A review of machine learning approaches for the discovery of thermoelectric materials

Thermoelectric materials based on porous hybrid and MOFs constitute a very recent development, with many new and undiscovered phenomena for the research field. These porous Hybrid materials and MOF (Metal-Organic-Framework) films represent modern designer materials that exhibit many requirements of a near ideal and tunable future thermoelectric (TE) material. It is anticipated that especially the inherent porosity can be used advantageously in the design and simulation predictions of future thermoelectric materials. In principle, porous Hybrid and MOFs materials can comply with most of the requirements of an ideal and tunable future thermoelectric (TE) material. In contrast to traditional semiconducting bulk thermoelectric materials, porous hybrid MOF templates can be used to overcome some of the constraints of physics in bulk thermoelectric materials. These porous hybrid systems are amenable for simulation and modeling to design novel optimized electron-crystal phonon-glass materials with potentially very high Figure of Merit (ZT) numbers. Porous MOF and Hybrid materials possess an ultra-low thermal conductivity, which can be further modulated by phonon engineering within their complex porous and hierarchical architecture to advance the thermoelectric figure of merit ZT. This class of novel materials offers new approaches to the design of complex and hierarchical porous structures, possessing ultra-low thermal conductivity, which are able to trap, and localize phonons, thus resulting in a new generation of thermoelectric materials with a high ZT number. This work is demonstrating recent results of MOF thermoelectric Materials and provides a future outlook to the search for the next generation thermoelectric porous Hybrid and MOF materials, which could be part of a green renewable energy revolution with novel materials of sustainable high ZT values.

Thermoelectric materials and devices fabricated by additive manufacturing

New and more efficient thermoelectric (TE) materials that make use of nanotechnology have been developed. These new materials, called quantum wells (QW), are composed of alternating layers of 10 nm thick silicon and SiGe films. They can be deposited by various techniques and magnetron sputtering was used to obtain uniform layered structures that exhibited no degradation of the TE properties or microstructure after thermal aging. For QW thin films, the heat and current flow are “in plane” and in this orientation all of the thermoelectric properties (the Seebeck coefficient, electrical resistivty, and the thermal conductivity) are improved to increase the TE Figure of Merit, ZT (see equation 3, p. 4, for the definition of Z). From the most recent QW test data, ZTs greater than 3 at room temperature have been obtained which constitutes a significant improvement over the state-of-the-art (SOTA) bulk thermoelectrics which have ZTs less than 1. QW materials have the best measured TE power factor (Seebeck coefficient squared divided by electrical resistivity) and, combined with low thermal conductivity substrates, should provide very high efficiency TE modules. The QW TE materials with ZTs greater than 3 lead to conversion efficiencies greater than 20 percent, which allows for much wider commercial applications, particularly in the applications such as the waste-heat recovery from truck engines, refrigeration, and air conditioning, where the SOTA bulk TE modules were shown to be technically feasible but economically unjustified due to low conversion efficiencies. With higher efficiency QW materials, these applications become economically attractive. For the refrigeration and air conditioning applications, the QW TE materials are predicted to have higher coefficients of performance (COP) than the SOTA vapor compression systems, with the additional advantages of having no compressors, no moving parts, no refrigerants, no vibrations, no noise, and practically no maintenance. With such significant advantages, it is very important to have independent confirmation of the QW TE properties that lead to such improved performance. Three independent researchers have confirmed the previously measured QW TE properties using conventional test techniques, and a totally new test technique was developed to measure the TE properties and performance and the results provided yet another confirmation of the superior TE performance of the QW materials versus the SOTA bulk thermoelectrics. The temperature range for the applications is anticipated to be as low as −150C to the upper temperature of 1000C, with the power generation capacity ranging from milliwatts to kilowatts and cooling capacity ranging from watts to several tons of refrigeration.

A series of Ba0.4In0.4Co4Sb12/Bi2Te2.7Se0.3 (FS/BT) thermoelectric (TE) materials were fabricated by a two-step spark plasma sintering method. The samples contained various numbers of gradient layers between FS and BT as follows: one gradient layer (1GL) with FS/BT volume ratio of 5:5, three GLs with ratios of 3:7, 5:5, and 7:3 (denoted as 3GLs-I with 3:7–5:5–7:3), 3GLs-II with 7:3–5:5–3:7, 5GLs-I with 3:7–4:6–5:5–6:4–7:3, and 5GLs-II with 2:8–3:7–5:5–7:3–8:2. The interfacial structure and mechanical properties of the FS/BT TE materials were investigated in this work. In FS/BT TE materials with no GLs, a large number of macroscopic cracks occurred on the FS bulk material side. It was discovered that designing and optimizing GLs between the FS and BT bulk materials could effectively relax the thermal stress induced by the large difference in coefficient of thermal expansion, eliminating the macroscopic cracks and resulting in a remarkable enhancement of the interfacial mechanical properties of the FS/BT TE materials. The flexural strength of the FS/BT TE material with 1GL reached 9.67 MPa, increased by 85% compared with that of the FS/BT TE material with no GLs. The present work indicates that increasing the BT content in the GL near to the FS bulk material side is an effective method to completely eliminate macroscopic cracking. The optimized gradient interface of the FS/BT TE material was 3GLs-II with FS/BT volume ratios of 3:7–5:5–7:3. The highest flexural strength reached 12.76 MPa, representing a 144% increase.

Patternable and flexible thermoelectric generators based on Bi2Te3/silk fibroin composites for temperature sensing and wearable applications

This paper describes the design, modeling, initial build and testing of a novel thermoelectric power generator (TPG), incorporating state of the art material technology with optimized thermal management. A numerical model simulates the operation of the device and facilitates its design. Advanced multi-parameter, gradient-based optimization techniques are used to better understand the interactions between various design variables and parameters in order to progress towards an optimal TPG design. The device, made up of a series of segmented elements each comprised of up to three different materials, combines thermal isolation in the direction of flow with high power density thermoelectric (TE) material integrated directly into the heat exchanger device. Electrical current runs parallel to the heat source and sink surfaces in the device, allowing the integration of the TE material with multiple geometric degrees of freedom. This design attribute coupled with the thermal isolation thermodynamic cycle, allows each element of the TE device to be optimized semi-independently. Each p- and n-type element can have different aspect ratios (cross-sectional area divided by thickness) so that each material layer of each element has the highest possible ZT for each temperature range. The increased design flexibility helps address TE material compatibility issues associated with segmented elements and fluid flow that ordinarily degrade performance. Eliminating the impact of thermal expansion mismatch while still maintaining excellent thermal and electrical contacts is also a design goal. Additional design considerations are also discussed, including electrical and thermal connector design and minimizing interfacial resistances. The device described is suitable for both waste heat recovery and primary power applications. Initial test results from prototype builds are discussed

Chapter 8 - New paradigm for efficient thermoelectrics

Dynamic mechanical analysis (DMA) techniques are commonly applied to characterize polymer-based materials - but little if at all to characterize semiconductor thermoelectric (TE) materials. TE materials may be coupled with polymeric materials in advanced thermoelectric devices, and the knowledge of TE material properties will be useful in the choice of materials for future applications. We have obtained DMA results for both n-type and p-type bismuth telluride based TE materials. We find that tan δ values, indicative of viscoelastic energy dissipation modes, approach the values for glassy or semi-crystalline polymers, and are larger by more than a whole order of magnitude than the tan δ of structural metals. DMA thermal scans show clear hysteresis-type effects and a correlation with differential scanning calorimetry thermal transitions. DMA properties as a function of frequency are briefly discussed. Our results show that DMA techniques are useful in the evaluation of thermophysical and thermomechanical properties of these TE materials and of assembled coolers. The viscoelastic effects we report may provide a damping mechanism for severe stresses inherent to service conditions of the TE coolers.

13 - Introduction to thermoelectricity

Electrochemically tuning the thermoelectric performance of flexible polythiophene films by ionic liquid

Thermoelectric materials — materials that exploit a phenomenon in which the application of heat to combination of certain metals induces an electric current- are emerging as potentially important systems for increasing fuel efficiency and decreasing emissions in future automotive vehicles. Thomas seebeck, a German physicist, first noticed the thermoelectric effect in 1821 when he discovered that a voltage was developed in a loop containing two dissimilar metals, provided the two junctions were maintained at different temperature. Some devices have emerged over the years to exploit that property, but have in general been hampered by low efficiency of about 5% with corresponding value of (ZT) equal to approx one (and high cost) Despite the fact that thermoelectric devices are compact, quite, rugged, stable and very reliable; thermo electrics have found only in niche applications and their wide widespread acceptance depends upon discovery of new materials with high conversion efficiency. The issue of thermoelectric efficiency for automotive application is examined from the standpoint of what material figures of merit (FOMs) will enable solid-state devices to compete with conventional electricity generator or vapor compression refrigerators. It is estimated that FOMs of (ZT) equal to or greater than 8 will be required. Some of the emerging new bulk materials may eventually reach that level of performance. How ever layered thin — film structures based on two — dimensional quantum wells and short period super lattices appear to have qualities will enable rapid attainment of FOMs that signal economic competitiveness.

It is of great significance to develop high-performance thermoelectric (TE) materials, because they can be used to harvest waste heat into electricity and there is abundant waste heat on earth. The conventional TE materials are inorganic semimetals or semiconductors like Bi2Te3 and its derivatives. However, they have problems of high cost, scarce/toxic elements, high thermal conductivity, and poor mechanical flexibility. Organic TE materials emerged as the next-generation TE materials because of their merits including solution processability, low cost, abundant element, low intrinsic thermal conductivity, and high mechanical flexibility. Organic TE materials are mainly conducting polymers because of their high conductivity. Both the conductivity and Seebeck coefficient depend on the doping level, and they are interdependent. Hence, the TE properties of polymers can be improved through doping/dedoping engineering. There are three types of doping forms, oxidative (or reductive) doping, protonic acid doping, and charge transfer doping. Accordingly, they can be dedoped by different approaches. In this article, we review the methods to dope and dedope p-type and n-type TE polymers and the combination of doping and dedoping to optimize their TE properties. Secondary doping is also covered, since it can significantly enhance the conductivity of some TE polymers.

An insight into β-Zn4Sb3 from its crystal structure, thermoelectric performance, thermal stability and graded material

Materials with thermoelectric (TE) efficiencies comparable to Carnot efficiency are desirable for applications in devices that use TE effects to produce electricity from heat. The figure of merit of a TE material is a measure of its efficiency. This quantity is determined by the intricate interplay of electronic and thermal transport parameters. Layered materials at low dimensions, such as 2D monolayers, have been in focus with regard to the quest for new TE materials. Group III–VI semiconductors in their 2D monolayers have exhibited fascinating TE properties. Naturally, it is expected that isoelectronic IV–V semiconductors will possess similar novelty. In this work, we have investigated the electronic, dynamical and TE properties of IV–V monolayer SiN, SiP, SiAs, SiSb and SiBi using first-principles electronic structure methods in conjunction with the semiclassical Boltzmann transport theory. Unlike previous studies, here the focus was to compute and analyse the effects of structural arrangements on TE properties. We found that upon consideration of an alternative stacking arrangement in the systems, significant changes occur in the transport phenomena, leading to a large figure of merit and thus TE efficiency. A comprehensive comparative analysis is presented to interpret the results. This work bears significance in the fact that it demonstrates a very important structure–property relationship aspect for these compounds that has been overlooked so far, and this approach may lead to the discovery of new TE materials with desirable efficiencies.

Thermoelectric materials and generators, enabling to convert thermal energy into electrical energy or vice versa, offer great potentials in solving the energy problem from an environmental-sustainable perspective. As one of emerging thermoelectric materials, tin telluride (SnTe) shows uniuqe characterisitcs, such as low-toxicity and eco-friendly behaviour. The recent trend shows that band engineering and nanostructuring can enable to achieve enhanced thermoelectric performance of SnTe in the temperature range from 300 to 800 K, which provides an alternative for toxic PbTe with the same operational temperature. This thesis firstly highlights the key strategies to enhance the thermoelectric performance of SnTe materials through band engineering, carrier concentration optimization, synergistic engineering and structure design. On the basis of the fundamental analysis, the underpinnings for the property improvement are elucidated and can boost the relevant research with a view to work on further performance enhancement of SnTe materials.While most of the reported work on SnTe uses conventional melting method, this thesis governs with the solvothermal synthesis method which has unique advantages over melting method. In solvothermal method, the reactant ions and/or molecules react in solution which can lead to different final structures of products even if the same reactants are used with the melting method. In addition to this, solvothermal method can yield well-controlled nanomaterials with low energy consumption. In theoretical perspective, it is important to see the electronic band structure of doped and undoped SnTe, hence, density functional theory calculations are preformed to see the effect of suitable dopants including In, In/Sr, In/Ag on the band structure and density of states (DOS) of SnTe. The success of the solvothermal method in this thesis with synergistic band engineering and structure engineering is summarized as followsTo improve the thermoelectric performance of pristine SnTe, single In dopant was introduced to modify the crystal structure and band structure of SnTe. In dopant creates InTe nanoprecipiate in the Sn1-xInxTe matrix and the structure of this nanoprecipitates has been clearly identified by extensive transmission electron microscopy analysis. It is found that the structure of InTe (a = 6.14 A) is face-centred cubic which is the similar crystal structure with pristine SnTe (a = 6.32 A). These nanoprecipitates together with the point defects and grain boundaries significantly reduce lattice thermal conductivity to ~0.45 W m-1 K-1. Density functional theory calculation shows that the distortion of DOS (resonance energy level) near the Fermi level leads to enhanced seebeck coefficient (S) from ~23 mV K-1 to ~88 mV K-1. Finally, a high power factor of ~21.8 mW cm-1 K-2 and a corresponding figure of merit, a figure of merit (ZT) of ~ 0.78 have been obtained in Sn0.99In0.01Te at 773 K.The co-dopants of In and Cd with extra Te were used to further improve the thermoelectric performance of pristine SnTe. In and Cd rich nanoprecipitates can be found in the SnTe matrix and can significantly ameliorate the thermal transport properties. Pisarenko plot shows higher S in the In/Cd co-doped SnTe compared with the pristine SnTe, revealing that the significant valence band convergence and the resonant energy effect co-exist in the Sn(CdIn)xTe1+2x. Consequently, a high ZT of ~1.12 is obtained at 773 K in the p-type SnIn0.03Cd0.03Te1.06.A systematic theoretical and extensive experimental analyses have been successfully perceived in the In/Sr co-doped SnTe. The synergistic band and structure engineering significantly improve the electrical and thermal transport properties of Sn1-3xInxSr2xTe. The contribution of different phonon scattering centers such as grain boundaries, point defects and nanoprecipitates on the reduction of the lattice thermal conductivity has been well understood by using phonon modelling calculations. Besides, strain field associated with the In/Sr rich nanoprecipates is also analyzed by geometrical phase analysis (GPA). As a result, a record high power factor of ~33.88 mWcm-1K-2 and a peak ZT of ~1.31 has been achieved at 823 K for the Sn0.925In0.025Sr0.05Te pellet.A solvothermal synthesis method was developed to increase the solubility of In/Ag co-dopant in SnTe. It is has been found that In/Ag co-doping with appropriate dopant ratio (In:Ag = 1:2) can significantly improve the valence band convergence and resonant energy effect by using extensive density functional theory calculations. High-density strain field, dislocations, point defects and grain boundaries are observed in the matrix, which significantly scatter heat carrying phonons and yield low lattice thermal conductivity in a whole temperature range. Consequently, a high peak ZT of ~1.38 at 823 K has been achieved in Sn0.85In0.05Ag0.10Te, outperforming most of SnTe-based materials.In summary, this thesis successfully demonstrates the effectiveness of the facile and reliable solvothermal method for synthesising high-performance SnTe based thermoelectric materials by using synergistic band engineering and structure engineering. The simulation investigations on electronic transport and the phonon modeling fundamentally illustrate the effect of the proposed concepts, which will direct the future development of other thermoelectric system.