bluuebunny/crossref_metadata_2025 huggingface dataset:
Electrical Conductivity of the Solid Solutions XNiO + (1 − X)Tm2O3 (0.01 X 0.05)
Authors: Cho Eun Kyung , Chung Won Yang , Kim Keu Hong , Choi Kyung Moon , Choi Jae Shi
Published: March 2005

NiO‐doped Tm2O3 systems (NDT) containing 1, 3, and 5 mol% NiO were found to be solid solutions by X‐ray diffraction (XRD) analysis. The lattice parameters (a) were obtained by the Nelson‐Riley method, and the values decreased with increasing dopant content. Thermal analysis showed that no phase transition occurred in the temperature range covered in this experiment. The electrical conductivities were measured in the range of temperatures from 400° to 1100°C and of oxygen partial pressures from 1 × 10−5 to 2 × 10−1 atm. The conductivity increases with temperature but there is a break in the conductivity curve, dividing it into two temperature regions. At the high temperatures of 700° to 1100°C, the activation energy (Ea) and oxygen partial pressure dependence of conductivity are found experimentally to be 1.4 to 1.5 eV and 1/n= 1/5.4, and the possible defects and charge carriers are suggested to be metal vacancies and electron holes, respectively. At the lower temperatures of 400° to 600°C, the Ea and 1/n values obtained are 0.8 to 0.9 eV and 1/n= 1/7.2 to 1/8.8, respectively. At the lower temperature, the NDT system seems to display a mixed conduction involving ionic conductivity due to diffusion of oxygen.
Saturated Solutions of Anhydrous Phases in the System Lime‐Silica‐Water: Example of β‐C2S
Authors: Barret Pierre , Bertrandie Danielle
Published: March 2005

Saturated solutions rerely form when the anhydrous constituents of aluminous and portland cement are stirred in water or in lime solutions of increasing concentrations. Apart from monocalcium aluminate, concentration of ions in solution cannot exceed maximum supersaturation with respect to the hydrate most likely to precipitate. The present work shows such a behavior for β‐C2S suspended in water and in lime solutions at low concentration. In more concentrated lime solutions, a short lifetime saturation state with respect to β‐C2S seems to be reached.
SiC‐Whisker‐Reinforced Cellular SiO2 Composites
Authors: Wu Mianxue , Messing Gary L.
Published: March 2005

Fabrication of lighweight, cellular ceramic composites by foaming sol–gels is presented. Foams of 20‐vol%‐SiC‐whisker‐reinforced SiO2 can be tailored with relative densities as low as 10%, with either open or closed cell structures. In addition to reducing drying shrinkage and thus gel cracking, whisker reinforcement significantly increases the strength of the composite foams relative to pure silica foams.
Characterization of Two Germania Gels of Different Coordination
Authors: Mukherjee Shyama P.
Published: March 2005

Two germania gels were synthesized by hydrolytic condensation of germanium ethoxide and by dissolution of germanium dioxide powder in ammonium hydroxide solution. The two gels were characterized using differential scanning calorimetry, X‐ray powder diffraction, surface area and weight loss data, and infrared spectroscopy. The results indicate that, in gels, germanium is capable of tetragonal as well as hexagonal coordination, resulting in different crystallization behaviors.
Electrical Properties and Phase Stability of a Zinc Ferrite
Authors: Kim Jin‐Ho , Yoo Han‐Ill , Tuller Harry L.
Published: March 2005

The electrical conductivity and theromoelectric power of a zinc ferrite of nominal composition 0.46ZnO·0.54Fe2O3 were examined as a function of oxygen partial pressure under equilibrium conditions at elevated temperatures. The isothermal variations of both transport properties were found to be closely correlated with PO2‐induced phase transitions from wustite to ferrite spinel to hematite with successively increasing PO2's. The region of thermodynamic stability of each phase was determined from either of the two property isotherms. Within the single‐phase field of the spinel ferrite, both properties are PO2 dependent. Applying the small polaron hopping model to these results, the PO2 and temperature dependencies of the octahedral ferrous and ferric ion concentrations were extracted and interpreted in terms of the buffering capability of the nominal A‐site cation and the variability of the partial molar enthalpy of oxygen with the progress of the oxidation reaction of Fe2+ to Fe3+.
Failure of Glass with Subthreshold Flaws
Authors: Choi S. R. , Ritter J. E. , Jakus K.
Published: March 2005

Conventional postthreshold crack analysis cannot be used to predict the strength and fatigue behavior of glass with subthreshold flaws. Therefore, a fracture mechanics model for failure of glass with subthreshold indentation flaws was developed. This model accounts for both the near‐ and farfield residual stresses associated with the indentation impression. It is shown that these stresses play a major role in the initiation and subsequent propagation of cracks that eventually cause failure. The model predicts “pop‐in” of a well‐developed crack and failure under continuous and discontinuous crack growth in both inert and fatigue conditions. The results of experiments with bare fused silica fibers with indentation subthreshold flaws in inert and fatigue (water) environments were in good agreement with the predictions by the model.
Computer Simulation of the Microstructure Developed in Reaction‐Sintered Silicon Nitride Ceramics
Authors: Ku Wenfu , Gregory Otto J. , Jennings Hamlin M.
Published: March 2005

A simulation model of the microstructure developed during the reaction sintering of ultrafine silicon powders is presented. The model employs interactive subroutines that describe particle compaction, sintering, and nitridation. The particle compaction model is based on a random particle packing model, whereas the sintering and reaction sintering models are based on the modified grain model and the sharp interface model (SIM). Microstructural changes due to the competition between chemical reaction and sintering are taken into account in the model. The results predicated by this model show good agreement with experimental data from previous studies.
Burner Rig Hot Corrosion of Silicon Carbide and Silicon Nitride
Authors: Fox Dennis S. , Smialek James L.
Published: March 2005

A number of commercially available SiC and Si3N4 materials were exposed to 1000°C for 40 h in a high‐velocity, pressurized burner rig as a simulation of an aircraft turbine environment. Na impurities (2 ppm) added to the burner flame resulted in molten Na2SO4 deposition, attack of the SiC and Si3N4, and formation of substantial Na2O. x(SiO2) corrosion product. Room‐temperature strength of the materials decreased as a result of the formation of corrosion pits in SiC and grain‐boundary dissolution and pitting in Si3N4.
Preparation of Cordierite below 1000°C via Bismuth Oxide Flux
Authors: Dupon Ryan W. , McConville Richard L. , Musolf Douglas J. , Tanous Adam C. , Thompson Mark S.
Published: March 2005

Bismuth oxide has been found to be a useful flux for the preparation of dense body cordierite (Mg2Al4Si5O18). The addition of as little as 2 at.% of bismuth ion to the appropriate stoichiometric mix of commercially available oxide starting materials gives fully dense cordierite after firing for 12 h at 1000°C. The lower‐temperature limit for this reaction is 960°C, with essentially no reaction occurring below this temperature. There is also a temperature‐dependent induction period prior to cordierite formation. The microstructure of the sintered body has the residual bismuth oxide flux distributed in discontinuous domains at the grain boundaries and at the triple points. The cordierite body has a dielectric constant of 5.0, unperturbed by the presence of the bismuth oxide.
Intergranular Fracture of Lithium Fluoride–22% Calcium Fluoride Hypereutectic Salt at 800 K
Authors: Raj Subramanium V. , Whittenberger J. Daniel
Published: March 2005

Constant‐velocity compression tests were conducted at 800 K on as‐cast LiF‐22 mol% CaF2 hypereutectic salt with engineering strain rates varying between 1.8 × 10−6 and 2.3 × 10−1 s−1. Considerable stain hardening was observed during the initial stages of deformation, and the true stress‐strain curves exhibited maxima. Plots of the true strain rate against the flow stress at the proportional limit and the peak stress exhibited a power‐law relation with stress exponents of 7.7. Microstructural examination of the deformed specimens showed extensive grain‐boundary cracking and cavitation. These results suggest that grain‐boundary cracking and interfacial sliding is important for cavity nucleation at the grain boundaries and at the LiF‐CaF2 interfaces, and cavity growth and interlinkage, which appear to depend on the morphological differences between different grain boundaries, occur through the preferential failure of the weaker LiF phase.
Optical and Mechanical Properties of Hot‐Pressed Cesium Iodide
Authors: Kim Hyoun‐Ee , Moorhead Arthur J.
Published: March 2005

Polycrystalline CsI disks were fabricated by hot‐pressing in a nitrogen‐purged glove box. Densification during hotpressing occurred by plastic flow resulting from lattice dislocation glide. Primary recrystallization and extensive grain growth were observed. Both the optical and mechanical properties of this material were significantly affected by grain growth, but in opposite ways. Transmittance increased and strength decreased as grain size increased. The hot‐pressed CsI had transmittance of about 85% in the extra‐long‐wavelength infrared range, a value equivalent to that of single‐crystal CsI. The flexural strength of the CsI that was hot‐pressed under conditions that minimized grain growth was about 8 times higher than that of single‐crystal CsI.
Reactions Which Form Both Silicide and Nitride Layers
Authors: Desu Seshu B. , Taylor J. Ashley
Published: March 2005

Reactions of Ti which form both a silicide and a nitride after rapid thermal annealing have been characterized using Auger depth profiling, X‐ray diffraction, and transmission electron microscopy. Reactions of Ti with Si annealed in a nitrogen ambient at temperatures above 700°C form a thin TiN layer on TiSi2 (C54). A similar structure is formed by annealing Ti deposited on thin Si3N4 films on Si in Ar at 1000°C. The reaction of nitrogen directly with TiSi2 begins at temperatures above 950°C, but at temperatures greater than 1050°C, TiSi2 completely reacts to form TiN. The reaction of Ti with thick Si3N4 films was studied in greater detail. At lower temperatures in Ar, mostly Ti5Si3 is formed at the interface, with some contaminant oxygen and nitrogen released from the reaction, uniformly dissolved throughout the unreacted Ti. Also, a very thin TiN layer exists at the Ti5Si3/Si3N4 interface. At higher temperatures, a three‐layered structure, TiN/TiSix/TiN/(unreacted Si3N4) develops, allowing a conducting layer to be formed on an insulator. A mechanism explaining the formation of the three‐layered structure is discussed. The important reactions are the fast dissolution of nitrogen into the unreacted Ti and the thermal instability of the Ti silicide/Si3N4 interface.
Dissolution of a Sphene Glass‐Ceramic, and of Its Component Sphene and Glass Phases, in Ca‐Na‐Cl Brines
Authors: Hayward Peter J. , Doern Diane C. , George Ian M.
Published: March 2005

Leaching experiments for up to 569 d have been performed to examine the dissolution in Ca‐Na‐Cl groundwater of a sphene (CaTiSiO5)‐based glass‐ceramic, a candidate material for immobilizing nuclear fuel recycle wastes. The experiments involved leaching of samples of a simulated‐waste‐loaded glass‐ceramic, doped with 22Na or 45Ca, in a synthetic groundwater at 25° and 100°C. The results are compared with those from separate leaching experiments with 22Na‐ or 45Ca‐doped samples of aluminosilicate glass and ceramic sphene, representing the component phases of the glass‐ceramic. The comparison supports a model in which the glass‐ceramic dissolution rate may be approximately derived from the weighted average of the separate dissolution rates of its component glass and ceramic phases. No synergistic effects in the leaching of the glass and ceramic phases in the glass‐ceramic were found.
Median Cracking of Brittle Solids Due to Scribing with Sharp Indenters
Authors: Cheng Weilli , Ling Elizabeth , Finnle Iain
Published: March 2005

The process of moving a loaded hard indenter across the surface of a brittle solid to produce a median crack is referred to as “scribing.” As a first step in predicting the depth of median cracking due to sharp indenters, an approximate plastic plane strain analysis is presented for the region under the indenter. This allows the horizontal force transverse to the scribing direction to be predicted. Taking this force and an approximate solution for the stress intensity factor for the crack geometry in scribing leads to a prediction for crack depth as a function of load. The residual stresses which arise on unloading are shown to be a strong function of indenter geometry and may act to extend or partially close the original median crack formed during loading. Reasonable agreement is found between prediction and experiment for six indenter configurations.
Point Defects and Surface Diffusion in Iron Oxide and Nickel Oxide
Authors: Lin Chang‐Ming , Stubican Vladimir S.
Published: March 2005

Diffusion of the 57Co isotope on Fe3O4 (110) and NiO (100) surfaces was investigated by the edge‐source method. The surface diffusion parameter, αDsδ, where α is the segregation factor, Ds the surface diffusion coefficient, and δ the thickness of the high‐diffusivity layer, was determined at 750°C for different partial pressures of oxygen. In both cases point defects strongly influenced surface diffusion. For Fe3O4 the vacancy mechanism is dominant at high oxygen activities and interstitial (or interstitialcy) mechanism dominates at low oxygen activities. For NiO the surface diffusion at low oxygen activities is influenced by aliovalent impurities and at high oxygen activities the strong influence of the intrinsic defects, nickel vacancies, on surface diffusion was observed. It was concluded that similar mechanisms operate during surface diffusion in the near surface layer and during diffusion in the lattice.
Ferroelectric Domain Structure of Lanthanum‐Modified Lead Titanate Ceramics
Authors: Demczyk Brain G. , Rai Raghaw S. , Thomas Gareth
Published: March 2005

The ferroelectric domain configurations in lanthanummodified lead titanate ceramics have been studied by transmission electron microscopy in conventional, analytical, and high‐resolution modes. Results indicate a preponderance of (101), twin‐related 90° domains of equilibrium width 50 to 100 nm. These values are consistent with those derived from consideration of elastic strain energy. Below a critical grain size of approximately 0.3 μm, single‐domain grains are found. A domain wall energy of 1 mJ/m2 was calculated based on this observation. Within such twins are occasionally found ordered domains, displaying little or no misorientation with respect to one another. In addition, (001), 180° domains were observed.
Inclusion‐Initiated Fracture Model for Ceramics
Authors: Sung Jason , Nicholson Patrick S.
Published: March 2005

The fracture of ceramics initiating from a typical inclusion is analyzed. The inclusion is considered to have a thermal expansion coefficient and fracture toughness lower than those of the matrix and a Young's modulus higher than that of the matrix. Inclusion‐initiated fracture is modeled for a spherical inclusion using a weight function method to compute the residual stress intensity factor for a part‐through elliptical crack. The results are applied to an α‐Al2O3 inclusion embedded in a tetragonal ZrO2 ceramic. The strength predictions agree well with experimental data.
Mechanical Healing of Potassium Chloride Single Crystals
Authors: Jang Syun M. , Lee Sanboh
Published: March 2005

The mechanical healing of KCl single crystals has been investigated under atmospheric pressure at room temperature. A crack was introduced by a utility knife and then fractured by tensile load. The healing process can be divided into three stages. The first and the second stages correspond to the progressive healing in fractures arising from the tensile load and the utility knife, respectively. The third stage happens when the compression load is still applied after the above two stages. The healing rate is related to the morphology of the fracture surfaces and the employed strain rate, but is independent of the compressive load. The effect of strain rate on fracture toughness for healed material is opposite to that for virgin material. The optimal repeated healing process was related to the applied load. Finally, the effects of γ‐ray irradiation and prior compression on healing are described.
Sintering of Spherical Glass Powder under a Uniaxial Stress
Authors: Rahaman Mohamed N. , De Jonghe Lutgard C.
Published: March 2005

The sintering of spherical borosilicate glass powder (particle size 5 to 10 μm) under a uniaxial stress was studied at 800°C. The experiments allowed the measurement of the kinetics of densification and creep, the viscosities for creep and bulk deformation, and the sintering stress which was found to increase with density. The data show excellent qualitative agreement with Scherer's theory of viscous sintering. In addition, the quantitative comparison between theory and experiment shows good agreement; the measured viscosity of the bulk glass was ∽1×109 P (∽1×108 Pa·s) compared to ∽3×109 P (∽3 Pa·s) obtained by fitting the data with Scherer's theory.
Structure of Sodium Aluminosilicate Glasses
Authors: Zirl David M. , Garofalini Stephen H.
Published: March 2005

A series of sodium aluminosilicate glasses composed of varying ratios (R) of Al2O3/Na2O (0.25 R 2.0) has been simulated with the molecular dynamics technique using a tetrahedral form of a three‐body interaction potential. Extrema in the activation energies for sodium diffusion and in the diffusion constants for all of the atomic species were observed for glasses with equal concentrations of Al2O3 and Na2O (R= 1.0). These changes corresponded to the minimum observed experimentally in the activation energy for electrical conductivity and to the maximum observed in the viscosity for glasses with compositions of R= 1.0. The coordination of aluminum remained 4 over the entire compositional range, negating the need to invoke a coordination change of aluminum to explain the changes in the physical properties. The changes to the simulated physical properties as R passed through the equivalence point were attributed to the elimination of nonbridging oxygen, to the introduction of oxygen triclusters, and to changes in the distribution of ring structures within the glass networks.
Raman Microprobe Investigation of Oxygen and Carbon Dioxide Dissolution from Bubbles in Silicate Glasses Containing Arsenic Oxide
Authors: Lee Sang Won , Condrate R. A.
Published: March 2005

Raman microprobe analysis was used to investigate the changes in the relative concentrations of oxygen and carbon dioxide found in bubbles in silicate glasses containing different amounts of arsenic oxide, sodium nitrate, and/or sodium nitrite after various heat treatments. As expected, analysis of the spectral data clearly indicated that addition of arsenic oxide to the glass accelerates the relative rate of oxygen dissolution with respect to carbon dioxide. Addition of either sodium nitrate or sodium nitrite to the glass composition containing arsenic oxide generates a relative rate of oxygen dissolution that is intermediate between those for the glass containing no refining agent and the silicate glass containing only arsenic oxide. This relative rate of oxygen dissolution is closely related to the amount of As3+ that is present in the glass.
Removal of Carbonaceous Residue with Wet Hydrogen in Greensheet Processing for Multilayer Ceramic Module: I, Residue Formation and Intrinsic Chemical Kinetics
Authors: Wall Donald R. , Sohn Hong Y.
Published: March 2005

An experimental and mathematical investigation of the removal of carbonaceous residue deposited on the ceramic substrate in the fabrication of multilayer ceramic modules has been conducted. Such carbonaceous residue is formed when the organic binder is pyrolyzed during the firing of greensheets. This Part I is concerned specifically with the formation of carbonaceous residue and the intrinsic kinetics of its subsequent removal in a mixture of water vapor with hydrogen. The reaction is of first order with respect to the amount of the carbonaceous residue and depends on the water vapor and hydrogen partial pressures according to a Langmuir‐Hinshelwood kinetic expression.
Effect of Ribbon Orientation on the Fracture Toughness of a Metallic‐Glass‐Ribbon‐Reinforced Glass‐Ceramic Matrix Composite
Authors: Vaidya Rajendra U. , Subramanian K. N.
Published: March 2005

A commercially available amorphous metal ribbon incorporated into a glass‐ceramic matrix was assessed from the standpoint of fracture toughness. The ribbon orientation (with respect to the long and short transverse sides) relative to the opening crack significantly affected the fracture toughness of such composites. The strong dependence of the fracture toughness on the ribbon orientation was correlated with the differences in the bending contributions of the ribbons in the different orientations. The effect of the ribbon orientation on the crack growth rate in the matrix was also investigated.
Growth of β‐Alumina (Na2O · 11Al2O3) Single Crystals by the Flux Method
Authors: Sumiyoshi Yoshihiro , Ushio Masumi
Published: March 2005

The crystal‐growth process and growth conditions of β‐alumina (Na2O · Al2O3) were investigated using the Na2B4O7‐Na3AlF6 flux method. β‐Alumina (electric fusion brick) was used as both nutrient and seed. Weight loss of the flux varied widely for various runs: ≅ 10 wt% of flux evaporated at 100 h, ≅ 17 wt% at 150 h, and 43 wt% at 600 h. When β‐alumina crystal was grown, only 20 wt% Na2B4O7 was added to the Na3AlF6 flux. The linear growth rates of the β‐alumina single crystal grown by an Na3AlF6‐20 wt% Na2B4O7 flux method at 1040°C and Δt= 18°C were ≅ 1.0 × 10−3 mm/h (a face) and ≅0.3 × 10−3 mm/h (c face). The β‐alumina single crystals grown were bounded by only c[001] and a[100] and were colorless and transparent.
Si3N4/SiC‐Whisker Composites without Sintering Aids: I, Quantitative Evaluation of Microstructure
Authors: Pezzotti Giuseppe , Tanaka Isao , Okamoto Taira
Published: March 2005

A microstructural evaluation of Si3N4 with 20 vol% SiC whiskers, fully densified by hot isostatic pressing (HIP) without sintering aids, is presented. The grain size and morphology of the matrix, the whisker aspect ratio after sintering, interfacial bonding, and the structural stability of reinforcement up to 2000°C are discussed. Image analysis provides quantitative information about whisker dispersion and orientation. It is pointed out that a whisker dispersion and orientation. It is pointed out that a whisker composite with a high degree of homogeneity and isotropy can be obtained by optimizing the mixing procedure and using HIP.
Diffuseness as a Useful Parameter for Relaxor Ceramics
Authors: Pilgrim Steven M. , Sutherland Audrey E. , Winzer Stephen R.
Published: March 2005

The low‐field electrical behavior of relaxor ferroelectric ceramics has been qualitatively described by many parameters, including diffuseness (δ), which characterizes the breadth of the transition peak and is roughly related to the standard deviation of the peak. Three major types of equations have been used to calculate δ: Gaussian distribution (originated by Smolenskii and Rolov et al.), power law (used by many authors), and variable power law (Uchino et al). In this work, the three calculational techniques are compared analytically and illustrated using dielectric data from ceramic 0.88Pb(Mg1/3Nb2/3)O3. 0.12PbTiO3. All three methods of analysis result in very linear plots; however, the calculated values of δ are significantly different. A method of limiting the temperature range to that corresponding to 2/3 of the maximum relative dielectric constant is suggested in order to decrease the deviations. This limitation is shown to give consistent reproducible diffuseness values without extraneous contributions arising from the relaxor nature of the material or from differing temperature ranges of measurement, thus allowing δ to be used as an effective comparative parameter for relaxor ceramics.
Residual Stress Determination Using Strain Gage Measurements
Authors: Tandon Rajan , Green David J.
Published: March 2005

A strain gage technique, which relates the prior residual stress state in a material to the strain data obtained by fixing a strain gage on one surface and grinding off the other, has been proposed previously. In the current work, a generalized solution for obtaining an arbitray residual stress profile from strain gage data is presented. Numerical analysis using the solution indicates that the formulation is insensitive to random errors of 10% or less in the experimental data. Based on the results of the analysis, a procedure for determining stress profiles from strain gage data is outlined. Experimental data for tempered glass was analyzed using the technique proposed. The stress profiles predicted are in good agreement with independent observations using indentation and strength data.
Neutron Powder Diffraction Studies of Ce3B2N4 and Isotypic RE3B2N4 Compounds (RE = La, Pr, Nd, MM)
Authors: Rogl Peter , Klesnar Helmuth , Fischer Peter
Published: March 2005

The crystal structure of Ce3B2N4 (space group Immm, α= 0.35653(5) nm, b= 0.63160(21) nm, c= 1.07131(50) nm) was derived from X‐ray powder photographs; precise atom parameters, atom occupancies, atom order, and interatomic distances were obtained from room‐temperature neutron powder diffraction data employing the Rietveld profile analysis method. The final reliability values of the refinement were RF= 0.059 and RI= 0.068. Boron atoms in triangular prismatic metal coordination [Ce6B] are at a distance of 0.1785(7) nm and form pairs. In a tetrakaidecahedral surrounding [Ce6B]BN2, each boron atom forms single bonds with two nitrogen atoms at a distance of 0.1464(4) nm. Nitrogen atoms thus are in rectangular pyramidal metal coordination [Ce5N] with additional boron atom completing a distorted octahedron [Ce5BN]. There are no direct N─N bonds. With respect to atom‐site occupancy, the crystal structure of Ce3B2N4 is isopointal with the structure type of CeCr2B6 (═(CeCr2)B2B4) and can furthermore be considered as a filled‐up version of the W2CoB2‐type ((W2Co)B2□4). Isotypic compounds, RE3B2N4, were synthesized with all the early and large rare‐earth metals (RE = La, Ce, Pr, Nd) and mischmetal (MM). Nd3B2N4 was found to be unstable at temperatures higher than 1400°C. No RE3B2N4‐phase was observed with the smaller rare‐earth elements Gd Lu, Sc, and yttrium.
Morphology of Phase Separation in AIN─Al2OC and SiC─AIN Ceramics
Authors: Kuo Shih‐Yee , Virkar Anil V.
Published: March 2005

Solid solutions in the AIN─Al2OC and SiC─AlN systems were fabricated by hot‐pressing powder mixtures in graphite dies. X‐ray diffraction showed the samples to be single phases of 2H structure. The samples were annealed between 1600° and 1900°C for up to 1000 h. In the SiC─AlN system, optical microscopy and X‐ray diffraction failed to reveal microstructural or phase changes. However, electron microscopy showed that samples had decomposed. Streaking of diffraction spots occurred along directions orthogonal to {012} planes (∼43° off the c axis), which is approximately the direction along which the elastic energy function is a minimum. The orientation‐dependent Young's modulus was also a minimum along this direction. In AIN─Al2OC, optical microscopy and X‐ray diffraction indicated the occurrence of decomposition. The precipitates were disk‐shaped with [001] orthogonal to the disks. The occurrence of decomposition along the [001] direction suggests that it is the elastically soft direction.
Effects of Inclusions on the Sintering Behavior of YBa2Cu3O6+x
Authors: Stearns Laura C. , Harmer Martin P. , Chan Helen M.
Published: March 2005

The sintering behavior of two types of heterogeneous compacts of YBa2Cu3O6+x was studied: (1) Soft agglomerates present in the starting powder were used to study the effect of rapidly densifying inclusions on the overall sample densification. In this case, the induced stresses caused severe cracklike damage in the sintered microstructure. (2) On the other hand, when nondensifying inclusions (same composition) were incorporated into the starting powder, no sintering damage was observed. Further, there was no retardation of densification or coarsening due to the presence of these dense inclusions, over a wide range of inclusion size. Several possibilities for this behavior are discussed, based on the distribution of stresses induced by differential sintering rates.
Modified Tape Casting Method for Ceramic Joining: Application to Joining of Silicon Carbide
Authors: Rabin Barry H.
Published: March 2005

A simple modified tape casting procedure has been developed for application to ceramic joining when the joining materials are in powder form. The method involves preparation of a slurry from the powder, solvent, and thermoplastic binder, and then casting directly onto the joining surface using a moving doctor blade. Handling of the tape prior to joining is not necessary: therefore, binder content is minimized, plasticizers are not required, and viscosity is controlled by solvent content. The utility of this technique for producing joints with thin, uniform interlayers is demonstrated for silicon carbide materials joined with TiC + Ni and SiC + Si.
Electrical Resistivity of Composites
Authors: McLachlan David S. , Blaszkiewicz Michael , Newnham Robert E.
Published: March 2005

Percolation and Bruggeman's effective media theories, as they apply to the electrical conductivity of composites, are reviewed, and a general effective media (GEM) equation, which combines most aspects of both percolation and effective media theories, is introduced. It is then shown that the GEM equation quantitatively fits electrical resistivity (conductivity) as a function of the volume fraction data for binary composites. The parameters used to fit the experimental data are the electrical resistivities of the two phases, the percolation threshold for the lower resistivity phase (ôc), and an exponent t. Preliminary work, showing how the GEM equation can be used to model the piezoresistivity of composites by postulating that ôc is a function of the independent variable, is also presented.
Validity of Using Mercury Porosimetry to Characterize the Pore Structures of Ceramic Green Compacts
Authors: Lee Howard H. D.
Published: March 2005

The ability of mercury porosimetry in characterizing particle packing and pore structures of dense ceramic green compacts is evaluated. Differences in intrusion spectra between the dry‐pressed and slip‐cast compacts demonstrate the utility of mercury porosimetry for particle‐packing analysis. Of the voids embedded in the matrix, only those at or extending to the matrix surface are detectable. Intrusion—extrusion spectra show hysteresis, but otherwise do not provide added insight into bulk defect structure.
Ractions of SiC with H2/H2O/Ar Mixtures at 1300°C
Authors: Jacobson Nathan S. , Eckel Andrew J. , Misra Ajay K. , Humphrey Donald L.
Published: March 2005

The reactions of a sintered α‐SiC with 5% H2/H2O/Ar at 1300°C were studied. Thermomchemical modeling indicates that three reaction regions are expected, depending on the initial water vapor or equivalently oxygen content of the gas stream. A high oxygen content (P(O2) > 10−22 atm) leads to a SiO2 formation. This generally forms as a protective film and limits consumption of the SiC (passive oxidation). An intermediate oxygen content (10−22 atm > P(O2) > 10−26 atm) leads to SiO and CO formation. These gaseous products can lead to rapid consumption of the SiC (active oxidation). Thermogravimetric studies in this intermediate region gave reaction rates which appear to be controlled by H2O gas‐phase transport to the sample and reacted microstructures showed extensive grain‐boundary attack in this region. Finally, a very low oxygen content (P(O2) < 10−26 atm) is thermochemically predicted to lead to selective removal of carbon and formation of free silicon. Experimentally low weight losses and iron silicides are observed in this region. The iron silicides are attributed to reaction of free silicon and iron impurities in the system.
Characterization of Textured Ceramics by Electron Paramagnetic Resonance Spectroscopy: II, Formation and Properties of Textured MgO
Authors: Boldä O. José Luis , Boatner Lynn A. , Abraham Marvin M.
Published: March 2005

A method for the formation of macroscopic quantities of faceted, nanophase MgO precursor powders has been developed that permits a degree of control over the particulate size distribution. By employing powders of this type it was possible to form textured MgO ceramics by means of cold‐pressing and sintering. The theoretical basis for the application of EPR techniques to texture analysis as developed in Part I of this work was used to establish the nature of the preferred orientation in these MgO ceramics. Both EPR and X‐ray analyses showed that preferred orientations of the 〈111〉 axes of the MgO microstructural grains occur along or near the direction of force used to compact the green body. EPR results also show that this texture develops during the sintering process and that no preferred orientation effects are present in the unsintered body formed by cold‐pressing the faceted, nanophase MgO powder. Effects of parametric variations such as sintering temperature, compaction force, and faceted particle size on the degree of preferred orientation in these sintered MgO ceramics were investigated using EPR techniques. The results show that the amount of texturing increases with increasing compaction force or temperature up to a limiting value and that it also increases with decreasing faceted particle size.
High‐Temperature Mechanical Properties of a Uniaxially Reinforced Zircon‐Silicon Carbide Composite
Authors: Singh Raj N.
Published: March 2005

Mechanical properties of a monolithic zircon ceramic and zircon‐matrix composites uniaxially reinforced with either uncoated or BN‐coated silicon carbide monofilaments were measured in flexure between 25° and 1477°C. Monolithic zircon ceramics were weak and exhibited a brittle failure up to about 1300°C. An increasing amount of the plastic deformation was observed before failure above about 1300°C. In contrast, composites reinforced with either uncoated or BN‐coated SiC filaments were stronger and tougher than the monolithic zircon at all test temperatures between 25° and 1477°C. The ultimate strength and work‐of‐fracture of composite samples decreased with increasing temperature. A transgranular matrix fracture was shown by the monolithic and composite samples tested up to about 1200°C, whereas an increasing amount of the intergranular matrix fracture was displayed above 1200°C.
A Compression‐Loaded Double Cantilever Beam Specimen
Authors: Gonzalez Armando C. , Pantano Carlo G.
Published: March 2005

In crack propagation studies, double cantilever beam specimens are typically loaded in tension or by an applied moment. A method of compression loading is described in which the stress intensity KI decreases with crack length. This new arrangement offers added crack stability over other techniques, as well as a convenient method of loading.
Surface Chemistry of Various Poly(vinyl butyral) Polymers Adsorbed onto Alumina
Authors: Howard Kevin E. , Lakeman Charles D. E. , Payne David A.
Published: March 2005

Organic polymeric materials, often used as ceramic processing aids to assist in the fabrication of green bodies, are thought to be of a temporary nature, nature, being easily removed by thermolysis at temperatures between 250° and 600°C. The interaction of various poly(vinyl butyral) binders with fine, high‐purity alumina powder was investigated using variable‐temperature Fourier‐transform infrared spectroscopy and thermal gravimetric analysis. It was found that there was interaction between the functionality of the polymers and the surface of the alumina powders. If these interactions are strong enough to persist to initial sintering temperatures, the polymers would not then be merely innocuous temporary processing aids, but could have an effect on the development of ceramic microstructures.
Preparation of Strontium Titanate Thin Film on Titanium Metal Substrate by Hydrothermal‐Electrochemical Method
Authors: Yoo Seung Eul , Hayashi Motoo , Ishizawa Nobuo , Yoshimura Masahiro
Published: March 2005

Polycrystalline SrTiO3 thin films have been prepared on Timetal substrates by the hydrothermal‐electrochemical method. The films were prepared at temperatures ranging from 100° to 200°C and under saturated vapor pressures in electrolytic solutions of 1N to 5N Sr(OH)2. A galvanostat was used to apply a direct current of 5 to 25 mA/cm2. The films had smooth and homogeneous surfaces without any visible pores or defects. The dielectric constant of the films was approximately 129 assuming 25 nm for film thickness.
Optical Basicity of Titanium(IV) Oxide
Authors: Bergman Åke
Published: March 2005

The relationship between phosphate and vanadate capacites and optical basicity, Λ, are used in order to settle the optical basicity of TiO2. Phosphate and vanadate capacity data indicate that the optical basicity of TiO2 is 0.55. The validity of this value is supported by the good agreement with the value derived from Pauling electronegativity.
Electroceramics from Source Materials via Molecular Intermediates: PbTiO3 from TiO2 via [Ti(catecholate)3]2‐
Authors: Davies Julian A. , Dutremez Sylvain
Published: March 2005

The precursor [NH4]2[Ti(catecholate)3] · 2H2O is known to react with Ba(OH)2· 8H2O in an acid/base process that generates Ba[Ti(catecholate)3] · 3H2O, a compound which undergoes low‐temperatue calcination to produce BaTiO3 powder. Attempts to develop similar routes to PbTiO3 have been frustrated, since lead(II) hydroxide does not exist. The amphoteric yellow PbO and the basic oxide, Pb6O(OH)64+, are both insufficiently basic to react with [NH4]2[Ti(catecholate)3] · 2H2O. Based on the large sizes of both the [Ti(catecholate)3]2‐ anion and the Pb2+ cation, a precipitation method has been developed in which lead nitrate and [NH4]2[Ti(catecholate)3] · 2H2O are added together in an aqueous medium causing precipitation and leaving only NH4NO3 in solution. The lead‐titanium‐catecholate complex that forms in this manner undergoes low‐temperature pyrolysis to produce PbTiO3. SEM indicates a submicrometer ultimate crystallite size.
Fiber‐Matrix Bond‐Strength Characterization of Silicon Carbide‐Silicon Carbide Materials
Authors: Abbé Francois , Chermant Jean‐Louis
Published: March 2005

The fiber‐matrix interface bond strength of a ceramicmatrix composite is quantitatively evaluated using an indentation technique at room temperature and after creep tests under vacuum. The observed decrease in frictional stress with increasing temperature is discussed in connection with such other data as rupture stress, importance of pullout, and geometric characteristics of the fibers.
Pressure and Temperature Dependence of the Raman Spectra of Zirconia and Hafnia
Authors: Kourouklis Gerasimos A. , Liarokapis Efthymios
Published: March 2005

The pressure dependence of the phonon frequencies in the isomorphic materials ZrO2 and HfO2, and the temperature dependence of the phonon frequencies of HfO2, were investigated by Raman spectroscopy in the ranges 0 to 16 GPa and 20 to 800 K. The comparison of the pressure dependence of the Raman spectra of the two materials helps to elucidate the correlation of their vibrations in the intermediate spectral region (240 to 410 cm−1). The high‐pressure phase of ZrO2 is stable up to 16 GPa and of tetragonal symmetry. HfO2 does not show any pressure‐induced phase transition up to 12 GPa, while the temperature effect on the phonon frequencies is similar to that of ZrO2.
Effects of Hydrolysis on the Kinetics of High‐Temperature Transformations in Aluminosilicate Gels
Authors: Li Dong X. , Thomson William J.
Published: March 2005

The reaction kinetics for the formation of mullite (3Al2O3· 2SiO2) from sol‐gel‐derived aluminosilicate gels prepared under various hydrolysis conditions were studied using dynamic X‐ray diffraction (DXRD) and differential thermal analysis (DTA). The DXRD experiments showed that the apparent single‐phase gels aluminosilicate gels were not completely single‐phase gels but a composite of a single‐phase gel (which has molecular‐scale mixing) and a diphasic gel (which has nanometer‐scale mixing). Mullite formation from these composite gels exhibits a two‐stage conversion, the first at about 980°C and the second at about 1220°C. Gels prepared by a slower hydrolysis rate tend to have a higher conversion after the first stage, therefore, better molecular‐scale mixing. Simultaneous formation of Al‐Si spinel and mullite was also observed at 980°C. This coincidence of mullite and spinel formation could explain some of the controversy in the literature concerning the 980°C thermal event.
Residual Stress in Alumina–Mullite Composites
Authors: Root John H. , Sullivan James D. , Marple Basil R.
Published: March 2005

Lattice strains have been measured by neutron diffraction from the alumina matrix in alumina–mullite composite cylinders. Infiltration processing results in a macroscopic variation of mullite concentration and residual stress throughout the samples. At the midheight of the cylinder, the stress component parallel to the cylinder axis is in a compression of −130 ± 30 MPa with respect to the value near the top surface. At the surfaces, the stress component parallel to the surface is in a compression of −130 ± 30 MPa with respect to values near the midheight of the cylinder. An overall tensile shift of the macroscopic stress profile in a fully infiltrated specimen is interpreted as a grain interaction effect between the alumina and mullite constituents. The positional dependence of the mullite phase concentration is also determined by neutron diffraction.
Wettability of Pyrolytic Boron Nitride by Aluminum
Authors: Chiaramonte Francis P. , Rosenthal Bruce N.
Published: March 2005

The wetting of pyrolytic boron nitride by molten 99.9999% pure aluminum was investigated by using the sessile drop method in a vacuum operating at approximately 660 μPa at temperatures ranging from 700° to 1000°C. The equilibrium contact angle decreased with an increase in temperture. For temperatures at 900°C or less the equilibrium contact angle was greater than 90°. At 1000°C a nonwetting‐to‐wetting transition occurred and the contact angle stabilized at 49°.
Internal Nucleation of Highly Undercooled Magnesium Metasilicate Melts
Authors: Cooper Reid F. , Yoon Woo Young , Perepezko John H.
Published: March 2005

Crystallization and vitrification in undercooled, fine magnesium silicate droplets, with compositions ranging from 34.5 ≤ wt% MgO ≤ 39.9, were examined following containerless drop tube processing. From an initial phase assemblage of a mixture of the metasilicate (MgSiO3) polymorphs orthoenstatite and clinoenstatite, three morphological powder types were observed following processing: unmelted shards, glass spheres, and melted/recrystallized spheres. The primary phase in the powders processed at a maximum temperature of ∼1650°C is the high‐temperature metasilicate polymorph protoenstatite, with metastable forsterite (Mg2SiO4) also appearing. The melted/recrystallized spheres have the uniform, submicrometer texture of a glass ceramic, decisively different from the surface crystallization textures normally seen for melts/glasses of these compositions. Transmission electron microscopy results indicate that the glass‐ceramic texture occurs because the process technique allows a liquid‐phase immiscibility to precede crystallization. The phases and textures developed during containerless solidification processing of these metsilicate compositions are analyzed thermodynamically; the minimum amount of undercooling required for amorphous phase separation is evaluated using the metastable extensions of the forsterite + liquid and the silica‐rich, twoliquid miscibility phase boundaries. The application of metastable phase diagram analysis is demonstrated as an effective guide for identifying potential compositions for development of novel glass‐ceramics.
Influence of Processing Parameters on the Strength of Fluoride Glass Fibers
Authors: Orcel Gérard , Biswas Dipak
Published: March 2005

The different factors affecting the tensile strength of heavy‐metal fluoride glasses are presented. Surface crystals were found to be responsible for the lower fiber strength. The tensile strength of fluoride glass fibers can be increased by controlling the preform surface and drawing in a dry atmosphere. Using lower draw temperature and dual polymeric coatings increases the strength. Finally, phosphate glass overclad on fluoride glass fiber can be used to prevent surface hydrolysis during the drawing process and increases the tensile strength of fibers due to formation of compressive stress on the outside of the glass fiber.
Carbothermal Reduction of the Molybdenum Oxide/Phenanthroline Complex
Authors: Ōya Asao , Kasahara Michika , Ohashi Humihiko , Ōtani Sugio
Published: March 2005

Samples of an MoO3/phenanthroline complex and an MoO3/carbon black mixture were heated below 1400°C in nitrogen to characterize the carbothermal reduction process of the complex. The conversion of crystalline phases by carbothermal reduction followed substantially the same process in both samples but occurred at a temperature 100° to 150°C lower in the complex than in the carbon black mixture. Two possible explanations for this difference are theorized: (1) The complex may provide more intimate contact between the MoO3 and the intercalated reducing material. (2) The intercalated reducing agents may have a higher reactivity than does the carbon black.
Probing the Nature and the Structure of Pores in Silica Xerogels by Water Sorption: The Tetramethyl Orthosilicate‐Hydrogen Chloride/Fluoride System
Authors: Malla Prakash B. , Komarneni Sridhar , Taguchi Hideki , Kido Hiroyasu
Published: March 2005

A series of silica xerogels were synthesized in a closed system by hydrolyzing tetramethyl orthosilicate (TMOS) with various concentrations of hydrochloric acid solution but without the addition of alcohol. The molar ratio of H2O to TMOS was fixed at 4.96. The gels were characterized by water and nitrogen sorption measurements. The gelation time peaked at 50 to 80 h with 5 × 10−2 to 10−1M HCl, apparently a consequence of the isoelectric point of silica. Surface area, pore size, and sorption capacity were, however, at a minimum for samples prepared with 5 × 10−2 to 10−1M HCl. The BET c constant and hence the net heats of sorption were at a maximum for these samples. Although water sorption of all the samples prepared with HCl exhibited moderate Type I isotherms, the concavity of the isotherms to the P/P0 axis increased with increasing concentrations of HCl up to 10−1M in accordance with the decreased pore size and increased heat of sorption. Addition of F− ions (HF or NaF) in the presence of 10−1M HCl steadily decreased the gelation time. Addition of F− in the absence of HCl, however, caused rapid gelation and an increase in pore size, nitrogen BET surface area, and hydrophobicity. Consequently, the shape of water isotherms gradually converted to Type IV and/or Type V. Adsorption and desorption measurements on differently activated gels exhibited various types of hystereses as a result of development of hydrophobie sites at as low as 200°C, inelastic distortion induced by adsorption, and chemical reaction and aging of gels when exposed to water vapor.