The instabilities of Si₃N₄ and Si₃N₄ ceramics during sintering were reviewed. As a result of the instabilities, Si₃N₄ and Si₃N₄ ceramics lose their masses or decompose. The major mass loss reaction of Si₃N₄ ceramics is as follows.
Si₃N₄(s)+3SiO₂(l)=6SiO(g)+2N₂(g)
Regarding mass loss, decreasing the sintering temperature by 30K corresponds to increasing N₂ pressure in the furnace by one order of magnitude. By being well acquired with the mass loss phenomena, sintering of Si₃N₄ ceramics in N₂ gas at 0.1MPa or air will become the candidate conditions for the mass production of Si₃N₄ ceramics. The research to reduce the total cost of Si₃N₄ ceramics was discussed.

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Amorphous based solid electrolytes are promising for application to solid-state batteries and other electrochemical devices. We have prepared a number of the electrolytes by rapid quenching of melts using a twin-roller and by mechanical milling of starting materials using a planetary ball mill. Three topics of silver or lithium ion conducting amorphous electrolytes prepared by these techniques are reported. The first one is the formation and characterization of α-AgI microcrystal-dispersed composite electrolytes, in which high temperature superionic phase of α-AgI is stabilized at room temperature in a superionic glass matrix. The second one is the structural investigation of lithium ion conducting oxysulfide glasses prepared by rapid quenching of melts. The last one is the mechanochemical preparation of high lithium ion conducting oxysulfide glasses and their application to solid-state secondary batteries.

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Proton conductivity of the P₂O₅-SiO₂ glasses was investigated at sub-zero to 120°C. The glasses prepared by a sol-gel method are porous, pore diameters in which are smaller than 5nm with an average diameter of 2.5nm. The electrical conductivities, measured under 100%RH, follow the Arrhenius equation with the activation energy of 10kJ/mol in the temperature between 120 and -20°C. Below -20°C, the adsorbed-water molecules are frozen to increase the activation energy and decrease the proton conductivity. We could conclude that our P₂O₅-SiO₂ glasses, exhibiting high protonic conductivity of-30mS/cm at 120°C, have potential for the electrolyte of the fuel cell operating at sub-zero to 120°C.

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Sol-gel synthesis, characterization and a possible explanation for the formation of TiO₂-modified SiO₂ mesoporous materials are described. The effects of TEOS-aging time and Ti-quantity have been investigated systematically. Surface area above 1000m²/g was obtained with low quantity of Ti-loading when SiO₂ gelation was only half completed. The higher sorption capacity for water was interrelated to higher acidity. Phase separation may occur in the Ti-rich region.

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Substitution of Dy rare earth ions in Ba(Ti, Zr)O₃ dielectric materials was studied, using X-ray diffraction. Dy³⁺ ions enter both the A- and B-sites of the perovskite structure, depending on whether there is a Ba or Ti-excess. There are two discrete mechanisms of occupancy: 1) all Dy ions enter only Ti-sites as acceptors when Dy concentration is below 2mol%; 2) Dy³⁺ enters Ba- and Ti-sites when Dy concentration is above 2mol%. The Dy_Ba¹⁺ donor is highly effective in improving the life stability by 10 orders of magnitude. Be-sides, oxygen vacancies being compensated by the existence of Dy_Ba¹⁺ which acts as a donor, impedance measurements indicated the existence of an insulating barrier layer, which could help to suppress electromigration of oxygen vacancies, thus the life stability at grain boundaries can be improved by Dy³⁺ incorporation into Ba-site-donors.

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Acceleration effect of the ternary carbonate of Li/Na/K on the CO₂-absorption rate of lithium zirconate was studied. Potassium carbonate additive in lithium zirconate powder clearly produces the absorption-rate acceleration as a result of the formation of the binary carbonate of Li/K, which melts at around 500°C. The experimental results obtained revealed that the lithium zirconate powder with the ternary carbonate of Li/Na/K (ternary-type lithium zirconate) shows a more accelerated absorption rate at 500°C, and particularly at 450°C, than does the powder with the binary carbonate of Li/K (binary-type lithium zirconate). At 500°C, the reaction-yield range of CO₂ absorption by a ternary-type lithium zirconate, which shows complete melting of the carbonate formed during the reaction, is 2.3times as long as that by a binary-type. Furthermore, even at 450°C, this ternary type has the range of the complete melting, which is as long as that by the binary-type at 500°C, whereas the binary-type does not have the range. The difference of the acceleration effect between the ternary and the binary carbonate was attributed to the difference of the length of this reaction-yield range.

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A new electrochromic material λ-MnO₂ thin film was prepared from LiMn₂O₄ thin film by acid treatment. LiMn₂O₄ thin film was prepared by dip coating method using manganese acetate tetra-hydrate (Mn(OCOCH₃)₄⋅4H₂O) and lithium acetate di-hydrate (Li(OCOCH₃)⋅2H₂O) as starting materials, and ethanol (C₂H₅OH), ethylene glycol monoethyl ether (C₂H₅OCH₂CH₂OH), 2, 4-pentanedione (CH₃COCH₂COCH₃) and water as solvents. λ-MnO₂ thin film showed electrochromism by application of voltage. The color of thin film changed from reddish brown to transparent.

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The liquid-phase diffusion coefficient of nickel nitrate in wet silica gels with different Si content as well as in mesoporous silica gel was measured by the flow-cell method, where absorbance of a gel plate placed in a flow cell was monitored as a function of time. For diffusion in the mesoporous silica gel with pore diameter of 12nm, the tortuosity was as low as 1.2, suggesting that the liquid-phase diffusion of nickel nitrate is not restricted in the large mesopores. In contrast, the diffusion coefficient in wet silica gels monotonously decreased with increasing Si content in the gels, and the tortuosity in the wet gel, 1.7-3.5, was larger than that in the mesoporous silica gel. The wet gel is considered to consist of crosslinked polymeric network of silica. The tortuosity in the wet gel would vary depending on the mesh size in the network and on the number of free side chain.

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Lattice deformation in monoclinic Sc₂(MoO₄)₃, induced by both thermal expansion and phase transition, has been investigated by a matrix algebra analysis. From 4K to 178K, one of the principal distortions of the monoclinic lattice (λ₁) invariably showed a positive coefficient of thermal expansion, while the other two distortions λ₂ and λ₃ showed negative thermal expansion in the temperature region of T<55K (λ₂) and T<35K (λ₃). Because of the bidimensional shrinkage along the latter two axes, volumetric contraction occurred below 30K. The λ₂-axis coincides with the crystallographic b-axis. The intersection angle between the λ₁-axis and the crystallographic c-axis was -78.8° at 11K, and it steadily decreased to -20.1° with increasing temperature up to 178K. The magnitude of lattice deformation nearly along [302] is negligibly small. Therefore, the lattice deformation induced by the monoclinic-to-orthorhombic phase transition at 178K was accompanied essentially by bidimensional expansion along [010] and nearly along [205], the magnitudes of which are, respectively, -0.6 and -0.9%.

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A spinel-carbon clinker has been prepared through a two-step heating process of MgO-Al compact in CO atmosphere. Al partially changes into intermediate phases during low temperature heating step. The phases further react with MgO and CO during high temperature heating step to form spinel and carbon. The precipitated carbon phase is almost amorphous and finely distributed throughout the spinel phase. Because of its low degree of crystallinity, the precipitated carbon shows an oxidation resistance lower than graphite. At higher temperatures, the effect of heating in CO gas on the clinker is negligible, but heating in Ar gas strongly reduces the clinker.

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GaN single crystals were synthesized by using the Na-Ga melt at 700-850°C and 1-5 MPa of N₂ for 200h. The formation of GaN was not observed at 800°C under 1 MPa of N₂, and at 850°C and 1.5MPa of N₂. GaN single crystals were obtained lower temperatures and/or the higher N₂ pressures. The morphology of the crystals tended to change from prismatic to platelet by decreasing the growth temperature and/or increasing N₂ pressure. Colorless transparent prismatic single crystals with a size of about 1.0±0.5±0.5mm³ were obtained by adopting a three-step heating process in which the temperature was increased from 750°C at 0.5°C/h to 800°C under 3MPa of N₂.

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Green sheets of SiC containing Al₂O₃-Y₂O₃-CaO sintering additives prepared by the doctor-blade method and polycarbosilane-impregnated 2D woven Hi-Nicalon cloths with BN-coating were used for the fabrication of SiC fiber-reinforced SiC composite by hot-pressing. Thermal conductivity of the composite was measured between room temperature and 1000°C. It increased with increasing bulk density, and was 5-11W/m·K at room temperature. The thermal conductivity of the composites showed a maximum at approximately 500°C or slightly increased with test temperature up to 1000°C. The composite fabricated in this study showed relatively low thermal conductivity in spite of low porosity. The reasons for the low thermal conductivity could be attributed to the SiC matrix consisting of fine SiC grains, the phase transformation of 3C-SiC to 4H-SiC, and the low thermal conductivity grain-boundary phases, in addition to the large amount of SiC fibers with low thermal conductivity.

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Calcium containing organic phosphorous compounds were synthesized through reactions of calcium acetate monohydrate with phenylphosphonic acid or phosphoric acid mono-n-butyl ester in 50vol% ethanol/water solution, and they were abbreviated as Phenyl Ca-P and Butoxy Ca-P, respectively. The effects of the phenyl and butoxy groups on the specific surface area, particle shape and thermal properties of calcium containing organic phosphorous compounds were investigated. As the amounts of added calcium acetate monohydrate (Ca/P) increased from 0.5 to 2 in the reaction solutions, the Ca/P ratios in calcium containing organic phosphorous compounds changed from 0.5 to 1. Calcium containing organic phosphorous compounds are self-assembled and have a layered structure. The interlayer distance of Phenyl Ca-P was shorter than that of Butoxy Ca-P. The Butoxy Ca-P particle was flake shaped, and the Phenyl Ca-P particle was an aggregate of tubular particles. The specific surface area of Phenyl Ca-P (30m²/g) was larger than that of Butoxy Ca-P (5-6m²/g). The thermal decomposition temperature of Phenyl Ca-P (300°C) was higher than that of Butoxy Ca-P (250°C). The crystal structure of calcium phosphate transformed from amorphous calcium phosphate to u-Ca₂P₂O₇ by the thermal decomposition of calcium containing organic phosphorous compounds. It was possible to prepare a porous material with a hydrophobic group by using phenylphosphonic acid as a starting material.

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The hydration rate of allophane and imogolite has been estimated by means of an adiabatic vapor absorption calorimeter. The hydration amount of allophane and imogolite were 16.6 and 19.4%, respectively, at dehydrated temperature of 100°C. The maximum hydration rate of allophane and imogolite were 0.063mmol/ g·s and 0.087mmol/g·s, respectively, at same dehydration temperature. When allophane and imogolite were dehydrated at temperatures below 100°C, the hydration rate of allophane was larger than that of Mg-A type zeolite, and the hydration amount of allophane was larger than that of B type silica gel. Both hydration rate and hydration amount of imogolite were larger than those of Mg-A type zeolite and B type silica gel. Therefore, allophane and imogolite are suitable for heat exchange absorbents of the heat pump and speedy drying desiccant.

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The formation and some properties of sintered kaolin honeycomb monolith with thick silicalite film, prepared by a hydrothermal reaction, were investigated. To enhance the adhesion strength of the silicalite film onto the honeycomb surface and the mechanical strength of the honeycomb body, the Malaysian kaolin was sintered at 1500°C and then treated in 1M NaOH solution at 180°C to form a partially porous mullite layer on the honeycomb. Silicalite film was hydrothermally prepared on porous honeycomb from colloidal silica, NaOH, tetrapropylammonium bromide and H₂O at 180°C. The effect of hydrothermal reaction time on the formation of the silicalite film onto the porous honeycomb surface and Teflon vessel was studied. Due to the formation of a porous mullite layer of 150 to 160μm in thickness on the honeycomb surface by leaching silica in the honeycomb, the adhesion strength of silicalite film on the honeycomb was improved, and a sintered kaolin honeycomb monolith with high strength which was coated with a silicalite film of 100 to 150μm in thickness at 180°C for 25 to 30h could be prepared by hydrothermal reaction.

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Developing texture is one possible way for improving the properties of ceramics. Texture in ferromagnetic materials can be tailored by a magnetic field, but it had been difficult to produce texture in diamagnetic and paramagnetic materials, such as silicon carbide and alumina, because their susceptibility is very small. A well-dispersed suspension of SiC (w)-Al₂O₃ was prepared and consolidated by slip casting in a high magnetic field. The Al₂O₃ matrix with a slight degree of orientation was observed in the green body and the degree of texture increased by annealing.

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Dielectric breakdown of titania ceramics was investigated with respect to the precurrent which is the current flow before breakdown. The precurrent behavior can be divided into two regions; an electron multiplication region in which carrier electron increases with a constant rate and a critical current increase just before the breakdown. Optical microscopic observation indicated that a part of titania ceramics melted during dielectric breakdown. Relation between critical current and applied electric field, as well as the electric power and heat calculated from the critical current, supported the observed melt of titania ceramics.

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In order to develop a novel synthetic method for MgAl₂O₄ powder, a mixture consisting of Al₂O₃ and MgCl₂ (1:10 molar ratio) was heated. As raw materials, two types of Al₂O₃; α-Al₂O₃ platelets and γ-Al₂O₃ powder and MgCl₂ were used. When the mixture consisting of α-Al₂O₃ platelets and MgCl₂ was heated at 750°C for 2h, MgAl₂O₄ powder with high crystallinity was formed. The formed Mg₂O₄ grains had a polyhedral shape (octahedron or pentahedron) 1-2μm in size and a maximum size of 3μm. When the mixture consisting of γ-Al₂O₃ powder and MgCl₂ was heated at 750°C for 2h, MgAl₂O₄ powder was formed, but its crystallinity was low. After heating the mixture at 1000°C for 2h, the crystallinity of MgAl₂O₄ became high and the size of the formed polyhedral MgAl₂O₄ grains was 3-4μm and they attained a maximum size of 10μm.

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A polycrystalline BaTiO₃ bulk ceramic with a preferred ‹111› orientation was fabricated by the reactive templated grain growth method. A green compact was made by tape-casting of slurry containing plate-like Ba₅Ti₁₇O₄₀ template particles and BaCO₃ powder. Single phase BaTiO₃ was obtained by calcining the green compact at 1000°C. The calcined compacts were sintered between 1250 and 1400°C. The degree of orientation increased with an increase in the sintering temperature. Growth of ‹111›-oriented BaTiO₃ grains originated from the Ba₆Ti₁₇O₄₀ particles is responsible for the increase in the degree of orientation.

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A new direct method for characterizing porosity in ceramic green bodies was developed. The high suitability of this technique was demonstrated by examining the structure of alumina granule compacts. The compacts were prepared by uniaxial pressing in atmosphere with different humidities. Clear images could locate minor differences in pore structure in the examined specimens.

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