Double-filled skutterudites of the type Yb_xCa_yCo ₄Sb₁₂: Synthesis and properties

Filled skutterudites based on CoSb₃ exhibit high ZT values due to the inclusion of filler atoms into voids that comprise the crystal structure of CoSb₃. These atoms act as electron-donating species that dope the parent material, thereby decreasing the electrical resistivity. Additionally, the loosely bound nature of the filler species acts to scatter heat carrying phonons, which reduce the thermal conductivity. Recently it has been reported that filler atoms from different chemical groups could be cofilled into CoSb₃, which further reduces the thermal conductivity, likely by scattering a wider spectrum of phonons. Presented here is the synthesis and transport property evaluation of a series of double-filled skutterudites of the type Yb_xCa_yCo₄Sb₁₂. Good power factors, S²/ρ comparable to Yb-filled skutterudites are obtained for samples with a high Yb to Ca filling ratio. Filling with Ca and Yb did not yield a significant reduction in the thermal conductivity and as a result the ZT values are not improved as compared to Yb-only filled skutterudites although they are much improved as compared to Ca-only filled.