MmFe4Sb12- and CoSb3-based nano-skutterudites prepared by ball milling: Kinetics of formation and transport properties

Experiments of mechanical alloying/milling of filled and unfilled skutterudites have been performed under various ball milling conditions in order to (a) optimise the preparation of nano-sized skutterudites and (b) to elucidate formation and/or decomposition of unfilled (CoSb3) and filled skutterudites MmyFe4Sb12 (Mm means ‘mischmetal’). State-of-the-art X-ray techniques have been used to evaluate the size distribution of the smallest crystallographically undisturbed regions (coherent-scattering-domains), which in many cases are significantly smaller than the physical size of grains. The influence of oxides in in situ precipitations on the stability of the nanostructures at 600 °C was investigated and compared with oxide-free nano-sized skutterudites. It was shown that at 600 °C the crystallites of nano-sized CoSb3 grow rapidly reaching micro-size after 90 h, whilst in situ oxide stabilized nanostructures MmFe4Sb12 with crystallite size below 200 nm do not show coagulation even after 600 h of heat treatment at 600 °C. The nano-sized oxide composite has a significantly lower lattice thermal conductivity resulting in an improvement of the thermoelectric figure of merit ZT740 K = 0.52 being about 20% higher than for oxide-free macro-crystalline MmyFe4Sb12 as reference. The composition of nano-composite was optimised for large-scale production in oxidizing atmosphere. Attempts to evaluate the dislocation density for ball-milled samples indicate that it is only slightly above the lower limit of resolution of the method of about 1012 m−2.