bluuebunny/crossref_metadata_2025 huggingface dataset:
Creep Behavior of a SiC‐Whisker‐Reinforced Alumina
Authors: Lin Hua‐Tay , Becher Paul F.
Published: March 2005

Creep tests in four‐point flexure loading configuration in air employing applied stresses of 37 to 300 MPa at temperatures of 1200°, 1300°, and 1400°C were performed on 20‐vol%‐SiC‐whisker‐reinforced alumina and unreinforced single‐phase polycrystalline alumina. The creep rate of polycrystalline alumina was significantly reduced through the addition of SiC whiskers, although strain to failure was lower. Transmission and scanning electron microscopy results suggest that substantial increase in the creep resistance in flexure of alumina composites originates from the retardation of grain‐boundary sliding by the SiC whiskers.
Phase Transformation Kinetics in the System Bismuth Oxide‐Zirconia
Authors: Takamori Takeshi , Shafer Merrill W.
Published: March 2005

The effect of ZrO2 addition on the phase transformation of Bi2O3 was studied with powder X‐ray diffraction and differential thermal analysis. Samples containing higher than a few mole percent ZrO2 were obtained as the metastable β phase (tetragonal) at room temperature when they were cooled at 20°C/min from 800°C where the δ phase (fcc) is stable. When the same samples were cooled at the same rate from 1000°C, which is above the solidus, they formed the stable α phase (monoclinic) at room temperature. Evidence for the relatively higher solubility of ZrO2 in the α phase is discussed to explain this observation.
Conventionally Prepared Submicrometer Lead‐Based Perovskite Powders by Reactive Calcination
Authors: Shrout Thomas R. , Papet Philippe , Kim Sunuk , Lee Gye‐Song
Published: March 2005

Submicrometer powders of various Pb‐based perovskites, including PbTiO3, PbZrO3, Pb(Zr0.53Ti0.47)O3, and Pb(Mg1/3Nb2/3)O3 were prepared by a reactive calcination process. Using only reagent‐grade raw materials and conventional processing techniques, highly reactive powders were produced by reacting the materials near the temperature of maximum volumetric expansion. At this point, the morphological development results in a skeletal‐type structure consisting of ultrafine particulates that can be readily broken down further by milling. Powder sizes less than 0.3 μm and as small as 70 nm generally only achievable using chemical processing techniques were achieved. The highly reactive powders allowed densification to occur at temperatures as low as ∼900°C with correspondingly small grain sizes. A model describing the physiochemical behavior and associated morphological development of Pb‐based perovskites was herein proposed.
Phase Composition and Compatibilities in the Bi‐Sr‐Ca‐Cu Quaternary Oxide System at 800°C in Air
Authors: Hong Byungsun , Hahn Joon , Mason Thomas O.
Published: March 2005

Subsolidus phase relations in the system (Bi‐Sr‐Ca‐Cu)‐O at 800°C were investigated via powder X‐ray diffraction and electron probe microanalysis of solid‐state reaction products. Two recently reported (Bi‐Sr‐Ca)‐O phases were detected in quaternary assemlages and had the approximate cation stoichiometries of 9115 and 825. The only truly quaternary compound detected is the superconducting phase with the cation stoichiometry 2212. Tie lines, tie triangles, and compatibility tetrahedra involving this phase and the 2201 superconducting phase were determined. Both superconducting compounds exhibit small but detectable Sr‐to‐Ca solubility and are Sr‐deficient with respect to the “ideal” formulas.
Microstructural Evolution During the Infiltration Treatment of Titanium Carbide‐Iron Composite
Authors: Chae Ki‐Woong , Chun Dong‐II , Kim Doh‐Yeon , Baik Young‐Joon , Eun Kwang‐Yong
Published: March 2005

The microstructural evolution of grain‐matrix interfaces in TiC‐Fe cermet has been observed and explained in terms of equilibration reaction and interfacial energy minimization. The coherency strain energy is assumed to be the driving force for enhanced dissolution of certain specific planes, which results in a very irregular grain shape. The equilibrium shape appears after heat treatment for a long period.
Origin and Growth Kinetics of Platelike Abnormal Grains in Liquid‐Phase‐Sintered Alumina
Authors: Song Huesup , Coble Robert L.
Published: March 2005

The grain‐growth behavior of Al2O3compacts with small contents (≤10 wt%) of various liquid‐forming dopants was studied. Equiaxed and/or elongated grains were observed for the following dopants: MgO, CaO, SiO2, or CaO + TiO2. The platelike grains, defined as the abnormal grains larger than 100 μm with an aspect ratio ≥5 and with flat boundaries along the long axis, were observed when the boundaries were wet with the liquid phase and the codoping satisfied two conditions of size and valence. These dopings were Na2O + SiO2, CaO + SiO2, SrO + SiO2, or BaO + SiO2. However, an addition of MgO to the Al2O3doped with CaO + SiO2resulted in the change of grain shape from platelike to equiaxial. Equiaxed grains were also observed for the MgO + SiO2doping, indicating that two conditions were necessary but not sufficient to develop the platelike grains. The fast growth rate of the platelike grains was explained by an increased interfacial reaction rate due to the codopants. AT the same time the codopants made the basal plane, which appeared as the flat boundaries, the lowest energy plane. The appearance of the platelike grains was favored in compacts with a small grain size and with a narrow size distribution at the onset of abnormal grain growth. Accordingly, the use of starting powders with a small particle size and narrow size distribution, smaller amounts of dopings, and high sintering temperature resulted in an increased number of the platelike grains.
Strengthening of Soda‐Lime Glass Rods by heat Treatment Alone and Heat Treatment plus Silanes
Authors: Daly Patrick , Byers Stanley
Published: March 2005

A study with soda‐lime glass rods showed that modulus of rupture values can be increased from approximately 124 × 106 Pa (18 ksi) to 276 × 106 Pa (40 ksi) by heat treatment. Dipping the rods into diethyldichlorosilane prior to heating at 593°C (1100°F) or exposing hot glass to the silane vapor following heat treatment increased strength an additional 30% over the average strength obtained by heating only at this temperature. Similar treatment with phenyltrichlorosilane or diethyldichlorosilane did not give strengths statistically higher than those obtained by heating alone.
Measurement of Stresses Using Fluorescence in an Optical Microprobe: Stresses around Indentations in a Chromium‐Doped Sapphire
Authors: Molis Steven E. , Clarke David R.
Published: March 2005

A technique for measuring stress (positive and negative) with a lateral spatial extent of approximately 2 μm is introduced. The technique, implemented using a Raman microprobe, is demonstrated with measurements of the frequency shift of the sharp, R‐luminescence lines (2Ā and Ē to 4A2 radiative transitions) in, and around, a hardness indentation in a 0.06‐wt%‐chromium doped sapphire. From the observed frequency shifts the stresses in regions sampled in the hardness impression, in the complex stress field surrounding it, and at the tip of a crack are measured.
Simple Interpretation of the Verwey Transition in Magnetite
Authors: Honig Jurgen M. , Spałek Jozef , Gopalan Prakash
Published: March 2005

The Verwey transition in magnetite is rationalized in terms of an electronic model involving correlated electron occupancy of nearest‐neighbor octahedral site pairs. This formalism can be restated in terms of order‐disorder theory. The site pair representation of the lattice is analyzed in terms of possible occupation states that represent respectively the ground state of the system as a trapped electron, a first excited state in which an electron can resonate between the two constituents of a site pair, and a second excited state involving two electrons on neighboring sites. The free energy of this representative assembly is then minimized to obtain the equilibrium configuration. The Verwey transition is driven by the Coulomb repulsive interaction between electrons on neighboring sites. In certain limiting cases one obtains results equivalent to the phenomenological Strässler‐Kittel model, which had been successfully used on an empirical basis to analyze both the first‐ and second‐order Verwey transitions that have been experimentally encountered with increasing departures from the ideal 4/3 ratio of oxygen/iron in magnetite.
In Situ Measurements of Bridged Crack Interfaces in the Scanning Electron Microscope
Authors: Rödel Jürgen , Kelly James F. , Lawn Brian R.
Published: March 2005

A device for in situ SEM examination of crack propagation during loading of compact tension specimens is described, with a specific demonstration on an alumina ceramic. The device facilitates direct qualitative observations of the inception and subsequent frictional pullout of grain‐localized bridges at the crack interface. Quantitative data on the bridging mechanism are obtained from measurements of the crack‐opening displacements behind the crack tip. The crack profile is found to be closer to linear than parabolic at the bridged interface. Deconvolution of these crack‐opening data allow for an evaluation of the closure tractions operative at the crack walls within the bridging zone, and thence the R‐curve.
Preparation of Ultrafine Silicon Nitride, and Silicon Nitride and Silicon Carbide Mixed Powders in a Hybrid Plasma
Authors: Lee Hyung Jik , Eguchi Keisuke , Yoshida Toyonobu
Published: March 2005

Ultrafine Si3N4 and Si3N4+ SiC mixed powders were synthesized through thermal plasma chemical vapor deposition (CVD) using a hybrid plasma which was characterized by the superposition of a radio‐frequency plasma and an arc jet. The reactant, SiCl4, was injected into an arc jet and completely decomposed in a hybrid plasma, and the second reactant, CH4 and/or NH3, was injected into the tail flame through multistage ring slits. In the case of ultrafine Si3N4 powder synthesis, reaction effieciency increased significantly by multistage injection compared to single‐stage injection. The most striking result is that amorphous Si3N4 with a nitrogen content of about 37 wt% and a particle size of 10 to 30 nm could be prepared successfully even at the theoretical NH3/SiCl4 molar ratio of ∼ 1.33, although the crystallinity depended on the NH3/SiCl4 molar ratio and the injection method. For the preparation of Si3N4+ SiC mixed powders, the N/C composition ratio and particle size could be controlled not only by regulating the flow rate of the NH3 and CH4 reactant gases and the H2 quenching gas, but also by adjusting the reaction space. The results of this study provide sufficient evidence to suggest that multistage injection is very effective for regulating the condensation process of fine particles in a plasma tail flame.
Thermal Analysis of Precursors of Barium Titanate Prepared by Coprecipitation
Authors: Fang Tsang‐Tse , Lin Horng‐Bin , Hwang Jim‐Bin
Published: March 2005

Thermal analysis was performed on coprecipitated materials and on individual components. The detailed decomposition schemes of coprecipitates and individual components are proposed and discussed. According to the proposed decomposition schemes, the values of the observed weight loss are in good agreement with those of the theoretical values of the coprecipitated materials and individual components. The results indicate that barium titanyl oxalate is an inserting compound, i.e., a structure of distorted barium hydrogen oxalate hydrate being inserted by Ti(OH)3+. The results also verify that copreciptation of barium and titanium ions in an oxalate aqueous solution at pH 7 is a mixture of BaC2O4· 0.5H2O and TiO(OH)2· 1.5H2O and coprecipitation of barium and titanium ions using the process of Yamamura et al. is a mixture of Ba(NO3)2 and Ti(OH)2C2O4.
Interfacial Segregation in Perovskites: II, Experimental Evidence
Authors: Desu Seshu B. , Payne David A.
Published: March 2005

Experimental evidence determined by scanning transmission electron microscopy, scanning Auger electron spectroscopy, and lattice parameter measurements is reported for interfacial segregation in pure and doped perovskite materials, e.g., BaTiO3 and SrTiO3. The segregation behavior for isovalent dopants (e.g., Sr2+, Ca2+, or Pb2+ at a Ba2+ site) is mainly associated with a strain energy contribution from ionic misfit. However, both strain energy and space charge contributions are important in determining the segregation behavior of aliovalent ions (e.g., La3+ or Nd3+ at a Ba2+ site and Fe3+ or Ni3+ at a Ti4+ site). Segregation depths for aliovlent dopants are typically 15 to 20 nm thick. Besides dopants, constituent alkaline‐earth ions also segregate to interfaces. Their segregation behavior is promoted by high temperatures, low partial pressures of oxygen, and the presence of acceptors. The results are explained in terms of a proposed theory of segregation presented in part I of the present series.
Kinetic Analysis of Solution‐Precipitation During Liquid‐Phase Sintering of Alumina
Authors: Kwon Oh‐Hun , Messing Gary L.
Published: March 2005

Densification controlled by solution‐precipitation during liquid‐phase sintering was analyzed for the aluminamagnesium aluminosilicate glass system. As a model system for liquid‐phase sintering, narrowly sized alumina powders and up to 20 vol% magnesium aluminosilicate glass samples were isothermally sintered at 1550° to 1650°C. Densification rate increases with increasing liquid content and sintering temperature but decreases with increasing density. For samples with >15% grain growth, the densification rate during the solution‐precipitation stage of sintering was proportional to (particle size)−2 and thus interface reaction‐controlled. Activation energies ranged from 270 to 500 kJ/mol over the relative density range of 66% to 96%, respectively. The low activation energy is attributed to densification by particle rearrangement, whereas the higher activation energy is due to densification controlled by interface‐reaction‐controlled solution‐precipitation. Intermediate activation energies are attributed to simultaneous densification by the two mechanisms.
Superplasticity of Hot Isostatically Pressed Hydroxyapatite
Authors: Wakai Fumihiro , Kodama Yasuharu , Sakaguchi Shuji , Nonami Tooru
Published: March 2005

Dense and translucent hydroxyapatite polycrystals (Ca10(PO4)6(OH)2 with a grain size of 0.64 μMm) were obtained by hot isostatic pressing at 203 MPa and 1000°C for 2 h in argon. The material exhibited superplastic elongation (>150%) in a tension test at temperatures from 1000° to 1100°C and at strain rates from 7.2×10−5 to 3.6 × 10−4 s−1. Extensive strain hardening was observed. The stress exponent of the yield stress was larger than 3.
Electronic Band Structure of Al2O3, with Comparison to Alon and AIN
Authors: French Roger H.
Published: March 2005

As the uses of Al2O3 and other ceramics expand into new and more demanding applications, it is increasingly important to understand their electronic structure and its relationship to properties. However, compared with metals, semiconductors, or alkali halides, our understanding of the electronic structure of ceramic materials is limited. There has been much recent progress in our understanding of the electronic structure of Al2O3, based on the applications of new experimental and theoretial methods. Vacuum ultraviolet spectroscopy and valence band photoemission spectroscopy coupled with pseudofunction band structure methods provide a comprehensive approach to study a wide variety of electronic structure issues of importance to ceramic materials. The high‐temperature electronic structure and its role in determining the high‐temperature, intrinsic, electronic conductivity gives us the ability to evaluate high‐temperature conductivity data, and supports the conclusion that Al2O3 is predominantly an electronic conductor at high‐temperatures. The strain dependence of the electronic structure, as embodied in the deformation potentials, provides a simple method to determine surface stresses and strains. The variation of the electronic structure in the family Al2O3‐AION‐AIN demonstrates the changes associated with the valence band chemistry of changing the anion from oxygen to nitrogen, and the bonding from mixed ioniccovalent in the direction of greater covalency. These changes in the anion valence bands lead to dramatic changes in the atomic and electronic nature of room‐temperature bimaterial interface formation for copper to Al2O3 or AIN. The application of this new methodology to develop our perspective on electronic structure and apply it to problems associated with temperature, stress, composition, or interface formation can improve our understanding of many critical questions in ceramics.
Whisker Toughening: A Comparison Between Aluminum Oxide and Silicon Nitride Toughened with Silicon Carbide
Authors: Campbell Geoffrey H. , Rühle Manfred , Dalgleish Brian J. , Evans Anthony G.
Published: March 2005

Two whisker‐toughened materials have been studied, with the objective of identifying the mechanisms that provide the major contribution to toughness. It is concluded that, for composites with randomly oriented whiskers, bending failure of the whiskers obviates pullout, whereupon the major toughening mechanisms are the fracture energy consumed in creating the debonded interface and the stored strain energy in the whiskers, at failure, which is dissipated as acoustic waves. The toughening potential is thus limited. High toughness requires extensive pullout and, hence, aligned whiskers with low fracture energy interfaces.
R‐Curve Behavior and Thermal Shock Resistance of Ceramics
Authors: Swain Michael V.
Published: March 2005

The influence of R‐curve behavior of ceramics on the strength degradation associated with thermal shock is explored. Of particular significance for this interdependence is the observed nonlinear stress‐strain behavior of materials that exhibit minimal strength degradation under severe thermal shock conditions. These two features, R‐curve behavior and nonlinear behavior, are incorporated into a fracture mechanics analysis to provide a framework with which to understand severe thermal shock of ceramics. This analysis enables the estimation of the crack growth due to thermal shocking and also the anticipated strength degradation. The influence of specimen size is also addressed, and it is shown that greater strength degradation is anticipated with increasing specimen size. Experimental results for an alumina‐zirconia composite material are presented to support the simple analysis.
Gypsum Dehydration During Comminution and Its Effect on Cement Properties
Authors: Goswami G. , Mohapatra B. , Panda J. D.
Published: March 2005

A mix containing fixed amounts of gypsum and ordinary portland cement clinker was ground in plant and laboratory ball mills to examine the effect of grinding in different mills on the cement properties. Mineralogical examination showed that in the plant mill (1200 cm×260 cm), because of the rising temperature during grinding, gypsum was dehydrated to hemihydrate, while there was no dehydration in the laboratory mill (45 cm×45 cm) grinding. Study of hydration of the cement pastes showed that the presence of hemihydrate increased the rate of ettringite formation. Hemihydrate also was rehydrated to gypsum and in the process retarded the setting times and reduced the strength development of the cement. This has been further confirmed by examination of cements prepared with gypsum, hemihydrate, and anhydrite.
Volatility Diagrams for Silica, Silicon Nitride, and Silicon Carbide and Their Application to High‐Temperature Decomposition and Oxidation
Authors: Heuer Arthur H. , Lou Victor L. K.
Published: March 2005

Volatility diagrams—isothermal plots showing the partial pressures of two gaseous species in equilibrium with the several condensed phases possible in a system—are discussed for the Si‐O and Si‐N systems, and extended to the Si‐N‐O and Si‐C‐O systems, in which the important ceramic constituents SiO2, Si3N4, Si2N2O, and SiC appear as stable phases. Their use in understanding the passiveactive oxidation transitions for Si, Si3N4, and SiC are demonstrated.
Metal Nitrate‐Urea Decomposition Route for Y‐Ba‐Cu‐O Powder
Authors: Varma Harikrishna , Warrier Krishna Gopakumara , Damodaran Alathoor D.
Published: March 2005

Y‐Ba‐Cu‐O powder, having desirable composition, particle size, compaction, and sintering properties, has been prepared by a novel combustion process involving metal nitrate‐urea decomposition. Single‐phase Y‐Ba‐Cu‐O is obtained by reacting the mixture of yttrium, barium, and copper nitrates in 123 stoichiometry with urea at 900°C for a period of 1 h. Following grinding in acetone the powder possessed an average particle size of 1 μm, a surface area of 42.2 m2/g, a bulk density of 2.7 g/cm3, and could be sintered to 92% theoretical density at 930°C with the resulting material having a Tc of 90 K.
Development of Superplastic Structural Ceramics
Authors: Chen I‐Wei , Xue Liang An
Published: March 2005

Superplastic structural ceramics (Y‐TZP, Al2O3, Si3N4, and their composites) that can withstand biaxial stretching to large strains have been developed recently. Microstructural design of these ceramics first requires an ultrafine grain size that is stable against coarsening during sintering and deformation. A low sintering temperature is a necessary, but not a sufficient, condition for achieving the required microstructure. In many cases, the selection of an appropriate phase, such as tetragonal phase in zirconia or α phase in silicon nitride, which is resistant to grain growth, is crucial. The use of sintering aids and grain‐growth inhibitors, particularly those that segregate to the grain boundaries, can be beneficial. Second‐phase particles are especially effective in suppressing static and dynamic grain growth. Another major concern is to maintain an adequate grain‐boundary cohesive strength, relative to the flow stress, to mitigate cavitation or grain‐boundary cracking during large strain deformation. Existing evidence suggests that a lower grain‐boundary energy is instrumental in achieving this objective. The selection of an appropriate phase and the tailoring of the grain boundary or liquid‐phase composition can sometimes drastically alter the cavitation resistance. Related observations on forming methods, forming characteristics, and sheet formability are also reviewed. The basic deformation characteristics are similar to diffusional creep and are dominated by grain‐boundary diffusion. However, deformation characteristics are frequently altered by interface reactions, secondphase hardening/softening, and dynamic grain‐growth‐induced strain hardening. Ductility and formability, on the other hand, are controlled by the flow stress and flaw distribution, not by deformation instability as in superplastic metals. Analytical models and empirical correlations are presented to describe various constitutive relations pertaining to superplastic ceramics.
Ceramic Coatings on Ceramic Substrates by Liquid Metal Transfer
Authors: Silverman Lance D. , Zdaniewski Wieslaw A. , Brungard Nancy L.
Published: March 2005

A variety of ceramic coatings have been put on ceramic substrates by the molten metal transfer technique. In the diffision coating process a reactant metal dissolved in a molten “transfer agent” metal reacts with a substrate to form the coating. Titanium dissolved in molten tin has been reacted with silicon carbide, aluminum nitride, and silicon nitride substrates to produce titanium carbide and titanium nitride coatings, respectively. Similarly tantalum metal has been reacted to form carbide or nitride coatings. The unreacted tin is finally removed by decantation followed by acid leaching.
Characterization of β‐Silicon Carbide by Silicon‐29 Solid‐State NMR, Transmission Electron Microscopy, and Powder X‐ray Diffraction
Authors: Carduner Keith R. , Shinozaki Sam S. , Rokosz Michael J. , Peters Charles R. , Whalen Thomas J.
Published: March 2005

Increased interest in ceramic materials, particularly for high‐temperature, high‐stress applications, has created the need for rapid and reliable analytical techniques to monitor microcrystalline structure of commercial ceramic powders. A comparative evaluation of commercially available β‐SiC powders is undertaken to analyze the potential of nuclear magnetic resonance (NMR) in the characterization of β‐SiC powder. NMR provides an acceptable, rapid method for characterization of powders both during powder manufacturing as well as for powder analyses priror to sintering studies. The results of transmission electron microscopy and X‐ray diffraction are correlated with the NMR spectra to explain some newly observed features in the NMR spectra of β‐SiC powders and to illustrate the sensitivity of NMR to microcrystalline disorder.
Local Structure of xAl2O3· (1 −x)NaPO3 Glasses: An NMR and XPS Study
Authors: Brow Richard K. , Kirkpatrick R. James , Turner Gary L.
Published: March 2005

A series of xAl2O3· (1 −x)NaPO3 glasses (0 ≤x≤ 0.275) were prepared and characterized by magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy and by X‐ray photoelectron spectroscopy (XPS). 27AI MAS NMR spectra reveal that: at low alumina contents (x < 0.125), Al is in dominantly octahedrally coordinated Al(OP)6 structural environments; in glasses with x > 0.125, Al(OP)4 environments are increasingly favored; and at x= 0.275, most Al is tetrahedrally coordinated. 5‐coordinated Al environments also appear to be present. Variations in the 31P MAS NMR spectra are consistent with bonding between neighboring P and Al polyhedra. Quantitative changes in the oxygen bonding, determined from the O 1s spectra obtained by XPS, also reflect these structural changes. The effects of alumina content on the relative concentration of nonbridging oxygen (PO−) are quantitatively described by a simple structural model derived from the MAS NMR data. The thermal expansion coefficients and glass transition temperatures are shown to be sensitive to these structural changes.
Temperature Dependence of Hardness in Yttria‐Stabilized Zirconia Single Crystals
Authors: Morscher Gregory N. , Pirouz Pirouz , Heuer Arthur H.
Published: March 2005

The temperature dependence of hardness and microcracking in single‐crystal 9.5‐mol%‐Y2O3‐fully‐stabilized cubic‐ZrO2 was studied as a function of orientation. Crack lengths increased with increased temperature up to 500°C; above 800°C, no cracks were found, indicating an indentation brittle‐to‐ductile transition of ∼800°C. The temperature dependence of hardness was reduced around 500°C. Etching studies to delineate the plastic zone around and below indents identified the operative slip systems. The role of dislocations and their interactions within the plastic zone on the hardness and indentation fracture behavior of cubic‐ZrO2 are discussed.
Oxidation of Silicon Carbide in Environments Containing Potassium Salt Vapor
Authors: Pareek Vinod , Shores David A.
Published: March 2005

At high temperatures in clean oxidizing environments, SiC forms a very protective SiO2 film, but, in environments containing low levels of gaseous alkali salt contaminants or where condensed salts may deposit on the surface, the resistance of the film is significantly reduced. Oxidation kinetics of SiC were measured by continuous thermogravimetric analysis in a controlled environment containing CO2, H2O, and O2 plus low levels of potassium‐containing salts. Potassium was found to be incorporated into the SiO2 scale and to significantly change its transport properties and its morphology. The rate of scale formation was found to increase directly in proportion to K in the scale. A change in mechanism was observed when water vapor was added to the reacting gas stream.
Temperature‐Dependent Indentation Behavior of Transformation‐Toughened Zirconia‐Based Ceramics
Authors: Tikare Veena , Heuer Arthur H.
Published: March 2005

Indentation behavior of Ce‐TZP, Y‐TZP, and Mg‐PSZ between room temperature and 1300°C was investigated. Hardness decreased with increasing temperature for all three materials, but indentation cracking increased with increasing temperature. The opposing temperature dependences are discussed in terms of dislocation and transformation plasticity.
Consolidation of Alumina—Zirconia Mixtures by a Colloidal Process
Authors: Kimura Toshio , Kaneko Yasuyuki , Yamaguchi Takashi
Published: March 2005

The relationship between the dispersion of colloidal powder particles in Al2O3–ZrO2 suspensions and the microstructures of consolidated compacts was examined. Suspensions were prepared from Al2O3 powder and ZrO2 sol with average particle sizes of 390 and 62 nm, respectively. The dispersion was controlled by pH and salt concentration adjustments. The compacts composed of completely separated Al2O3 and ZrO2 layers were obtained from well‐dispersed suspensions with pH values below about 4 and salt concentration of 0.0527M. An increase in pH or salt concentration resulted in macroscopically uniform compacts. The compacts made from suspensions with pH values above about 7, however, were composed of a mixture of Al2O3 and ZrO2 agglomerates, with one acting as a matrix and the other a dispersed phase. Suspensions with a pH value of 4.5 and optimum salt concentrations resulted in compacts with microscopically uniform microstructure. Above or below these salt concentrations, ZrO2 agglomerates were distributed in an Al2O3 matrix. The optimum concentration was dependent on solid content. In addition, the dispersion of mixed suspensions was compared with those of single‐component suspensions. The ZrO2 particles formed three‐dimensional networks during agglomeration, which reduced the component separation in suspensions and during consolidation.
Formation and Characterization of Amorphous Aluminum Nitride Powder and Transparent Aluminum Nitride Film by Chemical Vapor Deposition
Authors: Riedel Ralf , Gaudl Kai‐Uwe
Published: March 2005

Based on thermodynamic predictions that, in the gas‐phase system AlCl3/NH3, AlN can be produced up to the theoretical yield, the performance of a chemical vapor deposition (CVD) reactor suitable for continuous powder production is investigated. Under the applied reaction conditions and reactor configuration, fine spherical AlN powders and transparent AlN films could be preferentially obtained. The generated amorphous CVD AlN powder is characterized by a BET surface area of 23.5 m2/g. The powder deposition rate was determined to be 3.5 g/h.
Inorganic Fibers—A Literature Review
Authors: Cooke Theodore F.
Published: March 2005

A review of inorganic fibers other than asbestos, glass, and carbon fibers and optical and hollow fibers. Special emphasis is placed on high‐performance fibers. The manufacture, structure, properties, and uses of the selected fibers are discussed.
Reaction and Diffusion Kinetics During the Initial Stages of Isothermal Chemical Vapor Infiltration
Authors: Sheldon Brian W. , Besmann Theodore M.
Published: March 2005

Individual bundles of ceramic fibers were infiltrated with SiC to study the reaction and diffusion kinetics during isothermal chemical vapor infiltration (CVI). More uniform infiltration was observed in samples where baffles were placed in the reactor and when HCl was added to the inlet gases. The evolution of the microstructure was modeled using an analytical expression for impinging cylindrical fibers. The transport of reactants was treated using classical descriptions of molecular and Knudsen diffusion in a porous body and also by considering the existence of a percolation threshold. All of these models predict that infiltration should be more uniform than the results that were obtained experimentally. It is possible that this discrepancy occurs because mass transport is more complex than the descriptions that were used. However, a more likely explanation is that the deposition kinetics are significantly more complex than the simple first‐order reaction that was used for these models.
Effects of pH and Particle Size on the Processing of and the Development of Microstructure in Alumina–Zirconia Composites
Authors: Bleier Alan , Westmoreland C. Gary
Published: March 2005

The aqueous processing of binary suspensions that contain 0.50‐μm alumina and either 0.20‐ or 0.71‐μm zirconia was studied from pH 3 to 11, emphasizing colloidally stable systems. Differences in particle size and density can induce differential sedimentation and lead to a nonuniform oxide distribution in pellets prepared by pressure filtration. Uniformity improves when the zeta potentials of the unlike particles are of the same sign but differ greatly in magnitude, i.e., in the pH range 5 to 6, where the oxides experience a weak, mutual association. However, when they are more similar electrostatically, they do not mutually associate and uniformity suffers. This nonuniformity occurs with the adjustment of pH to ≪5 (pos ive particles) and the adsorption of soluble species that strongly alter the zeta potential, e.g., after the preexposure of zirconia to aluminum chloride (positive particles) and the addition of polyacrylic acid (negative particles). In these cases, either high viscosity or rapid consolidation are required to ensure a uniform distribution. The disparity in the particle number fraction, derived from the size differences, and the thermodynamic interparticle energies control the prefired microstructure and are central to the prediction of the proper processing conditions.
Some New Perspectives on Oxidation of Silicon Carbide and Silicon Nitride
Authors: Luthra Krishan L.
Published: March 2005

This study provides new perspectives on why the oxidation rates of silicon carbide and silicon nitride are lower than those of silicon and on the conditions under which gas bubbles can form on them. The effects on oxidation of various rate‐limiting steps are evaluated by considering the partial pressure gradients of various species, such as O2, CO, and N2. Also calculated are the parabolic rate constants for the situations when the rates are controlled by oxygen and/or carbon monoxide (or nitrogen) diffusion. These considerations indicate that the oxidation of silicon carbide and silicon nitride should be mixed controlled, influenced both by an interface reaction and diffusion.
Autoclave‐Free Formation of α‐Hemihydrate Gypsum
Authors: Zürz Alfred , Odler Ivan , Thiemann Felicia , Berghöfer Katarina
Published: March 2005

Flue gas gypsum may be converted to α‐hemihydrate by suspending the starting material in inorganic salt solutions of high concentration or in diluted sulfuric acid and maintaining the suspension at boiling, or near boiling temperature for a period of time. The crystal morphology of the produced α‐hemihydrate gypsum and, thus, its quality depend significantly on the composition of the liquid phase. Additions of different carbonic acids may improve the quality of the product distinctly. Under optimal working conditions the quality of the formed α‐hemihydrate gypsum is similar to that of a material produced in an autoclave.
Silicon Nitride–Silicon Carbide Refractories Produced by Reaction Bonding
Authors: Reddy Navuri Kishan , Mukerji Joydeb
Published: March 2005

Silicon nitride‐silicon carbide refractories have been prepared by reaction bonding. The percentage of silicon nitride in the refractory was varied from 5 to 45 wt%. The modulus of rupture of refractories increases with an increase of temperature, whereas thermal expansion decreases. These refractories are resistant to molten non‐ferrous metals but are attacked by molten cast iron.
Mullite/Alumina Particulate Composites by Infiltration Processing: III, Mechanical Properties
Authors: Marple Basil R. , Green David J.
Published: March 2005

The effect of incorporating mullite into alumina by an infiltration process on the mechanical properties was investigated. Data for Young's modulus, strength, and fracture toughness for various composite compositions were compared with those for the unreinforced matrix (alumina). Measurements of Young's modulus by a resonance technique showed that the addition of mullite decreased Young's modulus. Up to 14 vol%, these changes were close to those expected, but above this mullite content, the decrease was more dramatic and indicated specimen damage during processing. The addition of mullite led, in some cases, to increases of more than 60% in both the strength (biaxial flexure) and indentation fracture toughness. These increases have been attributed to the method of introducing mullite and the resulting residual compressive surface stresses. The strength of the indented composite bodies deviated from the ideal behavior, indicating the probability of R‐curve behavior in these materials.
High‐Temperature Characterization of Reaction‐Sintered Mullite‐Zirconia Composites
Authors: Descamps Philippe , Sakaguchi Shuji , Poorteman Marc , Cambier Francis
Published: March 2005

Mullite‐zirconia composites are prepared by reaction sintering of zircon and alumina, either using reaction and sintering additives (titania (T‐MZ) or magnesia (M‐MZ)) or starting from highly reactive powder mixtures obtained by an ultra‐rapid‐quenching technique (URQ‐MZ). In the latter case, the presence of large amounts of an amorphous phase in the quenched powders enables sintering without the use of additives. The various composites differ mainly by their microstructure—i.e., the mullite grains aspect ratio and grain size—and by the quantity and nature of impurities or additives. Within this work, the modulus of rupture and critical stress intensity factor of each material are measured from room temperature to 1200°C. Bending creep is also investigated for various temperature and stress ranges. For the three materials, the fracture toughness (KIC) decreases with temperature, according to tetragonal zirconia stabilization, up to 600° to 700°C, above which a significant increase occurs. This toughening effect is attributed to plastic relaxation ahead of the crack tip associated with a glassy phase at the grain boundaries. The reinforcement is more efficient for T‐MZ because of the higher viscosity of the glassy phase. For the same material, strength does not follow the KIC curve because of subcritical crack growth. Creep data are explained on the basis of two distinct mechanisms, depending on stress and temperature range. At high stresses and/or low temperatures, creep is associated with microcrack formation, whereas, at low stresses and/or high temperatures, data are in agreement with a solution‐precipitation mechanism accommodating grain‐boundary sliding with interface reaction as the limiting stage. Despite its larger amount of glassy phase, the M‐MZ ceramic shows the better creep resistance because of its particular microstructure—consisting of cross‐linked elongated mullite grains—which impedes grain‐boundary sliding.
The Preference of Silicon Carbide for Growth in the Metastable Cubic Form
Authors: Heine Volker , Cheng Ching , Needs Richard J.
Published: March 2005

A paradox is discussed concerning the growth of SiC polytypes from the vapor or the melt, based on recent ab initio quantum calculations of the relative energies of several polytypes in bulk. Why does the cubic (3C) structure grow in preference to all others, although the calculations indicate it is not the stable phase at any temperature? This can be explained from the calculations, with some further approximations, as due to the constrained equilibrium when adding one atomic double layer at a time to the growing crystal in the hexagonal direction without allowing rearrangement of the lower layers. The differing roles of donor and acceptor impurities are also discussed, with donors being found to favor the cubic structure.
Temperature Dependence of the Hardness of Nanocrystalline Titanium Dioxide
Authors: Guermazi Mohamed , Höfler H. Jurgen , Hahn Horst , Averback Robert S.
Published: March 2005

Vickers hardness of nanocrystalline (N‐) TiO2 was measured in the temperature range 25° to 1200°C and it was compared with that of polycrystalline (P‐) TiO2 in the same temperature range. N‐TiO2 showed a lower hardness than P‐TiO2 at temperatures up to 800°C which resulted in the lack of indentation crack traces for N‐TiO2 presumably implying more plasticity.
Cation Deficiency in Bi2(Sr,Ca)3Cu2O8+d
Authors: Nomura Shunji , Yamashita Tomohisa , Yoshino Hisashi , Ando Ken
Published: March 2005

X‐ray lattice parameter measurements of the Bi2‐x (Sr,Ca)3‐yCu2‐zO8+d system show that a continuously varying solid‐solution cation nonstoichiometry exists within the overall composition ranges x < 0.32, ‐0.2 < y < 0.32, and z < 0.25. A modulated structure along the b axis remains present over these composition ranges. Defects introduced by cation deficiency have been clarified to be vacancies by density measurements. The oxygen content determined by titration decreases proportionately with increasing cation vacancy concentrations. In Bi‐deficient compositions, the interstitial oxygen ion concentration decreased and the oxygen vacancy concentration increased with increasing concentration of Bi vacancies. The critical temperature, Tc, shows a maximum value at the stoichiometric cation composition and decreases to a slightly lower (5 to 10 K) constant value across the nonstoichiometric composition region with both increasing alkaline‐earth and Cu deficiency.
Direct Pyrolysis of Raw Rice Husks for Maximization of Silicon Carbide Whisker Formation
Authors: Krishnarao Raghavarapu Venkata , Godkhindi Mahadev Malhar , Mukunda Pudukotta Gopal Iyengar , Chakraborty Madhusudan
Published: March 2005

A new alternate route for the production of SiC from rice husks has been described. The proposed single‐stage process has the advantage of reduced time of production and a much higher yield of SiC whiskers than the conventional two‐stage process. Direct pyrolysis of raw rice husks was carried out in a graphite furnace under vacuum and the reaction products were analyzed through macroscopic examination, XRD, SEM, and chemical analysis. Effects of time and temperature on the relative yields of the various reaction products have been determined over a wide range. The pyrolysis of raw rice husks has been found to be accompanied by four different competitive processes, viz., formation of SiC whiskers, formation of SiC polycrystals, crystallization of amorphous silica, and graphitization of amorphous carbon. Crystallization of silica and formation of SiC(w) are dominant processes at 1150° to 1310°C. At higher temperatures formation of SiC polycrystals and graphitization of carbon are more favored. A multistep pyrolysis to 1310°C was found to yield a still higher amount of SiC(w) than direct pyrolysis.
Sinterability of Calcined Freeze‐Dried Alumina Powders
Authors: Wang Ling , Lloyd Isabel K.
Published: March 2005

The sinterability of alumina powders derived from freezedried, calcined aluminum sulfate is highly variable. Sintered densities of similarly prepared powders can range from <40% of theoretical density to 99% of theoretical density. Our results suggest that this variation is a result of the sensitivity of freeze‐dried powders to heating rate during calcination as well as calcination temperature, time, and atmosphere. Residual sulfur may also play a role in the sinterability.
Thermochemical Analysis of the Silicon Carbide‐Alumina Reaction with Reference to Liquid‐Phase Sintering of Silicon Carbide
Authors: Misra Ajay K.
Published: March 2005

The stabilities of different phases in the Si‐Al‐C‐O system are calculated from thermodynamic considerations with the objective of identifying the liquid phases formed during sintering of SiC in the presence of Al2O3. It is shown that a liquid phase can form at the sintering temperatures by the reaction of SiC with Al2O3. Depending on the carbon activity, the liquid can be either of the following: Al2O3+ Al4C3, SiC + Al4C3, or molten aluminum. The stability of the aluminosilicate melts that can form by the reaction of Al2O3 with the surface silica layer on SiC powders is also evaluated. Several factors that influence liquid‐phase sintering, such as the solubility of SiC in the melts and the generation of gases during sintering, are discussed. The results of the thermodynamic analysis are compared with the observed sintering behavior for SiC.
Thermal Expansion Anisotropy and Acoustic Emission of NaZr2P3O12 Family Ceramics
Authors: Srikanth Varanasi , Subbarao Eleswarapu C. , Agrawal Dinesh K. , Huang Chi‐Yuen , Roy Rustum , Rao Gutti V.
Published: March 2005

Most members of the NaZr2P3O12 (NZP) family possess low, near zero, overall thermal expansion coefficients. However, they also exhibit anisotropy of axial thermal expansion. Some compounds have opposite anisotropy; for example, the a parameter of CaZr4P6O24 contracts on heating and that of SrZr4P6O24 expands, while the c parameter expands for the Ca compound and contracts for the Sr compound. The anisotropy of the axial thermal expansion of these materials is believed to induce microcracking. The acoustic emission method was employed here to detect microcracking in ceramics due to the axial thermal expansion anisotropy. Acoustic signals were observed during cooling of the Ca and Sr compounds from 500°C, and Na and K compounds from 600°C. On the other hand, no acoustic emission signal is detected in Ca0.5Sr0.5Zr4P6O24 ceramics, in which the lattice parameters a and c remain nearly unchanged in the temperature range of room temperature to 500°C. Thus, a direct correlation between microcracking of ceramics and their anisotropic axial thermal expansion coefficients was established by employing acoustic emission monitoring techniques.
Emulsion Precipitation of Yttria‐Stabilized Zirconia for Plasma Spray Coatings
Authors: Sherif Fawzy G. , Shyu Lieh‐Jiun
Published: March 2005

Chemically uniform, high‐purity, yttria‐stabilized zirconia (YSZ) powders were prepared by emulsion hydrolysis of the metal alkoxides, acetates, and mixtures of alkoxides and acetates. Both the morphology and particle size of the powders can be controlled by varying the hydrolysis conditions. Spherical or granular powders with particle sizes ranging from submicrometer to a few hundred micrometers were obtained. X‐ray diffraction and EDX results showed that yttria was evenly distributed throughout the zirconia particles. The crystalline phase of the powders after calcination at 800°C was 100% nontransformable tetragonal. The powders were successfully sprayed by a plasma coating technique on stainless steel coupons, and the coatings were evaluated.
Effect of Processing Temperature on the Morphology of Silicon Carbide Whiskers
Authors: Zhou Yan‐Chun , Xia Fei
Published: March 2005

Two kinds of SiC whiskers were annealed at temperatures similar to those for the processing of ceramic matrix composites. The morphology and structure of the as‐received and postannealed whiskers were investigated by SEM, TEM, and XRD, and the influence of processing temperature on the mechanical properties of ceramic matrix composites was discussed.
Quantitative X‐ray Diffraction Determination of Leucite Thermal instability in Dental Porcelain
Authors: Mackert J. Rodway , Evans Amalia L.
Published: March 2005

Alterations in the leucite content of dental porcelain during thermal treatments required for fabrication of dental restorations produce proportional changes in the thermal expansion coefficient. Such thermal expansion changes can create a thermal expansion mismatch with the metal to which it is fused and can result in the fracture of the porcelain during fabrication. The purpose of this study was to determine whether changes in the leucite content of a dental porcelain resulting from thermal treatments could be detected via quantitative X‐ray diffraction. The thermal treatments investigated were isothermal soaks for 0, 4, 8, and 16 min at 500° and 750°C, or multiple firings of 1, 2, 4, 8, and 16 times. Quantitative X‐ray diffraction was performed on the heat‐treated porcelain specimens, and the data were subjected to linear regression analysis. A significant negative correlation (r=−0.92, p < 0.001) was found between leucite volume fraction and the number of firings. A significant positive correlation (r= 0.88, p < 0.001) was obtained between leucite volume fraction and duration of heat soak at 750°C. No relationship was found (r= 0.40, p= 0.127) between the leucite content and the duration of the heat soak at 500°C. Such leucite variations could be responsible for changes in porcelain—metal thermal compatibility.