Amorphous Si-C-O fiber (Nicalon) was oxidized under oxygen partial pressures from 0.25 to 1 at temperatures from 1073 to 1773K. The oxidation mechanism of Nicalon was proposed by carrying out the kinetic analysis of TG data as follows:
(1) In the earliest stage of oxidation, its rate obeyed both the linear rate equation for gas-phase diffusion control and the two-dimensional disc-contracting rate equation for reaction control. Both the rate constants, k_g and k_r, were proportional to the partial pressure of oxygen gas, p_O2. The apparent activation energies for gas-phase diffusion and for chemical reaction were estimated to be 50 and 45kJ/mol, respectively. The initial rate seems to be determined by both the gas-phase diffusion and chemical reaction.
(2) The oxidation rate at later stage was described by the two-dimensional disc-contracting rate equation for diffusion control. The proportionality between the rate constant, k_d, and p_O2 was found to hold. The apparent activation energy of 102kJ/mol was obtained.
(3) It is thought that oxygen molecules diffuse through the pores of SiO₂ film to oxidize Nicalon at the interface bett ween SiO₂ film and unoxidized core.

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A phase diagram in the three-component system C₃P-M-S was constructed. Abbreviations for oxide formulae are used as follows: C=CaO, M=MgO, P=P₂O₅ and S=SiO₂. Primary crystals are C₃P (α′, α, β), diopside and pigeonite on the C₃P-MS join, and C₃P, sc (magnesium silicocarnotite, C₃M₂PS) and forsterite on the C₃P-M₂S join. Periclase appeared on the MgO corner, and tridymite, cristobalite and two liquids region appeared on the SiO₂ corner. A eutectic with the lowest temperature 1245°C is located near the center portion where three crystals of β-C₃P, diopside and tridymite coprecipitate. The sc compound appeared in a spindle-shaped region and disappeared below 1360°C to form β-C₃P and forsterite. It is concluded that the system C₃P-M-S consists of 4 composition triangles of C₃P-M-sc, sc-M-M₂S, C₃P-M₂S-MS and C₃P-MS-S containing 7 invariant points and this system is pseudo-binary.

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A general sintering equation for a single mechanism is developed with respect to ΔL according to Taylor's expansion method, where ΔL is the shrinkage of a powder compact during sintering. The resulting equations indicate that effective shrinkage of the green compact, ΔL_i, influences the apparent time dependence, that is, the m-power dependence of ΔL. The n-power dependence of ΔL, defined using a conventional sintering model, appears only in the case where ΔL_i<<ΔL. If ΔL<<ΔL_i, sintering obeys a kinetic equation of m=1 irrespective of the n value, that is, regardless of the sintering mechanism that contributes to densification. The foregoing evaluation well explains the data that slight pre-sintering of a sinterable Al₂O₃ powder compact results in an appreciable decrease of the m value. The presently derived equations can be used to estimate the degree of neck growth in a green compact and thus evaluate the quality of sintering data.

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The static and dynamic fatigues of 72 and 74mass% Al₂O₃ mullite ceramics containing 10vol% ZrO₂ (72A-10Z, 74A-10Z) were measured at 1200°C, and the fracture toughness was evaluated from the size of a slow crack growth (SCG) region. The crack growth parameters N obtained by the static fatigue technique were 15 for 72A-10Z and 37 for 74A-10Z, On the other hand, the N values obtained by the dynamic fatigue technique were 23 for 72A-10Z and 30 for 74A-10Z. Fracture toughness (K_IC), obtained by the controlled surface flaw (CSF) method after the static fatigue measurement, was 2.5MPa⋅m^1/2 for 72A-10Z or 1.6MPa⋅m^1/2 for 74A-10Z, and was negligibly affected by the addition of ZrO₂. The fracture toughness of 72A-10Z obtained after the dynamic fatigue measurement varied considerably with stressing rate. The fracture toughness obtained through static and dynamic fatigue measurements showed the rising R-curve behavior. Enhanced fracture, toughness and R-curve behavior in 72A-10Z were attributed to the fracture of rodlike crystals and/or the viscoelastic flow of the glassy phase in the grain boundary.

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In this study, the dependence of inorganic filler on apatite-forming ability in simulated body fluid (SBF) was investigated for the composite cements of bioactive CaO-SiO₂-P₂O₅-CaF₂ glass, glass-ceramic A-W or sintered hydroxyapatite powders combined with bisphenol-a-glycidyl methacrylate/triethyleneglycol (Bis-GMA/TEGDMA) resin. All the composite cements showed apatite-forming ability, but with different rate of apatite formation and surface structure. The first one formed the outermost continuous apatite layer in the shortest period, but simultaneously formed a less dense layer under the apatite layer. The second one similarly formed an outermost continuous apatite layer without forming the low density layer, although the rate of apatite formation was somewhat slower compared with that of the first one. The third one discontinuously formed apatite only on individual hydroxyapatite grains. Such different characteristics among the inorganic fillers may determine both the bioactivity and the mechanical strength of the composite cements in vivo.

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Electrical properties of Nb-(Fe, Mn)-codoped BaTiO₃ ceramics showing a positive temperature coefficient of resistivity (PTCR) were studied by combining their diffuse reflectance measurements and electron spin resonance (ESR) spectroscopy. Nb-(Fe, Mn)-codoped samples, especially the Nb-Fe-codoped sample showed higher durability to reducing atmosphere than the single Nb-doped ones. It is assumed that Fe ions easily segregated at grain boundaries and contributed to the density of surface acceptor states, meanwhile localizing electrons in a form of Ti³⁺ balanced by oxygen vacancy which might stabilizing the chemisorbed oxygens. Mn and Fe acceptors still maintained their valence states and high ability of trapping electrons in the grain boundary after annealing. In addition, ESR signals of Fe³⁺ and Mn²⁺ in annealed samples were intensified above Curie temperature (T_c), indicating their close correlation with the phase transition from ferroelectric tetragonal to paraelectric cubic.

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The optimum heat-treatment process to grow single crystals of Bi₂SrTa₂O₉ (BiSTa), which have been attracting much attention as a material for capacitors of ferroelectric nonvolatile memories, was determined using a Bi₂O₃-excess self-flux method. The starting mixture (Bi₂O₃:SrCO₃:Ta₂O₅=79:10.5:10.5) was inserted into a double crucible made of platinum and alumina. Single crystals were grown at 1345°C and 1400°C in a vertical furnace. With a short hold time, the main product was α-Bi₂O₃ and with an excessively long hold time, SrTa₄O₁₁ and Sr₂Ta₂O₇ were also synthesized. For the purpose of successfully obtaining BiSTa crystals in a single-phase state, complete vaporization of the excess Bi₂O₃ flux in the double crucible was essential under an appropriate heat-treatment condition, such as a hold time of 2h at 1400°C and a cooling rate of 2°C/h to 1150°C. The obtained crystals were transparent and rectangular thin plate, with the long side of 300μm-1mm. The compositional ratio of the crystal was 1.91±0.05:1.27±0.05:2.00 for Bi:Sr:Ta, suggesting that not only is the crystal Bi poorer but Sr richer, compared to the stoichiometric ratio.

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Stacking faults in SiC are one of the important factors in treat SiC, since they affect physical and chemical properties. Ultra-pure β-SiC specimens with stacking faults were annealed at various temperatures for various duration in an Ar atmosphere under a condition without grain growth. Stacking faults decreased in two steps by heat treatment characterized by fast decrease and by slow decrease, respectively. The apparent activation energy for the former was estimated to be 182kJ/mol and that for the latter was estimated to be 260kJ/mol. The value of the latter corresponded to the activation energy for migration of carbon atoms in β-SiC. It was also observed that lattice strain decreased with decreasing stacking faults.

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The influence of phase transformation on grain growth behavior was investigated by annealing dense, fine-grained α- or β-silicon nitride ceramics with or without large α- or β-nuclei at 1700°C for 0.5 to 2h. In the case of α-materials, α-phase content decreased with annealing time, which resulted in the development of in-situ composite microstructures having a small number of large elongated grains in the fine-grained matrix. The large grains in α-materials were based on the grain growth from both formed β-nuclei through the phase transformation and added β-nuclei. In the case of β-materials, only the selective abnormal grain growth of added β-nuclei was observed. Aspect ratios of both matrix and large grains in α-materials were larger than those in β-materials. The grain-growth rate of added β-nuclei in α-materials was also higher than those in β-materials. The grain growth of silicon nitride should be accelerated by α/β phase transformation, which enhanced supersaturation of liquid phase due to the higher solubility of α-grains than that of β-grains and the formation of fine β-grains.

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The sintering rate for the initial stage of sintering was expressed as a function of temperature and relative density for two kinds of Al₂O₃ which differ in purity and mean particle size. Parameters in sintering rate equations were determined by sintering experiments under a uniaxial load. After it was confirmed that the remarkable grain growth was not observed in the temperature range of this experiment, sintering rate equations for two kinds of Al₂O₃ were determined. Calculated sintering rate gave fairly good agreement with the experimental data. Change in relative density during sintering were calculated and compared with experimental data. The difference between calculated values and experimental data remained less than 2% in all the cases. It became evident that change in density during the initial stage of sintering was able to predict for two kinds of Al₂O₃ with sintering rate equation based on visco-elastic analogy model.

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The sulfurization processes of the oxide phase existing on the surface of calcium sulfide powder were studied using thermogravimetric analysis, differential thermal analysis and an X-ray diffraction measurements. According to the results and to thermodynamic considerations, the mixture of calcium oxide and sulfur showed some weight loss starting at a temperature lower than the boiling point of sulfur. An X-ray diffraction peak of calcium sulfate was identified in this mixture specimen after heating to 1200°C under argon atmosphere. Sulfurization of calcium oxide by sulfur was thought to be occurred at temperatures between 200°C and 300°C from the viewpoint of thermodynamic considerations.

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For the purpose of characterizing concentrated slurry suspension applied in fine ceramics shaping process, an evaluation technique has been developed which measures time changes in balancing moment stemming from the centrifugal consolidation of particles. The dispersing system of alumina slurry has been controlled by adjusting the concentration of dispersant as well as the solids content. It is shown that the transient behavior of concentrated slurry in centrifugal field essentially consists of a consolidation and deformation process of network structure immediately formed after particle sedimentation is started. The mechanical strength of the network structure depends on the dispersing system of prepared slurry. It is also demonstrated that the consolidation process of the network structures can be expressed by strain parameter, e=e_∞{1-exp(-t/τ)}, which contains two characteristic values: the final equilibrium strain, e_∞, and the delay time, τ.

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A large volume of the perovskite-type CeMO₃ (M=Al, Ga) are synthesized by the arc-melting method with a horizontal-traveling hearth under Ar stream. The colour of the product for the M=Al is dark green. Evaporation of the raw materials is not observed. The single phase region of CeAlO₃ in volume of -3mm³ is obtained. In the case of M=Ga, the product is light grey in colour. The arc-melting procedure resulted in a loss of the raw material of Ga₂O₃, hence CeO₂ is remained and distributed in the matrix of CeGaO₃. As a result, the single phase region of CeGaO₃ is not obtained. Both of CeAlO₃ and CeGaO₃ are crystallized in tetragonal structure (space group: P4/mmm) and number of molecules per unit cell, Z=1. The lattice parameters of a=0.37669(9)nm and c=0.37967(7)nm for CeAlO₃, and a=0.3873(1)nm, c=0.3880(1)nm for CeGaO₃, are obtained.

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Aluminum nitride-hexagonal boron nitride ceramic composites were sintered by means of uniaxial hot pressing with changing the mixing conditions of raw materials. Mixed powders with a planetary-type milling had a larger surface area than those mixed with a rotary-type milling, which was a consequence of the fragmentation of the h-BN particles. The bulk density of the powder compact increased with increasing the specific surface area of the starting powders independent of the mixing conditions. The density of the isostatically cold pressed body reached about 74% of the calculated theoretical density. These results were thought to be caused by a bimodal particle-size distribution, suitable for dense compact during mixing, which also reduced the friction among particles during forming because of the excellent lubricant action of the h-BN. It was noticed that the maximum density in the sintered materials was achieved in correspondence of a specific surface area of the starting powders. The bulk densities of the sintered materials produced from small specific surface area powders increased with increasing the green density. On the other hand, the sintered density decreased because of the presence of fine h-BN particles which impeded the grain growth and the densification of AlN with a large specific surface area. It was considered that the densification of the composites is dominated by the densification of AlN.

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Sintering behavior of some fine-ground alumina powders with various particle sizes and lattice strains prepared by wet rotation ball milling method were investigated. It was found that easy-to-sintering of powders was dependent on the particle size and independent of the lattice strain; hence, it was taken that mechanochemical effects did not contribute to easy-to-sintering of powders. Platelike grains, which grew in the transverse to c-axis, were formed in the sintered body of all ground powders. These grains grew largely as the particle size of ground powders increased. And it could be suggested that there is a possibility of controlling the platelike grain size by the particle size of the powder.

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The effect of annealing on the distribution of OH content and defect structures in a disk shaped block of fused silica containing approximately 1500ppm of OH was investigated. Heating at 1160°C for 120h reduced the OH content near surface by about 60ppm, and absorption bands at 6.5, 5.8, 5.0 and 4.8eV were induced near the side surface. The amount of the reduction of OH content near the top surface was about 45ppm, and only the 6.5-eV band was induced.

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The silica gels derived from triethoxysilane and tetraethoxysilane were studied by Raman spectroscopy. The vibrational band at 492cm^-1, ordinally labeled D₁, was shifted to 506cm^-1 with the introduction of Si-H groups into the silica network. This suggests that this band is ascribable to the symmetric stretch oxygen vibration of three oxygen atoms connected to a silicon atom with a terminal group, as proposed by Mulder and Damen for fluorine-doped silica (D₀) [C. A. M. Mulder and A. A. J. M. Damen, J. Non-Cryst. Solids, 93, 387-394 (1987)]. However, a simple harmonic oscillator model, which Mulder and Damen successfully applied for fluorine-doped silica, does not explain the observed vibrational frequency. Judging from the structural similarity between gels derived from triethoxymethylsilane composed of four-membered siloxane rings and triethoxysilane, we consider that four-membered siloxane rings are more plausible than three oxygen atoms connected to a silicon atom with a terminal Si-H as the origin of the Raman band at 506cm^-1 of gels derived from triethoxysilane.

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Cryogenic specific heats are measured for the evaluation of the surface state of alumina powders. Specific heats of commercial powders are measured in a temperature range between 15K and 50K. For specific heat measurements, powder particles are covered by Apiezon N grease to increase the thermal conduction of powder compacted bodies. Excess specific heats are observed below 30K for each sample. For the commercial powders, the total entropy becomes higher as the specific surface area increases. The specific surface entropies of commercial powders are higher than those of porous samples. A possible reason of the decrease of entropy upon sintering process is the phase transformation of hydroxide alumina on the surface of commercial powders to other phases like the α-phase by sintering process. The weight change by the heat treatment at 1200°C is in agreement with the calculated weight change for the phase transformation, assuming that the surface thickness is about 1nm. The high entropy of commercial powders may be due to the presence of hydroxide alumina which exists on the surface of the alumina crystals and is not detectable by X-ray diffraction.

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The effects of pulsed electric current sintering (PECS) on the surface reaction of carbon-coated Si₃N₄ powder were studied. The carbon-coated Si₃N₄ powder was heat-treated at 1500°C in N₂ atmosphere for 0-60min. During the heat treatments, the carbon reacted with oxidation layer on the Si₃N₄ powder surface, and formed SiC and CO gas. By measuring the content of residual carbon, the rate constant was obtained and compared with that of hot press (HP) sintering. It was found that PEGS promoted the reaction about 3 times faster than HP. However, the reaction rate of PECS became the same rate as that of HP sintering when the carbon-coated Si₃N₄ was placed between insulating BN pellets with 2mm in thickness. Therefore, it is concluded that the promotion of the reaction in PECS method is caused by the electric conduction through carbon coated layer on the surface of the raw Si₃N₄ powder.

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Cubic zirconia base ceramic composites were fabricated by dispersing barium titanate with various contents. Mechanical strength increased on longitudinal poling and decreased on transverse poling. This strength change has been interpreted as being related to internal stresses generated at the matrix/dispersant interface.

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