The crystallization of high purity amorphous polymer-derived Si₃N₄ fiber, which was heat-treated with and without BN powders, was investigated. In the case without BN powder, the crystallization proceeds by vapor phase growth mechanism through SiO gas. The activation energy of 602kJ/mol was calculated. In the case with BN powder, the crystalline growth and crystallization through vapor phase was fairly suppressed. The crystallization proceeds by solid phase growth mechanism. The activation energy of 1273kJ/mol was calculated.
Durability of Si₃N₄ fiber in a ceramic matrix composite was estimated. Si₃N₄ fiber may remain amorphous at 1300°C for 5×10⁴h and at 1400°C for 6×10h, respectively.

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Dissolution of dental zinc phosphate cement was investigated in acetic, lactic, oxalic and citric acid solutions with different degrees of neutralization (α). The dissolution process was discussed based on the ion equilibrium in acid solution. Dissolution was controlled by H⁺ ion concentration and/or complexing ability of the acid anions. The release of Zn, Mg and P decreased with increasing α except for oxalic acid. In acid solutions with low complexing ability such as acetic acid, the unreacted ZnO and/or MgO particles dissolved preferentially by the attack of H⁺ ion and most of the dissolved Zn or Mg existed as hydrated ions. While in acids with high complexing ability such as citric acid and oxalic acid, both of the phosphate matrix and the unreacted ZnO particles simultaneously dissolved at lower degrees of neutralization. Zn and Mg dissolved to form stable complexes with those acid anions. At high degrees of neutralization, a driving force for the dissolution was complexing ability of acids. The extent of the cement matrix erosion increased, with increasing tendency of the stable complex formation. Hopeite, Zn₃(PO₄)₂⋅4H₂O crystallized on the surface of the cement immersed in acetic acid with α from 0 to 0.9 and zinc oxalate monohydrate, Zn(COO)₂⋅H₂O did in oxalic acid with α from 0 to 0.6.

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The redox behavior of chromium ions in alkali and mixed alkali borate melts was studied by cyclic and differential pulse voltammetries. Half wave potentials for reduction of Cr⁶⁺ to Cr³⁺ were determined in each melt. The half wave potentials shifted progressively to the negative side when alkali ions changed from lithium to sodium, potassium, rubidium and cesium in accordance with the order of the basicity of the melts. The half wave potential of the mixed lithium and cesium melts was more negative than that expected from the mean value of the corresponding potential values of lithium borate and cesium borate melts. This is due to the formation of excess non-bridging oxygens, as suggested by NMR investigation of the mixed alkali glasses.

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The dc conductivities of Bi₂O₃-SrO-V₂O₅ glasses prepared by cast quenching, together with that of glass-ceramics were measured and the conduction mechanism has been discussed. It was found that the glass-forming region is in the range Bi₂O₃ 0-9, SrO 9-50, and V₂O₅ 49-87mol% and the glass-transition temperature, T_g, is in the range 260-360°C. The conductivities of the glasses were ranged from 2.66-8.66×10^-7S⋅cm^-1 at room temperature and the conduction was attributed to small-polaron hopping at temperatures higher than 340K and was changed to variable-range hopping at temperatures between 340 and 170K. The conductivities of the glass-ceramics are about 10³-10⁴ times higher than those of the glasses at 300K. The conduction of the glass-ceramics also showed small-polaron hopping at temperatures higher than 170K, but a mixed conduction of small-polaron and variable-range hoppings at temperatures lower than 170K. The disorder energy, W_D, estimated from Mott's parameter analysis in variable-range hopping for the various glasses, was evaluated to be 0.15-0.22eV, which was larger than that for the glass-ceramics (0.12-0.15eV).

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Different structural porous ceramic filters were investigated by the nitrogen gas permeability through the filters which were immersed in ethanol bath. The proposed method can discriminate between symmetric and asymmetric pore structure from the nitrogen gas permeability, and further the pore size distributions can be calculated from the relationship between volumetric flow rate and pressure. The permeability of the symmetric filter can be described on the basis of the Hagen-Poiseuille equation and the calculated result of pore size distribution was in accordance with the data from the measurement using mercury penetration porosimetry. Also the pore size distribution ranges in the limit of pore size corresponding to the pressure in which the total permeable area increases remarkably. In the case of the asymmetric filter, a discontinuous point was shown in the relationship between exhaust pressure and volumetric velocity. It was shown that this discontinuity was due to the asymmetric pore structures. The asymmetric permeability was also described on the basis of the Hagen-Poiseuille equation. It was found from the comparison of the results between the gas and mercury penetration methods that the gas penetration method could be applied to the measurement of the pore size distribution of the skin part in the filter which indicated asymmetry.

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Sector-like crystals of magnesium hydroxide sulfate hydrate (trade name MOS·HIGE) were grown in a hydrothermal reaction at 180°C using MgSO₄ and NaOH aqueous solutions. Their morphology and crystal structure were investigated by X-ray diffraction, SEM and TEM. Typical morphology of the crystals was sector-like bundles, each of them grows three-dimensionally, not in a plane, in the opposite direction from its center. The element whiskers of the sector-like crystal were always parallel to the b-axis. In the initial stage of growth, a single crystallite composed of small whiskers growing along the b-axis nucleates in Mg(OH)₂ slurry. During the growth of long whiskers from the crystallite, their growth direction slightly inclines with one another because of the lattice mismatching between two whiskers caused by impurities such as Na, Ca, Si and so forth on the boundary of an element whisker. Sector-like crystals were examined by the X-ray diffraction method at various high temperatures. It is found that they decompose into MgSO₄ and MgO between 700°C and 1000°C. The diameter dependence of the tensile strength of the crystals was measured mainly at room temperature.

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Rotary bending fatigue and static fatigue tests were carried out for silicon nitride and silicon carbide ceramics at room temperature, with determining the fracture toughness values by SEVNB and SEPB methods. Two of the silicon nitride ceramics exhibit the great difference between the toughness values obtained from two methods, suggesting that there is a strong traction force in the propagating crack interface. From the observation of crack extension path, mechanisms of grain bridging and crack surface interlocking are concluded to be responsible for the strong traction. Materials, having a much greater SEPB toughness than that of SEVNB, exhibited considerable degradation in fatigue strength due to cyclic loads during rotary bending tests. This suggests that the damage in the crack surface causes degradation in fatigue strength. Analysis, assuming that cyclic loads affect the R-curve behavior, was consistent with the measured rotary bending fatigue behavior.

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The thermal decomposition processes of natural and synthesized gyrolite, which have a sheet silicate anion structure with Ca/Si molar ratio of 0.66, were compared mainly by ²⁹Si MAS NMR. In the two samples, the silicate anions has the same structure (protonated Q³), however their crystallinity was different. The protonated Q³ structure in both samples decomposed gradually below 800°C to form nonprotonated monomer (Q⁰) and chain silicate (Q¹ and Q²). Above 900°C, the natural gyrolite decomposed to pseudo-wollastonite and silica glass, while the synthesized one changed to pseudo-wollastonite and cristobalite via wollastonite. These differences are believed to be caused by the differences in their crystallinity, decomposition rate and the flux effect resulting from the glassification of the impurities.

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Reactivities of PAN and Pitch carbon fibers in hot-pressed carbon fiber reinforced SiC composite have been investigated. In the case of both fiber types a diffusion of Si into the center of the fibers which took place during the sintering process was observed. However, the concentration of Si in the Pitch fibers was lower than in PAN fibers, which changed shape during hot-pressing process of the SiC-C fiber composite. Cracks propagated along fiber/matrix interfaces in SiC-Pitch carbon fiber composite, and fibers were pulled out at the fracture surface. In the case of PAN fiber reinforced composites, cracks propagated across the fibers. The results indicated that Pitch fibers seem to be much more suitable for SiC-C composite compared to PAN fibers.

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Mullite crystallization behavior was studied using two kinds of gels obtained from the same compositional solutions from which mullite precursor fiber is spinnable. The gels were obtained by the following two different procedures: (1) Al(OPrⁱ)₃ and TEOS were simultaneously added to Al(NO₃)₃ containing aqueous solution, and (2) Al(OPrⁱ)₃ was added to Al(NO₃)₃ containing aqueous solution, then TEOS was added to these solutions. XRD experiments for both gels after heating at 1000°C showed that Al and Si mixing is away from the ideal homogeneity. The gel prepared by the latter procedure had less complete molecular-scale homogeneity of Al and Si mixing than that by the former procedure. Mullite crystal was detected after heating at 600°C in the former and at 800°C in the latter. These mullite particles were presumed to act as nuclei and to increase the crystallization rate at isothermal treatment. The difference of mullite crystallization temperature were interpreted from the differences of changes in radii of gyration for the concentration process of these solutions. The differences of mullite nucleation and growth from the gel specimen obtained by the different processes were attributed to these different solution structures, even though the raw materials had the same compositions.

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The mechanical properties of the bone china were evaluated in relation to the method of the glazing to the different body states and the different glaze compositions. The glazing was conducted by glazing to the green body, the biscuit body fired at 800°C, and the sintered body fired at 1240°C. The glaze compositions were raw glaze, fritted lead glaze, and fritted leadless glaze. The mechanical strength of the bone china was enhanced by the glazing to the sintered body with the fritted glaze. The surface roughness and the mechanical properties of the bone china were not dependent on the compositions, in lead glaze or leadless glaze. The mechanical properties were considered to be dependent on the growth of the interface layer by reacting at the glaze-body contact.

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The properties of MgO-C bricks containing 0 to 5% aluminum powder were investigated after heat-treatment at 1400°C under different heating conditions. When Al₄C₃, the reaction product of Al and C, reacts with CO and MgO to form MgAl₂O₄ and C, a weight increase of MgO-C brick containing Al should be obtained. However, the weight increase decreased with increasing heating rate and Al content. It was considered that Mg(g) diffused to the surface of brick and reacted with CO to form MgO, from the fact that the carbon powder contained MgO after heat-treatment. Mg(g) was generated by the reaction of MgO and Al or C in the brick. The decrease in weight increase was controlled by slow heating. The mechanical properties were almost the same regardless of the difference of weight increase between fast and slow heatings.

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The pyrolysis of two types of amorphous Si-Ti-C-O fibers (Tyranno T-1 (18%O) and T-2 (13%O) coated with oxide layers of various thicknesses was investigated in an argon stream at 1673K. The oxide layers thicknesses of 0.12, 0.21, 0.42 and 0.65μm were formed on the surface of the fibers by the heat treatment in an oxygen stream at 1673K. The fibers with oxide layers of various thicknesses were heated rapidly at 1673K and kept at this temperature in an argon stream continuously monitoring the weight change with a thermoba-lance. After cooling the crystal structure and morphology of the fibers were examined with X-ray diffraction and SEM observation, respectively. The pyrolysis of all T-2 fibers with oxide layers of the above thicknesses was suppressed significantly. On the other hand, the T-1 fiber with an oxide layer 0.12μm thick pyrolyzed completely, and growth and coalescence of β-SiC crystal were observed. The pyrolysis of the T-1 fiber with oxide layers of 0.21, 0.42 and 0.65μm thick was suppressed significantly, and this pyrolytic behavior was similar to that of the T-2 fiber. The oxide layer of fibers was removed by chemical etching treatment, then the microstructure of the etched fibers was observed with TEM and their tensile strength was measured at room temperature. As the thickness of oxide layer decreased, crystallization of β-SiC in the fiber was inhibited and the tensile strength of the fiber increased. At the same thickness of oxide layer, the tensile strength of the T-2 fiber was larger than that of the T-1 fiber.

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Silicon nitride whiskers have been prepared by reduction and nitridation of natural resources, occurred in Oita prefecture, such as Asono diatomaceous earth, Beppu Terra abla, Youra quartzite and casting sand, at 1370°C in an NH₃-N₂ stream on a carbon plate. Two types of Si₃N₄ whiskers, short needlelike whisker (inside whisker) and woollike whisker (outside whisker) were obtained from the inexpensive raw materials. The shape of these whiskers, however, varied with the intrinsic composition of the starting materials. The outside whiskers obtained from the casting sand were almost all composed of Si₃N₄ with a few whiskers composed mainly of Al, and which looked like abacus or connected hexagonal beads. The outside whiskers obtained from the four kinds of raw materials had droplets on one of their ends, indicating that the outside whiskers grow by the VLS mechanism. The droplets produced from diatomaceous earth and quartzite were mainly composed of Fe and Si. On the other hand, those from Terra abla were composed of Fe, Si and a small amount of Ti. In the case of the casting sand, two types of droplets, one composed of Fe and Si and the other composed of Cr, Fe and Si, were observed. These metals, which composed the droplets, were derived from metal oxides in the raw materials. The metal elements are thought to have migrated from the starting material to the outside whisker in a vapor phase.

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Behavior and test method of subcritical crack growth (SCG) at elevated temperatures were investigated on an alumina with 90% purity. The proposed test method enabled to measure SCG at temperatures over 1000°C using the piston displacement of testing machine and fractographic information. In order to evaluate SCG behavior based on the linear elastic fracture mechanics (LEFM), larger specimens are recommended, because a small scale creep (SSC) condition is required in expressing the crack tip singular stress field by LEFM parameter, i.e. stress intensity factor K. Since creep is dependent on both thermal and mechanical factors, requirement of SSC condition, which implies that the creep damage zone is localized at the crack tip region, may be influenced by those factors. Detailed observations of SCG behavior at 900 and 1050°C revealed that a main crack grew by coalescences with microcracks initiated in the creep damage zone at the crack tip. Thus, tortuosity of SCG path increased with increasing size of the creep damage zone. Temperature dependence of SCG characteristics was discussed in terms of the crack growth mechanism and creep deformation scale.

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In pyrolysis of PTC barium titanate precursor synthesized by the oxalate method, growth and morphological change of the particles formed were studied. Throughout the process, an aggregate particle was observed to be generally composed of a large number of crystallite particles. In the calcination process up to 1000°C, formation of various kinds of phases (dehydrated oxalate, amorphous phase and titanate) and grain growth of the titanate were observed in sequence, but the unique morphology and the size (100-200μm) of the aggregate particles, including the precursor particles, remained unchanged.

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The growth of emerald crystals by the flux evaporation method in the system MoO₃-B₂O₃ is reported. The crystal growth was conducted by heating a mixture of a solute (3BeO⋅Al₂O₃⋅6SiO₂+1wt% Cr₂O₃) and a flux (MoO₃-B₂O₃) at 1100°C, followed by holding at 1100°C for 10, 20, 25 and 30h. The evaporation loss of flux decreased gradually with increasing amount of B₂O₃ added to MoO₃ and increased with the holding time. The transparent emerald crystals were grown with the typical emerald-green color in size up to 1mm. The crystal size was dependent on the evaporation loss of flux. Hexagonal thin plate-like crystals were grown from an MoO₃-rich flux containing 0.15-0.60wt% B₂O₃. The plate-like crystals were bounded by the {0001} and {1010} faces. In rare cases, the small {1120} faces appeared. Crystals grown from the MoO₃ flux were twelve-sided prism in shape. Molybdenum trioxide containing a small amount of B₂O₃ was found to be a suitable flux for growing plate-like crystals of emerald.

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Zirconia (3mol% Y₂O₃) matrix composites reinforced with 20vol% α-alumina whiskers were prepared by spark plasma sintering (SPS) at 1200 to 1500°C under 30MPa for a short holding time of 5min. The room temperature mechanical properties were measured. The SPS brought about dense alumina whisker/zirconia composites at a sintering temperature lower than that in the hot-pressing process. The composites obtained by SPS had strengths, hardness and fracture toughness higher than those by hot-pressing under the same sintering temperatures. The zirconia matrix grains in the composites prepared by SPS grew larger than those by hot-pressing.

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In order to raise the power generation efficiency on a fuel cell, a perovskite type oxide powder was prepared using metal acetate. The synthesis was carried out at compositions between X=0.1 and 0.4 in La_1-XSr_XMnO₃. The average size of primary particles was 0.1μm. La_1-XSr_XMnO₃ (X=0.1, 0.2, 0.3, 0.4) were rhombohedral phases. The composition with X=0.3 showed the highest power generation activity. It was assumed that the power generation efficiency was raised by increase in the amount of oxygen vacancies and by improvement of p-type semiconductor characteristics.

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Bi₄Ti₃O₁₂ thin films were prepared by MOCVD method using triorthotolyl-bismuth, Bi(o-C₇H₇)₃, and di-isopropoxy-bis-(di-pivaloylmetanato)-titanium, Ti(i-OC₃H₇)₂(DPM)₂, as the metalorganic sources. Thin films obtained were characterized by XRD analysis and SEM observation. Bi₄Ti₃O₁₂ thin films with high c-axis orientation and most of single phase were deposited on Pt/Ti/SiO₂/Si and Pt substrates above 500°C in air. Use of newly developed Bi(o-C₇H₇)₃ and Ti(i-OC₃H₇)₂(DPM)₂ instead of normally used Bi(C₆H₅)₃ and Ti(i-OC₃H₇)₄ was advantageous in fabricating Bi₄Ti₃O₁₂ thin films with c-axis orientation at lower temperatures.

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