Showing papers in "Journal of the American Ceramic Society in 1997"

Three-Dimensional Computer Simulation of Portland Cement Hydration and Microstructure Development

Abstract: A three-dimensional computer model for the simulation of portland cement hydration and microstructure development has been developed. Starting with a measured particle-size distribution and a set of scanning electron microscopy images, a three-dimensional representation of a cement of interest is reconstructed, matching the phase volume fractions and surface-area fractions of the two-dimensional images. A set of cellular-automata rules is then applied to the starting microstructure to model the chemical reactions for all of the major phases during the evolving hydration process. The dissolution cycles used in the model have been calibrated to real time using a single set of parameters for two cements at three different water-to-cement ratios. Based on this calibration, there is excellent agreement between the model predictions and experimental measurements for degree of hydration, heat release, and chemical shrinkage. The degree-of-hydration predictions have been successfully applied to predicting the compressive strength development of mortar cubes for the two cements. The effects of temperature have been examined by performing hydration experiments at 15°, 25°, and 35°C and applying a maturity-type relationship to determine a single degree of hydration-equivalent time curve that can be compared to the model predictions. Finally, the computer model has been further extended to simulate hydration under sealed conditions, where self-desiccation limits the achievable hydration.

Paralinear Oxidation of CVD SiC in Water Vapor

Abstract: The oxidation kinetics of CVD SiC were monitored by thermogravimetric analysis (TGA) in a 50% H2O/50% O2 gas mixture flowing at 4.4 cm/s for temperatures between 1200 and 1400 C. Paralinear weight change kinetics were observed as the water vapor oxidized the SiC and simultaneously volatilized the silica scale. The long-term degradation rate of SiC is determined by the volatility of the silica scale. Rapid SiC surface recession rates were estimated from these data for actual aircraft engine combustor conditions.

Processing and Electromechanical Properties of (Bi0.5Na0.5)(1−1.5x)LaxTiO3 Ceramics

Abstract: Bismuth sodium titanate (Bi0.5Na0.5TiO3, BNT) with 0–6 at.% lanthanum was prepared by the conventional mixed oxide method. Each composition was calcined at 800–900°C for 2–5 h to form a pure perovskite phase. Green pellets were sintered at 1050–1150°C for 1–4 h to obtain dense ceramics with at least 95% of theoretical density. X–ray diffraction (XRD) showed phase distortion as lanthanum was added to this system. Meanwhile, a small amount of La was found to affect the grain size and had an influence on the poling conditions and electrical properties. The BNT–based composition with 1 at.% La doping provided a dielectric constant (K) of 560, a piezoelectric charge constant (d33) of 92 pC/N, and a hydrostatic piezoelectric coefficient (dh) of 72 pC/N.

Effect of Solvent on Titania Particle Formation and Morphology in Thermal Hydrolysis of TiCl4

Abstract: Titania powders were synthesized by the thermal hydrolysis of titanium tetrachloride with the yield of above 85% in a mixed solvent of n-propanol and water. The morphology of the precipitates was controlled by adjusting the volume ratio of n-propanol to water (RH ratio) of the mixed solvent. Precipitates obtained with an RH ratio of 0 were fine, and highly agglomerated. In contrast, an RH ratio of 3 resulted in precipitates consisting of uniform and discrete particles. According to observations of the zeta potentials of precipitates and the dielectric constants of solvents, the discrete particles obtained with an RH ratio of 3 resulted from the low zeta potential and dielectric constant. The result of Fourier transform infrared (FTIR) spectroscopy showed the chemical interaction of particle surface with the solvent alcohol, which decreased the zeta potential of precipitates with an increase of RH ratio. Adding hydroxypropyl cellulose (HPC) as a steric dispersant made it possible to reduce the size of particles to the submicrometer range. Effects of the solvent on the formation and morphology of the resulting particles were investigated.

Size Effects in Barium Titanate Particles and Clusters

Abstract: Clustering has an important effect on the tetragonal-cubic transformation of barium titanate (BaTiO3) particles. Small particles that would be cubic if they were by themselves can be tetragonal if they are in a cluster. The effects of clustering are shown in the behavior of the c/a ratio of the particles and the enthalpy change, ΔH, of transition as a function of particle size. The c/a ratio and the value of ΔH both decrease at a smaller particle size than those which are observed in samples where clustering is minimal. Our results are consistent with the observation that very small grains in polycrystalline samples can remain tetragonal even though the grain size is so small that it would be cubic if it were an individual particle. The transition temperature, TC, on the other hand, is insensitive to the particle size, which is similar to the observation in polycrystalline BaTiO3 that TC is insensitive to the grain size. The observed clustering effect is suggested to result from the reduction of depolarization energy of particles in clusters.

Sintering of Fine Oxide Powders: II, Sintering Mechanisms

Abstract: Conventional and new sintering mechanisms have been investigated using fine powders of CeO 2 and Y 2 O 3 of excellent sinterability. We have verified the validity of Herring's scaling law for 60%-84% relative density and found that it is consistent with grain-boundary-diffusion control. At lower densities, we have found that pores larger than the critical size, in the sense of Kingery and Francois, can still be sintered readily. This is rationalized by a new sintering mechanism based on particle repacking concurrent with particle coarsening, resulting in a higher packing factor. Very fine, surface-active powders that coarsen rapidly are uniquely capable of taking advantage of this new sintering mechanism, which along with their propensity to homogenization, accounts for their remarkable sinterability even at very low green densities.

Domain Growth in Pb(Mg1/3Ta2/3)O3 Perovskite Relaxor Ferroelectric Oxides

Abstract: Extensive studies that have been conducted on the Pb(Mg1/3Nb2/3)O3 (PMN) family of relaxor ferroelectrics have led to the establishment and acceptance of the“space-charge” model as a basis for explaining their structures and dielectric properties. In this model, the arrangement of the metal cations on the octahedral sites of the perovskite structure is interpreted in terms of the formation of nega-tively charged ordered nanodomains that are dispersed in a positively charged disordered matrix. The primary experi-mental support for this interpretation has come from the apparent absence of any growth of the domains or change in the degree of ordering as the heat treatment is extended. Here, we report on experiments that have been conducted on the tantalate relaxor, Pb(Mg1/3Ta2/3)O3(PMT), and its solid solutions with PbZrO3, in which the size of the do-mains and the degree of cation ordering have been in-creased by two orders of magnitude through annealing that has been conducted at a temperature of 1325°C. Moreover, fully ordered ceramics that are comprised of large domains retain relaxor behavior. These results cannot be explained by the space-charge model and support a charge-balanced, “random-site” model for the ordering of the metal cations

Hydroxyapatite/Hydroxyapatite‐Whisker Composites without Sintering Additives: Mechanical Properties and Microstructural Evolution

Abstract: Hydroxyapatite/0%-30% hydroxyapatite-whisker (HAp/0%-30%HAp(w)) composites have been fabricated by pressureless sintering, hot pressing, and hot isostatic pressing (HIP). Composites that were HIPed at 1000°-1100°C (2 h, 190 MPa) exhibited the best properties: relative densities of 97.0-99.5%, fracture toughness of 1.4-2.0 MPa·m1/2 (as compared with 1.0 MPam1/2 for the nonreinforced HAp matrix). Compressive pre-stressing and crack deflection contributed mostly to the increase of fracture toughness. Depending on processing conditions, grain growth in the HAp matrix and/or Rayleigh instability of the HAp whiskers were probably responsible for microstructural changes in the composites. The HAp/HAp(w) composites exhibited improved toughness, attaining the lower fracture-toughness limit of bone without a decrease of bioactivity and biocompatibility.

Equilibrium Shape of Internal Cavities in Sapphire

Abstract: The equilibrium shape of internal cavities in sapphire was determined through the study of submicrometer internal cavities in single crystals. Cavities formed from indentation cracks during annealing at 1600°C. Equilibrium could be reached only for cavities that were smaller than is approximately100 nm. Excessive times were required to achieve equilibrium for cavities larger than is approximately 1μm. Five equilibrium facet planes were observed to bound the cavities: the basal (C) {0001}, rhombohedral (R) {1¯012}, prismatic (A) {12¯10}, pyramidal (P) {112¯3}, and structural rhombohedral (S) {101¯1}. The surface energies for these planes relative to the surface energy of the basal plane were γR = 1.05, γA = 1.12, γP = 1.06, γS = 1.07. These energies were compared with the most recent theoretical calculations of the surface energy of sapphire. The comparison was not within experimental scatter for any of the surfaces, with the measured relative surface energies being lower than the calculated energies. Although the prismatic (M) {101¯0} planes are predicted to be a low-energy surface, facets of this orientation were not observed.

Microstructural Development of Silicon Carbide Containing Large Seed Grains

Abstract: Fine ({approximately}0.1 {micro}m) {beta}-SiC powders, with 3.3 wt% large ({approximately}0.44 {micro}m) {alpha}-SiC or {beta}-SiC particles (seeds) added, were hot-pressed at 1,750 C and then annealed at 1,850 C to enhance grain growth. Microstructural development during annealing was investigated using image analysis. The introduction of larger seeds into {beta}-SiC accelerated the grain growth of elongated large grains during annealing, in which no appreciable {beta}{yields}{alpha} phase transformation occurred. The growth of matrix grains in materials with {beta}-SiC seeds was slower than that in materials with {alpha}-SiC seeds. The material with {beta}-SiC seeds, which was annealed at 1,850 C for 4 h, had a bimodal microstructure of small matrix grains and large elongated grains. In contrast, the material with {alpha}-SiC seeds, also annealed at 1,850 C for 4 h, had a uniform microstructure consisting of elongated grains. The fracture toughnesses of the annealed materials with {alpha}-SiC and {beta}-SiC seeds were 5.5 and 5.4 MPa{center_dot}m{sup 1/2}, respectively. Such results suggested that further optimization of microstructure should be possible with {beta}-SiC seeds, because of the remnant driving force for grain growth caused by the bimodal microstructure.

Microstructural Evolution and Mechanical Properties of Si3N4 with Yb2O3 as a Sintering Additive

Abstract: Ytterbium oxide (Yb2O3) was used as a sintering aid to enhance the mechanical properties of silicon nitride (Si3N4) ceramics. The amount of Yb2O3 had significant effects on microstructural evolution and the composition of secondary phases at the grain boundary. When the Yb2O3 added was less than 8 wt%, small homogeneous grains were formed. At the grain boundary, crystalline Yb2Si2O7 was formed along with a glassy phase. As the amounts of Yb2O3 were increased to higher than 8 wt%, large elongated grains were developed in the fine matrix. In those cases, the grain boundary crystalline phase was changed from Yb2Si2O7 to Yb4Si2O7N2. Mechanical properties were influenced by these changes in microstructure and grain boundary phase. The fracture toughness increased with the Yb2O3 content up to 8 wt% and decreased slightly thereafter. The increase in fracture toughness was apparently due to the formation of the large elongated grains. When more than 8 wt% of Yb2O3 was added, interfacial debonding energy between the elongated grains and grain boundary phase became too large, resulting in a decrease in the fracture toughness. The room-temperature flexural strength was not significantly affected by the Yb2O3 content or the microstructure, other than in the case of 2 wt% addition. The high-temperature strength in nitrogen, however, increased steadily with Yb2O3 content. The highest strength, 870 MPa at 1400°C, was observed when 16 wt% of Yb2O3 was added. The increase in the high-temperature strength with Yb2O3 content was attributed to the formation of crystalline Yb4Si2O7N2 phase at the grain boundary.

Mesostructure of calcium silicate hydrate (C-S-H) gels in Portland cement paste : Short-range ordering, nanocrystallinity, and local compositional order

Abstract: Transmission electron microscopy studies of calcium silicate hydrate gels in Portland cement were performed. Selected area electron diffraction studies revealed diffuse rings, indicating the presence of short-range ordering. Investigations of various regions within the gel found strong fluctuations in the diffuseness of the rings ; corresponding local compositional probes showed the presence of large fluctuations in the local calcium:silicon ratio. Lattice imaging studies were then performed to determine the nature of the short-range ordering. These investigations revealed nanocrystalline regions within an amorphous matrix. The local composition within the nanocrystalline regions is believed to be homogeneous. However, strong fluctuations in the local compositional order parameter between various regions would seemingly exist. Optical diffractograms taken from individual nanocrystalline regions demonstrated the coexistence of both jennite and 1.4-nm tobermorite structural elements, supporting the essential features of the Taylor nanophasic model.

Influence of Spray Angle on the Pore and Crack Microstructure of Plasma-Sprayed Deposits

Abstract: To understand the influence of processing parameters on the microstructure of plasma-sprayed deposits, small-angle neutron scattering measurements were made of the processing-parameter-dependent specific surface area of the voids in gray alumina deposits. These studies indicate that the voids are in the form of pores between the splats and around inclusions or unmelted particles, and are also in the form of cracks within the splats which may develop during cooling. The porous volume increases as the angle between the spray gun and the substrate (the spray angle) decreases. This study also indicates that the cracks are preferentially oriented, and that the crack orientation also depends on the spray angle. The interlamellar pores, however, are preferentially oriented parallel to the substrate surface, and the orientation of the pore is independent of the spray angle.

Monoclinic-to-Tetragonal Phase Transformation in a Ceramic Rare-Earth Orthoniobate, LaNbO4

Abstract: The monoclinic-to-tetragonal phase transformation in LaNbO4 has been investigated by X-ray diffraction and dilatometry. Based on the experimental results and theoretical concerns (Landau theory and thermodynamics), it is suggested that this transformation is second order in nature.

Ignition phenomena and reaction mechanisms of the self-propagating high-temperature synthesis reaction in the titanium-carbon-aluminum system

Abstract: The effect of the addition of aluminum on the ignition and self-propagating high-temperature synthesis (SHS) reaction between titanium and carbon was experimentally investigated. Although TiC was the only product compound in the final product, the reaction between titanium and aluminum was believed to occur before the ignition of the reaction between titanium and carbon, as evidenced by a hump that appeared in the temperature profile during heating. The ignition temperature of the reaction between titanium and carbon was significantly decreased by the addition of aluminum. This was explained by the fact that the aluminum provides an easier route for reactant mass transfer, thus significantly increasing the reaction rate of the reaction between titanium and carbon. As the temperature was increased to beyond the melting point of aluminum during heating, aluminum melted and titanium dissolved into it. As the temperature was increased further (1050°C), the titanium-containing aluminum spread over the carbon particles. Ignition is believed to have occurred by the reaction of titanium and carbon at the interface by the diffusion of titanium through the aluminum melt to the interface. The rate of heating and the density of the reactant compact also affected the ignition temperature. These were explained by their influence on the extent of the capillary spreading of the titanium-containing aluminum melt on the carbon particles or by the formation of a TiAlx layer at the interface between the aluminum melt and the titanium particles. Fracture surfaces of the product showed three different types of morphology, i.e., discrete particles, groups of grains similar to sintered bodies, and unreacted reactant particles. A possible reaction mechanism was proposed that describes the ignition process and explains the formation of each type of product.

Structure and Nonlinear Optical Properties of Lanthanide Borate Glasses

Abstract: The third–order nonlinear optical susceptibility, χ(3), of lanthanide (lanthanum, praseodymium, neodymium, and samarium) borate glasses has been measured by the third harmonic generation method. The structure of the present glass system has been studied by infrared and Raman spectroscopic methods. The network structures of the present Ln2O3–B2O3 glasses have been confirmed to be basically similar to each other. Praseodymium, neodymium, and samarium borate glasses exhibit χ(3) values that are larger than lanthanum borate glasses, because of the optical resonance effect, in accordance with the f–f transition. Especially, the χ(3) value for 30Pr2O3·70B2O3 glass is 1.8 × 10−12 esu, which is a factor of ∼60 larger than that of SiO2 glass. This striking enhancement of χ(3) is mainly attributed to the large transition moment to the first excitation state.

Low‐Temperature Sintering and High‐Strength Pb(Zr,Ti)O3‐Matrix Composites Incorporating Silver Particles

Abstract: Lead zirconate titanate based composites containing silver particles were fabricated from commercially available PZT and Ag2O powder mixtures. Densification behavior and mechanical and dielectric properties were evaluated as a function of the volume fraction of silver. No unwanted reaction phases between the PZT matrix and silver could be detected in the X-ray diffraction analysis. The added silver particles were homogeneously dispersed in the matrix. The densification of PZT was substantially accelerated by the addition of silver particles. Mechanical properties, e.g., fracture toughness and fracture strength, were improved by the addition of silver particles. The relative dielectric constant also increased when the volume fraction of silver was increased. This observed enhancement in dielectric constant may be associated with the effective dielectric field developed by the existence of conducting silver particles.

Dielectric Temperature Characteristics of Cerium‐Modified Barium Titanate Based Ceramics with Core—Shell Grain Structure

Abstract: Various particle sizes of starting barium titanate were used to investigate the effect of chemical inhomogeneity on the dielectric temperature characteristics of a dilute solid solution of cerium-modified barium titanate. Transmission electron microscopy and energy dispersive spectroscopy confirmed three distinct areas in the modified barium titanate: a grain core, a grain shell, and gradient regions. The grain size and volume fractions of the grain core were somewhat proportional to the particle size of the starting barium titanate. The dielectric temperature characteristics will be explained in the present paper in terms of the three inhomogeneous regions. Actually, the dielectric temperature characteristics of the large grains (>3 μm) proved to be a function of variation in the T c values, whereas those of the small grains (<3 μm) were a function of both the T c values and the internal stress.

Fabrication, Mechanical Properties, and Electrical Conductivity of Co3O4 Ceramics

Abstract: Well-densified Co{sub 3}O{sub 4} ceramics (98.3% of theoretical) have been fabricated by the combined use of hot pressing (800 C/1 h/30 MPa) and hot isostatic pressing (880 C/2 h/196 MPa). Their Vickers hardness and fracture toughness are 10.3 GPa and 4.2 MPa{center_dot}m{sup 1/2}, respectively. They exhibit a high electrical conductivity of 3.35 {times} 10{sup 1}S{center_dot}cm{sup {minus}1} at 800 C.

Coprecipitation and Hydrothermal Synthesis of Ultrafine 5.5 mol% CeO2-2 mol% YO1.5ZrO2 Powders

Abstract: Ultrafine 5.5 mol% CeO2—2 mol% YO1.5ZrO2 powders with controllable crystallite size were synthesized by two kinds of coprecipitation methods and subsequent crystallization treatment. The amorphous gel produced by ammonia coprecipitation and hydrothermal treatment at 200°C for 3.5 h results in an ultrafine powder with a surface area of 206 m2/g and a crystallite size of 4.8 nm. The powder produced by urea hydrolysis and calcination exhibits a purely tetragonal phase. In addition, the powders crystallized by hydrothermal treatment exhibit high packing density and can be sintered at lower temperature (,1400°C) with nearly 100% tetragonal phase achieved.

Active electrochemical dissolution of molybdenum and application for room-temperature synthesis of crystallized luminescent calcium molybdate film

Abstract: Crystallized luminescent calcium molybdate (CaMoO4) film has been prepared on a molybdenum substrate in an alkaline solution containing calcium ions by active electrochemical dissolution of molybdenum at room temperature (25°C). The dissolution rate became faster with an increase of pH value. A high concentration of calcium (0.02M) and a high pH value (13) favored the reaction of film formation. The film showed only a single green emission at 536 nm with the excitation of 285 nm at liquid-nitrogen temperature (-196°C), strongly suggesting that it consisted of well-crystallized defect-free crystals.

On the Thermodynamic Stability of Amorphous Intergranular Films in Covalent Materials

Abstract: The thermodynamic origin, structure, and stability of the thin amorphous films commonly found in grain boundaries in covalent ceramics are investigated by molecular-dynamics simulation. to focus on the purely thermodynamic aspects, any kinetic effects associated with impurity-controlled interface chemistry are excluded by investigating pure silicon (described by the Stillinger-Weber three-body potential). For this single-component covalent model material, the authors demonstrate that all high-energy boundaries exhibit a universal amorphous structure, with a width of {approximately}0.25 nm, whereas low-energy boundaries are crystalline and much sharper. They also demonstrate that introduction of an amorphous film into a crystalline interface lowers the excess energy to a level similar to the energy of two bulk crystal-amorphous interfaces. The competition between a narrow crystalline boundary structure and a wider amorphous boundary structure is shown to be governed by the relative excess energies of the atoms in the grain boundaries and in the bulk amorphous phase. These observations suggest that, in principle, amorphous grain-boundary films do not require impurities for their stabilization and that, as first proposed by Clarke, an equilibrium grain-boundary phase of uniform thickness can be the result of purely thermodynamic rather than kinetic factors.

Effect of Composition on Mullitization Behavior of α-Alumina/Silica Microcomposite Powders

Abstract: The effect of composition on mullite formation was investigated using submicrometer composite particles that consisted of α-alumina cores and amorphous silica coatings. Powders with varying alumina/silica ratios were prepared by varying the thickness of the silica coating. The mullitization behavior was monitored using differential thermal analysis, X-ray diffractometry, and scanning electron microscopy. The results were consistent with previous studies that indicated that mullite forms initially by nucleation and growth within the siliceous phase, but also that chemical interdiffusion within the mullite grains is required to complete the reaction. The reaction rate in both stages was affected by the alumina/silica ratio. Available evidence has indicated that the first stage of the reaction is controlled by the dissolution of alumina in the siliceous phase. Results for the second stage suggested that alumina diffuses more rapidly through mullite than silica.

Low‐Temperature Chemical Synthesis of Lanthanum Monoaluminate

Abstract: One of the promising candidates for ferroelectric substrate materials, lanthanum monoaluminate (LaAlO3), was successfully synthesized by two separate chemical powder preparation techniques: (i) homogeneous precipitation from aqueous solutions containing urea (CH4N2O) in the presence of nitrate salts, and (ii) self-propagating combustion synthesis from aqueous solutions containing CH4N2O and the respective nitrate salts of lanthanum and aluminum. The submicrometer, spherical-like particles of the precursors were completely converted to pure LaAlO3 at 850C in the homogeneous precipitation route, and the same conversion temperature was observed to be 750C, which becomes the lowest temperature ever reported for the powder synthesis of a pure LaAlO3 phase. The materials were characterized by powder X-ray diffractometry, simultaneous thermogravimetric/differential thermal analysis, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. Structural refinements by Rietveld analysis showed that LaAlO3 was isostructural with BaTbO3 and had the space group R-3c, in contrast to the previously assumed space group of R-3m for this phase. The atomic positions in the structure of LaAlO3 were refined and presented for the first time, with respect to the present space group.

Polarization Dependence of the Cr3+ R-Line Fluorescence from Sapphire and Its Application to Crystal Orientation and Piezospectroscopic Measurement

Abstract: The room-temperature polarization characteristics of the ruby R lines have been studied by monitoring the fluorescence intensity as a function of excitation direction relative to the crystallographic axes of the ruby crystal. The R lines are strongly polarized in the basal plane but have no preferred vibration direction within the basal plane. However, the degree of polarization, P, of the R1 and R2 lines is not the same, being PR1=87% and PR2=62%. The phenomenon is explained in terms of the absorption anisotropy and the probability of spontaneous emission. The findings provide the basis for a high-spatial-resolution spectrophotometric method to determine the optical c-axis of chromium-doped Al2O3 crystals using the angular dependence of the intensity ratio of the R2 line to the R1 line. The application to the orientation determination of sapphire fibers embedded in composites and the corresponding residual stress components are discussed together with the possibility of measuring the texture in the Al2O3 polycrystalline thin films such as oxidation scales. Furthermore, the consequences of polarization orientation on piezospectroscopic measurements are discussed.

In–Situ Transmission Electron Microscopy Crystallization Studies of Sol–Gel‐Derived Barium Titanate Thin Films

Abstract: Barium titanate (BaTiO{sub 3}) thin films that were derived from methoxypropoxide precursors were deposited onto (100) Si, Pt/Ti/SiO{sub 2}/(100) Si, and molecular-beam-epitaxy-grown (MBE-grown) (100) BaTiO{sub 3} on (100) Si substrates by spin coating. The crystallization behavior of the amorphous-gel films was characterized using in-situ transmission electron microscopy heating experiments, glancing-angle X-ray diffraction, and differential thermal analysis/thermogravimetric analysis. Amorphous-gel films crystallized at a temperature of {approximately}600 C to an intermediate nanoscale (5--10 nm) barium titanium carbonate phase, presumably BaTiO{sub 2}CO{sub 3}, that subsequently transformed to nanocrystalline (20--50 nm) BaTiO{sub 3}. Random nucleation in the bulk of the gel film was observed on all substrates. In addition, oriented growth of BaTiO{sub 3} was concurrently observed on MBE-grown BaTiO{sub 3} on (100) Si. High-temperature decomposition of the intermediate carbonate phase contributed to nanometer-scale residual porosity in the films. High concentrations of water of hydrolysis inhibited the formation of the intermediate carbonate phase; however, these sols precipitated and were not suitable for spin coating.

Mechanism and Kinetics of the Carbothermal Nitridation Synthesis of α‐Silicon Nitride

Abstract: The carbothermal nitridation synthesis of {alpha}-Si{sub 3}N{sub 4} is studied using electron microscopy techniques (FEG/SEM and TEM) and chemical composition analysis to characterize the reaction at various degrees of conversion. The reaction follows a nucleation-growth mechanism. Without ``seed`` {alpha}-Si{sub 3}N{sub 4} in the precursor, the reaction rate is controlled by the formation of nuclei which are associated with a Si-O-C intermediate phase. In the presence of seed, the limiting step is growth of {alpha}-Si{sub 3}N{sub 4} onto the seed nuclei. Growth appears to follow a gas-phase route and is characterized by an irregular porous layer which grows onto the seed. The porous structure is the result of reaction around carbon particles which are consumed during the process. The presence of admixed seed Si{sub 3}N{sub 4} in the precursor formulation increases the reaction rate since the nucleation step is eliminated. An activation energy of E = 457 {+-} 55 kJ/mol for the overall reaction closely approximates that previously reported for the formation of SiO. This result, along with the finding that residual crystalline SiO{sub 2} is present at all stages of the reaction, indicates that the overall reaction rate is controlled by the reduction of SiO{sub 2}. Since reaction at themore » carbon and SiO{sub 2} contact points is fast, the rate-limiting step is most likely the gas-phase carbon reduction of SiO{sub 2} with CO.« less

Phase Equilibria in the Ga2O3In2O3 System

Abstract: Subsolidus phase relationships in the Ga2O3–In2O3 system were studied by X-ray diffraction and electron probe microanalysis (EPMA) for the temperature range of 800°–1400°C. The solubility limit of In2O3 in the β-gallia structure decreases with increasing temperature from 44.1 ± 0.5 mol% at 1000°C to 41.4 ± 0.5 mol% at 1400°C. The solubility limit of Ga2O3 in cubic In2O3 increases with temperature from 4.X ± 0.5 mol% at 1000°C to 10.0 ± 0.5 mol% at 1400°C. The previously reported transparent conducting oxide phase in the Ga-In-O system cannot be GaInO3, which is not stable, but is likely the In-doped β-Ga2O3 solid solution.

The α–β Transformation in Silicon Nitride Single Crystals

Abstract: Single crystals of {alpha}-Si{sub 3}N{sub 4} were annealed at 2,000--2,150 C. The {beta} phase was detected after annealing at 2,150 C only when the crystals were surrounded by MgO{center_dot}3Al{sub 2}O{sub 3} or Y{sub 2}O{sub 3} powders. On the other hand, no evidence of the {alpha}-{beta} transformation was found when the crystals were annealed without additives. The solution-precipitation mechanism was concluded to be the dominant factor in the {alpha}-{beta} transformation of Si{sub 3}N{sub 4}.

Formation of Lead Magnesium Niobate Perovskite from Niobate Precursors Having Varying Magnesium Content

Abstract: Formation of lead magnesium niobate (PMN) perovskite phase using a columbite type of precursor together with the addition of excess MgO has been investigated When MgO and Nb2O5 are reacted with 1:1 stoichiometry, a minor amount (}2%) of Mg4Nb2O9 is formed along with the columbite-type phase which in turn could lead to the formation of a minor amount of “free Nb2O5” in the precursor This may be the reason for the formation of small amounts of pyrochlore phase during the synthesis of PMN perovskite from the stoichiometric 1:1 precursor However, it has been found that the perovskite phase can easily be stabilized by the addition of a large excess of MgO in the precursor, although this leads to additional Mg-Nb-O phase Mg4Nb2O9, besides the columbite-type phase MgNb2O6 It has been shown that, after this precursor reacts with PbO to form the PMN perovskite, the excess Mg present in the Mg-rich niobate precursor separates out as MgO, as evidenced by X-ray diffraction, which can be removed by leaching with dilute nitric acid, thus forming pure PMN