The recent experimental results and current knowledge on the positron annihilation studies for a-Si and a-Si: H are reviewed. Positron annihilation measurements have been proved as useful and powerful methods to investigate vacancy-type defects and microvoids in solids, even for amorphous semiconductors. Randomness of the structure and hydrogenation for Si increase the size and the concentration of microvoids. In particular, the quadrivacancy-like microvoids can be seen in a-Si and the formation of positronium has been observed in sputtered a-Si: H. For a-Si: H prepared at high deposition rate, the substrate temperature is one of the important factors on the concentration of microvoids. The boron doping, however, scarcely affect the concentration of microvoids and spin density. Furthermore, the annealing temperature dependence of line shape parameters are different from undoped a-Si. These influences of boron doping can be attributed to the elimination of hydrogens from the structure.

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The Zr(O-nC₃H₇)₄ and Ca(CH₃COO)₂⋅H₂OH₂O-C₂H₅OH-HCl solutions were prepared and heated at 80°C for hydrolysis-condensation reaction of the alkoxide. The spinnability of the solutions was examined in the course of reaction. Following results have been obtained.
(1) The Zr(O-nC₃H₇)₄ solutions with the H₂O/alkoxide molar ratio ranging from 0.7 to 1.5 and C₂H₅OH/alkoxide molar ratio larger than 4 exhibited spinnability within 2 to 9 hours. The ZrO₂ gel fibers were successfully drawn.
The CaO-ZrO₂ gel fibers could also be drawn from the corresponding alkoxide-acetate solution with H₂O/alkoxide molar ratio of about 1 and C₂H₅OH/alkoxide ratio of 27.
(2) The solution composition region for spinnability is considerably limited, compared to the cases of making SiO₂ and TiO₂ gel fibers from Si(OC₂H₅)₄ and Ti(OC₃H₇)₄ solutions.
(3) Transformation of the tetragonal ZrO₂ to monoclinic ZrO₂ occurring in ZrO₂ fibers above 600°C caused the fracture of the fibers on heating. The addition of CaO to fibrous ZrO₂ increased the stability of precipitating tetragonal ZrO₂ and resulted in the avoidance of fracture of fibers by heating.
(4) It is considered that the linear type zirconoxane polymers consisting of ZrO₆ octahedra connected with each other by sharing their edges are formed in the spinnable Zr(O-nC₃H₇)₄ solutions.

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A unique porous material can be prepared by carbonizing montmorillonite/acriflavine complex (MAC). Control of its porous structure was attempted by freeze-drying MAC blocks having various water content. The porous structure was changed over a wide range (pore volume: 1-3ml/g, pore radius: 40-240nm) by water content in MAC. Particle size of raw montmorillonite also caused certain effects. When pore volume increased, pore size also increased, i.e., both properties could not be controlled independently. In spite of such changes, uniform pore size distribution and characteristic card-house structure were kept. The porous structure was stable up to 1073K under nitrogen.

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Surface modified alumina membranes were examined in terms of stability in water and filtration characteristics to apply the membranes for membrane bioreactors. Surface modifications were carried out by the treatment of octadecyldimethylchlorosilane (ODS), γ-aminopropyltriethoxysilane (γ-APS) and 1, 3-propane sultone (PS) to control the zeta potential and surface free energy. The introduced functional groups from the treatment of silane coupling agents such as ODS and γ-APS were stable in water. While sulfo groups from PS were hydrolyzed and reduced to 1/3 under reflux for 20 hours, it were stable enough at room temperature for 8 hours enough to operate a short time filtration examinations. The relationship between zeta potential and restration of membrane permeability by backflush was predicted from DLVO theory. Low zeta potential for membrane predicts better permeation characteristics. It is confirmed that surface modification to control the zeta potential for negative improves the clogging characteristics of membrane.

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A special attention has been paid to the low pressure plasma-spraying (LPPS) to obtain dense and thick ceramic coatings on metal components. In this study, the effect of LPPS on mechanical properties of Al₂O₃ coatings was compared with that of atmospheric plasma-spraying (APPS). The LPPS method improved the density, Vickers hardness, adhesive strength and wear resistance remarkably. When the spraying distance was 300mm, optimum conditions of LPPS for dense coatings were 500-600A and 60-65V.

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The effect of reducing gas mixture, H₂-N₂ or CO-N₂, on the cubic-hexagonal transition in BaTiO₃ has been investigated by X-ray diffraction, ESR, ESCA, PAS and chemical analysis. The firing temperatures of BaTiO₃ were 1380°C and 1500°C, and the transition temperature in air is already known to be 1460°C. The fractional conversion to the hexagonal phase increased with increasing concentrations of reducing gases, larger for samples fired in H₂-N₂ than those fired in CO-N₂ for the same concentrations of the reducing gases. The hexagonal phase in all samples increased proportionally to the amount of Ti³⁺ produced by reduction of Ti⁴⁺ ions, regardless of the reducing gas species and firing temperatures. It was found that the minimum amount of Ti³⁺ ions to stabilize the hexagonal BaTiO₃ at room temperature was 0.3% of total Ti ions. ESCA spectra due to Ti³⁺ were observed in the sample fired in 100% H₂ at 1380°C.

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The density of surface silanol groups in three kinds of porous glasses with different pore structures was determined by using CH₃Mgl, (CH₃)₃SiCl(TMS), (CH₃)₂Cl₂(DMS) and CH₃OH. The density ranged from 2.3 to 7.7μmol/m². The density determined by TMS was about 3μmol/m² regardless of the pore structures. The surface of porous glasses was boron-rich after exposure to air for a long period. But, washing with acid solution removed boron, and the surface contained only silicon. The number of silanol group of single type was dominant in all types of silanol groups and occupied 50-80% regardless of the pore structures. Geminal type comes second. But, when pore was small, the number of geminal type was almost the same as that of paired type.

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Diamond was synthesized by using the Fe-Ni-Cu alloy. As compared with the diamond-formation region obtained with the Fe-Ni alloy, the region shifted to the lower temperature and pressure side. This shift has been explained in terms of the change of melting temperature due to an addition to Fe-Ni alloy, because Cu itself has no catalytic effect on the transformation of graphite to diamond. However, the excess of Cu content in the alloy seemed to suppress the transformation. EDX analysis revealed that the chemical composition of inclusion in diamond crystals was similar to that of catalyst. Judging from the morphology of diamond, the formation was governed substantially by the dissociation-precipitation process of carbon in the melted alloy. The mechanism proposed here coincided with that of the catalytic reaction of Ni to form diamond.

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Preparation of AlN powders by vapor phase reaction of Al(i-Bu)₃-NH₃ system was studied by using a flow method. Mixing temperature of Al(i-Bu)₃-H₂ and NH₃ streams was 200°C and reaction temperatures were 600°-1400°C. AlN powders produced were not crystalline at low temperatures, but became slightly crystalline at high temperature. Particle size of AlN powders decreased with an increase of reaction temperature (600°C: 0.034μm, 110°C: 0.025μm, 1400°C: 0.017μm). The carbon content in AlN powders was minimum at around 1200°C. Heat treatment of as-synthesized AlN powders in H₂-NH₃ (10:1 in mol) at 1400°C for 2h decreased impurity oxygen and carbon from 18.8wt% to 4.5wt% and from 1.2wt% to 0.6wt% respectively. Crystallization of as-synthesized AlN powders by heat-treatment in the same atmosphere was appreciable above 1200°C, and heat treatment at 1400°C gave well crystallized AlN powders. Heat-treated AlN powders having the average particle size of 0.03μm densified to 90% theoretical without sintering aid by firing at 2000°C for 1h under N₂ of 10kg/cm².

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Si₃N₄ composite ceramics containing up to 50wt% AlN were fabricated by hot pressing using MgO as a sintering aid. Their mechanical and tribological properties were measured. Almost full density composites were obtained up to 10wt% AlN addition, but more addition increased porosity in composites. The fracture toughness decreased rapidly with increasing AlN content. The Vickers hardness of the composites up to 20wt% AlN content was as high as that of the Si₃N₄ ceramics without addition, but decreased appreciably with increasing AlN content over 20wt%. AlN addition hindered the α→β transformation of Si₃N₄, and AlN itself was transformed into the pseudo-polytype 16 H. The coefficient of friction decreased slightly with increasing AlN content. The specific wear rate decreased about 30% for SiC disk and 50% for Si₃N₄ disk, with 10wt% or more AlN addition. It is concluded that AlN addition was effective to improve the wear resistance of Si₃N₄ ceramics.

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Various hydrosols of rutile and anatase particles were prepared by aging or autoclaving of amorphous titanium hydroxide precipitated from solutions. Crystalline gel films were formed by drying the sols on a glass substrate and separated from the substrate for sintering. The particles in the films, both rutile and anatase, have a tendency to orientate with the tetragonal caxis parallel to the film plane even before sintering. The degree of orientation increased with grain growth during firing. Rutile polycrystalline thin films, about 10μm in thickness, were obtained by sintering at 1300°C.

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The sinterability of SiO₂-Al₂O₃ powders containing 20, 30, 40wt% Al₂O₃, synthesized by hydrolysis of metal alkoxide, was examined. The relationship between the chemical composition and the properties, such as thermal, electrical and mechanical properties, of the sintered bodies has been studied. These powders densified to almost full density at 1300°-1350°C for 4h without forming cristobalite. When heated above 1350°C, however, crystallization of cristobalite was recognized which resulted in the increase in thermal expansion of the sintered body due to α-β phase transformation of cristobalite. The densification rate increased with increasing SiO₂ content in the powder. The sintered bodies consisted of very fine acicular mullite particles and glassy matrix. The aspect ratio of the mullite particle increased with the decrease in Al₂O₃ content. The dielectric constants, thermal expansion coefficients and flexural strength of the sintered bodies containing no cristobalite phase were 4.3-5.3 (at 1MHz), 1.8-2.9×10^-6/C (30°-400°C) and 124-201MPa, respectively. These values seem to be more advantageous in application for the insulating substrate material than those of conventional alumina ceramics. The above mentioned values increased with the increase in Al₂O₃ content which determines the amount of mullite phase in the sintered body. Consequently, the optimum property values can be designed by adjusting Al₂O₃/SiO₂ ratio in the starting powder.

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Internal Friction of hot-pressed, pressureless sintered and HIPped silicon nitride was investigated at elevated temperatures by the resonance method. The temperatures at which internal friction increased abruptly corresponded to the temperatures at which Young's modulus and flexural strength decreased. The temperature dependence of internal friction in hot pressed silicon nitride shows a maximum at 1080°C, which disappeared by oxidation at 1080° and 1160°C. The decrease in internal friction was considered to decrease the creep deformation. The internal friction of HIPped silicon nitride increased exponentially with increasing temperature. This increase was considered as the background increase of the internal friction in every silicon nitride.

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A monoclinic phase is frequently seen on fractured surfaces of yttria stabilized tetragonal ZrO₂ polycrystals (Y-TZP). In this study, Y-TZP materials were ground by diamond grinding wheels of different grain sizes and the effects of grinding on the structure in the surface layers was examined from the viewpoint of the grinding mechanism. It was found that a compressive layer was formed on the ground surface and bending strength was increased by grinding under specific conditions.

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The authors developed a new powder forming technique, called as “Cyclic Cold Isostatic Pressing (C-CIP)”, by superimposing cyclic pressure wave onto a static hydraulic pressure. Using this method, we performed dry powder forming on two kinds of alumina powders which have different compaction characteristics, and measured the bulk density and the hardness of the green compacts. The green density increased remarkably as the number of repeating times of pressure increased. The rate of increasing density against the repeating times became larger as the maximum pressure became higher. The micro- Vickers hardness for slightly calcined body, which is closely connected with the green compacts, increased remarkably as the repeating times increased. Employing our new method, we can make the CIP pressure remarkably low in comparison with the ordinary CIP.

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