Solid-state Thermoelectric Devices Research Articles

Structural, electronic and thermoelectric properties of PbS, PbSe and their ternary alloy PbSe1−xSx

Solid-state thermoelectric devices offer the possibility of exploiting waste heat from engines and power plants and converting it into electrical energy. One of the greatest challenges in the development of thermoelectric material systems is to find new thermoelectric materials with high thermoelectric figures of merit ZT. In this work, the structural, electronic and thermoelectric properties of PbSe[Formula: see text]S[Formula: see text] ([Formula: see text], 0.25, 0.50, 0.75 and 1) semiconductors are investigated by applying density functional theory in a full potential linearized augmented plane wave method (FP-LAPW). Calculations of structural properties were completed using the generalized gradient approximation GGA of Perdew Burke and Ernzerhof PBE to get reliable lattice constant results with experimental values. The obtained electronic results show that the PbSe[Formula: see text]S[Formula: see text] material is a narrow band gap semiconductor. In addition, the thermoelectric properties are studied on the basis of the fully iterative solution of the Boltzmann transport equation. PbSe[Formula: see text]S[Formula: see text] had a high figure of merit indicating that our materials are promising candidates in thermoelectric applications.

First principles study on electronic, magnetic and optical properties of Gd2NiMnO6

The double perovskite oxide materials have various applications in light absorber, solid oxide fuel cells, electrode materials for electrochemical energy devices, solid state thermoelectric devices, etc. The calculations of electronic, optical and magnetic properties of Gd2NiMnO6 (GNMO) double perovskite are aimed in the present work. GNMO crystallizes in monoclinic structure with P21/n (14) space group. Implementing first principles density functional theory, we have reported the spin-polarised electronic band structure, density of states (DOS), optical absorption property and magnetic moment of GNMO double perovskite. Furthermore, the effect of on-site d-d Coulomb interaction energy (Ueff) on the electronic and optical properties were investigated by applying a range of Hubbard parameters from 0 to 6 eV on Ni-3d and Mn-3d orbitals within the local spin density approximation (LSDA) and generalised gradient approximation (GGA). Interestingly, on applying Ueff in this range, band gap of GNMO enhanced progressively in both majority and minority spins and also for Ueff = 4 and 6 eV implemented on Ni-3d and Mn-3d orbits, respectively, we observe the band gap value 1.16 and 2.12 eV for the spin-up and spin-down states which is in good agreement with the previously reported experimental band gap value (1.5 eV). In this regard, the electronic structure and light absorption of GNMO have been analysed for Ueff = 4 and 6 eV implemented on Ni-3d and Mn-3d orbits, respectively. The calculations revealed suitable narrow band gap and large visible light absorption coefficient in GNMO, which are extremely desired for the high photovoltaic performance.

Thermoelectric cooling materials.

Solid-state thermoelectric devices can directly convert electricity into cooling or enable heat pumping through the Peltier effect. The commercialization of thermoelectric cooling technology has been built on the Bi2Te3 alloys, which have had no rival for the past six decades around room temperature. With the discovery and development of more promising materials, it is possible to reshape thermoelectric cooling technology. Here we review the current status of, and future outlook for, thermoelectric cooling materials.

In Situ Synthesis of Conducting Polymers: A Novel Approach toward Polymer Thermoelectrics

The efficient conversion of thermal energy into electricity by means of durable and scalable solid-state thermoelectric devices has been a well stablished aim. Electrically conducting polymers have...

Will organic thermoelectrics get hot?

The generally low energy density from most heat sources—the Sun, Earth as well as most human activities—implies that solid-state thermoelectric devices are the most versatile heat harvesters since, unlike steam engines, they can be used on a small scale and at small temperature differences. In this opinion piece, we first discuss the materials requirements for the widespread use of thermoelectrics. We argue that carbon-based materials, such as conducting polymers and carbon nanotubes, are particularly suited for large area and low-temperature operation applications, as they are abundant, low-toxicity and easy to process. We combine experimentally observed macro-trends and basic thermoelectric relations to evaluate the major performance limitations of this technology thus far and propose a number of avenues to take the thermoelectric efficiency of organic materials beyond the state of the art. First, we emphasize how charge carrier mobility, rather than charge density, is currently limiting performance, and discuss how to improve mobility by exploiting anisotropy, high persistence length materials and composites with long and well-dispersed carbon nanotubes. We also show that reducing thermal conductivity could double efficiency while reducing doping requirements. Finally, we discuss several ways in which composites could further boost performance, introducing the concept of interface engineering to produce phonon stack-electron tunnel composites.This article is part of a discussion meeting issue ‘Energy materials for a low carbon future'.

Achieving High Thermoelectric Figure of Merit in Polycrystalline SnSe via Introducing Sn Vacancies.

Thermoelectric power generation technology has emerged as a clean "heat engine" that can convert heat to electricity. Recently, the discovery of an ultrahigh thermoelectric figure of merit in SnSe crystals has drawn a great deal of attention. In view of their facile processing and scale-up applications, polycrystalline SnSe materials with ZT values comparable to those of the SnSe crystals are greatly desired. Here we achieve a record high ZT value ∼2.1 at 873 K in polycrystalline Sn1-xSe with Sn vacancies. We demonstrate that the carrier concentration increases by artificially introducing Sn vacancies, contributing significantly to the enhancements of electrical conductivity and thermoelectric power factor. The detailed analysis of the data in the light of first-principles calculations results indicates that the increased carrier concentration can be attributed to the Sn-vacancy-induced Fermi level downshift and the interplay between the vacancy states and valence bands. Furthermore, vacancies break translation symmetry and thus enhance phonon scattering, leading to extralow thermal conductivity. Such high ZT value ∼2.1 is achieved by synergistically optimizing both electrical- and thermal-transport properties of polycrystalline SnSe. The vast increase in ZT for polycrystalline SnSe may accelerate practical applications of this material in highly effective solid-state thermoelectric devices.

(Invited) Nanostructures for Reduced Lattice Thermal Conductivity — Case Studies for Nanopores and Grain Boundaries

Solid-state thermoelectric devices have the ability to directly convert heat into electricity for power generation. A good TE material should possess a high electrical conductivity, a high Seebeck coefficient, and a low thermal conductivity. This requirement is hard to be satisfied within the same material. To address this issue, the nanostructuring approach has been widely used to reduce the lattice part of the thermal conductivity (kL ) but still maintain the bulk electrical properties. High TE performance has thus been achieved in various nanostructured materials, such as nanoporous thin films and different nanostructured bulk materials. To better understand the observed kL reduction by nanostructures, two major types of nanostructured materials are studies here, including nanoporous thin films and nanograined bulk materials. In the latter case, a super-flexible thin film was hot pressed onto a wafer to represent a grain boundary.

Thermally Chargeable All-Solid-State Flexible Supercapacitor for Waste Heat Harvesting and Storage

Low-grade thermal energy is an abundantly available, sustainable and clean energy source. Its source includes waste heat from power plants, ocean thermal energy, geo-thermal energy, distributed solar energy among others. Conventional solid-state thermoelectric devices are limited in application due to their high cost, intrinsically low Seebeck coefficient (10-100 μV/K) and inflexible nature. Thermogalvanic systems, also known as thermocells have recently been found to be a promising alternative. These devices using the temperature dependence of electrochemical potential of redox electrolytes such as aqueous ferri/ferrocyanide possess the advantages of simple design, direct thermal-to-electric energy conversion, continuous operation, low mainte­nance, zero carbon emissions and low cost. However, these devices need to be connected to energy storage devices (battery or capacitor) by a power management circuit in order to store energy. This leads to complexity in system design and operation. Here, we demonstrate an integrated thermal energy harvesting supercapacitor device that can convert a temperature gradient applied over a short period of time, into stored charge. The stored electrical energy can be used to power an external load long after the temperature gradient is disengaged. Furthermore, it is flexible, thin, lightweight and all-solid-state which makes it ideal for powering portable and wearable devices. Electrodes consisting of graphene petals (GP) grown on conductive carbon cloth by microwave plasma chemical vapor deposition (MPCVD) are sandwiched onto a polyvinyl alcohol (PVA)/potassium ferrocyanide/ferricyanide gel electrolyte film to build a supercapacitor device. The device exhibits a Seebeck coefficient of -1.21 mV/K, a high areal capacitance near 9 F cm-2 at a current density of 0.1 mA cm-2 and excellent charge retention capability.

Realizing High Figure of Merit in Phase-Separated Polycrystalline Sn1–xPbxSe

Solid-state thermoelectric technology, interconverting heat to electrical energy, offers a promising solution for relaxing global energy problems. A high dimensionless figure of merit ZT is desirable for high-efficiency thermoelectric power generation. To date, thermoelectric materials research has focused on increasing the material's ZT. Here we first fabricated phase-separated Sn1-xPbxSe materials by hydrothermal synthesis. We demonstrate that the simultaneous optimization of the power factor and significant reduction in thermal conductivity can be achieved in the phase-separated Sn1-xPbxSe material. The introduction of the PbSe phase contributes to improvement of the electrical conductivity and power factor of the SnSe phase. Meanwhile, nanoscale precipitates and mesoscale grains define all-scale hierarchical architectures to scattering phonons, leading to low lattice thermal conductivity. These two favorable factors lead to remarkably high thermoelectric performance with ZT ∼ 1.7 at 873 K in polycrystalline SnSe + 1% PbSe along the pressing direction, which is a record-high ZT for SnSe polycrystals. These findings highlight the prospects of realizing highly effective solid-state thermoelectric devices.

Long delay time study of thermal transport and thermal stress in thin Pt film-glass substrate system by time-domain thermoreflectance measurements

Solid state thermoelectric devices are desirable for various sustainable applications. Most of these thin film devices are multilayered structures which requires thorough understanding of thermal transport and thermal stress in order to solve thermal issues. In this paper the experimental technology as well as theoretical model of long delay time pump-probe study with four different illumination configurations have been developed to comprehensively characterize the thermal transport and thermal stress of thin film, substrate and the interface. Furthermore, 94-nm-thick Pt film-glass substrate system has been studied by applying time-domain thermoreflectance measurements under four illumination configurations with high signal quality. The obtained time-dependent temperature signal is superposed by the effects of multilayered thermal transport and thermal stress spontaneously, and the corresponding theoretical predictions match well with the experimental data in the whole delay time range. The determined thermal conductivity of the Pt film shows significant size effect.

Development of a Circular Thermoelectric Skutterudite Couple Using Compression Technology

Approximately, 55% of the energy produced from conventional vehicle resources is lost due to heat losses. An efficient waste heat recovery process will lead to improved fuel efficiency and greenhouse gas emissions. Thermoelectric generators (TEGs) are heat recovery devices that are being widely studied by a range of energy-intensive industries. Efficient solid-state thermoelectric devices are good candidates to reduce fuel consumption in an automobile. Thermoelectric materials have had limited automotive applications due to the automotive waste heat recovery temperature range, the rarity and toxicity of some materials, and the limited ability to mass manufacture thermoelectric devices from expensive TE materials. However, skutterudite is one class of material that has demonstrated significant promise in the transportation waste heat recovery temperature domain. Durability and reliability of the TEGs are the most significant concerns in the product development process. Cracking of the materials at hot-side interface is found to be a major failure mechanism of TEGs under thermal loading. Cracking affects not only the structural integrity but also the energy conversion and overall performance of the system. In this paper, cracking of thermoelectric material as observed in performance testing is analyzed using numerical simulations and analytic experiments. This paper shows, with the help of finite element analysis (FEA), the detailed distribution of stress, strain, and temperature is obtained for each design. Finite element (FE)-based simulations show the tensile stresses as the primary factor causing radial and circumferential cracks in the skutterudite. For a TEG design, loading conditions and closed-form analytical solutions of stress/strain distributions are derived. Scenarios with minimum tensile stresses are sought. These approaches yield the minimum of stress/strain fields which produce cracks. Finally, based on these analyses and computational fluid dynamics (CFD) studies, strategies in tensile stress reduction and failure prevention are proposed.

High-performance thermoelectric nanocomposites from nanocrystal building blocks.

The efficient conversion between thermal and electrical energy by means of durable, silent and scalable solid-state thermoelectric devices has been a long standing goal. While nanocrystalline materials have already led to substantially higher thermoelectric efficiencies, further improvements are expected to arise from precise chemical engineering of nanoscale building blocks and interfaces. Here we present a simple and versatile bottom–up strategy based on the assembly of colloidal nanocrystals to produce consolidated yet nanostructured thermoelectric materials. In the case study on the PbS–Ag system, Ag nanodomains not only contribute to block phonon propagation, but also provide electrons to the PbS host semiconductor and reduce the PbS intergrain energy barriers for charge transport. Thus, PbS–Ag nanocomposites exhibit reduced thermal conductivities and higher charge carrier concentrations and mobilities than PbS nanomaterial. Such improvements of the material transport properties provide thermoelectric figures of merit up to 1.7 at 850 K.

Thermogalvanic Waste Heat Recovery System in Automobiles

Waste heat recovery remains an inviting subject for research. Solid-state thermoelectric devices have been widely investigated, but their practical application remains challenging due to cost and inability to fabricate them in geometries that are easily compatible with heat sources. An alternative to such devices is thermogalvanic cells. Temperature difference between hot and cold electrodes creates a potential difference. Once they connect to a load, electric current and power are delivered, thus converting heat into electricity. Recent work in our laboratory has showed that an annular copper-copper sulfate thermogalvanic cell system, which was affixed conformingly onto a hot pipe to simulate car’s exhaust pipe, produced electricity in the order of tens of milliwatt per square meter. Therefore, the current study is essentially the continuation of that proof-of-concept work. We redesign and build new cells with increased electrodes’ surface area to generate more power. We also incorporate a flowing cell design to capture more waste heat from a car’s radiator. This study is a largely experimental approach to improve the power generation of the system, with the goal being to make possible future practical energy conversion devices for automotive industries. Figure 1

Theory of electrocaloric effect in a shape-changing container: gas in a nanotube.

Driven by applied voltage or Ohmic heating, bistable nanotubes filled with gas can transform between expanded and collapsed configurations and by doing so convert energy between mechanical, electrical, and thermal forms. The electrocaloric response, a reversible change of temperature in response to applied voltage, combines the advantages of a working fluid with the lack of internal interfaces characteristic of robust solid-state thermoelectric devices. Such devices could be constructed from any conductive two-dimensional atomically thin material wrapped into an appropriate geometry.

The 2014 Joseph W. Richards Summer Research Fellowship -- Summary Report: Thermogalvanic Waste Heat Recovery in Transportation Energy Systems

Waste heat recovery remains an inviting subject for research. Solid state thermoelectric devices have been widely investigated for this purpose, but their practical application remains challenging due to high cost and the inability to fabricate them in geometries that are easily compatible with heat sources. An alternative to solid-state thermoelectric devices are thermogalvanic cells.1-7 The temperature difference between the hot and the cold electrodes creates a difference in electrochemical potential of the redox couples at the electrodes. Once connected to a load, electrical current and power is delivered, converting thermal energy into electrical energy. The aim of this summer project is to extend ongoing research to study the feasibility of incorporating thermogalvanic systems into automobiles. A climate-controlled wind tunnel (Fig. 1) was built to provide equivalent conditions to the ambient air stream under the car. Temperature was controlled using a window air conditioner, while an air mover drove the flow up to 6 m s−1. A heat gun provided the equivalent of a low-temperature exhaust gas stream (~110 oC). The annular cell was bound by concentric copper pipes (electrodes) and CPVC bulkheads; all sealed with silicone. K-type thermocouples were attached with epoxy onto electrode surfaces to measure the electrode temperature, which were monitored and recorded by a Campbell Scientific CR23X Micrologger. The cell potential (E) was probed using a Fluke 8846A Digital Multimeter. An Elenco RS-500 resistor box (Rext) was connected in parallel to measure power output, P = E/Rext. A 0.7 M CuSO4 aqueous electrolyte was used, with 0.1 M H2SO4 as the supporting electrolyte. The Tcold was varied based on the quad-monthly average ambient air temperature (Tambient) in Phoenix, AZ of 31.6, 22.5, and 14.1 oC. Because thermal resistance from the hot air stream to the inner copper pipe is large relative to the thermal resistance of the cell, the Thot was measured to be less than the hot air stream’s temperature. Reducing this resistance would greatly improve the system performance. The experimental values of Thot and Tcold are shown next to their resultant plots in Fig. 2. These results showed that higher Tambient yielded a higher P, because of the higher average cell temperature, Tavg = (Thot+ Tcold)/2. This trend agreed with our previous study.8 Since the resistor could only be Fig 1. 3D CAD drawings of the wind tunnel and the cross-sectional diagram of the annular Cu/Cu2+ thermogalvanic cell.

Heat Transfer in Thermoelectric Materials and Devices

Solid-state thermoelectric devices are currently used in applications ranging from thermocouple sensors to power generators in space missions, to portable air-conditioners and refrigerators. With the ever-rising demand throughout the world for energy consumption and CO2 reduction, thermoelectric energy conversion has been receiving intensified attention as a potential candidate for waste-heat harvesting as well as for power generation from renewable sources. Efficient thermoelectric energy conversion critically depends on the performance of thermoelectric materials and devices. In this review, we discuss heat transfer in thermoelectric materials and devices, especially phonon engineering to reduce the lattice thermal conductivity of thermoelectric materials, which requires a fundamental understanding of nanoscale heat conduction physics.

Thermoelectric power generation in a thermoacoustic refrigerator

Commercially available solid-state thermoelectric devices may be used for their electrical power generation capabilities when coupled to a thermoacoustic refrigerator or heat pump. General performance characteristics as well as bulk thermal conductivity for a selection of thermoelectric elements was first found by using a two-plate apparatus to maintain a constant temperature difference across the element. Further studies of an element’s performance when placed in series with the thermoacoustic refrigerator’s heat exchangers will be presented. Design considerations for using thermoelectric elements in a no-moving parts electrical power generation scheme will be discussed.

SIZE LIMITS FOR REGENERATIVE HEAT ENGINES

This article explores the lower size limit placed on regenerative heat engines by thermodynamics and heat transfer. Information derived in this work has direct relevance to the development of mesoscopic heat engines that are based on standard gas cycles employing regeneration. A model is developed for the Stirling cycle that incorporates a regenerator effectiveness term and an axial conduction term, both of which are dependent on the length scale of the device. The thermal efficiency for the engine is determined in terms of the cycle temperature ratio, the expansion ratio, regenerator effectiveness, and a nondimensional term called the conduction parameter. Results from this study show that a small-scale heat engine fabricated from a low-thermal-conductivity material can be made with a length scale approaching 1 mm. Such a device would undoubtedly be composed of numerous microscale components. Below the 1-mm limit, efficiency suffers to such a degree that solid-state thermoelectric devices would become a better choice for a particular application.

A Novel Synthesis of Polycrystalline Bismuth Telluride

With the planned phase-out of chlorofluorocarbon (CFC) refrigerants within this decade, considerable interest has developed in altemative refrigeration and cooling technologies. It is possible that alternatives to CFC liquid-gas expansion systems such as solid-state thermoelectric devices could easily develop into a substantial future technology. Thermoelectric cooling devices based upon the Peltier effect have been used for many years in specialized applications. At present, these units do not achieve the performance currently available with CFCs. Typically, CFC systems operate near 40% of Carnot efficiency while the best thermoelectric systems reach only about 10% of Carnot efficiency.' Nonetheless, a significant thermoelectric cooling industry based on the Peltier effect has already developed. Certain semiconductor materials, particularly bismuth telluride-based alloys, are the materials of choice in modem thermoelectric coolers. These alloys are commonly made through metallurgical melt processing,2 Le., by comelting appropriate amounts of the pure elements in sealed vessels at temperatures above 600 C, mixing, and then subjecting the melts to controlled cooling. This batch-processing approach is both equipment and labor intensive, while thermoelectric elements cut from the solidified alloys tend to be somewhat fragile. An altemative approach, amenable to automated production, is the fabrication of thermoelectric elements from polycrystalline powder^.^^^ These powders are commonly obtained by crushing the solidified melts and sieving the resultant particulate material. Polycrystalline thermoelectric elements offer improved structural integrity, although they generally exhibit some degradation in thermoelectric performance due to the anisotropic nature of the matieral. As a step toward the development of an altemative route for the preparation of polycrystalline, Bi/Te-based thermoelectric materials, this publication describes a novel synthesis of the parent compound, bismuth telluride, BiaTe3. This method provides a simple, two-step process for the preparation of fineparticle bismuth telluride. The process features the roomtemperature coprecipitation of a bismuth telluride precursor in aqueous media, followed by its conversion to Bi2Te3 through hydrogen reduction. For scale-up to production levels, the coprecipitation reaction lends itself to continuous powder synthesis through the use of a chemical flow r e a ~ t o r . ~ A very recent publication by Groshens et al. describes another approach to the synthesis of Bi2Te3 and related materials by means of elimination reactions conducted in hexane at -30 OC.6