On the True Indium Content of In-Filled Skutterudites

Abstract

The incongruently melting single-filled skutterudite InxCo4Sb12 is known as a promising bulk thermoelectric material. However, the products of current bulk syntheses contain always impurities of InSb, Sb, CoSb, or CoSb2, which prevent an unbiased determination of its thermoelectric properties. We report a new two-step synthesis of high-purity InxCo4Sb12 with nominal compositions x = 0.12, 0.15, 0.18, and 0.20 that separates the kieftite (CoSb3) formation from the topotactic filler insertion. This approach allows conducting the reactions at lower temperatures with shorter reaction times and circumventing the formation of impurity phases. The synthesis can be extended to other filled skutterudites. High-density (>98%) pellets for thermoelectric characterization were prepared by current-assisted short-time sintering. Sample homogeneity was demonstrated by potential and Seebeck microprobe measurements of the complete pellet surfaces. Synchrotron X-ray diffraction showed a purity of 99.9% product with traces (≤0.1%) of InSb in samples of nominal composition In0.18Co4Sb12 and In0.20Co4Sb12. Rietveld refinements revealed a linear correlation between the true In occupancy and the lattice parameter a. This allows the determination of the true In filling in skutterudites and predicting the In content of unknown AxCo4Sb12. The high purity of InxCo4Sb12 allowed studying the transport properties without bias from side phases. A figure of merit close to unity at 420 °C was obtained for a sample of a true composition of In0.160(2)Co4Sb12 (nominal composition In0.18Co4Sb12). The lower degree of In filling has a dramatic effect on the thermoelectric properties as demonstrated by the sample of nominal composition In0.20Co4Sb12. The presence of InSb in amounts of ∼0.1 vol% led to a substantially lower degree of interstitial site filling of 0.144, and the figure of merit zT decreased by 18%, which demonstrates the significance of the true filler atom content in skutterudite materials.

Thermoelectric Properties of In<i>_x</i>FeCo₃Sb₁₂ Consisting Mainly of In-Filled <i>p</i>-Type Skutterudites

“…Though the spatial resolution of EDS analysis prevented us from quantitatively investigating the composition of nanoparticles, we speculate that those nanoparticles would be InSb because the formation of InSb nanoparticles in In-rich samples was reported in previous papers. 27,28) in the skutterudite phase increased with increasing x at x ≤ 0.7 and then decreased at x > 0.7, while the Fe/Co ratio of all samples were kept nearly constant at 0.25 ± 0.02. This result also indicates that the lling fraction limit of In would be 0.43, which was determined for the sample prepared at x 0.7 by means of EDS analysis.…”

Enhanced thermoelectric properties of Ga and In Co-added CoSb3-based skutterudites with optimized chemical composition and microstructure

“…Figure 2 shows the temperature dependences of the Seebeck coefficient S, electrical resistivity ρ, and power factor S 2 ρ −1 for the samples of (Ga 0. 8 47 The In 0.12 Co 4 Sb 12 sample, as a reference for comparison, was taken with consideration for In content close to the sample with x = 0.45. 47 The sample with nominal composition In 0.12 Co 4 Sb 12 , as a reference sample, was taken with consideration for In content of the sample with x = 0.45.…”

“…8 47 The In 0.12 Co 4 Sb 12 sample, as a reference for comparison, was taken with consideration for In content close to the sample with x = 0.45. 47 The sample with nominal composition In 0.12 Co 4 Sb 12 , as a reference sample, was taken with consideration for In content of the sample with x = 0.45. 47 The S values for all samples (except for x = 0.05) were negative across the entire temperature range between room temperature and 773 K. For the samples with x = 0.150.35, the absolute values of both S and ρ decreased with x, mainly due to increase in the carrier concentration caused by the introduction of Ga and In species.…”

“…47 The sample with nominal composition In 0.12 Co 4 Sb 12 , as a reference sample, was taken with consideration for In content of the sample with x = 0.45. 47 The S values for all samples (except for x = 0.05) were negative across the entire temperature range between room temperature and 773 K. For the samples with x = 0.150.35, the absolute values of both S and ρ decreased with x, mainly due to increase in the carrier concentration caused by the introduction of Ga and In species. In particular, the co-adding of Ga and In effectively gave rise to the enhancement of the absolute S compared to In single filling for CoSb 3 , which led to high S 2 ρ −1 .…”

“…Figure 3 shows the temperature dependences of the κ, κ lat , and zT for the samples of (Ga 0.8 In 0.2 ) x Co 4 Sb 12 (x = 0.35 and 0.45) and In 0.12 Co 4 Sb 12 . 47 The value of κ lat was obtained by subtracting κ el (= LT ρ −1 , where L was the Lorentz number = 2.45×10 -8 W Ω K -2 ) from the measured κ, resulting in the formula κ lat = κ−LT ρ −1 , which indicated that κ lat contained a bipolar contribution. It was clearly confirmed that the κ lat values for the samples with x = 0.35 and 0.45 were systematically lower than those obtained at In single filled CoSb 3 across the entire temperature range, the co-adding of Ga and In for CoSb 3 effectively scatters heat carrying phonon.…”