CoSb3-based skutterudite nanocomposites prepared by cold sintering process with enhanced thermoelectric properties

We show here for the first time the use of a cold sintering process (CSP) to sinter CoSb3-based thermoelectric materials. CSP at 150 {\deg}C for 90 min under a uniaxial pressure of 750 MPa yields pieces with a relative density of 86 %, which is increased to around 92 % after a post-annealing at temperatures > 500 {\deg}C in Ar atmosphere. The reported CSP produces Te doped-CoSb3 nanocomposites with similar morphological and structural characteristics to the starting nanopowders obtained by ball milling in air atmosphere. The post-thermal treatment induces grain coalescence and grain growth, crystallite size growth as well as compositional changes in the nanocomposite, decreasing the amount of the main phase, CoSb3, and increasing the weight of secondary phase, CoSb2, up to a 30 wt% at 600 {\deg}C. Remarkably, the average valence for the Co, Sb and Te absorbing atoms is neither transformed by the sintering process nor by the subsequent heat treatment. The functional response of the sintered thermoelectric nanocomposites exhibits a maximum figure of merit of 0.12(3) at room temperature for the nanocomposites sintered by CSP with a subsequent post-annealing at 500 {\deg}C. This is mainly due to its low thermal conductivity in comparison with similar powders sintered by other approaches, and it is explained by the morphological and structural properties. These findings represent an attractive alternative for obtaining efficient thermoelectric skutterudites by a scalable and cost-effective route.