High-Temperature Thermoelectric Properties of Ge-Substituted P -Type Nd-Filled Skutterudites

To clarify the influence of Ge substitution on the electrical transport properties of Nd-filled p-type skutterudites, a series of Nd0.9Fe2Co2Sb12−x Ge x compounds with compositions x = 0, 0.1, 0.2, 0.3, and 0.4 were synthesized by a solid-state reaction method followed by a spark plasma sintering process. The crystal structure of the prepared samples was characterized by x-ray diffraction analysis, revealing successful formation of skutterudites as main pure phase. The electrical and thermal transport properties were investigated as a function of Ge doping content for fixed Nd filler content. All samples possessed positive Seebeck coefficient, indicating effective p-type behavior. The lightly doped samples (x = 0.1 and 0.2) showed higher figure␣of merit over the entire temperature range, with the x = 0.2 sample showing the highest ZT value of 0.56 at 674 K, 40% higher than that of the Ge-free sample. The enhancement in ZT can be ascribed to the optimized carrier concentration and reduced thermal conductivity. However, further increase of the Ge content resulted in second phase in the matrix, lowering the overall ZT. The large enhancement of ZT through Ge doping indicates that these compounds may have great potential for application as p-type segments of thermoelectric devices.