Nanostructure features, phase relationships and thermoelectric properties of melt-spun and spark-plasma-sintered skutterudites

Reduction of thermal conductivity remains a main approach relevant to enhancement of figure-of-merit of most thermoelectric materials. Melt spinning combined with spark plasma sintering appears to be a vital route towards fine-grain skutterudites with improved thermoelectric performance. However, upon high-temperature processing the Fe4-xCoxSb12-based skutterudites are prone to decompose into multiple phases, which deteriorate their thermoelectric performance. In this study we addressed the effects of combined melt spinning and spark plasma sintering on the phase composition and microstructural properties of filled Fe4-xCoxSb12 as well as their influence on thermoelectric characteristics of these compounds. The crystallites of filled Fe4-xCoxSb12 were effectively reduced to sizes below 100 nm upon melt spinning, but also severe decomposition with weakly preserved nominal phase was observed. Spark plasma sintering of melt spun skutterudites resulted in even further reduction of crystallites. Upon short annealing and sintering the n-type materials easily restored into single-phase filled CoSb3 with nanoscale features preserved, while secondary phases of FeSb2 and Sb remained in p-type compounds. Relatively high figure-of-merit ZTmax of 0.9 at T ~ 400 °C has been gained in nanostructured YbxCo4Sb12, however, no significant reduction of thermal conductivity was observed. Abundant impurities in p-type filled Fe4-xCoxSb12 led to drastic drop in their ZT, which even further degraded upon thermal cycling.