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      Realizing a stable high thermoelectric zT ∼ 2 over a broad temperature range in Ge1−x−yGaxSbyTe via band engineering and hybrid flash-SPS processing

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          Abstract

          We report a remarkably high and stable thermoelectric zT ∼ 2 by manipulating the electronic bands in hybrid flash-SPSed Ga–Sb codoped GeTe.

          Abstract

          We report a remarkably high and stable thermoelectric figure of merit zT close to 2 by manipulating the electronic bands in Ga–Sb codoped GeTe, which has been processed by hybrid flash-spark plasma sintering. According to the experimental results and first-principles calculations, the vast enhancement achieved in the thermopower due to codoping of Ga (2 mol%) and Sb (8 mol%) in GeTe is attributed to a concoction of reasons: (i) suppression of hole concentration; (ii) improved band convergence by decreasing the energy separation between the two valence band maxima to 0.026 eV; (iii) Ga predominantly contributing to the top of the valence band in Ga–Sb codoped GeTe, despite the Ga-induced resonance state not being located at a favorable position near the Fermi level; (iv) active participation of several bands increasing the hole carrier effective mass; (v) facilitating band degeneracy by reducing the R3 mFmm structural transition temperature from 700 K to 580 K. The synergy between these complementary and beneficial effects, in addition to the reduced thermal conductivity, enabled the flash sintered Ge 0.90Ga 0.02Sb 0.08Te composition to not only exhibit a peak of zT of ∼1.95 at 723 K, but also to maintain/stabilize its high performance over a broad temperature range (600–775 K), thus making it a serious candidate for mid-temperature range energy harvesting devices.

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              Convergence of electronic bands for high performance bulk thermoelectrics.

              Thermoelectric generators, which directly convert heat into electricity, have long been relegated to use in space-based or other niche applications, but are now being actively considered for a variety of practical waste heat recovery systems-such as the conversion of car exhaust heat into electricity. Although these devices can be very reliable and compact, the thermoelectric materials themselves are relatively inefficient: to facilitate widespread application, it will be desirable to identify or develop materials that have an intensive thermoelectric materials figure of merit, zT, above 1.5 (ref. 1). Many different concepts have been used in the search for new materials with high thermoelectric efficiency, such as the use of nanostructuring to reduce phonon thermal conductivity, which has led to the investigation of a variety of complex material systems. In this vein, it is well known that a high valley degeneracy (typically ≤6 for known thermoelectrics) in the electronic bands is conducive to high zT, and this in turn has stimulated attempts to engineer such degeneracy by adopting low-dimensional nanostructures. Here we demonstrate that it is possible to direct the convergence of many valleys in a bulk material by tuning the doping and composition. By this route, we achieve a convergence of at least 12 valleys in doped PbTe(1-x)Se(x) alloys, leading to an extraordinary zT value of 1.8 at about 850 kelvin. Band engineering to converge the valence (or conduction) bands to achieve high valley degeneracy should be a general strategy in the search for and improvement of bulk thermoelectric materials, because it simultaneously leads to a high Seebeck coefficient and high electrical conductivity. ©2011 Macmillan Publishers Limited. All rights reserved
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                Author and article information

                Journal
                ICFNAW
                Inorganic Chemistry Frontiers
                Inorg. Chem. Front.
                Royal Society of Chemistry (RSC)
                2052-1553
                January 15 2019
                2019
                : 6
                : 1
                : 63-73
                Affiliations
                [1 ]Univ. Rennes
                [2 ]Ecole Nationale Supérieure de Chimie de Rennes
                [3 ]CNRS
                [4 ]ISCR – UMR 6226
                [5 ]F-35000 Rennes
                [6 ]IPR – UMR 6251
                [7 ]France
                [8 ]School of Engineering and Materials Science
                [9 ]Queen Mary University of London
                [10 ]London E1 4NS
                [11 ]UK
                Article
                10.1039/C8QI00703A
                e5dc718e-7a84-421c-bdc2-d00cf94db782
                © 2019

                http://rsc.li/journals-terms-of-use

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