Electronic Transport and Thermoelectric Properties of p-Type NdzFe4-xCoxSb12 Skutterudites

p-Type NdzFe4-xCoxSb12 (z = 0.8, 0.9, 1.0 and x = 0, 0.5, 1.0) skutterudites were synthesized by encapsulated melting and annealing, and consolidated with hot pressing. The effects of Nd filling and Co substitution for Fe (charge compensation) on the electronic transport and the thermoelectric properties of the skutterudites were examined. A few secondary phases such as Sb and FeSb2 were formed together with the skutterudite phase, but the formation was suppressed with increasing Nd and Co contents. It was confirmed that Nd was filled in the void and Co was substituted for Fe in all specimens, because the lattice constant increased with increasing Nd content and decreased with increasing Co content. The electrical conductivity of all specimens decreased slightly with increasing temperature, showing degenerate semiconductor characteristics. The Hall and the Seebeck coefficients showed positive signs, indicating that the major carriers were holes (p-type conduction). The Seebeck coefficients were increased due to the decrease in the carrier concentration with increasing Nd and Co contents, while the electrical conductivity and the thermal conductivity were decreased. As a result, the dimensionless figure-of-merit, ZT, was improved by Nd filling and Co substitution, and a maximum ZT = 0.91 was obtained at 723 K for Nd0.9Fe3.5Co0.5Sb12.