fundamentals of ceramic powder processing and synthesis

F u n d a m e n t a l s

of Ceramic

Powder

Processing

and Synthesis

This Page Intentionally Left Blank

Fundamentals of Ceramic Powder Processing and Synthesis

Terry A. Ring Department of Chemical and Fuels Engineering and Department of Materials Science and Engineering University of Utah Salt Lake City, Utah

?P A c a d e m i c Press San Diego New York Boston London Sydney Tokyo Toronto

Photo taken from Millot, G., La Science 20, 61-73 (1979). Please see Chapter 1 for more information.

This book is printed on acid-free paper. ( ~

Copyright 9 1996 by ACADEMIC PRESS, INC.

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United Kingdom Edition published by Academic Press Limited 24-28 Oval Road, London NW1 7DX

Library of Congress Cataloging-in-Publication Data

Ring, Terry. Fundamentals of ceramic powder processing and synthesis / by Terry

Ring. p. cm.

Includes index. ISBN 0-12-588930-5 (alk. paper) 1. Ceramic powders. I. Title

TP815.R56 1995 666--dc20 95-15418

CIP

PRINTED IN THEUNITED STATES OF AMERICA 96 97 98 99 00 01 MM 9 8 7 6 5 4 3 2 1

To Susan's understanding

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Contents

Preface xxi

I N T R O D U C T I O N

HISTORY, RAW MATERIALS, CERAMIC POWDER CHARACTERIZATION

1.1 General Concepts of Ceramic Powder Processing 4 References 6

Ceramic Powder Processing History and Discussion of Natura l Raw Materials

1.1 Objectives 7 1.2 Historical Perspective 8 1.3 Raw Materials 27

1.3.1 Natural Raw Materials 1.3.2 Synthetic Raw Materials

1.4 Selecting a Raw Material 40 1.5 Summary 41

References 41

27 34

C e r a m i c P o w d e r Character izat ion

2.1 Objectives 43 2.2 Introduction 43

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2.3 Powder Sampling 44 2.3.1 Sampling Accuracy 44 2.3.2 Two-Component Sampling Accuracy 45 2.3.3 Sampling Methods 46 2.3.4 Golden Rules of Sampling 46

2.4 Particle Size 48 2.4.1 Statistical Diameters 48 2.4.2 Mean Particle Size 52 2.4.3 Size Distribution Accuracy 55

2.5 Particle Morphology 56 2.5.1 Shape Factors 57 2.5.2 Shape Analysis 59 2.5.3 Fractal Shapes 60 2.5.4 Internal Porosity 62

2.6 Powder Density 63 2.7 SurfaceArea 64

2.7.1 First Layer Adsorption~Langmuir Adsorption 64

2.7.2 Multilayer Adsorption~BET Adsorption 2.8 Particle Size Distributions 66

2.8.1 Normal Distribution 68 2.8.2 Log-Normal Distribution 69 2.8.3 Rosin-Rammler Distribution 72

2.9 Comparison of Two-Powder Size Distributions Problem 2.5. Comparison of Two Size

Distributions 74 2.10 Blending Powder Samples 75

Problem 2.6. Mixing Two Log-Normal Size Distributions 76

2.11 Summary 78 Problems 78 References 79

CERAMIC POWDER S Y N T H E S I S

The Population Balance

3.1 Objectives 85 3.2 Microscopic Population Balance 86 3.3 Macroscopic Population Balance 87

Problem 3.1. Constant Stirred Tank Crystallizer 88

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3.4 Population Balances Where Length, Area, and Volume Are Conserved 89 3.4.1 Conservation of Length in the Batch

Grinding of Fibers 89 3.5 Population Balances on a Mass Basis 91

3.5.1 Population Balances on a Discrete Mass Basis 91

3.5.2 Population Balances on a Cumulative Mass Basis 92

3.6 Summary 93 3.6.1 Microdistributed Population Balance 93 3.6.2 Macrodistributed Population Balance 93 3.6.3 List of Symbols 93 References 94

Comminut ion and Classification of Ceramic Powders

4.1 Objectives 95 4.2 Comminution 96

4.2.1 Comminut ionEquipment 96 4.2.2 Energy Required for Size Reduction 101 4.2.3 Comminution Efficiency 102 4.2.4 Population Balance Models for Comminution

Mills 103 4.2.5 Array Formulation of Comminution 110

4.3 Classification of Ceramic Powders 115 4.3.1 Dry Classification Equipment 115 4.3.2 Classifier Fundamentals 117 4.3.3 Size Selectivity, Recovery, and Yield 123 4.3.4 Classifier Efficiency 124 4.3.5 Wet Classification Equipment 127

4.4 Comminution and Classification Circuits 129 4.5 Summary 135

Problems 136 References 136

Ceramic Powder Synthesis with Solid Phase Reac tant

5.1 Objectives 139 5.2 Introduction 140 5.3 Thermodynamics of Fluid-Solid Reactions 141

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5.4 Oxidation Reactions 144 Problem 5.1. Free Energy of Oxidation 145

5.5 Reduction Reactions 147 5.6 Nitridation Reactions 148 5.7 Thermodynamics of Multiple Reaction

Systems 148 Problem 5.2. What Is the Reaction Product When

A1 Metal Is Exposed to Air at 800~ 149

5.8 Liquid-Solid Reactions 151 5.9 Fluid-Solid Reaction Kinetics 151

5.9.1 Shrinking Sphere Model 157 5.9.2 Comparison with Kinetic Models 158 5.9.3 Kinetic Models Where Nucleation and

Growth Are Combined 161 5.10 Fluid-Solid Reactors 162

Problem 5.3. Conversion of a Size Mixture of Ceramic Powders 165

5.11 Solid-Solid Reactions 166 5.11.1 Vaporization of One Solid Reactant 167 5.11.2 Solid-Solid Interdiffusion 170


Liquid Phase Synthesis by Precipitation

6.1 Objectives 179 6.2 Introduction 180 6.3 Nucleation Kinetics 183

6.3.1 Homogeneous Nucleation 183 6.3.2 Heterogeneous Nucleation 189 6.3.3 Secondary Nucleation 192

6.4 Growth Kinetics 193 6.4.1 Stages of Crystal Growth 196 6.4.2 Diffusion Controlled Growth 196 6.4.3 Surface Nucleation of Steps 202 6.4.4 Two-Dimensional Growth of Surface

Nuclei 203 6.4.5 Screw Disclocation Growth 204 6.4.6 Summary of Growth Rates 208

6.5 Crystal Shape 210 6.5.1 Equilibrium Shape 210

Contents xi

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6.5.2 Kinetic Shape 212 6.5.3 Aggregate Shape 214 6.5.4 Crystal Habit Modification by

Impurities 216 6.6 Size Distribution Effects--Population Balance and

Precipitator Design 220 6.6.1 Continuous Stirred Tank Reactor 220 6.6.2 Batch Precipitation 226 6.6.3 Effect of Aggregation on the Particle Size

Distribution 229 6.7 Coprecipitation of Ceramic Powders 244

6.7.1 True Coprecipitation 244 6.7.2 Simultaneous Precipitation and

Coaggregation 246 6.8 Summary 249


Powder Synthesis with Gas Phase Reactants

7.1 Objectives 255 7.2 Introduction 256 7.3 Gas Phase Reactions 260

7.3.1 Flame 260 7.3.2 Furnace Decomposition 262 7.3.3 Plasma 262 7.3.4 Laser 262

7.4 Reaction Kinetics 263 7.4.1 Combination Reactions 265 7.4.2 Thermal Decomposition Reactions 267 7.4.3 Laser Reactions 268 7.4.4 Plasma Reactions 269 7.4.5 Complex Reaction Mechanisms 269

7.5 Homogeneous Nucleation 270 7.6 Collisional Growth Theory 275 7.7 Population Balance for Gas Phase Synthesis 7.8 Dispersion Model for Gas Synthesis Reactors

7.8.1 Single-Point Nucleation 284 7.8.2 Multipoint Nucleation 285

7.9 Population Balance with Aggregation 289 7.9.1 Rapid Flocculation Theory 290 7.9.2 A Physical Constraint on the Population

Balance 292 7.9.3 Other Numerical Models 295

278 280

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7.10 Quenching the Aggregation 296 7.10.1 Heat Transfer Quench 298 7.10.2 Gas Mixing Quench 300

7.11 Particle Shape 301 7.12 Summary 303


Other Ceramic Powder Fabrication Processes

8.1 Objectives 307 8.2 Spray Drying 307

8.2.1 Atomization 309 8.2.2 Droplet Drying 315 8.2.3 Gas-Droplet Mixing 327 8.2.4 Spray Dryer Design 330

8.3 Spray Roasting 331 8.4 Metal Organic Decomposition for Ceramic

Films 335 8.5 Freeze Drying 336

8.5.1 Problem: Freezing Time for a Drop 8.6 Sol-Gel Synthesis 340

8.6.1 Precursor Solution Chemistry 343 8.6.2 Film Formation 347 8.6.3 Gel Drying 349 8.6.4 Thermal Decomposition of Gels 8.6.5 Gel Sintering 350

8.7 Melt Solidification 351 8.8 Summary 353


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338

CERAMIC PASTE FORMATION-- MISE-EN PATE

Wetting, Deagglomeration, and Adsorption

9.1 Objectives 359 9.2 Wetting of a Powder by a Liquid 360

Problem 9.1. Spreading H20 on SiO2 364

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9.2.1 Smooth versus Rough Surfaces 366 Problem 9.2. Wetting of a Rough Solid

Surface 368 9.2.2 Partial Wetting of a Solid 368 9.2.3 Internal Wetting 368 9.2.4 Heat of Wetting 370 Problem 9.3. Solvent Selection 373

9.3 Deagglomeration 374 9.3.1 Ultrasonification 375

9.4 Adsorption onto Powder Surfaces 379 9.4.1 Gibb's Adsorption Isotherm for the

Liquid-Vapor Interface 380 9.4.2 Adsorption Isotherms for the Solid-Liquid

Interface 382 9.4.3 Binary Solvent Adsorption 384 9.4.4 Adsorption of Ions 386 Problem 9.4. Surface Change 394 9.4.5 Adsorption of Ionic Surfactants 398 9.4.6 Adsorption of Polymers 403 9.4.7 Selection of a Surfactant 410

9.5 Chemical Stability of a Powder in a Solvent 414 9.5.1 Stability in Water 414


Colloid Stability of Ceramic Suspensions

10.1 Objectives 421 10.2 Introduction 421 10.3 Interaction Energy and Colloid Stability 422

10.3.1 van der Waals Attractive Interaction Energy 422

Problem 10.1. Hamaker Constant 427 10.3.2 Electrostatic Repulsion 428 10.3.3 Steric Repulsion 445 10.3.4 Total Interaction Energy 466

10.4 Kinetics of Coagulation and Flocculation 467 10.4.1 Doublet Formation 467 Problem 10.2. Determine the Half-Life for Doublet

Formation for Various Initial Number Densities of Particles in Water 467

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10.4.2 Growth and Structure of Large Aggregate Clusters 475

10.4.3 Shear Aggregation 486 Problem 10.3. Critical Size for Shear

Aggression 487 10.5 Colloid Stability in Ceramic Systems 488 10.6 Summary 489


Colloidal Properties of Ceramic Suspensions

11.1 Objectives 495 11.2 Introduction 496 11.3 Sedimentation 497

Problem 11.1. Terminal Settling Velocity 499 11.3.1 Nonspherical Particle Settling 500 11.3.2 Hindered Settling 500 Problem 11.2. Hindered Settling Velocity 502 11.3.3 Centrifugal Sedimentation 503 11.3.4 Sedimentation Potential 503

11.4 Brownian Diffusion 504 11.4.1 Nonspherical Particle Diffusion 504 11.4.2 Fick's Laws for Diffusion 505 11.4.3 Equilibrium between Sedimentation and

Diffusion 505 Problem 11.3. Sedimentation Equilibrium 506 11.4.4 Rotational Diffusion 506

11.5 Solution and Suspension Colligative Properties 509 11.5.1 Osmotic Pressure of Electrolyte

Solutions 511 11.5.2 Osmotic Pressure of Polymer

Solutions 512 11.5.3 Osmotic Pressure of the Double Layer in a

Colloidal Suspension 513 11.6 Ordered Suspensions 516

11.6.1 Osmotic Pressure (and Other Thermodynamic Properties) of a Ceramic Suspension 517

Contents X V

IV

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11.6.2 Measurement of Ordered Array Structure 526

11.6.3 Defects in Ordered Arrays 527 11.6.4 Processing Effects on Order Domain

Size 529 11.6.5 Measurement of Ordered Domain Size by

Light Diffraction 530 11.6.6 Effect of Ordering and Domain Size on

Ceramic Processing 531 11.7 Summary 532


G R E E N B OD Y FORMA T I O N - - MISE-EN FORME

Mechanical Properties of Dry Ceramic Powders and Wet Ceramic Suspensions

12.1 Objectives 541 12.2 Introduction 542 12.3 Equations of Motion 543

12.3.1 Continuity Equation 543 12.3.2 Momentum Balance 544 12.3.3 Constitutive Equations for Dry

Powders 545 12.3.4 Constitutive Equations for Fluids

12.4 Ceramic Suspension Rheology 550 12.4.1 Dilute Suspension Viscosity 551 12.4.2 Rheology of Concentrated Ceramic

Systems 562 Problem 12.1. Hard Sphere Stress-Strain

Curve 569 12.4.3 Ceramic Paste Rheology 585

12.5 Mechanical Properties of Dry Ceramic Powders 590 12.5.1 Coefficient of Pressure at Rest 12.5.2 Compact Body 594 12.5.3 Plastic Body 595

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12.5.4 Yield Criteria for Packings 596 12.5.5 The Coulomb Yield Criterion 597 12.5.6 Yield Behavior of Powders at Low

Pressures 599 12.6 Summary 602


Ceramic Green Body Formation

13.1 Objectives 609 13.2 Introduction 610 13.3 Green Body Formation with Ceramic

Suspensions 612 13.3.1 Slip Casting 613 13.3.2 Filter Pressing 618 13.3.3 Tape Casting 620 13.3.4 Sedimentation Casting and Centrifugal

Casting 629 13.3.5 Electrodeposition 636 13.3.6 Dip Coating 638

13.4 Extrustion and Injection Molding of Ceramic Pastes 643 13.4.1 Flow in the Extruder 644 13.4.2 Flow in the Extrusion Die 646 13.4.3 Flow into the Injection Molding Die 651

13.5 Green Body Formation with Dry Powders--Dry Pressing 653 13.5.1 Tapped Density 654 13.5.2 DiePressing 656 13.5.3 Stress Distribution in the Ceramic

Compact 661 13.5.4 Deformation of Visco-Elastic Solids and

Fluids 667 13.5.5 Die Ejection and Breakage 667 13.5.6 Isostatic Pressing 671 13.5.7 Green Machining 673

13.6 Green Body Characterization 674 13.7 Summary 675


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PRESINTERING HEAT TREATMENTS OF DRYING A N D B I N D E R B URNO UT

Green Body Drying


14.2.1 Heat Transfer 686 14.2.2 Mass Transfer 687 14.2.3 Flow of Liquid in the Pores 689 14.2.4 Drying Shrinkage 690 14.2.5 Drying Induced Stresses 691

14.3 Sphere and Cylinder Drying 693 14.3.1 Boundary Layer Heat and Mass Transfer

Giving the Drying Rate for the Constant Rate Period 693

14.3.2 Shrinkage during the Constant Rate Period 695

14.3.3 Diffusion and Heat Conduction in the Porous Network Giving the Drying Rate for the Falling Rate Period 698

Problem 14.1. Drying Time Calculation 700 14.3.4 Cylinder Drying 702

14.4 Drying of Flat Plates 703 14.5 Warping and Cracking during Drying 705

14.5.1 Thermal Stresses Induced during Drying 708

Problem 14.2. Temperature Difference Induced Tensile Stress 712

14.5.2 Flow Stresses during Drying 713 14.5.3 Capillary Stresses 716

14.6 Characterization of Ceramic Green Bodies 718 14.6.1 Green Density 719 14.6.2 Uniformity of Microstructure

Mixedness 719 14.6.3 Green Body Strength 721


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VI

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Binder Burnout


15.2.1 Heat Transfer 731 15.2.2 Mass Transfer 732

15.3 Thermal Degradation of Polymers 733 15.3.1 Reaction Kinetics 737 15.3.2 Polymer Residues and Volatiles 738

15.4 Oxidative Polymer Degradation 738 15.4.1 Reaction Kinetics 749 15.4.2 Polymer Residues and Volatiles 750

15.5 Kinetics of Binder Burnout 752 15.5.1 Kinetics of Binder Oxidation 755 15.5.2 Kinetics of Volatiles Loss 758 Problem 15.1. 760 15.5.3 Kinetics of Binder Pyrolysis without

Oxygen 761 15.5.4 Kinetics of Carbon Removal 762 Problem 15.2. 765

15.6 Stresses Induced during Binder Burnout 767 15.6.1 Thermal Stresses Induced during Binder

Burnout 768 15.6.2 Stresses Due to Volatile Flow 770


S I N T E R I N G A N D F I N I S H I N G

Sintering

16.1 Objectives 781 16.2 Introduction 782 16.3 Solid State Sintering Mechanisms 785

16.3.1 Driving Force for Sintering 786 16.3.2 Sintering Kinetics by Stage 788 16.3.3 Effect of Green Density of Sintering

Kinetics 811

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16.3.4 Effect of Particle Size Distribution on Sintering Kinetics 812

16.3.5 The Effect of Fractal Aggregates on Sintering Kinetics 817

16.4 Grain Growth 824 16.4.1 Normal Grain Growth 827 16.4.2 Abnormal Grain Growth 840

16.5 ReactiveSintering 844 16.5.1 Sintering wtih a Liquid Phase 844 16.5.2 Solid State Reactive Sintering 860 16.5.3 Gas-Solid Reactive Sintering 861

16.6 Pressure Sintering 864 16.7 Cool Down after Sintering 867 16.8 Summary 869


Finishing

17.1 Objectives 875 17.2 Introduction 875 17.3 Ceramic Machining 876

17.3.1 Effect of Machining on Ceramic Strength 877

17.3.2 Effect of Grinding Direction on Ceramic Strength 878

17.3.3 Effect of Ceramic Microstructure on Strength 879

17.3.4 Grinding and Machining Parameters 17.4 Coating and Glazing 882 17.5 Quality Assurance Testing 883

17.5.1 Proof Testing 884 17.6 Nondestructive Testing 886 17.7 Summary 888

References 889

Appendix A Ceramic Properties 891 Appendix B Gamma Function 893 Appendix C Normal Probability Function Appendix D t Test 901 Appendix E Reduction Potentials 903 Appendix F Thermodynamic Data 905

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Appendix G Summary of Differential Operations Involving the V-Operator in Rectangular Coordinates (x, y, z) 915

Appendix H Summary of Differential Operations Involving the V-Operator in Cylindrical Coordinates (r, 0, z) 917

Appendix I Summary of Differential Operations Involving the V-Operator in Spherical Coordinates (r, 0, d~) 919

Appendix J Liquid Surface Tensions 921 Appendix K Drago E and C Parameters 925 Appendix L Hildebrand Solubility Parameter and

Hydrogen Bond Index 929 Appendix M Hydrated Cation Radii 935 Index 937

Preface

In the past 15 years ceramic powder processing and synthesis have undergone a transformation. Scientific and engineering methods have been applied in this field at a much higher level than ever before, allowing much greater control of properties than could be achieved previously. Ceramic systems are not simple and therefore these scientific and engineering methods had to achieve sufficient sophistication to be adaptable to this field. We now have many examples of the applica- tion of these scientific and engineering methods to ceramics. As a result these first examples can be explained to students of ceramics, who with this knowledge, can continue this evolution of sophistication in the fundamentals of ceramic powder processing and synthesis. This book was written in an attempt to do just that.

The organization of this book is explained in the introduction. Basi- cally, it is organized like a ceramic manufacturing facility starting with raw materials and ending with sintering and finishing. Various chapters contain problems within the text for illustration. At the end of each chapter, additional problems allow the reader to go into greater depth using the material presented in the chapter. These problems are not necessarily easy but the reader's efforts to resolve them will result in much greater knowledge of the material covered in the chapters.

Finally, I acknowledge the help of others in writing this book. Many long nights over a period of more than six years were spent writing this book and my family has suffered as a result. This book is dedicated to my understanding wife, Susan. Many people have helped me with concepts and ideas. Professor Alain Mocellain critiqued the outline of this book and made many useful suggestions. Dr. Paul Bowen, Dr.

xxi

x x i i Preface

Dennis Gallagher, Dr. Jacques Lemaitre, and the LTP-EPFL students performed the very important task of proofreading the manuscript. Dr. Bowen provided gentle guidance in areas where rewriting was required. Academic Press provided a long list of anonymous reviewers, one for each chapter; I am indebted to them for many helpful suggestions. Elizabeth Burdet worked diligently to minimize the other work in my laboratory so that sufficient time was available to write this book. Silvia Yvette helped with typing the first draft of this manuscript, Wilma Bunners made many of the more complex drawings found in the text, and my wife, Susan, read each chapter for English corrections. Many thanks to all.

Terry A. Ring

PART I INTR OD UC TION: HISTORY, RAW MATERIALS, CERAMIC POWDER CHARACTERIZATION

Many options are to be considered in organizing a book on the fundamentals of ceramic powder processing. One could organize a book along phenomenological lines (e.g., similar thermodynamics and reaction or diffusion kinetics) or along material classifications lines (e.g., oxides, carbides, and nitrides) or along material properties lines (e.g., structural ceramics and electronic ceramics). After considering the many possibilities, this book has been organized as if the reader were following a ceramic process in a factory from powder to final finished piece. Ceramic powder processing can take two traditional routes: one is a wet powder processing route, where the powder is mixed in the liquid and cast into the green body before firing; the second is a dry ceramic powder processing route, which consists of pressing the dry powder (with binder) into the green body and then firing. Both of these processing routes are shown in the Figure 1.1. This is the flow sheet for the computer controlled tile making facility for the ' INAX Corporation in Japan. Here they use these two routes, a wet paste-extrusion route and a dry-pressing route, to make ceramic tiles. These processing routes are also used

for modern ceramics such as tiles for the space shuttle's surface and electronic BaTiO3 capacitors.

In this figure one sees all the steps that go into making ceramics, starting with grinding the ceramic powders to develop a very fine particle-size distribution (the grinding circuits contain classification and recycle loops). This is

2 Part I Introduction

followed by putting the ceramic powder into liquid form, adding different additives to adsorb to the particle surface and prevent coagulation of the particles, as well as to adjust the rheology of the paste and provide a binder of the particles after consolidation. The paste is then dewatered to the best consistency for extrusion into the desired shape. The resulting green bodies are dried very slowly, then subject to binder burn-out t reatment at higher temperatures followed by sintering. Dur-

ing sintering, pores are removed from the ceramic body, leaving behind

a fully dense piece which must then be finished in some way (e.g.,

applying a glaze or grinding to size). This constitutes the wet route as shown in Figure 1.1. The outline of this book follows that sequence of

events very closely. As a result we have the following parts of this book:

F I G U R E 1.1 Ceramic tile manufacturing process. Photo courtesy of Inax Corp., Japan.

Part I Introduction 3

Part I. Introduction: History, Raw Materials, Ceramic Powder Characterization

Part II. Ceramic Powder Synthesis Part III. Ceramic Paste Formation: Mise-en Pdte Part IV. Green Body Formation: Mise-en Forme Part V. Presintering Heat Treatments of Drying and Binder

Burnout

Part VI. Sintering and Finishing

The parts are further broken down into chapters discussing the chemical, physical, and engineering fundamentals of each step of

the process. The other route for ceramic manufacturing, starting with dry

powders and pressing them with a polymer or binder, is discussed in this book in the different sections. This route will have in common with the wet processing methods the steps of powder synthesis and ceramic green body formation, binder burn-out, sintering, and finishing; thus, the reader interested in the dry powder processing route can follow this processing sequence by stepping over various materials which are not of interest. For the students particularly interested in ceramic part manufacture, Part II of this book, discussing ceramic powder synthesis, would be of less interest. As a result the student can start with the part three after reading the introduc- tory chapters in Part I on raw materials and ceramic powder characterization.

Each chapter is broken into sections with the first section always stating the objectives of the chapter, and the last section always providing a summary of the chapter. In the text, problems are worked to elucidate the points discussed. Finally at the end of each chapter there are unworked problems that the students can do for homework. The book attempts to provide a large list of references for specific concepts and ideas presented elsewhere, and we hope that the reader will refer to these references for the derivation of specific equations not presented. This book is highly mathematical in comparison with other texts in the field, because this field should be much more quantitative than heretofore presented. With these mathematics, the field of ceramic powder processing can become more quantitative in the future.

4 Part I Introduction

1.1 G E N E R A L C O N C E P T S OF C E R A M I C POWDER P R O C E S S I N G

Several general ideas are associated with ceramic powder processing. These general ideas have been generated after many years of research and have resulted in a philosophy of ceramic powder processing.

The first idea is that uniformity in the microstructure of a single phase ceramic is better for electrical and mechanical properties. This idea is based on the Griffith fracture theory for ceramics, where the strength of the ceramic is related to the largest flaw size. With a bigger flaw size, weaker single phase ceramics result. Uniformity is also important for electrical ceramics. For example, the final grain size distribution of BaTiO3 should be uniform to have the highest dielectric constant for ceramic capacitors or the highest piezoelectric coupling constants for actuators. In the case of the capacitor, the grains should be uniformly small; and in the case of the actuator, they should be uniformly large to achieve the orthorhombic crystal structure necessary for piezoelectricity, which is prevented for grains less than 1 micron in size for pure BaTiQ. The idea of uniformity remains for both large and small grains in the case of electrical properties. This is sometimes difficult when cannibalistic grain growth occurs during sintering, leading to a bimodal grain size distribution. For this reason, dopants are used to prevent cannibalistic grain growth.

Another idea is that the microstructural inhomogeneities that occur in casting a green body remain (or even get larger) during drying, binder burn-out, and sintering. Therefore, to obtain the best uniformity the casting process must be performed very carefully with suspensions that contain no bubbles or large pieces of polymers. In addition, the uniformity produced in the green body should not be destroyed by rough handling. In the case of drying and binder burn-out, huge volumes of gas, many thousands of times that of the green body itself, must leave the green body. This process puts tensile stress on the green body which can cause cracks. To prevent these cracks, drying and binder burnout conditions which are very slow are desirable. Uniformity is also extended from the green body casting down to the ceramic suspension utilized for casting. In this case uniformity of the particles used is important because larger and smaller particles segregate into different parts of the mold during casting of monophase ceramics, leading to

1.1 General Concepts of Ceramic Powder Processing 5

nonuniformity. This is the same reason why stable colloidal suspensions are used for casting to prevent packing inhomogeneities caused by aggregates. With composite ceramics which consist of two or more different phases, uniform mixing in the suspension is also important. This may be impossible if the two powders utilized have either different

densities or different particle-size distributions or both. For this reason, the suspension is often flocculated with polymers so that the well-mixed

nature of a suspension is preserved in the flocs..These flocs, with their inhomogeneous packing of particles, are then broken into homogeneous green bodies by pressing at high pressure.

The last general concept of ceramic powder processing is that smaller powders sinter to give smaller grains that give a stronger ceramic piece. This idea is again based on the Griffith fracture theory for ceramics, where the strength of the ceramic is related to the largest flaw size. Assuming homogeneity, a smaller grain size will result in a smaller

flaw size, leading to a stronger ceramic. The sintering times tl and t2 for two powders with the same chemistry but different particles sizes

rl and r2 is given by Herring's scaling law [1]:

t2 = [r2/rl]ntl

where n is a constant depending on the sintering mechanism. In the case of volume diffusion, n = 3. From the Herring scaling law, we see that, as the mean particle size is decreased, the time needed to sinter a ceramic piece is decreased.

These general concepts will play an important role in the selection of a process for the manufacture of a particular ceramic part and as a result these general concepts will be encountered again and again throughout this book.

Reference 1. Herring, C., J. Appl. Phys. 21, 301 (1950).

1 Ceramic Powder Processing History and Discussion of ~~atural Raw ?daterials

1.1 O B J E C T I V E S

This chapter will give the reader a historical perspective of the field of ceramic powder processing. This field has a long and rich history which in many ways is impossible to trace because it goes back to before writing. Nonetheless, there is a rich archaeological record of ceramic articles produced by different technologies from which we can learn a great deal. In addition, this chapter presents the raw materials used for ceramic manufacture both historically and in the present day. Fi- nally an overview of the organization of this book is presented. This book is organized like a ceramic factory, with powder synthesis and preparation first followed by paste preparation, forming, drying, binder burnout, and sintering.

7

8 Chapter I Ceramic Powder Processing History

1.2 HISTORICAL PERSPECTIVE

The first ceramic objects in the archaeological record are fired clay figures appear ing about 22,000 B.C. [1]. These figures were probably na tu ra l clay pieces shaped by hand into a humanoid form, allowed to dry, and placed in a fire. This ar t form gradual ly became used for more practical objects such as bowls and storage vessels on a much larger scale. This larger scale of production became an integral par t of the Chinese villages about 6000 B.C., where the ceramic kiln played a cen- tral role [17]. As a result, the fundamenta ls of ceramic powder processing, the title of this book, have been practiced for over eight mil lennia [1 ]. A highly developed ceramic technology was in place for ear thenware like tha t shown in Figures 1.1 [2] and 1.2 [2] well before the Bronze Age (about 4000 B.C. [3]), at a t ime when silkworm cultivation was also invented. These red pottery vases have a complex shape and are painted with black ornamenta l pat terns. Figure 1.2 was excavated at Pan-p'o,

FIGURE 1.1 Red pottery vase with a contracted waist, a black design on a red base that covers the earthen ware (brown). It was excavated at Lan-chou, Kansu, in 1958, and is 18.3 cm in height, from the third millenium B.C. Taken from "The Genius of China" [2 ].

1.2 Historical Perspective 9

FIGURE 1.2 Yang-shao bowl excavated in 1954-57 at Pan-p'o, Shensi, made of red pottery painted with black (carbon) triangles over a slip of white clay, 12.7 cm in height, from the fifth or fourth B.C. Taken from "The Genius of China" [2].

Shensi, China* with other objects tha t date it to 5000 B.C. or 4000 B.C. The gloss of the paint resul ts from burnishing the clay before firing. The deep red color suggests the use of a clay (i.e., kaoline, an aluminosilicate mineral) containing hemati te , a red iron oxide. This pot tery was fired in a kiln of a relatively advanced design, capable of t empera tures up to about 1000~ because only at t empera tures above 900~ does kaolinite sinter to reasonable s t rength [4,5]. By 3000 or 2000 B.C., a burnished black pottery was produced as shown in Figure 1.3 [6]. This type of pottery, excavated at Wei-fang, Shantung, China, is ei ther entirely black or has a black surface with a grey core. Analysis of the polished surface shows tha t it has only a higher concentration of carbon than at the core. Much debate has centered on how this polished surface was achieved. But, due to the presence of carbon in the clay body, the kiln design mus t have been sufficiently advanced to give a reducing atmosphere. This pot tery is s imilar to the present day Santa Clara pottery produced by the Indians in the southwestern USA.

* Note that all Chinese names have been romanized by the Wade-Giles system.


FIGURE 1.3 Tall beaker, tou, excavated in 1960 at Weifang, Shantung. It is of burnished black pottery, 16.1 cm in height, and from the third or second millenium B.C. Taken from "The Genius of China" [2].

The first archaeological record of bronze production in China comes from an Erlitou Culture (1700 B.C.) site in Henan, Shanxi, China [6]. Bronze is an alloy of copper and tin (although in ancient China, lead was also frequently used). The earliest known Chinese bronze object is shown in Figure 1.4, which has 92% copper and 7% tin. This wine cup displays the basic metalworking features of the Chinese Bronze Age, which are sharply different from Near Eastern and Western tradi- tions. This alloy is not an accident but a deliberate choice and indicates that a complex metallurgical infrastructure was in place to mine the ores of both metals and then smelt each ore to its respective metals.

1.2 Historical Perspective 1 1

FIGURE 1.4 Wine cup with tripod feet, of bronze, from the seventeenth century B.C. (one of the earliest so far known). Taken from "Treasures from the Bronze Age of China" [6].

Most important, this bronze vessel has seams which show it to have been cast from a mold made in four separated sections. This wine cup required a complex ceramic mold, which sets the early Chinese bronze technology apart from the lost-wax process used in the West.

The production of bronze is a major undertaking. Sources of copper and tin must be located and protected. The ore must be mined and the metal removed. In the case of copper, this is difficult because copper accounts for only a small fraction of the volume of the ore. In ancient China, the ores seems to have been crushed, liberated, separated, and smelted at the mines and then transported to communities for casting. The melting of large quantities of metal, primarily copper, Tm = 1085~ required elaborate kilns and huge fires of high intensity; skills that had developed out of the ceramic tradition. Casting required controlled cooling of the metal to avoid holes and cracks in the finished object; skills which also relied on the precise fitting ceramic molds. During the Shang Dynasty (1600 to 1027 B.C.), when writing was first developed, bronze metallurgy developed into a highly skilled technology. Shang bronze molds where made from loess, the wind-blown ochre- colored soil that covers much of the landscape of northern China. Loess is rich in micas, fine quartz, sodium feldspar, and alkaline minerals


FIGURE 1.5 (a) Diagram showing how early Chinese bronzes were formed: (1) the model, (2) the sections of the mold, and (3) schematic of completed vessel.

[3]. The natural clay content (mostly illite) in loess ranges between 10 and 20%, which is enough to give it plasticity when mixed with water [3]. The unique property of loess as a ceramic molding material is that it does not shrink much as it is dried and fired [3] to 900~ Also, it is porous after firing, allowing the bronze to degas into the mold during solidification. By the Warring States Period (475 to 221 B.C.), there is evidence of the prolific use of ceramic multipart piece molds shown in Figure 1.5 in the direct casting of bronze vessels and weapons.

In the Shang Dynasty, the first example of pottery covered with a high-fired feldspathic glaze [2] was observed. The body of this vase, shown in Figure 1.6, is of near stoneware hardness. The glaze, requiring kiln temperatures of 1200~ is spread uniformly over the whole body. This glaze technology then disappears from the archaeological record until the late fourth or early third century B.C.

China's first emperor, Ch'in Shih Huang Ti, in about 221 B.C. united the various warring states of China by providing a uniform code of law, standards of currency, written language, and weights and measures


FIGURE 1.5 (b) Actual completed vessel. Taken from "Treasures from the Bronze Age of China" [6].

and completed the separate ramparts of the Great Wall of China, some 1000 km long, as protection from northern invaders. During his reign, ceramic and bronze arts were also practiced to perfection. In his mausoleum 7000 life size terracotta soldiers (one shown in Figure 1.7) and horses made of fired loess were discovered.

On the eve of the Western Han Dynasty (206 B.C. to 8 A.D.), low-fired lead-fluxed glaze made its first appearance. This is the predecessor to the "polychrome" lead glasses of the Sui (581 to 618 A.D.) and T'ang Dynasties (618 to 906 A.D.). The colors of the lead glazes (i.e., brown, yellow, green, and blue) were produced by adding refined metal ores to the glaze mixture. A three-colored T'ang Dynasty vase, shown in Figure 1.8, is an example of this technology. These glazes were generally


FIGURE 1.6 Glazed pottery vase of high fired stoneware hardness, excavated in 1965 at Ming-Kung-lu, Cheng-chou, Honan. It is covered with a high-fried feldspatic glaze, 28.2 cm in height, and from the sixteenth or fifteenth century B.C. Taken from "The Genius of China" [2].

applied over a slip of white clay. During the T'ang Dynasty, the feldspathic glazes evolved into what is to become a long tradition of white porcelaneous ware like that shown in Figure 1.9. This glaze required firing at 1300~ This body has a glassy phase, filling the pores, giving a nonporous fired body. For this development, purified raw materials had to be used with a specific narrow range of chemical composition. This glaze was further refined into the subtle celedon green prolific in the Sung Dynasty (960 to 1279 A.D.). At this point the Chinese ceramics had reached one of its technological objectives, which was a porcelain


FIGURE 1.7 Life-size figure of a terracotta military commander in the mausoleum of the first emperor of China. Taken from "Treasures from the Bronze Age of China" [6].

body fused with a glossy, t ranslucent green glaze that rung when struck and looked like jade. With the porcelaneous ware of the Yuan Dynasty (1271 to 1368 A.D.) underglaze painting of cobalt blue (cobalt oxide fired to give cobalt silicate, which is blue [7]) and copper red (copper oxide fired to give metallic copper, which is red [7]), present in the delicate

F I G U R E 1.8 Tang Dynasty covered bowl, with lead glaze in green, brown, and yellow, excavated in 1958 at Loyang, Honan, 21 cm in height, from the first half of the 800s A.D. Taken from "The Genius of China" [2].

F I G U R E 1.9 White porcelaneous bowl, clear glazed with applied medallions, excavated in 1956 at Han-sen-chai, Near Sian, Shensi, 23 cm height, from the T'ang dynasty 667 A.D. Taken from "The Genius of China" [2].


leaf and floral motifs of the vase with cover shown in Figure 1.10, indicate a new technological level of excellence. This was to be exploited in the Ming Dynasty (1368 to 1644 A.D.) and t ransferred to Europe to become the Meisen and Delft pottery of the early 18th century. Control of the a tmosphere dur ing firing of these glazes (i.e., reducing conditions) was necessary to give the desired blue or red color and not simply black. The glazing technology culminated in the polychrome overglaze paints of the Ch'ing Dynas ty (1644 to 1911 A.D.), Figure 1.11 [8].

FIGURE 1.10 White porcelain vase and cover with underglaze decoration of incised dragons and blue glaze waves, excavated in 1964 in Pao-ting, Hopei, 51.5 cm height, from the Yuan dynasty, late 14th century A.D. Taken from "The Genius of China" [2].

18 Chapter 1 Ceramic Powder Processing History

FIGURE 1.11 Polychrome glazed vase decorated with flowers and insects in a peach branch, 51.4 cm height, from the Ch'ing dynasty, with a Ch'ien Lung mark, 1736-95. Taken from "A Handbook of Chinese Ceramics" [8].

Ceramic powder processing technology is discussed in the T'ao Shuo [9]. This text describes how kaolin raw materials had to be found and ground to the desirable size distribution. After grinding, the earth was washed and purified. This was done by mixing it with water in a large


earthen ware jar and stirring the mixture until all the organic impurities had floated to the top and were poured off. The resulting paste was next passed through a fine horsehair sieve and then into a bag made of two thicknesses of finely woven silk. Afterward, the paste was poured into several earthenware vessels, so that the excess water could run off. The paste was then allowed to sediment. The settled paste was further dewatered by wrapping it in a fine cotton cloth, and placing it in a bottomless wooden box resting on dry earthenware bricks. More bricks were piled on top of the cotton bag of paste to press and absorb more of the water, using both hydrostatic and osmotic pressure. When free of excess water, the paste was thrown on large stone slabs and turned over and over until it was ductile. The paste was worked into the green body shape by various techniques: coiled and layered by hand, thrown on a potter's wheel, slip cast or pressed into molds, or stamped. The green body was then dried slowly, so as not to crack it, and fired in a wood-burning kiln under oxidizing conditions at more than 900~

A stunning example of this type of technology are the 7000 life-size terracotta statues, each with a different face, of the army of the First Emperor of China, Emperor Ch'in (221 to 207 B.C.), at his grave site in Xian, China (see Figure 1.7). Historians believe there is an official document describing this ceramic powder processing technology that was among the official documents of the Ch'Hi state incorporated in 140 B.C. Indeed, updated copies of the Chou Li, an early encyclopedia of art and technology, shows wood block prints of the various processing steps. The wood block prints shown here are from "T'ao Shuo," Descrip- tion of Pottery [9], in six books by Chu Yen. This work describes 20 woodblock prints dating from 1743 with T'ang Ying, director of the Imperial Factory at Ching-t~, narrat ing a description of each print. Several of these woodblock prints are reproduced in Figures 1.12 and 1.13 showing green bodyt shaping methods, decorating, and firing in a wood-burning kiln.

Closely interacting with this earthenware technology were developments in metallurgy. Some of the metallurgical operations are described in the T'ien Kung K'ai Wu published in 1637 A.D. from which Figures 1.14 and 1.15 come [10]. Much of the glaze technology that sealed the outside of the porous earthenware structure and gave the body color and texture was a result of ceramic alloying of metal oxides, which were made available by metallurgical operations. The lead oxide flux glazes of the Han Dynasty (202 B.C. to 220 A.D.) and the T'ang Dynasty (618 to 906 A.D.) funeral ware were refined to give colors that

t Unfired ceramic body. Green due to color of purified kaolin when wet. Also from the Chinese word Qing with the definitions (1) fresh and (2) green.


FIGURE 1.12 Woodblock prints of the stamping of a pattern on the surface of a bowl (a) and the firing of a "dragon kiln" with several chambers (b). These kilns were so named because they snaked their way at a constant gradient up the contours of a hillside. Taken from T'ien Kun K'ai Wu, 1637 print from "Description of Chinese Pottery and Porcelain" [9].

included white, amber, yellow, green, and violet blue with a minutely cracked texture, analogous to the lead oxide glazes used today. The colors were obtained by adding refined metal oxides to the basic glaze formula and controlling the oxidizing or reducing conditions in the kiln.

This history of ceramic powder processing technology is only a brief description of the events in China. In fact, other parts of the world also contributed to the technological developments of ceramic powder processing. Table 1.1 lists the roots of the ceramic technologies throughout the world. Egypt played an important part in the development of faience about 4000 B.C. and glass making about 1500 B.C. In the 9th


FIGURE 1.12 (Continued)

century Baghdad played a role in developing tin glaze ware, to cite some examples.

One of the most interesting developments is that of porcelain manufacture [11]. Crude porcelain was first made during the T'ang dynasty in China (618 to 908 A.D.). This technology was carefully guarded by the Chinese but finally spread to Korea by the l l00s and to Japan by the 1600s. Marco Polo and other Western travelers described the Chinese porcelain to the Italian ruling class upon their return from the Far East, and they started importing pieces. Under royal patronage, alche- mists tried to discover how the material could be manufactured, but without chemical analytical methods success came only from trial and error. In 1575, under the sponsorship of de Medicis in Florence, soft paste porcelain was developed, a mixture of clay and ground glass, fired at 1200~ The French also produced soft paste porcelain at Rouen and St. Cloud in the 1600s. Later in the 1600s, this technology spread

22 Chapter I Ceramic .Powder Processing History

FIGURE 1.13 Woodblock prints of painting the ceramicware with cobalt underglaze patterns (a) and using a wheel for painting a circle on the rim of a bowl (a). In (b), two men are dipping the painted ware into a great bowl of glaze prior to firing. Taken from T'ien Kun K'ai Wu, 1637. Print from "Description of Chinese Pottery and Porcelain" [9].

to other parts of France (Chantilly, Mennecy, Vincennes, and Sevres) and to England (Chelsea, Bow, and Derby) in the mid-1700s. The secret of true porcelain was not rediscovered in Europe until 1707 by von Tschirnhaus (a mathematician) and B~ttger (a kidnapped alchemist), who were "employed" by Augustus the Strong of Saxony. Augustus the Strong's fascination with collecting Oriental porcelain nearly bank- rupted his kingdom. Using the crude scientific analysis of BOttger, Tschirnhaus recognized that true porcelain must be a mixture of natural materials and not ground glass as in soft paste porcelain. They ordered samples of clay from various parts of the kingdom and finally substituted ground feldspar for ground glass of the soft paste with a natural kaolin clay. Tschirnhaus and B~ttger established a true porcelain factory at Meissen near Dresden. The first major sales from this


FIGURE 1.13 (Continued)

factory took place at the Leipzig Fair in 1713. This technology spread quickly across Europe, fueled by the demand of the new fad of drinking tea, coffee, and chocolate.

Throughout this history the purity of the raw materials has been of the utmost importance. In ancient China the purification of raw materials was practiced for 8000 years with the use of purified white clay as a wash coat under designs. Ground feldspar of a closely controlled chemical composition was used for the very first glaze of the Shang Dynasty. During the Bronze Age, ceramic raw materials were first synthesized (e.g., lead oxide used for the basic glaze formula). In modern times, ceramic powder raw materials are still purified and synthesized by separate processes (e.g., Bayer process alumina, flame synthesis of titania, dead-burned magnesia) so that their purity is better controlled. This attention to raw material purity coupled with attention to the


FIGURE 1.14 Distillation of mercury in a retort, from T'ien Kun K'ai Wu, 1637. Re- printed by permission of the publishers from "Science in Traditional China" [10], copyright 9 1981 by the Chinese University of Hong Kong.

details of each process step are important attr ibutes of the development of ceramic powder processing.

In this brief overview of the first recorded ceramic powder processing, we find all of the at tr ibutes of this technology still with us today; for example, raw materials selection, grinding, size classification, raw material purification and blending, paste preparation, dewatering, green body formation, drying, and firing. In this regard ceramic powder processing is a very old art. Yet, in the last 10 years, enormous development has taken place in the scientific understanding of this very old art. Currently now that the speed with which these new developments emerge appears to be slowing down, it is time to reflect on what has been accomplished and where we stand scientifically. This book will give the state of the art of ceramic powder processing in the early 1990s. We should keep in mind that this field is still progressing and this book, like the official documents of the Ch'hi state incorporated

F I G U R E 1.15 Liquidation process for the separation of silver from copper by lead, which is later cupelled, from T'ien Kun K'ai Wu, 1637. Reprinted by permission of the publishers from "Science in Traditional China" [10], copyright 9 1981 by the Chinese University of Hong Kong.

TABLE 1.1 Outline of Ceramic History Showing the Main Lines of Technological Development

Prehistory of Ceramics -22,000 B.C. earliest known fired clay figures -8 ,000 B.C. fired earthenware vessels in Near East -6 ,000 B.C. slip coatings and clays prepared by decanting suspensions, ochre red and black decoration, manganese and spinel black pigments, control of oxidation, reduction during firing, impressed designs, rouletting, incised decoration, coil and slab construction, burnishing, joining paddle and anvil shaping, carving, and trimming -4 ,000 B.C. Egyptian faience 4,000-3,500 B.C. wheel throwing, earthenware molds, craft shops -1600 B.C. vapor glazing, prefritted glazes, lead glazes -1500 B.C. glass making, alkaline glazes -1000 B.C. glazed stoneware in China - 7 0 0 B.C. Greek black and red wares

Developments toward particular ceramic products

Soft-paste Hard-paste Tin-glazed porcelain porcelain ware Jasperware Stoneware

900s clay quartz ware in Egypt 1200s enameled minai ware 1400s white tile 1500s Isnik tile blue on white

wares 1575-1587 Medici porcelain 1600s Gombroon ware 1695 soft paste porcelain at St.

Cloud 1742 soft paste porcelain at

Chelsea 1796 Spode's English bone china 1857 Beleek frit porcelain

206 B.C.-221 A.D. (Han Dynasty in China)

White porcelain 618-906 (Tang Dynasty in China)

extensive porcelain exports to Europe

960-1279 (Sung Dynasty in China) celadon and Jun ware, cobalt blue and white porcelain

1368-1644 (Ming Dynasty in China) blue and white, reduced copper red and white porcelain extensively exported to Europe

1600s Arita ware 1600s Bottinger porcelain 1700s fine white semivitrous ware

in England 1800s Parian porcelain

900s tin-glazed ware in Bagh- d a d - l u s t e r painting

1300s majolica ware in Spain and Italy

1500s polychrome painting 1600s paintings of history and

stories 1700s faience in Europe 1700s blue and white Delftware 1900s hand-crafted tin glaze ware

1400s German stoneware, salt glazing

1400s English sipware 1600s fine terracotta 1700s turning by steam engine 1700s basalite cane ware 1764 Wedgewood jasperware

6000 B.C. earthenware 600 B.C. terracotta in Greece

1400s German stoneware, salt glaze, English slipware

1600s fine terracotta 1800s engine turning 1900s hand-crafted stoneware

1.3 Raw Materials 27

140 B.C., is only a description of the state of the art. Many problems have yet to be solved before ceramic powder processing can be developed into a mature field for all ceramics. Part of the reason for constant technological evolution is that this field will never be completennew ceramic compositions are always being developed (i.e., high temperature superconductors, piezoelectrics, varistors). In this regard ceramic powder processing will never be without challenging frontiers.

1.3 R A W MATERIALS

Since the Bronze Age both natural ceramic raw materials and synthetic raw materials have been used. Today synthetic raw materials are referred to as industrial minerals or specialty chemicals. Natural raw materials are those to which only physical separations are performed (e.g., clay soils from which organic raw materials are floated, feldspar rock ground to a particular size distribution). With this classification, a description of common ceramic raw materials will be given in the next part of this chapter.

1.3.1 N a t u r a l R a w M a t e r i a l s

1.3.1.1 Clays Clays were probably the first ceramic raw materials. Clay minerals

are fine-particle hydrous aluminum silicates, like those shown in Figure 1.16 [12], which develop plasticity when mixed with water. They have a wide range of chemical and physical characteristics but the common attribute of a crystalline layer structure consisting of electrically neu- tral aluminosilicate layers as shown in Figure 1.17. The platelike morphology gives easy cleavage, which leads to a fine particle size and a narrow particle-size distribution and allows the particles to easily move over one another. Clays perform two important functions in ceramic bodies. First, the plasticity of clay suspensions is basic to many of the forming processes commonly used to fabricate ceramic bodies; the ability of clay-water suspensions to be dewatered to give a shape with strength during drying and firing is unique. Second, clays fuse over a temperature range, depending on composition, that can be economically attained, to become dense and strong without losing their shape.

The most common clay minerals of interest to ceramists are based on the kaolin structure, A12(Si2Os)(OH)4. (The term kaolin comes for the name, Kao Ling, of a mountainous district, 20 miles northeast of Chingtechen, China, famous during the Tang and Sung Dynasties as a strong hold for outlaws [7]). The reason why kaolin is such a useful


FIGURE 1.16 Photomicrograph of kaolinite platelets, A12Si20~(OH)4. From Millot [12].

FIGURE 1.17 A platelet of kaolinite consists of a tetrahedral layers and octahedral layers superimposed. The summits of the tetrahedral layer and octahedral layers form a plane of oxygen atoms. The distance between the two units is 7/k. From Millot [12].

1.3 Raw Materials ~

~ 0 Chapter I Ceramic Powder Processing History

FIGURE 1.18 Common compositions in the ternary system MgO-A1203-SiO2. Taken from Kingery et al. [13], reprinted with permission from John Wiley & Sons, Inc. 9 1976, New York.

raw material is that above 500~ the crystallization of water evolves. Then it decomposes at 980~ to form fine-grained mullite, A16Si2013, in a silica matrix. Fur ther heating of kaolin gives rise to a growth of mullite crystals, crystallization of the silica matrix as cristobalite, and formation of a eutectic liquid at 1595~ as shown in the phase diagram in Figure 1.18 [13]. Reasonable strengths of the sintered kaolin ceramics can be obtained on firing between 900 and 1000~ where one can observe the viscous sintering [4,5] of the kaolin polymorphs produced above 900~ Common natural impurities (i.e., MgO, CaO, Na20, K20) in kaolin act as mineralizers, which promote the crystallization of different mineral phases and enhance strength in this temperature range [5].

Other clay minerals important in ceramics (and their chemical for- mulas) are

Kaolinite Ale(Si2Os)(OH)4 Halloysite A12(Si2Os)(OH)4- 2H20

1.3 Raw Materials 3 1

Pyrophyllite A12(Si2Os)2(OH) 2 Montmorillonite All.67Nao.33Mgo.33(Si2Os)2(OH) 2 Mica A12K(Sil.sAlo.5Os)2(OH)2 Illite A12_xMgxKl_x_y(Si1.5_yAlo.5

These minerals have different stacking of the silica and alumina layers, as well as, incorporating metal hydrates of Na, K, Mg, A1, or Fe between the silica and alumina layers. Clay minerals can also be characterized according to their morphological features including crystal habit (i.e., plates, rods, or rolled-up platelets) stacked in either a house of cards or blocklike aggregates giving a particle-size distribution.

1.3.1.2 Talc

A related natural raw material is talc, a hydrous magnesium silicate with a layer structure similar to clay minerals [14]. Talc has the chemical formula Mg3(Si2Os)2(OH)2 and is used as a raw material for making tile, dinnerware, and electronic components. Talc decomposes to give a mixture of fine-grained protoenstatite crystals, MgSiO3, in a silica matrix at 1000~ Further heating leads to crystal growth of enstatite (MgO 9 SiO2), which has a high thermal expansion coefficient. A liquid is formed at 1547~ At this temperature almost all of the talc melts because its composition is not far from the eutectic composition in the MgO-SiQ system (Figure 1.18). Reasonable sintered strengths can be obtained when talc is sintered at 1000~ The high thermal expansion coefficient of enstatite is used in glaze formulations to put the glaze into compression after firing, which prevents crazing. In some cases, saponite a hydrous magnesium aluminum silicate is used in place of talc because saponite is cheaper than talc.

1.3.1.3 F e l d s p a r

Feldspar is an anhydrous aluminosilicate containing K, Na, or Ca. The value of feldspar in ceramics is due to it being an inexpensive and water insoluble source of alkali. The major minerals of interest in this area are orthoclase, K(A1Si3)O s albite Na(A1Si3)Os, and anorthite Ca(A12Si2)Os. These minerals are widely abundant in nature. Feldspars are a major constituent of igneous rocks (e.g., granite contains about 60% feldspar). These minerals are used as a flux which forms a glass phase in either the ceramic body or the glaze. Figure 1.19 [15] shows the phase diagram for the ternary system K20-A1203-SiO2. In this phase diagram orthoclase (potassium feldspar) is shown to give a near eutectic composition which melts between 800 and 1000~ Feldspar provides alumina and alkali for the glass batch that is used for bottles, fiber glass, and television picture tubes. Feldspar is also the most widely used fluxing agent for ceramics and can be found in formulation for both bodies and glazes, as well as enamels.


FIGURE 1.19 Common compositions in the ternary system K20-A1203-SiO 2 from Schairer and Bowen [15].

1.3.1.4 Si l ica

Silica is both abundant and widespread in the earth's crust. In addition, it is one of the purest of the abundant minerals. The most commonly used forms of natural silica are quartz, cristobalite, diatomite. Quartz is the most common form of silica, which in natural form can have very large crystals > 10 cm with very high purity. Common sand is high in quartz. Silica sand shows sharply angular fragments of quartz [14]. It is an important constituent of igneous rocks such as granite and diorite. It is also found in most metamorphic rocks, constituting a major portion of sandstone. Quartz as a pure form is often found in veins in other rocks. Diatomite consists of the skeletons of diatoms, an ancient microbe about 10 ftm in diameter, see Figure 1.20. This material is widely found in bogs throughout the world, however, large deposits of diatomite are rather rare. Diatomite is used in insulating bodies to give pores smaller than the mean free path of air and for catalysts to provide a controlled microporous diffusion pathway. Silica is a very important raw material for ceramics. Its extensive use is due to its hardness, high melting point, low cost, and ability to form glass. Silica


F I G U R E 1.20 Silica, Si02, diatom skeleton, from a Cellite Corporation | advertisement.

is used in a ceramic body like aggregate is used in concrete: to provide a solid mass around which the glass phase can be used to bind the body together.

1.3.1.5 Wollastonite

Another source of water insoluble calcium is wollastonite (CaO 9 SiO2). Wollastonite is found in either a pure form or in association with garnet or calcite and dolomite. The impure wollastonite deposits must be beneficiated by optical sorting, high intensity magnetic separation, or froth flotation. In glazes, wollastonite may be used as a substitute for calcite, which reduces the volatiles and increases the gloss and texture of the glaze. Wollastonite deposits are known for their low iron content, which gives a glaze with an excellent fired color. In enamels, wollastonite acts as a na tura l frit to reduce gas evolution. It is also


used in ultralow ceramic insulating bodies and as an auxiliary flux in electrical insulators.

1.3.1.6 A l u m i n u m Minerals

Corundum (A1203) in its impure form is also known as emery, a common abrasive. Natural corundum has sharp highly angular particles [14]. Sillimanite minerals with the theoretical composition A12SiO 5 are also a source of alumina for refractories. They include alusite, sillimanite, and kayanite, which are common metamorphic minerals found in slates and schists. Alusite is the aluminosilicate mineral which is stable at low pressure and low temperature. Sillimanite is stable at high temperature, and kayanite is stable at high pressure. Kayanite (A1203 9 SiO2) is commonly used for mullite (3A1203 9 2SIO2) refractories and porcelain sparkplug insulators and has prismatic crystals with steplike fracture surfaces.

1.3.1.7 L i th ium Minerals

The important lithium minerals are spodumene (Li2A12Si4012), lepid- olite (LiKA12F2Si3Og), amblygonite (Li2F2A12P2Os), and petalite (LiA1 Si4010). Spodumene has prismatic and lath-shaped crystals. In a few cases, it is used in glaze formulations in a ground form; in other cases, a lithium salt is extracted and used in a pure form in glass, glazes, and acid resistant enamels. Lithium minerals are most often used as network modifiers in glass to increase melting efficiency and lower the thermal expansion coefficient of the glass, which increases durability.

1.3.1.8 F luor ine Minerals

For ceramic use, the most important mineral containing fluorine is fluorite (CaF2) which occurs in fluorspar. Natural deposits have a purity of 90-98% with silica as the principal impurity. Fluorite mineral powders have angular surfaces which result from cleavage and conchoidal fracture of the mineral [14]. Fluorspar is used in many forms of optical glass of low index of refraction and in enamels.

1.3.2 Synthetic Raw Materials

Synthetic raw materials are those produced by the chemical treat- ment of natural raw materials or by the chemical transformation of synthetic materials.

1.3.2.1 Trans formed Natural Raw Materials

Magnes i te and Calcite Magnesite is the mineral form of magnesium carbonate which has particles composed of aggregates of well- crystallized 1 t~m rhombohedra, many of which are in parallel align-


ment [14]. It is often associated with calcite (CaCO3), which is a mineral with well-defined rhomboidal or prismatic crystals [14]. The mixture of magnesium and calcium carbonate is the mineral dolomite [14]. Dolomite particles are rounded agglomerates composed of rhombohe- dral subunits. Magnesite, calcite, and dolomite can be calcined to drive off the CO2, leaving the respective metal oxides. Magnesite is used for refractories because it has one of the highest fusion points known (2800~ and is resistive to many metal slags. Calcite is used in ceramics, as well as ground limestone in glazes, enamels, and glass. Another source of calcium in ceramics is calcined gypsum a natural hydrous calcium sulfate mineral. Calcium and magnesium are network modifiers in glass, which improve the glass's resistance to chemical attack.

Barium Minerals Barite (BaSO4) and witherite (BaCO3) are commonly used to supply barium in ceramic formulations. Purified barium carbonate, made by dissolution and reprecipitation, is used most frequently in ceramic processes and as fluxing compounds in the glazes, glass, and enamels of electronic ceramics and in heavy clay products to prevent scumming. The use of these minerals have the drawback that upon heating they give off gas, which can cause cracks.

L e a d Mine ra l s The most common lead-containing mineral is galena (PbS) followed by anglesite (PbSO4) and cerussite (PbCO3). The two latter minerals result from the weathering of galena. The occurrence of galena deposits is unexpectedly high and spread throughout the world. Galena is roasted in air to give lead oxide. Red lead (Pb304) and white lead (PbCO3 9 Pb(OH)2) are commonly used as a basic flux. From the point of view of health, the lead should be reacted with silica to give insoluble PbSi205 . Lead is used in "crystal" stemwear, electrical glass for lighting and television picture tubes, and radiation-absorbing glass and in sanitary ware for enamels and glazes.

1.3.2.2 Synthetic Raw Materials--Specialty Chemicals

Alumina Alumina used in ceramics today is commonly obtained via the Bayer process. The Bayer process starts with gibbsite (A1203 9 3H20), which is a common soil mineral often found in association with hematite (Fe203). This raw material is leached with sodium hydroxide at high temperature and pressure and separated from the hematite which is insoluble. The resulting sodium aluminate solution is then allowed to precipitate gibbsite. This purified gibbsite is calcined to give alumina, A1203, which contains both well-formed hexagonal crystals and rounded agglomerated masses attached to the surfaces of the hexagonal surfaces [14]. Another synthesis method for alumina is to mix the sodium aluminate solution with an acid to lower its pH and thereby

3 6 Chapter 1 Ceramic Powder Processing History

FIGURE 1.21 Pseudo-boehmite gel produced by precipitation of alumina by acid-base neutralization. Versal | a Kaiser Aluminum Corp. product. Photo courtesy Ron Rigge.

precipitate a microcrystalline boehmite, which forms gel agglomerates similar to those shown in Figure 1.21. Aluminas are commonly used as catalytic substrates and as silicon chip substrates, as well as additives to glass. High-alumina ceramics are used as refractories for la- dle metallurgy.

C h r o m i a Chromite Cr2FeO 4 is the most commonly used chro- mium-containing mineral for ceramic formulations. This mineral has a spinel crystal structure, where the iron may be replaced by magnesium and aluminum. Chromite is used in ceramics largely as a refractory in the form of burned and chemically bonded bricks. For this purpose, a low-silica material is desired. When low silica is desired, chromic oxide is extracted from chromite by dissolution in acid, removal of the iron impurity by liquid-liquid extraction, and precipitation of the hydroxide, which is subsequently calcined to the oxide. Chromic oxide is used as a color additive to glazes and enamels and in ferrite production to give magnetic materials.

Magnes ia Magnesia (MgO) is produced from seawater or brine. In one process, the chloride brine is sprayed into a reactor where hot gasses convert the MgC12 solution to MgO and HC1. The MgO is slurried


with water, which reacts to form Mg(OH)2. The Mg(OH)2 is washed, thickened, filtered, and then calcined to produce magnesia. Magnesia produced in this way is composed of agglomerates of well crystallized 1 ~m platelets [14]. In another process, the magnesium chloride brine is reacted with strong base to precipitate Mg(OH)2, which is washed, thickened, filtered, and then calcined to produce magnesia. With in- creasing calcination time and temperature, the MgO crystallites increase in size. Magnesia calcined at 1400~ has a low chemical reactivity and is used exclusively in refractories because it has a high resistance to basic metallurgical slags.

Soda Ash, Caustic Soda In the Solvay process, soda ash is produced by reaction of salt (NaC1) with limestone (CaCO3) to produce soda ash and a calcium chloride salt solution. Ammonia enters the reaction process at various steps but is not consumed. Caustic soda is produced by electrolysis of NaC1 brine solutions, giving C12 gas and Na metal, which forms an amalgam with the Hg of the cathode. The amalgam is decomposed using water to form a sodium hydroxide solution, which is concentrated and precipitated to give anhydrous caustic soda. The glass container and flat glass industries use an extensive quantity of both soda ash (NaCO3) and caustic soda (NaOH) as a network mod- ifier to decrease the working temperature of the glass.

Ti tania Pigment grade titania is produced by the oxidation of titanium tetrachloride. Titanium tetrachloride is produced by the chlorination and selective distillation of ilmenite (FeTiO2) ore. The powder produced by the oxidation process consists of spherical particles (0.2-0.3 ~m in diameter [14]. Titania's high refractive index of 2.5 and its narrow submicron size distribution makes it a very good white pigment in glass and glaze.

Zinc Oxide Zinc oxide has a specific gravity of 5.6, sublimes at 1800~ is photoconductive, insoluble in water, soluble in strong alkali solutions, and in acid solutions. It is produced by one of two processes. One process vaporizes zinc metal and burns the vapor in air to give a fine spherical zinc oxide particles [14]. In the other process, the mineral form of zinc sulphide is roasted with carbon to reduce the ore to zinc metal, which in turn vaporizes to give a gas which is burned in air. Zinc oxide powder is used in the manufacture of glass, glazes, porcelain enamels, varistors, and magnetic ferrites. In glass, glazes, and enamels, zinc oxide offers great fluxing power, reduction of expansion, prevention of cracking and crazing, and enhanced gloss and whiteness.

3 8 Chapter I Ceramic Powder Processing History

Zirconia Common zirconium-containing minerals include badde- leyite (ZrO2) and zircon (ZrSiO4). Most of the zirconium-containing materials used in ceramics are extracted from zircon sands. This extraction is performed by chlorination of the silicious raw material and distillation of the mixed metal chloride gases. The separated zirconium chloride is then mixed with water and precipitated as the hydroxide or the hydroxychloride. Upon calcination, both the hydroxide and the hydroxychloride decompose to zirconium oxide. Calcined zirconia particles are composed of 0.1 t~m granules agglomerated into rounded ~20 t~m particles [14]. Zirconium produced in this way has 4% halfnia in it. To remove the halfnia, a liquid-liquid extraction must be performed on the zirconium chloride solution before precipitation. Zirconium oxide is used as an opacifier in glazes and enamels or as a refractory after fusion with lime, which acts as a stabilizer of the crystal phases present. Mixed with yittria the tetragonal phase of zirconia is stabilized, which transforms to monoclonic undergoing a 15% volume change, allowing ceramics to be transformation toughened by the presence of this phase.

Si l icon Carbide The Acheson process is used to produce large quantities of SiC. This process carbothermically reduces SiO2 to give SiC and CO(g) in a resistance furnace. In 36 hr at 2400~ the chemical reaction is complete. The SiC produced is 1 to 5 mm crystals of a-SiC and must be ground to the desirable particle size distribution. Low- purity silicon carbide is used in abrasive and refractory applications. High-purity silicon carbide is used for reaction bonded ceramics that require strength at high temperatures, high thermal conductivity, high thermal shock resistance, and a low thermal expansion coefficient. For the manufacture of high-performance ceramics by sintering or hot pressing, other methods of powder synthesis are used. Such processes include plasma-arc synthesis, batch reaction of silica and carbon in CO or inert gas, decomposition of polycarbosilanes, and chemical vapor decomposition. In addition, SiC whiskers are manufactured by the carburization of molten silicon. These single crystal whiskers are used in ceramic matrix composites.

Other Metal Carbides A host of other metal carbides are used in ceramic formulations. These include TaC, TiC, Cr3C2, VC, Mo2C, B4C, WC, and ZrC. These metal carbide powders are produced by carbothermal reduction of the relevant metal oxide or reaction of the relevant metal with carbon in CO or an inert atmosphere. These metal carbides are used as abrasives and in high-temperature wear applications.

Si l icon Nitr ide Silicon nitride is a synthetic raw material which is synthesized by various high-temperature reactions between 1000


and 1600~ The three most important methods of silicon nitride powder synthesis are

9 reacting silicon metal powder with nitrogen 9 reacting silica, nitrogen, and carbon 9 reacting chlorosilanes with a gas containing nitrogen (e.g., am-

monia).

Silicon nitride whiskers are also produced by variations of processing conditions in these synthesis methods. Silicon nitride is used for toolbits for cutting cast iron and other high-temperature wear parts including burner nozzels.

Other Metal N i tr ides Many other metal nitrides are used in ceramic formulations. These include A1N, TiN, VN, and BN. These metal nitride powders are produced by carbothermal reduction of the relevant metal oxide in a nitrogen-containing atmosphere or reaction of the relevant metal with a nitrogen-containing reducing atmosphere. These metal nitrides are used as abrasives and in high-temperature wear applications.

Bor ides Metal borides form another important class of ceramic powders, which include TiB2, BC, W2 B, and MoB. Borides have metallic characteristics, with high electrical conductivity and positive coefficient of electrical resistivity. They are produced either by reaction of the relevant metal with boron at a suitable temperature, usually in the range of 1100-2000~ or by reaction of a mixture of the relevant metal oxide and boron oxide with aluminium, magnesium, carbon, boron, or boron carbide followed by purification. Borides are used for electrically heated boats for aluminum evaporation and sliding electrical contacts, as well as abrasives and wear parts, including sandblast nozzels, seals, and ceramic armor plates. TiB 2 has been investigated for use as a nonconsumable replacement for the consumable graphite anode in the electrolytic reduction of alumina to aluminum metal.

Other R a w Mater ia l s This list of ceramic raw materials is by no means complete. A myriad of other raw materials are presently being used in ceramic formulations. New raw materials are being developed all the time to fulfill the need for better material properties and tailor ceramic powder properties to meet different ceramic processes. The tailoring of ceramic powders usually involves altering particle morphology or particles size distribution for use in a new ceramic powder process. Such new ceramic materials include metal silicides (e.g., NbSi2, V3Si, WSi2, and MoSi2) and metal sulphides (e.g., CdS2). These materials are synthesized in various ways by small-scale batch methods and are used for highly specific applications.


1.4 S E L E C T I N G A R A W M A T E R I A L

To select a ceramic raw material, it is necessary to know the final material properties demanded of the ceramic product and the ceramic process by which it will be fabricated. With the physical property information, it is possible to develop a list of raw materials that, after high temperature fabrication, will give the desired chemical formulation. This list of raw materials will next have to be considered in light of a particular ceramic process to be used, which may include, for example, powder mixing, slurry formation, slip casting, drying, binder burn-out, and reactive sintering. To prevent segregation in the ceramic green body, raw materials with similar particle morphology and size distributions should be used. Thus, the different raw materials necessary for the process must be compared to one another for particle size and shape compatibility. Sometimes surface chemistry compatibility is also important. The particle morphology and particle size distribution of a particular raw material depends on the method of powder synthesis. The fundamental principles of many of these powder synthesis methods will be discussed in the balance of this book to explain the reasons for the various particle morphologies and particle size distributions observed in natural and synthetic raw materials. These powder characteristics influence in what ceramic processes these powders can be used.

For the simple case of the processing a single ceramic powder, what

FIGURE 1.22 Micrograph of silicon nitride powder, SN-E10, from UBE Industries, Ltd. [16] with sedigraph size distributions for various grades of this powder with 3 m2/ gm (E03), 5 m2/gm (E05), and 10 m2/gm (El0).

References 4 1

type of powder should be chosen? Reflecting on the "General Concepts of Ceramic Powder Processing" discussed in the introduction to this chapter, a ceramic powder with high chemical purity and a uniform size distribution and particle morphology is the best choice. Only synthetic ceramic powders provide these characteristics. A good example of such a powder is shown in Figure 1.22. Here a silicon nitride powder with a >97% s-phase purity and a narrow size distribution of spherical particles is shown. Various grades of this powder corresponding to different particle size distributions are commercially available from UBE Industries, Ltd. [16].

1.5 SUMMARY

This chapter has reviewed the field of ceramic powder processing from a historical perspective. In addition, it has catalogued the various ceramic powder raw materials used to produce ceramics.

References

1. Smith, B., and Weng, W.-Go, "China--A History in Art," Gemini Smith Inc. Book. Doubleday, New York, 1972.

2. "The Genius of China," an exhibition of the archaeological finds of the People's Republic of China held by the Royal Academy, London 29 September 1973 to 23 January 1974.

3. Wood, N., New Sci. February, pp. 50-53 (1989). 4. Lemaitre, J., and Delmon, B., Am. Ceram. Soc. Bull. 59(2), 235 (1980). 5. Lemaitre, J., and Delmon, B., J. Mater. Sci. 12, 2056-2065 (1977). 6. "Treasures from the Bronze Age of China," an exhibit from the People's Republic of

China, The Metropolitan Museum of Art, Ballantine Books, New York. 7. Brankston, A. D., "Early Ming Wares of Chingtechen," p. 64. Vetch and Lee Ltd.,

Hong Kong, 1938. 8. Valenstein, S. G., "Handbook of Chinese Ceramics." Weidenfeld & Nicholson, Lon-

don, 1989. 9. Bushell, S. W., "Description of Chinese Pottery and Porcelain" (translation of "T'ao

Shuo"). Oxford Univ. Press, Oxford, 1977. 10. Needham, J., "Science in Traditional China." Harvard Univ. Press, Cambridge,

MA, 1981. 11. Anderson, K. J., MRS Bull., July, pp. 71-72 (1990). 12. Millot, G., La Science 20, 61-73 (1979). 13. Kingery, W. D., Bowen, H. K., and Ulhmann, D. R., "Introduction to Ceramics," 2nd

ed. Wiley (Interscience), New York, 1976. 14. McCrone, W. C., and Delly, J. G., "The Particle Atlas." Ann Arbor Sci. Publ., Ann

Arbor, MI. 15. Schairer, J. F., and Bowen, N. L., Am. J. Sci. 245, 199 (1947). 16. UBE Industries, Ltd., Ceramic Div., Tokyo Head Office, ARK MORI Building, 12-

32, Akasaka 1-chome, Minato-ku, Tokyo, 107 Japan. 17. Hobson, R. L., "Chinese Pottery and Porcelain." Dover, New York, 1976.

2 Ceramic Powder Charac ter i za t ion

2.1 O B J E C T I V E S

To characterize a ceramic powder, a representative sample must be taken. Methods of sampling and their errors therefore are discussed. Powder characteristics, including shape, size, size distribution, pore size distribution, density, and specific surface area, are discussed. Em- phasis is placed on particle size distribution, using log-normal distributions, because of its importance in ceramic powder processing. A quantitative method for the comparison of two particle size distributions is presented, in addition to equations describing the blending of several powders to reach a particular size distribution.

2.2 I2VTR OD UCTION

In all ceramic raw materials, both natural and synthetic, a powder with a particular chemical formula is the primary objective. Chemical

43

44 Chapter 2 Ceramic Powder Characterization

analysis of ceramic powders is performed by many techniques from X-ray fluorescence spectrometry and atomic absorption spectrometry to the wet chemical methods of titration. All of these techniques are subjects unto themselves, covered by other books and not be discussed further here. After satisfying this primary objective of chemical purity, other powder characteristics are important to optimize the powder to the requirements of the ceramic process in which it will be used. The diversity of these production methods calls for the choice of ceramic powders to be based on different characteristics. Beyond chemical purity, the most important characteristics for subsequent ceramic processing are particle morphology, particle size distribution, and surface chemistry. The surface chemistry of ceramic powders is extremely important for wet and dry processing methods and will be discussed in detail in a separate chapter. The characteristics of ceramic powders corresponding to their size and shape are discussed in this chapter. An excellent book that treats particle size measurement is one by Allen [1], from which many concepts used in this chapter are taken.

2.3 POWDER SAMPLING

Before any characteristics of a powder can be measured it is impera- tive to have a representative sample of the powder. This problem can be viewed in its true magnitude by considering that several tons of material will be analyzed on the basis of less than 1 gm of material. The ultimate that may be obtained in a representative sample is called the perfect sample; the difference between this perfect sample and the bulk can be established by a statistical method, described in the following problem.

2.3.1 Sampling Accuracy

P r o b l e m 2.1. D e t e r m i n e t h e S a m p l i n g Error

A glaze formulation has poor color when a finely ground silica powder has a fraction of iron impurities larger than 50 ppm by weight. Let us assume that a 10 gm sample is taken from a 10,000 kg batch. In this 10 gm sample, we find 40 ppm iron particles greater than 44 ftm by sieving.

The maximum sampling error, E, can be expressed as [1]

E-- +20i _ ~ (2.1)

2.3 Powder Sampling 45

where (r~ is the s tandard deviation intrinsic in the sample due to the sampling of 10 gm from a 10,000 kg batch and P is the weight fraction of material greater than 44/xm measured in the sample. The s tandard deviation due to sampling is determined by [1]

( s)11 2 ~ = [ W-~ . (Pw~ + ( 1 - P)w2) . 1-- Wbb (2.2)

where Ws and Wb are the weight of the sample and the bulk, respec- tively; w~ and w2 are the weights of individual grains, the metal impurity particles 1 assumed to be the density of iron with a diameter of 44 txm and 2 assumed to be silica 0.5 txm in diameter. In this example, P = 40 x 10 -6, Ws = 10 -2 kg, Wb = 10,000 kg, Wl = 3.5 x 10 -1~ kg, and w2 = 1.7 x 10 - ~ kg, giving a value of cr i = 2.37 x 10 -7 and an error of 1.19%. In addition to the error caused by using a small sample, we have the error in our analytical technique. For our example, when multiple iron impuri ty measurements were made on the same sample, the s tandard deviation of the impurities was ___4 ppm with a mean value of 40 ppm as before. This 10% analysis error will have an effect on the total error since the total s tandard deviation o't is

O. t (0.2 + __2,1/2 = ~'nJ (2.3)

where (rn is the s tandard deviation of the analysis technique (i.e., 4 10 -s for this example). Accounting for the error in our analysis in this problem, we find a total error (using equation (2.1) with (rt replacing (r~) is 20.0%. The analysis error (rn is, therefore, the most significant error in this example.

2.3.2 Two-Component Sampling Accuracy Any powder can be considered to be made up of two components,

the fraction above and below a certain size and assumptions made as to the weights of the individual grains in each of the two components. Equation (2.2) may then be used to determine the sampling accuracy of a single powder. Furthermore, if the particles are counted instead of weighed, a more general equation is applicable [1]:

(2.4)

where p is the fraction of particles greater than a certain size, Ns is the number of particles counted, and Nb is the number of paraticles in the bulk. (This equation is also used to determine the accuracy of public opinion poles.) It is obvious from the preceding equations, tha t the larger is the sample, the smaller is the sampling s tandard deviation.

46 Chapter 2 Ceramic Powder Characterization

2.3.3 Sampling Methods

Unfortunately, the size of a practical analytical sample is often minuscule compared to the bulk material being sampled and even the analytical sample is subject to a large degree of sampling variation. There are two ways to reduce this variation. One way is to make up a large laboratory sample from many increments of the bulk and divide the laboratory sample to produce an analytical sample. This laboratory sample is often retained for replicate analyses to determine the stan- dard deviation of the analytical method. The second way to reduce sampling variation is to take a number of replicate samples and mix them together to make an analytical sample.

A representative sample is difficult to obtain when one considers that

1. Particles encounter many types of segregation that will bias the sample.

2. Many different conditions are to be sampled.

Frequently, one must sample a continuous stream, batches, bags, heaps, hoppers, or trucks. The most important segregation-causing property is particle size, and this problem is exacerbated with flowing material. In a

fundamentals of ceramic powder processing and synthesis

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