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

碳化硼(B4C)等富硼化合物因其高化学稳定性和高热电优值(ZETA)而成为高温热电材料的候选材料。由于B_4C的塞贝克系数(α)强烈地依赖于结构缺陷，分散在B_4C基质中的第二相将使a值增大。在本研究中，我们选择了SiB_4、SiB_6、SiB_(14)、SiC作为高α材料，W_2B_5、TiB_2、YB_6作为高导电性材料。通过在熔融复合材料的凝固过程中利用共晶和共晶反应来控制分散体的微观结构，我们成功地改善了热电性能。在B-C-Si过晶系中，熔化后的样品含有SiB_(14)和Si三相。热处理使Si固溶于SiB_(14)和SiB_6中，形成SiB_4或SiB_6。B_4C-W_2B_5、_4C-碳化硅、B_4C-TiB_2体系均为准二元系，共晶组成分别为20、50-55、60和75mol%B4C，表现为细小的片层结构。少量第二相YB_6和TiB_2的加入使ZT值显著提高。在1100K时，B_4C-TiB_2和B_4C-YB_6共晶体系的最大ZT值为0.55。该值(ZT=0.55)是文献报道的富B_4C基复合材料中的最高水平。

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.