GeTe-based alloys have been intensively considered as p-type thermoelectrics for about 50 years, yet existing literature barely discussed the thermoelectric properties of pristine GeTe at high ...temperatures (300–800 K). This work first backs to a fundamental understanding on the thermoelectric transport properties inherent to p-type GeTe, based on more than 50 samples synthesized with expected carrier concentrations ranging from 1 × 1020 to 3 × 1021 cm–3. A thermoelectric figure of merit zT as high as ∼1.7 is found inherent to this compound when it is optimally doped with a Hall carrier concentration of 2.2 ± 10% × 1020 cm–3, offering a reference substance to expose the origins for the high zT in historical GeTe-based alloys. Guided by the above knowledge, further alloying Te with Se in samples with an optimal carrier concentration enables a reduction on the lattice thermal conductivity by ∼40% and eventually leads to a further enhancement on zT (up to 2.0) by ∼20%. This work demonstrates not only GeTe as an inherently high performance thermoelectric matrix compound but also its availability for further improvements by additional strategies.
Low-grade heat accounts for >50% of the total dissipated heat sources in industries. An efficient recovery of low-grade heat into useful electricity not only reduces the consumption of fossil-fuels ...but also releases the subsequential environmental-crisis. Thermoelectricity offers an ideal solution, yet low-temperature efficient materials have continuously been limited to Bi
Te
-alloys since the discovery in 1950s. Scarcity of tellurium and the strong property anisotropy cause high-cost in both raw-materials and synthesis/processing. Here we demonstrate cheap polycrystalline antimonides for even more efficient thermoelectric waste-heat recovery within 600 K than conventional tellurides. This is enabled by a design of Ni/Fe/Mg
SbBi and Ni/Sb/CdSb contacts for both a prevention of chemical diffusion and a low interfacial resistivity, realizing a record and stable module efficiency at a temperature difference of 270 K. In addition, the raw-material cost to the output power ratio in this work is reduced to be only 1/15 of that of conventional Bi
Te
-modules.
High-efficiency thermoelectric materials require a high conductivity. It is known that a large number of degenerate band valleys offers many conducting channels for improving the conductivity without ...detrimental effects on the other properties explicitly, and therefore, increases thermoelectric performance. In addition to the strategy of converging different bands, many semiconductors provide an inherent band nestification, equally enabling a large number of effective band valley degeneracy. Here we show as an example that a simple elemental semiconductor, tellurium, exhibits a high thermoelectric figure of merit of unity, not only demonstrating the concept but also filling up the high performance gap from 300 to 700 K for elemental thermoelectrics. The concept used here should be applicable in general for thermoelectrics with similar band features.
Compared to commercially available p‐type PbTe thermoelectrics, SnTe has a much bigger band offset between its two valence bands and a much higher lattice thermal conductivity, both of which limit ...its peak thermoelectric figure of merit, zT of only 0.4. Converging its valence bands or introducing resonant states is found to enhance the electronic properties, while nanostructuring or more recently introducing interstitial defects is found to reduce the lattice thermal conductivity. Even with an integration of some of the strategies above, existing efforts do not enable a peak zT exceeding 1.4 and usually involve Cd or Hg. In this work, a combination of band convergence and interstitial defects, each of which enables a ≈150% increase in the peak zT, successfully accumulates the zT enhancements to be ≈300% (zT up to 1.6) without involving any toxic elements. This opens new possibilities for further improvements and promotes SnTe as an environment‐friendly solution for conventional p‐PbTe thermoelectrics.
A combination of band convergence by alloying with MnTe and interstitial defect scattering by alloying with Cu2Te, each of which enables an ≈150% increase in the peak thermoelectric figure of merit, zT of SnTe, successfully accumulates the zT enhancements to be ≈300% without involving any toxic elements. This promotes SnTe as an eco‐friendly solution for p‐PbTe thermoelectrics.
To minimize the lattice thermal conductivity in thermoelectrics, strategies typically focus on the scattering of low-frequency phonons by interfaces and high-frequency phonons by point defects. In ...addition, scattering of mid-frequency phonons by dense dislocations, localized at the grain boundaries, has been shown to reduce the lattice thermal conductivity and improve the thermoelectric performance. Here we propose a vacancy engineering strategy to create dense dislocations in the grains. In Pb
Sb
Se solid solutions, cation vacancies are intentionally introduced, where after thermal annealing the vacancies can annihilate through a number of mechanisms creating the desired dislocations homogeneously distributed within the grains. This leads to a lattice thermal conductivity as low as 0.4 Wm
K
and a high thermoelectric figure of merit, which can be explained by a dislocation scattering model. The vacancy engineering strategy used here should be equally applicable for solid solution thermoelectrics and provides a strategy for improving zT.
Advancing thermoelectric n‐type Mg3Sb2 alloys requires both high carrier concentration offered by effective doping and high carrier mobility enabled by large grains. Existing research usually ...involves chalcogen doping on the anion sites, and the resultant carrier concentration reaches ≈3 × 1019 cm−3 or below. This is much lower than the optimum theoretically predicted, which suggets that further improvements will be possible once a highly efficient dopant is found. Yttrium, a trivalent dopant, is shown to enable carrier concentrations up to and above ≈1 × 1020 cm−3 when it is doped on the cation site. Such carrier concentration allows for in‐depth understand of the electronic transport properties over a broad range of carrier concentrations, based on a single parabolic band approximation. As well as reasonably high carrier mobility in coarse‐grain materials sintered by hot deforming and fusing of large pieces of ingots synthesized by melting, higher thermoelectric performance than earlier experimentally reported for n‐type Mg3Sb2 is found. In particular, the thermoelectric figure of merit, zT, is even higher than that of any known n‐type thermoelectric, including Bi2Te3 alloys, within 300–500 K. This might pave the way for Mg3Sb2 alloys to become a realistic material for n‐type thermoelectrics for sustainable applications.
Yttrium is found to be an effective dopant for obtaining electron concentrations as high as ≈1020 cm−3 in Mg3SbBi. This leads to extraordinary thermoelectric performance and high thermal stability.
Conventional strategies for advancing thermoelectrics by minimizing the lattice thermal conductivity focus on phonon scattering for a short mean free path. Here, a design of slow phonon propagation ...as an effective approach for high‐performance thermoelectrics is shown. Taking Ag8SnSe6 as an example, which shows one of the lowest sound velocities among known thermoelectric semiconductors, the lattice thermal conductivity is found to be as low as 0.2 W m−1 K−1 in the entire temperature range. As a result, a peak thermoelectric figure of merit zT > 1.2 and an average zT as high as ≈0.8 are achieved in Nb‐doped materials, without relying on a high thermoelectric power factor. This work demonstrates not only a guiding principle of low sound velocity for minimal lattice thermal conductivity and therefore high zT, but also argyrodite compounds as promising thermoelectric materials with weak chemical bonds and heavy constituent elements.
Low sound velocity leads to low lattice thermal conductivity, motivating the exploration of high‐performance thermoelectric Ag8SnSe6.
High-symmetry thermoelectric materials usually have the advantage of very high band degeneracy, while low-symmetry thermoelectrics have the advantage of very low lattice thermal conductivity. If the ...symmetry breaking of band degeneracy is small, both effects may be realized simultaneously. Here we demonstrate this principle in rhombohedral GeTe alloys, having a slightly reduced symmetry from its cubic structure, to realize a record figure of merit (zT ∼ 2.4) at 600 K. This is enabled by the control of rhombohedral distortion in crystal structure for engineering the split low-symmetry bands to be converged and the resultant compositional complexity for simultaneously reducing the lattice thermal conductivity. Device ZT as high as 1.3 in the rhombohedral phase and 1.5 over the entire working temperature range of GeTe alloys make this material the most efficient thermoelectric to date. This work paves the way for exploring low-symmetry materials as efficient thermoelectrics.
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•Rhombohedral GeTe shows a record thermoelectric figure of merit (zT ∼ 2.4)•This is enabled by the slight reduction of symmetry from its cubic analogs•The symmetry breaking splits bands but enables an overall band convergence•The principle expands low-symmetry materials as efficient thermoelectrics
Thermoelectric materials enable a heat flow to be directly converted to a flow of charge carriers for generating electricity. The crystal structure symmetry is one of the most fundamental parameters determining the properties of a crystalline material including thermoelectrics. The common belief currently held is that high-symmetry materials are usually good for thermoelectrics, leading to great efforts having historically been focused on GeTe alloys in a high-symmetry cubic structure. Here we show a slight reduction of crystal structure symmetry of GeTe alloys from cubic to rhombohedral, enabling a rearrangement in electronic bands for more transporting channels of charge carriers and many imperfections for more blocking centers of heat-energy carriers (phonons). This leads to the discovery of rhombohedral GeTe alloys as the most efficient thermoelectric materials to date, opening new possibilities for low-symmetry thermoelectric materials.
Cubic GeTe thermoelectrics have been historically focused on, while this work utilizes a slight symmetry-breaking strategy to converge the split valence bands, to reduce the lattice thermal conductivity and therefore realize a record thermoelectric performance, all enabled in GeTe in a rhombohedral structure. This not only promotes GeTe alloys as excellent materials for thermoelectric power generation below 800 K, but also expands low-symmetry materials as efficient thermoelectrics.
Due to point defect phonon scattering, formation of solid solutions has long been considered as an effective approach for enhancing thermoelectric performance through reducing the lattice thermal ...conductivity. The scattering of phonons by point defects mainly comes from the mass and strain fluctuations between the guest and the host atoms. Both the fluctuations can be maximized by point defects of interstitial atoms and/or vacancies in a crystal. Here, a demonstration of phonon scattering by interstitial Cu atoms is shown, leading to an extremely low lattice thermal conductivity of 0.5 W m−1 K−1 in SnTe‐Cu2Te solid solutions. This is the lowest lattice thermal conductivity reported in SnTe‐based materials so far, which is actually approaching the amorphous limit of SnTe. As a result, a peak thermoelectric figure of merit, zT, higher than 1 is achieved in Sn0.94Cu0.12Te at 850 K, without relying on other approaches for electrical performance enhancements. The strategy used here is believed to be equally applicable in thermoelectrics with interstitial point defects.
Thermoelectric SnTe alloying with Cu2Te shows an extremely low lattice thermal conductivity due to the strong phonon scattering by point defects.
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