Thermal Conductivity Reduction by Tuning the Rattler Fraction in a p-type Ce _yYb_1−yFe₃CoSb₁₂ Double-filled Skutterudite

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 R_1−yR′_yFe_xCo_4−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 Ce_yYb_1−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 Ce_0.6Yb_0.4Fe₃CoSb₁₂ 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 (Yb_yFe₃CoSb₁₂, and Ce_yFe₃CoSb₁₂) 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 Ce_y Yb_1−yFe₃CoSb₁₂ double-filler system. The Yb²⁺-filler effectively increased the electrical conductivity of the p-type Fe₃CoSb₁₂ filled skutterudite by modifying the carrier concentration.