Nanotechnology Boosts Thermoelectric Performance
Researchers from Boston College and MIT have turned to nanotechnology to achieve a 60-90% increase in the thermoelectric figure of merit (ZT) of a p-type quinary compound semiconductor known as a half-Heusler. ZT is a term scientists use to measure a material’s relative thermoelectric performance. The work by Xiao Yan and co-workers paves the way for a new generation of cleaner products, in applications as diverse as semiconductors, air conditioners, car exhaust systems and solar power technology. The material improved by the team was a half-Heusler p-doped bulk semiconductor compound with the chemical formula Zr0.5Hf0.5CoSb0.8Sn0.2. This material isan attractive prospect to the thermoelectric community due to its thermal stability, good mechanical sturdiness, non-toxicity and inexpensiveness. However, the application of half-Heuslers is limited due to their poor thermoelectric performance - peak ZT is around 0.5 at 700oC for bulk ingots.A good thermoelectric material should have a ZT above 1 across this temperature range. The scientists have increased the ZT value of p-type half-Heusler to 0.8 at 700oC. Moreover, according to Xiao Yan, their techniques are less time-consuming and cheaper than conventional ingot preparation methods. “This method is low cost and can be scaled for mass production. This represents an exciting opportunity to improve the performance of thermoelectric materials in a cost-effective manner,” said Zhifeng Ren of Boston College. The researchers formed alloyed ingots using arc melting, before creating nanopowders by ball milling the ingots. Application of hot pressing yielded dense bulk material. Transport property measurements and microstructural studies were performed on the nanostructured samples and bulk ingots. Results revealed that the ZT improves thanks mostly to the low thermal conductivity arising from enhanced phonon scattering at grain boundaries and defects in the material and partially to the high “Seebeck” coefficient. The Seebeck coefficient, or so-called thermopower, is a measure of the magnitude of an induced thermoelectric voltage with respect to the temperature difference across that material. “In other words, the resistance to heat flow increases without a degradation or even with an enhancement in the material’s electrical properties,” explains Gang Chen of MIT. The US team lead by Ren and Chen is still making continuous efforts in preventing grain growth during press, which accounts for them maintaining a large thermal conductivity of half-Heuslers. “Even lower thermal conductivity and thus higher ZT can be expected when average grain sizes are made smaller than 100 nm,” says Ren. Contributions to this work were also made by S. J. Poon from University of Virginia and T. M. Tritt from Clemson University. This work was published in the paper “Enhanced Thermoelectric Figure of Merit of p-Type Half-Heuslers” by Xiao Yan,Giri Joshi,Weishu Liu,Yucheng Lan,Hui Wang,Sangyeop Lee,J. W. Simonson, S. J. Poon, T. M. Tritt, Gang Chen,and Z. F. Ren, in Nano Letters, DOI: 10.1021/nl104138t.
Figure 1. TEMimages of hot-pressed nanostructured samples under (a) low and (b) high magnification. The inset in (a) is the selected area electron diffraction pattern showing the single crystalline nature of the individual grains. Temperature-dependent (c) lattice part of thermal conductivity, and (d) ZT of ball-milled and hot-pressed sample in comparison with that of the ingot.
