Polycrystalline samples of stoichiometric spinels, Mg_1+xTi_2-xO₄(0≤x≤1.0), and spinels with cation diversity, Mg_(1+x)yTi_2-xO₄ and Mg_1+xTi_(2-x)yO₄ (x=0.4215, y=0.95-1.05), and oxygen deficiency Mg_1+xTi_2-xO_4-δ(x=0.4215, δ=0.05-0.30) were prepared by solid-state reaction of MgO, TiO₂ and Ti to examine the effects of the composition and holding time on the structure of the spinel phase, the impurity phase formation behavior and the electrical properties. In addition to Mg₂TiO₄ (x=1) prepared at 1300°C under an ambient atmosphere, samples were prepared at 1250°C under an argon atmosphere. Compositional analyses revealed that volatilization of compounds and oxidation during firing were negligible. The variation in the unit cell parameter a of the spinel phase indicated solid-solution formation for all stoichiometric samples in spite of the presence of impurities. The impurity-phase formation behavior was strongly dependent upon synthetic conditions, x-value, and induced nonstoichiometry. Other than the insulating Mg₂TiO₄, resistivity measurements indicated that all samples were semiconducting below room temperature. No distinct variation in resistivity behavior was observed for samples with different holding time or with induced nonstoichiometry.

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Polished aluminium nitride substrates have imperfections of surface roughness in the form of cavities due to the loss of grains. Measurements using the Image Processing System and the Dimensional Measurement method, revealed that cavities with diameters larger than 10μm existed in quantities of about 22.5×10⁵ pieces on a substrate of 152mm² and that their depths were about 1.3μm. The cavities also had sharp edges and steep cliffs. The application of a thin coating of silicon oxide was studied for obtaining a smoother surface of the polished aluminium nitride substrate. The depth of the coated cavity decreased with increasing thickness of the film, and the shape of the cavities, especially their sharp edge and steep cliff could not be seen clearly as the thickness of the film increased. However, even if the film thickness was as great as 1.2μm, cavities of about 0.21μm depth remained on the coated substrate. A 10μm comb electric pattern could be made on the surface of the substrate coated with 0.5μm of film. All patterns exhibited 1856 blocks (a block has 29 lines) and were uniform without any broken portions.

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The essential requirement for an artificial material to bond to living bone is the formation of bonelike apatite layer on its surface in the living body. This apatite layer can be reproduced on its surface even in a simulated body fluid (SBF) with ion concentrations nearly equal to those of human blood plasma. In the present study, Ti metal was treated with various NaOH aqueous solutions, and apatite formation on the resultant metals were examined in SBF. A sodium titanate hydrogel layer was formed on the surface of Ti metal, when it was treated with NaOH solutions with concentrations higher than 0.5M at 60°C for periods longer than 24h. Thus treated metals exchanged Na⁺ ion in the surface layer for H₃O⁺ ion in SBF to produce a hydrated titania on their surfaces and to increase the degree of supersaturation with respect to the apatite of SBF. The hydrated titania induced the apatite nucleation and the increased supersaturation accelerated the apatite nucleation. Thus formed apatite nuclei spontaneously grow by consuming calcium and phosphate ions from SBF. These results indicate that bioactive metal can be obtained by a simple alkali treatment.

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Structures of (Ca_2-x/2_??__x/2)(Si_1-xP_x)O₄ crystals were examined as a function of x ranging from 0.03 to 0.40. All of the samples were heated at the stable temperature region of the α-phase and then quenched in water. The phase constitution at ambient temperature was classified into three categories according to the fraction of the α-to-α′_H transition. When the transition was completed as in the crystals with x≤0.100, the β- and α′_L-phases were obtained, the relative amounts of which were determined by the start and finish temperatures of the α′_L-to-β martensitic transformation. With 0.125≤x≤0.150, the α-to-α′_H transition was incomplete. During further cooling, the product α′_H-phase was inverted to the α′_L-phase, and the residual α-phase was inverted to the incommensurate phase successively. Because the start temperature of the α′_L-to-β transformation was lower than ambient temperature, the α′_L-phase was stabilized. With x≥0.175, all of the crystals were free from the α-to-α′_H transition. The crystals with 0.175≤x≤0.225 were made up exclusively of the incommensurate phase. A good correlation existed between the modulation wavelength (=N) and the P/(Si+P) ratio (=x) as N=4.134-1.56x (0.175≤x≤0.250). With 0.275≤x≤0.300, the crystals were isostructural with α-Ca₂SiO₄. The hexagonal phase with 0.350≤x≤0.400, probably a transition product from the α-phase, showed two-dimensional modulations along the α-axis with N=2 and along the c-axis with N=3.

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Microstructure of TiN/Si₃N₄ ceramics was tailored by using a cosintering process of their agglomerates, then structure-properties relations were examined by measuring mechanical property and electrical conductivity of TiN/Si₃N₄. The TiN/Si₃N₄ material using TiN and Si₃N₄ agglomerates showed duplex microstructure. This materials showed a sudden onset of electrical conduction at a lower TiN content than that of conventional composites. The duplex materials exhibited a higher fracture toughness than that of the normal composites, while other hand its strength remained at the same level of TiN. It is concluded that higher fracture toughness of duplex materials resulted from the complex fracture path at the interface between Si₃N₄ and TiN agglomerates.

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Densification process, microstructure and mechanical strength of the feldspathic porcelain bodies were investigated as a function of alumina addition A maximum flexural strength of 150MPa was obtained by addition of 50mass% of alumina and by subsequent firing at 1350°C. Packing ratio and flexural strength showed a linear correlation and its gradient (k-value), which indicates a degree of the effect for the reinforcement during densification, increased linearly with increasing amount of alumina. Particles of alumina added to the bodies retained the original shape and size after firing. It is considered that dispersion of alumina articles inproved the reinforcement of the bodies.

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Fine mullite powders were prepared from molecular-designed Al, Si-double alkoxide. The change in the lattice parameters of the mullite powders with different compositions was consistent with the phase diagram proposed by Prochazka et al., showing the high purity and homogeneity of the resultant mullite powders prepared in this study. The surface areas of the resultant mullite powders with different compositions decreased with increasing calcination temperatures, depending on the compositions. However, fine mullite powder with a stoichiometric composition maintained high surface area even after high-temperature calcination. The change in the surface area could explain from the crystallization behavior of precursors, which was affected by the compositions. Precursor powders with SiO₂-rich composition crystallized into mullite at relatively low temperature, resulting in the formation of amorphous SiO₂-rich phase which promoted the grain growth during calcination by the viscous sintering. On the other hand, precursor powders with Al₂O₃-rich composition mullitized, via ultra-fine intermediate spinel and/or alumina, leading to the high surface areas at lower temperatures. However, the surface areas of the Al₂O₃-rich precursor powders abruptly decreased by the calcination above 1100°C, due to the grain growth with the aid of the active ultra-fine alumina. As a result, only the precursor powder with a stoichiometric composition showed high surface area after high-temperature calcination.

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Friction and wear properties of TiB₂/SiAlON composite ceramics slid against SiAlON ceramics were investigated in a zinc plating bath and in air. Friction coefficient of TiB₂/SiAlON in a zinc plating bath was found to decrease monotonically from 0.5 to 0.1 when TiB₂ content was increased from 0 to 80mass%, but it did not change in air with increasing TiB₂ content and was about 0.5. No wear was observed for TiB₂/SiAlON although hard particles (dross) were present in the bath, because TiB₂/SiAlON were harder than the dross. Improved lubrication afforded by better wetting between TiB₂/SiAlON and the molten zinc may be the primary cause for the reduction of friction coefficient.

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Surface topography and mechanical characteristics of ceramic substrates is very important for the thermal spraying for solid oxide fuel cells. Usually, surfaces are eroded by blasting before thermal spraying to in crease the adhesive strength of coatings. The erosion tests were performed for three kinds of commercial ceramics (Al₂O₃, machinable ceramics: MACOR, glass) with changing impingement velocity and time. The eroded surfaces were evaluated by average surface roughness (R_a), fractal dimension (F. D.) and wear volume based on Evans'theory. It was concluded that the relations between effective wear rate and eroded surface topography depend on the crack types, and fractal dimension increases with fracture toughness of ceramics.

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The effect of nitrates as sintering additives on the densification of Si₃N₄ based ceramics by pressureless sintering was compared with that of oxide additives. The surface of Si₃N₄ particles milled with a methanol solution of Al and Y nitrates, was coated with a layer containing Al, Y and O. Homogeneous distribution of sintering additives increased the sinterability of Si₃N₄. Monolithic Si₃N₄ with nitrate additives could be sintered to 94% T. D. at 1500°C. Furthermore, after pressureless sintering at 1750°C for 2h in N₂, the bulk density of a Si₃N₄/20mass% SiC composite reached 95% T. D. with nitrate additives. The enhancement of densification in the case of nitrate additives was attributed to the enhancement of primary particle rearrangement due to the homogeneous distribution of liquid phase, a consequence of the homogeneous distribution of sintering additives, during sintering.

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The purpose of this work is to present thermoelectric and mechanical properties of Bi-Te polycrystalline materials. Semiconducting ceramics of p- and n-type bismuth telluride were prepared by a hot press method. High purity row materials of Bi, Sb, Te and Se were used for the alloy preparation and the sample with a dimension of 144mm in diameter and 35mm in hight was obtained. The tensile and compressive strengths of n-type were 20.6 and 118MPa, respectively, being independent of hot press directions. A thermomechanical analysis (TMA) was used to determine the thermal expansion coefficient of 1.59×10^-5K^-1 for n-type and 1.69×10_-5K^-1 for p-type. The thermal expansion coefficient was denpendent on the hot press direction. The thermoelectric modules were fabricated to determine their figure of merit of 2.3×10^-3K^-1 at 20°C. From the present observations, the polycrystalline bismuth telluride was found to have the same thermoelectric properties as zone-melting ones.

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A compatibility test of Y₂O₃ against molten reactive lanthanum metal was performed. Several pieces of sintered Y₂O₃ were immersed into molten lanthanum at 1513 and 1273K from 86.4ks (1d) to 5Ms (60d). It was observed that Y₂O₃ was corroded under these conditions, and that the maximum corrosion depth after 5Ms was about 160μm. The corrosion is considered to proceed through the dissolution of Y₂O₃ into molten lanthanum.

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Silicon oxynitride (SiO_xN_y) films were deposited by plasma-enhanced chemical vapor deposition (PCVD), using silane (SiH₄), nitrious oxide (N₂O) and nitrogen (N₂) as reactive gases. Studies on the mechanical properties and composition of the SiO_xN_y films indicated that an increase in N₂O flow rate leads to an increased SiO₂ molar fraction in SiO_xN_y films, a decrease in microhardness, and change from a compressive stress for Si-N rich films to a tensile stress for Si-O rich films. It was found that there was a good relationship between the SiO₂ molar fraction and the stress and the microhardness of the SiO_xN_y films formed under the various conditions of N₂O flow rate or pressure. Experimental results showed no significant dependence of the microhardness and the composition of the films on the N² flow rate, but the residual stress depended on the N² flow rate. The residual stress changed from compressive to tensile with increasing N₂ flow rate, indicating that the residual stress in SiO_xN_y film was controlled by N₂ flow rate, while the microhardness and the composition of the films were kept constant. This has been explained in terms of the bombardment effects of N₂ ions against the growing layers, which was confirmed by measuring the optical emission spectrum from a PCVD plasma of an SiH₄-N₂-N₂O gas mixture. A good correlation between the microhardness and the refractive index of the films deposited at constant substrate temperature was found, leading to the conclusion that fairly accurate estimation of microhardness of SiO_xN_y films with thicknesses of 1μm or less was possible through measuring the refractive index.

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Preparation procedure of Ba-ferrite thin films was as follows. The butanol solutions of Ba and Fe naphthenate in molar ratios of n: 12 (n=1.0, 1.5, 2.0 and 2.5) were coated on Al₂O₃ ceramic substrates, dried at room temperature and 110°C, and then thermally decomposed at 550°C. The above operation was repeated 11 times, and then finally the specimens were heated at various temperatures from 700 to 1400°C. Result of X-ray diffraction of the obtained Ba-ferrite thin films showed that the most suitable molar mixing ratio of the naphthenates and the temperature of the final heating were 1.5: 12 and 900°C, respectively. The thin film prepared under the above conditions had saturation magnetization of 134emu/cm³, residual magnetization of 73emu/cm³, and coercive force of 4.42kOe.

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Green compacts of high-purity silica powders with different particle sizes were formed by slip casting method and pressureless-sintered. Conditions to fabricate transparent silica glasses are presented using diagrams described by sintering-temperature and -time. The variation of linear shrinkage with time reveals that the sintering mechanism at its early stage follows the viscous flow of silica glass particles. The effect of firing schedule on residual OH concentration was studied for transparent sintered silica glasses. A green compact prepared by silica powders with a mean particle size of 1.6μm was fired at 1573K for 3h for dehydration and heated up to 1373K for densification; this typical fabrication condition results in a transparent sintered silica glass with <1ppm OH concentration.

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Effect of heat treatment on the tensile strength of Al₂O₃-SiO₂, Al₂O₃-SiO₂-ZrO₂ and Al₂O₃-SiO₂-Cr₂O₃ glass fibers was investigated. Like general glass fibers, the strength of these glass fibers decreased as the fiber diameters increased. There was little difference in tensile strength among three kinds of these fibers that had the same diameter but different in chemical composition, regardless whether they were heat-treated from 800 to 1100°C or not. By the heat treatment up to 900°C, these fibers consisted of only amorphous phase, irrespective of their chemical composition. By the heat treatment at 1000 and 1100°C, all these fibers partly crystallized. However, they still contained at least 35mass% of amorphous phase. Because the tensile strength of these fibers was independent of their heat treatment, their tensile strength can be governed mainly by the properties of amorphous phase in these fibers.

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Preparation of carbon fiber reinforced ceramic composite (CFRC) was tried by low temperature sintering, using a phosphate (ACP) mixture consisting mainly of aluminum phosphate as a matrix material. Carbon fiber (CF) strand (HP, 12K) was used as a reinforcing material. Prepreg sheets of CF impregnated with ACP were dried at 80°C. Laminated prepreg sheets were heated up to 200°C in a hot press, and then heated in N₂ at 500°C to prepare the CFRC. In this study, the prepreg sheets were heated under continuously generated steam, and its effects on mechanical properties and structure of the CFRC were investigated. Consequently, mechanical properties of CFRC were improved by heat treatment in moist air, showing a bending strength of 516MPa and a Young's modulus of 55GPa. Observation on its side surface indicated that few voids were left in the CFRC, which leads to the improvement of adhesiveness between prepregs.

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The solubilities of Cr₂O₃ and Al₂O₃ in CoO were determined at 1000-1600°C in air. In the CoO-Cr₂O₃ system, the solubilities were 3mol% at 1400°C and 5.2mol% at 1600°C. In the CoO-Al₂O₃ system, 0.6mol% at 1400°C and 1.6mol% at 1600°C. From the dependence of the lattice constant of CoO phase upon the concentration of M₂O₃, we conclude that the Co (II) sites in the CoO phase were displaced by M(III) cations, producing vacancies at other Co(II) sites.

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CaCO₃ powder with high purity was coated with a chelate compound of aluminum corresponded to 0.5mass% Al₂O₃. The CIPed compact of the coated powder was sintered at 1500°C for 2h in air to prepare CaO ceramics (“COAT” specimen). The distribution of Al in the CaO matrix for the COAT was different from that for the CaO ceramics (“MIX” specimen) prepared from the mixture of CaCO₃ and Al₂O₃ powder. The sinterability and slaking resistance of the CaO ceramics were greatly improved by addition of 0.5mass% Al₂O₃. The relative density and slaking resistance of the COAT specimen were higher than those of the MIX specimen.

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Cobalt silicate was synthesized by the reaction of cobalt nitrate with sodium metasilicate in aqueous solution. The reaction was carried out by dropwise addition of sodium metasilicate solution to a refluxed cobalt nitrate solution in 30min and the mixture was allowed to stand for 3h. The resultant powder was then sintered at 1200°C for 3h. The reaction in an aqueous solution can be assumed to proceed by the following process. Na₂SiO₃+2Co(NO₃)₂→H₂OCo₂SiO₄⋅3.5H₂O+2NaNO₃+2HNO₃ Co₂SiO₄⋅3.5H₂O→ΔCo₂SiO₄+3.5H₂O The precursor and sintered powders were analyzed by XRD, atomic absorption spectrometry and thermal analysis. The sintered sample was confirmed to be cobalt silicate, Co₂SiO₄.

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