High thermoelectric figure of merit in well optimized YbyCo4Sb12

Abstract

By implementing X-ray absorption spectroscopy among other techniques, ytterbium was inferred to display a valence of 2 in the thermoelectric Yb_yCo₄Sb₁₂ skutterudites (0.10 ≤ y ≤ 0.20). Consequently, the correlation PF(A³⁺) < PF(A²⁺) < PF(A¹⁺) between the valence of the filler atom and the power factor (PF) could be established in a generic set of A_yCo₄Sb₁₂ skutterudites (A = filler atom and electron donor). By grinding the Yb_yCo₄Sb₁₂ samples under inert atmosphere prior to densification by Spark Plasma Sintering, no spurious oxidation of Yb occurred, conversely to what happens when grinding in air. This led to the effective ytterbium concentration being strictly equal to the nominal one, up to the solubility limit determined to be y_lim = 0.41 ± 0.02 mol per formula unit when the Yb_yCo₄Sb₁₂ compounds are synthesized at 1073 K. The maximum PF = 4.2 mW m⁻¹ K⁻² at 300 K is reached in Yb_0.15Co₄Sb₁₂, for the first time at the theoretically predicted electron concentration n_opt = 1.8 ± 0.2 × 10²⁰ cm⁻³, thus evidencing the optimization of this compound. Finally, Yb_0.15Co₄Sb₁₂ also displays the best thermoelectric properties in the series, with a figure of merit ZT_max = 1.4 and PF = 5.5 mW m⁻¹ K⁻² at 750 K.