Papers by Author: Akira Kohyama

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Authors: Wen Yang, Hiroshi Araki, Somsri Thaveethavorn, Akira Kohyama, Jin Nan Yu, Hiroshi Suzuki, Tetsuji Noda
Authors: Akira Kohyama
Abstract: As one of the most important breakthrough in the field of SiC/SiC composite materials, the new process called Nano-powder Infiltration and Transient Eutectoid (NITE) Process has been developed. The outstanding total properties of the NITE SiC/SiC composites are presented. Then, the current efforts to make attractive GFR based on the NITE SiC/SiC composites and the technology R & D to make reactor components with the NITE SiC/SiC composites are provided together with our efforts on innovative reactor designs.
Authors: Ji Yeon Park, Seok Min Kang, Weon Ju Kim, Akira Kohyama
Abstract: To estimate the ion-irradiation effect on various types of SiCf/SiC composites, a silicon self-ion irradiation was performed at temperatures of 600 °C and 1200 °C and at doses of 5 dpa and 20 dpa, respectively. These SiCf/SiC composites were prepared by different processes such as CVI (chemical vapor infiltration), WA-CVI (SiC whisker assisted CVI) and hot-pressing (HP) method. Hardness was measured by a nano-indentation tester and microstructural changes were observed by TEM with SAD(selected area diffraction) technique for the specimens prepared by FIB (Focused Ion Beam) milling. The damage dose was calculated by the SRIM2003 code and then compared with microstructureal observation.
Authors: Shao Ming Dong, Yu Sheng Ding, Zhen Wang, Qing Zhou, Xiang Yu Zhang, Dong Liang Jiang, Akira Kohyama
Abstract: Hot pressing is an effective way to densify powder compacts, especially during the inclusion of a second phase, such as particles, whiskers or fibers. In the present study, SiC/SiC and C/SiC composites were prepared by hot pressing via liquid phase sintering. Nano-SiC powder was used for matrix formation with sintering additives. The effects of preparation conditions such as sintering temperature, pressure and matrix composition, on the microstructural evolution and mechanical behaviors were discussed. Using micro-SiC powder and nano-SiC powder for matrix formation, the interaction between fiber and matrix was characterized. Because the matrix compositions can be easily modified in the CMC-processing from powders by hot pressing, the SiC-BN matrix was also studied in the present experiment. The inclusion of BN can either improve the machinability or provide oxidation resistance to the composite. BN was derived through an in-situ reaction between boron acid and urea by hot-pressing. Boric acid and urea were solved into the ethanol and mixed with nano-SiC particles, and then infiltrated into the fiber bundles. Correlations among microstructures, properties and compositions will be discussed.
Authors: Hisashi Serizawa, Yuuki Asakura, Hirotaka Motoki, Daisuke Tanigawa, Masahiro Tsukamoto, Joon Soo Park, Hirotatsu Kishimoto, Akira Kohyama
Abstract: Silicon carbide fiber reinforced silicon carbide composite (SiC/SiC composite) is expected as a potential replacement of the current Zircaloy fuel cladding in the light-water reactor because of its many superiorities, where it is necessary to develop the end-cap seal of SiC/SiC composite cladding. In this research, the applicability of diode laser irradiation for joining between Zircaloy and SiC/SiC composite tubes was examined based on the caulking method. As the method for fitting two tubes, the screw cutting was conducted for the inner face of Zircaloy tube and the outer surface of SiC/SiC composite. In addition, as a method to improve the joinability of Zircaloy and SiC/SiC composite, titanium nanopowder was inserted into a gap between two tubes. The laser beam irradiation was circumferentially applied to the outer surface of Zircaloy tube. Although the mechanical caulking joint was successfully produced, the airtightness was insufficient for the end-caps of SiC/SiC composite cladding. However, the good adhesion between Zircaloy and SiC/SiC composite was partially produced as the result of the generation of all proportional solid solution between titanium and zirconium. Namely, it can be concluded that the circumferential diode laser irradiation with inserting titanium nanopowder between Zircaloy and SiC/SiC composite tubes would have a good potential for producing the caulking joint of two tubes.
Authors: Han Ki Yoon, Yi Hyun Park, Joon Soo Park, Akira Kohyama
Abstract: SiC has been extensively studied for high temperature components in advanced energy system and gas turbine because of its excellent high temperature mechanical properties and good thermal-chemical stability etc. However, the brittle characteristics of SiC such as low fracture toughness and low strain-to fracture still impose a severe limitation on practical applications of SiC materials. For these reasons, SiCf/SiC composites can be considered as a promising for various structural materials, because of their good fracture toughness compared with monolithic SiC. But, high temperature and pressure lead to the degradation of the reinforcing fiber during the hot pressing. Therefore, reduction of sintering temperature and pressure is key requirements for the fabrication of SiCf/SiC composites by hot pressing method. In the present work, monolithic Liquid Phase Sintered SiC (LPS-SiC) was fabricated by hot pressing method in Ar atmosphere at 1800oC under 20MPa using Al2O3, Y2O3 and SiO2 as sintering additives. The starting powder was high purity β-SiC nano-powder with an average particle size of 30nm. The characterization of LPS-SiC was investigated by means of SEM and three point bending test. Base on the composition of sintering additives-, microstructure- and mechanical property correlation, the compositions of sintering additives are discussed.
Authors: Han Ki Yoon, D.H. Kim, Yi Hyun Park, Yu Sik Kong, Akira Kohyama
Abstract: Reduced activation ferritic (JLF-1) steel is one of the leading candidates for blanket/first-wall structures in D-T fusion reactors. In fusion applications, structural materials will suffer the effects of repeated temperature changes. Therefore, a database of tensile strength and fracture toughness are very important as the temperature is 400 ° C and it's TIG welded joint. In this paper, fracture toughness JIC and KIC) and tensile tests were carried out at room temperature (RT) and at elevated temperature (400 ° C). The tensile properties of the TIG welding joint of JLF-1 steel was also investigated. It was shown that the tensile strength and fracture toughness values of the JLF-1 steel are slightly decreased with as increase in temperature. The fracture toughness values of JLF-1 steel at room temperature and at 400°C show excellent JIC values of about 530 kJ/m2 and 340 kJ/m2, respectively.
Authors: Sang Ll Lee, J.O. Jin, Akira Kohyama
Abstract: The characterization of RS-SiC and RS-SiCf/SiC composite materials fabricated by the reaction sintering process has been investigated, based on the detailed examination of their microstructures. In this composite system, Tyranno SA SiC fiber and Hi-Nicalon SiC fiber were used as reinforcing materials. The green bodies for RS-SiC and RS-SiCf/SiC materials were prepared with the complex matrix slurry of SiC and C particles. The density and the room temperature strength of RS-SiC material with the starting SiC particles of 0.3 µm showed about 3.1 Mg/m3 and about 520 MPa, respectively, even if there were large amount of residual silicon (about 19 %). The flexural strength of Hi-Nicalon/SiC composites greatly decreased at the temperature higher than 1000 􀀀
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