Influence of filler element and Ni-substitution on thermoelectric properties of multi-filled skutterudites

The influence of multi-filling on the thermoelectric properties was investigated for skutterudites (Ca,Sr,Ba,Mm)yCo4Sb12, where Mm is Mischmetal, a naturally mixed rare earth (Ce,La,Nd,Pr), which throughout this work was treated for convenience as one filler element. Regardless of the filling elements, triple filling shows lower electrical resistivity as well as lattice thermal conductivity and consequently a higher figure of merit ZT than single and double filling, indicating the positive influence of multi-filling on thermoelectric performance. Investigations of triple-filled alloys (Sr,Ba)AYbRCo4Sb12 point out that Yb-filling in multi-filled skutterudites entails better thermoelectric properties than Mm-filling. Also the ratio A:R is a critical parameter to achieve high ZT. Moreover, for (Sr,Ba,Yb)yCo4Sb12, a larger filling level y leads to lower electrical resistivity. Although Ni substitutions for Co in double-filled CayMmy(Co1-xNix)4Sb12 result in complicated compositional dependences for the electrical resistivity and the Seebeck coefficient, they exert almost no influence on the thermal conductivity, and the Ni-free sample shows the highest ZT in CayMmy(Co1-xNix)4Sb12, indicating a different impact of Ni with respect to unfilled (Co1-xNix)4Sb12.