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

β-Fe_<1-x>Ru_xSi_2固溶体是在真空石英管中于1100℃固相反应48h并随后在850℃退火168h合成的。获得组成范围为0*x*0.1的单相固溶体。通过Ru原子的部分取代，β相的热稳定范围扩展到更高的温度区域。 Cr或Co掺杂的B-Fe_1xRu_xSi_2的热电性能很大程度上取决于烧结条件。通过在3 GPa、800℃高压下烧结1 h，获得具有最佳热电性能的样品。垂直于施加压力的 X 轴的塞贝克系数优于平行于施加压力的 Z 轴的塞贝克系数。电导率的各向异性与Seebeck系数的结果相比呈现相反的趋势。对于p型和n型材料，发现最佳成分分别为Fe_<0.92>Ru_<0.05>Cr_<0.03>Si_2和Fe_<0.92>Ru_<0.05>Co_<0.03>Si_2。这些材料的功率因数 (sigmaalpha_2) 高于 β-FeSi_2 基材料。研究了热电性能各向异性的机制。因此，认为定向晶粒微观结构模型是最有可能引入热电性能各向异性的因素。在该模型中，极少量的杂质（FeSi、非晶硅等）位于晶界处。微观结构中的取向晶粒和杂质对于引入热电性能的各向异性起着重要作用。

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.