Thermal Conductivity Reduction by Tuning the Rattler Fraction in a p-type CeyYb1−yFe3CoSb12 Double-filled Skutterudite

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The rattling effect has been a major research topic in attempts to reduce high-frequency phonon conduction, especially for the skutterudite system. Filling skutterudite with rattler atoms with different localized frequencies (double-filling) has also been reported as an effective method to decrease the thermal conductivity. Optimization research for the double rattler effect is required, especially in the p-type R1−yR′yFexCo4−xSb12 double-filled skutterudite system (R, R′ = rare earth rattlers) because these need larger numbers of rattlers (≈ 1.0) than are needed in the n-type skutterudite case (≈ 0.3). We attempted to optimize the rattler fraction in a p-type CeyYb1−yFe3CoSb12 skutterudite system in this research. We combined a melt spinning (MS) process with a subsequent spark plasma sintering (SPS) technique to fabricate bulk nanocrystalline skutterudites. The resulting low thermal conductivity (2.20 Wm−1K−1 at 723 K) and higher electrical conductivity (912 Ω−1cm−2 at 723 K) of this Ce0.6Yb0.4Fe3CoSb12 double-filled system makes the maximum dimensionless figure-of-merit (ZT, 0.90 at 723 K) in the double-filled specimen higher than those in the single-filled skutterudite systems (YbyFe3CoSb12, and CeyFe3CoSb12) previously reported by us. We found that filling with the Ce4+-rattler was more effective than filling with the Yb2+ for reducing the lattice thermal conductivity in the Cey Yb1−yFe3CoSb12 double-filler system. The Yb2+-filler effectively increased the electrical conductivity of the p-type Fe3CoSb12 filled skutterudite by modifying the carrier concentration.