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