report
5.3.3 State of the art

Nanostructures

Theoretical works have proven that small, dimensionally-confined material can exhibit figure of merit better than 1[i],[ii]. ZT values of 2 or 3, i.e. 30% of Carnot efficiency are supposed to be reached, which will allow thermoelectric devices to be competitive with other electric generators.

2D structures such as superlattices[iii],[iv] have a better figure of merit than bulk materials. ZT=2.4 has been theoretically reported by A. Shakouri[v]. 1D structures such as nanowires[vi],[vii],[viii] have an even greater figure of merit. Indeed, on one hand, quantum confinement enhances Seebeck coefficients. The energy spectrum of the charge carriers can be modified thanks to the structure change due to small dimensions or to dispersions interaction with ions and phonons. On the other hand, the surface area increase improves the boundary phonons scattering and therefore reduces the thermal conductivity: the boundaries can be considered as a selective filter of the phonons wavelength. Moreover, when the film thickness is closed to the phonons wavelength, destructive interferences appear (for a review, read[ix]).

This predicted ZT enhancement has been experimentally demonstrated[x]. But for the moment, it seems that the theoretical value of ZT = 3 or more has never been reached.

These results have led to research on 0D structures such as nanoparticles. First it was proposed to embed nanoparticles and nanowires in a host matrix material[xi] or to mix two different kinds of nanoparticles[xii]. These nanocomposites exhibit reduced thermal conductivity. The challenge was to properly choose electronic properties of the materials used so that the electron transport properties are maintained[xiii]. More recently works on pressed Bi nanoparticles have been carried out.[xiv]

Last results concerned the nanostructuration of materials. Grinding BiSb telluride alloys into fine nanopowers and then pressing them into nanocrystalline ingots allows to increase the figure of merit to a value around 1.2 at room temperature and 1.4 at 100°C.[xv]

Materials

Concerning the materials, the most appropriate to low temperature uses are for the moment those based on group-IV tellurides. The matter is that these materials are noxious. So studies have been carried out to use other material such as those based on silicon. The problem remains that the best performance are obtained at high temperature (around 1200°K) as is shown on the following figure[xvi].

Evolution of the figure of merit with the temperature for different alloys

For the moment, those materials are laboratories ones and some are even just at modelling level. So the great challenge is to get to an industrial level and sometimes even to a lab prototype first. Another point is that most advances have been achieved at high temperature and that some low temperature improvements need to be studied.

Non toxic materials, like alloys of strontium titanate, are now intensely studied, particularly by Japanese research teams[xvii].

[i] Effect of quantum-well structures on the thermoelectric figure of merit; L. Hicks and M.S. Dresselhaus: Physical Review B 47, 12727 (1993)

[ii] Thermoelectric figure of merit of a one dimensional conductor; L. Hicks and M.S. Dresselhaus: Physical Review B 47, 16631 (1993)

[iii] Thin-film thermoelectric devices with high room-temperature figures of merit; R. Venkatasubramanian, E. Siivola, T. Colpitts and B. O'Quinn: Nature 413, 597 (2001)

[iv] Nanostructured thermoelectric materials and devices; T. C. Harman, P. J. Taylor, M. P. Walsh: US Patent 2002/0053359 A1 (2002)

[v] Heterostructure integrated thermionic coolers; A. Shakouri, J.E. Bowers: Applied Physics Letters 71, 1234 (1997)

[vi] Thermoelectric properties of superlattices nanowires; Y.-M. Lin and M.S. Dresselhaus: Physical Review B 68, 075304 (2003)

[vii] Enhanced thermoelectric performance of rough silicon nanowires; A.I. Hochbaum, R. Chen, R.D. Delgado, W. Liang, E.C. Garnett, M. Najarian, A. Majumdar and P. Yang: Nature 451, 163 (2008)

[viii] Silicon nanowires as efficient thermoelectric materials; A. I. Boukai, Y. Bunimovich, J. Tahir-Kheli, J.-K. Yu, W.A. Goddard III and J.R. Heath: Nature 451, 168 (2008)

[ix] Nanoscale thermal transport and microrefrigerators on a chip; A. Shakouri: Proceedings of the IEEE 94 (8), 1613 (2006)

[x] Quantum dot superlattice thermoelectric materials and devices; T. Harman, P. Taylor, M. Walsh and B. LaForge: Science 297, 2229 (2002)

[xi] Thermal Conductivity Reduction and Thermoelectric Figure of Merit Increase by Embedding Nanoparticles in Crystalline Semiconductors; W. Kim, J. Zide, A. Gossard, D. Klenov, S. Stemmer, A. Shakouri, A. Majumdar: Physical Review Letters 96, 045901 (2006)

[xii] Thermal conductivity modeling of periodic two-dimensional nanocomposites; R. Yang and G. Chen: Physical Review B 69, 195316 (2004)

[xiii] Nanostructured Thermoelectric Materials: From Superlattices to Nanocomposites; R. Yang and G. Chen: Materials Integration 18, 31 (2005)

[xiv] Thermoelectric properties of pressed bismuth nanoparticles; S.R. Hostlera, Y.O. Qua, M.T. Demkoa, A.R. Abramsona, X. Qiub and C. Burdab: Superlattices and Microstructures 43, 195 (2008)

[xv] High-Thermoelectric Performance of Nanostructured Bismuth Antimony Telluride Bulk Alloys: B. Poudel, O Hao, Y. Ma, Y. Lan, A. Minnich, B. Yu, X. Yan, D. Wang, A. Muto, D. Vashaee, X. Chen, J. Liu, M. S. Dresselhaus, G. Chen and Z. Ren; Science 320, 634 (2008)

[xvi] Microfabricated thermoelectric power-generation devices; J.-P. Fleurial, M.A. Ryan, A. Borshchevsky, W. Phillips, E.A. Kolawa, J.G. Snyder, T. Caillat, T. Kascich, P. Mueller; US Patent 6388185 B1 (2002)

[xvii] Thermoelectrics based on strontium titanate; H. Ohta: Materials Today 10(10), 44 (2007)