Synthesis of thermoelectric materials by simultaneous addition of several kinds of fine particles and improvement of their thermoelectric properties

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Several kinds of fine particles, such as insulating conductive and/or metallic conductive fine particles, were dispersed in the matrix of thermoelectric materials. The thermoelectric materials used in this study were β-FeSi_2, CoSb_3 and several thermoelectric oxides, and the effects of these fine particles addition of the thermoelectric performance, especially the thermal conductivity. In the case of the β-FeSi_2, it was found that a densely sintered body with the high figure of merit could be synthesized by SiC fine particle dispersion and Cu powder addition. The addition of rare earth oxides, which have low thermal conductivity and good chemical stability, was significantly effective on reducing thermal conductivity and increasing Seebeck coefficient, especially when Y_2O_3 powder was added. The improvement of Seebeck coefficient and the results of the Hall coefficient measurement suggest that the solution of a small amount of the rare earth element into the Fe site of the β-FeSi_2 matrix occurs. In the case of the CoSb_3, the dispersion of the metallic FeSb_2 and NiSb fine particles was quite effective on reducing the electrical resistivity, as well as the thermal conductivity, resulting in the significant improvement of the thermoelectric performance. The thermoelectric oxides, ZnO and Na_xCo_2O_4 were synthesized by the polymerized complex method (PCM). The PCM was found to promote the solution of doping element compared to the conventional solid state reaction method, expanding the possibility of enhancement of thermoelectric properties by doping. When the impurity phases precipitated, they were finely dispersed in the matrix, indicating that the PCM is also effective on reducing the thermal conductivity.