Thermoelectric properties of In1Co4Sb12+δ: role of in situ formed InSb precipitates, Sb overstoichiometry, and processing conditions

Abstract

In-filled skutterudites In_xCo₄Sb₁₂ have attracted much attention due to their relatively high thermoelectric performance, which, in turn, is attributed to the In atoms acting as rattlers in the skutterudite voids and to the formation of InSb precipitates when the In solubility limit is exceeded (0.22 ≤ x_max ≤ 0.27). In this work, to suppress the formation of the unwanted CoSb₂ phase and favor the formation of InSb precipitates, the following composition of In₁Co₄Sb_12+δ with In concentration much higher than the solubility limit and Sb overstoichiometry was used. Three sets of bulk specimens with a nominal composition of In₁Co₄Sb_12+δ were synthesized by conventional induction melting followed by (1) ball milling (BM) and spark plasma sintering (SPS), (2) BM and SPS followed by high-temperature annealing, and (3) melt spinning and SPS. The utilization of different sample preparation methods and processing conditions leads to samples with different microstructures and InSb precipitates of different shapes, sizes, and distributions. Thus, for all samples with the same nominal composition of In₁Co₄Sb_12+δ, zT_max varies from 0.7 to 1.3 at 673 K only because of microstructural modification. A maximum zT value of around 1.3 was obtained at 673 K for the sample prepared using induction melting followed by annealing, melt spinning, and SPS.