Showing papers on "Thermoelectric effect published in 2014"

Ultralow thermal conductivity and high thermoelectric figure of merit in SnSe crystals

Abstract: The thermoelectric effect enables direct and reversible conversion between thermal and electrical energy, and provides a viable route for power generation from waste heat The efficiency of thermoelectric materials is dictated by the dimensionless figure of merit, ZT (where Z is the figure of merit and T is absolute temperature), which governs the Carnot efficiency for heat conversion Enhancements above the generally high threshold value of 25 have important implications for commercial deployment, especially for compounds free of Pb and Te Here we report an unprecedented ZT of 26 ± 03 at 923 K, realized in SnSe single crystals measured along the b axis of the room-temperature orthorhombic unit cell This material also shows a high ZT of 23 ± 03 along the c axis but a significantly reduced ZT of 08 ± 02 along the a axis We attribute the remarkably high ZT along the b axis to the intrinsically ultralow lattice thermal conductivity in SnSe The layered structure of SnSe derives from a distorted rock-salt structure, and features anomalously high Gruneisen parameters, which reflect the anharmonic and anisotropic bonding We attribute the exceptionally low lattice thermal conductivity (023 ± 003 W m(-1) K(-1) at 973 K) in SnSe to the anharmonicity These findings highlight alternative strategies to nanostructuring for achieving high thermoelectric performance

The panoscopic approach to high performance thermoelectrics

Abstract: This review discusses recent developments and current research in high performance bulk thermoelectric materials, comprising nanostructuring, mesostructuring, band alignment, band engineering and synergistically defining key strategies for boosting the thermoelectric performance. To date, the dramatic enhancements in the figure of merit achieved in bulk thermoelectric materials have come either from the reduction in lattice thermal conductivity or improvement in power factors, or both of them. Here, we summarize these relationships between very large reduction of the lattice thermal conductivity with all-scale hierarchical architecturing, large enhanced Seebeck coefficients with intra-matrix electronic structure engineering, and control of the carrier mobility with matrix/inclusion band alignment, which enhance the power factor and reduce the lattice thermal conductivity. The new concept of hierarchical compositionally alloyed nanostructures to achieve these effects is presented. Systems based on PbTe, PbSe and PbS in which spectacular advances have been demonstrated are given particular emphasis. A discussion of future possible strategies is aimed at enhancing the thermoelectric figure of merit of these materials.

Organic Thermoelectric Materials: Emerging Green Energy Materials Converting Heat to Electricity Directly and Efficiently

Abstract: The abundance of solar thermal energy and the widespread demands for waste heat recovery make thermoelectric generators (TEGs) very attractive in harvesting low-cost energy resources. Meanwhile, thermoelectric refrigeration is promising for local cooling and niche applications. In this context there is currently a growing interest in developing organic thermoelectric materials which are flexible, cost-effective, eco-friendly and potentially energy-efficient. In particular, the past several years have witnessed remarkable progress in organic thermoelectric materials and devices. In this review, thermoelectric properties of conducting polymers and small molecules are summarized, with recent progresses in materials, measurements and devices highlighted. Prospects and suggestions for future research efforts are also presented. The organic thermoelectric materials are emerging candidates for green energy conversion.

Enhanced Thermoelectric Efficiency via Orthogonal Electrical and Thermal Conductances in Phosphorene

Abstract: Thermoelectric devices that utilize the Seebeck effect convert heat flow into electrical energy and are highly desirable for the development of portable, solid state, passively powered electronic systems. The conversion efficiencies of such devices are quantified by the dimensionless thermoelectric figure of merit (ZT), which is proportional to the ratio of a device’s electrical conductance to its thermal conductance. In this paper, a recently fabricated two-dimensional (2D) semiconductor called phosphorene (monolayer black phosphorus) is assessed for its thermoelectric capabilities. First-principles and model calculations reveal not only that phosphorene possesses a spatially anisotropic electrical conductance, but that its lattice thermal conductance exhibits a pronounced spatial-anisotropy as well. The prominent electrical and thermal conducting directions are orthogonal to one another, enhancing the ratio of these conductances. As a result, ZT may reach the criterion for commercial deployment along th...

A review on thermoelectric renewable energy: Principle parameters that affect their performance

Abstract: Developing thermoelectric materials with superior performance means tailoring interrelated thermoelectric physical parameters – electrical conductivities, Seebeck coefficients, and thermal conductivities – for a crystalline system. High electrical conductivity, low thermal conductivity, and a high Seebeck coefficient are desirable for thermoelectric materials. Therefore, knowledge of the relation between electrical conductivity and thermal conductivity is essential to improve thermoelectric properties. In general, research in recent years has focused on developing thermoelectric structures and materials of high efficiency. The importance of this parameter is universally recognized; it is an established, ubiquitous, routinely used tool for material, device, equipment and process characterization both in the thermoelectric industry and in research. In this paper, basic knowledge of thermoelectric materials and an overview of parameters that affect the figure of merit ZT are provided. The prospects for the optimization of thermoelectric materials and their applications are also discussed.

High thermoelectric performance in non-toxic earth-abundant copper sulfide.

Abstract: A new type of high performance thermoelectric material Cu_(2-x)S composed of non-toxic and earth-abundant elements Cu and S is reported Cu_(2-x)S surprisingly has lower thermal conductivity and more strikingly reduced specific heat compared to the heavier Cu_(2)Se, leading to an increased zT to 17

Point Defect Engineering of High-Performance Bismuth-Telluride-Based Thermoelectric Materials

Abstract: Developing high-performance thermoelectric materials is one of the crucial aspects for direct thermal-to-electric energy conversion. Herein, atomic scale point defect engineering is introduced as a new strategy to simultaneously optimize the electrical properties and lattice thermal conductivity of thermoelectric materials, and (Bi,Sb)2(Te,Se)3 thermoelectric solid solutions are selected as a paradigm to demonstrate the applicability of this new approach. Intrinsic point defects play an important role in enhancing the thermoelectric properties. Antisite defects and donor-like effects are engineered in this system by tuning the formation energy of point defects and hot deformation. As a result, a record value of the figure of merit ZT of ≈1.2 at 445 K is obtained for n-type polycrystalline Bi2Te2.3Se0.7 alloys, and a high ZT value of ≈1.3 at 380 K is achieved for p-type polycrystalline Bi0.3Sb1.7Te3 alloys, both values being higher than those of commercial zone-melted ingots. These results demonstrate the promise of point defect engineering as a new strategy to optimize thermoelectric properties.

High Thermoelectric Performance of p-Type SnTe via a Synergistic Band Engineering and Nanostructuring Approach

Abstract: SnTe is a potentially attractive thermoelectric because it is the lead-free rock-salt analogue of PbTe. However, SnTe is a poor thermoelectric material because of its high hole concentration arising from inherent Sn vacancies in the lattice and its very high electrical and thermal conductivity. In this study, we demonstrate that SnTe-based materials can be controlled to become excellent thermoelectrics for power generation via the successful application of several key concepts that obviate the well-known disadvantages of SnTe. First, we show that Sn self-compensation can effectively reduce the Sn vacancies and decrease the hole carrier density. For example, a 3 mol % self-compensation of Sn results in a 50% improvement in the figure of merit ZT. In addition, we reveal that Cd, nominally isoelectronic with Sn, favorably impacts the electronic band structure by (a) diminishing the energy separation between the light-hole and heavy-hole valence bands in the material, leading to an enhanced Seebeck coefficien...

Resonant bonding leads to low lattice thermal conductivity.

Abstract: Understanding the lattice dynamics and low thermal conductivities of IV-VI, V2-VI3 and V materials is critical to the development of better thermoelectric and phase-change materials. Here we provide a link between chemical bonding and low thermal conductivity. Our first-principles calculations reveal that long-ranged interaction along the 〈100〉 direction of the rocksalt structure exist in lead chalcogenides, SnTe, Bi2Te3, Bi and Sb due to the resonant bonding that is common to all of them. This long-ranged interaction in lead chalcogenides and SnTe cause optical phonon softening, strong anharmonic scattering and large phase space for three-phonon scattering processes, which explain why rocksalt IV-VI compounds have much lower thermal conductivities than zincblende III-V compounds. The new insights on the relationship between resonant bonding and low thermal conductivity will help in the development of better thermoelectric and phase change materials.

BiCuSeO oxyselenides: new promising thermoelectric materials

Abstract: BiCuSeO oxyselenides have recently acquired ever-increasing attention and have been extensively studied as very promising thermoelectric materials. The ZT of the BiCuSeO system was significantly increased from 0.5 to 1.4 in the past three years, which indicates that BiCuSeO oxyselenides are robust candidates for energy conversion applications. In this review, we first discuss and summarize the crystal structures, microstructures, electronic structures and physical/chemical properties of BiCuSeO oxyselenides. Then, the approaches that successfully enhanced the thermoelectric performances in the BiCuSeO system are outlined, which include increasing carrier concentration, optimizing Cu vacancies, a simple and facile ball milling method, multifunctional Pb doping, band gap tuning, and increasing carrier mobility through texturing. Theoretical calculations to predict a maximum ZT in the BiCuSeO system are also described. Finally, a discussion of future possible strategies is proposed to aim at further enhancing the thermoelectric figure of merit of these materials.

Thermoelectric properties of p-type polycrystalline SnSe doped with Ag

Abstract: Many IV–VI semiconductors tend to be good thermoelectric materials, these include all Pb chalcogenides as well as Pb-free SnTe: all of which crystallize in a NaCl cubic structure. Another group of IV–VI compounds form layered orthorhombic structures. SnSe is one of these compounds, whose transport properties as a polycrystalline thermoelectric material have rarely been studied. Here we present our study of p-type polycrystalline SnSe doped with Ag, prepared by melting and hot pressing. SnSe has anisotropic properties with hysteresis observed in resistivity between 300 and 650 K regardless of doping. Ag is not an ideal dopant but is able to increase the carrier density significantly, as a result a peak zT of 0.6 was observed at 750 K. Transport properties of doped SnSe can be explained with a single parabolic band model, which suggests promising potential for this compound together with its challenges.

Bismuth Telluride and Its Alloys as Materials for Thermoelectric Generation

Abstract: Bismuth telluride and its alloys are widely used as materials for thermoelectric refrigeration. They are also the best materials for use in thermoelectric generators when the temperature of the heat source is moderate. The dimensionless figure of merit, ZT, usually rises with temperature, as long as there is only one type of charge carrier. Eventually, though, minority carrier conduction becomes significant and ZT decreases above a certain temperature. There is also the possibility of chemical decomposition due to the vaporization of tellurium. Here we discuss the likely temperature dependence of the thermoelectric parameters and the means by which the composition may be optimized for applications above room temperature. The results of these theoretical predictions are compared with the observed properties of bismuth telluride-based thermoelements at elevated temperatures. Compositional changes are suggested for materials that are destined for generator modules.

Phonon-glass electron-crystal thermoelectric clathrates : Experiments and theory

Abstract: Type-I clathrate compounds have attracted a great deal of interest in connection with the search for efficient thermoelectric materials. These compounds constitute networked cages consisting of nano-scale tetrakaidecahedrons (14 hedrons) and dodecahedrons (12 hedrons), in which the group 1 or 2 elements in the periodic table are encaged as the so-called rattling guest atom. It is remarkable that, though these compounds have crystalline cubic-structure, they exhibit glass-like phonon thermal conductivity over the whole temperature range depending on the states of rattling guest atoms in the tetrakaidecahedron. In addition, these compounds show unusual glass-like specific heats and THz-frequency phonon dynamics, providing a remarkable broad peak almost identical to those observed in topologically disordered amorphous materials or structural glasses, the so-called Boson peak. An efficient thermoelectric effect is realized in compounds showing these glass-like characteristics. This decade, a number of experimental works dealing with type-I clathrate compounds have been published. These are diffraction experiments, thermal and spectroscopic experiments in addition to those based on heat and electronic transport. These form the raw materials for this article based on advances this decade. The subject of this article involves interesting phenomena from the viewpoint of not only physics but also from the view point of the practical problem of elaborating efficient thermoelectric materials. This review presents a survey of a wide range of experimental investigations of type-I clathrate compounds, together with a review of theoretical interpretations of the peculiar thermal and dynamic properties observed in these materials.

A review of thermoelectrics research – Recent developments and potentials for sustainable and renewable energy applications

Abstract: In recent years, thermoelectric (TE) devices have emerged as promising alternative environmental friendly applications for heat pumps and power generators since the environmental issues such as the global warming and the limitations of energy resources gradually drew worldwide attentions. Due to the green feature and distinct advantages, the thermoelectric technology have been applied to different areas in an effort of designing simple, compact and environmental friendly systems. The applied areas are extended from the earliest application on kerosene lamp to aerospace applications, transportation tools, industrial utilities, medical services, electronic devices and temperature detecting and measuring facilities. The application potentials of TE in directly conversing thermal energy into electrical power have been identified, especially for where the cost of thermal energy input need not to be considered, such as waste heat utilization, in the light of the present low efficiency of thermoelectric conversion. The capability of TE in producing thermal energy (in terms of cooling or heating) with the use of electrical power is also well identified. This paper reviews the status of the material development and thermoelectric applications in different areas and discusses the difficulties in terms of the commercialisations of advanced materials. Other than this, the main purpose of this paper is to present the great potential of achieving both environmental and economic benefits by exclusively utilizing thermoelectric applications in different areas. It also comes to the conclusion that the thermoelectric applications with the current conversion efficiency are economically and technically practical for micro/small applications. However, it would be transformed to a more significant green energy solution for improving the current environment and energy issues by using medium/large scale thermoelectric applications when the thermoelectric materials with a figure-of-merit over 2 come into commercial practice.

Two-dimensional molybdenum carbides: potential thermoelectric materials of the MXene family

Abstract: A newly synthesized family of two-dimensional transition metal carbides and nitrides, so-called MXenes, exhibit metallic or semiconducting properties upon appropriate surface functionalization. Owing to their intrinsic ceramic nature, MXenes may be suitable for energy conversion applications at high temperature. Using the Boltzmann theory and first-principles electronic structure calculations, we explore the thermoelectric properties of monolayer and multilayer M2C (M = Sc, Ti, V, Zr, Nb, Mo, Hf, and Ta) and M2N (M = Ti, Zr, and Hf) MXenes functionalized with F, OH, and O groups. From our calculations, it turns out that monolayer and multilayer nanosheets of Mo2C acquire superior power factors to other MXenes upon any type of functionalization. We therefore propose the functionalized Mo2C nanosheets as potential thermoelectric materials of the MXene family. The exceptional thermoelectric properties of the functionalized Mo2C nanosheets are attributed to the peculiar t2g band shapes, which are a combination of flat and dispersive portions. These types of band shapes allow Mo2C to gain a large Seebeck coefficient and simultaneously a good electrical conductivity at low carrier concentrations.

Enhanced thermoelectric performance and anomalous seebeck effects in topological insulators.

Abstract: For topological insulators, the thermoelectric performance is not an intrinsic material property, but can be tuned by changing the system size.

Interplay of spin-orbit torque and thermoelectric effects in ferromagnet/normal-metal bilayers

Abstract: We present harmonic transverse voltage measurements of current-induced thermoelectric and spin-orbit torque (SOT) effects in ferromagnet/normal-metal bilayers, in which thermal gradients produced by Joule heating and SOT coexist and give rise to ac transverse signals with comparable symmetry and magnitude. Based on the symmetry and field dependence of the transverse resistance, we develop a consistent method to separate thermoelectric and SOT measurements. By addressing first ferromagnet/light-metal bilayers with negligible spin-orbit coupling, we show that in-plane current injection induces a vertical thermal gradient whose sign and magnitude are determined by the resistivity difference and stacking order of the magnetic and nonmagnetic layers. We then study ferromagnet/heavy-metal bilayers with strong spin-orbit coupling, showing that second harmonic thermoelectric contributions to the transverse voltage may lead to a significant overestimation of the antidamping SOT. We find that thermoelectric effects are very strong in Ta(6 nm)/Co(2.5 nm) and negligible in Pt(6 nm)/Co(2.5 nm) bilayers. After including these effects in the analysis of the transverse voltage, we find that the antidamping SOTs in these bilayers, after normalization to the magnetization volume, are comparable to those found in thinner Co layers with perpendicular magnetization, whereas the fieldlike SOTs are about an order of magnitude smaller.

An electrochemical system for efficiently harvesting low-grade heat energy

Abstract: Efficient and low-cost thermal energy-harvesting systems are needed to utilize the tremendous low-grade heat sources. Although thermoelectric devices are attractive, its efficiency is limited by the relatively low figure-of-merit and low-temperature differential. An alternative approach is to explore thermodynamic cycles. Thermogalvanic effect, the dependence of electrode potential on temperature, can construct such cycles. In one cycle, an electrochemical cell is charged at a temperature and then discharged at a different temperature with higher cell voltage, thereby converting heat to electricity. Here we report an electrochemical system using a copper hexacyanoferrate cathode and a Cu/Cu(2+) anode to convert heat into electricity. The electrode materials have low polarization, high charge capacity, moderate temperature coefficients and low specific heat. These features lead to a high heat-to-electricity energy conversion efficiency of 5.7% when cycled between 10 and 60 °C, opening a promising way to utilize low-grade heat.

Electronic and thermoelectric properties of few-layer transition metal dichalcogenides.

Abstract: The electronic and thermoelectric properties of one to four monolayers of MoS2, MoSe2, WS2, and WSe2 are calculated For few layer thicknesses, the near degeneracies of the conduction band K and Σ valleys and the valence band Γ and K valleys enhance the n-type and p-type thermoelectric performance The interlayer hybridization and energy level splitting determine how the number of modes within kBT of a valley minimum changes with layer thickness In all cases, the maximum ZT coincides with the greatest near-degeneracy within kBT of the band edge that results in the sharpest turn-on of the density of modes The thickness at which this maximum occurs is, in general, not a monolayer The transition from few layers to bulk is discussed Effective masses, energy gaps, power-factors, and ZT values are tabulated for all materials and layer thicknesses

Assessment of the thermoelectric performance of polycrystalline p-type SnSe

Abstract: We report the evaluation of the thermoelectric performance of polycrystalline p-type SnSe, a material in which unprecedented values of the thermoelectric figure of merit ZT have been recently discovered in single crystals Besides anisotropic transport properties, our results confirm that this compound exhibits intrinsically very low thermal conductivity values The electrical properties show trends typical of lightly doped, intrinsic semiconductors with thermopower values reaching 500 μV K−1 in a broad temperature range An orthorhombic-to-orthorhombic transition sets in at 823 K, a temperature at which the power factor reaches its maximum value A maximum ZT of 05 was obtained at 823 K, suggesting that proper optimization of the transport properties of SnSe might lead to higher ZT values These findings indicate that this system represents an interesting experimental platform for the search of highly efficient thermoelectric materials

Origin of the high performance in GeTe-based thermoelectric materials upon Bi2Te3 doping.

Abstract: As a lead-free material, GeTe has drawn growing attention in thermoelectrics, and a figure of merit (ZT) close to unity was previously obtained via traditional doping/alloying, largely through hole carrier concentration tuning. In this report, we show that a remarkably high ZT of ∼1.9 can be achieved at 773 K in Ge0.87Pb0.13Te upon the introduction of 3 mol % Bi2Te3. Bismuth telluride promotes the solubility of PbTe in the GeTe matrix, thus leading to a significantly reduced thermal conductivity. At the same time, it enhances the thermopower by activating a much higher fraction of charge transport from the highly degenerate Σ valence band, as evidenced by density functional theory calculations. These mechanisms are incorporated and discussed in a three-band (L + Σ + C) model and are found to explain the experimental results well. Analysis of the detailed microstructure (including rhombohedral twin structures) in Ge0.87Pb0.13Te + 3 mol % Bi2Te3 was carried out using transmission electron microscopy and crystallographic group theory. The complex microstructure explains the reduced lattice thermal conductivity and electrical conductivity as well.

Solubility-Limited Extrinsic n-Type Doping of a High Electron Mobility Polymer for Thermoelectric Applications

Abstract: The thermoelectric properties of a highperformance electron-conducting polymer, (P(NDIOD-T2), extrinsically doped with dihydro-1H-benzoimidazol-2-yl (NDBI) derivatives, are reported. The highest thermoelectric power factor that has been reported for a solution-processed n-type polymer is achieved; and it is concluded that engineering polymerdopant miscibility is essential for the development of organic thermoelectrics.

High thermoelectric performance realized in a BiCuSeO system by improving carrier mobility through 3D modulation doping.

Abstract: We report a greatly enhanced thermoelectric performance in a BiCuSeO system, realized by improving carrier mobility through modulation doping. The heterostructures of the modulation doped sample make charge carriers transport preferentially in the low carrier concentration area, which increases carrier mobility by a factor of 2 while maintaining the carrier concentration similar to that in the uniformly doped sample. The improved electrical conductivity and retained Seebeck coefficient synergistically lead to a broad, high power factor ranging from 5 to 10 μW cm(-1) K(-2). Coupling the extraordinarily high power factor with the extremely low thermal conductivity of ∼0.25 W m(-1) K(-1) at 923 K, a high ZT ≈ 1.4 is achieved in a BiCuSeO system.

Perovskites for Solar Thermoelectric Applications: A First Principle Study of CH3NH3AI3 (A = Pb and Sn)

Abstract: Hybrid organic/inorganic CH3NH3AI3 (A = Pb and Sn) perovskites have been recognized as promising photovoltaic materials. Using ab initio calculations, we showed that these systems may also be promising materials for solar thermoelectric applications. We found that their large carrier mobilities mainly originate from a combination of the small effective masses of electrons and holes and a relatively weak carrier–phonon interaction. We propose that by tuning the carrier concentration to values of the order of ∼1018cm–3, the thermoelectric figure of merit of Sn and Pb based perovskites may reach values ranging from 1 to 2, which could possibly be further increased by optimizing the lattice thermal conductivity through engineering perovskite superlattices.

Shifting up the optimum figure of merit of p -type bismuth telluride-based thermoelectric materials for power generation by suppressing intrinsic conduction

Abstract: The abundance of low-temperature waste heat produced by industry and automobile exhaust necessitates the development of power generation with thermoelectric (TE) materials. Commercially available bismuth telluride-based alloys are generally used near room temperature. Materials that are composed of p-type bismuth telluride, which are suitable for low-temperature power generation (near 380 K), were successfully obtained through Sb-alloying, which suppresses detrimental intrinsic conduction at elevated temperatures by increasing hole concentrations and material band gaps. Furthermore, hot deformation (HD)-induced multi-scale microstructures were successfully realized in the high-performance p-type TE materials. Enhanced textures and donor-like effects all contributed to improved electrical transport properties. Multiple phonon scattering centers, including local nanostructures induced by dynamic recrystallization and high-density lattice defects, significantly reduced the lattice thermal conductivity. These combined effects resulted in observable improvement of ZT over the entire temperature range, with all TE parameters measured along the in-plane direction. The maximum ZT of 1.3 for the hot-deformed Bi0.3Sb1.7Te3 alloy was reached at 380 K, whereas the average ZTav of 1.18 was found in the range of 300–480 K, indicating potential for application in low-temperature TE power generation. Thermoelectric materials, which convert temperature differences and electric voltage into each other, serve in refrigeration or power generation applications. Currently, bismuth telluride (Bi2Te3) and its alloys are the most widely used thermoelectric materials. Tie-Jun Zhu, Xin-Bing Zhao and co-workers from Zhejiang University, China, have now investigated the effect of antimony (Sb) alloying on bismuth tellurides through a series of polycrystalline solid solutions of Bi2-xSbxTe3—where x varies between 1.4 and 1.8—prepared by hot deformation. Systematic tuning of the alloy composition showed that higher antimony content raised the material's optimal conversion temperature by repressing undesirable conduction. This effect arises from an increase in both the hole concentration and the band gap in the material. For a composition where x is 1.7, the alloy showed optimal performances at 380 kelvin—a suitable temperature for low-temperature power generation from the waste heat generated by industry or vehicles. The p-type bismuth telluride-based polycrystalline materials suiting for low-temperature power generations (near 380 K) have been obtained through Sb-alloying and HD, which suppresses the detrimental effect of intrinsic conduction at elevated temperature via increasing the hole concentration and band gap. The hot-deformed Bi0.3Sb1.7Te3 alloy, not usual composition Bi0.5Sb1.5Te3, shows a maximum ZT of 1.3 at 380 K, indicating a bright application potential in low-temperature power generations.

Most efficient quantum thermoelectric at finite power output.

Abstract: Machines are only Carnot efficient if they are reversible, but then their power output is vanishingly small. Here we ask, what is the maximum efficiency of an irreversible device with finite power output? We use a nonlinear scattering theory to answer this question for thermoelectric quantum systems, heat engines or refrigerators consisting of nanostructures or molecules that exhibit a Peltier effect. We find that quantum mechanics places an upper bound on both power output and on the efficiency at any finite power. The upper bound on efficiency equals Carnot efficiency at zero power output but decays with increasing power output. It is intrinsically quantum (wavelength dependent), unlike Carnot efficiency. This maximum efficiency occurs when the system lets through all particles in a certain energy window, but none at other energies. A physical implementation of this is discussed, as is the suppression of efficiency by a phonon heat flow.

Indium selenides: structural characteristics, synthesis and their thermoelectric performances.

Abstract: Indium selenides have attracted extensive attention in high-efficiency thermoelectrics for waste heat energy conversion due to their extraordinary and tunable electrical and thermal properties. This Review aims to provide a thorough summary of the structural characteristics (e.g. crystal structures, phase transformations, and structural vacancies) and synthetic methods (e.g. bulk materials, thin films, and nanostructures) of various indium selenides, and then summarize the recent progress on exploring indium selenides as high-efficiency thermoelectric materials. By highlighting challenges and opportunities in the end, this Review intends to shine some light on the possible approaches for thermoelectric performance enhancement of indium selenides, which should open up an opportunity for applying indium selenides in the next-generation thermoelectric devices.

Abnormally enhanced thermoelectric transport properties of SWNT/PANI hybrid films by the strengthened PANI molecular ordering

Abstract: Single-walled carbon nanotube (SWNT)/polyaniline (PANI) hybrid films were prepared by casting the suspension containing well-dispersed SWNTs and CSA-doped PANI. The electrical conductivity of the SWNT/PANI film at first increased with the increasing SWNT content and then decreased at high SWNT content, whereas the Seebeck coefficient increased monotonically in the present SWNT content range. Moreover, the electrical conductivity values of the SWNT/PANI composites were much higher than the values calculated based on the series-connected two-component mixture model, whereas the dependence of the Seebeck coefficient on the SWNT content fitted well with the mixture model. Thermal conductivities increased with the SWNT content, but the rate of increase was much lower than the values estimated using the mixture model. The maximum values of electrical conductivity and Seebeck coefficient of hybrid films were up to 769 S cm−1 and 65 μV K−1. Consequently, the maximum thermoelectric power factor and ZT value at room temperature reached 176 μW m−1 K−2 and 0.12, respectively. The optimal TE property of the SWNT/PANI hybrid film was remarkably higher than those of either individual component of the composite, and exhibits the highest values for inorganic–organic composite materials reported so far. XRD and Raman analyses revealed that the PANI molecules in the composite film had a more expanded conformation, and were more orderly arranged compared with both pure PANI bulk and pure PANI film. The abnormally enhanced thermoelectric performance is attributed to the highly ordered PANI interface layer on the SWNT surface, which formed by the synergetic effect of chain expansion by the chemical interactions between PANI and the solvent and chain-ordering due to the π–π conjugation between PANI and CNTs.

Enhanced thermoelectric performance of phosphorene by strain-induced band convergence

Abstract: The newly emerging monolayer phosphorene was recently predicted to be a promising thermoelectric material. In this work, we propose to further enhance the thermoelectric performance of phosphorene using the strain-induced band convergence. The effect of the uniaxial strain on the thermoelectric properties of phosphorene was investigated by using the first-principles calculations combined with the semiclassical Boltzmann theory. When the zigzag-direction strain is applied, the Seebeck coefficient and electrical conductivity in the zigzag direction can simultaneously be greatly enhanced at the critical strain of 5%, at which the band convergence is achieved. The largest $ZT$ value of 1.65 at 300 K is then conservatively estimated by using the bulk lattice thermal conductivity. When the armchair-direction strain of 8% is applied, the room-temperature $ZT$ value can reach 2.12 in the armchair direction of phosphorene. Our results indicate that strain-induced band convergence could be an effective method to enhance the thermoelectric performance of phosphorene.