Papers by Author: Shinobu Hashimoto

Abstract: Intra-type structure of ceramic matrix composites (CMCs) can improve the mechanical properties of ceramic materials. In this work, we used γ-alumina as a matrix including nano-pore for fabricating alumina/silicon carbide composites using pressure-less sintering and pulse electric current sintering (PECS) techniques. We added α-alumina as a seed in order to improve densification of this commercially available γ-alumina. The mixture was sintered from 1250 to 1450
by pressure-less sintering and PECS techniques. Densification of the specimen sintered by pressure-less sintering is very difficult and bulk density of the specimen sintered at 1450
showed lower than 3.0 g/cc. In the case of the PECS technique, the density was improved to 3.8 g/cc in seed added specimen sintered at 1450
. Maximum Vickers hardness and maximum fracture strength were obtained 17 GPa and 500 MPa for the seed added specimens sintered at 1450
, respectively.

Abstract: Aerogels are sol-gel derived mesoporous materials with low apparent density and large specific surface areas. The mechanical property evaluation of pulsed electric current sintered (PECS) alumina and its composite aerogels with nanosized metallic dispersions is an area of interest. In the present work alumina, alumina-copper, alumina-nickel, alumina-cobalt and alumina-tungsten aerogels have been synthesized by super critical drying of respective the sols. The samples were pre-calcined under controlled sintering conditions and pulsed electric current sintered at two different temperatures: (1) at a lower sintering temperature with short residence time (1250°C for 5min) and (2) at a high temperature with prolonged soaking time (1450°C for 30 min) and their mechanical properties were evaluated.

Abstract: Zr2Al3C5 has been successfully synthesized via solid state reaction between Al, ZrC and carbon powder at 1600
in vacuum. This complex carbide has very strong bond between metal atoms and carbon atoms. Thus, this material has a potential to be utilized as structural materials. Some properties of Zr2Al3C5 powder from solid-state reaction in vacuum had been tested. It was found that this powder was completely oxidized in air at 900
1 h, and can be hydrated in moist air. These drawbacks might come from the high reactivity of the powder due to synthesis in vacuum. Zr2Al3C5 powder from solid state reaction in vacuum was sintered at various temperatures from 1500
to 2000
under vacuum with pulse electric current sintering (PECS) and pressureless sintering. Zr2Al3C5 started to sinter at 1500
and got partially dense from 1700
. Physical properties and mechanical properties of this material were investigated and discussed.

Abstract: To improve fracture strength and fracture toughness in ceramic materials, we focused our attention on an intra-type structure of nanocomposites. We proposed new processing method for fabricating intra-type nanocomposites. In this work, Al2O3/Ni nanocomposites were fabricated using a soaking method and sintered by PECS(Pulse Electric Current Sintering) method. We also estimated seed effects on this system. Seeded nanocomposites showed high fracture strength and higher fracture toughness than non-seeded nanocomposites and monolithic alumina. The fracture strength of the seeded nanocomposites was more than 800MPa in all sintering temperature range. The maximum value of the fracture toughness was 5.5 MPa⋅m1/2 for the specimen sintered at 1350°C. The sintered specimens with high fracture strength and high fracture toughness were annealed from 800°C to 1000°C for 0 to 10 min. The specimen annealed at 800°C for 5 min showed the highest fracture toughness of 7.6 MPa⋅m1/2. This value is two times higher than that of the monolithic alumina.

Abstract: Mechanical destruction of acicular asbestos has been carried out as a process for decontamination. The dehydration reaction of asbestos occurred at 600°C and has been completed for 2 h at that temperature. This dehydration reaction of asbestos led to weakening of the mechanical properties of asbestos. After grinding, the microstructure of asbestos appeared no acicular shape. Further, this technique could be applied to asbestos-containing cement board. The spent cement board has about 10-20 mass% of asbestos (chrysotile: Mg3Si2O5(OH)4) and 5 mm thickness was used. After heating at 600°C for 2 h and grinding, the microstructure of the sample also appeared no acicular shape. Thus it was cleared that heating at 600°C and then grinding treatment for asbestos (chrysotile) has a grate important effect of mechanical and chemical destruction of asbestos in both cases of free asbestos and asbestos-containing cement board.

Abstract: Thermal shock resistances of commercially available aluminum nitride and alumina ceramics as used for the circuit substrate were evaluated by infrared radiation heating (IRH) technique. Thermal shock fracture toughness, R2c of these materials was estimated experimentally and theoretically using IRH technique at various ambient temperatures. Temperature dependence of thermal properties of the materials was taken into account for the temperature and the thermal stress analysis. Experimental values of thermal shock fracture toughness were in good agreement with the calculated values. Thermal shock fracture toughness decreased with elevated ambient temperature in both ceramics.