Thermoelectric properties of (Yb, Ca)x(Co, Fe)4Sb12 skutterudites

For n-type Yb_xCo₄Sb₁₂ skutterudites, Co and Yb were partially substituted by Fe and Ca, respectively, in order to improve its thermoelectric property. When the Yb content of Yb_xCo_4-yFe_ySb₁₂ is in the range of 0.3lestimesles 0.5 and the Fe content is 0.25 les y les 0.5, the absolute value of Seebeck coefficient increases with the increase of Fe content, while it decreases with the increase of Yb content. The substitution of Fe for Co makes thermal conductivity decreased, for example, from 4.4 W/mK of Yb_0.15Co₄Sb₁₂ to 2.4 W/mK of Yb_0.4Co_3.625Fe_0.375Sb₁₂ at 723 K, which is larger than the increase of electrical resistivity, resulting in improvement of dimensionless figure of merit ZT from 0.7 to 0.9 at 723 K. Further addition of Ca to the Yb_xCo_4-yFe_ySb₁₂ skutterudite makes its electrical resistivity and thermal conductivity much decreased so that its ZT is improved up to ZT = 1.1 in Yb_0.3Ca_0.1Co_3.75Fe_0.25Sb₁₂. Effect of Yb and Fe contents on ZT value is discussed.