Grain size effects on thermoelectric properties of hot-pressed CoSb 3

To get high ZT values, grain size optimization of hot-pressed CoSb ₃ is very important. Although single crystals of CoSb₃ have high electrical conductivities and moderate Seebeck coefficients, the ZT values are relatively low because of the high thermal conductivities. We tried to lower the thermal conductivity of hot-pressed CoSb by reducing the grain size. Mechanical alloying of the raw material powders of Co and Sb was the very effective method to reduce the grain size of hot-pressed CoSb₃. This paper shows the relations between the thermoelectric properties (Hall mobility, Seebeck coefficient, electrical conductivity and thermal conductivity) and the grain size. The thermal conductivity of hot-pressed CoSb₃ decreased with reducing the grain size. The value of 4.19 Wm^-1 K^-1, which is about 40% of the single crystals, was ultimately obtained at 293 K for the hot-pressed material with an average grain size of 0.9 μm. Although the larger grain size is preferable to attain a high ZT value at 293 K, because of the greater decrease of the carrier mobility than the thermal conductivity with reducing the average grain size, the highest ZT value of 0.10 was obtained at 673 K on the sample with an average grain size of 2.1 μm