Controlling reaction process to realize high thermoelectric performance in filled skutterudites

Filling Yb atoms into the nanovoid of skutterudites has been long the main strategy to achieve high thermoelectric performance. However, both the ZT and the Yb actual total filling fraction (ATFF) show great discrepancies among the literature. The underlying mechanisms behind such discrepancy still keep mysterious. Here, we theoretically and experimentally study the solid solution and phase transformation in the un-filled, single-filled, and multiple-filled skutterudites from a kinetics perspective. The ZT and ATFF discrepancies are thus explained by the complex reaction process. Our density functional theory calculations indicate that an extraordinarily large energy barrier exists in the solid solution process of Yb atoms in CoSb₃.The introduction of other filling atoms can further reduce the Yb diffusion rate, putting off the overall reaction process. As the reaction progresses, the ATFF first increases sharply and finally keeps constant, representing the complex phase transition behaviors from non-equilibrium microstructures to equilibrium microstructures. Through optimizing the synthesis route, we successfully promote the reaction process to the ultimate limit in the Yb_0.3Ca_0.1Al_0.1Ga_0.1In_0.1Co_3.75Fe_0.25Sb₁₂ sample. A remarkable theoretical conversion efficiency of 14.12% is achieved, exhibiting excellent stability and reproducibility as expected. Our results provide an intrinsic understanding of chemical doping from materials science essentials, which is applicable for designing other high-performance functional materials.