Synthesis and Thermoelectric Properties of Ce1-z Yb z Fe4-x Ni x Sb12 Skutterudites

p-Type Ce1-z Yb (z) Fe4-x Ni (x) Sb-12 (0.25 a parts per thousand currency sign z a parts per thousand currency sign 0.75 and 0.25 a parts per thousand currency sign x a parts per thousand currency sign 0.5) skutterudites were prepared by encapsulated melting and hot pressing, and the effects of filling and substitution (charge compensation) on the transport and thermoelectric properties were examined. The single-phase skutterudite could be obtained by Ce/Yb filling, and substitution of Ni for Fe. However, a small amount of FeSb2 was formed for Ce0.25Yb0.75Fe3.75Ni0.25Sb12. Carrier concentration and electrical conductivity were found to decrease with increasing filling fractions of Ce and substitution with Ni. The electrical conductivity of all samples decreased with increasing temperature, indicating that the Ce1-z Yb (z) Fe4-x Ni (x) Sb-12 skutterudites were degenerate semiconductors. All specimens showed positive Seebeck coefficients and Hall coefficients, confirming p-type conduction. The Seebeck coefficient increased with increasing temperature, and Ce and Ni contents, with the highest value being observed at temperatures ranging from 723 K to 823 K. The maximum power factor was obtained for Ce0.75Yb0.25Fe3.75Ni0.25Sb12, with a peak value of 2.9 mW/mK(2) at 723 K. The thermal conductivity increased at temperatures above 723 K, due to bipolar conduction. Though the lattice thermal conductivity increased with increasing filling fraction of Yb when the Ni content was x = 0.5, little difference was found regardless of the filling ratio of Ce/Yb when x = 0.25. However, the electronic thermal conductivity decreased with increasing Ce and Ni contents, resulting from the decrease in the carrier concentration. The dimensionless figure of merit (ZT) showed peak values at 723 K, due to the decrease in the Seebeck coefficient (or the power factor) and the increase in the thermal conductivity at high temperatures. The maximum ZT = 0.7 was achieved at 723 K for Ce0.5Yb0.5Fe3.5Ni0.5Sb12.