Improvements of thermoelectric properties of clathrate compounds by control of their crystal structures and chemical compositions

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We attempted to improve of thermoelectric properties of some clathrate compounds by control of their crystal structures and chemical compositions. The obtained results are mainly as follows:1. We have examined the influences of the kind of filled atoms on thermoelectric properties of n-type germanium clathrates. Firstly, we prepared potassium-filled K_8Ga_8Ge_<38> samples to compare their properties with those of Ba_8Ga_<16>Ge_<30>. Although the K_8Ga_8Ge_<38> had only a half number of Ga atoms for carrier compensation, the K_8Ga_8Ge_38 possessed a lower mobility than that of the Ba_8Ga_<16>Ge_<30>. However, the K_8Ga_8Ge_<38> had a larger effective mass, resulting in a higher power factor at room temperature. This result is consistent with their calculated band structures. Secondly, we prepared mixed clathrate Sr_<8-x>Ba_xGa_<16>Ge_<30> samples with various Ba-to-Sr ratios. Their thermoelectric properties varied systematically with the Ba content x. The electrical conductivity decreas … More ed with the Ba content x, while the activation temperature range and the temperature where the Seebeck coefficient reached the maximum value increased. The results may be associated with the band structures.2. We investigated some cationic clathrates to obtain a, high efficiency p-type thermoelectric material. Firstly, we successfully synthesized iodine-or tellurium-filled cationic clathrate compounds Sn_<38>Sb_8I_8, Ge_<38>Sb_8I_8, Ge_<30>P_<16>Te_8 and Si_<30>P_<16>Te_8 by sintering with mechanical alloying technique. The obtained samples were big in size enough to measure their properties, whereas cationic clathrates prepared by ampoule technique or high pressure technique are small. Secondly, we examined their thermoelectric properties. The obtained cationic clathrates Sn_<38>Sb_8I_8, Ge_<38>Sb_8I_8 and Ge_<30>P_<16>Te_8 possessed as low thermal conductivities as that of anionic clathrates such as Ba_8Ga_<16>Ge_<30> conductivities. Although the iodine-filled clathrates were n-type, the tellurium-filled clathrates Ge_<30>P_<16>Te_8 and Si_<30>P_<16>Te_8 showed p-type conductivity. In addition, the Si_<30>P_<16>Te_8 could be doped with carriers, resulting in a relatively high dimensionless figure of merit ZT of 0.45 for p-type materials. Less