Superhard ceramic materials such as diamond, cubic boron nitride and high-boron compounds are known to have a Vickers microhardness above 30 GPa and are expected to be applied to various kinds of cutting tools, molds or protective coatings. However, these materials are very difficult to form and sinter on account of their strong covalent bonding and the low diffusivity of their constituents in solids even at elevated temperatures. Therefore, special consolidation techniques are required to prepare bulk bodies or coatings of these materials. In the present paper, a series of the author's previous works on the preparation of superhard ceramic materials by chemical vapor deposition and high-pressure sintering are reviewed, and the mechanical and chemical/thermal properties are described and discussed in relation to their microstructures.

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The absorption spectra of Cr-containing Li₂O-SiO₂ transparent glass-ceramics was investigated. The crystalline phases precipitated in glass-ceramics are mainly Li₂O•2SiO₂ with a small amount of α-SiO₂. The percent crystallinity and crystal sizes are 40-77 and 20-42 nm, respectively. A strong absorption band ascribed to the charge transfer of Cr⁶⁺ ion and a weak d-d absorption band ascribed to Cr³⁺ ion were observed in the glass. However, the new absorption bands appeared at around 600-700 nm and the absorption intensity of these bands increased significantly (about 15 times) upon crystallization. It was suggested that the tetrahedrally coordinated Cr⁴⁺ ion might exist in Li₂O-SiO₂ transparent glass-ceramics.

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Porous diopside microspheres, CaO·MgO·2SiO₂, were prepared by heat treatment, at various temperatures, of spherical silica gel particles impregnated with Ca(NO₃)₂ and MgCl₂. Spherical silica gel particles were prepared by the sol-gel method in an O/W emulsion using tetraethyl orthosilicate. When the silica gel particles containing Ca(NO₃)₂ and MgCl₂ were heated at temperatures above 900°C, the results of X-ray powder diffractionmetry (XRD) and Fourier transform infrared spectroscopy (FT-IR) showed that the silica gels were crystallized into diopside, and scanning electron microscopy (SEM) observation revealed that a uniform porous structure was formed in the particles. The porosity in the diopside is believed to be caused by the mutual adhesion of smaller pores in the silica gel particles during the viscous sintering process.

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Self-organized nanograded SiO₂/TiO₂/Polymethyl methacrylate (PMMA) thin film was successfully prepared by only one coating from the combination of Tetraethyl orthosilicate (TEOS) and Titanium tetraisopropoxide (TTIP). Sufficient kinetics difference between two alkoxides was an important requirement for the processing of the graded thin film in this process. The film was colorless and transparent due to two gradual interfaces of three layers. The reflectance of the SiO₂/TiO₂/PMMA graded film was lower than 2% over a wide wavelength range, and this is attributed to the interference of lights reflected from both the SiO₂ layer and TiO₂ layer. Although further investigation and precise process control are required to further decrease reflectance, this film can be formed using only one coating process and has great potential as a new low reflective coating.

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Anatase (TiO₂)/silica (SiO₂) composite nanoparticles over the compositional range of SiO₂=0-70.9 mol% were directly formed from acidic precursor solutions of TiOSO₄ and tetraethylorthosilicate by hydrolysis under mild hydrothermal conditions at 200°C. With increasing SiO₂ content, the specific surface area of the as-prepared composite nanoparticles increased from 109 (SiO₂=0 mol%) to 277 m²/g (SiO₂=70.9 mol%), which was associated with an increase in the amorphous phase content. The composite nanoparticles with a unique microstructure, i.e., anatase crystallites 14-16 nm in size that exist separately due to the presence of an amorphous silica phase in the TiO₂/SiO₂ interface, showed enhanced photocatalytic activities for the decomposition of methylene blue (MB) in aqueous solution and better removal of NO gas in comparison with pure anatase-type TiO₂. The highest photoactivity was obtained when the SiO₂ content in the composites was 25.1 mol% for MB decomposition and 48.2 mol% for NO oxidation.

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The effect of CeO₂ additions into a PdO catalyst supported on χ-Al₂O₃ has been investigated. Changes in crystal structure have been examined in a methane atmosphere, in relation to catalytic activity for methane oxidation. The mass ratio Al₂O₃/PdO of the catalysts was kept constant at a value of 9, whereas the atomic ratio R=Pd/Ce was set to 0, 1, 2 and 5. The PdO-CeO₂/χ-Al₂O₃ catalyst with R=1 showed the best catalytic activity in methane oxidation. When the catalyst was heated at 300°C, Pd metal appeared under the O₂-deficient atmosphere, whereas PdO was reformed when the reduced catalyst was heated at 300°C in an atmosphere with an O₂-excess. Line broadening of X-ray diffraction patterns was detected for both the (110) plane of PdO and the (200) plane of the Pd formed after heating, suggesting that stacking faults were introduced into these planes by the loss of oxygens in the PdO/Pd lattice during the reaction with methane. As the (110) planes in PdO correspond to the (200) planes of the fcc Pd metal formed during reaction, in terms of the array of the constituent metal only, methane oxidation would easily proceed by way of these oxygen-deficient lattice planes.

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Si₃N₄/W composites with high wear resistance were fabricated by reduction reaction of tungsten boride (WB) under hot-press sintering in a nitrogen atmosphere. Composites sintered above 1850°C consisted of β-Si₃N₄, W, W₂B and W₂C. The Si₃N₄ matrix grains in the composite were composed of an elongated and bimodal structure similar to conventional Si₃N₄. Mechanical properties, i.e. fracture toughness and strength, were almost the same as monolithic Si₃N₄ ceramics produced under the same sintering conditions and with the same additive system.

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Detailed examination on growth direction, orientation relationship between two phases and the interface structure were carried out for an alumina/YAG (Yttrium Aluminum Garnet) melt-growth composite (MGC) prepared by the Bridgman method. Transmission electron micrograph revealed that this MGC had in coherent interfaces, and that alumina had a twin. It is suggested that the interface was dominated by the growth of YAG, and that close packed planes of the both phases were well mached, being parallel to the solidification direction.

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Linear thermal expansion and electrical conductivity of titanium substituted LaCrO₃ have been studied as ceramic interconnect materials in high-temperature solid oxide fuel cells. The transition temperature from orthorhombic to rhombohedral symmetry has a tendency to increase with decreasing Ti content in the perovskites. At Ti content ≥0.2, the phase transition was not observed in the temperature range from room temperature to 1273 K. Average linear thermal expansion coefficient of the Ti substituted perovskites increased with increasing Ti content and approaches that of 8 mol%Y₂O₃ stabilized ZrO₂ electrolyte. Electrical conductivity of the lanthanum chromites decreased with increasing Ti content.

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During thermal shock tests of porous materials by cooling media, the cooling media will infiltrate into the materials. Such as infiltration produces large effects on heat-transfer coefficients on the specimen surface and temperature/stress distributions developed with in the specimen. In this paper, we performed thermal shock tests for porous ceramics using compressed air cooling and water flow cooling to estimate the infiltration effect on temperature/stress distributions. The heat-transfer coefficients were estimated with a thermo-couple implanted in a silver specimen. The temperature distributions in the porous specimens under thermal shock were measured using several thermo-couples implanted in the specimens, and the temperature distributions were simulated using a transient heat-transfer equation, taking into account of the infiltration of the cooling media into the specimen. Thermal stress distributions in the specimens were calculated from the estimated temperature distributions. The results revealed that the temperature distribution in a porous specimen changes drastically compared with a dense specimen, and the maximum thermal stress on the cooled surface of the porous specimen was smaller than that of a dense specimen. These phenomena express the reason why porous materials have high thermal shock resistance despite having small fracture strength.

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The electrochemical corrosion of molybdenum electrodes in SiO₂-Al₂O₃-Na₂O-K₂O-SrO-BaO-ZrO₂ glass melts both with and without antimony was investigated at 1350°C and 1450°C. The corrosion degree of the electrodes, i.e., the decrease in the diameter of the electrode, increased with increasing antimony concentration and temperature. When an alternating current (AC) of 50 Hz was applied, the corrosion degree increased with increasing current density. Any corrosion on the electrodes could not be observed up to 1 A/cm² in the antimony-free glass melt. It was argued that the primary corrosion process is due to the oxidation of the electrodes by antimony ions in the glass melt both with and without AC supply. The AC supply accelerated the corrosion of the electrodes by the formation of molybdenum ions during anodic polarization, followed by the reduction of antimony ions rather than molybdenum ions during cathodic polarization. The increase in AC frequency, i.e., above 500 Hz, was effective in decreasing the corrosion degree of the electrodes.

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Chemical composition, crystal structure and fluorescence properties for scallop shells have been systematically investigated with the primary objective to convert scallop shells into functional materials. Scallop shells fired at 100-500°C show a white fluorescence, and these fluorescence intensity of the sample fired at 200°C is particularly high compared to other temperature. On the other hand, when fired at 800-1100°C, the shells show a blue-white fluorescence, and the fluorescence intensity of the sample fired at 1000°C is particularly high compared to other samples. As a result of XRD measurements, it is showed that the former samples are CaCO₃, and the latter samples are Ca(OH)₂.

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