Al₂O₃/aluminate composites were fabricated by a solid reaction in the La₂O₃-Al₂O₃ system. The formed phase showed elongated morphology that could be controlled by varying the hot-pressing temperature and time. Dense Al₂O₃/LaAl₁₁O₁₈ composites were fabricated by hot-pressing above 1500°C. When the hot-pressing temperature was 1400°C, Al₂O₃/LaAlO₃ composites were formed from starting composition. The grain size of monolithic alumina ceramics increased with increasing hot-pressing temperature, and marked grain growth occurred above 1600°C. However, the elongated LaAl₁₁O₁₈ phase suppressed the growth of alumina grains in the Al₂O₃/LaAl₁₁O₁₈ composites. Small LaAl₁₁O₁₈ grains existed in the alumina grains as an intragranular phase, while needlelike grains with a large aspect ratio existed at Al₂O₃ grain boundaries. The amount of in-situ formed LaAl₁₁O₁₈ phase increased with increasing amount of La₂O₃, and the aspect ratio of LaAl₁₁O₁₈ grains increased with increasing hot-pressing time.

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As fundamental research to raise the strength and fracture toughness of ceramics, the dispersibility of SiC whiskers, W (A), and carbon-coated SiC whiskers, W (AC), in eight organic solvents was evaluated by measuring the zeta potential and sedimentation velocity. Ethanol and 2-propanol were selected from these organic solvents, and the dispersibility was studied in detail. The electric charge of W (A) was positive in ethanol and negative in 2-propanol. On the other hand, the charge of W (AC) was negative in both solvents. The charge of their whiskers was a minimum (negative) by the addition of 0.001-0.002g/l dispersant for W (A) and 0.0002-0.001g/l for W (AC). The dispersibility of W (AC) was worse than that of W (A) in ethanol. The dispersibility of both SiC whiskers was improved by the addition of the dispersant to ethanol and 2-propanol. It was estimated that the dispersibility of W (A) and W (AC) was considerably affected by steric hindrance of the polymer used as a dispersant.

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The surface state of as-prepared AlN powders produced by direct nitridation and carbothermal methods was characterized by X-ray photoelectron spectroscopy (XPS) and X-ray excited Auger electron spectroscopy (XAES). In the as-prepared AlN powders, small peaks assigned to imide and amide groups were detected in the N1s XPS spectra of directly nitrided powders but not in those of the carbothermally treated powders. Both an oxidized surface phase and a parent AlN phase could be distinguished in the Al2p XPS and Al (KLL) XAES spectra of all the samples. The structural state of the oxidized surface phase was found to be similar to γ-Al₂O₃. The average thickness of the oxidized surface phase was determined to be about 0.5-0.6nm in directly nitrided AlN powders whereas it was 1.1nm in carbothermally treated AlN powders. The thickness of the oxidized surface phase increased with increasing storage times of the AlN powders in a container under ambient atmosphere. The increase of thickness with storage time of the oxidized surface phase in directly nitrided AlN powder was faster than in the carbothermally treated AlN powder. The oxidized surface phase changed from γ-Al₂O₃ in the as-prepared AlN powders to a hydroxide phase during storage.

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Ultrasonic mist pyrolysis was applied to electrode preparation for a solid oxide fuel cell (SOFC) reactor. La_1.8Al_0.2O₃ anode catalyst was prepared on heated YSZ substrate by the mist of aqueous solution containing lanthanum nitrate and aluminum nitrate generated by an ultrasonic transducer. The effect of spray times for periodic operation was investigated. Different anode catalyst structures were obtained by varying the spray time. These structures strongly affected the reactivity of oxidative coupling of methane. Short spray times gave thin layer structure of the anode catalyst which was suitable for this reaction.

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Oxygen doped Ca_1-xSr_xFeO_3-z (0≤x≤1.0, z=0.25-0) perovskite type oxides were prepared by treating grossly oxygen-deficient samples using an electrochemical oxidation technique at room temperature. Although the oxygen-deficient starting samples in the Ca-rich region (CaFeO_2.5 and Ca_0.8Sr_0.2FeO_2.5) had high resistivity, it was possible to dope oxygen when a high current and a high electrode potential were applied. However, there is no reproducibility in preparing them. Ca_1-xSr_xFeO_3-z in the Ca rich region (x=0.4-0.0) showed a gradual metal-insulator (MI) transition due to the charge disproportionation of Fe⁴⁺ (2Fe⁴⁺↔Fe³⁺+Fe⁵⁺) below room temperature. For the oxygen doping of the perovskite oxides, applying only a few hundred mV of electrode potential creates the same effect as a long annealing at high oxygen pressure of more than a few hundred MPa. Although the lack of stability after electrolysis is a weak point, the electrochemical reaction is a simple and easy way to creates the oxygen doped state of the perovskite oxides.

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The surface of polycrystalline alumina fibers was modified through the hydrolysis of Al-tributoxide (TBOA) or Si-tetraethoxide (TEOS). The former gave the partial deposition of granular materials, and the latter the entire coating of a thin film. Both of them could reduce the deterioration of the appearance of the fiber after being heated at 1400°C, while the geometrical patterns on the original fiber cropped up at that temperature. Particularly, the thin film on the TEOS-treated fiber which was thermally stable suppressed the outward diffusion of the inner amorphous Al species to hold the smoothness of the surface up to 1400°C. The thickness of the film was estimated to be 3nm after being dried at 90°C. This thermal stability decreased at 1500°C; that is, the patterns on the surface of the fiber cropped up.

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In situ Raman microprobe spectroscopy has for the first time provided direct experimental evidence that Si₃N₄ is toughened by virtue of local microscopic bridging stresses operated by unbroken grains, which resist crack opening upon external load. The bridging stresses may arise from elastic tractions of unbroken Si₃N₄ grains which bridge the crack in the very neighborhood behind its tip, or be of frictional nature at the grain interfaces in regions more far behind the tip. Particularly remarkable is the magnitude of the elastic bridging tractions which achieve the GPa order prior to microscopic fracture of the bridging site. Frictional tractions in correspondence of elongated Si₃N₄ grains may also locally achieve the order of several hundreds MPa, but they are shown to contribute toughening in a less effective way as compared to elastic bridges. Conventional fracture mechanics characterization and theoretical considerations are also provided which prove the rising R-curve effect in Si₃N₄ ceramics being explainable simply in terms of the microscopic crack bridging mechanism.

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Gels with interconnected porous morphology in micrometer range have been prepared in a silica-zirconia system containing 12.7 mass% of zirconia from metal alkoxides under coexistence of poly (ethylene oxide) (PEO) with an average molecular weight of 100000. The porous structure was formed when the transitional structure of spinodal decomposition was frozen-in by sol-gel transition of inorganic components. In the system, the driving force of phase separation was considered to be the repulsive interaction between solvent and PEO associated on inorganic oligomers. Although the addition of zirconium alkoxide has an effect to decrease the domain size of dried gel by the increase of condensation rate, the domain size could be controlled in the sizes ranging from 0.2 to 30μm by adjusting the compositions of other constituents in the reacting solution.

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The improvement of the sintering properties of a superconducting BaPb_0.75Bi_0.25O₃ ceramic by the introduction of point defects was examined. In order to introduce Ba vacancies into the structure, a mixture of BaCO₃, Pb₃O₄ and Bi₂O₃, with Ba-deficient nominal composition, was used as a starting material. The density of the specimen increased by using this nominally Ba-deficient mixture. However, it was revealed that the grain size in the sintered material was not affected by the amount of the nominal Ba deficiency and that a second phase, composed mainly of Bi and oxygen, was generated at grain boundaries. It was concluded that Ba vacancies could not be formed in BaPb_0.75Bi_0.25O₃ lattice and that the apparent high density and electrical resistivity, which was as high as 10^-1Ω⋅cm at room temperature, is attributed to the second phase formation at grain boundaries. when sintering under low oxygen partial pressure, the density of the superconducting BaPb_0.75Bi_0.25O₃ ceramic also increased. However, it was observed that the grain size of the BaPb_0.75Bi_0.25O₃ ceramic increased without the generation of a second phase at grain boundaries while sintering under reducing atmosphere. The dependence of the grain size on the oxygen partial pressure led to the suggestion that enhanced grain growth is governed by the increase of volume diffusion due to the introduction of oxygen vacancies, After annealing under oxygen to remove oxide ion vacancies, the specimen showed an electrical resistivity at room temperature as low as 10^-3Ω⋅cm and a sharp superconducting transition at 11.5K.

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Metastable phase relationship in the ZrO₂-YO_1.5, ZrO₂-TiO₂ and YO_1.5-TiO₂ systems has been investigated using precursors prepared by a sol-gel method through metal alkoxides. Well-crystallized phases were formed by heating the precursors at and above 1073K for 10h. Extended solubilities were obtained in the tetragonal zirconia, cubic fluorite, YO_1.5, ZrTiO₄, and Y₂Ti₂O₇ solid solution by heat treatments at 1073 and 1273K. Although the equilibrium Y₂Ti₂O₇ phase did not appear, a metastable cubic phase was obtained by heating Y_1-xTi_xO_1.5+x/2 (x=0.29 and 0.38) at 1073K.

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The effect of calcined kaolin grain size on firing process and physical properties of cordierite ceramics was studied using kaolin, talc and α-alumina as a starting materials. Formation temperature of cordierite crystals from the finest kaolin with average grain size, 0.51μm, was about 50°C lower than that from the most coarse kaolin with average grain size, 2.23μm. The sintering performance was improved by using the fine calcined kaolin powder, because diffusion and reaction progressed in firing process due to the increase of contact area of starting powders. Finer kaolin powders contributed to a decrease in porosity and to a densification of fired bodies, so that the bending strength was increased from 50 to 80MPa with a decrease in kaolin grain size from 2.23 to 0.51μm. The thermal expansion coefficient was constant, 1.6-1.8×10^-6/°C (50-900°C), for the fired bodies at 1300 and 1350°C independently of kaolin grain size.

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High-durability lead zirconate titanate (PZT) ceramics were developed in this study to improve both coercive field and mechanical strength under operation. Coercive field and displacement under preloading in PZT was increased by adding an elemental acceptor into B-sites. Mechanical strength was increased by obtaining a small grain size through a lowered sintering temperature using a pulverizing process. The temperature dependence of capacitance was decreased by transformation to tetragonal phase. The present investigation resulted in the development of high-displacement and high-durability PZT compositions.

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The accuracy in the quantitative phase analysis (QPA) of α- and β-silicon nitrides using the Gazzara method, the Rietveld method and the Whole-Powder-Pattern Decomposition (WPPD) method was examined. Integrated intensities, which were obtained by a profile fitting technique, and peak intensities were used for the Gazzara method. Experimental and analysis conditions were varied, and their influences on the accuracy of the QPA were examined. Model parameters in profile fittings, the Rietveld method and the WPPD method were optimized, and the standard settings for powder diffraction experiment were established. Under the optimized condition the three methods for the QPA gave errors, measured as the deviation from weighed value, of 0.9, 0.8 and 0.4 mass% at the maximum and of 0.3, 0.4 and 0.1 mass% on the average. On the other hand, the Gazzara method using peak-intensities gave a less accurate result ranging up to 6.4 mass%.

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Double oxide, Sr_xCa_1-xCuO_y has a layered structure in which Ca (Sr) is sandwiched between CuO₂ layers. This crystal structure is expected to give low friction due to its low shear strength between laminas. In this work, various kind of Sr_xCa_1-xCuO_y with x=0.0 to 1.0 were prepared from SrCO₃, CaCO₃ and CuO by sintering method. The compositions of these products were examined by X-ray diffraction. The sample powders were blended in oil (squalane) and grease, and their load carrying capacities were evaluated by a cylinderon-plate type tribometer and a four-ball machine, respectively. As a result of the tests, Sr_xCa_1-xCuO_y with x=0.0, 0.14, 0.90 and 1.0 showed low friction (about 0.1) and high load carrying capacity (4900N). It was found from SEM and EPMA results that the worn surface was smooth, and that a thin solid lubricating film containing Ca, Sr and Cu was formed on the rubbing counterface. Also, the oxides with x=0.9 and 1.0 showed superior properties under extreme pressure in four-ball machine testing.

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The effect of ball milling conditions on pulverization of the very small amount of large particles in silicon nitride powder was examined. In this paper, the wet sieve analysis and X-ray sedimentation method were used to measure the content of large particles in the ground powder which influence important properties of sintered ceramics such as fracture strength. As a result, it was made clear that the wet sieve analysis would be a reliable method to measure the content of just a few large particles in the ground powder. By applying this analysis to the ball-mill grinding of silicon nitride powder, it was found that the processing conditions required to grind the large particles were different from those to obtain high specific surface area as well as small average size of particles. This is because the impact force exerted onto the particles by the media balls is more effective on grinding large particles.

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We investigated the Si₃N₄-SiC ceramics prepared from Si-SiO-C mixed powder compacts. Si₃N₄ matrix and SiC particles were formed during the sintering process in the ceramics. During heat treatment at 1350°C in Ar, SiC particles were formed first and, after nitridation at 1400°C, self-composed Si₃N₄-SiC ceramics were obtained. In these ceramics, SiC particles about 100nm in diameter were dispersed in Si₃N₄ matrix particles. Yttrium which originate from sintered aide (Y₂O₃) was detected at the grain boundary between Si₃N₄ matrix and SiC particles intragranularly dispersed in the matrix. It is suggested that the SiC particles were dispersed in the Si₃N₄ during the α→β transformation or the grain growth of Si₃N₄ grains. Thermogravimetry (TG) and differential thermal analysis (DTA) results suggested that SiO and CO gases were concerned in the formation of the SiC particles.

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Hydroxyapatite whiskers were synthesized by heating the mixed solution of low concentration of calcium chloride (CaCl₂) and dipotassium hydrogenphosphate (K₂HPO₄⋅3H₂O) at 90°C. The whisker aggregates precipitated from the aqueous solution, and the morphology changed from the spherical form to arborescent form and cone-like form with increasing the initial pH of the aqueous solution. During the precipitation of the hydroxyapatite whiskers, the pH of the solution decreased and the solution tended to precipitate the monetite due to the release of H⁺ ions. To prevent the precipitation of monetite and synthesis the hydroxyapatite whiskers with a good efficiency, it is need to keep the pH of aqueous solution above 4.1 during the precipitation reaction.

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TiB₂-TiN_xC_y composite ceramics with high density, mechanical strength and electrical conductivity were prepared by the hot-pressing method with the solid phase reaction of TiN with B₄C powder. The TiN powder containing 0-20 mass%B₄C-additives was hot-pressed at 1950°C for 3.6ks under a pressure of 31.2MPa in ambient Ar gas. The 42.9 mass%TiB₂-TiN_xC_y composite derived by the addition of 16 mass%B₄C to TiN had a relative density of 98.9%, hardness of 25.4GPa, flexural strength of 850MPa, fracture toughness (K_IC) of 9.5MPa⋅m^1/2, Young's modulus of 505GPa and electrical conductivity of 5.4MS⋅m^-1. This composite showed excellent mechanical properties comparable with those of hot-pressed Si₃N₄ or SiC ceramics and an electrical conductivity comparable with metal. The composite is expected to be used as a ceramic mold material that can be machined by electrical discharge, corrosion-resistant electrode components, and electro-conductive high-temperature structural materials.

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Toughness of ceramic-matrix composites can be optimized through coatings the reinforcing fibers. Chemical vapor deposition (CVD) process is a key technology for fiber coatings. The mechanical properties of continuous silicon carbide fibers (Hi-Nicalon) have been studied as a function of the thickness of the pyrolytic boron nitride (P-BN) coating by CVD process. The compressive residual stress at the P-BN coating layer decreases slightly with increasing the coating thickness. It is considered that, with including the residual stress effect, cracking of the P-BN coating layer occurs first in a monotonic tensile fracture test. In addition no degradation of the silicon carbide-fiber in ultimate tensile strength is observed regardless of the P-BN coating thickness.

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The Na-β″-Al₂O₃ electrolyte possessing high sodium ion conductivity is a candidate material for secondary batteries and thereto-electric converters. Sintered compacts of β″-Al₂O₃ containing 0-10vol% of ZrO₂ were made from calcined powder consisting of β″-Al₂O₃ and β-Al₂O₃. The effects of the ZrO₂ addition on the normalized index f(β″) and microstructure of the sintered compacts were investigate, and the improvement of the density and normalized index was observed. It is therefore suggested that the ZrO₂ addition to β″-Al₂O₃ accelerates diffusion of the materials. The small amount of ZrO₂ is dissolved into the liquid which is generated during heat treatment. The dissolution ability or surface tension of that liquid appears to be influenced by the dissolution of ZrO₂, leading to improved sintering characteristics. The addition of ZrO₂ affects the microstructure, especially the grain growth of Al₂O₃. While abnormal grain growth of Al₂O₃ takes place in the case of 0.1vol% of ZrO₂ addition, such abnormal grain growth is prevented by a 1vol% of ZrO₂ addition. The concentration of Zr at the grain boundary, which decreases grain boundary mobility, was observed. The concentration of Zr is therefore considered to prevent abnormal grain growth.

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Silicon carbide (SiC) was synthesized from compacted pellet composed of Si powders and amorphous carbon powders through heating in solar furnace under controlled atmosphere (Ar or N₂). Under irradiation of the solar energy flux 1350kW/m² (ca. 1650°C in terms of measured temperature) for 30min, Si was completely converted to SiC in Ar atmosphere while the conversion was incomplete in N₂.

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