Complex impedance, relative permittivity and dielectric loss for the aqueous solutions and 5vol% alumina suspensions at pH 3.0-10.2 were measured at 17-27°C as a function of frequency of 100Hz-1MHz. Their electrical properties were well expressed by a parallel circuit with resistance (R) and capacitance (C). The pH dependence of resistance of solutions and suspensions was associated with the migration of coexisting ions and charged alumina particles, respectively. At the isoelectric point of alumina particles, both the resistivity of solutions and suspensions was comparable because of no migration of flocculated clusters of alumina particles. The capacitance of solutions resulting from the polarization of H₂O molecules decreased with the increasing conductivity. The effect of coexisting ions on this result was discussed. For the alumina suspensions, deformation of electrical double layer surrounding alumina particles was related to the permittivity. Finally, the time constant (τ=RC) was compared for aqueous solutions and alumina suspensions.

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Mixtures of calcined kaolin, calcined talc and alumina were ground to submicron particle size by ball milling, and the effect of particle size of the ground mixtures on the densification behavior of a cordierite sintered body was studied. As the particle size of the ground mixtures became smaller, the raw material powders were mixed more uniformly and the formation temperature of α-cordierite became lower. Dense and pore-free sintered bodies were obtained by firing ground mixtures with particle size smaller than 0.39μm at temperatures as low as 1200°C. Cordierite sintered bodies prepared from mixtures with particle size 0.39μm or smaller by grinding natural raw materials had bending strengths ranging from 140-160 MPa, and these values were larger than that of cordierite glass-ceramics reported by Hayashi et al. The thermal expansion coefficients of dense, high-strength cordierite bodies increased, because anisotropic thermal expansion could not lower the amount of thermal expansion in the pore-free body.

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The drying kinetics of the wet alumina pieces prepared by pressure filtration of ultrafine (average size 0.16μm) alumina powder in various dispersion conditions were studied. The effects of acidic dispersant and two drying control chemicals (DCC) were investigated by studying the change of contact angle, surface potential, green properties and drying kinetics. By appropriately controlling temperature and humidity, the drying kinetics can be divided into two stages, starting from an interfacial evaporation, then ending with a vapor diffusion through a porous matrix. The drying rate of the initial stage of the drying process is linearly dependent upon drying time, which will then change to a parabolic decrease behavior controlled by a diffusion-limited mechanism. These kinetic phenomena can be described by three parameters, including mass loss rate, J, mass transport coefficient, k, and diffusion coefficient, D. In association with the analysis of these kinetic parameters, the effects of drying with the change of temperature and relative humidity will be clarified. The results show that the kinetics is greatly improved and the drying can be reduced to only one step.

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Polycrystalline samples with nominal compositions K_1+xNb₄O₆ (0≤x≤1.0) were prepared by the solid-state reaction of KNbO₃, Nb₂O₅ and Nb. Powder X-ray diffraction analysis revealed that the KNb₄O₆-type compound was obtained as the major phase when x≥0.4. Transmission electron microscopy observations showed that when x=0.4, the KNb₄O₆-type compound contained intergrowth defects. In contrast, when x=1.0, practically no intergrowth defects were observed, although this sample contained KNbO₃ as an impurity phase. The relation between the nominal composition and the formation of the intergrowth defects is discussed.

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This paper presents a grindability data collected by surface grinding of Si₃N₄ caramics with a newly developed wheel with cutting edges of unified height. Grindability was evaluated according to the measurement of grinding force, specific grinding energy, surface roughness and material strength after surface grinding. The wheel with cutting edges of unified height produced small specific grinding energy and surface roughnes comparable to the usual resinoid-bonded wheels. As a result of an analysis of maximum grain depth of cut of the wheel with unified height of grain cutting edges offers smaller mean value of maximum grain depth of cut, consequently the above grindability were obtained.

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A dense silicon carbide matrix composite reinforced by Hi-Nicalon fibers with boron nitride coating was fabricated by slurry impregnation and subsequent reaction sintering process. Hi-Nicalon fibers were not stable in contact with molten silicon. Accordingly the effect of the fiber coating structure and the infiltrating metal composition on the mechanical properties of the composite was investigated. Selecting a boron nitride and silicon carbide dual coating structure and a boron-doped silicon as the infiltrating metal, a ceramic matrix composite with excellent fracture energy could be newly developed. The microstructure and the bending strength at room and high temperatures were also evaluated. Optimized composite showed cumulative failure mode both at room temperature and at 1573K. The failure behavior kept this cumulative fracture mode even after oxidation at 1573K for 360ks. Good oxidation resistance was thought to be caused by the presence of the dense SiC matrix.

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Boehmite samples were prepared from aluminium isopropoxide which was hydrolyzed in hot water under different conditions, and heat-treated at various temperatures below 1200°C. Amount of adsorbed moisture and specific surface area of the samples were measured. The amount of adsorbed moisture was maximum when the boehmite was in the dried gel state, which was almost the same as that of silica gel. The amount of adsorbed moisture slightly decreased up to the heat treatment temperature of 600°C. The adsorption feature was scarcely improved even when ultrasonic irradiation and freeze drying technique were applied. A quantitative analysis by differential scanning calorimetry (DSC) was attempted, and the following equation was derived to estimate the activation energy E of desorption of moisture; E≅nRT₁T₂/S₀⋅ΔS/ΔT Where n is the order of reaction, R the gas constant, S₀ the total area of endothermic curve fitting in desorption of moisture, T₁ and T₂ are the start and end temperatures of the reaction, and ΔS shows the area against the temperature change ΔS on the DSC curve. By the application of the above equation to the DSC data of samples heat-treated at 600°C, an activation energy of 1.7-2.7kJ/mol was obtained.

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Raman spectroscopy was used to analyze the crystal structure of Hot Isostatically Pressed silicon nitride (Si₃N₄ containing 0.5mol% of Y₂O₃, 0.5mol% of Nd₂O₃ and 1mass% of Si₃N₄ whiskers) to confirm whether microstacking faults occurred in the β-phase due to structural defects in the silicon nitride grains. With this technique, the presence or absence of a 514-520cm^-1 peak confirms whether or not such stacking faults have occurred. For the β-phase alone of orientation-controlled silicon nitride, a large grain diameter and high thermal conductivity were obtained. In contrast, for gas-pressure hot-pressed silicon nitride, the grain diameter was smaller and stacking faults occurred in the β-phase, even though the heat treatment process was performed at the same temperature. It was found that structural defects in the crystal grains caused phonon-impurity scattering, resulting in a decline in thermal conductivity. This indicates that structural defects in the grains must be eliminated to obtain high thermal conductivity. The results of this study suggest that Raman spectroscopy can be an effective technique for conducting structural analyses to determine the presence of such defects.

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A simple process was developed to produce transparent MgO ceramics. In this process, magnesium hydroxide was initially precipitated by means of dropping an ammonium solution to a magnesium chloride solution in a glass (pyrex) beaker at about 100°C. The resulting precipitate was kept in the aqueous solution for 3h at the same temperature. Sintering was carried out at 1600°C for 2h in vacuum. Aging the precipitate at about 100°C is a key step of the present method, in which silica and boron oxide were dissolved from the used beaker, and then major part of them were adsorbed or absorbed by the precipitate concerned. Coexistence of silica and boron oxide was one of the necessary conditions to produce high transparent MgO ceramics.

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One of the features of Mitsui-type solid oxide fuel cells (SOFC) is that the gas separators are made of MgO/MgAl₂O₄ composite ceramics, onto which several LaCrO₃ ceramics are joined so as to connect the cells electrically in series. Since one of the functions of the gas separator is the separation of fuel gas and oxidant gas, the joining interface of MgO/MgAl₂O₄ Composite and LaCrO₃ must maintain gas tightness at the operating temperature over 1200K for a long term. The authors developed a ceramic joining technique according to which a ceramics slurry is coated on the joining surfaces and then firing is performed at 1773K. The average bending strength of the joints was 65MPa at 1273K. The joining interface held gas tightness against four thermal cycles from 1773K to room temperature. The vicinity of the joining interface showed an altered quality due to the joining process. However, the altered area was not extended and the interface held gas tightness for not less than 8000h under the operating condition of the SOFC.

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The addition of surfactants of quaternary ammonium ions (QAI) to polynuclear hydroxoaluminum (PHA) ion solution having a OH/Al ratio of 2.50 led to transparent composite gels on drying. The composite gels were transformed into a single phase of γ-alumina upon heating above 600°C. The size of γ-alumina crystallites gradually increased with increasing temperature up to 900°C. The addition of QAI resulted in marked increases of specific surface area and pore volume of γ-alumina, giving a sharp monomodal pore size distribution. The pore sizes depend on the chemical species and the content of QAI; especially, the dependency of the pore sizes on the bulkiness of QAI suggests that the pore formation is determined by the so-called template mechanism of QAI micelles. These results indicate that the pore sizes of γ-alumina are precisely controllable in the range of 4-8nm by the PHA-QAI composite gel technique. The γ-alumina sheets thus obtained showed transparency due to the lack of macropores and the homogeneous distribution of uniform mesopores.

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The dispersibility of various commercial pseudoboehmite powders in nitric acid solution was investigated. In order to elucidate the factors that may influence the dispersibility, the physical and chemical properties of the pseudoboehmite powders were measured and observed, such as apparent density, surface area, porosity, water desorption temperature, microstructure, and point of zero charge in aqueous solution. The microstructure of the pseudoboehmite powders affected the dispersibility. The powder, composed of dense agglomerates of fine platelet primary particles, showed a good dispersion to form sol in 0.03M nitric acid solution. In contrast, the powder composed of porous agglomerates of wrinkled sheets, which are probably secondary particles, did not disperse well. In addition to powder properties, a drying treatment changed the dispersibility remarkably. It was observed that the dispersibility of pseudoboehmite powders decreased with increasing the drying temperature. The differential thermal analysis of the powder after the drying treatment exhibits a lower endothermic peak of moisture desorption. It is suggested that the dispersibility is related to the thickness of water molecule layer between particles.

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In order to develop the most suitable process for the preparation of low-oxygen SiC fiber, EB-cured PCS fibers were heated from room temperature to 1573K by using different combinations of heat treatment conditions such as heating rate, keeping temperature and keeping time. Subsequently, the fibers were exposed to 1873K in argon. While slow heating rate and long keeping periods were required for the improvement of thermal stability of fibers during heat treatment above 1200K, the heat treatment time could be shortened below 1200K. No change in specific resistivity and composition profile of fibers heat-treated under optimum conditions was observed before and after high-temperature exposure. In addition, fiber surface was dense and pore-free after high-temperature exposure. Consequently, the fibers showed a high level of strength both in the as-heat-treated state and after high-temperature exposure. For example, the tensile strength was 3.6GPa in as-heat-treated state and 2.2GPa after high-temperature exposure for the fiber which was prepared by heating at 600K/h below 1073K and at 50K/h above 1073K.

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The extruding characteristics and the packing structure of clay dough, and alumina dough mixed with water-soluble and water-insoluble plasticizers were investigated. The extrudability of the doughs, which were evaluated by the extruding pressure as a function of the packing ratio, became higher at higher volume of larger-sized pores reaching micrometed size. In the clay dough and the dough mixed with water-soluble plasticizers such as methylcellulose, the aggregates deformed into the larger-sized pores around each aggregate during extruding. In the dough mixed with water-insoluble plasticizers such as curdlan, the larger-sized pores based on curdlan gel, its deformation provided the fluidity of the particle. It was understood that the dough could be extruded by deformation of the aggregates or the gel acted as a buffer.

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Films of Cr-added V₂O₃ were prepared by reduction of sol-gel-derived V₂O₅ films at temperatures over 500°C in flowing H₂. Auger electron spectroscopy of the films showed that atoms of V and O were distributed homogeneously on thickness direction. From X-ray photoelectron spectroscopy of the films, it was found that the reduction was enhanced with an increase of the reduction temperature, however, even at 900°C reduction valence of V still interposed between +3 and +4. The specific resistivity (ρ)-temperature (T) curves of the films showed antiferromagnetic insulator (AFI)-metal (M) transitions, but not metal (M)-insulator (I) transitions. Grain diameters of Cr-doped V₂O₃ powders prepared in the same way of the films increased by sintering at 1000 to 1350°C for 5h in flowing mixture gas (95vol%N₂+5vol%H₂). The M-I transitions was observed in the ρ-T curves of these powders heated at 1200°C and higher. Absence of M-I transition of sol-gel-derived V₂O₃ films was considered to be due to small grain sizes and/or small crystallite size of films.

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The effects of whisker diameter and carbon-coating of whiskers on the flexural strength and the fracture toughness of SiC-whisker-reinforced mullite composites were studied, and quantification of a crack deflection was attempted. The flexural strength and the fracture toughness gradually increased with increasing the amount of SiC whiskers. The strength slightly decreased with increasing whisker diameter and after coating the SiC whiskers with carbon. In contrast, the fracture toughness slightly increased. The toughening of SiC-whisker-reinforced mullite composites is contributed to mainly by the increase of the elastic modulus by the addition of SiC whiskers with high elastic modulus and the occurrence of debond-bridging in the SiC whisker-mullite matrix interface, but is accompanied by slight crack deflection and whisker-pullout.

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Sn⁴⁺-Sn²⁺ redox in binary alkali silicate melts was studied by means of differential pulse voltammetry. The half-wave potential, E_1/2, of the reduction of Sn⁴⁺ to Sn²⁺ shifts toward the negative with changing the alkali metal ions from Li⁺ through Na⁺, K⁺ and Cs⁺, at a fixed concentration for each alkali metal oxide. Conversely, E_1/2 shifts to the positive with an increase in the content of each of the alkali oxide. This tendency for Sn⁴⁺-Sn²⁺ redox was different from those of Sb⁵⁺-Sb³⁺ and Cr⁶⁺-Cr³⁺ redox. The presence of Sb⁵⁺ and Cr⁶⁺ result in a strong acid, which leads to E_1/2 shift to the negative with an increase in basicity. These results for Sn ions can be interpreted in terms of complex formations of Sn⁴⁺ and Sn²⁺, in which the coordination numbers of the oxide ion for both ions do not differ significantly and their equilibrium constants for complex formation depend largely on the alkali species.

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Superplastic behaviors of silicon nitrides with and without α-phase were investigated on the tensile stress-strain curves. Despite of the much smaller grain size, the specimen containing 30vol% α-phase showed a higher deformation stress than the specimen without α-phase at an earlier stage of the deformation. As α-β transformation of the former specimen proceeded, the stress decreased and became lower than that of the latter. Thus, the higher deformation stress of the former at the earlier stage was attributed to the existence of α-phase.

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Physical and electrical properties of three types of Ag-Pd pastes, consist of different metal fine powders, a coprecipitated, an agglomerated alloy made by heat treatment and a dispersed alloy produced by improved pulverization method, have been studied. The dispersed alloy powder paste shows a high packing density (6.3g/cm³) of dried film compared to others. The film shows a lower expansion at around 500°C and a lower shrinkage from 700 to 1100°C and a lower electrical resistivity after firing. The results indicated that the paste having an improved pulverized Ag-Pd alloy is superior to other two for internal electrode material for multilayer ceramic device.

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