Microstructure engineering is an effective strategy to reduce lattice thermal conductivity (κl) and enhance the thermoelectric figure of merit (zT). Through a new process based on melt‐centrifugation ...to squeeze out excess eutectic liquid, microstructure modulation is realized to manipulate the formation of dislocations and clean grain boundaries, resulting in a porous network with a platelet structure. In this way, phonon transport is strongly disrupted by a combination of porosity, pore surfaces/junctions, grain boundaries, and lattice dislocations. These collectively result in a ≈60% reduction of κl compared to zone melted ingot, while the charge carriers remain relatively mobile across the liquid‐fused grains. This porous material displays a zT value of 1.2, which is higher than fully dense conventional zone melted ingots and hot pressed (Bi,Sb)2Te3 alloys. A segmented leg of melt‐centrifuged Bi0.5Sb1.5Te3 and Bi0.3Sb1.7Te3 could produce a high device ZT exceeding 1.0 over the whole temperature range of 323–523 K and an efficiency up to 9%. The present work demonstrates a method for synthesizing high‐efficiency porous thermoelectric materials through an unconventional melt‐centrifugation technique.
The melt‐centrifugation technique is demonstrated to be able to decrease the thermal conductivity while preserving the good electrical properties. By introducing a unique porous structure with microscale dislocation, ≈60% reduction in lattice thermal conductivity compared to conventional zone melted ingots is achieved. Such a method paves a new way for top‐down introduction of large porosity and dense dislocations in bulk materials.
Bismuth telluride is the working material for most Peltier cooling devices and thermoelectric generators. This is because Bi2Te3 (or more precisely its alloys with Sb2Te3 for p‐type and Bi2Se3 for ...n‐type material) has the highest thermoelectric figure of merit, zT, of any material around room temperature. Since thermoelectric technology will be greatly enhanced by improving Bi2Te3 or finding a superior material, this review aims to identify and quantify the key material properties that make Bi2Te3 such a good thermoelectric. The large zT can be traced to the high band degeneracy, low effective mass, high carrier mobility, and relatively low lattice thermal conductivity, which all contribute to its remarkably high thermoelectric quality factor. Using literature data augmented with newer results, these material parameters are quantified, giving clear insight into the tailoring of the electronic band structure of Bi2Te3 by alloying, or reducing thermal conductivity by nanostructuring. For example, this analysis clearly shows that the minority carrier excitation across the small bandgap significantly limits the thermoelectric performance of Bi2Te3, even at room temperature, showing that larger bandgap alloys are needed for higher temperature operation. Such effective material parameters can also be used for benchmarking future improvements in Bi2Te3 or new replacement materials.
The material properties underlying the exceptional thermoelectric performance of bismuth telluride are reviewed and compared with other notable materials. Using literature data and recent results, key parameters are identified to give insight into band structure engineering through alloying and thermal conductivity reduction by nanostructuring. Transport modeling is used to quantify these parameters and illustrate performance limitations.
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.
Alloying bismuth telluride with antimony telluride and bismuth selenide for
p
- and
n
-type materials, respectively, improves the thermoelectric quality factor for use in room temperature modules. As ...the electronic and thermal transports can vary substantially, the alloy composition is a key engineering parameter. The
n
-type Bi
2
Te
3-
x
Se
x
alloy lags its
p
-type counterpart in thermoelectric performance and does not lend itself as readily to simple transport modeling which complicates engineering. Combining literature data with recent results across the entire alloy composition range, the complex electronic structure dynamics and trends in lattice thermal conductivity are explored. Spin-orbit interaction plays a critical role in determining the position and degeneracy of the various conduction band minima. This behavior is incorporated into a two-band effective mass model to estimate the transport parameters in each band. An alloy scattering model is utilized to demonstrate how phonon scattering behaves differently on either side of the intermediate ordered compound Bi
2
Te
2
Se due to chalcogen site occupancy preference. The parametrization of the electronic and thermal transports presented can be used in future optimization efforts.
Alloys of Bi2Te3 and Sb2Te3 are the best performing p-type thermoelectrics near room temperature and have been the subject of extensive engineering efforts. Dramatic improvement is achieved by ...introducing defects that effectively scatter phonons and reduce thermal conductivity. Unfortunately, outstanding results are often difficult to reproduce as the process variables involved are difficult to control or possibly unknown. Here, a reproducible and controllable method of fabricating porous Bi0.5Sb1.5Te3+x is presented. While effective medium theory (EMT) predicts no benefit, improvements in the thermoelectric quality factor, B (which determines the maximum zT of a materials), were as high as 45% parallel to the pressing direction for a sample of roughly 20% porosity. The study of microstructural evolution with increasing porosity is facilitated by a combination of Scanning/Transmission Electron Microscopy (S/TEM) and Electron Backscattered Diffraction (EBSD). This study reveals a statistically significant shift in the distribution of grain boundaries favoring lower energy twins, which coincides with an increase in the presence of stepped twin boundaries. This work demonstrates the potential benefits of careful grain boundary engineering and the need for further detailed studies of the dependence of thermal and electrical transport on grain boundary structure and orientation in these alloys.
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•Developed a controllable and reproducible synthesis method for Te-rich Bi0.5Sb1.5Te3+x (BST).•Revealed that annealing causes swelling of BST which results in a porous material with enhanced thermoelectric performance.•Used a combination of electron microcopy techniques to characterize materials defects from the meso-to nanoscale.•Combined high-resolution images with statistical data to suggest the importance of Twin Boundary population and structure.
Microstructure engineering is an effective strategy to reduce lattice thermal conductivity (κ
) and enhance the thermoelectric figure of merit (zT). Through a new process based on melt-centrifugation ...to squeeze out excess eutectic liquid, microstructure modulation is realized to manipulate the formation of dislocations and clean grain boundaries, resulting in a porous network with a platelet structure. In this way, phonon transport is strongly disrupted by a combination of porosity, pore surfaces/junctions, grain boundaries, and lattice dislocations. These collectively result in a ≈60% reduction of κ
compared to zone melted ingot, while the charge carriers remain relatively mobile across the liquid-fused grains. This porous material displays a zT value of 1.2, which is higher than fully dense conventional zone melted ingots and hot pressed (Bi,Sb)
Te
alloys. A segmented leg of melt-centrifuged Bi
Sb
Te
and Bi
Sb
Te
could produce a high device ZT exceeding 1.0 over the whole temperature range of 323-523 K and an efficiency up to 9%. The present work demonstrates a method for synthesizing high-efficiency porous thermoelectric materials through an unconventional melt-centrifugation technique.
The thin film metallic glass (TFMG) is an effective diffusion barrier layer for PbTe-based thermoelectric (TE) modules. Reaction couples structured with Cu/TFMG/PbTe are prepared via ...sputter-deposition and are annealed at 673 K for 8-96 h. The transmission line method is adopted for the assessment of electrical contact resistivity upon the PbTe/TFMG, and the value remains in the range of 3.3-2.5 × 10−9 (Ω m2). The titanium-based TFMG remains amorphous upon annealing at 673 K for 48 h and effectively blocks the inter-diffusion by not having grain-boundaries, which only allows the bulk diffusion between the metal electrode and the TE substrate.
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