Study on Pressure Sintering of beta-FeSi_2 with highly Efficient Thermoelectric Properties

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beta-Fe_<1-x>Ru_xSi_2 solid solution was synthesized by solid state reaction at 1100ﾟC for 48h and subsequect annealing at 850ﾟC for 168h in an evacuated silica tube. Single phase solid solution was obtained in the composition range 0*x*0.1. The thermal stability range of the beta-phase extended to higher temperature region by partial substitution of Ru atom. The thermoelectric properties of Cr or Co-doped B-Fe_<1x>Ru_xSi_2 strongly depend on the sinthering conditions. The samples with optimum thermoelectric properties are obtaind by high-pressure sintering at 3 GPa and 800ﾟC for 1h. Seebeck coefficient for X-axis perpendicular to applied pressure was superior to that for Z-axis parallel to applied pressure. The anisotropy of electrical conductivity was opposite tendency comparing with the results of Seebeck coefficient. The optimum compositions are found to be Fe_<0.92>Ru_<0.05>Cr_<0.03>Si_2 and Fe_<0.92>Ru_<0.05>Co_<0.03>Si_2 for p-type and n-type materials, respectively. The power factors (sigmaalpha_2) of these materials are higher than that of beta-FeSi_2 based materials. The mechanism for the anisotropy of thermoelectric properties was examined. As a result, it is considered that the model of oriented-grain microstructure is most possible factor to introduce the anisotropy of thermoelectric properties. In this model, a very few amounts of impurities (FeSi, amorphous Si etc.) were located at grain boundaries. Both oriented-grains and impurities in microstructure play an important role to introduce the anisotropy of thermoelectric properties.