bluuebunny/crossref_metadata_2025 huggingface dataset:
Properties and Structure of Lead Fluorosilicate Glasses
Authors: COON JEFFREY , SHELBY JAMES E.
Published: March 2005

An extensive glass‐forming region has been determined for the lead fluorosilicate system. These glasses have a very high anionic (fluorine) conductivity. The value of 2 x 10‐4ω‐1.cm‐1 at slightly less than 200°C found for one glass is the highest anionic conductivity reported to date in a glass. Glass transition temperatures, thermal expansion coefficients, and UV‐visible spectra were also measured. Results of these measurements and melt‐history studies will be discussed and related to a proposed structural model for the lead fluorosilicate glasses.
Segregation of Magnesium and Calcium to the () Prismatic Surface of Magnesium‐Implanted Sapphire
Authors: MUKHOPADHYAY S.M. , JARDINE A.P. , BLAKELY J.M. , BAIK S.
Published: March 2005

Auger electron spectroscopy was used to study the segregation of magnesium and calcium to the prismatic plane of sapphire doped with Mg by ion implantation. Segregation behavior depended strongly upon the annealing atmosphere. Air annealing showed Mg segregation to the free surface whereas vacuum annealing resulted in no observable Mg cegregation above the detectability limit of our instrument. This is attributed to excessive vaporization of MgO at low oxygen pressures. In the absence of Mg on the surface, strong Ca segregation was observed although the bulk concentration of Ca was below 40 ppm, the total impurity level. The effective heat of segregation of Mg in air was found to be about ‐1.4 eV in the temperature range 1300° to 1550°C. The Ca coverage on the surface did not follow a Langmuir plot, probably because of Ca–Ca interaction at the larger surface concentrations. The implication of these results to the understanding of transport‐related properties of impurity‐doped alumina is discussed.
Computer Simulation of Failure in an Elastic Model with Randomly Distributed Defects
Authors: SROLOVITZ DAVID J. , BEALE PAUL D.
Published: March 2005

We analyze a simple model of failure in an elastic material with randomly distributed defects. The model that we employ is a two‐dimensional triangular lattice of springs. These springs are linearly elastic up to some small strain and irreversibly break at larger strain. Defects are introduced into the model by breaking a certain fraction of the springs before straining. During the simulation, a uniaxial strain is applied, the equilibrium configuration of the lattice is determined, and then the strain is incremented again. The failure stress is found to decrease as the fraction of springs removed is increased and decreases logarithmically with the linear dimension of the sample, L. The cumulative failure stress distribution is well fitted by the form F(σ) = 1 ‐ exp[‐cL‐2 exp(‐k/σμ)], where σ is the failure stress, σ is a constant between 1 and 2, and c and k are constants which are characteristic of the microscopic properties of the system.
Inclusion‐Size‐Independent Strength of Glass/Particulate‐Metal Composites
Authors: Troczynski Thomas B. , Nicholson Patrick S. , Rucker Carmen E.
Published: March 2005

Dispersed‐phase metallic inclusions limit brittie‐matrix defect size and contribute to toughness via plastic deformation. In this work, it is shown that composite strength depends on the dispersed‐phase volume fraction and not on the diameter of uniformly sized inclusions if the plastic deformation thereof is fully utilized. Experimental results for silicate glass/aluminum‐particle systems substantiate this finding. Particulate composites of 40 vol% Al and 40 vol% Al‐alloy soda–lime–silica glass exhibit 1.5‐ and 2‐fold strength increases, respectively. Both 20 vol% Al composites with 57‐ and 20‐μm particle size showed 1‐ to 3‐fold increases in strength, respectively. Model calculations of strength are overestimates and it is proposed that particle/matrix interfacial separation limits full utilization of the potential plastic work of deformation.
Calcium Segregation to Grain Boundaries in Alumina
Authors: COOK ROBERT F. , SCHROTT A. G.
Published: March 2005

The fracture surfaces of a series of CaO‐doped polycrystalline aluminas are examined by Auger electron spectroscopy (AES) and scanning electron microscopy (SEM). The AES results show a maximum in the detected calcium over the range of materials, whereas the SEM observations show a monotonic increase in the proportion of transgranular failure over the same range. AES measurements from as‐fractured and sputtered surfaces suggest that Ca is substituting for Al at the grain boundaries. By combining the AES and SEM measurements and compensating for the proportion of transgranular failure exposed on the fracture surfaces, the grain‐boundary concentrations of calcium are determined and are shown to be very different from the apparent concentrations suggested by AES measurements alone. The proportion of transgranular failure increases rapidly with grain‐boundary calcium concentration, although this concentration changes only slowly with changes in CaO‐dopant concentration and heat treatment. The driving force for segregation is concluded to be dominated by the misfit strain of the calcium ions substitutionally incorporated into the alumina lattice.
Grain‐Growth Transition During Sintering of Colloidally Prepared Alumina Powder Compacts
Authors: CAMERON CRAIG P. , RAJ RISHI
Published: March 2005

When the grain size in partially sintered compacts of alumina was measured as a function of density, we found that the grain‐growth behavior fell into two distinct regions. In the region where the porosity remained interconnected, grain growth was negligible; when the continuous pore network collapsed into isolated pores, grains grew rapidly. The transition in grain‐growth behavior was observed at approximately 90% of theoretical density. A simple phenomenological method for obtaining the transition in grain growth is suggested. It is based on the idea that an abrupt increase in grain size should be accompanied by a significant decrease in the rate of sintering since the sintering rate changes as the third or fourth power of the grain size. The method consists of fitting the sintering data to an exponential function. The transition then appears as a change in the time constant for the exponential. The low rate of grain growth in the region where the pores are interconnected contradicts earlier work in the literature where significant grain growth during intermediate‐stage sintering has been reported. This difference is explained in terms of the homogeneity of packing of our powder compacts, which were prepared by colloidal processing.
Pressure and Temperature Dependencies of the Direct‐Current Resistance of Semiconducting (Ba, Sr)TiO3 and (Ba, Pb)TiO3
Authors: Amin Ahmed , Holmes Michael B.
Published: March 2005

The direct‐current resistance of semiconducting compositions in the (Ba, Sr)TiO3 and (Ba, Pb)TiO3 systems has been measured under hydrostatic pressures from 0 to 35 MPa, and temperatures from 20° to 135°C. The temperature dependence of the hydrostatic piezoresistance coefficients is presented. Results are discussed in terms of barrier layer sensitivity to mechanical stress.
Preparation of Monodispersed Y‐Ba‐Cu‐O Superconductor Particles via Sol—Gel Methods
Authors: Sanjinéls Rosendo , Thampi K. Ravindranathan , Kiwi John
Published: March 2005

The high‐Tc superconducting material YBa2Cu3O7‐x was synthesized using a modified amorphous citrate or oxalate process. Phase‐pure superconducting powders were obtained by firing the citrate precursor to 900°C and the oxalate precursor to 950°C inflowing O2. The preparation is highly reproducible and leads to powders with excellent homogeneity and sinterability. The resulting material was examined by transmission electron microscopy, thermogravimetry, and X‐ray diffraction. Particle sizes for the superconductors obtained varied from 75 to 3300 Å (7.5 to 330 nm).
Infrared Spectroscopic Studies of the Hydrolysis Reaction During Leaching of Heavy‐Metal Fluoride Glasses
Authors: LE TOULLEC MICHEL , SIMMONS CATHERINE J. , SIMMONS JOSEPH H.
Published: March 2005

In this work, the interfacial reactions of heavy‐metal fluoride glasses with water have been investigated by Fourier transform infrared spectroscopy. High dilution and stirring conditions were maintained to prevent any influence of pH drift and reprecipitation. From comparisons of the integrated absorption peak areas for the O‐H stretching and H‐O‐H bending vibrations in the glass with those for molecular water, the contributions of metal hydroxyls formed in the corrosion layer can be separated from the total O‐H band. The comparison between the infrared spectra for leached samples and hydroxides of the glass components show the presence of Zr‐OH and La‐OH groups in ZrF4‐based glasses after leaching and drying and of Th‐OH groups in ThF4 glass, indicating the existence of F.‐/OH‐ ion exchange between the glass surface and water.
High‐Yield Synthesis of B4C/BN Ceramic Materials by Pyrolysis of Polymeric Lewis Base Adducts of Decaborane(14)
Authors: Rees William Smith , Seyferth Dietmar
Published: March 2005

Polymers of type [B10H12˙diamine]x (diamine=H2NCH2CH2NH2, (CH3)2NCH2CH2NH2, (CH3)2NCH2CH2N(CH3)2, etc.) have been found to be useful ceramic precursors. In a stream of argon, their pyrolysis gives B4C/BN; in a stream of ammonia, BN.
Effects of Composition and Environment on the Fracture of Fluoride Glasses
Authors: Freiman Stephen W. , Baker Theresa L.
Published: March 2005

Indentation‐crack‐length procedures were used to determine the critical fracture toughness (KIC) and the sensitivity to environmentally enhanced crack growth in heavy‐metal fluoride glasses of varying chemical composition. The data show that while KIC was more or less invariant with composition, some glasses were more susceptible to subcritical crack growth than others. The results are interpreted in terms of existing crack‐growth models. A technique for predicting KIC from fundamental atomic bonding and crystal‐structure data for these glasses is discussed.
Conductivity Activation Energy Relations in High‐Sodium‐Content Borate and Aluminoborate Glasses
Authors: MARTIN STEVE W.
Published: March 2005

Glasses were prepared having compositions xNa2O + (1 −x)B2O3 and xNa2O + (1 −x)(nB2O3·Al2O3) where n= 4.0 and 6.7, and their conductivities and activation energies were determined up to 70 mol% Na2O. Below 50 mol% Na2O, the conductivity activation energy decreases with Na2O content, which is attributed to a decreasing cation jump distance. Above 50 mol% Na2O, however, the activation energy increases with Na2O content. These measurements are the first to clearly show a conductivity activation energy increasing with alkali oxide content, an effect which is not yet explained by any of the current models of ionic conduction in glass.
Analytical Evaluation of Interfacial Shear Strength for Fiber‐Reinforced Ceramic Composites
Authors: HSUEH CHUN‐HWAY
Published: March 2005

A methodology is described to evaluate the shear strength at the fiber–coating interface. Stress analyses are performed for the following system: a fiber with a uniform coating in its central portion which is subjected to an axial tensile stress through its bare ends. The results show that the axial tensile stress in the coating decreases with the increase in the thickness of the coating and that the interfacial shear stress exhibits the opposite trend. Hence, a critical thickness of the coating can be defined depending on the properties of the material, such that when the coating thickness is less than the critical value, failure of the composites initiates from segmentation of the coating during tensile loading. Conversely, when the coating thickness is greater than the critical value, debonding of the interface occurs. Furthermore, the interfacial shear strength can be related to this critical thickness and the tensile strength of the coating.
Damage‐Enhanced Creep in a Siliconized Silicon Carbide: Mechanics of Deformation
Authors: CHUANG TZE‐JER , WIEDERHORN SHELDON M.
Published: March 2005

Continuum mechanics methods were employed to analyze creep deformation of a grade of siliconized silicon carbide at elevated temperatures. Three loading modes (tension, compression, and bending) are considered in this analysis. In tension, deformation is accompanied by cavitation at stresses in excess of a temperature‐dependent threshold level, resulting in bilinear power‐law creep. In compression, greater applied stresses are required to achieve the same rate of strain, and although bilinear creep behavior is also observed, a single power‐law creep equation was assumed to simplify the mathematical analysis of the flexure problem. Asymmetrical creep in siliconized silicon carbide leads to a number of unique features in flexural creep. At steady state, a threshold bending moment exists below which no damage occurs. The neutral axis shifts from the geometric center toward the compressive side of the specimen by an amount that depends on the level of applied stress. Cavitation zone shapes, which are predicted to develop in a four‐point bend specimen as a function of load, are found to be in qualitative agreement with those obtained experimentally. For transient creep under bending, the time‐dependent neutral axes for stress and strain do not coincide, although they do converge toward a single axis at steady state. Quantitative predictions are given for relaxation of tensile stresses at the outer fiber, reverse loading in the midplane region, and the growth of the damage zone toward the compressive side of the flexural specimen. This load redistribution leads to a prolonged transient stage as compared to its counterpart in uniaxial creep.
Rapid Calcination and Sintering of YBa2Cu3Ox Superconductor Powder Mixture in Inert Atmosphere
Authors: Rha Jong J. , Yoon Kyung J. , Kang Suk‐Joong L. , Yoon Duk N.
Published: March 2005

The rates of forming the superconducting YBa2Cu3Ox phase during the calcination of the Y2O3, BaCO3, and CuO powder mixture at 790° and 850°C are considerably enhanced when an inert atmosphere of N2 or He is used instead of O2. Sintering in an inert atmosphere also produces higher density and larger grain size than in O2. These results are consistent with the possibility of rapid atomic diffusion in tetragonal YBa2Cu3Ox due to either high oxygen vacancy concentration or expanded lattice in an inert atmosphere.
Titanate Ceramics for the Immobilization of Sodium‐Bearing High‐Level Nuclear Waste
Authors: BUYKX WILLIAM J. , HAWKINS KATE , LEVINS DESMOND M. , MITAMURA HISAYOSHI , SMART ROGER St. C. , STEVENS GEOFFRY T. , WATSON KENNETH G. , WEEDON DAVID , WHITE TIMOTHY J.
Published: March 2005

The phase chemistries and microstructures of titanate‐based ceramics containing simulated high‐level nuclear waste with varying sodium contents were compared. Incorporation of relatively low sodium levels (∼0.9 wt% Na2O) resulted in more complex phase assemblages. The principal hosts for sodium were hibonite and freudenbergite, and, when iron and sodium were present in combination, loveringite was also stabilized. During fabrication, oxygen potential was controlled by Ti‐TiO2 or TiH2‐Ti3O5 solid‐state buffers. These metal and hydride oxygen getters behaved similarly, neither disturbing the phase assemblages nor significantly altering the partitioning of waste elements between radiophases. It is believed that the hydrothermal stability of the sodium‐bearing ceramics (containing up to 2.7 wt% Na2O) will be comparable to sodium‐free material, since less durable sodium‐rich phases are encapsulated in a resistant matrix. Extensive formation of glassy phases may cause embrittlement at higher sodium loadings.
Analytical Electron Microscopic Studies of Doped Dicalcium Silicates
Authors: CHAN CHIN‐JONG , KRIVEN WALTRAUD M. , YOUNG J. FRANCIS
Published: March 2005

Dicalcium silicates having CaO/SiO2 molar ratios of 1.8 to 2.2 were sintered at 1450°C for 90 min with or without small quantities of dopants (K2O or Al2O3) and were air quenched. The microstructures of the fired samples were characterized using electron microscopy (SEM and TEM) and associated microanalytical techniques. There was no evidence for the existence of Ca1.8SiO3.8 or Ca2.2SiO4.2. Amorphous grain‐boundary phases were observed between grains and as inclusions within the grains; the amounts decreased as CaO/SiO2 ratios increased. The compositions of the amorphous phases were always rich in dopants and had a CaO/SiO2 ratio close to that of wollastonite. High levels of Al2O3 were observed to enter the β‐Ca2SiO4 grains under lime‐rich conditions (CaO/SiO2= 2.2) up to a saturation level of about 3.0 wt%. Some additional crystalline phases were observed to form depending on stoichiometry and dopant level.
Electrical Properties of Superionic Conducting Glasses in the Pseudobinary System CuI‐Cu2MoO4
Authors: MACHIDA NOBUYA , MINAMI TSUTOMU
Published: March 2005

Cu+‐ion‐conducting glasses were prepared in the pseudo‐binary system CuI‐Cu2MoO4. These glasses have high ionic conductivities at room temperature in the range 10−1 to 10° S·m−1, which are 10 to 20 times greater than those of the Ag+‐ion‐conducting glasses with the same iodide contents in the system AgI‐Ag2MoO4. The conductivities increase with increasing CuI content. The increase in the conductivity can be explained in terms of a decrease in the activation energy for conduction. The electronic conductivity was measured by the Wagner polarization method and indicates that the transport number of Cu+ ions in those glasses is 0.9996 and larger.
Sintering, Microstructure, Hardness, and Fracture Toughness Behavior of Y2O3‐CeO2‐ZrO2
Authors: DUH JENQ‐GONG , DAI HSING‐TAO , CHIOU BI‐SHIOU
Published: March 2005

A wet‐chemical approach is applied to derive fine powders with compositions 11 mol% CeO2‐ZrO2, 1 mol% YO1.5‐10 mol% CeO2‐ZrO2, 12 mol% CeO2‐ZrO2, and 2 mol% YO1.5‐10 mol% CeO2‐ZrO2 by the coprecipitation method. The characteristics of the as‐derived powders are evaluated through thermal analysis and electron microscopy. The sintering behavior of the calcined powders is carried out at 1400° and 1500°C for 1 to 10 h. Sintered density higher than 98% of theoretical is achieved for sintering at 1400°C for several hours. The as‐sintered density dependence on the sintering condition is related to the extent of tetragonal‐to‐monoclinic phase transformation as well as the associated microcracks. Partial substitution by Y2O3 in CeO2‐ZrO2 results in reduced grain size and tends to stabilize the tetragonal structure. Y2O3 is more effective than CeO2 with respect to the grain size refinement and tetragonal stability. In addition, Y2O3 substitution in CeO2‐ZrO2 increases the hardness, while it decreases the fracture toughness.
Stability of Phases in the Si‐C‐N‐O System
Authors: WADA HARUE , WANG MING‐JONG , TIEN TSENG‐YING
Published: March 2005

The stability of the phases in equilibrium is calculated for the Si‐C‐N‐O system in order to analyze and predict the reactions in ceramic whisker formation and sintering of silicon nitride composites. Equilibria among SiC, Si3N4, Si2N2O, SiO2, Si, and the gas phase are evaluated at different carbon activities, nitrogen pressures, and temperatures. Phase stability diagrams are constructed as a function of nitrogen and oxygen pressures for two levels of carbon activity. Silicon nitride becomes a stable phase with increasing nitrogen pressure or decreasing carbon activity and temperature, whereas silicon carbide becomes a dominant phase at lower nitrogen pressures or at higher temperatures when carbon activity is unity. The maximum sintering temperature of the SiC/Si3N4 composite is higher with an elevated nitrogen pressure or a reduced carbon activity.
Anionic Conduction in Lead Halosilicate Glasses
Authors: WANG YU‐HU , KOBAYASHI MASANORI , OSAKA AKIYOSHI , MIURA YOSHINARI , TAKAHASHI KATSUAKI
Published: March 2005

Electrical conductivity due to halide ions has been measured for the glasses in the system 30SiO2· (70 −x)PbO ·xPbX2 (0 < x < 17.2 mol%; X = F, Cl). The conductivity increases with x by several orders of magnitude for both series of glasses. The contribution of principal factors to the conductivity has been examined. The analysis of the activation energy on the basis of the Anderson‐Stuart model shows that the strain energy is a predominant term in the Cl glasses, while both the strain energy and the electrostatic energy have a comparable contribution in the F glasses. The decrease in the activation energy with increasing PbX2 content is related to the decrease in the shear modulus for the Cl glasses and to the increase in the permittivity for the F glasses.
Assessment of Strength by Young's Modulus and Porosity: A Critical Evaluation
Authors: DUTTA S.K. , MUKHOPADHYAY A.K. , CHAKRABORTY D.
Published: March 2005

Literature data on strength, porosity, and Young's modulus at room temperature of reaction‐bonded silicon nitride, sintered silicon nitride, and hot‐pressed silicon nitride have been fitted into available and proposed strength‐porosity relationships. In general, the Lewis method of iterative least‐squares fitting in these relationships has been found to be better than conventional linearized least‐squares fitting. Further a semiempirically proposed strength–Young's modulus relationship has been found to predict strength more precisely than the conventional strength‐porosity relationship.
Synthesis of Amorphous Boron Nitride by Pressure Pyrolysis of Borazine
Authors: Hirano Shin‐Ichi , Yogo Toshinobu , Asada Satoshi , Naka Shigeharu
Published: March 2005

Amorphous BN that contains hydrogen could be synthesized by pressure pyrolysis of borazine below 700°C at 100 MPa. The fraction of B and N bonded to H in the pyrolysis product could be controlled by changing the pressure pyrolysis condition. The pyrolysis product at 700°C and 100 MPa showed formation of B‐N‐B bonds of the hexagonal structure in the amorphous state with some B‐H and N‐H bonding remaining. Spherulitic BN could be prepared only by pyrolysis below 400°C and 100 MPa. The yield of amorphous BN from borazine was as high as about 60% by this pressure pyrolysis. Amorphous BN formed from borazine could be readily converted to cubic BN by reacting it with AIN at 1200°C and 6.5 GPa.
Interfacial Characterization of Silicon Nitride Powders
Authors: Bergström Lennart , Pugh Robert J.
Published: March 2005

The composition of the surface and the behavior in aqueous suspensions of three silicon nitride powders were investigated using electron spectroscopy for chemical analysis (ESCA), potentiometric titrations, leaching experiments, and electrophoretic mobility. ESCA shows that the as‐received powders have a surface‐layer composition similar to that identified as an intermediate state between silica and silicon oxynitride. The original differences in pHiep between the three powders disappears by aging the powders. The common pHiep of 6.8 ± 0.3 for the three powders is interpreted as the equilibrium pHiep for silicon nitride in aqueous suspensions.
Effects of Temperature and Strain Rate on the Plastic Deformation of Fully Dense Polycrystalline Y1Ba2Cu3O7−x Superconductor
Authors: Bussod Gilles , Pechenik Alexander , Chu Chung‐tse , Dunn Bruce
Published: March 2005

The knowledge of the steady‐state stress for plastic deformation as a function of temperature and strain rate is essential for hot‐forming superconducting material into commercially useful shapes. In this paper, results are presented on the experimental determination of the rheology of fully dense polycrystalline Y1Ba2Cu3O7−x superconducting material at temperatures ranging from 750° to 950°C and strain rates of 10−4, 10−5, and 10−6 s−1. The data are best fitted by a power law: ε(s−1)=8.9 × 10−17. (s−1) σ2.5 (Pa) exp [−2.01 × 105(J·mol−1)|RT]. X‐ray analysis shows that the superconducting material retains its phase composition after nearly 70% total strain of the sample. A strong anisotropy in the resistivity of the deformed samples is observed because of the development of a preferred orientation of the a or b axis of Y1Ba2Cu3O7−x orthorhombic perovskite single crystals perpendicular to the principal maximum compressive stress.
Synthesis of Uvarovite Garnet
Authors: Carda Juan , Monros G. , Escribano P. , Alarcon J.
Published: March 2005

A garnet with interesting optical properties is synthesized by means of the ceramic method and the formation of gels. Colloidal silica or tetraethoxysilane (TEOS) is used as a starting reagent in the latter method of synthesis, together with nitrate and chloride salts of the remaining components. Use is made of ultraviolet‐visible and infrared spectroscopy techniques and X‐ray diffraction in studying the evolution of the system. Formation of the garnet phase at lower temperatures is observed when using the gel methods, as well as the absence of chromates during the process in the samples which contained chlorides.
Superplastic Deformation of Zinc Sulfide Near Its Transformation Temperature [1020°]
Authors: Xue Liang A. , Raj Rishi
Published: March 2005

Polycrystalline ZnS was deformed in compression to large strains (up to 100%), near its α⇄β transformation temperature (1020°), at strain rates that ranged from 10−5 to 3 × 10−3 s−1. The flow stress showed a minimum near 1020°. The strain rate sensitivity and the grain size dependence of the flow stress suggest a superplastic mechanism of deformation. There was some evidence of dynamic recrystallization and stress‐induced β (fcc) to α (hcp) phase transformation. The polycrystals had a tendency to form in tergranular cavities at the higher temperatures and at faster strain rates.
Degradation of Polyolefin Blends Used for Ceramic Injection Molding
Authors: Wright Judith K. , Evans Julian R. G. , Edirisinghe Mohan J.
Published: March 2005

The degradation mechanisms of four polyolefins of differing molecular weight were studied by thermogravimetric analysis in order to identify the material properties which facilitate the extraction of ceramic injection molding vehicles. Three weight‐loss processes were identified and the corresponding activation energies were calculated. Low‐molecular‐weight vehicles were lost entirely by evaporation while higher‐molecular‐weight polymers underwent thermal degradation followed by evaporation of degration products. In oxidizing atmosphere the influence of oxidative degradation was confined to the surface region of moldings. It is shown that the summation of individual thermograms gives guidance on the likely weight loss behavior of blends.
Effect of Iron and Boron Carbide on the Densification and Mechanical Properties of Titanium Diboride Ceramics
Authors: Kang Eul Son , Jang Cheol Woo , Lee Chae Hyun , Kim Chong Hee , Kim Do Kyung
Published: March 2005

The effect of Fe and B4C on the sintering behavior and mechanical properties of TiB2 ceramics have been studied. Sintering was performed in an Ar atmosphere at 2000° using attrition‐milled TiB2 powder (mean particle size = 0.8 μm). When a small amount of Fe (0.5 wt%) was added, abnormal grain growth occurred and the sintered density was low. In the case of B4C added along with 0.5 wt% Fe, however, abnormal grain growth was remarkably suppressed, and the sintered density was increased up to 95% of theoretical. But with excess Fe addition (5 wt%), B4C grains did not act as a grain growth inhibitor, and B4C grains were frequently trapped in large TiB2 grains. The best mechanical properties were obtained for the TiB2–10 wt% B4C–0.5 wt% Fe ceramics, which exhibited a three‐point bending strength of 400 MPa and a fracture toughness of 5.5 MPa · m1/2.
Interfacial Bonding and Friction in Silicon Carbide [Filament]‐Reinforced Ceramic‐ and Glass‐Matrix Composites
Authors: Bright Jeffrey D. , Shetty Dinesh K. , Griffin Curtis W. , Limaye Santosh Y.
Published: March 2005

Interfacial shear strength and interfacial sliding friction stress were assessed in unidirectional SiC‐filament‐reinforced reaction‐bonded silicon nitride (RBSN) and borosilicate glass composites and 0/90 cross‐ply reinforced borosilicate glass composite using a fiber pushout test technique. The interface debonding load and the maximum sliding friction load were measured for varying lengths of the embedded fibers by continuously monitoring the load during debonding and pushout of single fibers in finite‐thickness specimens. The dependences of the debonding load and the maximum sliding friction load on the initial embedded lengths of the fibers were in agreement with nonlinear shear‐lag models. An iterative regression procedure was used to evaluate the interfacial properties, shear debond strength (Td), and sliding friction stress (Tf), from the embedded fiber length dependences of the debonding load and the maximum frictional sliding load, respectively. The shear‐lag model and the analysis of sliding friction permit explicity evaluation of a coefficient of sliding friction (μ) and a residual compressive stress on the interface (σ0). The cross‐ply composite showed a significantly higher coefficient of interfacial friction as compared to the unidirectional composites.
Effect of Residual Stresses on the Fracture of Ground Ceramics
Authors: Samuel Robert , Chandrasekar Srinivasan , Farris Thomas N. , Licht Robert H.
Published: March 2005

Grinding induces resiual stresses and cracks at and near the surfaces of ceramic workpieces. The residual stresses in several ground ceramics are measured using the curvature method. Four‐point bend tests are conducted to measure the fracture strength of the ground specimens. A fracture mechanics analysis includes the measured residual stress to calculate the grinding‐induced surface crack size. It is shown that the residual stresses sometimes have a significant effect on the strength‐controlling flaw size.
Thermomechanical Properties of Chemically Vapor Deposited Silicon Carbide Filaments
Authors: Brun Milivoj K. , Borom Marcus P.
Published: March 2005

The thermal expansion of filaments of CVD SiC was measured parallel and perpendicular to the filament axis. A discontinuity in the expansion versus temperature curve near the melting point of Si indicated the presence of free Si. The magnitude of the discontinuity was used to estimate the amount of the Si in the fibers. The presence of free Si is discussed in relation to the design and maximum‐use temperature of SiC‐filament‐reinforced ceramic composites.
Characterization of Lead Scandium Tantalate Ceramics Prepared by Precipitation Method
Authors: Feng Chude , Schulze Walter A.
Published: March 2005

Precipitation and coating techniques that proved successful for lead scandium niobate were tried with lead scandium tantalate. The Ta2O5 did not prove to be a useful substrate for the precipitating carbonates, but the resulting fine‐particle‐size powder lowered the sintering temperature to 1375° with only 0.4 wt% PbO loss. Annealed samples showed shifts in the dielectric response and sharpening of infrared and X‐ray diffraction patterns, but little development of super lattice.
Water Corrosion of YBa2Cu3O7−δ Superconductors
Authors: Fitch Lawrence D. , Burdick Vernon L.
Published: March 2005

Solid‐state sintering was used to make YBa2Cu3O7−δ superconducting bulk materials. Corrosion of the YBa2Cu3O7−δ superconductor material was investigated in a humid environment. The superconducting materials exhibited significant corrosion after 4 h at 80° and 100% relative humidity. A grain‐boundary phase was formed, and the percent superconducting phase in the material decreased by approximately 60%. The transition temperature (Tc) decreased with corrosion time. After 2 h of corrosion, Tc decreased from 87 to 81 K.
Ferroelastic Domain Switching in Polydomain Tetragonal Zirconia Single Crystals
Authors: Srinivasan Gajawalli V. , Jue Jan‐Fong , Kuo Shih‐Yee , Virkar Anil V.
Published: March 2005

As‐received, yttria‐doped (4.2 wt% Y2O3) single crystals of zirconia were heated to ≥2100°C in air. Cube‐shaped samples with faces perpendicular to 〈100〉 axes on the basis of the pseudocubic symmetry were cut from the crystals. X‐ray and electron diffraction indicated that the crystals are polydomain with [001] axes, on the basis of the tetragonal symmetry, in three mutually orthogonal directions (perpendicular to the cube faces). The cube‐shaped crystals were tested in compression at temperatures as high as 1400°C. X‐ray diffraction indicated that ferroelastic domains underwent reorientation (switching) in compression. Subsequently, notched samples with the long direction of the beams along 〈100〉 on the basis of the pseudocubic symmetry were fractured in three‐point bending at temperatures as high as 1000°C. X‐ray diffraction from fracture surfaces showed that domain reorientation had occurred and that no monoclinic phase was observed on fracture or ground surfaces. The fracture toughness at room temperature and at 1000°C was ∼12 and ∼8 MPa · m1/2, respectively. Preliminary experiments on polycrystalline tetragonal zirconia samples containing 5.4 wt% Y2O3 and sintered at ≥2100°C also showed no evidence of the monoclinic phase on fracture or ground surfaces. The toughness of the polycrystalline samples was typically 7.7 MPa · m1/2. These results indicate that ferroelastic domain switching can occur during fracture and may contribute to toughness.
Defect Chemistry of Relaxor Ferroelectrics and the Implications for Dielectric Degradation
Authors: Smyth Donald M. , Harmer Martin P. , Peng Ping
Published: March 2005

Degradation of the electrical resistance of perovskite‐related dielectrics under voltage–temperature stress is a function of the concentration of the most mobile ionic species, the oxygen vacancy. Sources of oxygen vacancies in simple perovskite oxides are reviewed, and the effects of the compositional complexity of the relaxor ferroelectrics are discussed. In particular, the possibility of compositional inhomogeneity that results from partial ordering of the cations on the octa hedral sites can greatly affect the properties of the continuous, disordered matrix that controls the degradation behavior. [Key words: ferroelectrics, electrical properties, defects, capacitors, perovskites.]
Poling of Lead Zirconate Titanate Ceramics and Flexible Piezoelectric Composites by the Corona Discharge Technique
Authors: Waller David , Iqbal Toriq , Safari Ahmad
Published: March 2005

In the conventional poling method, piezoelectric ceramics and composites are poled by applying a large dc voltage. Poling of composites having a polymer matrix with 0–3 connectivity is especially difficult because the electric field within the high‐dielectricconstant grains is far smaller than in the low‐dielectric‐constant polymer matrix. Therefore, very large electric fields are required to pole these types of composites. However, large electric fields often cause dielectric breakdown of the samples. In this study for improved poling, the corona discharge technique was used to pole piezoelectric ceramics, fired PZT composites, and 0.5PbTiO3· 0.5BiFeO3 0–3 polymer composites. An experimental setup for corona poling is described. The dielectric and piezoelectric properties of materials poled by the corona discharge technique were comparable to those obtained with the conventional poling method.
High‐Frequency Ultrasonic Characterization of Sintered Silicon Carbide
Authors: Baaklini George Y. , Generazio Edward R. , Kiser James D.
Published: March 2005

High‐frequency 60‐ to 160‐MHz ultrasonic nondestructive evaluation was used to characterize variations in density and microstructural constituents of sintered SiC bars. Ultrasonic characterization methods included longitudinal velocity, reflection coefficient, and precise attenuation measurements. The SiC bars were tailored to provide bulk densities ranging from 90% to 98% of theoretical, average grain sizes ranging from 3.0 to 12.0 μm, and average pore sizes ranging from 1.5 to 4.0 μm. Velocity correlated with specimen bulk density irrespective of specimen average grain size, average pore size, and average pore orientation. The attenuation coefficient was found to be sensitive to both density and average pore size variations, but was not affected by large differences in average grain size.
Growth Mechanism of Monodispersed ZrO2 Particles
Authors: Ogihara Takashi , Mizutani Nobuyasu , Kato Masanori
Published: March 2005

Monodispersed ZrO2 seed particles which were prepared by hydrolysis of zirconium alkoxide solutions were allowed to grow by further addition of zirconium alkoxide and water under conditions in which new particles do not nucleate and grow. The particle growth mechanism is presumed to be a surface reaction in which the rate‐determining step is a first‐order polynuclearlayer growth mechanism. With this method of powder preparation, it is possible to precisely control the particle size, and it may be useful for applications in ceramic processing.
Crack Deflection Process for Hot‐Pressed Whisker‐Reinforced Ceramic Composites
Authors: Liu Haiyan , Weisskopf Karl‐L. , Petzow Günter
Published: March 2005

A crack deflection model for two‐dimensional randomly arranged rods has been derived by making an appropriate modification to the model by Faber and Evans. This model predicts more effective toughening in certain directions than that for three‐dimensional randomly arranged rods. The theoretical predictions are compared with experimentally measured fracture toughness data of hot‐pressed whiskerreinforced ceramic‐matrix composites.
High‐Temperature Kelvin Probe in Application to Ceramic Materals: Investigation of Nickel Oxide
Authors: Nowotny J. , Sloma M. , Weppner W.
Published: March 2005

A work function analysis system (Kelvin probe) which operates at high temperatures and under controlled gas atmospheres has been constructed and its application in studies of surface properties of ceramic materials is discussed. The technique allows in situ characterization of solid–gas interfaces and monitoring of surface processes, such as structural rear rangements, phase transitions, equilibration processes, and defect segregations. Experimental data for NiO single crystals illustrate that high‐temperature work function (WF) data may be used for a description of the defect structure model in the boundary layer and its changes as a result of segregation of extrinsic defects. WF measurements were applied to monitor the surface reequilibration during redox processes at 780°C in gas atmospheres of controlled oxygen partial pressure (1.74 × 102 to 4.3 × 104 Pa). The power of the WF dependence on the oxygen partial pressure determined for the oxidation and reduction experiments changes from 1/6 within 10 h to 1/1.5 after 220 h of annealing. During this treatment the bulk‐related oxygen exponent remains unchanged and close to 1/6. The experimental results are discussed in terms of the NiO defect structure in relation to the crystalline bulk and the near‐surface layer. The results indicate that equilibration of the near‐surface oxide layer involves not only the propagation of the nonstoichiometry from the surface into the bulk but also the segregation of impurities. The second process leads to strong changes in composition and resulting interactions between the defects in the outer layer which no longer can be described by the ideal defect model. The effect of ceramic interfaces (surfaces and grain boundaries) on equilibration kinetics in the gas–solid systems has been considered.
Effect of YO1.5 Dopant on Unit‐Cell Parameters of ZrO2 at Low Contents of YO1.5
Authors: Toraya Hideo
Published: March 2005

The effect of YO1.5 dopant on unit‐cell parameters of ZrO2 (YO1.5=0 to 14.6 mol%) were examined by the X‐ray whole‐powder‐pattern decom‐position technique. The unit cell of monoclinic ZrO2 has the largest expansion along the direction perpen‐dicular to (100). The rate of increase of the unit‐cell volume of monoclinic ZrO2 with YO1.5 content is greater than that of tetragonal ZrO2 and comparable to that of cubic ZrO2.
Tensile Rupture Strength and Fracture Defects of Sintered Silicon Carbide
Authors: Ohji Tatsuki , Yamauchi Yukihiko , Kanematsu Wataru , Ito Shoji
Published: March 2005

The tensile strength distribution of sintered silicon carbide was measured at room temperature and 1300°C in air and fracture defects were characterized. The measured strength was compared with strength obtained from flaw characteristics and fracture toughness assuming a peripherally cracked spherical void model.
Shear and Densification of Glass Powder Compacts
Authors: Ducamp Véronique C. , Raj Rishi
Published: March 2005

Sinter‐forging types of experiments were carried out on powder compacts of glass. The shear and densification strains were measured simultaneously during the forging process. This information was analyzed to obtain the shear viscosity of the glass and the intrinsic sintering pressure, both as a function of density. The viscosity of the porous glass changed nonlinearly with density and was empirically fitted to an exponential function. The sintering pressure was found to increase with density; it was 100 kPa at ρ= 0.55 and 250 kPa at ρ= 0.90. In this range the sintering pressure could be adequately described in terms of a simple geometrical model. However, when the density approached 95%, the sintering pressure appeared to assume an uncertain value. The measurements were also used to calculate the viscous bulk modulus of the porous glass; these estimates agreed well with theoretical prediction.
Ancient Analogues of Modern Cement: Calcium Hydrosilicates in Mortars and Concretes from Gallo–Roman Thermal Baths of Western France
Authors: Rassineux François , Petit Jean Claude , Meunier Alain
Published: March 2005

The long‐term durability of cementbased materials over periods of time not directly accessible to laboratory experiments has to be thoroughly assessed to guarantee the safety of a radioactive waste repository. In this work, archaeological mortars and concretes have been studied to check whether their use as analogues of actual technological cements used for waste disposal is possible. Samples of mortar and concrete have been collected from several Gallo‐Roman thermae of western France; they were examined as petrographically thin sections with optical microscopy and were analyzed by X‐ray diffraction of microsamples (10−3 mg) and by electron probe microanalysis. The masonry cement appears to be entirely carbonated with large voids and fissures. In contrast, the use of lime with additional crushed brick or tile for the bath structure concrete and coating mortar induced pozzuolanic reactions. Despite the fact that the matrices are essentially composed of calcium carbonate, calcium aluminosilicates with compositions close to those of hydrogrossular, hydrated calcium silicates with composition and a structure comparable with those of calcium hydrosilicate formed in modern cement, and ettringite deposits in voids of concretes could also be identified. It is suggestea that these ancient artifacts can help identify very slow processes that rule cement alteration over long periods of time. The artifacts may be considered as possible analogues of present‐day materials whenever sufficient care is taken in interpreting and extrapolating data.
Mechanochemical Changes of Weinschenkite‐Type RPO4·2H2O (R = Dy, Y, Er, or Yb) by Grinding and Thermal Reactions of the Ground Specimens
Authors: Hikichi Yasuo , Sasaki Toshio , Murayama Kyouhei , Nomura Tsuyoshi , Miyamoto Masaaki
Published: March 2005

It is reported that, on mechanochemical treatment, weinschenkite‐type RPO4·2H2O (R = Dy, Y, or Er) gradually transforms into rhabdophane‐type RPO4· nH2O (n = 0.5 to 1) and weinschenkite‐type YbPO4·2H2O into xenotime‐type YbPO4, at room temperature in air. Rhabdophane‐type YPO4·0.8H2O and ErPO4·0.9H2O obtained by grinding weinschenkite‐type RPO4·2H2O (R=Y or Er) are new. The new rhabdophane‐type YPO4·0.8H2O and ErPO4·0.9H2O gradually transform to xenotime‐type YPO4 and ErPO4 when heated above 900°C (R = Y) and 700°C (R = Er) in air.
Selective Cation Exchange in Substituted Tobermorites
Authors: Komarneni Sridhar , Tsuji Masamichi
Published: March 2005

Selective cation exchange in tobermorites with three levels of Al3+ and Na+ substitution for Si4+ has been investigated. The cation exchange selectivity for a tobermorite with 2 mol% Al3+ and 0 mol% Na+ substitution increased as follows: Mg2+ > Ba2+ > Sr2+. Cation exchange in the above tobermorite is postulated to take place mainly from edge and planar surface sites and apparently from interlayer Ca2+ sites. Tobermorites with 10 and 15 mol% Al3+ (and Na+) substitution have additional Na+ exchange sites in the interlayers and show the following selectivity: Ba2+ > Sr2+ > Mg2+. The cation exchange selectivity in the substituted tobermorites can be explained by the hydrated radii of cations and the steric limitations of the ion exchange cavity in tobermorite; the latter was determined by 27Al and 29Si magic angle spinning nuclear magnetic resonance spectroscopy. These basic selective cation exchange studies of substituted tobermorites are of relevance in nuclear waste treatment and disposal.
Microhardness of Mullite at Temperatures to 1000°
Authors: Kollenberg Wolfgang , Schneider Hartmut
Published: March 2005

The microhardness of mullite single crystals was measured at temperatures to 1000°C. The hardness decreased from about 15 GPa at room temperature to about 10 GPa at 1000°C. Thus, the mullite at 1000°C had a higher hardness than sapphire, approaching the value of SiC, but at room temperature was softer than both materials.
Pressureless Sintering of Alumina‐Titanium Carbide Composites
Authors: Kim Young‐Wook , Lee June‐Gunn
Published: March 2005

The densification of Al2O3‐TiC composites is detrimentally affected by chemical reactions between Al2O3 and TiC. These reactions must be suppressed in order to promote sintering. In this study, the specific reactions occurring in Al2O3‐TiC composites were modeled, using thermodynamic calculations, and verified by experiments. The reaction between Al2O3 and TiC was suppressed by the use of specially prepared embedding powders allowing pressureless sintering to closed porosity. The Al2O3‐TiC composites were subsequently hot isostatically pressed to > 99% of theoretical density without encapsulation. Typical flexural strength and fracture toughness of Al2O3‐30 wt% TiC composites were 690 MPa and 4.3 MPa · m1/2, respectively.
Hardness and Depth‐Dependent Microstructure of Ion‐Irradiated Magnesium Aluminate Spinel
Authors: Zinkle S. J.
Published: March 2005

Stoichiometric polycrystalline magnesium aluminate spinel has been irradiated at 25° and 650°C with 2.4‐MeV Mg+ ions to a fluence of 1.4 × 1021 ions/m2 (∼35 dpa (displacement per atom) peak damage level). Microindentation hardness measurements and transmission electron microscopy combined with energy dispersive X‐ray spectroscopy measurements were used to characterize the irradiation effects. The room‐temperature hardness of spinel increased by about 5% after irradiation at both temperatures. There was no evidence for amorphization at either irradiation temperatures. Interstitial‐type dislocation loops lying on {110} and {111} planes with Burgers vectors along 〈110〉 were observed at intermediate depths (∼1 μm) along the ion range. The 〈110〉{111} loops are presumably formed from 〈111〉{111} loops as a result of a shear on the anion sublattice. Only about 0.05% of the calculated displacements were visible in the form of loops, which indicates that spinel has a high resistance to aggregate damage accumulation. The peak damage region contained a high density of dislocation tangles. There was no evidence for the formation of voids or vacancy loops. The specimen irradiated at 650°C was denuded of dislocation loops within ∼1 μm of the surface.