Papers by Author: Katsumi Yoshida

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: C-and BN-interphases on SiC fibers for unidirectional SiC_f/SiC composites were formed by EPD process, and their microstructure and mechanical properties were investigated. Whereas the C-SiC_f/SiC composites showed a pseudo-ductile fracture behavior with large amount of fiber pullout, the BN-SiC_f/SiC composites fractured in a brittle manner without fiber pullout in spite of sufficient thickness of BN interphase. It is inferred from the results of EDS that sintering additives would react with h-BN-interphase, and the interphase did not act effectively for toughening the SiC_f/SiC composites.

Abstract: The CNT/B₄C composite with Al₂O₃ additive was fabricated by hot-pressing following extrusion molding of a CNT/B₄C paste, and mechanical properties of the obtained composite were investigated. Many CNTs in the composite aligned along the extrusion direction from SEM observation. 3-points bending strength of the composite was slightly lower than that of the monolithic B₄C. Elastic modulus and Vickers hardness of the composite drastically decreased with CNT addition. Fracture toughness of the composite was higher than that of the monolithic B₄C.

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: For the transmutation of the very long half-lived isotopes which are separated from the spent nuclear fuels, it is necessary to find proper inert matrices these are stable under heavy neutron irradiation at high temperature. Silicon nitride ceramics is a candidate since it is very tolerant for heavy neutron irradiation and keeps relatively high thermal conductivity. For these reasons, we try to sinter Si3N4 ceramics containing large amounts of CeO2 as a simulant for Am2O3, a typical transuranium element. The low-temperature pressureless-sintering behavior of the ceramics and chemical and thermal properties of the obtained sintered bodies are reported.

Abstract: Silicon nitrides are often used as ladles, stalks, heater element protection tubes, etc., in the metal casting industry. A low wettability for molten metals is required for these purposes since wetting by molten metals leads to adhesion of solidified metals, which causes several problems. Surface structure is known to affect wettability. Thus, the present study attempts to fabricate silicon nitrides with controlled surface structures. Silicon nitrides, whose surfaces were covered with ordered hemispherical protrusions, were fabricated by the slip-casting technique. The cast bodies were sintered, and subjected to wettability tests using molten metals. For comparison, silicon nitrides with as-sintered and polished surfaces were also prepared. The surface with protrusions exhibited a lower contact angle compared to the as-sintered and polished surfaces. The contact angle depended on the diameter of the hemisphere; it was the largest at a diameter of 0.3 mm.

Abstract: Graphite powder was treated with lanthanum, aluminum and magnesium phosphate solution, and oxidation resistance of the obtained graphite powder was evaluated. Oxidation starting temperature and oxidation completion temperature of graphite powder treated with various phosphates were 50-100oC higher than those of as-received graphite powder. Graphite powder treated with small amount of lanthanum phosphate exhibited the higher oxidation starting temperature than graphite powder treated with aluminum and magnesium phosphates. LaP5O14 would partially exited on graphite powder, and protect the edge carbon atoms of graphite and reduce the reactivity of carbon atoms toward oxygen, resulting in improving the oxidation resistance.

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.