ICT2014 Abstracts

The use of advanced materials, combined with an optimized device geometry, has resulted in a significant improvement in thermoelectric device conversion efficiency. Three-stage cascade devices were assembled, consisting of nano-bulk Bi₂Te₃-based materials on the cold side, PbTe combined with improved TAGS-85 for the mid-stage, and half-Heusler alloys for the high-temperature top stage.  In addition, an area aspect ratio optimization process was applied in order to account for asymmetric thermal transport down the individual n- and p-legs. The n- and p-type chalcogenide alloy materials were prepared by high-energy mechanical ball milling and/or cryogenic ball-milling of elementary powders, with subsequent consolidation by high-pressure uniaxial hot-pressing.  The p-type nano- bulk alloys with ultra-low thermal conductivity exhibit a peak ZT of 1.7 between 27°C and 100°C.  Work was also carried out to apply the methods developed for p-type nano-bulk materials to corresponding n-type. We have shown that the nano-bulk Bi₂Te_3-xSb_x and Bi₂Te_3-xSe_x compositions  exhibit a unique mixture of nanoscale features that leads to an enhanced Seebeck coefficient and reduced lattice thermal conductivity, thereby achieving an average  ZT of ~1.26 and ~1.7 in the 27 ºC to 100 ºC range for the n-type and p-type materials, respectively.  Also, the addition of small amounts of selected rare earth elements has been shown to improve the ZT of TAGS-85 by 25%, compared with neat TAGS-85, resulting in a ZT = 1.5 at 400°C. The incorporation of these improved materials resulted in a device conversion efficiency of 26%, with a temperature difference of 750°C.  These results were shown to be reproducible across multiple devices.