Thermoelectric Properties Of Materials Research Articles

Thermoelectric Power Factor of Boron‐Doped Carbon Nanotubes Reinforced Cementitious Composites

In recent years, fossil fuel emissions and human activities have generated a lot of heat energy, causing environmental temperatures to rise. Thermoelectric cementitious composites that use temperature difference power generation to achieve the transformation of thermal and electrical energy can achieve a reduction in urban environmental temperature and alleviate resource depletion. However, the low thermoelectric conversion efficiency at this stage restricts its development. Herein, boron‐doped carbon nanotubes (B‐CNTs) are proposed to improve the thermoelectric properties of cement materials by doping boron into carbon nanotubes through the thermal diffusion method. At H3BO3: CNTs = 1:4, the synthesized B‐CNTs improve the Seebeck coefficient, which is mainly due to the increase in defect content and the appearance of additional phases around the Fermi energy level, leading to enhanced thermoelectric properties due to the increase in carrier scattering intensity. The thermoelectric optimum of the cement matrix composite with 7.0 wt% B‐CNTs content is 1.1 × 10−4, which is three times higher than that of the undoped carbon nanotube. This article provides a research idea to modify the thermoelectric properties of the material by doping the carbon material.

Mechanical properties of Sb2Te3 nanostructures under tensile loading

Thermoelectric research has become a prominent topic in recent years due to the mounting interest in providing alternative energy and reducing environmental pollution. The engineering applications of thermoelectric devices require high efficiency and mechanical robustness to ensure their reliability and stability. In this study, we investigated the material strength and failure mechanisms of Sb2Te3 nanofilms using molecular dynamics simulations. We designed a series of Sb2Te3 nanofilms to explore the effects of temperature, hole defects, and substrate on the mechanical properties of thermoelectric materials under tensile loading. Our findings provide valuable information for designing highly reliable and stable thermoelectric devices.

Investigations of microstructures and thermoelectric properties of TiNiSn half-Heusler compounds with micro- and nano-scale copper additions

The TiNiSn-based half-Heusler compounds have been considered as a promising thermoelectric material system for waste heat recovery applications. There have been numerous studies dedicated to improving its intrinsic low thermoelectric performance, and one strategy is to incorporate certain types of metal dopants into the host materials. Only a few research works have been focused on the effects of the dimensions of metal particles on the thermoelectric properties of materials. In this study, we investigate the roles of micro- (M) and nano-scale (N) Cu powders in affecting the microstructures and thermoelectric properties of TiNiCuxSn (x = 0−0.1) compounds. Owing to the ease of oxidation of Cu nanoparticles and the tedious preparation procedure, TiNiCux, NSn shows lower carrier concentration and electrical conductivity compared to the TiNiCux, MSn with the same nominal Cu content. The high-temperature Seebeck coefficient of TiNiCux, NSn gradually deteriorates as the temperature rises, partly due to the impurities-induced bandgap narrowing effect. The maximum power factor achieved for TiNiCux, NSn is about 20.2% lower than that for TiNiCux, MSn. Even though the reduced contribution of electronic component leads to a lower total thermal conductivity at low temperatures, the highest zT obtained in TiNiCux, NSn is markedly lower than that in the corresponding TiNiCux, MSn. These findings highlight the importance of investigating the dimensions of metal dopants in studies of the thermoelectric properties of materials.

Multiple fields generated by two dissimilar conductive punches on thermoelectric material

Exposed to high load concentrations environments such as mechanical, thermal, and electrical, thermoelectric equipment may be damaged by the contact surface during operation, resulting in equipment failure. The effects of two dissimilar rigid punches on the thermoelectric material layer under mechanical force and thermoelectric loading are investigated in this article. Using the Fourier transform, the problem is turned into a system of three singular integral equations containing the thermoelectric effect, and the collocation methods are used to solve it numerically. The effects of thermoelectric load, thermoelectric material layer thickness, and the distance between two punches on the electric current density, energy flux, and surface normal stress distribution are thoroughly discussed. The results show that when the two punches are close to each other, below the two punches, the electric current density, energy flux, and contact stress values are larger near the distal end and lower at the inner edge. The stresses at the inner edge of the two indices show opposite characteristics with an increasing of total energy flow and total current, whereas they are consistent at the outer edge. The results aid in the measurement of the multifield coupling properties of finite-thickness thermoelectric material using indenter experiments.

Theoretical investigation of the electronic structure and thermoelectric performance of 2D GeSb2Te4 and GeBi2Te4

Thermoelectric technology is attractive for waste heat recovery and clean energy; it is highly desired as a potential power supply. In this paper, we investigated the stability, elastic, electronic structures and thermoelectric properties of unexplored 2D materials, GeSb2Te4 and GeBi2Te4, which deliver high stability, acceptable cleavage energies (0.32–0.33 J/m2), and narrow band gaps of 0.80 and 0.69 eV. We also revealed that the septuple layers possess anisotropic electron and hole mobilities of about 1593.71/791.50 and 522.04/240.52 cm2/Vs, which result in high conductivities of 107–108 S/m, as well as desirable thermoelectric power factors of 28.08–70.382 mW/Km2. Besides, owing to the low group velocities and strong dissipative scattering for low-lying phonons, the materials inherit the low lattice thermal conductivities of 1.51 and 0.41 W/mK. As a result, their thermoelectric figure of merit reaches 1.60 and 1.70 at 300 K, and rises further to 3.80 and 4.16 at 700 K. Together, 2D GeSb2Te4 and GeBi2Te4 are suitable candidates for low-medium temperature thermoelectric application.

Advances in Flexible Thermoelectric Materials and Devices Fabricated by Magnetron Sputtering

Due to the direct conversion between thermal and electrical energy, thermoelectric materials and their devices exhibit great potential for power generation and refrigeration. With the rapid development of personal wearable electronics, the design of flexible inorganic thermoelectric materials and devices receives increasing attention. As one of the most mature thin‐film fabrication techniques, magnetron sputtering plays a key role in the fabrication of inorganic thermoelectric thin films and devices, but its progress is still not timely and comprehensively reviewed. Herein, recent advances in magnetron sputtering‐fabricated thermoelectric materials and devices are studied, including their thermoelectric properties, mechanical properties, and device design routes. The differences in the properties of thermoelectric materials under different sputtering conditions, as well as their underlying mechanisms, are carefully discussed. In the end, it is pointed out the challenges and future directions for magnetron sputtering‐prepared inorganic thermoelectric thin‐film materials and devices for practical applications. This review can serve as a useful reference to guide the design of inorganic thermoelectric materials and devices prepared by magnetron‐sputtering‐based deposition techniques.

Suppression of intervalley scattering and enhanced phonon anharmonic interactions in 2D Bi2TeSe2: Crystal-field symmetry and band convergence

The electron-phonon (el-ph) and phonon-phonon interactions play a key role in determining electronic and thermal transport properties, respectively, in promising two-dimensional (2D) semiconductor devices. In this study, we investigated el-ph interactions using Wannier-Fourier interpolation method and renormalized phonon scattering considering finite-temperature effects in Bi2TeSe2 monolayer. The results show that the optical phonon modes dominate the carrier scattering, where level repulsion induced by crystal field splitting and spin-orbit coupling (SOC) effect effectively suppresses intervalley scattering, leading to high hole mobility. Moreover, the strong anharmonicity in Bi2TeSe2 monolayer results in the temperature-dependent softening of its optical phonons, along with a corresponding variation in interatomic force constants (IFCs). As a result, the lattice thermal conductivity is remarkably low and exhibits weak temperature dependence. Finally, the predicted dimensionless thermoelectric figure of merit exceeds unity in the range of 200–800 K, indicating the potential of Bi2TeSe2 monolayer for thermoelectric applications. This work provides new insights into the elimination of intervalley scattering by crystal field splitting and SOC effects, and paves the way for the evaluation of thermoelectric properties of materials with complex scattering mechanisms and strong anharmonicity.

Enhanced power factor of expanded graphite phase change thermoelectric cement composites by metal oxides

Cement-based materials with thermoelectric effect can realize the conversion of waste heat in building environment, which is of great significance in energy conversion and utilization, and has become a research hotspot in recent years. In this study, three kinds of metal oxides were added into the phase change modified expanded graphite cement-based composites to investigate the impact of metal oxide content on the thermoelectric characteristics of cement composites. The findings shown that the thermoelectric properties of cement-based materials can be enhanced by the addition of metal oxides. The power factor increased to 3.15 μW·m−1·K−2, 3.60 μW·m−1·K−2, and 2.53 μW·m−1·K−2 after mixing ZnO, CuO, and TiO2. This is because the incorporation of metal oxides introduced a large number of interfaces into the cement, increased carrier scattering, strengthened the Seebeck coefficient of cement, and improved the power factor. This work presented a novel strategy to improving the thermoelectric properties of cement-based materials.

A wearable flexible battery using human body temperature differences

Ionic battery materials are widely used in wearable devices, especially ionic gel materials. In the face of a variety of ionic thermoelectric materials, it is a key issue to select the appropriate thermoelectric ones for different artificial heart power supply needs. In this study, it is aimed to establish a database of ionic thermoelectric materials for existing wearable devices, compare the battery performance under different boundary conditions and different materials based on the physical model of the battery, and select the optimal battery material suitable for an artificial heart. 72 kinds of ionic thermoelectric materials were identified through a literature review. The properties of ionic thermoelectric materials were comprehensively considered from the aspects of the Seebeck coefficient, electric conductivity, and power factor. The materials were determined by a thermoelectric figure of merit (ZT). The physical model of the battery was established by Solidworks. The three-dimensional physical field of thermoelectric materials was analyzed by COMSOL, so as to clarify the temperature and potential change characteristics of thermoelectric materials under different boundary conditions and different material types. Finally, the thermoelectric material with the largest electric potential energy was selected. Under a constant temperature boundary condition of 33 ℃ and 5 ℃, the maximum potential produced was 23.21 mV. By changing the materials and keeping other conditions unchanged, it was found that the potential generated by the six ion gel materials is mostly mV.

Performance degradation analysis and fabrication guidance of μ-TEG from material to device

Micro thermoelectric generator (μ-TEG) attracts more and more attention due to its small size and high power density. Many two-dimensional thermoelectric materials with high performance have facilitated the development of μ-TEG. However, the performance of μ-TEG fabricated by these great thermoelectric materials is significantly degraded due to size effect, interfacial effects (include contact effect and boundary effect) and structure effect. To accurately assess the performance degradation degree from material to μ-TEG and guide the device fabrication, an experiment-verified mathematical model considering interfacial and size effects is proposed. Firstly, the phonon/electron temperature distribution in thermoelectric leg of μ-TEG is analyzed to investigate the device-level thermoelectric properties of material. Then based on the device-level thermoelectric properties, the actual power generation performance model of μ-TEG is established to conduct the influence analysis of these effects (boundary, size, contact and structure effects) on material and device. Finally, the thermoelectric leg thickness (Hte) is optimized to realize optimal power generation. The study results reveal that boundary and size effects weaken the device-level thermoelectric properties, and the reduction trend is more obvious when Hte is smaller, especially when Hte ≤ 20 μm. The decrease from the material intrinsic figure of merit ((ZT)m) to the device figure of merit ((ZT)D) is owing to the boundary effect, structure effect and contact effect, and the dominant factor of this decrease changes from structure effect (Hte＜7 μm)to contact effect (Hte ≥ 7 μm) as Hte increases, which points to a main optimization direction for (ZT)D for different Hte. As for contact effect, the electrical contact resistivity (re,c) has a more significant impact on weakening the performance of μ-TEG than thermal contact resistivity (rk,c), and their optimization goals are explored (re,c ≤ 5.1 × 10−12 Ω·m2, rk,c ≤ 9.3 × 10−8 K·m2/W). At given electrical and thermal contact resistivity, there exists an optimal Hte for achieving the optimal power generation (Popt) and a large range of Hte for achieving 95%Popt, and the optimal Hte increases with increasing electrical and thermal contact resistivity. This study can reduce the processing difficulty and save time and economic costs of μ-TEG fabrication, which can avoid the blind fabrication of μ-TEG.

Enhanced thermoelectric properties of Cu3SbSe4-based materials by synergistic modulation of carrier concentration and phonon scattering

Cu3SbSe4, a copper-based sulfide free of rare earth elements, has received extensive attention in thermoelectric materials. However, its low carrier concentration restricts its widespread application. In this study, a microwave-assisted solution synthesis method was used to produce samples of Cu3SbSe4, which enabled the formation of CuSe in situ and increased the yield. Through the use of first-principles calculations, structural analysis, and performance evaluation, it was found that CuSe can enhance the carrier concentration and that induced nano-defects have a positive effect on reducing the lattice thermal conductivity. Moreover, doping with Sn decreases the band gap of the system and moves the Fermi level into the valence band, increasing the carrier concentration to 1.15 × 10−20 cm−3. Finally, the zT value of the Cu3Sb0.98Sn0.02Se4 sample was achieved at 1.05 at 623 K when the theoretical yield of a single synthesis was 10 mmol.

Neutron-irradiated effect on the thermoelectric properties of Bi2Te3-based thermoelectric leg

Thermoelectric (TE) materials working in radioisotope thermoelectric generators are irradiated by neutrons throughout its service; thus, investigating the neutron irradiation stability of TE devices is necessary. Herein, the influence of neutron irradiation with fluences of 4.56 × 1010 and 1 × 1013 n/cm2 by pulsed neutron reactor on the electrical and thermal transport properties of n-type Bi2Te2.7Se0.3 and p-type Bi0.5Sb1.5Te3 thermoelectric alloys prepared by cold-pressing and molding is investigated. After neutron irradiation, the properties of thermoelectric materials fluctuate, which is related to the material type and irradiation fluence. Different from p-type thermoelectric materials, neutron irradiation has a positive effect on n-type Bi2Te2.7Se0.3 materials. This result might be due to the increase of carrier mobility and the optimization of electrical conductivity. Afterward, the effects of p-type and n-type TE devices with different treatments on the output performance of TE devices are further discussed. The positive and negative effects caused by irradiation can cancel each other to a certain extent. For TE devices paired with p-type Bi0.5Sb1.5Te3 and n-type Bi2Te2.7Se0.3 thermoelectric legs, the generated power and conversion efficiency are stable after neutron irradiation.

Thermoelectric hotspot cooling using thermally conductive fillers

The commercial applications of thermoelectric coolers (TECs) are limited to niche markets owing to their low cooling efficiency. For hotspot cooling, in which the Fourier heat is added to the Peltier heat, thermoelectric (TE) materials should possess both large power factors and high thermal conductivities. However, the interdependence among TE material properties renders the development of such materials challenging. Herein, we provide a design strategy of TEC that addresses these material challenges at the device level. As proof of concept, we demonstrate that the cooling capability of filler-embedded TECs (F-TECs), which consist of TE legs with large power factors and a thermally conductive filler material, can be significantly improved compared to that of conventional TEC, owing to the 42.5% enhanced effective thermal conductance. Additionally, infrared imaging shows that the filler can decrease the temperature gradient and maximum temperature of the TE legs, which may improve the thermal stability of the TECs. Our proposed F-TEC concept can facilitate the development of effective TEC devices that can be widely used in hotspot cooling applications, such as in microprocessors, batteries and photovoltaic cells.

High-Performance MoS2/SWCNT Composite Films for a Flexible Thermoelectric Power Generator.

Single-walled carbon nanotube (SWCNT)-based thermoelectric materials have been extensively studied in the field of flexible wearable devices due to their high flexibility and excellent electrical conductivity (σ). However, poor Seebeck coefficient (S) and high thermal conductivity limit their thermoelectric application. In this work, free-standing MoS2/SWCNT composite films with improved thermoelectric performance were fabricated by doping SWCNTs with MoS2 nanosheets. The results demonstrated that the energy filtering effect at the MoS2/SWCNT interface increased the S of composites. In addition, the σ of composites was also improved due to the reason that S-π interaction between MoS2 and SWCNTs made good contact between MoS2 and SWCNTs and improved carrier transport. Finally, the obtained MoS2/SWCNT showed a maximum power factor of 131.9 ± 4.5 μW m-1 K-2 at room temperature with a σ of 680 ± 6.7 S cm-1 and an S of 44.0 ± 1.7 μV K-1 at a MoS2/SWCNT mass ratio of 15:100. As a demonstration, a thermoelectric device composed of three pairs of p-n junctions was prepared, which exhibited a maximum output power of 0.43 μW at a temperature gradient of 50 K. Therefore, this work offers a simple method of enhancing the thermoelectric properties of SWCNT-based materials.

Colloidal Synthesis of Multinary Alkali-Metal Chalcogenides Containing Bi and Sb: An Emerging Class of I-V-VI2 Nanocrystals with Tunable Composition and Interesting Properties.

The growth mechanism and synthetic controls for colloidal multinary metal chalcogenide nanocrystals (NCs) involving alkali metals and the pnictogen metals Sb and Bi are unknown. Sb and Bi are prone to form metallic nanocrystals that stay as impurities in the final product. Herein, we synthesize colloidal NaBi1-xSbxSe2-ySy NCs using amine-thiol-Se chemistry. We find that ternary NaBiSe2 NCs initiate with Bi0 nuclei and an amorphous intermediate nanoparticle formation that gradually transforms into NaBiSe2 upon Se addition. Furthermore, we extend our methods to substitute Sb in place of Bi and S in place of Se. Our findings show the initial quasi-cubic morphology transforms into a spherical shape upon increased Sb substitution, and the S incorporation promotes elongation along the <111> direction. We further investigate the thermoelectric transport properties of the Sb-substituted material displaying very low thermal conductivity and n-type transport behavior. Notably, the NaBi0.75Sb0.25Se2 material exhibits an ultralow thermal conductivity of 0.25 W·m-1·K-1 at 596 K with an average thermal conductivity of 0.35 W·m-1·K-1 between 358 and 596 K and a ZTmax of 0.24.

Synthesis and Thermoelectric Properties of La-doped n-type Mg3SbBi Materials

Mg3Sb2-based materials are very promising for thermoelectric applications at low temperatures, and are strong candidates to replace n-type Bi2Te3 for cooling and power generation. Substituting Sb atoms with chalcogen elements (S, Se, Te) is a typical method of n-type doping, while doping the Mg site with Group 3 elements (Y, Sc) and Lanthanides has also been studied. Unique advantages have been recently reported. In this study, a La-containing compound, LaSb, was used to fabricate n-type Mg3SbBi. The thermoelectric properties of polycrystalline Mg3LaxSbBi (0 ≤ &lt;i&gt;x&lt;/i&gt; ≤ 0.02) were investigated after synthesis by sequential processes of arc melting, ball milling, and spark plasma sintering. Undoped Mg3SbBi is p-type with poor thermoelectric performance, and switched to n-type with La doping. The electron concentration of Mg3LaxSbBi increased linearly with La content &lt;i&gt;x&lt;/i&gt;, reaching up to 9.4 × 10&lt;sup&gt;19&lt;/sup&gt; cm&lt;sup&gt;-3&lt;/sup&gt; at &lt;i&gt;x&lt;/i&gt; = 0.02. The power factor and the figure of merit were also maximized in Mg3La0.02SbBi, reaching 1.8 mW m&lt;sup&gt;-1&lt;/sup&gt;K&lt;sup&gt;-2&lt;/sup&gt; (573 K) and 0.89 (623 K), respectively. The lattice thermal conductivity decreased with increasing La content above ~500 K, and the minimum value of 0.73 W m&lt;sup&gt;-1&lt;/sup&gt;K&lt;sup&gt;-1&lt;/sup&gt; was obtained in Mg3La0.02SbBi. This study shows that La doping using LaSb provides a reliable method for n-type doping of Mg3Sb2-based materials.

Thermoelectric properties of CuO-LiCoO2‑La0.7Sr0.3MnO3 composition materials

For the widespread use of thermoelectric materials, it is necessary to reduce their cost, simplify the technology and increase the thermoelectric figure of merit, which is highly dependent on the Seebeck coefficient. Therefore, one of the promising directions is the search for composites for effective thermoelectric converters. In our article, three-component composites consisting of a conductive component of La0.7Sr0.3MnO3, (LSMO), and dielectric components: a mixture of CuO oxides and lithium cobaltite LiCoO2 have been prepared and experimentally investigated as such materials. The phase composition of the obtained samples was studied by X-ray diffraction, electron and optical microscopy. The thermoelectric properties of composite materials have been studied in the field of low temperatures from 30 to 250°C. The best results for Seebeck coefficient 550 μV / K and power factor 0.108 μW / (K2 ∙ m) are shown by experimentally selected compositions containing about 25 % by weight LSMO, 40 % CuO, 30 % LiCoO2, 5 % GeO2. However, the largest value of the power factor (1.859 μW / (K2 ∙ m) was achieved for the sample composition 77 % LSMO, 20 % CuO, 3 % GeO2 with Seebeck 310 μV / K. It should be noted that such three-phase samples have a “p”-type of conductivity. The presence of glass-forming germanium (3 – 5 %) oxide gives samples great mechanical stability.

Ultra-Low Lattice Thermal Conductivity Enables High Thermoelectric Properties in Cu and Y Codoped SnTe via Multi-Scale Composite Nanostructures

The eco-friendly thermoelectric material SnTe has recently gained immense attention due to its excellent thermoelectric properties and potential. However, its thermoelectric properties are limited by the inherent high carrier concentration (low Seebeck coefficient) and high thermal conductivity. Herein, we further introduce rare-earth element Y on top of the conventional Cu alloying to more effectively modulate its microstructure and thermal transport properties. The experimental results reveal that Y doping gives rise to band convergence, endowing Sn1–xYxTe-5%Cu2Te with an increased power factor of ∼22.14 μW cm–1 K–2. Meanwhile, the introduced abundant Y/Y2Te3 multiscale composite nanostructures in the matrix greatly enhance phonon scattering, resulting in an ultra-low lattice thermal conductivity of ∼0.43 W m–1 K–1. Consequently, these combined effects resulted in a peak ZT value of 1.27 at 823 K in Sn0.97Y0.03Te-5%Cu2Te. An enhancement of nearly 217% is achieved compared to pristine SnTe, which greatly improves the thermoelectric properties of SnTe-based materials.

First-principles research on the thermoelectric properties of NbCoGe based on the scattering mechanisms

ABSTRACT Half-Heusler compounds have excellent power generation performance at high temperatures. The scattering mechanism is an essential factor affecting the electrical transport properties of thermoelectric materials. In computational simulations, only the role of acoustic phonon scattering on the thermoelectric properties of materials is considered, whereas other scattering mechanisms are neglected. In this work, we investigate the thermoelectric properties of NbCoGe compounds with different combinations of acoustic deformation potential, polar optical phonon, and ionised impurity scattering mechanisms. The calculated results show that the ZT values of n-type and p-type NbCoGe compounds reach 8 and 2.8, respectively, when only acoustic deformation potential scattering is considered, indicating that this sole scattering is insufficient. The calculated ZT values of p-type NbCoGe compounds reach 1.8 at 1200 K and more than 1 at 800 K for p- and n-type NbCoGe compounds under the combined effect of the three scattering mechanisms due to their high–power factor. This provides solid theoretical guidance for the search for potentially high–temperature half-Heusler thermoelectric materials.

Realizing high thermoelectric performance for p-type SiGe in medium temperature region via TaC compositing

SiGe is recognised as an excellent thermoelectric material with superior mechanical properties and thermal stability in regions with high temperatures. This study explores a novel strategy for co-regulating thermoelectric transport parameters to achieve high thermoelectric properties of p-type SiGe in the mid-temperature region by incorporating nano-TaC into SiGe combined ball milling with spark plasma sintering. By optimizing the amount of TaC in the SiGe matrix, the power factors were significantly increased due to the modulation doping effect based on the work function matching of SiGe with TaC. Simultaneously, the ensemble effect of the nanostructure leads to a significant decrease in thermal conductivity. Thus, a high ZT of 1.06 was accomplished at 873 K, which is 64 % higher than that of typical radioisotope thermoelectric generator. Our research offers a novel strategy for expanding and enhancing the thermoelectric properties of SiGe materials in the medium temperature range.