Attempts to further enhance ZT in skutterudites via nano-composites

Abstract Skutterudites are known as excellent thermoelectric p- and n-type materials and have already achieved good efficiencies for the conversion of heat to electricity, but nevertheless researchers try to further enhance the figure of merit, ZT. In the present work the aim was to mix p- and n-type skutterudite powders with additives in order to produce an evenly dispersed distribution of sub micron grains, preferable small nano-particles, which enhance the scattering of the heat carrying phonons of different wavelengths and reduce thermal conductivity without changing electrical resistivity and Seebeck coefficient. Various quantities of three groups of materials (a) nonmetallic oxides (Al 2 O 3 ), (b) metallic oxides (Cu 2 O and La 1.85 Sr 0.15 CuO 4 ) and (c) metallic borides (Fe 2.25 Co 0.75 B and Ta 0.8 Zr 0.2 B) were added to industrially produced p-type (DD y Fe 3 CoSb 12 ) and n-type ((Mm,Sm) y Co 4 Sb 12 ) skutterudite powders. First the influence of pre-sieving and various ball milling conditions before hot pressing were studied, using Al 2 O 3 as additive. As a consequence of these studies pre-sieved powders and high-energy ball milling were used for all following experiments. The goal, to enhance ZT was not reached with Al 2 O 3 and Cu 2 O. La 1.85 Sr 0.15 CuO 4 was successful for the n-type, Fe 2.25 Co 0.75 B for the p-type skutterudites, although ZT-enhancement was small, but with Fe 2.25 Co 0.75 B the maximum ZT could be shifted to lower temperatures, a valuable information for device production. Much better results in respect to ZT values were gained with adding 0.5, 1.0 and 1.5 wt.% Ta 0.8 Zr 0.2 B to p-type DD y Fe 3 CoSb 12 . In this series it was possible to enhance ZT (from ZT ∼ 1.2 to ZT ∼ 1.3) as well as to significantly increase the thermal-electrical conversion efficiency η. In addition, we found that all boride additives enhanced the hardness, elastic moduli and fracture resistance.