Thermoelectric performance of mischmetal skutterudites MmyFe4−xCoxSb12 at elevated temperatures

Thermoelectric behaviour was studied for MmyFe4−xCoxSb12 with x = 0 and 1 (Mm is mischmetal) within the temperature range from 300 K to 800 K and was compared with data for corresponding CeyFe4−xCoxSb12. No decrease of thermal conductivity at high temperatures was observed changing from a single atom filler (Ce) to a multi-element filler (Mm). The electrical resistivity of MmyFe4−xCoxSb12 was found to be lower than or similar to that of CeyFe4−xCoxSb12. Accordingly the figure of merit for MmFe3CoSb12 exceeds 1 at temperatures above 600 K. The influence of several parameters such as impurity phases and grain size on the thermoelectric performance of MmyFe4−xCoxSb12 was investigated. FeSb2 and Sb have little effect on the figure of merit whilst the presence of MmSb2 and oxides reduces the thermoelectric performance, when these phases are segregated at the grain boundaries. The decrease of the crystallite size from micro scale (20–100 μm) to nano-scale (100–300 nm) in all cases improves the figure of merit by about 20%. Dislocations have no influence on ZT since the dislocation density after hot pressing is as low as 1012 m−2.