Chemical Processing of Ceramics

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Chemical Processing Ceramics edited

Burtrand I. Lee Clemson University Clemson. Carolina

Edward J. A. Pope MATECH Village, Calihrnia and

of City#

Marcel Dekker, Inc.

New York*Basel*Hong

Kong

Library of Congress Cataloging-in-Publication Data

Chemical processing ceramics edited by Burtrand I. Lee, Edward J . A. Pope. cm.p. Includes bibliographical references and index. ISBN 0-8247-9244-0 (alk. paper) 1. Ceramicmaterials. Lee,BurtrandInsung. II. Pope,Edward J . A. TP810.5.C48 1994 666-dc20 94-25396 CIP The publisher offers discounts on this book when ordered in bulk quantities. For more information, write to Special SalesProfessional Marketing at the address below. book is printed on acid-free paper. Copyright 0 1994 by Marcel Dekker, Inc. All Rights Reserved. Neither this book nor any part may be reproducedor transmitted in any formor by any means, electronic or mechanical, including photocopying, microfilming, and recording, or by any information storage and retrieval system, without permission in writing from the publisher. Marcel Dekker, Inc. 270 Madison Avenue, New York, New York 10016 Current printing (last digit): 10987654321 PRINTED IN T I E UNITED STATES OF AMERICA

Foreword

The value of a book is not only in its contents but in the creative idea it further encourages. These are a few ideas about the past and the future that occur to me as stimulated by chapters in this book. The volume is a fine resource of references to the use of chemistry in materials and should, indeed, encourage thinking about such things. The early use of chemistry for particular compounds was usually not with materials chemistry in mind but rather was chemistry for chemistry’s sake. The very early nature of much chemistry, now rightly readdressed for materials, appears to surprise many who believe (or wish it to be believed) that their field is new. Many chemical methods are naturally revived with new materials goals in prospect. Mechanochemical studies and aerogels are in vogue at the moment, the former more than 100 years old and the latter 60 years old. Only very ocoasionally will someone, with enough time and energy to review the early literature, produce a historical perspective as Hlavacek did for thermite processing also known as fast propagating reactions. we have come to realize that although the “spin” on chemistry for materials is new, the processes often have a long and steady historical progression. Also not new are the phenomena seen in nanostructures-all sol-gel (e.g., Chapter 13 “all aerogels are nanocrystalline”) and life chemistry is nanostructural, and chemists long have been able to prepare molecular clusters and nanocrystalline powers by vaporization or, better, by decomposition of precursors (typically hydroxides, carbonates, nitrates, acetates, citrates, and on). Even the recent production of biphasic or polyphasic nanostructures from polymers had been preceded. by the decomposition of mixed crystals [e.g., CaMg(Co3)2, dolomite, to CaO and MgO]. Carbon and S i c fibers (NicalonB) are nanostructural. In this book, nanocrystalline cobalt from 1966 is mentioned (Chapter 18). Naturally, the nanostructural works of chemists could not be directly examined before the advent of electron microscopes, but more indirect iii

iv

FOREWORD

methods (sometimes forgotten today) had given us a good knowledge of what was going on. Although chemical synthesis has a long history, many new, simple syntheses are being discovered and many more are mentioned in this book, e.g., the use of alkali iodides to make the molybdenum bronzes (Chapter 3) and creative uses of electrochemistry (Chapter 15). Many new structure types, some related to known types, are yet to be found by suitable new methods as discussed by Rao in Chapter 3. For example, many regular intergrowth structures are little lower in free energy of formation than the separate-layer types standing alone and so can be quite difficult to attain. Nevertheless they may have unique and useful properties. For every rule there are exceptions, and is peculiarly true in chemistry; it seems to be taken by many as gospel that precursorsto complex ceramics, chemically mixed at the molecular level, must always be preferable to other kinds of mixtures but “it ain’t necessarily However, of all the possible methods of chemical mixing, the default recourse to the Pechini method seems curious as it is hardly the simplest or most desirable method in most cases. To elaborate: if unwanted intermediates are produced at lower temperatures on the way to the desired compounds, then the purpose of molecular mixtures may be defeated. is true in some ferroelectrics and special routes involving deliberately unstable or transient intermediates can be preferable, as mentioned in Chapter 17. It was noted early that for the P-alumindmagnetoplumbitefamily, chemical mixing, which, indeed, produced products at lower temperatures (usually desirable), also led to syntactic intergrowth problems; this has recently been seen again in the high-temperature superconductors (HTSC). (There are no new ceramic phenomena in the HTSC.) In this case also, the production of special intermediates (e.g., Pb-doped-2122 en route to Pb-doped-2223) can be beneficial. Early alkoxide work was certainly done without modem materials in mind; rather the early sol-gel chemistry was with absorbents and catalyst supports in view. Much synthetic work has been done on alkoxides, but a peculiar theoretical schizophrenia exists about them and their relation to oxides. Studies show that bond lengths and angles are very similar in alkoxide clusters to those found in oxides and polyoxyanions. This is mentioned several times in the book (e.g., Chapters 1, 2, and 21). If it looks like a duck, etc., it probably is a duck, which is to say that the bonding in alkoxides is very similar to that in oxides. Alkoxides are not covalent and oxides ionic; they are similar. Modem X-ray methods to determine actual charges on “ions” in oxides show them to be very small. While modeling Madelung potential methods, etc., often work, they are as ballet or opera is to love, merely an artistic (and often useful) imaginative representation. This will become more apparent as workers are attracted to this field and as old shibboleths fade. Even with the extensive work on alkoxides described in this book, there are

FOREWORD

V

surprising new (old!) developments worth mentioning especially the use of polyhydric alcohols to attain unusual coordination states in silicon (viz. 5-fold coordinate with glycols and 6-coordinate with catechols). New synthetic possibilities will further enhance the power of alkoxide chemistry. The area of heterometallic alkoxides (as discussed in Chapter 2) is very important and there are many cheap ways of synthesizing them, including the reactions of chlorides and similar chemicals with already formed alkoxides. People who claim that alkoxides are too expensive to be used should realize that in any real commercial process the alcohols would be recycled. Difficulties with alkoxides-sol-gel, other than the silicon cases mentioned several times in this book, suggest to me that in the future syneresis, an area that surprisingly is little studied (mentioned in Chapter 17), should pay dividends. Those who may believesthat large-scale chemical methods are unlikely to be used for ceramics should consider the following: the earliest synthesis of Si3N4 was by the chemical reaction of Sic14 with ammonia and not the, perhaps, more familiar reaction of Si plus N2 (elemental Si was not readily available in the early days). Today the largest volume of Si3N4 powder (>300 tons per year) is made by that earliest reaction route and much of it in the future seems destined for automobiles. Carbothermal nitridation of Si02 to produce Si3N4 was only later done with fertilizers in mind. As discussed in Chapter 5, an unexpected related manifestation of this is the production of S i c by pyrolysis of rice hulls. It was for the longest time believed that p-Si3N4 was the high-temperature form, or could only be produced at higher temperatures than the a-form; chemical methods have revealed otherwise, as when pSi3N4 forms from prepolymers at 1150°C (Chapter 15). This exemplifies how chemical synthesis, often designed simply with the goal of making something, serendipitously reveals results in conflict with traditional wisdom that has been acquired via a different viewpoint. In chemical processing particular methods are rarely an answer to many problems but they may address niche uses of materials (the centrifugal pipe lining use of “thermite processing” is such a use). Although much early chemistry exists in the literature, and is not difficult to find, there is also the fact that much experimentation occurred that was never published. I know of all kinds of preliminary tests that were done and not followed up in the light of their perceived limited value at the time. It is inevitable that this ground will be gone over again and while the original workers cannot take credit for unpublished work, the modem “discoverers” should recognize that the oldsters knew much more than they let on, but inevitably they could not recognize the pertinence of their knowledge to the materials problems of today. Peter E. D.Morgan

This Page Intentionally Left Blank

Preface

Despite many recent advances in material science and engineering, the performance of ceramic components in severe conditions is still far below the ideal limits predicted by theory. Modem ceramics have been primarily the products of applied physics and parallel the developments of physical metallurgy. The emphasis on the relation between behavior and microstructure has been fruitful for ceramic scientists for several decades, It has been recently realized, however, that major advances in ceramics during the next several decades will require an emphasis on molecular-level control. Organic chemistry, once abhorred by ceramic engineers trained to define ceramics as “inorganic-nonmetallicmaterials,” has become a valuable source of new ceramics. It has recently become known that as the structural scale in ceramics is reduced from macro to micro and to nano crystalline regimes, the basic properties are drastically altered. brittle ceramic material has been shown to be partially ductile, for example. The impetus and the ultimate goal in chemical processing of ceramic materials are to control physical and chemical variability by the assemblage of uniquely homogeneous structures, nanosized second phases, controlled surface compositional gradients, and unique combinations of dissimilar materials to achieve desired properties. Significant improvements in environmental stability and performance should result from such nanoscale molecular design of materials. The unique properties of ceramics with comparable mechanical performance may be realized by the molecular level or nanoscale fabrication of the chemical building blocks of the materials. number of books are available that deal with the chemical processing aspect of ceramic materials but most of them are conference proceedings. This book is written by many authors who are actively involved in the field of chemical processing of ceramic materials. The authors are from the intemational materials community-Japan, Germany, Korea, France, India, and the United States, where they practice chemical principles in the fabrication of superior ceramic materials. vii

viii

PREFACE

This book presents current development and concepts in chemical techniques for production of state-of-the-art ceramic materials in a truly interdisciplinary fashion. The twenty-three chapters are divided into six parts reflecting topical groups. It first discusses the starting materials-how to prepare and modify them in the molecular precursor stage. Powders are the most heavily used form of starting ceramic materials. Synthesis, characterization, and behavior of ceramic powders are presented in Parts I1 and 111. In the fourth part, forming and shaping of ceramic components via sol-gel technique are discussed. Fabrication of nonoxide ceramics is covered in Part V. In the last part of this book, several specific examples of classes of ceramic materials fabricated by chemical processing including thin films, membranes, and superconductors are presented. These classes of examples are chosen on the basis of the current demand and active research. The topics of basic principles of sub-gel technique, sintering, and postsintering processing are not reviewed in volume since there are other excellent books dealing solely with these topics. Although this book is edited, it is organized to reflect the sequence of ceramic processing and the coherent theme of chemical processing for advance ceramic materials. Hence book is suitable as a supplementary textbook for advanced undergraduate and graduate courses in ceramic science and materials chemistry as well as an excellent reference book for practicing chemists and materials scientists and engineers. shown by some of the data presented in this book, the results from chemical processing are not yet up to the real applications of ceramic materials. It is evident that, through further research, the full potential of chemical processing for ceramic materials that perform up to the theoretical limit can be realized. It is the authors’ and the editors’ desire to bring the ceramist and chemist closer together to produce superior ceramic materials. Burtrand Lee Edward J. A. Pope

Contents

Foreword Peter E. D.Morgan Preface Contributors

iii vii xi

Precursor Chemistry

Molecular Design of Transition Metal Alkoxide Precursors Jacques Livage Metal Alkoxides for Electrooptical Ceramics Liliane G. Hubert-Pfalzgraf

3

II. Powder Synthesis and Characterization

3. Chemical Synthesis of Metal Oxide Powders C. N. R. Rao 4.

Multicomponent Ceramic Powders T. Mah, E. E. Hemes, and K. S. Mazdiyasni Chemical Synthesis of Nonoxides Christian Russel and Michael Seibold Techniques for Characterization of Advanced Ceramic Powders S. G. Malghan, P. S. Wang, and A. Hackley

Powder Processing

Colloid Interface Science for Ceramic Powder Processing Hyun M. Jang Ceramic Nonaqueous Particles Media in Burtrand I. Lee Synthesis and Dispersion of Barium Titanate and Related Ceramic Powders Ki Hyun Yoon and Kyung Hwa Jo

ix

CONTENTS

X

Rheology and Mixing of Ceramic Mixtures Used in Plastic Molding Beebhas C. Mutsuddy

239

W . Sol-Gel Processing Processing Monolithic Ceramics via Sol-Gel J. Phalippou 12. Bulk Optical Materials from Sol-Gel Edward J. Pope 13. Aerogel Manufacture, Structure, Properties, and Applications Jochen Fricke and Joachim Gross 14. Fractal Growth Model of Gelation Edward J. A. Pope

265 287 311 337

V. CeramicsviaPolymerChemistry 15. Nonoxide Ceramics via Polymer Chemistry Kenneth E. Gonsalves and Tongsan D. Xiao 16. Polymer Pyrolysis Masaki Narisawa and Kiyohito Okamura

359 375

VI. Processing of Specialty Ceramics 17. Processing of Lead-Based Dielectric Materials Hung C. Ling and Man F. Yan 18. Synthesis of Magnetic Particles Masataka Ozaki 19. Chemistry Processing of High-Temperature Superconductors Shin-Pei Matsuda 20. Preparation and Properties Tantalum Oxide Thin Films by Sol-Gel T. Ohishi 21. Crystalline and Amorphous Thin Films of Ferroelectric Oxides Ren Xu 22. Ceramic Membrane Processing C. Guizard, A. Julbe, A. Larbot, and L. Cot

Index

397 421

445

465

481 501

533

Contributors

L. Cot Centre National de la Recherche Scientifique, Montpellier, France Jochen Fricke Physikalisches Institut der Universiut, Am Hubland, Wurzburg, Germany Kenneth

Gonsalves University of Connecticut, Storrs, Connecticut

Joachim Gross Physikalisches Institut der Universitat, Am Hubland, Wurzburg, Germany

C. Guizard Centre National de la Recherche Scientifique, Montpellier, France V. Hackley National Institute of Standards and Technology, Gaithersburg, Maryland Hermes Wright Paterson Air Force Base, Ohio Liliane G. Hubert-PfalzgrafUniversitk de Nice-Sophia Antipolis, Nice, France Hyun Jang Pohang University of Science and Technology, Pohang, Republic Korea Kyung Hwa Jo* Yonsei University, Seoul, Korea Julbe Centre National de la Recherche Scientifique, Montpellier, France Larbot Centre National de la Recherche Scientifique, Montpellier, France Burtrand

Lee Clemson University, Clemson, South Carolina

Hung C. Ling AT&T Bell Laboratories, Princeton, New Jersey Jacques Livage Universite Pierre et Marie Curie, Paris, France *Currenr affiliation: Daewoo Corporation, Ltd.,

Seoul,

xi

xii

CONTRIBUTORS

l: Mah UES, Inc., Dayton, Ohio S. G. Malghan National Institute of Standards and Technology, Gaithersburg, Maryland Shin-Pei Matsuda Hitachi, Ltd., Ibaraki, Japan

K.

Mazdiyasni General Atomics, San Diego, California

P.

D. Morgan Rockwell Science Center, Thousand Oaks, California

Beebhas C. Mutsuddy Michigan Technological University, Houghton, Michigan Masaki Narisawa University of Osaka Prefecture,

Japan

T. Ohishi Hitachi, Ltd., Ibaraki, Japan Kiyohito OkamuraUniversity of Osaka Prefecture, Osaka, Japan Masataka Ozaki Yokohama City University, Yokohama, Japan

J. Phalippou Universitb de Montpellier 11, Montpellier, France Edward Pope MATECH, Westlake Village, California, and University of Utah, Salt Lake City, Utah C.

N.

Rao 1ndian.Instituteof Science, Bangalore, India

Christian Russel Universitat Jena, Jena, Germany Michael Seibold OSRAM GmbH, Augsburg, Germany Wang National Institute of Standards andTechnology, Gaithersburg, Maryland Tongsan D. Xiao University of Connecticut, Storrs, Connecticut Ren Xu University of Utah, Salt Lake City, Utah Man F. Yan AT&T Bell Laboratories, Murray Hill, New Jersey Ki Hyun YoonYonsei University, Seoul, Korea

I PRECURSOR CHEMISTRY


Molecular Design of Transition Metal Alkoxide Precursors Jacques Livage Universitk Pierre et Mane Curie Paris, France

I. INTRODUCTION Transition metal alkoxides are much more reactive toward hydrolysis and condensation than silicon alkoxides. This arises mainly from the larger size and lower ,electronegativity of transition metal elements. Coordination expansion becomes a key parameter that controls the molecular structure and chemical reactivity of these alkoxides. Hydrolysis and condensation rates of silicon alkoxides must be increased by acid or base catalysis, whereas they must be carefully controlled for the other metal alkoxides. The chemical modification of transition metal alkoxides leads to the development of a new molecular engineering. The chemical design of these new precursors allows the sol-gel synthesis of shaped materials in the form of fine powders, fibers, or films.

II. HYDROLYSIS AND CONDENSATION ALKOXIDES

METAL

Sol-gel chemistry is based on the hydrolysis and condensation of metal alkoxides M(OR),. These reactions can be described as follows [l]: M-OR M-OH

+ H20 ”-+ M OH + ROH hydrolysis + R 0 -M +M- 0 -M + ROH condensation

Condensed species are progressively formed, giving rise to oligomers, oxopolymers, colloids, gels, or precipitates.

LNAGE

Actually these reactions correspond to the nucleophilic substitution of alkoxy ligands by hydroxylated species XOH, as follows: M(OR), + AXOH _ j [M(OR),.AOX),I + xROH where X is hydrogen (hydrolysis), a metal atom (condensation), or even an organic or inorganic ligand (complexation). They can be described by a S$ mechanism [2]:

H” \ O-M-O’-R+

H

\

06+M”-OR/ X

/

X

Step 1

H” / XO-M--OXO-M+ROH \ R

Step 2

Step

1. The first step corresponds to the nucleophilic addition ofnegatively charged H06 groups onto positively charged metal atoms M&. It leads to an increase in the coordination number of the metal atom in the transition state. 2. The second step is a transfer, within the transition state, of the positively charged proton toward a negatively charged OR group. The positively charged protonated alkoxide ligand ROH is then removed.

The chemical reactivity of metal alkoxides toward hydrolysis and condensation mainly depends on the positive charge of the metal atom and its ability to increase its coordination number N [2]. Asa general rule, the electronegativity metal atoms decreases and their size increases when going down the periodic table, from the top right to the bottom left (Table 1). The chemical reactivity of the corresponding alkoxides toward hydrolysis and condensation then increases.

Table 1 Electronegativity x, PartialCharge Ionic Radius r, and Maximum Coordination NumberN of Some Metal Alkoxides

Alkoxide Si(OiPr)4 Ti(OiPr)4 Zr(0iPr)rl Ce(OiPr)4 PO(OEt)3 VO(OEt)3

X

N

0.64

MOLECULAR DESIGN

OF TRANSITION METAL ALKOXIDE

5

Silicon alkoxides are not very reactive. Gelation occurs within several days after water has been added [3]. Hydrolysis and condensation rates must be increased viaacid or base catalysis. The hydrolysis rate of Ti(OEt)4 ( h = lO-3/M/s) is about five orders of magnitude greater than that of Si(OEt)4 ( h = 5 lO-g/M/s). Cerium alkoxides are very sensitive to moisture. They must be handled with care in a dry atmosphere; otherwise, precipitation occurs as soon as water is present Alkoxides of highly electronegative elements, such as PO(OEt)3, cannot be hydrolyzed under ambient conditions [5], whereasthe corresponding vanadium derivatives VO(OEt)3 readily give gels upon hydrolysis [6].

MOLECULARSTRUCTURE OF TRANSITION METAL ALKOXIDES Silicon alkoxides have beenwidelyused for thesol-gelsynthesis of silicabased glasses andceramics. Silicon remains fourfold coordinated (N = in the precursor as well as in the oxide. All silicon alkoxides Si(OR)4 are therefore monomericandtetrahedral. Their reactivity decreases whenthe size of the alkoxy group increases; thisis mainly caused by steric hindrance factors, which play a major role during the formation of hypervalent silicon intermediates [l]. The situation is completely different with other metal alkoxides M(OR),. The coordination number N of the metal atom in the oxide M o a is usually larger than the oxidation state z. As a consequence, coordination expansion appears to be a general tendency for most metal alkoxides. Positively charged transition metal atomsMz+ tend to increase their coordination number by using theirvacant orbitals to accept electrons from nucleophilic ligands (nucleophilic addition). This currently occurs via oligomerization (OR bridges), solvation or the formation of oxoalkoxides (p-oxo bridges) [2].

A.

Oligomerization

Sharing alkoxy groups is the easiest way for metal alkoxides to increase the coordination of the metal atom without changing their stoichiometry. In pure alkoxides, coordination expansion currently occurs viathe formation ofOR bridges. Therefore oligomeric as well as monomeric molecular precursors can be found. Oligomerization depends on physical parameters (concentration and temperature) and chemical factors(solventand oxidation state of themetal atom or steric hindrance of alkoxide groups) [7]. Bulky secondary or tertiary alkoxy groups tend to prevent oligomerization, and the steric hindrance of alkoxy ligands appears to be a major parameter. Oligomeric species [Ti(0Et)4ln(n = 2 or 3) have been evidenced for titanium ethoxide whereas Ti(OiPr)4 and Ti(OtAm)4 remain monomeric (Fig. 1). X-ray absorption experiments at the Ti-K edge have been performed on the

LNAGE

'OEt

Molecular

of titanium alkoxides. (a) = TiOiPr4; (b) [Ti(oEt)4]3.

neat alkoxides in the liquid state [8]. Prepeaks can be seen in X-ray Absorption Near Edge Structure (=S) spectra just before the absorptionedge (Fig. 2a). They correspond to 1s + 3d electronic transitions and are very sensitive to the local symmetry around titanium. sharp prepeak is observed in Ti(OtAm)4, suggestingthatTi is fourfold coordinated, whereastheweakerprepeakin Ti(OEt)4 should correspondto a fivefold coordination. Moreover, two Ti-0 distances (1.82 and 2.05 A) and Ti to Ti correlations (3.12 A) can be observed in Ti(OEt)4, evidencing the oligomeric structure of this alkoxide. Such correlations are not seen in the Extended X-ray Absorption Fine Structure (EXAFS) spectrum Ti(OtAm)4, which exhibits a single Ti-0 distance (1.81 A; Fig. 2b). The chemical reactivity of metal alkoxides strongly depends on their molecular structure. Oligomeric titanium alkoxides, in which Ti has a higher coordination number, are less reactive, allowing better control of hydrolysis and condensation reactions. Monodispersed Ti02 powers are usually obtained from Ti(OEt)4 rather than Ti(OiPr)4 [9,10]. Oligomerization becomes more and more important as the N-Z difference increases, that is, as the oxidation state Mz+ decreases. Divalent metals give insoluble polymeric alkoxides [M(OR)2],, (#+ = Fe, CO, Ni, Cu, . . .). This was a real drawback for the sol-gel synthesis high-Tc superconducting ceramics, such as YBa2Cu307-~Bulky ligands, such as 2-(2-ethoxy-ethoxy)ethoxide had then to be used to prevent oligomerization and obtain soluble copper oxide molecular precursors [l l].

Metal alkoxides are not miscible with water, so that sol-gel reactions must be performed in the presence a common solvent, such as an alcohol. Coordination expansion can then also occur via solvation. Solvate formation is often

MOLECULAR DESIGN OF TRQNSITION METAL ALKOXIDE

7

Ti-0

.2 '

.¶

.6

I

.

4

.. :.

.

:i J

R

r

4.96

5.02

5

Ti-0

.9 -

1.2

-

!

-

Ti(OEtI4

.!/ i/

E

I J

4.96

I

4.98

5

5.02

2 X-ray absorption of titaniumalkoxides at the Ti-Kedge.(a) = X m S ; (b) = EXAFS (Fourier transform). observed when alkoxides are dissolved in their parent alcohol. The stability of such solvates increases with the size and the electropositive character of the metal, that is, when going down the periodic table [7]. Molecular complexity increases with the size the metal atom. Ti(IV), for instance, gives monomeric (Ti(OiPr)4 species, whereas Zr(IV) and Ce(1V) give dimeric species, [Zr(OniPr)4,iPrOnH]2and[Ce(OiPr)4(iPrOH]2. Such dimers can be crystallized from the solution. Their structure, studied byX-ray diffraction, shows that alcohol molecules are directly bonded to the metal atom to increase its coordination (Fig. 3) [12,13]. Because coordination expansion could occur either via alkoxide bridging or solvation, the molecular complexity of metal alkoxides can be tailored by an appropriate choice ofsolvent.[Zr(OnPr)41n oligomers are formed (n I4) in nonpolar solvents, such as cyclohexane, allowing slow hydrolysis rates and the formation of clear gels. Less condensed solvates are formed in propanol (n = 2); hydrolysis becomes much faster and leads to precipitation [14].

LNAGE Pro H

PrO

OPr

H OPr

Molecularstructure of [Ce(OiPr)4, PirOHIz.

C.

of Oxoalkoxides

Alkoxide groups can also bridge two different metal atoms, leading to the formation of heteroalkoxides, which are often used a precursors for the sol-gel synthesis of multicomponent ceramics [15]. Condensation can even go one step further, leading to p-oxo bridges via ether elimination. M-OR+RO-M"-0-"M+ROR This typically occurs when alkoxide solutions are heated under reflux to provide better mixing of the components. Condensation is favored by the smaller size of oxo ligands and their ability to exhibit highercoordination numbers, M"oxo ligands, for instance,havebeenbeenobservedin Na2FeaO(OCH3)18; 6CH30H [16]. Oxo bridges favor the coordination expansion of metal atoms, and oxoalkoxides are more stable thanthe corresponding alkoxides and of course less reactive toward hydrolysis and condensation [17]. Large and electropositive metals are known to give oxo-alkoxides, such as Pb40 (OEt)6. This Pb(I1) precursor undergoes complete dissolution in ethanol when Nb(OEt)5 is added, giving rise tothecrystallization of [Pb604(OEt)4][Nb(OEt)5]4.Such heterometallic alkoxides have the correct Pb/Nb stoichiometry for the sol-gel synthesis of PbMgln NbmO3 (PMN) ceramics [18]. Heterometallic alkoxides are often more soluble than their parent alkoxides, a property that can be advantageously used for the sol-gel chemistry of nonsoluble alkoxides. They also provide molecular precursors withthecorrect M I M stoichiometry inwhichsome M-0-M' bonds are already formed [15]. Alkoxide bridges are usually hydrolyzed during the sol-gel synthesis, but oxo bridges are strong enough not bebroken.CrystallineBaTiOscanbe obtained at rather low temperatures when a mixture of Ti(OiPr)4 and Ba(0iPr)z is heated under reflux before hydrolysis [19]. It was shown recently that heterometallic tetrameric species [BaTiO(OiPr)4,iPrOH]4arethen obtained in which Ti-O-Ba bonds are already formed in the solution [20].

MOLECULAR DESIGN OF TRANSITION METAL ALKOXIDE

9

W. CHEMICALMODIFICATIONOF TRANSITION METAL ALKOXIDES Most metal alkoxides are very reactive toward hydrolysis and condensation. They must be stabilized to avoid precipitation. The chemical control of these reactions is currently performed by adding complexing reagents that react with metal alkoxides at a molecular level, giving rise to new molecular precursors of different structure, reactivity, and functionality. Chemical modification is usually performed with hydroxylated nucleophilic ligands, such as carboxylic acids or P-diketones. In most cases complexation by XOH species can be described as a nucleophilic substitution, as follows: M(OR),

A.

+ xXOH +[M(OR),-AOX),I + xROH

CarboxylicAcids

Acetic acid (AcOH=CH3-COOH) is often used as an acid catalyst for the solgel synthesis of Si02. Adding a small amount of AcOH significantly decreases the gel time of silicon alkoxide systems. In similar conditions, this effect appears to be morepronouncedthan with inorganic acids, such as HCl [21]. However, the reverse effect is observed for Ti(OR)4. Adding acetic acid prevents precipitation and increases the gel time [22]. An exothermic reaction takes place when acetic acid is added to titanium alkoxide in a 1:l ratio. A clear solution is obtained. X-ray absorption experiments show that the coordination of titanium increases up to six(Fig. 4a), whereas two Ti-0 distances (1.80A and 2.06A) and Ti-Ti correlations (3.1 A) are observed on the ,Fourier transform of the EXAFS spectrum (Fig. 4b). Infrared spectroscopy can be used to study how acetate groups are bonded to tita-

(a)

Cb)

X-ray absorption at the Ti-K edge of pi(OiPr)3(OAc)]. (a) = XANES; (b) = EXAFS (Fourier transform).

WAGE

10

nium. The frequency shift between the two bands corresponding to the symmetric v, = 1450 cm" and antisymmetric v, = 1580 cm" stretching vibrations of the acetate group COO- is typical of bidentate bridging acetate groups (AV = 130 cm"). Acetate groups actually behave as complexing nucleophilic ligands and react with titanium alkoxides as follows: Ti(OiPr)4+ AcOH +[Ti(OiPr),(OAc)] + iPrOH During this stoichiometric reaction, the coordination number of Ti increases from four to six, and oligomeric species [Ti(OiPr)3(OAc)In ( n = 2 or 3 are formed (Fig. 5) Esterification occurs when more than 1 mol AcOH is added. Acetic acid in excess reacts with alcohol molecules released during complexation, providing the in situ generation ofwaterandgiving rise to more condensedspecies. Small oligomeric species are obtained in the presence of small amounts of water. Single crystals of hexameric Ti604(OiPr)~?(OAc)4,for instance, have been obtained upon aging an equimolar mixture of AcOH and Ti(OiPr)4 in a closed vessel (Fig. 6). Water, provided via esterification reactions arising from acetic acid in excess, leads to the slow hydrolysis of alkoxide groups, which are replaced by oxo bridges [23]. Most organic groups can be removed and clear transparent polymeric titanium dioxide gels are obtained in the presence of an excess of water. Upon hydroysis, alkoxy groups are removed first, whereas bidentate acetate ligands remain bonded to titanium. They prevent further condensation and increase the gelation time. The functionality of the new molecular precursor de-

5 Chemical modification = [Ti(OiPr)3(acac)].

titanium alkoxides. (a) = ri(OiPr)3

(b)


I1

0" OW Molecularstructure bridges (hatched circles).

of [Ti604(0iPT)12(OAc)4]:Ti(solidcircles);

creases, leading to the growth of anisotropic particles. TizO(0Ac)rj oxoacetates, made of chainlike polymeric species, are obtained when an excess of acetic acid is added

B.

P-Diketones

Strongly complexing P-diketones are currently employed to stabilize highly reactive metal alkoxides, such as W(OEt)6 Aluminum sec-butoxide, modified by ethylacetoacetate (etac), appears to be quite attractive as a precursor for the sol-gel synthesis of multicomponent ceramics, such as cordierite. Al(OsBu)z(etac) is more soluble and less reactive thanthe corresponding alkoxide Complexation is observed with acetylacetone (acacH=CH3-CO-CHzCO-CH3). Its enolic form contains hydroxyl groups and reacts with metal alkoxides as a chelating ligond. Oligomers are not readily formed, and for stoichiometric acacni = 1 ratio, the nucleophilic substitution leads to monomers in which Ti is only fivefold coordinated (Fig. 5b). Ti(OiPr)4+ acacH +Ti(OiP&(acac)

+ iPrOH

LWAGE Oligomeric compounds are formed only upon hydrolysis Titanium coordination becomes six as soon as water is added, whereas Ti-Ti correlations can be seen on the Fourier transform of the EXAFS spectrum. Strongly chelating acac ligands are not removed upon hydrolysis (except at low pH). Condensation is then prevented, and only small oligomers are formed [28].

V. MOLECULARENGINEERING OF METALALKOXIDE PRECURSORS Metal alkoxides react with nucleophilic complexing ligands. Two chemical parameters can then be used to design new molecular precursors. Metal alkoxides react with water molecules (hydrolysis) and nucleophilic species (complexation). The first reaction is followed by condensation and leads to the formation of larger oligomeric or polymeric species, whereas complexing ligands prevent condensation and favor the formation of smaller species. It therefore becomes possible to design sol-gel-derived materials by controlling the following two chemical parameters: the nature and amount of complexing additive XOH (x = X / M ) and the hydrolysis ratio (h = H20/M). large variety of oligomeric species can then be obtained upon hydrolysis and condensation. Molecular clusters, chain polymers, or colloidal particles can be synthesized, depending on the relative amount of hydrolysis and complexation. a general rule, more condensed species are obtained as x decreases and h increases.

A. HYDROLYSIS RATIO The hydrolysis and condensation of metal alkoxides M(OR)z leads to oligomers in which metal atoms tend to acquire their maximum coordination number. The formation of these species can be controlled by the hydrolysis ratio h = H20/M. In the presence of a large excess of water (h >> z). all alkoxide groups are removedand colloidal species are formed.Theylead to hydrous oxides MOzI2.xH20 similar to those synthesized from aqueous solutions. The adsorption and dissociation of water molecules at the oxide/water interface leads to the formation of charged particles. All alkoxide groups are not removed when h z and chain oxopolymers are formed. They can be advantageously employed for drawing fibers or making coatings. The dielectric constant of the organic solvent is rather low, the surface charge of polymeric particles is very small, and vander Waals interactions prevail. For very low hydrolyis ratios (h -c condensation is mainly governed by the formation of p o x 0 and alkoxo bridges. Solute molecular oxoalkoxides are then formed and can often be isolated as single crystals from the solution.

I3

MOLECULAR DESIGN OF T M S I T I O N METAL ALKOXIDE

Their structure appears tobe closely related to that of the corresponding polyanions formed in aqueous solutions. The first hydrolysis product ( h = 0.6) of titanium ethoxide, for instance Ti704(OEt)20, contains [Ti70241 units isostructural with MO7@& Larger species, such as Ti1008(0Et)24 ( h = 0.8) and Ti16016(OEt)32 (h = l), are formed when more water is added [30,31]. They have been obtained upon the controlled hydrolysis of Ti(OEt)4 and isolated as single crystals from the solution (Fig. 7). The molecular structure of these species in the solution has been studied by x-ray diffraction and 1 7 0 nuclear magnetic resonance using enriched water as a reagent [32]. Niobium ethoxide leads to Nb8010(OEt)20, which exhibits the same structure as the paratungstate [33]. In the presence of an organic base, NMwOH, a decamer, is obtained [Nblo028(NMe4)6*6H20], whichis isostructural with the decananadate polyanion (V10028)6- [34].

Complexation The chemical reactivity of metal alkoxides toward hydrolysis and condensation can be modified by complexation. Less electronegative alkoxide ligands are hydrolyzedpreferentially,whereasstronglybonded complexing groups are more difficult to removed. They prevent condensation, and gelation or precipitation rates decrease upon complexation. Complexing ligands behave as termination reagents. They cause the effective functionality toward condensation to be reduced, resulting in less condensed and more anisotropic polymeric species. Precipitation can then be avoided, and gelation is promoted.

b sis

7 Structure of somemolecularclustersobtained via thecontrolled hydrolyTi(OEt)4. (a) = [Ti704(0Et)20]; (b) = [Ti1008(OEt)24].

LNAGE The reaction of titanium alkoxides with acetylacetone leads to several molecular compounds.[TiO(acac)z]z is formed in the presence ofan excess of acetylacetone or upon hydrolysis of Ti(acac)2(OR)2. Single crystals have been isolated;andx-raydiffraction experiments show dimers with sixfold coordinated Ti atoms linked through oxygen atoms. Strongly complexing acac ligands cannot be hydrolyzed easily, and condensation does not go any further (Fig. 8) [35]. Lager molecular species can be obtained with smaller ainounts of acetylacetone. Single crystals of Tii8022(0Bu)26(acac)2were obtained recently [36]. They are made of 18 [Ti061 octahedra sharing edges or comers and correspond to x = 0.1. h = 1.2. Complexing acac ligands remain outside the Ti18022 core of the molecule (Fig. 9). These examples show that condensation can be tailored via complexation and hydrolysis. Gels are obtained in the presence of an excess of water and for small values of x (x I0.3, h 2 10). Clear sols are obtained when Ti(OnBu)4 is hydrolyzed in the presence of acetylacetone. The mean hydrodynamic diameter of colloidal particles was measured by quasi-elastic light scattering. It appears to increase from 2 to nm as the hydrolysis ratio increases from h = 1 to h = for x = 0.3). It decreases from to nm when the amount of acetylacetone increases from x = 0.3 to x = l (for h = [28]. Similar results have been found with zirconium alkoxides. Solvated dimeric species are formed when zirconium propoxide is dissolved in its parent alco-

OXO

Molecular of [TiO(acac)2]2 according to Ref. bridges (hatched); other atoms (C and 0) (open).

Ti (solid); p

-

MOLECULAR DESIGN OF TRANSITION METAL

Molecular structure

ALKOXIDE

15

[Ti18022(0Bu)26(acac)2](h = 1.2 and x =

hol. Tetameric Zr40(0nPr)1o(acac)4 species are obtained with acetylacetone (x = acacnr = 1) in the presence of a very small amount of water ( h = 0.2). They have been isolated as single crystals and their structure determined by x-ray diffraction (Fig. 10) [37]. Larger molecular clusters, such as [Zr1006(OH)4(0nPr)l8L6], with kallylacetoacetate, can be formed by decreasing x (x = 0.6) and increasing h (h = l) (Fig. l l ) [38]. Monodispersed zirconia powders have been prepared by Rinn and Schmidt [39] via the acid hydrolysis Zr(OiPr)4 in ethanol in the presence of hydroxypropylcellulose as a stabilizer avoid aggregation and acetylacetone as a complexing agent. Particles from about 200 to 10 nmin diameter were obtained depending on the relative amount each reagent. Dimericsolvated solute species [Cez(OiPr)s(iPrOH)z] are formed when cerium isopropoxide is dissolved in its parent alcohol. These solutions are very sensitive to moisture and can be stabilized with acetylacetone. Molecular clusters are formed when a small amount of water is added. Red crystals have been

LNAGE

16

Zr

(oxo bridge)

Molecularstructure

[Zr4PO(OPr)lo(acac)4]

=

and h =

grown from a solution in which x = acac/Ce = 2. They correspond to hexameric species [Ce604(0H)4(acac)l2] in which the oxohydroxo core (Fig. 12) is isostructuralwiththe inorganic analog [Ce604(OH)4(S04)6], whichwas observed in aqueous solutions of cerium sulfate [ 131. The hydrolysis of P-diketonate-modified cerium(IV) isopropoxide leads to the formation of colloidalsolutions or gels. The complexationratio (x = acac/Ce) appears to be the key parameter tailor the size of cerium oxide particles. Precipitation is observed when x 0.1, whereas sols are obtained when 0.1 < x 1. The mean hydrodynamicdiameter of these particles decreases from to 15 when x goes from 0.1 to 1. Only solute molecular clusters are formed when the complexing ratio becomes larger than 1 (1 Ix I2).

Organically modified silicates can be conveniently synthesized from precursors, such as R4Si(OR), which contain nonhydrolyzable Si-C bonds that organic moieties are not removed during the hydrolysis-condensation process. Organic groups R can behave as either network modifiers or network formers

MOLECULAR DESIGN

Zr

OF TRANSITION METAL

ALKOXIDE

17

0 (oxo & hydroxo bridges)

11 Molecular structure

[ZrloOs(OPr)ls(acac)6] (x = 1 and h = 0.8).

when polymerizable organic ligands are used. They allow the formation of an organic network in addition to the inorganic. The organic groups impart new properties to the organic network, such as flexibility, hydrophobicity, or refractive index The chemistry hybrid organic-inorganic gels is mainly developed around silicon-containing materials. Titanium and zirconium alkoxides are sometimes used as cross-linking reagents for the condensation of difunctional silicon precursors, such as [(CH3)2Si(OC2H5)2]. However, these alkoxides do not really behave as cross-linking agents. They lead to the formation of Poly Dimethyl Siloxane (PDMS)-Ti02 nanocomposites in which Ti02 nanoparticles are embedded in a PDMS matrix This certainty arises from the higher reactivity of these alkoxides and their well-known catalytic activity toward the condensation of siloxanes. Therefore, PDMS chains are formed on one side and Ti02 or ZrO2 nanoparticles on the other side. Si-O-Ti bonds have not yet been clearly evidenced. The catalytic role of titanium and zirconium alkoxides to-

18

LNAGE

12 Molecularstructure of [Ce604(OH)4(acac)l2].

ward the condensation of siloxanes is quite interesting for film formation. The presence of long chains in the film makes the material more flexible, allowing the deposition of crack-free transparent coatings several micrometers thick. Hybrid organic-inorganic compounds containing transition metal species were recently synthesized using organically modified polyoxometallates linked, together through W-O-Si-C bonds [42]. The organically modified precursor is obtained via thereaction of GSiW11039 witha trichlorosilane RSiCl3, R=vinyl (CH=CH2), allyl (CH2CH=CH2), or 3-methacryloxypropyl [-CH2(CH2)2OOCC(CH3)=CH2]. Each [SiW1104o(SiR)2]4- carries two binfuctional R groups, available for organic polymerization reactions performed in the presence of radical initiators. Organically modified transition metal oxide gels are difficult to synthesize. Transition metals have a lower electronegativity than silicon. The M-C bond becomes more polar and would be broken upon hydrolysis. Complexing hydroxylated organic ligands must then be used to form nonhydrolyzable M-0-C


19

bonds between the metal atom and the organic species. Complexation appears to be the only way to synthesize hybrid gels involving transition metal oxides. However, very few results have been reported so far in literature. According to the results reported previously, carboxylic acids R-COOH could be conveniently employed as complexing species. Prehydrolyzed titanium butoxide (H2013 = oligomers have been complexed with unsaturated organic acids, such as cinnamic acid(C6H5-CH=CH-COOH).Copolymerizationwasthenperformedwith styrene in the presence of benzoyl peroxide. Transparent brown polymers were thus obtained. They are very stable against water and cannot be dissolved in most organic solvents Acrylic or methacrylic acids can also be used as polymerizable complexing ligands They were recently reported for the sol-gel synthesis of zirconium oxides via the copolymerization of zirconium oxide sols and organic monomers Carboxylates are rather weak complexing ligands, however, and most of them are removed upon hydrolyis when an excess of water is added. Chelating P-diketones are stronger complexing ligands. Organically modified Ti02 gels, which exhibit photochromic properties, have been synthesized from an allyl acetylacetone-modified Ti(OnBu)4 alkoxide. A double polymerization process was initiated via the partial hydrolysis of alkoxy groups and the radical polymerization of allyl functions. The polymerization of the allyl function is slow, however, and organic polymerization is not very effective New approaches have been explored using functionalized chelates with both a strong chelating end and a highly reactive methacrylate group, such as aceto acetoxy ethyl methacrylate and methacryl amido salicylic acid (Fig. 13).

lor

HC

\

HC

I

0

13 Chelating ligands for the synthesis of Z r O 2 hybrid gels. (a) = Aceto acetoxy ethyl methacrylate (AAEM); (b) = methacyl amido salicylic acid (MASA).

LNAGE

Zirconium-oxopoly-polyAAEMcopolymers have been synthesized via the chemical modification of Zr(OnPr)4 precursors by M M . Both polymerization reactions were run simultaneously, leading to hybrid organic-inorganic polymers. The zirconium oxo core is made of oxoalkoxo AAEiM-modified species in which Zr is likely in a sevenfold coordination, as in monoclinic zirconia. Zirconium oxo species are chemically bonded to polymeric methacrylate chains via the P-diketo complexing groups The complexation ratio ( A A E W r ) seems to control the structure and texture of these hybrid gels, giving rise to more or less open structures. Organic and inorganic polymerization processes are presumably related, and such a hybrid material could be described as an interpenetrating polymer network (Fig. Both organic and inorganic polymerizations zirconium propoxide modified by methacryl amido salicylate were performed using similar procedures. This MASA ligand exhibits a stronger complexing power and a higher steric hindrance. Therefore, for equivalent complexation ratios (MASA substitutes two alkoxo groups), simultaneous polymerizations of methacryl amido salicylate-modified zirconium precursors lead to smaller zirconium oxopoly methacryl amido salicylate copolymers

14

organic-transitionmetaloxidenetwork.


21

Other routes could be used, such as the following: Copolymerizationbetweenhybridinorganic-organicsystems,zirconiumoxopoly-MM, andathirdorganicpolymerizablecomponent,suchas styrene. The complexation of zirconiumalkoxideswithligandscarryinganazobis group from which radical polymerization could be initiated Moreover,hybridorganic-inorganiccopolymerscouldbesynthesizedfrom many other metal oxide gels (rare earth, aluminum and others).

1. Brinker, C. J., and Scherer, G.W., Sol-Gel Science, Academic Press, New York, 1989. 2. Livage, J., Henry, M., and Sanchez, C. Prog. Solid Sate Chem., 259 (1988). 3. Klein, L., Annu. Rev. Muter. Sci., 15, 227 (1985). 4. Toledano, P., Ribot, F., and Sanchez, C., C.R. Acud. Sci. Fr., 311, 13 15 (1990). 5. Livage, J., Barboux, P., Vandenbore, M. T., Schmutz, C., and Taulelle,F., J. NonCryst. Solids 18 (1992). 6. Nabavi,M.,Sanchez,C.,andLivage, J., Eur. J. SolidStateInorg. Chem, 1173 (1991). 7. Bradley,D. C., Mehrotra, R. C.,andGaur,D.P. Metal Alkoxides, Academic Press,London,1978. 8. Babonneau, F., Doeuff, Leaustic, A., Sanchez, C., Cartier, C., and Verdaguer, M., Inorg. Chem., 3166 (1988). 9. Barringer, E. and Bowen, H. K., J. Am. Cerum. C-l99 (1982). 10. Barringer, E. Bowen, H. K., Langmuir, 1, 414 (1985). 11. Scozzafava, M. R., Rhine, W. E., and Cina, M. J., Better ceramics through chem697 (1990). istry, IV, Muter. Res. Soc. Symp. Proc., 12. Vaartstra, B. Huffman, J. C., Gradeff, P. Hubert-Pfalzgraf, L., Daran,J. C., Parraud, Yunlu, K., and Caulton, K. G.,Inorg. Chem., 3126 (1990). 1419 (1990). 13. Toledano, P., Ribot, F., and Sanchez, C., Acta Crystullogs, 14. Kundu D., and Ganguli, D., J. Muter. Sci. Lett., 5, 293 (1986). 969 (1990). 15. Caulton, K. G., and Hubert-Pfalzgraff, L., Chem. Rev., 16. Hegetschweiler, K., Schmalle, H. W., Streit, H. M., Gramlich, V., Hund, H. U., and Emi, I., Inorg. Chem., 1299 (1992). 17. Livage, J. and Sanchez, C., J. Non-Cryst. Solids 11 (1992). 18. Hubert-Pfalzgraf,L.,Papiemik, R., Massiani,M.C.,andSepte,B.,Betterceramics through chemistry, IV, Muter. Res. Symp. Proc., 393 (1990). Dolloff, R. T., and Smith, J. J. Am Cerum. Soc., 523 19. Mazdiyasni,K. (1969). 20. Yanovsky, I., Yanoskaya, M. I., Limar, V. K., Kessler, V. G., Turova, N. Y., and Struchkov, Y. T., J. Chem Chem. Cornmuc., 1605 (191). 21. Pope, E. J., and Mackenzie, J. D., J. Non-Cryst. Solids, 185 (1986). 22. Doeuff, Henry, M., Sanchez, C., and Livage, J., J. Non-Cryst. Solids, 206 (1987).

22

WAGE Doeuff,

Dromzee,Y.,Taulelle,

F., andSanchez,C.,

Znorg. Chem., 28,

Doeuff, Henry,M.,andSanchez,C., Muter. Res. Bull., 25, Unuma, H., Tokoka, T., Suzuki, Y., Furusaki, T., Kodiara, K., and Hatsushida, T., J. Mater. Sci. Lett., 5, Non-Cryst. Solids, 121, Babonneau,F.,Coury,L.,andLivage,J. Uaustic, A.,Babonneau,F.,andLivage,J., Chem. Mater., I , Uaustic, Babonneau, F., and Livage, J., Chem. Mater., I , Watenpaugh, K.,andCaughlan,C.N., Chem.Commun., Day, V. W., Eberslcher,Klemperer,W.G.,Park,C.W.,andRosenberg, F. Chemical Processing ofAdvunced Materials (L. L. Hench, and J. K. West, Wiley, New York, p. MossetA.,andGaly,J., C.R. Acad. Sci. Fr., 307, Day, V. W., Eberspacher, T. Klemperer, W. G., Park, C. W., and Rosenberg, F. J. Am. Chem. 113, Bradley, D. C., Hurthouse, M. B., and Rodesiler, P. F., J. Chem. Chem. Commun.,

Graeber, E. J. and Momson, B., Acta Crystallogr., B33, Smith,G.D.,Caughlan,C. N., andCampbell, J. Znorg. Chem., 11, Toledano,P., In, M.,andSanchez,C., C.R. Acad. Sci. Fr., 313, Toledano, P., In, M., and Sanchez, C., C.R. Acad. Sci. Fr., 31 1, Livage,J.Sanchez,C.,andToledano,P., Metaloxideclustersand colloids, Mater. Res. Symp. Proc., San Francisco 272, Rinn,G.,andSchmidt,H., CeramicTransactions (G.L.Messing,E.R.Fuller and H. Hausner, eds.), Am. Ceram. Soc., I, Schmidt,H., J. Non-Cryst.Solids, 73, Dir6, Babonneau, F., Sanchez, C., andLivage,J., J. Mater.Chem., 2, (

Judeinstein,P., Chem.Mater., 4, Suvorov,A.L.,andSpaski, Proc. Acad. Sci. USSR, 127, 44. Schubert, U., Arpac, E., Glaubitt, W., Helmerich, A., and Chau, C.,Chem. Mater., Nass, R., and Schmidt, H., Sol-Gel Optics, SPIE, 1328, Sanchez, C., Livage, J., Henry, M., and Babonneau, F., J. Non-Cryst. Solids, 100, Sanchez,C.,andIn,M.,

J. Non-Cryst. Solids 147-148,

2 Metal Alkoxides for Electrooptical Ceramics M a n e G. Hubert-Pfalzgtaf Universid de Nice-Sophia Antipolis Nice, France

INTRODUCTION Ceramics are one of the most recent classes of electrooptical materials. Table 1 summarizes the various compositions associated with transparent electrooptical ceramics [l]. These materials are mostly ferroelectric lead-containing titanates or niobates.Amongthe various types of precursors ofmetal oxides (e.g., carboxylates and nitrates), alkoxides have especially attractive features. These include high purity, the lability of the coordination sphere, which allows their tailoring according to specific applications, their easy transformation into oxides with formation of volatile by-products and their ability to form homogeneous solutions under a large variety of conditions, and, for multicomponent systems, via heterometallic alkoxides As a result of their versatility, they can meet the requirements for sol-gel as well as metal organic chemical vapor-phase deposition (MOCVD) applications This chapter deals with the synthesis and general properties of homo- and heterometallic &oxides that are or might be used in chemical routes to electrooptical ceramic materials. Some considerations concerning structure, reactivity, tailoring of their properties are also given. Emphasis is given to the most recent results.

II. HOMOMETALLICALKOXIDES For all elements involved in electrooptical ceramics, their homometallic alkoxides, of the general formula M(OR)n, or oxoalkoxides, MO(OR),, are either

on

24

HUBERT-PFALZGRAF

Table 1

Composition of ElectroopticalCeramics

Composition La)(Zr, (Pb, Ti103 PSN Ti103Nb)@ Pb(Sc, (Pb, La)(Hf, (Pb, Sr)(Zr, Ti)O3 (Pb, Ba, Sr)(Zr, Ti)@ (Pb, La, Li)(Zr, Ti)@ Ba, (Pb, Sn)(In, Zr, Ti)@ (Pb, M)(Zr, Ti)@; M = Bi, Sr (Pb, Bi)(Zr, Ti)03

PLZT M(Ta, Nb)03; M = Li, K PLHT Nb, Zr, Ti)@ PLMNZT PSZT (Pb, La)(Mg, PBSZT (Ba, BLTN Nb)@ La)(Ti, (Pb, PLLZT La)Nbz06 PBLN PSIZT (Pb, La)(Zn, Nb, Zr, Ti)@ PBLNZT PMZT (Sr. Ba)Nb206 SBN PBZT

commercially available or can readily prepared in high yield (strictly anhydrous conditions are required for theirhandling. Table 2 lists their general properties and main synthetic routes. '

A variety of synthetic pathways, depending on the starting material, are generally available for most elements. Their value can be quite different in terms of selectivity of the reaction and yield and purity of the resulting metal alkoxide. The most electropositive metals (alkaline, alkaline-earth metals, yttrium, and lanthanides) can be oxidized directly by alcohols. The rate of the reaction is largely dependent on the surface properties the metal and on the nature of the alcohol, especially its acidity and the bulkiness of the OR group. Thus the reaction proceeds easily with alkaline metals (Li, Na, and K), the primary alcohols being more reactive than the secondary or the tertiary. The same variation is observed for the alkaline-earth metals, the reaction being the least favored for magnesium In some cases, it might be necessary to catalyze the reaction by mercury or by gaseous ammonia. Ammonia was observed to be efficient toward barium when classic catalysts (Hg and 12) failed; it avoids contamination [49]. Activation of yttrium and lanthanides by trace amounts of mercury(I1) salts (chloride or acetate) is generally required, although sonochemistry can be a way to activate the metals in the absence of mercury derivatives [50]. Nonotable reaction is observed between yttrium and lanthanides and methanol, ethanol, or t-butanol, but the reaction proceeds well with isopropanol and functional alcohols, such as 2-methoxyethanol [51,52]. Synthesis of metal alkoxides directly from the metal can lead to degradation reactions either of the solvent (tetrahydrofuran, THF, for instance [ 181) or of the reactant 1131; they appear especially easy with barium. Alcoholysis reactions applied to alkoxides (alcohol interchange reactions) or to metal silylamides generally make it possible to overcome these limitations.

METAL ALKOXIDES FOR ELECTROOPTICAL CERAMICS

25

These reactions are generally achieved in the presence of an excess of alcohol, and thus the alkoxides might be obtained-in solution as solvates

[m.

M+ROHexcess+

M(OR)n(ROH)x+;H2

The lability of the alcohol molecules and thus the stability of the solvates is quite variable. Such formulas as Ba(OR)2 or M(OR)3 (M = In, Y, Ln) have been traditionally used to describe alkoxides of these electropositive elements. Many of these alkoxides have now been more precisely defined by x-ray diffraction studies, which have revealed quite different formulas. In fact, the removal of the alcohol molecules (under vacuum) often leads to oxoalkoxides. Pentanuclear oxo or hydroxo units seem to be a common feature for tri- and divalent metals and are illustrated by MsO(OiPr)13 (M= In, Sc; Ln = Y, Yb, . , .) and Bas(OH)(OR)s adducts [R = Ph, 3,5-tBuzCtjH3, CH(CF3)2], and even P-diketonates [53]. It is thus important to emphasize that the formulation, the properties, and the reactivity of metal alkoxides based on large elements may be different according to their “history”-used in situ or isolated [50]. Substitution reactions are another general approach to metal alkoxides, according to Eq. (2):

MG,

+ nROH +M(OR),(ROH), + nGH

(2)

G = NR2, R, H, Cl, . . . . These substitution reactions are especially attractive for obtaining a pure product when the by-product is volatile and thus the anionic ligand G, an amide group NR2, an alkyl group R, or a hydride H. Alcoholysis reactions of trimethylsilylamino derivatives M[N(SiMe3)2In are quite a versatile route to a variety of metal alkoxides, although they require the prior synthesis of the metal silylamide derivatives (commercially available only for Li and Na). This procedure is particularly attractive for metals whose alkoxides are poorly soluble, such as lead, zinc, or iron isopropoxides, since contamination with halide or alkali residues can be overcome. It also allows a large choice of R groups, and lanthanide tertiobutoxide derivatives, for instance, become accessible [39]. The use of appropriate (bulky or functional) alkoxide or aryloxide groups has also opened a way to soluble and/or volatile lead(II) and zinc derivatives [23,29]. Reactions according to Eq. (3) are generally extremely fast and may favor the formation of oxo derivatives, as observed for bismuth [U. N(SiMe3)2],

+ nROH+

M(OR),

+ nHN(SiMe3)2

(3)

Alcoholysis of metal alkyl derivatives is based on the elimination of a volatile by-product as well:

MR, + nR‘OH +M(OR’),

+ nRH

a g B

B B

B

Bm

e ....

% 8 .-

2M m

m

2m 0

8

z.. 3

x

2

.-9

c 8

28

e

d

d

- 0 -

"'-

B

29

HUBERT-PFALZGRAF

It is of practical use for metals whose alkyl derivatives commercially easily available, namely, lithium and magnesium. Metal hydrides can be used as starting material mainly for alkali and alkaline-earth metals. AlthoughZn(0R)z alkoxides derived from classic OR groups are insoluble soluble tetranuclear zinc alkyl or hydridoalkoxides [ZnX(OR’)]4 (X= R or H) can be obtained starting from zinc alkyls or hydride [30,31]. Finally, other general routes involve halides, mostly chlorides, as starting materials, according to Eq. (5):

M = alkaline metals. Substitution by alcohols in the presence of a base is the most general route to early transition metal alkoxides, especially for groups IV (Ti, Zr, and Hf) and V (Nb and Ta), and they are commercially available, at least for some R groups. Less labile halides (lead, bismuth, or lanthanide chlorides) require the use of alkali metal alkoxides to achieve substitution. Even the reaction may proceed with a poor yield and/or offer final products containing halide or alkali metals, the latter being present either as an impurity or as part of a definite product (heterometallic species). When halides are used as a starting material for metal alkoxides, contamination by chlorides can also result from a limited purification: variable amounts of Cl or Na are often found in commercial titanium isopropoxide Ti(OiPr)4. Substitution reactions applied to acetates generally result in a poor selectivity and thus a low purity of the metal alkoxides; their interest is thus limited despite the low cost of the‘acetates. Reactions between hydrated acetates and refluxing 2-methoxyethanol have been used as a way to obtain acetatoalkoxides in sol-gel processing, but most products are poorly characterized. Electrosynthesis by anodic dissolution ofthe metal in absolute alcohol appears for many elements to be a promising, inexpensive way to obtain large amounts of metal alkoxides [54]. The process goes smoothly and has good current yields Zr, for metals, such as scandium and lanthanides, and early transition metals (Ti, and Nb) using tetrabutylammonium bromide as an electrolyte. Oxo or hydroxo compounds are obtained for more electropositive metals (Mg and alkaline-earth metals), however, probably as a result of alcohol dehydration reactions. Finally, alcohol exchange reactions are a means of obtaining more appropriate precursor, starting from commercial or easily accessible metal alkoxides (see Sec. KC).


B.

31

PhysicalProperties and StructuralAspects

Most of the metal alkoxides of interest for electrooptical ceramics are solids (less often liquids) that can be purified by recrystallization, sublimation, or distillation. They are all moisture sensitive, and handling under an inert atmosphere and with anhydrous solvents is thus required. Their unequivocal characterization and formulation are best achieved by x-ray diffraction studies (on monocrystals). Studies on solutions (molecular weight data, nuclear magnetic resonance, NMR, with ‘H, 13C, or metal nuclei) are a means either to establish whether the solid-state structure is retained or, in the absence of x-ray data, to establish the molecular structure and eventually stereolability [48]. Mass spectrometry provides information on the stability of the oligomers or the heterometallic species in the vapor phase. The information gainedby infrared spectroscopy is limited; the technique is mostly useful for the identification of solvates M(OR)n(ROH)x (vOH absorption 3400-3100 cm-1 or of chemically modified (heteroleptic) alkoxides (probe for the vC0 stretching of P-diketonate or carboxylate ligands, for instance). The solubility and volatility of homoleptic alkoxides are mainly determined by the degree of oligomerization m,which depends on the nature of the metal (size and thus coordination number) and on the steric demand of theOR group. Methoxides are thus the least favorable. With the exception of magnesium, lead, and zinc and dn transition metals, isopropoxides are reasonably soluble for all metals of interest for electrooptical ceramics. The insolubility of the first can be overcome by addition of another metal alkoxide (see Sec. 111) or by using a more bulky ligand, such as aryloxide (2.6-di-tert-butylphenoxide)or triphenylsiloxide PhsSiO, or functional alcohols (for instance, OHC2H& with X = OR’ for alcoxyalcohols or X = NR’2 for alkanolamines), which provide intramolecular donor sites [52].Although the aryloxide or triphenylsiloxide group; generally lead to soluble species (often monomeric with a low coordination number for the metal), they might be of limited interest as a result of a low ceramic yield and high organic content of the resulting oxides. Fluorinated OR groups lead to an increase in volatility, reduce the tendency to hydrolysis, and favor the formation of adducts [19]. The bulky tert-heptyloxide groups (R = CMeEtiPr) generally represent a good means of achieving volatility [5]. The metals involved in the composition of electrooptical ceramics are large, oxophilic, and electropositive. High coordination numbers are thus generally required. Recent years have seen, as a result of x-ray studies, the reformulation of many alkoxides based on di-or trivalent large elements as oxo- or hydroxoalkoxides. It is wellknownthat M(OR)n alkoxides are often oligomersdimers, trimers, or tetramers-as a result of the OR group acting as a doublebridging (p2) or triple-bridging ligand (p3), which allows the metal to attain its usual coordination number. Oxo 02- or hydroxo OH- ligands are even more

HUBERT-PFALZGRAF versatile and can behave as p2 or ligands but can also accommodate four, five, or even six metals. As a result, oxo or hydroxo ligands appear to be a means of achieving the high coordination numbers required by the large di- or trivalent element in the absence of neutral ligands. Barium seems to be especially prone to formation of oxo or hydroxo derivatives, and this observation appears in the earlier literature as a tendency to give carbon-deficient or unstable metal alkoxides. The size effect of the mental is illustrated by comparison of Sr and Ba phenoxides: the strontium derivative is a tetranuclear nonoxo unit in which the metal is pseudooctahedral [16]; the barium derivative is a pentanuclear oxo or hydroxo aggregate, which allows Ba, which is larger than Sr, to be sevencoordinate for the basal metal atoms [ 181. Polydentate alcohols, such as triethanolamine, can encapsulate the metal and drastically improve the stability toward hydrolysis [14]. Figure 1 summarizes the basic [M(OR)& oligomers, as well as the oxo or hydroxo aggregates that are observed. Dimeric units (Fig. 1A) are illustrated by [M(OR)5]2 (M= Nb, Ta) or [M(OiPr)4(iPrOHh (M = Zr, Hf); [Pb(OtBu)2]3 and La3(OtBu)gL2 are typical examples of openand closed trinuclearunits (Fig. 1B and C), respectively; tetramers display a [Ti(OMe)4]4 type of structure (Fig. 1D); (also found for divalent or trivalent metals but with additional neutral ligands as for [Nd(OiPr)3(iPrOH)]4or Sr4(0Ph)8(PhOH)z(THF)6) or a cubanelike, (Fig. 1E) as for [K(OtBu)]4 or [ZnR(OR')]4 [S]. The structurally characterized penta- and hexanuclear aggregates are all oxo or hydroxo clusters.M4(p&o)o6 cores (Fig. 1F) havingan adamantanelike structure are found for lead and barium alkoxides. The M5(pS-o)Oy (Fig. 1G) core seems to display a special stability for large di- or trivalent metals. Hexanuclear aggregates derive from the pentanuclear by the addition of a sixth

Basicaggregates for M(OR),lm alkoxides or oxoalkoxides.


X = OR,R

E

G OR

OR

I

H

6R

Continued

metal (Fig. 1 H ) or by condensation of adamantane units. Oxo or hydroxo aggregates are mostly highly soluble despite their nuclearity-the OR groups ensure a lipophilic layer-and they are also sometimes volatile. NMR and mass spectrometry data have shown that these aggregates are generally retained in solution as well as in the gas phase. They result from C-0 bond cleavage reactions (with formation of alkenes) andor oxidation reactions (especially for

HUBERT-PFMZGRAF OR'

bridging-chelating

bridging P2 -

P2 - ? l 2

M

triply-bridging P3 -y'

triply- bridging P3

terminal

-q2 2

Coordination modes

2-methoxyethanol.

alkaline-earth metals) and are present before hydrolysis; in some cases pb(II)], further condensation is observed by recrystallization, especially when dialkyl ethers or siloxanes are easily formed [24,25]. 2-Methoxyethanol displays numerous coordination modes (Fig. 2) but is basically a bidentate and assembling ligand; [M(OC2H40Me)n]malkoxides are thus large oligomers (m= 9, M = Ca; m = 10, M = Y), sometimes infinite polymers [M = Bi, Pb(II)]. Despite the large values of the nuclearity, however, they are soluble (with the exception of lead) because of cyclic (M = Y) or compact structures (M = Ca) or dissociation of the polymer in solution, as observed for Bi [32]. Aryloxides are, with the exception of barium, either monomers or dimers [30,38]. Alkoxide ligands OR and to a lesser extent trialkylsiloxo R3SiO and aryloxo OAr are n donors [3]. l 7 donation is especially favored for do elements (Ti, I%,. Ta, and lanthanides). Asthe OR ligand undergoes oxygen p to metal d n'bonding, the M-0 distances can become very short for terminal OR groups (4 covalent radii); the M-0-C angles open up to nearly linear M-0-C units. The n bonding as well as the thermal stability of the M-0-C bond decreases with the metal oxidation state and by moving to the late transition metals and is illustrated by more acute M-0-C angles.

C. Chemical Modifications In addition to using bulky OR groups, reduction of the molecular complexity can also be achieved by a polar solvent. Since formation of complexes is quite

35


limited for metal alkoxides because of their preferred autoassociation, giving oligomers [48], one of the few appropriate ligand or polar solvents is an alcohol (6)]. This is illustrated by the fact that many metal alkoxides are obtained as solvates (the M-OHR interaction is often stabilized byH bonding with an adjacent OR group): the group IV isopropoxides (M(OiPr)4(ihOH)]z (M = Zr, Hf) are typical examples [42]. It should also be noted that since the structure may be different according to the solvent [niobium alkoxides (R = Me, Et) are dimers in toluene but are monomeric Nb(OR)s(ROH) adducts in the parent alcohol] [48], the reactivity can be modified, especially toward other metal derivatives (Fig. 3). 1 -[M(0R),lm m

+ xROH

1

[M(OR),(ROH),,],

m’ m

(6)

The electronegative alkoxo or aryloxo groups make the metal atoms highly prone to nucleophilic attack. The alkoxides easily react with the protons of a large variety of organic hydroxy compounds, such as alcohols, silanols, glycols, carboxylic acids, P-diketones (Fig. 3). These exchange reactions convert homoleptic M(OR), into heteroleptic alkoxides M(OR),-XZ, and thus reduce the functionality of the precursor (2= OR). They also illustrate the use of alkoxides as versatile starting materials and as precursors whose coordination sphere can be easily modulated for specific applications. Such modifications of the coordination sphere of the initial alkoxide are often achieved by the use of the so-called additives, which are in fact chemical modifiers [3,55a]. 1. Alcoholysis Alcoholysis reactions are generally performed to “tailor” the volatility or the rheology, to decrease the hydrolysis rates, or, finally, to increase the solubility of the metal alkoxides. As a synthetic route, alcoholysis reactions with classic alcohols are usually achieved n refluxing cyclohexane, the reactions being promoted by azeotropic distillation of the alcohol [2]. In contrast with silicon alkoxides, however, alcohol interchange reactions proceed to some extent (depending onthe OR group and on the stoichiometry) at room temperature for most elements [S]. This has been shown, for instance, by metal N M R on niobium and titanium alkoxides. “Uncontrolled” alcohol interchange reactions may thus occur when a metal alkoxide is used in an alcohol different from the parent alcohol. Such reactions lead to a modification (decrease) of the apparent functionality of the precursors, which in turn can affect the properties (morphology, porosity, and so on) of the resulting oxide. Functional alcohols, such as alcoxyalcohols-mainly 2-methoxyethanol-or alkanolamines (diethanolamine and triethanolamine), are actuallythemost commonly used to slow hydrolysis rates and thus to “stabilize” metal alkoxides. Although such exchange reactions occur at room temperature, Some heat-

36

HUBERT-PFALZGRAF

U

0

1 0

0

%

p:

0

:C:

0 E

ea

::

0 v ea

U

/-\

U E


37

ing may be necessary for completion of the reaction, especially if stoichiometric amounts of reactants are used. Thus the reaction between zirconium isopropoxide [Zr(OiPr)4(iPrOH)]2 and four equivalents of 2-methoxyethanol leads to Zr(OCZH40Me)3(0iPr) a room temperature. The presence offunctional alkoxo groups around the metals generally results in an increase in its coordination number, and thus hydrolysis becomes more difficult [57]. Alcoholysis reactions using 2-methoxyethanol generally also offer more soluble products [52]. already mentioned,this observation is no longer valid for lead(I1) since the methoxyethoxide derivative; are polymeric and insoluble [23]. The reactivity of a variety of glycols (CH&(OH)2 toward titanium alkoxides, zirconium isopropoxide, and niobium and tantalum ethoxides has been studied by Bradley et al.[2]. Complete substitution generally resultsinpoorly soluble products; such derivatives asM(OR),~X[O(CH~)~O]~ generally remain soluble and sometimes volatile. Further reactivity, for instance with acetylacetone, has established that the Ti-OR bond is more labile than the Ti-glycolate bond. a way of improving solubilTrialkylsilanols R3SiOH (R = Me, Et) are ity and controlling hydrolysis [58]; however, they have rarely beenused, probably as a result of their limited stability and thus commercial availability. 2. Reactions with Carboxylic Acid and P-Diketones Substitutionreactionswith carboxylic acids, such as acetic acid, or with Pdiketones, mainly acetylacetone,are other means of controlling hydrolysis rates by decreasing the functionality of the precursor. Carboxylic acids RCO2H are often added to the sol-gel system as an acid catalyst [59,60]. In fact, they are not merely a source of H+, but a chemical modifier, and carboxylate groups substitute for the OR groups. Although P-diketonate and acetate ligands both generally lead to an increase in the metal coordination number, their behavior is quite different. Carboxylates act as assembling and oxo donor ligands, and thus have a tendency to increase the nuclearity of the aggregates; diketones are chelating ligands and thus decrease the oligomerization. This behavior is well illustrated by the modification of titanium alkoxides.Reactionbetween [Ti(OEt)& and acetylacetone gives monomeric and dimeric species (according tox-ray absorption near-edge structurestudies[61], and condensation into hexanuclear oxo clusters is observed when acetic acid is used (x-ray structures of Ti6(~2-o)2(C13-o)2(oR)8(oAC)8 (R = Et. iPr, nBu) [46]. 3. Hydrolysis Hydrolysis reactions of alkoxides proceed by nucleophilic attack of the metal (7)], and polymerization occurs via the reactive hydroxoalkoxides (7b) and (7c)l. Hydrolysis-polycondensationreactions are governed by numerfactors (Table 3). The metals involved in the composition of electrooptical ceramics are electropositive and oxophilic, and thus the hydrolysis of their ho-

HUBERT-PFALZGRAF

38

Table HydrolysisParameters of Homo-andHeteroleptic Alkoxides Electronegativity of the metal and polarity of the M-0-Cbond Nature alkoxo groupR Modifies the molecular complexity Rate increases with chain lengthening Sensitive to hydrolysis TertiaryR > secondaryRprimaryR OR > OSiR3 pH (acid or basic catalysis) Solvent and dilution Temperature Degree hydrolysis h (h = [HzO]/M(OR)n) h < n: fibers, chains, coatings h 1: molecular clusters h > n: gels, tridimensional polymers Modified precursors M(OR)n-xZx(Z= OH, OAc, B-dik,...) Rate decreases with Functionality of the precursor (number the OR groups) Increase in metal coordination number Hydrolytic susceptibility OR > OAc > B-dik

moleptic alkoxides is often extremely facile and much easier than for silicon. Introduction of other ligands 2 slows hydrolysis rates and promotes anisotropic growth. M(OR),

+ H20

-

H\

0 - hf(OR),.1 H- --OR

2M(OH)(OR),-1+ (OR),-$4M(OH)(OR),-I + M(OR),*

/

> M(OH)(OR),-l(7a)

0 -M(OR),-1 + H20 (OR)n-lM- 0 “M(OR),-1 + ROH

(7b) (7C)

The early stages of the hydrolysis-condensationof [M(OR),Im can be characterized byx-ray diffraction for oxophilic metals (Table These include centrosymmetric polynuclear oxoalkoxides of titanium, zirconium, andniobium. Their structure is often related to that of polyoxoanions: Ti704(OEt)20 is isostructural with Mo70246, and Nbg010(OEt)20 has a cagelike Structure that compares well with that of the paratungstate H~W120421G.These oxoalkoxides are soluble as a result of their closo structure, and 1 7 0 NMR has been developed a technique to follow the hydrolysis of titanium(1V) alkoxides with

HUBERT-PFALZGRAF H217O. Since enrichment in 1 7 0 occurs selectively at their oxide (as opposed to alkoxide) oxygen sites, the different types of oxo ligands n= can be determined, the most encapsulated oxo ligands being the most shielded [63]. Similar types of molecular oxo clusters are observed for alkoxides modified by acetylacetone [65a,66]. Although the early stages of the hydrolysis-polycondensation are extremely rapid, removal ofall alkoxide ligands at room temperature generally requires quite a large excess of water [57]. Hydrolysis studies have also been performed on several lead(1I) alkoxides: Pb(OR)2 (R= CMefit, CHMeCH2NMe2) in THF [23]. The hydrate 3PbOeH20 was obtained at low temperature as the kinetic product; its conversion to massicot and then to litharge is catalyzed by an increase in the pH (which may be caused by alkali metal alkoxide residues). Since 3PbOeH20 is less stable than massicot and litharge, it could act as a more reactive component in ceramic oxide preparations.

D. Thermal Decomposition Thermolysis reactions have mostly been studied for group IV metal alkoxides, although metal oxide depositshavebeen obtained for many metals [5]. Oxoalkoxides are often formed asintermediates,but mechanistic studiesremain scarce [68]. The decomposition can be enhanced by hydrolysis as a result of either residual water on the surface or dehydration reactions of tertiary alcohols.

HETEROMETALLIC ALKOXIDES Another aspect of the lability of the metal alkoxide bond is the easy formation of heterometallic or mixed-metal species [4]. Such compounds have also been called “double alkoxides” in comparison with double salts; however, their description as heterometallic is more appropriate in view of their covalent character and the existence of species having three different metals. The formation of heterometallics is a means of overcoming the poor solubility of polymeric metal alkoxides (Zn and Pb), of achieving homogeneity at a molecular level for multicomponent systems, and of providing a better homogeneity of the final material. If they display a convenient volatility and stability, they can also be used as “single-source” precursors (in which the different elements are incorporated in a single molecule) for MOCVD applications and thus improve the transport of poorly volatile elements and reduce possible interdiffusion phenomena [5]. Table 5 collects the heterometallic alkoxides, associating elements present in the formulation of electrooptical ceramics, although the stoichiometry may be quite different from those required for materials.


41

Heterometallic alkoxides are generally obtained either by reactionbetween Lewis acids and bases or by condensation with elimination of volatile or insoluble by-products. 1. Lewis Acid-Base Reactions Formation of heterometallic alkoxides by simple mixing, often at room temperature and in nonpolar solvents, is one of the most general routes and corresponds to the "complexation" step in the sol-gel process. This general reaction [Eq. (8)] appliestonearlyallalkoxides,withthe exception thesilicon Si(OR)4:

yM'(0R')nI

+ M(0R)n +MM'y(OR)n(OR)nty

(8)

Determination of the stoichiometry of the reaction is best achieved if it is conducted in a solvent in which one of the reactants is poorly soluble. Since the heterometallic species is more soluble than the parent alkoxide, the progressive dissolution of the most insoluble species allows control of the stoichiometry between the two different metals. Metal N M R (7Li and 93Nb) may also be used as tool to follow the reaction, as shown for LiNb(OR)6 [91]. The following reactions, which all proceed at room temperature, are examples of this approach: L[Mg(OiPr)2], m M(OR)~ + "(OR)

+ 2Nb(OiPr)5 +MgNb2(OiPr)l2 250c > "'(OR)6

(9) (10)

M = Nb, Ta; M' = Li, Na, K, . . . Alkali metal alkoxides are often involved in the preparation of metal alkoxides [Eq.51; heterometallic alkoxides can thus be a side product if the stoichiometry of the reaction is poorly controlled ( o n ) . For electropositive metals, such barium, the alkoxide might be generated in situ: 2Ba + [Zr(OiPr)4(iPrOH)]2 dBa2Zr2(pOR)2(OR)18+ H2

(11)

Systematic studies the influence of the R group and the solvents on the stoichiometry of such reactions as Eq. (8) remain limited, although the Turova group has studied ternary-phase diagrams [M(OR)n, M'(OR)n, and solvent] and proposed compositions for many systems. The Ba-Ti system is one of the best characterized and has shown dependence on the R group (oxo compound with R = zW,nonoxo compound for R = Et) as well as on the stoichiometry of the reactants:

4

.. E

2

B C

z .^

e,

m C

%

52%

F? E

44

HUBERT-PFALZGRAF

[BaTi2(OEt)g(EtOH)4][H(OEt)2] +

+

Addition reactions similar to h.(8) can also be observed between anhydrous acetates and alkoxides. Such reactions can even occur at room temperature, but they may require heating (depending on the lability of the metal acetate bond) and/or be less selective and give redistribution products Fomometallic acetatoalkoxides, M(OR)n-dOAc)x] as well. Mg(0Ac)z + 2Nb(OiPr)s 250c > MgNb2(OAc)2(OiPr)lo

(14)

Although metal b-diketonates can be solubilized in the presence of a metal alkoxide and thus form heterometallic species, at least-as intermediates, most reactions are more in favor of the formation of redistribution products [M(OR)n-x(P-dik)x]as the final result [92]. 2. Elimination of Volatiles The formation of heterometallic alkoxides is sometimes driven by that of a volatile by-product, mainly a dialkyl ether. Typical examples are obtaining the oxoalkoxides Ba4Ti404(0iPr)16(iProH)~ and Pb6Nb404(0Et)24:

When acetates are involved, esters can be the volatile by-product and thus favor the formation of the mixed-metal species; approach can overcome the noncommercial availability of some alkoxides [89]: 3Pb(OAc).y3H20 + Zn(OAc)z.2HzO

> P~Zn~(OAc)4(0Cz&OMe)4 +

As a general observation, it is important to note that reaction between alkoxides and other metal derivatives, such as acetates, can be strongly dependent on the solvent and/or the addition order of the reactants [84]. Thus, no reaction is observed between anhydrous Pb(OAc)2 andNb(0Et)s in THF at room temperature, but the acetate dissolves readilyin toluene at room temperature in the presence of the niobium alkoxide with formation of a Pb-Nb species [go]. Another general feature is that in heterometallic acetatoalkoxides obtained by route, the metalintroduced as an alkoxideappearspredominant. More condensed species are obtained if the reaction is achieved at higher temperatures [93]: Pb(0Ac)z + [Zr(OiPr)4(iPrOH)]z 250c> PbZr3(~O)(~OAc)z(~OipT)5(OiPr)~ + '

(18 )


45

In some cases, the stoichiometry between the two metals M and M' in the isolated species is different from that of the reaction medium, suggesting that other metal species are present [Eq. (12)].

3. Elimination of an Insoluble By-product Such reactions are generally based on the elimination of an alkali salt according to (19): MCl,

+ M"M',(OR),

+ nM'C1

"+MIM'y(OR)y]n

(19)

M"= Li, Na, K, TI(I)3 Although they allow control

of the stoichiometry between the M and M', metals, they have not been much developed so far for species-associating metals involved in electrooptical ceramics. They are mostly limited totheuse of the complex alkoxide anionZrz(OiPr)s-,andLa-Zr species were recently built up by this route [87]. In contrast with the preceding synthetic routes based on mixing simple compounds, (19) implies the prior synthesis of a heterometallic alkoxide based on an alkali metal and further removal of the salt.

Mixed-metal alkoxides are generally covalent [4]. As a result, they are soluble in organic nonpolar solvents (often less in alcohols), generally more than the parent alkoxides. Some can besublimed without disproportionation, the volatility following the same variation as for the homometallic alkoxides (OtBu > OzR > OEt > OMe).Mass spectrometry, for instance, established thatheterometallic LiNb fragments were retained in the gas phase for LiNb(OEt)6. Their structure is mainly determined by the overall nuclearity of the aggregate (MxMy and derives from the basic structural moiety in which the different metals display the usual coordination numbers expected for the bulk of the OR [4]. Numerous mixed-metal homoleptic alkoxides for which the stoichiometry betweenthetwo metals is 1: 1 have a structurethat derives from that of [Ti(OMe)4]4 (Fig. 1D). &iNb(OEt)6], is an infinite polymer; this explains its poorsolubility,but it is favorable for obtaining fibers [70]. Trinuclear mixed-metal species generally display a triangular Mkf2(p3-OR)2(p-OR)3core (type Fig. IC), the coordination sphere eventually being completed by neutral ligand L as for SrTi2(0iPr)s(iPrOH)3. Association such triangular units via bridging OR groups or via incommon apex gives such compounds as [BaZr2(p-OR)4(OR)4]2(Fig. 4J) or a 1:4 stoichiometry, such as BaZr4(OR)i8' (type Fig. 4K), respectively [81]. Thestructure of the Pb-Nb oxoalkoxide, Pb6Nb404(OEt)24, derives from that of the lead, with the oxo ligands acting as donors toward Nb(OEt)5 moities (Fig. 4L), which in turn increase the lead coordination number by three double-bridged OR groups. Similarly,

HUBERT-PFALZGRAF

Ba4Ti404(0iPr)ia(iP1OH)~ (Fig. 4M) can be considered a Ba4(~-0)4tetrahedron with groups bearing Ti(OR), moeities. MgNb2(p-OAc)2(p-OiPr)4(0iPr)6 corresponds to an open polyhedron of the type shown in Fig. IB, the acetate ligands connecting the central Mg atom to the neighboring Nb ones. The presence of chelating or assembling ligands may modify the basic arrangement between the different metals, however.

Reactivity Data on the reactivity of mixed metal alkoxides are limited. Their reactivity remains dominated by the lability of the metal alkoxo bond, but a problem that needs to be addressed is that of the maintaining or not of the heterometallic unit, of the stoichiometry between the different metals, and finally of the ho-


47

mogeneity, at a molecular level when they are reacting. Dissociation into the parent alkoxides upon dissolution of the heterometallic species can be a spontaneous process, even in a nonpolar solvent,andoccurs, for instance, for Pb604(OEt)e[Nb(oEt)s]4, asshown by207Pb NMR [79].Suchdissociation phenomena can be promoted by a polar solvent (alcohol or depending on the nature of the metals, and thus the choice of the solvent becomes even more important than for homometallic alkoxides Common modifiers usedinthesol-gel process are functionalalcohols, acetic acid, and acetylacetone. Alcoholysis reactions are the most documented; KiNb(OEt)6Imfor instance, was converted to LiNb(OC2H40Me)rj in the presence of 2-methoxyethanol [91]. Assembling ligands, such as 2-methoxyethanol or acetic acid, seem more favorable than P-diketones for the stabilization of mixed-metal alkoxides. However, the poorly controlled addition of an additive may sometimes modify the mixed-metal species by “extracting,” at least partially, one metal as an insoluble product or as a complex. Information on the hydrolysis mixed-metal alkoxides is even scarcer than for the homometallic, Partial hydrolysis of [LiNb(OEt)6lo0leads to the dimeric heterometallic alkoxide [LiNbO(OEt)4(EtOH)]2, in whichboth the stoichiometry between the two metals and their coordination numbers are retained [93]. On the other hand, the partial hydrolysis of solutions of methoxyethoxides of Ba and Ti (1:l molar ratio) offers BwTi13018(OC2H40Me)24 (30% yield). Its structure corresponds to a tetrahedron ofBaO3unitssurimposedon a TiOs(Ti03)n core; this Ti13042 core is related to that of the aluminum salt [Al1304(0H)24(H20)]7+ [83].The important modification of the stoichiometry between the two metals illustrates the complexity the sol-gel chemistry of multimetallic systems and shows that important structural rearrangements can occur.

IV. APPLICATION TO CERAMICS

Sol-Gel Process PLZT and PNM are, with L i m o s and LiTaOs, the systems related to electrooptical ceramics that have been the most studied.Chemical low-temperature routes are particularly attractive for lead-containing materials, since lead oxide is quite volatile in comparison with other metal oxides; thus control the stoichiometry of lead-based materials is tedious. To overcome the difficulty inhandling alkoxides and their availability, commonly accessible salts, such as carboxylates (acetates or the more soluble ethylhexanoates), carbonates, nitrates, or hydroxides, have often been used in conjunction with metal alkoxides (mostly n-propoxides or n-butoxides for Ti and and ethoxides for Nb or Ta), the solvent being an alcohol. The poor re-

HUBERT-PFUGRAF activity of barium acetate is overcome by adding acetic acid [46a] or by using carbonate or, more often, barium hydroxide Ba(OH)~8Hz0[94]. Commercial ionic precursors are often available as hydrates, and this must betaken into account when speculation about the intermediates that may form is proposed. Carboxylate groups (acetates or 2-ethylhexanoates) are generally removed only at relatively high temperatures (400°C); they appear to favor porous materials and grain boundaries [95]. Additives for hydrolysis are either acids (0.1-0.2 M m03 or acetic acid) or ammonia solutions. Although leadoxide has also been used [96], acetate hydrates are the precursors most often used for the introduction of lead or lanthanum in the multimetallic systems required for PLZT materials,and they are often associated with the useof 2-methoxyethanol [97-991. This solvent appears as a way to achieve deshydratation by refluxing (boiling point 124"C), as well as condensation (elimination of organic esters as has been shown by infrared (vCOZ = 1730 cm-1) [loo]. Studies on the influence of the precursor remain limited. X-ray absorption fine structure analysis of the sol and gel precursors of PZT (lead acetate in methanol, n-propoxides of Zr and Ti, and acetic acid) have shown that an uniform composition exists in both states on a microscopic level with Ti-O-Z linkages [100a]. The choice of the titanium precursor Ti(OR)4, R = nF'r or iPr, was reported to have an effect on the microstructure development, particularly for PbTiOs films [97]. Clusters and grains were observed in the n-propoxide and were attributed to a less continuously cross-linked networks compared with films derived from the isopropoxide. Using the modified alkoxide Ti(OiPr)z(acac)z instead of Ti(OiPr)4 allowed an increase in the thickness of crack-free PbTiOs films up to 1 m under base-catalyzed conditions [loll. Using the same titanium precursor associated with lead acetate and pentane-lJ-diol instead 2of methoxyethanol permitted the production of films up to approximately in thickness by repeated coatings before fuing [ 1021. All alkoxide routes appear to be limited toobtaining PNM [88a] and lithium or potassium niobate or tantalate [102a]. For the latter, the formulation of the mixed-metal alkoxide MM'(OR)6, M = Nb, Ta, is in agreement with that of the material. Epitaxial thin films of LiNbO3 on sapphire could be obtained [103]. The useof a mixed-metal MgNbz(0R)lz species (R = Et [105], CzH40Me [104]) has been shown to promote the formation of the perovskite phase for PNM.

MOCVD Obtaining electrooptical thin films by MOCVD techniques has been less investigated, although volatile oxide precursors-alkoxides 'or p-diketonatesexist for nearly all elements. For PbTiOs films titanium isopropoxide has been

METAL ALKOXIDES FOR ELECTROOPTICAL CERAMICS associated with an alkyl derivative, PbEU using argon as the carrier gas. No additional oxygenwasnecessary. The film deposited at a quartz substrate heated at 500°C was shown to be conducting and dense, with good surface morphology and strong [l001 texture direction [106]. Pb(0tBu)z was used by Trundle and Brierley as the volatile lead source with Ti(OiPr)4, giving thick PbTiOs deposited at 450"C, annealed .in air at 800-900°C [107]. Although these authors could not deposit PbSc0.5Tm.503 (PST) directly from the metal precursors, the PST perovskite phase was obtained in atwo-stage process: deposition of cubic ScTaO4 by MOCVD using Ta(0Et)s and a scandium-fluorinated P-diketonate Sc(fod)3 (fodH = heptafluorodimethyloctanedione), followed by diffusion of PbO from a surface layer The heterometallic alkoxide LiNb(0R)a has the correct stoichiometry for depositing LiNbO3. Although LiNb(0R)a is volatile, control of the deposition parameters is sometimes hampered by disproportionation reactions. The problem has been overcome by using a lithium P-diketonate Lithd and Nb(0Me)s [5]. LiNbO3 has been deposited on a variety of substrates (-45O"C), but epitaxial layers require annealing in oxygen at higher temperatures.

V.

OUTLOOK

Metal alkoxides are readily available for all elements involved in electrooptical ceramics. The selection of appropriate OR groups-bulky or functional and thus polydentate-allows adjustment of their physical properties: solubility or volatility. An almost unlimited number of mixed-metal compositions is accessible under mild conditions, often at room temperature, by mixing metal alkoxides and carboxylates or P-diketonates. The molecular composition of these lutions can be quite complex and comprise mixed-metal species with different stoichiometries. To date, 2-methoxyethanol has been used predominantly for the sol-gel processing of thin layers and thus for the chemical modification of the metalalkoxides. However, methoxyethanol is teratogenic and can cause neurological and hematological damage, even at the ppm level. Alternative solgel systems, based for instance on other difunctional and polyfunctional alcohols and on hydroxyacids, for example, should be developed to promote the sol-gel process as a practical method. More systematic studies regarding the relationship between precursors, influence of additives, and properties of the final material are also required.

ACKNOWLEDGMENTS The author is grateful to the CNRS for financial support, and to the contributions and enthusiasm of coworkers, whose names are listed in the references.

50

HUBERT-PFALZGRAF

acacH acetylacetone (2,5-pentanedione) acetate OAc ArR R-substituted phenyl group:2,6-RzCaH3 ArR$ R-substituted phenyl group:2,6-Rz-4-MeCsH3 ArF phenylgroupsubstitutedbyCF3 2,4,6-(CF3)3C,jH2 Bz benzyl CH2C6Hs P-dikHP-diketone = (O)CRCH2C(O)R thdH 2,2,6,6-tetramethylheptane-3,5-dione

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HUBERT-PFALZGRAF Hubert-Pfalzgraf, L. G., Papiernik, R., and Massiani, M. C., to be published. Papiernik, R., Hubert-Pfalzgraf,L. G., andChaput,F.,Molecularroutesto ternarymetaloxides:A PbTi heterometallicisopropoxideasaprecursorto PbTi03, J. Non-Cryst. Sol., (in the press) Hirano, I., and Kato, K., Adv. Cerum. Muter., 3, Tripathi, U.M., Singh, and Mehrotra, R. C., Synthesis and characterization of some heterometallic isopropoxides of lanthanum with zirconium, Polyhedron, ZO,

Sakashita, Y . , Ono, T., Segawa, H., Tominaga, K., and Okada, M., Preparation and electrical propertiesof MOCVD-deposited PZT thin films,J. Appl. Phys., 88a. Munozaguado, M. F., Gregorkiewitz, M., and Larbot, Sol-gel synthesisof the binary oxide (Zr, Ti)Oz from the alkoxides and acetic acid in alcoholic medium, Muter. Res. Bull., Laaziz, I., Larbot, Julbe, Guizard, C., and Cot, L., Hydrolysis mixed titanium and zirconium alkoxides by an esterification reaction, J. Solid Sture Chem., to be published; Laaziz, I., Larbot, A., Foulon, 1. D., and Cot, L., zr2Ti404(oiPr)8(oiPr)2( OAC),~, submitted for publication. Francis, L. F., Payne, D. and Wilson, R., Crystal structure of a new lead zinc acetate alkoxide, PbzZnz(OCzH40Me)4(0Ac)4, Chem Murer., (

Veith,M.,Hans, J., Stahl, L., May,P.,Huch,V.,andSebald, Bimetallic alkoxygermanate(II)-stannate (II) andplumbate(II), Nuturjiorsch. B, 46b, Eichorst, Howard, K. E., and Payne, D. NMR investigations of lithium alkoxide solutions, J. Non-Cryst. Sol., Sirio, C., Poncelet, O., Hubert-F'falzgraf, L. G., Daran, l. C., and Vaissermann, J., Reactions between copper P-diketonates and metal alkoxides as a route sol-to uble and volatile copper(I1) oxide precursors: Synthesis and molecular structure of Cu4(p-3,nl-OC2H40iPr)4(acac)4 and (acac)Cu(p-OSiMe~)zAl(OSiMe~)~, Polyhedron, 11, Turova N. Y., Turevskaya, E. P., Kozlova, N. I., Rogova, T. V., Kessler, V. G.,

and Kucheiko,

I., Transition metal alkoxides-precursors of oxides materials,

J. Non-cryst. Sol., in the press

Thule, P. P., Raghavan, and Risbud, H., Comparison of Ba(OH)2, BaO and Baasstartingmaterialsforthesynthesis of bariumtitanatebythealkoxide Cerum Soc., method, J. Hsueh, C. C., and McCartney, M. L., Microstructural development and electrical properties of sol-gel prepared lead zirconate-titanate thin films,J. Muter, Res., 6, Yanovskaya, M.I., Turevskaya, E. P., Turova, N. Y., Dambekalne, M. Y . , Kolganova, N. V., Ivanov, Segalla, G., Belov, V. V., Novoselova, V., andVenevtsev, Y. N., Transparentceramic(Pb, La) (Zr, Ti)03 preparedby alkoxy technology, Inorg. Muter.,


57

97.Budd,K.D.,Dey, K.,andPayne,D.A.,Sol-gelprocessing PbTiO3, PbZrO3, PZT and PLZT thin films, Br. Ceram. Proc., 107 (1985). 98.Ramamurthi, D.,andPayne,D.A.,Structuralinvestigations prehydrolized precursors used in the sol-gel processing lead titanate,J. Am. Ceram. 2547 (1990). 99.Cheng,M. J., ZhaoZ.,andQuangD.,Preparation finePbTiO3powdersby hydrolysis of alkoxide, Chem. Mater., 3, 1006 (1991). 100. Seth, V. K., Schulze, W. A., and Condrate, R. A., Sr. Vibrational spectral characterization a lead lanthanum zirconate titanate during various stages sol1299 (1991). gel processing, Specrro. Lett., 100a.AhlfAanger, R., Bertagnolli,H.,Ertel, T., Kolb, U., PeterD.,Nass,R.,and Schmidt, H., First evidence the preformation of an inorganic network in solgel processing lead zirconate titanate, obtained by EXAFS spectroscopy,Ber. Bun. Gesel. Phys. Chem., 95, 1286 (1991). 101. Milne, J., and Pyke, H., Modified sol-gel process the production lead titanate films, J. Am. Ceram. 1407 (1991). 102. Philipps, J. F., and Milne, J., Diol-based sol-gel system for the production thin films of PbTiO3, J. Muter. Chem., 1, 893 (1991). 102a.Amini,M.M.,and Sooks, M.D.,Preparation of single-phaseKNbo3using 675. (1990). bimetallic alkoxides, Better Ceramics Through Chemistry W , 103. Nashimoto, K., and Cima, M. J., Epitaxial LiNbo3 thin films prepared by a solgel process, Mater. Lett., IO, 348 (1991). 104. Francis, L. F., Oh, Y. J., and Payne, D. A., Sol-gel processing and properties leadmagnesiumniobatepowdersandthinlayers, J. Muter. Scien., 5007 (1990). 105. Chaput, F., Boilot, J. P., Lejeune, M., Papiernik, R., and Hubert-Pfalzgraf, L. G., Low-temperature route to lead magnesium niobate, J. Am. Ceram. 1355 (1989). 106. Kwak, B. Boyd, E. P., and Erbil, E., Metalorganic chemical vapor deposition 1702 (1988). PbTiOs thin films, Appl. Phys. Lett., 107. Trundle,C.,andBrierley,C. J., Precursors thinfilmsoxidesbyPhoto102 (1989). MOCVD, Appl. Surf: Sci.


POWDER SYNTHESIS AND CHARACTERIZATION


Chemical Synthesis of Metal Oxide Powders C. N. R. Rao Indian Institute Science Bangalore, India

INTRODUCTION Synthesis of oxide materials provides an excellent case study of the contribution of solid-state chemists to materials synthesis Although tailoring oxides of the desired structure and properties remains the main goal of solid-state chemistry and materials science, it is not always possible to do One can but there is always the evolve a rational approach to the synthesis of solids element of surprise encountered not uncommonly. A well-known example of an oxide discovered serendipitiously is NaMo,06 containing condensed MO, octahedral metal clusters This was discovered in an effort to prepare the lithium analog of NaZn2M0308. Another such serendipitous discovery is that of the phosphorus-tungsten bronze Rb$8W320112, formed by the reaction of phosphorus present in the silica of the ampule during the preparation of the RbW bronze [5]. Novel solids of the type cu$o6s8, called Chevrel phases, were also discovered accidentally. There are many examples of rational synthesis. A good example is Sialon, in which Al and oxygen were partly substituted for Si and nitrogen in silicon nitride, Si3N,. The fast Na+ ion conductor Nasicon was synthesized based on understanding the coordination preferences of cations and the nature of oxide networks formed bythem. The zero-expansion ceramic C%.5Ti2P3012,possessing the Nasicon framework, was later synthesized based on the idea that the property of zero expansion would be exhibited by two- or three-coordination polyhedra linked in to leave substantial empty space in the network

61

RA0 Traditionally, oxides have been prepared by the so-called ceramic method, which involves mixing and grinding various powders and heating them at high temperatures, with intermediate grinding when necessary. The trend nowadays is to avoid such a brute force method to obtain better control of structure, stoichiometry, and phasic purity. Some of the chemical methods of synthesis of oxides that are especially noteworthy are (1) the precursor method, (2) coprecipitation and soft chemistry routes, the sol-gel method, intercalation and ion exchange, and topochemical methods. We examine examples of synthesis by some of these methods and briefly describe the synthesis of certain novel oxide materials, including superconducting cuprates, it should be noted that a variety ofconditions, often bordering on the extreme, such as very high temperatures or pressures, very low oxygen fugacities, and rapid quenching, have been employed in oxide synthesis. The low-temperature chemical routes, however, are of greater interest.

II. PRECURSOR METHOD In the ceramic method, diffusion distances between the reacting cations are rather large. The diffusion distances are markedly reduced by incorporating the cations in the same solid precursor. Synthesis of complex oxides by the decomposition of compound precursors has been known for some time. For example, thermal decomposition of L~CO(CN)~-SH,O and LaFe(CN),.H,O in air readily yields LaCo0, and LaFeO,, respectively. BaTiO, can be prepared by the thermal decomposition of Ba[TiO(C204),], and LiCrO, can be prepared from L I [ C ~ ( C ~ O ~ ) ~ ( H , O Unfortunately, )~]. compound precursors are not always easy to find. In such instances, precursor solid solutions can be used effectively. Carbonates of such metals as Ca, Mg, Mn, Fe, CO, Zn, and Cd are all isostructural, possessing the calcite structure. We can therefore prepare a large number of carbonate solid solutions containing two or more cations in different proportions [6,7]. The rhombohedral unit cell parameter uR of the carbonate solid solutions varies systematically with the weighted mean cation radius. Carbonate solid solutions are ideal precursors for the synthesis of monoxide solid solutions of rock salt structure. The carbonates can be decomposed in vacuum or in flowing nitrogen to yield single-phase solid solutions of monoxides of the type Mnl-pxO (M = Mg, Ca, CO, or Cd) of rock salt structure. Oxide solid solutions of M = Mg, Ca, and CO would require 770-970 K for their formation; those containing cadmium are formed at even lower temperatures. The facile formation of the oxides of rock salt structure by the decomposition of calcite carbonates is a result of the close (possibly topochemical) relationship between the structures of calcite and rock salt. The monoxide solid

CHEMICAL SYNTHESIS OF METAL. OXIDE POWDERS

63

solutions can be used as precursors for preparing spinels and other complex oxides. Besides monoxide solid solutions, a number of ternary and quaternary oxides possessing novel structures can be prepared by decomposing carbonate precursors containing the different cations in the required proportions, in air or oxygen. Thus, one can prepare Ca,Fe,O, and CaFe204 by heating the corresponding carbonate solid solutions in air at 1070 and 1270 K,respectively, for about 1 h.Ca,F%O, is a defect perovskite with ordered oxide ion vacancies and has the well-known brownmillerite structure (Fig. 1) with the Fe3+ ions in alternate octahedral (0)and tetrahedral (T) sites. Two new oxides of similar compositions, Ca$0,05 and CaCo204, have been prepared by decomposing the appropriate carbonate precursors in oxygen atmosphere around K. Unlike Ca,Fe,O,, in C%Mn,O,, anion vacancy ordering in the perovskite structure gives a square-pyramidal (SP) coordination around the transition metal ion (Fig.1). One can also synthesize complex oxides of the type Ca2FeCo05, Ca,Fe,,Mno~405, Ca3Fe2Mn08, and on, belonging to the family by the carbonate precursor route. In the Ca-Fe-0 system, there are several other oxides, such as CaFe407, CaFel2Olg, and CaFe204(FeO), (n = 1, 2, that can, in principle, be synthesized starting from the appropriate carbonate solid solutions and decomposing them in a proper atmosphere. good example a multistep solid-state synthesis achieved starting from carbonate solid solution precursors is provided by the Ca2Fez-$4nXO, series of oxides (Fig. 2). The structure of both the end members, Ca2F%0, and Ca2Mn,05, are derived from that of the perovskite (Fig. 1). Solid solutions be-

-C - L a -

b

A

-O (

Structure (a) C%F905 (brownmillerite) and (b) C%Mn,O,. Oxygen vacancies are shown by open circles.

RA0

I

JI

BM

2 Ca3F%MnOs (I) reduced topochemically to Ca2Fe4BMm05 transforms to the brownmillerite (BM) structure on annealing in

(m; (II)

tween the two oxides are expected to show oxygen vacancy ordered superstructures with Fe3+ in octahedral and tetrahedral coordinations and Mn3+ in square-pyramidal coordination, but they cannot be prepared by the ceramic method. These solid solutions have indeed been prepared starting from the carbonate solid solutions, Ca2F~-Mnx(C03),. The carbonates decompose in air around 1200-1350 K to give perovskitelike phases Ca2Fe--$fnxOcy (y < 1). The compositions of the perovskitelike oxides prepared from the carbonate precursors with x = 2/3 and 1 are Ca3Fe+4n8 and Ca3Fel,Mnl,08.,. X-ray and electron difraction patterns show that they are members of the homologous series of anion vacancy ordered superstructures with n = 3 (A3B308+J. Careful reduction of Ca3Fe2Mn08 andCa3Fe1.5Mn1.508.1in dilute hydrogen at 600 K yields Ca3Fe2Mn07.5 and Ca2FeMn05 (CagFe1.5Mn1.507.5),respectively (Fig. During this step, only Mn4+ in the parent oxides is topochemically reduced to Mn3+, and Fe3+ remains unreduced. The mostprobable superstructure of Ca3F%Mn07.5 involves SP, 0, andT polyhedra along the b direction. On heating in vacuum at 1140 K,however, it transforms to the more stable brownmillerite structure with only 0 and T coordinations (Fig. 2). A variety of complex metal oxides of perovskite and related structures can be prepared by employing hydroxide, nitrate, and cyanide solid solutions precursors as well [ 8 ] . For example, hydroxide solid solutions of the general formula Ln1-Jblx(OH)3, where Ln = La or Nd and M = Al, Cr, Fe, CO or Ni) and Lal-x-,"~"y(OH)3 (where M' = Ni and M" = CO or Cu), crystallizing in the

CHEMICAL. SYNTHESIS

OF METAL. OXIDE POWDERS

65

rare earth trihydroxide structure, can be decomposed at relatively low temperatures (-870 K) to yield LaNiO,, NdNiO,, LaNil-xCox03, LaNil-xCuxO,, and others. Anhydrousalkaline-earth metal nitrates A(N03)2 (A = Ca, Sr, Ba)and Pb(NO,), are isostructural. One can therefore readily prepare nitrate solid lutions of the formula Al+.Pbx(N03)2,which are ideal precursors for the preparation of such oxides as BaPbO,, B%PbO,, and Sr2Pb0,. Oxides of the type LaFe0.5C00.503and 5Nd&003, which cannot be madereadilyby the ceramic method, have been prepared by the decomposition of cyanide solid lutions: LaFeo,Coo.5(CN)6.5.H20 and L% 5N+.5C~(CN)6.5-H20, respectively.

LA2C0205 AND La2Ni205 BYTOPOCHEMICAL REDUCTION Both LaNiO, and LaCoO, crystallize in the rhombohedral perovskite structure, and the anion-deficient nonstoichiometry of these oxides is interesting. Occurrence of the homologous series La,Ni,O,,, on the basis of a thermogravimetric study of the decomposition of LaNiO, was proposed some time ago. It was not known, however, whether a similar series exists for cobalt. Controlled reduction of LaNiO,andLaCoO, in hydrogen shows the formation of La2Ni2O5 and L%Co205, representing the n = 2 members of the homologous series LaB,03,1 (B = CO or Ni). La2Ni205.can be prepared by the reduction of LaNiO, at 600 K in pure or dilute hydrogen [9] and La2C0205 by the reduction of LaCo0, in dilute hydrogen at 670 K. Both oxides can be oxidized back to the parent perovskites at lowtemperatures. Neither La2Ni2O5 nor La2C02O5 can be prepared by the solid-state reaction of La203 and the transitionmetaloxide.X-rayand electron diffraction data reveal that La2C0205 adopts the brownmillerite structure. The x-ray diffraction pattern of La2Ni205 is different from that of La2C0205 but could be indexed on a tetragonal cell related to cubic perovskite. The formation of La2C0205 and L%Ni205 by the reduction of LaCoO, and LaNiO,, respectively, is caused by the topochemical nature of the reduction process. The reduction of the high-temperature superconductor YB%Cu30, to YBa2Cu306 is also a topochemical reaction. Similarly, many of the reactions involving insertion of atomic species into host oxides are topochemical.

IV. MO,-xW,O,BYTOPOCHEMICALDEHYDRATION Many of the modem developments in solid-state chemistry owe much to the investigations carried out in MOO, and WO, the crystallographic shear planes being a major discovery. WO, crystallizes in a Reo3-like structure, but MOO,

66

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possesses a layered structure. MOO, can be stabilized in the WO, structure by partly substituting tungsten for molybdenum. These solid solutions can be prepared by the ceramic method (by heating MOO, and WO, in sealed tubes around 870 K) or by the thermal decomposition of mixed ammonium metallates. These methods do not always yieldmonophasicproducts,however, owing to the difference in volatilities of MOO, and WO,. We therefore sought to prepare Mo1,Wx03 by the topochemical dehydration of Mol,Wx03.H20. Topochemical dehydration, it is to be noted, is a gentle process. Since Mo03-H20 and WO,.H,O are isostructural, the solid solutions between them are prepared readily by adding a solution of MOO, and WO, in ammonia to hot M HNO,. The hydrates, Mol,Wx03.H20, crystallize in the same structure as Mo03.H20 and W03.H20, with a monoclinic unit cell. The hydrate solid solutions undergo dehydration undermild conditions (around K), yielding M o ~ - ~ W ~which O ~ , crystallize in the Reo3-related structure of WO,. Thermal dehydration of W03.H20 and M o , - ~ W ~ O ~ Hgives ~ O rise to oxides of Reo, structure. MOO, obtained by the dehydration of Mo03.H20, however, has a layered structure; this reaction is reported to be topotactic. The nature of the dehydration reaction was studied by an in situ electron diffraction study in which the decomposition occurs as a result of beam heating [IO]. Electron diffraction patterns clearly show how W 0 3 - H 2 0transforms to WO, topotactically with the required orientational relationships. The mixed hydrates, M O ~ - ~ W ~ O , . H also ~ O , undergo topotactic dehydration with similar orientational relations. What is interesting is that when the dehydration of Mo03.H20 is carried out under mild conditions (e.g., electron beam heating), MOO, in the Reo, structure is formed instead of the expected layered structure. The Reo, structure of MOO, is metastable and is produced only by topotactic dehydration under mild conditions. We believe that the preparation of Reo3 like MOO, by mild chemical processing is significant.

V. BRONZES OF MO,-flxO, REACTION

BYASOLID-STATE

Oxides of the type AxW03 (A = alkali metal) are known as tungsten oxide bronzes. These are readily prepared by the insertion of the alkali metal into WO,. The corresponding molybdenum bronzes are more difficult to prepare. High pressures and electrochemical methods are generally employed to thesize some of them. A simple solid-state reaction between an alkali iodide and MOO, or Mol,WxO3 (under dry conditions) has been found to yield such molybdenum oxide bronzes [l l]. The following reaction represents a simple means of making these bronzes:

CHEMICAL SYNTHESIS OF METAL OXIDE POWDERS

VI.

67

INTERCALATION

Intercalation chemistry has become an important aspect of solid-state chemistry, and a large variety of guest molecules are accommodated in the cavities, cages, channels, or interlayer spaces of host solids. Many oxides act as hosts, and interesting properties emerge from this property. Many of the intercalation reactions are topochemical in nature. One of the interesting intercalation reactions involving oxides is that involving chemical or electrochemical intercalation or disintercalation of alkali metal ions in oxides of the type LiMO,, where M = V or Co. When Li, for example, is removed from LiVO,, the resulting VO, remains in the metastable structure of the parent LiVO,. Other such metastable structures of oxides have been prepared by the preferential removal of intercalated species [l].

VII.

ION EXCHANGE

Ion exchange is employed effectively to synthesize new oxides [l]. Thus Na in p-alumina is readily exchanged by other cations. Such exchange reactions are well known in silicates, especially zeolites. Examples of simple ion-exchange reactions are as follows: U 1 0 2 + AgN03 p "AgA102 + KNO3 a - LiFeOz + CuCl +CuFeOz + LiCl NaCrO;! + LiN03 +LiCrOz + NaCl It has been possible to exchange Li in LiNbO, by protons to obtain HNbO,. Such alkali metal-proton exchange reactions are common in layered oxides (e.g., H2Ti307 and HLaNbO,).

VIII.

ALKALI FLUX METHOD

One of the early examples of the use of strong alkaline media for the synthesis of oxides is that of Pb2Ru2-.Pbx07y, which has the pyrochlore structure. The method stabilizes higher oxidation states of metals. Alkali carbonate fluxes have been traditionally used to prepare many oxides (e.g., LaNiO,). Recently, superconducting La,CuO, was prepared by the reaction of L%O, and CuO in a molten mixture of NaOH and KOH around K [12]. Superconducting Ba,_.K$iO, has been prepared by the use of molten KOH [13].

IX.

INTERGROWTH STRUCTURES

Several systems form chemically well-defined recurrent ordered intergrowth structure with large periodicities, rather than random solid solutions with vari-

68

RA0

able composition. However, the ordered intergrowth structures themselves frequently show the presence of wrong sequences. The presence of wrong sequences or lamellae is best revealed by a technique more suited to the study of local structure.High-resolution electron microscopy (HREM) enables direct examination of the extent to which a particular ordered arrangement repeats itself: the presence of different sequences of intergrowths, often of unit cell dimensions. Selected area electron diffraction, which forms an essential part of HREM, provides useful information (not generally provided by x-ray diffraction) regarding the presence of supercells caused by intergrowth or defect ordering. Many systems forming ordered intergrowth structures have come to be known in recent years [14]. These systems generally exhibit homology. If the perovskite structure is cut parallel to the (1 10) planes, slabs of the composition An-lBn03, are obtained; if these slabs are stacked, an extra sheet of A is introduced, giving rise to the family of oxides of the general formula A#n03n+2. Typical members of this family are Ca2Nb2O7 (n = 4) NaCa4Nb,017 ( n = 5), and Na,Ca,Nb,O, (n = 6). High-resolution electron microscopy and x-ray studies show that an ordered intergrowth structure with n = 4.5 with the composition NaCa8Nb903*corresponds to alternate stacking of n = 4 and n = 5 lamellae. In Fig. 3 we show the lattice image of the ordered intergrowth in NaCa8Nb903,. Between n = 4 and 4.5, a large number of or-

High-resolution electron microscopic image of the n = 4.5 memberinthe series.


69

dered solids are found, with the b parameter of the unit cell ranging anywhere from A in the = compound to a few thousand angstroms in longer period structures. These solids seem to belong to the class of infinitely adaptive structures. There is a family of oxides of the general formula Bi@A,1B,03,3, discovered by Aurivillius, in which the perovskite slabs, (An-1Bn03n+1)2-, octahedra thick, are interleaved by (Bi202)2+layers (Fig. Typical members of this family are Bi2W06 (n = l), Bi3Til,Wo.,0, (n = 2), Bi4Ti3CrO12(n = and Bi,Ti3Cro15 (n = andtheyhavebeeninvestigatedindetailby HREM. These oxides form intergrowth structure of the general formula Bi4Arn+&3,,, involving alternate stacking of two Aurivillius oxides with different values; the method of preparation' simply involves heating the compo-

eo

eo

Bi7M5021

Bi9M7027

"

4 The f i i t three members of a homologous series structures of Bi4Am+,&lm+f103~m+fl~4 formed by the Aurivillius family oxides, where is also takento be Bi. Bi cations are shown as filled circles and oxide ions as open circles. BO, are shown in polyhedral form.

RA0

70

nent oxides or carbonates of metals. Ordered intergrowth structures with (m,n) values of (1, 2), and (3, have been fully characterized by x-ray diffraction and HREM. What is most amazing is that such intergrowth structures with long-range order are indeed formed, although either member (mor n) can exist as a stable entity. These materials seem to be truly representative of recurrent ordered intergrowth. The periodicity found in the recurrent intergrowth solids formed by the Aurivillius family of oxides is indeed impressive. The relative ease withwhich WO, forms tetragonal, hexagonal, or perovskite-type bronzes by interaction with alkali and other metals is well known. The new family ofintergrowthtungsten bronzes (ITB) involving the intergrowth of nWO, slabs and one to three strips of the hexagonal tungsten bronze (HTB) is of relevance to our discussion here. In these intergrowth tungsten bronzes of the general formula MxW03, x is generally 0.1 or less, and depending on whether the HTB strip is one or two tunnels wide, ITB are classfied as belonging to (0, n) or (1, n) series (Fig. 5). HTB strips of two-tunnel width seem to be most stable in ITB, and many ordered sequences of the (0, n) and the (1, n) series have been identified. Recently, ITB phases of Bi have been characterized, and in this system the HTB strips are always one tunnel wide, Displacement of adjacent tunnel rows caused by the tilting of WO, octahedra often results in doubling of the long-period axis of the ITB. Evidence for the ordering of the intercalating Bi atoms in the tunnels has been found in terms of satellites around the superlattice spots in the electron diffraction patterns. Among the other systems exhibiting ordered intergrowth, special mention must be made of hexagonal barium femtes MpYq (M = BaFeI2Ol9 and Y = Ba2Me,2022, where Me is Zn, Ni, Mg, and so on). A large number of intergrowth structures of this family have been identified.

X.

SUPERCONDUCTING CUPRATES

The discovery of a superconducting cuprate with a T, above 77 K created a sensation in early 1987. Wu et al., who announced this discovery first, made measurements on a mixture of oxides containing Y, Ba, and Cu. In this laboratory, we independently worked on the Y-Ba-Cu-0 system on the basis of solid-state chemistry We knew that Y,Cu04 could not be made and that substituting Y with Ba in this cuprate was not the way to proceed (unlike in La2-€!axCu04). We therefore tried to make Y3Ba3Cu5OI4by analogy with the known La3Ba3Cu6OI4and varied the Y/Ba ratio as in Y3,rBa3+xC~6014. When x = 1, we obtained YBa2Cu3 (T, 90 K). We knew the structure had to be that of a defect perovskite from the beginning, because of the route we followed for the synthesis. We briefly examine some preparative aspects of the various types of cuprate superconductors [16]. The cuprates are ordinarily made by the traditional ce-

CHEMICAL, SYNTHESIS

OF METAL, OXIDE POWDERS

71

I I ,6)

5 The (1, n) intergrowth tungsten bronzes. Hexagonal tunnels of HTB strips separate the WO, slabs shown in polyhedral form. (After Kihlborg, 1979.)

ramic method (mix, grind, and heat), which involves thorough mixing of the various oxides and/or carbonates (or any other salt) in the desired proportion and heating the mixture (preferably in pellet form) at a high temperature. The mixture is ground again after some time and reheated until the desired product is formed, as indicated by x-ray diffraction. This method may not always yield the product with the desired structure, purity, or oxygen stoichiometry. Variof this method are often employed. For example, decomposing a mixture of nitrates has been found to yield a better product in the 123 compounds of the type YBa2Cu307 by some workers; some others prefer to use BaO, in place of BaC03 for the synthesis. The sol-gel method has been conveniently employed for the synthesis of 123 compounds, such as YBa2Cu30, and other cuprates. The sol-gel method

RA0 provides a homogeneous dispersion of the various component metals when a solution containing the metal ions is transformed into a gel by adding an organic solvent, such as glycol or an alcohol, often in the presence of other chemicals, such as organic amines. The gel is then decomposed at relatively low temperatures to obtain the desired oxide, generally in fine particulate form. Materials prepared by such low-temperature methods may need to be annealed or heated under suitable conditions to obtain the desired oxygen stoichiometry as well as the characteristic high T,. total of cuprates of the type YBa2Cu,08, lead cuprates, and bismuth cuprates have all been made by this method; the first two are particularly difficult to make by the ceramic method. Coprecipitation of all the cations in the form of a sparingly soluble salt, such as carbonate, ina proper medium, followed by thermal decomposition, has been employed by many workers to prepare cuprates. One of the problems with the bismuth cuprates is the difficulty in obtaining phasic purity (minimizing the intergrowth of the different layered phases). The glass or the melt route has been employed to obtain better samples. The method involves preparing a glass by quenching the melt; the glass is then crystallized by heating it above the crystallization temperature. Thallium cuprates are best prepared in sealed tubes (gold or silver). Heating T1203 with K) in a sealed tube a matrix the other oxides (already heated to is preferred by some workers. It is important that thalliumcuprates are not prepared in open furnaces since T1203, which readily sublimes, is highly toxic. obtain superconducting compositions corresponding to a particular copper content (number of Cu02 sheets) by the ceramic method, one often must start with various arbitrary compositions, especially with the T1 cuprates. The real composition of a bismuth or a thallium cuprate superconductor is not likely to be anywhere near the starting composition. The actual composition can be determined by analytical electron microscopy and other methods. Heating oxidic materials under high oxygen pressures or in flowing oxygen often becomesnecessary to attain the desired oxygen stoichiometry. Thus h 2 C u 0 4 and La&a,-,Sr,Cu2O6 heated under high oxygen pressure become superconducting,with T, of and K, respectively.With the compounds, one of the problems is that it loses oxygen easily. It therefore becomes necessary to heat the material in an oxygen atmosphere at an appropriate temperature below the orthorhombic-tetragonal transition temperature. Oxygen s t 6 ichiometry is not a problem with the bismuth cuprates, however. The perconductorswere first preparedunder high oxygenpressures. It was later found that heating the oxide or nitrate mixture in the presence of N%02 in flowing oxygen is sufficient to obtain compounds.Superconducting Pb cuprates, on the other hand, can only be prepared in presence of very little oxygen(N2witha small percentage of 02).For the electron superconductor Nd2-xCexCu04, it is necessary to heat the material in an oxygen-deficient at-


73

mosphere; otherwise, the electron given by Ce merely gives an oxygen-excess material. It may be best to prepare Nd2_,Ce,Cu04 by a suitable method (say, decomposition of mixed oxalates or nitrates) and then reduce it with hydrogen. Many thallium cuprates, as prepared by the sealed tube method, have excess oxygen; they become superconducting only on heating in vacuum or hydrogen. Several other novel strategies have been employed for the synthesis of perconducting cuprates. For example, a Eu-Ba-Cu alloy precursor has been oxidized to obtain EuBa$u,O, and hyponitrite precursor has been employed to prepare YBa,Cu,O, More interesting in the synthetic strategies are those in which structural and bonding considerations are involved in the synthesis. One such example is the synthesis of modulation-free superconducting bismuth cuprates Superconducting bismuth cuprates, such as Bi2CaSr2Cu208, exhibit superlattice modulation. Since such modulationhad something to do with the oxygen content in the Bi-0 layers and lattice mismatch, Bi3+ was substituted partly by Pb2+ to eliminate the modulation without losing the superconductivity.

XI.

CONCLUDINGREMARKS

The area of oxide synthesis has become trulyextensive, with newertypes of materials being prepared everyday by employing a variety of novel methods We have touched on only some of the important methods, and there are many more. For example, we have not discussed the commonly employed coprecipitation method (involving the simultaneous precipitation of the cations as carbonates and oxalates, for example) followed by decomposition or the sol-gel method. This method does not always give precursor solid solutions, but coprecipitated materials on decomposition readily give the desired oxides. Another novel method of preparing oxide powders is by the combustion method, involving the spontaneous ignition of a mixture of metal nitrates in presence of a fuel, such as urea or glycine This method can be employed to prepare powders of most oxides, including dielectrics and superconductors. The use of templatemolecules to synthesizeporous solids involving silicates andphosphates is noteworthy. The ingenuity with which properties of oxides are modified drastically by appropriate substitutions or by the modification of the structure forms an important part of synthetic strategies. Other methods of interest are those involving fine particles, vapor transport, and electrochemical methods

ACKNOWLEDGMENT The author thanks the Department of Science and Technology and the Indo-European Economic Community collaborative program for support.

74

REFERENCES 1. Rao, C. N. R., and Gopalakrishnan, J., New Directions in Solid State Chemistry, Cambridge University Press, London, 1989. 2. Rao, C. N. R., and Gopalakrishnan, J., Acc. Chem. Res., 20, 228 (1987). 3. Roy, R., Solid State lonics, 32133, 3, (1989). 3963, (1979). 4. Torardi, C. C., and McCarley, R. E.,J. Am. Chem. 5. Giroult, J. P., Goreand, M., Labbe, P. H., and Raveau B., Acta Crystallogr., 1336, 2570, (1980). . 6. Vidyasagar, K., Gopalakrishnan, J., and Rao, C.N.R., Inorg. Chem., 23, 1206, (1984). 7. Rao, C. N. R., Gopalakrishnan, J., Vidyasagar, K., Ganguli, K., and Ramanan, J. Mater. Res., 1, 280 (1986). 8. Vidyasagar, K., Gopalakrishnan, J., and Rao, C. N. R., J . Solid State Chem., 58, 29 (1985). 9. Vidyasagar, K., Reller, Gopalakrishnan, J., and Rao, C. N. R., J. Chem. Soc. Chem. Commun., 7 (1985). Gopalakrishnan, J., and Rao, C. N. R., J. Chem. Soc. 10. Ganapathi, L., Ramanan, Chem. Commun., 62 (1986). K., Gopalakrishnan, J., and Rao, C. N. R., J. Solid State Chem., 74, 11. Ganguli, 228 (1988). M., J . Am. Chem. 5214 12. Ham, W. K., Holland, G. F., and Stachy, (1988). 13. Schneemeyer, L. F., Thomas, J. K., and Siegrist, T., Nature, 335, 421 (1988). Rao, C.N.R., and Thomas, J. M., Acc. Chem. Res., 18, 113 (1985). and Sreed15. Rao, C. N. R., Ganguly, P., Raychaudhuri, K., Mohan Ram, R. har, K.,Nature, 326, 856 (1987). 16. Rao, C. N.R., Nagarajan, R., and Vijayaraghavan, R., Supercond. Sci. Technol., 6, 1(1992). 17. Matsuzaki, K., Inone, Kimura, H., Aoki, K., and Masumoto, K.,Jpn. J . App. Phys., 26, L1310 (1987). 18. Horowitz, H. Mclain, J., Sleight, W., Druliner, J. D., Gai, P. L.,Van Kavelaar, M. J., Wagner, J. L., Biggs, B. D., and Poon, J., Science, 243, 66 (1989). 19. Manivannan, V., Gopalakrishnan, J., and Rao, C.N.R., Phys. Rev., B.43, 8686 (1991). 20. Rao, C.N.R., Mater. Sci. Eng., in print (1992).

4 Multicomponent Ceramic Powders T. Mah UES,Inc., Dayton, Ohio

E. E. Hermes Wright Paterson Air Force Base, Ohio

K. S. Mazdiyasni General Atomics, San Diego, California

INTRODUCTION The chemical processing of ceramics, especially ceramic powder syntheses, has drawn a considerable amount of attention over the past two decades. The reason for this is the demand for reliable and advanced ceramic components for high-performance applications. This increased interest in the chemical synthesis of ceramic powders is illustrated by the vast number of publications and ongoing investigations [ 1-91. This chapter focuses only on the syntheses of oxide ceramic powders. The precursors of these powders are metal-organic compounds, mainly metal alkoxides. The different varieties of ceramic powders synthesized by mixed metal alkoxide precursors are the focal points of this chapter. Numerous references on the fundamental principles of sol-gel processing are available in the literature and within other chapters of this book [10-12]. This chapter is divided into three major sections: (1) metal alkoxide precursor synthesis methods used by the present authors, (2) silicate powder syntheses, and nonsilicate powder syntheses.

II. PRECURSOR SYNTHESIS In this section, only the following synthesis methods are described ammonia, metal/alcohol, metal halide/alcohol, ester exchange, and alcohol exchange. De75

76

MAHETAL..

tailed synthesis techniques for metal alkoxides can be found in Metal ides by Bradley et al. [ 101.

A.

Ammonia Method

The simplest andmost economic method for the large-scale productionof alkoxides involves the addition of anhydrous metal halides to a mixture anhydrous alcohol in a diluent (benzene, n-hexane, or toluene) in the presence of anhydrous ammonia [13,14]:

whereM is many of the transition metals, such as titanium, zirconium, hafnium, tantalum, and niobium, and R is the organic group. The transition metal tetrakis-isopropoxide is readily purified by fractional distillation or recrystallization. The removal of NH4Cl by filtration is always cumbersome and time consuming: the ammonia method may be carried out in the presence of amides or nitriles. For this particular method, the metal alkoxide separates out as the upper layer, and the ammonium chloride remains in the solution of the amide or nitile at the lower layer; thus, the filtration step is eliminated [13,15].

B.

Metal/Alcohol Reaction Method

The alkoxides of some metals interest, such as aluminum, yttrium, and the rare earths, can be made by reaction with alcohol using a reactioncatalyst (e.g., HgC12 and HgI2) [16-181: M + ROH

HgC12 > M(OR), exothermic

+ EH2 2

where n is the valence of metal M, and R is the organic group. Most alkali or alkaline-earth metal alkoxides can be prepared by this reaction without using reaction catalysts.

C.

Metal Halide/AlcoholReaction Method

typical example of this method is the synthesis con [19]: S i c 4 + 4ROH

exothermic

various alkoxides of sili-

'Si(OR)4 + 4HC1

where R is the ethyl, n-propyl, isopropyl, and

on, group.

MULTICOMPONENT CERAMIC POWDERS

D.

EsterExchangeReaction

valuable method for converting one alkoxide to another is an ester exchange reaction [20]. This method is particularly suited for the preparation of the tertiary butoxide from the isopropoxideand r-butyl acetate. The reaction is as follows: M(OR)4 + 4R’OOCCH3 _ _ j M(OR’)4 + 4ROOCCH3 where M is zirconium, titanium, hafnium, and thorium, for example, R is the isopropyl group, and R’ is the tert-butyl group. Since there is a large difference between the boiling points of the esters, the fractionation is simple and quite rapid. Another advantage appears to be the lower rate of oxidation of the esters compared with that of alcohol.

E.AlcoholExchangeReaction Substitution of other branched R groups with the lower straight-chain alcohols has been carried out in an alcohol interchange reaction [lo]:

M(OR)4 + 4R‘OH-

M(OR‘)4 + 4ROH

The distillation of azeotrope drives the reaction to completion. Compounds prepared by this method [21] include the secondary pentoxides, secondary hexoxides, secondary heptoxides, tertiary butoxides, tertiary heptoxides of transition metals, such as yttrium, and also rare earth elements. Increasing the molecular weight and branching of the attached organic group result in the rise in the melting point and stability toward hydrolysis and thermal decomposition. These characteristics are very important in controlling the hydrolysis of multicomponent mixed alkoxides.

ALKOXY-DERIVEDCERAMICPOWDERS The oxide ceramic powders produced through mixed-metal alkoxides have many advantages over powders prepared conventionally; some of the advantages are lower temperature processes, higher purity, more homogeneous distribution of constituents, finer particle size, and easier compositional alteration. Most metal alkoxides exposed to moisture and/or heat cause decomposition of the alkoxide and thus provide forming methods for fine ceramic powders (notable examples for decomposition are thermal decomposition and hydrolytic decomposition). Mazdiyasni[22,23] reported the direct pyrolysis of the metal alkoxides, which form very fine ceramic powders. The overall thermal decomposition of a liquid zirconium tertiary butoxide to ZrO2 is as follows: Zr(OR)4

*

ZrO2 + 2ROH + olefin

MAH ET AL.

78

The thermal decomposition method is very rapid, and the products formed are volatile olefins, alcohols, and fine ceramic powders. The hydrolytic decomposition of metal alkoxides, with subsequent dehydration, has been used to form many types of fine ceramic powders. The decomposition of the metal-organic is initiated via the hydrolytic reaction of water, with subsequent thermal dehydration of the resulting precipitates. The general decomposition is described as a two-step process:

M(OR), M(OH),

-

+ nH2O +M(OH), + nR(0H) MO,/;! + "nH 2 0 2

hydrolysis

dehydration

The hydrolysis reaction usually occurs at room temperature and dehydration occurs below 600°C. resulting in the formation of very fine ceramic particles, that is, 2-5 [22]. This method has been successfully used to make high-purity submicrometer-sized oxides from several metal alkoxides [23,24]. Focus on multicomponent oxide powder synthesis through the two-step hydrolysis and dehydration of metal alkoxides constitutes the remainder of this chapter. When mixtures of alkoxides are hydrolyzed, the different hydrolysis and polycondensation rates of each individual alkoxide can cause complications. These complications may lead to local inhomogeneities. If one species is hydrolyzed faster than the other, which is almost always the case, then differential precipitation occurs during hydrolysis, leaving the other species substantially unreacted. Examples of these are found in the following sections of this chapter. The classes of ceramic powder are divided into two categories (silicates and nonsilicates) and are discussed separately in the following sections.

Silicate Within the category of silicate powders there are two different varieties: glass and/or glass-ceramics and (2) crystalline. The first variety includes various compositions of glasses and glass-ceramics (e.g., lithium aluminosilicate and magnesium aluminosilicate), and mullite and zircon are typical examples of silicates that belong to the latter variety. 1. Multicomponent Silicate Glass and Glass-Ceramics Hydrolyses of multicomponent metal alkoxides were reported by Dislich [25,26] for silicate glass and by the present authors [27,28] for glass-ceramics. Dislich [25] synthesized the eight-component glass through a complexation of mixed alkoxides, followed by hydrolysis andcondensation. The processing flowchart for the synthesis of magnesium aluminosilicate (MAS) (with Li, Zr, and Nb) glass-ceramic oxide powder, hydroxides, and their mixtures for ce-


79

ramic matrix composite matrix material is shown in Fig. 1 [27]. Synthesis of the individual alkoxides was accomplished using one of the appropriate techniques previously described. The individual alkoxides combined into proportions to yield the desired oxide composition. Solid alkoxides, such as Mg(OC2H5)2 and AI(OC3H7)3,are mutually soluble in liquid alkoxide, such as Si(OCzH5)4. As the metal alkoxides are combined together and refluxed, the possibility exists that some of the alkoxides may form complex mutimetal-organic ligands distributed evenly throughout the solution. Within these mixedmetal alkoxides, the formation of double alkoxide between Mg(OCzH5)z and AI(OC3H7)3 is a strong possibility. In fact, the existence of the double alkoxide, MgA12(0R)g, is well known [10,29], indicating that the distribution of the multiple-metal hydroxides, in the solution after hydrolysis, may not be completely uniform at the molecular scale as a result of the preferential hydrolysis of some metal alkoxides and/or metal-alkoxide complexes. Although it has been documented [30] that the MgA12(OR)g double alkoxide does not break down to its constituents during hydrolysis, the resulting multicomponent metal hydroxides should have, at minimum, a uniform compositional distribution.

2. Multicomponent Crystalline Silicates There are a vast number of publications about mullite (3A1203*2Si02), ranging from synthesis and processing to its applications andproperties [31,32]. The interest and importance of mullite as a candidate material for various applications has been manifest by the number of symposiums on the topic. Hy-

1

Processingflowchart for thesynthesis of magnesiumaluminosilicate.

MAH ET AL.. drolytic decomposition of the mixed-metal alkoxide route to synthesize stoichiometric mullite powder was carried out by one of the present authors approximatelytwo decades ago [33].Aluminum tris-isopropoxide and silicon tetrakis-isopropoxide were synthesized. The mixed alkoxides were refluxed in excess isopropyl alcohol for 16 h before hydrolysis ensure thorough mixing. The hydroxyaluminosilicate was prepared by slowly adding the alkoxide solution to ammoniated triply distilled deionized water according to the reaction 6Al(OC3H7)3 + 2Si(OC3H7)4 + xH20 H20

+

NH3

> 2A13Si(OH)l3 .xHz0 + 26C3H7OH

The resulting hydroxyaluminosilicate was repeatedly washedwith propyl alcohol and dried in vacuum at 60°C for 16 h: vacuum 2A13Si(OH)13 F 3&03 60°C

*

dry iso-

2Si02 + 13H20

At this stage of preparation, the mixed oxide was amorphous to x-ray diffraction. Transmission electron microscopy ("EM) photomicrographs of the as-prepared powders, calcined at 600°C statically for 1 and 24 h and dynamically for 24 h, are shown in Fig. 2. In Fig. 2a, needlelike crystallites of the very fine asprepared particulates are evident. The crystallite growth (agglomeration of small particles followed by rapid growth) occurs during the calcination process, leading to very large but well-defined acicular or prismatic particulates [(Fig. 2b-d)l. The as-prepared powders are extremely active, with a surface area of -550 m2/g; however, the surface area is reduced to 280 m2/g when the powders are calcined at 600°C for 1 h. High-temperature x-ray diffraction (m) studies showed that the crystallization of stoichiometric mullite took place between 1185 and 1200°C. However, the previously mentioned hydrolytic reaction is very sensitive to experimental conditions, such as the presence acid or base catalysts, reaction temperature, and molar ratio of alkoxides to H2O. Three critical experimental parameters (pH, temperature, and reaction time) that affected the rate of hydrolysis were systematically studied by Paulick et al. (34). The volume ratio of 100 ml mixed alkoxide [Al(OC3H7)3 and tetraethylorthosilicate (TEOS)] solution to 500 m1of an H20/CH30H (3:l ratio) solution was used for all the experiments. It was found that in an acid environment (pH 2) the hydrolytic decomposition was completed within 7 h at room temperature. Stoichiometric mullite was the only product obtained, as evidenced by both elemental analysis and x-ray diffraction. Under alkaline conditions (pH lo), the degree of hydrolytic decomposition was both time and temperature dependent. The TEOS was only partially hydrolyzed even after heating at 70°C for 20 h. The oxide powder obtained was composed of 65% A1203 and 35% mullite. The degree of hydrolysis is only moderately affected by temperature, and it is not affected by the reaction time.


81

The difference between acid-versus base-catalyzed hydrolysis of mixed TEOS and Al(OC3H7)3 can be accounted for by the different reaction mechanisms of TEOS hydrolysis [l l]. Nucleophilic hydrolysis and condensation in alkaline solutions tend to produce highly cross-linked species, which may not be completely hydrolyzed. In contrast, in acid solution, the electrophilic reaction mechanism favors the production of weakly cross-linked species that tend to be completely hydrolyzed The specific surface area the as-calcined mullite powder obtained under acid hydrolysis conditions is m2/g. Clusters of particles develop during concentration of the hydroxide slurry. These particles polymerize on further drying, causing bound -OHgroups in the clusters, thereby resulting in hard agglomerates instead of fine powder. One can make a compromise through the partial hydrolysis of TEOS and then add AI(OC3H7)3 to accomplish complete hydrolysis and yet achieve fine powder sizes.

Nonsilicate few varieties of nonsilicate oxide ceramic powderssynthesizedthrough alkoxide processing are presented here. These oxide powders were developed for the use of electronic, optical, and high-temperature structural applications. For each material, we with a brief description of the synthesis, which is followed by powder characteristics (e.g., particle sizes, morphologies, and size distributions) and densification behavior andsome properties of dense material. 1. Spinel The precursor alkoxide syntheses for spinel (MgA1204) were carried out through the metal/alcohol reaction methodusing HgC12 as acatalyst. This methodwas described in the previous section. The two metal alkoxides, Al(OC3H7)3 and Mg(OC2H5)2, were mixed together and gently heated in isopropyl alcohol until solid Mg(OC2H5)2 was completely dissolved and reacted, producing a clear solution, and then allowed to cool to room temperature During the refluxing process, magnesium and aluminum double alkoxide was formed, The formation and structural characterization of the MgA12(0R)8 douThe double alkoxide was ble alkoxide can be found in references readily hydrolyzed by water, without acid or base catalyst. No differential hydrolytic decomposition mixed alkoxide was found in this system to hinder the formation of ultrafine homogenous spinel powder. The XRD analysis (Fig. 3) of the ultrafine powder, obtained after calcination at 550"C, shows amorphous characteristics. The powder was calcined at 1000°C for 1 h, and XRD analysis showed broad peaks of spinel, which is a characteristic diffraction pattern for fine particles. Distinct, sharp XRD peaks of spinel can be observed after the powder is hot pressed at 1550°C. Elemental analysis the powder calcined at 550°C gave a 1:l MgO/A1203 ratio. Emission spectrographic analysis of the as-calcined powder indicated that no measurable impurities were pre-

2 Transmissionelectronphotomicrographs of mullitepowders as-prepared, (b) calcined at for 1 h, (c) calcined for 24 h with tumbling, and (d) calcined for 24 h without tumbling.


83

MAH ET AL.


85

sent. The surface area of the as-calcined powder was 260 m2/g, which corresponds to an average particle size of 6.5 nm. The transmission electron photomicrograph of the as-calcined powder (Fig. 4) showed the particle size to be of the order of 10 nm, which is in agreement with the calculated value. The average particle size of the powder, heat treated at 1000°C for 1 h, increased to about 30 nm, and also the particles were faceted. 2. Electronic Ceramics A few multicomponent electronic ceramics, fabricated through metal alkoxide processing, are described in this section. PZT PLZT. The development of polycrystalline lead lanthanum zirconate-titanate (PLZT) electronic ceramic monoliths, which fully transmit incident light, requires methods for controlling stoichiometry, impurity content, porosity, grain size, and so on. Alkoxy-derived PLZT powders were prepared by hydrolytic decomposition of mixed-metal alkoxides [40]. The Zrand Ti alkoxides were synthesized by the ammonia method, and the lanthanum isopropoxide was synthesized by themetal/alcoholreaction method, which were described in the previous section. The Pb alkoxide was prepared by the reaction of anhydrous lead acetate, Pb(C2H302)2, with sodium isoamyloxide, NaOCsHl I [40]: Pb(C2H302)2 + 2NaOR

-

Pb(OR)2 + 2NaC2H302

where R is the isoamyl group. The solvated lead alkoxide solution was filtered to a clear, colorless solution. PLZT powder is typically prepared by dissolving the mixed-metal alkoxides in a mutual solvent, such as isoamyl alcohol, followed by hydrolytic decomposition to yield a nominal zirconate-titanate molar ratio of 65:35 containing 10atom%La. The preferred general formula of Pb1-~Lax(Zr,Ti~)1-~/403 is referred to in the literature[41,42]. The resulting hydroxide was washed repeatedly with high-purity water and then with isopropanol. The washed hydroxide was dried undervacuum at 60°C toyield white amorphous powder. The as-preparedPLZTpowderswere calcined at 500°C for 30 minutes to 1 h and then ground in a B4C mortar to break the large agglomerates. TEM photomicrographs of the powder calcined at 500°C for 15 min~tesand 1 h is shown in Fig. 5. The cubic symmetry of the material is quite evident; the electron diffraction patterns indicate a cubic structure, which is in agreement with the X R D analysis. Because of the rapid outward diffusion of PbO in this system, calcination at a higher temperature and for a longer time at 500°C invariably resulted in massive agglomeration or partial sintering of the particle. Thermograms (TG) for the as-prepared PLZT powders are shown in Fig. 6. The TG of the powders, unwashed and washed withisopropyl alcohol, was run inan ambient atmosphere from room temperature to 1000°C. Both types of

86

4 Transmission electron micrographs h. (a) 550°C and (b) 1000°C inair

MAHETAL..

dispersed spinel powder calcined at

5 Transmissionelectronmicrographs 500°C for (a) 15 minutes and (b)

of PLZT particulatescalcinedat

88

MAHETAL.

powder showed an initial 5% weight loss from 70 to 11O"C, caused mostly by the loss of alcohol. The additional weight loss observed within this temperature range is attributed to the loss of surface-absorbed water. The isopropanol washing was effective in removing this water, as evidenced by a difference of -15% in the weight loss in this region. Continued weight loss occurs as the temperature is increased and the remaining carbonaceous material is removed. The differential thermal analysis (DTA) curve for the as-prepared PLZT powder is also included in Fig. 6. Dehydration of the powder results in the endotherm observed at 110°C. The exothermic peak at -340°C is attributed to the nucleation and growth of the very fine as-prepared particles to large crystallites. No further peaks were observed while heating to 1200°C. When the ple was cooled and rerun, no additional peaks were noted, indicating retention the crystalline phase as formed. The microstructure, typical a PLZT body prepared by cold pressing and sinteringunderthestated experimental conditions, is showninFig.7a;the compact was thermally etched at 800°C for 30 minutes. The fine-grained microstructure is quite uniform, with internal and grain boundary porosity virtually nonexistent. Bodies fabricated similarly but sintered at higher temperatures

30

r

I

252 5.

-

200 . 2 v)

_"

-

I

!2

I

/ /

K L IO O -

"_"".

L

Bzz

S

O

; 0 2 b o 2('c) ;0&&4bosso I

TG

of

PLZT

TG and DTA of akoxy-derived PLZT powder.

I


89

(W 7 Scanning electron microscopy (SEM):typical microstructure PLZT prepared by cold pressing and sintering at (a) 1120OC for 8 h and (b) 1220°C for 4 h.

MAHETAL.

90

( -2 nm and highly repulsive at smaller separations. More recently, Lange and coworkers showed that a short-range repulsive potential can be also developed on the surfaces of A1203 particles. When sufficient salt was added to the slurry, it was noted that the dispersed suspension (as a result of long-range electrostatic repulsion at a low pH) was altered to what superficially appeared to be a coagulated network. Unlike a flocculated network produced by changing the pH to the PZC,the.particles in this attractive network (coagulated network) were easily arranged, as deduced from the rheological data. This suggests that the particles slip into the potential energy well of shallow depth to produce an attractive but nontouching particle network called a coagulated network, as schematically shown in Fig. They further demonstrated that the mixed A1203/ZrO2 green bodies formed from welldispersed suspensions exhibited significant phase segregation, whereas those formed with coagulated slumes under the presence of the short-range hydration potential exhibited uniform microstructures throughout. Since the control of rheological properties (e.g., shear thinning behavior with a suitable viscosity value) and the minimization of phase segregation (differential settling) are very important in the fabrication of ceramic composites from slumes, the processing scheme based on the weakly coagulated slurry seems to be a promising potential approach. Similarly, in the fabrication of a ceramic composite, Bleier and coworkers found that the uniformity of the relative phase distribution can be greatly improved when the potentials of the unlike particles are of the same sign but differ greatly inmagnitude. For example, in the fabrication of an

c

COLLOID INTERFACE SClENCE

189

~203-Zxfkcomposite using a colloidal processing route, the unlike particles (i.e., A1203 and ZrO2) experience a weak, mutual association in the pH range However, when they are more similar electrostatically (e.g., in magnitude of potential) at a lower pH, they do not mutually associate and the spatial uniformity of the ZrO2 phase in the resulting green body and sintered composite suffers They concluded that the observed phenomena are associated with a weak attractive interaction in the secondary minimum in the potential energy curve (Fig. and suggested that the secondary minimum for the interaction between the dissimilar oxide particles is a useful, quantitative guide for predicting conditions under which a homogeneous distribution of constitutive phases (also without extensive agglomeration) can be obtained. Since they also emphasized a weak attractive interaction between the nontouching particles in a shallow potential energy well, their approach very much resembles the previously discussed suspension processing that utilizes the concept of the short-range repulsive hydration potential. eliminate fundamentally problems associated with phase segregation and agglomeration in the fabrication of ceramic composite, Jang and Moon proposed another colloidal processing scheme in which a thermodynamic theory of interfacial electrochemical phenomena is applied. In this approach, a kinetically stable colloidal suspension for the matrix phase (after sintering) is first prepared. Then, precursor precipitates (e.g., zirconium hydroxide ina Al203-ZrO2 composite) for the dispersed phase are individually coated on the matrix-phase colloid particles. The selective, interface-induced nanometer (or submicrometer) scale coating of the precursor on the matrix-phase particle was termed surface-induced coating. In the fabrication of zirconia-toughened alumina (ZTA), for example, one should carefully examine the following three points to set up optimal conditions for the surface-induced coating: (1) the pH range for the formation of kinetically stable A1203 dispersion, (2) the pH range for the enhanced interfacial concentration of ionic species (e.g., zirconium cation and hydroxide anion) needed for a selective formation of the precursor phase for ZrO2 only at the interface; (3) the pH range in which the other two conditions overlap each other. The enhanced interfacial concentration of the ionic species for the surfaceinduced coating on the matrix phase particle was derived based on the interface electrochemical equilibrium and can be expressed as

c

where i can be either zirconium cation or hydroxide anion (for the fabrication of ZTA), is the concentration of the ionic species at a distance r from the charged A1203 surface, Ci(m) is the corresponding bulk concentration, and y ( r ) is the mean coulombic potential at r. @so~vrefers to the change in the sol-

190

JANG

vation energy of ionic species upon adsorption, and denotes any possible short-range chemical interaction free energy (or any free energy change other than the electrostatic and the solvation free energy). A semiquantitive calculation based on the preceding equation clearly indicated that the charged A1203 interface, under suitable conditions can induce a significant interfacial enrichment of both hydroxide and zirconiumions and thus provides a thermodynamic basis for selective surface-induced nucleation at the charged colloid interface. Based on the preceding proposition, homogeneous Al203-ZrO2 composite powder was fabricated by the surface-induced coating of the ultrafine precursor on the kinetically stable colloid particles of A1203 [58]. The composite prepared by this processing scheme was characterized by a uniform spatial distribution of the dispersed ZrO2 phase and by the absence of large ZrO2 grains formed fromhard ZrO2 agglomerates. The composite also showedahighly uniform grain size distribution ofboth the dispersed ZrO2andthe matrix A1203 phases. The uniform grain size distribution of the matrix phase indicates that the homogeneous coating of the ultrafine precursor particles effectively pins the A1203 grain boundaries. More recently, this processing scheme was also applied to the fabrication of Al203-ZrO2-SiCwhisker composites with uniform microstructures and large values of Klc (-12.5IMPa-m'R). The concept on which the surface-induced coating route is based is illustrated in Fig. 15 using the A1203-ZrO2-SiC whisker system as an example. Sacks and coworkers [60,61] proposed another novel approach for enhancing densification and controlling microstructure development of multicomponent ceramics and ceramic-ceramic composites based on the processing of composite particles via a sol-coating route. In this method, they prepared cores crystalline materials (A1203, Si3N4, Sic, and ZrO2) with outer coatings of amorphous silica. Silica coatings were made using the hydrolysis-condensation route of silicon alkoxides (e.g., tetraethylorthosilicate).Before deposition of the coating, the core powders were fractionated using colloidal size separation. Powder compacts prepared with composite particles showed enhanced densification caused by viscous flow of the amorphous coating and the increased threshold concentration for percolation (connected network) of the crystalline phase. An example is illustrated in Fig. 16, in which the core is alumina and the coating is silica. As shown schematically in Fig. 16, if the size of the composite particles is selected properly, this composite structure should viscously sinter at a relatively low temperature to form an alumina/siliceous glass composite. At a higher temperature, the core and coating would react to form mullite. This process was referred to as transient viscous sintering. The amorphous sol-coating approach was also successfullyused to fabricate mullite-matrix composites [60,61] by mixing the silica-coated alumina particles with other silica-coated particles (e.g., zirconia particles for mullite/zirconia composite and S i c particles for mullite/SiC composite).

I91

COLLOID INTERFACE SCIENCE Kinetically Stable and S i c 0 Colloidal Suspensions

Unifonn. Mixed Colloidal Suspension of Al&'SiCoN)

with Submicron Particles attached to Sic whisker via Heterointeraction

Nanometer Scale Surface-induced Coating of the Precursor for the Dispersed Phase

15 Homogeneousfabrication of an A1203-Zr02-SiCwhiskercomposite by surface-induced coating. The preparation of a kinetically stable mixed suspensionand the subsequent compression the electrical double layer, while maintaining the surface potentials near constant values, proved to be a useful strategy for the fabrica-

I92

JANG

-

+

+

___)

3Al~O,*2SQ(M~llit0)

16 Transient viscous sintering of mullite using amorphous silica-coated alumina particles.

tion of ceramic composites. For example, based on the electrokinetic properties of aqueous silica, boehmite, and zro;? dispersions, cordierite-Za composites were fabricated by a mixed colloidal processing route In this method, stable boehmite, silica, and Zro.2 suspensions were f i s t prepared separately and then mixed while continuously stirring at pH at which the 6 potential of the mixed suspension exhibits its maximum. The mixed suspension was quite stable mV). The addition of Mg(N@)2 solution to the mixed suspension then increased the ionic strength and caused it to gel quickly by the compression of the electrical double layer (decrease in the Debye-Hiickel length without significantly decreasing vo).This essentially eliminated the problems associated with the phase segregation (or differential settling) and produced a composite powder with a uniform spatial distribution of constitutive phases. The fabricated composite was characterized by a dense and homogeneous microstructure andbyauniform spatial distribution ofsubmicrometer-sized tetragonal Zro;! particles throughout the matrix This processing scheme was also successfully applied to the fabrication of S i c whisker-reinforced lithium aluminosilicate matrix composites

-

REFERENCES 1. Parsons, R.,Modern Aspects of Electrochemistry, Vol. 1 (J. 0”. Bockris and B. E. Conway, eds.), Butterworths,London, 1954, p. 103. 2. Smith, A. L., Electrical phenomena associated with the solid-liquid interface, Dispersion of Powders inLiquids (G.D. ParFtt, ed.),AppliedScience,London, 1985, p. 99. 3.Modi, H. andFuerstenau, D. W., Streamingpotentialstudies on corundum (a-Al203) in aqueous solutions of inorganic electrolytes,J. Phys. Chem., 61, (1957). 4. Parks, G. A., Theisoelectricpoints of solid oxides, solid hydroxides, andaqueous hydroxo complex system, Chem. Rev., 65, 117 (1965).

193

SCIENCE COLLOID INTERFACE Parks, G. A., and de Bruyn, P.

L., The zero point of charge of oxides, J. Phys.

Chem., 66,

Grahame, D. C., The electrical double layer and the theory of electrocapillarity, Chem. Rev., 41,

Loeb,A. L., Wiersema,P.H.,andOverbeek,J.T. G. The Electrical Double Layer Around a Spherical Colloid Particle, M.I.T. Press, Cambridge, MA, Levine, Mingins, J., and Bell, G. M., The discrete-ion effect in ionic doublelayer theory, J . Electroanal. Chem., 13, Wiese, G. R., James, R. 0.. and Healy, T. W., Discreteness of charge and solvation effects in cation adsorptionat the oxide/water interface,Disc. Faraday Soc., 52,

Yates, D. E., The structure of the oxide/aqueous electrolyte interface, Ph. D. Thesis, University of Melbourne, Australia, Hunter, R. J., Zeta Potential in Colloid ScienceLPrinciples and Applications, Academic Press, New York, Chaps. and Babchin,A. J., Chow, R. and Sawatzky, R. P., Electrokinetic measurements by electroacoustical methods, Adv. Colloid Interfac. Sci., 30, Overbeek, J. Th.G., The interaction between colloid particles, in Colloid Science (H. R. Kruyt, ed.), Elsevier, Amsterdam, p. BCruM, Y. G., and de Bruyn, P. L., Adsorption at the rutile-solution interface. I. thermodynamic and experimental study, J. Colloid Interfac. Sci., 27, Hunter, R. J., and Wright, H. J. L., Dependence of electrokinetic potential on concentration of electrolyte, J . Colloid Interjac. Sci., 37, Levine, and Smith, L., Theory of the differential capacity of the oxide/aqueelectrolyte interface, Disc. Faraday Soc., 52, Jang, H. M.,andFuerstenau,D.W.,Thespecificadsorptionofalkaline-earth cations at the rutile/water interface, Colloids Surfaces, 21, Bolt, G. H., Determination of the charge density of silica sols, J . Phys. Chem., 61,

Abendroth, R. P., Behavior of a pyrogenic silica in simple electrolytes, J. Colloid Interfac. Sci., 34,

20. Jang, H. M., The natureof counter-ion adsorption at the oxide/water interface, Ph. D. Thesis, University of California, Berkeley, Yates, D. E., Levine, and Healy, T. W., Site-binding model of the electrical double layer at the oxide-water interface, J . Chem. Soc. Faraday Trans. I , 70, Davis, J. A., James, R. O., and Leckie, J. O., Surface ionization and complexation at the oxide/water interface, J. Colloid Interfac. Sci., 63, Houchin, M. R., and Warren, L. J., Surface titrations and electrokinetic measurements on stannic oxide suspensions, J. Colloid Interfac. Sci., 100, Jang, H. M., and Fuerstenau, D. W., The nature of simple monovalent cation-silica interaction as reflected in the spin-lattice relaxation time of =Na, Langmuir, 3,

Bull, T. E., Nuclear magnetic relaxation of spin-312 nuclei involved in chemical exchange, J. Magn. Reson.,

I94

JANG Barringer,E. A., andBowen, H. K., Formation,packing,andsinteringof monodisperse Ti02 powders, J. Am. Ceram. Soc., Bamnger, E. A., Jubb, N., Fegley, B., Pober, R. L., and Bowen, H. K., Processing monosized powders, in Ultrastructure Processing of Ceramics, Glasses, and Composites (L. L. Hench and D. R. Ulrich, eds.), Wiley,New York, p. ner, R. K.,Inorganic colloids for forming ultrastructures, in Science of Ceramic Chemical Processing (L. L. Hench and D.R. Ulrich, eds.), Wiley, New York, p. Aksay, I. A., and Kikuchi, R., Structuresof colloidal solids, inScience of Ceramic Chemical Processing (L. L. Hench and D. R. Ulrich, eds.), Wiley, New York, p. Overbeek, J. T. G., Recent developmentsin the understanding of colloid stability, J. Colloid Interfac. Sci., 58, Hiemenz, P. C., Principles of Colloid andSurface Chemistry, 2nd ed., Marcel Dekker, New York, Chap. Gregory, J. The calculation of Hamaker constants, Adv. Colloid Interfac. Sci., 2, Visser, J., On Hamaker constants: A comparison between Hamaker constants and Lifshitz-van der Waals constants, Adv. Colloid Interfac. Sci., 3, Stol, R. J., and de Bruyn, P. L., Thermodynamic stabilization of colloids, J. Colloid Interfac. Sci., 75(1), Wiese, G. R., and Healy, T. W.,'Coagulation and electrokinetic behavior of Ti02 and A1203 colloidal dispersions, J . Colloid Interfac. Sci., 51, Reerink, H., and Overbeek, J. T. G., The rate of coagulation as a measure of the stability of silver iodide sols, Disc. Faraday Soc., 18, Wiese, G.R.,andHealy, T. W., Effect of particle size oncolloidalstability, J. Chem. Soc. Trans. Faraday Soc., Jang, H. M., and Lee, K. G., Effects of kinetic stability of colloidal dispersion on thegreenmicrostructureanddensificationbehavior of zirconia, in Ceramic Transactions Vol. 12, Ceram. Powder Science 111 (G. L. Messing, Hirano, and H. Hausner, eds.), Am. Ceram. Westerville, OH, p. Aksay,I.A.,andSchilling,C.H.,Colloidalfiltrationroutetouniform microstructures, in Ultrastructure Processing of Ceramics, Glasses,and Composites (L. L. Hench and D. R. Ulrich, eds.), Wiley, New York, p. Shih, W. Y., Aksay, I. A., and Kikuchi, R., Reversible-growth model: Clusterclusteraggregationwith finite bindingenergies, Phys. Rev. A, Zhao, J., and Harmer, M. P., Effect of pore distribution on microstructure development II. First- and second-generation pores, J. Am. Ceram. Soc.,

I

Yeh, T.-S.,and Sacks, M.D., Low-temperaturesintering of aluminumoxide, J. Am. Ceram. Soc., H., Ota, K., and Saito, H., Preparation cordierite ceramics from metal alkoxides (Part II). Sintering, Yogyo-kyokai-shi, Jang, H.M.,and Lee, H., Interfacial characteristics and colloidal stability of cordierite dispersion, Langmuir,

COLLOID INTERFACE SCIENCE

1

Napper, D. H., Polymeric Stabilization of Colloidal Dispersions, Academic Press, New York, Chapts. and Takahashi, A., and Nagasawa, M., Excluded volume of polyelectrolyte, J . Am. Chem. Kumagai, M., and Messing, G. L., Controlled transformation and sintering of a boehmite sol-gel by a-alumina seeding, J . Am. Ceram. Soc., Messing, G. L., Kumagai, M., Shelleman, R.A.,andMcArdle,J.L.,Seeded transformations for microstructuralcontrol in ceramics, Science of Ceramic Chemical Processing (L. L. Hench and D.R. Ulrich, eds.), Wiley, New York, p. Roy, R. A., and Roy, R., New metal-ceramic hybrid xerogels, in Abstracts Ann. Meeting Mater. Res. Boston, MA, p. Roy, R., Ceramics by the solution-sol-gel route, Science, Sacks, M. D., Lee H.-W., and Rojas, 0. E., Suspension processing of A1203/SiC whisker composites, J. Am. Ceram. Lee,H.-W.,and Sacks, M.D., Pressureless sintering of Sic-whisker-reinforced A1203 composites. I. Effect of matrix powder surface area, J . Am. Ceram. Lange, F. F. New interparticle potential paradigm for advanced powder processing, in Ceramics Transactions, Vol. 22, Ceramics Powder Science N (S.-I. Hirano, G. L. Messing, and H. Hausner, eds.), Am. Ceram. Westerville, OH, p. Pashley, R.M., DLVO and hydration forces between mica surfaces in LP, Na', K+,and CS+ electrolyte solutions: Correlation double-layer and hydrationforces with surface cation exchange properties, J . Colloid Znterfac. Sci., Velamakanni, B. V., Chang, J. C., Lange, F. F., and Pearson, D. New method for efficient particle packing via modulation of repulsive lubricating hydration forces, Lungmuir, Baik, Bleier, A., and Becher, P. F., Preparation of A1203-zfl2 composites by adjustment of surface chemical behavior, Mater. Res. Symp. Proc., Vol. Better Ceramics Through Chemistry (C.J. Brinker, D. E. Clark, and D. R. Ulrich, eds.), Mater. Res. Soc., Pittsburgh, PA, p. Bleier, A., and Westmoreland, C. G., Effects of pH and particle size on the processing of and the development of microstructurein alumina-zirconia composites, J . Am. Ceram. Soc., Jang, H.M., andMoon, J. H.,Homogeneousfabricationanddensification of zirconia-toughenedalumina(ZTA)composite by thesurface-inducedcoating, J . Mater. Res., Jang, H. M., Moon, J. H., and Jang, C. W., Homogeneous fabrication of Al2O3ZrOz-Sic whisker composite by the surface-induced coating, J . Am. Ceram. Soc., Sacks, M. D., Bozkurt, N., and Scheiffele, G. W., Fabrication mullite and mullite-matrix compositesby transient viscous sintering of composite powders, J . Am. Ceram. Soc., Sacks, M. D., Scheiffele, G. W., Bozkurt, N., and Raghunathan, R., Fabrication of ceramics and composites by viscous sintering of composite particles, Ceram-

JANG ics Transactions, Vol. 22, Ceramics Powder ScienceN (S.-I. Hirano, G. L. Messing, and H. Hausner, eds.), Am. Ceram. Westerville, OH, p. Lim, B. C., andJang, H.M.,Homogeneousfabricationanddensificationof cordierite-zirconiacomposites by amixedcolloidalprocessingroute, J . Am.

Ceram. Soc., Jang, H. M., Kim, K. and Jung, C. J., Development of Sic-whisker-reinforced lithium aluminosilicate matrix composites by a mixed colloidal processing route, J . Am. Ceram. Soc.,

Ceramic Particles,in Nonaqueous Media Lee Clemson University Clemson, Carolina

INTRODUCTION Properties of final ceramic products made from fine powders are often dependent on the state of dispersion of the paiticles in the liquid media. The particles interact with each other and with what is in the medium. Some of the liquid molecules can adsorb onto the solid surface. Some solid surfaces can be ionized to exhibit surface charges. Ceramic particles in organic liquid media behave differently from those in aqueous media. Many known ceramic-processing techniques involve fine particles dispersed in nonaqueous media. Some of the better known examples are tape casting of barium titanate and aluminum nitride powders and tape or slip casting, extrusion, and injection molding of ceramic high Tc superconductor powders. The theories of colloidal stability used for aqueous slurry systems may also apply to nonaqueous systems. According to the Derjaguin equation [l], the repulsive energy can be written as V, = 2 m y d In(l+ e-KH)

(1)

where: VR= electrostatic repulsive energy between two like particles E = dielectric constant of medium a = particle radius 197

LEE = electrical potential at the diffuse layer

H = interparticle distance K

= Debye-Huckel parameter

The attractive energy, on the other hand, is given by

vA -"12H

where: VA = attractive energy between two identical particles = Hamaker constant

The repulsive energy between two particles of size in a medium is a function of the dielectric constant of the liquid medium. This means a smaller VR for media of organic solvents with smaller than that of water. The dielectric constants of some common liquids are listed in Table 1. The Debye-Hiickel parameter K in (1) also depends on by

where:

e = electron change NA = Avogadro's number C = electrolyte concentration 2 = charge of the ionic species

.

The double-layer thickness 1 / is ~ therefore larger for larger E. However, the ionic strength, C in nonaqueous media is much smaller than that in aqueous media, hence larger VK. The attractive interaction depends largely on the Hamaker constant A, as shown in Eq. (2). The larger the value of A, the greater is the attractive energy between the particles. The net interaction energy is then the sum of VR and VA. Equations (1) and show that a better dispersion of fine particles must come from systems having large and small Fowkes and Lee and Rives [2,3] showed that ion formation in a nonaqueous medium is appreciably higher than predicted by Eq. (1). Steric stabilization of colloidal particles is achieved by adsorption of long-chain molecules on solid surfaces For an effective steric stability by the adsorbed molecules, there are several basic requirements: (1) long chain of the adsorbate molecule, (2) strong adsorption of the adsorbate on the solid particle surface, and (3) good solubility of the chain in themedium Regarding the chain length of a steric stabilizer, there is the question how

CERAMIC PARTlCLES IN NONAQUEOUS MEDIA Table

199

RelevantPhysicalProperties of Solvents

Dielectric Solvent Chloroform Ethanol Methylene chloride Water Tetrahydrofuran Acetone Ethyl acetate Toluene

Viscositya (ea-s) 0.58 l .2 0.45 1 .o 0.55 0.32 0.46 0.59

Ammd

Acidlbasec HPCPb constant 4.8 25 9.1 78.3 7.4 21.5 6.4 2.4

aAt 20°C. bHydrogen bonding cohesion parameter. cBased on Drago’s C and E values [K!], (strong) dcalculated Hamaker constants [5]. Source: Refs. 15 and 16.

(J

6.3 20 9.6 40.4 7.4 6.7 11

8.9 1.6

10-20)

A2 A3

7.6 6.4 7.4

B1 B2 B3 B4

6.6 6.7 8.4

4 (weak).

long the chain must be for effective steric stability. It has been shown by a number experimentalists that the chain length need not be so long predicted by the theory Relatively small molecules, such as fatty acids and some surfactants, are widely used as dispersants. Calvert et al. showed that the maximum packing density of silica suspension in hexane was achieved by a silane surfactant with chain length of carbon number 12. chain length longer than 12 did not improve the packing density. Surfactants that produce electrostatic charges on ceramic particles exhibit large changes in the viscosity and the suspension stability. When steric stabilization is combined with electrostatic stability, the results can be remarkable The more additives used in ceramic slurries for processing, the more complex interactions among the components in the system are expected. Better understanding these interactions is necessary to control particle dispersion and processing in nonaqueous media. The interactions are often indirectly assessed by the rheology of the suspension systems, the 6 potentials, and the green body characteristics. This chapter presents some of the results and an explanation of the rheological and related behavior of alumina, silica, and portland cement particles in various organic solvent media.

II.

RHEOLOGY AND ELECTROSTATICCHARGE

The surface alumina in aqueous environment is well characterized [7-lo]. It has amphoteric sites, that is, acidic sites of aluminum and basic sites of hy-

LEE droxyl. However, the isoelectric point and the point of zero charge of pH = 9 indicate that the surface is weakly basic in water. Although basic molecules can adsorb on an alumina surface via the acidic aluminum sites, more effective adsorption of dispersant molecules is expected for the hydroxyl groups via hydrogen bonding. Figure 1 shows the viscosity of vol% alumina suspensions having the mean particle size of 1.5 pm (Alcoa A-152) in three different organic liquids in the presence of wt% linolenic acid (LNA) per alumina. At low shear rates, they all exhibit shear thinning. Ethyl acetate (EtAc) exhibits the highest viscosity, followed by toluene (Tol). LNA, which is an acid, can adsorb on alumina via acid-base interaction or hydrogen bonding. The structural formula of LNA and those of other organic dispersants are given in Fig. 2. The interaction between the solvent and LNA cannot be responsible for the viscosity curves because the most basic solvent, tetrahydrofuran (THF) [1l], would interact with LNA the most. The acid-base interaction is based on Drago's work on Lewis acid-base interaction energies [12]. Fowkes [2,13,14] expanded the concept to ceramic processing. The relative acidity and basicity of solvents used are listed in Table The ceramic powders used are listed in Table 2. The rheogram in Fig. 1 suggests two possibilities: one is the largest amount of LNA adsorption onto an alumina surface in THF and the least in EtAc for steric stability. The other possibility is that the alumina surface is the most highly charged in THF and the least in EtAc for electrostatic stability. Our electrophoretic deposition experiments yielded alumina particles deposition on the positive electrode, which signifies that the alumina surface in THF was

E

>

IO

'

LNA per

0 3.4 6.8 10.2 13.6 20.4 23B (I/S)

Rheogram of alumina.

vol% alumina in threedifferentsolvents

with 5

201

CERAMIC PARTICLES IN NONAQUEOUS MEDIA

Linolenic Acid (LNA), C-18

H

OH €I$ = CH

-C

- 0 - C%CH - q - N - (CHJ,Si(OEt),

N-(3-Acryloxy-2-hydroxy-propyl)-3-aminopropyl~ethoxysilane(AHAS), 15 memberChain

\

“C-

l

x=14.3

2 Chemicalstructure of dispersantmolecules. negatively charged. In EtAc a smaller amount alumina was deposited on the positive electrode. This means that EtAc is weakly basic relative to the surface alumina, and THF is-more strongly basic. Alumina in toluene yielded an even smaller deposit of alumina on the positive electrode than in EtAc. This agrees with the basicity scale Drago shown in Table 1.

Table 2

Characteristics of CeramicPowders Mean particle Surface area alumina) (mVg

Source Powder Alumina A-l52 AKP-15 Silica Portland cement, type I

Alcoa Sumitomo Stiiber Holnam Santee

0.65 0.5 10

1.2 3.8 7.9 0.16

LEE Although toluene and EtAc are both listed as weak bases, this is the relative basicity to the solid surface. Hence, one can conclude that alumina is a weaker acid in toluene and EtAc, but a stronger acid in THF. The electrophoretic deposition of alumina in the presence of 5 wt% LNA per alumina yielded a deposit on the negative electrode in toluene and on the positive electrode in THF and EtAc. Although LNA is an acid (Fig. 2) relative to the alumina surface, the basic media THF and EtAc overrode the acidity of 5 wt% LNA, causing the negative charge on the alumina surface. In toluene, on the other hand, LNA reversed the charge on the alumina surface. This could be possible because the surface charge of alumina in toluene was nearly zero, shown by the very small amount of deposit of alumina particles on the positive electrode in the absence of LNA. Figure 3 sdows viscosities of 60 vol% AKP-l5 alumina suspensions as a function of shear rate in THF and chloroform (Chl), all with 5 wt% LNA. Both solvents in the presence of LNA exhibited that the suspensions are pseudoplastic with Chl as a better solvent than THF for alumina. A strong positive charge of the alumina surface in chlorofomLNA and a strong negative charge in THFLNA were indicated by the electrophoretic deposition. The surface charges and 6 potentials of alumina are determined primarily by the solvent media. As given in Table 1, choloroform has been classified as a strong acid and THF as a strong base, which is supported by the electrophoretic deposition results.

'p

a

E

0

200

Shear Rate (I/sec) 3 Rheogram per AKP-15 alumina.

60 vol% alumina in THF and

with 5

LNA

203

CERAMIC NONAQUEOUS PARTICLES INMEDIA

Figure is a rheogram of 60 vol% A-152 alumina in THF and toluene at low shear rates in the presence of 2.5 wt% per alumina of aminopolyisobutylene (APIB);(Fig. 2). AnAPIB molecule consists of an amine terminating group and cyclic dicarbonyl joined to a long alkyl chain. The amine functional groups are expected to behave as a base. The rheogram shows that toluene is a better solvent than THF for dispersing alumina at very high solids loading. In toluene, the alumina surface was shown to be negatively charged. In THF, it was charged weakly positively, indicated by a small deposit of alumina on the negative electrode. This disagrees with explanation that THF is a strong base in the presence of LNA; the surface charge of alumina is primarily affected by the liquid media, as seen in Fig. In the presence of APIB, however, the charge is reversed. This indicates significantly strong interactions between APIB and the alumina surface. Figure 5 is a rheogram of 60 vol% AKP-15 alumina in chloroform and in THF at higher shear rates in the presence of 5 wt% APIB. Aslightly lower viscosity of the slurry with THF than with chloroform is shown. A-152 alumina in methylene chloride was deposited in large amount on the negative electrode under an applied electrical field. This agrees with the acid nature of methylene chloride regardless of the nature of APIB. A smaller deposit of alumina in THF on the negative electrode indicates that the positive potential on the alumina surface in THF is smaller than the oxitive potentialin methylene chloride. Since chloroform is a stronger acid than methylene chloride [ll], the low viscosity of the alumina slurry in THF must be caused by a mechanism other than electrostatic, that is, steric stabilization. The sedimentation heights of alumina sediments, shown in Fig. indicate that a strong base "IF and a strong acid chloroform are equally effective in stabilizing alumina particles with LNA and with an aminosilane, AHAS (see

I 13.6

(Vs)

per

4 Rheogram of 60 vol% aluminain THF and toluenewith 2.5 wt% APIB alumina.

204

S E

LEE

IO

>

F

I

,O

(m)

5 Rheogram of 60 vol% alumina in THF per

chloroform with

APIB

alumina. -I

I

.3 .-x

20

Fig This is shown by the more compact sediments. Alumina with an photeric surface was stabilized by dispersants and strongly acid or strongly basic solvents by acombined steric and electrostatic mechanism. Hexane, which is a nonpolar solvent with no hydrogen bonding capability, is shown to be a poor solvent for AHAS, which is a highly polar dispersant. Hexane would introduce a low potential as well; hence the stability had to rely solely on the steric stabilization contribution. When the adsorbate AHAS is not highly soluble in the medium, the dispersant is ineffective as a steric stabilizer. Despite the high dielectric constant of EtOH, it is shown to be an ineffective medium with LNA. EtOH as a weakly acid solvent introduced nearly a zero potential

CERAMIC PARTICLES

205

IN NONAQUEOUS MEDIA

on alumina, although the adsorption isotherm showed a large amount of LNA, =20 mg/g Al203, on the alumina surface. Thus, for the most effective stabi-

lization, a strong electrostatic contribution in addition to the steric contribution becomes essential.

Silica The surface of silica is characteristically acidic because of the silanol groups. Figure 7 is a rheogram of 30 vol% silica suspensions in various media. EtOH exhibited the lowest viscosity and EtAc the highest for silica prepared by base hydrolyzing silicon tetraethoxide followed by calcination at 600°C. The potentials measured by electrokinetic sonic amplitude showed very a low negative value of -15 mV for silica in EtOH, as shown in Fig. 8. According to Drago (Table l), EtOH is a Lewis acid and is not expected to ionize the acid silica surface appreciably. The lowest viscosity must then be from a steric contribution by the adsorbed EtOH via hydrogen bonding and/or esterification (silicon ethoxide formation). A weakly basic solvent, EtAc did not adsorb and did not cause the silica surface to be highly charged. Acetone, as a more strongly basic solvent than EtAc, exhibited a lower viscosity than EtOH for silica. As shown in Fig. 8, the negative potential of silica in acetone is relatively high. In the presence of the organic dispersants LNA or AHAS, the viscosity of the 30 vol% silica suspension in acetone (Fig. 9) decreased substantially. This shows that AHAS is the more effective dispersant than LNA for silica in acetone, despite the small potential, less than around -10 mV in Fig. 8. Based on the chemical structure of AHAS given in Fig. 2, AHAS was believed to be a basic dispersant via the amino group, ester group, and ethoxy groups, representing a multifunctional adsorbate. The -OH group may act as an

1. This lastcharacteristic is sometimes encountered with filled systems when the filler concentration is very high, that the corresponding scarcity of a continuous phase makes it difficult for the dispersed particles to slide over one another. The lack of adequate lubrication by the continuous phase produces frictional voids and sets up structures among the particles under constraint. These structures become increasingly resistant to deformation as the applied shear rate is increased: the viscosity increases with increasing shear rate. Fluids in which no deformation occurs until a certain threshold shear stress is applied, in which upon the shear stress z becomes a linear function of shear rate The characteristics of the function are the slope (viscosity) and the shear stress intercept (yield value) The rheological expression for this type of material, known as a Bingham solid, is

241

PLASTIC MOLDING RHEOLOGY AND MIXING ORatenl with

Pdnt

t

with

Point

Shear Rate

Figure

-

Classification of non-Newtonianfluids.(After J.

Reed, Introduction

the Principles of Ceramic Processing, Wiley-Interscience, New York.)

Only when the applied stress is greater than the yield stress do viscous flow and shear deformation become manifest. Fluids that show a decrease in shear stress (hence in apparent viscosity) with time are termed thixotropic and are often observed with ceramic slurries. Rheologists have long believed that all fluids are viscoelastic in behavior. a result, the deformation of any fluid from the imposition of a stress is the sum of an elastic deformation, which is recoverable, and viscous flow, which is not recoverable. For fluids low viscosity at moderate rates of shear, the elastic recovery is extremely rapid and the relaxation time is extremely short. a result, the elastic portion of the deformation is too small to measure, and the fluid is considered simply viscous. When viscoelastic fluids are stressed, some of the energy involved is stored elastically, various parts of the system being deformed into new nonequilibrium positions relative to one another. The remainder is dissipated as heat, various parts of the system flowing into new equilibrium positions relative to one another. The elastic nature ofa fluid is characterized by dynamic mechanical or stress relaxation techniques. Dynamic mechanical (oscillatory) testing is a procedure inwhicha sample is sinusoidally strained and the resultant stress is measured. The she& stress varies with the same frequency as the shear rate

MATSUDDY

242

y but not in phase with it, the phase angle between and y being equal to n/2 for anon-Newtonianfluid. Because the normal stress differences are even functions of the shear rate, they vary at twice the applied frequency; they also suffer different phase shifts. Measuring the amplitudes of the stresses and their phase shifts with respect to y is a standard way of studying viscoelastic effects in non-Newtonian fluids. These measurements do not change with time unless the fluid exhibits thixotropy. In measuring viscoelastic effects with an oscillating plate method, the percentage of strain must be selected carefully, because the strain is proportional to the total angle at which the plate is turning. The larger the angle through which the plate oscillates, the greater is the strain. If the angle is too large, the fluid being measured may undergo internal cracking or segregation.

A.

Viscosity of HighlyFilledFluids

Figure 2 shows the flow curves for concentrated aqueous suspensions of 30 vol% 3Y-zirconia and 70 vol% alumina [l]. Figure 2 illustrates near-Newtonian flow characteristic of suspensions with filler concentrations as high as ~01%.Newtonian flow is generally encountered with highly filled suspensions when the polymeric binder concentration in the suspension is fairly low ( 4 ~01%)and the suspension is highly dispersed (discussed later). Figure 3 shows a gradual decrease in viscosity at different ceramic powder concentrations with

p

120

0 15

0

30

45

75

Bo

105

120

135

150

Shear Rate,

2 nia

curves for concentratedaqueoussuspensions of 30 vol%3Y-zircovol% alumina.

PLASTIC MOLDING RHEOLOGY AND MIXING

* x

4 l00

V/O 55v/O

V/O EEA

SIC + 4 5 V/O

V/O

..

Sic +

V/O

Sic + Sic +

V/O

EEA

V/O EEA

I

l000

Shear Rate (per

Decrease in viscosity at different concentrations of silicon carbide in EEA with increasing shear rate at increasing shear rate at temperature. The flow behavior is typically pseudoplastic. The power-law index n these fluids should be less than The suspension consists ofa silicon carbide powder (Starck and ethylene ethyl acrylate (EEA) polymer ~01%).Figure shows (shear stress)ll2 plotted against (shear rate)ln for vol% silicon carbide in polyethylene at The extrapolated intercept at zero shear rate gives the Bingham yield value .z, The general phenomenon dilatancy was observed by Reynolds in Reynolds observed that a mixture beach sand and water confined in a balloon would dilate if deformed; that is, the total volume the mixture would increase under pressure, causing the liquid level in the balloon to be lowered. In addition to the dilation, such a mixture would exhibit increasing resistance to stirring as the rate stirring was increased, until rupture the pseudosolid material that resulted occurred. A more general meaning the term “dilatant fluid” can be given by extending it to all materials showing increasingly apparent viscosity with an increasing rate shear. Sometimes this is referred to as inverted plasticity to differentiate between volumetric dilation versus no volumetric dilation. Typical dilatant flow curves for two concentrations alumina and paraffin wax are shown in Fig. Reynolds’ approach to dilatancy has some limitations. For example, Reynolds’ approach specifies hexagonal close pack-

" MATSUDDY

N

2m

0

Shear R a t e 1/2 ,

112

4 (Shear stress)ln against(shear rate)ln for 55 vol% siliconcarbidein 45 vol% polyethylene at 150°C.

ing. Such close packing is represented byavoid volume of approximately 28%. Therefore, it is expected that the volume concentration of dispersed, spherical particles must approach 72% for dilatancy to occur. In reality, dilatancy has been observed in fluids with solid concentrations well below 72% (Fig. 5). Besides, the approach does not account for other factors, such as the wetting characteristics of the suspension fluid for the dispersed solid, the pension fluid viscosity, and the effect of particle size of the dispersed solid.

B.

Viscoelastic Behavior of Highly Filled Fluids

Figure 6 shows plots of the storage modulus versus oscillation frequency for samples with S i c concentrations of 50, 57, and ~01%. the solids


I

I

1 1 1 1 .

i

I

I

I

I

I l l l j

Shear Rate,

5 Dilatant fin wax, 37 and 35

curves at 150°C.

concentrations of

alumina and paraf-

loading increases, at lower frequencies the storage modulus increases as a function of shear rate. The S i c filler increases the rigidity of the polymer. The increases in rigidity implies less mobility the macromolecular chains under the influence of the applied shear stress. In a highly concentrated suspension (say above 60 vol%), there is particle-particle interaction analogous to the chain entanglements in polymer solutions, showing a tendency for a rapid decrease in storage modulus that is indicative of nonuniform dispersion and agglomeration. This suggests that the storage modulus decreases because the agglomerates do not absorb much energy during interparticle movement.

C.

Influence of Temperature on Fluid Flow

Temperature has a significant effect on the flow behavior of liquids and slurries, including polymer melts, solutions, and filled suspensions. Viscosity de-

246

MATSUDDY I

109

-

IO8

-

I

I Ill1

I

I

I

I

I

I

V

V

V

V

m

m

m

V

m m Ig,

m

107

-

0

0

V

0

0

b

V

V/O

t

0-

m-

v-

lo5

104L1

-

I

I I 1 1 l l1 l

I

I

Sic +

V/O

5 0 V/O SIC

+

I

I

EEA

50"I/O EEA 57 V/O SIC + 4 3 V/O EEA 6 3 V/O sic + 37 V / O EEA

I I l l10 1

I

I Ill

Frequency,

6 Storage modulus versus oscillating frequency for 40, 50, 57, and 63 vel% silicon carbide with at creases with increasing temperature for both Newtonian and non-Newtonian fluids, primarily because of a decrease in intermolecular forces. The increased flexibility of a polymer backbone chain also contributes to reduce viscosity. The relation between viscosity and the temperature of a filled suspension can be expressed by the modified Eyring equation, which allows for variations in the free volume of the fluid structure:

In this equation h is Planck's constant, No is Avogadro's number, Vm is the molar volume the fluid phase, E is the activation energy needed to overcome the potential barrier between equilibrium positions, is a constant in the range 0.5-1.0, V0 is the van der Waals volume of the molecule, and V is the average volume per molecule in the fluid. Because the exponential form the viscosity equation, mixtures of ceramic powder in thermoplastic binder are only significantly sensitive to temperature change above 70°C In an injection molding operation, it is usual to plot In q versus T for a constant shear rate, not a In q versus 1/T plot at a


247

353 363 373 Temperature(K) cludes

7 Effect of temperature on viscosity at a constant rate of shear of vol% A1203 and vol% ethylene-vinyl acetate. (After Ref. 3).

in-

constant force. An example of one such plot for vol% A1203 loading in vol% ethylene-vinyl acetate copolymer is shown in Fig. 7. The linearity of sucha plot indicates the process sensitivity to temperature change, and the slope provides a measure of the activation energy for viscous flow. For a filled suspension, the effect of temperature on particle-particle interaction must be considered, in addition to the effect of temperature on inter- and intramolecular interaction in polymers. This temperature effect depends on the level of loading. If the temperature is sufficiently high to allow migration of binder from particle-particle interfaces, then the suspension may exhibit dilatancy in place of pseudoplasticity. The effect of temperature is similar to that of shear rate on polymer rheology, but its effect on particle-particle interaction is unpredictable. Both high temperature and high shear rate weaken polymerparticle interaction. However, it is important to remember that the extent of the effect (viscosity and interaction reduction) for a particular change in shear rate or temperature is not predictable. Both factors, exerting their effects at different rates, contribute to the complexity of the system.

D. Effect of FillerConcentration Concentration effects and interparticle forces are of rheological significance with injection molding mixes. Increasing the filler concentration (volume fraction) of a suspension increases the suspension relative viscosity = where is the suspension viscosity and q o is the continuous-phase viscosity.

MATSUDDY In the dilute region (CD O.l), is a linear function of CD and for spheres is found to obey the Einstein relation

(5)

q R= l + 2.5$ As CD increases, particle-particle interactions become significant and Eq. (5) takes the form of a general power-series equation in volume fraction [5]:

-=1+2.5t$+kt$'+ rl TO

+

The second-order coefficient k need not be a constant. In general, k is a complex function of the variables that determine particle interactions. DeBruijn [6] assumed that (6) was a quadratic and that the relative viscosity T R would become infinite when the volume fraction reached the value for cubic closepacked spheres. The value of k then becomes 4.7, but it increases slowly as the particles are more disordered. According to Vand's [7] assumption, however, if pairs of particles separate along rectilinear paths after collision, then k has a value of 7.4. A setback with this approach is that at higher concentrations a large number of terms in Eq. (6) are required to calculate T R , with the greatest influence moving to the terms with high powers of CD. Allowing for interaction between particles and for interactions between particles and continuous phase, Mooney [8] proposed the expression

and introduced k as l/CDmax.In the limit CD + 0, Eq. (7) reduces to 1 + B@. Thus, B can be interpreted as an Einstein coefficient and should be between and 5, depending on the degree of agglomeration and the extent of electrochemical forces between the particles. As CD + l/k, r ) + ~ therefore, k-* refers the maximum possible value of CD, that is, the value for which the suspension loses mobility, k is known as a crowding factor. Equation (7) provides a reasonable fit; as the data for an A1203-EVA (ethylene-vinyl acetate) copolymer (with M n = 3500 and melt flow index = 200, the melt flow index defined in terms of the amount of polymer that flows through a die in a given period of time: the higher the flow rate and thus the lower the viscosity, the higher the melt flow index) show the 4 2 0 3 had an irregular shape and agglomerated with a mean particle size 0.62 Relative viscosities at four powder loadings are given in Table 1. Taking logs of Eq. (7),

B$ lnqR = l-kt$

PLASTIC MOLDING RHEOLOGY AND MIXING Table 1

RelativeViscosity Determinedat a Shear of A1203-EVA Molding Mixes

Rate of

vol.% A1203

Relative viscosity

1.83 41.0 48.0 3.91 57.0 63.0

6.25 12.55 49.90

In 7

) ~

2.53 5.30

After Ref. 3.

A plot of this equation, shown in Fig. 8, can be used to find the slope at x, y, and

@m:

B 1 - 0.41/$, B 11.43 = 1 - 0.48/$, =

at at Y

Hence B = 2.46, which is a close fit to the Einstein constant 2S@m = 61.396, which is less than perfect close packing of the powder, an effect contributing to shape irregularity and agglomeration.

5 4 ~ -

1 -

0.1

0.3

0.5 0.6

Volume Fraction

8 Plot of t-

-l

m m

a (L

---- A2-PE.60

0-

I I

I

lo3

loc

IO*

lo6

WEIGHT (b)

12 (a) Infraredspectra ofpolyethyleneillustratingtheeffectofmixing, (After Ref. 11.) (b) Molecularweightdistribution of polyethylene andalumina-polyethylene mixes. (After Ref. 11.)

MATSUDDY

Region I

Mixing Time 13 Regionof optimum mixing time. (After Ref.

more, it is often necessary to premill the powder with a processing aid that modifies the surface characteristics of the powder and breaks down the agglomerates. brief account is presented here to amplify the role of processing aid(s) in achieving an agglomerate-free homogeneous mix. In general, dispersion forces give rise to an attraction between particles in a fluid whose polarizability differs from.that of the particles. The interaction potential is inversely dependent on particle separation, and the magnitude of the interaction is described by the Hamaker [l41 constant. The well-known DLVO theory describes the stabilization of aqueous suspensions of particles on the basis of coulombic repulsions between charged particle surfaces. It is not clear to what extent charged double layers are effective in nonaqueous suspensions. is the subject another chapter in this book. An alternative approach to overcoming these forces in organic fluids is to attach or graft polymer chains to the particle surfaces such that steric hindrance, the so-called entropic repulsion, prevents the particles from approaching one another more closely than the extended length of these chains in the surrounding media. The dispersion process can be illustrated in three distinct stages. First, wetting, in which the powdedair interface is replaced by the powderlfluid interface; second, mechanical disruption, as discussed earlier, in which some of the agglomerates are broken down into smaller particles; and third, stabilization of the resulting dispersion against flocculation.


257

Wetting is the displacement from a surface of one fluid by another. It therefore involves three phases, at least two of which must be fluids. Three types of wetting can be distinguished (1) spreading, (2) adhesional, and (3) immersional. For wetting, the function adhesion tension, y cos CP, is involved, where y is the surface tension of the liquid and CP is the contact angle of the liquid with the solid surface. The author used a simple method of measurement of adhesion tension of a number of processing aids in contact with powders. This was based on measurement the rate of unrestricted penetration the propowder in a small glass tube. The cessing aid into a column of A16-SG adhesion tension y cos CP is then estimated from '" -1=

t

r ycosq k2 271

where l is the distance of penetration in time t to the surface tension contact angle CP, and viscosity 71 of the advancing liquid, r is the radius of the capillaries between particles, and k is a tortuosity constant. In this equation, r/k2 is a constant depending upon the packing of the column and was eliminated by the use of a standard packing. Table 2 presents the calculated adhesion tension y cos CP for processing aids. It has been noted that the higher the measured adhesion tension, the more rapid is the rate of dispersion during the mixing operation. The technique is not suitable for use with polymers that are solid at room temperature, however, because a linear relation between the square of the distance of penetration and time is not attained. For high-melting polymers and waxes, the contact angles of polymers to ceramic substrates have been measured as a function of temperature and time using a goniometer.Figures 14 and 15 illustrate the effect of temperature on contact angle for different polymers [10,17]. Table presents some data for thermoplastic resins and waxes with alumina and silicon nitride substrates. The atactic polypropylene has given by far the best wettability of polymers and waxes. The paraffin and most of the microcrystalline waxes give low contact angles for both alumina and silicon niTable 2

CalculatedAdhesionTension for a Selected Processing Aidand A16-SG Alumina cos 0

Processing Adhesion aid tension Corn oil Camphorated cottonseed oil Peanut oil Olive oil Pine oil Olein Menhaden oil

22.0 17.3 12.4 8.5 7.6 20.7 24.8

TEMPERATURE

1

14 Contact angle of resin to alumina substrate (A) AF'P, mol(D) ecularweight; (B) M P , 6000 molecularweight;(C)low-densitypolyethylene; EVA vinylacetate);(E), (F) medium-densitypolyethylene; (G) polystyrene. (After Ref. 17.) tride, whereas the contact angles for polyethylene and polystyrene withalumina and silicon nitride are relatively high. Besides temperature, it can be assumedthatthesubstrate surface characteristics,suchas adsorbed species, porosity, cleanliness, and morphology, have a profound effect on the wetting of polymers and waxes. similar influence might be expected with fine powders. There is clearly scope for further work on the surface nature and wettability of fine powders, so that progress in the development of improved dispersability in injection molding mixes can be placed on a more rational basis. The mechanical dispersion as applied to mixing has already been discussed. The final stage of the dispersion process is that of stabilization of the dispersed particles against flocculation. It is not intended to give a detailed account of the theoretical basis of dispersion stabilization, because this is extensively covered inthe literature, at leastat lower particle concentrations in

259

PLASTIC MOLDING RHEOLOGY AND MIXING 60

Polyethylene/Alp03 50

40

30

20

10

0 0 20

10

40

70

60

TIME (mln)

15 Contact angle as a function of time for polyethylene on alumina substrate at different temperatures. (After Ref. IO.)

low-viscosity liquids (e.g., water or solvents). Very little information is available on dispersion stability suspensions with particle concentrations above 50 ~01%.An unstabilized dispersion flocculates by collision the primary particles and thereby reduces the number of particles P in the system: dP = 81tDRp’ -dt

where R is the collision radius the particles, D is the diffusion coefficient, and t is the time. D can be expressed

for particles radius in a fluid of viscosity 1 1 ,where k is the Boltzmann constant and T is the absolute temperature. It has been shown [l81 that the time required to reduce the number particles to half the initial value is given by the equation 412

=-

4KTP0

MATSUDDY Table

Wetting Angles

Thermoplastic Resins and Waxes with Ceramic

Substrates Contact angle 0, (“1 Thermoplastics and waxes

Si3N4

Temperature (“C)

Polyethylene NA-250 Polystyrene D-210 Atactic polypropylene Waxes Paraffin Shell 120 Microcrystalline 18654 Microcrystalline Shell 400 Microcrystalline C1035 Microcrystalline Mobil

200 200 160

-33 -40 -8

160 160 160 160 160

-15 -14 -30 -20 -20

-36

-

SiC3H. The fractions of the three units in some typical polycarbosilanes are listed in Table 2. Polytitanocarbosilane(PTC) is synthesized by heating a mixture of PDS and PBDPSO with titanium alkoxide. Cross-linking by the titanium compound occurs simultaneously with the conversion of PDS to polycarbosilane [3,19]. 3. Polysiloxanes By the pyrolysis of polymethylsiloxane generally used as silicon resin, a small amount of S i c ceramic is obtained. However, polydiphenylborosiloxane [20] or polydiphenyl-glycerosiloxane is available as a precursor inayield which was synthesized by dechlorination condensation of diphenyl-

NARISA WA AND OKAMURA Table

Fraction of Structural Unitsin Polycarbosilane

Polycarbosilane

Sic4

SiC3H

PC-B-5.5

0.40 0.44

PC-TMS

0.82

0.15 0.15 0.18

PC-A470 PC-B-3.2

0

0.41 0

Source: From Ref. 18.

dichlorosilane, boric acid,andglycerin. To achieve a greater ceramic yield with polysiloxane, thecarbon content in the polymermustbeincreased. A phenyl group is more appropriate than a methyl group for the side chain. In particular, PBDPSO is used for the binder in IR radiating coating materials, not only as a polycarbosilane synthesis accelerator. The structure of PBDPSO is C6H5

I

-si

I

/

0-

- o - B'

\

C6H5

0-

Recently, polysiloxane whose composition is (MeSiO1.5)0.75-x(PhSi00.5)~ (Me2ViSi00.5)0.25was synthesized by theDow Coming Corporation. This polymer is reported to be good as a sintering aid [21].

B.

PolymerPrecursors for Si3N4 Ceramics

Polysilazanes Polysilazanes for precursor polymers were synthesized from alkylchlorosilanes and alkylamines by Verbeek [22], and the process was characterized by Penn et al. The polymer was obtained by the polymerization of tris(N-methylamino)methylsilane [CH3Si(NHCH3)3], synthesized by the reaction of methylamine with methylmchlorosilane:

+

CH3SiC13 6CH3NH2 + CH,Si(NHCH,),

+3[CH3N+H3]C1-

A general formula for the polysilazane obtained is [CH3(CH3NH)Si(CH3N)]x [CH3Si(CH3N)1.sly, and its chemical structure is considered branched, partly incorporating a ring structure. Seyferth synthesized soluble polysilazanes in high yield by ammonolysis of SiH2Cb in polar solvents such as dimethyl ether or dichloromethane. The ammonolysis of CHsSiHC12 yields silazane oligomers, which can be poly-

381

POLYMER

merized by KH catalyst. The structure unit of polymer is -CH3SiH-NH-, and the -CHSiH-NH- rings are combined by SizNz units. soluble polysilazane was obtained by the ammonolysis ofan adduct between SiHzCIz and pyridine [25]. formula for the polymer is (SiHzNH)o.46(SiH2)0.36(SiI-I3)0.i~,and a structural model is shown in Fig. 3. This polymer is produced as the precursor for the Si3N4 fiber and Si-N coating by Tonen Corporation.

2. Polydisilazanes Polydisilylazanes have been synthesized by the reaction of mixeddisilanes with hexamethyldisilazane through silicon-chlorine/silicon-nitrogenbond redistribution reactions [10,26]. The formula [(cH3)2.6(si2)i.ONHi.5]ii was deduced during (reaction) at 250°C. 3. Nitridation of Polycarbosilanes Heat treating polycarbosilane in an NH3 atmosphere yields amorphous Si3N4 ceramics [27].If polycarbosilane is cured by oxygen before heat treating, amorphous Si-N-0 is obtained The oxygen content is correlated with the structure of the amorphous phase.

C.

Polymer Precursors for Other Types of Ceramics

The synthesis of boron nitride (BN) fiber has been attempted. The thermoplastic precursor for BN fiber was obtained by the reaction between B-tris(methy1amino)borazine and lauric amine [28,29]. Aluminum nitride polymer has been actively studied as the precursor for AlN ceramics [30,31].

3 Chemical structure

perhydroplysilazane. (From Ref. 25.)

NARISAWA AND OKAMURA

SYNTHESISOFCERAMICS FROM POLYMER PRECURSORS Various types of ceramic fibers, coatings, and moldings from precursor polymers are produced and commercialized on an industrial scale. The properties of ceramics are correlated with the microstructure formed in the thermal decomposition process. Detailed information about polymer pyrolysis is now required to control the qualities of the ceramics obtained. Figure shows weight loss curves for polycarbosilane, polysilazane, and polysiloxane, which are representative precursors for ceramics. The schematic mechanisms of each polymer pyrolysis are >IOOo"C

(-R,R,Si-CH,"),

>1OOo"C

(-R,R,Si-W-),

> SicI+,

> SiC+C

polycarbosilane

> S i c + C + N,

> Si,C,N,

polysilazane

The ceramic yield of polycarbosilane is excellent above 1600°C compared with the other polymers, but the yield is low below 1500°C. The thermal decomposition polycarbosilane in particular has been well studied because of its widespread use for fibers and coatings. Various kinds of ceramic fibers obtained from polycarbosilane are shown in Fig. 5.

A.Synthesis

of SICFibers

Figure shows the gas evolution and structural change in polycarbosilane during heat treatment The dotted line indicates that the polycarbosilane was cured in air. r

l

Temperature/'C Weight curves carbosilane (C). (From Ref.

plycarbosilane (A), polysilazane (B), and poly-

POLYMER PYROLYSIS

383

Electron ifradiation curing

Oxidation curing

/ \

/ \ Heating in NHS

in

Heating or in vacuum

J

I

Heating in NHJ

in

Heating or in vacuum

I

I

Silicon oxynltride fiber

Silicon carbide fiber (Si-C-0)

5 Preparation of

ceramics from polycarbosilane.

In the first step, the polycarbosilane is concentrated by distillation to remove low-molecular-weight components and to adjust its spinnability. A typical distillation temperature is 280"C, and the number-averaged molecular weight of the resulting polymer is 1500-2000. The polymer is spun into fibers by a melt spinning method. At this stage, the spun polycarbosilane fibers are very fragile. The spun fibers are cured by oxidation in air at a temperature ofup to 100-200°C or by 'y-ray or electron beam irradiation in a vacuum an inert atmosphere. The curing process is necessary to permit the conversion of polycarbosilane fiber to silicon carbide without softening during heat treatment. Oxidation curing is achieved by cross-linking the polycarbosilane with oxygen bonds, such as Si-0-Si or Si"c, whereas radiation curing forms direct Si-Si or Si-C bonds Heating polycarbosilane, whether cured or uncured, results in the evolution of H:! and CH4 gas as the heating temperature is increased. The thermal decomposition product from 500 to 800°C has a structure intermediate between that of organic and inorganic compounds. The product contains many radicals, and the pyrolyzed fiber keeps its flexibility. Although the thermal decomposition of cured polycarbosilane fiber. is completed in the neighborhood of 8OO"C,

384

NARISA WA AND OKAMURA

-c(

POLYMER

385

its strength continues to increase with temperature. Above 100O"C, the structure of the thermal decomposition product changes from the amorphous to the micro-crystalline state, and the tensile strength of pyrolyzed fiber achieves a maximum value at around 1200°C. The schematic representation of atomic arrangement in pyrolyzed Sic fiber through oxidation curing is showninFig.7. The formula for thefiber is Sic& (x = 1.2-1.4, y = 0.3-0.4). Above 1300"C, evolution of CO gas and hence weight loss occur with the p-Sic crystallization, resulting in a drop in strength. CO gas evolution around 1300°C occurs only with oxidation-cured polycarbosilane. Figure 8 shows the effect of electron curing on the mechanical properties of the fiber. Both Young's modulus and the tensile strength at high temperature are improved remarkably [35]. Raman spectroscopy is useful for studying @-Sic crystallite andCa-ring structure in Sic fiber above 1200°C [36], whereas infraredspectroscopy is useful for studying polymer structure below 1000°C. Raman spectra of oxidationcured Sic fibers obtained by heat treatment at various temperatures were measured, and these are shown in Fig. 9. The integrated Raman intensities of each components can be estimated from the width and amplitude of the individual curves in Fig. 9. The Sic intensity includes curves for the amorphous and crystalline states. The Sic component is almost constant to 1200"C, beyond which it decreases before increasing abruptly at 1500°C. Carbon components with a Ca-ring structure increase withheat treatment to 1400°Candthensuddenly decrease at 1500°C. These results suggest that excess carbon in Sic fibers precipitates to form carbon particles up to 1400"C, which then disappear above 1500°C. The CO gas evolution between 1400 and 1500°C may compete with the precipitation of the carbon. In the Sic fiber with the highest tensile strength (i.e., that treated at 1200"C), a small number of carbon and S i c microcrystallites (1-2 nm) are considered distributed uniformly in carbon-rich amorphous Sic [36]. The carbon component intheRamanspectravirtually vanishes for heat treatment temperatures > 1800"C, whereas the Si-0 line remains up to 1900°C. Thus the amount of excess carbon is expected to be less than or comparable to that of oxygen contained in Sic. Thus, from these combined experimental results, it is considered that the strength of S i c fibers decreases with the crystallization of p-Sic and the elimination of excess carbon and oxygen contained in the fibers. Recently, ESR spectra of Sic fibers and polycarbosilane without curing gave interesting information about the thermal decomposition process [37]. The quantity of radicalsin polycarbosilane shows two peaks at 800 and 1200°C during thermal decomposition, whereas ordinary organic polymers show only one peak [38]. The ESR spectra of Sic fiber cured by irradiation show behav-

386


Si: 7 Atomic arrangement in Sic fiber pyrolyzed by oxidative curing. iorsimilar to that polycarbosilane,exceptabsolute quantity radicals. fiber cured by oxygen shows only one large peak at the secHowever, ESR ond position, 1200°C. For polycarbosilane without curing, the peak width reachesamaximum ataround 700°C and aminimum at 1100°C. Beyond

387

POLYMER

r

300

3.01

rI

loot I

&

I

1200

1300

1400

1500

Temperaturd 'C

I

1200 1300 1400 1500

Temperature/'C

Tensile strength and Young's modulus Sic fibers. (a) Oxidation curing (02content 10.8%). (b) Electron curing( 0 2 content (c) Electron curing( 0 2 content 1.9%). (d) Electron curing (@ content < 0.5%). (From Ref.

11OO"C, the peak width gradually increases. The first peak at 800"C, indicating the existence of many dangling bonds, seems to correspond to the finishing point of structural change from organic to inorganic amorphous, and the second peak at 1200°C corresponds to the finishing point of H2 gas evolution and Cg-ring precipitation from carbon-rich amorphous Sic.

of SiC-TC Fibers SiC-Tic fibers have been obtained by the pyrolysis of a polytitanocarbosilane (PTC-0) PTC was synthesized by adding titanium alkoxide to polycarbosilane in xylene. The PTC was melt spun and cured in air at 180°C. The cured fibers were heated in N2 gas at temperatures in the range 800-1500°C. Ube Industries Ltd. has prepared Si-Tic-0 fiber (Tyranno) on an industrial scale Figure 10 shows the tensile strength, Young's modulus, and specific resistance of Tyranno fibers. Tensile strength is maximum for the 1300°C treatment compared with the maximum at 1200°C for S i c fibers. This superior heat resistance of Tyranno fiber seems to caused by the bonding of excess carbon with titanium or the increase in the overall bonding strength of the constituent elements in the fiber by the addition of titanium.


1

0

I

800 Raman

1400

2000

(cm”)

9 Ramanspectra of Sic fibersobtainedbytheheattreatment of polycarbosilanefibers at (a) 1000°C, (b) 1200”C,(c) 1400°C. (d) 1500”C,and (e) 1700°C (From Ref.

POLYMER PYROLYSIS

389

-

P

200 -

f !Vi i P

.-

2

:P (a)

0 700 900 110013001500 Temperature ("C)

(b)

Temperature

1l"I

P

*

1000

.

I

1200

1400

Temperature ("C)

(c)

(a)Tensilestrength, PTC fibers. (From Ref. 40.)

(b) tensile modulus, and (c)specificresistance

of

390

--


C. Synthesis of Silicon Nitride and Silicon Oxynitride Fibers from Polycarbosilanes

The nitridation of polycarbosilane (with ammonia) begins at about 500°C and terminates almost completely at about There is scarcely any carbon in the nitride obtained at Chemical analysis indicates its composition to be Si3N3.72, almost that pure silicon nitride, Si3N4 Up to the X-ray diffraction pattern nitride fiber is broad and characteristic the amorphous state, whereas those at indicate crystalline a-SisN4 (Fig. Both silicon nitride and silicon oxynitride fibers have been obtained by the heat

120O0C

13OO0C

20 (degrees)

X-ray diffraction patterns of silicon nitride obtained by the nitridation polycarbosilane. (From Ref. 34.)

of

POLYMER PYROLYSIS

391

treatment of electron-cured and oxygen-cured polycarbosilane fibers, respectively, in an NH3 gas flow [34]. Both nitride fibers are colorless, transparent to visible light, and amorphous.

D. Synthesis of Silicon Nitride Fibers from Polysilazanes Continuous stoichiometric silicon nitride fiber was produced by the pyrolysis of perhydropolysilazane. transparent colorless silicon nitridefiber, which has high strength, high modulus, and high thermal stability and is suitable to reinforce plastics, metals, glasses, and even ceramics, was obtained. The fiber was characterized by Fourier transform infrared (FTIR) spectroscopy, X-raydiffraction and 29Si NMR. The surface characterization of the fiber was also conducted using scanning electron microscopy, X P S , and FTIR [41].

E. Synthesis of Sic Whisker from a Biological Source Sic whisker has been obtained by the pyrolysis of rice shell. The precursor, rice shell, contains 15-20wt% Si02. The rice shell is heat treated at 700°C yields an intermediate. The Sic whisker, whose diameter is 0.1-.5 p,is obtained by pyrolysis of the intermediate at 1500°C. The whisker contains 75% a-Sic and 25% p-Sic. The ceramic yield is only 10%; however, a larger yield was achieved by using Fe catalyst [42].

F.

Synthesis ofSi3N4-SiCCeramics

Si3N4 ceramics using polycarbosilane have been prepared by Kurosaki Yogyo (Kuroceram-N). In the first step,polycarbosilane is mixed with Si powder with a diameter below p and cured by oxygen. By pyrolysis in a N2 atmosphere, the sample is converted into Si3N4 ceramics. Nitridation of Si yields Si3N4, and the pyrolysis polycarbosilane yields Sic and excess carbon. The excess carbon is trapped by unreacted Si and changed Sic. The ceramics obtained consist mainly of Si3N4 with a few percent Sic. By changing the quantity of polycarbosilane, the microstructure of the ceramic can be controlled through the form of the residualSic. microgap between Si3N4 and Sic crystallites gives flexibility to the material, and high thermal shock resistance is achieved.

G.

SynthesisofHeat-ResistantCoatings

Tyranno Coat was obtained from Tyranno polymer and widely used [43]. The virtures of Tyranno Coat are as follows: The polymer can be solubilized in organic solvent and used in the form of a solution or slurry.

N4RISAWA AND OKAMURA

Tyranno polymer

1000

Temperature/'C Weight

curve of Tyranno Coat (in air). (From Ref. 43.)

2. The polymer is thermoplastic and is easily manufactured because the softening point is about 300°C. 3. The polymer forms persistent plastic coatings below 400°C and changes into S i c ceramics above 500°C.

The weight loss of Tyranno Coat is shown in Fig. 12 compared with that of phosphate and polysiloxane. By dispersing ZrO2 particles in Tyranno Coat, highly radiative, highly heat resistant, and high thermal shock-resistant coating is obtained, which can avoid the radiation loss from surface reflection. Besides radiative coatings, Tyranno Coat is generally used to prevent the corrosion and oxidation of metals. Polyborodiphenylsiloxane is also used as a binder for infrared coatings. The coating is high in quality in particular for far-infrared rays. Recently, a coating of perhydropolysilazane on stainless steel has attracted attention. The perhydropolysilazane is converted into silicon nitride in an N H 3 atmosphere at 600°C or to a transparent siliconoxynitride in humid air beyond 150°C. This material is making progress in overcoming some weakness in the expansion mismatch to the coated metal and in contraction during pyrolysis.

W. CONCLUSIONS Several kinds of ceramics prepared from metallorganic polymers have been described. By controlling the precursor and conversion process, polymer precursor ceramics have the possibility to form novel types of structure, such as an interpenetrated microstructure, that is different from that of earlier ceramics.

POLYMER PYROLYSIS

393

Ceramic fibers are the most advancedmaterials in the polymer precursor ceramic field. These fibers have high tensile strength and heat resistance and are promising as reinforcement fibers for composites, such as fiber-reinforced plastics, metals, and ceramics. Detailed investigation of the structure and nature of precursor polymers and the conversion process are in progress. Many interesting phenomena occur during the conversion of polymers, and the study of these will expand not only with regard to ceramic fibers but to other ceramic materials with refractory, magnetic, and electronic properties. Therefore it can be predicted that new ceramics with a variety of properties will be prepared in the near future.

REFERENCES Yajima, Yajima,

Hayashi, J., and Omori, M., Chem Lett., Okamura, K., Hayashi, J., and Omori,M.,

J. Am. Ceram Soc., 59,

Yajima, Iwai, T., Yamamura, T., Okamura, K., and Hasegawa, Y., J. Mater. Sci., 16, Okamura, K., Sato, M., Hasegawa, Y., and Amano, T., Chem Lett., Prewo, K. M., Brennan, J. J., and Layden, G. K., Am. Ceram. Soc. Bull., 65, Cornie, J. A., Chinag, Y., Uhlmann, D. R., Mortensen, A., and Collins, J. M., Am. Ceram. Soc. Bull., 65, West, R., J. Organometal. Chem, 300, West, R., David, L. D., Djurovich, P. I., and Yu, H., Am. Ceram. Soc. Bull., 62, Baney, R. H., and Gaul, J. H., Jr., U.S. Patent Baney, R. H., in Ultrastructure Processing of Ceramics, Glasses and Composites (L. L. Henchand D. R. Ulrich.eds.),JohnWileyandSons,NewYork, p. Baney, R. H., Gaul, J. H., Jr., and Hilty, T. K., Organometallics, 2, Fritz, G.,Grobe,J.,andKummer, D., Adv. Inorg. Chem Radiochem., 77, Yajima, Omori, M., Hayashi, J., Okamura, K., Matsuzawa, T., and Liaw, C., Chem. Lett., Yajima, Hayashi, J., and Okamura, K.,Chem. Lett., Yajima, Hasegawa, Y., andOkamura,K.,andMatsuzawa, T., Nature, Schilling, C. L., Jr., and Williams, T. C., Polym. Preprints, Schilling, C. L., Jr., Br. Polym. J., 18, Hasegawa, Y., and Okamura, K., J. Mater. Sci., 21, Yamamura, T.,Polymer Preprints, 25(No Yajima, Hayashi, J., and Okamura, K., Nature, 226, Baney, R., Gaul, J., Jr., and Hilty, T., Mater. Sci. Res., 17,

NARISAWA AND OKAMURA 22. Verbeek, W., U.S. Patent 3, 853, 567 (1974). 23. Penn,B.G.,Ledbetter,F. E., m, Clemons,J.M.,andDaniels,J. G.,J. Appl. Polym 27, 3751 (1982). G. H.,in UltrastructureProcessing of Ceramics, 24. Seyferth,D.,.andWiseman, Glasses and Composites (L. L. Hench and D. R. Ulrich, eds.) , John Wiley and Sons, New York, 1984, p. 265. Isoda, T., and Tomizawa, T., Polym. Preprints, 407 25. Arai, M., Sakurada, (1987). 26. Gaul, Jr., J. H., U.S. Patent 4, 340, 619 (1982). 27. Okamura, K., Sato, M., and Hasegawa, Y., Ceram. Int., 13, 55 (1987). F’roc. Japan Intema28. Kimura,Y.,Hayashi,N.,Yamane,H.,andKitano,K., tional SAMPE Symposium, November 28 to December l, 1989. 29. Paine, R. T., JAPAN-US Joint Seminar on Inorganic and Organometallic Polymers, Nagoya, Japan, March 25-27, 1991, p. 78. 30. Hashimoto, N., Sawada, Y., Bando, T., Yoden, H., and Deki, J. Am. Ceram. 74, 1282 (1991). 31. Amato, C., Hudson, J., and Interrante, L. V., Mater. Soc. Symp. Proc., 168, 119 (1990). 32. Atwel, W. H., Bums, G. T., and Zank, G . A., private communication PolymerPreprints, 33. Okamura,K.,Sato,M.,Matsuzawa,T.,andHasegawa,Y., 25(No l), 6 (1984). 34. Okamura, K., Sato, M., and Hasegawa, Y., 6th World Congress on High Tech Ceramics (CIMTEC), Milan, Italy, June 23-28, 1986. 35. Okamura, K.,Sato, M., Seguchi, T., and Kawanishi, Proceedings of the Third InternationalConferenceonCompositeInterface(ICCI-III),May 21-24,1990, p. 209. 36. Sasaki,Y.,Nishida,Y.,Sato,M.,andOkamura,K., J. Mater. Sci., 22, (1987). 37. Shimoda, M.,Sugimoto,M.,Katase,Y.,Okamura,K.,andSeguchi,T., Muki Kobunsi Kenkyu Touronkui, 10, 76 (1991). and Lewis, I. C., Appl. Spectrosc., l), 52 (1982). 38. Singer, L. 39. Okamura,K.,Sato,M.,andHasegawa,Y., Proc. Fifth Int. Conf. on Composite Mater., San Diego, CA, July 29 to August 1, 1985, p. 535. 40. Yamamura, T., Hurushima, T., Kimoto, T., Shibuya, M., and Iwai, Y., 6th World Congress on High Tech Ceramics (CIMTEC), Milan, Italy, June 23-28, 1986. 41. Isoda, T., in Controlled Interphases in Composite Materials (H. Ishida, Elsevier, Amsterdam, 1990, p. 255. 42. Milewshi, J. V., Sandstrom, J. L., andBrown,W. in Silicon Carbide (R.C. Marshall, J. W. Faust, and J. R. C. E. Ryan, eds.), University of South Carolina Press, 1973, p. 634. 43. Nishihara, Y.,Kagaku to Kogyo, 309 (1987).

Part VI PROCESSING OF SPECIALTY CERAMICS


17 Processing of Lead-Based Dielectric Materials AT&T Bell Laboratones, Princeton, New Jersey

AT&T Bell Laboratories, Murray

New Jersey

INTRODUCTION Historically, BaTiOs-basedmaterials have been the dielectricsof choice for use in multilayer ceramic capacitors. BaTiOs has a perovskite crystal structure and exhibits ferroelectric behavior. At its Curie temperature, the dielectric constant E may exceed The effect of dopants on Curie point, microstructural control, and other processing parameters, such as powder conditioning (milling and solvents), green sheet processing (binder or casting), and firing temperatures and ambients, are reasonably understood. Multiple-cation substitution in the BaTiOs-based system has also been studied extensively withregardto preservation and cation ordering in the perovskite phase. In the 1950s, Smolenski and coworkers [ l 4 1 investigated many cation substitutions into PbTi03 in a search for new ferroelectric materials. In this kind of substitution, the general guidelines are that the ionic sizes should be comparable to those of ion and the combination must yield the same average charge as Ti& to maintain charge neutrality. Many such compositions take on the complex perovskite structure, and their properties have been compiled in Ref. [5]. With the promise of higher dielectric constants and lower firing temperatures, and hence lower electrode cost, lead-based perovskite compositions of the general form Pb(B1, have been studied extensively for application as capacitor dielectrics for the past 15 years. Of particular interest are compositions in which B1 is a divalent or trivalent cation such as Mg2+, Zn2+,Ni2+,

LING AND YAN

398

or Fe3+, and B2 is a high-valence ion, such as Ti&, Ta5+, or The most commonly studied is Pb(Mg1/3Nb2/3)01/3 (PMN). These compounds exhibit a broad maximum in the dielectric constant, and the temperature of the dielectric maximum also increases with the testing frequency (Fig. 1). These are known as the relaxor compounds. The origin of broad maximum ‘is postulated as caused by a distribution of Curie points resulting from microcompositional fluctuation in theB-sitecations.Onlytwo compositions have been found to show ionic ordering by thermal annealing, Pb(SclnTaln)O3 and Pb(SclnNbt/2)03 [6,7].In these compounds, thedielectric response varies from the normal behavior in the ordered state (as in BaTi03) to relaxor behavior in the totally disordered state. Another characteristic of relaxors is the frequency dispersion inthedielectricloss (DF), which occurs at a slightly lower temperature than thedielectric maxima. The dielectric loss is also slightly higher than that of the normal ferroelectrics. Beginning with the work by Ohno and Yonezawa on PFN-PFW* systems in the late 1970s [ 8 ] , many multicomponent dielectric systems have been evaluated and put into manufacture. Some of the patented compositions developed for multilayer capacitor (MLC)applicationwererecentlysummarized by Shrout and Dougherty [9].Other compositions were developed for piezoelectric sensors and electrostrictive actuator applications [lo].Most of the compositions used for capacitor dielectrics are based on PFN [S],PMN [ 11-14], or PZN [15]. An important issue in the preparation of the complex perovskite compositions is the appearance of a cubic pyrochlore phase, whichmay occur as a dominant or minor phase co-existent with theperovskite phase and may appear or disappear depending on the processing conditions. The pyrochlore is more likely to occur, usually in higher proportion when using the conventional processing method of ball milling the starting oxide powders. Since ferroelectric behavior is exhibited in the perovskite phase, the pyrochlore phase is generally perceivedas detrimental toattainingthedesirableproperties of a pure perovskite phase.Atlow pyrochlore concentrations, an approximate lineardecrease in E with pyrochlore content has been observed [13,16].Figure 2 shows that in the series of compositions we studied [13],the decrease in the dielectric constant E follows the relation E ,,

-@

(1)

where E m a = 17,200,k = 1.6 x 105, and p = volume fraction of pyrochlore. Compositional modification and/or alternative processing methods, usually spe-

LEAD-BASED DIELECTRIC MATERIALS

399 +7949: R3670-4 x = 0.2651; y =0.1566

W

Q

0 -40

I

I

-20

I

0

i

I

20

I

I

I

I

60

40

l

l

80

(C

(a)

1 Dielectricconstant anddissipationfactorversustemperatureatdifferent frequencies in a PbO-MgO-Nb205 composition. (From Ref. 13.)

LING AND YAN

2 Maximum dielectric constants at 1 kHz versus pyrochlore concentration in the PbO-MgO-Nb205 ternary composition. (From Ref. 13.) cific to a particular composition, may be developed to eliminate the pyrochlore phase from the end product. the other hand, nonferroelectric pyrochlores may serve as technologicallyusefuldielectricsin other applications,suchas temperature-stable dielectrics or microwave dielectrics. It is important to understand the structural relationship between the perovskite and pyrochlore phases in the Pb-based systems and to elucidate the thermodynamic and kinetic factors that mayinfluence the phase equilibrium. Understanding these willallow a more systematic method of tailoring dielectric compositions to applications. Another important area in dielectric processing is the formation of dense, sintered bodies and theformation of thin layers in the green (unfired) state for the fabrication multilayer structures. Densification and microstructural development during ceramic processing are influenced by the characteris-

LEAD-BASED DIELECTRIC MATERIALS

401

tics of the starting powder and the microstructure of the green compacts. In a review paper published 10 years ago, Yan [l71 discussed the factors impacting on the properties of sintered electronic ceramics. In general, small particle size and narrow particle size distribution are required for densification to full density. Furthermore, deagglomeration of very fine powders before forming green compacts is also extremely important. It was shown [l81 that BaTiOs with a very small particle size ended up with a low sintered density because of an inhomogeneous agglomeration in the fine powders. Defect-free green compacts with uniform microstructure before sintering are necessary for sintered ceramics with high mechanical strength and good electrical properties. Flaws in the green compacts will most likely be magnified during the sintering process. The origin of these flaws may include foreign particles in the milling process, binder accumulation and incomplete burnout, and lamination defects in multilayer structures In electronic ceramics these flaws can later become the initiation sites for failure, such as electrode migration under temperature-humidity-bias conditions [20,21],or physical cracking under surface mount assembly processes. Other than the desirable property of high dielectric constant, the technology to form very thin green sheets of dielectrics and to cosinter with metals at low temperatures ( under normal oxidizing preparation conditions. The partial substitution the La site by Sr, Ca, and Na also leads to the formation of a superconductive state, the highest Tc of 40 K being obtained in the (Lal-xSrx)2Cu04 system at = 0.075 (Ndl-xCex)2Cu04, whichhas a slightly different crystal structurefrom K2NiF4, was found to be a superconductor with electron carriers when prepared under mild reducing conditions, for example, a nitrogen atmosphere [ l].In case Nd3+ is partly replaced by C e b , introducing electron carriers. Superconductors inclass 111 have a nominal composition ofA1A'2B30x, which is characterized by the existence of two kinds of A-site ions (A and A') and one B-site ion. Typical of class 111 is Y1Ba2Cu30x with Tc = 90 K, first synthesized byWu et al. The A-site ions are Y3+ and Ba2+ [4].The crystal structure is a three-layered perovskite; that is, the repetition unit is three blocks the primary perovskite. The Y site can be partly or completely replaced by other rare earth elements, such as La, Nd, Sm, Eu, and giving rise to a 90 K class superconductors in all cases. From a chemical point of view the constancy of the critical temperature is surprising, since the rare earthelements change their valencies (Sm3+, Sm4+, E d + , and Eu3+) and ion sizes. This suggests that Tc depends only slightly on the nature of the rare earth elements as long as the crystal structure is preserved. The only exception is PrBa2Cu30x, which cannot be made superconductive although PrBa2CusOx is prepared in the (123) structure because of its rather high electrical resistance in the normal state. It is supposed, however, that there must be a synthetic condition under whichPrBa2Cu30xbecomes superconductive. The oxygen content 7 in

HIGH-TEMPERATURE SUPERCONDUCTORS

449

YlBa2Cu307 deviates largely from the nominal 9 in (AB03)3, indicating that two of the nine of oxygens are deficient in the formal perovskite structure. It may not be appropriate to call these high-temperature superconductors perovskite compounds. The Ba site in Y1Ba2Cu307 can be replaced by Sr, leading to the formation of a superconductor with Tc = 80 K in a metastable state. A superconducting Y1SnCu30x can be synthesized only under high oxygen pressure. When Pb is incorporated into the Y-Sr-Cu-0 system, it forms a stable superconductor (Y/Ca)-Sr-(Cu/Pb)-0 with Tc = 70-80 K [12]. Under certain conditions Y1Ba2Cu40x withTc = K, a derivative of Y (123), is formed. Superconductors in class N have the general formula (A/A')-(BIB')-0, which is characterized by having two kinds of A-site ions and two kinds of Bsite ions. Typical of class N are Bi2SnCa2Cu30x [5] and T12Ba2Ca2Cu30x [6],which are four-layered perovskites. The AIA' is Sr/Ca or BdCa and BIB' is Bi/Cu or TVCu. In the Bi-Sr-Ca-Cu-0 system there exist three variations, namely, the Bi(2223) phase with Tc = 105 K,Bi(2212) phase with Tc = 80 K, and the Bi(2201) phase with Tc = 20 K.These Bi superconductors contain double Bi-0 planes in their crystal structures. In the T1-Ba-Ca-Cu-0 system there arethree superconducting phaseswiththe double 1-0 plane, namely, T1 (2223) with Tc = 120 K, Tl (2212) with Tc = 110 K, and T1 (2201) with Tc = 80 K. The Tl-Sr-Ca-Cu-0 system forms two superconducting phases with the single Tl-0 plane, that is Tl (1223) with Tc = 105 K and T1 (1212) with Tc = 80 K, in a metastable state [7]. The metastable superconductors in the Tl-SrCa-Cu-0 system become stable when a partial substitution of T1 by Pb, Sr by Ba, and/or Y by Ca is performed [13]. The substitution means the adjustment of carrier concentrations, accompanied byan increase in Tc. An appropriate concentration of carrier is essential to attain the stable superconductive states. The Tl-Sr-Ca-Cu-0 system has a potential for use in magnets operable in liquid nitrogen for its high Tc and high critical magnetic field Hc [14].

SYNTHESIS AND BULK A.

SUPERCONDUCTING POWDER

Y(123) Phase of Y-Ba-Cu-0 System

The most common method of synthesizing the high-temperature superconductors in bulk form is shown in Fig. 3. Appropriate amounts of Y203, BaC03, and CuO are mixed and ground thoroughly. The mixed powder is precalcined in an electric furnace at 900-980°C for about 10 h in an oxygen or air atmosphere. The calcined powder is ground and then pelletized intoa disk form. The pellets are sintered at 975°C for 10 h under oxidizing atmosphere andthen cooled slowly to room temperature. It is very important to cool slowly in an oxygen or airatmosphere, especially passing the 400-600"C region, for at least 2 h. The resultant bulk has a chemical formula ofYlBa2Cu306.7-7, which

450

MATSUDA

I

Mixing Grinding

I

Calcination

I

Grin,ding

950°C,10h

I

I Sintering I 975OC,lOh 1 I Slow Cooling I -10h

3 Preparationprocedure bulk Y-Ba-Cu-0 superconductors.Calcination must be done in an oxidizing atmosphere.

shows the superconducting transition at 90-93"C, as shown in Fig. For the proofof superconductivity it is required to show both zero resistance and a Meissner effect (diamagnetism). It is seen in Fig. that both the resistivity and susceptibilitydrop sharply near90°C. The critical temperature depends strongly on the oxygen stoichiometry, the value of 6.7-6.95 normally obtained being smaller than the ideal value 7.0. When the oxygen stoichiometry is less than the sample shows no superconductivity [15]. The long annealing at 400-600"C in oxidizing atmosphere is necessary for the bulk materials to absorb oxygen. The absorption and desorption of oxygen are foundtobereversible.

B.

Bi (2223) and Bi (2212) Phases of Bi-Sr-Ca-Cu-0 System

Preparation of the Bi-Sr-Ca-Cu-0 superconductors is essentially the same as in the Y-Ba-Cu-0 system, except thecalcination temperature. The Bi (2223) phase is formed by firng at 850°C for 200 h in air atmosphere. shorten the firing time, theconventional practice is to replace 10-20 atom% Bi by Pb.The replacement induces the formation of liquid-phase Ca2PbO4, which accelerates the solid-state reaction. Lead atoms incorporated in the Bi sites. The Bi (2212) phase is more stable than the Bi (2223) phase and is formed by calcin-

HIGH-TEMPERATURE SUPERCONDUCTORS E

-

Tc 90K

q

E

iz2

5 -

Tc

U,

, \,, 50

35:

v)

0

451

v)

150 250 200

0

50

4 Superconductingpropertiesof Y-Ba-Cu-0 bulksample: change, (b) susceptibility change (diamagnetism, Meissner effect).

(a) resistivity

ing at 880°C for 2-10 h. At 880°C the Bi (2212) phase is partially melted, and therefore the reaction is considerably faster [16]. The Bi system contains a double Bi-0 plane in the crystal structure whose interplane strength is relatively weak and easily laminated. Crystallites in thick films ofBi superconductors prepared by the doctor blade or screen printing method are easily oriented by rolling or pressing because of the laminating property [17]. A high critical current density, for example, 105 Ncm2 at 4.2 K and 1-30 T of applied magnetic field, is obtained in wire and tape forms. Although the critical temperature of the Bi (2223) phase is much higher than the liquid nitrogen temperature 77 K, the Jc at 77 K is very low in the presence an external magnetic field of 1 T because of the low critical field Hc. The application of the Bi system to magnets is limited to temperatures below 20 K.

C.

TI (2223) Phase of TI-Ba-Ca-Cu-0 System

The Tl(2223)phase of the Tl-Ba-Ca-Cu-0 system with Tc = 122 K is prepared by calcining the corresponding mixture at 845-860°C for 5-10 h in air. Special care must be paid, since the mixture loses a few percent T1 through vaporization during calcination. Recalcined pellets are calcined in a crucible together with Tl (2223) powder to compensate for the T1 losses. At temperatures higher than 870°C Tl (2223) tends to decompose to Tl (2212) and CaCuOz. The Tl(2223) superconductor is much more easily formed than Bi (2223). The interplane bonding between the T1-0 planes is considerably stronger than that in Bi (2223). The dependence of Jc on the magnetic field has been studied using a polycrystalline Tl(2223) thin film on a single crystal of MgO prepared by the laser ablation method [18]. Platelike crystallites are aligned with the c plane on the MgO surface. As shown in Fig. 5, the Jc is 2 105 Ncm2 at 77 K in the absence an external magnetic field but decreases very rapidly in a magnetic field applied perpendicular to the c plane. The influence the magnetic field parallel to the c plane is rather small. The two dimensionality or

452

MATSUDA

5 Criticalcurrentdensity of polycrystalline "l (2223) thin film inmagnetic fields. Crystallite c planes are parallel to the MgO surface. Fields are applied parallel and perpendicular to the c plane. (From Ref. 14.)

anisotropy of the superconducting properties is clearly seen. The facts suggest that the Tl (2223) has no effective pinning centers at 77 K when a magnetic field is applied perpendicular to the c plane. The Hc of Tl (2223) is low at 77 K, although slightly higher than that of Bi (2223).

2.

TI (1223) Phase of TI-Sr-Ca-Cu-0 System

The Tl (1223) phase of the pure Tl-Sr-Ca-Cu-0 system is a metastable superconductor. Among many samples prepared, only a few samples show a weak superconductivity or Meissner signal and many others show no superconductivity [7]. This is a reason the T1-Sr-Ca-Cu-0 system is called a metastable superconductor. When Tl is partially substituted by Pb, Sr by Ba, or Ca by Y, the system becomes a stable superconductor [13]. It is thought that the pure T1Sr-Ca-Cu-0 system is too doped become superconductive.Reducing the carrier concentration by the substitution thereforeleads to a superconductive state. Examples of an optimum composition are as follows:

high-quality superconductor with Tc = 120 K is formed when a mixture of composition 1 is calcined at 875°C or composition 2 at 900°C for 8-30 h. The substitution of Sr by Ba leads to the formation of a liquid phase comprised

453


of Tl-Pb-Ba-Cu-0 at 877 K, reducing the temperature of sintering [18]. At temperatures above 890"C, Tl (1223) decomposes according to Tl(1223)+ Tl(1212)+CaCu02 The bulk samples prepared thus show crystallite size of2-3 When ples with larger grain sizes are required, the heat treatment must be done at 930-980°C with partial melting [19].

Single crystals high quality are required to study the fundamental physical and chemical properties of a superconductor. Single crystals of La2-xSrEu04 and YBa2Cu307 are prepared by a conventional traveling solvent floating zone (TSFZ)method [20]. A schematic diagram a TSFZ apparatus is shown in Fig. A focused infrared light is irradiated on a rod-shaped sample prepared by an ordinary sintering or a cold isostatic pressing method. The sample rod is

Vacuum

Computer

Traveling solvent floating zone apparatus. (From Ref. 20.)

MATSUDA moved slowly downward at a speed of0.2-5mm/h. The irradiated region is melted. To obtain better homogeneity in the liquid, the upper part of the rod is counterrotated against the lower part. single crystal of La1.8sSro.1sCu04 of 8 mm diameter and 100 mm length has been successfully prepared. Magnetization hysteresis of La (214) is shown in Fig. [21]. It is seen that AM has two peaks, the first at zero field and the second at higher fields. The phenomenon is called a peak effect, which is explained by the introduction of pinning centers as weak superconductive regions are converted to nonsuperconductive by the external magnetic fields. Crystal defects, such as oxygen deficiencies, cation concentration inhomogeneity, and inclusions, may play a role of pinning centers. The second peak shifts to higher fields as the temperature is lowered. The AM corresponds to the intragrain Jc as formulated in Bean’s model,

AM

J, =2Mm2

where

B.

is the thickness of a sample perpendicular to the magnetic fields.

Large-Grain Crystals of Y-Ba-Cu-0 System

Several methods of growing a large single-grain crystal of Y (123) have been developed based on partial melting. They are called melt textured growth [22], QMG(quenchandmelt growth) [23],and melt powdermeltgrowth[24], whose principal reaction is a peritectic reaction between a liquid-phase Ba-Cu0 and a solid-phase YzBaCuOs in a very slow cooling process. pseudobinary phase diagram of Y2CuOs-Ba3Cuso8 is shown in Fig. 8 [25]. liquid phase is formed above 980°Cin a mixture of Y203,BaO, and CuO,and YzBaCuOs decomposes at 1200°C.The preparation procedure of QMG process is showninFig.9. typical composition of20%richin Y203 from the YBa2Cu307 stoichiometry is heated and melted at 1500°C in a Pt crucible and quenched to room temperature very rapidly using two copper plates Y2BaCu05, BaO, and CuO + melt + Y203 The quenched melt is quickly heated to 1200°C for partial melting for 30 minutes: Melt -+ Y2BaCu05+ liquid (Ba,Cu,08) The peritectic reaction takes place during the slow cooling from lo00 to 940°C for 10 h. Y2BaCu05 +liquid(Ba,Cu,O,)

+ YBa,Cu,O,

+excess Y203

In the process it is crucially important to disperse the Y2BaCuOs (or Y203) particlesfinelyand to use a YzBaCuOs-(orY203)rich composition, which


-5

0

455

5

10

Bex [TI 7 Magnetizationhysteresis La-Sr-Cu-0single-crystal La1.82Sro.18CuO4: overdoped sample, magnetic fields parallel to c axis. (From Ref. 21.)

-

L Y*O3 +L

1200 -.............

2 a

-

I

211+L

2

g, E

F

123i-L

-

5YsOa

+2Bao

2n+123

211

3BaCuol 123

+2cuo

Pseudobinaryphasediagramalongtieline liquid; (211), Y2BaCuOs; and (123), YBa2Cu30x.

Y2BaCuOs-YBa2Cu30x. L,

MATSUDA

+L

I Time (hrs) Quench and melt process. Peritectic reaction takes place between IO00 and for 10 h.

eliminates liquid residues between grain boundaries. The QMG is later modified so that the initial melting is omitted when 2 wt%R02 is added to the mixture [26]. Staiting from Y2BaCuOs and Ba3CU508, a large single-grain YBa2Cu307 with the dimensions cm diameter and 2 cm thickness is successfully prepared. To grow a single-crystal YBa2Cu307, it is practical to seeda small SmBa2Cu307 crystal that has a higher melting point than Y (123) in the partial melt. The intragrain Jc of a single-crystal YBa2Cu307 prepared by QMG, shown in Fig. 10, is in the range of2-5 104 A/cm* at 77 K and 1 T [27]. The intragrain Jc is calculated from AM using Bean's equation. A large single grain can also be prepared by the unidirectional solidification method. The superconducting pellet behaves as a permanent magnet, which could be applied to the magnetic bearing and flywheel. The intergrain or transport Jc in polycrystalline Y (123) has been found to be poor. The insulating phases between Y (123) grains are yet to be identified to improve the transport Jc.

C.

Single Crystals of TI-Sr-Ca-Cu-0 System

Single crystals of the Tl (1223) phase of the Tl-Sr-Ca-Cu-0 system are prepared by a conventional flux method. An example of the starting composition is


I

I

0.5

Magnetic Field (T) Criticalcurrentdensity of Y-Ba-Cu-0largesingle-grainsamplesprepared by melt powder melt method. Three samples contain different amounts

where the last compound acts as a flux. The precalcined powder is loosely packed in an alumina crucible 3-4 cm in height, heated to 940°C, and cooled slowly to 850°C for 100 h. sintered bulk sample thus obtained is composed of T1 (1223), nonsuperconducting grains, such as Sr-Ca-Cu-0, and residues of liquid phases. Breaking the bulk by a hammer, single crystals with 0.5-1 mm size are found and collected under microscope. A photograph of a single crysSeven square sheets ofT1 (1223) tal of Tl (1223) is shown in Fig. 11 [28]. crystals are stacked with c planes parallel. The thickness of each sheet was 0.05 mm. The critical temperature was found to be 114 K. An anisotropy of magnetization was measured using the single crystal andis shown in Fig. 12. Even at K and 1 T, there exists a large magnetization hyteresis, indicating a large intragrain Jc. The intragrain Jc in the c plane is calculated >5 x 104 Akm2 at K and 1 T, which is much greater than in T1 (2223) and Bi (2223) and in the same range as Y (123). The linkage between Tl (1223) grains has been found to be stronger than that of Y (123) [29]. Since the magnetic hysteresis extends to 8 T, Tl (1223) could find applications in magnets generating 1-5 T and operated in liquid nitrogen.

Scanningelectronmicroscopeimageof

T1 (1223) singlecrystal.

B B

300

200

2a,

0

0

00

-10 -200 -4

-2

0

2

4

(T)

6

-20

-4

0

2

(T)

12 Magnetization hysteresis of TI (1223) single-crystal. Anisotropy is clearly seen for the applied magnetic field direction. (From Ref. 28.)

SUPERCONDUCTORS HIGH-TEMPERATURE

V.

FABRICATIONOF SUPERCONDUCTINGWIRES

Several methods, such as the drawing, screen printing, plasma spray, and dip coating, have been tried to fabricate a wire from the oxide superconductors. The drawing method is widely used to fabricate a long wire (-1 km) from NbTi and Nb3Sn superconductors. It is not practical to make a magnet unless a wire whose transport Jc exceeds Ncm2 in the presence of T magnetic fields is developed. To attain the high Jc the following four points, as illustrated in Fig. 13, are crucially important: 1. Densificationof superconducting core 2. Clean grain boundaries 3. Alignmentof crystallites Superconducting core withhigh intragrain Jc

Among these it is most difficult to eliminate weak links at the grain boundaries. The drawing method combined with the rolling process, which is one of the mostpromisingmethods to fabricate tape-shaped wire, is showninFig. For the sheath material, a Ag tube that allows oxygen diffusion into the core is generally used. Powders the oxide superconductor are packed into a Ag tube with 4-6 mm diameter and mm wall thickness. The diameter of

7

" "

I

: I

Impurities,

a

/ Oxygen Deficient Phase I IClean Boundary1

i

J

Threeimportantstepstoincreasethecriticalcurrentdensity conducting wire from oxide superconductors.

super-

TPSr-Ca-Cu-O Powder

list9 Ag Tube f

l

Q

W

Outer Diameter

Outer Diameter

6mm

2.8mm

Thickness 0.3mm

Packing

Drawing

Rolling

Figure

Fabricationmethodfortape-shapedwirewith

an oxidesuperconductor

core.

loo000

"E

h

I

5T : T

%

4

6

8

Magnetic Field (T) Figure Transportcriticalcurrentdensity of T1 (1223). TI (2223), and Bi (2223) tape in magnetic fields applied perpendicular to the tape surface. Critical field H*:TI (1223) >> T1 (2223) > Bi (2223).

461


the tube is reduced to 1-3 mm in successive drawings and then rolled to a tape with 0.2-0.5 mm thickness. Taking into considerations the melting point of Ag (960"C), the tapes are heat treated at temperatures below 910°C for h in air atmosphere. For the Bi (2212) and Bi (2223) phases, crystallites automatically aligned in the rollingprocess because of their laminating property between c planes [31]. the other hand, randomly oriented crystallites are observed in the Y (123), Tl (1223), and Tl (2223) tapes [32]. The transport Jc at 77 K of the Bi (2223), Tl(1223), and Tl (2223) tapes in magnetic fields is shown in Fig. 15 [33]. Magnetic fields are applied perpendicular to the tape surface. In a zero magnetic field the Jc is in a range of 2-5 104 Ncm2 at 77 K. It is seen that the Jc drops very sharply, anorder of magnitude, at fields less than 0.1 T in all three tapes. At fields more than 1 T the Jc of Tl (1223) stays relatively constant; those of Bi (2223) and Tl (2223) further decrease. The sharp drop at low fields is supposedly caused by the weak links between superconducting grains. Since T1 (1223) has a higher irre-

/

I

."

I

-

I

1

MAGNETIC FIELD (T) 16 State-of-the-art presentation of critical current density of metal and oxide superconductingwirein 1992. MetalsuperconductorsareusedonlyinliquidHeand oxide superconductors in liquid NZ and He.

MATSUDA versibility magnetic field H*,the Jc value extends over 8 T. The transport Jc should approach the intragrain Jc if crystallites are aligned with the c plane parallel to the tape surface. The status of Jc in is shown in Fig. which compares the oxide superconductors with the metal superconductors. The transport Jc the oxide su. perconductors at 77 K is more than one order of magnitude lower than that of the metal superconductors at K. The Bi wire will find an application in the very high field magnets over 20 T cooled by liquid helium and Tl in magnets between and T cooled by liquid nitrogen. Superconducting magnets operable in liquid nitrogen will be used widely in manyfields of industry whenever a high magnetic field is required.

REFERENCES 1. Bednorz, J. G., and Miiller, K. A., Z Phys., BM,189 (1986). 2. Uchida, Takagi, H., Kitazawa, K., and Tanaka, Jpn. J. Appl. Phys., L1 (1987). Takagi,H.,Kitazawa,K.,andTanaka, Jpn. J. Appl. Phys., 3. Uchida, L151 (1987). 4. Wu, M. K., Ashburn, J. R., Torng, C. J., Hor, P. H., Meng, R.L., Gao, L., Huang, Z. J., Wang, Y. Z., and Chu, C. W., Phys. Rev. Lett., 908 (1987). 5. Maeda,H.,Tanaka,T.,Fukutomi,M.,andAsano,T., Jpn. J. Appl. Phys., L209 (1988). 6. Sheng, Z. and Herman, A. M., Nature, 138 (1988). Sheng, Z. Z., Kiehl, W., Bennett, J., El Ali, A., Marsh, D., Mooney, G. D., Aram1738 mash, F., Smith,J.,Viar, D., andHerman,A.M., Appl. Phys. Lett., (1988). 7. Matsuda, Takeuchi, Soeta, A., Suzuki, T., Aihara, K., and Kamo, T., Jpn. J. Appl. Phys., 2062 (1988). Matsuda, Takeuchi, A., Suzuki, T., Aihara, K., and Kamo, T.,in Advances in Superconductiviv, (K. Ishiguro, ed.) Springer-Verlag,Tokyo (Proc. Int. Symp. Superconductivity, August28-31, 1988, Nagoya, Japan), 1988, p. 804. 8. Matsuda, P., Shokubai (Jap. J. Cataly.), 279 (1989). 9. Sleight, A. W., Gillson, J. L., and Bierstedt, P. E.,Solid State Commun., 17, 27 (1975). 10. Cava, R. J., Batlogg, B., Krajewski, J. J., Farrow, R.,Rupp, L. W., White, A. E., Short, K., Peck, W. F., and Kometani, T., Nature, 814 (1988). 11. Tokura, Y., Takagi, H., and Uchida, Nature, 345 (1989). 12. Oda,M.,Murakami,T.,Enomoto,Y.,and Suzuki, M., Jpn. J. Appl. Phys., L804 (1987). 13. Soeta, A., Suzuki, T., Takeuchi, Kamo, Usami, K., and Matsuda, P.,Jpn. J. Appl. Phys., L 1186 (1989). A.,Yuasa,T.,Inoue, N., Aihara,K.,andMatsuda, 14. Kamo, T., Doi,T., P., Appl. Phys. Lett., 59, 3186 (1991).


15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25.

26. 27. 28. 29. 30. 31. 32. 33.

463

Doi, T., Nabatame, T., Kamo, T., and Matsuda, S. P.,Supercond. Sci. Technol., 488 (1991). Cava,R. J.,Batlogg, B., Chen,C.H.,Rietman, E. A., Zahurak, M., and Werder, D., Phys. Rev., B36, 5719 (1987). Morgan, P.E.D., Piche, J. D., and Housley, R. M., Physicu C, 191, 179 (1992). Kase, J. I., Togano, K., Kumakura, H., Dietderich, D. R., Irisawa, N., Morimoto, T., and Maeda, H., Jpn. J. Appl. Phys., 29, L1096 (1990). Nabatame, T., Watanabe, K., Awaji, S., Saito, Y., Aihara, K., Kamo, T., and Matsuda, S. P.,Jpn. J. Appl. Phys., L1041 (1992). Doi, T., Okada,M.,Soeta,A.,Yuasa, T., Aihara, K.,Kamo,T.,andMatsuda, S. P., Physicu C, 183, 67 (1991). Ozkan, N., Glowski, B. A., Robinson, E. A., and Freeman, P. A., J. Muter. Res., 6, 1829 (1991). Kimura, T., Kishio, K., Kobayashi, T., Nakayama,Y.,Motohira,N.,Kitazawa, K., and Yamafuji, K., Physicu C, 192, 247 (1992). Jin, S., Tiefel, T. H., Sherwood, R. C., Davis, M. E., van Dover, R. B., Kammlot, G. W., Fastnacht, R. A., and Keith, H. D., Appl. Phys. Lett., 52, 2047 (1988). Murakami, M., Morita, M., Doi, K., Miyamoto, K., and Hamada, H., Jpn. J. Appl. Phys., L399 (1989). Murakami, M., Mod. Phys. Lett., B4, 285 (1990). Murakami, M., Flux pinning melt textured processed YBCO superconductors and their applications, in Studies of High Temperature Superconductors, Vol. 9 (A. V. Narlikar, ed.), Nova Science, New York, 1991, pp. 1. Ogawa, N., Hirabayashi, I., and Tanaka, S., Physicu C, 177, 101 (1991). Morita, M., Tanaka, M., Takebayashi, S., Kimura, K.,Miyamoto,K.,and Sawano, K., Jpn, J. Appl. Phys., 30, L813 (1991). Matsuda, S. P.,Soeta, Doi, T., Aihara, K., and Kamo, T., Jpn. J. Appl. Phys., L1229 (1992). Sasaoka, T., Nomoto, A., Seido, M., Doi, T., and Kamo, T., Jpn. J. Appl. Phys., L1868 (1991). Matsuda, S., Okada, M., Morimoto, T., Matsumoto, T., and Aihara, K., Muter. Res. Symp. Proc., 99, 695-698 (1988). Wilhelm, M., Neumuller, H. W., and Ries, G., Physicu C, 185-189, 2399 (1991). (Proc. Int. Conf. M2S-HTSC 111, Kanazawa, Japan, July 1991). Okada, M., Nabatame, T., Yuasa, T., Aihara, K., Seido, M., and Matsuda, S. P., Jpn. J. Appl. Phys., L2747 (1991). Matsuda, P.,Doi, T., Soeta, A., Yuasa, T., Inoue, N., Aihara, K., and Kamo, T., Physicu C, 185-189,2281 (1991) (Proc. Int. Conf. M2S-HTSC III, Kanazawa, Japan, July 1991).


Preparation and Properties of Tantalum Oxide Thin Films by Sol-Gel T. Ohishi Hitachi, Ltd. Ibaraki, Japan

INTRODUCTION Dielectric thin films are used in a variety of modern electronic devices, such as thin-film condensers and large-scale integrated (LSI) and electroluminescent (EL) devices [l]. Demands are strong for thin films having high dielectric constants and good breakdown-voltage characteristics for use in electron devices with superior performance and high degrees of integration There are several ways of preparing these films. Sputtering as a physical method of preparation and CVD (chemical vapor deposition) as a chemical method are well known. However, these methods have drawbacks; they require expensive vacuum equipment, and manufacturing of films with a large surface area or multicomponent films is difficult. The sol-gel method, which is a chemical method of film preparation, has received much attention for its advantages over the sputtering and CVD methods & it uses simple equipment and can produce large area and multicomponent films with high homogeneity at comparatively low temperatures. This chapter discusses the preparation of tantalum oxide thin films using the sol-gel method, which we believe is promising as a practical method for producing dielectric thin films whose dielectric constants and breakdown voltages are both comparatively high, Tantalum oxide films are good candidates for insulators in LSI devices, EL devices, and film capacitors Reports on the for-

OHISHI mation of fine particles of tantalum oxide prepared by the sol-gel method are avalable, but a detailed report on thin-film preparation is lacking.

OXIDE

II.

The basic reactions in the sol-gel method are the hydrolysis and condensation reactions shown here, which should proceed at a moderate rate.

+

Ta(OR)5 H,O

+ (R0)4Ta- OH + ROH

(R0)4Ta- OH + Ta(OR),

hydrolysis (R = alkyl group)

+ (RO),Ta - 0 - Ta(OR), + ROH

condensation

Factors influencing these reactions include the type of alkoxide reagents, the types of solvents and catalysts, the concentration, and the reaction temperature In practice, these factors affect each other in complicated ways, and ensuring that these reactions proceed ideally is not easy. It is necessary to identify the most influential factors and to optimize the reaction conditions. Dense transparent films are essential for insulation in electron devices. To produce high-quality films, a transparent sol suitable for coating must be prepared. The process for preparing these thin films is shown in Fig. 1. Ta (OC2Hs)s wasused as starting material. To the ethanol solution of Ta(OC2H5)5, the ethanol solution of H20 necessary for hydrolysis wasaddedslowly. After about 15 minutes a white spherical precipitate formed. Spectral and chemical

No HCI I

Proper Amount

HCI

I Excess HCI

Spin Coating

t i & Heat Treatment

Transparent Films

Syntheticprocedure for Ta2oS thin films.

TANTALUM OXIDE THIN FILMS BY SOL-GEL

467

analysis revealed that it had a structure similar to that of Ta(0H)s. The hydrolysis reaction between Ta(OC2Hs)s and H20 proceeded faster than the condensation reaction, and the alkoxy groups bonded to the Ta all appeared to be replaced with hydroxyl groups. When the correct amount of HC1 was added, however, no precipitates formed, and a uniform sol solution was obtained. The addition of HCl balanced the consecutive processes of the hydrolysis and condensation reactions. When excess HC1 was added, however, the sol solution became a turbid white. The reason for is thought to be particle growth at the high ionic strength. The factors with the greatest influence on these reactions are clearly the ratios of Ta(OC2Hs)s, H20, and HC1. Examining the ratios of these three ingredients in detail revealed that a clear sol solution was best achieved with proportions of5-18 mol% Ta (OC2H6)s,82-95mol% H20, and mol% HC1. Spin coating of the substrate surface with any of the three solutions (the one with the white precipitate, the clear sol solution, and the turbid white sol) produces a thin film. The films could then be densified through heat treatment. The thickness of the film can be controlled through the concentration of the sol, the number of revolutions per minute during spin coating, and the number of layers of spin coating before firing.

RESULTS AND DISCUSSION A.

Scanning Electron Microscope Observation of Surface and Cross-sectional Structure of Taz05 Thin Films

Figure 2 shows scanning electron microscope (SEM) photographs of the surface of thin films obtained from the three solutions. Spherical particles 1 4 in diameter can be observed in film (Fig. 2a) obtained from the solution with the white precipitates and no HCl. Film prepared from the clear sol with 1 mol% HCl added (Fig. 2b) is smooth and transparent. Excess HCl produced the white turbid sol, which resulted in the film in Fig. 2c. The smoothness is preserved here, but the film is opaque. The surfaces of the three films after firing at 800°C are shown in Fig. 3. The particles in Fig. 3a have shrunken visibly. No change is apparent in Fig. 3b, but the film in Fig. 3c is peeling. The reason for this peeling is not clear, but the cracks may be caused by vaporization of the excess HCl during firing. Thus, the thin film obtained from the clear sol prepared with precisely controlled quantities of Ta(OC2H5)s, H20, and HC1 exhibits favorable structural properties between the room temperature and 800°C. Figure shows SEM micrographs of the cross sections of a thin film prepared using the sol-gel method. For comparison, a film prepared using the sputtering method is also shown. fine film prepared by the sol-gel method was

4

470

OHISHI

c

BY SOL-GEL

TANTALUM OXIDE THIN

471

formed on the IT0 substrate, but there is no definite border between the IT0 film and the Ta2O5 film prepared by the sputtering method and the film thickness is not uniform. This is because, during sputtering, the IT0 film itself was sputtered off. The conductivity of the IT0 film thus decreases to two-thirds its originalvalue after the Ta205 film is formed. This phenomenon is well known. It is not seen in films prepared by the sol-gel method, there is no adverse influence on the first coating film by Ta205 film.

B.

Characteristics of TasOs Thin

We investigated how the properties of Ta205 films prepared by the clear sol vary according to the firing temperature. 1. Infrared Spectra Figure shows the infrared absorption spectra of Ta205 films dried at room temperature and fired at 200, 400, and 600°C. The room temperature film displays a wide variety of absorptions attributed toTa-0 stretching vibrations and Ta-O-Ta bending vibrations in the range of 400-1000 cm-1, indicating the formation of Ta205. In addition, -CH stretching vibrations and -CH2- rocking vibrations for ethoxy groups (-0C2H5) around 2900 and 1400 cm-1 suggest that

I

4000

l

l

3000

1600 1200

800

400

(cm-’) Figure 5 Infraredspectra of TazOs thin films after firing at

temperatures.

OHISHI

472

unreacted organic materials remain in the film. Also, absorptions at 3500 and 1600 cm-1 indicate the presence of water. As the firing temperature increases, absorption caused by organic materials and water decreases and is not observed in films fired at temperatures over X-ray Diffraction Analysis The x-ray diffraction patterns of Ta20s films fired at temperatures from room temperature to are shown in Fig. 6. At room temperature and 600"C, no distinct peaks are observed; the material is amorphous. appear beginning at indicating that the film has begun to crystallize. 800"C, the peaks attributable to orthorhombic p-Ta205 are visible, and the film is completely crystallized. This crystallization temperature is much higher than the for Ta2O5 films prepared using the sputtering and CVD methods 2.

Differential Thermal Analysis Figure 7 shows the results differential thermal analysis (DTA) of the film prepared at room temperature. A wide exothermic peak around and a

I

20

I J

40

60

80

28 X-ray diffraction patterns of Ta205 thin films after firing at various temperatures.

TANTALUM OXIDE THIN FILMS BY SOL-GEL sharp peak at can be seen. The infrared spectrum and x-ray diffraction results indicate that organic materials remain in the Ta2O5 film and that crystallization begins at over These facts suggest that the wide exothermic peak around in the DTA curve is attributable to the combustion of the organic materials and the sharp peak at is caused by crystallization of the amorphous film. 4. Refractive Index Figure 8 shows the relationship between firing temperature and the refractive indices. The refractive index for the room temperature film is 1.73, but value increases with the firing temperature, reaching 1.96 at For firing is 1.90. temperatures above 500"C, however, the value decreases, and at The initial increase in the refractive index is presumed to correspond to the increase in the film's densification as the unreacted organic materials bum The decrease in the refractive index at higher temperatures is thought to be caused by the lower density as the film crystallizes. This is discussed in detail .with the films' microstructure.

ElectricalProperties and Microstructure Figure 9 shows the relationship between firing temperature and the dielectric constant of the Ta205films. For the film prepared at room temperature, the dielectric constant is e = 12. This value increases with the firing temperature, for example, E = 28 at At temperatures above 400"C, the value decreases slightly. The rise in the dielectric constant between 200 and is striking. This is attributed tothe elimination of organic materials with low dielectric constants from the Ta205 films. According to the infrared spectra, organic materi-

0

z W

0

200

400

600

800

TEMPERATURE("C1 7 DTA

for Ta205 thin film.

1000

474

OHISHI

L I

0

200

400

600

800

TEMPERATURE("C) Refractive index of Ta205 thin film versus firing temperature.

i

0 i, -l

W n

0

I

I

I

I

200

400

600

800

J

TEMPERATURE("C) 9 Dielectric constant of Ta205 thin film versus firing temperature.

TANTALUM OXIDE THIN FILMS BY SOL-GEL

475

als are present in the film fired at 200"C, but no absorption by organic materials is apparent in the film. The DTA results also show heat generation at around from the combustion of organic materials. The dielectric constant & = 28 of the film is equal to that of films prepared by sputtering. The relationshipbetween firing temperature andthebreakdown voltage characteristics of Ta205 films is shown in Fig. 10. The breakdown voltage of the room temperature film is 2.5 MV/cm, and value increases with the firing temperature. It is about 3.0 MV/cm at 400 and Above this, however, it drops sharply. For a firing temperature of it is 1.6 MV/cm, and for is only 0.6 MV/cm. This abrupt drop in the breakdown voltage is thought to be caused by changes in the microstructure of the films during heating. No notable change in the surface of the Ta205 film isvisible by SEM observation. Thus the changes must be very small. We therefore examined the microstructure the Ta2O5 films using transmission electronmicroscopy (TEM). Figure 11 showshowtheTa2O5 film microstructure changes withfiring temperature, as observed through TEM. The thin film prepared at room temperature is a conglomeration fine spherical particles. The film has the same spherical structure, but the size of particles is slightly larger. It is pre-

(3

50 >

S0

n

m

I

0

200

I

400

600

800

TEMPERATURE("C) Breakdown voltage of Ta205 thin film versus firing temperature.

E M micrographs Ta205thin films (16.7 nm)heatedat peratures: (a) room temperature, (b) 400"C, (c)

tem-

TANTALUM OXIDE THIN FILMS BY SOLGEL

i

477

478

OHISHI

sumed that the particles grow during firing. The electron diffraction patterns for both show halo patterns; both are amorphous. The structure of the film fired at is quite different from the spherical structure. Furthermore, its selective area electron diffraction pattern shows a clear diffraction grating pattern, indicatingthatcrystallization is taking place.Some places inthis film still show halo patterns, however, depending on the area examined by TEM. Thus this is apparently a mixture of amorphous and crystalline materials. The film consists of thin flakes and thereare pinholes. The finestructure of the film is not preserved. The electron diffraction pattern showed a perfect diffraction grating pattern, indicating that the film has entirely crystallized. The microstructure of Ta205 films thus varies greatly depending on the firing temperature. The fine structure is preserved through but disappears with crystallization. Therefore, the sudden drop in the breakdown voltage for firing temperatures above is caused by degradation of the fine structure and the appearance of pinholes with rearrangement of the microstructure during crystallization. firing temperature of is clearly best for preparing Ta205 film with both high dielectric constants and favorable breakdown voltage characteristics.

IV.

CONCLUSIONS

Tantalum oxide thin films were prepared using the sol-gel method, and their spectralandelectricalpropertieswere examined. The following conclusions were derived:

2.

3. 4.

5.

To obtain a fine, transparent film, HCl should be added during the hydrolysis of tantalum &oxide. An amount between and 4.0 mol% is best. Tantalum oxide films prepared by the sol-gel method are amorphous from room temperature to and are completely crystallized at Thin films prepared at room temperature contain organic materials,but these can be eliminated by firing at or above. The dielectric constant of the thin films increases as the organic materials are eliminated. For films processed at 400"C, = The breakdown voltage of the film increases with the firing temperature This value drops sharply for and is about 3.0 MVkm for films at films fired above This is attributed to degradation of the fine structure as a result of crystallization. To obtain a tantalum oxide thin film with a high dielectric constant and favorable breakdown voltage characteristics, it is bestto fire the film at

TANTALUM OXIDE THIN FILMS BY SOLGEL

479

(a) Ohta, K., Yamada, K., Shimizu, K., andTarui,Y.,Quadruplyself-aligned VLSI dynamic stackedhigh-capacitance RAM usingTa2Oshigh-density memory, IEEE Trans. Electron Devices, 368 (1982). (b) Tiku, K., Choice of dielectrics for " E L displays, IEEE Trans. Electron Devices, 31, 105 2. Asai, Trends in megabit DRAMS, IEDM Tech. Dig., I , 6 (1984). Melnick,B.M.,Araujo,C.A.,Mcmillan, L. M.,Carver,D.A..andScott.

J.F.,Recentresultsonswitching,fatigueandelectricalcharacterizationof sol-gelbased PZT capacitors,in hoc. SecondSymp. on IntegratedFerroelectrics, 1990, pp. 79-93. Sheppard, L.M., Advances in processing of ferroelentric thin films, Am. Ceramic Soc. Bull., 71, 85, (1992).

Yamagishi, K., and Tarui, Y., Photo-CVD of tantalum oxide film from pentamethoxytantalumfor VLSI dynamicmemories, Jpn. J. Appl.Phys., 25, L306 (1986). Shinriki, H., Kisu, T., Kimura, Nishioka,Y.,Kawamoto,Y.,andMukai, K., Capacitortechnologycompatiblewithadvanced VLSI process, in Symp. VLSI Technology Tech. Dig., San Diego, 1988, pp. 29-30. Ogihara, T., Ikemoto, T., Mizutani,N., and Kato, M., Formation of monodispersed Ta20s powers, J. Mater. Sci., 21, 2771 (1986). Jean, J., Synthesis of monodispersed Ta20s powers, J. Mater. Sci., 25, 1013 (1990). 6. Brinker, C. J., and Scherer, G.W., Sol-Gel Science, Academic Press, San Diego, (1990). 7. Okada, M., Preparation and properties of TazOs thin film prepared by CVD and sputteringmethod(Japanease), Denki Kagaku (Electrochemistry), 53, 109 (1985).


21 Crystalline and Amorphous Thin Films of Ferroelectric Oxides Xu University of Utah Salt Lake City, Utah

INTRODUCTION Chemical and physical processing techniques for ferroelectric thin films have undergone explosive advancement in the past few years (see Ref. 1, for example). The use of PZT (PbZr1-~TixO3)family ferroelectrics in the nonvolatile and dynamic random access memory applications present potentially large markets [2]. Other thin-film devices based on a wide variety of ferroelectrics have also been explored. These include multilayer thin-film capacitors [3], piezoelectric or electroacoustic transducer and piezoelectric actuators piezoelectric ultrasonic micromotors [7], high-frequency surface acoustic devices [8,9], pyroelectric intrared (IR) detectors [lo-121, ferroelectric/photoconductive displays [13], electrooptic waveguide devices or optical modulators [14], and ferroelectric gate and rneWinsulatodsemiconductortransistor (MIST) devices [15,16]. Recently, efforts have been devoted to the fabrication and characterization of PbZr~~TixO3 family thin films for their potential applications in nonvolatile memory devices (See Ref. 17, for example). Partly because of the convenient stoichiometry control during processing, it was found that chemical methods, such as sol-gel and metal organic decomposition (MOD), are superior to physical means in many aspects. appreciate better the science and technology of ferroelectric thin-film fabrication, it is important to give a brief account of the past efforts and the present and, it is hoped, shed some light on the future.

xu The first report of a wet chemical processing of ferroelectric thin film was by Fukushima et al. in 1975 [18]. They reported the use of a mixed alkoxide and organic salt precursors in the fabrication of BaTiOs film. Application of sol-gel processing for the PZT thin films was started in 1984 by Wu et al. [l91 and Fukushima et al. [20] and followed by Budd et al. in 1985 [21]. More recently, continuing efforts in the processing of PZT family thin films by sol-gel and MOD methods can also be found in the literature [22-271. Meanwhile, chemical processing of thin films of other ferroelectric oxides resulted in remarkable progresses. One now can make ferroelectric thin films with crystallinities ranging from polycrystalline to texture-oriented polycrystalline to epitaxial in nature. These include (1) polycrystalline films by Hirano and Kat0 [28] for LiNbOs, Xu et al. [29] for Sr1-xBaxNb206 (SBN), Francis and Payne [30] for PMN-PT, Swartz et al. [31] for Pbl-,, LayZr~-~TixOs, and Nazeri-Eshghi et al. [32] for KNbO3; (2) texture-oriented films byXu et al. [33,34] for SBN and LiNbOs and by Hagberg et al. [35] for LiNbOs; and (3) epitaxial films by Partlow and Greggi [36], Hirano and Kat0 [37], and Xu et al. [34] for LiNbO3, Cheng et al. [38] for KNbO3, and Hirano et al. [39] for PbTiOs. In the following sections, we attempt to discuss some important aspects of sol-gel processing technique through a few model systems. Examples are given to illustrate the formation of various morphologies.Enoughdetails are cussed to aid those readers interested in sol-gel processing of other multicomponent oxide thin films. Finally, we devote one section to the fabrication and characteristic of amorphous thin films of a few ferroelectric compositions made by sol-gel. We further present some phenomenological observations and our preliminary attempts to understand them.

II. It is now appropriate to differentiate the sol-gel method from MOD. In general, we consider a process sol-gel if it involves these two characteristic chemical reactions: (1) hydrolysis of at least one precursor compound and (2) polycondensation of the hydrolyzed precursor compound. MOD process involves the thermal decompositions of all precursor compounds physically condensed onto substrates through rapid solvent evaporation. Therefore, methods using all alkoxide andpartially alkoxide precursors are considered sol-gel processing, and those using precursors other than alkoxides are MOD. One last interesting point to emphasize is that one can useall alkoxide precursors ina MOD process. This is possible when neither of the two characteristic reactions for sol-gel is involved and the reactions used to form the oxide is simply thermal decomposition of the metal alkoxides.

FERROELECTRIC OXIDE

THIN FILMS

483

Like any other thin-film deposition techniques, sol-gel processing is essentially a mass transport process. The transformation of a liquid solution to a solid crystalline film is accomplished through three steps: Precursor materials are dissolved in a homogeneous solution, thus assuring molecular-level mixing of different precursor compounds. 2. Mass transport is completed upon spin or dip coating of a thin layer of the solution onto the substrate surface. Because of a combined process of hydrolysis-evaporation-polycondensation,a thin layer of amorphous gel film is formed on the substrate. 3. The as-deposited thin film together withthesubstrate is thenheatedto cause densification and crystallization of the film. simple schematic diagram is shown in Fig. where a similar diagram for MOD is also included for comparison. Note that for sol-gel and MOD steps 1 and are similar, but step 2 for MOD is more physical than chemical in nature in that the formation of the amorphous film on the substrate is due solely to the evaporation of solvent and the disordered precipitation of solutes from the supersaturated solution. Ferroelectrics of interest are often multicomponent oxides. The metal elementsin concern do not always show enough solubilityingivensolvent. Therefore, the choice of precursor compounds and the dissolution procedures, as well as their behaviors toward moisture and heat, are all important aspects to consider. We select a few representative ferroelectric systems to illustrate the practices reported on the solution preparations.

A.

PbZr~-~TixOs and Pb1-yLa~Zrl-~TixO3 Systems

Homogeneous solutions containing Pb, Zr, Ti, and La in the correct proportion can be made with soluble precursors in alcoholic solvents. Table lists examples of precursors and the corresponding references. The choices of precursor compounds for Zr, Ti, and La in these examples are mainly for solubility reasons. The use of methoxyethanol as solvent has two effects: first, it is a chelating agent, which prompts the dissolution of lead acetate, and second, it is an ideal solvent with enough volatility to be used in practical coating procedures.

B.

S r i - ~ a d b n O sSystem

SBN is a solid solution system containing strontium, barium, and niobium oxides. The sol-gel solution preparation requires special precautions. This is because the alkaline-earth metal alkoxides dissolve in alcohols slowly and they are extremely moisture sensitive. Although alkoxides of both barium and stron-

xu Sol-gel:

Solution

1

Step 2.

Step 3.

I

Spin’Dip Coating

I

Hydrolysis-Polycondensation

Treatment Heat

I

Hydrolysis-Polycondensation

MOD

Step 2.

I 7:z:iI

Step3.1

Heat Treatment

Evaporation

l

Thermal Decomposition

Sol-gel and MOD methods.

tium can be made easily by allowing reactions between the metal powders directly with the corresponding anhydrous alcohol, the solution thus made is difficult to preserve even under a dry atmosphere. Precipitation problems may arise because of either slow dissolution of Ba(OR)2 and Sr(OR)2 in thesolvent or the hydrolysis of the alkoxide to form less soluble hydroxides. On the other hand,reacting these metal powderswith an alcoholicsolution of Nb(OR)5 helps the dissolution of the alkaline-earth alkoxides caused by the formation of double-alkoxide complexes, for example BaNb2(OC2H5)nr which are readily soluble in the parent alcohol Xu et al. prepared an ethanol solution of 1 mol of 60% Sr(OC2H5)2 + Ba(OC2H5)2 and 2 mol Nb(OCzH5)5 by reacting strontium and barium

FERROELECTRIC OXIDE THIN FILMS Table

ecursor

485

Common Precursors Used for PZT and PLZT Thin Films

Metal

232-Ethylhexanoate 22, 20,n-Propanol 19, Acetate Ethoxide Neodecanoate zr n-Propanol n-Propoxide Acetylacetonate 20 26 n-Butoxide Ti n-Propanol i-Propoxide n-Butoxide ZEthylhexanoate 40 La

Pb

Methoxyethanol 2521, 26 40 19,21-23,25,40 19.21-23,25,40 20,26

metals with anhydrous ethanol and subsequently mixing with Nb(OC2&)5 lution in ethanol.

LiNbOs System This is one of cleanest and simplest systems with which to work. Lithium and niobium ethoxides are readily soluble in ethanol. These same precursors and solvents are used widely by several research laboratories. However, the sol-gel processing procedure deviates markedly. Two types of solutions are made for LiNbOs thin-film deposition. The first contains stoichiometric LiOC2H5and Nb(OC2H5)5 witha certain amount of H20 addedin the form of dilute HOC2H5 solution and gels within a few days The second type contains no preadded water and is stable for a year It was argued that the &oxides involved here are quickly hydrolyzed within a few minutes by the ambient moisture Therefore, no water addition is necessary, except the moisture in the ambient during spin and dip coating. Further, we show in the latter part of this chapter that such an arrangement is a better way of controlling hydrolysis.

D.

KNb03 System

Nazeri-Eshghi et al. were the first to report sol-gel processing of KNbO3 ceramic powders. Their method was later adopted for the deposition of epitaxial thin films by Cheng et al. The precursors usedwere KOC2H5 and Nb(OC2H5)s. The preparation procedure is fairly simple. Potassium metal is allowed to react with anhydrous ethanol, and the solution is then mixed with Nb(OC2H5)s. The solutions are usually stable for a few weeks. It was found that the addition of small amount of 2-ethylhexanoic acid helps to modify the

ca

xu hydrolysis behavior of the solution, and subsequently better coating was obtained

Other reported ferroelectric systems include PbTiOs BaTi03 and PMN-PT Solution preparation for these systems is similar to the procedures already discussed. The reader should easily find precursors and appropriate procedures according to these examples.

CRYSTALLIZATION After the homogeneous solutions are prepared, simple coating procedures are used to deposit thin gel films by either dip or spin coating on substrates. Alcoholic solutions are generally capable of wetting all oxide substrates or metalsemiconductor substrates with a thin oxide layer on the surface. simple capillary tube experiment was performed to provide semiquantitative data on therelative surface tensionbetween alcoholic solutionsandcommon substrates. Table 2 lists the results. We use the product of the surface tension with the cosine the contact angle to represent the afinity of the solution to the corresponding substrates. By normalizing all data against the value for ethanol on a VWR Microslide, it was found that the relative surface tensions ethanol on all substrates tested are comparable in value. Therefore, it was concluded that alcoholic solutions are a good solution system for coating on oxide substrates. After coating of the gel films on substrates, the thin films together with the substrates are usually slowly heated to a few hundred degrees Celsius and maintained at that temperature for a few hours to cause crystallization of the ferroelectric crystalline phase. Depending on the typeof substrate and the processing procedures, crystalline films with different morphologies can be obtained. The

Table 2 Relative Surface Tension cos 8 HOCfl5 on Various Substrates

Substrates

VWR Microslides Silicon (1 11) Fused Sapphire (0.12) LiNbOs

COS

ely COS e 1

1.14 1.14

FERROELECTRIC OXIDE THIN FILMS

487

most common product is polycrystalline film with no apparent preferred crystallographic orientations. We present a few examples in the following section.

A.

PolycrystallineFilms

Polycrystalline films are usually obtained when any one of the following substrate types is used polycrystalline substrates, amorphous substrates, and single-crystal substrates with large lattice mismatch. For example, LiNbO3 [34] and SBN [44] grownon silicon (1 11)and(100) are polycrystalline. SBN grown on fused silica was also polycrystalline [29]. PZT thin films were grown on a large numbers of substrates, nearly all of which were polycrystalline in nature (see Ref. 45, for example). When polycrystalline films are the inevitable result, problems associated with the formation of grain boundaries, impurities, second phases, and on, are the central concern. For PZT family ferroelectrics, one of the main problems is the fatigue of the materials after large numbers of repetitive switchings, which is attributed to the domain wall pinning on defect structures, second phases, and grain boundaries [46]. A special heat treatment technique, rapid thermal annealing, is commonly used to reduce extensive grain growth, which was believed to improve the fatigue behavior of PZT thin films [45]. When optical application is the target for these polycrystalline films, scattering of light by the grain boundaries, second phases, and other defect structures are to be addressed. In general, the problems are far from being resolved at present.

B. Texture-OrientedFilms Texture-oriented films may be obtained under several special conditions. First, when single-crystal substrates with fairly large lattice mismatch were used, the film grown over the surface may be highly preferentially oriented.For example, for LiNbO3 grown on a sapphire (012)face, the x-ray diffraction patternsof the film indicated two principal directions only, the (012) and (300) [34]. Alternatively, when an amorphous substrate was used, it was found that because of the sensitivity of the ferroelectric crystallites toward the electrical field, the films can be grown with a preferred orientation if small a dc bias field is applied along the substrate surface direction during the heat treatment. This was reported by Xu et al. [33] for SBN grown on fused silica. Figure 2 shows this result. The x-ray diffraction pattern shows significant enhancement of peak intensities for (130), (121), (131), and (620)for the tungston bronze structure.These represent oxygen close-packing planes preferentially lined up along the external field direction. third type of textural orientation was obtained in the arrangement in which LiNbO3 film was grown on platinum [35]. It was found that with a rapid heattreatmentprocedure,LiNbO3preferentiallyorientedwiththe(006) face parallel to the substrate surface. The cause of this orientation effect is not clear.

xu (Sr0.60Ba0.40)Nb206 t h i n film on fused silica, 850 "C. 1 h; w i t h E = I kVlmm. thickness: A

E 1

6 40

0

I60

(Sr0.60Ba0.40)Nb206 thin film on fused silica, 850 "C. I h; without E . thickness: A

I20

40

- .

I

I

I

Diffractionangle

1

I

(&g)

2 X-raydiffractionpatterns ofSro.6Bao.4Nb206thin films grownonfused silica. Sample heated in the presence of a dc field showed preferential orientation, and peaks with indices and were significantly enhanced. (AfterXu et al.

C.

Epitaxial

When single-crystal substrates with a small lattice mismatch are used, sol-gel produces epitaxial films for a few ferroelectric systems.Although epitaxial growth of crystalline films from an amorphous layer has been observed in the amorphous silicon to silicon transformation, sol-gel epitaxy onlybegan to emerge as a possible fabrication technique in the last few years. Hirano and Kat0 were the first to observe the epitaxial growth of LiNbO3 on the sapphire 10) face Xu et al. found the epitaxial growth of L i m o 3 on the LiTa03 (1 10) face and the LiNbOs (006) face. Epitaxial KNbO3 was reported

489


on a SrTiO3 substrate and PbNbo.02 (Zro.52 Ti0.48)0.9803on sapphire (012) Figure 3 shows a set of x-ray diffraction patterns of LiNbO3 grown on showed various single-crystal substrates. LiNbO3 on silicon (1 11) and polycrystalline diffraction patterns but onsapphire (012) showed only principal diffraction peaks, (012) and On LiTaOs (110) and LiNbOs (006) the films grown were clearly epitaxial. The electron diffraction pattern and the energy-dispersive spectroscopic (EDS) analysis of composition at the interface LiNb03LiTa03 is showninFig. The diffractionpatternshowstheview along the [221] zone axis; EDS shows the elemental distribution of Nb5+ and

LN/Si(lII)

5

=

LN/LT( I I O )

I

LN/LN(006)

70

30

TWO THETA

3 X-raydiffractionpatternsof Limo3 films grownonvarioussingle-crystalsubstrates.PolycrystallinediffractionpatternswerefoundforLiNbo3onsilicon (1 11) and silicon (100). Highly oriented LiNbog was found on sapphire (012). Epitaxial LiNb03 was found on LiTaOs (110) and Limo3 (006).

0.5

0.4

0.2

0. l

0.5

0.0 0.0

2.0

0.0

2.0

ENERGY (IO3eV)

4 (a) Electron diffraction pattern of zone at the interfacial area between LiNbO3 and LiTaO3 substrate. (b) EDS peaks of Nb(La, Le), Ta(Ma, Mp), and across the interface with a lo00 beam size. On the substrate side only Ta(Ma, Me) and were found; on thefilm side the Nb(La, LP) peak is predominant, with only a small peak corresponding to Ta(Ma, Mp).

491

FERROELECTRIC OXIDE THIN

Tas+ across the interface line. The lattice mismatch between the film being grown and the substrates are probably responsible for the various degrees of orientation. Table summarizes the percentage of lattice mismatches for L i m o 3 with various single-crystal substrates. In addition to a lattice-matched substrate, sol-gel epitaxy also requires excellent stoichiometry control of the solution. Most of the demonstrated sol-gel epitaxies were performed at relatively low temperatures. Therefore, molecularlevel homogeneity is essential. To appreciate fully the delicate features solgel epitaxy, it is now appropriate compare sol-gel processing with other thinfilm deposition techniques. In the attempt to achieve optical signal processing, modulation, amplification, and memory functions in integrated circuits similar to those on electrical signals by semiconductor devices, integration of ferroelectric devices is the ultimate goal. However, to achieve integration of microscopic devices based on materials as complex as oxide ferroelectrics, which are predominantly multicomponent metal oxide compounds, reliable thin-film deposition techniques are critically needed. One of the most important aspects of multicomponent oxide thin-film deposition is the control of stoichiometry. We can now to understand the existing deposition techniques by analyzing the precursors. First, we introduce the terms compositionally “proper” and“improper”precursors.A compositionally proper precursor has on the molecular level precisely the same metal-metal ratio ( M ” ’ ) as the oxide compound one wishes to prepare. An improper precursor does not possess the correct ”/”‘ ratio on the molecular level. It may or may not possess the correct M’/”’ ratio on a larger scale, for instance, microscopic (or nanoscopic) scale, as in a micrometer-sized powder mixture. We immediately conclude that most of the existing deposition techniques use improper precursors. For example, such techniques as metal oxide chemical vapor deposition, multiple-target sputtering, and multiple-target evaporation use improper precursors. In these cases, single metal organic compounds or single metal oxide targets are used for the deposition, and thus arise the associated difficulties in stoichiometry control.

Table

LatticeParametersandMismatchesBetween LiNbOs and Single-Crystal Substrates Crystal Limo3 LiTa03 Sapphire Silicon Silicon

CH (A)

-

aH (A)

Mismatch (%)

-

xu These are often caused by the variation in evaporation rate, sticking coefficients, and inhomogeneous spatial distribution of components in the deposition apparatus. Single-target techniques, such as sputtering, evaporation, and laser ablation, use a composite target with overall correct stoichiometry or sometimes the compound itself in polycrystalline form. However, if we consider the precursors after the liquid vapor or plasma has left the target, the liquid, vapor or plasma in these cases becomes improper before the moment of arrival on the substrate. In other words, the precursors lose the correct M'/"' ratio on the molecular level. Therefore, these depositions are in nature similar to other processes using improper precursors. One mightconsider these deposition techniques as having an improper deposition mode. Sol-gel processing, on the other hand, uses proper precursors and can facilitate a completely proper deposition, for example for the epitaxial growth of LiNbO3 thin films. Precursors used in this case are LiOC2Hs and Nb(OC2Hs)s. double-alkoxide LiNb(OC2Hs)6 is prepared in alcoholic solution by extensive refluxing of a mixture of LiOC2Hs and Nb(OC2Hs)s The solution ofLiNb(OC2Hs)fj is a proper precursor. In addition, the double-alkoxide LiNb(OC2Hs)6 has nearest neighbor atomic arrangement remarkably similar to that of a crystalline LiNbO3, as shown separately in Fig. 5a and 5c. This lution is then used to coat thin films directly onto substrates. When the hydrolysis is controlled, the complex structure can be preserved during the solgel processing as a result of the steric hindrance of the bridging -0C2Hs groups to hydrolysis. In particular, if no water was added to the solution before the

5 Nearest neighbor shucturesof

double-ethoxide LiNb(OC&)6 in solu-

tion; (b) amorphous LiNbo3; and (c) crystallineLimos.


493

deposition, the hydrolysis occurs only during and after the spin or dip coating. Therefore, the preservation of such nearest neighbor structures is accomplished through immediate polycondensation after the double alkoxides are partially hydrolyzed by the ambient moisture. The as-deposited amorphous gelfilm, having a nominal composition LiNb0~x(OC2H5)2~, presents the nearest neighbor structure shown in Fig. 5b. Uponheating to a few hundred degrees Celsius L i m o 3 is crystallized through a local relaxation rather than extensive longdistance diffusion. Therefore, such sol-gel processing constitutes proper deposition with proper precursors. The mechanism of sol-gel epitaxy has not yet been explored in detail. The thermodynamic driving force for epitaxy is probably different from that for common solid-state epitaxy. Miller and Lange [49] studied a simpler system, zro;? epitaxial growth, and proposed a simplistic model for the epitaxy. It is generally considered that the nucleation in a gel film is three rather than two dimensional according to the Avrami model of thermal analysis. This suggests that the epitaxy occurs after nucleation. The Miller and Lange model is consistent with such an argument. A common problem associated with all singlecrystal films derived from sol-gelepitaxy, including LiNbO3[34,36,37], KNbO3 [38], PbTi03 [39], PNZT [47], and ZrO2 [49], is the porous and defective nature of the films. No effective solution to this problem has yet been reported.

W. AMORPHOUSFILMSOF FERROELECTRICS One of the direct outcomes of the proper deposition of a proper precursor by sol-gel processing is the possibility of preparing a structurally controlled amorphous thin film. Ferroelectricityhas been commonly associated with crystalline materials, although it has been postulated that this phenomenon can occur in an amorphous solid [51]. In the past,attempts were reported to demonstrate experimentally thepossibility of amorphous ferroelectricity.For example, a glassy L i m o 3 was produced by rapidly quenching molten LiNbO3 [52]. A dielectricanomalywasobserved in theglassyLiNbO3,attributed to possible amorphous ferroelectricity. Similarly, glassy LiNbO3, when doped with iron, showed an anomalous temperature dependence of resonance, quadruple splitting,and center shift, characteristic of a ferroelectricphasetransition[53]. Radio frequency sputtered amorphous LiNbO3 [54,55] and PbTiO3 [56] also showed anomalous dielectric behaviors below the crystallization temperature. However, the issue of amorphous ferroelectricity remains highly controversial because of the lack of concrete physical evidence for the existence of such a phenomenon. A more realistic approach to this controversy is probably to look at the materials from a strictly phenomenological point of view. We know that sol-gel processing is particularly convenient for the preparation of amorphous

xu films on a wide variety of substrates. It is worthwhile to compare systematically the properties of an amorphous film of ferroelectric composition with those of a crystalline film.

A.

Experimental Results

Amorphous LiNbO3 films made by sol-gel processing were subjected to a series of characterizations [57]. It was found that an amorphous LiNbO3 film obtained by heating the gel film at 100°C for 2 h showed P-E hysteresis with remnant polarization PT = 10 $/cm2 and coercive field Ec = 110 kV/cm. Electron diffraction of such film showed a diffuse ring pattern characteristic of an amorphous nature. These are shown in Fig. 6 in which the scale for E is 147 kV/cm division and that for P is 5.6 pC/cm2 division. Further measurement showed a pyroelectric coefficient of 8 pC/cm2 K at 28°C. Note that for singlecrystal LiNbO3, PT = 50 pC/cm2 and the pyroelectric coefficient was reported to be 20 pC/cm* K [l]. Further, a piezoelectric resonance was observed at similar frequency range for both amorphous and crystalline LiNbO3, characteristic of a ferroelectric material [57]. systematic comparison of the electrical and optical properties of amorphous and crystalline Pb(Zr0.52Tio.48)03 was reported by Xu et al. [57,58] It is noticeable that the amorphous film has lower values than the crystalline film for all parameters, consistent with the observations in a LiNbO3 system. The most remarkable characteristic of amorphous Pb(Zro.52Ti0.48)03 film is the exceptionally high pyroelectric coefficient. Figure 7 shows the pyroelectric current of an amorphous Pb(Zr0.52Tio.48)03 film measured 2, 44, and 162 h after poling at 150°C. The film remained highly pyroelectric several months after the original poling. It is thus possible to use these amorphous films in some applications, in this case pyroelectric heat sensors, for which crystalline ferroelectrics were the typical materials of choice. We have shown that although amorphous ferroelectricity as a physical phenomenon is not fully demonstrated, amorphous films of ferroelectric oxides have shown a number of useful properties that may warrant further studies of their structure and properties. It is possible to use them in limited areas in place of crystalline ferroelectrics.

B.

Structural Model

model was proposed by Xu et al. [57] to elucidate the preceding observations in an amorphous material. we have discussed, the nearest neighbor structure of the amorphous LiNbO3, as shown in Fig. 5b, consists of building blocksoflithium-andniobium-centeredoxygen octahedra sharing a face. When many of these building blocks come together, there are clusters of octahedral pairs in the amorphous film in the form shown in Fig. 8a. The schematic


6 (a) P-E hysteresis loop of anamorphous LiNbo3 film coated on a goldplated silicon wafer with a platinum top electrode (at 60 Hz; scale x axis; 147 kV/cm division, and y axis, 5.6 pC/cm2 division. (b) Electron diffraction pattern of the amorphous LiNbO3 film; the diffuse ring indicated the amorphous nature of the

xu 60

o 2hafterpoling o 44hafterpoling A 162hafterpoling heating after (24O0C/O

Q v

, !

E

.-

40

L

P)

e

P)

20

Sample: Au amorphous PZT(52/48) film Au. Electrode area= 3.14 X 10-%m2. PZT thickness= 0.3pm. Poling condition: d.c. field= irkVImm, T = 0 20

40

80

60

100

Temperature (%)

7 Pyroelectriccurrent of heated at 350°C for 1 h, and the thickness

PZT

film

gold. Filmwas

A. (After Xu et al. [58].)

representation of these clusters shows oriented octahedral pairs within the cluster; adjacent clusters may possess a different overall orientation. By comparing with a typical crystalline LiNbO3 structure (Fig. 8b), we make the following observations: (1) each individual cluster may possess a unique polarization throughtheinteraction of neighboring octahedral pairs; thepolarization within a cluster may be smaller than in the crystallinestructure of comparable size; (3) the order within a cluster need not be that of a crystalline order, since the sequence neednot be that of oxygen octahedra centered with Nb-Li-vacancy-Nb-Li-vacancy; the adjacent clusters may interact to favor one polarization direction to allow projection of polarizations along a unique macroscopic direction;and whensubjecttoanexternalelectricalfield,the 57) may be polarizations of individual clusters (referred to as ferrons in Ref. reversed to allow macroscopic polarization along the external field direction. These observations are preliminary: the model is also our first attempt to understand the observations. Although ferroelectricity can be consistent with an amorphous structure in theory, to be able to demonstrate such phenomenon unequivocally is by no means an easy task. However, the preceding discussion may be helpful in shedding light on future efforts in the sense that it suggests a possible avenue to prepare “structurally controlled” amorphous materials, which may be essential to the preparation of any amorphous material with “locally dialectically soft” structural units, as proposed by Lines After all,

8 (a) clusters orientedoctahedralpairsand (b) typicalarrangement the octahedral pairs in crystalline LiNbO3.

xu ferroelectricity relies on the existence of reversible electric dipoles, which by andlarge are constructed through a certain degree of shortto intermediate range order.

V. It has been demonstrated that sol-gel processing can be used to grow ferroelectric thin films with a wide range of qualities: they are polycrystalline, oriented, epitaxial, and amorphous. The advantages of sol-gel over other methods include convenient stoichiometry control, low heat treatment temperature, and simple processing procedures. One could easily grow and study ferroelectric thin films of all varieties with simple laboratory equipment. However, it is important to be aware that for the sol-gel films compete with those from other deposition techniques, especially integrated ferroelectric device applications, a systematic analysis of current deposition techniques, which may provide insight into the future development of the sol-gel method, is critically needed. Areas in need of improvement include (1) the nanopores and associated defect structures in the sol-gel epitaxial films; (2) better understanding and systematic study of the solution structure and its effect onsol-gel processing; and more rigorous structural and property investigationsof the amorphous films made by sol-gel and the correlation between these. Sol-gel processing is a very promising method for the fabrication of multicomponent oxide thin films. There are still many unanswered questions and many obstacles to overcome. Nonetheless, there is little doubt that a brighter future lies ahead for this technique given the vast numbers areas of demonstrated applicability for this method and the degree of success reported to date.

REFERENCES 1. Xu, Y . H.Ferroelectric Materials and Their Applications, North-Holland, Amsterdam, 1991. Ferroelectric Thin Films (E. R. Myers and A. Kingon, eds.), Materials Research Society, Pittsburgh, (1990). 2. Chapman,D. W., J. Vac. Sci.Technol., 9, 425(1972).Pazde Araujo, C. McMillan, L. D., Melnick, B. M., Cuchiaro, J. D., and Scott, J. F., Ferroelectrics, 241 (1990). Feuersanger, A. E., in Thin Film Eielectrics (F.Vratny, Electrochem. New York, (1969), p. 209. 4. Destefanis, G. L.,Gaillaiard, J. P.,Ligeon, E. L.,Valette, Farmery, B. W., Townsend P. E., and Perez, A., J. Appl. Phys., 420 (1979). 5. Foster, N. F.,J. Appl. Phys., 50, 7898 (1979). 6. Mansingh, A., Ferroelectrics, 69 (1990). 7. Udajakumar, K. R., Bart, F., Flynn, A. M., Chen, J., Tavrow,L. Cross,


8. 9. 10.

11. 12. 13. 14. 15. 16. 17. 18. 19.

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32. 33. 34.

L. E., Brooks, R. A., and Ehrlich, D. J., Proc. IEEE Micro Electro Mechanical Systems (Nara, Japan), (1991), p. 109. Castellans, R. N., and Feinstein, L. G., J. Appl. Phys., 50, 4406 (1979). Adachi,H.,Mitsuyu,T.,Yamazaki, O., andWasa, K., J. Appl. Phys., 60, 736 (1986). Takayama, R., Tomita, Y., Lijima,K.,andUeda,I., J. Appl.Phys., 61, 411 (1987). Glass, A. M., and Abrams, R. L., J. Appl. Phys., 4455 (1970). Takayama, R., Tomita, Y., Lijima, K., and Ueda, I., J. Appl. Phys., 61,411 1987. Chapman, D. R., and Mehta, P. R., Ferroelectrics, 3, 101 (1972). Webster, J. C., and Zernike, F., Ferroelectrics, IO, 249 (1976). Wu, Y., Ferroelectric, 11, 376 (1976). Wu, Y., IEEE Trans. on Electron. Devices, ED-21, 499 (1974). Symp.Proc. lst, 2ndand3rdInt.Symp.Integr.Ferroelect.,ColoradoSprings, Colorado, 1989,1990,1991. Fukushima, J., Kodaira, K., Tsunashima, A., and Matsushita, T.,Yogyo Kyoaishi, 204 (1975). Wu, E., Chen, K. C., and Mackenzie, J. D., in Better Ceramics Through Chemistry (C.J. Brinker, D. E. Clark, and D. R. Ulrich, North-Holland,New York, 1984, p. 169. Fukushima, J., Kodaira, K., and Matsushita, T., J. Mater. Sci., 595 (1984). Budd, K. D., Dey, K., and Payne, D. A., Br. Ceram. Soc. Proc., 36,107 (1985). Chen, K. C., Janah, A., and Mackenzie, J. D., in Better Ceramics Through Chemistry Il, (C. J. Brinker, D. E. Clark, and D. R. Ulrich, eds.), Materials Research Society, Pittsburgh, (1986), p. 731. Lipeles, R. Coleman, D.J.,andLeung,M. in Better Ceramics Through Chemistry, II, (C. J. Brinker, D. E. Clark, and D. R. Ulrich, eds.), Materials Research Society, Pittsburgh, (1986), p. 665. Dey, K., and Zuleeg, R., Ferroelectrics, 108, 37 (1989). Dana, Etzold, K. F., and Clabes, J., J. Appl. Phys., 4398 (1991). Tohge, N., Takahashi, and Minami, T.,J. Am Ceram. Soc., 74, 67 (1991). Sayer, M.,inSymp. Proc. 3rd Int. Symp. Integr. Ferroelect., Colorado Springs, Colorado, (1991), pp. 1-9. Hirano, and Kato, K., Adv. Ceram. Mater., 2, 142 (1987). Xu,R.,Xu,Y.H.,Chen, C. J., andMackenzie,J. D., J. Mater. Res., 5, 916 (1990). Francis, L. F., and Payne, D. A., in Ferroelectric Thin Films (E. R. Myers and A. Kingon, Materials Research Society, Pittsburgh, (1990), p. 173. SW&, L., Bright, J., Melling, P. J., and Shrout, T. R., Ferroelectrics, 108, 71 (1990). Nazeri-Eshghi, A., Kuang, A.X., and Mackenzie, J. D., J. Mater Sci., 25, 3333 ( 1990). Xu, Y. H., Chen, C. J., Xu,R., and Mackenzie,J. D., Phys. Rev. B., 41,35 (1991). Xu, R., Xu, Y. H., and Mackenzie, J. D., in Symp. Proc. 3rd Intern. Symp. Integr. Ferroelect., Colorado Springs, Colorado, (1991). p. 561.

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35. Hagberg,D.

Eichorst,D. J.. andPayne,D.A.,in Sol-gel Optics (J. D. MackenzieandD. R. Urich, eds.),SPIE Roc. 1328, SanDiego,California, (1990), p. 466. 36. Partlow, D. P., and Greggi, J., J. Muter. Res., 2, 595 (1987). andKato,K., Muter. Res. Symp. Proc., 155, 181 (1989). 37. Hiano, H., and Mackenzie, J. D., Muter. Res. Symp. Proc., 271 38. Cheng,C.H.,Xu,Y.

(1992). 39. Hirano, S., Yogo, T., Kikuta, K., Kato, K., Sakamoto, W., and Ogasahara, in 93rd Annual Meeting Exposition of the American Ceramic Society, 43-SV-91, Cincinnati, Ohio, 1991. 40. Vest, R. W., and Xu, J., Ferroelectrics, 93, 817 (1989). Merul Albxides, Academic 41. Bradley,D.C.,Mehrotra,R.C.,andGaur,D.P., Press, London, (1978). p. 306. 42. Eichorst, E., and Payne, D. A., in Sol-Gel Optics, (J. D. Mackenzie and D. R. Wrich, SPIE Roc. 1328, San Diego, California, (1990), p. 456. 43. Xu, R., Ph.D. Dissertation, University of California, 1992. 4 4 . Chen, C. J., Xu, Y. H., Xu, R., and Mackenzie,J. D., J. Appl. Phys., 1763 (1991). 45. Proceedings of lst, 2nd. 3rd and 4th International Symposium on Integrated Ferroelectrics, Colorado Springs, Colorado, 1989-1992. 46. Ramesh,R.,Chan,W. K., Wilkens, B., Sands, T., Tarascon, J. M.,Keramidas, V. G., and Evans, J. T.,Integrated Ferroelectrics, I , 1 (1992). 47. Barlingay, C. K., and Dey, K.,Appl. Phys. Lett. 1278 (1992). 538 (1988). 48. Hirano, and Kato, K.,J. Non-Cryst. Solid., 49. Miller, K. T., and Lange, F. F., J. Muter. Res., 5, 151 (1990). 50. Miller, K. T., and Lange, F. F., J. Muter. Res., 2387 (1991). 51. Lines, M. E., Phys. Rev. B, 15, 388 (1976). 52. Glass, A. M., Lines, M. E., Nassau, K., and Shiever, J. W., Appl. Phys. Lett., 31, 249 (1977). 53. Engelmann, H., Kraemer, N., and Gonser, U., Ferroelectrics, 127 (1989). Rao, K. J., and Rao, C. N. J., Muter. Res. 54. Varma, K. R. B., Harshavardhan, K. Bull., 315 (1985). 55. Kitabatake, M., Mitsuyu, T., and Wasa, K., J. Appl. Phys., 1780 (1984). 56. Kitabatake, M., Mitsuyu, T., and Wasa, K., J. Non-Cryst. Solid., 53, 1 (1982). 57. Xu, R., Xu, Y. H., and Mackenzie, J. D., in Sol-Gel Optics I1 (J. D. Mackenzie, ed.), SPIE Roc. 1758, San Diego, California, 1992. 58. Xu, Y. H., Cheng, C. H., Xu, R., and Mackenzie, J. D., Muter. Res. Symp. Proc., 271, (1992).

22 Ceramic Membrane Processing C. Guizard, A. Julbe, A. Larbot, and L. Cot Centre National de la Recherche Scientifique Montpellier, France

I.

,

INTRODUCTION

The use of inorganic membranes in separation technology is relatively new and has given rise to much interest in recent years. This is a result of the inherent properties of inorganic membrane materials, which are generally more stable chemically, structurally, and thermally than organic materials. Ceramic membranes represent a distinct class of inorganic membranes. Other classes consist of such membrane materials as glasses [1,2], carbon and metals and organic-inorganic polymers This chapter focuses on the chemical processing of ceramic membranes, which has to date constituted the major part of inorganic membrane development. Before going further into the ceramic aspect, it is important to understand the requirements for ceramic membrane materials intermsofporous structure, chemical composition, and shape. In separation technologies based on permselective membranes, the difference in filtered species ranges from micrometer-sized particles to nanometer-sized species, such as molecular solutes or gas molecules. One can see that the connected porosity of the membrane must be adapted to the class of products to be separated. For this reason, ceramic membrane manufacture is concerned with macropores above 0.1 pm in diameter for microfiltration, mesopores ranging from 0.1 pm to 2 nm for ultrafiltration, and nanopores less than 2 nm in diameter for nanofiltration, pervaporation, or gas separation. Dense membranes are also of interest for gas 501

502

GUlZARD ET

separation. These membranes must work in liquid or gaseous media, usually under harsh conditions. For this reason the major type of ceramics used in ceramic membrane manufacturing consists of refractory oxides-alumina, zirconia, or titania. Nevertheless, manyother ceramic materials, including cordierite, mullite, silicon carbide, silicon nitride, and silica, and also borosilicate glasses, have been mentioned as suitable materials for inorganic membrane preparation Concerning the shape of ceramic membranes, two main geometries have largely been investigated: tubular and flat membranes. Recently, alumina hollow fibers were proposed by DuPont in the microfiltration range. Membranes designed for the separation and concentration of various species must work in a cross-flow mode in which the liquid to be filtered circulates across the surface of the membrane while the permeatedliquid passes through the membrane perpendicular to the feed flow direction. Provided that the interactions between the feed liquid and the membrane can be minimized, the cross-flow mode prevents the accumulation of the retained products at the membrane surface and slows fouling phenomena. Returning to the ceramic aspect of inorganic membranes, different structural levels are concerned with the expected selectivity of the different categories of a ceramic membrane. Macropores in ceramic membrane supports are derived from micrometer-sized particles processed through conventional shaping and sintering methods, leading to porous bulk materials. Submicrometer-sized pores in the microfiltration range are obtained through submicrometer particle sintering. At this stage the system can no longer be considered a bulk material, and coating processes must be involved in membrane preparations [7,8]. With respect to pore size, “1 m’’constitutes the borderline between bulk materials and thin-film shaped materials to obtain high flow rates through the membranes.Finally, ceramic nonfilters are included in the newtechnologyof nanophase ceramic development [9,10]. After rapidly reviewing the historical aspects of inorganic membranes and outlining the basic principles in the preparation of porous ceramic materials, conventional methods adapted to the preparation of porous ceramic supports are first developed. The second part considers the new ceramic processing techniques that can be used advantageously in ceramic membrane preparation. Processing of submicrometer powders, sol-gel technologies, and chemical vapor deposition are some of the many advanced technologies that can be applied to ceramic membrane processing. The most adapted characterization techniques for each class of porous ceramic membrane are also described.

HISTORICAL BACKGROUND Workon inorganic membranes started withVycor glass membranes, which were studied in the mid-1940s[l]. In fact, two periods preceded the current de-

SSING MEMBRANE CERAMIC

503

velopment of inorganic membranes. The first was related to the separation of uranium isotopes by the gaseous diffusion process applied to UF6. The challenge was to select membrane materials able to work in avery aggressive chemical medium. Moreover, the radioactive environment required very reliable membranes. After the oil crisis in 1973, ceramic oxide-based supports were proposed and produced by two companies, Ceraver (a subsidiary of the CGE group) and Euroceral (a joint venture between Lafarge and Norton), with the view to providing the Eurodif plant in France with m2 membranes. The membranes were coated by SFEC (a subsidiary of CEA in France). The nature of the membranes still operating in Eurodif remains classified, but until today very reliable behavior has been observed. few years later (1982), nuclear energy programs were considerably decreased and the project for a second diffusion plant was abandoned. This was the end of the first period, and the Euroceral, Ceraver, and SFEC production plants were shut down. The second period consisted of the development of microfiltration and ultrafiltration inorganic membranes as a consequence of the know-how accumulated by the companies that built the gaseous diffusion plants. The inorganic membrane concept for the filtration of liquids was first developed by Carre (a subsidiary of DuPont) in the 1960s, with a dynamic (nonpermanent) zirconium hydroxide membrane on a stainless steel support, and by Union Carbide in the 1970s, with ceramic oxide layers coated on carbon supports. The first commercial wholly cross-flow filtration system equipped with an inorganic membrane was manufactured by SFEC (now TECH-SEP) in 1978. The Carbosep membranes from SFEC were made up of zirconia porous layers coated on a macroporous carbon support. In the 1980s, Ceraver (now SCT) developed a range of alumina microfiltration membranes (Membralox), introducing the concept of multichannel support. This was the starting point for the important development scheduled for inorganic membranes in the 1990s. A comprehensive review of the current advance in inorganic membranes is given in arecent book by Bhave [l l].

The previous chapters in this book present current developments in ceramic material processing with emphasis on new technologies. Most of these concepts are of interest to the preparation of both ceramic porous supports and supported ceramic membranes, showing that these kinds of materials enter the class of high-technology ceramics. An additional point is that, compared with other ceramic materials, the porous structure is the main aspect. In high-technology ceramic processing, a porous material usually appears as an intermediate structural stage that must be further densified, unlike ceramic membranes,

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in which a tailored porous structure appears to be key characteristicof the final material. The connected porosity in ceramic supports and membranes results from a process in which no pressure assistance is provided during sintering. Focusing on the voids, not on the solid part of the ceramic, the size, volume, and shape of the pores become evident characteristics of the ceramic material. The basic idea is that the porosity is directly related to the size and arrangement of individual particles in the fired material. Consequently, each filtration field (micro-, ultra-, and nanofiltration) consists of a different class of porous ceramic membranes in which the particle size must be adjusted. As a general rule, an ideal packing of monosized quasi-spherical particles generates interparticle voids for whichthesize, shape, andporousvolume (porosity) depend on the chosen arrangement model. Figure l a shows two different ways of packing spherical particles. This generates a connected porosity. Figure l b shows the corresponding shapes of the pores. With some ideal packings, such as the hexagonal compact mode, straight channels (tortuosity of 1) can be observed. All deviation from these ideal packings broadens the pore size distribution. In practice, these ideal arrangements are not observed in ceramic membranes and particle packing can be assumed to be more or less randomly arranged with a tortuous porosity in the 3040% range. When particles deviate from the spherical shape, an oriented porous structure can be obtained. This is the case for alumina ulafiltration membranes obtained from boehmite plate crystals [12]. Another phenomenon affecting the porous characteristics of ceramic membranes is the particle size evolution during sintering as a function of temperature and time. general tendency is forthe individual ceramic grains and pores to increase in size with increasing temperature and duration of the sintering process. An example is given in Sec. V.A. Figure 10 shows the evolution of pore size as a function of sintering temperature for an alumina membrane [13]. Ceramic membranes are not prepared with ideal particles, and so phenomenon is exacerbated when the starting particles exhibit a broad size distribution, because smaller particles are swallowed up by larger particles during the sintering process. The flux performance of a ceramic membrane is directly related to the porosity, which must be as high as possible without sacrificing mechanical strength. Porosity can be improved by a partial agglomeration of the particles at the initial stage of the process, but pore size distribution is increased as well. This is a contradictory approach because a good flux performance needs high porosity but good separation selectivity requires a narrow pore size distribution. Thus, a good ceramic membrane is a balance between these two requirements. Considering porous supports, the starting materials are ceramic powders a few micrometer in size. With these, one can generate a bulk structure withpore diameters larger than 1 pm. Because of the low surface area of these powders, high temperatures (up to 1600°C) are required for sintering. This kind of ce-

CERAMIC MEMBRANE PROCESSING

505

(a) Two different packing modes for spherical particles. (b) Corresponding pore shapes.

ramic material obeys the general rules of conventional ceramic processing. On the contrary, the design of supported membranes must be considered in a different way. Supported membranes differ from bulk supports by being thin-film shaped structures obtained through an appropriate coating process. Moreover, because the physical characteristics (including pore size, porosity, and surface area) define the application area for these membranes, different starting materials and processing techniques must be considered. Concerning microfiltration, in which pores of less than 1 pm are needed (typically 0.1-1 pm), suspensions of submicrometer powders must be processed using slip-casting or tape-casting techniques, depending on the support shape (flat or tubular). Smaller pore sizes are required for ultrafiltration. In this case pores result from the packing of colloidal particles, which cannot

GUIZ4RD ET behandled as drypowders. Instead they are formed in aqueous media and maintained as stable suspensions throughout the process. For this purpose, new techniques like the sol-gel process are of vital importance. Membranes exhibiting pore diameters down to 3 nm (present limit for commercial membranes) can be created. New developments of process are underway on the laboratory scale with the objective nanoporous membrane production. The development of ceramic nanofilters is closely related to the very innovative work on nanophase material. Sections V.B and C are specially devoted to recent work in this field.

W.

PREPARATION AND CHARACTERISTICS OF CERAMIC SUPPORTS

Most inorganic membrane supports exhibit a tubular shape. This is awelladapted geometry for cross-flow filtration in which the feed stream is circulated across the surface of the membraneand the permeated flux passes through the membrane in a perpendicular direction. Stainless steel, carbon, and ceramic are the most frequently used materials in the preparation of supports. shown in Fig. 2, tubes or multichannel substrates can act as membrane supports. well-designed support must be mechanically strong, and its resistance to fluid flow must be very low. Aiming at enhancing flux performances, multilayered substrates have been prepared that exhibit an asymmetric structure

permeate

multichannel porous ceramic

2 ramic;

ceramicsdesigned for cross-flowfiltration: (a) tube-shaped cemultichannel ceramic.

CERAMIC MEMBRAIVE PROCESSING

507

with an increasing pore size gradient, generally from inside to outside the tube. An example of a ceramic support made of several alumina porous layers is given in Fig. The bulk ceramic substrates are formed by the extrusion of a ceramic paste derived from a ceramic powder with a regulated grain size. This is a wellknown shaping method used in conventional ceramic processing. The shape of the extruded green body depends, as shown in Fig. 4, on the geometry of the die used. Tubular or multichannel supports can be produced in this way. To obtain an asymmetric support in a one-step process, two layers can be successfully coextruded in the manufacturing stage. The general formulation of ceramic pastes used in support manufacturing can be defined as a mixture of four basic compounds: a ceramic powder, an organic and/or an inorganic binder, a lubricant, and water. Because the ceramic porous structure depends on the shape of individual grains and the way they are packed, different factors can affect the two major characteristics of membrane supports, mechanical strength and porosity. The pseudoplastic behavior of the paste during extrusion is responsible for an exponential dependence of extrusion velocity versus applied pressure. High pressures to increase support permeability and strength have been emphasized in the literature These can be linked to a better (more dense) particle pack-

l

3 Cross-sectionalimage of anasymmetricceramic SCT (France).

porous substrate from

508

GUIZARD ET AL.

tubular

support

Two different dies used for ceramic support extrusion.

ing at high extrusion pressures and velocities as a result of high shear forces High extrusion velocities can be that are able to break up agglomerates achieved provided that the rheological properties of the paste have been carefully adjusted during the formulating step by adding the proper quantities of water and organic additives. Thermal treatment of the extruded green bodies can be carried out in two stages: (1) the drying step, with elimination of water at temperatures lower than lOo"C, under a controlled atmosphere, and (2) the firing step, with decomposition of organic additives and sintering of the ceramic up to 1200-1600°C. Concerning plate-shaped ceramics, tape casting (or thedoctor blade process) is an important forming operation commonly used to prepare multilayer capacitors and packages in the electronic industry [16]. This technique was recently investigated for the preparation of flat ceramic filtration supports [17]. Tape casting is a low-cost process for the manufacture of large-area thin ceramic sheets of controlled thickness andhighquality.Mosttape-casting processes with a milling procedure in which ceramic powders are mixed with a solvent and a dispersant in a ball mill. mixing and homogenization step then occurs in which binders and plasticizers are added to the low-viscosityslurry. The homogeneous, well-dispersed,and concentrated slurry is degassed and spread on a flat movingcarrier surface using the doctor blade process. Figure is a schematic representation of the doctor blade apparatus. Thickness control isa function of several parameters, including viscosity (which canbe adjusted by solvent content or temperature), castingcarrier

CERAMIC MEMBRANE PROCESSING Micrometer screws

0

509 5

L ” J

,Substrate

film

-Aluminium

Heating elements

5 Doctor bladeapparatus. (From Ref. 17)

speed, doctor blade gap setting, and reservoir depth behind the doctor blade After drying, the film is removed from the support andthenusually stamped to shape and further stacked into laminated configurations. Finally, burnout of organic polymers and sintering are performed under controlled temperatures. Three characterization methods are usually applied to ceramic supports: 1. Related to the ceramic structural properties, the porous texture can be imaged by scanning electron microscopy. 2. Mechanical strength can be measured using pressure tests. Highly pressure resistant supports (up to bar) with a multichannel geometry are now commercially available. The integrity of the support is checked by controlling air (or N2) bubble emission through the porous structure of the support immersed in a liquid, such as water or alcohol. The gas pressure at which bubbles are detected can be related to the size of cracks (or capillary-shaped pores) using calibration method according to the Laplace equation, COS

0

where is the mean diameter of the crack (or pore), Ap is the overpressure with respect to the atmospheric pressure, is the surface tension at the airniquid interface, and 0 is the wetting angle the liquid on the ceramic material. An air bubble penetrates the support through a void when the bubble radius is equal to (or smaller) than that of the void; this means that the contact angle is zero. This bubble-point method has become a dard technique used by suppliers to measure the largest active pores within

GUIZARD ET AL..

510

membranes. Table 1gives an indication ofthe pressure required for a given pore radius (using water as the wetting medium). If small pores are present, it is necessary to apply such high pressures because the surface tension of water/air is relatively high. Water must often be replaced by alcohol (i-propanol is often used as a standard liquid). This method has been approved as an American Society for Testing and Materials procedure (F316). 3. Finally, liquid permeability can bemeasured to evaluate the flux performance of the support, the aim being to obtain a porous material with a very low resistance to the filtered liquid. Once the supports exhibit suitable properties, they can be used as the base porous material in supported membrane processing. The successive layers acting as active membranes can be deposited on either the inner or the outer surface of tubes or supports. Starting from the same support, microfiltration, ultrafiltration, and nanofiltration performance is achieved by superimposing successive layers, yielding an asymmetric structure with porosity percolating from one layer to another. The main condition for a successful process is that the firing temperature of the final layer must be sensibly lower than the firing temperature of the supporting layer.

V.

SUPPORTEDCERAMIC MEMBRANES:SYNTHESIS AND CHARACTERIZATION

The effectiveness of a membrane depends both on its ability to separate particles or molecules in a selective way and on the flux that can be achieved across this membrane. Although some membranes, such as glass membranes, exhibit a symmetrical structure, in most cases they are asymmetric, consisting of several layers coated on a bulk porous support and with a gradual decrease in pore size (Fig. 3). The main advantages of an asymmetric structure are high fluxes and the ability to have tailored membranes made of materials different from the

Table

RelationBetweenPressureand Pore Radius Using Wateras the Wetting Medium in the Bubble-Point Method Pressure (MPa)

radius Pore

.o 0.1

0.145

PROCESSING MEMBRANE CERAMIC

51

support material. Ceramic supports are not the only category of material on which ceramic membranes can be deposited. Porous carbon or metal supports can also be used for this purpose. Depending on the mean pore size to be obtained for the membrane, an appropriate preparation method must be used.

A.

Ceramic Powder Method for Microfiltration Membranes

Microfiltration layers with pore diameters larger than can be made by coextrusion with the support in a one-step process. For this purpose a special die geometry is needed in which two concentric shells of paste (one for the support and the other for the microfiltration layer) are simultaneously extruded. Ceramic membranes with pore diameters in the 0.1-1 range can be prepared through aslip-casting method using asubmicrometer powder as the starting material. Membrane processing consists of preparing a slurry of the powder in water andthen slip casting the suspension onto the surface the support. Organic additives like binders or plasticizers are added to the slurry to adjust the rheological behavior of the coated layer. An example a zirconia microfiltration membrane coated on a porous stainless steel support is shown in Fig. 6. Two different microfiltration mem-

Cross-sectional image of a zirconia microfiltration membrane coated on a stainless steel macroporous support.

a

512

GUIZQRD ET AL..

branes have been coated on metal support using submicrometer zirconia powders exhibiting surface areas of 20 and 40 m2/g. These powders were dispersed in water, with polyvinyl alcohol and ethyleneglycol chosen as a binder and a plasticizer,respectively, to control the rheological properties of the slurry. When slip cast on porous stainless steel tubes, the suction force created on leaving slurry in contact with the support macropores in diameter) forced the dispersed particles to concentrate at the macropore entrance and a layer of packed particles. Once layer was dried at a controlled atmosphere and temperature to prevent crack formation, the sintering stage was performed at 1OOO"C under hydrogen atmosphere to preserve the metal support from oxidation. Two membranes with mean pore diameters of 0.1 and 0.2 were obtained, depending on the surface area of the original powder (Fig. 7).

B.

Sol-GelApproachfor Ultrafiltration Membranes

When, to satisfy ultr&iltration, nanofiltration, or gas separation requirements, the required pore size is under 0.1 the ceramic powder approach is no longer viable. Indeed, individual particles yielding pore diameters smaller than 0.1 cannot be handled by powder processing. In fact, particles of this type enter the category of colloids and must be maintained as a stable suspension during the process. indicated in the introduction, the sol-gel method is a very suitable way to produce mesoporous and nanoporous membranes. The lat&r is elaborated in the next section.

7 Pore size distribution mercury porosimetry.

zirconiamicrofiltrationmembranesmeasuredby


513

There are two main routes for the sol-gel process, which are described in Fig. 8. The route on the left side is known as the colloidal(or particulate) route. It involves the reaction of a metal salt or hydrated oxide with excess water. This yields a precipitate of gelatinous hydroxide colloidal particles, which can be redispersed through a peptization reaction using an electrolyte. The particle sizes in the resulting sol are typically a few tens of nanometers and are suitable to create mesopores down to 2-3 nm in diameter. The basic principles of soldestabilizationwiththeaim of producing these mesoporousmembranes were described in one our previous papers and widely exploited by a The porous structure and pore size distribution are number of authors governed by particle aggregation at the sol The peptization phenomenon used to prepare a colloidal sol is based on electrostatic interactions according

I

controlled hydrolysis

peptization

W (colloidal species)

WDERS

-..lc

\

I

I

Sol (polymeric species)

/

1

Sol-Gel transition

GEL LAYER

1

Drying

QRGANIC-MORGANIC MEMBRANE

1

Firing

PURE INORGANIC MEMBRANE

8 The

sol-gel routesusedto produce inorganicmembranes.

GUIZARD ET

514

to DLVO theory [21,22]. The repulsion forces that prevent particle aggregation in the sol are a result of the electrical double layer caused by the amphoteric behavior of most of the oxide surfaces. Electrophoresis allows us to determine the particle stability range through particle mobility measurement as a function of pH and electrolyte concentration. When the pH value is decreased or increased away from the isoelectric point (EP), a maximum stability for the sol is observed with high repulsive forces between particles. Near the IEP a flocculation phenomenon occurs, resulting in gel or precipitate formation. shown in Fig. 9, a change in particle mobility versus pH results in an evolution of particle stacking, which influences pore size and the structure of the membrane. Based on the control of these phenomena, tailored pore structures have been produced for titania and zirconia membranes This concept was also applied to alumina membranes, which were the first to be described in the literature. The pore sizes were in the nm range. Drying and firing steps can also affect the porous structure in different ways. The influence of drying conditions was revealed to be of prime importance to silica membranes Concerning the firing step, Fig. illustrates the influence of temperature on pore size evolution for an alumina membrane. Recent attempts in the literature to correlate the characteristics of bulk xerogels with those of coated membranes are questionable, mainly because the

+

ity

Influence particlechargeandmobilityin a colloidal sol on theporosthe resulting material. V = potential energy, H = interparticle distance.

515

CERAMIC MEMBRANE PROCESSING 15

I

55 nm

10

#

*

500

Influence

1000

the tiring temperature on the pore size evolution

an alu-

mina membrane.

environment of the drying and firing steps of a coated layer differs markedly from that of a bulk material. Because of the influence of the capillary forces exerted by the support, the coated sol coalesces in a very dramatic way, yielding a gel that cannot be compared to the corresponding bulk gel in terms of pore structure and porous volume. Moreover, during the drying step capillary forces that result from solvent evaporation are oriented in a direction perpendicular to the surface of the film, resulting in an important diminution of the layer thickness.Normally these capillary forces are responsible for the destruction of a bulk xerogel. Basic mechanisms involved in sol-gel derived coating formation (during aging, drying, and firing treatments) have been extensively developed by Brinker and Sherrer

C.

Ceramic Nanofilters by the Sol-Gel Process

Recentlymuch attention has beenpaid to ceramic membranes exhibiting a nanoporous structure with the aim of new membrane processes for the nanofiltration of liquids [26], pervaporation gas separation or catalysis

GUIZ4RD ET AL.

516

[29]. These membranes are achievable using the concept of nanophase ceramics. According to literature, this new class of materials can result from the emphasis of some new ceramic processes, such as the condensation of gaseous atomic clusters [30] or the sol-gel process [31]. This last method, which has beensuccessfully applied toultrafiltrationmembranes,wasusedrecently to prepare ceramic nanofilters. Nanophase materials deal both with the nanometer-sized particle and with the nanometer pore size aspects. The nanopore aspect is central to membrane technologies because of the need for selective separation processes at the molecular level. To understand the importance of nanostructures in microsieving membranes, the basic structure of nanophased ceramics must be briefly described. Because the particles are extremely small, one to a few tens of nanometers, an important fraction of the atoms is found in or very near the interface between grains, as reported in Table 2 [32]. Figure 11 isa schematic representation of a nanophase material. One can see that individual grains in the nm range induce a biphasic material with an interfacial phase between the grains and a residual nanoporosity, evidenced by positron lifetime spectroscopy [33]. Transmissionelectronmicroscopy is also a well-adapted technique for nanoscale structure characterization, as illustrated later. Concerning membranes, new separation capabilities are expected for these materials. The molecular sieving effect caused by connected nanopores can be applied tothe separation of molecules with molecular weights smaller than 1000. The key properties of such membranes are based on the preponderant effect of activated diffusion in nanopores, however. This phase transport phenomenon derives from the nanophased ceramic concept and classes these membranes among those materials expected to be crucial in the areas of modern technology, such as environmental protection, biotechnology, and the production of effect chemical. Recent advances in sol-gel science allowed to go further in controlling the individual particle size at the sol stage, resulting in individual nanoscale grains in the ceramic. This new development is based on the chemical modification of both metal salt and metal alkoxide precursors to modify their reactivity and to achieve a nanopore structure for the membranes [34]. Some ex-

Table 2

Atom Repartition in aMaterial as a Function the Individual Grain Size

Meangraindiameters(nm) 5

% Atomsinthegrainboundary


nanophasematerialaccordingto

517

Ref.

amples are given to show the interest of this method in ceramic nanofilter preparation. Zirconia Nanofiltration Membranes Prepared by the Colloidal Route. In this case zirconia sols obtained from zirconium oxychloride have been used for the preparation of inorganic membranes active in nanofiltration. Previously described conditions to achieve high selectivity and flux during nanofiltration can be expressed in terms of the structural requirements of the membrane material. Here the two main parameters are the phase stability of zirconia and crystallite size. If zirconia exhibits suitable basic properties .as a membrane material, different polymorphs .(tetragonal, monoclinic, and cubic) can be encountered starting from the amorphous state obtained at room temperature. neutron diffraction study performed on this material showed the evolution of crystallization kinetics and crystallite growth along the transformation sequences under different firing atmospheres (air, nitrogen, and a hydrogen-nitrogen mixture). Several phenomena have been evidenced that can affect the basic properties of the membrane. Tetragonal crystallites of less thm nm, obtained from the amorphous state at are very suitable to generate nanopores of less than 2 nm after sintering at 500°C. When raising the firing temperature to obtain a microporous sintered layer, the tetragonaVmonoclinic phase transition occurs, changing the crystallite size abruptly from nm to more than nm. Depending on the firing atmosphere, transition temperature can be shifted from 550°C under air to under nitrogen or hydrogen and nitrogen atmosphere. In each case individual tetragonal particle size can be maintained under nm provided that the firing temperature does not exceed the transition temperature, as shown in Fig.

51

GUIZARD ET

30 300

I

400

500

TEMPERATURE ("C)

600

12 Influence the f ~ n atmosphere g on the evolution size versus temperature for the tetragonal phase.

700 zirconia crystallite

transmission electron microscope (TEM) observation of the ZrO2 nanocrystallites obtained at 500°C is given in Fig.13. Homogenous, quite spherical, consolidated particles of about nm in size can be observed. This clearly shows that, to maintain nanoporosity inside the membrane, an upper limit for membrane sintering exists. Because these results, a zirconia nanofilter has been obtained by coating a 1 thick layer on a microfiltration zirconia layer. The separation performance of this membrane, characterized with model solutes in aqueous media, is in the nanofiltration range

2. Lanthanum Oxychloride Catalytic Membranes These membranes have been prepared from homogeneous sols based on lanthanum chloride aqueous solutions. Two routes have been compared for the sol preparation. In the first, ammonia was added to a stirred aqueous solution of lanthanum chloride; a complex mixture of lanthanum hydroxide and/or basic salts intermediate species was obtained in the form of an opalescent sol. In the second route, an organic complexant (acetic acid) was used to create soluble species around the working pH (pH = 8). Ammonia addition to the modified solution led to clear sols containing probably smaller particles than those obtained by the first preparation method. Sol formation required a binder and a plasticizer to achieve the desired behavior of the cast films during the drying and firing treatments. Here polyvinylic alcohol fulfilled these functions and led to crack-free ceramic layers. Typical processing consisted of a drying step at 110°C to avoid the crystallization of stable carbonate species and then heat

SING MEMBRANECERAMIC

519

13 TEM image of zirconia nanoparticles forming a nanofiltration membrane obtained from a zirconium oxychloride precursor.

treatment at 800°C for 5 h in an N2 atmosphere to obtain a lanthanum oxychloride porous thin film. LaOCl crystallizes at about 400°C in the tetragonal system and is still stable at 800°C. The first LaOCl layer (prepared without acetic acid) was used as a support for the deposition of a second finer membrane from a modified sol. Figure 14 is a scanning electron microscope observation of the cross section of a supported double layer. Very small particles (around m in size) have been evidenced by E M ; N2 adsorption and desorption measurements have revealed a pore diameter distribution with a maximum at around 1.5 m. The catalytic performance of such a material for oxidative coupling of methane has been described elsewhere [35] as a function of preparation conditions. Silica Membranes from Polymeric Sols The preparation of submicrometer monodisperse particles by the controlled hydrolysis of metal alkoxides has been widely investigated for a number of ceramic materials. In particular, sol-gel processes involving the formation of particulate materials or glass-precursor gels from silicon alkoxides, among them tetraethylorthosilicate (TEOS) and tetramethoxysilane, have received considerable attention in the literature. The potential of TEOS sols to form crack-free nanoporous thin films suitable for use as membranes has been investigated and is used as an example in this section. Starting from TEOS in ethanolic solution, the alkoxide hydrolysis was performed with water under acid conditions. Indeed, compared withbase-catalyzed hydrolysis reaction,acid catalysis is known to lead to smaller polymers with a low degree of cross-linking [36] and

GUlZARD ET

520

Cross-sectionalimage

a double-layer lanthanum oxychloride mem-

brane.

is thus a most interesting starting material for the preparation of microporous materials. An organic binder (e.g., polyvinylic alcohol or polyoxyethylene) was added to the sol before coating to prevent crack formation during drying and firng treatments. After elimination of the organic products at a supthick) was obtained. This ported crack-free amorphous silica thin film (0.2 membrane was deposited on a mesoporous silica membrane, described in Ref 37. A TEM observation of the top layer is presented in Fig. 15 and shows nonspherical grains with a length around 6 nm and 2 nm wide. The corresponding NZ adsorption isotherms carried out on top layer are characteristic of microporous materials. The Dubinin-Astakhov pore volume plot in Fig. 16 shows a pore radius distribution in the nanometer range. The specific surface measured for this membrane is about 700 m2/g. Titania and Zirconia Membranes Prepared by the Polymeric Route Titanium and zirconium propoxides can be used as precursors for the preparation of nanoporous titania and zirconia membranes. avoid the precipitation of inhomogeneous hydroxide particles during the hydrolysis step, the alkoxide reactivity can be modified with acetylacetone (acacH). This chelating agent reacts readily with transition metal alkoxides, as follows [34]: M(OR), + acacH + M(OR),(acac) + ROH with M = Ti or


521

Figure 15 TEM image of silica nanoparticles in a TEOS-derived membrane.

This ligand then acts as a functionality locker when substoichiometric hydrolysis ratios are used. A ratio acacH/M greater than 1 prevents precipitation and leads to stable colloids or gels. Consequently, with a good formulation choice, sols can be prepared in air without precipitation. Either titania or zirconia ultrafiltration layers have been used as supports for these membranes. Figure 17 is a cross-sectional image of a zirconia microporous membrane that is 0.2 pn thick. The supported layers obtained after treating at 500°C exhibited crystallized structures (anatase for titania and a tetragonal metastable form for zirconia) and revealed a very fine texture from TEM observation. For Ti with a Ti/acac ratio = 1, grains with defined faces can be observed whose size is around 20 nm. For Zr with a Zr/acac ratio = 2, a finer texture was obtained than with titania, with a mean grain size of around 4 nm. In the two cases, powder x-ray diffraction (Sherrer formula) was used to measure an individual crystal size, which is in good accordance with TEM observations. For the zirconia membrane the N2 adsorption and desorption isotherm in Fig. 18a shows a microporous domain with a 1.7 nm centered pore size distribution (Fig. 18b).

D. Characterization of Mesopores and Micropores in Ceramic Membranes Many characterization techniques developed for the characterization of mesoporous and microporous materials have been adapted to membrane characterization (e.g., mercury porosimetry, adsorption and desorption isotherms, and thermoporometry). These techniques are related to morphological parameters

522

GUIZARD ET AL.

0.028

2 0.024

0 &I

0.008 0.004

0.000 5

6

7

8

9

10

11

12

13

EQUIVALENT PORE RADIUS

14

15

(A)

Figure 16 Dubinin-Astakhov differential pore volume plot obtained from N2 adsorption-desorption measurements on a microporous silica membrane.

[38] and can be defined as static characterization techniques in which the sample has no active role. A series of specific techniques for the characterization of active pores in membranes (e.g., permeability of gases and liquids) have been also developed that involve the entire filtration element, not only samples. These dynamic techniques, in which the membrane plays an active role, are usually nondestructive and yield permeation-related parameters. A very precise characterization of the porous texture of materials is often difficult because of interactions between the tested material and the method but also because this approach requires the use of a model for the pore system (idealized pore shape assumption). Basic principles of the main characterization techniques of membranes are described here. 1. Mercury Porosimetry Mercury porosimetry is a well-adapted method to characterize the pore size and pore size distributions of nlicrofiltration ceramic membranes. In this method,


523

Figure 17 Cross-sectional image of a zirconia microporous membrane obtained from a modified alkoxide precursor and deposited on a zirconia ultrafiltration layer.

which is a variation of the bubble-point method, mercury is forced into the pores of a dry sample. The quantity of mercury forced into the pores is determined very accurately at each pressure; it can be measured by means of the capacity variation induced by the reduction in a Hg column height connected to the measuring cell. The relationship between the applied pressure P and the pore rp is given by the modified Laplace equation (Washburn equation),

Because mercury does not wet the membrane, the contact angle 8 is greater than 90" and cose has a negative value. A widely accepted value for oxides is eHg/oxide = 140" and mg/air = 0.48 N/m. Very high pressures are needed for pores in the nanometer range. A pore diameter of 4 nm corresponds to a pressure of about 400 MPa, which may damage the ceramic layer. 2. Gas Adsorption and Desorption Technique [39] This technique can be considered a standard method in the science of porous ceramics and catalysts. It is based on the principle that inside a small pore a gas can condense to a liquid at a relative pressure lower than unity; this introduces the capillary condensation theory. The adsorption and desorption isotherms of an inert gas are determined as a function of the relative pressure @re1 = p/po, i.e., the ratio between the applied pressure and the saturation pressure). N2 is often used as adsorption gas, and the experiments are carried out at the boiling liquid nitrogen temperature (at 1 bar). The adsorption isotherm

SING MEMBRANE CERAMIC

525

is determined by measuring the quantity of N2 adsorbed for each value of PR]. Adsorption at a low relative pressure. Ata certain minimum pressure, the smallest pores are filled with liquid nitrogen. As the pressure is increased further, 1arger.pores are filled, and near the saturation pressure, all the pores are filled. The total pore volume is determined by the quantity of gas adsorbed near the saturation pressure. Desorption occurs when the pressure is decreased from the saturation pressure.Manymesoporous systems exibit distinct adsorption-desorption behaviors, which lead to a characteristic hysteresis loop. The curve shape is linked to different geometrical factorsthat rulethe adsorption and desorption processes. The reason for this hysteresis is that capillary condensation occurs differently in adsorption and desorption. Because of the concave meniscus of the liquid in the pores, N2 evaporates at a lower relative pressure because the vapor pressure of the liquid is reduced. The lowering of the vapor pressure for a capillary of radius rk is given by the Kelvin relationship

the contact angle being assumed to be zero. The pore radius rp may be calculated from rp = rk + t, where t is the thickness of the adsorbed layer of vapor in the pores (estimated from calibration curves). This method is generally not very accurate in layers with a large pore size distribution and without a definite pore geometry. The Kelvin equation is useful to explain the hysteresis phenomenonand to calculatethe pore volume and pore area distributionsas a function of pore diameters (e.g., BJH method). Nevertheless, this Kelvin equation is based on thermodynamic considerations and can only be applied to a pore radius between 15 and In microporous systems, various methods have been developed for the determination of microporous volume and pore size distribution, but the validity of pore sizes is often ambiguous.

3. Thermoporometry This method is based on the microcalorimetric analysis of solid-liquid transformations in porous materials Since the system of water-filled pores has the closest resemblance to the practical situation of membrane filtration, the solid-liquid transition of water is often used for membrane pore size analysis. Because of the strong curvature of the solid-liquidinterface present within small pores, a freezing (or melting) point depression of the water (or ice) occurs. A full thermodynamic description of this phenomenon is given in Ref. According to this concept, the size of a confined ice crystal (which is set by the size of the pore) is inversely proportional to the degree of undercooling, whereas the pore volume is directly related to the apparent transition energy.

GUIZARD ET AL. A differential scanning calorimeter is used to obtain a solidification (or melting) thermogram, from whicha pore size distribution can be extracted. By comparing the solidification and melting processes, thermoporometry can also be used to determine a thermodynamic pore shape factor, which varies generally from 1 (spherical pores) 2 (cylindrical pores). 4. Permeabilityand Retention Measurements Typically, liquid permeabilities are obtained with water as permeate and expressed in terms of Lhm2-bar. Gas permeabilities are often expressed in terms of air or nitrogen permeability. For gases, membrane pore size also affects the transport mechanisms through the pores [27]. Manufacturers tend to characterize membranes by means of rejection measurements with reference molecules, such as dextrans, proteins, or polyglycols. A parameter extensively used for membranes characterization is the cutof value, which is defined as the lower limit of solute molecular weight for which the rejection is at least 90%. We must keep in mind, however, that rejection measurements always depend on the type of solute (shape and flexibility of the macromolecular solute), the membrane (its interaction with the solute), and the process parameters used (pressure, cross-flowvelocity,geometryof the test cell, and concentration and type of solute). In particular, concentration or polarization, pore blocking, and fouling phenomena affect rejection measurements very significantly. Some other techniques involving membrane permeability have been developed that have not yet been extensively used. Concerning gas permeability, a method called permeametry has been developed based on Adzumi equation. It consists of measuring the variation in membrane permeability when favoring either molecular (Knudsen) or laminar (Poiseuille) flow regime. A mean pore radius is obtained with this method. Another more recent technique, called permporometry (or liquid displacement porosimetry), is based on the controlled blocking of pores by condensation of a vapor, present as a component of a gas mixture, and the simultanemeasurement of the gas flux through the membrane [42,43]. By measuring the gas transport through the membrane upon decreasing relative vapor pressure, the size distribution of the active pores can be found in the limit validity of the Kelvin equation. The calculation of the number of pores can be performed by assuming a Knudsen type of transport regime. A liquid-liquid displacement technique (biliquid permporometry) was also found to be well adapted to the characterization of meso- and macroporous membranes [M]. It is based on the combined principles of bubble pressure and solvent permeability. In this case the applied pressure P and the flux J through the membrane are measured simultaneously. The recorded P and J values, introduced in the Laplace equation directly, give the pore equivalent radius and

PROCESSING MEMBRANE CERAMIC

52 7

the distribution of permeability versus pore radius. Pore number and pore area distributions versus pore size can be obtained by using Hagen-Poiseuille (or Kozeny-Carman) relationships. 5. Techniques Under Development number of techniques were developed recently to solve some of the several problems linked to classic technique principles (low sensitivity, necessary pore shape assumptions, lack of validity for pores smaller than 1.5 nm, and others). A nuclear magnetic resonance spin-lattice relaxation technique was recently successfully demonstrated on a number of types of porous media. The basic principle is that the portion of pore fluid near a pore wall undergoes spin-lattice relaxation in a magnetic field faster than pore fluid removed from the pore wall [45]. Small-angle scattering techniques (small-angle x-ray and neutron scattering) have recently become established as versatile probes for the determination of structures of porous solids [46,47]. The size, shape, concentration, and surface area of inhomogeneities, such as pores, can be studied. Positron lifetime spectroscopy has been shown to be a good means of investigating the structural levels of nanocrystalline materials [48]. Different annihilation sites (dislocations, micropores, and mesopores) have been attributed to the different measured positron lifetimes.

VI.

NEW TECHNIQUESUNDERDEVELOPMENT IN CERAMIC MEMBRANE PROCESSING

Separation processes using membranes arise as a key technology in many industries (e.g., chemical, biotechnological, and water and wastewater treatment). For achieving the goal of more efficient processes, higher performance and more cost effective membrane materials are needed. Because of their excellent inherent properties, a rapid development of inorganic membranes is expected over the next years, andnewthin-film deposition technologies are in progress with the aim of producing these new inorganic membranes. Electrophoresis was recently described as a promising method to produce inorganic membranes [49]. An electrical field is applied to deposit charged particles of dispersed mineral solids in water. Negatively or positively charged resins are used to coat the surface of the particles so they are transported to the electrode surface. Electrophoresis is therefore limited to conductive substrates that can serve as an electrode, but complicated shapes are easily accommodated. mentioned earlier, new developments in gas separation, catalysis, and petroleum industries have stimulated interest in microporous inorganic membranes suitable for high-temperature applications and with the property of high

GUIZARD ET AL.

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permselectivity. Physical vapor deposition methods that have been extensively developed for the electronic industries can be transposed to membrane preparationwith the objective of yielding anew generation ofmetal or ceramic membranes Several methods are of interest to produce these thin inorganic films active in separation processes. In fhennal evaporation,solid materials are first vaporized by heating at sufficiently high temperature in vacuum, and then a thin film is formed on a cooler substrate by condensation of the vapor. This technique can be used for the deposition of thin layers made of such metals Ag or Pt that are active in gas separation. Cathodic sputtering can also be avantageously used to deposit thin films of Ag and Pd on Ag supports. In this case materials are ejected atomistically from a target material by bombarding high-energy positive ions, such as Ar ions, and then condensed on a substrate to form a thin solid film. In chemical vapor deposition (CVD), chemically reacting gases or gaseous precursors are vaporized and decomposed under temperature to produce nonvolatile reaction products that deposit on the substrate. The low-temperature plasma-enhanced chemical vapor deposition (PECVD) process is also of interest to deposit inorganic thin layers on substrates that cannot be heated at high temperatures. Instead of temperature, plasma or microwaves are used to perform decompositionof the volatile precursors undervacuum. For example, nonporous silica glass, which is known for its excellent selectivity for hydrogen, is a good candidate for gas separation (28). Deposition of this kind of thin solid films using PECVD of Si02 can be envisaged on various porous supports such as ceramics, glasses, or polymers. These new developments show that ceramic membrane processing is fully involved in recent progress in ceramic materials and can be considered a hightechnology ceramic manufacturing industry.

Nordberg,M. E., Properties

some Vycor-Brand glasses, J. Am. Ceram.

Schnabel, R.,and Vaulout, W., High pressure techniques with porous glass membranes, Desalination, Bauer, J. M., Elyassini, J., Moncorge, G., Nodari, T., and Totino, E., New developments applicationsof carbon membranes, Key Eng. Mater.. 61/62, McCaffrey, R. R.,McAtee, R. E., Grey, E., Allen, C. A., Cumming, D. G., Appelhans, A. D., Wright, R. B.,andJolley, J. G.,Inorganicmembrane technologies. Separ. Technol., and Cot, L., A new generation membranes based on 5. Guizard, C., Larbot, organic-inorganic polymers in TCIM Proceedings, Montpellier, France, pp. 55-64.

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6. Chan, K. K., and Browstein, A. M., Ceramic membranes: growth, prospect and opportunities, Ceram. Bull., 70 Charpin, J., Bergez, P., Valin, F., Barnier, H., Maurel, A., and Martinet, J. M., InHigh organicmembranes:Preparation,characterization,specificapplications. Tech Ceramics (P. Vincenzini, ed.), Elsevier, Amsterdam, p. Larbot, A., Fabre, J. P., Guizard, C., and Cot, L., Inorganic membranes obtained by sol-gel techniques, J. Memb. Sci., 39, Burggraaf, A. J., Keizer, K., and Van Hassel, B. A., Ceramic nanostructure materials, membranes and composite layers, Solid Stare Ionics, 32/33, Guizard, C., Julbe, A., Larbot, A., and Cot, L., Nanostructures in sol-gel derived materials. Application to the elaboration of nanofiltration membranes, Key Eng. Mater.,

Bhave, R. R., (ed.), Inorganic membranes. Synthesis, Characteristics and Applications, Van Nostrand-Reinhold, New York Leenaars, A. F. M., and Burggraaf, A. J., The preparation and characterization of alumina membranes with ultra-fine pores. 2. The formation of supported membranes, J. Coll. Interfac. Si., Larbot, A., Alary, J. A., Guizard, C., and Cot, L., New inorganic ultrafiltration membranes: Preparation and characterisation,Int. J. High Technol. Ceram, 3, Terpstra, R. A., Bonekamp, B. C., and Veringa, H. J., Preparation, characterizamition and some properties of tubular alpha alumina ceramic membranes for crofiltration and as a support for ultrafiltration and gas separation membranes, Desalination,

Alfotd, N. M., Birchall, J. D., and Kendall, K., High-strength ceramics through colloidal control to remove defects, Nature, 330, Mistler, R. E., Tape casting: The basic process for meeting the needs of the electronic industry, Ceram. Bull., 69(6), Simon, C., Bredesen, R., Raeder, H., Seiersten, M., Julbe, A., Monteil, C., Laaziz, Key Eng. I., Etienne, J., and Cot, L. Tape casting of flat ceramic membranes, Mater. 65, Anderson,M.A.,Gieselmann,M.J.,andXu, Q., Titania and alumina ceramic membranes, J. Memb. Sci., 39, Zaharescu M., Pirlog, C., Vasilescu, A., Crisan, D., and Braileanu, A., Inorganic membranes by the sol-gel method, Rev. Roum. Chim., 35(7-g), Xu, andAnderson,M.A.,Synthesisofporositycontrolledceramicmembranes, J. Mat. Res., Parks, G. A., The isoelectric points of solid oxides, solid hydroxides, and aqueous hydroxo complex systems, Chem. Rev., Overbeek, J. T. G., The rule of Schulze and Hardy, Pure Appl. Chem., Larbot, A., Fabre, J. P., Guizard, C., Cot, L., and Gillot, J., New inorganic ultrafiltration membranes: Titania and zirconia membranes;J. Am. Ceram. Soc., Larbot, A., Julbe, A., Guizard, C., and Cot, L., Silica membranes by the sol-gel process, J. Memb. Sci., 44,

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GUIZARD ET AL. Brinker, C. J., and Scherer, G. W., Sol-Gel Science, Academic Press, New York, Guizard, C., Ajaka, N., Garcia, F., Larbot, A., and Cot, L., New membranes for the hyperfiltration of small molecules. Influence of the mesoporous structure on separationandfractionationperformances,in Proc. of V FiltrationCongress, Nice, France, pp. Ulhorn, R. J. R., and Burggraaf, A. J., Gas separation with inorganic membranes, in Inorganic Membranes (R. R. Bhave, ed.), Van Nostrand-Reinhold, New York, p. Klein, L. C., and Giszpenc, N., Sol-gel processing for gas separation membranes, Ceram. Bull.,

Armor, J. N., Catalysis with permselective membranes, Appl. Catal., Siegel, R. W., Nanophase ultrafine grained materials, Mater. Sci. Forum,

Guizard, C., Julbe, A., Larbot, A., and Cot, L., Nanostructures in sol-gel derived materials. Application to the elaboration of nanofiltration membranes; Key Eng. Mater.,

Siegel, R. W. Nanophase materials assembled from atomic clusters,

MRS Bull.,

15(10),

Schaefer, H. E., Wiiurschum,R.,Birringer,R.,andGleiter, H., Structureof nanometer-sizedpolycristallineironinvestigatedbypositronlifetimespectroscopy, Phys. Rev., B38(14), Sanchez,C.,andLivage,J.,Sol-gelchemistryfrommetalalkoxideprecursors. New J. Chem., 14,

Julbe,A.,Chanaud, P., Larbot, L., Guizard,C.,Cot,L.,Mirodatos,C.,and Borges, H., Lanthanum oxychloride catalytic membranes,Key Eng. Mater., Brinker,C.J.,Keefer,K. D., Schaeffer, D. W., Assink, A.,Kay,B. D., and Ashley, C. Sol-gel transition in simple silicatesII,J. Non-Cryst. Solids, (1

Larbot,A.,Julbe,A.,Guizard,C.,Cot,L.Silicamembranesbythesol-gel process, J. Memb. Sci., 44, Cuperus, F. P.,Bargeman, D., and Smolders, C. A., Characterizationof UF membranes,membranecharacteristicsandcharacterizationtechniques, Adv.Colloid Interfac. Sci.,

Gregg, J., and Sing, K. W., Adsorption, Surface Area and Porosity, Academic Press, London, Eyraud,C.,Quinson,J.F.,andBrun,M.,Theroleofthermoporometryinthe study of porous solids, in Characterization of Porous Solids, Elsevier, Amsterdam, p. Charpin, J., and Rasneur, B., Caract6risation de la texture poreuse des matCriaux, Tech. Ing., (France). Eyraud, C., Application of gas-liquid permporometry to characterization of inorganic ultrafilters, inProc. Europe-Japan Cong. Memb. Mernb. Processes (E.Dridi and M. Nakagaki, eds.), p.


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43. Mey-Marom, A., and Katz, M. J., Measurement of active pore size distribution of microporous membranes-a new approach, J. Memb. Sci., 119 (1986). 44. Capannelli, G.,Becchi, I., Bottino, A., Moretti, P., and Munari, Computer driven porosimeter for ultraf3tration membranes;Characterisation of Porous Solids, Elsevier, Amsterdam, 1988. p. 283. 45. Glaves, C. L., and Smith, D. M., Membrane pore analysis via NMR spin lattice relaxation experiments, J. Memb. Sci., 46, 167 (1989). 46. Ramsay, J. D. F., and Avery, R. G.,Neutron scattering investigation of adsorption processes in modelporous systems, in Characterisation of Porous Solids II, Elsevier, Amsterdam, 1991, p. 235. 41. Long, G.G.,Krueger, Gerhardt, R. A., and Page, R. A., Small-angle neutron scattering characterizationof pmcessinghicrostructure relationships in the sintering of crystalline and glassy ceramics, J. Mater. Res., 6(12), 2706 (1991). H., Structureof 48. Schaefer, H. E., Wiiurschum,R.,Birringer,R.,andGleiter, nanometersizedpolycrystallineironinvestigatedbypositronlifetimespectroscopy, Phys. Rev. B, 38(14), 9545 (1988). 49. Alary, J. A., Bauer, J. M., and Boudier, G.,Synthesis of new inorganic membrane byelectrocoatingprocess,in Proc. International Congress Membranesand Membrane Processes, Chicago, 1990, pp. 555-58. 50. Ilias, and Govind, R., Development of high temperature membranes for membrane reactor: An overview, AIChE Symp. Ser., 85(268), 18 (1989).


Index

Acetic acid, 9, 37 Acetyl acetone, 11, 37, 520 ABO3,68,445,447 AnBn03n-l,63,445 An-lBn03n+29 68 Acid-base reactions Lewis, 41 Acrylic acid, 19 Aerogels, 311, 315, 331 acoustic attenuation in, 322 acoustic properties of, 323, 330 alumina, 3 11 applications 327 catalysts, 331 Cherenkove detectors, 327 gas filters, 331 laser fus,ion targets, 332 photoluminescent light sources, 33 1 solar panels, 3 18 thermal insulation, 328

[Aerogels] thermal insulation windows, 327 tiles, 327 base-catalyzed, 323 Bragg scattering, 324 Brillouin scattering, 321 carbon, 312,314,320,330 as supercapacitors, 330 carbon black-doped, 320 characterization: Bragg peaks, 18 light scattering, 317, 324, NMR, 316 SAXS, 316, 323 skeletal, 316 structure, 315 density, 322,324 fractal range, 323 gas filters, 331 Guinier region, 317 insulating foams, 3 11, 319, 328

533

534

[Aerogels] iron oxide, 311 loss tangent, 324 mass fractal dimension, 317 mechanical properties of, 320 acoustic attenuation methods, 321, 324 pulse-echo method, 321 melamine-formaldehyde, 3 14 mixed oxide, 312 modulus of rupture (MOR), 322 nanostructure, 325 one-step silica, 318 optical properties, 319, 325 interferometric method, 327 Rayleigh scattering, 325, 344 organic, 313, 332 resorcinol-formaldehyde,3 14, 318 phonon mean free paths, 322 photon mean free path, 326 porosity, 322 quality factor, 324 Raman-Nath diffraction, 322 refractive index, 326, 344 silica, 266, 3 11 skeletal densities of silica, 316 sound velocities, 321, 322, 330 Kelvin-Voigt model, 324 thermal insulation of, 265, 328 thermal properties, 3 19 stationary hot-plate, 320 thermal conductivities, 319, 329 titania, 3 11 trap, 332 two-step silica, 3 18 Young's modulus of, 324 Alcohols alcoholysis, 35

INDEX [Alcohols] exchange reactions, 30,75, 77, 216 polydentate, 32 Alkali flux method, 67 Alkaline-earth metal nitrates, 65 Alkanolamines, 31, 35 Alkoxides chemically modified, 3 1 double, 40, 79 heteroleptic, 3 1 heterometallic, 41 homoleptic, 3 1 metal, 24, 25, 30,76,405 mixed metal, 75, 77 [M(OC2H40Me)nlm, 34 niobium, 35 precursor, 16 silicon, 3 transition metal, 5 16 Alkoxy alcohols, 35 groups, secondary, tertiary, 5 ligands, 4 Allyl function, 19 &Alumina, 67 Aluminum alkyls, 117, 365 Aluminum chloride, 365 Aluminum etac, 11 Aluminum hydride, 116 Aluminum hydroxide, 184 Al(OC3H7)3, 79, 80, 81 [A11304(oH)24(H20)17+, 47 Aluminum nitride, 10, 115, 122, 361,365,381 Al(Et)3, 365 N-alkylamino alanes, 365 cationic impurities, 115 formation of, 121

INDEX [Aluminum nitride] oxygen content, 123 powder, 197 sintering aids, 115 TGA profiles, 121 Aluminum oxide boehmite, 190 EVA mix, 248 polystyrene with, 258 rheology, 199 S i c whisker composite, 187, 190 suspension, 176, 180, 181 ZrO2-SiC composites, 190 Aluminum sec-butoxide, 11,278 Ammonia, 24, 108 ammonolysis of dichlorosilane, 36 1 method, 76 Aryloxide, 3 1 A, WO3,66 Azeotropic distillation, 35, 77 Barium &oxides, 217,484 hydroxide, 217 metal, 217 BaCo3,71 Barium ferrites hexagonal 70 particles, 426 . platelet, 430 Bag(OH)(OR)g, 25 BaPb03,65 Ba2PbO4,65 BaPbXBil_,03 (BPB), 44 Ba phenoxide, 32 Bal-xKxBi03, 67,447 BaTi03.627 231 -based dielectric, 401 -based materials, 397

535

[BaTi03] crystal structure, 139 powder, 90 S~2Og-d0ped,93 BaTi system, 41, 217 sol-gel derived, 218 BaTi[(OiPr)4 iPrOH14, 8

Ba4Ti404(0iPr)16(iPrOH)x,44, 46 B ~ ~ ~ ( c I - O R ) ~ ( O45R ) ~ I ~ , Benzoyl peroxide, 19 Binder amount of, 250 ethylene ethylacrylate (EEA), 243 ethylene vinylacetate, 247 GPC of, 252 molten organic, 250 oxidation of, 252 polymeric, 243 stability of, 251 thermoplastic, 246, 257 waxes, 257 Bi2An_1Bn03n+3,69 Bi2CaSr2Cu208,73 Bismuth cuprates, 73,449, 450 Bi(2212), 449,450,451 Bi(2223), 449,450 B(N,C) ceramics, 364 Borazoles, 118 Boron nitride also Nonoxide ceramics), 118 borazine, 363 boric acid, 363 ceramic yields, 118 fibers, 371, 381 hexagonal, 118 trichloroborazine, 363 urea, 363 Boron polymers, 112, 118, 363, 364

536 Boronylpyridine, 112 Brownmillerite, 63

Ca2Co204.63 Ca2Co205,63 CaCu02,453 ca2Fe2-xMnx(co3)4, Ca2Fe204,62 Ca2Fe205,62 Ca2Mn205,63 Ca2Nb207,68 Cag.5Ti2p30 12,61 Capacitor dielectric, 398, 465 lamination defects, 401 MLC, 398,416 Capillaries, 257 force, 515 tortuosity constant, 257 Carbides, transition metal 120 Carbon content, 122 excess, 110 precursor, 106 Carbothermal reduction, 105, 106 Carboxylic acids, 9, 19, 35 Catalysis or catalyst, 24, 515 acid, 3 base, 3 Ceramic membranes, 501 alumina (Membralox), 503,514 biphasic materials, 516 calibration, 509 Carbosep, 503 catalyic, oxidative coupling of methane, 519 charaterization, 521 Adzumi equation, 526 bubble-point method, 523 gas adsorptioddesorption technique, 523 Kelvin equation, 525

INDEX [Ceramic membranes] Laplace equation, 523 microcaloric analysis, 525 mercury porosimetry, 521, 522 N M R , 527 permeability, 526 positron life time spectroscopy, 527 rejection measurements, 526 small angle scattering, 527 texture, 522 thermoporometry, 521,526 Washburn equation, 523 fabrication, 504 acetyl acetone, 520 by colloidal, 513 by sol-gel, 512 cathodic sputtering, 528 CVD, 528 electrophoresis, 527 extrusion, 507 firing atmosphere, 5 17 inorganic binders, 507 organic additives, 508, 5 11 organic complexants, 518 physical vapor deposition, 528 plasticizers, 508 rheology, 508 slip casting, 505, 511 tape casting, 505,508 thermal evaporation, 528 Tilacac ratio, 521 Zrlacac ratio, 521 zirconium oxychloride, 5 17 flux performance, 504 macropores, 501 mesopores, 501, 512 nanofilters, 502, 506, 516 nanopores, 501,512 porosity, 501

INDEX [Ceramic membranes] supports, 503 carbon, 503 ceramic porous, 503, 504 multichannel, 503, 506 multilayered, 506 stainless steel, 503 Vycor glass, 502 zirconia, 503, 514, 517, 520 Ceramic powders, 129, 143 binder mixture, 239 characterization, 129 density, 131 hafnia, 94,96,97 morphology, 131, 136 nonsilicate, 75, 81 oxide, 75 physical properties of, 131 PLZT,85,88 polydisperse, 131 porosity, 131 silicate, 75, 78 silicon nitride, 129 size distribution, 131 specific surface area, 131 synthesis, 75, 359 YAG, 98 zirconia, 93, 97 Ceramic fibers, 106,359,382 BN, 371,381 graphite, 360, 371 green, 362 Sic, 375,385 silicon nitride, 375, 381 silicon oxynitride, 375, 390 Si-Ti-C-O,375 Tyranno, 375,387 Ceramic yields, 109, 119, 362 of boron carbide, 119 of boron nitride, 118 of polycarbosilane, 382

537 Cerium alkoxide, 5 isopropoxide, 15 Ce2(0ifi)8(mH)2,15 Ce604(0W4(acac)l2, 16 CFC in aerogel, 31 1, 329 blown polyurethane, 320, 328 Chemical modification, 3, 34 of alkoxide, 3 in ceramic membranes, 516 by complexation, 3 Chemical vapor deposition (see also CVD), 24,40, 48, 108, 118 Chevrel phases, 61 B-Chloroborazoles, 118 Chlorosilanes, 108, 114 Chromium-containing precursors, 122 CrN, 122 Cinnamic acid, 19 Coatings heat resistant, 391 infrared, 392 methods dip, 467 spin, 467 perhydropolysilazane, 392 tyranno, 391,392 C-0 bond cleavage, 33 Coercive force, 423 remnant, 423 coercivity, 425 Colloidal ceramics acid-base interaction, 200 gels from, 278 for membranes, 5 13 particles, 198 processing, 189 seeds, 187 Colloidal suspensions, 133, 164, 174,425

538

[Ceramic membranes] Doppler shift, 165 electrophoretic mobility, 182 electrostatic stabilization, 172 electrosteric stabilization, 186 ESA, 165,205 flocculation, 144 lyophobic sol, 172, 174 maghemite, 425 magnetite, 425 microemulsion, 434 polyelectrolytes, 186 stability, 144, 157, 172, 224 steric stabilization, 185, 186 zeta potential, 146, 164, 172, 176, 199,202 Columbite, 405 MgNb206,408 route, 406 Combustion method glycine, 73 urea, 73 Complexation step, 41 of zirconium akoxide, 21 Complexing additives, 12 Complexing reagent, 9 Composites alumina, 190 carbon-carbon, 364 ceramic-ceramic, 190 cordierite-ZrO2, 192 lithium aluminosilicate matrix, 192 molecular, 300 mullite matrix, 190 nano-, 300 Sic whisker, 190 zirconia, 190 Condensation, 3 CuFeO2,67 CU,MO&, 61

INDEX

Coprecipitation route, 62 Cordierite, 11, 178, 181, 275 Cracks aging liquid, 268 in gels, 266, 288, 314 pressure gradient, 269 routes to avoid, 272 solvent exchange, 269, 273 supercritical liquid, 269 Cross-linking, 362, 371 agents, 17 degree of, 115 of polycarbosilane, 383 Crystalline silicates, 79 Crystallite sizes, 123 Curing autoclave, 377 kinetics, 366 methods, 372 by oxidation, 385, 391 Raman spectra, 385 ring opening, 377 CVD, 23,40,48, 108, 295, 359, 465,528 plasma-enhanced, 360, 528

Darcy’s law, 270 liquid flux, 270 pressure gradient, 270 Density, 134 packing, 179, 199 relative, 181 skeletal, 3 18 Derjaguin equation, 197 Design chemical, 3 molecular, 3 Dielectric microwave, 400 multilayer capacitors, 481 thin films, 465

INDEX Dielectric constant of lead dielectrics, 41l of medium, 146, 163 of organic solvent, 12 of Ta205,473 B-Diketones, 9, 11, 35, 37 Diimid precipitation, 112 Dimethylchlorosilane, 108 Dip coating, 459,493 wetting for, 486 Direct pyrolysis, 77 Direct synthesis, 106 Disintercalation, 67 of alkali metal ions, 67 Dispersants, 252 acrylic acid, 217 amino groups, 205 amino polyisobutylene (APIB), 203,208 aminosilane (AHAS), 201, 206, 208 Darvan C,41 1 linolenic aid, 200, 206 mixed, 212 PMMA, 230,232,234 DLVO theory, 172, 174, 187,210, 256,514 Double alkoxides, 40,79 Ba-Nb, 484 Li-Nb, 492 Mg-AI, 79,81 Y-AI, 98 Double layer, 157, 160, 256, 514 adsorption, 158 adsorption density, 159 adsorption energy, 163 Coulombic potential, 160 Debye-Huckel length, 161, 172, 198 diffuse layer, 160,' 178 diffuse layer potential, 169

539

[Double layer] DLVO theory, 172, 174, 187, 210 electroneutrality, 160 electrostatc repulsion, 163 GCSG model, 160, 163 Helmholtz plane, 161 hydrodynamic slipping plane, 169 ionization, 158 Nernst equation, 165, 166 OHF' potential, 164 Poisson-Boltzmann equation, 160 relaxation, 178 shear plane, 164 specifically adsorbed ions, 163 Stem plane, 161 Stem potential, 172 surface charge, 158, 164 thickness, 146, 198 zeta potential, 172 Drying autoclave, 291, 314, capillary condensation, 272 capillary stress, 270, 288 control chemical additives, 273 formamide, 273,279 glycerol, 273 oxalic acid, 273, 279 chlorination, 294 curvature, 270 Darcy's law, 270 dehydroxylating agent, 294 differential strain, 270 of gels, 269, 275, 288 hypercritical or supercritical conditions, 272, 315, 318 Laplace's law, 269, 275 shrinkage, 266,279

540

Electrochemical method, 73 Electroluminescent devices, 465 Electrooptical ceramics, 23 Kerr effect, 299 Electrosynthesis, 30 of polymeric precursors, 123 Enolic form, 11 Epitaxial thin films, 48,488 Epitaxy, 488,492 sol-gel, 488, 491,493 solid-state, 493 Ester exchange, 75,77 Esterification, 10 2-(2-Ethoxy-ethoxy)ethoxide,6 Ethylacetoacetate, 11 EuBa$u307,73 EXAFS spectra, 6 , 9 frequency shift,

Ferroelectrics, 23,48 1 amorphous, 493 devices, 481 electric dipoles, 498 materials, 482 KNbO3,482,485,488 LiNbO3, 482,485 LiTa03,489 phase transition, 493 PLZT, 483 PMN-PT, 482,486 PNZT, 489 PT, 482,486 SBN, 482,483,487 oxides, 482 thin film preparation, 481 Ferric oxides cobalt modified, 428,429 B-FeOOH, 426 y-FeOOH, 426 &Fe203 also Hematite), 168

INDEX Flocculation, 144, 256, 258 Fluoride compound NH4F, 277 HF, 277 Fourier transform infrared spectroscopy (FTIR,see also Infrared), 99, 147,320,367,368 adsorption spectra, 220 diffuse reflectance, 144 IR active, 144 photoacoustic, 144 spectra of boron nitride, 118 powders, 143 Fractal, 337 chain entnanglements, 347 cluster-cluster aggregation, 347, 350 cluster nucleation, 337 cluster size, 346, 354 density, 339 diffusion limited growth, 338 dimension, 339 Eden growth model, 337 gel network, 354 growth, 337 models, 350 rate factor, 348 number density, 341 post gelation reaction, 345 radial growth rate, 346 radius, 340 radius of gyration, 339 shape factor, 342 surface, 337 transmission intensity, 348 Freeze drying, 217,219

Gas-phase synthesis, 106, 108 GC/mass spectroscopy, 367

INDEX Gelation, 5 Gel optical composites: inorganic dopants, 298 laser dyes, 298 NLO,299 perylene-silica-PMMA, 299 silica-PMMA, 298 Gels amorphous films, 483 aging, 268,288 alkoxide-derived silica, 289 bloating, 281 bulk xerogels, 5 14 capillary forces, 275 chlorination treatment, 281 colloidal, 278,289 connectivity, 274 cracking of, 266 dehydroxylating agent, 281 densification of, 266 drying, 288 filler phases, 279 fluorinated, 277 fracture of, 268 helium atmosphere, 28 1 He-02 mixture, 28 1 HF catalyzed silica, 347 hydrothermal aging, 291, 292 liquids formamide, 272,273,279 dimethyl formamide, 272,279 mechanical strength, 266 microbubbles in, 281 microporosity, 274 monolithicity in, 265, 270, 273 network of, 268, 279 organic, 3 14 permeability, 266, 268, 270, 273,274 polymeric, 279 network, 346 polymerization, 268

541

[Gels] powder, 275 silica, 288 sintering of, 280 foaming during, 281 syneresis, 268 thermal expansion coefficient of liquid, 268 wettability, 274 Gel permeation chromatography, 252,368 Gel time, 9 of silicon alkoxide, 9 Glycols, 35 Gradient index (GRIN) by ion exchange, 303 lenses, 287, 303 titanium aluminosilicate glasses, 305 Grain growth: homogeneous, 2 16 inhomogeneous, 215 Gravity sedimentation, 132 Stoke’s law, 132 Green density, 266 Hafnia (HfO2),94,96 Hamaker constant, 172, 198,210, 256 polarizability, 21 1, 256 Hematite, 168, 426,437 Heteroalkoxide, 8 High Tc superconducting ceramics, 6, 197, 445 Homometallic alkoxides, 23 Hydrolysis, 3 acid, 81 base catalyzed, 5 19 of organometallic polymers, 363 polycondensation, 37 rate, 12, 35, 217 reaction, 78

INDEX Hybrid compounds organic-inorganic, 18 Hydrolytic decomposition, 78 of metal alkoxides, 78 Hydrothermal transformation, 428, 437 Hyponitrite precursors, 73 Imino alanes, 116,366 poly-, 117, 118, 119 Indium tin oxide (ITO) film, 471 Infrared absorption spectra, 220 diffuse reflectance, 144 hydrogen bond, intensity of, 228 spectra of Bag 2Pb0 gTiO3,221,228 binders, 252 boron nitride, 118 silica, 28l Ta2O5 films, 471 Injection molding, 197, 246 Inorganic dopants, 298 Inorganic membranes, 501 fabrication by sol-gel, 502 supports, 503 supported, 505 tortuosity of channel, 504 Interface analysis calorimetry, 147 complexed ions, 145 in concentrated suspension, 165 counter ions, 145 diffuse layer, 145 dispersed particles, 144 Doppler shift, 165 electrolyte concentration, 146 extended X-ray absorption fine structure, 147 potentiometric titration, 167, 168 streaming potential, 158 surface species, 147 in suspension,

Intercalation, 62, 67 Interfacial phenomena, 157, 187 electrochemical, 189 free energy, 174 Hamaker constant, 172 ion pairs, 169 at metauwater, 157 negative surface sites, 158 positive surface sites, 158 Intergrowth structure, 67 Ion exchange, 62,67 Isoelectric point (IEP),133, 146, 164, 182,200 pH, 146,200,225,228,230 point of zero charge (PZC), 147, 159, 167, 174,200,224 PZR, 164, 182, 183 Lability of alcohol, 25 of alkoxide bond, 40 La2-xBaxCu04, 70,445,447,448 La2Cal-xSrxCu06, 72 La3(0tBu)gk, 32 LaCoOg, 62 L ~ ~ C U O67~ + ~ , LaFeO3,62 L*e0.5co0.5(cN)6.5 H209 65 H209 65 %.5Ndo.’jCo(CN)6.5 LaNiO3,65 LNI-~M,(OH)~, Laser ablation, 492 composite target, 492 single target, 492 Laser dyes, 298 coumarin-314T, 380 fluorescein, 300 poly(p-phenylene vinylene), 300 rhodamine B, 300 rhodamine 6G, 300 Lattice parameter, 136 Lead acetate, 85 Lead alkoxides, 40

INDEX Lead-based electronic ceramics, 397 Curie temperature, 41 1 dissipation factor, 41 1 Goldschmidt tolerance factor, 403 PCN, 398,403,412 P m , 398,402 PEW, 398,403 phase stability, 401 PMN, 398,402,411,412,482 PNN, 398,403 PT, 398,403,411,412 relaxor, 403,416 Lead cuprates, 72 pbl-xbx(zr Ti,)l-x/403, 85 Lead niobates, $3 PbgNb404(0Et)24,44,45,47 Pb/Nb stoichiometry, 8 Pb4o(oEt)6, 8 [PB60(0Et)4lINb(OEt)gl4,8 Lead oxide (PbO): columbite method, 410 excess route, 416 MgO, 410 rich grain boundary, 415 vaporization, 216,404,408 Pb2Ru2-xPbx07-y.67 Lead titanates, 23, 217, 397 Ligand, 38 double-bridging, 3 1 triple-bridging, 3 1 Light scattering Fraunhofer and Mietheories, 133 Rayleigh equation, 133 LiCro2, 67 LiM02, 67 Lithium niobate, 47,482 EDAX of, 489 XRD of, 489 LiNb(OR)6, 41, 45 LiTa03,47

543

LiVO2,67 Low-temperature method, 72,216

Maghemite, 425 aspect ratio, 43 1 elongated, 425 spindle-type, 43 1 Magnetic bacteria, 434,439 critical field, 432 dipole, 424 moment, 424 orientation of, 423 dispersion, 433 domain, 423 ferro fluid, 433 fluid, 433 interaction energy, 424 medium, 430 particles, 421 properties, 422 recording materials, 424 recording media, 425 Magnetic particles bioparticles, 439 composite, 438 vinylferrocene, 438 chelating agent, 435 chromium dioxide ((2109,429 coated with silica, 432 dispersion, 433 oleic acid, 433 elongated, 43 1 longitudinal recording system, 43 1 FeOOH, 426 goethite, 426,428 hematite, 426,437 plastic ferromagnetism, 437 spindle-type, 43 1,435 iron powder, 428 iron nitride, 430

INDEX [Magnetic particles] Fe@, 430 Fe16N2 fih,430 Fe/N ratios, 430 maghemite, 425 magnetite, 425,433 spherical, 435 magnetotactic bacteria, 434,439 geomagnetic field, 439 MnFeO4,434 microemulsion, 434 polymeric microsphere, 439 unilamellar vesicles, 434 Magnesium aluminosilicate (MAS), 78 MgM2(0R)g, 7 9 , ~ Magnesium ethoxide, 79, 81 MgNb2(0Ac)2(0iW 1 0 , 4 4 4 6 Magnetic tapes audio, 425 video, 425 Magnetization domain, 423 remnant 423 saturation, 423,425 Melt processing, 369 Melt spinning, 371, 383 Membrane filtration gas separation, 512 microfiltration, 501,504 layers, 5 11 zirconia, 511, 518 nanofiltration, 501,504 ultrafWation, 501, 504, 512, 516 alumina, 504 Membrane permeability, 526 Adzumi equation, 526 biliquid permporomerty, 526 Hagen-Poiseuille relationship, 527

[Membrane permeability] Kozeny-Carman relationship, 527 laminar (Poiseuille), 526 molecular (Knudsen) flow regime, 526 permeability vs pore radius, 27 permporometry, 526 rejection measurements, 526 Mercury, 24 HgC12, 76 HgI2, 76 porosimetry, 136 Metal acetates, 405 Metal carbonates, 62 Metal halidehlcohol, 75 Metal hydrides, 30 Metal hydroxides, 405 Metal organic chemical vapor deposition (MOCVD), (see also CVD), 23,48, 108 Metal-organic compounds, 75 decomposition (MOD), 481,482 Metal oxides, 61,75 rare earth, 93 Metal silylamides, 24 Methacrylic acid, 19 Methacryl amido salicylic acid, 19, 20 Methanol, 3 12 2-methoxyethanol, 34.49 Microscopy optical, 132 scanning electron, 132 techniques, 136 transmission electron, 132 Microwave dielectric, 400 Mixed oxide method, calcined 217, 218,221 Mixing, 250 analysis of mixedness, 260

INDEX Mixing] degree of mixedness, 260 mechanism, 250 critical stress, 251 dispersive, 250 laminar, 250 molecular diffusion, 250 orientation of agglemerates, 25 1 process tolerance capability, 252 rate of, 261 simulation of, 261 Monte Carlo technique, 261 MoO3,66 M01-~W,03,65,66 Molten salt synthesis method, 217, 218,221 Monolithic ceramics aerogels, 270 silica, 265 xerogels, 266, 270, 271 Mullite (3Al203 2Si02), 79, 80, 190,275 Multicomponent ceramics, 8, 216 glass-ceramics, 78 oxides, 483 silicate glass, 78 thin films, 498 Nanoscopic cluster, 3 13 Nasicon, 61 Near-net-shape, 287 Nd2-xCexCu04, 72 NdNiOg, 65 Nd(OiPr)3(iPrOH)4, 32 Niobium alkoxides, 484 ethoxide, 13 oxide (Nb2O5), 406,408,409 polymers, 120

545

ml0028(NMe4)6 6H20, 13 m8010(oEt)20, 13 Nicalon", 375 ESR spectra of, 385 excess carbon in, 385 fiber, 385 Nitridation of polycarbosilane, 390 rate, 107 temperature, 107 Nitrides, transition metal, 120 Nonaqueous medium, 185, 197 Nonoxide ceramics, 105 aluminum nitride, 105, 115,361, 363,365 boron carbide, 105, 112, 361, 364 boron nitride, 118, 361, 363 composites, 359 elemental analysis, 370 microstructure, 369 silicon carbide, 105, 115, 361 silicon nitride, 105, 112, 115, 129,361 surface analysis, 370 titanium carbide, 363 titanium diboride, 361, 363, 366 titanium nitride, 361,363 Nuclear magnetic resonance (NMR), 171,368,527 2 7 ~ 1 , ~ for aerogels, 316 Knight shift, 139 magic angle spinning (MAS), 139 nuclear magnetogyric ratio, 138 nuclear spin number, 138 31P, 139 powders, 130, 138 ulse Fourier transform, 139 MAS, 139

INDEX muclear magnetic resonance] spin relaxation times, spin-spin interaction, tetramethyl silane (TMS), Nucleophilic addition, attack, ligands, 5 substitution, Oligomerization, 5 Oligomers, Organic amines, Ormosil, optical metallic quatum dots, photochromic molecules, photochromism, semiconductor quantum dots, Oxidation curing, high temperature, Raman spectra for polycarbosilane, Oxophilic, Oxopolymers, alkoxides, 8 , bridges, 8, ligands, 8 Oxygen stoichiometry,

Paratungstate, Particles acoustic field, dispersion of, electrokinetics, sonic amplitude electrolyte concentration, electrophoresis,

[Particles] electrophoretic dispersion, hydrodynamic shear plane, in liquids, magnetic, monodispersed, quasi-spherical, sedimentation potential, spherical, ultrasonic wave, zeta potential of, Particle surface, acid-base equilibria, interaction, adsorption density, amphoteric dissociation, charge density, complex, counter ions, DLVO theory, metal-hydroxo complexes, potential, potential determining ions, site-binding model, surface sites, zeta potential, Particulate acicular, prismatic, 80 Peptization, Percolation, theory, Perovskite, phase, structure, Pervaporation, bonding,

INDEX PLZT, alkoxide derived, DTA of, powders, PMN ceramics, Phenolic resins, Photochromic properties, Piezoelectric actuators, resonance, tranducers, Plastic forming, Polarization coercive field in ferroelectric, ferrous, remnant, Polyacrylonitrile (PAN), Polyborazylene, Polyborosilazanes, Polycondensation via dechlorination, in gels, rate of, Polymers (see also Polysilanes) acrylic acid, atactic polypropylene, branched-ring, carbon-based, chain, mobility, cross-linking, entanglements in, ethylene ethylacrylate (EEA), ethylene vinylacetate (EVA), grafted chains, high-melting, linear,

547

[Polymers] melts, metal-organic, organo-silicon, PMMA, polyacrylonitrile (PAN), polyborosilazane, polyethylene, polysilazane, polysiloxanes, polystyrene, polytitanosilane (PTC), preceramics, pyrolysis, ring, sterically hindered, structure, thermoplastic resin, Poly01 process, Polyoxo anions, Polyoxometallates, Polysilanes, borodiphenyl siloxane, carbosilanes, oxygen cured, dimethyl silane, dimethylsiloxane (PDMS), silastyrene, silazanes, siloxanes, titanosilane (PTC), Polyvinyl borazine, Pore BET adsorption, macro- ,meso- , and micro- , mercury porosimetry, porosity,

548

Pore1 radius, 18 size, 269, 273, 315 Porous structure;503 bubble-point method, 509 capillary shaped, 509 concentric shell, 11 diameter, 506 Dubinin-Astakov pore volume, 520 layers, 507 oriented, 504 total pore volume, 525 Portland cement, 199,208 slumes, 209 surface, 209 tensile strength of cast, 209 Potassium hydride, 114, 115,381 [K(OtBu)lq, 32 K4SiW11039, 18 Powder characterization, 129 A E S , 140, 141, 142 atomic absorption spectroscopy, 136 bulk composition, 136 density, 134 inductively coupled plasma, 136 milling, 405 morphology, 136 NMR and EPR, 137, 140 size, 131 surface area, 134 XPS, 141 XFS, 137 XRD, 140 Precursors improper, 49 1 method, 62 proper, 491 organosilazane, 361 Primary alcohols, 24 Primary particles, 131

INDEX Processing aids, 200, 217, 243, 230,252,256 Pyrochlore phase, 398,402 structure, 67 Pyroelectric coefficient, 49 1 current, 494 heat sensors, 494 Pyrolysis, 77, 109, 361, 362, 371-376 or organometallic polymers, 360 of polysilanes, 110 of polysilazanes, 114, 115 of polyvinyl pentaborane, 364 rate of gas evolution, 363 of B-triamino-Ntriphenyborazine, 363 PZT, 48,481,482

Raman spectroscopy, 143 frequency, 144 Rayleigh scattering, 144 surface silica, 144 Rapid thermal annealing, 487 Rayleigh scattering, 144, 298 ReO3,66 Rheology, 144,239,369 Bingham solid, 240 Brookfield shear rate, 208 for ceramic membranes, 508 continuous-phase, 247 dilatant, 240, 243 drag force, 250 Einstein relation, 249 constant, 249 elastic deformation, 241 fluidity, 250 frictional voids, 240 of gels, 274 inverted plasticity, 243 Newtonian flow, 240

INDEX [Rheology] nonNewtonian, 240,242 properties, 188,226, 360 pseudo plastic, 240 and related behavior, 199 Reynolds number, 250 shear rate, 240 thixotropic, 241 threshold shear stress, 240 viscoelastic, 241 viscosity, 240 of ZR02lAl203, 188 RbxP8W3201 129 61 Rock salt structure, 62 Schlenk vacuum line, 363 Screen printing, 45 1,459 Sialon, 61 Silanols, 35 Silica aerosil, 277 Cab-0-Sil, 276 with carbon black, 320 coating, 190 colloidal, 276, 288 condensed, 3 13 electrokinetics, 192 Eu-doped, 294 fluorescence spectra, 296 fiber-optic amplifiers, 293 fumed, 273 gel, 288 residual silanols, 294 glasses, 287 infrared spectra of, 281 La-dopd, 293 laser glasses, 293 Ludox", 273 modified ormocer, 279 ormosil, 279,287,300,302 Nd-doped, 293

549

[Silica] quartz transformation, 273 rheology, 199 Stober, 273 surface, 205 uranium fluorescence, 296 Silicates grain boundary phase, 129 GRIN glasses, 305 potassium, 276 sodium, 276,312 surface silanol, 182 Silicon &oxide, 5, 80, 190 sodium coupling reaction, 365 TEOS, 519 Silicon carbide, 361 auger spectrum of, 142 fibers, 375, 385 ESR spectra of, 385 excess carbon in, 385 filler, 245 Nicalona, 375 polytypes, hexagonal 108 powder, 243 from rice shell, 391 13-silicon carbide, 108, 385 silicon nitride composites, 115 Tic composit fibers, 387 whiskers, 391 composites, 190, 192 Si(CH3)3Cl, 109 Si(CH3)3CH=CH2, 109 Silicon monomers dichlorosilane, 377 dimethyldichlorosilane,377 HSiCH3C12,380 dodecamethylcyclohexasilane, 379 phenylmethyl dichlorosilane, 377 methyltrichlorosilane, 380

550

Silicon nitride, 105, 112, 115, 129, 258,261 auger spectrum of, 142 coatings, 59 fibers, 375,381 a and B mixture, 361 substrates, 257 XRD of, 390 Si-0-C, 383 Si-0-Si, 383 Si-0-Ti, bonds, 17 Siloxanes, 17 Si-Ti-C-0 (Tyranno) fibers, 375, 387 Silylamid derivatives, 25 Sintering additives, 105, 115, 320 of B% 2Pb0 gTiO3,230 of gels; 272,'280 B-AgA102,67 Slip casting, 505,511 NaCaqNb5017,68 Na2Fe60(CH3)18 6CH30H, 8 NaMo406, 61 Sodium sulfur battery, 329 NaZn2Mo308.61 Sol-gel chemistry, 3 derived B% 2Pb0 gTiO3, 225-230 derived powders, 216 epitaxy, 488,491,493 in ferroelectrics, 48 1,482 homogeneity, 405 hydrolysis, 482 under basic conditions, 267 for membranes, 502,512 method, 62.7 1,215,465 multiphasic, 187 polycondensation, 265,482 rate of, 267 process, 41,75, 265,287,492

INDEX [Sol-gel] routes, 121 synthesis, 3 thin film, 465 Solid-state chemistry, 61 Solvation, 6 Solvents, 24 acetone, 204,205 enolization of, 206 basicity scale, 201 benzene, 23 1,232 chloroform, 202,278 l-decanol, 278 ethanol, 204,205 ethyl acetite,'2W good, 230 hexane, 204,23 1,232 methylene chloride, 203 mixed, 2 12 nonpolar, 41 organic media, 199,432 for PMMA, 231 0-point, 231,232 solubility parameter, 230 THF, 200 toluene, 200 Sonochemistry, 24 Spin coating, 467, 493 contact angle, 486 oxide substrates, 486 wetting, 486 Spinel (MgA1204), 81 Stability colloidal, 224 dispersion, 144,432 electrostatic, 172, 200 electrosteric, 86 ratio, 176 of solvates, 25 steric, 185, 186, 198 barrier, 278 hindrance, 5

INDEX

[Stability] repulsion, 185 stabilization, 185, 186, 198 stabilizer, 204 Steric stabilization, 185, 186, 198 chain length of stabilizer, 198 entropic repulsion, 256 Storage modulus vs. oscillation frequency, 244 Sr2PbO4,65 Sr phenoxide, 32 sr4(oph)~(phoH)2(THF)6. 32. srTi2(oiPr)8(iProH)3,45 Substitution reactions, 25 Superconducting cuprates, 62, 70, 197,445 Bi(2212), 449 Bi(2223), 449 copper oxide, 447 grain boundaries, 459 La(214), 447,448 La2xSrxCu4, 453,454 preparation, 453 melt textured growth, 454 quench and melt growth (QMG),454 traveling solvent floating zone (TSFZ), 453 PrBa2Cu30x, 448 SmBa2Cu307,456 superconducting grains, 46 1 T1(1223), 447,452 weak links, 459 Y(123), 65,70-72, 139, 447-449 Y2BaCuOg,454 Superconducting wires, 459 intergrain Jc, 459 transport Jc, 459 weak link, 459 Superconductivity,446 BCS theory, 446

551

[Superconductivity] Bean’s equation, 456 critical current density, 447 diamagnetism, 450 electron-phonon interaction, 446 isotope effect, 447 magnetization hysteresis, 457 Meissner effect, 450 transport Jc, 456 Surface acidic sites, 200 acidity-basicity, 209 amphoteric, 199 area, 134 basic sites, 199 of cement, 209 charge, 158, 164 contact angle, 257 force apparatus, 188 induced coating, 189 of mica, 187 -OHgroup, 205 of silica, 205 sites, 157 species, 147 tension, 257, 510 Surface area of B% 2Pb0 gTiO3,219 BET, i34,2i9 internal and external, 134 of lanthanum oxycloride membrane, 520 permeametry, 134 Surfactants for stabilization, 133 Suspensions filled, 247 interparticle forces, 247 mixed, 191 stability, 144, 157, 172, 197 viscosity, 240 Suspension stability, 144, 157, 172, 197,258

552

[Suspension stability] attractive energy, 198 Born repulsion, 173 Derjaguin equation, 197 electrostatic stabilization, 172 Eliers & Korff rule, 175 flocculation, 144 Hamaker constant, 172 kinetically stable mixed, 191 potential energy barrier, 176 primary maximum, 173 rapid coagulation, 176 secondary minimum, 173, 189 slow coagulation, 175 Smoluchowski limit, 176 stability ratio, 176

Tantalum alkoxide, 466 Tantalum oxide, 465 breakdown voltage, 478 dielectric constant, 473 electron diffraction pattern of, 478 films, 471 infrared spectra of, 472 microstructure, film, 475 refractive index of, 473 &Ta205,472 Tape casting, 45 1,508 for ceramic membranes, 505 doctor blade, 45 1, 508 Teratogenic, 49 Tertiary butoxide, 77 Tetraethylorthosilicate or tetraethoxysilane (TEOS), 80,278,519 Tetramethoxysilane (TMOS), 278, 312,519 Texture-oriented films, 487 epitaxial 488 of LiNbO3,492

INDEX [Texture oriented films] nearest atomic arrangement, 492 lattice mismatch, 488,491 KNBO3,488 substrates, 488,489 zro2,493 Thallium cuprates, 72,447, 449, 45 1-453,456,457 Tl2O3,72 Thermal anlysis DSc, 369 DTA, 368,472 TGA, 367 TMA, 368 Thermal decomposition, 72,78, 383 of Ba[TiO(C204)2], 62 Of LaCO(CN)6 5H20,62 of Li[Cr(C204)2(H20)21, 62 of metal alkoxides, 482 Thermalysis reactions, 40 Thin films epitaxial, 488 gel, 486 . ITO, 471 multilayer capacitors, 481 preferred crystallographic orientation, 487 preparations, 467 dip coating, 459 sol-gel, :482 spin coating, 467 sputtering, 467 substrates, 486 tantalum oxide, 465 thickness, 467 transparent, 466 Titanates, powder, 90 Titanium albxides, 14, 19, 32, 278

553

INDEX Ti alkoxy oxoacetates, 10 Titanium compounds, 366 acetilides, 120 carbide, 120 diboide, 361, 363, 366 nitride, 120 precursors, 122 tetrachloride, 120 tetrakis-dialkylamine, 120 TiNmiC solid solutions, 122 Ti containing monomers, 366 Ti02 gels, 10 powder, monodispersed, 6 TiN, 107 Ti20(0Ac)6, 11 Ti(acac)2(0R)2, 14,48 Ti-glycolate bond, 37 Ti-O-Ba bonds, 8 Titration conductometric, 147 potentiometric, 147, 167 Topochemical dehydration, 65 methods, 62 reactions, Trialkylsi1oxo.n donor, 34

S-Triamino-N-triphenylbonine, 363 B-Trichloroborazine, 363 Triphenyl boron, 112 W(OEt)6, 11 WO3,66 bronze, 66 hexagonal, 70 intergrowth, 70

Uranium hexafluoride, 503 isotopes, 503

Video recorder, high-definition, 430 Viscosity, 240 alumina suspension, 200 of supension, 342

Wetting, 257 adhesional, 257 contact angle, 257 immersional, 257 spreading, 257 surface tension, 257 Wolframite, 405 AlNb04, 408 CrNbO4,408 W-0-Si-C bonds, 18

XANES spectra, 6 X-ray photoelectron spectroscopy, 130, 140-143,370 binding energy, 141 photoelectrons, 141 Scofield cross-sectional values, 141 Sic, 142 Si3N4, 142 Si02, 142, 143 X-ray powder diffraction, 140,367 pattern Ta20.3472 X-ray Sedigraph, 132

YAG (Y3Al5O12) (see also Yttrium aluminum garnet), 98,100 Yttrium aluminum garnet, 98,100 YBa2Cu307 (YBCO), 65,70-72, 139 447,449

554

Zero-expansion ceramic, 61 Zinc alkoxides, 7, 30 ZnNb2O6,408 Zirconia alumindsilicon carbide composites, 190 dispersion, 192 green bodies, 180 powder, monodispersed, 15 submicron, 93, 97 toughened alumina with (ZTA), 189

INDEX [Zirconia] Y203-stabilized. 94, 97 Zirconium borides, 366 Zirconium organ0 complex, 15, 37 &oxides, 14 polymers, 120 Zirconium-oxopoly-PA, 20, 21 -methacry1 amidosilicylated copolymers, 20

Chemical Processing of Ceramics

Chemical Processing of Ceramics (Materials Engineering, 8)

Chemical Processing of Ceramics, Second Edition (Materials Engineering)