Thermoelectric properties of La-filled CoSb3 skutterudites

La-filled skutterudites LazCo4Sb12 (z = 0, 0.1, 0.2, 0.3, 0.4) were prepared by encapsulated melting and hot pressing. The filling effects of La on the transport and the thermoelectric properties of skutterudites were examined. All specimens were transformed to skutterudite phases through subsequent annealing. The intrinsic CoSb3 had a high positive Seebeck coefficient, but a thermoelectric figure of merit, ZT, of lower than 0.1 due to its low electrical conductivity and high thermal conductivity. However, La-filled CoSb3 showed degenerate semiconductor behavior and high electrical conductivity, and exhibited n-type conduction because of the excess electrons donated from the La fillers. The Seebeck coefficient increased and then decreased slightly with increasing temperature, and it decreased with increasing La filling content due to increased carrier concentration. The thermal conductivity was reduced significantly by La filling, mainly due to the decrease in the lattice thermal conductivity. ZT = 0.35 was obtained in the temperature range from 623 to 823 K for La0.2Co4Sb12.