Thermoelectrics

Thermoelectric energy conversion exploits the thermal energy carried by charges (electrons and holes). A thermocouple is one example of a temperature difference being used to directly generate electrical power. The efficiency of thermoelectric power generators is determined by the nondimensional figure-of-merit, ZT=S2σT/k, where S is the Seebeck coefficient, σ the electrical conductivity, k the thermal conductivity, and T the absolute temperature. An approach to increasing thermoelectric efficiency is to increase the ratio of electrical to thermal conductivity, σ/k. Thermoelectric materials can be used for power generation as well as solid-state refrigeration.

Figure 1: Click to enlarge

Until recently, the maximum attainable ZT value was around one. However, significant advances have been made over the last 10 years as new thermoelectric materials have been developed through optimization of electron and phonon transport (Figure 1). One particularly fruitful and exciting approach has been the use of nanostructures to reduce phonon thermal conductivity while electrical conductivity is maintained. Our nanocomposites have achieved record-high ZT values in the SiGe and BiSbTe material systems.

Nanostructured Thermoelectrics

Previous studies done by our research group suggest that phonon thermal conductivity reduction in nanostructures is caused by phonon boundary scattering at interfaces. An effective way to increase the number of interfaces is mixing two different nano sized particles. This nano-composite approach may enable us to batch fabricate thermoelectrics for large scale applications.

Experimentally, our current work is focused on characterizing samples synthesized by Prof. Zhifeng Ren's group at Boston College. We measure thermal conductivity, electrical conductivity, and the Seebeck coefficient. We are also using pump-probe transient thermoreflectance to study phonon transport in these materials. Theoretically, we are using Monte Carlo simulation of phonon thermal conductivity, developing models for electron transport in nanocomposites, and interpreting our experimental data to guide the material synthesis. We have introduced electron and phonon grain boundary scattering models for use with the Boltzmann transport equation which are successful in explaining the observed data.

Selected References

  1. Goldsmid, H.J., 1964. Thermoelectric Refrigeration, Plenum Press, New York.
  2. R. Venkatasubramanian, E. Silvona, T. Colpitts, and B. O'Quinn, Nature 413, 597 (2001).
  3. T.C. Harman, P.J. Taylor, M.P. Walsh, and B. E. LaForge, Science 297, 2229 (2002).
  4. Chen, G., 2001. Phonon Transport in Low-Dimensional Structures, Semiconductors and Semimetals 71: 203-259.
  5. Yang, R.G., and Chen, G., 2004. "Thermal conductivity modeling of periodic two-dimensional nanocomposites", Phys. Rev. B, 69: 195316.
  6. Bed Poudel, Qing Hao, Yi Ma, Yucheng Lan, Austin Minnich, Bo Yu, Xiao Yan, Dezhi Wang, Andrew Muto, Daryoosh Vashaee, Xiaoyuan Chen, Junming Liu, Mildred S. Dresselhaus, Gang Chen, and Zhifeng Ren, High Thermoelectric Performance of Nanostructured Bismuth Antimony Telluride Bulk Alloys, Science (Research Article), Vol. 320, pp. 634-638, May 2, 2008 (first appeared in Science Express Article 1156446, March 20, 2008)
  7. G.H. Zhu, H. Lee, Y.C. Lan, X.W. Wang, G. Joshi, D.Z. Wang, J. Yang, D. Vashaee, H. Guilbert, A. Pillitteri, M.S. Dresselhaus, G. Chen, and Z.F. Ren, Increased phonon scattering by nanograins and point defects in nanostructured silicon with a low concentration of germanium, Physical Review Letters, Vol. 102, 196803, 2009.
  8. Y. Lan, B. Poudel, Y. Ma, D. Wang, M.S. Dresselhaus, G. Chen, and Z.F. Ren, Structure study of bulk nanograined thermoelectric bismuth antimony telluride, Nano Letters, Vol. 9, pp 1419-1422, 2009.
  9. A.J. Minnich, M.S. Dresselhaus, Z.F. Ren, and G. Chen, Bulk nanostructured thermoelectric materials: current research and future prospects, Energy and Environmental Science, Vol. 2 pp. 466-479, 2009.
  10. G. Joshi, H. Lee, Y. Lan, X. Wang, G. Zhu, D. Wang, R. Gould, D.C. Cuff, M.Y. Tang, M.S. Dresselhaus, G. Chen, and Z. Ren, High Thermoelectric Figure of Merit in Nanostructured p-type Silicon Germanium Bulk Alloys, Nano Letters, Vol. 8, pp. 4670-4674, Dec. 2008.
  11. Yi Ma, Qing Hao, Bed Poudel, Yucheng Lan, Bo Yu, Dezhi Wang, Gang Chen, and Zhifeng Ren, Enhanced Thermoelectric Figure-of-Merit in p-type Nanostructured Bismuth Antimony Tellurium Alloys Made from Elemental Chunks, Nano Letters, Vol. 8, No. 8, pp. 2580-2584 (2008)