Thermoelectric Energy Conversion Research Articles

Development of the anodized aluminum substrates for thermoelectric energy converters

Operation of the thermoelectric converters requires hot- and cold-side insulating plates with low thermal resistance. In this work, we developed anodized aluminum substrates with physical properties accommodated for thermoelectric converters, which can be a possible alternative to the expensive and brittle commercial ceramics. The XRD, SEM, and EDS analyses were employed to investigate the quality of the prepared samples. The thermal resistance of the developed substrates shows much lower values compared with the commercial ceramics indicating the beneficial effect of the anodized aluminum substrates on the heat transfer through the thermoelectric module. The temperature-dependent breakdown voltage of the Al2O3 anodic layer was estimated as a key parameter that defines the thermoelectric module's workability. For the measurement of the thermal breakdown voltage of dielectric layers, the temperature-controlled testing setup was developed. With the help of the Walther-Fock theory and the measured data, the analytical expression that determines the thickness of the Al2O3 dielectric layer as a function of the anodization voltage and temperature is proposed. Considering Seebeck voltage generated by a thermoelectric module, the desired thickness of the Al2O3 layer can be defined to prevent the destruction of the dielectric substrate during the thermoelectric module operation. The application of the presented results can significantly accelerate the development of mechanically strong, low-cost, and highly efficient thermoelectric energy converters.

Thermal design of solar thermoelectric generator with phase change material for timely and efficient power generation

Solar thermoelectric energy-generation technology is being developed to mitigate the limitations of solar cells. Thermal management is essential to creating highly efficient and stable solar thermoelectric generators (STEGs). Phase change materials (PCMs) can be used to improve the performance of STEGs. In this study, we numerically investigate the heat transfer, thermal energy storage, and thermoelectric energy conversion in an STEG with PCMs (STEG-PCM). Based on a parametric study, we present thermal design guidelines for the configurations of an STEG-PCM and the thermal properties of PCMs by considering both the thermoelectric generation performance and thermal stability of the STEG-PCM. We also show that the PCM-impregnated structure is the most crucial component of the STEG-PCM in the thermal design based on a sensitivity analysis. To find the optimal design of a PCM-impregnated structure, topology optimization is performed for the STEG-PCM focused on daytime thermoelectric generation performance. The topology-optimized PCM-impregnated structure shows a thermoelectric generation performance close to that of an STEG-PCM whose PCM-impregnated structure is assumed to be an ideal mixture of PCM and framework material. This study explains how PCMs affect thermal energy transport and conversion in an STEG and sheds light on a highly efficient and stable STEG system.

Approximation of Composition and Temperature Dependent Heat Conductivity and Optimization of Thermoelectric Energy Conversion in Silicon–Germanium Alloys

We analyze the efficiency as thermoelectric energy converter of a silicon–germanium alloy with composition and temperature dependent heat conductivity. The dependency on composition is determined by a non-linear regression method (NLRM), while the dependency on temperature is approximated by a first-order expansion in the neighborhood of three reference temperatures. The differences with respect to the case of thermal conductivity depending on composition only are pointed out. The efficiency of the system is analyzed under the assumption that the optimal energy conversion corresponds to the minimum rate of energy dissipated. The values of composition and temperature which minimize such a rate are calculated as well.

Theoretical Understanding of thermoelectric energy conversion efficiency in Lead-Free halide double perovskites showing intrinsic defect tolerance

Thermoelectric devices based on the Seebeck effect can directly convert waste heat to electricity, but the demand for better thermoelectric materials remains unfulfilled. The lead-free halide double perovskites have recently shown promises for their stability and environment-friendly nature, but impacts from intrinsic defects (vacancies and anti-sites) on their thermoelectric performance remains elusive. Combing first-principle calculations, the Boltzmann transport theory, and the defect formation theory, we investigate the thermoelectric properties of lead-free double perovskite Cs2NaInCl6, taking the intrinsic defects into consideration. Our results demonstrate that the pristine Cs2NaInCl6 presents thermodynamic, mechanical, and dynamic stability. The band edges are mainly comprised of the electrons from In and Cl atoms. Furthermore, the double perovskite has an ultra-low lattice thermal conductivity and a high Seebeck coefficient, while showing a small charge carrier relaxation time and electric conductivity. Promisingly, the maximum ZT values can reach as high as ∼ 1.36 and ∼ 1.44 at 800 K temperature, respectively, with optimal extrinsic carrier concentrations of ∼ 1.9 × 1019 and 5.8 × 1020 cm−3 for N- and P-type carriers (holes and electrons), and the maximum power generation efficiency can reach ∼ 14% (for P-type) when the hot-side temperature is 800 K. Among various intrinsic defect types, we determine the preferable defect types in Cs2NaInCl6 based on the minimal defect formation energy criteria. The free carriers can be switched from N- to P-type under these two preferable defects, respectively, with Cl and Na vacancies. Even under a very high defect concentration of 2.1 × 1019/cm3 considered here, the extra carrier concentrations induced by the defects are only ∼ 1017 cm−3 at 800 K, showing strong defect tolerance in carrier concentration. This work suggests that lead-free halide double perovskites are promising thermoelectric materials, and they show strong intrinsic defect tolerance that prevents a negative impact on the extrinsic doping-controlled carrier concentrations.

Phonon anharmonicity and thermal conductivity of two-dimensional van der Waals materials: A review

Two-dimensional (2D) van der Waals (vdW) materials have extraordinary thermal properties due to the effect of quantum confinement, making them promising for thermoelectric energy conversion and thermal management in microelectronic devices. In this review, the mechanism of phonon anharmonicity originating from three- and four-phonon interactions is derived. The phonon anharmonicity of 2D vdW materials, involving the Gr\"uneisen parameter, phonon lifetime, and thermal conductivity, is summarized and derived in detail. The size-dependent thermal conductivity of representative 2D vdW materials is discussed experimentally and theoretically. This review will present fundamental and advanced knowledge on how to evaluate the phonon anharmonicity in 2D vdW materials, which will aid the design of new structures and materials for applications related to energy transfer and conversion.

Intrinsic ultra-low lattice thermal conductivity in orthorhombic BiSI: An excellent thermoelectric material

Eco-friendly chalcohalide compounds composed by heavy elements are the interesting candidates for thermoelectric energy conversion applications due to their intrinsically low lattice thermal conductivity, abundance and semiconducting behavior. This piqued the present study to investigate BiSI chacohalide for its feasibility in thermoelectric conversion through first principles computations. The deep insights into mechanical and lattice dynamical properties exposes the stability of BiSI. The transport properties of BiSI as a function of carrier concentration builds noticeable power factor of 1.3 mW m−1 K−2 (300 K) and 1.9 mW m−1 K−2 (500 K). The calculated lattice thermal conductivity of BiSI is ∼0.2 W m−1 K−1 at 300 K. This low kL can be resulted from phonon-phonon scattering process promoted by strong coupling between acoustic-optical branches. Moreover, the optimal figure of merit of 1.1–2.5 is observed at 300–500 K for both p- and n-type BiSI. The current study provides evidence for orthorhombic BiSI as a potentially beneficial thermoelectric material for room-to-mid temperature applications and it also accomplishes a gravitation towards pertinent experimental investigations.

EVERAL NOTES ON THE LATTICE THERMALCONDUCTIVITY OF FRACTAL-SHAPED NANOPARTICLES

Using the additive technologies in the production of nanoparticle-fabricated three-dimensional materials has become one of the most promising ways of obtaining effective and low-cost thermoelectric energy converters. Nanostructuring provides a route to modifying selectively the transport properties which determine the materials thermoelectric efficiency. In this paper, we have shown one more effect which consists in a significant dependence of the contribution of lattice vibrations to the thermal conductivity coefficient of a nanoparticle (its reducing is required in practice) on its morphology for nanoparticles of a pure substance. The particle morphology has been specified by the values of its effective diameter, fractal dimension and surface roughness.Using nanoparticles of pure bismuth at low temperatures as an example, we have demonstrated a notable decrease in the lattice thermal conductivity in “complicating” the particle morphology. In the final section, we have presented methods of calculating characteristics of nanoparticle ensembles, the methodology of measuring the fractal dimension experimentally also being discussed.

Galvanomagnetic and Thermoelectric Properties of Bismuth Films Doped with Tin

This paper presents the experimental study of the galvanomagnetic and thermoelectric properties of thin bismuth films doped with tin. The amount of tin is 0.06 at. % with the thickness ranged within 250-800 nm, and it is deposited on mica-muscovite substrates in vacuum up to 1·10-5 mm Hg. The galvanomagnetic and thermoelectric coefficients of all presented films are measured in the temperature range of 77-300 K in a magnetic field of up to 0.65 T. It is found that the classical size effect in the films occurs due to mobility of electrons being restricted by the thickness of the film. A characteristic maximum of temperature dependence of relative transverse magnetoresistance in the temperature range of 150-200 K is observed. A change in the sign of the Seebeck coefficient at the temperature of 175 K is found. It can be explained by the temperature change ratio of the electron and hole components contributions to galvanomagnetic and thermoelectric phenomena, and the contribution of holes at the L, T points of the Brillouin zone. The positive values of differential thermoelectric power in bismuth films doped with tin can become the basis for searching for the possibility of creating a p-branch of thermoelectric energy converters in the low-temperature area. The obtained results of measurements can be used for creation of a low-dimensional bismuth-based structures with a controlled hole concentration.

Energy Harvest in Ferromagnet‐Embedded Surface Acoustic Wave Devices

AbstractSurface acoustic wave (SAW) filters are at the center of the radio frequency module in mobile terminals. With the increase of SAW frequency and the miniaturization of devices, the heat generation becomes prominent in SAW devices, providing a potential platform for thermal energy harvesting. Here ferromagnetic materials are embedded into SAW delay lines to demonstrate that thermal energy can be harvested to generate robust transverse electrical signals via anomalous Nernst effect, which are comparable with the anomalous Hall signals when an electric current is intensively applied. The transverse voltage exhibits nonreciprocal features, both amplitude and polarity are dependent on the propagation direction of SAW in contrast to its longitudinal counterpart, which only changes the amplitude but remains polarity with opposite SAW direction. The finding provides a different platform for thermoelectric energy conversion and opens a window for thermal energy harvesting and acoustic or heat flow sensing in SAW devices.

Giant reduction of thermal conductivity and enhancement of thermoelectric performance in twinning superlattice InAsSb nanowires

Semiconductor nanostructures hold great promise for high-efficiency waste-heat recovery exploiting thermoelectric energy conversion. They could significantly contribute to the implementation of environmentally friendly energy sources and to the realization of self-powered biomedical wearable devices. A crucial thermoelectric material requirement is a reduced thermal conductivity together with good electrical transport properties. In this work we demonstrate a drastic reduction of the thermal conductivity in III-V semiconductor nanowires as a result of the introduction of periodic crystal-lattice twin planes during growth. The electrical and thermal transport of these nanostructures, known as twinning superlattice nanowires, are probed and compared with their twin-free counterparts, showing a one order of magnitude decrease of thermal conductivity while maintaining unaltered electrical-transport properties and Seebeck coefficients. This leads to tenfold enhancement of the thermoelectric figure of merit, ZT. Our study reports for the first time the complete experimental measurement of electrical and thermal properties in twinning superlattice nanowires, demonstrating their emergence as a novel class of nanomaterials of great potential for high-efficiency thermoelectric-energy harvesting.

Energy harvesting from human body heat using highly flexible thermoelectric generator based on Bi2Te3 particles and polymer composite

Energy harvesting technology is in high demand for self-powered devices. Flexible thermoelectric generators (f-TEGs) have attracted considerable attention for application in wearable electronics, which can be applied to healthcare and smart clothing. However, previous f-TEGs still have limitations such lack of novel research focus concerning its wearable applications and complicated and high-cost fabrication procedures. This study reports an f-TEG comprising Bi2Te3-based thermoelectric composite films on a flexible substrate to demonstrate thermoelectric energy conversion from the human body. To verify the thermoelectric energy harvesting efficiency of the energy harvester based on temperature gradients, an f-TEG was developed by placing p-type Bi0.5Sb1.5Te3 (BST) of 133 μWm−1K−2 and n-type Bi2Te2.7Se0.3 (BTS) of 124 μWm−1K−2 thermoelectric films. The fabricated f-TEG harvested an output voltage of 18.13 mV, current signal of 2.74 μA, and output power of ≈ 12.6 nW at ΔT = 25 K. The theoretical thermoelectric potential distribution of the f-TEG was confirmed by finite element analysis using multiphysics simulation. Furthermore, we investigated the biological output performance of the f-TEG on the human body, which showed the output performance of ≈ 0.38 μA and ≈ 2.3 mV. This study presents the possibility of harvesting thermal energy from human body heat, which will be useful for improving wearable device applications.

High performance solid-state thermoelectric energy conversion via inorganic metal halide perovskites under tailored mechanical deformation

High performance solid-state thermoelectric energy conversion via inorganic metal halide perovskites under tailored mechanical deformation

Linear and nonlinear thermoelectric transport in a quantum spin Hall insulators coupled with a nanomagnet

Thermoelectric effects in quantum systems have been focused in recent years. Thermoelectric energy conversion study of systems with edge states, such as quantum Hall insulators and quantum spin Hall insulators, is one of the most important frontier topics in material science and condensed-matter physics. Based on the previous paper (Gresta in Phys Rev Lett 123:186801, 2019), we further investigated the linear and nonlinear thermoelectric transport properties of helical edge states of the quantum spin Hall insulators coupled with double nanomagnet, calculated the Seebeck coefficients S_c and the thermoelectrical figure of merit ZT, discussed the influence of the length of the nanomagnet and the relative tilt angle of component of the magnetization perpendicular on the thermoelectric coefficients (S_c and ZT), and summarized some meaningful conclusions in the linear response regime. In the nonlinear regime, we calculated the equivalent figure of merit ZT_M and the power-generation efficiency eta in different length of the nanomagnet, obtain the temperature difference of achieving optimal thermoelectricity. The results of this paper further confirm that the setup can indeed be used as a device for achieving high performance thermoelectric.

New and Recent Results for Thermoelectric Energy Conversion in Graded Alloys at Nanoscale.

In this article, we review the main features of nonlocal and nonlinear heat transport in nanosystems and analyze some celebrated differential equations which describe this phenomenon. Then, we present a new heat-transport equation arising within the so-called thermomass theory of heat conduction. We illustrate how such a theory can be applied to the analysis of the efficiency of a thermoelectric energy generator constituted by a Silicon–Germanium alloy, as the application and new results for a nanowire of length nm, are presented as well. The thermal conductivity of the nanowire as a function of composition and temperature is determined in light of the experimental data. Additionally, the best-fit curve is obtained. The dependency of the thermoelectric efficiency of the system on both the composition and the difference of temperature applied to its ends is investigated. For the temperatures K, K, and K, we calculate the values of the composition corresponding to the optimal efficiency, as well as the optimal values of the thermal conductivity. Finally, these new results are compared with recent ones obtained for a system of length mm, in order to point out the benefits due to the miniaturization in thermoelectric energy conversion.

Ba1/3CoO2: A Thermoelectric Oxide Showing a Reliable ZT of ∼0.55 at 600 °C in Air.

Thermoelectric energy conversion technology has attracted attention as an energy harvesting technology that converts waste heat into electricity by means of the Seebeck effect. Oxide-based thermoelectric materials that show a high figure of merit are promising because of their good chemical and thermal stability as well as their harmless nature compared to chalcogenide-based state-of-the-art thermoelectric materials. Although several high-ZT thermoelectric oxides (ZT > 1) have been reported thus far, the reliability is low due to a lack of careful observation of their stability at elevated temperatures. Here, we show a reliable high-ZT thermoelectric oxide, Ba1/3CoO2. We fabricated Ba1/3CoO2 epitaxial films by the reactive solid-phase epitaxy method (Na3/4CoO2) followed by ion exchange (Na+ → Ba2+) treatment and performed thermal annealing of the film at high temperatures and structural and electrical measurements. The crystal structure and electrical resistivity of the Ba1/3CoO2 epitaxial films were found to be maintained up to 600 °C. The power factor gradually increased to ∼1.2 mW m–1 K–2 and the thermal conductivity gradually decreased to ∼1.9 W m–1 K–1 with increasing temperature up to 600 °C. Consequently, the ZT reached ∼0.55 at 600 °C in air.

Thermoelectric power generation in the core of a nuclear reactor

Thermoelectric energy converters offer a promising solution to generate electrical power using heat in the nuclear reactor core. Despite significant improvements in thermoelectric efficiency of nanostructured materials, the performance of these advanced materials has yet to be demonstrated in the harsh radiation environment of a reactor core. Herein, we demonstrate a thermoelectric generator (TEG) made from nanostructured bulk half-Heusler (HH) materials generating stable electrical power density > 1140 W/m2 after 30 days in the MIT Nuclear Research Reactor under an unprecedented fast-neutron (>1 MeV) fluence of 1.5 × 1020 n/cm2. Despite an initial degradation due to irradiation damage when operating under relatively low temperatures, our TEG showed a 20-fold increase in power output when operating under high temperature due to in-situ annealing and resulting thermoelectric property recovery. First-principles modeling indicates that a chemically disordered metallic phase was formed under irradiation at lower temperatures, resulting in a drastic degradation in thermoelectric properties, while at sufficiently high temperatures the system returned to the initial chemically ordered HH phase and the thermoelectric properties recovered. Transmission electron microscopy and electron diffraction demonstrated that the chemically disordered phase was formed upon ion irradiation, confirming the prediction from first-principles simulations. The results suggest that with proper control over the TEG operating temperatures, the nanostructured bulk TEGs could produce stable electrical power and operate indefinitely in the core of a nuclear reactor.

Dynamic doping and Cottrell atmosphere optimize the thermoelectric performance of n-type PbTe over a broad temperature interval

High thermoelectric energy conversion efficiency requires a large figure-of-merit, zT, over a broad temperature range. To achieve this, we optimize the carrier concentrations of n-type PbTe from room up to hot-end temperatures by co-doping Bi and Ag. Bi is an efficient n-type dopant in PbTe, often leading to excessive carrier concentration at room temperature. As revealed by density functional theory calculations, the formation of Bi and Ag defect complexes is exploited to optimize the room temperature carrier concentration. At elevated temperatures, we demonstrate the dynamic dissolution of Ag2Te precipitates in PbTe in situ by heating in a scanning transmission electron microscope. The release of n-type Ag interstitials with increasing temperature fulfills the requirement of higher carrier concentrations at the hot end. Moreover, as characterized by atom probe tomography, Ag atoms aggregate along parallel dislocation arrays to form Cottrell atmospheres. This results in enhanced phonon scattering and leads to a low lattice thermal conductivity. As a result of the synergy of dynamic doping and phonon scattering at decorated dislocations, an average zT of 1.0 is achieved in n-type Bi/Ag-codoped PbTe between 400 and 825 K. Introducing dopants with temperature-dependent solubility and strong interaction with dislocation cores enables simultaneous optimization of the average power factor and thermal conductivity, providing a new concept to exploit in the field of thermoelectrics.

Bacterial cellulose-based dual chemical reaction coupled hydrogel thermocells for efficient heat harvesting

Reasonable and efficient utilization of low-grade thermal energy in nature is the choice for sustainable energy development. We demonstrate a bacterial cellulose (BC) hydrogel thermocell (TEC) based on BC electrolyte combined with carbon fiber paper and copper composite electrode sheets. The large specific surface area of carbon fiber paper provides a large number of active sites for thermoelectric ions, which drives the redox reaction inside the electrolyte and stimulates the chemical reaction between the electrolyte and the electrode. The combination of the two chemical reactions significantly improves the thermoelectric performance of the thermocell. The thermopower of the BC-TEC reaches 5.9 mV·K−1 at a temperature difference of 50 K. The TEC consisting of 6-units in series produces an open-circuit voltage of about 2 V and a peak power of 535 μW. The TEC shows new potential and prospects in ambient thermoelectric energy conversion by rationally designing the power generation principle.

Recent Advances in 2D Material/Conducting Polymer Composites for Thermoelectric Energy Conversion

AbstractTwo‐dimensional material (2DM)/conducting polymer (CP) composites with excellent electrical transport behavior, functional multidimensional structure, and good flexibility have attracted widespread attention as potential composite thermoelectric (TE) materials in the field of energy conversion. Appropriate preparation strategies determine the interface microstructure of 2DM/CP composites, which would influence the optimization degree of TE performance and practical applications. This review describes the recent progress in preparing advanced 2DM/CP composites by different design strategies for improving TE performance. In addition, some typical 2DM/CP composites applied to TE generators are also presented. Finally, other challenges and opportunities toward high‐performance 2DM/CP composites are discussed, including precise structural regulation, an in‐depth understanding of the interfacial transport mechanism, and the development of practical 2DM/CP‐based flexible TE devices.

Insights into Low Thermal Conductivity in Inorganic Materials for Thermoelectrics.

Efficient manipulation of thermal conductivity and fundamental understanding of the microscopic mechanisms of phonon scattering in crystalline solids are crucial to achieve high thermoelectric performance. Thermoelectric energy conversion directly and reversibly converts between heat and electricity and is a promising renewable technology to generate electricity by recovering waste heat and improve solid-state refrigeration. However, a unique challenge in thermal transport needs to be addressed to achieve high thermoelectric performance: the requirement of crystalline materials with ultralow lattice thermal conductivity (κL). A plethora of strategies have been developed to lower κL in crystalline solids by means of nanostructural modifications, introduction of intrinsic or extrinsic phonon scattering centers with tailored shape and dimension, and manipulation of defects and disorder. Recently, intrinsic local lattice distortion and lattice anharmonicity originating from various mechanisms such as rattling, bonding heterogeneity, and ferroelectric instability have found popularity. In this Perspective, we outline the role of manipulation of chemical bonding and structural chemistry on thermal transport in various high-performance thermoelectric materials. We first briefly outline the fundamental aspects of κL and discuss the current status of the popular phonon scattering mechanisms in brief. Then we discuss emerging new ideas with examples of crystal structure and lattice dynamics in exemplary materials. Finally, we present an outlook for focus areas of experimental and theoretical challenges, possible new directions, and integrations of novel techniques to achieve low κL in order to realize high-performance thermoelectric materials.