Two separate research collaborations have recently reported advances in the efficiency of thermoelectric materials in converting heat to electricity. A collaboration including researchers from Boston College, MIT, the University of Virginia and Clemson University have achieved a peak ZT (thermoelectric figure of merit) of 0.8 at 700 °C (973 K), in half-Heusler alloys—about 60% higher than the best reported ZT of 0.5 for the materials and possibly good enough for consideration for waste heat recovery in automotive exhaust systems. The study by Yan et al. was published in the ACS journal Nano Letters.
And a team from Northwestern University and the University of Michigan reported experimentally achieving a ZT of 1.7 at ~800 K (527 °C) by structurally modifying lead telluride. The high thermoelectric figure of merit is expected to enable the conversion of 14% of heat waste to electricity. The study by Biswas et al. was published in Nature Chemistry.
|ZT of ball-milled and hot-pressed sample in comparison with that of the ingot. Credit: ACS, Yan et al. Click to enlarge.|
Yan et al. Half-Heusler alloys (ferromagnetic intermetallic alloys with a particular composition and face-centered cubic crystal structure) have been identified as prospective materials for high temperature thermoelectric power generation due to their high temperature stability and abundance if their dimensionless thermoelectric figure of merit (ZT) could be made high enough. One of the challenges with half-Heuslers is their high thermal conductivity (reported higher than 4W m-1 K-1) which diminishes the thermoelectric effect.
Yan et al. used a nanocomposite approach using ball milling and hot pressing to embed nanostructures and reduce the grain size of the material to achieve their peak ZT of 0.8. The improvement comes from a simultaneous increase in Seebeck coefficient and a significant decrease in thermal conductivity due to nanostructures, they said.
The samples were made by first forming alloyed ingots using arc melting and then creating nanopowders by ball milling the ingots and finally obtaining dense bulk by hot pressing.
Although we have achieved a significant enhancement in ZT of p-type half-Heusler alloys, there remains much room for further improvement. The average grain size of 100-200 nm of our hot pressed bulk samples is much larger than the 5-10 nm of the ball-milled precursor nanopowders, which is why the lattice thermal conductivity is still very high. If we can preserve the grain size of the original nanopowders, a much lower thermal conductivity and thus a much higher ZT can be expected. Besides boundary scattering, minor dopants may also be introduced to enhance the alloy scattering, provided that they do not deteriorate the electronic properties. The ZT values we report here are very reproducible within 5% from run to run on more than 10 samples made under the similar conditions.—Yan et al.
This work paves the way for half-Heuslers to become competitive candidates for use in automotive exhaust systems and other power generation systems, Xiao Yan, a research associate in Prof. Zhifeng Ren’s group at Boston College, suggested.
|ZT as a function of temperature for PbTe–SrTe samples doped with 1% Na2Te. The ZT of a control sample with no SrTe is also shown for comparison. Credit: Nature Chemistry, Biswas et al. Click to enlarge.|
Biswas et al. Although nanostructuring in bulk materials can dramatically reduce thermal conductivity (enhancing the thermoelectric effect), it can simultaneously increase the charge carrier scattering, which has a detrimental effect on the carrier mobility (dampening the thermoelectric effect), Biswas et al. note in their paper.
The team experimentally achieved concurrent phonon blocking and charge transmitting via the endotaxial placement of SrTe (strontium telluride) nanocrystals at concentrations as low as 2% incorporated in a PbTe (lead telluride, a well-known thermoelectric material for use in the temperature range 400-900 K) matrix doped with Na2Te (sodium telluride).
This effectively inhibits the heat flow in the system but does not affect the hole mobility, allowing a large power factor to be achieved. The crystallographic alignment of SrTe and PbTe lattices decouples phonon and electron transport and this allows the system to reach a thermoelectric figure of merit of 1.7 at ~800 K.—Biswas et al.
It has been known for 100 years that semiconductors have this property that can harness electricity. To make this an efficient process, all you need is the right material, and we have found a recipe or system to make this material.—,Mercouri Kanatzidis, the Charles E. and Emma H. Morrison Professor of Chemistry at Northwestern
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 (2010) Enhanced Thermoelectric Figure of Merit of p-Type Half-Heuslers. Nano Lett., Articles ASAP doi: 10.1021/nl104138t
Kanishka Biswas, Jiaqing He, Qichun Zhang, Guoyu Wang, Ctirad Uher, Vinayak P. Dravid and Mercouri G. Kanatzidis (2011) Strained endotaxial nanostructures with high thermoelectric figure of merit. Nature Chemistry (2011) doi: 10.1038/nchem.955