Synthesis and Thermoelectric Properties of p-Type Double-Filled Ce_1−z Yb_zFe_4−x Co _x Sb₁₂ Skutterudites

p-Type Ce_1−z Yb_zFe_4−x Co _x Sb₁₂ skutterudites were prepared by encapsulated melting and hot pressing. The thermoelectric and transport properties were examined as a function of Ce/Yb double filling fraction and amount of Co substitution. The Hall coefficients and Seebeck coefficients were positive, implying that all specimens had p-type conduction. Carrier concentration decreased with increasing Co and Yb content. Electrical conductivity decreased and Seebeck coefficients increased with increasing Co content, because of charge compensation. In contrast, electrical conductivity increased and Seebeck coefficients decreased with increasing Yb filling fraction, because of the valence difference between Yb^{2 or 3+} and Ce^{3 or 4+}. The lattice thermal conductivity of the double-filled skutterudites was lower than that of the single-filled skutterudites. The thermoelectric properties of the double-filled skutterudites were improved substantially; the maximum power factor, PF = 3.2 mW m⁻¹ K⁻², was achieved at 823 K for Ce_0.25Yb_0.75Fe_3.5Co_0.5Sb₁₂ and the maximum dimensionless figure of merit ZT = 0.87 was achieved at 723 K for Ce_0.75Yb_0.25Fe_3.5Co_0.5Sb₁₂.