Conventional Thermoelectric Research Articles

Topological insulator nanoribbons – A new paradigm for high thermoelectric performance

Thermoelectric (TE) materials, with the ability to convert heat directly to electricity, have attracted worldwide increasing attention due to the strong urge for clean energy today. However conventional TE devices suffer from the low conversion efficiency for decades due to the trade-off between TE parameters. Novel quantum materials such as topological insulators with unique boundary states protected topologically against backscattering offer a new way for designing high-performance TE devices. Here we demonstrate that topological insulator nanoribbons, which exhibit unconventional TE behaviors, can greatly improve the TE efficiency. As an example, we calculate the TE properties of the ZrTe5 nanoribbon through the Boltzmann transport theory with the electronic bands and the thermal conductivity obtained from ab initio calculations. The dramatic difference in the scattering intensity between the in-gap topological edge states and the bulk states originates several unusual TE behaviors: (I) The electrical to electronic thermal conductivity ratio can be several times larger than that predicted by the Wiedemann-Franz law. (II) The reduced Seebeck coefficient shows an anomalous opposite sign to that of normal semiconductors with the magnitude much larger than unity. (III) By properly introducing defects, the thermal conductivity can be significantly reduced without noticeably deteriorating the electrical conductivity. (IV) Under appropriate strain, the electrical to electronic thermal conductivity ratio could be tens of times larger due to the bulk gap narrowing. These results indicate that a figure of merit larger than ten is highly likely.

High-Performance Monolithic Photovoltaic–Thermoelectric Hybrid Power Generator Using an Exothermic Reactive Interlayer

A high-performance monolithic photovoltaic–thermoelectric hybrid generator with a photothermal conversion layer is introduced. The layer, a thin acrylic film dyed with black colored AZO dye, effectively absorbs photons in the near-infrared region. The AZO dye supports an exothermic reaction through nonradiative relaxation. The hybrid generator was fabricated through monolithic integration. A thermoelectric (TE) module was fabricated directly on the back surface of the photovoltaic (PV) module without using the ceramic substrate typically employed in conventional TE modules, so that thermal resistance between the PV module and TE module could be minimized. The 51.35 cm2 fabricated hybrid generator showed a power conversion efficiency of about 22.5% with an open-circuit voltage of 0.94 V and a maximum power of 1.15 W under standard AM 1.5G illumination. The hybrid power generator with the photothermal conversion layer showed about 40% higher output power compared to the control PV only, about 16% higher tha...

Thermoelectric Properties of PEDOT:PSS

AbstractNowadays, organic thermoelectric (TE) materials have attracted considerable attention due to their unique merits, e.g., light‐weight, high mechanical flexibility, nontoxicity, easy availability, and intrinsically low thermal conductivity. Among the organic/polymer TE materials reported so far, poly(3,4‐ethylenedioxythiophene):poly(styrenensulfonate) (PEDOT:PSS) is extensively investigated because it is water‐processable, thermally stable, and can be highly conductive. Over the past few years, the TE properties of the PEDOT‐based TE materials are continuously improved. With rational design, some PEDOT:PSS‐based materials have achieved high ZT values comparable to the conventional inorganic TE materials like bismuth telluride at room temperature. This paper reviews the recent breakthroughs for PEDOT:PSS‐based TE polymers and composites. The strategies for achieving high‐performance PEDOT:PSS‐based TE materials and the corresponding underlying mechanism are specifically discussed. The TE devices fabricated by the PEDOT:PSS‐based TE materials are also presented, in terms of their fabrication/assembly technique, device configuration and device performance. With all the exciting progress made in the PEDOT:PSS‐based TE materials, the further development and practical applications of the high‐efficient organic TE materials as flexible TE module devices and wearable electronics can be greatly anticipated.

High thermoelectric power factor of ytterbium silicon-germanium

Metal silicide-based thermoelectric (TE) materials have attracted attention owing to low toxicity and high chemical stability. Here, we demonstrate that ytterbium silicon-germanium, Yb(Si1−xGex)2−δ, shows a large Seebeck coefficient (S) accompanied by metal-like high electrical conductivity (σ) attributed to the intermediate valence behavior of Yb (Yb2+/Yb3+). We revealed that x = 0.5, i.e., YbSiGe, is the best composition with the highest power factor (S2σ) of 3.6 mW m−1 K−2 at room temperature, which is comparable to those of conventional TE materials, such as Bi2Te3.

Modeling, Simulation, Fabrication, and Characterization of a 10-<inline-formula> <tex-math notation="LaTeX">$\mu$ </tex-math> </inline-formula>W/cm2 Class Si-Nanowire Thermoelectric Generator for IoT Applications

We propose a planar device architecture compatible with the CMOS process technology as the optimal current benchmark of a Si-nanowire (NW) thermoelectric (TE) power generator. The proposed device is driven by a temperature gradient that is formed in the proximity of a perpendicular heat flow to the substrate. Therefore, unlike the conventional TE generators, the planar short Si-NWs need not be suspended on a cavity structure. Under an externally applied temperature difference of 5 K, the recorded TE power density is observed to be $12~\mu \text{W}$ /cm2 by shortening the Si-NWs length and suppressing the parasitic thermal resistance of the Si substrate. The demonstration paves a pathway to develop cost-effective autonomous internet-of-things applications that utilize the environmental and body heats.

Temperature dependent thermoelectric properties of cuprous delafossite oxides

The use of nanostructured delafossite oxides in thermoelectric (TE) applications has attracted a great interest due to their high performance and long-term stability at elevated temperatures. Cuprous delafossites, CuMO2 (M = Al, Cr, Fe, Ga, Mn), compared to conventional TE materials, such as Bi2Te3, PbTe and SiGe, are non-toxic and more earth abundant. In particular, CuAlO2 compound shows a great potential for high performance thermoelectric materials.In this work, a systematic study of temperature dependent TE properties of cuprous delafossite materials, CuAlO2, is reported. The optimization of the TE properties has been realized by controlling nanostructure size around 80 nm CuAlO2 powder was prepared using a solid-state synthesis method at ∼1373 K in nitrogen/air atmosphere. The nanostructure size was controlled by a high energy ball milling process. Reducing the particle size of nanostructured bulk materials decouples interdependent electron and phonon transport and results in a lattice thermal conductivity decrease without deteriorating electrical conductivity. The high effective mass plays a dominant role in the high Seebeck coefficient and low electrical conductivity. The power factor reached ∼0.78 × 10−5 W/mK2 at 780 K. Temperature dependent TE properties, including Seebeck coefficient, electrical conductivity, and thermal conductivity are analyzed. The processing-structure-property correlation of these materials are discussed.

(Invited) Ionic Thermoelectric Effects and Thermally Chargeable Solid-State Supercapacitors

Thermally-induced ion transport has been recently reported to exhibit extremely large thermally induced voltage in contrast to conventional thermoelectrics mainly utilizing thermally-driven electron transport. Here comparative experimental studies about the thermoelectric behaviors of different types of solid-state ionic conductors have been carried out. Their thermopower values have been found to be on the order of 1~10 mV/K. Based on the transport studies, novel methods for generating a large voltage from a temperature gradient and simultaneously storing electrical energy have been developed without losing the benefit of solid-state no-moving part devices like conventional thermoelectrics. Arrays of thermally chargeable supercapacitors have been batch-fabricated, and connected together to linearly raise the output voltage up to 2.1 V, suggesting excellent suitability for roll-to-roll mass manufacturing and practical implementation of thermal energy harvesting. The outcomes suggest that it may be feasible to eliminate needs for battery replacement and wired connections to power lines for various distributed electronic systems whenever and wherever a temperature gradient is present.

Advances in Thermoelectric Mg3Sb2 and Its Derivatives

AbstractOver the past couple of decades, thermoelectric Mg3Sb2 and its derivatives have attracted increasing attention for thermoelectric applications. This is enabled by the richness in composition for manipulating both electronic and thermal properties and by the intrinsic low lattice thermal conductivity. With existing efforts on these materials, the thermoelectric figure of merit has been significantly improved to compete with conventional thermoelectrics, while many of these materials keep the compositions cheap and less‐toxic elements only. Here, not only the control of defects, band structure, electronic transport properties, and lattice thermal conductivity for these materials, but also the proven strategies on transport property manipulation are summarized. These strategies are well demonstrated for advancing thermoelectric Mg3Sb2 and its derivatives, and the principles used are believed to be equally applicable for many other thermoelectric materials. In addition, perspectives for possible further advancements in this class of thermoelectric materials are shown.

Flexible thermoelectric power generation system based on rigid inorganic bulk materials

Herein, we demonstrate that conventional inorganic materials can be used in wearable systems despite their bulky and rigid nature. In particular, we proposed a bracelet-like modular design of a thermoelectric (TE) module with a heat sink made of rigid inorganic bulk materials. This device is referred to as the flexible TE system (FTES). Experiments and theoretical analyses were performed to verify whether the FTES performs like a conventional TE module even though it is flexible and wearable. In addition, we performed experiments while the FTES was worn on a subject’s wrist for body heat harvesting. The FTES produced a usable power output even when the wearer was at rest. When the subject was running at a slow pace, the FTES generated approximately 80μW. Our analyses indicate that this value can be further enhanced through future design improvements. Nevertheless, based on the experimental and analytical results, the FTES can deliver a usable power output. This research therefore demonstrates the possibility of using high-performance bulk materials in the design of wearable devices.

Dopant Induced Impurity Bands and Carrier Concentration Control for Thermoelectric Enhancement in p-Type Cr2Ge2Te6

Our previous work demonstrated that Cr2Ge2Te6 based compounds with a layered structure and high symmetry are good candidates for thermoelectric application. However, the power factor of only ∼0.23 mW/mK2 in undoped material is much lower than that of conventional thermoelectrics. This indicates the importance of an electronic performance optimization for further improvements. In this work, either Mn- or Fe-substitution on the Cr site is investigated, with expectations of both carrier concentration control and band structure engineering. First-principle calculations indicate that an orbital hybridization between d orbitals of the doping atom and the p orbital of Te significantly increases the density of states (DOS) around the Fermi level. In addition, it is found that Mn doping is more favorable to improve the electrical properties than Fe doping. By tuning the carrier concentration via Mn doping, the peak power factor rises rapidly from 0.23 mW/mK2 to 0.57 mW/mK2 at 830 K with x = 0.05. Combined with the...

High Power Density Electrochemical Thermocells for Inexpensively Harvesting Low-Grade Thermal Energy.

Continuously operating thermo-electrochemical cells (thermocells) are of interest for harvesting low-grade waste thermal energy because of their potentially low cost compared with conventional thermoelectrics. Pt-free thermocells devised here provide an output power of 12 W m-2 for an interelectrode temperature difference (ΔT) of 81 °C, which is sixfold higher power than previously reported for planar thermocells operating at ambient pressure.

Flexible thermoelectric foil for wearable energy harvesting

Flexible thermoelectric (TE) devices have been of rapidly growing interest for long-lasting and maintenance-free wearable power source that makes use of the temperature difference between human skin and ambient environment. Despite the high TE performance, conventional inorganic TE semiconductors, such as Bi2Te3, skutterudites, are restricted for this application due to their non-flexibility structure and non-scalable manufacturing techniques. In this paper, we report large-area free-standing TE foil with several centimeters in size through a scalable, cost-effective and solution-based approach for flexible thermoelectric devices. The foil is made by self-assembling two-dimensional hybrid superlattices of TiS2 layers and hexylamine molecules. Through a Lewis base-acid reaction, electrons are transferred from the hexylamine molecules into the TiS2 layers, making the material n-type and generating a large power factor of 0.23mW/mK2, comparable to the best p-type conducting polymers. The flexible foil shows superior bending deformation tolerance, even better than the conducting polymers. A power output of 32µWcm−2 for a 15-µm-thick foil were obtained under a temperature difference of 20K at ambient temperature, high enough to drive the wearable nanowatt-to-microwatt level circuit chips or devices.

(Invited) Electrodeposition of PbTe Thick Films for Micro Thermoelectric Devices

Heat is the natural by-product of energy conversion processes. Of the 4.25 × 1020 J of energy the United States consumes every year, more than 60% is wasted in the form of heat. Therefore, waste heat recovery is a crucial step to improve the energy generation and utilization efficiency. Thermoelectric (TE) materials, which can directly convert rejected or waste heat into usable electric power, has been extensively developed for this issue. Thick-film-based devices have advantages over conventional TE module because of its compact size. By shrinking the size of thermoelectric devices, it not only allows the device to operate under smaller temperature gradients, but by so doing it expands its capability to handle a wider range of thermal and power management microelectronic systems. The combination of electrochemical deposition of compound semiconductors (metal tellurides) with standard integrated circuit technique makes fabrication of thermoelectric microdevices possible. In our work, we demonstrated a rapid deposition process (180 µm/h) in alkaline solution to synthesize dense thick PbTe film (>50 µm) for thermoelectric applications. Compared to the commonly used acidic baths with deposition rate about 4 µm/h, our electrodeposition process enhanced the film growth rate 48 folds higher. The electrodeposition mechanism of PbTe was examined. The thermoelectric property of thick PbTe films were optimized by introducing energy barriers from crystal grain boundary and tellurium nanoinclusion, which was achieved by tailoring composition and crystal structures of the thick PbTe films. Additionally, control over the composition and crystal structures was realized by tuning the electrodeposition conditions, such as the ratio of [TeO3 2-], [Pb2+], [EDTA4-]/[Pb2+], solution pH, and applied potential, as well as post-annealing process.

Thermally Chargeable Solid‐State Supercapacitor

Ubiquitous low‐grade thermal energy, which is typically wasted without use, can be extremely valuable for continuously powering electronic devices such as sensors and wearable electronics. A popular choice for waste heat recovery has been thermoelectric energy conversion, but small output voltage without energy‐storing capability necessitates additional components such as a voltage booster and a capacitor. Here, a novel method of simultaneously generating a large voltage from a temperature gradient and storing electrical energy without losing the benefit of solid‐state no‐moving part devices like conventional thermoelectrics is reported. Thermally driven ion diffusion is used to greatly increase the output voltage (8 mV K−1) with polystyrene sulfonic acid (PSSH) film. Polyaniline‐coated electrodes containing graphene and carbon nanotube sandwich the PSSH film where thermally induced voltage‐enabled electrochemical reactions, resulting in a charging behavior without an external power supply. With a small temperature difference (5 K) possibly created over wearable energy harvesting devices, the thermally chargeable supercapacitor produce 38 mV with a large areal capacitance (1200 F m−2). It is anticipated that the attempt with thermally driven ion diffusion behaviors initiates a new research direction in thermal energy harvesting.

Flexible and thermostable thermoelectric devices based on large-area and porous all-graphene films

Thermoelectric (TE) generators, being able to convert temperature gradients into electricity, are an appealing eco-friendly energy harvesting technology. In order to broaden the applications of TE materials, many researchers have focused their efforts towards flexible TE materials to substitute the commercial rigid TE devices. Most of these works used polymers as flexible TE materials, but their lower efficiency and melting point in comparison to inorganic TE materials are limiting factors for many applications. Here, we report the fabrication of a flexible and thermostable TE generator based on porous all-graphene films. The output power of the generator is up to 0.43 μW for a temperature gradient of 75 K. In addition, it runs at 550 K higher temperature than current flexible TE devices, which allows its application at much higher temperatures than conventional flexible TE.

Improving thermoelectric energy harvesting efficiency by using graphene

This study is aimed at enhancing the efficiency of a thermoelectric (TE) energy harvesting system by using a thick graphene layer. This method is a simple yet effective way to increase the temperature gradient across a conventional TE module by accelerating heat dissipation on the cold side of the system. Aqueous dispersions of graphene were used to prepare a 112-μm thick graphene layer on the cold side of the TE system with aluminum as the substrate material. The maximum efficiency of the proposed system was improved by 25.45 %, as compared to the conventional TE system, which does not have a graphene layer. Additionally, the proposed system shows very little performance deterioration (2.87 %) in the absence of enough air flow on the cold side of the system, compared to the case of the conventional system (10.59 %). Hence, the proposed system, when coupled with the latest research on high performance TE materials, presents a groundbreaking improvement in the practical application of the TE energy harvesting systems.

Spin Seebeck Power Conversion

Spin caloritronics is the science and technology to control spin, charge, and heat currents in magnetic nanostructures. The spin degree of freedom provides new strategies for thermolelectric power generation that have not yet been fully explored. After an elementary introduction into conventional thermoelectrics and spintronics, we give a brief review of the physics of spin caloritronics. We discuss spin-dependent thermoelectrics based on the the two-current model in metallic magnets as well as the spin Seebeck and Peltier effects that are based on spin wave excitations in ferromagnets. We derive expressions for the efficiency and figure of merit ZT of several spin caloritronic devices.

Heat shrink formation of a corrugated thin film thermoelectric generator

A thin film thermoelectric (TE) generator with a corrugated architecture is demonstrated formed using a heat-shrink fabrication approach. Fabrication of the corrugated TE structure consists of depositing thin film thermoelectric elements onto a planar non-shrink polyimide substrate that is then sandwiched between two uniaxial stretch-oriented co-polyester (PET) films. The heat shrink PET films are adhered to the polyimide in select locations, such that when the structure is placed in a high temperature environment, the outer films shrink resulting in a corrugated core film and thermoelectric elements spanning between the outer PET films. The module has a cross-plane heat transfer architecture similar to a conventional bulk TE module, but with heat transfer in the plane of the thin film thermoelectric elements, which assists in maintaining a significant temperature difference across the thermoelectric junctions. In this demonstration, Ag and Ni films are used as the thermoelectric elements and a Seebeck coefficient of 14μVK−1 is measured with a maximum power output of 0.22nW per couple at a temperature difference of 7.0K. We then theoretically consider the performance of this device architecture with high performance thermoelectric materials in the heat sink limited regime. The results show that the heat-shrink approach is a simple fabrication method that may be advantageous in large-area, low power density applications. The fabrication method is also compatible with simple geometric modification to achieve various form factors and power densities to customize the TE generator for a range of applications.

Analytical and Numerical Studies of Heat Transfer in Nanocomposite Thermoelectric Coolers

Thermoelectric coolers (TEC) can produce a cooling effect (in refrigerator mode) or a heating effect (in heat pump mode) using electrical energy input. Performance characteristics of typical TECs are poor when compared to the traditional cooling system (e.g., a vapor compression system). However, nanostructuring of thermoelectric (TE) materials can generate high-performance TE materials (e.g., high Seebeck coefficient, low thermal conductivity, and high electrical conductivity), and such materials show the promise to improve the performance of TEC. The main objective of this paper is to investigate the effect of nanocomposite TE materials and surface to surrounding convection heat transfer on the thermal performance of TEC. The mathematical model developed in this paper includes Fourier heat conduction, Joule heat, Seebeck effect, Peltier effect, and Thomson effect. This model also includes temperature-dependent transport properties. Governing transport equations are solved numerically using the finite element method to identify temperature and electrical potential distributions and to calculate heat absorbed and the coefficient of performance (COP). Heat absorbed and COP are also calculated using a simplified 1D analytical solution and compared with numerically obtained results. An optimum electric current is also calculated for maximum heat absorption rate and maximum COP for fixed geometric dimensions and variable convection heat transfer coefficients. An Increase in the convection heat transfer coefficient increases the optimum electric current required for maximum heat absorption rate and maximum COP. For the materials considered, the results show that COP of TEC can be increased by approximately 13 ± 1% if nanostructured TE materials are used instead of the conventional TE materials.

Performance analysis of a thermoelectric cooler with a corrugated architecture

A thermoelectric (TE) cooler architecture is presented that employs thin film thermoelectric elements on a plastic substrate in a corrugated structure sandwiched between planar thermal interface plates. This design represents a hybrid of a conventional bulk TE device and an in-plane thin film TE design. This design is attractive as it may benefit from low cost thin-film processing in a roll-to-roll fashion onto low-cost plastics substrates while maintaining a cross-plane heat flux for large area applications and a geometry that assists in maintaining a significant temperature difference across the thermoelectric elements. First, the performance of a single thermocouple is analyzed and the effect of the parasitic heat loss through the plastic substrate is examined. The performance of an array of thermocouples is then considered and the effects of various geometric parameters are analyzed with particular focus on the packing density of thermoelectric legs. The results show that while the coefficient of performance (COP) is comparable to a conventional bulk element TE cooler, the cooling power density drops off dramatically with a decrease in stacking angle of the legs. A comparison is then made between the heat sink demands of the hybrid TE design and a conventional bulk TE device where it is found that the lower cooling power density of the hybrid TE results in a reduction of heat sink demands as compared to bulk TE modules. The modeled performance suggest that the hybrid TE device may be advantageous in low cooling power density applications over relatively large areas where the low-cost nature of the device is maximized and less elaborate heat sink designs work effectively, cumulatively improving cost competitiveness.