bluuebunny/crossref_metadata_2025 huggingface dataset:
Physical Thermodynamic Model of Alkali and Alkaline‐Earth Fluorozirconate Glass‐Forming Melts
Authors: Grande Tor , Aasland Siv , Julsrud Stein
Published: July 2010

A physical model for the thermodynamics of glass‐forming alkali and alkaline‐earth fluorozirconate melts is presented that takes into consideration the strong negative deviation from ideal behavior that is manifested by deep eutectic wells in the phase diagrams and large exothermic heats of mixing. The model is based on chemical equilibria between anionic complexes with six‐ to eight‐fold coordination of zirconium and a simple Temkin model for the free energy of mixing. Calculated phase diagrams and liquidliquid heats of mixing agree well with the experimental data. The model predicts six‐fold coordination of zirconium in glass‐forming melts, which implies that the fragile nature of fluorozirconate glasses can be explained by a significant increase in the coordination of zirconium when the melt is quenched to the glassy state.
High‐Thermal‐Conductivity Aluminum Nitride Ceramics: The Effect of Thermodynamic, Kinetic, and Microstructural Factors
Authors: Jackson T. Barrett , Virkar Anil V. , More Karren L. , Dinwiddie Ralph B. , Cutler Raymond A.
Published: July 2010

Improvement in the thermal conductivity of aluminum nitride (AlN) can be realized by additives that have a high thermodynamic affinity toward alumina (Al2O3), as is clearly demonstrated in the aluminum nitride‐yttria (AlN‐Y2O3) system. A wide variety of lanthanide dopants are compared at equimolar lanthanide oxide:alumina (Ln2O3: Al2O3, where Ln is a lanthanide element) ratios, with samaria (Sm2O3) and lutetia (Lu2O3) being the dopants that give the highest‐ and lowest‐thermal‐conductivity AlN composites, respectively. The choice of the sintering aid and the dopant level is much more important than the microstructure that evolves during sintering. A contiguous AlN phase provides rapid heat conduction paths, even at short sintering times. AlN contiguity decreases slightly as the annealing times increase in the range of 1–1000 min at 1850°C. However, a substantial increase in thermal conductivity results, because of purification of AlN grains by dissolution‐reprecipitation and bulk diffusion. Removal of grain‐boundary phases, with a concurrent increase in AlN contiguity, occurs at high annealing temperatures or at long times and is a natural consequence of high dihedral angles (poor wetting) in liquidphase‐sintered AlN ceramics.
Synthesis of Barium Hexaferrite by the Oxidation of a Metallic Barium–Iron Precursor
Authors: Ward Greg A. , Sandhage Kenneth H.
Published: July 2010

Tapes of the hard magnetic ferrite, BaFe12O19, were synthesized by the oxidation of metallic barium and iron precursors. Milled mixtures of barium flakes and iron powder (Ba:Fe atomic ratio = 1:12) were sealed within fugitive silver tubes and then compacted and formed into tapes by rolling at room temperature. Oxidation and postoxidation heat treatments were conducted over the temperature range 300°–1250°C (the silver was removed after heat treatment at 900°C). Phase‐pure BaFe12O19 tapes were produced after heat treatments at peak temperatures of is greater than or equal to $1080°C. Such tapes exhibited room‐temperature magnetization values of is greater than or equal to $65.0 emu/g in an applied field of 16.5 kOe (1.32 × 106 A/m). A coercivity of 3.44 kOe (2.75 × 105 A/m) was achieved in a fine‐grained (submicrometer) BaFe12O19 tape that had been annealed for 24 h at a peak temperature of 1080°C.
Effect of Oxide Devitrification on Oxidation Kinetics of SiC
Authors: Ogbuji Linus U. J. T.
Published: July 2010

The effect of oxide crystallinity on the oxidation rate of silicon carbide was investigated. CVD SiC coupons were oxidized in a clean TGA reactor system at 1300°C initially in flowing oxygen, immediately annealed in argon, and then reoxidized, all in situ and without cooling (so as to preserve oxide integrity). The parabolic rate constants determined for the preoxidation regime (of mainly amorphous oxide film) and reoxidation regime (of devitrified oxide) were compared. The oxidation rate decreased by a factor of }30 following full oxide crystallization induced by the anneal.
Growth Rate of Diamond on Polycrystalline 〈110〉 Diamond Substrates from Carbon Disulfide in Hydrogen by Hot‐Filament‐Assisted Chemical Vapor Deposition
Authors: Barber Greg D. , Yarbrough Walter A.
Published: July 2010

The growth rates for the deposition of diamond by hot‐filament‐assisted chemical vapor deposition using carbon disulfide (CS2) in hydrogen and methane (CH4) in hydrogen were investigated on polished, 〈110〉 textured, polycrystalline diamond substrates. The carbon concentration in each system was varied from 1% to 2.5%. The deposition rate increased proportionately with carbon concentration for both systems. The CH4‐H2 system gave higher growth rates with an essentially constant difference in rate between CS2 and CH4. Possible explanations for this are discussed with the most likely either a catalytic effect of sulfur or sulfur‐containing species on gas‐phase homogeneous atomic hydrogen recombination, or the surface chemisorption of relatively less labile sulfur‐containing species at the diamond surface.
Thermal Evolution and Sintering Behavior of a 2:1 Mullite Gel
Authors: Osendi Maria Isabel , Miranzo Pilar
Published: July 2010

The thermal evolution of a mullite gel of composition 2Al2O3·SiO2 has been investigated. The gel crystallized at 1300°C into an alumina‐rich mullite and corundum, instead of single‐phase 2Al2O3·SiO2 mullite. The amount of Al2O3 that dissolved in the mullite structure has been determined in the 1300–1780°C temperature range by measuring the mullite lattice parameters. A maximum limit for the amount of Al2O3 in solid solution has been observed. Densification of the gel powders has been analyzed up to temperatures of 1780°C. The microstructure of dense materials always showed the presence of residual Al2O3 particles.
Particle Size Control of Barium Titanate Prepared from Barium Titanyl Oxalate
Authors: Park Zee Hoon , Shin Hyo Soon , Lee Byung Kyo , Cho Sang Hee
Published: July 2010

Barium titanates formed from decomposition of barium titanyl oxalates (BT‐oxalates) exhibit the same powder state as that of the Bt‐oxalates, so that controlling the particle size and shape of the starting BT‐oxalates is necessary. In the present study, BT‐oxalates were precipitated from aqueous solution under various aging conditions: aging fluids, temperature, and time. The particle size of the spherical BT‐oxalates was controlled at 0.4 μm by aging at 25°C for 3 h, and the diameter of the barium titanate formed was similar to that of the BT‐oxalates.
Hydrothermal Preparation and Morphology Characteristics of Y3Fe5O12
Authors: Cho Yong S. , Burdick Vernon L. , Amarakoon Vasantha R. W.
Published: July 2010

The influences of processing parameters such as mineralizer, temperature, and nonstoichiometry on reaction product and morphology were investigated in the hydrothermal synthesis of yttrium iron garnet, Y3Fe5O12 (YIG). A cubic YIG phase was synthesized successfully under hydrothermal conditions of 200°C and 6 h from yttrium and iron hydroxide coprecipitates obtained by adjusting the pH to 10.5 using NH4OH. The other mineralizer, NaOH, was not effective in forming the garnet phase. Nonstoichiometric compositions utilizing excess Y content tended to increase the yield of the garnet phase, but did not affect the morphology. Different morphology characteristics were observed by changing the synthesis temperature. Isometric YIG particles disappeared at a relatively high temperature of 250°C, resulting in irregular star‐shaped particles of YIG due to the preferential orientation during particle growth.
Electron Spin Resonance Analysis of Lattice Defects in Polycrystalline Aluminum Nitride
Authors: Nakahata Seiji , Sogabe Kouichi , Matsuura Takahiro , Yamakawa Akira
Published: July 2010

Electron spin resonance was used in the present study to detect lattice defects in an aluminum nitride lattice. The ESR spectra were obtained from polycrystalline AlN with various thermal conductivities. Measurement of the AlN g values clearly indicated that the obtained ESR signals arose from electrons trapped by nitrogen vacancies. The ESR study revealed that thermal conductivity increases with an increase in the number of electrons trapped by nitrogen vacancies. The explanation for that phenomenon is that the thermal conductivity of AlN increases with a decreasing concentration of oxygen impurities incorporated into the AlN lattice, and the concentration of nitrogen vacancies changes inversely with the concentration of oxygen impurities.
Cavitational Strain Contribution to Tensile Creep in Vitreous Bonded Ceramics
Authors: Lofaj František , Okada Akira , Kawamoto Hiroshi
Published: July 2010

Analysis of the role of cavitation during uniaxial creep deformation in vitreous bonded ceramics reveals that the cavity volume contributes only to the strain in the direction parallel to the tensile stress regardless of the shape and orientation of cavities. Creep asymmetry results from the fact that cavitation preferentially contributes to axial tensile strain while the strain observed under the same conditions in compression is produced only by volume‐conserving mechanisms. The contribution of cavitational strain in the axial tensile strain is equal to the volume fraction of cavities and proportional to the difference between tensile and compressive strains in the axial direction. The density change method and a newly proposed method based on the difference in the axial strains were used for separating the cavitational from the true tensile strain in self‐reinforced silicon nitride. Both methods consistently revealed more than 90% contribution of cavitation to the total tensile strain. Cavitation is concluded to be the dominant mechanism of tensile creep deformation in vitreous bonded ceramics because the reported volume fractions of cavities during their deformation are usually in the range of 70–90% of tensile strain.
Edge Stresses in Alumina/Zirconia Laminates
Authors: Sergo Valter , Lipkin Don M. , Portu Goffredo De , Clarke David R.
Published: July 2010

Using the technique of fluorescence piezospectroscopy, we determine the distribution of thermal residual stresses across the edges of three laminated alumina/zirconia composites. We develop a methodology for separating the measured stress state into microstresses that result from grain‐to‐grain thermal mismatch and macrostresses that result from lamination‐induced thermal mismatch between individual plies. Comparison between the measured edge‐stress distributions and those calculated based on a simple force‐superposition model shows good agreement, indicating that the laminate system is well approximated as linear elastic. Given the experimental confirmation of significant edge stresses in multi‐ply laminates, the possibility of failure initiating at composite edges must be considered in the design of surface‐compressed laminate structures with the aim of mediating the detrimental effect of surface flaws.
Effect of Shear Stress on the Atomistic Structure of a Grain Boundary in Strontium Titanate
Authors: Kienzle Oliver , Ernst Frank
Published: July 2010

We have studied the atomistic structure of a σ3 (111) grain boundary in strontium titanate by high‐resolution transmission electron microscopy (HRTEM). Quantitative evaluation of HRTEM images reveals that shear stresses, which may originate from processing or external loading, have an important effect on the translation state of the two adjacent grains and on the ion positions at the grain boundary. While under low shear stress the boundary exhibits mirror symmetry with respect to the boundary plane and a comparatively large free volume, high shear stress transforms the structure to a “lock‐in” configuration, which has no mirror symmetry and a smaller excess volume. These results suggest that internal or external stresses may significantly alter the charge transport properties of SrTiO3.
Dislocation Cores and Hardness Polarity of 4H‐SiC
Authors: Ning Xian‐Jie , Huvey Nicolas , Pirouz Pirouz
Published: July 2010

The hardness of opposite basal faces of 4H‐SiC single crystals has been measured in the temperature range 25°‐1200°3C. A strong hardness anisotropy between the silicon‐terminated (0001) and carbon‐terminated (0001) faces of this polar crystal has been found. Transmission electron microscopy investigation of the dislocations in the plastic zone of the 1200°3C indentations shows that they lie predominantly on the basal planes parallel to the indented face, and the extra‐half planes of the nonscrew dislocations originate from the indented face. It is also found that, when the (0001) Si‐terminated face is indented, the dislocations are either widely dissociated, with the width of the stacking fault ribbon much larger than the equilibrium value, or else they are single leading partials, with the corresponding trailing partials absent. In this case, all the leading partials are found to have a silicon core. On the other hand, the dislocations in the plastic zone of the carbon‐terminated face are in the form of dissociated dislocations, with the width of the associated stacking fault ribbons appreciably less than the equilibrium value. Moreover, the leading partials of éhese dissociated dislocations have a carbon core. The results indicate that the hardness of the polar basal faces of 4H‐SiC at elevated temperatures is partly determined by the nature of the dislocation cores nucleated by the indentation process. It is argued that this is due to the influence of the core on the generation and glide of the leading partial dislocations.
Mechanisms of High‐Temperature Creep of Fully Stabilized Zirconia Single Crystals as a Function of the Yttria Content
Authors: Gómez‐García Diego , Martínez‐Fernández Julián , Domínguez‐Rodríguez Arturo , Castaing Jacques
Published: July 2010

High‐temperature creep behavior of yttria fully stabilized zirconia single crystals with various yttria contents in the interval 9.4 ≤x≤ 21 mol% has been studied. The mechanical data have been collected as a function of both the temperature and the yttria concentration. The creep behavior has been explained in terms of a transition between a dislocation viscous glide and a dislocation climb controlled mechanism, making use of a generalized version of a classical model for alloys proposed by Burton. The yttria concentration dependence of the creep rate has been connected with results on cationic diffusivity, with a possible influence of impurities.
Agglomeration during the Drying of Fine Silica Powders, Part II: The Role of Particle Solubility
Authors: Maskara Alok , Smith Douglas M.
Published: July 2010

The effect of particle solubility and the dissolution rate on agglomeration was studied by drying silica and titania particles from aqueous slurries with pH values in the range of 2–12. The agglomerate strength and strength distribution were measured by a calibrated ultrasonic force, and the strength increased as the solubility and dissolution rate increased. Two silica powders of different particle size (60 nm and 500 nm) were studied, and smaller‐sized particles formed stronger agglomerates. The drying rate of the powders was varied by using spray drying and tray drying, and slower drying was shown to lead to higher agglomerate strength. The agglomerate strength of titania powder (insoluble in water) was independent of pH, whereas the agglomerate strength of silica was dependent on pH. It was concluded that the solubility and dissolution rate are important parameters that govern the strength of agglomerates.
Effect of Ordering‐Induced Domain Boundaries on Low‐Loss Ba(Zn1/3Ta2/3)O3‐BaZrO3 Perovskite Microwave Dielectrics
Authors: Davies Peter K. , Tong Jianzhu , Negas Taki
Published: July 2010

Small substitutions of BaZrO3 into Ba[(Zn,Ni)1/3Ta2/3]O3 are utilized in the commercial preparation of low‐loss perovskite microwave dielectrics. The structures of a series of these phases with substitution levels ranging from 1% to 5% BaZrO3 were examined using high‐resolution TEM. For ≤ 2.15% BaZrO3 the solid solutions retain the ordered “1:2” structure of the Ba[(Zn,Ni)1/3Ta2/3]O3 end‐member but are comprised of small ordered domains whose size decreases as the Zr content is raised. The decrease in the size of the domains parallels a decrease in the processing time required to access a low‐loss state. Although for pure Ba[(Zn,Ni)1/3Ta2/3]O3 reductions in the degree of cation order produce a large increase in the dielectric loss, the Zr‐substituted ceramics retain a very low loss. We believe the low losses of the 1:2 ceramics are derived from the stabilization of the ordering‐induced domain boundaries via the partial segregation of the Zr cations. For substitutions between 3% and 5% BaZrO3 the size of the ordered domains continues to decrease but the system undergoes an abrupt transformation to a cubic “1:1” ordered structure with a doubled perovskite repeat. The structures of these phases have been interpreted using a “random layer” model in which one site is occupied by Ta and the other by a random distribution of Zn, Zr, and the remaining Ta cations, i.e., Ba{[Zn(2‐y)/3Ta(1–2y)/3Zry]1/2[Ta1/2]}O3. Although the ordering is confined to nano‐sized domains, these ceramics also exhibit low losses, again reflecting the relative stability of the domain boundaries. In this case we believe the low losses reflect the effectiveness of the random layer in stabilizing the anti‐phase boundaries.
Aqueous Processing of Sintered Reaction‐Bonded Silicon Nitride: I, Dispersion Properties of Silicon Powder
Authors: Hackley Vincent A. , Paik Ungyu , Kim Bong‐Ho , Malghan Subhas G.
Published: July 2010

An aqueous‐based system (Si‐Al2O3‐Y2O3‐Fe2O3) for processing sintered reaction‐bonded silicon nitride (SRBSN) was investigated with an emphasis on chemical control of suspension component interactions. Chemical stability and dispersion properties of a commercial silicon powder were characterized using electroacoustic, adsorption isotherm, and rheological measurements. The interactions of silicon with nitriding agent, sintering aids, dispersants, and binder were considered. The effects of pH, electrolyte, aging, particle size, and solids loading were examined. The suspension properties of the silicon powder were influenced by the native oxide film and powder treatment history. The silicon—oxide composite particles exhibit dispersion behavior similar to silica, characterized by a negative surface potential above pH 2. A method to improve the dispersion and homogeneity of suspension components based on the use of quaternary amine dispersants is proposed.
Reaction Sequence in the Preparation of NiAl2O4 Spinel—Mullite Composites by Sol—Gel
Authors: Sales Manuela , Valentín Carla , Alarcón Javier
Published: July 2010

NiAl2O4 spinel—mullite composites were prepared by simultaneous replacement of Al by Ti and/or Ni in 3:2 stoichiometric mullite. Specimens having nominal compositions 3(Al2‐2xNixTixO3) · 2SiO2 (x= 0, 0.025, 0.05, 0.2) and 3(Al2‐xMxO3) · 2SiO2 (M = Ni2+ or Ti4+ and x= 0.05) were synthesized by sol—gel techniques, which provide homogeneous gels in the SiO2‐Al2O3 system. Gel structures investigated by infrared (IR) spectroscopy revealed the formation of Al‐O‐Si bonds in dried gels. The reaction sequence of gel‐derived glasses, previously obtained by preheating gels at 750°3C for 3 h, was evaluated by X‐ray powder diffraction (XRD) and ultraviolet—visible (UV—vis) spectroscopy. All samples crystallized at around 1000°3C from an amorphous state, but unexpectedly the first crystalline phase was Al‐Si spinel in all aluminum‐substituted specimens; i.e., a change in the sequence of reaction with respect to the 3:2 stoichiometric mullite was produced. NiAl2O4 spinel was almost simultaneously detected. Two processes of mullite crystallization were observed. The temperature of formation of mullite was the lowest for the higher substituted sample. The microstructure of the final NiAl2O4 spinel—mullite composites found in all Ni‐containing samples after annealing for 96 h was examined by scanning electron microscopy with energy dispersive X‐ray analysis (SEM/EDX), which revealed the presence of small particles of NiAl2O4 spinel dispersed in a mullite matrix. For annealed compositions with the larger Al replacement, i.e., when x= 0.2, a small amount of very small Al2TiO5 particles was also detected by XRD and transmission electron microscopy (TEM).
Effect of Porosity and Grain Size on the Microwave Dielectric Properties of Sintered Alumina
Authors: Penn Stuart J. , Alford Neil McN. , Templeton Alan , Wang Xiaoru , Xu Meishing , Reece Michael , Schrapel Kevin
Published: July 2010

The real part of the permittivity (ε) and the tan δ of sintered alumina (Al2O3) at about 9 GHz have been measured. The dielectric properties have been examined as a function of purity, pore volume, and sintered grain size. The tan δ is found to depend very strongly on the pore volume, purity, and grain size. ε is far less sensitive to impurities and grain size. The dependence of ε on porosity can be described by simple mixture models as expected. A model of losses in single crystals cannot be extended easily to these materials where extrinsic factors such as porosity, random crystal orientation, grain boundaries, microcracks, and impurities dominate. These factors have been studied in an attempt to describe the tan δ and ε of sintered polycrystalline alumina. In this work, the tan δ for alumina has been studied in near‐theoretical density ranges between 9.1 times 10‐5 and 2.4 times 10‐5 depending on grain size.
High‐Temperature Automotive Catalytic Nitrogen Oxide Reduction over Copper–Lanthanum–Alumina
Authors: Ozawa Masakuni , Suzuki Suguru , Toda Hajime
Published: July 2010

The synthesis and nitrogen oxide (NO) removal activities of composite powders in the copper‐lanthanum‐alumina(Cu‐La‐Al2O3)system were studied, in regard to their high‐temperature application as an automotive lean‐burn exhaust catalyst. The solid‐state reaction between the copper, the lanthanum, and the Al2O3support, at elevated temperature, was examined by X‐ray diffractometry and electron‐spin resonance. Lanthanum impregnation into the Al2O3support successfully helped to form active composite powders that could remove nitrogen oxides and consisted of gamma‐Al2O3and LaAlO3, after heat treatment at 900°C, and mainly CuLaAl11O19 at 1000°C. Cu2+cations were observed to be isolated in the present powders, even after heat treatment up to 1000°C. The evaluation of NO removal activity over lanthanum‐modified copper‐Al2O3 catalysts that were subjected to harsh heat treatment was performed using a model exhaust‐gas mixture of NO, carbon monoxide (CO), propene (C3H6), carbon dioxide (CO2), water vapor (H2O), and an excess of oxygen (O2). The catalyst that was composed of 20 mol% copper, 10 mol% lanthanum, and Al2O3 and heat treated at 900° and 1000°C in air showed NO removal conversion efficiencies of 14% and 8%, respectively, under the lean‐burn conditions of an air:fuel (A/F) ratio of 18 and a space velocity (SV) of 100000 h‐1. Lanthanum‐modified copper‐Al2O3 powders are potentially useful as a NO removal catalyst, when applied to an automotive lean‐burn exhaust.
Formation and Stability Regions of the High‐Temperature Fluorite‐Related Phase in the R2O3‐Ta2O5 System (R = La, Nd, Sm, Ho, Er, and Yb)
Authors: Yokogawa Yoshiyuki , Yoshimura Masahiro
Published: July 2010

Solid‐state reaction and annealing of melts of samples in the R2O3‐Ta2O5system (R is a rare‐earth element, i.e., La, Nd, Sm, Ho, Er, and Yb) revealed a defect‐fluorite phase (F‐phase) at high temperatures. The formation region of this F‐phase was in the region of ˜70‐80 mol% R2O3for small rare‐earth ions, such as erbium and ytterbium, but only in the region of ˜80 mol% R2O3at temperatures of >1800°C for large rare‐earth ions, such as the lanthanum‐through‐samarium series. This F‐phase exhibited disordered cation and anion sublattices, such as (R0.8Ta0.2)(O1.7box0.3). The F‐phase decomposed to R2O3 and an ordered phase–R3TaO7 (orthorhombic weberite)–through a eutectic reaction at temperatures in the range of 1500°‐1700°C for gadolinium or larger rare‐earth ions, whereas the F‐phase was stable at 1500°C for the small rare‐earth ions (the erbium‐through‐ytterbium series).
Atomistic Structure of Calcium Silicate Intergranular Films in Alumina Studied by Molecular Dynamics Simulations
Authors: Blonski Slawomir , Garofalini Stephen H.
Published: July 2010

Molecular dynamics simulations of calcium silicate (CaSiO3)intergranular films that were formed during the liquid‐phase sintering of alumina (Al2O3)ceramics were conducted. A constant‐pressure algorithm was used in the simulations to accommodate changes in the sample size during heat treatment and tensile tests. A model of the grain boundary that was wetted by glass was created by melting the silicate film between two Al2O3surfaces with the basal orientation. Samples with different film thicknesses and CaO contents were studied. The presence of an ordered interface in the atomistic structure of the mostly amorphous films was revealed. Calcium additives segregated preferentially into the ordered SiO2/Al2O3interface regions. Increased addition of calcium further promoted the ordering and increased stability of the films. Tensile strength was evaluated and showed an increase with low calcium additions, followed by strength reduction at higher CaO additions. Two modes of fracture were observed in the simulations.
Effect of Yttrium and Lanthanum on the Final‐Stage Sintering Behavior of Ultrahigh‐Purity Alumina
Authors: Fang Jianxin , Thompson A. Mark , Harmer Martin P. , Chan Helen M.
Published: July 2010

Final‐stage sintering has been investigated in ultrahigh‐purity Al2O3 and Al2O3that has been doped individually with 1000 ppm of yttrium and 1000 ppm of lanthanum. In the undoped and doped materials, the dominant densification mechanism is consistent with grain‐boundary diffusion. Doping with yttrium and lanthanum decreases the densification rate by a factor of ˜11 and 21, respectively. It is postulated that these large rare‐earth cations, which segregate strongly to the grain boundaries in Al2O3, block the diffusion of ions along grain boundaries, leading to reduced grain‐boundary diffusivity and decreased densification rate. In addition, doping with yttrium and lanthanum decreases grain growth during sintering. In the undoped Al2O3, surface‐diffusion‐controlled pore drag governs grain growth; in the doped materials, no grain‐growth mechanism could be unambiguously identified. Overall, yttrium and lanthanum decreases the coarsening rate, relative to the densification rate, and, hence, shifted the grain‐size‐density trajectory to higher density for a given grain size. It is believed that the effect of the additives is linked strongly to their segregation to the Al2O3grain boundaries.
Phase Relations in the Aluminum Carbide–Aluminum Nitride–Aluminum Oxide System
Authors: Qiu Caian , Metselaar Rudi
Published: July 2010

The phase diagram of the pseudobinary Al4C3‐AlN system was predicted using thermodynamic models. It was combined with previous thermodynamic descriptions of the pseudobinary Al4C3‐Al2O3 and AlN‐Al2O3systems, and then thermodynamic properties of the pseudoternary Al4C3‐AlN‐Al2O3system were assessed by modeling the Gibbs energy of the various phases. An ionic‐liquid model was applied to the liquid phase and a compound‐energy model was applied to the solid‐solution phase (2H) that formed between Al2OC and AlN. A series of isothermal sections of the system were calculated in the temperature range of 1000°‐2100°C, and reasonable phase relations were established. The calculated isothermal section at 1600°C showed satisfactory agreement with experimental results. The calculated liquidus projection in the Al2O3‐rich corner can explain the experimental observation of the microstructure after solidification very well. However, the present description cannot account for the phase segregation inside the solution phase but could be modified in the future, when more experimental information would be available.
Predicted Effects of Interfacial Roughness on the Behavior of Selected Ceramic Composites
Authors: Parthasarathy Triplicane A. , Kerans Ronald J.
Published: July 2010

Potential effects of interfacial roughness in ceramic composites were studied using a model that included the progressively increasing contribution of roughness with relative fiber/matrix displacement during debonding of the fiber/matrix interface. A parametric approach was used to study interfacial roughness in conjunction with other parameters such as the strength, radius, and volume fraction of the fiber. The progressive roughness contribution during initial fiber/matrix sliding caused a high effective coefficient of friction, as well as an increased clamping stress, which led to rapidly changing friction with increasing debond length. Calculated effects implied a potentially significant contribution to the behavior of real composite systems and the necessity for explicit consideration in the interpretation of experimental data to understand composite behavior correctly. In a tension test, the Poisson's contraction of the fiber may negate the effects of roughness, allowing an “effective constant shear stress” (tau) approximation. This was evaluated using a piecewise linear approximation to the progressive roughness model in an analysis of composite stress‐strain behavior; for the Nicalon/SiC system, the effective tau value was lower than the values that would be obtained from fiber pushout tests and/or matrix crack spacings.
Micromechanics of Failure Waves in Glass: I, Experiments
Authors: Espinosa Horacio D. , Xu Yueping , Brar Natchater S.
Published: July 2010

Plate and bar impact experiments were performed on glass to investigate the mechanisms responsible for recently observed failure waves. In the present work we report observations showing that the so‐called failure wave is actually a propagating boundary of damaged material. It initiates at the specimen surface, without appreciable incubation time, and propagates to its interior. In‐material gauge measurements show that the spall strength and shear resistance of the material are drastically reduced behind the failure wave front. The shear resistance interferometrically measured in pressure‐shear experiments is in agreement with the in‐material gauge records. Normal stress measurements performed close to the impact surface show a progressive reduction in normal stress behind the failure wave. This feature suggests that the inelastic process responsible for the reduction in shear strength has well‐defined kinetics. Recovery experiments performed on confined soda‐lime glass rods reveal extensive material fragmentation. Microscopy studies performed on fracture surfaces show features commonly seen in glass fractured under tensile loading. Post‐test X‐ray experiments reveal that the material retains its amorphous structure. Two interpretations of the failure process, consistent with the above features, are discussed. The first mechanism is the initiation of microcracks, at the glass surfaces being subjected to compressive tractions, which propagate along surfaces of maximum shear to its interior. Another mechanism, based on the inhomogeneous nature of plastic flow in amorphous materials, is the initiation of shear‐induced flow surfaces at the impact plane which are punched into the bulk of the material. In the latest, microcracks nucleated at the intersection of these shear flow surfaces are believed to be responsible for the dramatic reduction in spall strength behind the failure wave. The experimental observations herein presented are especially suitable for the formulation and examination of constitutive models describing damage evolution behind the so‐called failure waves.
Micromechanics of Failure Waves in Glass: II, Modeling
Authors: Horacio D. Espinosa , Yueping Xu , Brar Natchater S.
Published: July 2010

In an attempt to elucidate the failure mechanism responsible for the so‐called failure waves in glass, numerical simulations of plate and rod impact experiments, with a multiple‐plane model, have been performed. These simulations show that the failure wave phenomenon can be modeled by the nucleation and growth of penny‐shaped shear defects from the specimen surface to its interior. Lateral stress increase, reduction of spall strength, and progressive attenuation of axial stress behind the failure front are properly predicted by the multiple‐plane model. Numerical simulations of high‐strain‐rate pressure‐shear experiments indicate that the model predicts reasonably well the shear resistance of the material at strain rates as high as 1 × 106/s. The agreement is believed to be the result of the model capability in simulating damage‐induced anisotropy. By examining the kinetics of the failure process in plate experiments, we show that the progressive glass spallation in the vicinity of the failure front and the rate of increase in lateral stress are more consistent with a representation of inelasticity based on shear‐activated flow surfaces, inhomogeneous flow, and microcracking, rather than pure microcracking. In the former mechanism, microcracks are likely formed at a later time at the intersection of flow surfaces. in the case of rod‐on‐rod impact, stress and radial velocity histories predicted by the microcracking model are in agreement with the experimental measurements. Stress attenuation, pulse duration, and release structure are properly simulated. It is shown that failure wave speeds in excess to 3600 m/s are required for adequate prediction in rod radial expansion.
Electrical‐Impulse‐Induced Fracture of Zinc Oxide Varistor Ceramics
Authors: Vojta Agnes , Clarke David R.
Published: July 2010

When varistors are subject to very short (˜1 μ s), high‐current pulses, they occasionally fracture. We attribute the occurrence of fracture to stress waves, created by inertial forces in response to the rapid Joule heating, causing the propagation of preexisting microstructural flaws. The amplitude of the stress oscillation is shown to be proportional to the second derivative of temperature with respect to time which, in turn, is directly related to the rate of increase of power. An upper limit for the magnitude of the peak stress is derived in terms of material parameters, varistor geometry, and the maximum electrical power. From the magnitude of the peak stress, a critical defect size is estimated which will cause fracture under given pulse conditions. Conclusions for the design and testing of varistors are drawn.
Evidence for a Phase Transition of β‐Gallium Oxide at Very Low Oxygen Pressures
Authors: Fleischer Maximilian , Höllbauer Leo , Born Eberhard , Meixner Hans
Published: July 2010

Depending on the operating temperature, gas sensors that are based on n‐type‐semiconductor, polycrystalline gallium oxide (Ga2O3) thin films are used to detect oxygen (at temperatures, T, of 850°C) or reducing gases (T 900°C). At high temperatures (T 900°C), beta‐Ga2O3has an oxygen deficiency in the crystal lattice that is in dynamic equilibrium with the oxygen in the surrounding atmosphere. Variations in the conductivity of the sensor are caused by variations of the concentration of ionized oxygen vacancies. Therefore, a reduction in the proportion of oxygen or an increase in the concentration of reducing gases in the atmosphere in which the sensor is located leads to an increasing number of conducting electrons and, hence, an increasing conductivity. During a research project to investigate the long‐term stability of thin beta‐Ga2O3 films in a variety of strongly reducing atmospheres at T > 600°C, a previously unknown phenomenon has been observed when measurements on low oxygen partial pressures (pO210‐10 Pa (10‐15bar)) have been made. A sharp decrease in sensor conductivity, by several orders of magnitude, is observed each time when pO2 is reduced to a value of <10‐15bar at temperatures in the range of ˜750°‐1000°C. The reason for this may be a phase transition in the β‐Ga2O3 layer. However, attempts to freeze the new state with subsequent identification by X‐ray diffractometry have not succeeded in identifying the new phase.
Influence of Phosphorus, Barium, and Lead Compositional Variation on the Chemical Durability of a Multicomponent Glass System
Authors: Luo Wen , Buechele Andrew C. , Fu Sabrina S.
Published: July 2010

Glasses were prepared by systematic additions of P2O5, BaO, and PbO to glasses consisting of Al2O3‐B2O3‐(BaO)‐CaO‐Fe2O3‐K2O‐MgO‐Na2O‐(PbO)‐(P2O5)‐SiO2, and their chemical durability was examined by the EPA TCLP test. The addition of about 1 mol% P2O5 to a glass containing 5 mol% PbO increases the normalized lead leach rate (NLLR) by about 40 relative%, but increasing P2O5 to 2 mol% decreases the NLLR to about the NLLR obtained without P2O5. A similar, but more dramatic, behavior with changes in the NLLR of about 300 relative% is obtained by sequential combined additions of 1 mol% each of P2O5 and BaO to a glass containing 1 mol% PbO.
Fabrication and Mechanical Properties of Continuously Graded MoSi2–ZrO2(2Y) Materials Using Wet‐Molding
Authors: Arata Kenji , Takeuchi Nobutaka , Yoshinaka Masaru , Hirota Ken , Yamaguchi Osamu
Published: July 2010

Continuously graded MoSi2‐ZrO2(2Y) materials with high density (97.5% of theoretical) have been fabricated by uniaxial wet‐molding, followed by hot pressing (1000°C/1 h/30 MPa) and hot isostatic pressing (1400°C/2 h/196 MPa). Their composition profiles are greatly influenced by the viscosity of mixed solutions of glycerin and ethanol used as a dispersion medium; a linear compositional gradient from MoSi2/ZrO2(2Y) 70/30 to 20/80 mol% is obtained from the solution (50/50 vol%) with a viscosity of 20 mPa s. Vickers hardness (Hv) and fracture toughness (KIC) increase from 9.7 to 12.4 GPa and from 5.1 to 12.5 MPa m1/2, respectively, with increasing ZrO2(2Y) composition.
Structure Evolution of Highly Crystallized BaWO4 Film Prepared by an Electrochemical Method at Room Temperature
Authors: Cho Woo‐Seok , Yoshimura Masahiro
Published: July 2010

Highly crystallized BaWO4 films have been prepared on a tungsten substrate in an alkaline solution containing barium ions by an electrochemical method with a constant direct current density of 1 mA/cm2 at room temperature (25°C). The average grain size was about 13 μm, and the thickness about 9 μm after a treatment time of 35 min. The dependence of cell voltage on deposition time was divided into three steps: conduction, anodic oxidation, and breakdown steps. The BaWO4 film formed during the first step. Electrochemical dissolution of metal tungsten occurred with an accompanying positive change of overpotential in the first step. The crystallization of BaWO4 was characterized by three‐dimensional nucleation. In the second step, an amorphous tungsten oxide film formed, thereby increasing the potential. An electrical breakdown occurred in the third step, and the breakdown voltage (about 90 V) was practically the same as those of anodic tungsten oxide films.
Mechanical Damping Arising from Dislocation Motion in Sapphire and Ruby Crystals
Authors: Pezzotti Giuseppe , Ota Ken'ichi
Published: July 2010

Internal friction was measured up to the melting point (i.e., Tm≈ 2050°C) in sapphire and two ruby single crystals (chromium content of 0.1 and 0.5 wt%), as a function of strain amplitude, temperature, and composition. The frequency range of the measurements was 6.7–17 Hz. Static (torsional creep) experiments also were performed up to very high temperatures, to provide further phenomenological insight about the mechanism of crystal deformation. Results of dynamic and static experiments were analyzed and discussed in terms of dislocation motion along the basal (0001) plane, perpendicular to which the torsion axes of the crystals were oriented. Particular emphasis was placed on isolating the role of chromium concentration, in regard to impeding dislocation motion and multiplication. An equivalent spring‐dashpot model, which attempts to represent the damping results quantitatively, also has been given.
High‐Temperature Raman Spectra and Thermal Expansion of Wollastonite
Authors: Swamy Varghese , Dubrovinsky Leonid S. , Tutti Faramarz
Published: July 2010

Unpolarized single‐crystal Raman spectra in the temperature interval of 298–1235 K and powder X‐ray diffractometry data from room temperature up to 1295 K have been obtained from a natural triclinic wollastonite of almost‐ideal composition. Room‐temperature spectra, along with the temperature dependencies of nine major Raman modes, are presented. The temperature shifts of the Raman modes indicate a discontinuity in the temperature range of 950–980 K. No corresponding changes are observed in the temperature dependencies of the lattice parameters. Thermal expansion coefficients for the lattice parameters in the temperature range of 298–1295 K are given.
Microstructure Refinement of Sintered Alumina by a Two‐Step Sintering Technique
Authors: Lin Frank J. T. , de Jonghe Lutgard C. , Rahaman Mohamed N.
Published: July 2010

For a few oxide ceramics, the use of an initial precoarsening step prior to densification (referred to as two‐step sintering) has been observed to produce an improvement in the microstructural homogeneity during subsequent sintering. In the present work, the effect of a precoarsening step (50 h at 800°C) on the subsequent densification and microstructural evolution of high‐quality alumina (Al2O3) powder compacts during constant‐heating‐rate sintering (4°C/min to 1450°C) was characterized in detail. The data were compared with those for similar compacts that were sintered conventionally (without the heat treatment step) and used to explore the mechanism of microstructural improvement during two‐step sintering. After the precoarsening step, the average pore size was larger, but the distribution in pore sizes was narrower, than those for similar compacts that were sintered conventionally to 800°C. In subsequent sintering, the microstructure of the precoarsened compact evolved in a more homogeneous manner and, at the same density, the amount of closed porosity was lower for the compacts that were sintered by the two‐step technique, in comparison to the conventional heating schedule. Furthermore, a measurably higher final density, a smaller average grain size, and a narrower distribution in grain sizes were achieved with the two‐step technique. The microstructural refinement that was produced by the two‐step sintering technique is explained in terms of a reduction in the effects of differential densification and the resulting delay of the pore channel pinch‐off to higher density.
Grain‐Boundary Defect Chemistry of Acceptor‐Doped Titanates: Inversion Layer and Low‐Field Conduction
Authors: Vollmann Markus , Hagenbeck Rainer , Waser Rainer
Published: July 2010

The electronic and ionic conduction across grain‐boundary (GB) space‐charge depletion layers in acceptor‐doped SrTiO3 ceramics have been investigated by admittance analysis in the time domain. The dependence on the acceptor concentration, the oxygen partial pressure during equilibration, and the temperature dependence of the GB conductivity are discussed and interpreted in terms of a defect‐chemistry model of the GB region using a numerical simulation technique.
Hardening in Creep of Alumina by Zirconium Segregation at the Grain Boundary
Authors: Wakai Fumihiro , Nagano Takayuki , Iga Takeo
Published: July 2010

The effect of zirconium segregation on hardening in the creep of fine‐grained alumina was studied by using the tensile creep test. To avoid the effect of zirconia particle dispersion on creep, 100‐ppm‐zirconium‐doped alumina and 1000‐ppm‐zirconium‐doped alumina were fabricated by using a zirconium‐containing precursor. The scanning transmission electron microscopy/energy‐dispersive X‐ray spectroscopy study revealed that the zirconium was segregated at the alumina grain boundary. Doping even as little as 100 ppm of zirconium caused the hardening effect. The creep rate was further reduced by increasing the amount of zirconium dopant. Although the stress exponent of 2 was not affected by zirconium segregation, the apparent activation energy of the creep was found to be increased, from 520 kJ/mol for undoped alumina to 670 kJ/mol for 100‐ppm‐zirconium‐doped alumina and 760 kJ/mol for 1000‐ppm‐zirconium‐doped alumina. It was suggested that grain‐boundary sliding was accommodated by impurity‐drag‐controlled diffusional creep.
Role of Microstructure in Hertzian Contact Damage in Silicon Nitride: I, Mechanical Characterization
Authors: Lee Seung Kun , Wuttiphan Sataporn , Lawn Brian R.
Published: July 2010

In this Part I of a two‐part study of Hertzian indentation in silicon nitride we characterize irreversible contact damage as a function of microstructure. Three controlled silicon nitride microstructures are examined, representing a progression toward greater long‐crack toughness: fine (F), bimodal with predominantly equiaxed α grains; medium (M), bimodal with mostly β grains of intermediate size; and coarse (C), with almost exclusively elongated β grains. An effect of increasing the microstructural heterogeneity in this sequence is to suppress ring cracking around the indenter, ultimately to a degree beyond that expected from increased toughness alone. Along with the crack suppression is a parallel tendency to enhanced damage accumulation beneath the indenter, such that the contact in the coarsest microstructure is predominantly quasi‐plastic. The characterization of damage includes the following: determination of indentation stress‐strain curves, to measure the level of quasi‐plasticity; measurement of threshold loads for the initiation of ring cracking and subsurface yield, to quantify the competing damage processes; and measurement of characteristic dimensions of the ensuing cracks and deformation zones in their well‐developed stages. These quantitative results are considered in terms of formal contact mechanics, along with finite element modeling to generate the essentially elastic‐plastic fields in the different silicon nitride structures. This contact mechanics description serves also as the basis for subsequent analysis of strength degradation in Part II. Implications concerning microstructural design of silicon nitride ceramics for specific applications, notably bearings, are considered.
Nucleation and Growth Characteristics of Cavities during the Early Stages of Tensile Creep Deformation in a Superplastic Zirconia–20 wt% Alumina Composite
Authors: Owen David M. , Chokshi Atul H. , Nutt Steven R.
Published: July 2010

Constant‐stress tensile creep experiments on a superplastic 3‐mol%‐yttria‐stabilized tetragonal zirconia composite with 20 wt% alumina revealed that cavities nucleate relatively early during tensile deformation. The number of cavities nucleated increases with increasing imposed stress. The cavities nucleate at triple points associated largely with an alumina grain, and then grow rapidly in a cracklike manner to attain dimensions on the order of the grain facet size. It is suggested that coarser‐grained superplastic ceramics exhibit lower ductility due to the ease in formation of such grain boundary facet‐cracks and their interlinkage to form a macroscopic crack of critical dimensions.
Alkoxy‐Derived KTiOPO4 (KTP) Fibers
Authors: Noda Ken‐ichi , Sakamoto Wataru , Yogo Toshinobu , Hirano Shin‐ichi
Published: July 2010

Crack‐free and dense KTiOPO4 (KTP) fibers could be successfully synthesized through metallo‐organics. A homogeneous, drawable KTP precursor solution was prepared from the reaction of (nBuO)2P(O)(OH), Ti(OEt)4, and KOEt. KTP precursor fibers crystallized to a single‐phase KTP at 650°C for 0.5 h. KTP fibers were a few centimeters in length and 10–150 μm in diameter. The density of the KTP fibers was higher than 2.97 g/cm3 (>98% of theoretical density).