Tuning the nature of charge carriers by controlling dual substitution in single-filled thermoelectric skutterudite, Yb-CoSb3

Abstract

Fe and Mn have been substituted for Co to study Yb0.3Fe0.7−xMnxCo3.3Sb12 alloys with x = 0.2 and 0.5. These alloys were made by simple vacuum alloying and hot pressing. Both the alloys were found to be essentially single phase with no second phase in agreement with earlier reports. The Seebeck coefficient of Fe-rich p-type alloy was found to increase with increasing temperature reaching a maximum value of 136 μVK−1 at 723 K while that of the Mn-rich n-type alloy increases to 249 μVK−1 at 520 K. The Seebeck coefficient in both alloys decreases after exhibiting a maximum, indicative of bipolar conduction with a small band gap. The electrical conductivity shows weak metallic behavior for the p-type alloy and degenerate semiconducting behavior for the n-type alloy. The charge carrier concentration in the p-type alloy (4.7 × 1019 cm−3) is lower by an order of magnitude compared with that in the n-type alloy (4.4 × 1020 cm−3) but with an order of magnitude higher mobility. The resultant maximum power factor for p-type is therefore ~ 0.83 mWm−1 K−2 compared with 1.3 mWm−1 K−2 for the n-type. The thermal conductivity of n-type, however, is found to be twice that of the p-type resulting in a maximum zT of 0.41 at 723 K for the n-type alloy compared with 0.42 at 723 K for the p-type alloy. These results show that compositional tuning can be used to make either p-type or n-type skutterudites with the same base composition and earth abundant elements.