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. Angewandte Reviews builds up inside the material in response to the temperature gradient can power an external consumer load.[79a] The efficiency of thermoelectric (TE) conversion is largely determined by ZT=S2T/1k, in which ZT is the TE figure of merit of the materials and S, 1, k, and T are the Seebeck coefficient, electrical resistivity, thermal conductivity of the material, and absolute temperature, respectively.[81] A large ZT value corresponds to efficient TE conversion and is thus desired for practical applications, for which TE conversion must be competitive with conventional power generation.[82] It may be possible to make TE conversion competitive as an energy source by reducing the thermal conductivity of the material used, increasing the electrical conductivity, and maximizing the concentration of charge carriers. Intensive efforts in the search for and engineering of materials with optimized TE characteristics have followed two main approaches: the development of advanced bulk TE materials and the investigation of low-dimensional material systems.[83] 3.3.1. Advanced Bulk Thermoelectric Materials A synergistic “rattling” effect enabled by void-filling by extrinsic guest atoms has resulted in significant enhancement of the performance of skutterudite-based TE devices. For example, a ZT value of 1.25 was achieved at elevated temperatures by simultaneously reducing the lattice thermal conductivity and further optimizing the carrier concentra- tion.[84] A similar strategy has also been investigated in clathrates and half-Heusler intermetallic alloys,[79c, 83c] with recent progress yielding Z T = 1.35 at 900 K for Ba8Ga16Ge30 and ZT=0.4 for n-type clathrates at room temperature.[85] These results demonstrate the potential of such materials to replace conventional high-temperature TE materials, such as p-type Si–Ge alloys, in future applications.[86] Metal oxides have also been investigated for high-temperature TE appli- cations, with promising results, in particular for p-type TE materials.[87] Research in the design of various derivatives of chalcogenide compounds, which are conventionally the most widely used TE materials for near-room-temperature and mid-temperature applications, has yielded TE devices with superior performance:[88] not only has TE performance in conventional temperature ranges been improved, but the application of these materials has been extended to even lower temperatures. Despite the potential exhibited by bulk TE materials and the progress made, the low efficiency of current TE devices prohibits their wider application. Recent efforts in the fabrication of nanostructured bulk TE materials, however, have led to significant progress in the enhancement of TE conversion. Bulk nanocomposite TE materials have shown the most promise for commercial applications owing to the feasibility of their large-scale production and their compat- ibility with existing TE-device configurations.[79a,b] Nanocom- posites have caused a remarkable increase in the Z T value in various materials systems[88c,89] owing to the fact that the nanostructures within these materials are smaller than the phonon mean free path while larger than the charge-carrier mean free path. As a result, the scattering of phonons at the interfaces is stronger than the scattering of charge carriers.[79a] Z. L. Wang and W. Wu High-performance solar thermoelectric generators (STEGs) based on bulk nanostructured Bi2Te3 alloys were recently described. With an efficiency in the solar conversion of thermal into electric power of around 4.6 %, a value seven to eight times higher than the previously reported best value for a flat-panel STEG, these STEGs exhibit potential for practical large-scale applications (Figure 5).[90] Although much progress has been made, many factors, such as the control of nanostructure formation and mechanisms for the enhanced TE performance of nanocomposites, need to be better understood. Furthermore, high-performance TE mate- rials need to be developed without the incorporation of toxic materials, such as tellurium and lead, for sustainable and environmentally friendly applications. Figure 5. Structure of solar thermoelectric generators (STEGs) based on bulk nanostructured materials. a) STEG cell; b) schematic illustra- tion of thermal concentration; c) photograph of a real STEG device (top view; from Ref. [90], Copyright 2011 Nature Publishing Group). 3.3.2. Low-Dimensional Nanostructured TE Materials Inspired by previous theoretical predications[91] and enabled by the recently acquired ability to synthesize nano- structured materials of various dimensions,[92] new effective approaches for the modulation and improvement of the TE &&&& www.angewandte.org 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2012, 51, 2 – 24 These are not the final page numbers! Ü ÜPDF Image | Self-Powered Nanosystems
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