Room Temperature Thermoelectric Material Research Articles

Study on the possibility of band gap widening of thermoelectric semiconductor α-SrSi2 by isoelectronic elements incorporation

The material α-SrSi2 has considerable application potential as a near room temperature thermoelectric material. However, it has a very narrow band gap which can vanish due to impurities, defects, and grain boundaries. To obtain guidelines for controlling the band gap and improving the thermoelectric properties, we have studied the substitution of isoelectronic impurities (Mg, Ca, and Ba for Sr, and C, Ge, and Sn for Si) through first-principles calculations using the Gaussian-Perdew-Burke-Ernzerhof hybrid functional. From these calculations, it was found that:(1) The band gap change due to Ca, Ba, Ge, and Sn substitutions is almost the same as that predicted from the change in the lattice constant of pure α-SrSi2.(2) Substitution with C was energetically unfavorable and therefore high levels of substitution would be difficult to achieve. However, it was expected to increase the band gap, contrary to the prediction from the decrease in the lattice constant.(3) An increase in the Mg substitution of the Sr site from 0 at. % to 8 at. % caused a decrease in the lattice constant from 0 % to −1.189 %

Complex defect formation in Fe doped γ-CuI: Enhancement of thermoelectric properties via band engineering

The doping effect of Fe atoms on CuI, a potential candidate for room temperature thermoelectric materials, was investigated due to the lack of studies for the aspect of band engineering. The addition of Fe atoms was observed to enhance the p-type performance of CuI, although Fe is more suitable as a donor material. Through Mossbauer spectroscopy and Density Functional Theory (DFT) calculations, it was determined that only about 5 % of the added Fe atoms simply substitute Cu sites, while most Fe atoms occupy interstitial sites, acting as defects and promoting Cu vacancies. Despite the trace amount of Fe only contribute to magnetic properties, it was found that multiple defects and deformation in the electronic band structure caused the effective mass to nearly double. As a result, electrical conductivity increased by an order of magnitude, while the decrease in the Seebeck coefficient was less than half, demonstrating a violation of the Pisarenko relation. Additionally, the incorporation of Fe atoms induced nano structuring behavior, increasing boundary scattering in CuI and significantly reducing thermal conductivity. Fe could be suggested as a suitable dopant for the "phonon-glass-electron-crystal" strategy.

Into the Void: Single Nanopore in Colloidally Synthesized Bi2Te3 Nanoplates with Ultralow Lattice Thermal Conductivity.

Bi2Te3 is a well-known thermoelectric material that was first investigated in the 1960s, optimized over decades, and is now one of the highest performing room-temperature thermoelectric materials to-date. Herein, we report on the colloidal synthesis, growth mechanism, and thermoelectric properties of Bi2Te3 nanoplates with a single nanopore in the center. Analysis of the reaction products during the colloidal synthesis reveals that the reaction progresses via a two-step nucleation and epitaxial growth: first of elemental Te nanorods and then the binary Bi2Te3 nanoplate growth. The rates of epitaxial growth can be controlled during the reaction, thus allowing the formation of a single nanopore in the center of the Bi2Te3 nanoplates. The size of the nanopore can be controlled by changing the pH of the reaction solution, where larger pores with diameter of ∼50 nm are formed at higher pH and smaller pores with diameter of ∼16 nm are formed at lower pH. We propose that the formation of the single nanopore is mediated by the Kirkendall effect and thus the reaction conditions allow for the selective control over pore size. Nanoplates have well-defined hexagonal facets as seen in the scanning and transmission electron microscopy images. The single nanopores have a thin amorphous layer at the edge, revealed by transmission electron microscopy. Thermoelectric properties of the pristine and single-nanopore Bi2Te3 nanoplates were measured in the parallel and perpendicular directions. These properties reveal strong anisotropy with a significant reduction to thermal conductivity and increased electrical resistivity in the perpendicular direction due to the higher number of nanoplate and nanopore interfaces. Furthermore, Bi2Te3 nanoplates with a single nanopore exhibit ultralow lattice thermal conductivity values, reaching ∼0.21 Wm-1K-1 in the perpendicular direction. The lattice thermal conductivity was found to be systematically lowered with pore size, allowing for the realization of a thermoelectric figure of merit, zT of 0.75 at 425 K for the largest pore size.

Assessing Effects of van der Waals Corrections on Elasticity of Mg3Bi2-xSbx in DFT Calculations.

As a promising room-temperature thermoelectric material, the elastic properties of Mg3Bi2-xSbx (0 ≤ x ≤ 2), in which the role of van der Waals interactions is still elusive, were herein investigated. We assessed the effects of two typical van der Waals corrections on the elasticity of Mg3Bi2-xSbx nanocomposites using first-principles calculations within the frame of density functional theory. The two van der Waals correction methods, PBE-D3 and vdW-DFq, were examined and compared to PBE functionals without van der Waals correction. Interestingly, our findings reveal that the lattice constant of the system shrinks by approximately 1% when the PBE-D3 interaction is included. This leads to significant changes in certain mechanical properties. We conducted a comprehensive assessment of the elastic performance of Mg3Bi2-xSbx, including Young's modulus, Poisson's ratio, bulk modulus, etc., for different concentration of Sb in a 40-atom simulation box. The presence or absence of van der Waals corrections does not change the trend of elasticity with respect to the concentration of Sb; instead, it affects the absolute values. Our investigation not only clarifies the influence of van der Waals correction methods on the elasticity of Mg3Bi2-xSbx, but could also help inform the material design of room-temperature thermoelectric devices, as well as the development of vdW corrections in DFT calculations.

Polymer-mixed Sb2Te3/Te nanocomposites exhibiting p-type to n-type conduction reversal and thermal conductivity reduction

Sb2Te3-based materials are potential room-temperature thermoelectric materials. In the present work, we choose polycrystalline Sb2Te3/Te nanocomposites and utilize Poly Methyl Methacrylate (PMMA) to reduce the thermal conductivity of Sb2Te3 samples. PMMA and polycrystalline Sb2Te3/Te were well mixed using ball milling. Pellets have been made by the cold press method. Thermoelectric transport properties of Sb2Te3/Te nanocomposites: composition, microstructure, and analysis are found to be influenced by PMMA. With increasing PMMA concentration a p-type to n-type transition has been observed because there are fewer charge carriers or the composites have a higher resistance. It is also observed that the thermal conductivity of Sb2Te3/Te nanocomposites decreases as the PMMA increases. This research paves the way for making the best thermoelectric materials by reducing thermal conductivity through the use of polymers.

Room-Temperature High-Performance Thermoelectric Bi0.6 Sb0.4 Te: Elimination of Detrimental Band Inversion in BiTe.

Room-temperature thermoelectric materials are the key to miniaturizing refrigeration equipment and have great scientific and social implications, yet their application is hindered by their extreme scarcity. BiTe exhibiting strong spin-orbit coupling peaks ZT at 600 K. Herein, we discover the synergy effect of Sb doping in BiTe that eliminates the detrimental band inversion and leads to an overlap of conduction band (CB) and valence band that significantly increases the S from 33 to 124 μV K-1 . In addition, this effect enhances the μ from 58 to 92 cm2 V-1 s-1 owing to the sharp increase in the CB slope along the Γ-A in the first Brillouin zone. Furthermore, Sb doping increases the anharmonicity, shortens the phonon lifetime and lowers κlat . Finally, Se/Sb codoping further optimizes the ZT to 0.6 at 300 K, suggesting that Bi0.6 Sb0.4 Te1-y Sey is a potential room-temperature TE material.

Temperature-Driven Twin Structure Formation and Electronic Structure of Epitaxially Grown Mg3Sb2 Films on Mismatched Substrates.

Mg3Sb2-based compounds are one type of important room-temperature thermoelectric materials and the appropriate candidate of type-II nodal line semimetals. In Mg3Sb2-based films, compelling research topics such as dimensionality reduction and topological states rely on the controllable preparation of films with high crystallinity, which remains a big challenge. In this work, high quality Mg3Sb2 films are successfully grown on mismatched substrates of sapphire (000l), while the temperature-driven twin structure evolution and characteristics of the electronic structure are revealed in the as-grown Mg3Sb2 films by in situ and ex situ measurements. The transition of layer-to-island growth of Mg3Sb2 films is kinetically controlled by increasing the substrate temperature (Tsub), which is accompanied with the rational manipulation of twin structure and epitaxial strains. Twin-free structure could be acquired in the Mg3Sb2 film grown at a low Tsub of 573 K, while the formation of twin structure is significantly promoted by elevating the Tsub and annealing, in close relation to the processes of strain relaxation and enhanced mass transfer. Measurements of scanning tunneling spectroscopy (STS) and angle-resolved photoemission spectroscopy (ARPES) elucidate the intrinsic p-type conduction of Mg3Sb2 films and a bulk band gap of ~0.89 eV, and a prominent Fermi level downshift of ~0.2 eV could be achieved by controlling the film growth parameters. As elucidated in this work, the effective manipulation of the epitaxial strains, twin structure and Fermi level is instructive and beneficial for the further exploration and optimization of thermoelectric and topological properties of Mg3Sb2-based films.

Deformable thermoelectric sponge based on layer-by-layer self-assembled transition metal dichalcogenide nanosheets for powering electronic skin

In this study, we fabricated mechanically deformable thermoelectric sponges comprising transition metal dichalcogenides (TMDs) and polyethyleneimine (PEI) through a layer-by-layer (LBL) self-assembly technique for a thermoelectric power supply for electronic skin. Chemically exfoliated molybdenum sulfide (MoS2) and niobium diselenide (NbSe2) were prepared as p- and n-type room-temperature thermoelectric materials, respectively, and deposited on a melamine sponge via electrostatic bonding with PEI to obtain stable mechanical stretchability and low thermal conductivity. Five bilayers of LBL self-assembled thermoelectric sponges exhibited an enhanced thermoelectric performance and figure of merit, which resulted from the improvement in the Seebeck coefficient compared with that of pristine chemically exfoliated TMDs owing to the energy filtering effect and the extremely low thermal conductivity owing to the phonon scattering effect at several created interfaces and the porous structure of the sponge. Additionally, the thermoelectric sponges showed mechanical stability during operation under stretching and compression and mechanical durability over 10,000 cycles under 30% tensile strain. Finally, based on the proposed thermoelectric sponge, a power patch that can be installed on the back of a hand to produce electrical energy in real time was successfully demonstrated.

Thermoelectric Silver-Based Chalcogenides.

Heat is abundantly available from various sources including solar irradiation, geothermal energy, industrial processes, automobile exhausts, and from the human body and other living beings. However, these heat sources are often overlooked despite their abundance, and their potential applications remain underdeveloped. In recent years, important progress has been made in the development of high-performance thermoelectric materials, which have been extensively studied at medium and high temperatures, but less so at near room temperature. Silver-based chalcogenides have gained much attention as near room temperature thermoelectric materials, and they are anticipated to catalyze tremendous growth in energy harvesting for advancing internet of things appliances, self-powered wearable medical systems, and self-powered wearable intelligent devices. This review encompasses the recent advancements of thermoelectric silver-based chalcogenides including binary and multinary compounds, as well as their hybrids and composites. Emphasis is placed on strategic approaches which improve the value of the figure of merit for better thermoelectric performance at near room temperature via engineering material size, shape, composition, bandgap, etc. This review also describes the potential of thermoelectric materials for applications including self-powering wearable devices created by different approaches. Lastly, the underlying challenges and perspectives on the future development of thermoelectric materials are discussed.

Enhancements of thermoelectric performance in n-type Bi2Te3-based nanocomposites through incorporating 2D Mxenes

Bi2Te2.7Se0.3 compound has been considered as an efficient n-type room-temperature thermoelectric (TE) material. However, the large-scale applications for low-quality energy harvesting were limited due to its low energy-conversion efficiency. We demonstrate that TE performance of Bi2Te2.7Se0.3 system is optimized by 2D Ti3C2Tx additive. Here, a 43% reduction of electrical resistivity is obtained for the nanocomposites at 380 K, originating from the increased carrier concentration. Consequently, the g = 0.1 sample shows a maximum power factor of 1.49 Wmm−1K−2. Meanwhile, the lattice thermal conductivity for nanocomposite samples is reduced from 0.77 to 0.41 Wm−1K−1 at 380 K, due to the enhanced phonon scattering induced by the interfaces between Ti3C2Tx nanosheets and Bi2Te2.7Se0.3 matrix. Therefore, a peak ZT of 0.68 is achieved at 380 K for Bi2Te2.7Se0.3/0.1 wt% Ti3C2Tx, which is enhanced by 48% compared with pristine sample. This work provides a new route for optimizing TE performance of Bi2Te2.7Se0.3 materials.

Maximizing the performance of n-type Mg3Bi2 based materials for room-temperature power generation and thermoelectric cooling

Although the thermoelectric effect was discovered around 200 years ago, the main application in practice is thermoelectric cooling using the traditional Bi2Te3. The related studies of new and efficient room-temperature thermoelectric materials and modules have, however, not come to fruition yet. In this work, the electronic properties of n-type Mg3.2Bi1.5Sb0.5 material are maximized via delicate microstructural design with the aim of eliminating the thermal grain boundary resistance, eventually leading to a high zT above 1 over a broad temperature range from 323 K to 423 K. Importantly, we further demonstrated a great breakthrough in the non-Bi2Te3 thermoelectric module, coupled with the high-performance p-type α-MgAgSb, for room-temperature power generation and thermoelectric cooling. A high conversion efficiency of ~2.8% at the temperature difference of 95 K and a maximum temperature difference of 56.5 K are experimentally achieved. If the interfacial contact resistance is further reduced, our non-Bi2Te3 module may rival the long-standing champion commercial Bi2Te3 system. Overall, this work represents a substantial step towards the real thermoelectric application using non-Bi2Te3 materials and devices.

2D GeP3 and blue P: promising thermoelectric materials for room- and high-temperature applications

Two dimensional GeP3 and blue P were investigated as a thermoelectric material using the Boltzmann model. Our study suggested that GeP3 is a good candidate as a room temperature thermoelectric material.

A comprehensive review on Bi2Te3‐based thin films: Thermoelectrics and beyond

AbstractBi2Te3‐based materials are not only the most important and widely used room temperature thermoelectric (TE) materials but are also canonical examples of topological insulators in which the topological surface states are protected by the time‐reversal symmetry. High‐performance thin films based on Bi2Te3 have attracted worldwide attention during the past two decades due primarily to their outstanding TE performance as highly efficient TE coolers and as miniature and flexible TE power generators for a variety of electronic devices. Moreover, intriguing topological phenomena, such as the quantum anomalous Hall effect and topological superconductivity discovered in Bi2Te3‐based thin films and heterostructures, have shaped research directions in the field of condensed matter physics. In Bi2Te3‐based films and heterostructures, delicate control of the carrier transport, film composition, and microstructure are prerequisites for successful device operations as well as for experimental verification of exotic topological phenomena. This review summarizes the recent progress made in atomic defect engineering, carrier tuning, and band engineering down to a nanoscale regime and how it relates to the growth and fabrication of high‐quality Bi2Te3‐based films. The review also briefly discusses the physical insight into the exciting field of topological phenomena that were so dramatically realized in Bi2Te3‐ and Bi2Se3‐based structures. It is expected that Bi2Te3‐based thin films and heterostructures will play an ever more prominent role as flexible TE devices collecting and converting low‐level (body) heat into electricity for numerous electronic applications. It is also likely that such films will continue to be a remarkable platform for the realization of novel topological phenomena.

CuI/Nylon Membrane Hybrid Film with Large Seebeck EffectSupported by the International Scientific and Technological Innovation Cooperation Project between the Governments of Key National R&D Program of China (Grant No. 2018YFE0111500), and the National Natural Science Foundation of China (Grant No. 51972234).

Room-temperature thermoelectric materials are important for converting heat into electrical energy. As a wide-bandgap semiconductor material, CuI has the characteristics of non-toxicity, low cost, and environmental friendliness. In this work, CuI powder was synthesized by a wet chemical method, then CuI film was formed by vacuum assisted filtration of the CuI powder on a porous nylon membrane, followed by hot pressing. The film exhibits a large Seebeck coefficient of 600 μV · K−1 at room temperature. In addition, the film also shows good flexibility (∼95% retention of the electrical conductivity after being bent along a rod with a radius of 4 mm for 1000 times). A finger touch test on a single-leg TE module indicates that a voltage of 0.9 mV was immediately generated within 0.5 s from a temperature difference of 4 K between a finger and the environment, suggesting the potential application in wearable thermal sensors.

Excellent Room-Temperature Thermoelectricity of 2D GeP3: Mexican-Hat-Shaped Band Dispersion and Ultralow Lattice Thermal Conductivity.

Although some atomically thin 2D semiconductors have been found to possess good thermoelectric performance due to the quantum confinement effect, most of their behaviors occur at a higher temperature. Searching for promising thermoelectric materials at room temperature is meaningful and challenging. Inspired by the finding of moderate band gap and high carrier mobility in monolayer GeP3, we investigated the thermoelectric properties by using semi-classical Boltzmann transport theory and first-principles calculations. The results show that the room-temperature lattice thermal conductivity of monolayer GeP3 is only 0.43 Wm−1K−1 because of the low group velocity and the strong anharmonic phonon scattering resulting from the disordered phonon vibrations with out-of-plane and in-plane directions. Simultaneously, the Mexican-hat-shaped dispersion and the orbital degeneracy of the valence bands result in a large p-type power factor. Combining this superior power factor with the ultralow lattice thermal conductivity, a high p-type thermoelectric figure of merit of 3.33 is achieved with a moderate carrier concentration at 300 K. The present work highlights the potential applications of 2D GeP3 as an excellent room-temperature thermoelectric material.

Polypyrrole sheets composed of nanoparticles as a promising room temperature thermo-electric material

Polypyrrole (PPy) is a highly conducting polymer with favorable room temperature (RT) thermoelectric (TE) properties. Recently, nanostructure forms of this polymer have been produced with various morphologies by using different surfactants; however, the effect of these morphologies on the TE figure of merit and TE performance has not been well described. In particular, the TE properties of PPy sheets/films composed of nanoparticles (SNPs) have not yet been investigated. In this work, PPy with various morphologies was synthesized using different surfactants. Nanoparticles, nanotubes, continuous sheets and SNPs were produced and characterized by well-known techniques. The structural and TE properties of the as-synthesized nanomaterials were investigated. The obtained results show that SNPs have improved TE properties compared to nanotubes and nanoparticles. The latter two forms have either higher or lower electrical and thermal conductivities than those of SNPs, which suppress their TE performance. The produced SNPs were able to overcome these limitations, as they have high electrical and low thermal conductivities, which result in a high figure of merit and enhanced TE performance. The best SNPs was found to have a power factor, thermal conductivity and figure of merit of 0.17 μV/m⋅K2, 0.132 W/mK2 and 0.4 × 10−3 at room temperature, respectively. These results might be useful for developing future TE materials and devices.

Memristive behaviour of electrodeposited bismuth selenide

We demonstrate memristive functions of the room temperature thermoelectric material Bi-Se fabricated by electrochemical deposition in combination with active electrode Ag-based contacts. We present microfabrication steps for creating polymer molds for the growth of compact memristive films. The electrical measurements show clear forming-free resistive switching behaviour for the electrochemically deposited material while temperature dependent measurements point to Schottky and space-charge-limited-current conduction mechanisms. For the envisioned combination of the memristors with a thermoelectric device we can conclude that electrodeposition is a viable method to produce silver-chalcogenide based non-volatile memristors for co-integration in zero power sensors.

Electronic, optical, and thermoelectric properties of Janus In-based monochalcogenides

Inspired by the successfully experimental synthesis of Janus structures recently, we systematically study the electronic, optical, and electronic transport properties of Janus monolayers In2 XY (X/Y = S, Se, Te with X ≠ Y) in the presence of a biaxial strain and electric field using density functional theory. Monolayers In2 XY are dynamically and thermally stable at room temperature. At equilibrium, both In2STe and In2SeTe are direct semiconductors while In2SSe exhibits an indirect semiconducting behavior. The strain significantly alters the electronic structure of In2 XY and their photocatalytic activity. Besides, the indirect–direct gap transitions can be found due to applied strain. The effect of the electric field on optical properties of In2 XY is negligible. Meanwhile, the optical absorbance intensity of the Janus In2 XY monolayers is remarkably increased by compressive strain. Also, In2 XY monolayers exhibit very low lattice thermal conductivities resulting in a high figure of merit ZT, which makes them potential candidates for room-temperature thermoelectric materials.

First-principles investigation of the electronic and thermoelectric properties of SiGe doped with Sn and one percent B

Silicon-Germanium (SiGe) has recently attracted much interest as a room temperature thermoelectric (TE) material for converting heat into electrical output power. With the advantage of silicon being non-toxic, cost-effective, and abundant on earth, a silicon base TE material has a promising future. A first-principle calculation based on the fully self-consistent Korringa-Kohn-Rostoker method with the coherent potential approximation (KKR-CPA) to treat several forms of chemical disorders of SiGe by Sn-doping was carried out. In SiGe1-xSnx, as the Sn content increases the Fermi level shifts to the conduction band edge. Similarly, in Si1-xSnxGe, a high Sn content (x = 0.4 to 0.9) results in the Fermi level shifting to the conduction band edge. On the contrary, a low amount of Sn content (x = 0.1 to 0.3) causes the Fermi level to fluctuate between the conduction band and the valence band states. With the addition of 1% Boron impurity to the alloys Si1-xSnxGe and SiGe1-xSnx, the number of carriers (electron and hole) states was enhanced by 0.05 states/eV. This makes the alloys Si0.3Sn0.69B0.01Ge and SiGe0.4Sn0.59B0.01 promising for application as n-type electrodes in a thermoelectric generator (TEG).

Enhanced power factor and thermoelectric performance for n-type Bi2Te2.7Se0.3 based composites incorporated with 3D topological insulator nanoinclusions

n-type Bi2Te2.7Se0.3 (BTS) alloy is a state-of-the-art room temperature thermoelectric material used for refrigeration commercially. However, wide application is limited by its low performance. Here, we show that incorporation of nano-sized 3D topological insulator (3D-TI) Bi2Se3 in Bi2Te2.7Se0.3 bring about a large electron mobility and increased electrical conductivity owing to the enhanced contribution of topologically protected conducting surface states as well as large Seebeck coefficient due to energy dependent carrier scattering. Besides, the incorporation of 3D-TI nanoinclusions leads to enhanced phonon scattering. As a result, addition of 0.3 vol% of Bi2Se3 in BTS results in ~ 36% increase in power factor and ~ 16% decrease in total thermal conductivity, leading to a large figure of merit ZTmax = 1.28 (at 375 K) and a large average ZTave = 1.16 (at T = 300–525 K), which are 62% and 59% greater than that of BTS, respectively.