Conventional Thermoelectric Devices Research Articles

Multifunctional energy harvesting and storage textile technology based on thermionic effect

The development of effective dual-function energy harvesting/storage technologies is a milestone demand for the emerging generation of low-consumption autonomous electronics. This work reports the development of a multifunctional thermionic power textile device merging thermal energy harvesting and electrochemical energy storage for application in self-powered systems. An everyday cost-effective non-conductive natural fabric was transformed into an all-solid-state textile-based thermally-chargeable supercapacitor (T-TCSC) via dyeing with multiwalled carbon nanotubes and subsequent assembly with H3PO4-doped poly(vinyl alcohol) (PVA) solid-gel polyelectrolyte.When subjected to an out-of-plane temperature gradient, the T-TCSC was able to convert the thermal energy into electrical output and store the harvested energy. A Soret coefficient of 1.85 mV K−1 was obtained, which is the highest value achieved so far for TCSCs developed on non-conductive textile substrates reported in literature and ∼10× higher than those of conventional thermoelectric devices. Additionally, a maximum output potential of ∼30 mV was achieved for ΔT = 24.7 K. Concerning the energy storage performance, an energy density of 9.39 μW h cm−2 was obtained at 1.62 mW cm−2 power density.The smart T-TCSC outputs demonstrated its potentialities as a two-in-one self-chargeable textile device that can harvest the thermal energy generated by a temperature gradient, convert it into electrical energy and simultaneously store it.

A self-healable, recyclable, and flexible thermoelectric device for wearable energy harvesting and personal thermal management

Flexible thermoelectric devices (f-TEDs) can realize direct energy conversion between heat and electricity, which hold great prospects in wearable power source and personal thermal management. However, conventional f-TEDs made from intrinsically flexible thermoelectric materials have low power density, and elastomer sealed bulk thermoelectric materials-based f-TEDs can hardly achieve active cooling. Additionally, these f-TEDs usually are not self-healable and recyclable, which easily occur fractures in wearable applications. Here, we developed a self-healable and recyclable f-TED by integrating dynamic covalent thermoset polyimine with liquid metal and thermoelectric legs. This f-TED achieves a normalized power density of 1.54 μW·cm−2·K−2, and can deliver a record 13.8 °C on-body cooling effect with a high coefficient of performance (COP) at 3.91 under 7 °C temperature difference, which leads to a low power consumption of ∼ 38 W for the cooling of regular human body. Based on the f-TED, we further developed a personal thermal management system, which can keep body within comfort zone at different surrounding temperatures, also realize healthcare function for fever or sprained ankle. Compared with conventional thermoelectric devices, this f-TED can simultaneously achieve self-healability, recyclability, flexibility, large normalized power density, and high on-body cooling effect with low power consumption. Such f-TED could open up new opportunities to develop devices for wearable energy harvesting and personal thermal management.

Doping as a tuning mechanism for magnetothermoelectric effects to improve zT in polycrystalline NbP

Weyl semimetals combine topological and semimetallic effects, making them candidates for interesting and effective thermoelectric transport properties. Here, we present experimental results on polycrystalline NbP, demonstrating the simultaneous existence of a large Nernst effect and a large magneto-Seebeck effect, which is typically not observed in a single material at the same temperature. We compare transport results from two polycrystalline samples of NbP with previously published work, observing a shift in the temperature at which the maximum Nernst and magneto-Seebeck thermopowers occur, while still maintaining thermopowers of similar magnitude. Theoretical modeling shows how doping strongly alters both the Seebeck and Nernst magnetothermopowers by shifting the temperature-dependent chemical potential, and the corresponding calculations provide a consistent interpretation of our results. Thus, we offer doping as a tuning mechanism for shifting magnetothermoelectric effects to temperatures appropriate for device applications, improving $zT$ at desirable operating temperatures. Furthermore, the simultaneous presence of both a large Nernst and magneto-Seebeck thermopower is uncommon and offers unique device advantages if the thermopowers are used additively. Here, we also propose a unique thermoelectric device that would collectively harness the large Nernst and magneto-Seebeck thermopowers to greatly enhance the output and $zT$ of conventional thermoelectric devices.

A theoretical prediction of novel Janus NiSX (X = O, Se, Te) Monolayers: Electronic, optical, and thermoelectric properties

Using density functional theory and semi-classical Boltzmann transport theory were employed to investigate the electronic structure, phononic and optical properties, dynamic stability, and thermoelectric characteristics of Janus NiSX (X = O, Se, Te). The electronic band structure of NiSTe indicates metallic behavior while NiSO and NiSSe show semiconducting features with indirect bandgap. Phononic properties and dynamic stability analyses reveal the stability of Janus Ni compounds. These compounds can be utilized as promising wave reflectors in the infrared (IR) range. Moreover, NiSO and NiSSe are good optical absorbers in the range of far ultraviolet (FUV). NiSTe exhibits isotropic absorption in the visible as well as FUV ranges. Thermoelectric calculations at room temperature show high Seebeck coefficients of 650 and 550 µV/K for NiSO and NiSSe, respectively. The large power factor of 20.83 × 1010 and 25.16 × 1010 W/msK2 are respectively found for n-type NiSO and NiSSe. The results suggest NiSO and NiSSe as promising thermoelectric materials with figure of merit of one (ZT ∼ 1). Consequently, NiSO and NiSSe can serve as efficient alternatives to conventional thermoelectric devices. The results of the current research can be used in the design of novel nanoelectronic devices as well as providing an excellent database for future studies.

A flexible spring-shaped architecture with optimized thermal design for wearable thermoelectric energy harvesting

Despite the rapid advancements made for thin-film thermoelectric materials, the rational design of corresponding thermoelectric devices (TEDs) raises the urgent need for thermal optimization to meet the requirement of efficient utilization of the temperature gradient vertical to the device plane. Herein, a three-dimensional spring-like thermoelectric device (S-TED) with fundamantal dual elastomer layers and air gaps is proposed. The novel device architecture not only inherits excellent flexibility and compressibility, but also enables the capability of harvesting waste heat under vertical temperature gradient. Detailed analysis of thermal modeling indicates that optimization is considered in three aspects compared to conventional TEDs, including the strengthened contact for more efficient interfacial heat transfer, the hindered heat transfer across the device body via the construction of thermally-insulating air gaps, and the effective heat sink that facilitates the rapid heat transfer from device to the ambient. The as-fabricated S-TED delivers a high output power of 749.19 nW under a vertical gradient of 30 K with only three pairs of p-n couple, corresponding to high output power of 416.22 nW cm -2 compared to other reported flexible TEDs. The design strategy conceived in this work will promote the development of high-performance TEDs for flexible and wearable applications. • A 3D spring-shaped architecture for flexible thermoelectric devices. • Thermal optimization ensures the effective building up of temperature gradient. • Based on in-plane properties, novel devices utilize vertical temperature gradient. • Design strategy can be extended to most thin-film thermoelectric materials. • High potential in flexible and wearable applications.

Thermal and Mechanical Analyses of Compliant Thermoelectric Coils for Flexible and Bio-Integrated Devices

AbstractThree-dimensional coil structures assembled by mechanically guided compressive buckling have shown potential in enabling efficient thermal impedance matching of thermoelectric devices at a small characteristic scale, which increases the efficiency of power conversion, and has the potential to supply electric power to flexible bio-integrated devices. The unconventional heat dissipation behavior at the side surfaces of the thin-film coil, which serves as a “heat pump,” is strongly dependent on the geometry and the material of the encapsulating dissipation layer (e.g., polyimide). The low heat transfer coefficient of the encapsulation layer, which may damp the heat transfer for a conventional thermoelectric device, usually limits the heat transfer efficiency. However, the unconventional geometry of the coil can take advantage of the low heat transfer coefficient to increase its hot-to-cold temperature difference, and this requires further thermal analysis of the coil in order to improve its power conversion efficiency. Another challenge for the coil is that the active thin-film thermoelectric materials employed (e.g., heavily doped Silicon) are usually very brittle, with the fracture strain less than 0.1% in general while the overall device may undergo large deformation (e.g., stretched 100%). Mechanical analysis is therefore necessary to avoid failure/fracture of the thermoelectric material. In this work, we study the effect of coil geometry on both thermal and mechanical behaviors by using numerical and analytical approaches, and optimize the coil geometry to improve the device performance, and to guide its design for future applications.

Thermoelectric properties of cubic Ba-substituted strontium disilicide, Sr1-xBaxSi2, with Ba content above solid solubility limit

In the interest of the sustainable development of society, it is preferable to construct next-generation thermoelectric devices using materials from earth-abundant elements. In conventional thermoelectric devices for low temperature applications, Bi2Te3-based materials that consists of scarce elements have been used. Cubic strontium disilicide (SrSi2) is a potential thermoelectric material for low temperature applications that consists of earth-abundant elements. To enhance the value of dimensionless thermoelectric figure of merit ZT of cubic SrSi2, partial substitution of Sr atoms with Ba atoms was examined. Cubic Ba-substituted SrSi2, Sr1-xBaxSi2 (x = 0.00, 0.09, and 0.19), was synthesized by spark plasma sintering at 250 MPa and 1023 K, and its thermoelectric properties, such as electrical resistivity ρ, Seebeck coefficient S, and thermal conductivity κ, were measured at temperatures ranging from 10 to 380 K. The sample with Ba content x above the Ba solid solubility limit in cubic SrSi2 (0.13) was successfully synthesized. With increasing x, ρ, and S increase while κ decreases, leading to an increase of ZT with x. ZT exhibits a maximum at a temperature Tmax, which shifts from 313 to 357 K when x increases from 0 to 0.19. The maximum ZT value for cubic Sr0.81Ba0.19Si2 in the present study was calculated to be 0.21 at 357 K. The ZT value obtained is higher than previously reported values for cubic Sr1-xBaxSi2 and cubic Sr1-xCaxSi2. These results demonstrate that cubic SrSi2 has properties suitable for low temperature applications.

Geometrical Structure Optimization Design of High-Performance Bi2Te3-Based Artificially Tilted Multilayer Thermoelectric Devices

Artificially tilted multilayer thermoelectric devices (ATMTDs) are a kind of thermoelectric device that can directly convert heat into electricity based on transverse thermoelectric effect. Although the devices have a simplified multilayer structure, their geometrical optimization is a complicated task. In this work, n-type Bi2Te2.7Se0.3 and p-type Bi0.1Sb1.9Te3 materials, which have the most important commercial applications in conventional thermoelectric devices, were selected as the component materials to assemble a promising high-performance Bi2Te3-based ATMTD. A numerical analysis method was employed to optimize the device length, device width, and thickness of the component materials. The results revealed that a large device width/length ratio, a small device aspect ratio, and a small thickness of component materials are favorable for achieving a high conversion efficiency. The temperature and charge distributions inside the ATMTD are studied based on finite element simulation. The nonuniform distribution of temperature field inside the device strongly depends on the thermal conductivity of component materials. The accumulation of a transverse electric field, accompanied with the cancellation of longitudinal electric field, is a consequence of different electric field distributions in the two component materials. This work provides a better understanding on the anisotropic electrical and thermal transport behaviors in transverse thermoelectric devices.

Free-standing flexible multiwalled carbon nanotubes paper for wearable thermoelectric power generator

Most of the thermoelectric research, these days, is being focussed towards development of flexible thermoelectric power generators (TEG) for wearable applications. Organic thermoelectric materials being flexible and solution processable, though, seem promising cannot render themselves for designing of conventional thermoelectric devices which require both p- and n-type of legs; due to lack of air-stable and flexible n-type materials. This work shows the possibility of conversion of a p-type flexible free-standing multiwalled carbon nanotubes (MWCNTs) paper into n-type on treatment with polyethylenimine (PEI). X-ray photoelectron spectroscopy results suggest that ‘as-grown’ MWCNTs attained n-type nature due to electron donation by imine group of PEI. Power factors of ~0.2 and ~0.06 μW/mK2 observed respectively for p- and n-type MWCNTs papers, owing to extremely low thermal conductivity (~0.05W/mK), resulted in figure-of-merit (ZT) of 1.27 × 10−3 and 3.02 × 10−4 at 68 °C. A prototype TEG designed using ‘as-grown’ and ‘PEI-modified’ MWCNTs as p- and n-type thermoelements respectively exhibited output of 227 μV/7.6 μA for a temperature difference of 40 °C. In short, facile scalability of MWCNTs when collaborated with such a low cost, environment friendly method that can easily modify its conduction to n-type can certainly open opportunities for scalable production of flexible roll-to-roll type wearable thermoelectric modules.

Enhancing Thermophotovoltaic Performance Using Graphene-BN- InSb Near-Field Heterostructures

Graphene---hexagonal-boron-nitride---InSb near-field structures are designed and optimized to enhance the output power and energy efficiency of the thermophotovoltaic systems working in the temperature range of common industrial waste heat, $400~\rm K \sim 800~\rm K$, which is also the working temperature range for conventional thermoelectric devices. We show that the optimal output electric power can reach $3.5\times10^{4} \rm\ W/\rm m^2$ for the system with a graphene---hexagonal-boron-nitride heterostructure emitter and a graphene-covered InSb cell, whereas the best efficiency is achieved by the system with the heterostructure emitter and an uncovered InSb cell (reaching to $27\%$ of the Carnot efficiency). These results show that the performances of near-field thermophotovoltaic systems can be comparable with or even superior than the state-of-art thermoelectric devices. The underlying physics for the significant enhancement of the thermophotovoltaic performance is understood as due to the resonant coupling between the emitter and the cell, where the surface plasmons in graphene and surface phonon-polaritons in boron-nitride play important roles. Our study provides a stepping stone for future high-performance thermophotovoltaic systems.

High-Performance μ-Thermoelectric Device Based on Bi2Te3/Sb2Te3 p-n Junctions.

A flexible and ultralight planar thermoelectric generator based on 15 thermocouples composed of n-type bismuth telluride (Bi2Te3) and p-type antimony telluride (Sb2Te3) legs (each with 400 nm thick) connected in series, on 25 μm thick Kapton substrate, was fabricated with impressive power factor values of 2.7 and 0.8 mW K-2 m-1 (at 298 K) for Bi2Te3 and Sb2Te3 films, respectively. The p-n junction thermoelectric device can generate a maximum open-circuit voltage and output power of 210 mV and 0.7 μW (3.3 mW cm-2), respectively, for a temperature difference of 35 K, which is higher than the one observed for a conventional thermoelectric device with metallic contacts for p-n junctions. The results were combined with numerical simulations, showing a good match between the experimental and the numerical results. The current density versus voltage (J-V) characteristics of the fabricated p-n junctions revealed a diode behavior with a turn-on voltage of ≈0.3 V and an impressive rectifying ratio (I+1V/I-1V) of ≈2 × 104.

A new approach for body heat energy harvesting

A new approach for body heat energy harvesting was proposed, and the theoretical performance has been demonstrated for wearable health care sensors as a target. One of the most important issues in the wearable system is a power supply. Humans keep their body temperature constant as long as they are alive. If we use thermal energy effectively, we could get the energy no matter when and where we go. However, thermal energy has hardly been used as power supply because thermoelectric conversion efficiency is very low. Quite different from the conventional thermoelectric device, the new approach uses rotation of turbine by stack effect. The driving force for the turbine is the pressure difference (ΔP) created by stack effect in a closed system. To obtain high performance, the device parameters are optimized by theoretical analysis. The obtained power density is 97.2 mW/cm3 when the outside air temperature is 277.15 K, and the device size is 5 mm × 2 mm × 10 mm. The energy density of the device is five or more times larger than that of a solar cell. The energy device can be assembled directly inside the various kind of wearable health care sensors. Therefore, the new approach for the body heat energy harvesting is a promising power supply means for the wearable system.

CuPt Alloy Thin Films for Application in Spin Thermoelectrics

Spin thermoelectrics represents a new paradigm of thermoelectricity that has a potential to overcome the fundamental limitation posed by the Wiedmann-Franz law on the efficiency of conventional thermoelectric devices. A typical spin thermoelectric device consists of a bilayer of a magnetic insulator and a high spin-orbit coupling (SOC) metal coated over a non-magnetic substrate. Pt is the most commonly used metal in spin thermoelectric devices due to its strong SOC. In this paper, we found that an alloy of Cu and Pt can perform much better than Pt in spin thermoelectric devices. A series of CuPt alloy films with different Pt concentrations were deposited on yttrium iron garnet (YIG) films coated gadolinium gallium garnet (GGG) substrate. Through spin Seebeck measurements, it was found that the Cu0.4Pt0.6/YIG/GGG device shows almost 3 times higher spin Seebeck voltage compared to Pt/YIG/GGG under identical conditions. The improved performance was attributed to the higher resistivity as well as enhanced spin hall angle of the CuPt layer.

Effect of diatomaceous earth on thermoelectric elements using multi-walled carbon nanotubes

Thermoelectric power generation is one of the expected new renewable energy in the future. However, the power generation capacity of thermoelectric devices is poor. In this research, we focused on utilizing diatomaceous earth to improve the performance of thermoelectric devices because it has low thermal conductivity. Our thermoelectric devices based on multi-walled carbon nanotube and diatomaceous earth have improved the open-circuit voltage about 30% compared with the conventional thermoelectric devices based on multi-walled carbon nanotube only.

A flexible photo-thermoelectric nanogenerator based on MoS2/PU photothermal layer for infrared light harvesting

Recently, ambient energy harvesting has attracted a great deal of attention to develop clean and renewable energy technologies. In this work, a flexible photo-thermoelectric nanogenerator (PTENG) based on MoS2/PU photothermal film and Te/PEDOT thermoelectric layer has been demonstrated for harvesting environmental infrared (IR) light. The MoS2/PU photothermal film is carefully designed to exhibit excellent flexibility, transferability, and photothermal property. By integrating the photothermal layer with a Te/PEDOT thermoelectric device, the PTENG can produce electrical output without a spatial temperature gradient which is necessary for conventional thermoelectric device. Under IR light illumination, a high temperature difference can be generated across the device, and hence a potential difference established between two electrodes based on a coupling of photothermal effect and Seebeck effect. This type of PTENG exhibits numerous advantages, such as flexible, shape-adaptive, light-weight, and simple-fabrication, which may have a great potential of application in photo-thermoelectric energy harvesting for wearable electronics and implantable electronics.

Thermoelectrochemical cells based on Li+/Li redox couples in LiFSI glyme electrolytes.

Thermoelectrochemical cells (TECs) provide conspicuous advantages, including a high Seebeck coefficient (Se), design flexibility, and low cost compared with conventional thermoelectric devices. Here, we investigated TECs employing Li metal electrodes (Li-TECs) and a series of glyme (CH3O[CH2CH2O]nCH3, n = 1-4, nG) solvents with 0.5-3.0 M lithium-imide salts (lithium bis [fluorosulfonyl]imide, LiFSI, and lithium bis[trifluoromethane sulfonyl]imide, LiTFSI). The Se value and power performance of Li-TECs markedly depend on the nature of glyme solvents and Li salt concentration. The dependency of Se on the solvation structure of the Li-ions is examined via Raman measurements, and the internal resistance of Li-TECs is analyzed using electrochemical impedance spectroscopy. Notably, a Li-TEC with 1.0 M LiFSI 1G displays about two times higher power density and about eight times higher conversion efficiency than a conventional Cu-TEC utilizing aqueous electrolytes, which is ascribed to the high Se value and low thermal conductivity of the former. In addition, for a Li-TEC with 1.0 M LiFSI 1G, the low-temperature performance is examined to assess its practical feasibility.

Spin-Caloritronic Batteries

The thermoelectric performance of a topological energy converter is analyzed. The H-shaped device is based on a combination of transverse topological effects involving the spin: the inverse spin Hall effect and the spin Nernst effect. The device can convert a temperature drop in one arm into an electric power output in the other arm. Analytical expressions for the output voltage, the figure-of-merit (ZT) and energy converting efficiency are reported. We show that the output voltage and the ZT can be tuned by the geometry of the device and the physical properties of the material. Importantly, contrary to a conventional thermoelectric device, here a low electric conductivity may in fact enhance the ZT value, thereby opening a path to new strategies in optimizing the figure-of-merit.

29pm3-PN-29 バイメタルMEMS方式高スループット放射計の開発

We have been developing a novel bimetal MEMS calorimeter as a high-sensitivity photothermal detector having large size for high-throughput optical radiometry. Prototypes of the Si/Al bimetal MEMS sensors of up to 5 mm × 5 mm in active area, based on the design optimized by finite element simulation, were fabricated via SOI processes. Their thermo-mechanical displacements can be measured with a Fabry-Perot interferometer. Noise equivalent power (NEP) of the sensor system was estimated to be several tens of nW at room temperature and atmospheric pressure, which is comparable to the conventional thermoelectric devices. Further reduction of the NEP toward sub-nW would be possible by high-sensitivity interferometry and appropriate noise reduction.

Convective Heat Transfer and Contact Resistances Effects on Performance of Conventional and Composite Thermoelectric Devices

The performance of Π shaped conventional and composite thermoelectric devices (TEDs) applied to waste heat recovery by taking the Fourier heat conduction, Joule heating, and the Peltier and Thomson effects in TE materials is investigated using analytical solutions. The TE legs built with semiconductor materials bonded onto a highly conductive interconnector material in a segmented fashion is treated as the composite TED, whereas the legs merely made from semiconductors is treated as the conventional TED. The top and bottom surfaces of TEDs are subjected to convective heat transfer conditions while the remaining surfaces exposed to ambient are kept adiabatic. The effects of contact resistances, convective heat transfer coefficients, and TE leg heights L on TEDs' performance are studied. An increase in electrical and/or thermal contact resistance and a decrease in heat transfer coefficients are resulted in a decrease in power output P0 and conversion efficiency η. Depending on the contact resistances and convective heat transfer loads, the optimum L where a maximum Po occurs is obtained typically in the range of 1–4 mm. For TE leg size greater than optimum L and TED operating under higher convective heat transfer conditions, the composite design exhibited better power output and lower conversion efficiency compared to conventional design. The effects of interconnector lengths and cross-sectional area on the composite TED's characteristics are also investigated. An increase in a length and a decrease in a cross-sectional area of the interconnector decreases the composite TED's performance. However, based on the increase of the interconnector's electrical resistance in relation to the device's total internal resistance, the composite TED exhibited both negligible and significant change behavior in P0.

New device architecture of a thermoelectric energy conversion for recovering low-quality heat

Low-quality heat is generally discarded for economic reasons; a low-cost energy conversion device considering price per watt, $/W, is required to recover this waste heat. Thin-film based thermoelectric devices could be a superior alternative for this purpose, based on their low material consumption; however, power generated in conventional thermoelectric device architecture is negligible due to the small temperature drop across the thin film. To overcome this challenge, we propose new device architecture, and demonstrate approximately 60 Kelvin temperature differences using a thick polymer nanocomposite. The temperature differences were achieved by separating the thermal path from the electrical path; whereas in conventional device architecture, both electrical charges and thermal energy share same path. We also applied this device to harvest body heat and confirmed its usability as an energy conversion device for recovering low-quality heat.