Papers by Author: Masamitsu Imai

Abstract: In this study, SiC/SiO₂ core-shell nanowires (SiCNWs) were fabricated by thermal evaporation method without any catalyst, using pre-oxidized silicon powder and methane (CH₄) gas as precursors. The reaction temperature was 1340°C in an inert atmosphere. The SiCNWs produced by this process had a single crystal β-SiC core ranging from 20 to 80 nm in diameter and low-crystalline SiO₂ shell about 10-20 nm thick, and up to 1 mm long. The exhaust gas from the production system was analyzed by gas chromatography and the growth activity of SiCNWs was captured by digital camera during a soaking period. From the results, CO gas was detected only when SiCNWs were growing and was not found when methane gas was stopped. It was clarified that CO gas was produced as a by-product during the formation of SiCNWs. The formation mechanism of SiCNWs synthesized by thermal evaporation method was suggested to be oxide-assisted growth mechanism.

Abstract: Effects of simultaneous addition of SiO2 and gadrinium oxide on densification of SiC ceramics were examined, and relation between microstructure and their mechanical properties were discussed. Total 11wt% of Gd2O3 and SiO2 were mixed with fine -SiC powder. The weight of Gd2O3 in (Gd2O3 + SiO2) were set as 0, 20, 40, 60, 80 and 100%. The mixture was hot-pressed at 1950oC under 40 MPa applied pressure for 1 h. In the case of 40Gd2O3 and 80Gd2O3 compositions, the effect of sintering temperature from 1900 to 2000oC was also examined. The bulk density increased with increasing Gd2O3 content at the sintering temperature of 1950oC. Bending strength of the sintered bodies also improved with increasing Gd2O3 content generally, but at 40Gd2O3 composition, the maximum over ~800 MPa was observed. Young’s modulous, Vickers hardness and fracture toughness also increased with increasing Gd2O3 content. The distribution of grain boundary phase was not homogeneous. Evaporation of additives, mainly SiO2, caused non-homogeneous distribution of grain boundary phase between outside and inside of sintered bodies. High temperature bending strength of 80Gd2O3 specimen was superior than that of 40Gd2O3 specimen.

Abstract: Alpha- or beta-Si3N4 powder with larger grain size was uses as starting material, and the effect of heat-treatment on thermal conductivity of Si3N4 ceramics using MgO, Y2O3 and SiO2 as sintering additives was investigated in terms of their microstructure and the amount of grain boundary phase. Most of the components derived from sintering additives existed as glassy phase in sintered Si3N4. After heat-treatment at 1950oC for 8 h, the amount of glassy phase significantly decreased, and then small amount of glassy phase existed in Si3N4 ceramics was crystallized as Y2O3 and Y2Si3N4O3. In the case of Si3N4 ceramics using SN-7 powder, thermal conductivity of heat-treated Si3N4 was around twice of the value of sintered Si3N4, and the thermal conductivity was increased from 41.4 to 87.2 W/m•K due to not only the reduction of grain boundary phase but also the grain growth. In the case of Si3N4 using SN-F1 powder, thermal conductivity of Si3N4 ceramics was also significantly increased from 36.0 to 73.2 W/m•K after heat-treatment. In this case, the reduction of grain boundary phase mainly affected the thermal conductivity of Si3N4 ceramics because the grain size of heat-treated Si3N4 was nearly the same as that of sintered Si3N4. The reduction of grain boundary phase from Si3N4 was effective for the improvement of their thermal conductivity in addition to grain growth of Si3N4.

Abstract: Silicon nitride (Si3N4) ceramics have been interested for electrical substrate applications, because the ceramics can be made highly mechanical strength, fracture toughness, electrical resistivity and high thermal conductivity. Generally, relatively large amount of additives are required to obtain dense Si3N4 ceramics. During sintering, additives react with SiO2 including surface oxide of Si3N4 raw powder to form a liquid phase. Most of liquid phase changed into glassy phase during cooling down. In this study, Si3N4 ceramics were fabricated by gas pressure sintering. Yttrium oxide (Y2O3), silica (SiO2), and magnesia (MgO) were used for liquid-phase-enhanced sintering process. Dense materials were sintered by this process, but their thermal conductivities were not so high (30-40 W/m·K). Therefore, post-sintering heat-treatment process was performed to reduce the excess amount of glassy phase. An additive system (3 mass% SiO2 with 3 mass% MgO and 1-5 mass% Y2O3) was selected as the sintering aid. These ceramics could be sintered to almost full density at relatively low temperature as 1650oC for 2 h under 0.1 MPa-N2 without packing powder. The resulting materials have high bending strength, about 1 GPa, when 5mass% of Y2O3 was added. Based on the creation of low temperature pressureless sintering without packing powder, a novel two-step sintering (once firing) was proposed. The two-step sintering conducted by sintered at 1650oC under 0.1 MPa-N2 for 2 h for densification in the first step. Followed by heated up to and kept at 1950oC for 8 h under 1.0 MPa-N2 in the second step. The Si3N4 ceramics could be fabricated with relatively high thermal conductivity of 90 W/m·K. Mass loss, microstructure, mechanical properties, oxygen content and chemical composition were discussed.

Abstract: Monazite(LaPO4)-coated alumina-fiber/alumina-YAG (Y3Al5O12) matrix composites were fabricated by in-situ coating of monazite followed by hot-pressing, and the effects of coating and sintering condition on mechanical properties of the composite were examined. Alumina powder and YAG powder (weight ratio, 95:5) were used as raw materials for green sheets, which was fabricated by tape casting technique. Monazite was synthesized by the in-situ reaction of La(NO3) solution with H3PO4 on the surface of fibers. After slurry infiltration into the coated fiber bundles, the fiber cloths were laminated with the green sheets alternately, then they were heat-treated, finally sintered by hot-pressing at various temperatures. The mechanical properties of the composites were changed by the fabrication conditions. Non-brittleness of the composites reduced with the increase of sintering temperature. The composites sintered at 1200oC showed the highest Weibull modulus and pseudo-ductility.

Abstract: In this study, Tyranno SA fiber cloth was coated with carbon black and SiC powder containing sintering aids by means of electrophoretic deposition method, and SiC/SiC composites with three different fiber volume fractions were fabricated using the Tyranno SA cloth by hot-pressing at 1700oC. The sufficient formation of the SiC matrix between each fiber could be observed. The composite fractured in non-brittle manner, and bending strength decreased with increasing fiber volume fraction. The crack propagation and fracture behavior depended on the fiber volume fraction. These differences in bending strength and fracture behavior would be caused by the difference in the interfacial bonding between fiber cloth and the matrix.