Low thermal conductivity materials for advanced thermoelectric applications.

Abstract

In this work the thermoelectric properties of pure and doped half-Heusler compounds have been studied over a wide temperature range from 2K to 1050K. We have successfully reduced the thermal conductivity by a factor of three through isoelectronic alloying of Zr and Hf on the M site. A semimetal to semiconductor transition was found for pure half-Heusler compounds between 100K to 150K. The transition temperatures depend strongly on the purity of the starting materials. A small amount of doping (less than 2 at%) with Sb and Bi reduces the electrical resistivity by nearly two orders of magnitude without significantly decreasing the thermopower. Our high temperature (300K to 1050K) studies show a broad maximum in the power factor around 750K which exceeds 35muW/cm-K<super>2</super> for the Zr_0.5Hf_0.5NiSN_0.99Sb_0.01 sample. The estimated <italic>ZT</italic> value is about 0.5 at 750K. In order to rival the current best thermoelectrics in this temperature range, further reduction of the thermal conductivity needs to be achieved. Fe doping on the Co site of skutterudites was also studied in a series of samples of the form Co_1--xFe_xSb₃ with x = 0, 0.5, 1, 2, 5 and 10 at%. Our magnetometry study and Hall data indicate that Fe takes on a magnetic low-spin <italic>d</italic><super>5</super> electronic configuration in the lattice of skutterudites. This is in contrast to Co, which has enough outer shell electrons to fully populate the non-bonding electron orbitals, giving a non-magnetic spin-paired <italic>d</italic><super> 6</super> electronic configuration. This is also in contrast to Fe in phosphide skutterudites in which Fe does not carry a moment. This difference explains why the (Fe, Sb)-containing skutterudites do not superconduct while the (Fe, P)-containing skutterudites do. The number of valence electrons available for Fe-Sb bonds in CoSb₃ is the same as that for Co, namely three. The thermal conductivity of CoSb₃ is dramatically suppressed with Fe doping due to an increase in lattice defects, most likely vacancies on the Co site of the skutterudite structure. The suppression of thermal conductivity by Fe doping partially accounts for the high figure of merit in filled skutterudite compounds.