Thermoelectric properties of p-type partially double-filled (Pr1−z Nd z ) y Fe4−x Co x Sb12 skutterudites

Abstract

p-Type partially double-filled (Pr1−zNdz)yFe4−xCoxSb12 (z = 0.25, 0.75, y = 0.8, 0.9, and x = 0, 0.5, 1.0) skutterudites were synthesized by encapsulated melting and annealing and consolidated by hot pressing. The effects of Pr/Nd partial double-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 marcasite FeSb2 were formed together with the skutterudite phase, but the formation was suppressed with increasing Co content. Unlike for complete filling with Pr/Nd, the (Pr,Nd)Sb2 phases were not formed by partial filling with Pr/Nd. Successful Pr/Nd double filling in the voids and Co substitution for Fe were confirmed because the lattice constant increased with increasing filling fraction and Pr/Nd ratio, and decreased with increasing Co content. The electrical conductivity slightly decreased with increasing temperature, showing degenerate semiconductor characteristics, while the Hall and the Seebeck coefficients showed positive signs, indicating that holes were the major carriers (p-type conductions). The Seebeck coefficient increased due to a decrease in the carrier concentration with increasing Pr and Co contents while the electrical conductivity decreased. The lattice thermal conductivity decreased because phonon scattering was enhanced by Pr and Nd double filling, which led to low thermal conductivity, but partially double-filled specimens did not have a significant effect on the reduction of lattice thermal conductivity compared with the completely double-filled specimens. The maximum dimensionless figure of merit, ZT = 0.84, was obtained at 723 K for (Pr0.75Nd0.25)0.8Fe3CoSb12.