The work-of-fracture (WOF) tests were conducted for hot-pressed silicon nitride at high temperatures above 1000°C to investigate displacement rate (D-rate) dependence of effective fracture energy, and then the bridging stress at a crack interface was estimated from the increment of fracture resistance with crack extension (R-curve behavior) to discuss its correspondence to tensile strength. The effective fracture energy and the bridging stress increased with lowering D-rate, and then decreased, probably due to the activated pulling out work of grains and its sensitivity to change in D-rate. The bridging stress was shown to correspond to a great part of tensile strength in the range where a plasticity was seen in the stress-displacement (S-D) curve, implying a large contribution of grain bridging toward bearing an applied tensile stress during slow crack extension.

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The densification of SiC-B-C composites doped with TiC, ZrC, HfC, VC, NbC, TaC or WC were investigated by pressureless sintering. Densities over 90% TD (theoretical density) were achieved by addition of TaC by sintering at a relatively higher temperature. TaC particles suppressed the grain boundary movement. Addition of 10mol% TaC doubled the toughness of composites under the same sintering conditions. Exposed TaC particles were oxidized severely at 1500°C in air, and formed Ta₂O₅ polymorphisms.

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The dynamic fatigue tests of nine structural ceramics were carried out at temperatures ranging from 800°C to 1500°C in vacuum. The same tests in air were conducted for two ceramics. In ceramics which has a glassy phase in grain boundaries, cracks propagate through grain boundaries due to softening of the glassy phase. A fatigue parameter of ceramics at elevated temperatures is degraded when MgO is used as a sintering additive. In reaction sintered Si₃N₄ and HP-SiC with B and C additives, a subcritical crack growth is not observed but the bending strength of HP-SiC is degraded due to the generation of pores by surface oxidation. It seems that life-time of HP-Si₃N₄ under the static load is predictable from its dynamic fatigue behaviour.

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Stoichiometric and non-stoichiometric hydroxyapatites (Ca/P=1.55, 1.60 and 1.67), prepared by the hydrolysis of brushite or monetite, were densified by the HIP treatment at 100°-800°C and 320-850MPa. The temperature-pressure region for densification depends on Ca/P ratios. At 600MPa, the lowest temperature of the densification is 300°C for Ca/P=1.55, 400°C for 1.60 and 800°C for 1.67. In the case of calcium-deficient hydroxyapatite, the densification temperature decreased with increasing crystal water content. In the case of Ca/P=1.55, the lowest temperature for densification is 500°C at 600MPa for fully dried starting powder, of which “n” in Ca_10-xH_x(PO₄)₆(OH)_2-x⋅nH₂O is nearly equal to zero.

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Tantalum Boride powders were synthesized by the solid state reaction between tantalum and amorphous boron. Five Boride phases (Ta₂B, Ta₃B₂, TaB, Ta₃B₄ and TaB₂) were formed by heat treatment of the mixed powders at 1000°-1800°C for 0-120min. Formation behavior of these borides was dependent on the heat-treatment temperature, in which structurally stable phases formed in order from low to high temperatures. Single phases of TaB₂, TaB and Ta₃B₄ were obtained by the heat-treatment of the mixed powders with corresponding stoichiometric compositions at 800°, 900° and 1800°C, respectively. Sintered compacts of TaB and TaB_2+x were prepared by a high-pressure sintering of synthesized TaB and TaB_2+x powders at 4GPa and 1600°C for 15min. The bulk density of TaB was 13.51g/cm³ (relative density: 95.1%) and the Vickers microhardness was 2550kg/mm². The homogeneous range of the solid solution TaB_2+x was 0<x<0.55, the density and the microhardness of which increased with increasing amount of boron in solid solutions.

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Effect of metal alkoxides as grinding aids in grinding of electrofused alumina powders and the reaction mechanisms were investigated. Dry grinding experiments with and without grinding aids were made using a vibration mill up to 320h. Particle size distribution and specific surface area of powders were measured to estimate the degree of grinding. The degree of dispersion-cohesion of ground powders was evaluated from an angle of repose and particle shape observation by SEM. Surface tension of metal alkoxide liquids and contact angle between metal alkoxide liquids and sintered alumina surface were also measured. Addition of grinding aids caused acceleration of grinding and dispersion for their ground powders. The following two major reasons were considered for these effects. (1) Surface tension of metal alkoxide liquid is smaller than that of water. (2) Hydrolysis of alkoxide excludes water, which tends to cause cohesion of powders, and also forms butyl alcohol which acts as grinding aid itself.

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Acoustic emission signals of sintered mullite were measured in 4-point bending tests. The specimens were prepared from a high-purity mullite powder and sintered at 1650°C for 2h. The 2ch AE signals were detected by the piezoelectric sensors on the both end surfaces of 4-point bending specimens (3mm×4mm×38mm). Detected waves were recorded in a digital wave memory, and then analyzed with a computer to obtain AE location and cumulative AE counts data. In addition, the SEM fractography and the fracture mechanics evaluation were carried out to determine the fracture origins. The experiments revealed the relation between type of fracture origins and AE signals, and new fracture models were proposed to explain the experimental results.

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A slurry was made from zirconia powder (yttria-stabilized zirconia powder), aluminium hydroxide sol (boehmite sol), binder and plasticizer, and was formed to green sheet by the doctor blade method. The green sheet was sintered in the temperature range of 1300° and 1500°C in air. The optimum alumina content to obtain a good ZrO₂/Al₂O₃ thin sheet was 10-15wt%. The thin sheet sintered at 1400°C was characterized by the thickness of 100μm, density of 4.9g/cm³, bending strength of 486MPa, modulus of elasticity of 167GPa, and resistivity of 2-4×10¹²Ω⋅cm.

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Yb₂O₃-Fe₂O₃ powders were prepared from ytterbium and iron chloride solutions by a homogeneous precipitation method where urea was used as precipitant. The precipitation process was carried out stepwise. At first the iron hydroxide was precipitated and then the ytterbium hydroxide was formed. In the firing process of precipitated powder the ytterbium orthoferrite was formed initially from the iron oxide and ytterbium oxide, and monophasic ytterbium iron garnet was obtained above 1200°C. The reactivity and sinterability of the powder obtained by the homogeneous precipitation method were better than that of oxide mixture. The ytterbium iron garnet contained up to 0.1-0.2mol% iron oxide in excess of the stoichiometric composition.

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β-sialon with Z=1(+4), Z=2(+4) and Z=3(+4) (Si_6-ZAl_ZO_ZN_8-Z: Z=0-4.2, +4:4eq% excess oxygen over the each Z number) from a powder mixture of α-Si₃N₄, α-Al₂O₃, AlN and SiC was hot pressed to produce β-sialon-SiC composites. The hot-pressed β-sialon-SiC composites were oxidized in an electric furnace at 1300°C for 100 hours. Bending strength, weight gain, surface roughness and optical microscopic observation of sintered materials were carried out after oxidation. The results are obtained as follows:
(1) The weight gain of the composite decreased slightly by the addition of SiC.
(2) The bending strength of the composites was slightly improved after oxidation.
(3) Weight gain of the composites was little increased as the Z number increased, but pitting was observed on the surface of Z=2(+4) and Z=3(+4) β-sialon-50wt% SiC composites.

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Infrared (IR) sensing properties were investigated for a positive temperature coefficient (PTC) thermistor with a composition of Ba_0.621Sr_0.336Ca_0.040La_0.003TiO₃+2mol% SiO₂+0.05mol% MnO (the Curie point was 25°C), operated under the self-regulating heating conditions. The results were compared with those of a detector element with the Curie point of 122°C. The sensitivity operated under the self-regulating heating conditions measured at various ambient temperatures in the range from 0° to 30°C was several times larger than that of the detector element with the Curie point of 122°C. Lowering the operating temperature of the detector by using a PTC thermistor with a low Curie point was shown to be effective for improving the sensitivity.

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Spherically aggregated ZrO₂ particles of about 1μm with narrow size distribution were obtained by hydrothermal treatment of highly concentrated ZrOCl₂ solutions at 200°C following the heat-treatment at 300°C in air to dry. The aggregates are joined each other and are easily collapsed and dispersed to ultrafine particles of smaller than 10nm by adding water before drying. The powder composed of the aggregates maintaining the shape as well as formed at the stage of the hydrothermal treatment was obtained when the ‘double structured vessel’ was employed for the hydrothermal treatment.

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Thermal diffusivities of molten boron oxide, which is one of the favorable liquid capsule materials for producing GaAs or GaP single crystals, have been measured systematically in the temperature range between 1000 and 1500K using a three layered laser flash method. The thermal diffusivity of molten boron oxide were found to decrease with increasing content of P₂O₅ or In₂O₃ additives.

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The oxidation of TiB₂-Al₂O₃ composites prepared by pressureless sintering at 1900°C was investigated. TiB₂-Al₂O₃ composites were oxidized at 1000°C for 36h in air, and weight changes were measured to estimate the oxidation resistance of the composites. The composites showed good oxidation resistance with increasing amount of Al₂O₃. The weight gain at 1000°C for 36h in air of the TiB₂ specimen was about 20mg/cm², that of the composite containing 50wt% Al₂O₃ was about 3mg/cm². TiB₂-90wt% Al₂O₃ composite had excellent oxidation resistance.

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