Application of Silicon Carbide-Boron Carbide Composite Ceramics for Functionally Graded Thermoelectric Conversion Materials

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Boron carbide (B_4C) is a good candidate material for high-temperature thermoelectric conversion at more than 1200K.In the present study, we tried to improve the thermoelectric performance of B_4C by controlling microstructure making B_4C-based composites. Firstly, we prepared SiC-B_4C composites by arc-melting., since SiC has good oxidation resistance and superior thermoelectric properties. A typical lamella texture was observed at SiC content of 40 to 50 mol%, and we confirmed that SiC-B_4C composites are a quasi-binary system. The optimum SiC content was 40 mol%, and its thermoelectric figure of merit value (ZT) was about 0.1 at T=1100K.Secondary, we found that the dispersed phase in the B_4C should be highly electrically conductive in order to improve the thermoelectric performance. Then we prepared TiB_2-B_4C composites. The TiB_2-B_4C composites are also a quasi-binary system. The lamella texture was observed at TiB_2 content of 15 to 25 mol%. The highest ZT values was attained at TiB_2 content of 6 mol%, and its ZT value was 0.5 at T=1100K.This value is almost equivalent to the required level for practical thermoelectric devises.The next issue which we should try is to develop a functionally graded thermoelectric materials which consist of the TiB_2-B_4C composites, Si-Ge alloys, PbTe and Bi_2Te_3.