Tuning phonon transport spectrum for better thermoelectric materials

The figure of merit of thermoelectric materials can be increased by suppressing the lattice thermal conductivity without degrading electrical properties. Phonons are the carriers for lattice thermal conduction, and their transport can be impeded by nanostructuring, owing to the recent progress in nanotechnology. The key question for further improvement of thermoelectric materials is how to realize ultimate structure with minimum lattice thermal conductivity. From spectral viewpoint, this means to impede transport of phonons in the entire spectral domain with noticeable contribution to lattice thermal conductivity that ranges in general from subterahertz to tens of terahertz in frequency. To this end, it is essential to know how the phonon transport varies with the length scale, morphology, and composition of nanostructures, and how effects of different nanostructures can be mutually adopted in view of the spectral domain. Here we review recent advances in analyzing such spectral impedance of phonon transport on the basis of various effects including alloy scattering, boundary scattering, and particle resonance.     

Enhancement of high temperature thermoelectric properties of intermetallic compounds based on a Skutterudite IrSb3 and a half-Heusler TiNiSb

Phonon glass and electron crystal (PGEC) thermoelectric materials have been expected to be a new class of thermoelectric materials for high temperature applications. Among the efforts to optimize the high temperature thermoelectric properties of various PGEC thermoelectric materials, recent experimental works on the Skutterudite IrSb₃ and half-Heusler TiNiSb intermetallic compounds are presented herein by which the material design concept for high energy conversion efficiency, i.e. a high figure of merit, is suggested. It is revealed that the thermoelectric efficiency of IrSb₃ can be increased by the decrease of lattice thermal conductivity due to the rattling effect of La atoms filled in the structural vacancies of the Skutterudite crystal structure. In the half-Heusler TiNiSn, high temperature thermoelectric properties are improved by Hf substitution to the Ti sites by reducing lattice thermal conductivity and also by Sb doping to increase power factor. It is concluded that the proper alloy designing for controlling crystal structure and carrier concentration could enable these intermetallic compounds to exhibit a high potential for elevated temperature thermoelectric applications.     

Enhancement of high temperature thermoelectric properties of intermetallic compounds based on a Skutterudite IrSb3 and a half-Heusler TiNiSb

Phonon glass and electron crystal (PGEC) thermoelectric materials have been expected to be a new class of thermoelectric materials for high temperature applications. Among the efforts to optimize the high temperature thermoelectric properties of various PGEC thermoelectric materials, recent experimental works on the Skutterudite IrSb₃ and half-Heusler TiNiSb intermetallic compounds are presented herein by which the material design concept for high energy conversion efficiency, i.e. a high figure of merit, is suggested. It is revealed that the thermoelectric efficiency of IrSb₃ can be increased by the decrease of lattice thermal conductivity due to the rattling effect of La atoms filled in the structural vacancies of the Skutterudite crystal structure. In the half-Heusler TiNiSn, high temperature thermoelectric properties are improved by Hf substitution to the Ti sites by reducing lattice thermal conductivity and also by Sb doping to increase power factor. It is concluded that the proper alloy designing for controlling crystal structure and carrier concentration could enable these intermetallic compounds to exhibit a high potential for elevated temperature thermoelectric applications.     

Thermoelectric GeTe with Diverse Degrees of Freedom Having Secured Superhigh Performance

Driven by the ability to harvest waste heat into reusable electricity and the exclusive role of serving as the power generator for deep spacecraft, intensive endeavors are dedicated to enhancing the thermoelectric performance of ecofriendly materials. Herein, the most recent progress in superhigh‐performance GeTe‐based thermoelectric materials is reviewed with a focus on the crystal structures, phase transitions, resonant bondings, multiple valance bands, and phonon dispersions. These features diversify the degrees of freedom to tune the transport properties of electrons and phonons for GeTe. On the basis of the optimized carrier concentration, strategies of alignment of multiple valence bands and density‐of‐state resonant distortion are employed to further enhance the thermoelectric performance of GeTe‐based materials. To decrease the thermal conductivity, methods of strengthening intrinsic phonon–phonon interactions and introducing various lattice imperfections as scattering centers are highlighted. An overview of thermoelectric devices assembled from GeTe‐based thermoelectric materials is then presented. In conclusion, possible future directions for developing GeTe in thermoelectric applications are proposed. The achieved high thermoelectric performance in GeTe‐based thermoelectric materials with rationally established strategies can act as a reference for broader materials to tailor their thermoelectric performance.     

Entropy as a Gene‐Like Performance Indicator Promoting Thermoelectric Materials

High‐throughput explorations of novel thermoelectric materials based on the Materials Genome Initiative paradigm only focus on digging into the structure‐property space using nonglobal indicators to design materials with tunable electrical and thermal transport properties. As the genomic units, following the biogene tradition, such indicators include localized crystal structural blocks in real space or band degeneracy at certain points in reciprocal space. However, this nonglobal approach does not consider how real materials differentiate from others. Here, this study successfully develops a strategy of using entropy as the global gene‐like performance indicator that shows how multicomponent thermoelectric materials with high entropy can be designed via a high‐throughput screening method. Optimizing entropy works as an effective guide to greatly improve the thermoelectric performance through either a significantly depressed lattice thermal conductivity down to its theoretical minimum value and/or via enhancing the crystal structure symmetry to yield large Seebeck coefficients. The entropy engineering using multicomponent crystal structures or other possible techniques provides a new avenue for an improvement of the thermoelectric performance beyond the current methods and approaches.     

Recent advances in thermoelectric materials

Thermoelectric materials are crucial in renewable energy conversion technologies to solve the global energy crisis. They have been proven to be suitable for high-end technological applications such as missiles and spacecraft. The thermoelectric performance of devices depends primarily on the type of materials used and their properties such as their Seebeck coefficient, electrical conductivity, thermal conductivity, and thermal stability. Classic inorganic materials have become important due to their enhanced thermoelectric responses compared with organic materials. In this review, we focus on the physical and chemical properties of various thermoelectric materials. Newly emerging materials such as carbon nanomaterials, electronically conducting polymers, and their nanocomposites are also briefly discussed. Strategies for improving the thermoelectric performance of materials are proposed, along with an insight into semiconductor physics. Approaches such as nanostructuring, nanocomposites, and doping are found to enhance thermoelectric responses by simultaneously tuning various properties within a material. A recent trend in thermoelectric research shows that high-performance thermoelectric materials such as inorganic materials and carbon nanomaterials/electronically conducting polymer nanocomposites may be suitable for power generation and energy sustainability in the near future.     

Synthesis and properties of Na<Subscript> <Emphasis Type="Italic">x</Emphasis> </Subscript>CoO<Subscript>2</Subscript> (<Emphasis Type="Italic">x</Emphasis> = 0.55, 0.89) oxide thermoelectrics

Na_xCoO₂ (x = 0.55, 0.89) sodium cobaltites have been prepared by solid-state reactions; their structural parameters have been determined; their microstructure has been studied; and their thermal (thermal expansion, thermal diffusivity, and thermal conductivity), electrical (electrical conductivity and thermoelectric power), and functional (power factor, thermoelectric figure of merit, and self-compatibility factor) properties have been investigated in air at temperatures from 300 to 1100 K. The results demonstrate that, with increasing sodium content, the electrical conductivity and thermoelectric power of the materials increase and their thermal conductivity decreases. As a result, the power factor and thermoelectric figure of merit of the Na_0.89CoO₂ ceramic at a temperature of 1100 K reach 0.829 mW/(m K²) and 1.57, respectively. The electron and phonon (lattice) contributions to the thermal conductivity of the ceramics have been separately assessed, and their linear thermal expansion coefficients have been evaluated.     

Nanoporous Si as an efficient thermoelectric material

Room-temperature thermoelectric properties of n-type crystalline Si with periodically arranged nanometer-sized pores are computed using a combination of classical molecular dynamics for lattice thermal conductivity and ab initio density functional theory for electrical conductivity, Seebeck coefficient, and electronic contribution to the thermal conductivity. The electrical conductivity is found to decrease by a factor of 2-4, depending on doping levels, compared to that of bulk due to confinement. The Seebeck coefficient S yields a 2-fold increase for carrier concentrations less than 2 x 10(19) cm(-3), above which S remains closer to the bulk value. Combining these results with our calculations of lattice thermal conductivity, we predict the figure of merit ZT to increase by 2 orders of magnitude over that of bulk. This enhancement is due to the combination of the nanometer size of pores which greatly reduces the thermal conductivity and the ordered arrangement of pores which allows for only a moderate reduction in the power factor. We find that while alignment of pores is necessary to preserve power factor values comparable to those of bulk Si, a symmetric arrangement is not required. These findings indicate that nanoporous semiconductors with aligned pores may be highly attractive materials for thermoelectric applications.     

New Directions for Low‐Dimensional Thermoelectric Materials

Many of the recent advances in enhancing the thermoelectric figure of merit are linked to nanoscale phenomena found both in bulk samples containing nanoscale constituents and in nanoscale samples themselves. Prior theoretical and experimental proof‐of‐principle studies on quantum‐well superlattice and quantum‐wire samples have now evolved into studies on bulk samples containing nanostructured constituents prepared by chemical or physical approaches. In this Review, nanostructural composites are shown to exhibit nanostructures and properties that show promise for thermoelectric applications, thus bringing together low‐dimensional and bulk materials for thermoelectric applications. Particular emphasis is given in this Review to the ability to achieve 1) a simultaneous increase in the power factor and a decrease in the thermal conductivity in the same nanocomposite sample and for transport in the same direction and 2) lower values of the thermal conductivity in these nanocomposites as compared to alloy samples of the same chemical composition. The outlook for future research directions for nanocomposite thermoelectric materials is also discussed.     

聚3, 4-乙烯二氧噻吩: 聚苯乙烯磺酸盐基柔性复合热电材料研究进展

热电材料可以实现热能与电能的直接转化,是一种安全环保的新型能源材料。近年来,随着可穿戴电子设备的发展,柔性热电材料成为研究人员关注的焦点。传统无机热电材料具有优异的热电性能,但由于自身固有的脆性,限制了在柔性领域的发展。聚3, 4-乙烯二氧噻吩: 聚苯乙烯磺酸盐(PEDOT: PSS)具有高电导率、低热导率和良好的柔性,在柔性热电领域具有巨大的潜力。当选择合适的无机填料与PEDOT: PSS进行复合,可以得到优异的热电性能和良好的力学性能。本文综述了PEDOT: PSS基纳米复合薄膜的最新进展,并详细介绍了提高PEDOT: PSS基纳米复合薄膜热电性能的有效方法。最后,本文总结了实现高性能PEDOT: PSS基柔性热电材料的途径及面对的挑战。      

Extraordinary Thermoelectric Performance Realized in n‐Type PbTe through Multiphase Nanostructure Engineering

Lead telluride has long been realized as an ideal p‐type thermoelectric material at an intermediate temperature range; however, its commercial applications are largely restricted by its n‐type counterpart that exhibits relatively inferior thermoelectric performance. This major limitation is largely solved here, where it is reported that a record‐high ZT value of ≈1.83 can be achieved at 773 K in n‐type PbTe‐4%InSb composites. This significant enhancement in thermoelectric performance is attributed to the incorporation of InSb into the PbTe matrix resulting in multiphase nanostructures that can simultaneously modulate the electrical and thermal transport. On one hand, the multiphase energy barriers between nanophases and matrix can boost the power factor in the entire temperature range via significant enhancement of the Seebeck coefficient and moderately reducing the carrier mobility. On the other hand, the strengthened interface scattering at the intensive phase boundaries yields an extremely low lattice thermal conductivity. This strategy of constructing multiphase nanostructures can also be highly applicable in enhancing the performance of other state‐of‐the‐art thermoelectric systems.     

热电材料的第一性原理高通量研究

热电材料是一种新型能量转换材料, 在温差发电或通电制冷等领域具有广泛应用。热电优值ZT值是衡量热电材料能量转换效率的关键参数, ZT值要求热电材料具有优异的电输运性能及较低的热导率。传统第一性原理热电材料研究往往关注少量样本下的电热输运性质理解与优化, 很难得到系统性的规律, 也不利于新体系的设计优化。材料基因组计划力求通过大数据、高通量手段去加速材料设计与发现, 具有广阔的发展前景。在热电材料研究领域, 第一性原理高通量计算也将在新材料预测与性能优化等方面起到越来越重要的作用。另一方面, 高通量研究也带来了新的挑战, 譬如电热输运性质的高通量算法发展、大数据分析手段等等, 这些方面的问题决定了高通量方法在材料应用中的效率与准确性。本文综述了热电材料中现有的电热输运性质高通量计算方法, 介绍了这些方法具体的应用案例, 并对高通量与热电材料结合的未来发展趋势进行了展望。     

Recent progress in thermoelectric nanocomposites based on solution-synthesized nanoheterostructures

Thermoelectric materials,which can convert waste heat into electricity,have received increasing research interest in recent years.This paper describes the recent progress in thermoelectric nanocomposites based on solution-synthesized nanoheterostructures.We start our discussion with the strategies of improving the power factor of a given material by using nanoheterostructures.Then we discuss the methods of decreasing thermal conductivity.Finally,we highlight a way of decoupling power factor and thermal conductivity,namely,incorporating phase-transition materials into a nanowire heterostructure.We have explored the lead telluride-copper telluride thermoelectric nanowire heterostructure in this work.Future possible ways to improve the figure of merit are discussed at the end of this paper.     

Enhanced thermoelectric performance in Cl-doped BiSbSe3 with optimal carrier concentration and effective mass

Possessing inherently low thermal conductivity,BiSbSe3 is a promising thermoelectric material for medium temperature.Therefore,to substantially optimize the thermoelectric performance of BiSbSe3,researchers mainly focus on the strategies to improve its electrical transport properties.Among these strongly coupled thermoelectric parameters,carrier concentration and effective mass are two intrinsic variables to decisively affect the electrical transport properties.In this work,Cl as a donor dopant is effec-tive to provide extra electrons in n-type BiSbSe3,and the carrier concentration and effective mass can be well optimized simultaneously with increasing Cl content owing to the multiple conduction bands in BiSbSe3.What's more,maximum weighted mobility～53 cm2 V-1 s-1 is obtained in Cl-doped BiSbSe3,which contributes to a largely enhanced power factor～4.8 μW cm-1 K-2 at room temperature and out-performs other halogen-doped BiSbSe3 samples.Finally,combining the significantly enhanced power factor and maintained low thermal conductivity,a maximum ZT～1.0 is achieved in Cl-doped BiSbSe3 at 800 K.     

Interface driven energy filtering of thermoelectric power in spark plasma sintered bi(2)te(2.7)se(0.3) nanoplatelet composites

Control of competing parameters such as thermoelectric (TE) power and electrical and thermal conductivities is essential for the high performance of thermoelectric materials. Bulk-nanocomposite materials have shown a promising improvement in the TE performance due to poor thermal conductivity and charge carrier filtering by interfaces and grain boundaries. Consequently, it has become pressingly important to understand the formation mechanisms, stability of interfaces and grain boundaries along with subsequent effects on the physical properties. We report here the effects of the thermodynamic environment during spark plasma sintering (SPS) on the TE performance of bulk-nanocomposites of chemically synthesized Bi(2)Te(2.7)Se(0.3) nanoplatelets. Four pellets of nanoplatelets powder synthesized in the same batch have been made by SPS at different temperatures of 230, 250, 280, and 350 °C. The X-ray diffraction, transmission electron microscopy, thermoelectric, and thermal transport measurements illustrate that the pellet sintered at 250 °C shows a minimum grain growth and an optimal number of interfaces for efficient TE figure of merit, ZT～0.55. For the high temperature (350 °C) pelletized nanoplatelet composites, the concurrent rise in electrical and thermal conductivities with a deleterious decrease in thermoelectric power have been observed, which results because of the grain growth and rearrangements of the interfaces and grain boundaries. Cross section electron microscopy investigations indeed show significant grain growth. Our study highlights an optimized temperature range for the pelletization of the nanoplatelet composites for TE applications. The results provide a subtle understanding of the grain growth mechanism and the filtering of low energy electrons and phonons with thermoelectric interfaces.     

Solid‐State Explosive Reaction for Nanoporous Bulk Thermoelectric Materials

High‐performance thermoelectric materials require ultralow lattice thermal conductivity typically through either shortening the phonon mean free path or reducing the specific heat. Beyond these two approaches, a new unique, simple, yet ultrafast solid‐state explosive reaction is proposed to fabricate nanoporous bulk thermoelectric materials with well‐controlled pore sizes and distributions to suppress thermal conductivity. By investigating a wide variety of functional materials, general criteria for solid‐state explosive reactions are built upon both thermodynamics and kinetics, and then successfully used to tailor material's microstructures and porosity. A drastic decrease in lattice thermal conductivity down below the minimum value of the fully densified materials and enhancement in thermoelectric figure of merit are achieved in porous bulk materials. This work demonstrates that controlling materials' porosity is a very effective strategy and is easy to be combined with other approaches for optimizing thermoelectric performance.     

Thermal conductivity modeling of core-shell and tubular nanowires

The heteroepitaxial growth of crystalline core-shell nanostructures of a variety of materials has become possible in recent years, allowing the realization of various novel nanoscale electronic and optoelectronic devices. The increased surface or interface area will decrease the thermal conductivity of such nanostructures and impose challenges for the thermal management of such devices. In the meantime, the decreased thermal conductivity might benefit the thermoelectric conversion efficiency. In this paper, we present modeling results on the lattice thermal conductivity of core-shell and tubular nanowires along the wire axis direction using the phonon Boltzmann equation. We report the dependence of the thermal conductivity on the surface conditions and the core-shell geometry for silicon core-germanium shell and tubular silicon nanowires at room temperature. The results show that the effective thermal conductivity changes not only with the composition of the constituents but also with the radius of the nanowires and nanopores due to the nature of the ballistic phonon transport. The results in this work have implications for the design and operation of a variety of nanoelectronic devices, optoelectronic devices, and thermoelectric materials and devices.     

Phonon Transport of Zigzag/Armchair Graphene Superlattice Nanoribbons

Nanostructured thermoelectric materials are promising for modulating physical properties to achieve high thermoelectric performance. In this paper, thermal transport properties of armchair/zigzag graphene superlattice nanoribbons (A/Z graphene SLNRs) are investigated by performing nonequilibrium molecular dynamics simulations. The target of the research is to realize low thermal conductivity by introducing single-vacancy point defects. Our simulations demonstrate that the thermal conductivity of A/Z graphene SLNRs depends nonmonotonically on periodic length. In addition, introducing single-vacancy point defects into the superlattice nanoribbons could decrease the phonon tunneling in superlattice nanoribbons, so that the thermal conductivity could be reduced further. Furthermore, a monotonic dependence of the thermal conductivity of A/Z graphene SLNRs with length of zigzag part in periodic length is discovered. This phenomenon is explained by performing phonon property analysis. Our simulations deliver a detailed phonon transport in A/Z graphene SLNRs and provide useful guidance on how to engineer the thermal transport properties of A/Z graphene SLNRs for applications of nanoribbon-related devices in thermoelectrics.     

Thermally insulative thermoelectric argyrodites

Ag/Cu-based argyrodites have recently received extensive attention as promising thermoelectric materials. This can be largely understood by the hierarchical chemical bonding structure that allows the existence of high-concentration and highly mobile Ag⁺/Cu⁺ ions distributed in the rigid polyanionic framework, since the diffusive cations enable an effective phonon scattering for phononically insulating as that in woods while the rigid framework ensures an electronic charge transport as that in trivial thermoelectric semiconductors. This review focused firstly on the synthesis of both single-crystalline and poly-crystalline argyrodites as well as the chemical compositions, phases and crystallographic features; secondly on the origins of the low lattice thermal conductivity including crystal structure complexity, weak bonded ions, strong lattice anharmonicity and low sound velocity; and thirdly on the electronic band structure characteristics including effective mass and band degeneracy for an evaluation on the electronic transport properties, as well as performance stability. These together lead a perspective on future development of thermoelectric argyrodites to emphasize the enhancement of electronic performance such as by carrier concentration and by manipulating the electronic band structure.     

多掺杂协同调控碲化锡热导率和功率因子提升热电性能

碲化锡(SnTe)是一种碲化铅的无铅替代物, 在热电领域有广阔的应用前景。但是, 纯相碲化锡样品具有较高的热导率与较低的塞贝克系数, 导致热电性能较差。本研究通过多重掺杂可以显著降低热导率, 提升塞贝克系数, 从而提升热电性能。SnTe热压样品的晶格热导率随着Se和S的引入明显降低,比如SnTe_0.7S_0.15Se_0.15室温下晶格热导率仅为0.99 W•m^-1•K^-1。透射电子显微镜显示, SnTe掺杂样品内存在大量的纳米沉淀相与晶格位错。在此基础上, 掺杂In在价带顶引入共振态大幅提高了样品的塞贝克系数。实验表明通过多重掺杂可以有效提升碲化锡的热电性能, 其中样品Sn_0.99In_0.01Te_0.7S_0.15Se_0.15在850 K时峰值ZT值达到0.8, 这说明碲化锡的确是一种有应用前景的中温区热电材料。