Rotary bending and static fatigue tests were carried out on sintered Si₃N₄ at room temperature to study the effect of loading conditions on the fatigue behavior. It is confirmed that the fatigue behavior under rotary bending was different from that under static fatigue. The value of n under rotary bending was much smaller than that under static fatigue. The assumed fatigue limit under rotary bending was also lower than that under static fatigue. However, the characters of mirror-like regions observed on the fractured surfaces after both fatigue tests were almost the same as those after the static test. If the fracture initiation part was near the specimen surface, the mirror-like region was roughly semicircular, and the fracture propagated radially from the region. The size of each mirror-like region, determined from optical micrographs at a magnification of 25, was correlated with the applied stress regardless of the loading condition. Stress intensity factor at the deepest point in the mirror-like region was also almost constant. It is considered that the fatigue failure occurred when the size of mirror-like region reached a critical value which was dependent on the magnitude of applied stress, and that the difference of fatigue behavior under rotary bending and static fatigue was due to the subcritical crack growth behavior in the mirror-like region.

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This research made reference to the surface energy as the driving force of sintering and studied the influences of eleven inorganic additives on the densification and the microstructure of sintered Al₂O₃ compacts. TiO₂-doped Al₂O₃ compact was found to have the highest densification rate of all the compacts studied, and resulted in the reduction of the sintering time. The effect of Fe₂O₃ on the enhancement of Al₂O₃ grain growth was greater than that of densification. PbO strongly retarded the densification of Al₂O₃. SiO_2- and CaO-doped Al₂O₃ compacts yielded fine grained microstructure, but these two additives were found to be detrimental to densification rate. MgO impeded Al₂O₃ grain growth at 1400°C. The existence of trace amount of alkali-metal oxides significantly affected the final grain size of Al₂O₃ compacts when sintered at 1500°C, although the effect at 1400°C was slight. When compared with the densities of Al₂O₃ compacts where deionized water was used during ultrasonic dispersion of the mixtures before centrifugal sedimentation, the densities of Al₂O₃ compacts obtained by methanol in ultrasonic dispersion were found to be lower, and the resulting grain sizes were smaller with inhomogeneous microstructures after sintering as well.

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Influences of humidity and temperature on the weathering of soda-lime-silica glass substrates coated with sol-gel derived TiO₂-SiO₂ and pure SiO₂ films have been studied, and the weathering mechanism of the glass substrates has been discussed. The weathering of glass substrates coated with the films was drastically accelerated with increasing humidity and with increasing temperature in the environment. The weathering of the glass substrates coated with sol-gel derived films proceeded by the ion exchange of hydronium ions (or protons) from atmosphere with sodium ions in the glass substrates and by the subsequent reactions of the sodium ions with carbondioxide and water in the atmosphere on the surface of the coating films. It was found that the sol-gel derived TiO₂-SiO₂ coating films provide the substrates with better durability in the environment of high temperature and high humidity compared with the pure SiO₂ coating films. The apparent activation energy for the weathering of the glass substrates coated with the 9TiO₂⋅91SiO₂ (in mol%) films was estimated to be 68kJ·mol^-1 under 90% R. H.

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The thermal decomposition and the phase transformations of ammonium aluminum carbonate hydroxide (AACH) to α-alumina have been studied. Amorphous-, γ- and θ-alumina were identified as intermediate products. A TTT diagram for the process of the thermal decomposition and the phase transformations was determined. Kinetic analysis of the θ- to α-aluminas transformation by the Avrami-Erofeev equation showed that the transformation occurred by the nucleation and growth mechanism. Results of transmission electron microscopy (TEM), however, suggested that the transformation occurred by the synchro-shear mechanism. The maximum particle size of θ-alumina and the minimum particle size of α-alumina were approximately 30 and 40nm, respectively. A model of the transformation was presented from the viewpoint of crystallography.

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Oxidation test of SiC whisker (SiC(w)) itself and four composites of Al₂O₃, Y-TZP, 3Al₂O₃⋅2SiO₂ and ZrO₂⋅SiO₂ matrices reinforced with SiC(w) was carried out in the temperature range from 800° to 1200°C. SiC (w) oxidized at higher than 800°C, and it was almost completely converted into SiO₂ by oxidation at 1300°C for 100h. Oxidation of SiC (w) in the oxide matrices became heavier in the order, 3Al₂O₃⋅2SiO₂, Al₂O₃, ZrO₂⋅SiO₂, Y-TZP, and was thought to be controlled by diffusion (of oxygen in matrices) step. Although the reaction between oxide matrices and SiO₂ formed by oxidation of SiC (w) was observed, the effect of this reaction on the oxidation of SiC (w) in oxide matrices was not clarified in this work. Degradation of strength was caused by oxidation of SiC (w). The oxide materials which have low oxygen diffusibility at high temperatures should be selected as the matrices for oxide composites with SiC (w).

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Changes in the structure and permeability of porous magnesia ceramic with a TiO₂-Al₂O₃ matrix with a TiO₂/Al₂O₃ molar ratio of 0.96 were studied at different matrix contents. The shape of pores became complicated and pores tended to have fractal nature as the matrix content increased. It was proposed that the change of pore shape was caused by the increase in the amount of the matrix and its reaction with MgO grains. Both mean pore diameter and permeability showed a maximum at a matrix content, 10wt% and 15%, respectively. On the other hand, the fractal dimension of pore shape changed from 1.39 at 5wt% to 1.65 at 20wt% of matrix. The fractal dimension changed sharply between 10-15wt%. The permeability was found to agree well with that of a nonspherical particle system by using pore size modified by the its fractal dimension.

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β, β″-Alumina precursors were prepared under the various synthetic conditions using aluminum isopropoxide and sodium ethoxide. Crystallization of these precursors with heating was examined by differential thermal analysis, powder X-ray diffraction and infrared absorption analyses. Two types of crystallization processes were observed for these precursors, which were controlled by the amount of acetic acid added in the precursor synthesis. At a low molar ratio of acetic acid/aluminum isopropoxide (Ac/Al=0.35), γ-alumina type phase with broad peaks in X-ray diffraction pattern was formed as a primary crystalline phase. On the other hand, λ-Na₂O⋅XAl₂O₃ phase was formed at a high Ac/Al ratio (=3.50). Both phases changed to β, β″-alumina structure at higher temperatures (-1200°C). The β- or β″-alumina phase with structural defects along c-axis was obtained from the γ-alumina type phase showing broad peaks. The lattice constant c₀ of the highly defective β- or β″-alumina decreased with increasing Na content of the precursor. On the other hand, a mixture of β- and β″-alumina phases was formed from the λ-Na₂O⋅XAl₂O₃ phase. The volume fraction of β″-alumina in (β+β″) phases was not affected by heat treatment conditions or Na content of precursors. The lattice constant c₀ of the β″-alumina in this mixture was independent of the composition of the precursor, while that of β-alumina decreased with increasing Na content.

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Four-point shear bending tests were carried out at room temperature to study the fracture criteria for the large crack of sintered silicon nitride under a mixedmode. The data of stress intensity factor under mode I and II agreed with the G-criterion. Ring-on-ring tests and ball-on-ring tests were also performed to investigate the fracture behavior of sintered silicon nitride under tension-tension equibiaxial stresses. The experimental results were compared with the perdicted strength based on the Weibull statistical theory and various fracture criteria. The strength under tension-tension stresses agreed with the predicted strength based on shear sensitive criteria for natural flaw. In the previous work, the strength of sintered silicon nitride under tension-compression biaxial stresses agreed with the predicted strength based on shear insensitive criteria. In this paper, it was proposed that the modified G-criterion is applicable to the prediction methodology for fracture strength of ceramic component under both tension-tension and tension-compression biaxial stresses.

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Microstructure of pessureless sintered bodies of h-BN fabricated using an activated powder by mechanochemical treatments has been examined by SEM and TEM observations. The BN compacts obtained by the pressurless sintering showed three dimensionally bonded network structure consisted of platy BN microcrystals. The interconnected regions between two particles are continuous in microstructure without any orientational relationship between them. The interconnected network, which would improve properties of BN bodies, was characterized for the pressureless sintered bodies.

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Crystallization behavior of multi-component non-alkali silicate glasses has been studied by DTA and SEM observation. The experimental results obtained in the present study are summarized as follows. (1) On the DTA curve of SiO₂-Al₂O₃-CaO-ZnO-TiO₂-B₂O₃ glass two or three exothermic peaks emerged, one (T_c1) is in the temperature range between 840° and 880°C, responsible for precipitation of anorthite (CaO⋅Al₂O₃⋅2SiO₂) and gehlenite (2CaO⋅Al₂O₃⋅SiO₂), and the others at T_c2=850°-920°C range, responsible for precipitation of willemite (2ZnO⋅SiO₂) and titanite (CaO⋅TiO₂⋅SiO₂). (2) Heating at a fixed temperature induced the crystallization of anorthite, followd by precipitation of gehlenite, willemite and titanite phases. (3) SEM observation showed surface crystallization on glass specimens. The crystallization peak temperatures on the DTA curve shifted downward as the specific surface area increased. (4) A crystallization process was proposed, which includes three steps; (a) a phase separation of glass into a SiO₂-Al₂O₃-CaO rich glass phase and a SiO₂-CaO-ZnO-TiO₂ rich glass phase, (b) precipitation of anorthite and gehlenite from the former, and (c) precipitation of willemite and titanite from the latter. (5) The possible roles of glass constituents playing in the crystallization process were speculated in terms of viscosity change (T_g, T_d) and compositional difference (ΔS_f) between the primary phase to be precipitated and the bulk glass.

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MgO particles were coated with a chelate compound of aluminum and heated in air to prepare Al₂O₃-coated MgO particles for modifying surface properties. The dissolution of MgO in water was suppressed by the Al₂O₃ coating. The amount of dissolved Mg ion was minimum after heating at 500°C. The Al₂O₃-MgO layer 10-30nm thick was observed on the surface of the MgO particles heated at 500°C. Fine particles 10nm in diameter were seen in the coating layer and they aggregated into clusters 300-500nm in diameter after heating at 1000°C. It was also found that the adherence of coating layer to MgO particles decreased after heating at 1000°C compared with those heated at 500°C.

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A new measuring system of elastic properties at low temperatures from 4.2K was constructed by using the cubic resonance method. F-doped silica glasses developed for optical fiber were employed to obtain the elastic constants and their temperature and pressure coefficients. It was found that the elastic constants decreased and that the abnormal behavior of temperature and pressure coefficient was enhanced with an increase in fluorine content. These were explained in terms of the open structure of silica glass.

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Infrared sensing properties were investigated on thermal-type IR sensors using semiconducting ceramic monofiber of SiC (Nicalon), and PAN-based carbon with different diameters (7-130μm). Thermal time constant of the monofiber IR sensor increased with increasing fiber diameter. An IR sensor made of SiC monofiber with a low resistivity (18Ω·cm) and of a short length (1mm) showed a small thermal time constant of 1.6ms. The output voltage increased with increasing radiation energy of IR source and decreasing surrounding temperature. PAN-based carbon monofiber having a resistivity above 200Ω·cm detected an IR radiation from human hand with a steady output voltage at room temperature. Semiconducting ceramic monofibers were found to be promising materials for IR sensors.

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AlN polycrystalline specimens were wet-polished with Al₂O₃ and Cr₂O₃ abrasives. A flat surface with a roughness of 100nm was achieved, while the surface layer was heavily damaged. High-voltage electron microscopy revealed that the damaged and/or dislocated layer was about 0.5μm thick and a few dislocations propagated deeper. The source of dislocation such as free surface and grain boundary was discussed.

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To investigate effects of grain size and temperature on the fracture toughness (K_IC) of polycrystalline alumina ceramics, the K_IC of high purity polycrystalline alumina having various grain sizes was measured using a single edge notched beam technique from room temperature to 1500°C. The material with grain size of 2μm revealed the largest K_IC values over the temperature range studied. The K_IC values of polycrystalline alumina decreased with increasing grain size. The decreasing rate of K_IC values with increasing grain size changed around 22μm at room temperature and 500°C and around 16μm from 1000° to 1500°C. The K_IC values of polycrystalline alumina having grain sizes over the range studied decreased with increasing temperature. For the grain size up to 42μm, the decreasing rate of K_IC values at temperature up to 1000°C was smaller than that above 1000°C. On the other hand, the K_IC values of the polycrystalline alumina having grain size of 70μm decreased slightly with increasing temperature and tended to increase at 1500°C. This trend might be attributed to be related to the interaction between fracture surfaces.

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Glasses doped with small sized-CdS crystals with a significant quantum size effect were prepared by the sol-gel process. Gels synthesized through the hydrolysis of a complex solution of Si(OC₂H₅)₄ and Cd(CH₃COO)₂⋅2H₂O were heated at 500° to 700°C, then allowed to react with H₂S gas to form silica glasses doped with fine CdS microcrystals. The formation of CdS crystals was controlled by diffusion of H₂S gas into the porous glass and the diffusion coefficient was from 10^-18 to 10^-16cm²/s at 100° to 200°C as determined from the measurement of weight gain of glass. The size of CdS microcrystals increased with heat-treatment time, reaching 2 to 6nm in diameter, which was dependent on the heat-treatment temperature and CdO content in glass. The optical absorption edge exhibited a blue shift compared to that of a bulk CdS crystal and its energy shift was reciprocally proportional to the square of the crystal size. In the photoluminescence spectra, two peaks were observed at 2.0 and 2.5eV, the intensity of which at 2.0eV decreased as the heat-treatment time was increased. It was considered that the emission at 2.0eV, pronounced at low temperature of 77K, was due to the carrier recombination via the deep level due to oxygen impurity in CdS crystals.

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Thin films of piezoelectric lead zirconate titanate (PZT) were prepared by the dip-coating method in the sol-gel process. Lead acetate, zirconylnitrate and titanium tetra-n-butoxide were used as raw materials. Water, acetic acid and tri-ethylene glycol were used as solvents. A thin film was prepared by dipping a substrate having a thin layer of ITO electrode on a silica glass plate. Thin films having single-phased PZT were obtained by heat-treatment at 600°C for 2h in air. The film thickness was 0.155μm per dipping, and PZT films 1-3μm thick were prepared by multiple deposition technique. Au thin layer by PVD method was used as upper electrodes. The dielectric constants and losses for the composition ratio (PZ/PT) of 53/47 were 1380 and 0.03, respectively by heat-treatment at 750°C for 2h. The spontaneous polarization, the remanent polarization and the coercive field were 21μC/cm², 11μC/cm² and 34kV/cm, respectively. These films prepared by the sol-gel process were shown to have more excellent dielectric properties and transparency than the conventional powder method.

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The manufacturing conditions, such as the amount of BeO sintering aid, the temperature and the pressure of hot-pressing on SiC ceramics having high thermal conductivity and high electrical resistivity were studied. SiC ceramics with addition of 2.0wt% of BeO hotpressed at 2050°C, 30MPa for 1h gave the high thermal conductivity of 270W/m·K and the high electrical resistivity of 4×10¹³Ω·cm. The effects of impurities were also studied. The BN addition decreased the thermal conductivity, while AlN caused the decrease of the electrical resistivity.

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Iron spinel (FeAl₂O₄) was heated at 1200°C under various low PO₂ and decomposed into spinel phase, metallic iron and α-alumina. For the spinel phase, the lattice constant was measured by powder X-ray diffraction and the distribution of the iron ions was measured by Mössbauer spectroscopy. The lattice constant decreased with increasing heating time and the decrease was more pronounced at lower PO₂. Some iron ions in the spinel phase were reduced to metallic iron, which formed a separate phase, resulting a spinel phase containing vacancies. The ratio of the iron ions in the tetrahedral site to the total number of the iron ions in the spinel phase decreased in the process of the decomposition. It was considered that the iron ions in the tetrahedral site became less stable and easier to be reduced with the contraction of the crystal lattice.

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