bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of Active Oxidation on the Flexural Strength of α‐Silicon Carbide
Authors: Kim Hyoun‐Ee , Moorhead Arthur J.
Published: March 2005

The effects of oxygen partial pressure (Po2) on the oxidation behavior and room‐temperature flexural strength of sintered α‐SiC were investigated. Groups of flexure bars were exposed at 1400°C to flowing Ar containing various levels of oxygen (Po2 ranging from 7.5 × 10−7 to 1.5 × 10−4 MPa). The changes in weight, flexural strength, and surface morphology of the samples were strongly influenced by the Po2 level. When the Po2 was higher than 3 × 10−5 MPa, SiO2 was formed on the surface (i.e., passive oxidation occurred) and the strengths of the samples were not significantly affected. However, when the Po2 was lower than 2 × 10−5 MPa, material loss occurred (active oxidation), decreasing the weight and strength of the samples. Both the reduction in strength and the weight loss resulting from active oxidation were proportional to the Po2. An approximately 50% reduction in strength was observed in the SiC after oxidation for 20 h at a Po2 of 1.5 × 10−5 MPa, a level that is slightly lower than the Po2 at which the transition from active to passive oxidation occurs. Large pits formed during exposure were responsible for the reduction in strength.
Influence of Abrasive Properties on Residual Stresses in Lapped Ferrite and Alumina
Authors: Chandrasekar Srinivasan , Kokini Klod , Bhushan Bharat
Published: March 2005

The effect of abrasive‐particle properties on surface‐finishing residual stresses, surface finish, and material removal rates during the lapping of ferrite and aluminium oxide is quantified experimentally. It is shown that lapping with softer abrasives and smaller particles results in lower compressive residual stresses near the surface and improved surface finish. These results demonstrate possible methods for controlling surface finishing residual stresses in ceramics while at the same time achieving dimensional accuracy.
Pyrolytic Deposition of Carbon Films Containing Nitrogen and/or Boron
Authors: Maya Leon , Harris Lawrence A.
Published: March 2005

Carbon films containing nitrogen and/or boron were prepared in a closed system by the pyrolysis of appropriate precursor materials. The decomposition products of the pyrolysis were identified and the resulting films were characterized by chemical and physical means. The electrical conductivity of the heteroatom‐containing films is similar to that of pure amorphous carbon films. Boron nitride was present in the C‐N‐B film as revealed by XPS and IR spectroscopy. Oxygen appeared to have a role in the adherence of the films to the silica substrate.
Morphology of Platelike Abnormal Grains in Liquid‐Phase‐Sintered Alumina
Authors: Song Huesup , Coble Robert L.
Published: March 2005

The microstructural features of platelike grains in liquid‐phase‐sintered Al2O3 were investigated. The flat boundaries of platelike grains wetted with a liquid phase were basal planes. After impingement of platelike grains, flat boundaries progressively changed to curved boundaries which consisted of both basal and rhombohedral planes as microscopic facets. Second‐phase particles were observed for most doped samples. Small‐angle subgrain boundaries inside platelike grains were also observed.
Cluster Models for Electronic Structure of Oxide Ceramics
Authors: Ellis Donald E. , Guo Jun , Lam Daniel J.
Published: March 2005

Recent progress in using cluster models to calculate electronic structure of oxide ceramics such as transition‐metal monoxides, zirconia, α‐alumina, ruby, and copper‐based superconducting oxides is reviewed. The self‐consistent field local density theory is used to find potentials, energy levels, and wave functions for a fragment consisting of N atoms embedded in the infinite solid. The single‐particle spectra are compared with photoelectron and optical data, and with X‐ray absorption and emission. Total energy calculations provide cohesive energies for prediction of relative stability of defect structures and lattice relaxation in the vicinity of impurities. A brief description is given of surface‐structure calculations for an α‐alumina (0001) surface.
Grain‐Boundary Chemistry of Barium Titanate and Strontium Titanate: I, High‐Temperature Equilibrium Space Charge
Authors: Chiang Yet‐Ming , Takagi Touichi
Published: March 2005

Direct observations using scanning transmission electron microscopy (STEM) of the grain‐boundary chemistry of selectively doped SrTiO3 and BaTiO3 show the predominant solute segregation in both systems to be that of acceptors (negative effective charge). Appreciable donor segregation is not observed even at lattice concentrations as high as 10 mol%. Donor and acceptor codoped materials show segregation of the acceptor only. The results are consistent with a grain‐boundary space‐charge distribution consisting of a positive boundary and negative space charge. All grain boundaries examined also show an excess of Ti relative to the A‐site cations, suggesting that the positive boundary charge is at least partially accommodated by an excess of Ti ions. The sign and magnitude of the electrostatic potential appear to be remarkably insensitive to changes in lattice defect structure with solute doping. Grain‐boundary chemistry appears dominated by space‐charge segregation, in contrast with the predictions of recent atomistic simulations which neglect the space‐charge potential.
Internal Precipitation of Molecular Oxygen and Electromechanical Failure of Zirconia Solid Electrolytes
Authors: Virkar Anil V. , Nachlas Jesse , Joshi Ashok V. , Diamond Jordan
Published: March 2005

The phenomenon of electromechanical failure of zirconia has been examined. It is shown that spatial variation of electronic conductivity in a predominantly oxygen ion conductor can lead to internal precipitation of molecular oxygen and its pressurization. Internal pressures high enough to crack zirconia can develop if the electronic conductivity is higher on the oxygen exit side. The calculations demonstrate that the kinetics of pressurization are very sensitive to the relative magnitudes of the electronic and the ionic conductivities. Oxygen was electrolyzed at 800°C through zirconia solid electrolytes with lanthanum strontium manganate (LSM) electrodes under an applied dc potential of 1.0 V. Spalling of the solid electrolytes was observed. Composite disk electrolytes were fabricated such that part of the disk contained TiO2 as a dopant which was added to enhance the electronic conductivity. The electrolytes exhibited severe cracking and pitting when the TiO2‐doped region was on the oxygen exit side. No degradation occurred when the TiO2‐doped region was on the oxygen entry side. Implications regarding the use of mixed conductors as dense electrodes in electrochemical systems are discussed.
Interfacial Segregation in Perovskites: I, Theory
Authors: Desu Seshu B. , Payne David A.
Published: March 2005

Based on thermodynamic principles a theory for equilibrium interfacial segregation is proposed for perovskite materials, and this theory is applied to BaTiO3. An approach developed by Frenkel and refined by Kliewer and Koehler is extended to undoped ternary oxide materials such as BaTiO3. The approach uses regular solution approximations and considers space charge effects as the major driving force for segregation. The analysis based on this model indicates the presence of a negative space charge potential (−0.1 V at 800°C) at the surface of pure BaTiO3. The model also predicts cation enrichment at the interface. The thickness of the space charge layer decreases with increasing temperature, and calculated values agree well with experimental results. Since both elastic and electrostatic driving forces are important for dopant/impurity segregation, an approach where the grain boundary is considered to be a two‐dimensional phase, in equilibrium with the three‐dimensional phase of the grain, proves useful. Solving for the impurity/dopant segregation ratio is case specific and requires knowledge of the charge neutrality conditions as well as the strain energy contribution.
Characterization of Barium Titanyl Oxalate Tetrahydrate
Authors: Yen Fu‐Su , Chang Chen T. , Chang Yen‐Hwei
Published: March 2005

The XRD pattern and the thermal behavior of barium titanyl oxalate tetrahydrate (BTOT) were investigated. BTOT crystallizes in the monoclinic system, 2/m, with unit cell dimensions of a= 14.954 Å, b= 19.332 Å, c= 13.947 Å, and β= 106.43°. Unit cell content (Z) is 12 and the Bravais lattice is P. Theoretical density is 2.31 g/cm3. At a relatively low temperature (∼60°C), BTOT starts to dehydrate, resulting in a less‐than‐calculated weight loss of ignition. Barium titanate powder obtained by calcining the oxalate exists as the cubic perovskite phase, instead of the tetragonal phase at low temperatures. This happens when the particle size of the crystals is smaller than ∼ 30 nm. As the crystal coarsens, barium titanate powder transforms to the tetragonal state.
Correlation between Neutron Diffraction Measurements and Thermal Stresses in a Silicon Carbide/Alumina Composite
Authors: Tomé Carlos N. , Bertinetti María A. , MacEwan Stuart R.
Published: March 2005

Sintering of Al2O3 powder and SiC fibers at high temperatures leads to a state of high internal stresses in the composite material at room temperature because of the difference in thermal contraction between both materials. These stresses are calcualted in this work using the scheme of the elastic elliposidal inclusion of Eshelby and regarding the fibers as prolate ellipsoids. The predicted strain in the fibers is used to calculate variations in the spacing of crystallographic planes and the results are compared against experimental measurements obtained with neutron diffraction by other authors. We show that in order to correlate the measurements with the actual stress state, it is necessary to account for the elastic anisotropy and the proper crystallographic structure of the fibers. We demonstrate that the interpretation also depends on the distribution of fiber orientations inside the matrix and that results can be substantially changed depending on the assumptions employed.
Diffusion Bonding of Zirconia/Alumina Composites
Authors: Nagano Takayuki , Kato Hidezumi , Wakai Fumihiro
Published: March 2005

ZrO2/Al2O3 composites with from 0% to 100% Al2O3 content were diffusion bonded at 12.5 MPa for 30 min in the temperature range 1450° to 1500°C. Under appropriate bonding conditions, a bonding strength greater than 1000 MPa was achievable between dissimilar materials with different thermal expansion coefficients.
V‐Notch Technique for Single‐Edge Notched Beam and Chevron Notch Methods
Authors: Awaji Hideo , Sakaida Yoshihisa
Published: March 2005

The technique of a V‐shaped notch with very sharp root radius was used with the single‐edge notched beam and chevron notch methods to evaluate fracture toughness of ceramics. These methods are improved techniques using a V‐shaped diamond wheel. The single‐edge precracked beam and chevron notch techniques were also used to compare with these methods. The material used is a gas‐pressured Si3N4. The fracture toughness measured by these techniques showed quite small scatters and coincided with one another quite well, except for the usual chevron notch method.
Energy Storage in Ceramic Dielectrics
Authors: Love Gordon R.
Published: March 2005

Historically, multilayer ceramic capacitors (MLC's) have not been considered for energy storage applications for two primary reasons. First, physically large ceramic capacitors were very expensive and, second, total energy density obtainable was not nearly so high as in electrolytic capacitor types. More recently, the fabrication technology for MLC's has improved significantly, permitting both significantly higher energy density and significantly lower costs. Simultaneously, in many applications, total energy storage has become smaller, and the secondary requirements of very low effective series resistance and effective series inductance (which, together, determine how efficiently the energy may be stored and recovered) have become more important. It is therefore desirable to reexamine energy storage in ceramics for contemporary commercial and near‐commercial dielectrics. Stored energy is proportional to voltage squared only in the case of paraelectric insulators, because only they have capacitance that is independent of bias voltage. High dielectric constant materials, however, are ferroics (that is ferroelectric and/or antiferroelectric) and display significant variation of effective dielectric constant with bias voltage. The common ferroelectric materials, whether based upon barium titanate or lead manganese niobate (PMN), in the high‐field limit, exhibit an energy storage which increases linearly with bias voltage. Mixed phase, ferroelectric plus antiferroelectric, dielectrics from the lead lanthanum zirconate titanate (PLZT) system, as predicted theoretically, show the best energy density at low to moderate fields. Surprisingly, maximum energy storage is not obtained in high dielectric constant materials but in those materials which display intermediate dielectric constant and the highest ultimate breakdown voltages.
Jonker “Pecir” Analysis of Oxide Superconductors
Authors: Su Ming‐Yih , Elsbernd Christine E. , Mason Thomas O.
Published: March 2005

Data for the normal state of the superconducting oxide systems YBa2Cu3O6+y, and the nonsuperconducting system La3Ba3Cu6O12.5+y, when plotted as thermoelectric coefficient versus logarithm of conductivity, exhibit Jonker “pear” behavior, confirming the semiconducting character of these materials. The symmetry of the plots indicates similar conduction parameters (density‐of‐states (DOS), transport constants, and mobilities) for p‐ and n‐type mechanisms. Band gaps on the order of 0.5 eV are obtained. DOS—mobility products for the above‐mentioned systems and La2‐xBaxCuO4 and Bi2Sr2CaCu2O8 scale roughly according to the density of copper ions in these materials and fall in the range 1020 to 1021 (cm·V·s)−1. Assuming DOS equal to copper content results in mobilities on the order of 0.15 cm2·V−1· s−1. These results are discussed in terms of possible semiconduction mechanisms.
Evolution of the Formation of Inorganic Polymers in the CaO—SiO2—P2O5 System Using Metal Alkoxides
Authors: Agrawal Dinesh K. , Maslowski Anthony R. , Adair James H.
Published: March 2005

Inorganic polymers provide structural integrity to cements. The CaO—SiO2—P2O5 system has not been studied extensively, but it is important to both cementitious and biological systems. The hydrolytic decomposition of homogeneous mixtures of tetraethyl orthosilicate and calcium isopropoxide with additions of anhydrous phosphoric acid in isopropyl alcohol was used to evaluate the formation of inorganic polymers. XRD, DTA, and TGA were used to characterize the decomposition products. The formation of inorganic polymers at room temperature and pressure was used as a criterion to establish cement‐forming compositions within the CaO—SiO2—P2O5 system.
Influence of Stoichiometry on the Microstructure and Positive Temperature Coefficient of Resistivity of Semiconducting Barium Titanate Ceramics
Authors: Lin Tsai‐Fa , Hu Chen‐Ti , Lin I‐Nan
Published: March 2005

The influence of stoichiometry, i.e., Ba/Ti ratio, on the microstructure and positive temperature coefficient of resistivity (PTCR) characteristics of BaTiO3 was investigated. Fine‐grain microstructures are obtained for Ba‐rich, stoichiometric, low‐temperature‐sintered, Ti‐rich materials. The room‐temperature resistivities (ρRT) of the fine‐grain Ti‐rich samples are large (>108Ω·cm). Excess Ba2+ ions can decrease the ρRT, by more than 2 orders of magnitude, because of the compensation of barium vacancies near the grain‐boundary regions. Rapid cooling after sintering can also decrease ρRT (⋍100×) and is ascribed to the suppression of reoxidation. Large‐grain microstructures and low ρRT, on the other hand, are generally observed for Ti‐rich and Al2O3‐SiO2‐TiO2‐added samples after sintering at a temperature higher than the corresponding eutectic point.
Oxidation Kinetics of Zirconium Carbide at Relatively Low Temperatures
Authors: Shimada Shiro , Ishil Tadao
Published: March 2005

The isothermal oxidation of ZrC powders was carried out at relatively low temperatures of 380° to 550°C at oxygen pressures of 1.3, 2.6, and 7.9 kPa under a static total pressure of 39.5 kPa, achieved by mixing with argon, using an electromicrobalance. The oxidation kinetics are described by the diffusion‐controlled Jander's equation, following rapid oxidation in the early stage. Two activation energies were obtained: 138 kJ · mol−1 below about 470°C and 180 kJ · mol−1 above that temperature. The high‐ and low‐temperature oxidation mechanisms are discussed in connection with the crystallization of cubic ZrO2, accompanied by the generation of cracks, as well as the formation of carbon in the hexagonal diamond form in the product phase.
Mullite Whiskers from Precursor Gel Powders
Authors: Ismail Mohamed G. M. U. , Arai Hiroshi , Nakai Zenjiro , Akiba Tokuji
Published: March 2005

A mullite precursor sol was prepared by mixing boehmite and silica sols. On addition of 3.5 wt% F− ion as a 47% solution of HF, the mullite sol was gelled. The dried gel was ball‐milled and calcined at 1400°C for 1 h in an airtight container. Mullite whiskers grew on the (111) plane along the 〈001〉 direction.
Fabrication of High‐Curie‐Point Barium‐Lead Titanate PTCR Ceramics
Authors: Wang Shiang‐Hua , Hwang Ford‐Son , Tseng Tseung‐Yuen
Published: March 2005

High‐Curie‐point semiconducting barium‐lead titanate positive temperature coefficient of resistivity (PTCR) ceramics of composition Ba0.897Pb0.1La0.003TiO3 and Ba0.5Pb0.5La0.003TiO3 were prepared. The starting powders were synthesized by reacting commercial BaTiO3, PbO, and TiO2. To avoid the nonstoichiometry due to the volatilization of Pb during the sintering process, a lead atmosphere sintering approach with PbTiO3 as packing powder was used. The samples being fabricated by this method show a PTCR effect of 3 to 4.5 orders of magnitude above the Curie point. The curie points were about 180°C for Ba0.897Pb0.1La0.003TiO3 and about 360°C for Ba0.497Pb0.5La0.003TiO3.
Overlap Model for Chemical Vapor Infiltration of Fibrous Yarns
Authors: Currier Robert P.
Published: March 2005

Attention is focused on fabrication of fiber‐reinforced ceramic composites by chemical vapor infiltration. A model is proposed for infiltration of a cylindrical fiber bundle, or yarn strand, comprised in turn of small cylindrical fibers. Along any cross section perpendicular to the yarn axis, the centers of the fibers are assumed to be randomly distributed throughout the cross‐sectional area without fiber overlap. As infiltration/densification proceeds, growth is assumed to occur via sequential deposition of uniform layers such that the fiber‐matrix composite consists of growing cylinders whose edges eventually overlap. Based on the random overlap model, expressions for key time‐dependent properties are developed for both the reaction‐limited case and for cases with significant diffusional limitations. An analytical solution to the resulting equations is obtained for the reactionlimited case whereas numerical solutions are required for the diffusion‐limited case. The effects of geometric, kinetic, and diffusional parameters on the infiltration dynamics are explored.
Preliminary Model of Radiation Damage in Ceramics
Authors: Stoller Roger E.
Published: March 2005

A preliminary model of radiation damage in binary ceramic oxides has been developed. The model described here accounts in an approximate way for some of the major differences between metallic alloys and ceramics that are believed to be responsible for the fact that ceramic materials appear in some cases to behave differently than metallic alloys when exposed to displacive irradiation. The model considers the influence of the existence of a second lattice and the additional constraint of stoichiometric point defects absorption by dislocation loops on the concentrations of point defects that would be observed at steady state in an irradiated ceramic. These point defect concentrations are then used to compute various measures of the sensitivity of these materials to the type of microstructural evolution that is observed in irradiated metals. Initial results indicate that both the lattice and stoichiometry effects can help to mitigate radiation damage in ceramics. However, the effect is not necessarily large. In agreement with recent data, the results indicate that at least some ceramic oxides may exhibit a sensitivity to displacement damage that is similar to metals.
Tensile Fracture Toughness of Ceramic Materials: Effects of Dynamic Loading and Elevated Temperatures
Authors: Suresh S. , Nakamura T. , Yeshurun Y. , Yang K.‐H. , Duffy J.
Published: March 2005

Experimental methods are presented for the determination of fracture initiation toughness of ceramics and ceramic composites in pure tension under quasi‐static and dynamic loading conditions over a range of temperature spanning 20° to 1300°C. Circumferentially notched and cyclic fatigue precracked rods of a variety of ceramic materials were subjected to quasi‐static tensile fracture (rate of stress intensity factor loading, K1∼ 0.1 MPa ∼ m1/2∼ s−1) in an electroservohydraulic test machine and to dynamic tensile fracture (K1∼ 106 MPa ∼ m1/2· s−1) using a modified tensile Kolsky (split‐Hopkinson) bar. For the quasi‐static and dynamic fracture tests at elevated temperatures, the ceramic specimen was inserted into an air furnace where either friction grips or stress wave loading outside the furnace subjected the specimen to fracture. Dynamic finite‐element analyses of the circumferentially notched cylindrical rod have been conducted to develop the optimum specimen dimensions and test procedures for the measurement of dynamic fracture toughness at ambient and elevated temperature. Experiments conducted on Al2O3, Si3N4, and SiC, and an Al2O3‐25 vol% SiC whisker composite at room temperature indicate that the dynamic to quasistatic fracture initiation toughness ratio KId/KIc is in the range of 1.1 to 1.4. Elevated‐temperature fracture tests for the polycrystalline Al2O3 of 3‐μm average grain size reveal that KId is only mildly sensitive to temperature over a range of 20° to 1100°C, whereas it suffers a precipitous drop above 1100°C. Over the temperature range 20° to 1300°C, the ratio KId/KIc is found to be in the range 1.2 to 1.5. Scanning electron microscopy observations show that failure above 1100°C usually evolves by the nucleation, growth, and coalescence of cavities. The mechanisms of elevated‐temperature quasi‐static and dynamic fracture in polycrystalline Al2O3 are examined and possible causes for the apparently higher dynamic fracture initiation resistance are discussed. The significance and limitations of the proposed experimental techniques are highlighted.
Grain‐Size Dependence of Thermal‐Shock Resistance of Yttria‐Doped Tetragonal Zirconia Polycrystals
Authors: Ishitsuka Masayuki , Sato Tsugio , Endo Tadashi , Shimada Masahiko , Ohno Hideo , Igawa Naoki , Nagasaki Takanori
Published: March 2005

Thermal‐shock fracture behavior of yttria‐doped tetragonal zirconia polycrystals (Y‐TZP) of various grain sizes was evaluated by the quenching method using water as the quenching solvent. The tetragonal‐to‐monoclinic phase transformation behavior of Y‐TZP around cracks introduced by thermal stress was investigated by using Raman microprobe spectroscopy. The critical quenching temperature difference (ΔTc) of Y‐TZP ceramics increased from 250° to 425°C with increasing grain size of zirconia from 0.4 to 3.0 μm, while the fracture strength decreased from 900 to 680 MPa. The improvement of ΔTc of Y‐TZP with increasing grain size of zirconia corresponded with the quantity of tetragonal‐to‐monoclinic phase transformation around cracks introduced by thermal stress.
High‐Vacancy‐Content Magnetites and Zinc‐Bearing Ferrites from Iron(III) Tartrate in Strongly Alkaline Solution
Authors: Tamaura Yutaka , Kodama Tatsuya , Itoh Tomoyuki
Published: March 2005

High‐vacancy‐content magnetites with particle size in the range of 20 to 150 nm can be precipitated as stable products in the clear and strongly alkaline solution of iron(III) tartrate and dextrose at 100°C, although Fe(III) ions in alkaline solution have been reported previously to form only α‐FeO(OH) and α‐Fe2O3. When Zn(II) ions were present in the reaction solutions, high‐vacancy‐content Zn(II)‐bearing ferrites represented by (ZnFe2O4)x‐(Fe3O4)y‐(γ‐Fe2O3)z, where x+y+z= 1, were obtained in the strongly alkaline solutions. The Fe(II) ions, which were formed by the reduction of Fe(III) ions with dextrose, accelerate the crystallization of the spinel structure.
Diffusionless Cubic‐to‐Tetragonal Phase Transition and Microstructural Evolution in Sintered Zirconia–Yttria Ceramics
Authors: Zhou Yu , Lei Ting‐Chuan , Sakuma Taketo
Published: March 2005

Microstructural changes associated with the diffusionless cubic‐to‐tetragonal phase transition (c–t′) in sintered ZrO2–Y2O3 ceramics with 4 to 20 mol% Y2O3 are investigated. Ceramics containing 4 to 7 mol% Y2O3 that are quickly cooled from high temperatures experience cubic‐to‐tetragonal transformation by a diffusionless mechanism. The TEM diffraction pattern of the transformation product, t′‐ZrO2, reveals (112) reflections that are suppressed in the c‐ZrO2 phase. The microstructural features of the t′‐ZrO2 phase include antiphase boundaries and a twinned substructure in the (112) dark‐field images. The (112) reflections result from the displacement of oxygen atoms in the lattice during phase transformation. With increasing Y2O3 content, the antiphase domain size, the tetragonality of the lattice, and the intensity of the (112) reflections decrease. When the Y2O3 content reaches 8 mol% or more, the (112) reflections appear, but the dark‐field image shows no antiphase boundary, and the tetragonality decreases to 1. In ceramics with 14 to 20 mol% Y2O3, no suppressed reflection appears because the c–t transformation is fully suppressed. A model of the lattice structure is calculated and the intensity of diffraction determined experimentally; on this basis, the mechanism of diffusionless c–t’transformation is discussed.
X‐ray Photoelectron Spectroscopy of Cadmium Tin Oxide Ceramics in As‐Fired and Electrochemically Reduced Forms
Authors: Hashemi Tooraj , Illingsworth Jenny , Golestani‐Fard Farhad
Published: March 2005

X‐ray photoelectron spectroscopy (XPS) has been employed to investigate the chemical nature of samples of dicadmium stannate (Cd2SnO4) in the as‐fired, electrochemically reduced, and reoxidized states. The reduction of Cd2SnO4 was found to be associated with a dramatic color change from bright yellow to dark green, a phenomenon commonly known as the electrochromic effect. Both quantitative XPS results and binding energy measurements proved that, upon exposure of the reduced ceramic bodies to air, the Sn2+ to Sn4+ transition readily took place to produce the intermediate compound, Cd2SnO3 with divalent tin. Prolonged exposure to the atmosphere did not result in further progress of reoxidation extending to monovalent cadmium. However, complete reoxidation of the reduced samples was possible by annealing in air at 350°C for a short period of time, e.g., 3 h, by which the original features of the as‐fired state such as color and electrical conductivity were restored. The results also showed that reoxidized samples at high temperature assume the same XPS characteristics as those of as‐fired ceramics.
Atmospheric Effects on Compressive Creep of SiC‐Whisker‐Reinforced Alumina
Authors: Lipetzky Pete , Nutt Steven R. , Koester David A. , Davis Robert F.
Published: March 2005

A SiC‐whisker‐reinforced alumina composite was crept in compression at 1200° to 1400°C in an air ambient and in nitrogen. The data were described by a power‐law‐type constitutive relation. The measured value of the stress exponent was n= 1 at 1200°C and n= 3 at 1300° and 1400°C in both ambients. TEM observations were correlated with the measured creep response to determine active deformation mechanisms. Values of n= 1 were associated with diffusional creep and unaccommodated grain‐boundary sliding, while values of n= 3 were associated with increased microstructural damage in the form of cavities. Experiments conducted in circulated air resulted in higher creep rates than comparable experiments in nitrogen. The accelerated creep rates were caused by the thermal oxidation of SiC and the resultant formation of a vitreous phase along composite interfaces. The glassy phase facilitated cavitation, weakened interfaces, and enhanced boundary diffusion.
Dissolution Rates of Silicate Glasses in Water at pH 7
Authors: Perera Gunarathna , Doremus Robert H. , Lanford William
Published: March 2005

Dissolution rates of different glasses in buffered solutions of constant pH of 7 were measured by weight change, profilometry, and ion implantation with Rutherford backscattering. Rates with different techniques agreed within experimental error. Natural obsidian glass dissolved most slowly, at a rate comparable with those of quartz and crystalline aluminosilicate minerals. Commercial soda–lime glass containing alumina dissolved slowly, at about the same rate as vitreous silica; soda–lime silicate glasses both commercial and laboratory without alumina dissolved much more rapidly. Pyrex borosilicate glass dissolved at a rate intermediate between those of soda–lime silicate glasses with and without alumina; at room temperature Pyrex borosilicate glass dissolved about 100 times faster than a commercial soda–lime glass containing alumina. We suggest that surface structure is the main factor determining the relative dissolution rates of silicate glasses. Glasses with transformed surface layers caused by hydration dissolve most rapidly; phase separation and openness of the glass structure are also important factors.
Chemically Induced Migration of Liquid Films and Grain Boundaries in TiN—Ni—(TiC) Alloy
Authors: Baik Young‐Joon , Eun Kwang Yong
Published: March 2005

Liquid films and grain boundaries in the TiN–Ni system migrate when carbon atoms are added as TiC. Ti(NC) solid solution grows at the expense of TiN solid through liquid film migration (LFM), and its general characteristics are similar to those of the previously investigated Mo–Ni and W–Ni systems. But, in this system, precipitating solid–liquid interfaces of migrating liquid films show faceting, which is caused by orientation‐dependent interface‐controlled reactions. With an increase of added TiC, the migration rate increases parabolically with the observed lattice parameter difference between the TiN and Ti(NC) solid, which is in agreement with the prediction of the coherency strain model. Chemically induced grain‐boundary migration (CIGM) also occurs, together with LFM. The results in this study show that LFM and CIGM occur in a system having relatively strong covalent bonding.
Effect of Storage Time on the Rheological Behavior of Oil Well Cement Slurries
Authors: Barbic Lenart , Tinta Vence , Lozar Bojan , Marinkovic Velibor
Published: March 2005

The objective of this study was to determine the effect of storage time on the rheological behavior of three fresh cements grained at different temperatures. Although rheology was affected, proton pulsed NMR measurements showed that specific surface areas remained unchanged; i.e., there was no additional cross‐linking between cement particles due to prehydration or hydration products. The two techniques allowed us to differentiate between properties which account for mutual attraction and repulsion of particles and those which are attributed to hydration and changes in surface area.
Development of Reaction‐Bonded Electroconductive Silicon Nitride‐Titanium Nitride and Resistive Silicon Nitride‐Aluminum Oxide Composites
Authors: Yasutomi Yoshiyuki , Chiba Akio , Sobue Masahisa
Published: March 2005

Reaction‐bonded Si3N4· TiN and Si3N4· Al2O3 composites were successfully fabricated by heating mixed powder compacts of Si and TiN or Si and Al2O3 in a nitrogen atmosphere. The former showed electrical conductivity, owing to the presence of TiN. An electrical resistivity of 2.6 × 10−5Ω· m was obtained for the Si3N4· TiN composite with 70 vol% TiN. The composite with 20 vol% TiN showed an electrical resistivity of 0.22 Ω· m and a bending strength of 460 MPa. On the other hand, the Si3N4· Al2O3 composite had insulating properties. The use of an appropriate amount of resin binder resulted in a higher green density and, consequently, a higher bending strength. Moreover, electroconductive Si3N4· TiN/resistive Si3N4· Al2O3 complex ceramics could be fabricated by heating green compacts composed of two different portions, one composed of mixed powders of Si and TiN and the other of Si and Al2O3. Attainment of such complex ceramics was attributed to the small dimensional change at the nitriding stage, under 0.3% and the similarity of the thermal expansion coefficients of the two composites.
Thermochemical Studies in Selected Metal–Carbon–Oxygen Systems
Authors: Wittmer Dale E. , Temuri Maqsood Z.
Published: March 2005

Carbon–carbon composites have unique physical and chemical properties but are limited by poor oxidation resistance at elevated temperatures in an oxidizing environment. Coatings based on metal carbides and metal oxides are currently being used for limited oxidation protection at temperatures exceeding 1200°C. However, problems exist for all current coatings and potential future coatings because of thermochemical instability at elevated temperatures. Thermochemical equilibrium calculations for reactions between oxides and carbides of the same metal system were used to predict the stability of the associated interfaces. The systems examined were carbon and oxygen in combination with five metals (Si, Ti, Ta, Hf, and Zr). The SiO2/SiC system was determined to be the best choice up to about 1700°C, because of optimization of properties, while at higher temperatures, HfO2/HfC and ZrO2/ZrC systems appear to be more stable thermodynamically, although high thermal expansion coefficient and oxidation rate, for each carbide, may make them less physically compatible.
Crack Initiation in Unidirectional Brittle‐Matrix Composites
Authors: Kim Ran Y. , Pagano Nicholas J.
Published: March 2005

This paper describes the initiation of matrix cracking for glass and glass‐ceramic matrix composites reinforced with small‐diameter silicon carbide and carbon fibers under uniaxial tensile loading. Acoustic emission, replication, and optical microscopy in conjunction with stress‐strain curves are employed to detect the initiation of matrix cracking. The proportional limit of the stress‐strain curve is found to overestimate the initiation of matrix cracking in the material systems studied. The matrix cracking iniates at axial strains from 0.07% to 0.15%. The ACK model overestimates the initiation of the matrix cracking for the material systems studied in this paper.
Copper ⇌ Alkali Ion Exchange of Alkali Aluminosillcate Glasses in Copper‐Containing Molten Salt: I, Monovalent Copper Salt, CuCl
Authors: Yoko Toshinobu , Nishiwaki Toshio , Kamiya Kanichi , Sakka Sumio
Published: March 2005

Cu+⇌ R+ (R = Li, Na, and K) ion exchange experiments were conducted for 20R2O·10Al2O3·70SiO2 glasses in molten CuCl at 550°C in air and nitrogen atmospheres. The depth profiles of the copper incorporated into glasses were determined with an electron microprobe X‐ray analyzer. The total amount of diffusing copper, Mt, strongly depended on the type of alkali ion in the glass and the ion‐exchange atmosphere; i.e., Mt increased with increasing cationic size in the order Li < Na < K and Mt was greater in air than in nitrogen. The Cu ⇌ R+ ion exchange kinetics are discussed in detail.
Nitrogen‐Containing Tetragonal Zirconia
Authors: Cheng Yibing , Thompson Derek P.
Published: March 2005

The present paper gives evidence for the replacement of oxygen by nitrogen when partially stabilized tetragonal zirconia is reacted with nitrogen above 1400°C to give a tetragonal product but in the t′ (nontransformable) form. No transformation toughening is therefore expected from this nitrogencontaining tetragonal phase. These results may explain why less‐effective toughening is obtained in nitrogen‐containing ceramic‐based composites than in equivalent zirconia‐alumina and other oxide‐based systems.
Effect of Processing on Mechanical Properties of Platelet‐Reinforced Mullite Composites
Authors: Nischik Carmen , Seibold Michael M. , Travitzky Nahum A. , Claussen Nils
Published: March 2005

SiC‐ and Al2O3‐platelet‐reinforced mullite‐matrix composites were fabricated by both conventional powder processing and a pressure filtration route with constant filtration rate. Consolidation during the pressure filtration experiment was monitored as a function of time with respect to actual pressing stages. The packing density of green bodies was strongly affected by the different processing methods. Either fracture toughness (3.2 MPa · m1/2) or bending strength (344 MPa) increased, depending on surface conditioning of the SiC platelets. Pressure filtration was shown to decrease processing flaws in the platelet‐reinforced bodies leading to substantial improvement of the mechanical properties.
Influence of Residual Impurities on Phase Partitiioning and Grain Growth Processes of Y‐TZP Materials
Authors: Stoto Tiziana , Nauer Marco , Carry Claude
Published: March 2005

Microstructural evolution at 1450°C of two 3 mol% Y2O3─ZrO2 tetragonal polycrystals (Y‐TZP) containing different residual impurity amounts has been investigated by analytical and scanning electron microscopy in order to clarify the role of the amorphous intergranular phases. Phase partitioning is interpreted as a process based on dissolution/reprecipitation reactions involving mass transport via the liquid impurity phase. Grain growth is inhibited by yttria migration before the phase equilibrium is reached, then increases drastically.
The Density of Titanium(IV) Oxide Liquid
Authors: Dingwell Donald B.
Published: March 2005

The density of TiO2 liquid in equilibrium with air has been measured at 1875° to 1925°C using an Ir double bob Archimedean method. The melt density data have been combined with data extrapolated from the CaSiO3─TiO2 join at 1600°C. A combined fit to these density data yields the following description of the density of liquid TiO2: ρ= 3.7611 ‐ 0.00028T(°C), in the temperature range of 1600° to 1925°C. This expansivity value is consistent with those obtained on TiO2‐rich melts using a Pt‐based system at lower temperature and with multicomponent oxide data. The similarity between the volume of liquid TiO2 and that of crystalline rutile implies a dominantly octahedral coordination of Ti in the liquid state.
Estimation of Interfacial Shear in Ceramic Composites from Frictional Heating Measurements
Authors: Cho Chongdu , Holmes John W. , Barber James R.
Published: March 2005

A new approach for estimating the interfacial frictional shear stress in fiber‐reinforced ceramics is presented. The approach is based upon measurement of the temperature rise which occurs during the cyclic loading of ceramic composites. This temperature rise, which is caused by the frictional slip of fibers within the composite, is related to the level of frictional shear stress which exists along debonded interfaces. To illustrate the technique, the interfacial shear stress in a unidirectional Nicalon‐fiber calcium aluminosilicate matrix composite was determined at ambient temperature.
Theory of Mechanical Properties of Ceramic‐Matrix Composites
Authors: Curtin William A.
Published: March 2005

A theory is presented to predict the pullout work and ultimate tensile strength of ceramic‐matrix composite (CMC) materials tested under uniaxial tension as functions of the underlying material properties. By assuming that the fibers fracture independently and that global load redistribution occurs upon fiber fracture, the successive fragmentation of each fiber in the multifiber composite becomes identical to that of a single fiber embedded in a homogeneous large‐failure‐strain matrix, which has recently been solved exactly by the present author. From single‐fiber fragmentation, the multifiber composite distribution of pullout lengths, work of pullout, and ultimate tensile strength are easily obtained. The trends in these composite properties as a function of the statistical fiber strength, the fiber radius and fill fraction, and the sliding resistance τ between the fibers and the matrix easily emerge from this approach. All these properties are proportional to a characteristic gauge length δc and/or the associated characteristic stress σ, with proportionality constants depending only very weakly on the fiber Weibull modulus: the pullout lengths scale with δc, the work of pullout scales with σcδc, and the ultimate strength scales with σc. The key length δc is the generalization of the “critical length,” defined by Kelly for single‐strength fibers, to fibers with a statistical distribution of strengths. The theory also provides an interpretation of fracture‐mirror measurements of pulled‐out fiber strengths so that the in situ key strength σc and Weibull modulus of the fibers can be determined directly. Comparisons of the theoretical predictions of the ultimate tensile strength to literature data on Nicalon/lithium aluminum silicate (LAS) composites generally show good agreement.
Tensile and Fatigue Behavior of Silicon Carbide Fiber‐Reinforced Aluminosilicate Glass
Authors: Zawada Larry P. , Butkus Lawrence M. , Hartman George A.
Published: March 2005

The onset of damage accumulation in ceramic‐matrix composites occurs as matrix microcracking and fiber/matrix debonding. Tension tests were used to determine the stress and strain levels to first initiate microcracking in both unidirectional and cross‐ply laminates of silicon carbide fiber‐reinforced aluminosilicate glass. Tension–tension fatigue tests were then conducted at stress levels below and above the matrix cracking stress level. At stress levels below matrix microcracking, no loss in stiffness occurred. At stresses above matrix cracking, the elastic modulus of the unidirectional specimens exhibited a gradual decrease during the first 10 000 cycles, and then stabilized. However, the cross‐ply material sustained most of the damage on the first loading cycle. It is shown that fatigue life can be related to nonlinear stress–strain behavior of the 0° plies, and that the cyclic strain limit was approximately 0.3%.
Spontaneous Polarization Screening Effect and Trap‐State Density at Grain Boundaries of Semiconducting Barium Titanate Ceramics
Authors: Wang Da Yu , Umeya Kazumasa
Published: March 2005

The grain‐boundary trap‐state density and the polarization screening effect were studied for a series of semiconducting PTCR barium titanate ceramic samples with different manganese (Mn) additives and different thermal treatments. The grain‐boundary resistance and capacitance data were measured by the ac complex impedance method. The grain‐boundary data obtained were analyzed using a simple double‐depletion‐layer model and an absolute‐zero‐temperature approach for the Fermi distribution function for the boundary trap states. The energy density distributions of the boundary trap states were found to be V‐shaped for the energy range studied, from 0.35 to 1.35 eV, as measured from the conduction band downward. The “neutral” Fermi level at the grain boundary is taken as 1.35 eV and the bulk Fermi level is taken as 0.15 eV from the conduction band. For the samples without the Mn additive, the PTCR effect is controlled by the trap densities located near 0.35 eV. The trap centers are believed to be barium and oxygen vacancies, or chemisorped oxygens. For the samples with Mn additives, the trap densities increase dramatically near 1.35 eV and play a dominant role in the PTCR effect. These trap centers are believed to be Mn4+ ions at the titanium sites. The charge‐compensation effect of spontaneous polarizations on the trap charges was found to be linearly proportional to the total amount of trap charges at that temperature.
Dissolution Rates of Commercial Soda–Lime and Pyrex Borosilicate Glasses: Influence of Solution pH
Authors: Perera Gunarathna , Doremus Robert H.
Published: March 2005

Dissolution rates of two commercial soda—lime silicate glasses and Pyrex borosilicate glass (Corning 7740) have been measured as a function of pH. The dissolution rate was constant with time and increased with increase in pH from 4 to 9. The activation energy for dissolution was nearly constant with pH for the Pyrex borosilicate glass, and decreased somewhat as pH increased for the soda—lime glasses. The openness of the glass surface influences strongly the rate of dissolution. The solubility of silica as a function of pH increases at pH above 8 but does not seem to be quantitatively related to dissolution rate. Other possible influences on this rate are briefly considered.
Theoretical Analysis of the Fiber Pullout and Pushout Tests
Authors: Kerans Ronald J. , Parthasarathy Triplicane A.
Published: March 2005

The fiber pullout and pushout tests have been analyzed to predict the load‐displacement behavior in terms of fiber/matrix interface parameters. The effects of residual axial strain in the fiber and fiber surface topography were included. The residual axial strain was found to be a significant parameter. It is shown that the interface failure can be progressive or catastrophic. In the case of a progressive failure of the interface, the load‐displacement curve is nonlinear. The portion of the curve from above the first nonlinearity to near the peak load can be predicted in terms of parameters of the interface, viz., the friction coefficient, the radial stress at the interface, the fracture toughness of the interface, and the residual axial strain in the fiber. Values for these parameters can be obtained from a single loaddeflection curve. The peak load and load drop, which are usually reported, are found not to be directly relatable to any interface property, since the length of the last portion of the fiber to debond is influenced by end effects and hence not easily predicted. However, for data which describe the peak load as a function of initial embedded length, that factor can be eliminated and the data reduced to yield the relevant interface parameters. In pullout, the peak and friction loads saturate with large specimen thickness. Catastrophic failure is favored when the debond initiation load is high or when residual stress is low. Finally, a methodology to extract interface parameters from experimental data is suggested.
Processing and Characterization of Thin Films of the Two‐Layer Superconducting Phase in the Bi─Sr─Ca─Cu─O System: Evidence for Solid Solution
Authors: Golden Steve J. , Bloomer Tamara E. , Lange Fred F. , Segadaes Ana M. , Vaidya Kaushik J. , Cheetham Anthony K.
Published: March 2005

Thin films in the Bi─Sr─Ca─Cu─O system have been synthesized from liquid ethylhexanoate precursors by spin pyrolysis. An extensive solid‐solution range was found for the two‐Cu‐layer phase through the study of c‐axis‐oriented, single‐phase thin films fabricated on single‐crystal, (100)‐oriented, MgO substrates. All two‐layer compositions were excess in Bi and deficient in Sr + Ca relative to the ideal 2212 composition and showed an overall cation deficiency. The solidus temperature and c lattice parameter were found to vary systematically with composition. Sharp superconductive transitions were obtained in the case of a number of different compositions with Tc varying between 72 and 84 K. Evidence for significant compositional heterogeneities within single‐phase two‐layer thin films was found and the implications for superconductivity are discussed. Compositions within the solid‐solution range gave single‐phase, c‐axis‐oriented films over a wide temperature range extending from 730°C to an upper, solidus (or peritectic) temperature (780° to 840°C) which is dependent on the initial starting composition. A model has been developed that describes the formation of the two‐layer phase from a fugitive liquid.