Synthesis of nanocrystalline filled skutterudites by mechanical alloying

We report on the preparation of nanocrystalline filled skutterudites in the series Ce_yFe_4-xCo_xSb₁₂ by mechanical alloying. X-ray diffraction and energy dispersive X-ray spectroscopy reveal that skutterudites can be prepared with an homogeneity range larger than what can be obtained by powder metallurgy (PM). Interestingly, no after-milling thermal treatment is required to prepare essentially single phase samples (~90-95%). This is an important difference with skutterudites prepared by PM which only form after long term annealing below the peritectic decomposition temperature. Differential thermal analysis shows that grain growth occurs above ~350degC and that the peritectic decomposition temperature is similar to PM- (i.e. 750degC for x = 1). Grain sizes are in the 12-30 nm range as deduced from Rietveld refinements of X-ray diffraction data and scanning electron microscopy. Cerium is trivalent in nanocrystalline filled skutterudites, as it is in samples prepared by PM. Both p-type and n-type skutterudites can be obtained but values of the Seebeck coefficient are of the same order of magnitude as in PM samples only for p-type skutterudites