The long-standing popularity of thermoelectric materials has contributed to the creation of various thermoelectric devices and stimulated the development of strategies to improve their thermoelectric performance. In this review, we aim to comprehensively summarize the state-of-the-art strategies for the realization of high-performance thermoelectric materials and devices by establishing the links between synthesis, structural characteristics, properties, underlying chemistry and physics, including structural design (point defects, dislocations, interfaces, inclusions, and pores), multidimensional design (quantum dots/wires, nanoparticles, nanowires, nano- or microbelts, few-layered nanosheets, nano- or microplates, thin films, single crystals, and polycrystalline bulks), and advanced device design (thermoelectric modules, miniature generators and coolers, and flexible thermoelectric generators). The outline of each strategy starts with a concise presentation of their fundamentals and carefully selected examples. In the end, we point out the controversies, challenges, and outlooks toward the future development of thermoelectric materials and devices. Overall, this review will serve to help materials scientists, chemists, and physicists, particularly students and young researchers, in selecting suitable strategies for the improvement of thermoelectrics and potentially other relevant energy conversion technologies.

Advanced Thermoelectric Design: From Materials and Structures to Devices

Thermoelectric technology generates electricity from waste heat, but one bottleneck for wider use is the performance of thermoelectric materials. Manipulating the configurational entropy of a material by introducing different atomic species can tune phase composition and extend the performance optimization space. We enhanced the figure of merit (zT) value to 1.8 at 900 kelvin in an n-type PbSe-based high-entropy material formed by entropy-driven structural stabilization. The largely distorted lattices in this high-entropy system caused unusual shear strains, which provided strong phonon scattering to largely lower lattice thermal conductivity. The thermoelectric conversion efficiency was 12.3% at temperature difference ΔT = 507 kelvin, for the fabricated segmented module based on this n-type high-entropy material. Our demonstration provides a paradigm to improve thermoelectric performance for high-entropy thermoelectric materials through entropy engineering.

High-entropy-stabilized chalcogenides with high thermoelectric performance.

High-entropy ceramics (HECs) are solid solutions of inorganic compounds with one or more Wyckoff sites shared by equal or near-equal atomic ratios of multi-principal elements. Although in the infant stage, the emerging of this new family of materials has brought new opportunities for material design and property tailoring. Distinct from metals, the diversity in crystal structure and electronic structure of ceramics provides huge space for properties tuning through band structure engineering and phonon engineering. Aside from strengthening, hardening, and low thermal conductivity that have already been found in high-entropy alloys, new properties like colossal dielectric constant, super ionic conductivity, severe anisotropic thermal expansion coefficient, strong electromagnetic wave absorption, etc., have been discovered in HECs. As a response to the rapid development in this nascent field, this article gives a comprehensive review on the structure features, theoretical methods for stability and property prediction, processing routes, novel properties, and prospective applications of HECs. The challenges on processing, characterization, and property predictions are also emphasized. Finally, future directions for new material exploration, novel processing, fundamental understanding, in-depth characterization, and database assessments are given.

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High-entropy ceramics: Present status, challenges, and a look forward

High entropy ceramics are novel materials with no less than four different cations or anions. The development of high entropy ceramics follows the ‘configurational entropy stabilized single phase’ concept, which was first demonstrated for high entropy metal alloys in 2004. The advantages of high entropy ceramics are their compositional and structural diversity, and many of them have a band gap, which makes them potential functional materials for a wide range of applications. They have recently generated significant interest with the publication of 70+ related papers since 2015. In this review we have summarized the recent progress in this rapidly growing field. We emphasize the progress by researchers to answer the following three fundamental questions for high entropy ceramics: (1) which combinations of cations or anions can be synthesized as single-phase materials; (2) are the component elements truly random down to the atomic scale; and (3) what new physics, properties and applications will the incorporation of multi-elements elements bring. These fundamental questions are still open at this stage and warrant further studies. The objective of this review is to give a comprehensive overview of the literature to date on high entropy ceramics and to guide further investigation in this emerging field.

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Review of high entropy ceramics: design, synthesis, structure and properties

Due to the nature of their liquid-like behavior and high dimensionless figure of merit, Cu2 X (X = Te, Se, and S)-based thermoelectric materials have attracted extensive attention. The superionicity and Cu disorder at the high temperature can dramatically affect the electronic structure of Cu2 X and in turn result in temperature-dependent carrier-transport properties. Here, the effective strategies in enhancing the thermoelectric performance of Cu2 X-based thermoelectric materials are summarized, in which the proper optimization of carrier concentration and minimization of the lattice thermal conductivity are the main focus. Then, the stabilities, mechanical properties, and module assembly of Cu2 X-based thermoelectric materials are investigated. Finally, the future directions for further improving the energy conversion efficiency of Cu2 X-based thermoelectric materials are highlighted.

https://espace.library.uq.edu.au/view/UQ:4704b7d/UQ4704b7d_OA.pdf

Promising and Eco-Friendly Cu 2 X-Based Thermoelectric Materials: Progress and Applications

DOI: 10.1002/aenm.201802116 TE material research has attracted intense interest over the past few decades.[1] The TE performance of a material is measured by zT = σS2T/κ, where T, σ, S, and κ are the absolute temperature, electrical conductivity, Seebeck coefficient, and total thermal conductivity, respectively. Typically, κ = κel + κph, where the κel and κph are the carrier and lattice thermal conductivity, respectively. Since σ, S, and κel are adversely interrelated whereas the κph is relatively independent of σ, S, and κel, the stride toward high zT is in line with a two-pronged strategy, coined by Slack as “electron-crystal phonon-glass” (ECPG):[2] i) decoupling σ, S, and κel through band structure engineering toward a high power factor (PF) = σS2;[3,4] and ii) suppressing the κph via all-scale hierarchical microstructures.[5–7] Rooted in the core effects of high entropy alloys (HEAs), entropy engineering enables a synergy of band structure engineering and multiscale hierarchical microstructures through high entropy alloying. HEAs typically refer to the solid solutions in which more than five principal elements each in 5–35% molar ratio compete for the same crystallographic site, yielding high entropy of mixing and a wider variety of exciting properties.[8] HEA is a subset of multielement-doped materials. Neither the doping process nor the resulting composition would differentiate a The core effects of high entropy alloys distinguish high entropy alloying from ordinary multielement doping, allowing for a synergy of band structure and microstructure engineering. Here, a systematic synthesis, structural, theoretical, and thermoelectric study of multi-principal-element-alloyed SnTe is reported. Toward high thermoelectric performance, the entropy of mixing needs to be high enough to make good use of the core effects, yet low enough to minimize the degradation of carrier mobility. It is demonstrated that high entropy of mixing extends the solubility limit of Mn while retaining the lattice symmetry, the enhanced Mn content elicits multiscale microstructures. The resulting ultralow lattice thermal conductivity of ≈0.32 W m−1 K−1 at 900 K in (Sn0.7Ge0.2Pb0.1)0.75Mn0.275Te is not only lower than the amorphous limit of SnTe but also comparable to those thermoelectric materials with complex crystal structures and strong anharmonicity. Co-alloying of (Sn,Ge,Pb,Mn) also enhances band convergence and band effective mass, yielding good power factors. Further tuning of the Sn excess yields a state-of-the-art zT ≈1.42 at 900 K in (Sn0.74Ge0.2Pb0.1)0.75Mn0.275Te. In view of the simple face-centeredcubic structure of SnTe-based alloys, these results attest to the efficacy of entropy engineering toward a new paradigm of high entropy thermoelecrics.

Entropy Engineering of SnTe: Multi‐Principal‐Element Alloying Leading to Ultralow Lattice Thermal Conductivity and State‐of‐the‐Art Thermoelectric Performance

Here a progressive hot deformation procedure that endows the benchmark n-type V2VI3 thermoelectric materials with short range disorder (multiple defects), long range order (crystallinity), and strong texture (nearly orientation order) is reported. Not only it is rare for these structural features to coexist but also these structural features elicit the synergistic compositional– mechanical–thermal effects, i.e., a profound interplay among the counts, magnitude, and temperature of hot deformation in relation to the as formed point defects, dislocations, textures, strain clusters, and distortions. Using progressively larger die sets and relatively low hot deformation temperature, rich multiscale microstructures concurrently with a high level of texture comparable to that of zone melted ingot are obtained. The strong donor-like effect significantly increases the majority carrier concentration, suppressing the detrimental bipolar effect. In addition, the multiscale microstructures yield an ultralow lattice thermal conductivity ≈0.31 W m−1 K−1 at 405 K. A record zT ≈ 1.3 at 450 K are attained in progressively hot deformed n-type Bi1.95Sb0.05Te2.3Se0.7 through the synergistic effects. These results not only promise a better pairing between n-type and p-type legs in device fabrication but also bring our understanding of n-type V2VI3 alloys and hot deformation technique to a new level.

Synergistic Compositional–Mechanical–Thermal Effects Leading to a Record High zT in n‐Type V2VI3 Alloys Through Progressive Hot Deformation

The Bi2Te3-based alloys have been commercialized for the applications of energy harvesting and refrigeration for decades. However, the commercial Bi2Te3-based alloys produced by the zone-melting (ZM) method usually show poor mechanical strength and crack problems as well as the sluggish figure of merit ZT, especially for the less-progressed n-type samples. In this work, we have simultaneously enhanced the thermoelectric and mechanical performance of the one-step spark plasma sintering (SPS)-derived n-type Bi2Te2.7Se0.3 alloys just by doping a small amount of superconducting material MgB2 where Mg and B atoms can play significant roles in carrier density optimization and hardness enhancement. Besides the optimization of carrier density, the MgB2 doping can also increase the carrier mobility but decrease the lattice and bipolar thermal conductivity, leading to a peak ZT of 0.96 at 325 K and an average ZT of 0.88 within 300-500 K in the 0.5% MgB2-doped Bi2Te2.7Se0.3 (BTSMB) alloys. The peak ZT and average ZT of our optimized BTSMB samples are comparable and higher than those of the state-of-the-art commercial ZM ingot. Moreover, the optimized BTSMB sample also exhibits almost 70% enhancement in hardness compared with the ZM ingot. Our results demonstrate the great potential of the MgB2 doping strategy for mass production of SPS-derived Bi2Te3-based alloys in one-step sintering.

Simultaneous Enhancement of the Thermoelectric and Mechanical Performance in One-Step Sintered n-Type Bi2Te3-Based Alloys via a Facile MgB2 Doping Strategy.

Positive Effect of Ge Vacancies on Facilitating Band Convergence and Suppressing Bipolar Transport in GeTe‐Based Alloys for High Thermoelectric Performance

Lipeng Hu

Papers

Entropy Engineering of SnTe: Multi‐Principal‐Element Alloying Leading to Ultralow Lattice Thermal Conductivity and State‐of‐the‐Art Thermoelectric Performance

Synergistic Compositional–Mechanical–Thermal Effects Leading to a Record High zT in n‐Type V2VI3 Alloys Through Progressive Hot Deformation

Simultaneous Enhancement of the Thermoelectric and Mechanical Performance in One-Step Sintered n-Type Bi2Te3-Based Alloys via a Facile MgB2 Doping Strategy.

Positive Effect of Ge Vacancies on Facilitating Band Convergence and Suppressing Bipolar Transport in GeTe‐Based Alloys for High Thermoelectric Performance

Leveraging Deep Levels in Narrow Bandgap Bi0.5Sb1.5Te3 for Record-High zTave Near Room Temperature