bluuebunny/crossref_metadata_2025 huggingface dataset:
Direct Observation and Analysis of Indentation Cracking in Glasses and Ceramics
Authors: Cook Robert F. , Pharr George M.
Published: March 2005

A review of the observations of indentation‐induced fracture suggests that there is no simple generalization which may be made concerning crack initiation sequences. Here, we investigate the material dependence of the initiation sequence of indentation cracks (cone, radial, median, half‐penny, and lateral) using an inverted tester allowing simultaneous viewing of the fracture process and measurement of the indeter load and displacement during contact. Two normal glasses, two anomalous glasses, and seven crystalline materials are examined. Key results include (i) direct evidence that the surface traces of cracks observed at indentation contacts are those of radial cracks, rather than median‐nucleated half‐penny cracks (at least for peak contact loads <40 N) and (ii) that, in crystalline materials, radial cracks form almost immediately on loading of the indenter, in anomalous glasses at somewhat greater loads, but in normal glasses during unloading. A detailed consideration of the stress fields arising during indentation contact predicts material‐dependent initiation sequences, in agreement with observations, particularly those of radial crack formation on loading for materials with large modulus‐to‐hardness ratios. In addition, a new, unexplored crack system is demonstrated, the shallow lateral cracks, which appear to be responsible for material removal at sharp contacts.
Formation of High‐Tc Superconducting Bi‐Pb‐Sr‐Ca‐Cu Oxide Films by Spray Pyrolysis of an Oxalate Suspension
Authors: Chen Fueng‐Ho , Tseng Tseung‐Yuen
Published: March 2005

A Bi‐Pb‐Sr‐Ca‐Cu oxide superconducting thick film, exhibiting Tc(onset)= 120 K and Tc(zero)= 100 K, can be obtained by spray pyrolysis of oxalate suspension on the (100) MgO substrate, using the partial‐melting state at 860°C for 15 h, a rapid‐quenching process to 500°C, and a postannealing treatment for 1 h at 835°C. These processes are believed to enhance the volume fraction of 2223 phase and increase the Tc(zero). The films show preferred orientation and adhere well to the substrate.
Rapid Thermal Processing of Pb(Mg0.7Zn0.3)1/3Nb2/3O3 Multilayer Ceramic Capacitors
Authors: Landin Steven M. , Schulze Walter A.
Published: March 2005

Fast firing, with total sintering times of between 3 and 15 min, was utilized to process multilayer ceramic capacitors of dielectric composition Pb(Mg0.7Zn0.3)1/3Nb2/3O3. This dielectric composition has been studied extensively because of its diffuse phase transition, which is intrinsic to relaxors, and also its high dielectric constant. The fast‐firing procedure, which did not utilize any prevention for PbO volatilization, has been shown to be a feasible method for the processing of these components. Sintering times in the range of 12 to 15 min at 1100°C resulted in components with weak field dielectric constants in excess of 12 000 and grain sizes in the range of 1 μm or less. The development of a pyrochlore phase at the component surfaces was less influential on the dielectric performance of the components because of the internal nature of the electrodes.
Nature of Abnormal Grain Growth in (Ba, Nb)‐Doped TiO2 Ceramics and Effect of Crystal Phases in Starting Powders
Authors: Wu Jenn‐Ming , Sung Hsiao‐Miin
Published: March 2005

The effect of crystal phase in the starting powder of (Ba, Nb)‐doped TiO2 ceramics on abnormal grain growth was investigated. The abnormal grain growth took place mainly during the cooling period after the second phase precipitated. The extent of abnormal grain growth is higher in an anatase powder sample than in a rutile powder sample and is highest at an intermediate rate of cooling for both powders. The nature of abnormal grain growth is suggested to be the Ba flow resulting from the concentration gradient of Ba between neighboring grains as the second phase precipitated. The reason for the greater abnormal grain growth in the anatase powder sample is proposed to be the higher dissolution of Ba in the grain which can be explained by the more open crystal structure of anatase with respect to rutile. The amount of second phase on the surface is greater than that in the interior, especially in the anatase powder sample. Characteristic features of the structure of anatase were calculated.
Hot Isostatic Pressing and High‐Temperature Strength of Silicon Nitride‐Silica Ceramics
Authors: Zeng Jianren , Yamada Osamu , Tanaka Isao , Miyamoto Yoshinari
Published: March 2005

Dense Si3N4 ceramics containing 2.5 and 20 wt% SiO2 were hot isostatically pressed by a glass‐encapsulation method. Their fracture strengths showed no degradation up to 1400°C, despite the presence of a large amount of SiO2 glass. Moreover, in the case of Si3N4‐20 wt% SiO2, plastic deformation of as much as 2.5% was observed during bending tests at the strain rate ε˙= 1.5 × 10−4/s. The α‐ to β‐phase‐transformation rate decreased with increasing SiO2 content.
Oxidation Kinetics of Aluminum Nitride
Authors: Suryanarayana Darbha
Published: March 2005

Thermal oxidation kinetics of aluminum nitride (AlN) powders, having fine‐ and coarse‐particle‐size distributions, were studied using thermogravimetric analysis (TGA). The kinetics showed dependence on the particle‐size parameter, and the experimental TGA data were curve fitted using empirical mass relations employing both linear and parabolic models. The simulations predicted mixed kinetics in AlN oxidation.
Residual Stress State of Brazed Ceramic/Metal Compounds, Determined by Analytical Methods and X‐ray Residual Stress Measurements
Authors: Iancu Otto T. , Munz Dietrich , Eigenmann Bernd , Scholtes Berthold , Macherauch Eckard
Published: March 2005

The residual stress state of brazed ceramic/metal compounds is described by means of X‐ray residual stress determinations and analytical calculations using a model of three elastic infinite plates. It is shown that the residual stress state of the soldered compound depends on the materials combination and on the geometrical conditions. The combination of X‐ray residual stress measurements and analytical calculations allows decisions on whether the assumption of a linear elastic model, based on elementary bending theory, is valid for the particular compounds.
Grain‐Boundary Migration in Nonstoichiometric Solid Solutions of Magnesium Aluminate Spinel: II, Effects of Grain‐Boundary Nonstoichiometry
Authors: Chiang Yet‐Ming , Kingery W. David
Published: March 2005

The grain‐boundary chemistry of magnesium aluminate spinel solid solutions MgO·nAl2O3 in which grain growth measurements were reported in part I has been investigated in order to understand the mechanism of grain‐boundary migration. It is found that although segregation of impurity Ca and Si is common, much larger deviations in grain‐boundary stoichiometry are present. There is an excess of Al and O relative to Mg at grain boundaries in all compositions. Grain‐boundary migration appears to be rate‐limited by solute drag from intrinsic defects accommodating lattice nonstoichiometry, rather than by extrinsic solutes, consistent with the observed impurity tolerance of grain‐boundary mobility. Different rate‐limiting defects are proposed for magnesia‐rich and alumina‐rich spinels.
Interdependence of Phase Chemistry, Microstructure, and Oxygen Fugacity in Titanate Nuclear Waste Ceramics
Authors: Buykx William J. , Levins Desmond M. , Smart Roger St. C. , Smith Katherine L. , Stevens Geoffry T. , Watson Kenneth G. , Weedon David , White Timothy J.
Published: March 2005

Titanate ceramic waste forms were prepared using several combinations of calcination atmosphere (N2, N2‐3.5% H2, H2) and metallic redox buffers (Ni, Fe, Ti, Al) to examine the dependence of microstructure and durability upon oxygen activity. It was found that the microstructures and phase assemblages were mostly insensitive to the fabrication method, although in detail systematic changes were recognized. The correlation of aqueous durability with oxygen fugacity was not straightforward, because of density variations in the hot‐pressed ceramics. These fluctuations in density dominated the dissolution characteristics of the waste forms and sometimes obscured the more subtle changes associated with redox potential. It is concluded that although the best durability is achieved at lower fugacities (i.e., Ti metal buffer and H2 calcination atmosphere), a satisfactory product can be produced using any of the preparative routes examined, provided the material is completely densified.
Influence of Core‐Shell Grains on the Internal Stress State and Permittivity Response of Zirconia‐Modified Barium Titanate
Authors: Armstrong Timothy R. , Buchanan Relva C.
Published: March 2005

BaTiO3 modified with grain‐boundary resident ZrO2 (≤ 2 wt%) results in substantial chemical modification of the microstructure when sintered at 1320°C for 2 h. These modified regions were seen as core‐shell grains in the micro‐structure and were accompanied by an increase in the internal stress level. The shells had a variable composition (Ba(Ti1−xZrx)O3) and a tetragonal structure. An expansion mismatch between the core and the shell placed the core in compression resulting in a decrease in the lattice parameter and a pseudocubic structure with respect to the unmodified BaTiO3. The combination of core‐shell grains and increased internal stress resulted in an essentially flat permittivity response with temperature, a condition consistent with a distribution of Curie points within the ceramic.
Improved Equation of the Continuous Particle Size Distribution for Dense Packing
Authors: Zheng Jingmin , Johnson Paul F. , Reed James S.
Published: March 2005

The Furnas model describes the discrete particle size distribution for densest packing. Using a model that considers a continuous particle size distribution for the densest packing to be a mixture of infinite Furnas discrete particle size groups, an equation for the cumulative particle size distribution providing the densest packing was derived. Monosize particles with different shapes have a different packing pore fraction. One parameter in the equation is the pore fraction of packed monosize particles; the particle size distribution for achieving densest packing is a function of this pore fraction. A reduced form of this equation is also presented as a working equation. The equation derived here is compared to the modified Andreasen equation for dense packing. An equation and the correlated graph for calculating theoretically the geometric mean particle size and an equation for calculating the specific surface area of the particle size distribution of the improved equation are also derived.
Application of Zinc Oxide Varistors
Authors: Gupta Tapan K.
Published: March 2005

This paper deals with the application of ZnO varistors—an area which has not been treated in a systematic way in the literature. The paper starts with a brief description of the fundamental properties comprising the electrical behavior as well as the physics, chemistry, and microstructure of the varistor. These properties then form the basis for defining the application parameters that are directly related to the nonlinear current‐voltage characteristics of the varistor. This paper provides a detailed description of these parameters and their relation to microstructure and the processing of the varistor. Finally, a discussion is presented on the reliability of the varistor by considering a grain‐boundary defect model which explains both the instability and the stability under use conditions.
High‐Temperature Creep and Cavitation of Polycrystalline Aluminum Nitride
Authors: Jou Zuel Chown , Virkar Anil V.
Published: March 2005

Dense, polycrystalline AlN samples of grain size between 1.8 and 19 μm were fabricated by hot‐pressing. Bar‐shaped samples were subjected to creep in four‐point bending under static loads in nitrogen atmosphere. The outer fiber stress was varied between 20 and 140 MPa and the temperature between 1650 and 1940 K. Steady‐state creep rate, dɛ/dt was proportional to σnd−m where the stress exponent, n, was between 1.27 and 1.43 and grain‐size exponent, m, between ∼ 2.2 and ∼ 2.4. The activation energy for creep ranged between 529 and 625 kJ/mol. Both round (r type) and wedgeshaped (w type) cavities were observed in electron micrographs of the deformed samples. No noticeable change in the dislocation density was observed. Contribution of cavitation to the creep rate was estimated using an unconstrained cavity model. Based on this study it is concluded that the dominant mechanism of creep in polycrystalline AlN is diffusional.
Better Sintering through Green‐State Deformation Processing
Authors: Cameron Craig P. , Raj Rishi
Published: March 2005

Aqueous colloidal suspensions of alumina were processed in the dispersed state and the flocculated state by controlling the double‐layer interactions between the particles. Repulsive particle forces led to high packing densities but the green bodies were mechanically so weak that they were unable to retain their shape (the dispersed case). Attractive forces led to good green strength but the packing density was low and the particles were agglomerated (the flocculated case). The agglomerated structure of the flocced specimens could be fragmented by mechanical deformation of the green compact; the deformation was carried out under a superimposed hydrostatic pressure of less than 1 MPa. The flow stess of the flocculated structures depended on the deformation rate, and on the magnitude of the superimposed hydrostatic pressure. The flow stress was 2.5 kPa at a strain rate of 0.1 s−1. Deformation processing of the flocced structures increased the green (relative) density from 0.51 to 0.62. The sintering behavior of underformed and deformation‐processed flocced structures was studied. Deformation‐processed green bodies sintered more rapidly and yielded a final grain size that was smaller and more uniform than that obtained from the undeformed specimens. The ability to homogenize and densify the packing of flocculated structures by deformation processing suggests new opportunities in green‐state processing, for example (i) uniform mixing of more than one kind of particle or particles and fibers, and (ii) net shape forming by injection molding or extrusion, without the use of organic binders.
Pore Drag and Pore‐Boundary Separation in Alumina
Authors: Rödel Jürgen , Glaeser Andreas M.
Published: March 2005

Microdesigned interfacial pore structures were used to study pore drag and pore‐boundary separation in Al2O3. This approach allows the creation of pore arrays containing pores of controlled size and spacing at well‐defined singlecrystal seed/polycrystalline matrix interfaces, and enables experimental determination of the peak pore velocity. From the peak pore velocity, values of the surface diffusion coefficient pertinent to sintering can be extracted. At 1600°C, the surface diffusion coefficient is ∼1 × 10−7 cm2/s for undoped Al2O3 and ∼4 × 10−7 cm2/s for MgO‐doped Al2O3. The values appear to be insensitive to the seed orientation for the two seed orientations studied. The results suggest a strong influence of pore spacing on the separation condition in undoped Al2O3, and a diminished influence in MgO‐doped Al2O3. Quantitative agreement between theoretically predicted and experimentally observed separation/attachment conditions was obtained.
Combustion Synthesis of Silicon Nitride‐Silicon Carbide Composites
Authors: Agrafiotis Christos C. , Lis Jerzy , Puszynski Jan A. , Hlavacek Vladimir
Published: March 2005

The feasibility of synthesizing silicon nitride‐silicon carbide composites by self‐propagating high‐temperature reactions is demonstrated. Various mixtures of silicon, silicon nitride, and carbon powders were ignited under a nitrogen pressure of 30 atm (∼ 3 MPa), to produce a wide composition range of Si3N4‐SiC powder products. Products containing up to 17 vol% of SiC, after being attrition milled, could be hot‐pressed to full density under 1700°C, 3000 psi (∼ 21 MPa) with 4 wt% of Y2O3. The microhardness and fracture toughness of these composites were superior to those of the pure β‐Si3N4 matrix material and compared very well with the properties of “traditionally” prepared composites.
Processing Impurities as Phase Assemblage Modifiers in Titanate Nuclear Waste Ceramics
Authors: Buykx William J. , Levins Desmond M. , Smart Roger St. C. , Smith Katherine L. , Stevens Geoffrey T. , Watson Kenneth G. , White Timothy J.
Published: March 2005

The tolerance of titanate nuclear waste ceramics to fluctuations in the concentrations of processing contaminants was monitored using X‐ray diffraction, electron microscopy, secondary ion mass spectrometry, and analysis of dissolution liquors. Several waste forms were fabricated, all of which contained idealized Purex waste simulant and, in addition, varying quantities of the common waste stream impurities P2O5, MgO, Fe2O3, Na2O, and SiO2. Incorporation of the oxides individually stabilized new phases including monazite (P2O5), pseudobrookite (MgO), loveringite (Fe2O3), freudenbergite (Na2O), and pollucite (SiO2)—only the latter phase deleteriously affected waste form performance by promoting cesium dissolution. However, when the processing contaminants were added simultaneously, a number of synergetic effects, particularly the stabilization of a soluble glassy phase, resulted in elemental losses which were an order of magnitude greater for some matrix and radwaste species. It was found that up to 25 wt% of the idealized Purex waste could be incorporated in the waste without diminution of its properties.
Polymorphic Phase Transformation of Lead Monoxide and Its Influence on Lead Zirconate Titanate Formation
Authors: Chen San‐Yuan , Cheng Syh‐Yuh , Wang Chien‐Min
Published: March 2005

The polymorphic phase transformation of PbO from massicot to litharge phase is easily produced by mechanical force. The transformation is relatively quick by wet ball‐milling. The additions of TiO2 and ZrO2 tend to retard it. A second‐order exponential relation is observed between the fractional conversion and the milling time for pure PbO, PbO‐TiO2, and PbO‐ZrO2‐TiO2 systems with a rutile constituent. This exponential relation is changed into a first‐order logarithmic relation when anatase instead of rutile is used. Although the PbOss phase is observed irrespective of the phase of the PbO in the raw materials, this PbOss peak is more obvious when massicot PbO or anatase TiO2 is used. The formation of PbOss phase is also dependent on the reactivity of the ZrO2, and its presence can affect the formation and piezoelectric properties of PZT ceramics.
Kinetics of Constrained‐Film Sintering
Authors: Garino Terry J. , Bowen H. Kent
Published: March 2005

Results of shrinkage measurements on constrained and corresponding unconstrained films of materials that densify by viscous flow or solid‐state diffusion mechanisms have been presented. Constrained‐film shrinkage was measured in situ during sintering using laser reflectance; unconstrained‐film shrinkage was measured using scanning electron microscopy (SEM). Data for unconstrained films were fitted to densification kinetics expressions for the operative densification mechanism. Constrained‐film shrinkage behavior predicted by the Scherer‐Garino model,1using expressions that fit data for the unconstrained case, was compared to actual data for the constrained films. Data for the viscous sintering case fit the predictions made by the model well, but data for densification by solid‐state diffusion deviated from predicted behavior even when grain growth was accounted for, possibly because of pore growth caused by local densification.
Piezoresistivity in Polymer‐Ceramic Composites
Authors: Yoshikawa Shoko , Ota Toshitaka , Newnham Robert , Amin Ahmed
Published: March 2005

Composite materials composed of randomly dispersed semiconducting ceramic particles in an insulating polymer matrix show a pronounced change in resistivity with pressure. Different amounts of iron oxide (Fe3O4) powder and antimony‐doped tin oxide (SnO2:Sb) powder were dispersed in an epoxy polymer matrix to form pressure‐sensitive composites. In each family of materials, an insulator‐to‐semiconductor transition is observed in agreement with percolation theory. Composites within a certain range of filler content showed substantial piezoresistive effect under both uniaxial and hydrostatic pressure in which sensitivity is controlled by the choice of filler material and the volume fraction. The effect of temperature on the piezoresistance effect was also examined. Piezoresistors made from Fe3O4 composites showed larger temperature changes than those filled with Sb‐doped SnO2.
Theoretical Models for Binder Burnout
Authors: Calvert Paul , Cima Michael
Published: March 2005

The kinetics of binder burnout, from a ceramic green body, are considered for the case of an “unzipping” binder which decomposes to produce a volatile monomer. The process is considered to fail if the concentration of monomer in the green body exceeds that in equilibrium with vapor at 1 atm (≅105 Pa), when an internal bubble would be expected to form. Steady‐state diffusional calculations and computer simulations explore the size and temperature dependence of the process and are in agreement. The model suggests that it is not feasible to burn out a large flat piece greater than about 3 mm thick, without going to very long times of burnout. The kinetics are significantly improved if porosity develops in the piece during the early stages of decomposition, as opposed to the retreat of the binder into the piece on a uniform front.
Formation and Properties of Calcia‐Calcium Fluoride‐Alumina Glasses
Authors: Shelby James E. , Lord Christine E.
Published: March 2005

The glass formation region has been determined for the CaO‐CaF2‐Al2O3 ternary system. The glass transformation temperature of these glasses decreases as the CaF2 concentration increases, whereas the thermal expansion coefficient and electrical conductivity increase. MAS‐NMR (magic‐angle‐spinning nuclear magnetic resonance) indicates that the aluminum ions in these glasses are in tetrahedral coordination. The structures of these glasses are proposed to be analogous to those of similar silicate glasses, with aluminum ions replacing silicon ions and fluorine acting as a nonbridging anion.
Hot Hardness Behavior of Yttrium Sialon Ceramics
Authors: Ekström Thommy , Persson Jeanette
Published: March 2005

The hot hardness of polycrystalline single‐phase α‐ or β‐ sialon ceramics declines with increasing temperature, but the measured Vickers hardness (HV1) at 1100°C is still about 1550 and 1300 for the α‐sialon and the low‐substituted β‐sialon materials, respectively. The hardness of ‘composite’β‐ or α‐β‐sialon ceramics containing a high volume fraction of glassy phase is lower at all temperatures and drops significantly above about 900°C.
Computer Simulation of Final‐Stage Sintering: I, Model Kinetics, and Microstructure
Authors: Hassold Gregory N. , Chen I‐Wei , Srolovitz David J.
Published: March 2005

A Monte Carlo model for simulating final‐stage sintering has been developed. This model incorporates realistic microstructural features (grains and pores), variable surface difusivity, grain‐boundary diffusivity, and grain‐boundary mobility. A preliminary study of a periodic array of pores has shown that the simulation procedure accurately reproduces theoretically predicted sintering kinetics under the restricted set of assumptions. Studies on more realistic final‐stage sintering microstructure show that the evolution observed in the simulation closely resembles microstructures of real sintered materials over a wide range of diffusivity, initial porosity, and initial pore sizes. Pore shrinkage, grain growth, pore breakaway, and reattachment have all been observed. The porosity decreases monotonically with sintering time and scales with the initial porosity and diffusivity along the grain boundary. Deviations from equilibrium pore shapes under slow surface diffusion or fast grain‐boundary diffusion conditions yield slower than expected sintering rates.
Control of Radioactive Waste Glass Melters: I, Preliminary General Limits at Savannah River
Authors: Bickford Dennis F. , Applewhite‐Ramsey Amy , Jantzen Carol M. , Brown Kevin G.
Published: March 2005

A ceramic, Joule‐heated, slurry‐fed melter will be used to vitrify Savannah River Site high‐level waste at the Defense Waste Processing Facility (DWPF) in Aiken, SC. The entire vitrification process must be performed remotely and the borosilicate host glass must accommodate a variety of waste compositions. The control bases for such a melter are unique and complex. DWPF operations are tightly constrained by processing parameters and by product specifications. Part I discusses the melter control considerations which have been identified to facilitate compliance with these constraints. These considerations include melter temperature, glass composition, product durability, waste loading limits, glass redox control, and glass cooling requirements.
Removal of Carbonaceous Residue with Wet Hydrogen in Greensheet Processing for Multilayer Ceramic Module: II, Reaction in Large Modules Influenced by Pore Diffusion and Mass Transfer
Authors: Sohn Hong Y. , Wall Donald R.
Published: March 2005

An experimental and mathematical investigation of the removal of carbonaceous residue deposited on the ceramic substrate in the fabrication of multilayer ceramic modules has been conducted. Such carbonaceous residue is formed when the organic binder is pyrolyzed during the firing of greensheets. This paper is the second of a two‐part series that reports on and discusses the principal findings of the investigation. In this paper we describe the removal of carbonaceous residue from large blocks of the multilayer ceramic module in which the diffusion of gaseous species may play a role. A mathematical model is developed using the Law of Additive Reaction Times for fluid‐solid reactions, which incroporates the effects of intrinsic chemical kinetics, pore diffusion, and external mass transfer. This law gives an approximate solution to the governing equations which describe the removal of carbonaceous residue from the MLC module during firing.
Nitridation of the Sol–Gel‐Derived Titanium Oxide Films by Heating in Ammonia Gas
Authors: Kamiya Kanichi , Nishijima Tomoaki , Tanaka Katsuhisa
Published: March 2005

Dip‐coated sol–gel‐derived TiO2 films on an alumina substrate were converted to nonstoichiometric titanium nitride (TiNx (x≦ 1)) films by heating at approxmately 1000°C in NH3 gas. TiO2 films made from TiO2 sols prepared from Ti(O–i‐C3H7)4 and stabilized by diethanolamine were more easily nitrided than those from sols containing HCl as a deflocculant reagent. This appears to be a result of the more porous structure of the former films.
Nanometer‐Sized ZrO2 Particles Prepared by a Sol–Emulsion–Gel Method
Authors: Ramamurthi Sangeeta D. , Xu Zhengkui , Payne David A.
Published: March 2005

ZrO2 powder was prepared by a sol–emulsion–gel method at temperatures below 140°C from ZrO(NO3)2·nH2O. The asprepared powder was amorphous, but crystallized into the tetragonal structure by 600°C. The metastable tetragonal powder (600°C) was comprised of ultrafine 4‐ to 6‐nm size particles. On heat treatment, the tetragonal form completely transformed into the monoclinic state at 1100°C. Preliminary studies indicate good sinterability with densities greater than 94% at 1100°C and with a grain size of 0.25 μ.
Fracture Toughness of Ceramic Precracked Bend Bars
Authors: Bar‐On Isa , Beals James T. , Leatherman Gary L. , Murray Catherine M.
Published: March 2005

Fracture toughness was measured for four ceramic materials using precracked bend bar specimens. The effect of the precracking parameters, used for the bridge indentation method on fracture toughness values, was determined. Excellent agreement was obtained between fracture toughness values measured by this method and values obtained by other techniques.
Phase Equilibria in the Systems SrO‐CuO and SrO‐1/2Bi2O3
Authors: Hwang Nong M. , Roth Robert S. , Rawn Claudia J.
Published: March 2005

The phase equilibria of the systems SrO‐CuO and SrO‐1/2Bi2O3 were studied by X‐ray diffraction analysis of quenched powder samples. The compounds SrCuO2 and Sr2CuO3 melt incongruently at 1085° and 1225°C, respectively. The newly found compound Sr6Bi2O9 decomposes at 965°C into SrO and Sr3Bi2O6 melts incongruently into SrO and liquid at 1210°C. SrBi2O4 undergoes a phase transition at ∼825°C, and although both are nonstoichiometric, the low‐temperature phase is slightly poorer in SrO with 33.5 mol% SrO than the high‐temperature phase.
Stability of Monoclinic and Orthorhombic Zirconia: Studies by High‐Pressure Phase Equilibria and Calorimetry
Authors: Ohtaka Osarnu , Yamanaka Takamitsu , Kume Shoichi , Ito Eiji , Navrotsky Alexandra
Published: March 2005

The stability fields of two high‐pressure polymorphs of ZrO2 (ortho I and ortho II) were determined by both calorimetry and phase equilibrium experiments. Enthalpies of transition were measured by transposed temperature drop calorimetry. The entropies of transition and slopes of phase boundaries were calculated using the measured enthalpies and free energies calculated from the results of phase equilibrium experiments. From the thermodynamic measurements, it is seen that the entropy increases and the volume decreases during the monoclinic–ortho I transition, whereas both the entropy and the volume decrease during the ortho I–ortho II transition. Accordingly, the gradient of phase boundaries, dP/dT, is negative in the former and positive in the latter. These trends are consistent with those of phase equilibrium experiments.
Transient Hot‐Strip Method for Simultaneous Determination of Thermal Conductivity and Thermal Diffusivity of Refractory Materials
Authors: Log Torgrim
Published: March 2005

The transient hot‐strip method has been used for determining thermal conductivity and thermal diffusivity of a refractory brick. The technique is based on recording the transient temperature increase of a 25‐μm‐thick, 8‐mm‐wide, and 70‐mm‐long iron strip clamped between two sample halves and heated with a constant direct current. The method is accurate to within 5% in the thermal conductivity and to within 10% in the thermal diffusivity in the temperature range 20° to 700°C.
Strength of Nicalon Silicon Carbide Fibers Exposed to High‐Temperature Gaseous Environments
Authors: Kim Hyoun‐Ee , Moorhead Arthur J.
Published: March 2005

The effects of exposures to high‐temperature gaseous atmospheres on the strength of Nicalon SiC fibers were investigated. The exposure conditions were as follows: (1) H2 with various PH2O for 10 h at 1000° and 1200°C, and (2) air for 2 to 100 h at 800° to 1400°C. Individual fibers were tested in tension following each exposure. The strengths of the fibers were strongly influenced by the exposure atmosphere and temperature, but less affected by time at temperature. When exposed in air, a SiO2 layer was formed on the surface, minimizing the degradation of strength. However, this beneficial effect was negated under conditions in which the SiO2 layer became too thick. The most severe degradation resulted from exposure to a reducing atmosphere, presumably due to the reduction of SiO2 inherent in the fibers.
Poly(methylsilane)—A High Ceramic Yield Precursor to Silicon Carbide
Authors: Zhang Zhi‐Fan , Babonneau Florence , Laine Richard M. , Mu Ying , Harrod John F. , Rahn Jeffrey A.
Published: March 2005

Preceramic polymers offer exceptional potential for low‐temperature processing of both oxide and non‐oxide ceramics. In addition, shapes such as fibers, films, and membranes that are not commonly available using standard processing techniques are readity available using preceramic polymers. In non‐oxide ceramics, the ceramic products generally available from preceramics do not exhibit all of the typical properties associated with the same materials produced by standard, high‐temperature processing approaches. In part, this appears to be because there are very few preceramic polymers that lead to high‐purity, single‐phase materials. Poly(methylsilane), (–[MeHSi]x–), produced from MeSiH3, can be used to produce relatively pure, bulk SiC at temperatures below 1000°C. The transformation process from polymer to ceramic is followed by 29Si NMR and diffuse reflectance IR. The polymer first undergoes a major rearrangement from poly(silane) to poly(carbosilane) at 400°C. Above 400°C, the resulting poly(carbosilane) decomposes to a hydrogenated form of SiC as shown by spectroscopic analysis of the 600°C material. Further heating, to 1000°C for 1 h, provides very narrow 29Si peaks indicative of β‐SiC mixed with small amounts of α‐SiC polytypes. Chemical analysis, when coupled with the 29Si and XRD results, suggests that poly(methylsilane) produces resonably pure, nanocrystalline SiC at temperatures much lower than previously observed for other SiC preceramic polymers.
Inversion Domain Boundaries in Aluminum Nitride
Authors: Westwood Alistair D. , Notis Michael R.
Published: March 2005

Inversion domain boundaries (IDBs) have been identified in undoped, hot‐pressed AIN. The microstructure and micro‐chemistry of the IDBs have been studied using conventional transmission electron microscopy, convergent‐beam electron diffraction, and analytical electron microscopy. Two distinct IDB morphologies are present: a planar variant which lies on the basal plane (0001), and a curved variant which does not possess a particular habit plane, but portions of the boundary are often seen lying on one of the {1011} planes. The boundaries exhibit α‐like fringe contrast, indicating that a translation exists across the boundary. The displacement vectors RF for the planar and curved boundaries have been investigated using both two‐beam and multiple‐beam techniques. Microchemical analysis has revealed oxygen segregation to the planar IDB; when present on the curved IDB, oxygen is at a lower concentration than in the planar case. Lattice fringe imaging and long‐exposure selected area electron diffraction patterns have indicated the presence of thin, platelike precipitates at the planar IDBs. Sintering and annealing studies indicate that oxygen is necessary for the formation of the planar IDBs and that oxygen is not uniformly distributed along the curved IDBs.
High‐Temperature Wetting of Sapphire by Aluminum
Authors: Ownby P. Darrell , Li Ke Wen K. , Weirauch Douglas A.
Published: March 2005

Sessile drop studies of molten aluminum on single‐crystal sapphire substrates were conducted to investigate the effects of atmosphere on contact angle, substrate reactions, and interfacial crystal growth. Unlike previous investigations performed briefly in a vacuum environment in a temperature range within 600°C of the aluminum melting point, these experiments were conducted at higher temperatures (1200° to 1600°C) and at 1‐atm total pressure over longer experimental times to more closely approach equilibrium conditions. A continuously flowing buffered gas system utilizing high‐purity metered mixtures of hydrogen and helium in combination with a thoria ceramic electrolyte sensor were employed to achieve variations of the oxygen partial pressure from 10−19 to 10−15 atm while continuously maintaining the total pressure at 1 atm. At constant temperature, it was found that neither the oxygen partial pressure nor the crystallographic orientation of the sapphire substrate had a significant effect on the observed contact angles. A continuous decrease of acute contact angles and a single reaction ring characterized the 8‐h experiments without the alternating spreading and contracting behavior repeatedly reported in the literature. This phenomenon can be attributed to the lower rate of metal evaporation and interfacial reaction at the higher total gas pressure and yet extremely low oxygen partial pressure of these experiments. Profilometric analysis of sapphire substrates subsequent to the removal of the quenched sessile drops indicates a reduction in metal–solid interaction due to the closer approach to equilibrium than in previous studies. An epitaxial orientation with respect to the substrate was observed in α‐alumina crystallite formation at the metal–ceramic interface. Experimental evidence suggests that it was formed by a nucleation and growth process during the cooling period.
Preparation of Colloidal Boehmite Needles by Hydrothermal Treatment of Aluminum Alkoxide Precursors
Authors: Buining Paul A. , Pathmamanoharan Chellappah , Jansen J. Ben H. , Lekkerkerker Hendrik N. W.
Published: March 2005

Fairly monodisperse colloidal boehmite fibrils with a high aspect ratio were synthesized by hydrothermal treatment at 150°C of an acidified aqueous alkoxide solution, prepared by adding an aqueous HCl solution to an aluminum alkoxide precursor. The average particle length could be controlled between about 100 and 500 nm by varying the initial amounts of alkoxide and acid. Using two different alkoxides in a 1:1 molar ratio yielded the most needlelike product, having a particle length standard deviation of 40%. The boehmite particles were polycrystalline and contained 0.14 mol of excess H2O per mol of AlOOH, bound to the particle surface.
Vapor Pressure of Barium and Copper Components in YBa2Cu3O7−x Superconductor Ceramics
Authors: Sata Toshiyuki , Sakai Kazuo , Tashiro Suguru
Published: March 2005

Purified air is passed over a specimen of YBa2Cu3O7–x at 890°C; the vaporized substances are condensed in a pure alumina tube, then subjected to inductively controlled plasma analysis. Vapor pressure values of 2.5 × 10−5 Pa for BaO(g), 1.2 × 10−4 Pa for Cu(g), and 2.2 × 10−5 Pa for CuO(g) are obtained under 2.1 × 104 Pa (0.21 bar) of oxygen pressure. No Y vapor is detected at this temperature.
Role of Specimen Geometry in the Effect of Fiber Orientation on the Thermal Conductivity of a Uniaxial Carbon‐Fiber‐Reinforced Aluminoborosilicate Glass‐Matrix Composite
Authors: Bhatt Hemanshu , Donaldson Kimberly Y. , Hasselman D. P. H. , Chyung Kenneth , Taylor Mark P.
Published: March 2005

A study concerning the effect of fiber orientation on the thermal conductivity of a uniaxial carbon‐fiber‐reinforced borosilicate glass was conducted. For thin specimens, and thick specimens with specimen sides cut parallel to the fiber direction, the dependence of thermal conductivity on fiber orientation showed excellent agreement with theoretical behavior predicted for a composite infinite in extent. In contrast, the thermal conductivity data for rectangular thick specimens fell well below the data for the thin and angled specimens. For fiber orientation of 45° and higher, at which the heat was directed toward the side of the specimen rather than across, the thermal conductivity values showed excellent agreement with theoretical behavior predicted for a finite composite strip with insulated sides.
Enhanced Fracture Toughness in Layered Microcomposites of Ce‐ZrO2 and Al2O3
Authors: Marshall David B. , Ratto Joseph J. , Lange Fred F.
Published: March 2005

Laminar composites, containing layers of Ce‐ZrO2 and either Al2O3 or a mixture of Al2O3 and Ce‐ZrO2, have been fabricated using a colloidal method that allowed formation of layers with thicknesses as small as 10 μm. Strong interactions between these layers and the martensitic transformation zones surrounding cracks and indentations have been observed. In both cases, the transformation zones spread along the region adjacent to the layer, resulting in an increased fracture toughness. The enhanced fracture toughness was observed for cracks growing parallel to the layers as well as for those that were oriented normal to the layers.
Chemical and Microstructural Changes at Alkali‐Resistant Glass Fiber–Cement Interfaces
Authors: Ylmaz Veyse T. , Lachowski Eric E. , Glasser Fred P.
Published: March 2005

Portland cement matrices have been reinforced by 5 wt% of “CemFIL 1′’alkali‐resistant glass fiber and isothermally wet cured for up to 1 yr at 20° and 55°C. Degradation of mechanical properties occurs as a consequence of chemical attack on the fibers. Various stages of the degradative process are followed by electron microscopy and analysis. The original composite microstructure is described in terms of a solid fiber reinforcing system. However, the fibers gradually become hollow, leaving a concentric shell of cement‐fiber reaction product which still gives a useful measure of reinforcement. These changes, together with fiber shortening arising from local impingement of Ca(OH)2 crystals, give rise to what is termed a hollow cylinder reinforcement system. The hollow shell structures consist of gel and semicrystalline material; glass is absent or nearly so. The role of zirconia in stabilizing the hollow cylinder structures is described. It is considered that the microstructure developed at ∼20°C, while not as effective a reinforcement as solid fibers, still provides useful tensile reinforcement relative to plain paste matrices.
Nucleation and Crystallization of Na2O · 2CaO · 3SiO2 Glass by Differential Thermal Analysis
Authors: Xu Xiaojie J. , Ray Chandra S. , Day Delbert E.
Published: March 2005

The nucleation and crystallization of the Na2O · 2CaO · 3SiO2 (NC2S3) glass were studied by differential thermal analysis (DTA), and a (nucleation rate—temperature)‐like curve was determined by plotting either the reciprocal of the temperature corresponding to the crystallization peak maximum, 1/Tp, or the height of the crystallization peak, (δT)p, as a function of nucleation temperature, Tn. The temperature where nucleation can occur for this glass ranges from 550° to 650°C and the temperature for maximum nucleation is 600°± 5°C. Both temperatures are in excellent agreement with those determined by the classical technique of nucleation followed by isothermal crystallization. The activation energy for crystallization, Ec, for this glass is the same for surface and/or bulk crystallization, and is 370 ± 15 kJ/mol. The analysis of the crystallization data with the Kissinger equation yelds the correct value for Ec only when crystal growth occurs on a fixed number of nuclei. When a majority of the nucleation occurs during the DTA measurements, a modified Kissinger equation must be used to calculate Ec. Ec is also independent of the heating rate when determined using a single‐crystallization‐peak analysis technique. The single‐peak analysis technique is useful for a rapid determination of Ec or when only a small amount of sample is available.
Creep and Creep Fracture in Hot‐Pressed Alumina
Authors: Robertson A. Gordon , Wilkinson David S. , Cáceres Carlos H.
Published: March 2005

The creep and creep fracture behavior of two hot‐pressed aluminas are presented, for both flexural and tensile testing. Steady‐state power‐law creep is observed with a stress exponent of about 2 for each material. Three distinct fracture regimes are found. At high stress in flexure, fracture occurs by slow crack growth with a high stress dependence of the failure time. At intermediate stresses, in both flexure and tension, creep fracture occurs by multiple microcracking after modest strains. Failure times exhibit a modest stress dependence (stress exponent of 2.5 in tension and 3 in flexure), with a constant failure strain equal to 0.09. The failure times are considerably longer in flexure than in tension, because of the constraint imposed on crack growth by the bending geometry. We conclude that flexure cannot be used for creep lifetime assessment, even in simple, single‐phase materials such as Al2O3. At low stresses, in tension, failure also exhibits a modest stress dependence but with a much higher failure strain. The material shows the onset of super‐plastic behavior.
Damage and Fracture Mechanisms During High‐Temperature Creep in Hot‐Pressed Alumina
Authors: Wilkinson David S. , Cáceres Carlos H. , Robertson A. Gordon
Published: March 2005

The mechanisms responsible for creep damage accumulation and fracture have been examined in two commercial hot‐pressed aluminas. Differences between the two materials can be ascribed to minor compositional variations. Three damage regimes have been identified, depending on stress. However, in all three regimes, failure is controlled by crack propagation. At high stress, a single crack, nucleated at a processing flaw, controls failure. These cracks grow in a linear elastic stress field. At intermediate stresses, crack tip stresses relax, and many microcracks are nucleated. They grow and link under strain control. The details of this process differ under tension and bending, thus invalidating the flexure test as a means of establishing creep life, even in simple, single‐phase materials. At the lowest stress, extensive cavitation, with relatively little microcrack development, is observed. However, failure continues to be dominated by the growth of cracks. The material is damage tolerant and can be thought of as superplastic. We find that processing flaws (primarily large grains) control the creep life at all stresses. These should therefore be carefully controlled in materials aimed at high‐temperature structural applications.
Model for the Ferroelectric Transition in Nonstoichiometric Lithium Niobate and Lithium Tantalate
Authors: Birnie Dunbar P.
Published: March 2005

A model is developed for the ferroelectric‐to‐paraelectric phase transition in LiNbO3 and LiTaO3. The model is consistent with literature reports of nonstoichiometry, point defect formation, and phase transformation behavior. The model uses lithium Frenkel‐defect‐based order‐disorder as the phase transformation mechanism. The phase transformation (Curie) temperature is predicted to decrease linearly with deviation from stoichiometry in agreement with experimental observations.
Preparation and Infrared Characterization of Thioborate Compounds and Polycrystals
Authors: Martin Steve W. , Bloyer Donald R.
Published: March 2005

Polycrystals and compounds in the series xNa2S + (1 −x)B2S3 have been prepared for the first time in the range 0 x 0.75. Infrared spectroscopy has been used to examine the extent to which the structures of the thioborate systems are similar or dissimilar to their oxide analogues. The complexity of the IR spectra systematically increases and then decreases as the composition is changed away from and then up to another compound. This behavior is used to propose the existence of a new compound in the series, 2Na2S · B2S3, sodium dithioborate. The other known compounds in this series, Na2S · B2S3 and Na2S · 3B2S3, were isolated in this same way. the IR spectra do not suggest the compound Na2S · 2B2S3 forms. Evidence is found, however, for the formation of tetrahedral borons, isolated thioboroxyl rings, and orthoborate triangles. These results are used to conclude that there are significant similarities between the structures and compounds that form in the alkali thioborate and borate systems.
Copper ⇌ Alkali Ion Exchange of Alkali Aluminosilicate Glasses in Copper‐Containing Molten Salt: II, Divalent Copper Salts, CuCl2 and CuSO4
Authors: Yoko Toshinobu , Nishiwaki Toshio , Kamiya Kanichi , Sakka Sumio
Published: March 2005

The ion‐exchange mechanism between copper and alkali ions, when 20R2O · 10Al2O3· 70SiO2 (R = Li, Na, and K) glasses are immersed in divalent copper‐containing molten salts in air and nitrogen at 550°C, has been investigated. In molten CuCl2, the ion‐exchange behavior in both air and nitrogen was very close to that in molten CuCl in air reported previously. This is explained by assuming that CuCl2 decomposes into CuCl and Cl2 at 550°C and the Cu+ ions thus formed mainly diffuse in glasses to replace alkali ions, where Cl2 acts as an oxidizing agent just like oxygen. In the case of molten CuSO4─2SO4, a small amount of Cu+ which is present in the molten state plays a primary role in the Cu ⇌ R+ ion exchange process, although the contribution of direct Cu2+⇌ 2R+ ion exchange cannot be ignored.