Chemical Processing of Ceramics (Materials Engineering, 8) [1 ed.] 0824792440, 9780824792442, 9780585399973

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Chemical Processing of

Ceramics

zyx zyx zy zyxwvu zyxwv zyxw zyxwv zyxw zyxwv zy zyxw MATERIALS ENGINEERING

1. Modern Ceramic Engineering: Properties, Processing, and Use in Design. Second Edition, Revised and Expanded, David W. Richerson

2. lntroductianto Engineering Materials: Behavior, Properties, and Selection, G. T. Murray 3. Rapidly SolidifiedAlloys: Processes StructuresApplications, 6

edit ed by H0 ward H. Liebermann

4. Fiber and Whisker Reinforced Ceramics forStructuralApplicationS , David Belit skus 5. Thermal Analysis of Materials, Robert F. Speyer 6 . Friction and Wear of Ceramics, edited by Said Jahanmir 7. Mechanical Properties of Metallic Composites, edit ed by Shoj iro Ochiai 8. Chemical Processing of Ceramics, edited by Burt rand 1. Lee and Edward J. A. Pope 9. Handbook of Advanced Materials Testing, edit ed by Nicholas P. Cheremisinof f andPaul N. Cheremisinoff

Additional Volumes in Preparation

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Chemical Processing of

Ceramics edited 14y

Burtrand I. Lee Clemson University Clemson. South Carolina

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Edward J. A. Pope MATECH Westlake Village, Calihrnia and University of Utah Salt LakeCity#Utah

Marcel Dekker, Inc.

New York*Basel*Hong

Kong

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Library of Congress Cataloging-in-Publication Data

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Chemical processingof ceramics I edited by Burtrand I. Lee, Edward J . A. Pope. cm.p. Includes bibliographical references and index. ISBN 0-8247-9244-0 (alk. paper) 1. Ceramicmaterials. I.Lee,BurtrandInsung. II. Pope,Edward J . A. TP810.5.C48 1994 666-dc20 94-25396 CIP The publisher offers discounts on this book when ordered in bulk quantities. For more information, write to Special SalesProfessional Marketing at the address below. This book is printed on acid-free paper.

Copyright 0 1994 by Marcel Dekker, Inc. All Rights Reserved.

Neither this book nor any part may be reproducedor transmitted in any formor by any means, electronic or mechanical, including photocopying, microfilming, and recording, or by any information storage and retrieval system, without permission in writing from the publisher. Marcel Dekker, Inc. 270 Madison Avenue, New York, New York 10016 Current printing (last digit): 10987654321 PRINTED IN T I E UNITED STATES OF AMERICA

Foreword

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

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zy zyxw zyx zyxw zyxwz FOREWORD

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

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FOREWORD

V

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

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Peter E. D.Morgan

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Preface

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

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zyxwv vii

viii

zy zyxwvu zyx zy zyxw zy PREFACE

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

z zyxwvut zyxwvu Contents

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Foreword Peter E. D.Morgan Preface Contributors 1.

iii vii xi

Precursor Chemistry

1. Molecular Design of Transition Metal Alkoxide Precursors

Jacques Livage 2. Metal Alkoxides for Electrooptical Ceramics Liliane G. Hubert-Pfalzgraf

3

23

II. Powder Synthesis and Characterization

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

61

4.

75

Multicomponent Ceramic Powders T. Mah, E. E. Hemes, and K. S. Mazdiyasni 5. Chemical Synthesis of Nonoxides Christian Russel and Michael Seibold 6. Techniques for Characterization of Advanced Ceramic Powders S. G. Malghan, P. S. Wang, and V. A. Hackley 111.

105

129

Powder Processing 7.

Colloid Interface Science for Ceramic Powder Processing Hyun M. Jang 8. Ceramic Nonaqueous Particles Media in Burtrand I. Lee 9 . Synthesis and Dispersion of Barium Titanate and Related Ceramic Powders Ki Hyun Yoon and Kyung Hwa Jo

157

197

215

ix

X

zy zyxwvuts zyxwv zyxwvu zyxwvu zyxwvu CONTENTS

10. Rheology and Mixing of Ceramic Mixtures Used

in Plastic Molding Beebhas C. Mutsuddy

239

W . Sol-Gel Processing

11. Processing of Monolithic Ceramics via Sol-Gel

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

265 287

311 337

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

359 375

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

397 421

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Index

445

465

481

501

533

zyxwvut zyx zyxwvu zyxw zyxwv zyxwvu Contributors

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

Kenneth E. Gonsalves University of Connecticut, Storrs, Connecticut Joachim Gross Physikalisches Institut der Universitat, Am Hubland, Wurzburg, Germany

C. Guizard Centre National de la Recherche Scientifique, Montpellier, France V. A. Hackley National Institute of Standards and Technology, Gaithersburg, Maryland

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

Liliane G. Hubert-PfalzgrafUniversitk de Nice-Sophia Antipolis, Nice, France Hyun M . Jang Pohang University of Science and Technology, Pohang, Republic of Korea Kyung Hwa Jo* Yonsei University, Seoul, Korea

A. Julbe Centre National de la Recherche Scientifique, Montpellier, France A. Larbot Centre National de la Recherche Scientifique, Montpellier, France Burtrand 1. Lee Clemson University, Clemson, South Carolina

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Hung C. Ling AT&T Bell Laboratories, Princeton, New Jersey

Jacques Livage Universite Pierre et Marie Curie, Paris, France *Currenr affiliation: Daewoo Corporation, Ltd.,

Seoul, Korea

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xii

CONTRIBUTORS

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l: Mah UES, Inc., Dayton, Ohio

S. G. Malghan National Institute of Standards and Technology, Gaithersburg, Maryland Shin-Pei Matsuda Hitachi, Ltd., Ibaraki, Japan

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

P. E. D. Morgan Rockwell Science Center, Thousand Oaks, California

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

T. Ohishi Hitachi, Ltd., Ibaraki, Japan

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Kiyohito OkamuraUniversity of Osaka Prefecture, Osaka, Japan Masataka Ozaki Yokohama City University, Yokohama, Japan

J. Phalippou Universitb de Montpellier 11, Montpellier, France

Edward J. A. Pope MATECH, Westlake Village, California, and University of Utah, Salt Lake City, Utah C.

N. R. Rao 1ndian.Instituteof Science, Bangalore, India

Christian Russel Universitat Jena, Jena, Germany

Michael Seibold OSRAM GmbH, Augsburg, Germany

P. S. Wang National Institute of Standards andTechnology, Gaithersburg, Maryland Tongsan D. Xiao University of Connecticut, Storrs, Connecticut Ren Xu University of Utah, Salt Lake City, Utah Man F. Yan AT&T Bell Laboratories, Murray Hill, New Jersey Ki Hyun YoonYonsei University, Seoul, Korea

I PRECURSOR CHEMISTRY

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

I. INTRODUCTION

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

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II. HYDROLYSIS AND CONDENSATION OF METAL ALKOXIDES

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

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

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

3

4

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Actually these reactions correspond to the nucleophilic substitution of alkoxy ligands by hydroxylated species XOH, as follows:

M(OR), + AXOH _ j [M(OR),.AOX),I + xROH where X is hydrogen (hydrolysis), a metal atom (condensation), or even an organic or inorganic ligand (complexation). They can be described by a S$ mechanism [2]:

H” \ O-M-O’-R+

H

\

06+M”-OR/ X

/

X

Step 1

H” / XO-M--OXO-M+ROH \ R

Step 2

Step 3

1. The first step corresponds to the nucleophilic addition ofnegatively charged H06 groups onto positively charged metal atoms M&. It leads to an increase in the coordination number of the metal atom in the transition state. 2. The second step is a transfer, within the transition state, of the positively charged proton toward a negatively charged OR group. 3. The positively charged protonated alkoxide ligand ROH is then removed.

The chemical reactivity of metal alkoxides toward hydrolysis and condensation mainly depends on the positive charge of the metal atom 6nn and its ability to increase its coordination number N [2]. Asa general rule, the electronegativity of metal atoms decreases and their size increases when going down the periodic table, from the top right to the bottom left (Table 1). The chemical reactivity of the corresponding alkoxides toward hydrolysis and condensation then increases.

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Table 1 Electronegativity x, PartialCharge 6, Ionic Radius r, and Maximum Coordination NumberN of Some Metal Alkoxides

Alkoxide

X

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

1.74 1.32 1.29 1.17 2.11 1S 6

6

0.32 4 0.60 0.64 7 0.75 8 0.13 4 0.46 6

?-(A)

N

0.40

0.64 0.87 1.02 0.34 0.59

6

z zyxwv zy z zyxwvu zyxwv zyxwvu zyxwvutsrqpo zyxwvu zyxwvu

MOLECULAR DESIGN

OF TRANSITION METAL ALKOXIDE

5

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

111. MOLECULARSTRUCTURE OF TRANSITION METAL ALKOXIDES

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

A.

Oligomerization

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

6

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'OEt

Figure 1

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zyxwvuts zyxwv zyxw zyxwvu Molecular structure of titanium alkoxides. (a) = TiOiPr4; (b) [Ti(oEt)4]3.

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

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B.

Solvation

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

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MOLECULAR DESIGN OF TRQNSITION METAL ALKOXIDE

7

Ti-0

.2 '

.¶

.6

.3

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.. :.

.

:i

:g J

1 w nm

J

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E

r

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R

5

Ti-0

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.6

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Ti(OEtI4

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4.98

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5

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5.02

Figure 2 X-ray absorption of titaniumalkoxides at the Ti-Kedge.(a) = X m S ; (b) = EXAFS (Fourier transform).

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

8

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Pro H

PrO

OPr

H OPr

Figure 3 Molecularstructure of [Ce(OiPr)4, PirOHIz.

C.

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Formation of Oxoalkoxides

Alkoxide groups can also bridge two different metal atoms, leading to the formation of heteroalkoxides, which are often used a precursors for the sol-gel synthesis of multicomponent ceramics [15]. Condensation can even go one step further, leading to p-oxo bridges via ether elimination. M-OR+RO-M"-0-"M+ROR

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

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z zyxw zyxwv zyxwvu zy zyxwvu zy

MOLECULAR DESIGN OF TRANSITION METAL ALKOXIDE

9

W. CHEMICALMODIFICATIONOF TRANSITION METAL ALKOXIDES

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

A.

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

CarboxylicAcids

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

(a)

Cb)

zyxw

Figure 4 X-ray absorption at the Ti-K edge of pi(OiPr)3(OAc)]. (a) = XANES; (b) = EXAFS (Fourier transform).

10

zyxwvuts z zyxwv zyxwvu zyxwvutsrq zyxwv zy WAGE

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

z

zyxwv

Figure 5 Chemical modification of titanium alkoxides. (a) = ri(OiPr)3 (0&)]2; = [Ti(OiPr)3(acac)].

(b)

zyxw zy

MOLECULAR DESIGN OF TRANSITION METAL ALKOXIDE

I1

z zyx zyxwvuts zyxwv zy 0"

OW

Figure 6 Molecularstructure bridges (hatched circles).

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

oxo

zyxwvu

creases, leading to the growth of anisotropic particles. TizO(0Ac)rj oxoacetates, made of chainlike polymeric species, are obtained when an excess of acetic acid is added [24].

B.

zy zyxwvu zyxwv

P-Diketones

Strongly complexing P-diketones are currently employed to stabilize highly reactive metal alkoxides, such as W(OEt)6 [25]. Aluminum sec-butoxide, modified by ethylacetoacetate (etac), appears to be quite attractive as a precursor for the sol-gel synthesis of multicomponent ceramics, such as cordierite. Al(OsBu)z(etac) is more soluble and less reactive thanthe corresponding alkoxide [26]. Complexation is also observed with acetylacetone (acacH=CH3-CO-CHzCO-CH3). Its enolic form contains hydroxyl groups and reacts with metal alkoxides as a chelating ligond. Oligomers are not readily formed, and for a stoichiometric acacni = 1 ratio, the nucleophilic substitution leads to monomers in which Ti is only fivefold coordinated (Fig. 5b). Ti(OiPr)4+ acacH +Ti(OiP&(acac)

+ iPrOH

12

zyxwvuts z zyx zyxw zyx zyx zy LWAGE

Oligomeric compounds are formed only upon hydrolysis [27]. Titanium coordination becomes six as soon as water is added, whereas Ti-Ti correlations can be seen on the Fourier transform of the EXAFS spectrum. Strongly chelating acac ligands are not removed upon hydrolysis (except at low pH). Condensation is then prevented, and only small oligomers are formed [28].

V. MOLECULARENGINEERING OF METALALKOXIDE PRECURSORS

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

zyxwv zyxwv z

A. HYDROLYSIS RATIO

zyxw

The hydrolysis and condensation of metal alkoxides M(OR)z leads to oligomers in which metal atoms tend to acquire their maximum coordination number. The formation of these species can be controlled by the hydrolysis ratio h = H20/M. In the presence of a large excess of water (h >> z). all alkoxide groups are removedand colloidal species are formed.Theylead to hydrous oxides MOzI2.xH20 similar to those synthesized from aqueous solutions. The adsorption and dissociation of water molecules at the oxide/water interface leads to the formation of charged particles. All alkoxide groups are not removed when h I z and chain oxopolymers are formed. They can be advantageously employed for drawing fibers or making coatings. The dielectric constant of the organic solvent is rather low, the surface charge of polymeric particles is very small, and vander Waals interactions prevail. For very low hydrolyis ratios (h -c 1) condensation is mainly governed by the formation of p o x 0 and alkoxo bridges. Solute molecular oxoalkoxides are then formed and can often be isolated as single crystals from the solution.

zyxw

z zy zyxwv zy zyxw zyxwvut zyx I3

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

Their structure appears tobe closely related to that of the corresponding polyanions formed in aqueous solutions. The first hydrolysis product ( h = 0.6) of titanium ethoxide, for instance Ti704(OEt)20, contains [Ti70241 units isostructural with MO7@& [29]. Larger species, such as Ti1008(0Et)24 ( h = 0.8) and Ti16016(OEt)32 (h = l), are formed when more water is added [30,31]. They have been obtained upon the controlled hydrolysis of Ti(OEt)4 and isolated as single crystals from the solution (Fig. 7). The molecular structure of these species in the solution has been studied by x-ray diffraction and 1 7 0 nuclear magnetic resonance using enriched water as a reagent [32]. Niobium ethoxide leads to Nb8010(OEt)20, which exhibits the same structure as the paratungstate [33]. In the presence of an organic base, NMwOH, a decamer, is obtained [Nblo028(NMe4)6*6H20], whichis isostructural with the decananadate polyanion (V10028)6- [34].

B.

Complexation

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

b

zyxwvuts

Figure 7

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

14

zyxwvutsr zy zyxwvut zyxwvu zyxwvuts zyxwvu zyxwv zyxwv zyxwv zyxwv z LNAGE

The reaction of titanium alkoxides with acetylacetone leads to several molecular compounds.[TiO(acac)z]z is formed in the presence ofan excess of acetylacetone or upon hydrolysis of Ti(acac)2(OR)2. Single crystals have been isolated;andx-raydiffraction experiments show dimers with sixfold coordinated Ti atoms linked through oxygen atoms. Strongly complexing acac ligands cannot be hydrolyzed easily, and condensation does not go any further (Fig. 8) [35]. Lager molecular species can be obtained with smaller ainounts of acetylacetone. Single crystals of Tii8022(0Bu)26(acac)2were obtained recently [36]. They are made of 18 [Ti061 octahedra sharing edges or comers and correspond to x = 0.1. h = 1.2. Complexing acac ligands remain outside the Ti18022 core of the molecule (Fig. 9). These examples show that condensation can be tailored via complexation and hydrolysis. Gels are obtained in the presence of an excess of water and for small values of x (x I0.3, h 2 10). Clear sols are obtained when Ti(OnBu)4 is hydrolyzed in the presence of acetylacetone. The mean hydrodynamic diameter of colloidal particles was measured by quasi-elastic light scattering. It appears to increase from 2 to 40 nm as the hydrolysis ratio increases from h = 1 to h = 4 for x = 0.3). It decreases from 40 to 4 nm when the amount of acetylacetone increases from x = 0.3 to x = l (for h = 4) [28]. Similar results have been found with zirconium alkoxides. Solvated dimeric species are formed when zirconium propoxide is dissolved in its parent alco-

zyxwvu zyx

Figure 8 Molecular structure of [TiO(acac)2]2 according to Ref. 35: Ti (solid); p OXO bridges (hatched); other atoms (C and 0) (open).

-

zyxwv z

MOLECULAR DESIGN OF TRANSITION METAL

Figure 9

ALKOXIDE

15

zyxwvuts zyxwv zyxw zyx zyx zy Molecular structure

of

[Ti18022(0Bu)26(acac)2](h = 1.2 and x = 0.1).

z zyxwv

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

16

z zyxwvuts LNAGE

zyx zyxwvu zyxwvu zyxwvu zyxwv zyxwv zy b

0

Zr

Figure 10

e

0 (oxo bridge)

zyxw

Molecularstructure of [Zr4PO(OPr)lo(acac)4] (X = 1 and h = 0.2).

grown from a solution in which x = acac/Ce = 2. They correspond to hexameric species [Ce604(0H)4(acac)l2] in which the oxohydroxo core (Fig. 12) is isostructuralwiththe inorganic analog [Ce604(OH)4(S04)6], whichwas observed in aqueous solutions of cerium sulfate [ 131. The hydrolysis of P-diketonate-modified cerium(IV) isopropoxide leads to the formation of colloidalsolutions or gels. The complexationratio (x = acac/Ce) appears to be the key parameter to tailor the size of cerium oxide particles. Precipitation is observed when x e 0.1, whereas sols are obtained when 0.1 < x e 1. The mean hydrodynamicdiameter of these particles decreases from 450 to 15 A when x goes from 0.1 to 1. Only solute molecular clusters are formed when the complexing ratio becomes larger than 1 (1 Ix I2).

zy

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

zyxwvu z

MOLECULAR DESIGN

0

OF TRANSITION METAL

ALKOXIDE

17

zyxwvut zyxwvu zyxw zyxwv zyxw zyxwvut zyxwv zy Zr

Figure 11

e

0 (oxo & hydroxo bridges)

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

zyxwvu

when polymerizable organic ligands are used. They allow the formation of an organic network in addition to the inorganic. The organic groups impart new properties to the organic network, such as flexibility, hydrophobicity, or refractive index [40]. The chemistry of hybrid organic-inorganic gels is mainly developed around silicon-containing materials. Titanium and zirconium alkoxides are sometimes used as cross-linking reagents for the condensation of difunctional silicon precursors, such as [(CH3)2Si(OC2H5)2]. However, these alkoxides do not really behave as cross-linking agents. They lead to the formation of Poly Dimethyl Siloxane (PDMS)-Ti02 nanocomposites in which Ti02 nanoparticles are embedded in a PDMS matrix [41]. This certainty arises from the higher reactivity of these alkoxides and their well-known catalytic activity toward the condensation of siloxanes. Therefore, PDMS chains are formed on one side and Ti02 or ZrO2 nanoparticles on the other side. Si-O-Ti bonds have not yet been clearly evidenced. The catalytic role of titanium and zirconium alkoxides to-

18

zyxwvuts z LNAGE

zyxwvut zyxwvu zyxwv zyxwz

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

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

z zy zyxwv zyxw zyx zyxw

MOLECULAR DESIGN OF TRANSITION METAL ALKOXIDE

19

bonds between the metal atom and the organic species. Complexation appears to be the only way to synthesize hybrid gels involving transition metal oxides. However, very few results have been reported so far in literature. According to the results reported previously, carboxylic acids R-COOH could be conveniently employed as complexing species. Prehydrolyzed titanium butoxide (H2013 = 1) oligomers have been complexed with unsaturated organic acids, such as cinnamic acid(C6H5-CH=CH-COOH).Copolymerizationwasthenperformedwith styrene in the presence of benzoyl peroxide. Transparent brown polymers were thus obtained. They are very stable against water and cannot be dissolved in most organic solvents [43]. Acrylic or methacrylic acids can also be used as polymerizable complexing ligands [a]. They were recently reported for the sol-gel synthesis of zirconium oxides via the copolymerization of zirconium oxide sols and organic monomers [45]. Carboxylates are rather weak complexing ligands, however, and most of them are removed upon hydrolyis when an excess of water is added. Chelating P-diketones are stronger complexing ligands. Organically modified Ti02 gels, which exhibit photochromic properties, have been synthesized from an allyl acetylacetone-modified Ti(OnBu)4 alkoxide. A double polymerization process was initiated via the partial hydrolysis of alkoxy groups and the radical polymerization of allyl functions. The polymerization of the allyl function is slow, however, and organic polymerization is not very effective [46]. New approaches have been explored using functionalized chelates with both a strong chelating end and a highly reactive methacrylate group, such as aceto acetoxy ethyl methacrylate and methacryl amido salicylic acid (Fig. 13).

zy lorzyx z

HC

\

HC

zyxw I

0

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

20

zyxwvuts z zyxw LNAGE

Zirconium-oxopoly-polyAAEMcopolymers have been synthesized via the chemical modification of Zr(OnPr)4 precursors by M M . Both polymerization reactions were run simultaneously, leading to hybrid organic-inorganic polymers. The zirconium oxo core is made of oxoalkoxo AAEiM-modified species in which Zr is likely in a sevenfold coordination, as in monoclinic zirconia. Zirconium oxo species are chemically bonded to polymeric methacrylate chains via the P-diketo complexing groups [47]. The complexation ratio ( A A E W r ) seems to control the structure and texture of these hybrid gels, giving rise to more or less open structures. Organic and inorganic polymerization processes are presumably related, and such a hybrid material could be described as an interpenetrating polymer network (Fig. 14). Both organic and inorganic polymerizations of zirconium propoxide modified by methacryl amido salicylate were performed using similar procedures. This MASA ligand exhibits a stronger complexing power and a higher steric hindrance. Therefore, for equivalent complexation ratios (MASA substitutes two alkoxo groups), simultaneous polymerizations of methacryl amido salicylate-modified zirconium precursors lead to smaller zirconium oxopoly methacryl amido salicylate copolymers [47].

zyxw zyx

zyxw

Figure 14

A hybrid organic-transitionmetaloxidenetwork.

zyxwv zy zy zyxwvu zyxw

MOLECULAR DESIGN OF TRANSITION METAL ALKOXIDE

21

Other routes could be used, such as the following:

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

zyx zyx zyxwvu zyxwv zyxwv zyxwvu zy

REFERENCES

1. Brinker, C. J., and Scherer, G.W., Sol-Gel Science, Academic Press, New York, 1989. 2. Livage, J., Henry, M., and Sanchez, C. Prog. Solid Sate Chem., 18, 259 (1988). 3. Klein, L., Annu. Rev. Muter. Sci., 15, 227 (1985). 4. Toledano, P., Ribot, F., and Sanchez, C., C.R. Acud. Sci. Fr., 311, 13 15 (1990). 5. Livage, J., Barboux, P., Vandenbore, M. T., Schmutz, C., and Taulelle,F., J. NonCryst. Solids 147-148, 18 (1992). 6. Nabavi,M.,Sanchez,C.,andLivage, J., Eur. J. SolidStateInorg. Chem, 28, 1173 (1991). 7. Bradley,D. C., Mehrotra, R. C.,andGaur,D.P. Metal Alkoxides, Academic Press,London,1978. 8. Babonneau, F., Doeuff, S., Leaustic, A., Sanchez, C., Cartier, C., and Verdaguer, M., Inorg. Chem., 27, 3166 (1988). 9. Barringer, E. A., and Bowen, H. K., J. Am. Cerum. Soc., 65, C-l99 (1982). 10. Barringer, E. A., Bowen, H. K., Langmuir, 1, 414 (1985). 11. Scozzafava, M. R., Rhine, W. E., and Cina, M. J., Better ceramics through chemistry, IV, Muter. Res. Soc. Symp. Proc., 180, 697 (1990). 12. Vaartstra, B. A., Huffman, J. C., Gradeff, P.S., Hubert-Pfalzgraf, L., Daran,J. C., Parraud, S., Yunlu, K., and Caulton, K. G.,Inorg. Chem., 19, 3126 (1990). 13. Toledano, P., Ribot, F., and Sanchez, C., Acta Crystullogs, C46, 1419 (1990). 14. Kundu D., and Ganguli, D., J. Muter. Sci. Lett., 5, 293 (1986). 15. Caulton, K. G., and Hubert-Pfalzgraff, L., Chem. Rev., 90, 969 (1990). 16. Hegetschweiler, K., Schmalle, H. W., Streit, H. M., Gramlich, V., Hund, H. U., and Emi, I., Inorg. Chem., 31, 1299 (1992). 17. Livage, J. and Sanchez, C., J. Non-Cryst. Solids 145, 11 (1992). 18. Hubert-Pfalzgraf,L.,Papiemik, R., Massiani,M.C.,andSepte,B.,Betterceramics through chemistry, IV, Muter. Res. Soc. Symp. Proc., 180, 393 (1990). 19. Mazdiyasni,K. S., Dolloff, R. T., and Smith, J. S., J. Am Cerum. Soc., 52, 523 (1969). 20. Yanovsky, A. I., Yanoskaya, M. I., Limar, V. K., Kessler, V. G., Turova, N. Y., and Struchkov, Y. T., J. Chem Soc. Chem. Cornmuc., 1605 (191). 21. Pope, E. J., and Mackenzie, J. D., J. Non-Cryst. Solids, 87, 185 (1986). 22. Doeuff, S., Henry, M., Sanchez, C., and Livage, J., J. Non-Cryst. Solids, 89, 206 (1987).

22

zyxwvuts z zyxwvutsrq zyxwvu zyxwv zyxwvu zyxw zyxwvu zyxwv zyx

23.

WAGE

Doeuff, S., Dromzee,Y.,Taulelle,

F., andSanchez,C.,

Znorg. Chem., 28, 4439

(1989).

24. Doeuff, S., Henry,M.,andSanchez,C., Muter. Res. Bull., 25, 1519 (1990). 25. Unuma, H., Tokoka, T., Suzuki, Y., Furusaki, T., Kodiara, K., and Hatsushida, T., J. Mater. Sci. Lett., 5, 1248 (1986). 26. Babonneau,F.,Coury,L.,andLivage,J. Non-Cryst. Solids, 121, 153 (1990). 27. Uaustic, A.,Babonneau,F.,andLivage,J., Chem. Mater., I , 240 (1989). 28. Uaustic, A., Babonneau, F., and Livage, J., Chem. Mater., I , 248 (1989). 29. Watenpaugh, K.,andCaughlan,C.N., Chem. Commun., 2,76 (1967). F. S., 30. Day, V. W., Eberslcher,Klemperer,W.G.,Park,C.W.,andRosenberg,

Chemical Processing ofAdvunced Materials (L. L. Hench, and J. K. West, eds.), Wiley, New York, 1992, p. 257. 31. MossetA.,andGaly,J., C.R. Acad. Sci. Fr., 307, 1747 (1988). 32. Day, V. W., Eberspacher, T. A., Klemperer, W. G., Park, C. W., and Rosenberg, F. S., J. Am. Chem. Soc., 113, 8190 (1991). 33. Bradley, D. C., Hurthouse, M. B., and Rodesiler, P. F., J. Chem. Soc. Chem. Com-

mun., 1112 (1968). 34. Graeber, E. J. and Momson, B., Acta Crystallogr., B33, 2136 (1977). 35. Smith,G.D.,Caughlan,C. N., andCampbell, J. A., Znorg. Chem., 11, 2989 (1972). 36. Toledano,P., In, M.,andSanchez,C., C.R. Acad. Sci. Fr., 313, 1247 (1991). C.R. Acad. Sci. Fr., 31 1, 1161 (1990). 37. Toledano, P., In, M., and Sanchez, C., 38. Livage,J.Sanchez,C.,andToledano,P., Metaloxideclustersand colloids, Mater. Res. Soc. Symp. Proc., San Francisco 272, 3 (1992). 39. Rinn,G.,andSchmidt,H., CeramicTransactions (G.L.Messing,E.R.Fuller and H. Hausner, eds.), Am. Ceram. Soc., I, 23 (1988). 40. Schmidt,H., J. Non-Cryst.Solids, 73, 681 (1985). 41. Dir6, S., Babonneau, F., Sanchez, C., andLivage,J., J. Mater.Chem., 2, 239 (1992). 42. Judeinstein,P., Chem.Mater., 4, 4 (1992). 43. Suvorov,A.L.,andSpaski, S. S., Proc. Acad. Sci. USSR, 127, 615 (1959).

zyxw

44. Schubert, U., Arpac, E., Glaubitt, W., Helmerich, A., and Chau, C.,Chem. Mater., 4, 291 (1992). 45. Nass, R., and Schmidt, H., Sol-Gel Optics, SPIE, 1328, 258 (1990). J. Non-Cryst. Solids, 100, 46. Sanchez, C., Livage, J., Henry, M., and Babonneau, F., 650 (1988). 47. Sanchez,C.,andIn,M., J. Non-Cryst. Solids 147-148, 1 (1992).

z zy zy

2 Metal Alkoxides for Electrooptical Ceramics M a n e G. Hubert-Pfalzgtaf

zyxwv Universid de Nice-Sophia Antipolis Nice, France

1.

INTRODUCTION

Ceramics are one of the most recent classes of electrooptical materials. Table 1 summarizes the various compositions associated with transparent electrooptical ceramics [l]. These materials are mostly ferroelectric lead-containing titanates or niobates.Amongthe various types of precursors ofmetal oxides (e.g., carboxylates and nitrates), alkoxides have especially attractive features. These include high purity, the lability of the coordination sphere, which allows their tailoring according to specific applications, their easy transformation into oxides with formation of volatile by-products [2,3], and their ability to form homogeneous solutions under a large variety of conditions, and, for multicomponent systems, via heterometallic alkoxides [4]. As a result of their versatility, they can meet the requirements for sol-gel as well as metal organic chemical vapor-phase deposition (MOCVD) applications [5]. This chapter deals with the synthesis and general properties of homo- and heterometallic &oxides that are or might be used in chemical routes to electrooptical ceramic materials. Some considerations concerning structure, reactivity, and tailoring of their properties are also given. Emphasis is given to the most recent results.

zyxw zyxw zyxwv

II. HOMOMETALLICALKOXIDES For all elements involved in electrooptical ceramics, their homometallic alkoxides, of the general formula M(OR)n, or oxoalkoxides, MO(OR),, are either 23

on

24

zyxwvut z zyxwvu zyx zyx zyxwvut zyxwv zyxwv zyxw zyxw zyxwv HUBERT-PFALZGRAF

Table 1

Composition of ElectroopticalCeramics

Composition

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

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

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

A.

'

Synthesis

A variety of synthetic pathways, depending on the starting material, are generally available for most elements. Their value can be quite different in terms of selectivity of the reaction and yield and purity of the resulting metal alkoxide. The most electropositive metals (alkaline, alkaline-earth metals, yttrium, and lanthanides) can be oxidized directly by alcohols. The rate of the reaction is largely dependent on the surface properties of the metal and on the nature of the alcohol, especially its acidity and the bulkiness of the OR group. Thus the reaction proceeds easily with alkaline metals (Li, Na, and K), the primary alcohols being more reactive than the secondary or the tertiary. The same variation is observed for the alkaline-earth metals, the reaction being the least favored for magnesium [6,7]. In some cases, it might be necessary to catalyze the reaction by mercury or by gaseous ammonia. Ammonia was observed to be efficient toward barium when classic catalysts (Hg and 12) failed; it also avoids contamination [49]. Activation of yttrium and lanthanides by trace amounts of mercury(I1) salts (chloride or acetate) is generally required, although sonochemistry can be a way to activate the metals in the absence of mercury derivatives [50]. Nonotable reaction is observed between yttrium and lanthanides and methanol, ethanol, or t-butanol, but the reaction proceeds well with isopropanol and functional alcohols, such as 2-methoxyethanol [51,52]. Synthesis of metal alkoxides directly from the metal can lead to degradation reactions either of the solvent (tetrahydrofuran, THF, for instance [ 181) or of the reactant 1131; they appear especially easy with barium. Alcoholysis reactions applied to alkoxides (alcohol interchange reactions) or to metal silylamides generally make it possible to overcome these limitations.

25

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

[m.

(113:

M+ROHexcess+

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

(1)

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

MG,

z

z zyxwv z zyx zy zyxwvu zyxwvu

METAL ALKOXIDES FOR ELECTROOPTICAL CERAMICS

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

(2)

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

+ nROH+

zyxw

M(OR),

+ nHN(SiMe3)2

(3)

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

MR, + nR‘OH +M(OR’),

+ nRH

(4)

a g m

B

z"

E

zyx

z"z"

B B

P Q

z"

B

P

E

zyxw zyxwvu zyxwvu

Bm

%

e ....

m

8 2 .-aM m 0

c )

2m 0 c)

.-9ax

2

x

8 z.. 3

zyxwvuts

zyxwvuts zyxwvutsrq zyxwvu zyxwvuts zyxwvut zyxwvu zyxw zyxwvu zyxwvu zyxwvu zyxw

c a 8

27

28

Q

zyxwv

d

s

d

e

zyxwvut zyxwv zyxwv zyxwvu

i

0 .P - 0 -

M

zyx v)

"'-

zyxwvuts

zyxw zyxw

B

s

29

30

zyxwvuts zy zyx zyxwv zyxw zyxw zyxwv zyxwvu HUBERT-PFALZGRAF

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

M = alkaline metals.

zyx

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

z zyx

METAL ALKOXIDES FOR ELECTROOPTICAL CERAMICS

B.

PhysicalProperties and StructuralAspects

31

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

zyxw zyx zyxwvut zyxw

32

zyxwvuts zy zyxw HUBERT-PFALZGRAF

versatile and can behave as p2 or p3 ligands but can also accommodate four, five, or even six metals. As a result, oxo or hydroxo ligands appear to be a means of achieving the high coordination numbers required by the large di- or trivalent element in the absence of neutral ligands. Barium seems to be especially prone to formation of oxo or hydroxo derivatives, and this observation appears in the earlier literature as a tendency to give carbon-deficient or unstable metal alkoxides. The size effect of the mental is illustrated by comparison of Sr and Ba phenoxides: the strontium derivative is a tetranuclear nonoxo unit in which the metal is pseudooctahedral [16]; the barium derivative is a pentanuclear oxo or hydroxo aggregate, which allows Ba, which is larger than Sr, to be sevencoordinate for the basal metal atoms [ 181. Polydentate alcohols, such as triethanolamine, can encapsulate the metal and drastically improve the stability toward hydrolysis [14]. Figure 1 summarizes the basic [M(OR)& oligomers, as well as the oxo or hydroxo aggregates that are observed. Dimeric units (Fig. 1A) are illustrated by [M(OR)5]2 (M= Nb, Ta) or [M(OiPr)4(iPrOHh (M = Zr, Hf); [Pb(OtBu)2]3 and La3(OtBu)gL2 are typical examples of openand closed trinuclearunits (Fig. 1B and C), respectively; tetramers display a [Ti(OMe)4]4 type of structure (Fig. 1D); (also found for divalent or trivalent metals but with additional neutral ligands as for [Nd(OiPr)3(iPrOH)]4or Sr4(0Ph)8(PhOH)z(THF)6) or a cubanelike, (Fig. 1E) as for [K(OtBu)]4 or [ZnR(OR')]4 [S]. The structurally characterized penta- and hexanuclear aggregates are all oxo or hydroxo clusters.M4(p&o)o6 cores (Fig. 1F) havingan adamantanelike structure are found for lead and barium alkoxides. The M5(pS-o)Oy (Fig. 1G) core seems to display a special stability for large di- or trivalent metals. Hexanuclear aggregates derive from the pentanuclear by the addition of a sixth

zyx zy zyxwvu z

Figure 1

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

METAL ALKOXIDES FOR ELECTROOPTICAL CERAMICS

E

zy

z zyx 33

zyx X = OR,R

PR

zyxwvu zyxw G

M5°14

OR

OR

I

I

zyxw zyxwvuts H

Figure 1

6R

Continued

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

34

zyxwvutsz HUBERT-PFMZGRAF

OR'

zyxw zyxwv zyx

bridging-chelating

bridging P2 - q'

P2 - ? l 2

M

triply-bridging P3 -y'

zyxwvut zyxwvu zyxwvu zyxw

triply- bridging P3

terminal

-q2

Figure 2

Coordination modes of 2-methoxyethanol.

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

zyxwv zy z zyxwvut zy

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

z zyxwvu zyxw z zyxwvu zyxwv zyx zyxw zy zy 35

METAL ALKOXIDES FOR ELECTROOPTICAL CERAMICS

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

+ xROH

1 2 [M(OR),(ROH),,],

m’ m

(6)

The electronegative alkoxo or aryloxo groups make the metal atoms highly prone to nucleophilic attack. The alkoxides easily react with the protons of a large variety of organic hydroxy compounds, such as alcohols, silanols, glycols, carboxylic acids, P-diketones (Fig. 3). These exchange reactions convert homoleptic M(OR), into heteroleptic alkoxides M(OR),-XZ, and thus reduce the functionality of the precursor (2= OR). They also illustrate the use of alkoxides as versatile starting materials and as precursors whose coordination sphere can be easily modulated for specific applications. Such modifications of the coordination sphere of the initial alkoxide are often achieved by the use of the so-called additives, which are in fact chemical modifiers [3,55a].

1. Alcoholysis Alcoholysis reactions are generally performed to “tailor” the volatility or the rheology, to decrease the hydrolysis rates, or, finally, to increase the solubility of the metal alkoxides. As a synthetic route, alcoholysis reactions with classic alcohols are usually achieved n refluxing cyclohexane, the reactions being promoted by azeotropic distillation of the alcohol [2]. In contrast with silicon alkoxides, however, alcohol interchange reactions proceed to some extent (depending onthe OR group and on the stoichiometry) at room temperature for most elements [S]. This has been shown, for instance, by metal N M R on niobium and titanium alkoxides. “Uncontrolled” alcohol interchange reactions may thus occur when a metal alkoxide is used in an alcohol different from the parent alcohol. Such reactions lead to a modification (decrease) of the apparent functionality of the precursors, which in turn can affect the properties (morphology, porosity, and so on) of the resulting oxide. Functional alcohols, such as alcoxyalcohols-mainly 2-methoxyethanol-or alkanolamines (diethanolamine and triethanolamine), are actuallythemost commonly used to slow hydrolysis rates and thus to “stabilize” metal alkoxides. Although such exchange reactions occur at room temperature, Some heat-

36 HUBERT-PFALZGRAF

zyxwvu U

0

v

0

X

0

V

% X

ea X U

n

x m

0 v

p:

n

ea

:: zyxwvut

::

C

W

0

x e p:

zyxwv zyxw zyxw 1zyx zyxwvut zyx zyxwv zyxw p:

h

0 E

v

/-\

E

U

z zyxwvu zyxwv zy zyxw zyx zy

METAL ALKOXIDES FOR ELECTROOPTICAL CERAMICS

37

ing may be necessary for completion of the reaction, especially if stoichiometric amounts of reactants are used. Thus the reaction between zirconium isopropoxide [Zr(OiPr)4(iPrOH)]2 and four equivalents of 2-methoxyethanol leads to Zr(OCZH40Me)3(0iPr) a room temperature. The presence offunctional alkoxo groups around the metals generally results in an increase in its coordination number, and thus hydrolysis becomes more difficult [57]. Alcoholysis reactions using 2-methoxyethanol generally also offer more soluble products [52]. As already mentioned,this observation is no longer valid for lead(I1) since the methoxyethoxide derivative; are polymeric and insoluble [23]. The reactivity of a variety of glycols (CH&(OH)2 toward titanium alkoxides, zirconium isopropoxide, and niobium and tantalum ethoxides has been studied by Bradley et al.[2]. Complete substitution generally resultsinpoorly soluble products; such derivatives asM(OR),~X[O(CH~)~O]~ generally remain soluble and sometimes volatile. Further reactivity, for instance with acetylacetone, has established that the Ti-OR bond is more labile than the Ti-glycolate bond. Trialkylsilanols R3SiOH (R = Me, Et) are also a way of improving solubility and controlling hydrolysis [58]; however, they have rarely beenused, probably as a result of their limited stability and thus commercial availability.

2. Reactions with Carboxylic Acid and P-Diketones Substitutionreactionswith carboxylic acids, such as acetic acid, or with Pdiketones, mainly acetylacetone,are other means of controlling hydrolysis rates by decreasing the functionality of the precursor. Carboxylic acids RCO2H are often added to the sol-gel system as an acid catalyst [59,60]. In fact, they are not merely a source of H+, but a chemical modifier, and carboxylate groups substitute for the OR groups. Although P-diketonate and acetate ligands both generally lead to an increase in the metal coordination number, their behavior is quite different. Carboxylates act as assembling and oxo donor ligands, and thus have a tendency to increase the nuclearity of the aggregates; diketones are chelating ligands and thus decrease the oligomerization. This behavior is well illustrated by the modification of titanium alkoxides.Reactionbetween [Ti(OEt)& and acetylacetone gives monomeric and dimeric species (according tox-ray absorption near-edge structurestudies[61], and condensation into hexanuclear oxo clusters is observed when acetic acid is used (x-ray structures of Ti6(~2-o)2(C13-o)2(oR)8(oAC)8 (R = Et. iPr, nBu) [46]. 3. Hydrolysis Hydrolysis reactions of alkoxides proceed by nucleophilic attack of the metal M s . (7)], and polymerization occurs via the reactive hydroxoalkoxides [Eqs. (7b) and (7c)l. Hydrolysis-polycondensationreactions are governed by numerous factors (Table 3). The metals involved in the composition of electrooptical ceramics are electropositive and oxophilic, and thus the hydrolysis of their ho-

38

z zyxwvu zyxwv zyxwv zyxw zyxwvu zyxw HUBERT-PFALZGRAF

Table 3 HydrolysisParameters of Homo-andHeteroleptic Alkoxides

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

zyxwvuts

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

+ H20

-

H\

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

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

/

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

zy zyx

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

(7b) (7C)

The early stages of the hydrolysis-condensationof [M(OR),Im can be characterized byx-ray diffraction for oxophilic metals (Table 4). These include centrosymmetric polynuclear oxoalkoxides of titanium, zirconium, andniobium. Their structure is often related to that of polyoxoanions: Ti704(OEt)20 is isostructural with Mo70246, and Nbg010(OEt)20 has a cagelike Structure that compares well with that of the paratungstate H~W120421G.These oxoalkoxides are soluble as a result of their closo structure, and 1 7 0 NMR has been developed as a technique to follow the hydrolysis of titanium(1V) alkoxides with

39

40

zyxwvutsr z zyxw zyx zy zy zyxwv zy HUBERT-PFALZGRAF

H217O. Since enrichment in 1 7 0 occurs selectively at their oxide (as opposed to alkoxide) oxygen sites, the different types of oxo ligands (h,n = 2- 4) can be determined, the most encapsulated oxo ligands being the most shielded [63]. Similar types of molecular oxo clusters are observed for alkoxides modified by acetylacetone [65a,66]. Although the early stages of the hydrolysis-polycondensation are extremely rapid, removal ofall alkoxide ligands at room temperature generally requires quite a large excess of water [57]. Hydrolysis studies have also been performed on several lead(1I) alkoxides: Pb(OR)2 (R= CMefit, CHMeCH2NMe2) in THF [23]. The hydrate 3PbOeH20 was obtained at low temperature as the kinetic product; its conversion to massicot and then to litharge is catalyzed by an increase in the pH (which may be caused by alkali metal alkoxide residues). Since 3PbOeH20 is less stable than massicot and litharge, it could act as a more reactive component in ceramic oxide preparations.

D. Thermal Decomposition

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

111.

HETEROMETALLIC ALKOXIDES

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

zyxwvu

zyxwvu zyx z zyxw z zyxwvuts zyx zyxwvu zyx zyxw zyxw

METAL ALKOXIDES FOR ELECTROOPTICAL CERAMICS

A.

41

Formation of Heterometallic Alkoxides

Heterometallic alkoxides are generally obtained either by reactionbetween Lewis acids and bases or by condensation with elimination of volatile or insoluble by-products.

1. Lewis Acid-Base Reactions Formation of heterometallic alkoxides by simple mixing, often at room temperature and in nonpolar solvents, is one of the most general routes and corresponds to the "complexation" step in the sol-gel process. This general reaction [Eq. (8)] appliestonearlyallalkoxides,withthe exception of thesilicon Si(OR)4:

yM'(0R')nI

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

(8)

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

M(OR)~ + "(OR)

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

(9)

(10)

M = Nb, Ta; M' = Li, Na, K, . . . Alkali metal alkoxides are often involved in the preparation of metal alkoxides [Eq.51; heterometallic alkoxides can thus be a side product if the stoichiometry of the reaction is poorly controlled ( o n ) . For electropositive metals, such as barium, the alkoxide might be generated in situ:

zyxw

2Ba + [Zr(OiPr)4(iPrOH)]2 dBa2Zr2(pOR)2(OR)18+ H2

(11)

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

5 E .e

4 .. h E

zyxwv zyxw zyxwvuts zyxw zyxw P

0

C

2

B

zyx zyxwvuts

zyx zyx zyxwv

Y

e,

.^

z m C

.e

8

%

F? E

zyxwv zyxwvuts zyxwvu zyxw

S X -

52%

43

44

zy

zyxwvut z zy HUBERT-PFALZGRAF

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

+

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

(14)

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

zyx zy

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

> P~Zn~(OAc)4(0Cz&OMe)4 + (17)

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

zyxwvuts

Pb(0Ac)z + [Zr(OiPr)4(iPrOH)]z 250c> PbZr3(~O)(~OAc)z(~OipT)5(OiPr)~ + '

(18 )

zy zyx zyxwv zyxw zyx zyxwvu zy

METAL ALKOXIDES FOR ELECTROOPTICAL CERAMICS

45

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

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

+ M"M',(OR),

+ nM'C1

"+MIM'y(OR)y]n

(19)

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

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

B.

PropertiesandStructuralAspects

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

zyxwv zyxw zyxwv

46

zyxwvuts z HUBERT-PFALZGRAF

zyxw zyx zyxwvu

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

zyxwv Reactivity

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

zy zyxw zyx zyxw

METAL ALKOXIDES FOR ELECTROOPTICAL CERAMICS

47

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

zyxwvu zyxw

IV. APPLICATION TO CERAMICS A.

Sol-Gel Process

PLZT and PNM are, with L i m o s and LiTaOs, the systems related to electrooptical ceramics that have been the most studied.Chemical low-temperature routes are particularly attractive for lead-containing materials, since lead oxide is quite volatile in comparison with other metal oxides; thus control of the stoichiometry of lead-based materials is tedious. To overcome the difficulty inhandling alkoxides and their availability, commonly accessible salts, such as carboxylates (acetates or the more soluble 2ethylhexanoates), carbonates, nitrates, or hydroxides, have often been used in conjunction with metal alkoxides (mostly n-propoxides or n-butoxides for Ti and Zr and ethoxides for Nb or Ta), the solvent being an alcohol. The poor re-

zyxwvu zyxwv

48

zyxwvutsr zy HUBERT-PFUGRAF

activity of barium acetate is overcome by adding acetic acid [46a] or by using carbonate or, more often, barium hydroxide Ba(OH)~8Hz0[94]. Commercial ionic precursors are often available as hydrates, and this must betaken into account when speculation about the intermediates that may form is proposed. Carboxylate groups (acetates or 2-ethylhexanoates) are generally removed only at relatively high temperatures (400°C); they appear to favor porous materials and grain boundaries [95]. Additives for hydrolysis are either acids (0.1-0.2 M m03 or acetic acid) or ammonia solutions. Although leadoxide has also been used [96], acetate hydrates are the precursors most often used for the introduction of lead or lanthanum in the multimetallic systems required for PLZT materials,and they are often associated with the useof 2-methoxyethanol [97-991. This solvent appears as a way to achieve deshydratation by refluxing (boiling point 124"C), as well as condensation (elimination of organic esters as has been shown by infrared (vCOZ = 1730 cm-1) [loo]. Studies on the influence of the precursor remain limited. X-ray absorption fine structure analysis of the sol and gel precursors of PZT (lead acetate in methanol, n-propoxides of Zr and Ti, and acetic acid) have shown that an uniform composition exists in both states on a microscopic level with Ti-O-Z linkages [100a]. The choice of the titanium precursor Ti(OR)4, R = nF'r or iPr, was reported to have an effect on the microstructure development, particularly for PbTiOs films [97]. Clusters and grains were observed in the n-propoxide and were attributed to a less continuously cross-linked networks compared with films derived from the isopropoxide. Using the modified alkoxide Ti(OiPr)z(acac)z instead of Ti(OiPr)4 allowed an increase in the thickness of crack-free PbTiOs films up to 1 m under base-catalyzed conditions [loll. Using the same titanium precursor associated with lead acetate and pentane-lJ-diol instead 2of methoxyethanol permitted the production of films up to approximately 5pm in thickness by repeated coatings before fuing [ 1021. All alkoxide routes appear to be limited toobtaining PNM [88a] and lithium or potassium niobate or tantalate [102a]. For the latter, the formulation of the mixed-metal alkoxide MM'(OR)6, M = Nb, Ta, is in agreement with that of the material. Epitaxial thin films of LiNbO3 on sapphire could be obtained [103]. The useof a mixed-metal MgNbz(0R)lz species (R = Et [105], CzH40Me [104]) has been shown to promote the formation of the perovskite phase for PNM.

B.

zyxw zyxw zyxw zyxw zy zyxwvu

MOCVD

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

zyx zy zy zy zyxw zyxw zyx zyxwv zyxwvu

METAL ALKOXIDES FOR ELECTROOPTICAL CERAMICS

49

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

V.

OUTLOOK

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

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

50

zyxwvuts z zyxwvu HUBERT-PFALZGRAF

zyxw zyxwv zyxwv zyxwvu

ABBREVIATIONS

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

REFERENCES

1. Haertling, G. H.,Electronic Ceramics (L. M. Levinson, ed.), Dekker, New York, 1988, Chap. 7). 2. Bradley,D.C.,Mehrotra,R.C.,andGaurD. P.,MetalAlkoxides, Academic Press, London, 1978. 3. Hubert-Pfalzgraf, L. G., Alkoxides as molecular precursors for oxide-based inorganic materials. Opportunities for new materials, New J. Chem., 11, 663 (1987). 4. Caulton, K. G., and Hubert-Pfalzgraf, L. G., Synthesis, structural principles, and reactivity of heterometallic alkoxides, Chem Rev., 90, 9-995 (1990). 5. Bradley, D. C., Metal alkoxides as precursors for electronic and ceramic materials, Chem. Rev., 89, 1317 (1989). 6. N.Y. Turova, V. A., Kozunov, E. P., Turevskaya, E. P., and Novoselova, A. V., Magnesium alkoxides, Russ. J. Inorg. Chem, 188, 327-329 (1973). 7. Lutz, H. D., Infrared and powder diffraction studies of Mg(OMe)2, Ca(OMe)2, Sr(0Me)z and Ba(OMe)z, Z. Anorg. Allgem. Chem, 365, 100 (1969). 8. Purdy, A. F., George, C. F., and Callahan, J. H., New alkoxides of copper and the alkaline and alkaline metals. Crystal structure of Na2CU[OCH(CF3)2]4, Inorg. Chem., 30, 2812-2819 (1991). 9. Rees, W. S., and Moreno, D. A., Preparation of monomeric Ba([O(C2&0)nCH3]2 (n = 2, 3), ambient temperature liquid barium compounds, J. Chem. Soc. Chem. Commun., 1759 (1991). 10. Calabrese, J., Cushing, M. A., and Ittel,S. D. Sterically hindered magnesiumaryloxides, Inorg. Chem, 27, 867 (1988). 11. Roesky, H. W., Scholz, M., and Noltemeyer, M.,Reaction between 2,4,6-tris(trifluoromethy1)phenolwithmaingroupandtransitionmetalderivatives(Li,Na, Ng, Ca, Ba, Ge, Sn and Ti, W, Mn, Cd), Chem. Ber., 123, 2303-2309 (1990). 12. Hitchcock, P. B.,Lappert,M.F.,Lawless,G.A.,andRoyo, B., The synthesis and structure of the alkaline earth metal organic compounds[M(OAr)2(thfln] (1) M = Ca, n = 3 or 0 M = Ba, n = 4 and [Ca(NRz)(pNR2)(thf)]2 and the x-ray structure of (1) Ar = C6HztBu2-2, 6-Me4, J. Chem. Soc. Chem. Commun., 1141 (1990). 13. Caulton,K.G.,Chisholm,M. H., Drake, S. R.,andFolting,K.,Synthesisand

zyxwvuts zyxwvu zyxwvu zyxw zy z

zyx zy zy zyxwvu

METAL ALKOXIDES FOR ELECTROOPTICAL CERAMICS

51

zyx zyxwvu zyxw zyxw zy zyxwvu

molecular structure of a mononuclear barium aryloxide-ethanolamine complex, Ba(2,6-tBu2CaH30)2(0HC2H4NMe2)4,2C7H8, exhibitingextensivehydrogen bonding, Znorg. Chem., 30, 1500 (1991). 14. Poncelet, O., Hubert-Pfalzgraf, L.G., Toupet, L., and Daran, J. C.,Molecular precursors of alkaline structure synthesis earths: ofand [N(C2H4O)(C2H4OH)2]2Ba,2EtOH, the first structurallycharacterizedbarium alkoxide, Polyhedron, 10, 2045-2050 (1991). 15. Caulton, K. G., Chisholm, M. H., Drake, S. R., and Streib, W. E., Synthesis and structure of Ba3(0SiPh3)6(THF),OSTHF, a trinuclear barium siloxide containing coordinate barium ions, Angew. Chem. Int. Ed. Engl., 29, 1483 (1990). Coan, P. S., Streib, W. E., and Caulton, K. G. A triangular heterometallic siloxide containing barium, Inorg. Chem., 30, 5019-5023 (1991). 16. Caulton, K. G., Chisholm, M.H., Drake, S. R., and Streib, W. E., Facile synstructural and thesis principles of strontium the phenoxide Sr4(OPh)8(PhOH)2(6, Inorg. Chem., 29, 2707 (1990). 17. Tesh, K. F., and Banusa, T. P. Formation and solid state structure of a tetranuclear oxoaryloxide cluster of barium IBa40(0C6H4CH2NMe2)3-2,4,6)6], 3toluene, J. Chem. Soc. Chem.Commun., 879-881(1991). 18. Caulton, K. G., Chisholm, M. H., Drake, S. R., and Huffman, J. C., Examples of molecular aggregates of barium supported by aryloxide and alkoxide ligands, J. Chem. Soc. Chem. Commun., 1498 (1990). 18a. Miele, P., Foulon, J. D., Hovnanian, N., Durand, J., and Cot, L., J. Chem. Soc. Chem. Commun., to be published. 19. Labrize, F., and Hubert-Pfalzgraf, L. G., unpublished results. 20. Caulton, K. G., Chisholm, M. H., Drake, S. R., and Huffman, J. C., Preparation, crystal and molecular structureof a hydrocarbon soluble volatile oxo-alkoxide of barium, H4Ba6(~la-O)(OC2H4OMe)l4, J. Chem. Soc. Chem.Commun., 1750 (1990). 21. Cetinkaya, B., Gunriiku", I., Lappert, M.F., Atwood, J. L., Rogers, R. D., and Zaworotko, M. J., Bivalent germanium tin and lead 2,6-di-tert-butylphenoxides and the crystal and molecular structures of M(OC6H2Me-4tBu-2,6)2 (M = Ge or Sn), J. Am. Chem. Soc., 102, 2088 (1980). 22. Veith, M., and Tallner, P., Cyclic azastannylene. W Reactivity of a 1,3-diaza-2sila-4, 2-stannylene with dienes, J. Organometal. Chem., 246, 219 (1983). 23. Goel, S. C., Chang, M.Y., andBuhro W. E.Preparation of sixlead (n) dialkoxides. X-ray structures of [Pb(OC2H40Me)2Im and Pbs(OtBu)6 and hydrolysis studies, Inorg. Chem., 29, 46404646 (1990). 24. Papiernik, R., Hubert-Pfalzgraf, L. G., and Massiani, M. C., Synthesis, characterization and reactivity of lead(II) alkoxides and oxoalkoxides: Condensation to oxoalkoxides as a general structural feature, Polyhedron, 10, 1657 (1991). 25. Gaffney, C., Harrison, P. G., and King, T. J., The crystal and molecular structure of adamanta-M-oxo-hexakis (p-triphenylsi1oxy)-tetralead(II), J. Chem. SOC. Chem.Commun., 1251(.1980). 26. Yanovskii, A. A., Turova, N. Ya, Turevskaya, E. P., and Struchkov,Y. T., Structure of the first hydrolysis product of lead(I1) isopropoxide. Trans. 8, 153 (1982).

52

zyxwvuts z zyxwv z zyxwvu HUBERT-PFALZGRAF

27. Harrison,P.G.,Haylett, 28.

29. 30.

31. 32.

33. 34.

35.

36.

37.

B. J., andKing,T. J., X-raycrystalstructure of Sn604(OMe)4: An intermediate in the hydrolysis of tin(II) dimethoxide, J. Chem. Soc. Chem. Commun., 112 (1978). Turevskaya, E. P., Ya Turova, N., and Novoselova, A. V., Trans. IN. A M . Nuuk SSSR, Seriyu Khimicheshyu, 8, 1667 (1968). Goel, S. C., Chang, M. Y., and Buhro, W. E., Preparation of soluble and volatile zinc dialkoxides. X-ray crystal structures of an (amido)zinc alkoxide and a homoleptic zinc enolate Inorg. Chem, 29, 4646 (1990). Geerts,R. L., Huffman, J. C.,andCaulton, K. G., Soluble zinc bisaryloxides, Inorg. Chem., 25, 1803 (1986). Neils, T. L. L., and Burlitch, J. M., A soluble alkoxyzinc hydride, [HZnOCMe3]4. Synthesis and reactions with copper(I) alkoxides,Inorg. Chem., 28, 1607 (1989). Massiani, M. C., Papiernik, R., Hubert-Pfalzgraf, L. G., and D m , J. C., Molecular precursors of bismuth oxide. Synthesis structure and of [B~~(OC~H~OM~)~(OC~HLIOM~)~]-, aone-dimensionalribbon-likechain, J. Chem. Soc. Chem. Commun. , 301 Massiani, M.C., Papiernik, R., Hubert-Pfalzgraf, L. G., and Daran, J. C., Molecular precursors of bismuth oxides: b-Diketonates and alkoxides. Molecular structure of [Bi2(0C2H40Me)6], and of Bi(OSiPh3)3(THF)3, Polyhedron, 10, 437 (1991). Evans, W. J., Hain, J. H., and Ziller, J. W., Synthesis and molecular structure of Bi(OCaH3tBuzh, J. Chem. Soc. Chem. Commun., 1628 (1989). Matchett, M.A., Chang, M.Y., and Buhro, W. E., Soluble and volatile alkoxides of bismuth. The first structurally characterized bismuth trialkoxide: @3i(p,nlOC2&0Me)4(0C2fiOMe)~]~,Inorg. Chem., 29, 358 (1990). Sauer, N. N., Garcia, E., and Ryan, R., Soluble and volatile precursors for the preparation of super-conducting films, Better Ceramics throughChemistry W, Mater. Res. Soc. Symp. Proc., 180, 921 (1990). Bradley, D. C., Chudzynska, H., Frigo, D. M., Hursthouse, M. B., and Mazid, M. A., A penta-indium oxo alkoxide cluster with a central 5-co-ordinate oxygen. Preparation and X-ray crystal structure of (InOiPr)5(p2-0iPr)4(p3-OPr)4(~1-50), J. Chem. Soc. Chem. Commun. 1258 (1988). Bradley, D. C., Chudzynska, H., Frigo, D. M., Hammond, M. E., Hursthouse, M. B., and Mazid, M. A., Pentanuclear oxoalkoxide clusters of scandium, yttrium, indium and ytterbium. X-ray structureof MsO(OiPr)13 (M = In, Yb), Polyhedron,

zyxwv

zy zyxwv zyxwvut

9, 719-726 (1990). 37a. Chattejee, S., Bindal, S. R.,andMehrotra,R.C.,Indiumalkoxides, J. Indiun Chem. Soc., 53, 867 (1976). 38. Hitchcock, P. B., Lappert, M. P., and Singh, A., Three and four-co-ordinate hy-

drocarbon-solublearyloxides of scandium,yttriumand the lanthanides:X-ray crystal structure of tris(2,6di-t-butyl-4-methylphenoxo)scandium, J. Chem. Soc. Chem. Commun., 1499 (1983). 39. Bradley, D. C., Chudzynska, H., Hursthouse, M. B., and Motevalli, M., Volatile tris-tertiary alkoxides of yttrium and lanthanum. The X-ray structure of La3(0tBu)9(tBuOH)2, Polyhedron, 10, 1049-1059 (1991). 40. Evans, W. J., Golden, R. E., and Ziller, J. W., A comparative synthetic and struc-

z zyxwv zyxwv zyxw zyxwvu zyxwvu zy zyxwvu zyxwvu zyxwvuts

METAL ALKOXIDES FOR ELECTROOPTICAL CERAMICS

41. 41a. 42.

43.

44. 45. 46.

46a. 47. 48. 49.

50.

51. 52. 53. 54.

53

tural study of triphenylmethoxide and triphenylsiloxide complexes of the early lanthanides, including X-ray crystal structures of Laz(xPh3)a and Ce~(OSiPh3)6,Inorg. Chem., 30, 4963-4968 (1991). McGeary, M. J., Coan, P. S., Folting, K., Streib, W. E., and Caulton K. G., Yttrium and lanthanum silyloxide complexes, Inorg. Chem., 30, 1723 (1991). Mehrotra, R. C., Transition metal alkoxides, Adv. Inorg. Chem. Rudiochem., 26, 269 (1983). Vaartstra, B. A., Huffman, J. C., Gradeff, P. S., Hubert-Pfalzgraf, L. G., Daran, J. C., Parraud, S., Yunlu K., and Caulton, K. G., Alcohol adducts of alkoxides: Intramolecular hydrogen bonding as a general structural feature, Inorg. Chem., 29, 3126-3131(1990). Kozlova, N. I., andTurova, N. Y.,Zirconiumisopropoxide, Russ. J. Inorg. Chem., 25, 1188 (1980). Fay, R. C., Zirconium and hafnium, in Comprehensive Coordination Chemistry (G. Wilkinson, ed.), Pergamon, London, Chap. 32, 1987, p. 390. Menge,W.M.P.B.,andVerkade,J.G.,Monomericanddimerictitanatranes, Inorg. Chem, 30, 4628 (1991). Doeuff, S., Dromzee, Y., Taulelle, F., and Sanchez, C., Synthesis and solid- and liquid-state characterization hexameric aof cluster titanium(IV): of Ti6(p2~)(~3-o)2(p~t-oC4H9)2(OC(4H9)6(oAc)8, Inorg. Chem., 28, 4439 (1989). Mosset,A.,Gautier-Luneau I., Galy,J.,Strehlov,P.,andSchmidt,H.,Sol-gel processed BaTi03; structural evolution from the gel to the crystalline powder,J. Non-Cryst. Solids, 100, 339 (1988). Laaziz, I., Larbot, A., Guizard,C.,Durand,J.Joffre,J.,andCot,L.,Crystal structure of Ti604(0iPr(g(OAc)g,Acta Crystallogr. C46, 2332 (1990). Riess, J. G., and Hubert-Pfalzgraf, L. G., Niobium pentamethoxide, a multi-faced study of an early transition metal alkoxide, Chimia, 30, 481 (1976). Drake, S. R., and Otway, D. J., The synthesisof metal organic compounds of calcium,strontiumandbariumbyammoniagassaturatedetherealsolutions., J. Chem. Soc. Chem. Commun., 517-519 (1991). Caulton, K. G., Chisholm, M. H., Drake, S. R., and Folting, K., Inorg. Chem., 30, 1500 (1991). Poncelet, O., Sartain, W. J., Hubert-Pfalzgraf, L. G., Folting, K., and Caulton, K. G.,Chemistryofyttriumtriisopropoxiderevisited;characterizationandcrystal Inorg. . Chem., 28, 263 (1989). structure of Y5(p5-O)(p3-OiPr)4(p2-OiPr)4(OiPr)5 Mehrotra, R. C., Singh, A., and Tripathi, U. M., Recent advances in alkoxo and Chem. Rev., 91, aryloxochemistryofscandium,yttriumandlanthanides, 1287-1303 (1991). Poncelet, 0..Hubert-Pfalzgraf, L. G., Daran, J. C., and Astier, R., Alkoxides with polydentatealcohols:Synthesisandstructure of [Y(OCfl40Me)3]10,ahydrocarbon soluble cyclic decamer, J. Chem. Soc. Chem. Commun., 1846 (1989). Turnipseed, S. B., Barkley, R. M., and Sievers, R. E., Synthesis and characterization of alkaline-earth-metal P-diketonate complexes used as precursors for chemInorg. Chem, 30, 1164 (1991). ical vapor deposition of thin-film superconductors, Shreider, Turevskaya,E. P., Kozlova, N. and Turova, N. Y., Direct electrochemical synthesis of metal alkoxides, Inorg. Chim. Acta, 53, L73 (1981).

zyx

54

zyxwvutsr zy zyxwvuts zyx zyx zyxwvu zyxwvuts zyxwvu zyxwv

55.

HUBERT-PFALZGRAF

Chisholm, M. H., Drake, S. R., Naiini, A. A., and Streib, W. E., One-dimensional ribbon chains [M(OtBu)tBuOH], and the cubane [M(OtBu)]s M = K, Rb, polyhedron, 10, 337 (1991).

55a.Guglielmi, M., andCarturan,G.,Precursorsforsol-gelpreparations,

Non-crys-

tall. Sol., 100, 16 (1988).

Sanchez, C., andLivage, J., Sol-gelchemistryfrommetalalkoxideprecursors, New J. Chem, 14, 513 (1990). 57. Hubert-Pfalzgraf,L.G.,Poncelet, O., andDaran, J. C.,Tailoredmolecularpre-

56.

cursors of yttrium oxide using functional alcohols and acetylacetone as modifiers, BetterCeramicsThroughChemistry N Mat.,Res.Soc.Symp. Proc., 180, 73 (1990).

58.

Plouxviel, J. C., Boilot, J. P.,Poncelet, O., Hubert-Pfalzgraf, L. G.,Lecote,A., Dauger, A., and Beloeil,J. C., An aluminosiloxane as ceramic precursor,J. NonCryst. Sol., 93, 277 (1987).

Phule, P. P., Deis, T. A., and Dindiger, D. G., Low temperature synthesis of ultrafine LiTaOs powders, J. Mater. Res., 6, 1567 (1991). J., Sol-gelroutetoniobium 60. Griesmar,P.,Papin,G.,Sanchez,C.,andLivage, pentoxide, Chem. Mater., 3, 335 (1991). 61. Leaustic,A.,Babonneau, F., and Livage, J., Structural investigation of the hydrolysis-condensation of titanium alkoxides Ti (OR)4 (R= iPr, Et) modified by acetylacetone 1. Studyofthealkoxidemodification, Chem. Mater., I , 240

59.

(1989).

zyxwvu zyxwvu zyx zyxwvu

62.

Schmidt, R.,Mosset, A., and Galy, J., New compounds in the chemistry of group 4 transitionmetalalkoxides.Synthesisandmolecularstructureof two polymorphs of Ti16016(OEt)32 and refinement of Ti704(OEt)20, J. Chem. Soc., Dal-

63.

Day, V. W., Eberpachere, T. A., Klemperer, W. G., Park, C. W., and Rosenberg, F. S., Solution, structure, elucidation of early transition metal polyoxoalkoxides using 1 7 0 nuclearmagneticresonancespectroscopy, J. Am.Chem. Soc., 113,

64.

Morosin, B., CrystalstructureofZr1308(0Me)36,

ton Trans., 1999 (1991).

8190- 8192 (1991). (1977).

Acta Crystallegr., 33B,303

65.

Kessler, V. G.,Turova,N.Y,Yanovskii,Belokon,A.I.,andStruchkov,Structure of Nb8010(OEt)20 and nature of metal oxoalcoholates,Zh. Neorg. Khim., 36,

65a.

Toledano, P., In, M., and Sanchez, C., Synthesis and structure of the compound T i ~ ~ ( ~ ~ - O ) ~ ( ~ O ~ ( ~ ~ - O ~ o ( ~ ~ - O ~ ( ~ - O n B u ) R. ~ ~Acad. ( O n BSci., u ) ~13, ~(acac)~,C

1662 (1991).

1247 (1991).

Toledano,P., In, M., and Sanchez, C., Co. R. Acad. Sci., 5, (1990). Parraud, S., Hubert-Pfalzgraf, L. G.,andFloch, H.,Stabilizationandcharacterization of nanosize niobium and tantalum oxidesols. Optical applications for high power lasers, in Ultrastructure and Processing of Ceramics, Glasses and Composites, L. Hench (ed.), Wiley, New York, (1992). 68. Nandi,M.,Rhubright,D.,andSen,A.,Pyrolytictransformationofmetalalkox-

66. 67.

zy zyxwvut

zyxw zyxwvu zyxwv zyxw zyxwvut zyxwv zyxw

METAL ALKOXIDES FOR ELECTROOPTICAL CERAMICS

55

ides to oxides: Mechanistic studies. Pyrolysis of homoleptic titanium alkoxides, Inorg. Chem., 29, 3065 (1990).

69. Mehrotra,R.C.,Agrawal,M.M.,andKapoor,P.N.,Alkali-metalhexa-alkoxides of niobium and tantalum, J. Chem. Soc. A, 2673 (1968). 70. Eichhorst, D. J., Payne, D. A., Wilson, S. R., and Howard, K. E., Crystal structure of LiNb((0Et)a: A precursor for lithium niobate ceramics, Inorg. Chem., 29, 1458-1459 (1990). 71. Yanovskii, A. P., Turevskaya, E. P., Turova, N. Y., and Struchkov, Y., Structure of lithium ethoxo-niobate crystal and molecular structure of its partial hydrolysis product, Transl. Koord. Khim., 11, 110 (1985). 72. Goel, S., Goel, A. B., and Mehrotra, R. C., Double ethoxides of niobium with alkaline earth metals, Syn. React. Inorg. Metal-Org. Chem., 6, 251 (1976). 73. Boulmaaz, S., and Hubert-Pfalzgraf, L. G., to be published. 74. Kessler, V., and Turova, N. Y., private communication. E., Sol-gelpro75. Hayes, J. H.,Gururaja,T.R.,Geoffroy,G.L.,andCross,L. Mater. Lett., 5, 396 (1987). cessing of 0.9Pb(Zn1/3Nb~3)03-0.09PbTi03, B., Het76. Hubert-Pfalzgraf,L.G.,Papiernik,R.,Massiani,M-C.,andSepte, erometallic alkoxides as precursors to multicomponent oxides, Bener Ceramics Through Chemistry W , Mat. Res. Soc. Symp. Proc., 180, 393 (1990). 77. Turevskaya,E.P.,andTurova,N.Y.,Thestudyoflanthanumandscandium alkoxoniobates. Formation, polymorphism,Koord. Khim., 10, 1357 (1984). 77a. Fukui, T., Sakurai, C., and Okuyama. Lower temperature preparation of Pb(Mg1nNbu3)03 with a perovskite structure by the complex alkoxide method, J. Non Cryst. Sol., 134, 293 (1991). 78. Hubert-Pfalzgraf, L. G., and Riess, J. G., Isolation of a mixed niobium-tantalum alkoxide, Inorg. Chem., 15, 1196 (1976). 80. Hampden-Smith,M. J., Williams, D. S., andRheingold,A.L.,Synthesisand characterization of alkali-metal titanium alkoxide compounds MTi (Oipr)~(M = Li,Na,K):Single-crystalx-raydiffractionstructureof[(LiTi(OiPr)=&, Inorg. Chem., 29, 40764081 (1990). 81. Vaartstra, B. A., Huffman, J. C., Streib, W. E., and Caulton, K. G., Incorporation of barium for the synthesis of heterometallic alkoxides. Synthesis and structures of [saZr2(OiPr)10]2 and Ba[Zs(OiPr)slz, Inorg. Chem., 30, 3068 (1991). 81a. Ritter, J. J., Roth, R. S., and Blendell, J. E., Alkoxide synthesis and characterization of phases in the barium-titanium oxide system, J. Am. Ceram. Soc., 69, 155 (1986). 82. Yanovski, A. I., Yanovskaya, M. I., Limar, V. K., Kessler, V. G., Turova, N. Y., and Struchkov, Y. T., Synthesis and crystal structure of double barium-titanium isopropoxideBwTi404(0R)16(ROH)~, J. Chem Soc. Chem. Commun., 1605 (1991). 83. Campion, J. F., Payne, D. A., Choe, H. K., Maurin, J. K., and Wilson, S. R., Synthesis of bimetallic barium titanium alkoxides as precursors for electrical ceramics. Molecular structure the of new barium titanium oxide alkoxide BiLiri13012(OC2H40Me)24,Inorg. Chem., 30, 3245 (1991).

zyxwvuts zyxwvut zy

56

zyxwvuts zy zyxw zyxwvu zyxwvu zy zyxwvutsrq zyxwv HUBERT-PFALZGRAF

Hubert-Pfalzgraf, L. G., Papiernik, R., and Massiani, M. C., to be published. Papiernik, R., Hubert-Pfalzgraf,L. G., andChaput,F.,Molecularroutesto ternarymetaloxides:A PbTi heterometallicisopropoxideasaprecursorto PbTi03, J. Non-Cryst. Sol., (in the press) (1992). 86. Hirano, S. I., and Kato, K., Adv. Cerum. Muter., 3, 503 (1988). 87. Tripathi, U.M., Singh, A. , and Mehrotra, R. C., Synthesis and characterization of some heterometallic isopropoxides of lanthanum with zirconium, Polyhedron,

84. 85.

ZO, 949 (1991).

88.

zyxwv zyxwvu

Sakashita, Y . , Ono, T., Segawa, H., Tominaga, K., and Okada, M., Preparation and electrical propertiesof MOCVD-deposited PZT thin films,J. Appl. Phys., 69, 8352 (1991).

88a. Munozaguado, M. F., Gregorkiewitz, M., and Larbot,A., Sol-gel synthesisof the binary oxide (Zr, Ti)Oz from the alkoxides and acetic acid in alcoholic medium, Muter. Res. Bull., 27,87 (1992), Laaziz, I., Larbot, A. , Julbe, A., Guizard, C., and Cot, L., Hydrolysis of mixed titanium and zirconium alkoxides by an esterification reaction, J. Solid Sture Chem., to be published; Laaziz, I., Larbot, A., Foulon, 1. D., and Cot, L., zr2Ti404(oiPr)8(oiPr)2( OAC),~, submitted for publication. 89. Francis, L. F., Payne, D. A. , and Wilson, S. R., Crystal structure of a new lead zinc acetate alkoxide, PbzZnz(OCzH40Me)4(0Ac)4, Chem Murer., 2,645- 647 (1990).

90.

Veith,M.,Hans, J., Stahl, L., May,P.,Huch,V.,andSebald, A. , Bimetallic alkoxygermanate(II)-stannate (II) andplumbate(II), Z. Nuturjiorsch. B, 46b, 403- 424 (1991).

Eichorst, Howard, K. E., and Payne, D. A. , NMR investigations of lithium alkoxide solutions, J. Non-Cryst. Sol., 121, 773 (1988). 92. Sirio, C., Poncelet, O., Hubert-F'falzgraf, L. G., Daran, l. C., and Vaissermann, J., Reactions between copper P-diketonates and metal alkoxides as a route sol-to uble and volatile copper(I1) oxide precursors: Synthesis and molecular structure of Cu4(p-3,nl-OC2H40iPr)4(acac)4 and (acac)Cu(p-OSiMe~)zAl(OSiMe~)~, Poly-

91.

93.

zyxwvu z

hedron, 11, 177 (1992). Turova N. Y., Turevskaya, E. P., Kozlova, N. I., Rogova, T. V., Kessler, V. G.,

and Kucheiko, S. I., Transition metal alkoxides-precursors of oxides materials, J. Non-cryst. Sol., in the press (1992).

Thule, P. P., Raghavan,S., and Risbud, S. H., Comparison of Ba(OH)2, BaO and Baasstartingmaterialsforthesynthesis of bariumtitanatebythealkoxide method, J. Am. Cerum Soc., 70, C108 (1987). 95. Hsueh, C. C., and McCartney, M. L., Microstructural development and electrical properties of sol-gel prepared lead zirconate-titanate thin films,J. Muter, Res., 6,

94.

2208 (1991).

96.

Yanovskaya, M.I., Turevskaya, E. P., Turova, N. Y., Dambekalne, M. Y . , Kolganova, N. V., Ivanov, S. A. , Segalla, A. G., Belov, V. V., Novoselova, A. V., andVenevtsev, Y. N., Transparentceramic(Pb, La) (Zr, Ti)03 preparedby alkoxy technology, Inorg. Muter., 23, 584 (1983).

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METAL ALKOXIDES FOR ELECTROOPTICAL CERAMICS

57

97.Budd,K.D.,Dey, S. K.,andPayne,D.A.,Sol-gelprocessing of PbTiO3, PbZrO3, PZT and PLZT thin films, Br. Ceram. Proc., 36, 107 (1985). 98.Ramamurthi, S. D.,andPayne,D.A.,Structuralinvestigations of prehydrolized precursors used in the sol-gel processingof lead titanate,J. Am. Ceram. Soc., 73, 2547 (1990). 99.Cheng,M. J., ZhaoZ.,andQuangD.,Preparation of finePbTiO3powdersby hydrolysis of alkoxide, Chem. Mater., 3, 1006 (1991). 100. Seth, V. K., Schulze, W. A., and Condrate, R. A., Sr. Vibrational spectral characterization of a lead lanthanum zirconate titanate during various stages of solgel processing, Specrro. Lett., 24, 1299 (1991). 100a.AhlfAanger, R., Bertagnolli,H.,Ertel, T., Kolb, U., PeterD.,Nass,R.,and Schmidt, H., First evidence of the preformation of an inorganic network in solgel processing of lead zirconate titanate, obtained by EXAFS spectroscopy,Ber. Bun. Gesel. Phys. Chem., 95, 1286 (1991). 101. Milne, S. J., and Pyke, S. H., Modified sol-gel process for the production of lead titanate films, J. Am. Ceram. Soc., 74, 1407 (1991). 102. Philipps, J. F., and Milne, S. J., Diol-based sol-gel system for the production of thin films of PbTiO3, J. Muter. Chem., 1, 893 (1991). 102a.Amini,M.M.,and Sooks, M.D.,Preparation of single-phaseKNbo3using bimetallic alkoxides, Better Ceramics Through Chemistry W , 180, 675. (1990). 103. Nashimoto, K., and Cima, M. J., Epitaxial LiNbo3 thin films prepared by a solgel process, Mater. Lett., IO, 348 (1991). 104. Francis, L. F., Oh, Y. J., and Payne, D. A., Sol-gel processing and properties of leadmagnesiumniobatepowdersandthinlayers, J. Muter. Scien., 25, 5007 (1990). 105. Chaput, F., Boilot, J. P., Lejeune, M., Papiernik, R., and Hubert-Pfalzgraf, L. G., Low-temperature route to lead magnesium niobate, J. Am. Ceram. Soc., 73, 1355 (1989). 106. Kwak, B. S., Boyd, E. P., and Erbil, E., Metalorganic chemical vapor deposition of PbTiOs thin films, Appl. Phys. Lett., 53, 1702 (1988). 107. Trundle,C.,andBrierley,C. J., Precursors for thinfilmsoxidesbyPhotoMOCVD, Appl. Surf: Sci. 36, 102 (1989).

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POWDER SYNTHESIS AND CHARACTERIZATION

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

1.

INTRODUCTION

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Synthesis of oxide materials provides an excellent case study of the contribution of solid-state chemists to materials synthesis [1,2]. Although tailoring oxides of the desired structure and properties remains the main goal of solid-state chemistry and materials science, it is not always possible to do so. One can evolve a rational approach to the synthesis of solids [3], but there is always the element of surprise encountered not so uncommonly. A well-known example of an oxide discovered serendipitiously is NaMo,06 containing condensed MO, octahedral metal clusters [4]. This was discovered in an effort to prepare the lithium analog of NaZn2M0308. Another such serendipitous discovery is that of the phosphorus-tungsten bronze Rb$8W320112, formed by the reaction of phosphorus present in the silica of the ampule during the preparation of the RbW bronze [5]. Novel solids of the type cu$o6s8, called Chevrel phases, were also discovered accidentally. There are many examples of rational synthesis. A good example is Sialon, in which Al and oxygen were partly substituted for Si and nitrogen in silicon nitride, Si3N,. The fast Na+ ion conductor Nasicon was synthesized based on understanding the coordination preferences of cations and the nature of oxide networks formed bythem. The zero-expansion ceramic C%.5Ti2P3012,possessing the Nasicon framework, was later synthesized based on the idea that the property of zero expansion would be exhibited by two- or three-coordination polyhedra linked in to leave substantial empty space in the network [3].

61

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Traditionally, oxides have been prepared by the so-called ceramic method, which involves mixing and grinding various powders and heating them at high temperatures, with intermediate grinding when necessary. The trend nowadays is to avoid such a brute force method to obtain better control of structure, stoichiometry, and phasic purity. Some of the chemical methods of synthesis of oxides that are especially noteworthy are (1) the precursor method, (2) coprecipitation and soft chemistry routes, (3) the sol-gel method, (4) intercalation and ion exchange, and (5) topochemical methods. We examine examples of synthesis by some of these methods and briefly describe the synthesis of certain novel oxide materials, including superconducting cuprates, it should be noted that a variety ofconditions, often bordering on the extreme, such as very high temperatures or pressures, very low oxygen fugacities, and rapid quenching, have been employed in oxide synthesis. The low-temperature chemical routes, however, are of greater interest.

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II. PRECURSOR METHOD

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

CHEMICAL SYNTHESIS OF METAL. OXIDE POWDERS

z zy zyx zy zyx zy zyx zy 63

solutions can be used as precursors for preparing spinels and other complex oxides. Besides monoxide solid solutions, a number of ternary and quaternary oxides possessing novel structures can be prepared by decomposing carbonate precursors containing the different cations in the required proportions, in air or oxygen. Thus, one can prepare Ca,Fe,O, and CaFe204 by heating the corresponding carbonate solid solutions in air at 1070 and 1270 K,respectively, for about 1 h.Ca,F%O, is a defect perovskite with ordered oxide ion vacancies and has the well-known brownmillerite structure (Fig. 1) with the Fe3+ ions in alternate octahedral (0)and tetrahedral (T) sites. Two new oxides of similar compositions, Ca$0,05 and CaCo204, have been prepared by decomposing the appropriate carbonate precursors in oxygen atmosphere around 940 K. Unlike Ca,Fe,O,, in C%Mn,O,, anion vacancy ordering in the perovskite structure gives a square-pyramidal (SP) coordination around the transition metal ion (Fig.1). One can also synthesize complex oxides of the type Ca2FeCo05, Ca,Fe,,Mno~405, Ca3Fe2Mn08, and so on, belonging to the A#,O,,, family by the carbonate precursor route. In the Ca-Fe-0 system, there are several other oxides, such as CaFe407, CaFel2Olg, and CaFe204(FeO), (n = 1, 2, 3), that can, in principle, be synthesized starting from the appropriate carbonate solid solutions and decomposing them in a proper atmosphere. A good example of a multistep solid-state synthesis achieved starting from carbonate solid solution precursors is provided by the Ca2Fez-$4nXO, series of oxides (Fig. 2). The structure of both the end members, Ca2F%0, and Ca2Mn,05, are derived from that of the perovskite (Fig. 1). Solid solutions be-

zyxw zy zyxw -

-C - L a -

b

(0 )

A

-O

( b)

Figure 1 Structure of (a) C%F905 (brownmillerite) and (b) C%Mn,O,. Oxygen vacancies are shown by open circles.

64

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I

JI

BM

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

(m; (II)

tween the two oxides are expected to show oxygen vacancy ordered superstructures with Fe3+ in octahedral and tetrahedral coordinations and Mn3+ in square-pyramidal coordination, but they cannot be prepared by the ceramic method. These solid solutions have indeed been prepared starting from the carbonate solid solutions, Ca2F~-Mnx(C03),. The carbonates decompose in air around 1200-1350 K to give perovskitelike phases Ca2Fe--$fnxOcy (y < 1). The compositions of the perovskitelike oxides prepared from the carbonate precursors with x = 2/3 and 1 are Ca3Fe+4n8 and Ca3Fel,Mnl,08.,. X-ray and electron difraction patterns show that they are members of the A,,Bn03,,-1 homologous series of anion vacancy ordered superstructures with n = 3 (A3B308+J. Careful reduction of Ca3Fe2Mn08 andCa3Fe1.5Mn1.508.1in dilute hydrogen at 600 K yields Ca3Fe2Mn07.5 and Ca2FeMn05 (CagFe1.5Mn1.507.5),respectively (Fig. 2). During this step, only Mn4+ in the parent oxides is topochemically reduced to Mn3+, and Fe3+ remains unreduced. The mostprobable superstructure of Ca3F%Mn07.5 involves SP, 0, andT polyhedra along the b direction. On heating in vacuum at 1140 K,however, it transforms to the more stable brownmillerite structure with only 0 and T coordinations (Fig. 2). A variety of complex metal oxides of perovskite and related structures can be prepared by employing hydroxide, nitrate, and cyanide solid solutions precursors as well [ 8 ] . For example, hydroxide solid solutions of the general formula Ln1-Jblx(OH)3, where Ln = La or Nd and M = Al, Cr, Fe, CO or Ni) and Lal-x-,"~"y(OH)3 (where M' = Ni and M" = CO or Cu), crystallizing in the

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CHEMICAL. SYNTHESIS

OF METAL. OXIDE POWDERS

65

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

Ill. LA2C0205 AND La2Ni205 BYTOPOCHEMICAL REDUCTION

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

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IV. MO,-xW,O,BYTOPOCHEMICALDEHYDRATION

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

66

zy zyxwvu zy RA0

possesses a layered structure. MOO, can be stabilized in the WO, structure by partly substituting tungsten for molybdenum. These solid solutions can be prepared by the ceramic method (by heating MOO, and WO, in sealed tubes around 870 K) or by the thermal decomposition of mixed ammonium metallates. These methods do not always yieldmonophasicproducts,however, owing to the difference in volatilities of MOO, and WO,. We therefore sought to prepare Mo1,Wx03 by the topochemical dehydration of Mol,Wx03.H20. Topochemical dehydration, it is to be noted, is a gentle process. Since Mo03-H20 and WO,.H,O are isostructural, the solid solutions between them are prepared readily by adding a solution of MOO, and WO, in ammonia to hot 6 M HNO,. The hydrates, Mol,Wx03.H20, crystallize in the same structure as Mo03.H20 and W03.H20, with a monoclinic unit cell. The hydrate solid solutions undergo dehydration undermild conditions (around 500 K), yielding M o ~ - ~ W ~which O ~ , crystallize in the Reo3-related structure of WO,. Thermal dehydration of W03.H20 and M o , - ~ W ~ O ~ Hgives ~ O rise to oxides of Reo, structure. MOO, obtained by the dehydration of Mo03.H20, however, has a layered structure; this reaction is reported to be topotactic. The nature of the dehydration reaction was studied by an in situ electron diffraction study in which the decomposition occurs as a result of beam heating [IO]. Electron diffraction patterns clearly show how W 0 3 - H 2 0transforms to WO, topotactically with the required orientational relationships. The mixed hydrates, M O ~ - ~ W ~ O , . H also ~ O , undergo topotactic dehydration with similar orientational relations. What is interesting is that when the dehydration of Mo03.H20 is carried out under mild conditions (e.g., electron beam heating), MOO, in the Reo, structure is formed instead of the expected layered structure. The Reo, structure of MOO, is metastable and is produced only by topotactic dehydration under mild conditions. We believe that the preparation of Reo3 like MOO, by mild chemical processing is significant.

zyx zyxwv zy zyxw

V. BRONZES OF MO,-flxO, REACTION

BYASOLID-STATE

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

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CHEMICAL SYNTHESIS OF METAL OXIDE POWDERS

VI.

67

INTERCALATION

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

VII.

ION EXCHANGE

Ion exchange is employed effectively to synthesize new oxides [l]. Thus Na in p-alumina is readily exchanged by other cations. Such exchange reactions are well known in silicates, especially zeolites. Examples of simple ion-exchange reactions are as follows: U 1 0 2 + AgN03 p "AgA102 + KNO3 a - LiFeOz + CuCl +CuFeOz + LiCl NaCrO;! + LiN03 +LiCrOz + NaCl

It has been possible to exchange Li in LiNbO, by protons to obtain HNbO,. Such alkali metal-proton exchange reactions are common in layered oxides (e.g., H2Ti307 and HLaNbO,).

VIII.

ALKALI FLUX METHOD

zyx zyx

One of the early examples of the use of strong alkaline media for the synthesis of oxides is that of Pb2Ru2-.Pbx07y, which has the pyrochlore structure. The method stabilizes higher oxidation states of metals. Alkali carbonate fluxes have been traditionally used to prepare many oxides (e.g., LaNiO,). Recently, superconducting La,CuO, was prepared by the reaction of L%O, and CuO in a molten mixture of NaOH and KOH around 520 K [12]. Superconducting Ba,_.K$iO, has been prepared by the use of molten KOH [13].

IX.

INTERGROWTH STRUCTURES

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

68

zyxwvut zy zyxwvu zyxwv zy zy zyxwvu z RA0

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

zy

Figure 3 High-resolution electron microscopic image of the n = 4.5 memberinthe series.

A,,B,,O,,,

zyxwvu z zyxwvu zyxwvut zyxwv zyxwvu zyxwv zy z

CHEMICAL SYNTHESIS OF METAL OXIDE POWDERS

69

dered solids are found, with the b parameter of the unit cell ranging anywhere from 58.6 A in the n = 4.5 compound to a few thousand angstroms in longer period structures. These solids seem to belong to the class of infinitely adaptive structures. There is a family of oxides of the general formula Bi@A,1B,03,3, discovered by Aurivillius, in which the perovskite slabs, (An-1Bn03n+1)2-,n octahedra thick, are interleaved by (Bi202)2+layers (Fig. 4). Typical members of this family are Bi2W06 (n = l), Bi3Til,Wo.,0, (n = 2), Bi4Ti3CrO12(n = 3), and Bi,Ti3Cro15 (n = 4), andtheyhavebeeninvestigatedindetailby HREM. These oxides form intergrowth structure of the general formula Bi4Arn+&3,,, + n03(rn+ involving fl)+ 6; alternate stacking of two Aurivillius oxides with different n values; the method of preparation' simply involves heating the compo-

0 0

eo 0

eo 0 0

zyxwvutsrq

zyxwvutsrqpo

Bi7M5021

zyxw

zyxwvut Bi9M7027

"

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

70

zyxwvut z zyxwvu zy zy zyxwvu zyxwvu zyxw RA0

nent oxides or carbonates of metals. Ordered intergrowth structures with (m,n) values of (1, 2), (2, 3), and (3, 4) have been fully characterized by x-ray diffraction and HREM. What is most amazing is that such intergrowth structures with long-range order are indeed formed, although either member (mor n) can exist as a stable entity. These materials seem to be truly representative of recurrent ordered intergrowth. The periodicity found in the recurrent intergrowth solids formed by the Aurivillius family of oxides is indeed impressive. The relative ease withwhich WO, forms tetragonal, hexagonal, or perovskite-type bronzes by interaction with alkali and other metals is well known. The new family ofintergrowthtungsten bronzes (ITB) involving the intergrowth of nWO, slabs and one to three strips of the hexagonal tungsten bronze (HTB) is of relevance to our discussion here. In these intergrowth tungsten bronzes of the general formula MxW03, x is generally 0.1 or less, and depending on whether the HTB strip is one or two tunnels wide, ITB are classfied as belonging to (0, n) or (1, n) series (Fig. 5). HTB strips of two-tunnel width seem to be most stable in ITB, and many ordered sequences of the (0, n) and the (1, n) series have been identified. Recently, ITB phases of Bi have been characterized, and in this system the HTB strips are always one tunnel wide, Displacement of adjacent tunnel rows caused by the tilting of WO, octahedra often results in doubling of the long-period axis of the ITB. Evidence for the ordering of the intercalating Bi atoms in the tunnels has been found in terms of satellites around the superlattice spots in the electron diffraction patterns. Among the other systems exhibiting ordered intergrowth, special mention must be made of hexagonal barium femtes MpYq (M = BaFeI2Ol9 and Y = Ba2Me,2022, where Me is Zn, Ni, Mg, and so on). A large number of intergrowth structures of this family have been identified.

zyxw zyxw zyxwvu

X.

SUPERCONDUCTING CUPRATES

The discovery of a superconducting cuprate with a T, above 77 K created a sensation in early 1987. Wu et al., who announced this discovery first, made measurements on a mixture of oxides containing Y, Ba, and Cu. In this laboratory, we independently worked on the Y-Ba-Cu-0 system on the basis of solid-state chemistry [15]. We knew that Y,Cu04 could not be made and that substituting Y with Ba in this cuprate was not the way to proceed (unlike in La2-€!axCu04). We therefore tried to make Y3Ba3Cu5OI4by analogy with the known La3Ba3Cu6OI4and varied the Y/Ba ratio as in Y3,rBa3+xC~6014. When x = 1, we obtained YBa2Cu3 (T, 90 K). We knew the structure had to be that of a defect perovskite from the beginning, because of the route we followed for the synthesis. We briefly examine some preparative aspects of the various types of cuprate superconductors [16]. The cuprates are ordinarily made by the traditional ce-

zyxwv

2:

zyxwvu z

CHEMICAL, SYNTHESIS

OF METAL, OXIDE POWDERS

71

zy z zyxwvu zyxwvu I I ,6)

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

z

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

72

zyxwvutsr z RA0

provides a homogeneous dispersion of the various component metals when a solution containing the metal ions is transformed into a gel by adding an organic solvent, such as glycol or an alcohol, often in the presence of other chemicals, such as organic amines. The gel is then decomposed at relatively low temperatures to obtain the desired oxide, generally in fine particulate form. Materials prepared by such low-temperature methods may need to be annealed or heated under suitable conditions to obtain the desired oxygen stoichiometry as well as the characteristic high T,. A total of 124 cuprates of the type YBa2Cu,08, lead cuprates, and bismuth cuprates have all been made by this method; the first two are particularly difficult to make by the ceramic method. Coprecipitation of all the cations in the form of a sparingly soluble salt, such as carbonate, ina proper medium, followed by thermal decomposition, has been employed by many workers to prepare cuprates. One of the problems with the bismuth cuprates is the difficulty in obtaining phasic purity (minimizing the intergrowth of the different layered phases). The glass or the melt route has been employed to obtain better samples. The method involves preparing a glass by quenching the melt; the glass is then crystallized by heating it above the crystallization temperature. Thallium cuprates are best prepared in sealed tubes (gold or silver). Heating T1203 with a matrix of the other oxides (already heated to 1100-1200 K) in a sealed tube is preferred by some workers. It is important that thalliumcuprates are not prepared in open furnaces since T1203, which readily sublimes, is highly toxic. To obtain superconducting compositions corresponding to a particular copper content (number of Cu02 sheets) by the ceramic method, one often must start with various arbitrary compositions, especially with the T1 cuprates. The real composition of a bismuth or a thallium cuprate superconductor is not likely to be anywhere near the starting composition. The actual composition can be determined by analytical electron microscopy and other methods. Heating oxidic materials under high oxygen pressures or in flowing oxygen often becomesnecessary to attain the desired oxygen stoichiometry. Thus h 2 C u 0 4 and La&a,-,Sr,Cu2O6 heated under high oxygen pressure become superconducting,with T, of 40 and 60 K, respectively.With the 123 compounds, one of the problems is that it loses oxygen easily. It therefore becomes necessary to heat the material in an oxygen atmosphere at an appropriate temperature below the orthorhombic-tetragonal transition temperature. Oxygen s t 6 ichiometry is not a problem with the bismuth cuprates, however. The 124 superconductorswere first preparedunder high oxygenpressures. It was later found that heating the oxide or nitrate mixture in the presence of N%02 in flowing oxygen is sufficient to obtain 124 compounds.Superconducting Pb cuprates, on the other hand, can only be prepared in presence of very little oxygen(N2witha small percentage of 02).For the electron superconductor Nd2-xCexCu04, it is necessary to heat the material in an oxygen-deficient at-

zyxw zyxwvu zyx

zyxw

z zy zyxwv zyxw zyxw zy

CHEMICAL SYNTHESIS OF METAL OXIDE POWDERS

73

mosphere; otherwise, the electron given by Ce merely gives an oxygen-excess material. It may be best to prepare Nd2_,Ce,Cu04 by a suitable method (say, decomposition of mixed oxalates or nitrates) and then reduce it with hydrogen. Many thallium cuprates, as prepared by the sealed tube method, have excess oxygen; they become superconducting only on heating in vacuum or hydrogen. Several other novel strategies have been employed for the synthesis of superconducting cuprates. For example, a Eu-Ba-Cu alloy precursor has been oxidized to obtain EuBa$u,O, [17], and hyponitrite precursor has been employed to prepare YBa,Cu,O, [18]. More interesting in the synthetic strategies are those in which structural and bonding considerations are involved in the synthesis. One such example is the synthesis of modulation-free superconducting bismuth cuprates [19]. Superconducting bismuth cuprates, such as Bi2CaSr2Cu208, exhibit superlattice modulation. Since such modulationhad something to do with the oxygen content in the Bi-0 layers and lattice mismatch, Bi3+ was substituted partly by Pb2+ to eliminate the modulation without losing the superconductivity.

XI.

CONCLUDINGREMARKS

The area of oxide synthesis has become trulyextensive, with newertypes of materials being prepared everyday by employing a variety of novel methods [20]. We have touched on only some of the important methods, and there are many more. For example, we have not discussed the commonly employed coprecipitation method (involving the simultaneous precipitation of the cations as carbonates and oxalates, for example) followed by decomposition or the sol-gel method. This method does not always give precursor solid solutions, but coprecipitated materials on decomposition readily give the desired oxides. Another novel method of preparing oxide powders is by the combustion method, involving the spontaneous ignition of a mixture of metal nitrates in presence of a fuel, such as urea or glycine [20]. This method can be employed to prepare powders of most oxides, including dielectrics and superconductors. The use of templatemolecules to synthesizeporous solids involving silicates andphosphates is noteworthy. The ingenuity with which properties of oxides are modified drastically by appropriate substitutions or by the modification of the structure forms an important part of synthetic strategies. Other methods of interest are those involving fine particles, vapor transport, and electrochemical methods

1201. ACKNOWLEDGMENT The author thanks the Department of Science and Technology and the Indo-European Economic Community collaborative program for support.

74

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REFERENCES

1. Rao, C. N. R., and Gopalakrishnan, J., New Directions in Solid State Chemistry, Cambridge University Press, London, 1989. 2. Rao, C. N. R., and Gopalakrishnan, J., Acc. Chem. Res., 20, 228 (1987). 3. Roy, R., Solid State lonics, 32133, 3, (1989). 4. Torardi, C. C., and McCarley, R. E.,J. Am. Chem. Soc., 101, 3963, (1979). 5. Giroult, J. P., Goreand, M., Labbe, P. H., and Raveau B., Acta Crystallogr., 1336, 2570, (1980). . 6. Vidyasagar, K., Gopalakrishnan, J., and Rao, C.N.R., Inorg. Chem., 23, 1206, (1984). 7. Rao, C. N. R., Gopalakrishnan, J., Vidyasagar, K., Ganguli, A. K., and Ramanan, A. , J. Mater. Res., 1, 280 (1986). 8. Vidyasagar, K., Gopalakrishnan, J., and Rao, C. N. R., J . Solid State Chem., 58, 29 (1985). 9. Vidyasagar, K., Reller, A., Gopalakrishnan, J., and Rao, C. N. R., J. Chem. Soc. Chem. Commun., 7 (1985). 10. Ganapathi, L., Ramanan, A., Gopalakrishnan, J., and Rao, C. N. R., J. Chem. Soc. Chem. Commun., 62 (1986). 11. Ganguli, A. K., Gopalakrishnan, J., and Rao, C. N. R., J. Solid State Chem., 74, 228 (1988). 12. Ham, W. K., Holland, G. F., and Stachy, A. M., J . Am. Chem. Soc., 110, 5214 (1988). 13. Schneemeyer, L. F., Thomas, J. K., and Siegrist, T., Nature, 335, 421 (1988). 14. Rao, C.N.R., and Thomas, J. M., Acc. Chem. Res., 18, 113 (1985). 15. Rao, C. N. R., Ganguly, P., Raychaudhuri, A. K., Mohan Ram, R. A., and Sreedhar, K.,Nature, 326, 856 (1987). 16. Rao, C. N.R., Nagarajan, R., and Vijayaraghavan, R., Supercond. Sci. Technol., 6, 1(1992). 17. Matsuzaki, K., Inone, A. , Kimura, H., Aoki, K., and Masumoto, K.,Jpn. J . App. Phys., 26, L1310 (1987). 18. Horowitz, H. S., Mclain, S. J., Sleight, A. W., Druliner, J. D., Gai, P. L.,Van Kavelaar, M. J., Wagner, J. L., Biggs, B. D., and Poon, S. J., Science, 243, 66 (1989). 19. Manivannan, V., Gopalakrishnan, J., and Rao, C.N.R., Phys. Rev., B.43, 8686 (1991). 20. Rao, C.N.R., Mater. Sci. Eng., in print (1992).

zyxwvut zyxwvu zyxwvut zyxwvu

4

z zyx zy zy zyxwv zyxwvu Multicomponent Ceramic Powders T. Mah

UES,Inc., Dayton, Ohio

E. E. Hermes

Wright Paterson Air Force Base, Ohio

K. S. Mazdiyasni

General Atomics, San Diego, California

1.

INTRODUCTION

zyxwvu zyxwvu

The chemical processing of ceramics, especially ceramic powder syntheses, has drawn a considerable amount of attention over the past two decades. The reason for this is the demand for reliable and advanced ceramic components for high-performance applications. This increased interest in the chemical synthesis of ceramic powders is illustrated by the vast number of publications and ongoing investigations [ 1-91. This chapter focuses only on the syntheses of oxide ceramic powders. The precursors of these powders are metal-organic compounds, mainly metal alkoxides. The different varieties of ceramic powders synthesized by mixed metal alkoxide precursors are the focal points of this chapter. Numerous references on the fundamental principles of sol-gel processing are available in the literature and within other chapters of this book [10-12]. This chapter is divided into three major sections: (1) metal alkoxide precursor synthesis methods used by the present authors, (2) silicate powder syntheses, and (3) nonsilicate powder syntheses.

II. PRECURSOR SYNTHESIS

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

zy zy zyxwv zyxwv zyxwv zy

76

MAHETAL..

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

A.

Ammonia Method

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

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

B.

Metal/Alcohol Reaction Method

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

HgC12 > M(OR), exothermic

+ EH2 2

zyx

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

C.

Metal Halide/AlcoholReaction Method

A typical example of this method is the synthesis of various alkoxides of silicon [19]:

S i c 4 + 4ROH

exothermic

'Si(OR)4 + 4HC1

where R is the ethyl, n-propyl, isopropyl, and so on, group.

zyxw z zyxw zyxwvu zyxwvu zy zyxwvu zyxwvu

MULTICOMPONENT CERAMIC POWDERS

D.

77

EsterExchangeReaction

A valuable method for converting one alkoxide to another is an ester exchange reaction [20]. This method is particularly suited for the preparation of the tertiary butoxide from the isopropoxideand r-butyl acetate. The reaction is as follows:

M(OR)4 + 4R’OOCCH3 _ _ j M(OR’)4 + 4ROOCCH3

where M is zirconium, titanium, hafnium, and thorium, for example, R is the isopropyl group, and R’ is the tert-butyl group. Since there is a large difference between the boiling points of the esters, the fractionation is simple and quite rapid. Another advantage appears to be the lower rate of oxidation of the esters compared with that of alcohol.

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

M(OR)4 + 4R‘OH-

M(OR‘)4 + 4ROH

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

111.

zyxwv

ALKOXY-DERIVEDCERAMICPOWDERS

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

*

S ZrO2

+ 2ROH + olefin

78

zyxwvu zy zy MAH ET AL.

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

M(OR),

-

+ nH2O +M(OH), + nR(0H)

hydrolysis

zyxwvu z zyxw zy zyxwvu zy

M(OH),

MO,/;! + "nH 2 0 2

dehydration

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

A.

Silicate Pow ders

Within the category of silicate powders there are two different varieties: (1) glass and/or glass-ceramics and (2) crystalline. The first variety includes various compositions of glasses and glass-ceramics (e.g., lithium aluminosilicate and magnesium aluminosilicate), and mullite and zircon are typical examples of silicates that belong to the latter variety.

1. Multicomponent Silicate Glass and Glass-Ceramics Hydrolyses of multicomponent metal alkoxides were reported by Dislich [25,26] for silicate glass and by the present authors [27,28] for glass-ceramics. Dislich [25] synthesized the eight-component glass through a complexation of mixed alkoxides, followed by hydrolysis andcondensation. The processing flowchart for the synthesis of magnesium aluminosilicate (MAS) (with Li, Zr, and Nb) glass-ceramic oxide powder, hydroxides, and their mixtures for ce-

MULTICOMPONENT CERAMIC POWDERS

79

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

2. Multicomponent Crystalline Silicates

zy

zy

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

zyxwvuts

Figure 1

Processingflowchart for thesynthesis of magnesiumaluminosilicate.

80

zyxwvut zy zyx zy zyxwvut zy MAH ET AL..

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

zyxwvu

6Al(OC3H7)3 + 2Si(OC3H7)4 + xH20 H20

+

NH3

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

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

*

dry iso-

2Si02 + 13H20

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

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MULTICOMPONENT CERAMIC POWDERS

81

The difference between acid-versus base-catalyzed hydrolysis of mixed TEOS and Al(OC3H7)3 can be accounted for by the different reaction mechanisms of TEOS hydrolysis [l l]. Nucleophilic hydrolysis and condensation in alkaline solutions tend to produce highly cross-linked species, which may not be completely hydrolyzed. In contrast, in acid solution, the electrophilic reaction mechanism favors the production of weakly cross-linked species that tend to be completely hydrolyzed [35]. The specific surface area of the as-calcined mullite powder obtained under acid hydrolysis conditions is 197 m2/g. Clusters of particles develop during concentration of the hydroxide slurry. These particles polymerize on further drying, causing bound -OHgroups in the clusters, thereby resulting in hard agglomerates instead of fine powder. One can make a compromise through the partial hydrolysis of TEOS [36- 391 and then add AI(OC3H7)3 to accomplish complete hydrolysis and yet achieve fine powder sizes.

B.

Nonsilicate Pow ders

A few varieties of nonsilicate oxide ceramic powderssynthesizedthrough

alkoxide processing are presented here. These oxide powders were developed for the use of electronic, optical, and high-temperature structural applications. For each material, we start with a brief description of the synthesis, which is followed by powder characteristics (e.g., particle sizes, morphologies, and size distributions) and densification behavior andsome properties of dense material.

1. Spinel The precursor alkoxide syntheses for spinel (MgA1204) were carried out through the metal/alcohol reaction methodusing HgC12 as acatalyst. This methodwas described in the previous section. The two metal alkoxides, Al(OC3H7)3 and Mg(OC2H5)2, were mixed together and gently heated in isopropyl alcohol until solid Mg(OC2H5)2 was completely dissolved and reacted, producing a clear solution, and then allowed to cool to room temperature [34]. During the refluxing process, magnesium and aluminum double alkoxide was formed, The formation and structural characterization of the MgA12(0R)8 double alkoxide can be found in references [10,29,30]. The double alkoxide was readily hydrolyzed by water, without acid or base catalyst. No differential hydrolytic decomposition of mixed alkoxide was found in this system to hinder the formation of ultrafine homogenous spinel powder. The XRD analysis (Fig. 3) of the ultrafine powder, obtained after calcination at 550"C, shows amorphous characteristics. The powder was calcined at 1000°C for 1 h, and XRD analysis showed broad peaks of spinel, which is a characteristic diffraction pattern for fine particles. Distinct, sharp XRD peaks of spinel can be observed after the powder is hot pressed at 1550°C. Elemental analysis of the powder calcined at 550°C gave a 1:l MgO/A1203 ratio. Emission spectrographic analysis of the as-calcined powder indicated that no measurable impurities were pre-

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Figure 2 Transmissionelectronphotomicrographs of mullitepowders (a) as-prepared, (b) calcined at 6OOOC for 1 h, (c) calcined for 24 h with tumbling, and (d) calcined for 24 h without tumbling.

MULTICOMPONENT CERAMIC POWDERS

83

84

MAH ET AL.

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MULTICOMPONENT CERAMIC POWDERS

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sent. The surface area of the as-calcined powder was 260 m2/g, which corresponds to an average particle size of 6.5 nm. The transmission electron photomicrograph of the as-calcined powder (Fig. 4) showed the particle size to be of the order of 10 nm, which is in agreement with the calculated value. The average particle size of the powder, heat treated at 1000°C for 1 h, increased to about 30 nm, and also the particles were faceted.

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2. Electronic Ceramics A few multicomponent electronic ceramics, fabricated through metal alkoxide processing, are described in this section. PZT and PLZT. The development of polycrystalline lead lanthanum zirconate-titanate (PLZT) electronic ceramic monoliths, which fully transmit incident light, requires methods for controlling stoichiometry, impurity content, porosity, grain size, and so on. Alkoxy-derived PLZT powders were prepared by hydrolytic decomposition of mixed-metal alkoxides [40]. The Zrand Ti alkoxides were synthesized by the ammonia method, and the lanthanum trisisopropoxide was synthesized by themetal/alcoholreaction method, which were described in the previous section. The Pb alkoxide was prepared by the reaction of anhydrous lead acetate, Pb(C2H302)2, with sodium isoamyloxide, NaOCsHl I [40]: Pb(C2H302)2 + 2NaOR

Pb(OR)2 + 2NaC2H302

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

86

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Figure 4 Transmission electron micrographsof dispersed spinel powder calcined at (a) 550°C and (b) 1000°C inair for 1 h.

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Figure 5 Transmissionelectronmicrographs 500°C for (a) 15 minutes and (b) 1 h.

of PLZT particulatescalcinedat

88

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

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MULTICOMPONENT CERAMIC POWDERS

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

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( 98% of theoretical density (t.d.) [92]. The oxygen contents of the pyrolyzed powders were in the range of 1.5-2.0 wt% for aluminum nitride, calcium cyanamide, and the transition metal compounds investigated. The electrochemical synthesis of polymeric precursors is characterized by a broad versatility. Most of the metals can be handled easily and electrolytically dissolved and transferred into polymericprecursors; thus this processing scheme seems to be a general approach and an alternative to already known methods. In addition, all precursor solutions can be mixed with one another, and therefore a wide range of new composites or solid solutions is possible in the near future.

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124

Table 1

NonoxidesDerivedbyElectrochemicallySynthesizedPrecursors

Metal dissolved Ceramic yield AI Ti

zr

(%)

42 20

Calcination gas Product NH3 NH3 N2 NH3 NH3 N2

AIN TiN Ti(C, N) ZrN CrN

Ar

C4C2 TaC CaCN2 MgCN2

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Ta Ca Mg Y

41 40

31 34 21 39

NH3 NH3 NH3 NH3

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REFERENCES

1. Seyferth, D., Wiseman,G. H., Schwark, J. M., andYu,Y.-F., Am. Chem. Soc., 71, 143-155 (1988). 2. Wynne, K. J., and Rice, R. W., Annu. Rev. Mater. Sci., 14, 297-334 (1984). 3. Rice, R.W., Am. Ceram. Soc. Bull., 62, 889-892 (1983). 4. Pouskopuleli, G.,Ceram. Int., 15, 213-229(1989). 5. Walker, B. E., Rice, R.W., Becher, P. F., Bender B. A., and Coblenz, W. S., Am. Ceram. Soc. Bull., 62, 916-923 (1983). 6. Billy, M., Keramische Nitride und Oxinitride, Handbuch der Keramik II K 2.10, Verlag Schrnid, Germany, (1990). 7. Szweda, A., Hendry, A., and Jack, K. H.,Proc. Br. Ceram. Soc., 31, 107 (1981). 8. Mehrwald, K. M., Ber. Dtsch. Keram. Ges., 46, 57-64 (1969). 9. Harris, L. A., Kennedy, C. R., Wei, G. C., and Jeffers, F. P., J. Am. Ceram. Soc., 67, (1984) p. C 121. 10. Wei, G. C., Kennedy C. R., and Harris, L. A., Am. Ceram. Soc. Bull., 63, 1054 (1984). 11. Kuroda, K., Tanaka, Y., Sugahara, Y., and Kat0 C., in (C. J. Brinker, D. E Clark,

.

and D. R. Ulrich, eds.), Better Ceramics Through Chemistry Ill, M R S Proc. Vol. 121, Pittsburgh, PA, (1988), 575-580. 12. Belau, A., Muller, G., Ber. Dtsch. Keram Ges., 65, 122 (1988). 13. Haggerty, J. S., Lightfoot, A., Ritter, J. E., Nair, S. V., and Gennari, P., Ceram. Eng. Sci Proc., 9, 1073 (1988). 14. Grieco, M. J., Worthing, F. L., andSchwartz,B., J . Electrochem. Soc., 115, 525-531 (1968). 15. Kirnura, I., Hotta, N., Nukui,H.,Saoito,N.,andYasukawa, S., J . Mater.Sci. Lett., 7, 66-68 (1988). 16. Prochazka, S., and Creskovich, C., Am. Ceram. Soc. Bull., 57, 579 (1978).

zyxwvu zyxw z zyxwvut zyxw

CHEMICAL. SYNTHESIS OF NONOXIDES

i25

17. Bauer, R. A., Smulders, R., Brecht, J. G. M., van der Put, P. J., and Schoonman, J., J. Am. Cerum. Soc., 72, 1301 (1989). 18. Vissokov, G. P., and Brakalov, L. B., J . Muter. Sci., 18, 201 1 (1983). 19. Ishizaki, K., Egashira, T., Tanaka, K., and Celis, P. B., J . Muter. sei., 24, 3553 (1989). 20. Baba, K., Shohata, N., and Yonezawa, M., Appl. Phys. Lett., 54, 2309 (1989). 21. Ho, P., Buss, R. J., and Loehman, R. E., J. Muter. Res., 4, 873 (1989). 22. Okabe, Y., Hojo, J., and Kato, A., Yogyo Kyokuishi, 85, 173 (1977). 23. Schulz, O., Kanziora, D.,and Hausner, H.,(eds.) Ceramic Powder Processing Science, Roc. 1st Int. Conf. Ceramic Powders Processing Science, American Ceramics Society, Westerville, OH, 1988. 24. Somiya, S., Suzuki, K., and Yoshimura, M., Adv. Cerum., 21, 279 (1987). 25. Volpe, L., and Boudart, M., J. Solid State Chem., 59, 332 (1985). 26. Exell, S. F., Roggen, R., Gillot, J., and Lux, B., in 2nd Znt. Conf. Fine Particles, (W. E. Kuhn ed.), Electrochem. Soc., Pennington, NJ, (1974) p. 165. 27. Hojo, J., Oku, T., and Kato, A., J . Less Common Met., 59, 85 (1978). 28. Kipling, S. F., J . Chem. Soc., 125, 2291-2297 (1924). 29. Yajima, S., Hayashi J., and Omori, M., Chem. Lett., 931 (1975). 30. Yajima, S., Okamura, K., Hayashi,J.,and Omori M., J. Am. Cerum. Soc., 59 324-327 (1976). 31. Yajima, S., Hayashi, J., Omori, M.,and Okamura, K., Nature, 261, 683-685 (1976). 32. Ishikawa, T., Shibuya, M., and Yamamura T., J. Muter. Sci., 25, 2809 (1990). 33. Schilling, C. L., Wesson, J. P., and Williams, T. C., Am. Cerum. Soc. Bull., 62, 912-915 (1983). 34. Schilling, C. L., Br. Polym. J., 18, 355-358 (1986). 35. Schilling, C. L., Wessel, J. P., and Williams, T. C., J . Polym. Sci. Polym. Symp., 70, 121-128(1983). 36. Peroz, M., Ann. Chim. Phys., 44, 315 (1830). 37. Klemser, O., and Naumann, P., Z. Anorg. Allg. Chem., 298, 134-141 (1959). 38. Mazdiyasni K. S., and Cooke C. M., J. Am. Cerum. Soc., 56, 628-633 (1973). 39. Segal, D. L., Br. Cerum. Trans.. J., 85, 184-187 (1986). 40. Segal, D.L., Chem. Znd., 544-545 (1985). 41. Stock, A., and Somieski, K., Ber. Dtsch. Chem. Ges., 54, 740-758 (1921). 42. Larsson, E., and Bjellerup, L., J. Am. Chem. Soc., 75, 995-997 (1953). 43. Laineet, R. M., Blum, Y. D., Chow, A., Hamlin, R., Schwartz, K. B., and Rowecliffe, D.J., Polymer Preprints, 28, 393-395 (1987). 44. Barant, V., Orgunosilicon Compounds, Academic Press, New York,1965,pp. 77-8 1. 45 Blum, Y.D., Schwartz,K.B.,and Lain, R.M., J. Muter. Sci., 24, 1707-1718 (1989). 46. Matsubayashi, S., Saiko, G., and Kubo, H., in Ceramic Powder Processing Science, (H. Hausner, G. C. Messing, and A. Hirano, eds.), Deutsche Keramische Gesellschaft, Koln, 1989, pp. 825-831. 47. Seyferth, D., Wiseman, G. H., and Prud’Homme, C., J . Am. Cerum. Soc. Commm., C-l3 (1983).

zyxwv zyxwvu zyx zyxwvut zyxwvu zyxw

zyxwvut

#

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48. Tamio, S., and Hiroyuki, T., FR 2 583 744-A1 (1986). 49. Legrow, G. E., Lim, T. F., Lipowitz, J., and Reaoch, R. S., Am. Ceram.Soc. Bull., 66, 363-367 (1987). 50. Seyferth, D., and Wiseman, G. H., J. Am. Ceram Soc., 67, C-l32 (1984). 51. Seyferth,D.,andWiseman,G.H.,in UltrastructureProcessing of Ceramics, Glasses and Composites, (L. L. HenchandD.R.Ulrich, eds.), Wiley Interscience, New York, 1984, p. 265. Jackson,T. B., andCutler, R.A., J. Am.Ceram. Soc., 72, 52. Virkar, A.V., 2031-2042 (1989). 53. Schwetz, K. A., in Progress in Nitrogen Ceramics, (F. L. Riley, ed.), Martnius Nijhoff, Boston, MA (1983), pp. 245-252. 54. Buhr, H., Muller, G., Wiggers, H., Aldinger, F., Foley, P., and Roosen,A., J. Am. Ceram. Soc., 74, 718 (1991). 55. Jaschek, R., and Russel, C., Coat. Surf. Technol., 45, 99-103 (1991). 56. Teusel, I., and Russel, C., J. Mater. Sci., 25, 3531-3534 (1990). 57. Jaschek, R., and Russel, C., J. Non-Cryst. Solids, 135, 236-241 (1991). 58. Jaschek, R., and Russel, C., Thin Solid Films, 208, 7-10 (1992). 59. Baker, R. T., Belt, J. D., Reddy, G. S., Roe, D.C., Staley, R. H., Tebbe, F. N., and Vegas A. J., in M R S Symp. Proc., Vol. 121, Better Ceramics Through Chemistry III (C. J. Brinker, D. F. Clark, and D. R. Ulrich, eds.), Pittsburgh, PA, 1988, pp. 471476. 60. Wiberg, E., and Amberger,E., Hydrides of the Elements of the Main Group I-N, Elsevier, Amsterdam, 1971. 61. Wiberg, E., and May, A., Z. Naturforsch., lob, 229-238 (1955). 62. Einarsrud, M. A., Rhine, W. E., and Cima, M. J. Proc. ECerS (G. de With, R. A. Terpstra,andR.Metselaar,eds.),Elsevier,London,NewYork 1989, pp. 1.38-1.42. 63. Ochi, A., Bowen, H. K., and Rhine, W. E., in MRS Symp. Proc., Vol. 121, Better Ceramics Through ChemistryIll (C. J. Brinker, D. F. Clark, and D. R. Ulrich, eds.), Pittsburgh, PA, 1988, pp. 663666. Jensen,K.F., Chem. Mater., I , 339-343 6 4 . Gladfielder, W. L., Boyd, D.C.,and (1989). 65. Sugahara, Y.,Onuma, T., Tanegashima, O., Kuroda, K., andKato, C., J. Jpn. Ceram. Soc., 100, 101-103 (1992). 66. Interrante, L.V., Carpenter E, L. E., Whitmarsh, C., Lee, W., Garbanskas, M., and Slack, G. A., in M R S Symp. Proc. Vol. 73, Better Ceramics Through Chemistry I1 (C. J. Brinker D. F. Clark, and D. R. Ulrich, eds.), Pittsburgh, PA, 1986, pp. 359-366. 67. Lappert, M. F., Metal and Metalloid Amides, J. Wiley & Sons, New York, 1980, p. 99. 68. Maya, L., Adv. Ceram. Mater., I , 150-153 (1986). 69. Frigo, D. M., Reuvers, P. J., Bradley, D.C., Chudzynska, H., Meinena,H.A., Kraaijkamp, J. G., and Timmer, K., Chem. Mater., 3, 1097-1101 (1991). 70. Cucinella, S., Dozzi, G., Busetto, C., andMazzei,A., J. Organometal. Chem., 113, 223-243 (1976).

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SYNTHESIS CHEMICAL.

OF NONOXIDES

127

71. Schleich, D. M., U.S. Patent 4,767,607 (1988). J . Mater.Sci.Lett., 9, 222- 224 72. Riedel, R., Petzow,G.,andKlingebielU., (1990). 73. Maya, L., and Angelini, P., J. Am. Ceram. Soc., 73, 297 (1990). R., Ceram. Eng. Sci.Proc., 1171 74. Bender,B. A., Rice,R. W., andSpann,J. (1985). 75. Tanigushi, I., Harada, K., and Maeda, T., Japan Kokai 76 53 OOO (1986). 76. Narula, C. K., Paine, R. T., and Schaeffer, R., in (C. I. Brinker, D. E. Clark, and D.R.Ulrich,eds.), M R S Pr. Vol. 73, BetterCeramicsThroughChemistry I1 Pittsburgh, PA, 1986, pp. 383- 388. Chem. Mater., 3, 77. Paciorek,K. J.L., Nakahara,J. M., andHoferkamp,L.A., 83- 87 (1991). 78. Rye, R. R., Tallant, D. R., Borek, T. T., Lindquist, D. A., and Paine, R. T., Chem. Mater., 3, 286- 293 (1991). 79. Fazen, P. J., Remsen, E. E., and Sneddon, L. G., Polymer Preprints, 32, 544-545 (1991). 80. Paine, R. T., Janik, J.F., Borek, T. T., Lindquist, D. A., Duesler, E. N., Smith, D. M., Kodas, T. T., and Datye, A. K., Polymer Preprints, 32, 546- 547 (1991). W., in M R S Proc.Vol. 121, BetterCeramics 81. Seyferth, D., andSmithRees,

zyxw zyxwvu zy

82. 83. 84.

85. 86. 87. 88. 89. 90. 91. 92.

93.

94. 95.

zyx

Through Chemistry Ill (C. J. Brinker, D. E. Clark, and D. R. Ulrich, eds.),Pittsburgh, PA, 1986 pp. 449- 454. Seyferth, D., and Rees, W. S., Chem. Mater., 3, 1106- 1 116 (1991). Schroeder, H., Reiner, J. R., and Knowles, T. A., Inorg. Chem., 2, 393 (1963). Niedenzu, K., and Buschbeck, K.-C., Gmelin Handbook of Inorganic Chemistry, 8th ed., Vol. 54, Boron Compounds: B-H Compounds, Springer, Berlin, 1979, pp. 151 ff. Maya, L., in Better Ceramics Through Chemistry III (C. J. Brinker, D. E. Clark, and D. R. Ulrich, eds.) M R S hoc. Vol. 121, Pittsburgh, PA, 1988, pp. 455- 460. Seyferth, D., and Mignani, G., J. Mater. Sci. Lett. 7, 487- 488 (1988). Lappert, M. F., Power, P. P., Sanger, A. R., and Srivastava, R. C.,Metal and Metalloid Amides, Ellis Horwood, Chinchester, 1980. Brown, G. M., and Maya, L., J . Am. Ceram. Soc., 71, 78- 82 (1988). Maya, L., Inorg. Chem., 25,4213- 4217 (1986). Maya, L., Inorg. Chem., 26, 1459- 1462 (1987). Maya, L.,in M R S Symp. Proc. Vol. 73, Better Ceramics Through Chemistry I1 (C. J. Brinker, D. F. Clark, and D. R. Ulrich, eds. Pittsburgh, PA, 1986, p. 401. Seibold, M., andRussel, C., inMRSSymp. Proc. Vol. 121, BetterCeramics Through Chemistry I l l (C. J. Brinker, D. F. Clark, and D. R. Ulrich, eds.), Piasburgh, PA, 1988, pp. 477- 482. Seibold, M., Viemeusel, U., and Russel, C., in Ceramic Powder Processing Seience (H.Hausner,G.C.Messing,and S. Hirano,eds.),DeutscheKeramische Gesellschaft, Koln, 1989, pp. 173- 179. Seibold, M., and Russel, C., J . Am. Ceram. Soc., 72, 1503- 1505, (1989). Russel,C.,Hofmann,T.,Kulig,M.,andSeibold., Silikattechnik, 40, 425- 429,

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This Page Intentionally Left Blank

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Techniques for Characterization of Advanced Ceramic Powders S. G. Malghan, P. S. Wang, and V. A. Hackley

National Institute of Standards and Technology Gaithersburg, Maryland

1.

zy

zy zyx

IMPORTANCE OF POWDERCHARACTERIZATION

The practical performance of a ceramic component, the microstructure of the ceramic, and powder behavior during processing are strongly dependent on the physical and chemical characteristics of starting powders [l]. In addition, in the manufacture of advanced ceramic components, detailed information on powder characteristics is required to achieve reproducibility and cost competitiveness. The choice of starting powder characteristics depends on the intended microstructure and application of the final ceramics. For instance, although total oxygen may be one of the most important measurements in silicon nitride powders because oxygen participates in the formation of a grain boundary silicate phase, the total Y2O3 content may be the most desired parameter in zirconia powder processing because it controls phase composition and transformation. The requirements of powder characterization are complicated by the fact that a large number of properties must be defined to understand its characteristics completely. A review of Table 1 shows that this task is not simple because the powders are complex materials consisting of numerous features. Therefore, ceramists are faced with the question of what characteristics of powders to measure for a given process. Commonly, an answer to this question lies in the development of parametric relationships between the powder properties and their influence on the performance of the ceramic. The next question is how to measure the powder characteristics. A program has been in progress

zyxwv zyxwv z 129

130

zyxwvu zy zy zyxwvu zyxwvuts zyxwvu MALGHAN ET AL.

Table 1 Properties of AdvancedCeramic

Powders That Constitute Their Characterization

Physical properties Specific surface area Primary particle size and size distribution Agglomerate size and size distribution Porosity, total quantity and pore size distribution Density Phase composition Crystalline phases, quantity and identification Amorphous material, quantity Chemical composition Major element concentration Minor impurities (10 ppm to =l%) Trace impurities (10 ppm) Inorganic elements Organic elements Composition of impurities Surface composition Major elements Minor elements Trace elements Inorganic species Organic species Crystallinity

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since 1985 under the auspices of the International Energy Agency to develop procedures for the analysis of ceramic powders [ 2] .An overall goal in this task is to define procedures that provide repeatable and reproducible data. The development of standard reference materials (SRM) stands at the forefront of this task since the SRM are necessary for the improvement of measurement quality [3]. In this chapter, we address the available methods for the characterization of ceramic powders, and selected methods have been briefly described. Primarily, for these methods, a description of operating principles, type of data to be obtained, and measurement limitations are presented. In addition, nuclear magnetic resonance (NMR) and x-ray photoelectron spectroscopy ( X P S ) techniques have been described in more detail because their treatment, as applied to powders, is not available elsewhere. Thermal techniques, such as differential thermal analysis, are not covered because of their specialized use on powders. For interested readers, a number of references have been cited for additional information.

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CHARACTERIZATION OF ADVANCED CERAMIC

POWDERS

I31

II. PHYSICAL PROPERTIES

The major physical properties of ceramic powders constitute size distribution of primary particles and agglomerates, specific surface area, density, porosity, and morphology (e.g., shape, texture, and angularity).

A.SizeDistribution

In the measurement of particle or agglomerate size distribution, the major distinction is that agglomerates are made of primary particles. Hence, often there is a need to determine the true size of the underlying primary particles. In such cases, the size distribution of particles may be determined by a number of techniques, as shown in Table 2, which lists the commonly used methods [3]. With

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Table 2 Commonly Used Methods of Particle Size Analysis, Nominal Size Ranges, and Measurement Parameters

Nominal particle Measurement

Method

Coarse particles > 10 pm, sieving Dry Wet Fine particles < 10 pm Field scanning Optical microscopy Electron microscopy Gravity sedimentation Pipette Photoextinction X-ray absorption Radiation scattering, Laser diffraction, scattering Stream scanning Resisitivity Optical Ultrasonic attenuation Column hydrodynamic chromatography Sedimentation fieldflow fractionation Laser Doppler velocimetry Centrifugal sedimentation Photoextinction Mass accumulation X-ray absorption

>l0

Geometric

>2

0.5- 1000

Image

0.01-10

1-100 0.5-100 0.1-130

Stokes

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0.03-900

Geometric

0.05-500

Dynamic/Stokes

1-500 1 0 0 1 m 0.1-1 .o 0.01-1.0 0.01-3.0 0.05-100 0.05-25

0.1-5

Dynamic/Stokes

132

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zyx MALGHAN ET AL..

such a large number of choices, the selection of a system for a given application depends on a number of criteria, including the size range of the application, throughput of the instrument, accuracy, precision, reproducibility, resolution, versatility, cost, and manufacturer support [4]. The three methods described in the following text are microscopy, gravity sedimentation, and light diffraction. Their selection is based purely on their technical diversity.

1. Microscopy Three commonly used direct viewing techniques are optical, scanning electron, and transmission electron microscopy. Optical microscopy is used in the size range 1-150 pm for the determination of morphology, agglomeration, and size distribution using automated counting devices. Despite significant improvements in optics, a major disadvantage is its depth of focus, which is about 0.5 pm at x1000 [5]. Scanning electron microscopy (SEM) is a versatile technique in which a beam of electrons at 5-50 keV scans the specimen surface. The resultingx-rays, backscattered electrons, andsecondary electrons are detected and analyzed by a number of techniques. Magnifications of up to ~100,000can be achieved at resolutions finer than 20 nm. Depth of focus and magnification are inversely proportional. Automated image analysis systems are available to analyze particle size distribution.However, the major issues remain sample preparationand dispersion of powder, that is, particles tend to agglomerate when deposited on SEM stubs and thereby do not provide complete and true data for primary particles. Intransmission electron microscopy (TEM), 5-0.001 pm particles deposited on thin membranes are examined at 10- 100 times better resolution than that in SEM. TEM is more often used as an analytical tool for surface characterization and texture rather than for routine particle size distribution.

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2. Gravity Sedimentation The underlying principle behind the gravity sedimentation method is Stokes’ law, which describes the relationship between the settling velocity of particles in a fluid medium of known density and viscosity [6]. A number of instruments are available in which the settling velocity of particles is measured by x-ray absorption, light absorption, and density changes. One such instrument is the Sedigraph* by the Micromeritics Corporation in which the particle size distribution is determined by x-ray absorption [6]. This instrument offers an excel-

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*Certain trade names and company products are mentioned in the text or identified in illustrations to specify adequately the experimental procedure and equipment used. In no case does such identification imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the products are necessarily the best available for the purpose.

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lent measurement capability in the 0.2-50 pm range of a relatively concentrated suspension containing up to 6% by weight powder in an appropriate dispersion fluid. The data are presented as a continuous plot of percentage weight Of particles smaller than the stated size or other convenient formats. heparation of an appropriate dispersion is a key step in obtaining repeatable data [7]. The dispersion preparation for a given powder requires two Sets of data:(1) the amount and intensity of energy application to break agglomerates into primary particles; and (2) the pH at which the isoelectric point, the point at which the particles carry a net zero charge, occurs in a given aqueous solvent containing a surfactant for stabilization of particles. For each powder, it is necessary to develop an appropriate data for these parameters.

3. Light Scattering Instruments based on light scattering employ a laser beam to radiate particles in a stream, and the resulting scattered light is analyzed using the Fraunhofer and Mie theories to obtain size distribution data [g]. When the size of particles is very small with respect to the wavelength of light or when the refractive index of the particles is very close to that of the dispersion medium, such as that of a liquid to form a suspension, the relatively simple equations of Raleigh and Gans can be employed. For colloidal suspensions, however, it is necessary to resort to the Mie theory, which includes the restrictive conditions that the particles be spherical and intrinsically isotropic. With the advent of desktop computers, the application of the Mie theory has become a reality for polydisperse systems. Mie formulated equations to delineate light scattering by particles by considering electrical fields within and outside each particle. The equations for scattering intensity for each scattering angle involved refractive index difference between particle and suspending medium, the wavelength of incident light, and the spherical diameter of the particle. For particle diameter in the range of %o-10 times the wavelength of incident light, scattering from different portions of the particle is out of phase, which results in interference and reduced intensity. The net effect is an angular distribution of scattered light in the forward direction. This information is utilized in Mie theory computations. A number of instruments are available by which 0.03-900 pm particles can be measured in a matter of several minutes from dispersed suspensions. One major advantage of these systems is their ability to handle particulate systems containing different densities and refractive indices. A requirement of these instruments is the refractive index data, which may not be readily available for many powders. If the light-scattering particulate systems are not monodisperse, the particle size distribution obtained from these systems represents an average value, related but not identical to the weight-average particle size distribution. These averages are expected to differ depending on particle shape, particle size distribution, and degree of anisotropy.

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B.

Specific SurfaceArea

Particles consist of both internal and external surface area. The external surface area represents that caused by exterior topography, whereas the internal surface area measures that caused by microcracks, capillaries, and closed voids inside .the particles. Since the chosen surface area technique should relate to the ultimate use of the data, not all techniques are useful for fine powders. The commonly used approaches are permeametry and gas adsorption according to the Brunauer, Emmet, and Teller (BET) equation [g]. Because of simplicity of operation and speed of operation, permeametry methods have received much attention. The permeametry apparatus consists of .a chamber for placing the material to be measured and a device to force fluid to flow through the powder bed. The pressure drop and rate of flow across the powder bed are measured and related to an average particle size and surface area. Especially for porous powders, permeametry data include some internal surface area, thus decreasing their value. In the BET method, the volume or weight of adsorbed gas as a function of partial pressure is measured. Most commonly, NZgas is adsorbed by the powder at liquid N2 temperature from a gas stream of NZand He. The NZadsorbed and later desorbed is measured by thermal conductivity in one type of equipment. The BET equation describes the equilibrium between vapor and adsorbate in which multilayer adsorption occurs. The BET equation is typically written in the form PIP0

._

1 '

+

"

V(1- PIP,)

V,C

(C - 1)P& vmc

where PIP0 is relative pressure (the ratio of the equilibrium pressure to the saturation pressure at a specific temperature), V is the amount of gas adsorbed, and Vm represents V for monolayer coverage. The constant C is related to the energies of adsorption and gas liquefaction. Sample preparation is the key to obtaining reproducible data. Sufficient outgassing, minimal surface contamination, and the absence of microporosity are important aspects of the proper analysis of surface area. BET measurements are carried out in either singlepoint or multipoint mode, in which multipoint data are normally higher and more representative than those of the single point [2].

C.

Density

Theoretical, true, and tap density are three types of densities associated with ceramic powders. Theoretical density is determined by atomic composition and lattice parameters; tap density is determined by a prescribed procedure, which

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CHARACTERIZATION OF ADVANCED CERAMIC POWDERS

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consists of measuring the volume of a known weight of powder after a certain number of mechanical taps. Therefore, tap density provides a relative measure of fill density or degree of compactibility of the powder. In the determination of true density, the powder volume is determined by the volume of gas displaced by a known weight of powder. Helium pycnometry is the commonly used method for this application. Helium is the most frequently used gas because of its inertness and small size, which enables it to penetrate even the smallest pores. Therefore, contribution tothe volume by pores can beaccounted for the measurement of true density.

D. Porosity

Porosity in a powder can come from both closed and open pores. Two primary methods for the determination ofporosityare gas adsorption andmercury porosimetry. It is assumed that gas adsorption is favored in small capillaries because ofthe overlapping surface potentials, which result in capillary condensation.Pores from 1.5 to 100 nm in diameter are determined using the Kelvin equation, which relates capillary radius r to the ratio of the vapor pressure P and the equilibrium vapor pressure of the same liquid over a plane surface Po, as follows:

where y is the surface tension of the liquid, V is the molar volume of the liquid, 8 is the contact angle between the liquid and the wall, R is the gas constant, and T is the absolute temperature [lo]. Adsorption in the microporosity range ( r c 1.5 nm) often exhibits a Langmuir typeof isotherm, normally a characteristic of monolayer adsorption. The second principal method for porosity measurement is mercury porosimetry, in which a nonwetting liquid, such as mercury, is forced into capillaries of radius r. The force Foutcaused by interfacial tension y is calculated as F , ~ = 2 m y COS e

(3)

where 8 is the contact angle for mercury. The applied pressure P is related to the force driving the mercury Fin into a capillary and is given by -Fin

= P m2

Equating the two forces results in the Washburn equation [l l]:

(4)

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Mercury porosity measurements are carried out in the pressure range 14 Pa (ambient) to 415 Pa, which corresponds to pore radii from 2 nm to 200 pm.

E.

Morphology

The morphological analysis of particles constitutes the measurement of size, shape, and texture, which describes the surface profile of a particle image. The primary methods of morphology analysis are microscopy techniques, depending on the size of particles. In recent years, a large number of software packages for processing image data fromautomated microscopes have enabled faster accumulation and interpretation of data. However, the major drawbacks of microscopic techniques are representativeness of the sample examined and two-dimensional examination of the particle surface. The data from the shortest dimension may not be adequately represented.

111.

BULKCHEMICAL COMPOSITION

A large number of techniques are available, depending on the specific need to

analyze major chemical components, minor components, nonmetallic impurities, and metallic impurities (see Table 3). Further, the applicability of a technique depends on the concentration of impurities in the powder, a problem of considerable magnitude for ultrapure powders. A generalproblemwith the techniques'that require dissolution of the powders is their resistance to chemical attack and lack of complete dissolution. The two major methods of placing the powders in solution are acid dissolution and flux decomposition followed by dissolution [2,3]. The nonavailability of standard reference powders and procedures is a serious drawback to all the analysis methods. It is not possible to cover all the available techniques; therefore, only selected techniques with applicability to bulk and minor impurities are described here.

A.

InductivelyandDirectlyCoupledPlasma

These techniques, used for qualitative and quantitative measurements, utilize excitation of atoms in a plasma to obtain their characteristic emissions that are analyzed by photodetection. The powder is dissolved, and the resulting solution i s injected into the plasma. The fast speed of analysis and applicability to a wide range of elements are strong points, but these methods are subject to errors resulting from powder dissolution. Currently, a number of industrial laboratories are addressing this issue.

B.

Atomic AbsorptionSpectroscopy (AAS)

A A S is a versatile method in which an analyte is atomized in a flame, thus emitting spectral lines. The characteristic spectral lines that correspond to the

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Table 3 Methods for BulkChemicalandImpurityAnalysis of Metallic and Nonmetallic Impurities in Ceramic Powders Bulk chemical analysis X-ray fluorescence spectroscopy Atomic absorption spectroscopy Inductively coupled plasma emission spectroscopy Direct-current plasma emission spectroscopy Arc emission spectroscopy Gravimetry Combustion Kjeldahl Impurities Neutron activation analysis Mass spectrometry Electrochemical Coulometry Selective-ion potentiometry Potentiometric titration Argentometric Ion chromatography Nuclear magnetic resonance Electron paramagnetic resonance

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energy required for an electronic transition from the ground state to an excited state are emitted. The absorption of radiation from the light source depends on the population of the ground state, which is directly proportional to solution concentration. The absorption is measured by the difference in transmitted signal in the presence and absence of the test element [ 121. However, this technique is also subject to errors resulting from powder dissolution.

C.

X-rayFluorescenceSpectroscopy

The powder in the form of fine particulates or dissolved in solution is excited in an x-ray source, and the characteristic fluorescence intensity is analyzed for qualitative and quantitative data. Standards are required for the conversion of x-rayintensities to absolute concentrations. The preparation of appropriate standards that include background similar to that in the solution tobe analyzed is critical to obtaining accurate data.

D.

Nuclear Magnetic Resonance and Electron Paramagnetic Resonance (EPR)

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In principle, NMR utilizes the atomic nuclear transition, induced by radio frequency irradiation, between two quantized nuclear energy levels in a magnetic

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138

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AL.

field. This transition caused by resonance in a magnetic field occurs in all atomic nuclei, except in atoms with even numbers of both mass and atomic number. The nuclear spin numbers are zero for atoms withboth mass and atomic numbers that are even, and consequently, nuclear energy splitting does not occur in a magnetic field and these atoms are not active in NMR. All other isotopes in the periodic table are NMR active. ‘H, W , 15N,19F,27Al, W i , 31P, 47Ti, 63Cu, 69Ga, 89Y, and 91Zr are some examples. For an atom with a nuclear spin number I , there are 2 I + 1 quantized nuclear energy levels: -I, -(I - l), . .. (I - l), I. These levels are practically equal in energy outside a magnetic field. If a magnetic field is applied to this atom, however, the nuclear energy split into 2 I + 1 levels separated by (yh/27c)H, where H is the magnetic field strength, h is the Planck constant, and y is the nuclear magnetogyric ratio. The nuclear energy E is expressed by

E=--

rh m,H 2x

where MI = I, (I - l), .. . , -(I - l), -I. If a sample is irradiated by a radiofrequency energy equal to the energy separation, resonance occurs, the nuclei at the ground state are excited to a higher energy state, and an absorption signal is observed. The NMR is therefore an absorption spectroscopy, not an emission spectroscopy (e.g., X P S ) . The magnetogyric ratio is a constant for a specific isotope of interest. However, the magnetic field H is an effective field to the nuclei including the applied external field, the field induced by the electrons around the nuclei, and even the field produced by other parts of the molecule. For Si atoms, for example, 95.3 atom% are 28Si, which have a nuclear spin I = 0 and are not NMR active ( 2 I + 1 = 1, only one energy level; therefore, no resonance can occur). There are 4.7 atom% 29Si in natural abundance, however, and 29Si has a nuclear spin I = H. In an external magnetic field of 9.4 T, for example, the 29Si nuclear energy is split into two levels separated by approximately 79.5 MHz, with mr = H and -H and with H lower in energy. In addition, the actual magnetic field experienced by the 29Si nuclei in Sic, Si3N4, and Si02 is different and not exactly 9.4 T because the electron density and distribution around the Si nuclei in these compounds are different. This difference makes it possible for us to observe the absorption signals at different field strengths and is the “chemical shift.” Chemical shift is a comparative value and is most often referenced to tetramethylsilane [TMS, Si(CH3)4]. This compound can be used as a reference to three frequencies: ‘H, W , and 29Si. The frequency difference between the sample and that of TMS is often divided by the frequency of the spectrometer to give a value in parts per million (ppm), which is spectrometric frequency independent. Other factors, such as spin-spin interaction with neighboring atoms, affect Si nuclear energy level splitting. For nuclei with I 2 1, the nuclear quadrupole

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CHARACTERIZATION OF ADVANCED CERAMIC POWDERS

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effect also alters the energy level splitting. A more in-depth coverage of N M R theory is found elsewhere [13,14]. N M R spectroscopists are also interested in nuclear spin relaxation times. The relaxation time measures the time required for an excited nucleus to return to the ground state. Two types of relaxation times are involved: spin-lattice relaxation time T i , the time constant for thermal equilibrium between the nuclei and crystal lattice, and spin-spin relaxation time T2, the time constant for thermal equilibrium between nuclei themselves. Information on molecular dynamics can be obtained from these relaxation times. Generally, Ti = T2 for lowviscosity liquids and Ti >> T2 for solids. A combination of information in molecular dynamics (from relaxation times), molecular structure (from spinspin interaction), molecular identification (from resonance frequency and chemical shift), and spin density (from signal intensity) make the NMR an extremely versatile tool. The application of NMR spectroscopy to ceramic powders is a relatively new area, but it has potential. N M R application in solids is difficult because of large dipole-dipole interactions that result in line broadening and thus poor resolution. However, by the application of modem instrument technology and discoveiy of pulsed Fourier transform N M R , scientists have developed various line-narrowing techniques to improve resolution [15]. One of the most often utilized techniques is magic angle spinning (MAS). Molecules in solids are not mobile and tumbling as they are in the liquid or gaseous state. This induces large dipolar interactions between two spin centers. If we physically spin the sample, at several kilohertz, the anisotropic part of the nuclear dipole is eliminated. The dipole-dipole interaction is a function of (3 cos2 8 - l), where 8 is the angle between the static magnetic field and internuclear vector. Setting this term to zero, the “magic” angle is 54”44’ [16]. N M R has been applied most successfully for high-temperature superconductors, YBa2Cu307-6 [17-211. The studies involve mainly 89Y,63Cu, and 65Cu NMR and nuclear quadrupole resonance of this compound below, above, and at the critical temperature. Nuclear relaxation, Knight shift, and crystal structure are often examined at these temperatures. 29Si and *7Al N M R have been applied to silicates, aluminosilicates, and surface-adsorbed molecules on ceramic materials [22]. N M R can also be a powerful technique to determine the surface area [23], porosity, pore size, and molecular diffusion into the crystallattice [24]. The technique is basedon the different relaxation times (or linewidths) of the mobile molecules and adsorbed molecules, the latter losing certain degrees of freedom. The crystal structure of B a T i e has been characterized by 137Ba, 47Ti, and 49Ti NMR at various temperatures up to the Curie point [25]. Structural changes in the alkaline-earth silicate glasses induced by phosphorus were also studied by 31P and 29Si NMR [26]. 29Si MAS NMR is a very useful technique to study the crystal-phase composition and transition in silicon nitride and carbide [27,28]. The a phase of

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MALGHAN ET AL..

these compounds has more than one Si crystal site; while p phase has only one. This yields different line patterns for different phases. MAS NMR has been used to study high-temperature reactions, sintering, the formation of surface oxide and suboxide, and reaction products with the sintering aid [29- 341. The theory of EPR (also known as electron spin resonance) is very similar to that of the NMR except the frequency falls into gigahertz region because the mass of electrons is smaller. Electrons are negatively charged, so that the splitting of energy is inverted in its order compared with the nuclei [35]. Since only molecules with unpaired electrons are EPR active, the application of EPR to ceramics is limited.However,EPR is a unique technique for characterizing powders with dangling bonds (as in carbon atoms), impurities, and defects.

E. Combustion

This technique is limited to such elements as carbon, nitrogen, and oxygen that can be liberated in gaseous form during decomposition. The concentration of gas is determined by a property of the gas. Commonly a flux is used to aid the combustion [3,36].

IV.

PHASE COMPOSITION

A.

X-rayPowderDiffraction(XRPD)

This is the most widely used method for the determination of the phase composition of powders. The x-ray diffractometer contains a source of monochromatic x-rays that irradiate the sample and are diffracted from atomic planes and detected. The angle of diffraction of x-rays by the crystalline planes is characteristic of the crystal structure, and the intensity of scattered radiation is characteristic of the atomic composition. In recent years, automated data processing has enabled higher accuracy andspeed. A number of problems are encountered in the quantitative determination of phases in fine powders. Some of these are overlap of phase peaks (e.g., in silicon nitride), orientation of grains, and presence of coarse particles. The last produces distortion of the diffraction data. A number of standard reference materials for XRPD have been developed for use in improving the quality of data [37].

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V. A.

SURFACECHEMICAL COMPOSITION BY SPECTROSCOPY X-ray Photoelectron Spectroscopy and X-ray-Induced Auger Electron Spectroscopy (XAES)

X P S (also known as electron spectroscopy for chemical analysis) and XAES provide not only the surface elemental composition but also reveal the oxida-

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CHARACTERIZATION OF ADVANCED CERAMIC POWDERS

141

tion states of the elements. X P S , especially, has emerged as one of the most important techniques for studying the chemistry of the ceramic-powdersurface as a result of oxidation at high temperatures. In X P S , when soft x-rays, such as MgKa (1253.6 eV), impinge on a powder surface, photoelectrons with binding energy BE are ejected from the surface with kinetic energy (KE). The relationship among the irradiation energy hv (1253.6 eV in the case of MgKa), KE, and BE is as follows: KE=hv-BE-eS

(7)

where is the work function of the spectrometer. Since is a constant for a specific spectrometer, hv is known and KE can be measured; therefore, BE can be calculated. Often, residual carbon in the form of hydrocarbon (-CH) is found on the powder surface as a contaminant, and the C 1s photoelectron peak at 284.5 eV binding energy is used as a reference to calculate the binding energies of other signals. From the binding energy of the emitted electrons, the surface elemental composition and chemical state can be derived. Detailed information on surface analysis techniques, comparison of XPS and Auger electron spectroscopy (AES), and photoelectron binding energies can be found in the respective references [3840]. For quantitative surface composition, the photoelectron cross sections (or atomic sensitivity factors) are required to convert the signal intensities to atomic ratios. In other words, one photoelectron from C 1s gives a different spectral intensity from that of a Si Is, for example. Scofield cross-sectional values are often used for this purpose [41]. When two spectral signals from different oxidation states of an element (for example, Si 2p from Si3N4 and its surface oxide Si02) can be resolved because of a large difference in binding energy, the surface film thickness can be calculated if the photoelectron mean free paths data are available [4247]. When photoelectrons are emitted from the core level as a result of a photoelectronic process, the surface becomes ionized and unstable. The ion relaxes its energy by drawing an outer electron to fill the inner orbital vacancy left by the photoelectron, and a second electron is emitted by the excess energy. This second emitted electron is an Auger electron (more precisely, x-ray-induced Auger electron, X A E S , or bremsstrahlung-excited Auger electron). A comparison of the photoelectron and Auger processes is shown in Fig. 1. It can be concluded that the kinetic energy of a photoelectron is directly proportional to the energy of irradiation, but the kinetic energy of an Auger electron is independent of the radiation source and possesses kinetic energy equal to the difference between the energy of the initial ion and the doubly charged final ion. The energy of neither electron can exceed the energy of the ionizing photons. Sometimes the difference in kinetic energies of two Auger electrons from two different oxidation states is larger than that of the photoelectrons, and thus the

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PHOTOELECTRON

PHOTON

0

0

0

f

0/

K OR 1s

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/

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i

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Figure 1 Comparison of photoelectron and Augerprocesses.

Augerspectrum provides better resolution for surface chemical studies, as shown for S i c and Si3N4 powders [42,43,48]. S i c and Si3N4 powders have been studied extensively by X P S or x-ray-induced AES [42,43,48-571. Powders, whiskers, and platelets were treated at elevated temperatures, and surface analysis was carried out by measuring Si 2p X P S or Si KLL AES intensities for Si02 and S i c (or Si3N4). Subsequently, surface oxide film growth rates were measured and surface oxidation activation energies were calculated [42,43,48-511. In some cases, the effects s f the presence of ymia, a sintering aid, and boron, an impurity, were studied [43,48,49]. The effects on surface oxidation chemistry caused by different powder processing routes were also studied [50]. Surface composition, including oxygen and oxynitide concentration, is important information required for nonoxide powder processing. In a recent study, several commercial silicon nitide powders were analyzed for total oxygen at high temperatures, and the “surface” oxygenwasmeasured by AES. The “bulk” oxygen was then calculated based on these data [52]. Commercial S i c

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CHARACTERIZATION OF ADVANCED CERAMIC POWDERS

143

powders were examined for surface composition and contamination by X P S [53,54]. Ultrafine powders grown bya radiofrequency plasma process were found to have a thin oxide layer; those grown in a vapor-liquid-solid process had a thicker silica layer [53]. Surface-sensitive techniques, such as XPS and AES, also contribute to the understanding of moisture effects, surface coating, and interfacial chemistry of ceramic powders [S-591.

B.

Infrared and RamanSpectroscopy

For a polyatomic molecule containing n atoms, the total degree of freedom is 3n and there are 3n - 6 modes of vibration if the molecule is nonlinear. This shows that molecular vibration in a polyatomic molecule can be very complex. Infrared (R) and Raman are two complementary techniques to study molecular vibration and identify unknown species. There are two fundamental types of molecular vibrations: stretching and bending. Stretching, in which twoatoms increase or decrease their distance but remain in the same bond axis, can be either symmetrical or asymmetric. Bending is a molecular deformation and can be scissoring, rocking, wagging, or twisting. Figure 2 shows these vibrations for a group of atoms. Molecular bending generally requires less energy than stretching and is thus observed at lower frequency. The frequency of stretching depends on the bond strength and mass of the atom attached and can be calculated from

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where 2) is stretching frequency in cm-1, c is the velocity of light, Mx and M y are the mass in grams of the two atoms attached to the bond, and k is the force constant in dyneshentimeter of the bond.

Asymmetric

Symmetric

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king

Scissoring

'Figure 2 Vibrations for a group of atoms.

Twisting

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Each of these vibrations has a characteristic frequency and can occur at quantized frequencies only. When IR light of the same frequency is incident on the molecule, the energy is absorbed by the molecule and the amplitude of the particular mode increases. However, this absorption occurs only if this vibrational mode can cause a change in the molecular dipole. Consequently, not all vibrational modes are IR active and the molecular symmetry plays a key role in the reduction of IR spectrum patterns. In addition to these fundamental vibrations, overtone peaks may also be observed with much reduced intensity at two, three times, and so on, the wave numbers, the sum of twoor three times the wave numbers, or the difference between two wave numbers. Detailed IR spectroscopic theory and group theory can be found elsewhere [60- 62]. Surface moisture is a problem of concern in ceramic powders, and IR has been used to characterize the surface groups of -OHand -H [58,63,64]. IR was also applied to characterize chemically bound hydrogen in chemical vapordeposited silicon nitride at various ammonia-silane ratios [65]. Surface silicon dioxide on S i c powders was determined by photoacoustic IR and diffuse reflectance IR spectroscopy [66,67]. IR spectroscopy was also used to study the surface oxidation of S i c and Si3N4 [68,69]. Raman spectroscopy is complementary to R.The vibration that is inactive to IR, because ofa high degree of molecular symmetry or lack of dipole change, may be detected by Raman. Raman depends on a change in the polarizability during the vibration instead of the electric dipole moment. When a monochromatic light with frequency v impinges on a molecule, the scattered light has the same frequency if the scattering is elastic. This is Rayleigh scattering. However, some of the light is scattered with v f v’ because of inelastic interactions. Lines with lower frequency are known as Stokes lines And are formed by the loss of energy to the molecules during scattering. Lines with higher frequency are known as anti-stokes lines and are formed by energy gain from the molecules during scattering. The Raman frequency v’, is completely independent of the incident light v. Rather, it is characteristic of the molecular vibration energy. By measuring the characteristic Raman frequency, one can identify the unknown molecules [60]. Application of Raman scattering to ceramic powder characterization is a new research area, and the data are limited.

VI.INTERFACEANALYSISIN

SUSPENSIONS

A suspension of finely dispersed particles is thermodynamically unstable; this system is inclined to lower its free energy through flocculation. The stability and rheology of a powder in a suspension depend on the nature of the solid/ solution interface, particularly on the electrical properties of this region [70]. This interface may be described as consisting essentially of two layers (Fig. 3):

z

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CHARACTERIZATION O F ADVANCED CERAMIC POWDERS Shear Plane

0

0 0 Solution

145

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Figure 3 The solid/aqueous solution interface and electrical double layer for a hydrolyzed oxide surface: G , charge density; v, electrostatic potential; +, cation; -, anion, 0 , surface; c, compact layer; d, diffuse layer.

a compact layer near the surface consisting of potential determining adsorbed species and complexed ions, balanced by a diffuse layer of counterions in solution. Suspension stability is a consequence of mutual repulsion between similarly charged double layers. It is therefore desirable to measure the electrical potential as a function of solution or powder conditions to optimize slurry properties during wet processing. A complete surface chemical characterization should also include information about the type and density of surface sites and the interaction of solution species with the surface. In this section, we cover only in situ techniques for the analysis of aqueous powder dispersions, because these systems are most relevant to ceramic powder processing. The focus is on electrokinetic methods for the measurement of particle electrostatic potential. In addition, a brief overview of available surface chemical characterization techniques is given. Additional details on this topic are given by other authors in this book.

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A.

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ElectrokineticCharacterization

At some distance from the particle surface (usually identified as the beginning of the diffuse layer in Fig. 3), a hydrodynamic shear plane exists that is characterized by the potential. The magnitude of ( is directly related to dispersion stability [71]. For oxides, hydroxides, and related materials, is strongly influenced by solution pH and electrolyte concentration and may be modified by surface-active species, such as oxyanions and polyelectrolytes. The key parameter characterizing a powder surface is the isoelectric point pHg. Under pristine conditions (i.e., no surface contamination), PHiep defines the solution pH at which = 0 and the particles exhibit a net surface charge of zero. This point can be identified from an acid-base titration curve in which is plotted against pH. Shifts in PHiep may result from changes in surface chemistry that occur during aging or chemical treatment of powders [ S ] , for instance, or caused by the chemical adsorption of solution species [72]. The potential may be obtained from measurements of particle mobility using electrokinetic techniques, such as electrophoresis or sedimentation potential [70]. Electrophoresis, the standard technique for submicrometer particles, is based on the movement of charged particles in response to an applied electrical field. Optical scattering methods are used to measure the distribution of particle velocities for a given field strength, and may then be calculated using the Henry equation,




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Figure 5 Rheogram of 60 vol% alumina in THF and chloroform with 5 W% APIB per AKP-15 alumina. 6 0 -I Y

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Fig 2). This is shown by the more compact sediments. Alumina with an amphoteric surface was stabilized by dispersants and strongly acid or strongly basic solvents by acombined steric and electrostatic mechanism. Hexane, which is a nonpolar solvent with no hydrogen bonding capability, is shown to be a poor solvent for AHAS, which is a highly polar dispersant. Hexane would introduce a low 5 potential as well; hence the stability had to rely solely on the steric stabilization contribution. When the adsorbate AHAS is not highly soluble in the medium, the dispersant is ineffective as a steric stabilizer. Despite the high dielectric constant of EtOH, it is shown to be an ineffective medium with LNA. EtOH as a weakly acid solvent introduced nearly a zero potential

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CERAMIC PARTICLES

205

IN NONAQUEOUS MEDIA

on alumina, although the adsorption isotherm showed a large amount of LNA,

=20 mg/g Al203, on the alumina surface. Thus, for the most effective stabi-

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

B.

Silica

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


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Figure 12 Rheogram of 35 vol% silica in Chloroform and THF with 5 wt% LNA, AHAS, and APIB.

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ological behavior. The adsorbent silica is acidic and the solvent and dispersant are basic. This combination is shown to be ineffective for dispersing silica. AHAS, on the other hand, is shown to be effective in both choloroform and THF. This may be because of the amphoteric nature of AHAS, discussed previously. Aminopolyisobutylene is also shown to be effective in both chloroform and THF, although not as low in viscosity as AHAS. APIB was previously mentioned as a basic dispersant for the amino groups and carbonyls. It may be that the basic APIB adsorbs on silica very well and is very soluble in both chloroform and THF. To consider the three-way interactions between dispersant-solvent,solvent-solid surface, and dispersant-solid surface, one must consider the relative acidity or basicity of all the participants. Is chloroform a stronger acid than silica? Is THF a stronger base than APIB? Such questions are relevant and need to be answered. It may be speculated that silica is a stronger acidthan chloroform; hence the basicAPIB preferentially interacts with silica over chloroform. If APIB is a stronger base than THF for silica, then it preferentially interacts with silica over THF. However, the exact relative acidity or basicity of the three interacting participants, that is, APIB, solvent, and silica, is unknown. Hence, it is difficult to assess the degree of the interaction quantitatively.

C.

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Portland Cement

It is well known that portland cement is strongly basic because of the large proportion of calcium hydroxide. Figure 13 shows the viscosity of cement slurries in various liquids as a function of Brookfield shear rates. At 24 vol% cement in organic solvents, it showed lower viscosities than that in water. A strongly basic liquid medium, THF, exhibited the lowest viscosity with Newtonian be-

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Figure 13 Rheogram of 24 vol% portland cementin various media.

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CERAMIC NONAQUEOUS PARTICLES INMEDIA

havior. This is also difficult to assess based simply on the Drago and Fowke's acid-base interaction theory, because the cement surface is basic and the THF medium is also basic. Using the relative acid-base scale in Table 1, THF is acid compared with cement and introduces a larger 1 / with ~ a smaller ionic strength and higher electrostatic contribution than for the cement surface in water to stabilization of the suspension [Eqs. (1) and (3)]. In the presence of LNA, EtOH, which most resembles water chemically, shows the 'higher viscosity in Fig. 14. The cement in all other liquid media exhibitedlowviscosities. In particular, the strong acid chloroform and strong base THF show Newtonian low viscosities. The reasoning given for silica in Fig. 12 must apply here as well, that is, the relative acidity-basicity in the suspension among the three participants. When water is added to the cement slumes in an organic medium, chloroform in the presence of LNA (Fig. 15), the viscosity changed little up to 10 vol%of H20. A water volume above this destroys the stability. Figure 16 shows the relative diametral tensile strength for specimens from cement dispersed in various liquid media. The diametral tensile strengths were determined by applying compressive loadings diametrically on cylindrical specimens 3 x 3 cm in diameter and height formed from 41. 4 vol% cement and 17 vol% organic solvent in water. The relative strengths were calculated by dividing the measured values by the strength of the specimen formed from 100% water. The strength of the cement body formed from chloroform-water showed more than a threefold higher strength and a slightly higher strength than EtOH and water. This shows the effectiveness of nonaqueous liquid in forming superior cement components through improved dispersion.

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Figure 14 Rheogram of 24 vol% portlandcementin LNA per cement.

various

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Figure 15 Viscosity of 35 vol%portlandcementwi Ith 5 wt% LNA in Chloroform as a function of water content.

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Figure 16 Relative diametral tensile strengths of 27 day cured portland cement cast from various liquid media.

111.

HAMAKERCONSTANT

In Deryagin, Landau, and Verwey (DLVO) theory the magnitude of attractive energy is represented by the Hamaker constant A, as shown in Eq. (2). Two like particles in a liquid medium exhibit attraction arising from van der Waals forces. The net Hamaker constant representing the attraction between the two particles in the medium is given by the geometric mean of the two materials in Eq. (4):

CERAMIC PARTICLES

zy z zyxwvu zyx z zy zy z zy

IN NONAQUEOUS MEDIA

21 I

where the subscripts p p = particle-particle and m m = medium-medium. For a small App/m, that is, better dispersion, one wants the value of App to be close to that of A-. The A- values calculated from the surface tension of the liquid [2,5] are listed in Table 1. If the dispersoid particles are covered by another material, the value of App approaches that of the adsorbate material. In this case, for small App/m, one can select a dispersant having a value ofA close to thatof the liquid medium. Unfortunately, accurate determination of the Hamaker constant for a given material system is not yet well established. Since the Hamaker constant is a function of the polarizability of a molecule, LNA, having three double bonds,would have a relatively high value (>S) [5,17]. Plots of the viscosity of alumina suspension as a function of the Hamaker constant of the liquid medium, made by Rives [IS], show that the viscosity minimum at a Hamaker constant = 9 for LNA in acidic solvents and = 8 for LNA in basic solvents. The values of Am in Table 1 show that such solvents as toluene, methylene chloride, chloroform, and THF would be better media for smaller APplm and water the worst. This practice should apply to mixed solvent systems to tailor the Hamaker constant, solubility, electrostatic charge, and so on. This was presented in our recent work elsewhere [19].

IV.

SUMMARY AND CONCLUSIONS

Ceramic particles can be well dispersed in nonaqueous solvent via electrostatic repulsive forces. In the presence of organic dispersants, the electrostatic repulsive forces can be augmented by steric hindrance from the adsorbed dispersant molecules. The combined hindrances to attraction and coagulation can be very effective in dispersing fine particles and increasing solids loading. For alumina powders, THF is found to be a good medium with an acid dispersant LNA. Chloroform, which is an acidic solvent, is found to be an even better solvent for alumina dispersion with LNA. For a basic dispersant, APIB, toluene is found to be good choice of solvent as well as TI-IF.Chloroform is also found to be a good choice. For silica powders, ethanol is a good medium, but in the presence of an aminosilane, AHAS, the more basic solvents acetone and THF are better dispersionmedia.A strong acid Chloroform was also found to be good with AHAS. For a basic dispersant, APTS, both THF and Chloroform are good dispersion media. THF and Chloroform were found to be excellent in obtaining a low viscosity of cement slurries. Cement specimens formed from 16.7% chloroform in water produced a 3.4-fold stronger cement cast. It may be possible to reduce

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the amount of water currently in use in the field to improve the strength of cement concretes. For better dispersion, the relative acid-base interactions of the three participants-liquid medium-adsorbate, adsorbate-solid surface,andsolidsurfaceliquid medium-should be considered simultaneously rather than one pair at a time. To reduce the attractive forces between the particles, tailoring the effective Hamaker constant of the particle surface can be made by choice of dispersant and solvent. Mixed dispersants and/or mixed solvents may be considered for the purpose. Depending on the choice of dispersant, the role of dispersants may be combined with therole of binders to decrease the content of the organic additives.

ACKNOWLEDGMENTS

The author acknowledges the contributions of data from his graduate and undergraduate students at Clemson University, C. Calhoun, J. Rives, and U. Paik. The financial support from the National Science Foundation and the State of South Carolina is also gratefully acknowledged.

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REFERENCES

1. Shaw, D. J., Introduction to Colloid and Surface Chemistry, 3rd ed., Butterworths, London,1985,p.186. 2. Fowkes, F. M.,Dispersionofceramicpowdersinorganicmedia,in Ceramic Powder Science Advances in Ceramics, Vol. 21 (G.L. Messing, K. S. Mazdiyasni, J. W. McCauley, and R. A. Haber, eds.), Am. Ceram. Soc., Westerville, OH, 1997, p. 41 1. 3. Lee, B. I., and Rives, J. P., Dispersion of alumina in nonaqueous media,Colloids Surfaces, 56, 25 (1991). 4. Napper, D. H.,Polymeric Stabilization of Colloidal Dispersions, Academic Press, NewYork,1993. 5. Johnson,R. E., andMomson,W. H., Jr., Ceramicpowderdispersioninnonaqueous systems, in Ceramic Powder Science, AdvancesinCeramics,Vol.21 (G. L. Messing, K. S. Mazdiyasni, J. W. McCauley, and R. A. Haber, eds.), Am. Ceram. Soc., Westerville, OH, 1987, p. 323. 6. Calvert, P. D., Lalanandham, R. R., Parish, M. V., Fox, J., Lee, H., Pober, R. L., Tormey, E. S., and Bowen, H. K., Dispersion of ceramic particles in organic liquids, in Proc. Mae. Res. Soc. Symp., Vol. 73, (C. J. Brinker, D. E. Clark, and D. R. Ulrich, eds.), Materials Research Society, Pittsburgh, PA, 1986, pp. 579-584. 7. Griffiths, D. A., and Fuerstenau, D. W., Effect of pH and temperature on the heat of immersion of alumina, J. Colloid Interfac. Sci., 80, 271 (1981). 8. Robinson, M., Pask, J. A., and Fuerstenau, D. W., Surface charge of alumina and magnesia in aqueous media, J. Am. Cerarn. Soc., 47, 516 (1964).

zyxw z zyxwvu zyxwv zyx zyxw zyxwvut zyxwvu zyxw zyxwv zyxwvuts

CERAMIC PARTICLES IN NONAQUEOUS MEDIA 9. 10.

11. 12. 13.

213

Lee, B. I., and Hench, L.L., Electrophoretic behavior and surface reactionssolof gel derived alumina, Colloids Sufaces, 17, 2 1 (1987). Kiselev, A. V., and Lygin, V. I., Infiared Spectra of Sugace Compounds, Wiley and Sons, New York, 1975, p. 254. Okuyama, M., Garvey,G., Ring, T. A., and Haggerty, J. S., Dispersion of silicon carbide powders in nonaqueous solvents, J. Am. Ceram. Soc., 72, 1918'(1989). Drago, R. S., Vogel, G. C., and Needham, T. E., A four parameter equation for predicting enthalpies of adduct formation,J. Am. Chem. Soc., 93, 6014 (1971). Fowkes,F.M.,Acid-basecontributionstopolymer-fillerinteractions, Rubber

Chem. Technol., 57, 328 (1984). 14. Fowkes, F. M., and Mostafa, M. A., Acid-base interactions in polymer adsorption, Ind. Eng. Chem. Product Res. Dev., 17, 3 (1978). 15. Barton, A. F. M., CRC Handbook of Solubility Parameters and Other Cohesion Parameters, CRC Press, New York, 1983. 16. Weast, R. C. (ed.), CRC Handbook of Chemistry and Physics, 52nd ed., Chemical Rubber Co., Cleveland, OH, 1971. 17. Osmond, D. W. J., andWaite,F.A.,Thetheoreticalbasisforthestericstabi-

18.

lization of polymer dispersions prepared in organic media, in Dispersion Polymerization in Organic Media (K.E. J. Barrett, ed.), Wiley and Sons, New York, 1975, pp. 9 4 . Rives, J. P.,Dispersion of alumina powder in nonaqueous media via steric and electrostaticrepulsiveforces, MS. Thesis,ClemsonUniversity,Clemson,SC,

zyxwvutsrq

1990. 19. Lee, B. I., and Paik, U., Dispersion of alumina and silica powders in nonaqueous media: Mixed solvent effects, Ceram. h r . , 19, 241 (1993).

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9 Synthesis and Dispersion of Barium Titanate and Related Ceramic Powders Ki Hyun Yoon and Kyung Hwa Jo* Yonsei University Seoul, Korea

1.

zz

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To improve the electrical and mechanical properties of advanced electronic ceramics, it is known that the methods of powder preparation and control of the starting materials, that is, grain size and size distribution, have a direct effect on the material properties. There are various kinds of chemical methods for ceramic powder preparation via a liquid phase, which make it easy to control the properties of the powder product and to prepare fine particles. The sol-gel method has some merits, such as highpurity, homogeneity, stoichiometric composition, and infinite flexibility [1,2] using a variety of organic raw materials [3,4]. However, it is necessary to reduce the agglomeration and to control the interaction between particles in suspension. The high reactivity results in shortening the sintering cycle and developing stability against inhomogeneous grain growth. Therefore, research on suspension behavior is necessary, but this can proceed only at a finite pace. The extent to which individual particles exist as aggregates obviously has a major influence on the behavior of the suspension during processing and on the properties and performance of the final product. Therefore, the primary emphases are on the preparation and dispersion of fine powders. The extension of most of the general concepts to high-density

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*Current affiliation: Daeww Corporation, Ltd., Seoul, Korea

215

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and homogeneous grain growth need to be assured [5]. The sintered samples, which are homogeneously packed with fine spherical particles, have high density, a reduced sintering cycle, and enhanced stability against abnormal grain growth. It is believed that by preparation of fine powders and manipulation of the dispersion characteristics, a uniformly packed, high green density compact with stable dispersion will result. From controlling the dispersion characteristics, enhancement of sintered density and stability from abnormal grain growth may be expected.

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II. PREPARATION OF FINE POWDERS: PREPARATION BY THE SOL-GEL METHOD

Whena multicomponent alkoxide is used, the multicomponent or complex alkoxide can be prepared by reacting a combination of single alkoxides or by adding soluble inorganic salts to single or complex alkoxides. The species from an inorganic salt can be incorporated into both multicomponent alkoxides and the alkoxide sol structure itself. If the atom or ion from the salt is to be incorporated into the sol, it must be properly dispersed and then reacted to form the multicomponent oxide. Utilizing an inorganic salt in this manner requires obtaining a stable solution of the alkoxide and the salt. Metal salts are less expansive than alkoxide, so the use of metal salts compensates for a disadvantage of the sol-gel method. Barium titanate is a well-known dielectric material of technological importance for high-technology ceramics and is normally synthesized byconventional solid-state reaction. For the sol-gel-derived powder, the high surface area of the dried gel results in very high reactivity, which permits low-temperature processing, Therefore, the weight loss that is a problem inthe BaTiOg-PbTiO3 system may be reduced during calcination since vaporization of the PbO is expected to be less. By starting with well-mixed solutions or sols, chemical homogeneity on the molecular scale can be obtained. High purity can be maintained because the successive steps common to many conventional ceramic processes can be avoided [6]. The nature of the solvent can likewise play a critical role in the synthesis. The solvents are not inert with respect to the reaction sequence. It is important to choose solvents for the complete solubility of alkoxides or salts. Solvents play a direct role through the process of alcohol exchange. The solvent and raw materials were dissolvedcompletely with vigorous stirring before the hydrolysis reaction at a certain temperature. The solution was refluxed for many hours. Generallythis temperature is near the boiling point of the solvent. The solvent must be dried before use to prevent partial hydrolysis. Because the alkoxide and salts are extremely moisture and carbon dioxide sensitive [7], they may be handled under an N2 atmosphere. The early work of

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SYNTHESIS AND DISPERSION OF BaO-Ti02

217

Mazdiyasni [S] utilized barium isopropoxide derived from high-purity Ba metal; much earlier, Flaschen [g] used Ba(OH)2 as a starting material. Ritter et al. [lo] studied the alkoxide-based synthesis of phases in aBaO-Ti02 system. Inorganic water-soluble salts, such as Bach and Ba(N03)2, are considered but not investigated because of the potential problems of crystallization of these reagents during drying .of the gels and contamination of the final electroceramic by the residual anionic species [1 l]. For PbTi02, there are two possible structures of product formed by the reaction of a lead salt and a titanium alkoxide [12]. The hydrolysis is performed for metal alkoxide precusors. It is performed by introducing the alkoxide into excess water under vigorous stirring. The purity of water seems to be very important as well, so hydrolysis is performed generally with deionized water. During hydrolysis, the input method of water is steam or slow dripping. The total amount of water for hydrolysis has an effect on particle size [13]. The rate of hydrolysis depends on the nature of both the metal ions and the alkoxide groups. The hydrolysis rate generally decreases with increasing length of the n-alkyl chain and with increasing branching of the alkyl group, presumably for steric reasons [14]. It is also expected that the hydrolysis rate should increase as the metal ion is more electropositive. In the sol-gel method, the amount of acid usedfor peptization influences the shape and size of the particle [3]. Acid and base additions are generally specified in terms of pH. In this case, however, it is observed that the type of acid plays a much more important role than the pH of the system. The anions of the acid must be noncomplexing or very weakly complexing with metal ions at these low catalytic concentrations. The amount of acid in relation to the metal must not be large enough to prevent the formation of a continuous metal bonding through oxygen. For example, acrylic acid used for peptization in the (Ba, Pb)TiOs system has an ionization constant [4] of 5.6 x 10-5 at 25°C (generally others have ionization constants below 10-4). Thus, the acrylic acid satisfied the general requirement for an acid for peptization: it must not be too large to prevent the formation of a continuous bonding of metalions of the alkoxide throughhydroxides. After hydrolysis and peptization, gel is formed. The last step, drying, plays a very important role in maintaining a fine powder and has direct effects on the calcination step. The relation between the drying method and the surface area of powders is important. There are many drying methods, such as air, vacuum, and freeze drying. This point is illustrated with the examples of (Ba0.2Pbo.s)Ti03. In powder preparation, the surface reactivity, that is, the specific surface area, plays a very important role. Specific surface areas are often determined by the BET (Brunauer, Eminett, and Teller) method. Typical BET plots of the sol-gel, CM0 (calcined mixed oxide method), and MSS (molten salt synthesis method) [l51 derived powders are shown in Fig. 1.

218

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Figure 1 BET plotfor(Bao.2Pbo.8)TiO3: (triangles) sol-gelderived (freeze dried); (squares) sol-gel derived (vacuum dried); (filled circles) CM0 derived (air dried); (open circles) MSS derived (air dried).

Table 1 shows that the specific surface area is dependent on the synthesis method for the powders. In general, sol-gel-derived (Bao.2Pbo.8)Ti03 powders have specific surface areas greater than those of CMO-derived powders. Generally, the smaller the particle size, the greater is the specific surface area. Thus, it can be expected that the particle size of sol-gel-derived powders is smaller than that of CMO- and MSS-derived powders. When sol-gel-derived powders are dried,the surface areas of air-dried,vacuum-dried,and freezedried powders are 36.4,51.9, and 54.6 m2/g respectively. Freeze-dried powders have the highest value. These results are similar to the value for vacuum-dried powders prepared only from alkoxides by Mazdiyasni et al. [l61 Freeze-dried powders, after heat treating at 600"C, exhibited a surface area value of 19.9 m2/g, which is higher than those obtained by, Mazdiyasni [161 and Rehspringer and Bernier [17], 15 and 16.8 m2/g, respectively, both preparedusing only alkoxide. Metallurgical micrographs for sol-gel-derived (Bao.2Pbo.8)Ti03 prepared by air drying (Fig. 2) reveal that the air-dried powder has the strongest

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SYNTHESIS AND DISPERSION OF BaO-Ti02 Table 1

BET Values(SurfaceArea) for (Bao.flb0.8jTiO3

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powder Calcined Dried gel

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agglomerate and the freeze-dried powder has the weakest agglomerate. The results agree with the BET values shown in Table 1. The freeze-drying method may be divided into freezing, sublimation, and desorption steps. The sublimation step is affected by the equilibrium condition of composition versus temperature and pressure. The advantages of this method are that it is easy to control particle size, composition, and input of minor components in the systems homogeneously, low-temperature processing is possible, and evaporation of volatile elements is prevented . The disadvantages are the need for a cold trap at very low temperatures and a vacuum system, so this method is difficult to apply to mass production. The resulting monosized and submicrometer particles are easier to process into uniform green microstructures, which results in easier control of the microstructure during densification. This means uniformityof the particle size

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Figure 2 Metallurgicalmicroscopephotographs for sol-gel-derived (Bao.flbf~.8) TiO3: (a) air drying, (b) freeze drying, (c) vacuum drying.

220

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and distribution of the voids, which can be measured by a particle size or pore size analyzer. Figure 3 shows the BaTiO3-PbTiO3, particle size distributions as a function of synthesis method. In Fig. 3, the sol-gelderived powders have the narrowest particle size distribution and smallest mean diameter. The particle size and distribution can also be observed with scanning and transmission electron microscopy (SEM and TEM). X-ray diffraction of (Ba0.2Pbo.8)Ti03 in Fig. 4 shows that the powder is initially amorphous. Samples were calcined at various temperatures, and a progressive reduction in peak width with increasing calcination temperature can beseen. The x-ray diffraction patterns of solgelderived andCMO-derived(Ba0.2Pbo.8)Ti03 are shown inFig. 5. For CMO-derived powders, the characteristic peaks of unreacted BaC03 are found at 600"C,andthey decrease with increasing the treatment temperature to 900°C. At 9OO"C, the powder is completely a single phase. The degree of crystallinity appears relatively good even at 600°C in the solgel method, indicating that this method has a synthesis temperature 200°C lower than that of the CM0 method. Tine infrared absorption spectra have been obtained for powder samples dispersed in pressed KBr disks to identify the molecular structure of their components, specifically organic materials. Figure

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diameter [ pm)

Figure 3 Particle size distributions of ( B ~ I - J . ~ P ~ o .prepared ~ ) T ~ Oby ~ differentmethods. (a) Sol-gel, (b) CMO,(c) MSS.

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SYNTHESIS AND DISPERSION OF BaO-Ti02

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Figure 4 X-ray diffraction patterns of sol-gel-derived (Ba0.2Pb0.8)Ti03:(a) 60O0C, (b) 500°C, (c) 400"C, (d) 200°C.(e) 100"C, (0 dried gel.

6 shows the infrared (IR) absorption spectra of (Bao.2Pbo.8)Ti03 heated to various temperatures. The absorption bands around 3450 cm-1 (peak 1) are caused by the stretching vibration of -0-H bonds. With increasing heat treatment, a single C bond diminishes at lo00 cm-1 in size and a double C bond at 1650 cm-l.Peaks2and 3 shift to lower wave numbers, and the intensity of the bands decreases with increasing temperature, suggesting gradual evaporation of the residual organic compounds. The absorption band (peak 4) corresponds to the inorganic metal characteristic peak in the sol-gel-derived BaTi03-PbTi03 solid solution. The broad absorption band at 530 cm-1 [l81 corresponds to the characteristic peaks of BaTiOs. Lead titanate possesses two characteristic absorption bands at 580 cm and 400 cm-1 [19]. Peak 4 is broad and appears to include a PbTiOs peak in the region; there is a large component of Pb in the composition of (Ba0.2Pbo.8)Ti03. The breadth of peak 4 narrows with increasing heat treatment temperature.The sharper peak indicates strengthening of the bonds[20,21]. In the same frequencyrange, the sharp peaksuggestsan increase in crystallization. Figure 7 shows infrared absorption spectra compared withCMO-derived,MSS-derived,and sol-gel-derived (Ba0.2Pbo.8)Ti03. For

222

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60

50

40

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20

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Figure 5 X-ray diffraction patterns of (Ba0.2Pl~o.g)Ti03:(a) 600°C, 1 h (sol-gel derived), @) 900°C. 2 h (CM0 derived),(c)600°C, l h (CM0derived); * (circles) BaC03.

CMO-derived powders heated to 900°C, for MSS-derived powders heated to 8OO0C, and for sol-gel-derived powderheated to 600°C, the characteristic peaks of the solid solutions indicate complete transition to the final oxide. The IR results show a tendency similar to that of the x-ray diffraction patterns mentioned before. The formation of a fully crystalline product appears to be essentially complete at a relatively lowtemperature. Higher temperatures promote grain growth, resulting in an increase in average crystallite size but with no further crystallization. One can also observe a degree of grain growth by SEM and

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SYNTHESIS AND DISPERSION OF BaO-Ti02

223

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Figure 7 Infraredtransmissionspectra of (Ba1).2Pb&8)Ti03afterheating(a) 90OoC, 2 h (CM0derived), (b) 800"C, 6 h (MSS derived),(c) 600"C, 1 h (sol-gel derived). (From Ref. 7.)

224

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Figure 8 SEM photographs of (Bm.flbo.8)Ti03 prepared by variousmethodsand sintered at 1200°C (bar = 1 p): (a) CM0 derived, (b) sol-gel derived (nondispersed), (c) sol-gel derived (dispersed with ;2% PMMA).

"EM. Homogeneous and fine particles lead to a good sintered body. In a good sintered body, grain grows homogeneously. Figure 8 shows SEM photographs of BaTi03-PbTi03 solid solution from various synthesis methods. In the solgel-derived specimens, the grain size is smaller compared with CMO-derived specimens. However, in the well-dispersed system, which is explained in the next section, the grain grows homogeneously, resulting in high density.

111.

DISPERSIONCHARACTERISTICS OF FINEPOWDERS

A.

Dispersion Characteristics at Various pH Values

The electrophoretic mobility is proportional to the 6 potential and is thus a qualitative measure of colloid stability. Therefore, to find how to prevent aggregation and accelerate dispersion, the variation in suspension behavior with pH, which determines the surface charge, and the effect of dispersion on the sintered specimens are the subjects of this study. l, potential measurements can provide important guidance on the preparation of colloidal dispersions. A plot of 6 potential against pH for the sol-gel-derived (Bao.2, Pbo.8)Ti03 gel is given in Fig. 9 [22]. The point of zero charge (PZC) is found to occur at (Ba2+, Pb2+, and T i 4 ion concentrations of 2.0 x 10-7, 5.01 x 10-7, and 1.0 x l0-8M, respectively, or &, = - log [Ba2+] = 6.7, ppb = - log [Pb2+] = 6.3, and h i = -log [Ti&] = 6.0 [23]. A point of zero charge (6 = 0) with positive and negative charge branches is shown in Fig. 9. The 6 potential simply decreases to zero with an increase in pH to 7 and becomes increasingly negative with further increases in pH. Thermogravimetric analysis of (Ba0.2Pbo.8)Ti03 powders is shown in Fig. 10.The drastic weight loss is attributed to dehydration and the

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SYNTHESIS AND DISPERSION OF BaO-Ti02

225

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Figure 9

c potential as a function of pH for sol-gel-derived (Bao.zPbo.s)Ti03.

loss of volatile organic solvent residue from the solution at high temperature for pH 11and 13. At the isoelectric point, pH 7, the weight loss is small. Mazdiyasni [24] studied the preparation of fine particles and their applications to perovskite materials. In this report, a well-dispersed colloidal system with good reactivityaccelerated the removalof organic residue from the solution and maintained the deagglomerative conditions. Extensive reviews of the charge potential behavior of colloidal systems have been given by Hunter [25]. The most important dissociable groups are of the strong acid (sulfate or sulfonate), weak acid (sulfite or carboxyl), weak base (amine), and strong base (quaternary ammonium) types, and they may occur alone at the surface or in various combinations. These combinations form zwitterionic surfaces, and then the driving force removed from the system is replaced by repulsive and interactive forces. The specific surface areas [22] of sol-gel-derived (Bm.2Pb0.8)Ti03powders are 5 m2/g for pH 7 and 98- 100 m2/g for pH 11 and 13. That pH 7 is found to be highly agglomerated, and those at pH 11 and pH 13 indicate good dis-

226

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zyxwvuts zyxwv zy zyxwvu

Figure 10 Thermogravimetry function aas of pH for sol-gel-derived (Ba0.2Pbo.g)Ti03:(solidline) pH 1; (dash-dotline) pH 3; (dash-doubledot) pH 5; (dashed) pH 7;(circles) pH 9; (squares) pH 11; (triangles) pH 13. (From Ref. 22.)

persion. Metallurgical microscope photographs of the sol-gel-derived (Bao.2Pbo.8)Ti03at various pH are shown in Fig. 11. The results correlate with the weight loss and BET values. For all operations that involve suspensions of powders in liquids, the processing behavior is dominated by the rheological properties of the suspension. Suspension rheology is determined largely by the agglomeration of particles within the liquid. Most suspensions of ceramic interest are shear thinning, and their properties depend upon plastic viscosity. Plastic viscosity depends on the state of agglomeration. Figure 12 shows a plot of viscosity as a function of shear rate and pH for sol-gel-derived (Bm.2Pb0.8)Ti03 powders. In the samples at pH 3-9, the shear-thinning phenomenon is apparent, demonstrating that

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,

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.

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Figure 11 Metallurgical microscope photographs of sol-gelderived (Bao.2Pb0.8)Ti03:(a)pH 1, (b) pH 3, (c) pH 5, (dl pH 7, (e) pH 9, (0 pH 11, (g) pH 13. (From Ref. 22.)

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'p

0 .0

Y

=

0.02 0.01

0

zyxwvut zyxwvu zyx 10

30 40 Shear Rate (sec" )

20

50

60

Figure 12 Suspension viscosity plotted against shear rate for sol-gel-derived (Bao.$b0.8)Ti03:(opencircles)pH 1; (filledcircles) pH 3; (open triangles)pH 5; (filled triangles) pH 7 ; (open squares) pH 9; (filled squares) pH 11; (exes) pH 13. (From Ref. 22.)

227

228

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there is a weak agglomeration in the suspension. At higher pH or greater negative potential value, the viscosity decreases with increasing shear rate. One of the most obvious effects is the creation of large voids caused by poor packing around aggregates and huge voids caused by the bridging of aggregates. Such voids, much larger than the surrounding grains, cannot be removed during sintering: there is no driving force for elimination of such oversized pores. A more subtle effect occurs when the packing density is not uniform, having denser regions and more porous regions. This leads to locally inhomogeneous sintering and microstructures with completely dense patches and more porous areas. In the dense regions, there is no porosity to impede grain boundary migration, and grain growth is rapid. Sintering kinetics is affected by the state of agglomeration as well. X-ray diffraction patterns of the calcined powders and the sintered specimens of the sol-gel-derived (Ba0.2Pbo.8)Ti03 are shown in Fig. 13. In the pH 1 pattern, not only the characteristic peaks of the solid solution but also second-phase peaks are found from the HCl used to adjust the dispersion characteristics. For the pH 13 specimen, however, only the characteristic peaks of the solid solution appear. At low pH, the degree of crystallinity is reduced and the powder is not synthesized completely. Figure 14 shows infrared absorption spectra of sol-gel4erived (Bao.2Pbo.8)Ti03 calcined powder for various dispersives in pH. The intensity of the hydrogen bond and the characteristic bond of the organic compound in the dried gel decrease as the pH is increased. Peaks 2 and 3 become sharper with an increase in pH and shift to higher frequencies as the crystalline phase becomes more stable. Figure 15 shows the apparent and relative density for

c

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70

20

70

2 8 (de g)

20

Figure 13 X-ray diffractionpatterns of sol-gelderived (Bao.2Pbo.s)TiO3dispersed in (a) pH 1, (b) pH 7, (c) pH 13.

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4000 3OOO 1500 2000

1000

500 400

Wa ve num m (cm”)

Figure 14 Infraredtransmission of sol-gelderived and [email protected])Ti03: (a) pH 1, (b) pH 7 , (c) pH 13.

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Figure 15 Apparentand relativedensityplottedagainstpHforsol-gel-derived (Bao.2Pbo.8)Ti03 sintered at 1200°C for 0.5 h.

229

230

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zy YOON AND JO

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I

Figure 16 SEM photographs of(Ba0.2Pbo.g)Ti03sinteredat pH 1, (b) pH 7, (c) pH 13. (Bar = 1 pm.)

1200°C for 0.5 h: (a)

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(Ba0.2Pbo.8)Ti03 sintered at 1200°C for 0.5 h as a function of pH. At pH 7, where the dispersion is poor, the density is the lower than at all other pH levels. The effect on the sintering mechanism of agglomerate size and particle size inyttria-stabilized zirconia hasbeenstudied by Rhodes [26]. According to Rhodes’ report, an agglomerate with a very open arrangement of crystallites may develop large pores or pore clusters and leave large lenticular voids that are difficult to close. These results decrease the density. Reeve [27] predicted a similar phenomenon in BeO. At pH 11 and 13, with good dispersion characteristics, the sintered density is high. SEM photographs of the sol-gel-derived (Ba0.2Pb0.8)Ti03as a function of pH are shown in Fig. 16. At pH 13, the grain size is homogeneous; aggregation appears and the sinterability decreases at pH 7.

B. Dispersion Characteristics as a Function of Polymethyl Methacrylate (PMMA) Solvents should have a solubility parameter close to that of the organic polymer chain to maximize extension of the attached chain into the liquid. The extent of polymer adsorption from a solution is determined by the balance of three interactions: polymer-solvent, polymer-adsorbent, and solvent-adsorbent. In good solvents (i.e., solvents in which polymer-solvent contacts are energetically favored over polymer-polymer and solvent-solvent contacts, called 0 solvents), the polymer chains on one particle repel the polymer on another. In contrast, a poor solvent is such that segment-segment contacts are energetically favored, leading to association of the polymer molecules. Since osmotic pressure is a colligative property, intermolecular association leads to fewer osmot-

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SYNTHESIS AND DISPERSION OF BaO-Ti02

ically active species in solution, and this leads to a negative deviation from ideality. The polymer is tied to the surface at a number of points, but for some of its length it is able to extend into the solution. Segments attached to the surface form trains, which are separated by loops: theends of the polymer are usually able to extend into the solutions as tails. Complete dissolution of the polymer allows the outer segments to extend into the solution and the inner segments to become attached to the particle surface. Polymers can be good dispersants only in suitable solvents. In BaTiOsand related material systems, PMMA is a popular dispersant [28]. PMMA is dissolved completely when nhexane and benzene are used as the dispersing medium for this system [28]. The results, illustrated in Fig. 17 in terms of sedimentation density, show an optimum dispersion in the 70% benzene and 30% hexane mixture. Above 40% n-hexane, the 8 point is reached and the sedimentation density decreases drastically. The 8 point is as follows: with gas molecules, polymer chains can show either positive or negative deviations from ideal behavior. It follows that a temperature can be found at which the solvency leads to ideal osmotic pressure behavior, at least up to a polymer concentration of a few percent. Under 8 solvency conditions, polymer molecules ofamillion molecular weight behave as if they are ideal small molecules. At this temperature, the effects of attractive and repulsive interactions balance one another. This implies

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F :

8 c

50

e

401

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zyxwvut zy zyxw I

100 0

90

10

I

I

Be nze ne ( % 1 H e x a ne (%)

I

I

60 40

50 50

I

Figure 17 Sedimentation of (Bao.zPbo.8)TiO3powderdispersedinsolutions of various PMMA contents in benzene-hexane mixtures: (a)0%.(b) OS%, (c) 1.O%, (d) 1 S%, (e) 2.0%, (f) 3.0%.

232

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that the polymer chains can telescope one another without change in the Gibbs free energy. Thus, the 8 point represents the transitional point with respect to segmentsolvent interactions. At this point, the polymer segments change from exhibiting a net mutual repulsion to a net mutual attraction. A fuller description of this change would involve consideration of segment-segment, segment-solvent, and solvent-solvent interactions. The adsorption of polymer molecules at aninterface, which determines the segment density distribution function, is critically dependent upon many factors, which include [29] (1) the chemical constitution of the polymer, (2) the chemical nature and geometrical shape of the interface, (3) the chemical composition of the solvent, (4) the mode of attachment of the polymer chains to the surface, and (5) the surface density of the polymer molecules at the interface. It is the subtle interaction of these diverse factors that determines the conformation of a polymer at an interface and thus the steric interactionina particular system. The effect ofPMMA concentration on (Ba0.2Pbo.8)Ti03 sedimentation density in 70% benzene and 30% hexane is shown in Fig. 18. The results show the best dispersion at about 2% PMMA concentration, with a slight decrease in packing at higher concentrations, which can also affect weight loss during firing. Dehydration and removal of the organic residue increased with increasing the concentration of polymer to a critical concentration, above which these decreased. The weight loss is not proportional to the concentration of polymer as a dispersant. Poly(viny1 alcohol) in aqueous solution takes up much space, which results in brisk dehydration. This can be confirmed by Fig. 19. Optical micrographs for sol-gel-derived (Ba0.2Pbo.8)Ti03powders dispersed in 70% benzene and 30% hexane mixture as a function of PMMA concentration are shown here. Optimum dispersion ap-

r

0

zyxwvu zyxwvuts zyxwv

l 2 PMMA concentration ( %

3

Figure 18 Effect of PMMA content on (Ba0.2Pbo.8)Ti03 sedimentationin 70% benzene and 30% hexane mixture.

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SYNTHESIS AND DISPERSION OF BaO-Ti02

233

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Figure 19 Metallurgicalmicrographsfor sol-gelderived (Bac~Pbr~)Ti03 powder dispersed in 70% benzene and 30% hexane as a function of PMMA content: (a) 0%, (b) OS%, (c) 1. 0%, (d) 1.5%, (e) 2.0%, (0 3.0%.

pears at about 2% PMMA concentration. This result supports the preceding theory. There are numerous ways of determining the concentration of the polymer in the adsorptionmedium. These include (1) gravimetric analysis by direct weighing of the polymer after vaporization of the solvent, (2) spectrophotometric analysis using the visible, infrared, and ultraviolet regions of the spectra, (3) the refractometric method, (4) viscometric methods, (5) densometric methods, (6) titrimetric analysis, (7) use of radioisotopes, and (8) measurements of the change in other physical constants, such as turbidity, and dipole moment. Viscosity measurement is used to determine the quality of dispersion in suspensions. A well-dispersed system usually shows a Newtonian viscosity with no pseudoplasticity. The viscosity of such a system is lower than that of poorly dispersed systems. A plot of viscosity against dispersant concentration typically shows a rapid decrease in viscosity and then a leveling at a concentration required for monolayer coverage of dispersant on particle surfaces. At higher dispersant concentrations the viscosity either rises again or remains low. The degree of agglomeration in dilute particle suspensions can also be found

234

zz zyxw z YOON AND JO

by particle size measurements using a photon correlator; agglomeration leads to an increase in the average apparent particle size. Figure 20 represents plots of suspension viscosity against shear rate for solgel-derived (Ba0.2Pb0.8)Ti03 as a function of PMMA concentration. The viscosity decreased with increasing shear rate in the 0% PMMA solution. In this chapter the suspension with no polymer addition shows extensive low shear rate aggregation, as indicated by the high shear-thinning behavior. The viscosity becomes independent of shear rate, that is, Newtonian behavior observed with an increase in the polymer. For the interaction between polymers and a mica sheet, explained in detail by Klein [30], the viscosity of the suspension with polymer washigher than that of the suspension without polymer. The reason for this is the high molecular weight of the polymer itself. Molecular packing and sedimentation density play a crucial role in determining the characteristic of the green and sintered specimens. The high packing density satisfies the conditions for a pore-free and dense, sintered body. Figure 21 shows the apparent and relative densities of sol-gel-derived (Bao.2Pbo.8)Ti03 sintered at 1200°C for 0.5 h after dispersion in PMMA solution. The sintered density increases with increasing the PMMA concentration to 2% PMMA, above which it decreases. The highest sedimentation density at 23% concentration is char-

Y

zyxwvuts

zyxwv zyxwvu zyxwvu zyxw zyx zy zyxwvuts Shear Ra t e (sec")

Figure 20 Suspensionviscosityplottedagainst shear rate for sol-gelderived (Bao.zPbo.8)Ti03 as a functionof PMMA content: (a) 0%,(b) OS%, (c) 1.0%, (d) 1.5%, (e) 2.0%, (f) 3.0%. (From Ref. 28.)

235

SYNTHESIS AND DISPERSION OF BaO-Ti02

I

0

1

0

z

0

zyxwvut zyxwvu zyxwvu zyxwvuts zyxwvu l 2 3 PMMA concentration (% )

Figure 21 Apparentandrelativedensityplottedagainst gel-derived (Bao.2Pbo.8)Ti03 sintered at 1200°C for 0.5 h.

PMMA content for sol-

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acteristic of a well-dispersed suspension. Addition of a polymer enhances the strength, flexibility, and workability of ceramics in its green state, before sintering. It might be preferable for the polymer to function not only as a dispersant but also as a binder. The effect of a dispersant on green and sintered specimens has been studied by Calvert et al. [31]. The agglomeration causes inhomogeneous grain growth and void formation. The residual pores and agglomerates in the sintered body act as microflawsthatadversely affect mechanical and dielectrical properties. InFig. 22 the dielectric constant of (Ba0.2Pbo.8)Ti03sintered at 1200°C for 0.5 h is given as a function of PMMA concentration. The packing density increases with increasing PMMA concentration [28]. A good packing density results in high sintered density and high dielectric constant. However, at higher PMMA concentrations (>2%), poor dispersion behavior causes decreasing density and results in a decrease in the dielectric constant.

236

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zy zy zyxwvuts YOON AND JO

r

zyxw zyxwvu zy zy zyxwv zyxwvu zy zyxwvu zyxwvu zyxwvut zyxwvu Temperature (OC)

Figure 22 Dielectric constantplottedagainsttemperatureforsol-gelaerived (Bao.2Pbo.g)Ti03sinteredat 1200°C for 0.5 h withaPMMAcontentof OS%, (c) 1.0%, (d) 1.5%, (e) 2.0%, (f) 3.0%.

(a) 0%, (b)

REFERENCES

1. Sakka, S., Sol-gel synthesis of glasses: Present and future, Am. Cerum. Bull. 6 4 , 1463 (1985). 2. Segal, D. L.,Sol-gel processing: Routes to oxide ceramics using colloidal dispersionsofhydrogenoxidesandalkoxideintermediates, J. Non-Cryst. Solids, 63, 183 (1984). 3. Dislich, H., and Him, P., History and principles of the sol-gel process, and some new multicomponent oxide coatings, J. Non-Cryst. Solids, 48, 11(1983). 4. Jo, K. H., Yoon, K. H., Preparation of sol-gel derived (Bao.zPbo.g)Ti03 powders, Muter. Res. Bull., 24, 1 (1989). 5. Ueyama, T., Wada, H., and Kanako, N., Pulverization and dispersion technique foragglomeratedparticlesofaluminapowderinaslurry, J. Am Cerum Soc. Commun., 71, C- 74 (1988). 6. Afsten, N. J., Sol-gel derived transparent IR-reflecting IT0 semiconductor coatings and future applications, J. Non-Cryst. Solids, 63, 243 (1984).

zyxwvutsr zyxw z zyxwvuts zyxw zyxwvuts

SYNTHESIS AND DISPERSION OF BaO-Ti02

237

7. Jo, K. H., and Yoon, K. H., Characteristics of the (Bal-xPh)Ti03 powders prepared by various synthesis methods, J. Kor. Cerum. Soc., 27, 127 (1990). 8. Mazdiyasni, K. S., Fine praticle perovskite processing,Am. Cerum. Soc. Bull., 63, 591 (1984). 9. Flaschen, S. S., Preparation of BaTiOs by chemical methods, J. Am. Cerum. Soc., 77, 6194 (1955). 10. Ritter, J. J.. Roth, R. S., and Blendell, J. E., Alkoxide precursor synthesis and characterizationofphaseinthebarium-titaniumoxidesystem, J. Am.Cerum. Soc., 69, 155 (1986). 11. Phule,D.P.,andRisbud, S. H.,Sol-gelsynthesisofbariumtitanatepowders Adv.Cerum. Mat., 3, 183 usingbariumacetateandtitanium(1V)isopropoxide, (1988). 12. Grukovich, S. R.,andBlum, J. B., UltrastructureProcessing of Ceramics, Glasses and Composites, John Wiley and Sons, New York, 1984, p. 152. K., Formation,packing,andsinteringof 13. Baninger,E.A.,andBowen,H. monodisperse Ti02 powders, J. Am. Cerum. Soc., 65, C-l99 (1982). Metal Alkoxide. Academic 14. Bradley,D. C., Mehrotra,R.C.,andGaur,D.P., Press, NewYork, 1970, p. 55. 15. Yoon, K. H., Oh, K. Y., and Yoon, S. O., Influence of synthesis methods of the FTCR effect in semiconducting BaTi03, Mat. Res. Bull., 21, 1429 (1986). 16. Mazdiyasni, K. S., Dolloff, R. T., and Smith, J. S., III, Preparation of high purity submicron barium titanate powders, J. Am. Cerum. Soc., 52, 523 (1969). 17. Rehspringer,J. L., and Bemier, J. C., Mat. Res. Soc. Symp. Proc., Pittsburgh, PA., 1986, p. 67. 18. Last, J. T., Infrared-absorption studies on barium titanate and related materials, Phys. Rev., 105, 1740 (1957). 19. Congshen, Z., Lisong, H., Fuxi, G., and Zhonghong, J., Low temperature synthe-

zyx zyxwvu zyxw zyxwv

zyxwv

sis of ZrO2-TiO2-SiO2 glasses from Zr(N03)45H20, Si(OC2H5)4 and Ti(OC4H9)4 by the sol-gel method, J. Non-Cryst. Solids, 63, 106 (1984). 20. Gottardi,V.,Gulielini, M., Bertoluzza,A.,Fagnano,C.,andMorelli,M.A., Ramanandinfraredspectraonsilicagelevolvingtowardglass, J. Non-Cryst. Solids, 48, 117 (1965). Mat. Sci. Eng., 44, l(1980). 21. Bowen, H. K., Iron diffusion in iron-aluminate spinel, 22. Jo, J. H.,Kim,E. S., and Yoon, K. H., Dispersion characteristics of sol-gel derived (Bao.flb0.8)Ti03 at various pH, J. Mat. Sci., 25, 880 (1990). 23. Lauf, R. J., and Bond, W. D., Fabricationof high field zinc oxide varistor by solgel processing. Am. Cerum. Soc. Bull., 63, 278 (1984). on measure24. Mazdiyasni, K. S., Effects of dispersion of barium titanate powder ments in an electrical-sensing-zone particle size counter, Am. Cerum. Soc. Bull., 57, 448 (1978). 25. Hunter, R. J., Zetapotential in Colloid Science, Academic Press, New York, 1981, p. 134. on sintering yittria-stabi26. Rhodes, W. H.,Agglomerateandparticlesizeeffects lized zirconia, J. Am. Cerum. Soc., 6 4 , 19 (1981). 27. Reeve, R. D., Non-uniform shrinkage in sintering,Am. Cerum. Soc. Bull., 42,452 (1963).

zyxwv zyxwvuts

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28. Jo, K. H., and Yoon, K. H., Dispersion characteristics of sol-gel derived (Bao.2pbo.8)Ti03 as a function of PMMA, J. Mat. Sci., 26, 809 (1991). 29. Napper,D. H.,Polymeric Stabilimtion of Colloid Dispersions, Academic Press, NewYork,1983, p. 197. 30. Klein, J., Forces between mica surfaces bearing layer of absorbed polystyrene in cyclohexane, Nature, 288, 248 (1980). 31. Calvert, P., Lanardham, R., Parish, M., and Tormey, E.,Dispersants in Ceramics, JohnWileyand Sons, New York, 1984, p. 249.

10

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

Michigan Technological University Houghton, Michigan

1.

INTRODUCTION

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The plastic forming of ceramic shapes is based on the application of external forces to a mixture of ceramic powder and binder. The plastic mixture deforms and flows under applied stresses. The external forces cause the plastic mix to be adjusted to any die or mold, which dictates the eventual shape. Therefore, the flow behavior of such mixes during plastic forming has a major effect on the quality of the ceramic parts. Again, the flow stability of the mix during forming depends on the homogeneity of the ceramic powder and binder mixture. The primary focus of this chapter is to describe the factors that influence flow behavior and mix homogeneity of a plastic mixture containing a high volume fraction of ceramic powder as a filler and a relatively small volume fraction of organic polymeric binder. The emphasis is placed on mixes in which flowability is achieved only above the ambient temperature.

II. RHEOLOGY The flow of a plastic mass through a die or into a mold is dictated by its rheological behavior. Rheology describes the deformation and flow of a material under the influence of applied stresses. The rheological response of a fluid is generally expressed as viscosity. The viscosity of a fluid is a measure of the

239

240

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internal resistance offered to the relative motion of different parts of the fluid. Viscosity is defined as Newtonian when the shearing force per unit area z between two parallel planes of fluid in relativemotion is proportional to the velocity gradient dvldx between the planes; in other words, dv

z= q-

dx

where:

q = coefficient of viscosity z = shear stress = forcelarea = Nlm2 = Pa dvldx = shear rate y = m/s/m = S-1 q = dynamic viscosity = zly, (Pa=s)

For many fluids, especially if highly concentrated and/or if the particles are asymmetric, deviations from Newtonian flow are observed.The main causes of non-Newtonian flow are the formation of a structure throughout the fluid and the orientation of asymmetric particles caused by the velocity gradient. Non-Newtonian fluids may be classified (shown in Fig. 1) as follows:

Pseudoplastic, in which the shear stress depends on the shear rate alone. Power law, in which the shear stress is not a linear but an exponential function of shear rate. The rheological expression for a power-law fluid is T = Ky"

(2)

where n is the power-law index and K is known as the coefficient of vis(1) if n = 1, or unity, the cosity. Here, we have three different situations: fluid is characterized as Newtonian; (2) if n < 1, less than unity, the fluid is considered pseudoplastic; and (3) the fluid is characterized as dilatant if n > 1. This lastcharacteristic is sometimes encountered with filled systems when the filler concentration is very high, so that the corresponding scarcity of a continuous phase makes it difficult for the dispersed particles to slide over one another. The lack of adequate lubrication by the continuous phase produces frictional voids and sets up structures among the particles under constraint. These structures become increasingly resistant to deformation as the applied shear rate is increased: the viscosity increases with increasing shear rate. Fluids in which no deformation occurs until a certain threshold shear stress is applied, in which upon the shear stress z becomes a linear function of shear rate y. The characteristics of the function are the slope (viscosity) and the shear stress intercept (yield value) zy The rheological expression for this type of material, known as a Bingham solid, is 2 "Z y

= qy

zyxw zyx (3)

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PLASTIC MOLDING RHEOLOGY AND MIXING ORatenl with Yield Pdnt

t

Q) Q)

2 ti

Pseudoplastic with

Yield Point

Shear Rate

Figure 1 Classification of non-Newtonianfluids.(After J.

S. Reed, Introduction to

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

Only when the applied stress is greater than the yield stress do viscous flow and shear deformation become manifest. Fluids that show a decrease in shear stress (hence in apparent viscosity) with time are termed thixotropic and are often observed with ceramic slurries. Rheologists have long believed that all fluids are viscoelastic in behavior.

As a result, the deformation of any fluid from the imposition of a stress is the

sum of an elastic deformation, which is recoverable, and viscous flow, which is not recoverable. For fluids of low viscosity at moderate rates of shear, the elastic recovery is extremely rapid and the relaxation time is extremely short. As a result, the elastic portion of the deformation is too small to measure, and the fluid is considered simply viscous. When viscoelastic fluids are stressed, some of the energy involved is stored elastically, various parts of the system being deformed into new nonequilibrium positions relative to one another. The remainder is dissipated as heat, various parts of the system flowing into new equilibrium positions relative to one another. The elastic nature ofa fluid is characterized by dynamic mechanical or stress relaxation techniques. Dynamic mechanical (oscillatory) testing is a procedure inwhicha sample is sinusoidally strained and the resultant stress is measured. The she& stress z varies with the same frequency as the shear rate

242

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y but not in phase with it, the phase angle between z and y being equal to n/2

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

A.

zyxwv zyxw zy z

Viscosity of HighlyFilledFluids

Figure 2 shows the flow curves for concentrated aqueous suspensions of 30 vol% 3Y-zirconia and 70 vol% alumina [l]. Figure 2 illustrates near-Newtonian flow characteristic of suspensions with filler concentrations as high as 56 ~01%.Newtonian flow is generally encountered with highly filled suspensions when the polymeric binder concentration in the suspension is fairly low ( 4 ~01%)and the suspension is highly dispersed (discussed later). Figure 3 shows a gradual decrease in viscosity at different ceramic powder concentrations with

p

E: v,

zyxwvutsrq zyxwvutsr 120

60

0

zyxwvuts 0

15

30

45

60

75

Shear Rate,

Bo

105

120

135

150

sec"

Figure 2 Flow curves for concentratedaqueoussuspensions of 30 vol%3Y-zirconia and 70 vol% alumina.

PLASTIC MOLDING RHEOLOGY AND MIXING 10000

A

v)

z zyxw 243

zy zyxw zyxw zyx zy zy zy

zyxwvutsrqpon * x

4 I

l00 100

0

..

60 V/O

55v/O

Sic + 4 0 V/O EEA

SIC + 4 5 V/O

5 0 V/O

4 0 V/O

EEA

Sic + 5 0 V/O EEA

Sic + 6 0 V/O EEA

zyxwvutsrqpon

zyxwvu I

r?

10000

l000

Shear Rate (per

S)

zyxw zyxwvutsr zyxwvu

Figure 3 Decrease in viscosity at different concentrations of silicon carbide in EEA with increasing shear rate at 150°C.

increasing shear rate at 150°C temperature. The flow behavior is typically pseudoplastic. The power-law index n of these fluids should be less than 1. The suspension consists ofa silicon carbide powder (Starck A-10) and ethylene ethyl acrylate (EEA) polymer (40-60 ~01%).Figure 4 shows (shear stress)ll2 plotted against (shear rate)ln for 55 vol% silicon carbide in polyethylene at 150°C. The extrapolated intercept at zero shear rate gives the Bingham yield value .z, The general phenomenon of dilatancy was observed by Reynolds in 1885. Reynolds [ 2] observed that a mixture of beach sand and water confined in a balloon would dilate if deformed; that is, the total volume of the mixture would increase under pressure, causing the liquid level in the balloon to be lowered. In addition to the dilation, such a mixture would exhibit increasing resistance to stirring as the rate of stirring was increased, until rupture of the pseudosolid material that resulted occurred. A more general meaning of the term “dilatant fluid” can be given by extending it to all materials showing increasingly apparent viscosity with an increasing rate of shear. Sometimes this is referred to as inverted plasticity to differentiate between volumetric dilation versus no volumetric dilation. Typical dilatant flow curves for two concentrations of A-16SG alumina and paraffin wax are shown in Fig. 5 . Reynolds’ approach to dilatancy has some limitations. For example, Reynolds’ approach specifies hexagonal close pack-

N

"

zyxwvutsr z

zyxwv

244

MATSUDDY

zyxwvu zyxwvutsrqp zyxwvutsr

2m a

0

zyxwvut zyx zyxwvu Shear R a t e 1/2 , Se c112

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

zy zyxw

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

B.

Viscoelastic Behavior of Highly Filled Fluids

Figure 6 shows plots of the storage modulus versus oscillation frequency for samples with S i c concentrations of 40, 50, 57, and 63 ~01%.As the solids

PLASTIC MOLDING RHEOLOGY AND MIXING

245

zyxwvutsrqp zyxwvutsrqp zyxwvutsrq z zyxwvutsrqp zyxw zyxwv zyxw I

10

zyxw zy

1

1

I 1 1 1 1 1 .

i 02

I

Shear Rate, Se c

I

-1

I

I

I l l l j 10

Figure 5 Dilatant flow curves for two concentrations of A-16SG alumina and paraffin wax, 37 and 35 ~ 0 1 %.at 150°C.

zyxw

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

C.

Influence of Temperature on Fluid Flow

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

246

z zyxwvutsr zyxwv zy zyxwvutsr zyx zy zy

r

I

109

-

IO8

- ii

b 106

I

I I I Ill1

I

I

1

I

I 111'1

I

I IIIII-

-

V

V

V

V

m

m

m

V

m m mmIg,

P

h

m a 107 v

z

MATSUDDY

P@

zyx zyx zy tzyxwv -

0

0

54

0

0

-

V

X

-

X

X

X-

0-

m-

1

I

4 0 V/O

Sic + 6 0 V/O EEA

5 0 V/O SIC

+

50"I/O

EEA

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

zyx zyxwvu v-

lo5

104L1

V

X

I I 1 1 l l1 l

I

1

I I I I l l10 1

I

I

I I I Ill

Frequency, d l =

Figure 6 Storage modulus versus oscillating frequency for 40, 50, 57, and 63 vel% silicon carbide with EEA at 200°C.

creases with increasing temperature for both Newtonian and non-Newtonian fluids, primarily because of a decrease in intermolecular forces. The increased flexibility of a polymer backbone chain also contributes to reduce viscosity. The relation between viscosity and the temperature of a filled suspension can be expressed by the modified Eyring equation, which allows for variations in the free volume of the fluid structure:

zyxwvu zyx

In this equation h is Planck's constant, No is Avogadro's number, Vm is the molar volume of the fluid phase, E is the activation energy needed to overcome the potential barrier between equilibrium positions, y is a constant in the range 0.5-1.0, V0 is the van der Waals volume of the molecule, and V is the average volume per molecule in the fluid. Because of the exponential form of the viscosity equation, mixtures of ceramic powder in thermoplastic binder are only significantly sensitive to temperature change above 70°C [3]. In an injection molding operation, it is usual to plot In q versus T for a constant shear rate, not a In q versus 1/T plot at a

z zyxwv zyxw

PLASTIC MOLDING RHEOLOGY AND MIXING

247

zyxw zyxwvut zy zy zyxwvu 353 363 373 Temperature(K)

Figure 7 Effect of temperature on viscosity at a constant rate of shear of 100 S-’ . includes 55.0 vol% A1203 and 45 vol% ethylene-vinyl acetate. (After Ref. 3).

constant force. An example of one such plot for 55.0 vol% A1203 loading in 45 vol% ethylene-vinyl acetate copolymer is shown in Fig. 7. The linearity of sucha plot indicates the process sensitivity to temperature change, and the slope provides a measure of the activation energy for viscous flow. For a filled suspension, the effect of temperature on particle-particle interaction must be considered, in addition to the effect of temperature on inter- and intramolecular interaction in polymers. This temperature effect depends on the level of loading. If the temperature is sufficiently high to allow migration of binder from particle-particle interfaces, then the suspension may exhibit dilatancy in place of pseudoplasticity. The effect of temperature is similar to that of shear rate on polymer rheology, but its effect on particle-particle interaction is unpredictable. Both high temperature and high shear rate weaken polymerparticle interaction. However, it is important to remember that the extent of the effect (viscosity and interaction reduction) for a particular change in shear rate or temperature is not predictable. Both factors, exerting their effects at different rates, contribute to the complexity of the system.

zy zy zyxw

D. Effect of FillerConcentration

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

248

zyxwvutsr z zyxwv zyxwvu zyxw zyxw

zyxwvu MATSUDDY

zyxw zyxwvu zyxwv zy

In the dilute region (CD O.l), q~ is a linear function of CD and for spheres is found to obey the Einstein relation [4]

(5)

q R= l + 2.5$

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

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

+

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

and introduced k as l/CDmax.In the limit CD + 0, Eq. (7) reduces to 1 + B@. Thus, B can be interpreted as an Einstein coefficient and should be between 2.5 and 5, depending on the degree of agglomeration and the extent of electrochemical forces between the particles. As CD + l/k, r ) + ~ m ; therefore, k-* refers to the maximum possible value of CD, that is, the value for which the suspension loses mobility, k is known as a crowding factor. Equation (7) provides a reasonable fit; as the data for an A1203-EVA (ethylene-vinyl acetate) copolymer (with M n = 3500 and melt flow index = 200, the melt flow index defined in terms of the amount of polymer that flows through a die in a given period of time: the higher the flow rate and thus the lower the viscosity, the higher the melt flow index) show the 4 2 0 3 had an irregular shape and agglomerated with a mean particle size 0.62 pm. Relative viscosities at four powder loadings are given in Table 1. Taking logs of Eq. (7),

B$ lnqR = l-kt$

zyxwv zyxw z zyxwvu zyxwvuts zyxwvuts zyxw zy zyxwv zyxwvu

PLASTIC MOLDING RHEOLOGY AND MIXING

249

Table 1

RelativeViscosity Determinedat a Shear Rate of 100s-1 of A1203-EVA Molding Mixes vol.% A1203

1.83 41.0 48.0 3.91 57.0 63.0

Relative viscosity 6.25 12.55 49.90

In 7

) ~

2.53

5.30

Source: After Ref.

3.

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

@m:

B 1 - 0.41/$, B 11.43 = 1 - 0.48/$, 7.5 =

zyx zyx zyxwvut zyxw zyxwv at x

at Y

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

5 CI

F

4 ~ -

v

E3

-

2 -

1 -

0.1 0 . 2 0.3

0 . 4 0.5

0.6

0.7

Volume Fraction

Figure 8 Plot of 1OOo"C

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

> SicI+,

> Si,C,N,

> SiC+C

>16000c > 1 6 oooc

polycarbosilane

> S i c + C + N,

polysilazane

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

A.Synthesis

of SICFibers

Figure 6 shows the gas evolution and structural change in polycarbosilane during heat treatment [33]. The dotted line indicates that the polycarbosilane was cured in air. r

l

Temperature/'C

Figure 4 Weight loss curves for plycarbosilane (A), polysilazane (B), and polycarbosilane (C). (From Ref. 32.)

zy

zyxwvut

POLYMER PYROLYSIS

383

zyxwvu zyxw zyxwvu zyxwv

Electron ifradiation curing

Oxidation curing

/ \

/ \

Heating in NHS

Heating in Ar or in vacuum

Heating in NHJ

Heating in Ar or in vacuum

I

J

I

I

Silicon oxynltride fiber

Silicon carbide fiber

zyxwvuts zyxwv (Si-C-0)

Figure 5 Preparation of

various ceramics from polycarbosilane.

In the first step, the polycarbosilane is concentrated by distillation to remove low-molecular-weight components and to adjust its spinnability. A typical distillation temperature is 280"C, and the number-averaged molecular weight of the resulting polymer is 1500-2000. The polymer is spun into fibers by a melt spinning method. At this stage, the spun polycarbosilane fibers are very fragile. The spun fibers are cured by oxidation in air at a temperature ofup to 100-200°C or by 'y-ray or electron beam irradiation in a vacuum or an inert atmosphere. The curing process is necessary to permit the conversion of polycarbosilane fiber to silicon carbide without softening during heat treatment. Oxidation curing is achieved by cross-linking the polycarbosilane with oxygen bonds, such as Si-0-Si or Si"c, whereas radiation curing forms direct Si-Si or Si-C bonds [34]. Heating polycarbosilane, whether cured or uncured, results in the evolution of H:! and CH4 gas as the heating temperature is increased. The thermal decomposition product from 500 to 800°C has a structure intermediate between that of organic and inorganic compounds. The product contains many radicals, and the pyrolyzed fiber keeps its flexibility. Although the thermal decomposition of cured polycarbosilane fiber. is completed in the neighborhood of 8OO"C,

zyxwv zyxwvu

384

NARISA WA AND OKAMURA

zyxwvu zy

z zyxwv

zyxwv

-c(

a

d 0

zy zyxwvuts zyxwvut

POLYMER

385

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

386

Si:

zyx zy

NARISAWA AND OKAMURA

zyxw zyxwvuts zyxwvu

Figure 7 Atomic arrangement in Sic fiber pyrolyzed by oxidative curing.

iorsimilar to that of polycarbosilane,exceptabsolute quantity of radicals. However, ESR of fiber cured by oxygen shows only one large peak at the second position, 1200°C. For polycarbosilane without curing, the peak width reachesamaximum ataround 700°C and aminimum at 1100°C. Beyond

r 3.01

zy zyx

zy zyx 387

POLYMER

300

rI

d)

zy zyx zyx

zyxwvut z loot I T

1200

1300

1400

1500

&

I

I

I

I

T

z

1200 1300 1400 1500

Temperaturd 'C

Temperature/'C

Figure 8 Tensile strength and Young's modulus of Sic fibers. (a) Oxidation curing (02content 10.8%). (b) Electron curing( 0 2 content 4.0%). (c) Electron curing( 0 2 content 1.9%). (d) Electron curing (@ content < 0.5%). (From Ref. 43.)

zyxwv

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

B.

Synthesis of SiC-TC Fibers

SiC-Tic fibers have been obtained by the pyrolysis of a polytitanocarbosilane (PTC-0) [39]. PTC was synthesized by adding titanium alkoxide to polycarbosilane in xylene. The PTC was melt spun and cured in air at 180°C. The cured fibers were heated in N2 gas at temperatures in the range 800-1500°C. Ube Industries Ltd. has prepared Si-Tic-0 fiber (Tyranno) on an industrial scale [40]. Figure 10 shows the tensile strength, Young's modulus, and specific resistance of Tyranno fibers. Tensile strength is maximum for the 1300°C treatment compared with the maximum at 1200°C for S i c fibers. This superior heat resistance of Tyranno fiber seems to be caused by the bonding of excess carbon with titanium or the increase in the overall bonding strength of the constituent elements in the fiber by the addition of titanium.

zyxw

388

zyxwvuts zyx z NARISAWA AND OKAMURA

z zyxwvu zyxwvut zyxwv zyxwv zyx 1

0

I

200

800 1400 Raman shift (cm”)

2000

Figure 9 Ramanspectra of Sic fibersobtainedbytheheattreatment of polycarbosilanefibers at (a) 1000°C, (b) 1200”C,(c) 1400°C. (d) 1500”C,and (e) 1700°C (From Ref. 36.)

POLYMER PYROLYSIS

-2

200 -

zyxw zyxwv zyxw zyxwvu zyxwv U

-a Y)

3

U

.-

f !Vi zyxwvu zyxwv i P :P P

2 100Y)

C

r-"

,

(a)

zyxzyxw

zy

zy zyx P zyxwv 389

*

800

(c)

I

1000

I

,

I

.

I

L

.

I

1200

zyxwvuts I

I

1400

Temperature ("C)

Figure 10 (a)Tensilestrength, PTC fibers. (From Ref. 40.)

,

(b)

Temperature ( "C)

1l"I

I

0 700 900 110013001500 Temperature ("C)

(b) tensile modulus, and (c)specificresistance

of

390

zyx z zy zyxw zyxwvu zyxw zyxwv

--

NARISAWA AND OKAMURA

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

The nitridation of polycarbosilane (with ammonia) begins at about 500°C and terminates almost completely at about 800°C. There is scarcely any carbon in the nitride obtained at 1400°C. Chemical analysis indicates its composition to be Si3N3.72, almost that of pure silicon nitride, Si3N4 [39]. Up to 1300"C, the X-ray diffraction pattern of nitride fiber is broad and characteristic of the amorphous state, whereas those at 1400°C indicate crystalline a-SisN4 (Fig. 11). Both silicon nitride and silicon oxynitride fibers have been obtained by the heat

120O0C

13OO0C

20 (degrees)

zyxwv

Figure 11 X-ray diffraction patterns of silicon nitride obtained by the nitridation polycarbosilane. (From Ref. 34.)

of

POLYMER PYROLYSIS

z zyxw zyxwv zyx zyxwv zyxw 391

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

D. Synthesis of Silicon Nitride Fibers from Polysilazanes

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

E. Synthesis of Sic Whisker from a Biological Source

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

F.

Synthesis ofSi3N4-SiCCeramics

Si3N4 ceramics using polycarbosilane have been prepared by Kurosaki Yogyo (Kuroceram-N). In the first step,polycarbosilane is mixed with Si powder with a diameter below 10 p and cured by oxygen. By pyrolysis in a N2 atmosphere, the sample is converted into Si3N4 ceramics. Nitridation of Si yields Si3N4, and the pyrolysis of polycarbosilane yields Sic and excess carbon. The excess carbon is trapped by unreacted Si and changed Sic. The ceramics obtained consist mainly of Si3N4 with a few percent Sic. By changing the quantity of polycarbosilane, the microstructure of the ceramic can be controlled through the form of the residualSic. A microgap between Si3N4 and Sic crystallites gives flexibility to the material, and high thermal shock resistance is achieved.

G.

SynthesisofHeat-ResistantCoatings

Tyranno Coat was obtained from Tyranno polymer and widely used [43]. The virtures of Tyranno Coat are as follows: 1.

The polymer can be solubilized in organic solvent and used in the form of a solution or slurry.

zyx z

N4RISAWA AND OKAMURA

7 .,

L

0

zyxwv zyxwv zyxwv Tyranno polymer

zyxwvu zyxwvu zyxw zy

zyxwvutsrqpon 250

500

750

1000

Temperature/'C

Figure 12 Weight loss curve of Tyranno Coat (in air). (From Ref. 43.)

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

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

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

z zy zyxwv zyxwv zyxwvu zyxw

POLYMER PYROLYSIS

393

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

zy zyxwvu zyxwvu zy zyxw

REFERENCES

1. Yajima, S., Hayashi, J., and Omori, M., Chem Lett., 931 (1975). 2. Yajima, S., Okamura, K., Hayashi, J., and Omori, M., J. Am. Ceram Soc., 59, 324 (1976). 3. Yajima, S., Iwai, T., Yamamura, T., Okamura, K., and Hasegawa, Y., J. Mater. Sci., 16, 1349 (1981). 4. Okamura, K., Sato, M., Hasegawa, Y., and Amano, T., Chem Lett.,2059 (1984). 5. Prewo, K. M., Brennan, J. J., and Layden, G. K., Am. Ceram. Soc. Bull., 65, 305, 322 (1986). 6. Cornie, J. A., Chinag, Y., Uhlmann, D. R., Mortensen, A., and Collins, J. M., Am. Ceram. Soc. Bull., 65, 293 (1986). 7. West, R., J. Organometal. Chem, 300, 327 (1986). 8. West, R., David, L. D., Djurovich, P. I., and Yu, H., Am. Ceram. Soc. Bull., 62, 899 (1983). 9. Baney, R. H., and Gaul, J. H., Jr., U.S. Patent 4,310, 651 (1982). 10. Baney, R. H., in Ultrastructure Processing of Ceramics, Glasses and Composites (L. L. Henchand D. R. Ulrich.eds.),JohnWileyandSons,NewYork, 1984, p. 235. 11. Baney, R. H., Gaul, J. H., Jr., and Hilty, T. K., Organometallics, 2, 859 (1983). D., Adv. Inorg. Chem Radiochem., 77, 349 12. Fritz, G.,Grobe,J.,andKummer, (1965). 13. Yajima, S., Omori, M., Hayashi, J., Okamura, K., Matsuzawa, T., and Liaw, C., Chem. Lett.,551 (1976). 14. Yajima, S., Hayashi, J., and Okamura, K.,Chem. Lett.,521 (1977). T., Nature, 273, 15. Yajima, S., Hasegawa, Y., andOkamura,K.,andMatsuzawa, 525 (1978). 16. Schilling, C. L., Jr., and Williams, T. C., Polym. Preprints, 25, 1(1984). 17. Schilling, C. L., Jr., Br. Polym. J., 18, 355 (1986). 18. Hasegawa, Y., and Okamura, K., J. Mater. Sci., 21, 321 (1986). 19. Yamamura, T.,Polymer Preprints, 25(No 3), 8 (1984). 20. Yajima, S., Hayashi, J., and Okamura, K., Nature, 226, 521 (1977). 21. Baney, R., Gaul, J., Jr., and Hilty, T., Mater. Sci. Res., 17, 253 (1984).

394

zyxwvuts z zy zyxwvu zyxwvu zyx zyxw zy NARISAWA AND OKAMURA

22. Verbeek, W., U.S. Patent 3, 853, 567 (1974). 23. Penn,B.G.,Ledbetter,F. E., m, Clemons,J.M.,andDaniels,J. G.,J. Appl. Polym Sci., 27, 3751 (1982). G. H.,in UltrastructureProcessing of Ceramics, 24. Seyferth,D.,.andWiseman, Glasses and Composites (L. L. Hench and D. R. Ulrich, eds.) , John Wiley and Sons, New York, 1984, p. 265. 25. Arai, M., Sakurada, S., Isoda, T., and Tomizawa, T., Polym. Preprints, 28, 407 (1987). 26. Gaul, Jr., J. H., U.S. Patent 4, 340, 619 (1982). 27. Okamura, K., Sato, M., and Hasegawa, Y., Ceram. Int., 13, 55 (1987). F’roc. 1st Japan Intema28. Kimura,Y.,Hayashi,N.,Yamane,H.,andKitano,K., tional SAMPE Symposium, November 28 to December l, 1989. 29. Paine, R. T., JAPAN-US Joint Seminar on Inorganic and Organometallic Polymers, Nagoya, Japan, March 25-27, 1991, p. 78. 30. Hashimoto, N., Sawada, Y., Bando, T., Yoden, H., and Deki, S., J. Am. Ceram. Soc., 74, 1282 (1991). 31. Amato, C., Hudson, J., and Interrante, L. V., Mater. Res. Soc. Symp. Proc., 168, 119 (1990). 32. Atwel, W. H., Bums, G. T., and Zank, G . A., private communication PolymerPreprints, 33. Okamura,K.,Sato,M.,Matsuzawa,T.,andHasegawa,Y., 25(No l), 6 (1984). 34. Okamura, K., Sato, M., and Hasegawa, Y., 6th World Congress on High Tech Ceramics (CIMTEC), Milan, Italy, June 23-28, 1986. 35. Okamura, K.,Sato, M., Seguchi, T., and Kawanishi, S., Proceedings of the Third InternationalConferenceonCompositeInterface(ICCI-III),May 21-24,1990, p. 209. 36. Sasaki,Y.,Nishida,Y.,Sato,M.,andOkamura,K., J. Mater. Sci., 22, 443 (1987). 37. Shimoda, M.,Sugimoto,M.,Katase,Y.,Okamura,K.,andSeguchi,T., Muki Kobunsi Kenkyu Touronkui, 10, 76 (1991). 38. Singer, L. S., and Lewis, I. C., Appl. Spectrosc., 36(No l), 52 (1982). 39. Okamura,K.,Sato,M.,andHasegawa,Y., Proc. Fifth Int. Conf. on Composite Mater., San Diego, CA, July 29 to August 1, 1985, p. 535. 40. Yamamura, T., Hurushima, T., Kimoto, T., Shibuya, M., and Iwai, Y., 6th World Congress on High Tech Ceramics (CIMTEC), Milan, Italy, June 23-28, 1986. 41. Isoda, T., in Controlled Interphases in Composite Materials (H. Ishida, e d . ) , Elsevier, Amsterdam, 1990, p. 255. 42. Milewshi, J. V., Sandstrom, J. L., andBrown,W. S., in Silicon Carbide (R.C. Marshall, J. W. Faust, and J. R. C. E. Ryan, eds.), University of South Carolina Press, 1973, p. 634. 43. Nishihara, Y.,Kagaku to Kogyo, 40, 309 (1987).

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Part VI PROCESSING OF SPECIALTY CERAMICS

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Processing of Lead-Based Dielectric Materials Hung C. Ling AT&T Bell Laboratones, Princeton, New Jersey

M a n F. Yan

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1.

INTRODUCTION

Historically, BaTiOs-basedmaterials have been the dielectricsof choice for use in multilayer ceramic capacitors. BaTiOs has a perovskite crystal structure and exhibits ferroelectric behavior. At its Curie temperature, the dielectric constant E may exceed 20,000. The effect of dopants on Curie point, microstructural control, and other processing parameters, such as powder conditioning (milling and solvents), green sheet processing (binder or casting), and firing temperatures and ambients, are reasonably understood. Multiple-cation substitution in the BaTiOs-based system has also been studied extensively withregardto preservation and cation ordering in the perovskite phase. In the 1950s, Smolenski and coworkers [ l 4 1 investigated many cation substitutions into PbTi03 in a search for new ferroelectric materials. In this kind of substitution, the general guidelines are that the ionic sizes should be comparable to those of W + ion and the combination must yield the same average charge as Ti& to maintain charge neutrality. Many such compositions take on the complex perovskite structure, and their properties have been compiled in Ref. [5]. With the promise of higher dielectric constants and lower firing temperatures, and hence lower electrode cost, lead-based perovskite compositions of the general form Pb(B1, &)Os have been studied extensively for application as capacitor dielectrics for the past 15 years. Of particular interest are compositions in which B1 is a divalent or trivalent cation such as Mg2+, Zn2+,Ni2+,

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398

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

= E0

-@

(1)

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

LEAD-BASED DIELECTRIC MATERIALS

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

1 0

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Figure 2 Maximum dielectric constants at 1 kHz versus pyrochlore concentration in the PbO-MgO-Nb205 ternary composition. (From Ref. 13.)

cific to a particular composition, may be developed to eliminate the pyrochlore phase from the end product. On the other hand, nonferroelectric pyrochlores may serve as technologicallyusefuldielectricsin other applications,suchas temperature-stable dielectrics or microwave dielectrics. It is important to understand the structural relationship between the perovskite and pyrochlore phases in the Pb-based systems and to elucidate the thermodynamic and kinetic factors that mayinfluence the phase equilibrium. Understanding these willallow a more systematic method of tailoring dielectric compositions to applications. Another important area in dielectric processing is the formation of dense, sintered bodies and theformation of thin layers (