Synthesis and thermoelectric properties of p-type- and n-type-filled skutterudite RyMxCo4−xSb12(R:Ce,Ba,Y;M:Fe,Ni)

Single-phase RyMxCo4−xSb12(x=0–3.0,y=0–0.7) compounds filled by Ce, Ba, and Y, and substituted by Fe and Ni are synthesized by using the solid-state reaction method and melting reaction method. The structure and the thermoelectric properties of RyMxCo4−xSb12 are investigated systematically. The thermal parameter (B) of Ba and Ce filled in Sb-icosahedron voids in the skutterudite structure is larger than that of Sb and Co (Fe). The maximum filling fraction of Ce and Ba (ymax) for RyFexCo4−xSb12 increases with the increasing Fe content, and it is found that the maximum filling fraction of Ba (ymax) is greater than that of CeyFexCo4−xSb12. The filling atoms Ba, Ce, and Y in Sb-icosahedron voids can reduce the lattice thermal conductivity of RyMxCo4−xSb12 compounds remarkably, and the lattice thermal conductivity decreases in the order of ionic radii decreasing of Ba2+, Ce3+, and Y3+. When Ce and Ba filing fraction is 0.3–0.4, the lattice thermal conductivity of RyFexCo4−xSb12 compounds reaches a minimum value. The lattice thermal conductivity can be greatly reduced by substituting Co with Fe or Ni, and compared with Fe substitution, the substituted atoms Ni are more effective in reducing the lattice thermal conductivity. The filling atoms Ba, Ce, and Y, and the substituted atoms Fe and Ni influence electrical transport properties of RyMxCo4−xSb12 compounds significantly. The carrier concentration and electrical conductivity of p-type RyFexCo4−xSb12 increase with the increasing Fe content but decrease with the increasing R filling fraction. At the same carrier concentration, electrical conductivity of p-type BayFexCo4−xSb12 is larger than that of p-type CeyFexCo4−xSb12. Electrical conductivity of n-type BayNixCo4−xSb12 increases with the increasing Ni content remarkably. The Seebeck coefficient of p-type RyFexCo4−xSb12 increases with the increasing Ce and Ba filling fraction and with the decreasing Fe content, and Seebeck coefficient of n-type BayNixCo4−xSb12 decreases with the increasing Ni content. In the present study, the obtained maximum ZT values reach 1.1 and 1.25 for p-type Co-rich Ce0.28Fe1.5Co2.5Sb12 (at 750K) and n-type Ba0.30Ni0.05Co3.95Sb12 (at 900K), respectively.