23rd Annual Conference on

Composites, Advanced Leramics,

Materials, and Structures: A

Ersan Ustundag Gary Fischman Editors

January 25-29, I999 Cocoa Beach, Florida

Published by The American Ceramic Society 735 Ceramic Place Westerville, OH 4308 I

0 I999 The American Ceramic Society ISSN 0 196-62 I9

The page is intensily left blank

23rd Annual Conference on

Composites, Advanced Ceramics,

Materials, and Structures: A

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23rd Annual Conference on

Composites, Advanced Leramics,

Materials, and Structures: A

Ersan Ustundag Gary Fischman Editors

January 25-29, I999 Cocoa Beach, Florida

Published by The American Ceramic Society 735 Ceramic Place Westerville, OH 4308 I

0 I999 The American Ceramic Society ISSN 0 196-62 I9

Copyright I999 by The American Ceramic Society. All rights reserved.

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Cover photo: "Serial sections sawcut at I mm spacing from the undamaged portion of a CST composite test specimen," is courtesy of TL. Jessen, A.J. Kee, RK. Everett, B.A. Bender, ICE. Simmonds, and A.B. Gettmacher, and appears as figure Z(fj in their paper "Application- Specific CFCMC Design Using 2 0 Structural Simulations," which begins on page 77.

Contents 23rd Annual Conference on Composites,Advanced Ceramics, Materials, and Structures: A

... Preface ......................................................... .xi11

Sturting Muten'uls und Processes Synthesis and Properties of Erbium Oxide Single Crystals . ... .3 1.1. Petrovic, RS. Romero, D. Mendoza A.M. KuWa RC. Hoover; and KJ. McClellan

Barium Titanate and Barium Orthotitanate Powders through an Ethylene Glycol Polymerization Route ........ .I I S.J. Lee, M.D. Biegalski, and W.M. Kriven

Gelcast Forming of PZT Materials ..................... I9 1.0. Kiggans Jr,T.N.Tiegs, F.C. Montgomery, LC. Maxey, and H.T. Lin

A New Route to Hexaluminate Ceramics via a Novel Transmetalation Reaction ......................... .27 RL. Calender and A.R Barron

Rheological Properties of Ceramic Formulations for Extrusion Freeform Fabrication ..................... .35 RVaidyanathan, J.L. Lombardi, B.Tennison, S. Kasichainula, and I? Calvert

Low-Temperature Carbonization of W-Co Salts Powder .... .45 G.Q.Shao,B.L.Wu,M.KWei,X.LDuan,J.RXie,RZ.Yuan,~F.Wu, and D.L. Zhou

Composites Development Processing and Properties of an Aluminum Oxidel Aluminum Nitride Composite ...................... .53 G.A. Gilde, J.C. LaSalvia and 1.1. Swab

Processing and Properties of a Nicalon-Reinforced Zirconium Phosphate Composite Radome Faceplate ....... .6l B.A. BendecTL Jessen, and S. Browning

A Submicron-Scale Duplex Zirconia and Alumina Composite by Polymer Complexation Processing ......... .69 S.J. Lee and W.M. Kriven

V

Application-Specific CKMC Design Using 20 Structural Simulations ......................... .77 TL Jessen, A j Kee. RK Everett, B A. Bender; ICE Simmonds, and A B Geltmacher

Oxidative Degradation Behavior of Polycarbosilane-Derived

A Urano,A Saeki. M Takeda. and A Yokoyama

On the Way t o Cost-Effective Oxidation Protection Techniques of CMCs. Case Study of Tyranno-Hex Materials . . . .95 M Drissi-t4abti.Y Kanno, K Suzulo, K Nakano, and S Sakakibara

Investigation of Microwave Behavior of Silicon Carbide/

K S Leiser and DE Clark

Microwave-Induced Combustion Synthesis of Tic-AI,O,

D Atong and DE Clark

Methods for Joining Silicon Carbide Composites for High-Temperature Structural Applications . ............. I I 9 C A Lewinsohn, RH Jones, M Singh,T Shibayama,T Hinokr, M Ando, andA Kohyama

Silicon Carbide Fibers ............................ .85

High Alumina Cement Composites ................... I03

Composites .................................. .I I I

Comings Development Formation of Interface Coatings on SIC and Sapphire Fibers Using Metal Doped Carboxylate-Alumoxanes ...... .I27 R L Callender and A R Baron

The Growth and Structure of Nanocrystalline

I Kosacki. M Shumsky, and H U Anderson

Protective Coatings for lnframd Materials .............. I45 J G Lee E D Case M A Crimp, J Malik and DK Reinhard

Grain Growth and Tensile Strength of 3M Nextel 720"

R S Hay E E Boakye, M D PetryY Berta. KVon Lehrnden, and J Welch

Sol-Gel Synthesis of Zircon-Carbon Precursors and

E Boakye R S Hay, M D Petry and T A Parthasarathy

Dissolution, Reactions, and Diffusion in the SiCICITiB, and SiCICITiN + Liquid Silicon Systems at 1450°C ........ I73

Zr02:Y Thin Films .............................. .I35

after Thermal Exposure .......................... I53

Coatings of Nextel 720" Fiber Tows . ................ I65

PJ Holrnes. RD Sisson Jr, and M Singh

vi

Micmstructural Assessment Evaluation of Microstructure for SiC/SiC Composites

S SuyamaY ltoh, S Nakagawa, and N Tachikawa

Mechanical and Microstructural Properties of Nextel 720" Relating to I ts Suitability for High-Temperature Application in CMCs ................ I 9 I C. Mrlz, j. Goerrng, and H Schnerder

Effect of Dopants on Anisotropic Grain Growth in Oxide-Matrix Materials ........................... I99 TN Tiegs, M R Snyder. F.C Montgomery, J.L Schroeder. and D W. Coffey

Using Mercury Intrusion Method .................... I8 I

Advanced Materials Partnership: Gelcast Silicon Nitride Turbomachinery Technology to Commercialization Cost Modeling and Analysis for Advanced Structural Silicon Nitride Turbomachinery Ceramics ............. ,209 J M Schoenung

Gelcasting Advancement for Manufacturing Scale-up ..... .2 I7 j J Nick D Newson, B Draskovich, and 0 0. Omatete

Gelcasting Automation for High-Volume Production of Silicon Nitride Turbine Wheeis ..................... .225 J J Nick D Newson, R Masseth, S Monette. and 0 0 Omatete

Gelcast Slurry Enhancement ...................... .233

Improved Gelcasting Systems ..................... .24 I

00 Omatete and M Colic

0 0 Omatete and j J Nick

Structure-Property Relationships Mechanical Behavior of a Hi-Nicalon" Sic Composite Having a Polycarbosilane Derived Matrix . . ............ .25 I F I Hurwitz. A M Calomino, andT.R McCue

Comparison of the Tensile, Creep and Rupture Strength Properties o f Stoichiometric Sic Fibers . . . . . . . . . . . . . . . ,259 H M Yun and J A. DiCarlo

Thermal Conductivity of CVI and PIP SiClSiC Composites .. .273 RYarnada,TTaguchr, J. Nakano, and N. lgawa

vii

Reflectance of CVD TiB, Films ..................... ,281 W.J. Lackey and B.N. Beckloff

Thin, Adherent Oxide Coatings on Bismaleimide Polymeric Substrates ........................... .289 C. Mukhejee, E.D. Case, and A.Y Lee

Part Charucterization and Anulysis Analysis of the Indentation-Quench Test for Ceramics ..... .301 IKM. Collin and D.]. Rowcliffe

Computational Analysis of Crack Growth in Composite

H. Murakawa, H. Serizawa and Z.Q. Wu

Front-Flash Thermal Imaging Characterization of Continuous Fiber Ceramic Composites ............... .3 17 C. Deemer; J.G. Sun,W.A. Ellingson, and S. Short

Materials Using Lennard-Jones Type Potential Function .... .309

Non-Contact Ultrasonic NDE of CarbodCarbon Composite ... .325

Voroni Element Analysis of Functionally Graded Materials .. .333

Using NDE Techniques ........................... .34 I

B.RTittman, M.C. Bhardwaj, L.Vandervalk and I. Neeson

S.B. Biner

Damage Characterization of a Si,N,-BN Fibrous Monolith

J.L Finch, J.M. Staehler; L.f? Zawada W.A Ellingson, J.G. Sun, and C.M. Deemer

Effects of Fiber Coating Composition on Mechanical Behavior of Silicon Carbide Fiber-Reinforced Celsian Composites .................................. .353 N.F! Bansal and ].I. Eldridge

Flexural and Compressive Strengths and Room-Temperature CreeplRelaxation Properties of Plasma-Sprayed ZrO,-8 wt% Y,O, ................... .365 S.R Choi. D. Zhu, and RA Miller

In sku Observation of Stable Crack Propagation in Toughened Silicon Nitride Ceramics ................. .373 YTakigawa, K Hiramatsu, and H. Kawamoto

Active Metal Brazing of Various Types of Si,N, to Cu and Cu-Alloy ............................... ,381 M. Heim, D.G. Ulmer; andV.A. Greenhut

... V l l l

Properties and Testing A Modified Flaw-Size Toughness Technique ............. .393 ].A. Griggs,T]. Hill, and 1.1. Mecholsky JI:

Relationships among Changes in Electrical Conductivity, Thermal Conductivity, Thermal Diffusivity, and Elastic Modulus for Microcracked Materials ................. .40 I E.D. Case

Fracture Behavior of High-Thermal Conductive Aluminum Nitride ............................. .41 I J.Tatami, K. Komeya,T Meguro, S. Iwasawa, and M. Komatsu

Creep Deformation and Rupture Behavior of Laminated Metal-Metal Matrix Composites .................... .4 I 9 S.0. Biner

High-Temperature Compressive Deformation of Si,N,/BN Fibrous Monoliths ....................... .427 J.L. Routbort, KC. Goretta, E.T. Park D. Singh, ]. Finch, J. Staehler; L Zawada, and G.E. Hilmas

A New Approach for Interface Sliding Shear Resistance in A$O, Fiber-Reinforced AI,O, Matrix Composite ........ .435 H. Kakrsawa and Y Kagawa

... .443 7: Senda, F.Yano, J. Drennan, E.Yasuda, and RC. Brad

Evaluation of Monazite Fiber Coatings in Dense

KA. Kel1er.T.A ParthasarathyT. Mah, M.K. Cinibulk, and E.E. Boakye

High-Temperature Uniaxial Creep Behavior of a Sintered, in si tu Reinforced Silicon Nitride Ceramic ............. .463 Q. Wei, J. Sankar. and J. Narayan

Why It Is Necessary t o Determine Each Fiber Diameter When Estimating the Parameters of the Distribution of

E. Lara-Cunio and C.M. Russ

Br i t t le to Ductile Transition in Sliding Wear of Alumina

Matrix Composites ............................. .45 I

Fiber Strengths ............................... .47 I

Fiber Diameter Variation-Sample Preparation and Anatysis ... .48 I G.E.Youngblood, C.R Eiholzer; C.A. Lewinsohn, RH. Jones, A. Hasegawa, and A. Kohyama

Damage Evolution and Residual Stresses in Plasma-sprayed

J.P Singh, B. Nair; D. Renusch, M. Sutaria and M. Grimsditch Zirconia Thermal Barrier Coatings .................. .487

ix

Re/iubi/ity und Life Prediction Fracture Toughness of Ceramics Using the SEVNB Method: Fiat Results of a joint VAMASIESIS Round Robin . ....... .495 J. Kubler

Stress Intensity Factor Coefficients for Chevron-Notched Flexure Specimens and a Comparison Fracture

J. Salem, L. Ghosn, M. Jenkins, and G. Quinn

Standard Reference Material 2 100: Ceramic Fracture

Toughness Methods ............................ .503

Toughness ................................... .5 13 G.D. Quinn, R. Gettings, and K. Xu

Specimen Geometry Effect on the Determination of Slow Crack Growth Parameters of Advanced Ceramics in Constant

S.R. Choi and ].I? Gyekenyesi

Specimen Size Effect on the Creep of Si,N, . . .......... .535 A.A. Wereszczak M.K. Ferber, AS. 6ames, andTI? Kirkland

In situ Determination of Constituent Properties and Performance in an Oxide-Oxide CFCC ................ .545

Predictions of the Inert Strength Distribution of Si,N, Diesel Valves ............................. .555 M.J. Andrews, A.A. Wereszuak. and K Breder

Determination of Strength for Reliability Analysis of Multilayer Ceramic Capacitors ..................... .565 K Breder, A.A. Wereszuak. L RiestecTP Kirkland, and RJ. Bridge

Machining Damage and Reliability Analysis of Glass Specimens Using the CARESlLife Design Code . ......... .573 F.A. Holland JL S.R Choi, and N.N. Nemeth

Assessment of Hypervelocity Impact Damage of the

J.E. Ritter. K. Jakus, and J. Lankford, JK

Reliability and Lifetime Predictions for Ceramic

Flexural Stress-Rate Testing at Elevated Temperatures . ..... .525

C.X. Campbell and M.G. Jenkins

Space Shuttle Windows .......................... .58 I

Components ................................. .589 V.RVedula, S.J. Glass, S.L. Monroe, M.K. Neilsen, and C.S. Newton

Multi-Laboratory Round Robin Results for a Single CFCC: Input for Test Standards, Design Codes, and Databases . ... ,595 M.G. Jenkins, PLVanLandeghen, E. Lam-Curzio, S.T. Gonczy, and L.P Zawada

X

Comparative Investigation of Intra- and Interlaboratory Mechanical Tests of Flexural, Tensile, and Shear Behavior in a CFCC ................................... .605 i?L.VanLandeghen and M.G. Jenkins

Flexure Testing of NicalonTM 2-D Fiber-Reinforced SylramicTM S-200 Ceramic Composites: A Multi-Laboratory Round-Robin Test .............................. .6 I 5 S . 1 Gonczy and M.G. Jenkins

Mechanical Behavior of BN-Coated Hi-Nicalon TM Sic Fiber-Sic Matrix Composite at Room and High

S.Q. Guo,Y Kagawa, M.Takeda, H. Ichikawa, M. Fujikura, and R.Tanaka

Damage Tolerance of Coating Free SiTiCO (Tyranno?

T Mamiya,Y Kagawa,T Nakayasu, M. Sato, and TYarnamura

Temperatures ................................ A25

Sic Matrix Composites .......................... .63 I

xi

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Preface

The International Conference on Engineering Ceramics and Structures-the 23d Annual Cocoa Beach Conference and Exposition drew a diverse group of people interested in ceramics for various reasons. Scientists and engineers from government, industry, and academia joined to discuss advanced ceramics from materials preparation to commercial- ization.Three successful symposia focused the program into their subject areas, and the general conference topics assured that a broad perspective of the field was discussed.

The three symposia gave a unique character to the program.They were:

Advanced Synthesis and Processing Materials Behavior under Extreme Conditions, which focused on innovative approaches in materials synthesis and processing in which extreme conditions are present or imposed. Most of these papers are included in these volumes.

Commercialization and Use of Engineering Ceramics, which focused on ways to increase or improve the interactions necessary to bring engineering ceramics to market in a timely fashion.

*The Larry Hench Sympsium on SurFdce-Active Processes in Materials (papers published in Ceramic Tmnsactions Volume 101) brought a diverse group of engineers, medical practi- tioners and scientists together to discuss surface active materials in honor of a major pioneer in these areas.

These three symposia and the general topics offered perspectives of ceramics that ranged from materials modeling to technology transfer and market development. Such a range of topics created a dynamic program that attendees enjoyed greatly. These vol- umes are a tribute to that meeting.

Finally, we would like to thank those that helped make this conference possible: the sym- posium chairs who worked to organize their programs, session chairs who kept things running smoothly on stage, and the staff who kept things running smoothly. We would also like to thank the efforts of the Advanced Composite Working Group who ran a conference concurrently and worked hard so we could create the highest value pro- gram to participants of both conferences.

Gary Fischman Ersan Ustundag

... X l l l

Starting Moterials and Processes

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SYNTHESIS AND PROPERTIES OF ERBIUM OXIDE SINGLE CRYSTALS

J.J. Petrovic, R.S. Romero, D. Mendoza, A.M. Kukla, R.C. Hoover, K.J. McClellan, Group MST-8, Los Alamos National Laboratory, Los Alamos, NM 87545

ABSTRACT Erbium oxide (ErzO3, erbia) is a highly stable cubic rare earth oxide with a

high melting point of 2430 OC. Because of this, it may have potential applications where high temperature stability and corrosion resistance are required. However, relatively little is known about the properties of this oxide ceramic. We have employed a xenon optical floating zone unit with a temperature capability of 3000 OC to grow high quality single crystals of erbia. The conditions for single crystal growth of erbia have been established. The mechanical properties of erbia single crystals have been initially examined using microhardness indentation as a function of temperature.

INTRODUCTION

of formation of any binary oxide ceramics (1 ,2). This means that they possess excellent high temperature stability and corrosion resistance, making them candidate materials for potential elevated temperature applications requiring these characteristics, such as fixturing which must be immersed in molten metals. These oxides have high melting points, possess a cubic crystal structure, and exhibit complete solid solubility with each other.

Over the past few years we have been focusing on the study of Er2O3 (3,4). The purpose of the present investigation was to grow high quality single crystals of erbium oxide and to initially investigate the mechanical properties of these erbia single crystals.

The rare earth oxides Er2O3, Yz03, and Sc2O3 have the lowest free energies

MATERIALSANDPROCEDUFN

Single Crystal Growth The high melting point of erbia, 2430 OC, makes the synthesis of single

crystals relatively difficult. However, high quality erbia single crystals can be produced using a xenon optical floating zone apparatus (3,4). The xenon optical floating zone unit is shown schematically ( 5 ) in Figure 1. It consists of a high

To the extent authorized under the laws of the United States of America, all copyright interests in this publication are the property o f n e American Ceramic sodety. Any duplication, re rodudion, or re ublication of this ublication or any part thereof, without the express written consent of The American Ceramic L i e t y or fee pai!to the Copyright &earance Center, is prohibited.

3

power xenon lamp which is situated within an ellipsoidal mirror cavity. This

Figure 1 : Schematic of xenon optical floating zone single crystal growth apparatus (5).

allows the xenon lamp’s optical power to be focused at a spot for the heating and growth of single crystals. Seed and feed rods are positioned vertically within a sealed quartz tube along a line containing the focal point, and can be translated vertically with respect to the focal point. These rods are also counter- rotated in order to increase stirring action and convective heat transfer. In operation, the counter-rotating seed and feed rod tips are located at the xenon lamp focal point, then the lamp power is gradually increased until the rod tips become molten. At this point,

the rod tips are brought together, and a stable molten zone between them is established. Once this molten zone is stable, the rods are then translated vertically in tandem, so that the molten zone propagates along the rod length, thus producing a single crystal. This approach has the major advantage of being containerless, which is important due to the high melting temperatures required for erbia. Melting can also occur in controlled atmospheres, established within the quartz tube that contains the seed and feed rods.

powders (Rhone-Poulenc, 99.5% purity relative to rare earth elements). These rods were approximately 66 % dense. In previous work (4), it was determined that a reducing atmosphere of 94% Ar-6% H2 at a slight overpressure of 76 KPa (1 1 psi) was effective in minimizing deleterious erbia “flaking” effects during melting in an air atmosphere, which complicated single crystal growth. This “flaking” may be due to vaporization and redeposition of erbium at the very high temperatures of erbia melting. A 94% Ar-6% H2 atmosphere was employed for the present work.

Erbia seed and feed rods were cold isostatically pressed from erbia

4

Microhardness Testing Erbia single crystals obtained using the xenon optical floating zone

technique were studied using a Nikon QM-2 high temperature microhardness unit. This unit has the capability to perform Vickers microhardness indentations fiom room temperature to 1500 O C , using diamond or sapphire indenters in a vacuum environment. Erbia single crystal specimens of suitable size (5 mm x 5 mm x 10 mm) were oriented crystallographically using Laue x-ray back reflection techniques, and then prepared using a slow speed diamond saw. The face of the specimen to be indented was polished down to a 0.25 pm diamond finish.

Using a 1000 gm load, Vickers indentations were made from room temperature to 1400 OC at 200 OC intervals, to characterize microhardness and indentation fracture toughness as a function of temperature. Indentation fracture toughness was calculated using the following relationship (6):

K, = (0.016) ( E / H ) l n (P / c3') (1)

where K, = fracture toughness, E = elastic modulus, H = hardness, P = indentation load, and c = crack length. Polycrystalline values of the elastic modulus of erbia as a function of temperature were employed in the fracture toughness calculations (7).

RESULTS AND DISCUSSION

Er20J Single Crystals

Figure 2: Unit cell of Er203. Large atoms are Er. Small atoms are 0.

The crystal structure of erbium oxide (Erz03) is body centered cubic (bcc), with a lattice parameter of 10.55 angstroms (8). The unit cell, shown in Figure 2, contains 80 atoms, 32 erbium atoms and 48 oxygen atoms. This crystal structure is essentially a modified fluorite structure, with one-fourth of the fluorite oxygen sites vacant.

5

Single Crystal Growth The use of an ArM2 atmosphere during crystal growth significantly

reduced the deleterious “flaking” effects. However, it also caused the erbia melt to become more opaque to the xenon radiation. This in turn caused the melt to absorb more radiation at the surface and severely limit the amount of light radiation transmitted to the center, which caused both the molten and solidification interfaces to become more conical. During crystal growth, these cones would come into contact with each other and produce mechanical instabilities within the melt.

minimized using a combination of high counter-rotation rate, lamp power setting, and crystal growth rate. Conditions for the synthesis of erbia single crystals using 6.2 mm diameter seed and feed rods were established at a 55 rpm counter-rotation

The instabilities resulting from feed rodseed rod conical contacts were

Figure 3 : Er203 single crystal.

speed, power setting of 2.86 kW which produced a ratio of molten zone neck diameter to mne length of 0.4-0.8, and a growth rate of at least 20 d o u r . Surface cracks which penetrated approximately 0.5- 1 .O mm from the rod surface towards the center were observed. However, below this surface crack level, the erbia single crystal was sound and of high quality. An example of the best erbia single crystal produced to date is shown in Figure 3. The diameter is approximately 5 mm, while the length is 70 mm. Pronounced growth striations can be seen on the crystal surface. The preferred growth direction using polycrystalline erbia powder seed and feed rods was 411,.

was observed that the erbia turned from pink to black in color when melted in the reducing Arm2 environment. The material had remained pink when melted in an air environment. This suggests that Er203 is susceptible to substoichiometry effects when heated at elevated temperatures. Limited work in the literature confirms that erbia becomes substoichiometric to a level of Er202.978 when

During crystal growth experiments, it

6

melted in a vacuum or reducing environment (9). It was observed that the heating of black erbia in air at 1600 OC for a few hours caused the erbia to return to its original pink color. This strongly indicates that the substoichiometry effects are due to the loss of oxygen (9).

Indentation versus Temperature

temperature using the Nikon QM-2 high temperature microhardness tester. These two specimens were synthesized according to the erbia single crystal growth conditions described in this paper, and were from the same single crystal growth run. Specimen ET-98-11-2 was in the as-synthesized condition, and was black in color, indicating that it was substoichiometric. Specimen ET-98- 1 1-1 was heat treated at 1600 "C for 4.5 hours in air following single crystal synthesis, and was pink in color, indicating that it was more stoichiometric. The indentation plane of these specimens was { 1 lo}, with the perpendicular side faces oriented on { 1 1 1 }

Two erbia single crystal specimens were tested as a function of

10 r

tET-98-11-1 (stoichiornettic) . MT-98-11 -2 (substoichiornetric) t \\

0 200 400 600 800 lo00 1200 1400

Temperature (C)

Figure 4: Er,O, single crystal microhardness versus temperature. Average

and { 1 12) faces.

are shown in-Figure 4. Microhardness generally decreased with increasing Vickers microhardness (1 000 gm load) data as a function of temperature

7

temperature, by roughly a factor of three between room temperature and 1400 "C. Stoichiometry appeared to have little effect on hardness.

0.5 .

0 200 100 600 600 1000 l a 0 1400

Temperature f'C)

Figure 5 : Er203 single crystal indentation fracture toughness versus temperature. Average standard deviation is 0.23 MPa mlR.

Figure 5 shows indentation fracture toughness as a k c t i o n of temperature. Room temperature fracture toughness levels were in the range of 1.8-2.5 MPa mln. Fracture toughness values of stoichiometric and substoichiometric Er203 were similar as a function of temperature. Toughness values decreased to less than 1 MPa mIn in the temperature range of 200- 1000 "C, then increased above 1000 "C. The increase in fracture toughness above 1000 "C is likely due to the onset of plastic deformation processes. However, the reasons for the reduction in toughness between room temperature and 200 OC are unclear at the present time.

CONCLUSIONS High quality Er203 single crystals were grown using a xenon optical

floating zone single crystal growth apparatus. Erbia single crystal growth occurred in an Arm2 atmosphere, at growth rates of 20 d o u r or greater, and at power levels that maintain a molten zone neck diameterheck length ratio of 0.4-

8

0.8. As-synthesized erbia single crystals were black in color, as a result of substoichiometry produced by melting in a reducing atmosphere, due to loss of oxygen. Annealing the black crystals at 1600 “C in air returned them to a stoichiometric state and pink color.

The hardness of erbia single crystals decreased by approximately a factor of three from room temperature to 1400 OC. Room temperature hardness was in the range of 9 Gpa. Stoichiometry level had little effect on hardness. Room temperature fracture toughness values were in the range of 1.8-2.5 MPa mlR. Toughness values decreased to less than 1 MPa mln in the temperature range of 200- 1000 OC, then increased above 1000 OC.

ACKNOWLEDGEMENTS

Laboratory under its University Programs student activity. Support for staff and experimental work was provided by the DOE Office of Basic Energy Sciences, Division of Materials Science.

Student support for this work was provided by the Los Alamos National

REFERENCES 1. J.P. Coughlin, “Contributions to the Data on Theoretical Metallurgy, MI. Heats and Free Energies of Formation of Inorganic Oxides”, U.S. Bureau of Mines Bulletin 542, (1954).

2. 0. Kubaschewski and C.B. Alcock, Metallurgical Thermochemistry, 5* Edition (1 979).

3. A. Neuman, M. Platero, R.S. Romero, K.J. McClellan, and J.J. Petrovic, “Fabrication and Properties of Erbium Oxide”, Ceram. Eng. Sci. Proc., l 8 , 3 7 (1 997).

4. A.D. Neuman, M.J. Blacic, M. Platero, R.S. Romero, K.J. McClellan, and J.J. Petrovic, “Mechanical Properties of Melt-Derived Erbium Oxide”, Ceram. Eng. Sci. Proc., 2 , 4 2 3 (1 998).

5. S. Kimura and K. Kitamura, “Floating Zone Crystal Growth and Phase Equilibria: A Review”, J. Am. Ceram. S O C . , ~ ~ , 1440 (1992).

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6. G.R. Anstis, P. Chantikul, B.R. Lawn, and D.B. Marshall, “A Critical Evaluation of Indentation Techniques for Measuring Fracture Toughness: I, Direct Crack Measurements”, J. h e r . Ceram. SOC., 64,533 (1981).

7. W.R. Manning and 0. Hunter Jr., “Elastic Properties of Polycrystalline Yttrium Oxide, Holmium Oxide, and Erbium Oxide: High-Temperature Measurements”, J. h e r . Ceram. Soc., 52,492 (1969).

8. L. Eyring, “The Binary Rare Earth Oxides”, Chapter 27 in Handbook on the Physics and Chemistry of Rare Earths, Vol. 3, Non-Metallic Compounds, Eds. K.A. Gschneidner Jr. and L. Eyring, North-Holland Publishing Co., Amsterdam, c. 1979.

9. A.E. Miller and A.H. Daane, “Preparation of a New Type of Nonstoichiometric Rare-Earth Oxide”, J. Inorg. Nucl. Chem., 27, 1955 (1 965).

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BARIUM TITANATE AND BARIUM ORTHOTITANATE POWDERS THROUGH AN ETHYLENE GLYCOL POLYMERIZATION ROUTE

Sang-Jin Lee, Michael D. Biegalski and Waltraud M. Kriven Department of Materials Science and Engineering University of Illinois at Urbana-Champaign Urbana, IL. 6 180 1

ABSTRACT Nano-size and pure barium titanate and barium orthotitanate powders were

synthesized by simple. mixed-oxide, ceramic processing using ethylene glycol. Barium nitrate and titanium isopropoxide were directly dissolved in ethylene glycol in stoichiometric proportions. The organic/inorganic precursors were crystallized to tetragonal barium titanate and monoclinic barium orthotitanate, after calcining at 700 "C for 1 h and at 1200 "C for 1 h, respectively. A dense barium titanate microstructure which had a nano-scale grain size was obtained by sintering at 1200 "C for 2 h. In barium orthotitanate, a phase transformation from orthorhombic to monoclinic symmetry occurred. The transformation temperature and volume change were determined by dilatometry. The high temperature orthorhombic phase was retained down to room temperature by addition of at least 6 wt% MgO stabilizer.

INTRODUCTION Barium titanate (BaTi03) has a large polarization and a high dielectric constant,

whose value usually ranges from 650 to 8,500 depending on the particle size and impurities.I4 Barium orthotitanate (Ba2Ti04) has two crystalline structures which are orthorhombic and mon~clinic.~ The orthorhombic form has been reported to exist at high temperatures and the monoclinic form at room temperature. The BagTi04 could be expected to undergo a displacive transformation above room temperature.

Barium titanate powder is difficult to obtain by a conventional ceramic process.' This is due to the formation of many intermediate compounds which reduce the purity of the barium titanate itself. Chemical methods are preferred because they can yield powders that are very pure. Previous ex eriments have yielded BaTi03 powders of small particle size using various methods. 2-6~181 Barium orthotitanate powder is very hard to synthesize because the powder is not as stable as barium titanate. A common way to produce this powder is through physical mixing of TiOz and BaC03 and heating at 1350 OC.12 Controlled hydrolysis of barium and titanium alkoxide precursors can also be used to synthesize Ba2Ti04.7

The method used in this experiment is based on complexation-polymerization using ethylene glycol (HOCH2CH20H, EG), as an organic carrier. The polymerization agent, EG in this method, acts to capture the metal ions through a polymerization-complexation

To the extent authorized under the laws of the United States of America, all copyright interests in hs publication are the property of The American Ceramic society. Any duplication, re rcduction, or re ublication of this ublication or any part thereof, without the express written consent of The American Ceramic &ety or fee pailto the Copynght&earance Center, is prohibited.

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mechar~ism.~, '~. '~ This serves to decrease the mobility of metal ions and constrain the system to prevent precipitation of cation species and agglomeration. At the optimum amount of polymer, the metal ions are dispersed in solution and a homogeneous polymeric network is formed. Titanium isopropoxide is not soluble in water, so other sol gel methods can not be used. This method also has a distinct advantage over other sol- gel methods in that it is easier and cheaper.

In this study the two powders were synthesized by the ethylene glycol method, and the sinterability of the BaTi03 and the phase transformation behavior of the Ba2Ti04 were examined.*

EXPERIMENTAL PROCEDURE

(1) Powder Synthesis Barium nitrite (Ba(N02)2*H20, reagent grade, Alfa Aesar Chem. Co., Ward Hill,

MA) and titanium (IV) isopropoxide (Ti(OC3H7)4, reagent grade, AIfa Aesar Chem. Co., Ward Hill, MA) were dissolved in stoichiometric proportions in liquid-type ethylene glycol (Fisher Chemical, FW:62.07, Fair Lawn, NJ). The amount of ethylene glycol was calculated using a ratio of total weight of metal ions from cation sources to weight of ethylene glycol. Several ratios ( I : 1, 1:3, 1:5 and 1:7) were tried. The barium nitrite was first added to the ethylene glycol and heated to 80 "C. while mixing, until it was fully dissolved. Then, the titanium (IV) isopropoxide was slowly added, while stirring. The solution was then allowed to gel for 48 h in a drying oven at 50 "C. The dried gel was then calcined on a zirconia (3Y-TZP) substrate, in a box furnace, at various temperatures. The calcined barium orthotitanate powders were kept in a dessicator under vacuum to prevent reaction with air.

The stabilization of the high temperature phase of Ba2Ti04 was investigated using 3, 6 and 10 wt?/o of MgO. The stabilizer was added to the sol in the form of magnesium nitrate hexahydrate (Mg(N03)2-6H20, reagent grade, Aldrich Chemical, Milwaukee WI). In the case of barium titanate, the calcined powder was ball-milled with zirconia media for 12 h. Isopropyl alcohol was used as a solvent for milling. The ball-milled BaTi03 powders and as-calcined Ba2Ti0, powders were uniaxially pressed at 20 MPa followed by cold iso-static pressing at 170 MPa for 10 min. The pellet-shaped green compacts of barium titanate and barium orthotitanate were sintered at 1200 "C for 2 h in an air atmosphere.

(2) Characterization The crystallization behavior of the gel-type powders was examined as functions of

calcining temperature and holding time, using a X-ray diffractometer (Rigaku/USA, Dmax, automated powder diffractometer, Danvers, MA) with CuKa radiation (40 kV, 40 mA). The morphology of the crystallized powders and grains after sintering were examined by scanning electron microscopy, (SEM, Hitachi S530, Hitachi, Japan). The specimens were mounted on an aluminum stub and Au-Pd sputtered. To observe the grain size of sintered barium titanate, a cross section was polished and then thermally etched at 1100 "C for 10 min. The thermal expansion behavior at the phase transformation of sintered barium orthotitanate samples was determined with a recording dilatometer (Netzsch Dilatometer, 402E, Germany) and DSC up to 1200 "C, at a heating

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rate of 5 OC/min. The dielectric constant of the sintered barium titanate pellet was characterized with a HP4275A impedance analyzer operated at room temperature.

RESULTS AND DISCUSSION

(1) Barium titanate The metal ion sources (barium nitrite and titanium (IV) isopropoxide) were

dissolved in ethylene glycol after heating to 80 OC. However, in the case of the solution which had the 1 : 1 ratio, some precipitation occurred, despite long stirring times. The amount of ethylene glycol was not enough to dissolve the cation sources without any solvent media. Table I lists the XRD results and particle sizes of the crystallized powders after calcination at each temperature for 1:3, 1 5 and 1:7 ratios. The powder derived from the 1 :3 ratio crystallized to tetragonal BaTi03 at the lowest temperature of 700 "C after 1 h. The other precursors required higher calcining temperatures to crystallize. Figure 1 shows the SEM morphologies of the crystallized powders calcined at their lowest crystallization temperatures. The SEM micrograph of the powder prepared from the 1 :3 ratio sol revealed nanosized particles of about 100 nm in size, with a narrow particle size distribution. The powder derived from the 1 :5 solution resulted in a larger particle size of approximately 200 nm. This was attributed to higher activation energies for particle size growth at the higher crystallization temperatures. The morphology of the powder made by the 1 :7 solution consisted of broken spheres and had a wide particle size distribution. The excess polymer caused inhomogeneity in the precursor due to entanglements of the polymer chains. This resulted in serious agglomeration, large particle sizes and a wide particle size distribution. In the case of the 1:7 ratio powder, 1250 "C was needed for full crystallization, due to the inhomogeneity in the system. The broken sphere morphology was produced by trapped gases, such as CO and COz that expanded when heated, and fractured the spherical shapes.'

Table I. Phase development of barium titanate powders at various calcination temperatures and average particle size corresponding to each lowest crystallization temperature.

Mixing ratio 7OOOC 900°C 125OOC Particle size

1 :3 Tetragonal Tetragonal Tetragonal 70- 150

1:5 Amorphous Tetragonal Tetragonal 150-250

1 :7 Amorphous Amorphous Tetragonal Inhomogeneous

Figure 2 is an SEM micrograph of the polished and thermally etched surface of densified BaTi03 derived from the 1:3 ratio route. The pellet had a 97% relative density and a grain size of about 200 nm after sintering at 1200 OC for 2 h. The densification temperature was notably lower than reported to The dielectric constant for the disc-type, sintered BaTi03 reached 2,100 at room temperature.

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Fig. 1 . SEM micrographs of barium titanate powders calcined at their lowest crystallization temperatures (Table I). (a) powder from 1 :3 solution calcined at 700 OC for Ih, (b) powder from 1 5 solution calcined at 900 "C for Ih, and (c) powder from 1:7 solution calcined at 1250 "C for Ih.

(2) Barium Orthotitanate The B Ti04 was found to decompose when exposed to air at ambient

temperatures?2 Hence, the calcined powder was kept in a dessicator under vacuum. The

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