Traditional Thermoelectric Materials Research Articles

Thermoelectric characteristics of X_2YH_2 monolayers (X=Si, Ge; Y=P, As, Sb, Bi): a first-principles study

Ever since global warming emerged as a serious issue, the development of promising thermoelectric materials has been one of the main hot topics of material science. In this work, we provide an in-depth understanding of the thermoelectric properties of X_2YH_2 monolayers (X=Si, Ge; Y=P, As, Sb, Bi) using the density functional theory combined with the Boltzmann transport equation. The results indicate that the monolayers have very low lattice thermal conductivities in the range of 0.09−0.27 Wm^{-1}K^{-1} at room temperature, which are correlated with the atomic masses of primitive cells. Ge_2PH_2 and Si_2SbH_2 possess the highest mobilities for hole (1894 cm^2V^{-1}s^{-1}) and electron (1629 cm^2V^{-1}s^{-1}), respectively. Si_2BiH_2 shows the largest room-temperature figure of merit, ZT=2.85 in the n-type doping ( sim 3times 10^{12} cm^{-2}), which is predicted to reach 3.49 at 800 K. Additionally, Si_2SbH_2 and Si_2AsH_2 are found to have considerable ZT values above 2 at room temperature. Our findings suggest that the mentioned monolayers are more efficient than the traditional thermoelectric materials such as Bi_2Te_3 and stimulate experimental efforts for novel syntheses and applications.

A flexible thermoelectric film based on Bi2Te3for wearable applications

In recent years, with the development of the Internet of Things (IoT) and wearable technology, the research and exploration of thermoelectric materials have been greatly promoted. However, traditional thermoelectric materials are rigid and brittle. Thermoelectric devices made of these materials usually cannot be closely attached to the heat and cold sources that provide temperature differences, thus limiting the application of thermoelectric materials. Therefore, manufacturing new high-performance flexible thermoelectric devices is still a huge challenge. In this work, polyimide/copper (PI/Cu) substrate was deposited by electron deposition technology. The flexible thermoelectric thin film device was fabricated by bonding [Formula: see text]-type and [Formula: see text]-type bismuth telluride (Bi2Te[Formula: see text] slurries onto the PI/Cu substrate. Then, the PDMS film was coated on the device to make the device waterproof and oxidation resistant. The output voltage and maximum power of this device, at the temperature of 80 K, reach 97.5 mV and 60 uW, respectively. After 200 cycles of cyclic bending experiments, 90% high conductivity retention can be maintained. It demonstrates that the new flexible thermoelectric thin film has good flexibility and excellent stability. This work provides a simple method for the preparation of flexible thermoelectric thin films and opens up a new way for its application in the sensing equipment and wearable device of the IoT.

Comparison between the thermoelectric properties of new materials: The alloy of iron, vanadium, tungsten, and aluminum (Fe2V0.8W0.2Al) against an oxide such as NaCO2O4

An analysis of the thermoelectric characteristics of certain recently discovered materials is carried out in this investigation. The alloy of iron, vanadium, tungsten, and aluminum (Fe2V0.8W0.2Al) applied to a silicon crystal is compared to new inorganic thermoelectric materials, which are mosly oxides like NaCO2O4. For both materials, the thermoelectric effects, Seebeck effect, Peltier effect, Thomson effect, and Kelvin relations are described. The cooling rate’s influence on the energy balance is also assessed. The traditional thermoelectric materials provided are mostly made up of toxic, rare and/or expensive elements, which makes large-scale thermoelectric generator integration difficult. In recent decades, research has shifted toward the development of novel materials with a better price-to-performance ratio. Despite a low conversion yield, the family of oxides offers significant benefits in this respect, which are particularly evident at high temperatures. The findings of our study indicated that Fe2V0.8W0.2 applied to a silicon crystal has good thermoelectric characteristics. A sufficient merit factor was found in the new substance under investigation.

Integrated, Highly Flexible, and Tailorable Thermoelectric Type Temperature Detectors Based on a Continuous Carbon Nanotube Fiber.

As possible alternatives to traditional thermoelectric (TE) materials, carbon nanomaterials and their hybrid materials have great potential in the future application of flexible and lightweight temperature detection. In this work, an integrated, highly flexible, and tailorable TE temperature detector with high performance has been fabricated based on a continuous single-walled carbon nanotube (SWCNT) fiber. The detector consists of more than one pairs of thermocouples composed of p-type SWCNT fiber and n-type SWCNT hybrid fiber in situ doped by polyethylenimine. Due to the node contact mechanism of the detection, the sensitivity of the detector can be improved with the increase of the number of p-n thermocouples, independent of the length of the thermocouple. The temperature detection process of the detector has been studied in detail. In particular, the integrated and flexible detector can be divided into several sub-detectors easily by cutting, illustrating the prospect of large-scale preparation of this kind of novel temperature detectors. Its high flexibility ensures the detector to maintain excellent detection performance after 15000 bending circles. Furthermore, the as-designed TE type temperature detector demonstrates a great application promise for real-time temperature detection and temperature change sensing even in complex surface and harsh environment.

Wearable Electronics Based on the Gel Thermogalvanic Electrolyte for Self-Powered Human Health Monitoring.

There is always a temperature difference of more than 10 degrees between the human body, as a sustainable heat source, and the ambient temperature. Converting body heat into electricity that in turn is used to drive personal medical electronics is of significance in smart wearable medicine. To avoid the frangibility and complex preparation of traditional thermoelectric materials, we fabricated a gel electrolyte-based thermogalvanic generator with Fe3+/Fe2+ as a redox pair, which presents not only moderate thermoelectric performance but also excellent flexibility. With a micropore-widespread polyvinylidene fluoride diaphragm implanted in the gel, a thermal barrier was created between the two halves, effectively improving the Seebeck coefficient by reducing its thermal conductivity. Considering the superior temperature response of the gel, a self-powered body temperature monitoring system was established by conformally affixing it to the forehead. Meanwhile, the gel patch with a high specific heat capacity can effectively cool down fever patients. This work may offer a new train of thought for exploiting self-powered wearable medical electronics by scavenging low-grade body heat.

Enhancing Near-Room-Temperature GeTe Thermoelectrics through In/Pb Co-doping.

The traditional thermoelectric material GeTe has drawn much attention recently because of the reported high thermoelectric performance of the rhombohedral phase in low-temperature ranges, where the split L and Σ band can be reconverged to have a small energy offset and thus high density of state effective mass according to the rhombohedral angle. In addition, In doping in GeTe is also reported to enhance the density of effective mass and therefore increase the Seebeck coefficient because of the induced resonant levels. In this work, In and Pb are doped in GeTe, and In doping leads to an increase in the rhombohedral angle and thus enhanced density of state effective mass in addition to the resonant effect. However, the improved Seebeck coefficient result from In doping is compensated for by a sharp reduction of Hall mobility, leading to no significant enhancement of electronic performance in the rhombohedral phase. By further Pb/Ge doping in the matrix Ge0.95In0.05Te for the optimization of carrier concentration and reduction of lattice thermal conductivity (as low as 0.7 W/mK), a zT as high as ∼1.2 at 550 K and average zT of ∼0.75 between 300 and 550 K are realized in this work, demonstrating GeTe as a promising candidate for near-room-temperature application.

An Update Review on N-Type Layered Oxyselenide Thermoelectric Materials.

Compared with traditional thermoelectric materials, layered oxyselenide thermoelectric materials consist of nontoxic and lower-cost elements and have better chemical and thermal stability. Recently, several studies on n-type layered oxyselenide thermoelectric materials, including BiCuSeO, Bi2O2Se and Bi6Cu2Se4O6, were reported, which stimulates us to comprehensively summarize these researches. In this short review, we begin with various attempts to realize an n-type BiCuSeO system. Then, we summarize several methods to optimize the thermoelectric performance of Bi2O2Se, including carrier engineering, band engineering, microstructure design, et al. Next, we introduce a new type of layered oxyselenide Bi6Cu2Se4O6, and n-type transport properties can be obtained through halogen doping. At last, we propose some possible research directions for n-type layered oxyselenide thermoelectric materials.

Synthesis and characterization of non -stoichiometric SrTiO2.8 thermoelectric materials at high temperature and high pressure

SrTiO3(STO) is a thermoelectric (TE) material with a wide band gap. It is usually either sintered or annealed in a reducing atmosphere to obtain semiconducting behavior. In this work, non-stoichiometric SrTiO2.8 was synthesized by the high temperature and high pressure (HPHT) method in a reducing environment to obtain improved electrical and thermal properties. The reducing environment was obtained by the application of high pressure and due to the strong ability of Ti to capture oxygen at high temperatures. The thermoelectric properties of the samples synthesized at high temperature and pressure were studied by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and UV spectroscopy. The results show that with increasing of synthesis pressure, the band gap of the samples decreases, improving their electrical properties; at the same time, the high pressure also changes the micro-morphology of the samples and improves their thermal properties. The zT value of the sample synthesized at 5 GPa reached the maximum value of 0.20 at 973 K. It was found that the mechanical strength of the synthesized samples is superior to that of the traditional alloy-based thermoelectric materials. Our work shows that HPHT provides a new variable of pressure for the optimization of the synthesis process of TE materials and provides a new approach for the synthesis and performance optimization of thermoelectric materials.

Chemically Switchable n-Type and p-Type Conduction in Bismuth Selenide Nanoribbons for Thermoelectric Energy Harvesting.

Realizing switchable n-type and p-type conduction in bismuth selenide (Bi2Se3), a traditional thermoelectric material and a topological insulator, is highly beneficial for the development of thermoelectric devices and also of great interest for spintronics and quantum computing. In this work, switching between n-type and p-type conduction in single Bi2Se3 nanoribbons is achieved by a reversible copper (Cu) intercalation method. Density functional theory calculations reveal that such a switchable behavior arises from the electronic band structure distortion caused by the high-concentration Cu intercalation and the Cu substitution for Bi sites in the host lattice. A proof-of-concept in-plane thermoelectric generator is fabricated with one pair of the pristine n-type and intercalated p-type Bi2Se3 nanoribbons on a microfabricated device, which gives rise to an open-circuit voltage of 4.8 mV and a maximum output power of 0.3 nW under a temperature difference of 29.2 K. This work demonstrates switchable n-type and p-type electrical conduction in Bi2Se3 nanoribbons via a facile chemical approach and the practical application of nanoribbons in a thermoelectric device.

Cover Feature: Thermoelectric Converters Based on Ionic Conductors (2/2021)

Ionic thermoelectric materials are able to convert heat energy into electric energy under a temperature gradient through ion rearrangement at the electrode-liquid interface and directional migration of ions in the bulk phase. Compared with traditional electronic thermoelectric materials, they have a higher Seebeck coefficient and lower thermal conductivity, as well as the advantages of high flexibility, low cost, low environmental pollution and self-healing ability. This Minireview proposes three different thermoelectric conversion mechanisms of ionic thermoelectric materials. In addition, several representative examples of ionic thermoelectric materials are summarized to demonstrate their applications and challenges. More information can be found in the Minireview by Yapei Wang et al.

Research progress of Cu<sub>2</sub>Se thin film thermoelectric properties

Thermoelectric (TE) materials can directly realize the mutual conversion between heat and electricity, and it is an environmentally friendly functional material. At present, the thermoelectric conversion efficiencies of thermoelectric materials are low, which seriously restricts the large-scale application of thermoelectric devices. Therefore, finding new materials with better thermoelectric properties or improving the thermoelectric properties of traditional thermoelectric materials has become the subject of thermoelectric research. Thin film materials, compared with bulk materials, possess both the two-dimensional macroscopic properties and one-dimensional nanostructure characteristics, which makes it much easier to study the relationships between physical mechanisms and properties. Besides, thin film are also suitable for the preparation of wearable electronic devices. This article summarizes five different preparation methods of Cu<sub>2</sub>Se thin films, i.e. electrochemical deposition, thermal evaporation, spin coating, sputtering, and pulsed laser deposition. In addition, combing with typical examples, the characterization methods of the film are summarized, and the influence mechanism of each parameter on the thermoelectric performance from electrical conductivity, Seebeck coefficient and thermal conductivity is discussed. Finally, the hot application direction of Cu<sub>2</sub>Se thin film thermoelectrics is also introduced.

Liquid Thermocells Enable Low-Grade Heat Harvesting

Recently, two Science papers tackled the challenge of harvesting low-grade heat via liquid thermocells. One combines TGC and TDC to significantly enhance thermopower by an order of magnitude, and the other reveals the selectively thermosensitive crystallization/dissolution at two electrodes. These intriguing and inspiring advances in terms of materials, mechanisms, and performances are briefly introduced, and some remaining challenges and future directions are outlined for further thermopower improvement and practical implementation.

Mechanical reliability, thermal stability and thermoelectric performance of the transition-metal nitride CrN

ABSTRACTThe mechanical reliability and thermal stability of thermoelectric materials are often the most neglected, while they will greatly affect the reliability and performance of thermoelectric devices when used in extreme environments. In this study, a nanostructured chromium nitride (CrN) thermoelectric material with a ZT value ∼0.12, exhibiting both high mechanical reliability and thermal stability, was prepared using a simple chemical method. Its hardness, reaching 7.21 GPa, is much higher than that of most other traditional thermoelectric materials. Thermogravimetric analysis shows that CrN remains stable at 873 K. Moreover, friction and wear tests demonstrate the high wear resistance and low friction coefficient (∼0.42) of CrN. The high thermal stability and good mechanical properties are expected to offer a wide range of opportunities for the thermoelectric application of CrN in some extreme environments, such as collision avoidance systems and outer space.

Enhanced Thermoelectric Performance of Zr1-xTaxNiSn Half-Heusler Alloys by Diagonal-Rule Doping.

Although Sb doping is regarded as the most effective method to regulate the carrier concentration within the optimum range for ZrNiSn-based half-Heusler (HH) alloys, the resulting thermal conductivity remains high. Hence, the aim of this study was to investigate the effect of "diagonal-rule" doping; that is, the Zr site was displaced by Ta, which can simultaneously enhance the electrical conductivity and reduce the lattice thermal conductivity. The solid-solubility limit of Ta in the ZrNiSn matrix was determined to be x = 0.04. The highest ZT, 0.72, was achieved at 923 K for Zr0.98Ta0.02NiSn. In addition, ZTavg increased by 10.2% for Zr0.98Ta0.02NiSn compared with that for ZrNiSn0.99Sb0.01 at 873 K, which was mainly attributed to the reduced lattice thermal conductivity of Zr0.98Ta0.02NiSn. These results suggest that Ta doping is more effective than Sb doping in ZrNiSn-based HH alloys. In addition, the microhardness of Zr1-xTaxNiSn was substantially improved with increasing Ta content and was also much higher than that of other traditional thermoelectric materials.

Various Performance Datum that Affect the Working of TEG

“Waste heat recovery with thermoelectric power generators can improve energy efficiency & provide distributed electricity generation.” The strategy of how to recover this heat depends on the temperature of waste heat gases and economics involved. The energy lost in exhaust gases cannot be fully recovered. However, much of the heat could be recovered & loss minimizes by adopting ideal performance conditions at the system level. The performance of TEG relies on more factors than traditional Thermoelectric (TE) material performance metrics alone, Positioning within the automotive system, Module Structure and Electrical Performance of one whole Thermoelectric (TE) system decides the efficiency of heat recovered. This review discusses the performance of TEG in different practical cases & what could be the best arrangement of the array of modules, Placement or Positioning & Conditions for a TEG setup to work in an Automotive System."

Topological current for transverse electrical and thermal conductivity in thermoelectric effect

Thermoelectric efficiency of the traditional thermoelectric material is low, which restricts the large scale applications. Recently, the developing of the topological insulator provides a new opportunity to get high thermoelectric efficiency material. There are two effects in topological insulator: anomalous Hall and Nernst effect, which have contribution to the transport properties. Because of anomalous Hall and Nernst effect the electrical thermal conductivity have transverse parts, which affect the Seebeck coefficient. However, the transverse parts can be expressed by Berry curvature. By using of φ-mapping topological theory, the Berry curvature is studied and we find there is topological vortex in the momentum space. The Bloch wave function is zero at the topological vortex. Finally, the relationships between the topological vortex and the transverse electrical and thermal conductivity is given and how the topology affects the Seebeck coefficient is researched in detail.

New Sb2Te3-xSex Monolayers with High Electron Mobilities and Wide Absorption Range.

As a traditional thermoelectric material with high thermoelectric performance at room temperature, antimony telluride (Sb2Te3) has been widely used in energy applications like power generation and refrigeration. By employing the "atomic transmutation" method, three new kinds of Sb2Te3-based monolayers of α-Sb2Te2Se, α-Sb2TeSe2, and β-Sb2TeSe2 are designed, which are expected to possess a high thermoelectric performance due to the quantum-confinement effects. In this work, by using the ab initio calculations, we systematically study electronic structures, vibration modes, optical, transport, and thermoelectric properties for the three kinds of monolayers and find that they are indirect-band-gap semiconductor materials with chemical and thermodynamic stability up to 700 K (α-Sb2TeSe2) or 900 K (α-Sb2Te2Se and β-Sb2TeSe2). The band gaps are around 1.0 eV with five nearly degenerate peaks in valence bands. The three Sb2Te3-xSex monolayers possess high electron mobilities larger than 1000 cm2/(V s), and the maximum zT values of α-Sb2Te2Se/α-Sb2TeSe2/β-Sb2TeSe2 are 0.70/0.71/0.65 at 300 K, respectively. For optical properties, the three Sb2Te3-xSex monolayers possess a wide absorption range from the blue region to the ultraviolet region. Compared with Sb2Te3, the three new kinds of monolayers possess a wider range of absorptions, higher mobilities, and thermoelectric performances, which may lead to promising applications in thermoelectric devices and saturable absorbers.

Energy Conversion by Nanomaterial-Based Trapezoidal-Shaped Leg of Thermoelectric Generator Considering Convection Heat Transfer Effect

Thermoelectric generators (TEGs) can harvest energy without any negative environmental impact using low potential sources, such as waste heat, and subsequently convert that energy into electricity. Different shaped leg geometries and nanostructured thermoelectric materials have been investigated over the last decades in order to improve the thermal efficiency of the TEGs. In this paper, a numerical study on the performance analysis of a nanomaterial-based (i.e., p-type leg composed of BiSbTe nanostructured bulk alloy and n-type leg composed of Bi2Te3 with 0.1 vol % SiC nanoparticles) trapezoidal-shaped leg geometry has been investigated considering the Seebeck effect, Peltier effect, Thomson effect, Fourier heat conduction, and surface to surrounding irreversible heat transfer loss. Different surface convection heat transfer losses (h) are considered to characterize the current output, power output, and thermal efficiency at various hot surface (TH) and cold surface (TC) temperatures. Good agreement has been achieved between the numerical and analytical results. Moreover, current numerical results are compared with previous related works. The designed nanomaterial-based TEG shows better performance in terms of output current and thermal efficiency with the thermal efficiency increasing from 7.3% to 8.7% using nanomaterial instead of traditional thermoelectric materials at h = 0 W/m2K while TH is 500 K and TC is 300 K.

Phonon-enhanced photothermoelectric effect in SrTiO3 ultra-broadband photodetector

The self-powered and ultra-broadband photodetectors based on photothermoelectric (PTE) effect are promising for diverse applications such as sensing, environmental monitoring, night vision and astronomy. The sensitivity of PTE photodetectors is determined by the Seebeck coefficient and the rising temperature under illumination. Previous PTE photodetectors mostly rely on traditional thermoelectric materials with Seebeck coefficients in the range of 100 μV K−1, and array structures with multiple units are usually employed to enhance the photodetection performance. Herein, we demonstrate a reduced SrTiO3 (r-STO) based PTE photodetector with sensitivity up to 1.2 V W−1 and broadband spectral response from 325 nm to 10.67 μm. The high performance of r-STO PTE photodetector is attributed to its intrinsic high Seebeck coefficient and phonon-enhanced photoresponse in the long wavelength infrared region. Our results open up a new avenue towards searching for novel PTE materials beyond traditional thermoelectric materials for low-cost and high-performance photodetector at room temperature.

Recent Progress in Thermoelectric Materials Based on Conjugated Polymers.

Organic thermoelectric (TE) materials can directly convert heat to electricity, and they are emerging as new materials for energy harvesting and cooling technologies. The performance of TE materials mainly depends on the properties of materials, including the Seebeck coefficient, electrical conductivity, thermal conductivity, and thermal stability. Traditional TE materials are mostly based on low-bandgap inorganic compounds, such as bismuth chalcogenide, lead telluride, and tin selenide, while organic materials as promising TE materials are attracting more and more attention because of their intrinsic advantages, including cost-effectiveness, easy processing, low density, low thermal conductivity, and high flexibility. However, to meet the requirements of practical applications, the performance of organic TE materials needs much improvement. A variety of efforts have been made to enhance the performance of organic TE materials, including the modification of molecular structure, and chemical or electrochemical doping. In this review, we summarize recent progress in organic TE materials, and discuss the feasible strategies for enhancing the properties of organic TE materials for future energy-harvesting applications.