U Michigan team boosting efficiency of thermoelectric materials; potential of reaching 15-20% efficiency could result in practical applications
|Example of the structure of a partially filled CoSb skutterudite. Source: C. Uher. Click to enlarge.|
Researchers at the University of Michigan and Hong Kong University of Science and Technology are working to improve the thermoelectric (TE) capabilities of filled skutterudites to the point that they could improve the performance of thermoelectric materials from about 5% efficiency to about 15 or 20%, according to Massoud Kaviany, professor in the Department of Mechanical Engineering, and co-author of a paper on the work published in the journal Physical Review Letters.
At that level of efficiency, TE materials become useful in practical applications such as waste heat recovery in vehicles, Kaviany noted. Earlier this year, the Directorate for Engineering at the National Science Foundation in partnership with the US Department of Energy Vehicle Technologies Program issued a solicitation for proposals for thermoelectric waste heat recovery devices for automotive applications. (Earlier post.)
Skutterudites are one of several promising novel thermoelectrics—materials that convert a heat differential to electricity—that have been developed and pursued for more than a decade, notes Ctirad Uher, co-author of the PRL paper and author of a chapter on Skutterudite-based thermoelectrics in the 2006 Thermoelectrics Handbook. The basic skutterudite is a cubic structure of the formula MX3, where M=CO, Rh, Ir and X=P, As, Sb. Skutterudites are some of the highest mobility semiconductors, and have outstanding electronic properties. However, for thermoelectrics, their thermal conductivity is too high, Uher notes.
Efficient thermoelectric materials want to conduct electricity well, and conduct heat poorly. Filling the skutterudite voids with the right material can dramatically reduce the thermal conductivity, and optimize the thermoelectric properties.
In their work, the team showed that the phonon conductivity of filled skutterudites is greatly influenced by the topology of the filler species. (Basically, scattering phonons lowers thermal conductivity, earlier post.) The team identified certain configurations of barium that drastically increase the skutterudites’ thermoelectric efficiency.
We predict (ab initio) the phase diagram of BaxCo4Sb12 and find several stable configurations of Ba ordering over the intrinsic voids. The phonon conductivity predicted using molecular dynamics shows a minimum in the two-phase mixture regime, dominated by significantly reduced long-range acoustic phonon transport.—Kim et al.
We knew that skutterudites are promising materials. But we did not know what features we could manipulate to maximize the conversion of heat into electricity. In this paper, we propose that certain configurations of the filler element barium will be very effective in lowering the materials’ thermal conductivity and thus increasing their conversion efficiency.
This is an important advancement in the sense that it provides guidance for the experimentalists to focus as they try to synthesize highly efficient thermoelectric materials.—Ctirad Uher
Car companies are extremely interested in this technology, Uher said. The ideal environments for these materials are spots where large differences in temperatures exist. One such place is the pipe system of a car between the motor and the catalytic converter.
The research is funded by the University of Michigan’s Center for Solar and Thermal Energy Conversion, which is funded by the US Department of Energy's Office of Basic Energy Sciences.
We explained the physics of these materials for the first time. This will help to advance the development of these materials. If you are designing them based on fundamental physics and materials and not just by trial and error, then you need to know how they work so you can avoid haphazard experimentation.
Hyoungchul Kim, Massoud Kaviany, John C. Thomas, Anton Van der Ven, Ctirad Uher, and Baoling Huang (2010) Structural Order-Disorder Transitions and Phonon Conductivity of Partially Filled Skutterudites. Phys. Rev. Lett. 105, 265901 doi: 10.1103/PhysRevLett.105.265901
Ctirad Uher, Recent Progress in the Development of N-type Skutterudites, 2009 DoE Thermoelectrics Applications Workshop
Chapter 34. Skutterudite-Based Thermoelectrics Ctirad Uher, Thermoelectrics Handbook Macro to Nano; Edited by D. M. Rowe CRC Press 2006. Pages 34-1–34-17 doi: 10.1201/9781420038903.ch34