Abstract: The great pressure of energy shortage has made CoSb₃ materials with excellent mechanical stability in the mid-temperature region favored for the integration of thermoelectric devices. However, their excessive lattice thermal conductivity and poor Seebeck coefficient lead to low energy conversion efficiency. Filling Yb into the lattice void to optimize the band structure and regulate the chemical potential is an indispensable means for improving the thermoelectric properties of CoSb₃-based materials, while the phase structure and thermoelectric properties vary with the preparation process. This motivates the current work to focus on the influence of annealing temperature on the microstructure and thermoelectric properties of Yb-filled CoSb₃. Experimental analysis and theoretical model elucidated that an increase in annealing temperature can optimize the Yb filling fraction, which simultaneously manipulates the band structure as well as chemical potential, resulting in an excellent electrical property. Furthermore, the phase and microstructure characterization clarify that the annealing temperature can effectively affect the grain size. The complex grain boundary induced by grain refinement, more filled Yb atoms and precipitates strongly scatter wide-frequency phonons, significantly suppressing the lattice thermal conductivity. As a result, a superior dimensionless figure of merit (ZT) value of ∼1.33 at 823 K and an average ZT_ave of ∼0.9 (323-823 K) were achieved in the Yb_0.4Co₄Sb₁₂ sample annealed at 923 K, and the calculated conversion efficiency could reach ∼13%. This work provides a unique paradigm to improve thermoelectrics in the filled CoSb₃-based skutterudites by annealing engineering.

Key words: CoSb3 skutterudite, Annealing engineering, Grain refinement, Phonon scattering, Thermoelectric optimization