TE Materials Research Articles

Physical Metallurgy Inspired Nano‐Features for Enhancement of Thermoelectric Conversion Efficiency

AbstractThermoelectric (TE) materials are useful for the conversion of heat flux into electrical power and for heat removal and are, therefore, technologically important. Development of TE materials involves controlling their thermal and electrical transport, and both are sensitive to the finest features of the material microstructure. Major turning points recorded in TE material research are enabled due to adopting concepts from physical metallurgy. These classical approaches, originally developed to explain plastic deformation mechanisms in metallic systems, are now being employed to elucidate thermal and electrical transport in highly efficient TE materials, and serve as tools to manipulate their transport properties. The present Progress Report reviews recent studies combining experimental and computational approaches aiming to elucidate the role of 0D, 1D, 2D, and 3D lattice defects, and to harness them to enhance TE performance.

Structural Reliability Evaluation of Thermoelectric Generator Modules: Influence of End Conditions, Leg Geometry, Metallization, and Processing Temperatures

Structural reliability of thermoelectric generators (TEGs) in medium to high temperature applications still remains a challenge. Currently there are no established standards for evaluating the thermomechanical reliability of TEGs during new material and module development phases in the industry. In this work we implemented a procedure for evaluating the reliability of brittle TE materials based on the common practice in the ceramics industry. The probabilistic thermomechanical reliability of TEG couples and modules is evaluated using Weibull analysis. Finite element simulations were carried out to study the influence of TEG couple parameters such as leg geometry, dimensions, spacing, metallization thickness, and processing conditions on the stresses, and the structural reliability evaluations were conducted based on the TE material stresses. Results showed that the boundary conditions and processing temperatures have significant influence on the TE material and metallization stresses and overall device reliability. The external compressive load significantly enhanced structural reliability. Results also showed that among geometrical parameters the leg length has a dominating influence on the overall stresses compared to the leg cross-sectional shapes. The cross-section geometry of legs showed considerable effect at shorter lengths, and the influence diminished with increased lengths. The proposed approach for evaluating the structural reliability could be integrated with performance optimization techniques for developing optimal and reliable TEG devices.

The effects of Ag nanoparticles on the thermoelectric properties of Ag2Te-Ag composite fabricated using an energy-saving route

We measure electricity consumption and thermoelectric property of Ag2Te-Ag composites fabricated using green synthesis at room temperature followed by evacuated-and-encapsulated sintering at 500 °C for 10 h. As compared to Ag2Te and Ag2Te-Ag composites fabricated using other methods, we elucidate the effects of Ag nanoparticles on electronic transport of composites at temperatures between structural transition and 525 K based on the Matthiessen's rule and Kohler formula. We also present detailed methods of calculating the Lorenz number with single Kane band model and further deduce the lattice thermal conductivity. As a result, we find that electronic contribution dominates the temperature dependence of the total thermal conductivity of the composites. For energy consumption of heat treatment, the energy-saving approach in this study is only 15–18% of two typical procedures using melting process reported in the literature. If considering consolidation step reported in the literature, our procedure is more economic. We also demonstrate that the energy-saving route of fabricating the Ag2Te-Ag composite leads to a zT value of 0.68 at 573 K, which is comparable to or even higher than that of Ag2Te fabricated using high-energy consumption routes. It is more sensible for fabricating TE materials using less energy consumption.

Tilt-structure and high-performance of hierarchical Bi1.5Sb0.5Te3 nanopillar arrays

The uniquely tilted nanopillar array favorably influence carrier and phonon transport properties. We present an innovative interfacial design concept and a novel tilt-structure of hierarchical Bi1.5Sb0.5Te3 nanopillar array comprising unique interfaces from nano-scaled open gaps to coherent grain boundaries, and tilted nanopillars assembled by high-quality nanowires with well oriented growth, utilizing a simple vacuum thermal evaporation technique. The unusual structure Bi1.5Sb0.5Te3 nanopillar array with a tilt angle of 45° exhibits a high thermoelectric performance ZT = 1.61 at room temperature. The relatively high ZT value in contrast to that of previously reported Bi1.5Sb0.5Te3 materials and the Bi1.5Sb0.5Te3 nanopillar array with a tilt angle of 60° or 90° evidently reveals the crucial role of the unique interface and tilt-structure in favorably influencing carrier and phonon transport properties, resulting in a significantly improved ZT value. This method opens a new approach to optimize nano-structure film materials.

Spinodal Superlattices of Topological Insulators

Spinodal decomposition is proposed for stabilizing self-assembled interfaces between topological insulators (TIs) by combining layers of iso-structural and iso-valent TlBiX2 (X = S, Se, Te) materials. The composition range for gapless states is addressed concurrently to the study of thermodynamically driven boundaries. By tailoring composition, the TlBiS2–TlBiTe2 system might produce both spinodal superlattices and two-dimensional eutectic microstructures, either concurrently or separately. The dimensions and topological nature of the metallic channels are determined by following the spatial distribution of the charge density and the spin-texture. The results validate the proof of concept for obtaining spontaneously forming two-dimensional TI-conducting channels embedded into three-dimensional insulating environments without any vacuum interfaces. Since spinodal decomposition is a controllable kinetic phenomenon, its leverage could become the long-sought enabler for effective TI technological deployment.

Effect of annealing on microstructure and thermoelectric properties of hot-extruded Bi–Sb–Te bulk materials

The effect of annealing on the microstructure, thermoelectric properties and hardness of the hot-extruded Bi–Sb–Te materials has been investigated systematically to optimize their thermoelectric and mechanical properties. The mechanically alloyed powder was consolidated by hot extrusion at either 340 or 400 °C, followed by annealing in a temperature range of 260–400 °C. The microstructure of the annealed samples contained submicron grains with preferred (001) texture. As annealing temperature increased, the small-angle grain boundaries (SAGBs) increased because the increased amount of Te-rich and Sb-rich phases inhibits the movements of dislocations and SAGBs. The submicron microstructure led to a low thermal conductivity, for example, ~ 0.9 W/mK after annealing at TA ≥ 380 °C. The Seebeck coefficient highly depended on carrier mobility in addition to carrier concentration. For the extruded samples prepared at a lower extrusion temperature of 340 °C, the mobility increased significantly after annealing, resulting in great enhancements in the Seebeck coefficient and electrical conductivity. A peak ZT value of 0.94 and high hardness were simultaneously obtained under the conditions of hot extrusion at 340 °C and annealing at 380 °C. It seems that the combination of low-temperature extrusion and high-temperature annealing is an effective route to prepare high-performance Bi2Te3-based materials.

MoSbTe for high-speed and high-thermal-stability phase-change memory applications

Mo-doped Sb1.8Te materials and electrical devices were investigated for high-thermal-stability and high-speed phase-change memory applications. The crystallization temperature (tc = 185 °C) and 10-year data retention (t10-year = 112 °C) were greatly enhanced compared with those of Ge2Sb2Te5 (tc = 150 °C, t10-year = 85 °C) and pure Sb1.8Te (tc = 166 °C, t10-year = 74 °C). X-ray diffraction and transmission electron microscopy results show that the Mo dopant suppresses crystallization, reducing the crystalline grain size. Mo2.0(Sb1.8Te)98.0-based devices were fabricated to evaluate the reversible phase transition properties. SET/RESET with a large operation window can be realized using a 10 ns pulse, which is considerably better than that required for Ge2Sb2Te5 (∼50 ns). Furthermore, ∼1 × 106 switching cycles were achieved.

Angular dependence of vortex instability in a layered superconductor: the case study of Fe(Se,Te) material

Anisotropy effects on flux pinning and flux flow are strongly effective in cuprate as well as iron-based superconductors due to their intrinsically layered crystallographic structure. However Fe(Se,Te) thin films grown on CaF2 substrate result less anisotropic with respect to all the other iron based superconductors. We present the first study on the angular dependence of the flux flow instability, which occurs in the flux flow regime as a current driven transition to the normal state at the instability point (I*, V*) in the current-voltage characteristics. The voltage jumps are systematically investigated as a function of the temperature, the external magnetic field, and the angle between the field and the Fe(Se,Te) film. The scaling procedure based on the anisotropic Ginzburg-Landau approach is successfully applied to the observed angular dependence of the critical voltage V*. Anyway, we find out that Fe(Se,Te) represents the case study of a layered material characterized by a weak anisotropy of its static superconducting properties, but with an increased anisotropy in its vortex dynamics due to the predominant perpendicular component of the external applied magnetic field. Indeed, I* shows less sensitivity to angle variations, thus being promising for high field applications.

Effect of annealing process on the properties of undoped and manganese2+-doped co-binary copper telluride and tin telluride thin films

The binary semiconductor materials Cu1.81Te and SnTe materials, without and with manganese (Mn2+) doping, were prepared by dropping a Cu-Sn-Te solution on a commercial glass substrate to fabricate a co-binary thin film. The characteristics, optical, and electrical properties of undoped and Mn2+-doped Cu1.81Te/SnTe thin films were investigated with variations in the annealing process. The XRD results confirmed the films consisted of the co-binary orthorhombic phase materials Cu1.81Te and SnTe, and that for all annealing temperatures from 50 to 400 °C an amorphous structure became prevalent in the Mn2+-incorporated co-binary thin films. The optical parameters and electrical performance varied with the annealing temperatures and Mn2+ doping, showing alterations in the properties of the co-binary film. These co-binary thin films have feasibility for real applications in surface analysis, electro-optical materials, solar selective surfaces, and photovoltaic thermal devices.

Enhancing Thermoelectric Performances of Bismuth Antimony Telluride via Synergistic Combination of Multiscale Structuring and Band Alignment by FeTe2 Incorporation.

It has been a difficulty to form well-distributed nano- and mesosized inclusions in a Bi2Te3-based matrix and thereby realizing no degradation of carrier mobility at interfaces between matrix and inclusions for high thermoelectric performances. Herein, we successfully synthesize multistructured thermoelectric Bi0.4Sb1.6Te3 materials with Fe-rich nanoprecipitates and sub-micron FeTe2 inclusions by a conventional solid-state reaction followed by melt-spinning and spark plasma sintering that could be a facile preparation method for scale-up production. This study presents a bismuth antimony telluride based thermoelectric material with a multiscale structure whose lattice thermal conductivity is drastically reduced with minimal degradation on its carrier mobility. This is possible because a carefully chosen FeTe2 incorporated in the matrix allows its interfacial valence band with the matrix to be aligned, leading to a significantly improved p-type thermoelectric power factor. Consequently, an impressively high thermoelectric figure of merit ZT of 1.52 is achieved at 396 K for p-type Bi0.4Sb1.6Te3-8 mol % FeTe2, which is a 43% enhancement in ZT compared to the pristine Bi0.4Sb1.6Te3. This work demonstrates not only the effectiveness of multiscale structuring for lowering lattice thermal conductivities, but also the importance of interfacial band alignment between matrix and inclusions for maintaining high carrier mobilities when designing high-performance thermoelectric materials.

Thermoelectric performance of n-type (PbTe)1−x(CoTe)x composite prepared by high pressure sintering method

Composites of nominal composition (PbTe)1−x(CoTe)x (x = 0–0.18) were fabricated by high pressure (6 GPa) sintering (773 K) method. The thermoelectric performances were investigated in the temperature range of 293–773 K. The experimental results show that CoTe utilized as the secondary phase can remarkably enhance the TE properties of PbTe, of which the highest ZT value reaches 0.88 at 473 K when x = 0.14. The enhancement of TE performance owes much to its high electric conductivity of CoTe. Meanwhile, the high pressure sintering (HPS) samples consist of nanoparticle, which significantly enhances the boundary scattering on carriers, decreases thermal conductivity, and increases Seebeck coefficient. All the results indicate that HPS method and the addition of CoTe-composite are effective methods to enhance the thermoelectric performance of PbTe as a potential TE material.

XXII Международный симпозиум Нанофизика и наноэлектроника", Нижний Новгород, 12-15 марта 2018 г. Расчет состояний многозарядных примесно-дефектных центров в эпитаксиальных слоях Hg-=SUB=-1-x-=/SUB=-Cd-=SUB=-x-=/SUB=-Te

AbstractA method for calculating the states of multivalent donors and acceptors in Hg_1 –_ x Cd_ x Te materials is developed. The ionization energies of deep acceptor and donor centers in epitaxial Hg_1 –_ x Cd_ x Te films are calculated. The calculation method takes into account the influence of both the valence band and the conduction band on the states of impurity-defect centers. The calculations of energies for the levels of tetravalent acceptors and donors associated with crystalline structure defects indicate the intercenter nature of lines observed previously in the photoluminescence spectra of Hg_1 –_ x Cd_ x Te films.

Experimental Approaches to Improve Thermal Stability of Organic Phase Change Properties

This study investigates the thermal behavior of Polyolefin containing Paraffin and Nano Hydrated aluminum silicate Al2Si2O5 (OH) 4 (Kaolin) particles to enhance store energy at ambient temperature. The hybrid Nano composite is based on polyolefin PE as a matrix, whereby paraffin wax and Kaolin were hot blended at varying concentrations. In addition Carbon Nanotube (CNTs) was added in different relative low concentrations to improve the thermal transition among the polymer matrix, since polymer domains are considered as isolator. The composite was prepared by melt mixing using a Brabender Plasrograph and a Two Role Mill. Thermal properties of the composite were determined using DSC and Melt flow Index. Because TES materials are subjected to melting and freezing during life time, multiple extrusion tests to simulate the degradation process of the composite were carried out. FTIR was applied to determine the degradation effect and investigate microstructure changes of the composite. The results obtained demonstrate that the blend shows a tendency to be thermally active at low temperatures. DSC tests evidenced a decrease in melt tempera-ture as a result of increasing Kaolin content and some changes in the latent heat of the compound.

Thermoelectric properties of GeTe and Sb2Te3 calculated by density functional theory

The GeTe and Sb2Te3 system materials are compound very narrow band gaps semiconductors in the energy range 0.20-2.80 eV and highly regarded as good performance TE materials. We calculated thermoelelctric properties of GeTe and Sb2Te3 system by using the density functional therory (DFT) and Boltzmann transport therory calculations based on QUANTUM ESPRESSO and BoltzTraP package. The Seebeck coefficient, electrical conductivity, thermal conductivity and power factor dependent on Fermi energy were predicted for future experimental of Ge-Sb-Te materials.

Thermoelectric Properties of Texture-Controlled (GeTe) x (AgSbTe2)100−x (x = 75, 80, 85, and 90) Alloys Fabricated by Gas-Atomization and Hot-Extrusion Processes

In this study, p-type (GeTe) x (AgSbTe2)100−x : TAGS-x (where x = 75, 80, 85, and 90) thermoelectric materials were fabricated by a combination of gas atomization and a hot-extrusion process, and the effects of chemical composition on microstructure, thermoelectric, and mechanical properties were investigated. The extruded samples exhibited higher relative densities (> 99%), and a significant orientation degree parallel to the extrusion direction with fine and homogeneous microstructure was observed. The hardness of extruded samples was around 200–260 kgf/mm2, which indicates that they have much better mechanical properties than most other TE materials. The power factor of the extruded samples showed excellent values; the maximum power factor achieved was 3.81 × 10−3 W/mK2 for TAGS-90 at 723 K due to an effective combination of the Seebeck coefficient and electrical conductivity.

Preparation and Thermoelectric Properties of Graphite/Bi0.5Sb1.5Te3 Composites

Bismuth telluride zone-melting alloys are the most commercially used thermoelectric materials. However, the zone-melting ingots have weak machinability due to the strong preferred orientation. Here, non-textured graphite/Bi0.5Sb1.5Te3 (G/BST) composites were prepared by a powder metallurgy method combined with cold-pressing and annealing treatments. The composition, microstructure, and thermoelectric properties of the G/BST composites with different mass percentages of G were investigated. It was found that G addition could effectively reduce the thermal conductivity and slightly improve the electrical properties of the BST, which resulted in a large enhancement in the figure-of-merit, ZT. The largest ZT for the xG/BST composites with x = 0.05% reached 1.05 at 320 K, which is increased by 35% as compared with that of the G-free BST materials. This work provided an effective method for preparing non-textured Bi2Te3-based TE materials with a simple process, low cost, and large potential in scale production.

High Performance Thermoelectric Materials: Progress and Their Applications

AbstractThermoelectric (TE) materials have the capability of converting heat into electricity, which can improve fuel efficiency, as well as providing robust alternative energy supply in multiple applications by collecting wasted heat, and therefore, assisting in finding new energy solutions. In order to construct high performance TE devices, superior TE materials have to be targeted via various strategies. The development of high performance TE devices can broaden the market of TE application and eventually boost the enthusiasm of TE material research. This review focuses on major novel strategies to achieve high‐performance TE materials and their applications. Manipulating the carrier concentration and band structures of materials are effective in optimizing the electrical transport properties, while nanostructure engineering and defect engineering can greatly reduce the thermal conductivity approaching the amorphous limit. Currently, TE devices are utilized to generate power in remote missions, solar–thermal systems, implantable or/wearable devices, the automotive industry, and many other fields; they are also serving as temperature sensors and controllers or even gas sensors. The future tendency is to synergistically optimize and integrate all the effective factors to further improve the TE performance, so that highly efficient TE materials and devices can be more beneficial to daily lives.

Sc solubility in p-type half-Heusler (Ti1-cScc)NiSn thermoelectric alloys

Global climate changes, due to rise of combustion of fossil fuels and greenhouse gas emissions, are becoming a major environmental concern, which can be approached by utilizing of waste heat into usable electrical energy, as in thermoelectric, TE, heat to electricity conversion. Among the recently developed TE materials classes, half-Heusler, HH, alloys gain a lot of attention due to their combined high electronic TE potential with a high temperature stability. So far the most efficient HH compositions are n-type. In the current research, based on ab-initio DFT and statistical thermodynamic calculations as well as an experimental validation, the temperature dependent solubility limit of Sc in HH (Ti1-cScc)NiSn alloys is reported. It is shown that below the solubility limit p-type TiNiSn rich single phases are expected, while beyond the solubility limit a phase separation into ScNiSn and p-type TiNiSn rich phases is favored. This finding is of a high significance upon designing of TE devices with similar n- and p- type legs, as required for practical applications, with an addition of very interesting sub-micron phase-separation features, enabling scattering of phonons and enhancing of the TE performance.

Mott theory predicted thermoelectric properties based on electronic structure of Bi and Sb atoms substituted PbTe material

In this work, thermoelectric properties of Bi and Sb atoms substituted PbTe material were predicted by Mott theory through electronic structure calculation. This calculation has been carried by the first-principles DV-Xα molecular orbital method based on Hartree-Fock-Slater approximation. The Pb14Te13, Pb13SbTe13 and Pb13BiTe13 small clusters with a cubic rocksalt structure (Fm-3m; 225) were designed to be performed PbTe, Pb0.75Sb0.25Te and Pb0.75Bi0.25Te materials, respectively. The electronic structure showed that the high symmetry crystal structure, spin energy levels, partial spin density of states and electron charge density. The energy gap and Fermi level have been obtained from energy levels and density of state to be evaluated of electrical conductivity and Seebeck coefficient within Mott's theory predication.

Skutterudite-Based Thermoelectric Technology for Integration into a Proposed Emmrtg for Space Power Applications

The overall objective of the Skutterudite Technology Maturation (STM) project at NASA’s Jet Propulsion Laboratory (JPL) is to advance JPL-developed skutterudite (SKD) technology to a point where it can be considered for use in a proposed enhanced Multi-Mission Radioisotope Thermoelectric Generator (eMMRTG). The goal is to be prepared for potential flight unit development readiness by end of FY2018. Conversion efficiency values on the order of 9% have been demonstrated for SKD-based un-segmented couples when operating at a hot junction of 600C and a cold junction of 200C. This represents ~ a 25% improvement over the conversion efficiency of PbTe/TAGS MMRTG couples at beginning-of-life (BOL). The STM project entered its third year at the beginning of FY2016. This talk will highlight the material and device challenges associated with the maturation of this technology. During the first two years of the project, JPL and Teledyne Energy Systems Inc. (TESI) have collaborated to complete the initial technical technology transfer to TESI. Manufacturing capabilities for SKD TE materials have been established at TESI. TESI has also established the manufacturing capabilities for couples and module thermal insulation. First iteration SKD couples have been fabricated and tested at TESI. Key findings from this initial Phase of the technology maturation project will be summarized. TESI and JPL are currently developing a second iteration of couples and module thermal insulation, incorporating lessons learned from the development and testing of the first iteration of couples and insulation. Progress to date on the development of this second iteration of couples will be presented and discussed. The eMMRTG life performance prediction approach and development status will also be presented.