Thermoelectric Conversion Performance Research Articles

Enhancement of Transverse Thermoelectric Conversion by Interface‐Induced Spin Current in Ferromagnetic Metal/Nonmagnetic Insulator Hybrid‐Structure

AbstractTransverse thermoelectric conversion phenomena including the anomalous Ettingshausen effect (AEE) and anomalous Nernst effect (ANE) in magnetic materials are actively investigated to realize versatile cooling and energy harvesting technologies. However, further improvement of the thermoelectric performance of AEE and ANE is still required, and most research efforts have focused on material exploration. Here, a new approach to improve the transverse thermoelectric conversion performance through interface engineering is reported by focusing on the transverse thermoelectric phenomena that output heat currents. A longitudinal charge current in a ferromagnetic metal Ni/nonmagnetic insulator Bi2WO6 hybrid‐structure induces a larger transverse heat current than that of AEE in a Ni single‐layer. It is indicated that the enhancement of the transverse thermoelectric conversion is due to the generation of a heat current concomitant with a spin current induced at the Ni/Bi2WO6 interface. This finding demonstrates that the interface of a magnetic metal/nonmagnetic insulator has a potential to improve the transverse thermoelectric performance, extending the applicability of nonmagnetic insulators to the field of spin caloritronics and thermoelectrics.

Thermally-driven electrokinetic power generation system utilizing microporous NiO wicks and nanoporous AAO membranes for low-grade heat harvesting

Low-grade heat (T<100 ℃) exists ubiquitously in the environment, but it is difficult to harvest and utilize due to existing technology’s limitations. It would be significant to harvest this kind of energy through effective acquisition techniques. Here, we propose a micro/nano porous membranes low-grade heat harvesting system based on the thermoelectric effect, which uses the temperature gradient to induce the separation and transmission of electrolyte solution ions in the porous membrane to convert low-grade thermal energy into electrical energy. Two power generation materials of microporous nickel oxide (NiO) wicks and nanoporous anodic aluminum oxide (AAO) membranes were investigated experimentally. In the porous NiO wick experiment, it is observed that the wick significantly improves the thermoelectric conversion performance. In particular, when the temperature difference is 50 ℃, the open circuit voltage of the system reaches 215 mV, which is about 300 % higher than the case without the porous NiO wick. In addition, in the AAO porous membranes experiment, we found that at the low concentration, by collecting ambient heat with a temperature difference of 5 ℃, the system obtained the output power of about 6 nW, which improved the 2 ∼ 3 orders of magnitude compared with the previous research reports. This work offers a promising method for developing high-performance and low-cost low-grade heat harvesting systems.

Performance evaluation of AMTEC/TEG coupling system for nuclear power space stations in space exploration

For deep space exploration, the performance of an alkali metal thermoelectric converter (AMTEC) can be improved by utilizing the thermoelectric generator (TEG) subsystem. In this work, a coupling system comprised of an AMTEC and a TEG is proposed, in which the waste heat produced by the AMTEC top cycle is used by the TEG bottom cycle cooled by a heat pipe radiator. A general mathematical model is built to analyze the output power and efficiency of each subsystem and the whole system. Finally, the effect of critical parameters on the output power and efficiency are evaluated, including the current density of AMTEC, dimensionless current i of TEG, geometrical structure, and operating environment. The results show that the combined AMTEC/TEG system is 4.35% and 15.39% higher than the efficiency and output power obtained when using the TEG subsystem to recycle the waste heat from the AMTEC condenser. The thermoelectric conversion performance of the AMTEC/TEG system is related to that of the AMTEC subsystem while less affected by the TEG subsystem. The present work could provide a new route for constructing long-distance nuclear space power stations, rovers, and lunar bases in extreme environments.

Self‐Powered Vibration Frequency Monitoring Device for the Grid Based on Triboelectric Nanogenerator and Micro Thermoelectric Generator

AbstractThe galloping of transmission lines (GTLs) has a significant impact on the development of smart grids. However, traditional vibration frequency monitoring devices for transmission lines suffer from issues such as low measurement accuracy, high environmental requirements, and the inability to achieve self‐powering. A self‐powered vibration frequency monitoring method is proposed based on frequency‐sensing triboelectric nanogenerator (F‐TENG) and micro thermoelectric generators (MTEG). Models for vibration frequency sensing based on F‐TENG and energy capture based on MTEG are established. A flexible self‐powered sensing prototype, integrating F‐TENG, MTEG, and a Signal processing and Energy harvesting Circuit (SEC), is fabricated. Additionally, an innovative solvothermal method for the preparation of MTEG materials is presented, resulting in Bi2Te3‐based thermoelectric materials with significantly high thermoelectric conversion performance. Experimental results demonstrate that within the range of 0.1–5.1 Hz, F‐TENG can precisely perceive the frequency of GTLs, with a maximum error of 1.274%. MTEG achieves a maximal open‐circuit voltage of 3.282 V. Finally, the SEC unit is designed to couple the outputs of F‐TENG and MTEG for frequency calculation and wireless transmission to a microcontroller. This device provides an efficient solution for monitoring the frequency of GTLs and offers robust support for the stability of the smart grid.

Mesoscopic combinatorial design of anisotropic graphitic carbon with ordered porous frameworks for thermoelectric conversion

The incorporation of carbon materials can effectively enhance the performance of thermoelectric materials. By optimizing the thermoelectric properties of the carbon scaffold, it is possible to further improve the thermoelectric performance of the prepared carbon-based composites. To elevate the thermoelectric performance of the carbon scaffold, this work introduces a novel mesoscopic design strategy, utilizing natural flake graphite to create a three-dimensional ordered porous framework. Highly controllable anisotropic conductivity and thermal conductivity of the porous graphite skeleton were achieved, by combining sizable sodium chlorine templates (37.5–250 μm) with a natural flake graphite matrix (50–150 μm). A porous graphite with high electrical conductivity and low thermal conductivity along the vertical axis was obtained by carefully regulating the parameters. The sample demonstrated exceptional thermoelectric conversion performance, achieving a thermoelectric power factor of 12.2 μW m−1 K−2 and a figure of merit value of 28.3 × 10−4. Furthermore, the thermoelectric property of the samples could be greatly improved by adding a small amount of ferric chloride, with a thermoelectric power factor reaching 41.9 × 10−4 and a thermoelectric figure of merit reaching 81.4 μW m−1 K−2. This work provides an effective scheme for the mesoscopic regulation of porous graphite carbon structures and offers a new strategy for enhancing the performance of thermoelectric generators.

A Rolled Organic Thermoelectric Generator with High Thermocouple Density

AbstractThe surge in the number of distributed microelectronics and sensors requires versatile, scalable, and affordable power sources. Heat‐harvesting organic thermoelectric generators (TEGs) are regarded as potential key components of the future energy landscape. Recent advances in the performance of organic thermoelectric materials have made practical applications of organic TEGs more feasible than ever before, yet the challenges of designing and fabricating organic TEGs suitable for real scenarios are scarcely addressed. Specifically, small sensors and wearables demand for micro‐thermoelectric generators (µTEGs) with high power density architectures and small form factors, while typical demonstrations of organic TEGs are characterized by &lt; 10 thermocouples (TCs) per cm2. This work presents a rolled, organic µTEG architecture combining large‐area, solution‐based deposition techniques, such as inkjet and spray‐coating, and an ultrathin parylene substrate to achieve a thermocouple density of 1842 TCs cm−2. Such demonstrative µTEG reaches a thermoelectric conversion performance of 0.15 µW cm−2 at ΔT = 50 K. Such power output is well in line with finite element method simulations, which highlight the benefit of the architecture and show that remarkable power densities, in the mW cm−2 range at ΔT = 10 K, are realistically achievable with geometrical improvements and already ongoing advancements in organic thermoelectric inks.

Efficiency Enhancement in Ocean Thermal Energy Conversion: A Comparative Study of Heat Exchanger Designs for Bi2Te3-Based Thermoelectric Generators.

This research focuses on enhancing the efficiency of Bi2Te3-based thermoelectric generators (TEGs) in ocean thermal energy conversion (OTEC) systems through innovative heat exchanger designs. Our comparative study uses computer simulations to evaluate three types of heat exchangers: cavity, plate-fins, and longitudinal vortex generators (LVGs). We analyze their impact on thermoelectric conversion performance, considering the thermal energy transfer from warm surface seawater to TEGs. The results demonstrate that heat exchangers with plate-fins and LVGs significantly outperform the cavity heat exchanger regarding thermal energy transfer efficiency. Specifically, plate-fins increase TEG output power by approximately 22.92% and enhance thermoelectric conversion efficiency by 38.20%. Similarly, LVGs lead to a 13.02% increase in output power and a 16.83% improvement in conversion efficiency. These advancements are contingent upon specific conditions such as seawater flow rates, fin heights, LVG tilt angles, and locations. The study underscores the importance of optimizing heat exchanger designs in OTEC systems, balancing enhanced heat transfer against the required pump power. Our findings contribute to a broader understanding of materials science in sustainable energy technologies.

Effect analysis on energy conversion enhancement of porous medium micro-combustor and thermoelectric system and its optimization

The energy conversion of micro combustion system has become the main research direction at present. However, due to the small size of micro-combustors, they face challenges such as unstable combustion and difficulty in effectively converting generated thermal energy into usable energy. This work mainly studies the thermoelectric conversion system based on porous medium micro-combustor, which mainly uses the high energy density of micro-combustor to replace the traditional battery energy and improve the working time of small equipment. And this work also establishes the coupling model of porous medium micro combustor and thermoelectric module. By analyzing the performance parameters of the micro thermoelectric system in different flow rate, the results show that at low flow rates, semiconductor materials with better thermoelectric performance, such as Bi2Te3, can be used to improve the energy conversion efficiency of the system. However, the low heat release from combustion leads to lower maximum output power. At high flow rates, there is a significant improvement in the thermoelectric conversion performance of the system, but it is limited by the temperature of the thermoelectric materials. To optimize the impact of these factors on micro-thermoelectric systems, the optimization methods were analyzed as: (1) The temperature uniformity of micro-combustor wall can be improved by using mixture with low equivalence ratio and high flow rate; (2) enhancing the thermal conductivity of the micro-combustor and porous medium to improve the thermal conductivity of micro thermoelectric system; (3) Using CH4-H2 mixed combustion to reduce combustion temperature and ensure that thermoelectric materials work within the normal temperature range; (4) The segmented thermocouple is used to improve the thermoelectric performance of thermoelectric module. The results show that using a mixture with low equivalence ratio and high flow rate can achieve a maximum conversion efficiency of 5.51%. Using mixed combustion to reduce combustion temperature can increase the maximum output power of the system to 57.8 W.

Performance analysis of diesel particulate filter thermoelectric conversion mobile energy storage system under engine conditions of low-speed and light-load

The thermal heat from diesel particulate filter (DPF) can generate electrical energy through the thermoelectric generator (TEG) which can be stored in mobile battery power energy storage system (MBPES). The DPF-TEG of MBPES system is a new technology proposed in this study, which is made up of the DPF system, heat exchanger (HEX), the thermoelectric module (TEM) and the battery. The effects of low-speed and light-load operating conditions on the DPF-TEG of MBPES system were studied. The low regeneration temperatures (823 K, 873 K, 923 K) and the high regeneration temperatures (1606 K, 1696 K, 1786 K) were selected to explore the thermoelectric conversion performance of DPF-TEG of MBPES system. The maximum output power is increased 55% with the velocity change from 6 m/s to 8 m/s at low regeneration temperatures. The DPF-TEG of MBPES system contains the 48 TEMs with external load 19.2 Ω, which can generate the maximum power output of 4.99 w/6 w/7.12 w of 823 K/873 K/923 K, 32.09 w/36.76 w/41.75 w of 1606 K/1696 K/1786 K. The output voltage of the 4 TEMs increased from 0.015837 V to 0.016172 V per 10 K temperature difference. The output power grew from 0.017714 w to 0.08057 w.

Modeling and property study of thermoelectric converter based on subwavelength photothermal absorption structure

Achieving light absorption and conversion in a wide wavelength region and large angles of incidence is an effective way to improve the efficiency of thermoelectric devices. In this work, the thermoelectric converter device with a sub-wavelength photothermal absorption structure is designed, and the thermoelectric device is adapted to medium and high-temperature working environments by introducing PbS and SiGe thermoelectric materials. By changing the material of the surface heat-absorbing layer and studying the adjustment effect of the shape and size of the sub-wavelength photothermal absorption structure on reflectivity, the perfect absorption of sunlight with a wide wavelength region and large angles of incidence is realized, and the maximum photothermal conversion is achieved. Furthermore, the qualitative and quantitative relationships between the thermal absorption and thermoelectric conversion performance of PbS and SiGe thermoelectric materials and the physical parameters of surface subwavelength photothermal absorption structures are studied, so as to maximize the absorption and conversion of thermal energy into electrical energy. This work provides research ideas for the design of perfect thermoelectric devices.

Tuning Thermoelectric Conversion Performance of BiSbTe/Epoxy Flexible Films with Dot Magnetic Arrays.

Embedding of magnetic functional units into the thermoelectric (TE) materials has been demonstrated to be an effective way to enhance the TE conversion performance. However, the magnetic functional units in TE materials are all randomly distributed. In this paper, to explore the effect of the ordering of the magnetic functional units on TE conversion performance, a series of BiSbTe/epoxy flexible thermoelectromagnetic (TEM) films with dot magnetic arrays were successfully prepared by a two-step screen printing combined with a hot pressing process. TEM films with dot magnetic arrays can achieve high carrier mobility, while the carrier concentration increases due to large coercivity. Therefore, its electrical conductivities are significantly improved on the condition that it maintains a high Seebeck coefficient. The TEM film with hexagonal-dot magnetic arrays exhibits the best electrical transport properties, for which the room-temperature power factor reaches 1.51 mW·m-1·K-2, increased by 33.6 and 36.1% as compared with those of the pristine TE film and the TEM film with a continuous magnetic layer, respectively. This work provides a new way to enhance the TE conversion performance of flexible TEM films through the ordered magnetic arrays.

Optimizing the Transient Performance of Thermoelectric Generator with PCM by Taguchi Method

Phase change material (PCM) is an effective thermal management method to improve the thermoelectric conversion performance of a system. PCM can not only absorb excessive thermal energy at high temperature to protect the thermoelectric module (TEM) and increase the maximum available temperature range, but also compensate for intermittent energy to extend the working time of the TEM. In the paper, the transient performance is improved by adding PCM to a traditional thermoelectric generator (TEG) system. Due to the low thermal conductivity of PCM, metal fins are used to improve the thermal conductivity of PCM. To achieve maximum efficiency of the TEG system, the Taguchi method is employed. Four factors are heat source thermal power, PCM type, height of the PCM box, and filling ratio of the PCM, respectively. The results show that heat source thermal power has the greatest effect, and PCM has the least effect on the conversion efficiency of the TEG system. Conversion efficiency from thermal to electricity is about 1.472% during 2300 s of the heating and cooling stages.

Optimal design of thermoelectric properties of graphene nanoribbons with 5-7 ring defects based on Bayesian algorithm

Owing to the huge degree of freedom of structure, the optimal design of thermoelectric conversion performance of defective graphene nanoribbons is one of the difficulties in the field of materials research. In this paper, the thermoelectric properties of graphene nanoribbons with 5-7 ring defects are optimized by using non-equilibrium Green's function combined with Bayesian algorithm.The results show that the Bayesian algorithm is effective and advantageous in the search of graphene nanoribbons with 5-7 ring defects with high thermoelectric conversion efficiency. It is found that the single configuration with the best thermoelectric conversion performance can be quickly and accurately searched from 32896 candidate structures by using Bayesian algorithm. Even in the least efficient round of optimization, only 1495 candidate structures (about 4.54% of all candidate structures) need to be calculated to find the best configuration. It is also found that the thermoelectric value &lt;i&gt;ZT&lt;/i&gt; (about 1.13) of the optimal configuration of 5-7 ring defective graphene nanoribbons (21.162 and 1.23 nm in length and width, respectively) at room temperature is nearly one order of magnitude higher than that of the perfect graphene nanoribbons (about 0.14). This is mainly due to the fact that the 5-7 ring defects effectively inhibit the electron thermal conductivity of the system, which makes the maximum balance between the weakening effect of the power factor and the inhibiting effect of the thermal conductivity (positive effect). The results of this study provide a new feasible scheme for designing and fabricating the graphene nanoribbon thermoelectric devices with excellent thermoelectric conversion efficiencies.

Relation between Electronic Structure and Thermoelectric Properties of Heusler-Type Ru2VAl Compounds

We investigated Heusler-type Ru2VAl, a candidate material for next-generation thermoelectric conversion, by first-principle calculations of its thermoelectric conversion properties and direct experimental observations of its electronic structures, employing photoemission and infrared spectroscopy. Our results show that Ru2VAl has a wider pseudogap near the Fermi level compared to Fe2VAl. Accordingly, a higher thermoelectric conversion performance can be expected in Ru2VAl at higher temperatures.

Progress on the performances and applications of supercapacitors for thermoelectric conversion

<p indent="0mm">The comprehensive utilization rate of energy in China is less than 40%, and most of the unused energy is lost in the form of heat energy. Improving the utilization rate of heat energy has become one of the urgent problems in the world. In the past few decades, immense efforts have been made to explore and develop alternative technologies to capture low-grade heat. Thermoelectric technology, as an effective way to realize low-grade heat recycling, provides a simple and environmentally friendly solution for the direct conversion of low-grade heat to electric energy. Among them, supercapacitors are favored by researchers because of their excellent electrochemical performances and convenient functions of thermoelectric conversion. Therefore, this paper summarizes the latest progress on the performances and applications of supercapacitors for thermoelectric conversion. First, based on the analysis of the research progress for thermoelectric conversion technology at home and abroad, we discuss the composition and types of supercapacitors, including the electrical double-layer capacitors, pseudocapacitors, and asymmetric hybrid capacitors. When thermoelectric conversion is carried out in supercapacitors, thermally-induced effects including thermocapillary effect and Soret effect will occur at the solid-liquid interface. The thermocapillary effect will change the pore size at the interface, which will affect the adsorption of ions. The occurrence of the Soret effect will cause migration of ions in the electrolyte, resulting in a potential difference. As for the characterization of the performance of the supercapacitor for thermoelectric conversion, related evaluation metrics are obtained by changing the temperature of electrochemical environments on the basis of other characterization parameters. Second, we introduce the progress of the performance of supercapacitors in thermoelectric conversion. The power density of the supercapacitors with thermoelectric conversion under temperature differences does not meet the ideal requirements, and the liquid type supercapacitors have the problem of easy leakage. In recent years, a large amount of research has been conducted to improve the electrode material and electrolyte. The electrolyte types are mainly based on ion concentration, cation/anion size and the addition of nanoparticles in electrolyte solution, while the selection of electrode materials is mainly based on metal materials with high work function and carbon materials with a large specific surface area. At the same time, major improvements have been made in selecting the appropriate electrolyte (solid electrolyte or gel electrolyte) to construct supercapacitors with high thermoelectric conversion performance. Afterwards, we review the progress on supercapacitors for thermoelectric conversion without temperature differences, which mainly starts from the presence or absence of external power sources. Moreover, the relationships between structural design and economic cost are highlighted through a comprehensive analysis of the literature. On one hand, the cost can be reduced by reducing occurrence of side reactions and removing the ion exchange membrane while the external power supply is connected. On the other hand, the external power supply can be removed to simplify the structure and reduce the cost. Third, we introduce the latest developments in the application of supercapacitors based on thermoelectric conversion in various fields. In the field of wearable electronic products, supercapacitors can be charged by using the body heat to extend the use time. In terms of cooling of electronic components, supercapacitors can be used as auxiliary equipment of other thermal management systems to enhance local heat transfer capabilities. As for power batteries, supercapacitors use the waste heat generated by the power system to perform thermoelectric conversion, which improves the electrical performance of the car to a certain extent and prevents a large amount of waste heat from accumulating and burning the battery. Finally, we discuss the major challenges faced by supercapacitors based on thermoelectric conversion, along with future prospects.

The maximum efficiency enhancement of a solar-driven graphene-anode thermionic converter realizing total photon reflection

Thermionic converters show highly efficient thermoelectric conversion performance. Compared to metals, graphene materials exhibit excellent optical and electrical properties. A novel solar-driven graphene-anode thermionic converter is proposed, in which a photon reflector is attached to the graphene-anode to reduce dissipation. Expressions for the power output density and efficiency of the system are derived by considering the major irreversible dissipation. The performance characteristics of the system are comprehensively evaluated, including the effects of the voltage output, current density, area ratio of the absorber to the cathode, and solar concentration factor. The results show that the solar-driven graphene-anode thermionic converter with a photon reflector achieves better performance than the case without reflectors, and outperforms the solar-driven metal-anode thermionic converter. Under the same conditions of 3000 solar concentrations, the maximum power output density and efficiency of the proposed system attain 777.2 Wcm−2 and 44.56%, which increase 24.33% and 115.2%, respectively, compared to those of the solar-driven metal-anode thermionic converter. Besides, the maximum efficiency and power output density of the system at different concentrations and corresponding optimal values of key parameters are calculated. The optimum operating region of the proposed system is determined and the optimum selection criteria of key parameters are provided.

Excellent thermoelectric performance of open framework Si24 nanowires from density functional based tight-binding calculation

Open framework Si24 is a cage-like silicon allotrope with a highly anisotropic character and predicted to possess fascinating optoelectronic and thermoelectric performance. In this paper, we systematically investigated the thermoelectric conversion performance of Si24 nanowires (Si24NWs) through using nonequilibrium Green's function method as implemented in the density functional based tight-binding framework. The calculations reveal that Si24NWs possess superb electronic transport properties, e.g., the Seebeck coefficient could approach 2.67 mV/k at room temperature (several times greater than that of diamond silicon nanowires). Meanwhile, evident anisotropic thermal transport is also observed in Si24NWs, where the room temperature phonon thermal conductance of Si24NW-[100]/[010]/[001] is 1.36/0.27/0.38 nW/k, respectively. Through analyzing the phonon spectra of Si24NWs, we explain the origin of such an anisotropic thermal transport property. By combining the electron and phonon transport properties, the thermoelectric properties of Si24NWs are predicted and the ZT value of Si24NW-[100]/[010]/[001] could, respectively, approach 0.18/1.28/0.85 at room temperature. In order to achieve better thermoelectric performance, the Si/Ge core–shell structures are constructed (based on Si24NW-[010]). The results show that the core–shell structure is indeed a viable and efficient way to improve the thermoelectric conversion efficiency (the figure of merit could be boosted to 2.2 at room temperature). The findings presented in this paper shed light on the thermoelectric performance of Si24NWs and could provide a helpful guideline for designing and fabricating excellent thermoelectric devices based on Si24NWs.

Constructing Layered MXene/CNTs Composite Film with 2D–3D Sandwich Structure for High Thermoelectric Performance

AbstractReasonable heterostructure design can effectively improve the thermoelectric performance of composite materials. High‐conductivity and high‐stability MXenes are expected for the application of thermoelectric conversion benefiting from their diverse electronic properties. In this work, the 2D MXene and 1D SWCNTs are employed to construct layered composite films with 2D–3D sandwich structures in the vertical direction by the wet‐chemical assembly strategy. As a result, sandwich Ti3C2Tx–SWCNTs–Ti3C2Tx film shows a higher power factor than that of Ti3C2Tx/SWCNTs film with interlaced structure, further 25 times higher than that of Ti3C2Tx film, which mainly due to the considerable electrical conductivity (750.9 S cm−1) and enhanced Seebeck (−32.2 µV K−1) coefficient. This study provides a feasible and valuable reference for expanding the application field of MXenes‐based materials and reasonably improving their thermoelectric conversion performance.

Study of thermal insulation materials influence on the performance of thermoelectric generators by creating a significant effective temperature difference

Thermoelectric generator are attractive heat engines because they can convert heat directly into electricity via Seebeck effect. However, the energy conversion efficiency of today’s thermoelectric generators is significantly lower than that of conventional mechanical engines. Materials and devices are the main direction of its improvement. Although thermoelectric material has been making some progress, actual thermoelectric devices has remained stagnant with rather poor efficiencies. In this paper, a mathematical model containing heat losses ignored by previous studies are developed and used to discuss how to reduce heat losses to improved thermoelectric performance. In order to reduce and eliminate the impact of heat loss and study the effects of heat loss, the segmented thermoelectric modules filling with different insulation fillers are built to verify the accuracy of the model. Self-made experimental instruments are developed to study of the effect of reducing module heat loss on thermoelectric conversion performance. The results show that the filling of insulation materials makes contribution to the improvement of thermoelectric performance, the efficiency of aerogel filled module is 8.225% higher than that of unfilled module, and the efficiency of thermoelectric module can be improved up to 9.12% by adding ideal thermal insulation materials. The results also showed that the performance of TE module depends on the thermal conductivity of filling materials and its’ filling rate. Those results provided some practical guidelines for the design and improvement of practical thermoelectric modules.

Phonon transport and thermoelectric properties of semiconducting Bi2Te2X (X = S, Se, Te) monolayers.

Confinement or dimensionality reduction is a novel strategy to reduce the lattice thermal conductivity and, consequently, to improve the thermoelectric conversion performance. Bismuth and tellurium based low-dimensional materials have great potential in this regard. The phonon transport and thermoelectric properties of Bi2Te2X (X = S, Se, Te) monolayers are systematically investigated by employing density functional theory and the Boltzmann transport equation. The calculated lattice thermal conductivity of these 2D systems ranges from ∼1.3 W m-1 K-1 (Bi2Te2Se) to ∼1.5 W m-1 K-1 (Bi2Te3) for a 10 μm system size at room temperature and considering spin-orbit coupling in harmonic force constants. This remarkably low lattice thermal conductivity is attributed to small group velocities and enhanced anharmonic phonon scattering rates. A detailed analysis is presented in terms of mode-level phonon group velocities, anharmonic scattering rates and phonon mean free paths. Our results reveal that the thermal transport in these 2D systems is dominated by in-plane transverse acoustic modes. Additionally, the thermal conductivity can be further reduced by decreasing the sample size due to phonon-boundary scattering. The thermoelectric properties including the Seebeck coefficient, power factor and electrical conductivity are calculated using the semi-classical Boltzmann transport equation within the rigid band approximation. The low thermal conductivities coupled with their high carrier mobilities lead to good thermoelectric power factors. With optimal carrier doping, a figure of merit ∼0.6 can be achieved at room temperature, which increases to ∼0.8 at 700 K, thus making them promising candidates for thermoelectric applications.