Handbook of Advanced Ceramics: Materials, Applications, Processing and Properties

Handbook of Advanced Ceramics VOLUME II

Processing and their Applications

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Handbook of Advanced Ceramics VOLUME II

Processing and their Applications

EDITORS SHIGEYUKI SOMIYA (Editor-in-Chief ) Professor Emeritus, Tokyo Institute of Technology, Tokyo, Japan

FRITZ ALDINGER Max-Planck-Institut für Metallforschung, Stuttgart, Germany

NILS CLAUSSEN Technische Universität Hamburg-Harburg, Germany

R ICHARD M. SPRIGGS Alfred University, New York, USA

K ENJI UCHINO The Pennsylvania State University, USA

K UNIHITO K OUMOTO Nagoya University, Japan

MASAYUKI K ANENO Japan Fine Ceramics Association, Tokyo, Japan

Amsterdam  Boston  Heidelberg  London  New York  Oxford Paris  San Diego  San Francisco  Singapore  Sydney  Tokyo

This book is printed on acid-free paper. Copyright © 2003, Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. ACADEMIC PRESS An Imprint of Elsevier 84 Theobald’s Road, London WC1X 8RR, UK http://www.elsevier.com ACADEMIC PRESS An Imprint of Elsevier 525 B Street, Suite 1900, San Diego, California 92101-4495, USA http://www.elsevier.com ISBN 0–12–654640–1 A catalogue record for this book is available from the Library of Congress A catalogue record for this book is available from the British Library Typeset by Newgen Imaging Systems (P) Ltd, Chennai, India Printed and bound in Great Britain by MPG Books, Bodmin, Cornwall 03 04 05 06 07 MP 9 8 7 6 5 4 3 2 1

To The late Professor Emeritus E. F. Osborn The Pennsylvania State University University Park, Pennsylvania, USA

To Professor Emeritus J. A. Pask University of California, Berkeley, California, USA

To Professor Emeritus Günter Petzow Max Planck Institut für Metallforschung, Pulvermetallgisches Laboratorium Shigeyuki S¯omiya

This Page Intentionally Left Blank

CONTENTS

Preface Acknowledgments List of Contributors

ix xi xiii

Part 1: Functional Ceramics 1.1 Insulating Ceramics/High Thermal Conductive Ceramics

3

Kazunori Koga

2.1 Semiconductive Ceramics

25

Hideaki Niimi and Yukio Sakabe

3.1 Ionic Conductors/ Oxygen Sensors

37

Tessho Yamada

3.2 Ceramic Fuel Cells

59

J. Fleig, K. D. Kreuer and J. Maier

4.1 Piezoelectric Ceramics

107

Kenji Uchino

5.1 Dielectric Ceramics

161

Yukio Sakabe

6.1 Magnetic Ceramics

181

Takeshi Nomura

7.1 Optoelectroceramics

199

Hajime Haneda

8.1 Superconductive Ceramics

241

Kazumasa Togano vii

viii

Contents

Part 2: Engineering Ceramics 9.1 High-Temperature High-Strength Ceramics

267

Kaoru Miyahara, Yasuhiro Shigegaki and Tadashi Sasa

10.1 Porous Ceramics for Filtration

291

Toshinori Tsuru

11.1 Ceramic Bearing

313

Hiroaki Takebayashi

11.2 Cutting Tools

333

Mikio Fukuhara

11.3 Decorative Ceramics

347

Mikio Fukuhara

12.1 Ceramic Materials for Energy Systems

355

Hiroshi Nemoto

13.1 Extruded Cordierite Honeycomb Ceramics for Environmental Applications

367

Toshiyuki Hamanaka

14.1 Ceramics for Biomedical Applications

385

Tadashi Kokubo, Hyun-Min Kim and Masakazu Kawashita

15.1 Ceramic-Matrix Composites

417

Akira Okada

16.1 Functionally Graded Materials

445

Lidong Chen and Takashi Goto

17.1 Intelligent Ceramics—Design and Development of Self-Diagnosis Composites Containing Electrically Conductive Phase

465

Hideaki Matsubara, Yoshiki Okuhara, Atsumu Ishida, Masayuki Takada and Hiroaki Yanagida Index

479

PREFACE In 1989 Shigeyuki Somiya, ¯ the Editor-in-Chief of this book, published Advanced Technical Ceramics (Academic Press, Inc.; original publication in Japanese, 1984). Well over a decade has passed without the appearance of an authoritative new title on the ever-changing subject of Advanced Ceramics. The purpose of this book is to provide an up-to-date account of the present status of Advanced Ceramics, from fundamental science and processing to application. The Handbook of Advanced Ceramics has an internationally renowned group of contributing editors. They are well known throughout the world in their fields of study. These editors discussed the contents and chose the authors of each of the book’s chapters very carefully. The chapters consist of review and overview papers written by experts in the field. Up until about 50 years ago, ‘ceramics’ were considered to be porcelains, bottle glass, sheet glass, refractory bricks, enamels, cements, lime, gypsum and abrasives. In recent years the field of ceramics has broadened and expanded. Ceramics are now used in new fields of research as well as in the old fields. This handbook describes these developments and the new processes and applications. The handbook will enable the reader to understand the present status of Ceramics and will also act as an introduction, which may encourage further study, as well as an estimation of the role advanced ceramics may have in the future. The handbook is a two-volume set. Part I deals with Materials Science and Part II with Processing and Applications. Part I serves as an introduction to the basic science, raw materials, forming, drying, sintering, innovative processing, single crystal growth, machining, joining, coating, fracture mechanics, testing, evaluation, etc. Part I is intended to provide the reader with a good understanding of the new techniques in advanced ceramics, such as thin films, colloidal processing, active and passive filler, pyrolyses process and precursor derived ceramics, as well as providing a template for the deposition of ceramics from aqueous solutions. Part II deals with more recent processes and applications and functional and engineering ceramics. The engineering ceramics covered in this book were developed within the last decade. The functional ceramics covered include electro-ceramics, optoelectro-ceramics, superconductive ceramics, etc. ix

x

Preface

as well as the more recent development of piezoelectric ceramics and dielectric ceramics. The use of ‘Engineering’ Ceramics, introduces entirely new fields to be considered. These include mechanical properties, decorative ceramics, environmental uses, energy applications, bioceramics, composites, functionally graded materials, intelligent ceramics and so on. The term Advanced Ceramics is opposite in meaning to ‘Traditional’ or ‘Classical’ Ceramics. In the past, Advanced Ceramics were often confused with New or Newer Ceramics, Modern Ceramics, Special Ceramics and so on. Furthermore, Fine Ceramics, at least in the USA and Europe, is synonymous with Fine Grain Ceramic Products and/or Fine Grain Porcelain; Fine Ceramics in Japan is similar to what we understand as Advanced Ceramics. So for this edition, the term Advanced Ceramics was chosen as the most suitable title for a book providing an in-depth survey of the current state of Ceramics Science and its applications. It is the editors’ wish that this book will provide the reader with a detailed understanding of the many applications of Advanced Ceramics in both today’s world and in that of the future. The editors wish to thank all those who participated in the preparation of this book such as authors, publishers and copyright owners in Europe, USA, Asia and the rest of the world. Fritz Aldinger Nils Claussen Masayuki Kaneno Kunihito Koumoto Shigeyuki Somiya ¯ (Editor-in-Chief) Richard M. Spriggs Kenji Uchino

ACKNOWLEDGMENTS

First and foremost, I would like to thank all the co-editors, Fritz Aldinger, Nils Claussen, Richard M. Spriggs, Kenji Uchino, Kunihito Koumoto and Masayuki Kaneno for their valuable suggestions with regard to the chapters, authors, and their editorial assistance. Without their help, this book would not have been possible. Especially, Fritz Aldinger, Nils Claussen, and Richard M. Spriggs gave me good advice for this book. Mr Kaneno offered secretarial assistance. Second, I wish to thank all the publishers who gave permissions to authors to reproduce and use the materials from their original published papers. Third, many authors wrote their chapters within a short time in spite of their tight schedule. I would like to express my gratitude to all these authors. I would also like to extend my appreciation to Ms Amanda Weaver and her group at Elsevier, Oxford for their contribution to the publishing works. Finally, I was able to study abroad in the USA under the Fulbright Exchange Program and in Germany under the scholarship program by Max Planck Institut für Metallforschung, Pulvermetallgisches Laboratorium. Without these experiences, I would not have made it as Editor-in-Chief of this book. I thank all my professors and friends around the world who have made this possible. Shigeyuki Somiya ¯ Editor-in-Chief

xi

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CONTRIBUTORS

LIDONG CHEN, Shangai Institute of Ceramics, Academic Sinica, 1295 Ding-Xi Road, Shangai 200050, China J. FLEIG, Max-Planck-Institut für Festkörperforschung, Heisenbergstrasse 1, 70569 Stuttgart, Germany MIKIO FUKUHARA, Development of Special Products, Toshiba Tungaloy Ltd., Sugasawa, Tsurumi, Yokohama 230-0027, Japan TAKASHI GOTO, Institute for Materials Research, Tohoku University, Katahira 2-1-1, Sendai 980-8577, Japan TOSHIYUKI HAMANAKA, Ceramics Business Group, NGK Insulators Ltd., 2-56 Suda-cho, Mizuho-ku, Nagoya 467-8530, Japan HAJIME HANEDA, Advanced Materials Laboratory, National Institute of Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan ATSUMU ISHIDA, Japan Fine Ceramics Center, 2-4-1 Mutsuno, Atsuta-ku, Nagoya 456-8587, Japan MASAKAZU KAWASHITA, Graduate School of Engineering, Kyoto University Sakyo-ku, Kyoto 606-8501, Japan HYUN-MIN KIM, Department of Ceramic Engineering, School of Advanced Materials Engineering, Yonsei University, 134, Shinchon-dong, Seodaemun-gu, Seoul 120-749, Korea KAZUNORI KOGA, Corporate R&D Group for Components & Devices, Kyocera Corporation, 6 Takeda Tobadono-cho, Fushimi-ku, Kyoto 612-8501, Japan TADASHI KOKUBO, Graduate School of Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan K. D. KREUER, Max-Plank-Institut für Festkörperforschung, Heisenbergstrasse 1, 70569 Stuttgart, Germany xiii

xiv

Contributors

JOACHIM MAIER, Max-Planck-Institut für Festkörperforschung, bergstrasse 1, D-70569 Stuttgart, Germany

Heisen-

HIDEAKI MATSUBARA, Japan Fine Ceramics Center, 2-4-1 Mutsuno, Atsuta-ku, Nagoya 456-8587, Japan KAORU MIYAHARA, Technical Development, Ishikawajima-Harima Heavy Industries Co., Ltd., 1 Shin-nakahara-cho, Isogo-ku, Yokohama 235-8501, Japan HIROSHI NEMOTO, BIU, NGK Insulators Ltd., 2-56 Suda-cho, Mizuho-ku, Nagoya 467-8530, Japan HIDEAKI NIIMI, Murata Manufacturing Co. Ltd., Yasu 520-2393, Japan TAKESHI NOMURA, Materials Research Center, TDK Corporation, Narita 2868588, Japan AKIRA OKADA, Japan Fine Ceramics Center, 2-4-1 Mutsuno, Atsuta-ku, Nagoya 456-8587, Japan YOSHIKI OKUHARA, Japan Fine Ceramics Center, 2-4-1 Mutsuno, Atsuta-ku, Nagoya 456-8587, Japan YUKIO SAKABE, Murata Manufacturing Co. Ltd., Yasu 520-2393, Japan TADASHI SASA, Technical Development, Ishikawajima-Harima Heavy Industries Co., Ltd., 1 Shin-nakahara-cho, Isogo-ku, Yokohama 235-8501, Japan YASUHIRO SHIGEGAKI, Technical Development, Ishikawajima-Harima Heavy Industries Co., Ltd., 1 Shin-nakahara-cho, Isogo-ku, Yokohama 235-8501, Japan HIROAKI TAKEBAYASHI, EXSEV Engineering Department, Koyo Seiko Co., Ltd., Kokubu Tojyo-machi, Kashihara 582-8588, Japan MASAYUKI TAKADA, Japan Fine Ceramics Center, 2-4-1 Mutsuno, Atsuta-ku, Nagoya 456-8587, Japan KAZUMASA TOGANO, Institute for Materials Research, Tohoku University, 2-1-1, Katahira, Aoba-ku, Sendai 980-8577, Japan TOSHINORI TSURU, Department of Chemical Engineering, Hiroshima University Higashi-Hiroshima 739-8527, Japan KENJI UCHINO, Materials Research Laboratory, The Pennsylvania State University University Park, PA 16802-4801, USA TESSHO YAMADA, Sensor Division, Automotive Components Group, Engineering Department, NGK Spark Plug Co. Ltd., Komaki 495-8510, Japan HIROAKI YANAGIDA, Japan Fine Ceramics Center, 2-4-1 Mutsuno, Atsuta-ku, Nagoya 456-8587, Japan

PART

Functional Ceramics

1

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Handbook of Advanced Ceramics S. Somiya ¯ et al. (Eds.) Copyright © 2003 Elsevier Inc. All rights reserved.

CHAPTER 1

1.1 Insulating Ceramics/High Thermal Conductive Ceramics KAZUNORI KOGA Corporate R&D Group for Components & Devices, Kyocera Corporation, 6 Takeda Tobadono-cho, Fushimi-ku, Kyoto 612-8501, Japan

1.1.1 GENERAL REMARKS

1.1.1.1 MATERIALS FOR PKG Because of its high thermal conductivity, high mechanical strength, good insulation characteristics, moderate dielectric properties and high chemical durability, alumina (HTCC: high-temperature co-fired ceramics) is the most popular ceramics material for semiconductor packages. However, for power devices like power amplifier for base station or for satellites, higher thermal conductivity material is required to dissipate the heat generated in the devices. To meet this requirement, aluminum nitride (AlN), which has high thermal conductivity (TC) and a low thermal expansion coefficient comparable to that of Si, has been adopted for packages requiring high thermal dissipation. Another market trend, toward higher power, higher working frequencies and lower power consumption, requires reduction of the resistivity of conductors in co-fired packages. To meet this requirement, glass ceramics (LTCC: lowtemperature co-fired ceramics) with silver or copper conductors have been developed. Kyocera has been conducting research and development on AlN and LTCC for more than 10 years, and has produced many kinds of AlN and LTCC products, such as Cer-Quad and multilayer packages, in addition to thin film substrates. Now, we have developed three materials for packages. The first is a novel AlN material (AN75W) that can co-fired at low temperature to reduce cost. The second is a novel LTCC that has a high thermal coefficient of expansion close to that for FR-4. The third is also a novel LTCC that has low permittivity 3

4

K. Koga

Tape casting Sheet cutting Punching

Via filling Screen printing Lamination Cutting Co-firing Ni–Au plating FIGURE 1.1.1 Process flow for multilayer ceramic package fabrication.

and low loss tangent at high frequency. In this section, we describe these new materials.

1.1.1.2 PROCESS FLOW Figure 1.1.1 shows the process flow for a co-fired multilayer ceramic package. There are many steps to produce a multilayer package; however, there are only a few differences among different materials. Of course, material composition, metallize composition and process condition is different for each. Among these steps, metallizing is an especially critical technology for package production.

1.1.2 ALUMINUM NITRIDE

1.1.2.1 MATERIAL PROPERTIES Table 1.1.1 shows properties of the new and standard aluminum nitride compositions, compared with multilayer alumina [1]. The main difference between

1.1

5

Insulating Ceramics/High Thermal Conductive Ceramics TABLE 1.1.1 Properties of AlN AlN

Al2 O3

AN75W

AN242

A-440

Thermal conductivity

75

150

14

Dielectric constant (1 MHz) tan δ × 10−4 (1 MHz) Volume resistivity ( cm at 20◦ C)

8.6 6 >1014

8.7 1 >1014

9.8 24 >1014

Flexural strength (MPa) Young’s modulus of elasticity (GPa)

400 310

400 320

400 310

250 AN75W Tan  (× 10–4)

200

AN242

150 100 50 0 1

10

100

1000

10 000

100 000

Frequency (MHz) FIGURE 1.1.2 Frequency dependence of dielectric loss of AlN.

these materials is in thermal conductivity, which is 75 W/mK for AN75W and 150 and 14 W/mK for standard AlN and Al2 O3 , respectively. Other fundamental properties are rather similar. AlN, however, has a characteristic dielectric dispersion at high frequencies [2]. Figure 1.1.2 shows the frequency dependence of the dielectric loss (tan δ) of AN75W and AN242. The dielectric loss of AlN shows a maximum at a few gigahertz. This phenomenon is due to the piezoelectricity of AlN, and the peak frequency inversely depends on crystallite size. As the crystallite size of AN75W is smaller than that of AN242, the peak (dispersion) frequency of AN75W is correspondingly higher.

1.1.2.2 TECHNOLOGY FOR MULTILAYER AlN PACKAGES Concerning the cost, tape casting and co-firing processes of AlN package are particularly expensive compared with those of Al2 O3 packages, since the cost

6

K. Koga

Sheet resistance (mΩ/SQ)

50 40 30 20 10 0 Residual carbon content

FIGURE 1.1.3 Sheet resistance as a function of residual carbon content in co-fired AlN.

of raw material is high and a significantly higher firing temperature is needed. We have specifically developed AN75W for low-temperature co-firing. It is not so difficult to sinter AlN below 1700◦ C by combining rare earth and alkaline-earth compounds as sintering aids. However, as the sintering temperature is reduced, the chemical durability, especially to alkaline solutions, becomes much worse. Moreover, residual carbon content in the package makes a strong effect on the sheet resistance of the refractory metal conductors (Fig. 1.1.3). In addition, the concentration of residual carbon depends on organic system in tape and sintering condition. The sintering aids and sintering conditions for AN75W have been developed to optimize the manufacturability, cost, material properties, chemical durability and metallization characteristics. The magnitude of any cost reduction depends on package size and design. In general, however, packages manufactured using AN75W are much cheaper than those produced with standard grades of aluminum nitride.

1.1.2.3 THERMAL RESISTANCE The thermal conductivity of packaging materials will influence its adoption in consumer and other applications. Figure 1.1.4 shows the thermal resistance (θj-a) of a package (OD: 8 mm2 ; thickness: 0.762 mm; cavity: 4 mm sq; power consumption: 0.5 W) alone and mounted on a PC board (FR-4: 50 mm2 , 1.5 mm thick) as a function of the thermal conductivity of the ceramic (Al2 O3 , AlN: AN75W and AN242). The thermal resistance of the AN75W package is almost identical to that of the AN242 package especially after mounting onto the PC board.

1.1

7

Insulating Ceramics/High Thermal Conductive Ceramics

Thermal resistance (K/W)

150 125 PKG

100

PKG on PCB 75 50

0

25

50 75 100 125 150 Thermal conductivity (W/mK)

175

FIGURE 1.1.4 Thermal resistance as a function of ceramic thermal conductivity.

Thermal resistance (K/W)

35 30 25 20 15 10 0

25

50

75

100

125

150

175

Thermal conductivity (W/mK) 30 mm2 50 mm2

40 mm2

FIGURE 1.1.5 Thermal resistance as a function of thermal conductivity for packages measuring 30, 40 and 50 mm2 .

Figure 1.1.5 shows the thermal resistance (θj-a) of other packages (OD: 30, 40 and 50 mm2 ; thickness: 1.0 mm; power consumption: 3 W) as a function of thermal conductivity. In this example, two Al2 O3 ceramics are included. The thermal resistance of these packages decreases with increasing package size. This is due to the increased thermal capacity of the package. As in the earlier example, the thermal resistance of the AN75W packages is superior to alumina and comparable to that of the AN242 packages. With increasing package size, however, the effect of the higher thermal conductivity of AN242 is more pronounced than in the smaller outlines. With larger formats, the thermal path

8

K. Koga

Thermal resistance (K/W)

35 1W 3W 5W

30

25

20

15

0

25

50

75

100

125

150

175

Thermal conductivity (W/mK) FIGURE 1.1.6 Thermal resistance as a function of ceramic thermal conductivity and input power.

length becomes longer. Thus, thermal conductivity exerts a greater influence on thermal resistance in larger packages. Figure 1.1.6 shows the effect of package power dissipation, again as a function of ceramic thermal conductivity (OD: 40 mm2 ; thickness: 0.762 mm). The thermal resistances of the AN75W and AN242 packages show only a slight difference in the power consumption range from 1 to 5 W.

1.1.2.4 RELIABILITY We have performed reliability tests on AlN packages produced with both AN75W and AN242. Seal hermeticity, external visual and electric performance, including plating durability, were evaluated after high-temperature storage, low-temperature storage, temperature cycling, and thermal shock. No AlN package has shown any reliability issues. Thus, we confirm that both AN75W and AN242 can be applied for consumer products where high reliability is required.

1.1.2.5 SUMMARY The characteristics of Kyocera’s AlN packages are summarized as follows; 1 AN75W packages have been demonstrated. This material is low temperature co-fireable and can be produced at a cost less than standard AlN.

1.1

Insulating Ceramics/High Thermal Conductive Ceramics

9

2 As a result of its fine grain size, AN75W exhibits a low dielectric loss at a few gigahertz. The loss maximum is shifted to higher frequency relative to AN242, which has a coarser crystallite size. 3 The thermal resistance of AN75W packages is comparable to that measured on packages produced from high TC AlN, especially for small outline packages mounted on PWB. 4 Both AN75W and AN242 packages have demonstrated high reliability. In conclusion, we believe that AN75W is applicable to packages requiring both high thermal dissipation and low cost, for example in consumer products.

1.1.3 LTCC WITH HIGH THERMAL COEFFICIENT OF EXPANSION

1.1.3.1 INTRODUCTION Due to the ever increasing I/O counts for IC devices, packaging trends have been changing to surface mountable area array second-level interconnection, namely BGA and CSP. The driving force for these types of second-level mounting is described as follows [4]. 1 Higher wiring density: smaller packages, thinner packages, lighter packages, 2 higher performance: electrical performance, thermal performance, higher I/O counts, 3 lower cost. A surface mounting technology (SMT) package, such as BGA, has low height interconnection between the substrate and the PWB. When we have a big difference of TCE between the substrate and the PWB, BGA and CSP packages receive more severe shear strain, damaging the reliability of solder joints, compared with PGA. For the second-level mounting of ceramic package on the PWB, this shear strain is a big problem, since alumina ceramics has TCE of 7 ppm/◦ C while the TCE of a typical PWB, FR-4 board is 12–16 ppm/◦ C as is illustrated in Figure 1.1.7. By an FEM analysis for the second-level interconnection, we found that the stress yielded by the TCE mismatch between the substrate and the PWB reaches a minimum value when the TCE is around 11 ppm/◦ C for a flip chip assembly type packages [5]. We developed a high TCE ceramic material, in which the TCE and the Young’s modulus are 11.5 ppm/◦ C and 114 GPa, respectively.

10

K. Koga

At low temperature

BGA, CSP

Substrate (alumina), TCE = 7 ppm/°C

Crack

Crack

PWB, TCE = 12–16 ppm/°C TCE mismatch (PWB > alumina) => strain => low reliability FIGURE 1.1.7 Schematic diagram of TCE mismatch between substrate made of alumina material and PWB.

The FEM analysis has also shown that the stress of the first-level interconnection in the encapsulated flip chip bump is comparably same to that of the alumina package which has high first-level interconnection reliability. We evaluated the solder joint reliability of BGA and CSP by TCT. We confirmed that we could obtain approximately three to ten times longer fatigue life by this high TCE ceramic material package than that of an alumina material package [5]. However, in the case of wire bonded chip assembly type CSP with a potting compound, we found that a sufficient reliability was not achieved compared to life predictions from FEM analyses [6]. Since the shrinkage of the potting compound is larger than that of the ceramic substrate in this assembly, the substrate warped upwards, while the PWB warped downwards. Solder balls at the package corner was most largely displaced due to this warping mechanism. This observation suggests that the reliability largely depends on the TCE mismatch between the substrate and the potting compound. We have developed new high TCE ceramic material according to the FEM results, and evaluated the solder joint reliability between CSP and PWB by using this new material.

1.1.3.2 NEW HIGH TCE CERAMIC MATERIAL FOR WIRE BONDED CHIP ASSEMBLY TYPE CSP WITH POTTING COMPOUND According to the target value of TCE, we developed a new material of high TCE ceramics by modifying the 11.5 ppm/◦ C ceramic material’s composition. Table 1.1.2 shows properties of the newly obtained high TCE caramic material.

1.1

11

Insulating Ceramics/High Thermal Conductive Ceramics TABLE 1.1.2 Characteristic Properties of 13 ppm/◦ C Material, Compared to 11.5 ppm/◦ C Material and FR-4 Item

Unit

Bulk density Dielectric constant (1 MHz) TCE (40–400◦ C) Flexural strength Young’s modulus

g/cm3 — ppm/◦ C MPa GPa

11.5 ppm/◦ C material

13 ppm/◦ C material

2.6 5.8

2.6 5.3

11.5 230 114

13.0 200 110

FR-4 — 5.5 12–16 430 24

The properties of this material are almost same to the 11.5 ppm/◦ C material with the exception of TCE. The TCE is 13 ppm/◦ C, which is in the range of PWB’s (FR-4: 12–16 ppm/◦ C) and that of the potting compound’s (10–30 ppm/◦ C). The dielectric constant is 5.3 at 1 MHz, lower than 9.8 of alumina. The Young’s modulus is 110 GPa, approximately one-third that of alumina. Also, copper conductor is co-firable.

1.1.3.3 RELIABILITY OF THE NEW HIGH TCE CERAMIC MATERIAL 1.1.3.3.1 Evaluation of Temperature Cycling Test for wire bonded chip assembly type CSP We evaluated the solder joint reliability by TCT. TCT sample was a wire bonded chip assembly type CSP with potting compound, mounted on a PWB (FR-4). The TCT condition was −40◦ C (10 min)/125◦ C (10 min). Size of CSP is 13 mm × 13 mm × 0.4 mm. CSPs were made of the 11.5 ppm/◦ C material and the 13 ppm/◦ C material for comparison. The construction of the CSP is 196 pins, 0.8 mm pad. Pitch, 0.35 mm pad diameter and 14 × 14 array. The construction of the PWB is 1.6 mm thickness, 0.8 mm pad pitch, 0.4 mm pad diameter with two copper conductor layers. Figure 1.1.8 shows a photograph of the TCT sample of the CSP mounted on the PWB. Daisy-chained copper conductors in these package was cofired. The CSP was mounted on the PWB by using eutectic solder balls (37Pb/63Sn) and paste. The height of solder joints was approximately 0.45 mm. Figures 1.1.9 and 1.1.10 show cross-sectional SEM photographs of solder joints at a corner of CSP after 1000 cycles, for 13 and 11.5 ppm/◦ C substrates, respectively. In the case of the 13 ppm/◦ C material, there is no crack. In the other case of the 11.5 ppm/◦ C ceramic material, failed solder joints are observed. Figure 1.1.11 shows TCT (−40 to 125◦ C) results for CSPs made of the 11.5 and the 13 ppm/◦ C material. The package failure was defined as 50%

12

K. Koga

FIGURE 1.1.8 Photograph of TCT sample made of the new high TCE material mounted on the PWB.

FIGURE 1.1.9 Photograph of cross-sectional SEM of solder joints of TCT (−40 to 125◦ C) samples of wire bonded chip assembly type CSP made of the 11.5 ppm/◦ C material after 1000 cycles.

increase of electrical resistance compared to the initial one. The number of samples were 10 for both cases. In the case of 11.5 ppm/◦ C material CSP, the first failure is observed at 300 cycles. In the case of the 13 ppm/◦ C material CSP, the first failure occurred at 1750 cycles. 1.1.3.3.2 Evaluation of package reliability for high TCE ceramic CSP We evaluated a CSP package reliability of the high TCE ceramic material of 11.5 ppm/◦ C. We evaluated a wiring line resistivity between a chip bonding pad to a land pad, and an insulation resistivity between neighbor wiring lines. We evaluated those changes after package reliability tests of thermal shock, high temperature, high humidity with no bias, high temperature, high humidity with

1.1

13

Insulating Ceramics/High Thermal Conductive Ceramics

FIGURE 1.1.10 Photograph of cross-sectional SEM of solder joints of TCT (−40 to 125◦ C) samples of wire bonded chip assembly type CSP made of the 13 ppm/◦ C material after 1000 cycles.

Cumulative failures (%)

100 11.5 ppm/°C 13 ppm/°C

80 60 40 20 0 0

500

1000

1500

2000

2500

3000

Thermal cycles FIGURE 1.1.11 Solder joint reliability by TCT (−40 to 125◦ C) for wire bonded chip assembly type CSP made of the high TCE ceramic materials.

bias, high temperature and thermal cycling test. Table 1.1.3 shows the condition of package reliability tests. Tables 1.1.4 and 1.1.5 show the results for circuit resistance and insulation resistance, respectively. In all cases, we found very low changes after package reliability tests, and high reliability of CSP comparable to alumina package.

14

K. Koga TABLE 1.1.3 Package’s Reliability Test Condition Item

Condition

Number

Thermal shock High temperature, high humidity (1) High temperature, high humidity (2) High temperature Thermal cycling test

−65 to 150◦ C 85◦ C, 85% no bias 85◦ C, 85% 5.5 V 150◦ C −65◦ to 150◦ C

20 20 20 20 20

TABLE 1.1.4 Results of Package’s Reliability Test for Circuit Resistance Item

Result

Thermal shock

0 cycle (400 m) 500 cycles ±1.1% 0 h (400 m) 1000 h ±3.5% 0 h (400 m) 1000 h ±4.3% 0 h (400 m) 1000 h ±3.3% 0 cycle (400 m) 1000 cycles ±4.1%

High temperature high humidity (1) High temperature high humidity (2) High temperature Thermal cycling test

TABLE 1.1.5 Results of Package’s Reliability Test for Insulation Resistance Item

Result

Thermal shock

0 cycle >1 × 1011  500 cycles >1 × 1011  0 h >1 × 1011  1000 h >1 × 1010  0 h >1 × 1011  1000 h >1 × 1010  0 hr >1 × 1011  1000 h >1 × 1011  0 cycle >1 × 1011  1000 cycles >1 × 1011 

High temperature high humidity (1) High temperature high humidity (2) High temperature Thermal cycling test

1.1.3.4 DISCUSSION We have developed 11.5 ppm/◦ C material [5]. Ikemizu et al. [6] reported that by using this material the ceramic fine pitched ball grid array package and

1.1

15

Insulating Ceramics/High Thermal Conductive Ceramics

PWB warped into opposite directions due to large shrinkage of the potting compound. TCE of package is smaller than that of the potting compound. Therefore, substrate warps upwards, while the PWB warps downwards with this assembly. The solder ball fracture occurred at 300 cycles. This thermal fatigue life is too short compared to the prediction of an FEM analysis. We have developed new high TCE ceramic material of 13 ppm/◦ C. This material and 11.5 ppm/◦ C material have almost same properties with the exception of TCE. By using the 13 ppm/◦ C material, the TCE mismatch between the substrate and the potting compound decreases, and the improvement of solder joint reliability is obtained as shown in Figure 1.1.11. Figure 1.1.12 shows height of solder joints between substrate and PWB along a diagonal direction of CSP. The horizontal axis is the position in the diagonal line of CSP. The vertical axis is the height of solder joints. This distribution of the height represents the warpage of CSP. At the beneath of Si-die, both curves are flat, and at both ends they warp upwards. The 13 ppm/◦ C material CSP shows a smaller warpage than the 11.5 ppm/◦ C material CSP. This result verifies the FEM analysis and the evaluation of TCT. The warpages may be caused also by firing distortion of the substrate and/or shrinkages by solidification of the potting compound. We also evaluated a combination of organic CSP and PWB [6]. The secondlevel reliability between organic CSP and PWB was sufficient since their warpage occur in the same direction. It shows, however, a larger warpage than the ceramic CSP. A large stress at Si-die may be induced by this warpage. Tsukada has done some simulation and experiments concerning the joint reliability of

Height of solder joints (μm)

600 550 500 450 400 11.5 ppm/°C 13 ppm/°C

350 300 0

2

4

6

8 10 12 Position (mm)

14

16

18

FIGURE 1.1.12 Height of solder joints between substrate and PWB in the diagonal direction of CSP.

16

K. Koga

Cumulative failures (%)

100 11.5 ppm/°C 13 ppm/°C

80 60 40 20 0 0

500

1000 1500 2000 Thermal cycles

2500

3000

FIGURE 1.1.13 Solder joint reliability by TCT (−40 to 125◦ C) for flip chip assembly type CSP made of the high TCE ceramic material.

a plastic BGA [7]. A large warpage of the plastic BGA is caused by the TCE mismatch between the Si-die and the substrate, since the organic substrate has high TCE and low Young’s modulus. Due to this warpage, thermal fatigue life of a chip assembled BGA is shorter than that of no chip assembled BGA. Young’s modulus of the high TCE ceramic material and a BT resin are 110 and 20–25 GPa, respectively. Since the high TCE ceramic material is more rigid than the organic material, this material has an advantage in this subject. A substrate of flip chip assembly type CSP has to be 11.5 ppm/◦ C material as was reported [5]. When we evaluated the solder joint reliability of the CSP with no potting compound, the first failure occurs at 2500 cycles, as shown in Figure 1.1.13. Flip chip assembly type CSP has a sufficient reliability with the 11.5 ppm/◦ C material. Even if the reliability of the second-level interconnection was improved by using 13 ppm/◦ C material, the TCE mismatch between the Si-die and the substrate shall be increased, and the stress at a flip chip bump becomes large. The first-level interconnection gets weaken by the larger TCE mismatch, and becomes harder to be reinforced by an underfill material. Therefore, the 13 ppm/◦ C material shall be used for a wire bonded chip assembled type CSP, and the 11.5 ppm/◦ C material is suitable for a flip chip assembly type CSP.

1.1.3.5 CONCLUSIONS We found that the equivalent plastic strain generated by the TCE mismatch among Si-die, substrate, potting compound and PWB drastically decreases

1.1

Insulating Ceramics/High Thermal Conductive Ceramics

17

FIGURE 1.1.14 Photograph of BGA and CSP packages co-fired with copper conductors.

as TCE of the substrate increases from 11.5 to 13 ppm/◦ C. We developed a high TCE material of 13 ppm/◦ C, and confirmed by TCT that a wire bonded chip assembly type CSP made of this material has sufficient second-level reliability.

1.1.3.6 FUTURE PACKAGE APPLICATION Based on the present studies by these two high TCE materials, we believe that we can apply these materials for SMT packages of various shapes. Figure 1.1.14 shows such application samples of BGA, CSP packages cofired with copper conductors at low temperature.

1.1.4 LTCC WITH LOW PERMITIVITY AND LOW LOSS TANGENT AT HIGH FREQUENCY FOR MICROWAVE APPLICATION

1.1.4.1 INTRODUCTION The ceramic package used for microwave applications requires following properties; (a) lower dielectric constant and lower loss tangent in the radio frequency

18

K. Koga

range; (b) lower resistivity conductor; (c) thermal expansion coefficient of the ceramic material close to that of semiconductor chips; and (d) high reliability of hermeticity. We have developed a new LTCC package that meets these requirements. In this section, we will describe the properties of newly developed LTCC material and copper conductor cofired with LTCC material and the reliability of the package.

1.1.4.2 CHARACTERISTICS OF MATERIAL 1.1.4.2.1 LTCC material A new LTCC material was designed to be able to sinter under 1000◦ C because of co-firing with copper conductor. The LTCC is composed of lead-free, −Al2 O3 − −MgO− −ZnO− −B2 O3 system glass and ceramic fillers. In order to SiO2 − satisfy electrical and thermal properties, we adjusted the amount of crystalline phases precipitated after sintering [8]. Figure 1.1.15 shows the X-ray diffraction pattern of this LTCC material. The major crystalline phases are SrAl2 Si2 O8 , ZnAl2 O4 and SiO2 . The major characteristics of this material are shown in Table 1.1.6. The coefficient of thermal expansion is 7.5 ppm/◦ C in the range of 40–300◦ C. This value is close to that of GaAs chips which are mainly used for microwave applications. Thermal conductivity and flexural strength and volume resistivity are as good as conventional LTCC material. Because fine leak rate by helium gas through this material is less than 1 × 10−9 Pa m3 /s, it is possible to measure the hermeticity of the package by fine leak method. Dielectric constant and loss tangent were determined by dielectric resonator method using an HP 8757C network

Intensity

SiO2 SrAl2Si2O8 ZnAl2O4

10

20

30 2 (degree)

40

50

FIGURE 1.1.15 X-ray diffraction pattern of new LTCC.

1.1

19

Insulating Ceramics/High Thermal Conductive Ceramics TABLE 1.1.6 Characteristics of New LTTC Item

Unit

Developed material

Dielectric constant (30 GHz) Loss tangent (30 GHz) Volume resistivity Coefficient of thermal expantion (40–300◦ C) Thermal conductivity Flexural strength Fine leak rate

— —  cm ppm/◦ C

6.0 0.029  1014 7.5

W/mk MPa Pa m3 /s

1.5 200 105 . LiNbO3 and LiTaO3 belong to an isomorphous crystal system and are composed of oxygen octahedron. The Curie temperatures of LiNbO3 and LiTaO3 are 1210 and 660◦ C, respectively. The crystal symmetry of the ferroelectric phase of these single crystals is 3m and the polarization direction is along c-axis. These materials have high electromechanical coupling coefficients for surface acoustic wave. In addition, large single crystals can easily be obtained from their melt using the conventional Czochralski technique. Thus both materials occupy very important positions in the surface acoustic wave (SAW) device application field.

4.1.1.2.2 Polycrystalline Materials Barium titanate (BaTiO3 ) is one of the most thoroughly studied and most widely used piezoelectric materials. Just below the Curie temperature (120◦ C), the vector of the spontaneous polarization points in the [001] direction

4.1

113

Piezoelectric Ceramics

(tetragonal phase), below 5◦ C it reorients in the [011] (orthrhombic phase) and below −90◦ C in the [111] direction (rhombohedral phase). The dielectric and piezoelectric properties of ferroelectric ceramic BaTiO3 can be affected by its own stoichiometry, microstructure, and by dopants entering onto the A or B site in solid solution. Modified ceramic BaTiO3 with dopants such as Pb or Ca ions have been developed to stabilize the tetragonal phase over a wider temperature range and are used as commercial piezoelectric materials. The initial application was for Langevin-type piezoelectric vibrators. Piezoelectric Pb(Ti,Zr)O3 solid solutions (PZT) ceramics have been widely used because of their superior piezoelectric properties. The phase diagram for the PZT system (PbZrx Ti1−x O3 ) is shown in Figure 4.1.2. The crystalline symmetry of this solid-solution system is determined by the Zr content. Lead titanate also has a tetragonal ferroelectric phase of perovskite structure. With increasing Zr content, x, the tetragonal distortion decreases and at x > 0.52 the structure changes from the tetragonal 4mm phase to another ferroelectric phase of rhombohedral 3m symmetry. The line dividing these two phases is called the morphotropic phase boundary (MPB). The boundary composition is considered to have both tetragonal and rhombohedral phases coexisting together. Figure 4.1.3 shows the dependence of several piezoelectric d constants on composition near the MPB. The d constants have their highest values near

500 Cubic

Temperature (°C)

400

a a

a 300 Tetragonal

Morphotropic phase boundary

200 c

Ps 100

a

Rhombohedral a Ps

a

a

0 0 10 PbTiO3

20

30

40

50

60

70

80

90

Mol% PbZrO3

FIGURE 4.1.2 Phase diagram of lead zirconate titanate (PZT).

100 PbZrO3

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K. Uchino

800

dij (× 10–12 C/N)

600

d15

400 d 33 200 –d 31 0 48

50

52

54

56

58

60

Mol% PbZrO3 FIGURE 4.1.3 Dependence of several d constants on composition near the morphotropic phase boundary in the PZT system.

the MPB. This enhancement in piezoelectric effect is attributed to the increased ease of reorientation of the polarization under an applied electric field. Doping the PZT material with donor or acceptor ions changes its properties dramatically. Donor doping with ions such as Nb5+ or Ta5+ provides soft PZTs, like PZT-5, because of the facility of domain motion due to the resulting Pb-vacancies. On the other hand, acceptor doping with Fe3+ or Sc3+ leads to hard PZTs, such as PZT-8, because the oxygen vacancies will pin domain wall motion. Subsequently, PZT in ternary solid solution with another perovskite phase has been investigated intensively. Examples of these ternary compositions are: PZTs in solid solution with Pb(Mg1/3 Nb2/3 )O3 , Pb(Mn1/3 Sb2/3 )O3 , Pb(Co1/3 Nb2/3 )O3 , Pb(Mn1/3 Nb2/3 )O3 , Pb(Ni1/3 Nb2/3 )O3 , Pb(Sb1/2 Sn1/2 )O3 , Pb(Co1/2 W1/2 )O3 , Pb(Mg1/2 W1/2 )O3 , all of which are patented by different companies. The end member of PZT, lead titanate has a large crystal distortion. PbTiO3 has a tetragonal structure at room temperature with its tetragonality c/a = 1.063. The Curie temperature is 490◦ C. Densely sintered PbTiO3 ceramics cannot be obtained easily, because they break up into a powder when cooled through the Curie temperature due to the large spontaneous strain. Lead titanate ceramics modified by adding a small amount of additives exhibit a high piezoelectric anisotropy. Either (Pb, Sm)TiO3 [9] or (Pb, Ca)TiO3 [10] exhibits an extremely low planar coupling, that is, a large kt /kp ratio. Here,

4.1

Piezoelectric Ceramics

115

kt and kp are thickness-extensional and planar electromechanical coupling factors, respectively. Since these transducers can generate purely longitudinal waves through kt associated with no transverse waves through k31 , clear ultrasonic imaging is expected without “ghost” caused by the transverse wave. (Pb,Nd)(Ti,Mn,In)O3 ceramics with a zero temperature coefficient of SAW delay have been developed as superior substrate materials for SAW device applications [11]. 4.1.1.2.3 Relaxor Ferroelectrics Relaxor ferroelectrics can be prepared either in polycrystalline form or as single crystals. They differ from the previously mentioned normal ferroelectrics in that they exhibit a broad phase transition from the paraelectric to ferroelectric state, a strong frequency dependence of the dielectric constant (i.e. dielectric relaxation) and a weak remanent polarization. Lead-based relaxor materials have complex disordered perovskite structures. Relaxor-type electrostrictive materials, such as those from the lead magnesium niobate–lead titanate, Pb(Mg1/3 Nb2/3 )O3 –PbTiO3 (or PMN–PT), solid solution are highly suitable for actuator applications. This relaxor ferroelectric also exhibits an induced piezoelectric effect. That is, the electromechanical coupling factor kt varies with the applied DC bias field. As the DC bias field increases, the coupling increases and saturates. Since this behavior is reproducible, these materials can be applied as ultrasonic transducers which are tunable by the bias field [12]. Recently, single-crystal relaxor ferroelectrics with the MPB composition have been developed which show tremendous promise as ultrasonic transducers and electromechanical actuators. Single crystals of Pb(Mg1/3 Nb2/3 )O3 (PMN), Pb(Zn1/3 Nb2/3 )O3 (PZN) and binary systems of these materials combined with PbTiO3 (PMN–PT and PZN–PT) exhibit extremely large electromechanical coupling factors [13, 14]. Large coupling coefficients and large piezoelectric constants have been found for crystals from the morphotropic phase boundaries of these solid solutions. PZN–8% PT single crystals were found to possess a high k33 value of 0.94 for the (001) crystal cuts; this is very high compared to the k33 of conventional PZT ceramics of around 0.70–0.80. 4.1.1.2.4 Polymers Polyvinylidene difluoride, PVDF or PVF2, is piezoelectric when stretched during fabrication. Thin sheets of the cast polymer are then drawn and stretched in the plane of the sheet, in at least one direction, and frequently also in the perpendicular direction, to transform the material to its microscopically polar phase. Crystallization from the melt forms the non-polar α-phase, which can be

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K. Uchino

[CH2CF2]n Carbon Fluorine z

Hydrogen y

x FIGURE 4.1.4 Structure of polyvinylidene diflouride (PVDF).

converted into the polar β-phase by a uniaxial or biaxial drawing operation; the resulting dipoles are then reoriented through electric poling (see Figure 4.1.4). Large sheets can be manufactured and thermally formed into complex shapes. The copolymerization of vinilydene difluoride with trifluoroethylene (TrFE) results in a random copolymer (PVDF–TrFE) with a stable, polar β-phase. This polymer need not be stretched; it can be poled directly as formed. A thickness-mode coupling coefficient of 0.30 has been reported. Piezoelectric polymers have the following characteristics: (a) small piezoelectric d constants (for actuators) and large g constants (for sensors); (b) light weight and soft elasticity, leading to good acoustic impedance matching with water or the human body; and (c) a low mechanical quality factor QM , allowing for a broad resonance band width. Such piezoelectric polymers are used for directional microphones and ultrasonic hydrophones. 4.1.1.2.5 Composites Piezocomposites comprised of a piezoelectric ceramic and a polymer phase are promising materials because of their excellent and readily tailored properties. The geometry for two-phase composites can be classified according to the dimensional connectivity of each phase into 10 structures; 0–0, 0–1, 0–2, 0–3, 1–1, 1–2, 1–3, 2–2, 2–3 and 3–3 [15]. A 1–3 piezocomposite, such as the PZTrod/polymer composite is a most promising candidate. The advantages of this composite are high coupling factors, low acoustic impedance, good matching to water or human tissue, mechanical flexibility, broad bandwidth in combination with a low mechanical quality factor and the possibility of making undiced arrays by structuring the electrodes. The thickness-mode electromechanical coupling of the composite can exceed the kt (0.40–0.50) of the constituent ceramic, approaching almost the value of the rod-mode electromechanical coupling, k33 (0.70–0.80) of that ceramic [16]. Acoustic impedance is the square

4.1

Piezoelectric Ceramics

117

root of the product of its density and elastic stiffness. The acoustic match to tissue or water (1.5 Mrayls) of the typical piezoceramics (20–30 Mrayls) is significantly improved by forming a composite structure, that is, by replacing some of the heavy, stiff ceramic with a light, soft polymer. Piezoelectric composite materials are especially useful for underwater sonar and medical diagnostic ultrasonic transducer applications. 4.1.1.2.6 Thin Films Both zinc oxide (ZnO) and aluminum nitride (AlN) are simple binary compounds with a Wurtzite-type structure, which can be sputter-deposited as a c-axis oriented thin film on a variety of substrates. ZnO has large piezoelectric coupling and thin films of this material are widely used in bulk acoustic and surface acoustic wave devices. The fabrication of highly oriented (along the c-axis) ZnO films have been studied and developed extensively. The performance of ZnO devices is limited, however, due to their low piezoelectric coupling (20–30%). PZT thin films are expected to exhibit higher piezoelectric properties. At present the growth of PZT thin films is being carried out for use in microtransducers and microactuators.

4.1.2 PRESSURE SENSORS/ACCELEROMETERS/ GYROSCOPES One of the very basic applications of piezoelectric ceramics is a gas igniter. The very high voltage generated in a piezoelectric ceramic under applied mechanical stress can cause sparking and ignite the gas (Fig. 4.1.5). There are two means to apply the mechanical force, either by a rapid, pulsed application or by a more gradual, continuous increase. Piezoelectric ceramics can be employed as stress sensors and acceleration sensors, because of the direct piezoelectric effect. Figure 4.1.6 shows a three-dimensional (3D) stress sensor designed by Kistler. By combining an appropriate number of quartz crystal plates (extensional and shear types), the multilayer device can detect 3D stresses [17]. Figure 4.1.7 shows a cylindrical gyroscope commercialized by Tokin (Japan) [18]. The cylinder has six divided electrodes, one pair of which are used to excite the fundamental bending vibration mode, while the other two pairs are used to detect the acceleration. When the rotational acceleration is applied about the axis of this gyro, the voltage generated on the electrodes is modulated by the Coriolis force. By subtracting the signals between the two sensor electrode pairs, a voltage directly proportional to the acceleration can be obtained.

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K. Uchino

(b) F A Electrode L

15 Output voltage (kV)

(a)

10

5

Polarization 0

0

20 40 Time (μs)

60

FIGURE 4.1.5 (a) Gas igniter and (b) output voltage.

x z

1

+ – – + + – – + + – – +

+ – – + + – – + + – – +

+ – – + + – – + + – – +

+ – – + + – – + + – – +

+ – – + + – – + + – – +

y

2 x 3 z 4 y

FIGURE 4.1.6 3D stress sensor (by Kistler).

The converse electrostrictive effect—the stress dependence of the permittivity—is also used in stress sensors [19]. A bimorph structure provides superior stress sensitivity and temperature stability. A measuring system with a bimorph structure, which subtracts the static capacitances of two dielectric ceramic plates, has been proposed [19]. The capacitance changes of the top and bottom plates have opposite signs for uniaxial stress and the same sign for temperature deviation. The response speed is limited by the capacitance measuring frequency to about 1 kHz. Unlike piezoelectric sensors, electrostrictive sensors are effective in the low-frequency range, especially DC.

4.1

119

Piezoelectric Ceramics

Vibrator

Lead

Holder

Support

FIGURE 4.1.7 Cylindrical gyroscope (by Tokin).

4.1.3 PIEZOELECTRIC VIBRATORS/ULTRASONIC TRANSDUCERS

4.1.3.1 PIEZOELECTRIC RESONANCE 4.1.3.1.1 The Piezoelectric Equations When an electric field is applied to a piezoelectric material, deformation ( L) or strain ( L/L) arises. When the field is alternating, mechanical vibration is caused, and if the drive frequency is adjusted to a mechanical resonance frequency of the device, large resonating strain is generated. This phenomenon can be understood as a strain magnification due to accumulating input energy, and is called piezoelectric resonance. Piezoelectric resonance is very useful for realizing energy trap devices, actuators, etc. The theoretical treatment is as follows. If the applied electric field and the generated stress are not large, the stress X and the dielectric displacement D can be represented by the following equations: xi = sEij Xj + dmi Em ,

(20)

X εmk Ek

(21)

Dm = dmi Xi +

where i, j = 1, 2, . . . , 6; m, k = 1, 2, 3. These are called the piezoelectric equations. The number of independent parameters for the lowest symmetry trigonal crystal are 21 for sEij , 18 for dmi

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K. Uchino

X and 6 for εmk . The number of independent parameters decreases with increasing crystallographic symmetry. Concerning the polycrystalline ceramics, the poled axis is usually denoted as the z-axis and the ceramic is isotropic with respect to this z-axis (Curie group C∞v (∞m)). The number of non-zero matrix X X , and ε33 ). elements in this case is 10 (sE11 , sE12 , sE13 , sE33 , sE44 , d31 , d33 , d15 , ε11

4.1.3.1.2 Electromechanical Coupling Factor Next let us introduce the electromechanical coupling factor k, which corresponds to the rate of electromechanical transduction. The internal energy U of a piezoelectric vibrator is given by summation of the mechanical energy UM (= ∫ x dX) and the electrical energy UE (= ∫ D dE). U is calculated as follows, when linear relations Eqs 20 and 21 are applicable: U = UM + UE =



 1 1

sEij Xj Xi + dmi Em Xi 2 i, j 2 m,i 

1 1 X + dmi Xi Em + εmk Ek Em . 2 i, j 2

(22)

k,m

The s and E terms represent purely mechanical and electrical energies (UMM and UEE ), respectively, and the d term denotes the energy transduced from electrical to mechanical energy or vice versa through the piezoelectric effect. The coupling factor k is defined by: √ k = UME / UMM · UEE .

(23)

The k value varies with the vibrational mode (even in the same ceramic sample), and can have a positive or negative value. Note that this definition is equivalent to the definition provided in section on “Piesoelectric figures of merit”: k2 = (stored mechanical energy/input electrical energy) or k2 = (stored electrical energy/input mechanical energy). 4.1.3.1.3 Longitudinal Vibration Mode Let us consider the longitudinal mechanical vibration of a piezoceramic plate through the transverse piezoelectric effect (d31 ) as shown in Figure 4.1.8. If the polarization is in the z-direction and x–y planes are the planes of the electrodes,

4.1

121

Piezoelectric Ceramics

z

y

w b 0

Pz

L

x

FIGURE 4.1.8 Longitudinal vibration through the transverse piezoelectric effect (d31 ) in a rectangular plate.

the extentional vibration in the x-direction is represented by the following dynamic equation: ρ(∂ 2 u/∂t2 ) = F = (∂X11 /∂x) + (∂X12 /∂y) + (∂X13 /∂z),

(24)

where u is the displacement of the small volume element in the ceramic plate in the x-direction. The relations between stress, electric field (only Ez exists) and the induced strain are given by: x1 = sE11 X1 + sE12 X2 + sE13 X3 + d31 Ez , x2 = sE12 X1 + sE11 X2 + sE13 X3 + d31 Ez , x3 = sE13 X1 + sE13 X2 + sE33 X3 + d33 Ez , x4 =

(25)

sE44 X4 ,

x5 = sE44 X5 , x6 = 2(sE11 − sE12 )X6 . When the plate is very long and thin, X2 and X3 may be set equal to zero through the plate. Since shear stress will not be generated by the electric field Ez , Eq. 25 is reduced to: (26) X1 = x1 /sE11 − (d31 /sE11 )Ez . Introducing Eq. 26 into Eq. 24, and allowing for x1 = ∂u/∂x and ∂Ez /∂x = 0 (due to the equal potential on each electrode), leads to a harmonic vibration equation: (27) −ω2 ρsE11 u = ∂ 2 u/∂x 2 . Here, ω is the angular frequency of the drive field, and ρ is the density. Substituting a general solution u = u1 (x)ejωt + u2 (x)e−jωt into Eq. 26, and with the boundary condition X1 = 0 at x = 0 and L (sample length), the following solution can be obtained: ∂u/∂x = x1 = d31 Ez [sin ω(L − x)/v + sin(ωx/v)/sin(ωL/v).

(28)

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K. Uchino

Here, v is the sound velocity in the piezoceramic which is given by v = 1/ ρsE11 .

(29)

When the specimen is utilized as an electrical component such as a filter or a vibrator, the electrical impedance [(applied voltage/induced current) ratio] plays an important role. The current flow into the specimen is described by the surface charge increment, ∂D3 /∂t, and the total current is given by:  L  L X D3 dx = jωw [(ε33 − d231 /sE11 )Ez + (d31 /sE11 )x1 ] dx. (30) i = jωw 0

0

Using Eq. 28, the admittance for the mechanically free sample is calculated to be: (1/Z) = (i/V) = (i/Ez t) LC LC E [1 + (d231 /ε33 s11 )(tan(ωL/2v)/(ωL/2v)], = (jωwL/t)ε33

(31)

where w is the width, L the length, t the thickness of the sample, and V the LC is the permittivity in a longitudinally clamped sample, applied voltage. ε33 which is given by LC X = ε33 − (d231 /sE11 ). (32) ε33 The piezoelectric resonance is achieved where the admittance becomes infinite or the impedance is zero. The resonance frequency fR is calculated from Eq. 31, and the fundamental frequency is given by fR = v/2L = 1/(2L ρsE11 ). (33) On the other hand, the antiresonance state is generated for zero admittance or infinite impedance: LC E 2 2 s11 = −k31 /(1 − k31 ). (ωA L/2v) cot(ωA L/2v) = −d231 /ε33

The final transformation is provided by the definition, X k31 = d31 / sE11 ε33 .

(34)

(35)

The resonance and antiresonance states are described by the following intuitive model. In a high electromechanical coupling material with k almost equal to 1, the resonance or antiresonance states appear for tan(ωL/2v) = ∞ or 0 [i.e. ωL/2v = (m − 1/2)π or mπ (m: integer)], respectively. The strain amplitude x1 distribution for each state (calculated using Eq. 28) is illustrated in

4.1

123

Piezoelectric Ceramics

Resonance m=1

Antiresonance Low coupling High coupling m=1

m=2

m=2

FIGURE 4.1.9 Strain generation in the resonant or antiresonant state.

Figure 4.1.9. In the resonance state, large strain amplitudes and large capacitance changes (called motional capacitance) are induced, and the current can easily flow into the device. On the other hand, at antiresonance, the strain induced in the device compensates completely, resulting in no capacitance change, and the current cannot flow easily into the sample. Thus, for a high k material the first antiresonance frequency fA should be twice as large as the first resonance frequency fR . In a typical case, where k31 = 0.3, the antiresonance state varies from the previously mentioned mode and becomes closer to the resonance mode. The low-coupling material exhibits an antiresonance mode where capacitance change due to the size change is compensated completely by the current required to charge up the static capacitance (called damped capacitance). Thus, the antiresonance frequency fA will approach the resonance frequency fR . The general processes for calculating the electromechanical parameters X ) are described below: (k31 , d31 , sE11 , and ε33 1. The sound velocity v in the specimen is obtained from the resonance frequency fR (refer to Figure 4.1.10), using Eq. 33. 2. Knowing the density ρ, the elastic compliance sE11 can be calculated. 3. The electromechanical coupling factor k31 is calculated from the v value and the antiresonance frequency fA through Eq. 34. Especially in lowcoupling piezoelectric materials, the following approximate equation is available: 2 2 /(1 − k31 ) = (π 2 /4)( f /fR ) k31

( f = fA − fR )

(36)

X , the d31 is calculated through Eq. 35. 4. Knowing the permittivity ε33

Figure 4.1.10 shows observed impedance curves for a typical k material (PZT 5H, k33 = 0.70) and a high-k material (PZN–PT single crystal, k33 = 0.90). Note a large separation between the resonance and antiresonance peaks in the high-k material, leading to the condition fA = 2fR .

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K. Uchino

(a)

fA = 465 kHz = 1.3fR

Impedance

k33 = 0.70

fR = 360 kHz Frequency (b) k33 = 0.90 Impedance

fA = 584 kHz = 2fR

1/C0 fR = 295 kHz Frequency FIGURE 4.1.10 (a) Impedance curves for a reasonable k material (PZT 5H, k33 = 0.70); and (b) a high-k material (PZN–PT single crystal, k33 = 0.90).

4.1.3.2 EQUIVALENT CIRCUITS OF PIEZOELECTRIC VIBRATORS The equivalent circuit for the piezoelectric acuator is represented by a combination of L, C and R. Figure 4.1.11a shows an equivalent circuit for the resonance state, which has a very low impedance. Cd corresponds to the electrostatic capacitance, and the components LA and CA in a series resonance circuit are related to the piezoelectric motion. For example, in the case of the longitudinal vibration of the above rectangular plate through d31 , these components are represented by 2 LA = (ρ/8)(Lb/w)(sE2 11 /d31 ),

CA = (8/π

2

)(Lw/b)(d231 /sE11 ).

(37) (38)

The component RA corresponds to the mechanical loss. In contrast, the equivalent circuit for the antiresonance state of the same actuator is shown in Figure 4.1.11b, which has high impedance.

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(a)

(b)

LA Cd

Gd

Cf

CA RA

GB

CB

LB

FIGURE 4.1.11 Equivalent circuit of a piezoelectric device for: (a) the resonance: and (b) the antiresonance states.

Elastic vibrator

Piezoceramic

FIGURE 4.1.12 Piezoelectric buzzer.

4.1.3.3 PIEZOELECTRIC VIBRATORS In the use of mechanical vibration devices such as filters or oscillators, the size and shape of a device are very important, and both the vibrational mode and the ceramic material must be considered. The resonance frequency of the bending mode in a centimeter-size sample ranges from 100 to 1000 Hz, which is much lower than that of the thickness mode (100 kHz). For these vibrator applications the piezoceramic should have a high mechanical quality factor (QM ) rather than a large piezoelectric coefficient d; that is, hard piezoelectric ceramics are preferable. For speakers or buzzers, audible by humans, devices with a rather low resonance frequency are used (kilohertz range). Examples are a bimorph consisting of two piezoceramic plates bonded together, and a piezoelectric fork consisting of a piezodevice and a metal fork. A piezoelectric buzzer is shown in Figure 4.1.12, which has merits such as high electric power efficiency, compact size and long life.

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4.1.3.4 ULTRASONIC TRANSDUCERS Ultrasonic waves are now used in various fields. The sound source is made from piezoelectric ceramics as well as magnetostrictive materials. Piezoceramics are generally superior in efficiency and in size to magnetostrictive materials. In particular, hard piezoelectric materials with a high QM are preferable. A liquid medium is usually used for sound energy transfer. Ultrasonic washers, ultrasonic microphones for short-distance remote control and underwater detection, such as sonar and fish finding, and non-destructive testing are typical applications. Ultrasonic scanning detectors are useful in medical electronics for clinical applications ranging from diagnosis to therapy and surgery. One of the most important applications is based on ultrasonic echo field [20, 21]. Ultrasonic transducers convert electrical energy into mechanical form when generating an acoustic pulse and convert mechanical energy into an electrical signal when detecting its echo. The transmitted waves propagate into a body and echoes are generated which travel back to be received by the same transducer. These echoes vary in intensity according to the type of tissue or body structure, thereby creating images. An ultrasonic image represents the mechanical properties of the tissue, such as density and elasticity. We can recognize anatomical structures in an ultrasonic image since the organ boundaries and fluid-to-tissue interfaces are easily discerned. The ultrasonic imaging process can also be done in real time. This means we can follow rapidly moving structures such as the heart without motion distortion. In addition, ultrasound is one of the safest diagnostic imaging techniques. It does not use ionizing radiation like X-rays and thus is routinely used for fetal and obstetrical imaging. Useful areas for ultrasonic imaging include cardiac structures, the vascular systems, the fetus and abdominal organs such as liver and kidney. In brief, it is possible to see inside the human body without breaking the skin by using a beam of ultrasound. Figure 4.1.13 shows the basic ultrasonic transducer geometry. The transducer is mainly composed of matching, piezoelectric material and backing layers [22]. One or more matching layers are used to increase sound transmissions into tissues. The backing is added to the rear of the transducer in order to damp the acoustic backwave and to reduce the pulse duration. Piezoelectric materials are used to generate and detect ultrasound. In general, broadband transducers should be used for medical ultrasonic imaging. The broad bandwidth response corresponds to a short pulse length, resulting in better axial resolution. Three factors are important in designing broad bandwidth transducers; acoustic impedance matching, a high electromechanical coupling coefficient of the transducer, and electrical impedance matching. These pulse echo transducers operate based on thickness mode resonance of the piezoelectric thin plate. Further, a low planar mode coupling coefficient, kp , is beneficial for limiting

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Piezoelectric element Backing

Matching layer

Ultrasonic beam

Input pulse FIGURE 4.1.13 Basic transducer geometry for acoustic imaging applications.

energies being expended in non-productive lateral mode. A large dielectric constant is necessary to enable a good electrical impedance match to the system, especially with tiny piezoelectric sizes. There are various types of transducers used in ultrasonic imaging. Mechanical sector transducers consist of single, relatively large resonators and can provide images by mechanical scanning such as wobbling. Multiple element array transducers permit discrete elements to be individually accessed by the imaging system and enable electronic focusing in the scanning plane to various adjustable penetration depths through the use of phase delays. Two basic types of array transducers are linear and phased (or sector). A linear array is a collection of elements arranged in one direction, producing a rectangular display (see Figure 4.1.14). A curved linear (or convex) array is a modified linear array whose elements are arranged along an arc to permit an enlarged trapezoidal field of view. The elements of these linear type array transducers are excited sequentially group by group with the sweep of the beam in one direction. These linear array transducers are used for radiological and obstetrical examinations. On the other hand, in a phased array transducer, the acoustic beam is steered by signals that are applied to the elements with delays, creating a sector display. This transducer is useful for cardiology applications where positioning between the ribs is necessary.

4.1.3.5 RESONATORS/FILTERS When a piezoelectric body vibrates at its resonant frequency, it absorbs considerably more energy than at other frequencies resulting in a dramatic decrease

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W

(a) L

T

Vibrator element (b) Piezoelectric vibrator

Backing

Structure of an array-type ultrasonic probe FIGURE 4.1.14 Linear array type ultrasonic probe.

in the impedance. This phenomenon enables piezoelectric materials to be used as a wave filter. A filter is required to pass a certain selected frequency band or to block a given band. The band width of a filter fabricated from a piezoelectric material is determined by the square of the coupling coefficient k, that is, it is nearly proportional to k2 . Quartz crystals with a very low k value of about 0.1 can pass very narrow frequency bands of approximately 1% of the center resonance frequency. On the other hand, PZT ceramics with a planar coupling coefficient of about 0.5 can easily pass a band of 10% of the center resonance frequency. The sharpness of the passband is dependent on the mechanical quality factor QM of the materials. Quartz also has a very high QM of about 106 , which results in a sharp cut-off to the passband and a well-defined oscillation frequency. A simple resonator is a thin disk type, electroded on its plane faces and vibrating radially, for filter applications with a center frequency ranging from 200 kHz to 1 MHz and with a bandwidth of several percent of the center frequency. For a frequency of 455 kHz, the disk diameter needs to be about 5.6 mm. However, if the required frequency is higher than 10 MHz, other modes of vibration such as the thickness extensional mode are exploited, because of its smaller size. The trapped-energy type filters made from PZT ceramics have been widely used in the intermediate frequency range for applications such as the 10.7 MHz FM radio receiver and transmitter. When the trapped-energy phenomena are

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Electrode

Ceramic plate Top

Bottom

FIGURE 4.1.15 Schematic drawing of a trapped-energy filter.

utilized, the overtone frequencies are suppressed. The plate is partly covered with electrodes of a specific area and thickness. The fundamental frequency of the thickness mode of the ceramic beneath the electrode is less than that of the unelectroded portion, because of the extra inertia of the electrode mass. The lower-frequency wave of the electroded region cannot propagate into the unelectroded region. The higher-frequency overtones, however, can propagate away into the unelectroded region. This is called the trapped-energy principle. Figure 4.1.15 shows a schematic drawing of a trapped-energy filter. In this structure the top electrode is split so that coupling between the two parts will only be efficient at resonance. More stable filters suitable for telecommunication systems have been made from single crystals such as quartz or LiTaO3 .

4.1.4 SURFACE ACOUSTIC WAVE DEVICES A surface acoustic wave (SAW), also called a Rayleigh wave, is essentially a coupling between longitudinal and shear waves. The energy carried by the SAW is confined near the surface. An associated electrostatic wave exists for a SAW on a piezoelectric substrate, which allows electroacoustic coupling via a transducer. The advantages of SAW technology are [23, 24]: 1. The wave can be electroacoustically accessed and tapped at the substrate surface and its velocity is approximately 104 times slower than an electromagnetic wave. 2. The SAW wavelength is on the same order of magnitude as line dimensions produced by photolithography and the lengths for both short and long delays are achievable on reasonably sized substrates.

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SAW Input

Output

Interdigital electrode Piezoelectric substrate FIGURE 4.1.16 Fundamental structure of a SAW device.

There is a very broad range of commercial system applications which include front-end and intermediate frequency (IF) filters, community antenna television (CATV) and video cassette recorder (VCR) components, synthesizers, analyzers and navigators. In SAW transducers, finger (interdigital) electrodes provide the ability to sample or tap the wave and the electrode gap gives the relative delay. A SAW filter is composed of a minimum of two transducers. A schematic of a simple SAW bidirectional filter is shown in Figure 4.1.16. A bidirectional transducer radiates energy equally from each side of the transducer. Energy which is not associated with the received signal is absorbed to eliminate spurious reflection. Various materials are currently being used for SAW devices. The most popular single-crystal SAW materials are LiNbO3 and LiTaO3 . The materials have different properties depending on the cut of the material and the direction of propagation. The fundamental parameters considered when choosing a material for a given device applications are SAW velocity, temperature coefficients of delay (TCD), electromechanical coupling factor and propagation loss. Surface acoustic waves can be generated and detected by spatially periodic, interdigital electrodes on the plane surface of a piezoelectric plate. A periodic electric field is produced when an RF source is connected to the electrode, thus permitting piezoelectric coupling to a traveling surface wave. If an RF source with a frequency, f , is applied to the electrode having periodicity, d, energy conversion from an electrical to mechanical form will be maximum when f = f0 = vs /d,

(39)

where vs is the SAW velocity and f0 is the center frequency of the device. The SAW velocity is an important parameter determining the center frequency. Another important parameter for many applications is temperature sensitivity. For example, the temperature stability of the center frequency of SAW bandpass

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Piezoelectric Ceramics

filters is a direct function of the temperature coefficient for the velocity and the delay for the material used. The first-order temperature coefficient of delay is given by: (1/τ ) · (dτ/dT) = (1/L) · (dL/dT)(1/vs ) · (dvs /dT),

(40)

where τ = L/vs is the delay time and L is the SAW propagation length. The surface wave coupling factor, ks2 , is defined in terms of the change in SAW velocity which occurs when the wave passes across a surface coated with a thin massless conductor, so that the piezoelectric field associated with the wave is effectively short-circuited. The coupling factor, ks2 , is expressed by: ks2 = 2(vf vm )/vf ,

(41)

where vf is the free surface wave velocity and vm the velocity on the metallized surface. In actual SAW applications, the value of ks2 relates to the maximum bandwidth obtainable and the amount of signal loss between input and output, which determines the fractional bandwidth as a function of minimum insertion loss for a given material and filter. Propagation loss is one of the major factors that determines the insertion loss of a device and is caused by wave scattering at crystalline defects and surface irregularities. Materials which show high electromechanical coupling factors combined with small temperature coefficients of delay are generally preferred. The free surface velocity, v0 , of the material is a function of cut angle and propagation direction. The TCD is an indication of the frequency shift expected for a transducer due to a temperature change and is also a function of cut angle and propagation direction. The substrate is chosen based on the device design specifications which include operating temperature, fractional bandwidth, and insertion loss. Piezoelectric single crystals such as 128◦ Y–X (128◦ -rotated-Y-cut and X-propagation)—LiNbO3 and X–112◦ Y (X-cut and 112◦ -rotated-Ypropagation)—LiTaO3 have been extensively employed as SAW substrates for applications in VIF filters. A c-axis oriented ZnO thin film deposited on a fused quartz, glass or sapphire substrate has also been commercialized for SAW devices. Table 4.1.2 summarizes some important material parameters for these SAW materials. A delay line can be formed from a slice of glass such as PbO or K2 O doped SiO2 glass in which the velocity of sound is nearly independent of temperature. PZT ceramic transducers are soldered on two metallized edges of the slice of glass. The input transducer converts the electrical signal to a shear acoustic wave which travels through the slice. At the output transducer, the wave is reconverted into an electrical signal delayed by the length of time taken to travel around the slice. Such delay lines are used in color TV sets to introduce a delay of approximately 64 μs and are also employed in videotape recorders.

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TABLE 4.1.2 SAW Material Properties Cut–propagation direction

k2 (%)

TCD (ppm/C)

v0 (m/s)

εr

ST–X 128◦ Y–X X112◦ –Y (110)–001

0.16 5.5 0.75 0.8

0 −74 −18 0

3158 3960 3290 3467

4.5 35 42 9.5

Ceramic PZT–In(Li3/5 W2/5 )O3 (Pb, Nd)(Ti, Mn, In)O3

1.0 2.6

10 1400◦ C). 161

162

Y. Sakabe TABLE 5.1.1 Dielectric Properties of the Typical High-Q Materials Dielectric

εr (1 MHz)

TCC (10−6 /◦ C)

Q (1 MHz)

TiO2 MgTiO3 CaTiO3 SrTiO3 La2 O3 –2TiO2 ZnO–TiO2 Bi2 O3 –2TiO2 MgTiO3 –CaTiO3 BaO-4TiO2 –TiO2 2MgO-SiO2 –SrO–BaO–TiO2 BaTiO3 –Nd2O3 –TiO2 CaTiO3 –La2 O3 –TiO2 SrTiO3 –CaTiO3 –Bi2 O3 –TiO2 CaTiO3 –La2 O3 –Bi2 O3 –TiO2 BaTiO3 –SrTiO3 –La2 O3 –TiO2

90 17 150 240 35–38 35–38 104–110 17–45 35–65 6–13 35–87 100–150 240–300 145–210 360–650

N750 P100 N1500 N3300 P60 N60 N150 P100–N150 N15–N500 P100–N1000 P100–N330 N470–N1000 N1000–N2000 N750–N1500 N3300–N4700

>5000 >5000 >5000 >1500 >5000 >1500 >2000 >5000 >3000 >5000 >2500 >3000 >1500 >2500 >1500

εr , dielectric constant; TCC, temperature coefficient of K; Q, quality factor.

Today, on the other hand, glass ceramics are widely used for ceramic multilayer substrate with Ag and Cu as an inner conductor. About 40–50% of grass elements such as Al2 O3 , SiO2 , MgO and alkali-earth elements compose the dielectrics, which can sinter at relatively lower temperature (5 × 106 A/cm2 at 77 K in 0 T.

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257

Most works on the thin films of high-temperature superconductors have been carried out for Y-123 and their families of RE-123, because of relative easiness of synthesizing high-quality films. Various deposition techniques have been examined on various substrate such as MgO, ZrO2 , SrTiO3 , LaAlO3 , and NdGaO3 . Substrate wafers 4 in. in diameter can now be obtained. Heteroepitaxial multiplayer structures of Y-123 and semiconducting PrBCO or insulating perovskite (SrTiO3 , LaAlO3 ) have also been made. At present time, Y-123 seems to be most promising for microelectronic device applications. Very recently, Y-123 film with extremely high quality for wide area was successfully synthesized by triphase epitaxy method. Advances have been also made in the fabrication of BiSrCaCuO films. Bi-2212 is better than Bi-2223 for the fabrication of high-quality thin films due to its thermodynamic stability. The most important issue in the fabrication of highquality Bi-2212 films is to avoid the intergrowth of homologous structures such as Bi-2201 and Bi-2223.

8.1.5 APPLICATIONS The most important reason for evaluating the new high-Tc ceramic superconductors for practical application in electric power systems and electronic devices is that the critical temperature of these materials is above the boiling point of liquid nitrogen (77 K). At temperatures of 77 K or higher, considerable simplification and cost savings for the refrigeration system using liquid nitrogen are obtained compared with refrigeration using liquid helium. Another feature of the high-Tc ceramic superconductor is the higher upper critical field, Hc2 , over those of conventional practical superconductors of Nb–Ti and Nb3 Sn. This makes the high-Tc superconductors attractive for the generation of magnetic field even when they are used in lower temperatures of liquid helium or refrigerator operation.

8.1.5.1 POWER APPLICATION Huge amounts of energy are lost in various electric power applications due to the presence of electrical resistance of conventional wires such as copper wire. Electric power systems made from the coils wound by high temperature superconducting wires are expected to have reduced volume and mass, enhanced performance and improved operating efficiency as a result of their larger currents, generated higher fields and smaller resistance losses. Therefore, various kinds of electric power systems are being developed by using high temperature superconductors as shown in Figure 8.1.15.

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K. Togano

Generator in power plant MRI of hospital

Transformer, micro-SMES

Fault current limiter

Transmission cable underground

Transformer, magnetic separation, motor in factory

FIGURE 8.1.15 Various kinds of power applications being developed by using high-temperature oxide superconductors.

Major advantages of motors and generators made from the high-temperature ceramic superconducting wires are reduced weight and size as well as higher efficiency. However, alternating current loss in the superconductor may limit the use of high-temperature superconducting wire in certain motor types. Transformers using high-temperature superconducting wires are a more realistic application. In addition to higher efficiency, less flammability and less environmental impact are also great advantages of the high-temperature transformers. Figure 8.1.16 shows the 500 kVA transformer developed by the group of Kyushu University using Bi-2223 multifilamentary tape [19]. Transmission and distribution cables made from the superconductors are expected to have increased current capacities. The application of hightemperature transmission cables is particularly attractive in dense urban area where the demand for electric power is rapidly increasing while the space is extremely limited and most underground ducts are filled to capacity. The replacement of conventional cable with high-temperature superconducting cable is planned in Japan and the United States. Figure 8.1.17 shows the 7-m prototype high-temperature transmission cable developed by Sumitomo Electric Co. and TEPCO using Bi-2223 wire [20]. High-temperature ceramic superconductors are also promising for the application of fault current limiter, which protects the power transmission system from the damages caused by unanticipated power disturbances. The high-temperature superconducting current limiter has tremendous benefits of

8.1

Superconductive Ceramics

259

FIGURE 8.1.16 500 kVA transformer made of Bi-2223 coils [19].

enhanced operating capacity, increasing safety, reliability and power quality, because it can limit the fault current without adding impedance to the circuit during normal operation. Figure 8.1.18 shows the schematic drawing of inductive type fault current limiter, in which the high-temperature superconducting wire is used for the winding of a coil. The use of high-temperature superconducting films such as YBaCuO film is also being developed for the application of fault current limiter. Bulk superconductors of YBa2 Cu3 O7 are being used for flywheel energy storage. Conventional flywheel has a problem of the energy loss in the bearing. One solution of this problem is to use the repulsive force between the high-temperature superconductor and permanent magnet. Figure 8.1.19 shows a conception of the flywheel energy storage using high-temperature bulk superconductors. Superconducting magnet bearings have demonstrated losses of 10−2 –10−3 W/kg for a 2000 rpm rotor.

8.1.5.2 HIGH MAGNETIC FIELD GENERATION High-temperature superconductors have an important role to play in the development of superconducting magnet. Since Bi-2212 and Bi-2223 tapes show

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FIGURE 8.1.17 A 7-m prototype high-temperature transmission cable system and cut model of the cable made by using Bi-2223 tapes [20].

excellent high-field performance at the temperatures below 20 K, they are being used for the construction of “cryogen-free” conduction-cooled superconducting magnet operated by refrigerator. Figure 8.1.20 shows a 10 T superconducting magnet made by using Bi-2212 multifilamentary wire for windings. The current

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Superconductive Ceramics

Cryostat (liquid nitrogen)

Current

Iron core

Cu-wire winding

Superconduting wire winding

FIGURE 8.1.18 Schematic drawing of inductive type high-temperature superconducting fault current limiter.

Magnetic bearing Motor/generator Power converter system

Flywheel Permanent magnet assembly High-temperature bulk superconductor assembly

Vacuum pump

Liquid nitrogen container

FIGURE 8.1.19 Conception of flywheel energy storage using high-temperature bulk superconductors.

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FIGURE 8.1.20 Cryocooled 10 T superconducting magnet made by using Bi-2223 tapes (by courtesy of Hitachi and Hitachi Cable).

leads of high-temperature superconductors also play an important role in the development of helium-free conduction cooled superconducting magnets. There are several attempts to generate extremely high magnetic field in liquid helium using BiSrCaCuO superconducting wires. Their idea is to use conventional metallic superconducting coils of Nb–Ti and Nb3 Sn as the outer magnets and the BiSrCaCuO coil as the insert magnet as shown in Figure 8.1.21. In this case, the insert magnet plays the role of booster to generate the magnetic field well over 23 T, which can never be attained by conventional metallic superconductors. Projects using 23.5 T superconducting magnets for 1 GHz nuclear magnetic resonance analyzing system (1 GHz NMR) are under progress at the Tsukuba Magnet Laboratory of NIMS [21], Japan and HFML of Florida State University, USA.

8.1.5.3 ELECTRONIC DEVICES The application of high-temperature oxide superconductors for electronic devices is being successfully performed for superconducting quantum

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Probe

NMR spectroscopy analyzing system

BiSrCaCuO magnet Nb3Sn magnet

Nb–Ti magnet

FIGURE 8.1.21 Configuration of high field superconducting magnet using Bi-2212 insert magnet. The magnet is being used for 1 GHz NMR system.

FIGURE 8.1.22 Chip of high-Tc SQUID [22].

interference device (SQUID) magnetometers and filters for wireless telecommunications. These systems are being developed by using YBa2 Cu3 O7 thin films. Figure 8.1.22 shows the chip of high-Tc SQUID [22]. The high-Tc SQUID has a great potential for applications in medical diagnostics, nondestructive tests and geological survey.

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The so-called “intrinsic Josephson junction” in BiSrCaCuO superconductors has also potential to be used for new type of superconducting device [23]. The crystal structure of BiSrCaCuO superconductors is the alternative stacking of superconducting oxide layer containing CuO sheet and nonsuperconducting (or weakly superconducting) oxide layer. This means the structure naturally contains the stacked series arrays of SIS junction. This intrinsic Josephson junction would be useful for high-frequency applications such as Josephson voltage standards, mixers, and oscillators. For such applications, large and high-quality single crystal is required.

REFERENCES 1. Bednorz, J. G., and Mueller, K. A. (1986). Z. Phys. B 64: 189. 2. Uchida, S., Takagi, H., Kitazawa, K., and Tanaka, S. (1987). Jpn. J. Appl. Phys. 26: L1. 3. Wu, M. K., Ashburn, J. R., Torng, C. J., Hor, P. H., Meng, P. L., Gao, L., Haang, Z. J., Wang, Y. Q., and Chu, C. W. (1987). Phys. Rev. Lett. 58: 908. 4. Maeda, H., Tanaka, Y., Fukutomi, M., and Asano, T. (1988). Jpn. J. Appl. Phys. 27: L209. 5. Shen, Z. Z., and Hermann, A. M. (1988). Nature 332: 138. 6. Shilling, A., Cantoni, M., Gao, D., and Ott, H. R. (1993). Nature 365: 56. 7. H. Suzuki et al. (1996). J. Phys. Soc. Jpn, 65: 1529. 8. Cava, R. J., Batlogg, B., Krajewski, J. J., Farrow, R., Rupp, L. W. Jr, White, A. E., Peck, W. F. Jr, and Kometani (1988). Nature 332: 814. 9. Michel, C., Hervieu, M., Borel, M. M., Grandin, A., Deslands, F., Provost, J., and Raveau, B. (1987). Z. Phys. B 68: 421. 10. Yamada, Y., Fukushima, N., Nakayama, S., Yoshino, H., and Murase, S. (1987). Jpn. J. Appl. Phys. 26: L865. 11. Kase, J., Irisawa, N., Morimoto, T., Togano, K., Kumakura, H., Dietderich, D. R., and Maeda, H. (1990). Appl. Phys. Lett. 56: 970. 12. Kase, J., Togano, K., Kumakura, H., Dietderich, D. R., Irisawa, N., Morimoto, T., and Maeda, H. (1990). Jpn. J. Appl. Phys. 29: L1096. 13. Dimos, D., Chaudhari, P., Manhart, J., and Legoues, F. K. (1988). Phys. Rev. Lett. 61: 219. 14. Iijima, Y., Tanabe, N., Kohno, O., and Ikeno, Y. (1992). Appl. Phys. Lett. 60: 769. 15. Goyal, A., Norton, D. P., Budai, J. D., Paranthanman, M., Specht, E. D., Kroeger, D. M., Christen, D. K., He, Q., Saffian, B., List, F. A., Lee, D. F., Martin, P. M., Klabunde, C. E., Hartfield, E., and Sikka, K. (1996). Appl. Phys. Lett. 69: 1795. 16. S. Jin et al. (1989). Phys. Rev. B 37: 7859. 17. M. Murakami et al. (1989). Jpn. J. Appl. Phys. 28: 1189. 18. Y. Shiohara et al. (1997). Mater. Sci. Eng. R19: 1. 19. Funaki, K., Iwakuma, M., Takeo, M., Yamafuji, K., Suehiro, J., Hara, M., Konno, M., Kasagawa, Y., Itoh, I., Nose, S., Ueyama, M., Hayashi, K., and Sato, K. (1997). IEEE Trans. Appl. Supercond. 7: 824. 20. Fujikami, J., Saga, N., Ohmatsu, K., Shibata, T., Isojima, S., Sato, K., Ishii, H., and Hara, T. (1996). Advances in Superconductivity VIII, p. 862, Tokyo: Springer-Verlag. 21. K. Inoue et al. (1996). Proceedings of the 16th CEC/ICMC, Kitakyushu, p. 1103. 22. Sumitomo Electric Hightechs, High Tc SQUID, Catalog http://www.shs.co.jp. 23. Kleiner, R., Steinmeyer, F., Kunkel, G., and Mueller, P. (1992). Phys. Rev. Lett. 68: 2394.

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Engineering Ceramics

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Handbook of Advanced Ceramics S. Somiya ¯ et al. (Eds.) Copyright © 2003 Elsevier Inc. All rights reserved.

CHAPTER 9

9.1 High-Temperature High-Strength Ceramics KAORU MIYAHARA, YASUHIRO SHIGEGAKI and TADASHI SASA Ishikawajima-Harima Heavy Industries Co., Ltd., Shin-nakahara-cho, Isogo-ku, Yokohama 235-8501, Japan

9.1.1 INTRODUCTION The energy crisis in the 1970s made the industrialized countries aware of the importance of the effective utilization and conversion of energy for the first time. Since the late 1980s, the efficient utilization of resources and the protection of environments have been widely recognized as the most crucial issues in the global scale. As one of the most promising solutions to the problems, attention has been given to ceramics with high temperature durability due to their highly probable contribution to the heat-engine efficiency improvements. From this point of view, tremendous efforts have been carried out in the area of science and engineering of structural ceramics for high-temperature applications through the late 1970s to 1990s. In the present chapter, the developments in science and technology of hightemperature high-strength ceramics for heat-engine applications, especially those related with silicon nitride and silicon carbide materials, are summarized with special emphasis on certain typical examples.

9.1.2 SILICON-BASED CERAMICS AS HIGH-TEMPERATURE HIGH-STRENGTH MATERIALS The improvements in the efficiency of heat-engines generally require the structural materials to be utilized under severer operation conditions. Typically in the case of gas turbine engines, the turbine inlet temperature (TIT) has continued to increase for the improved efficiency, which has concurrently required structural materials with higher temperature durability. The TIT of the most advanced large-scale gas turbines has already far exceeded the melting point of the super-alloys, which are currently utilized for these high-temperature components with tremendous air cooling. 267

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TABLE 9.1.1 Silicon Nitride and Carbide as High-Temperature High-Strength Ceramics Characteristics as substance

Characteristics as material

Characteristics for heat engine components

Compound between Si and N/C

Abundance as element materials Oxidation to form SiO2 layer with low O permeability Low specific gravity

Abundance as resources Potential low cost High oxidation resistance

High-temperature deformation resistance

High fracture strength High creep resistance High fatigue resistance High rigidity Low thermal deformation Thermal shock resistance Wear resistance

Covalent bonding

High Young’s modulus Low thermal expansion High hardness

Light weight

The general requirements to the high-temperature structural materials for heat-engines such as gas turbines and diesel engines are as follows: • high fracture strength from ambient to high temperatures, especially high strength per density; • high fatigue strength from ambient to high temperatures; • high thermal shock and thermal fatigue resistance; • high creep resistance to high temperatures; • high oxidation and corrosion resistance; • high wear resistance; • high impact resistance. Although many kinds of materials are classified as ceramics, only limited materials are capable of simultaneously satisfying the conditions listed above. Silicon nitride and silicon carbide are the most promising ceramic materials from this point of view. Table 9.1.1 summarizes the characteristics of silicon nitride and silicon carbide as high-temperature high-strength ceramics.

9.1.3 FABRICATION AND MICROSTRUCTURE CONTROL OF SILICON-BASED MONOLITHIC CERAMICS

9.1.3.1 SILICON NITRIDE Silicon nitride is a highly covalent material, which means bulk diffusion rate is too low to give densification. Therefore, sintering additives are used to obtain the fully desified material. The added metal oxides (MgO, Al2 O3 , Y2 O3 , rare

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earth oxides, etc.) and metal nitride form a liquid phase by the reaction with Si3 N4 and surface SiO2 on Si3 N4 powder at high temperatures. Liquid phase sintering is considered to be composed of the three stages, namely, rearrangement, solution–reprecipitation and grain growth [1]. Silicon nitride has two crystal structures, α and β. α-Silicon nitride, which is a stable phase at lower temperatures is generally used as a raw powder, where the amount of initial β-Si3 N4 nuclei and the particle size largely influence the sintering behavior and the final microstructure. At the first stage, the raw powder rearranges to give more close packing by a capillary force of liquid phase. Shrinkage rate depends on the characteristics of the raw powder and the liquid phase. Second, small Si3 N4 particles dissolve into the liquid phase and then solutes diffuse and reprecipitate on the larger particles. α-Si3 N4 transforms to β-Si3 N4 during this stage. The rate of shrinkage is controlled by either interface reaction or diffusion through the liquid phase. The final stage is a grain growth, where residual pores are eliminated and the further grain growth proceeds. The grain boundary energy drives the grain growth, which is called Ostwald ripening. The application of mechanical and gas pressure during sintering is also effective to increase density and control the microstructure [2]. Hot-pressing (HP), where an uniaxial pressure is applied, is one of the most effective sintering method of simple shape parts. Complex shape parts can be densified by hot-isostatic-pressing (HIP), which utilizes isostatic pressure of gas. The other important point is the temperature limitation due to the decomposition of Si3 N4 , which depends on the temperature. Serious decomposition occurs at a temperature of 1800◦ C under ambient nitrogen atmosphere. Gas pressure sintering (GPS) is often applied to suppress the decomposition at higher temperatures. Silicon nitrides have composite microstructure, consisting of rod-like large grains and equiaxial small grains. Final grain size and morphology also are affected by the sintering additives. Most additives remain at the grain boundary after sintering usually as glassy phase, which strongly affect the thermal and mechanical properties of the sintered body. In general, glassy phase is located at triple points, which are called glass pockets, and along a grain boundary between two grains. The glass pockets can be changed to refractory crystallized phases by post-sintering heat treatment. Figure 9.1.1 shows SEM micrograph of the typical microstructure of Si3 N4 obtained by using the sintering additives: (a) Y2 O3 –Al2 O3 , (b) Yb2 O3 –SiO2 . The sintered body containing Yb2 O3 has a relatively coarser microstructure compared to that containing Y2 O3 owing to the higher melting point of eutectic systems (Y(Yb)–Al–Si–O–N). Linear fracture mechanics predicts that strength and fracture toughness have a proportional relationship in the case of the same size of the fracture origin. However, high strength and high fracture toughness cannot be attained

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(a)

(b)

FIGURE 9.1.1 SEM micrographs of sintered silicon nitrides: (a) Y2 O3 –Al2 O3 ; (b) Yb2 O3 –SiO2 .

FIGURE 9.1.2 Relationship between strength and fracture toughness of typical silicon nitride.

concurrently in a single material made by a conventional processing technique. The dotted line in Figure 9.1.2 shows this limited relationship between strength and toughness [3, 4]. Recently, the new strategy of microstructural design has been proposed to attain the high strength and the high fracture toughness. Hirao et al. [5, 6] reported that the well-controlled microstructure was obtained by adding β-seeds as nuclei, and tape casting or extrusion technique introduced

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FIGURE 9.1.3 SEM micrographs of the microstructure of the highly anisotropic silicon nitride: (a) parallel; (b) perpendicular.

high orientation into the sintered body. Figure 9.1.3 shows the anisotropic microstructure, which clearly indicates the aligned rod-like grains. These materials enabled high strength and high fracture toughness concurrently.

9.1.3.2 SILICON CARBIDE Although silicon carbide has a covalent bond, SiC can be densified by solid state sintering. A combination of boron and carbon is used as typical sintering additives [7]. The added carbon is considered to react with SiO2 on the surface of silicon carbide particles, and boron is thought to increase grain boundary diffusion rate. The liquid phase sintering is also applied by using the similar additives as Si3 N4 [8]. Silicon carbide has two crystal structures, α and β type, where β-SiC is the low-temperature phase. The transformation during sintering and the particle size of the powder largely affect the microstructure and the sintering behavior. SiC produced by solid state sintering has no heterogeneous phase at the grain boundary, which leads to better oxidation resistance and high temperature strength than SiC produced by liquid phase sintering. However, SiC by the solid state sintering has lower toughness and lower thermal shock resistance, compared to SiC or Si3 N4 having the heterogeneous phase at the grain boundary.

9.1.4 MECHANICAL PROPERTIES OF Si-BASED MONOLITHIC CERAMICS

9.1.4.1 SPONTANEOUS FRACTURE Monolithic ceramic materials, as typical brittle materials, possess very limited plastic deformation capability. The lack of the capability of release of local stress concentration results in extremely high sensitivity of the materials to the

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microstructure defects. Besides the typical defects, that is, voids, inclusions or surface flaws, the heterogeneities in the grain microstructure such as elongated grains also have to be carefully taken into consideration. According to linear fracture mechanics, the fracture stress of the material of containing a defect with size a is described by the following equation: √ (1) σf = KIC /Y a where KIC is the critical stress intensity factor or fracture toughness and Y the geometrical factor. The dependence of the fracture stress on the size and the geometry of the defect gives statistical nature to the fracture behavior. The statistical distribution of the spontaneous fracture strength of many ceramic materials is reasonably well described by the one-parameter Weibull distribution described below.   (2) Pf = 1 − exp − (σf /σ0 )m where Pf is the accumulative failure probability, σf the fracture strength, σ0 the characteristic stress parameter, and m the statistical parameter (Weibull parameter).

9.1.4.2 TIME-DEPENDENT DEFORMATION AND FRACTURE Many ceramic materials also show time-dependent mechanical behavior. Glasses are known to show sub-critical crack growth (SCG) in the presence of humidity. Silicon nitrides with metal oxide additives also show SCG at elevated temperatures where the crack growth occurs along the grain boundaries. SCG under static stress is often called as static fatigue. While sintered silicon nitrides show noticeable static fatigue (Fig. 9.1.4), silicon carbides with B and C show little SCG even at high temperatures (Fig. 9.1.5) [9]. SCG rate is dependent on the stress intensity factor KI . The typical behavior is shown in Figure 9.1.6. SCG is found to start above a certain threshold value of KI called ‘fatigue limit’, and reaches a certain level of crack growth rate which increases as a function of KI described by the following equation: da/dt = AKIn

(3)

where A and n are constants. The combinations of Eqs. 1–3 give the following equation as the life of a ceramic component under steady state static fatigue as a function of applied stress. 2 (4) tf =  (n−2)/2  (n − 2)AY n σ n ai where ai is the size of the initial crack.

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FIGURE 9.1.4 Static fatigue strength of sintered silicon nitride (four-point bending, 1250◦ C). Reprinted with kind permission of Fine Ceramics Technical Research Association.

FIGURE 9.1.5 Static fatigue strength of sintered silicon carbide (four-point bending, 1400◦ C). Reprinted with kind permission of Fine Ceramics Technical Research Association.

SCG rate steeply increases as KI approaches the critical value KIC , leading to spontaneous fracture. In the case of sintered silicon nitrides at high temperatures, the fatigue limit is considered as the threshold between the crack healing and the crack growth. Many ceramic materials also show creep deformation at elevated temperature. Among the several creep mechanisms studied or proposed for ceramic materials, sintered silicon nitride with metal oxide additives is considered to show creep with grain boundary sliding mechanism. At the final stage of the

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FIGURE 9.1.6 Sub-critical crack growth in silicon nitrides.

FIGURE 9.1.7 Fracture map of sintered silicon nitrides.

creep, the formations of cavities at grain triple points and their coalescence lead to the accelerated deformation rate typical to the final stage. The timedependent deformation or fracture behavior of the ceramics described above are further described by the fracture map. A typical example of the fracture map of sintered silicon nitride is shown in Figure 9.1.7.

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FIGURE 9.1.8 R-curve behavior of sintered silicon nitride with enhanced fracture toughness.

9.1.4.3 ANELASTICITY In the case of fracture of polycrystalline materials, grain boundaries generally play a role of crack branching or bowing, which results in higher fracture toughness of polycrystalline materials than that of single crystals. Especially in the case of polycrystalline silicon nitride with grains of high aspect ratios, process-zones with a mechanism of grain pull-out and bridging play an important role to give R-curve behavior and enhanced fracture toughness as shown in Figure 9.1.8. The material developments with higher toughness will be discussed in the following sections.

9.1.4.4 OXIDATION Silicon nitride and silicon carbide are thermodynamically unstable in oxygenrich atmosphere, although they are exceedingly oxidation resistant compared with super-alloys. Their oxidation resistance basically relies on the oxidation product layer on the surface. The oxidation of silicon-based ceramics under relatively high oxygen partial pressure is called passive oxidation, where silica or silica-based oxide layer formed on the surface plays the role of oxidation protection and fairly high durability of the materials are expected. The compositions of the grain boundary phase have to be carefully considered since they strongly influence on the oxidation resistance of the silicon nitride or carbide materials. On the other hand, the exposure of silicon nitride and carbide to low oxygen partial pressure atmosphere has to be carefully avoided, since volatile SiO is

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formed and poor oxidation resistance of the materials called active oxidation takes place. The oxidation is more enhanced in high-velocity streams as are encountered in gas turbines, where high-velocity gas retards the formation of surface protection films. The effect of aggressive atmosphere has to be also taken into considerations in the evaluation of the time-dependent fracture behavior such as fatigue or creep.

9.1.5 TOUGHENING OF Si-BASED CERAMICS BY FIBER REINFORCEMENT Si-based ceramics have a remarkable potential for various structural applications at high temperatures as already described. However, their catastrophic fracture behavior sometimes limit further use of Si-based ceramic materials. Therefore, considerable efforts have been made in order to improve the toughness of Si-based ceramics especially by whisker and fiber reinforcement. Silicon nitride reinforced with silicon carbide whisker is one of the material systems that has been most extensively investigated, because superior mechanical properties and heat resistance can be expected. An example of the fracture toughness of hot-pressed silicon nitride with SiC whisker reinforcement is shown in Table 9.1.2. These materials, which have a different orientation of whiskers achieved by cold-pressing or extrusion in powder forming, show a improved fracture toughness by whisker reinforcement [11]. Silicon nitride can be toughened by the enhanced acicular grain growth caused by heat treatment at relatively high temperature. These heat treatments often cause strength degradation, because elongated grains tend to act as fracture origins. As whisker reinforcement usually inhibits the growth of the matrix grains, this approach has a potential to simultaneously achieve higher strength and toughness. TABLE 9.1.2 Fracture Toughness of Whisker-reinforced Si3 N4 and Unreinforced Si3 N4 (after Ref. [11]) Whisker orientation

Forming process

Hot-pressing temperature (◦ C)

Fracture toughness (MPa m1/2 )

Two-dimensional Unidirectional Two-dimensional Unidirectional Unreinforced

Cold-pressing Extrusion Cold-pressing Extrusion Cold-pressing

1700 1700 1750 1750 1750

8.2 ± 0.4 10.0 ± 0.2 8.0 ± 0.3 10.2 ± 0.3 7.5 ± 0.6

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Since toughening can be achieved by bridging of whiskers and crack deflection at whiskers, various microstructural aspects, namely morphology, distribution and orientation of whiskers and so on, are considered to affect the toughening performance. Among these microstructural aspects, a property of the interface between the whiskers and matrix is primary factor to determine the toughness. Effect of interface is obviously shown in the fracture surface of silicon carbide reinforced with silicon carbide whiskers (Fig. 9.1.9). When carbon coating was introduced on the whiskers, carbon interface was formed between the whiskers and the matrix. Consequently, a remarkable interaction between crack and whiskers is observed on the fracture surface, while the materials without carbon-coating show smooth typically brittle fracture surface. Figure 9.1.10 shows the fracture resistance also obtained in silicon carbide reinforced with the carbon-coated SiC whiskers. These materials have

(a)

(b)

FIGURE 9.1.9 Fracture surface of SiC whisker-reinforced SiC ceramics: (a) with carbon coating; (b) without carbon coating.

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10 40% whisker addition

KR(MPa m1/2)

8

6

30% whisker addition 4

2

0

1.0

1.5

2.0

Pre-crack length (mm) FIGURE 9.1.10 Rising R-curve behavior of whisker-reinforced SiC ceramics fabricated by extrusion process.

an unidirectional orientation of whiskers, which was achieved by an extrusion forming of compounds containing SiC powder and whiskers. These materials show an obvious rising R-curve behavior and relatively high toughness, which is considered to be enhanced by the unidirectional orientation of whiskers. Figure 9.1.11 shows strength degradation of a monolithic and a whiskerreinforced silicon carbide after a pin-on-disk test, in which a test contact load was applied with a ceramic pin, and then the specimen was slid at a constant speed. Reinforced material shows little strength degradation, while monolithic material degrades steeply at relatively low or load. As contact stress or foreign object damage is most probable in the actual applications, whisker reinforcement is expected to improve the performance. For the further increase of toughness, continuous fiber reinforcement has been employed. In this category, silicon carbide reinforced with silicon carbide fibers, which is usually fabricated by the densification process using chemical vapor infiltration (CVI) or impregnation and pyrolysis of organosilicon polymers, is one of the most attractive materials. In these materials, the interface is also essentially important. As shown in Figure 9.1.12, appropriate thickness of carbon interface introduced by CVI process leads to a nonlinear fracture.

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Normalized strength

1

0.5

: Monolithic SiC : Whisker-reinforced SiC 0

0

0.5

1

1.5

2

Pin applied load (kN) FIGURE 9.1.11 Normalized strength of a monolithic SiC and a whisker-reinforced SiC after pin-on-disk test (four-point bending strength after pin-on-disk test was normalized by the initial four-point bending strength).

Displacement FIGURE 9.1.12 Flexural stress–strain behavior of continuous SiC fiber-reinforced SiC.

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Reinforcement

None

Whisker

Continuous fiber

1

10

100

1000

10 000

100 000

2)

Fracture energy (J/m

FIGURE 9.1.13 Fracture energy of monolithic, whisker- and fiber-reinforced SiC ceramics.

Fracture energy of the aforementioned silicon carbide materials are compared in Figure 9.1.13. Significant toughening is thus possible in fiber-reinforced materials.

9.1.6 LAMINATED COMPOSITE STRUCTURE WITH ENHANCED FRACTURE RESISTANCE Multilayered ceramic composites have attracted attention in recent years to overcome the brittleness of ceramic materials. An alternate layered composite composed of different materials has enhanced fracture resistance and/or damage tolerance, as reported in ZrO2 /Al2 O3 , SiC/C systems and so forth. The large difference in the layer properties leads to improved mechanical properties. On the other hand, the difference of shrinkage and thermal expansion coefficient causes the subsequent delamination or cracking through the sintering stage. If the aimed properties are distinguished between each layer, strengthening and toughening mechanism should arise even in the same material system. From this point of view, the multilayered silicon nitride composed of dense and porous layers has been developed. Porous silicon nitride has lower elastic moduli, compared to dense silicon nitride, where a distinction of elastic property between the layers is realized. The whisker was used, both for anisotropic grain growth and for retarding the densification of porous layer by percolation. Figure 9.1.14 shows the SEM micrographs for the typical microstructure incorporating 70 vol% whisker for

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(a)

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(b)

FIGURE 9.1.14 SEM micrographs of multilayered silicon nitrides: (a) dense layer; (b) porous layer.

FIGURE 9.1.15 Load–displacement curves in SENB test.

the porous layer and 5 vol% for the dense layer. The duplex layered structure was successfully fabricated and the anisotropic microstructure was clear in the both layers. The layers with sintering additives were sintered to nearly full density and no cracks or delaminations was observed, neither within the layers nor at the interfaces. The porous layer showed unique microstructure, where rod-like grains were aligned in the casting direction and anisotropic pores were generated by tightly tangled elongated grains. The microstructure and layered structure can be controlled by modifying the distribution of the sintering additives and the content of whisker. The fracture of the layered composite originated in the dense layer on the tensile surface owing to lower strain to failure, compared to the porous layer. Actually, a fracture origin was always confirmed in the dense layer. The strength behavior strongly depended on the thickness of the dense layer. The material with the thin dense layer presented high strength, over 1 GPa.

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FIGURE 9.1.16 SEM micrograph of the crack path (side view).

The difference in the elastic properties between the layers caused the zigzag R-curve behavior. The porosity in the porous layer directly affected the fracture behavior. Figure 9.1.15 shows the load–displacement curves in the SENB test for the materials with porosity in the porous layer of 30 and 16%. The high porosity clearly contributed to the higher fracture resistance due to the enhanced delamination in the porous layer (Fig. 9.1.16).

9.1.7 FABRICATION TECHNOLOGY OF MONOLITHIC CERAMIC COMPONENTS FOR HEAT ENGINES

9.1.7.1 FABRICATION TECHNOLOGY OF CERAMIC COMPONENTS The applications of structural materials to various utilizations such as heat engines require the forming technology developments of the materials to desired size and geometry as the components. A forming process is generally composed of three elements: a certain raw material with shaping capability, a shaping process, and a consolidation process of the raw material. In the case of the metallic components, the most popular forming process is casting, where the raw material is liquid metal, the shaping process is pouring into molds, and the

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consolidation process is solidification of the molten metal. The casting process can be also applied to ceramic materials except that such materials as silicon nitride or silicon carbide sublimate at high temperatures instead of melting. The application of the casting process is, however, quite limited in ceramic materials due to the fact that the melting of ceramic materials usually requires extremely high temperatures and also that the microstructures of the materials obtained by this process generally show considerably coarse grain structures, which lead to poor mechanical properties. The most popular forming process for ceramics is powder forming followed by sintering, which enables the processing of ceramics at reasonable temperatures and also the formation of reasonably fine grain structures. Figure 9.1.17 shows the general flow sheet of the fabrication process of ceramic materials for machinery components. The shaping process of the raw material powder is especially crucial to obtain high-quality and low-cost components. Different component geometries require different forming technologies. Major forming technologies for typical component geometries are summarized in Table 9.1.3.

-

FIGURE 9.1.17 Fabrication process of ceramic components.

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Component geometry

Isostatic pressing + (machining)

Two-dimensional shape or simple semi-two-dimensional shape Thick or large shape Three-dimensional hollow shape Three-dimensional solid shape Three-dimensional precise shape Plate-like shape Small simple shape Tubular or planar shape Thin plate shape

Slip casting (drain casting) Slip casting (solid casting) Injection molding Die pressing Extrusion Tape casting

9.1.7.2 APPLICATIONS OF Si-BASED CERAMICS TO HEAT ENGINES The initial target of the application of silicon-based ceramics to heat engines was automotive engine turbochargers, which was started in the 1970s, and silicon nitride turbocharger rotors for diesel engines and gasoline engines were commercialized in the 1980s (Fig. 9.1.18). The injection molding method has been developed and applied to the fabrication of the ceramic turbocharger rotors. Further, several components of automotive diesel engines have been commercialized successively, including igniter plugs, swirl chambers, cams, and valves. The cold isostatic pressing method followed by machining has been applied to these components. The major hot-section components in diesel engines such as piston-head, piston-cylinder are still in the process of development. There have been a number of programs in the United States, the European Union, and Japan for the development of gas turbine engines with ceramic components since the 1970s. Many of the programs have been for automotive engine applications. From late 1980s to 1990s, the development of larger-size gas turbine engines for co-generation applications have been started in the United States and in Japan. Figure 9.1.19 shows the ceramic components utilized in a gas turbine engine of 300 kW-class developed for co-generation applications, which is a singleshaft engine with a two-stage axial-flow type turbine and a single-can type combustion chamber installed with a shell-tube type heat exchanger. Especially, the turbine rotor is composed of ceramic blades inserted to a metallic disk (Fig. 9.1.20), and the turbine nozzle assembly is composed of individual vanes supported by a retainer ring. The ceramic turbine blades and the turbine nozzle vanes have been fabricated by the injection molding method. The combustion

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FIGURE 9.1.18 Automotive turbocharger with a silicon nitride rotor.

chambers have been fabricated by the cold isostatic pressing method followed by machining, and especially in case of the turbine nose cones (Fig. 9.1.21), the joining technology by cold isostatic pressing combined with machining has been developed and applied. The heat-exchanger tube assemblies shown in Figure 9.1.22 have been fabricated by the extrusion method combined with joining technology. Improvements in the properties and reliability of silicon-based ceramic components as well as those in the technologies of designing and assembling of them have enabled the extremely high engine efficiency of 35% or more with the 300 kW-class gas turbine systems installed with ceramic components, which usually show efficiencies of around 20% with metallic components.

FIGURE 9.1.19 300 kW-class gas turbine CGT301 for co-generation applications and ceramic components utilized in the engine system.

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FIGURE 9.1.20 Ceramic turbine blades inserted to a metallic disk.

FIGURE 9.1.21 Ceramic turbine nose-cone.

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FIGURE 9.1.22 Ceramic heat-exchanger tube assembly.

9.1.8 CONCLUSIONS Silicon-based ceramics, especially sintered silicon nitride materials have shown extraordinary advancements in the past decades. Not only the improvements in the high-temperature mechanical properties of the materials but also the technologies for the applications to machinery components have shown considerable advancements, that is, the forming technologies of precise complex shapes, the evaluation and the quality control technologies of defectsensitive ceramic components, the design technologies suited to brittle ceramic components. Further improvements in the technologies and developments of newer applications are expected to proceed steadily hereafter in the world of

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engineering ceramics, although the extensive commercialization of structural ceramics for heat engines, which many ceramists had once dreamed of, has not yet come to fruition.

REFERENCES 1. Jack, K. H. (1976). Review: Sialons and related nitrogen ceramics. J. Mater. Sci. 11: 1135–1158. 2. Mitomo, M., and Tajima, Y. (1991). Sintering, properties and application of silicon nitride and Sialon ceramics. J. Ceram. Soc. Jpn. 99: 1014–1025. 3. Kawashima, H., Okamoto, H., Ymamoto, H., and Kitamura, A. (1991). Grain size dependence of the fracture toughness of silicon nitride. J. Ceram. Soc. Jpn. 99: 320–323. 4. Yoshimura, M., Nishioka, T., Yamakawa, A., and Miyake, M. (1995). Grain size cotrolled high-strength silicon nitride ceramics. J. Ceram. Soc. Jpn. 103: 407–408. 5. Hirao, K., Ohashi, M., Brito, M. E., and Kanzaki, S. (1995). Processing strategy for producing highly anisotropic silicon nitride. J. Am. Ceram. Soc. 78: 1687–1690. 6. Kondo, N., Suzuki, Y., and Ohji, T. (1999). Superplastic sinter-forging of silicon nitride with anisotropic microstructure formation. J. Am. Ceram. Soc. 82: 1067–1069. 7. Prochazka, S., and Charles, R. J. (1973). Am. Ceram. Soc. Bull. 52: 885–888. 8. Suzuki, K., and Sasaki, M. (1986). Fundamental Structure Ceramics. Trra Sci. Pub. Co. 9. Inamura, T., Suzuki, A., Hayashi, S., and Shigegaki, Y. (1992). Static and cyclic fatigue. Final Report of Fine Ceramics for Future Industries Research Program, Fine Ceramics Research Association, pp. 753–775. 10. Sakida, T., Tanaka, S., Mikami, T., Tatsuzawa, M., and Taoka, T. (1999). Development of the Ceramic Gas Turbine Engine System (CGT301). ASME 99-GT-104, pp. 1–7. 11. Goto, Y., and Tsuge, A. (1993). Mechanical properties of unidirectionally oriented SiCwhisker-reinforced Si3 N4 fabricated by extrusion and hot-pressing. J. Am. Ceram. Soc. 76: 1420–1421.

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Handbook of Advanced Ceramics S. Somiya ¯ et al. (Eds.) Copyright © 2003 Elsevier Inc. All rights reserved.

CHAPTER 10

10.1 Porous Ceramics for Filtration TOSHINORI TSURU Department of Chemical Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan

10.1.1 INTRODUCTION A general definition of a membrane is that it is “a selective barrier between two phases” [1]. Therefore, using membranes, the feed is separated into two streams, that is, the retentate and permeate streams, as shown in Figure 10.1.1. Either the retentate or the permeate could be product stream, depending upon types of membranes used and the feed stream. The permeate stream is the product stream, if the solvent is purified by removing solutes using a membrane which allows the permeation of solvent and retains the permeation of solutes, such as in the desalination of seawater. If the purpose of the separation process is the concentration of solutes, then the retentate becomes the product. Membrane separation, which is a relatively new separation process, has been commercialized in the last two decades. The majority of membrane materials which have been commercialized thus far are polymeric. Porous ceramic membranes have great potential for opening up new types of applications to which polymeric membranes cannot be applied. This review will summarize the present status and a potential application of porous ceramics as materials for membrane separation. Retentate

Feed

Membrane Permeate

FIGURE 10.1.1 Membrane separation process (feed stream is divided into retentate and permeate).

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10.1.2 MEMBRANE AND MEMBRANE SEPARATION PROCESS Membrane separation processes can be categorized based on the phases of the feed and permeate, as well as the types of driving force for the separation, as shown in Table 10.1.1 [1]. For the case where both the feed and permeate streams are a liquid phase and the driving force is a pressure difference between the two phases, the separation process is referred to a filtration process such as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO), depending upon the pore sizes. It should be noted that membranes, the pore sizes of which are larger than 10 μm, are usually categorized as filters. Gas separation is conducted in the gas phase for both feed and permeate stream. For the case of pervaporation, the feed stream is a liquid phase, while the permeate stream is a gas phase by evacuation. Other separation processes (electrodialysis, dialysis, membrane distillation) are also conducted in the liquid phase, but ceramic porous membranes have not yet been applied to these types of separation processes. The separation mechanism is mainly controlled by the sieving effects, where solutes which are smaller than the pore sizes of membranes permeate through the porous membranes. Another mechanism is the affinity of solutes for membrane materials, but this mechanism plays an important role in cases where the pore size is relatively small. Most applications have been carried out using polymeric membranes, while inorganic membranes, including ceramic membranes, have been utilized to

TABLE 10.1.1 Membrane Processes Based on Phase and Driving Force Membrane process

Phase

Driving force

Pore size

Note

Separation of particles Separation of macromolecules Separation of low MW solutes (MWCO 200–1000) Reject electrolytes

Feed

Permeate

Microfiltration

L

L

P

0.1–10 μm

Ultrafiltration

L

L

P

2–100 nm

Nanofiltration

L

L

P

1–2 nm

Reverse osmosis

L

L

P