Grain size dependence of transport properties of nano-engineered thermoelectric CoSb3

Nano-engineered skutterudite CoSb₃ was synthesized by a chemical alloying method. A set of samples with different grain size were processed upon annealing. The physicochemical characterizations of these samples were carried out with respect to their phase purity and microstructure, particularly the porosity and grain size. Their transport properties were characterized by measuring Seebeck coefficient, electrical and thermal conductivities in a temperature range of 300 - 650 K. The measurement procedures were standardized for absolute accuracy and reproducibility carried out in two laboratories. It has been observed that the measurement data of transport properties have a significant dependence upon the grain size of the samples as it reduced toward the nano-scale.,he thermal conductivity data have demonstrated the significant size dependence, with a decrease by an order of,magnitude for nano-grained samples, compared to the literature value of κ = 10 W/m.K for highly annealed sample with micron grain size. The electrical conductivity has shown a similar reduction, but to a less extent, upon decreasing the grain size. On the other hand, the size dependence of Seebeck coefficient is less remarkable though notable. As a result, ZT value is still enhanced with decreasing the gain size. The highest ZT value obtained at 611 K was 0.17, much higher than the literature value of 0.1 for a single crystal and polycrystalline samples with micron grain size.