Recent developments of superplastic ceramics brought about various possibilities for manufacturing techniques in ceramic industry. Superplastic forming (SPF) and SPF concurrent with diffusion bonding (SPF/DB) of metals have been already accepted and used in aerospace industry. The similar developments can be anticipated in ceramic technology also. This paper will give an overview on fundamentals and characteristics of SPF/DB method, and will make a comparison between techniques for metals and that for ceramics. The diffusion bonding of ZrO₂/Al₂O₃ composites, mullite/ZrO₂ composite, and Al₂O₃/TiN composite is described as examples. The fabrication of functionally gradient material will be demonstrated as an application of SPF/DB for ceramics.

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By a progressive weight percent substitution of CaO for SiO₂, the effect of CaO addition on the crystallization processes of Li₂O-Al₂O₃-SiO₂-TiO₂ glasses was studied. The crystallization rate was inversely proportional to CaO content. The isothermal kinetics of crystallization of β-spodumene from the Li₂O-CaO-Al₂O₃-SiO₂-TiO₂ glasses was investigated using quantitative X-ray diffraction (XRD) method. For samples in which CaO contents are 5, 9 and 13wt%, the activation energies for crystal growth were 249.4kJ/mol, 151.9kJ/mol and 189.1kJ/mol, respectively. The growth morphology parameter m=2 was found, showing rod-like growth.

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Local structure around silicon and nitrogen atoms in Na-Si-O-N oxynitride glasses has been studied by means of ²⁹Si MAS NMR spectroscopy. The NMR spectra have been analyzed on the assumption that the resonances of singly nitrided silicate tetrahedra shift towards less shielded direction by about 15ppm compared with those of non-nitrided ones. It has been estimated that the number of silicon atoms around nitrogen is about 2.5, suggesting that about a half of the total nitrogen atoms may be bonded to two silicon atoms.

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Uni-directionally oriented 20vol%-SiC-whisker/Al₂O₃ composite (UD-comp) was fabricated by the sol-gel method from Al(OC₃H₇)₃ and by its extrusion. Mechanical properties of UD-comp were compared with those of 2-dimensional randomly oriented 20vol%-SiC-whisker/Al₂O₃ composite (2DR-comp) made by conventional hot-press sintering. Fracture toughness K_IC value of UD-comp was measured to be 7.1MPa⋅m^1/2 which was larger than that of 2DR-comp (5.6MPa⋅m^1/2). Improvement in fracture toughness by 2.3 times was realized over the fracture toughness K_IC 3.1MPa⋅m^1/2 of monolithic Al₂O₃ fabricated by the same sol-gel method. The same tendency was observed in effective fracture energy. These results indicate that whiskers aligned vertically to a crack were more effective than inclined whiskers for toughening by whisker bridging and pull-out. No increment in bending strength was obtained in UD-comp and its value was smaller than that of 2DR-comp. This result is attributable to the aligned direction of residual thermal stress.

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Fracture strength of silicon nitride was studied under biaxial stress states. Multiaxial fracture distribution functions by several fracture criteria were employed to analyze tension/torsion, compression/torsion test. The most appropriate criterion was Shetty's empirical equation (parameter c=1.4) and the modefied G criterion (parameter β=0.9). The fracture strength of a disk for a spin test was estimated by the equivalent normal stress of Shetty's empirical equation and the modified G criterion. Good agreement between the experimental data and the estimated values were obtained.

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To clarify the effect of plasma spraying on cyclic fatigue crack growth behavior, the single edge notched (SEN) specimen of SUS 316 stainless steel with plasma-sprayed alumina deposit on one side has been fatigued. Fatigue tests were conducted in push-pull loading with a servo electro hydraulic fatigue testing machine at a stress ratio of R=-1 and a frequency of 30Hz. Microscopic examination of the front rim contour of growing cracks demonstrated that at the stress amplitude of 46MPa fatigue cracks fairly delayed on the alumina plasma-sprayed surface side as compared with those on the non-sprayed one. However, the plasma spraying had no effect on crack growth behavior at 130MPa. It was concluded that the retardation of crack growth on the sprayed surface side was closely connected with the grit blasting which was carried out prior to plasma spraying and depended on cyclic stress amplitudes. Fatigue cracks in the alumina deposit were observed to develop linking those microcracks which were formed in grains and on grain boundaries near the tip of a main crack under cyclic loading. Fracture surface of the alumina deposit was characterized by the mixed brittle mode of transgranular and intergranular fracture. In addition, microstructure of the alumina deposit was examined by X-ray and transmission electron microscope techniques. It was found that the plasma-sprayed alumina deposit was composed of the η-phase which often contained small α-phase grains and a number of pores were included in some grains. These pores have been mostly observed with microcracks which were formed around a main crack.

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The anisotropies of the mechanical properties of SiC whisker (SiC (w))/Si₃N₄ composite ceramics prepared by hot pressing under 15MPa at 2073K for 7.2ks were investigated by using three kinds of specimens (VV, PV, PP), in which the tensile surface and longitudinal axis are vertical (V) or parallel (P) to the hot press direction. The results were compared with those of hot-pressed Si₃N₄ monolith (M) and SiC (p)/Si₃N₄ composite ceramics (P). The results obtained were as follows; (1) Both the flexural strength (FS) and fracture toughness (K_IC) of the composites were nearly the same for VV and PV specimens, and the properties of PP specimens were fairly lower than those of VV or PV specimens. The higher the aspect ratio of SiC (w), the larger the anisotropy of K_IC. The anisotropies of K_IC of the composites were larger than those of M and P ceramics. For both composites and monolith, the anisotropy of hardness was very small in comparison with those of FS and K_IC. (2) The anisotropy of mechanical properties of the SiC (w)/Si₃N₄ composites was ascribed to the orientation anisotropies of acicular β-Si₃N₄ grains as well as of SiC (w). (3) The anisotropy of FS was larger than that of K_IC, which was attributed to the larger size of the fracture source of PP specimens.

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A skin layer which was generated near the surface of hot-pressed silicon nitride ceramics was studied from microscopic viewpoint. The ceramics was composed of acicular β-Si₃N₄ grains and glassy grain boundary which was liquid at hot-pressing temperature. The thickness of skin layer increased with an increase in the amount of sintering additive up to 8wt%. The amount and viscosity of the liquid phase and the applied pressure in the hot-pressing influenced strongly the microstructure development of the silicon nitride ceramics. These factors affected the α→β phase transformation and the formation of acicular β-Si₃N₄ grains. A compositional difference of glassy phases was observed between the skin layer and the bulk of Si₃N₄ ceramics. A mechanism of the generation of skin layer was discussed on the basis of experimental results.

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ZnO was sintered successfully by microwave. A ZnO powder compact was placed in a quartz tube and exposed to 2.45GHz microwave (0.1-0.8kW) for 30min. Response of ZnO was examined by measuring the surface temperature of specimens. When the power input was sufficient, the specimens were heated to 800°-1000°C within a few minutes. The temperature was nearly constant during the exposure, and did not exceed 1000°C. In this case, the specimens were sintered to almost theoretical densities. If the power input was not sufficient, however, the specimens were not heated at all and no shrinkage was observed. The response of ZnO was quicker for larger specimens. The densities and the microstructures of ZnO sintered by microwave and conventional furnace were compared.

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Effects of microstructure on fracture toughness were investigated. Using the equivalent inclusion method, the change in Gibbs' free energy was calculated for a relatively large body containing a main crack and many spherical (or disk-shaped) inclusions with a Young's modulus (or thermal expansion coefficient) different from that of the matrix. Applying the Griffith's fracture criteria to the model, the effects of inclusions on fracture toughness were formulated. The fracture toughness increased with the increase in volume fraction of inclusions with a larger Young's modulus (or thermal expansion coefficient) than that of the matrix. The fracture toughness decreased with the increase in volume fraction of inclusions with a smaller Young's modulus (or thermal expansion coefficient) than that of the matrix. The difference in thermal expansion coefficient influenced the fracture toughness more strongly than that in Young's modulus. The present analysis explains the tendencies of the experimental results reported previously.

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Preparation of HfN fine powders from mixtures of HfO₂ and Mg by the thermite method has been investigated by three different methods; (1) direct synthesis in a N₂ gas flow, (2) reduction of HfO₂ and subsequent nitridation of α-Hf, and (3) direct synthesis in a mixed N₂+Ar gas flow. The HfN fine powders of single phase were synthesized by the methods (2) and (3). In the former case, the powder mixture of Mg/HfO₂=6 or 7 (molar ratio) was heated at 1000°C for 1h in N₂ after heating at 800°C for 1h in Ar. In the latter case, the same powder mixture was heated at 1000°C for 1h in a N₂-Ar flow containing 75 to 90vol% nitrogen. On the other hand, the preparation of HfN fine powders of single phase by the method (1) was very difficult, because nitridation of Mg proceeded preferentially in the reduction-nitridation process. The particle size of HfN with the composition of HfN_0.92 prepared at 1000°C for 1h was 50-300nm.

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This work was done in order to prepare (La_1-xSr_x)₂MnO₄ (0.35≤x≤0.65) with K₂NiF₄ structure and to investigate their electrical properties. The La-rich (La_1-xSr_x)₂MnO₄ has been successfully prepared by a solid-state reaction method in vacuum. The lattice parameters of (La_0.5Sr_0.5)₂MnO₄ were the same as those of La₂CuO₄. This indicates that the Mn-O bond shrank in the a-b plane and the lattice expanded along the c-axis in the K₂NiF₄ structure by the co-operative Jahn-Teller effect. However, the lattice parameters increased along the a-axis and decreased along the c-axis as x deviates from x=0.5. This tendency differs from that of the (La_1-xSr_x)₂CuO₄ superconductor. The temperature dependence of electrical resistivity of (La_1-xSr_x)₂MnO₄ obeyed the Arrhenius law. The result showed that (La_1-xSr_x)₂MnO₄ (0.35≤x≤0.65) is a semiconductor.

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Diffusion of PVA through pores of an Al₂O₃ compact was measured by means of “diffusion from a stirred solution to a spherical compact powder”. The porosity and the diameter of the compact, and the volume of PVA aqueous solution did not markedly affect the measured value of diffusion coefficient within the region of the present study, thereby the diffusion coefficient of PVA in this system can be measured reproducibly by present method. The diffusion coefficient of PVA through pores in an Al₂O₃ compact was much smaller than that in a PVA aqueous solution. In the calculation of the diffusion coefficient, not only modification of the equilibrium PVA concentration in the sphere, but also modification of the pores through which PVA diffuses must be taken into account. Effect of adsorption of PVA on the apparent diffusion coefficient was discussed.

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Dynamic viscoelastic properties of aqueous alumina suspensions containing acrylic resin were studied in the drying process in tape casting. Suspensions with pH7.5 and pH10.4 were used. The solid content of suspensions was changed by vacuum drying or adding water. In a suspension with pH7.5 the storage modulus, tanδ, and viscosity increased drastically at the solid content of 80.5wt%. Below the critical solid content, the yield stress of suspension was nearly zero, and the suspension showed Newtonian flow behavior. On the other hand, in a suspension with pH10.4, the storage modulus and viscosity increased gradually at the smaller critical solid content of 76.0wt%. Below the critical solid content, the suspension had a high yield stress and showed shear-thinning flow behavior. Green sheets from a suspension with pH10.4 showed a low packing density. It was also found that the storage modulus of suspension above the critical solid content was a linear function of the strength of the resultant green sheet.

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To study the effect of compaction pressure on the internal structure of sintered alumina, alumina green bodies were formed at 20-300MPa and sintered at 1350°C for 1h. The internal structure was examined with an optical microscope of transmission mode on thinned specimens. Isolated pores were found to be nonuniformly distributed in sintered bodies forming a circular array of pores. Their origin was ascribed to the crack-like pores in green bodies. The pores reduced the fracture strength of sintered bodies.

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Syntheses of high-thermal-expansion ceramics from NaAlSiO₄ were investigated. NaAlSiO₄ has two crystal polymorphic forms; nepheline and carnegieite. Mean axial thermal expansion coefficients of nepheline and carnegieite were 16.9×10^-6/°C (from room temperature to 800°C) and 15.2×10^-6/°C (from room temperature to 600°C), respectively. High density ceramics of nepheline and carnegieite were fabricated from a carnegieite powder calcined from 1300°to 1350°C followed by sintering at 1250°C and at 1300°C, respectively. They had the bending strength of ca. 100MPa and linear thermal expansion coefficients of 16.4×10^-6/°C and 15.0×10^-6/°C, respectively.

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Coarse β-silicon nitride powder of low purity, which was used as a refractory material, was tested as a raw powder for the usage of engineering structural materials. Y₂O₃-Nd₂O₃-doped β-Si₃N₄ was sintered at temperature from 1700°to 1900°C for 4h under 10MPa N₂, and the effects of impurities and grain size on the strength, fracture toughness and microstructure were investigated. A dense material was obtained by firing at 1900°C with addition of 2 to 4mol% Y₂O₃-Nd₂O₃. The material containing 2mol% oxides fired at 1900°C had a strength of 600MPa and a fracture toughness of 5.3MPa√m. Iron impurity decreased the strength and Weibull modulus. Coarse particles in raw powder suppressed the grain growth, so that the microstructure of self-reinforced composite, which contains in-situ developed elongated rod-like grains, were not obtained.

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Powders of a superconductor oxide, Bi₂Sr₂CaCu₂O_y (BSCC, 2212), were packed in a copper tube and hotpressed in the temperature range from 500°to 800°C for 1h under 200kg/cm² aiming at fabrication of superconducting wire using copper sheath. The reactions between the copper and the BSCC were investigated. The reaction began above 600°C, and a red-brown reaction layer of 70μm was observed at 800°C. Powder mixtures of the oxide and copper were hot-pressed. Results of EPMA and XRD of the reaction layer showed that the 2212 phase decomposed to a 2201 phase and CuO and then to Cu₂O accompanied by the oxidation of the copper tube. For prevention of such reactions, electroplating of gold or silver was applied to the inner face of the copper tube. The reactions were suppressed by electroplating 20μm of gold or 40μm of silver. But a semiconductive temperature change of the electrical resistivity was seen for 60μm of Ag. Finally, in the results of hot-pressing at 700°C for 1h flowing oxygen into the tube at 1.5kg/cm² using 20μm Au or 40μm Ag on copper, zero resistance was not shown for Au-electroplating, but it was obtained at 22K for the Ag-plating. Thus, a possibility of fabrication of copper sheath wire with silver-plating holding oxygen pressure in the copper tube during an extrusion or a hot-pressing process was confirmed.

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Carrier functions of nine kinds of oxide ceramics for immobilization of enzymes have been examined. Mullite ceramics were the most suitable material for the immobilization of enzyme and stabilization of immobilized enzyme activity. Control of the pore structure and SiO₂/Al₂O₃ ratio of mullite ceramics was carried out for improving the efficiency of carrier for immobilization of enzymes. Immobilized enzyme activity was highest for the stoichiometric mullite sintered at 1400°C, which was formed by adding liquid paraffin to the mixed SiO₂/Al₂O₃ powder prepared from colloidal sol.

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Powders of garnet-type Gd₃Al₅O₁₂, which have not been prepared, were obtained by an amorphous citrate process. A precursor gel was prepared by dehydration of sol which was obtained by heating a mixture of gadolinium nitrate and aluminum nitrate gently with citric acid. The decomposition process of the precursor was analyzed using TG-DTA and FT-IR. The precursor gel was heated at 1000°C for 30min in air to remove organic compounds, then heated at 1100° to 1600°C for 2h in air. Single phased garnet-type Gd₃Al₅O₁₂ powders were obtained by heating only at 1400°C for 2h under the experimental conditions examined. Perovskitetype GdAlO₃ was obtained by heating below 1400°C. At 1600°C, the garnet decomposed into GdAlO₃ and Al₂O₃. The crystallite size and the lattice constant of the Gd₃Al₅O₁₂ single-phase powder were 30nm and 1.2107nm, respectively.

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