High-performance three-stage cascade thermoelectric devices with 20% efficiency

Abstract

The use of advanced materials 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 and enhanced TAGS-85 [(AgSbTe2)(15)(GeTe)(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 low-temperature stage materials, nano-bulk Bi2Te3-x Sb (x) and Bi2Te3-x Se (x) , 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 similar to 1.26 and similar to 1.7 in the 27A degrees C to 100A degrees C 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 conventional or neat TAGS-85, resulting in a ZT = 1.5 at 400A degrees C. The incorporation of these improved materials resulted in a peak device conversion efficiency of similar to 20% at a temperature difference of 750A degrees C when corrected for radiation heat losses and thermal conduction losses through the lead wires. These high-efficiency results were shown to be reproducible across multiple cascade devices