To improve mechanical properties of TiO₂-added zirconia-toughened alumina (ZTA), ZTA powders containing (Zr, Ti) O₂ were sintered in an atmosphere of N₂+5%H₂. The effect of TiO₂ content on the microstructure and mechanical properties of ZTA was investigated. Experimental results showed that Ti ions diffused to grain boundaries and into Al₂O₃ grains, thereby enhancing the densification of ZTA powders sintered in N₂+5%H₂, but the grain growth of ZTA was less than that of a sample sintered in air. Al₂TiO₅ dispersed in a ZTA sintered body containing ≥5.5mol% TiO₂ was considered to increase the residual stress. These results indicate that ZTA composites containing TiO₂ sintered in N₂+5%H₂ have superior bending strength and toughness to those sintered in air.

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Lead zirconate titanate solid solution (PZT) was synthesized in potassium hydroxide (KOH) solution under hydrothermal conditions. The relationship between the morphology of the products and the concentrations of starting materials and of KOH were investigated. At 150°C, at least 100mol·m^-3 of PbO and 2000mol·m^-3 of KOH was necessary to form cubelike PZT particles. A higher KOH concentration reduced the size of PZT particles. In order to form PZT nuclei, Pb concentration must be increased supersaturation with respect to PZT. The Pb concentration is restricted by the saturation of PbO which depends on KOH concentration. Therefore the number of nuclei was determined by the KOH concentration when undissolved PbO solid existed at the PZT nucleation stage. Undissolved PbO acted as the buffer of the precipitation of PZT, and PZT particles grew by consuming the Pb supplied from the dissolved PbO. The morphology and the size of PZT particles can be controlled by the amount of PbO and the KOH concentration.

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Oxygen-defective LaMnO_3-x with twin structure was studied using an HREM. On the basis of direct observation with a 200-kV HREM and with the aid of computer simulation, an optimum imaging condition was found, where metal La and Mn atoms and oxygen atoms were imaged as dark spots and bright spots, respectively. The through-focus observation and intensity profile observed from the image contrast support the view that the oxygen vacancies could be detected from the intensity distribution of bright spots in the observed image, which represents the positions of the oxygen atoms. A structure image under the optimum imaging condition reflected the twin structure with oxygen vacancies in LaMnO_3-x. Twin structures were found to consist of two kinds of twin planes, (110) and (112), including oxygen vacancies.

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Fracture mechanics and electron microscopy studies were systematically conducted on a dense Si₃N₄ material reinforced with high-aspect-ratio SiC platelets. This model system contained only an amorphous SiO₂ phase at the multigrain pockets and along the internal interfaces. Two different procedures (and their empirical combination) were followed to gradually weaken the grain interfaces and, hence, deliberately change the fraction of intergranular fracture within the material upon crack propagation. These procedures were: 1) the gradual addition of a fluorine impurity, which remains segregated at the grain boundaries after sintering, and 2) the adoption of a “quenching” process under high pressure. Quantitative high-resolution and analytical electron microscopy provided fundamental insight into the internal structure of the intergranular SiO₂ phase doped with fluorine and the corresponding microcracking processes. Quantitative fractography allowed to estimate the apparent cohesive energy of the internal interfaces of the composite and to relate it to the macroscopic material toughness.

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Fine and uniform silicon nitride powders having different α-content were prepared from commercial submicron powders by centrifugal sedimentation. The densified pre-sintered materials were fabricated from these powders by hot-pressing. The fine powders with similar particle size distribution showed the same densification behavior during hot-pressing. The microstructures of pre-sintered materials of the fine powders showed the fine and homogeneous microstructures, and the similar one irrespective of α content. The pre-sintered materials were annealed at 1800°C for 1h under 1MPa nitrogen atmosphere. The annealed material from the fine β powder showed the fine and homogeneous microstructure. On the other hand, the material from the fine α powder developed “in-situ composite” microstructure consisting of large grains and fine matrix grains by annealing. Because, the abnormal grain growth was caused by the α- to β-phase transformation during annealing. While, these results of the fine powders were compared with those of the commercial submicron powders.

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The influence of (Ni, Co) Al₂O₄ spinel precipitates on the grain growth in (Ni, Co) O ceramics with 1.45mol%, Al₂O₃ was investigated in the temperature range 1280 to 1380°C. Smaller precipitates preferentially dissolved and larger precipitates grew with increasing temperature and holding time, therefore, the growth mechanism of spinel precipiates was considered to be Ostwald ripening. The matrix grain size increased linearly as a function of particle size of spinel at 1280°C and 1360°C. Furthermore, the slope increased with increasing temperature, which indicates that the grain growth inhibition effect became weak. It is believed that higher solubility of Al₂O₃ into the matrix at higher temperature decreases the volume fraction of the spinel precipitates inhibited the grain growth of matrix.

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The mechanism of mirror-finish grinding of Si₃N₄ ceramics was studied under different grinding conditions using a diamond grinding wheel with large grain size (#800, #1200). The main results are summarized below. (1) A good mirror-finished surface with smaller surface roughness was obtained under the grinding conditions of high grinding force and large theoretical wheel cutting distance. (2) By an ESCA analysis of the mirror-finished surface, it was found that a surface layer comprised mainly of silicon and oxygen was generated, and that Si₃N₄ ceramics with extensive coverage of the surface layer had a good mirror-finish-ground surface. Judging from the above results, it was concluded that the good mirror-finish-ground surface was a result of the plasticity produced by the grinding heat at the contact zone between abrasive grains and the workpiece.

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Palladium powders were prepared from aqueous solutions of Pd(NO₃)₂, PdCl₂ and Pd(NH₃)₄Cl₂ by ultrasonic spray pyrolysis in nitrogen flow and the effects of starting materials and preparation conditions on the properties of prepared powders were investigated. Submicron powders were obtained from PdCl₂ because it melts at 680°C and disintegrates on evolving gas. Powders prepared from Pd(NH₃)₄Cl₂ often had a bimodal particle size distribution and irregular shapes because the porous particles of Pd formed at about 300°C underwent either sintering or breaking up to pieces at higher temperatures, resulting in a mixture of large and small particles. Spherical solid powders were prepared from Pd(NH₃)₄Cl₂ only when the synthesis condition was carefully controlled within a narrow range. Dense spherical powders were obtained in a wide range of preparation condition by using Pd(NO₃)₂. Pd(NO₃)₂ hydrolyzes in heated droplets and then dissociates to PdO. At higher temperatures Pd particles formed by the dissociation of PdO sinter and densify successively to form single-crystal-like solid particles. Pd powders prepared from Pd(NO₃)₂ and Pd(NH₃)₄Cl₂ oxidize less when they are heated in air.

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Effect of sintering temperature, atmosphere and concentration of Pr oxide on the grain growth and grain boundary formation of Pr-doped ZnO was investigated. Grain size of ZnO was maximized at composition of Pr 0.5mol% and grain growth rate and weight loss rate of 0.5mol% Pr-doped ZnO increased drastically above 1350°C, regardless of the sintering atmosphere. The morphology of grain boundary in samples fired above 1350°C was strongly affected by the sintering atmosphere. The grain growth rate was proportional to 1/n-th power of time, with n=4.5 between 1300°C and 1400°C and n=2.2 at 1500°C. An exothermic peak in DTA was observed at 1380°C, which indicated the formation of a liquid phase.

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Thermal conductivity of sintered silicon nitride has been improved by grain growth of β-Si₃N₄. β-Si₃N₄ containing 0.5mol% Y₂O₃ and 0.5mol% Nd₂O₃ was gas-pressure sintered at 1973 to 2473K and the microstructures and the thermal conductivities were investigated. The materials sintered at temperatures higher than 2173K had a microstructure of “in-situ composite” with smaller β-Si₃N₄ matrix grains and a small amount of elongated β-Si₃N₄ grains. The grain size increased with increasing sintering temperature. Room temperature thermal conductivity increased with increasing sintering temperature; 122W·m^-1·K^-1 was produced by sintering at 2473K-this value was about two times higher than the values reported up to this time. Higher thermal conductivities were established by growth of Si₃N₄ grains and decrease in the amount of two-grain junctions.

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The effect of seeding on microstructure development and mechanical properties of silicon nitride was investigated by the use of two kinds of morphologically regulated β-Si₃N₄ single crystal particles. Silicon nitrides with bimodal grain-size distribution were obtained by seeding. The seeding of an appropriate amount of β-Si₃N₄ particles with an appropriate size allowed the development of optimal microstructures, where large elongated grains grown from the seed particles were highly dispersed. As a result, fracture toughness increased retaining the high strength levels of about 1GPa. On the other hand, specimens with large amount of seed particles presented lower strength owing to the coalescence of large elongated grains. The increase in fracture toughness can be associated with diameter as well as volume fraction of well dispersed large β-Si₃N₄ grains, assuming toughening by the bridging mechanism.

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The influence of spray-dry slurry flocculation on the structure of sintered silicon nitride, especially on the pore structure and size distribution, was investigated as a function of slurry pH. The flocculation of spray-dry slurry considerably affected the character of spray-dried granules and also the microstructure of sintered bodies. Decreasing the slurry pH to a certain value (e.g., 7.9) caused slurry flocculation and reduced granule density as well as the diametral compression strength of granule. The reduction of granule strength contributed to the decrease in the large pore population in sintered bodies and also to the increase in its modulus of rupture (MOR). However, the population of pores with size less than 3μm increased an order of magnitude by decreasing the slurry pH from 10.8 to 7.9. The microscopic observation of spray-dried granule showed flocculated clusters consisting of several particles in a low pH slurry system. The void space in/between the clusters was not removed during forming and sintering and remained in the sintered body possibly because of the hard agglomeration of particles. Numerous remaining micropores in sintered body appeared to be the origin of relatively low strength of sintered bodies (i.e., 1000MPa) which contained no clear fracture origin in the flocculated slurry system.

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Semiconducting ceramics of p-type bismuth telluride by hot pressing the milled powders of (Bi₂Te₃)_0.25(Sb₂Te₃)_0.75 particle size of 1.9μm and 0.29mass% impurity oxygen was fabricated. These ceramics showed a maximum figure of merit of 2.4×10^-3K^-1 at 25°C when the grain size was 5μm because their thermal conductivity apparently became the lowest possibly due to enhanced phonon scattering at grain boundaries. Their thermoelectric properties did not exhibit anisotropy due to their isotropic microstructure with no grain orientation.

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A novel method for preparing ceramic precursors via homogeneous mixing of different metal ions from aqueous solutions is presented. Using sodium polyacrylate-polyethyleneimine interpolymer complex, precursor coprecipitates containing 99-95mol% zirconium and 1-5mol% yttrium ion were prepared. Yttrium-stabilized zirconium oxides synthesized from the precursors were produced by heating at 800°C for 5h. This method was exemplified to be convenient and useful for the preparation of some ceramic precursors.

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Submicron semiconductive epitaxial BaTiO₃ thin film was prepared on (100) MgO substrate. An epitaxially grown BaTiO₃ film prepared by MOCVD was coated with ethanol solution of LaCl₃ and heated under a reducing atmosphere at 800-1200°C. The resistivity was less than 1Ω·cm and the carrier concentration was in the order of 10¹⁸cm^-3. The film was n-type semiconductor and the resistivity increased abruptly with the increase in temperature above 600°C in air because of the decrease of the carrier concentration.

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