Application of boron carbide-based composite ceramics to thermoelectric conversion materials

碳化硼（B_4C）等富硼硼化物具有较高的化学稳定性和较高的热电性能系数（ZETA），是高温热电材料的候选材料。由于B_4C的Seebeck系数（alpha）与结构缺陷有很大关系，因此在B_4C基体中分散第二相会使a值增大。采用高导电性材料制备复合材料可提高电导率，本研究选用SiB_4、SiB_6、SiB_<14>、SiC作为高α材料，W_2B_5、TiB_2、YB_6作为高α材料。在熔融复合材料凝固过程中，利用共晶和共晶反应控制分散体的微观结构，成功地改善了材料的热电性能。在B-C-Si完美体系中，熔点试样包含SiB_<14>和Si三个相。试样热处理后，Si固溶成SiB_<14 bb_0，形成SiB_4或SiB_6。siguma随热处理而减小，但a在特定热处理时间达到最大值。在1100K时，包晶体系ZT值最大，为0.4。B_4C- w_2b_5、_4C-SiC、B_4C- tib_2体系均为准二元体系，共晶成分分别为20、50-55、60和75 mol%B_4C，具有良好的片层结构。少量的第二相如YB_6和TiB_2的加入使ZT值显著提高。B_4C-TiB_2和B_4C-YB_6共晶体系在1100K时ZT值最大，为0.55。该ZT值（ZT=0.55）是目前文献报道的富硼硼化物中最高的。

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.