Jennifer Jordan (Ga Tech)

SHOCK COMPACTION SYNTHESIS OF TITANIUM-SILICON CARBIDE (Ti3SiC2) Jennifer L. Jordan and Naresh N. Thadhani School of Materials Science and Engineering Georgia Institute of Technology, Atlanta, GA 30332 The synthesis of the ternary carbide of titanium and silicon (Ti3SiC2) is being investigated using Ti, SiC, and graphite precursors. The shock compression experiments are performed using an 80 mm diameter single-stage gas gun with a three capsule plate-impact recovery fixture. The recovered shock compressed samples show varying amount of reaction product including TiCx and Ti3SiC2. With subsequent heat treatment at 1600 oC for 4 hours, there was formation of up to 43 vol% of the ternary carbide phase. The TiC phase shows a lattice parameter corresponding to either a non-stoichiometric phase or containing dissolved Si. In this paper, the synthesis of the ternary carbide of Ti and Si using a non-equilibrium shock compression approach will be presented. abstracts

Tim Perham, (Berkeley )

"A Novel Invasion/Percolation Model for Co-Interpenetrating Composites". ABSTRACT A novel simulation technique has been developed for estimating the threshold values of critical material and process parameters in the fabrication of Co-Interpentrating Ceramic-Metal Composites. It is known that the processing times in these systems must be minimized to prevent the formation of "core-shell" morphologies which limit the transport of reactive metal to the infiltration front. With percolation statistics the trade-offs present in the processing of these systems can be explored on a statistically significant basis. For example, processing at high temperatures (thus decreasing the viscosity and increasing the reactivity) increases the growth rate of product phases in the pore space, which leads to plugged porosity that can slow the infiltration rate. Processing at high overpressures, however, causes viscous instabilities, which lead to trapped porosity and an inhomogeneous final product. A novel Invasion/Percolation model is presented which uses Monte Carlo methods to simulate the effect of the interface transformation rate. The additional thermodynamic driving force, apparent in the reactive wetting process, can then be dynamically linked to the product phase formation in the pore space that could seal off the infiltration core from the reactive metal species.
 

D.C. Pender (UConn)

PROCESSING, MODELING AND CONTACT MECHANICAL PROPERTIES OF SILICON NITRIDE-BASED FGMS D.C. Pender* and N.P. Padture, Department of Metallurgy and Materials Engineering, Institute of Materials Science, University of Connecticut, Storrs, CT, 06269 A.E. Giannakopoulos and S. Suresh, Department of Materials Science, MIT, Cambridge, MA, 02139 A novel idea for cone crack suppression by engineering a surface-gradient in elastic modulus to produce a functionally graded material (FGM), is applied to silicon nitride-based ceramics. This type of structure was first shown to be effective in suppressing cone cracks in alumina-based ceramics. Aspects of the processing, which affect the nature of the gradient, as well as the spherical indentation conditions, which simulate different contact situations, will be discussed. Finite element modeling of the indentation of these structures has been performed to elucidate the mechanics of contact damage evolution at elastically graded surfaces. Furthermore, the generic nature of these graded structures leads to a discussion of new types of functionally graded materials, which may make use of this technique for contact-damage resistance.

Swarnima Deshpande ***Not attending, as of June 30 1999 (UConn)

EFFECT OF SEEDING ON THE MICROSTRUCTURAL EVOLUTION IN LIQUID-PHASE-SINTERED SiC S.A. Deshpande* and N.P. Padture, Department of Metallurgy and Materials Engineering, Institute of Material Science, University of Connecticut, Storrs, CT, 06269 F.L Cumbrera and A. Luis, Departmento de Fisica, Universidad de Extremadura, Badajoz 06071, Spain Pressureless liquid-phase sintering of b-SiC starting powder involves b F a phase transformation, which is responsible for the growth of elongated-grain reinforcements within the microstructure. External a-SiC seeds were added to the starting b-SiC powder to determine if these seeds acted as centers for grain coarsening. Addition of 5% external a-SiC has a profound effect on the microstructural evolution in these ceramics. Growth occurs mainly on these seeds in all directions resulting in a more equiaxed microstructure. The effect of a-SiC seeds on both, the aspect ratios of the grains and the grain-size distributions in the final microstructure will be discussed.

Dehui (David) Qu (UConn)

Net Shape Processing for Ceramic High Temperature Superconductor YBaCuO for the Novel Microwave Resonators Today's best conventional filtering techniques in wireless communication systems, including cavity pre-selectors, inter-digital or band-pass filters, cavity notch filters, and especially ceramic and ferrite loaded filters, do not perform well enough to provide the level of filtering that will be required by the dense, heavily used systems.  In particular, the fundamental problem for radio frequency (RF) communications is that as channels increase and they are placed more closely together, the present generation of RF equipment loses its ability to discriminate signals between channels.  More selective filters with lower insertion losses for dense radio network must be designed to satisfy the rapidly increasing demand in wireless communications.  The ceramic high temperature superconductor (HTS) materials offer promise in this regard by virtue of the extremely low surface resistance.  While the commercialization of cellular base station filters using high temperature superconductor is a very active development area, but the current material processing techniques have short-comings that limit the further improvement of the system performance, such as better selectivity and making various components with different geometry and shapes.
In this presentation, we will report a novel microwave cavity resonator, which has been made for the first time using HTS single domain YBaCuO The resonator consists of two parts, one of which is a single domain material with cylindrical hollow cup in it, and the other is a plate used as a cover to complete the cavity structure. The single domain YBaCuO is a two-phase superconducting composite material with a single crystal structure. In order to accomplish the design of the resonator, a net shape processing (NSP) technique has been developed, which can produce a smooth surface in the cavity for the propagation of microwave and avoid post-machining after processing.  The disadvantage of post-machining is that, since the top part of domain has the highest density that provides the required RF properties, machining will remove this high quality portion of material and leave a porous surface in the cavity.   The resonator was characterized by measuring the quality factor (Q value) using network analyzer and swept frequency frequency technique.  The Q value achieved on the single domain YBaCuO , microstructure and the dependence of Q value on processing parameters, such as oxygen content and annealing procedure. 
The significance of this research is that a new approach has successfully been developed to apply the ceramic high temperature superconductor YBaCuO into wireless communication system and the current result shows that the system performance will be greatly improved.

Dave Saylor (CMU)

David M. Saylor and Gregory S. Rohrer Department of Materials Science and Engineering Carnegie Mellon University Pittsburgh, PA 15213 We have developed a procedure to extract the misorientation dependence of the relative grain boundary free energy in crystalline solids. The development of a completely automated EBSP based mapping technique, the mesoscale interface mapping system (MIMS), allows the interfacial geometry and crystallography of large quantities of grain boundary triple junctions in polycrystalline materials to be determined. We have applied the MIMS technique to an MgO polycrystal, in which the microstructure was equilibrated at 1600=B0C. To extract the grain boundary energy function, = it is assumed that Herring=92s local equilibrium condition for triple = junctions holds and that the grain boundary energy is independent of the boundary plane. A series of symmetric generalized spherical harmonic functions is used to approximate the energy function, and the unknown coefficients of = the series are determined by fitting the observations to the local equilibrium condition. The resulting function is the consistent with the = Read-Shockley model for boundaries with low angle misorientations, and low sigma

Kevin Schlichting (UConn)

"Failure modes of Plasma Sprayed Thermal Barrier Coatings." Plasma sprayed thermal barrier coatings (TBCs) have become the mainstay for thermal protection in turbine engines. In order to lower costs and increase engine component life, manufactures are constantly looking for longer life cycles from their TBCs. Plasma sprayed TBCs are known to fail in the ceramic just above the thermally grown oxide (TGO). It is unclear what causes this failure to occur. Stress due to the thermal expansion mismatch between the oxide and the ceramic at the rough bond coat interface as well as stress due to subsquent growth of the TGO with cycles are thought to be the major factors for TBC failure. The technique of laser fluorescence was used to measure stress in the TGO and a correlation between measured stress and failure mechanisms will be presented.

Jong-kook Park (Mich. State)

High Resolution Micro-Machining/Patterning of CVD Diamond Films for MEMS

J.K. Park (1), V.M. Ayres (2), J. Asmussen (2) and K. Mukherjee (1)

(1) High Energy Laser Processing Laboratory Dept. Materials Science & Mechanics Michigan State University, East Lansing, MI 48824

(2) Electronic and Surface Properties of Materials Center Dept. Electrical & Computer Engineering Michigan State University, East Lansing, MI 48824

Excellent mechanical and electrical properties make chemical vapor deposited (CVD) diamond films uniquely qualified for applications to microelectromechanical systems (MEMS). To fully utilize the excellent properties in the MEMS applications, micro-machining/patterning on diamond films is primarily required. Laser ablation technique, which utilizes a very rapid laser pulse with high intensity, is a unique method for micro patterning and structuring film materials. However, the pulsed laser ablation induces plasma over the irradiation spot. Formation and expansion of the plasma give rise to thermal damages, such as molten material ejection and deposition, transformed layers, and rippled surfaces. These thermal damages have been major obstructions for high resolution micromachining of CVD diamond films. To implement laser ablation technique to the micro-machining/patterning of diamond films, a systematic investigation is primarily required on the plasma-diamond interaction. This investigation is mainly focused on novel approaches to avoid the thermal damages from the plasma. Behavior of plasma is diagnosed according to dynamic gas streaming, which effectively alters the environment of the laser-induced plasma. Practical fabrication of components for MEMS will be accompanied with the fundamental studies to verify the feasibility of the high resolution micromachining on CVD diamond films.

Tania Bhatia (UConn)

Tania Bhatia, Huiwen Xu and Nitin Padture Department of Metallurgy and Materials Engineering University of Connecticut 97 North Eagleville Road Storrs, CT 06269-3136 The key to obtaining in situ toughened silicon carbide lies in developing a microstructure that consists of elongated a-SiC grains in a YAG matrix. The processing strategy employed involves pressureless sintering of b-SiC powders with Al2O3 and Y2O3 at temperatures substantially lower (1850-2000?C) than those employed for solid state sintering (> 2100?C). The Al2O3 and Y2O3 form an in situ liquid phase in which the b to a transformation occurs concomitant with grain growth results in elongated a grains in the final microstructure. It has been found that the nature of the starting powders plays a vital role in determining the final aspect ratio. Starting with a-SiC powders results in a equiaxed microstructure while starting with b powders results in abnormally elongated grains. The polytype and microstructural evolution in b powders from two different sources (with different stacking fault density) are found to vary significantly. Based on detailed x-ray diffraction evidence and preliminary TEM evidence, it has been found that the b (3C) to 4H (one of the a polytypes) is the transformation controlling the aspect ratio of the grains. It is attempted to understand and explain the transformation/coarsening mechanism that determines the microstructural evolution in in situ toughened ceramics.

Guoping He (New Mexico, Tech) (***Withdrawn July 19, 1999)

PHASE TRANSFORMATIOM OF Si3N4 DURING CERAMIC PROCESSING

Guoping He and Deidre A. Hirschfeld,Materials Engineering Department, New Mexico Tech, Socorro, NM87801

Structural characteristics of alpha/beta-Si3N4 crystals are summarized. Both of them are hexagonal but differ in that the lattice distance in the direction of the crystallographic c-axis. With increasing temperature, the alpha-phase becomes increasingly unstable with respect to beta-Si3N4. The mechanisms of alpha- to beta-phase transformation and densification are discussed. The liquid-phase sintering through the dissolution-reprecipitation process can be used to describe the phase transformation and densification. The alpha- to beta-Si3N4 phase transformation is a reconstructive transformation of secondary coordination. Effect factors, such as properties of starting powders, sintering additives, and processing conditions, on the alpha- to beta-phase transformation have been analyzed. Finally, the microstructural characteristics of dense Si3N4 ceramics with superior mechanical properties are summarized. The most important consequence of the alpha- to beta-phase transformation in Si3N4 is the morphological changes observed in the grains. The alpha-grains are equiaxed, whereas the beta-grains are elongated. The Si3N4 ceramics have an interlocking microstructure of the elongated (acicular) beta-grains or whiskers, which is believed to be responsible for the superior mechanical properties of Si3N4 ceramics.

Andrey Soukhojak (MIT)

ELECTRORHEOLOGY OF ELECTROMECHANICALLY ACTIVE MATERIALS Andrey N. Soukhojak and Yet-Ming Chiang Department of Materials Science and Engineering. Massachusetts Institute of Technology, Cambridge, MA 02139. The diversity and often mixed character of electromechanical behavior of high-strain electromechanically active materials (EMAM), e.g. relaxor ferroelectrics, requires a phenomenological framework for the analysis and comparison of different materials/compositions. An appropriate universal phenomenological model of EMAM should be able to describe observed pure and mixed cases of ferroelectric, antiferroelectric, paraelectric, ferroelastic, and paraelastic behaviors with the possibility of relaxation phenomena in electric and elastic subsystems. The presence of elastic and viscous elements in electric and elastic subsystems of EMAM suggests that electrorheological models should be used to represent different types of electromechanical responses. The electrorheological models have been successfully used to describe dynamic properties of liquid crystals, including ferroelectric ones. We present the first (to the best of our knowledge) attempt to apply electrorheology to model dynamic electromechanical response of EMAM. We show applicability of our model to describe versatile behavior of real EMAM.

Jesus Chapa (Colo School of Mines)

MECHANICAL PROPERTIES OF GRADED Cu/W JOINTS

Jesus Chapa, Colorado School of Mines
Ivar Reimanis, Colorado School of Mines

Mechanical properties of Cu/W joints are investigated through a combination of numerical simulations and experiments. Residual stress distributions are manipulated by the strategic placement of interlayers, as shown by finite element modeling (FEM). Guided by the FEM studies, a variety of graded joint architectures have been produced by powder metallurgy methods. The fracture and deformation of these joints is examined and assessed with respect to the predicted residual stress distribution and fracture criteria for the interlayer materials.

Jack Smith (Penn)

Analysis of the TiO2(110)(2x3) Reconstruction Using STM Aided by Image Simulation
J. Smith and D.A. Bonnell ; University of Pennsylvania

STM image contrast of transition metal oxides have been compared to experimental images to solve the atomic structure of the TiO2 (110) (2x3) reconstruction. The calculation is based on wave functions that are varied to match first principle results for two ideal structures and then extended to arbitrary structures on these surfaces. The procedure was successful in distinguishing between two possible atomic terminations of the TiO2 (110) 2x3 surface and is proposed as a general approach to interpreting STM images of systems with ionic/partially covalent bonding and complex surface structures.

Jian Luo (MIT)

Jian Luo and Yet-Ming Chiang
Department of Materials Science and Engineering,
Massachusetts Institute of Technology, Cambridge, MA.

Stable Nanometer-Thick Surface Phases in Ceramics

Bismuth-enriched films have been observed to form a constant thickness (~1.5 nm) on the surfaces of ZnO and Fe2O3. These films appear to be a discrete phase coexisting at thermal equilibrium with two bulk phases in a binary system, being thermodynamically allowed as a surface phase. Correspondingly, the films are found to be compositionally distinct from any equilibrium bulk phase, and appear to have a "thermodynamically-determined" thickness. The stability of the surface phase does not follow the bulk phase diagram, i.e. the amorphous films are observed to form in samples equilibrated above and below the eutectic temperature, and in unsaturated samples. Furthermore, surface film formation is anisotropic, with {11-20} facets exhibiting films, while {10-10} facets are devoid of films. A thermodynamic model is presented, with the goal of defining selection criteria for predicting systems that will form such films. This phenomenon is relevant to the understanding and control of nanometer-thick surface coatings, activated sintering, and supportedoxide catalysts.

Boskar Bramarutu, (Penn. State)

Susan Sinnott, (U. of Kentucky, no support)
COMBINED EXPERIMENTAL AND THEORETICAL STUDY OF THE ATOMIC SCALE STRUCTUREAT NICKEL/CUBIC ZIRCONIA INTERFACES

Susan B. Sinnott, Ma Yong, and Elizabeth C. Dickey
Department of Chemical and Materials Engineering, University of Kentucky,
Lexington, Kentucky 40506-0046

High-resolution microscopy has been coupled with first principles atomistic calculations to understand the structure and orientation relationships of a nickel film deposited on (100) cubic zirconia. Reflection high-energy electron diffraction and electron backscattering diffraction was used along with orientation imaging microscopy. Two orientations were found that lead to a near coincidence site lattice (NCSL) between nickel and the cubic zirconia but they do not always form the smallest possible NCSL. Using the experimentally derived structural models as input, first-principles density functional calculations were performed. The bonding at the interface was studied and local coordination and electronic structure determined from the calculations.

*Supported by the National Science Foundation (DMR-9976851)

Yoed Tsur (PSU)

THE ROLE OF AMPHOTERIC DOPANTS IN BARIUM TITANATE BASED DEVICES

Yoed Tsur and Clive A. Randall
Materials Research Laboratory
The Pennsylvania State University
University Park, PA 16802-4800 USA

Recently, multilayer ceramic capacitors based on BaTiO3 dielectrics have been processed at production scale with nickel electrodes. The properties in terms of dielectric constant, loss, dielectric breakdown strength and degradation are comparable to the more costly precious metal devices. In order to sinter the dielectric material without oxidizing the electrodes, the sintering is done in reducing atmosphere. The dielectric material is modified by various dopants, in order to maintain its electronic properties. In particular, it was found that some "magic" trivalent dopants are important to improve the dielectric breakdown strength of the devices. In this presentation we will first show (experimentally) that these "magic" dopants are amphoteric. The underlying point defect chemistry will be described. We will then present a hypothesis on the role of amphoteric defects in improving the lifetime of the devices, in connection with grain boundary segregation and diffusion of oxygen vacancies.enter for Dielectric Studies, M

Perena Gouma (Ohio State)

Structural Stability of High Temperature CO Gas Sensors Based on Titania

P. I. Gouma, M.J. Mills, and M. Madou NSF Center for Industrial Sensors and Measurements, The Ohio State University, Columbus, OH 43210

Abstract:

Titania is an important electronic ceramic material for use in diverse applications, such as gas sensors, catalysts, dielectrics, and ceramic membranes. It exists as several polymorphic phases, most commonly as the rutile and anatase phases. Powders and nano-crystalline thin films of anatase-type titania intended for use in high-temperature (600°C-800°C) gas sensing devices have been studied. The structural stability of the sensing systems at these elevated temperatures has been evaluated. The findings of this work raised stability concerns particularly for the thin film sensors. Exposure of the nanocrystalline anatase films (having initial grain size of 8nm) to temperatures >400°C resulted in the nucleation and subsequent rapid grain growth of rutile grains (final grain size >300nm). In-situ hot stage experiments in the TEM were carried out which revealed details about the rutile nucleation and abnormal grain growth processes in this system. Controlling the rutile nucleation process or using dopants that inhibit the transformation in the system are proposed ways to achieve stability for high temperature titania sensors.

Yuri D. Tretyakov (Moscow State)

Deterministic Chaos Approaches for Optimal Organization of a Reaction Zone during Synthesis of Multicomponent Ceramics Deterministic Chaos Approaches for Optimal Organization of a Reaction Zone during Synthesis of Multicomponent Ceramics

Yu. D. Tretyakov,Inorganic Chemistry Division, Department of Chemistry,
Moscow State University, Moscow, 119899, Russia
Phone +7-(095) 939-2074, E-mail yudt@inorg.chem.msu.ru

One of the most important direction of modern ceramic materials engineering is connected with new chemically complicated oxide materials including superconducting cuprates, GMR manganates and quantum paraelectric REE titanates. Reproducibility of composition, structure and properties of such materials depends strongly on precursors modification and type their transformation. At any steps of evolution from precursors to final product the whole system is far from equilibrium state and could be described by some sort of bifurcation diagram with one or few bifurcation points corresponding to intermediate phase transformations. This kind of evolution would be reasonable considered in frames of deterministic chaos approach. It means that

- system behavior gets progressively nonpredictable upon evolution

- the more complicated the system the faster it passes in deterministic chaos region

- the longer process the more rigorous demands to the initials state of the system

According to accounts above mentioned consideration ceramics engineering could be provided due to optimal organizing of reaction zone including application of:

- the maximum homogenous precursors, preferably the solutions, melts, vapors of precursors with molecular bonds

- the optimal treatment of precursors assuming formation of non-crystalline intermediate products or formation of crystal product at as low temperatures as possible due to synergetic action of low pressures, intensive grinding, ultrasonic and extra-high freuency techniques

- template providing preferably formation of prearranged structure of the product

Matthew Henrichsen

Phase Transformation and Shrinkage Rates in Ultra-Rapid
RF-Plasma Pass-Through Sintering of Beta Alumina

Matthew Henrichsen, Jinha Hwang, Vinayak Dravid, D. Lynn Johnson

Ultra-rapid phase transformation to beta" alumina and high sintering rates in RF-plasma sintering have been reported previously. We will compare phase transformation rates in tubes formed from varying precursor phases. Additionally instantaneous shrinkage rates will be presented. Widely separated double maxima have been observed in the instantaneous shrinkage rate. The conditions which result in a double maxima will be presented.

Paul Dearhouse, UC Berkeley

The stress intensification factor as discussed by Beere [5], and Vieira and Brook [4] has been investigated with the use of Surface Evolver [10]. The stress intensification factor is a useful parameter that describes the load bearing area in a porous ceramic compact which experiences pressure due to sintering and/or external loading. Its effect on sintering stress and current sintering theories will be discussed. The present study is based on an approach by Carter [9], where a cubic space-filling cell was used to represent a porous compact. Dihedral angle and density were varied- from 15-90? and ~45-95% theoretical, respectively- and the stress intensification factor was determined for each case. Additionally, the geometry dependence of the stress intensification factor was explored over the range of densities and dihedral angles. Stress intensification factors were obtained for elongated and compressed cells by either increasing or decreasing the height of the cell with respect to width and length. The low dihedral angle regime is considered for applications to liquid phase sintering. Results are compared with previous calculations and observations of other authors.

[4] J. M. Vieira, R. J. Brook, ?Kinetics of Hot-Pressing: The Semilogarithmic Law?, J. Am. Cer. Soc., 67 [4] 245-49 (1984).

[5] W. Beere, ?Diffusional Flow and Hot-Pressing: A Study on MgO?, J. Mater. Sci., 10 pp. 1434-40 (1975).

[9] Carter, W.C. (Edited by: Chen, L.-Q.; Fultz, B.; Cahn, J.W.; Manning, J.R.; Morral, J.E.; Simmons, J.) ?Surface Evolver as a tool for materials science research?, Proceedings of Materials Week '96. Mathematics of Microstructure Evolution, Cleveland, OH, USA, 29 Oct.-2 Nov. 1995.) Warrendale, PA, USA: TMS, 1996. p.1-14.

[10] K. A. Brakke, ?The Surface Evolver?, Experimental Mathematics, 1(2):141-165, (1992)

A.V. Lukashin (Moscow State University, Russia)

In the present work the new method of synthesis functional nanostructured materials was suggested. This method is based on use anion- and cation-substituted layered double hydroxides (LDH) as a precursors with subsequent chemical modification. It combines simplicity of chemical methods of synthesis and possibility of obtainment two- and one-dimensional nanoparticles in ceramic matrixes. Layered double hydroxides is the class of layered inorganic compounds with general formula M'1-xM''x(OH)2[(anion)x/nmH2O], where M' and M'' are metals in the oxidation state +2 and +3 respectively, and anionn- is almost any anion, which does not form stable complexes with M' and M''. Their structure consists of positively charged hydroxide layers [M'1-xM''x(OH)2]x+, which are bonded by negatively charged anions placed within the interlayer space. Due to the relatively large interlayer distance, anions in LDHs are readily exchanged and it is possible to obtain species with practically stoichiometric content of an appropriate anoinn-. When anionn- undergoes some reaction, hydroxide layers of LDH restrict the reaction area. Thus, interlayer space of LDH may serve as a two-dimensional nanoreactor. The chemical modification of substituted LDH allows to obtain wide range of materials, having various functional properties. Suggested method was successfully used for prepare magnetic nanomaterials (M/matrix, MOx/matrix; M = Fe, Co, Ni; matrix = Mg1-xAlxO1+x/2) and semiconductors quantum dots (PbS/matrix, matrix = Mg1-xAlx(OH)2[(anionn-)x/n mH2O]).

Miladin Radovic (Drexel, 2 posters)

Mechanical Properties of Ti3SiC2

Tamer El-Raghy and Michel W. Barsoum

Department of Materials Engineering, Drexel University, Philadelphia, PA 19104

Inn this study we report on the mechanical behavior of Ti3SiC2 in the 25-1300 oC temperature range. In particular, the compressive and flexural response of fine-grained, FG, (3-5 (m) and coarse-grained, CG, (100-200 (m) samples are compared.. At all temperatures, the FG material exhibits higher strengths in both compression and bending than the CG material. At room temperature, both microstructures exhibit excellent damage tolerant properties. Up to about 1200 °C, the fracture is brittle for both microstructures; at 1300 °C high levels of plasticity (> 20 %) are observed. Although the CG material was not susceptible to thermal shock (up to 1400 oC), the FG material was found to thermal shock gradually between 750 oC and 1000 oC. The compressive response of very coarse (1-4 mm), oriented, polycrystalline samples was also tested. Orienting the grains results in plastic deformation even at room temperature. When the basal planes are oriented in such a way that allows for slip, deformation occurs by the formation of shear bands. When the slip planes are parallel to the applied, load deformation occurs by a combination of delamination and kink band formation, as well as by shear band formation. Results presented here provide key evidence for two concepts in the mechanical behavior of Ti3SiC2: (a) inelastic deformation in Ti3SiC2 is assisted by damage formation in the form of grain boundary cracks, kinking and delamination of grains, and; (b) the initiation of damage does not result in catastrophic failure because Ti3SiC2 exhibits a remarkable capacity to confine the spatial extent of that damage.
Key words: Ti3SiC2, compressive properties, flexure properties, high temperature, plasticity, damage tolerance, thermal shock resistance

Tensile and Tensile-Creep Properties of Ti3SiC2

Miladin Radovic?, Michel W. Barsoum, Tamer El-Raghy J. Seidensticker** and S. Wiederhorn**

Department of Materials Engineering, Drexel University, Philadelphia, PA 19104
**National Institute for Standards and Technology, Gaithersburg, MD 20899

Although significant progress has been achieved in understanding the mechanical behavior of bulk, polycrystalline Ti3SiC2 in compression and flexure, as far as we are aware there are no reports in the literature dealing with its mechanical response under tension. In this study, we report on the functional dependence of the tensile response of fine-grained (3-5 µm) Ti3SiC2 on strain rates in the 25-1300 oC temperature range. Results presented here shows that the tensile response of Ti3SiC2 is a strong function of strain rate and temperature. Increasing the testing temperatures and/or decreasing the strain rates leads to large tensile plastic deformation. A high value of strain rate sensitivity (0.42-0.56) was obtained from the tensile tests and confirmed by strain rate jump/drop test and stress jump creep tests. That value is equal to, or greater than, the strain rate sensitivity of most superplastic ceramics. The deformation of fine-grained Ti3SiC2 also has another aspect in common with superplastic ceramics - larger elongation to failure (> 25 %) that are typically observed in ceramics. The large elongations to failure appear to result from a high degree of damage and damage tolerance and not from a structure that remains self-similar throughout deformation (as in superplasticity). Similarly, large strains to failure (e.g. 12% at 1100 °C and 25 MPa) were also observed tensile creep tests carried out in the 1000 °C to 1200 °C temperature range. Furthermore, the fact that the steady state creep rates (at any given testing temperature and stress), are significantly lower in tension than in compression is further evidence that a high level of accumulated damage in the form of voids and cavities, rather than plastic deformation in the classic sense, is responsible for the high strains to failure in tensile creep tests. These results are another manifestation of the unusual damage tolerance of these materials.

Key Words: Ti3SiC2, tensile properties, high temperature, strain rate sensitivity, damage tolerance, tensile-creep properties

Sergei V. Kalinin (Penn)

Ferroelectric Phase Transformations and Surface Potential of BaTiO3 (100) by Scanning Probe Microscopy

Sergei V. Kalinin, Dawn A. Bonnell
The University of Pennsylvania

In the present research we applied tapping mode AFM and scanning surface potential microscopy to the characterization of BaTiO3 (100) surface. The general characteristics of domain-induced surface topography and it's relation to surface potential distribution are presented. The origins of potential contrast on ferroelectric surfaces are also briefly discussed. We also present the results of the variable temperature SPM measurements. The domain induced surface corrugations were found to disappear above the Curie temperature, in full agreement with theoretical expectations. Unusual relaxation of surface polarization after the ferroelectric transitions is also observed.

Edwin Garcia, MIT , Carter

Catherine Bishop, MIT, Carter

Valeria, Cannillo, MIT/Modena ,Carter (No Support needed)

Dylan Morris (Minnesota)

Nuno Reis (Manchester)

Solid Freeform Fabrication (SFF) of Ceramics by Inkjet Printing

During the past year we have been developing new hot-melt ceramic inks to realize direct deposition of ceramic parts. These are complex suspension formulations of low melting point substances, surfactants and dispersants, optimized to match the window of rheological jettability for high solid contents (40-50 vol% particulate). We have also characterized the drop and deposit formation as a function of fluid properties and driving parameters, by means of stroboscopic imaging. We have recently fabricated green parts (30 vol% alumina dispersed in paraffins) by direct jet deposition and proved the viability of jetting 40 vol% with these systems. Work is being undertaken to improve our SFF platform for higher particulate contents.

Don Ellerby (Waiting List for Posters, June 20)

PROCESSING AND MECHANICAL PROPERTIES OF METAL-CERAMIC COMPOSITES WITH CONTROLLED MICROSTRUCTURE FORMED BY REACTIVE METAL PENETRATION

D. T. Ellerby and R. K. Bordia, University of Washington, Seattle WA, and K. G. Ewsuk, Sandia National Laboratories, Albuquerque NM.

Compared to monolithic ceramics, metal-reinforced ceramic composites offer the potential for improved toughness and reliability in ceramic materials. As such, there is significant scientific and commercial interest in the microstructure and properties of metal-ceramic composites. Considerable work has been conducted on modeling the toughening behavior of metal reinforcements in ceramics; however, there has been limited application and testing of these concepts on real systems. Composites formed by newly developed reactive processes now offer the flexibility to systematically control metal-ceramic composite microstructure, and to test some of the property models that have been proposed for these materials. In this work, the effects of metal-ceramic composite microstructure on resistance curve (R-curve) behavior, strength, and reliability were systematically investigated.

Al/Al2O3 composites were formed by reactive metal penetration (RMP) of aluminum metal into aluminosilicate ceramic preforms. Processing techniques were developed to control the metal content, metal composition, and metal ligament size in the resultant composite microstructure. Quantitative stereology and microscopy were used to characterize the composite microstructures, and then the influence of microstructure on strength, toughness, R-curve behavior, and reliability, was investigated. To identify the strength limiting flaws in the composite microstructure, fractography was used to determine the failure origins. Additionally, the crack bridging tractions produced by the metal ligaments in metal-ceramic composites formed by the RMP process were modeled.

Due to relatively large flaws and low bridging stresses in RMP composites, no dependence of reliability on R-curve behavior was observed. The inherent flaws formed during reactive processing appear to limit the strength and reliability of composites formed by the RMP process.

This investigation has established a clear relationship between processing, microstructure, and properties in metal-ceramic composites formed by the RMP process. RMP composite properties are determined by the metal-ceramic composite microstructure (e.g., metal content and ligament size), which can be systematically varied by processing. Furthermore, relative to the ceramic preforms used to make the composites, metal-ceramic composites formed by RMP generally have improved properties and combinations of properties that make them more desirable for advanced engineering applications.

This work was supported by the US Dept. of Energy under Contract DE-AC04-94AL85000. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the US DOE. Work at the University of Washington was provided by the NSF under contract number NSF-DMR 9257027.

Maiorova and Mudretsova

CERAMIC HEAT FLOW DETECTOR FOR INVESTIGATION OF SOLID STATES
S.N.Mudretsova, A.F.Maiorova Moscow State University Department of Chemistry, 119899 Moscow, Russia

The study of a heat flow by means of sensitive ceramic detector is suggested. The method is based on the use of electrochemical cells with solid state electrolyte. It was used the separate gaseous space of electrodes configuration where both electrodes (gaseous and solid state) contain a chemical element, an ion of which determines the selective conduction of the electrolyte.

The mechanism of detector operation is based on measuring of a EMF change of an electrochemical cell versus time while the heat process inside the cell is on. By itself, the source of the heat flow being investigated is not a part of a cell process and only affects the temperature changes of the process. Thus, the source determines the variation of the electrochemical cell potential only during heat transformation. In general, the electrochemical detector can be built according to the following scheme:

current lead - solid state electrode - ceramic electrolyte - gaseous electrode - object at question - current lead.

The detector was constracted on the basis of the electrochemical cell of the type: Pt; CuO,Cu2O | O = | Pt,O2 (air) , where CuO,Cu2O - an equilibrium solid state mixture of oxides; O = - 0.85ZrO2. 0.15YO 1.5 - ceramic electrolyte in a shape of a tube closed at one end; Pt -platinum current leads. The value of a temperature sensitivity (DEcell / DT) for electrochemical cell is more than 10 times higher than DE / DT for a thermocouple (type "S") which is used at elevated temperatures in microcalorimetric detectors.

The processes of reduction and oxidation, specific heats , temperatures and heat transformation of some metalls and oxides were investigated by this method.

Seungbum Hong

Direct evidences supporting the major contribution of the cantilever-sample capacitive force to the sustained vibration in dynamic contact mode electrostatic force microscope

KAIST

Electrostatic force microscope (EFM) has been reported to show a sustained vibration of the tip even in contact mode for hard samples like triglycine sulfate or Si wafer covered with native oxide when ac modulation voltage is applied between the tip and the sample [Phys. Rev. B 58, 5078 (1998)]. To reveal the origin of this sustained vibration, we measured the static deflection of the tip as a function of dc bias voltage (Vdc) applied to the sample (n type Si wafers covered with native oxide or thermal oxide) while small ac voltage (Vacsinwt) was applied to the tip. Also, the amplitude (Aw) and the phase (Fw) of the first harmonic (w) signal coming from the tip vibration were measured as a function of Vdc. It was found that the static deflection changed from attractive to repulsive movement as we changed the mode from non-contact to contact. Moreover, it was revealed that Fw showed a 180-degree phase shift regardless of Vdc when changing the mode from non-contact to contact. Our results imply that the displacement of the tip-cantilever system in contact EFM cannot be explained by the tip movement mainly induced by the tip-sample capacitive force. Instead, it is concluded that the cantilever-sample capacitive force mainly contributes to the observed sustained vibration of the tip-cantilever system in contact mode. Furthermore, the contact potential difference (CPD) between the Si wafer and the cantilever was measured from Aw vs. Vdc plot and its dependence on the tip-sample distance was discussed

Richard K. Holman (MIT, Waiting List)

Improving Component Resolution and Surface Finish in Ceramics 3DP(TM)

Richard K. Holman, Scott A. Uhland, Michael J. Cima, and Emanuel Sachs

Three Dimensional Printing, or 3DP(TM), is a solid freeform fabrication (SFF) technique developed at MIT. In Ceramics 3DP(TM) a ceramic layer 30 to 100 microns thick is deposited by rastering a jet of slurry across a substrate. This slip casts and is dried to form a cohesive layer of powder bed. Next a digital cross section of the component (extracted from a 3D solid CAD model) is sent to the control software, and a two dimensional image of the component's cross section is "ink jet printed" with binder solution into the powder bed layer, defining the part. The next layer is then deposited and printed, etc. until the entire component has been formed. This type of forming technique allows for essentially any level of complexity, limited only by the resolution of the printing process and the layer height in the build direction. The size and shape of the primary building block, or "primitive", are largely defined by the characteristics of the binder solution and of the binder deposition process. This primitive is critical in determining the resolution and surface finish possible with the 3DP(TM) process. Studies are underway to determine which processes and properties are the controlling factors in defining the primitive size and shape. This work involves droplet impact, post-impact phenomena, and droplet infiltration. Key variables include binder solution viscosity, surface tension, contact angle, droplet size, and droplet velocity.

Scott A. Uhland (MIT, Waiting list)

PTM Part Retrieval via Redispersion

Scott A. Uhland, Richard K. Holman, Michael J. Cima and Emanuel Sachs
Massachusetts Institute of Technology, Cambridge, MA 02139

Products which incorporate ceramic components frequently demand properties and shapes that challenge contemporary forming techniques. The Three-Dimensional Printing (3DPTM) process has been modified to incorporate colloidal science for the fabrication of these fine ceramic parts. Dielectric components were built using a sequential layering process of a ceramic powder bed followed by ink jet printing of a binder. An important processing step in 3DPTM is removing the printed components from the powder bed. The part retrieval process plays a key role in the resulting shape and therefore the properties of the ceramic parts. Part retrieval is achieved through the redispersion of the powder bed. The addition of a redispersant to the slurry, polyethylene glycol, enables the redispersion of the ceramic powder bed. Processing conditions, i.e. powder bed chemistry and the chemistry of the redispersing liquid, must be controlled throughout the process due to their strong influence the redispersion behavior of the powder bed. The processing technology has been used for the fabrication of dielectric RF components with extremely tight dimensional tolerances. The dielectric characterization of cylindrical resonators, coaxial resonators, and RF filters was performed on dry-pressed and 3DPTM formed components in order to examine the effects of processing technique on performance.

Ivar Reimanis (Colo. School of Mines)

Deformation Behavior of a Graded Nickel-Alumina Composite Measured with Phase Shifted Moire Interferometry
Ivar Reimanis and Andrew Winter Colorado School of Mines
and
Eric Steffler Idaho National Engineering and Environmental Laboratory
Phase shifted moire interferometry (PSMI) was used to measure the deformation of a discretely layered functionally graded nickel/alumina composite loaded in compression. Finite element analysis predicted thermal residual stresses and subsequent mechanical loading displacements. FEA results are qualitatively in agreement with PSMI measurements of the displacement evolution for increasing compressive loads. The implications of these displacement fields with respect to inhomogeneous deformation of functionally graded materials is discussed.

S. P. Chen (LANL)

Ideal cleavage planes in MoSi2
S. P. Chen,
Theoretical Division, Los Alamos National Laboratory, Los Alamos, NM 87545
Results of first-principles calculations on several cleavage planes c11b-MoSi2 will be presented and compared with available experimental data on fracture toughness on (001) and (110) planes. The similarities and differences between the theoretical and experimental results together with their implications will be discussed.

Malinda M. Tupper (MIT)

Fabrication and Assembly of Micron-Scale Ceramic Components
Malinda M. Tupper, Matthew E. Rosenthal, Michael J. Cima
MIT, Dept. of Materials Science and Engineering
Miniaturization and integration of ceramic components into devices is becoming an important field as new ceramic materials and applications are being developed. A novel method for forming micron-scale ceramic components has been developed in the Ceramics Processing Research Laboratory. Piezoelectric fibers, alumina microturbines, and ferrite inductor cores have been formed by this process. The piezoelectric fibers are used in the manufacture of active fiber composites. We have begun to manufacture interconnected arrays of fibers as an initial approach to the assembly or integration of these small components into useful structures.

Ivar Reimanis (Colorado School of Mines)

Thermo-Mechanical Behavior of a YAG Boundary
Maria Peters, Ivar Reimanis, Colorado School of Mines
and John Petrovic, Los Alamos National Laboratory
The thermo-mechanical behavior of Y3Al5O12 (YAG) is examined by conducting thermal grooving experiments on grain boundaries in addition to elevated temperature boundary sliding experiments. Model grain boudaries were synthesized by diffusion bonding two YAG single crystals to form an extremely flat sigma-3 symmetric tilt boundary. Compression tests in air up to 1750C resulted in boundary sliding. Grain boundary thermal grooving was measured with AFM to assess the role of diffusion in grain boundary sliding. SEM and TEM were used extensively to characterize the boundary and study the role of dislocations in the sliding process.

Uma Sampathkumaran (Case Western, Scholarship promised by DB?)

Ceramic Oxide Thin Film Growth Promoted by Organic Self-Assembled Monolayers

Uma Sampathkumaran, Mark R. De Guire and Arthur H. Heuer
Department of Materials Science and Engineering
Case Western Reserve University

Organic self-assembled monolayers (SAMs) have been shown to be effective surfaces for promoting the deposition of uniform, pore-free, nanocrystalline, thin (typically <1 micron) oxide films from aqueous solutions at low temperatures (< 100C). We have used this approach to deposit oxide films on a variety of substrates, inlcuding single crystal silicon, silica glass, pigment powders, single crystal saphhire and titanium. SAMs have also been deposited and characterised on silicon carbide and silicon-germanium substrates for the first time. Crystalline films of TiO2, ZrO2, SnO2, In2O3, FeOOH and AlOOH have been formed directly from solutions at temperatures below 100C. Films of ZnO, Y2O3, ZrTiO4, Y2O3-ZrO2, Fe3O4 and gamma-Fe2O3 have been formed using modest heat treatments. The characteristics of the nanocrystalline oxide films and those of the SAMs, as well as the novel capabilities of the SAM like photpatterning and contact printing will be discussed. Our current understanding of the role of these ordered organic surfaces in the film formation process and their influence on the microstructure of the films will be presented. The SAM approach to ceramic film formation offers several advantages over vapor-deposition and sol-gel routes, such as lower processing temperatures, the ability to coat non-planar substrates and powders, less costly equipment and resuced environmental impact through the use of aqueous precursors. The patterning capabilities of the SAM are utilised to direct the oxide film formation with the elimination of post-processing steps.