| Co-Investigator(Kenkyū-buntansha) |
MASUMOTO Hiroshi Tohoku Univ., Inst.for Mat.Res., Res.Associate, 金属材料研究所, 助手 (50209459)
CHEN Lidong Tohoku Univ., Inst.for Mat.Res., Assoc.Professor, 金属材料研究所, 助教授 (90280883)
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| Research Abstract |
Boron rich borides such as boron carbide (B_4C) are a candidate for the high-temperature thermoelectric material because of their high chemical stability and high thermoelectric figure-of-merit (ZETA). Since the Seebeck coefficient (alpha) of B_4C strongly depends on the structural defects, the a values would increase by dispersing second phases in the B_4C matrix. The electrical conductivity (siguma) may also increase by making composite with highly conductive materials, In the present research, we have chosen SiB_4, SiB_6, SiB_<14>, SiC as high alpha materials, and W_2B_5, TiB_2, YB_6 as high siguma materials. We have successfully improved the thermoelectric performance by controlling the microstructure of dispersoids using the eutectic and pertectic reactions during the solidification process from the melted composites. In the case of B-C-Si pertectic system, as-melted specimens contained three phases of SiB_<14> and Si. Heat-treatments of the specimens caused the solid solution of Si into SiB_<14> and the formation of SiB_4 or SiB_6. The siguma decreased with the heat-treatment in general, but the a indicated the maximum at a specific heat-treatment time. The greatest ZT value in the peritectic system was 0.4 at 1100K.The B_4C-W_2B_5, _4C-SiC, B_4C-Tib_2 systems were all quasi-binary, whose eutectic compositions were 20, 50-55, 60 and 75 mol%B_4C, respectively, indicating fine lamella structures. The addition of small amount second phases such as YB_6 and TiB_2 resulted in the significant increase of ZT values. The greatest ZT values in the eutectic system were 0.55 at 1100K for the B_4C-TiB_2 and B_4C-YB_6 systems. This value (ZT=0.55) of the present B_4C-based composites are the highest level among the boron-rich borides reported in the literatures.
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