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

Based on the concept of band bending at metal/semiconductor interfaces as an energy filter for electrons, we present a theory for the enhancement of the thermoelectric properties of semiconductor materials with metallic nanoinclusions. We show that the Seebeck coefficient can be significantly increased due to a strongly energy-dependent electronic scattering time. By including phonon scattering, we find that the enhancement of $ZT$ due to electron scattering is important for high doping, while at low doping it is primarily due to a decrease in the phonon thermal conductivity.

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Theory of enhancement of thermoelectric properties of materials with nanoinclusions.

Manipulation of Band Structure and Interstitial Defects for Improving Thermoelectric SnTe

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

Thermoelectric energy conversion is an all solid-state technology that relies on exceptional semiconductor materials that are generally optimized through sophisticated strategies involving the engineering of defects in their structure. In this review, we summarize the recent advances of defect engineering to improve the thermoelectric (TE) performance and mechanical properties of inorganic materials. First, we introduce the various types of defects categorized by dimensionality, i.e. point defects (vacancies, interstitials, and antisites), dislocations, planar defects (twin boundaries, stacking faults and grain boundaries), and volume defects (precipitation and voids). Next, we discuss the advanced methods for characterizing defects in TE materials. Subsequently, we elaborate on the influences of defect engineering on the electrical and thermal transport properties as well as mechanical performance of TE materials. In the end, we discuss the outlook for the future development of defect engineering to further advance the TE field.

Defect engineering in thermoelectric materials: what have we learned?

SnTe alloys, which have the same crystal structure as PbTe, have attracted increasing attention. Here, we demonstrate that the synergistic effect of band structure modification and chemical bond softening can be realized simultaneously in In & Mn doped SnTe bulk alloys. The Seebeck coefficient and power factor are synergistically improved by co-doping of In and Mn. In doping is known to introduce a resonance level. Mn doping reduces the separation of light- and heavy-valence bands. The combination of these effects significantly enhances the Seebeck coefficient at room temperature owing to around a factor of five times increase in the band effective mass. The reduction of thermal conductivity is from the decrease of both the electronic and phononic parts. The electronic thermal conductivity is decreased by the increase in defect scattering, as can be confirmed by the carrier mobility. The force constant of the bonds around the Te site is decreased due to the co-doping of In & Mn, which indicates that the chemical bonds are softened, which leads to low sound velocity and lower lattice thermal conductivity. As a result, the peak thermoelectric figure of merit, zT = 1.03 has been achieved for Sn0.89In0.01Mn0.1Te at 923 K. This strategy of using the synergistic effect of band structure modification and chemical bond softening could be applicable to other thermoelectric materials.

Enhancement of the thermoelectric performance of bulk SnTe alloys: Via the synergistic effect of band structure modification and chemical bond softening

Compositing is viewed as a promising strategy to synergistically enhance the thermoelectric and mechanical properties of materials as compared with the element doping method. Herein, different volume percentages of nano SiC (1, 3, 5, and 10 vol%) powders were firstly composited with pristine SnTe. The SiC particles combined well with the SnTe matrix and dispersed homogeneously. The lattice interface effect increases the electrical conductivity up to 700 S m−1 at room temperature due to the increased carrier concentration with addition of 1 vol% SiC. Along with the decreased lattice thermal conductivity (0.67 W m−1 K−1 at 823 K), the maximum figure of merit for SnTe-1 vol% SiC is ∼0.7 which is 1.7 times that of pristine SnTe. And this value is comparable to the results obtained by traditional chemical doping. Additionally, the hardness for all SiC composited samples is also significantly enhanced. Thus nano SiC compositing is a promising way to enhance both the thermoelectric and mechanical properties of SnTe.

Simultaneous enhancement of thermoelectric and mechanical performance for SnTe by nano SiC compositing

Micro/nanotopographical modification of biomaterials plays an essential role in the improvement of implants’ cytocompatibility. In this paper, hierarchical micro/nanostructured surface was constructed to improve the titanium alloy cytocompatibility, which combined sandblasting, acid etching, and alkali heat treatment. The surface features were characterized using scanning electron microscope (SEM), laser scanning microscope, X-ray diffraction (XRD), and contact angle goniometer. A series of biological tests were carried out to evaluate cytocompatibility of bio-titanium alloy in vitro, including cell adhesion, counting, proliferation, osteocalcin expression, alkaline phosphatase (ALP) activity, and extracellular matrix mineralization. The results show that bio-titanium alloy with micro/nanostructured surface provided a beneficial interface which could promote the growth of osteoblasts. A uniform nanowire layer with high hydrophilicity was superposed on microstructures. Meanwhile, the biological tests indicate that hierarchical micro/nanostructures with micropits and nanowires could significantly promote osteoblasts’ initial adhesion, proliferation, and differentiation. Moreover, the underlying mechanism of interaction between implants and cells was also discussed. This work proposes an effective approach to fabricate micro/nanostructured surface on titanium alloy for cytocompatibility improvement.

Fabrication of hierarchical micro/nanotopography on bio-titanium alloy surface for cytocompatibility improvement

Lead telluride (PbTe) is an excellent thermoelectric material in the intermediate temperature zone and has been applied to deep space exploration, waste heat recovery and other fields. However, the low thermoelectric conversion efficiency of the n-type PbTe alloys limits its applications. Here, the thermoelectric performances have been enhanced in n-type PbTe alloys through trace bismuth (Bi) and iodine (I) co-doping. The Pb1-xBixTe1-xIx (x = 0.00%, 0.05%, 0.10%, 0.20% and 0.50%) alloys are synthesized in the single phase compounds by a stepwise synthesis method. The carrier concentration has reached an optimal concentration range within the order of 1019 cm-3. The highest absolute Seebeck coefficient of 244 μV/K is obtained for 0.05% doped alloy at 730 K. The highest absolute Seebeck coefficient leads to high power factor for 0.05% doped, especially in low- and middle-temperature range. The highest power factor ~ 25 μW/K2cm has been obtained at 329 K. Complex micro-scale grain boundaries and point defects strongly increase the phonon scattering and then lead to the lowest lattice thermal conductivity of 0.64 W/mK at 674 K for x = 0.50%, which is 26% lower than that of pristine PbTe. As a result, the highest figure of merit, zT ~ 0.9 has been determined in 0.20% doped samples at 725 K. Moreover, the highest average figure of merit, zTave ~ 0.7 has been achieved in 0.05% doped samples in the 323 - 723 K temperature range, which is about two or three times higher than reported for single Bi or I doped PbTe samples.

https://iopscience.iop.org/article/10.1088/1361-6463/ab7d6c/pdf

Teng Wang

Papers

Enhancement of the thermoelectric performance of bulk SnTe alloys: Via the synergistic effect of band structure modification and chemical bond softening

Simultaneous enhancement of thermoelectric and mechanical performance for SnTe by nano SiC compositing

Fabrication of hierarchical micro/nanotopography on bio-titanium alloy surface for cytocompatibility improvement

Trace bismuth and iodine co-doping enhanced thermoelectric performance of PbTe alloys

Geometric structural design for lead tellurium thermoelectric power generation application