Flexible Thermoelectric Modules Research Articles

High performance magnesium-based plastic semiconductors for flexible thermoelectrics

Low-cost thermoelectric materials with simultaneous high performance and superior plasticity at room temperature are urgently demanded due to the lack of ever-lasting power supply for flexible electronics. However, the inherent brittleness in conventional thermoelectric semiconductors and the inferior thermoelectric performance in plastic organics/inorganics severely limit such applications. Here, we report low-cost inorganic polycrystalline Mg3Sb0.5Bi1.498Te0.002, which demonstrates a remarkable combination of large strain (~ 43%) and high figure of merit zT (~ 0.72) at room temperature, surpassing both brittle Bi2(Te,Se)3 (strain ≤ 5%) and plastic Ag2(Te,Se,S) and organics (zT ≤ 0.4). By revealing the inherent high plasticity in Mg3Sb2 and Mg3Bi2, capable of sustaining over 30% compressive strain in polycrystalline form, and the remarkable deformability of single-crystalline Mg3Bi2 under bending, cutting, and twisting, we optimize the Bi contents in Mg3Sb2-xBix (x = 0 to 1) to simultaneously boost its room-temperature thermoelectric performance and plasticity. The exceptional plasticity of Mg3Sb2-xBix is further revealed to be brought by the presence of a dense dislocation network and the persistent Mg-Sb/Bi bonds during slipping. Leveraging its high plasticity and strength, polycrystalline Mg3Sb2-xBix can be easily processed into micro-scale dimensions. As a result, we successfully fabricate both in-plane and out-of-plane flexible Mg3Sb2-xBix thermoelectric modules, demonstrating promising power density. The inherent remarkable plasticity and high thermoelectric performance of Mg3Sb2-xBix hold the potential for significant advancements in flexible electronics and also inspire further exploration of plastic inorganic semiconductors.

Chemically Transformed Ag2 Te Nanowires on Polyvinylidene Fluoride Membrane For Flexible Thermoelectric Applications.

Flexible thermoelectric devices of nanomaterials have shown a great potential for applications in wearable to remotely located electronics with desired shapes and geometries. Continuous powering up the low power flexible electronics is a major challenge. We are reporting a flexible thermoelectric module prepared from silver telluride (Ag2 Te) nanowires (NWs), which are chemically transformed from uniquely synthesized and scalable tellurium (Te) NWs. Conducting Ag2 Te NWs composites have shown an ultralow total thermal conductivity ~0.22 W/mK surpassing the bulk melt-grown Ag2 Te ~1.23 W/mK at ~300 K, which is attributed to the nanostructuring of the material. Flexible thermoelectric device consisting of 4 legs (n-type) of Ag2 Te NWs on polyvinylidene fluoride membrane displays a significant output voltage (Voc ) ~2.3 mV upon human touch and Voc ~18 mV at temperature gradient, ΔT ~50 K, which shows the importance of NWs based flexible thermoelectric devices to power up the low power wearable electronics.

Chemically Transformed Ag2Te Nanowires on Polyvinylidene Fluoride Membrane For Flexible Thermoelectric Applications

AbstractFlexible thermoelectric devices of nanomaterials have shown a great potential for applications in wearable to remotely located electronics with desired shapes and geometries. Continuous powering up the low power flexible electronics is a major challenge. We are reporting a flexible thermoelectric module prepared from silver telluride (Ag2Te) nanowires (NWs), which are chemically transformed from uniquely synthesized and scalable tellurium (Te) NWs. Conducting Ag2Te NWs composites have shown an ultralow total thermal conductivity ~0.22 W/mK surpassing the bulk melt‐grown Ag2Te ~1.23 W/mK at ~300 K, which is attributed to the nanostructuring of the material. Flexible thermoelectric device consisting of 4 legs (n‐type) of Ag2Te NWs on polyvinylidene fluoride membrane displays a significant output voltage (Voc) ~2.3 mV upon human touch and Voc ~18 mV at temperature gradient, ΔT ~50 K, which shows the importance of NWs based flexible thermoelectric devices to power up the low power wearable electronics.

Optimizing Waste Heat Conversion: Integrating Phase-Change Material Heatsinks and Wind Speed Dynamics to Enhance Flexible Thermoelectric Generator Efficiency.

Flexible thermoelectric generators (FTEGs) have garnered significant attention for their potential in harnessing waste heat energy from various sources. To optimize their efficiency, FTEGs require efficient and adaptable heatsinks. In this study, we propose a cost-effective solution by integrating phase-change materials into FTEG heatsinks. We developed and tested three flexible phase-change material thicknesses (4 mm, 7 mm, and 10 mm), focusing on preventing leaks during operation. Additionally, we investigated the impact of wind speed on the output performance of FTEGs with a flexible phase-change material heatsink. The results indicate that the appropriate flexible phase-change material thickness, when integrated with considerations for wind speed, demonstrates remarkable heat-absorbing capabilities at phase-change temperatures. This integration enables substantial temperature differentials across the FTEG modules. Specifically, the FTEG equipped with a 10 mm thick flexible phase-change material heatsink achieved a power density more than four times higher when the wind speed was at 1 m/s compared to no wind speed. This outcome suggests that integrating phase-change material heatsinks with relatively low wind speeds can significantly enhance flexible thermoelectric generator efficiency. Finally, we present a practical application wherein the FTEG, integrated with the flexible phase-change material heatsink, efficiently converts waste heat from a circular hot pipe into electricity, serving as a viable power source for smartphone devices. This work opens exciting possibilities for the future integration of flexible thermoelectric modules with flexible phase-change material heatsinks, offering a promising avenue for converting thermal waste heat into usable electricity.

Integration of Active Clothing with a Personal Cooling System within the NGIoT Architecture for the Improved Comfort of Construction Workers

Intense physical activity and high ambient temperature cause construction workers to be exposed to an increased risk of overheating, especially in the summer season. Personal cooling systems have great potential to support workers’ thermoregulation and reduce this risk. In particular, solutions based on the thermoelectric effect can provide high cooling effectiveness and ergonomics at the same time. In this paper, a newly developed active clothing solution with flexible thermoelectric modules intended for outdoor activities is presented. The active clothing was subjected to utility tests on a treadmill under laboratory conditions with the participation of potential end users. A comparison of results from cooled and uncooled places indicated a reduction in local skin temperature of as much as 2.7 °C. Moreover, a gradual decrease in temperature in the uncooled place during the experiment was observed. Based on the positive results from this evaluation, the personal cooling system was integrated into active clothing within the ASSIST-IoT NGIoT reference architecture. This allows contextual and personalized adjustment of the cooling power to be provided using AI techniques and, additionally, by using data from a weather station and a smartwatch. Training procedures and models for the AI system are proposed, with special attention paid to the privacy aspect.

Lignin containing cellulose nanofiber/Ag2Se nanocomposite films: a promising material for thermoelectric film generators.

This work deals with the fabrication of lignin containing cellulose nanofiber (LCNF)/Ag2Se films for thermoelectric applications. Ag2Se nanoparticles were synthesized within the LCNF network through in situ methods, employing Na2SeO3 and AgNO3 along with microwave energy treatment. LCNF/Ag2Se films fabricated with two LCNF : Ag2Se weight percent ratios (i.e., 50 : 50 and 30 : 70) were used to construct a flexible thermoelectric module. The obtained Ag2Se nanoparticles displayed a uniform size distribution in the LCNF network with smaller dimensions from the microwave energy treated group. The microstructure of LCNF/Ag2Se films was improved by hot-pressing, leading to enhanced film density thermoelectric properties. At a differential temperature of 50 K, films with 50% and 70% of Ag2Se exhibited output voltages of 18 and 21 mV; and Seebeck coefficients of -60 and -70 μV K-1 at 350 K, respectively. When microwave energy was applied, the films at 50% and 70% Ag2Se showed highest output voltages of 19 and 33 mV, respectively, and Seebeck coefficients of -63.3 and -110 μV K-1 at 350 K. The low-cost fabrication process associated with this module opens a pathway for applications such as energy harvesting.

Evaluation of Performance and Power Consumption of a Thermoelectric Module-Based Personal Cooling System—A Case Study

Thermoelectric (TE) technology is promising for reducing thermal discomfort of workers during their routine professional activities. In this manuscript, a preliminary evaluation of a newly developed personal cooling system (PCS) with flexible TE modules is presented based on an analysis of cooling efficiency and power consumption. For this purpose, tests with human participation were performed involving the monitoring of local skin temperature changes and electrical parameters of the controller. Thanks to TE cooling, a significant reduction of local skin temperature was observed at the beginning of the experiment, reaching as much as 6 °C. However, the observed effect systematically became weaker with time, with the temperature difference decreasing to about 3 °C. Cooling efficiency stayed at the same level over the ambient temperature range from 25 °C to 35 °C. The obtained results showed that a proper fitting of the PCS to the human body is a crucial factor influencing the PCS cooling efficiency.

Transferrable, Large‐area and Highly Conductive PEDOT : PSS Film and Its Application for Flexible Thermoelectric Generators

AbstractA facile method was demonstrated to prepare large‐area (∼100 cm2), conductive poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate (PEDOT : PSS) dense film via post‐treatment with sulfuric acid at 160 °C on a glass substrate. This PEDOT : PSS dense films exhibited a high electrical conductivity of up to 1800 S cm−1 without significantly compromising with the high humidity level, and could be easily transferred onto various flexible substrates such as plastics and paper with excellent adhesion. The conductive PEDOT : PSS dense films can be made a flexible and foldable thermoelectric module, showing an output voltage of 2.2 mV at ΔT=26 K. On the other hand, PEDOT : PSS thin films, which were prepared using a similar method, could be transferred onto the polyethylene terephthalate (PET) substrate and suitable to serve as flexible transparent electrodes for optoelectronic devices. Our simple method would hold great promise for scalability and mass production of large‐area, freestanding and conductive PEDOT : PSS polymer film for a variety of applications.

From Brittle to Ductile: A Scalable and Tailorable All-Inorganic Semiconductor Foil through a Rolling Process toward Flexible Thermoelectric Modules.

Inorganic thermoelectric (TE) materials with outstanding capacity for energy conversion are expected to be promising eco-friendly and renewable power sources, but they are always intrinsically brittle, restricting their development in flexible TE electronics. Therefore, we have developed a facile manufacturing method of large-scale all-inorganic silver chalcogenide foils and flexible TE generators in this work. A rolling process, as an effective and facile molding technique, is applied in ductile TE materials. The figure-of-merit for flexibility of this free-standing foil is in the range of 0.02-0.13, suggesting the superior flexibility of the all-inorganic TE foils. A high TE figure-of-merit ZT of 0.47 at room temperature is reached for Ag2S0.45Se0.45Te0.1, which is one of the most promising room-temperature ZTs among flexible TE materials. A proof-of-concept flexible TE generator based on silver chalcogenide foils achieves an open-circuit voltage of 1.19 mV and an output power density of 1.8 mW/m2 with a temperature difference of 2.7 °C across the TE leg, showing great potential in heat-to-electricity conversion.

Interfacial Effect Boosts the Performance of All‐Polymer Ionic Thermoelectric Supercapacitors

AbstractIonic thermoelectric supercapacitors (ITESCs) have recently been developed for converting low‐grade waste heat into electricity. Until now, most reports of ITESCs have been focused on the development of electrolytes, which then have been combined with a specific electrode material. Here, it is demonstrated that the electrode is not only critical for electrical energy storage but also greatly affects the effective thermopower (Seff) of an ITESC. It is shown that the same ion gel can generate a positive thermopower in an ITESC when using gold nanowire (AuNW) electrodes, while generating a negative thermopower when using poly(3,4‐ethylendioxythiophene):polystyrene sulfonate (PEDOT:PSS) electrodes. The achieved negative sign of the Seff could be attributed to the Donnan exclusive effect from the polyanions in the PEDOT:PSS electrodes. After examining the thermovoltage, capacitance and charge retention performance of the two ITESCs, it is concluded that PEDOT:PSS is superior to AuNWs as electrodes. Moreover, a new strategy of constructing an ionic thermopile of multiple p‐ and n‐type legs is achieved by series‐connecting these legs with same electrolyte but different electrodes. Using interfacial effect at ionic gels/PEDOT:PSS electrode interface, an enhanced thermoelectric effect in ITESCs is obtained, which constitutes one more step towards efficient, low‐cost, flexible, and printable ionic thermoelectric modules for energy harvesting.

A simple method for fabricating flexible thermoelectric nanocomposites based on bacterial cellulose nanofiber and Ag2Se

Flexible thermoelectric (FTE) devices have become attractive in recent years since they can be utilized as a power generator for wearable and portable electronics. This work fabricated FTE nanocomposites from bacterial cellulose (BC) and Ag2Se via an easy and inexpensive method. The blended BC was thoroughly mixed with Ag2Se powders before casting onto a filter paper via vacuum filtration, followed by oven-drying and hot-pressing. Phase formation of Ag2Se in the BC nanofiber network was confirmed by x-ray diffraction and energy dispersive spectroscopy. SEM images revealed the distribution of Ag2Se particles in the BC matrix. The Ag2Se particles were densely packed for large Ag2Se concentrations in the BC/Ag2Se nanocomposite. Thermoelectric measurements found that the electrical conductivity (σ) and Seebeck coefficient (S) varied with the Ag2Se proportion due to the changes in the carrier concentration and carrier mobility. The maximum σ of 5.7 × 104 S/m and S of −80 μV/K were observed at room temperature (RT), yielding the power factor (PF) of ∼300 μW/mK2. This PF value is comparable to other FTE materials, but the process used in this research is much simpler. The thermal conductivity was 0.56 W/mK at RT. Moreover, the BC/Ag2Se nanocomposites were highly flexible and could be attached to curved surfaces. In addition, the FTE module was constructed from BC/Ag2Se uni-leg elements, which could generate an output power of 0.28 μW. In addition, the simple fabrication process makes the BC/Ag2Se nanocomposite readily expandable to an industrial scale for modern FTE devices.

Flexible in-plane thermoelectric modules based on nanostructured layers ZnO and ZnO:In

In this study, with the aim of developing flexible in-plane thermoelectric module we deposited via SILAR method 0.3–1.4 thick nanostructured layers of undoped and indium doped zinc oxide on Kapton polyimide (PI) substrates and thus prepared ZnO:In/PI and ZnO/PI samples. We analyzed surface morphology, crystal structure, chemical composition, optical, electrical and thermoelectric properties of ZnO and ZnO:In thin films grown via SILAR on PI substrates. It was shown that ZnO:In/PI samples are consisted of flower-like ZnO:In microstructures with rod petals having pencil-like ends. ZnO/PI samples have lily flower-like ZnO microstructures with rounded petals. Despite the fact that crystal structure and optical properties of the nanostructured ZnO:In and ZnO films were almost the same, their morphology determined the electrical and thermoelectric properties. More compact 1.4 µm thick ZnO layer in sample ZnO/PI provided the best thermoelectric power factor ≈ 8 µW/(m·K2) due to its lower resistivity at near-room ≈ 0.15 Ω cm. Among four samples of the prepared flexible in-plane thermoelectric modules with single legs in the form of PI strips 3 cm long and 0.5 cm wide, covered with the semiconductor film ZnO or ZnO:In and having thin-film Al ohmic contacts at the edges, the TE module based on ZnO/PI showed the best output characteristics. At the temperature difference 50 К, its output power density reaches 10 µW/cm2, which is commensurate with the up-to-date values for solid miniature and flexible TEGs.

Experimental and Theoretical Investigation of the Effect of Filler Material on the Performance of Flexible and Rigid Thermoelectric Generators.

Thermoelectric generators have found many applications where the heat source can be either flat or curved. For a curved heat source, flexible thermoelectric generators are generally used. A filler material with low thermal conductivity can provide additional mechanical support to the thermoelectric module and can reduce convection and radiation losses. Herein, the effect of three different filler materials on the output performance of rigid and flexible thermoelectric generators is investigated. At first, theoretical models are derived and the experimental study validated the models. The experimental study revealed that the flexible thermoelectric modules outperformed the rigid modules; this is due to the reduction of the number of thermal junctions in the flexible modules and due to the differences in the thermal conductivities of the flexible and rigid substrates. Likewise, among TE modules without filler/with air between the TE legs, with polyurethane foam filler material, and with polydimethylsiloxane filler material, air has the lowest thermal conductivity, and therefore, the thermoelectric generator without filler generates higher output power and higher power density than when the other two filler materials are used. For the fixed temperature gradient, the highest power densities for the flexible and rigid thermoelectric generators without filler are 155 and 137.7 μW/cm2 for temperature gradients of 10.8 and 10.3 °C, respectively.

High-performance flexible thermoelectric modules based on high crystal quality printed TiS2/hexylamine

ABSTRACT Printed electronics implies the use of low-cost, scalable, printing technologies to fabricate electronic devices and circuits on flexible substrates, such as paper or plastics. The development of this new electronic is currently expanding because of the emergence of the internet-of-everything. Although lot of attention has been paid to functional inks based on organic semiconductors, another class of inks is based on nanoparticles obtained from exfoliated 2D materials, such as graphene and metal sulfides. The ultimate scientific and technological challenge is to find a strategy where the exfoliated nanoparticle flakes in the inks can, after solvent evaporation, form a solid which displays performances equal to the single crystal of the 2D material. In this context, a printed layer, formed from an ink composed of nano-flakes of TiS2 intercalated with hexylamine, which displays thermoelectric properties superior to organic intercalated TiS2 single crystals, is demonstrated for the first time. The choice of the fraction of exfoliated nano-flakes appears to be a key to the forming of a new self-organized layered material by solvent evaporation. The printed layer is an efficient n-type thermoelectric material which complements the p-type printable organic semiconductors The thermoelectric power factor of the printed TiS2/hexylamine thin films reach record values of 1460 µW m−1 K−2 at 430 K, this is considerably higher than the high value of 900 µW m−1 K−2 at 300 K reported for a single crystal. A printed thermoelectric generator based on eight legs of TiS2 confirms the high-power factor values by generating a power density of 16.0 W m−2 at ΔT = 40 K.

A Comparative Analysis of Thermoelectric Modules for the Purpose of Ensuring Thermal Comfort in Protective Clothing

In recent times, more and more workers are exposed to thermal stress due to climate changes and increased ambient temperature. Demanding physical activities and the use of protective clothing are additional sources of thermal load for workers. Therefore, recent research has focused on the development of protective clothing with a cooling function. Phase change materials, air or liquid, were mainly used for this purpose; only a few publications were concerned the use of thermoelectric modules. This publication analyzes the influence of such factors as supplied current, ambient temperature, and the type of heat sink on the amount of heat flux transferred by a thermoelectric cooler (TEC) and the electric power consumed by it. In the course of laboratory tests, a flexible thermoelectric module and three heat sink variants were tested. For this purpose, a polymer TEGway heat sink, a metal one, and a self-made one based on a superabsorbent were used. The research showed that at a temperature of 30 °C and above, the amount of the heat flux transferred by a TEC with a total area of 58 cm2, and an active area of 10 cm2 should be expected to be from 1 W to 1.5 W. An increase in ambient temperature from 20 to 35 °C caused a significant reduction in the heat flux by about 1 W. The results obtained indicated that the type of heat sink affects the heat flux drawn by the TEC to a statistically significant extent. The heat sink using the evaporation effect turned out to be the most efficient.

Degeneration of power output of a flexible and wearable thermoelectric module under bending fatigue

Strength and energy conversion efficiency of thermoelectric (TE) thin films upon cyclic mechanical loading are major concerns for flexible thermoelectric generator (TEG) applications. A comprehensively analysis model for the performance evaluation and fatigue failure of a TEG is constructed in this paper. The analytical solutions of temperature field, stress field, fatigue life, power output and energy conversion efficiency of the flexible TEG under cyclic loading are presented. A bending experiment is carried out to validate the analytical model and to fitting the model parameters. The results show that a continuing increase of electrical resistance and decrease of power output under the cyclic bending. The electrical resistance and power output of the TEG varies with the bending radius. The thickness ratio of the substrate to the TE film has a great influence on power output and energy conversion efficiency. A critical bending radius can be obtained from the model to provide a guideline for future flexible TEG design.

Intelligent light-driven flexible solar thermoelectric system

Solar thermoelectric generation (STEG) is an excellent and environmentally-friendly way to convert thermal energy into electricity by utilizing Seebeck effect of thermoelectric material. However, how to overcome the rigidity and large output fluctuation of traditional STEG is the key to realize its large-scale application in self-powered devices. Herein, a smart light-driven flexible STEG system was designed and constructed through simple layered structure of VO2 flexible film and carbon nanotube (CNT) thermoelectric modules. Based on the optical control characteristics of VO2 flexible film due to the thermal crystal phase transformation, and the light capture, photo-thermal-electric conversion capabilities of CNT flexible film, the smart light-driven flexible STEG system exhibits good flexibility, light-thermal-electrical conversion and stable voltage output. In the above system, CNT, polyvinylidene fluoride (PVDF) as raw material and polyethyleneimine (PEI) as electron donor, p- and n-type flexible thermoelectric films and modules were constructed; VO2 ultrafinepowder was dispersed in polyvinylpyrrolidone(PVP), and the optical control VO2 flexible film was obtained by simple spin coating on the flexible substrate. The system is so flexible that can be folded or bended to different shapes to adapt to different application environments. Furthermore, the intelligent light-driven flexible STEG system can obtainrelatively stable voltage to prevent the damage of equipment caused by large voltage fluctuation and has great application potential in civil micro-self-powered devices.

Highly flexible and excellent performance continuous carbon nanotube fibrous thermoelectric modules for diversified applications* *Project supported by the National Key Research and Development Program of China (Grant No. 2018YFA0208402), the National Natural Science Foundation of China (Grant Nos. 11634014, 51172271, and 51372269), and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA09040202).

A highly flexible and continuous fibrous thermoelectric (TE) module with high-performance has been fabricated based on an ultra-long single-walled carbon nanotube fiber, which effectively avoids the drawbacks of traditional inorganic TE based modules. The maximum output power density of a 1-cm long fibrous TE module with 8 p–n pairs can reach to 3436 μW ⋅ cm−2, the power per unit weight to 2034 μW ⋅ g−1, at a steady-state temperature difference of 50 K. The continuous fibrous TE module is used to detect temperature change of a single point, which exhibits a good responsiveness and excellent stability. Because of its adjustability in length, the flexible fibrous TE module can satisfy the transformation of the temperature difference between two distant heat sources into electrical energy. Based on the signal of the as-fabricated TE module, a multi-region recognizer has been designed and demonstrated. The highly flexible and continuous fibrous TE module with excellent performance shows a great potential in diversified applications of TE generation, temperature detection, and position identification.

High-Power All-Carbon Fully Printed and Wearable SWCNT-Based Organic Thermoelectric Generator.

In this study, we introduce the fabrication process of a highly efficient fully printed all-carbon organic thermoelectric generator (OTEG) free of metallic junctions with outstanding flexibility and exceptional power output, which can be conveniently and rapidly prepared through ink dispensing/printing processes of aqueous and low-cost CNT inks with a mask-assisted specified circuit architecture. The optimal p-type and n-type films produced exhibit ultrahigh power factors (PFs) of 308 and 258 μW/mK2, respectively, at ΔΤ = 150 K (THOT = 175 °C) and outstanding stability in air without encapsulation, providing the OTEG device the ability to operate at high temperatures up to 200 °C at ambient conditions (1 atm, relative humidity: 50 ± 5% RH). We have successfully designed and fabricated the flexible thermoelectric (TE) modules with superior TE properties of p-type and n-type SWCNT films resulting in exceptionally high performance. The novel design OTEG exhibits outstanding flexibility and stability with attained TE values among the highest ever reported in the field of organic thermoelectrics, that is, open-circuit voltage VOC = 1.05 V and short-circuit current ISC = 1.30 mA at ΔT = 150 K (THOT = 175 °C) with an internal resistance of RTEG = 806 Ω, generating a 342 μW power output. It is also worth noting the remarkable PFs of 145 and 127 μW/mK2 for the p-type and n-type films, respectively, at room temperature. The fabricated device is highly scalable, providing opportunities for printable large-scale manufacturing/industrial production of highly efficient flexible OTEGs.

High thermoelectric efficiency in electrodeposited silver selenide films: from Pourbaix diagram to a flexible thermoelectric module

High thermoelectric efficient is obtained in electrodeposited silver selenide films and a unileg flexible thermoelectric module is fabricated with a maximum power density of 138.6 mW m−2 at 19 K.