, combined with experimental and analytical evaluation of microwave cavities, forms an integral part of microwave heating studies. A defendable view is that most microwave heating applications simply require a box with suitably applied microwave energy. Computers can also be thought of on a wider front, for example in knowledgebased systems to aid design of system specification, or neural networks for on-line

10

Reader

process control. The following presents facets of what can be obtained through computation and measurement. In some applicators, single-mode being a good example, analytical expressions can be processed to yield a good description of heating operation. In other cases, analytical expressions are most awkward or even impossible to formulate, and here simulation is an invaluable tool allowing users to see what is happening inside cavities [9]. Main published methods applied to this field are the finite element method (FEM) in the frequency (FEFD) and in the time domains (FETD), the finite difference time domain method (FDTD), the transmission line matrix method (TLM), the method of moments (MOM), and the method of lines (MOL). Several pictorial examples of loaded and unloaded cavities can be found in [9] which demonstrate the properties of analytical, computational (FEM) and measurements studies. Metaxas [10] provides a helpful summary of techniques applied to microwave heating. Attention to validation of codes should always be given, as it is not uncommon to obtain plausible but quite inaccurate results. Groups such as the Institute for High Frequency Techniques and Electronics in Karslruhe Germany eg. [14] and others are coupling electromagnetic and thermal modelling within codes such as FDTD to simulate conventional, microwave and combined heating. This is no mean task and will certainly make significant contributions to optimised systems as the research matures. An important requirement will be knowledge of varying load dielectric properties with temperature. Knowledge based systems (KBS) can be thought of as the use of databases and specialist knowledge to emulate the thought process of an expert. An example quoted [10] is EHEAT: A package which examines options available to customers when considering applications which involve solely the use of HF. Apart from advising on the choice of equipment to be used, the package includes an economic assessment based on a cost-benefit analysis which gives payback periods for the various options. On-line process control is, and will be, an area significantly affected by computers. The development of advanced sensors (for example giving reliable indications of either localised or volumetric moisture content in wood), coupled with sophisticated control techniques has much scope. In this regard, approaches such as the use of neural networks, which can be trained, may be of assistance particularly under nonlinear conditions. An interesting example of neural networks is the determination of full penetration in laser welding [10].

Comments on Safety and EMI/EMC, and Practical Matters As this paper has an element of “state-of-the-art” the question of safety and electromagnetic interference (EMI) and EMC will be introduced with reference to related issues associated with SPS (solar space power), a concept first proposed in 1968 by Glaser (see [15]). In his well-referenced policy review article on the subject, Osepchuk [15] presents an account of electromagnetic energy safe usage and interference and sketches the worldwide towards international harmonization of

Understanding Microwave Heating Systems

11

standards. Osepchuk stresses the need for international standard setting, which must now be regarded as inevitable. In terms of safety, published recommendations in the microwave band mostly vary between 1 and 10 mW/cm2 and should be checked by a competent body in any given application. In terms of interference, the most recent out-of-band emission limits are found in the latest edition of CISPR 11 (see [6]), which is the standard on ISM equipment generated by the special committee under the auspices of the International Electrotechnical Commission (IEC). In safety and EMI/EMC terms, manufacturers and users of microwave heating equipment must expect to comply with more tightly defined legislation. Apart from source harmonic content, conducted emissions, applicator unintentional leakage (for example through seams), the design of chokes at product entry ports must enjoy specific attention. Vale, Meyer and Palmer [16, 17] have applied some advanced computational and measurement techniques to this subject. Shielding in general is an important constraint with financial implications. Meredith [18] provides useful sections on procedures for testing high power installations and equipment safety, and on economics and specification of industrial microwave equipment. When industrialising equipment, choice of material bears consideration. For applicator and waveguide walls, Metaxas and Meredith [7] suggest that for fairly heavily loaded conditions (Q-factor < 100), non-magnetic stainless steel is an excellent material - hard and basically free of corrosion problems. For lightly loaded conditions (Q-factor > 200), wall currents are such that stainless steel heats significantly. Materials of lower resistivity such as aluminium or copper should then be considered. Thuéry [8] states that it is imperative that supports, conveyor-belts, partitions, water seals, joints, etc., be made from very low loss dielectrics. The list of acceptable materials is in fact very restricted: polyethylene, polypropylene, polytetraflourethylene (PTFE, Teflon) and silicones [8]. Osepchuk [4] adds that under some circumstances, for example elevated temperatures, materials such as ULTE, polysulfone and composites, could be included. Where localised high power density conditions prevail, electric arcing and corona discharges are possible. Vacuum conditions exacerbate the problem. Precautions include: sharp angles and metals objects are to be avoided, quarter-wave traps can be used to reduce current to zero at critical points, soldered surfaces must be smooth and the enclosure must be free of traces of iron filings or dust. Industrially, sources and applicators usually exist in hostile environments and may need to be dust-proof, waterproof, resistant to corrosion by salt and acid, wide variations in temperature, shocks, and vibrations. Safety cut-outs to guard against mis-use are also needed [8].

State-of-the-Art and the “Best Applicator for the Job” Intimated in the preceding discussion is that the physics governing the behaviour of HF heating systems is really quite well understood and that no new dramatic

12

Reader

discoveries should be expected. This does not mean that new systems, through insightful and lateral application of thought, will not continually be found. Metaxas suggests that a state-of-the-art system making use of an RF or microwave applicator would certainly be a hybrid system where hot air, steam or other conventional heat is used, pressure or vacuum control may be provided, a cooler device may be used, and perhaps some kind of heat recovery process may be involved in the input or output of the system [10]. Thuéry [8] cautions that microwaves do not provide a universal solution, but should be considered whenever all other processes fail to solve an industrial problem - microwaves then become unique and offer considerable savings compared to other processes. Osepchuk and Decareau predict quite new developments (section 1). My own perspective is that there is scope within each element of the whole system for state-of-the-art features to be advanced. By way of illustration, consider the blocks of Figure 2, beginning with the source. This was originally the domain of tube devices. Thuéry [8] identifies that solid-state oscillators in microwave appliances has been the subject of a number of patents since the seventies. The power outputs of silicon transistors are of the order of 100 W at 950 MHz and 15 W at 2.45 GHz. These powers are likely to double in the near future [8] and should lead to an increase in the use of solid-state sources particularly in the medical and domestic applications. Power combiners could be used to obtain sufficient source levels. The cellular phone industry requirements have driven many of the developments in the field. This would fall directly in line with EMC trends requiring improved emission control and stability. In terms of the applicator and control blocks, there are several developments that are presently reported and that will continue to take place. Energy distribution within the load can always be optimised through computational and empirical study for a specific purpose. For the computational investigations, the determination of load constitutive properties as a function of temperature will need to be refined. With this knowledge, coupled electromagnetic and thermal codes will yield valuable information for process control. The work at Karlsruhe [14] is an example. In addition to this knowledge, clever control methodologies can be applied. One such method is the phase control method of Meier and de Swardt [19] whereby low power injection locking of magnetrons permits energy deposition to be actively moved through a load. Concerning overall system construction some novel work has been published by Vale, Meyer and Palmer [16, 17] on optimised choke design. Here mode cancellation principles and mode-matching analysis combined with generic algorithms are employed. 2.45 GHz chokes have been built and recently tested. This will be reported at the conference. Modern developments also include available industrial combinational heating such as the Microwave-Assisted Gas Firing (MAGF) unit reported by Bond [4], where applications in the ceramics field, requiring roughly 15% of microwave (896 MHz in industrial units) energy, result in faster throughputs, improved product quality and reduced flourine emissions. Krieger’s remark at the beginning of this paper [4] adds weight to these combinational approaches, emphasizing the minimum usage of microwave energy. Krieger provides further prudent counsel [4] that the critical items are related to what the customer must live with, including

Understanding Microwave Heating Systems

13

issues such as: how well conveyor-belts track in the oven; ease of loading products; cleaning; dealing with condensation, dust, fumes and hot air re-circulation; preventing fires and arcing (a tough issue). Schiffmann [5] examines criteria for successful adoption of new microwave applications, barriers to their adoption and improving the likelihood of success. His adage of “why microwaves?” rather than “microwaves!” suggests a sensible starting point. As to the question of “what is the best applicator for the job?”, academic interests of careful understanding of applicator behaviour must be offset with the industrial savoir-faire of the previous paragraph. Where microwaves can be demonstrated to provide unique advantages, trade-offs between product field profiles, economics and quality must guide the designer.

Conclusion It was suggested to me that more might be said by way of conclusion. This indeed would be appropriate in a defined area of dielectric heating. My view is that in some of these areas one best solution will be apparent and in others, several good choices may be made. I have thus presented perspectives on the subject. We must apply our minds appropriately, mindful of the physics and practicalities, to develop “state-of-the-art” microwave heating systems.

Acknowledgements J Binner for first contacting me to suggest this paper, M Willert-Porada, A C Metaxas (Ampere President) and J Binner for commenting on the paper’s development throughout, and J M Osepchuk, R V Decareau, R F Schiffmann, B Krieger, J von Hagen, N Tran, John Zimmerly and M Bond for substantial inputs and correspondence. Present and past colleagues and students in the Stellenbosch Electro-Heat Group including J B de Swardt, S M Bradshaw, T V Chow Ting Chan (co-author of recent book, [9]), J W Gerber, P Siebritz, I M Meier, M Rimbi, E van Wyk; Colleagues in our EEM Group including J H Cloete, D B Davidson, K D Palmer and P Meyer; Technical Colleagues W Croukamp, J C J Greyling, P Kruger and U Buttner.

Literature [1] S H Gold and G S Nusinovich, “Review of High-Power Microwave Source Research”, Rev. Sci. Instrum, 68 (11), Nov. 1997, pp. 3945 - 3974. [2] R Sieber, P Eberhard and P U Gallmann, “Heat Treatment of Milk in Domestic Microwave Ovens”, Int. Dairy Journal, 6 (3), Mar. 1996, pp. 231 - 246.

14

Reader

[3] J M Osepchuk, “A History of Microwave Heating Applications”, IEEE Trans. MTT, 32 (9), Sept. 1984, pp. 1200 - 1223. [4] Private communications with: J Binner, M Bond, R V Decareau, B Krieger, A C Metaxas, J M Osepchuk, R F Schiffmann, N Tran and M Willert-Porada. [5] R F Schiffmann, “Microwave Processes for the Food Industry” in “Handbook of Microwave Technology for Food Applications” edited by A K Datta and R C Anantheswaran, Marcel Dekker, New York (2001). [6] IEC CISPR 11, Industrial, Scientific and Medical (ISM) Radio-Frequency Equipment Electromagnetic Disturbance Characteristics - Limits and Methods of Measurement, 3rd edition, 1997, p. 13. [7] A C Metaxas and R Meredith, "Industrial Microwave Heating", Peter Peregrinus Ltd., London, UK, ISBN: 0-906048-89-3, 1988. [8] J Thuéry, edited by E H Grant, “MICROWAVES: Industrial, Scientific and Medical Applications”, Artech House Publishers, Boston and London, ISBN: 0-89006-448-2, 1991. [9] T V Chow Ting Chan and H C Reader, “Understanding Microwave Heating Cavities”, Artech House Publishers, Boston and London, ISBN: 1-58053-094-X, June 2000. [10] A C Metaxas, “Foundations of Electroheat: A Unified Approach”, Wiley, Chichester, UK, ISBN: 0-471-95644-9, 1996. [11] G Roussy and J A Pearce, “Foundations and Industrial Applications of Microwaves and Radio Frequency Fields: Physical and Chemical Processes”, Wiley, Chichester, UK, ISBN: 0 471 93849 1, 1995. [12] A W Guy, “History of Biological Effects and Medical Applications of Microwave Energy,” IEEE Transactions on Microwave Theory and Techniques, 32 (9), Sept. 1984, pp. 1182-1200. [13] R I Neophytou and A C Metaxas, “Characterisation of Radio Frequency Heating Systems in Industry Using a Network Analyser”, IEE Proc. Sci. Meas. Technol., 144 (5), Sept. 1997, pp. 215 - 222. [14] J Haala, J v. Hagen and W Wiesbeck, “Fast Implementation of Heat Radiation in a Self-Consistent FDTD Analysis Tool for Microwave and Hybrid Ovens”, Applied Computational Electromagnetics Society Journal, March 2001, pp. 215 - 222. [15] J M Osepchuk, “Microwave Policy Issues for Solar Space Power”, Space Policy, Vol. 16, Issue 2, May 2000, pp. 111-115. [16] C A W Vale and P Meyer, “Waveguide Chokes for Microwave Heating Applications”, MTT/EMC Section, SATCAM 2000, Lord Charles Hotel, Somerset West, Cape Town, South Africa, Sept. 2000, 4 pages, CD-ROM ISBN: 0-620-26497-7. [17] C A W Vale, P Meyer and K D Palmer, “A Design Procedure for Bandstop Filters in Waveguides Supporting Multiple Propagating Modes”, IEEE Trans. Microwave Theory Tech., Boston, Vol. 48, No. 12, Dec. 2000, pp. 2496 - 2503. [18] Roger Meredith, “Engineers’ Handbook of Industrial Microwave Heating”, IEE, London, UK, ISBN: 0-85296-916-3, 1998. [19] I Meier and J B de Swardt, "Synthesis of Heating Patterns by Interference of Microwaves”, International Journal of Electronics, 87 (6), pp.725 - 734, 2000.

Millimeter-Wave-Sources Development: Present and Future Manfred Thumm1,2 and Lambert Feher1 1

Forschungszentrum Karlsruhe, Institut für Hochleistungsimpuls- und Mikrowellentechnik (IHM), Postfach 3640, 76021 Karlsruhe, Germany 2 Universität Karlsruhe, Institut für Höchstfrequenztechnik und Elektronik (IHE), Kaiserstr. 12, 76128 Karlsruhe, Germany

Introduction From the point of view of standard microwave technology at the frequencies 0.915 GHz and 2.45 GHz, the need for using higher ISM frequencies like 5.85 GHz and 24.15 GHz for materials processing industrial applications has to be carefully verified with respect to special physical/engineering advantages or to limits the standard microwave technology meets for a specific problem. Costs evaluation of industrial millimeter (mm)-wave systems have to be competitive, not only to conventional heating, but also to standard microwave solutions. In general, the physical and technological advantages of mm-waves compared to standard microwaves (decimeter waves) are very well known: x Better coupling of nonmetallic, dielectric materials to the electromagnetic field due to the higher frequency and the increased loss tangent (stronger absorption and therefore faster heating). x Finer grain sizes (control of microstructure), enhanced mechanical or electrical properties. x Highly increased spatial field homogeneity in the applicator due to the shorter wavelength (applicators with stochastic field distribution). x Millimeter waves can be focused and targeted much better (spatial selective processing), optical transmission and antenna techniques are applicable. x Higher densities and dissociation rates in plasma-chemical processes are achievable. However, a crucial point for new industrial mm-wave systems at very high ISM frequencies is the availability of affordable sources with appropriate power, size, efficiency and costs. Fig. 1 schematically demonstrates the competition of conventional and high frequency (HF) systems. Recent publications showed a strong increase on development of power generators at the ISM frequency of 24.15 GHz. The present paper reports on the stateof-the-art and future of modern 24.15 GHz sources like magnetron, extended in-

16

Thumm

teraction oscillator (EIO), klystron, travelling wave tube (TWT) and gyrotron for industrial processing of materials.

Need for technical (process) sophistication/ Product quality

Research Mm-Wave Technology

Microwave Technology

Conventional Heating Technology

Development

System Costs

Fig. 1. Achievable process/product quality related to system costs for micro-, mm-wave and conventional heating systems

24.15 GHz Millimeter-Wave Sources The most common microwave sources for industrial applications are the magnetron (e.g. kitchen microwave oven) and the klystron (e.g. radar applications). At higher frequencies, the EIO, the travelling wave tube (TWT) and the gyrotron attract due to their unique properties. This paper tries for an elementary comparison of the significant differences of these vacuum electron tubes (single component) at 24.15 GHz with typical power levels < 10 kW (industrial power level) in terms of the tube's size, weight, efficiency and accelerating voltage requirements for a single component. Magnetrons Magnetrons are low-cost, efficient cross-field microwave oscillators used for generation of continuous-wave (CW) power. L-band (0.915 GHz) tubes deliver up to 90 kW, S-band (2.45 GHz) devices up to 30 kW at efficiencies of approximately 75% and 65%, respectively [1, 2]. However operating at frequencies around

Millimeter-Wave-Sources Development: Present and Future

17

24 GHz leads to power densities which limit the power output of a single device to approximately 0.2 kW at an efficiency of 35% [2]. The size of this magnetron would be 10 x 10 x 10 cm and the weight less than 2 kg.

Klystrons The state-of-the-art of high-power CW multi-cavity klystron amplifiers for industrial applications is summarized in Table 1 [1]. Table 1. High-power CW multi-cavity klystron amplifiers [1].

The primary factor that limits the maximum output power and efficiency of high frequency (24.15 GHz) klystron amplifiers is thermal detuning of the output and penultimate cavities due to HF heating. CPI has tested an air cooled permanent magnet focused klystron as high as 2 kW, CW at a center frequency of 29.312 GHz for earth-satellite communications. Design studies at CPI [3] show that a 4.5 kW, CW device at 24.15 GHz with a saturated gain of 53 dB and an efficiency of 30% (45% with multi-stage depressed collector) should be feasible. The size would be 20 x 20 x 27 cm and the weight 32 kg. Travelling Wave Tubes (TWT) In TWT amplifiers the periodic-permanent magnet focused electron beam experiences a long lasting and intensive interaction with a travelling wave in a broadband slow-wave structure. The overall efficiency is increased by the use of multi-stage depressed collectors.

18

Thumm

Wire and tape helix devices provide bandwiths up to an octave, whereas high output power coupled-cavity tubes have a bandwith of approximatly 7% (2.8 kW, 8 GHz, Gain = 55 dB from Hughes) [1]. Due to the very high costs of generators employing TWT amplifier tubes, we will not further consider this type of microwave tube in this paper. Nevertheless, broadband microwave tubes provide a great advantage in materials processing compared to single frequency tubes, since with them it is very easy to get a homogeneous microwave power density in an applicator. This is shown in Fig. 2 where the relative power density using a frequency modulated TWT (46 GHz) is compared to that of a fixed frequency magnetron (2.45 GHz) [4]. Lambda Technologies, Inc. of Morrisville, USA, has dedicated its process development activities to the commercialization of this patented microwave technique for processing advanced materials [5]. This processing method, known as Variable Frequency Microwave (VFM) Energy, has demonstrated to overcome the inherent problems found in attempts to apply conventional microwave technology based on fixed frequency irradiation to these applications. Lambda has focused on developing its technology for curing polymeric adhesives used in semicondductor and electronics manufacturing, which include, but are not limited to, flip-chip underfill, glob top, dam and fill, die attach, encapsulation, post mold cure and structural adhesion. Extended Interaction Oscillator (EIO) The EIO is a single-cavity device with a segmented drift tube. It can be seen as a coupled-cavity TWT with strong cavity-to-cavity coupling (hybrid tube). EIO's with depressed collector from CPI and the University of Hsinchu in Taiwan [6] achieve CW powers of 1-1.2 kW with an efficiency around 25% at frequencies of 28 and 24 GHz, respectively. The size of the devices is 10 x 10 x 20 cm and the weight is approximately 6 kg. Gyrotrons

Gyrotrons are microwave oscillators based on the Electron Cyclotron Maser (ECM) instability [1, 2]. The free energy for microwave generation is the rotational energy of a weakly relativistic helical electron beam (1<Jd1.2) in a longitudinal cavity magnetic field. A net transfer of energy from the gyrating electrons to the electromagnetic field in the interaction circuit occurs as a result of azimuthal phase bunching when the wave frequency is slightly larger than the relativistic electron cyclotron frequency or one of its harmonics. Many other types of microwave sources are also based on the ECM instability, such as the gyroklystron, gyro-twystron, gyro-travelling wave amplifier (gyro-TWT) or the gyro-backward wave oscillator (gyro-BWO).

Millimeter-Wave-Sources Development: Present and Future

19

Fig. 2. 2.4 GHz to 7.5 GHz broadband microwave heating [4].

Recently, gyrotrons have also been successfully utilized in materials processing (e.g. sintering of advanced nanocrystalline- and piezo-ceramics, surface hardening or dielectric coating of metals and alloys) as well as in plasma chemistry [8, 9]. Such technological applications require gyrotrons with the following parameters: f > 24 GHz, Pout = 10 - 30 kW, CW, K > 30%. The present state-of-the-art of in-

20

Thumm

dustrial CW gyrotrons for technological applications in the frequency range from 23 to 84 GHz is summarized in Table 2 [7, and references given there]. Table 2. Performance parameters of present industrial CW gyrotron oscillators for technological applications [7]. Institution CPI, Palo Alto CPI, NIFS Palo Alto, Toki GYCOM / IAP, Nizhny Novgorod

MITSUBISHI, Amagasaki

Frequency [GHz]

Cavity Mode

Output Mode

Power [kW]

Efficiency [%]

V [kV]

Magnet

28 28(2:c) 60 84

TE02 TE02 TE02 TE15,3

TE02 TE02 TE02 TEM00

15 10.8 30 50

38 33.6 38 14

40 30 40 80

roomtemp. roomtemp. cryo.mag. cryo.mag.

24.15(2:c) 24.15 23(2:c)

TE11 TE32 TE12

TE11 TE32 TE12

28.3(2:c) 30(2:c)

TE12 TE02

TE12 TE02

35 37.5 83 28(2:c)

TE02 TE62 TE93

TEM00 TEM00 TEM00

3.5 36 13 28 12 10 30 10-40 20 10-40

25 50 50 32 24 42 35 30-40 35 30-40

12 33 25 25 25 26 26 25-30 30 25-30

roomtemp. roomtemp. roomtemp. roomtemp. PM, 68 kg roomtemp. roomtemp. cryo.mag. cryo.mag. cryo.mag.

TE02

TE02

15

38.7

40

PM, 600 kg tapered B

Comparison of Different Millimeter-Wave Sources Fig. 3 shows a summary of potential 24.15 GHz, CW sources for technological applications and the Figs. 4 and 5 demonstrate an elementary comparison of these vacuum electron tubes (single component) with typical power levels < 10 kW in terms of tube's size, weight, efficiency and accelerating voltage requirement. In terms of produced mm-wave power, the gyrotron is the most efficient source at 24.15 GHz. With a single stage depressed collector efficiencies of 50% - 60% can be achieved. The EIO shows here the lowest performance. Another clear disadvantage of the EIO is obvious, if you consider the necessary high accelerating voltage per power. The gain in compact dimensions and weight is reduced by the need of oversized power supplies.As a result to their physical interaction principles, the EIO and the magnetron are most efficient for compact applications, where the component size is strictly limited. If one considers the component’s weight related to its microwave power, the EIO concept as well turns out to be the most lightweight source at 24.15 GHz (see Fig. 4). The use of gyrotrons appears to be of interest if one can realize a relatively simple, low cost device which is easy to use (such as a magnetron). Gyrotrons with low magnetic field (operating at the second harmonic of the electron cyclotron frequency) which can be provided by a permanent magnet system, low anode voltage, high efficiency and long lifetime are under development.

Millimeter-Wave-Sources Development: Present and Future

Magnetron

EIO

Klystron

21

Permanent-Magnet Gyrotron (30 GHz) IAP

2 kg

6 kg

32 kg

90 kg

200 W

1-1.2 kW

3-4.5 kW

> 10 kW

24 GHz

24 GHz

24 GHz

30 GHz

Fig. 3. Various possible 24.15 GHz, CW millimeter-wave sources.

Fig. 4. left: Power vs. size [W/cm3],

right: Power vs. weight [W/kg]

Fig. 5. left: Efficiencies of 24.15 GHz tubes, right: Accelerating voltage vs. power [kV/kW]

22

Thumm

Commercial Gyrotron Millimeter-Wave Systems At present, mm-wave power in CW operation of about 10 - 20 kW and, when necessary 100 kW, which can be of interest for industrial applications, can be only produced with gyrotrons. The first 10 kW 30 GHz gyrotron system, designed for the purpose of high temperature processing of materials was developed in collaboration of the Institute of Applied Physics (IAP) in Nizhny Novgorod, Russia, and the Research Center Karlsruhe (FZK) [10]. The system comprises the following parts (Fig. 6): gyrotron, oil-cooled electro-magnet system, quasi-optical and mode-converting transmission line, microwave furnace (applicator), temperature acquisition system and PC-based control system. Today, the 10 - 15 kW systems produced jointly by IAP and GYCOM, Ltd. (Nizhny Novgorod, Russia) are installed at research centers in USA and China. Later, several 10 kW, 28 GHz gyrotron systems were produced by CPI (USA) and Fuji Dempa Kogyo Corp. (Japan) and operate now at leading scientific centers of Japan and Australia. Some of the Fuji Dempa Kogyo systems employ Mitsubishi gyrotrons with permanent magnet (see Table 2). Low consumed power, decreased electron beam voltage and the use of switchmode power supplies make the new 3.5 kW, 24 GHz system of the IAP [11] compact and particularly convenient for university laboratory applications. The high lead of automatization of the system permits the user to be focused completely on materials science research and not on microwave engineering.

Fig. 6. Gyrotron oscillator technological system at Research Center Karlsruhe (FZK) (P = 15 kW, f = 30 GHz, CW).

Millimeter-Wave-Sources Development: Present and Future

23

Conclusions The present paper reports on the state-of-the-art and future of modern 24.15 GHz sources like magnetron, extended interaction oscillator (EIO), klystron and gyrotron for industrial processing of materials. In terms of produced mm-wave power, the gyrotron is the most efficient source. With a single stage depressed collector efficiencies of 50% - 60% can be achieved. The use of gyrotrons appears to be of great interest if one can realize a relatively simple, low cost device which is easy to use (such as a magnetron). Gyrotrons with low magnetic field, operating at the second harmonic of the electron cyclotron frequency, which can be provided by a permanent magnet system, low anode voltage, high efficiency and long lifetime are under development. 3.5 kW, 24.15 GHz and 10 - 15 kW, 28 GHz and 30 GHz technological gyrotron systems are commercially available. At lower power levels magnetrons (d 0.3 kW), EIO's (1 kW) or klystrons (2 - 4 kW) seem to be feasible.

References [1] Thumm, M. (1998): Moderne Mikro- und Millimeterwellenquellen für die Materialprozeßtechnik, in "Mikrowelleneinsatz in den Materialwissenschaften, der chemischen Verfahrenstechnik und in der Festkörperchemie", Monika Willert-Porada, ed., Shaker Verlag, Aachen, 2-28. [2] Richardson Electronics GmbH (2001), www.industrielle-mikrowelle.de Wright, E.L. (2000), private communication and www.cpii.com [3] Lauf, R.J., D.W. Bible, A.C. Johnson, C.A. Everleigh (1993): 2 to 18 GHz broadband microwave heating systems, Microwave Journal, November, 24-34. [4] Lambda Technologies, Inc. (2001), www.microwave.com [5] Chen, H.Y., L. Chen, M.H. Tsao, K.R. Chu (1999): A novel Ka-band extended interaction oscillator, Proc. 24th Int. Conf. Infrared and Millimeter Waves, Monterey, CA, USA, Paper TU-E4. [6] Thumm, M. (2001): Novel applications of millimeter and submillimeter wave gyrodevices, Int.J. Infrared and Millimeter Waves, 22, 377-386. [7] Gapanov-Grekhov, A.V., V.L. Granatstein, eds. (1994): Applications of high-power microwaves, Artech House, Boston, London. [8] Thumm, M. (1997): in "Generation and application of high power microwaves", eds. R.A. Cairns and A.D.R. Phelps, SUSSP 48, Inst. of Physics Publishing, Bristol, Philadelphia, 305-323. [9] Bykov, Yu., A. Eremeev, V. Flyagin, V. Kaurov, A. Kuftin, A. Luchinin, O. Malygin, I. Plotnikov, V. Zapevalov, L. Feher, M. Kuntze, G. Link, M. Thumm (1995): Gyrotron installation for millimeter-wave processing of materials, Proc. ITG-Conference Vacuumelectroncis and Displays, Garmisch-Partenkirchen, 103-108. [10] Bykov, Yu., G. Denisov, A. Eremeev, M. Glyavin, V. Holoptsev, I Plotnikov, V. Parlov (2001): 3.5 kW 24 GHz compact gyrotron system for microwave processing of materials, this book.

3.5 kW 24 GHz Compact Gyrotron System for Microwave Processing of Materials Yu. Bykov, G. Denisov, A. Eremeev, M. Glyavin, V. Holoptsev, I. Plotnikov, V. Pavlov Institute of Applied Physics, Russian Academy of Sciences, Nizhny Novgorod, Russia

Introduction A number of technological processes, primarily high-temperature ones, may benefit from employing microwave power at frequencies much higher than the frequencies traditionally used today in industrial applications (d 2.45 GHz). Summarizing briefly the results of studies of processes implemented with the use of the millimeter-wave (mm-wave) power of frequencies 24 - 100 GHz, that have been accomplished by research groups in Germany, Japan, Russia, U.S.A., one may highlight several specific features of the mm-wave processing. Since the power absorbed in the unit volume of material is, at least, proportional to the frequency even materials of very low loss such as high pure alumina can be heated effectively with mm-waves. In an applicator with the ratio of L/Oa50-100 (L is the dimension of applicator, O is the wavelength of radiation), several hundred modes are excited simultaneously. The superposition of the electromagnetic fields of these modes results in rather high uniformity of the microwave energy distribution in the whole volume of applicator. The uniform distribution of microwave energy opens the way for the volumetric heating of large-size specimens and for reproducible heating of many specimens in one batch in such processes as ceramics sintering. An effect of temperature runaway inherent in heating of materials with sharp temperature dependence of microwave absorption becomes less of a problem in the mm-wave range [1]. All these factors have played the decisive role in the experiments described [1, 6] where examples of the mm-wave sintering of large-size (~20 cm) and intricate shape specimens of high pure alumina are given. The processes based on surface heating of materials become possible with the use of mm-wave radiation [2, 5]. When radiation is focused into a spot of about O2, an intensity of 104 - 105 W/cm2 is achievable with mm-wave sources. So high intensity makes the mm-wave beam processing much like infrared laser processing with due regard to the difference in interaction of materials with mm-wave and infrared radiation.

3.5 kW 24 GHz compact gyrotron system for microwave processing of materials

25

At present, the mm-wave power in continuous wave regime of about tens and, when necessary, hundred kilowatts, which can be of interest for industrial applications, can be produced with gyrotrons. The first 10 kW 30 GHz gyrotron system, designed purposely for high-temperature processing of materials was developed at the Institute of Applied Physics (IAP), Russia in 1993. Its detailed description is given elsewhere [3, 4]. Today, the 10 / 15 kW systems produced jointly by IAP and GYCOM, Ltd. (Nizhny Novgorod, Russia) are installed at the research centers of Germany, USA, and China. Later, several 10 kW 28 GHz gyrotron systems were produced by the Fuji Dempa Kogyo Corp. (Japan) and operate now at leading scientific centers of Japan. Despite many groups are involved into the research in this field, the stage of development of the mm-wave processing of materials is rather far from commercialization. Evidently, this fact is a particular case of the more general problem of commercialization of microwave technologies. It is well known how many roots have this problem [7]. In accord with the discussion [7], it is safe to assume that microwave or mm-wave technologies have a chance to replace the traditional ones, if only the replacement does lead to drastic improvement in properties of materials and /or in economic validity. Therefore, when considering the line of investigation for application-driven mm-wave processes, it is of paramount importance to pursue not so a comparison between microwave (or traditional) and mmwave processes, as a search of radically new processes in which the use of mmwave radiation is indispensable. The aim of the work reported here was to develop a gyrotron system capable of becoming a versatile, flexible and user-friendly tool for laboratory research into the mm-wave processing of materials. The experience accumulated over a number of years shows that investigations of the vast majority of processes can be successfully conducted at the power level well below 3 kW, when the frequency of microwaves is about 24 GHz or higher. Low consumed power, decreased electron beam voltage and the use of switch-mode power supplies make the developed 3.5 kW 24 GHz system compact and particularly convenient for laboratory application. The operation at the frequency of 24 GHz allocated for applications facilitates direct transfer of the results of laboratory research to the industrial scale. The level of microwave engineering of the System permits the user to be focused completely on the materials science research.

Gyrotron System The gyrotron system (Fig. 1) is designed as an integrated set of the following principal components: x a 24 GHz 3.5 kW gyrotron, the source of the mm-wave power, x a transmission line for transport of mm-wave power to an applicator, x an applicator of the volume of about 80 litres for operation in the 10-4 – 1.5 bars pressure range, equipped with changeable mirrors,

26

Bykov

x a temperature diagnostics and PC-based control subsystems for automatic/manual modes of processing, x a set of power supplies, x a microprocessor-based unit maintaining failure-free performance of the system.

Fig. 1. The 3.5 kW 24 GHz gyrotron system

Gyrotron. The gyrotron operates on the TE11 mode. The gyrotron has a diode electron gun. The output circular waveguide has a diameter of 20 mm. The output power of the gyrotron is smoothly varied in the range from 0.1 kW to 3.5 kW by variation of the electron beam voltage. In the regime of materials processing the output of high voltage power supply is governed by a PC-based control system. The fine adjustment of the electron gun for optimal performance of gyrotron is achieved using the additional coil mounted over the gun. The electron beam current is stabilized at any pre-set value by a specialized power supply having a feedback loop. The dependencies of the output microwave power and efficiency on the electron beam current is shown in Fig. 2. The gyrotron operates at the 2nd harmonic of the electron cyclotron resonance. This allows reduction of the energy consumption by the water-cooled main magnet almost 4 times as compared with performance at the fundamental electron cyclotron resonance. In addition, the power supply and cooling system for such a magnet becomes appreciably more compact and much less operationally strained. The gyrotron operates with effi-

3.5 kW 24 GHz compact gyrotron system for microwave processing of materials

27

ciency about 23% in the regime of the maximal output power at the electron beam voltage as low as 12 kW. 25 20

3

15 2 10 1

Efficiency, %

Output power, kW

4

5

0

0 0

0.5

1

1.5

Beam current, A Fig. 2. The dependence of the output microwave power (-*-) and efficiency (-|-) of gyrotron on the electron beam current

Transmission line and microwave applicator. The output mm-wave power is transported from gyrotron to the applicator through a transmission line built up of the oversized circular waveguide components. The transmission line includes the following components: a polarizer, a waveguide bend, a mode filter, a mode converter, and a launcher. The mode filter protects the gyrotron against the microwave power that can be scattered back from the applicator. The mode filter is cooled with flowing water and furnished with a set of temperature sensitive elements and calibrator, which allows measuring the back-scattered microwave power and using this parameter as an indicator of the gyrotron operation. The mode converter transforms the operating mode of gyrotron, TE11, into a quasigaussian wave-beam. Further on, the wave beam illuminates the surface of a mirror that is installed in the upper part of the applicator (Figs. 3a, 3b). An efficiency of microwave power transport in the transmission line exceeds 95%. The gyrotron system features new application-driven capabilities. Both volumetric heating by uniformly distributed microwave energy and surface heating with a focused wave beam of high intensity can be performed in its applicator. This enhanced capability is achieved by the alternative use of changeable mirrors for directing microwaves to the volume of applicator. To perform surface heating of samples (Fig. 3a), the focusing mirror is installed that concentrates microwave power in the spot with size of about (2O)2 located in the central region of applicator. At maximal output power the intensity of the wave beam in the focal spot is about 1 kW/cm2, which is close to the threshold intensity for ignition of the near-surface microwave discharge at surface heating of most of dielectrics.

28

Bykov

Fig. 3. Schematic diagram of surface (a) and volumetric (b) heating.

High uniformity of the microwave energy distribution in an applicator is an essential prerequisite for the volumetric heating of large-size samples and for reproducible heating of many samples in one batch in such processes as ceramics sintering. The applicator used in the system can be treated as an untuned multimode cavity with the ratio (L/O)3 a5·104. A ray tracing analysis has been conducted to optimize the geometry of the inner wall of the applicator and the position of the rotating wavebeam scatterer. A water-cooled aluminum insert with specific geometry of walls is mounted in the applicator when a process based on volumetric heating is to be performed. High uniformity of the microwave power distribution is achieved owing to high Q-factor of cavity, special geometry of walls of the insert, and the use of moveable wave beam scatterer. A water-cooled dummy load is installed instead of mirror into the upper part of applicator for measurement of the total microwave power. The load is furnished with a set of thermosensors, and AC heaters for calibration. Temperature diagnostics and process control. The system is supplied with a PC-based 12-channel control subsystem, which makes it possible to control the materials processing in both automatic and manual modes. A code, specially developed on the basis of LabWindows 5.0, supports monitoring of both the temperature of material under processing and the major gyrotron operation parameters: voltage and current of the electron beam, output mm-wave power, and the DC current in the main magnet. The high-temperature thermocouples and optical pyrometer can be used to measure the temperature of material under processing. The control subsystem adjusts the output power of gyrotron so that the actual temperature of the material undergoing processing follows the temperature-time schedule of heating, which is preset by the operator. The accuracy of maintaining the temperature of ceramic samples at the hold stage of sintering (T | 1600°C) is no less than 0.2% when either the thermocouple or the optical thermometer are used for measurements. Auxiliary equipment. The gyrotron facility is equipped with the sub-system for closed-cycle water cooling of the gyrotron and main magnet. This makes the gyrotron facility independent on the purity of water available at the laboratory. The

3.5 kW 24 GHz compact gyrotron system for microwave processing of materials

29

facility has also a microprocessor based sub-system for control over parameters in the power and water supply lines that are of vital importance for the operation of the gyrotron system. This device secures the start-up of the gyrotron system in accordance with a pre-set algorithm, and automatic shut-off of the main power supplies in case of failure in the water-cooling and/or AC power source.

Materials processing On the whole, the results of an investigation into processing of materials on the developed gyrotron system have proved its potential to extend markedly the field of application of the millimeter-wave radiation. The system proved a reliable and versatile instrument for research and pilot tests of the millimeter-wave-assisted processes. High-temperature ceramics sintering, curing of polymers, rapid annealing of semiconductor wafers were among the processes that demonstrated both high uniformity of microwave energy distribution in the applicator and high efficiency of using the microwave power of enhanced frequency in heating of various materials. As an illustration, shown in the Fig. 4 is a sintered aluminabased ceramic insulator 115 mm in diameter and 105 in height.

Fig. 4. An alumina-based ceramic insulator sintered in a gyrotron system. For comparison an unsintered specimen is shown on the right

The specimen was sintered at a temperature of 1580qC with a 10 min soak time. No distortion of shape was observed in the sintered specimen. Shown in Fig. 5 is a control system screen display at the end of sintering of high pure alumina disk 50 mm in diameter. It should be noted that, despite high pure alumina has very low microwave absorption, the maximal power at sintering did not exceed one kilowatt.

30

Bykov

Fig. 5. A control system screen display at the end of high pure alumina disk sintering.

References [1] Bykov Yu, Rybakov K, Semenov V (2001) High-temperature microwave processing of materials. J. Phys. D: Appl. Phys. 34 : R55-R75 [2] Bykov YuV, Gol'denberg AL, Flyagin VA (1990) The possibilities of material processing by intense millimeter-wave radiation. In : Snyder WB, Sutton WH, Iskander MF, Johnson DL (eds) Microwave processing of materials II, Material Research Society, Pittsburgh, PA, pp 41-42 [3] Bykov Yu, Eremeev A, Flyagin V, Kaurov V, Kuftin A, Luchinin A, Malygin O, Plotnikov I, Zapevalov V (1995) The gyrotron system for ceramics sintering. In: Clark DE, Folz DE, Oda SJ, Silberglitt R (eds) Microwaves: Theory and applications in materials processing III, The American Ceramic Society, Westerville, OH, pp 133-140 [4] Bykov Yu, Denisov G, Ereemev A, Gol'denberg A, Holoptsev V, Luchinin A, Semenov V (1999) Upgraded gyrotron system for millimeter-wave processing of materials. In: Proceedings of the 29th European microwave conference, Munich, Germany, l: 123-126 [5] Dekster MMG, Paton BE, Sklyarevich VE (1993) Gyrotron processing of materials. MRS Bulletin 18 : 58-63 [6] Saji T (1996) Microwave sintering of large products. In: Iskander MF, Kiggans JO, Bolomey J (eds) Microwave processing of materials V, Material. Research Society, Pittsburgh, PA, pp 15-20 [7] Tinga WR (1997) Microwave and RF energy utilization - an experts and audience perspectives. In : Clark DE, Sutton WH, Lewis DA (eds) Microwaves: Theory and application in materials processing IV, The American Ceramic Society, Westerville, OH, pp 715-725

Design Guidelines for Applicators Used in the Microwave Heating of High Losses Materials Juan V. Balbastre, E. de los Reyes, M. C. Nuño and P. Plaza

Abstract Multiple port travelling wave applicators and distribution networks are tested for improved microwave heating of materials with high dielectric loss factors, based on simulation using a commercial code, ANSYS¥ 5.6. Single point feed applicators, phase shifted by a distribution network appear to be superior over multiple feeding point applicators.

Introduction This work is focused on development of applicators for microwave heating of materials with high dielectric loss factors. As it is well known, microwave heating is caused by the electromagnetic power absorption inside dielectric materials due to polarisation phenomena [1]. For high loss materials, since the electric field decays exponentially as the electromagnetic waves penetrate inside them, the heat generated at the material surface will be greater than into the material body, following the local electrical field strength. If the thermal conductivity of the material is low, temperature gradients will develop, that can affect the physical properties of the material. In applications, where temperature uniformity is important, this behaviour can restrict the feasibility of microwave heating. Furthermore, undesired thermal phenomena, like hot spots and thermal runaway, can be found in high loss materials subjected to microwave heating. Hot spots occur when the electrical field intensity is higher somewhere inside the dielectric as compared to the surrounding material and the temperature rises there faster than in other parts of the material. If the loss factor increases with temperature at the same time, thermal runaway, that leads to localised or complete destruction of the material will occur. Microwave heating of nylon or food defrosting are some examples of materials where runaway processes can appear. More examples and a detailed description of the runaway problem can be found in [1]. In order to avoid such unwanted effects, particular methods for applicator design were developed in the past, to enable either a homogeneous distribution of the electric field in the parts to be heated or to provide measures to better distribute the thermal energy evolution with time. Research work, carried out in the Microwave Heating Group at the Technical University of Valencia, derived some new

32

Balbastre

ideas, based on simulation, for an appropriate design of applicators for this kind of materials.

Simulation parameters Microwave heating of a 5 cm diameter and 1 cm height cylindrical sample of material with relative permittivity Hr = 13–j3.5 has been considered as a representative example for the simulation. The dielectric constant corresponds to a powder material with high water content, typical of some food products. Different applicators have been analysed by means of a Finite Element Method (FEM) to obtain the electric field amplitude in a closed region, using the ANSYS¥ 5.6 electromagnetic field simulator. All calculations are based on an operating frequency of 2.45 GHz and a TE10 excitation, assumed as the only mode existing at the cavity ports with a 1 V/m peak to peak amplitude electric field. A magnetic wall is used in order to reduce the number of unknowns in the FEM simulation by exploitation of the symmetry of the problem and shortening the simulation time without loss of accuracy.

Results and discussion Applicators with a different number of incident fields were analysed. Simulation results on the electric field distribution in V/m in a one-port travelling wave applicator are shown in Figure 1. In this case a matched standard WR340 waveguide was taken as an example. The waveguide is fed from the right and the dielectric sample is placed in the middle of the waveguide using a cylindrical holder made of PTFE, a microwave transparent material with Hr = 2.56–j0.0001. In Figure 1 a detail of the field inside the sample is included. It can be seen that, since the sample is smaller than the waveguide, the standing wave is negligible. However, the field distribution inside the sample is not uniform: the electric field is higher in the surface region of the sample than in its centre. This non-uniformity in the field distribution, due to the absorption of electromagnetic power in the surface, will produce a non-uniform heating, since only a residual part of the electromagnetic power will be transformed into heat in the body of the sample. An improvement of the field distribution uniformity is achieved by reducing the waveguide height to 1 cm. This reduction of the waveguide dimensions does not affect the normal operation, since the TE10 cut-off frequency, propagation constant and impedance do not vary with the waveguide height.

Design Guideliness for Applicators Used in The Microwave Heating

33

0.02 V/m

1.44 V/m Fig. 1. Electric field strength (V/m) inside a one-port travelling wave guide applicator, based on a WR340 waveguide with a cylindrical sample of high loss material in the middle of the waveguide. The waveguide is fed from the right side with a TE10 mode. Electric field strength in the waveguide ranges between 0.017 V/m to 1.44 V/m.

In Figure 2, the electric field strength, in V/m in the waveguide and inside the sample for a TE10 incidence from the left side of the image is shown, revealing significant differences with respect to the previous configuration. The electric field does not vary in the vertical direction, because both the geometry and the excitation are invariant in this direction. However, since in this case the sample occupies a significant portion of the waveguide volume, a strong standing wave appears in the left part of the waveguide. The electric field decays due to the electromagnetic energy absorption inside the material sample as the wave propagates across it from left to right, with a higher electric field strength on one side of the sample. Consequently, heating homogeneity will not be improved. Both example offer an interesting insight into the problem of designing applicators for microwave heating of high loss materials, however, they are of very limited practical use unless the thermal diffusion inside the material compensate the strong field differences inside the material. In a further simulation approach for a more uniform field distribution inside the material, the same non-standard wave guide is fed simultaneously from the two sides, using the TE10 mode. The results are shown in Figure 3.

34

Balbastre

0.003 V/m

1.65 V/m Fig. 2. Electric field strength inside a non-standard 86 mm wide, 10 mm high waveguide with a cylindrical sample of high loss material in the middle of the waveguide. The waveguide is fed from the left side with a TE10 mode. Electric field strength in the waveguide ranges between 0.003 V/m to 1.65 V/m

0.004 V/m

y 1.77 V/m Fig. 3. Electric field strength (V/m) inside a non-standard 86 mm wide, 10 mm high waveguide with a cylindrical sample of high loss material in the middle of the waveguide. The waveguide is fed from both the left and right sides with a TE10 mode. Electric field strength in the waveguide ranges between 0.004 V/m to 1.77 V/m.

The detailed picture of the sample reveals a standing wave inside the sample, produced by the field interactions. This is due to the fact, that the sample radius is

Design Guideliness for Applicators Used in The Microwave Heating

35

slightly bigger than one wavelength inside the material at the operating frequency. One maximum appears in the centre and two other near the surface of the sample in the propagation direction. However, in the x direction there is a variation of amplitude that can not be suppressed since it is due to the transverse variation of the TE10 mode. It should be pointed out, that in this configuration only a small fraction of the incident power from one of the two generators is absorbed by the sample, and all the remaining microwave power will be transmitted to the other generator.

0° phase shift 0.05 V/m

1.60 V/m

Fig. 4. Electric field strength inside the cross applicator with 0º phase shift between the two arms. Electric field strength in the waveguide ranges between 0.05 V/m to 1.60 V/m.

Without additional means this applicator concept is of only limited practical value. Although the two-port applicator shown in Figure 3 may not be applicable for processes where very uniform temperature distribution in low thermal conductivity materials is needed, the basic idea of an optimised applicator design for a more uniform field distribution can be further developed, e.g., by the use of more incident fields and by combining them to obtain the desired field uniformity. As an additional optimisation parameter, a phase shift between the incident fields is introduced, as shown in Figures 4-8. The basic geometry consideration of this approach is to combine two sections of a waveguide with an 86 mm x 10 mm cross-section to form a cross, as shown in Figure 4. For the calculation, the incident field at each port has 1 V/m peak to peak amplitude, like in the previous examples, while the phase is modified in order to achieve a homogeneous field distribution inside the applicator. In Figures 5, 6, 7 and 8 the field distribution inside the dielectric sample was calculated for different phase shifts: 0º, 45º, 90º, 105º, 120º, 135º and 180º, respectively. By introducing a phase shift, a great diversity of field patterns can be obtained, some of them with quite good uniformity.

36

Balbastre

However, the field distribution is quite sensitive with respect to the phase shift, and for some particular values the field uniformity can be severely reduced, as shown in Figure 8.

Fig. 5. Electric field amplitude inside the sample for a cross applicator with 0º and 45° phase shift between its arms. The amplitude of the electric field ranges between the values given in the figure.

Fig. 6. Electric field amplitude inside the sample for a cross applicator with 90º and 105° phase shift between its arms. The amplitude of the electric field ranges between the values given in the figure

In Figure 9 simulation results are presented for one possible alternative, with a single feeding point and a distribution network that can provide the necessary phase shifts to obtain a satisfactory field profile. For practical purposes, the power level needed and the internal loss of the distribution network will represent the key components of a real optimisation of such an applicator concept. Therefore, a sufficient control of the relative phases must be guaranteed, if such a solution is to be implemented in an actual applicator. Using several independent power oscillators does not guarantee the phase coherence needed in this applicator for real applications. As with the two-port travelling wave guide applicator, the efficiency of power consumption must be taken into account.

Design Guideliness for Applicators Used in The Microwave Heating

37

For a small sample in the cross-applicator feed from four ports the electromagnetic power that will be transformed into thermal energy will be small, although the loss factor is high. Therefore, a major portion of the energy will be transmitted to the generators, causing a serious problem with respect to the lifetime of the hardware besides the poor energy efficiency of such a process.

Fig. 7. Electric field amplitude inside the sample for a cross applicator with 120º and 135° phase shift between its arms. The amplitude of the electric field ranges between the values given in the figure.

Fig. 8. Electric field amplitude inside the sample for a cross applicator with 180º phase shift between its arms. The amplitude of the electric field ranges between 0.008 V/m and 0.51 V/m.

Therefore, a single fed applicator should be favored, with some additional measures to improve the homogeneity of the electric field distribution, as shown in Figure 9.

38

Balbastre

0.005 V/m

1.58 V/m Fig. 9. Optimised single fed applicator for high loss materials. The amplitude of the electric field shown in the figure ranges between 0.005 V/m and 1.58 V/m.

Conclusions Microwave heating of high loss materials is a quite complex task, because of the limited electrical field uniformity within the material to be heated. Particularly for large samples of low thermal conductivity materials electrical field distribution will completely control the homogeneity of microwave heating, with an increased probability of developing hot-spots or thermal runaway. Based on simulation, the usefulness of multiple ports in travelling wave guide applicators is tested, with respect to sample size and phase shifts. Some general guidelines for applicators design are presented that can help to arrive at an improved electric filed distribution in the high loss material sample. The results achieved for electrical field amplitude analysis using a commercial code, ANSYS¥ 5.6, are discussed with respect to the efficiency of energy utilisation with respect to hardware life time in case of multiple port systems or energy consumption by a distribution network in case of phase shifting devices.

References [1] A. C. Metaxas A C, Meredith R J (1987) Industrial Microwave Heating. Peter Peregrinus Ltd, London.

Design Parameters of Multiple Reactive Chokes for Open Ports in Microwave Heating Systems J. M. Catalá-Civera, P. Soto, V.E. Boria, J. V. Balbastre, and E. de los Reyes Microwave Heating Group, Technical University of Valencia, Camino de Vera s/n, E-46022 Valencia, Spain

Abstract Corrugated chokes provide one of most effective techniques to avoid leakage through open ports in microwave heating systems. The traditional analysis and design of these filters is based on monomode equivalent representations of the elements integrated in the structure. These representations usually lead to approximate responses, particularly with multiple corrugations, which recommend the use of multimode or more complete simulation techniques if highly accurate responses are desirable. Here, the effect of the designing parameters dimensions of these corrugated structures in the final response of the filter are analysed comparing the results given by a full-wave simulator with those provided by the approximate method. These effects as well as others, such as the effect of the material in the electrical specifications of the filters are discussed in this paper and design guidelines are recommended as a consequence of this study.

Introduction In microwave heating systems, there is a major concern about the hazardous effect of microwave energy leakage on human tissues since human tissue is highly receptive to microwave radiation. Continuous flow microwave heating systems are very prone to radiation because they incorporate open ports trough the material enters and leaves the applicator. To preserve the radiation of these open-ended waveguide systems into permissible levels, one of the most widely used techniques is based on reactive corrugated chokes. Figure 1 shows the schematic of a typical doubly corrugated choke. As depicted, the filter basically consists of a periodic configuration of slots in longitudinal and transversal directions, thus shaping a periodic structure of square blocks on the top of the waveguide. Reactive corrugated chokes reflect back the energy escaped from the microwave applicator

40

Catalá-Civera

by offering an open circuit to such energy; thus, also improving the overall efficiency of the system. Considering that this structure is essentially isotropic, the filter will have nearly the same characteristics for TEM waves propagating though it in any direction parallel to the web [1]. As a result, the filter can reflect back the set of modes TEmo since these modes can be decomposed into two of such TEM wave components. Using this procedure, typical open port dimensions can reach till 5 or 6 cm. in height.

Fig. 1. Schematic of a doubly corrugated choke

Analysis and design of doubly corrugated filters The design and analysis of multiple corrugated filters have traditionally been performed by an approximate method based on a monomode equivalent circuit of short-circuited E-plane T-junctions interconnected with waveguides modelled by simple transmission lines [1, 2]. Figure 2 shows the equivalent T-junction with the main designing parameters of the filter, namely, the port height (g), the stub width (b) and the stub height (d).

d g

b

Fig. 2. Equivalent E-plane T-junction of the corrugated choke

A typical example of this procedure is used next to determine the attenuation behaviour of a doubly corrugated filter for an open port height (g) of 14.88 mm. The filter is composed by 5 rows of 3 square blocks. Figure 3 shows the transmission response of the filter obtained with the mentioned monomode approach next to measurements of the structure carried out by a Network Analyser.

Design Parameters of Multiple Reactive Chokes

41

Fig. 3. Simulations and measurements of a doubly corrugated choke

We can appreciate a good agreement between measurements of the structure and results obtained with the traditional method mainly at low frequencies, however at higher frequencies, the traditional response largely separates from measurements. Obviously, for singly corrugated chokes, where there is only one mode propagating over the structure, the monomode approximation can achieve good results. Unfortunately, this procedure, for doubly or multiple corrugated chokes, does not provide precise results since the effect of the mutual interactions of the modes in the corrugations invalidates the monomode representation. This result reveals the necessity of employing multimode or full-wave simulation methods to obtain more accurate responses of these multiple corrugated structures. A multimode analysis method based on the GAMs (Generalised Admitance Matrix) representation was reported in [3] and [4] for the simulation of this type of structures offering a very accurate and fast tool for the characterization of multiple corrugated chokes. With this technique, several filters were developed and manufactured and their electrical specifications experimentally verified with measurements performed by Automatic Network Analyzers and in anechoic chambers. For instance, the same structure previously analysed with the monomode approach is simulated with the GAM technique and results are also represented in Figure 3. An excellent agreement can be observed now between simulation and measurements in all the frequency band. From such figure, it can be stated that this proposed analysis is extremely accurate. The same type of analysis may be achieved as well with the aid of electromagnetic simulators. In this case, the simulator employed is CONCERTO and makes use of the FDTD method. Figure 4 shows the response of a doubly corrugate filter for an open port of 58 mm in height. As depicted, results are not so accurate that those previously presented with the GAM modal technique, but it is also possible to obtain good responses at a reasonable time.

42

Catalá-Civera

Fig. 4. Response of a doubly corrugated choke simulated with FDTD.

However, both methods, modal technique and electromagnetic simulators, are analysis techniques, from which it is possible to obtain the response of the structure over a frequency range, starting from a previous design. They are not designing techniques. Nevertheless, with the aid of this full-wave simulators, it is possible to analyze the behavior of the monomode approach in function of each geometric parameter in the final response of the filter and therefore, extract designing parameters very useful to design these corrugated filters in a simple way.

Parametric analysis of doubly corrugated chokes

Effect of the open port height (g) and stub width (b). The situation analyzed first is the effect of the open port height (g) and the stub width (b) on the filter response. As described in previous section, the open height, (g parameter in Figure 2), restricts the operation range associated only to the propagation of the TEm0 modes and may reach till O0/2 (60 mm). On the other hand, the influence of the stub width (b) is showed in Figure 5, where 4 structures for an open port of 86 x 20 mm with different block size are simulated with the full-wave method, and comparing results with the monomode approximation. As shown in Figure 5 lower frequencies of the filter curves are again properly determined by the monomode approach, whereas higher frequencies present more deviations respect to the full-wave analysis. However, as can be seen all responses include 2.45 GHz as central frequency and we can also appreci-

Design Parameters of Multiple Reactive Chokes

43

ate an increment of the filter bandwidth directly related with the increment of the stub width b. As a conclusion, from these simulations, by only using the monomode approach, it is possible to properly predict low frequencies of the filter and the bandwidth of the attenuation curve. 0

Transmission (dB)

-10

w

g

l

b

d

86.36 86.36 86.36 86.36

20 20 20 20

28.79 28.79 28.79 28.79

5 10 15 20

28.07 28.07 28.07 28.07

b1 b2

-20

b3

-30

b4

-40

-50

b1 b2 b3 b4

-60

2

2.2

2.4

2.6

2.8

3

3.2

Frequency (GHz)

Fig. 5. Effect of stub with b in the final response of the choke.

Effect of the open port height (g) and the stub height (d). The influence of these values has been checked manufacturing a doubly corrugated choke with the possibility of modifying the physical dimensions. The filter manufactured, depicted in Figure 5a, is composed of 5 rows of 3 cylindrical rods, instead of square blocks, that can penetrate in the waveguide at a desired depth. Figure 6 displays the attenuation response of the filter in function of the rod penetration in the waveguide for rods of 10 mm diameter. From the figure, we can appreciate that the central frequency of the filter is shifted with the penetration of the rods, since d is also modified, and the attenuation and bandwidth of the filter are altered as well. Moreover, despite they are not represented, simulations of these structures result in large differences between the full-wave simulator and the monomode approach even at lower frequencies. Thus, taking into account these effects not appropriately predicted by the monomode analysis, these scenarios (low d/g values) should be avoided.

44

Catalá-Civera

Fig. 6. Effect of stub height in the final response of the choke.

Effect of the dielectric load in the filter response The effect of the dielectric load, corresponding to the material entering and leaving the microwave applicator, directly depends upon its dielectric properties. These dielectric properties, mainly the dielectric constant, reduces the cutoff frequency of the waveguide fragment (open port) and a set of modes that do not propagate with the empty waveguide, can propagate now and invalidate the protection given by the choke. It is important to keep in mind that doubly corrugated chokes avoid leakage only of TEmo modes. On the other hand, for high loss dielectrics, material itself absorbs part of the microwave power available at the open port, thus improving the behavior of the filter. Design parameters of multiple reactive chokes for open ports in microwave heating systems. From this results, it is possible to extract some very simple guidelines to design corrugated chokes. First of all, the material to be processed determines the dimensions of the open port. Once these dimensions have been fixed, and taking into account the dielectric properties of the load, it must be identified the set of modes propagating within such a structure. Then, l, namely, the number of rectangular post per section, should be chosen as close as possible to O0/2 (60 mm). This parameter usually gives an indication the slope of the response at lower frequencies. Next, select d near 30 mm, corresponding to a quarter of wavelength (O0/4). This parameter (d), tunes the central

Design Parameters of Multiple Reactive Chokes

45

frequency approximately at 2.45 GHz. As depicted in Figures 5, where d = 28.07 mm. and Figure 6, where d varies from 10.18 to 25.18 mm, small changes in this parameter yield to small changes in the central frequency, which hardly modifies the attenuation behavior of the filter taking into account the considerable bandwidth of the response. The last designing parameter b, the stub width of the equivalent T-junction, is the only value that has to be careful selected. Since the bandwidth of the filter mainly depends on this parameter (see Figure 5), large b values are preferable. With all these parameters and the equivalent electric values obtained from the equivalent circuit of the E-plane T-junction [1, 2], we can directly compute the response of the filter. We can expect in practice about 15/20 dB per section of corrugated choke. Example: Design of a doubly corrugated choke These guidelines have been followed to design a filter for an open port of g = 20 mm and w = 570 mm. Electrical specifications requires 70 dB of attenuation level in the stop band. Stub height is fixed to 30 mm. and 10 corrugations are chosen to select l close to 60 mm. From these values, the monomode approach is used to obtain the attenuation response of the filters. Figure 7 shows this attenuation for five different configurations of stub width. As expected from previous analysis, it is possible to appreciate from the figure, the bandwidth dependence with b. Simulation b3 in Figure 7 was selected as suitable in order to match designing specifications. 0

b3

Transmission (dB)

-10

w

g

l

b

d

570 570 570 570 570

20 20 20 20 20

57 57 57 57 57

10 20 32 40 50

30 30 30 30 30

b1 b2 b3 b4 b5

b2

-20

b4

-30

b1

-40 -50 -60 -70

b5 -80 -90 1

1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

2.8

3

Frequency (GHz)

Fig. 7. Monomode responses of the filter in function of the stub width.

These design parameters were then introduced in the electromagnetic simulator and the full-wave attenuation response is displayed in Figure 8 for the first three propagation modes, compared to the monomode approximation. As shown in the

46

Catalá-Civera

figure, there is a very good agreement between them, which validates the designing procedure followed to obtaining the parameters of the filter. 0

TE10 TE20 TE30

Transmission (dB)

-10 -20 -30 -40 -50 -60 -70 -80 -90

1

w (mm)

570

g (mm)

20

l (mm)

57

d (mm)

30

b (mm)

32

Sections

4

1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

2.8

3

Frequency (GHz)

Fig. 8. Attenuation level of the filter simulated with the full-wave method and the monomode approach.

Conclusions The theoretical method based on monomode approximations traditionally used in the analysis and design of corrugated filters may not provide accurate solutions when these structures present multiple corrugations. Proposed techniques well based on the multimode GAMs representation or well based on electromagnetic simulators can offer the required alternative to characterize these corrugated chokes with high accuracy. Additionally, with an exhaustively use of these full-wave simulators it is possible to analyze the effect of each designing parameter of the corrugated structure in the final response of the filter allowing to establish the validity margin of the monomode approximation. Taking into account this analyzed restrictions, a simple procedure based on the monomode approach to design multiple reactive chokes for open ports in microwave heating systems has been described. As an example of this fast and simple procedure, a doubly corrugated choke has been designed. The behavior of the filter obtained from the monomode approach is very similar to that obtained using a full wave simulator, thus showing the good design guidelines derived from the rigorous full wave analysis.

Design Parameters of Multiple Reactive Chokes

47

References [1] G. Matthaei, L. Young, and E. M. T. Jones, 'Microwave Filters, Impedance-Matching Networks. and Coupling Structures. Norwood, MA: Artech House, 1980. [2] A. L. Vankoughnett, J.G. Dunn, ‘Doubly corrugated chokes for microwave heating systems’, Journal of Microwave Power, 1973, 8, (1), pp. 101-110. [3] P. Soto, V.E. Boria, J.M. Catalá-Civera, N. Chouaib, and B. Gimeno, “Efficient modal analysis of corrugated chokes for microwave heating systems”, 7th International Conference on Microwave and High Frequency Heating, Valencia (Spain), pp. 27-30, September 1999. [4] P. Soto, V.E. Boria, J.M. Catalá-Civera, N. Chouaib, M. Guglielmi, and B. Gimeno, “Analysis, design and experimental verification of microwave filters for safety issues in open-ended waveguide systems“, IEEE Transactions on Microwave Theory and Techniques, Vol. 48, No. 11, pp. 2133-2140, November 2000.

Microwave High-Power Four Post Auto-Matching System Pedro Plaza, Antoni J. Canós, Felipe L. Penaranda-Foix and Elias de los Reyes Departamento de Comunicaciones, Universidad Politécnica de Valencia, Camino de Vera s/n, E-46022 Valencia, Spain

Abstract In industrial applications, slight modifications in the size, shape, or dielectric properties in the material may result in a very inefficient application from an energetic point of view, since microwave cavities are usually very sensitive. Therefore, adjustable devices that can re-adapt the applicator behaviour simultaneously with the processing of materials can offer unique alternative to improve the energetic performance of these systems. In this work an auto-matching system based on a four-post matching network has been designed and implemented. The developed system has been constructed in WR340 waveguide, with four motorised metallic posts, whose penetration depth inside the waveguide can be modified. This penetration depth is calculated from S11 measurements performed by a high-power six-port reflectometer and then the system is monitored and fine-adjusted all along the process.

Introduction The matching process is a problem of great interest in microwave heating applications, because we are dealing with very high powers and it is very important to avoid power losses. In addition to this, it is important in the matching progress to be adaptative, because the load can change dynamically and then the system must be able to rematch the network. Despite there are lots of references in matching networks -for example we have two important books like [1] or [2]- very few works about adaptatives networks can be found in the literature -see [3] and [4]-. These two articles present a matching method that do not cover all the Smith Chart, so we can not match all the loads. And another important fact is to remember that we are dealing with high powers, then we must avoid using some kind of apertures and small distances in order to prevent the arch. Therefore, our interest

Microwave High-Power Four Post Auto-Matching System

49

is to match any load in the Smith Chart taking into account the presence of high powers. The most traditional matching network consists of three ideal stubs. But in real problems it is very difficult to get an ideal stub that can vary in a dynamic way. A very useful system to get a dynamic stub is the metallic post introduced in a rectangular waveguide. This post can be mechanized very easily and it can be made like a screw in order to change their penetration depth and the its electrical behaviour. The equivalent circuit was introduced by Marcuvitz [5] and it consist of a T-network that can be seen in Figure 1. The values of the series and parallel elements are calculated as a function of the penetration depth. But, due to the presence of the series elements, the stubs are not ideal and then a three post system in unable to math all the Smith Chart. Therefore we are going to use a four stub matching network, with four posts separated 3Og/8 ,Og/4 and 3Og/8 (see Figure 3). In order to get the main objective, in following sections we will see how to characterize the metallic posts, then a matching strategy will be presented and, finally, we will see some results and some conclusions. jXa

jXa

jXb

T

T

Fig. 1. Equivalent circuit of a metallic post

Post characterization The equivalent T-network of the metallic post is shown in Figure 1. Its electrical behaviour has been measured with a Vectorial Network Analyser, in order to ensure the best characterization of the full matching system. The results have been processed and, comparing the measured values with the equivalent circuit, we can obtain the values of Xa and Xb. These values are shown in Figure 2, for a frequency of f = 2.45 GHz and a post diameter of 24 mm, for a set of different depths. The values can be interpolated by a polynomial function for each frequency and diameter. Then, the two elements of the equivalent network are: X a 0.0057 ˜ h  0.0014 (1) Xb

0.0001˜ h

3



 0.0034 ˜ h 2  0.0097 ˜ h  0.0145

where h is the penetration depth.

1

(2)

50

Plaza

j1 j0.5

j2

h=0 mm 0

0.5

1

2

Xa h=26 mm Xb

-j0.5

-j2 -j1

Fig. 2. Reactances of metallic post as a function of the depth

A-B

T4

C-D

d1

T3

E-F

d2

T2

G-H

d1

T1

Fig. 3. Four post matching network

Matching process The matching strategy in a four stubs matching system is based on the results shown in Figures 4, 5, 6 and 7. Figure 4 shows the impedances that can be adapted with only one stub. Then, the previous three posts must drive the load to this zone. Figure 5 shows all the impedances that can be adapted with two posts, with a 3Og/8 line between them. Then, the previous two posts –screws- must drive the load to this wider zone than before. Figure 6 shows the load that can be adapted adding a third post Og/4 far away. The matching area is now wider. Finally, Figure 7 shows the zone that can be adapted with a fourth post collocated 3Og/8 away from the third post. With this last post we can see that the whole Smith Chart can be matched. This was one of main objectives stated above: to match all the Smith Chart.

Microwave High-Power Four Post Auto-Matching System

51

j1 j0.5

0

j2

0.5

1

2

-j0.5

-j2 -j1

Fig. 4. Matched zone with one post

j1 j0.5

0

j2

0.5

1

-j0.5

2

-j2 -j1

Fig. 5. Matched zone with two posts

The matching process will consist of calculating the depths for each post, starting with the post that is nearer the load –the fourth post- and driving the load to the matching zone of the posterior posts. The final algorithm will be the following: initially, we measure the load and determine if a matching process is needed. In this case, a coarse matching process starts, consisting on an initial calculations of the starting depths.

52

Plaza

j1 j0.5 j0.4

j2

j0.3 j0.2

0

0.2 0.30.40.5

1

2

-j0.2 -j0.3 -j0.4 -j0.5

-j2 -j1

Fig. 6. Matched zone with three posts

Fig. 7. Matched zone with four posts

After this coarse algorithm, a fine algorithm is used. The coarse algorithm drives our load to a very good matched point in the Smith Chart. The fine one consists of moving the first two posts in a dynamic way, up and down, in order to get the better reflection coefficient. This fine algorithm is able to match the network even when the load changes, so we can readapt the system in an adaptative way. When the load changes too much, the fine algorithm is not able to match the

Microwave High-Power Four Post Auto-Matching System

53

network. When this happens, the systems resets the post –penetration depth zeroand the coarse algorithm starts again. This only happens when the load changes are very important. If we are in an industrial and continuous process, the load will not change too much, so this procedure is enough to match the load.

Results The whole system is shown in Figure 8, where we can see that the four posts have been motorised to change the penetration depth. Some results are shown in Figures 9 and 10. Figure 9 shows a static process. Initially the load has a poor-matched reflection coefficient (S11 is about -8 dB). The matching process starts in point P1 and in point P2 the coarse algorithm finishes. Now the reflection coefficient is about -20 dB (it means that we have about 1% of energy losses). After this coarse algorithm the fine one starts and we get a reflection coefficient of about -35 dB (about 0.03% of energy losses). The total time was about 70 seconds. Figure 10 shows the same initial process than before, but after some seconds, the load has changed its value. The figure shows two changes. We can see that in both cases the fine algorithm has got to readapt the system very fast.

Fig. 8. Physical implementation of the whole matching system

54

Plaza

-5 P1

-10 -15

P2

|*| [dB]

-20

-25

-30 -35

-40 0

50

100

150

200 Samples

250

300

350

400

Fig. 9. Static matching process

-5 Load variations -10 -15

|*| [dB]

-20

P1

-25

P2

-30 -35 -40 -45 0

Matching 100

200

300

400

Samples

Fig. 10. Dynamic matching process

500

600

700

Microwave High-Power Four Post Auto-Matching System

55

Conclusions A variable matching system has been constructed. It is able to match the whole Smith Chart dynamically. The results are good and the system can be used in almost any industrial process.

References [1] G. Roussy, J.A. Pearce. Foundations and Industrial Applications of Microwave and Radio Frequency Fields. John Wiley & Sons. [2] G. Matthaei, L. Young, E.M.T. Jones. Microwave Filters, Impedance-Matching Networks, and Coupling Structures. Artech House Books. [3] W. Wyslouzil, A.L. VanKoughnett. Automated Matching of Resonant Microwave Systems. Journal of Microwave Power, 8 (1), 1973. [4] W. Wyslouzil, A.L. VanKoughnett. An Automatic Tuner for Resonant Microwave Heating Systems. Journal of Microwave Power, 6 (1), 1971. [5] N. Marcuvitz. Waveguide Handbook. IEEE Electromagnetic waves series 21.

Design of an Applicator for Processing of Nanoscale Zeolite/Polymer Composites with Superposed Static Magnetic Field Ralph Schertlen, Stefan Bossmann, Werner Wiesbeck

Abstract Microwave heating is a favoured tool for fast and efficient material processing. One of the biggest advantages is the fact that heat is produced directly within the material. This effect is combined in the new MicrowaveMagnetoApplicator (MMA) described in this paper with the action of a static magnetic field. The design of the MW-processing unit with a superposed static magnetic field is reported in detail. The magnetic field effect is shown for processing of a polymer-doped zeolite.

Introduction The field of applicator design for microwave heating of food, materials, and selected industrial products has been subject of comprehensive reports [1, 2]. Two main directions can be defined: on the one hand the standard application branch with multi-mode ovens and on the other hand the tailored applicator branch with either resonant structures or matched power input. The increasing awareness of microwave heating advantages for chemical processing calls for new concepts in applicator design. A completely new design is presented in this paper, where a matched applicator is combined with a superposed static magnetic field in order to align magnetosensitive particles in the reaction mixture.

Motivation Zeolites are cage structures which can be used as form-selective catalyst materials or as template-containers for polymerisation reactions [3]. The polymers have been found to interact weakly with magnetic fields. To verify these finding experimentally, a reaction of certain zeolite powders, filled in their cage with organic monomers, was selected, because different products should be obtained, depending upon the presence or absence of a superposed magnetic field.

Design of an Applicator for Processing of Nanoscale Zeolite/Polymer Composites

57

Therefore, a reactor for fast synthesis under controlled superposition of a magnetic field was necessary. The paper describes the design of the microwave as well as the magnetic part of such an reactor and the steps required to merge both systems. The applicator design has to meet some main process requirements, e.g., a homogeneous electromagnetic power distribution inside the material to be processed and an equal distribution of processing temperature for all zeolite nanoparticles. Furthermore, a fast and easy assembling of the applicator and an easy access to the powder are required.

Results The applicator is designed in a modular way, with a stand-alone rectangular microwave part inserted into a coil for the static magnetic field, as shown in Figure 1 and 2. Assembly and separation is easy, by inserting the microwave applicator into the magnetic field coil, shown in Figure 1. For operation of the device, a safety cage is used, with the Microwave Magneto Applicator shown in Figure 3.

Details of the microwave applicator The applicator containing the powder mixture is designed for operation in the dominant mode (TE10). The volume of the powder container is approximately 50 ml. Numerical field simulation ensured the appropriate choice of the dimensions of the waveguide, to achieve safe TE10-operation, although opposite to the assumption of an undisturbed field, the loaded material has a relative complex permittivity and loss slightly higher than the embedding PTFE. Two different approaches for field homogeneity inside the reaction mixture were followed: a single tube milled into the PTFE as a container for the organically doped zeolite powder and a multiple layer-waveguide, cutting the electric field into sub-waveguides, in order to re-feed electrical field intensity to the reacting mixture in a certain distance from the power connector. The layers have different length , as shown in Figure 4. As the applicator operates in TE10-mode, the particular wave guided by each layer should superpose constructively with the field already present in that region. Although with the single tube a homogeneous power distribution was not fully achieved, the improvement in homogeneity of power distribution within the zeolite powder using the more complex shape was low. Because of the much easier manufacturing, the tube structure is therefore favoured over the layer concept.

58

Schertlen

Fig. 1. Magnetic part of the applicator: coil, power supply and temperature sensor.

Fig. 2. Stand-alone microwave applicator part of the MMA, coaxial N-connector for the microwave input on the left side.

Fig. 3. Merged Microwave Magneto Applicator, MMA system in a safety cage.

Temperature measurements of the powder are taken by an optic sensor through a drilling at one side of the applicator, as shown in Figure 1. The processing temperature should be kept at 100°C. The homogeneity of power density distribution

Design of an Applicator for Processing of Nanoscale Zeolite/Polymer Composites

59

is calculated with a normalising bar graph method [4] and measured with the help of a temperature sensitive paper, turning from white to black at 90°C. The results are shown in Figure 5 for the single tube applicator without additional layers.

Fig. 4. TE10-applicator-concept with guiding layers.

Details of the magnetic subsystem and the MMA To create a static magnetic field, an appropriate coil is chosen. Limitations are the maximum current which is responsible for the maximum magnetic field, and the thermal resistance of the insulation on the windings, which is heated by ohmic loss of the coil. The coil, shown in Figure 1, is mounted on an rectangular aluminium carrier. The size and shape is adjusted to the microwave applicator, so that the PTFE-applicator fits in the carrier without gaps. Inside the coil, the applicator needs no fixation as there are no moving parts. The length and number of windings of the coil are calculated analytically by assuming the validity of the equation for the magnetic field in a long coil [5]. Inside the coil, the maximum magnetic flux at a current of 10 A is about 40 mT. Safe operation is possible with a maximum current of 5 A, causing a maximum temperature of the coil of about 80°C after half an hour of operation. After reaching a coil temperature of 80°C the voltage increases from 28 V at 20°C to almost 35 V. This leads to a total power consumption of 165 W for heat losses. The MMA consisting of both units is shown in Figure 3. Each of the subsystems is equipped with its own power source. This allows an arbitrary variation of the strength of the magnetic field within the zeolite powder.

Operation of the MMA-unit The zeolite powder-filled microwave applicator is shown in Figure 5. The material to be processed is zeolite powder doped with methylmetacrylate. A commercial microwave source is used for processing. The processing temperature is controlled manually by adjusting the microwave power. During heating up, maximum microwave power is needed, which after reaching the processing temperature can be reduced to keep the temperature level stable and compensate the heat losses only.

60

Schertlen

Fig. 5. Calculated (upper) and measured (lower) power density distribution in the applicator

The reaction mixture was processed with and without a static magnetic field superposing the microwave field. First results are shown in Figure 6. The scanning electron micrographs show a more or less arbitrary agglomerated powder in the left part of the figure, obtained upon heating without the superposed magnetic field. The product at the left side of Figure 6 is obtained upon heating with the static magnetic field superposing the microwave field.

Conclusion A new type of chemical syntheses can be performed, using the Microwave Magneto Applicator developed by our group. First results are shown on processing polymers staring with organically doped zeolite powders. A static magnetic field is superposed during microwave heating in this applicator. This allows to process nanomaterials with unique morphology. Additional processing options are possi-

Design of an Applicator for Processing of Nanoscale Zeolite/Polymer Composites

61

ble by applying a magnetic field during microwave heating. First results of material processing are very promising.

Zeolite-Powder

Fig. 6. Microwave applicator filled with the organically doped zeolite powder.

Fig. 7. SE-micrographs of the product obtained after microwave heating the metacrylatedoped zeolite, without (left) and with (right) a static magnetic field superposed during the heating process.

References [1] A.C. Metaxas; Foundations of Electroheat; John Wiley & Sons; 1996 [2] A.C. Metaxas, R.J. Meredith; Industrial Microwave Heating; Peter Peregrinus; 1983 [3] G. Fu, C.A. Fyfe, W. Schwieger, G.T. Kokotailo, Structure Organisation of Alumosilicate Polyanions with Surfactants, Optimisation of Al-Incorporation in Alumosilicates Meso-Structural Materials, Angew. Chem., 34 (1995)1489. [4] R. Schertlen, Y. Venot, C. Stenzel, W. Schwieger, W. Grill, R. Herrmann, M. Schmachtl, H. Toufar, J. Caro; An Applicator for Homogeneous and Gradient Heating of High Lossy Dielectric Zeolites; 7th International Conference on Microwave Heating; Valencia, Spain, Sept. 13-17, 1999, pp. 161-164 [5] Hering, Martin, Stohrer; Physik für Ingenieure; VDI-Verlag; 1992

Measurement Techniques for Microwave and RF Processing Georges Roussy Laboratoire de Spectroscopie et des Techniques Microondes, Université Henri Poincaré – Nancy I, BP 239. 54506 Vandoeuvre les Nancy CEDEX (FRANCE).

Tools for Process Evaluation For the feasibility tests, the LSTM and SAIREM have developed special installations called DIELECMETRE, at Microwave and High Frequency. The idea is to irradiate a sample of the product and to follow the process as a function of the applied field and time. The sample is selected to be of small volume, so that the field is permanently maintained as homogeneous as possible, whatever the state of the processing. One takes care to properly define the external conditions applied to the sample (temperature of gases which flow through it, temperatures of surfaces in contact with it, etc...). Microwave Dielecmeter The diagram of the microwave installation is given in Figure 1. The sample (1 cm3), is located in a tube, in the center of a RG112-U guide, in front of a variable short-circuit piston, whose position is constantly adjusted during the processing, so that the electric field is maximum on the sample. The sample is thermally insulated with a foam cylinder made of silica. Two thermocouples measure the temperature of the sample and of the wall. The incident power and the reflected power are measured by a directional coupler and two bolometers. The field applied to the sample is measured with a probe placed in the wall of the piston and the standing wave distribution is determined by four diode detectors, located at Og/8 from each other. A computer collects all measurements and calculates the electric irradiation conditions according to a procedure which we detail below. When the material has very low losses, a coupling circuit, which puts the applicator in resonance and improves the power transfer between the generator and the product, is placed in front of the applicator.

66

Roussy

Fig. 1. Diagram of the microwave Dielecmeter.

Working of the four probe sensor The diodes measure the modulus of the local electric field Vi

ª Pin k i «1  « ¬

U

2

§ d ·º  2 U cos ¨T  2S i ¸ » ¨ O g ¸¹ »¼ ©

i = 1 to 4

(1)

Pin is the power of the incident wave ki is a proportionality factor Ue jT is the complex reflexion coefficient of the applicator measured in the original plane, from which each detector is at a distance di. Og is the wavelength of the propagating wave inside the guide. Knowing these parameters (di, ki), and the Vi, one should be able to calculate, U, T, and Pin. But the four equation system with three unknown factors is badly conditioned. One defines a matrix [A], which relates the vector

Pin (1  U 2 ) Pin U cos T Pin U sin T 0

and the vector

V1 V2

. >A@ is a singular matrix. But, the twelve

V3 V4

terms Di,j (other than those of the last line),can be calculated by a least squares fitting while measuring Vi for 45 different positions of a movable short-circuit piston, placed as a load. One can also include in this calculation, the coefficients de-

Measurement Techniques for Microwave and RF Processing

67

scribing the non-linearity of the detectors. The calibration procedure is automatic. It works in two minutes. The results are stored in the memory of the computer, by noting the "power level" of the generator; because it is known that the frequency of the generator depends on it, and that the Di,j slightly depend on the frequency. The calculation of the absorbed power by the sample is then immediate: Pa

Pinc 1  U 2

(2)

The process can then be described as a function of time, with Pinc constant (and one checks that the incident power is maintained constant), by plotting the curves, H'(t), H"(t), T(t). One can also exploit the results obtained with constant applied electric field steps. High frequency Dielecmeter functioning The operation of the HF Dielecmeter is similar. Its diagram is represented in Figure 2.

Fig. 2. Diagram of the HF Dielecmeter.

The applicator is a cylindrical circular capacitor, whose electrodes can be thermally insulated. V-I sensors are used at various places. The amplitude of the field (or the voltage applied to the cell) during the process, is adjusted by the capacitance (J) of the coupling system. All the mixers of the sensors have a common reference signal, which is provided by the pilot of the generator. V-I sensors are corrected for the defects of offset, amplifier gain, and phase shift. The calibration is done on by measuring several known impedances: the matched load and several others (SC, OC, capacitors with high power resistance). The calibration amounts to calculating by least squares the complex coefficients

68

Roussy

A*, B*, C*, which relate measured and known values of Z*. Finally, the sensor is calibrated to make power measurements. If W is the voltage applied to a complex impedance Z*, inserted at the extremity of a line of characteristic impedance Zc, the incident and absorbed power levels are: 2

Pin

W Z*  Z c 4Z c Z*

Pab

W Z*  Z c 4Zc Z*

2

2

(3) 2

(4)

Permittivity computation

Permittivity measurement follows the same procedure as for measurement with a vector network analyzer. The equation :

U*m

U vide

1  (B' jB" )(H*  1) 1  (B' jB" )(H*  1)

(5)

is used. U*m is the complex reflexion coefficient of the applicator when it contains the product. U*vide is that of the empty applicator. B' and B" are two parameters calculated with different materials (liquids generally) whose permittivity is known.

Modeling of Processes Instrumentation and modeling of processes are closely related. Various aspects are reciprocal. The physicochemical interpretation of the information provided by the Dielecmeters, (experiments with a homogeneous field, for example), characterizes the evolution of the material, and the progress of the process. In an industrial facility, advancing of a process is followed by electrical or thermal measurements, which are interpreted with the assistance of previously obtained data. Modeling a process establishes the equations which it follows; i.e. the ensemble of relations which allow calculating the temperature and the degree of processing according to the external conditions and the applied electromagnetic field. We will illustrate the analysis below, by taking as an example, the high frequency polymerization of a composite (glass / DGEBA resin).

Measurement Techniques for Microwave and RF Processing

69

Basis of resin polymerization modeling

The fundamental equations of the model are well-known. One defines the local temperature, at the time t: T(x, y, z, t); the square of the modulus of the electric field: E2(x,y, z, t); the degree of cure : D (x, y, z, t). The equations are 1) the kinetic equation dD E k (D) exp (6) dt RT dD is the rate of polymerization dt k(D) is the rate constant E is the polymerization activation energy 2) the heat equation with electromagnetic and chemical sources because the reaction is exothermic wT wD Uc p  ’.( /’T ) H 0 H" E 2 Z  ('H ) (7) wt wt U is the specific mass cp is the specific heat / is the thermal conductivity H" is the imaginary part of the permittivity of the product 'H is the enthalpy of the reaction

3) The heat exchanged by the polymer with its environment, imposes that the derivative of the temperature normal to the surface, be proportional to the variation in temperature wT  hSTS  Text 0 (8) o wn Modeling the process means defining (and justifying) how the coefficients k, E, U, cp, /, H", 'H, depend (possibly) on D and T by testing if each coefficient can be parameterized by formulas k

k 0  k 1D  ...

(9)

/ / 0  /1D  ...

(10)

etc... and by calculating the parameters by a least squares method. The problem does not have a satisfactory solution if one takes it blindly without precaution.

70

Roussy

Fig. 3. Temperature, H' and H" curves, as function of time.

Measurement Techniques for Microwave and RF Processing

71

The absorbed power varies, although the amplitude of the field is constant, because viscosity, and permittivity of the material, change during curing. The temperature increases because the dissipated power increases at the beginning of the process, and because the reaction is exothermic. Thus, the temperature is stabilized at the end because the imaginary part of the permittivity of the cured material is lower than that of the initial material. The problem is to interpret all these results. If the simplified equations of the model (with a variable z) were valid, the problem would be well-known in control system theory. The variables are T and D, and the variable which controls the system is the absorbed power since Pa(t) makes it possible to calculate T(z,t) and D(z,t) wT w 2T wD (11) Uc  / 2 Pa ( t )  ( 'H ) wt wz wt wT wz

 h T( z

re)

 T0 0

(12)

E RT (z)

(13)

z re

wD wt

k (D) exp

The first simplifying idea is to suppose that the temperature is constant in the sample volume of the Dielecmeter experiments. A first set of parameters is so obtained. Supposing that thermal conductivity / and the specific heat c are linear functions of D, and independent of the temperature.

/ c

1  D /(0)  D/(1) 1  D c(0)  Dc(1)

(14) (15)

and that dD dt

a1(D  a 2 ) a 3 (100  D) a 4 exp

E RT

(16)

Agreement of acceptable quality can be obtained. If the temperature is not supposed homogeneous (and it is not in the experiment), the problem is much more difficult, because it is necessary to distribute the power Pabsorbed(t) between the various sections according to the thickness of the sample, and the local value of the permittivity. To do so, it is necessary to know the law of variation of the complex permittivity of the material. Many suggestions have been made. A formula describing H*(D,T), according to the law of clusters, with parameters di such as :

72

Roussy

H*

d0 

d 1  d 2D 1 d 6 ª d 4  d 5D · º § ¸ » «1  jZ exp¨ d 3  T © ¹ ¼» ¬«

d7

(17)

never gives good results. On the other hand, a formula derived from the dielectric mixture law (Looyenga n = 1/2), which uses the permittivity of glass, that of the non cured resin, that of the cured resin and that of the air whose volume varies with the pressure, is acceptable. Moreover, the total real part of the industrial material permittivity varies little, so that the number of parameters is overabundant.

H

* n

>

W v H*v  W a  W r 1  D H *r (0)  DH *r (1) n

H' = constant with D and T 2 3 H *r (0) = b0+b1T+b2T +b3T

n

n

@

(18)

2 H*r (1) = c0+c1T+c2T

One confirms that the parameterization of k(D), with a single value of the activation energy of the curing reaction is acceptable. Figures 4 and 5 illustrate these results.

Fig. 4. Curves of the evolution of the dielectric losses as a function of T and D

Measurement Techniques for Microwave and RF Processing

Fig. 5. Discussion of the Dielecmeter curves.

73

74

Roussy

Optimization of the Operation of Industrial Facilities. It was previously seen that one can simulate the processing of a material resulting from the application of an electromagnetic field, once a precise and robust model is known. The optimal industrial facility is then that which provides the distribution which leads to the best results, taking into account the technological and financial constraints of the problem. Numerical analytical method

In the following paragraph, a different optimization method for obtaining the best operating conditions of a microwave installation is described. The method is completely passive. It consists in recording simultaneously a given result, and data during operation of the machine, and then correlating the result with the data to optimize the conditions. The recorded data called variables can be the power delivered by the generator, the amplitude of the applied field in several places and at several times, or the duration of the process if it varies, the instantaneous value of one capacitance of the matching box. For a polymerization process, one can record the maximum value of the obtained permittivity, the total energy supplied by the field during the cycle (integral of Pa over the course of time). The result can quantify the quality of the process, or a performance, or any output. It can be the temperature in the center of a face of a small barquette, at the time when it comes out of the defrosting installation. It can also be the difference in temperature between the center and the edge of a barquette if the inhomogeneity of the process is significant. When a great number of variables and results are collected, a result can be correlated with the measured variables. The relations which relate the variables to this result can be found. The experimental conditions can be determined so that the result and the variables take the desired values. It follows that a computer can control the processing by maintaining the measured variables within specified limits during the processing. A criterion of stopping of the machine is defined. Moreover, the computer which accumulates much information, and therefore can increase its basic data, improve knowledge of the model and ensure the operation of the installation with increasing rigour. Process analyzer

The statistical analysis of the correlations between the results and the variables is easier when measurements are more precise. The LSTM laboratory is building an apparatus called a process analyzer, especially conceived to record the principal electric variables during operation of a microwave installation. The diagram of the analyzer is given in Figure 6.

Measurement Techniques for Microwave and RF Processing

75

Fig. 6. Diagram of a process analyzer.

It measures the complex reflexion coefficient *

and the transmission factors ( t 1 and

U*

at the input of the applicator

t *2 ), at two places which the user can choose

in the applicator. The analyzer is connected to a directional coupler which provides a reference signal and supplies four double mixers. Three of them treat the signals

U* , t 1*

et

t *2 , the fourth one is used to measure the characteristics (am-

plitude and phase) of the transmission coefficient of a known reference circuit under the same conditions. The originality of the assembly is its use of a mechanical phase-shifter, which modulates the phase of the LO signals, 12 u 15 degrees, every ten seconds. The computer records the eight output signals of the mixers and filters them in synchronism with the modulation. It calculates the amplitudes and the average phases of the four double mixers. Moduli and phases of

U* , t 1*

and

t *2

are thus meas-

ured by comparison with the attenuation and the phase of the known circuit. The

76

Roussy

obtained mean values are relative to the frequency domain which is covered by the generator, even if the characteristics of the generator vary, or if the generator does not have a stable frequency. In this way,

U* , t 1*

and

t *2

are estimated with good

accuracy, in spite of the bad quality of the generator, and detailed calibration of the double mixers is avoided.

Conclusion Microwave and high frequency techniques have reached their maturity. One can scientifically manage any application, logically, (with more or less success according to the means implemented). It is now possible to measure the performances of an installation and evaluate the limits, if not to predict them, before building it. HF and microwave techniques are based on a small set of concepts. But the assumptions upon they are built are strict, they are often badly taught, badly popularized, badly transmitted to the industrial world. It is necessary to consider them again with rigour. I hope that this paper has helped you to advance in this direction.

Dielectric Characterisation of High Loss and Low Loss Materials at 2450 MHz Andrew Y.J Lee1 and V. Nguyen Tran2, 1 2

Microwave Research Laboratory, RMIT, Australia IRIS, Swinburne University, Australia.

Introduction Dielectric characterization is an essential step in the simulation of the propagation of electromagnetic waves in a wireless environment. The dielectric constant and loss factors have profound influence on the performance of wireless systems. Likewise, dielectric characterization is equally important in the microwaveheating field. It helps to understand the penetration depth, the microwave absorption and the heating rate. Numerous methods of measuring dielectric constant and loss have been developed to date. Some require expensive equipment, such as an Automatic Network Analyzer (ANA) and some rely on basic equipment, such as an impedance bridge or a slotted line. To a specialist, dielectric characterization should be accurate to as many decimal points as possible. The principle of these measurements are described in the chapter called “Dielectric constant” from Microwave Measurement Handbook, vol. II, edited by Sucher and Fox [1]. Recent publications Baker-Jarvis et al [2, 3, 4] describe some of these techniques in detail. To a microwave heating application engineer, scientist or technologist, coming from a non-microwave background, most if not all, characterization methods are complex and overpowering. The open ended coaxial probe, a recent development, has become popular because it is easy to use with an ANA. Its major advantages are in the speed and the broadband nature of measurement, enabling the investigation of temperature and time variation effects as well as any frequency resonance effects. However, there are several drawbacks with this method. The sample must be homogeneous and optically flat to less than 25 Pm to obtain good results. The sampling size is very small. It is not usually accurate for low loss dielectric materials. There is also the problem with lift-off where minute air gaps between the probe and the dielectric material can influence the accuracy of the measurement [2]. The open shielded coaxial probe [3] enables the measurement of low loss material but it also requires a homogeneous and carefully prepared sample. Baker-

78

Lee

Jarvis et al [3] goes as far as to suggest that the outer dimension of the sample should be metallised to achieve better results. The transmission/reflection method [4] using an ANA is the most comprehensive technique and it can be used to measure low loss and high loss materials with good accuracy. However, it requires meticulous preparation to obtain good characterization. Regular re-calibration may be necessary because the waveguide flanges may have to be disconnected and re-connected for every measurement to insert the sample. We investigate the two-position method by Surber et al [5] and the infinite line method by Altschuler [1]. Both methods provide considerably accurate results quickly with standard waveguide components and a sample holder.

Dielectric Characterization

Two position method

We used a rectangular waveguide to achieve a better average result. The waveguide of interest, at 2450 MHz, is WR284, WR340, and WR430 and is set up as shown in Fig. 1. For medium and low loss materials such as bricks, plasterboard and dry food materials, we used a method of measurement by using a direct short circuit and provide a correct length for an open circuit.

Fig. 1. Waveguide sample holder connected to an adapter.

Dielectric characterisation of high loss and low loss materials at 2450 MHz

79

We then measure the short circuit (Ysc) and open circuit (Yoc) admittances. According to [1, 5], the dielectric properties of a material is obtained from the products JH = YscYoc. Typical measured results are shown in Table 2. JH = YscYoc = 1/(ZscZoc)

(1)

JH = gH + jbH

(2)

Hrc = [(gH +(Og/Oc)2)/(1 + (Og/Oc)2)]

(3)

Hrcc = -bH/[1 + ((Og/Oc)2]

(4)

where Hrc = dielectric constant and Hrcc = loss factor. With an ANA, the dielectric characterization can be simplified to the point of placing the short circuit over the sample in the waveguide and then shifting it to a distance equal to one quarter of the guide wavelength to create an open circuit as shown in Table 1. Table 1.

Waveguide WR284 WR340 WR430

o/c length 57.1 43.0 36.8

Table 2.

Ramp angle (degrees) 90 45 30 15 6

|(S11)| 0.856 0.709 0.276 0.063 0.01

VSWR 12.90 5.90 1.80 1.13 1.02

Infinite Line method

Because most food materials have a high moisture content, which is tantamount to a high dielectric loss if the sample length is many times the penetration depth, placing a short circuit and an open circuit will have no effect on the material’s admittance. As shown in Fig. 1, infinitely long sample is justified if a short circuit at the open end of the loaded waveguide does not cause a change to the reflection coefficient. According to Altschuler [1], the dielectric properties are then given by:

80

Lee

(1/(1+(Og/Oc)2))(r – j(tan(N(D – DR)))) Hr = [(1/(1+(Oc/Og)2))] + [] (1–jrtan((N(D – DR))))

(5)

where N = 2S/Og, Og = guide wavelength, Oc = cutoff wavelength, r = VSWR measured by any impedance meter, N (D-Dr) represents the phase shift derived from the shifting of the minimum position relative to the reference short circuit.

Calibration All ANAs possess inherent system errors. To obtain better accuracy, these errors have to be removed. This is done by representing the ANA as an ideal instrument connected to a load impedance via a reciprocal error box as seen in Fig. 2.

Fig. 2. A representation of an ANA.

The error box transforms the measured reflection coefficient * to *L. The error box has four unknowns, the S11, S12, S21, S22, but S21 = S12. The three unknown Sparameters can be solved by three known terminations, such as a short circuit, open circuit and a match load as shown in Somlo and Hunter [7]. *L = S11 +[(S212*)/(1-*S22)]

(6)

The accuracy of the three terminations determine the accuracy of the measuring system. The short circuit is to be ideally placed at (-1,0) on the polar display, the open circuit at (1,0) and the match load at the centre (0,0).

Dielectric characterisation of high loss and low loss materials at 2450 MHz

81

It is easy to obtain a good short circuit and open circuit terminations but the matched load termination must have a return loss better than -40 dB to get 0.0001 error results. All industrial load terminations have a maximum VSWR of 1.15, corresponding to a reflection coefficient of 7%. This is not good enough. We used HFSS to model the reflection coefficient of several load terminations using water (76-j10) as absorbing material at T = 90, 45,15 and 6 degrees shown in Fig. 3. The modeling of a 90 degrees termination though impractical is used only as a guide for comparison. Table 2 shows a comparison of the magnitudes for the reflection coefficient at 2.45 GHz. Fig. 4 shows a matched termination simulated for frequencies from 1.8 GHz to 2.6 GHz at 6 degrees ramp angle to the absorbing termination.

Ramp termination

Waveguide

T Fig. 3. Matched termination design.

Fig. 4. Return loss of the precision matched termination used.

Results and Discussion Tables 3 and 4 show the results of the dielectric properties of several medium and low loss materials obtained using the above procedures.

82

Lee

Table 3. Dielectric characteristics of food materials. Materials

Mushroom Ginger Garlic Onion Potato Minced Beef Corn Kernels Rice(dry) Pasta Yellow split beans Sesame Seed Flour

Density (g/cm3)

Dielectricconstant

Loss factor

0.85 0.86 0.72 0.97 ----0.78 0.82 0.832 0.73

--------1.03 1.16 1.59 1.38 1.58 1.3

45.0 55.0 68.0 65.0 59.9 39.9 3.1 3.3 3.2 2.5

7.800 10.200 13.000 12.100 19.205 11.474 0.418 0.445 0.392 0.169

22.4 22.4

0.60 0.75

-----

1.9 2.2

0.009 0.126

22.4 22.4 22.4

0.33 0.38 0.33

-------

1.4 1.7 1.8

0.003 0.100 0.112

Moisture Content (%) 94.0 93.4 67.0 93.9 79.0 70.0 11.4 12.2 9.0 8.3

Temperature (oC) 22 22 22 22 4.2 2.5 22.4 22.4 22.4 22.4

3.7 11.8 5.1 9.7 11.2

Breadcrumbs Oats Wheat germ

Bulkdensity (g/cm3)

Table 4. Dielectric characteristics of low and medium loss materials. Materials Teflon Untreated Pine Styrofoam Wet Cement Block Wet Brick Plaster Board

Moisture content(%) --11 --15 10 ---

Density (g/cm3) 2.26 0.52 0.01 -------

Dielectric constant 2.07 2.62 1.09 14.90 3.30 2.10

Loss factor 0.04 0.54 0.02 6.60 0.60 0.12

The results show that the combination of the two-position and the infinite line method [5] is suitable for all types of materials, low loss to high loss. Clean surfaces within the waveguides and contact between surfaces of the connectors were an important factor in obtaining good results as stated in Yeo et al [6]. They should ideally be silver or gold plated and polished to a smooth finish. Wall losses in the waveguides must be accounted for as discussed by von Hippel [8]. When a measurement is made at the open circuit position with reference to the short circuit, its position is critical. If the open circuit is not a quarter wavelength from the short circuit, errors in the measurement will occur. We investigated the level of error in the measurement (G|Hr|/Gx) where G|Hr| is the change in relative dielectric properties and Gx is the change in distance relative to the correct open circuit position. We combined (3) and (4) to form:

Dielectric characterisation of high loss and low loss materials at 2450 MHz

[(1/(jzotan(Exo)))(1/(jzotan(Ex)))+(Og/Oc)2] Hr(x) =  1+(Og/Oc)2

83

(7)

where Hr = dielectric properties, xo = short circuit position, x = open circuit position, zo = characteristic impedance and E = 2Sf. In solving the differential for (7), all other constants and are simplified by k to: G|Hr|/Gx = -k/[sin2(Ex)]

(8)

Change in Dielectric properties

Change in Dielectric Properties of rice vs delta x

Loss factor

0,3 0,25

Dielectric constant

0,2 0,15

Polynomisch (Loss factor)

0,1 0,05 0 -4

-2 -0,05 0

2

4

Polynomisch (Dielectric constant)

Delta x

Fig. 5. Rate of change of (G|Hr|) versus deviation (Gx) from open circuit position of a rice sample.

Fig. 5 shows the deviation of the dielectric properties of rice when the open circuit position is moved by a distance Gx. The present method has several advantages over the open ended and open shielded coaxial line probe: (a) The sample size is much bigger, which can account for any inhomogeneity and provide a better average characterization without grinding or mincing most materials. (b) The sample surface need not be very flat as in the case of the open-ended probe. (c) It is simpler to calibrate and measure materials without the intricate preparation needed for the open-shielded coaxial line probe. (d) It is inexpensive to setup. (e) With little modification, the procedure can characterize both solids and liquids alike.

84

Lee

Conclusion We have developed a simple waveguide technique for characterizing dielectric properties of food materials for microwave heating. Our results show that we have achieved reasonable accuracy in half the time as shown in Table 3 and 4. The application can be used for liquid and solid materials, especially bulky items like wheat grain, rice or dry corn kernels that otherwise would have to be ground up for the open shielded coaxial line probe. The grinding procedure affects the integrity of the material and maybe the dielectric characteristics of the original sample. The infinite line procedure can be used to determine the dielectric characteristics of food materials like potatoes, meat items that possess high moisture content. Other food materials can utilize the two-position method. Samples that have still have to be ground to fit into the sample holder can use a mixing formula to attain their solid material permittivity.

Acknowledgment This research has been possible through an ARC SPIRT scholarship and the active support of ERICSSON Australia.

Literature [1] Sucher and Fox (ed), “Handbook Of Microwave Measurements”, vol II, Dielectric constant, Polytechnic Press, 1963, pp. 502 & 512. [2] Baker-Jarvis J., Janezic M.D., Domich P.D., Geyer R.G., “Analysis of an open-ended coaxial probe with lift-off for nondestructive testing” Instrum. and Meas., IEEE Tran., vol: 43 Issue: 5 , Oct. 1994, pp. 711-718. [3] Baker-Jarvis J., Janezic M.D., Jones C.A., “Shielded open-circuited sample holder for dielectric measurements of solids and liquids” IEEE Trans., Instrum. and Meas., vol: 47 Issue: 2 , April 1998, pp. 338 –344. [4] Baker-Jarvis J., Vanzura E.J., Kissick W.A., “Improved technique for determining complex permittivity with the transmission/reflection method.” IEEE Trans., MTT, vol: 38 Issue: 8, Aug. 1990, pp.1096 –1103. [5] Surber, Jr. W. H. and Crouch, Jr. G. E., “Dielectric Measurement Methods for Solids at Microwave Frequencies J. App. Phys. Vol.19, Dec.1948, pp.1130-1139. [6] Yeo T.S., Tran V.N., Kooi P.S., and Leong M.S. “Microwave Complex Permittivity of Rain Water”, Asia Pacific Microwave Conference, Adelaide, Aug. 1992, pp.617-620. [7] Somlo P.I. & Hunter J.D., “Microwave Impedance Measurement”, Peter Peregrinus Ltd., London.,UK, 1985, pp. 30-31. [8] von Hippel, A. R. (ed) “Dielectric materials and applications”, MIT Press, 1966, p. 86.

European Regulations, Safety Issues in RF and Microwave Power Walter Van Loock Ghent University, B-9000 Ghent, Belgium

Abstract Most of the radio spectrum is being exploited by commercial telecommunication services. In Europe, directives and many standards dealing with equipment electro-magnetic compatibility (EMC) and with human exposure to electromagnetic fields (EMF) are threatening all other use of the radio spectrum. Shortcomings of the standards are discussed and compared with the actual electromagnetic environment. The standards are likely to slow down high frequency and microwave power applications. Recommendations are presented for improvement.

Introduction Today most of the radio spectrum is being exploited by commercial telecommunication services and information technology. All other applications are denoted as ISM (industry, science, medicine) which means the use of radio frequencies for industrial and domestic processing, in scientific research and in medicine [1]. The medical use of the radio spectrum which is usually bound to medical supervision in clinics, is not considered here. The selection of a radio frequency for a specific application can be based on many arguments and an optimum frequency may exist. However, regulations and laws dictate the frequency to be used. The world wide framework for radio frequency management is provided by the ITU (Int. Telecommunication Union) Radio Regulations (ITU-R) as adopted by the World Radiocommunication Conferences (WRC-95 and -97), formerly WARCS, World Administrative Radio Conferences [2]. The international treaty ITU-R or the “Radio Regulations” together with criteria and recommendations provide a general basis for the use of the spectrum and to prevent interferences. The IEC (International Electrotechnical Commission) provides a world wide regulatory basis for access to the radio frequencies and to

86

Van Loock

prevent interferences in more detail, in particular with the issue of EMC, electromagnetic compatibility. In Europe, the EMC Directive 89/336 EC (European Commission), explains electromagnetic compatibility [3]. This directive, in force since 01/01/96, requires that all new manufactured goods placed on the market within the European Union must comply with it. EMC implies emission and immunity requirements which are given in many EN (European Normative) standards for devices, appliances, installations and systems. On going concerns about human safety when exposed to electromagnetic energy has generated international safety guidelines (ANSI, IEEE, WHO, ICNIRP,…). This framework provides a basis for national guidelines and standards for human exposure. In Europe, this electromagnetic compatibility problem is referred to “safety levels for human exposure to electromagnetic fields, EMF”. A key question for ISM users of the electromagnetic spectrum is to what extend the new standards ensure continuation of scientific research and useful applications. In particular, for high frequency and microwaves heating applications, the understanding of the interactions with materials is essential not only from a scientific point of view but also for to foster technical innovation for example in processing techniques and industrial and residential application of this technology. In Europe and in its member states the standards are complicated and limit the radio spectrum use for high frequency and microwave heating. Compliance with the standards involves higher investment and maintenance costs and therefore resulting in a slow down of future research and applications. Important standards for equipment EMC and for human safety in EMF are discussed and compared with the actual electromagnetic environment. Some recommendations for improvement are presented.

ISM equipment and the radio spectrum For ISM and in particular for scientific research, access to all radio frequencies is desirable. Such a demand is becoming extremely difficult because the spectrum is almost exclusively predestinated for radio-communication. The high frequency and microwave ISM bands for Europe, according to ITU-R, are listed in Table 1. As can be seen, only a limited number of frequency bands are available and there is a large gap between 40 and 2450 MHz. Suppose a research project for ceramics requires 10 kW at 400 MHz which is not an ISM frequency. The EMC standard EN 55011 sets the radiated emission limit at this frequency to 40 dBuV/m at 10 m distance. Isotropical radiation of 10 kW in space produces 155 dBuV/m at the same distance. Compliance with the standard dictates therefore a shielding effectiveness of 115 dB. This value is known as a high quality performance of a shielded room. In practice, double shielding will be necessary: equipment shielding and location of the equipment in a shielded room increasing the cost of the project.

European Regulations, Safety Issues in RF and Microwave Power

87

Table 1. ISM bands for Europe, according to ITU-R (1998)

Center frequency 13.560 MHz 27.120 MHz 40.680 MHz 2.450 GHz 5.800 GHz 24.125 GHz

Frequency band 13.530 - 13.567 MHz 26.975 - 27.283 MHz 40.660 - 40.700 MHz 2.400 - 2.500 GHz 5.725 - 5.875 GHz 24.000 - 24.250 GHz

The ISM bands used to be “unlicensed bands” with the privilege of unlimited radiation. In the past, services such as telecommunication in an ISM band had to accept potential disturbances caused by ISM equipment. Business pressure is now affecting parts of the ISM bands and also the privilege of unlimited emission. ITU-R does not list ISM bands in the mm range. Millimetres waves are difficult to shield, involving more costs for scientific research and making industrial mm applications are almost impossible. A new development is Bluetooth, a wireless connection that will make communication cables obsolete [4]. Bluetooth is a specification for low cost, short range wireless devices. It uses frequency-hopped multiple access, time-divisionduplexes radios in the ISM band 2.4 – 2.5 GHz. The problem of interferences on the wireless connections caused by microwave ovens is being investigated and now well documented in literature [6 – 8]. Technical innovative measures to cope with the in band unstable emissions of domestic ovens are not considered. Instead of choosing adjacent bands, recommendation are given for limiting the ISM band to a few megahertz and reducing the level of oven leakage [9]. The success of the domestic oven as a commercial inexpensive and useful high technological consumer product is based on the freedom to radiate with an unstable power source. Without this freedom, microwave ovens, microwave power applications and ISM industry would not exist. Removal of ISM bands can kill future expansion in all these areas and ever threaten existing business. The mission of the Radiocommunication Sector of ITU is to ensure the rational, equitable, efficient and economical use of the radio frequency spectrum. In the latest edition of ITU-R, the ISM bands have no longer priority for ISM use and the privilege of unlimited in band radiation has degraded to recommendation to administrations for minimal radiation. Clearly, the specific problem of ISM use is technically not handled on an equitable basis.

ISM equipment and the radio environment Systems for high frequency and microwave heating usually involve high power and unintentional radiation in space. Transmitter antennas of radio stations radiate intentionally radio frequencies in an specified allocated frequency bandwidth. When high powers are involved transmitters produce emissions in adjacent bands

88

Van Loock

and numerous unintentional emissions at spurious frequencies. This fact is well known and it is not compatible with regulations or radio laws but it is tolerated by administrations world wide. Analogous to the problem of transmitter stations and the interference tolerance, administrations should consider conditional agreement for the use of a non-ISM frequency in Europe for example 920 MHz. The licensing should be based on a code of good practice and a limited local non-harmful interference must be tolerated. Organizations and business involved in ISM are urged to use pressure to obtain such licensing agreements. High frequency and microwave installations rarely cause EMC problems because of the unstable frequencies. Today, digital techniques in communication and information technology are not likely to be disturbed. In case of a complaint then an administrative demand for assessment of compliance and adjustment should be possible. Clear produces should be available in the framework and the standards. The unstable emissions from 915 MHz installations have been shown not to disturb telecommunication equipment provided the level is limited to 60 dBuV/m at 30 m distance. In spite of the world wide success of 0.9 GHz ISM equipment, for large scale industrial processing, a relaxation of the emission limit according to the state of the technique is not yet obtained in Europe. A proven and safe way for avoiding interferences is to monitor the electromagnetic emissions at a reasonable distance from the installation. Electromagnetic compatibility standards are conceived for low power electronic consumer products and these standards are not compatible with high power equipment such as the domestic oven and large high frequency and microwave systems. In the Europe, the relevant standards are: the EN 55011 which designates the ISM bands (see Table 1, except for the UK where 896 MHz is allowed provided the emissions are limited), classifies the equipment and gives complicated emission limits; the EN 60555 and EN 61000 to limit the harmonics and voltage fluctuations on the power line. Detailed shortcomings and the consequences when complying with the conducted low and high frequency emissions, and the problems of the radiated emissions are described elsewhere.

Human safety in the radio environment The draft standard ENV 50166 developed to protect humans from the hazards of electromagnetic fields (EMF) for the range 0 – 300 GHz has been withdrawn and partly replaced by the European Specification ES 59005 of 1998 [10] and the publication of 12 July 1999 [11]. Many international accepted guidelines dealing with human safety in electromagnetic fields (EMF) exist. Although the historical basis is different, the guidelines of WHO (World Health Organization) are considered as the framework to protect workers and the general public for the development of national and international standards. The last decade, much confusion has arisen from mobile tele-

European Regulations, Safety Issues in RF and Microwave Power

89

phone technology. Some countries in Europe have adopted in new standards more stringent levels than in the conventional guidelines such as WHO. For ISM use, in particular for high frequency and microwave heating systems when open systems such as tunnels are involved, this evolution is important. Safety guidelines usually explain that a time averaged whole body SAR (specific absorption rate) of about 4 W/kg is the limit above which known and scientific proven negative biological effects are generated, see for example WHO [12]. This level of 4 W/kg is sometimes called the risk level. For workers, a safety factor of 10 is used giving the accepted basic restriction of 0.4 W/kg. For the general public, a factor 5 is applied so that the basic restriction for the general public becomes 0.08 W/kg. The basic restrictions are used to calculate the incident power densities and field strength. For the interesting ISM frequencies, the incident power densities and field strength considered safe for whole body exposure are given in Table 2. Applied to antennas of mobile telephone base stations, safe distances from the antenna are usually less than 1 m in the direction of the main lobe. This distance reduces to a few decimetres for other directions. These facts are well documented in literature. Table 2. General accepted safety levels

power density, W/m2 electric field, V/m workers public workers public ________________________________________________________________ 27 MHz 10 2 61 19 900 MHz 22.5 4,5 92 41 2 400 MHz 45 9 130 58

Frequency

The logic of deciding for 4 W/kg is not correct. For example in the WHO document, see p. 21: In normal thermal environments, an SAR of 1 – 4 W/kg for 30 minutes produces average body temperature increases of less than 1°C for healthy adults. Thus, an occupational RF guideline of 0.4 W/kg SAR leaves a margin of protection against complications due to thermally unfavourable environmental conditions. Clearly, 1 to 4 W/kg produces an average temperature increase of 1°C in healthy adults. In normal conditions, it is believed that the body of a healthy adult can cope with this increase. Therefore the basic restriction should be 1 W/kg and not 4 W/kg. This is the standard procedure of good practise for generating a safety standard. It implies that a minimum safety factor of 4 has to be applied on all the EMF guidelines of WHO and on all others. Furthermore, the WHO guidelines and many other guidelines do not give a definition of “normal conditions”. The difference between the power density levels for the workers and the public, the factor 5, is too small, because workers are supposed to be adults in good condition and able to execute coordinated operations. There are many other arguments such as other than adults, in extreme conditions of temperature and humidity and

90

Van Loock

not sufficient ventilation and abnormal clothing. Also, individuals having an extremely poor thermoregulation do exist. The actual standard for workers is in the range of the sensitivity threshold. The standard to protect the general public is only a factor 10 below the incident power density which one can feel for example on his hand. Metal plates can give standing waves. Resonant waves can exist between metal walls, typically in factories. Complaints such as uncomfortable feeling are therefore likely not only in an industrial environment but also in the vicinity of antennas where people have to live 24 h a day. Therefore, the guidelines have to be reduced. Simple calculations reveal that the levels in the standard are too conservative. Also, it is not appropriate to distinguish between the exposure of workers and of the general public and to specify levels higher by a factor 5 for the former. The technical details of recent accidents with workers in the vicinity of mobile telephone base station antennas, clearly prove this statement. Workers had to be hospitalised with severe hyperthermia problems. Therefore a safety factor of 100 on the general accepted levels is necessary, see Table 3. Some countries have implemented such a safety factor to protect the general public in the vicinity of stationary antennas. Table 3. A safety factor of 100 on the general accepted safety levels

power density, W/m2 electric field, V/m workers public workers public ________________________________________________________________ 27 MHz 0.1 0.02 9 4 900 MHz 0.23 0.045 13 6 2 400 MHz 0.5 0,1 13.7 6.1

Frequency

Local SAR in the body For local exposures of parts of the body, such as hands, very high levels are permitted. It is easy to show that this specifications is only valid when using low power devices such as hand held telephones with a total power output not exceeding a few watts. Computer studies and phantom simulations usually demonstrate a hot spot in the brain when using a mobile phone. Suppose a GSM of max 2 W and that this amount is absorbed in 1 kg of brain. Together with the metabolic heat at rest, the total specific power in the brain is 3 W/kg, which corresponds with a level of moderate activity such as walking. The hot spot and the absorption can cause problems. First, the local heat stress in the brain does not exist in nature and there are no sensors for temperature control. Uncomfortable feeling such as headacke or symptoms as loss of memory are likely to occur.

European Regulations, Safety Issues in RF and Microwave Power

91

Conclusions Compliance with the equipment standards usually requires shielding, filtering and expensive measurement procedures. The higher overall investment costs may slow down future electromagnetic heating applications. Therefore, the standards should be simplified and relaxed. Authorities should tolerate 0.9 GHz installations according to a code of good practice which avoids interferences. Europe has a guideline for safe human exposure to electromagnetic fields. However, the levels for continuous exposure to electromagnetic energy are too conservative. The SAR-values and the corresponding power density levels are too high for the safety of workers and the general public. All safety levels are to close to the sensitivity threshold. People tend to complain and do not feel comfortable.

References [1] CENELEC, 1991. EN 55011 Limits and methods of measurements of radio disturbance characteristics of industrial, scientific and medical (ISM) radio-frequency equipment, Brussels 1991 [2] ITU-R Radio Regulations, edition 1998, International Telecommunication Union, Geneva. [3] European Commission, “Guideliness on the application of Council Directive 89/336/EEC on the approximation of the laws of the Member States relating the electromagnetic compatibility, 1989. [4] D. Rosener, “Bluetooth integration poses challenges for developers”, Microwaves & RF, May 2000, pp. 55-65. [5] C.R. Buffler and P.O. Risman, “Compatibility issues between Bluetooth and high power systems in the ISM bands”, Microwave Journal, July 2000, pp. 126-134. [6] E. Rejman, “Bluetooth puts bite on mobile communications”, Microwave Journal July 2000 pp. 110-124. [7] T. Kowada et al., “Interference on wide-band digital communication by disturbances in the GHz band”, Proc. EMC Conference, Tokyo 1999 pp 317-322. [8] A. Kamerman et al, “Microwave oven interference on wireless LANs operating in the 2.4 GHz ISM band, Revue HF 2000. [9] J.M. Osepchuk, “The Bluetooth threat to Microwave Equipment”, Microwave World, Vol. 20, no 1, May 1999 pp.4-5. [10] ENV 50166. 1995, European Prestandard ENV 50166-1: January 1995 - Human Exposure to Electromagnetic Field – Low frequency (0 Hz to 10 kHz). European Prestandard ENV 50166-2: January 1995 - Human Exposure to Electromagnetic Field – High frequency (10 kHz to 300 GHz) [11] European Communities Counsel Recommendation of 12 July 1999 on the limitation of the geneal public to electromagnetic fields (0 Hz to 300 GHz), 1999/519/EC, p 59-68. [12] WORLD Health Organisation, Electromagnetic Fields (300 Hz to 300 GHz), Environmental Health Criteria 137, Geneva 1993.

Future Prosperity of Industrial, Scientific and Medical (ISM) Applications of Microwaves David Sánchez-Hernández1 and José M. Catalá-Civera2 1

Grupo de Ingeniería de Microondas, Radiocomunicaciones y Electromagnetismo, Departamento de Tecnologías de la Información y Comunicaciones. Universidad Politécnica de Cartagena, , Antiguo Hospital de Marina, Campus Muralla del Mar. E-30202 Cartagena, Spain. 2 Grupo de Calentamiento por Microondas (GCM), Departamento de comunicaciones. Universidad Politécnica de Valencia. Camino de Vera, S/N, E46071 Valencia, Spain.

Introduction Computing and wireless communications technologies have developed at a stunning rate over the last few years, and nowadays there is a great deal of enthusiasm in general public and telecommunications industry about the potential for convergence and new applications. Two years ago, a TV station in Dallas, Texas (USA), accidentally crippled the Baylor University Medical Center when it began testing digital TV signals. Part of Baylor’s wireless patient monitoring telemetry, which operated in the same band, experienced interference. No patients were injured, but the event produced the Federal Communications Commission (FCC) to find a dedicated chunk of spectrum for medical telemetry, and that was 14 MHz for the Wireless Medical Telemetry Service (WMTS), broken up into three bands; 608 - 614, 1395 - 1400 and 1429 - 1432 MHz. The FCC did not specify transmission standards, and it is up to hospitals to prevent interference within the WMTS from competing technology. Part of the allocated upper band is currently used by government radars, which will not go completely off-line until 2009. In its communication, the FCC predicted that the old bands (UHF and VHF) will become increasingly troublesome to use in medicine (they are expected to be use by Digital TV by 2003, police and other private-Land Mobile Radio users). Hence, the only alternative is to move WMTS to the unprotected but much larger industrial, scientific and medical (ISM) band (84 MHz band centred at 2.45 GHz), which is also home to devices using IEEE 802.11b wireless Ethernet standards. This is a clear example about how ISM applications can be considered in the large communications markets of the USA, Europe and Japan. The aim of this paper is to bring up the current problems and future prosperity of ISM applications,

Future prosperity of Industrial, Scientific and Medical applications of microwaves

93

particularly those related to the fierce competition for the use of the spectrum. Some of the big threats are outlined, and possible actions to be undertaken by the ISM industry and academia are indicated.

The big threat: Wireless LANs BluetoothTM is a known star by now in this fierce scenario. BluetoothTM, pioneered by Ericsson, is a short-range (30 m) wireless network, and it operates in the 2.45 GHz ISM band using frequency-hopping spread-spectrum. Much later, Nokia, IBM, Toshiba, TDK, Alcatel and Intel joined Ericsson and mapped out the basics of the specification and launched a Special Interest Group (SIG) to promote development of the technology. Today, there are more than 2000 members of the BluetoothTM SIG. The BluetoothTM protocol standard has been designed as a generic standard upon which multiple wireless applications will be developed, and is gaining popularity as a ‘cord-killer’ between devices, opening up a wide range of new and exciting proximity-based applications and services for the telecommunications industry. In December 2000, experts predicted BluetoothTM to happen not much before 2002. Predictions have demonstrated its non-viability in the wireless market before, and in February 2001, Ericsson announced the commercial availability of its PBA 313 01 BluetoothTM radio module, operating in the 2.4 2.5 GHz ISM band, with a Class 2 RF output power (outputs 2.5 mW (+4 dBm) at maximum). Version 2 of BluetoothTM is expected by mid-2001. Similarly, prognosticators have forecast a possible 200 million BluetoothTM -enabled devices in 2003, and up to 670 million by 2005. ‘Imagine a cooker –this could be a microwave oven1– that sends a message to the TV that you are watching to tell you that your meatloaf is done’, writes Anders Edlund, marketing director for BluetoothTM at Ericsson Mobile Communications, reports. 7Layers is one of the first independent BluetoothTM qualification bodies to be appointed. In five years, the market for wireless BluetoothTM devices should surpass € 3.22 billion, as it can be observed from Figure 2.

Fig. 1. BluetoothTM wireless connectivity.

1

This is a personal comment

94

Sánchez-Hernández

Market will surpass € 3.22 billion by 2005 1350

1400 1200 1000 800 600 400 200 0

780 420 75 0.3

2000

2001

180

2002

2003

2004

2005

Fig. 2. BluetoothTM enabled equipment (mil., Source: Cahners In-Stat Group, Mobilian Strategic Marketing, 2000).

The USA-based Institute of Electrical and Electronics Engineers (IEEE) has introduced a series of standards for wireless LANs. The first for the 2.45 GHz band, IEEE 802.11, catered for data rates of 1 - 2 Mbit/s. This was followed by IEEE 802.11b, which gives a data rate of 11 Mbit/s. The IEEE 802.11b standards have widespread backing from USA telecommunications industry, including the Wireless Ethernet Compatibility Alliance, whose membership includes major telecom industry players. IBM, Motorola, Proxim and others are presently supporting a standard called HomeRFTM, which has a much more limited range and is much slower than IEEE 802.11b, but it is also much less expensive and easier to install. Like BluetoothTM and IEEE 802.11b, HomeRFTM shares the 2.45 GHz spectrum. Yet, this market is really competitive, and Motorola, for example, has recently acquired the Denmark-based Digianswer, one of the leading players in BluetoothTM design and engineering. Prior to the acquisition, both Toshiba and IBM had committed to deals with Digianswer. Very much like BluetoothTM, IEEE 802.11b or HomeRFTM, RWRTM (Rooftop Wireless Router) is gaining confidence, sponsored by Nokia, and also running in this ‘unlicensed’ 2.45 GHz band. The RWRTM does not even need a Line-Of-Sight (LoS) link to the service provider, and it is just necessary to see other node in the net. RWR is then a self-configurable and self-heal system as new customers connect to the local net. The RWR can be moved since the radio portion of the gear is separate from the router portion and can be independently re-engineered. Fortunately for the ISM industry and academia, there are already clear signs of the traditional USA/Europe/Japan split for wireless LANs technologies, as it can be observed from Table 1.

Future prosperity of Industrial, Scientific and Medical applications of microwaves

95

Other challenging technologies Likewise, new standards, technologies and partnerships are driving a global boom in the radio-frequency identification (RFID) industry. Comprising an antenna, transceiver and transponder, RFID tags and labels are fast becoming an important technology. Revenues world wide are expected to total € 1.72 billion in 2002 and reach € 8.05 billion by 2006. Large companies like Philips, Texas Instruments and Motorola are beginning to dominate the production of high-volume systems. The European preference is for active tags, which use battery power to broadcast information to the tag reader. Regional differences apply for frequency bands. In Europe, the 420 - 460 MHz band has been favoured for UHF RFID. In North America, the 902 - 928 MHz band is used. Table 1. International IEEE 802.11 Frequency Allocations.

Geography Regulatory Range USA, Europe, and most 2.400 to 2.4835 GHz other countries Spain 2.445 to 2.475 GHz France 2.4465 to 2.4835 GHz

RF Channels f=2.402 + k MHz, k=0,...,78 f=2.449 + k MHz, k=0,...,22 f=2.454 + k MHz, k=0,...,33

Despite these differences, a combination of new standards and technologies is promising to bring Europe and North America close together, to great disregard of ISM applications, since the recent RFID frequency allocations in Europe at around 869 MHz. Future allocations will be defined in part by the ISO 18000 standard for item management and area interface. The standard is currently under development and covers RFID in five regions of the radio spectrum, defining the appropriate parameters for air-interface communications. These regions are (sig) the sub135 KHz, 13.56 MHz, 2.45 GHz, 5.8 GHz and UHF bands. Intermec, Philips, Gemplus, Texas Instruments, SCS Corporation, Zebra Technologies, Symbol Technologies are some of the companies that have already announced developments of RFID smart labels and tags in the 915 MHz and 2.45 GHz bands.

Fig. 3. RFID systems track goods and luggage in transit.

96

Sánchez-Hernández

What future can we expect for the ISM band ? Although over 300 million domestic and industrial microwave devices already in use today, oven sales at approximately 8 - 10 million units per year, and a number of ISM applications steadily increasing, health and industrial applications do not lead to a business industry large enough to compete with giant telecommunications operators. UK-based analyst ARC Group predicts that world-wide revenues for broadband fixed-wireless services will reach € 45 billion in 2005 -a 53-fold increase from the 2000 figure of € 849 million-, with more than 27 million broadband fixed-wireless installations, mostly in the USA and western Europe. These are rather large figures for the ISM industry and the possibilities for spectrum interference are out of doubt. Never before has spectrum interference faced the potential for millions of microwave devices interfering with hundreds of millions of communications devices, so ISM future and prosperity are at risk, and this is in the short term, unless some actions are undertaken and both ISM industry and academia join efforts. Perhaps one key action would be the approval by Health Care Governmental Institutions to adopt ISM products as the seal for that technology’s approval in medicine, which in turn could lead to a greater protection over big communication business. Another possibility is to convince regulatory bodies to exploit other parts of the spectrum free of interference. The sub-band from 2.4 to 2.5 GHz designated as ISM (industrial, scientific and medical) has had spectrum priority for over 50 years, transmitters and de-regulated links are an unprotected secondary service, so they must tolerate any interference. In fact, despite the cellular industry’s explosive growth, rather large segments of the spectrum are still underexploited. Modern technology leads to a more efficient use of the spectrum, and that could undoubtedly boil down to a freeing-up process of the spectrum. But this is not happening, and the less spectrum is used by TV or voice channels, the more channels can be delivered, or at least that is the way the broadcasting and telephone companies think of. In cellular communications, for example, the use of adaptive antennas and real-time digital signal processing techniques allow for capacity enhancement. The result is the ability to conduct many more conversations in any given cell and with the same amount of radio spectrum. Ultra-wideband and Timemodulated Ultra-wideband (UWB and TM-UWB) systems, which combine bandwidths in excess of 1 GHz with very low power spectral densities (PSDs) are currently attracting growing interest as a mean of wrestling additional capacity from the already heavily utilised store of wireless bandwidth. UWB devices are yet to be regulated and even considered within the EU, while the USA has started the process of setting up a regulatory framework [1 - 2]. A few companies have already tested UWB devices operating in the 2.4 – 2.5 GHz band. Another good example of the possible exploitation of other portion of the spectrum is the Local Multipoint Distribution System (LMDS). Europe has earmarked spectrum for broadband access systems (like LMDS) in the 40.5 – 43.5 GHz. The bandwidth allocation is substantial, but it is an enormous challenge for the engi-

Future prosperity of Industrial, Scientific and Medical applications of microwaves

97

neering design point of view to get commercial products operating in this range in the short term, while offering low cost and meeting the performance required for a broadband network. It is rather easier to use lower ‘unlicensed’ ISM frequency bands. Although some companies are already testing 43 GHz LMDS devices [3], no EU government has yet licensed this band.

Fig. 4. The spectrum of an UWB transmission.

Fig. 5. An LMDS node.

The third generation Universal Mobile Telecommunication System (UMTS) is another lawyer in court. The € 35.43 billion raised in the UK and the € 12.000 millions raised in Spain in their respective auctions is a clear example about the health of mobile business. With ever-increasing mobile services and emerging BluetoothTM devices, the combination of both could be fatal for ISM applications. Christian From, industry marketing manager for Nokia Mobile Phones, is confident that all new mobile handsets will be BluetoothTM-enabled ‘at some point’. figures of one billion units within three years are routinely thrown up, and 70% of Ericsson handsets will be BluetoothTM-enabled within a similar time frame. But not everything is in wireless market favour. The growing concern of the effect of electromagnetic fields (a theme well known by most of ISM scientists) radiated by cellular base-stations over humans is playing a key role in the deploying process. One of BluetoothTM chinks in its armour is the fact that it has the potential to interfere with other short-range communications technologies –specifically IEEE 802.11b–, and its inherently less secure design than line-of-sight technologies like IrDA, the infra-red standard for data transfer, although IEEE has recently appointed a committee to address the diverse WLAN systems coexistence problem. Likewise, BluetoothTM may also be more susceptible to stray electromagnetic interference (EMI) from other devices, particularly wireless LANs, although this is not good news for the ISM industry.

98

Sánchez-Hernández

Regarding BluetoothTM-like technologies, the ISM issue is focused from a different prospective by Orinoco, Lucent’s wireless LAN product range. ‘One possibility for compatibility is to employ the non-emitting time percentage of operating microwave ovens. Orinocco squeezes information between these bursts, so the connection is not lost’, said Harry Stevens, Lucent Technologies’ wireless networks business manager for Europe. But that strategy will not work near multimagnetron installations. This is still to be seen. In any case, ETSI in Europe and the FCC in the USA have yet to consider the implications of a 2.45 GHz device inside a 900, 1800 or 1900 MHz device.

Fig. 6. Lucent’s Orinoco range is said to minimise interference from microwave ovens.

The challenge to ISM has only just started. ‘European mobile operators will consolidate or disappear, and UMTS will be remembered as the trigger that imploded Europe’s mobile industry’, said Lars Godell, telecom analyst at Forrester’s office in The Netherlands. ‘Consolidation will leave only five groups serving all mobile users in Europe by 2008. Scale will become a key success factor as grim profitability prospects and huge capitals requirements take their roll’, predicted Godell. We have clear figures regarding the future of the wireless industry (see Table 2), but is there some similar about ISM markets?. An in depth analysis of this market and its potential through research activities is envisaged. Table 2. The Telecom market in Europe (figures in % GNP)

IT DE IE FR DK ES FI PT NL US Information 3.31 2.73 3.51 2.75 2.63 4.26 3.32 4.84 3.29 2.76 Technologies Telecommu- 1.71 2.59 1.88 2.92 3.17 1.88 2.88 1.56 3.25 4.53 nications

Future prosperity of Industrial, Scientific and Medical applications of microwaves

99

The Spanish scenario The problem of spectrum ‘absorption’ is of particular importance in Spain. In the Catalonia region (Spain), a recent study performed by the Catalonian Secretary for the Information Society, mobile telephony reached 58.3% of all homes, and 18% had two handsets [4]. This number reached 90% when home income were higher than E2525 per month. In comparison, TV sets reached 99.99% of all homes. The Telecommunications sector represents 4.8% of the GNP in Catalonia. This number decreases to 4.26% in Spain (EITO 2000). With the exception of Portugal (4.8%), this is the largest number within the EU. As an example, only a few ISM product companies operate in Catalonia (i.e. SAIREM Ibérica). In fact, the Spaniards prefer mobile phones to any other technology, including the microwave oven [5] (see Table 3). 60% of the population has a mobile phone, and out of the other 30%, 7% answered that they would buy one within the next two or three months. Spain is also the most active European country as far as Napster file exchange is concerned [6]. 22,5% of all home PCs connected to Internet in Spain exchange Mp3 file through Napster, while this value was down to 10.7% in Germany, not-very-closely following up. Hence, the upcoming of Mobile Internet is another serious threat for ISM applications in Spain. Table 3. Use of diverse technologies in Spain (Numbers in %).

SERVICE/DEVICE

HOME

AT WORK

BOTH

Fixed telephone Wireless hadset Answering machine Fax Mobile phone Internet/e-mail

87.3 22.9 34.2 3.8 60.1 10.3

42.5 6.8 14.6 18.6 180. 12.3

37.8 3.3 7.7 1.6 16.0 3.4

NOT AVALAIBLE 8.1 73.5 58.9 79.3 37.9 80.8

In an effort to regulate the fast growing Spanish telecommunications market, the Spanish Ministry of Science and Technology created in 1999 the Comisión del Mercado de las Telecomunicaciones (CMT), in a similar way to other telecommunications-control bodies in Europe and USA (i.e. FCC), which is only concerned with frequency allocation and competence rules for the telecom industry. No contact between ISM industry or academia and CMT has been produced yet, and the few attempts performed by the interuniversity research group at Valencia and Carthagene have got a dilatory response. Another system entering the busy ISM road in Spain is the LMDS, which uses directional antennas in a similar way to more conventional cellular systems. Although originally designed around 28 GHz, LMDS also operates at 3.5 GHz. In 13th march 2000, two days before general elections, the Spanish government allocated and auctioned six LMDS licenses, three around 26 GHz (Broadnet, Sky Point and Banda26) and another three around 3.5 GHz (FirstMark, Abrared and Banda-Ancha). The 3.5 GHz-Band license by FirstMark is currently operating in

100

Sánchez-Hernández

26 cities, expecting to reached 177 by 2004. It is obvious the deploying 10 km separation base-stations with no need of Line-Of-Sight communication is easier that doing it at 26 GHz. The effect of electromagnetic radiation of human health is currently a big issue in most Spanish cities. The problem is so big with general concern that the National Ministry of Science and Technology is preparing a Royal Order with the National regulations for the installation of mobile communications base stations. The Order, currently under the public opinion stage, basically creates a control body that will take care of the safety certificate for the installations, following the European Council recommendation [7]. The problem is of particular importance in the Region of Murcia (Spain), where several Townhalls have decided to stop giving away installation licenses for new base stations until the Royal Order from the Ministry of Science and Technology is officially approved. In fact, the Townhall of Murcia has unilaterally decided to uninstall more than 30 base stations, and one operator has decided to go to justice. In an unprecedented decision, the Regional Judge Office declared valid the Initial Judge Order to uninstall one Retevision (a Spanish mobile communications operator) antenna site, and gave orders to comply with the Order in 13th march 2001. Similarly, the same Judge Office obliged Iberdrola (a Spanish electric supply company) to pay € 3600 as a compensation to a family that lived under an electric transformer ministation for 10 years, ordering Iberdrola to take immediate action to avoid any presence of electromagnetic radiation within the family home. The public concern regarding EMF radiation effects on human health, surprisingly, could have a positive impact on ISM applications. A clear example, apart from the one expressed in the introduction, is the report of Prof. Hawkins from the University of Alaska, who had a complaint about ISM interference (he is doing research on microwave power transmission MPT) with a neighbouring wireless LAN operator. After being informed that MPT at 2.45 GHz has priority (USA) over unlicensed communications networks, the WLAN company filed a formal interference complaint to FCC. The FCC, however, acknowledged in a communication that ISM at 2.45 GHz has unrestricted emissions, with priority over unlicensed communication uses [8]. Yet, previously the FCC suggested that MPT might not be classified as an ISM application, and many more complaints from WLAN companies may come.

Conclusions As a summary, the 2.45 GHz ISM band is already pretty crowed only with wireless communication technologies. Although Europe has also allocated three bands in the 5 GHz region for wireless LANs, some European countries have already reserved parts of the 5 GHz band for military use, and regulations also vary across Europe for indoor and outdoor use. Hence, the 2.45 GHz band attracts most of the investment and research for future communications, and as regulators impose

Future prosperity of Industrial, Scientific and Medical applications of microwaves

101

stricter demands on wireless LANs, the need for consolidation of standards becomes increasingly apparent. The beauty of the 2.45 GHz band is the fact that is, according to telecom companies, cleared for unlicensed data-communication use in most countries, and hence the standard is portable and interoperable. However, regulations controlling the use of this band vary, and some countries do not allow public commercial use, which makes a difference to the wireless services that can be offered around the globe. France is not a good example. Spain has just open up, and there is an everdecreasing number of countries still keeping the 2.45 GHz out of the commercial communications business. In a effort to survive, ISM industry must take an important step and join forces. The ISM industry and academia Observatory is required as a project where the dimension and future potential of all ISM applications is urgently needed. Recently, the German Federal Criminal Investigation Agency has found fake mobile phones that are really guns, which can only be detected when you realise they are heavier. They have four chambers to the gun selected by pressing 5, 6, 7 or 8 in the keypad while pushing the connect button fires it. One of the killed innocent victims will be, undoubtedly unless we get going, ISM applications.

Fig. 7. Mobile health risk.

References [1] FCC, ‘Notice of Inquiry (NOI), Revision of Part 15 of the Commission’s Rules Regarding Ultrawideband Transmission Systems’, 1 September 1998. [2] FCC, ‘Notice of Proposed Rule Making (NPRM), Revision of Part 15 of the Commission’s Rules Regarding Ultra-wideband Transmission Systems’, 11 may 2000. [3] T. Fowler, ‘Mesh networks for broadband access’, IEE Review, January 2001, pp. 1722. [4] Estadísticas de la Sociedad de la Información Cataluña 2000. Secretaría para la Sociedad de la Información. Departamento de Universidades, Investigación y Sociedad de la Información. Generalitat de Cataluña. Abril 2000 (in Spanish).

102

Sánchez-Hernández

[5] S. Del Campo, ‘Actitud de los españoles ante las nuevas tecnologías’, estudio sociológico para Fundes, July 2000 (in Spanish). [6] Source: NetValue. January 2001. [7] Official Diary of the European Communities. Recommendation of the European Council, 12 July 1999, regarding the exposure of general public to electromagnetic fields (0 Hz to 300 GHz). [8] Microwave World. Edited by International Microwave Power Institute, USA. Vol. 21, no. 2, pp. 5, Fall 2000.

Electric Field Measurements for CommerciallyAvailable Mobile Phones Antonio Martínez-González1, Ángel Fernández-Pascual2 and David SánchezHernández1 1

Grupo de Ingeniería de Microondas y Radiocomunicaciones Departamento de Tecnologías de la Información y las Comunicaciones Universidad Politécnica de Cartagena, Antiguo Hospital de Marina, Campus Muralla del Mar, 30202 Cartagena, Spain. 2 ICEM, Ingeniería de Compatibilidad Electromagnética Departamento de comunicaciones, Universidad Politécnica de Valencia Camino de Vera, 14, Valencia, Spain.

Abstract Nowadays, mobile communications technology is a growing market that, continuously, tends towards a more fashionable, amusing and attractive services. But the fast spreading of these radio-networks is seen in most cases as a plague for citizens who feel their health threatened. In this sense, the growing public concern regarding human exposure to EM fields has lead to a wide range of ‘absorbent’ gaskets available in the market. In this paper, electric field values for several commercial handsets with and without ‘absorbent’ gaskets have been measured in an ENAC certified semi-anechoic chamber. Results clearly show that no reducing electric field effect is observed for the gaskets.

Introduction Recently the European Committee for Electrotechnical Standarization (CENELEC) has issued a recommendation on the limitation on exposure of the general public to electromagnetic fields from 0 Hz up to 300 GHz [1]. The European Committee provides the reference levels that guaranty that basic restrictions are not exceeded. Following the pre-standard, various countries in Europe are currently regulating and adapting their legislation, and many absorption related studies have been published [2, 3] In that sense, the inter-university research group of the Technical Universities of Valencia and Cartagena has carried out some measurements over real commer-

104

Martínez-González

cially available mobile phones. Several of these handsets for mobile communications purposes have been measured inside a semi-anechoic chamber simulating, a free space propagation situation. The measurement process agrees with European standard EN 61566:1997, ‘Measurements of exposure to radio frequency electromagnetic fields. Field strength in the frequency range 100 kHz to 1 GHz’, and has been performed in an ENAC enabled site (certified 190/LE443). A photograph of the semi-anechoic chamber can be observed in Figure 1.

Fig. 1. The ENAC certified semi-anechoic chamber

Measurements of radiated electric field were made in the frequency range of 900 MHz. In absence of human model, results can be considered as a worst case situation due to the lack of scattered fields and the electric field values are appropriate to compare with reference levels. In order to obtain higher values for electric field, mobile phones have been fixed to transmit the maximum radiated power during a 6 minutes period. Peak and averaged values have been obtained. The identity of the samples submitted to the test has been reserved because they are not the object of the study. The real object of study are the absorbent pieces announce by manufacturers that have effects over the radiation fields induced inside the users of mobile phones. A good example is depicted in Figure 2 for the Zeropa butterfly. Therefore the samples are identified as MP1 (Mobile Phone 1) and MP2.

Fig. 2. The Zeropa butterfly ‘emf absorbent’

Electric Field Measurements for commercially-available mobile phones

105

Measurements have been completed both with and without the insertion of little pieces near the antenna feed point. These pieces are usually announced by manufacturers as electromagnetic field absorbent gaskets and therefore as a good alternative to avoid the harmful ('they say') radiation from mobile phones. At first, Electric Field intensity has been measured for the two samples MP1 and MP2. Also measurements of Electric Field including "absorbent elements" for MP1 sample have been achieved.

Test Conditions and Results For the measurements, each of these terminals was installed inside the semianechoic chamber on a non-metallic table situating the speaker directly in front of the electric field probe. Measurements of radiated electric field were made at 925 MHz. As has been mentioned, and in order to obtain higher values for electric field, mobile phones have been fixed to transmit the maximum radiated power during a 6 minutes period. The FP4000A (Amplifier Research) isotropic probe was employed for the measurements (10 KHz to 1 GHz), and the examining period was set to six minutes with an audio input signal of a continuous frequency sweep between 100 Hz and 1 kHz under normal operating conditions. After the measuring period has passed, both peak and average field levels are obtained. Figure 3 shows the electric field values for the handsets without absorbers. Results show a lower electric field level registered for handset number 2. It can be seen that the differences between models are about a few Volts per meter. In both cases radiation levels conform to the established limits set at 925 MHz. In Figure 4 the Electric Field measured taking into account the absorber effect is depicted only for mobile phone MP1. MP1

MP2 50 45

40

40

35

35

30

30 E (V/m)

E (V/m)

50 45

25

25

20

20

15

15

10

10

5

0

5

50

100

150 200 Tiempo (seg.)

250

300

350

0

50

100

200 150 Tiempo (seg.)

250

300

350

Fig. 3. Electric Field versus time for handsets MP1 and MP2 without absorbent gaskets.

The worst model is around 20 V/m below the reference level. Likewise, measured results show clearly that no protection effect is observed when publicised absorbing objects are used. Fields levels for mobile phone number 1, the worst case, are unaltered when absorbing objects are used, therefore they can be considered completely unprofitable.

106

Martínez-González

MP1 with LIT protection

45

40

40

35

35

30

30

25 20

25 20

15

15

10

10

5 0

MP1 with ZEROPA butterfly

50

45

E (V/m)

E (V/m)

50

5

50

100

150 200 Tiempo (seg.)

250

300

350

0

50

100

150 200 Tiempo (seg.)

250

300

350

Fig. 4. Measurements with two different absorbent gaskets for MP1.

REFERENCES [1] Council Recomendation of 12 July 1999 on the Limitation of Exposure of the General Public to Electromagnetics Fields (0 Hz – 300 GHz), Official Journal of the European Communities. [2] Martínez González, A.M. and Sánchez Hernández D., ‘ SAR evaluation for picocell environments with the presence of metallic objects in the user body close to the handset antenna’ XIII International Conference on Microwave and High Frequency Heating, Valencia, Spain, September 1999. [3] Spanish Ministry of Science and Technology. ‘Proyecto de Real Decreto por el que se aprueba el reglamento de desarrollo de la Ley 11/1998, de 24 de abril, General de Telecomunicaciones, en lo relativo a las servidumbres, a los límites de exposición y otras restricciones a las emisiones radioeléctricas’, in public consultancy until 18th January 2001.

Use of the Dielectric Properties to Detect Protein Denaturation S. A. Barringer and C. Bircan Department of Food Science and Technology, The Ohio State University, 2015 Fyffe Road, Columbus, OH 43210-1007 U.S.A.

Abstract When proteins denature, they change their water and salt binding capacity, expelling or binding water, and thus should change the dielectric properties of the protein-containing food item. The dielectric properties of samples were measured from 25 - 100°C at 300-2450 MHz and compared to DSC results. Our objective was to determine whether denaturation of collagen in several common muscle foods, lipovitellins in egg yolk, ovalbumin in egg white, and E-lactoglobulin, Dlactalbumin and bovine serum albumin in whey protein, could be detected based on their dielectric properties. The dielectric constant and loss factor increased at the collagen denaturation temperature in muscle foods and decreased at the lipovitellin denaturation temperature in egg yolk. In egg white only the dielectric constant decreased during denaturation, while in whey protein only the dielectric loss factor decreased at the denaturation temperature.

Introduction Dielectric properties determine the response of a material to an electromagnetic field. The inability of the molecules to instantaneously align with the applied electromagnetic field leads to dissipation of electromagnetic energy, usually causing heating. The dielectric properties are characterized by a complex number consisting of a real portion, the dielectric constant or H’, and an imaginary portion, the dielectric loss factor or H”. The dielectric constant is an indication of the polarizability of the molecules and their ability to store electrical energy. The dielectric loss factor is related to the energy absorption and dissipation of electromagnetic energy from the field [1]. Typically, water, ash and temperature are parameters included in predictive equations. Using literature values, for meat at 2 - 3 GHz and 0 - 70qC, the following equations were developed by [2]:

108

Barringer

H’= -52 – 0.03T + 1.2mwater + (4.5 + 0.07T)msalt H”=-22 – 0.013T + 0.48mwater + (4 + 0.05T)msalt A similar study using literature values developed the following equations for meat [3]: H’=8.5452 + (1.0707 – 0.0018485T)mwater + 4.7947mash H”=– 3.599 + (3.447 – 0.0187T + 0.000025T2)mwater + mash(-57.093 + 0.231T) However, it has been reported that changes in physical state could affect dielectric properties as well, particularly if the polarizability properties of the material change. For example, the dielectric constant and loss factor changes after the heating of gluten protein-starch mixtures [4]. The dielectric loss of whey protein solutions increases after denaturation [5]. The dielectric properties change as the electromagnetic field is oriented perpendicular or parallel to meat fibers [6]. When starch gelatinizes, the dielectric loss factor increases but the dielectric constant shows no change [7]. Therefore, denaturation, gelatinization and physical structure affect the dielectric properties, for certain foods. The denaturation process severely changes the configuration of many proteins. In particular, when proteins denature, they change their water and salt binding capacity, expelling or binding water, and thus should change the dielectric properties of the food. Therefore it should be possible to detect protein denaturation by measuring their dielectric properties. Either the dielectric constant or loss factor may be more useful, depending on whether the protein is more strongly interacting with the water or with the salt, respectively. For proteins that do not change the mobility of either water or salt upon denaturation, the dielectric properties would not be expected to show any change during denaturation. Our objective was to determine which proteins cause a detectable change in the dielectric properties when they denature. Samples tested include beef, chicken breast, chicken thigh, salmon, cod, perch, egg yolk, egg white, ovalbumin and whey protein concentrate. Differential scanning calorimetry, DSC, was used to determine the denaturation temperature of the proteins. DSC is the most common method for determining the temperature at which denaturation occurs, by measuring the exothermic and endothermic heat flow that accompany structural changes [8].

Materials and Methods

Materials

Commercially available raw beef, chicken breast, chicken thigh, salmon, cod and perch were used. Samples were taken from these materials by cutting a 2.5 cm diameter cylindrical plug to fit the sample holder, for dielectric property measurement. Commercially available eggs were used. Egg yolk and egg white were separated from each other and sampled by pouring into the container. Twenty percent

Use of the Dielectric Properties to Detect Protein Denaturation

109

whey protein solutions were prepared from 80% whey protein concentrate (Lando-Lakes, St. Paul, MN). 25 g whey protein was gradually added to 75 g water and stirred with a stirring bar for 30 min. Whey protein solutions were also prepared with the addition of 5 or 15% sugar, or 2% salt. The pH was reduced from 6.25 to 4 with 0.1 N HCl for the low pH samples. Individual whey protein solutions: Elactoglobulin (10%), D-lactalbumin (20%) and bovine serum albumin (10%), were prepared by the same procedure used for the whey protein solutions. Separate samples were taken from the same muscle or beaker, to determine both dielectric properties and denaturation temperature by DSC. Both tests, including replicates, were performed typically on the same day, but at least during the same week for any given sample. Dielectric property measurement

An open ended coaxial probe and network analyzer were used to measure dielectric properties (85070B and 8752C, Hewlett-Packard Company, Denver, CO). The probe was mounted with the ground flange facing up. An o-ring sealed the probe into a hole in the bottom plate. Another o-ring seals the bottom to a 2.5 cm inner diameter stainless steel jacketed sample holder attached to a silicon oil temperature bath. A third o-ring seals the sample holder to the lid. The bottom and lid are screwed together with eight screws. The resulting sample holder allows the sample to be heated to above the boiling point without any loss of water. Samples plus the expelled juices were weighed before and after heating to ensure there was no loss of water during heating. The sample fills the entire sample holder so there is minimum head space for the steam to collect. The probe and the cable were fixed so they could not be moved during sample measurement. A calibration was done using a short, air and water, before each set of experiments then checked to insure the calibration was stable. The dielectric properties were automatically calculated from the phase and amplitude of the reflected signal by the computer. A scheme of the experimental setup is shown in Figure 1. Computer

Temperature controller Frequency constant

RTE Neslab

Silicon oil bath

loss

Sample in jacketed holder Network Analyzer Probe

Fig. 1. Experimental set up for measuring dielectric properties

110

Barringer

The dielectric properties were measured at 300 to 2450 MHz from room temperature to 100 qC. Samples were heated up to 115qC then cooled and heated again without removing the sample from the container to determine the dielectric properties of the denatured solutions. All the measurements were repeated at least three times and were reproducible r 6%. DSC measurement

Hermetic aluminum pans were used for sample measurement for DSC. The pan and lid were weighed and approximately 12 mg of the sample was placed into the pan with a syringe. Pans were sealed with a sample encapsulating press using crumpling kit # 900680-902 (TA Instruments, Inc., New Castle, DE). A reference of approximately 7mg distilled water was prepared in a second pan. The pans were weighed to determine the exact weight. Tweezers were used to place the sample pan and the reference into the DSC cell (DSC 2920, TA Instruments, Inc., New Castle, DE). The heating rate was 10qC per min and the temperature was scanned from 20 to 95qC. The peak endotherm temperatures are reported. A typical DSC trace for beef is shown in Figure 2.

Fig. 2. DSC of raw beef

Use of the Dielectric Properties to Detect Protein Denaturation

111

Results and Discussion The dielectric properties and DSC denaturation temperatures were determined for several different protein foods. First, muscle foods were measured and compared to the denaturation temperatures of myosin, collagen and actin to determine which protein is responsible for the change in dielectric properties. The muscle foods include both white and red meat with various water binding capacities so that the proteins denature at different temperatures. Second, eggs, both white and yolk were measured. Egg yolk is an unusual system since it contains 31% lipid, causing the native protein to be folded with hydrophobic groups exposed rather than buried inside the protein. Unlike muscle proteins, these proteins change from a solution to a gel when denatured. Third, whey protein solutions were studied at approximately the same concentration of protein as occurs in the muscle or eggs. The whey protein solutions were studied with various additives to change the denaturation temperature and to confirm that the changes in the dielectric properties match the DSC results as the denaturation temperature is changed. Muscle Foods

Raw beef was heated to 115°C as shown in Figure 3. The raw samples showed a transition, or increase, in both the dielectric constant and loss at approximately 70°C. The sample was then cooled and reheated as shown in Figure 4. The reheated sample showed no transition in the dielectric properties, indicating that an irreversible change was taking place when 70°C was reached for the first time. 90 915 MHz 80

70 915 MHz

60

H'

50

2450 MHz

40 2450 MHz 30

H"

20

10

0 10

20

30

40

50

60

70

80

90

100

110

120

o

Temperature, C

Fig. 3. The dielectric properties of fresh beef during heating, at 915 to 2450 MHz

112

Barringer 80 70

H' 60 50 40

fresh beef, constant

H"

fresh beef, loss factor

30

reheated, constant

reheated, loss factor

20 10

20

30

40

50

60

70

80

90

100 110 120

Temperature, C

Fig. 4. The dielectric constant and loss factor as a function of temperature, for fresh and reheated beef. 915 MHz

As shown from the comparison with the results of the DSC measurements, given in Table 1, a good match with the denaturation temperature of collagen is found. The dielectric properties of various muscle samples were measured during heating and compared to DSC-determined denaturation temperatures and literature values for protein denaturation to determine which protein was responsible for the change, as shown in Table 1 and Figure 5. Both the dielectric constant and loss factor increased at the collagen or connective protein’s denaturation temperature for beef, chicken breast, chicken thigh, perch, cod and salmon, but showed no change at the actin or myosin denaturation temperatures. This increase in the dielectric constant and loss factor is likely caused by denaturation of the collagen surrounding the muscle fiber bundles. As the collagen denatures, it causes the muscle to shrink, squeezing out water and dissolved ions. This “juice”, consisting of water and dissolved salts, may have caused the increases in the dielectric constant and loss factor. The linear decrease in the dielectric constant and increase in the loss factor during reheating is typical of salt solutions [1]. Since the “juice” remained unbound by the protein after the sample was heated, the values for the reheated samples are high and show no transition. Above the denaturation temperature of 70qC, the values for the raw and reheated samples are similar.

Use of the Dielectric Properties to Detect Protein Denaturation

113

Table 1. Comparison of DSC (literature and measured) denaturation temperatures and transition temperature in the dielectric properties for muscle proteins.

Sample

Beef [8]

Protein

Myosin Collagen Actin Chicken breast Myosin [9, 10] Stromal or connective Actin Chicken thigh Myosin [10, 11] Collagen or connective Actin Salmon [12] Myosin and collagen ? Actin Cod [12] Myosin and collagen Actin Perch Actin

Denaturation temperature, ˚C Literature DSC 54-58 59 65-67 71 71-83 83 57-58 62

Dielectric property change, ˚C H’ H” 70-75

70-75

64-65 78-81 57-60

70 82 63

70-75

70-75

63-69 76-79 45-57 65 76-77 45-55 73-74

70 80 50 63 79 75

70-75

70-75

65-70

65-70

55-60

55-60

50-55

50-55

-

71

Beef and chicken thigh are red meat, while chicken breast, perch, cod and salmon are white meat. The water holding capacity of the muscles are also different. These differences cause the connective proteins, which is mainly collagen, to denature at different temperatures, as shown in Table 1. The different muscles were chosen to show that the increase in the dielectric constant and loss factor consistently appeared at the same temperature as the collagen or connective proteins denatured.

114

Barringer

H'

80 70 cod, e'

60

cod, e" perch, e'

50

perch, e"

salmon, e' salmon, e"

40

H"

chicken breast, e' chicken breast, e" chicken thigh, e'

30

chicken thigh, e"

20 20

40

60

80

100

o

Temperature, C

Fig. 5. The dielectric constant and loss factor as a function of temperature, for cod, perch, salmon, chicken breast, and thigh. 915 MHz

Egg Yolk and White

The protein in eggs coagulates during denaturation, forming a gel. The dielectric constant and loss factor of egg yolk showed a transition at the temperature which corresponds to the denaturation of lipovitellin, the major protein in egg yolk. In Table 2 a comparison of DSC and dielectric measurement results is shown for the major proteins in egg. However, unlike the muscle foods, the denaturation corresponds to a decrease rather than an increase in the dielectric constant and loss factor, as shown in Figure 6. In the case of lipovitellin, denaturation does not expel water, but rather the protein unfolds and forms a solid gel, resulting in greater water binding. Therefore a decrease in water dipole mobility can be assumed. Thus the denaturation of this protein would decrease the dielectric constant and loss factor due to binding of the water. The yolk contains 31% lipid by weight, thus causing the low values for the dielectric constant, compared to the muscle. In egg white and purified ovalbumin, the dielectric constant also decreased at the denaturation temperature, as shown in Figure 7. Again, denaturation causes the protein to gel, binding water. When the mobility of ions is unchanged, and only a small amount of water is bound, the loss factor shows no change. In egg white, the loss factor showed no change, possibly because the formation of the ovalbumin gel only inhibited the mobility of the water, which most strongly affects the dielectric constant, and not the mobility of ions, which most strongly affect the loss factor.

Use of the Dielectric Properties to Detect Protein Denaturation

115

Table 2. Comparison of DSC (literature and measured) denaturation temperatures and transition temperature in the dielectric properties for egg proteins.

Sample

Protein

Denaturation temperature, ˚C Literature DSC 65 71 84 85 92.5 90 84

Egg white [13] Conalbumin Ovalbumin Albumin Egg yolk [13] Lipovitellin

Dielectric property change, ˚C H’ H” 80-85 80-85 80-85

45 40 300 MHz, H"

35 30

H'

300 MHz, H'

2450 MHz, H'

25

515 MHz,H"

20 730 MHz, H"

H"

945 MHz, H"

15

1160 MHz, H" 1375 MHz,H" 1590 MHz,H" 1805 MHz,H" 2020 MHz, H" 2235 MHz, H" 2450 MHz, H"

10 5 10

20

30

40

50

60

70

80

90

100

o

Temperature, C Fig. 6. The dielectric constant and loss factor of egg yolk at 300 to 2450 MHz

110

116

Barringer

H', Dielectric constant

72

Ovalbumin, 300 MHz Reihe3 Ovalbumin, 2450 MHz Reihe4 Egg white, 300 MHz Reihe1 Egg white, 2450 MHz Reihe2

67

62

57

52

47 15

25

35

45

55

65

75

85

95

Temperature, oC Fig. 7. The dielectric constant and loss factor of egg white and ovalbumin at 300 and 2450 MHz

Whey protein solutions

Whey protein solutions were tested with various additives known to change the denaturation temperature. The dielectric loss factor of the whey protein shows a small decrease above 75°C, the same temperature at which E-lactoglobulin denatures, as shown in Figure 8 and concluded from the results of the DSC measurements given in Table 3. When 15% sugar is added, both the DSC-measured temperature of denaturation and the temperature of the dielectric loss factor transition are increased by approximately 5qC. The dielectric properties of the three proteins which make up the greatest percentage of whey, E-lactoglobulin,D-lactalbumin and bovine serum albumin were measured. Since E-lactoglobulin is 50% of the protein in whey protein concentrate, it would be expected to have the greatest effect on the dielectric properties of whey solutions. The E-lactoglobulin had a similar curve to the whey protein without sugar, and the dielectric loss factor increased at the temperature of denaturation, as shown in Table 3. The dielectric properties of the pure solutions of the two other major proteins in whey, D-lactalbumin and bovine serum albumin, also increased at their denaturation temperatures, as shown in Table 3. When salt or acid was added to the whey protein solution, the denaturation temperature increased, as determined by DSC, as shown in Table 3. A corre-

Use of the Dielectric Properties to Detect Protein Denaturation

117

sponding increase in the temperature at which the dielectric constant increased was also measured. With the addition of salt or acid, the dielectric loss factor showed no change at the denaturation temperature. It can be assumed that the ionic loss component from the salt becomes so large that changes in the dipole loss from water binding are no longer detectable. However, with the addition of ions the dielectric constant becomes sensitive enough to detect the denaturation of the protein. Table 3. Comparison of the dielectric and DSC results for whey protein solutions with various additives.

Sample WPC solution WPC + 5% sugar WPC + 15% sugar WPC + 2% salt WPC at pH 4 E-lactoglobulin D-lactalbumin Bovine serum albumin

Denaturation, °C By H’ or H” By DSC 75-80 79 75-80 79 80-85 82 83.8 81 85-90 86 75-80 79 70-75 75 85-90 88

14 1 3 ,5 15% sugar

13

no sugar

H", dielectric loss factor

1 2 ,5 12 1 1 ,5 11 1 0 ,5 10 9 ,5 0

10 20 30 40 50 60 70 80 90 10 11 0 0 T e m p e ra tu re , o C

Fig. 8. The dielectric loss factor of whey protein solutions with and without 15% sugar added

118

Barringer

Conclusions The dielectric properties are affected by the denaturation of the proteins collagen, lipovitellin, ovalbumin, and E-lactoglobulin. In egg and muscle foods heated in an enclosed system with no drip loss, denaturation causes a significant change in the dielectric properties. The dielectric properties increased if denaturation made the water and ions more mobile, and decreased if the water and ions became less mobile.

Literature [1] Mudgett RE (1986) Dielectric Properties of Food. In: Decareau RV (ed) Microwaves in the food processing industry. New York: Academic Press, Inc. pp 14-57 [2] Calay RK, Newborough M, Probert D, Calay PS (1995) Predictive equations for the dielectric properties of foods. Int. J. Food Sci. Technol. 29:699-713 [3] Sun E, Datta A, Lobo, S (1995) Composition-based prediction of dielectric properties of foods. J. Microwave Power Electromagnetic Energy. 30(4):205-212 [4] Umbach S L, Davis EA, Gordon J, Callaghan PT (1992) Water self- diffusion coefficients and dielectric properties determined for starch-gluten-water mixtures heated by microwave and by conventional methods. Cereal Chem. 69(6):637-642 [5] Barringer SA, Fleischmann, AM, Davis EA, Gordon J (1995) The dielectric properties of whey protein as indicators of change in polymer mobility. Food Hydrocolloids. 9(4):343-348 [6] Bengtsson EN, Melin J, Remi K, Soderlind S (1963) Measurements of the dielectric properties of frozen and defrosted meat and fish in the frequency range 10-200 MHz. J. Sci. Food Agric. 14:592-604 [7] Miller LA, Gordon J, Davis EA (1991) Dielectric and thermal transition properties of chemically modified starches during heating. Cereal Chem. 68 (5):441-448 [8] Findlay JC, Barbut S. Thermal Analysis of Meat. (1990) In: Harwalkar RV, Maya CY, editors. Thermal Analysis of Foods. New York: Elsevier Applied Science. pp 92-125 [9] Murphy RY, Marks BP, Marcy JA (1998) Apparent specific heat of chicken breast patties and their constituent proteins by differential scanning calorimetry. J. Food Sci. 63:88-91 [10] Kijowski JM, Mast MG (1988) Thermal properties of proteins in chicken broiler tissues. J. Food Sci. 53(2):363-366 [11] Kijowski JM, Mast MG (1988) Effect of sodium chloride and phosphates on the thermal properties of chicken meat proteins. J. Food Sci. 53(2): 367-369 [12] Ofstad R, Egelandscal B, Kidman S, Myklebust R, Olsen RL, Hermansson AM (1996) Liquid loss as effected by post mortem ultrastructural changes in fish muscle: cod muscle (Gadus Morhua L) and Salmon (Salmo salar). J. Sci. Food Agric. 71(3):301312 [13] Barbut S, Findlay JC (1990) Thermal Analysis of Egg protein. In: Harwalkar RV, Maya CY, (eds) Thermal Analysis of Foods. New York: Elsevier Applied Science. pp 126-148

Sandalwood Microwave Characterisation and Oil Extraction V. Nguyen Tran Microwave Research Laboratory, IRIS, Melbourne, Victoria, Australia

Introduction: Sandalwood timber and its oil are highly valued in East Asia in the form of joss sticks and incense in religious ceremonies in temples and pagodas. Sandalwood oil has also been popular in homes as an ointment. Sandalwood timber is highly prized by furniture and ornament makers. Recently it has become popular in the perfume and cosmetic industry and in the practice of aromatherapy [1]. In Australia, in the past, sandalwood oil was used as an effective medicine against gonorrhoea, until the discovery of anti-biotic, and as an active disinfectant on the genito-urinary mucous membrane. Another current medical application is to destroy Staphyloccus aureus and Candida albicu,s much like Australian tea tree oil (Melaleuca alternfolia) [2]. The traditional method of extracting sandalwood oil is steam distillation, which takes up to 72 hours per batch. Solvent extraction using Xylene and sandalwood fines between 40 and 120 mesh is another current method, but it is not in favour because of the solvent residue left in the extracted oil. A sample of steam extracted sandalwood oil from the Australian species (Santalum spicatum) contains 35% D and E santalol. This is good because it is well recognised in the perfume industry that the more santalol present in the oil the better. We are investigating an alternative method of distilling sandalwood oil using microwave energy, in view of its penetrating property into a thermal insulating material and its immediate and volumetric heating. Therefore in contrast to the slow stewing operation of the conventional steam distillation, the microwave process is expected to provide a better yield and higher quality oil because microwaves can reach into oil cavities and cell walls almost immediately. In order to optimise the microwave distillation process, we characterise the dielectric properties of sandalwood materials to gain a better understanding of their behaviour under microwave processing. We used four methods of dielectric property measurements.

120

Tran

Methods of measuring the dielectric properties of sandalwood Many techniques, and variations, have been developed for measuring the dielectric properties of a material. Since the introduction of the Automatic Network Analyzer (ANA), there have been some faster methods of measurement, compared to a well known slotted line measuring technique used by von Hippel [3]. We investigate at least four methods: (1) open ended coaxial line method; (2) open shielded coaxial line method; (3) Transmission/Reflection method; (4) open and short circuit method. Open ended coaxial line method:

As the name implies, this method uses the open end of a piece of coaxial line placed against, or immersed in, a material to measure its dielectric properties. The open-ended probe is depicted in Fig. 1. Basically it makes use of the broadband response of a coaxial line and the small capacitance due to the fringing field at the end. The internal fringing capacitance Cf is not affected, while the external fringing capacitance Co is loaded with the dielectric properties of the material to be measured. The network analyser or similar equipment is used to record the reflected signal. Because the signal variation can be small, to obtain accurate measurements, system errors must be eliminated through calibration. Depending on the diameter of the coaxial line, the open ended coaxial method [4] can measure the dielectric properties over a broad band of frequencies, because the fringing capacitance is found to remain constant over a wide frequency range. This method is most suitable for a large dielectric constant and medium to high loss factor and preferably the sample material should be liquid or semi-liquid. A solid sample must be optically flat to obtain sensible values, because the fringing fields at the microwave frequencies only extend a very short distance from the end. The best accuracy achievable for the loss factor is r 0.01. Minute air gaps between the sample and the probe may cause considerable errors. For low frequency measurements, the centre conductor is extended as a short monopole. Capacitance approximation provides a simple way of representing the open end, and this works well for frequencies up to 20 GHz. The method can be improved to extend to higher frequencies by modelling the field at the open end of the sensor, as reported by Baker-Jarvis et al [5]. At present, field modeling is used in a manual mode because of the computational demand. But there is no doubt that in the near future, automatic measurements using field modelling will become available. Field modelling also provides information on lift-off, when the probe cannot be placed flat against the material to be measured. Tran et al [6] used the open ended probe to characterise vegetables and food from 0.1 to 10 GHz. Referring to Fig. 1,a 3.6 mm coaxial line, at 2.45 GHz, Co = 0.022 pF, Cf = 0.001 pF and Go = 2.3x10-7, therefore the assumption that Go ~ 0 is valid. The input admittance of the probe is given by:

Sandalwood Microwave Characterisation and Oil Extraction

Y/Yo = jZCfZo + jZHHCoZo + GoZoH5/2

2a

121

(1)

2b

Zo

Cf

Co

Go

Fig. 1. Open ended coaxial line probe and equivalent circuit

Where 2a = inner diameter, 2b = outer diameter, Cf = internal fringing capacitor, Co = external fringing capacitor, Go = radiation conductance and Zo = 1/Yo = characteristic impedance of the coaxial line (usually 50 :). A calibrated automatic network analyser measures the complex reflection coefficient S11. Since Y/Yo =(1-S11)/(1+S11) and from (1), H is equal to: H = (jZCtZo)-1.(1-S11)/(1+S11).(1+Cf/Co) - Cf/Co

(2)

Where

Ct = Cf + Co, H = Ho Hr and Hr = Hr'-jHr" Hr' = dielectric constant and Hr" = loss factor According to Hewlett Packard [7], a 3.6 mm coaxial line open ended probe can be used to measure the dielectric constant up to 100, and the loss factor greater than 0.05, when the flatness is at least 25 microns, the sample diameter is greater than 20 mm and sample thickness is greater than 20 mm/—Hr. Baker-Jarvis et al [5] extends the capability of the open ended coaxial line probe, by using appropriate terminations under the material such as short circuit, open circuit or dielectric terminated geometry. An open circuit allows measurements in a strong magnetic field, and an open circuit in a strong electric field. In general, the accuracy of this method is r 2% for Hr and Hr" ! r 0.01. This method is suitable for oils and flat solids. Open shielded coaxial line method:

This method involves extending the outer conductor well past the inner conductor as reported by Bussey [8]. Thus the inner conductor becomes shielded and hence the radiation conductance becomes zero. Basically the probe is equivalent to a capacitor, which can be varied by varying the length of the inner conductor. According to Bussey [8], such a probe has been used to measure dielectric properties

122

Tran

from 0.1 to 18 GHz. Unlike the short circuit line, which was used extensively by von Hippel [3] the open termination of this probe must be properly characterized and represented to achieve the desired accuracy. The open circuit admittance can be represented by at least one of three methods depending on preference: (1) open circuit capacitance; (2) extension of the centre conductor; (3) termination by a TM evanescent mode.

'L L

2b

2a Fig. 2. Open shielded coaxial line probe

The second method is the simplest when the sample holder is treated as a series of coaxial line sections terminated by an open circuit because only a calculated extension 'L needs to be added to the physical length as shown in Fig. 2. The extension of the inner conductor is given by: 'L = (b-a)(0.6034+0.9464x2+18.19x5.127)

(3)

for x = b/O  0.3 A calibrated automatic network analyser measures the reflection of coefficient at the measurement plane, which is then transformed to S11 at the interface of the sample to obtain the admittance Ym: Ym = (1-S11)/(1+S11) = Yo tanh(Jd)

(4)

Ym = calibrated admittance of the sample. Yo = characteristic admittance of the coaxial line = 2S(HeP P 1/2/ln(b/a) d = corrected length of the inner conductor J = (-Z2PH 1/2, H = Ho Hr and Hr = Hr'-jHr" Yo and J contain H and when the sample is non magnetic: P = Po. Equation (4) is transcendental and may be solved by either the Newton-Raphson's method or the Muller's method to yield an infinite number of solutions. To determine the correct answer, a priori knowledge of the dielectric properties must be available. Where

Sandalwood Microwave Characterisation and Oil Extraction

123

Otherwise two samples having different lengths must be used [3]. The correct answer is the intersection of the two solution curves. Since the centre conductor is extended unlike the open-ended coaxial line probe, the length of the centre conductor must be correctly selected for achieving minimum error for the dielectric properties. The general rule is to use a short length for high dielectric constant and high frequencies, and a long length for low dielectric constant and low frequencies. Estimates for an appropriate length can be obtained using the following analysis. By letting z =j Zd—HH/c, where c = velocity of light and W = Zd/c (1+S11)/(1-S11) One has: ztanh(z) = W

(5) (6)

The behaviour of the solutions for equation (6) has been studied by Gelinas et al [9]. To gain an insight into selecting the correct length to use, the following procedure could be used. First approximate ztanh(z) using the binomial theorem: ztanh(z) | z2 | W for |z|  0.3 or z ~ —W giving: H = -jc/(Zd) (1-S11)/(1+S11) By taking the relative errors of (7): |'HH|/|H| = 'd/d + 2 |'S11|/(|1+S11||1-S11|)

(7) (8)

The correct length d of the inner conductor is the length at which |'HH|/|H| is minimum. For instance: When 'd 1 and thus more fit parameters), these errors still can be reduced.

1000

H“

100

10

0

3.1

6.0

Frequency [GHz]

Calibrated (validated) oil fraction [%]

Fig. 4. The reproduction of the previously excluded spectrum (2.86 mol/l kitchen-salt, 30 vol% oil) by the basic components succeeds within experimental error bars without needing a new principal vector.

60 calibration

40

validation 20 0

0

20

40

60

oil fraction [%] Fig. 5. Cross validation of calibrated and predicted oil volume fractions.

136

Regier

With these results the principle procedure for a dielectric spectroscopy based oil volume fraction determination of salt containing oil-in-water-emulsions should be as shown in Figure 6: - Measurement of the dielectric spectra of the calibration emulsions - Data analysis by a principal component analysis - Correlation of the weights of the first principal components with the oil volume fraction - Dielectric characterisation of the enmulsion with unknown composition - Target testing of its spectrum - Determination of the oil volume fraction using the correlation function.

Calibration emulsions

Dielectric Spectroscopy Principal components

Principal Component Analysis

Loadings

Emulsion to be characterized

Target Testing

Correlation of Loadings with Oil Volume Fraction Fig. 6. Determination of the oil volume fraction by dielectric spectroscopy and principal component analysis.

Summary and Outlook In this work the dielectric spectroscopy (especially the frequency depending loss factor) of oil-in-water-emulsions with varying salt content were used to determine their oil fraction. Instead of using the dielectric constant at a fixed frequency the whole dielectric spectrum between 0.2 and 6 GHz was analysed by a principal component analysis approach. It could be shown, that three principal factors are sufficient to reproduce the measured spectra within experimental error. The dielectric spectroscopy and principal component analysis based oil fraction determination yielded oil contents with an absolute mean relative error of 7% and 11% for the calibration and the validation, respectively. This is already achieved by using only 3 principal components and the simplest linear correlation model.

Dielectric Spectroscopy and Principal Component Analysis

137

Up to now the experiments (the emulsification and the dielectric measurements) were performed batchwise. In further experiments the integration of the coaxial line reflectory probe and the PCA should be tested in a continuous emulsification process for online-indicating the oil fraction of the emulsions.

Acknowledgements The authors thank the Deutsche Forschungsgemeinschaft (DFG) for financial support in projects Schu 335/18 1-3.

References [1] Danner, T., Regier, M., Schubert, H., Volume Fraction Determination of Oil-in-Water Emulsions by Dielectric Spectroscopy Using Principal Component Analysis, 35th Annual Microwave Symposium Proceedings, ISSN1070-0129, 35-40, (2000) [2] Malinowski, E.R., Howery, D.G., Factor Analysis in Chemistry, J. Wiley & Sons, (1980) [3] Regier, M., Oberflächenuntersuchungen an YBa2Cu3O7-G-Einkristallen, Au-TiO2LIGA- und Ni-W-Schichten, Diploma thesis, University of Karlsruhe, (1998). Readers interested in the software code should contact the author. [4] Hasted, J. B., Aqueous Dielectrics, Chapman and Hall, (1973)

Microwave Non-Destructive Evaluation of Moisture Content in Liquid Composites in a Cylindrical Cavity at a Single Frequency J. M. Catalá-Civera, A. J. Canós, F. Peñaranda-Foix and E. de los Reyes Microwave Heating Group, Technical University of Valencia, 46022 Valencia, Spain.

Abstract A dynamic microwave sensor system able to monitor compositional changes of an alcohol used in footwear industry in a gluing process was designed and tested. The sensor provides on-line information about the process by continuously detecting the water content of the alcohol through microwave non-invasive measurements. It is applicable for industrial processes.

Introduction In most physical or chemical processes, like e.g. cure, polymerisation, mixing, dehydration or degradation of materials or mixtures, where structural changes are involved, the complex permittivity of the specimen is affected as well. Therefore real-time measurements of the permittivity changes through dielectrometry can be directly correlated with the main parameters of the process, like moisture content, degree of cure and degree of degradation, eliminating the need for performing “end of pipe” quality control after the process has been completed. In polymerisation reactions, it is very important to control the concentration of the different monomer components to ensure the polymer structure. This may be accomplished by measuring the concentration of monomers during the progress of the reaction. For instance, polyol is a highly reactive basic monomer commonly used for obtaining polymers in the footwear industry that tends to absorb water, thus modifying the proper stoichometric relationship in the reaction. Since changes in the composition of a reagent, e.g. by water absorption, involve variations in its dielectric properties, instantaneous and non-destructive permittivity measurements can be used to monitor the purity and composition of the material. Dielectric characterisation of materials is not an easy task. Many methods have been developed and used for measuring both permittivity and permeability of ma-

Microwave Non-Destructive Evaluation of Moisture Content

139

terials [1, 6, 7]. The most sensitive and accurate results are generally obtained using resonant cavity methods [1, 6]. In these techniques, the permittivity values are usually determined by measuring the cavity response in a frequency range, and obtaining the resonance frequency and Q factor of the resonator. In principle, completely filled cavities are more sensitive than partially filled structures. However, if the material losses are not small, Q-factor values in completely filled cavities may be too low to find and measure the resonance peaks accurately. For these reasons partially filled cavities should be preferred, where rod-shaped solids, or liquids in tubular cells can be introduced through holes in the end plates of the resonator. Partially filled sensors can be completely non-intrusive, and the leakage of radiation through the holes of the covers is minimised. However, generally it is not possible to find a simple formula relating the shifts in both, resonant frequency and Q-factor, with permittivity. In addition to this, sweeping frequency techniques involve complicated and expensive equipment, like e.g. network analysers. Since the scope of the work presented here is to monitor the degradation process in the liquid, only changes in the original response need to be detected, instead of an accurate determination of permittivity and, consequently, frequency and Q-factor. Then, in order to simplify the circuitry, only phase measurements at a fixed frequency are required. The corresponding equipment would be cheaper and rugged, applicable for industrial use due to real-time, precise but from the operators point of view significantly simplified measurement technique. In this work a design procedure of a microwave cavity sensor is presented. A coaxially-loaded resonant cylindrical cavity is analysed to evaluate the parameters governing the sensing ability of the device. Based on these results the response of one-port cylindrical sensor can be simulated. With the aid of this simulation tool the design parameters can be optimised in order to obtain a high sensitivity in the monitoring of the composition of the relevant substance.

Model of a cylindrical cavity sensor The sensor under analysis is a resonant cylindrical cavity, in which a dielectric sample holder, e.g. a tubular PTFE pipe, containing the sample of material under test is introduced along the symmetry axis, extending over the complete length of the cavity, as depicted in Figure 1. The thickness of PTFE pipe is assumed to be very small and for sake of simplicity it will be neglected in the analysis. Appropriated structures have been used to eliminate possible radiation through the cover holes and pipes [5]. An electromagnetic analysis of a cavity without coupling network, is firstly performed. Once the relevant parameters, unloaded resonance frequency, fu, and quality factor, Qu, have been obtained, the effect of the coupling system will be introduced by using a circuital representation of the cavity in the vicinity of a resonance [4]. Thus the response can be simulated for different design parameters concerning the resonant cavity, the dielectric sample and the excitation circuit.

140

Catalá-Civera

The structure under analysis consists of an air-filled circular metallic waveguide of radius U =a, closed by metallic walls, as shown in Figure1.

Fig. 1. Partially filled cylindrical cavity.

It contains a non-magnetic, linear, homogenous and isotropic dielectric rod of thickness U = c of a relative complex permittivity Hr, placed in the symmetry axis and extending over the complete cavity height, h. In these structures, standard TE or TM modes cannot generally satisfy the boundary conditions. Another set of modes called hybrid modes or HEM (IEEE) must be used for the analysis. With & & the aid of auxiliary vector potentials A and F [3] in cylindrical co-ordinates, the transcendental equation (1) is obtained after solving the boundary conditions at the interface [2]. ª J mc x S c y º 1 R mc  y º ª J mc x   m « »˜« » ¬ xJ m x H r yRm  y ¼ ¬ xJ m x yS m  y ¼

ª 1 1 1 º ª 1 1 º m2 « 2  »˜« 2  2 » 2 H r y »¼ «¬ x y »¼ «¬ x

(1)

Here J m and Ym are mth-order Bessel functions of first and second kind, respectively, J mc and Ymc are the derivatives of the previous functions with respect to U, and x = EUs, y = EU0, being EUs and EU0 the cutoff wavenumbers at the sample and air zones, respectively. Therefore:

Z 2H 0 P 0 Z 2H 0H r P 0

E U2 0  E z2

(2a)

E U2s  E z2

(2b)

Functions Rm and Sm satisfy the boundary condition at U = c with



Rm E U 0 U













Ym E U 0 c J m E U 0 U  J m E U 0 c Ym E U 0 U



(3a)

Microwave Non-Destructive Evaluation of Moisture Content



Sm E U0 U













Ymc E U 0 c J m E U 0 U  J mc E U 0 c Ym E U 0 U



141

(3b)

The solutions of Eq. (1), ordered by cutoff frequency, provide the different modes HEMmn in the coaxially-loaded circular waveguide. In the particular case of m = 0, Eq. (1) can be split into two equations for the TM0n and TE0n modes, (Eq. (4a) and (4b), respectively). If Ez = 0 then E U2 0 H r E U2s , and Eq. (1) can also be written as J 0c x 1 R0c  y xJ 0 x H r yR0  y J 0c x S 0c  y xJ 0 x yS 0  y

(4a) (4b)

By applying the boundary conditions at the end plates, E z

pS / h is obtained

for HEMmnp modes, where p is an integer number. By using Eq. (2), the resonance frequency can be expressed as:

fr

2

1

§ y· § pS · ¨ ¸ ¨ ¸ ©a¹ © h ¹

2S H 0 P 0

2

§ j f u ¨¨1  2Qd ©

· ¸, ¸ ¹

(5)

where the Qd factor includes the losses of the material. The unloaded Q-factor, Qu, can be calculated as: Qu1

Qu01  Qd1 ,

(6)

where Qu0 includes the wall losses of the cavity. The coupled cavity can be represented by an equivalent resonant circuit in the vicinity of a resonance, as shown in Fig. 2. The cavity is fed by a predominantly magnetic coupling system (current loop), so a parallel RLC circuit (first Foster form) is used [8], where L0 , C 0 , R0 are internal parameters of the resonator, which are related with fu and Qu by f u





1 / 2S C 0 L0 , Qu

R0 C 0 / L0 . The

excitation circuit is supposed to be lossless, and is represented by an ideal transformer and a normalised equivalent serial reactance, Xs. The coupling network changes the resonance frequency to a new value [4]: fl

§ kx · f u ¨¨1  s ¸¸ , © 2Qu ¹

(7)

142

Catalá-Civera





where k is the coupling coefficient, k r0 1  x s2 , and the equivalent reactance and internal resistance are normalised to the characteristic impedance of the transmission line feeding the resonator, Z c , in the way x s X s Z c , r0 n 2 R0 Z c . The excitation network also affects to the Q-factor of the system, which is lowered to a new value, Ql: Ql1

Qu1  Qe1 ,

(8)

where Qe is the external Q-factor, Qe Qu N , that mainly depends on mode, location and area of the loop. The reflection coefficient can be expressed as: *i

with *d

­° 1 · § *d ® 1  2 ˜ ¨1   j 2QeG l ¸ k ¹ © °¯

 jx s  1 1 

jx s , G l

1 ½

° ¾, °¿

(9)

Z  Z l Z u .

The values of Xs and Qe can be considered constants with frequency in the vicinity of the resonance and they are taken as input design parameters for the development of a dynamic microwave sensor system, since they can be fixed with the position and dimensions of the loop, for a given resonant mode.

Fig. 2. Circuital representation of a resonant cavity.

Sensor design By using the expressions (1) - (9), the theoretical resonator response can be simulated and the relation of various parameters, like e.g. working frequency and mode, cavity/sample relative dimensions, coupling coefficient, etc., on the final response of the cavity, i.e. dynamic range, sensitivity, resolution, frequency stability, etc., can be analysed. The sensor can be then optimised around a desired

Microwave Non-Destructive Evaluation of Moisture Content

143

permittivity value of the material under test. Therefore, the design procedure starts with a dielectric characterisation of the material under the conditions to be monitored within a real process and with the expected processes of change. Material and process characterisation

Changes in the dielectric properties of polyol caused by water absorption in its molecular structure were first investigated. It has been shown that small water contents of around 4% deteriorate the desired properties of the monomer. Thus, the effect of moisture content on the permittivity of polyol was measured adding small percentages of water to the material in pure state, with steps of 0.5% in volume and leaving the solution homogeneously mixed. The dielectric properties were determined using a resonator technique, making use of a cavity designed in the TM010 mode around 3 GHz [5]. The measured complex permittivity of polyol as a function of the water content is plotted in Figure 3. Both, dielectric constant and loss factor, increase with water content, following typical quadratic behaviour H rc 3.97  0.2025 X  0.03125 X 2 , H rcc 1.27  0.1175 X  0.02375 X 2 , X being the water content (% volume) in the mixture.

Fig. 3. Complex permittivity of polyol samples with water content

Cavity design

The characteristics of the sensor are very dependent of the choice of the resonant mode. Modes with the electric field maximum in the centre, e.g. TM0np or TE11p,

144

Catalá-Civera

improve the sensitivity of the sensor. Changes in the sample may affect various modes differently, and adjacent modes may come close to the used mode and cause interference, or even the order of the modes may be changed, thus causing a confusion of resonance peaks. To avoid these risks the resonant mode and the dimensions of the resonator should be chosen with care. If the sensor is short, the lowest mode is TM010 and there is a wide frequency separation to the next modes. Therefore, this mode is very practical for industrial sensors. Several simulations of the sensor response were performed with the measured permittivity values. As an example, the reflection coefficient of a resonator of radius a = 70.9 mm versus permittivity at different values of frequency is plotted in Figure 4, for the resonant mode TM010 and c/a = 0.07. As typical values for the coupling network, xs = 0.1, Qe 15 were considered. The resonance circles corresponding to different values of permittivity are depicted with dashed lines, constant frequency points are linked with solid lines. An increase in the dielectric constant, H rc , involves a spin of the resonance circles in clockwise sense. If the loss factor, H rcc , is increased, the coupling coefficient, k, and therefore the diameter of the resonance circle is decreased. When changes are produced in both H rc and H rcc , both effects mentioned above are observed simultaneously. These effects are very pronounced at certain margin of frequencies and practically unappreciable outside. Phase increments of the reflection coefficient from initial state to final state of the material, 'M , can be calculated in order to optimise the sensor. Phase increments as a function of frequency for several values of c a , and Qe = 4 are plotted in Figure 5.

Fig. 4. Evolution of the reflection coefficient of the resonator at Smith chart, for a = 70.9 mm, TM010 mode, c/a = 0.07, xs = 0.1 and Qe = 15.

A value of c/a = 0.19 was found as an appropriate ratio that may produce large phase deviations, 'M 175 $ at 1.257 GHz. Figure 6 shows the evolution of the

Microwave Non-Destructive Evaluation of Moisture Content

145

phase of the reflection coefficient at this frequency with the water content, for different c/a ratios. From this figure, the sensitivity can be optimised around a given degradation range.

Fig. 5. Phase differences of the reflection coefficient between pure and water containing polyol.

Fig. 6. Phase of reflection coefficient vs. water content of the polyol.

146

Catalá-Civera

Microwave transducer system

A simple system can be used to follow the phase changes of the reflection coefficient at the working frequency, as shown in Figure 7. An oscillator generates a microwave signal, which feeds the resonant cylindrical cavity by means of a current loop. With a double directional coupler, incident and reflected signals from the sensor are obtained. These signals are applied to an I/Q demodulator, after being converted to a lower frequency (IF) and filtered. From I/Q outputs (dc signals) phase response of the cavity can be displayed in real time with the aid of a computer.

Fig. 7. Schematic of the microwave transducer.

Experimental results of the sensor The designed sensor was tested in the laboratory, where a continuous line with the liquid continuously flowing through a PTFE pipe was prepared. The contamination process of the pure polyol was simulated by water injections in the pipe. The evolution of the sensor response is plotted in Figure 8, where the steps of adding water are indicated with the corresponding percentages in volume. As shown in Figure 8, the phase of reflection coefficient is very sensitive to water content. Since water is directly injected into the pipe, a high concentration goes through the sensor during each of the adding process and sharp and momentary changes on the response can be seen. After the water injection the phase takes

Microwave Non-Destructive Evaluation of Moisture Content

147

its original value and as polyol is mixed with water, the response follows the degree of water absorption and dissolution in polyol. 250

Phase response of the microwave transducer (º)

240 230 220 210 200 190

+0.5%

180

+0.5%

+0.5%

+0.5%

+0.5%

170 160 150 1

115 229 343 457 571 685 799 913 1027 1141 1255 1369 1483 1597 1711 1825 1939 2053

Time (s)

Fig. 8. Phase response of the sensor. Water additions are displayed with % in volume

Conclusions A dynamic microwave sensor for the continuous monitoring of the degradation degree of an alcohol commonly used in the footwear industry has been successfully designed and manufactured. The design procedure was performed with the aid of a simulation tool developed from the analysis of a partially filled resonant cylindrical cavity. The influence of the design parameters on the final sensor response can be previously analysed from simulations, so the sensor can be optimised around a desired degree of degradation of the material. The results are extremely promising since the sensor response is able to detect very small percentages of water content in the material composition. The use of phase monitoring at a single-frequency in the resonator during the degradation process was found as a simple, cheap, rugged and very accurate mechanism of control, showing the unique capabilities of microwaves to follow changes in chemical reactions.

References [1] Afsar MN, Birch JR, Clarke RN (1986) The measurements of the properties of materials. Proc IEEE 74: 183-199 [2] Catalá-Civera JM, Devece C, Peñaranda-Foix FL, De los Reyes E (1999) Analysis of coaxially-loaded cylindrical cavities for measuring the complex permittivity of dielectric rods. Proceedings of 7th International Conference on Microwave and High Frequency Heating. AMPERE. pp 67-70

148

Catalá-Civera

[3] Collin RE (1990) Field theory of guided waves. IEEE Press [4] Kajfez D, Hwan EJ (1984). Q-Factor measurement with network analyzer. IEEE Trans. Microwave Theory Tech 32: 666-670 [5] Li S, Akyel C, Bosisio RG (1981) Precise calculations and measurements on the complex dielectric constants of lossy materials using TM010 cavity perturbation techniques. IEEE Trans. Microwave Theory Tech 29: 1041-1047 [6] Nyfors E, Vainikainen P (1989) Industrial microwave sensors. Artech House [7] Sucher M (1963) Dielectric constants. In: Sucher M, Fox J (ed) Handbook of Microwave Measurements vol 2, 3rd edn. Polytechnic Press, New York. [8] Sun EY, Chao SH (1995) Unloaded Q measurement-The critical-points method. IEEE Trans. Microwave Theory Tech 43: 1983-1986

Millimeter Wave Spectroscopy of AluminaZirconia System Saburo Sano1, Akihiro Tsuzuki1, Kiichi Oda1, Toshiyuki Ueno2, Yukio Makino2 and Shoji Miyake2 1

Ceramics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 2266-98 Anagahora, Shimoshidami, Moriyama-ku, Nagoya 463-8560 Japan. 2 JWRI, Osaka University, 11-1 Mihogaoka, Ibaraki 567-0047 Japan.

Abstract It is important to know the dielectric properties of ceramics at high frequency millimeter wave region as the basis of practicing the millimeter wave sintering of ceramics. For this purpose, dielectric measurements by the millimeter wave spectroscopy were done for alumina-zirconia system compact bodies. The measurements were done using tow measuring systems, a BWO millimeter wave spectroscopy system and a millimeter wave vector network analyzer system. As an example of this study, it was obtained that the relationship between the dielectric constant and the alumina/zirconia ratio was almost linear at W-band.

Introduction Recently, some attempts to use high frequency millimeter wave, such as 60 GHz or 83 GHz, have been performed for the microwave sintering of ceramics [1]. For such purpose, it is important to know the dielectric properties of ceramics at high frequency millimeter wave region. For this attempt, resonant cavity, coaxial fixture, wave-guide fixture [2], free space fixture [3] and so on are used. As the frequency of microwave becomes higher the wavelength become shorter, therefore, the size of measuring fixture must be small when measuring is done at high frequency millimeter wave region. It is difficult to make small measuring fixtures like resonant cavity, wave-guide fixture and so on for the measurement at millimeter wave region. However, the free space method has advantages since the method is based on the quasi-optical behavior of millimeter wave. Authors reported dielectric measurement results of ceramics by a free space method using a BWO (Backward Wave Oscillator) millimeter wave spectroscopy system [4].

150

Sano

In this study, dielectric measurement results for alumina-zirconia system compact bodies measured by two measuring systems, a BWO millimeter wave spectroscopy system and a millimeter wave vector network analyzer system, were done. The measuring fixture is composed like an optical spectroscopy system. As the objective samples, alumina-zirconia system compact bodies were used. In this paper, the results of dielectric behavior measurements of different alumina/zirconia ratio samples measured at a frequency range from 50 to 110 GHz will be shown.

Experimental Alumina (AES-11C: Sumitomo Chemical Industries Co. Ltd.) and zirconia (TZ-3Y: Tosoh corporation, partially stabilized by 3 mol% yttria) were used as the raw materials. Different alumina/zirconia ratio samples were prepared by the slip casting process. The sizes of as formed bodies were 52mm in diameter and about 6mm in thickness. Samples were dried for few days and calcined at 800 degree C for 7.2 ks in air. Then the samples were sintered at various temperatures. Obtained samples have compositional variety, from 0 to 100% of zirconia, and variety of density, form 50 to 98% of theoretical density. For measurements, a BWO millimeter wave spectroscopy system [4] and a millimeter wave vector network analyzer system were used. The millimeter vector network analyzer system consists of a free space fixture and a HP8510C network analyzer as shown in Figure 1. As shown in Figure 1, Teflon lenses and an iris are arranged between the antenna of transmitter/receiver module and the sample. The millimeter wave is adjusted as become parallel beam at the sample.

Fig. 1. Schematic illustration of free space time domain measuring system with millimeter wave vector network analyzer.

The millimeter wave beam is restricted within diameter of 30 mm by the iris. By using this system, free space-time domain reflection measurements at W-band

Millimeter Wave Spectroscopy of Alumina-Zirconia System

151

(from 75 to 110 GHz) are performed. By using the BWO millimeter wave spectroscopy system, dielectric measurements are done at the frequency range from 50 to 85 GHz.

Results and discussion

Reflection voltage / mV

Figure 2 shows a free space-time domain reflection measurement (S11 response) result without sample at W-band. There are no peaks in the reflection profile since the measurement was done just after a system calibration. Figure 3 shows a free space-time domain reflection measurement result with an alumina sample. The sample was set at a position as shown in Figure 1. The distance from the calibration plane and the sample surface (front side) was about 280 mm. Sample size is 47 mm in diameter and 6.09 mm in thickness. The sample density is 97% of theoretical density. As seen in the figure, there are many reflection peaks except from the alumina sample. The reason of many peaks is thought that the millimeter wave beam conditions were affected by the setting of alumina sample and reflections begun to occur at many places like at the lens, at the iris and so on. As shown in Figure 3, the biggest reflection peak is reflection from the front surface of the alumina sample and second one is from the back surface. Figure 4 is the magnified profile of Figure 3. As shown in this figure, the reflection peak from the front surface is 1.875 ns and one from the back surface is 2.00025 ns.

200

100

0 -0.5

0.0

0.5

1.0

1.5

2.0

2.5

Time / ns Fig. 2. Free space-time domain measurement result without sample.

3.0

3.5

Sano

Reflection voltage / mV

152

reflection from front surface

200

reflection from back surface

100

0 -0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Time / ns

Reflection voltage / mV

Fig. 3. Free space-time domain measurement result with alumina sample.

200

reflection from front surface 1.875ns reflection from back surface 561.36mm 2.00025ns 599.66mm

100

0 1.8

1.9

2.0

Time / ns

2.1

2.2

Fig. 4. Magnified profile of Figure 3.

By multiplying the light velocity to the time, the distance from the calibration plane is obtained. The calculated distances were shown in the figure. The difference between the reflection from the front surface and the one from the back surface is twice of the sample thickness measured by millimeter wave (tmm). The apparent thickness (ta) can be measured by using a caliper. From the apparent thickness and the sample density (U), we can obtain the equivalent thickness ( teq = ta x U ). teq is the net thickness of the sample excluding pores. The value of tmm includes the thickness of pores in the sample. Hence, we need to calculate the net thick ness measured by millimeter wave (tnm). tnm can be calculated by next equation. tnm = tmm – ta x (1-U)

(1)

From the equivalent thickness and the net thickness measured by millimeter wave, the sample refractive index (n) can be calculated.

Millimeter Wave Spectroscopy of Alumina-Zirconia System

153

n = tnm / teq

(2)

The dielectric constant of the sample expressed by the next equation.

H = n2

(3)

For the alumina sample shown in Figure 4, tnm = 18.97 mm and teq = 5.91 mm. Hence, the dielectric constant is calculated to be 10.3. Figure 5 shows the relationship between the volume ratio of zirconia in alumina-zirconia system samples and the dielectric constants of sample by free space-time domain reflection measurements at W-band. As shown in the figure, there is a linear relationship between the alumina/zirconia ratio and the dielectric constant at W-band.

Dielectric constant / -

40

30

20

10

0

0

20

40

60

80

100

Ratio of zirconia / vol% Fig. 5. Relationship between ratio of zirconia in alumina-zirconia compact body and dielectric constant measured by free space-time domain method at W-band.

Dielectric constants were also measured by a BWO millimeter wave spectroscopy system at the frequency range from 50 to 85 GHz. Obtained dielectric constants for pure alumina and zirconia (3 mol% yttria partially stabilized zirconia) were 9.8 and 32.8, respectively. The values are almost same with the values obtained by the free space-time domain measurement. Similar to the result of Figure 5, the relationship between the volume ratio of zirconia in alumina-zirconia system samples and the dielectric constants of sample by the BWO millimeter wave spectroscopy system showed linear relation.

154

Sano

Conclusions Dielectric measurement of the alumina-zirconia system compact bodies have been done by using a BWO millimeter wave spectroscopy system and a millimeter wave vector network analyzer system (free space-time domain method). Obtained results are as follows. (1) Dielectric constants of ceramics at millimeter wave region were successfully measured by using a BWO millimeter wave spectroscopy system or a millimeter wave vector network analyzer system (2) Dielectric constant values obtained by the BWO millimeter wave spectroscopy system were almost same with the values obtained by the free spacetime domain measurement. (3) It was obtained that the relationship between the dielectric constant and the volume ratio of alumina-zirconia system is linear at W-band.

Literature [1] Sano, S., Makino, Y., Miyake, S., Bykov, Y. V., Eremeev, A. G., and Egorov, S. V., “30 and 83GHz Millimeter Wave Sintering of Alumina“, Materials Science Letters, 19, 2247-2250, (2000) [2] Harris, N. H., Chow, J. R., Eisenhat, R. L., and Pierce, B. M., “Dielectric Properties of ceramics at Microwave Frequency“, Ceramic Transactions, American Ceramic Society, 21, 235-242, (1991) [3] Hollinger, R. D., Varadan, V. V., Varadan, V. K., and Ghodgaonkar, D. K., “FreeSpace Measurement of High-Temperature, Complex Dielectric Properties at Microwave Frequency“, Ceramic Transactions, American Ceramic Society, 21, 243-250, (1991) [4] Sano, S., Hotta, Y., Banno, T., Tsuzuki, A., Miyake, S., Makino, Y. and Ueno, T., “Dielectric Measurement of Ceramic Compact Bodies by Millimeter Wave Spectroscopy“, Proceedings of International Conferees on Microwave Chemistry, 9-12, (2000)

A Modified Cavity Perturbation Technique for Measurement of the Dielectric Constant of High Permittivity Materials. Sheila Oree Department of Physics, Faculty of Science, University of Mauritius, Réduit, Mauritius.

Abstract A technique for complex permittivity measurements in the microwave region is presented. In this method, a cylindrical cavity is loaded along its central axis with a concentric lossy material. The resonance condition of the structure is obtained from the exact field theory solution and is coupled to the experimental measurements of resonant frequency and Q-factor to provide Hc and Hs values. Accurate results are obtained using relatively large samples even for high permittivity or very lossy materials, provided the resonance mode selected has an electric field minimum along the central axis of the cavity. An additional advantage of the formulation is that it does not require any prior calibration with known standards.

Introduction Measurements involving cavity perturbation are among the most precise methods for determining complex microwave permittivity and permeability. In the classical cavity perturbation technique for permittivity measurements, a resonant cavity is loaded with a dielectric sample and the consequent shift in resonant frequency and the change in Q-factor are observed. Cylindrical TM0n0 cavities are very often used with samples in the form of long and thin cylinders placed along the cavity’s central axis where the electric field amplitude is a maximum. This method has several limitations, the most significant one being the need for long and thin specimens of precise dimensions. This factor is particularly critical when considering high permittivity or high loss materials (food products, acids), where needlelike samples may be required. Parkash et al. [1] extended the classical cavity perturbation technique to include thicker specimens whose length is less than the cavity height. However, the volume of material used remains low and there is degradation in the measurement accuracy.

156

Oree

The objective of the present analysis, therefore, is to propose a measurement method for complex permittivity, which can afford samples of much larger diameter and volume. Instead of using the usual TM0n0 mode, a TM210 resonance having minimum amplitudes of both the electric and magnetic fields in the region of the central axis is used. This configuration enables interaction of the fields over a larger volume of the material while still allowing a sharp resonance.

Theory Figure 1 shows a view of the loaded cylindrical cavity in one dimension. The material medium 1 of unknown relative permittivity H1 has a radius r1, whereas a medium of relative permittivity H2 is assumed elsewhere inside the resonant cavity whose radius is r2. It is supposed that the cavity walls are perfectly conducting and that both media are non-magnetic, non-conducting, linear, homogeneous and isotropic. For simplicity in the ensuing theoretical discussion, the presence of coupling devices for cavity excitation is neglected.

Fig. 1. View of the loaded cavity in one dimension

A detailed electromagnetic analysis of this problem in cylindrical co-ordinates can be found in [2]. Consequently, only a brief account of the results will be given here in the case of TMmn0 modes. In these modes the electric field is purely axial and the magnetic field lies in the transverse plane. Moreover, the resonant frequency is independent of cavity height. Simplified field expressions of the nonzero components of electric and magnetic field for the TMmn0 modes at resonance are:

Ez

HT

Hr

j

k i2 Z m rki cosmT

k i3

P 0Z

Z m' rk i cos mT

ki2 m ' j Z m rki sin mT P 0Zr

(1)

(2)

(3)

A Modified Cavity Perturbation Technique

157

where the subscript i designates the region of the cavity under consideration and ki represents the corresponding wave number:

ki

2SQ Hi c

(4)

For lossy materials, Q the complex resonant frequency can be interpreted in terms of the physically measurable quantities, resonant frequency Q and quality factor Qs.

1 ·

§

In region 1: In region 2:

¸¸ Q Q 0 ¨¨1  j 2 Q © s ¹

Z m u

(5)

AJ m u

Z m u BJ m u  CYm u

where Jm(u) and Ym(u) are the mth order Bessel functions (with complex argument) of the first and second kind respectively and A, B, C are scalars. In order to provide a complete description of the cavity fields at resonance, the complex resonant frequency must be evaluated. The resonance condition of the structure can be evaluated by enforcing boundary conditions on the axial (z) and ortho-radial (T) components of electric field. It can be written in the compact form of the singular matrix equation:

0

J m r2 k 2

Ym r2 k 2

k1 J m r1k1  k 2 J m r1k 2  k 2Ym r1k 2

0

(6)

k12 J m' r1k1  k 22 J m' r1k 2  k 22Ym' r1k 2 Due to the assumed infinite conductivity of the cavity walls in our formulation of the problem, the quality factor Qs appearing in equation (5) results from dielectric losses only. In actual practice, however, the cavity walls are lossy and Qs must be deduced from the quality factors of the loaded cavity QL and that of the unloaded cavity Q according to [3]:

1 Qs

1 1  QL Q

(7)

Configuration of the measurement system A two port cylindrical brass cavity of inner diameter 215 mm and inner height 50 mm was constructed and was operated in the vicinity of the resonant frequency

158

Oree

(2281 MHz) of the TM210 mode. Two small coupling loop antennas were inserted inside the cavity through diametrically opposite small holes on its side. These antennas were then connected by precision coaxial lines to the reflection and transmission ports of a HP 8752A network analyser. A weak coupling coefficient of around 0.1 at each port was necessary to achieve a good compromise [4] between a high Q-factor and a wide dynamic range of the transmitted signal. The Q-factor of the empty cavity was 3640. Before proceeding with the measurements, it is important that we model the fields inside the cavity to ascertain that the perturbations introduced are small.

Structure of the cavity fields In this part, the cavity fields are calculated at the TM210 resonance frequency for the measurement cavity (with r2 = 107.5 mm and H2 = 1). Figures 2, 3 and 4 show the variation with radial distance of the normalized amplitude, at TM210 resonance, of the electric and magnetic field components. The fields are shown inside the unloaded cavity and after loading by a lossless cylindrical rod. A rod of radius r1 = 7.85 mm having relative permittivity H1 = 60 is considered in these examples. Here, the extent of the perturbation (which depends on sample radius and permittivity) has deliberately been made rather large for illustration purposes. For easy comparison of the curve shapes, normalization is achieved by separately scaling the field component values so that their maximum is set to 1.

Loaded

1

Normalized electric field, |Ez|

Unloaded 0,8

0,6

0,4

0,2

0 0

0,02

0,04

0,06

0,08

0,1

r (m)

Fig. 2. Calculated normalized electric field in the cavity at TM210 resonance. (a) Unloaded cavity. (b) Cavity loaded with sample having H1 = 60 and r1 = 7.85 mm.

A Modified Cavity Perturbation Technique

159

1.2 Unloaded

Normalized value of |Ht|

1

Loaded

0.8

0.6

0.4

0.2

0 0

0.02

0.04

0.06 r (m)

0.08

0.1

0.12

Fig. 3. Calculated normalized ortho-radial component of magnetic field in the cavity at TM210 resonance. (a) Unloaded cavity. (b) Cavity loaded with a sample having H1 = 60 and r1 = 7.85 mm.

1.2 Loaded Unloaded

Nomrmalized value of |Hr|

1

0.8

0.6

0.4

0.2

0 0.00

0.02

0.04

0.06 0.08 r (m)

0.10

0.12

Fig. 4. Calculated normalized radial component of the magnetic field distribution inside the cavity at TM210 resonance. (a) Unloaded cavity. (b) Cavity loaded with a material having H1 = 60 and r1 = 7.85 mm.

From the illustrations, it is clear that the TM210 resonant mode has minimum electric and magnetic fields along the cavity axis. This feature implies that small

160

Oree

sample insertion holes can be made at the centre of the lids of the cavity without causing significant error.

Dielectric losses The presence of dielectric losses in medium 1 causes a decrease of the normalized electric field profile inside the sample. This effect is illustrated in Fig. 5 for cylindrical samples having real part of permittivity equal to 60 and various loss tangents. As might be expected, the quality factor of the resonant mode is largely dependent on both the field profile inside the sample and its loss tangent. It is noteworthy that the penetration of the electric field inside the sample decreases when Im(H1) increases. This increased shielding effect of the sample from the electric field means that the Q-factor Qs does not necessarily fall monotonically with increase in dielectric loss.

0,08

H Normalized value of |Ez|

60 - j 0 0,06

60 - j 30 60 - j 50 1 - j0

0,04

0,02

0 0

0,002

0,004

0,006

0,008

r (m)

Fig. 5. Normalized electric field profile inside various lossy samples with Re(H1) = 60. The unperturbed cavity field distribution is also shown for comparison.

Processing of resonance data The values of r1, r2, H2, QL, Q and Q being either measurable or known, it is clear that equation (6) depends only on parameter H1. A non-trivial solution can henceforth be found [5] numerically for H1. A computer program for deriving the

A Modified Cavity Perturbation Technique

161

real and imaginary parts of the permittivity H1 has been written using the Maple software. The program has been used to generate a chart (Fig. 6) showing the variations of the Q-factor Qs and resonant frequency shift ǻf in terms of permittivity of a sample of radius 7.3 mm.

Fig. 6. Chart giving the complex permittivity of the sample of radius r1 = 7.3 mm in terms of the Q-factor Qs and resonant frequency shift ǻf (kHz) of the cavity. The contour lines for Qs are labeled in italics and those of ǻf are in small bold characters.

An analysis of Figure 6 shows that a sample having Re(H1) = 60, will display negative values of ǻf if it has a low value of tan į, but as tan į is increased ǻf increases and eventually becomes positive. A fall in the value of Qs also accompanies the increase of tan į, but at still higher values of tan į, the sense of variation of Qs is reversed. The sensitivity of the method varies in different regions of the Re(H1) vs. tan į chart. Therefore caution must be exercised in choosing the proper sample radius to suit the material’s dielectric characteristics.

162

Oree

Results and discussion The measurement method can be applied to solids but also to liquid samples placed in thin-walled tubes having low permittivity and low losses. For a cylindrical sample holder of arbitrary size and dielectric properties, a slightly more complex model consisting of three concentric dielectric layers might be required [2]. To validate the method, measurements have been carried out in the cavity on pure water placed in a thin walled borosilicate test tube of inner radius 7.3 mm. Although no account was made for the presence of the test tube and sample insertion holes, results obtained showed deviations from accepted values of less than 3% for Hc and less than 3% for Hs. The cavity was also tested with aqueous NaCl solutions of various concentrations. The conductivity ı of the solution was modeled as contributing to the equivalent relative permittivity through the term Hs + jı/H0Ȧ. The results displayed in Table 1 show that although the results obtained are accurate for low salinity solutions there is loss in measurement accuracy for high salinity solutions. This is attributed to the existence of regions of space charge concentration in the sample (ȡ  0) which cannot be neglected when ı is large. A much more comprehensive approach is required to care for samples with high ionic conductivities. Table 1: Measured complex permittivity of water and aq. NaCl solutions at 2281 MHz Solution

Pure water Solution 1 Solution 2 Solution 3

Salinity Conductiv[% NaCl] ity [S/m] 0 1.4 2.18 2.4 3.56 4.15 5.81

Measured Measured Lit. [6] Hcc

Hc 76.4 71.5 67.4 61.9

Hcc

Hc 9.6 28.2 43.8 63.8

Lit. [6]

78.8 74.4 72.0 67.8

9.8 25.7 36.1 53

Conclusion We have shown that dielectric measurements can be performed at a location of electric and magnetic field minima using a cavity perturbation technique. Accurate measurements of complex permittivity are possible using relatively large samples (r1 | 6-9 mm at 2281 MHz) even for very lossy or high permittivity materials. The method has also been applied successfully to measure the equivalent permittivity of low salinity NaCl solutions and could find applications in the measurement of dielectric properties of soups, drinks and other food products.

A Modified Cavity Perturbation Technique

163

Acknowledgements I am grateful to Professor Claude Marzat, Laboratoire MASTER, ENSCPB - Université Bordeaux I, France, for kindly providing laboratory facilities for the experimental part of this work.

References [1] A. Parkash, J.K. Vaid and A. Mansingh, “ Measurement of dielectric parameters at microwave frequencies by cavity perturbation technique,” IEEE Trans. Microwave Theory Tech., vol.-MTT – 27, pp. 791-795, Sept. 1979. [2] A. Poinsot, J.C Joly, “ Fréquences propres d’une cavité hyperfréquence cylindrique contenant un échantillon diélectrique coaxial,” L’onde Electrique, vol. 52, fasc. 5, pp. 223 – 227, May 1972. [3] R.E Collin, “ Foundations for microwave engineering,” 2nd ed., McGraw-Hill, 1992. [4] V. Pohl, D. Fricke, A. Mühlbauer, “ Correction procedures for the measurement of permittivities with the cavity perturbation method,” J. Microwave Power and Electromagnetic Energy, vol. 30, No. 1, 1995. [5] D. Gershon, J.P Calame, Y. Carmel and T.M. Antonsen, Jr, ”Adjustable resonant cavity for measuring the complex permittivity of dielectric materials,” Review of Scientific instruments, vol. 71, No. 8, 2000. [6] A. Stogryn, “Equations for calculating the dielectric constant of saline water,” IEEE Trans. Microwave Theory Tech., vol.-MTT – 19, pp.733 -736, 1971.

Finite Elements in the Simulation of Dielectric Heating Systems G.E Georghiou, R.A Ehlers, A. Hallac, H. Malan, A.P. Papadakis and A.C. Metaxas EUG, Engineering Department, University of Cambridge, Cambridge CB2 1PZ, England, UK.

Abstract This paper outlines the continuous development within the EUG in computational codes for dielectric heating as well as for the study of coronas and arcs.

Introduction Finite elements have been used extensively in the simulation of rf and microwave heating problems both in the frequency and time domain [1, 2]. This paper concentrates on recent developments and extensions of the basic code for simulating dielectric heating and the instigation of streamers and coronas. Specifically we will discuss the modelling of thin metallic films using a discontinuous electric field method, the optimisation of a mode-stirred microwave applicator, the incorporation of a Navier Stokes solver which is required to study the corona to arc transition and the extension of the FE-FCT (Finite Element - Flux Corrected Transport) method to three dimensions.

Discontinuous Field Modelling of Thin Films The original EUG code has been modified by incorporating surface integrals in both the frequency and time domain methods, initially focussed on the modelling of wall losses but was thereafter extended to incorporate thin film surfaces. The ability to model the influence of a thin metallic surface using finite elements has useful applications in the microwave food packaging industry. Package designers are able to obtain insight into the performance of a package and food product subject to microwave fields where metallic structures may either be used to shield

168

Georghiou

regions of the food that heat rapidly and are prone to burning or alternatively to focus energy into cold spot zones. The standard impedance boundary condition (SIBC) used in the study of wall losses [3] was modified in an attempt to model the properties of a thin film surface. However, it was not possible to represent the component of transmitted energy through a film using the SIBC, the accuracy of which decreased with increasing transmitted energy. A solution was found in the use of a resistive sheet boundary condition which, unlike the SIBC, takes into consideration the film thickness and hence could account for the transmitted component, despite an offset which effects its accuracy. The lack of accuracy in the resistive sheet is a result of a single finite element surface element being used for both the incident and transmitted fields. In practice, a thin film represents a discontinuity in the field components that reduces with decreasing film thickness. A discontinuous sheet finite element has thus been introduced such that field components either side of the film are separated thus accounting for the field attenuation within the film [4]. The conductivity and thickness of the thin film metal are the key factors for controlling the thin film properties, either of which can be used to characterise a reflective or transmittive film. In order to illustrate this, a plastic (2.5 – j0.01) rectangular load was covered with a thin film sheet and simulated using both a reflective and transparent scenario. Placing the load in the centre of the cavity and applying two planes of symmetry, it is possible to consider only one quarter of the problem domain and hence minimize the computation time. In this particular example, the total number of tetrahedral elements within a quarter of the domain totalled 48471, of which the mesh in the load was approximately 5% (2220 elements). The simulation required 10 minutes to complete on an Intel PII 400 MHz system. Such use of symmetry is particularly useful especially for very low Q simulations where the load mesh may take up to 30% of the total mesh size. Where the use of symmetry is not possible, a low Q cavity simulation may exceed a million elements. As this size of mesh exceeds the memory limitations of many stand-alone computers in use today, the option of parallelizing the numerical implementation should be considered [5]. The normalised electric field plot on the load surface is shown in Figure 1 where a reflective film covers the load, hence preventing any field penetration. When the film is made transmittive, the field penetration into the load can be seen in Figure 2. Full details and results using the discontinuous field method can be found in [4].

Optimising a mode-stirred applicator As is well known, a mode stirrer creates a variety of field patterns at the different positions it assumes, which in combination provide a smoother total distribution within the loaded microwave applicator. To date, stirrers have generally been designed by empirical methods, and here their performance is analysed using the finite element time domain method.

Finite elements in the simulation of dielectric heating systems

169

The mode stirrer investigated is shown in Figure 3. It consists of a single blade of metal mounted at an angle of 28.5 degrees at the top part of the multimode applicator. The symmetrical test load is placed at the bottom part of the cavity and has a permittivity of 8.3-j0.4. An unstructured mesh of tetrahedral edge elements is used to discretise the domain in each stirrer position, giving around 430,000 free degrees of freedom. A sinusoidal excitation is employed in the waveguide, and 41 frequency cycles are required to reduce the field difference between consecutive cycles to less than 5 percent.

Fig. 1. Horizontal slice across multimode cavity and load (centre) surface for a load covered with a reflective thin film (by kind permission of the JMP).

Fig. 2. Horizontal slice across multimode cavity and load (centre) surface for a load covered with a transmittive thin film (by kind permission of the JMP).

170

Georghiou

Fig. 3. Mode stirrer used for the simulations

From the steady state electric field and the material properties, the power distribution in the load can be computed. To improve the power delivery in the load, it is necessary to quantify the quality of a particular distribution. The power density on the surface of a square load may be sampled at points on a regular grid, and the resulting values stored in a matrix. For a particular power distribution stored in a square matrix with g rows and columns the average power is found simply as [6]:

Pavg

§ g ¨¦ ¨ ©j1

g

¦p

ij

i 1

· 1 ¸ 2 ¸g ¹

(1)

where the power density at point i,j in the grid is given by pij. A figure for the quality of the distribution, or smoothness, may then be obtained as:

S

ªg «¦ ¬j 1

g

¦p

ij

i 1

º 1  Pavg » 2 ¼ g Pavg

(2)

This figure gives a quantitative measure of the quality of a particular distribution, and it can therefore be minimised using an optimisation procedure, if suitable variable parameters are chosen. It remains unchanged if the distribution is multiplied by a constant, so that power distributions may be compared without having to consider scaling factors. To find the power distribution in a mode-stirred applicator, the patterns at different positions of the stirrer are computed and then combined to give the total distribution. If patterns at m different positions are computed, and the result for pattern l stored in matrix Pl then the combined pattern Pcomb may be written as:

Finite elements in the simulation of dielectric heating systems m

Pcomb

¦k P l

171

(3)

l

l 1

The coefficients kl are used to scale the power distributions before addition. An evenly revolving mode stirrer will spend equal amounts of time at each position, in which case the coefficients will all be equal to one. It is assumed that power output and frequency of the source is not affected by the impedance of the applicator, as will be the case if an iso-circulator is present.

Fig. 4. Power distribution on the load surface with (a) no mode stirrer, (b) an evenly rotating stirrer and (c) using an optimal combination of stirrer positions

The field is computed at 30 degree intervals from 0 to 330, with the stirrer rotating clockwise if viewed from the top. The pattern obtained by combining these 12 distributions using equation 3 is shown in Figure 4(b) with the smoothness figure as computed with equation 2 beneath it. Compared with the pattern obtained in the loaded multimode applicator with no stirrer shown in Figure 4(a), it can be seen that the distribution is more uniform. This is confirmed by a reduction in the smoothness figure from 0.63 to 0.45. To obtain an optimised distribution, it is observed that the summation in equation 3 does not have to be carried out with uniform coefficients. Practically, this would require that the mode stirrer rotate irregularly, spending unequal amounts of time at the different positions using a special control algorithm of the motor drive. A Matlab implementation of the Sequential Quadratic Programming method is used as non-linear optimisation algorithm. It allows the values for the coefficients in equation (3) to be found that yields the most uniform power distribution. The function in equation (2) was found to vary smoothly with changes in the coefficients. Since the stirrer is simulated at twelve positions, twelve coefficients can be varied to obtain the optimum distribution. Figure 4 shows the smoothness as a function of each of the coefficients in turn, with the other eleven held steady at their optimal values. The optimal value for the coefficient being varied is indicated with an asterisk, from which it can be seen that the algorithm has indeed found a minimum point. The optimised power distribution at the surface of the

172

Georghiou

load is added to Figure 3 where is can be seen that the smoothness factor has now been reduced further to 0.33. The deviation from the average power if the optimised stirrer is used would be nearly half that of the cavity with no stirrer. Mode stirrers can be very effective in improving the quality of the power distribution pattern in a microwave oven, and numerical analysis can allow the performance evaluation of a particular structure. With numerical optimisation, the distribution produced by a specific stirrer design can be further adjusted for a particular load by using a weighted combination of the patterns at different stirrer positions. In future it might be possible to design a versatile applicator that can effectively process different loads, simply by varying the rotation of one or more mode stirrers.

Fig. 5. Power distribution smoothness as a function of the optimising coefficients, with optimal points indicated.

Towards the simulation of arcs The simulation of coronas and streamers has been actively studied using a new finite element flux corrected transport method, (FE-FCT), in two dimensional structures. So far this entailed solving the continuity equations for electrons, positive and negative ions together with Poisson’s equation [7, 8]. This work is a necessary step in studying the transition from a corona or a streamer to a fully developed arc, where the neutral gas properties cannot any

Finite elements in the simulation of dielectric heating systems

173

longer be assumed to be constant. For example, it is usually assumed that the temperature of the neutral gas particles is gradually increasing as we progress from a corona to a spark or an arc. It is therefore essential to extend the basic numerical FE-FCT code for coronas and streamers by including a Navier Stokes equation solver. Although this remains the goal, it is imperative first to validate the Navier Stokes solver before one proceeds to couple it to the continuity and Poisson’s equations. Furthermore, to fully characterise the arc in complex geometrical configurations the FE-FCT method should ultimately be solved in three dimensions. This section will summarise the results so far with the basic FE-FCT code, it will show a bench mark test validation case as regards the Navier Stokes equations and will also present a test case for solving the continuity equations in three dimensions. Coronal Development The early results with the FE-FCT method were compared with a similar technique using finite differences (FD-FCT) and shown to give superior performance for streamer propagation in that the unstructured nature of the finite element grid allows concentration of finer meshes where needed, thus enabling very good results to be obtained with far fewer elements. These results were studied in terms of the peak applied voltage to the resulting current development and light output. A typical set of results is shown in Figure 6 (a) and (b) where a peak voltage of 2.5 and 3 kV was applied to a point plane electrode configuration of gap distance 1 cm at 40 MHz. A summary of the two results is as follows: With the start of the simulation, the seed electrons in the gap, caused by cosmic rays, move rapidly into the high field region near the needle. If the peak applied voltage is high enough, then at some part of the cycle (around the peak value of the applied voltage) the voltage exceeds the breakdown value and the free electrons acquire enough energy to ionize atoms and create more free electrons and positive ions, forming the initial avalanche. This activity occurs around the needle region where the positive ions begin to accumulate, as they do not have enough time to drift. During the positive cycle, the electrons move towards the needle and become absorbed when they strike it. As a result, an amplification of the current, for part of the cycle near the peak voltage is obtained. When the electrons become absorbed the current reduces once again. In the negative cycle, electrons are produced due to secondary emission from the needle electrode and drift towards the gap and hence feed the gap with electrons for the next cycle. Consequently, a gradual increase in electron density, positive ion density and hence current is observed. On the other hand, if the applied voltage is not high enough to cause breakdown, then the seed electrons do not multiply and hence no corona is observed Instead, the current remains at very low values and decays, which reflects the very small number of electrons remaining in the gap. This suggests that the corona onset predicted by our model is in the region 2.5 - 3 kV at a frequency of 40 MHz. The predicted onset agrees with the experimental results in air at atmospheric pressure [9].

174

Georghiou

Fig. 6. a) Current development at a peak applied voltage of 2.5 kV. Solid line, current waveform, --- : applied voltage , b) Current development at a peak applied voltage of 3 kV. Solid line, current waveform, --- : applied voltage

Euler bench mark test A two dimensional Navier Stokes solver was developed for the study of neutral gas dynamics in gas discharges. The Euler equations, which are a simplified form of Navier Stokes equations, were validated using a shock tube or Riemann test case in air where analytical results are available.

Fig. 7. Schematic representation of the Riemann shock tube.

Figure 7. shows the different regions formed within a shocktube. At the beginning of the simulation, there are two regions in the tube separated by a diaphragm in the centre of the shock tube. The two regions referred to as left (l) and right (r) regions have initially different pressures and densities. The velocities are assumed to be zero on both sides at the beginning of the simulation. When the diaphragm is removed, due to the difference of pressure, a normal shock wave will propagate from the high-pressure (l) to the low-pressure region

Finite elements in the simulation of dielectric heating systems

175

(r) towards the plane CD with velocity q. Simultaneously, an expansion wave will propagate from the low to the high-pressure region towards the plane AB. Furthermore, a contact discontinuity will be created due to the difference in densities across the diaphragm, which will propagate in the same direction as the shock wave, but with a smaller velocity v.

Fig. 8. Comparison of density’s analytical and numerical results using three different meshes at two different instants in time t = 0.1 and 0.5 s. – Mesh 1: 500 nodes, Mesh 2: 1000 nodes and Mesh 3: 5000 nodes.

Fig. 9. Comparison of analytical and numerical results for energy using three different meshes at two different instants (t = 0.1, 0.5 s). – Mesh 1: 500, Mesh 2: 1000 and Mesh 3: 5000 nodes.

The Euler equations are solved for a normal shock wave propagating in a shock tube. The number of time steps used is 50000 and the time interval between each step is taken to be 0.00001 s. Three different meshes were used and results were taken at instances t = 0.1 and 0.5 s. Figures 8 and 9 show respectively the analyti-

176

Georghiou

cal and numerical solution of density and pressure for three different meshes. The graphs show that the capturing of the steep gradients is improved for finer meshes and the good agreement between numerical and analytical results is apparent.

FE-FCT in three dimensions Finally, in this section, we present results for the solution of the charge continuity equations in three-dimensions in cartesian coordinates, to demonstrate the capabilities of the FE-FCT method, before coupling these to the electromagnetic equations for the solution of gas discharge problems. The test case descibed here concerns propagating a block of amplitude 1 along the diagonal of the domain with unity speed. The solution is shown in Figure 10 at three different time instances: at time zero, t = 0.28 s and t = 0.56 s. The domain consists of about 162000 tetrahedral elements and there are approximately 30 nodes per line. As expected the unit block propagates with minimum distortion along the diagonal.

Fig. 10. Propagation of a block of amplitude 1 along the diagonal of the domain with unity speed, using the FE-FCT method

From the above tests, the capability of the code to deal with fully three dimensional problems efficiently and accurately is evident. This gives us confidence that it can be readily employed in the solution of gas discharge problems in three dimensions.

Conclusions The discontinuous field method was found to give superior results as opposed to the resistive sheet or the SIBC in the study of thin metallic films placed within ap-

Finite elements in the simulation of dielectric heating systems

177

plicators used in electromagnetic heating. Further, applicators using mode stirrers can be optimised, using the finite element method by ensuring that the stirrer spends more time in preferred positions thus minimising the likelihood of hot or cold spots within the workload. The simulation of a fully developed arc moves one step nearer having validated the Euler's equations and tested the charge continuity equations in three dimensions using the FE-FCT method.

References [1] A.C. Metaxas, "Radio frequency and microwave heating; a perspective for the millennium", IEE Power Engineering Journal, Vol 14 No 2, pp. 51-60, 2000. [2] A.C. Metaxas, “Designing dielectric heating equipment using numerical techniques”, Metallurgia, pp. 24-28, 2001. [3] R.A. Ehlers, D.C. Dibben and A.C. Metaxas, “Inclusion of wall losses in the numerical simulation of microwave heating problems”, Proceedings, 7th International Conference on Microwave and High Frequency Heating, Valencia, p49-52, September 1999. [4] R.A. Ehlers and A.C. Metaxas, “Finite element modelling of thin metallic films for microwave heating”, Proceedings, 8th International Conference on Microwave and High Frequency Heating, Bayreuth, September 2001. [5] H. Malan and A.C. Metaxas, “Implementing a finite element time domain program in parallel”, IEEE Microwaves and Antennas Magazine, Vol 42, No 1, pp. 105-109, 2000. [6] H. Malan, “Parallel finite element analysis for microwave heating systems”, PhD Thesis, University of Cambridge, 2000. [7] G.E. Georghiou, R. Morrow and A.C. Metaxas, “Two-dimensional simulation of streamers using the FE-FCT algorithm”, Journal of Physics D: Applied Physics, Vol 33, L27-32, 2000. [8] G.E. Georghiou, R. Morrow and A.C. Metaxas, “A two-dimensional, finite element, flux-corrected transport algorithm for the solution of gas discharge problems”, Journal of Physics D: Applied Physics, Vol 33, pp 2453 – 2466, 2000. [9] G.E. Georghiou, R. Morrow and A.C. Metaxas, “Simulation of the coronal development in air at radio frequency: The effects of attachment, secondary emission and diffusion”, IEE Proc. Science Measurements and Technology, Vol 147, No 2, 2000.

Examination of Contemporary Electromagnetic Software Capable of Modeling Problems of Microwave Heating Vadim V. Yakovlev Department of Mathematical Sciences, Worcester Polytechnic Institute, Worcester, MA, USA

Introduction Despite all the progress in numerical mathematics and computational technologies, computer simulation of processes and systems of microwave power engineering remains a new and unexplored arena for most practitioners. Engineers dealing with microwave non-communication applications currently seem to lack not specific technical data but general information on modern computational opportunities. At the same time, a number of modeling tools do allow one to get valuable data about the characteristics of the considered system prior to constructing a physical prototype. The goal of the present paper is to update the database of the modern electromagnetic (EM) software suitable for the modeling of microwave heating and outline a few conceptual and practical issues associated with the efficient use of these simulators.

Software Database The database of the EM software available in the market and applicable to the majority of problems of microwave (MW) heating has been recently introduced in [5]. The selection criteria for this database were set up to identify high frequency (HF) 3D simulators able (as a minimum) to determine return losses and compute the power dissipated in the processed material. From Fall 2000, when [5] was sent to press, to the time of writing the present paper (July 2001), the contents of the database have noticeably changed. The market for the modern EM modeling software is very dynamic due to strong competition among the vendors. Since all the solvers were originally developed for the communication and high-speed electronics, these rapidly growing sectors are permanently demanding more adequate and sophisticated computations.

Examination of Contemporary Electromagnetic Software

179

Many features of these newly developed tools can be of help for the practice of MW power engineering. Nowadays, the list of pieces of software suitable for this field includes 17 names of full-wave 3D EM simulators. These commercially available codes are produced by 16 vendors from 7 countries in Europe, North America, and Japan. Table 1 contains the references to the solvers, which, to our mind, deserve the intent look of the engineers designing the microwave heating systems. Other codes from this database not shown in this table may currently be not as suitable as these ones: some of them run only under UNIX operating system (EMFlex by Weidingler Associates, Inc.), others are present only on the regional markets (like the Japanese codes MAGNA/ TDM and JMAG-Works). The term “actual use” means that the code has been used at least once in some R&D or industrial microwave heating projects; “potential use” indicates that the solver has passed the selection criteria, but the examples of its application in modeling of MW thermal processing are unknown. Among the kernel computational methods, Finite Element Method (FEM) and Finite Difference Time Domain (FDTD) dominate; Transmission Line Method, usually considered quite similar to FDTD, is also available. FEM algorithms have been limited to the problems that are electrically not large because of the necessity to use too much memory. Nevertheless, because of their ability to accurately approximate complex structures with curved boundaries, two or three years ago the simulators based on the FEM could still be found more attractive. However, today the time domain algorithms associated with the techniques for overcoming the difficulty conforming to curved surfaces (the FIT with the Perfect Boundary Approximation suggested by CST for Microwave Studio (MWS) and the conformal FDTD developed by QWED and implemented in QuickWave-3D (QW3D) may appear preferable for many classes of problems in MW heating typically involving objects with complicated boundaries. These algorithms are able to handle larger problems, need less memory, are generally quicker than FEM algorithms, and are capable of naturally animating the field and power propagation in the structures. User-friendly intuitive graphical interface can be named among other advantages of MWS. QW3D has been recently supplemented by a number of options specifically appropriate for modeling of microwave heating [1]. Examples of successful uses of this software in modeling of systems of MW power engineering have been reported in [3, 4]. One particular option is noteworthy. Some EM codes from the database could be joined with the programs solving a thermal problem in the framework of a common computational process in which the results of the EM computations are used as input data for solving the heat conduction equation. For instance, MAFIA and Multiphysics can be easily connected with the corresponding thermal simulators available from the same vendors. The two solvers run subsequently and are governed by the user through the common interface.

180

Yakovlev

Table 1. Modern Modeling Software Applicable to Simulation of the Test Problems (as of September 2001).

Vendor and Code

Kernel Method; Operating System Ansoft Corp. Finite Element Method; www.ansoft.com UNIX, Windows HFSS 8.0 95/98/2000/NT4 ANSYS, Inc. $40-45,000 Finite Element Method; www.ansys.com call vendor UNIX, Windows Multiphysics 6.0 95/98/2000/NT4 $30-50,000 Finite Integration TechCST, GmbH www.cst.de 14% nique; MAFIA 4.1, UNIX, Windows Microwave Studio 3.2 95/98/2000/NT4 Windows 95/98/2000/NT4 Faustus Scientific Corporation $10-20,000 Transmission Line www.faustus.ca 20% Method; MEFiSTo-3D Pro 2 Windows 95/98/2000/NT4 Flomerics Electromagnetics Division $49,500 Transmission Line www.micro-stripes.com 12% Method; Micro-Stripes 5.6 Windows 95/98/2000/NT4 IMST, GmbH $12-20,000 Finite Difference Time www.imst.de from $1.5K Domain Method; UNIX, EMPIRE 2.2 Linux, Windows 95/ 98/2000/NT4 Matra Systèmes & Information $30-35,000 Time Domain Finite Volwww.emc2000.org (with interface) ume Method; Windows EMC2000-VF 15% 95/98/2000/NT4 Remcom, Inc. $15,000 Finite Difference Time www.remcom.com $3K Domain Method; UNIX, XFDTD 5.1 Windows 95/98/2000/ NT4 QWED from $15,000 Conformal Finite Differwww.qwed.com.pl 15% ence Time Domain QuickWave-3Da 2.1 Method; Windows 95/98/ 2000/NT4 Zeland Software, Inc. $20,000 Finite Difference Time www.zeland.com 15% Domain Method; Windows FIDELITY 3.0 95/98/2000/NT4 a

License & Maintenance $42,000 12%

Also distributed by Vector Fields, Inc. under the name Concerto Optimization Systems Associates, Inc. (1983-1997) c By Altair Engineering, Inc., www.altair.com d Engineous Software, www.engineous.com e Basic Heating Module b

Examination of Contemporary Electromagnetic Software

181

Table 1. (Continued) Status in Microwave Power Engineering Actual use. Actual use. Actual use.

Features of Performance, System Requirements, etc. SAR (for plane wave). OSAb optimization. PC-toUNIX simulations. Eigenmode solver for anisotropic materials. Compatibility with other ANSYS products. Coupled HF/thermal solution, import of major CAD models, advanced animation. SAR. MAFIA: up to 20 mil. cells. Optional temperature analysis. Microwave Studio: PBA, non-uniform meshing, AutoCAD and ACIS export/import, CAD design, optimizer, multithread solver.

Actual use.

SAR. 20 MB free hard-drive space from minimum installation. Multithread solver.

Potential use.

SAR. ACIS-based interface. Non-uniform meshing. Parallel solver functionality. Thin films. Minimum 400 MHz processor and 1000MB free disk space. SAR. Auto CAD import (limited to 3D boxes). 300 MB hard-disk space.

Actual use.

Potential use.

SAR. 4 GB hard-disk space. HyperMeshc interface.

Actual use.

SAR. iSIGHT optimizationd. Multiprocessor for FDTD

Actual use.

SAR. 45 MB hard-drive space for typical installation. Non-uniform conformal meshing, ACIS export/import, AutoCAD import, optimization. Optional multithread solver & BHMe. SAR. Non-uniform meshing, 1 GB hard-disk space.

Potential use.

Possibility of coupling exists also in MWS: its state-of-the-art VBA interface is open to quick user-specified modifications and connections with other computational tools. However, except for one project with MAFIA successfully conducted by Battelle several years ago (a microwave oven for drying of piles of wet books), no other attempts to couple the EM codes with the compatible thermal simulators are known.

182

Yakovlev

Currently, the licenses cost from 10 to 50 thousand USD (with the average 31 thousand), and the maintenance varies from zero (for 2 years) to 7 thousand (with the average 3.9 thousand) per year. The present study was not supposed to bring justifications of the costs of the licenses; the financial information is given here just for preliminary orientation. However, the general trend appears to be as follows: the price tends to be higher when the work on the software took more resources and was based on a larger investment into the development of the user interface. The latter, for example, makes MWS interface more convenient and easier to learn and use than the QW3D’s one. At the same time, it has to be mentioned that appealing automated functions cannot guarantee adequacy and accuracy of modeling and have to be under permanent control of the user from which sufficiently high qualification in electromagnetics and numerical mathematics is certainly expected. To complete the brief review of the database, it is important to emphasize that nowadays developments in HF EM computational technologies are very fast, so some data presented here, particularly related to extensions and supplementary functions, could become antiquated even before this paper is printed. For the updated data, the interested readers should contact the software vendors.

Test Problems To estimate the efficiency and check out the computational characteristics of the software in the database, a few test problems resembling typical constructions for microwave thermal processing have been formulated to be modeled by those simulators. For example, there were: 1. a 2.45 GHz 1 kW non-standard microwave oven [4] with a uniform spherical potato centered on a circular shelf of a finite thickness (Fig. 1, a); 2. a 915 MHz industrial oven excited by two rectangular waveguides, featuring the bottom formed by two inclined planes, and containing a conveyor belt with the set of hamburgers. It was supposed that the examined codes would be checked as to how they got the patterns of the electric field and the dissipated power and how they computed matching, coupling and some other parameters. While problem (2) turned out to be complicated enough for collecting more or less “uniform” results suitable for comparison, the computational data obtained for problem (1) allow us to reveal some conceptual issues in the modern comprehensive modeling of systems of microwave heating.

Examination of Contemporary Electromagnetic Software

183

Results Preprocessing All the programs in Table 1 have capabilities to generate a concise 3D view of the scenario. Particularly convenient and flexible graphical functions are available in the ACIS-based interfaces and the commercial post-processor HyperMesh (Fig. 1, b). An alternative concept resides in providing parametarized libraries of typical elements and scenarios. This seems to be the approach taken by QWED.

Fig. 1. Microwave oven with a spherical potato (permittivity H = 65 – i20) on a microwavetransparent shelf (H = 2.55 – i0) (a) and its reproduction by EMC2000-VF /HyperMesh (courtesy of Aerospatiale Matra, 2000) (b).

184

Yakovlev

Reflections When considering potential benefits from computer modeling, microwave heating engineers typically wonder how a particular simulator could help improve heating uniformity, but do not always appreciate the possibility to calculate the reflections both from the entire construction and its certain elements. The system is usually considered well-designed and properly operated if less than 10% of the energy supposed to be delivered to the cavity is lost due to all kinds of reflections [2]; in other words, the level of a return loss less than -10 dB is supposed to be small enough. Taking control over this characteristic is fairly important: the reflections

Fig. 2. Return loss in the oven in problem (1) computed by Microwave Studio (courtesy of CST of America, 2001) (a) and QuickWave-3D (courtesy of QWED, 2001) (b).

Examination of Contemporary Electromagnetic Software

185

could be very high for some particular configurations of the cavity and parameters of the load; in addition, the cavities used for MW thermal processing are usually characterized by resonances. These factors may remarkably decrease the efficiency of the whole system. All the codes in Table 1 (except EMC2000-VF) are capable of computing Sparameters of an analysed structure and thus determining S11 (the reflection coefficient) in the frequency range adjacent to the operating frequency and by this to clarify the important property of the cavity and its feed(s). The return loss in the oven (1) have been simulated by MWS and QW3D. In the simulations performed on regular PCs, the first program used about 1.085 million cubic cells, while the second used approximately 1.002 million. In both cases, the applied mesh was non-uniform with smaller cell size within the potato (1.0 mm in MWS, 1.2 mm in QW3D) and larger in air (5 mm both in MWS and QW3D). The two curves are shown in Fig. 2. The divergence in the value of S11 at 2.45 GHz is less than 0.2%. The graphs are very similar, so we can be confident that they both describe the reflections in the system quite adequately. The tiny differences in the curve shape can be attributed to the minor distinctions in meshing and in the number of time iterations in the course of simulations. At 2.45 GHz, the system is characterized by the large return loss (about 4.4 dB, that means about 37% of energy is lost) whereas there is a strong resonance in the immediate neighborhood: in the range with a width of 4 MHz, S11 is less than -15 dB, which means that the energy loss at these frequencies is fairly low (not more than 3%). The peak, however, is too narrow, so this could bring no particular profit, but rather cause instability in the operation of the system: the range of the magnetron frequency deviation is typically about 50 MHz, and the width of the magnetron output spectrum may be up to 100 MHz. Upon getting a characteristic of S11 like the one shown in Fig. 2 one may conclude (even prior to simulation of the dissipated power) that it would be unfeasible to build a prototype with the considered configuration because of its possible low efficiency. Since the position and the form of the resonance is primarily governed by the cavity dimensions, further computations (particularly with the use of optimisation options) could help find the dimensions of a system with reasonably low reflections (say, |S11| < -10 dB) in a wider frequency range (about 50 - 100 MHz) around 2.45 GHz. Electric Field While for computation of S-parameters a circuit should be considered as excited by a pulse of frequency spectrum, simulation of the fields requires the sinusoidal excitation at a particular frequency. Although the structure of the electric field does not explain how the energy is released in the load, knowledge of the field pattern is essential for understanding of processes in microwave heating systems. The time-domain algorithms implemented in the simulators in Table 1 deal with direct numerical solutions of Maxwell’s equations and thus handle the fields varying in time.

186

Yakovlev

Fig. 3. The vertical component of the electric field in the vertical cuts through the center of the potato (a, b) and in the horizontal cut (10 mm above the oven’s bottom) (c, d): instantaneous fields by Microwave Studio (courtesy of CST of America, 2001) (a, c) and field envelopes by QuickWave-3D (courtesy of QWED, 2001) (b, d).

The fields therefore can be naturally animated and visualized for different phases. The patterns of the electric field at 2.45 GHz obtained by MWS and QW3D (using

Examination of Contemporary Electromagnetic Software

187

the meshes described in the previous section) and presented in Fig. 3 look quite similar and allow one to identify the mode as quasi-TE331.

Fig. 4. SAR patterns in the mutually perpendicular cuts through the center of the potato: Microwave Studio (courtesy of CST of America, 2001) (a), EMC2000-VF/HyperMesh (courtesy of Aerospatiale Matra, 2000) (b), QuickWave-3D (courtesy of QWED, 2001) (horizontal, (c); vertical, (d)).

Due to the high dielectric constant of the potato, the field is much stronger in air which confirms the point that characteristics of the oven in problem (1) depend on the cavity dimensions rather than on the material properties. Dissipated Power, SAR, Energy Coupling To show how microwave energy associated with the determined field is released in the potato, simulation of the dissipated power or SAR is required. Fig. 4 presents the SAR patterns within the product computed by MWS, QW3D, and EMC2000-VF. Since the potato is uniform, the patterns of the dissipated power look similar, differing only in scale. It is seen that there is a strong “hot spot” in the center of the potato; in accordance with the QW3D simulation, the magnitude’s max/min ratio in the vertical pattern (Fig. 4, c) is about 120. The result appears to be consistent with the focusing effect in spherical objects well described in literature for the plane wave. MWS and QW3D generate practi-

188

Yakovlev

cally identical patterns whereas EMC2000-VF suggests higher level of the dissipated power on the bottom surface of the sphere. This discrepancy can be attributed to the fact that the latter code does not simulate the modal excitation of the cavity (the dominant TE10 mode of the waveguide feed), which in fact takes place, but approximates it by the plane wave whereas MWS and QW3D reproduce the waveguide modal excitation scrupulously. As seen from Figs. 3, 4, post-processor functions of MWS and EMC2000VF/Hypermesh allow the user to get convenient quasi-3D views showing patterns in the mutually perpendicular coordinate planes. Conformal FDTD computation performed by QW3D receives conformal visualization only in the horizontal plane (Fig. 4, b) whereas the display in the vertical plane is simplified and does not show the actual shape. The percentage of the power absorbed by the processed material with respect to the power generated by the magnetron was rigorously calculated by QW3D through an average power dissipated in a sinusoidally excited system. In oven (1), it was found to be equal to 67.8% of the power delivered to the cavity. The coupling C can also be approximated after the first run of the simulator with the pulse excitation from the formula (1): 2

(1)

C # (1  S11 )100%

where | S11 | is the module of the reflection coefficient at 2.45 GHz, without the need of analysing a sinusoidally excited system. From QW3D‘s computation of reflection, C appears to be 63.4%, i.e., the divergence with the rigorous computation is about 9%. The possibility of estimation of coupling brings another argument in favor of computation of reflection preceding the runs for the field and SAR patterns. Temperature Patterns In the beginning of 2001, the version of QW3D designed specifically for microwave power engineering was released. It includes the so-called Basic Heating Module (BHM), which allows the user to compute and visualize the temperature patterns in the processed material taking into account the fact that its material properties (complex permittivity, density U, and specific heat capacity c) are changed as functions of dissipated power. BHM does not deal with the heat conduction problem, but computes temperature T at the moment m+1 in accordance with the formula (2):

T m 1 ( x, y, z ) T m ( x, y, z )  T m ( x, y , z ) 

P ( x , y , z ) 't m

(2)

m

U ( x , y , z ) c ( x, y , z ) H m 1 ( x, y, z )  H m ( x, y , z )

U m ( x , y , z )c m ( x , y , z )

,

Examination of Contemporary Electromagnetic Software

189

where H is the enthalpy density, and 't is the assumed user-specified time of heating at a particular steady-state with constant average power P. However, the adequacy of this approach appears to be reasonable for many applications, so the presence of such an extension in the EM solver obviously provides more opportunities for detailed and advanced modeling of processes of microwave heating. Yet, the functions offered by this module surpass the options that the present study suggests as sufficient for the valuable analysis of the formulated test problems; these functions are therefore not considered here.

Conclusion This paper has presented the updated database of the modern EM simulators suitable for modeling of problems of microwave power engineering. The collected results have shown the advanced capabilities of the time domain solvers (Microwave Studio and QuickWave-3D) and emphasized their practical usefulness in modeling reflections, the electric field, dissipated power, and energy coupling. Both simulators perform very well in the basic EM simulation. Currently, QuickWave-3D appears to be particularly useful for designers of applied and industrial systems of microwave heating due to a number of implemented specific extensions and functions beneficial for the field (such as field envelopes, energy coupling, BHM, and others). Strong dedication of the vendor to the field is also noteworthy. The study did not have the aim of comparison of the kernel computational methods implemented in the simulators, but it rather focused on their current technical adjustments to the needs of the field of microwave power engineering. With the wider participation of other software in solutions of the suggested test problems, more specific information about particular features of the solvers could be revealed. Evaluation of the EM modeling tools is going to be continued in the framework of comprehensive benchmarking, which means solving a typical and meaningful microwave heating problem by the different simulators with the subsequent experimental validation. The way the simulators were used for the analysis of problem (1) can be beneficial for other systems of microwave power engineering. It appears to be feasible to start computer analysis with the pulse excitation and getting reflections in the frequency range adjacent to the operating frequency. This should give a general understanding of the EM processes in the system and perhaps suggest certain changes in the initial design – for instance, if return loss in the adjacent frequency range is characterized by strong resonance(s). Then the excitation has to be switched to the sinusoidal one so that the field and the dissipated power could be computed and visualized. A convolution technique implemented in Microwave Studio allows one to extract the sinusoidal fields at the several frequencies from the broadband calculation, so just one simulation is necessary to obtain both the behaviour of S-parameters in the frequency range and the harmonic field at a user defined operating frequency.

190

Yakovlev

References [1] Celuch M, Gwarek W (2000) Advanced features of FDTD modeling for microwave power applications. (Paper presented at the 35th Microwave Power Symposium, Montreal, Canada) [2] Edgar RH, Osepchuk J (2001) Consumer, commercial, and industrial microwave ovens and heating systems. In: Datta AK, Anantheswaran RC (eds) Handbook of Microwave Technology for Food Applications, Marcel Dekker, pp 215-277 [3] Stillesjo F, Solbrand A (2001) Dynamic field tuning and heating studies in a microwave chemistry applicator, In: Proc. 36th Microwave Power Symposium, San Francisco, CA, April 2001, pp 37-40 [4] Risman PO (1998) A microwave oven model. Examples of microwave heating computations. Microwave World 19 (Summer): 20-21 [5] Yakovlev VV (2001) Commercial EM codes suitable for modeling of microwave heating – a comparative review. In: Van Reinen U, Gunther M, Hecht D (eds) Scientific Computing in Electrical Engineering, Lecture Notes in Computational Sciences and Engineering 18, Springer, pp 87-96

A Hybrid Approach for Resolving the Electromagnetic Fields Inside a Waveguide Loaded with a Lossy Medium Viktor Vegh, Ian W. Turner Centre in Statistical Science and Industrial Mathematics, School of Mathematical Sciences, Queensland University of Technology, GPO Box 2324 Brisbane Q4001, Australia.

Abstract In this work a novel hybrid approach is presented that uses a combination of both time domain and frequency domain solution strategies to predict the power distribution within a lossy medium loaded within a waveguide. The problem of determining the electromagnetic fields evolving within the waveguide and the lossy medium is decoupled into two components, one for computing the fields in the waveguide including a coarse representation of the medium (the exterior problem) and one for a detailed resolution of the lossy medium (the interior problem). A previously documented cell-centered Maxwell’s equations numerical solver can be used to resolve the exterior problem accurately in the time domain. Thereafter the discrete Fourier transform can be applied to the computed field data around the interface of the medium to estimate the frequency domain boundary condition information that is needed for closure of the interior problem. Since only the electric fields are required to compute the power distribution generated within the lossy medium, the interior problem can be resolved efficiently using the Helmholtz equation. A consistent cell-centred finite-volume method is then used to discretise this equation on a fine mesh and the underlying large, sparse, complex matrix system is solved for the required electric field using the iterative Krylov subspace based GMRES iterative solver. It will be shown that the hybrid solution methodology works well when a single frequency is considered in the evaluation of the Helmholtz equation in a single mode waveguide. A restriction of the scheme is that the material needs to be sufficiently lossy, so that any penetrating waves in the material are absorbed.

192

Vegh

Introduction Over the last two decades, researchers in the field of computational electromagnetics (CEM) have explored a number of numerical techniques to resolve the electromagnetic fields inside waveguide and cavity structures [1 - 5]. The popular finite-difference time-domain (FD-TD) method originally proposed by Yee [6] provides very accurate results on structured domains, however it is not straightforward to migrate the scheme to an entirely unstructured mesh. The FD-TD method has been used previously in the literature with great success to simulate microwave heating problems [5, 7]. It is proposed here to use a hybrid approach that combines the frequency and time domain numerical solvers to resolve the power distribution inside a waveguide loaded with a lossy medium. Typically, the FD-TD method when applied to computing the power distribution in a lossy dielectric load can be a computationally intensive (in terms of CPU time and memory) solution scheme. This research work aims to establish a scheme that can predict the heating distribution inside a lossy medium, both accurately and efficiently. To achieve this goal, a new hybrid approach is developed whereby the problem of determining the electromagnetic fields evolving within the waveguide and the lossy medium is decoupled into an exterior problem for computing the fields in the waveguide, including a coarse representation of the medium and an interior problem for a detailed resolution of the lossy medium. This scheme is demonstrated on structured grids, so that the accuracy of the developed method can be compared to the exact solution. Since the motivation behind this research is to develop numerical schemes that can be used on unstructured grids, a time-domain scheme that allows arbitrary shaped mesh elements needs to be implemented in future research. Here, an existing cell-centered time domain numerical solver for the Maxwell’s equations is used in the free-space component of the domain to resolve the electromagnetic field behaviour [8]. This cell-centered numerical solver is discussed in the section labelled Numerical Solution of the Maxwell’s Equations. In the past, a number of researchers in the field of electromagnetics and microwave heating have used frequency domain strategies to predict the power distribution inside lossy media [9, 10]. However, it was highlighted that when the analysis is performed on the whole domain of the waveguide, including the load or material, the decomposition when formulated into a system of linear equations yields a coefficient matrix that is highly ill-conditioned. Adaptive iterative techniques that use left and right preconditioning have to be utilized to obtain electric field solutions inside the material. Although, it is possible to condition (via left and right preconditioning) a matrix that performs well numerically, a small change in the layout of the problem can cause the system to break down numerically. The strict ill conditioning in the coefficient matrix is generally related to the implementation and spatial location of the boundary information. The hybrid method proposed here aims to utilize schemes to their full advantage in the time and frequency domains. The time-domain solver is used to predict boundary information via the discrete Fourier transform (DFT). This procedure is

A Hybrid Approach for Resolving the Electromagnetic Fields Inside a Waveguide

193

normally fast, due to the fact that the time-stepping constraints of the time-domain solver are restricted by only the free-space properties. Given that the boundary information at a material interface can be obtained accurately and efficiently, the Helmholtz equation can be resolved using a mesh imposed on the domain of the load alone. The boundary information obtained from DFT is used as the material boundary condition in the solution of the Helmholtz equation. The resulting system of equation that governs the physical phenomena in the material is better conditioned than a system that is representative of the domain of the free-space and the load. Such a system of linear equations is solved using a Krylov subspace method [11]. Normally for multi-mode cavities, the solution to the Helmholtz problem should be obtained for a number of frequencies within the vicinity of the dominant mode. This is necessary if representative electric field behaviour inside the load is to be attained. Numerical Solution of the Maxwell’s Equations – Exterior Problem In free-space, the time-domain numerical method is used to obtain the solution of the microwave applicator on a coarse grid that satisfies the stability requirements for the external region of the computational domain, excluding the lossy material. To do this, a cell-centred numerical solver is used to predict the electric and magnetic field components for discrete cells [8]. All of the components of the electric and magnetic fields are located at the same cell-centred spatial location. For a given cell, interpolation is used to estimate the facial unknowns. To establish the cell-centred scheme, the Maxwell’s equations have to be revisited and formulated to cater for electric and magnetic field components at the cellcentres: wB , wt D İ E, J

’uE  B

µ 0 H,

’uB ı E,

İ

wD  J, wt İ 0 İ c, ı Ȧ İ 0 İ cc.

(1)

The Maxwell’s equations (1) are transformed into a surface-volume representation. To obtain this form, together with certain vector product properties have been used for the derivation: wB wD (2) w V, n u H w S ³³³ w V  ³³³ J w V. ³³S n u E wS ³³³ ³³ w t wt V S V V From (2), the discrete in space form of the Maxwell equations can be deduced directly. For a given cell in the computational domain, the electromagnetic behaviour can be represented at a point p as: wBp wt



1 ¦ n u E F ǻ SF , ǻV Fȗ p

w Dp wt

1 ¦ n u H F ǻS F  J p . ǻV Fȗ p

(3)

To resolve (3) in terms of time, a staggered Leapfrog discretisation is adapted to the left-hand sides of (3). The resulting system of equations is expressed as:

194

Vegh n  12

Hp

E pn 1

n  12

Hp



ǻt ¦ n u (E F ) n ǻS F , µ 0 ǻV Fȗ p

2İ  ı ǻ t n 2ǻ t n 1 Ep  ¦ n u (H F ) 2 ǻSF , 2İ  ı ǻ t (2İ  ı ǻ t)ǻV Fȗ p

(4)

where, the magnetic and electric fields at a given cell-centre have been staggered half a time step apart. In (4), n is the unit outward normal to face F, and ȗp is the set of faces that constitute the pth cell in a computational domain. 'SF and 'V are the surface area of a particular face in ȗp and the volume of the pth cell, respectively. On structured grids, the facial values can be obtained by simply averaging the cell-centre values of the cells common to that face. Such averaging of the cell values provides a second order in space and second order in time numerical scheme. The method in (4) is usually referred to as a staggered Leapfrog discretisation (SLF) to Maxwell’s equations. Numerical Solution of the Helmholtz Equation – Interior Problem Numerous applications for computing the electromagnetic fields using frequency domain solution strategies for the purpose of heating have been employed in the past [7, 9, 10]. In this section, the frequency domain equations are outlined, and brief descriptions of the different terms are provided. In time-harmonic form, the electromagnetic behaviour can be represented as a set of coupled curl equations: ’ u H (ı  j Ȧ İ) E, ’uE

(5)

j Ȧ µ H.

In (5), by taking the curl of the latter equation, and substituting it into the first expression, the following Helmholtz equation for the electric field that satisfies the divergence criteria is obtained: ’ 2 E  k 2 E 0, k2

(6)

µ 0 Ȧ( Ȧ İ  ı i).

Note that since the interest lies in computing the power distribution, it is necessary only to determine the electric field inside the lossy medium. On structured grids, (6) can be discretised using the standard (for example, second-order) finitedifference stencil to obtain a system of linear equations. This yields a finitevolume method for the spatial discretisations. When written in matrix form, the resulting large, sparse complex system of equations can be solved using the GMRES method [11]. Other methods can be used with and without preconditioning to obtain the solution to the discrete system governed by (6). Since Ȧ is frequency dependent, for multi-mode waveguides and cavities, the system obtained from the discretisation of (6) has to be resolved for a number of distinct frequencies. The number of frequencies used in the evaluation of the power distribution depends on the different modes existent in the microwave heating apparatus. Normally, a

A Hybrid Approach for Resolving the Electromagnetic Fields Inside a Waveguide

195

number of discrete frequencies have to be taken in the neighbourhood of the dominant frequency to identify the power distribution within the medium. The solutions at these discrete frequencies are then collated, to obtain the electric field behaviour inside the material or load. The power distribution is calculated from the electric field inside the domain of the material using the following equation: Pp

2

ı Ep .

(7)

The scope of this research is limited to finding the power distribution inside the lossy medium. If required, the power distribution can be coupled with the forced heat equation to obtain heating inside the material. Summary of the Hybrid Method The proposed hybrid model combines a time dependent solver with the Helmholtz equation, and calculates the power distribution from the electric field Helmholtz equation. In this case, the time dependent equations are solved everywhere, even inside the material, to maintain consistency of the solution. It can be shown that for lossy media on a fairly coarse mesh, it is possible to capture the time dependent unknowns at an interface between free-space and the dielectric load. Generally, the computed time dependent solution contains fairly large errors inside the load, but it maintains a good approximation to the solution in the free-space region, and particularly at the material interface. For a multi-mode waveguide or cavity, by using discrete Fourier transform, it is possible to take a band of discrete frequencies in the neighbourhood of the dominant frequency and transform the time dependent solution to the frequency domain (typically, for a period of the wave). This has to be performed once the time dependent solution has assumed a plane wave form. During the time stepping of the numerical solver at the interface between the free-space and the dielectric load, the time dependent electric fields are mapped to their equivalent fields in the frequency domain. Once the electric field on the material boundaries at a particular frequency has been computed, the Helmholtz equation is solved implicitly at that discrete frequency. Since all of the frequency dependent electric fields are computed using DFT on the faces of the material, the resulting system of linear equations is complex, also in this case it is sparse and banded. To find the solution to such a matrix system, a reliable iterative solver without preconditioning is utilized (GMRES) [11]. The Hybrid scheme makes the assumption of a single dominant mode inside the microwave heating apparatus. For microwave heating, the frequency bandwidth is narrow, hence the assumption of a single dominant frequency inside the microwave applicator makes the time-harmonic solution feasible.

196

Vegh

Results A simple study to demonstrate the potential of the Hybrid method has been considered and investigated. The implementation of a loaded waveguide study is illustrated. The results are compared to the exact solution to assess the accuracy of the method. The SLF scheme outlined in a previous section was used to generate the frequency domain boundary information for the Helmholtz equation. The time-domain information at material interfaces was converted to the frequency domain using a number of frequencies in the neighbourhood of the dominant TE10 2.45 GHz mode. It was observed that for this study, the dominant frequency could accurately capture the exact solution. The incident TE10 wave was smoothed using a Gaussian function: gauss (t )

­  ( Tt  3) 2 °e ® °¯ 1

t d 3T , t ! 3T

(8)

where, T is the period of the wave. The waveguide is dimensioned 0.1 x 0.05 x 0.4 m3. A load of size 0.1 x 0.05 x 0.1 m3 was placed at the short circuit end of the waveguide. At z = 0.1 metres, the incident field was imposed to propagate the electromagnetic fields. The material was discretised using a mesh of size 40 x 20 x 30 cells (ie. 24 000 cells).

Fig. 1. The Hybrid method compared to the exact solution (dielectric property 2-0.5i); (a) exact solution, and (b) Hybrid - 2.45 GHz.

For this study, the standard conducting wall boundary conditions were implemented. Any impinging reflected waves in the scattered field region of the guide were absorbed using a perfectly matched layer (PML) absorbing boundary condition [12]. In Figs. 1 and 2, the Hybrid method is compared to the exact solution. The Hybrid method has been implemented using the dominant 2.45 GHz frequency. In the Figures, the power distribution is calculated for a material with properties H r 2.0  0.5 j and H r 3.0  j , respectively.

A Hybrid Approach for Resolving the Electromagnetic Fields Inside a Waveguide

197

Fig. 2. The Hybrid method compared to the exact solution (dielectric property 3-i); (a) exact solution, and (b) Hybrid - 2.45 GHz.

The solution has been normalised (ie. scaled by the largest value), so that the scheme can be compared using the same legend and any similarities in the solution between the Hybrid method and the exact solution can also be demonstrated. For heating purposes, the magnitude of the power distribution is less important than the location of the hot spots, and hence, the focus concerns the analysis of the locations of the hot spots. It can be seen from the figure that at the sole 2.45 GHz frequency Hybrid solution, the power distribution is obtained fairly accurately, and the numerical approximation obtained from the Hybrid method tends to be the one observed by the exact solution. Rigorous amplitude analysis for the Hybrid scheme is left to future research. Computationally, the classical FD-TD method is very fast. Though, the Hybrid scheme uses a coarser grid outside the domain of the material, and for this reason, when the Hybrid solution is computed for a single frequency, the time to obtain the solution is comparable to the FD-TD method.

Conclusions This research work illustrates that a cell-centred Maxwell’s equations numerical solver with Leapfrog time integration can be used to obtain the frequency domain boundary condition information via the discrete Fourier transform at the material interfaces. This boundary information is then used to resolve the Helmholtz equation for the electric fields at several frequencies within the lossy medium. At the dominant frequency, the Hybrid method was used to solve for the electric fields inside a microwave heating apparatus. The electric fields are then directly related to the power distribution inside the lossy medium. For this simple case study, it is evident from the results that the Hybrid method can capture the electromagnetic field behaviour inside the material for lossy media more than adequately. This new solution methodology initiates future work on unstructured domains for hybrid solvers for multi-mode cavities, where decoupled domains for both the

198

Vegh

free space and the lossy medium can be utilised for predicting the power distribution inside a lossy arbitrarily shaped material loaded inhomogeneously within a microwave cavity structure. The impact of using preconditioning in the solution strategy of the Hybrid method is currently under investigation. Future work will also investigate numerical solvers that use higher order approximations for the cell face unknowns. In that work, material interface boundary treatments using dielectric properties will also be considered and treated. Further investigation on preconditioners will also be carried out.

References [1] Mur, G., Absorbing Boundary Conditions for the Finite-Difference Approximation of the Time-Domain Electromagnetic-Field Equations. IEEE Trans. Microwave Theory Tech. EMC-23, 1981. 4: p. 377-382. [2] Madsen, N.K. and R.W. Ziolkowski, Numerical solution of Maxwells' equations in the time domain using irregular nonorthogonal grids. Wave Motion, 1988(10): p. 583-596. [3] Shankar, V. and A.H. Mohammadian, A Time-Domain, Finite-Volume Treatment for the Maxwell Equations. 1990: p. 128-145. [4] Dibben, D.C., Numerical and Experimental Modelling of Microwave Applicators, in Department of Engineering. 1995, University of Cambridge. p. 188. [5] Zhao, H., Computational Models and Numerical Techniques for Solving Maxwell's Equations: A Study of Heating of Lossy Dielectric Materials Inside Arbitrary Shaped Cavities, in Centre in Statistical Science and Industrial Mathematics. 1997, Queensland University of Technology: Brisbane. [6] Yee, K.S., Numerical Solution of Initial Boundary Value Problem Involving Maxwell's Equations in Isotropic Media. IEEE Trans. Antennas Propagat., 1966. 14: p. 302-307. [7] Jia, X. and P. Jolly, Simulation of Microwave Field and Power Distribution in a Cavity by a Three-Dimensional Finite Element Method. Jounral of Microwave Power, 1992. 27(1): p. 11-22. [8] Vegh, V. and I.W. Turner, Comparison of Time Domain Numerical Solvers for the Propagation of a Gaussian Pulse Inside a Rectangular Waveguide. Presented at CTAC2001 Conference (Brisbane), paper under review for a special issue of the ANZIAM Journal, 2001. [9] Madsen, N.K. and R.W. Ziolkowski, A three-dimensional modified finite volume technique for Maxwell's equations. Electromagnetics, 1990(10): p. 147-161. [10] Dibben, D.C. and A.C. Metaxas, Time Domain Finite Element Analysis of Multimode Microwave Applicators Loaded with Low and High Loss Materials. Intl. Conf. Microwave and High Energy Heating, 1995. 1-3.4. [11] Y. Saad, M.H.S., GMRES: a generalized minimum residual algorithm for solving nonsymmetric linear systems. SIAM Journal of Scientific and Statistical Computing, 1986. 7: p. 856-869. [12] Vegh, V., I.W. Turner, and H. Zhao, Effective cell-centred time-domain Maxwell's equations numerical solvers for the purpose of microwave heating. Under review for the Journal of Applied Mathematical Modelling, 2001.

A Novel FDTD System for Microwave Heating and Thawing Analysis with Automatic Time-Variation of Enthalpy-Dependent Media Parameters Malgorzata Celuch-Marcysiak, Wojciech K.Gwarek, Maciej Sypniewski Institute of Radioelectronics, Warsaw University of Technology, 00-665 Warsaw, Poland.

Introduction Over the last decade, the finite difference time domain (FDTD) method for electromagnetic analysis has been a subject of a large research effort, and has consequently reached the level of practical applicability to microwave engineering problems. To a large extent it can replace hardware experiments, and in many cases offers additional advantages. With regard to microwave power applications, it allows quick and nearly arbitrary changes of the applicator and load geometry, and a free choice of the electric and magnetic properties of all items - that would be very costly and time-consuming, if not impossible, in experimental set-ups. Moreover, it facilitates the study of all electromagnetic field components, all over the computational domain, which leads to better understanding of absolute and relative importance of the separable heating phenomena. Despite the above advantages, the use of FDTD in microwave power engineering and research is not so widespread as, for example, in microwave communications. In terms of attitude towards FDTD, the various branches of thermal processing industry seem to exhibit now what the communications industry exhibited a decade ago: a mixture of interest in new capabilities with skepticism regarding the adequacy for "real" problems and the ease of use. To a large extent this attitude may stem out of specific economical and societal conditions discussed elsewhere [1]. However, it additionally has an undeniable technical background, which is also the background for this work. One should remember that in communications there were (and there are) industrial giants, who have for many years financed a large volume of focussed research and software developments. It is one of the factors that have successfully led to the current state of art in FDTD knowledge and FDTD solvers, but it has also biased the scope of implemented models, techniques, and software operating regimes. The idea behind this paper, and behind our more general research launched about two years ago, is to adapt our existing FDTD software package [2] so as to provide fully consistent, fast, and reliable analysis of microwave heating prob-

200

Celuch-Marcysiak

lems. Three essential features have been identified and are planned to be successively developed: enthalpy-dependent media parameters; load movement and rotation; and coupling of the electromagnetic analysis with heat flow. In general, we do not feel in a position to evaluate the order of importance of these three features, and we also think that the order may vary between various applications. However, from the software development perspective, we have concluded that the implementation of the variable media parameters should be a starting point as it is also a precondition for the consistent implementation of moving loads and coupling to heat transfer. Hence, this is what the present paper concentrates on. All standard FDTD solvers assume that media parameters remain constant throughout the analysis. This assumption is not met when we model practical microwave power processes since constitutive parameters of all typical foodstuffs, timber, rubber and other treated materials vary substantially as a result of heat dissipation, temperature rise and/or phase changes. The analysis of such processes with constant parameters produces inconsistent results. Multiple iterative simulations, with media parameters modified manually in accordance with the previously simulated power distribution, are a tedious task and can only be conducted very roughly. For example, we can divide the load into a few subregions and assign to them different physical parameters; but we cannot, within any reasonable time, manually assign different parameters to each of the thousands or even millions of FDTD cells. A novel FDTD system for microwave heating and thawing applications presented herein automatically modifies all physical media parameters as a function of dissipated power, in each FDTD cell separately. It accepts arbitrary userdefined variation of parameters versus either enthalpy or temperature. This new functionality of FDTD appears basic for further heating-oriented developments, and we therefore call the new part of the software Basic Heating Module (BHM). The whole system will be further referred to as FDTD-BHM. In extension to the Conference abstract, this paper discusses the assumptions behind and the operation of FDTD-BHM in more technical detail. Our objective is to ensure the relevance of the new system for microwave power research and engineering, and thus the readers' comments in this regard will be appreciated. The paper also includes two simulation examples, which involve thawing and heating of bread and beef. These confirm the efficiency and flexibility of FDTD-BHM for practical microwave power processes.

Development of flexible media description A starting point for this work is to decide how media parameters should be described and provided to the software. We must note that various practically heated materials exhibit very different and sometimes very complicated thermal characteristics. Techniques for measuring material data under controlled thermal conditions, as well as those for interpolating between the measured points, are likely to constitute proprietary information of potential FDTD-BHM users. Thus to main-

A novel FDTD system for microwave heating and thawing analysis

201

tain generality of the software while providing maximum flexibility of media description, we have decided that FDTD-BHM will communicate with a library of text files prepared and expandable by the user. Each file medium.pmo contains tabulated parameters of the medium versus a selected independent variable, representing the thermal conditions. Another issue is the choice of the independent variable. In many heating applications this may be temperature, and in such a case the medium.pmo file will contain a listing of medium parameters at specified temperature points. However, temperature-dependence is poorly conditioned around phase changes. Then the microwave power is absorbed, enthalpy of the medium increases, and parameters rapidly change, while temperature remains nearly constant. In such cases, it is more meaningful to list medium parameters versus enthalpy density. Thus the format of *.pmo files allows either enthalpy- or temperaturedependence. The choice of the independent variable can be different for different media. To further discuss the file format, let us refer to bread.pmo and beef.pmo shown in Fig. 1 and Fig. 2, respectively. Each independent or dependent variable is recognised by its keyword on top of the data column. Either Temperature or Enthalpy column must exist; if both exist, then the actual media modification is performed versus enthalpy, while temperature data is only used for the display of temperature distribution.

Fig. 1. Example of bread.pmo file for FDTD-BHM

The following physical parameters can be modified by FDTD-BHM: permittivity (EPx, EPy, EPz), conductivity (SIGx, SIGy, SIGz), permeability (MUx, MUy, MUz), magnetic loss (MSIGx, MSIGy, MSIGz), density (Density), and specific heat capacity (SpecHeat). Any or all of those can be listed in any or all of the used *.pmo files, and are recognised by keywords indicated in brackets. The user can list as many enthalpy (or temperature) points as are known and relevant for a particular application. The only requirement of FDTD-BHM is that the points are listed in the order of increasing enthalpy (or temperature). Linear

202

Celuch-Marcysiak

interpolation will be applied between any two listed points, and flat extrapolation outside the listed range.

Fig. 2. Example of beef.pmo file for FDTD-BHM

Utilising and adapting the FDTD solver Our existing electromagnetic FDTD software [2] comprises two functional blocks: Editor and Simulator. Editor allows to specify the geometry of the analysed scenario, either by manual drawing, or by picking up parameterised elements from the libraries, or by importing CAD files. It is also used for defining excitation and post-processing data as well as default media parameters. Finally, it generates the conformal FDTD mesh. Simulator is the conformal FDTD solver supplemented with signal postprocessing capabilities and extensive display functions. It includes sophisticated FDTD models, which have been originally developed for communications but then proven relevant and useful for microwave power applications [3, 4]. Interesting examples include: x conformal approximation of arbitrary shapes of cavities and loads, inhomogeneous media, media boundaries, wire probes, and metal edges, x electric and magnetic loss modelling, x user-controlled input power, x access to absolute values of fields and dissipated power, x extraction of average dissipated power, also in the case of out-of phase modes, x freeze-of-state, which allows to terminate the analysis at any point and re-start from the saved field distribution.

A novel FDTD system for microwave heating and thawing analysis

203

To adapt the software to the FDTD-BHM operation, only minor changes are needed in Editor: it should allow the introduction of initial temperature and default values of specific heat for all heated media. Major technical changes are needed in Simulator. First of all, please note that Simulator always starts its operation by converting the shape and material data into the so-called lcsm matrices (we shall call this stage the lcsm compilation), with separate entries for each FDTD cell and field components. In standard operation, all the necessary material data comes from Editor. In FDTD-BHM, Simulator will need to extract appropriate values of media parameters from *.pmo files, taking into account enthalpy or temperature of each cell. Moreover, when media parameters are modified due to enthalpy or temperature increase, the lcsm matrices will require re-compilation.

Operating the FDTD-BHM system Electromagnetic analysis with FDTD-BHM proceeds in the following steps: 1. We run the FDTD analysis with sinusoidal excitation until the electromagnetic steady-state is reached. 2. We produce the 3D pattern of average dissipated power Pm(x,y,z). 3. We upgrade the enthalpy (or rather enthalpy density) distribution by: Hm+1(x,y,z) = Hm(x,y,z) + Pm(x,y,z) 'W

(1)

where 'W is the assumed (user-defined) time of heating at a particular steadystate, with constant average power Pm(x,y,z). Please note that enthalpy increases in all lossy media. Moreover, within one medium, each FDTD cell will typically have different enthalpy. 4. We upgrade the temperature distribution in each FDTD cell, in one of the two ways: a) If a medium filling the cell has its *.pmo file, and both temperature and enthalpy are listed in the file, a value of temperature corresponding to the value of enthalpy is read from the file: Tm+1(x,y,z) = T [Hm+1(x,y,z)]

(2)

No reference is made to specific heat. b) In other cases, a new value temperature is calculated using specific heat Cm and density U m : Tm+1(x,y,z) = Tm(x,y,z)+ P(x,y,z) 'W / (U m (x,y,z) Cm(x,y,z) ) = = Tm(x,y,z) + [Hm+1(x,y,z) - Hm(x,y,z)] / (U m (x,y,z) Cm(x,y,z))

(3)

204

Celuch-Marcysiak

If *.pmo file exists and contains the listing of specific heat, a value of Cm at enthalpy Hm or temperature Tm is read from the file. Otherwise a default constant C as set in Editor is used. The same rule applies to density. 5. We repeat the process of lcsm compilation. For each FDTD cell we take media constitutive parameters corresponding to the cell's current enthalpy Hm+1 or temperature Tm+1 . There are three ways of detecting current values of all media parameters. Since these ways are the same for each medium parameter, let us consider Hz as an example: a) If a medium filling the cell has its *.pmo file, Hz is listed in the file, and enthalpy is listed in the file, a value of Hzm+1 corresponding to the value of enthalpy Hm+1 is read from the file:

Hzm+1(x,y,z) = Hz [Hm+1(x,y,z)]

(4)

No reference is made to temperature or default settings for Hz made in Editor. b) If a medium filling the cell has its *.pmo file, Hz is listed in the file, but enthalpy is not listed (which means that temperature must be listed), a value of Hzm+1 corresponding to the value of temperature Tm+1 is read from the file:

Hzm+1(x,y,z) = Hz [Tm+1(x,y,z)]

(5)

c) If a medium filling the cell does not have its *.pmo file, or Hz is not listed in the file, a default constant value of Hz (as defined Editor) is maintained. 6. We resume the FDTD electromagnetic analysis starting with the previously obtained steady-state fields, but using the new lcsm parameters, until a new steady state is reached. One sequence of steps 1...6 will be further called a thermal iteration. The process can be iteratively repeated. A majority of the above tasks are performed automatically by FDTD-BHM. The user interaction is necessary at the level of decision making and concerns:  deciding when the steady state has been reached (end of step 1), by either watching the fields and power, or based on previous experience with a similar scenario,  activating the average power calculations (start of step 2),  deciding that the average power pattern remains unchanged (end of step 2), and activating the "heating" part of the process with a particular "heating time" 'W. Steps 3, 4, 5 are then performed automatically.

A novel FDTD system for microwave heating and thawing analysis

205

Remarks on the choice of "heating time" 'W The choice of heating time 'W at each "thermal" iteration is very important. This choice determines the convergence, and the rate of converge, of the modelling: Too big 'W may cause immediate divergence - at hot spots of an intermediate electromagnetic steady state, it may produce temperature rise above the highest temperature that would ever be reached in the medium. This will obviously happen in a self-controlled thawing process, where the frozen parts of the food absorb more energy than the already thawed parts. x Too small 'W means that the complete heating process is sub-divided into many "heating iterations", each requiring many FDTD iterations. The process will converge, but the computing time will be long. It is assumed that the user will be able to set an appropriate value of 'W based on previous experience with (and understanding of) the particular substance heating. To this end, it is important to note that 'W simply equals to the physical heating time in seconds, with no additional scaling factors involved. This convenient arrangement follows from the fact that the physical power levels can be rigorously controlled in the FDTD-BHM software. For example, let us consider heating in a 800 W microwave oven. By setting the sinusoidal source amplitude to —800 = 28.28 in Editor, the user ensures that the Simulator will work with the source of 800 W available power, producing correct levels of dissipated power in the load. Moreover, FDTD-BHM is prepared to assist the users with establishing appropriate 'W by numerical experiments. Especially when modelling new media or unusual heating systems, the user is strongly recommended to prepare graphical plots of the media characteristics versus enthalpy or temperature, and to correlate these plots to the outcomes of the numerical process. The following options are available FDTD-BHM: 1. The user can watch the levels of average dissipated power over the scenario and choose so that the resulting increase of enthalpy / temperature will nowhere cause a "jump" over local maxima / minima on the media characteristics. 2. After performing a thermal iteration (steps 3, 4, 5 above) the user can verify that choice by checking the new values of media parameters, which can be displayed by the software. 3. If the changes of media parameters produced by one thermal iteration with 'W1 are negligible, the user can make another "thermal" iteration with 'W2 , before re-starting the EM analysis (i.e., repeat steps 3, 4, 5 before going to step 6). This is equivalent to one "thermal" iteration with 'W1 +'W2 . 4. If the changes of media parameters produced by one thermal iteration with 'W1 are too big, so that important intermediate effects seem to have been skipped, the user can make another "thermal" iteration with -'W2 (0aH  bH T  293 K  1@

(2)

The constants in this equation were taken from [1]: aH = 10, bH = 0.002 /K (for alumina), and the dependence on U was assumed plausibly. The imaginary part of the dielectric constant is described by the electric conductivity with the following dependencies on temperature and relative density § T

·

3

(3)

¸¸ (2 U  1) V V 0 ¨¨ © 293K ¹

with V0 = 6.54E-6 /(:m) for alumina at a frequency 2.45 GHz from [1], and with a plausible dependence on relative density. Maxwell's equation is solved together with the heat conduction equation UU cT ’ ˜ O’T  Q (4) 0

Here U0 is the density of the pore-free material, c is the specific heat per unit mass; for alumina is U0c = 4E6 J/(Km3). Further, Q is the heat produced per volume and time by the absorbed electromagnetic power Q VE

2

(5)

and O is the heat conductivity, which depends on temperature [6] and relative density [4] according to

O

2U  1 aO  bO T

(6)

with aO = -0.0024 Km/W and bO = 1.08E-4 m/W [6]. Finally, heat exchange between adjacent bodies by radiation is taken into account, but only in an approximate way. Evolution equations for the relative density, U, and for the grain radius, R, are taken from a detailed solid-state sintering model [5]. They have the following general form, and parameters for alumina are given in [3] f ( U , R, T )

(7)

R g ( U , R, T )

(8)

U

212

Riedel

Numerical solution for axisymmetric resonant cavities For axisymmetric problems the harmonic electric field has the form E(r , I , z , t ) E (r , z ) exp(iZt  imI )

(9)

with integer m, so that in Maxwell's equation one can replace w/wI = im. The resulting axisymmetric form of Maxwell's equation is solved numerically using a finite difference scheme. The solution is particularly simple for m = 0, since in that case the differential (and the finite-difference) equations for EI decouple from those for E r and E z . Solving for EI gives the transverse electric modes TE0np. Only for this case, solutions are presented here. As an example, Figure 1 shows a cavity with antennas to feed in the microwave energy, with a ceramic sample that is to be sintered, with ceramic susceptors and with insulation made of low density fibre-alumina ceramic.

Susceptor Ceramic

Insulation

Microwaves

Antenna

Fig. 1. Cylindrical cavity with a ceramic sample

Solutions without heat conduction and sintering In a first step the dielectric properties of all materials in the cavity are assumed to be time independent. The numerical values were chosen to be those of alumina at 1000°C. Figure 2 shows the response of the cavity when exited at the lower antenna. As the figure shows, the empty cavity exhibits sharp resonance lines. The resonance frequencies calculated numerically agree accurately with the analytic solutions [2]. If the cavity is loaded as shown in Figure 1, the resonance lines broaden and they are shifted to lower frequencies. Figure 3 shows the electric field

Simulation of Microwave Sintering with Advanced Sintering Models

213

distributions for the TE013 mode in the empty and in the loaded cavity. In the presence of the dielectric materials the central field maximum is enhanced at the expense of the two other maxima.

Fig. 2. Resonance spectrum of the cavity shown in Figure 1 with and without dielectric load

Fig. 3. Distribution of the electric field in the empty and the loaded cavity (TE013 mode)

Solutions with heat conduction and sintering The calculation is started at a certain uniform temperature (e.g. room temperature). Maxwell's equation is solved, and the program seeks the maximum of the desired resonance line (e.g. TE013). The excitation amplitude is normalized such that a

214

Riedel

predefined total power is absorbed in the cavity. Then an explicit, Euler forward, time integration procedure is started: From Eqs. (7) and (8) the increments and the updated values of density and grain radius are calculated, then the new temperature distribution is calculated from the heat conduction equation using the heat conductivity for the updated density, and finally Maxwell's equation is solved with the updated dielectric properties. Usually the cavity will run out of the resonance line when the temperature rises. Hence the program adjusts the frequency to keep the cavity in resonance. Attempts to achieve that goal by tuning the phase angle at the upper antenna were not successful, since the effect of the phase turned out to be insufficient to compensate the change of the dielectric properties. After each time step the excitation amplitude is normalized to keep the absorbed power at the desired value. The solution of Maxwell’s equation requires matrix inversion. This is computionally more expensive than the explicit time integration of the ordinary differential equations (7) and (8) and of the heat conduction equation. On the other hand, the explicit integration requires small time steps to remain stable. Hence the electric field distribution, together with the adjustment of the resonance frequency is re-calculated only after typically 100 increments of the time integration scheme. To demonstrate the capabilities of the program, the sintering of coarse and finegrained alumina was modelled. The initial grain radius is 1 µm in the coarsegrained material and 50 nm in the fine-grained material. The relative green density is 55% in both cases. The parameters of the sintering model are given in [3]. Figure 4 shows the temperature distribution in the cavity for the coarse-grained material, if the absorbed power is 300 W. In the ceramic (the hatched area in the center of the furnace) the temperature has values between 950 K and 1550 K.

Fig. 4. a) Temperature distribution for 300 W microwave power after 1500 s. The axis of rotation is on the left. Hatched areas are the sintering ceramic and the susceptors. b) Maximum and average temperature in the (coarse-grained) ceramic

This is far too low to sinter the coarse-grained material. The temperature in the ceramic reaches a certain saturation after about 1500 s when the radiation losses balance the absorbed microwave power (Fig. 4b).

Simulation of Microwave Sintering with Advanced Sintering Models

215

Fig. 5. Temperature distribution for 1 kW microwave power. Maximum and average temperature in the (coarse-grained) ceramic

Fig. 6. Temperature distribution for 500 W microwave power. Maximum and average temperature in the (fine-grained) ceramic

Fig. 7. Relative density and grain radius (in m) in the ceramic after sintering.

If the power is increased to 1 kW the temperature in the ceramic becomes very inhomogeneous (Fig. 5). There is a hot spot where the temperature reaches the melting point of alumina (2330 K) after 240 s, whereas the average temperature in

216

Riedel

the ceramic is less than 1250 K, which is insufficient to start sintering. This is a typical example for a runaway instability with excessive local over-heating. On the other hand, a very homogeneous temperature can be achieved with the fine-grained material and heating power 500 W (Fig. 6). As Fig. 6b shows, there is an initial trend towards localization, but as the fine-grained ceramic starts to sinter, the heat conductivity increases, and the heat is distributed homogeneously. After about 800 s the temperature becomes nearly time independent and it is sufficient to sinter the fine-grained material. The result is a product with high density and fine grain size at the end of the sintering cycle (Fig. 7).

Conclusions Microwave sintering of ceramics is modelled by solving Maxwell’s equations, the heat conduction equation and sintering equations simultaneously. The heat conduction equation and the equations for solid-state sintering are integrated by explicit time integration. After a certain number of time steps the electric field distribution and the resulting heat production are re-calculated by solving Maxwell’s equations with a finite difference program in the frequency domain. It was demonstrated that this coupled approach allows to predict runaway instabilities and to describe the influence of the sintering process on the evolution of the instability. For example, a coarse-grained alumina ceramic cannot be sintered with the considered TE-mode, since the microwave power is either too low to achieve the sintering temperature, or so high that a runaway instability occurs. A fine-graind ceramic, on the other hand, can be sintered homogeneously to full density with very little grain coarsening.

References [1] Evans NG, Hart, NA, Hamlyn MG (1999) Dielectric property measurements in the design of industrial scale hybrid kiln. British Ceramic Transactions 98:93-99 [2] Jackson JD (1962) Classical electrodynamics. Wiley, New York London Sydney [3] Kraft T, Coube O, Riedel H (2001) Numerische Simulation des Pressens und Sinterns Grundlagen und Anwendungen, Fraunhofer Institut für Werkstoffmechanik, Freiburg, http://www.iwm.fhg.de/arbeitsgebiet/lb/thema_lb41/pressen-sintern.pdf [4] Raether F, Zimmer J, Springer R (1999) Improved sintering of alumina by new in-situ measuring methods. In: Proceedings of the 9th Cimtec-World Ceramics Congress 1998, Ceramics: Getting into the 2000‘s – Part B, Techna Srl., pp 711–720 [5] Riedel H, Blug B (2001) A comprehensive model for solid state sintering and its application to silicon-carbide. In: Chuang T-J, Rudnicki JW (eds) Multiscale Deformation and Fracture in Materials and Structures, The J.R. Rice 60th Anniversary Volume. Kluwer Academic Publishers, Dordrecht Boston London, pp 49-70 [6] Schulz B (1988) High temperature thermal conductivity of irradiated and non-irradiated D-Al2O3. J Nuclear Mater 155-157:348-351

Finite Element Modelling of Thin Metallic Films for Microwave Heating R.A. Ehlers and A.C. Metaxas Department of Engineering, University of Cambridge, Trumpington Street, Cambridge CB2 1PZ, UK.

Abstract A variety of surface boundary conditions are examined for the purpose of modelling thin metallic film characteristics in microwave heating systems. The numerical formulation for the inclusion of each boundary condition is presented in a frequency domain finite element formulation. Verification is provided in a single and multimode context with the aid of analytical and thermal methods respectively.

Introduction Thin metallic films with thicknesses in the order of micrometres to nanometres exhibit a range of reflective and transmittive properties that is a function of the film conductivity. The use of such properties has been seen in applications of packaging requirements for food preparation in microwave ovens. Antennas, which have reflective properties are used where certain food products are known to heat vary rapidly and require some form of shielding. In contrast thereto, susceptors, which possess properties of field absorbtion at thicknesses in the region between that of reflective and transmittive properties, are used as additional heat sources. Numerical modelling of metallic film properties in microwave fields provides a convenient, time and cost effective design tool for packaging specifications. Methods for modelling film boundaries have appeared in the literature for the FDTD time domain [1]. Although surface impedance conditions have been applied successfully in a broad range of electromagnetic applications for finite elements [2], there has been little emphasis on microwave heating. Four surface boundary methods are presented in this paper - the SIBC (Standard Impedance Boundary Condition), the Resistive Sheet, the Modified Resistive Sheet and the Discontinuous Sheet. Their implementation is shown in the frequency domain finite element method which is based on the use of edge elements.

218

Ehlers

Numerical formulation

Extension of the Standard Impedance Boundary Condition The first order impedance boundary condition used to implement wall losses is extended to include a thin film impedance such that the impedance at one side of the metal, Zin is represented as that of a terminated lossy transmission line [3]

Z in

Z film

Z 0  Z film tanh(JW ) Z film  Z 0 tanh(JW )

(1)

where Zo is the impedance of air, W is the film thickness and the film impedance, Zfilm is defined as :

Z film

ZP 2V

(1  j )

(2)

for metallic films. The terms P and V are the film permeability and conductivity respectively. The complex propagation constant is given as J = D + E, where

D

Z

E The effective loss factor is

PH 0H '

Z

He

1

2

PH 0H '

H e2

1

H e2

2 H '' V  ZH 0H

ZH 0

(3)

1

H' '

1

(4)

.

The formulation for the Helmholtz equation requires that an additional surface integral be added where the thin film is to be evaluated :

1

2 *

³ (’ u\ ).(’ u E)d:  Z H ³\ .Ed:

P0 :

:

 jZ ³\ .J s d:  jZ ³\ .(n u H )d*1 where

H

*

'

H 0 (H  j

(5)

*1

:

V  ZH 0H '' ZH 0

) , Js is the source current density, : is the

domain volume and M is the weighting function. The input impedance, Zin, given by equation (1), is related to the field tangential components by :

Finite Element Modelling Of Thin Metallic Films For Microwave Heating

E x (Z )

Z in (Z ) H y (Z )

219

(6)

The Resistive Sheet Boundary Condition The resistive sheet boundary condition replaces the thin film by a single sheet of resistance which is a function of the film thickness, W. It ignores components of volume currents normal to the thin film layer and the electric field is assumed to remain continuous across layer, that is, there is no tangential component variation within the layer. The magnetic field however is discontinuous across the layer. Hence the accuracy is inversionally proportional to the layer thickness where an upper limiting constraint exists on the layer thickness based on the wavelength in the film, W = 0.1Ofilm. The impedance of a resistive sheet, Zres, is a complex term defined as :

Z res where

Hr

j ZH 0W (H r  1)

(7)

H '  jH e is the relative permittivity of the film layer.

The resistive sheet is modelled by supplementing the Helmholtz equation with an additional surface integral that encompasses the film impedance and the electric field which interacts with the film.

1

P0 :³

(’ u \ ).(’ u E)d:  Z 2H * ³\ .Ed:

 jZ ³\ .J s d:  jZ :

:

³

*res

1 (n u E).(n u \ )d*res Z res

(8)

The Modified Resistive Sheet Boundary Condition The modified resistive sheet is similar to the resistive sheet in that it uses a single surface element to relate the electric fields on either side of the metallic film. However, it differs in the use of a single impedance based on transmission line theory to relate the reflected and transmitted components of the field. The Helmholtz equation is thus similar to equation 8 with a modified impedance value over the surface, *modres [4].

1

2 *

³ (’ u\ ).(’ u E)d:  Z H ³\ .Ed:

P0 :

§ 1 D ·¸  jZ ¨  \ .Ed*mod res ¨Z ¸* ³ Z d ¹ mod res © 0

:

 jZ ³\ .J s d: :

(9)

220

Ehlers

Where

P0 H0

Z0

cos(k dieW ) 

, and D

1 §¨ jZP ·¸ sin(k dieW ) for a Z 0 ¨© k die ¸¹

film of thickness, W. The wave number of the sheet is given as

k die

k 0 P (H '  jH e'' ) and k 0 is the free space wave number.

The Discontinuous Sheet Boundary Condition Implementing a Discontinuous Sheet Boundary Condition requires that the mesh is split on the surfaces where the thin film is defined. The mesh generator is not responsible for the task of splitting the mesh, although it is still necessary for the mesh generator to include the surfaces along which a film will exist when designing the domain geometry as is also required for the other implementations that have already been discussed. Unlike the resistive sheet boundary condition, the effect of splitting the mesh results in both the electric and magnetic fields being discontinuous across the thin film layer [5]. A thin film element is thus required to relate the field components on either side of the film using transmission line theory. The use of a discontinuous sheet has been found to produce accurate results and indeed better than the method of extending the Standard Impedance Boundary Condition presented earlier [3]. The tangential electric fields on each side of the layer, Etan1 and Etan2, are related to the tangential magnetic fields, Htan1 and Htan2, as follows:

ª E tan 1 º «E » ¬ tan 2 ¼

ª Z 11 «Z ¬ 12

 Z 12 º ª n u H tan 1 º  Z 11 »¼ «¬n u H tan 2 »¼

(10)

The impedances Z11 and Z12 are given as

Z 11

where k

 jZP k tan(kW ) '

Z (H 0 H  j

Z 12

V  ZH 0 H '' Z

)P

 jZP k sin(kW )

(11)

Finite Element Modelling Of Thin Metallic Films For Microwave Heating

221

Verification

Analytical Verification Analytical verification is provided using the forward travelling waveguide shown, with a mesh, in Figure 1 where an absorbing boundary provides a reflectionless termination at the far end of the waveguide (z = 40 cm). The source is located at the near end (z = 0 cm) and the film to be tested placed in the centre (z = 20 cm). The portion of the waveguide in which reflection and transmission takes place is shown in dark and light grey respectively. The following analytical expressions for the reflection, R, transmission, T and absorption, A power coefficients are provided by [6]:

Fig. 1. Travelling wave applicator (8.6 cm x 4.3 cm x 40 cm) used for thin film analytical verification.

R

§ § P J ·§ P J · § P J ¨ ¨1  0 2 ¸¨1  2 ¸  ¨1  0 2 ¨ ¨ P J ¸¨ P J ¸ ¨ P J 2 0 2 ¹ 2 ¹© © ¨© ¨ § P J ·§ P J · § P J ¨ ¨1  0 2 ¸¨1  2 ¸  ¨1  0 2 ¨ ¨ P J ¸¨ P J ¸ ¨ P J 2 0 2 ¹ 2 ¹© © ©©

·§ P 2J ¸¨1  ¸¨ P J 0 2 ¹© ·§ P 2J ¸¨1  ¸¨ P J 0 2 ¹©

·  2J W ¸e 2 ¸ ¹ ·  2J W ¸e 2 ¸ ¹

· ¸ ¸ ¸ ¸ ¸ ¸ ¹

2

(12)

222

Ehlers

T

§ ¨ ¨ J W 4e ¨ ¨ § P J ·§ P J · § P J ·§ P J · ¨ ¨1  0 2 ¸¨1  2 ¸e J 2W  ¨1  0 2 ¸¨1  2 ¸eJ 2W ¨ P J ¸¨ P J ¸ ¨ ¨ P 2J ¸¨ P 0J 2 ¸ 2 0 2 ¹ ¹ ¹© © ¹© ©©

· ¸ ¸ ¸ ¸ ¸ ¸ ¹

2

(13)

A 1 R  T where P0 and P2 are the permeability of free space and the film respectively, whilst the propagation constants in the TE10 mode for air, J and the film, J2 are defined as follows: 2

2

J

§S · 2 ¨ ¸  Z P 0H 0 ©a¹

J2

§S · 2 * ¨ ¸  Z P 2H ©a¹

(14)

The numerical reflection coefficient is obtained by evaluating the difference in the electric field on two planes that cover the cross-section of the waveguide. Figure 2 shows the reflection coefficient for each of the four methods at a film conductivity of 1.326e5 S/m. It is evident that the SIBC fails as the film thickness decreases whereas the resistive and modified resistive conditions have a defined offset with the analytical result [7].

Fig. 2. Numerical verification of four different boundary conditions.

Figure 3 shows how the reflection coefficient varies as a function of the film conductivity for the most accurate method, the discontinuous sheet.

Finite Element Modelling Of Thin Metallic Films For Microwave Heating

223

Fig. 3. Reflection coefficient variation with film conductivity for the discontinuous sheet

Thermal Verification Verification for a multimode example is provided using a typical practical application of a food package, where a thermal image of the heated food surface within the package can be compared with a numerical power density image. The multimode oven (30 cm x 40 cm x 35 cm) containing the food package is shown in Figure 4(a). The package is loaded with mashed potato (48.86 - j 20.34), a simulation of which is shown in Figure 4(b).

Fig. 4. (a) CAD model of multimode oven and food package; (b) Normalised absolute electric field distribution (left) and power density distribution (right) in the oven on a plane at the food surface in a package without metallic film (package is seen in region between -5 cm and 5 cm on both axis).

224

Ehlers

Fig. 4. (c) Infra-red image of heated food surface inside a package with metallic film; (d) Normalised absolute electric field distribution (left) and power density distribution (right) in the oven on a plane at the food surface in a package with metallic film (package is seen in region between -5 cm and 5 cm on both axis; Frequency domain modelling of food package with a thin film lining. [Figures (a) and (c) by kind permission of J. of Microwave Power and Electromagnetic Energy]

When the package is lined with aluminium foil on its vertical sides, where the foil has a conductivity of 3.43E7 S/m and a thickness of 50 Pm, the effect of the foil focussing energy to the package centre is evident as shown in Figure 4(d). The numerical implementation of the foil, the outcome of which is seen in Figure 4(d), makes use of the discontinuous sheet boundary condition. This compares favourably with the thermal image in Figure 4(c). The slight offset of the hotspot in the package centre is also seen to correspond between the thermal plot and the power density plot. This offset is due to an unsymmetrical field distribution within the cavity brought about by an off-centre waveguide input into the cavity.

Conclusions The paper has studied the implementation of four various boundary conditions in an attempt to model thin film surfaces within microwave cavities. The extension to the SIBC that was implemented for the purposes of modelling wall losses is seen to be inappropriate when modelling thin films. Both the resistive and modified resistive sheets have been shown, in a forward travelling waveguide, to have an offset correspondence with the reflection coefficient trace obtained by analytical means. The discontinuous sheet has the highest accuracy of the four methods studied and has been validated in single and multimode systems by means of analytical and thermal techniques.

Finite Element Modelling Of Thin Metallic Films For Microwave Heating

225

References [1] QWED. Quickwave-3D electromagnetic simulator. Finite Difference Time Domain software - www.qwed.com.pl. [2] J.L. Volakis, A. Chatterjee, and L.C. Kempel. Review of the Finite-Element Method for Three-Dimensional Electromagnetic Scattering. Journal of the Optical Society of America, 11(4):1422-1433, April 1994. [3] F. Alessandri, G. Baini, G. D'Inzeo, and R. Sorrentino. Conductor Loss Computation in Multiconductor MIC's by Transverse Resonance Technique and Modified Perturbational Method. IEEE Microwave and Guided Wave Letters, 2(6):250-252, June 1992. [4] F. Bocquet, L. Pichon, A. Razek, and G. Tanneau. 3D FEM Analysis of Electromagnetic Wave Scattering from a Dielectric Sheet in EMC Problems. IEEE Transactions on Magnetics, 34(5):2791-2794, September 1998. [5] S. Van den Berghe, F. Olyslager, and D. De Zutter. Accurate Modeling of Thin Conducting Layers in FDTD. IEEE Microwave and Guided Wave Letters, 8(2):75-77, February 1998. [6] R.L. Ramey and T.S. Lewis. Properties of Thin Metal Films at Microwave Frequencies. Journal of Applied Physics, 39(3):1747-1752, February 1968. [7] R.A. Ehlers and A.C. Metaxas. Application of the resistive sheet in finite element microwave heating systems. Accepted for publication in the Journal of Microwave Power and Electromagnetic Energy, JMPEE, Vol 36 No 2, 77-87, 2001.

Analysis of Coupled Electromagnetic and Thermal Modeling of Pressure-Aided Microwave Curing Processes J. M. Catalá-Civera2, J. Monzó-Cabrera1, A. J. Canós2, F. L. Peñaranda-Foix2 1

Departamento de Tecnologías de la Información y Comunicaciones, Universidad Politécnica de Cartagena, Campus Muralla del Mar. 30202 Cartagena, Spain.; 2 Microwave Heating Group, Technical University of Valencia, 46022 Valencia, Spain.

Abstract In this work microwave heat generation in materials surrounded by several layers of dielectrics is studied. Particularly, the pressure-aided microwave rubber vulcanisation process, where the use of dielectric moulds to exert the proper pressure over the rubber samples is essential for this application. Temperature distributions of rubber and mould materials have been simulated using a discretisation strategy based on the FDTD technique. Simulation results show that dielectric and thermal properties of the pressing moulds are of the utmost importance in order to obtain the desired temperature profiles along the volume of the rubber and therefore to achieve a uniform degree of vulcanisation.

Introduction Microwave and pressure-aided curing of polymers and rubber compounds is a very complex process that involves molecular and structural changes of these materials under a coupled electromagnetic and thermal process [1, 2]. Additionally, in some sectors (e.g. footwear industry) the vulcanization process is carried out with the simultaneous application of external pressure through dielectric moulds to avoid the formation of porosity and to conform the desired pattern sample. This process is very dependent on the dielectric properties of the materials involved, the spatial microwave density distribution in the microwave applicator and the thermal conditions around the rubber sample imposed by the dielectric mould used to apply the pressure. Since vulcanizing processes are activated by temperature, the different spatial temperature distribution inside the samples might yield to different degrees of vulcanization in the rubber and consequently influence the final

Analysis of Coupled Electromagnetic and Thermal Modeling

227

quality of the cured material. Therefore, the design of the dielectric mould is essential to obtain uniform temperature profiles into the vulcanized rubber. The study of pressure-aided microwave vulcanization of rubber samples placed inside dielectric moulds was formerly reported in [3]. In this work, a three-zone cylindrical cavity was developed to concentrate the electric field distribution over the rubber, overcoming the disturbance effect of dielectric moulds in the cavity. Microwave power was delivered to the cavity by means of a PID controller, which allowed to maintain the desirable level of heating during the time that the vulcanization process was taking place. Despite of most rubber samples were successfully vulcanized in this small applicator, some experimental tests demonstrated the existence of several non-uniformities in the curing profiles in these vulcanized samples under certain conditions of treatment and mainly for some specific rubber compositions (specially those low thermal conductivity rubber compounds). In this work, to understand better these effects and to equalize the undesirable uneven temperature profiles, which produce lower quality vulcanized rubber, the thermal process coupled with the electromagnetic processing of these materials has been simulated. Two different situations are studied: pressure-assisted microwave vulcanization using microwave-transparent moulds (such as PTFE) and the effect of absorbent moulds, which in turn absorb microwave energy and thus contribute to the heating of rubber.

Simulation Scheme The problem regarding pressure-aided microwave curing of rubber samples surrounded by dielectric molds is the modeling of multilayer dielectrics immersed in an electromagnetic field. Figure 1 shows a general scheme for this situation where different materials with diverse dielectric and thermal characteristics are depicted. Those structures are in close contact between them due to the applied pressure and as a result, heat flux is continuous through the materials interface. Temperature evolution in the rubber can be analysed with the aid of the socalled heat equation (1), [4]. For microwave-transparent layers, internal heat generation is negligible but, if materials are microwave-absorbent, volumetric heat generation (2) must be taken into account and related to the electric field and the material dielectric properties in order to compute the temperature time evolution (see key to symbols in Appendix A): As described in [3], the electric field can be considered constant along the sample and moulds, due to the specific design of the three-zone cylindrical cavity and, consequently, the complexity of the calculations can be reduced to the resolution of a single partial differential equation. The thermal and dielectric properties of the materials are also considered constant with the temperature. At the external surface of the mould, which is in contact with air, a convective boundary condition is applied as declared in (3). On the surface interface between two materials, the heat flux must be continuous as expressed in (4).

228

Catalá-Civera

Fig. 1. General scheme for simulations of rubber samples surrounded by dielectric molds.

Q kT ’ 2T  gen U ˜ cP U ˜ cP &2 Q gen 2Sf oH oH ' ' E

wT wt

 kT ˜

(1) (2)

wT ht ˜ (TS  Tair ) wx S

(3)

wT wT  kT 2 ˜ wx S1 wx S2

(4)

 kT1 ˜

In order to solve these expressions with the configuration of Figure 1, a FDTD (Finite Difference Time Domain) discretisation algorithm has been implemented. In this case, the partial differential equation of (1) has been discretised by using the Cranck-Nicholson strategy [5].

Simulation Results Figures 1 and 2 show temperature evolution of a rubber sample completely enclosed by a microwave transparent dielectric mould, simulated with the FDTD technique described above. When exerting pressure by means of a these materials, such as PTFE, there is not internal heat generation within it, so microwave energy

Analysis of Coupled Electromagnetic and Thermal Modeling

229

is transformed into heat only in the rubber sample. This generated heat is transmitted to the mould owing to thermal diffusion and, because of this, the outer parts of the rubber sample loose energy and experiment a lower temperature rise than the inner parts. However, temperature differences in the rubber greatly depend on its thermal conductivity. In Figure 1, plotted for a rubber sample of ktrubber = 0.1 W/mºC, it is possible to appreciate temperature gradients of 50ºC between center and surface, meanwhile in Figure 2, plotted for a thermal conductivity eight times higher, the energy can be redistributed more easily within the rubber and temperature gradients are flatter (around 15ºC of difference). Simulation parameters for these figures are given in Table 1 and spatial temperature profiles are provided for each twenty-second interval.

Fig. 2. Spatial distribution of temperature within the rubber sample and the PTFE mould considering PID temperature of 155ºC , ktrubber = 0.1 W/mºC

From these results, it is concluded that in order to obtain good quality vulcanized rubber with these transparent moulds, heat migration through the interface rubber-mould must be minimized. One possible alternative is the pre-heating of the transparent moulds, which would reduce heat losses by thermal conduction at the contact interface. This situation has been simulated and depicted in Figure 3. As shown, temperature profiles are flatter and the temperature at the interface of the sample higher than the temperature represented in Figure 1. Nevertheless, this solution seems not to be sufficient for low-thermal conductivity rubber samples and other kind of solutions must be explored. Moreover, practical problems could appear when manipulating these hot materials in an experimental installation.

230

Catalá-Civera

Fig. 3. Spatial distribution of temperature within the rubber sample and the PTFE mould considering PID temperature of 160ºC , ktrubber = 0.8 W/mºC.

The other alternative chosen consists of including an intermediate and absorbent mould between the transparent mould and the rubber sample. The fact of placing this absorbent mould can avoid thermal migration, which happens now at the interface between the absorbent and the transparent mould, giving more uniform temperature profiles. Then, if thermal and dielectric properties of the absorbent mould are properly selected, it is really possible to obtain a high degree of uniformity of temperatures along the volume of the rubber sample and therefore obtain a high quality vulcanised material. This is the simulation depicted in Figure 4 for an absorbent material of loss factor 0.078 and the conditions given in Table 1. Additionally, the fact of introducing an absorbent mould reduces the energetic efficiency of the system, since an extra amount of energy is needed to heat up the absorbent mould and it is not employed directly in the sample curing process. As a consequence, the less mass used in the absorbent mould the higher the energy efficiency. However, a compromise must be taken between efficiency and mould resistance, a situation that depends greatly on the material under consideration. In order to validate the uni-dimensional results and to observe the bidimensional effects caused by the use of these moulds, 2D simulations have also been carried out for the situations depicted in Figures 1 and 4, respectively. Figure 5 shows a comparison of bi-dimensional temperature distribution between the fact of using only transparent moulds or transparent and absorbent moulds to pressure the rubber. As it can be perceived in the figure, the employment of such absorbent mould moves away the thermal migration to the surface between PTFE and absorbent mould, and consequently temperature gradients in the rubber are uniform enough to obtain uniform enough vulcanised patterns.

Analysis of Coupled Electromagnetic and Thermal Modeling

231

Fig. 4. Spatial distribution of temperature within the rubber sample and the pre-heated PTFE mould considering PID temperature at 160ºC, ktrubber = 0.1W/m ºC.

Fig. 5. Spatial distribution of temperature within the rubber sample, the absorbent dielectric and the PTFE mould for simulation conditions are listed in Table 1.

232

Catalá-Civera

Table 1. Simulation conditions

Initial microwave power Mould thermal conductivity (PTFE) Mould density (PTFE) Mould specific heat (PTFE) Rubber thermal conductivity Rubber density Rubber specific heat Rubber dielectric constant Rubber Loss factor Microwave-absorbent mould’s thermal conductivity Microwave-absorbent mould’s density Microwave-absorbent mould’s specific heat Microwave-absorbent mould’s dielectric constant Microwave-absorbent mould’s loss factor Convective heat transfer

450 Watts (W) 0.195 W/mºC 1172 kg/m3 2150 J/kg 0.1, 0.8 W/mºC 1000 kg/m3 2000 J/kg 2,3 0.1 2 W/mºC 1000 kg/m3 1500 J/kg 3 0.078 0.1 W/m2ºC

Fig. 6. Comparison of temperatures distribution of rubber samples enclosed with microwave transparent (a) and absorbent moulds (b).

Conclusions In this work the thermal problem associated to pressure-assisted microwave vulcanization has been analyzed simulating these structures with a FDTD discretisa-

Analysis of Coupled Electromagnetic and Thermal Modeling

233

tion scheme. The utilization of conventional transparent moulds has been proved as an inefficient solution, specially for low-thermal conductivity compounds, since it leads to non-uniform temperature profiles due to conductive thermal migration at the interface rubber-mould. A proposed solution, is the use of intermediate microwave-absorbent moulds. This configuration has proved its efficacy as it provides the required energy to obtain very uniform temperature-spatial distributions along the sample more independently of the thermal characteristics of the sample. List of abbreviations cp Specific heat E Electric field intensity fo Frequency ht Convective heat transfer coefficient kT Thermal conductivity Qgen Microwave generated volumetric heat t Time T Temperature Ts Surface temperature Air temperature Tf x Distance to axis origin Greek Symbols H0 Air dielectric constant H’’ Loss factor U Density

References [1] Thuery J (1992) Microwaves: Industrial, Scientific and Medical Applications. Artech House, Inc. Norwood, Massachussets: 188-203 [2] Wei JB, Shidaker T, Hawley MC (1996) Recent progress in microwave processing of polymers and composites. Composites Science and Technology, TRIP Review, 4(1): 18-24 [3] Catalá-Civera JM, Giner-Maravilla S, Sánchez-Hernández D, De los Reyes E (2000) Pressure-aided rubber vulcanization in a ridged three zone cylindrical cavity, Journal of Microwave Power and Electromagnetic Energy 35: 92-104 [4] Metaxas AC, Meredith RJ (1983) Industrial Microwave Heating, Peter Peregrinus Ltd, London [5] Holman JP (1980) Transferencia de Calor. Mc Graw Hill.

Selective Heating and Moisture Levelling in Microwave-Assisted Drying of Laminar Materials: An Explicit Model J. Monzó-Cabrera1, A. Díaz-Morcillo1, J. M. Catalá-Civera2, E. de los Reyes2 1

Departamento de Tecnologías de la Información y Comunicaciones, Universidad Politécnica de Cartagena, Campus Muralla del Mar, E-30202 Cartagena, Spain, 2 Departamento de Comunicaciones, Universidad Politécnica de Valencia, E46022 Valencia, Spain.

Abstract In the drying processes microwave energy can be applied in order to achieve a more uniform moisture content distribution enhancing, consequently, the final product quality. In many cases, experimental drying tests are carried out in the laboratory stage to characterise the moisture-levelling. However, based on the knowledge of dielectric properties vs. moisture content it should be possible to find out whether application of microwave heating is beneficial for moisture levelling or not. In this work, an explicit model of microwave selective heating and drying of laminar materials is presented and validated with experimental tests over leather samples. The validation of the model shows a very close relation between experimental and simulated curves and demonstrates that moisture levelling and selective heating for laminar materials can be explained based on the material dielectric properties and the microwave field intensity under certain conditions. The results indicate that as a consequence of moisture leveling drying curves show a fast convergence, leading to a more uniform moisture distribution within the hide.

Introduction Drying processes are widely used in many industries as final or intermediate stages of production. In the developed countries, the energy used in industrial drying is a high percentage of the whole process energy due to the inefficiency of the traditional drying methods and the high amount of energy necessary to evaporate water [1]. In addition, products must be manufactured with an increasing quality and speed, which forces firms to look for alternatives to traditional drying

Selective Heating and Moisture Levelling in Microwave-Assisted Drying

235

processes that usually are slow and inefficient. Microwaves can provide, thanks to their characteristics, an increased and an improvement of quality, mainly for materials with a low thermal conductivity (thermal insulators), by substituting the traditional conductive heat transmission with the volumetric heat generation related to dielectric properties [2]. One of the major advantages of using microwaves in the drying process is the development of selective drying and heating. This kind of drying allows the wetter zones of the material to loose water and heat faster due to their stronger energy absorption. In this way, microwave drying yields materials with a very smooth moisture-content distribution [3]. In the present work, moisture levelling and selective heating in microwaveassisted drying are studied theoretically and the obtained expressions are validated with experimental tests in order to cover the reference shortage found in the literature.

Theoretical Study Whenever a model is developed, it is necessary to assume several considerations to simplify the real situation. These assumptions, however, should be as close as possible to reality in order to achieve good results. In this model, the next assumptions are made: 1. Electric field is constant along each laminar sample. Although this condition seems to be very restrictive, it can be achieved easily in industrial ovens for laminar materials by using mode stirrers or sample movement. 2. Dielectric properties behaviour vs. moisture content must be known. 3. In the constant drying period, almost all the electromagnetic energy is used in order to evaporate the sample moisture content, so it is supposed that the samples increase their temperature slowly meanwhile. Development of a Moisture Leveling Model In the constant drying rate period it has been assumed that nearly all the microwave power absorbed in the sample is converted into water evaporation. This assumption means that the energy lost by heat convection and conduction is small when compared to the energy spent in taking away the internal moisture content as supported by the results in [4]. As a general rule, the conversion of microwave into evaporation can be modelled as indicated in equation (1). It is not possible to assume that H’’ stays constant along the drying process because, as indicated in [5, 6], dielectric properties decrease strongly with declining moisture contents. In fact, this is the reason why selective heating and moisture levelling takes place in microwave-assisted drying. On the other hand, the efficiency of water evaporation depends, clearly, on the

236

Monzó-Cabrera

drying stage considered [6] and, therefore, drying efficiency must be considered as a time-dependent function (see key to symbols in Appendix):

 U ˜ 'H v

wX wt

K (t ) ˜ 2Sf oH oH ' ' ( X ) E

2

(1)

Then, if the dielectric properties of the material to be irradiated vs. moisture content are known and the behaviour of the efficiency can be modelled as a function time, equation (1) can be solved by variable separation as showed in equation (2). This leads to an analytical expression of moisture content vs. time.

wX ³ H''(X )

2Sf oH o E U ˜ 'H v

2

³ K (t ) ˜ wt

(2)

These equations must be applied to specific materials and microwave ovens that are characterised, by the loss factor and the efficiency. Recently, some selective drying and heating tests [3] and the dielectric properties [6] for leather samples have been reported and, consequently, the developed models can be validated for this material. In the case of leather, the behaviour of the loss factor vs. moisture content has been modelled here as a linear function:

H ''( X ) b ˜ X

5.4855 ˜ X

(3)

Regarding to the drying efficiency, it is considered constant in the constant drying rate stage and linearly increasing in the heating-up period in accordance to the model in [6]. This means that the loss of efficiency in the falling drying rate period is only due to the decrement of the loss factor in each sample. Equation (4) divides this behaviour into the two mentioned periods:

K (t )

­K x ° t t  th ® th °¯K x t t th

(4)

By using (2) (3) and (4), an explicit expression has been found for the drying stages under consideration. The selective drying curves are described in (5):

Selective Heating and Moisture Levelling in Microwave-Assisted Drying

X (t )

2 ­ K X bSf oH o E 2 t )  Xe ° ( X o - X e )exp(U'H v t h ° 2 2 ® § · °( X - X )exp(- K X bSf oH o E t ) exp¨  K X b2Sf oH o E (t  t ) ¸  X o e h e h ¨ ¸ ° U'H v U'H v © ¹ ¯

237

t  th t t th

(5)

Selective Heating Modelling As described before, in order to model selective heating it is assumed that in the heating up period heat losses due to convection and conduction are negligible. This is a reasonable assumption as experimental tests show that heating time is very short to make convection losses appreciable. Taking this into account, the differential equation that rules selective heating is given by equation (6),

U ˜ cp

wT wt

2

2Sf oH oH ' ' (t ) E K h (t )

(6)

where Kh is the time dependent ratio of energy expended in heating the material vs. the global absorbed energy, being

Kh (t ) KT  K (t )

(7)

If the temperature dependence of the material’s loss factor is negligible, then the time dependence of temperature can be calculated from (6) and (7), by taking into account the loss factor and the moisture content time dependence, which are expressed in equations (3) and (4). By separating variables in equation (6), the following expression can be obtained (see Appendix for temperature model coefficients):

T (t )

­ § exp(at 2 )  1 · S ¸¸ t  th °°To  cKT  erf (  at )  cK X ¨¨ 4a 2ath © ¹ ® °K  BD >exp(C (t  t ))  1@  BX (t  t ) t t th h e h °¯ C

being erf the error function.

(8)

238

Monzó-Cabrera

Experimental Setup Leather samples, used in all the drying tests, were Nubuck ND chrome tanned hides. The initial mass of the samples was approximately 200 grams and its average dry mass was near 100 grams. To obtain the dielectric properties of leather, the samples were measured using a WR-340 slotted waveguide and a HP-8720B automatic network analyzer (ANA) as shown in Figure 1. Hence, the scattering coefficients of a symmetrically positioned leather sheet inserted along the broad wall of the waveguide, were measured using the procedure described in [7]. With these measurements, the relative dielectric constant and the loss factor of leather can be expressed as a function of moisture content. Microwave selective drying tests were carried out in a combined microwavehot air oven as presented in [3]. To prepare the moisture leveling tests, the leather samples were cut in four similar parts, some of them were partially dried in a conventional oven, while some others were soaked in order to obtain higher initial moisture contents. The assumption of a constant electric field within each sample was obtained in the cavity thanks to several metallic and mobile sheets placed inside the oven. Optical fibers were chosen to measure leather temperatures so as not to perturb the electric fields and consequently minimize the interference over the measurements. For the moisture leveling experiments, four samples with different initial moisture content were inserted in the microwave-assisted drier. Weight losses of leather samples were measured every minute in a high precision scale (Ohaus 110). Although a very constant field distribution was achieved inside the microwave applicator thanks to the metallic-mobile sheets, the position of the samples were changed every minute to minimize a possible influence of the field patterns inside the oven. For the selective heating tests the temperature of leather samples was measured at ten-second intervals, and the measurements were made at the center of the hides. These tests were carried out at a microwave power of 700 watts and an air temperature of 34ºC.

Results In Figures 1 and 2, the experimental data of moisture leveling tests and the simulation results are shown. Several considerations must be taken into account. First of all, it can be observed that the heating-up period is not appreciated for samples with an initial moisture content less than 0.8 (dry basis). On the contrary, this period is very pronounced for samples with higher initial moisture content indicating that the heating-up period length depends on initial moisture content. The simulation results show that the higher the initial moisture content, the higher the field intensity within the sample, which suggests that the microwave energy absorption is much higher for wet samples than for dry ones in this case. It also can be appreciated that both experimental and simulated drying curves converge to a very similar equilibrium value despite the different initial moisture

Selective Heating and Moisture Levelling in Microwave-Assisted Drying

239

content. This fact involves that wetter samples dry more quickly because of their higher microwave energy absorption, which is related directly to the loss factor and the electric field intensity. The good matching between experimental and simulated values also shows that, for laminar materials, energy conversion (from microwave energy to water evaporation) can be considered the main phenomenon that drives the drying process, pushing diffusion and liquid migration to a second place. 2 1.8

Simulation values Xo1= 1.846, E01= 4.7E3 V/m Xo2= 1.808, E02= 4.7E3 V/m Xo3= 0.731, E03= 3.5E3 V/m Xo4= 0.464, E04= 3.3E3 V/m

1.6

X (dry basis)

1.4

1.2

1 0.8

Xo { Initial moisture content + Experimental

0.6

Air

0.4

0

Temperature:34

ºC

0.2 0

1

2

7

6 5 4 time (minutes)

3

9

8

10

Fig. 1. Model simulations vs. experimental moisture levelling data. Microwave Power = 700 W 1.4 1.2

Simulation values Xo1= 1. 401, E01= 4E3 V/m Xo2= 0.583, E02= 3.6E3 V/m Xo3= 0.319, E03= 3 E3 V/m

X (dry basis)

1 0.8

Xo { Initial moisture + Experimental values

0.6

Air Temperature: 34 ºC

0.4 0.2 0

0

2

4

6 time (minutes)

8

10

12

Fig. 2. Model simulations vs. experimental moisture levelling data. Microwave Power

= 540 W.

In Figure 3, selective heating tests are depicted versus the simulated curves. Two stages can be clearly distinguished in the plot: the first one corresponding to the heating up period and the second one occurring in the constant rate drying stage. In the first stage, the curves show that the higher the initial moisture content, the larger the heating rate of the sample, which reaffirms the idea that the dielectric properties play a very relevant role in the heating process. The duration of

240

Monzó-Cabrera

this period is also moisture dependent, increasing with higher initial moisture contents, a result that agrees with Figures 1 and 2. In this period, the samples’ temperature increases continuously according to their dielectric properties and the electric field distribution inside the microwave oven. 75

70

65

Temperature (º C)

60

55

x Xo1 = 1.2145 Xo2 = 0.7607 Xo3= 0.5769

50 45

Simulation values: E01= 5.5E3 V/m, KT=1, KX=0.998 E02= 4.8E3 V/m, KT=1, KX=0.982 E03= 4.0E3 V/m, KT=1, KX=0.963

40 35 30 25

Air Temperature: 34 ºC 0

100

200

300

400

500

time(seconds)

Fig. 3. Model simulations vs. experimental selective heating data. Microwave Power = 700 watts.

In the constant drying rate period, however, the microwave energy is no longer employed in heating the sample but in evaporating the internal moisture of the sample. That is the reason why the heating of the samples stops or, at least, decreases its heating rate.

Conclusion The good matching between the simulated temperatures and the experimental values highlight the importance of a proper characterization of the driving mechanisms of microwave-assisted drying. In this case, splitting the drying process of laminar materials into a heating up period and a constant drying period, and taking into account the moisture content dependence of the leather samples, has been sufficient to achieve a good matching between the experimental tests and the proposed model.

Selective Heating and Moisture Levelling in Microwave-Assisted Drying

Appendix fo Frequency (2.45 GHz) t Time th Heating-up period delimiting time X Moisture content (dry basis) XeEquilibrium moisture content U Density H’ Dielectric constant H’’Loss factor K(t) Evaporation efficiency function KX Evaporation efficiency at constant-drying rate period 'Hv Latent heat of evaporation cp Specific heat To Initial temperature Kh(t) Material heating efficiency as a function of time KT Global energy efficiency (absorbed power/delivered power)

a

-(ʌ ˜ fo ˜ İo ˜ b ˜ Ș x ˜ E 2 ) (ȡ ˜ Hv ˜ th )

B

2ʌ ˜ fo ˜ İo ˜ b ˜ (Șt -Ș x ) ˜ E 2 ȡ ˜ (c ps  c pw X(t))

c

2ʌ ˜ fo ˜ İo ˜ X o ˜ E 2 ȡ(c ps  c pw X(t))

C

2ʌ ˜ fo ˜ İo ˜ b ˜ Ș x E 2 ȡ˜ Hv

§ Ș ˜ ʌ ˜ fo ˜ İo ˜ b ˜ th ˜ E 2 · ¸¸ D (X o -X e ) ˜ exp¨¨ - x ȡ ˜ Hv © ¹

241

242

Monzó-Cabrera

References [1] X Jia, S. Clements and P Jolly, Study of Heat Pump Assisted Microwave Drying, Drying technology , 11(7), pp.1583-1616, (1993). [2] Roussy, G. and Pearce, J. A., Foundations and Industrial applications of Microwave and Radio frequency Fields, (1994). [3] J. Monzó-Cabrera, A. Díaz-Morcillo, J. M. Catalá-Civera, E. de los Reyes. Heat and mass transfer characterisation of microwave drying of leather. Proceedings of the12th International Drying Symposium, (2000). [4] J. Monzó-Cabrera, A. Díaz-Morcillo, J. M. Catalá-Civera, E. de los Reyes. Heat Flux and Heat Generation Characterisation in a Wet-Laminar Body in Microwave-Assisted Drying: an Application to Microwave Drying of Leather. Int.Comm. Heat Mass Transfer, vol. 27, no.8, 1101-1110, (2000). [5] Metaxas A.C. y Meredith R.J., Industrial Microwave Heating. Peter Peregrinus Ltd, (1983). [6] J. Monzó-Cabrera, A. Díaz-Morcillo, J. M. Catalá-Civera and E. de los Reyes. Study of kinetics of combined microwave and hot air drying of leather. Journal of the Society of Leather Technologists and Chemists. Vol. 84 (1), pp 38-44, (2000). [7] J.M. Catalá-Civera, F. Peñaranda-Foix, D. Sánchez-Hernández and E. de los Reyes, Precise dielectric properties determination of laminar-shaped materials in a partiallyfilled waveguide, IEEE AP-S International Symposium, Orlando,(1999), pp. 19421945.

Microwave Heating of Ready Meals – FDTD Simulation Tools for Improving the Heating Uniformity B. Wäppling-Raaholt1, P. O. Risman2 and T. Ohlsson1 1

SIK, the Swedish Institute for Food and Biotechnology, Box 5401, SE-402 29 Göteborg, Sweden 2 Microtrans AB, Box 7, SE-438 21 Landvetter, Sweden

Abstract A method is discussed, based on multiple finite difference time domain (FDTD) simulations for optimising the microwave heating distribution when heating food products in a microwave oven. Optimisation is by means of multi-linear regression in a four-dimensional subspace. The method leads to better possibilities to reduce too strongly and weakly heated regions, which is important from a microbiological as well as a quality point of view. The method is now applied in a project where it is helpful in the design and further development of ready meals intended for microwave heating.

Introduction It is of vital importance that the temperature distribution in microwave heated food products becomes as even as possible, in order to avoid underheating as well as overheating of parts of the food item. A related, common quality problem of microwave heated ready meals is that the different food components are heated differently in the microwave oven. From a culinary and microbiological point of view all regions should reach at least 70°C, while heating to 100°C and subsequent overheating of some food components or parts cause drying-out with resulting loss of taste, nutritional quality, texture and appearance. Today’s theoretical knowledge as well as practical experience on preparation and design of ready meals have resulted in some improved methods used in parts of the microwave and food industries to obtain a more even microwave heating distribution. Furthermore, advanced simulation and optimisation methods based on electromagnetic modelling have been developed into useful tools for such purposes. Complete optimisation of food products, microwaveable containers or ovens for a

244

Wäppling-Raaholt

certain application might require hundreds or thousands of simulations. In such cases, repetitive FDTD simulations would be too time-consuming. At SIK, the Swedish Institute for Food and Biotechnology, a multi-disciplinary simulation and optimisation method has been developed, which overcomes this problem by reducing the time required for finding an optimised design of a microwave heating appliance or food product in each specific application. By gradually reducing the number of optimisation variables under consideration, the microwave heating homogeneity may be optimised with respect to several user-defined parameters. The method is also useful as a guide to more efficient and time-saving heating experiments. The general method described here is currently applied in a project where several ready meal producers as well as a microwave oven company participate. The focus is on finding the optimal design of the food products and containers, rather than of the microwave heating appliance. Comparisons with practical experiments, where the temperature distribution in the food product is measured with infra-red thermography and fibre optic temperature sensors, illustrate how to improve the quality of microwave heated ready meals. In this paper, the heating uniformity is optimised for two different types of ready meals, firstly a compact ready meal (lasagna) and secondly a multicomponent ready meal consisting of three homogeneous food materials (mashed potatoes, cooked carrots, and minced beef) with variable shapes and relative positions, in a thin microwave transparent container. The results indicate that adding the feature of computational optimisation to the direct numerical modelling provides a new product development tool for strengthening the competitiveness of in particular the ready meal and food packaging industries.

Materials and methods It is of primary importance to find the relevant optimisation variables and adapt these to available computational resources. A set of FDTD simulations, arranged by means of a fractional factorial design, may be performed to find those variables which give the most pronounced effects on a target function. Effects of changes of a number of such variables, which are exceeding a threshold value for grid reduction and time step computational errors, described in the following section, are here defined as significant effects. Variables of interest in this application are among others overall shape, specific dimensions, and position in the cavity. FDTD simulations The electromagnetic field distribution as well as the dissipated microwave power within the two different food products heated in a common household microwave oven of type Whirlpool MD 121 was calculated. The particular oven choice was for simple but typical cavity and feed, without complicated embossings, etc.

Microwave Heating of Ready Meals

245

The electromagnetic model was implemented numerically in a threedimensional space, assuming constant material properties. An FDTD algorithm [e.g. 3] solving for the six electromagnetic field components was employed. The two Maxwell curl equations in derivative form were formulated in the time domain, and expressed in a linearised form by means of central finite differentiation. Only nearest neighbour interactions needed be considered as the fields were advanced temporally in discrete time steps over spatial cells of rectangular shape (other cell shapes are possible as well). The FDTD algorithm was implemented as a C++ code by Sundberg [7], though modified here in order to obtain a quantitative measure of what variables will give significant effects, as related to grid reduction as well as time step computational errors. For this purpose, the following time step error norm was defined, as shown in equation (1-1), where Pn+1(i,j,k) and Pn(i,j,k) denote the power density at a point ((i+½)'x, (j+½)'y, (k+½)'z) in the full-cycle intervals of the nth and (n+1)th time steps, respectively. Further, M is a set of points located within the food load. Equation (1) also gives an indication of the number of time steps needed for the sinusoidal excitation to reach stationary conditions in each particular simulation run. A suitable value for the maximum time step error H1, as given in equation (1), was found by relating it to the grid size. Adhering to this limitation prevents computational “noise” and similar artefacts. For this purpose, a set of simulations was performed with consecutive changes of 10% grid size. The grid reduction computational accuracy was defined as the relative least square error of the power density according to equation (2). This accuracy was thereafter used as a grid reduction computational error norm. In equation (2), H2 is a given value of the maximum grid reduction error, P'x(i,j,k) and P0.9'x(i,j,k) denote the power density at a point ((i+½)'x, (j+½)'y, (k+½)'z) for grids with cell sides 'x and 0.9'x, respectively.

¦ (P

Time step error

n 1

(i, j , k )  P n (i, j , k ))

( i , j ,k )M

¦P

n 1

(i, j , k )

2

(1)

d H1

( i , j ,k )M

¦ (P

Grid reduction error

'x

(i, j , k )  P 0.9 'x (i, j , k ))

( i , j ,k )M

¦P

'x

(i, j , k )

2

d H2

(2)

( i , j ,k )M

The parameter H1 was then set equal to H2, see equations (1) and (2). A lower limit for significant effects was thereby determined, since the parameter H1=H2 was here selected as threshold value of what parameters will give significant effects on the target function, which may be a measure of the evenness of the power distribution in the food load, according to equation (3).

246

Wäppling-Raaholt

1 Ft arg et where

imax jmax

jmax

imax

¦ 1 i 1

sim (i , j ) (i , j )

¦ (w

P

 P krepr ) 2

j 1, ( i , j )k repr

,

P krepr Pkrepr

1 imax jmax

imax

jmax

i 1

j 1

¦ ¦w

(3)

x

(i , j ) (i , j )

The cell size must be large enough to keep resource requirements manageable, yet small enough to permit accurate results at the highest frequency of interest, and to limit result artefacts by the geometrical discretisation. In this paper, the grid size ranges between 0.5 mm and 4 mm, which corresponds to a grid reduction error of 7.1%. Furthermore, for stationary conditions an error in the power density of 4.3% in a “hot” spot is obtained, as compared to the value obtained by a model with no discretisation errors. Optimisation of the heating uniformity Since optimisation of the heating uniformity would require a large number of simulations, a method was developed to find the optimal settings of several variables in a computationally efficient way, in order to achieve a more uniform relative heating homogeneity. The quality of the microwave heating process was quantified primarily as in [8, 9], i.e. by its ability to provide a uniform microwave power distribution in the food item, though here also particularly taking into account its ability to avoid “hot” and “cold spots” by introducing weighting factors. These factors are normalised to 1.

¦w

(i , j )

1

( i , j )M , krepr

The target function Ftarget, which describes the homogeneity of the power distribution in a set of representative planes [8, 9] k of the food product, is defined according to equation (3), where w(i,j) are weight factors. These weighting factors are real numbers between 0 and 1, giving smaller weight to values of P(i,j) in cells next to the boundaries in order to reduce the influence from computational errors, while giving larger weight to areas where “hot” and “cold” spots in the food item are located. In the latter areas, the weight factors have values ranging from 0.5 to 1. This target function does however not consider the influence of heat conduction, which will smear out and reduce the severity of smaller and closer “hot” or “cold” spots more than large and distant ones.

Microwave Heating of Ready Meals

247

The heating uniformity in the food product is optimised by minimising Ftarget:

F Target

o x

min food

,

r

F Target

food

such, that the optimisation variables: dimensions, altogether described by the vector xfood, and placement of the food components rfood, are varied according to Tables 1 and 2, respectively, subject to the following constraints: x (i,j)  a representative plane k, where the set of points located in the food, (i,j), is varied according to the changes of the optimisation variables x there is only one food material at each cell for each iteration. In equation (3), Psimulated(i,j) denotes the power density in points (i,j) for plane k, in a set of FDTD simulations. The power density Psimulated(i,j,k) is calculated at the point ((i+½)'x, (j+½)'y, (k+½)'z) for grids with cell sides 'x, 'y, and 'z in the x, y and z directions, respectively. The FDTD simulations Psimulated(i,j,k) are performed as a setup of simulations, arranged by means of a fractional factorial design [2]. In such a design, the simulation where all optimisation variables take their centre values, will be referred to as a reference simulation.

G target

D F target, simulated  (1  D ) F target, measured

D  >0 ,1@

(4)

In order to relate the optimisation problem to simulations as well as to measurements, the target function Gtarget is proposed (eqn. (4)), where Ftarget, measured is defined analogous to Ftarget, simulated, though based on a set of reference measurements. These reference measurements are performed with infrared thermography (IR SnapShot®, model 525, Infrared Solutions Inc.) as snapshots of the temperature profile in two representative (horizontal) k planes and one representative (vertical) j plane of the original food load, repeated ten times. The representative planes are selected analogous to [8, 9], taking into account that no cell should be considered more than once. There are several reasons for this choice of planes. In order to minimise the number of points used for the computations and also for facilitating direct comparisons between the numerical and experimental results, plane cuts in the food loads should obviously be used. The choice of these planes must be based on some assumptions on the general behaviour of microwave heating. x The first choice is correlation to the cavity volume modes. A horizontal plane near the top of the largest flat load, the beef patty, is used. This cut will also pass through the upper full-diameter part of the mashed potato item, and through the near-axis part of the carrots. The added advantages then become a correlation to possible centre focussing by diffraction in these items. x The second choice is correlation to the under-heating of foods, by LSM (trapped surface waves) between the load underside and the cavity bottom, as-

248

Wäppling-Raaholt

sisted by the glass shelf [5]. This is by a horizontal cut close to the bottom of the loads, an added advantage being that all loads become included. x The third choice is correlation with hot or cold spots at centres of rounded loads with diameters in the range of the present beef patty and mashed potato item [4], and possible overheating of higher food objects. These are caused by diffraction phenomena and cavity volume mode balancing, respectively. A vertical cut through the axis of (only) the mashed potato item load provides the best possibility for recording these kinds of unevenness; the height of the beef patty is so low that sufficient detection in it is provided by its two horizontal cuts. Since there are common lines with the two horizontal cuts, only the cut area above the top horizontal cut is considered. Three representative planes are selected accordingly in the lasagna item. These were made in the same way as above, for the same reasons. For the case of the multi-component ready meal, the placement and size of the individual components are varied in order to minimise the maximum value of the target function Ftarget, for the individual components (mashed potatoes, cooked carrots, and minced beef, respectively), applied in the three representative planes in the food components. All thermographs are converted into Matlab™ matrixes, and post-processed there to find the reference temperature profile, as well as the reference power profile Preference(i,j,krepr). The power density Preference(i,j,krepr) is, quite analogous to Psimulated(i,j,krepr), given at the point ((i+½)'x, (j+½)'y, (krepr+½)'z) for grids with cell sides 'x, 'y and 'z in the respective coordinate direction. The microwave oven and food products The oven cavity is rectangular with only minor embossings and has the following dimensions: width (left-right) 335 mm, depth 290 mm, and height 186 mm. A circular glass turntable with diameter 280 mm, thickness 6 mm, and permittivity 6–j0 is located with an airspace of 15 mm above the bottom. The oven has one significant microwave feed opening (cross sectional dimensions 79 mm (width) × 27 mm (height), with its centre located 68 mm above the oven bottom, at half the cavity depth). The nominal frequency of the standard magnetron is 2455 MHz; no frequency pulling effects by impedance mismatching are assumed. Two different types of ready meals are considered: a compact ready meal, lasagna, and a three-component ready meal. The lasagna is placed in a thin plastic container of rectangular shape (nominal dimensions are width 165 mm, depth 115 mm, and height 30 mm), with rounded corners (radius of curvature = 15 mm). It is composed of three layers of lasagna pasta plates (permittivity = 55–j21), with intermediate layers of Bologna sauce (permittivity = 60–j22), and a Bechamel and cheese sauce (permittivity = 51–j18) on top. The multi-component ready meal consists of mashed potatoes (permittivity = 61–j17), cooked carrots (permittivity = 70–j12), and minced beef (permittivity = 52–j20). The permittivities at 40qC and 2800 MHz are employed, from

Microwave Heating of Ready Meals

249

measurements with a cavity perturbation technique (TM012 mode cavity). The error caused by permittivity dispersion and actual temperature rises is considered acceptable, since the error in the real part enters all equations as a square root, and the loss factor contributions by dipole relaxation and ionic conductivity largely cancel [1]. The three-component meal is placed on a thin (permittivity = 1; since its thickness is 1 mm, it is assumed to have the same influence on the calculations as air) plastic tray of radius 100 mm. The shape of the minced beef item is flat circular (hamburger), the shape of the mashed potato portion is flat circular with a hemispherical top, and the shape of the two equal carrots is of square cross section with continuing half cylindrical ends. The nominal radius of the mashed potato component is 32 mm, the nominal radius of the hamburger-shaped beef patty is 44 mm, and the dimensions of the block part of the cooked carrots are 50 mm × 20 mm × 20 mm (the total length thus becomes 70 mm). Since the food components are originally placed in a symmetric manner, symmetry considerations were made in the corresponding FDTD simulations in order to reduce the computational effort. The oven and load symmetry with a centre vertical right-left symmetry plane was used. Therefore, only the front half of the oven cavity with food item was modelled. This was accomplished by introducing a magnetic wall at the symmetry plane. The minced beef item is nominally placed horizontally with its axis at the cavity half-depth symmetry plane, to the left on the plastic tray and with its left edge held fixed in relation to the cavity walls. The mashed potato component is placed with its centre as the beef item, and with a fixed distance of 15 mm to the nearest edge of the minced beef item. The carrots are placed symmetrically, according to Figure 1.

Fig. 1. The composition of the multi-component ready meal.

The fields have to be advanced one step at a time in an iterative process until stationary conditions are reached. Since the initially incident fields in this application may need to propagate tens of wavelengths to reach time-harmonic conditions, the total number of time steps may be quite large. For this oven, the number

250

Wäppling-Raaholt

of cells is of the order of 600000. In the lasagna case, about 6000 time steps were needed, while for the multi-component ready meal approximately 17000 time steps were required. The difference is due to the load geometries. More time steps are necessary for resonant loads; fewer for lossy loads. Since the stability criterion requires the number of time steps per cycle (or wavelength) to be approximately inversely proportional to the size of the smallest cell, there is also an influence by this factor for smaller loads with more complicated load geometry. Fractional factorial designs of FDTD simulations In optimisation problems, the total number of possible combinations would be too large to allow for the use of a complete factorial design. Since each factor involves at least two levels, i.e. the total number of combinations is at least 2n, this may happen even with only a moderate number of factors. As n is increased, 2n increases geometrically. Instead, only a fraction of all possible combinations are included in a fractional factorial design of a simulation series. Such a design aims at concluding, subject to appropriate constraints, which factors are significant by an optimal utilisation of the available resources. At a first stage, a setup of repetitive simulations was performed, arranged by means of a fractional factorial design at two levels. For the two different types of ready meals, the variables under consideration are as given in Tables 1 and 2, respectively. It was decided to vary the height of the lasagna by varying the distance between the pasta plates, i.e. the thickness of the Bologna sauce layers, within the overall interval given in Table 1. Other variables of interest are varied within the intervals given in the Tables. Table 1. The factors for the lasagna case are varied within the intervals given in the Table. All linear measures are given in millimeters. Variable

Interval

Centre value

Load width wload

[150, 180]

165

Value after optimisation 159

Load depth dload [100, 130] =wload,1+dload,1wload,2=165+115-wload Load height; [26, 34] Distance between [8, 12] pasta plates Radius of curvature at [10, 20] the edges Constraints: wload+dload is held constant

115

121

30 10

28 9

15

Unchanged

From the fractional factorial design of simulations, those factors which give the largest significant effects on the target function are obtained for each type of ready meal. For the lasagna, the largest significant effects were found to be due to the

Microwave Heating of Ready Meals

251

load height, the load width, and the co-varying load depth. For the multicomponent ready meal, significant effects were obtained from variations of the radii of the mashed potato, as well as of the hamburger-shaped minced beef component. For each ready meal, a reduced number of significant factors were further investigated in a full factorial design. An extension of this full factorial design, a so called central composite design [2], was then carried out at five levels for these factors (2 factors, in both the lasagna and the multi-component cases, respectively). This formed a basis for the optimisation procedure, since the best conditions known from the full factorial design were selected as a starting point for optimisation. By multilinear regression in a two-dimensional subspace, the two most pronounced significant factors were then used as optimisation parameters when minimising Ftarget. (eqn. (3)).The target function Gtarget (eqn. (4)) may be used analoguously, in order to relate the optimisation to heating experiments. The number of required simulations was 23-1+1+22+(52–23–22) = 22, as compared to 53 = 125 for all possible combinations at the same resolution. The horizontal interval of variation was chosen to be so small that consumers would not find it deviating from a typical product. The co-variation of width and depth was such that a food producer would be able to use a rather unchanged food weight per package. Table 2. Optimisation intervals for the variables under consideration in the multicomponent ready meal. All linear measures are given in millimeters. Variable (food component) Radius of minced beef patty rbeef Placement of the cooked carrots, I

Optimisation interval

Value after optimisation

rbeef [38.5, 49.5]

49 or 39

Rotation 90 degrees around vertical axes containing their centre points. rpotato[25, 35]

Unchanged

27 or 34 Radius of the mashed potato component rpotato Constraints: Distance between central parts of beef item and potato component is held constant =rbeef+rpotato+15 The volume of the potato component is held constant The left edge of the minced beef item is fixed in relation to the cavity

For the lasagna case, the dimensions (width, depth, height), and the radius of curvature are varied within the intervals as given in Table 1, subject to the constraint that the circumference of the load is held constant. From the fractional factorial design it was found that the effects on the target function from changing the corner radius was much less than the effects from the dimension variables. The corner radius of the load edges was thereafter held constant (15 mm). The reasons for this choice are based on practical experience from microwave heating as well as computational quantification of edge overheating [6]. It can also be theoreti-

252

Wäppling-Raaholt

cally explained by the circumferential length not changing much with changes of the corner radius, and the exposure of the corner regions to many wave polarisations when the load is rotating as in the normal oven. For the multi-component ready meal the variables are selected as in Table 2. By means of a fractional factorial design the variables rbeef (radius of the hamburgershaped minced beef), radius of the mashed potato component rpotato, and placement of the cooked carrots I were varied according to Table 2. It was concluded that the largest significant effects were due to rbeef and rpotato. In a central composite design [2] these variables were further investigated, the result forming a basis for optimisation by multi-linear regression in two dimensions. The main reason for the choice of basic geometries of the food items is that they should be as different as possible, and still realistic: large flat (“2D”), rounded (”3D”) and elongated (“1D”). Each of these represents a category and there is no reason to introduce the categories as variable. Therefore, just the linear dimensions of each item is to be changed; in this work, this is only done for two of the items (the length of the carrots is not changed). Simulation results In Figure 2, the microwave heating profiles for the lasagna case is given. Before optimisation overheating of the edges as well as underheating at two “cold spots” within the load are apparent. In the optimised design “hot spots” as well as “cold spots” are much less pronounced. In the case of the lasagna ready meal, one optimal design was found, corresponding to wload = 159 mm (Table 1. For comparison, Figure 2 c) illustrates the microwave power distribution in a load where wload = 136 mm, which gives an extremely uneven heating, with distinguishable areas where overheating occurs. It is clearly seen in the figures that there are two depth-directed dominating bands of stronger heating. These must be caused by a hybrid TEy 51p mode, due to the horizontal dimensions of the cavity and the feed orientation and location. With a width-directed periodicity of 335/5 = 67 mm, one finds that the optimum corresponds to 2.4 such periods and the worst case to 2.0 periods. Hence, an explanation involve a reasoning on the poorer (and less favourable) coupling of the desired hybrid mode to the load when this is exactly 2 mode field periods wide, resulting in stronger multimode fields which typically cause edge overheating. For the multi-component ready meal, the optimal heating uniformity was obtained for rbeef (the radius of the hamburger-shaped minced beef) = 49 mm and rpotato (the radius of the mashed potato component) = 34 mm; Figure 3 b). Other designs, which also resulted in good heating uniformity, though sub-optimal ones, were those where rbeef = 39 mm; rpotato = 27 mm and where rbeef = 39 mm; rpotato = 34 mm, respectively.

Microwave Heating of Ready Meals

253

Fig. 2. Microwave power distributions in lasagna for a) the initial settings (wload = 165 mm) and b) the optimal settings (wload = 159 mm). For comparison, Figure c) illustrates the microwave power distribution in a load where wload = 136 mm, which gives an extremely uneven heating. The microwave heating uniformity has been optimised (Figure b), with respect to the dimensions of the food product. The colder areas are blue; while the hotter areas are red.

Fig. 3. The microwave heating profile for a multi-component ready meal a) before and b) after optimisation of the heating uniformity. The components are: mashed potato (left), hamburger-shaped minced beef (right), and cooked carrots (two smaller loads). The size and placement of the food components have been changed in accordance with Table 2, in order to optimise the heating uniformity. The microwave power distribution in a “worst case” c) load, which gives significantly uneven heating (rbeef = 44 mm; rpot = 31 mm) is included for comparison.

It is more difficult to provide explanations to the best and worst results in this case, since there are both diffraction phenomena caused by the load itself and cavity volume mode phenomena.

254

Wäppling-Raaholt

One clear and explicable phenomenon is the load diffraction by the flat round meat patty; a simple free space plane wave excitation of this type of load by a wave similar in polarisation and vertical equivalent angle of incidence shows that the tendency for a central cold spot is minimal at about 100 mm diameter. The same phenomenon is shown here. There should be a tendency for a central hot spot in the mashed potato load, by the diffraction phenomenon commonly called “the exploding egg effect”. However, it seems as cavity volume mode coupling phenomena of the same type as for the lasagna load are also in action here and dominate the picture; the overall distance between the leftmost and rightmost load parts in the multiload worst case is 88+32+15 = 135 mm, which coincides with the worst value for that load.

Conclusion The simulation and optimisation method was used for optimising the microwave heating uniformity, with respect to a number of parameters, which are related to the food product. A more uniform heating uniformity of the food product was obtained for two different types of loads. The method is generally applicable, since it is possible to start out from parameters that are relevant for each particular application. The results are promising enough to indicate that the method has a great potential to facilitate optimisation of food products, and food packages, as well as microwave heating appliances in a close future, meeting quality aspects as well as microbiological safety requirements. The results presented here indicate that worst and best cases of evenness, with very significant differences, may fall within changes of linear horizontal dimensions by less than 25 mm. This scale is of course related to the free-space wavelength – or rather a quarter of that, which may be the distance between a maximum and a minimum of a standing wave. What may cause even smaller dimensional changes to give large result fluctuations is that some load diffraction phenomena are related to quarter-wave changes of the load periphery rather than diameter, and that displacement current flows between adjacent food items may change quite strongly also for very modest changes of distance between the items. It is therefore concluded that a linear placement and dimensional resolution of 2 mm may be necessary in the optimisation work. As exemplified above, the particular cavity volume mode properties of a microwave oven may have a very significant influence on the result – what is in practise here a co-optimisaton of results. If ovens behave quite differently, development of ready meals will have to rely on tests in multiple microwave ovens with different cavity dimensions, etc. This is also what is done now in the food industry. Since oven designs may differ in a number of respects, many ovens are needed in practise. The question on how these ovens can be selected – and in particular how poorly performing ovens can be characterised and their performance aspects be simplified

Microwave Heating of Ready Meals

255

in food portion optimisation work– is, however, outside the scope of this presentation. What has been shown here is that microwave modelling can be an increasingly strong tool, and by its capability of combination with efficient optimisation methods and “seeing the microwaves” is likely to become even stronger.

References [1] Bengtsson, N. and Risman, P.O. (1971) Dielectric properties of foods at 3 GHz as determined by a cavity perturbation technique. II. Measurements on food materials, Journal of Microwave Power, 6(2): 107-124. [2] Box, G.E.P. et al. (1978) Statistics for Experimenters, an Introduction to Design, Data Analysis and Model Building, John Wiley and Sons, Inc., 653 pp. [3] Kunz, K.S. and Luebbers, R.J. (1993) The Finite Difference Time Domain Method for Electromagnetics, CRC Press, Boca Raton, Florida, 448 pp. [4] Ohlsson, T. and Risman, P.O. (1978) Temperature Distribution of Microwave Heating - Spheres and Cylinders, ”, Journal of Microwave Power, 13(4): 303-310. [5] Risman, P.O. (1994) Confined modes between a lossy slab load and a metal plane as determined by a waveguide trough model, Journal of Microwave Power and Electromagnetic Energy, 29(3): 161-170. [6] Sundberg, M.(1998) An investigation of the edge overheating effect for high permittivity dielectrics; paper F in Sundberg, M., (1998) Analysis of Industrial Microwave Ovens, Ph.D. thesis, Department of Electromagnetics, Chalmers University of Technology, Göteborg, Sweden. [7] Sundberg, M.; Risman, P.O.; Kildal, P.-S.; Ohlsson, T. (1996) “Analysis and Design of Industrial Microwave Ovens Using the Finite Difference Time Domain Method”, Journal of Microwave Power and Electromagnetic Energy, 31(3): 142-157. [8] Wäppling-Raaholt, B.; Galt, S.; Ohlsson, T. (1999) FDTD Simulation of a Microwave Heating Process: Effects of Food Parameters, Book of Proceedings of the AMPERE Conf., MW and HF Heating 1999, Valencia, Spain. [9] Wäppling-Raaholt, B. and Risman, P.O. (2000) FDTD Simulation of a Microwave Heating Process: Effects of Oven Parameters on Heating Uniformity, 3rd Int. Conf. on Predictive Modelling, 2000, Leuven, Belgium.

Novel and Traditional Microwave Applications in the Food Industry H. Schubert and M. Regier Institut für Lebensmittelverfahrenstechnik, Universität Karlsruhe, Germany.

Abstract This paper gives an overview over microwave heating applications in industry and research from the beginning to up-to-date developments. It describes the benefits and advantages of microwaves compared to conventional techniques, but also the disadvantages and problems of the them, that often hindered them to become commercially successful for a longer time.

Introduction The total sales number of microwave ovens in the United States stays at a constant level of approximately 10 million per year. The corresponding number in Europe is in the same range. These enormous sales rates of household ovens together with the estimated number of 225 million microwave ovens spread in the industrialised world point to the importance of microwave heating today. Compared to households, microwave processing is not frequently used in food industry. Although many applications have been claimed to be successful with advantages compared to alternative techniques, the number of working installations is estimated to be still less than 1000 [1].

History The development of dielectric heating applications in food industry started in the radio frequency range in the 1930s [2]. The desired energy transfer rate enhancement led to an increased frequency: the microwaves. The first patent, describing an industrial conveyor belt microwave system was issued in 1952 [3], however its first application started 10 years later. This was caused by the need for high power microwave sources to be developed.

260

Schubert

The first major applications were finish drying of potato chips, pre-cooking of poultry and bacon, tempering of frozen food and drying of pasta [4]. Whereas the first applications were only temporarily successful, since the quality enhancement due to the microwave process could quickly be achieved by a more economic improvement of the conventional technique, the other techniques survived and are still successful in industrial application. More recent applications are microwave pasteurisation, sterilisation, combined processes like microwave vacuum drying and puffing, but also the combination of microwaves with jet impingement of hot air or halogen bulbs.

Classification Generally, the use of microwave energy in industrial food processing can be classified into six unit operations: (re)heating, baking and (pre)cooking, tempering, pasteurisation and sterilisation, and dehydration. In spite of these distinctive objectives, they are established by a simple temperature enhancement. Nevertheless for each application the advantages and disadvantages differ and have to be taken into account. In industrial applications due to the desired high throughputs mostly a continuous processing is needed, so that continuous microwave applicators have to be developed. The industrial ovens may be categorised into high power single magnetron and low power multi-magnetron devices. For single mode units as shown in Figure 1 only a single source is reasonable, whereas for multi-mode, near field or travelling wave systems both types are possible, where multi-source devices are claimed to yield a more homogeneous heating pattern. Movement of the product to be heated and/or wave stirrers are also used for the same reason. On the other side an inhomogeneous field pattern can be desired, to achieve, for example, high heating rates in a special product area as shown in Figure 1. A typical conveyor belt oven with alternative openings and power sources is shown in Figure 2. The openings of the continuous system have to be engineered to the specified problem, due to the existence of leakage radiation, which is limited in the US and in Western Europe by law to 5 mW/cm2. These values can be easily fulfilled for fluids or small granular material by small product in- and outlet sizes (cm-range) together with additional dielectric load in the corresponding area. For larger products, in- and outlet gates to close the microwave application room should be taken into consideration. In most cases conveyor belt ovens work as near-field or multi-mode devices, already due to the relatively large dimensions and the corresponding high mode density. Physically similar to the near field device is a travelling wave set-up as shown in Figure 3: The material to be heated is introduced by wall slots, that do not cut wall current lines and do not exceed certain dimensions to avoid the above mentioned electromagnetic leakage [6] and the waveguide has to be terminated by a matched load.

Novel and Traditional Microwave Applications in the Food Industry

261

Fig. 1. A TM010 microwave resonator as an example for a single mode heating device, adapted from [5]

Fig. 2. Schematic view of a continuous conveyor belt microwave tunnel, adapted from [5]

262

Schubert

heated material matched load waveguide

microwave source

heated material

cold material cold material

Fig. 3. Schematic view of a meandering waveguide applicator as an example for a travelling wave device, adapted from [6]

Applications in the food industry Due to the very large number of microwave ovens in households, the food related industry not only uses microwaves for processing but also develops products and product properties especially for microwave heating. This way of product enhancement is called product engineering or formulation. A prominent example is the microwave popcorn. Possible methods to achieve a desired, for example uniform, temperature distribution within the food to be heated are changes in product properties like ingredients, geometrical set-up or special packagings. So, containers with metal foil coverings containing product-suited apertures or susceptors have been developed to yield browning or crisping effects. A more detailed overview to this field can be found in [7]. Although the importance of the development of microwavable products cannot be overestimated, we now want to focus on microwave processes in the food industry. Baking and cooking Detailed references to the baking process of bread, cakes, pastry etc. by the help of microwaves on industrial scale can be found in [8]. An enhanced throughput is achieved by an acceleration of the baking where the additional space needs for microwave power generators are negligible. Often the microwaves in baking are

Novel and Traditional Microwave Applications in the Food Industry

263

used in combination with conventional or infrared surface baking; this avoids the problem of the lack of crust formation and surface browning. An advantage of the combined process is the possible use of European soft wheat with high alpha-amylase and low protein content. In contrast to conventional baking microwave heating inactivates this enzyme fast enough (due to a fast and uniform temperature rise in the whole product) to prevent the starch from extensive breakdown, and develops sufficient CO2 and steam to produce a highly porous [9]. One difficulty to be overcome was a microwavable baking pan, that is sufficiently heat resistant and not too expensive for commercial use. By 1982 patents had been issued overcoming this problem by using metal baking pans in microwave ovens [10, 11]. The main use of microwaves in the baking industry today is the microwave finishing, when the low heat conductivity lead to considerable higher baking times in the conventional process. A different process, that also can be accelerated by application of microwave heating is (pre-)cooking. It has been established for (pre)cooking of poultry [9], meat patties and bacon. Microwaves are the main energy source, to render the fat and coagulate the proteins by an increased temperature. In the same time the surface water is removed by a convective air flow. Another advantage of this technique is the valuable by-product namely rendered fat of high quality, which is used as food flavorant [12]. Tempering Another important industrial microwave application is the tempering of foods, where the temperature of frozen foods is raised from below –18°C to temperatures just below the melting point of ice (approximately –2°C). At these temperatures the mechanical product properties are better suited for further machining operations, (e.g. cutting or milling). Whereas the time for conventional tempering strongly depends on the low thermal conductivity of the frozen product and can be in the order of days for larger food pieces, e.g. blocks of butter, fish, fruits or meat, by using microwaves (mostly with 915 MHz due to their larger penetration depth) the tempering time can be reduced to minutes or hours [13]. The space required is diminished to one sixth of the conventional system [14] and the microbial growth can be reduced or even stopped by low environmental temperatures. Besides there is an enormous reduction in drip loss by the microwave process. In microwave tempering processes the heating uniformity and the control of the end temperature are very important, since a localised melting would be coupled to a thermal runaway effect.

264

Schubert

Drying In drying the main cause for the application of microwaves is the acceleration of the processes, which are (without using microwaves) limited by low thermal conductivities, especially in products of low moisture content. Correspondingly sensorial and nutritional damage caused by long drying times or high surface temperatures can be prevented. The possible avoidance of case hardening, due to more homogeneous drying without large moisture gradients is another advantage. Two cases of microwave drying are possible, drying at atmospheric pressure and that with applied vacuum conditions. Combined microwave-air-dryers are more widespread in the food industry, and can be classified into a serial or a parallel combination of the both methods. Applied examples for a serial hot air and microwave dehydration are pasta drying [4] and the production of dried onions [15] whereas only intermittently successful in the 1960s and 1970s was the finish drying of potato chips. The combination of microwave and vacuum drying also has a certain potential. Microwave assisted freeze drying is well studied, but no commercial industrial application can be found, due to high costs and a small market for freeze dried food products [16]. Microwave vacuum drying with pressures above the triple point of water has more commercial potential has microwave vacuum drying with pressures above the triple point of water. Microwave energy overcomes the problem of very high heat transfer and conduction resistances, leading to higher drying rates. These high drying rates correspond also to lower shrinkage and to the retention of water insoluble as shown in Figure 4. In parsley, for example, most of essential oils are present as a separate phase with high boiling temperature. For fast drying conditions (high microwave energy input) only the small amount of volatile essential oils, that is dissolved is lost, whereas there is not enough time to resolve the remaining oil in the separated phase [17]. In contrast the retention of water soluble aromas, as in apples, is not as advantageous, since the microwave energy generates many vapour bubbles, so that the volatile aromas have a large surface to evaporate. Nevertheless, the low pressures limit the product temperatures to lower values, as long as a certain amount of free water is present and this helps to retain temperature sensitive substances like vitamins, colours etc. So, in some cases the high quality of the products could make also this relative expensive process economical. Microwave vacuum dehydration is used for the concentration or even powder production of fruit juices and drying of grains in short times without germination [4]. Newly and successfully applied is the combination of pre-air-drying, intermittent microwave vacuum drying (called puffing) and post-air-drying. It is predominantly used to produce dried fruits and vegetables, with improved rehydration properties [18]. After the form is stabilised by case hardening due to conventional air-drying, the microwave vacuum process opens the cell structures (puffing) due to the fast vaporisation of water and an open pore structure is generated. The subsequent post-drying reduces the water content to the required value.

Novel and Traditional Microwave Applications in the Food Industry

265

fresh parsley freeze dried, 1,3mbar, 40°C microwave dried MW, 72 W / 100g MW, 158 W / 100g MW, 376 W / 100g MW, 628 W / 100g MW, 860 W / 100g 20 60 100 120 0 40 80 Amount of essential oil relative to fresh parsley [%] Fig. 4. Comparison of aroma (essential oils) retention of different drying processes, adapted from [17]

Pasteurisation and Sterilisation Studies of microwave assisted pasteurisation and sterilisation have been motivated by the fast and effective microwave heating of many foods containing water or salts. A detailed review can be found in [19]. Although, physically non-thermal effects on molecules are very improbable, early works seemed to show just these effects. But in most cases the results claimed could not be reproduced, or they lacked an exact temperature distribution determination. The improbability of non-thermal effects becomes clear, when the quantum energy of photons of microwaves, of a thermal radiator and the energy of molecular bonds are compared. The quantum energy of a photon of f = 2.45 GHz is defined by E = hf | 1*10-5 eV, the typical energy of a photon radiated from a body of 25°C § 298 K equals E = kT § 0.26 eV and the energy of molecule bonds are in the eV-range. Since the collection of energy with time for bound electrons are forbidden by quantum mechanics, only multi-photon processes, which are very unlikely, could yield chemical changes. Recently Lishchuk also showed, that even a deviation of the energy distribution of water molecules from the conventional Boltzmann distribution can not be proved [20]. More thinkable is the induction of voltages and currents within living cell material, where eventual consequences are still in discussion [21]. Due to the unquestioned thermal effects of microwaves, they can be used for pasteurisation and sterilisation. Studied applications of microwave pasteurisation or sterilisation cover prepacked food like yoghurt [22] or pouch-packed meals as well as continuous pasteurisation of fluids like milk [4, 19]. Due to the corre-

266

Schubert

sponding product properties either conveyor belt systems or continuous resonator systems are invented. The possibly high and nearly homogeneous heating rates, also in solid foods (heat generation within the food) and the corresponding short process times, which helps preserving a very high quality yield advantages of microwave compared to conventional techniques. The crucial point in both processes is the control and the knowledge of the lowest temperatures within the product, where the destruction of microorganisms has the slowest rate. Due to the difficult measurement or calculation of temperature profiles it is still very seldom industrially used [4].

Important parameters for microwave processing Essential for the modelling of microwave processes are the dielectric properties of the material to be processed, its composition and temperature [23]. Various methods for the determination of dielectric properties of food are used, for example the open ended coaxial line reflectory probes or resonator methods. In the last few years the modelling of microwave processes has been increased in number, strongly. On one hand the numerical methods for electromagnetic and thermodynamic modelling have become more efficient, on the other hand the performance of computers is also growing tremendously, making it possible to include more details in the models and more extended calculation grids with higher resolution.

80

without NaCl 1 mol/l NaCl

60

2 mol/l NaCl

H´

Fricke-Mudgett

40 20 0

0

20

40 60 80 oil concentration [%]

100

Fig. 5. Dielectric constant (2.45 GHz, 20°C) of salt containing oil-in-water emulsions together with the best describing mixture equation (the Fricke-Mudgett-model), adapted from [24]

Novel and Traditional Microwave Applications in the Food Industry

267

Due to the changes of the material properties during the microwave processing, also models predicting the temperature or the composition dependence of the materials become important. Astonishingly, even relatively simple models are able to predict the dielectric properties of non-interacting mixtures with a reasonable accuracy of a few percents. As an example the dielectric properties of oil-in-water emulsions with varied salt content are shown in Figures 5 and 6 together with the best predicting formula [24].

parallel Wiener

120

H´´ 80 40 0

0

20

40 60 80 oil concentration [%]

100

Fig. 6. Dielectric loss factor (2.45 GHz, 20°C) of salt containing oil-in-water emulsions together with the best describing mixture equation (the parallel Wiener-model), adapted from [24]

H“ ‘, H

70 60 50 40 30 20 10 00

H‘ H“

1

2 3 4 X /(g H2O / g TM)

5

6

Fig. 7. Dependency of dielectric properties of apple tissue on water content, adapted from [17]

Even more complicated than heating is microwave drying. Most important here is the water distribution within the food, since this water content strongly influ-

268

Schubert

ences the material properties like dielectric properties, as shown in Figure 7 or heat conductivity. A unique and elegant tool for the non-invasive measurement of water and/or temperature distributions is the magnetic resonance imaging method. Examples are depicted in Figures 8a-c, where the water distribution within an apple tissue cylinder during the drying process is vsualized. By using a water sample as calibration reference the data cannot only be used qualitatively for estimating the drying homogeneity but also quantitatively.

Fig. 8. a-c: Schematic view of a NMR imaging (upper left), false colour represantaion of the water distribution within the shown slice (upper right) and the axial water distribution during a drying process.

Nott [25] also shows quantitative results of temperature distributions by MRI. Although this technique is relatively expensive, due to its unique possibilities, it

Novel and Traditional Microwave Applications in the Food Industry

269

has the potential to help to optimise not only microwave food processes by using the largely increasing modelling power.

Summary and Outlook Whereas microwave ovens are widely spread in households and are established there as devices, daily used, in industry the distribution of microwave processes is still far away from such high numbers, compared with their undisputable high potential. The successful microwave applications range over a great spectrum of all thermal food processes. The most prominent advantages of microwave heating are the reachable acceleration and time savings and the possible volume heating, leading to high product quality. Possible reasons for the failure of industrial microwave applications range from high capital and energy costs, which have to be counterbalanced by higher product qualities, over the conservatism of the food industry and relatively low research budgets, to the lack of cooperation between microwave and food engineers and of sufficient microwave heating models and their possible numerical solution. The latter has been partly overcome by the performance of computers which makes it possible to calculate increasingly realistic models by numerical methods. An essential prerequesit for realistic calculations is the determination of dielectric properties of food substances by experiments and theoretical approaches. Nevertheless the numerical calculations have to be tested by experiments, which yield the real temperature distributions within the product. This task is really important especially in pasteurisation and sterilisation applications. Whereas more conventional temperature probe systems, like fibre optic probes, liquid crystal foils or infrared photographs only give an incomplete information about the temperature distribution within the whole sample, probably the magnetic resonance imaging has the future potential to give very useful information about the heating patterns. The breakthrough of the microwave technology in the food industry has been predicted many times before, but it was delayed every time up to now. Although we think that the microwave potential in the food industry is far from being completely used, we are therefore cautious to predict this breakthrough for the next few years.

References [1] Ohlsson, T.; Bengtsson, N., ‘Microwave Technology and Foods’, Advances in Food and Nutrition Research, 43, 65-140, (2001) [2] Püschner, H. A., Heating with Microwaves, Berlin, Philips Technical Library , (1966) [3] Spencer, P., Means for Treating Foodstuffs, U. S. Patent 2,605,383, 605, 383, (1952)

270

Schubert

[4] Decareau, R. V., Microwaves in the Food Processing Industry, Orlando, Academic Press Inc., (1985) [5] Regier, M.; Schubert, H., ‘Microwave Processing’ in: Richardson, P., ‘Thermal Technologies in Food Processing’, Woodhead Publishing, 178-207, (2001) [6] Roussy, G; Pearce, J. A., Foundations and Industrial Applications of Microwaves and Radio Frequency Fields, Chichester, J. Wiley, 193, (1995) [7] Decareau, R. V., Microwave Foods: New Product Development, Trumball, Food & Nutrition Press Inc., (1992) [8] Rosenberg, U.; Bögl, W., ‘Microwave Thawing, Drying and Baking in the Food Industry’, Food Technology, June 1987, 85-91, (1987) [9] Decareau, R. V., 'Microwave Food Processing Equipment Throughout the World', Food Technology, June 1986, 99-105, (1986) [10] Schiffmann, R. F.; Mirman, A. H.; Grillo, R. J., Microwave Proofing and Baking Bread Utilizing Metal Pans, U. S. Patent 4,271,203, (1981) [11] Schiffmann, R. F., Method of Baking Firm Bread, U. S. Patent 4,318,931, (1982) [12] Schiffmann, R. F., ‘Food Product Development for Microwave Processing’, Food Technology, June 1986, 94-98, (1986) [13] Edgar, R., ‘The Economics of Microwave Processing in the Food Industry’, Food Technology, June 1986, 106-112, (1986) [14] Metaxas, A. C., Foundations of Electroheat, Chichester, J. Wiley & Sons, (1996) [15] Metaxas, A. C.; Meredith, R. J., Industrial Microwave Heating, London, Peter Pelegrinus Ltd, (1983) [16] Knutson, K. M.; Marth, E. H. and Wagner, M. K., ‘Microwave Heating of Food’, Lebensmittel-Wissenschaft und -Technologie, 20, 101-110, (1987) [17] Erle, U., Untersuchungen zur Mikrowellen-Vakuumtrocknung von Lebensmitteln, PhDThesis, Universität Karlsruhe, Aachen, Shaker Verlag, (2000) [18] Räuber, H., ‘Instant-Gemüse aus dem östlichen Dreiländereck’, Gemüse, 10'98, (1998) [19] Rosenberg, U.; Bögl, W., ‘Microwave Pasteurization, Sterilization, Blanching and Pest Control in the Food Industry’, Food Technology, June 1987, 92-121, (1987) [20] Lishchuk, S. V. and Fischer, J., ‘Velocity Distribution of Water Molecules in Pores under Microwave Electric Field’, International Journal of Thermal Sciences, Vol. 40(8), 717-723, (2001) [21] Sienkiewicz, Z., ‘Biological Effects of Electromagnetic Fields’, Power Engineering Journal, 12 3, 131-139, (1998) [22] Bach, J., ‘Multiple Frequency Method for Prolonging Shelf Life of Milk Products’, Deutsche Milchwirtschaft, 28, 1376-1377, (1977) [23] Lovenson, C., ‘The Why's and How's of Mathematical Modelling for Microwave Heating’, Microwave World, 11 1, 14-23, (1990) [24] Ghio, S.; Regier, M. and Schubert, H., ‘Prediction of dielectric properties of microwave-vacuum dried foods’, 8th International Conference on Microwave and High Frequency Heating, Bayreuth, Germany, Book of Abstracts, 116-117, ISBN 3-00-0083561, (2001) [25] Nott, K. P.; Hall, L. D.; Bows, J. R.; Hale, M.; Patrick, M. L., ‘Three-Dimensional MRI Mapping of Microwave Induced Heating Patterns’, International Journal of Food Science and Technology, 34, 305-315, (1999)

Microwave Drying: Process Engineering Aspects SM Bradshaw Department of Chemical Engineering, University of Stellenbosch, South Africa

Abstract Microwave drying is characterised by rapid heating and the development of high pressure and temperature inside the solid. This leads to rapid moisture transport, with significant potential time savings, although internal pressure must be limited to avoid sample rupture. Modelling indicates that the power and moisture content from which microwaves are applied can be optimised in economic terms. The decision to use microwaves in a drying application should be based on adequate return on investment while design of the dryer is best tackled in conjunction with mechanical specialists.

Introduction Most manufactured solids are dried at least once in their manufacture, making drying one of the most important unit operations in the process industries [1]. Indeed, 20% of industrial energy consumption is used for drying [2]. When unbound surface moisture is removed from a solid, external conditions dominate the drying rate, which remains constant until the critical moisture content has been reached and dry spots appear on the solid surface. Internal movement of moisture then becomes the controlling factor in the drying rate. From this point on the drying rate continues to decrease until the equilibrium moisture content is reached. Of the many methods of drying, the most common method of drying particulate, sheet-form or pasty solids is by convection [3]. Heat for evaporation is supplied by convection to the exposed surface of the material and the evaporated moisture is removed by the drying medium. During the falling rate period, convective drying becomes increasingly less effective. This is because transfer of energy from the surface to the interior (to establish a driving force for moisture migration) is by the relatively slow process of conduction, which leaves the surface hotter than the interior. Drying thick sections rapidly with convective drying is difficult as it is difficult to achieve internal vaporisation. Frequently the result is case-hardening of the object. Achieving drying uniformity can also be a problem.

272

Bradshaw

In order to meet product specifications, material is over-dried in this situation but this leads to higher energy consumption and poor product quality [4]. Provided the moisture contained in the solid absorbs microwave energy, exposure of the body to microwaves allows volumetric energy dissipation, thus permitting the development of high internal temperature and pressure and so high rates of moisture transport to the surface. Indeed, the heat flux to sustain the vaporisation of moisture in the centre of the body no longer requires a temperature gradient, and it is possible to sustain vaporisation with only a moderate temperature gradient. In fact, cooling of the surface due to the convection establishes a favourable gradient even when the internal temperature is less than the boiling point of the liquid. Microwaves have the potential to reduce drying time (and hence increase throughput and therefore profit), to improve product quality (less offspecification material), to offer energy savings and in some cases allow manufacture of dried products which cannot be made in any other way.

Features of Microwave Drying Observations of microwave drying have revealed a number of interesting phenomena, inter alia the lack of a constant rate period, liquid pumping and catastrophic sample rupture. Microwave drying is characterised by rapid heating and the development of high pressure and temperature inside the solid. Insight into these phenomena is vital to ensure proper design and operation of microwave drying installations. Sophisticated drying experiments have included internal pressure transducers that have allowed measurement of internal pressure as well as trays to catch water “pumped” from the solid during the early stages of high power microwave drying [5, 6]. Models have typically involved the simultaneous solution of dynamic mass and energy balances in one dimension coupled with an electromagnetic model, often for illumination of a slab by a plane wave without reflection [6 - 10]. These models, validated by experiment, are invaluable in providing insight into the phenomena occurring during microwave drying. In particular, the development of high pressure within the solid, the rapid migration of liquid and vapour and the rapid expulsion, or pumping, of liquid, especially during the early phases of microwave drying have been well explained. An understanding of these phenomena is necessary in order to ensure proper design of microwave drying equipment to cope with, e.g. the build-up of surface water due to liquid pumping or to ensure that internal pressure generation does not cause product rupture. The ability of microwaves to drive moisture increases with increasing diffusivity and at high powers it is possible to get inverse moisture gradient for materials with high diffusivity [10]. This is undesirable and suggests that advanced control strategies might be needed for such materials. Many of these concepts can be conveniently represented using the concept of the identity drying card, or IDC [5]. The IDC is a plot of internal pressure and temperature at a fixed location within the solid (frequently the midpoint) in which time is an implicit parameter along the curve. Typical IDC's are shown for con-

Microwave Drying: Process Engineering Aspects

273

vective steam and combined convective and microwave drying. Note that here the curves are represented schematically, and that the drying dynamics are not immediately apparent. Use of time markers along the curves would allow these aspects to be considered as well.

Fig. 1. Schematic Identity Drying Cards for steam and microwave drying (after [5])

For the case of steam drying, the well-known periods of heat up, constant rate drying (at 100°C in this case) and falling rate drying can be seen. The IDC for microwave drying looks quite different. In general, the pressure developed during microwave drying is considerably higher than in convective drying. The liquid streaming period occurs at the maximum pressure located on the saturated vapour pressure curve. The pressure then decreases as vapour flow replaces liquid flow and this occurs along the saturated vapour pressure curve. The period of increasing temperature begins within the hygroscopic range and, for materials that are moderately lossy and flammable, shows that power must be switched off to prevent burning. Vapour condensation results in a slight underpressure followed by cooling. It is possible to use simple and analytically tractable models to describe the pressure, at least qualitatively, and to predict parts of the IDCs.

274

Bradshaw

The designer who is considering using microwaves drying is faced with a number of important design decisions. It is necessary to decide on the power, the moisture content from which microwaves will be applied, and the appropriate material thickness, if this is a variable. Engineering judgement suggests that microwaves are likely to be most advantageous at the start of the falling rate period when internal moisture transport becomes rate controlling. It was found [10] that the time saving benefits of microwave drying were more pronounced when drying to the end of the pendular regime rather than only into the funicular regime. Use of microwave energy to remove bound moisture is inadvisable, as the bond energy not easily broken by microwave energy. Microwave application should start in the funicular regime, although there will be an upper limit to this, as shown in [7]. Interestingly, there have been some applications in which microwaves have been used exclusively (without convection) to avoid over-rapid drying and resultant cracking [11]. If the aim of the microwave drying process is moisture levelling, then knowledge of the variation of loss factor with moisture content is vital. For high moisture contents dH"/dX should be large and should reduce sharply at lower moisture contents. When moisture levelling is the aim it is important to start application of microwaves before the slope starts to level out, otherwise the moisture levelling benefit will not be realised. Note that the shape of the loss factor curve as a function of moisture content is dependent on material orientation for anisotropic materials and can vary with frequency. Often it is found that RF drying shows an effective loss factor variation that is more appropriate for moisture levelling. Constant et al. [7] modelled drying of various woods and cement and their simulations allow conclusions to be drawn on the economic effect of parameters such as incident power, moisture content at which microwaves are added and material thickness. One way of considering this is to look at potential time savings, as one of the reasons for using microwaves is the potential time savings that can be achieved. Typically these savings, which are directly related to increased profit through increased production, must be traded off against increased capital expenditure. Constant et al. [7] showed that increasing the incident power yields diminishing returns in terms of time savings (there is an upper bound) and that there is an upper limit in terms of moisture content at which microwaves should be applied. The same is true for thickness: the time savings for a given power increase with increasing thickness, but there is an upper limit. To take this analysis further, as a first approximation we can assume that time savings are directly proportional to increased revenue, and also that (above a certain threshold) power is directly proportional to capital expenditure. This means that it should be possible to obtain an optimum power to maximise profitability. The same is likely to be true for moisture content. We also expect that there will be a penalty in terms of increased energy costs, but we would expect that, for reasonable operation, energy costs are somewhat less than product value. By scaling the ratio between time savings and power reported in [10], to represent return on investment, it is possible to generate curves showing how return on investment is likely to vary with incident power and moisture content from which

Microwave Drying: Process Engineering Aspects

275

microwaves are applied. The figure shows schematically the return on investment that could be expected as a function of incident microwave power when microwaves are applied from different moisture contents. The curves suggest that there is likely to be an optimum incident power (visible for the case where microwaves are applied from 10% moisture) and that the return on investment appears to tend to an asymptotic value with this same moisture content. In fact, we could expect that there would also be an optimum moisture content from which to apply microwaves. 35 XMW = 10%

Return on Investment %

30

XMW = 20% XMW = 30%

25

XMW = 40%

20 15 10 5 0 0

5

10

15

20

Power absorbed mW /cm2

Fig. 2. Schematic representation of return on investment as a function of incident power and moisture content from which microwaves are applied

Several works (e.g. [12]) have shown that the kinetics of microwave drying can be represented by a simple exponential term X X 0 exp(  kt n ) where k and n are fitted functions of specific microwave power.

Appropriate Use of Microwave Drying The key to any successful microwave implementation is appropriate use of the technology to solve a real problem. Usually appropriate use means profitable operation, although there can sometimes be other drivers such as environmental regulations which mitigate for the use of microwaves. Some of the generic characteristics of drying problems favouring microwave assistance are:

276

Bradshaw

Table 1. Generic and specific features favouring microwave drying

Problems with conventional process Slow drying Uneven drying Environmental pressures Space restrictions High value products

Specific feature lending themselves to microwave Moisture levelling Thick sections Insulating materials Thermally sensitive materials

From the discussion of microwave drying phenomena it is apparent that the following process design issues should be considered: Table 2. Key process design issues in microwave drying

Process design issue Moisture content from which to apply microwaves Microwave power Internal pressure Moisture pumping Material thickness Moisture content at which to stop microwaves Moisture levelling

Important feature There will be either an optimum or an upper economic limit There will be an economic optimum or an upper limit This must not exceed internal strength of the material Care taken to ensure air quantity is adequate to evaporate the liquid Time savings better for thicker material, there will probably be an upper economic limit Pendular regime Appropriate dielectric property dependence on moisture content required. Apply microwaves from above the point where gradient in dH"/dX changes

Dryer Selection Issues Dryer selection is a difficult task because there are no established quantitative measures by which to judge alternatives, and for any given application there will usually be several suitable types [13]. A number of simple tree methods are available [14], which are qualitative and based on feed type (and do not include microwave dryers). There is still need for quantification in this regard. Steps have been made for convection, rotary and fluidised bed dryers, by giving the minimised total annual cost in terms of constructional and operational parameters [13].

Microwave Drying: Process Engineering Aspects

277

As of yet, no such analysis has been done for microwave assisted dryers, although Metaxas [15] reports an expert system to aid in selecting dielectric dryers. This means that, for the time being, selection of microwave assisted dryers will largely rely on expert knowledge of the process and assessed microwave benefits. For many industries, ignorance means that microwave dryers cannot yet be considered to be commodity items. The final choice of dryer will always be one of profitability. This is discussed in the following section.

Microwave drying economics Once it has been decided that the material is suitable for high frequency processing it is vitally important to make a preliminary assessment of the economics. Determination of this before entering an expensive test programme can save considerable effort that would otherwise have been wasted for those applications that are not economically viable. It is possible to obtain a crude estimate of the heat demand for a dryer from the latent heat of vaporisation of the liquid being removed. The actual heat demand in a real dryer will be greater than the latent heat of vaporisation, unless the wet bulb temperature reaches the boiling point and if the evaporated moisture is removed as condensate. This is unlikely in a real dryer. For well-designed convection dryers for granular materials, a heat demand of 1 kWh/kg moisture removed is reasonable [14]. Van Loock [2] suggests a heat demand for microwave dryers of 1.75 kWh/kg moisture. A microwave dryer with applicator could be expected to cost between $4000 and $7000 /kW while the capital cost of a convection system could be about an order of magnitude less [2]. In industrial practice, microwave drying systems have been used in which the fraction of the heat demand supplied by microwave energy ranges from 5% to complete application of microwave energy. With these figures it is possible to obtain a rough idea of the capital investment as a function of the microwave power. This must be balanced against added value, e.g. the increased profit resulting from reduced drying time. Note that the economic drivers vary from process to process. Estimating operating costs is a simple matter, depending on the electricity cost and including a tube replacement cost of about $0.02 /kWh. Note that in countries with cheap fossil fuel available for combustion, electrical technologies struggle to compete. Frequently we are faced with comparing alternative investments, e.g. whether to invest in a combined microwave hot air dryer, or to purchase what is likely to be a cheaper convection-only dryer. In such a situation it is important to remember that we should always choose the minimum investment that achieves functional results while yielding the desired return. This can be expressed in another way by saying that the more expensive investment should be accepted only if the incremental return on the additional investment also meets the desired return [16]. If this condition is not met it is better to invest the additional capital in the bank. Profitability can be assessed by any of the standard methods, such as payback time

278

Bradshaw

or return on investment for screening calculations or internal rate of return for more definitive estimates.

Equipment Design Drying equipment comes in many different types and the nature of the feedstock largely dictates the choice. If microwaves were to be retrofitted then this would largely determine the structural parameters. Many industrial dryers are continuous belt dryers; these are perhaps most easily suited to retrofitting. For grass roots designs there is more flexibility and both the nature of the feedstock and desired microwave characteristics can be used in determining dryer design. In this situation belt dryers are frequently favoured with the applicator being either a multimode cavity, or lightly resonant slotted feed type. It is important to note that it is perhaps best to consider the unit as a dryer for the product rather than as a microwave unit, and that commercialisation will be easier when microwave equipment is considered as commodity that can be purchased (as far as the end user is concerned) off the shelf. It is also very important to note industry-specific requirements, e.g. in the food industry equipment to be cleanable. Attention needs to be given to fire hazards and choice of belt materials is important. Working with machine designers at this point in the design is crucial. An example of a key issue that can frequently be overlooked when designing a dryer is that of air recirculation. At low drying temperatures the ideal heat demand can be considerably greater than the latent heat, and this is especially so when there is no recycle of humid air. Increasing the air inlet temperature and increasing the recycle ratio can decrease the heat demand. However, it is important to note that there is a trade-off between capital and energy, and that capital costs can typically be reduced (for conventional dryers) by operating at moderate conditions. A cursory inspection of microwave dryers suggests that infrequent use is made of humid air recycling, probably due to ignorance of its benefits. Scale up of dryers is difficult in general. It is usually best to make sure the experimental dryer looks as much like the real one as possible. A typical laboratory dryer approximating an industrial tunnel dryer is shown in Figure 3, while Table 3 indicates measurements required for full scale design.

Microwave Drying: Process Engineering Aspects

FLANGE FOR M.W. SOURCE

SLOTTED WAVEGUIDE

SCALE BRICKS

HEATING ELEMENTS

CENTRIFUGAL BLOWER

VALVE

TROLLEY STRUCTURE

Fig. 3. Typical laboratory microwave dryer

Table 3. Typical pilot plant data for dryer design (after [17])

Air velocity to convey material pneumatically, m/s Feed rate, kg/h Product rate, kg/h Initial moisture content, % by weight Final moisture content, % by weight Ambient air temperature, °C Ambient air humidity, kg/kg Inlet air temperature, °C Outlet air temperature, °C Material feed temperature, °C Material discharge temperature, °C Percentage solids recovery Material recycle ratio Air recycle ratio

279

280

Bradshaw

Selected Applications The number of applications of microwaves in drying is considerable, particularly in the food industry, where the high value of the products more easily justifies the expense. Examples include drying of sugar cubes to save time, energy and space, drying of bananas and drying of apples and mushrooms. In the following sections a few recent interesting applications are discussed to give an idea of the scope of application. A microwave-heated rotary kiln has been successfully applied on a commercial scale in Brazil to the drying of pulped coffee beans [18]. Hot air drying was used to dry from 50 to 30% moisture (wb) after which a hot air microwave rotary oven dried to the desired 11 - 13%. For a 50 kW unit producing 250 kg/h of dried product a payback time of 2½ years was predicted. Pilot scale work on drying of moulded ceramics [11] has shown that considerable reduction in demoulding and drying times can be achieved. The reduction in demoulding time meant that the number of castings per week could be increased considerably. With conventional demoulding, only 9 casting per week could be performed, while with microwave demoulding 22 casting per week were possible. It was also found that there was increase mould life and a lower reject rate. This lead to a reduction in mould use of 55 - 75%. This has major implications throughout the mould life cycle. A 6 kW dryer was predicted to pay for itself in 2 years. Of particular interest is the EA Technology batch dryer. This batch dryer operates with a single 2.45 GHz source providing only 6 kW of microwave power. The microwave source provides about 5% of the total power of the dryer, which means that the capital cost component for the microwave side, which is normally prohibitively expensive, is very little in this case.

References [1] Kisakürek, B., Flash Drying, in Majumdar, A.S., ed., Handbook of Industrial Drying, vol. 1, Marcel Dekker, New York (1995) [2] Van Loock, Energetics and economics of microwave and high-frequency drying applications, Microwave and High Frequency Heating 1997, Fermo, 421-424 (1997) [3] Mujumdar, A.S. and Menon, A.S., Drying of Solids, in Mujumdar, A.S., ed., Handbook of Industrial Drying, vol. 1, Marcel Dekker, New York (1995) [4] Kumar, P. and Mujumdar, A.S., Microwave drying: effects on paper properties, Drying Technology, 8(5), 1061-1087 (1990) [5] Perré, P, Drying with internal vaporisation: introducing the concept of the Identity Drying Card (IDC), Drying Technology, 13(5-7), 1077-1097 (1995) [6] Constant, T., Moyne, C. and Perré, P., Drying with internal heat generation: theoretical aspects and application to microwave heating, AIChE J., 42(2), 359-368 (1996)

Microwave Drying: Process Engineering Aspects

281

[7] Constant, T., Perré, P. and Moyne, C., Microwave drying of light concrete: from transport mechanisms to explanation of energy savings, Drying '92, ed. A.S. Mujumdar, Elsevier, 617-626 (1992) [8] Turner, I.W. and Jolly, P.G., Combined microwave and convective drying of a porous material, Drying Technology, 9(5), 1209-1269 (1991) [9] Hernandez, J.-M. and Puigalli, J.-R., simultion of drying of coniferous wood using various processes, Int. Chem. Eng., 34(3) 339-350 (1994) [10] Turner, I.W., Puigalli, J.R. and Jomaa, W., A numerical investigation of combined microwave and convective drying of a hygroscopic porous material, Trans IchemE 76(A), 193-209, (1998) [11] Jansen, W.J.L., Visscher, K., Beuse, R. and Weber, J., Implementation of dielectric drying in the ceramics industry, 7th International Conference on Microwave and High Frequency Heating, Valencia, 481-484 (1999) [12] Tomas, S. and Skansi, D., Microwave drying of a consolidated slab of raw clay, Chem. Biochem. Eng. Q., 8(2) 63-67 (1994) [13] Kiranoudis, C.T., Maroulis, Z.B. and Marinos-Kouris, D., Design and operation of industrial dryers, AIChE J., 42(11) 3030-3040 (1996) [14] Keey, R.B., Drying of loose and particulate materials, Hemisphere, New York (1992) [15] Metaxas, A.C., Foundations of electroheat, John Wiley and Sons, Chichester (1996) [16] Peters, M.S. and Timmerhaus, K.D., Plant design and economics for chemical engineers, McGraw-Hill, 3rd ed. (1980) [17] Williams-Gardner, A., Industrial Drying, Leonard Hill, London (1971) [18] Cunha, M.L., Canto, M.W. and Marsaioli Jr., A., Drying pulped coffee cherry beans by means of heated air assisted by microwaves, Microwaves: Theory and application in materials processing IV, Ceramic Transactions, 80, 641-649 (1997)

Quality of Microwave Heated Multicomponent Prepared Foods Suvi Ryynänen Department of Food Technology, P.O. Box 27, FIN 00014 University of Helsinki, Finnland.

Introduction

Heating multicomponent foods with microwaves is problematic because of nonuniform energy absorption, which leads to uneven temperature distribution in the food. This is to a large extent due to different thermal and dielectric properties of the components. Also the boundaries between different components cause reflections of microwaves and thus can be a source of uneven heating [7, 5]. Uneven heating has adverse effects on both sensory and microbiological quality. There is a British recommendation of minimum heating, which ensures that every point of the food reaches a temperature of 70ºC for two minutes [3]. The aim of this recommendation is to ensure the microbiological safety of the product. On the other side, when the food is pathogen free the minimum temperature requirement is based on consumers' acceptance on most appropriate temperature, and this requirement is more difficult to define [6]. On the both cases, however, control of heating uniformity is essential. Many factors affect the heating uniformity, for example dielectric properties of the components and by physical factors of the foods [5]. Temperature distribution in food during and after microwave heating is different from that using conventional heating. Cold and hot spots were found near each other. Therefore, temperature should be measured with several methods for a more reliable record of temperature distribution [1, 10]. This study had three objectives: 1) to investigate the dielectric properties of food ingredients and to choose possibly promising ingredients for recipe modifications, 2) to examine factors affecting the microwave heating uniformity of multicomponent foods (ready meal, hamburger), and 3) to study how the temperature and uneven temperature distributions are perceived by consumers.

Quality Of Microwave Heated Multicomponent Prepared Foods

283

Materials and Methods Dielectric properties of different starches and food components with varying salt concentrations were measured by a cavity perturbation technique at temperatures at about 2.75 GHz over the temperature range from 3 to 95ºC [7]. TM010 cavity was used for measuring the dielectric properties of hamburger bread (bun) and TM012 cavity for those of the starch solutions (concentrations from 5 to 30%) and the other food components. Experimental material used in temperature measurements were real foods: a ready meal with four components, and a hamburger (a layered food). Temperature was measured by a fiber-optic system (Luxtron models 755 and 790, Luxtron Corporation, USA) during microwave heating and by ordinary thermocouples (with a “temperature hedgehog” at 20 points) after microwave heating. Infrared imaging (Agema type T 870 connected to a computer) was used to determine surface temperatures of hamburgers. The water content of the hamburger bun during microwave heating was monitored by a measurement system using near infrared light. The instrument has two measurement probes made of dielectrically inert material. The probe is 3 mm in diameter and the measured sample volume is 2-3 mm3. The instrument has to be specially calibrated for each food material to be measured, as the instrument is very sensitive to both the structure and the temperature of the sample [15]. The microwave oven used in temperature measurements was a domestic Philips M610 "Space Cube" (microwave output power was 600 W according to IEC, 1000 g water load test). The effect of chemical (NaCl and encapsulated salt) or chemical and physical modifications (geometry and arrangement of food components) on the foods were investigated. Fig. 1 shows all the different arrangements and geometries of the ready meals. A consumer panel was used to examine how temperature affected the pleasantness of meal components and how the more or less uneven temperature distribution of two microwave heated meals with different arrangement of components (arrangements A1 and A4 in Fig. 1) was perceived. A backward multiple regression analysis of the results was used to build up equations linking the meal pleasantness to individual ratings. All data were evaluated by analysis of variance (SystatW5, Systat Inc, and SPSS 8.01, SPSS Inc.)

284

Ryynänen

Fig. 1. Arrangement and geometry of the ready meal. A tray with one compartment, B with two and C with three compartments.

Results and Discussion The measurement of dielectric properties of starches showed that the differences in the permittivities of various starch solutions (potato, wheat, corn, and waxy corn) and the differences between gelatinized and nongelatinized starches at high water content were very small. Their dielectric behavior mostly follows that of water. That means that, in practice, the variations between the different starches were too small to be significant for recipe modifications [13]. For the recipe modifications of the hamburger and the ready meal two salts were chosen as salts had very pronounced effect on the dielectric properties of the measured foods. The temperature measurements showed, however, that the chemical modifications had either no effect or they had only some combined effects with other factors although the dielectric properties varied much [10, 12]. The most important factors in modifying the temperature distribution of a ready meal were arrangement and geometry of the meal components. The saltiness did not affect significantly the heating rates of the meal components but the higher salt content of mashed potato and sauce increased the mean temperature of some components. The component with most variation in final temperature was mashed potato. It had lower mean temperature than the other components and the temperature distribution could be very uneven. Temperatures after microwave heating in

Quality Of Microwave Heated Multicomponent Prepared Foods

285

the mashed potato near the edges of the tray were over 70ºC but in the middle mostly between 55 and 65ºC. The edge heating effect may be used to advantage but also packaging of products should be considered more [10]. Recipe modifications of a hamburger had relatively small effect on temperature; a low NaCl content in the bun resulted in a faster heating rate, which was undesirable [12]. IR-thermographs showed that especially the center of the bun had a very high temperature and the center was the coldest area in the meat patty. The IR-thermographs of the cross section of meat patty indicated that saltier bun decreased the inside temperature of patty. The heating rate values indicated that the center part of the bun reached 100°C within about 30 seconds while the center areas of the meat patty were still cold (even only 15°C) and the bun became soggy and unpleasant. Depui [4] proposed that the fast temperature rise together with fast water transport cause a texture which differs from conventionally reheated bread. Rogers and others [9] reported that most of the changes observed during heating of bread were associated with changes in gluten, which was likely to be involved in the toughening of bread during microwave heating. They suggested that toughness is not simply a function of moisture content of the bread after reheating, but depends on the method of reheating. Yamauchi and others [16] concluded that the rapid hardening of microwave heated bread was mainly caused by hardening of starch. Water transport at five different points of the hamburger bun (Fig. 2a) during microwave heating was studied. The fastest heating rates were in the center of the bun and the lowest at the meat patty surface (Fig. 2b). The same figure shows the water content at the measured points after microwave heating of 120 and 160 seconds. The highest moisture content was in the area between the center and the surface of the meat patty. At the outer surface moisture could escape to the circulating air and during the heating the bun lost about 10% water.

Fig. 2. The five points where water content and temperature were monitored in the hamburger bun. a) a cross section of the bun and b) the heating rates and water content at the measured points after microwave heating of 120 and 160 seconds.

Calibration of the water measurement system is very critical. Although the instrument was calibrated for the used bread material and the temperature range, the results showed too high water contents for the hamburger bun compared to the gravimetric measurements, the true content being presumably 15-20% units lower. The instrument is very sensitive both to the structure and the temperature but it

286

Ryynänen

may be sensitive also to the time-temperature combination of the sample. The calibration procedure was developed for a conventional oven. As the system includes metal it could not be used in a microwave oven. As the temperature rise was not as fast as it would have been in a microwave oven, the structure dependence could have been different from that of our calibration [11]. The sensory study [14] showed that serving temperatures significantly affected all rated sensory attributes. The foods used in this study are normally eaten warm and, according to Zellner and others [17], consumers may even reject food if the serving temperature is not appropriate. Cardello and Maller [2] showed that the acceptability of foods was a function of the temperature at which the food is normally served. Although temperature significantly affects the sensory quality of a food, it is the combination of temperature and other sensory properties, which determine liking of a food. Temperature may induce changes also in other sensory properties, which in turn can affect liking. The overall pleasantness of the meal components correlated strongly with the appropriateness of temperature. It correlated also with the intensity of odor and flavor which, on the other hand, correlated with appropriateness of temperature. However, serving temperature is one of the main factors, which affect pleasantness of food. Large differences in temperature of mashed potato and carrots between the two meal types were measured instrumentally in the earlier research [10]. The mean temperature of mashed potato varied from 65 to 75ºC and that of carrots from 70 to 85ºC depending of the arrangement of the components. This sensory research indicated the same tendency but the differences between the meals were not significant. Possibly the overall temperature of the meal was perceived as so high that it was difficult to detect the differences. The regression models of the overall pleasantness were rather similar for both meal types. The three dominating factors were the pleasantness of flavor of the meat patty, pleasantness of the appearance of the whole meal, and the perceived temperature uniformity of the whole meal. This indicates that the temperature uniformity of microwave heated meals is also important to consumer acceptance but it is not easy to separate it from the other sensory properties, like flavor of the components. Regardless of temperature differences, the overall pleasantness of the two microwave heated meals with different arrangements was judged to be similar. Serving temperature is crucial to the pleasantness of food but, to the extent that it varied in microwave heating, it did not have a major impact on the acceptability of the meal [14].

Conclusions The most important factors in providing microwave heating uniformity of multicomponent foods are the arrangement of food components, geometry of products and packages. The temperature distribution could be balanced partly by taking advance of edge and corner heating intensification. In contrast, chemical modifications (saltiness) did not notably affect the heating uniformity.

Quality Of Microwave Heated Multicomponent Prepared Foods

287

The NIR moisture measurement instrument, which was proven to be suitable for measuring local water content during conventional heating, was found to be very sensitive to both the structure and the temperature of the sample. It seemed that structural changes during microwave heating differ from those during conventional heating and, thus, the calibration procedure should be developed and tested separately for measuring water diffusion in a microwave oven. Serving temperature clearly affected sensory attributes and the pleasantness of foods. However, while instrumental measurements indicated large differences in temperatures between two microwave heated ready meals, the consumer panel perceived only small differences in some sensory attributes. Regardless of temperature differences, the overall pleasantness of the two microwave heated meals with different arrangement of meal components was judged to be similar. Small temperature differences, especially at high temperatures (e.g. common food serving temperatures), may be very difficult to perceive in the mouth. Serving temperature is crucial to the pleasantness of food but, to the extent that it varied in microwave heating, it did not have a major impact on the overall pleasantness of the meal. Possibly, if the problem of uneven heating recurs it may become a nuisance. Therefore control of heating uniformity is essential.

References [1] Bows JR (1989) The Influence of the Thermal, Electrical and Physical Properties on the Quality of Food Heated by Microwaves. (Technical Memorandum No 527. Campden Food & Drink Research Association) [2] Cardello AV, Maller O (1982) Acceptability of water, selected beverages and foods as a function of serving temperature. J Food Sci 47: 1549-1552. [3] Department of Health & Social Security (1980). Guidelines on pre-cooked chilled foods. HMSO: London. [4] Depui H (1993) Återuppvärmning av bröd i mikrovågsugn (Reheating of bread in microwave oven)(in Swedish). M.Sc thesis. Chalmers Tekniska Högskola and SIK.34 p. [5] George RM, Burnett S-A (1991) General guidelines for microwaveable products. Food Control, 2 (1): 35-43. [6] James SJ (1993) Factors affecting the microwave heating of chilled foods. Food Sci Technol Today 7 (1): 28-36. [7] Mudgett RE (1986) Electrical Properties of Foods. In: Rao MA, Rizvi SSV (eds) Engineering Properties of Foods. Marcel Dekker Inc, New York, pp 329-390. [8] Risman PO, Bengtsson NE (1971) Dielectric properties of food at 3 GHz as determined by a cavity perturbation technique. I. Measuring technique. J Microwave Power 6: 101-106. [9] Rogers DE, Doescher LC, Hoseney RC (1990) Texture characteristics of reheated bread. Cereal Chem 67(2): 188-191. [10] Ryynänen S, Ohlsson T (1996) Microwave heating uniformity of ready meals as affected by placement, composition, and geometry. J Food Sci 61(3): 620-624. [11] Ryynänen S, Thorvaldsson K (1999) Diffusion of water in a microwave heated hamburger measured by a fiber optic NIR-instrument. In: Catalá-Civera JM, Peñaranda-

288

Ryynänen

Foix FL, Sánchez-Hernández D, de los Reyes E (eds) 7th International Conference on Microwave and High Frequency Heating 1999. Book of Proceedings. 13-17 September 1999, Valencia, Spain, pp 537-539. [12] Ryynänen S, Risman PO, Ohlsson T (2001) Microwave heating uniformity of a hamburger: modeling and the effect of recipe modifications. Manuscript. [13] Ryynänen S, Risman PO, Ohlsson T (1996) The dielectric properties of native starch solutions. J Microwave Power Electromagnetic Energy 31(1):50-53. [14] Ryynänen S, Tuorila H, Hyvönen L (2001) Perceived temperature effects on microwave heated meals and meal components. Submitted to Food Service Technol. [15] Thorvaldsson K (1998) Diffusion of Water in Foods during Heating. Ph.D. thesis. Department of Food Science, Chalmers University of Technology and SIK - The Swedish Institute for Food and Biotechnology. Sweden. [16] Yamauchi H, Kaneshige H, Fujimura M, Hashimoto S, Ohya K, Hirakawa T, Kobayashi T (1993) Role of starch and gluten on staling of white bread treated with microwave-heating. J Jap Soc Food Sci Technol 40(1): 42-51. [17] Zellner DA, Stewart WF, Rozin P, Brown JM (1988) Effect of temperature and expectations on liking for beverages. Physiol Behav 44: 61-68.

Sensory Evaluation of Dried Bananas Obtained from Air Dehydration Assisted by Microwaves Sousa, W.A.1; Pitombo, R.N.M.1; Da Silva, M.A.A.P.2; Marsaioli, Jr., A.3 1

Biochemical & Pharmaceutical Technology Dept/FCF/USP, São Paulo, SP, Brasil. 2 Nutritional & Food Planning Dept./FEA/UNICAMP, Campinas, SP, Brasil. 3 Food Engineering Dept./FEA/UNICAMP, Campinas, SP, Brasil.

Abstract

Ripened bananas of 3 kg water/kg dry matter were dried to 0.2 kg water/kg dry matter (|17% w.b. or water activity < 0.73) final moisture, by using a microwave domestic oven adapted for a bench scale drying operation. Drying curves were built, half of them based on peeled fruit samples without perforations, the other half applied to perforated samples. Drying times obtained in the first case averaged 6 hours, shortening to 5 hours for the latter case. Decreasing drying rates were observed for all experiments. Microwave processed samples were sensory evaluated and compared to conventionally processed commercial samples. Consumers acceptance tests of the product and its purchase intention were conducted and have shown high rates for the microwave dried samples, lowest rates for the commercial samples. It was also observed that for obtaining a dried banana of higher sensory standard it is necessary to apply a microwave power density averaging 428 r55 W/kg banana during the first two hours of drying and 358 r76 W/kg banana during the remaining hours. As a conclusion, the microwave process drying time could be reduced by up to ten times in comparison to the conventional drying process. The acceptance tests of the product and its purchase intention have confirmed the technical feasibility of obtaining the product and also the excellent marketing potential for the new technological option.

Introduction Brasil is the main banana producer in the world, although great part of the production is being wasted, mainly because normal commercialization standards for fresh fruit have not been reached yet. Furthermore, alternative ways of utilizing the surplus fresh fruit for industrialization are scarce, besides the fact that most of the existing plants maintain inadequate processing conditions, incompatible with

290

Sousa

obtaining good quality banana derivatives. One of them, the dried banana, is mostly consumed in the internal market and is generally produced by the small manufacturing establishments located near the banana growing areas. It has not reached yet the required uniformity and quality levels claimed every time more by the demanding consumers, probably due to such processing deficiencies, like too high temperatures, extremely delayed drying times and inefficient application of the local energetic sources (fuel oil, gas and firewood). One of the greatest disadvantages of the conventional food drying is the low energetic efficiency due to the long decreasing drying rate time, caused by the low food thermal conductivity, resulting of course in a slow internal heat transfer during the conventional heating process (Adu & Otten, 1996; Feng & Tang, 1998). In order to eliminate such problems, as well as to keep the nutritious and sensory properties of the product, in addition to its rapid and efficient thermal process, the use of the energy of microwaves as applied to food processing has been increased, specifically concerning to the drying processes. Drying by microwaves is faster, more uniform and energetically efficient as compared to the conventional process. Moisture removal is accelerated, as long as heat is generated internally by means of friction among molecules, having no strong dependency on external convective conditions created by the heated air. It should also be taken into consideration, under the same energetic expenditure, that just 20 to 35% of the equipment physical space is necessary when microwave is applied to the process, in comparison to the conventional process alone (Maskan, 2000). In more recent years much research work has been developed on drying assisted by microwaves as an alternative method for a great variety of food products, like fruits, vegetables, seasonings and breakfast foods. Food products processed by microwaves have been reported as of superior quality, best aroma and color, besides as being obtained under considerable energetic economy and also under a reduced processing time as compared to the conventional drying process (Maskan, 2000). Only a few papers could be pointed out on microwave drying of bananas, like those by Maskan (2000); Garcia et al. (1988); Schubert et al. (1996); Nijhis et al. (1998).

Objectives The present work aimed at verifying the optimal microwave power density range, expressed as W/kg banana, combined with the more convenient processing time, at a certain limited product temperature, to be applied in order to produce dried bananas of highest commercialization standard [final moisture within 16 to 17% (w.b.) and a water activity smaller than 0.73]. Other targets were to build drying curves and drying rate curves, determine overall processing times and check whether exists any influence when drying peeled fruit samples without perforations or perforated samples. Instrumental color and texture characteristics of the

Sensory Evaluation of Dried Bananas Assisted by Microwaves

291

product samples were evaluated and also compared to those of the dried product obtained by conventional processes, besides testing the product acceptance, the purchase intention, color, texture and sweetness indexes of the new product by the consumer market.

Materials and Methods Bananas of the variety “nanicão” (Musa acuminata, subgroup Cavendish), of ripening stage graded 7, were used in the experiments. Before drying, the integral peeled fruits were divided into two groups (A and C). Samples A were treated with a 4% citric acid solution for 10 min., whereas samples C were perforated by using a tool made up of a set of plastic pins (1 mm dia., 5 mm away from one another in a square distribution pattern) and then treated with a 1% sodium metabisulfite solution for 10 min.. Another group of samples (B), acquired as dried bananas in commerce, was used for being evaluated comparatively by the same analyses assigned to the products A and C. The drying operation was developed in bench scale, by using a microwave domestic oven, equipped with double wave emission, 950 W of maximum power, 2.45 GHz frequency (Brastemp DES, Brasil), and adapted with an additional exhausting fan located at the back wall of the oven in order to efficiently remove all the moisture generated from the drying operation. Drying curves were determined by the gravimetric method, consisting of weighing the samples at every 20 minutes interval, by the use of a semi analytical balance (Ainsworth DE-3100D, USA). One colorimeter model Tristimulos ColorQuest II/Hunter Lab was used for the instrumental color evaluation of the samples. Readings were adjusted as to reflectance, illuminant D65 and angle of observation of 10°, having as evaluated parameters L* (luminosity), a* (red intensity) and b* (yellow intensity), with results expressed in the CIE system. One computerized texturometer model TA-XT2 Texture Analyzer (SMS), with double compression cycle and 2 mm/s speed, acrylic cylinder of 2.5 cm dia. and 50% compression ratio, was used for the instrumental texture evaluation of the samples, under a constant 25°C room temperature. The acceptability test was carried out on a laboratory scale (Stone & Sidel, 1985) using 52 potential consumers, selected as a function of the degree with which they liked and consumed dried banana. First, the individuals evaluated the samples overall acceptability using a structured hedonic scale (1 = disliked extremely, 9 = liked extremely). Consumers also evaluated their intention to purchase each sample using a purchase intention scale (0 = certainly I would not buy this sample; 4 = certainly I would buy this sample). Results were worked by variance analysis - ANOVA (factors: sample and consumers), Tukey test - using the Statistical Analysis System (SAS Institute, 1985) and histograms of frequency. Consumers were also required to judge the samples color, texture and sweetness using a “just-right-scale” ranging from –4 = extremely less dark/sweet/soft

292

Sousa

than ideal to +4 = extremely darker/sweeter/softer than ideal. Results were analyzed by histograms of frequency.

Results and Discussion

Drying The ripened bananas, with 3.0 kg water / kg dry matter (75% w.b.), were dried to the previously established final moisture at the ratio 0.205 kg water / kg dry matter (|17% w.b.), corresponding to a water activity under 0.73 for the final dried product. Drying curves and drying rate curves were built for a total of 14 experiments, 7 of them based on peeled fruit samples without perforations, the other 7 applied to perforated samples. Each curve corresponds to a different applied microwave power, expressed as power density (W/kg banana), as is shown in Figures 1 to 4. The values of Xe were assumed as zero for the microwave drying, like in the work of Maskan (2000). The power density (W/kg banana) for every experiment is also plotted as a function of the average product moisture (kg water/kg dry matter), according to Figures 5 and 6. On analyzing the drying curves on Figures 1 and 2, it becomes evident that the power density influences strongly the length of the drying times, which is in agreement with the results obtained by Maskan (2000), Nijhis et al. (1998), Schubert et al. (1996) and Garcia et al. (1988), although these authors have dried bananas cut in slices of different thickness and used fixed values for power levels in their experiments. The drying times obtained for the whole fruits ranged from 4 to 8 hours (Figure 1), changing to 3.5 to 7 hours for the perforated bananas (Figure 2), that means a not very drastic average time reduction as would be expected. Decreasing drying rates were observed for all experimental curves (Figures 3 and 4). These same results had already been achieved by the above mentioned authors. There were no substantial differences between the curves for perforated bananas from those for the bananas without perforations. It can be noticed that the decreasing tendency of the microwave drying rate curves contrasts with the initial constant rate of the conventional drying curves, because in the former case the rate of microwave absorption due to the smaller absorptive capacity of a material of decreasing moisture is always falling.

Sensory Evaluation of Dried Bananas Assisted by Microwaves

293

1

0,8 1

(X - Xe)/(Xo - Xe)

2

0,6

3 4 5

0,4

6 7

0,2

0 0

60

120

180

240 300 Time (min)

360

420

480

540

Fig. 1. Drying curves for bananas without perforations submitted to different power densities (W/kg banana) 1

(X - Xe)/(Xo - Xe)

0,8 8 9

0,6

10 11

0,4

12 13 14

0,2

0 0

40

80

120

160

200 240 280 Time (min)

320

360

400

440

Fig. 2. Drying curves for bananas with perforations submitted to different power densities (W/kg banana)

294

Sousa

dX/dT (kg water/kg dry matter*min)

0,04 0,035 0,03 1

0,025

2

0,02

3

0,015

5

4 6

0,01

7

0,005 0 0

0,3

0,6

0,9 1,2 1,5 1,8 2,1 2,4 Xaverage (kg water/kg dry matter)

2,7

3

Fig. 3. Drying rate curves for bananas without perforations submitted to different power densities (W/kg banana)

dX/dT (kg water/kg dry matter*min)

0,035 0,03 0,025 8

0,02

9 10

0,015

11 12

0,01

13

0,005

14

0 0

0,3

0,6

0,9 1,2 1,5 1,8 2,1 2,4 Xaverage (kg water/kg dry matter)

2,7

3

Fig. 4. Drying rate curves for bananas with perforations submitted to different power densities (W/Kg banana)

Figures 5 and 6 illustrate how the power density was applied decreasingly with the smaller average banana moisture. One important question to be pointed out is concerned to the existence of two drying periods : the first one going from 0 to 2 hours; the second from 2 hours to the end of drying. Depending on the power density on the first period , there can be the occurrence of emptiness inside the product, fractures or even an openness of the material, as a function of the internal temperature rising of the sample in such a level as to generate vapors responsible for the higher internal pressures, that is why the product temperature should be

Sensory Evaluation of Dried Bananas Assisted by Microwaves

295

kept during this first period in the range 50 to 60oC. On the second period there prevail other types of occurrences, like changes of color and flavor (sugar caramelization), burned spots, heterogeneity of shrinkage and texture differences.

Power density (W/kg banana)

600 550 500 450

1

400

2 3

350

4

300

5

250

6 7

200 150 0

0,3

0,6

0,9

1,2

1,5

1,8

2,1

2,4

2,7

3

Xaverage (kg water/kg dry matter)

Fig. 5. Power density (W/kg) as functions of average banana moisture (kg water/kg dry matter) for 7 drying processes (bananas without perforations) 700 650 Power density (W/kg banana)

600 550 8

500

9

450

10

400

11

350

12

300

13 14

250 200 150 0

0,3

0,6

0,9 1,2 1,5 1,8 2,1 2,4 Xaverage (kg water/kg dry matter)

2,7

3

Fig. 6. Power density (W/kg) as functions of average banana moisture (kg water/kg dry matter) for 7 drying processes (bananas with perforations)

Color and flavor changes, as well as burned product are caused by temperature rises due to excessive power densities; therefore, it is advisable to maintain the

296

Sousa

product temperature in the range 80 to 90°C . Other imperfections might be associated to an uneven electric field strength distribution due to the presence of.standing waves, to a deficiency of the internal ventilation system or as a consequence of internal moisture level differences provoked during the first drying period Instrumental color Table 1. Mean values (15 points) of Luminosity (L*), red (a*) and yellow (b*), for samples A and C (processed by microwaves), and B (commercial sample). L*1

a*1

b*1

A (citric acid added)

50.71a

11.94a

19.57a

B

40.66c

3.06c

3.95b

C (sodium metabisulfite added)

52.93b

8.16b

18.21a

Samples

1

Mean values showing common letter in the same column indicate samples that did not differ among them.

The results appearing in Table 1 were analyzed by ANOVA, to the significance level p d 0.05, revealing that the samples processed by microwaves (A and C) have shown much brighter color than the commercial sample (B), as can be verified by the values of luminosity L*, where sample C exhibited a value of L* = 52.93, whereas the value of sample A was L* = 50.7, a significant difference existing between these samples to the tested significance level p. The analysis of the chromaticity values a* (red) and b* (yellow) of the samples showed a higher reddish color for sample A than for sample C, a significant difference existing between these samples to the tested significance level p as to a* (red). As concerns to b* (yellow), the difference was not significant between samples A and C and the results indicate sample A and C to possess a higher yellowish component as compared to sample B. The analysis of parameters L*, a* and b* as a whole has demonstrated that sample A exhibited a little darker coloration than sample C, which was the brighter among all samples.

Sensory Evaluation of Dried Bananas Assisted by Microwaves

297

Texture profile Table 2. Mean values (20 determinations) of the Texture Profile Analysis (TPA), 50% compression ratio, for samples A and C (processed by microwaves), and B (commercial sample) Sample A B C

Hardness 9.395r1.711 14.648r4.352 6.539r1.155

Springiness 0.666r0.076 0.606r0.078 0.657r0.106

Cohesiveness 0.472r0.033 0.437r0.057 0.448r0.044

Chewiness 2.984r0.797 3.925r1.573 1.930r0.479

Adhesiveness -0.021r0.026 -0.306r0.165 -0.103r0.074

The results shown in Table 2 correspond to average means for all of 20 determinations of TPA (Texture Profile Analysis), accomplished by using 50% compression ratio. These numbers demonstrate a significant difference between the microwave processed samples and the commercial sample, for all of the analyzed attributes : it can be noticed the lack of uniformity of the attributes for the commercial sample, attested by the high value of the standard deviations. For the samples processed by microwaves it is confirmed smaller values for hardness, chewiness and adhesiveness. Sample C showed smaller hardness and chewiness as compared to sample A, what can be justified by the presence of perforations made on the fresh fruit before processing. The commercial sample presented a high degree of hardness, chewiness and adhesiveness, as well smaller values of springiness. The high values of hardness and chewiness found in the commercial sample indicate an over-process of drying and that the final moisture content is too low (less than the recommended for a dried product with a water activity aw < 0.73), besides the fact that a high adhesiveness and a low springiness values may be indicative of sugar caramelization due to the high temperature of the conventional processing. Samples Acceptability Table 3 shows the samples acceptability means and indicates that sample A presented the highest acceptability, differing from all remaining samples at p < 0.05. On the other hand, sample B obtained the lowest acceptability mean, differing from the remaining samples at p < 0.05. The distribution of the overall acceptability values given by consumers to the three samples of dried bananas is shown in Figure 7. It can be seen that for sample A the majority of the consumers attributed values ranging from 6 (liked slightly) to 8 (liked very much), suggesting they liked this sample; whereas for sample B, most consumers attributed values ranging from 2 (disliked very much) to 4 (disliked slightly), indicating they did not like this sample. Sample C showed an intermediate acceptance among consumers.

298

Sousa

Table 3. Mean values for overall acceptance of the dried banana samples

Sample A C B

Average 6.5192a 5.4038b 3.5962c

*Samples of same index do not differ from one another at p d 0.05 16 Sample A

Frequency (no. of tasters)

14

Sample B

12

Sample C

10 8 6 4 2 0 1

2

3

4

5

6

7

(1=disliked extremely, 9=liked extremely)

8

9

Fig. 7. Frequency histogram for overall acceptance

As to the purchase intention it can be noticed by looking at Figure 8 that sample A obtained the highest intention of purchase among the consumers. Sample C has also shown good purchase intention, with similar values to those of sample A, showing low rejection index. In contrast, sample B had the highest frequency of grade zero (62%), indicating that consumers are unlike to buy that product. The degree with which the samples color matched the consumers expectations can be seen in the Figure 9, where sample A received the highest rate of grade 0 (ideal color). The sample C exhibited a color slightly less dark than ideal or, in other words, the consumer considered its color brighter than the ideal. The sample B presented most grades between 3 and 4 (much darker/extremely darker than the ideal), suggesting this sample was too dark in the consumers opinion. In Figure 10 it can be observed that sample A presented the ideal sweetness for the consumer, showing 62% of grade 0 (ideal sweetness). The sample C has also shown ideal sweetness, but rated smaller than that of sample A. The sample B was less sweet than the ideal, with a high frequency of grades –2 and –1 (moderately/much less sweet than the ideal). The results of sample B might indicate that other attributes like bitter taste, smoke flavor and smoke aroma could be masking the sweet flavor.

Sensory Evaluation of Dried Bananas Assisted by Microwaves

299

Frequency (no. of tasters)

35

Sample C

30

Sample B

25

Sample A

20 15 10 5 0 0

1

2

3

4

0=certainly I would not buy this sample, 4=certainly I would buy this sample

Frequency (no. of tasters)

Fig. 8. Frequency histogram for purchase intention 18

Sample A

16

Sample B

14

Sample C

12 10 8 6 4 2 0 -4

-3

-2

-1

0

1

2

3

4

Categories (-4=extremely less dark than the ideal; 0=ideal; 4=extremely darker than the ideal)

Fig. 9. Frequency histogram for color ideality

The frequency histogram for texture for the three dried banana samples shown in Figure 11 reveals that the samples A and C had an ideal texture, whereas the sample B was less soft than the ideal, that means, the consumers considered the sample B too hard.

300

Sousa

Fig. 10. Frequency histogram for sweetness ideality

The level of acceptability for the microwave processed samples was very good, showing better performance for sample A, to which the citric acid 4% solution was added for 10 minutes before the drying operation, thus producing a final product of a light caramel color. On the other hand the sample C, to which the metabisulfite 1% solution was added for 10 minutes, presented a color tending to the yellow, suggesting that could reside there another reason for the better acceptance of sample A. Sample A

Frequency (no. of tasters)

20

Sample B Sample C

15 10 5 0 -4

-3

-2

-1

0

1

2

3

Categories (-4=extremely less soft than the ideal; 0=ideal; 4=extremely softer than the ideal)

Fig. 11. Frequency histogram for texture ideality

4

Sensory Evaluation of Dried Bananas Assisted by Microwaves

301

Conclusions It can be concluded that it is possible to obtain products of excellent sensory quality by using microwave during the banana drying process, provided an adequate power density level be applied to the product. It was observed that the average drying time of 6 hours to produce samples A did not differ too much from that spent to obtain samples C, which averaged 5 hours. Not least important it was also observed that the microwave drying processing times have been reduced by up to ten times when compared to the conventional drying process.

Notation W X Xe Xo

Watt moisture content (kg water/ kg dry matter) at any time equilibrium moisture content (kg water/ kg dry matter) initial moisture content (kg water/ kg dry matter)

Literature [1] Adu, B., Otten, L.. Effect of increasing hygroscopicity on the microwave heating of solid foods, Journal of Food Engineering, vol. 27, pp.35-44, (1996). [2] Decareau, R.V., Peterson, R.A.. Microwave processing and engineering, Chichester: Ellis Horwood, 224 p., (1986). [3] Drouzas, A. E., Schubert, H.. Microwave application in vacuum drying of fruit, Journal of Food Engineering, vol. 28, pp.203-209, (1996). [4] Feng, H., Tang, J.. Microwave finish drying of diced apples in a spouted bed, Journal of Food Science, vol. 63, pp. 679-683, (1998). [5] Funebo, T., Ohlsson, T.. Microwave-assisted air dehydration of apple and mushroom, Journal of Food Engineering, vol. 38, pp. 353-367, (1998). [6] Garcia, R., Leal, F., Rolz, C.. Drying of bananas using microwave and air ovens, International Journal of Food Science and Technology, vol. 23, pp. 73-80, (1988). [7] Khraisheh, M. A. M., Cooper, T. J. R.. Magee, T. R. A., Shrinkage characteristics of potatoes dehydrated under combined microwave and convective air conditions, Drying Technology International, vol. 15, pp. 1003-1022, (1997). [8] Lin, T. M., Durance, T. D., Scaman, C. H.. Characterization of vacuum microwave air and freeze dried carrot slice, Food Research International, vol. 4, pp. 111-117, (1998). [9] Maskan, M.. Microwave/air and microwave finish drying of banana, Journal of Food Engineering, vol. 44, pp. 71-78, (2000). [10] Meilgaard, M., Civille, G. V., Carr, B. T.. Sensory evaluation techniques, Cap. 9: Affective tests: consumer tests and in-house panel acceptance tests, 2nd ed., CRC Press, Florida, (1998). [11] Moskowittz, H. R.. Product Testing and Sensory Evaluation of Foods, Westport Food and Nutrition Press, Inc., 605 p., (1983).

302

Sousa

[12] Mudgett, R.E.. Microwave properties and heating characteristics of foods, Food Technology, vol. 40, no. 6, pp. 84-93, (1986). [13] Mudgett, R.E.. Microwave food processing, Food Technology, vol. 43, no.1, pp.117126, (1989). [14] Nijhuis, H. H., Torringa, H. M., Muresan, S., Yuksel, D., Leguijt, C., Kloek, W. Approaches to improving the quality of dried fruit and vegetables, Trends in Food Science &Technology, vol. 9, p.13-20, (1998). [15] Owusu-Ansah, Y.J.. Advances in microwave drying of foods and food ingredients, Canadian Institute of Food Science and Technology Journal, vol.24, no.3, pp.102-107, (1991). [16] Prabhanjan, D. G., Ramaswamy, H. S., Raghavan, G. S. V.. Microwave-assisted convective air drying of thin layer carrots, Journal of Food Engineering, vol. 25, pp. 283293, (1995). [17] Ren, G., Chen, F.. Drying of American ginseng panax quinquefolium roots by microwave-hot air combination, Journal of Food Engineering, vol. 35, pp. 433-443, (1998). [18] Senise, J.T.. A utilização de rádio frequência e microondas na eletrônica industrial, Revista Brasileira de Engenharia Química, vol. 8, no.1, pp. 51-56, (1985). [19] Schubert, H., Drouzas, A. E.. Microwave application in vacuum drying of fruits, Journal of Food Engineering, vol. 28, pp. 203-209, (1996). [20] Shiffmann, R. F.. Microwave and dielectric drying, In: Mujundar, A.S., Handbook of Industrial Drying, Marcel Dekker, New York, pp. 327-356, (1987). [21] Stone, H., Sidel, J.L.. Descriptive analysis, cap. 6, In: Stone, H., Sidel, J.L., Sensory evaluation practices, Academic Press, London, pp. 202-226, (1985). [22] Tulasidas, T. N., Raghavan, G. S. V., Norris, E. R.. Effects of dipping and washing pre-treatments on microwave drying of grapes, Journal of Food Process Engineering, vol. 19, pp. 15-25, (1996). [23] Von Hippel, A.R.. Dielectrics and waves, 2nd ed., MIT Press, Massachusetts, 284 p., (1995). [24] Yongsawatdigul, J., Gunasekaran, S.. Microwave-vacuum drying of cranberries: Part II, Quality evolution, Journal of Food Processing and Preservation, vol. 20, pp.145156, (1996).

Acknowledgements The authors are indebted to FAPESP (Fundação de Amparo à Pesquisa do E.S.Paulo) for the scholarship connected to this work.

Microwave Method for Increasing the Permeability of Wood and its Applications G. Torgovnikov and P. Vinden School of Forestry, University of Melbourne, Creswick, Victoria 3363, Australia

Introduction A number of wood species, particularly hard wood, have a very low permeability causing problems during timber processing. These include, very long drying times, large material losses after drying, expensive drying processes, and extreme difficulty in impregnating timber with preservatives and resins. Therefore it is essential for the timber industry to have a method which can provide an increase in wood permeability. The main elements of wood structure include tracheid, libriform fibres, vessels and ray cells. Ray cells, which occupy 5 to 35% of the total wood volume, have thinner walls than the cell walls of main tissues of wood (fibres) and run in a radical direction from pith to cambium of the tree stem. Ray cells are the weakest cells in wood, and under high internal pressure can be ruptured preferentially, forming pathways for easy transportation of liquids and vapour. The net result is a substantial improvement in radial wood permeability. The green wood moisture content varies between species and between sapwood and heartwood. It can range between 40 to 250%. Therefore green wood readily absorbs microwave (MW) energy. This results in a very high release of energy from within the material. Intensive MW power applied to the wood generates steam pressure within the wood cells. Under high internal pressure the weak ray cells are ruptured to form pathways for easy transportation of liquids and vapours in the radial direction. An increase in the intensity of the MW energy applied to the wood increases the internal pressure, resulting in the forming of narrow voids in the radiallongitudinal planes. The number of cavities, their dimensions and distribution are controlled by the intensity of MW energy supplied. A several thousand-fold increase in wood permeability in the radial and longitudinal directions can be achieved in species, previously found to be impermeable liquids and gases (Vinden and Torgovnikov, 1998, 2000). Other physical properties and technological attributes are also improved. The include the physical properties of permeability, reduced heat conductivity (better heat insulation), reduced shrinkage and swelling, and improved acoustic properties (better sound insulation) and the technological

304

Torgovnikov

properties of improved impregnation and liquid uptake, improved sawing, and improved drying. It is possible to use microwave modification of wood to enhance existing processes and to develop entirely new processing and product options for wood. It establishes opportunities for developing a number of new industrial applications including rapid preservative treatment of heartwood of softwoods, the treatment of refractory wood species with preservatives, rapid drying of hardwoods, and new wood-based material, „Vintorg“, production.

MW modified wood – Torgvin The application of very intensive microwaves can give rise to a novel wood product, Torgvin, which has a multitude of cavities oriented in the radial/longitudinal planes. Furthermore, materials may be produced with modified and unmodified zones by irradiating selected areas of the sample, or by using intermittent or pulse irradiation. Torgvin has very high permeability, increased flexibility, altered shrinkage and mechanical properties, and lower densities compared to unmodified wood. Torgvin has been successfully produced from a number of different wood species including Radiata pine, Douglas fir, English oak, Messmate, Mountain Ash, Jarrah and Manna Gun. Torgvin properties of some species in comparison to unmodified wood properties

Density and volume The oven-dry wood density reduction and volume increase following Torgvin production varies between species as indicated below: Radiata pine Douglas fir Messmate Mountain Ash

from 0 to 15.5% from 0 to 9.4% from 0 to 13.4% from 0 to 11.1%

The range in density reduction is also a function of MW power as indicated in Fig 1.

Microwave Method for Increasing the Permeability of Wood and its Applications

305

Density reduction, %

8 6 4 2 0 0

200

400

600

800

1000

1200

MW energy, MJ/m³

Fig. 1. Reduction in density (percent) of Messmate wood as a function of MW energy supplied to the wood (moisture contents of 8 – 10%, conveyor speed 6.5 mm/s).

Permeability The permeability of wood is highly variable; therefore the uptake of CCA (copper-chrome-arsenic-solution) was used as an index of changing permeability after MW treatment. Uptakes (L/m3) after pressure impregnation (Bethell treatment): Douglas fir heartwood Radiata pine Messmate Yellow Stringybark posts

Control 60 – 90 120 – 140 18 46

After MW conditioning 375 – 426 361 – 516 192 – 255 340 – 400

Mechanical properties of Torgvin MW modification of wood results in loss of the mechanical strength. The magnitude of strength loss is determined by the intensity of MW irradiation, the speed of the conveyor, and the exposure time. An increase in MW powder leads to a progressive reduction in both MOE (modulus of elasticity) and MOR (modulus of rupture), as indicated in Table 1.

306

Torgovnikov

Table 1. Percentage residual MOE and MOR in Messmate as a function of MW power, conveyor speed and sample orientation.

Conv. Speed (mm/s)

MOE Tangential dir.

MOE Radial dir.

MOR Tangential dir.

MOR Radial dir.

Control 36 kW 48 57

6.5 6.5 6.5

100 62 50 44

100 88 85 82

100 46 20

100 56 41 30

Control 36kW 48 57

12 12 12

100 83 77 -

100 86 82 -

100 85 80 76

100 85 80 76

The residual MOE in Messmate in the tangential direction ranged from 44 – 83% (of initial MOE) and 82 – 88% in the radial direction. The residual MOR at a conveyor speed of 6.5 mm/s in the tangential direction ranged from 20 – 40% (of initial MOR) and 30 – 56% in the radial direction. An increase in conveyor speed to 12 mm/s provided an average residual MOR in the tangential and radial directions of 76 – 85% (of initial MOR). Thus investigations into the influence of MW wood modification on mechanical properties show that intensive MW treatment can reduce MOE in the range 35 – 90% (of initial MOE) and MOR in the range 20 – 94% (of initial MOR). The minimum reductions in Radiata pine and Messmate strength properties after MW wood modification (at observable permeability increases) are summarised in Table 2. Table 2. Minimum losses in MOE and MOR to effect changes in wood permeability

Radiata pine Messmate

MOE Tangential dir 26 17

MOE Radial dir 4 12

MOR Tangential dir 23 15

MOR Radial dir 6 15

Energy consumption Electrical energy consumption for Torgvin production ranges from 120 to 300 kW/m3 depending on the required degree of modification, species, timber density and moisture content.

Microwave Method for Increasing the Permeability of Wood and its Applications

307

Microwave conditioning of timber for preservative treatment The low permeability of many species does not allow proper preservative treatment. The results of microwave conditioning of Douglas fir and Radiata pine heartwood with cross-sections of 45 x 90, 70 x 70 and 90 x 90 mm, indicate that it is possible to totally impregnate the cross-sections of these species. Improvements in permeability were obtained in the radial, tangential and longitudinal grain directions. This was achieved by destroying or rupturing ray cell tissue, by resin softening and mobilisation, and by the formation of large numbers of cavities in the radial/longitudinal grain direction. Improvements in permeability (as indicated by preservative absorption) indicate that preservative uptakes range from 375 – 426 L/m3 compared to 60 – 90 L/m3 for controls. The best results are obtained at the maximum supplied MW power of 54 – 57 kW and timber speed of 1.5 – 2.1 cm/sec. The MW energy consumption needed to make Douglas fir heartwood permeable is 117 kWh/m3. Investigations into potential for microwave modification of hardwoods including English Oak (Quercus robur), Yellow Stringybark (Eucalyptus muelleriana), Victorian grown Ash (Eucalyptus regnans) and Messmate (Eucalyptus obliqua), indicate that ray cells can be ruptured with the rapid generation of steam during microwave conditioning. The application of more intensive microwaves can result in the controlled formation of microchecks at the interface of ray tissue and longitudinal fibres. The thinner cell walls of ray tissue combined with lower wood tensile strength in the tangential direction (2 - 3 times lower than in the radial direction) results in the formation of micro-voids in the radial/longitudinal direction. Relatively low pressures are required to form micro-voids when the wood is hot. Microwave irradiation of Yellow Stringybark posts with diameters ranging from 60 – 100 mm, resulted in the improved heartwood penetration of preservatives and the relaxation of stress such that there was no check formation following drying of the post. High Preservative absorption (copper-chrome-arsenic) ranging from 340 – 400 L/m3 was achieved, indicating greatly improved permeability, compared to controls which achieved 46 L/m3. Microwave conditioning followed by soaking in creosote resulted in total creosote penetration throughout the crosssection. Creosote uptakes ranged from 119 – 169 kg/m3. Control of creosote uptake was achieved by changing the soaking time. The process for treating hardwood posts from green comprises conveyor belt microwave conditioning and soaking. No pressure treatment plant is required.

Rapid drying of hardwoods Many commercial hardwood drying operations impose an extended period of slow air drying to reduce the occurrence of drying defects (checking and collapse) prior to kiln drying. MW conditioning of eucalypts provides an opportunity for kiln drying immediately after sawing, potentially without the need for steam condi-

308

Torgovnikov

tioning. The reduction in drying time for hardwoods provides a reduction in associated capital, space, energy and labour costs whilst the reduction in drying defects can increase yields of high quality timber by approximately 5%. Microwave pretreatment of Victorian grown Messmate timber reduces drying times by a factor of 5 - 10 times that of unmodified wood. Intensive microwave conditioning increases the permeability of the green wood, overcoming the propensity of the wood to collapse during kiln drying. Commercial drying trials with MW modified Messmate sawn timber were undertaken to estimate the economic advantages of MW conditioning. Messmate timber measuring 28 x 90, 45 x 90 and 90 x 90 mm were processed using a MW installation (frequency 0.922 GHz), a MW power range of 7.5 – 54 kW and timber feed speed of 5.3 – 14 mm/sec. Moisture losses after MW conditioning ranged from 20 – 35% m.c. The subsequent moisture content of timber before kiln drying was 60 – 70% m.c. The results of kiln drying at a commercial installation (Black Forest Sawmills, Victoria) using conventional hardwood drying schedules (40°C, 65% RH) are summarized in Table 3. Table 3. Acceleration of kiln drying of microwave modified timber: Drying time reduction for Messmate (E. obliqua) in convection kiln

Product

Microwave Kiln schedule condition- Temp RH% ing . °C

Parquet One side of boards board modi28 x 92 fied mm Timber Full cross 28 x section 92mm modified Timber Full cross 90 x 90 section mm modified

MC

Final MC

Drying time

Acceleration in drying

45

45-55

60-70%

8-12%

7 days

4-5 times

70

50

60-70%

8-12%

4 days

10 times

45

63-70

60-70%

8-12%

42 days

MW energy consumption for timber conditioning prior to drying ranges from 80 – 160 kWh/m3. A cost analysis of MW pre-drying conditioning of Messmate timber for a plant output of 20,000 m3/year indicates a cost of approximately AU$ 30 /m3 for electricity costs of AU$ 0.077 /kWh. The modified properties of Torgvin open up a number of new fields for the application of wood material. One application for Torgvin is the production of a new bio-composite material called Vintorg.

Microwave Method for Increasing the Permeability of Wood and its Applications

309

Vintorg material Vintorg is manufactured from modified wood, Torgvin by impregnating the modified wood with resin and curing in a press. Vintorg has strength properties similar to laminated veneer lumber (LVL). The process is particularly suitable for using the low value core wood of species like ‚Radiata pine and Douglas fir, but to date we have found the process suitable for any species. Vintorg has the following advantages: 1. High strength properties (comparable with LVL) 2. Natural appearance and structure 3. Good dimension stability 4. The use of low grade material (for example heartwood of Radiata pine or Douglas fir) 5. Simple technology compared with LVL 6. Cheap production. Production process An outline of the process used for Vintorg production from radiata heartwood is shown in Figure 2. Vintorg production is much simpler compared to laminated veneer lumber (LVL) production. The manufacturing stages of Vintorg production include: 1. 2. 3. 4. 5.

Microwave timber modification: the MW installation is used for increasing the heartwood permeability of softwoods and reducing the wood moisture content from 30 – 40% to 10% Resin soaking: used for timber impregnation with a resin in a cold bath with a cross conveyor. Pre-pressing: for removing surplus resin after impregnation. Hot pressing: used for resin curing and cross section formation . Sawing and finishing of the material.

The success of this technology arises from microwave modification which renders any wood species totally permeable to resin impregnation. The microwaves have identified weak planes in the wood which are subsequently bonded with resin under pressure to provide stronger wood of the same dimensions as the unmodified wood. On-going research is investigating the improvements in strength that can be achieved by increasing the density of wood by compressing Torgvin to smaller cross sectional volumes than the original unmodified wood.

310

Torgovnikov Flow diagram of Vintorg Production line continuous taper-press

A-A

Sawn timber Microwave applicator Hot Press

Resin bath Re-sawing

Pre-press

Finished product

Section A-A

Fig. 2. An outline of the process used for Vintorg production

Resin consumption ranges from 6 - 15%. However, on going research is investigating the potential for reducing resin consumption and using alternative resins and treatments to modify the visual characteristics of Vintorg. Vintorg production costs The estimate costs of Vintorg production compared to laminated veneer lumber (LVL) are summarized in Table 4. Table 4. Comparison of Vintorg and LVL production costs (Radiata pine heartwood)Production costs, including leased capital costs (AU$/m³)

Output, (m³/year) 20,000 30,000 40,000 50,000 80,000

LVL 669 576 484 381

Vintorg 355 - 366 338 - 349 -

Substantial economies are possible in the manufacture of Vintorg compared to laminated veneer lumber production. The process also lends itself to small-scale production.

Microwave Method for Increasing the Permeability of Wood and its Applications

311

Conclusions The study of wood modification using MW treatment demonstrates selective ray cell modification, and the formation of a number of narrow cavities in the radial /longitudinal planes. The volume of the cavities and their dimensions can be controlled. Such wood treatment increases the permeability of wood in the radial and longitudinal directions and modifies the properties of the wood. MW treatment on different wood species (Radiata pine, Douglas fir, English oak, Messmate, Mountain Ash, Jarrah, Manna Gum) transforms the wood into a new material, Torgvin, which has altered properties. It is possible to use microwave modification of wood to enhance existing processes and to develop entirely new processing and product options for wood. It establishes new opportunities for developing a number of new industrial applications: x Rapid preservative Treatment of heartwood of softwoods x The treatment of refractory wood species with preservatives x Rapid drying of hardwoods x New material – „Vintorg“ production A cost analysis of MW conditioning of different species of timber in Australia indicates costs of approximately AU$ 20 – 60 /m3 depending on electricity costs. These costs are reasonable for industry and can provide the commercialisation of MW method for wood modification.

References [1] Vinden P., and Torgovnikov G. (1998). A method for increasing the permeability of wood. Patent Appl. Intern. Publication NoWO 99/64213 [2] Vinden P. and Torgovnikov G. (2000) the physical Manipulation of Wood Properties Using Microwave. Proceedings (p.240-247), International Conference of IUFRO, The Future of Eucalypts for Wood Production, 19-24 March, Tasmania, Australia.

Acknowledgements The authors would like to acknowledge the encouragement and assistance given by Dr. Kwame Asumadu, Executive Director of the Forest and Wood Products Research and Development Corporation of Australia in the development of this microwave technology.

Selective Heating of Different Grain Parts of Wheat by Microwave Energy E. Pallai-Varsányi1; M.Neményi2; A.J.Kovács2; E.Szijjártó1 1

Univ.Kaposvár Research Institute of Chemical and Process Engineering Veszprém, H- 8201 Veszprém, P.O. Box 125. Hungary 2 West-Hungarian University, Faculty of Mosonmagyaróvár Institute of Agricultural, Food and Environmental Engineering, H-9200 Mosonmagyaróvár, Vár u.2. Hungary.

Abstract The aim of author’s research work was to investigate the effect of process parameters and microwave heat treatment condition on the inactivation of D-amylase enzyme being in wheat grains, responsible for decomposition of starch molecules without considerable damage of the gluten content. The task was furthermore to clear up the possibilities of selective moistening of the different wheat grain parts containing the D-amylase enzyme and the valuable gluten protein. For checking the results Magnetic Resonance Imaging method was used.

Introduction Industries continually make effort to reduce manufacturing costs and to improve the quality of its products. Food manufacturers are under constant pressure to modify and extend the range of their products and to reduce additives such as artificial colours, flavours and preservatives in order to meet consumer demand. The use of microwave energy is more efficient than the traditional heating [1]. Microwaves are used for rapidly supplying a large amount of energy. The advantages of electromagnetic heating are mainly a result of the internal heat generation. Microwave energy exhibits unique properties such as the great penetration depth and first of all the selective heating of different product components having different dielectric properties. In food processing it is of special interest to see how electromagnetic energy can influence the activity of enzymes and microorganisms. In case they can be selectively activated or inactivated this may prevent the need of a sever heat treatment and may have a positive effect on the product quality.

Selective Heating of Different Grain Parts of Wheat by Microwave Energy

313

The dielectric properties of food depend on its constituents. Water, proteins and carbohydrates are among the polar molecules that line up in a microwave electric field. The protein content of wheat is of great importance in respect of the germination capacity and baking quality. This latter depends considerably on the gluten protein content and also on the D-amylase enzyme activity responsible for decomposition of starch molecules being in wheat grains. If the D-amylase content is too high, heat-denaturation , that is enzyme inactivation must be performed, but without notable damage of gluten protein. The main difficulty is due to the fact that Damylase is one of the most heat resistant enzymes (up to 80 - 90°C) while the gluten is very thermolabile. As a result the water absorbing capacity of proteins and their capacity for swelling decreases. The degree of denaturation depends also on the moisture content of the grains. The thermal stability of proteins decreases with the increase of the moisture content [2]. The aim of research work was to investigate the effect of process parameters, and of the heat treatment conditions on the inactivation of the harmful D-amylase enzyme without considerably heat damage of the gluten content. The task was furthermore to clear-up the possibilities of selective enzyme inactivation and even to promote it. For this reason selective moistening was carried out. To check the results Magnetic Resonance Imaging (MRI) was used.

Experiments The microwave heat-treatments were carried out in a domestic microwave oven (Philips M734, stereo mode, 2.45 GHz). By a supplementary microwave power control unit definite microwave power values could be set from 50 Ws up to 750 Ws by 12 steps. The temperature of samples was measured by infrared thermometer (AMIR 7814) immediately after the microwave was switched off.

Wheat grain as model material Grains are living biological products which germinate and respire also. Wheat consists mainly of pericarp seed coat, aleuron layer, germ and of endosperm (Figure 1.). The embryo of the germ is the principal part of the seed. It contains the highest amount of fat, protein, and a large amount of enzymes. The endosperm contains the highest amount of carbohydrate in the form of starch, furthermore gluten protein. This latter can be found also in the aleuron-layer. The initial moisture content has an important effect on the microwave energy absorption of the grains. Therefore, in most cases the grains have to be moistened to some extent depending on the initial moisture content.

314

Pallai-Varsányi

Fig. 1. Structure of wheat kernel

It is to be noted that the moisture content of different grain parts (germ, endosperm, aleuron layer) is different and depends highly on the conditioning time (Figure 2.).

Moisture content (weight %)

120

Starchy endosperm Aleurone layer Germ ( embryo )

100 80 60 40 20 0 0 10 20 30 40 50 60

Time of conditioning Fig. 2. The effect of conditioning time on the moisture content of different grain parts

Selective Heating of Different Grain Parts of Wheat by Microwave Energy

315

On the base of this dependence selective moistening of different grain parts was applied. For this reason, to promote selective microwave energy absorption, that is to promote the heating of wheat germ containing the harmful D-amylase enzyme in contrary to the endosperm and to the aleuron layer containing the gluten protein, suitable conditioning time must be set. The microwave treatments were carried out using wheat samples of high D-amylase content (1.5 - 1.6%).

Experimental conditions Microwave power: 200 WS…1000 Ws Specific microwave power (W/g): 1.0…10.0 Moisture content of samples: 10 wt%…25 wt% Conditioning time: 5…16 hours On the basis of preliminary microwave treatments it was stated that the highly heat resistant D-amylase can be inactivated without considerable gluten damage only under special treatment conditions. Microwave heating experiments were performed to investigate the effect of the following treatment methods on the D-amylase activity and on the gluten content: selective microwave energy absorption promoted by selective moistening, continuous microwave heating with postcooling, interrupted microwave treatment with and without air cooling during the irradiation pauses, ¾ microwave treatment under “heat shock” conditions. ¾ ¾ ¾

Qualification of treated samples To qualify the wheat samples the following analitycal data were determined: moisture content, wet gluten content, D-amylase content (activity), bakery value. To study the moisture intake of the different characteristic parts of wheat grain depending on the conditioning (soaking) time, Magnetic Resonance Imaging (MRI) method was used. Nuclear magnetic resonance imaging is one of the few experimental techniques which has the capability to study mass-transfer in biological systems noninvasively and non-destructively [3], [4]. The wheat grains were soaked in water for 4, 8, 12, 16 and 20 hours. The images were obtained using a Bruker Avance 360 MHz spectrometer with a 15 mm inside diameter RF coil resonator. The acquisition time was 17 minutes 7 sec. Images from each kernel were taken in two orientations: one in the longitudinal

316

Pallai-Varsányi

(major) axis and an other in the lateral (minor) axis. The images show different perspectives of the germ and the endosperm of the kernels. A traditional oven method (130°C for 19 hrs, ASAE Standard S352,2) was used to measure the moisture content of the kernels after the different soaking times.

Results

MRI examinations Figures 3a-e show the MRI images of wheat kernels in two operations after 4,8,12,16 and 20 hours of soaking respectively. The proton density (moisture content) is expressed in gray scale. The highest proton density corresponds to the light grey colour. Parts where the moisture content was too low to be distinquished from the noise caused by the inhomogeneity of magnetic field were coloured by white.

Fig. 3. MRI images of a wheat kernel ; left - longitudinal, right - lateral a: after 4 hours of soaking , b: after 8 hours of soaking

Selective Heating of Different Grain Parts of Wheat by Microwave Energy

317

Fig. 3, cont. MRI images of a wheat kernel ; left - longitudinal, right - lateral c: after 12 hours of soaking , d: after 16 hours of soaking e: after 20 hours of soaking

Conclusions were drawn on the base of visually investigation of the image sequences, e.g. by considering the brightness of the images which is proportional to the proton density. The water intake of different grain parts depending on the conditioning time was checked also by measuring the moisture content of the manually separated germ and endosperm. In Table 1 are summarized the moisture content data of the whole grains while Table 2 shows the water intake of different grain parts depending on the conditioning time. On the base of visually investigation of MRI pictures the following conclusions could be drawn: After 4 hours soaking the germ showed the highest moisture content. Lower moisture was detected in the hairy parts and in the bran. The driest part was the endosperm. After 8 and 12 hours of soaking the moisture content increased pri-

318

Pallai-Varsányi

marily in the germ and secondarily in the hairy part of the kernel through the bran. The middle of the endosperm remained dry. After 16 hours of soaking the inner endosperm started to absorp moisture from the germ. Figure 3e shows that water starts to penetrate to the endosperm not only from the germ as in the shorter soaking times but through the bran as well. Table 1. Moisture content data of the whole grains

Soaking time Hours 4 8 12 16 20

Moisture content (db.) % 40.32 46.99 51.58 58.31 65.22

Table 2. Water intake of different grain parts depending on the conditioning time

Grain parts Endosperm

Germ

Soaking time hours 1 3 6 10 1 3 6 10

Moisture content % 6.1 12.4 15.5 23.9 15.5 24.5 37.5 39.9

The measured data of Table 2 confirm the moisture intake of different grain parts depending on the conditioning time, represented in Figure 2 and correspond with the water intake tendency established on the base of MRI pictures. Microwave heat treatments The microwave heat treatments were carried out under conditions given in chapter 2.2. To investigate the possibility of selective microwave energy absorption promoted by selective moistening, the initial moisture content of the wheat samples was set by moistening at different soaking times. The most important representative measuring data are summarized in Table 3.

Selective Heating of Different Grain Parts of Wheat by Microwave Energy

319

Table 3. Microwave heating of wheat

Continuous microwave treatment without postcooling tmw Tend Tpost tcond Sample Mw-power W/g Mi code W (%) (h) (s) (°C) (°C) 1 200 1 12 220 70,5 109 2 200 1 12 125 59,1 81 3 1000 5 12 45 83,5 117 4 1000 5 15 2 45 81,3 119 5 1000 5 15 6 45 79,0 118 6 1000 5 15 12 45 77,4 116 7 1000 5 15 12 20 65,3 110 8 9 10 11 12 13 14

750 750 750 750 750 750 750

Continuous microwave treatment with postcooling 4,7 12 85 110,5 4,7 12 50 88,3 4,7 18 2 50 85,5 4,7 18 12 50 86,5 15,0 18 2 30 91,5 15,0 18 6 30 90,5 15,0 18 12 30 90,8 -

Interrupted microwave treatment with cooling Mi tcond Np tper Sample Mw-power W/g Tper code W (%) (h) (s) (°C) 15 750 4,7 12,0 3 70 100,5 16 750 4,7 18,0 2 3 70 99,5 17 750 10,0 18,0 2 2 20 63,0 18 750 10,0 18,0 6 2 20 61,6 19 750 10,0 18,0 12 2 20 63,1

Gld (%) high 3,5 9,8 10,6 9,1 8,3 3,4

Ad (%) 18,8 0 63,5 65,2 67,5 68,2 16,1

high 8,3 9,5 7,2 4,1 3,0 1,9

85,5 21,0 24,9 26,5 38,8 36,5 34,0

Gld

Ad

(%) high high 1,2 0,74 0

(%) 50,9 56,2 20,3 23,5 28,4

Marks: Mi – initial moisture content; tcond – conditioning (soaking) time; tmw – time of mw-treatment; tper – sum of mw-treatment periods; Tend – temperature of the sample measured at the end of mw-treatment; Tpost – maximum temperature of the sample measured after the end of mw-treatment; Np – number of treatment periods; Gld – denaturation of gluten; Ad – decrease of D-amylase activity

On the basis of the obtained experimental data the following conclusions were made: ™ The inactivation of D-amylase enzyme without gluten damage, carried out by continuous mw-treatment without postcooling could not be realized ( see tests 1 - 5.) because of the so-called “post-processing temperature rise (PPTR), except of “heat shock conditions” (see test 7.). ™ Both the D-amylase inactivation and gluten denaturation are promoted by the increase of initial moisture content ( see tests 3 - 4, and 9 - 10).

320

Pallai-Varsányi

™ Results of tests 5 - 6, and 12 - 14 show the promoting effect of the selective moistening of wheat grains that results in selective microwave energy absorption. ™ The best results were obtained under heat-shock conditions and selective moistening of wheat grains ( using conditiong-soaking-times of 12 hours). Nevertheless it must be emphasized that these results are moderate in respect of D-amylase inactivation because it is necessary to make a compromise between the enzyme activity decrease and the acceptable gluten degradation. However microwave heat treatment of wheat could be considered advantageous taking into account that at the same time also improvement in baking quality and decrease of microbial contamination can be obtained.

Conclusion On the basis of experimental results it was stated that inactivation of the harmful D-amylase enzyme being in wheat grain can be realized without notable gluten degradation by selective microwave heating under heat-shock conditions using selective moistening.

Acknowledgement Financial support from the Hungarian National Scientific Research Fund (OTKA T030386) and from the EU-Copernicus programme (PL967048) is gratefully acknowledged.

Literature [1] Sakac, M., Ristic, M. and Lavic, J.: Effects of Microwave Heating on the Chemiconutritional Value of Soybeans. Acta Alimentaria 1996, 25 (2) pp.163-169. [2] Pallai-Varsányi, E., Szijjártó, E.et al: Effect of Microwave Heating on the D-Amylase Activity and Baking Quality of Wheat. Proc.of the 7th International Conference on Microwave and High Frequency Heating, Valencia (Eds.: Catala-Civera, J.M., PerandaFoix, F.L., Sanchez-Hernandez, D.) Servicio de Publicaciones, Valencia, 1999. pp.325-328. [3] McCarthy, M.J. (1994) Magnetic Resonance Imaging in Foods. Charman and Hall, Inc. New-York. [4] Song, H., Litchfield, J.B. et al (1992). Threedimensional Microscopic MRI of Maize Kernels During Drying. J. Agric. Engng. Res. 53, pp. 51-69.

Microwave in situ Remediation of Soils Polluted by Volatile Hydrocarbons D.Acierno1, A.A.Barba2, M.d'Amore2 ,V.Fiumara3, I.M.Pinto4, A.Scaglione3 1

D.I.M.P., University of Naples "Federico II" D.I.C.A., 3D.I.I.I.E., University of Salerno 4 University of Sannio, Faculty of Engineering 2

Abstract Microwaves as a tool for remediating soils contaminated by Volatile Organic Compounds (VOC) appear to be attractive for the advantages they offer, such as reduced thermal gradients, selectivity, possibility of in situ operation, time and money saving. The Microwave Induced Steam Distillation process (MISD) has been performed, to remediate a naphthalene polluted soil, using a closed applicator and an open applicator prototype designed and constructed for in situ treatments. Results showing the feasibility and advantages of this technique are presented and discussed. In particular they show that MISD can be effective for in situ treatment of the VOC polluted soils if contamination does not interest excessively deep layers.

Introduction Conventional decontamination of soils polluted by volatile organic compounds (VOC) is performed by mechanical removal of the contaminated layer, followed by off-line treatment (incineration-disposal, or cleanup and replant) [1]. This kind of treatment is characterized by high costs due to the enormous amount of solid movement and by the highly hazardous nature of the removed material. Recently alternative techniques (bacteria utilization, soil vapour extraction, microwave induced steam distillation) have been investigated which would allow VOC's removal by in situ treatment of the soil [1, 2, 3, 4]. In particular, microwave induced steam distillation (MISD) when compared with the other techniques, seems to offer several advantages including shortened treatment times, reduced environmental impact (as a consequence of reduced soil manipulation) and accurate control of the process parameters [4].

322

Acierno

MISD is based on stripping phenomena due to the water vapour phase action on the pollutants in the soil matrix. Water vapour phase is generated via microwave heating of water added soil. In our previous works [5, 6] we discussed preliminary microwave decontamination experiments carried out on small samples of polluted soil in closed applicators. We found that microwave heating occurs in layers progressively deeper as steam distillation proceeds and allows the pollutant to be removed in the whole sample. Obviously the limit of microwaves penetration is related to the loss factor of the soil matrix. Starting from results obtained in closed applicator we studied the feasibility of MISD treatments to remediate large contaminated areas directly in situ and designed a slot-array open applicator which was shown to be able to achieve this aim [7]. In this paper we resume the results obtained in closed applicator, the design criteria of the open applicator and the results obtained using a prototype of the open applicator.

Materials and Methods

Soil samples Due to its large availability at constant composition, experiments were performed on commercial (gardening) soil samples which were previously chemically and physically characterized. The composition is reported in Table 1. The soil was contaminated by naphthalene (solid at room temperature and water insoluble) using a solution of naphthalene in methanol. Table 1. Gardening soil characteristics Proximate analysis (% on weight basis) organic matter mineral matter

Properties 35 39

organic nitrogen

1

organic carbon

25

PH

5.5 - 6

bulk density Sauter diameter

0.32 g/cm3 300 Pm

Blumenerde Mould supplier

After the microwave treatment, a Soxhlet was employed for extracting the residual naphthalene from the soil and a gas chromatograph was used for the analysis. The effectiveness of MISD treatment on the soil was evaluated by measuring temperature, moisture contents and residual naphthalene profiles in the sample

Microwave In Situ Remediation Of Soils Polluted By Volatile Hydrocarbons

323

depth. Temperature measurements were performed by progressively inserting a thermocouple into the sample after the treatment; both moisture and residual naphthalene contents were determined by analyzing small amounts of soil in assigned positions of the sample. Closed applicator A 2450 MHz, 800 W maximum power, home style microwave oven was used to treat the gardening soil samples. The sample holder was a steel cylinder (diameter = 10 cm, height = 12 cm) which allowed the sample to be irradiated from the top only. The holder was internally insulated to avoid heat conduction through the metallic wall. Open Applicator In order to perform in situ MISD remediation the open applicator should have the following main characteristics: i) good trade-off between the extension of the irradiated area and the incident microwave power density; ii) minimization of the power radiated in undesired directions; iii) good power handling and fault tolerance. To meet such requirements we designed an open applicator consisting of a planar array of waveguide longitudinal resonant slots. The applicator was obtained by placing side by side three distinct waveguide slot linear arrays (sub-arrays); each sub-array is coupled by tilted slots (coupling slots) to an upper waveguide connected to the microwave source. In order to achieve maximum radiated power in the direction normal to the array plane (broadside array), the position along the waveguide axis of both the coupling and radiating slots has been chosen so that all radiating slots are excited in phase. The excitation level of the slots was chosen according to a binomial sequence, in analogy to the usual technique used to reduce the side lobes of antenna far-field radiation pattern. To optimize applicator efficiency (i.e. to obtain the impedance matching condition) proper values for both the offset from the waveguide axis of the radiating slots and the angular tilt of the coupling ones were determined by following a classical method [8]. An applicator prototype, working at 2.45 GHz, was built in aluminum. More details about design and prototype construction are given in [7]. In order to evaluate the electromagnetic behavior of the applicator, measurements of the Voltage Standing Wave Ratio (VSWR) at the applicator input section were performed by using a vector network-analyzer HP85070B. Measurements were carried out at frequencies ranging from 2 GHz to 3 GHz by placing the applicator 4 cm above the surface of a soil sample filling completely a large wood holder (120x100x9 cm). VSWR measurements were performed without naphthalene addition, soil dielectric properties depending essentially on moisture contents [5]. 30 minute long microwave treatments were carried out at 2.45 GHz by using a 1.9 kW microwave power source (magnetron), with the same arrangement used in VSWR measurements.

324

Acierno

Results

110

100

100

90

90

80

80

70

70

temperature

60

60 humidity

50

40

40 30 20 10 0

50

30

1 min 2 min 3 min 4 min 0 1 2 (bottom)

3

Humidity [%]

Temperature [°C]

Figures 1 and 2 refer to treatments performed in closed applicator. Fig. 1 shows temperature and humidity profiles at different exposure times. It is manifest that, by increasing the exposure time, the MISD reduces water content in surface layers allowing the microwave heating to get to deeper layers into the soil.

20 10 4

5

6

7

Depth, cm

8

9

10

11 12 (surface)

0

Fig. 1. Temperature and residual humidity profiles in closed applicator treated samples as a function of depth. Initial humidity: 40%.

Fig. 2 shows the results of decontamination runs. The profiles highlight the stripping effect of the MISD on the contaminant. All the polluted samples have a starting naphthalene concentration of 2000 ppm and the same initial humidity, but the time of exposure to microwaves ranges from 3 to 10 minutes. For the samples exposed to microwaves for short times (3 or 5 minutes), naphthalene is significantly stripped out from superficial layers, whereas it is still contained in a considerable amount in the deeper layers. When the sample is exposed to microwaves for a longer time, (7 to 10 minutes), microwaves can reach the deeper layers, and the naphthalene is quite completely removed from the sample. Fig. 3 shows the measured VSWR of the open applicator prototype. Measurements were performed on soil samples with different moisture contents (data reported in figure refer to a sample with humidity of 66%). Near the operating frequency (2.45 GHz), the VSWR increases as the humidity decreases but keeps lower than 2.5, corresponding to an efficiency value K = 81%.

Microwave In Situ Remediation Of Soils Polluted By Volatile Hydrocarbons

325

100 10 min

7 min

5 min

3 min

90 80

Residual naphthalene [%]

70 60 50 40 30 20 10 0

0 1 2 (bottom)

3

4

5

6

7

Depth [cm]

8

9

10 11 12 (surface)

Fig. 2. Decontamination profiles of residual naphthalene in closed applicator treated samples as a function of depth. Initial concentration: 2000 ppm; initial humidity: 40%. 100 10 9 8 close-up 7 6 5 4 3 2 1 0 2.30 2.35 2.40 2.45 2.50 2.55 2.60

90

VSWR

80 70

VSWR

60 50

Frequency [GHz]

40 30 20 10 0 2.0

2.1

2.2

2.3

2.4

2.5

2.6

Frequency [GHz]

2.7

2.8

2.9

3.0

Fig. 3. Measured VSWR of the open applicator prototype irradiating a soil sample with initial humidity of 66%. Distance from soil surface: 4 cm.

Figures 4, 5 and 6 refer to treatments performed using the open applicator prototype on soil with 66% initial moisture contents. Due to the binomial tapering of the slot excitation levels, the incident power density (and consequently the micro-

326

Acierno

wave heating) is very high in the soil portion immediately under the central slots of the applicator, and decays outward. 100 90 80

Temperature [°C]

70 60 50 40 30 time of exposure: 30 minutes initial humidity: 66% initial temperature: 25°C

20 10 0

0 1 (bottom)

2

3

4

5

6

7

Depth [cm]

8 9 (surface)

Fig. 4. Temperature profiles in open applicator treated samples.: central area, x: intermediate area; „: peripheal area 100 90

Residual humidity [%]

80 70 60 50 40 30 time of exposure: 30 minutes initial humidity: 66% initial temperature: 25°C

20 10 0

0 1 (bottom)

2

3

4

5

Depth [cm]

6

7

8 9 (surface)

Fig. 5. Humidity profiles in open applicator treated samples.: central area, x: intermediate area; „: peripheral area.

The results reported here refer to averaged values measured in three different concentric areas of the sample: a central (very hot, approximately 10 x 10 cm wide), an intermediate (hot, external dimensions 25 x 25 cm) and a peripheral area (not very hot, external dimensions 35 x 35 cm).

Microwave In Situ Remediation Of Soils Polluted By Volatile Hydrocarbons

327

In Figures 4 and 5 the temperature and residual moisture contents profiles in each of the three areas are plotted. In the central area, where the incident power density is high, the microwave heating is strong and the surface layers significantly leak moisture. In the peripheral area the incident power density is lower than in the central one, the microwave heating reduces and the treatment is essentially limited to the surface layers only. Due to the low heating, surface layers retain a moisture contents a little lower than the initial one; this residual humidity further reduces the strength of the field reaching the deeper layers of the sample. In the intermediate area the temperature profile is close to the one measured in the central area but has the maximum value in the middle of the sample; in fact, the higher residual moisture contents reduces the strength of the microwave field reaching the bottom of the sample. 1000

Residual concentration [ppm]

900

time of exposure:30 minutes initial concentration: 4000 ppm

800 700 600 500 400 300 200 100 0

0 1 (bottom)

2

3

4 5 Depth [cm]

6

7

8 9 (surface)

Fig. 6. Naphthalene concentration profile in the central area of open applicator treated samples.

Fig. 6 shows the residual naphthalene contents in the central area of a contaminated sample (initial naphthalene concentration 4000 ppm) after the microwave treatment. The effectiveness of the treatment is clearly observable. Both the surface and intermediate layers are nearly decontaminated while only a reduced amount of naphthalene is left in the deeper layers.

Conclusions The results show that microwave induced steam distillation can be effective for in situ treatment of the VOC polluted soils if contamination does not interest exces-

328

Acierno

sively deep layers. In particular, results obtained by an open applicator prototype represent, to the best of our knowledge, the first reported evidence of microwave VOC removal using an open applicator such as required by a real in situ treatment.

References [1] Hyman M. and Bagaasen L. (1997) Chem. Eng. Prog., Special Issue on Site Remediation, pp. 22-42 [2] Windgasse G. and Dauerman L. (1992) J. of Microwave Power and Electromagnetic Energy, vol. 27, (1), pp. 23-32 [3] Regan A.H. et al. (1995) Int. Microwave and HF Heating Conf. Proc., St. John’s College, Cambrige, England, pp. D3.1- D3.4 [4] Price S.J. et al. (1997) Proc. of Air & Waste Managment Association's 90th Annual Meeting & Exhibition, Toronto, Canada, pp. FA115 [5] Acierno D. et al. (1998) Proc. CHISA’98, Praha, Rep. Ceca, P5- 217, pp. 1-10, CDROM Czech Society of Chemical Engineering [6] Acierno D. et al. (2000) In: Application of the microwave technology, Series of Monographs on Materials Science, Engineering and Technology, Leonelli C. et al. Eds., Mucchi, Modena, ISBN 88-7000-346-9, ISSN 1120/7302, pp. 11-23 [7] Acierno D. et al. (2000) In: Application of the microwave technology, Series of Monographs on Materials Science, Engineering and Technology, Leonelli C. et al. Eds., Mucchi, Modena, ISBN 88-7000-346-9, ISSN 1120/7302, pp. 37-48 [8] Collin R.E. (1985) Antennas and Radiowave Propagation, Mc Graw-Hill, pp. 265-268

Bio-dielectric Soil Decontamination J.P.M. Janssen-Mommen, W.J.L. Jansen KEMA, Arnhem; the Netherlands

Abstract Soils, polluted with organic contaminants, can be cleaned using dielectric heating, by volatilisation of the contaminants. Soil temperatures needed with this method are in general quite high, between 90 and 160oC, leading to relative high electric energy demands and subsequently high energy costs. To lower the costs of energy, the feasibility of combining dielectric heating with biological decontamination was investigated. The cleaning temperatures are then far below the temperatures needed for volatilisation, resulting in lower energy costs. The purpose of dielectric heating is to raise the soil temperature to such a level that the activity of the microorganisms, present in the polluted soil, is at its highest. This leads to a new decontamination method, called bio-dielectric in-situ remediation of soils. This paper describes the results of an in-situ pilot-test on oil contaminated soil with this method.

Introduction Dielectric heating can be used for in-situ cleaning of soil, polluted with organic contaminants, by volatilisation of the contaminants. The vapour developed is removed from the soil by surface collection and in this way recovered for destruction. This method needs soil temperatures which are in general quite high, between 90 and 160oC, leading to relative high electric energy demands and subsequently high energy costs [1, 2, 7]. An alternative for dielectric heating is biological in-situ cleaning of soils by micro-organisms; especially soils contaminated with organic components, i.e. oil [8]. Biological decontamination is a low-cost and promising technology but has some disadvantages. One of them is that it takes several years to decrease pollution to acceptable levels depending on the solubility and biodegradability of the contamination. Furthermore, biological in-situ cleaning is only applicable for more or less impermeable soils, in which it is possible to inject enough nutrients and oxygen in the soil to stimulate the activity of the micro-organisms.

330

Janssen-Mommen

In order to lower the energy costs of soil remediation via dielectric volatilisation, the feasibility of combining dielectric heating with biological decontamination was investigated. The assumption was that, with cleaning temperatures far below the temperatures needed for volatilisation, lower energy costs could be obtained. Furthermore, an increase of the decontamination rate of micro-biological cleaning was expected due to elevated temperatures and the resulting higher microbiological activity compared with the usual biological cleaning conditions. The purpose of dielectric heating is to raise the soil temperature to the optimum activity level of micro-organisms, normally present in the polluted soil. The combination of the two existing remediation methods leads to a new cleaning method, called bio-dielectric decontamination of soil [5].

Preliminary tests and economics To evaluate the possibilities of in-situ bio-dielectric decontamination of soils, a series of preliminary laboratory experiments with dielectric heating were undertaken. It was first established that a volume of 0.25 m3 of contaminated soil could be maintained at a temperature of 30oC within 1oC, this in combination with a reasonable amount of bacterial activity. Biodegradation at 30°C seems higher than at 10°C but was difficult to quantity due to the heterogeneity of the contamination in the soil [3]. Furthermore the economics of the method were evaluated. The extra costs of dielectric heating, on top of biological decontamination, were estimated to be in the order of EUR 12 per ton ground. The total average costs of remediation with different in-situ and ex-situ technologies are between EUR 140 and 270 per ton. This means that the dielectric method can be economically interesting and offers an alternative for biological decontamination given the advantages of a faster process. The method seems especially suitable for soils impermeable to water, when biological cleaning cannot be accelerated by other means (air sparging, etc.). The most important effect of bio-dielectric cleaning is that the contamination, as a result of the higher soil temperatures, is more available for decomposition, by increasing both the volatility of the contaminants (contaminants are less captured in the soil matrix) and microbial respiration rates. An extra effect is the enhanced activity of the micro-organisms at these higher temperatures. On the basis of these positive results an in-situ pilot test in the field was planned.

Selecting a pilot test site To evaluate the bio-dielectric decontamination method in-situ, a location in the Netherlands was selected, where the soil was contaminated with organic components, especially mineral oil. The selection criteria for the test site were as follows: -

soil contaminated with mineral oil (both light and heavy fractions)

Bio-Dielectric Soil Decontamination

-

331

heterogeneous profile of the soil (sand, loam or clay, gravel, etc.) contamination present in a saturated zone no metal parts in the soil which could disturb the electric field no telephone lines or communication cables nearby the possibility of placing electrodes in the soil

A location satisfying these criteria was found in the northern part of the Netherlands; an old railway-yard on a heterogeneous sandy soil (sand and layers of loam). The soil was contaminated with diesel oil and aromatic hydrocarbons such as benzene, toluene, ethylbenzene and xylene. Results of soil research from 1990 showed that the contamination reached a depth of 4.5 m below ground level and that concentration of oil was around 5500 mg/kgds (dry solid).

Results and discussion of laboratory tests Before a field test at the location was started, laboratory tests were performed using contaminated soil samples taken from the ground of the railway-yard. These laboratory tests showed that a substantial acceleration of biological decontamination of the soil could be obtained by dielectric heating of the soil to a temperature of 30oC. The availability of the contamination for micro-organisms, was increased as well as the rate of decay. In particular, the availability of mineral oil was considerably improved by a factor 2. This is shown in Table 1 where the mass of oil removed from soil is given, for soil dielectrically heated to 30°C and soil at a temperature of 10°C. At higher temperatures the oil is less enclosed in the soil matrix and is therefore also more available for biological cleaning. Table 1. Availability of contamination in column studies [4].

column 1 column 2 column 3 average

dielectric heated by 30°C time water oil removed (days) through from colcolumn umn (mg) (litre) 41 4,7 29 33 5,3 13 53 8,9 3 6,3 15

Time (days) 59 59 59

reference by 10°C water oil rethrough moved from colcolumn (litre) umn (mg) 5,2 15 4,4 4 5,0 0,6 4,8 6,5

Furthermore it was discovered that the biodegradation at 30°C was about 30% higher than at 10°C. This is shown in Table 2 where the concentration of mineral oil is given during six weeks of biodielectric cleaning and biological cleaning only. After six weeks the reduction in oil content with biodielectric cleaning is almost three times higher then with biological cleaning only.

332

Janssen-Mommen

Table 2. Oil concentrations in mesocosms in reference and dielectric heated soil [4].

Treatment

time after start (weeks)

concentration mineral oil (mg/kgds) Bio-dielectric cleaning: dielectric heated soil (30°C) 0 2900 ± 130 2 1533 ± 150 4 1350 ± 70 6 1650 ± 60 Biological cleaning only: reference soil (10°C) 0 2500 ± 120 3 2333 ± 150 6 2133 ± 380

reduction mineral oil (%)

47 ± 5 53 ± 4 43 ± 2

7±6 14 ± 3

Both results, more availability of contaminants and higher remediation rates, mean that in practice, by using bio-dielectric cleaning, it should be possible to remove mineral oil contamination two times faster compared to biological decontamination only, at ambient soil temperatures. Based on these results the project was continued with an in-situ pilot test at the old railway-yard.

Results and discussion of field test

Check of present existing pollution level on site To check if the oil was still present (a go or no-go for the pilot test) the present oil content of the soil was measured again in June 2000 between a depth of 0 and 2 m. The results were a concentration of mineral oil between 1300 and 6100 mg/kgds and a water content of 12 to 18%. These results were definitely positive for continuation with the pilot test. Pilot test set-up A pilot test set-up was developed for bio-dielectric decontamination. Figure 1 shows a scheme of the test set-up; the applied electrode configuration is shown in Figure 2.

Bio-Dielectric Soil Decontamination

333

Container

2m

soil earth electrodes

earth electrodes

high voltage electrodes

Fig. 1. Test Setup

1m

1m

Fig. 2. Electrode Configuration

Four in line high voltage electrodes were used, covering a distance of about 0.5 m. The high voltage electrodes were surrounded by 12 earth electrodes (Figure 2) at 1 m distance from the high voltage electrodes. All electrodes were 2 meters long. Ground water level was about 7 m below ground surface level, far below the electrodes. The earth electrodes were interconnected with an aluminium meshed plate, which acted as an electric ground plane and was also used for screening. For the last function the aluminium-meshed plate was extended around the high voltage electrodes. A 27 MHz 10 kW generator was placed in a container, above the electrodes and the meshed aluminium plate. The container was placed about half a meter above the ground surface to enable connection of the high voltage electrodes to the RF generator. The gap between the container and the ground and the high voltage connection in the container, was screened by means of a wired mesh to prevent leakage of the electric field and to prevent disturbance of the measuring and control equipment placed in the container. This screen was also connected to the aluminium ground plane. In Figure 3 and 4 the container and the connection of the RF generator to the electrodes, together with the applied screening, are shown.

334

Janssen-Mommen

Fig. 3. Container on the location

Fig. 4. RF installation (generator, generator screen and connection) in the container

Bio-Dielectric Soil Decontamination

335

Temperature measurements and control Temperature of the soil was measured with a Luxtron measurement system with four optical sensors. The temperature signal was also used to regulate the on/off cycle of the RF generator, so that an average temperature of 30oC in the ground was obtained. The measured temperatures were collected with a data acquisition system by remote control using a cellular telephone connection. During the first tests large temperature differences were observed. Figure 5 shows the results of typical temperature profiles near the RF electrode (distance 10 cm) and between the RF and the earth electrodes (50 cm from RF electrode). 35

Temperature (°C)

30

25

20

15 0

20

40

60

80

100

120

140

160

180

200

Depth (cm) Temperature near RF-electrode

Temperature between RF-electrode and earth electrode

Fig. 5. Temperature profile as a function of the depth below ground level

It is obvious that a "hot" layer was present at about 50 cm below ground level. Because the temperature was quite steady between a depth of 100 and 200 cm, it was decided to regulate the temperature to 30°C, of the measurement signal from a depth of 150 cm. At first the temperature was regulated with a cycle of 4 hours on and 20 hours off. However, this resulted in large differences in temperature between the "hot" layer (52°C) and the other ground layers (28 to 36°C, see Figure 6). With a more frequent cycle the temperature of the "hot" layer can be maintained below 40°C. Figure 7 shows the results of more frequent cycles. Cycles between 4/10 (four minutes on, 10 minutes off); 6/20 and 10/60 were used. Several times a heating time of 20 minutes was used, this resulted in unacceptable temperatures, above 45 oC. A cycle below 6/20 was used for the pilot test.

336

Janssen-Mommen Date: 30 July 2000 60

50

Temperature (oC)

hot layer

40 top layer

30

control layer container

20

10

0 0

20

40

60

80

100

120

140

160

180

200

Time (hours)

Fig. 6. Temperature in time with a 4/20 cycle. (top layer = 30 cm bs, hot layer = 50 cm bs, control layer = 150 cm bs, container = air temperature in container) Date: 11 Aug 2000 60

50

Temperature (oC)

hot layer

40 top layer

30

control layer container

20

10

0 0

5

10

15

20

25

30

Time (hours)

Fig. 7. Temperature in time with a more frequent cycle. (top layer = 30 cm bs, hot layer = 50 cm bs, control layer = 150 cm bs, container = air temperature in container)

Bio-Dielectric Soil Decontamination

337

Mesocosms Because of the heterogeneity of the soil and the irregular distribution of oil concentrations in the soil (concentrations, measured in June 2000 between a depth of 0 and 2 m, varied between 1300 and 6100 mg/kgds) it was not possible to measure biodegradation in the soil itself. Therefore mesocosms were used. For the preparations of mesocosms, samples of the soil to be treated were taken and were homogenised by quartering and coning. Oil concentrations of six mesocosms were directly measured to determine the start value of the oil content. The other mesocosms were placed in perforated polyethylene tubes (length 1.5 m, diameter 3 cm); the tubes were filled with homogenised ground of the soil to be treated. Eight tubes with mesocosms were placed in the soil to be treated with bio-dielectric cleaning, this is called the "dielectric" site. To have a reference for the results of the "dielectric" site, a biological decontamination site was used. Four tubes with mesocosms were placed in the reference test site. The reference site was located outside the electric field. In this reference site biological cleaning took place under natural conditions of the soil. Test conditions and measurements during the decontamination test The duration of the bio-dielectric decontamination test was about 40 days. During this time the soil was heated to about 30°C to a depth of about 2 m. The temperature in the soil was quite homogeneous, except for one particular layer, where a maximum temperature around 40°C was reached (Figure 7). During the test the soil, in both the dielectric and the reference site, was kept moist by adding water together with a nutritious fluid containing NH4Cl and K2HPO4 to stimulate the activity of the micro-organisms. This prevented the soil for drying out and at the same time, as a result of the nutrition, the circumstances for an optimal biodegradation of oil by micro-organisms were enhanced. Temperature, oxygen- and carbon dioxide-concentration, rate of decay of contamination and the availability of the contamination were measured. Oxygen and carbon dioxide are a measure for the activity of the micro-organisms. To determine the rate of decay of the oil concentration and the availability of the contamination, mesocosms were taken out during the test and the samples were analysed. Test results The test started in July 2000; the first mesocosms were taken for analysis from the test sites in August. After the samples were taken, interference problems with the temperature measurement system occurred, caused by imperfect screening. Because of that, the temperature of the soil raised to 60°C. It was checked, if the high temperatures had a negative effect on the presence of micro-organisms. A bacterial count was made on samples from the soil by means of the MPN method (Most Probable Number). From the results it was concluded that the number of micro-

338

Janssen-Mommen

organisms was not effected by the temperature rise. The interference problems were solved; further mesocosms were taken for analysis in October and November 2000. Figure 8 shows the results of the oil concentration measurements from the analysed mesocosms; both from the "dielectric" test site and the reference test site. Oil concentrations in mesocosms in dielectric test site Mineral oil [mg/kg ds]

3000

17-08-'00

2000

05-10-'00

1000

31-10-'00

22-11-'00

mesocosms - start value mesocosms / 0.65 m-bs mesocosms / 1.0 m-bs mesocosms / 1.35 m-bs

0 0

20

40

60

80

100

120

140

160

Time in days after start project [26 juni 2000] (bs: below surface level)

Oil concentrations in mesocosms in reference test site Mineral oil [mg/kgds]

3000

17-08-'00

05-10-'00

31-10-'00

22-11-'00

2000

1000

mesocosms - start value mesocosms / 0.65 m-bs mesocosms / 1.0 m-bs mesocosms / 1.35 m-bs

0 0

20

40

60

80

100

120

140

160

Time in days after start project [26 juni 2000] (bs: below surface level)

Fig. 8. Concentration of mineral oil (mg/kg dry solid) in the mesocosms in time during the test

In Figure 8 no clear decrease in oil concentration in the "dielectric" test site can be observed in contrast with the more decreasing trend of oil concentrations in the reference field. The standard deviation of the oil concentration in the mesocosms was very high and ranged between 115 to 600 mg/kgds (or 5 - 40%). In column

Bio-Dielectric Soil Decontamination

339

studies a decrease in oil concentration, at a temperature of 30°C, was be found in the order of 15 and 35 mg/kg/day for soils with a diesel range organic concentration of 500 and 5000 mg/kg [6]. These results were confirmed by the laboratory test results (see Table 2). Nevertheless is obvious from Figure 8, that even an increase in oil concentration can be observed. An explanation for this could be that by heating the soil, groundwater containing oil was transported upwards, thereby increasing the oil content of the ground below the surface were the samples for the measurements were taken. Between October and November a slight decrease of oil content is found, although not very conclusive. It was also determined that the higher temperatures of the ground did not lead to higher concentrations of microorganisms in the ground compared with the reference field. Probably other factors have limited bacterial growth such as lack of oxygen and nutrition. The amount of water was not a limiting factor, during the pilot clean up the water content varied between 15 and 30%. During the test the energy consumption was measured. The average electric energy consumption during clean up was 0.6 kWh per day. The energy needed for heating the soil to a temperature of 30°C and the energy needed to compensate for the thermal losses to the surroundings, are both included in this consumption figure. For a clean up of 40 days this means a total energy consumption of 25 kWh and a specific loss of 6.5 W/m2 surface area. This heat loss is lower then expected; 20 W/m2. This value is obtained by calculating the heat loss to the surface of the soil, as given in [9], from a rectangular volume, with a higher temperature then the surrounding soil.

Conclusions The trend in oil concentration of the samples taken during the clean-up by dielectric heating does not give conclusive evidence of accelerated biodegradation, as higher oil concentrations were found after 3 weeks of cleaning. This is in contradiction with the laboratory test results, which were performed on samples of soil taken from the ground of the pilot test site. The results of this laboratory test showed clearly that the remediation rate with dielectric cleaning was a factor three higher then with biological cleaning only. An explanation could be that by heating the soil, groundwater is transported upwards, thereby increasing the oil concentrations in the test area. Therefore during bio-dielectric remediation also the oil concentrations on greater depths should be monitored. After six weeks the oil concentrations are slightly decreased. However because of the spread in results, no affirmative conclusions can be drawn. Therefore a longer period of testing seems necessary. Large temperature differences in the soil can occur due to differences in soil material. This temperature difference can be prevented by proper control of the generator on/off cycle. The heat loss is lower (6.5 W/m2) than estimated (20 W/m2); hence the extra costs of dielectric heating, on top of biological decontamination, will not exceed

340

Janssen-Mommen

EUR 12 per ton ground. Therefore bio-dielectric soil decontamination is economically interesting and offers an alternative for dielectric heating as well as biological cleaning. Summarised; bio-dielectric soil remediation seams feasible, temperature differences can be controlled, the availability of oil for biodegradation shows an increase.

Acknowledgement The work reported herein was performed in co-operation with IWACO and supported by SBNS, Colpitt and HWZ. We would like to thank Drs M.C.M. Bruijs for his valuable comments on the manuscript.

References [1] Dev H, Bridges JE, Sresty GC (1984) Decontamination of hazardous waste substances from spills an uncontrolled waste sites by radio frequency in situ heating. Hazardous Material Spills Conference 1984, pp 57-64 [2] Edelstein WA, Iben IET, Mueller OM, Uzgiris EE, Philipp HR, Roemer PB (1994) Radio frequency heating for soil remediation: Science and engineering. Environmental progress, Vol 13, no. 4, pp 247-252 [3] Hoek EE van der, Lodder P, Tiemens AH (1997). Biodiëlektrische bodemreiniging. Resultaten van de biodiëlektrische verwarming van twee heterogene bodems ter bevordering van in-situ biologische reiniging.(in Dutch), KEMA report 42023-KET/PTE 97-3000. [4] Hoek EE van der, Venhuis LP, Doelman P, Moolenaar SW (1998) Biodiëlektrische bodemreiniging. Fase I, het aantonen van versnelde afbraak door substantiële verhoging van beschikbaarheid (in Dutch). KEMA report 998550017-KPG 98-5067. [5] Hoek EE van der, Weultjes IJW, Jansen, WJL, Enoch GD (1998) Werkwijze en inrichting voor de bio-diëlektrische reiniging van verontreinigde grond (in Dutch) (Operation and method for bio-dielectric remediation of contaminated soil). Dutch patent owned by KEMA, patent number 1004114 of 02.06.1998 [6] Marley MC, Price SL, Kasevich RS, Droste EX, Acomb L, Fosbrook C, Wallace M, LeFrancois T (1999). Pilot Test at Fort Wainwright, Alaska of Radio Frequency Heating System for Enhanced Bioremediation. Abstracts and Presentation "In Situ Thermal Treatment for Remediation of DNAPLS". US EPA, December 14 and 15, 1999, Philadelphia, PA and Edison, NJ. [7] Price Sl, Kasevich RS, Johnson MA, Wiberg D, Marley MC (1999). Radio frequency heating for soil remediation. Journal of the Air and Waste Management Association. Vol 49, February, pp 136-145 [8] Schlegel HG (1988). General microbiology, Cambridge University Press, Cambridge. [9] Wong HY (1977) Heat transfer for engineers (Handbook of essential formulae and data). Longman Group limited, London

Waste Treatment under Microwave Irradiation A. Corradi1, L. Lusvarghi1, M. R. Rivasi1, C. Siligardi1, P. Veronesi1, G. Marucci2, M. Annibali2, G. Ragazzo3 1

Dept. of Materials and Environmental Engineering, University of Modena and Reggio Emilia, Via Vignolese 905, 41100 Modena, Italy 2 ENEA- ERG-SIEG, Centro Ricerche Casaccia, Via Anguillarese 301, 00060 Roma, Italy 3 ITREC – Centro Ricerche ENEA Trisaia, Strada Statale Ionica 106, 75026 Rotondella (Mt), Italy

Abstract Waste treatments to induce stabilization or recovery of waste materials is an important part of modern research efforts. Microwave processing has proved to be a powerful tool to convey energy exactly where it is needed by the process, as well as to allow operation in peculiar environments, even in remote-controlled modality. The present work is a summary of three years of joint research between Modena’s University and ENEA investigation regarding microwave assisted thermal treatments lead on wastes of different nature, performed at the 2.45 GHz ISM frequency. The heat-treatments regarded the inertisation and vitrification of asbestos and the stabilization in a glassy matrix of a multi-oxide mixture simulating nuclear waste. Either single-mode or multi-mode applicators were used during the preliminary tests, and hybrid heating was exploited, if necessary. In some cases, additives ensuring a better microwave coupling were used to improve the process speed or the overall yield.

Introduction Many industrial activities involve the creation and subsequent disposal of waste, which represents a noticeable cost in terms of money and pollution. Moreover, sometimes waste materials are hazardous as well, i.e. materials containing asbestos or byproducts of nuclear plant. In this case, regulatory procedures are particularly restrictive, to guarantee the safety of the operators, and the choice of an inertization process becomes a compromise between safety issues, energetic evaluations and economical aspects. Thus, the waste treatment has to be evaluated

342

Corradi

not only considering the process itself, but also bearing in mind the possible destination of the final product. The disposal of waste materials is now becoming a very serious problem, since in recent years the great increase of their production was not matched by a corresponding rise in the number of authorised dumps. Moreover, the existing regulation does not always allow all kind of waste material to be recycled, especially if harmful or hazardous materials are involved [1]. But considering the present year production of wastes like ashes, or the wide spread presence on the territory of asbestos containing materials, it seems impossible to handle this environmental issue only by disposal in dumps. To face this situation, it is necessary to study and develop alternative ways to treat and re-use the components of waste materials, for instance converting them in secondary raw materials and, if possible, restoring them to accomplish the task they were initially meant for. Waste, even if originated by the same manufacturing process, and thus belonging to the same category (i.e. ashes, nuclear waste, asbestos containing materials, etc.), can be regarded as a multi-component material having a wide range of compositions, and usually it is the presence of only some of these components that makes all the mixture a product to be disposed of. Thus, a process allowing selective treatment of the "unwanted" portion of the waste, and to do this volumetrically, could represent an enormous advantage in terms of time and money, especially as far as materials presenting low thermal conductivity are concerned [2]. Microwaves can be an interesting candidate to fulfill the need for this kind of processes, and this is particularly true if the matrix of the waste materials exhibits dielectric properties significantly different from those of the unwanted components.

Asbestos inertization and vitrification The samples investigated belong to panels used for building insulation, made of serpentine asbestos containing a small amount of an organic binder [3]. The thermal behaviour of the samples was determined by means of thermal analysis (DTA, Netzsch, STA 409) on the as-received samples. Three main groups of peaks can be individuated: x The first pair of exothermic events, at T1 = 288°C and T2 = 326°C, corresponds to the decomposition and destruction of the organic fabric used to pack the mineral; x The second pair are endothermic events, at T3 = 648°C and T4 = 680°C, which corresponds to the release of the hydroxyls groups from the serpentine mineral; x The last exothermic peak, at T5 = 803°C, indicates a change of the crystalline structure.

Waste treatment under microwave irradiation

343

Fig. 1. A) SEM micrograph of the as-received asbestos sample; B) thermal analysis of an as-received sample of serpentine asbestos

Thus, the treatments must reach at least 803°C to attain major crystalline structure changes [4]. To speed up the microwave treatment, a very small amount of a microwave-sensitive activator mixture was added to the sample, which was irradiated with 1000 W power at 2.45 GHz in a multimode cavity (Radatherm VPMS, Variable Power Microwave System[5]) for 13 minutes. Different percentages of additives were used, ranging form 0.3 to 3 wt%, covering the surface of rectangular and circular samples of 0.015 m2 area and thickness varying from 0.01 to 0.07 m, the latter obtained by superposition of multiple layers, as in the normal building practice. The temperature was measured by a Pt-30%Rh/Pt-6%Rh type B thermocouple positioned between two layers of asbestos. After the treatment, the sample surface looked different from the as-received one, even though maintaining its original shape and size. Due to the kind of heating, the sample was not homogenous, being the sides cooler than the inside. However, the use of alumina fibers insulation allowed the temperature not to drop drastically in that region. XRD analyses showed the formation of a newly formed crystalline phase, forsterite (Mg2SiO4; file ICDD n° 34 - 189), suggesting an effective sample inertization, since forsterite is an harmless magnesium silicate. Moreover, the fibrous structure of the asbestos, too, was affected by the thermal treatment, as shown in Figure 2. The forsterite can be already considered a secondary raw material, and a source for magnesium, to be used, after milling, in the production of traditional ceramic bodies like tiles and bricks. This process represents a good alternative to conventional heating inertization methods, which suffer from the very low thermal conductivity of asbestos, and, compared to the treatments meant to immobilize asbestos fibers in polymeric matrix, it can be considered an ultimate solution. At the moment, a thorough costinvestigation can not be conducted, since the microwave assisted inertization process is not fully optimized, nor the developed equipment is. If the thermal treatment is prolonged further on, reaching higher temperatures, a more drastic modification in the crystalline structure occurs, and the sample loses its shape, too. In fact, during the heat-treatment, the sample became a viscous liquid and assumed a black colour. Many millimetre-sized open pores are present on the surface, probably due to gas evolution in the molten phase. This treatment

344

Corradi

transforms the original whole original asbestos sample into pure forsterite [5]. This treatment at higher temperature is an even more drastic way to eliminate asbestos fibrous structure, as it can be observed in Figure 3. These preliminary tests, aimed at verifying the feasibility of the microwave assisted inertization and/or vitrification of asbestos, allowed the determination of the proper conditions of power, temperature, time and additives to be used on a larger microwave open applicator prototype installed in the ITREC ENEA laboratories. The prototype is equipped with three 6 kW water cooled magnetrons, giving a total maximum power output of 18 kW at 2.45 GHz. The output is controllable, independently for each magnetron, in 300 W steps. Three optional flexible waveguides constitute the transmission line, feeding an open end rectangular applicator. The applicator is mounted on a translating support, and the support itself can be installed on a remote controlled unit on wheels. This equipment allows the operator to perform the microwave treatments from a remote position, thus avoiding the contact with harmful materials and possible exposure to microwaves. However, the microwave leakage from the applicator is reduced by the presence of a series of shielding grids surrounding the applicator and leaning on the horizontal or vertical surface to be treated. An outer layer of microwave absorbing material offers a further attenuation. Figure 4 shows the prototype in the operating conditions, after removing the protective grids. The prototype, used at maximum power, allowed the complete inertization or vitrification in 1 - 5 minutes, depending on the presence of additives, for a single layer of material, 100 x 150 x 10 mm in size. = serpentine = forsterite

counts

Asbestos

m.w. Heating 5

A

25

45

65

B

Fig. 2. A) SEM micrograph of the microwave-treated sample B) comparison between the XRD pattern of the sample before (asbestos) and after (m.w.heating) the microwaveassisted inertization process

Waste treatment under microwave irradiation

345

Fig. 3. A) SEM micrograph of the microwave-vitrified sample B) comparison between the XRD pattern of the sample before (a) and after (b) the microwave heat treatment

Fig. 4. 18 kW open end applicator heating an asbestos panel- left: single large asbestos layer on alumina fibers; right: inertization of the asbestos layer in progress (applicator shielding removed, equipment remote controlled)

Glass encapsulation of simulated nuclear waste Nuclear waste encapsulation in a glassy matrix is a well known application, but the conventional requires long times to ensure glass melting and homogenization. The waste to be inserted in the glassy matrix usually is a mixture of metal oxides which are added as powders. The glass, having a composition suitable to prevent leaching of the trapped oxides, is provided under the form of spheres of 1 mm diameter [6]. The different particle size of the matrix and of the powders does not provide an homogenous mixture, and milling of the glass spheres is considered not cost-effective.

346

Corradi

The tests were performed on a mixture of oxides simulating the real composition and chemical behaviour of the radioactive waste. The composition of the mixture, in percentage referred to the glass weight, is reported on Table 1, where the elements Cs, Th and U are omitted. Table 1. Composition of the metal oxides mixture Oxide Mn Mo Zr Pr Ni La Ce Co(II) Te St Ba Nd

wt % 0.72 1.7 1.65 0.44 0.33 0.9 0.93 0.12 0.23 0.33 0.6 1.59

Preliminary tests involved the choice of a suitable refractory support, able to withstand rapid heating but almost transparent to microwaves. Cordierite crucibles were suitable for the application, while alumina and porcelain ones presented thermal shock resistance problems. The selected crucibles, having a cylindrical shape ( inner diameter 25 mm, height 50 mm), allowed with ease the retention of 200 g of the starting mixture. Further tests were performed to determine the heating behaviour of the glassy matrix. It was observed, using either multi mode or single mode applicators and a directional coupler, that the absorption of microwaves is particularly poor at low temperatures, while, as expected, it increases once the glassy transition temperature is reached [7]. Thus, heating tests performed on 200 g glass samples in cordierite crucible, using 500 and 1000 W power, lasted up to 40 minutes due to the difficulties in raising the temperature in the early stages. The addition of the oxide mixture did not cause a significant increase in the heating behaviour of the whole system, but introduced strong perturbation on the heating process [8], due to arcing or localized superheating. Temperature differences between the central melted region of the system and the outer regions were as high as 450°C, as measured by quartz optical fibres. By modulating the power output of the microwave furnaces, it was possible to melt the glass and allow the oxide mixture to be trapped in the matrix. However, the macroscopical temperature differences observed during the heating of the system had their counterparts in the melt, presenting regions having different oxide content, which made the usual leaching tests almost meaningless.

Waste treatment under microwave irradiation

347

The presence of strong microwave couplers, added to the system as powers, sped up the heating process and allowed to slightly compensate the temperature differences, but interesting results were obtained only after pre-heating the glass and the mixture at 600°C. In this case, more than 85% of the 1000 W forward power is absorbed by the system, which melts after 6 minutes. Lowering the forward power, it is possible to keep the temperature constant and allow the melt to homogenize for the time required [7]. Leaching tests conducted on the more homogenous samples, however, showed that only partial encapsulation could be achieved, probably due to the persistence of segregation regions, near to the bottom of the edged crucible walls. The experimental results suggest the use of hybrid systems, or, as an alternative, to use the glass melt as a heat generator to raise the temperature of new glass and mixture to add to the melt, thus allowing a fairly high power transfer from the microwave generator to the source, and the proper melt homogenization.

Conclusions Nuclear waste and asbestos containing materials have been successfully treated using microwave heating. The processes studied sometimes are in direct competition with analogous conventional treatments, but the latter are characterized by long processing times, due to the low thermal conductivity of the treated waste materials. Microwave volumetric heating, in fact, was used to rapidly raise the temperature of asbestos, or glasses. To perform some heat treatments, a dedicated equipment has been developed and constructed, for instance to allow remote-controlled operation in potentially harmful environments or to maximize the power transfer and allowing measurements to be taken or the cavity environment control. Finally, most of the materials resulting from the microwave assisted thermal treatments can be re-used as they are or recycled in different applications: as an example asbestos inertization lead to forsterite-rich raw materials suitable for the use in the ceramic industry.

348

Corradi

References [1] Oda S.J., Microwave remediation of hazardous waste: a review, in Microwave processing of materials III, vol 269, 453-464, MRS, 1992. [2] Metaxas A.C. & Meredith R.J. In: Industrial microwave heating, Peter Peregrinus Ltd, London 1993, pp. 26-49. [3] C. Leonelli, C. Siligardi, G.C. Pellacani, G. Gherardi, G. Marucci, “Inertization process of asbestos containing materials with microwave energy” Sept. 1998, Italian Patent, IT1302348 [4] Marucci G., Annibali M., Carboni G., Gherardi G., Ragazzo G., Siligardi C., Veronesi P., Lusvarghi L., Rivasi M.R., "Characterization of microwave inertized asbestos containing materials", Materials Engineering Monograph, 115-125, Mucchi editore, Modena (IT), 2000. [5] Veronesi P.,Leonelli C., Corradi A.B., Annibali M., "Variable power microwave system for high temperature materials treatment", Materials Engineering Monograph, 4954, Mucchi editore, Modena (IT), 2000. [6] Wicks G.G., Nuclear waste glasses, Treatise on materials science and technology, Glass IV, vol. 26, 57-118, Academic Press Inc., 1985. [7] Kolberg U., Roemer H., Microwave heating of glass, in Microwaves: theory and application in material processing V, edited by Clark D.E., Binner J,G,P., Lewis D.A., The American Ceramic Society Westerville, Ohio (USA), 527-533, Ceramic Transactions III, 2001. [8] Roussy G, Bennani A., Thiebaut J.M., Temperature runaway of microwave irradiated materials, Journal of applied physics, 62 [4], 1167-1170, 1987.

Environmental Aspects of Microwave Heating in Polyelectrolyte Synthesis E. Mateescu1, G. Craciun1, D. Martin1, D. Ighigeanu1, M. Radoiu1, I. Calinescu2 and H. Iovu2 1

National Institute for Lasers, Plasma and Radiation Physics, Electron Accelerators Laboratory, #111 Atomistilor St., P.O.Box-MG36, 76900 Bucharest, Romania 2 Polytechnic University of Bucharest, Industrial Chemistry Faculty, #149 Calea Victoriei St., R-71102 Bucharest, Romania

Introduction Due to the ever growing ecological problems we are facing in our country, the acrylamide copolymers radiation technology was first developed on a semiindustrial scale with gamma rays (GR) and then with electron beam (EB) sources at the Institute of Atomic Physics (IAP)– Bucharest [1, 2, 3]. The acrylamide copolymers are used as flocculation agents for treating a large variety of aqueous suspensions in metallurgical industry, potable water treatment (polyacrylamide of low degree of hydrolysis and with very low residual monomer contents) and as coagulation aids for industrial wastewater treatment [4, 5, 6, 7]. For an industrial scale production, the problem of reducing the electrical energy consumption as well as the technology's cost is especially important. In view of this argument we have attracted during the last year to the microwave (MW) energy use, which is less expensive, for the polyelectrolytes preparation. The analysis of the experimental results concerning the acrylamide and acrylic acid copolymer (PA type polyelectrolyte) parameters demonstrated that MW heating always produces high water solubility (low values for Huggins constant, kH = 0.015 - 0.5) but median values for intrinsic viscosity (Kintr = 3 dl/g - 7 dl/g) and conversion coefficient (CC = 75% - 95%) while ionizing radiation gives higher Kintr and CC values. Thus, our interest was focused upon the methods, which lead to the optimization of Kinr (or molecular weight Mw), and CC of the polyelectrolytes obtained by microwave heating. High intrinsic viscosity values are desired because they provide good flocculation of the suspended particles leading to small sedimentation time and small size of the sediment volume put down in the treated wastewater [9].

350

Mateescu

Methods and materials For the preparation of acrylamide-acrylic acid copolymers our interest was focused on the basic optimization of the characteristics involved in wastewater treatment, such as, conversion coefficient (CC), intrinsic viscosity (Kintr) or average Mw and linearity coefficient given by Huggins' constant (kH). The values of Kintr, Mw and kH are established from the following relations: Kred = Kintr + CtgD

(1)

Kred = KspC-1 = (Krel -1) C-1

(2)

tgD = '(Kred) '-1(C)

(3)

kH = tgD(Kintr)-2

(4)

Kintr = K (Mw)

(5)

(Mark-Houwink-Sakurade relation for a polymer in aqueous solution of 1 N NaNO3 at 30°C, pH = 7) where: C = polymer concentration (%) in very dilute solutions (under 0.1%) of 1 N NaNO3 at 30°C and pH = 7; Kred = reduced viscosity; Ksp = specific viscosity; D is the angle between the plot of Kred against C and a parallel axis to the C axis; Krel = relative viscosity established from appropriate physical measurements with Hoppler viscometer BH-2 [8]. For acrylamide-acrylic acid copolymers: K = 1.34 10-3; a = 0.57 for Kintr given by dl g-1 and Mw by atomic mass units (a.m.u.). The acrylamide-acrylic acid copolymers exhibit good water solubility only for kH < 0.5. Preparation of PA polyelectrolytes is based on copolymerization of the aqueous solutions containing appropriate mixtures of acrylamide and acrylic acid monomers and certain chemical agents, such as initiators (I) for the monomer conversion optimization, chain transfer agents (CTA) for the cross-linked structure diminution and complexing agents (CA) for the impurities inhibition. The PA polyelectrolyte characteristics may be influenced by the following factors: chemical composition of the solutions to be irradiated and irradiation conditions (quantity of absorbed energy per unit mass). In order to simultaneously obtain high Kintr, high CC and good water solubility of PA type polyelectrolytes obtained by microwave heating, special experiments have been performed. The experiments were carried out using the following conditions: 550 W microwave power, samples of 75 ml aqueous solution of 40% (36% acrylamide and 4% acrylic acid) total monomer concentration (TMC), 0.25%-1.5% initiator concentration (solution of 2% sodium peroxide), 0.05%-0.3% CTA concentration (solution of 10% sodium formiate) and 0.1% CA concentration (solution of 5% ethylene diamine tetra acetic acid).

Environmental Aspects of Microwave Heating in Polyelectrolyte Synthesis

351

Results and discussions

Intrinsic viscosity, Kintr (dl/g)

9.5

90

9.0

80

8.5

TMC=40% CTA=0.2% NaCl=0%

8.0 7.5

70 60 50

7.0

40

6.5

30

6.0

20

5.5 5.0 84

10

86

88

90

92

94

96

98

Conversion coefficient, CC (%)

From experimental observations, it was found that CC increases gradually with I concentration rise (Fig. 1a). Always, Kintr exhibits a maximum value associated with a minimum value for kH, for all irradiation modes (Fig. 1b). Kintr(I=0.25%) Kintr(I=0.5%) Kintr(I=0.7%) Kintr(I=1%) Kintr(I=1.5%)

CC (I=0.25%) CC (I=0.5%) CC (I=0.7%) CC (I=1%) CC (I= 1.5%)

100 102

Microwave irradiation time (s)

1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0.2

10 8 6 4 2

0.4

0.6

0.8

1.0

1.2

1.4

0 1.6

Intrinsic viscosity, Kintr(dl/g)

Huggins constant, kH

a) Kintr(Tir=85s) Kintr(Tir=90s) Kintr(Tir=95s) Kintr(Tir=100s)

kH (Tir=85s) kH(Tir=90s) kH(Tir=95s) kh(Tir=100) TMC=40% CTA=0.2% NaCl=0

Initiator concentration (%)

b) Fig. 1. a) The effects of microwave irradiation time and initiator concentration upon Kintr and CC for samples without NaCl, b) Huggins constant and intrinsic viscosity versus microwave irradiation time

The rise of the CTA concentration overcomes the cross-linked effect and provides good water solubility but leads to the lower values for Kintr. CC suffers little change with CTA concentration increasing. CC increases lightly with microwave irradiation time, reaches a maximum value and then lightly decreases.

Mateescu

100

Intrinsic viscosity, Kintr (dl/g)

10 TMC=40% CTA=0.2% I=0.5%

9 8

80 60 40

7 Kintr (NaCl=0) Kintr (NaCl=8%)

6

20

CC (NaCl=0) CC (NaCl=8%)

5

90

95

100

105

110

115

120

0

Conversion coefficient, CC (%)

352

Microwave irradiation time (s) Fig. 2. The effects of NaCl on Kintr and CC

Intrinsic viscosity, Kintr (dl/g)

9

80

8

60 Kintr (I=0.5%) Kintr (I=0.7%) Kintr (I=1%)

7

TMC=40% 6 CTA=0.2%

95

20

CC (I=0.5%) CC (I=0.7%) CC (I=1%)

NaCl=8% 5 90

40

100

105

110

115

120

0

Conversion coefficient, CC (%)

100

10

Microwave irradiation time (s) Fig. 3. The effects of microwave irradiation time and initiator concentration upon Kintr and CC for samples with NaCl

The additional use of NaCl, in the range of 4% - 8%, to the samples preparation, increases markedly CC and Kintr (Fig. 2 and Fig. 3). CC and Vint. of the samples containing NaCl exhibit higher values than the samples without NaCl (Fig. 2). Kint increase with I concentration, reaches a maximum value and then decreases (Fig. 3). The value of I concentration at which Kintr exhibits an optimum value de-

Environmental Aspects of Microwave Heating in Polyelectrolyte Synthesis

353

pend on CTA concentration, NaCl concentration and irradiation conditions (microwave power level and microwave irradiation time). Kintr decreases gradually with microwave irradiation time, reaches a minimum value afterwards lightly increases, reaches a maximum value and then lightly decreases (Fig. 2 and Fig. 3). Irradiation conditions have a markedly effects upon polyelectrolyte characteristics. Three microwave irradiation time ranges were identified for 550 W microwave power and 75 ml samples. In the first irradiation time range (80 s - 90 s), the polymerization process is incomplete under microwave irradiation and continues afterwards for un uncontrollable time duration when the samples are taken away from the microwave oven. In this time range CC and Kintr gradually increase, reach an optimum value and then markedly decrease. Also, in this time range the samples exhibit the best values but uncontrollable. Our present research is focused to the methods of controlling the polymerization process, which develops after irradiation time period. In the second time range (90 s - 105 s), the polymerization process begins violent in the microwave oven and continues, for a short time period, after that the samples are removed from the microwave oven. In the last irradiation time period, the polymerization process begins and finishes in the microwave oven. The samples obtained in this time range exhibits reproductive and acceptable characteristics when certain chemical compositions are used. According to the above mentioned experimental results the main conclusion is that the additional use of NaCl markedly rises CC (near 99%) and Kintr (>8). We notice that, Huggins constant remains small (0.015 - 0.5) for all samples containing NaCl (good water solubility). Further studies are necessary to establish the optimum operational condition for the microwave induced polymerization with high efficiency, low costs and good polyelectrolyte characteristics. The acrylamide-acrylic acid copolymers can be used in aqueous solutions in a concentration of 0.05 - 0.1%. In certain cases (very contamined wastewaters) these copolymers are used together with Al2(SO4)3 or FeSO4 but the consumption of these classical electrolytes may be reduced to 50% - 75%. Thus, for each liter of wastewater from a slaughterhouse (“INCAF”- Ploiesti, Romania) we used 4 ml acrylamide-acrylic acid copolymer solution of 0.1% + 2 ml Al2(SO4)3 solution of 10% instead of classical treatment with 4 ml Al2(SO4)3 solution of 10%. Also, for each liter of wastewater from a vegetable oil plant (“SOLARIS” – Bucharest, Romania) we used 4 ml acrylamide-acrylic acid copolymer solution of 0.1% + 4 ml Al2(SO4)3 solution of 20% instead of classical treatment with 8 ml Al2(SO4)3 solution of 20%. The fatty substances, matters in suspension, chemical oxygen demand (COD) and biological oxygen demand over 5 days (BOD5) were reduced, in comparison with classical treatment, as is shown in Table 1. Also, sedimentation time was around four times smaller and sediment volume was 60% smaller than values obtained by using Al2(SO4)3.

354

Mateescu

Table 1. Comparative results obtained by using classical treatment and combined treatment for wastewaters from the slaughterhouse “INCAF”-Ploiesti, Romania (A) and for wastewaters from the vegetable oil plant “SOLARIS”-Bucharest, Romania (B). Initial value Quality indicator Matters in suspension Biological oxygen demand Chemical oxygen demand Fatty maters

Unit

Allowed value

Final value with classical treatment

Final value with combined treatment

A

B

A

B

A

B

mg l-1

300

766

1020

496

971

112

50

mgO2 l-1

300

1500

2300

1050

1400

205

298

mgO2 l-1

500

479.7

4664

321.3

2889

100.7

395

mg l-1

20

332

3497

275

82

32

8

Conclusions According to the above mentioned experimental results the main conclusion is that the additional use of NaCl markedly rises CC (near 99%) and Kintr (>8) while Huggins constant remains small (0.015 - 0.5) for all samples containing NaCl (good water solubility). Further studies are necessary to establish the optimum operational condition for the microwave induced polymerization with high efficiency, low costs and good polyelectrolyte characteristics. The estimation of processing rates for a microwave installation of 2 kW used for MW heating is about 30 kg/hour. This production could satisfy the PA required quantity per month for a wastewater treatment plant of 250 l/s capacity.

Environmental Aspects of Microwave Heating in Polyelectrolyte Synthesis

355

References [1] Martin, D., Fiti, M., Dragusin, M., Radu, A., Cojocaru, G. (1995) Low-power-high energy electron accelerators for irradiation in polymeric systems. Radiat. Phys. Chem. Vol. 45. No.4, 615-621 (1995). [2] Martin, D., Dragusin, M., Radoiu, M., Radu, A., Oproiu, C., Cojocaru, G. (1996) Polymers for waste water treatment. Progr. Colloid Polym. Sci. 102, 147-151(1996). [3] Martin, D., Dragusin, M., Radu, A., Radoiu, M., Oproiu, C., Cojocaru, G., Marghitu, S. (1996) IAP linacs in applied research. Nucl. Instr.and Meth. in Phys.Res. B 113, 106-109 (1996). [4] Narkis, N., Ghattas, B., Rebhun, M., Rubin, A. J. (1991) The mechanism of Flocculation with Aluminium Salts in Combination with Polymeric Flocculants as Flocculants Aids. Water Supply, Vol. 9, 37-44 (1991). [5] Fetting, J., Ratnaweera, H., Odegaard H. (1991) Synthetic Organic Polymers as Primary Coagulants in Wastewater Treatment, Water Supply, Vol. 9, 19-26 (1991). [6] Selvapathy, P., Reddy, M. J. (1992) Effects of Polyelectrolytes on Turbidity Removal. Water Supply, Vol. 10, No. 4, 175-178 (1992). [7] Edzwald, J. K (1993) Coagulation in Drinking Water Treatment; Particles, Organics, and Coagulants, Water Science &Technology, Vol. 27, No. 11, 21-35 (1993) [8] Flory, J. P. (1978) Determination of Molecular Weights, In Principles of Polymer Chemistry, Cornell University Press, Ithaca and London, 267-`314 91978). [9] McCormick, Charles L., Hester, R. D., Morgan, S. E., Safieddine, A. M. (1990) WaterSoluble Copolymers 30. Effects of Molecular Structure on Drag Reduction Efficiency. Macromolecules 23.8, 2124-2131 (1990).

Role of Microwave Radiation on Radiopharmaceuticals Preparations Enrico Gattavecchia, Elida Ferri, Biagio Esposito, Alberto Breccia Complex Unit of the Institutes of Chemical, Radiochemical and Metallurgic Sciences, University of Bologna, Bologna, Italy

Introduction The activating effects of microwave (MW) application on several organic reactions were repeatedly demonstrated since the early reports in 1986 and nowadays they are definitively recognized by the scientific community. Some of these reactions were very difficult or even impossible to perform by using traditional procedures [12]. The majority of these interesting results were undoubtedly ascribed to the particular mechanisms by which MW treatment increases the temperature of absorbing materials, i.e., the thermal effect. Nevertheless, at least in some cases the debate on the involvement of the so-called ”non thermal” effects is still opened, since the solely heating effect cannot explain all the experimental results, for example unexpected yields of reaction or changes in the reaction kinetics. Our use of MW in radiochemical research is aimed on exploitation of the positive effect of microwave radiation to develop alternative synthesis methods for radio-pharmaceuticals or, in general, radio-labeled molecules. The experimental research is backed up by theoretical studies in the attempt to define which effects accounted to the use of microwave radiation were actually involved in the reactions under study. Different possible mechanisms affecting the chemical systems were assumed: -

high energy production in particular areas (hot spot formation) [1] electron spin alignment in radical structures as studied by E.P.R. [3] catalytic effects as a consequence of dipole formation [6].

The increase of the reaction rate can be of fundamental importance in radiochemistry when it leads to a reduction of the labeling time and to an increase of specific activity of the end products, especially in the case of the short-life isotopes used in radiodiagnosis. In any case it is useful to reduce the exposure time to radioactive sources.

360

Gattavecchia

In fact, the main application of MW treatment in this field has been in the production of compounds used in Positron Emission Tomography (PET), labeled by positron emitting nuclides with extremely short half-life [8, 10], or in the preparation of radio-derivatives that must be immediately available when necessary [11]. Even few minutes saving in the preparation time of these radio-drugs can be of fundamental importance [7]. Different reactions were investigated with respects to improvements by MWtreatment: 99mTc-MAG3 synthesis, rifamicine labelling under MW irradiation, indomethacin and nitroimidazoles hydrolysis reactions. 99mTc is the most widely used nuclide to produce diagnostic radiopharmaceuticals, thanks to its ideal characteristics for in vivo use. It is easily available, with a half-life of 6 hours and a decay process which occurs through an isomeric transition with emission, at the 89%, of a J radiation, (EJ=0.140 MeV).

Experimental 99m

Tc-MAG3 and 99mTc-labelled Rifamicine

The mercaptoacetyltriglycine (MAG3), Figure 1, labeled by 99mTc through the formation of a chelation complex, is very useful in diagnostic kidney scintiscan. The reagents for the complex synthesis are already available as a commercial kit [5]. The suggested procedure includes heating of the mixture in boiling water bath, at 100°C. The labeling mechanism is a trans-chelation; the complex formation require the reduction of pertechnetate by a reducing agent, generally SnCl2. During the first step the reduced 99mTc makes a complex with tartrate, followed by the transchelation during boiling treatment, to form the complex with MAG3. The MAG3-complex is shown in Figure 1.

Fig. 1. Left: Mercaptoacetyltriglycine, (MAG3); right: MAG3-TcO complex

This complex is more stable as compared to the tartrate, but its formation occurs with a slower kinetics. The final injectable solution obtained by this method may contain residual Sn2+ and various labeled impurities. Such impurities are assumed to be fragments of the

Role of Microwave Radiation on Radiopharmaceuticals Preparations

361

MAG3 molecule produced during the boiling treatment. They decrease the labeling yield, and, even worse, make the solution unusable when their amount is higher than 5%. In fact, when the radiochemical purity is lower then 95%, the product cannot be used in vivo because the presence, among others, of a lipophylic component that is absorbed through the liver and excreted by the gall bladder. Another part of these impurities is absorbed by the kidneys, unfortunately, because the behavior of these fragments is completely different from the main product, a less clear scintiscan is obtained [2] In order to improve the results of the transchelation reaction, which apparently is very sensitive to heating, direct MW heating (MW oven at 2450 MHz frequency and 850 W power) was employed and the results were compared with results of the traditional procedure, using a boiling water bath. The reagents of a MAG3 commercial kit (Mallinckroot medical, B.Y., Petten, Holland) were employed, and aliquots were heated at various times, 5 - 40 min, in a boiling water bath. The boiling time suggested in the instructions was 10 min. Samples from the same initial kit solution were directly irradiated in the MW oven at 2450 MHz frequency and 850 watt power, for 30, 90 and 210 sec. In both cases the labelled products were tested by radiochromatography. Rifamicine is an antibiotic molecule with a loop forming structure, theoretically able to complex ions. For this reason the chosen radionuclide was again 99mTc. A reduced form, rifamicine SV, was also used. The labeling of this molecule was requested to study its pharmacokinetics in vivo after oral administration, in particular its absorption at the intestinal level. Rifamicine was dissolved in a watermethanol solution (1:2 v/v). Again reactions were carried out in a water bath as well as by MW-direct irradiation. In both cases a temperature of 60 - 70°C was applied. SnCl2 was added as reducing agent and the presence of labeled products was tested by radiochromatography. Hydrolysis reactions Since many labeling procedures require a hydrolysis step, the influence of direct MW-irradiation of the reaction mixture on the kinetics of some well known hydrolysis reactions, such that of indomethacin and of the NO2 group of various nitroimidazoles was studied. The hydrolysis rate of all compounds was evaluated spectrophotometrically. The temperatures reached during the microwave treatment was meausred by a thermocouple and calibrated by using capillaries filled with substancies of known and sharp melting point. The Arrhenius activation energy, Ea values were calculated and compared for reactions with and without direct microwave irradiation. The experiments were performed in a parallel way, in a temperature range between 50 and 80°C. Misonidazole and Metronidazole solutions were set at pH 12 and 13, respectively. Indomethacin, due to the lower stability of the solution was set at pH 9. The rate constants of the reactions were used to prepare the Arrhenius plots and to evaluate the activation energy and the preexponential factor K0.

362

Gattavecchia

Results

99m

Tc-MAG3

As shown in Figure 2, MAG3-TcO complex formation is accelerated upon MW-irradiation as compared to heat treatment in a boiling water bath. The microwave treatment reduces the time for a complete labeling: 3 min of MW treatment instead of the 10 min of boiling water bath.

A

COUNTS

800

400

0 0

20

0

20

RF x 100

60

80

100

B

COUNTS

400

200

0 -20

80

100

RF x 100

Fig. 2. Radiochromatograms of MAG3 + TcO4- -mixture. A: after 2 min of treatment in boiling water bath. B: after 90 s under MW. Peak at RF = 0 is the reduced technetium; RF = 80 MAG3-TcO complex, RF = 95 [TcO4 ]-.

Moreover, the complete labeling, under microwave treatment, was reached at a temperature at least 20°C lower in comparison with the traditional procedure. Both these differences are surely decisive in reducing the probability of MAG3-SH degradation and the consequent formation of unwanted labeled by-products. All radiochromatograms of MW treated samples show a single peak corresponding to

Role of Microwave Radiation on Radiopharmaceuticals Preparations

363

the desired complex. A radiochemical purity higher than 99% was reached after 3 min of irradiation. In samples treated by the traditional procedure labeled impurities were found in some cases at very high levels (45%). Thermal decomposition of the MAG3 itself was investigated by HPLC-MS on untreated samples of MAG3 and on samples after heat treatment: 40 min in boiling water bath and after 3.5 min of MW irradiation. From the untreated MAG3 only the intact, deprotonated pseudomolecular ion [M-H]- is detected. After heating, in addition to the expected ions: [M-H]-; at m/z 365.2 MAG3, at m/z 262,0 MAG3SH and from its acetate adduct at m/z 321.6, new ions were detected at m/z 211.6, 231.1, 253.2, 255.0, 283.6 and 342.9, respectively. According to a previous identification of MAG3-disulfide as a possible by-product originated from oxidation of the S-unprotected ligand during the kit preparation [9], we could identify these ions as sodium and sodium acetate adducts of the double charged [M-2]2- ion at m/z 283.6 and 342.9, respectively, generated from MAG3-disulfide [Mw= 524.0]. Similarly, ions at 211.6 and 253.2 could be identified as sodium and sodium + acetonitrile adducts of the pseudomolecular [M-H]- ion. Peaks corresponding to m/z 231.1 and 255.0 were not assigned. Opposite to these numerous products, ESMS analysis of MAG3 sample treated with MW irradiation showed only the expected pseudomolecular ion from MAG3-SH.

99m

Tc-labelled Rifamicine

The results of labeling experiments on rifamicine are summarized in Figure 3. With freshly prepared 99mTc by heating the reaction mixture in a water bath labeling almost failed. The yield was extremely low, reaching not more than 20%. When the experiment was repeated employing 99mTc prepared 24 - 48 h before use, surprising results were obtained: the older the 99mTc, the higher the labeling yield, based on the total decay time. This is opposite to what is expected theoretically, with the better yield achievable by using 99mTc with high activity, it means freshly prepared. Under MW irradiation very good labeling yield results were obtained, independently of the activity, e.g., fresh or old 99mTc used. The medium yield was at least 20% higher than the best results obtained by using traditional heating. The excellent labeling results achieved upon direct MW-irradiation of the reaction mixture are further supported by the fact, that the structure of rifamicine was unchanged after treatment and that the complex formed by this way remained stable for the time required to perform the pharmacokinetics studies. As already described in the introduction, different hypotheses can be taken into account in order to explain these surprising experimental results.

364

Gattavecchia

B

A 140

120

120

Tc-Red

Tc-L

100

COUNTS

COUNTS

100 80

Tc-L

60 40

80

Tc-RED

60 40

20

20

0 0

0

20

40

60

RF x 100

80

-20

100

20

0

40

60

RF x 100

80

100

D

C

400 160

Tc-L

140

Tc-L

300

COUNTS

100

COUNTS

120

Tc-RED

80 60 40

200

100

Tc-RED

20

0

0 -20

0

20

40

60

80

100

0

20

RF x 100

40

60

80

100

RF x 100

Fig. 3. Radiochromatograms of Rifamicine complex (99m Tc-L) yield using fresh (A) and 48 h old (B) 99m Tc. C: after 1 min under MW; D:after 3 min, using fresh 99m Tc.

The reduction of pertechnetate by MW irradiation was confirmed by the following experiment: adding to a MW irradiated TcO4- solution a spot of fresh pertechnetate and performing the usual TLC, it is possible to distinguish two peaks, as shown in Figure 4. The two overlapped peaks correspond to the original TcO4-, the single peak on the left to a reduced form, probably the TcO2+. 400

COUNTS

300

200

100

0 0

20

40

60

80

100

RF x 100

Fig. 4. Radiochromatogram of a MW treated pertechnetate solution after addition of a spot of untreated TcO4- . The peak at Rf =80 is a reduced form, the two peaks on the right are the residual pertechnetate in the irradiated solution and the spot of fresh, untreated, solution.

Role of Microwave Radiation on Radiopharmaceuticals Preparations

365

The hydrolysis reaction of indomethacin and that of the NO2 group of various nitroimidazoles are simple pseudo-first order reactions, because of the excess of one of the reagents, the H2O. This condition simplified the theoretical kinetics models. Absorbance spectra, repeated at various time intervals and perfectly overlapped have shown that no changes in the reaction mechanism occurred under microwave irradiation, both for nitroimidazoles and indomethacin. The kinetic data are shown in Figure 5. On the contrary, the reaction rate is increased under irradiation with microwaves in the case of indometacine, the hydrolysis rate can be even 50 time higher than using traditional heating, respectively, as shown in Figure 6. The increase of reaction rate for this compound is inversely proportional to the reaction temperature, whereas for nitroimidazoles the reaction rate increases with increasing temperature.

Fig. 5. Hydrolysis kinetics of Metronidazole (left) and Indomethacin (right) as a function of heating method.

Fig. 6. Arrhenius plots of Metronidazole (left) and Indomethacine (right) hydrolysis.

It is clear from the above reported graphs that MW-irradiation causes a significant change in the activation energy of the hydrolysis reaction of indomethacin.

366

Gattavecchia

Discussion Different hypotheses are taken into account to explain the influence of the heating method on the labeling results with 99mTc in a trans-chalcogenation reaction and in complex formation with rifamicine. Tc forms besides the oxidation state +7 (pertechnetate) other oxoions, with +5, +4 and +3 ions [4]. It can quite easily be reduced to the metal. Both reactions require reduction of the starting pertechnetate. Generally, Sn2+ is used as reducing agent. However, at the same time red-ox reactions with water may take place as well, due to water radiolysis. Therefore, the results will be analyzed with respect to reactions, which would increase the amount of the reduced technetium-ions: 1. In old 99mTc solutions a significant amount of H2O2, produced by water radiolysis, can be present. Hydrogen peroxide could react with the pertechnetate, yielding reduced technetium ions and increasing the labeling efficiency for rifamicine. However, at the concentrations of 99mTc and of H2O2 present in the systems investigated here, 10-9 M and 10-7 M, respectively, this reaction is improbable. 2. Another hypothesis is based on the presence of “hot atoms”, high energy atoms produced during the decay of radioactive nuclides by the recoil effect. In the case of 99mTc the total recoil energy can be calculated to equal 800°C; this energy is available and effective to reduce the pertechnetate. 3. Based on these two effects, a similar mechanism is assumed to be responsible for the high labeling yield under MW-irradiation. In the following reactions, attempts are made to include the effect of direct microwave irradiation into the hypothetical reaction pathways. During direct MW-heating of the reaction mixture, “hot spots” of yet unknown temperature could be generated, that mimic the effects of “hot atoms”, supplying the energy requested to reduce the pertechnetate-ions. In both cases it should be possible to obtain technetium (+3), able to bound rifamicine. Upon MW-irradiation the "hot species" is “activated” H2O, which would be the reducing agent of [TcO4]- to [TcO2]+, as suggested in Equation (1) and (2): MW irradiation H 2 O    o H 2 O

TcO4  H 2O o TcO2  2OH  

(1)

1 O2 2

( 2)

More exactly, “hot spots” generated by MW-irradiation or "hot atoms" from decay of radioactive nuclides could trigger the radio-labeling by promoting at least one of the needed reductive steps. Such a mechanism would explain the high labeling yield upon direct MW-irradiation of the reaction mixture and the observed reduced influence of the actual 99m Tc- activity on the labeling yield. For the “hot atoms” in aqueous solution we postulate particular chemical reactions, that can occur in an aqueous solution containing 99mTc.

Role of Microwave Radiation on Radiopharmaceuticals Preparations

367

99m - 99* TcO4 = TcO4 99* 99

99 + TcO4 +H2O= TcO2 +2OH + 1/2 O2

+ 99m - 99m + 99 TcO2 + TcO4 = TcO2 + TcO4

99m - 99m R+ TcO4 = TcO2 99 - 99 R+ TcO4 = TcO2 99m + 99m + TcO R+ TcO2 = 99 + 99 + R+ TcO2 = TcO 99m + 99m TcO + L = TcL 99

+ 99 TcO + L = TcL

Sn2+-ions are R and rifamicin is L. When a pertechnetate solution is directly irradiated by MW, reducing species could be as indicated by the following reactions: [99mTcO4]- o [99mTcO2]+ upon MW-irradiation [99TcO4]- o [99TcO2]+

upon MW-irradiation

The proposed reaction series was tested by computer simulation. The results are shown in Figure 7. In addition, the simulated reactions were in agreement with experimental results. As already described in the results, a significant change in the activation energy of the hydrolysis reaction of indomethacin performed under MW irradiation occurs. It means an influence on the transition state that cannot be ascribed to a heating effect. Nitroimidazoles did not show such changes for the Ea. Only the k0 values were influenced. We assume, that in both cases the hydrolysis can occur through two competitive pathways involving intermediates that differ in their polarity, as shown in Figure 8. Reactions (1) and (2) describe hydrolysis of indomethacin, (3) and (4) the hydrolysis of the -NO2 group. In this case we suppose that the way involving the more polar intermediate (2) is preferred under MW-irradiation, because the electromagnetic field stabilizes this dipolar molecule. The change in the reaction mechanism would explain the difference in the Ea. Nitroimidazoles could follow a similar mechanism, but because the reactions were carried out at pH 12 or 13, the reaction path (3) is predominating. A stabilizer effect on reaction path (4) is not strong enough to change the equilibrium.

368

Gattavecchia

Tc-Red

Tc-Red

0,5

0,8

radiochemical yield

radiochemical yield

1,0

0,6

0,4

0,2

0,4

0,3

Tc-L 0,2

0,1

Tc-L 0,0 0

10

20

30

40

50

0,0

60

0

10

20

time (min)

30

40

50

60

time (min)

0,7

0,8

Tc-L

Tc-L

0,5

radiochemical yield

radiochemical yield

0,6

0,4 0,3 0,2

Tc-Red

0,1 0,0

0

10

20

30

time (min)

40

50

60

0,6

0,4

0,2

0,0 0

Tc-Red 10

20

30

40

50

60

time (min)

Fig. 7. Computer simulation graphs of the complex (Tc-L) formation yield. Fresh pertechnetate was irradiated by MW at various time (0, 2, 4, 10 min) and the reagents were added after the irradiation time. At T = 0 the Tc-L yield was equal to 0.

Fig. 8. Hydrolysis pathways involving intermediates that differ in their polarity.

Role of Microwave Radiation on Radiopharmaceuticals Preparations

369

Conclusions Advantages of the MW-irradiation in comparison with classical heating methods are confirmed in all studies here reported. In radiochemistry microwave heating showed itself very useful to solve several problems specific of this field. In addition to the well known effect of shortening the time interval to complete a reaction, MW-irradiation enables a less aggressive treatment of the reagents, extremely important when unstable molecules must be labelled and a high purity of the product is requested. The introduction of MW treatment proved to be the right way to obtain labelled rifamicine, otherwise only unsatisfactorily results were obtained. The theoretical attempts to explain these various effects take into account a non thermal effects of MW, since some of our experimental data cannot be explained, till now, by thermal activation.

References [1] Baghurst DR, Mingos DMP (1992) Superheating effects associated with microwave dielectric heating. J Chem Soc Comm 674: [2] Brandau W, Bubeck B, Eisenhut M, Taylor DM (1988) Technetium-99m labeled renal function and imaging agents: III Synthesis of 99mTc-MAG3 and biodistribution of byproducts. Appl Radiat Isot 39: 121-129 [3] Buchachenko AL, Frankevich EL (1994) Chemical generation and reception of radio and microwaves. VCH Publisher, New York [4] Busey RH, Sprague ED, and Bevan RB, (1969), J. Phys. Chem., 1039 [5] Fritzberg AR, Kasina S, Eshima D, Johnson DL (1986) Synthesis and biological evaluation of technetium-99m MAG3 as a hippuran replacement. J Nucl Med 27: 111116 [6] Hajek M (1997) Application of microwave energy in catalysis. In: Breccia A., Metaxas AC (eds) Microwave and high frequency heating. Principles and chemical applications. Lo Scarabeo, Bologna, pp 85-95 [7] Hung JC, Wilson ME, Brown ML, Gibbons RJ (1991) Rapid preparation and quality control method for Technetium 99-m-2 metoxy-isobutyl-isonitrile (Technetium 99-msestamibi). J. Nucl Med 32: 2162-2168 [8] Hwang DR, Dence CS, Gong J, Welch MJ (1991) A new procedure for labelling alkylbenzenes with 18 F fluoride. Appl Radiat Isot 42-3: 1043-1047 [9] Noll B, Johannsen B, Spies H (1995) Sources of radiochemical impurities in the 99m Tc/S-unprotected MAG3 system. Nucl Med Biol 22: 1057-1062 [10] Stone-Elander S, Elander N (1993) Fast chemistry in microwaves fields: nucleophilic 18F-radiofluorinations of aromatic molecules. Appl Radiat Isot 44: 889-893. [11] Thorell JO, Stone-Elander S, Elander N (1992) Use of microwaves-cavity to reduce reaction times in radiolabelling with 11C-cyanide. J Labelled Cpd Radiopharm 31: 207-217 [12] Whittaker AG, Mingos DMP (1994) The application of Microwave heating to chemical syntheses Journal of Microwave Power and Electromagnetic Energy 29:195-214

Fast Synthesis of Biodiesel from Triglycerides in Presence of Microwaves C. Mazzocchia1, A. Kaddouri1, G. Modica1, R. Nannicini2 1

Industrial Chemistry and Chemical Engineering Department, Polytechnics of Milan, P.zza L. da Vinci 32, 20133 Milano, Italy 2 E.N.E.A., Pisa, Italy

Abstract Microwaves irradiation has proved to be a faster method than the traditional ones for transesterification reactions with high activity and yields to biodiesel in the alcoholysis of triglycerides with methanol; it prevents the degradation of the products and needs a shorter reaction time together ith a lower energy consumption.

Introduction The transesterification of natural triglycerides (eg oils and fats), which serve as key reagents to vital products >1 - 4@ in the chemical industry, is a highly desired goal. These are employed to obtain lots of products used in a wide variety of industrial processes. The most significant products obtained by transesterification, which involve the use of millions of tons of fats and oils a year, are soaps, long chain carboxylic acids, detergents, mono and diglycerides >5@, methyl esters of fatty acids, additives for foods, cosmetics, pharmaceuticals and alternative fuels for diesel engines (BioDiesel) >5 - 7@. Several processes are normally employed for catalytic transesterification, but these reactions must be carried out at high temperatures and pressures >5@ which reduce selectivity, are energy demanding and suffer from by-products presence. One of the objectives of this work was to use microwave irradiation as an easy and fast method for BioDiesel synthesis through triglycerides transesterification with methanol. The reactions were carried out either at atmospheric or under pressure, in order to compare their results with the ones conducted with conventional heating. Another objective of this work was the study of the effects of the mole ratio alcohol-

Fast Synthesis of Biodiesel from Triglycerides in Presence of Microwaves

371

oils and the amounts of the catalyst on the conversion in FAME (Fatty Acid Methyl Esters).

Experimental

Transesterification procedure Transesterification reactions of triglycerides were carried out without the use of solvents. To compare the action of microwaves on the alcoholysis reactions (transesterification reaction of rape and sunflower oils using methyl alcohol) some of the tests were carried out by conventional heating (') at different temperatures and pressures, others under microwave irradiation ( MW ) at the same temperature. In each test stirring was performed with a magneto rotating at 650 rpm unable to emulsify and disperse methanol in oil, which was solely used to mix mass reaction and to equalise its temperature. Apparatus Tests under microwaves irradiation were conducted with a Milestone Ethos 1600 oven working at 2.45 GHz, with a power up to 1000 W. Products separation and analysis Once the reaction was completed the mass was cooled at room temperature and treated with 96 - 98% sulphuric acid to reach pH 6,5 - 7. This operation is essential to block the extent of reaction, as it converts the soap (sodium salt of fatty acid) into free fatty acid. The reaction products was separated in two layers: the light layer is essentially the BioDiesel (fatty acid methyl esters) in which are dissolved methanol, glycerine and non-reacted mono-di-glycerides and triglycerides; the heavy one is made up of glycerine dissolved in the methanol containing fatty acid methyl esters and also non-reacted mono-di-glycerides too. The two layers were processed separately under vacuum in order to remove the methanol. At this point each of the two layers form other two different phases. A light phase containing Biodiesel and the heavy phase containing glycerine and mono-di glycerides The two light layers (fatty acid methyl esters) were separated from the two heavy layers containing glycerine and mono- di- glycerides. The homogeneous layers were weighed to perform a mass balance, and then analysed by GC and volumetric analysis with periodic acid titration as to determine the quantity of fatty acid methyl esters, glycerine and monoglycerides.

372

Mazzocchia

Results and discussion The results of the transesterification reactions of rape seed and sun flower oils are reported in Table 1 to 3. In Table 1 have been compared the results obtained either with microwave (MW) or conventional heating. All tests were conducted at atmospheric pressure, at the boiling-point of methanol and with the ratio alcohol/oil of 30/1, which is definitely higher than the stoichiometric one of 3/1. To study the effect of the catalyst amount (solid NaOH 99.5%) on oil conversion into FAME, different amounts of catalysts were used: 0.01% in run 1, 0.1% in runs 2, 3, 5 and 1% in runs 4 and 6. Table 1. Compared results of the transesterification tests of rape seed oil with methyl alcohol carried out under microwave irradiation (MW) and conventional heating (').

Reaction time [min]

Heating (MW)

Heating (')

FAME Conversion (%)

FAME Conversion (%)

1 2 3 4 Run Oil 0,5 89 RO 1.0 91 RO 1,5 90 RO 2,0 92 RO 3.0 92 95 RO 5.0 94 RO 6,0 RO #99 13 RO 13,5 RO 15,5 RO 16,0 RO # 99 16,0 SO # 99 16,0 26 SO 17,5 RO 22,5 RO 27,5 RO 42,5 RO 72,5 RO 0,01 0,1 0,1 1,0 Catalyst % (w/w) Operating conditions: P=1bar, T = 338K, cat. %(w/w) NaOH/oil oil/methanol mole ratio = 1/30 RO rape seed oil SO sunflower oil

5

6

87 90 90

90 91 92

91

94 96

93 96 # 99 0,1

# 99 1,0

The tests conducted under MW with sunflower oil showed that, after 16 minutes, the conversion into FAME reaches only the 26% employing the 0.01% of the catalyst amount (run 1); on the contrary, using the 0.1% (runs 2 and 3) the conversion reaches the 99% both with sun flower and rape-seed oils and increasing the amount to 1% (run 4), the 99% is reached after 6 minutes.

Fast Synthesis of Biodiesel from Triglycerides in Presence of Microwaves

373

When the reaction is carried out with conventional heating (runs 5 and 6) oil conversion attains 99% in 72.5 min (run 5) using the 0.1% of catalyst amount and 42.5 minutes (run 6) using 1%. The effect of the catalyst amount on the conversion is more marked under MW treatment than with traditional heating. Actually, comparing the time necessary for reaching the 99% conversion in runs 4 vs. 3 and runs 6 vs. 5, it can be observed that when a larger amount of catalyst is employed the heating time is respectively 2.6 and 1.7 times shorter under MW irradiation than by conventional heating. A comparison between the reaction times of both the treatments to get the same conversion with the same catalyst amount, shows that they decrease by 4.5 times in runs 2 vs. 5 and by 7 in runs 4 vs. 6. Runs 1 - 3 (Table 2), conducted with conventional heating, show that the FAME conversion progressively increases when the alcohol-oil ratio is augmented; besides, run 2 vs. run 6 (Table 2) carried out under MW, prove that it is possible to achieve the same results in a shorter time (ca. 4.5 times). Table 2. Results of the effects of mole ratio methanol/oil and catalyst quantity on the transesterification of rape seed oil carried out under microwave irradiation (MW) and conventional heating (') Run

Trigl/MeOH mole ratio

Catalyst % (w/w)

Reaction time min.

Heating ' MW

FAME % (w/w)

1

1/3

0,3

72,5

'

89,87

2

1/9

0,3

72,5

'

98,60

3

1/18

0,3

72,5

'

99,66

4

1/3

0,3

16

MW

94,08

5

1/9

0,1

16

MW

96,32

6

1/9

0,3

16

MW

98,70

Operating conditions P = 1 bar, T = 338 K, cat. % (w/w) NaOH/ rape seed oil stoichiometric mole ratio triglycerides/methanol: 1/3

Using a stoichiometric MeOH-oil (run 1 vs. run 4) the reaction is obtained in a shorter time and the conversion passes from 89.87 to 94.08%. The conversion obtained under MW in run 5 (Table 2) is the same as those obtained with conventional heating (Table 1, run 5), but the reaction, done with a methanol-oil ratio of 9/1 vs. 30/1 is reached in a reduced time ( 3.7 times). Table 3 reports runs carried out under pressure both using MW (runs 1 and 2) and conventional heating (run 5). Once again it is shown that a larger catalyst amount allows a shorter reaction time using microwaves (run 1 vs. 2), whose effects still prove advantageous if we compare run 1 and run 5: the latter has been carried out at the same temperature under conventional heating with a catalyst amount which is ten times bigger, a time three fold longer and a greater molar ratio than in the former performed under MW.

374

Mazzocchia

Finally if we compare the results of runs 3 and 4 with those of runs 1 and 2 (Table 3) and with run 6 (Table 2) we can see that with MW heating the reaction time is reduced by 36 working under pressure, and by 5.6 at atmospheric pressure. Table 3. Results of the transesterification tests of rape seed oil with methyl alcohol carried out under microwave irradiation (MW) and conventional heating (') to obtain Biodiesel (FAME mixture) Run

Process

Trigl/M eOH (Mole ratio)

1

Laboratory Under pressure 1 step

1/9

0,1

2

Laboratory Under pressure 1 step

1/9

Catalyst Reaction % (w/w) time [min]

T [°C]

Pressure [bar]

Heating ' MW

¦ C14C20 %

6,5

103

3,5

MW

98,45

0,5

2,5

92-95

3

MW

98,75

3

Industrial 1 step(a)

1/9

1-2

90

65

1

'

93,84

4

Industrial 3 step(b)

1/9

1-2

90

65

1

'

97,95

5

Laboratory Under pressure 1/30 1,0 21 103 3,5 ' 1 step (a) one step industrial process for biodiesel production for domestic heating use (b) three steps industrial process for biodiesel production for auto-traction use

98,0

The effects of microwaves are essentially developed on polar groups present in the molecules and produce an energy concentration released as local heat which increases both the reciprocal solubility between not very soluble compounds (in this case methanol and triglycerides ) and the reaction rate. These effects become more evident when the reaction intermediates start forming, namely the mono and diglycerides containing polar groups which strongly interact with MW. The increasing of reciprocal solubility by MW irradiation has been evidenced by adding a small quantity of phenolphthalein to a sodium hydroxide previously dissolved in a methanol solution so as to give the methanolic solution rose-pink colour. After starting the reaction at the methanol boiling-point temperature it was observed that MW interacted only with methanol inducing a violent ebullition while oil remained cold. Two distinguishable layers were clearly observed in the liquid phase: the upper layer, the rose-pink coloured methanol and the lower one, the yellow-coloured oil. Presently (1 - 2 minutes), under MW irradiation, appeared a rose-pink coloured homogeneous monophase which hested up fastly and evenly. Immediately after stopping the MW action and the stirring, a fast demixing of the solutions took place, accompanied by an inversion of the position of two phases. This meant that the reaction had occurred since the methanol dissolving

Fast Synthesis of Biodiesel from Triglycerides in Presence of Microwaves

375

the glycerine, had a higher specific weight than the oil and the FAME; on the contrary, throughout conventional heating, two distinct not limpid phases were present. MW heating also proved advantageous in the separation of the reaction products. After neutralising the solution containing the reaction products with sulphuric acid (96 - 98%), the solution was transferred to a flask separator for stratification and were obtained two phases consisting of the FAME and a glycerinemethanol mixture. The reactions performed with conventional heating need more time for the stratification of the solutions as the time may vary from 48 to 100 hours while with MW heating, less than 12 h are required. It is a complex phenomenon which may depend on the extent of reaction, since MW select the molecules to be heated preferentially; thus they are more reactive to the methanol and to mono-di glycerides ones and less reactive to no polar substances such as triglycerides. Another two advantages of microwaves are the fast separation (few hours) of the reaction products in two layers due to the small amount of mono and diglycerides which have good emulsifying properties, and the low stirring, needed for the reaction, limiting the emulsion between the products.

Conclusions Microwave irradiation allows the synthesis of methyl esters (BioDiesel) and the high conversion of triglicerides in few minutes. The stirring process doesn’t appear to be so notable under MW irradiation as it is with conventional heating.The energy needed for the transesterification under microwaves irradiation is very low, eg 100 g of rape-seed oil is totally converted into biodiesel in 150 seconds using a MW energy of 150 W. The conversion reached under microwave in one step is higher and may be compared to the one reached industrially at atmospheric pressure in three steps. The scale-up of the process can be realised employing low-power MW generators; actually it is possible to produce 16 kg/h of FAME using 1 kWh, with a cost incidence of 0,008 Euro/kg FAME (KWh cost is 0,129 Euro).

Acknowledgements The MURST-COFIN financial support (Application of the microwave technology to physical-chemical processing involving solids 1999 - 2000) is gratefully acknowledged

376

Mazzocchia

References [1] A. Loupy. et al, The synthesis of esters under microwave irradiation using dry-media conditions, Can. J. Chemistry, 71, 90 (1993) [2] Method for preparing polyol fatty acid polyesters by transesterification. United States Patent n°5,596,085. [3] A continuous transesterification method for preparing polyol polyesters. PCT/US96/07799. [4] E. Ahn, M. Koncar, M. Mittelbach, R. Marr. A low-waste process for the production of biodiesel. Separation Science and Technology, 30(7-9), p.2021 [5] N. O. V. Sonntag. Glycerolysis of fats and methyl esters. Journal of American Oil Chemists Society (1982), vol.59, n°10, pp.759 [6] C. C. Akoh, B. G. Swanson. Base catalysed transesterification of vegetable oils, Journal of Food Processing & Preservation, 12 (1998), 139. [7] K. Scharmer. Verwendung von Kraftstoffen aus Pflanzenolen unter Umweltgesichtspunkten, Raps 11/1, 2 (1993).

Alteration of Esterification Kinetics Under Microwave Irradiation L. A. Jermolovicius, B. Schneiderman and J. T. Senise Laboratório de Microondas. Instituto Mauá de Tecnologia.,Pça. Mauá, 1, São Caetano do Sul, SP, 09580-900, Brazil.

Introduction At the 7th International Conference on Microwave and High Frequency Heating (Valencia, Spain, 1999) experiments were reported showing a velocity increase of esterification of maleic anhydride with 2-ethylhexanol-1, under microwave irradiation [1]. Experiments were performed in a continuous stirred tank reactor in order to overcome the difficulty of measuring temperature with conventional thermometers or thermocouples subject to microwave irradiation. This paper describes a new set of experiments, aiming at a better understanding of the chemical kinetics of microwave enhanced esterification. The acquisition of a microwave compatible fiberoptic digital thermometer solved the question of temperature measurements in the presence of microwaves, making it possible to perform the same esterification reaction in a batch reactor, with shorter operation times and consuming less raw materials than required by using a continuous reactor.

Esterification reaction The main reaction for plasticizer production is the esterification reaction. This kind of reaction involves two reactants, one alcoholic and another acidic, as carboxylic acids. It occurs in a reversible mode, this means that while products are formed they react with themselves recovering the initial raw materials, establishing a dynamic equilibrium. Exemplifying with maleic acid esterification with 2ethylhexanol-1 represented by chemical equations 1 and 2. In the plasticizer industry, anhydrides are used rather than acids, because the reaction between anhydrides and alcohols is irreversible. This substitutes the reversible reaction represented by chemical equation 1 by an irreversible reaction represented by chemical equation 3.

378

Jermolovicius

CH - COOH k1 CH-COO- CH2-CH-(CH2) 3-CH3 " + HO-CH2-CH-(CH2) 3-CH3 " | + H 20 CH-COOH CH2-CH3 CH - COOH | k2 CH2-CH3 (chem. eqn 1) COO- CH2-CH-(CH2) 3-CH3 CH| k3 " CH2-CH3 + HO- CH2-CH-(CH2) 3-CH3 CH-COOH | k4 CH2-CH3

CH2-CH3 | CH-COO- CH2-CH-(CH2) 3-CH3 " + H 2O CH-COO-CH2-CH-(CH2)3-CH3 | CH2-CH3 (chem. eqn 2)

CH - CO " \ " O + HO- CH2-CH-(CH2) 3-CH3 " / | CH - CO CH2-CH3

k5 ----------->

CH-COO- CH2-CH-(CH2) 3-CH3 " | CH-COOH CH2-CH3 (chem. eqn 3)

The technical limitation to this reaction is the equilibrium at chemical equation 2. This step limits the concentration of monoester according to equation 1, where C: molar concentration, and K: equilibrium constant, and consequently reduces the diester production. Cdiester .CH 2O k (1) Cmonoester.Calcohol The solution adopted by the chemical industry to increase the yield of the diester, in this case where the boiling points of raw materials and diester are higher than the boiling point of water, is to distil reaction water simultaneously with the esterification. To increase the efficiency of this water withdrawal it is usual in plant operation to use an inert solvent to form an azeotropic mixture with water. Xylene is usually employed for this intent. After distilled the vapors of the azeotropic mixture are condensed and the azeotrope is broken. Water and xylene are decanted. Xylene is returned to the reactor and water is withdrawn. In this way, the equilibrium step is displaced to the product formation side and the reaction evolves to a pseudo irreversible reaction represented by chemical equation 4.

k6 CH-COO- CH2-CH-(CH2) 3-CH3 " | + HO- CH2-CH-(CH2) 3-CH3 ------> | CH-COOH CH2-CH3 CH2-CH3

CH2-CH3 | CH-COO- CH2-CH-(CH2) 3-CH3 " +H2O n CH-COO- CH2-CH-(CH2) 3-CH3 | CH2-CH3 (chem. eqn 4)

Alteration of Esterification Kinetics Under Microwave Irradiation

379

The industrial 2-ethylhexanol-1 or dioctyl maleate manufacture may be represented by two reactions in series, specifically the reactions of the chemical equations 3 and 4: k5 k6 MALEIC ANHYDRIDE ---> MONOOCTYL MALEATE ---> DIOCTYL MALEATE (chem. eqn. 5)

Considering that the order of reaction and velocity constants of both reactions are unknown and the desired product is the last of the series, it is possible to assume a simplification as showed in chemical equation 6. CH2-CH3 CH - CO | " \ k7 CH-COO- CH2-CH-(CH2) 3-CH3 " O + 2 HO- CH2-CH-(CH2) 3-CH3 -------> " + H2 O n " / | CH-COO-CH2-CH-(CH2) 3-CH3 CH - CO CH2-CH3 | CH2-CH3 (chem. eqn 6)

Supposing that this global reaction is an elementary reaction, then it is possible to represent it by the following velocity equation:

 rA



dCA dt

k .CA..CB 2

(2)

where CI: molar concentration of reactant „I“, t: time (min), k: reaction velocity constant (mol-2.min-1). This simplification allows an easier approach for an initial study of the influence of microwaves over esterification.

Equipment The chemical kinetics determinations were done in a well-stirred batch reactor. It was a 500 mL Pyrex kettle vessel with internal baffles and a top cover with four holes 24/40. A mechanical stirrer with a turbine paddle was used to ensure a good mixing of reagent materials. The shaft was made of Pyrex rod and the paddle of Teflon. The stirrer’s explosion proof electrical motor was assembled over the reactor top. The mixing velocity was measured by a stroboscopic digital tachometer. A total reflux Allinh condenser (Pyrex, 60 cm) with a Dean Stark flask (25 mL) was assembled at the vessel top. The condenser was cooled with water at room temperature. The sample collector was a 2 mm inside diameter glass tube immersed into the reagents and with the other side connected to a vacuum vessel. It was fitted at the same hole used to feed the reagents. The temperature of reagents was monitored by a Nortec Handy Flex digital fiberoptic thermometer sensor protected by a thermometric well of glass. The temperature control was done by an internal „U“ tube of 6 mm diameter, where kerosene cooled at 30 oC was circulated at a convenient flow rate. This „U“ tube was fitted at the same hole where the thermometric well was fitted.

380

Jermolovicius

This assembled reactor was enclosed in an aluminum multimode cavity with mode stirrer, connected to a microwave variable-power generator of 2.45 GHz. The irradiated and reflected power of the microwave emission were measured by a directional coupler and a HP - 435B-power meter. During experiments with electrical heating, the reactor was wrapped up with a rheostat controlled 180 W electrical tape.

Procedure The experiment was statistically designed to search the influence of microwave on the reaction. An experiment of 24 complete factorial, was conducted at random without restriction for order of execution comparing electric heating with a 200 W microwave irradiation at 140°C and with 300 W microwave irradiation at 145°C. The parameters of this statistical design were the nature of heating (electric or microwave power), the microwave power (200 and 300 W microwave), the catalyst concentration (0.1 and 0.2% over the reactant feed), 0.5 and 1 M concentration of maleic anhydride, 1 and 2 M concentration of 2-ethylhexanol-1. Table 1 shows this statistical design of experiments. The temperatures, 140 and 145°C, were chosen empirically by means of a preliminary experiments: execute a first esterification with electric heating and determine the temperature reached in the reactor and a second esterification with microwave heating at 300 W of microwave power. This 300 W power microwave was selected from previous works with polystyrene [2], where a probable threshold value for microwave power to influence a chemical reaction was observed. The reactor was fed with the necessary quantities of maleic anhydride, ptoluene sulfonic acid and dry xylene to prepare their solution at the Table 1 specified concentrations. The charge volume was around 250 mL. The heating was started at maximum intensity (300 W of microwave or total power of heating tape). The charge was mixed by the mechanical stirrer at 600 rpm. This heating was maintained until the charge temperature reached 137 – 138°C, when the heating is set to the desired power value. The electrical power for heating tape was regulated by a rheostat. The microwave power absorbed by the reactor cavity was regulated measuring the incident and reflected power of microwaves. The necessary quantity of 2-ethylhexanol-1 was conditioned, simultaneously, in an electrically heated decantation funnel and heated. When the temperature of the alcohol reached the same temperature as the charge, it was added to the anhydride solution. The time count was started simultaneously. This pre-heating of raw materials was done because it was observed in a preliminary experiment that 12 minutes to heat the reagents from the room temperature to the reaction temperature were sufficient. The reaction temperature control was manual. The temperature was limited by the power applied to the reactor, either with electric tape heating or microwave heating. A fine control of temperature was done by circulation of kerosene throughout the reactor „U“ shape heat exchanger. Two complete sets of experi-

Alteration of Esterification Kinetics Under Microwave Irradiation

381

ments were done. One at 145ºC with electric heating and 300 W microwave. Other at 140ºC with electric heating and 200 W microwave irradiation, and 300 W microwave at 145ºC. The maleic anhydride quality was different for each set of experiments. Table 1. Experimental Results 1# Set 2# Set t/C Ao(min.)

Variables Values d

c

b

a

140ºC

Elect.

0.1

0.5

1

Elect.

0.1

0.5

2

Elect.

0.1

1

Elect.

0.1

Elect. Elect. Elect.

145ºC

1# Set 2# Set Reaction order

145ºC

140ºC

145ºC

145ºC

19.1

21.7

3.10

4.73

7.62

3.40

4.10

1

26.6

24.5

4.90

3.70

1

2

1.16

5.14

4.60

5.20

0.2

0.5

1

12.7

16.1

4.50

3.70

0.2

0.5

2

2.79

5.99

3.10

3.10

0.2

1

1

5.67

30.6

5.50

3.80

1.87

3.60

3.30

0.99

Elect.

0.2

1

2

0.93

200W-MW

0.1

0.5

1

18.3

3.90

200W-MW

0.1

0.5

2

3.95

5.70

200W-MW

0.1

1

1

12.2

5.70

200W-MW

0.1

1

2

1.20

4.80

200W-MW

0.2

0.5

1

12.2

3.60

200W-MW

0.2

0.5

2

2.60

3.10

200W-MW

0.2

1

1

5.40

5.60

200W-MW

0.2

1

2

0.87

300W-MW

0.1

0.5

1

22.4

17.8

2.70

2.10

300W-MW

0.1

0.5

2

9.21

10.8

3.40

3.10

300W-MW

0.1

1

1

18.4

11.1

4.70

5.20

300W-MW

0.1

1

2

14.4

3.36

3.20

4.10

300W-MW

0.2

0.5

1

16.9

16.8

2.60

3.30

300W-MW

0.2

0.5

2

4.33

8.90

3.20

3.90

300W-MW

0.2

1

1

13.9

12.3

4.40

4.50

300W-MW

0.2

1

2

26.1

2.33

2.30

2.90

3.30

a: molar concentration of 2-ethylhexanol-1. b: molar concentration of maleic anhydride. c: concentration of catalyst (percent over reactants feed). d: heating conditions. t/CAo (min.): time per unit of initial concentration of maleic anhydride to obtain 50% conversion of maleic anhydride.

The vapor of xylene and water azeotrope was condensed in the Allinh condenser. The condensed azeotrope brakes in two phases: an upper with the xylene and a lower with the water. These phases were decanted in the Dean Stark flask. The condensed xylene was returned to the reactor and the water drawn and dis-

382

Jermolovicius

carded. At the beginning of each esterification, the Dean Stark flask was filled up with dry xylene. During the esterification the water level in the Dean Stark was maintained at a maximum level of 2 mL by intermittent drawing off the water. The evolution of the reaction was monitored by titration of residual maleic anhydride with 0.1 N solution of potassium hydroxide. Samples were taken at initial time, one minute after start, 3 minutes after start and each 3 subsequent minutes until 18 minutes of reaction. The sample was chilled to room temperature. From this sample an aliquot part of 2 mL was taken with graduated pipette. This aliquot was titrated with potassium hydroxide. The conversion of the titration result to maleic anhydride molar concentration is: CAM = 0.025.fc.Vtit

(3)

where CAM: molar concentration of residual maleic anhydride, fc: correction factor of the potassium hydroxide solution, Vtit: volume of 0.1 N potassium hydroxide titrated (mL).

Results The experimental data produced a collection of tables of reaction times and molar concentrations of residual maleic anhydride. With these data graphics of concentration as a function of time were plotted and derived to prepare tables of time and reaction velocity values. From those data two kinds of measured parameters were determined. The „nominal pseudo global order“ of the simplified esterification and the „time per unit of initial concentration of reactant to obtain a 50% conversion of maleic anhydride“. Global order because the individual orders of reaction for both reactants are not expressed, but added in one global order. Pseudo order because the actual process is a series of two reactions, one of them reversible, and is assumed a simplified irreversible reaction as demonstrated above in this paper. Nominal order because the adopted method to search the chemical kinetics equation is concerned with an unknown and complex mechanism of reaction which is assumed as a „nth“ order reaction [3]. Then the velocity may be expressed by:

 rA



dCA dt

kC An

where, nz1

(4)

which after separation and integration gives:

k

C 1An  C 1A0n (n  1)t

(5)

Alteration of Esterification Kinetics Under Microwave Irradiation

383

Where CAo: molar concentration of reagent A at initial time. This method consist in choosing a value for order „n“ and applying to equation 5 to determine the velocity constant „k“. The „n“ value that minimizes the variation in „k“ is the desired value of „n“ [3]. The „time per unit of initial concentration“ is obtained throughout the equation, which represents the batch reactor [3]:

t C A0

³

X Af

X Ao

dX A  rA

(6)

where XA: conversion of reactant „A“ and -rA: velocity of consumption of reactant „A“. This equation may be integrated by numerical methods, as Simpson’s rule, using experimental data of reaction rate (first derivative of concentration as a function of time). The time per unit of initial concentration represents the batch reactor design equation and takes into consideration the initial concentration of reactant measured and the kinetic equation of the processed reaction. From these aspects it is more related to the chemical kinetics than the chronological time of reaction. Table 1 shows these data treatments.

Discussion These experimental results were submitted to a variance analysis by Yates method, for variance on reaction order and for time per unit of initial concentration. Time to obtain 50% conversion Starting the discussion of statistical results with the „time per unit of initial concentration to obtain a 50% conversion of maleic anhydride“. In the first set of data (140ºC, electric heating and 200 W microwave), a 99% confidence level for the significance of the molar concentration of 2-ethylhexanol-1 influence, a 95% for the catalyst amount and a 90% for the interaction between both were observed with 5 degrees of freedom for the error and 1 for the variables. The concentration of maleic anhydride and the kind of heating did not reach some significant level. Apparently, in this case of low microwave power (200 W) no sensible nonthermal effect of the microwave irradiation was observed. In the second set of data analysis (145ºC, electric heating and 300 W microwave), a 99.9% confidence level for the significance of the molar concentration of 2-ethylhexanol-1 influence, a 90% for the nature of heating, a 95% for the interactions between the nature of heating with the molar concentration of 2ethylhexanol-1 and maleic anhydride, and a 90% for the interaction between the concentrations of 2-ethylhexanol-1 and maleic anhydride were observed with

384

Jermolovicius

5 degrees of freedom for error and for the variables. Probably the catalyst influence, in this case reached its saturation of influence on the reaction rate. Applying Yates method to 200 W and 300 W microwave power data from the first set of experimental results a 99% confidence level for the significance of microwave power increase (from 200 to 300 W), for molar concentrations of 2ethylhexanol-1 and maleic anhydride was observed, and also a 90% for the interaction between 2-ethylhexanol-1 and maleic anhydride. Global order of reaction Initially the nominal global pseudo order data was submitted to a comparison of means, by Students „t“ test, between the first set of data (140°C with electrical heating and 200 W microwave power) and between the data of the second set (145ºC, with electric heating and 300 W microwave power). In both universes the two groups of data (electric heating and microwave heating) can not be considered different. Data for 200 W and 300 W microwave power were also submitted to a comparison of means, by Students „t“ test. Now, these two groups of data can be considered different with a 95% confidence level. For 140°C with electric heating or 200 W microwave the mean value of the global order is 4.5, and for 145°C with 300 W microwave is 3.3. Applying the Yates method for order of reaction to the first set of experiments (140ºC, electric heating aand 200 W microwave) a 95% confidence level for the molar concentration of maleic anhydride influence, a 95% for the interactions between the molar concentration of maleic anhydride with 2-ethylhexanol-1 and with catalyst amount, a 90% for the 2-ethylhexanol-1 were observed with 5 degrees of freedom for error and for the variables. In the second set of data (145ºC, electric heating and 300 W microwave), a 95% confidence level for the significance of the molar concentration of maleic anhydride influence, a 95% for the interaction between the molar concentration of 2-ethylhexanol-1 and maleic anhydride with catalyst amount, a 90% for the interaction between 2-ethylhexanol-1 with maleic anhydride were observed with 5 degrees of freedom for error and for the variables. Applying Yates method to 200 W and 300 W microwave power data from the first set of experimental results a 99% confidence level for the significance of microwave power increase (from 200 to 300 W) and for the interaction between concentration of 2-ethylhexanol-1 and maleic anhydride were observed, also a 95% for molar concentration of maleic anhydride and catalyst amount, and 90% for 2-ethylhexanol-1 concentration and its interaction with catalyst amount.

Conclusion These experimental results show an alteration in the significance profile of the influences on reaction order and on reaction time of maleic anhydride esterification

Alteration of Esterification Kinetics Under Microwave Irradiation

385

with 2-ethylhexanol-1, when a more drastic irradiation (300 W) with 2.45 GHz microwaves was done. This alteration introduces a strong significance for the microwave power effect over reaction order and reaction time. This induces to believe a probable threshold value for microwave power, above which microwaves may influence the reaction mechanism.

Literature [1] Senise, J.T., Jermolovicius, L.A. Performance of Continuous Esterification of High Esters Under Microwave Irradiation. 7th ICMHFH, Valencia, Spain, September 1999. [2] Jermolovicius, L. A., Schneiderman, B., Senise, J. T. Alteration of Emulsion Polymerization Kinetics Under Microwave Irradiation. 2nd ICMicrowave Chemistry, Antibes, France, September 2000. [3] Levenspiel, O. Chemical Reaction Engineering. 2 ed, USA, J.W., 1972.

Acknowledgements The authors would like to thank Professor L. Sartorio and E. R. de Castro, A. Selmikaitis and R. M. Botelho for technical support. Support from Instituto Mauá de Tecnologia and Fundação de Amparo à Pesquisa do Estado de São Paulo is gratefully acknowledged.

Multistep Microwave-Assisted Solvent-Free Organic Reactions: Synthesis of 1,6Disubstituted-4-Oxo-1,4-Dihydro-Pyridine-3Carboxylic Acid Benzyl Esters Mauro Panunzio1, Maria Antonietta Lentini1, Eileen Campana2, Giorgio Martelli2, Paola Vicennati2 1

C.S.F.M.-C.N.R, Dipartimento di Chimica "G. Ciamician" Via Selmi 2, I-40126 Bologna, Italy. 2 I.Co.E.A-C.N.R. Via Gobetti 101, I-40129 Bologna, Italy.

Introduction The first examples for rapid synthesis of organic compounds using microwave ovens were reported by [1]. Since then a number of papers appeared concerning microwave-assisted organic reactions and the number of articles continues to grow quickly [2 - 8]. The main advantages of microwave-assisted organic synthesis are shorter reaction times, minimum waste and generally higher yields. A particularly attractive feature of the microwave technique is the possibility of carrying out reactions in the absence of solvents. Within a general program on the utilisation of microwave enhanced organic reaction for the synthesis of biologically active compounds, in this paper we wish to report our results on the use of microwave technique in a multi-step synthesis of 1,6-disubstituted-4-oxo-1,4-dihydropyridine-3-carboxylic acid benzyl esters 9. Such compounds are of some interest for their biological activity. They present antibacterial [9 - 12] and CNS stimulatory activity while the 6-methyl derivatives and related N-unsubstituted compounds are side-chain constituents in penicillins and cephalosporins possessing high activity vs Gram-negative organisms [13 - 15]. On the other hand 4-oxo-1,4dihydro-3-pyridine carboxylic acid derivatives have been studied, by a Glaxo research group, as inhibitors for DNA-Gyrase [16]. Our multi-step approach, summarised in Schemes 1 and 2, takes into account the readily accessibility of phosphonate 1 [17] and the application of our recent developed synthesis of acetoacetate esters by means of microwave technique [18] (Figure 1).

Multistep Microwave-Assisted Solvent-Free Organic Reactions

387

Results and Discussion The phosphonate 1, which is the milestone precursor, has been prepared by known route from commercially available 2,2,6-trimethyl-4-methylene-4H-[1,3]dioxine (LDA 1.3 eq, THF -78°C then quenching with hexachloroethane 1.76 equiv., THF, 76% yield)17 followed by treatment of chloride derivative with the potassium salt of diethylphosphite (potassium tert-butoxide 4.0 eq, /ether-DMF 12:1) at room temperature (1 h) and acidic work-up (HCl). Wittig reaction of 1 with aldehydes 2 (LiHMDSA, THF, -78°C) gave rise to the adducts 3. Dioxi-4-one ring opening, upon solvent-free microwave irradiation, [18, 19] in the presence of benzyl alcohol in an open-vessel microwave system, gave the corresponding benzyl esters 4 in almost quantitative yields. Treatment of 4-en-3-keto-benzyl esters 4 thus obtained with dimethylformamide dimethylacetal, under microwave solventfree conditions results in the formation of the 2-dimethylaminomethylene derivatives 5. O

O O

EtO

O P

EtO

O O

1

O

i R

O

O

iii

ˆii

O

O

Ph

O

3

R

R

4

O

Ph

N

5

Reagents and conditions: i: LiHMDSA, RCOH (2),THF, 0°C; ii: Benzyl alcohol, MW, 300W; iii: Dimethylformamidedimethylacetal, MW, 300W.

Fig. 1. Preparation of intermediate 5.

Treatment of 5 with amines 6, once again under microwave irradiation, gave rise to the compounds 7, which, upon microwave irradiation, cyclizes to the compounds 8. It must be stressed that substitution of dimethylformamide with the amines 6 and subsequent ring closure to the 4-oxo-1,4,5,6,-tetrahydro-pyridine carboxylic acid benzyl esters 8, may be performed in a one pot two-step procedure depending on the irradiation time. Finally compounds 8 were oxidised to the target compounds 9 by DDQ (Dicyanodichloroquinone) following literature proce16 dure. It is interesting to note that our procedure is compatible with highly functionalized and enantiomerically pure compounds (9d-f). In this case the ring closure gave rise to an almost single diastereoisomer, to which an anti-configuration between the C5-C5'was tentatively assigned on the basis of the coupling constant value (approximately 6 Hz), contamined by a 5 - 10% of the syn isomer (J = 3.3 Hz) except in the case where cyclopropylamine has been used (Compounds 8f and d-8f, Figure 3), for which a 1:1 mixture of the two diastereoisomers was obtained.

388

Panunzio

O

O

O N

R

O

O

O O

Ph HN

R

i

O R

ii

R1

5

Ph

O

O

N

O

Ph R

iii

Ph

N R1

R1

8

7

O

9

Reagents and Conditions: i: R1 NH 2 (6), MW 225W; ii: MW, 225W; iii: DDQ, Toluene, 80°C 6

a

b

c

f

R1 OH

O

OC H3

OMe

Fig. 2. Synthesis of target 9

Since this interesting diastereoselectivity is beyond the scope of this paper, it will be object of a forthcoming one. Oxidation of compounds 8 thus obtained results in the formation of single unsatured derivatives 9 (Figure 2). From the results reported in Table 1 it is evident that this protocol based on microwave-assisted solvent-free organic reaction is very efficient. Yields are at least equivalent, or even slight better than those described in literature and experimental conditions are sufficiently mild. The procedure avoids solvents and reactions are complete in very short time under safe conditions. Extension of this work to more challenging cases is currently in progress in our laboratories. Table 1. Dihydro-Pyridine 9a-i produced via procedure shown in Figure 2 *

b

*

c (13%) d (61%) e (95%) f (95%)

g

*

h

*

*

i

7

a

8

a (79%) b (60%) c (72%) d (47%) e (67%) f (60%)

g (54%)

h (70%) i (70%)

9

a (80%) b (92%) c (82%) d (90%) e (81%) f (78%)

g (63%)

h (80%) i (60%) S

R OTIPS O

R1 OH OMe

OTIPS

OTIPS

O OCH3

OCH3

OH

OMe

OMe

*: This product was not isolated since product 8 was directly obtained from 5

Multistep Microwave-Assisted Solvent-Free Organic Reactions O

O

O O

Ph

TIPSO

OTIPS 7f

O O

H

N

O

H

N

Ph +

TIPSO 8f

O O

H H

389

Ph

9f

N d-8f

Fig. 3. Compounds 7f, 8f and d-8f.

References [1] Gedye, R.; Smith, F.; Westway, K.; Ali, H.; Baldiserta, L.; Labergel, L.; Roussel, J. Tetrahedron Lett. 1986, 27, 279. [2] de la Hoz, A.; Diaz, -. O., A.; Moreno, A.; Langa, F. Eur. J. Org. Chem. 2000, 36593673. [3] Langa, F.; De La Cruz, P.; De La Hoz, A.; Diaz-Ortiz, A.; Diez-Barra, E. Contemp. Org. Synth 1997, 373-386. [4] Dittmer, D. C. Chem. Ind.-London 1997, 780-784. [5] Galema, S. A. Chem. Soc. Rev. 1997, 26, 233-238. [6] Strauss, C. R.; Trainor, R. W. Aust. J. Chem. 1995, 48, 1665-1692. [7] Caddick, S. Tetrahedron 1995, 51, 10403-10432. [8] Loupy, A.; Petit, A.; Mathe, D. Synthesis 1998, 1213. [9] Ger. Offen. 2 , 1979. [10] Abstr., C. 1979, 91, 2112273h. [11] JPn. Kokai Tokkyo Koha JP 61 1979. [12] Abstr., C. 106 1987, 156278q. [13] Yamada, H.; Tobiki, H.; Jimpo, K.; Gooda, K.; Takeuchi, Y.; Ueda, S.; Kamatau, T.; Okuda, T.; Noguchi, H.; Irie, K.; Nagagome, T. J. Antibiot. 1983, 36, 532. [14] DeJhon, D.; Domagala, J. M.; Hsakell, T. H.; Heifetz, C. L.; Huang, C. G.; Kaltenbromn, J. S.; Krolla, U. J. Antibiot. 1985, 38, 372. [15] Sakagami, K.; Iwamatsu, K.; Atsumi, K.; Hatanaka, M. Chem. Phar. Bull. 1990, 38, 3476-3479. [16] Bassini, C.; Bismara, C.; Carlesso, R.; Feriani, A.; Gaviraghi, G.; Marchioro, C.; Perboni, A.; Shaw, R. E.; Tamburini, B.; Tarzia, G.; Xerri, L. Il Farmaco 1993, 48, 159-189. [17] Boeckmann, R. K. J.; Thomas, A. J. J. Org. Chem. 1982, 47, 2823-2824. [18] Gianotti, M.; Martelli, G.; Mendozza, M.; Panunzio, M.; Campana, E. Synthetic Commun. 2000, 30, 1725. [19] Panunzio, M.; Castiglioni, E.; Campana, E.; Favi, G.; Vicennati, P. Application of Microwaves Technique to Organic Synthesis; Acierno, D., Leonelli, C. and Pellacani, G. C., Ed.; Mucchi Editore: Modena, 2000, pp 83-92.

Recent Applications of Microwave Power for Applied Organic Chemistry Bernd Ondruschka and Matthias Nüchter Institute of Technical Chemistry and Environmental Chemistry, University of Jena, D-07743 Jena, Germany

Introduction Use of microwave radiation as an unconventional energy source has been attempted by different research groups world-wide for more than 10 years. Different applications, including reaction engineering and separation steps [1], environmental engineering [2], and product syntheses of fine chemicals [3] are among the areas already known. After showing the innovative nature of microwave-assisted processing in chemical synthesis [4] solutions are now needed for appropriate reaction setups. As a contribution to this development, our institute started a close collaboration with a microwave equipment supplier (Microwave Labor Systems (MLS) Ltd, Leutkirch, Germany) with the aim on systems for scale-down strategies using parallel syntheses and combinatorial chemistry as well as systems for scale-up of microwave assisted processes, particularly in the pharmaceutical, cosmetics, and fine chemical area.

Scale-down and parallel syntheses A constantly increasing demand for new active substances in the pharmaceutical and fine-chemical industry facilitated some 15 years ago activities towards the development of new synthesis strategies, e.g., parallel synthesis and combinatorial chemistry [5]. This process is still on-going, however, the limitations inherent to these new synthesis concepts are meanwhile well known [6]. In parallel and in combinatorial synthesis the number of parallel working reactors can not be further increased, because a homogeneous energy transfer to a large number of parallel reactors is still not satisfactorily solved [7]. Processes, which require high temperatures to achieve short reaction times are another example of the poor “state of the art” of chemical synthesis tools. Usually high boiling solvents are applied, which have to be separated from the product.

Recent Applications of Microwave Power for Applied Organic Chemistry

391

Such high boiling solvents severely restrict the work-up and purification methods applicable to such reaction mixtures. On the other hand, low boiling solvents increase the reaction times, which limits the utilization of parallel synthesis. Further development in the field of parallel synthesis and combinatorial chemistry will crucially depend upon the availability of certain fine chemicals to perform model reactions. Such chemicals are needed on a 100 g scale, which is to small for industrial production and to large and time consuming if classical laboratory synthesis techniques and equipment is used. In order to develop new synthesis equipment, suitable for parallel and combinatorial approaches as well as for medium size production scale of different chemicals, new laboratory scale set-ups of the commercially available microwave chemistry equipment (MLS, Leutkirch, Type ETHOS 1600) were developed together with the equipment producer. The aim of this activity is to transfer the above mentioned new synthesis concepts into a particular type of equipment and to perform complete tests using such a lab-system. Doing so, the results of parallel and combinatorial as well as medium scale synthesis can be validated and made accessible for general use in other laboratories. As already tested to be successful a reactor system with a rotor-like configuration is applied for 6 - 10 parallel reactions (HPR 1000/6 as well as HPR 1000/10). This type of microwave heating equipment was originally developed for digestion of samples. Now it is extremely useful to perform chemical reaction upon microwave heating. By means of this set up the preliminary experiments for scale-up of transesterification of linalool with carbonic acid anhydride, shown in Figure 1 were successfully conducted. O OH

O

R

+

O

O R

O

Base

R

+

R

OH O

Fig. 1. Transesterification of linalool with carbonic acid anhydride; the particular reaction conditions are as follows: additive:alkali carbonate; 18 mmol/mole linalool; residence time: 15 - 20 min; molar ratio of carbonic acid anhydride to linalool = 2,0 - 2,3 : 1

By further improvement of the “rotor” reactor and adjustment to the needs of synthesis-chemists, the reaction system MultiPrep 36/P, capable to process 36 single devices simultaneously was developed. The rotors are arranged in two circles, as shown in Figure 2. The glass vials have a net volume of around 40 m and allow reaction conditions of 160°C and 5 bars excess pressure. Each device is placed in a chamber and mantled by a thin transparent teflon-coat. Gas tight fittings and a safety valve are included. If needed, a magnetic stirrer can be introduced. It is possible, to perform numerous reaction-types in the microwave-field as parallel-reactions.

392

Ondruschka

Fig. 2. Microwave-system ETHOS MR with MultiPrep-reactor for 36 singles-reactors

By means of this ETHOS set-up we were able to test parallel reactions under selected conditions. First, homogeneity of heating was tested, by repetition of identical reaction conditions. Numerous reaction types were than tested:  nucleophilic substitution  esterification  ester condensation  acetalisation / ketalisation  Biginelli reaction [8]  Hantzsch reaction [9]. After the reproducibility of the results was established for all 36 single devices and for numerous reaction conditions, the system was employed for a more advanced study of microwave-assisted parallel synthesis, with selected variation of reaction mixtures in the 36 devices.

Microwave assisted esterification, parallel reactions The results of esterification-reactions for 6 different n-alcohols with acetic acid (6 x 6 = 36 devices) are summarised in Figure 3. This figure shows the reliability of the used system and the comparability of yields of corresponding n-alkyl acetate. The yield-deviations between the outer (1. - 20. device) and the inner ring (21. - 36. device) are small and can be caused not only by temperature effects but by a sligthly varyiing composition od the starting materials, e.g., chemical, water content of the used n-alcohols or by the different chemical reactivity of the nalcohols as well. In particular, a slightly higher water content of n-butanol in comparison with n-hexanol could be responsible for the lower yield of the corresponding acetate.

Recent Applications of Microwave Power for Applied Organic Chemistry

393

100,00 C3OH

C6OH

95,00 C4OH C8OH

C10OH

C12OH

yield [ma%]

90,00

85,00

80,00

75,00

70,00 1

2

3

4

5

6

7

8

9

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

Reactor number

Fig. 3. Esterification of 6 different n-alcohols with acetic acid in ETHOS MultiPrep 36/P (30 min and 120°C) .

Following this series of experiments, a much more complex reaction was studied, the Biginelli reaction. The reaction is explained in Figure 4. To test the applicability of the microwave assisted synthesis reactor for heterogeneous reaction mixtures, the aldehyde component was changed. X

X

O

O

H NH2

O

+

3

R

2

R

H

O

N

3

R

+ HN

O

O Z

1

R

2

R

N

Z

1

R

4-Aryl-3,4-dihydro2(1H)-pyrimidon-carboxylat

Fig. 4. Schematic representation of the Biginelli reaction

The Biginelli reaction was studied with 6 different benzaldehydes, in the as described ETHOS MultiPrep 36/P reactor. The reaction conditions were kept constant at 30 min and 100°C. The yield for all 6 systems, again distributed in the outer and inner rotor, are shown in Figure 5.

394

Ondruschka 100,00 90,00

benzalde salizylaldehyde

p-dimethylamino benzaldehyde

80,00

m-nitrobenzaldehyde

yield [ma%]

70,00 60,00

veratrumaldehyde o-chlorobenzaldehyde

50,00 40,00 30,00 20,00 10,00 0,00

1

2

3

4

5

6

7

8

9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

reactor number

Fig. 5. Results of the Biginelli reaction with 6 different benzaldehydes in the MultiPrep 36/P-ETHOS-system. Reaction conditions 100°C, 30 min.

The successful implementation of a multicomponent reaction (Ugi type) promoted further research towards parallel synthesis by variation of 2 components in a base. Again 6 different aldehydes were used and combined with urea or thiourea. A group formation was planned (6 x 2 x 3) and tested. Again, to insure comparability each reaction mixture was placed both in the inner and the outer rotor ring of the MultiPrep system. The reaction products were isolated and analysed with respect to yield. As in the previous reactions, the yields correspond to the relevant reactivity of the aldehydes.

Microwave assisted combinatorial chemistry After the feasibility of microwave-assisted reactor system was established on parallel reactions, the next series of experiments was designed to test “real” combinatorial chemistry in this reactor system. Because of the large number of samples, a different containment for the reaction mixtures was developed. The vials were replaced by deep well plates, suitable for probe sampling by use of a pipetting automate. As a result of this development a new type of microwave assisted reactor was built, the CombCHEM system, which enables the use of one or several deep well plates. On principle, use of 24, 48, 96 and any multiple number 6 vials is possible. Working at pressure of several bar is possible.

Recent Applications of Microwave Power for Applied Organic Chemistry

395

The homogeneity of the microwave field distribution in the cavity of the CombCHEM system was tested by the same method as for the MultiPrep. Test experiments revealed the same reaction mixture similar or identical yield for every vial. Again, small deviations are found which are most probably caused by the sample preparation without an automatic dosage. For test purposes, the esterification of hexanoic acid with n-hexanol and octanoic acid with octanol was performed. Finally, the ComChem system was validated by the Hantzsch reaction, which is schematically shown in Figure 6. The performance of the reactor system is demonstrated by the results of the MCR type, shown in Figure 7. 2

R O

H5C2

C

O

O

H

+ 1

R

O

O

O O

NH3

O

C2H5

-3 H2O

H5C2

2

R

H

O

O

O 1

R

1

R

C2H5

1

N

R

H Ox.

1

R = methyl 2

R = 2-hydroxy-phenyl (range 1)

O

3-chloro-phenyl (range 2) H (range 3)

H5C2

2

R

O

O

O 1

phenyl (range 4)

R

N

C2H5

1

R

Fig. 6. Schematic representation of the Hantzsch-reaction

1 00 90 80

yield [max]

70 60 50 40 30 20 10 0

1

2

3

4

range 1

5

6

7

8

9

10 11 12

r ange 2

13 14 15 16 17 18

range 3 r e a c t or n um be r

19 20 21 22 23 24 range 4

Fig. 7. Yield-results of a Hantzsch reaction in the ETHOS CombCHEM system (24-deep well plate, 20 min and 110°C)

396

Ondruschka

As found before, it is possible to distinguish the reactivities of the staring aldehydes. The yield-deviations are very small. For the evaluation, both the dihydropyridines as well as the corresponding by-product pyridine derivatives were examined.

Scale-up The aim of further work is the transfer of laboratory developed microwaveassisted processes into pilot and industrial scale. The advantage of the reaction acceleration and the fast energy transfer found in laboratory scale reactions needs to be verified in larger batch as well as in continuous reactions. First attempt to develop equipment suitable for contineous microwave assisted reactions were successful, as shown in Figure 8 (a). The ETHOS Pilot 4000 is capable to cover the range from lab- to pilot-scale and continuous- reactions at with up to 100 mole feed scale. By utilization of the jointly developed (MLS Ltd. Milestone) reactor system MPR 2000, shown in Figure 8 (b), a further extension of the applicability of the microwave assisted synthesis is possible.

Fig. 8. (a) ETHOS Pilot 4000 equipment with a contineous reactor; (b) ETHOSMPR 2000 reactor

This MW-system is a “dual” system, useable both as a batch and also - in the main application purpose - as a continuous system with reactors of different diameters and with magnetic or mechanical stirrer. The spectrum of residence times becomes adjustable in microwave field by such means. The reactor system was tested by performing esterifications and transesterifications reaction, which are easy to analyse and where larger quantities of feeds are

Recent Applications of Microwave Power for Applied Organic Chemistry

397

available. Particularly the transesterification from rapeseed oil to rapeseed methyl ester, cf. equation 3 was of interest. The multiple runs of the reaction have shown the utilization of continuous regimes in the microwave-supported pilot setup. 1) R-COOH + R1-OH o R-CO-O-R1 + H2O 1 o R-CO-O-R1 + R-COOH 2) (R-CO)2O + R -OH o R-CO-O-R2 + R1-OH 3) R-CO-O-R1 + R2-OH In the near future, by means of an on-line-HPLC and the long time stability of the reaction system reaction optimization will be attempted.

Acknowledgment The authors thank the firm MLS GmbH Leutkirch / Germany for the intensive cooperation in development of the microwave systems and reactor design.

References [1] M. Nüchter, B. Ondruschka, A. Jungnickel, U. Müller, J. Phys. Org. Chem. 2000, 13, 579-586. [2] U. Nüchter, B. Ondruschka, H. G. Struppe, M. Nüchter, Chem. Technik 1995, 50, 249252 [3] for example: a) Ch. R. Strauss, R. W. Trainer, Aust. J. Chem. 1995, 48, 1665-1692, b) R. J. Varma, Green Chem. 1999, 43-55, c) A. K. Bose, B. K. Banik, N. Lavlinskaja, M. Jayaraman, M. S. Manhas, CHEMTEC 1997, 18-24.

[4] for example: a) S. Caddick, Tetrahedron 1995, 51, 10403-10432, b) S. G. Deng, Y. S. Lin, Chem. Eng. Sci. 1997, 52, 1563-75, c) N. Elander, J. R. Jones, S.-Y. Lu, S. Stone-Elander Chem. Soc. Rev. 2000, 29, 239-49. [5] a) I. Ugi, J. Prakt.Chem.-Chem.-Ztg. 1997, 339, 499-516, b) R. Schlögl, Angew. Chem. 1998, 110, 2467-70. [6] E. H. Ohlstein, R. R. Ruffolo, J. D. Elliott, Ann. Rev. Pharmacol. Toxicol. 2000, 40, 177-91. [7] R. P. Hertzberg, A. P. Pope, Curr. Opinion Chem. Bio. 2000, 4, 445-51. [8] a) P. Biginelli, Gazz. Chim. Ital. 1893, 23, 360-416, b) C. O. Kappe, Tetrahedron 1993, 32, 6937-63. [9] a) A. J. Hantzsch, Liebigs Ann Chem. 1892, 215, 1-47, b) N. Iqbal, C. R. Triggle, E. E. Knaus, Drug. Dev. Res. 1997, 52, 120-30. [10] Autorenkollektiv, Organikum: Organisch-chemisches Grundpraktikum, 20. Aufl., Wiley-VCH, Weinheim 1999, 519.

Liquid Phase Catalytic Hydrodechlorination of Chlorobenzene Under Microwave Irradiation Marilena T. Radoiu1, Ioan Calinescu2, Diana I. Martin1, Rodica Calinescu2 1

National Institute for Lasers, Plasma and Radiation Physics, Electron Accelerator Laboratory, R-76900 Bucharest, Romania 2 Polytechnic University of Bucharest, Industrial Chemistry Faculty, , R-71102 Bucharest, Romania

Introduction There is an ongoing need in finding better methods of destruction of the organic wastes containing halogen atoms since conventional incineration of these wastes has been recognized to be associated with the unexpected formation of more harmful by-products such as polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans. The major methods of destruction of these compounds are thermal and catalytic destruction with catalytic destruction favored economically because of the lower temperatures involved in conversion of the chlorinated component into easily trapped inorganic, leaving the initial compound in a less toxic recyclable form. In recent years, the process of hydrodechlorination has been realized by using a hydrogen-donor and a catalyst. Several hydrogen-donors as alcohol have been used in combination with homogeneous and heterogeneous supported noble metal catalysts. This method is recognized as a facile and efficient procedure. However, the practical application of catalysts to the dehalogenation of organic halides is always accompanied by the problem of the deactivation of the catalyst. In heterogeneous catalytic systems with solid catalysts, the activity change may be caused by absorption of the halogen, halogenation of catalyst [1], surface composition change of catalyst [9] and formation of oligomers and coke [2]. For practical use, finding a method/catalyst that maintain the activity for a prolonged time is an essential problem. Moreover, commercial catalysts must overcome interference by concomitants contained in organic wastes, because industrial wastes sometimes consist of mixtures of organic compounds including compounds undesirable for the catalytic reaction. The aim of this study was to examine the chlorobenzene hydrodeclorination in heterogeneous liquid phase under conventional and microwave heating. Catalysts and reagent have been tested from point of view of their activity, reactivity and selectivity.

Liquid Phase Catalytic Hydrodechlorination of Chlorobenzene Under Microwave

399

Experimental The catalytic dehalogenation reaction was carried out under microwave irradiation, at atmospheric pressure and reflux temperature of the reaction mixture formed by 2-propanol, chlorobenzene, NaOH and catalyst. The proposed reaction scheme is as follows: CH3-CH(OH)-CH3 ĺ CH3 –C(O)-CH3 + 2H* C6H5-Cl + 2H* ĺ C6H5-H + HCl HCl + NaOH ĺ NaCl + H2O C6H5-Cl + CH3-CH(OH)-CH3 + NaOH ĺ C6H5-H + CH3 –C(O)-CH3 + H2O

where H* indicates the hydrogen-transfer species

(1) (2) (3) (4)

Materials and catalyst A commercial alumina-supported palladium (5 wt%) catalyst (Viromet S.A., Romania) was used for the dechlorination reaction. The catalyst was analyzed by electronic microscopy using a Philips XL 20 device and its structure and composition are shown in Fig. 1. Catalyst was used as obtained. All chemicals were of high-purity grade and used without further purification. A commercial microwave oven properly modified was used as microwave applicator. Modifying the oven magnetron supply produced the variable power, up to 850 W [6].

Fig. 1. Catalyst structure and composition

Reaction procedure The catalytic hydrodehalogenation reaction was performed at reflux temperature of 2-propanol (~83°C) and 1 atm with no use of molecular hydrogen. In this transformation 2-propanol is the source of hydrogen, including hydrogen transfer from 2-propanol to organic halides [8].

400

Radoiu

A characteristic of this system is that the substitution of halogen by hydrogen occurs selectively without hydrogenation of the aromatic ring. The reactions were carried out in a 100 ml two-neck Pyrex vessel fitted with water-cooled condenser and mechanic stirrer. Mixture of 0.1 g catalyst, 25 ml 2-propanol, 0.25 ml (100 mmol/l) chlorobenzene and 0.3 g (300 mmol/l) NaOH was stirred and irradiated by microwaves in the range of power output 200 - 300 W. Capillary GC was used in order to analyze concentrations of chlorobenzene and benzene after reaction. The analysis was performed on a Fissons 8330 gas chromatograph with flame ionization detector (FID) and hydrogen as carrier gas. Products were separated on a 30 m x 0.32 mm x 0.25 Pm WCOT fused silica nonpolar capillary column, CP-Sil 8 CB.

Results and discussion

Influence of the catalyst amount The catalytic dechlorination of a mixture containing 100 mmol/l chlorobenzene was carried out in a solution of NaOH (300 mmol/l) in 2-propanol using different catalyst amounts at reflux temperature of the mixture. Results are presented in Table 1 and Fig. 2. Table 1. The catalyst amount influence on the dehydrodecholrination of chlorobenzene XMW [%] XMW/ XCH Time XCH [%] [min] 0 0 0 0 15 12.1 15.0 1.25 0.05 30 18.7 21.8 1.17 45 20.0 22.0 1.10 60 20.8 22.0 1.06 0 0 0 0 15 20.5 39.8 1.94 0.10 30 36.0 71.5 1.99 45 41.2 83.9 2.02 60 46.3 95.0 2.05 0 0 0 0 15 30.0 62.5 2.08 0.15 30 47.5 95.2 2.00 45 54.2 98.6 1.82 60 60.0 98.9 1.64 XCH chlorobenzene conversion by conventional heating, XMW chlorobenzene conversion by microwave heating. Catalyst amount [g]

The initial concentration of chlorobenzene was 100 mmol/l. The chlorobenzene concentration decreased with the formation of benzene, the reaction rate in mi-

Liquid Phase Catalytic Hydrodechlorination of Chlorobenzene Under Microwave

401

crowave heating being 1.06 to 2.08 times higher than in conventional heating. The selectivity of the transformation of chlorobenzene to benzene in the described transformation was always 100%. X [%]

100 80

0.05g CH

60

0.10g CH

40

0.15g CH

20

0.05g MW 0.10g MW

0 0

15

30

45

60

0.15g MW

Time [min]

Fig. 2. Influence of the catalyst amount on the dehydrodechlorination of chlorobenzene by conventional (CH) and microwave (MW) heating

Influence of NaOH amount Different molar ratios of NaOH to chlorobenzene were tested for the liquid-phase hydrodechlorination of chlorobenzene and the results obtained are presented in Table 2 and Fig. 3. 100 X [%] 80

1.0/1.0 CH 1.5/1.0 CH 3.0/1.0 CH

60

4.5/1.0 CH

40

1.0/1.0 MW

20

1.5/1.0 MW

0

3.0/1.0 MW 0

15

30

45

60

4.5/1.0 MW

Time [min]

Fig. 3. Molar ratio of NaOH/chlorobenzene influence on the dehydrodechlorination of chlorobenzene by conventional (CH) and microwave (MW) heating

Firstly, the effect of addition of stoichiometric (to chlorobenzene) NaOH to the reaction mixture was studied. Thus, in theory all HCl formed during reaction can be neutralized. As shown in Table 2, in conventional heating the maximum conversion/reaction rate were reached by working with an excess of NaOH (NaOH/chlorobenzene 1.5/1 mol/mol). Further addition of NaOH decreased cata-

402

Radoiu

lytic activity. These results could be considered surprising, since the neutralization of the HCl formed was expected to increase the reaction rate. This behavior can be explained by taking into account the increase of the alkaline pH of the reaction medium that modifies the porous system and solubilises the Pd0 [1] and by the formation of NaCl that covers the surface of the catalyst and by this, reduces its activity [9]. Table 2. Influence of the molar ratio NaOH/Chlorobenzene on conversion rate upon microwave and conventional heating NaOH/chlorobenzene [mol/mol] 1.0/1.0

1.5/1.0

3.0/1.0

4.5/1.0

Time [min] 0 15 30 45 60 0 15 30 45 60 0 15 30 45 60 0 15 30 45 60

XCH [%] 0 19.0 23.5 26.6 29.0 0 24.6 40.0 48.0 52.1 0 20.5 36.0 42.0 46.3 0 22.0 28.7 30.5 32.1

XMW [%] 0 19.5 38.7 45.6 49.8 0 30.8 52.5 63.5 69.8 0 39.8 71.5 83.9 95.0 0 51.2 69.3 80.0 88.2

XMW/ XCH 0 1.02 1.64 1.71 1.72 0 1.25 1.31 1.32 1.34 0 1.94 1.99 2.00 2.05 0 2.33 2.41 2.62 2.74

When transformations of chlorobenzene has been carried out under microwave conditions, significant differences in reaction rates and in reaction course have been recorded. The addition of NaOH gave higher reaction rate/conversion of chlorobenzene. As can be seen from Table 2, the microwave enhancement of the reaction was in the range of 1.02 to 2.74. Activation of the catalyst Our attention was focused on the increase of the catalyst activity by electric heating. The activation was performed using an conventional heating in an electric oven. Mixtures of 0.255 ml chlorobenzene, 25 ml 2-propanol, 0.1 g in- and activated catalyst and 0.3 g NaOH was stirred and irradiated by microwaves at 300 W. Results are presented in Table 3. When the dechlorination reaction of chlorobenzene was carried out in the presence of the activated catalyst, the dechlorination rate was significantly affected compared with that in the presence of the fresh catalyst – Fig. 4. The tolerance of the catalyst for water has practical advantages because industrial wastes sometimes contain water.

Liquid Phase Catalytic Hydrodechlorination of Chlorobenzene Under Microwave

403

Table 3. Activation of the catalyst influence on the transformation of chlorobenzene by microwave heating Conditions of activation Inactivated

Weight lost [g] 0

2 h at 105 0C, electric oven

7.25

2 h at 180 0C, electric oven

12.50

Time [min] 15 30 15 30 15

30

XMW [%] 39.8 71.5 22.5 28.9 4.7

9.4

Fig. 4. Influence of the catalyst ctivation on dehydrodechlorination of chlorobenzene by microwave heating

Conclusions Heterogeneous catalytic dehydrochlorination of chlorobenzene by protons transfer was studied in a solution of NaOH in 2-propanol in the presence of a commercial alumina-supported Palladium (5 wt%) catalyst. Heating of the reaction mixture to the reflux temperature, ~83°C was done by conventional methods (electric heating) and by microwave irradiation. It was established that microwaves had a strong effect on the catalytic process from point of view of both the reaction rate and activity of the catalyst – Fig. 5. Despite of the gradual decrease of the catalytic activity due probably to the formation of HCl and accumulation of NaCl on the surface of the catalyst, this decrease was just slightly observed in the microwave conditions. This observation is very important and it can be explained by either accepting material-wave interactions leading to thermal effects (connected to dipolar and charge space polarizations) and specific (non-purely thermal) effects resulting from variations in activations parameters, enhancements in molecular impacts and possibly high localized microscopic temperatures [3, 5]. The presence of 5 - 12% of water in the catalyst has a positive effect on the reaction rate in the presence of microwaves.

404

Radoiu

Fig. 5. Heterogeneous liquid-phase catalytic dehydrohalogenation of chlorobenzene under conventional (CH) and microwave (MW) conditions

References [1] Aramendia MA, Burch R, Garcia IM, Marinas A, Marinas JM, Southward BWL, Urbano FJ (2001) The effect of addition of sodium compounds in the liquid-phase hydrodechlorination of chlorobenzene over palladium catalysts. Appl Catal B 31: 163-171 [2] Bae JW, Park ED, Lee JS, Lee KH, Kim YG, Yeon SH , Sung BH (2001) Hydrodechlorination of CCl4 over Pt/J-Al2O3: Effects of reaction pressure and diluent gases on distribution of products and catalyst stability. Appl Catal A 217:79-89 [3] Hajek M, Radoiu MT (2000) Microwave activation of catalytic transformation of tbutylphenols. J Mol Cat A 160: 383-392 [4] Kameoka K, Myiadera T (2000) Catalytic dechlorination of aromatic chlorides with noble-metal catalysts under mild conditions: approach to practical use, Appl Catal B: Environmental 27: 97-104 [5] Loupy A, Perreux L, Petit A (2001) Solvent-free microwave assisted organic synthesis. In: Microwaves: Theory and Application in Materials Processing V. Ceramic Trans 3: 163-172 [6] Radoiu MT, Calinescu I, Chipurici P, Martin DI (2000) Microwave heating in the hydrogen peroxide oxidation of benzene on zeolite catalysts. J MW Power 35: 86-90 [7] Ukisu Y, Ikimura S, Uchida R (1996) Catalytic dechlorination of polychlorinatedbiphenyls with carbon-supported noble-metal catalysts under mild conditions. Chemosphere 33: 1523-1530 [8] Ukisu Y, Miyadera T (1997) Hydrogen-transfer hydrodehalogenation of aromatic halides with alcohols in the presence of noble-metal catalysts. J Mol Catal A 125: 135142 [9] Ukisu Y, Kameoka S, Miyadera T (2000) Catalytic dechlorination of aromatic chlorides with noble-metal catalysts under mild conditions:approach to practical use. Appl Catal B 27: 97-104

Conventional and New Solvent Systems for Microwave Chemistry Jens Hoffmann, Antje Tied, Matthias Nüchter and Bernd Ondruschka Institute of Technical Chemistry and Environmental Chemistry, University of Jena, D-07743 Jena, Germany

Introduction Since approximately 15 years microwave-assisted processes are intensively studied in chemical research [1]. In lab-scale experiments numerous reactions and technologies were successfully tested [2], naturally with not too much attention on general issues, like e.g., energy efficiency upon up-scaling, system safety, and continuous reaction engineering. Domestic microwave ovens or their modifications are usual the instrument of choice in organic laboratories [3]. For chemical applications specially designed systems, with means for control of reaction parameters, magnetic or mechanical stirring, and continuous power supply to evenly irradiate the reaction mixtures [4], are expensive in comparison to usual laboratory appliances. However, only such innovative microwave systems are capable of providing the information needed for process up-scaling and introduction of microwave processing into chemical industry. Because the vast majority of results reported from laboratory microwave reaction experiments is collected from small scale (mmol or few milligrams of chemicals) experiments, basis considerations about dielectric loss and heating behaviour of the chemicals are not taken into account, when discussing the results. Phenomenological and “yield of product” -oriented statements predominate. However, it should be stated, that the use of domestic microwave equipment on few grams or milligrams of a reaction mixture [5] is equivalent with the use of a very high energy dose. From the view point of an industrial use for a reaction which has shown benefits in terms of yield and product quality when microwave radiation is applied, such a high energy consumption could cause numerous problems, e.g., regarding the energy efficiency, safety, size and shape of the reactor (depending upon the penetration depth of microwave radiation into reaction mixtures), measurement and control of reaction parameters like temperature, residence time, pressure, energy distribution within reactor system. Furthermore, the choice of materials for reactors and periphery is different and to some extent limited upon application of microwave radiation as compared to conventional chemical engi-

406

Hoffmann

neering. As part of a general study aiming at the scale-up of microwave applications to industrial chemical synthesis, heating behaviour of polar solvents and solvent mixtures was systematically investigated upon microwave heating to gain knowledge about the energy efficiency of such processes as compared with conventional thermal behaviour.

Heating behaviour of polar solvents exposed to a microwave field The commonly used microwave frequency in domestic ovens is 2.45 GHz. Therefore, in order to relate the large number of experiments known from literature to the actual heating behaviour of different polar solvents, for all experiments the same frequency of 2.45 GHz is used. A 500 g sample of the solvent was exposed to different levels of microwave power in the cavity. The experiments were performed in a commercially available microwave chemical synthesis reflux apparatus (ETHOS MR, glass reflux apparatus). The results are shown in Figure 1.

temperature [°C]

120 100

190 sec 250 sec 380 sec

80 850 sec

60 40

300 W 700 W

20

0 -100

100

300 500 time [sec]

500 W 900 W

700

900

Fig. 1. Heating behaviour of water (500 g, 27.77 moles) as function of different microwave power levels (2.45 GHz) at temperatures of 25 to 100°C (reflux)

In a second step, the amount of the solvent was varied at and the microwave power level is kept constant. The results are shown in Figure 2. From the slope of a heating curve at a fixed microwave power the amount of microwave power converted into heat can be re-calculated, if the temperature and the mass of the solvent is accurately known. A further improvement by taking into account the “distortion” of the microwave field by the solvent is achieved from the heating experiments with various amounts of a solvent exposed to a fixed microwave power. This method can be applied to all stable solvents. Taking the calorimetric values of the solvent heat capacity from the literature, it is possibly to determine the consumed power microwave power and to relate it to

Conventional and New Solvent Systems for Microwave Chemistry

407

the total microwave power. An “efficiency” factor for different solvents can be defined for most of the solvents used in a laboratory microwave synthesis apparatus.

Q

m ˜ c ˜ 'T

PTh PMw

K

(1)

(2)

temperature [°C]

120 100 80 60 40 20

500 g

400 g

300 g

200 g

0 0

50

100

150

200

250

300

time [s] Fig. 2. Heating behavior of different quantities of water until boiling at 700 W microwave power level (ETHOS MR, glass reflux apparatus)

1,00

efficieny factor

0,90 0,80 0,70 0,60 0,50

energy [W]

0,40 0

200

400

water n-octanol Polynomisch (water) Polynomisch (n-octanol)

600

800

1000

n-butanol n-decanol Polynomisch (n-butanol) Polynomisch (n-decanol)

Fig. 3. Efficiency degree for 500 g of solvent at different microwave power levels (ETHOS MR, glass reflux apparatus); the lines represent a polynomial fit

The relation between this “efficiency factor” and the microwave power level is shown in Figure 3 for the same weight of different solvents. It is clearly seen, that all polar solvents show a similar heating efficiency due to microwave absorption.

408

Hoffmann

At low power levels of course a larger inaccuracy of such calculations comes from heat losses not accounted for. However, it is obvious, that for achieving a high efficiency of heating a short processing time at as high as possible microwave power levels should be chosen. A natural limit for such processing parameters is given by the electric brake down of the solvent. Although the basic background for microwave heating is known since decades, its use for discussion of microwave-assisted processes in the organic chemistry is very limited. The superiority of processing at very high local electric field strength levels is clearly see from the basic equations relating heating rates and microwave field strength. According to [6, 7], the heating curve as well as the absorbed microwave energy are proportional to square of electric field strength.

P

2 1 ZH ´´ ~E~ 2

(3)

'T 't

V Uc

2 ~E~

(4)

P: absorbed microwave power; V: DC-conductivity; Z: angular frequency; U density; H´´imaginary part of the dielectric constant; c molar heat capacity; ~E~ local electrical field strength

Of course, for chemical reactions, analysis of the molar quantities is more important than the mass of a solvent. Therefore it would be very useful to develop “molar efficiency” factors for different solvents an ambitious task, when one recalls the problems to calculate the dielectric properties of a substance from its molecular features, e.g., dipole moments and polarisability. Therefore, only a very qualitative picture can be expected from recalculation of the heating curves to molar quantities. From the polynomial fit of the heating curves in Figure 1 and 2 a “theoretical” molar heating rate was calculated and compared with experimental results obtained from heating experiments with 500 g of the solvent. The results are shown in Figure 4 and Table 1. The lowest molecular weight has water; therefore the largest number of molecules is present per volume to convert by some collective “viscous” motion the energy of the electromagnetic field into heat. The alcohol molecules should be less efficient in terms of molar heating, by their lower number. For low power levels this idea is quite reasonably reflected by the results shown in Table 1, reflecting a large time difference for heating up the solvent. The “alcohol molecules” need more time to generate heat in a certain mass of the solvent by absorption of microwaves than water molecules. It is well known, that not the movement of isolated individual molecules is responsible for heat generation upon microwave absorption in liquids, but the collective movement inside the liquid. The results in Figure 4 clearly show, that at a high power level of microwave radiation less efficient “molar” heating occurs and furthermore, a “saturation” is observed for all solvents. This is not really surprising, because with increasing temperature the dielectric loss of liquids is decreasing. An alternative interpretation is possible by comparing the energy content of a 50°C “hot” liquid (eV~kT) with the photon energy of the microwave radiation.

Conventional and New Solvent Systems for Microwave Chemistry

409

300,0

n-decanol

250,0

time [sec]

200,0

n-octanol

150,0

n-butanol

100,0

Water 50,0

0,0 0,0

100,0

200,0

300,0

400,0

500,0

600,0

700,0

800,0

900,0

1000,0

energy [W]

Fig. 4. Calculated molar heating time based on fitted curves of experimental heating curves for 500 g solvent

Table 1. Comparison of calculated with experimentally determined (500 g runs) molar heating times for selected solvents (ETHOS MR, glass reflux apparatus) Microwave Energy [W]

Time [s] Water calc.

Time [s] Water exp.

200

Time [s] n-Butanol calc. 123

Time [s] n-Butanol exp. 141

Time [s] n-Octanol calc. 220

Time [s] n-Octanol exp. 243

65

79

110

137

300

31

500

14

25

33

39

54

80

700

9

15,5

21

28

34,5

46

900

7

12

16

23

27

32

1000

12

22

31

Hence, there is for each solvent a macroscopic efficiency factor, calculated from 'TxCp which can be used to judge about the “primary energy use” of a chemical synthesis performed using microwave heating. Here high microwave power levels and short processing times are recommended. A statement regarding the “molecular” efficiency of a solvent is not possible. At high power levels the thermal motion is predominant and “activation” by microwave radiation seem s to be less efficient.

410

Hoffmann

Heating behaviour of ionic liquids in microwave field Ionic liquids are one of the intensively examined groups of compounds [8, 9] for solvent promoted catalysis reactions. They are mainly quaternary ammonium or phosphonium compounds, with a melting point below 100°C [9]. The ammonium group is often attached to a imidazolium or pyridinium ring. The counter ions are large, area-filling anions, like BF4, PF6, benzenesulfonium, and trifluoromethylsulfonium. Examples of typical cations are shown in Figure 5.

+ N

R´

N + N

R

+ R P R R

Ammonium cation

Phosphonium cation

R´

R Pyridinium cation

Imidazolium cation

R´

+ R N R R

Fig. 5. Cations of ionic liquids

Because of the ionic structure a strong absorption of microwave radiation can be expected from ionic liquids. The anion-cation arrangements form inner dipols, therefore besides ionic conduction as loss mechanism responsible for microwave absorption also ion rotation can be assumed. In Table 2 the ionic liquids used in this study are listed, along with the abbrevaitions used in the figures. In Table 3 examples some frequently used ionic liquids are shown. Table 2. The ionic liquids used in this study [BMIM][PF6] [BMIM][BTA] [EMMIM][BTA] [EMIM][BTA] [MMIM][BTA]

1-Butyl-3-methyl-imidazolium-hexafluorophosphate 1-Butyl-3-methyl-imidazolium-bis-((trifluoromethyl)sulfonyl)amide 1-Ethyl-2,3-dimethyl-imidazolium-bis-((trifluoromethyl)sulfonyl)amide 1-Ethyl-2-methyl-imidazolium-bis-((trifluoromethyl)sulfonyl)amide 1,3-Dimethyl-imidazolium-bis-((trifluoromethyl)sulfonyl)amide

Table 3. Examples of ionic liquids and their melting points as well as typical alkylation compounds used for synthesis of chemicals in such solvents Ionic liquid

Mp. [°C]

Alkylation compound

1-Butyl-3-methylimidazolium-chloride 1,3-Diethyl-imidazoliumbromide

65 - 69

Butylchloride

53

Ethylbromide

Conventional and New Solvent Systems for Microwave Chemistry 1-Butyl-3-methylimidazolium-trifluoroacetate

16

Trifluororacetic acid methylester

Octyltriphenylphosphonium-p- 70 - 71 toluene-sulfonate

p-Toluenesulfonic acid octylester

Tributylmethylammonium-ptoluene-sulfonate

p-Toluenesulfonic acid methylester -

62

411

For preliminary testing of the applicability of these solvents to synthesis using microwave radiation the same equipment is employed as for the polar solvent testing. Besides pure ionic liquids mixtures of such compounds with non-polar solvents were investigated. The results of microwave heating experiments are shown in Figure 6. Since in most cases less than one mole of an ionic liquid was available, the measured heating time is normalized to one mole for all used solvents.

45 40

25 20 15

time [min]

35 30

10 5 0

200 energy [W]

400

[MMIM][BTA] [EMIM][BTA] [EMMIM][BTA] [BMIM][BTA] [BMIM][PF6]

Fig. 6. Heating times of ionic liquids to reach a temperature of 140°C by application of 200, 300 or 400 W microwave power. Normalized to 1 mole, starting temperature 40°C.

Surprisingly, the anion, e.g., hexafluorophosphate or sulfonic acid, as well as the cation composition and structure has an influence on the microwave heating behaviour, as seen in Figure 6 particularly for low to medium power levels. At high power levels the difference is less pronounced – the same qualitative behaviour is found for polar solvents. Small differences in the chain length, e.g., ethyl- or butyl- as well as in the side groups, e.g., mono-methyl or dimethyl, cause differences in microwave absorption. From the viewpoint of “structural parameters” responsible for dipole rotation as well as viscosity the elongation of a chain or the addition of a side group could be important.

412

Hoffmann

Time [min]

In a second type of experiments, the influence of small quantities of ionic liquids on the heating behaviour of nonpolar, microwave transparent (non-absorbing) solvents, like toluene or cyclohexane was tested. Different ratios of the nonabsorbing solvent and the ionic liquid were employed. The results are shown in Figure 7.

12 11 10 9 8 7 6 5 4 3 2 1 0 100 : 1

Solvent-Ratio

100 : 3

100 : 5

Fig. 7. Dependence of heating times for a temperature raise of 'T = 80 K on microwave power level, for a mixture of toluene with different ionic liquids. The toluene-ionic liquid ratio is varied from 100:1 to 100:3 to 100:5. Each mixture is exposed to 300; 400; and 500 W microwave power, respectively (columns from front to back). The composition of the ionic liquid is (from left to right): [MMIM][BTA]; [EMIM][BTA]; [BMIM][BTA]; [BMIM][PF6]; [EMMIM][BTA];.see Table 2 for full names.

The few % (1 - 5) additive of a ionic liquid to toluene dramatically reduces the time to raise the temperature by 80 K. This means, very efficient microwave absorption is possible in such “diluted” ionic compound. Comparison with data shown in Figure 6 for the pure ionic liquids reveals the degree of efficiency of microwave heating in this dilute ionic liquid solution. Although the molar heat capacity of toluene is most probably only half of the one for the ionic liquids, still a remarkable increase of heating rate remain, e.g., 5% [MMIM][BTA] in toluene heats up by 80 K at 500 W in less than 2 minutes; the 100% (20 x more) ionic liquid heats up by 100 K with 400 W power in 5 minutes. It can be speculated, that the penetration depth of microwaves into such a diluted solution of an ionic liquid is much higher than into the pure solvent. Therefore efficient utilisation of the applied microwave power can occur. The application of small amounts of ionic liquids as “internal heating elements” in chemical reaction using microwave heating could become a very attractive field of F&E.

Conventional and New Solvent Systems for Microwave Chemistry

413

Summary Heating experiments on different solvents and ionic liquids were carried out in a commercially available microwave-synthesis system (ETHOS 1600, MLS Ltd.), with the aim to define parameters relevant to scale-up of microwave heating for in industrial chemical synthesis. Such heating experiments are useful to estimate the macroscopic energy efficiency of a solvent by calorimetric calculations. A certain reactor volume has to be defined experimentally for each equipment, to exploit an optimum heating rate in the reaction. The heating rate in real systems is not linearly increasing with increased microwave output power, due to heat loss, limited penetration depth of the radiation and due to reflection of microwave power. Therefore steady state conditions will be reached at certain reactor volume/power level ratio, depending upon the applicator used. Compared with classical polar solvents ionic liquids can be heated very efficient by microwave absorption and, particularly when diluted, transfer the volumetrically generated heat to non-polar solvents. Acknowledgement The authors thank Mr. W. Lautenschläger (MLS Ltd. Leutkirch, Germany) for the cooperation in development of microwave systems and reactor design. B. O. and M. N. thank Dr. P. Wasserscheid (RWTH Aachen, Germany) for interested discussions and donation of some ionic liquids.

References [1] a) S. Caddick, Tetrahedron 1995, 51, 10403-32, b) S. G. Deng, Y. S. Lin, Chem. Eng. Sci. 1997, 52, 1563-75, c) N. Elander, J. R. Jones, S.-Y.Lu, S. Stone-Elander, Chem. Soc. Rev. 2000, 29, 239-49. [2] D. M. P. Mingos, A. G. Whittaker, in: Chemistry under extreme or non-classical conditions, R. van Eldik, C. D. Hubbard (eds.), Wiley and Spektrum Akad. Verlag, New York and Heidelberg 1997, 479-514. [3] for example: a) L. Lami, B. Casal, L. Quadra, J. Merino, A. Alvarez, E. Ruiz-Hitzky, Green Chemistry 1999, 199-204, b) S. Balalaie, N. Menati, Synth. Commun. 2000, 30, 869-75, c) D. S. Bose, B. Jayalakshmi, A. V. Narsaiah, Synthesis 2000, 67-68. [4] a) M. Nüchter, B. Ondruschka, A. Jungnickel, U. Müller, J. Phys. Org. Chem. 2000, 13, 579-86, b) M. Nüchter, B. Ondruschka, A. Tied, W. Lautenschläger, LaborPraxis 2001, 25, 28-31. [5] for example: a) J. A. Seijas, M. P. Vazques-Tato, M. M. Martinez, Synlett 2001, 87577, b) G. W. Kabalka, L. Wang, R. M. Pagni, Synlett 2001, 676-78, c) S. K. Pandey, K. K. Awasthi, A. K. Saxena, Tetrahedron 2001, 57, 4437-42. [6] C. A. Vriezinga, J. Appl. Physics 1999, 85, 3774-79.

414

Hoffmann

[7] C. Gabriel, S. Gabriel, E. H. Grant, B. S. J. Halstead, D. M. P. Mingos, Chem. Soc. Rev. 1998, 27, 213-23. [8] J. D. Holbrey, K. R. Seddon, Clean Products and Processes 1999, 1, 223-36. [9] P. Wasserscheid, W. Keim, Angew. Chem. Int. Ed. 2000, 39, 3773-89. [10] J. Pernak, A. Czepukowicz, R. Pozniak, Ind. Eng. Chem. Res. 2001, 40, 2379-83.

State of the Art of Microwave Applications in the Food Industry in the USA Robert F. Schiffmann R.F. Schiffmann Associates, Inc., 149 West 88 Street, New York, NY 100242401, USA

Introduction Microwave heating was rapidly adopted by the food industry in the early 1950's. The first means of conveying materials through a microwave oven was patented by Percy Spencer in 1952 [1]. However, it awaited the development, by Cryodry in 1962, of successful means for choking conveyer tunnel openings before food processing applications became practical. For nearly thirty years following the invention of the microwave oven in 1945, industrialists and microwave equipment manufacturers predicted a rosy future for microwave processing, while very few expected that microwave ovens would become popular home appliances. Numerous papers such as those by Jeppson [2] and Shelton [3] as well as in a special edition of the Journal of Microwave Power [4] predicted large industrial sales of equipment for microwave processing. Alas, reality was quite different, as seen in Table 1: Table 1. Comparison of Consumer and Industrial Microwave Heating Markets in the United States: (all numbers are approximate) Consumer Ovens Number of units installed 180 million Total sales value ($ US) 27 billion a Installed megawatts (out108,000 c put) a $150 per oven. b $3000 - $5000 per kilowatt output. c 600 watts per oven.

Industrial Systems < 500 500 million b < 100

How could the experts by so wrong? What all failed to recognise is that speed and convenience were powerful driving forces for consumers, leading to the success of the microwave ovens as food reheaters, not cooking appliances. However, speed and convenience have little meaning in industry where the financial bottom line and return on investment (ROI) are the powerful motivators for adopting new processes. Microwave heating, in the eyes of the processor, is just another unit

418

Schiffmann

manufacturing process, albeit one with unique heating abilities. But uniqueness alone is not a reason for using microwaves. As we shall see, there have been many attempts at microwave food processes but only a few have succeeded for the long term. Along with this processing review, I will give some of the reasons for success and failure and some suggestions to act as guidelines for future work

Early History of Microwave Food Processes During the 1960's and early 1970's, there were a number of very significant microwave processing systems operating in the United States, shown in Table 2, none of which are still operating. Table 2. Significant Microwave Food Processes 1960 - 1975 (numbers are approximate) Process Post drying of potato chips Precooking of chicken parts Donut proofing Donut frying Pasta drying

Total Units Installed 100+ 3 24 6 20

All were commercially successful, some with significant technical problems.They resulted in many tons of product sold into food service or retail, and many millions of dollars of sales. Yet, none of these is still operating. The following review briefly describes the processes their technology, their benefits and why they are no long being used. Post drying of potato chips Microwave drying was applied at the end of a conventional frying process for the manufacture of potato chips (or potato crisps). It provided a means for processors to control the final product color. Previously, it was often necessary to over-fry them in order to reach the very low moisture level required (1 - 3%), and that resulted in too dark a color. Potato chip color is affected by the level of sugar present in the potato, and that, in turn, is a result of the storage conditions and type of potato. Using microwave process it was possible to fry to nearly the desired color, remove the chips from the fryer and finish drying them with microwaves to the desired moisture content, with little change in color. These were 915 MHz systems employing conveyorized multimode cavities. What happened to eliminate the microwave process is the subject of an excellent article by O'Meara [5]. Several things occurred: x There were technical problems with the dryers – the often poor load often led to fires within the dryer, or scorching of the chips.

State of the Art of Microwave Applications in the Food Industry in the USA

419

x Fryer manufacturers made significant improvements in temperature uniformity and uniformity creating better process and color control; so there was no need for microwaves. x The nature of the potato chip market changed. Large manufacturers, such as Frito Lay, controlled the market including the selection, growing and storage of more tolerant potatoes that were less likely to contain high sugar content, and, hence, less likely to become too dark. Precooking of chicken parts Millions of pounds of chicken are annually precooked in the United States, for sale as fried, breaded pieces to consumers, or non-fried, breaded pieces for food service use. The precooking is usually done using steam or hot water cooking. Cooking times vary from 30 minutes to 2 hours. The yield is generally less than 100%. (Yield is calculated by dividing the weight of the breaded pre-cooked pieces by the raw chicken weight). There were three industrial microwave systems, two of which produced chicken pieces at a rate of 2500 to 3000 pounds per hour. In both of these the chicken pieces were segregated by size transported through one of two multimode cavities and cooked using 30 to 80 kilowatts. at 2450 MHz. One system employed magnetrons; the other klystrons (30 kilowatts each). The third system consisted of a resonant-cavity tunnel using a single 25 kilowatt 915 MHz source. In all cases the cooking operation used a combination of microwaves and steam. Processing costs were approximately $0.02 per kilogram of finished chicken. Yield was generally greater than the conventional methods, and an economic comparison by May [6] showed the microwave process to be $0.02 to $0.06 per kilogram less expensive than the standard procedures. The reasons these systems are no longer operating are varied and not completely satisfying. In the case of the klystron systems, tube failure was the major reason for the processes short operating life. Klystrons were expensive and the lack of isolators and circulators, which were not available at that time, caused them to encounter damaging high reflected power, often from the system condensate on the waveguide windows. Another major factor was that the yield increase was, in a sense, a phantom with little real monetary value. Chicken is sold by the piece and increasing the yield increases the final weight of these pieces without creating a new piece – so the yield advantage was not easy to capitalize upon. Finally, there was the element of a process using mysterious (and in some minds dangerous) microwaves at a time when people didn't yet have microwave ovens in their homes – so there was reluctance to use this process. Yet, one of these systems ran successfully for well over 10 years producing many millions of pounds of well received product.

420

Schiffmann

Donut Proofing The science of baking is extraordinarily complex making the technology involved in drying or cooking appear to be quite simple. One family of baked products is defined as "yeast-raised". This means that a dough containing yeast is made to form a finely divided gas-bubble structure which is then cause to expand prior to baking or frying. This expansion step is known as "proofing". Conventional donut proofers use low humidity warm air (approximately 60° to 65°C) for a period of 40 to 50 minutes prior to frying. In the mid-1960's a microwave donut proofing system was developed by Schiffmann, et al [7] which resulted in the installation of a large number of commercial installations in bakeries throughout the United States and Canada, with a few units in England and Spain, as well. These multimode conveying systems proofed donuts in four minutes at a production rate of 400 to 2000 dozen per hour using simple multimode conveyors, employing several 2.5 kilowatt – 2450 MHz generators. The advantages of the microwave technique over the conventional was enormous. The extreme reduction in proofing time resulted in great floor space savings; the simple straight-through easily controlled and cleaned conveyor replaced a much larger highly complex, difficult to control and clean conventional conveyor. There was also a significant reduction in labor and a lower capital cost! The finished fried donuts were superior to the conventional, in many ways. Interestingly, the process was developed internally by the largest manufacturer of donut manufacturing machinery – including proofers – and bakery mixes, which they sold to bakeries all over the world: DCA Food Industries. The R&D group worked for over a year to create a suitable mix to proof dough in short time, thereby creating an entirely new science of dough formulation. By comparison the development of the microwave equipment was a trivial task. The significance of a food company doing its own internal development of a microwave process cannot be over-emphasized. It could not have happened in any other way. Yet, the process is no longer used in donut manufacturing. The reasons are complex and have nothing to do with the quality of the product or process. The major factor was that it was an innovation long before its time – the industry was not ready for it and the sales department preferred selling their conventional equipment. Also, there were internal changes within DCA and the driving forces behind the use of the microwave process left the company. Donut frying Another class of baked products are those whose expansion depends upon the interaction of chemical leavening agents (usually sodium bicarbonate plus an organic phosphate) to produce the foam-like gas cell structure. Cakes and donuts are typical examples. The combination of microwaves with conventional donut frying technology in order to produce superior, high volume, lower fat donuts, was also developed by the R&D laboratory at DCA Food Industries. Here, not only was the development of the donut mix a complex problem, but the addition of a micro-

State of the Art of Microwave Applications in the Food Industry in the USA

421

wave applicator to a donut fryer required a great deal of unique engineering, both microwave and conventional. These systems operated at 2450 MHz, again using multiple 2.5 kilowatt generators. The reasons for microwave fryers no longer being used, despite many product and profit advantages are similar to those for the microwave proofer. There also were problems surrounding the marketing of the systems. The fear of microwave radiation was manifested by the deliberate sabotage of two large systems by bakery personnel who were worried for their safety. Pasta drying This process employed controlled humidity, warm air and microwaves in a series of applicators to dry macaroni and other "short goods" in one to two hours instead of 8 to 24 hours. Pasta is extremely difficult to dry by ordinary means because high temperature will cause case hardening and cracking of the surface as the moisture is removed, therefore, conventional dryers operate at low temperatures for long processing times. Microwaves are able to heat the internal structure, thereby pumping water to the surface; hence the much shorter drying time. Since macaroni contains eggs and the conventional dryers operate at 35º to 40ºC, there can be a microbial problem as Salmonella incubates at these temperatures, a problem overcome by the microwave drying process. This microwave process reduced the moisture content of the pasta from 30% to 12% in three stages. All the microwave systems operated at 915 MHz. There are few, if ay still operating, largely due the changes in ownership of the equipment companies involved.

Microwave Food Processing Systems Today Despite many attempts to develop various food processing applications, only two have become really successful, if success is measured in terms of numbers of systems installed and operating. These are microwave tempering, and microwave cooking of bacon. Microwave Tempering This is, by far, the most successful and one of the oldest commercial applications of microwaves. It has been successfully and commercially applied to frozen meat, fish, vegetables, fruit and butter. Tempering is the heating of a frozen food materials to a temperature of -2° to -3°C, and definitely below the thawing temperature of the product. All attempts at thawing frozen foods, on an industrial scale, fail because of non-uniform and runaway heating caused by the enormous difference in dielectric loss factor between water and ice ( a ratio of approximately 4000 to 1). However, raising the temperature of frozen food materials to the tempering

422

Schiffmann

temperatures will soften the tissues or matrix enough for further processing such as slicing, dicing, grinding and shaping. A typical application is the tempering of frozen beef so it may be made into hamburger patties. Cartons of frozen beef, 30 to 50 kilograms in weight, travel through the microwave tempering tunnel for ten to twenty minutes, after which the tempered meat is removed from the carton and placed in chopper/grinders to be made into hamburger meat which is then mechanically formed into patties, and then frozen prior to sale. These systems operate at 915 MHz. While there are some smaller batch units operating, most are conveyorized multimode cavities with large tunnel openings, long choke sections and are usually 50 to 100 kilowatts in output power. A typical 100 kilowatt system will process approximately 3200 kilograms per hour of 90% lean beef, raising the temperature from –18°C to –3°C. The microwave equipment cost is approximately $250,000, which results in a total operating cost of $44.50 per hour, or approximately $0.03 per kilogram of meat, including amortization, electrical, tube replacement and maintenance costs [8]. The major development of this process was done by the Industrial Microwave Division of Raytheon, which continues to dominate the market under the Amana corporate name. They began in the 1960's by setting up a meat processing facility in the Raytheon plant, using a meat scientist on staff to research and develop the process. It then set up numerous microwave tempering systems at meat storage facilities around the United States at which companies test and evaluate large quantities of frozen products. The process is extraordinarily successful because it meets a major processing requirement in a way that cannot be matched by conventional heat forms. Nonmicrowave tempering consists of storing the frozen products for one to four days (opposed to minutes in the microwave process), in what are often relatively unsanitary conditions resulting in large amounts of drip, weight, and yield loss. The long tempering times means a great deal of money is tied up in inventory resulting in significant financial loss. One can say that microwave tempering represents the ideal, unique application of microwave heating impossible to match any other way. There are approximately 300 microwave tempering systems operating in the United States. Bacon cooking There are 30 to 40 large industrial microwave bacon cookers operating in the United States. The process was first introduced in the early 1970's but has gone through numerous technical developments in applicator design, conveying systems and power distribution. Today's systems usually operate at 400 to 500 kilowatts at 915 MHz launched into a series of interconnected multimode applicators – a modular form of construction. Such power usually processes 50,000 to 60,000 slices per hour, or a raw weight of approximately 900 kilograms per hour. Usually two microwave generators of 75 kilowatts each are coupled via waveguide through top launch apertures into each module. All the generators are located remotely, often in a separate room adjacent to or above the cooker, because of the

State of the Art of Microwave Applications in the Food Industry in the USA

423

large quantities of hot water and steam used for sanitation in meat processing plants. The conveyors are usually articulated polypropylene belts, one below the product to convey it and one above to hold it down and keep it flat. The finished bacon is packed and frozen for distribution into restaurants and fast food establishments where it is easily reconstituted in an oven, on the grill or under infrared lamps. Approximately 150 to 200 million kilograms of raw bacon, or 12 x 109 slices, is processed annually in the United States [9]. There is, however, a dark cloud hanging over this process – fires. There have been at least four major microwave bacon cooker fires in the United States and England, causing severe damage and, in at least one case, burning the plant down. This author discussed the problem in detail at the last AMPERE conference [10] and an expanded paper has been subsequently published [11]. Major modifications are required in the fire suppression systems and conveyors in order to deal with the problems. Other microwave food processing applications The focus of this paper has been upon microwave processes that resulted in numerous installations, rather than one's and two's. There have been many of the latter, some of which are still operating. Some of these are [9]: x Pre-cooking of sausage patties x Drying of vegetables x Drying of snacks x Drying of low or no-fat potato chips – at one time, this process had many installations operating producing no-fat potato chips. But the company declared bankruptcy due to poor consumer acceptance. A new low-fat potato chip process seems to be having some success with the high-end market. x Baking of snacks

Barriers to the adoption of microwave food processes I have spent over 40 years in the food industry, and most of those have involved microwave process development. I was one of those who thought the adoption of microwaves by industry was "a sure thing". While I was wrong, I also learned many of the pitfalls are that either inhibit the adoption of a microwave process, or causes it to have only a limited lifetime of success: 1. Product throughput is very large in the food industry: most processing systems operate at 500 to 5,000 kilograms per hour. So, any unit process must be able to operate at this capacity. Most microwave systems are not suitable for this. For example, the pasteurization of fruit juices may work well in the laboratory. But, when scaled up to typical industrial throughputs of thousands of liters per hour, they fail for technical and economic reasons.

424

Schiffmann

2. The intrinsic value of most food products is very low, while microwave processing costs are high. As a rule of thumb, the intrinsic value of the food must be at least two to four dollars per kilogram to even consider its adoption. In the United States any addition of cost to the manufacture of a product (ingredients, packaging, processing, transportation, etc.) is multiplied by 4 to 5 in its final retail selling price. In other words, if a microwave process adds $0.02 to the cost of manufacture, the selling price will increase by $0.10. Unless the market is exclusively controlled by that processor, such a price increase may be impractical in a competitive situation. An alternative would be to reduce the manufacturing cost somewhere else, which manufacturers are very reluctant to do. Only a very significant improvement in a product may lead a manufacturer to consider this. And, "improvement" is relative – if the product is "improved" but different it is likely to fail. 3. Manufacturers are rarely willing to change a product's formulation. It may be necessary, for example, for the microwave process to require formulation changes in order to work best. But most manufacturers consider their product formulas to be sacred and refuse to change them. 4. Many food systems are complex and beyond the technical capabilities of microwave equipment manufacturers. This is accompanied by the lack of microwave knowledge by the food processor, who is reluctant to learn it, but expects it and often pilot equipment to be provided free by the microwave people. Since all the microwave equipment companies are small, or small divisions of large companies, this is not practical This usually leads to abandonment of the process R&D. 5. Food processors want proprietary rights to any process they fund or are actively involved in developing. This is contrary to the best interests of the microwave equipment manufacturer, whose major business interest is to sell as many systems as possible. 6. Innovation leads to further innovation. The successful replacement of a conventional system with a microwave process is likely to lead to developmental work to improve the conventional systems until it is able to compete with and replace the microwave process. The early potato chip fryers is a good example. 7. Food processors are not in love with microwaves. They really couldn't care less about how to manufacture a product as long as they get the product results they want and maximize profit. This often leads to disappointment when the microwave manufacturer presents the new microwave process for adoption. 8. Speed may create serious processing and cost problems: Microwave researchers seem to be enchanted by speed. However, very high production speed usually leads to one or both or two serious problems: x The process becomes hard to control, often manifested by non-uniform temperature or other physical manifestations. x Doubling or tripling a unit process means that all other equipment needs to be sized to deal with the higher speed. Suddenly a $1 million microwave process demands the purchase of an additional $1 million in auxiliary up and downstream equipment, and the microwave process dies on the spot.

State of the Art of Microwave Applications in the Food Industry in the USA

425

9. There is a 40 year history of microwave processing, not to be ignored. I am appalled that many researchers have investigated the same things over and over again simply because they haven't searched the literature. There has been some excellent work in the past that shouldn't be ignored.

Conclusion Despite the many negative remarks in the foregoing, I remain positive about the adoption of microwave processes by the food industry. However, a great deal of development and economic analysis is necessary for anything to be successful; and it should be done up front, before building and testing expensive equipment. I believe we will soon see new microwave systems in the baking and meat processing industries in particular. However, I doubt we will ever achieve the lofty dreams we all had 30 years ago.

References [1] Spencer PL (1952) Means for treating foodstuffs, US Patent 2, 605, 383 [2] Jeppson MR (1968) The evolution of industrial microwave processing in the United States. J Microwave Power 3(1):29-38 [3] Shelton E (1966) Devices for the generation of microwave power for industrial processing. J Microwave Power 1:21-27 [4] Schiffmann RF (1973) The applications of microwaves to the food industry in the United States. J Microwave Power 8(2):137-142 [5] O'Meara JP (1972) Why did they fail? J Microwave Power 8(2): 167-172 [6] May KN (1969) Applications of microwave energy in preparation of poultry convenience foods. J Microwave Power 4(2) 54-59 [7] Schiffmann RF et al (1971) Applications of microwave energy to doughnut production. Food Technology 25:718-722 [8] Schiffmann RF (2001) Microwave processes for the food industry. in: Datta AK and Anantheswaran RC (eds) Handbook of Microwave Technology for Food Applications. Marcel Dekker, Inc. New York pp 299-337 [9] Edgar RH and Osepchuk JM (2001) Microwave and heating systems. in: Datta AK and Anantheswaran RC (eds) Handbook of Microwave Technology for Food Applications. Marcel Dekker, Inc. New York pp 215-277 [10] Schiffmann RF (2001) Fires in microwave and RF heating systems: causes and prevention. in: Clark DE and Binner JGP (eds) Theory and Application in Materials Processing V, Proceedings of the Second World Congress on Microwave and Radio Frequency Processing [11] Schiffmann RF (1999) Fires in microwave and RF heating systems. Presented at 7th International Conference on Microwave and High Frequency Heating, AMPERE, Valencia, Spain

Microwave Vacuum Drying in the Food Processing Industry G. Ahrens, H. Kriszio, G. Langer Battelle Ingenieurtechnik GmbH, Eschborn, Germany

Abstract Heating and drying of food by microwaves is a well established technology, and it is well known, that the combined application of microwave energy and vacuum creates food products with properties comparable to freeze drying, e.g. in terms of instant properties, but in shorter time and thus, at lower costs. Developmental work was done at Battelle Ingenieurtechnik with the objective, to use this technology not only for the processing of pasty and liquid products, but also for bulk products and to establish it on an industrial scale. It resulted in a successful industrial application for the production of vegetables as ingredients for „instant“ soups and sauces, of crispy and crunchy fruits as ingredients for e.g. muesli, yoghurt etc. and entirely new products like e.g. fruit snacks, vegetable snacks or other kinds of snacks. The development was started after an extensive market analysis which revealed the demand for the products as mentioned above. The development includes simulations in order to maximize power density and, at the same time, to avoid plasma discharges in the vacuum cavity. A laboratory plant and, finally, a full scale unit was designed and constructed, which successfully went into operation. In the meantime, four more stations of the same size, including additional equipment like pre-dryer, post-dryer and some preparation components have been installed for the processing of vegetables, fruits and berries. This paper describes, how our microwave vacuum dryer had been developed and which results were obtained. A perspective on further development is given as well.

Microwave Vacuum Drying in the Food Processing Industry

427

Objective

Market demands In nearly all modern and highly employed societies exists a big demand for snack products and convenient food. Because of their leisure-time behaviour many people agree to the consumption of these high end food products or product ingredients, which include the following food categories: x Snacks, i.e. “fast food” snacks for immediate consumption, produced from fruits, vegetables etc. x Snacks, i.e. “fast food” snacks for immediate consumption, produced from fruits, vegetables etc. x Instant products as components of fast cooking meals, soups, sauces, etc. Benefits of the new drying technology All of the above mentioned market demands could be met by the microwave vacuum drying technology because of the following benefits: x The products obtain excellent rehydration properties because of the volumetric vaporisation of water, which is a constituent of most food stuffs. When evaporated simultaneously through the whole product piece volume, the water forms capillars inside the product pieces, which produce a high porosity and therefore allow an easy rehydration. In many cases gaining porosity is associated with an expansion of volume, so this process is called „puffing“ sometimes. The instant properties are crucial for the application of dried food in the composition of ready meals and additives used in modern citchens. x The same effect of gaining porosity enables the production of crispy fruit and vegetable snacks. x In comparison to conventional air belt drying the application of microwaves under vacuum conditions allows fast volumetric drying of the product pieces at relatively low temperatures. So vitamins, taste, flavour and natural colours are conserved very well. x In comparison to other advanced drying technologies (i.e. freeze drying) microwave vacuum drying is is more economic, as drying progress is much faster and thus allows a higher througput for the same plant dimensions. x The combination of microwave vacuum drying or puffing and conventional airbelt pre-drying into a continuous production line is economically most advantageous.

428

Ahrens

Development

The challenge For process optimization of the microwave vacuum dryer the following challenges have to be solved. Maximization of the dissipated power density in the product The dissipated power density s can be described by the following formula, where E is the electrical field strength in the product, N is the specific conductivity, Hr the relative permittivity and tan G the loss tangens of the product, while f is the microwave operating frequency, in Europe normally in the ISM-Band around 2.45 GHz: s

1 ˜N ˜ E 2

2

S ˜ f ˜ H 0H r ˜ tan G ˜ E

2

(1)

The formula shows, that the dissipated power is proportional to the square of the electrical field strength in the product, which should be maximized. Limitation of product layer thickness The product layer thickness is restricted by penetration depth and thermodynamic behaviour of the product. It should be less than the penetration depth d, which may be calculated by the following formula, where O0 is the free space wavelength (122 mm for 2.45 GHz), Hr the relative permittivity and tan G the loss tangens and of the product: d

O0 S ˜ tan G

Hr

(2)

For Hr = 15, tan G = 0.5 and O0 = 122 mm the penetration depth (for which the field strength is reduced by the factor e) becomes d = 20 mm. This value may be further reduced by the thermodynamic behaviour of the product, e.g. if the heated product has a bad thermal conductivity and the center of the layer heats up to much. Minimization of the product processing temperature Because of quality aspects the foodstuff should not be heated up to much. A typical temperature limit is 60°C. Because of that reason drying has to be carried out

Microwave Vacuum Drying in the Food Processing Industry

429

under vacuum conditions, where the cavity pressure p is low enough, that the boiling point Tb of the water is reduced below the limiting temperature and thus restricts product temperature as long as the product contains liquid water. The diagram Fig. 1 shows, that the pressure should not exceed 200 mbar to keep the product temperature below 60°C. On the other hand condensation of the nearly pure water atmosphere in the cavity should be prevented. If we assume a cavity wall temperature of 30°C, the boiling point Tb should not exceed this temperature. So the vacuum pressure p must not exceed 50 mbar. 120,00 100,00 Tb in °C

80,00 60,00 40,00 20,00 0,00 0,00

100,00

200,00

300,00

400,00

500,00

p in mbar

Fig. 1. Boiling point Tb of water as a function of pressure p

Avoidance of glow discharge The critical value of electrical field strength Ed for glow discharge is also a function of pressure p in the microwave vacuum cavity. Fig. 2 (derived from [1]) shows this relation for the ISM-Bands around 915 and 2450 MHz. 1600 1400 Ed in V/cm

1200 f = 915 MHz f = 915 MHz f = 2450 MHz f = 2450 MHz

1000 800 600 400 200 0 0

20

40

60

p in mbar

Fig. 2. Critical field strength Ed as a function of pressure p

Fig. 2 shows, that the critical field strength grows nearly proportional to pressure for pressures above 5 mbar. The maximum values of the electrical field strength, and therefore the highest probabilities of glow discharge, are to be ex-

430

Ahrens

pected around the waveguide port windows, at peaks and edges of the metallic structure of the cavity and possibly at the surface of the product layer. To avoid glow discharge, the maximum value of the electrical field strength in the processing cavity and around waveguide port windows must be less than Ed, and, to get an acceptable dissipated power density, the pressure in the cavity should as high as possible. These contradictory demands require optimisation. Evenness of product quality To obtain an evenly agreeable product quality, each volume element of the product layer should absorb the same amount of dissipated electromagnetic energy during its residence time in the microwave cavity. This means, that the field distribution in the cavity has to be optimized. High efficiency This requirement means, that as much as possible of the generated microwave power should be absorbed by the product to be dried. So the reflection of microwave power due to mismatch of waveguide ports and the transmission of power from one waveguide to the others (scattering coefficients) should be minimized. Furthermore the losses in cavity walls, waveguide components and waveguide windows due to ohmic losses should be minimized. Avoidance of microwave radiation hazards To avoid hazards to personnel and electronic or measurement equipment, following countermeasures should be arranged: x Interlock surveillance of the outer boundaries of the microwave cavity, generators and of high voltage equipment x Installation of microwave chokes in the cavity’s ports, i.e. the openings, through which the product is conveyed into and out of the cavity x Appropriate design of all penetrations for control and measurement equipment, i.e. pyrometers for temperature measurement, optical windows, equipment for movement and control of conveyor belts etc. x To avoid electromagnetic interference, appropriate equipment has to be used in all electromagnetically irradiated parts of the plant. Additional process requirements The following requirements should be met, that each product could be treated in its individually best way: x Control of the dissipated energy density in product x Control of product temperature x Control of product layer thickness x Control of vacuum pressure in microwave treatment cavity

Microwave Vacuum Drying in the Food Processing Industry

431

To avoid damage of the plant and the processed product, each glow discharge should be rapidly monitored and extincted. Such discharges are highly improbable in our microwave stations, but could occur because of faulty product characteristics. Our solution To meet all of the above mentioned requirements by a microwave vacuum dryer, we continued with a theoretical and an experimental approach. The theoretical approach was used to obtain more information about interdependences of different parameters and to get a good starting etsimate for the optimization of the whole process. The following tasks had been worked off: x Estimation of the expected dielectric properties of the product by using heuristic approaches: Available databases and literature values of measured product properties were used and inserted into reported formulae [2] for the estimation of dielectric properties of mixtures, e.g. the formulae of Rayleigh, Bergmann, Maxwell-Wagner and others. So the prospective dielectric properties of product layers where obtained, which are accumulations of predried, but still moist granules and pieces. x Estimation of the cavity’s electromagnetic parameters by using electrodynamic FDTD simulation code: This code was used to obtain the S-parameters of the waveguide ports and chokes as well as the field strength distribution in the cavity, the dissipated power density in the product and the wall losses. The simulation results were used to optimize all parts of the plant, which are to be designed according to microwave design criteria. x Estimation of the maximum applicable power as a function of cavity pressure: Design had been optimized so far that, in terms of the plant‘s specification, the necessary microwave power could be applied to the product. x Design of penetrations for control and measurement equipment x Estimation of the temperature distribution in the product by using thermodynamic simulation code: Design had been optimized so far, that the evenness of temperature distribution meets the product quality requirements. x Design and development of equipment for the detection of glow discharges in the vacuum cavity and initiation of countermeasures After all, the theoretical work had been completed by an experimental approach which consists of the following steps: x Construction of a test cavity in original scale, to verify the values of theoretical approach x Performance of components tests to verify the following acceptance conditions: - Operability of microwave generators and waveguide components - Absence of glow discharges under the specified operational conditions - Sufficiently low leakage of chokes - Reliability of glow discharge detection - EMC tests

432

Ahrens

Results

Structure of a production line and production costs Fig. 3 shows the structure of a continuous production line, which fits all requirements mentioned above.

Fig. 3. Structure of a production line

It consists of the following processing steps: x Peeling, Cutting: The raw product, i.e. vegetables, has been washed before and is peeled and cut to pieces of a specified shape. x Pre-Drying: The product pieces are predried conventionally to remove most of the moisture. This conventional step could be powered by cheaper primary heat energy and therefore helps to save costs and energy. The product will not be damaged during this conventional processing step, as it still contains enough water. x MW-Puffing: In this step the wanted valuable properties, such as crispyness, instant characteristics, conservation of vitamines, etc., are achieved by microwave treatment of the product under vacuum conditions. x Post-Drying: In this additional drying step the water activity of the product is reduced so far, that long term storage is possible. The post-drying could be done in a separate conventional post-dryer or - alternatively – by microwave vacuum processing. In the latter case MW-puffing and post-drying should be integrated in the same microwave vacuum drying unit. The mass flow within a production line is shown in Fig. 4. It is obvious, that mass loss is concentrated on product preparation (losses through peeling and coring) and the evaporation of water in the pre-dryer. Fig. 5 gives an estimation of the relative production costs of different drying methods, showing that the combination of conventional drying and microwave drying is a cost-cutting approach for the production of high-quality products.

Microwave Vacuum Drying in the Food Processing Industry

Dried Product

Dry Matter Water Content Mass Loss % Moisture

5% 5% 5%

Post-Dryer

433

20% Pre-Dryer

35% 88% 88%

Raw Product 0

500

1000

1500

2000

2500

Mass Flow in kg/h

Fig. 4. Mass Flow Within Production Line (Example Apples)

Freeze Drying Microwave Vacuum Puffing Air Drying with integrated Microwave Vacuum Puffing Air Drying 0

50

100

150

200

Production Costs (rel. Units)

Fig. 5. Relative production costs of different drying methods

Process lines installed Two process lines and a laboratory plant are installed in Zittau / Saxonia (East Germany): x The production plant for primarily vegetables and apples, consists of two microwave stations, equipped with an installed microwave power of 60 kW each. It also includes equipment for product preparation (washing, peeling, cutting), a conventional air belt pre-dryer, a conventional air belt post-dryer, packaging equipment and a number of connecting belts. The capacity of this plant is 200 kg/h output of dried product. x One of the microwave stations of this plant is shown in Fig. 6. The product is fed by a steeply rising conveyor belt (right edge of the figure), and enters the cylindrical vacuum chamber of the station through an air lock. Inside the station it is evenly distributed on a belt and continously conveyed through the micro-

434

Ahrens

wave section. The generators are inside the cabinet on top of the cylindrical vacuum chamber. After microwave treatment the dried product enters the stabilization section (left part of the figure), where it further cools down and gets a more resistable structure, before it leaves the vacuum chamber through another air lock in the basement of the factory (lower left edge of the figure). x A second production plant for primarily berries, in particular diced strawberries, has the same size and capacity. It is expected to be put into operation in autumn 2001 with frozen strawberries. The plant has additional equipment for the stabilisation of the strawberries, which else would become a slurry after thawing. x The laboratory plant for bulk products consists of one microwave station with an installed microwave power of 4 kW. It also includes an air belt pre-dryer and an air belt post-dryer. The capacity of this plant is 5 kg/h output of dried product. The microwave station of this plant is shown in Fig. 7. The structure of the station is almost the same like that of the production plant (product feed at the upper left side of the figure), but the microwave section is equipped with only two smaller generators, which can be seen on top of the central part of the cylindrical vacuum chamber.

Fig. 6. Microwave-vacuum-puffing plant, Zittau/Germany

Fig. 7. Laboratory Microwave Station

Microwave Vacuum Drying in the Food Processing Industry

435

Perspective Our work is going on in development of new food product applications as well as in equipment development of our microwave vacuum technology. The development of new food product applications is carried out in close contact to our potential clients. In the moment our efforts are focused on the development of new crispy snacks and snack ingredients as well as of microwave drying methods for liquid and pasty products, which should obtain excellent instant properties. Objectives are the improvement of existing and the invention of new interesting products, which can be produced by microwave vacuum treatment exclusively or more economically. The equipment development focuses on the optimization of our concept in terms of qualitative and economical improvements, and the adaption of the equipment to certain applications. In this connection we are using the experience, which we gained with the already running plants. Objective is the improvement of our drying method, thus confirming microwave vacuum technology, at least for certain products, to be the best alternative in comparison to other high-level drying methods, such as vacuum and freeze drying.

Literature [1] Jacques Thuéry; Microwaves: Industrial, scientific and medical applications, edited by Edward H. Grant; King’s College, London; Artech House, Boston, London (1992); chapter 4.3.2.5 [2] Georges Roussy, John A. Pearce: Foundations and industrial applications of microwave and radio frequency fields; physical and chemical processes; John Wiley & Sons, Chichester, New York, Brisbane, Toronto, Singapore (1995); chapter 8.5

Development of an Industrial Solid Phase Polymerization Process Using Fifty-Ohm Radio Frequency Technology Joseph W. Cresko1, L. Myles Phipps2, Anton Mavretic3 1

Electrotechnology Applications Center, Bethlehem, PA 18020 Advanced Polymer Technologies, Warren, PA 16365 3 Advanced Energy, Incorporated, Voorhees, NJ 08043, USA 2

Abstract The development of an industrial radio frequency (RF) solid phase polymerization (SPP) process is described. A unit operation for SPP of polyamides (nylons) has been designed that accelerates the SPP reaction with RF energy, while removing the condensation polymerization reaction by-products (water) to ensure the reaction equilibrium continues toward the desirable higher molecular weight product. Since RF heats volumetrically, the result is a tighter molecular weight distribution resulting from better temperature homogeneity. Further, the continuous application of RF energy can significantly increase the rate of nylon-6 and nylon-6,6 condensation polymerization in the solid phase. Initial studies indicate RF energy gives an 80% reduction in processing time over SPP techniques that utilize convection heating.

Introduction Typical polymerization reactions are accomplished when reactants are fluid, e.g. as a gaseous phase monomer, or more likely in the liquid phase (polymer solutions and polymer melts). However, condensation polymerization [1, 2] reactions that build molecular weight in polyamides such as polycaprolactam can be advanced at elevated temperatures when the material is in the solid phase, known as solid-phase polymerization (SPP). SPP is typically performed as an independent unit operation subsequent to the initial polymerization reaction since excessive reactor residence times would be required to achieve the high SPP molecular weights. Further, the high number-average molecular weights cause the melt viscosity to rise rapidly, which interferes with efficient stirring and heat transfer within the polymer melt resulting in increased energy consumption and polymer

Development of an Industrial Solid Phase Polymerization Process

437

uniformity problems. Hence, SPP by convection heating methods is performed on finished resin, but this operation has its own limitations. Traditional, industrial convection heating methods of SPP are rate limited by poor thermal conductivity coupled with slow moisture diffusion in the polymer. This manifests itself by yielding undesirable variations in molecular weight in the bulk resin as well as in individual resin pellets, which adversely affects the processing and properties of the final polymeric product. Our RF SPP research has focused on nylon, specifically nylon-6 and nylon-6,6 for a variety of reasons. First, nylon has an adequate dielectric loss factor (tan G), which is not necessarily the case for all condensation polymers (see Table 1). Second, since SPP of nylons has been commercialized utilizing conventional heating sources, data on that process is available in the literature, including the patent literature. Third, the mechanism of SSP in nylons is reasonably well understood [3, 4] as are the kinetics to a somewhat lesser extent [5]. This research into the SPP of nylon forms a basis for understanding the mechanisms that enable accelerated molecular weight increases via RF energy over conventional methods. Finally, nylons are produced in large quantities for applications as engineered plastics, and there is a large commercial demand for performance improvements of these products. Table 1. Typical Industrial Condensation Polymers *(loss tangent data compiled

from various sources: [6, 7, 8, 9]) Polymer Type (Common Names) Cellulose (wood, fiber) Polyamide Polyester Polyurethane Polysiloxane Phenol-formaldehyde Urea-formaldehyde Melamine- formaldehyde Polysulfide Polyacetal Polycarbonate

Source

By-Product

Natural Synthetic Synthetic Synthetic Synthetic Synthetic Synthetic Synthetic

Water Water, HCl Water, alcohol, HCl None Water Water Water Water

Loss Factor* (@ 25°C, 10 MHz) 0.07 - 0.8 0.09 - 0.16 0.04 - 0.05 0.35 0.001 - 0.009 0.18 - 0.2 0.027 - 0.2 0.2 - 0.23

Synthetic Synthetic Synthetic

NaCl Water HCl

0.0014 – 0.0041 0.0048 - 0.025 0.007 - 0.03

Solid Phase Polymerisation Condensation polymerization that occurs between the glass transition temperature and melting temperature is referred to as solid-phase polymerization (SPP) or solid-state polymerization (SSP) since the reaction progresses when the polymer is in the solid-phase. The SPP of semi-crystalline polymers such as polyesters, polyacrylamides and polyamides are industrially important reactions that can yield very long chain lengths.

438

Cresko

It is common industrial practice to perform SPP on thermoplastics after they have been processed into resin pellets and chips or otherwise post-processed into parts. This operation is done primarily to take advantage of the improved physical and mechanical properties that result from the polymer chain growth. Since high viscosity polymer is produced by condensation reactions, it is important to consider the rate-determining steps when designing a process based on SPP: x Chemical reaction of the polymeric reactants. x Diffusion of condensation by-products within the polymeric matrix. x Diffusion of condensation by-products from the surface of the bulk polymer. These dynamics are important from several design perspectives. First, the size and shape of the polymer being processed are critical to the reaction and diffusion processes. Heat transfer through and diffusion from the polymer are very different when processing relatively small resin pellets versus treating a relatively large finished part. SPP of resin pellets or chips can be handled as a bulk, free-flowing product, but finished parts handling depends upon shape and size. Our research has focused on SPP of nylon-6 and nylon-6,6. We have determined that radio frequency (RF) energy accelerates the reaction and diffusion dynamics within the solid phase, where the result is a significant reduction in time over traditional convection SPP techniques for polyamides. Polymerization of NYLON-6 Polyamides such as nylon-6 (also referred to as PA-6) and nylon-6,6 (PA-66) are prepared from their monomers by a series of reactions. While these reactions can be generalized for the production of polyamides that utilize the larger monomeric diamines and monomeric dicarboxilic acids in their synthesis, the polymerization reactions of nylon-6 require that a single cyclic lactam, specifically caprolactam (CPL), is ring opened. The polymerization reactions of nylon-6 are as follows (where n = number of monomeric units in the chain): Step 1: Ring Opening: CPL + H2O Æ H2N-(CH2)5-COOH (6-aminohexanoic acid, 6-AHA) (monomer, n = 1) Step 2: Addition: 2 (6-AHA) Æ H2N-(CH2)5-CO-NH-(CH2)5-COOH + H2O, and (dimer, n = 2) dimer + 6-AHA Æ H2N-((CH2)5-CO-NH)2-(CH2)5-COOH + H2O, and (trimer, n = 3) trimer + 6-AHA Æ H2N-((CH2)5-CO-NH)3-(CH2)5-COOH + H2O, etc. (tetramer, n = 4) Generally, this addition produces chain-lengths of n = 5-20 units. Then the reaction changes character somewhat, and polycondensation reactions begin to predominate:

Development of an Industrial Solid Phase Polymerization Process

Step 3: Polycondensation: pentamer + hexamer Æ undecamer (n = 11) + octamer Æ nonadecamer (n = 19) + H2O, etc.

439

+ H2O, and undecamer

These reactions are performed on the industrial scale either by batch or continuous processes. In both cases, the conversion of CPL to nylon-6 is generally carried out to produce material with a degree of polymerization (DP) corresponding to values of up to n = 175. In practice, DP is readily assessed by dissolving the nylon-6 in formic acid, and measuring the viscosity of the solution. Formic acid dissolves, but does not degrade the linear nylon-6 macromolecule. Typical nylon-6 product has a relative viscosity in formic acid, i.e. an FAV (8.4 wt% solution of nylon-6 in 90% formic acid) of 40 - 50 units. While this DP is sufficiently high for many applications such as carpeting and textiles, there exist a growing number of engineering applications where a significantly higher molecular weight, hence higher measured FAV (100 - 200 or more), is required. Four points regarding these reactions are relevant to the industrial manufacture of polyamides in general, and to the SPP of nylon-6 resins and products in particular: x The reactions that occurs in all of the steps are fully reversible, with equilibrium constants somewhat greater than 1. It is important to keep the reaction shifted toward the products. The reactions in steps 2 and 3 are identical in principle. However, since only one monomer unit is added for each addition reaction and free reaction sites decrease as the smaller molecules are consumed, the rate of polymer chain growth continuously decreases. Alternatively, polycondensation reactions occur between relatively large molecules that result in higher molecular weight increases per reaction event. These rapid increases are quickly offset by reaction kinetics due to the relative scarcity of available end groups available for reaction. x The optimal design of a SPP unit operation must be based upon reaction thermodynamics and kinetics. In all cases of either addition or condensation reactions, the formation of the amide linkage is associated with the release of a water molecule. The removal of this water, both at the molecular level and the macroscopic level, becomes an important determinant of the rate and the degree of polymerization that occurs [10]. It is reasonably well established that these two amidation reaction rates are determined by the rate of water removal. This has been shown for reactions that occur in the melt, in the solid phase or even in the form of tiny droplets [11].

x The SPP process must remove the evolving reaction water to ensure the reac-

tion equilibrium favors the products, and the efficiency of the water removal impacts the SPP rate. Since all of the reactions described are in equilibrium, the finished nylon-6 contains about 10% of a mixture of unreacted CPL, ring-opened CPL (6-AHA), oligomers (in the n = 3 - 10 range) and reaction water (this end-reaction equilibrium

440

Cresko

is different for different varieties of polyamides). This mixture must be removed since it adversely affects the properties of the nylon as well as the process economics. Removal of the undesirable mixture is usually done by an aqueous leach step, followed by drying to remove the resulting 12 - 15% water trapped within the nylon-6 resin. x An optimal SPP unit operation must integrate into the overall nylon production process; great potential exists for significant gains in efficiency of the overall nylon manufacturing process. Solid Phase Polymerisation by Radio Frequency energy The polycondensation reactions will determine the configuration (primary chemical bonds) of the macromolecules, but the conformations (rotational arrangements) and resultant morphology of the structure are in large part dependent on the overall processing scheme. Nylons are semi-crystalline, with regions of amorphous and crystalline regions dependent upon processing conditions. The deployment of RF energy to drive the SPP polymerization reactions must principally address the diffusion of condensation by-products from the polymer, which depends upon the geometry of the polymer product receiving RF SPP, since diffusion of condensation by-products is the primary rate limiting factor. The RF power absorbed by the bulk polymer is described by the well-known equation [12]: 2 (1) P v f E Hcc where f is the frequency (s-1), E is the electric field strength (Vm-1), and H” is the imaginary component of the complex permittivity (dielectric loss). The dielectric loss is a function of the temperature of the material, and the temperature of SPP of nylon-6 is in the range of 190 – 210°C. As long as the reaction water is removed during the SPP process (and the SPP temperature remains below the melt temperature of nylon-6, ~220 – 225°C), the reaction progresses and runaway heating is not observed. Processing must occur under an inert atmosphere to prevent oxidation under the high heat conditions. Dry nitrogen is continuously swept past the material during processing to prevent oxidation and to remove the reaction water. RF energy drives the diffusion of condensation by-products within the polymeric matrix, and the dry nitrogen atmosphere drives diffusion of condensation by-products from the surface of the nylon-6. FIFTY-OHM RF TECHNOLOGY Fifty-ohm solid state RF generator technology (equipment supplier: Advanced Energy, Inc. Voorhees, NJ) is preferred for control and precision power delivery [13, 14]. The equipment applies RF energy of absolutely fixed frequency through an automatic impedance matching network, which matches the constant imped-

Development of an Industrial Solid Phase Polymerization Process

441

ance output of the RF power supply (50 ohms) to the impedance of the overall load (applicator and product). Changes to the overall load characteristics can commonly occur during start-up or during material changes. The use of fifty-ohm technology gives significant feedback control advantages since the automatic matching network adjusts for load impedance variations. Also, since frequency does not drift as with older free-running oscillator systems, there is no difficulty complying with the allowable industrial, scientific and medical (ISM) bands. The applicator design is based on product characteristics and the overall unit operation.

Fig. 1. Schematic of Static Test Bed Reactor

Experimental Results

Batch Processing Various nylon resins and parts were treated by RF SPP in a batch, static bed test reactor. The batch reactor is constructed as a RF transparent box (15 cm deep by 30 cm long by 30 cm wide) with a removable sealing lid, constructed of Garolite grade G-7 (a silicone/fiberglass composite laminate), as shown in Figure 1. An open-weave Kevlar cloth shelf suspends a layer of resin or parts 3 centimeters from the bottom of the box. Nitrogen flows in from under the shelf, and exits above the shelf at the opposite end of the inlet. Nylon flow rates are typically 20 60 ft3/min. Radio frequency energy at 13.56 MHz is applied from the outside of the box by parallel plate electrodes, and the temperature of the nylon is monitored by fiber optic probes. Table 2 describes the batch tests performed and the results of the formic acid viscosity tests performed before and after SPP for resins. Ta-

442

Cresko

ble 3 summarizes that information for treated parts, as well as results of preliminary mechanical testing. Table 2. Experiments and SPP results from static bed reactor for resins Run #

Polymer (Manufacturer) nylon-6 homompolymer (Allied Signal) nylon-6/nylon-6,6 copolymer (AlliedSignal) nylon-6 homopolymer (EMS Grilon) nylon-6, unlubricated (Dupont Zytel 101) nylon-6,6, lubricated (Dupont Zytel 101L)

R-1 R-2 R-3 R-4 R-5

Temp. [°C] 200 - 205

Time (hrs.) 6

FAV before SPP 73

FAV after SPP 145

182*

5

93

172

205

6

47.5

141

200

6

50

1,646

200

6

48

200

*Note: low melting point material Table 3. Experimental results from SPP in the static test reactor for various parts Run #

Polymer

P-1

nylon-6,6

Part

T (°C)

Time (hrs.)

machine 200 10 screws Comments: 43% increase in tensile strength P-2 nylon-6,6 hex-head 200 10 screws P-3 nylon-6,6 braided 235 4 fiber twine P-4 nylon-6,6 hex nut 230 6 w./ 30% glass fill Comments: no FAV correction made for glass fill P-5 nylon-6,6 hose clamp 230 6 w./ 2% car. black P-6 nylon-6,6 cable tie 230 6 58% increase in tensile strength P-7

nylon-6,6

wing nut

230

6

FAV Bef. SPP 41

FAV after SPP 545

41

545

57

398

19

81.2

46

312 46

46

363

233

Comments: 8% increase in shear strength of threads

Continuous Processing The RF SPP of resins in pellet or chip form, and certain small, pre-formed parts (i.e. injection-molded fasteners), are ideally processed in a steady-state, continuous operation. A vertical radio frequency (VRF) design is utilized that is based upon a pair of nested, cylindrical electrodes [15]. The free-flowing product is heated in the annular space between the electrodes. Product can be recycled

Development of an Industrial Solid Phase Polymerization Process

443

through the RF zone in an inert nitrogen atmosphere. Temperature-sensing devices mounted at different locations along the vertical height and around the circumference of the unit provide feedback control to a 13.56 MHz, fifty-ohm RF power supply. The unit was loaded with ~350 lbs. of Capron L (Allied Signal), heated to the target temperature with RF power input of 3 - 8 kW, then power was reduced to 1.5 – 2.5 kW to maintain the target SPP temperature. Results from a sample run are shown in Table 4.

Conclusion Radio frequency (RF) solid-phase polymerization (SPP) has been demonstrated to increase the degree of polymerization of the polyamides nylon-6 and nylon-6,6. Results of batch and continuous RF SPP of nylon resins and parts are presented. RF SPP processes for the batch treatment of large parts, and the continuous treatment of bulk nylon resins have been developed. These are cost effective, industrial methods that result in higher product strength, at a significantly lower cost than with current SPP techniques. Table 4. Experimental SPP results from continuous VRF reactor Time [h] Start

T [°C] 25

FAV 45

4 5 6 7 8

200 (+/-5) 200 (+/-5) 200 (+/-5) 200 (+/-5) 200 (+/-5)

62 69 78 84 86

Comments Start preheat; heat to SPP temp. under recycle & N2. Maintain recycle & N2; take sample. “ “ “ Stop reaction; start cool-down.

References [1] Carothers, W.H., J. Am. Chem. Soc. Vol. 51, pg. 2548, 1929. [2] Odian, G., “Principles of Polymerization, 3rd Edition,” pg. 1-4, J. Wiley & Sons, NY, 1991. [3] Kohan, M.I., Ed. “Nylon Plastics Handbook,” pg. 28 Hanser/Gardnier Publications, Inc. Cincinnati USA, 1995. [4] Zimmerman, J., J. Poly. Sci. Vol. 2B, pg. 955, 1964. [5] Nelson, W.E., “Nylon Plastics Technology,” pg. 49-51 Newnes-Butterworths, 1976. [6] Union Internationale d’Electrothermie (UIE), “Dielectric Heating for Industrial Processes,” 1992. [7] Electric Power Research Institute (EPRI), Final Report EM-4949-Project 2416-21, Dielectric Heating in Industry, prepared by Thermo Energy Corp., Palo Alto California, March 1987.

444

Cresko

[8] Shackelford, J.F. et al, “CRC Practical Handbook of Material Selection,” CRC Press, Boca Raton Florida, 1995. ISBN: 0-8493-3709-7 [9] Von Hippel, A., “Dielectric Materials and Applications,” Cambridge Massachusetts, M.I.T. Press, 1966. [10] Mallon and Ray, Journal of Applied Polymer Science, Vol. 69, pg. 1203, 1998. Leventis, Panagioitou, and Flagan, U.S. Pat. 5269980, 1993. [11] A.C. Metaxas,, “Foundations of Electroheat,” John Wiley & Sons, Inc. Chichester, UK, 1996. [12] A. Mavretic, E. Gergin and J. Stach, “Materials Processing by RF Power,” pp. 561-567 Proceedings of the 7th International Conference on Microwave and High Frequency Heating, Valencia, Spain Sept. 13-17, 1999. ISBN: 84-7721-781-5 [13] A. Mavretic an J. Allen., “Integration of Fifty Ohm Technology for Materials Processing,” pp. 267-273 Proceedings of the 6th International Conference on MW & HF Heating, Fermo, Italy Sept. 9-13, 1997. [14] J.W. Cresko, L.M. Phipps and N.L. Brown, “A Vertical Radio Frequency Design as a Unit Operation for the Continuous Processing of Free-Flowing, Bulk Granular or Particulate Products,” Proceedings of the 2nd World Congress on Microwave and Radio Frequency Processing, Orlando, FL April 2000. Published in: “Microwaves: Theory and Application in Materials Processing V,” Ceramic Transactions Vol. 111, April 2001. ISBN: 1-57498-103-X

RF World Tour Jean-Paul Bernard SAIREM, Vaulx-en-Vellin, France

Introduction Radio frequency started to be developed in the fifties and from that time more and more industrial applications were found. Today it is widely used for welding as well as for textile, wood, paper working industries and cardbox manufacturing. In the food processing industry this technology is sharply developing, particularly for biscuits but also for tempering or thawing frozen blocks. It has also started to be used for composites (preheating and gluing). For surface treatment by plasma its use is rapidly increasing particularly for semi-conductors. The huge sales of domestic microwave ovens during the last 15 years have led to a worldwide knowledge of microwaves, while radio frequency was staying unknown. But inversely radio frequency is much more used in industry and surpasses microwaves in many fields. In the field of Research & Development there are far fewer private or public research laboratories that are using radio frequency compared to the ones using microwaves. As a result the research subjects and above all the publications in scientific or specialized magazines are rather limited.

Frequencies Two frequencies are mainly used : 13.56 MHz and 27.12 MHz. The 6.78 MHz frequency is sometime used, particularly for wood industry as well as the 40.68 MHz frequency in the field of analysis.

Technology There are two main types of applicators : 1. system with condensor’s plate, 2. system with alternate bars.

446

Bernard

The first system is used either in batch, or for continuous equipment with conveyor. The second system is only used in continuous and mainly with thin products. For more than 10 years the 50 : radio frequency technology developed by SAIREM and more recently by PETRIE TECHNOLOGIES has contributed to improve some industrial processes through a better control of parameters and a more flexible use.

Manufacturers The main manufacturers of radio frequency systems for industrial heating are the following : -

NEMETH Engineering, USA, PETRIES TECHNOLOGIES, United Kingdom, PSC, USA, RADIO FREQUENCY Corp., USA, RADYNE-STRAYFIELD, United Kingdom, SAIREM, France, STALAM, Italy.

Industrial Applications

Welding Today the first field using radio frequency is welding. There are a large number of manufacturers all over the world (several hundreds) . This particular application could be treated alone. We do not study it today because it stands apart from the other applications that will be mentioned hereafter. Wood Radio frequency is widely used today by many firms for speeding up gluing of wood flooring or wood core plywood. Cardbox manufacturing and paper working industry They use large quantities of glue for the manufacture of envelopes or forms. Compared to traditional technologies such as hot air or more recent ones such as infra-

RF World Tour

447

red, radio frequency has the advantage to heat only the glue line or paving without damaging the support and within a very short treating time. This allows a line treatment and a line automatization. Radio frequency is very used for the paper working industry. In the field of cardbox manufacturing there are some installations for correction of moisture profile in cross direction, but competition with infra-red technology is very hard. Textile Radio frequency is very used in this industry particularly for drying : - yarn reels, - yarn pachass, - tops, - stockings and tights, - hants, slivers and loose stock. Radio frequency is also used by manufacturers of glass fibres reels that need to be dried. Food processing industry Today radio frequency is mainly used for drying biscuits during the last step of manufacture. For tempering and thawing radio frequency is now conquering manufacturers. Indeed compared to microwaves at 915 MHz this technology gives much better results as for temperature homogeneity. Above all it allows to reach a temperature near 0°C (-2/0°C) while keeping the product’s quality. Combined with hot air radio frequency is also used for cooking some products. Recently radio frequency has started to be used for pasteurization of liquid or semi-liquid products. Composites – ceramics – foundry Some applications exist for car industry : 1. gluing of back door, 2. roof window encapsulation, 3. baking of mould cores, 4. mould drying, 5. catalytic converters.

448

Bernard

Plasma

Surface treatment by plasma Radio frequency is used for exciting gases that will be used for deposit or etching. The semi-conductor industry is an activity widely using radio frequency. This application stands apart from the traditional radio frequency heating and could also be the subject of a conference.

Some Industrial Installations

Textile Main manufacturers : - NEMETH ENGINEERING, USA, - PSC, USA, - RADIO FREQUENCY CORP., USA, - RADYNE-STRAYFIELD, USA, - STALAM, Italy.

Fig. 1. Example of an equipment at STALAM Frequency : 27.12 MHz, power : 50 kW. Size : length = 9 m, width = 2,40 m, height = 3,3 m. Production : 500 kg/h for synthetics, 200 kg/h for wool and cotton blends

RF World Tour

Drying of fibre glass yarns Manufacturers : - NEMETH ENGINEERING, USA, - PSC, USA, - RADIO FREQUENCY CORP., USA, - RADYNE-STRAYFIELD, USA, - STALAM, Italy.

Fig. 2. Example: RADYNE-STRAYFIELD, 3-zone tunnel, 3 x 40 kW

Fig. 3. Example: PSC: 100 kW RF dryer with monorail conveyor

449

450

Bernard

Wood There are many manufacturers in the world and mainly in Italy, USA, Finland, United Kingdom, South Africa and Germany.

Fig. 4. Example: wood floor manufacturing, MK IMPIANTI, Italy/ SAIREM, France, 50 : RF press, 30 kW.

Cardbox manufacturing and paper working industry Manufacturers : - GEAF, Italy, - NEMETH ENGINEERING, USA, - RADIO FREQUENCY CORP., USA, - RADYNE-STRAYFIELD, USA, - SAIREM, France,

Fig. 5. Example: SAIREM, 15 kW equipment

RF World Tour

451

Fig. 6. Example: RADYNE-STRAYFIELD, 27,12 MHz RF oven, 12 kW, Drying of water based adhesives.

Food processing industry Manufacturers : - PETRIE TECHNOLOGIES, United Kingdom, - RADIO FREQUENCY CORP., USA, - RADYNE-STRAYFIELD, USA, - SAIREM, France - STALAM, Italy.

452

Bernard

A. Tobacco

Fig. 7. Example: PETRIE TECHNOLOGIES for IMPERIAL TOBACCO, drying of cigars, moisture reduction (5 to 6%), power: 20 kW, frequency: 13.56 MHz 50 :, hot air: 80°C, size: 5,5 m x 1,5 m x 2 m.

Fig. 8. Example: SAIREM, drying of 400 double small cigars per minute, power 8 kW

RF World Tour

453

B. Tempering, thawing

Fig. 9. Example: SAIREM, 50 : RF tunnel, 3 zones, 27.12 MHz, thawing of poultry fillets from –20°C to –2/0°C, 3 x 35 kW, size: length : 15 m, width: 2 m, height: 2 m, capacity: 750 kg à 1 T/hour.

Fig. 10. Example: SAIREM, 50 : RF tunnel, 50 kW, tempering of fish fillets from –20°C to –3/-2°C, power: 50 kW, frequency: 27.12 MHz, capacity: 800 to 2 T/hour, size: length: 6 m, width: 1 m, height: 2 m.

454

Bernard

C. Biscuits

Fig. 11. Example: RADYNE-STRAYFIELD, 85 kW RF tunnel, Moisture profile of cereals and biscuits.

Fig. 12. Example: STALAM, biscuits line, 2 x 60 kW

RF World Tour

455

Fig. 13. Example PETRIE TECHNOLOGIES, ARFA system, 27.12 MHz tunnel, 4 x 40 kW, hot air: 200°C.

Composites – Ceramics – Foundry Manufacturers : - NEMETH ENGINEERING, USA, - PSC, USA, - RADIO FREQUENCY CORP., USA, - SAIREM, France.

Fig. 14. Example: SAIREM, 50 : RF equipment for car industry, back door gluing, 27.12 MHz, 20 kW.

456

Bernard

Fig. 15. Example: SAIREM, roof window encapsulation, 27.12 MHz, RF 50 :, 20 kW

Fig. 16. Example: PSC, baking of mould cores.

RF World Tour

457

Miscellaneous

Fig. 17. Example: PSC, 200 kW tunnel, polyurethane foam dryer

Fig. 18. Example: SAIREM, polyurethane foam dryer, 50 : RF tunnel /hot air, 50 kW, 27.12 MHz

458

Bernard

CONCLUSION Radio frequency has a great future, because it meets the manufacturers requirements when microwaves and other technologies do not prove their efficiency. The development of the 50 : technology has led to a positive evolution of radio frequency in some fields thanks to a better control of parameters. Today the scientific community has to take a more important part and to get more involved by proposing much more research subjects including the radio frequency technology as heating mean. This will lead to the development of new industrial processes

THANKS I would like first to thank : - Professor Georges ROUSSY, Director of the Laboratory of Spectroscopy and Microwave Technics at Nancy University, - Professor Ricky METAXAS, Director of the Engineering Department at Cambridge University, for the information they gave to me, as well as all the companies who accepted to participate in this RF World Tour and authorized me to present in their name a large number of RF industrial installations. Again many thanks to all of you.

How the Coupling of Microwave and RF Energy in Materials can Affect Solid State Charge and Mass Transport and Result in Unique Processing Effects John H. Booske and Reid F. Cooper University of Wisconsin, Madison, WI 53706, USA

Introduction Investigation and application of electromagnetic field heating spans many decades, many frequency regimes, and many media types. To understand the interaction of electromagnetic fields and media is to understand the influence of electromagnetic fields on charge and mass transport. We therefore begin our discussion with a brief review of classical fundamentals of electromagnetic field interactions with matter. To simplify and focus the discussion, we will restrict our attention to classical (non-quantum) interactions. This means that we will ignore those high frequency interactions that involve quantum energy state transitions usually associated with high frequency fields with high energy per photon. Specifically, we will focus on interaction mechanisms relevant to frequencies below 1 THz, although some of the same mechanisms apply to radiation fields at higher frequencies. From a classical perspective, electromagnetic fields heat substances by inducing or modulating the motion of electrical charges (electrons, ions, molecular electric dipoles, or atomic magnetic dipoles). This influence on the motion of charges results from either electric or magnetic field forces on mobile or bound charges in the medium, as shown in Eq. (1) and (2).

Felec

Fmag

qE

(1)

qv u B

(2)

With insulating, nonmagnetic materials (predominantly bound charge, negligible mobile charge and without magnetic dipoles), the electric field induces motion of electric dipoles. This may involve localized dipole motion or collective dipole

462

Booske

motion (coupling to phonons) [1, 2]. A conventional description of electric dipole “heating” would say that the oscillating electric field induces molecular dipoles to either precess or rapidly flip polarity, heating the surrounding medium by “friction” with neighboring molecules. However, in dense media (liquids or solids), it is more often the case that the dipole re-orientations result from regular, localized, breaking and reforming of inter-atomic bonds, similar to processes described in [1, 2]. A classical description of magnetic materials adopts the view that atomicallybound electron motions (“orbits” and “spins”) constitute microscopic current loops, which, of course, are (atomic) magnetic dipoles. Through the magnetic force of Eq. (2), an oscillating magnetic field exerts an oscillating polarity torque on these magnetic dipoles, inducing dipole reorientation effects. In “soft” magnetic materials, this leads to heating similar to that of electric dipole reorientation discussed in the preceding paragraph. When “hard” (ferro)magnetic materials are involved, the important dipole re-orientations occur near domain boundaries. This action promotes domain boundary movement in the presence of crystal imperfections, inclusions, cavities, and anisotropies, and ultimately leads to an additional, irreversible form of energy transfer associated with hysteresis. When mobile charges are present in the medium, the electric field force induces longer-range motion of the mobile “monopolar” charges. This leads to “ohmic” or “joule” heating wherein the moving charges (currents) collide with neighboring atoms and molecules, enhancing the medium’s thermal agitation energy. This form of heating is occasionally neglected in discussions of dielectric heating, but it can be appreciable, even in conventionally insulating media [2]. It is much more commonly associated with conducting or semi-conducting media. When the material is highly conducting, the induced mobile charge currents can be quite substantial this is often referred to as “induction heating” [3]. Efficient induction heating of the material requires the local magnetic field to be much stronger than the electric field. This often leads to an alternative description crediting the induced charge motion to the much stronger magnetic field. Indeed, the mathematics can be worked out quite satisfactorily from this point of view. However, the fundamental physical force responsible for moving the charges into the induced oscillating currents is the (much weaker) electric field’s influence, a point that is occasionally misunderstood. Magnetic induction heating is, in the end, an electric field effect. The thermal agitation (“heating”) of the atoms and molecules enables reaction processes (synthesis, decomposition, bonding, sintering, drying) to occur on timescales attractive for practical applications. However, in addition to thermal agitation, complete reaction of a volume of substances requires mass transport, to bring unreacted species into close proximity, as illustrated in Fig. 1. Such mass transport results from either thermo-electrochemical driving forces or from macroscopic mechanical mixing. In fluid media, mixing can be quite substantial, either through natural convection, boiling-induced turbulence, or deliberate external stirring. In these instances, the process kinetics will probably be chemical-reactionrate-limited, rather than transport-rate-limited. However, in materials (solid phase) processing, macroscopic mixing is rare, and mass transport requires solid state dif-

How the Coupling of Microwave and RF Energy in Materials

463

fusion (a relatively slow process) in response to thermo-electrochemical driving forces. Hence, materials’ processing kinetics will more often be transport-ratelimited.

Fig. 1. Complete reaction of a volume of substance requires transport as well as thermal agitation (heating)

Altered reaction processes In the most general sense, all “heating” occurs from either atomic collisions between the surface of the object and a hotter environment (conduction, convection) or electromagnetic heating (radiant heating is incoherent electromagnetic heating, primarily via infrared frequency radiation that is absorbed at the medium’s surface). When electromagnetic fields can penetrate significantly into the interior of the medium, (which often occurs during microwave and RF heating), the induced charge motions occur internally. Such field penetration therefore enables “volumetric” heating. Endothermic reactions can be powerfully affected by volumetric heating. In conventional furnace heating, endothermic reactions can be rate-limited by the need to conduct heat from the surface inwards. The interior is precluded from full heating and reaction until the endothermic reaction at the surface is completed. Volumetric heating by high frequency electromagnetic radiation can “bypass” the heat transfer rate limitation and produce enhanced reaction rates. If the heated medium is permitted to attain a thermal steady state, then volumetric heating combined with conductive dissipation of the internally absorbed energy will produce inverted temperature profiles, with the interior hotter than the surface. An excellent example of this is illustrated in Fig. 2, taken from [4]. Cylindrical alumina powder compacts undergoing microwave sintering were “seeded” with small magnesia crystallites. Post-sintering analyses of the magnesiaaluminate spinel layer thicknesses at the magnesia-alumina grain interfaces pro-

464

Booske

duced a highly reliable, internal temperature indicator. Taking samples from various internal locations in the sintered sample revealed the inverted temperature profile shown in Fig. 2(b). This dramatically contrasts with the uniform radial temperature profile of Fig. 2(a), obtained from samples sintered in a conventional, resistive-element furnace. distance from specimen center (mm) 1600

1600

temperature (C)

temperature (C)

(a)

(b)

1550

1550

1500

1500

1450

0

2

4

6

distance from specimen center (mm)

8

10

1450

0

2

4

6

8

10

distance from specimen center (mm)

Fig. 2. Local internal temperature versus radial distance from specimen center for (a) conventionally- and (b) microwave-heated MgO-Al2O3 composite powder compacts. The conventionally-heated case was processed at a furnace soak temperature of 1500°C. The microwave-heated specimen was processed at a surface-measured temperature of 1500°C. Dashed curves are only for visual aid and are not based on theoretical models or explicit curve-fits.

Inverted temperature profiles can provide unique processing effects. Conventional heating can result in the surface reaction completing before the interior is fully reacted. Surface pores may close prematurely, preventing mass transport of gas reactants to the center needed to complete the reaction of the interior. With inverted temperature profiles, microwave-heated materials can allow the reactant gases to permeate the specimen and diffuse to the hot center until it is fully reacted. Excellent illustrations of this effect are provided in Refs. [4] and [5]. Reference [4] describes how this effect can facilitate thorough processing in chemical vapor infiltration and reaction-bonded silicon nitride. Reference [5] describes how the same effect can be used to successfully sinter bulk high-Tc YCBO superconductors with uniform and full oxygen content. In heterogenous media, localized “internal” coupling to specific “susceptible” regions may produce unique effects. It may explain, for example, why microwave heating of grain to destroy weevil infestations is more effective at high (mmwave) frequencies [6]. It has also been cited to explain successful gold film eutectic bonding of semiconductor wafers [7] under conditions where direct wafer-towafer bonding was problematic. Care must be taken, however, in drawing such conclusions about selective internal heating and micro- or mesoscopic temperature gradients with microwave or RF energy. Excess thermal energy accumulated

How the Coupling of Microwave and RF Energy in Materials

465

within highly-absorbing phases tends to rapidly diffuse to surrounding, connected, low-absorbing regions. During rapid heating transients, irregular microscopic or mesoscopic temperature distributions are theoretically possible, depending on the heating rate and the internal thermal diffusion timescales. However, once the system has achieved thermal steady state, internal thermal energy must dissipate by conduction to the object’s macroscopic surface followed by conduction, convection, or radiation to the cool walls surrounding the object. In this situation, the only significant, physically possible temperature gradient is a smooth, macroscopic, inverted temperature profile such as illustrated in Fig. 2(b) [8]. It should be noted that unusual thermal gradients can occur when internal thermal energy can rapidly diffuse to a nearby macroscopic surface and radiate to the walls more effectively than it can diffuse to other regions within the specimen. This explains the ability to control the axial temperature profile in microwave-heated metal-ceramic functionally gradient materials [9]. It also explains why microwave or RF heating of thin semiconductor wafers can result in significant, irregular, temperature nonuniformity across the wafer if proper care is not taken during applicator design. In addition to heating, electromagnetic fields can influence reactions through direct modulation of particle motion, and, in some cases, modification of mass transport. Compelling evidence of such “non-thermal” influence continues to accumulate in the literature. However, unequivocal evidence of these effects is much more limited because it is very difficult to acquire. Experiments whose interpretation relies on accurate knowledge of the internal sample temperature remain subject to question, owing to the well-known challenges of obtaining accurate temperature measurements during microwave or RF heating. To date, the authors are familiar with four published examples in microwave or RF processing of materials where evidence of non-thermal effects is robustly immune from questions of temperature measurement inaccuracies [3, 11 - 13]. In Ref. [11], comparisons of microwave- and conventionally-sintered alumina emphasized clear differences in closed and open porosity or specific free surface area as a function of sample density. As illustrated in Fig. 3, between 59-64% theoretical density, the microwave-sintered specimens realize increased density at almost constant free surface area. In contrast, the conventionally-sintered specimens increase density only if there is a simultaneous decrease in free surface area. These data, along with other measurements [11], give strong evidence that microwave sintering of these samples was more effective at removing closed porosity than conventional sintering. Because of the unique way this data was analyzed, one can conclude, free from any reliance on temperature knowledge, that the microwave heated samples followed different reaction kinetics from the conventional-sintered samples, owing to a non-thermal microwave field influence.

466

Booske

Fig. 3. Reduction of free surface area and porosity in microwave- and conventionallysintered 99.7% Al2O3 [courtesy of M. Willert-Porada].

A similar “temperature-independent” data analysis has been conducted on microwave and RF-annealed B-implanted silicon wafers [3]. The objective for nextgeneration microelectronics integrated circuit (IC) manufacturing is to obtain boron doping profiles in Si wafers that are very shallow and with a high activation (high percentage of the B atoms sitting on substitutional Si lattice sites). Using secondary ion mass spectroscopy (SIMS) to measure the B concentration profile and four-point-probe surface resistance as a measure of the B activation, a comparison has been made between microwave or RF rapid annealing versus halogen heat lamp rapid annealing in Fig. 4. Figure 4 displays the 1 x 1018 cm-3 B concentration point versus the surface resistance of B-implanted wafers. There are also two curves in Fig. 4. The upper curve represents the “best results” limit circa 2000, compiled by SEMATECH, for heat lamp rapid annealing. For IC’s with smaller devices (e.g., 100 nm or 70 nm scale devices), one needs to push below this best-results-barrier to points further “southwest”. During the last couple of years, researchers have established that controlling the O2 concentration in the annealing ambient improves the results by controlling surface oxide growth which in turn controls B diffusion. This is illustrated by the open circle data points in Fig. 4, which have been lamp-annealed in a 33 ppm concentration of O2 and atmospheric pressure N2 [12]. The second curve, (connecting the filled circle data points) represents preliminary results obtained with microwave and RF rapid annealing in air (a sub-optimal oxygen concentration). These data represent a significant improvement over the SEMATECH barrier standard for lamp annealing. Moreover, it is expected that further improvements (further movement towards the southwest corner) will be observed in future microwave and RF experiments when the oxy-

How the Coupling of Microwave and RF Energy in Materials

467

gen concentration is precisely controlled to ~ 30 - 35 ppm. The fact that the microwave and RF results show higher activation for lower diffusion (i.e., are southwest of the prior comparable results with lamp annealing) implies a nonthermal influence of the electromagnetic fields during processing. The fact that this conclusion is derived from a resistance-versus-diffusion figure ensures that it is immune from concern about accurate temperature measurement. 10000

Sematech barrier standard BF2+, 425 C/s, 33 ppm B+, microwave and RFO2, lamp-anneal

Rs (ohms/sq.)

BF2, microwave and RFanneal, air (UW) PLAD, microwave anneal, air (UW)

1000

180nm 100nm 70nm

50n

100 10

20

30

130nm

40

50

60

70

Junction Depth (nm) @ 1E18atoms/cm

80

3

Fig. 4. Sheet resistance versus junction depth for lamp-based RTP and a new microwave and RF annealing technique. The solid black “SEMATECH” line represents the best achievable as of 1999 - 2000 with lamp-based RTP with high (> 10,000 ppm) concentrations of oxygen. The improvements with lamp-RTP represented by the black (open) circles have been achieved through a discovery that controlling the O2 concentration to 30 35 ppm in the annealing ambient minimizes the dopant diffusion. The red curve represents the new capabilities of microwave and RF annealing, but with high O2 concentrations. Further improvements in the microwave and RF annealed results are expected during experiments with controlled 30 - 35 ppm levels of O2. “Improvement” is represented by movement of the data points towards the bottom left-hand or “southwest”corner of the figure.

In research described in Ref. [13], a small YBa2Cu3O7 sintered pellet was placed at the center of a pressed YbBa2Cu3O7 powder compact and heated in a polarized microwave-frequency electromagnetic field, as illustrated in Fig. 5(a). After heating, the sample was sectioned, and the extent of Y-Yb chemical interdiffusion at the internal interface was examined as a function of interface orientation relative to the electric field. Since this is another example of a solid state reaction that is transport limited, one can characterize the extent of interdiffusion with an effective diffusion coefficient. Figure 5(b) is a polar plot of the relative, effective diffusion coefficient and the angle between the electric field vector and the direc-

468

Booske

tion of chemical interdiffusion. In this data, it is evident that transport parallel to the electric field’s orientation was much greater than transport perpendicular to the electric field’s orientation. In terms of the effective diffusion coefficient, the difference is remarkably between a factor of five to greater than 10. This measurement is a relative comparison using a small analysis volume within a single heated specimen. Consequently, it is another example of evidence of nonthermal microwave field influence on solid state reactions for which questions of accurate temperature measurement are not relevant.

Fig. 5. Measurements of Y-Yb chemical interdiffusion in a polarized microwave radiation field. (a) illustration of the configuration. (b) polar plot of relative, effective diffusion coefficient versus electric field orientation, showing much greater transport along the direction of the electric field than transverse to the field. Conventional thermal processing would have produced a uniform (circular) polar plot. [courtesy of G. Whittaker].

Finally, Ref. [14] describes a series of experimental and theoretical investigations that revealed the first known mechanism whereby microwave fields can nonthermally enhance ionic diffusion in materials and thereby accelerate solid state reaction kinetics. By measuring the instantaneous response of ionic current in a halide salt ceramic crystal to short pulses of microwave radiation (see Fig. 6), it was shown that microwave fields can induce a significant driving force for ionic transport [15]. Reference [16] proposed a mechanism, including a theoretical description, of a ponderomotive driving force (pmf) that arises when high frequency electric fields modulate ionic currents near surfaces where abrupt changes in ionic mobility occur. Such surfaces can include macroscopic free surfaces or microscopic grain boundaries. Reference [17] describes a carefully-coordinated series of

How the Coupling of Microwave and RF Energy in Materials

469

experiments and theoretical modeling efforts verifying the existence of the pmf. Reference [18] describes the results of numerical modeling that indicate the magnitude of the pmf’s influence can be large enough to noticeably enhance the transport and kinetics in solid state reactions with comparatively weak thermochemical driving forces (e.g., intermediate state sintering or late stage, chemical interdiffusion).

Fig. 6. Oscilloscope traces showing the effect of microwave irradiation on ionic currents in a NaCl crystal at 150°C. In both frames, the upper trace is the ionic current and the lower trace is the microwave power. Horizontal scale is 1 ms/div. (a) ionic current results from a 5 ms application of a 10 V bias across the crystal which is enhanced during the 400 Ps microwave pulse. (b) no bias voltage is applied, yet still an ionic current pulse is observed during the 400 Ps microwave irradiation pulse.

The characteristics of the pmf are entirely consistent with the observations illustrated in Figs. 3 and 5 and described in Refs. [11] and [13], respectively. Explanations for the enhanced annealing results of Fig. 4 are still under investigation.

Summary Microwave and RF fields heat materials by inducing motion of charged particles through the action of electric or magnetic forces. Consequently, internal penetration of the fields enables volumetric heating of many materials that are only sur-

470

Booske

face-heated in conventional furnaces. This volumetric heating can accelerate endothermic reactions that are rate-limited by heat transport from the material’s surface in conventional processing. The inverted temperature profiles that result from volumetric heating can provide unique processing effects. Conventional heating can result in the surface reaction completing before the interior is fully reacted. Surface pores may close prematurely, preventing mass transport of gas reactants to the center needed to complete the reaction of the interior. With inverted temperature profiles, microwave-heated materials can allow the reactant gases to permeate the specimen and diffuse to the hot center until it is fully reacted. In addition to heating, electromagnetic fields can influence reactions through direct modulation of particle motion, and, in some cases, modification of mass transport. The strongest evidence includes experiments where microwave- or RF-heated data can be compared with conventionally-heated data without reliance on an exact knowledge of the internal temperature. Several examples of such data exist, including results of a comprehensive study revealing a microwave-field-induced driving force that enhances ionic transport in materials. New results indicating microwave- and RF-field enhancements of annealing kinetics in B-doped Si are under investigation to advance scientific understanding and for their application value to advanced integrated circuit manufacture.

References [1] B. Meng, B. Klein, J. Booske, and R. Cooper, Phys. Rev.B 53 [19], 12,777-12,785 (1996). [2] B. Meng, Ph.D. Thesis, Electrical Engineering, University of Wisconsin-Madison, 1995. [3] K. Thompson, J. Booske, Y. Gianchandani, and R. Cooper, this Conference. [4] D.J. Skamser, J.J. Thomas, H.M. Jennings, and D.L. Johnson, J. Mater. Res., Vol. 10, 3160-3178 (1995); D.J. Skamser and D.L. Johnson, MRS Symp. Proc. Vol. 347, 325330 (1994). [5] J.G.P. Binner and I.A.H. Al-Dawery, Superconductor Science and Technology, Vol. 11, 449-457 (1998). [6] R. Plarre, S.L. Halverson, W.E. Burkholder, T.S. Bigelow, M.E. Misenheimer, J.H. Booske, and E.V. Nordheim, Proceedings of ANPP-4th International Conference on Pests in Agriculture, Montpellier 6-8 January, 1997. [7] N.K. Budraa, H.W. Jackson, M. Barmatz, W.T. Pike, and J. Mai, Proceedings of the 7th International (AMPERE) Conference on Microwave and High Frequency Heating, 463-466 (1999). [8] see for example, D.L. Johnson, J. Amer Ceram. Soc. Vo. 74, 849-850 (1991). [9] R. Borchert, M. Willert-Porada, Ceram. Trans., Vol. 80, 491-498 (1997). [10] K. Thompson, J.H. Booske, R.F. Cooper, Y.B. Gianchandani, and S. Ge, Ceram. Trans. Vol. 111, 391-398 (2001). [11] M. Willert-Porada, Ceram. Trans., Vol. 80, 153-163 (1997); S. Vodegel, Ph.D. Thesis, University Dortmund (1993).

How the Coupling of Microwave and RF Energy in Materials

471

[12] W. Lerch, B. Bayha, D.F.Downey, E.A. Arevalo; Proceedings of the 199th Annual Mtg, Electrochemical Society, Symposium on Rapid Thermal Processing, (Washington, DC, March 2001). [13] G. Whittaker and L. Cronin, Proceedings of the 2nd International AMPERE Conference on Microwave Chemistry (Antibes, 2001), pp. 291-294. [14] J.H. Booske, R.F. Cooper, S.A. Freeman, K. Rybakov, and V. Semenov, Phys Plasmas 5, 1664-1670 (1998). [15] S.A. Freeman, J.H. Booske, and R.F. Cooper, Phys. Rev. Lett. 74, 2042-2045 (1995). [16] K.I. Rybakov and V.E. Semenov, Phys. Rev. B, Vol. 49, 64 (1994). [17] V.E. Semenov, K.I. Rybakov, S.A. Freeman, J.H. Booske, and R.F. Cooper, Phys. Rev. B, 55, 3559 - 3567 (1997). [18] S.A. Freeman, J.H. Booske, and R.F. Cooper, J. Appl. Phys. , 83, 5761-5772 (1998).

Enhanced Mass and Charge Transfer in Solids Exposed to Microwave Fields V.E. Semenov, K.I. Rybakov Institute of Applied Physics, Russian Academy of Sciences, Nizhny Novgorod, Russia

Introduction Microwave heating is widely exploited in processes of thermal treatment of materials. Its well known advantages are based on the volumetric nature of microwave energy deposition [5, 6]. Generally the use of microwaves helps reduce energy consumption and processing time, which is important for industrial applications. During the last decade there have been many reports on microwave enhanced process rates. A special term, “microwave effect”, was even invented to denote the peculiar features in the course of microwave-assisted processes, as compared to their conventional counterparts. Still, the real reasons for many observed microwave effects are far from being understood completely. The specific non-thermal action of microwaves is discussed in almost all processes such as ceramics sintering and joining, chemical synthesis, annealing of semiconductor materials, etc. By non-thermal action we mean the effects which cannot be explained based on the evolution and spatial distribution of temperature in the material and which therefore are caused directly by the electromagnetic field. In fact, of all proposed microwave effects known to us, only molecular agitation, which may enhance chemical reactions, and modification of transport phenomena in the solids subjected to microwaves have the non-thermal nature [10]. The former mechanism was seriously criticized by Stuerga [21], whereas the latter found a confirmation [8, 20]. This paper reviews some experiments which indicate or suggest the nonthermal nature of microwave-induced phenomena, and discusses possible reasons for the enhanced mass and charge transport in the solids exposed to microwave fields.

Enhanced Mass and Charge Transfer in Solids Exposed to Microwave

473

Experiments To justify a claim for having observed a non-thermal microwave effect, an experiment should satisfy definite conditions. Specifically, any uncertainty in the temperature measurements must be minimized, and the influence of temperature gradients on the phenomenon under investigation must be avoided. The latter can be achieved when a local effect (that takes place within a small region) is considered or when the dependence of an effect on microwave parameters (intensity or direction of the field) is studied for the same temperature distribution. A detailed review of all experiments that satisfy the conditions formulated above is beyond the scope of this paper. Rather, the examples of experiments discussed below just illustrate the existence of the non-thermal microwave effects on transport phenomena in solids. To our knowledge, the first report of the field-induced mass transport was made by Geguzin and co-workers [14], who directly observed the motion of a pore in a NaCl crystal along the static, externally applied electric field. In these experiments the thermal action of the field was negligible due to poor electric conductivity of the crystal and low field strength (70 V/cm). Besides, a direct proof for the nonthermal nature of the effect was its correlation with the direction of the electric field vector. This study has demonstrated that the electric field can produce a driving force for mass transport in the crystalline solid. The pore motion is explained in [14] on the basis of a model with different mobilities of the positively and negatively charged vacancies on the pore surface and in the bulk. This model provides not only qualitative, but also quantitative interpretation of the experimental results. The field-induced mass transport has also been observed in a number of experiments using microwaves. In the basic study [11] Janney and co-authors explored the influence of microwave field on the diffusion of the oxygen isotope 18O in alumina single crystals. The observed effect was local, since the evolution of the spatial distribution of isotope concentration was investigated within a thin layer of thickness on the order of 1 µm in which the temperature could be considered constant. It has been found that the widening of the 18O-enriched layer proceeds faster in the presence of microwave field than under conventional heating at the same temperatures. Yet, the form of the spatial distribution of the isotope concentration under microwave heating remains typical for a diffusion-controlled process. On this basis the authors of [11] have concluded that the microwave field modifies oxygen diffusivity, D, in single-crystalline alumina. They have also established that under microwave heating the temperature dependence of the diffusivity, D (T), has an Arrhenius form, D v exp (-Q/T), with the activation energy, Q, considerably smaller than under conventional heating. Similar conclusions about microwave-enhanced atom diffusion were made in a number of other papers. In [13] the microwave influence on the annealing process of thin amorphous silicon films was studied. Again, the effect was local due to small thickness of the films, which ensured that temperature variations were negligible. It has been found that microwave annealing not only accelerates crystalli-

474

Semenov

zation in the films, but also enhances the process of hydrogen removal which is known to be diffusion-controlled under conventional heating. The authors of [15] have discovered that microwaves enhance the process of bulk oxygenation of dense YBCO ceramics. The temperature in these experiments was independently verified by measuring the equilibrium oxygen content which is highly temperature-sensitive. In [24] the microwave influence on the healing of cracks introduced to relatively thin plates of polycrystalline alumina by Vickers indentation was analyzed. The rate of this process is also governed by the diffusion flows of atoms in the immediate vicinity of the crack. Therefore its observed acceleration means that the mass transport is enhanced within a small region where the variations of temperature are negligible. Assuming that this effect is caused by a microwave enhancement of the diffusivity, the authors of [24] have investigated its dependence on the microwave power and temperature and obtained somewhat paradoxical results. In particular, the diffusivity was found to increase monotonically with microwave power. However, in contrast to [11], the activation energy for diffusion at low microwave power was larger than under conventional heating. Yet, with an increase in the microwave power the activation energy did not grow further, as it might be expected, but decreased. The problem of microwave effects is especially popular among the researchers who study the sintering of ceramic materials. Certainly, the densification of powder materials at sintering is closely related to mass transport in each powder particle. However, in many cases it is difficult to make any definite conclusions about the non-thermal microwave influence on mass transport on the basis of the observed enhancement of sintering because of insufficiently detailed experimental investigation. In fact, there have been not many purposeful studies of the possible non-thermal effects. A detailed investigation of the densification rate on the microwave power was accomplished by Wroe and Rowley [25]. Identity of the temperature-time schedules was ensured by simultaneously using microwave and conventional heating with controlled power of each. Specially mentioned should be the method of turning microwave power on and off in the course of sintering while keeping the temperature-time schedule unchanged. The results prove that the observed enhancement of densification correlates with the microwave power and not the temperature and therefore is of a non-thermal nature. The results of a comparative study of the closed porosity evolution at microwave and conventional sintering [23] are of special importance for understanding the mechanism of the non-thermal microwave effect. It is known that the size of sufficiently large pores surrounded by a large number of grains cannot be reduced by capillary forces [12]. Therefore the changes in the evolution of the closed porosity under microwave heating observed in [23] not only evidence for the changes in mass transport, but suggest that microwaves produce a new driving force for this process. Similar conclusions can be drawn from the studies of the microwave influence on the formation and decomposition of solid solutions [9, 22]. A direct observation of non-thermal microwave mass transport enhancement has been made recently in the experiments on microwave heating of thin amorphous alumina membranes with a regular structure of cylindrical pores with a di-

Enhanced Mass and Charge Transfer in Solids Exposed to Microwave

475

ameter of tens of nanometers [4]. Small thickness of the membranes (10 – 20 µm) eliminated the influence of temperature gradients. The observed enhancement of the closure process of the pores of such a small diameter has demonstrated that non-thermal effects can be strong enough to compete with very high capillary stresses typical for such pores.

Theory The above listed experimental findings, along with the other results not mentioned here, is a convincing evidence in favor of the existence of non-thermal microwave effect on the mass transport in solids. Based on very general notions, two alternatives for the physical mechanism of such effect can be formulated: (1) a change in the transport coefficients or mobilities under the action of the field, or (2) a new driving force created by the field. The idea about the change in the transport coefficients appears very attractive and has often been put forward in the experimental works cited above. However, until present no convincing models have been suggested that would be capable of describing quantitatively such an influence of the field on the material [5, 8]. The attempts to observe the enhancement of ion mobility in ionic crystals exposed to microwaves were also not successful [3, 8]. Instead, in the study of the influence of microwave field on the dc electric current flowing in an ionic crystal the existence of an additional microwave-induced driving force was established [8]. The nature of a similar force exerted by a dc electric field is well known. It is this (Coulomb) force that causes the drift of charged particles. Under microwave field such a drift is oscillatory and can lead to a non-zero net motion of particles only if there are some effects that rectify the alternating currents. Theoretical studies have demonstrated that such rectification is possible [16, 17] in the vicinity of structural nonuniformities, such as surfaces or grain boundaries. Two different mechanisms of rectification have been proposed. One is based on the supposed asymmetry of permeability of the surface/boundary for the flows of particles crossing it in the inward and outward direction [16]. The rectification efficiency in this case cannot be determined without the additional data on the surface/boundary properties, but in principle it may reach values close to 100%, which is more than enough for the mass transport enhancement observed in experiments. The second mechanism of rectification has been considered in detail in [2, 16, 17]. It relies on the natural formation of oscillating space charge perturbations near the surface/boundary and does not require any additional assumptions. In the case of ionic crystals calculations lead to the following expression, Eq. (1), for the net ponderomotive force acting on the charged vacancies in a unit volume: & & f UE , (1) & where U is the volumetric density of the oscillating space charge, E is the oscillating electric field vector, and the angular brackets denote averaging over a period of the microwave field.

476

Semenov

An analysis shows that a similar to (1) component of the net volumetric force exists for any type of charge carriers in the material. In particular, for the dipoles which determine the electric properties of many dielectric materials, the expression for the net volumetric force has the form of Eq. (2), & & & Hs  Hf H  Hf & · E2 1 & § UE  ’  f E ˜ ¨¨ ’ ˜ s E ¸¸ , (2) 2 2 8S 4S 1 Z W © 1  Z2 W 2 ¹ where Z is the field frequency, W is the time of dipolar relaxation in the Debye model, and H s , H f are low-frequency (at ZW  1 ) and high-frequency ( ZW !! 1 ) values of the dielectric permittivity of the material. & The characteristic magnitude of the volumetric force f is not large (a measure of the corresponding stresses is the radiation pressure of the electromagnetic field, & E 2 8S , which under the conditions of most microwave processing experiments does not exceed 0.1 Pa). However, in contrast to mechanical stresses this force is applied directly to the mobile particles, e. g., to the vacancies in an ionic crystal. In addition, the component of the force associated with the space charge is localized in the near-surface region, where the particle mobility is considerably higher than in the bulk. Therefore the efficiency of mass transport generation by ponderomotive forces may be up to 8 – 9 orders of magnitude higher than that by mechanical stresses. The main conclusion from the general analysis of the microwave ponderomotive influence on mass transport phenomena can be formulated as follows. The ponderomotive effect can be noticeable only due to its selectivity. Therefore, it can be observed in the materials where there is a small fraction of defects (vacancies or other) which are mobile and possess a significantly higher electric susceptibility compared to an average site in the crystalline lattice. A necessary condition for the ponderomotive effects is the presence of macroscopic structural nonuniformities in the material, such as free surfaces and/or boundaries between separate crystalline grains. According to Eq. (3), the mass flux under these conditions is proportional to: & D E2 H cc 2 , (3) T 8S H c 2  H cc 2 where D is the diffusivity of mobile defects, Hc , Hcc are real and imaginary parts of the complex dielectric permittivity of the material, respectively. Under the microwave heating the temperature dependence of mass transport intensity can differ significantly from that observed under conventional heating. In particular, when the dielectric losses are low ( H cc  H c ), the apparent activation energy for mass transport can exceed the value typical for the diffusion mass transport. On the contrary, when the dielectric losses are high ( H cc t H c ), the apparent activation energy for mass transport can be lower than the value typical for the diffusion mass transport.

Enhanced Mass and Charge Transfer in Solids Exposed to Microwave

477

Discussion A key moment of every theoretical model is the comparison of its predictions with the experimental results. Below we return to the experiments considered above as evidence for the non-thermal microwave enhancement of mass transport in order to find out whether or not they can be interpreted based on the model of ponderomotive action of the microwave field. It would be natural to start such an analysis with those experiments which suggest the existence of an additional driving force for transport phenomena under microwave heating [8, 9, 14, 22, 23, 25]. As mentioned above, the observed in [14] motion of pores in NaCl single crystals is completely understood within the frame of usual notions about the drift of charged vacancies in a dc electric field. In this sense, this is a linear effect with respect to the electric field strength. The model of ponderomotive action accounts for more subtle effects that are quadratic with respect to field (cf. (3)). Such nonlinear effects in the case being considered lead to the deformation of the pore, specifically, to a decrease in its dimension along the electric field vector. Such deformation was actually seen in the experiment [14], although the authors did not discuss that. Unfortunately, the absence of experimental data on the magnitude of this deformation does not allow us to quantitatively check applicability of the ponderomotive model to these experiments. A detailed comparison of the theory with experiment became possible in the study of microwave-induced dc currents in ionic single crystals [8]. For this purpose, additional experiments and calculations were undertaken [20] which have shown that the ponderomotive model is capable of describing the sophisticated transient dynamics of the current induced by microwaves in different ionic crystals. Theoretical investigations of the ponderomotive effect in the vicinity of neck contacts between adjacent grains in ceramics [1, 2] have shown that there exists an additional factor of its enhancement related to the electric field amplification in this region. Since only the field component which is perpendicular to the intergrain boundary is amplified, the ponderomotive effect manifests in a preferential direction regardless of the field vector orientation with respect to the boundary. As a result, the atoms are prevailingly driven from the boundary region to the adjacent free surfaces of grains [2], which means expansion of the intergrain contact area and hence accelerates sintering. Similar considerations can be invoked for the analysis of mass transport in the vicinity of a pore surrounded by crystalline grains. The ponderomotive action of the field in this case facilitates closure of the pores which are otherwise thermodynamically stable and do not tend to get smaller under conventional heating [19]. The ponderomotive model is thus capable of explaining qualitatively the observed decrease in the amount of closed porosity under microwave heating [23]. Certain conclusions regarding the influence of ponderomotive forces on the formation and decomposition of solid solutions can be drawn from a consideration of the model of a spatially inhomogeneous mix of two metal oxides. Let the ions of these metals have different mobilities, e. g., the mobility of the cation A1 be

478

Semenov

higher than the mobility of the cation A2 . Then the microwave ponderomotive action will exert a force on the negatively charged vacancies directed towards the region with higher concentration of the cations A2 . In the bulk this will only insignificantly redistribute vacancy concentration. However, near the free surface of the crystal this force will cause mass flows that corrugate this surface and simultaneously enhance the degree of inhomogeneity in the distribution of cation concentration in the near-surface layer (Fig. 1).

E

E0 cosZt

development of surface corrugation mass transport due to ponderomotive effect perturbation with a higher concentration of A1+ (more mobile)

perturbation with a higher concentration of A2+ (less mobile)

Fig. 1. Ponderomotive mass transport in a spatially inhomogeneous mix of two metal oxides with different cation mobility

On the other hand, as it was reported previously [18], the corrugation of the solid surface is unstable in the microwave electric field directed along the surface. This means that the ponderomotive mass transport will be further enhanced due to corrugation. Thus one can conclude that the ponderomotive force promotes decomposition of solid solutions but only in the near-surface layer. In principle, the results of the experimental study of spinodal decomposition [22] can be interpreted in this manner. It is possible that similar effects are also responsible for the reversible solid solution formation which was observed in [9]. The process of densification of powder materials in the sintering of ceramics is too complicated to permit direct theoretical analysis of the possible influence of the microwave ponderomotive forces on it. Besides, the values of the parameters that determine the efficiency of the ponderomotive action on mass transport are most often unknown. Therefore, there still exists a considerable uncertainty in the interpretation of the enhanced sintering observations. Yet, a correlation can be noticed between the possible magnitude of the ponderomotive effect and the observed enhancement of densification. In fact, based on the data on the dielectric losses in zirconia ceramics provided by the authors of [25], one might expect a microwave enhancement of sintering to manifest itself in this case, and this was actually observed. On the other hand, in the experiments with pure alumina ce-

Enhanced Mass and Charge Transfer in Solids Exposed to Microwave

479

ramics [7] the measured value of H cc was too low for the ponderomotive effects to occur, and no enhancement in densification was found. Simple estimates show that with typical microwave field strengths the space charge-based ponderomotive effect cannot compete with capillary stresses in the processing of materials with pores of a nanoscale size. Such competition can be successful only in the case of full rectification of the alternating vacancy flows [4], which can occur only in the case of significant asymmetry of the surface permeability for the inward- and outward-directed vacancy flows [16]. Such properties of surfaces are known insufficiently. The observation of microwave-enhanced closure of nanosized pores [4] stands as a call for further research in this area. As noted above, there has been a number of experimental observations of nonthermal microwave effect on mass transport, the results of which have been considered as evidence in favor of the hypothesis of microwave-enhanced diffusivity [11, 13, 15, 24]. Since no clear physical mechanism has been proposed for such an effect, we may attempt to interpret these experimental results on the basis of the ponderomotive model. Such an interpretation has the best prospects in the analysis of the crack healing under microwave heating [24]. First, in the vicinity of the crack edge a considerable amplification of the electric field takes place. Second, we deal with a localized mass transport near a pronounced structural nonuniformity, which are favorable conditions for the ponderomotive effect to occur. At last, a pure diffusivity-based interpretation leads in this case to a controversy in the dependence of the activation energy for diffusion on the intensity of microwaves. The diffusivity-based interpretation appears to be quite natural for the processes of oxygenation of YBCO ceramics [15] and removal of hydrogen impurities from amorphous silicon films [13]. However, the experimental data presented in these papers are generally insufficient for making unambiguous conclusions about diffusivity enhancement under microwave heating. In fact, the processes considered in both cases are not purely volumetric but associated with the phenomena on the free surface. Under these conditions the rates of both processes depend not only on the volumetric diffusivity, but also on the boundary conditions at the surface, i. e., its permeability for oxygen and hydrogen atoms. The ponderomotive action of the field intensifies vacancy flows near the surface of materials. Therefore, it may, in principle, change the permeability of the surface for the atoms of gases significantly. Further research is needed to interpret these phenomena more precisely. In [11] the volumetric processes of oxygen atom transport were studied in a material with uniform microstructure. The use of oxygen isotopes in sapphire also ensures uniform dielectric properties of the material. Under such conditions no appreciable ponderomotive effects can be expected. Besides, the detailed investigation of the spatial distribution of isotope concentration confirmed that the transport phenomena under study obey the diffusion laws both under conventional and microwave heating, with the activation energy for diffusion being noticeably lower in the latter case. The results of this study thus are unlikely to be explained on the basis of ponderomotive effects. Therefore, the search for the possible physical mechanism(s) of the microwave field influence on atom diffusivity should continue.

480

Semenov

Conclusion Summarizing the research on the non-thermal action of microwave field on mass and charge transport in solids, we state that despite the appreciable progress achieved both in the experimental identification of such effects and in understanding some of their mechanisms, the physics of these phenomena is still far from being clear. The proposed ponderomotive mechanism of microwave field action remains to this date the only quantitative model that can be correlated with experimental data. However, in many cases its experimental verification requires more detailed knowledge of the materials’ properties. Besides, it appears that most probably there exists more than one mechanism of non-thermal microwave action. This field is still wide open for fundamental research, and its future results will be of crucial importance for applications.

References [1] Birnboim A, Calame JP, Carmel Y (1999) Microfocusing and polarization effects in spherical neck ceramic microstructures during microwave processing. J Appl Phys, 85: 478-482 [2] Booske JH, Cooper RF, Freeman SA, Rybakov KI, Semenov VE (1998) Microwave ponderomotive forces in solid state ionic plasmas. Phys Plasmas 5: 1664-1670 [3] Bykov YuV, Eremeev AG (1996) Electrical conductivity of Al2O3- and ZrO2- based ceramic materials under microwave heating. Fiz Khim Obr Mater No. 6: 114-121 (in Russian) [4] Bykov YuV, Egorov SV, Eremeev AG, Rybakov KI, Semenov VE, Sorokin AA, Gusev SA (2001) Evidence for microwave enhanced mass transport in the annealing of nanoporous alumina membranes. J Mater Sci 36: 131–136 [5] Bykov YuV, Rybakov KI, Semenov V E (2001) High temperature microwave processing of materials. J Phys D: Appl Phys 34: R55–R75 [6] Clark DE, Sutton WH, Lewis DA (1997) Microwave processing of materials. In: Clark DE, Sutton WH, Lewis DA (eds) Microwaves: theory and application in materials processing IV (Ceramic Transactions, vol 80). The American Ceramic Society, Westerville, pp 61–96 [7] Fliflet AW, Bruce RW, Fischer RP, Lewis D, Kurihara LK, Bender BA, Chow G-M, Rayne RJ (2000) A study of millimeter-wave sintering of fine-grained alumina compacts. IEEE Transactions on plasma science 28: 924–935 [8] Freeman SA, Booske JH, Cooper RF (1995) Microwave field enhancement of charge transport in sodium chloride. Phys Rev Lett 74: 2042–2045 [9] Getman OI, Panichkina VV, Skorokhod VV, Shevchenko EA, Holoptsev VV (2001) Densification and diffusion processes in the Ba–Sr titanate system under microwave sintering. This volume [10] Jacob J, Chia LHL, Boey FYC (1995) Thermal and non-thermal interaction of microwave radiation with materials. J Mater Sci 30: 5321–5327 [11] Janney MA, Kimrey HD, Allen WR, Kiggans JO (1997) Enhanced diffusion in sapphire during microwave heating. J Mater Sci 32: 1347–1355

Enhanced Mass and Charge Transfer in Solids Exposed to Microwave

481

[12] Lange FF (1984) Sinterability of agglomerated powders. J Am Ceram Soc 67: 83–89 [13] Lee JN, Choi YW, Lee BJ, Ahn BT (1997) Microwave-induced low-temperature crystallization of amorphous silicon thin films. J Appl Phys 82: 2918–2921 [14] Lifshitz IM, Kossevich AM, Geguzin YaE (1967) Surface phenomena and diffusion mechanism of the movement of defects in ionic crystals. J Phys Chem Solids 28: 783– 798 [15] Rowley AT, Wroe R, Vazquez-Navarro D, Lo W, Cardwell DA (1997) Microwaveassisted oxygenation of melt-processed bulk YBa2Cu3O7-G ceramics. J Mater Sci 32: 4541–4547 [16] Rybakov KI, Semenov VE (1994) Possibility of plastic deformation of an ionic crystal due to the nonthermal influence of a high-frequency electric field. Phys Rev B 49: 64– 68 [17] Rybakov KI, Semenov VE (1995) Mass transport in ionic crystals induced by the ponderomotive action of high-frequency electric field. Phys Rev B 52: 3030–3033 [18] Rybakov KI, Semenov VE (1996) Possibility of microwave-controlled surface modification. In: Iskander MF, Kiggans JO, Bolomey J-C (eds) Microwave processing of materials V (Materials Research Society Symposium Proceedings, vol 430). Materials Research Society, Pittsburgh, pp 435–440 [19] Rybakov KI, Semenov VE (1998) Non-thermal effects in microwave sintering of ceramics. In: Ceramics: getting into the 2000’s, Part C. Techna, Faenza [20] Rybakov KI, Semenov VE, Freeman SA, Booske JH, Cooper RF (1997) Dynamics of microwave-induced currents in ionic crystals. Phys Rev B 55: 3559–3567 [21] Stuerga DAC, Gaillard P (1996) Microwave athermal effects in chemistry: a myth’s autopsy. J Microwave Power and Electromagnetic Energy 31: 87–113 [22] Willert-Porada M (1996) Microwave effects on spinodal decomposition. In: Iskander MF, Kiggans JO, Bolomey J-C (eds) Microwave processing of materials V (Materials Research Society Symposium Proceedings, vol 430). Materials Research Society, Pittsburgh, pp 403–409 [23] Willert-Porada M (1997) A microstructural approach to the origin of “microwave effects” in sintering of ceramics and composites. In: Clark DE, Sutton WH, Lewis DA (eds) Microwaves: theory and application in materials processing IV (Ceramic Transactions, vol 80). The American Ceramic Society, Westerville, pp 153–163 [24] Wilson DA, Lee K-Y , Case ED (1997) Diffusive crack-healing behavior in polycrystalline alumina: a comparison between microwave annealing and conventional annealing. Mater Res Bull 32: 1607–1616 [25] Wroe R, Rowley AT (1996) Evidence for a non-thermal microwave effect in the sintering of partially stabilized zirconia. J Mater Sci 31: 2019–2026

Thermal Runaway and Hot Spots Under Controlled Microwave Heating V.E. Semenov, N.A. Zharova Institute of Applied Physics, Russian Academy of Sciences, Nizhny Novgorod, Russia

Introduction RF and microwave heating is associated with a volumetric generation of heat caused by dissipation of electromagnetic energy inside material. In these processes the spatial distribution of the heating power density, W, is determined by the electric field strength, E, and electric conductivity, V, of material:

W

VE 2

(1)

Electromagnetic absorptivity of most dielectric materials increases sharply with temperature, T. Respectively the rate of RF or microwave heating increases in the course of this process given constant field intensity. Therefore the process of heating acquires nonlinear nature that makes its control difficult. Specifically, when an increase in V(T) is stronger than the increase of energy losses from the heated sample, then one faces an out-of control temperature growth known as the temperature runaway [3]. Usually this thermal instability is accompanied by the formation of a spatially inhomogeneous temperature distribution with separate peaks known as hot spots [4]. The problems of thermal runaway and hot spots are widely discussed in the literature related to the RF and microwave processing of materials (see, for example, [1, 2, 7, 8]). In most papers the attention is paid to the analysis of thermal instability under the conditions of a preassigned power schedule. At the same time it may seem almost evident that the thermal runaway can be avoided using appropriate control of electromagnetic power, i.e. variation of the evolution of E2(t) depending on temperature. Specifically, this possibility was pointed out in [6] where a temperature time derivative feedback loop was considered to control the process of microwave heating. However, full control of the thermal runaway is possible only when the maximum temperature of the sample is available for use in the control feedback loop. As shown in this paper, the temperature at the sample surface cannot be used for this purpose in all cases. The possibility of such control of the thermal runaway stands in close relation with the heating regime, thermal insulation arrangement, and the properties of material.

Thermal Runaway and Hot Spots Under Controlled Microwave Heating

483

Model description This paper presents results of numerical simulations of thermal runaway under the conditions of controlled microwave heating. The spatio-temporal evolution of the temperature field was described by the thermal conduction equation containing a volumetric heat source:

UC

wT wt

’ O ’ T  V T E 2 t

(2)

where U, C, and O stand for density, specific heat capacity, and thermal conductivity of material. Calculations were carried out for a temperature field with spherical symmetry: T=T(r,t). Within the model considered it was assumed that the heat flux is proportional to the temperature perturbation at the sample surface:

wT wr

 r R

G T s  T 0 R

(3)

where R is the sample radius, Ts is the temperature at the sample surface (Ts = T(R,t)), T0 stands for undisturbed temperature of the sample, and the factor G simulates properties of the thermal insulation arrangement. Following [5], a power-law temperature dependence of the material’s electromagnetic absorptivity,

V vTE ,

was considered. Two different regimes of heating were studied. The 2

const . The secfirst one simulated heating with fixed microwave power: E ond regime simulated controllable microwave heating. Within this model the temporal evolution of the electric field intensity was adjusted to maintain an increase of the surface temperature Ts at a constant rate up to definite maximum value Tm and then to sustain it at this maximum level.

Results of numerical simulations The results of numerical simulations are presented below using dimensionless variable such as x = r/R, T= T/T0, W= Ot/UCR2. The dimensionless equation of heat conduction and the respective boundary condition are

wT wW

1 w § 2 wT · E ¨x ¸  qT x 2 wx © wx ¹ wT wx

 G T s  1 x 1

(4)

(5)

484

Semenov

where q V T 0 E 2 R 2 O T 0 is normalized microwave power, T s Ts T0 is normalized surface temperature of the sample. Some important information about the peculiarities of heating can be obtained from an analysis of its steady state. A stationary solution of Eqs. (4)-(5) depends significantly on the parameters E and G. When nonlinearity is weak (i.e. E < 1) there is only one stationary solution for each particular value of power, q (see Fig.1a). Within a range 1 < E < 5 there are two stationary modes (Fig. 1b) differing in the value of temperature, Tc, at the sample center for small level of power (q < qmax) but stationary solution is impossible if q > qmax. When nonlinearity is strong (E > 5) the systems becomes very sensitive to the quality of thermal insulation. In this case the stationary solutions are qualitatively similar to those shown in Fig. 1b if G is less than a definite critical value Gc(E) shown in Fig. 2. However, for G > Gc the picture is quite different (see Fig. 3). It should be also noted 1

15.8

1

10

6.31

Tc, Ts(q)

Tc, Ts(q)

10

2 3.98

6.31

2

3.98 2.51

2.51

1.58

q 3.98

2.51

1.58

a

10

6.31

1 0.001

q 0.1

0.01

b

1

Fig. 1. Normalized stationary temperature at the center, Tc (curve 1), and at the surface, Ts (curve 2), of a sample vs. normalized microwave power, q: G = 1; (a) E = 0.5, (b) E = 3.

that for two particular cases (E = 1 and E = 5) the stationary solutions can be expressed in explicit analytical form. When E = 1,

T

Tc





sin x q , x q

Tc

q cos

G q q  G  1 sin

q

(6)

In this case the normalized power, q, must be less than qmax determined as the least positive root of equation q max  G  1 tan q max 0 . When E = 5,

T

Ts

Ts

T s  G T s  1 1  x

2



,

q

3G T s  1 >G  1  G T s @

T s6

(7)

The stationary temperature distribution is quasi-uniform over a sample if the thermal nonlinearity is weak or the thermal insulation is sufficiently good provided the following inequality (Eq. (8)) is fulfilled:

Thermal Runaway and Hot Spots Under Controlled Microwave Heating 1 0.95

Gc(E) 15.8

0.9

Tc, Ts(q)

10

0.85

0.8

0.75

1

6.31 3.98 2.51

0.7

2

1.58

0.65 0.6

485

5

5.2

5.6

5.4

5.8

1

E

Fig. 2. Function Gc(E). When G < Gc the stationary surface temperature Ts can be infinitely high whereas in the opposite case the possible value of Ts is limited from above.

0.001

q 0.01

0.1

1

Fig. 3. Normalized stationary temperature at the center, Tc (curve 1), and at the surface, Ts (curve 2), of a sample vs normalized microwave power, q: G = 1, E = 6.

(8)

E G  G  2

In this limiting case the stationary solution of Eqs. (4) - (5) can be presented in the approximate form given below:

T | T c  GT x 2 , q|

6GT

T cE

,

GT q max |

G T c  1 G2 E 1 6 G E  1

(9)

EE

A similar simplified model can be used to analyze temporal evolution of temperature that yields an equation for Tc:

dT c dW

q T cE 

6 G T c  1 G2

(10)

This simplified model is very close to that considered by Kriegsmann [3]. According to Eq. (10), only the stationary mode with the smaller value of temperature, Tc, (see Fig. 4) is stable when E > 1 and power is constant. In particular, it implies that in a heating regime with fixed power one is unable to obtain a steady temperature exceeding threshold value T th E E  1 if E > 1. At the same

time the stationary mode with T ! T th is not difficult to stabilize using a heating regime with controllable power [6]. For instance, it is possible to vary the power following the dependence q(Tc) shown on Fig. 4.

486

Semenov c

3

2.5

2.5

Tc

Ts

2. 2

1.5 1.5

1.

0.01

0.02

0.03

q

Fig. 4. Stationary temperature of the sample center (equation (10)) vs. normalized power, q. (E = 6, G = 0.1). Dotted line shows a regime of power control q(Tc) that makes it possible to get the higher (unstable) temperature value.

1 0

5

10

Tc

Fig. 5. Normalized stationary surface temperature, Ts vs. a temperature, Tc, at the sample center. G = 1, E = 6.

The above results of the analysis of a simplified model have been confirmed by numerical solution of Eqs. (4)-(5). The calculations have shown that any stationary mode of temperature can be stabilized using an appropriate control of a power, q, even if the inequality (8) is violated and the temperature distribution is far from uniformity. When E < 5 or E > 5 but G < Gc the stabilization is possible even if a control of a power is based on a temperature at the sample surface. However, in the opposite case (E > 5 and G > Gc) such type of power control allows to stabilize only the stationary modes with the temperature limited from above. These modes exhibit a growing dependence of Ts on Tc (see Fig. 5 and Fig. 3). To stabilize the stationary modes with a decreasing function Ts(Tc) it is necessary to use the power control based on the temperature at the sample center. In numerical simulations a particular attention was paid to a special regime of microwave controllable heating which is widely exploited in practice. Within this regime the power, q, was adjusted to provide a constant rate of the surface temperature increase ( d T s d W const ) up to a definite value Tm and to sustain the equality Ts = Tm further. Most calculations were carried out taking Tm = 4. The main results of this study can be formulated as follows. When E d 3 any heating rate was found to be stable. As can be seen from (Figs. 6a, 6b) the temperature difference between the center and the surface of the sample increases with an in crease in the heating rate. Nevertheless, the maximum temperature remains finite in the course of heating. In the case of E t 4 and G < Gc a threshold value of the heating rate was found to exist for a stable regime.

Thermal Runaway and Hot Spots Under Controlled Microwave Heating 7

10

6

1

1

3

Tc(W TsW

Tc(W TsW

8

2

2

5

2

6

4

1 0

487

2

4

0.2

0.4

a

0.6

0.8

W

0 0

0.1

0.2

b

0.3

0.4

W

Fig. 6. Temporal dynamics of normalized temperatures Tc (curve 1) and Ts (curve 2) in the heating regime with controlled power schedule for the parameter values G = 1, E = 3. The case (a) corresponds to heating at a constant rate dTs/dW 4 and the case (b) to dTs/dW 32

Actually, slow heating was stable, similar to the case of E d 3, whereas fast heating resulted in the thermal runaway at the sample center (see Fig. 7). The stronger the thermal nonlinearity (the higher a value of E) the smaller the threshold value of the heating rate (cf. Fig. 7 and Fig. 8). Finally, when E > 5 and G > Gc any heating rate was found to be unstable (see Fig. 9). Such a peculiarity is completely consistent with the above results related to stationary solutions of Eqs. (4) - (5). Actually, for sufficiently high values of E and G there is no stationary solution with Ts = 4. As can be seen from the Figs. 7b, 8b, and 9 the observed temperature runaway takes place under the conditions of completely controlled temperature at the sample surface. Therefore this phenomenon cannot be explained by poor control of the surface temperature within the model used. The calculated temperature distribution demonstrates a strong narrow peak at the sample center (Fig. 10). This peak is separated from the rest temperature distribution and its evolution becomes independent on the surface temperature at the final stage of the thermal runaway.

488

Semenov

12

25

10

20

Tc(W TsW

Tc(W TsW

1

8

15

6

2

10

4

1

5

2 0 0

0.2

0.4

a

0 0

W

0.8

0.6

0.1

0.2

b

2 0.3

0.4

W

Fig. 7. Temporal dynamics of normalized temperatures Tc (curve 1) and Ts (curve 2) in the heating regime with controlled power schedule for the parameter values G = 1, E = 4. The case (a) corresponds to heating at a constant rate dTs/dW 4.8 and the case (b) to dTs/dW 4.9. Maximum value of the surface temperature Tm = 4 was not achieved in the latter case due to thermal runaway at the center. 20

15

1

Tc(W TsW

Tc(W TsW

15

10

2

5

0 0

1

10

0.5

1

1.5

a

2

2.5

2

5

W

0 0

0.5

1

b

1.5

W

Fig. 8. Temporal dynamics of normalized temperatures Tc (curve 1) and Ts (curve 2) in the heating regime with controlled power schedule for the parameter values G = 1, E = 5. The case (a) corresponds to heating at a constant rate dTs/dW 4. The case (b) corresponds to heating at a constant rate dTs/dW 1.45. Maximum value of the surface temperature Tm = 4 was not achieved in case (b) due to thermal runaway at the center.

Thermal Runaway and Hot Spots Under Controlled Microwave Heating

489

8

100

6

1

60

4

W = 0.5

40

2

3

W = 0.4

W = 0.3

20

2 1 0

W= 0.5056

80

5

T(r)

Tc(W TsW

7

5

10

15

W

Fig. 9. Temporal dynamics of normalized temperaturesTc (curve 1) and Ts (curve 2) in the heating regime with controlled power schedule for the parameter values G = 1, E = 6. Heating at a constant rate dTs/dW = 0.1In this case it is not possible to get the surface temperature higher than 2.7.

0 0

0.2

0.4

W = 0.2 0.6

0.8

r

Fig. 10. Temporal dynamics of radial distribution of normalized temperature in the same heating regime as in Fig. 7b.

Conclusion Numerical simulations of the thermal runaway have been carried out within the simplest model of spherically symmetrical temperature distribution over a sample undergoing microwave heating. It was shown that the thermal runaway can be avoided under conditions of controllable microwave heating. However when a control of microwave power is based on the surface temperature of a sample, a stable regime of heating can only be achieved if an increase of the electromagnetic absorptivity of a sample material with temperature growth is not too sharp. In the opposite case it is necessary to use a very good thermal insulation arrangement. In the intermediate case a development of the thermal runaway is sensitive to the rate of microwave heating.

Acknowledgement This research is supported in part by the Civilian Research and Development Foundation (grant # RE1-2065), International Science and Technology Center and

490

Semenov

Japan as its funding party under project # 1607, and Russian Foundation for Basic Research (grant # 00-02-17318).

References [1] Coleman CJ (1991) On the microwave hotspots problem. J Austral Math Soc B33:1-8 [2] Koh M, Singh RK, Clark DE (1995) Temperature distribution considerations during microwave heating of ceramics. In: Clark DE, Sutton WH, Lewis DA (eds) Microwaves: Theory and Application in Materials Processing III (Ceramic Transactions, vol 59) The American Ceramic Society, Westerville, pp 313-321 [3] Kriegsmann GA (1992) Thermal runaway in microwave heated ceramics. A one dimensional model. J Appl Phys 71:1960-1966 [4] Kreigsmann GA (1994) Growth and stabilization of hot spots in microwave heated ceramic fibers. In: Iskander MF, Kiggans JO, Bolomey J-C (eds) Microwave Processing of Materials IV (Materials Research Society Symposium Proceedings, vol 347) Materials Research Society, Pittsburgh, pp 473-478 [5] Pincombe AH, Smyth NF (1994) Microwave heating of materials with power law temperature dependencies. IMA J Appl Math 52:141-176 [6] Senko H and Tran VN (1997) Control of microwave induced thermal runaway using temperature derivative feedback. In: Clark DE, Sutton WH, Lewis DA (eds) Microwaves: Theory and Application in Materials Processing IV (Ceramic Transactions, vol 80) The American Ceramic Society, Westerville, pp 241-250 [7] Thomas JR, Unruh WP, Vogt GJ (1994) Mathematical model of thermal spikes in microwave heating of oxide ceramic fibers. In: Iskander MF, Kiggans JO, Bolomey J-C (eds) Microwave Processing of Materials IV (Materials Research Society Symposium Proceedings, vol 347) Materials Research Society, Pittsburgh, pp 363-368 [8] Tran N and Piotrowski A (1997) Advances in the modeling of microwave RF and hot air drying of materials. In: Clark DE, Sutton WH, Lewis DA (eds) Microwaves: Theory and Application in Materials Processing IV (Ceramic Transactions, vol 80) The American Ceramic Society, Westerville, pp 201-216

Densification and Diffusion Processes in the Ba,Sr-Titanate System Under Microwave Sintering O.I. Getman1, V.V. Panichkina1, V.V. Skorokhod1, E.A. Shevchenko1, V.V. Holoptsev2 1 2

Institute for Problems of Material Science, Kiev, Ukraine Institute of Applied Physics, Nizhny Novgorod, Russia

Abstract The densification and diffusion processes in the Ba,Sr-Titanate system under microwave (MW) heating (frequency 30 GHz) and under conventional (CV) heating were studied. The BaTiO3-SrTiO3 system was selected as a model system for the investigation of unusual diffusion process under MW heating. The samples were prepared from the fine particle (0.1 - 0.3 Pm.) powder equimolar blends of components and were sintered at 1000 - 1300°C in air under MW and CV heating. Kinetics of a solid solution formation was studied by X-ray analysis. The porous structure of samples was studied by mercury porosimetry. The correlation between the local densification, coarsening of porous structure and solid solution formation is shown.

Introduction The results of numerous experiments demonstrate a considerable decrease in the sintering time and, in a number of cases, in the sintering temperature under microwave heating as compared with sintering under conventional heating. There are also many experimental results indicative of enhanced diffusion processes under microwave heating [1]. Finding an explanation for the enhanced mass transport and, in particular, for the accelerated densification under microwave sintering of ceramic materials is a challenging problem. The authors of [2] suggested the existence of non-thermal action of the microwave field on mass transport processes in ceramic materials. The authors showed that the vacancies that are forming in surface layers of ionic crystals placed in a microwave field would move. Estimates indicate that in this case stresses are comparable with capillary pressures arising in a powder compact at sintering if the

492

Getman

size of powder particles are is on the order of 1 Pm. Such a displacement of vacancies under a non-thermal action of the microwave field should result in a macroscopic mass transport and accelerated densification of a porous sample. One of the indicators of a macroscopic mass transport during sintering may be the diffusion process in binary systems. Using a comparison of the results obtained under microwave and conventional heating we have studied the character of densification and the process of formation of solid solutions by mutual diffusion in the BaTiO3-SrTiO3 powder system, whose components feature an unlimited mutual solubility [3].

Experimental procedure Samples for the study were prepared from an equimolar mixture of BaTiO3 and SrTiO3 powders with an average particle size of 0.1 - 0.3 Pm. Cylindrical samples (10 mm diameter, 5 mm high) were compacted in steel moulds by double pressing. The samples show an initial porosity of 36.8%. The samples were sintered in air under microwave heating, using microwave radiation at a frequency of 30 GHz [4] and under conventional heating in a temperature range of 1000 - 1300qC for 15 min – 4 hours. A separate sample was used for every treatment condition. The kinetics of solid solutions formation was studied by XRD analysis. The samples were characterised by the volumetric and local shrinkage. The average pore size was determined by mercury porosimetry. The microstructure of samples was studied by electron microscopy.

Results The results on the (Ba,Sr)-Titanate solid solution content determined by XRD are summarized in Table 1. Table 1. (Ba,Sr)-Ttitanate solid solution content vs. temperature and time of annealing at the microwave (MW) and conventional (CV) heating. 15 min CV MW 1100   1200   1300 94 100 -* - no measurements T, °C

(Ba, Sr) titanate solid solution content, mol% 30 min 1 hour 2 hours CV MW CV MW CV MW 11 7.5 -* 14 13 12 59 46 76.5 70 84 65 94 100 98 89 98 97

4 hours CV MW 24 12 87 58 100 100

The diffusion interaction between BaTiO3 and SrTiO3 begins at the temperature above 1100qC under both microwave and conventional heating. Below 1100qC the interaction is negligible. In all cases the barium component disappears

Densification and Diffusion Processes in the Ba,Sr-Titanate System

493

Solid solution content, %mol

faster than the strontium one, as a result of the well-known Kirkendall effect in this system [5]. Under conventional heating, the content of the formed solid solution increases with temperature (from 1100qC to 1300qC) and with the hold time (from 15 min to 4 hours). At a temperature of 1300qC, the content of the solid solution reaches 94 mol% after 15 min of holding. The results obtained on samples after annealing at 1200qC under microwave heating differ significantly from the respective results of conventional annealing. In contrast to the conventional annealing, the solid solution content at microwave heating depends on temperature non-monotonically. After 1 hour of holding, the solid solution content was 70 mol%, whereas the content of solid solution in the samples annealed for 2 and 4 hours turned out to be less, 65 and 58 mol% respectively (Fig. 2). The same tendency was observed at 1100qC. 100 90 80 70 60 50 40 30 20 10 0

CV MW 0

1

2

3

4

5

Time, h

Fig. 1. Kinetics of (Ba,Sr)-Titanate solid solution formation at 1200°C

The effective diffusion coefficient Deff was calculated with the formula (1):

Deff

9 S ˜W 2

(1)

where S is interface area, W is the hold time [6]. The effective diffusion coefficient is 8·10-15 m2/s for the samples annealed 15 min at 1300°C under MW heating when the solid solution content is 100 mol% (Table 1), S = 1 m2/cm3 and W = 15 min. At 1300°C the 131Ba diffusion coefficient in BaTiO3 is 3.5·10-17 m2/s and the 89 Sr diffusion coefficient in SrTiO3 is 7.8˜10-14 m2/s [7]. The decrease in the solid solution content with an increasing annealing time can be caused by its decomposition. Such a process can occur when the instability at small fluctuations of the composition, inherent in a material, results in a spinodal decomposition [8]. Formation of two-phase solid solutions was found at synthesis of a double titanate from nanosized TiO2 powders and barium and strontium

494

Getman

chloride solutions [9]. However, we do not have data on spinodal decomposition of the solid solution in a BaTiO3-SrTiO3 system.

Fig. 2. XRD (211) patterns of BaTiO3 – SrTiO3 powder samples sintered at 1200°C under microwave and conventional heating, the hold times are shown.

The pore sizes in the samples increase under both microwave and conventional heating (Fig. 3), which evidences that local densification proceeds in microvolumes of samples.

Densification and Diffusion Processes in the Ba,Sr-Titanate System

495

100 80

%

60 40

Initial

20

CV MW

0 0

1

2

3

4

Pore size, Pm

Fig. 3. Integral pore size distributions for (Ba,Sr)-Titanate samples after sintering at 1300°C, 30 min

Using the mercury porosimetry data, the local shrinkage ('V/V)loc of samples was calculated with the formula [10]

§ 'V · ¨ ¸ © V ¹loc

ª § R ·3 § 'V 1 · 2 º ¸ », «1  ¨ 0 ¸ ˜ ¨¨1  ˜ V 4 0 ¸¹ » «¬ © R ¹ © ¼

(2)

where 'V/V is the volumetric shrinkage; R0 and R are the initial and the current average pore radius; 40 is the initial porosity of the compact. The results of volumetric ('V/V) and local ('V/V)loc shrinkage, the content of BaTiO3 - SrTiO3 solid solution (C), and the average pore size via temperature and time of annealing are given in Table 2. Table 2. Volumetric ('V/V) and local ('V/V)loc shrinkage, content of BaTiO3 - SrTiO3 solid solution (C) and average pore size via temperature and time of annealing MW 1200°C 1300°C Time, min 30 60 120 240 15 30 0.1 0.1 0.1 0.1 1.1 1.2 'V/V, % 35.0 20.1 25.2 27.0 23.9 -** ('V/V)loc, % C, % 46 70 65 58 100 100 1.6 0.78 0.88 0.93 0.85 -** Pore size*, Pm * ,the initial average pore size of the samples was 0,6 Pm, -**, no measurements

CV 1200°C 1300°C 30 240 15 0.3 5.0 3.7 27.0 29.0 -** 59 87 94 -** 0.92 1.4

30 8.2 35 94 -**

In all cases the values of the local shrinkage exceed drastically those of the volumetric shrinkage and correlate with the content of the formed solid solution. As seen from Table 2, under conventional heating the local shrinkage increases with an increasing volumetric shrinkage.

496

Getman

Under microwave heating at 1200qC no volumetric shrinkage takes place in samples, 'V/V d 0.1%, whereas the local shrinkage reaches 27%. At the temperature of 1300°C local shrinkage achieves 35%, which is close to a practically complete densification in microvolumes. At the same time a volumetric shrinkage remains low. The content of solid solution reaches 100%, but the volumetric shrinkage of samples is less than 1.2%, in other words the porosity of the sintered samples remains practically on the initial level. Thus, the microwave annealing process is characterised by the local shrinkage. Under microwave heating of (Ba,Sr)-Titanate powders the accelerated densification at sintering occurs in microvolumes of samples. As a result, a coarsening porous structure is developed and the volumetric densification and the diffusion processes are inhibited. The preparation of non-agglomerated powder compacts and optimization of the conditions of microwave treatment are necessary to suppress the dominant local densification of samples. The samples annealed under microwave and conventional heating differ by the microstructure. The microstructure of the all conventionally sintered samples is more developed and includes the clear rectified grain boundaries contrary to the unformed grain boundaries in the samples sintered under MW heating. The characteristic microstructures are on the Fig. 4.

Fig. 4. Microstructures of (Ba,Sr)-Titanate samples after sintering at 1200°C 4 hours at conventional (a) and microwave (b) heating

The reason of significant difference between the results of forming of solid solution under microwave and conventional annealing of (Ba,Sr)-Titanate system remains unknown and requires the further investigations.

Densification and Diffusion Processes in the Ba,Sr-Titanate System

497

Resume The diffusion phenomena occurring at the sintering of an equimolar mix of fine barium and strontium titanate powders under microwave and conventional annealing have been studied. It is demonstrated that they depend largely on the processes of densification and porosity structure transformation in the samples. It has been found that an intense local densification occurs under MW heating, which manifests as an increase in the average pore size. The amount of the formed titanate solid solution in the samples sintered under microwave annealing correlates with the values of local densification and occurs to be equal or slightly lower than in the samples sintered under conventional annealing. The local densification and coarsening of the porous structure cause the retardation of the volumetric densification process due to a decrease in the capillary stresses and stabilisation of the density of the samples at a high level, close to its starting value.

References [1] Rothman S.J (1994) Critical assessment of microwave-enhanced diffusion. In: Iskander MF, Lauf RJ, Sutton WH (eds) Microwave processing of materials IV (Mater Res Soc Symp Proc v. 347, Pittsburgh, PA), pp 9-18 [2] Rybakov K.I, Semenov V.E (1994). Phys Rev, B 49, pp 64–68 [3] Basmajian J.A, DeVries RC (1957) J Am Ceram Soc, vol 40, pp 373-376 [4] Bykov Yu, Eremeev A, Flyagin V, Kaurov V, Kuftin A, Luchinin A, Malygin O, Plotnikov I, Zapevalov V (1995) The gyrotron system for ceramics sintering. In: Clark DE, Folz DF, Oda SJ, Silberglitt R (eds) Microwaves: theory and application in materials processing III (Ceram Trans, vol 59, The American Ceramic Society, Westerville, OH), pp 133-140. [5] Gopalan S., Virkar AV (1995) J Am Ceram Soc, vol 78, pp 993-998. [6] Solonin SM (1994) Powder Metallurgy, vol 3/4, pp 37-41 (in Russian) [7] Freer R (1980) J Materials Science, vol 15, pp 803-824 [8] Martin JW, Doherty RD (1976) Stability of microstructure in metallic systems, Cambridge Univ Press, England [9] Roeder RK, Slamovich EB (1999) J Am Ceram Soc, vol 82, pp 1665-1675 [10] Skorokhod VV, Solonin YM (1993) Powder metallurgy, vol 12, pp 25-30 (in Russian)

Observation of the Microwave Effect on the Diffusion Behavior in 28 Ghz Millimeter-Wave Sintered Alumina Toshiyuki UENO1, Yukio MAKINO1 and Shoji MIYAKE1, Saburo SANO2 1

Joining and Welding Research Institute, Osaka University, Ibaraki, Osaka 5670047, Japan 2 National Industrial Research Institute of Nagoya, Nagoya, Aichi 462-4457, Japan

Abstract Microwave effect on diffusion behavior was investigated by examining the relation between the enhanced densification of the 28 GHz millimeter-wave-sintered alumina and the change of optical absorption peak of doped Cr3+ ions. Among two optical absorption peaks due to Cr3+ ions, the peak I due to the 4A2o4E electronic transition clearly showed a different dependence on the relative density for both millimeter-wave- and conventionally sintered aluminas, respectively. For the millimeter-wave-sintered alumina, a large shift of the peak I was observed against a slight change of the relative density from 80%TD (theoretical density) to 90%TD. For the conventionally-sintered alumina, only a sight shift of the peak I was observed irrespective of a large change of the relative density from 68%TD to 90%TD. Assuming that the same diffusion stage of Cr3+ ions in alumina can be detected by observing the peak I at the same wavelength, the enhanced densification is estimated to be about 18%TD in millimeter-wave-sintered alumina.

Introduction Application of millimeter-wave heating to ceramic sintering is very promising since millimeter-wave energy is easily available by the spread of gyrotron system. Until now, various characteristics of microwave heating such as rapid and internal heating have been indicated [1], and, among these characteristics, the enhanced densification, which is very important for sintering ceramics, has been explained by the enhanced diffusion due to so-called “microwave effect”.

Observation of the Microwave Effect on the Diffusion

499

From tracer experiments in sapphire, reported in the literature [2, 3], enhanced densification of alumina in millimeter-wave sintering was assumed to arise from the decrease of the activation energy of oxygen diffusion. Similar rapid densification, observed in the microwave sintering of aluminazirconia composites [4] was found to be further enhanced by the usage of 28 GHz millimeter-wave in comparison with the usage of 2.45 GHz microwave. The enhanced diffusion due to the presence of microwaves has been theoretically explained by the “non-thermal effect” of the high frequency electromagnetic field [5]. However, no direct observation of enhanced diffusion has been reported except for the already mentioned tracer experiment of 18O-oxygen in alumina [3]. In the present study, the densification behavior due to the enhanced diffusion in millimeter-wave sintering was investigated by detecting the diffusion stage of chromium ions in alumina using the optical absorption method, which can detect the difference of the crystal field around chromium ions.

Experimental Procedures High purity D-alumina (AKP-20, Sumitomo Chemical Co., average size; 0.55 Pm) was used as a starting powder. Chromium sesquioxide (High Pure Chemical Co., average size 3 Pm) was used as the additive oxide. The amount of Cr2O3 added to D-alumina was fixed at 0.5 wt%. After mixing these powders by ball milling for 6 h, rectangular powder compact with the dimension of 20 mm x 20 mm x 8 mm were formed by slip casting method. Before sintering the powder compacts were calcined at 800°C for 1 hr in an electric furnace. Millimeter-wave sintering of the high purity alumina with 0.5 wt% Cr2O3 was performed by using a sintering equipment (Fuji Denpa Kogyo Co., FGS-10-28) consisting of an applicator and a 28 GHz gyrotron-source. Sintering of the high purity alumina was done at the temperature ranging from 1200°C to 1500°C at the interval of 100°C. Sintering time was varied from 10 min to 60 min. Heating and cooling rates were 20°C/min and 60°C/min, respectively. Nitrogen and air were used as the atmospheres for millimeter-wave and conventional sintering experiments, respectively. The density of sintered aluminas containing chromium oxide was estimated by measuring the weight and the dimensions of the sintered bodies. Optical absorption spectra of chromium ions dissolved into alumina were measured in the range from 340 nm to 700 nm by the reflectance method using integrating sphere accessory.

500

Ueno

Results and Discussion

Densification behaviour and Cr-diffusion The relative density of sintered aluminas with 0.5 wt% Cr2O3 is shown in Table 1. Comparison of the relative density of aluminas sintered at the same conditions with millimeter-wave heating and conventional heating reveals, that the former shows several %TD (theoretical density) higher values than the latter. Table 1. Relative densities (%TD) of aluminas with 0.5 wt% Cr2O3 sintered by the millimeter-wave heating and conventional heating methods

10 min 30 min 60 min

Millimeter-wave heating 1200°C 1300°C 1400°C 81.5 88.5 90.5 84.5 — — 86.5 — —

1200°C 67.9 — —

Conventional heating 1300°C 1400°C 1500°C 75.9 82.7 87.0 — 83.2 — — 88.0 —

Although trivalent chromium ions can dissolve in D-alumina, because Cr2O3 has the same corundum structure, peak positions of absorption spectra from trivalent chromium ions in alumina depend on the content of Cr2O3 in the alumina crystal, due to the change of the crystal field around Cr3+ ions in the alumina lattice [6, 7]. The optical absorption peaks can be clearly explained from the crystal field [8] and the content of Cr3+ ions in the local region. The diffusion behavior of chromium in alumina can therefore be analysed by estimation of the Cr2O3 content from the peak position, using a calibration curve. In the present study, Cr2O3 content in local regions of alumina during sintering is expected to change from 100% Cr2O3 in the form of particle before sintering to a very low content close to the added 0.5 wt% Cr2O3 after almost complete sintering. Accordingly, the compositional dependence of the optical absorption due to Cr3+ ions in equilibrated samples is required as the standard spectra, before examining the state of chromium ion in the sintered alumina. The standard optical absorption spectra were obtained from D-alumina with several different Cr2O3 contents sintered for 180 min at 1500°C by the conventional method. As shown in Fig. 1, the two peaks were observed in the visible region and the peak positions shifted to a higher wavelength with decreasing Cr2O3 content. The drastic change corresponded to the color change reported in the previous paper [6], where the color change was found to occurre in the range from 10 wt% Cr2O3 to 35 wt%. If the Cr2O3-content was less than 10 wt% in alumina, the positional shift of the optical absorption peaks was small but still pronounced enough to be detectable. The optical absorption spectra were examined in the aluminas with 0.5 wt% Cr2O3 sintered at 1200°C, 1300°C and 1400°C for 10 min by both millimeter-wave and conventional heating methods.

Absorbance(arb.units)

Observation of the Microwave Effect on the Diffusion

501

(1) (2) (3) (4) (5) 400

500

600

700

Wavelength, Fig. 1. Optical absorption spectra of standard aluminas with various Cr2O3contents, (1) 0.5 wt% Cr2O3, (2) 5 wt% Cr2O3, (3) 10 wt% Cr2O3 (4) 20 wt% Cr2O3, (5) pure Cr2O3

Interrelation between sintering temperature, density and optical absorption in Cr-doped alumina As shown in Fig. 2, one optical absorption peak (denoted as peak I) was observed near 560 nm and the other peak (denoted as peak II) was observed in the range from 410 nm to 370 nm. When the peak I in the millimeter-wave-sintered alumina was compared with that in conventionally-sintered alumina, the positional difference was more clearly observed with decreasing the sintering temperature. On the other hand, no systematic temperature dependence of peak II was found. The optical absorption spectra were first examined in the aluminas with nearly the same density, not necessarily the same sintering temperature. Particularly, alumina-0.5% Cr2O3 sintered at 1200°C with the millimeter-wave heating method is compared with alumina sintered at 1400°C using the conventional heating method. As shown in Fig. 3, the position of the peak I in the conventionally-sintered alumina scarcely depended on the sintering time, whereas the position in the millimeter-wave-sintered aluminas shifted to the longer wavelength with decreasing the sintering time. On the other hand, no systematic dependence was found in the behavior of peak II in both aluminas.

Ueno

1400oC

Absorbance (arb. units)

502

1300oC

1200oC Pure Cr2O3 400

500

700

600

Wavelength, O[nm]

Absorbance (arb.units)

Fig. 2. Optical absorption spectra of Cr3+ ions in 0.5% Cr2O3-doped alumina sintered for 10 min by the millimeter-wave heating (full lines) and conventional heating (broken lines).

60 min. U (MW)=3.45 U (conv)=3.51 30 min. U (MW)=3.37 U (conv)=3.32 10 min. 400

500

600

U (MW)=3.25 U (conv)=3.30

700

Wavelength, O[nm]

Fig. 3. Optical absorption spectra of Cr3+ ions in aluminas sintered at 1200°C by the millimeter-wave heating(full lines) and at 1400°C by conventional heating (broken lines). [U is the density of the respective alumina.]

Examining the relation between the relative density of alumina and the position of peak I, two different curves were obtained for millimeter-wave-sintered and conventionally-sintered aluminas, respectively. The density of the millimeter-

Observation of the Microwave Effect on the Diffusion

503

wave-sintered alumina increased gradually along with a decreasing wavelength of the peak I. On the other hand, the density of the conventionally-sintered alumina increased drastically along with a decrease of the wavelength of peak I. The peak shift (wavelength) is recalculated to Cr2O3 content using the compositional dependence of the peak-position shown in Figure 1. The results of this calculation are shown in Fig. 4. There is a clear difference in local concentration of Cr2O3 at a certain density level of the alumina, when millimeter-wave sintered Cr-doped alumina is compared with conventionally sintered maetrail. In the millimeter-wave-sintered alumina regions with higher concentration than 24 wt% Cr2O3 exist even after the density of alumina exceeded 85%TD. In the conventionally sintered Cr-doped alumina the local concentration of Cr2O3 is much lower not only in a comparably dense material but even at a density of approximately 70%TD. In the conventionally-sintered alumina a Cr-concentration as low as 14 wt% Cr2O3 is found, even before the density of the alumina reaches 70%TD. The evolution of chromium diffusion can be also examined vice versa. Assuming that the wavelentgh of peak I is reflecting the “stage“ of Cr-diffusion into the alumina lattice, densities of aluminas, in which peak I is observed at the same wavelength are plotted along the shift of this peak. 100

Millimeter-wave sintering Conventional sintering

Relative density [%TD] Relative density,[%TD]

90

80

' TD=18.6%

70

60

0

10

20

30

40

Local concentration of Cr 2O 3 , [wt%] Fig. 4. Dependence of relative density on the local concentration of Cr2O3 in aluminas estimated by the shift of the peak I

For example, the appearance of peak I around 565 nm corresponds to the diffusion stage in which the “dilution” of Cr2O3 is quite advanced and regions with the average concentration of about 14 wt% Cr2O3 predominate in the alumina.

504

Ueno

Examining millimeter-wave-sintered alumina with an optical absorption peak I at 565 nm reveals that the densification of such an alumina is about 18%TD higher in relative density than that of conventionally-sintered alumina, as shown in Fig. 4. Thus, an enhanced densification of alumina was detected by the positional change of peak I reflecting the stage of dissolution of Cr3+ ions in the alumina lattice. Electronic transitions behind the optical spectra Unlike the peak I, the position of the peak II was not changed systematically with both sintering temperature and time. The reason is probably attributed to the existence of a small trigonal field in D-alumina. Details are shown in Fig. 5.

Fig. 5. Energy splitting of the electronic levels of trivalent chromium ion in Oh symmetry by the change to lower symmetries. (The possible transitions are shown by the solid arrows. The arrow drawn by the broken line shows the forbidden transition.

The trigonal field splits the 4T2g level in the cubic field into 4A1 and 4E levels, and 4T1g level in the same cubic field into 4A2 and 4E levels, respectively. When the small trigonal field has C3v symmetry, the electronic transition between 4A2 level of the ground state, 4A2 (G), and the 4A1 level is forbidden by selection rules, as shown in Figure 5 [9]. Accordingly, peak I arises from the only electronic transition from 4A2 (G) to 4 E, whereas peak II arises from two electronic transitions from 4A2 (G) to 4E and from 4A2 (G) to 4A2. Thus, the absence of a systematic dependence of the peak II on temperature and time of the sintering process, which is accompanied by the dissolution process of Cr-ions in the alumina lattice is attributed to the overlap of the two peaks, which arises from two electronic transitions due to the existence of C3v distortion in D-alumina.

Observation of the Microwave Effect on the Diffusion

505

Other factors influencing Cr-diffusion Besides the difference in heating: millimeter-wave or conventional heating, the sintering atmosphere was different. Conventional sintering was performed in air, millimeter-wave sintering in N2-atmosphere. The influence of the different oxygen content in the sintering atmosphere on the dissolution process of chromia in alumina has not been analyzed yet.

Summary Enhancement of diffusion in millimeter-wave sintering was investigated by examining the relation between the densification behavior of alumina and the positional change of the optical absorption of Cr3+ ions doped in alumina. Two optical absorption peaks due to Cr3+ ions were observed near 565 nm and between 370 nm and 410 nm, in both millimeter-wave- and conventionally-sintered aluminas, respectively. The former peak (peak I) showed a clear dependence of the peak wavelength on the relative density of the doped alumina. The density of the millimeter-wave-sintered alumina increased gradually along with the wavelengthdecrease of peak I. On the other hand, the density of the conventionally-sintered alumina increased drastically with the wavelength-decrease of peak I. Comparing with the densification behavior of conventionally-sintered alumina, the enhanced densification in the millimeter-wave-sintered alumina was estimated to be about 18%TD assuming the same diffusion stage of Cr3+ ions in alumina is present at a certain wavelength of peak I.

References [1] W.H.Sutton; Ceramic Bulletin, 68(1989)376-386. [2] M.A.Janney and H.D.Kemrey; Mat. Res. Soc. Symp. Proc., Vo;.189(1991)215-227. [3] M.A.Janney, H.D.Kemrey, W.R.Allen and J.O.Kiggans; J.Mater. Sci., 32(1997) 13471355. [4] H.D.Kemrey, J.O.Kiggans, M.A.Janney and R.L.Beatty; Mat. Res. Soc. Symp. Proc., Vo;.189(1991)243-255. [5] K.I.Rybakov and V.E.Semenov; Phys. Rev., B52(1995)3030-3033. [6] C.P.Poole, Jr.; J. Phys. Chem. Solids, 25(1964)1169-1182. [7] C.P.Poole, Jr. and J.Itzel, Jr.; J. Chem. Phys., 39(1963)3445-4355. [8] For example, “Some Aspects of Crystal field Theory”, by T.M.Dunn, D.S.McClure and R.G.Pearson, Harper & Row, New York, 1965, Chap.1 and Chap.2. [9] A.S.Marfunin; “Physics of Minerals and Inorganic Materials”, Springer, Berlin, 1979, Chap.2, p.70 and Chap.6, p.198.

Dilatometer Measurements in a mm-Wave Oven G. Link, S. Rhee1, M. Thumm2 Forschungszentrum Karlsruhe GmbH, Institut für Hochleistungsimpuls- und Mikrowellentechnik, POBox 3640, 76021 Karlsruhe, Germany; 1 now with NIH Transducer Resource Center, Pennsylvania State University, USA 2 also University of Karlsruhe, IHE, Kaiserstraße 12, 76128 Karlsruhe, Germany

Abstract For detailed investigations on the sintering behavior of ceramics in the mm-wave field a dilatometer system was designed and integrated into an un-tuned, highly overmoded mm-wave applicator. A commercially available dilatometer with an inductive displacement transducer and an alumina sensor rod was modified in an appropriate way to avoid microwave leakage out of the applicator. The dilatometer system allows in-situ measurements of the sample's extension and shrinkage during heating cycles up to a temperature of 1600°C. The densification behavior of various nanoscaled ceramics has been studied and compared with results collected by a conventional resistant heated dilatometer.

Introduction Dilatometry is a standard method of characterization in material science and development, e.g., to measure thermal expansion coefficients, shrinkage during sintering processes or phase transformations. The principle of the dilatometer is based on measurements of the sample length as a function of temperature. The temperature change is achieved by heating or cooling through a programmed cycle. One of the most crucial steps in the processing of modern high-performance ceramics is the heating process for sintering. Due to the low penetration depth of IRradiation in a standard resistance heated or gas fired sintering furnace, temperature gradients are induced in the ceramic parts, with a hot surface and a colder interior in the green bodies. At low to medium temperatures the temperature can be equilibrated by a time consuming thermal conduction process only. Such temperature gradients inherent in conventional heating techniques lead to thermal stresses, inhomogeneous shrinkage or even to destruction of the sample, unless an optimized time-temperature program with low heating rates is applied.

Dilatometer Measurements in a mm-Wave Oven

507

The use of microwaves allows direct transfer of energy into the materials, where it is converted to heat. Therefore high heating rates and a significant reduction of the processing time are possible. Furthermore, the densification process of ceramic bodies seems to be enhanced by sintering in a microwave field, as demonstrated by several authors [1, 2] allowing a reduction of the sintering temperatures and dwelling times as compared to processing in a standard sintering furnace. Because of obvious economical benefits such as energy conservation, reduced cycle time, reduced operating space and improvement of the environment the use of microwave technology is very attractive. In addition microwave technology gives the unique possibility to influence microstructure (e.g. grain size) and physical properties of the ceramic materials.

Experimental

Fig. 1. Photograph of the dilatometer system (left) and schematic representation of the mmwave applicator (right).

At the Forschungszentrum Karlsruhe, Germany, a compact gyrotron system has been established in order to investigate technological applications in the field of high temperature materials processing by means of millimeter wave (mm-wave) radiation [3]. The mm-wave output power of up to 15 kW is coupled into an untuned applicator of about 100 l volume via an advanced quasi-optical transmission line. The heating process is fully computer controlled in that way, that the temperature measured with a thermocouple at the sample surface is following any preset time-temperature program. A calibration of the thermocouples with the melting point of gold revealed an error of less than 1%.

508

Link

For more detailed investigation of sintering various functional and structural ceramics a specific dilatometer setup as shown in Figure 1 has been designed and integrated into the mm-wave applicator of the compact gyrotron system. For this purpose a commercially available dilatometer of the type L75 (Linseis Company, Germany) has been used and modified in order to prevent leakage of mm-wave radiation out of the applicator. The alumina ceramic rods used in the dilatometer sensor head were exchanged by metallic components in the cold part of the applicator system where it is fed through the applicator wall. Calibration Procedure The dilatometer signal is not only a function of the sample temperature but also a function of the temperature distribution in the dilatometer sensor head. Therefore, for precise measurements a calibration experiment has to be performed. This calibration experiment has to show the intrinsic temperature behavior of the dilatometer system for a process identical to the process which will be applied to the material under test. The data from such a preprocess have to be taken into account for precise evaluation of the experiment with the ceramic sample. With conventional heating the development of the temperature distribution in the dilatometer system is identical for any material under test. Therefore in conventionally heated dilatometers such a calibration experiment can be performed with an unloaded system or with a calibration standard with known thermal expansion behavior. In a microwave heated dilatometer calibration of the system is more difficult. Since microwave heating is always selective, the development of temperature distribution strongly depends on the dielectric properties of the material under test as well as the surrounding materials. This means that the temperature distribution of an empty dilatometer system is different as compared to the temperature distribution with the material under test. Furthermore, temperature evolution at different electromagnetic field strength will result in different heating rates. Under such circumstances a reasonable calibration is difficult if not impossible at all. The only solution for relevant calibration is to use a ceramic material identical to the material under test. Calibration experiments have been performed with pure alumina with a low dielectric loss factor and also with the more lossy ZrO2. Both materials are fully sintered so that no densification can be expected. A basic difference in the resulting dilatometer data with these materials can be seen in Figure 2. Whereas the lossy ZrO2 sample shows a thermal expansion while power is switched on, the low loss alumina seems to shrink (see bottom part of Figure 2). This behavior is related to different temperature distributions (see middle part of Figure 2). For the low loss alumina the temperature Tin measured at the sample surface is lower than the temperature Tout at the outer surface of the surrounding alumina tube during the heating cycle.

Dilatometer Measurements in a mm-Wave Oven 1,2

1,2

1,0

1,0

0,6

0,8

0,4

0,6

dl/lo in %

dl/l0 in %

0,8

0,2 0,0 -0,2 -0,6 0

500

0,4 0,2

5°C/min 10°C/min 20°C/min

-0,4 -0,8

5 °C/min 10 °C/min 20 °C/min

0,0

1000

-0,2

1500

0

300

600

900

1200

Temperature in °C

Temperature in °C

140

20 5 °C/min 10 °C/min 20 °C/min

5 °C/min 10 °C/min 20 °C/min

120 100 Tin-Tout in °C

0 Tin - Tout in °C

509

-20 -40

80 60 40 20 0

-60 300

600

900

1200

1500

-20

8

300

600

900

1200

8

Power dl/lo

6 4

Power in kW - dl/lo in %

Power in kW - dl/lo in %

0

Temperature in °C

Temperature in °C

6

Power dl/lo

4 2

2

0

0 Time in a.u.

Time in a.u.

Fig. 2. Characteristics of dilatometer for Al2O3 (left) and ZrO2 (right) samples. Calibration with dense material (top); temperature gradients during calibration experiment (middle); dilatometer signal under mm-wave power pulses (bottom).

This can be explained by a more pronounced thermal expansion of the outer tube compared to the sample resulting in a virtual shrinkage. In the case of the lossier ZrO2 the sample is hotter and therefore shows stronger thermal expansion than the surrounding sample holding alumina tube. The virtual thermal expansion

510

Link

is enhanced with higher heating rates due to the higher power input as can be seen in Figure 2 (top).

Results

Nanoscaled zirconia One of the materials under test is nanoscaled 3 mol% Y2O3 doped zirconia powder produced by flame pyrolysis at Inocermic GmbH, Hermsdorf, Germany. The nanopowder has a specific surface area of 25.8 m2/g corresponding to an average particle size of about 37 nm. The motivation for sintering this material by mmwaves is densification with minimal grain growth resulting in a nanoscaled microstructure. Here the mm-wave sintering inherent advantages such as volumetric heating and enhanced densification which allow to apply a fast sintering process are exploited. 5 °C/min MWS 10 °C/min MWS 20 °C/min MWS 5° C/min KS

Linear Shrinkage in %

0

-5

-10

-15

-20 700

800

900

1000

1100

1200

1300

1400

Temperature in °C

Fig. 3. Linear shrinkage for zirconia samples at three different heating rates upon mmmicrowave sintering and, for comparison, upon conventional sintering

A comparison of dilatometer experiments with mm-wave heating (MWS) and conventional heating (KS) demonstrates, that densification occurs at temperatures about 150°C lower with mm-wave heating (see Figure 3). Increasing the heating rate from 5°C/min to 20°C/min leads to an additional temperature decrease of about 50°C.

Dilatometer Measurements in a mm-Wave Oven

511

A possible explanation for this effect might be the already explained temperature gradient between the outer tube and the sample. At temperatures above 800°C, the onset of shrinkage, the temperatures Tin measured at the sample surface are up to 100°C higher than the temperature Tout measured at the outer surface of the surrounding alumina tube. If there are similar temperature gradients present in the small samples (10 x 3 x 3 mm3), the temperature shift of the onset can be due to a non-representative temperature measurement. But the significant temperature shift of about 150°C at a heating rate of 5°C/min can hardly be explained by temperature gradients and resulting measurement errors. This may indicate a basic difference in sintering kinetics with mm-wave sintering.

Fig. 4. Microstructures of MWS (top) and KS (bottom) zircomia samples.

Investigations of microstructure evolution confirm this indication [4]. Whereas microstructures of samples conventionally sintered at 1300°C show grain sizes in the range of 150 - 200 nm, mm-wave samples sintered to similar density at 1200°C reveal average grain sizes less than 100 nm.

512

Link

Alumina, Al2O3 Further dilatometer experiments have been performed with two different grades of alumina powers, a pure submicron D-alumina with d50 = 150 nm and nanoscaled Jalumina with an average particel size of about 30 nm, doped with Mg and Ti, respectively. The D-modification is thermally and chemically stable whereas in the J-alumina a temperature dependent phase transformation occurs. This results in a density change of about 10%. It can be observed as the first step in the dilatometer signal superimposing the shrinkage (see right part of Fig. 5). With Mg doped alumina this effect is more pronounced than with Ti-doping. The comparison of the densification behavior under mm-wave irradiation and conventional sintering shows a shift of densification to lower temperatures of about 50°C as compared to KS for the nanoscaled alumina, whereas the submicron D-alumina reveals no remarkable difference. Compared to the shrinkage of ZrO2 samples, which showed a remarkable shift of the shrinkage onset to lower temperatures, the pure alumina samples reveal an opposite behavior (see left part of Fig. 5). The onset of densification with a heating rate of 20°C/min. occurs about 30°C higher than with 5°C/min. Similar results are known from conventional dilatometer experiments. It can again be explained by the temperature gradients in the dilatometer system in Figure 4 (right). As the temperature Tout at the dilatometer wall is higher compared to the sample temperature Tin the temperature difference increases with increasing heating rates. This is similar to the behavior under conventional conditions where heat penetrates from the sample surface into the interior. The dielectric absorption in the dilatometer tubes is higher than in the sample. 2

0,02

0

0,00 -0,02

-4

-0,04

-6 -8 -10 -12

-0,06

dl/lo

linear shrinkage in %

-2

5 °C/min MWS 10 °C/min MWS 20 °C/min MWS 5 °C/min KS

-0,08 -0,10

-14

-0,12

-16

-0,14

-18 1000

1100

1200

Temperatur in °C

1300

1400

KS J-Al2O3/Mg KS J-Al2O3/Ti MWS J-Al2O3/Mg MWS J-Al2O3/Ti

-0,16 900

1000

1100

1200

1300

1400

Temperature in °C

Fig. 5. Dilatometer experiments with high purity D-Al2O3 (left) and nanoscaled and doped J-Al2O3 (right) in comparison with conventional heating.

This results in a virtual shift of shrinkage onset to higher temperatures with higher heating rates, assuming that temperature gradients in the small sample under test are in the same direction.

Dilatometer Measurements in a mm-Wave Oven

513

Conclusion It has been demonstrated that a dilatometer system can be a useful tool for detailed investigations of densification and phase transformations under mm-wave irradiation. The temperature development of the mm-wave dilatometer loaded with materials with different dielectric properties has been investigated carefully. Although calibration measurements are nearly impossible with pure mm-wave heating, the accuracy of the system is sufficient to demonstrate differences in densification under mm-wave radiation compared to conventional heating. Further improvement of the system accuracy will be achieved by application of a hybrid heating system where temperature gradients and the reported problems can be controlled.

References [1] M.A. Janney, H.D. Kimrey; Diffusion Controlled Processes in Microwave-Fired Oxide Ceramics; MRS Symp. Proc., Vol. 189, Microwave Processing of Materials II, ed. by W.B. Snyder, W.H. Sutton, Pittsburgh, PA, (1991) 215-227. [2] R. Roy, D. Agrawal, J.P. Cheng, M. Mathis; Microwave Processing: Triumph of Applications-Driven Science in WC-Composite and Ferroic Titanates; Microwaves: Theory and Application in Materials Processing IV, ed. D.E. Clark, W.H. Sutton, D.A. Lewis, Ceramic Transactions Vol. 80, (1997) 3-26. [3] Yu. Bykov, A. Eremeev, V. Flyagin, V.Kaurov, A. Kuftin, A. Luchinin, O. Malygin, I. Plotnikov, V. Zapevalov, L. Feher, M. Kuntze, G. Link, M. Thumm, Gyrotron Installation for millimeter-wave processing of materials, Vakuumelektronik Displays form VDE-Verlag GmbH, Berlin Offenbach, Vol. 132, (1995) 103-108. [4] S. Rhee; G. Link; L. Feher; M. Thumm; High power millimeter waves for sintering of nanostructured ceramics. ed. Liu, S.; Digest of 25th Internat. Conf. on Infrared and Millimeter Waves, Beijing, China, September 12-15, 2000, IEEE Press, (2000) 42526.

In Situ Determination of Shrinkage Under Microwave Conditions J. Bossert1, C. Ludwig1, J.R. Opfermann2 1 2

Friedrich-Schiller-Universität Jena, Technisches Institut, Jena, Germany NETZSCH-Gerätebau Selb, Germany

Abstract Using a dilatometer it is possible to correlate sintering kinetics with the high heating rates applied under microwave conditions. A rapid temperature increase indicates effective microwave absorption. In situ measurements of shrinkage are suitable to distinguish, whether pure microwave sintering takes place or if sintering is mainly caused by radiation-heating from preheating media. Standard measuring equipment was used, therefore all information normally available from dilatometer investigations can be obtained.

Introduction The sintering process is generally linked to a reduction of specimen volume, resulting from a reduction of porosity, driven by surface energy reduction. Grain growth, caused by the driving force of grain boundary length reduction, often accompanies the sintering process. Whereas the reduction of porosity is desired, grain growth is unwanted in most ceramic materials, because of reduced mechanical properties of coarse grained ceramics. Furthermore, fast and inhomogeneous shrinkage may cause macroscopic deformation and crack formation due to inhomogeneous mechanical stresses. Sinter kinetics and grain-growth are correlated with temperature, therefore heating and sintering should be stopped when no further shrinkage takes place. The determination of shrinkage behaviour depending on temperature is therefore one of the most important investigations to control the sinter-process. Shrinkage rate as well as densification rate are experimental data used for investigating sinter mechanisms [1, 2]. The use of dilatometer investigation is well established for this purpose. However, for dilatometer-measurements when comparing microwave heating and sintering with conventional heating some major differences have to be

In Situ Determination of Shrinkage Under Microwave Conditions

515

taken into account, particularly with respect to certain measurement techniques as well as to materials. Thermocouples, generally used in conventional dilatometers for temperature measurements as well as ceramic sample holders are a source of problems, when microwave radiation with a frequency of 2.45 GHz is applied for heating. Uncontrolled absorption of microwave energy and change of field distribution in the applicator may occur or even generation of a plasma, which can destroy the sample holder. Therefore, the material of the sample holder must be able to withstand a high thermal gradient developing between the sample and the thermal insulation or an cooling system as well as be insensitive to thermal shock caused by an accidental microwave plasma. Most technical problems of a dilatometer suitable for use in a microwave sintering apparatus are therefore linked to the sample holder and the integration into the microwave applicator.

Experimental A standard dilatometer system (NETZSCH, Selb, Germany) was used as measurement unit. The conventional tube-like sample holder was replaced by thin alumina rods. The advantage of using thin rods is the better thermal shock resistance because of the small dimension and the much lower mass for unwanted interaction with the microwave filed. Furthermore, inside a tube a microwave plasma can ignite, which would destroy the sample holder. The dilatometer was mounted on a slide in order to adjust the sample in the applicator. A multi-mode cavity fed with microwave radiation of 2.45 GHz frequency was used as microwave applicator. The experiments were carried out at a constant power level of 800 W. In this cavity an alumina-fiber casket was used for thermal insulation of the material to be sintered, and SiC-plates were used for preheating. A pyrometer (KTR 1085 Maurer Optoelektronik, Germany) was used for temperature measurement. In order to avoid large temeparture gradient between the sintered sample and the cavity interior, a high frequency heating system (Linn Highterm) with a carbon susceptor can be installed instead of the alumina-fiber casket system. The system is described in more detail elsewhere [3]. The powder samples were pre-compacted in an uniaxial press at 15 MPa and additionally compacted in an cold isostatic press at 600 MPa (KIP 300, WeberPressen, Germany). The samples had a cylindrical shape (15mm in length and 1213 mm in diameter). The ratio of surface to volume was about 0.46, which was found to be favourable for the microwave sintering experiments [4]. A hole, 5 mm in diameter, was drilled in the centre of the green body in order to measure the accurate temperature. The principle of the measuring system can be seen in Figure 1. In order to vary the sintering activity of the ceramic compacts, titania powder prepared by different hydrolysis technique and further modified by different calcination temperatures, from 600°C to 950°C, was employed [3, 5]. The powders were

516

Bossert

characterised by specific surface area measurements (BET, Gemini Micromeritics, Germany).

pyrometer dilatometer

push rod

drill hole

sample

microwave directional beam

sample holder rods Fig. 1. Scheme of the dilatometer with microwave heating

Results

Shrinkage upon microwave heating In the system used the TiO2-ceramic green body and the SiC-plates can absorb microwave energy and convert it into heat. By keeping the microwave power at a constant level, microwave absorption will preferentially occur in the material with the higher dielectric loss. At low temperatures the SiC plates will be heated by microwave absorption. This can be seen from the temperature measurements. As long as the SiC-plates absorb more microwave energy than the TiO2-ceramic body, the temperature at the surface of the TiO2-sample is higher than inside the body, in the hole drilled into the sample. This can be observed visually as well as by means of the pyrometer. As soon as the dielectric loss of the TiO2-sample has increased to a certain level due to the temperature increase, the titania sample will absorb microwave energy and become hotter inside the ceramic body than at the surface. The difference between surface temperature and the temperature inside the body, measured in the hole, is significant and reaches up to 120 K, as shown in Figure 2a. A progressive temperature increase occurs, typical for TiO2 but not found in this temperature range for dense Al2O3 used as standard, as shown in Figure 2b. At this temperature dense alumina is a much poorer absorbing material than SiC or TiO2 and therefore is not directly heated with microwaves of 2.45 GHz frequency. Instead, it is slowly heated by the SiC plates.

In Situ Determination of Shrinkage Under Microwave Conditions

517

The rapid temperature rise of the TiO2-ceramic indicates thermal runaway. The higher efficiency of microwave absorption at higher temperatures can be seen from the reflected power, which decreases sharply, as shown in Figure 3 (a). Because at higher temperature heat loss from the surface is increased by T4, the sample finally reaches a stationary surface temperature. (a) (b) 1400

1500

250

1400

temperature [°C]

1200 1100

150 temperature outside

1000 100 900 50

800 700

0

600

1300 1200

temperature [°C]

200

temperature inside

temperature difference [K]

1300

1000 TiO2

900

Al2O3

800 700 600

temperature difference

500

500

-50 0

1100

0

5 10 15 20 25 30 35 40 45 50

5

10 15 20 25 30 35 40 45 50

time [min]

time [min]

Fig. 2. (a) Temperature inside and at the surface of the TiO2-ceramic and temperaturedifference; (b) Heating behaviour of TiO2-ceramic as compared to dense alumina (standard)

1500

2

46

1400

0

1300

-2

1200

-4

1100

-6

1000

-8

900

-10

800

-12

700

-14 -16

temperature [°C]

1300

44

1200

42

1100

40

1000

38

900

36

800

34

700

temperature [°C]

48

1400

reflected MW-power [mV]

1500

600

32

600

500

30

500

0

5

10 15 20 25 30 35 40 45 50

time [min]

linear shrinkage [%]

In general, the onset of shrinkage could be observed at the same time as the microwave absorption occurred in titania, as shown in Figure 3 (b). It can be seen that 90% of shrinkage takes place within 5 minutes. (a) (b)

-18 0

5

10 15 20

25 30 35 40 45 50

time [min]

Fig. 3. (a) Decrease of reflected power upon sharp temperature increase (thermal runaway) inside the TiO2-ceramic; (b) Onset of shrinkage upon sharp temperature increase due to volumetric microwave heating.

518

Bossert

Influence of particle size on shrinkage Different types of titania powder were prepared in order to investigate the influence of particle size on microwave sintering and shrinkage. Table 1 shows the specific surface area if TiO2 calcined at different temperatures. The specific surface could be varied from about 50 m2/g to 1.3 m2/g. The microwave absorption behaviour and consequently, the slope of the heating curve and shrinkage depend strongly upon the specific surface area, as shown in Figure 4. Table 1. Specific surface area of the TiO2-powders depending upon calcination temperature calcination temperature [°C]

spec. surface [m2/g]

600 650 700 750 800 850 900 950

49,98 37,66 28,17 15,42 8,05 4,09 3,27 1,28

1500 1400

temperature [°C]

600°C

650°C

1300

700°C 750°C

1200

800°C

1100 1000

reference

900

850°C

800

900°C

700

950°C

600 500 0

2,5

5

7,5

10

12,5

15

17,5

20

22,5

25

time [min] Fig. 4. Influence of the specific surface area on the onset-temperature for efficient microwave heating

The results shown in Figure 4 suggest, that powders with a high specific surface area absorb microwave energy at significantly lower temperature than those with a low specific surface area. A possible explanation for such behaviour could be the significantly increased fraction of surface atoms in materials with large specific surface area as compared to conventional powders. Assuming, that surface

In Situ Determination of Shrinkage Under Microwave Conditions

519

atoms show a different dielectric behavior than volume atoms, a decrease of absorption temperature and increase of microwave heating could occur. Furthermore a phase transformation takes place in the temperature range from 600°C to 800°C: anatas changes into rutile. Therefore the shrinkage slope at 600°C represents the microwave sintering behaviour of pure anatase and the slope at 900°C that of pure rutile, see Figure 5. The differences might be caused not only by the different amount of surface atoms, but also by the different dielectric properties of anatas and rutile as well as by the exothermic transformation process itself. 2 800°C

0 850°C

linear shrinkage [%]

-2 900°C

-4

950°C

-6 -8 -10 650°C

-12 -14

700°C

-16

750°C

-18 0

10

20

30

40

50

time [min]

Fig. 5. Shrinkage of titania calcined at different temperatures

Influence of specific surface area on sintered density It is well known, that reduction of surface energy is the driving force of the sinter process. Therefore, accelerated shrinkage, as shown in Figure 4 for finer powders is probable. The important property is, however, the final density. The finer powders which were calcined at 700°C reach a final density of 97% of theoretical density, whereas the powder calcined at 950°C only reaches a density of 89%. An interesting effect can be seen when the derivative of shrinkage and temperature slope is plotted in one diagram. As shown in Figure 6 and Figure 7, differences of the powders with a specific surface of 37 m2/g and 15 m2/g are seen not in the heating but rather in the densification rate. Whereas the maximum heating rate for both powders is of approximately the same value, the maximum densification rate is reached at lower temperature for the powder with the higher specific surface area. As a consequence, the powder calcined at 650°C sinters mainly while being heated by the neighbouring SiC plates, whereas the powder calcined at 750°C is really microwave sintered.

Bossert

(b): calzined at 750°C 2

450

0

400

0

400

350

-2

300

-4

250

-6

200

-8

150 100

-10

50

-12

0

-14

-50 0

2,5

5

7,5 10 12,5 15 17,5 20 22,5 25

time [min]

densification rate [%/min]

450

heating rate [K/min]

densification rate [%/min]

(a): calzined at 650°C 2

350

-2

300

-4

250

-6

200

-8

150 100

-10

50

-12

heating rate [K/min]

520

0

-14

-50 0

2,5

5

7,5

10 12,5 15 17,5 20 22,5 25

time [min]

Fig. 6. Heating rate and densification of titania with (a) 37 m²/g specific surface area; (b) 15 m²/g specific surface area

Conclusion As with conventional sintering, dilatometric shrinkage measurements are a useful tool to control and optimise the sinter process. In microwave applications, especially when hybrid heating is applied, in situ measurement of shrinkage allows a distinction between sintering by microwave heating, accompanied by “microwave-effects“ and conditions of conventional heating. Temperature measurement is a very crucial part of microwave sintering, therefore it is recommended that the temperature is measured inside the sample via a pyrometer.

References [1] Schatt, W.: Sintervorgänge VDI-Verlag 1992, p 83-105 [2] Palmour III, H. and Hare, T.M. : In Sintering `85, Ed. G. C. Kuczynski, D. P. Uskokovic, H. Palmour III and M. M. Kistic. New York and London: Plenum Press 1987, 17 [3] Ludwig, C.: PhD-Thesis, University of Saarbrücken 1998, p12-16. [4] Bossert, J., Ludwig, C.: Microwave sintering of titania, Key Engineering Materials Vols. 132-136 (1977) pp. 1022-1026 [5] Khalil, T., Untersuchungen im ternären System Al2O3-SiO2-TiO2 zur Herstellung biokeramischer Phasen, Scientific Series of the International Bureau, Forschungszentrum Jülich, Vol. 26, P 90-102 [6] Bossert, J., Ludwig, C.: Einfluß der spezifischen Oberfläche auf das Erwärmungs- und Sinterverhalten von TiO2 im Mikrowellenfeld, Werkstoffwoche 98, Band 7, Symposium 9 Keramik 1999, p 77-82.

Multistable Behaviour in Microwave Heating of Ceramics J. R. Thomas1, Jr., Xiaofeng Wu1, W. A. Davis2 1

Mechanical Engineering Department Bradley Department of Electrical Engineering Virginia Polytechnic Institute & State University, Blacksburg, VA 24061-0238, USA

2

Introduction In 1991, G. A. Kriegsmann [2] published a paper which predicted, on the basis of an asymptotic analysis, the existence of an S-shaped response curve relating steady-state temperature to microwave power when a material, whose electrical conductivity depends exponentially on temperature, is heated in a microwave field (Fig. 1). This result has been obtained in numerous theoretical studies since that time [1, 3 - 9, 11], but apparently has never been verified experimentally. Stuerga, et al. [10] provided indirect experimental evidence in the form of a change of slope in a plot of temperature vs. time during microwave heating of frozen water (ice). This result is not a complete verification of the theoretical prediction, however, since it did not provide information about steady-state conditions. We have attempted to produce direct experimental evidence and have found two branches of the curve. The highest temperature branch would apparently be above the melting point of the materials heated.

Experiments Our apparatus consists of a 3 kW power supply and magnetron at 2.45 GHz, reflector and dummy load, 2 m of WR340 waveguide with a 4-stub tuner, and a TE10n cavity with an adjustable end-wall short. The cavity is connected to the waveguide through a flange with iris; a moveable wedge-shaped Teflon plug in the waveguide helps smooth the transition to the cavity through the iris. A schematic of the apparatus is shown in Fig. 2, and a schematic representation of the cavity is shown in Fig. 3.

522

Thomas

The materials heated in the experiments consist of cylindrical mullite rods 4.5 mm in diameter. They were inserted across the cavity at one of the field maxima in the TE103 mode as shown in Fig. 3. The cavity includes two parts: the waveguide and the adjustable short. The position of the short plane ranges from 5.7 cm to 15.28 cm if measured from the junction. 3500

3000

(3)

Temperature, K

2500

2000

1500

(2) 1000

(1)

500

0

5

10

15

20

25

30

35

Field (kV/m)

Fig. 1. Typical S-shaped curve of steady-state temperature at a given field value for microwave heating of a material whose dielectric loss increases with temperature. Pyrometer

Power Meters

Water Load

iris Magnetron

Circulator

Waveguide Extension

Stub Tuner

4-port Coupler

Waveguide with wedge

Cavity

Adjustable Short

Fig. 2. Schematic of experimental setup. A d ju sta b le S h o r t

S h ort P osition

Fig. 3. Schematic of TE103 mode cavity.

R od

P robe

Ir is

Multistable Behaviour in Microwave Heating of Ceramics

523

Experimental Results The experimental results for mullite are shown in Figs. 3 and 4. The output of the power generator is set to 600 W, which is not changed during the experiment. The only adjustable component in the system is the position of the shorting plane. The experiment starts from the point at which the cavity is highly detuned. As Figs. 3 and 4 show, at the beginning of the experiment the short plane position is at 15.2 cm and the temperature of the rod is the room temperature. When we adjust the short to decrease the length of the cavity, the temperature of the rod increases, as well as the electric field intensity at the plane of the rod. Critical point a is the point at which the maximum electrical field intensity is reached. From this point, the electric field intensity always decreases no matter how we change the short plane position. If we change the short plane position to increase the length of the cavity, then temperature and electric field intensity decrease simultaneously to a new equilibrium point on the same curve. If we very gently adjust the short to decrease the length of the cavity further, the temperature of the rod will increase but the electric field intensity will also decrease, leading to a new equilibrium point at higher temperature. If we continue decreasing the length of the cavity, the field in the cavity will collapse at critical point b. Both the temperature of the rod and the field intensity will drop dramatically, leading to equilibrium at a much lower temperature.

Fig. 4. Behavior of the rod temperature when the position of the shorting plane is adjusted.

This process is like a reverse thermal runaway process. To heat the rod from this lower temperature point, we must increase the cavity length. The temperature and the field intensity will both increase along another curve that does not coin-

524

Thomas

cide with the original one. At critical point c, another electric field intensity maximum is encountered.

Fig. 5. Behavior of the electric field strength for the same experiment shown in Fig. 3.

Fig. 6. Two branches of the S-curve , obtained by plotting the data shown in Figs. 4 and 5

If we increase the length of the cavity from point c, a temperature excursion occurs. The temperature continues increasing and the field intensity continues de-

Multistable Behaviour in Microwave Heating of Ceramics

525

creasing until they reach an equilibrium point that has much higher temperature and the electric field is much lower. Notice that at the end of this process the field intensity decreases rather than increases. If we continue to decrease the length of the cavity, the temperature will decrease but the field intensity will increase. The equilibrium point curve will coincide with the original curve. In Fig. 6, the equilibrium points from Figs. 4 and 5 are collected to plot Temperature vs. electric field strength. This shows clearly branches (1) and (2) of the S-curve sketched in Fig. 1. Typical Heating Processes There are 4 typical heating processes at low power levels. To illustrate the processes clearly, each is discussed and plotted on the S-curve in the following paragraphs. Decreasing the Cavity Length The process starts from an equilibrium point, and is illustrated in Fig. 7. When the cavity length is decreased, the field intensity increases immediately while the temperature of the rod is nearly unchanged.

Fig. 7. Behavior of electric field and temperature for decreasing cavity length, illustrated for two different starting points.

526

Thomas

When the rod heating begins, the cavity is detuned by the increasing dielectric loss of the rod resulting from the heating process. If the starting point is below the critical point a, this process will end at an absolutely stable point. If the starting point is above the critical point a, the process will end at a relatively stable point. It is well known all equilibrium points above the critical point a are unstable as viewed from the energy balance since it cannot tolerate any small perturbation. But the process is governed by the energy balance as well as Maxwell's equations. When the system attempts to escape from the equilibrium point, changes in the cavity field will force it back. Thus it will oscillate around the equilibrium point. Increasing the Cavity Length As illustrated in Fig. 8, when increasing the cavity length, the rod can be heated or cooled depending upon the starting point. If the starting point is below the critical point c, the cavity will continue being tuned and the temperature increases. If the starting point is above the critical point, the cavity is detuned first. Then as the temperature increases, the cavity is tuned.

Fig. 8. Behavior of electric field and temperature for increasing cavity length, illustrated for two different starting points.

Multistable Behaviour in Microwave Heating of Ceramics

527

Thermal Excursion If the cavity length is increased form critical point c, a rapid temperature excursion occurs in a rather complicated process. Although the temperature increases almost continuously, the field intensity does not vary monotonically. As illustrated by Fig. 9, the cavity is first highly tuned and the temperature increases rapidly. When the field intensity reaches its maximum value, the temperature increases dramatically during a very short time. From the experiment results, the temperature increases more than 500 degrees in less than half a minute, causing the cavity to become detuned. At the end of the process, the temperature drops slightly to the equilibrium curve.

Fig. 9. Behavior of the electric field and temperature when the cavity length is increased from critical point c. The temperature excursion mimics thermal runaway, but a stable equilibrium point is achieved at high temperature.

Reverse Thermal Runaway If we decrease the length of cavity from the critical point b, a “reverse thermal runaway” occurs. The field strength decreases slightly, but the temperature drops precipitously, as illustrated in Fig. 10.

528

Thomas

Fig. 10. Rapid temperature decrease following a decrease in cavity length from critical point b.

Discussion and Conclusions In our experiments on heating of ceramic rods in a TE103 cavity, we have discovered three clearly identifiable critical tuning points, and have explored the heating behavior resulting from changes of cavity length from each of these. Two branches of the well-known S-curve first identified by Kriegsmann [2] have been observed. In Ref. [3], Kriegsmann finds the upper (3rd) branch of the S-curve for two cases: (1) for an exponential electrical conductivity law and a relatively thick sample, the greatly increasing conductivity at higher temperatures decreases the internal field due to the skin effect, requiring a much larger external field to induce further temperature increases; (2) for an Arrhenius conductivity model, the electrical conductivity itself saturates at high temperatures, thus a much larger external field is required to increase the sample temperature. Since mullite does not obey the Arrhenius law for electrical conductivity, and our samples are relatively thin (4.5 mm diameter), neither of these effects come into play. Thus we do not expect to see the upper branch of the curve. Contrary to most conclusions from theoretical modeling, we find that branch (2) of the S-curve is at least locally stable for the mullite rods we used in our experiments. In future work, we intend to heat different materials which might lead to branch (3) on the multivalued S-curve.

Multistable Behaviour in Microwave Heating of Ceramics

529

Acknowledgements This work was supported in part by the U. S. National Science Foundation through Grant DMI-9622326, and by equipment grants from the Virginia Tech ASPIRES program. Loan of equipment by the Los Alamos National Laboratory is gratefully acknowledged.

References [1] Hill JM, Marchant TR (1996) Modelling microwave heating. Appl Math Modelling 20: 3-15 [2] Kriegsmann GA (1991) Microwave heating of ceramics: A mathematical theory. In: Clark DE, Gac FD, Sutton WH (eds) Microwaves: Theory and Applications in Materials Processing, Ceramic Transaction 21, American Ceramic Society , Cincinatti, OH, pp 117-183 [3] Kriegsmann GA (1991) Thermal runaway in microwave heated ceramics: A onedimensional model. J Appl Phys 71:1960-1966 [4] Kriegsmann GA (1993) Microwave heating of dispersive media. SIAM J Appl Math 53: 655-669 [5] Kriegsmann GA (1997) Cavity effects in microwave heating of ceramics. SIAM J Appl Math 57: 382-400 [6] Kriegsmann GA (1997) Hot spot formation in microwave heated ceramic fibers. IMA J Appl Math 59:123-148 [7] Liu B., Marchant TR (1999) The microwave heating of two-dimensional slabs with small Arrhenius absorptivity. IMA J Appl Math 62:137-166 [8] Marchant TR, Liu B (1998) The steady-state microwave heating of slabs with small Arrhenius absorptivity. J Engrg Math 33:219-236 [9] Pelesko JA, Kriegsmann GA (1997) Microwave heating of ceramic laminates. J Engrg Math 32: 1-18 [10] Stuerga D, Zahreddine I, More C, Lallemant M (1993) Bistable behaviour in microwave heating: the first experimental evidence. C R Acad Sci Paris 316:901-906. [11] Vriezinga CA (1996) Thermal runaway and bistability in microwave heated isothermal slabs. J Appl Phys 79: 1779-1783

Microwave Sintering of Silicon Nitride Ceramics Kiyoshi Hirao, Mark I. Jones, Manuel E. Brito and M. Toriyama Synergy Materials Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Shidami Human Science Park, Shimo-shidami, Nagoya 463-8687, JAPAN.

Introduction In recent years, microwave heating has been attracting increasing attention for sintering of ceramic materials [1 - 2]. Compared to conventional heating, microwave heating is characterized as an energy saving process, which results from volumetric and rapid heating of materials by microwaves and from shorter sintering times. On the other hand, it has been reported that interactions between microwaves and ceramics are significantly different among the diverse kinds of ceramic. Therefore, in ceramic materials composed of substances with large difference in microwave absorption efficiency, it is expected that a material with unique microstructure, which leads to excellent properties unattainable by conventional heating process, can be fabricated through localized heating by microwaves. Silicon nitride is well known as a high temperature structural ceramic having excellent mechanical and thermal properties. In general, silicon nitride is sintered with the aid of a liquid phase, which is generated by the reaction of impurity silica and oxide additives during heating. As the amount of liquid phase reaches to about 10% in volume, silicon nitride is regarded as a two-phase composite constituted by Si3N4 and a secondary liquid phase. In addition, D(low temperature form) to E (high temperature form) phase transformation, as well as grain growth of E-Si3N4 during sintering, proceed by mass transfer through the liquid because silicon nitride has very low self-diffusion coefficients even at high temperatures. From this point of view, microwave sintering of silicon nitride ceramic is of interest, in addition to the economic standpoint. A large number of studies have been made on microwave sintering of silicon nitrides [3 - 10]. It has been shown that microwave heating enhanced densification [3, 4, 8], phase transformation [4, 8] and grain growth [3, 4, 6, 8], compared to the conventional heating. However, little argument about the microstructure evolution by microwave heating has been conducted based on the nature of liquid phase sintered silicon nitride. In this paper, recent results on microwave heating of silicon nitride at 28 GHz are summarized, and the phase transformation, densification

534

Hirao

and grain growth behavior during microwave heating are discussed compared to conventional heating method.

Densification behavior in microwave sintered Si3N4

Characteristics of raw powder and microwave sintering of Si3N4 It is well recognized that the characteristics of raw Si3N4 powder such as E phase content and grain size distribution significantly affect the microstructure of liquid phase sintered silicon nitride. In this series of investigation, two kinds of D-Si3N4 powders (Grade E-10 and E-5, Ube Industries Ltd., Japan) were subjected to microwave sintering. Table 1 summarizes the characteristics of these raw powders provided by the supplier. The E-10 powder is characterized with fine particles and relatively large amount of E-phase content in comparison with the E-5 powder. It should be noticed that particle density of E-Si3N4 for E-10 powder is about 25 times higher than that for E-5 powder. Table 1. Characteristics of starting raw D-Si3N4 powder

Powder E-10 E-5

BET 11.2 m2/g 5.3 m2/g

E phase content 4.0 mass% 1.5 mass%

Particle density of E-Si3N4

15.5 N/m3 0.6 N/m3

The powder compact of D-Si3N4 with oxide additives of 5 mass% Y2O3 and 5 mass% spinel (MgAl2O4) was sintered in a nitrogen atmosphere of 0.1 MPa, in an applicator fed by a 28 GHz Gyrotron source as reported previously [11]. The sample was placed on a BN plate and located inside the chamber surrounded by porous alumina insulation. Sintering was carried out at temperatures ranging from 1200 to 1850qC with 1 hour holding at sintering temperature, and the temperature was monitored by a W-Re thermocouple embedded in the center of the sample. Sintering behavior under microwave heating was examined by evaluating density, phase composition and microstructure. For the sake of comparison, the green compact was also sintered by a resistance-heating furnace (hereafter referred to as conventional furnace) under the same sintering conditions. Densification and phase transformation The variations of density and E-phase content of the samples from E-10 powder are shown in Figs. 1-(a) and (b). It is clear that the samples produced by microwave sintering exhibit enhanced densification and phase transformation when compared to those produced by conventional sintering. The samples from E-5

Microwave Sintering of Silicon Nitride Ceramics

535

powder also show the similar densification behavior (Figs. 2-(a)and (b)). The microwave sintering of silicon nitrides with Y2O3 and MgAl2O4 additive system, in terms of transformation rates and densification, could be achieved at temperatures in the range of 200qC lower than those of conventionally sintered samples. These results are compatible with the results reported by Kim et al. [8] that microwave sintering of silicon nitride with Al2O3 and Y2O3 additives using a 2.45 GHz source brought about temperature improvements of about 200qC. 100

(a)

90

MW

80 70

Conventional

60 50 1000

Beta-phase content [%]

Realative density (%)

100

(b)

80 60

Conventional

40 20 0 1000

1200 1400 1600 1800 Sintering temperature (ºC)

MW

1200 1400 1600 1800 Sintering temperature (º C)

Fig. 1. Variation of (a) relative density and (b) E phase content with sintering temperature of Si3N4 from E-10 powder. 100

(a)

90

MW

80 70

Conventional

60 50 1100

1300 1500 1700 1900 Sintering temperature (ºC)

Beta-phase content [%]

Realative density (%)

100

(b)

80

MW

60 40

Conventional

20 0 1100

1300 1500 1700 1900 Sintering temperature (º C)

Fig. 2. Variation of (a) relative density and (b) E phase content with sintering temperature of Si3N4 from E-5 powder

536

Hirao

Fig. 3. Power requirement for heating the sample at 1450°C by microwave

The power required to heat the sample using microwaves up to 1450qC is shown in Fig. 3. The microwave power increased with increasing setting temperature. However a drastic decrease in microwave power was observed at a temperature of approximately 1250qC, and thereafter the microwave kept lower values. When microwaves are irradiated to a green body of silicon nitride ceramic, because of poor coupling between Si3N4 and microwave it is at first absorbed substantially by the oxide additives, and then by the liquid phase [3]. Such relaxation of the microwave power required to heat the silicon nitride ceramics is, therefore, caused by the formation of the liquid phase by the reaction between oxide additives, impurity SiO2 in Si3N4 and Si3N4 [3]. The phase diagram for Y2O3-Al2O3MgO-SiO2-Si3N4 system has not been reported, but it is reasonable to think that in this investigation the liquid phase generates around 1250qC when referred to ternary phase diagrams among these compounds. Rapid densification of microwave sintered samples is observed at around a temperature of 1300qC (Fig. 1-(a)), and, in addition, phase transformation began to occur at this temperature (Fig. 1-(b)). These results suggest that the enhanced densification and phase transformation achieved in the microwave sintering is closely related to the preferential absorption of microwaves by the liquid phase. Microstructure evolution Microstructures of microwave sintered samples from E-10 and E-5 powders are shown in Figures 4 and 5 respectively, along with those of conventionally sintered samples. At lower sintering temperatures, microstructures of microwave sintered samples were quite different from those of conventionally sintered samples be-

Microwave Sintering of Silicon Nitride Ceramics

537

cause of differences in phase compositions: the microwave sintered sample at 1550qC, for example, was composed of characteristic rodlike grains of E-Si3N4 while the conventionally sintered sample was composed of mainly equaxial grains of D-Si3N4. For microwave sintered samples, grain size of the sample from E-5 powder is much larger than that from E-10 powder. This is attributed to the fact that during D to Ephase transformation newly formed E phase is grown mainly on the preexisting E grains [12]. Therefore, the sample from E-5 powder with low E particle density resulted in the coarse microstructure after phase transformation. At higher temperatures where phase transformation of conventionally sintered silicon nitride was completed, microstructures of microwave and conventionally sintered samples were quite similar.

Fig. 4. Microstructures of silicon nitrides from E-10 powder.

Fig. 5. Microstructures of silicon nitrides from E-5 powder.

538

Hirao

Furthermore, the microstructure of microwave sintered samples at higher temperatures exhibit little grain growth when compared to the sample at lower temperatures. It can be seen that growth of E grains in length direction, that is the preferential growth direction of E-Si3N4 crystal, is inhibited by the impingement of neighboring grains. The size distribution of larger elongated E grains after complete phase transformation in the each specimen is rather uniform. Such grain size distribution is also one of the reasons for the unchanged microstructure even when sintered at high temperatures by microwave.

Grain growth during microwave annealing

Fabrication of seeded silicon nitride It is recognized that a well distinguished bimodal microstructure composed of large elongated grains and smaller grains leads to high fracture toughness of silicon nitride. Therefore, enhancement of E-Si3N4 grain growth is an important subject for development of toughened silicon nitride. It has been shown that seeding with morphologically regulated E-Si3N4 single crystal particles is an effective strategy for developing such bimodal microstructure [13]. Figure 6 shows microstructure of hot-pressed silicon nitride with 5 mass% Y2O3, 2 mass% Al2O3 and 2 vol% seed particles. In order to investigate the effect of microwave heating on grain growth behavior, microwave annealing of this seeded silicon nitride was carried out [6, 14]. Large difference in grain size between large elongated grains from seed and fine grained matrix can provide driving force for further growth of large grains during solution reprecipitation process. Grain growth behavior during annealing The microstructure development of seeded silicon nitride during annealing is characterized, concentrating on the growth behavior of seed particles. Seeded silicon nitride exhibits a dual modality in grain distribution, in which larger grains developed from seeds are distributed in the fine matrix grains grown from E-Si3N4 nuclei present in the raw powder. It is easy to distinguish larger grains from fine matrix grains [6, 14]. Figure 7 shows the effect of annealing time on length and diameter of selective large grains grown from seeds. The results for samples annealed using a conventional furnace are also shown in this figure. In both specimens grain diameter exhibited little change, while appreciable growth in the length direction was observed. It is evident that microwave annealing dramatically enhances the growth in the length direction compared to the annealing by conventional furnace. Well faceted side surfaces and rounded tips of large elongated grains imply that growth in the width and length directions are controlled by inter-

Microwave Sintering of Silicon Nitride Ceramics

539

facial reaction and diffusion controlled kinetics respectively [15]. It is, therefore, speculated that enhanced grain growth of length direction by microwave heating is closely related to the preferential heating of the liquid phase, which enhances mass transfer through the liquid phase.

Fig. 6. Microstructure of seeded silicon nitride fabricated by hot-pressing.

Fig. 7. Effect of annealing time on growth of selected grains.

Summary Application of microwave heating to the processing of silicon nitride ceramics showed enhanced densification and phase transformation during sintering, and, in addition, enhanced grain growth during annealing (solution/reprecipitation stage) in the case when appropriate grain size distribution is provided. It is speculated that these enhanced mass transfer phenomena are closely related to the preferential heating of the liquid, which enhances mass transfer through the liquid phase. From this point of view, glass chemistry might affect the sintering behavior of silicon nitride during microwave heating.

540

Hirao

References [1] Clark DE, Sutton WH, Lewis DA (1997) Microwave processing of materials. In : Clark DE, Sutton WH, Lewis DA (eds.) Theory and Application in Materials Processing IV. The American Ceramic Society, Westerville, OH, pp 61-96. [2] Roy R, Agrawal D, Cheng JP, Mathis M (1997) Microwave processing: Triumph of applications-driven science in WC-composites and ferroic titanates. ibid., pp3-26. [3] Tiegs TH, Kiggans JO, Kimrey HD (1991) Microwave sintering of silicon nitride. Ceram. Eng. Sci. Pro., 12: 1981-1992. [4] Kim YC, Koh SC, Kim DY, Kim CH (1995) Effect of microwave heating on the sintering of silicon nitride-doped with Al2O3 and Y2O3. In: Clark DE, Folz DC, Oda SJ, Silberglitt R (eds.) Microwaves: Theory and Application in Materials Processing III. The American Ceramic Society, Westerville, OH, pp 415-422. [5] Kiggans JO, Tiegs TN, Davisson CC, Morrow MS and Garvey GJ (1996) Scale-up of the nitridation and sintering of silicon preforms using microwave heating. In: Iskander MF, Kiggans Jr JO, Bolomey JC (eds.) Microwaves Processing of Materials V. Mater. Res. Soc. Proc., Materials Research Society, pp3-8. [6] Hirota M, Brito ME, Hirao K, Watari K, Nagaoka T, Toriyama M (1996) Grain growth behavior during microwave annealing of silicon nitride. ibid., pp 441-445. [7] Xu G, Zhuang H, Li W and Wu F (1997) Microwave sintering of Į/ȕ-Si3N4. J. Eur. Ceram. Soc. 17 : 977-981. [8] Kim YC, Kim CH, Kim DK (1997) Effect of microwave heating on densification and DĺE phase transformation of silicon nitride. J. Eur. Ceram. Soc. 17 : 1625-1630. [9] Hirota M, Brito ME, Hirao K, Watari K, Toriyama M, Nagaoka T (1997) Microwave sintering of silicon nitride with rare earth sesquioxide additives. In : Clark DE, Sutton WH, Lewis DA (eds) Microwave : Theory and application in materials processing IV. The American Ceramic Society, Westerville, OH, pp 515-522. [10] Valecillos MC, Hirota M, Brito ME, Hirao K., Toriyama M (1998) Microstructure and mechanical properties of microwave sintered silicon nitride. J. Ceram. Soc. Jpn. 106 : 1162-1166. [11] Jones MI, Valecillos MC, Hirao K, Toriyama M, Densification behavior in microwave sintered silicon nitride at 28 GHz. J. Am. Ceram. Soc., J. AM.Ceram. Soc. 84 : 242426. [12] Dressler W, Kleebe HJ, Hoffmann MJ, Ruhle M, Petzow G (1996) Model experiments concerning abnormal grain growth in silicon nitride. J. Eur. Cer. Soc. 16 : 3-14. [13] Hirao K. Nagaoka T, Brito ME, and Kanzaki S (1994) Microstructure control of silicon nitride by seeding with rodlike E-silicon nitride particles. J. Am. Ceram. Soc. 77 : 1857-1862. [14] Hirota M, Valecillos MC, Brito ME, Hirao K, Toriyama M (2000) Enhancement of grain growth in silicon nitride by 28 GHz microwave annealing. J. Ceram. Soc. Jpn. 108 : pp321-323. [15] Kitayama M, Hirao K, Toriyama M and Kanzaki S (2000) Modeling and simulation of grain growth in Si3N4 III. Tip shape evolution. Acta Materialia 48 : 4635-4640.

Novel Materials Processing by Millimeter-Wave Radiation - Present and Future Shoji Miyake Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan.

Abstract Results of extensive research on mm-wave materials processing conducted at JWRI, Osaka University are described. Modification of diamond films drastically improves their microstructure, resulting in a strong cathode luminescence, while for SrTiO3-films the dielectric constant of as-deposited films increases from 70 to about 260, a value comparable with the bulk material post annealed at 300°C. In the study of low temperature ceramics sintering, a new sintering aid containing Yb2O3 is applied, yielding a highly dense (bending strength and fracture toughness of 900 MPa and about 7 MPa m1/2, respectively) Si3N4 sintered at a 200 - 400°C lower temperature as compared with the conventional method. Here, “selective mm-wave heating” of Yb2O3 around the grain boundary is assumed to promote liquid phase sintering. In the sintering of pure Al2O3 a bending strength of 800 MPa was found upon mm-wave sintering as compared to 400 MPa for conventional sintering at a similar density and grain size. For pure Al2O3 doped with Cr2O3 an enhanced densification is clearly observed in the case of mm-wave sintering at a fixed Cr3+ volume diffusion. For AlN sintered for less than 60 min with mm-wave radiation a high thermal conductivity of about 180 W/mK was obtained at a temperature by about 300°C lower than in conventional sintering. Finally, a highly oxidation-resistant (Ti, Cr)N composite (weight gain less than 5 g/m2 under oxidation at 1000°C for 60 min) is obtained by mm-wave sintering of Ti- and Cr2N-powder mixtures in N2 gas environment. Near-future trends and expectations are discussed for mm-wave application in materials processing.

Introduction Microwave processing of materials has been studied extensively in the 1960s typically using 2.45 GHz microwave radiation. Fabrication of commercially available samples with high reproducibility was not successful, mainly due to dominant

542

Miyake

thermal runaway and/or very small sample size available. This situation changed when high power mm-wave tubes (gyrotron) were developed in 1970s, within the nuclear fusion research activities. Microwave processing again became one of the topics in the materials research with frequent international seminars, conferences all over the world and with many published proceedings and books [1, 2]. Great progress in the theoretical analysis of the specific microwave effects to materials [3] was achieved, facilitating ongoing interdisciplinary studies among physicists, chemists, electrical engineers, materials scientists, biomedical engineers. We have been studying mm-wave materials processing from around 1990 and obtained various fruitful results mainly on the sintering of bulk ceramics and modification of thin films including several international joint works. In this report we demonstrate our progress of research using 28 GHz and 60 GHz mm-wave radiation and describe further step of research expecting efficient applicability to the next generation world-wide engineering.

Modification of Thin Films

Modification of CVD Diamond Films Strongly focused and pulsed mm-wave radiation with an energy density of several 10 kW/cm2 was used to modify properties of diamond films prepared by thermal and/or plasma CVD [4]. The film thickness was about 500 Pm and the 60 GHz mm-wave pulse duration was fixed at 5 ms with a repetition of 5 Hz. The sample temperature increased with time and reached a constant value at a fixed power of the mm-wave radiation. The total irradiation time was 30 min. Figure 1 shows variation of the peak shift and full width half maximum of the Raman diamond peak around 1332 cm-1 with heating temperature.

Fig. 1. Temperature influence on the shift and width of the Raman peak at 1332 cm-1 for mm-wave treated CVD diamond films. The value at 0°C is for the starting material before irradiation.

Novel Materials Processing by Millimeter-Wave Radiation

543

For mm-wave irradiated samples the width becomes smaller when the temperature exceeds 1000°C. This result clearly indicates that defects and distortions in the diamond films were reduced by the mm-wave radiation treatment. In the asdeposited films no cathode luminescence (CL) was observed due to high concentration of defects, while clear CL was found in the samples with mm-wave irradiation, which supports the data in Fig. 1. Modification of PVD SrTiO3 Films Another application of mm-wave radiation to the modification of thin films was studied in SrTiO3 films prepared by plasma PVD process [5, 6]. Films were deposited on Si wafers and/or Pt/Ti/SiO2/Si multi-layer substrates at a temperature below 200°C with a thickness of about 100 nm by a mirror-confinement type ECR plasma sputtering system. On Si substrates films had little crystallization but on multi-layer substrates they were sufficiently crystallized even at such a low temperature. After deposition films deposited on Si substrates were heated by mm-wave radiation up to 327°C and, as shown in Fig. 2, they could be crystallized at a temperature of 277°C which was 100°C lower than by conventional electric furnace heating. While films on multi-layer substrates did not show remarkable change in their crystallization by the mm-wave post-annealing up to 400°C, since they were sufficiently crystallized already at as-deposited condition.

Fig. 2. Variation of XRD patterns of SrTiO3 Fig. 3. Dielectric constant of SrTiO3 films films with annealing temperature versus electric field strength at 100 kHz

It was clarified, however, that electric properties of the films on multi-layer substrates drastically changed by the mm-wave annealing. The results are shown in Fig. 3. The dielectric constant of as-deposited and of mm-wave heated samples up to 300°C is given as a function of applied electric field at a frequency of

544

Miyake

100 kHz. From the original value of about 70 in the as-deposited state a drastic increase up to 260 by the mm-wave heating is observed. These results indicate that post annealing process using mm-wave radiation is a new and very efficient method of modifying film microstructure and its property.

Low Temperature Sintering of Bulk Ceramics

Si3N4 with New Sintering Aids Si3N4 is one of the promising ceramic candidates for high temperature applications. In the sintering of this material sintering aids are frequently used such as Y2O3, MgO and Al2O3. These additives promote atomic diffusion to achieve sufficient densification forming a liquid inter-granular phase. We have made experiments on sintering of Si3N4 containing Y2O3 and Al2O3 additives by mm-wave heating method and obtained ceramics with a high bending strength of about 1GPa, with finer grains and less weight loss at a temperature of 1700°C which was lower by about 50°C compared with the conventional electric furnace heating method [7]. Selection of sintering aids is also important to improve mechanical properties of the bulk ceramics and Yb2O3 is expected as one of the promising additives. With this aid, however, it is difficult to enhance the densification in the conventional sintering process. We intended to overcome this problem by applying a mm-wave sintering process, since Yb2O3 has a high absorption to microwave radiation, and we succeeded in obtaining well sintered ceramics at a temperature remarkably lower than by the conventional method [8]. Figure 4 shows the variation of relative density of Si3N4 with sintering temperature, for different amounts of the sintering aid Yb2O3. To obtain the same relative density for the same composition of the samples, a temperature difference ranging from 200 to 400°C between mm-wave sintering and conventional sintering is observed. Additionally, in the conventional sintering the density at a fixed temperature decreases with the contents of Yb2O3, but in the mm-wave sintering it does not show a monotonic variation. This behavior was checked in more detail as shown in Fig. 5, where the temperature to densify Si3N4 up to 96%TD is shown as a function of the additive composition. In the figure “7Yb-1Al” means the composition of 7 wt% Yb2O3 – 1 wt% Al2O3, for instance. It is interesting to note that in the case of mm-wave sintering the temperature shows a non-linear variation with a minimum densification temperature of about 1500°C at a composition of 6.4Yb-1.6Al, whereas only monotonic decrease of the temperature is observed in the case of conventional sintering. The shape of the densification curve for mm-wave sintering is found to be quite similar to the liquidus-solidus line in the Yb2O3 – Al2O3 phase diagram, as indicated in Figure 5.

Novel Materials Processing by Millimeter-Wave Radiation

545

Fig. 4. Relation between relative density and Fig. 5. Densification temperature of Si3N4 versus Yb2O3 sintering temperature of Si3N4

Fig. 6. Bending strength and fracture toughness of Si3N4 as function of sintering temperature; Yb2O3 is used as sintering aid.

Generally it is believed, that oxide additives and SiO2 form a liquid phase on the surface of Si3N4 –grains during sintering and the liquid phase with Yb2O3 has stronger mm-wave absorption than matrix Si3N4, by which the liquid temperature would be locally higher than the temperature of the matrix phase in the mm-wave sintering. On the other hand in the conventional sintering this kind of temperature difference would not occur and only a linear decrease of the sintering temperature with an increasing amount of Yb2O3-additive was observed. Thus, the observed peculiar variation of the sintering temperature upon Yb2O3-additive content for mm-wave sintering is suggested to come from “selective heating” inherent to the mm-wave process.

546

Miyake

Figure 6 shows bending strength and fracture toughness of 5 wt% Yb2O3 3 wt% Al2O3 Si3N4 samples as a function of sintering temperature. The bending strength has higher values compared with that (550 MPa) of conventionally sintered sample and reaches to about 900 MPa for the ceramic sintered at 1750°C. While the fracture toughness shows a higher value for the ceramic sintered at 1600°C. These results make us suggest that selection of appropriate sintering aids with high mm-wave absorption and composition for liquid phase formation at lower temperature will bring about a superior Si3N4 bulk ceramics by mm-wave sintering process.

High Strength Al2O3 In pure Al2O3 sintered by microwave radiation reduction of the sintering temperature has been observed in many studies and suppression of the grain growth is typically expected in this process. Sometimes contradictory results, however, have been reported [9, 10] without remarkable difference in the grain growth, by which we studied microstructures and mechanical properties of mm-wave sintered Al2O3 in comparison with those by conventional sintering [11]. A similar densification curve was typically obtained for different sintering times when samples were soaked at 1250°C by mm-wave method and at 1500°C by conventional one, and we compared microstructures and mechanical properties of these samples.

Fig. 7. SEM image of sintered pure Al2O3

Novel Materials Processing by Millimeter-Wave Radiation

547

Fig. 8. Bending strength of pure Al2O3 versus sintering temperature

Figure 7 shows SEM photographs of Al2O3 samples of over 97%TD sintered with a soaking time of 60 min. We find that the grain size of both samples is similar and has a value of 2 – 3 Pm. Additionally the crystallite size of the samples was determined from the full width at half maximum of the XRD peak from the (113) plane of Al2O3. However it also indicated almost no difference for both sintering methods. The bending strength was measured by the 3-point bending test, as shown in Fig. 8. It reaches to a value between 600 and 800 MPa in the case of mm-wave sintering , while that of conventional sintering is lower and ranges between 400 and 600 MPa. Indeed this difference of the bending strength between mm-wave and conventional sintering is not attributed to the difference of the grain size, but probably due to the difference in the microstructure of the grain boundary. Detailed analysis by TEM observation will give us more refined understanding on the peculiar non-thermal effect of mm-wave heating process concerned with grain boundary structure of Al2O3. Diffusion of Cr3+ in Al2O3 Sintering For Yb2O3-Si3N4-ceramics we described a unique behavior of selective heating in the mm-wave sintering process of Si3N4. In this section another unique feature of mm-wave sintering is demonstrated in the heating of Al2O3 samples containing 0.5 wt% Cr2O3.

548

Miyake

Enhanced diffusion due to microwave action has been theoretically explained by the pondero- motive effect [3]. In this report we demonstrate a unique densification behavior by mm-wave radiation, in which optical absorption spectroscopy is applied to analyze the diffusion stage of Cr+ ions in the Al2O3 lattice [12].

Fig. 9. Relative density of Al2O3 versus con- Fig. 10. Variation of thermal conductivcentration of Cr2O3 ity of AlN with sintering temperature

The optical absorption spectra of Al2O3 containing Cr2O3 has typically 2 peaks around 410 nm and 560 nm and they shift to larger wavelength with increasing content of Cr2O3, from which we can estimate local concentration of Cr2O3 in Al2O3. In Figure 9 the relation between relative density and local concentration of Cr2O3 is shown, estimated from the shift of the longer wavelength absorption peak in Al2O3 ceramics sintered by mm-wave and conventional methods. A large difference is observed in the relative density at a constant concentration of Cr2O3, which indicates a remarkably enhanced densification of Al2O3 in the case of mmwave sintering. AlN with High Thermal Conductivity AlN is expected to be a promising heat-sink material for electronic devices. Theoretical thermal conductivity of AlN is calculated to about 320 W/m K, but in sintered AlN a value of about 180 W/m K is commonly found.

Novel Materials Processing by Millimeter-Wave Radiation

549

Fig. 11. Thermal conductivity of AlN versus sintering temperature

Fig. 12. XRD patterns of sintered Ti-Cr-N system at various nitrogen pressures

In the conventional sintering method it was necessary to heat samples up to 1900°C – 2000°C for 120 min to obtain sufficiently dense specimens. While in the mm-wave sintering using Y2O3 additive the sintering temperature of 1750°C was

550

Miyake

sufficient and the soaking time was below 60 min [13]. Figure 10 shows the dependence of thermal conductivity of the sample with Y2O3 additive on the soaking time by mm-wave sintering for 2 types of materials. The conductivity reaches a value of about 180 W/m.K even at a short soaking time of 40 min. When Yb2O3 is applied as sintering aid instead of Y2O3 the sintering temperature for obtaining full density decreased further to 1650°C which is over 100°C lower than for Y2O3 as sintering aid. In Figure 11 variation of thermal conductivity on the sintering temperature is shown for 2 kinds of sintering aids with the same vol%, for mm-wave sintering at 60 min holding time. It is interesting that the conductivity retains similar values over a wide range of sintering temperatures. From these results we demonstrate that application of mm-wave sintering with Yb2O3 aids brings high quality AlN at a short sintering time below 60 min at a sintering temperature as low as 1600°C – 1700°C, which is about 300°C lower than for conventional sintering.

Preparation of Composite Materials from Ti/Cr2N Powders To find an applicability of mm-wave heating to materials containing metallic species we made experiments of sintering new composite materials of Ti-Cr-N system from powder mixtures of metallic Ti and Cr2N [14]. It was possible to obtain well densified sintered samples containing up to 70 mol% Ti at a sintering temperature of 1350°C and a sintering time of 20 min. These sintered samples were composed of (Ti, Cr)N, Cr2N with some amount of metallic Cr as shown in Fig. 12, where samples with 51 mol% Ti were sintered under several N2 gas pressures. We also studied oxidation behavior of these samples and succeeded in obtaining quite high oxidation resistance with small weight gains less than 5 g/m2 in air at 1000°C for 60 min. From these results we think it is possible to fabricate varieties of composite materials including large amount of metal powders at low temperature and short sintering times by mm-wave heating.

Conclusion and Future Expectation Results of extensive research on mm-wave materials processing conducted at JWRI, Osaka University were briefly described. In the modification of thin film properties we conducted 2 kinds of experiments. The one is the mm-wave irradiation to diamond sheets prepared by CVD method and the other is irradiation to SrTiO3 films prepared by PVD. In the former study Raman spectra of diamond films indicated improvement of their microstructure by the irradiation of high energy density mm-wave radiation. While in the SrTiO3 preparation we could obtain well crystallized films at a low temperature of 300 - 400°C by the post annealing with mm-wave radiations and the di-

Novel Materials Processing by Millimeter-Wave Radiation

551

electric constant of the films reached to a value of about 260 which is comparable with the bulk one (300). In the low temperature sintering of ceramics and/or their composites we could find various attractive results. In the case of Si3N4 we obtained highly dense sintered samples by applying new sintering aids containing Yb2O3 at a temperature lower by about 200 - 400°C in comparison with the ones by the conventional method. The bending strength and fracture toughness reached to a value of 900 Pa and about 7 MPam1/2, and “selective heating” of Yb2O3 by mm-wave around the grain boundary was found to promote densification process in its liquid phase resulting in the remarkably low temperature sintering of Si3N4. In the sintering of pure Al2O3 comparison of the bending strength of mm-wave sintered Al2O3 with that of conventionally sintered one was done at a similar density and grain size. The result indicated a big difference with a value of 800 MPa in the former case and 400 MPa in the latter. While in the sintering of Al2O3 diffusion behavior of Cr3+ ions into Al2O3 was investigated by optical absorption spectroscopy. It was verified that enhanced densification of Al2O3 was clearly observed in the case of mm-wave sintering at a fixed Cr3+ volume diffusion, which supports the importance of grain boundary structure in improving the mechanical structures of Al2O3 under the influence of mm-wave radiation, as well as in understanding unique mm-wave effect. In the sintering of AlN a high thermal conductivity of about 180 W/mK was obtained at a lower temperature by about 300°C with a very short sintering time below 60 min as compared with the conventional method. As an example of composite materials sintering, we tried to synthesize Ti-Cr-N compound by 28 GHz radiations starting from the heating of metal Ti and Cr2N mixed powders under N2 gas environment. We could succeed to obtain (Ti, Cr)N composite with high oxidation resistance with a small weight gain less than 5 g/m2 in oxidation at 1000°C for 60 min. Above mentioned fruitful results encourage us to expect development of an energy saving and environmentally conscious new novel materials processing system using high power mm-wave radiations and following research activities will be expected in the near future. 1) Development of commercial processing system using 24 GHz radiation which is allowed for industrial usage, 2) Development of commercially available high temperature ceramic parts, 3) Non-thermal reatment of various thin films or small parts of electronics devices, 4) Synthesis of new nano-composite bulk materials with microstructure, 5) Synthesis of functionally gradient composite materials for high temperature application, 6) Basic research on new chemical processes appearing in mm-wave interaction with materials, 7) Basic research on the electrical properties of various ceramics in the wide range of temperature through interaction with mm-wave radiations.

552

Miyake

Acknowledgement The author would like to express sincere appreciation to Prof. Y. Makino for his continuing interest and discussion on this research and to Dr. S. Takeda for his strong interest and support in the research of modifying diamond films, as well as to Mr. S. Baba for his support in conducting research on SrTiO3 films. Thanks are given to Mr. T. Ueno and Mr. T. Yoshioka for their support to this research. He also thanks Dr. T. Mori for his help in preparing the manuscript. Thanks are also given to researchers of Kanagawa High-Technology Foundation (KTF) for their strong support in the characterization of many samples fabricarted.

References [1] Microwave Processing of Materials I – V, MRS Symp. Proc. (Pittsburg, PA, MRS, 1988 – 1996) [2] Microwave Theory and Application in Materials Processing I – V, Ceramic Trans. (Westerville, OH, ACS, 1991 – 2001). [3] K. I. Rybakov and V. E. Semenov, Phys. Rev. B49 (1994) 64. [4] N. Abe and S. Miyake, Kagaku to Gijutsu no Ishizue ( Seisan Gijutsu Shinkou Kyoukai, Osaka 1999)p. 173. [5] S. Baba, K. Numata, H. Saito, M. Kumagai, T. Ueno, B. Kyoh and S. Miyake, Thin Solid Films, 390 (2001) 70. [6] S. Baba, K. Numata and S. Miyake, Sci. Technol. Adv. Mater. 1 (2001) 211. [7] T. Ueno, S. Kinoshita, Y. Setsuhara, Y. Makino, S. Miyake, S. Sano and H. Saito, Key Eng. Mater. Vol 161 – 163 (1999) p. 45. [8] T. Ueno, H. Saito, S. Sano, Y. Makino and S. Miyake, Proc. 2000 Powder Metallurgy World Congress (PM 2000), part 1 (2000) p. 753. [9] T. Meek, R. D. Blake and J. J. Petrovic, Ceramic Eng. Sci. Proc. 8 (1987) 861. [10] R. W. Bruce, A. W. Flifled, R. P. Fischer, D. Lewis, III, B. A. Bender, G> M. Chow, R. J. Rayne, L. K. Kurihara and P. E. Schoen, Ceramic Trans. 80 (1997) 287. [11] Y. Makino, S. Miyake, S. Sano, H. Saito, B. Kyoh, H. Kuwahara and A. Yoshikawa, 8th Int. Conf. on Microwave and High Frequency Heating, Sept. 3 – 7, 2001, Bayreuth). [12] T. Ueno, Y. Makino, S. Miyake and S. Sano, see this volume [13] T. Matsumoto, Y. Makino and S. Miyake, Proc. 2000 Powder Metallurgy World Congress (PM 2000), Part 1(2000) p. 663. [14] T. Matsumoto, Y. Makino and S. Miyake, J. Mater. Sci. 36 (2001) 693.

Correlation Between Densification and E - Phase Formation at Microwave Sintering of Si3N4 Ceramics O. I. Getman1, V. V. Panichkina1, V. V. Skorokhod1, I. V. Plotnikov2, V. V. Holoptsev2 1 2

Institute for Problems of Material Science, Kiev, Ukraine Institute of Applied Physics, Nizhny Novgorod, Russia

Introduction Despite the considerable progress in experimental studies of microwave ceramics sintering, many problems remain unsolved. One of the most challenging to solve is the sintering of silicon nitride-based ceramics. Silicon nitride - based ceramics is some of the most promising materials for high-temperature and severe environment applications. It is well known that the Si3N4 ceramics sintered with large amount of oxide additives, such as Al2O3 and Y2O3, lose their strength at temperatures above 800 - 900°C [15]. The well-recognized methods for obtaining Si3N4 ceramics with improved high-temperature properties use reduction in the amount of oxide additives that promote liquid-phase sintering of silicon nitride, and replacement of yttrium oxide with other rare-earth oxides having higher melting point. Both methods use expensive sintering techniques, such as hot pressing or hot isostatic pressing. The development of alternative methods for the sintering of Si3N4 ceramics at normal pressure of nitrogen is a highly important task. The main distinguishing features typical for microwave sintering of oxide ceramics have been confirmed for silicon nitride-based materials as well. Accelerated densification and increased final density of the sintered samples have been observed for the Si3N4 ceramics with rather large amounts (~10%) of oxide additives (Al2O3, Y2O3) [6, 9]. These results have inspired further research in the microwave processing of high performance Si3N4 ceramics. A wide spectrum of rareearth oxides used as additives was investigated in [7]. The Si3N4 ceramics with Al2O3 + Yb2O3 additives with density higher than 98% were obtained at an increased pressure of nitrogen (d 9 bars) [13]. As for the ceramics with reduced additive content, it has been demonstrated that the advantages of microwave sintering disappear with the decrease in the overall amount of oxides.

554

Getman

In the present study, the processes of densification and phase transformation of silicon nitride-based materials with various amount and composition of oxide additives are compared for identical regimes of microwave and conventional heating.

Experimental Methods The green body specimens were prepared from D-Si3N4 powder (Ube Co. Ltd., SN-E10, average size of particles 0.55 Pm), Y2O3 (Shin-Etsu Chemical Co., SU grade, 0.85 Pm), Yb2O3 (Shin-Etsu Chemical Co., RU grade, 1.04 Pm), and DAl2O3 (Sumitomo Chemical Co., AKP-20, 0.57 Pm). The slip-casting procedure described elsewhere [12] was used to obtain the following compositions Si3N4 + 3 wt% Al2O3 + 5 wt% Y2O3 Si3N4 + 3 wt% Al2O3 + 5 wt% Yb2O3 Si3N4 + 1.5 wt% Al2O3 + 2.5 wt% Yb2O3

(type A), (type B), (type C).

The green density was about 50% for the materials of all types. The experiments on microwave sintering were performed at the 30 GHz, 10 kW gyrotron facility of the Institute of Applied Physics (Nizhny Novgorod, Russia) [2]. The microwave power applied to the furnace was adjusted by the PC-based control system to provide a pre-set regime of heating. The rectangular 10 x 10 x 4 mm samples were positioned in a thermal insulation casket as shown in Fig. 1. The temperature was measured by a (W+5% Re  W+20% Re) thermocouple, which contacted the top of sample in its center. No drop down in the thermocouple readings was detected at switching the microwave power off. This proved that there was no influence of the microwave field on the temperature measurement. The thermocouple was calibrated by measuring the melting temperature of a small copper plate (1083°C) heated with microwaves; the accuracy of temperature measurement at this point was ~1%. Conventional sintering was performed at the Institute of Material Science (Kiev, Ukraine) in a muffle oven. All experiments were conducted in nitrogen at a pressure of 1 bar. The main series of microwave and conventional sintering experiments was carried out at a constant heating rate of 60°C/min. Additionally, microwave heating at rates of 30 and 90°C/min was performed. At microwave heating, once the pre-set temperature was achieved, the microwave power was switched off and the sample cooled down together with the thermal insulation at a rate of about 40 to 90°C/min. In the muffle oven the cooling rate was 60°C/min. The density of the processed samples was determined by measuring their dimensions and weighing. The relative content of the D- and E-phases of Si3N4 was determined by XRD analysis [5]. The diffractograms were obtained from the side of samples that had been in contact with the thermocouple, after removing the near-surface layer of thickness of about 0.5 mm. The pore size distribution was obtained by mercury porosimetry.

Correlation Between Densification and ȕ - Phase Formation

555

Fig. 1. Schematic diagram of thermal insulation casket used in microwave processing

Results and Discussion

Densification of materials The density of samples heated to different maximal temperatures was characterized by the compaction parameter K = (V0-V)/(V0-Vm), where V0 and V are the initial and current volume of sample, respectively, and Vm is the volume of the same mass of poreless material [11]. As seen from Fig. 2, at microwave heating the compaction parameter depends mainly on the amount of additives, while at conventional heating it depends on the chemical composition of additives. Upon microwave heating, the C type material densifies at temperatures approximately 150°C higher than the B type material. Upon conventional heating, this difference is about 50°C. The curves of densification for all three types of materials are shifted to lower temperatures by 100 - 150°C upon microwave heating. This temperature shift exceeds significantly the possible experimental error of temperature measurement. To reveal the reason for acceleration of densification at microwave heating, let us consider the equation of densification at the initial stage of the liquid-phase sintering, (1/U dUedt =9JKr, where U is density, J is surface tension, K is viscosity of the material, r is radius of powder particles [8]. An increase in the density achieved at a given time, which, at a fixed heating rate, is equivalent to a given temperature, means the increase of the parameter J/K. It is not known whether microwaves can influence surface tension and viscosity of materials. It has been speculated [10] that the decrease in viscosity can be the reason of accelerated microwave sintering of Si3N4 ceramics. In the cited and other publications it was assumed that the reduction of viscosity is caused by local

556

Getman

overheating of intergranular material, as an effect of predominant absorption of microwave energy by the secondary glass phases. The selectivity of microwave heating of Si3N4 ceramics is apparent and has been proved by a study of its dielectric properties [4]. However, predominant absorption of microwave energy by intergranular layers cannot cause their considerable overheating due to fast thermal conduction over the grain size scale.

Relative densification

1.0 0.8

A-type/MW B-type/MW C-type/MW A-type/Conv. B-type/Conv. C-type/Conv.

0.6 0.4 0.2 0.0 1500

1600

1700

1800

0

Temperature, C Fig. 2. Compaction parameter (relative densification), K, versus temperature at microwave and conventional heating at a heating rate of 60°C/min

The temperature difference over the sample as a whole, which exists as a result of volumetric absorption of microwave radiation, also cannot explain the observed shift of densification curves. In the case of homogeneous absorption of microwave energy and homogeneous energy losses from the surface of the sample the temperature difference between the center and the surface of the sample can be easily estimated as 'T | (V˜«(dT/dt)«˜R)/2FS, where (dT/dt)- is the cooling rate, R is the sample size, V is the sample volume, F is the thermal diffusivity, and S is the surface area. For a sample of R = 1 cm, F | 5˜10-3 cm2/s, and experimentally measured rates of cooling, 'Ɍ | 15°C, which is much less than the observed value of the temperature shift. Thus, we come to the conclusion that the shift of densification curves results not from the temperature differences occurring upon microwave and conventional heating but probably from a direct influence of microwaves on the viscosity of a given ceramic composition. An additional argument in favor of this speculation follows from a consideration of the specific microwave power absorbed in the samples, Pabs. The values of Pabs, which were determined according to the procedure described in [3], are given in Table 1 for the B type material heated to a temperature of 1600°C at different heating rates. As seen from Fig. 3, an increase in the heating rate results in acceleration of densification at the initial stage of sintering. It should be noted that such behavior is in contrast with the behavior typical for both solid-phase microwave sintering of

Correlation Between Densification and ȕ - Phase Formation

557

oxide ceramics and conventional sintering, where an increase in the heating rate leads to a reduction in the densification rate [14]. Table 1. Heating Rate, °C/min Pabs, W/cm3

30

60

90

1.84

2.75

3.65

The temperature dependence of the compaction parameter K for the B type material at different heating rates is given in Fig. 3.

Relative densification

1.0 0.8 0.6

30 degrees/min

0.4

60 degrees/min

0.2

90 degrees/min 0.0 1500

1600

1700

1800

0

Temperature, C Fig. 3. Compaction parameter (relative densification), K, versus temperature for the B type material at different heating rates

Phase transformation As seen from Fig. 4, the DÆEphase transformation also accelerates at microwave heating. The rate of phase transformation is higher in the A type material, containing yttrium oxide, at both microwave and conventional heating. It is known that phase transformation in silicon nitride occurs as a result of dissolution of the D-phase grains in the liquid intergranular phase, nucleation of the E-phase, and growth of the E-phase grains. Liquid phase formation in the Al2O3-SiO2-Y2O3 system occurs at about 1650°C, whereas in the Al2O3-SiO2-Yb2O3 liquid formation needs higher temperatures, by about 50°C [1]. As can be concluded from the E-phase amount of samples sintered at different maximum temperatures, (Fig. 4), the temperature of formation of the E-phase upon conventional heating is consistent with these data. Upon microwave heating, the temperature for E-phase formation is decreased by 100°C, at least in the materials with large amount of additives.

558

Getman

It is known that formation of the liquid phase increases the absorption of microwave energy in the Si3N4 ceramics. The onset of the liquid phase formation is therefore marked by a drop of the microwave power needed to maintain a constant rate of heating (Fig. 5).

E-phase contents, %

70 60 50

A-type/MW B-type/MW C-type/MW A-type/Conv. B-type/Conv. C-type/Conv.

40 30 20 10 0 1550

1600

1650

1700

1750

1800

0

Temperature, C

4500

2000 1800 1600 1400 1200 1000 800 600 400 200 0

3000

1500

Power, W

0

Temperature, C

Fig. 4. The E-phase amount in samples sintered at a heating rate of 60°C/min

0 0

500

1000

1500

2000

Time, sec

Fig. 5. Microwave power needed to maintain a constant heating rate of (60°C/min) for an A type material sample

Significant retardation of phase transformation occurs upon a reduction in the content of oxide additives in the Si3N4-Al2O3-Yb2O3 system, as seen in Figure 5 for the C-type material. This is probably caused by the smaller amount of the liquid intergranular phase forming in this material.

Correlation Between Densification and ȕ - Phase Formation

559

Pore structure

Average Pore Size,Pm

The driving force for densification is inversely proportional to the average size of pores. An analysis of the pore size distribution allows one to describe phenomenologically the difference between the results of microwave and conventional sintering and explain the poor sinterability of the Si3N4 ceramics with a reduced amount of oxide additives (Fig. 6). 0.25

0.20

B-type/Conv. B-type/MW

0.15

C-type/Conv. C-type/MW

0.10 1500

1600

1700

1800

0

Temperature, C Fig. 6. Average pore size vs. temperature. Initial average pore size is 0.20 Pm and 0.17 Pm for the type B and C materials, respectively

Samples with a large amount of additives heated to 1500°C using microwave heating show a smaller average pore size of 0.17 µm as compared to conventionally heated samples with an average pore size of 0.20 Pm (Fig. 6). Upon further heating the average pore size decreases slowly, and a narrow distribution of pore and grain sizes is maintained during the entire process of densification. Upon conventional heating the average pore size increases before the onset of densification and remains larger than using microwave heating during the active phase of densification. This fact can be considered as an additional proof of an earlier formation of an intergranular liquid phase upon microwave heating as compared with the conventional process. The average pore size in the type C material increases at both microwave and conventional heating. In addition, the proportion of large pores increases upon microwave sintering. These data suggest that the difference in the porous structure evolution of type B and C materials is caused by a different amount of the liquid phase formed during microwave heating. The amount of intergranular liquid phase in the type C material is not sufficient for homogeneous mass transport in the bulk of the sample. Inevitable agglomeration in the green samples results in a faster local densification of agglomerates and growth of large pores. It results in a decrease of capillary tension and slower densification of the sample as a whole. Probably, highdensity Si3N4 ceramics with small amount of additives can be obtained at micro-

560

Getman

wave heating with the use of external pressure that would compensate the decrease in capillary tension. On the other hand, the observed difference in the rate of densification at microwave and conventional heating allows one to expect that the pressure needed for this combined type of processing will be less than the pressure typically used in hot pressing and hot isostatic pressing.

Acknowledgement The authors thank Dr. Yu. Bykov for helpful discussions.

References [1] Ashby MF, Verral RA (1973) Diffusion-accomodated flow and superplasticity. Acta met., 21, 2, pp. 149-163 [2] Bykov Yu, Eremeev A, Flyagin V, Kaurov V, Kuftin A, Luchinin A, Malygin O, Plotnikov I, Zapevalov V (1995) The gyrotron system for ceramics sintering. In: Clark DE, Folz DF, Oda SJ, Silberglitt R (eds) Microwaves: theory and application in materials processing III (Cer Trans, v.59, The American Ceramic Society, Westerville, OH), pp. 133-140 [3] Bykov Yu, Eremeev A., Holoptsev V. (1996) Comparative study of Si3N4 - based ceramics sintering at frequencies 30 and 83 GHz. In: Iskander MF, Kiggans JO, Bolomey JC (eds) Microwave processing of materials V (Mater Res Soc Symp Proc v. 430, Pittsburgh, PA), pp. 613-618 [4] Clarke DR, Ho WW (1983) Effect of intergranular phases on the high-frequency dielectric losses of silicon nitride ceramics. In: Yan MF, Hener AH (eds) Advances in ceramics, v. 7, Additives and interfaces in electron ceramics (The American Ceramic Society, Columbus, OH), pp. 246-252 [5] Gazzara CP, Messier DR (1977) Determination of phase content of Si3N4 by X-ray diffraction analysis. In: Ceramic Bulletin, v. 56, pp. 777-780 [6] Hirota M, Brito M, Hirao K, Watari K, Toriyama M, Nagaoka T (1996) Grain growth behavior during microwave annealing of silicon nitride. In: Iskander MF, Kiggans JO, Bolomey JC (eds) Microwave processing of materials V (Mater Res Soc Symp Proc v. 430, Pittsburgh, PA), pp. 441-445 [7] Hirota M, Brito M, Hirao K, Watari K, Toriyama M, Nagaoka T (1997) Microwave sintering of silicon nitride with rare earth sesquioxide additions. In: Clark DE, Sutton WH, Lewis DA (eds) Microwaves: theory and appliation in materials processing IV (Cer Tran, v. 80, The American Ceramic Society, Westerville, OH), pp. 515-522 [8] Kingery WD (1976) Introduction to ceramics. New York, Wiley. [9] Tiegs TN, Kiggans JO, Kimrey HD (1990) Microwave processing of silicon nitride. In: Snyder WB, Sutton WH, Iskander MF, Johnson DL (eds) Microwave processing of materials II (Mater Res Soc Symp Proc v. 189, Pittsburgh, PA), pp. 267-272 [10] Tiegs TN, Kiggans JO, Lin HT, Willkens CA (1994) Comparison of properties of sintered and reaction-bonded silicon nitride fabricated by microwave and conventional

Correlation Between Densification and ȕ - Phase Formation

561

heating. In: Iskander MF, Lauf RJ, Sutton WH (eds) Microwave processing of materials IV (Mater Res Soc Symp Proc v. 347, Pittsburgh, PA), pp. 501-506 [11] Tikkanen MH, Makipirtti SA (1965), In: Inter Journal of Powder Metallurgy, vol 1, No 1, pp 15-19 [12] Ueno T, Kinoshita S, Setsuhara Y, Makino Y, Miyake S, Sano S, Saito H (1999) Millimeter-wave sintering of Si3N4. In: Key Engineering Materials, vols 161-163, pp 4548 [13] Ueno T, Makino Y, Miyake S, Sano S, Saito H (1999), 3rd intern. nano ceramic forum and 2nd intern. symp. on intermaterials (NCF 3 and IMA'99), Seoul, Korea, pp. 146152 [14] Wang J, Raj R (1990) Estimate of the activation energies for boundary diffusion from rate-controlled sintering of pure alumina, and alumina doped with zirconia or titania. In: Journal of the American Ceramic Society, vol 73, No 5, pp. 1172-1175 [15] Ziegler G, Heinrich J, Wotting G (1987) Relationships between processing, microstructure and properties of dense and reaction-bonded silicon nitride. In: Journal of Material Science, No 22, pp 3041-3086

Sintering Behaviour and Mechanical Properties of Microwave Sintered Silicon Nitride Mark I Jones, Maria-Cecilia Valecillos, Kiyoshi Hirao, Manuel E. Brito, Motohiro Toriyama Synergy Materials Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Nagoya 463-8687, Japan

Introduction Microwave sintering is a technology that has received considerable interest, for both oxide [1, 2] and non-oxide ceramics [3, 4]. This is partly due to the fact that microwave sintering is reported to be capable of producing material microstructures and, therefore, properties different to those produced by conventional sintering. This phenomenon is often termed the “microwave effect” [5], although there is still debate as to whether the reported enhancements in sintering are thermal or non-thermal in nature. The heating mechanism in microwave sintering is fundamentally different to that of conventional sintering, where heat is transferred to the surface of the sample by radiation from the furnace heating elements and then conducted through the bulk material. In microwave sintering, the material couples with the microwaves and absorbs the electromagnetic energy, transforming it into heat. This interaction between the electric field and the sample means that the heating is volumetric [6]. This method of heating the material, coupled with heat losses from the sample surface, means that the temperature profile within the material is opposite to that of conventionally fired materials, with the centre being the location of highest temperature [7]. In this work, the effect of microwave heating on the sintering behaviour, microstructural development and mechanical properties of silicon nitride, a low loss ceramic where the heating is associated with absorption by the oxide additives used to aid sintering [8], has been assessed and compared with identical samples sintered conventionally.

Sintering Behaviour and Mechanical Properties of Microwave Sintered Si3N4

563

Experimental work

Powder preparation Specimens were prepared from D-Si3N4 starting powder (Grade E-10 Ube Industries Ltd, Japan), with oxide additives of 5.00 mass% Y2O3 (RU-P, Shin-Etsu Rare Earth Ltd., Japan), 3.58 mass% Al2O3 (AKP-50, Sumitomo Chemical Ltd., Japan) and 1.42 mass% MgO (1000A, Ube Industries Ltd, Japan). ). The relative amounts of Al2O3 and MgO were chosen so as to give an overall composition of 5% by mass of the spinel (MgAl2O4). The powders were mixed and planetary milled in methanol, before being dried by evaporation and vacuum oven heating. The dried powders were sieved through a 250 Pm mesh and uniaxially pressed under a pressure of 7.6 MPa before being cold isostatically pressed under 500 MPa. Sintering Microwave sintering was carried out in a nitrogen atmosphere of 0.1 MPa, in an applicator fed by a 28 GHz Gyrotron source (FDS-10-i28, Fuji Dempa Kogyo Ltd. Japan). Sample insulation included the use of silicon carbide plates, which absorb the microwaves at low temperatures, and produce uniform heating, as reported previously [9]. The samples were located inside the gyrotron in a Si3N4/BN powder bed within a double walled boron nitride crucible arrangement and surrounded by porous Al2O3. Sintering was carried out at temperatures ranging from 1100 to 1750qC with 1h holding at temperature. The temperature was monitored by a WRe thermocouple embedded in the centre of the sample. The heating rate was approximately 10°Cmin –1, and following the holding period the samples were allowed to cool in the applicator at an average cooling rate of 20°Cmin-1. Identical samples were sintered conventionally under the same heating cycles for comparison. Characterisation Sintered specimens for microstructural analysis were polished through successive grades of silicon carbide and finished with 0.5 Pm diamond slurry. Plasma etching was carried out in CF4 (Plasma reactor PR-41, Yamato Science Co. Ltd. Japan). Microstructural characterisation was carried out by scanning electron microscopy, SEM, (JSM-6340, JEOL Ltd, Japan). Phase analysis was carried out by X-ray diffraction (Model RAD-RB, Rigaku Ltd, Japan), and the relative amounts of D and E Si3N4 were determined following the method reported by Gazzara and Messier [10]. Bulk density was measured by the Archimedes method in water. Vickers Hardness was carried out with an indentation load of 98 N. Fracture strength of

564

Jones

the sintered samples was determined by 4-pt bend (JIS R1601), and fracture toughness was assessed by the indentation fracture method (JIS R1607).

Results and discussion Figure 1 shows the density and DoE transformation behaviour of the microwave and conventional sintered samples. In the microwave sintered samples, density of over 97% theoretical density was obtained at a sintering temperature of 1450qC, whereas in the conventionally sintered material, the same density was not achieved until 1650qC. The transformation was also enhanced in the microwave sintered materials, with full transformation occurring at temperatures over 100qC lower than that in the conventional material. 100 100

Density / % TD

80 60 60 40

40

20 0 1000

Microwave Conventional 1100

1200

1300

1400

1500

1600

1700

1800

20

ESi3N4 Content / wt.%

80

0 1900

Sintering Temperature / oC

Fig. 1. Densification and DoE transformation curves for microwave and conventionally sintered Si3N4 ceramics. Filled symbols represent density and open ones show E content. Microwave sintering resulted in around 200°C reduction in required sintering temperature.

The Vickers hardness and fracture toughness of selected samples are plotted against sintering temperature in Figure 2. Both the conventional and microwave sintered samples showed increasing fracture toughness with sintering temperature, up to values of around 6 MPam½. This was accompanied by a decrease in hardness from 17 GPa to around 15 GPa at the highest sintering temperature. Although the absolute values of the two sets of samples were similar, again in the case of the microwave sintered materials the maximum properties were achieved at temperatures around 200qC lower than the conventional material.

20

10

18

9

16

8

14

7

12

6

10

5

8

4

6

3

4

Microwave Conventional

2 0 1100

1200

1300

1400

1500

1600

1700

1800

2 1

565

Fracture Toughness / MPa m 1/2

Vickers Hardness / GPa

Sintering Behaviour and Mechanical Properties of Microwave Sintered Si3N4

0 1900

Sintering Temperature / oC

Fig. 2. Vickers hardness (filled symbols) and fracture toughness (open symbols) of the sintered materials. Similar properties were achieved in the microwave sintered materials at temperatures around 200°C lower than those sintered conventionally.

From these two figures it can be observed that microwave sintering of Si3N4, with the Y2O3, Al2O3, MgO additive system, results in a sintering enhancement of around 200°C, in both densification and transformation, and in maximum values of mechanical properties, when compared to conventionally sintered material. Other authors have noted similar reductions in sintering temperature for Si3N4 ceramics sintered with Y2O3 and Al2O3 [11]. The room temperature bending strength of the microwave sintered samples is given in Table 1. As with the fracture toughness, the strength increased with sintering temperature, again showing maximum values at a sintering temperature of 1650°C. Table 1. Room temperature bending strength of the microwave samples Sintering Temp (°C) 1200 1375 1450 1600 1650 1750

E-Si3N4 (wt% ) 10 17 18 78 100 100

Bending Strength (MPa) 471 475 442 691 738 729

The question that always arises in the application of microwave radiation to the sintering of ceramic materials is the accuracy of the temperature measurement. Temperature measurement is undoubtedly a non-trivial problem, but whilst the enhancements in sintering are described simply in terms of a reduction in sintering temperature, this debate will continue. The issue, then, is whether microwave enhanced effects are simply to be described in terms of the reduction in sintering

566

Jones

temperature which leads to similar microstructures, 'T, as suggested by some authors [12] or whether a difference can be observed in the densification mechanism between conventionally sintered and microwave sintered samples. During the sintering of Si3N4, the D-Si3N4 starting powder is dissolved in the liquid phase that forms between the oxide additives and the SiO2 present on the surface of the Si3N4 powder. This is followed by precipitation of E Si3N4 on preexisting E nuclei in the material. Difference in the growth rates on different crystallographic planes in E-Si3N4 results in one-dimensional growth in the length direction [13]. The DoE transformation process thus provides the possibility of discussing the ceramics at the same stage of sintering, assuming that the densities are comparable. Figure 3 shows the microstructures of the conventionally and microwave sintered samples at the same stage of sintering, i.e. at similar density and stage of the transformation process.

Fig. 3. SEM micrographs of the microstructure in the conventionally (a, b) and microwave sintered (c, d) materials at the same stage of the DoE transformation. Development of elongated E grains were observed at a much earlier stage in the transformation process in the case of microwave sintering.

From this figure it can be seen that elongated E-Si3N4 grains were observed in the microstructure at a much earlier stage in the transformation process during microwave sintering. Such grains were observed at E contents as low as 10% in the case of the microwave sintered samples. At full transformation the microstructures were similar, although this may be as a result of increased grain impingement in the case of the microwave material, restricting further growth. It has been reported [14] that microwave sintering of silicon nitride resulted in microstructures that were significantly refined in comparison with conventionally sintered material. It was suggested in this work that this may be due to the selec-

Sintering Behaviour and Mechanical Properties of Microwave Sintered Si3N4

567

20

10

18

9

16

8

14

7

12

6

10

5

8

4

6

3

4

Microwave Conventional

2 0 0

20

40 60 E Si3N4 Content / Wt. %

80

2 1

Fracture Toughness / MPa m1/2

Vickers Hardness / GPa

tive heating of the oxide sintering additives resulting in a higher E nucleation rate caused by a temperature gradient driven dissolution process. However most researchers report enhanced grain growth for microwave sintered Si3N4 [4, 8, 15], and it is possible that selective heating of the grain boundary phase could actually lead to a lower nucleation rate due to dissolution of the pre-existing E nuclei in the starting powder. It has been shown that small E particles can dissolve during the solution precipitation process due to changes in the equilibrium concentrations of Si3N4 in the liquid phase at the particle surface [16] due, for example, to the growth of an adjacent larger crystal. The critical size, below which E particles dissolve, is shifted to higher values with increased nitrogen solubility in the glass phase [17]. Since the solubility of nitrogen in the glass increases with temperature [18], localised heating of the grain boundary phase, during microwave sintering, could therefore lead to fewer nucleation sites, and the possibility of the enhanced development of the precipitated grains. Despite the fact that the use of microwave sintering led to enhancement in the densification and transformation of the silicon nitride ceramics, the trend in mechanical properties of the two sets of samples were very similar (Fig. 2), both showing increasing fracture toughness and decreasing hardness with sintering temperature. The bending strength of the microwave sintered samples also increased with sintering temperature. These results are of course related to the formation of E Si3N4 and the development of elongated grains, which contribute to toughening mechanisms such as crack bridging [19]. The mechanical properties of the samples as a function of E content are shown in Figure 4. Both sets of samples exhibited the maximum in fracture toughness at a temperature corresponding to the end of the DoE transformation process. Further sintering tends to reduce fracture toughness by decreasing the aspect ratio of the elongated grains [20]. The fracture toughness of the microwave sintered specimens was slightly higher than that of the conventional, possibly due to the enhanced development of such elongated grains.

0

100

Fig. 4. Mechanical properties of the sintered bodies as a function of E Si3N4 content.

568

Jones

Summary Silicon nitride ceramics with Y2O3, Al2O3 and MgO as sintering aids were produced by microwave sintering at 28 GHz. The densification, DoE transformation and maximum mechanical properties of these samples were observed to occur at temperatures of around 200°C lower than conventionally sintered material. In addition the microstructural development showed differences, with elongated E grains being observed at a much earlier stage of the transformation process. The fracture toughness and strength of the sintered bodies increased with E content for all samples, showing maximum values at the end of the transformation process, and the enhancement in the development of elongated E grains with microwave sintering resulted in slightly higher toughness values.

Acknowledgements This work has been carried out under the STA Fellowship Program managed by the Japan Science and Technology Corporation (JST) in co-operation with the Japan International Science and Technology Exchange Center (JISTEC).

References [1] Wroe R, Rowley AT (1996) Evidence for a non-thermal microwave effect in the sintering of partially stabilized zirconia. J Mater Sci 31:2019-2026 [2] Janney MA, Kimrey HD, Schmidt MA, Kiggans JO (1991) Grain growth in microwave annealed alumina. J Am Ceram Soc 74:1675-1681 [3] Hu G, Zhuang H, Li W, Wu F (1997) Microwave sintering of D/E-Si3N4. J Eur Cer Soc 17:977-981 [4] Kim YC, Kim CH, Kim DK (1997) Effect of microwave heating on densification and Do E phase transformation of silicon nitride. J Eur Cer Soc 17:1625-1630 [5] Clark DE, Folz DC, West JK (2000) Processing materials with microwave energy. Mat Sci Eng A287:153-158 [6] Sutton WH (1989) Microwave processing of ceramic materials. Cer Bull 68:376-386 [7] Birnboim A, Carmel Y (1999) Simulation of microwave sintering of ceramic bodies with complex geometry. J Am Ceram Soc 82:3024-3030 [8] Tiegs TN, Kiggans JO, Kimrey HD (1991) Microwave sintering of silicon nitride. Cer Eng Sci Proc 12:1981-1992 [9] Jones MI, Valecillos M-C, Hirao K (2001) Role of specimen insulation on densification and transformation during microwave sintering of silicon nitride. J Ceram Soc Jpn 109:762-766 [10] Gazzara CP, Messier DR (1977) Determination of phase content of Si3N4 by X-ray diffraction analysis. Cer Bull 56:777-780 [11] Kim YC, Koh SC, Kim DK, Kim CH (1995) Effect of microwave heating on the sintering of silicon nitride-doped with Al2O3 and Y2O3. In: Clark DE, Folz DC, Oda SJ,

Sintering Behaviour and Mechanical Properties of Microwave Sintered Si3N4

569

Silberglitt R (eds) Microwaves: Theory and application in materials processing III. American Ceramic Society, Westerville, OH, pp 415-22. [12] Boch Ph, Lequeux N (1997) Do microwaves increase the sinterability of ceramics? Solid State Ionics 101-103:1229-1233 [13] Hoffmann MJ (1994) Analysis of microstructural development and mechanical properties of Si3N4 ceramics. In: Hoffmann MJ, Petzow G (eds) Tailoring of mechanical properties of Si3N4 ceramics. Kluwer Academic Publishers, Netherlands, pp 59-72 [14] Plucknett KP, Wilkinson DS (1993) Microstructural characterization of microwave sintered silicon nitride ceramics. Mat Res Soc Proc 287:289-294 [15] Hirota M, Brito ME, Hirao K, Watari K, Toriyama M, Nagaoka T (1997) Microwave sintering of silicon nitride with rare earth sesquioxide additions. In: Clark DE, Sutton WH, Lewis DA (eds) Microwave: Theory and application in materials processing IV. American Ceramic Society, Westerville, OH, pp 515-522 [16] Dressler W, Kleebe H-J, Hoffmann MJ, Rühle M, Petzow G (1996) Model experiments concerning abnormal grain growth in silicon nitride. J Eur Cer Soc 16:3-14 [17] Kitayama M, Hirao K, Toriyama M, Kanzaki S (1999) Control of E Si3N4 crystal morphology and its mechanism (Part 1)-effect of SiO2 and Y2O3 ratio. J Ceram Soc Jpn 107:930-934. [18] Kleebe H-J (1997) Structure and chemistry of interfaces in Si3N4 ceramics studied by transmission electron microscopy. J Ceram Soc Jpn 105:453-475 [19] De Arellano-Lopez AR, McMann MA, Singh JP, Martinez-Fernandez, J (1998) Microstructure and room-temperature mechanical properties of Si3N4 with various D/E phase ratios. J Mat Sci 33:5803-5810 [20] Pyzik AJ, Carroll DF, Hwang CJ (1993) The effect of glass chemistry on the microstructure and properties of self reinforced silicon nitride. Mat Res Soc Proc 287:411416

Millimeter-Wave Sintering of High Pure Alumina – Microstructure and Mechanical Properties Yukio Makino1, Shoji Miyake1, Saburo Sano2, Hidenori Saito3, Bunkei Kyoh4, Hideki Kuwahara4 and Akinobu Yoshikawa4 1

Joining and Welding Research Institute, Osaka University, 11-1, Mihogaoka, Ibaraki, Osaka, 567-0047, Japan. 2 National Institute of Advanced Industrial Science and Technology, 2266-98 Anagahora, Shimoshidami, Moriyama-ku, Nagoya, Aichi, 463-8560, Japan 3 Kanagawa High-Technology Foundation, 3-2-1, Sakato,Takatsu, Kawasaki, Kanagawa 213-0012, Japan 4 Kinki University, 4-1, 3 chome, Kowakae, Higashiosaka, Osaka, 577-0818, Japan

Abstract Characteristics of millimeter-wave (MM) sintering were investigated in high pure alumina from the standpoints of their structural and mechanical properties, comparing with the conventionally-sintered alumina. Nearly fully-densified pure alumina was obtained at 1250°C in the MM-sintering, indicating the decrease of about 250°C in the sintering temperature in comparison with the conventional sintering. From SEM observation and XRD measurement, it was found that similar grain sizes ranging from 2 Pm to 3 Pm and similar crystallite sizes were detected in both aluminas, which were sintered at 1250°C by MM-heating and at 1500°C by conventional heating. Further, remarkable growth of crystallites was observed in the MM sintering at 1200°C. MM-sintered aluminas showed the average bending strength higher than 800 MPa, whereas conventionally-sintered aluminas showed only the average bending strength less than 600 MPa. It is concluded that the high bending strength of MM-sintered alumina is not attributed to the differences of both grain and crystallite sizes.

Introduction Recently, heating processes based on millimeter-wave(MM) radiation have been widely interested in sintering of various ceramics on account of its inherent characteristics such as the weak temperature dependence of dielectric constant of ceramics in the MM region and easily designing of applicator [1]. It has been also

Millimeter-Wave Sintering of High Pure Alumina

571

suggested to be able to suppress grain growth by the effect due to electromagnetic field, so-called “microwave non-thermal effect”. Further, many investigators have reported that ceramics powder compacts can be densified at a lower temperature than those in conventional methods [2 - 4]. In our previous works on MM sintering [5, 6], we have reported that nearly full densification of alumina-zirconia and silicon nitride was obtained at fairly lower temperatures than those in their conventional sintering. Thus, the capability of MM sintering has been indicated in various ceramics and their composites. However, the MM effect on mechanical properties, especially on the relation of mechanical properties with microstructure is still unclear. In order to clarify the unsolved problems on MM sintering, it is necessary to examine the effect of MM heating on the microstructure and mechanical properties by selecting wellcharacterized ceramic powder. In the present study, high pure alumina was selected among various candidate ceramics because alumina is one of the most popular and its well-characterized commercial powder is easily available. In pure aluminas sintered by both the MM and 2.45 GHz radiation, the lowering of densification temperature has been observed in the previous papers [7, 8]. On the suppression of grain growth, however, contradictory results have been reported in MM sintering of alumina [9, 10], whereas no remarkable difference has been observed in the 2.45 GHz microwave sintering [11]. Further, the microstructure and mechanical properties of MM-sintered alumina have been not well investigated as far as the authors examined. In the present study, sintering of high pure alumina was performed by both MM and conventional heating in order to clarify characteristics of MM sintering from structural and mechanical standpoints.

Experimental Procedures Alumina powder (AKP-20; average size; 0.55 Pm, Sumitomo Chemical Industries Co. Ltd.) was used as the high pure alumina. Powder compacts were prepared by the slip casting method. The sizes of as-cast bodies were 52 mm in diameter and 6~8 mm in thickness. These as-cast bodies were dried at room temperature and calcined at 800°C for 60 min. in air. These calcined bodies were sintered with MM heating and conventional methods, respectively. MM sintering was done in nitrogen of 0.1 MPa at 1200°C and 1250°C. Heating and cooling rates were fixed at 20°C/min.. A high power 28 GHz gyrotron generator combined with multimode applicator (Fuji Denpa Kogyo, FGS-10-28) was used for MM sintering. In the MM sintering, alumina fiberboard and BN board were used for thermal insulation. Temperature of the specimen was measured by contacting a Mo-sheathed thermocouple of W/Re with the under side of the specimen. Conventionally sintered aluminas were made by using an electric furnace with same sintering conditions, except for the sintering temperature (1500°C) and the usage of air. Densities of sintered aluminas were measured with Archimedean method. Microsructures of these aluminas were observed by a field emission SEM (Hitachi,

572

Makino

S-4500). Bending strength of sintered alumna was determined by the three-point bending test in accordance with JIS R-1601. Further, crystallite size of sintered alumina was estimated from XRD line broadening of the (113) peak of D-alumina using the following Scherrer’s equation; (1)

D=0.9O/EcosT

where D, O, E andTarethe crystallite size(Å), wavelength of x-ray(Å), full width at half maximum(rad.) and diffraction angle(deg.), respectively.

Results and Discussion First, temperature dependence of the densification of alumina was examined in both millimeter-wave and conventional heating methods. It was found that MM heating enable to sinter the high pure alumina AKP-20 at about 250°C lower temperature than that in the conventional heating. Therefore, further experiments were mainly performed at 1250°C for MM sintering and at 1500°C for the conventional sintering. Dependence of densification on the sintering time is shown in Fig. 1, together with the result on MM sintering at 1200°C. Except for the results on shorter sintering time than 20 min., similar densification curves were observed in both MM and conventionally sintered aluminas. Thus, the high pure alumina (AKP-20) can be densified over 97%TD at 1250°C by MM heating. Further, nearly full density can be obtained at a short sintering time around 30 min.

Relative density (%TD)

100

95

90 MM sintering 1200oC

MM sintering 1250oC

Conv sintering 1500 oC

85

0

30

60

90

120

Sintering time (min)

Fig. 1. Dependence of relative densities on the sintering time in the high pure aluminas sintered with millimeter-wave and conventional heating methods.

Grain sizes of MM- and conventionally-sintered aluminas were examined from the SEM observation. The grain sizes of aluminas MM-sintered at 1250°C ranged

Millimeter-Wave Sintering of High Pure Alumina

573

from 2 Pm to 3 Pm and these sizes were similar to those of aluminas conventionally-sintered at 1500°C. Typical SEM photographs are shown in Fig. 2. While, as shown in Fig. 2, the grain sizes of aluminas were suppressed up to 1 Pm when the sintering temperature was 1200°C, irrespective of slightly lower densities of their aluminas than those sintered at 1250°C. Remarkable suppression of grain growth in MM-sintered alumina was reported by Bruce et al. [10], though the sub-micron size alumina (AKP-50) finer than AKP-20 was used as the starting powder. According to their results, grain size was 1.0 Pm in the alumina sintered at 1495°C in oxygen. When the grain sizes of MM-sintered AKP-20 aluminas are compared with those reported by Bruce et al., we can obtain the relation between grain size and relative density, as given in Fig. 3. The data for MM-sintered AKP-20 aluminas are plotted between the data for the conventionally sintered aluminas and MM-sintered AKP-50 aluminas. The result is quite reasonable because the average grain size of AKP-20 alumina (0.55 Pm) is about twice as large as that of AKP-50 (0.25 Pm).

Fig. 2. SEM photographs of aluminas sintered by millimeter-wave and conventional heating methods.

Subsequently, the bending strengths of sintered aluminas measured by the three-point bending test are shown in Fig. 4. Average bending strengths higher than 800 MPa were obtained in the aluminas MM-sintered at 1250°C, except for the case in 0 min., while conventionally sintered alumina showed the bending strength less than about 600 MPa. Further, the maximum bending strength over 900 MPa was obtained in MM-sintered alumina. As described above, it is indicated from the SEM observation that the sizes of alumina MM-sintered at 1250°C are similar to those of aluminas conventionally-sintered at 1500°C. Accordingly, similar bending strengths must be obtained from these aluminas if the grain size is predominantly effective for the bending strength. However, a great difference of more than 200 GPa was observed in the bending strength. Therefore, it is concluded that the great difference is not attributed to the difference of grain size. The

574

Makino

reason for high bending strengths of MM-sintered aluminas probably arises from the structural difference in grain and/or grain boundary. 10 AKP-20(MM/28GHz,1250oC) AKP-20(MM/28GHz,1200oC) AKP-50(MM/35GHz)

Grain size, [Pm]

After Coble

1

0.1

50

60

70

80

90

100

Relative density, [%TD]

Fig. 3. Relation between grain size and relative density in the aluminas sintered by the millimeter-wave heating. (Data on AKP-50 and after Coble are quoted from [10].) 1000

Bending strength (MPa)

800

600

400

MM sintering 1200 oC

200

MM sintering 1250 oC Conv sintering 1500 oC

0 0

30

60

90

120

150

Sintering time (min.)

Fig. 4. Bending strengths of high pure aluminas(AKP-20) sintered by the millimeter-wave and conventional heating methods.

In order to examine the difference in grain structures of MM- and conventionally-sintered aluminas, their crystallite sizes were estimated from the full width at half maximum of XRD peak due to (113) plane of D-alumina using the Scherrer’s

Millimeter-Wave Sintering of High Pure Alumina

575

equation. As given in Fig. 5, no remarkable difference was observed between the crystallite sizes of aluminas sintered at 1250°C by MM heating and at 1500°C by conventional heating. Roughly speaking, the similar crystallite size corresponds to the similar order of subgrain size. Accordingly, the alumina grain with a similar number of crystallite shows a similar order of strength. Thus, it is suggested that the higher bending strengths of MM- sintered aluminas than those of conventionally-sintered aluminas is not caused by the strength of grain itself but by others reasons such as strengthened grain boundary. The formation of well-crystallized boundary may be suggested as indicated in the previous paper [12]. 90 1200oC(MM)

Crystallite size, D[nm]

80

1250oC(MM) 1500oC(conv)

70

60

50

40

88

90

92

94

96

98

100

Relative density, [%TD]

Fig. 5. Dependence of crystallite size on relative density in aluminas obtained from millimeter-wave and conventional sintering methods.

In XRD measurement of the aluminas sintered at 1200°C by MM heating, remarkable growth of crystallite size was found at a longer sintering time than 60 min.. Further, suppression of grain growth (up to 1 Pm at most) was also found from SEM observation of the same aluminas in spite of their slightly lower densities than those sintered at 1250°C. Thus, the quite different effect of MM heating was observed in the grain and crystallite growths, respectively. In other words, MM heating enhances the coalescence of crystallites slightly below the critical temperature, which lies in between 1200°C and 1250°C in pure alumina, whereas it enhances the grain growth just above the critical temperature. Finally, it is indicated that the millimeter-wave process has the capability of controlling the microstructure of ceramics as well as the structure of grain itself.

576

Makino

Summary Characteristics of millimeter-wave (MM) sintering was investigated by examining the difference of structural and mechanical properties in high pure aluminas sintered by MM and conventional heating methods. Decrease of densification temperature for the pure alumina sintered by MM-heating was around 250°C, comparing with that in conventional sintering. Similar densities with more than 97%TD (theoretical density) were obtained at 1250°C by MM-heating method and at 1500°C by conventional heating method. It was found that these aluminas showed similar grain sizes ranging from 2 Pm to 3 Pm from the SEM observation. Similar crystallite sizes were also detected from the measurement of line broadening of D-alumina (113) XRD peak. MM-sintered aluminas showed the average bending strength higher than 800 MPa, whereas conventionally-sintered aluminas showed only the average bending strength less than 600 MPa. It is concluded that the high bending strengths of MM-sintered aluminas are not attributed to the differences of both grain and crystallite sizes. Further, it is indicated that MMheating enhances the coalescence of crystallites slightly below the critical temperature, which lies in between 1200°C and 1250°C.

References [1] T.Saji, Meter. Res. Soc. Proc., Vol.430(1996)p15-20. (Microwaves: Theory and Application in Materials Processing V, ed by M.F.Iskander et al.) [2] M.A.Janney and H.D.Kimrey; Meter. Res. Soc. Proc., 189(1991) p.215-227. (Microwave Processing of Materials II, ed. by W.B.Snyder et al.) [3] H.D.Kimrey, J.O.Kiggans, M.A.Janney and R.L.Beatty; Meter. Res. Soc. Proc., 189(1991) p.243-255. (Microwave Proc. of Materials II, ed. by W.B.Snyder et al.) [4] Y.C.Kim, S.C.Koh, D.K.Kim and C.H.Kim; Ceramic Transactions Vol.59(1995) p415-422. (Microwaves: Theory and Application in Materials Processing III, ed. by D.E.Clark et al. ) [5] Y.Makino, T.Ohmae, Y.Setsuhara, S.Miyake and S.Sano; Key Engineering Materials Vol.161-163(1999)41-44. [6] T.Ueno, Y.Makino, S.Miyake, S.Sano and H.Saito; 3th Int. Nano Ceramic Forum and 2nd Int. Symp. on Intermaterials(NCF 3 & IMA ’99), June, Seoul, Korea, 1999, p.146152. [7] Z.Xie, J.Yang and Y.Huang; Materials Letts, 37(1998)215-220. [8] Y.Makino, T.Ueno, T.Matsumoto and S.Miyake; Japn. J. Appl. Phys., 40(2001)10801082. [9] T.T.Meek, R.D.Blake and J.J.Petrovic; Ceramic Eng. Sci. Proc., 8(1987)861-871. [10] R.W.Bruce, A.W.Fliflet, R.P.Fischer, D.Lewis,III, B.A.Bender, G.-M. Chow, R.J.Rayne, L.K.Kurihara and P.E.Schoen; Ceramic Trans., Vol.80(1997)287-293. [11] Z.Xie, J.Yang, X.Huang and Y.Huang; J.Euro. Ceram. Soc., 19(1999)381-387. [12] J.Koike, S.Tashima, S.Wakiya, K.Maruyama and H.Oikawa; Mater. Sci. and Eng., A220(1996)26-34.

Microwave Sintering of Large-Size Ceramic Workpieces S. V. Egorov, N. A. Zharova, Yu. V. Bykov, V. E. Semenov Institute of Applied Physics, Russian Academy of Sciences, Nizhny Novgorod, Russia

Introduction Volumetric nature of microwave heating makes it possible to increase the heating rate of large-size objects. At the same time temperature gradients are known to be inherent in microwave heating and increase with a sample size. Therefore a problem of microwave sintering of large-size ceramic specimens is a challenging one to solve. In principle, one can reduce undesirable temperature gradients increasing a thickness of thermal insulation. However, this approach is applicable to the sintering of relatively small ceramic samples used in laboratory study but hardly valid for industrial application. Another method, already used on the industrial scale [5], is the use of additional heating from outside of material under processing, so-called hybrid heating. The similar purpose can be achieved using special, highly absorbing microwave radiation susceptors around the sample [3]. In the latter case it is possible to reduce temperature gradients without additional sources of energy. Finally, one can suggest using such material in the thermal insulation layer that absorbs microwave energy itself thereby providing the necessary reduction in the thermal flow from the sample. The solution requires optimization over many variables and should be found individually in each particular case. The experimental way to this solution is time- and labor-consuming. Therefore, computer simulation appears to be an important step of the optimization procedure. This paper focuses on the results of numerical simulation for the millimeter-wave sintering process in a supermultimode cavity and comparison of these results with the respective experimental data.

Experimental setup The experiments on millimeter-wave sintering of large-size specimens were performed in a 30 GHz 10 kW gyrotron system [1, 2] with the cavity volume above 100 liters. The temperature of the sample was measured by Pt - Pt-Rh thermocou-

578

Egorov

Al2O3 fibrous insulation

x x

Al2O3 -sample

Pt  Pt-Rh thermocouples 240mm

50mm

ples. To test the results of numerical modeling, a series of sintering experiments was done with alumina-based ceramic samples of simple shape (rectangular plate with size 130 u 130 u 16 mm3). In these experiments the temperature was measured not only on the surface of the sample but also in its center (see Fig. 1). The surface temperature data, Ts, were used to control the gyrotron output power to achieve any desirable heating–cooling schedule. The measured temperature difference between the center and the surface, 4 = Tc-Ts, as well as the power required to maintain the targeted temperature-time schedule, were compared with the respective data from calculations. The sample was positioned inside the thermal insulation arrangement made up as a box (parallelepiped) with the dimensions 300 u 300 u 240 mm3 and wall thickness 50 mm (see Fig.1).

solid Al2O3 plate 300x300mm

Fig. 1. The schematic diagram of the sintering experiment arrangement (the sample positioned in the thermal insulation box)

Various types of fibrous alumina with different microwave absorptivity were used for the walls of the thermal insulation box. The preliminary experiments have shown that better uniformity of the surface temperature was achieved when an air gap existed between the sample and the inner surface of the insulation box. At the same time a temperature difference between the top and bottom faces of the sample was revealed in the first experiments (the top was hotter than the bottom). Subsequently, an additional plate of dense alumina ceramics was placed beneath the sample as it is shown in Fig. 1, to equalize the temperature.

Software description The numerical simulation was based on the MICROS code developed recently by the authors [6] for modeling microwave heating and densification of a compacted

Microwave Sintering of Large-Size Ceramic Workpieces

579

powder sample inside an overmoded cavity fed by a beam of millimeter-wave radiation. Within this code the spatial distribution of the electromagnetic field intensity throughout the volume of the cavity containing the ceramic sample was computed by the method of averaged geometric optics [4]. The code accounted for the changes in the dimensions of the sample and its dielectric properties in the course of sintering. The computed data on the field intensity distribution were used by another part of the code, which solved a 3D heat conduction equation taking into account volumetric heat sources due to microwave absorption inside both the sample and thermal insulation arrangement. To calculate the rate of the sample densification, the code used the dependence of density on temperature that was obtained in previous experiments with the same ceramic material and the same heating schedule (Fig. 2). A solution of the thermal problem was performed taking into account heat exchange between the sample and inner surface of the insulation box caused by both thermal radiation and convection. The intensity of the latter process was assumed to be proportional to the temperature difference between the surfaces of the sample and the insulation box. Similar to the experimental system the running code adjusted the input microwave power to implement any prescribed regime of heating, i.e. a temperature schedule at a fixed point of the sample. The data on thermal conductivity and thermal capacity of the insulation material were taken from separate experiments. The respective data for the ceramic material were obtained using the parameters of dense alumina available from the literature corrected with regard to the relative density of ceramic material. The first stage of calculations used simple dependencies of microwave absorptivity of the ceramic material on its density and temperature. These data only allowed us to estimate roughly the level of microwave absorptivity of the thermal insulation material that is necessary to avoid excessive temperature drop over the sample (see Fig. 3). Within this study a comparison between the results of calculations and experimental measurements was used to get more accurate data both on the thermal and dielectric properties of the ceramic material. Fig. 4 demonstrates the degree of agreement between the experimental and computed results achieved after such a fit of the ceramic properties. The obtained data can be further used to optimize the process of microwave sintering of any ceramic sample of arbitrary shape prepared from the same material. An experimental result of the optimized sintering procedure is presented on Fig. 5, where a photograph of a sintered alumina “spindle” (with 170 mm in height, 85 mm in the largest diameter, and about 1 kg in weight) is shown. This sample was successfully sintered (without noticeable shape distortion) using the millimeter-wave heating.

580

Egorov

density of sample (kg/m3)

360 340 320 300 280 260 240 220 200 0

500

T,C

1500

1000

Fig. 2. The experimentally derived dependence of density on temperature. 350

4, C

300 250

200

1

150 100 2

50 0 -50

0

1000

500

1500

T, C Fig. 3. Evolution of the temperature difference 4 vs. Ts computed for low (curve 1) and relatively high (curve 2) microwave absorption in the material of the thermal insulation. Both curves were obtained for the same heating rate dTs/dt = 10°C/min

Microwave Sintering of Large-Size Ceramic Workpieces 300

Temperature, C

Power, W

581

3000

250

2500

200

2000

150

1500

3

100

1000

2

50 0 -50

500 0

1 0

2000

6000

4000

8000

10000

t, s

Fig. 4. Measured (experimental data shown by triangles) and calculated (curve 1) evolution of the temperature difference 4 vs. time t. Presented are the results of calculation obtained with the fitted ceramic properties. Curve 2 is the optimized heating schedule Ts(T)/10 (the heating rate was increased at an intermediate value of temperature in order to reduce the temperature difference). The input power schedule is shown by asterisks (measured) and curve 3 (computed).

Fig. 5. A 1 kg, high pure Al2O3-specimen sintered in a 30 GHz gyrotron system at the sintering temperature 1600°C and a hold time of 20 min

582

Egorov

References [1] Bykov Yu, Eremeev A, Flyagin V, Kaurov V, Kuftin A, Luchinin A, Malygin O, Plotnikov I, Zapevalov V (1995) The gyrotron system for ceramics sintering In: Clark DE, Sutton WH, Lewis DA (eds) Microwaves: Theory and Application in Materials Processing III, pp 133-140 [2] Bykov Yu, Denisov G, Ereemev A, Gol'denberg A, Holoptsev V, Luchinin A, Semenov V (1999) Upgraded gyrotron system for millimeter-wave processing of materials In: Proceedings of the 29th European Microwave Conference, Munich, Germany, vol 1, M-TuP1, pp 123-126 [3] De A, Ahmad I, Whitney ED, and Clark DE (1991), Microwave (hybrid) heating of alumina at 2.45 GHz: I. Microstructural uniformity and homogeneity In: Clark DE, Sutton WH, Lewis DA (eds) Microwaves: Theory and Application in Materials Processing (Ceramic Transactions, vol 21), pp 319-328 [4] Semenov VE, Pozdnyakova VI, Shereshevskii IA, and Zharova NA (1999) Microwave field distribution inside an oversized cavity. Comparison of calculations within the ray tracing method with exact solutions. In: Catala-Civera JM, Penaranda-Foix FL, Sanchez-Hernandez D, Reyes E (eds) Proceedings of the 7th International Conference on Microwave and High Frequency Heating, Valencia, Spain: Servicio, pp 57-60 [5] Wroe R (1997) Microwave -assisted firing of ceramics In: Clark DE, Sutton WH, Lewis DA (eds) Microwaves: Theory and Application in Materials Processing IV (Ceramic Transactions, vol 80), The American Ceramic Society, Westerville, pp 671-678 [6] Zharova NA, Rybakov KI, Semenov VE, Egorov SV (2001). Computer simulation of millimeter-wave sintering of ceramic and composite materials. In: Clark DE, Sutton WH, Lewis DA (eds) Microwaves: Theory and Application in Material Processing V (Ceramic Transactions, vol 111) The American Ceramic Society, Westerville, pp 11-18

Microwave Assisted Sintering of Al2O3 S. Leparoux1, G. Walter1; Th. Lampke2, B. Wielage2 1

Freiberg University of Mining Technology, Institut of Thermodynamics, Freiberg, Germany 2 Chemnitz University of Technology, Institut of Composite Materials, Chemnitz, Germany

Introduction Firing or sintering of ceramics is one of the most critical stages of ceramic processing. In order to avoid damage to the specimen due to thermal stresses, a precisely controlled homogeneous heating is required. The optimized combination of microwave (MW) and conventional heating – called Microwave Hybrid Heating (MHH) - should be a suitable way to prevent or reduce the thermal stresses within the specimen. It is considered that microwave energy operates directly in the specimen while the conventional energy is first applied to the surface of the sample and its center is heated by thermal conduction. The conventional heating accomplishes two roles. On one hand, it can be used to achieve the temperature range where the material (e.g. Al2O3, TiO2, etc.) begins to absorb microwave energy. On the other hand, when both energy sources are applied, their balance (tuning) must be controlled to prevent surface heat loss which causes temperature differences between There are two ways to apply the microwave energy to the material in order to influence the temperature dependent dielectric properties: 1. “Consecutive microwave assisted firing” The material is first heated by conventional energy to achieve the temperature range where microwave energy can be absorbed. Starting from this point, microwave energy is applied. 2. “Additive microwave assisted firing” Simultaneous heating by conventional and microwave energy with controlled ratio between the two energy sources. In both cases, the ratio used must be determined for every material as it depends on the temperature and the dielectric properties. It usually changes during the sintering process and it has to be optimized to achieve uniform temperature and to avoid excessive energy consumption (thermal runaways, hot spots) [1]. The principal material benefit of the microwave assisted firing process is the nearly homogeneous, fine grain size due to the improved temperature uniformity within the components [2, 3]. On the other hand, the benefit for the production it-

584

Leparoux

self is very important. Less failure in production and reduction in process-time leads to high energy efficiency and therefore to reduced costs. Most of these advantages have been confirmed in this work on Al2O3.

Experimental Procedure

Apparatus In a lab-scale Linn microwave furnace (Linn High Therm GmbH, Germany), a gas burner with maximum power of 5 kW has been installed as additional conventional heating source. The 6 kW, 2450 MHz cavity was thermally insulated with aluminosilicate fibre boards in order to reduce thermal losses. With respect to previous works on low absorbing insulating materials [4], Rath KVS 164 material was chosen. The effective sintering chamber was of cylindric shape with both diameter and height of 150 mm and was located within the microwave cavity. The temperature measurement of the product was done in the range of 600 to 2000°C by a pyrometer. Thermocouples were not used in order to prevent perturbations of the microwave field [5]. A view inside the sintering chamber and on the microwave furnace is given in Figure 1.

Fig. 1. a) sintering chamber of KVS 164

b) microwave furnace

Procedure Alusuisse Martinswerk alumina powder (Martoxid) with a primary particle size of 0.6 to 0.8 µm was used. Samples of 25 mm diameter were made by cold isostatic pressing of Al2O3 powder to ensure constant green density (60%TD). The cylinder obtained was cut with a diamond saw into 10 mm thick samples. The heating schedule of the MHH-furnace was optimized with respect to the following parameters: x sintering temperature,

Microwave Assisted Sintering of Al2O3

585

soaking time, ratio between radiant and microwave energy. A heating rate of 30 K/min was chosen to minimize the processing time i.e. the energy costs. It is known that Al2O3 absorbs microwave energy of 2.45 GHz frequency at temperatures above 900 – 1000°C [6]. Thus the microwave power was not applied before it was really effective for the chosen material. x x

Results and discussion In order to achieve the highest density, the optimized sintering process parameters were 1550°C sintering temperature and a holding time of 30 min. The samples were heated up to 1000°C by the traditional heat source (e.g. the gas flame) before the microwave energy was applied. Influence of the ratio MW-power/burner power The suitable ratio between these two energy sources (ratio of input power) was determined. Figure 2 shows the different heating schedules as compared to conventional heating of Al2O3. 1800 1600

temperatur in °C

1400 1200 1000 800 600 400

25 % MW Max 50% MW

200

12 % MW conventional

0 0

50

100

150

200

250

300

350

400

time in min

Fig. 2. Influence of the MW-contribution on the heating schedule; starting temperature for microwave heating ~950°C (see Table 1 for details)

First, a maximum process time reduction of 4 hours as compared to conventional heating can be observed for the MHH schedule with a constant ratio of 25% microwave energy. In fact this ratio appeared to be the best combination to obtain a uniform heating of the sample without failure. A ratio of only 12% microwave

586

Leparoux

heating power resulted in a decrease of the heating rate i.e. the heat absorption of the sample. The efficiency of dielectric heating and its influence on the heating rate was further analysed by application of microwave power at two different temperatures: 1200 and 1400°C, as shown in Figure 3. 1600

1400

temperatur in °C

1200

1000 Application of MW power 800

600

400 25% MW

200

MW at 1400°C MW at 1200°C

0 0

20

40

60

80

100

120

140

160

180

time in min

Fig. 3. Impact of MW-heating at 1200°C and 1400°C

As the same ratio (25% MW power) is used in both cases, the heating rate and so the absorbed energy is nearly the same even if the temperature increase takes place at different time. In Table 1 the different sintering conditions are summarized. Table 1. Variation of the MW-Input energy Sample 1 2 3 4 5 conventional

Soak temperature [°C]/soak time [min] 1500/30 1500/30 1500/30 1500/30 1500/30 1600/120

Part of MW-in put energy [%] 25 25-50 12 25 25 -

Starting temperature for MW-Power 968 926 1300 1200 1400 -

The influence of these different sintering conditions on the achieved material properties is first examined by density and Vickers hardness HV 0.05 measurements. The results are given in Table 2. Both the measured density and the hardness HV 0.05 are nearly comparable to the conventional sintered sample. The density achieved is about 95% of the theoretical one. The higher soaking temperature and time explains the slightly higher density of the conventional sintered sample. Due to the properties of the chamber material (max. permissible temperature 1550°C), it was not possible to reproduce this heating schedule in the microwave furnace.

Microwave Assisted Sintering of Al2O3

587

Table 2. Material properties as a function of heating schedule Sample 1 2 3 conventional

% theor. Density 95 93 94 97

Hardness HV 0.05 1661 1505 1541 1659

Nevertheless, a constant ratio of 25% microwave energy appeared to be the best combination to obtain a uniform heating of the sample without failure. In this case, the heating rate could be raised up to 30 K/min without damaging the specimens. A further increase of the amount of microwave power input does not lead to improved material properties. From grain size distribution throughout the sample cross-section an inverse temperature profile can be assumed in the conventionally sintered specimen, while sample 1 (MHH-sintering) had a nearly homogenous one.

Fig. 4. SEM-picture of a conventionally sintered sample, a) microstructure at the periphery, b) microstructure in the core

Fig. 5. SEM-pictures of a MHH-sintered sample with 25% MW power, a) microstructure at the periphery , b) microstructure in the core

A more homogeneous microstructure of the MHH-sintered material is confirmed by the microstructural analysis of the samples which revealed a fine-

588

Leparoux

grained homogeneous morphology throughout the whole body whereas a conventional heated sample showed strongly coarse-grained morphology at the surface. Figures 4 a, b and 5 a, b demonstrate the remarkable microstructural difference between alumina ceramics conventionally and microwave assisted sintered. As the pores in the triple points indicate, the sintering process in the MHH sample seems not to be completed. Higher sintering temperature should lead to less porosity. Influence of the sintering temperature In order to achieve a temperature increase of 50 K it is necessary either to accept a decrease in the heating rate or to increase the microwave ratio at the beginning of the sintering process (approx. 1200 - 1300°C). Figure 6 shows the performed heating schedule with 30 min soaking time. 1600

1400

temperatur in °C

1200

1000

800

600

400

25 % MW 25 % MW to 1550°C

200

30 % MW at 1200°C

0 0

50

100

150

200

250

300

time in min

Fig. 6. Influence of MW power on the heating rate

In order to maintain a constant heating rate of 30 K/min, the amount of microwave power added had to be increased from 25 to 30%. In Table 3 the results from the material investigations are shown. Table 3. Material properties for samples MHH-sintered at different temperatures

Sample 1

Sample 6

Sintering temperature [°C]

1500

1550

% of Theoretical Density

95

97

Hardness HV 0,05 Variant

1661 7,3 %

1752 6,0 %

Microwave Assisted Sintering of Al2O3

589

In fact, an increase in sintering temperature of only 50 K has improved the density of the sample from 95 to 97% of the theoretic density. Furthermore, the measured hardness has increased. No difference in the microstructure was detected. This heating schedule is recommended for MHH processing of alumina ceramics in this furnace. Influence of the soaking time The soaking time was increased from 30 to 120 min in order to improve the sinter density of the specimens. The sintering temperature of 1500°C remained constant. Table 4 shows the resulting material properties. Table 4. Material properties of MHH-sintered samples for different soaking times

Sample 1

Sample 7

soak time [min]

30

120

% theor. density

95

96

1661 7,3 %

1651 8,3 %

Hardness HV 0,05

Variant

Fig. 7. SEM-pictures of Al2O3, 25% MW, Tsoak: 1500°C, soaking time 120 min

The SEM-pictures shown in Figure 7 clearly indicate a fully sintered structure without unacceptable grain coarsening (grain diameter < 2 µm). Comparable grain sizes exist both in the core as well as in the periphery of the specimen, indicating a densely sintered, fine-grained and homogeneous structure of the specimen.

Conclusions The technique of Microwave Hybrid Heating (MHH) with a gas burner as conventional heating source can be used to sinter alumina ceramics. No interaction of

590

Leparoux

the flame and the exhaust fumes with the microwaves could be detected. Furthermore, the combination of dielectric and traditional heating of the sample is confirmed as beneficial for the temperature uniformity leading to the desired properties of the ceramic such as fine grain, high density and adequate mechanical strength. This can be achieved with a net reduction in processing time - up to 4 hours or ~60% less than conventional sintering - and with a reduction in energy consumption of about 30 to 50%. So, it can be claimed that this microwave assisted approach to sintering of ceramics is very successful from both the material and the energy-economic point of view. Of course, the dielectric properties of the ceramic material must be suitable for that technique (e.g. high absorption of microwave power without generation of skin effects as observed with SiC). In future, it would be interesting to examine other ceramics in order to define the group of material where this approach is of much benefit.

Acknowledgement Research MHH have been sponsored by AIF. The authors wish to thank all sponsors and project partners for their assistance.

References [1] M. Willert-Porada, A Microstructural Aproach to the Origin of Microwave Effects in Sintering of Ceramics and Composites, Ceram. Trans. Vol. 80, 153 – 163, 1997 [2] Hart, N.A. et a.: The Development of a Combined Mikrowave and Electric Radiant Heating System for Processing Ceramics. 2. Workshop on Microwave Processing of Materials, Proceedings, Karlsruhe, 1997 [3] Wroe, F.C.R.: Microwave-Assisted Processing of Materials. Patent GB 9316616.3, 1993 ; WO 95/05058, 1995 [4] Daneke, N.; Krause, H.; Walter, G.: Mikrowelleneignung von Wärmedämmstoffen bei hohen Temperaturen. Freiberg, Berg- und Hüttenmännischer Tag, 1999 [5] Daneke, N.; Schilm, J.; Krause, H.; Walter, G.: Möglichkeiten und Grenzen der Temperaturmessung im Mikrowellenfeld. In: Bathen, D.; Schmidt-Traub, H. (Hrsg.): Innovative Energieträger in der Verfahrenstechnik. Aachen: Shaker, 2000. -ISBN 3-82656944-X [6] Daneke N., Zesch U.: Anforderungen an die Anlagen-und Verfahrenstechnik für die Mikrowellensinterung, Forschungsbericht 50/659, Forschungsgemeinschaft Industrieofenbau e.V., 1999

Absorption of Millimeter Waves in Composite Metal-Ceramic Materials A. G. Eremeev1, I. V. Plotnikov1, V. V. Holoptsev1, K. I. Rybakov1, A. I. Rachkovskii2 1

Institute of Applied Physics, Russian Academy of Sciences,46 Ulyanov St., Nizhny Novgorod 603950 Russia 2 Russian Federal Nuclear Center, Sarov, Russia

Introduction The fabrication of metal-ceramic composites, including functionally graded materials, is an important new application area for microwave processing. In order to implement the microwave heating of these materials it is necessary to take into account their microwave absorption properties. These vary greatly depending on the nature and concentration of metal constituents, density and temperature. The goal of this study is to assess the absorptivity of composite metal-ceramic materials at millimeter-wave frequencies in a broad range of temperatures. Experimental data are compared with the results of theoretical calculations. The study provides practical information for the implementation and optimization of the millimeter-wave processing of metal-ceramic composite materials.

Theory Microwave absorption in composite metal-ceramic materials is generally due to both dielectric relaxation in the ceramic component of the material and the Ohmic losses produced by the conduction currents in the metal particles. These currents are driven by the alternating electric field induced by the magnetic component of the microwave field. To account for all electromagnetic energy losses, it is necessary to describe the composite metal-ceramic material by both the complex magnetic permeability, P P c  iP cc , and dielectric permittivity, H H c  iH cc . The power absorbed from microwaves per unit volume of material, w, is obtained as a sum of dielectric- and magnetic-type losses: w

we  w m

Z Z 2 2 H cc E  P cc H , 8S 8S

(1)

592

Eremeev

where Z is angular frequency of microwaves, E and H are amplitudes of the microwave electric and magnetic field, respectively. The effective magnetic permeability of the composite material, P , depends on the size and shape of metal particles, their concentration, conductivity of metal and the microwave frequency. If the stationary magnetic permeability of the ceramic and metal is 1, the effective magnetic permeability of the composite material can be expressed as P 1  4SF 1  4SC m D ,

(2)

where F is magnetic susceptibility of the material, which is equal to the product of metal concentration, Cm (per unit volume of material), and magnetic polarizability of an individual metal particle in the alternating magnetic field , D (per unit volume of metal). For a spherical metal particle in a uniform periodic magnetic field the complex magnetic polarizability, D , is obtained analytically [1] as

D 

3 § 3 3 · ¨1  2 2  cot ka ¸ , 8S © k a ka ¹

(3)

where a is radius of the particle, k 1  i G , G c 2SVZ is the magnetic field penetration depth in the metal (skin depth), c is light velocity, and V is the conductivity of metal. The effective dielectric permittivity of the composite material, H , depends on several factors including the dielectric properties of its components, their relative content, shape of particles, etc. Subject to certain limitations, it can be obtained within the effective medium approximation [2]. All components of the composite are considered as spherical inclusions in the material with the sought effective permittivity, which is found as a root of the equation 3H

¦ C j H j  H 2H  H j

0,

(4)

j

where Cj is volumetric fraction and Hj is dielectric permittivity of each component of the composite: metal, ceramic, and voids. The dielectric permittivity of the metal has the prevailing imaginary part, H m i 4SV Z . The absorbed microwave power can be most easily assessed by considering the incidence of a plane electromagnetic wave onto a slab of material with the obtained effective dielectric permittivity and magnetic permeability [1]. As a result, temperature dependencies of microwave absorptivity of metal-ceramic composite materials are obtained.

Absorption of millimeter waves in composite metal-ceramic materials

593

Experimental The samples for the experimental study were compacted by cold isostatic pressing from the mixtures of alumina and nickel powders with the mass content of nickel, XNi, varying in the range 0 - 80 wt%. The powder particle size was about 2 - 3 µm for alumina and 2 - 6 µm for nickel. The samples had cylindrical shape, 10 mm in diameter and 6 mm in height. The density of the samples was in the range 56 - 61% of the theoretical density. A sample was positioned inside a thermal insulation box and heated alone in the cavity of a 30 GHz millimeter-wave gyrotron system [3] to temperatures between 200 and 1200ºC. The temperature of the sample was controlled by a thermocouple. The heating rate was typically 30ºC/min. The microwave power input into the cavity was measured in the transmission line by a calorimetric method. The absorptivity, K, was determined from the rates of heating and cooling, dT dt  and dT dt  , immediately before and after the millimeter wave shutoff. The equations for the energy balance in this case can be written as c eff mdT dt 

KP  Q ; c eff mdT dt 

Q ,

(5)

where ceff and m are effective thermal capacity and mass of the sample, P is the microwave power incident on the sample, Q is the rate of heat loss from the sample. The thermal capacity, ceff, was determined for each sample as ceff = XNi cNi + (1 - XNi ) cAl2O3. Due to large dimensions of the cavity, the field pattern inside it was governed by absorption in its walls. Therefore, the proportion of the incident power on the sample, P, to the (measured) power input to the cavity, Pinp, was approximately the same with all samples regardless of their composition: P = JPinp. Excluding Q from Eqs. (5), the relative absorptivity, KJ, was obtained.

Results and discussion As follows from an analysis of the result for magnetic polarizability of metal particles, D (Eq. (3)), the magnetic losses reach the maximum when the particle radius, a, is on the order of the skin depth, G. For a > G D cc v a 1V 1 2 [1]. For nickel at a frequency of 30 GHz, the value of the skin depth varies (due to its temperature-dependent conductivity) from 1.0 µm at 200ºC to 1.8 µm at 1200ºC. Shown in Fig. 1 are the results of calculation of the absorbed millimeter-wave power for three nickel particle sizes, 1, 3, and 10 µm, and for five mass contents of nickel (equal to those used in experiment). It can be seen that millimeter-wave absorptivity increases drastically with addition of metal particles. Among the three nickel particle sizes tested, the strongest absorption is achieved with 3 µm particles. The absorptivity exhibits relatively weak dependence on temperature, although this dependence becomes somewhat more pronounced at higher contents of metal.

594

Eremeev

Absorbed Power / Incident Power

0.20

0.16

Ni particle size: 1 µm 40 wt. % Ni

0.12

20 wt. % Ni

0.08

10 wt. % Ni

0.04

5 wt. % Ni 0 wt. % Ni

0.00 200

400

600 800 Temperature, C

1000

1200

Absorbed Power / Incident Power

0.5

Ni particle size: 3 µm

0.4

40 wt. % Ni 0.3 20 wt. % Ni 0.2

10 wt. % Ni 5 wt. % Ni

0.1

0 wt. % Ni 0.0 200

400

600 800 Temperature, C

1000

1200

0.4

Absorbed Power / Incident Power

Ni particle size: 10 µm 40 wt. % Ni 0.3 20 wt. % Ni

0.2 10 wt. % Ni 0.1

5 wt. % Ni

0 wt. % Ni 0.0 200

400

600 800 Temperature, C

1000

1200

Fig. 1. Calculated absorbed millimeter-wave power in a slab of Ni–Al2O3 composite material vs. temperature, composition, and Ni particle size. Millimeter-wave frequency 30 GHz, slab thickness 6 mm, data on the properties of materials from [4, 5].

Power in Magnetic Losses / Total Absorbed Power

Absorption of millimeter waves in composite metal-ceramic materials 1.00

40 wt. % Ni 0.80 20 wt. % Ni 0.60 10 wt. % Ni Ni particle size: 1 µm 0.40

5 wt. % Ni

0.20

Power in Magnetic Losses / Total Absorbed Power

200

400

600 800 Temperature, C

1000

1200

1.00 40 wt. % Ni 0.96 20 wt. % Ni 0.92 10 wt. % Ni 0.88 Ni particle size: 3 µm

0.84 5 wt. % Ni 0.80 200

Power in Magnetic Losses / Total Absorbed Power

595

400

1.00

600 800 Temperature, C

1000

1200

40 wt. % Ni 0.95 20 wt. % Ni 0.90 10 wt. % Ni 0.85 Ni particle size: 10 µm

0.80 5 wt. % Ni 0.75 200

400

600 800 Temperature, C

1000

1200

Fig. 2. Calculated ratio of the power in magnetic losses to the total absorbed millimeter-wave power in a slab of Ni–Al2O3 composite material vs. temperature, composition, and Ni particle size. Millimeter-wave frequency 30 GHz, slab thickness 6 mm, data on the properties of materials from [4, 5].

Figure 2 illustrates the proportion of magnetic losses in the total millimeterwave absorption of the composite material. It can be seen that in most cases magnetic losses are prevailingly responsible for the absorption. Only in the samples with low metal content and metal powder particle size significantly different from

596

Eremeev

the skin depth, the dielectric and magnetic losses are comparable at high temperature. This reflects the fact that the dielectric losses of ceramics grow sharply with temperature, while the conductivity of metal decreases smoothly. The experimental results on absorptivity measurements are shown in Fig. 3. In agreement with the theoretical predictions, maximum absorption is observed in the samples with 20 and 40 wt% of nickel. The temperature dependence of absorptivity is also significant only on these samples.

Absorbed power / Input power, 10-3

14 12 10 8 6 4 2

0 wt. % Ni 20 wt. % Ni

5 wt. % Ni 40 wt. % Ni

10 wt. % Ni 80 wt. % Ni

0 0

200

400

600

800

1000

1200

1400

Temperature, C

Fig. 3. Measured relative absorbed millimeter-wave power in the samples of Ni–Al2O3 composites vs. temperature and composition.

Although the theoretical and experimental results are generally consistent with each other, there still is a discrepancy between them, which is especially noticeable in the low-temperature region. One possible reason for this is low accuracy of the used method for millimeter-wave power measurement at low power levels. This has probably led to underestimation of the input power and therefore to overestimation of absorptivity at low temperatures. Another source of discrepancy is in the assumption of spherical shape and uniform size of metal particles adopted for the theoretical model. The microscopic investigation of the compacted samples has shown that nickel particles agglomerate, deform and assume irregular shape in the course of compaction. In addition, the particles are distributed over size. Both these factors may influence the effective dielectric properties of the composite material and thereby the structure of the electromagnetic field inside it. An indication that this actually occurs is present in the experimental data. In the experiment, the sample with the 80 wt% nickel content exhibited noticeable millimeter-wave absorption. The density of this sample was 56% of the theoretical density, which gives the volumetric fraction of nickel of about 36 vol%. In the model of equal-sized spheres this value of volume fraction is higher than the percolation threshold, which means that the metal particles become all connected and the sample should act as made of pure metal, i. e., reflect almost all microwaves and absorb almost nothing.

Absorption of millimeter waves in composite metal-ceramic materials

597

Finally, it would be unrealistic to expect exact quantitative agreement between the theory and experiment because the calculations used the plane wave approximation, whereas actual samples had cylindrical shape and their entire surface was irradiated.

Conclusion Millimeter-wave absorption in composite metal-ceramic materials has been analyzed theoretically and measured experimentally. The absorptivity of such materials has been shown to originate prevailingly from the magnetic-type losses. The absorptivity increases with the metal content in the material and depends on the size of metal particles, reaching a maximum when the size is on the order of the skin depth. The results of experimental measurements of absorptivity of nickel– alumina composite materials are generally consistent with theoretical calculations.

Acknowledgment This research was made possible in part by the support from the International Science and Technology Center and Japan as its funding party under project # 1607.

References [1] Landau LD, Lifshits EM, and Pitaevskii LP (1984) Electrodynamics of continuous media, 2nd ed. Pergamon, New York [2] See, for example, Bergman DJ, Stroud D (1992) Physical properties of macroscopically inhomogeneous media. In: Ehrenreich H and Turnbull D (eds) Solid state physics: advances in research and applications, vol 46. Academic Press, New York, pp. 147–269 [3] Bykov Y, Eremeev A, Flyagin V, Kaurov V, Kuftin A, Luchinin A, Malygin O, Plotnikov I, Zapevalov V (1995) The gyrotron system for ceramics sintering. In: Clark DE, Folz DC, Oda SJ, Silberglitt R (eds) Microwaves: theory and applications in material processing III (Ceramic Transactions, vol 59). Amer. Ceram. Soc., Westerville, pp 133–140 [4] Ho WW (1988) High-temperature dielectric properties of polycrystalline ceramics. In: Sutton WH, Brooks MH, Chabinsky IJ (eds) Microwave processing of materials (Materials Research Society Symposium Proceedings, vol 124). Materials Research Society, Pittsburgh, pp 137–148 [5] Grigor’ev IS, Meylikov EZ (eds) (1991) Fizicheskie velichiny: spravochnik (Physical quantities: a handbook). Energoatomizdat, Moscow (in Russian)

Microwave Sintering of PM Steels F. Petzoldt1, B. Scholz1, H. S. Park2, M. Willert-Porada2 1

Fraunhofer-Institute for Manufacturing and Advanced Materials (IFAM), Bremen, Germany; 2 University of Bayreuth, Chair of Materials Processing, Bayreuth, Germany

Introduction Powder metallurgical (PM) process technology is used to produce a wide range of structural components for different applications e.g. automotive parts. The basic steps in the traditional process are those of powder production, compaction of powders in a pre-form and sintering. The heat can be generated by different means like gas burner heating [1], electrically resistance heating [2], application of microwaves [3 - 6] or laser energy [7 - 9]. Sintering means to heat the pre-form to a temperature below the melting point of the major constituent. During sintering the powder particles lose their identities essentially through diffusion processes and the required properties are developed. The powder metallurgy process is neither energy nor labor intensive, it conserves material, it is ecologically clean and it produces components of high quality and with homogeneous and reproducible properties. The major tonnage growth of the industry has been in iron and alloy steel structural parts. The potential of using microwave energy as heat source for sintering of ceramics has been studied over the last 15 years, with industrially oriented concepts of hybrid heating by means of microwave assisted firing [10]. In case of electrically conductive materials, like e.g., cemented carbides and metallic materials a "technical pre-judgement" existed among the sintering community because due to bulk electric conductivity of metals microwave heating is limited to the thin "eddy currents" surface near area of the workpiece. As the pre-forms consist of dense packed and deformed powders coated with organic additives or oxide layers, the PM parts show a significant penetration depth and heating ability towards microwave radiation at the technically interesting frequency of 2.45 GHz [4, 6]. Scope of the study The EPMA in co-operation with IFAM, University of Bayreuth, PM Technology AB and a consortium of powder metallurgical component producing companies from all over Europe carried out a feasibility study to ascertain whether mi-

Microwave Sintering of PM Steels

599

crowave sintering is beneficial for advanced sintering concepts of PM steels. The following companies were actively participating in the consortium: AMES SA, Spain; Dansk Sintermetall, Denmark; Federal Mogul, France/UK; GKN Sinter Metals, Germany; Hilti AG, Lichtenstein; Höganäs AB, Sweden; Mahler GmbH, Germany; MIBA Sintermetall AG, Austria; Plansee AG, Austria; Polyfour GmbH, Germany; Powdrex Ltd., UK; Quebec Metal Powders Ltd, Germany; Schunk Sintermetalltechnik GmbH, Germany; Sinterstahl GmbH, Germany; SKF Nova, Sweden). In this study a direct comparison of sintering with microwave (MWS) and with conventional heating furnaces (CS) is carried out. Properties like density, hardness, tensile and yield strength, ductility were tested on tensile bars. Additionally real components from the different companies were sintered and characterized after MWS and CS with respect to density and dimensional stability.

Experimental

Materials and methods Two steel-powders were used for the investigations. The powder Distaloy AE was supplied by Höganäs AB and the powder MSP 1.5 Mo by Quebec Metal Powders Ltd. The Distaloy AE powder contains 3.75% Ni; 1.43 % Cu, 0.5% Mo, Fe base mixed with 0.5% C-UF and 0.6% lubricant. The MSP 1.5 Mo powder is pre-alloyed and contains 1.54% Mo, 0.77% C, Fe base and 0.6% lubricant. Tensile test bars (DIN ISO 2740) were pressed with a pressure of 700 MPa and 500 MPa. The samples were sintered at IFAM in the conventional furnace or by application of microwaves at the University of Bayreuth. The same heating and cooling rates of 10 K/min as well as the same sintering times were chosen for both types of sintering. A gas-atmosphere of 95% Ar / 5% H2 was used for sintering of Distaloy AE, an atmosphere of 95% N2 / 5% H2 for sintering of MSP 1.5 Mo. The density of sintered samples was measured by Archimedes method by immersing in water (ISO 3369). The Vickers Hardness HV10 (ISO 50 133) of all samples was determined, and tensile tests were carried out according to DIN EN 10 002-1. Every plotted value with the standard deviation is the average of at least five single measurements. The corresponding samples - microwave and conventionally sintered – were prepared and etched in the same way to compare the microstructure of the differently sintered materials. Micrographs were taken from the center and the near surface area of the samples. To study the fracture surfaces of the tensile test samples the Scanning Electron Microscope LEO 438VP was used.

600

Petzoldt

Microwave sintering, MWS For microwave sintering a batch type “Hot Wall Resonator” Molybdenum furnace, build and operated at University Bayreuth is used. The furnace is equipped with 12 kW Mo-resistance heating elements, capable of heating the Molybdenum resonator to 1500°C, and 9 kW output power microwave heating at a frequency of 2.45 GHz, feed from two rectangular wave guides [11]. The conventional heating was applied for drying the thermal insulation materials prior to sintering. For sintering only the microwave system was used. To achieve stable heating conditions by microwave absorption, the steel parts were placed in a microwave transparent thermal insulation casket inside the 0.2 m³ Molybdenum process chamber, as shown in Figure 1. As sampleholder materials Alsint 99.7 (Al2O3-ceramic) or Mullite is used. For temperature measurements a pyrometer (Keller PZ 20 AF1, 20 - 2000°C) is used. The emission coefficient of the samples was experimentally determined by calibration using the melting temperature of a gold and a copper wire tightly contacted at the surface of the PM-steel parts.

Fig. 1. Schematic description of the microwave sintering set up.

Conventional sintering A batch type resistance heated furnace was used for the conventional sintering. The heating element and the thermal insulation consist of graphite. Temperature measurements were taken with a W/Re thermocouple. The thermocouple W/Re was calibrated by the melting points of copper and cobalt. A dense alumina sample holder was used for sintering steel samples.

Microwave Sintering of PM Steels

601

Results and discussion

Tensile Test Bars

MSP 1,5 Mo The as pressed (700 MPa axial pressure) density of the tensile test bars is 7.09 g/cm³. As shown in Figure 2, a different densification behavior is observed for MWS as compared to CS. The density of the conventionally sintered (CS) samples slightly increases with increasing temperature, reaching a maximum value of ~7.2 g/cm³ for a sintering temperature of 1192°C, whereas the density upon microwave sintering (MWS) reaches this maximum value at the lowest temperature applied - 1030°C - and remains constant with increasing temperature.

MWS

CS

300

7,0

200

6,8

100

6,6

0 1000

1050

1100

1150

1200

Hardness HV

Density [g/cm³]

7,2

1250

Temperature [° C]

Fig. 2. Densification (left) and hardness (right) of MSP 1.5 Mo tensile bars The results of hardness measurement indicate a more pronounced difference between the MWS and CS samples. As shown in the right part of Figure 2, MWS samples reach the hardness of ~ 200 HV10 after sintering at 1030°C, conventionally sintered samples need sintering at 1100 - 1150°C to reach a similar hardness value. From the viewpoint of advanced applications of PM part, tensile strength, yield strength as well as elongation are very important. For the tensile and yield strength values of the differently sintered parts again a maximum value is achieved at very low sintering temperatures for MWS, slightly decreasing with increasing sintering temperature. Opposite to this, for the conventional sintering a maximum strength

602

Petzoldt

is achieved upon sintering at 1190°C, with lower values at decreased as well as increased sintering temperature, as shown in Figure 3. 700

MWS, tensile strenght CS, tensile strength

600

Strength [MPa]

500 400 300

MWS, yield strenght

CS, yield strength

200 100 0 1000

1050

1100

1150

1200

1250

1300

Temperature [° C]

Fig. 3. Tensile strength and yield strength of MSP 1.5 Mo, MWS and CS tensile test bars; compaction of green parts at 700 MPa. The elongation corresponding to the tensile strength values shows a very surprising dependence upon strength for MWS samples, as shown in Figure 4.

Elongation [%]

5

4

3

2

1

0 1000

1050

1100

1150

1200

1250

Temperature [°C]

Fig. 4. Elongation upon tensile testing of MSP 1.5 Mo, MWS and CS test bars; compaction of green parts at 700 MPa From a value 5fold higher for MWS as compared to CS at low sintering temperature elongation decreases significantly with increasing sintering temperature, although the strength is only slightly decreasing. The MW sintered samples show an elongation of up to 2.6% at a sintering temperature of 1032°C, but only 0.6% at 1145°C, the conventionally sintered samples display an opposite behavior, show-

Microwave Sintering of PM Steels

603

ing an elongation of 0.6% after sintering at 1081°C and 3.48% at 1243°C. By microwave sintering test bars are obtained with a tensile strength of ~500 - 550 MPa and an elongation >1% after sintering at 1100°C whereas conventionally such a materials needs 1200°C. Microstructure investigation reveals bainitic phase composition and no strong differences in grain size and shape between MWS and CS samples at 1145°C and 1142°C, respectively, as shown in Figure 5.

Fig. 5. Light microscope photographs of polished and etched samples of MSP 1.5 Mo: left MWS, 30’1145°C; right CS 30’ 1142°C.

Fig. 6. SEM-photographs of fracture surfaces of sintered MSP 1.5 Mo samples: left MWS, 30’1145°C; right CS 30’ 1142°C. This supports the results of mechanical testing, because for these two materials very similar mechanical properties were found as well. Further support is given by the fractography results, as shown in Figure 6. In the beginning of the study, in some cases the MW sintered samples were slightly decarburised in the outer area, because of the different measures to control atmoshere quality upon MWS as compared to CS.

604

Petzoldt

Mechanical properties developing to a better values at lower sintering temperatures upon MWS was also found for green parts pressed at 500 MPa axial pressure only. In the case of samples pressed with 500 MPa the properties (hardness and strength) are at a lower level on account of the lower density. Distaloy AE There is a significant difference between the MSP and the Distaloy powder. The powder MSP is prealloyed and only carbon and the lubricant is added. The Distaloy powder is produced by a different process, yielding a mixture of Fe together with alloying elements as Cu , Mo and Ni only partly dissolved in the matrix of the FE-powder. 800

Strength [MPa]

Tensile 600

CS

400

200

0 1000

Yield

MWS

1050

1100

1150

1200

1250

Temperature [°C]

Fig. 7. Results of mechanical testing for Distaloy AE tensile test bars, tensile and yield strength.

From the results shown in Figure 7 and 8 it is evident, that a difference exists between MWS and CS parts. Although hardness and strength increase similarly for both sintering methods with increasing sintering temperature modestly due to the solution of copper and the simultaneous development of the microstructure typical of Distaloy AE, the slightly higher tensile strength of the MWS samples corresponds to a significantly higher elongation ( more than 25% ) of the MW sintered samples. A comparison of fracture surfaces of the MW and conventionally sintered samples indicates a slightly larger contact area between the grains for the MWS test bar, therefore more deformation yielding a higher elongation seems probable. Tests of reproducibility carried out with Distaloy AE showed good results.

Microwave Sintering of PM Steels

605

5 MWS

Elongation [%]

4

3

2 CS

1

0 1000

1050

1100

1150

1200

1250

Temperature [°C]

Fig. 8. Mechanical testing of Distaloy AE tensile test bars, elongation Industrial Parts The most important question to be answered by the study pointed towards application of MWS for geometrically demanding industrial PM-parts. Eight different parts were supplied by different companies in the as pressed state. The compaction pressure was adjusted to the one applied for tensile test bars. Selection of the parts occurred following the potential risks of a strong high frequency electromagnetic field present during sintering in terms of plasma ignition and limitation of penetration depth. Therefore parts with sharp edges, like gears, and very different wall thickness as well as holes were selected. The same heating and cooling rates as for the tensile test bars were chosen for the sintering of the larger parts. The atmosphere was always 95% H2 / 5% N2. The microwave sintering temperature was about 80°C lower than the sintering temperatures of conventional sintering, but the properties and the microstructure were similar. As shown in Figure 9, all investigated parts of different shapes and sizes could be sintered by application of microwaves with good retention of shape. The parts (1) and (2) consist of MSP 1.5 Mo and the part (3) of Distaloy AE. AMES SA The parts were MW sintered for 30´ at 1058°C and conventionally sintered for 30´ at 1048°C. The density of the parts were identical. Section densities did not show any remarkable difference between MW and conventionally sintered parts. Lower densities were found at the edges of the small holes in both cases. A similar size and distribution of pores was found for MWS and CS.

606

Petzoldt

Fig. 9. Industrial parts after MWS; from left to right AMES SA (1), GKN Sinter Metals GmbH (2), MIBA Sintermetal AG (3).

GKN Sinter Metals GmbH bracket There is no difference in density of the MWS bracket after sintering for 15´ at 1023°C as compared to CS parts sintered for 15´ at 1101°C. No difference is found in the micrographs of the unetched samples, with respect to size, shape and distribution of pores. But there were some differences in the micrographs of the etched samples. Large parts of the surface of the MW sintered samples were decarburised up to a depth of 300 µm. Due to the lower MW sintering temperature, the MW sintered sample had a slightly finer microstructure than the conventionally sintered sample. The cooling rates were identical in both sintering processes. In the MW sintered sample no molten areas could be detected. The microwave sinterability of a component with big differences in the wall thickness is proven with this part. MIBA Sintermetall AG synchronizer hub For the synchronizer hub made of Distaloy AE MWS was performed for 30´ at 1032°C, CS for 30´ at 1111°C. Both sintered parts show an identical density. No difference could be found in the micrographs of the unetched samples. Surprisingly, non-dissolved Copper particles were not found in MWS-samples, although the MW sintering temperature was much lower than the melting point of copper. After etching, a decarburised surface layer became visible in MWS samples, reaching a depth of 200 µm. Again, the decarbusrisation was avoided in later sintering experiments by careful drying of the thermal insulation prior to MWS. The microstructures of the MW and C-sintered parts are identical in the center areas. During MW sintering of batches of the larger parts plasma ignition occurred more frequent as compared to the small tensile test bars. A correlation with the higher power levels applied in order to keep the heating rate high was found as well as an influence of the geometry of the parts. Such plasma incidents are detectable by small molten areas at highly curved surfaces as well as on sharp gear tooth. Parts of geometries without sharp edges are easier to sinter by using MW only. The microstructures of the MW and conventionally sintered samples are very similar although the MWS temperatures were much lower than the CS tempera-

Microwave Sintering of PM Steels

607

tures. This is a promising result which corresponds to the results obtained with tensile test bars.

Conclusions From the mechanical testing results, shown in Figures 2 - 4 and 7 - 8, evidence is given for an influence of the microwave heating on the sintering process. Different mechanisms can be assumed for MSP and Distaloy AE: 1. microwave heating can increase the activity of carbon or strengthen the sintering in the D-phase. The high hardness of the MW sintered samples seems to prove that carbon is already solved at lower temperatures. 2. Because of field strength effects, microwave fields can increase the reactivity of gaseous species in the pores and facilitate dissolution of Cu. Both steel powders, Distaloy AE and MSP 1.5 Mo, can be sintered by microwave heating only, without additional heat sources from resistant heaters placed outside of the Molybdenum cavity. For tensile test bars a satisfactory level of reproducibility was achieved. However, larger scatters in the individual mechanical properties are found upon MWS as compared with CS. Decarburisation of surface areas during MW sintering is closely related to the microwave furnace technology and can be avoided by hot gas purging of the cvold thermal insulation before starting MWS. All industrial parts of different shapes, sizes and thickness could be MW sintered. Comparable properties of MWS and CS are achievable at 50 - 80°C lower sintering temperature for MWS. In the case of MSP the mechanical properties of the MWS samples are much better than those of the conventionally sintered samples at low sintering temperatures. For Distaloy AE parts a similar though less pronounced effect is found. MWS at lower sintering temperatures could therefore contribute to lower manufacturing costs and an increased product throughput. The sintering temperature of conveyor belt furnaces used in industry for this kind of material is limited to 1120 to 1150°C. This actual restriction limits the selection and mass of alloying elements. New PM steels could be sintered by means of MWS in such furnaces by exploitation of the lower sintering temperature of MWS as compared to CS, found in this work.

References [1] D. Geldner, Sinteröfen für die P/M-Industrie, Pulvermetallurgie-Schlüssel zur Effizienzsteigerung, Pulvermetallurgie in Wissenschaft und Praxis, Bd. 17, Hrsg. H. Kolaska, S. 283-284 (2001) [2] H. Endres, Widerstandserwärmung zur Wärmebehandlung und zum Schmelzen, in Industrielle Elektrowärmetechnik, Ed. A. Mühlbauer, Vulkan-Verlag Essen, 1992, p. 232ff, contineous electrically heated sintering furnaces

608

Petzoldt

[3] T. Gerdes, M. Willert-Porada, Microwave sintering of metal-ceramic and ceramicceramic composites, MRS Proc. Volume 347, pp. 531-537 (1994) [4] Kolaska, Rödiger, Willert-Porada, Gerdes, “Microwave Sintering”, Ger. Pat. Appl. P 43 40 652.1, (1993), DE 43 40 652 A1, PCT/DE95/00548 [5] R. Roy, DK. Agrawal, S. Gedevanishvili, J. Cheng, Full sintering of powdered-metal bodies in a microwave field, Nature, Vol. 399, S. 668-670 (1999) [6] M. Willert-Porada, H.-S. Park, Heating and Sintering of Steel Powders with Microwaves at 2.45 GHz Frequency – Relation Between Heating Behaviour and Electrical Conductivity, Ceram. Trans., Vol. 111, 459-470 (2001) [7] A. Simchi, F. Petzoldt, H. Pohl, H. Löffler, Direct Laser Sintering of Low Alloy P/M Steel, PM Science & Tech.Brief (2000) [8] A. Simchi, F. Petzoldt, H. Pohl, H. Löffler, A New Steel Powder Mixture for Direct Laser Sintering, Rapid Prototyping Tooling Newsletter 4, 4-5, (2000) [9] G. Veltl, A. Oppert, F. Petzoldt, Warm Flow Compaction Process for Complex Shaped PM Parts, Proceeding Powder Metallurgy World Congress Exhibition, Kyoto, Japan (2000) [10] R. Wroe, Microwave Assisted Firing of Ceramics, Ceram. Trans. Vol. 80, 671-678 (1997) [11] M. Willert-Porada, W. Bartusch, G. Dhupia, G. Müller, A. Nagel, G. Wötting, Material and Technology Development for Microwave Sintering of High Performance Ceramics, in “Ceramics-Processing, Reliability, Tribology and Wear, Euromat Vol. 12, Ed. G. Müller, Wiley-VCH, p. 87-93 (2000)

Formation of Functionally Graded Cemented Carbides by Microwave Assisted Sintering in Reactive Atmospheres R. Tap1, M. Willert-Porada1, K. Rödiger2, R. Klupsch3 1

University of Bayreuth, Department of Materials Processing, Germany Widia Valenite, Essen, Germany 3 University of Dortmund, Department of Materials Science, Germany 2

Introduction In the field of metal machining for future "dry" turning applications tools with improved wear in high speed turning operations are required. The more rapid processing of high quality materials with reduction of lubricant and cooling agents requires the development of new cutting tools, with thick coatings or graded structures, suitable for increasing the heat conductivity, the toughness as well as the adhesion between the substrate and the functionally graded structure. Recently, cutting tools of identical composition of the basic cemented carbide as well as the coating showed a significantly enhanced wear resistance, when the basic cemented carbide insert was sintered by microwaves (abbreviated as MWS) prior to coating [1]. A doubled cutting length was achieved, as shown in Figure 1. It was assumed, that sintering by microwaves influences the surface near composition of the cemented carbide in a different way as conventional sintering (abbreviated by CS) does [2]. Moreover, by application of microwave heating sintering of cemented carbides is performed in ambient pressure in different inert or reactive atmospheres [3]. In the conventional process reduced pressure is required, therefore not only the chemical activity of the components will be affected but also compositional changes due to evaporation of Cobalt will become less important upon MWS as compared to CS. Aim of the work In order to understand the physical and chemical processes underlying this improvement in properties, a systematic study was undertaken on microwave sintering of cemented carbide and cermet grades susceptible to the formation of Ticarbonitride coatings, when sintered in reactive atmospheres. Such grades are

610

Tap

known to develop a compositional gradient when heated under nitrogen atmosphere for prolonged time after the densification has taken place [4, 5].

Fig. 1. Comparison of cutting performance between a microwave sintered and conventionally coated (MWS, coated) as well as conventionally sintered and conventionally coated (CS) cemenetd carbide tool bit, courtessy of WIDIA GmbH [6].

Application of nitrogen at quite high temperatures to cemented carbides sintered conventionally under reduced pressure leads to numerous chemical reactions as well as directional diffusion towards the region with a high nitrogen activity. Different gradient structures have been found, as shown in Figure 2 with respect to the morphology and composition within a distance of 5 - 50 µm in the near surface region of a cutting tool bit.

Fig. 2. Classification of FGM-cemented carbides obtained from reactive atmosphere sintering, according to [5].

All four types of structures reflect the depletion or enrichment of cubic carbonitrides within the surface near zone of a cutting tool bit. The present study was aimed at modification of the known process by application of microwave sintering. In particular, an increased growth rate as well as a

Formation of Functionally Graded Cemented Carbides

611

selective variation of the gradient composition for similar grades of cemented carbides was of interest. Upon microwave sintering compositional gradient formation was observed even in "classical" cemented carbides, as reported in literature [2]. Therefore, combining microwave sintering with reactive atmospheres was expected to yield new functionally graded cemented carbides and cermets.

Experimental Different grades of cemented carbides and cermets (WIDIA, Germany) were employed, with different amounts of Ti- and Ta,Nb-carbides or nitrides in addition to WC and Cobalt, as shown in Table 1. For comparison a Ti,Ta,Nb-free grade was used (THM). Sintering was performed in a Molybdenum-cavity multimode microwave oven, equipped with resistant heating elements, described in previous work [7]. Temeparture meausrements are taken with a pyrometer. The applied microwave power has a frequency of 2.45 GHz, a maximum power of 9 kW, fed into the oven by two rectangular waveguides. The influence of the starting composition on the gradient formation upon conventional sintering is known from literature [6]. Table 1. Starting composition of the different grades of cemented carbides and cermets (Widia GmbH, Essen)

Sample THM 20 21 22 23 24

WC-content high high high high high low

Ti(C,N) none high (N) low low (N) low (N) high (N)

Ta,Nb none none low low high high

Hard phase WC Ti(C,N) Ti(C,N) Ti(C,N) Ti(C,N) Ti(C,N)

Binder Co, 6 Co, >6 Co, 6 Co, >6 Co, >6 Co, >6

To induce directional diffusion, microwave heating and sintering was usually started in Ar or Ar/H2 at ambient pressure, followed by pressure reduction for atmosphere exchange and completion of densification in N2-atmosphere. The conditions for atmosphere exchange are 200 mbar/1000°C achieved by active pumping for a certain period of time, followed by pressure increase to ambient with nitrogen gas. At lower pressure plasma ignition occurs, at higher pressure insufficient exchange of the atmosphere was observed, causing very low growth rates for the compositional gradient. More details are given in [8]. A typical sintering and microwave heating profile is shown in Figure 3. Quite low reflected power levels were adjusted by means of a HOMER (High Ohmic Impedance Analyser, Mügge, Germany).

612

Tap

reflected power *

T [°C]

1200

3000

forwarded power ’

2000

800

1000

400

6 11

96

81

71

61

51

36

0 16

0

0

Microwave Power [W]

Characterisation of the sintered FGM-tool bits was performed by density measurements using the immersion method, XRD, SEM-EDX and light microscopy for phase and microstructure analysis. Cutting experiments were performed by Widia to characterise the FGM-functionality.

Time [min] Fig. 3. Typical microwave sintering profile with atmosphere exchange at 1000°C.

Results and discussion

Gradient formation The experimental results achieved from MWS of TiC- or TiN- as well as TaN- or TaC-doped grades reveal a significantly increased growth rate for the compositional and morphology gradient as well as differences in the composition and microstructure of the graded layers as compared to CS in reactive atmospheres. A microstructure with an extremely thick as well as an thin graded layer structure is shown in Figure 4 and Figure 5. Large WC-rich grains have developed, which are too coarse to achieve good cutting or turning performance in grade 24. Upon heating at6 1550°C in N2-atmosphere a supersaturated (W,Co,Ti)-(C,N)-melt forms, which decomposes upon further dissolution of Nitrogen picked up from the atmosphere, into a WC-rich and a Ti(C,N) crystalline phase. Carbon deficiency usually causes crystallisation of large grains, as shown in Figure 4. A more promising compositional gradient is formed in grade 22 under comparable MWS conditions, as shown in Figure 5. This is particularly interesting, because according to the starting composition given in Table 1, grade 22 has only low contents of expensive Ti, Ta, Nb-additives, opposite to grade 24, which is highly doped by these hard phases.

Formation of Functionally Graded Cemented Carbides

613

Fig. 4. SEM-EDX as well as light microscope image (right) of an extremely thick compositional gradient formed in grade 24. Ti is enriched in the top part of the large WCcrystallite layer, Co is depleated.

Fig. 5. SEM-EDX (left) as well as light microscope image (right) of an optimised compositional gradient formed in grade 22, with a dense Ti(C,N) layer on top of a “striation”-like Co-enriched layer.

Fig. 6. Schematic description of the correlation between grade composition (see Table 1) and gradient type formed upon MWS as compared to CS.

A summary of the results achieved by MWS as compared to CS experiments performed in a different laboratory [5] for the same cemented carbide grades, is

614

Tap

schematically shown in Figure 6. It is evident, that different graded layers are formed upon MWS as compared to CS for a certain carbide composition. More experiments are needed to distinguish between pressure related effect – vacuum sintering in the CS technology, ambient pressure in MWS – and microwave heating effects. Turning performance For turning operations CS materials sintered at Widia were used and compared with MWS materials sintered in the University laboratory. As shown in Figure 7, the graded cutting tool inserts made from grade 22 show improved wear resistance as compared to the graded material obtained from a conventional sintering process of the same grade- However, for grade 21 the opposite behaviour is found, with an even better lifetime of the cutting tool CS-21 as compared to MWS-22. Turning Operation

Material: CK45N Tool Bit: SNUN 120408 Vc = 180 m/min; ap = 2,0 mm; f = 0,2 mm/U

20

Lifetime of Cutting Tool in min

18 16 14 12

Grade Nr. 22

10 8

Grade Nr. 21

6 4 2 0 MW

konv.

MW

konv.

Fig. 7. Comparison of turning performance of microwave and conventionally sintered cemented carbides with a compositional gradient of hard phases in the surface near area.

Conclusion Development of a compositional gradient upon MWS of cemented carbides containing Ti-, Ta- or Nb-carbide or nitride additives in reactive nitrogen atmosphere is possible, although opposite to the CS process, sintering occurs at ambient pressure. Careful exchange of the Ar-atmosphere by reducing the pressure fol-

Formation of Functionally Graded Cemented Carbides

615

lowed by purging and fill up with nitrogen is needed to achieve high growth rates for the graded layers upon MWS. Within this study, no plasma was ignited to modify the surface of the cutting tools, therefore the lowest applied pressure was 200 mbar. Preliminary turning performance tests show, that by MWS a good quality functionally graded material is obtained based on a not expensive starting composition. Formation of a compositional gradient strongly depends upon the chemical activity of Ti within the cemented carbide tool bit. At present no clear conclusion with respect to pressure related and heating profile related processes can be drawn.

Acknowledgement We thank the German Science Foundation for financial support of this project, in the programme TFB 12, TP 4.

References [1] H. Westphal, V. Sottke, R. Tabersky, H. van den Berg, U. König, Neue Hartstoffbeschichtungen auf Basis der Carbonitride von Titan und Zirkon, Proc. 14. International Plansee Seminar, Vol. 3 (1997), C7, 55-62. [2] T. Gerdes, Microwave Sintering of Metal-Ceramic Composite Materials, PhD-Thesis, University of Dortmund, 1996 (ISBN 3-18-343205-6) [3] M. Willert-Porada, T. Gerdes, K. Rödiger, H. Kolaska, Microwave Sintering of Hardmetals and Ceramics, in „Advances in Hard Materials Production“, ed. by European Powder Metallurgy Assoc., Euro PM 96, (ISBN 1 899072 03 9), S. 69-76 (1996) [4] H. Suzuki, K. Hayashi, Y. Taniguchi, Trans. Jap. Instit. Metals, 1981, 22 (11), 758 [5] W. Lengauer, J. Garcia, K. Dreyer, I. Schmid, D. Kasel, H.-W. Daub, G. Korb, L. Chen, Diffusion-Controlled Fabrication of Functionally-Graded Cermets and Hardmetals, Proc. PM99, Turin; Advances in Powder metallurgy and particulate materials, Vol. 3, S. 10-85-10-96 (1999) [6] K. Rödiger, K. Dreyer, T. Gerdes, M. Willert-Porada, Microwave Sintering of Hardmetals, J. of Refractory Metals and Hard Materials, 1999 [7] M. Willert-Porada, W. Bartusch, G. Dhupia, G. Müller, A. Nagel, G. Wötting, Material and Technology Development for Microwave Sintering of High Performance Ceramics, in “Ceramics-Processing, Reliability, Tribology and Wear, Euromat Vol. 12, Ed. G. Müller, Wiley-VCH, p. 87-93 (2000) [8] M. Willert-Porada, R. Klupsch, A. Schmidt, K. Dreyer, K. Rödiger, Herstellung neuer Hartmetalle mit Gradientenmikrostruktur durch Mikrowellensintern, Materialwissneschaft und Technik, 2/2002

Microwave Plasma Synthesis of Ceramic Powders Dieter Vollath, D. Vinga Szabó Forschungszentrum Karlsruhe, Institut für Materialforschung III, P.O.Box 3640, D-76021 Karlsruhe, Germany

Introduction Non-agglomerated fine ceramic powders are important for any technical application demanding an extreme uniform structure. The difficulty of synthesis for this type of materials increases with decreasing particle size of the powder, particularly in the case of powders with particle sizes less than 10 nm. Synthesis of a composite material, homogenous on almost molecular scale, is most difficult. Pyrolytic processes are well suited for the synthesis of powders with highest quality. Conventionally, such processes are performed in tubular furnaces [1, 2] or in chemical combustion flames [3 - 6]. As long as the targeted particle size is not below approximately 100 nm, these processes work satisfactorily. For smaller particles, agglomeration and residence time become crucial process parameters and conventional methods are no longer suitable. At this point, application of microwave plasma processes offers a significant advantage. The use of microwave plasma can follow two different strategies: i) It can be applied like a flame for pyrolytic processes [7]. In this case, an aqueous solution of a nitrate, chloride, or other compound that may dissociate and form an oxide under elevated temperature condition is sprayed into the “flame”, to evaporate the water and form the ceramic powder particles. By application of a microwave plasma combustible gases are avoided, improving the safety of operation for flame pyrolysis and the purity of the product. ii) The second application leading to unique products comes from the fact that the species in a microwave plasma are partly dissociated and ionized [8, 9]. This reduces the reaction temperature and activates reactions going on at significantly lower temperature as compared to conventional processing, such as in tubular furnaces. In this case, proper controlling of the reaction conditions leads to a nanoscaled powder with narrow particle size distribution. In summary, as compared to pyrolytic processes in conventional tubular furnaces, the application of microwave plasma process offers the advantages of a lower reaction temperature, shorter residence time in the reaction zone and an inherent mechanism that avoids agglomeration of the as produced particles. These

620

Vollath

advantages justify higher costs for investments as they improve the quality of the product significantly.

Physical and technological background

The microwave plasma and its generation The specific characteristic of the microwave plasma is its relative low temperature as compared to chemical flames, dc, or ac plasma. The reason for this behavior is the energy transfer in a microwave plasma that is proportional 1/f 2 (f: microwave frequency) and 1/m (m: mass of the charged particle) [10]. The microwave plasma consists – like any other plasma – of electrons, positively charged ions, radicals, and neutral gas molecules. Therefore, the energy transfer is primarily to the electrons with their low mass. The energy transfer to other species in the plasma is due to collision processes with the high energetic electrons. Analyzing the energy transfer in a microwave plasma in detail, one realizes a strong additional dependence on the gas pressure. Therefore, gas pressure, energy input, and frequency are handles to adjust the conditions for synthesis that are optimal for the desired chemical reactions and the product. The temperature is determined directly after the plasma zone. Depending on the experimental conditions, adjusting of the temperature is possible in a range from 150 to 900°C. Water Load Circulator

Magnetron Directional Couplers

Tri-StubTuner

TE11 Cavity

3 dB Divider Reaction Tube Water Load

Fig. 1. Active and passive components of a microwave system for supplying two consecutive cavities for the synthesis of coated nanoparticles with microwave power.

For synthesis, a tube made of quartz glass crossing either a TE01 [7 - 9] or a resonant TE11 cavity [11, 12] contains the microwave plasma. These cavities are

Microwave Plasma Synthesis of Ceramic Powders

621

tuned with tri-stub-tuners and, additionally, in the case of the TE01 cavity with a sliding short. The advantage of a TE01 cavity is its simplicity in design. This has to be seen in the context of severe disadvantages: Poor microwave efficiency, because the system always operates in the ignition mode, and difficult tuning with two independent tuning elements. The TE11 cavity, a more sophisticated design, is relatively expensive. High microwave efficiency and the fact that plasma ignition needs no special tuning compensate the higher price. Microwave frequencies suitable for these applications are 0.915 and 2.45 GHz. For both frequencies, well established in industrial application, high power magnetrons are available for reasonable prices. Figure 1 displays a schematic overview of the arrangement of the active and passive microwave components. Experimental set-up As plasma gas mixtures of argon or nitrogen with oxygen, hydrogen etc, depending on the intended product, are used. Possible products are oxides, nitrides, sulfides, selenides, or in special cases metals and carbides. Depending on the required particle size of the product, the precursors are introduced as solution in a liquid or as vapor. The solvents may be organic ones or water. Aqueous solutions lead to largest particles, whereas the introduction of the precursor as vapor leads to nanoparticulate material. Solutions are sprayed directly into the microwave plasma using a two-phase nozzle. Particularly with aqueous solutions, it is quite difficult to find operating conditions that avoid the formation of hollow powder particles. In general, when the precursor is in an aqueous solution, the plasma acts like a flame. The situation is different for a vaporized precursor or one dissolved in an organic solvent. In these cases, prior to the reaction, the precursor vaporizes completely. Therefore, in contrast to conventional synthesis, the formation of the particles, in general nanosized ones occurs after ionization and dissociation of the reactive components in the plasma, forming molecules of the desired compound. Following homogenous nucleation by random collision of two or more molecules, further collisions with molecules occur, leading to the growth of the particles. The particles leave the reaction zone with electrical charges of the same polarity. This avoids agglomeration of the particles enabling to coat each one in a second reaction step. The experimental set-up depicted in Figure 1 reflects such a system for the synthesis of coated nanoparticles [12, 13]. Such a process leads to coated nanoparticles, an ideal starting material for nanocomposites with the best possible homogeneity. Applying this process, it is possible to design nanocomposites exhibiting new properties or new combinations of properties. Conventional processes for powder synthesis never result in such a broad range of products with specially designed new properties.

622

Vollath

The products

Products obtained from solutions Powder particles obtained from aqueous solutions of salts are spherical. The actual size and the size distribution of the particles are determined by the properties of the nozzle used for spraying. Additionally, the precursor and its concentration are selected to minimize the risk of hollow sphere formation. Although it may be time consuming to find appropriate conditions for the synthesis, this is a point of great importance. Figure 2 (a) shows a TEM-image of powder particles of ZrO2(Y2O3) produced by this process using nitrate solutions as precursor. It is obvious that the operating conditions were inappropriate because most of the particles are hollow. In Figure 2 (b) a TEM-image of a solid particle out of a batch with better quality is shown.

Fig. 2. ZrO2(Y2O3) particles produced by pyrolysis of metalnitrates in aqueous solutions in a microwave plasma [7] (a) non-optimized process conditions; (b) optimized process conditions.

The spherical shape of the particles is characteristic for properly selected spraying conditions. In this case, the water is evaporated completely before the nitrate dissociated to form the oxide. It is obvious that the dried, water-free precursor was melted prior to dissociation. Starting powders for more complex ceramics, as e.g. for the two-phase system ZrO2(Y2O3)/Al2O3 require perfect blending, with controlled segregation of the two phases. In this case, one may also use aqueous solutions of the nitrates as precursor. For such compositions, a pyrolytic process using microwave plasma is of great advantage. A typical example for a multi-phase product is shown in Fig-

Microwave Plasma Synthesis of Ceramic Powders

623

ure 3. In this TEM-micrograph the core-shell structure of the particle is clearly visible. The core consists of stabilized zirconia, the shell of alumina. Such an arrangement caused by the segregation of the two immiscible phases is the most homogenous that can be obtained with particles of that size.

Fig. 3. ZrO2(Y2O3)/ Al2O3 particles produced by pyrolysis of the nitrate in a microwave plasma. Please note the thin layer of precipitated Al2O3 at the surface [7].

This process may be different using organic precursors [14]. If the dissolved precursor evaporates together with the solvent, formation of particles occurs by gas phase reactions. A detailed description of this case follows in the next chapter. Products obtained from vapor phase reactions

An entirely new class of products becomes available from gas phase reactions in microwave plasma. Such processing conditions allow combining the advantages of a gas phase reaction and of the plasma activation, to synthesize nonagglomerated particles with a narrow particle size distribution at high throughput rates. The basic principles are contradictory: in gas phase reactions a high production rate is obtained only with high particle density in the gas stream, which usually promotes particle agglomeration. Additionally, to avoid sintering of the particles, it is necessary to work at the lowest possible temperature, however kinetic limitation of the chemical reactions may occur. Dissociation and ionization of the reactants in the microwave plasma avoids kinetic hindrances. At significantly reduced reaction temperatures in microwave plasma an additional effect of the electrostatic field is exploited – the repelling forces between particles carrying electric charges of the same sign inhibit agglomeration. Presumably, nanoparticle formation in microwave plasma occurs in the following steps [15]: x Ionisation and dissociation of the reactive components of the plasma. x Reaction of the dissociated species forming molecules of the intended compound. x Homogenous nucleation of the particles by random collision of two or more molecules.

624

Vollath

x Growth of the nuclei by further collision with molecules. x Further growth by coagulation of the particles. The increase of the electrical charge of the particles with size limits this coagulation process. Figures 4 (a) and 4 (b) display TEM-micrographs of typical particles produced by the microwave plasma process. Zirconia, ZrO2 shown in Figure 4 (a) is crystallized in the cubic fluorite structure. In contrast, alumina, Al2O3 shown in Figure 4 (b) is amorphous as nanoparticle with sizes below ca. 8 nm.

Fig. 4. (a) Zirconia nanoparticles exhibiting remarkable narrow size distribution of the different particles [8]. (b) Alumina nanoparticles. As this material is not crystallized, the particles appear “cloudy” [8].

The particles leave the reaction zone non-agglomerated, carrying electric charges repelling each other. Therefore, it is possible to coat each particle individually in a second reaction step. Typical examples are given in Figure 5 (a) and 5 (b). In Figure 5 (a), the core of the particle consists of a zirconia kernel coated with alumina. In this example, the zirconia core is crystallized, whereas the alumina coating is glassy. In this micrograph, there is a good contrast between the core and the coating. This is different in Figure 5 (b), showing a Fe2O3 kernel coated with ZrO2. Kernel and coating are crystallized in different structures. The striation visible in Figure 5 (b) represents the lattice of the coating. The reduced visibility of the lattice fringes in the center of the particle is due to the Fe2O3 core with a different lattice. In this situation, in many cases misfit dislocations are visible in the lattice image of the coating [13, 16]. By exploitation of both phenomena of microwave plasma gas phase reactions, (i) charging of the particles and (ii) the low temperature, it becomes possible to coat the particles with a polymer [17]. A typical example, maghemite, Fe2O3 coated with PMMA is shown as TEM-image in Figure 6. In this case, the polymer coating has two purposes: It acts as a distance holder between the particles and as a “binder“ enabling to sinter these composites at temperatures as low as 100 to 150°C.

Microwave Plasma Synthesis of Ceramic Powders

625

Fig. 5. (a) Zirconia, ZrO2 particle coated with alumina, Al2O3. The lattice image is stemming from the crystalline zirconia core; the alumina coating is amorphous, showing no structural features [13]. (b) Maghemite, J-Fe2O3 particle coated with zirconia. The lattice fringes belong to the zirconia coating. The crystalline maghemite core blurs the lattice fringes in the center of the particle [13].

Fig. 6. Maghemite particles coated with PMMA [17].

Conclusions The microwave plasma is very useful for the synthesis of ceramic powders, independently of the intended product. It is applicable for pyrolysis of nitrates or chlorides in aqueous solutions to obtain fine-grained, sub-micron powders or to produce nanosized ceramic particles by gas phase reactions. In all of these cases the advantages of the microwave plasma are: x Low reaction temperature x Electrically charged particles x High production rate

626

Vollath

These advantages together make it possible to obtain products that are superior to ones obtained by any other method. In special, the main characteristic of a narrow particle size distribution and the possibility of coating the particles with a second ceramic or a polymer are unmatched.

References [1] W. Chang, G. Skandan, H. Hahn, S. C. Danforth, B. H. Kear, Chemical vapor condensation of nanostructured ceramic powders, Nanostructured Mat. 4 345-351 (1994). [2] W. Chang, G. Skandan, H. Hahn, S. C. Danforth, B. H. Kear, H. Hahn, Chemical vapor processing and applications for nanostructured ceramic powders and whiskers, Nanostructured Mat. 4 507-520 (1994). [3] G. P. Fotou, S. E. Pratsinis, Aerosol-made titania for photocatalytic destruction of organics, J. Aerosol Sci. 26 Suppl.1, S227-S228 (1995). [4] C.-H. Hung, J. L. Katz, Formation of mixed oxide powders in flames: Part I: TiO2-SiO2, J. Mater. Res. 7, 1861-1869 (1992). [5] C.-H. Hung, P. E. Miquel, J. L. Katz, Formation of mixed oxide powders in flames: Part II: SiO2GeO2 and Al2O3TiO2, J. Mater. Res. 7, 1870-1875 (1992). [6] H. K. Kammler, S. E. Pratsinis, Electrically assisted flame aerosol synthesis of fumed silica at high production rates, Chem. Eng. and Proc. 39, 219-227 (2000). [7] D. Vollath, K. E. Sickafus, Synthesis of ceramic oxide powders by microwave plasma pyrolysis, J. Mater. Sci. 28, 5943-5948 (1993). [8] D. Vollath, K. E. Sickafus, Synthesis of nanosized ceramic powders by microwave plasma reactions, Nanostructured Mat. 1, 427-437 (1992). [9] D. Vollath, K. E. Sickafus, Synthesis of nanosized ceramic nitride powders by microwave supported plasma reactions, Nanostructured Mat. 2, 451-456 (1993). [10] A. D. MacDonald, Microwave breakdown in gases, John Wiley & Sons, New York, (1966). [11] A. Möbius, M. Mühleisen, German Patent DEGM 29512436.9 (1995). [12] D. V. Szabó, D. Vollath, W. Arnold, Microwave plasma synthesis of Nanoparticles: Application of microwaves to produce new materials, Ceramic Transactions Vol. 111 (2000), in the print. [13] D. Vollath, D. V. Szabó, Nanocoated particles: A special type of ceramic powders, Nanostructured Mat. 4, 927-938 (1994). [14] D. Vollath, K. E. Sickafus, Synthesis of ceramic oxide powders in a microwave device, J. Mater. Res. 8, 2978-2984 (1993). [15] D. Vollath, D. V. Szabó, J. Haußelt, Synthesis and properties of ceramic nanoparticles and nanocomposites, J. Europ. Ceram. Soc. 17, 1317-1324 (1997). [16] D. V. Szabó, D. Vollath, Characterization of ceramic nanoparticles and nanocomposites by high resolution electron microscopy, Advances in Science and Technology Vol. 3B, 1443-1450 (1995). [17] D. Vollath, D. V. Szabó, Synthesis and properties of ceramic-polymer composites, Nanostructured Mat. 12, 433-438 (1999).

Microwave and Conventional Hydrothermal Synthesis of Zirconia Doped Powders F. Bondioli1, C. Leonelli1, C. Siligardi1, G.C. Pellacani1, S. Komarneni² 1

Department of Materials and Environmental Engineering, University of Modena and Reggio Emilia, Italy ² 205 Materials Research Laboratory, The Pennsylvania State University, University Park, Pennsylvania, USA

Abstract Hydrothermal synthesis of powders is a very attractive process to directly prepare submicrometer- and nanometer-sized crystalline powders because of reduced contamination and low synthesis temperature. The application of microwave radiation during the process enhances the reaction kinetics by 1-2 orders of magnitude. Nanosized Pr-doped zirconium oxide powders were prepared by adding NaOH to a zirconyl chloride aqueous solution under microwave-hydrothermal conditions. The properties of the powders produced are compared with those of powders obtained by conventional hydrothermal synthesis.

Introduction Over the years ZrO2 ceramics have been largely used because of their chemical and physical properties. In order to achieve such desirable properties, the synthesis conditions must be well controlled to obtain fine powders with a narrow particle size distribution to enhance reactivity and densification. Among the various methods, the crystallization under hydrothermal conditions [1] is an interesting process for direct preparation of submicrometer and nanometer sized crystalline powders with reduced contamination at low synthesis temperature. The introduction of microwaves during the hydrothermal synthesis increases the kinetic of crystallization by 1-2 orders of magnitude [2, 3]. This characteristic results are of particular importance for preparation of ceramic solid solutions. In fact, the high temperatures usually necessary for their synthesis, are seriously limiting the preparation, the study and the use of such materials. Therefore, there is a growing interest in the development of non-conventional methods to synthesize ceramic solid solutions at low temperature and reduced reaction time.

628

Bondioli

The aim of this work has been to verify the effect of the microwave application upon hydrothermal synthesis by comparing the properties of (Zr,Pr)O2 powders obtained from the microwave processing route with those observed on conventionally synthesized powders. Praseodymium oxide has been chosen [4] for several reasons:  For being one of the materials under investigation in the field of oxygen storage materials  For undergoing oxygen exchange at a temperature lower than those recorded for cerium oxide  For its oxygen storage capacity not being diminished by a high temperature of sintering [5].

Experimental procedure

Materials The solution composition of ZrOCl2, 0.5 M, and Pr(NO3)3 was adjusted such to obtain a stoichiometric theoretical formula corresponding to Zr0.9Pr0.1O2. The solution was neutralized with 1 M NaOH to pH = 10 and poured in the hydrothermal apparatus. Apparatus The microwave-assisted syntheses were conducted in a microwave digestion system (Model MARS5, CEM, North Carolina, USA). The system uses microwave radiation at a frequency of 2.45 GHz and is controlled by both temperature and pressure (Pmax = 14 atm). The reaction vessel was connected to a pressure transducer that monitors and controls the pressure during synthesis. After preliminary tests, microwave-hydrothermal treatments were conducted at 200°C for different time (30 min - 4 hrs). The time, pressure/temperature, and power were computercontrolled. The conventional syntheses were conducted at 200°C for different time (1 - 24 hrs) in an electric oven using bombs with metal bodies and removable PTFE liners (Parr Instrument Company, Illinois, USA). After both synthesis reactions, the powders were filtered, washed and dried. Powder characterization All the synthesized products were analyzed with a computer-assisted X-ray powder diffractometer (Model PW3710 Philips, Eindhoven, Netherlands) using Cu-

Microwave and Conventional Hydrothermal Synthesis

629

KD-radiation. The X-ray diffraction (XRD) patterns were collected in a 2T range of 25 - 90q at room temperature, with a scanning rate of 0.005q/s and a step size of 0.02q. Crystallization behavior of samples was studied by thermogravimetric analysis (TG) in air at a heating rate of 10qC/min (Model STA 409, Netzsch, Selb, Germany) [6]. The sample morphology was examined by transmission electron microscopy (TEM) (Model JEM 2010, Jeol, Tokyo, Japan). Specimens were prepared by dispersing as synthesized powders in distilled water and than placing a drop of suspension on a copper grid covered with a transparent polymer followed by drying.

Results and discussion To investigate the nature of the obtained powders, all the samples were subjected to powder X-ray diffraction. The stabilizing effect of the praseodymium ion on the zirconia tetragonal phase, by forming the (Zr,Pr)O2 solid solution, is evidently underlined in Figure 1. The XRD pattern reported is characteristic independently of the synthesis methods used. In particular, the absence of alkali ions in the last washing water indicated the efficacy of the washing step.

Fig. 1. XRD patterns of microwave(a) and conventionally (b) treated powder (200°C, 2 hrs chosen as representative sample)

In order to optimize the synthesis conditions, the powders obtained at different soaking time by both synthesis methods were characterized by thermogravimetric analysis. In Figure 2 the typical behavior of the weight loss as a function of temperature is reported. The observed weight losses are due to loss of both chemically coordinated and physisorbed water and to removal of the oxo-bridging and non-

630

Bondioli

bridging structural hydroxyl groups [6]. Concomitant with this process crystallisation takes place. The crystallization degree of the obtained (Zr,Pr)O2-solid solution was analyzed indirectly by measuring the weight loss upon TG-analysis in the 50 1100°C temperature range. As shown in Figure 3, microwave treatment increases both reaction rate and crystallinity of the powders. 2 hrs microwave treatment seems to be sufficient to obtain a 90% crystallization of the solid solution while the conventional treatment takes 12 hrs in order to obtain the same result.

Weight Loss (%)

-1 -3 -5 -7 -9 -11 -13 -15 0

300

600

900

1200

Temperature (°C) Fig. 2. TG curve of microwave treated powder (200°C, 2 hrs as representative sample)

Crystallinity (%)

100

b)

95

a)

90 85 80 75 70 0

4

8

12

16

20

24

time (hrs) Fig. 3. Crystallinity evolution for (a) conventional and (b) microwave hydrothermal prepared powders, deduced from TG-measurements

TEM observations of particles revealed spherical shaped particles with no agglomeration and grain size dimension lower than 10 nm in the samples obtained

Microwave and Conventional Hydrothermal Synthesis

631

by microwave thermal treatments (Figure 4a). Instead the conventional powders obtained by a soaking time longer than 12hrs showed a remarkable agglomeration of nano-sized crystallite (Figure 4b).

Fig. 4. TEM micrographs (12000 X) of (a) conventional and (b) microwave hydrothermal prepared powders

Conclusions Nanostructured (Zr,Pr)O2 (Pr = 10 mol%) powders were synthesized via conventional and microwave hydrothermal methods. The application of microwaves leads to shorter processing schedules and, at the same time seems to be an efficient way to improve powder quality, enhancing powders crystallinity.

Acknowledgements The financial support given by MURST-PRIN Contract, “Application of the microwave technology to physical-chemical processing involving solids- 1999 2000” is gratefully acknowledged. Moreover, the authors would like to thank Ms. S. Braccini for performing the synthesis experiments.

632

Bondioli

References [1] S. Somiya, T. Akiba, Hydrothermal zirconia powders: a bibliography, J. Eur. Ceram. Soc., 19, 81-87, (1999) [2] S. Komarneni, M.C. D’Arrigo, C. Leonelli, G.C. Pellacani H. Katsuki, Microwavehydrothermal synthesis of nanophase ferrites, J.Am.Ceram.Soc, 88 (11), 3041-44, (1998) [3] F. Bondioli, A.M. Ferrari, C. Leonelli, C. Siligardi, G.C. Pellcani, Microwavehydrothermal synthesis of nanocrystalline zirconia powders, J. Am. Ceram. Soc (2001) in press. [4] C.K. Narula, J.E. Allison, D.R. Bauer, H.S. Gandhi, Materials chemistry issues related to advanced materials applications in the automotive industry, Chem. Mater. 8, 9841003, (1996). [5] A.D. Logan, M. Shelef, Oxygen availability in mixed cerium/praseodymium oxides and the effect of noble metals, J. Mater. Res., 9, 468-72, (1994). [6] G. Dell’Agli, A. Colantuono, G. Mascolo, The effect of mineralizers on the crystallization of zirconia gel under hydrothermal conditions, Solid State Ionics, 123, 87-94, (1999)

Microwave Decomposition of Metal Alkoxides to Nanoporous Metal Oxides – A Mechanistic Study F. Bauer, T. Schubert, M. Willert-Porada Chair of Materials Processing, University of Bayreuth, Bayreuth, Germany

Abstract Synthesis of nanoscaled TiO2 and ZrO2 by pyrolysis of metal alcoholate – paraffin emulsions using microwave heating (Colloidal Microwave Pyrolysis, CMP) as well as conventional heating (Colloidal Conventional Pyrolysis, CCP) is described. The ceramic yield of the inorganic products, the pore structure, grain size and phase composition of the nanoscaled oxides is influenced to a lesser extend by the heating method as by the organic by-products. Different reaction paths are found for Ti-alcoholate as compared to Zr-alcoholate. A mechanism is proposed for the decomposition of each alcoholate, showing that microwave heating enhances the catalytic activity of the oxide powders generated during the reaction.

Introduction Oxides of titanium and zirconium are important materials, used as powders for catalysts and pigments as well as starting materials for ceramics important in high temperature functional or structural devices. The application area depends upon the phase composition, e.g., rutile or anatase in case of TiO2 and monoclinic-, tetragonal- or cubic-ZrO2 in case of zirconia. On industrial scale oxides of titanium are obtained hydrometallurgically by leaching of minerals with different acids, whereas ZrO2 is produced by plasma decomposition of ZrSiO4. Modification of the phase composition and powder morphology is limited in these processes. Depending on the manufacturing process quite different material properties are generated. Nanoscaled TiO2- and ZrO2– powders have attracted attention for new applications, e.g. n-TiO2 as photocatalyst in dye based photovoltaic elements, the so called Grätzel solar cells, and n-ZrO2 as superacid catalysts, when combined with mineral acids [1]. The photocatalytic properties of titania depend on its phase composition, particle size and surface area. Quantum size effects on the band gap

634

Bauer

width, which can be exploited in electro-optical devices, become increasingly important for small particles. The aim of the study was to synthesise nano-TiO2 and nano-ZrO2 powders by pyrolysis of commercially available alcoholates, with a high degree of reproducibility of powder properties. Based on previous work on Colloidal Microwave Processing, CMP [2], different heating methods were applied in order to identify the decomposition mechanism and find key parameters of the pyrolysis process governing the powder properties.

Experimental

Pyrolysis experiments Commercially available alcoholates of titanium and zirconium (Ti(O-n-C4H9)4, Ti-butylate, Chempur; Zr(O-n-C4H9)4 C4H9OH, Hereaus, both Germany) were applied. All operations are performed in Argon-atmosphere, to prevent hydrolysis and oxidation. As solvent Paraffin 60 - 90 is used (Fluka, Germany). Xylene (Merck, Germany) is used for removal of solvent and non volatile by-products after the pyrolysis reaction. Pyrolysis experiments were accomplished either by a 200 W resistant heating pad or in a domestic Siemens microwave oven, equipped with a 870 W, 2.45 GHz magnetron. The set-up is shown in Figure 1. exhaust gas

T

T

water cooling

thermocouple water cooling

microwave cavity

Argon in

Argon in

movable thermocouple

exhaust gas

volatile fraction (B) high b.p. fraction (A)

high b.p. fraction (A) volatile fraction (B)

T

thermal insulation

Fig. 1. Experimental set-up for conventional heating (left) and microwave heating (right) pyrolysis experiments

In a typical experiment a 200 ml flask is charged with 20.0 g of paraffin, 66.0 ml of titanium alcoholate or 80.0 ml of the zirconium butylate complex, and the mixture is heated under dry argon atmosphere until the reaction is completed at approximately 588 K for the Ti-compound and 663 K for the Zr-compound. The temperature inside the flask is measured with a type K thermocouple, with the mi-

Microwave Decomposition of Metal Alkoxides

635

crowave power switched off for a short period of time in case of microwave heating. A slight constant argon gas stream is passing above the reaction mixture and through cooling traps at ambient temperature and at 195 K (-78°C, dry ice in alcohol), in order to collect the volatile by-products of the pyrolysis reaction, as shown in Figure 1. In addition, samples of the exhaust gas were directly collected and analyzed for organic compounds by mass spectroscopy. Product purification and analysis The oxide powders were purified by hot extraction with xylene several times to remove the paraffin, filtered under argon and dried for 3 h at 380 K. High molecular weight impurities were removed under vacuum at 480 K, the non volatile organic residue by calcination of the powder for 3 h at 480 K in air. The volatile products fraction (B in Figure 1), collected at 195 K, was warmed up to room temperature and analyzed as a gas. The high boiling point fraction of volatile products (A in Figure 1) was divided by vacuum distillation up to 450 K into a lower boiling point colorless liquid and a yellowish residue. The gaseous and liquid decomposition products were identified by means of GC-MS (Hewlett-Packard HP 5890, Finnigan MAT 95), HPLC-UV-VIS (SpectralSystems P 2000) and 1H-13C-NMR (Bruker AC 300). For quantitative measurements of the liquid and gaseous fractions a Hewlett-Packard HP 6890 GC with HP-Plot Q column and a Carlo-Erba Series 4160 GC with DB-5 column was used. As standards mixtures of liquids and purchased gas standards from Messer Griessheim, Germany were used. The composition of the organic residue was analysed by TGA-FTIR (Netzsch STA 409). The metal oxide powders were characterised by N2-BET specific surface area measurements and Hg-porosimetry (Micromeritics ASAP 2010), XRD pattern (Philips X-Pert, Cu KD radiation) and electrical conductivity (Solartron 1285 potentiostat).

Results

Pyrolysis experiments The progress of the pyrolysis reaction can be followed by recording the temperature profile and correlating it with the amount of volatile products collected as liquids in calibrated traps. As shown in Figure 2 for the pyrolysis of Ti-butylateparaffin mixtures a certain “induction” period of time after the decomposition temperature is reached is needed to start the pyrolysis reaction. There is a remarkable difference between microwave and conventional heating with respect to the length of this induction-period in case of Ti-butylate decomposition, as shown in Figure 2. Although in both cases a nearly identical heating rate is applied, high

636

Bauer

Ti-butylat microwave

400

100 90

300

80

200

70

conventional

High b.p. fraction (A)

60 50

100

40

0

30

Low b.p. fraction (B)

20

-100

10

-200 0

20

40

60

80

100

120

140 160

0

Volume of volatile by-products [ml]

Temperature of the reaction mixture [°C]

boiling point volatile products appear after 20 minutes at 300°C from the conventionally heated reaction mixture, whereas upon microwave heating almost two hours of heating are needed to start the decomposition. The time to complete the pyrolysis, however, is only 30 minutes upon microwave heating and twice as long for conventional heating. The overall amount of volatile by-products is nearly the same for both heating methods.

Heating time [min]

microwave

Zr-butylate

100

400

90

300

80 70

200

High b.p. fraction (A)

60

100

50 40

conventional

0 -100 -200

30

Low b.p. fraction (B)

20 10

0

20

40

60

80

100

120

140

0

Volume of volatile by-products [ml]

Temperature of the reaction mixture [°C]

Fig. 2. Time-temperature-volatile product evolution profiles of Ti-butylate-paraffin pyrolysis upon conventional and microwave heating.

Heating time [min]

Fig. 3. Time-temperature-volatile product evolution profiles of Zr-butylate-paraffin pyrolysis upon conventional and microwave heating

For the Zr-butylate-paraffin mixture a completely different result is obtained, as shown in Figure 3. It is not possible to achieve identical heating rates for the conventional and microwave heating, most probably because of the additional butanole-molecule in Zr(O-n-C4H9)4 C4H9OH as compared to the solvent free Ti-

Microwave Decomposition of Metal Alkoxides

637

butylate, leading to a different microwave heating behaviour as well as decomposition reactions [2]. Microwave absorption is much more efficient for Zr-butylate as for the Ti-compound. Removal of the co-ordinated alcohol-molecule is an endothermal process, which starts in the Zr-butylate-paraffin mixture after 5 minutes of microwave heating, at a temperature around 200°C. In the conventionally heated mixture this process starts at the same temperature, but because of the endothermal reaction it takes 4 times as much time to complete this step by surface heating of the reaction mixture as compared to volumetric microwave heating. As visible from the change in slope for the volume of the volatile products curve in Figure 3 decomposition is proceeding in two steps, independent of the heating method. Heating is stopped as soon as the amount of volatile products remains constant. In the microwave heated Zr-butylate mixture rapid cooling occurs, whereas in the Ti-butylate mixture slow cooling is observed, as shown in Figure 2 and 3. From this finding it could be assumed, that the organic as well as inorganic products of the pyrolysis remaining in the paraffin matrix are better microwave absorbers in case of the Zr-butylate decomposition as compared to the Ti-butylate reaction. Product purification and analysis

Inorganic products As described in the experimental part, the nanoscaled powders were purified from non volatile residue by solvent extraction, followed by drying and calcining. In Table 1 the physical properties of the powders are summarised. For comparison, a commercially available titania-powder is shown. Independent of the heating method, nanoscaled ZrO2 and TiO2 powders are obtained, with comparable primary crystallite size, pore size and phase composition in case of TiO2, but a much lower crystallinity in case of the conventionally heated Zr-butylate. There are subtle differences in the ceramic yield, which is higher for the conventionally heated reaction mixtures, however on the expense of a lower specific surface area (TiO2 and ZrO2) and phase homogeneity as well as crystallinity (ZrO2). A more pronounced difference is found for the colour of the TiO2-powders obtained from conventional as compared to microwave heating of the reaction mixture. As described in the “organic products” part, high molecular weight secondary reaction products are responsible for this effect. These results indicate a more selective nucleation step of powder synthesis for the microwave heated mixtures, with some loss of the starting alcoholate caused by the very high heating rate and the short reaction period as compared to conventional heating in case of Zr-butylate.

638

Bauer

Table 1. Properties of the oxides obtained from Ti- and Zr-butylate pyrolysis in paraffin as solvent by conventional ( C ) and by microwave (MW) heating Quantity Organic residue/ %: Ceramic yield after calcining / %: BET-surface m2/g: BET-average pore diam. / nm XRD-crystal size nm Colour Phase composition

TIO2-C 22,8 95

ZrO2-C i. A. 82

269,1 3,7

-

4,5 Pale yellow

white

anatase

t, (m)

TIO2-MW ZrO2-MW TIO2-P25* 9,1 12,6 92 76 338,9 3,2 7,0 Dark yellow to orange anatase

151,0 2,2

49,2 -

2,2 white

78,0 white

t

anatase, rutile

* commercial product

Organic products Much more information about the pyrolysis itself can be drawn from analysis of the organic products. The collected fractions A and B of volatile materials were investigated with respect to composition, structure and concentration of organic substances. The variety of chemical compounds found in the volatile by-products is astonishing, as shown in Table 2, 3 and 4. Among the by-products are highly volatile alkanes and alkenes (butane, butene), carbonyl compounds, aldol-condensation products (poly 1,3-butadiene) and oligomers typical for Ziegler-Natta type polymerisation reaction products, like e.g., polybutylenes. In Table 2 the ratio of the alipahtic product butane is compared with the carbonyl compound ratio. The heating method has no large effect, in contrast to the large influence of the metal: Ti-butylate decomposition follows a different mechanism as compared to Zr-butylate pyrolysis, with a large amount of butane and carbonyl compounds evolving from the pyrolysis mixture in case on Ti-butylate. Table 2: Butane and carbonyl compounds M Ti Zr

method MW conv. MW conv.

Butane / mol% 36 31 4.0 2.0

Carbonyl Compounds / mol% 22 24 1.0 1.0

Another piece of information comes from the equivalent value of the main products of pyrolysis in case of Ti-butylate as compared to Zr-butylate. In Table 3 these values are shown as function of the heating method. The largest contribution of organic by-products belongs to the group of unsaturated butenes, with a characteristic isomeric distribution, shown in Table 4. High molecular weight com-

Microwave Decomposition of Metal Alkoxides

639

pounds from condensation reactions make up the third large group of decomposition products. Table 3. Equivalent value of the main products of the decomposition of Ti- and Zr- n butylate Decomposition CCP CMP method Product name TIO2-C ZRO2-C TIO2-MW n-Butanea 0,31 0,02 0,36 trans-2-Butene 0,02 0,01 0,15 cis-2-Butene 0,02 0,01 0,11 1-Butene 0,17 0,61 0,36 1-Butanol 1,17 0,94 1,33 n-Butanal 0,17 0,01 0,03 Di-Butylether 0,11 0,05 0,42 2-Ethyl-trans-hex-20,07 0,00 0,19 en-1-one Oktenes 0,12 0,11 0,19 Dodekenes 0,15 0,02 0,12 Sumb 2,50 2,34 3,46 a: liberated C4-fragment equivalent per mole metal alcoholate b: sum of all products, some of them are not mentioned in the table

ZRO2-MW 0,04 0,00 0,00 0,99 1,18 0,01 0,11 0,00 0,21 0,02 2,90

Table 4. Isomeric distribution of butenes Metal

Heating method

1-Butanol [mol%]

1-Butene [mol%]

2-Butenes Ratio 1:2 [mol%], cis butene and trans Ti CMP 133 36 26 1.4 CCP 117 17 4 4.3 Zr CMP 118 99 0 >100 CCP 94 61 2 30.5 a: thermodynamic equilibrium at 25°C for free carbocations present

Theoret. a Ratio 1:2 0.03 0.03 0.03 0.03

The significant difference of the isomeric distribution of butenes during CMP and CCP as compared to the thermodynamically most probable distribution shows that the nanoscaled powders generated during the pyrolysis act as catalytic active surface in the formation of butenes, and that the catalytic activity of n-ZrO2 upon microwave heating is higher than upon conventional heating. In case of n-TiO2 the effect is less pronounced, for CCP as well as for CMP. However, upon pyrolysis of Ti-butylate aldol-condensation products with conjugated double bonds appear, with an increasing amount upon CMP as compared to CCP. Organic secondary products As shown in Table 1, the nano-TiO2-powders are yellow, independently of the heating method, with increased colour intensity in case of microwave heating. Opposite to this, both nano-ZrO2 powders are white. Closer inspection of the decomposition product pattern, shown in Figure 4 and Figure 5, reveals among the

640

Bauer

secondary products compounds containing up to at least 3 or 4 conjugated carbon double bonds, due to aldol condensation of butyraldehyde, e.g., 2-ethyl-trans-hex2-en-1-one1 , for the Ti-butylate reaction only. The colour differences between nTiO2 and n-ZrO2 clearly correspond to the equivalent value distribution of secondary reaction products. In case of the white nano-ZrO2 powders no conjugated carbon double bonds containing secondary products are found, whereas synthesis of nano-TiO2 by a pyrolysis reaction is accompanied with the formation of such by-products. “As synthesised” titania powder can be whitened using ultraviolet radiation (UV) in air and argon atmosphere, due to photolysis of conjugated double bonds. Also treating the titania with peroxides bleaches the powder by hydroxylation of double bonds. The decomposition patterns of the Ti- and Zr-butylate shown in Figure 4 and 5 display at the y-axis C4 – “equivalents”, based on the four butylate groups of tetravalent metal alcoholates. Compounds containing 8 carbons therefore appear with the double intensity in the graph and the overall sum of all products, including unconsidered products and non-volatiles is 4. decomposition patterns of titanium butylate 1,40 TIO2-C

C4 - equivalents

1,20

TIO2-MW

1,00 0,80 0,60 0,40 0,20 0,00 n-butane

trans-2butene

cis-2butene

1-butene 1-butanol

butanal

butyl ether

2-ethyltrans-hex2-en-1one

Fig. 4. Decomposition pattern of the organic by-product during colloidal microwave pyrolysis (CMP) and colloidal conventional pyrolysis (CCP) of Ti-butylate

1 The UV-VIS spectrum of the condensation products of the titanium butylate show at least three absorption bands with maxima below 200 nm (strong), at 215 nm (strong, SS ) and at 280 nm (weak, nS  It can be attributed to an aldehyde with a conjugated carbon carbon double bond (ethylhexenal). In order to determine the chromophoric groups, the condensation products were separated by means of HPLC and absorption was measured at two wavelengths (220, 300 nm) simultaneously. Peaks that elapsed after the same time at both wavelengths with an intensity ratio between 500 to 1500 H220H300) may indicate aldehydes with conjugated double bonds. This was observed for the three main products of the chromatograms. From this observations it can be concluded that aldol condensation as secondary reaction significantly contributes to the high molecular weight fraction.

Microwave Decomposition of Metal Alkoxides

641

decomposition patterns of zirconium butylate 1,40 ZRO2-C

C4 - equivalents

1,20

ZRO2-MW

1,00 0,80 0,60 0,40 0,20 0,00 n-butane

trans-2butene

cis-2butene

1-butene

1-butanol

butanal

butyl ether

2-ethyltrans-hex2-en-1one

Fig. 5. Decomposition pattern of the organic by-product during pyrolysis of Ti-butylate

Discussion

Mechanism of alkoholate pyrolysis Mechanistic studies of metal-alcoholate pyrolysis reactions are complicated by the difficulty to achieve a constant and reproducible induction period. This is known from previous work on the decomposition reactions of alcoholates [2, 3], and was also observed for the Ti- and Zr-butylate upon CMP as well as CCP in the work reported here. Till today no clear explanation exists for this effect, but there is increasing evidence, that traces of water my strongly influence the induction period. A mechanism for the decomposition of metal alcoholates was first proposed by Bradley et al. [3], followed by newer studies on this topic [4]. The overall reaction of metal alcoholate under conditions of constant pressure is assumed to be: M(OR)4 Æ MO2 + 2 ROH + 2 alkene

(1)

A chain reaction is proposed, with formation of an alcohol molecule followed by dehydration of the alcohol into an alkene and H2O. Water is than causing hydrolysis of the alcoholate to yield another alcohol molecule, which again decomposes to water and alkene. If the reaction is performed without removal of water (sealed reactor), complete dehydration of the alcohol, as verified by means of V.P.C. analysis and IR spectroscopy occurs, as shown in Equation (2). M(OR)4 Æ MO2 + 2 H2O + 4 Alkene

(2)

Nandi et al. [4] identified an additional reaction path during vacuum flash pyrolysis of titanium alkoxides, shown Scheme 1. According to Nandi et al. the forma-

642

Bauer

tion of carbonyl compounds is due to E-hydrogen abstraction [5] leading to a change in the oxidation state of Ti4+ to Ti2+. The E-hydrogen abstraction reaction is already known from the decomposition of group-VIII-transition metal monoalcoholates, into a metal hydride and a carbonyl compound [5]. (RO)2Ti

O CH2

CH3

E-H

O

R

(RO)2Ti O

CH

CH3 +

H (RO)2Ti + ROH

O

abstr.

R

Scheme 1: Decomposition reaction of Ti-alcoholates during vacuum flash pyrolysis, after [4] to form Ti2+- compounds and carbonyl compounds

Occurrence of such Ti2+- compounds could explain the formation of butane by an unimolecular reaction upon reduction of butanol, butene or butoxy groups, as shown in Scheme 2. (RO)2Ti

H

O

(RO)2Ti O

+

Butane

C4H9

Scheme 2: Alternative decomposition reaction of Ti(II)-compound to yield butane.

Another unimolecular reaction to be considered is the direct formation of an ether, as shown by Equation (3). Ti(OR)4 Æ (RO)2TiO + ROR

(3)

Pyrolysis upon CMP and CCP In contrast to the reaction conditions reported by Nandi et al. [4], volatile reaction products are likely to have an extended dwell time in the reaction mixture upon CMP and CCP, because of the surrounding paraffin solvent. Accordingly, butene is observed in our experiments. Among the organic by-products of the pyrolysis reactions performed within this study a large amount of additional compounds was found, as shown in Table 2, 3 and 4 and in Figure 4 and 5, particularly for the decomposition of Ti-butylate. The pattern of the decomposition by-products of Zr-butylate is in good agreement with the mechanism according to Equation (1). Remarkably a nearly theoretical ratio of alcohol to butene is found, indicating a very smooth decomposition. For the decomposition of Zr-butylate, a main reaction according to Equation (4) could therefore be assumed.

Zr>O  nC 4 H 9 @4 l Zr O O  nC 4 H 9 2  C 4 H 9 OH  C 4 H 8

(4)

In addition, direct formation of a di-butylether according to Equation (3) could contribute to the organic by-producta spectrum.

Microwave Decomposition of Metal Alkoxides

643

Decomposition of Ti-butylate is more complicated and includes not only reactions according to Equation (1) but quite a contribution from reactions shown in Scheme 1, 2 and 3 as well as in Equation (3). These reactions are more pronounced upon CMP as compared to CCP, yielding: 1) high amount of butane evolving during the reaction 2) ethylhexenone, octenes and higher molecular weight compounds as byproducts In addition, for both, Ti- and Zr-alcoholate, the isomeric distribution of the butenes does not agree with the one expected from thermodynamic equilibrium which predicts 74% trans-butene, 23% cis-butene, 3% 1-butene (at 25°C) if free carbocations were present. Instead, formation of 1-butene is preferred. Butane formation is always accompanied by the formation of carbonyl compounds, according to Scheme 1 and 2. Such compounds are very reactive and contribute to the formation of secondary products. The amount of organic residue increases with increasing partial pressure of monomers, like e.g., butene and butyraldehyde, reaching values of 10 wt% when the reaction is carried out with an inert purge gas stream or 25 wt% without a purge gas. High molecular weight molecules could evolve from Ziegler-Natta type polymerisation (poly butylene) and aldol condensation (poly 1,3-butadiene), as suggested in Scheme 3. Such compounds contribute to the insoluble organic residue trapped in the nano-TiO2 and ZrO2.

Butene

' Ti(IV)

' Butanal

*

n

*

H+

- H2O

n

*

O Scheme 3: Formation of high molecular weigt compounds by secondary reactions of butene and butanal

From the isomeric distribution of the butenes, shown in Table 4, strong evidence is given that the formation of butenes takes place at the surface of a catalytic active site of the metal oxide precursor or the nanoscaled metal oxide itself, because the isomer distribution significantly differs from the thermodynamic equilibrium, Furthermore, differences of the distribution depending on the metal are evident, with a lower deviation found for the less acidic oxide TiO2. The observation that the decomposition of titanium butylate furnishes a much higher amount of carbonyls and alkanes is in good agreement with the strongly reduced tendency of Zirconium to form a 2+ oxidation level as compared to Titanium.

644

Bauer

Conclusion Pyrolysis of Zr- and Ti-butylate in paraffin solution is a suitable method for the reproducible synthesis of nano-ZrO2 and TiO2. The inorganic product quality is higher upon microwave heating of the reaction mixture (CMP) as compared to conventional heating (CCP). This could be an effect of volumetric heating as well as due to the fact, that the dielectric loss of the reaction mixture decreases upon formation of the metal oxide. Forthermore, removal of the major organic primary reaction product – butanol – proceeds much faster upon microwave heating than by conventional heating. Surprisingly, a significant difference is found for the mechanism of the pyrolysis of Zr(O-nBu)4 as compared to Ti(O-nBu)4, the later being complicated by formation of a larger amount of organic high molecular weight secondary products, difficult to separate from the inorganic powder. The decomposition of the titanium precursor is more complex since an intermediate change in the oxidation state of titanium can take place. Secondary reactions are influenced by the heating method, which most probably enhances the catalytic activity of the nano-oxide powder in case of microwave heating.

Acknowledgements The authors thank BITÖK, University Bayreuth, DFG-0339476 D, for perfoming the gas analysis and Ludolf Eusterbrock and Jürgen Hacker, Chair for Ceramic and Composite Materials, University Bayreuth, for UV, HPLC-UV and TGA-IR measurements. Financial support of DFG, Wi-856-13-1 is gratefully acknowledged.

References [1] V. F. Stone Jr. and R. J. Davis, Chemistry of Materials 10, 1468 (1998)

[2] Ch Gerk. C-W Schmidt, A Niesenhaus, and M. Willert-Porada, Ceram. Trans. Vol. 80, 387-392 [1997]. [3] D. C. Bradley and M. M. Faktor, Trans. Faraday Soc. 55, 2117 (1959) [4] M. Nandi, D. Rhubright, and A. Sen, Inorganic Chemistry 29, 3065 (1990) [5] C. F. de Graauw, J. A. Peters, H. van Bekuum, and J. Huskens, Synthesis 10, 1007 (1994).

Characterization of SiC Produced by Microwaves Juan Aguilar, Zarel Valdez, Ubaldo Ortiz, Javier Rodríguez. Universidad Autónoma de Nuevo León, FIME AP. 076 F, San Nicolás de los Garza, N.L., México

Abstract Characterization of silicon carbide produced in our laboratory from a silicon oxide and graphite mixture by means of microwaves as an energy source is presented, with respect to process parameters such as microwave power and achieved temperature. Using a magnetron operating at 2.45 GHz at output power up to 2000 W for times as long as 25 minutes temperatures in the order of 2000°C are reached. The SiC phase composition was determined by means of X-rays diffraction, the powder morphology by scanning electron microscopy. Most of the product is ESiC, in agreement with the reaction temperature close to this polytype growth regime. Depending upon the heating rate, different particle morphology, from coarse grains to whiskers occurs, although the phase composition is unchanged. A comparison with SiC from an electrical resistance heating process and with commercial SiC for fired products is shown.

Introduction Silicon carbide (SiC) is one of the products of a carbothermal reaction between silica (SiO2) and carbon (C). One of the aspects relevant in industrial SiC production technology is the temperature. The Acheson process achieves temperatures in the range of 2200°C - 2500°C [1], although at early stages it could be as high as 2700°C with zones at around 1800°C [2]. Depending on the location in the furnace the heating conditions are different, therefore the product is mainly SiC, but with different polytypes. Production of SiC by means of microwaves as an energy source has been reported in the literature [3]. The heating rate of the reaction mixture depends strongly on the permittivity, which is related to wave-matter interactions. It is known that graphite absorbs microwaves to such an extent, that it has been used as an auxiliary to facilitate the production of MgAl2O4 spinel at laboratory scale [4]. The superior heating ability of graphite is evident, when comparing the heating of a 20 g of silicon oxide sample exposed to 2.45 GHz microwaves at 800 W for

646

Aguilar

300 seconds with a graphite sample under the same nominal conditions, as shown in Figure 1 and 2. 500 450

Temperature (°C)

400 350 300 250 200 150 100 50 0 0

50

100

150

200

250

300

Time (sec)

Fig. 1. Temperature against time of 20 g of silicon oxide exposed to 2.45 GHz microwaves at 800 W for 300 seconds. Maximum heating rate was 7°C/s. 900 800

Temperature (°C)

700 600 500 400 300 200 100 0 0

50

100

150

200

250

300

Time (sec)

Fig. 2. Temperature against time of 20 g of graphite exposed to 2.45 GHz microwaves at 800 W for 300 seconds. Maximum heating rate was 21°C/s.

These tests were conducted in a multimode cavity and the nominal power is not the actually applied to the sample. However, it is evident that heating with microwaves is feasible and that a variety of conditions can be obtained by increasing the applied power, indeed SiC has been found at processing temperatures as low as 727°C [3]. The issue of SiC-synthesis is related to the difficulty in obtaining a pure polytype. SiC can be found in more than 74 polytypes [5]. The cubic structure SiC-3C (because the three-layer stacking sequence and the cubic symmetry) is defined as E-SiC and exists in the temperature regime of 105 Tonns/year) FBR-processes, which do not require extreme purity and sophisticated process control, for CVD applied in fluidized beds new heating technology is important, in order to significantly improve coating yield and homogeneity, e.g., by adding to the good heat and mass transfer the unique feature of selective heating of the bed particles by dielectric loss within the volume of the fluidized bed. In existing high purity fluidized bed processes, used for drying of pharmaceutical products or of selected minerals, temperature is rather low. In contrast, CVD-processes usually require high temperatures in the range of 800 - 1200°C [6]. Conventional heating of FBR`s by reactor wall or fluidised gas heating would increase contamination of the bed material by abrasion from hot reactor walls and formation of by-products from CVD-reaction with the wall material or within the overheated gas mixture.

CVD-Processes in Microwave Heated Fluidized Bed Reactors

721

The paper presents experimental results on microwave heating within such new CVD-FBR-processes for two different applications: the production of solar grade silicon and the coating of carbon materials with ceramics. Fluidized bed technology In order to understand dielectric heating of fluidized beds, some fundamentals of this technology are summarized. A fluidized bed is formed by an upwards streaming fluid passing through a particle bed. If the gas velocity is lower than minimum fluidization velocity all particles are touching each other, the bed is not moved but rather “fixed”, as shown in Figure 1. At higher gas velocities the bed begins to move and the pressure drop decreases. In most technical systems formation of gas bubbles is observed at higher gas velocities. If the bubble diameter reaches the reactor dimension, the bed begins to slug. With a further increase of the gas velocity the particles leave the reactor, pneumatic transport occurs. Fixed Bed

Minimum Fluidization

Bubbling Bed/ Slugging

Pneumatic Transport

Gas

Gas

Gas

Gas

Fig. 1. Different stages of fluidization in a FBR.

Conventional and microwave heating of fluidized beds Different heating methods for fluidized beds have been investigated up to now. These methods can be divided into indirect and direct heating of the bed. The common method of indirect heating is resistance heating of the reactor walls. If the heat input from the surface of the reactor is not sufficient, a heat exchanger can be installed directly in the fluidized bed, although hydrodynamic and abrasion behavior are influenced negatively. Particularly for drying processes preheating of the fluidization gas is applied. However, if the fluidization gas is hotter than the

722

Gerdes

particles itself, reactive gasses needed for CVD would undergo homogeneous gas phase reaction instead of heterogeneous nucleation and CVD at the particle surface. As most important direct heating method for processes with large mass flow exothermic reactions are used, which release energy directly into the fluidized bed in order to reach a temperature level not achievable economically by indirect heating methods. The best example is coal gasification, where a partial oxidation is executed, which however limits the selection of the gas atmosphere and the yield. In case of CVD only highly exothermic reactions could be used for direct heating, because of dilution of the CVD-gases with the fluidization gas. Other methods of direct bed heating use dielectric loss, e.g., capacitive heating, microwave or RF-heating. The main difference between direct and indirect heating is the direction of heat flux: when heating the reactor wall or the gas, the heat has to be transferred from the wall to the fluid and/or from the fluid to the particles. Direct heating means heat flux from the particles to the fluid and to the walls. In both cases temperature gradients are the driving force for the overall heat flux. Although on principle direct dielectric heating of the bed should be possible without a macroscopic temperature gradient, practically due to the heat losses through the wall an inverse temperature gradient may develop between the bed and the walls of the FBR. Microwave heating of fluidized beds requires sophisticated solutions for feeding high power microwaves into the FBR, due to the dusty environment inside the reactor.

Fig. 2. Feeding microwaves into fluidized beds through the reactor wall: left and centre, multimode; right, monomode applicator.

A useful approach to solve this problem in a pilot plant installation with 75 kW magnetrons operating at 915 MHz frequency using wave guides has been presendted by the Canadian company EMR, for roasting ores in a microwave heated fluidized bed reactor [7]. Other methods to feed microwaves into fluidized beds are shown in Figure 2.

CVD-Processes in Microwave Heated Fluidized Bed Reactors

723

Production of electronic and solar grade Silicon by CVD-Processes Chemical Vapor Deposition is the general description for deposition of a solid on the surface of a substrate, whereby the deposited material comes from precursor gases. Applications include thin films and coatings formed by e.g., pyrolysis, reduction, hydrolysis, oxidation to form metals, intermetallics, nitrides, carbides and oxides, to mention only few processes [6]. One of the most important applications is synthesis of electronic grade Silicon, abbreviated as EG-Si and coating of silicon wafers for semiconductor production. State of the Art production of semiconductor-Si proceeds via a CVD process based on Silane, SiH4 or Trichlorosilane, Cl3SiH, by heterogeneous nucleation at the surface of a resistant heated Si-rod at temperatures from 900°C to 1200°C [8]. Laboratory scale attempts to use FBR-technology for EG-Silicon production started almost 25 years ago [9]. Application of microwave heating was suggested at the very beginning of this development [10]. The main advantage of the CVD-process in a fluidized bed reactor besides energy saving (250 kWh/kg Silicon in the Siemens-Process [8]) is the well defined granulate size obtained from the FBR, therefore no grinding of the Silicon for further thermal treatment is needed. This is of particular importance for solar grade Silicon, abbreviated as SG-Si, which up to now is mainly used as polycrystalline material and has no EG-Si-independent feed stock [11, 12]. The major disadvantage of the FBR technology for CVD of Silicon is the difficulty to establish a high yield and stable operation conditions, because of the competing reaction paths, based on heterogeneous and homogeneous nucleation of Silicon from the precursor pyrolysis, as shown in Figure 3. Homogeneous nucleation

SiH4

Sig

Diffusion CVD growth

Si(s) granules

Scavenging of fines

Condensation

Coalescence

Attrition of large particles

Si(s) fines

Fig. 3. Competing paths for conversion of Silane, SiH4 to silicon by heterogeneous nucleation (left part of the scheme) and homogeneous nucleation (right part of scheme).

By homogeneous nucleation very fine Silicon particles are formed, which are either removed from the reactor by pneumatic transport or deposited at the reactor walls.

724

Gerdes

The unwanted dust formation depends directly upon the process temperature and precursor concentration [13, 14], as shown schematically in Figure 4. Decrease of the overall gas temperature would be beneficial for reduction of homogeneous decomposition, but at the same time the growth rate of Silicon would also be reduced [13]. Different mechanisms for dust formation are currently under investigation: some authors assume dust formation in the bubble phase of the fluidized bed [13, 14], others in the colder upper part of the FBR [15]. In practice, deposition of dust on the reactor walls and unstable process conditions prevent a continuous operation of the CVD-FBR and reduce the yield of the process.

Fig. 4. Schematic representation of CVD growth rate of silicon from Silane, SiH4, as a function of gas temperature and of fines (Si-dust) formation due to homogeneous nucleation in different temperature and Silane concentration regimes (after experimental and modelling data from [13, 14]).

In order to improve stable growth of seed particles into granules by heterogeneous nucleation of Si from SiH4 or other precursors, selective overheating of the seed particle surface could be applied. This is the aim of the present study on the development of a microwave assisted CVD-FBR process. The improved utilisation of the precursor should also contribute to a significant reduction of the energy consumption and costs for polycrystalline Silicon solar cells, in order to meet the cost figures required for a fully competitive future photovoltaic technology based of polycrystalline Silicon. A recent study asks for an EG-Si-independent SG-Silicon feed stock with a capacity of 5000 t/year at a price level of 12 – 13 €/kg Silicon [11]. Other, completely different production technologies are also intensively investigated, e.g., Upgraded Metallurgical Silicon, UMG [12]. Coating of fibers and particles by microwave assisted FBR-CVD The only well established industrial application of CVD in FBR for coating of different particulate materials is the deposition of a thin layer of catalytic active metals or oxides on the surfcae of a porous substrate material [16]. The microporous areas are accessible for the precursor, therefore full utilization of the inner surface of the particle is possible. Opposite to this, protective coating or surface

CVD-Processes in Microwave Heated Fluidized Bed Reactors

725

activation of fibers and particles used as dispersion in metal matrix composites, MMC`s or in carbon reinforced composites, CFC`s is a new area for FBR-CVD. Application of microwave heating to CVD-coating of short carbon fibers in a rotary kiln has been investigated recently [17]. In fluidized bed processes utilization of a microwave plasma for coating particles has been of major interest since almost 20 years, up to now without break-through towards commercialization [18, 19], mainly due to problems with reactor scale up. Interest in such a technology is still high, because of numerous advantages expected from microwave heating of particles and short fibers in a fluidized bed reactor for CVD. First of all, microwave heating is expected to yield uniformly coated particles for materials which in particulate form can easily be heated with microwaves, like e.g., carbon fibers, porous carbons, SiC, WC, TiC and finely divided metals. In this case selective microwave heating of the particles could reduce coating of the reactor wall and homogeneous nucleation, improving the yield and purity of the coated fibers. Furthermore, because the particles are moving, on principle volumetric heating of the fluidized bed should be possible, which could significantly reduce the residence time and increase the throughput.

Experimental

Microwave heated FBR for CVD of Silicon A medium scale hybrid heated fluidized bed reactor shown in Figure 5 was used for investigation of SG-silicon fluidized bed heating behavior at 2.45 GHz.

Fig. 5. Hybrid heated fluidized bed reactor with resistant heating and 2.45 GHz microwave sources.

726

Gerdes

Microwaves are introduced into the FBR through a microwave transparent reactor wall. The outer reactor is made of steel, the quartz in-liner has a diameter of 10 cm and a height of 100 cm. Outside of the quartz in-liner resistive heating elements with 3 kW power are arranged for indirect heating of the bed. Inside the steel-FBR a microwave transparent alumina ceramic fiber board is applied as thermal insulation for the in-liner. Microwaves at a frequency of 2.45 GHz can be introduced into the FBR by four waveguides. For the present study only two sources with 2 kW each were used. Different fluidization gas velocities were tested: 50, 70 and 90 Nl/min. For temperature measurements inside the inliner a newly designed rotatable, “fishbone-like” multi-wire Ni-Cr-thermocouple is used, grounded with a special flange to the steel reactor so as to enable measurements upon microwave irradiation [20]. By slowly rotating this device, a 3-dimensional temperature distribution within the FBR is recorded during microwave as well as conventional heating. Additional axially movable metal shielded thermocouples were positioned in the thermal insulation. As bed material solar grade silicon granules from different suppliers were employed. Argon and Ar/H2 is used as fluidization gas. Microwave heated FBR for plasma assisted CVD-coating of carbon materials For the laboratory scale FBR-set up a quartz tube with 40 mm diameter and 140 mm bed height is placed inside a microwave cavity made from brass, as shown in Figure 6

Fig. 6. Fluidized bed reactor in a monomode applicator for coating of fibers and particles: left schematic view, right installation.

CVD-Processes in Microwave Heated Fluidized Bed Reactors

727

The tube is passing with the fluidised bed zone through an overmoded singlemode cavity (150 x 150 x 400 mm). A magnetron source (6 kW, 2.45 GHz) is attached to the cavity by a wave guide; tuning is achieved by means of stub tuners in the wave guide and a sliding short in the cavity. Cut off tubes are attached to the cavity in order to prevent microwave leakage. The transmission line is equipped with a circulator, the absorbed microwave power was calculated from the difference of forwarded and reflected power. In a first set of experiments the influence of fluidization velocity and microwave power level on microwave plasma ignition was studied. In a second series of experiments microwave heating was used for coating of carbon materials with TiC. For coating experiments temperature is measured through a microwave tight window by a pyrometer pointing to the surface of the quartz reactor within the fluidized bed zone. The fluidizing gas and the precursor is introduced from the bottom of the reactor, using a porous quartz plate build into a stainless steel flange for gas distribution. The reactor tube is equipped with a cyclone, in order to collect pneumatically transported powder, and with a gas washer for removal of the acidic by-products of the CVD-precursor reaction. As precursor material TiCl4 is employed (Merck, >99%, Tboiling = 136°C). It is added to the fluidized gas (Ar + 5 vol% H2) through an evaporator, as shown in Figure 6. The substrate materials are short carbon fibres (SGL, Meitingen, Germany; L = 500 µm, I10 µm) or activated porous carbon powders (Fluka or Merck, Germany, fraction 100 – 500 µm agglomerate size, 720 – 950 m²/g BET-surface). The following main reaction takes place on carbon substrates in a hydrogencontaining atmosphere:

TiCl 4  2 H 2 o Ti  4 HCl Carbon o TiC

'H reaction

198,9 kJ

mol

For gas velocities between 0.024 and 0.04 m/s and an absorbed power of 1230 – 1400 W a coating temperature in the range of 850°C to 930°C is reached. The Ticontent of the coated fibers and powders is estimated indirectly, by combustion of the material and weighing of the TiO2 formed. Characterization of the coated materials was performed by X-ray diffraction, XRD (XPert, Philips) Scanning Electron Microscopy and Energy Dispersiv Analysis of X-rays, SEM-EDX (Jeol/Inka, Oxford Instruments) and by measurement of the specific surface area (BET, Micromeritics).

Results and Discussion

Microwave heating a FBR for CVD of SG-Silicon For the CVD-FBR application to produce SG-Silicon the aim of the study was to understand the microwave heating behavior of different Si powders. As shown in

728

Gerdes

Figure 7, calculation of penetration depth at 2.45 GHz into bulk Silicon gives a clear evidence for a limited penetration depth of the high frequency radiation, depending upon the purity of the Silicon powder. Common size for Si-seeds is in the range of tenths- to few mm, therefore no limited penetration into individual seeds is expected. With respect to volumetric hetaing of a fluidized Silicon bed, penetration depth is also influenced by the fluidized bed density, resulting from a certain gas velocity. In Figure 7 calculated values are given for a typical SG-Si fluidized bed. At high bed density penetration depth is strongly reduced. As shown in Figure 8, for one particular grade of silicon seeds in a reactor made of microwave transparent material heating of the fluidized bed can be limited to the area near the reactor wall, further decreasing with increasing bed density and temperature. Therefore, lower frequency, e.g., 915 MHz, or an alternative reactor concept should be used for scaling up the CVD-Si synthesis by means of a microwave heated FBR. 50

High purity Si Low purity Si

3 2,5 2

T = 20°C

1,5

Frequency = 2,45 GHz

1 0,5 0 13 10

1014

1015

1016

1017

Dopant atoms

1018

1019

Penetration depth [cm]

Penetration depth [cm]

3,5

USi=100 :cm, T=20°C

40

Frequency 2.45 GHz 30 20 10 0 20

[cm-3]

30

40

50

60

Relative bed density [%]

Fig. 7. Penetration depth of microwave radiation as function of the electrical conductivity of silicon seeds (left part) and depending on relative fluidized bed density (right). 12

Penetration depth [cm]

2 x 1014 dopant atoms cm-3 10 8 915 MHz, 35% bed density 6 2,45 GHz, 35 % bed density

4

2,45 GHz, 100 % bed density

2 0

0

200

400

600

800

1000

Temperature [°C]

Fig. 8. Penetration depth of microwave radiation into a Si-particle bed at different frequency and with increasing temperature of the fluidized bed.

CVD-Processes in Microwave Heated Fluidized Bed Reactors

729

Another very important process parameter for CVD of Si in a FBR is the homogeneity of temperature. A comparison between both temperature distributions measured by thermocouples in the Si-FB is shown in Figure 9. The efficient dielectric heating of Si-seeds is visible from the temperature distribution developing upon microwave irradiation. Depending on the arrangement of the wave guides relative to the FBR-in-liner a hotter zone in the upper part of the microwave heated bed and a cold zone near the gas distributor is observed. In the case of the conventional heating a lower temperature at the top of the bed is measured, in accordance with the heat loss to the fluidizing gas stream. The hot zone achieved by microwave heating along the height of the bed is beneficial for the CVD-process, because of a synergetic effect: the high particle velocity in the bed facilitates a high heat transfer, the efficient heat transfer facilitates the pyrolysis, yielding a high reaction rate in this zone. Silicon-seeds which growth fast in this zone will move to the lower part of the FBR, where they will be continuously removed. Conventional

Microwave

360

400

580-585

Temperature [°C]

360

320

635-640 320

Position above the gas distributor in [mm]

630-635

280

625-630 620-625

240

600-605

280

600-605

615-620

240

610-615

200

605-610

200

600-605

160

595-600

120

590-595 585-590

160

580-585

120

595-600

575-580

80

80

570-575 565-570

40

47

0

47

Radial Position [mm]

47

vN2 = 50 NL/min

0

40 47

Radial Position [mm]

Fig. 9. Radial temperature profile in a conventional (right) and a microwave (left) heated fluidized bed reactor filled with a certain grade of SG-Si granules as bed material, at the same fluidization conditions. The lower temperature in the center indicates the thermocouple immersed into the fluidized bed [21].

Very important is the radial temperature gradient over the reactor diameter. In the case of microwave heating a hot zone is formed in the central upper region of the

730

Gerdes

FBR. A strong decrease of temperature in the quartz in-liner and the typical temperature decrease in the thermal insulation is observed. The experimental data match with the calculation, with exception of the temperature within the thermal insulation. Because the alumina fiber insulation itself is heated by microwaves at higher temperatures, the temperature in the insulation is higher than the value calculated from thermal conductivity data provided by the supplier, as shown in Figure 10. Using resistance heating of the reactor walls, a 30°C lower fluidized bed temperature is reached, however, it is necessary to overheat the in-liner by more than 100°C as compared to the bed. In order to achieve this, the resistance heating elements are operated at their limit-temperature of about 800°C. As shown in Figure 11, comparison of conventional heating with microwave heating reveals an in-liner wall temperature of about 150 K less for microwave heating. Conventional Heating 900

600

800

Temperature [°C]

Temperature [°C]

Microwave heating 700

measured

500 400 300

Quartz In-liner

Fluidized Si-Bed

calculated

200 100

Insulation -40

-20

0

20

40

60

80

calculated

700

measured

600 Resistance heater

500 Fluidized Si-Bed

400 300

Quartz In-liner

200

Insulation

100

100

120

-40

-20

0

Position [mm]

20

40

60

80

100

120

Position [mm]

Temperature difference [K]

Fig. 10. Comparison of the temperature profile across the FBR in a microwave and a conventional heated silicon bed. 150

Resistance heating

100 50 Position above the gas distributor [mm] 0

90

140

190

240

290

-50 Microwave heating

-100

Fig. 11. Temperature difference between the silicon bed and the in-liner as a function of the bed height.

Because of this, and the strong temperature dependence of the Silicon deposition rate on temperature, the Silicon deposition on the reactor walls can be lowered to about ¼ in the case of microwave heating as compared to conventional heating.

CVD-Processes in Microwave Heated Fluidized Bed Reactors

731

Although the best CVD-process conditions for Si deposition on the seeds can be achieved by exclusive microwave heating, the highest economic benefit is expected from a hybrid heating system. In continuous operation a temperature gradient between the bed particles and the reactor wall can only become stationary when heat is transferred from the interior to the exterior. The electrical efficiency of resistant heating is higher as compared to microwave heating, therefore such heat loss should be compensated by resistant heating. Processing conditions for microwave dielectric and microwave plasma heating of a FBR

Si-particles in Nitrogen Plasma ignition

5000 4000 3000 2000 1000 0

0

1

2

3

4

5

6

7

8

Multiple of minimum fluidization velocity [-]

Absorbed microwave power [W]

Absorbed microwave power [W]

The FBR-operation conditions influence strongly the efficiency of microwave heating, as shown in the first part of the results for the Si-FBR with respect to penetration depth and power requirements to achieve a certain temperature. In addition, microwave heating of the fluidized bed by dielectric losses of the bed particles is expected to be limited in terms of the maximum applied microwave power by plasma ignition due to electric break down of the gas atmosphere. Surprisingly, as shown in Figure 12 for Argon and Nitrogen as fluidization gas, the dielectric heating limit is not only influenced by the microwave power level but also by the fluidization velocity. It can be assumed, that at high fluidization gas velocity the break down voltage of the gas atmosphere is reduced by pressure fluctuations, due to large bubble formation and pneumatic transport, as depicted in Figure 1, and by electrostatic charging of dust particles as well as by evaporation of ionisable, volatile substances. Si-particles in Argon 5000

Plasma ignition

4000 3000 2000 1000 0

0

0,4

0,8

1,2

1,6

Multiple of minimum fluidization velocity [-]

Fig. 12. Plasma ignition upon „dielectric“ microwave heating of Si-particles in a FBR.

Therefore, microwave heating of a fluidized particle bed to very high temperatures most probably will not be a “dielectric” heating process, but rather a “combined” dielectric and microwave plasma heating process, because of evaporation and further decrease of the break down resistance of the atmosphere by the particle bed dynamics. For CVD-synthesis of Si as well as for CVD-coating of different materials such processing conditions could be even more beneficial than “pure” dielectric heating.

732

Gerdes

Microwave assisted FBR-CVD coating of carbon materials Microwave assisted CVD of TiC on carbon proceeds by microwave heating of the carbon particles and fibers fluidized at 0.03 m/s Ar-5%H2 charged with 2.3 x 10-5 mol/l TiCl4 vapour at 900°C for 5 minutes. SEM-EDX of the coated products reveals two important features: 1) the nucleation of Ti is heterogeneous and governed by the surface roughness of the carbon fibers, 2) infiltration of the Ti-precursor into porous carbon particles is not restricted by microwave heating, as shown in Figure 13.

Fig. 13. Left part (top and bottom: SEM-EDX of TiC on short carbon fibers from SGL, showing the nucleation to occur at structural imperfections of the fiber; Right part (top and bottom) SEM-EDX of TiC inside pores of an activated carbon (Merck), with a penetration depth of ~100 µm into the porous carbon template.

By means of EDX only traces of oxygen and chlorine could be detected, showing that removal of by-products is facilitated by the fluidization of the particulate material. The specific surface area is increased from 0.98 to 1.65 m²/g in case of the short carbon fibers, indicating that the coating is not a dense film but rather a nanosized particulate coating. The average Ti- content of the coated fibers is 0.85 wt%. For the activated carbon the specific surface decreased due to coating and pore closure from 720 to 310 m²/g. The average Ti-content is 5.1 wt%. For the coated activated carbon from Merck a surface decrease from 950 to 800 m²/g and an average Ti-content of 3.4 wt% was found. The formation of crystalline TiC was confirmed by XRD.

CVD-Processes in Microwave Heated Fluidized Bed Reactors

733

Conclusion Application of microwave heating to FBR-CVD is beneficial in different processes: - Direct heating of Silicon seeds by absorption of microwave radiation at a frequency of 2.45 GHz and 915 MHz is possible in the temperature regime relevant for Si-CVD. This could enhance heterogeneous nucleation of Si on Silicon seeds on the expense of homogeneous nucleation, leading to an improved process control and yield. - For SG-Si synthesis in a FBR an optimized temperature distribution in the reaction zone is achievable, with a large potential to reduce homogeneous deposition and dust formation in the upper part of the fluidized bed even for higher concentration of the precursor. Contamination related to high reactor wall temperature could be also reduced. - For coating of strengthening materials, like e.g., carbon fibres and porous carbon particles, microwave heating of the FBR improves the coating efficiency and has the potential for high throughput. However, succesful scale up of microwave heated CVD-FBR processes requires a thorough investigation of microwave specific parameters, like e.g., penetration depth of the radiation into the fluidized bed, plasma ignition at high microwave power levels due to pressure variations and electrostatic charging, and reliability of microwave coupling into the dusty environment of a fluidized bed reactor. No general solution can be provided, rather an optimized solution should be developed for each particular process. Sensitivity to temperature dependent dielectric and discharge properties has to be fully implemented into the process control, in order to arrive at a successful microwave heated FBR´s on industrial scale.

Acknowledgement InVerTec is gratefull for financial support through the AG SOLAR programme of Land Northrhine-Westfalia, contract number 254 117 98.

References [1] S.T. Sie, R. Krishna, Fundamentals and selection of advanced Fischer-Tropsch reactors, Applied Catalysis A, 186, 55-79 (1999). [2] R.E. Bahu, Fluidized bed dryer scale-up, Drying Technology 12 [1&2], 329-339 (1994) [3] H. Bradley Eldrege, D.C. Drown, Solid flow and mixing model for a fluidized bed coating and calcination process, Chemical Engineering Science 54, 1253-1264 (1999).

734

Gerdes

[4] S.P. Chakraborty, P.K. Tripathy, I.G. Sharma, D.K. Bose, Thermal decomposition of ammonium polymolybdate in a fluidised bed reactror, J. of Alloys and Compounds, 238, 18-22 (1996). [5] M. Sciazko, H. Zielinski, Circulating fluid-bed reactor for coal pyrolysis, Fuel and Energy Abstracts, 37 [3], 204 (1996). [6] M. J. Hampden-Smith and T. T. Kodas, Chemical Vapor Deposition of Metals: Part 1: An Overview of CVD Processes, Chemical Vapor Deposition 1995, 1, No.1, VCH Verlagsgesellschaft mbH, Weinheim, p 9ff. [7] J.M. Tranquilla, Method and apparatus for optimisation of energy coupling for microwave treatment of metal ores and concentrates in a microwave fluidized bed recator, US Pat. No. 6,074,533 (June 2000) and patents cited there. [8] W. Zulehner, D. Huber, Crystals Growth, Properties and Applications, Vol. 8, Springer Verlag 1982, p. 90ff. [9] T. Kojima, Production of polycrystalline silicon in a fluidized bed, Trends in Chemical Engineering, 2, 159173 (1994). [10] Y. Pong, S. Yongmok, Method of preparing a high purity polycrystalline silicon using a microwave heating system in a fluidized bed reactor, US Patent 4,900,411 (1990) [11] P. Woditsch, W. Koch, Solar grade silicon feedstock supply for PV industry, Solar Energy Materials & Solar Cells 72, 11-26 (2002). [12] D. Sarti, R. Einhaus, Silicon feedstock for the multi-crystalline photovoltaic industry, Solar Energy Materials & Solar Cells, 72, 27-40 (2002). [13] T. Furusawa, T. Kojima, H. Hiroha, Chemical vapour deposition and homogeneous nucleation in monosilane pyrolysis with interparticle spaces: application of fines formation analysis to fluidized bed CVD, Chem. Eng. Sci., 43, 2037-2042 (1988). [14] T. Kojima, T. Kimura, M. Matsukata, Development of numerical model for reactions in fluidized bed grid zone-application to chemical vapor deposition of polycrystalline silicon by monosilane pyrolysis, Chem. Eng. Sci., 45 [8] 2527-2534 (1990). [15] B. Caussat, M. Hemati, J.P. Couderac, Silicon deposition from Silane and Disilane in fluidized bed: Part I Experimental Study; Part II, Theoretical Analysis and Modelling, Chem. Eng. Sci., 50 [22] 3615-3624; 3625-3635 (1995). [16] G. Ertl; H. Knötzinger; J. Weitkamp; Preparation of Solid Catalysts; Wiley-VCH; 1999; page 150ff, 427-459. [17] Daimler Chrysler AG, Method for manufacturing of coated short fibers, DE 19828843, 1998 [18] M. Matsukata, K. Suzuki, K. Ueyama, T. Kojima, Development of a microwave plasma fluidized bed reactor for novel particle processing, Int. J. of Multiphase Flow, 20 [4], 763-773 (1994). [19] M.Karches, Ch.Bayer, Ph.Rudolf von Rohr, A circulating fluidised bed for plasmaenhanced chemical vapour deposition on powders at low temperatures, Surface and Coatings Technology, 116-119 (1999) 879-885 [20] P. Bahke, Verfahrensentwicklung zur mikrowellenunterstützten Beheizung von SiWirbelschichten, Studienarbeit, März 2000, Universität Dortmund, FB Chemietechnik, p. 17. [21] P. Bahke, Verfahrensentwicklung zur mikrowellenunterstützten Beheizung von SiWirbelschichten, Studienarbeit, März 2000, Universität Dortmund, FB Chemietechnik, p. 29-34.

Processing of Carbon-Fiber Reinforced Composite (CFRP) Materials with Innovative Millimeter-Wave Technology Christian Hunyar1, Lambert Feher1 and Manfred Thumm1,2 1

Forschungszentrum Karlsruhe GmbH, Institut fur Hochleistungsimpuls- and Mikrowellentechnik, P.O. Box 3640, D-76021 Karlsruhe, Germany 2 University of Karlsruhe, Institut fur Höchstfrequenztechnik and Elektronik, Kaiserstr. 12, D-76128 Karlsruhe, Germany

Abstract The use of carbon fiber reinforced composite materials (CFRP) in aerospace industries is increasing due to their unique combination of characteristic features such as light weight, high specific mechanical strength etc. Currently the main obstacle for widespread industrial implementation is high manufacturing costs caused by the necessity of curing at elevated temperatures of 100 - 200°C. Heating the CFRP materials to these temperatures in a conventional furnace is an energy consuming and therefore costly procedure. This paper presents a heating procedure by means of millimeter-waves. Advantages of this method are presented along with theoretical considerations and numerical simulations of the heating process. Experimental verification by millimeter-wave cured CFRP slabs are shown along with results of numerical simulations.

Introduction The characteristic features of CFRP such as high specific tensile and flexural strengths, and high moduli as well as their vibration stability make them suitable for structural components in transportation systems, especially in aerospace industries. In contrast to metallic parts which are machined from bulk and assembled the final form of a fiber component is directly shaped during the manufacturing process. Additionally the material properties can be influenced by the manufacturing route. For fiber reinforced plastic materials, the matrix and fiber material are inexpensive (only 30% of processing costs). The preparation of the material, the

736

Hunyar

control of the production process as well as it's duration and energy consumption are the determining factors for costs and quality of the resulting components. For materials with a high electrical conductivity like carbon fiber reinforced plastic (CFRP) the use of microwaves and especially mm-waves for processing has been reported to offer unique benefits [1]: • selective heating of composites - and therefore direct energy deposition in material (an advantage due to the low thermal conductivity of CFRP) • high heating rates • significant reduction of the processing time • lower energy consumption • better process control (no overheating) Experiments on the processing of CFRP samples in a 30 GHz gyrotron system which are shown in this paper demonstrate these advantages on a smaller scale and enable insights into the design requirements of a microwave heating system. Parallel to these experiments, theoretical studies on the heating process of fiber composite materials are conducted which take the anisotropy of the materials' properties in account.

Background and Fabrication Issues Two common forms of carbon fiber materials are woven carbon fibers and fabric made of carbon fiber strands bundled together with polymer threads. Carbon fiber layers are stacked with varying orientation to obtain a multiaxial fiber compound. So called wet techniques are employed to fabricate high quality fiber composites because they allow production of large (lateral dimensions > 10 m) structures with good laminate qualities [2]. The flexible fiber material is shaped into moulds and backfilled with a resin. Curing takes place at temperatures between 100 - 200°C. A common method to implement this process is the so called resin transfer moulding (RTM) [3]. The fiber material is completely enclosed in an evacuated mould in which the resin is injected before heating. The differential pressure (DP-)RTM [4] is a modification of this procedure which does not use a closed mould but only a simple forming tool offering more flexibility and easier handling. The mould is sealed with a polymer foil and then evacuated so that the material is pressed on the forming tool by the pressure difference to the outside atmosphere. This pressure difference is also utilized for injecting the resin into the fiber weave and the conventional process takes place in an autoclave employing hot air or infrared heating. Due to the dielectric properties of CFRP very good coupling occurs in the millimeter-wave frequency band of the electromagnetic spectrum [1]. This enables heating of large workpieces directly and homogeneously without the timeconsuming heating of the complete furnace interior while the flexible polymer foil is penetrated without significant losses which makes the (DP-)RTM very suitable for use with the mm-wave heating.

Processing of Carbon-Fiber Reinforced Composite (CFRP) Materials

737

Experimental Investigations To verify the feasibility of mm-wave processing of CFRP weave and woven fabric samples were produced in a 30 GHz gyrotron system. The mm-waves are generated in a gyrotron vacuum tube that provides an output power of up to 15 kW CW. The mm-wave radiation of the gyrotron is guided through a quasi-optical mirror system into the applicator. This system was already successfully used for mm-wave processing of many different types of other materials such as ceramics [5].

Fig. 1. Schematic of the experimental setup used to produce the CFRP samples.

The setup of the mm-wave applicator for the experiments is depicted in Fig. 1. A hexagonal structure made of an aluminum plate was designed to ensure a homogeneous field distribution inside the applicator [6] as can be seen in Fig. 2.

Fig. 2. Distribution of the electromagnetic field in the hexagonally shaped applicator (left) of the gyrotron system in comparison to a cylindrical geometry (right).

738

Hunyar

The processed CFRP samples are square shaped (150 mm x 150 mm) and have a thickness of up to 4 mm. Six to eight layers of weave of fabric were placed on a flat CFRP mould and sealed with polymer foil. They were backfilled with a three component epoxy resin (LY556, HY917, DY070, produced by Vantico). It is important, that a homogeneous loading of fiber material with resin and prevention of voids has to be achieved. The process temperature was measured with a thermocouple in direct contact to the sample. The power output of the gyrotron was computer controlled so performance of a give temperature profile was possible. Samples were processed by using the pressure difference to inject the preheated resin as it is shown in fig. 1 and processing it with the above temperature profile. The fiber material was heated with a rate of about 20 K/min up to 80°C and held there for one minute. Subsequent to the injection of the resin backfilling the entire sample it was heated to the curing temperature of 120°C. Fiber volume contents of about 60%, similar to conventionally produced samples were achieved. In contrast to conventional processing methods the microwave heated samples were held for only 10 minutes versus 120 minutes with considerably higher heating rates, 20 K/min versus 4 K/min (see also Sigle [4]). In the case of conventional processing, overheating of the sample caused by the exothermic reaction during the curing process may occur. To date no differences in the properties of the mm-wave cured samples compared to the conventionally produced ones have been found, although further in depth testing of the mechanical properties has yet to be performed. Figure 3 shows the nearly flawless surface of a sample.

Fig. 3. Contrast enhanced photograph of the surface of a CFRP sample produced with mmwave processing.

Modeling of Microwave Heating Heating of matter with microwaves is a rather complex process. It is not only depending on dielectric and thermodynamic properties of the material but also on the frequency and electromagnetic field distribution as well as boundary conditions like thermal radiation, convection or conduction [7]. Due to the very small time scale of the resonant electromagnetic field in comparison to the thermal

Processing of Carbon-Fiber Reinforced Composite (CFRP) Materials

739

diffusion the temperature problem can be numerically decoupled from the field calculations. This approach is applied within the three dimensional THESIS3D code [8] that uses a FDTD (finite differences time domain) numerical model combined with an analytic field solution to solve the nonlinear heat conduction equation in geometries of rectangular parallelepipeds. The code was successfully used to model aspects of the temperature distribution of several isotropic and homogeneous materials, especially ceramics [9], under the influence of microwaves. General Approach We will first consider a general approach to the problem which can be directly derived from the anisotropic case with the same heat flow and dielectric parameters for all directions as described in [10]. Heat flow density j T (x,t) (x denotes space, t time) in the substrate is determined by the Fourier equation: jT (x, t) = - Vˆ T (x,T) grad T (x, t),

(1)

where Vˆ T (x,T) is the general thermal conductivity at the temperature T(x,t). From the differential formulation of the energy conservation one can deduce the continuity equation div j T (x, t) + cv (x, T)U(x, T) w T(x, t) = peff (x, t). (2) wt with c V (x,T) the heat capacity and U (x,T) the density of the material. peff (x,t) is the density of power summed up over all influences at t and x. In case of electromagnetic heating of a substrate in which an exothermal chemical reaction can take place, it can be split up in the terms peff (x,t) = pconv(x,t) + pcond(x,t) + prad(x,t) + pelec(x,t)+ pchem (x,t),

(3)

which describe the convection (at the edges of the material), conduction and radiation heat losses or gains, as well as the electromagnetic power input and the thermal influences of the exothermal chemical curing reaction. The combination of Eq. 1 and Eq. 2 leads to c V (x,T) U (x,T) w T(x,t) - ’ ( Vˆ T (x,T) ’T(x,t)) = peff (x,t) (4) wt the nonlinear heat conduction equation which determines the temperature field for the material. With given material parameters it is possible to calculate the electromagnetic field to get the power density terms and the differential equation problem can be numerically solved with FDTD methods. A homogeneous electrical field in the applicator [10] can be assumed as a boundary value for the sample in the experimental setup mentioned in section 3. For a rectangular geometry with this boundary condition the Helmholtz equation can be deduced from the Maxwell equations and has the form

740

Hunyar

'E  kˆ 2 E

0,

(5)

which represents the stationary field E(x) and can be analytically solved.

kˆ 2

Z2 §

· i ¨¨ Hˆ  Vˆ ¸¸ , c © ZH 0 ¹ 2

(6)

is the complex value of the squared wavelength vector, dependent on the electrical permittivity, e, and the electrical conductivity, Vˆ , both at the frequency v( Z =2 S v) of the electromagnetic field. The electromagnetic power density at a location in the sample can be calculated with pelec(x,t ) = 1 E ( x , t ) Vˆ (x,t ) E ( x , t ) . (7) 2 Anisotropic Material The extension of the basic equations to anisotropic materials can be achieved by replacing all the scalar values that determine material parameters which are influenced by the anisotropy by their tensor equivalents. So thermal conductivity, electrical permittivity and electrical conductivity are tensorial values and can be expressed as a matrix. Their orthogonal form separates in parallel and perpendicular terms to the orientation of the fiber (here shown on the example of the thermal conductivity):

Vˆ T

0 0 · §V T – ¨ ¸ 0 ¸, ¨ 0 VT I ¨ 0 0 V TA ¸¹ ©

(7)

VT – is the value parallel to fiber, VT I is the value within fiber layer and perpendicular to fiber, VT A is the value perpendicular to both (and it is usually the same as VT I). The in the isotropic case separated differential equations for the electrical field (Eq. 5) and the temperature (Eq. 4) will now form a system of coupled differential equations. A solution can be found by transforming the problem from the original coordinate system (x,y,z) in a coordinate system (x',y',z') that is chosen relative to the fiber orientation. As can be seen in Fig. 4, this is achieved by rotating the original coordinate system by the angle T. The x'-axis is oriented parallel to the fiber direction and the y'-axis perpendicular to it, the z'-axis remains oriented in the direction of z. The equations separate in the rotated system and taking in account the modified boundary conditions an analytical solution can be found. The resulting vector values for the electric field are transformed back to the original coordinate system and represent a solution of the anisotropic problem.

Processing of Carbon-Fiber Reinforced Composite (CFRP) Materials

741

Fig.4. Coordinate system oriented in the direction of the fiber system.

The calculation method has been implemented for the analytical solver of the Helmholtz equation calculating electrical field and electromagnetic power density in the material with only one fiber layer. Temperature Distribution in a Controlled Process It has already been shown that the anisotropic heating of the material affects the temperature distribution in the sample [11]. After that a complete complete process was simulated for a sample with the isotropic relative dielectric constant of er = 30 and an electrical conductivity of

V3

100 : 1m 1 and V T A

VTA

1 V3 100

parallel to the fibers, perpendicular to them and perpendicular to the fiber layer in direction of the z-axis. Fig. 5 shows the temperature distribution at several times of the process in the middle (z = 10 mm) layer of the short side of a sample with the dimensions of 40 mm x 40 mm x 20 mm (calculated in a grid of 20 x 20 x 10 voxels) and a 45° fiber orientation. A heating rate of 30 K/min and a holding temperature of 150°C were selected. The anisotropy of the electromagnetical coupling causes an anisotropy in the developement of the temperature distribution which manifests particularly in the higher temperature of the corners in fiber direction. The overall homogeneity of the temperature in the sample is increasing over the time. This can be clearly observed in Fig. 6 where the variance of the temparature in the sample is getting smaller over the length of the process. The extremal values which are also plotted in Fig. 6 correspond to the temperature in the middle and on the fiber direction corners of the sample. As can be seen, especially the corner temperature peaks in that process and is supposably the place where the chemical curing reaction will start. The diagram in Fig. 7 [1] shows the result of an experimental setup in which several temperatures throughout the height of a sample were measured. It was

742

Hunyar

heated with the same parameters as used in the simulation above. The sample itself was larger than the simulated one but had the same height. The central temperature was used for controlling the process.

Fig. 5. Developement of the temperature in the x-y-plane of a simulated CFRP sample.

In comparison to the simulation, the phenomenology of the heating process is predicted even with only a partial implementation of the anisotropic material properties. The growing difference of the outside and inside temperatures in the heating phase can be observed as well as the convergence of these temperatures after reaching the curing temperature.

Conclusion and Future Work The experimental studies in this paper have shown that mm-wave processing in a 30 GHz gyrotron system can be used to produce CFRP samples with an DP-RTM technique. The advantages over conventional heating processes is the selective heating of the material which leads to shorter processing times, higher heating rates and reduces the overall energy consumption. Overheating of the material was not observed. After visual inspection and determination of the fiber volume content the homogeneity and quality of the cured samples is comparable to conventionally produced samples but further testing and characterization will be necessary. The numerical calculation of the temperature distribution in CFRP samples within a single fiber layer was performed with the THESIS3D simulation tool. Although up to now only the anisotropic nature of the electrical conductivity is implemented in the computational model it can already be seen that the different dielectric parameters in the direction of the fiber and perpendicular to it affect the resulting temperature developement. The start of the curing reaction will be

Processing of Carbon-Fiber Reinforced Composite (CFRP) Materials

743

induced in the fiber direction corners of the material. Primary results of the simulation show immediately the phenomenology of the experiments.

Fig. 6. Simulated temperature distribution throughout the height of a CFRP sample. The fiber orientation is marked.

Fig. 7. Experimentally determined temperature distribution in a rectangular CFRP sample.

The implementation of the thermodynamic properties of the exothermal curing process and the anisotropic heat conduction as well as multi-layered materials with different orientations of fibers will be the next step in the development of the simulation tool. We expect to gain further insights in the mechanism of mm-wave processing of anisotropic materials, especially CFRP. Especially knowledge over the spatial and temporal distribution of the curing reaction over the sample will deliver enhancements for the process.

744

Hunyar

References [1] Feher L., Thumm M. (2001) Millimeter-Wave Processing of Composite Materials. In: Proceedings of the 2nd IEEE International Vacuum Electronics Conference, Noordwijk, Netherlands, 2-4 April, 2001, 83-84 [2] Sigle C. (1999) Ein Beitrag zur kostenoptimierten Herstellung von großflächigen Hochleistungsverbundbauteilen, Ph. D. Thesis, Technical University of Braunschweig, Germany [3] Wadsworth M. (1989) Resin Transfer Molding of Composite Structures, Aerospace Engineering, Dec., 23-26 [4] Sigle C., Herrmann A. S., Pabsch A. (1997) Low Cost Manufacturing of Composites for Highly Loaded Structures, as Demonstrated by DifferentialPressure-ResinTransfer-Moulding-Technology. In: Proceedings of the 5th European Conference on Advanced Materials, Processes and Applications, Maastricht [5] Link G., Feher L., Thumm M. (1999) Hot Wall 30 GHz Cavity for Homogeneous High Temperature Heating. In: Proceedings of the 7th International Conference on Microwave and High Frequency Heating, Valencia, 165-168 [6] Feher L., Link G., Thumm M. (1997) Optimized Design of an Industrial MillimeterWave Applicator for Homogeneous Processing of Ceramic Charges. In: Breccia A., De Leo R., Metaxas A. C. (eds.), Proceedings of the Conference on Microwave and High Frequency Heating, 1997, 443-446 [7] Feher L., Thumm M. (1996) Modeling of Millimeterwave Materials Processing. In: Proceedings of the 21st International Conference on Infrared and Millimeter Waves, Contributed Paper AW2, Berlin [8] Feher L., Link G., Thumm M. (1996) The MiRa/THESIS-Code Package for Resonator Design and Modelling of Millimeter-Wave Material Processing. In: Symposium Proceedings, Spring Meeting of the Materials Research Society, Microwave Processing of Materials V, 430, 363-368 [9] Feher L., Link G., Thumm M. (1999) Electrothermal Heating Effects and Temperature Gradients in Microwave Processed Materials. In: Proceedings of the 7th International Conference on Microwave and High Frequency Heating, Valencia, 435-438 [10] Feher L. (1997) Simulationsrechnungen zur verfahrenstechnischen Anwendung von Millimeterwellen fur die industrielle Materialprozeßtechnik Ph. D. Thesis, University of Karlsruhe, FZKA-5885 Report, Forschungszentrum Karlsruhe, Germany [11] Hunyar C., Feher L., Thumm M. (2001) Processing of Carbon-Fiber Reinforced Composite (CFRP) Materials with Innovative Millimeter-Wave Technology for Aerospace Industries. In: Brandt H. E. (ed.), Proceedings of the International Society for Optical Engineering, Intense Microwave Pulses VIII, 2001, Orlando, 4371, 111118

Basic Research and Industrial Production Using the Spark Plasma System (SPS) Mamoru Omori Laboratory for Advanced Materials,Institute for Materials Research, Tohoku University, 2-1-1Katahira, Aoba-ku, Sendai 980-8577, Japan

Abstract A spark plasma system (SPS) is characterized by an electric source of direct pulsed current. Powders in a die are heated by this current and compacts with high performance are obtained. The unique features of SPS can be expressed in terms of five factors, i.e., spark plasma, plasma impact, electric field, electric current and rapid heating. Spark plasma is also applied for organic reaction. Consolidation of ceramics and metal powders is advantageously conducted by the effect of electric fields. Crystal growth is accelerated by the electric current on the surface of powders. The rapid heating results in the formation of solids with nanosized crystals. Furthermore, production costs can be decreased.

Introduction A spark plasma system (SPS) is the alternative description of spark plasma sintering (SPS) [1], known also as plasma activated sintering (PAS) [2]. SPS is characterized by an electric source of direct pulsed current, which is similar to that of an electric discharge machine. There is a slight difference with regard to the electric source between SPS and PAS. The direct pulsed current of the SPS described in this paper can be continuously applied, from start to finish of the process. Opposite, the direct pulsed current of PAS is limited to 1000 seconds, after which it changes to another electric source, e.g., direct current + direct pulsed current. PAS equipment has not been produced for several years. SPS was developed for sintering of materials in plasma and in an electric field [3, 4]. Using an original machine, beryllium parts for airspace applications were synthesized by Lockheed Missiles & Space Company [5]. However, the use of SPS was confined to some materials only, because not many machines were sold commercially. Modified SPS’s were produced for sale in late 1980s after the original patent expired [1, 2].

746

Omori

Innovative materials, which could not be prepared by conventional heat treatment have been obtained for ceramics, metals and polymers. SPS includes various functions such as spark plasma growth, spark plasma joining, spark plasma reaction and spark plasma consolidation. These functions are based on five factors:     

Spark plasma Plasma impact Electric field Electric current and Rapid heating.

It is not fully understood, what these factors depend on nor how they affect materials prepared by SPS. Considerable experimental data are needed to elucidate these uncertain factors. Some approximate explanations for the relation between these factors and the material being processed are under discussion [6] and will be considered in this paper. In an electric discharge machine gaps exist between the two electrodes and high-energy plasma is generated, which can cut and shape electric conductors. Opposite to this, the die alignment of SPS does not allow any gaps, and highenergy plasma cannot be induced. It is thus reasonable that the plasma of SPS has not been identified directly. However, a low-energy plasma among powder particles could be caused by the direct pulsed current. Although such a spark plasma cannot remove oxide film from metal surfaces, it still could facilitate elimination of adsorbed gases. Furthermore, organic bonds could be excited by the spark plasma, to yield a synthesis method for advanced materials and the consolidation of polymers. Plasma impact inevitably takes place together with the generation of plasma. The mechanical strength of this impact is correlated with the intensity of plasma energy. The impact of the spark plasma is weak and is insufficient to break oxide films and to induce the diffusion of materials. Electric fields and electric currents are considered as effective factors for sintering, joining and crystal growth. They influence atomic mobility in metals, which is called electromigration [7]. High density dc electric current either continuously or as short duration pulses can significantly enhance the plastic deformation rate in metals and ceramics. An electric field enhances the glide mobility of dislocations in ceramics. The accelerated dislocation mobility is termed electroplasticity [8]. Electromigration and electroplasticity can decrease the sintering temperature and increase the sintering rate. Electric currents lacking high density can run over the surface of semiconductor and insulator powders in the graphite die. The surface current is responsible for the crystal growth in semiconductors and insulators. There are no insulators and heating elements with large heat capacity, and the graphite die is heated directly by an electric current. The consolidation is rapidly carried out so as not to induce grain growth and results in the production of materials consisting of nanosized crystals.

Basic Research and Industrial Production Using the Spark Plasma System (SPS)

747

Basic research

Sintering SPS is advantageous for sintering of metals and ceramics, as demonstarted from examples of previous work. Al metal powders can not be sintered by hot pressing because the oxide film on Al particles retards the diffusion of Al. A fine Al powder can be sintered by SPS [9], to yield the same properties of the sintered Al alloy as those of the Al alloy prepared by hot forging [10]. AlN powder can be sintered without a Y2O3 sintering additive [11]. The consolidation of AlN is regulated by the migration of cation vacancies [12]. Electric fields can affect the migration. Graphite or carbon is one of the hard-to-sinter materials and is consolidated at the high pressure of 0.5 GPa. A high density of 1.8 - 1.9 g/cm3 and 2.0 - 2.2 g/cm3 are obtained under 2000°C and near 3000°C, respectively by using carbon powders heated at more than 1500°C [13]. Charcoal prepared at low temperature contains small amounts of hydrogen, oxygen and other elements, which are bonded with carbon. A charcoal powder prepared from wood at 700°C can be consolidated by SPS to a density of 1.63 g/cm3 applying 49 MPa pressure at 2080°C [14]. This consolidation is not carried out by hot pressing and may be related to the excitation of chemical bonds, except for carbon-carbon bonds. Natural graphite particles of 75 - 106 Pm are sintered at 1800°C under a pressure of 40 MPa by SPS and result in a dense compact with a high density of 2.02 g/cm3. Natural graphite contains 15 wt% ash (36 at% Si, 4 at% K, 1 at% Ca, 2 at% Fe, 57 at% Cu) [15], and it is not clear if sintering depends on the elements in the ash. An eutectic composite consists of twined single crystals and is usually prepared by crystal growth techniques of unidirectional solidification. Oxide eutectic composites are characterized by high temperature strength. An eutectic composite of Al2O3-ZrO2(8.8Y2O3) holds its strength from room temperature till 1600°C [16]. The strength of an Al2O3-Y3Al5O12 eutectic composite is not decreased at high temperatures. Such materials are not considered as candidate for applications in e.g., turbine blades, because it is hard to apply unidirectional solidification to the fabrication of large and near-net-shaped parts at high temperature. Development of suitable processing methods could therefore open the way for such applications. Eutectic composite powders with less than 44 Pm particle size are prepared by conventional techniques and consolidated by SPS. The density of the consolidated composite is critical for achieving the high strength, e.g., a density over 99% is needed to attain a strength of 607 MPa. Figure 1 shows, that the consolidated composite is free from pores and has a eutectic structure. The strength of the composite is decreased at 1400°C, but still 75% of the strength is maintained at this temperature [17]. Sintering by SPS is monitored by the

748

Omori

shrinkage of samples. Cessation of shrinkage marks the end of sintering and is judged by a monitoring meter. Because the eutectic powder is sintered close to its melting temperature, to determine the consolidation temperature is difficult. The shrinkage monitor is very useful to determine the accurate temperature of sintering and to achieve full densification.

Fig. 1. SEM-image of SPS-consolidated eutectic ceramic; the scale bar is 10 µm.

Consolidation of metallic glass powder Amorphous materials are characterized by the absence of long-range order and are crystallized by heat treatment. The consolidation of amorphous powders permits the production of advanced materials with adjustable structure. Compacts consisting of nanosized crystals are prepared from the consolidated amorphous powder material by controlled heating. Properties of the resulting material are thought to be significantly altered by the ordering and size of crystals. A metallic glass is an amorphous metal, with a supercooled liquid stability in a wider temperature range than twenty or thirty degrees. Consolidation of glass metal powders at temperatures below the crystallization temperature is impossible by hot pressing. Applying SPS, the powder is expected to be solidified by viscous flow of supercooled liquid in the amorphous solid. Co40Fe22Nb8B30 metallic glass powder , which is a soft magnetic alloy with a wide range of supercooled liquid of 81 K and is crystallized at 954 K, is consolidated by SPS. The sintering conditions are 925 and 935 K, at no MPa and at a holding time of 0.5 to 2 min, respectively without crystallisation, as shown in Figure 2. The density of the consolidated alloy is as high as 99.8%. The X-ray diffraction patterns of the powder and the compact shown in Fig. 2. consist of broad diffraction peaks, typical for amorphous solids. Consolidation at 945 K results in a crystalline compact. The soft-magnetic properties of the amorphous compact are superior as compared to the crystalline one [18].

Basic Research and Industrial Production Using the Spark Plasma System (SPS)

749

Fig. 2. X-ray diffraction of Co40Fe22Nb8B30 metallic glass powder before SPS-compaction (top graph) and after SPS-compaction at 925 K

Chemical reaction The generation of a spark plasma in powders during the treatment by SPS is often questioned, because there is no evidence of visible light and ultraviolet light emission in the process of sintering by SPS. Oposite to ceramics and metals, organic compounds are sensitive to plasma and light. Therefore, presence of spark plasma could be diagnosed by changes occurring to several organic materials. Some example of such processes are known. An insoluble polymonomethylsilane undergoes rearrangement from Si-Si to Si-C-Si bonds and becomes soluble. A thermosetting polyimide powder is not solidified by heat, but dense bodies result from SPS. This solidification is based on the excitation of chemical bonds such as ether- or imido-groups. These chemical reactions do not result from heating and ultraviolet light [6]. Therefore it can be concluded, that spark plasma consisting of energy different from IR- or UV-light is certainly generated by the pulsed direct current. Etching of organic fibers Plasma etching of organic fibers is a conventional technique. The spark plasma of SPS etches the surfaces of polyethylene and polypropylene fibers consisting of hydrocarbon. The spark plasma cuts C-C bonds, but does not generate carbon [6]. This result gives further evidence that the spark plasma is similar to low energy plasma and that its energy is higher than that of ultraviolet light.

750

Omori

Crystal growth The peritectic compound of a CoSb3 system is characterized by a high efficiency of thermoelectric performance. The crystal growth of CoSb3 has been carried out by SPS using CoSb3 powder. A single crystal of 10 mm in length is obtained by slow heating of 1°C/min from 600° to 720°C [6]. The surface diffusion of atoms or molecules may be a dominant factor for the crystal growth and is thought to be promoted by the skin current on powders and/or accelerated by the electric field during SPS. In such process atoms and molecules could migrate slowly on the surface of crystals.

Industrial application

Joining of steel By classical methods it not possible to make curved channels inside a die. Such channels can be obtained by using SPS for joining of appropriately designed halfparts. As shown in Figure 3 (a), channels of half width are carved on the surface of steel blocks (Cr: 12.00 - 14.00 wt%, C: 0.30 wt%) and is contacted with a counterpart. Both are heated in a graphite die at 800°C for 10 min at 30 MPa in a vacuum by SPS. To remove mechanical stresses developed during this treatment, the joined steel part is annealed at 1000°C in a vacuum. The completed part subjected to a cooling test is shown together with a half-cut segment in Fig. 4., which clearly shows the channel for water cooling inside the segment.

Fig. 3. A part with inner channels is made from two parts (a) half parts with carved channels (b) full size article made from half-part blocks and cross section

The two blocks can be also joined without the graphite die at 980°C at 2.6 MPa. No difference in strength is found between joined steel and the block itself. The tensile strengths of joined steels of SUS304 (Ni: 8.00 - 10.50%, Cr: 18.00 - 20.00%) and SKD 61 (C: 0.32 - 0.42%, Si: 0.80 - 1.20%, Mn: 0.50%,

Basic Research and Industrial Production Using the Spark Plasma System (SPS)

751

Cr: 4.50 - 5.50%, Mo: 1.00 - 1.50%, V: 0.80 - 1.20%) is 551 MPa and 695 MPa, respectively. Such a joined die is used at high temperature for rapid casting and exposed to fast heating and cooling [19]. Preform of Al alloy with fine crystal Starting from inexpensive Al alloy, powders with fine structure, prepared by a spinning water atomization process dense compacts are produced. Two powder compositions were prepared: Al- 12 wt%Si - 0.8 wt%Fe - 2 wt%Cu - 0.5 wt%Zn and Al- 17 wt%Si - 2 wt%Fe - 1 wt%Ni - 2 wt%Mg - 1 wt%Cu - 0.5 wt%Mo. The fabrication by hot forging yields dense compacts as does SPS. The experimental conditions are indicated in Table 1. Table 1. Sintering and strength of Al- 17%Si compacts

SPS Forging

Sintering Pressur [MPa]; Temperature [K]; Time [Minutes] 100; 773; 15 1000; 773; 60

Tensile strength [MPa] Preform 320 - 340 290 - 360

Tensile strength [MPa] Superplastic working 410 410

Good mechanical properties can be achieved by SPS although the consolidation by SPS occurs at a pressure of one tenth as low as that by hot forging. Furthermore, the strength of the hot-forging product is anisotropic, but that of the SPS product is homogenous.

a

b

(a)

(b)

Fig. 4. SEM-image of Al-Si-alloy (a) preform; (b) shaped by superplastic working. The scale bar is 3 µm.

The microstructure of both products is shown by SEM-images in Figure 4 (a) and (b). The crystallite size is smaller than 1 Pm, which is adequate for

752

Omori

superplastic working. The fine structure is present in the consolidated preform as well as after superplastic-working. No crystal growth occurs during the SPS process. The final shaped materials with fine structures have high strength and are used for cylinder heads of automobile engines. Although SPS is a batch process which seems to be inefficient for producing low-cost articles the rapid heating and the excellent quality of the product can decrease the production cost. This SPS process is to be incorporated into an industrial production system [20, 21]. WC body without Co Industrial part made from WC are fabricated using Co-metal as binder and sinering additive. Co and WC form a eutectic E-phase which melts at temperature < 1400°C. The binder phase exists as soft islands together with the hard WC dispersion and reduces the hardness of the hard metal. Pure WC powder is sintered above 1900°C by SPS. The sintered compact has excellent mechanical properties, e.g., a hardness of 24 GPa of Vickers hardness, and a high toughness of 6 MPa˜m/2 [6]. The surface of the pure WC body can be polished to a mirror surface free from fine irregularities. Such a material is useful for production of aspheric lenses, which require a die with high hardness and a mirror surface [22]. In wire drawing oil lubricants are used in some cases. A porous WC body manufactured by SPS can be used as die for this technology, because of a sufficient strength attained during SPS, probably due to creation of tight bonds between WC crystals [6]. WC-Co/steel functionally graded material The application of WC-Co-hard metal is greatly expanded by joining with steels. Direct connection of a hard metal with steel is difficult to accomplish, because of thermal expansion mismatch and lack of wettability. The functionally graded material concept (FGM) can solve these problems. Three layers, composed of a S35C steel plate (C: 0.32 – 0.38 wt%, Mn: 0.60 – 0.90wt%), WC- 40 wt%Co powders and WC- 25 wt% Co powders, respectively are sintered and joined by SPS. This FGM is welded to drilling bits and used for stabilizing their forward direction in oil-well pipes. In conventional stabilization, WC(Co) buttons are embedded in a steel stabilizing block. The steel not covered between buttons is exposed to wear by the environment and limits seriously the operation life of the tool [23].

Basic Research and Industrial Production Using the Spark Plasma System (SPS)

753

References [1] M. Tokita, “Trends in Advanced SPS Spark Plasma Sintering Systems and Technology”, J. Soc. Powder Tech. Jpn., 30 [11], 790-804 (1993), Japanese). [2] M. Ishiyama, “Plasma Activated Sintering (PAS) System”, Proc. 1993 Powder Metall. World Congress, Ed. Y. Bando and K. Kosuge., Kyoto, Jpn. Soc. Powder and Powder Metall. Japan (1993) 931-934. [3] K. Inoue, “Electric-Discharge Sintering”, U S Patent, No. 3,241,956 (1966). [4] K. Inoue, “Apparatus for Electrically Sintering Discrete Bodies”, U S Patent, No. 3,250,892 (1966). [5] R. W. Boesel, M. I. Jacobson and I. S. Yoshioka, “Spark Sintering Tames Exotic P/M Materials”, Mater. Eng., 70 [4], 32-35 (1969). [6] M. Omori, “Sintering, Consolidation, Reaction and Crystal growth by the Spark Plasma System (SPS)”, Mater. Sci. Eng., A287 [2], 183-188 (2000). [7] R. E. Hummel and H. B. Huntington, Ed, “Electro- and Thermo-Transport in Metals and Alloys”, AIMA, New York, (1977). [8] H. Conrad, “Electroplasticity in Metals and Ceramics”, Mater. Sci. Eng., A287 [2], 276-287 (2000). [9] G. DeGroat, “One-Shot Powder Metal Parts”, Am. Machinist, 109 [11], 107-109 (1965). [10] T. Nagae, M. Nose and M. Yokota, “Spark Plasma Sintering of Ar Gas Atomized High Si-Al Alloy Powder”, J. Jpn. Soc. Powder and Powder Metall., 43[10], 1193-1197 (1993) (Japanese). [11] J. R. Groza, S. H. Risbud and K. Yamazaki, “Plasma Activated Sintering of Additivefree AlN Powders to Near-Theoretical Density in 5 Minutes”, J. Mater. Res., 7[10], 2643-2645 (1992). [12] T. Sakai and M. Iwata, “Effect of Oxygen on Sintering of AlN”, J. Mater. Sci., 12, 1659-1665 (1977). [13] K. Yamane, S. Ishihara and H. Okuda, “Electric and Thermal Properties of Wood Charcoal Made by Spark Plasma Sintering”, Tanso No. 182, 95-100 (1998) (Japanese). [14] M. Inagaki, “Graphitization under High Pressure” Tanso, No. 129, 68-80 (1987) (Japanese). [15] S.Hoshii, A. Kojima and M. Goto, “Rapid Baking of Graphite Powders by the Spark Plasma Sintering Method”, Carbon, 38, 1896-1899 (2000). [16] C. O. Hulse and J. A. Batt, “Effect of Eutectic Microstructures on the Mechanical Properties of Ceramic Oxides”, Final Tech. Rept. UARL-N910803-10, May 1974; NTIS AD-781995/6GA; 140pp. [17] T. Isobe, M. Omori and T. Hirai, unpublished data [18] T. Itoi., T. Takamizawa Y. Kawamura and A. Inoue, “Fabrication of Co40Fe22Nb8B30 Bulk Metallic Glasses by Consolidation of Gas-Atomized Powders and their SoftMagnetic Properties”, submitted to Scripta Mater. [19] Y. Miyasaka, “Solid-State Joining by SPS and the Application”, Proceedings of 5th Symposium on Spark Plasma Sintering, Sendai, Japan, A7-A9 (2000) (Japanese). [20] N. Kuroishi, “High-Strain-Rate Superplasticity in Sintered Preforms Produced by Plasma Sintering”, Proc. NEDO International Symposium on Functionally Graded Materials, Tokyo, Japan, 67-74 (1999).

754

Omori

[21] N. Kuroishi, “Fabrication of Billet by SPS for High-Strain-Rate Superplastic Working”, in Proceedings of 5th Symposium on Spark Plasma Sintering, Sendai, Japan, A10-A11 (2000) (Japanese). [22] Catalog of Sumitomo Coal Mining Co. Ltd., I-29-3 Oji, Kita-ku, Tokyo 114-8513, 213-0012, Japan. [23] K. Tsuda, A. Ikegaya, T. Miyagawa and Y. Suehiro, “Application of Graded Cemented Carbide Sinter-Bonded on Steel to Drilling Tool”, J. Jpn. Soc. Powder and Powder Metall., 47 [5], 564-568 (2000) (Japanese).

Combined Processes, Laser Assisted Microwave Processing and Sintercoating M. Willert-Porada1, T. Gerdes1, Ch. Gerk2, Ho-Seon Park1 1 2

Chair of Materials Processing, University Bayreuth, Bayreuth, Germany; now with H.C. Starck, Goslar, Germany

Introduction The fundamental difference between conventional heating by convection, conduction and infrared radiation as compared to microwave heating becomes evident when upon high temperature microwave processing of materials opposite to conventional processing thermal runaway [1] and plasma ignition occurs. Temperature dependent material properties as well as modern microwave technology are the origin of such effects. For almost any material, in the solid as well as in the fluid phase, dielectric loss is increasing and dielectric strength is decreasing with increasing temperature, therefore the mechanism for absorbtion of microwave radiation will vary within a certain temperature range. Although the photon energy of microwave radiation in the frequency range of 0.3 to 300 GHz is very low, µeV to meV, efficient “heating” is possible, because of the numerous structural moieties which accumulate and convert the low energy photons into rotational or vibrational motion of e.g., lattice point defects, ions, molecules and clusters. With increasing temperature the number of defects increases, in addition, thermal energy becomes available to overcome the energy barrier for motion, therefore the majority of dielectric materials will exhibit a non-linear increase of dielectric loss with temperature. This causes “thermal runaway”, melting and evaporation, if microwave power level is not adjusted accordingly. As soon as evaporation of ionisable atoms or molecules occurs, arcs or microwave plasma will develop. The discharge becomes often sustained by a localized electrical filed enhancement connected with dielectric inhomogeneity of the solid materials. Modern microwave sources contribute to this effect – they deliver intense, coherent and almost monochromatic microwave radiation. Therefore locally very high electrical field intensity might build up by constructive interference of the microwave radiation. The electrical feld strength distribution can be calculated on the macroscopic dimension of the objects to be heated but up to now no simulation at the microstructure level is possible, therefore predictive tools for “combined” heating due to the appearance of different microwave absorption mechanisms are

756

Willert-Porada

missing. Full implementation of the temperature dependent electric properties of real materials, like e.g., ceramics, powder metals and composites is not possible yet, although significant progress with respect to thermal field implementation has been made over the last decade [2 - 5]. For most real materials microstructural homogenity can not be assumed. A large scatter in dielectric properties will always be present in porous green parts made from ceramic, metal or composite powders, with the pores bearing a gas of limited dielectric strength and the solids developing vapor pressure at elevated temperature. In order to calculate an average electrical field distribution, the microstructural dimensions can be neglected as “scattering” objects. To enable reliable processing conditions and systems for process control for future industrial scale microwave heating and sintering, new experimental and modelling methods for thermal runaway and plasma control are needed. As a general approach, “combined heating” by a systematic and intentional application of microwave plasma upon microwave processing of materials is therefore proposed, based on results of microwave sintering of ceramics, cemented carbides, metals and functionally graded materials collected over almost 10 years of research activity [6 - 15]. Microwave plasma upon microwave sintering Microwave sintering up to now is seen as direct heating by microwave absorption within the solid or a temporary liquid phase, not as an in-direct heating by a microwave gas discharge or a filamentous plasma. Efforts to scale-up microwave sintering revealed the almost ubiquitous plasma formation at high microwave power levels. Therefore, different processes were tested, aiming at an intentional use of “combined heating” by microwave plasma in addition to dielectric, ohmic and induction heating within one sintering schedule, and using one microwave frequency. Such combined heating would enable integrated processes, e.g., microwave sintering with an intentional grain size or compositional gradient, sintering followed by coating, surface modification before and after densification, infiltration followed by sintering [6 - 15]. In the paper only three examples will be discussed: 1. “Micro-Plasma” assisted microwave heating and sintering of PM-metals and cemented carbides [8 - 12] 2. Microwave plasma as a bulk “temporary pre-heating” for microwave sintering of Al2O3 and other oxide ceramics in a cold wall cavity [6, 7] 3. Laser Assisted Microwave Plasma processing, LAMP [13 - 15]. In Figure 1 the contribution of a microwave plasma to the microwave heating of PM-steel parts is shown schematically, and by IR-camera pictures taken during the heat up of the samples [9]. Similar effects will occur, when cemented carbides are exposed to high microwave power levels [8]. For sintering of ceramic coatings on a metal substrate spatial control of a microwave plasma is needed. Therefore the concept of a bulk “temporary preheating” is not applicable, because both, the coating and the substrate would be

Combined Processes, Laser Assisted Microwave Processing and Sintercoating

757

heated. “Micro-Plasma” is not applicable for powders adhered on the surface of bulk metals due to the low electrical field strength at the metal-ceramic interface.

Fig. 1. Bottom: schematic representation of microwave plasma contribution to microwave heating of PM-steels; upper row shows from left to right camera pictures of steel powder at T < 600°C and tensile test bar upon microwave heating [9].

In order to spatially confine the microwave plasma to the coating region only, LAMP was developed. A laser is used for localized evaporation of ionisable atoms to ignite a microwave plasma and also to sustain it close to the surface while processing the coating [15]. Microwave plasma upon melt processing Electronic as well as ionic conduction limits the penetration depth of microwave radiation into a material. Particularly in ceramic and silicate melts ionic conduction is very high, therefore volumetric heating is restricted to few µl volume at T > 1500°C. At the high temperature needed for melting of ceramic eutectics heat loss is significant, if the environment is colder. In order to compensate this by microwave absorption in a small volume, a high microwave power level is needed, which exceeds the dielectric strength of the atmosphere, therefore a plasma is ignited. Such conditions were applied for melting and directional solidification of Membrane Electrode Assemblies, MEA for SOFC [13]. Eutectic powders used for SOFC-anodes and multifunctional coatings for TBC were prepared either by microwave plasma processing or by LAMP [13 - 15].

758

Willert-Porada

Experimental

Microwave plasma upon microwave sintering and sintercoating For microwave sintering of steel and cemented carbide a molybdenum or a graphite cavity is used. The details are given in [17, 18]. Microwave plasma is generated by pressure reduction and increase of microwave power in case of cemented carbides. For steel sintering microwave power level is close to atmosphere ionization at ambient pressure in Ar or N2/H2 at a temperature above 1000°C, as shown in Figure 1. In Figure 2 a set-up is shown for microwave sintering of D-Al2O3 ceramics in a cold wall 2.45 GHz microwave furnace, by pre-heating the ceramic green part using a microwave gas discharge inside the casket.

Fig. 2. Left; top, multi mode cold wall cavity (Brands Ofenbau GmbH); right top, casket system in a quartz cylinder (indicated by the white line) used for microwave plasma preheating of ceramics in a cold wall cavity ; left and right, bottom, visualization of the carbon heating and the plasma without the casket [6, 16, 19].

To ignite and sustain the discharge, usually < 1 g of a carbon fibre cloth with a specific electrical conductivity of 103 :-1m–1 is fixed at the inner side of the casket. By microwave absorption in air the carbon is heated and starts to burn within the casket. At increased microwave power a CO2/CO-microwave plasma is ignited and sustained until the ceramic samples reach a temperature of 900 1000°C. The sample temperature is measured through a quartz window by a

Combined Processes, Laser Assisted Microwave Processing and Sintercoating

759

pyrometer, with the plasma switched off due to short interruption of the microwave power. Air is purged to the casket in order to remove the remaining carbon by combustion. Further heating proceeds by direct microwave heating of the alumina. Multi mode cold wall cavities with and without rotating table were used, with total microwave power of 2 kW (Püschner Mikrowellentechnik, rotating table) or 18 kW (Brands Ofenbau GmbH) at 2.45 GHz. To keep the plasma close to the samples, a box made of 2 layers of porous alumina board is applied [6, 16, 19]. Microwave plasma upon melting Two methods were employed for melt processing: single mode cavity microwave plasma and LAMP. Melting and crystallization of the eutectic ceramics and of silicate glasses was performed by heating the samples in air at atmospheric pressure in a rectangular single mode cavity operating in the H102 mode with a maximum power density of 3 x 104 W/m3. The wave-guide is equipped with an AutoTuner system (HOMER, Cober-Muegge).

Fig. 3. Single mode applicator used for microwave plasma melting and for LAMP [13].

In order to reduce temperature gradients in ceramic samples SiC was used as moderate additional heat source. Temperature was monitored with a pyrometer (band-width 1.1 - 1.7 µm, 200 - 2000°C). The incident MW power was varied between 0.2 - 1 kW, the dwell time at melting temperature between several seconds and 15 minutes. The set-up is shown in Figure 3. For LAMP the set-up and atmosphere conditions as for monomode microwave plasma experiments were used. The pyrometer is replaced by the coupling fiber for Nd:YAG laser light (1.064 µm, 4.5 x 108 W/m² maximum pulsed power, 90 W continuous power). Temperature measurements upon LAMP were made with a light pipe and the laser switched off. In order to move the laser beam and/or the laser assisted microwave plasma over the sample surface and adjust the optimal laser velocity, an automated x-y table was used, with a spatial resolution of 1 µm

760

Willert-Porada

and a total accessible area of 10 x 10 cm. For smaller areas the laser beam was moved over the sample by means of the focussing optics. Materials Oxide ceramic: Commercial Al2O3 is used for the ceramic samples (A16, Alcoa, 99.6% D-Al2O3). The powder is sieved and filled into rubber molds for Cold Isostatic Pressing in water. After CIP at 60 MPa the samples are presinterd, to yield rectangular green plates 88 x 35 x 20 mm in size. PM-steels: as described in [17]. Cemented carbides: as described in [18]. Thermal barrier coatings: ~375 µm thermally sprayed 7 wt%Y-ZrO2 on Nimonic 80A, cylindrical samples with Ø 40 mm, height 10 mm (provided by the Chair of Materials Technology, Prof. Bach, University of Dortmund). On top of the sprayed coating an additional 0.5 – 2 mm layer of 80 wt% ZrSiO4-20 wt% NiO is slip cast from an ethanol slurry. SOFC: different binary or quasi-binary ceramic powders (Y-ZrO2, SZ from TOSOH, others from CHEMPUR) were mixed by ball milling for 15 minutes to yield eutectic compositions, as shown in Table 1. The powders were dried and axialy pressed to pellets with f 13 mm, height 0,1 – 1 mm. Table 1. Composition of the eutectic mixtures [20] Composition [mol%] 77 NiO, 23 YSZ 69 NiO, 31 Y2O3 55 NiO, 45 Gd2O3 74 MnO, 26 YSZ

Eutectic Temperature [°C] > 1800 1700 1540 1540

Such pellets were employed on the top of sintered 20x20x0,2 mm3 tape caste YSZ foils (KERAFOL, Germany). Upon melting a reaction zone is developed between the eutectic melt and the substrate, which causes change of composition and facilitates solidification. All materials were characterised by XRD (Philips, XPert), optical microscopy (Zeiss LSM), SEM-EDX (Jeol, Oxford Instr.), electrical impedance measurements (for SOFC-materials) and oxidation stability in air (for TBC). Grain size distribution in oxide ceramics was calculated from the SEM-pictures of thermally etched samples, pore size distribution was measured by Hg-Intrusion.

Combined Processes, Laser Assisted Microwave Processing and Sintercoating

761

Results and Discussion

Plasma assisted microwave sintering and sintercoating

Microwave sintering of PM-metals Microwave sintering of PM-steel in a cold wall cavity is a combined heating process, as visible from the strong influence of the material used as supporting plate for the PM-parts on the heating rate. In Figure 4 the experimental results with different materials are shown.

Fig. 4. Upper part, left; heating profile upon microwave sintering of a batch of PM-steel parts using the same microwave power level and casket to PM-steel weight ratio but different supporting ceramic materials. Upper part right, assignment of DC-conductivity of the PM-parts to the slope of the heating curve [9]. Bottom: IR-camera image of microwave discharge starting from a high-E-field position, molten area on the tooth gear.

Differences in heat capacity and heat conductivity of these materials can be neglected, because heat loss is governed by the casket, which was identical for each type of supporting plate material. In a typical experiment the weight ratio of PM steel parts to the casket was 1:10, e.g. 350 g PM parts and 3500 g casket. In addition, when the change in slope of the heating curves is compared with the DCconductivity of the steel parts at the relevant temperature level, no correlation with the increasing metallic conductivity but rather with a temperature typical for microwave absorption of ceramics is found. Furthermore, the fact, that the difference between porous and dense alumina as support plate is rather small (no sign of

762

Willert-Porada

Elongation [%]

Cumulative Pore Volume [mL/g]

thermal runaway typical for dense alumina) indicates the contribution of plasma discharge heating, confirmed by the IR camera images (see also Figure 1). Therefore the change in slope of the T/t-curve could reflect different contribution of direct microwave absorption, ceramic „hot plate “ heating and plasma discharge heating to the overall sintering profile. The microwave sintered PM-parts also show less larger pores as compared to conventionally sintered materials, sintered at 1042°C (MW) and 1045°C (conventional) for 30 minutes, as shown in Figure 5. Results of tensile testing for these materials correlate with the porosity measurements, showing higher elongation for the microwave sintered material. Contribution from gas discharge to removal of porosity upon microwave sintering could be assumed. 0.016 0.014 0.012 0.01 0.008 0.006 0.004 0.002 0

conv. Sintering 0,01 - 1,0 Diameter [µm]

MW-Sintering 1,0 - 5,0

3

conventionally sintered microwave sintered MSP 1.5Mo, 700MPa

2 1 0 1032

1060 1080

1102 1112

1145 1150

1192

Sintering Temperature [°C]

Fig. 5. Left, pore volume and pore size distribution measured by Hg-porosimetry for MSP 1.5 Mo-PM parts; right, elongation of MSP 1.5 Mo test bars sintered by microwaves and conventionally [21].

Microwave sintering and sintercoating of ceramics and cemented carbides For cemented carbides and alumina ceramics combined heating is applied for the development of new material properties, whereas for PM-steel improvement of the basic microstructure at lower costs than with existing technology is the main goal so far. Al2O3 ceramics are very important for wear applications, therefore a grain size gradient with small grains in the surface near region and larger grains in the interior could provide new combinations of strength and toughness. Therefore, microwave sintering at 2.45 GHz upon a permanent temperature gradient in the cavity, leading to dense ceramics with smaller grain size at the surface and larger grain size in the center is expected to improve the mechanical performance of the Al2O3-ceramic parts. Because at 2.45 GHz microwave absorption in alumina is efficient only at higher temperature, controlled microwave plasma discharge is used for pre-heating of alumina ceramics. As shown in Figure 6, the discharge preheating leads to a grain size gradient of almost 10 mm depth, corresponding to 25% of the volume of the ceramic plate, with gradually increasing grain size from

Combined Processes, Laser Assisted Microwave Processing and Sintercoating

763

5.0 µm to 7.5 µm in the central part of the ceramic plate. This is surprising, because a high sintering temperature of 1700°C and a dwell time of 1 hour was applied, therefore grain growth in the whole plate could be expected. As compared to conventional sintering, the dwell time has to be doubled to arrive at a comparable grain size. By application of SiC as pre-heating element for microwave sintering with 2.45 GHz frequency, rather an inhomogeneous grain size gradient is developed upon comparable sintering conditions [6]. Graded microstructure can also be generated in cemented carbides, by application of a reactive atmosphere, as described in [18] but even more pronounced, by a plasma discharge, as shown in Figure 6.

Fig. 6. Influence of microwave plasma upon microwave sintering on microstructure gradient formation in alumina ceramics (left) and in cemented carbide (right).

The strong atmospheric microwave plasma, ignited and sustained by an high microwave power level in Ar/H2 as sintering atmosphere removes Cobalt from the surface near region, which is beneficial for following coating by nitrides or diamond [18]. However, as shown in Figure 6, a thick, very dense metallic Cobalt

764

Willert-Porada

layer can develop, which would prevent full densification of the cutting tool pretreated in this way by a microwave plasma [8, 22]. Therefore, for practical purposes the plasma etching is performed at reduced pressure, as described in [11]. On the other hand, such a dense coating would be of interest for Hot Isostatic Pressing as compaction method. Plasma assisted microwave melting Eutectic ceramic melts with melting points close or even above 1800°C (see Table 1 for details) have been prepared as coatings on the top of 8 mol% Yttria stabilized Zirconia (YSZ) foils using 2.45 GHz microwave heating combined with a microwave plasma discharge. The heating process is divided three steps, as shown in Figure 7. Temperature [°C]

2000

melting temperature

1600

step 1

step 2

1200 threshold

near surface gas breakdown

800 400 0

MW off 0

50

100

150

200 250 Time [sec]

300

350

400

Fig.7. Typical heating profile of a combined 2.45 GHz, single mode applicator microwave dielectric and microwave plasma heating process to melt a pellet of NiO/YSZ eutectic on top of a 8Y-ZrO2 substrate.

First, moderate heating with medium power (< 500 W) up to a threshold temperature typical for each ceramic occurs. When the threshold temeparture is reached, MW poweris increased to 500 - 1000 W and a discharge is ignited close to the surface of the sample. The heating mechanism is changed from volumetric to surface heating. Than a dwell time is applied by reducing the MW power to approx. 200 - 400 W and holding the melting temperature. The power intensity for heating the surface is about 107 W/m2, which is comparable with low power laser processing. It should be emphasized, that the temperature rise due to the gas breakdown always stops just above the melting temperature. This process of “self regulation” is not understood yet, but it is broadly applicable, e.g., to estimate the melting temperature of an unknown systems with a deviation of r 50 K. During the dwell time dissolution/precipitation processes occur at the interface between the melt and the ceramic substrate. The main part of the incident MW power is absorbed in the discharge. Only 200 - 300 W are necessary for discharge maintaining. Therefore, decreasing the incident MW power after melt formation reduces the temperature of the substrate, because the melt is consuming all power.

Combined Processes, Laser Assisted Microwave Processing and Sintercoating

765

The eutectic lamellae near to the YSZ substrate crystallize perpendicular due to the directional heat flow through the cold substrate. Upon directional cooling, a graded, micro-structured material is obtained, as shown in Figure 8 for NiO-ZrO2 on ZrO2. Such materials could be used as an Anode in a SOFC, by reduction of NiO to Ni. Furthermore, Ni can be etched away and the elongated pores filled with a cathode material, like La(Sr)MnO3, as shown in Figure 9. Unfortunately, the nucleation of a NiO-layer upon crystallization of the melt on ZrO2-substrates prevents such applications [13].

Fig. 8. Graded and micro-structured composite materials for SOFC-Membrane-Electrode Assembly, obtained by combined microwave heating to generate a thin melt film on a ZrO2-electrolyte membrane [23].

Fig. 9. Potential application of eutectic composites as MEA-materials for SOFC.

LAMP-Processing of coatings The combined dielectric and microwave discharge heating is limited to a hot spot of a small melt pool. But in order to coat larger areas, the hot spot should be mobile. This is achieved by combining laser heating with 2.45 GHz microwave heating, as in LAMP. Laser energy can be applied with a high spatial resolution because the laser beam is easy to control. On the other hand, the depth of a Nd-YAG or other laser heated zone is smaller than the penetration depth of microwaves at 2.45 GHz. Therefore, laser heating is used to evaporate from a very narrow zone a small amount of ionisable atoms and reduce locally the pressure by heating the

766

Willert-Porada

surface and expanding the surface bound gas layer. When doing so in presence of a strong microwave field, surface near gas discharge can be ignited and sustained upon movement of the laser beam over the surface of the substrate. Details of the discharge and process control are given in [13]. In Table 2 the threshold temperature for discharge ignition as recorded experimentally in the LAMP set-up shown in Figure 3 is given for different ceramic coatings. The corresponding partial pressure of metal atoms in neutral atmosphere and in air is calculated. Table 2. Threshold temperatures for discharge ignition and partial pressures of metal atoms for different ceramic coatings.

Ceramic

Threshold temperature [°C]

NiO 1200 ZnO 850 MgO 1450 SnO2 1210 SiO2 1370-1470 *at 1 atm total pressure

Metal atom pressure in Metal atom presneutral atmosphere* [bar] sure at 0.2 bar O2 [bar] Ni ˜ 3.15 10-8 8.82 ˜ 10-12 1.50 ˜ 10-11 Zn ˜ 4.48 10-8 -8 Mg ˜ 3.25 10 9.28 ˜ 10-12 -8 1.05 ˜ 10-14 4.58 ˜ 3.15 10 ?SiO

Fig. 10. Upper part, optical image of thermally sprayed ZrO-TBC on Nimonic-Alloy after glazing and sealing by means of LAMP. Laser sintering is shown for comparison. Bottom part, results of oxidation testing in forced air, furnace temperature 1450°C, ceramic surface temperature 1200°C, metal at 300°C [13].

As described in the experimental part, thermally sprayed coatings were glazed and compacted at the surface by laser melting and LAMP. As shown in Figure 10,

Combined Processes, Laser Assisted Microwave Processing and Sintercoating

767

LAMP yields a dense coating, with improved oxidation resistance in air (1200 °C surface temperature, 1450°C furnace temperature) as compared to the thermally sprayed coating. Only LAMP provides an overlapping trace, needed for a smooth sealing of the surface.

Conclusion Combined heating by simultaneous or sequential application of microwave dielectric heating and microwave discharge heating in one process is a promising concept for the development of integrated thermal treatments for almost any class of materials. Sintering can be combined with coating and micro-structuring of metals and ceramics, as has been shown in detail. Furthermore, new material properties can be achieved and multifunctional composites can be processed. Further development is required in particular for improved process control and increased basic knowledge about the physical properties of materials which govern the variation of heating mechanisms during combined heating.

Acknowledgement The financial support of DFG, Wi-856/3 (SOFC) and of the Bavarian Ministry of Science, Research and Arts (LAMP) is gratefully acknowledged (contract BXVI). M.W.P. is indebted to Brands Ofenbau GmbH for the temporary donation of the cold wall cavity furnace.

References [1] V.M. Kenkre, L. Skala, M.W. Weiser: Theory of Microwave Interactions in Ceramic Materials; The Phenomenon of Thermal Runaway, J. Mat. Sci. 26, 2483-89 (1991) [2] M. Celuch-Marcysiak, W. K.Gwarek, M. Sypniewski, A Novel FDTD System for Microwave Heating and Thawing Analysis with Automatic Time-Variation of Enthalpy-Dependent Media Parameters, p. 199, this book [3] V.E. Semenov, N.A. Zharova, Thermal Runaway and Hot Spots Under Controlled Microwave Heating, p. 482, this book. [4] Vadim V. Yakovlev, Examination of Contemporary Electromagnetic Software Capable of Modeling Problems of Microwave Heating, p. 178, this book [5] G.E. Georghiou, R.A. Ehlers, A. Hallac, H. Malan, A.P. Papadakis and A.C. Metaxas, Finite Elements in Simulation of Dielectric Heating systems, p. 167, this book [6] M. Willert-Porada, S. Vodegel, Verfahren zur Sinterung von hochschmelzenden anorganischen Stoffen mit geringen dielektrischen Verlusten mittels Mikrowellenstrahlung, P 4224974.0, 1992

768

Willert-Porada

[7] S. Vodegel, Mikrowellen-Sintern von Aluminiumoxid, VDI, Nr. 354, Reihe 3, 1994 (ISBN 3-18-335403-9) [8] T. Gerdes, Mikrowellensintern von metallisch-keramischen Verbundwerkstoffen, PhD Thesis, University of Dortmund, 1996; VDI, 432, Reihe 5 1996 (ISBN 3-18-343205-6) [9] H.S. Park, M. Willert-Porada, Comparative Study of Casket Materials and Temperature Measurement Methods upon Microwave Sintering of PM-Steel, S. 280ff, ISBN 300-008356-1 (2001) [10] M. Willert-Porada, H.-S. Park, Heating and Sintering of Steel Powders with Microwaves at 2.45 GHz Frequency – Relation Between Heating Behaviour and Electrical Conductivity”, Ceram. Trans., Vol. 111, p. 459-470 (2001) [11] M. Willert-Porada, R. Klupsch, A. Schmidt, K. Dreyer, K. Rödiger, Herstellung neuer Hartmetalle mit Gradientenmikrostruktur durch Mikrowellensintern, Mat.-Wiss. U. Werkstofftech. 32,1-14(2001), Klupsch, Tap, FGM, this book [12] Willert-Porada, T. Gerdes, R. Borchert, “Application of Microwave Processing to Preparation of Ceramic and Metal-Ceramic FGM”, “Proc. of the 3rd International Symposium on Functional Gradient Materials”, FGM 94, ed. B. Ilschner, N. Cherradi, Presses Polytechniques et universitaires romandes, EPFL-Ecublens, 1015 Lausanne, 1994, S. 15-20 (1994); R. Borchert, M. Willert-Porada “Eigenschaften drucklos im Mikrowellenfeld gesinterter Metall-Keramik Gradientenwerkstoffe” in “Verbundwerkstoffe und Werkstoffverbunde”, Hrsg. G. Ziegler, DGM-Inf. Ges. mbH, 1996, 73-77 (1996). [13] Ch. Gerk, Herstellung keramischer Eutektika durch simultanen Laser- und Mikrowelleneinsatz, Fortschrittsberichte VDI, Reihe 5, Nr. 619 (2001), PhD-Thesis, U of Bayreuth, December 2000. [14] Gerk, Willert-Porada, DE 199 511 43.8, (1999), Laser Assisted Microwave Sintering [15] Ch. Gerk, M. Willert-Porada, Laser assisted microwave processing as a new tool in ceramic processing, Ceram. Trans., Vol. 111, 451-458 (2001) [16] R. Klupsch, Dotierungseinfluß auf das Gefüge von mikrowellengesintertem Aluminiumoxid, BSc Thesis, Univeristy of Dortmund, 1997. [17] F. Petzoldt, B. Scholz, H. S. Park, M. Willert-Porada , Microwave Sintering of PM Steels, p. 598, this book [18] R. Tap, M. Willert-Porada, K. Rödiger, R. Klupsch , Formation of Functionally Graded Cemented Carbides by Microwave Assisted Sintering in Reactive Atmospheres, p. 609, this book [19] Yearly report, BMBF-MATECH-Project 03 N 5010 A0, Werkstoff- und Technologieentwicklung zum Mikrowellensintern von Hochleistungskeramik, 1997, 1998. [20] W. Kurz, P.R. Sahm, Gerichtet erstarrte eutektische Werkstoffe, Springer Verlag Berlin-Heidelberg (1975) [21] H.S. Park, Mikrowellensintern von Metallen, PhD Thesis, in preparation [22] DE 000004340652C2, DE 196 01 234.1 (1996), ”, DE 196 54 371.1 (1996), DE 197 09 527.5 (1997) [23] Ch. Gerk, M. Willert-Porada, Microwave Processing of Eutectic Oxide Melts – a new Tool in Solid State Chemistry, Proc. of the 2nd Int. Conference on Microwave

Chemistry, AMPERE, Antibes 327-330 (2000), Ch. Gerk, M. Willert-Porada, New Composite Ceramic Powders for SOFC Anodes from a Eutectic Ceramic Melt Process, Proc. of the 10th Intl. IUPAC Conference, High Temperature Materials Chemistry (2000)

Author Index Acierno, D. Aguilar, Juan Ahrens, G. Annibali, M. Bahke, P. Balbastre, Juan V. Barba, A.A. Barringer, S.A. Bauer, F. Bernard, Jean-Paul Bircan, C. Bondioli, F. Booske, John H. Boria, V.E. Bossert, J. Bossmann, Stefan Bradshaw, S.M. Breccia, Alberto Brito, Manuel E. Bykov, Yu. Calinescu, I. Calinescu, Rodica Campana, Eileen Canós, A.J. Catalá-Civera, J.M. Celuch-Marcysiak, Malgorzata Cooper, Reid F. Corradi, A. Craciun, G. Cresko, Joseph W. Da Silva, M.A.A.P d'Amore, M. Danner, T. Davis, W.A. de los Reyes, E. Denisov, G. Díaz-Morcillo, A. Dikhtyar, V. Egorov, S.V. Ehlers, R.A Eremeev, A. Esposito, Biagio Eusterbrock, L. Feher, Lambert Fernández-Pascual, Ángel Ferri, Elida Fiumara, V. Gattavecchia, Enrico

321 645 426 341 720 31, 39 321 107 633 445 107 627 461, 673 39 514 56 271 359 533, 562 24, 577 349, 398 398 386 48, 138, 226 39, 92, 138, 226, 234 199 461, 673 341, 703 349 436 289 321 129 521 31, 39, 48. 138, 234 24 234 687 577 167; 217 24, 591 359 710 15, 681, 695, 735 103 359 321 359

770

Author Index

Georghiou, G.E Gerdes, T. Gerk, Ch. Getman, O.I. Ghio, S. Gianchandani, Yogesh Glyavin, M. Grosse-Berg, J. Gwarek, Wojciech K. Hallac, A. Hasznos-Nezdei, M. Hirao, Kiyoshi Hoffmann, Jens Holoptsev, V.V. Hunyar, Christian Ighigeanu, D. Iovu, H. Jansen,W.J.L. Janssen-Mommen, J.P.M. Jerby, E. Jermolovicius, L.A. Jones, Mark I. Kaddouri, A. Klupsch, R. Komarneni, S. Kovács, A.J Kriszio, H. Kuwahara, Hideki Kyoh, Bunkei Langer, G. Lee, Andrew Y.J. Lentini, Maria Antonietta Leonelli, C. Leparoux, S. Link, G. Ludwig, C. Lusvarghi, L. Makino, Yukio Malan, H. Marsaioli, A. Jr. Martelli, Giorgio Martin, D. Martínez-González, Antonio Marucci, G. Mateescu, E. Mavretic, Anton Mazzocchia, C. Metaxas, A.C. Miyake, Shoji Modica, G. Monzó-Cabrera, J. Nannicini, R. Neményi, M. Nguyen Tran, V. Nüchter, Matthias Nuño, M. C. Oda, Kiichi

167 720, 755 755 491, 553 129 673 24 710 199 167 661 533, 562 405 24, 491, 553, 591 735 349 349 329 329 687 377 533, 562 370 609 627 312 426 570 570 426 77 386 627, 703 583 506 514 341 149, 498, 570 167 289 386 349, 398 103 341 349 436 370 167; 217 149, 498, 541, 570 370 226, 234 370 312 77, 119 390, 405 31 149

Author Index Ohlsson, T. Omori, Mamoru Ondruschka, Bernd Opfermann, J.R. Oree, Sheila Ortiz, Ubaldo Pallai-Varsányi, E. Panichkina, V.V. Panunzio, Mauro Papadakis, A.P. Park, H.S. Pavlov, V. Pellacani, G.C. Penaranda-Foix, F.L. Petzoldt, F. Phipps, L. Myles Pinto, I.M. Pitombo, R.N.M Plaza, P. Plaza, Pedro Plotnikov, I.V. Rachkovskii, A.I. Radoiu, M. Ragazzo, G. Reader, H.C. Regier, M. Rhee, S. Riedel, Hermann Risman, P.O. Rivasi, M.R. Rödiger, K. Rodríguez, Javier Roussy,Georges Rybakov, K.I. Ryynänen, Suvi Saito, Hidenori Sánchez-Hernández, David Sano, Saburo Scaglione, A. Schertlen, Ralph Schiffmann, Robert F. Schneiderman, B. Scholz, B. Schubert, H. Schubert, T. Semenov, V.E. Senise, J. T. Shevchenko, E.A. Siligardi, C. Skorokhod, V.V. Soto, P. Sousa, W.A. Svoboda, Jiri Sypniewski, Macie Szabó, D. Vinga Szabó, L.P. Szabó, S.

243 745 390, 405 514 155 645 312, 661 491, 553 386 167 598, 755 24 627 48, 138, 226 598 436 321 289 31 48 24, 553, 591 591 349, 398 341 3 129, 259 506 210 243 341 609 645 65 472, 591 282 570 92, 103 149, 498, 570 321 56 417 377 598 129, 259 633, 651 472, 482, 577 377 491 341, 627, 703 491, 553 39 289 210 199 619 661 661

771

772

Author Index

Szijjártó, E. Tap, R. Thomas, J.R. Jr. Thompson, Keith Thumm, Manfred Tied, Antje Torgovnikov, G. Toriyama, M. Tsuzuki, Akihiro Turner, Ian W. Ueno, Toshiyuki Valdez, Zarel Valecillos, Maria-Cecilia Van Loock, Walter Vegh, Viktor Veronesi, P. Vicennati, Paola Vinden, P. Vollath, Dieter Walter, G. Wäppling-Raaholt, B. Wielage, B. Wiesbeck, Werner Willert-Porada, M. Wu, Xiaofeng Yakovlev, Vadim V. Yoshikawa, Akinobu Yu, X. Zharova, N.A. Ziegler, G.

312 609, 720 521 673 15, 506, 681, 695, 735 405 303 533, 562 149 191 149, 498 645 562 85 191 341, 703 386 303 619 583 243 583 56 598, 605, 633, 651, 710, 720, 755 521 178 570 129 482, 577 710

Subject Index 2.45 GHz

5pp

30 GHz 24.15 GHz

5, 21-25, 491, 492, 554, 577, 581, 593-595, 682-686, 697, 701, 736, 737, 742 15-17, 20-22, 681, 701

13,56 MHZ

674-676

915 MHz

5, 88, 95, 111-114, 182, 263, 418-422, 429, 447, 681, 701, 722, 728, 733

A acrylamide copolymers

349

activation energy

69, 72, 361, 365, 367, 473-479, 499

adhesiveness

297

admittance

79, 120, 122, 124

agglomeration

559, 619, 621, 623, 631, 656

aircraft

695-698, 701

AlN

541, 548-551, 747

alpha-amylase

263

Į-Si3N4

534, 537, 554, 563, 566,

Į-SiC

646, 759, 761, 763

alumina

8, 24, 29, 30, 149, 150-154, 211-216, 343-346, 399, 403, 463, 465, 473, 474, 478, 480, 481, 498, 500-512, 515-517, 534, 561, 568579, 582, 584, 588, 589, 593, 597, 600, 623-625, 647, 650, 682, 691, 712, 714, 716, 726, 730, 759-763 714

alumina green parts alumina membranes

475, 480

alumina powder

463, 571, 584, 712

alumina single crystals

473

Al2O3

216, 404, 464, 466, 480, 509, 512, 516, 517, 520, 535-541, 544548, 555-557, 558, 563, 565, 568, 578, 581-589, 594-596, 600, 622-625, 682, 706, 709, 710, 714, 715, 718, 743, 747, 756, 758, 760, 762, 763 714, 715

Al2O3 green parts Al2O3-ceramics

710

Al2O3-Y3Al5O12

743

Al2O3-ZrO2(8.8Y2O3)

743

aluminium

11, 28, 59, 110, 224, 323, 333, 380, 681, 695, 701

amorphous silicon films

473, 479

amorphous titania

659

Ampere law

674

anatase

519, 633, 638

774

Subject Index

annealing ANSYS-software

29, 466, 467, 469, 470, 472-481, 492-497, 538, 539, 540, 543, 544, 550, 560, 673, 678, 679 31, 32, 38, 718

antenna

8, 15, 89, 95, 100, 105, 106, 150, 212, 214, 323, 675-678, 692

anti-icing

695-701

aperture

48, 262, 422

applicator design

3, 6, 31, 35, 56, 57, 127, 422, 441, 465

arcing

5, 7, 11, 13, 346, 658, 757

aromatherapy

119, 127

aromatic hydrocarbons

331, 710, 712

arrhenius form

473

asbestos

341-345, 347, 348

atomic collisions

463

automatic impedance tuner

689

automatic network analyzer

9, 41, 77, 120, 238

average pore size

492, 495, 497, 559

axisymmetric problems

211

azeotropic mixture

378

B backward wave oscillator

18, 150

baking

260, 262, 263, 270, 313, 320, 420, 423, 425, 447, 456

ȕ-cages

666, 667

bending strength

541, 544-547, 549, 551, 565, 567, 570, 572-576

benzaldehyde

394, 711, 716, 717

ȕ-Si3N4

533, 534, 537, 538, 540, 565, 566, 568

ȕ-SiC

645, 646, 647, 648, 650

BF2

467, 678, 679

bimodal microstructure

538

binder

342, 611, 624, 706, 707, 710-719, 752

bio-dielectric decontamination

337

biological activity

386

blackbody radiation

692

bolometers

9, 65

boron

466, 563, 678, 680

boron doping

466

boron nitride

563

borosilicate

162, 703

breakdown

5, 173, 263, 626, 764

bricks

78, 279, 343

bulk ceramics

542, 544, 546

butanon

711, 715, 717

C calcination

515, 518, 635, 704, 705, 720

Subject Index calibration emulsions

130, 136

capillary stresses

475, 479, 497

capillary tension

559, 560, 562

CAPRI

697- 699

caramelization

295, 297

carbon fibre

697, 727, 733, 758

carbon fibre reinforced composite

697

carbon support

404, 652, 653, 654, 656, 657, 658, 659, 660

carbothermal reaction

645

cardbox manufacturing

445, 446, 447, 450

catalyst

372, 398, 404, 633, 644, 651, 655, 659, 720

catalyst activity

402, 659, 660

catalyst structure

399

cathode luminescence

541, 543

cell-centred finite-volume method

191

cellular phone

12

cement

82, 274, 707

775

cemented carbide

609, 613, 615, 756, 758, 760, 762, 763

ceramic

ceramic fiber board

12, 24, 28, 29, 210-216, 343, 474, 491, 506-513, 514-521, 533, 562-569, 577-582, 583-590, 591-597, 619-627, 633-644, 673-679, 682, 686, 687, 691-693, 703-709, 710-719, 721, 726, 737, 739, 745-753, 756-767 726

ceramic glazed stoneware

708

ceramic pigment

703-705

ceramic powders

619, 621, 623, 625, 626, 706, 760

ceramic tile

703, 704, 707

CFRP

686, 697-701, 732-743

charge and mass transport

461

charged vacancies

473, 475, 477, 478

chemisorption

653, 659

chewiness

297

chokes

11, 12, 14, 39, 41-47, 430, 431

clay minerals

707

Co

752

CO

758

coagulation aids

349

coal gasification

720, 722

coalescence coating

575, 576, 652, 653, 723 19, 609, 610, 624-626, 657, 658, 660, 687, 688, 691, 693, 704, 720-733, 755-767 6

coaxial cable coaxial guide

687

coaxial probe

77, 84, 109, 123, 127

776

Subject Index

collagen

107, 111- 113, 118

colloidal microwave processing

634, 652, 654- 656, 660

combinatorial chemistry

390, 391, 394

compaction parameter

555- 557

complete factorial

250, 380

complex permittivity

57, 71, 84, 128, 140, 143, 147, 155, 156, 161-163, 188, 440, 684

composite curing system

684

composite materials

550, 551, 582, 583, 591, 592, 597, 615, 644, 681-685, 695, 701, 735, 736, 765 14, 192

computational electromagnetics concrete

281, 687, 688, 690, 692, 693

condensation polymerization

436, 437

conductive currents

6, 591

conductive dissipation

463

conductivity

31, 35, 38, 69, 71, 133, 157, 162, 168, 201, 206, 207, 210, 214, 216-218, 222-224, 227, 229, 232, 233, 235, 249, 263, 290, 303, 342, 343, 347, 408, 428, 437, 473, 480, 482, 483, 528, 541, 548-551, 579, 592, 593, 596, 598, 608, 609, 635, 674-676, 678, 682-684, 695, 699-701, 708, 718, 729, 731, 740-743, 760-762 396

contineous reactor convection

211, 268, 521, 669, 736,

convective drying

228, 235, 237, 271, 272, 274, 276, 277, 308, 436-438, 462, 463, 465, 579, 684, 712, 718, 738, 739, 755 271, 273, 281

conventional colloidal processing

657

conventional firing

707, 708,

conventional syntheses

628

conveyor belt

8, 11, 13, 182, 259-261, 266, 307, 430, 433, 607

conveyorized multimode cavities

418, 422

cooling rate

499, 554, 556, 563, 565, 571, 599, 605, 606, 679

core-shell structure

623

corona

11, 167, 172, 173, 757

coupling to phonons

462

Cr2O3

499-503, 541, 547, 548, 755

crotonaldehyde

711, 715, 717

crystal crystal growth

6, 269, 462, 468, 469, 473, 475, 476, 478, 481, 499, 500, 505, 538, 567, 569, 638, 648, 650, 652, 653, 661, 662, 667, 668, 704, 732, 741, 745-752, 759, 765 745-752

crystalline habitus

704

crystallisation

612, 630, 703, 704, 730, 748, 749, 765

crystallite size

547, 570, 572, 574, 575, 576, 637, 751

CVD-process

720-731

cylindrical waveguide

8

D DC electric field

475, 477

Subject Index debindering

716-719

Debye model

476

deformation of the pore

477

dehalogenation

398, 399

dehydration

138, 260, 264, 289, 301, 641, 661, 663, 664, 668

dehydration process

663, 664

de-icing

695-701

777

delamination

657, 679, 695, 700, 701

demoulding

280

denaturation

107-109, 111-118, 313, 319

densification

474, 479, 498, 499, 504, 505, 510, 533, 534, 536, 539, 541, 548, 551, 556, 572, 748, 756, 764 703, 704

devitrification dewaxing

706, 709,

diamond films

541, 542, 543, 550, 552

diastereoisomer

387

dielecmeter

65-68, 71, 73

dielectric composite panel

697

dielectric heating

13, 167, 169, 171, 173, 175, 177, 259, 329-331, 339, 340, 369, 443, 462, 586, 721, 722, 729, 731, 767 476, 591, 592

dielectric permittivity dielectric properties

dielectric relaxation

5-10, 44, 48, 79-84, 107-127, 129-133, 138-143, 149-162, 198, 212-214, 226-230, 234-255, 266-270, 282-288, 312, 323, 342, 408, 431, 479, 508, 513, 519, 556, 579, 583-590, 592-597, 691, 697, 704, 706, 718, 733, 736, 738, 756 591, 669

dielectrometry

138

diffusion

diffusion behavior

33, 118, 177, 229, 239, 287, 288, 427, 438, 440, 465-468, 473476, 479-481, 491-493, 495-501, 503, 505, 513, 533, 539, 541, 544, 547, 548, 551, 560, 561, 598, 611, 615, 660, 673, 679, 695, 700, 723, 739, 746, 747, 750 498, 500, 551

diffusion-controlled process

473

dipole re-orientation

462

directional coupler

9, 65, 75, 146, 346, 380, 620

doping technology

678

drilling

58, 687-693, 752

drying

4-7, 129, 181, 234-243, 259-270, 271-281, 282-288, 289-302, 303-311, 320, 337, 418-423, 426-435, 440, 447-452, 462, 490, 600, 606, 629, 637, 687, 705, 720, 721 138, 142, 147

dynamic microwave sensor

E effective diffusion coefficient

467, 468, 493

effective medium approximation

592

E-H tuner

689

electric dipoles

461

778

Subject Index

electric field amplification

477

electric field force

462

electric field homogenity

713

electric susceptibility electrical conductivity

476 480, 521, 528, 608, 635, 682-684, 699, 700, 728, 736, 740-742, 758 684, 740

electrical permittivity electrochemical activity

655

electro-erosion

687

electromagnetic compatibility

85

electromagnetic energy

10, 33, 86, 91, 107, 118, 163, 224, 225, 233, 235, 255, 288, 312, 328, 369, 430, 481, 482, 562, 591, 650, 675 15, 18, 32, 68, 74, 91, 105-108, 192, 197-199, 227, 244, 245, 367, 408, 461, 467, 472, 476, 499, 508, 571, 579, 596, 605, 676, 684, 700, 737-740 91, 177, 312, 463, 682, 700, 739

electromagnetic field electromagnetic heating electromagnetic induction heating

673

electromagnetic interference

10, 97, 430

electromigration

746

electronic grade silicon

723

electroplasticity

746

EMC

4, 6, 10, 11, 12, 14, 85, 86, 88, 91, 225, 431

emulsifying properties

375

endothermic reactions

463, 470

energy dissipation

272, 674, 676

energy losses

50, 482, 556, 591

environmental engineering

257, 312, 341, 390, 627

Euler equations

174, 175

eutectic

464, 747, 748, 752, 759, 760, 764, 765

eutectic bonding

464

external stirring

462

F Faraday’s law

673, 674

FDTD simulations

243, 244, 247, 249, 250, 684, 691, 739

FDTD solvers

199, 200

feldspar

703, 707

fiberoptic thermometer

244, 379

fibre reinforced composite

683, 697

fibrous alumina

578

field distribution fifty-ohm technology

9, 15, 32, 33, 35-38, 159, 160, 202, 209, 214, 216, 223, 224, 227, 238, 240, 244, 395, 430, 515, 582, 713, 714, 737, 738, 756 441

finish drying

260, 264, 301, 418

finite difference time domain

10, 179, 180, 192, 199, 225, 228, 243, 255, 739

finite element method

10, 32, 177, 179, 180, 198, 217, 225

Subject Index

779

flocculation agents

349

fluidized bed

720-733

flux corrected transport method

172

food food reheaters

4-14, 31-32, 56, 78-84, 107-118, 120, 155-162, 167-168, 190, 200-205, 217-224, 243-257, 259-270, 278, 280, 282-288, 289-302, 312-320, 370-376, 417-425, 426-435, 445-451 417

formaldehyde

437, 711, 715, 717

four probe sensor

66

fractional factorial design

244, 247, 250-252

fracture toughness

538, 541, 545, 546, 551, 564-568,

free dipole rotation

667

free space-time domain

151, 152, 153, 154

freeze drying

4, 264, 426, 427, 433, 435

frequency counters

9

frit

703, 704

frozen food

260, 263, 421

fuel cells

651, 653, 655, 657, 659, 660

functionally graded material

591, 752, 756

fundamental research

480

G gas burner

584, 589, 598

gas chromatography

711

gas phase reaction

623, 624, 625, 722

gel-casting

710

GFRP (Glass Fibre Reinforced 697-701 Composite) glass 6, 8, 68, 72, 206, 248, 268, 345-348, 379, 391, 406-409, 437, 442, 447, 449, 539, 556, 567, 569, 620, 662, 663, 668, 687, 690-692, 697, 703, 704, 713, 714, 748, 749, 759 glass fibre reinforced composite 697-701 glass transition temperature

437

glaze

703-705, 707-709, 766

glow discharge

429, 430, 431

gluing

138, 445-447, 455

gluten

108, 118, 285, 288, 312-320

grain growth

510, 514, 533-540, 546, 560, 567-569, 571-575, 652, 660, 746, 763 534, 538, 539, 587, 760

grain size distribution graphical interface

179

graphite

600, 645-650, 746, 747, 750, 758

green body

515, 516, 536, 554, 706, 710, 717

green part machining

710

green processing

710, 711

780

Subject Index

gyrotron

16, 20-30, 497-499, 507, 508, 513, 534, 542, 554, 560, 563, 571, 577, 578, 581, 582, 593, 597, 736-738, 742

H halogen lamp

673

hard non-conductive materials

687, 688

hard wood

303

healing of cracks

474

heat capacity

188, 201, 407, 408, 412, 483, 718, 739, 746, 761

heat equation

69, 195, 227, 691, 692

heat exchange

69, 211, 380, 579, 712, 721

heat transfer

200, 208, 232, 233, 264, 290, 340, 436, 438, 463, 684, 718, 729

heating iterations

205

heating pattern

206, 208, 260, 676, 678, 682

heating systems heating uniformity

3-14, 16, 23, 39-47, 51, 89, 167-177, 179, 185, 190, 205, 217, 225, 425, 731, 736 6, 8, 184, 243-255, 263, 282-288

heat-sink material

548

Helmholtz equation

191, 193-197, 218, 219, 684, 739, 741

high frequency

4, 5, 6, 10, 15, 17, 47, 65-68, 76, 85-91, 106, 147, 149, 177, 178, 270, 277, 280, 281, 288, 298, 320, 369, 377, 425, 444, 461, 463, 468, 470, 476, 481, 499, 515, 552, 560, 582, 605, 659, 669, 673, 681, 686, 695, 700, 728 711

high pressure liquid chromatography homogeneous absorption

556

homogeneous microstructure

587

homogeneous nucleation

621, 624

hot air re-circulation

13

hot spot

90, 187, 207, 208, 215, 246, 254, 359, 529, 687-691, 765

hot water cooking

419

hot-pressed silicon nitride

538

household ovens

259

human safety

86, 88

humidity profiles

324, 326

hybrid heating

341, 513, 520, 577-583, 589, 598, 682, 710-717, 719, 725, 731

hybrid model

195

hybrid modes

8, 140

hybrid system

4, 12, 347

hydrogen impurities

479

hydrogen removal

474

hydrothermal synthesis

627-632, 705

hyperthermia

7, 90

I impedance matching

8, 47, 51, 323, 440, 689, 691, 692

impedance measurement

84, 760

Subject Index impedance parameters

781

9

implantation damage annealing

673

incineration

321, 398

induced oscillating currents

462

induction heating

462, 673, 680, 756, 757

inertisation

341

infinite line method

78, 79, 82

infrared

244, 263, 679, 736, 755

infrared thermography

244

injected matrix resin

686

injection locking in-situ cleaning

12 329

insulation box

578, 579, 593

insulator

29, 673, 746, 673

interdiffusion

467-469, 673

interslot spacing intrinsic

9 349-352, 424, 508, 676

intrinsic carrier concentration

676

ion conduction

667

ion mobility

475

ionic liquids

410- 413

ionic transport

468, 470, 679

ISM frequencies

5, 11, 14, 15, 85-91, 92-101, 341, 428, 429, 441, 681, 687, 701

isotope concentration

473, 479

J junction depth

467, 678

junction formation

673, 678-680

K klystrons

17, 419

knife-edge delamination tests

679

L laminar materials

234, 235, 239, 240

LAN

91, 93, 94, 97, 100, 101

laser

10, 24, 133, 598, 608, 687, 693, 755-760, 764-766

leakage

11, 39, 44, 87, 139, 260, 333, 344, 431, 506, 508, 697-699, 713, 727 697-699

leakage wave applicator leather

238, 242

leavening agents

420

linear effect

477

liquid crystal polymer

652, 653

liquid phase formation

546, 557, 558

liquid phase sintering

541, 553, 555

782

Subject Index

liquid pumping

272

lithium

703

local multipoint distribution

96

local overheating

216, 556, 660, 701

lossy dielectric medium

692

low carnegieite

665, 668

low loss ceramic

562, 716

L-type matching network

675

M magnesia-aluminate spinel

464

magnetic dipoles

461, 462

magnetic field

18, 20, 22, 56, 57, 59-61, 121, 156, 158, 159, 162, 193, 210, 219, 316, 461, 462, 592, 675 59, 673- 675

magnetic flux magnetic fusion program

682

magnetic induction

462, 673, 680

magnetic losses

593, 595, 596

magnetic permeability

210, 591, 592

magnetron

15-22, 185, 188, 248, 260, 261, 323, 344, 399, 521, 522, 620, 634, 645, 647, 675, 687-689, 697, 722, 726, 727 377-384

maleic anhydride mass spectroscopy

466, 635, 711

mass transfer

242, 315, 533, 539, 718, 720

mass transport

461-470, 473-481, 491, 492, 559

matched termination

80, 81

Maxwell's equations

185, 191-198, 216, 526, 739

mechanical phase-shifter

75

mechanical properties mechanical stresses

304-306, 438, 514, 540, 544, 546, 562-576, 603-607, 706, 738, 751, 752 476, 514, 750

membrane-electrode-assembly

651

mesocosms

332, 337, 338

metal

8, 90, 168, 169, 202, 218, 225, 255, 262, 263, 270, 286, 331, 345, 346, 366, 398, 404, 465, 477, 478, 550-599, 608, 609, 615, 628, 633, 635, 637-639, 640-644, 651-659, 661, 662, 687, 688, 691, 693, 695, 696, 700, 701, 723-726, 735, 745-749 640, 641

metal alcoholates metal particles

591-593, 596, 597, 653, 657

metal powder

595, 745, 747, 748

metal-ceramic composites

591

metallic glass

748, 749

metallic glass powder

748, 749

metallurgical industry

349

metallurgy

497, 552, 561, 598, 608, 615

metalorganic compounds

653

Subject Index method of lines

783

10

method of moments

10

MgAl2O4

534, 535, 563, 645

MgO-Al2O3 composite

464

MgOHCl

707

microbiological quality

282

microelectromechanical systems

673

microscopic current loops

462

microwave absorbent moulds

233

microwave absorption

24, 29, 77, 210, 292, 408-413, 514-518, 533, 579, 580, 591, 600, 637, 699, 712, 755-758, 761, 762 4, 6, 7, 198, 260, 713

microwave applicator microwave assisted firing

583, 598, 608

microwave assisted gas firing

12, 710

microwave assisted machining

693

microwave assisted synthesis

393, 396, 628, 651-660

microwave cavity

675-677, 716, 726

microwave conditioning

307, 308

microwave de/anti-icing

698, 700

microwave decontamination

322

microwave drill

687-693

microwave enhanced synthesis

660

microwave extraction

126

microwave freeze drying

4

microwave heat treatment

312-320, 345, 661-663

microwave heated fluidized bed

720-722

microwave hybrid heating

583, 589, 717

microwave induced DC currents

477

microwave induced driving force

475

microwave induced phenomena

472

microwave plasma

515, 619-626, 725, 727, 731, 755-759, 762-763

microwave pyrolysis

633, 640, 717

microwave regime

675

microwave tempering

263, 421, 422

microwave vacuum dryer

426, 428, 431

millimeter waves

15-22, 24-28, 506-513, 541-551, 682, 685, 699, 701, 735

mineralizer

705

mixed metal catalysts

651

mobilities

473, 475, 478

mode stirrer

8, 168-177, 235, 380, 699

modeling modeling tools

68, 69, 80, 120, 123, 178-180, 182, 184, 189, 190, 209, 225-227, 229, 231, 233, 288, 469, 490, 528, 540, 578, 680, 738 178, 189

moisture content

7, 82, 138, 139

784

Subject Index

moisture gradient

272

moisture-levelling

234

molar heating

408, 409

molecular agitation

472

monomode

39-46, 699, 722, 726, 759

monopole antenna

692

motion of charged particles

469

motion of electrical charges

461

movable center electrode

689

multilayer dielectrics

227

multi-layer substrates

543

multimode

7, 8, 28, 169, 278, 343, 380, 646, 675, 705-707, 722

multimode conveyors

420

N nanocomposites

621, 626

nano-Pt

652

nanoscaled metal oxide

643

nanoscaled oxides

633

nanoscaled powders

637, 639

nanoscaled TiO2

633, 639

naphthalene

321-325, 327

natural triglycerides

370

Navier Stokes

161, 173, 174

near-field applicators

7, 8

near-field concentrator

687

near-surface region

476

nickel powders

593

nickel-alumina composite

597

nonlinear effects

477

non-zero net motion

475

nucleation rate

567

nucleation step

637

nucleophilic substitution

392

nylon resins

441, 443

O ohmic collisions

674

ohmic contact interdiffusion

673

ohmic heating

696

ohmic losses

430, 591

oil fraction

129, 131, 134-137

one-dimensional growth

566

opacification

704

Subject Index open circuit admittance

122

open-end coaxial waveguide

689

optical devices

9, 634

optical sensors

335

optimized heating schedule

581

organic binder

342, 706

organic by-products

633, 638, 642

organic wastes

398

out-of-band emission

11

ovalbumin

107- 118

785

overcooked meals

206

overmoded

506, 579, 712, 713, 727

oxidation oxidation-resistant (Ti, Cr)N

363, 366, 388, 404, 440, 541, 550, 551, 634, 642-644, 652, 710, 719, 722, 723, 760, 766, 767 541

oxide additives

533, 534, 536, 545, 553-559, 562-566

oxygen diffusivity

473

oxygen isotope

473, 479

oxygenation

474, 479, 481

P paper working industries

445

parallel synthesis

390, 391, 392, 394

particle coating

734

pasteurisation

260, 265, 269

penetration depth

persistent organic compounds

6, 48-50, 77, 79, 263, 312, 405, 413, 428, 506, 592, 598, 605, 728, 731-733, 757, 765 32, 57, 68, 69, 71, 72, 74, 84, 127, 128, 138-140, 143, 144, 147, 155, 156, 158, 160, 162, 163, 169, 183, 188, 201, 207, 219, 248, 249, 255, 428, 440, 476, 591, 592, 645, 684, 740 710

phantom simulations

90

permittivity

phase transformation

506-513, 519, 533-540, 554-558, 568

photocatalyst

633

photon energy

409, 755

photovoltaic elements

633

pigments

633, 703, 704, 705

plasma

15, 19, 398, 426, 445, 515, 563, 605, 611, 619-627, 633, 687, 725733, 745-752, 755-764 745

plasma activated sintering plasma impact

745, 746

plasma PVD process

543

plasterboard

78

plastic

168, 248, 249, 291, 481, 687, 692, 693, 735, 736, 746, 751

plastic deformation

481, 746

platinum precursor

652, 653

786

Subject Index

Poisson equation

684

polar solvents

406, 408, 412, 413

polishing

703, 707

polishing sludge

703, 707

polishing tools

707

polyacrylic acid

711, 712, 714

polyamides

436, 437, 438, 439, 440, 443

polycondensation reactions

438, 439, 440

polycrystalline

474, 481, 597, 723-724

polycyclic aromatic hydrocarbons

712

polyethyleneglycol

710, 712, 714

polymerization

68, 69, 74, 350, 353, 354, 385, 436-443

polypropylene belts

423

polytype

645, 646, 647, 648

polyvinylalcohol

710, 712, 714

porcelain

346, 703, 704, 707

porcelain stoneware

703, 707

pore size distribution

495, 554, 559, 760, 762

positron emission tomography

360

powder injection moulding

710

power combiners

12

power loss density

5

power oscillators

6, 36

pre-cooking

260, 423

precursor

387, 621-623, 643, 644, 652-658, 660, 704, 723-727, 732, 733

pressure-aided curing

226

principal component analysis

129-137

processing kinetics

463

protein denaturation

107, 108, 112

puffing

260, 264, 427, 432-434,

pyrolysis

633-644, 653, 710-719, 723, 729

pyrometer

28, 515, 516, 520, 522, 584, 600, 611, 647, 676, 712, 713, 718, 727, 759

Q Q-factor

11, 28, 139, 141, 142, 148, 155, 158, 160, 161

quarter-wave traps

11

quartz

620, 675, 707, 711-715, 726, 727, 730, 758

R radial tuning stub

675

radiation pressure

476

radiative loss

676

radioactive nuclides

366

radioactive waste

346

Subject Index radiodiagnosis

359

radiolysis

366

radio-pharmaceuticals

359

randomises

8

rapid heating

6, 271, 272, 346, 465, 533, 673, 745, 746, 752

rare-earth oxides

553

rate-limited

463, 470

reaction engineering

385, 390, 405

reaction kinetics

359, 439, 465, 468, 627

ready meal

243-254, 282-287, 427

rectification

475, 479

reduced additive content

553

reflectometer

48, 689

reheated bread

285, 287

rehydration properties

264, 427

787

remote-controlled actuators

691

resin resistance heating

68, 69, 72, 307, 309, 310, 437, 438, 440, 441, 682, 685, 686, 711, 736, 738, 760 534, 712, 714, 721, 730

resistive sheet boundary condition

168, 219, 220

resonant applicator

7, 675

retardation of phase transformation 558, 561 RF applicators

6

RF band

5

RF frequencies

678

RF generators

6

RF interference

689, 693

rubber vulcanisation

226

runaway instabilities

210, 216

rutile

519, 633, 638

S safety saltiness

7, 10, 11, 47, 57, 58, 85-91, 100, 254, 282, 341, 391, 405, 421, 619, 689, 693, 696 284, 286

sandalwood oil

119, 125, 127

sandalwood timber

119

sapphire

479, 481, 499

seeded silicon nitride

538, 539

selective heating

234-241, 312, 545, 547, 551, 567, 652, 720, 736, 742

self-diffusion coefficients

118, 533

SEMATECH barrier curve

678, 679, 680

semi-anechoic chamber

103, 104

semi-conducting media

462

semiconductor

29, 464, 465, 472, 673, 678, 680, 723, 746

788

Subject Index

sensory properties

129, 286, 290

sensory quality

286, 301

serum albumin

107, 109, 116, 117

shock wave

174, 175

short cut tubes

713

silicon silicon carbide

12, 109, 216, 466, 473, 479, 481, 673-677, 679, 680, 687, 691, 692, 707, 708, 720-733 563, 645-648, 650, 707-708

silicon nitride

464, 533-540, 553, 554, 557, 560-562, 566-569, 571

silicon production

723

silicon wafer

466, 673-679, 691, 723

simulation

9, 10, 31-33, 36, 38- 41, 45, 57, 77, 139, 147, 167-169, 171-175, 177, 178, 180, 185, 187, 189, 198, 200, 209-211, 213, 215, 216, 223, 226-232, 238-240, 243-245, 247, 250, 252, 254, 255, 367, 368, 431, 540, 568, 577, 578, 582, 691, 718, 735, 742, 743, 755 8, 675, 705, 712, 727, 759, 764,

single-mode sintering

sintered alumina

19, 24-30, 149, 210-216, 462-469, 472-481, 491-497, 498-505, 506-513, 514-520, 533-540, 541-551, 553-561, 562-569, 570-576, 577-582, 583-590, 597, 598-608, 609-615, 623, 628, 687, 703707, 710, 745-749, 751, 756, 758, 761-763, 766, 767 465, 498-501, 503-505, 570-576, 579

sintering aids

544, 546, 550, 551, 568, 704

sintering of ceramic site-selective deposition

149, 216, 474, 478, 481, 491, 531, 533, 551, 565, 568, 582, 583, 590, 598, 710, 756 657, 658, 660

skin depth

592-597, 674

slotted waveguide

7, 8, 9, 238, 261

soaking time

307, 315, 316, 318, 319, 320, 547, 550, 584, 588, 589, 629, 631

sodium ion

661, 663, 665, 667, 668

solar cells

633, 724

solar grade silicon

721, 723, 726

solar space power

4, 10, 14

solid phase polymerization

436, 437, 443

solid solutions

474, 478, 492, 493, 627

solid state reactivity

703-705

solid-phase microwave sintering

557

solid-state sintering

211, 216

solvent-free organic reaction

386, 388

space charge

162, 475, 476, 479

spark plasma

745-749

spectrum analysers

9

SrTiO3-films

541

standard impedance boundary con- 168, 217, 218, 220 dition steady-state 189, 203, 204, 442, 521, 522, 529, 676, 692 steam drying

273

Subject Index steam extraction

789

126

steel

11, 492, 598-608, 711, 726, 727, 751-753, 756-762

sterilisation

260, 265, 269

streamers

167, 172, 173, 177

strengthened grain boundary

575

structural bound water

665, 667, 668

structural ceramic

508, 533

supported catalyst

655, 656, 659

surface boundary methods

217

surface integrals

167

surface moisture

271, 285

surface permeability

479

surface tension

555

susceptors

212, 214, 217, 262, 580

T tan į

161, 428, 437, 699, 703, 704

TE01 mode

8, 620, 621

TE10 mode

32-35, 46, 57, 59, 188, 222

TE11 mode

8, 20, 26, 27, 143, 620, 621

TE12 mode

12, 20

telemetry

92

telescopic coaxial concentrator

689

temperature field

483, 739

temperature gradients

31, 210, 229, 230, 464, 473, 475, 506, 509, 511-513, 577, 701, 706, 718, 722, 729, 731, 759, 762 463, 464, 470, 682, 729, 730, 738

temperature profile temperature measurement

676, 712, 713, 726, 759

temperature measurement inaccuracies temperature runaway

465

temperature sensing devices

443

tempering

6, 260, 263, 421, 422, 445, 447, 453, 685

tensor

683, 684, 740

theoretical resonator response

142

thermal agitation energy

462

thermal barrier coating

688, 691, 693, 760

24, 348, 482, 487, 661, 662, 667, 669

thermal conduction equation

483

thermal conductivity

233, 235, 263, 290, 342, 343, 347, 428, 437, 483, 541, 548-551, 579, 682, 683, 695, 701, 708, 718, 730, 736, 739, 740 483-485, 489, 507, 515, 554, 555, 571-580, 593, 600, 606, 607, 634, 726, 730 10

thermal insulation thermal modelling thermal runaway thermal steady state

31, 38, 263, 482, 483-490, 517, 523, 527, 529, 542, 583, 663, 667, 687, 691, 759, 755, 756, 762 463, 465

790

Subject Index

thermal stresses

506, 583

thermogravimetric analysis

629, 711

thermoplastic

685

thermoregulation

90

thin film

167, 181, 224, 481, 542, 543, 551, 723

thin metallic film

167, 176, 177, 217

third firing process

704

TiCl4

727, 732

time domain algorithms

179, 185

time domain solvers

189

titanium alkoxides

641

titanium carbide, TiC titanium nitride, TiN

612, 725, 727, 732 508, 511, 512, 612

titanium oxide, TiO2 titanium precursor

137, 493, 516-518, 520, 583, 626, 633, 634, 637-640, 643, 644, 653, 656-658, 660, 706, 727 633-644

TM01 mode

8, 249, 261, 283, 675, 677

TM10 mode

521, 522, 528

TM11 mode

8, 675, 677

toluene

331, 388, 412, 711, 716, 717

transceiver

95

transmission line matrix

10

transmitter stations

88

transponder

95

transport coefficients

475

travelling wave

7, 221

tunnel

261, 278, 417, 419, 422, 449-457

tungsten carbide, WC

725, 752, 763

turbulence

462

U ultraviolet light

749

unimolecular reaction

642

Universal Mobile Telecommunica- 97 tion System unlicensed bands 87

V vacancies

473, 475-478, 491, 492, 747

vacuum drying

260, 264, 301, 302, 426, 427, 432

ventilation system

296

Vickers hardness

563-565, 567, 586, 599, 752

viscosity

71, 349-352, 412, 436, 438, 439, 442, 555-557, 685

vitrification

341, 342, 344, 703, 704

volatilisation

329, 330

Subject Index volumetric heating

791

von Hippel

24, 27, 28, 119, 347, 463, 470, 510, 644, 608, 688, 699, 701, 710, 718, 725, 757 82, 84, 120, 122, 127, 302, 444

VSWR

9, 79, 80, 81, 23, 324, 325

W wafer bonding

673

wall losses

82, 141, 167, 168, 177, 218, 224, 431

wall slots

260

wave equation

691

waveguide

wax burnout

7-9, 11, 14, 26, 27, 32-35, 39, 43, 44, 47-49, 51, 57, 78, 79, 81, 84, 124, 140, 141, 169, 188, 191-193, 195-198, 206, 221, 222, 224, 238, 242, 255, 260-262, 279, 323, 419, 422, 430, 431, 521, 522, 689, 697, 699, 713, 726 5, 7, 9, 15, 24, 35, 44, 66, 79, 82, 124, 149, 219, 250, 254, 428, 498, 500-503, 505, 548, 572, 688, 691, 740 703

WC

513, 540, 610-612, 725, 752, 763

welding

445, 446, 610

wavelength

wheat grains

312, 313, 315, 320

wireless

91-94, 97, 98, 100, 101

wood wood modification

7, 10, 281, 303-309, 311, 323, 437, 445, 446, 450, 687, 692, 693, 747 306, 311

WR340 waveguide

32, 33, 48, 521, 689

X xylene

119, 331, 378, 380-382, 634, 635, 711, 716, 717

Y Y2O3 Yb2O3

510, 534-536, 538, 540, 544, 548-550, 553, 554, 557, 563, 565, 568, 569, 622, 623, 747, 760 541, 544, 545, 546, 547, 550, 551, 553, 554, 557, 558

YBa2Cu3O7

137, 467, 481

YCBO superconductors

464

Z zeolite

56, 57, 59, 60, 61, 404, 661-669

Ziegler-Natta type polymerisation

638, 643

zirconia, ZrO2

479, 624, 625, 627, 703, 706, 747, 760, 764