Thermoelectric Legs Research Articles

Wearable Respiration Sensor for Continuous Healthcare Monitoring Using a Micro‐Thermoelectric Generator with Rapid Response Time and Chip‐Level Design

AbstractThermoelectric generators (TEG) serve as excellent passive wearable sensors for monitoring human body heat. However, a micro‐TEG (μTEG) with chip‐level size, rapid response, and high and stable responsivity is desired for real‐time and full‐time respiration monitoring to predict and diagnose breath‐related diseases. In this study, a thin‐film compact μTEG is elaborately designed by combining an ultrathin vertical structure for rapid heat conduction and a horizontal high‐integration density for transient response and a high filling rate. The device integrated with 28‐pair micro thermoelectric (TE) legs is fabricated on an aluminum nitride (AlN) substrate, which is patterned using ultrafast laser direct writing with embedded bottom contacts and TE legs. This unique design of the proposed μTEG provides a rapid response of 8 ms and chip‐level size of 1.9 mm × 2.7 mm × 400 μm for easy wearability. Additionally, application scenarios of real‐time respiration monitoring are demonstrated by mounting the μTEG under the nostril and near the mouth. The recorded airflow signals are displayed precisely with distinct features separating the nose and mouth breathing. Thus, the study presents a subtle and wearable respiration sensor for real‐time and full‐time human physiological signal acquisition.

Feasibility of high performance in p‐type Ge1−xBixTe materials for thermoelectric modules

AbstractGeTe is a promising candidate for the fabrication of high‐temperature segments for p‐type thermoelectric (TE) legs. The main restriction for the widespread use of this material in TE devices is high carrier concentration (up to ∼ 1021 cm−3), which causes the low Seebeck coefficient and high electronic component of thermal conductivity. In this work, the band structure diagram and phase equilibria data have been effectively used to attune the carrier concentration and to obtain the high TE performance. The Ge1−xBixTe (x = 0.04) material prepared by the Spark plasma sintering (SPS) technique demonstrates a high power factor accompanied by moderate thermal conductivity. As a result, a significantly higher dimensionless TE figure of merit ZT = 2.0 has been obtained at ∼ 800 K. Moreover, we are the first to propose that application of the developed Ge1−xBixTe (x = 0.04) material in the TE unicouple should be accompanied by SnTe and CoGe2 transition layers. Only such a unique solution for the TE unicouple makes it possible to prevent the negative effects of high contact resistance and chemical diffusion between the segments at high temperatures.

Flexible Bi2Te3-based thermoelectric generator with an ultra-high power density

• A flexible thermoelectric generator with newly structure design was fabricated. • The generator consists of rigid Bi 2 Te 3 legs sandwiched by two flexible substrates. • The upper substrate is diced into blocks to ensure the generator some flexibility. • A specific power density of 13.1 mW/cm 2 is achieved for the generator. • The output power of the generator remains almost unchanged after 7400 bends. Environmental energy harvesting based on flexible thermoelectric generator (f-TEG) provides an ideal micro-power source to drive the node sensors for the Internet of Things (IoT) and wearable electronics. However, the existing flexible thermoelectric devices have some disadvantages, such as low strength, large thermal resistance, complex manufacturing processes, and low reliability. Here, we report a high-performance f-TEG with a dimension of 2 × 16 × 0.6 mm 3 , encapsulating 18 pairs of thermoelectric legs with the size of 0.38 × 0.38 × 0.38 mm 3 . The flexible polyimide (PI) films with patterned electrodes act as the substrates, of which the upper one is cut into blocks to ensure the generator a minimum bending radius of 4.5 mm. The f-TEG can produce an open circuit voltage of 236 mV and output power of 4.19 mW under a temperature difference of 50 K, which remain almost unchanged even after 7400 times of bending tests. The power density reaches up to 13.1 mW/cm 2 at ΔT = 50 K, and the normalized output power density is 5.26 μW/cm 2 ·K 2 . These results are among the best performances reported for f-TEGs, which opens a new avenue for flexible micro-power system design and promotes the development of the next generation self-powered sensors and charge-free electronic devices.

Design of a scalable, flexible, and durable thermoelectric cooling device for soft electronics using Kirigami cut patterns

A flexible, soft thermoelectric cooling device is presented that shows potential for human cooling applications in wearable technologies and close-to-body applications. Current developments lack integration feasibility due to non-scalable assembly procedures and unsuitable materials for comfortable and durable integration into products. Our devices have been created and tested around the need to conform to the human body which we have quantified through the creation of a repeatable drape testing procedure, a metric used in the textile industry. Inspired by mass manufacturing constraints, our flexible thermoelectric devices are created using commercially available materials and scalable processing techniques. Thermoelectric legs are embedded in a foam substrate to provide flexibility, while Kirigami-inspired cuts are patterned on the foam to provide the drape necessary for mimicking the performance of textile and close to body materials. In total, nine different configurations, three different fill factors and three different Kirigami cut patterns were fabricated and inspected for thermal characterization, mechanical testing, flexibility and drape. Our studies show that adding Kirigami patterns can increase the durability of the device, improve the flexibility, decrease the drape coefficient, and have <1% of impact on cooling performance at higher fill factors (>1.5%), reaching temperature differences up to 4.39 °C ± 0.17 °C between the hot and cold faces of the device. These thermoelectric cooling devices show great flexibility, durability, and cooling for integration into soft cooling products.

Energy Conversion Efficiency of Thermoelectric Power Generators With Cylindrical Legs

Abstract Thermoelectric power generators (TEGs) have been attracted increasing attention due to their capability of converting waste heat into useful electric energy without hazardous emissions. Many theoretical models to conduct their performance analysis are developed based on the generalized heat transport theory. However, most of them are assumed that the TEGs are thermally isolate from the surroundings except for the heat exchange at hot and cold reservoirs. This paper develops a theoretical model to study the performance of TEGs with cylindrical legs, and the influence of convective heat loss between lateral surfaces of legs and ambient environment is considered. Analytical solutions for temperature distribution inside the TEG, power output and energy conversion efficiency are obtained by using eigenfunction expansion method. A new dimensionless impact factor H is introduced to capture the convective heat effect, and the maximum energy conversion efficiency can be evaluated by the figure of merit, impact factor H and temperature ratio of heat sink to hot source for a well-designed TEG. There exists an optimal leg’s height for maximum energy conversion efficiency when the convective heat loss on lateral surfaces of thermoelectric legs and electrode thermal resistance are considered. The conclusions provided in this paper will be very helpful in the designing of high-performance TEG devices.

Improving the oxidation resistance of thermoelectric Mg2Si leg with silica coating

Due to the volatile and oxidation nature of Mg, Mg2Si based thermoelectric legs degrade faster when exposed to ppm level of oxygen during mid-temperature range operation. Here, we have established a modified sol–gel technique to get an oxidation-resistant SiO2 coating on the Mg2Si thermoelectric legs. The cyclic thermal ageing stability, oxidation resistance, and thermoelectric properties of the SiO2 coated Mg2Si have been investigated. The SiO2 coated Mg2Si is stable after ageing for 200 h at 823 K in air, and there is no degradation in the thermoelectric properties due to atmospheric exposure. It is thus a promising technique to increase the life span of Mg2Si based thermoelectric materials for waste heat recovery in automobiles and heavy industries.

A prototype thermoelectric module based on p-type colusite together with n-type nanostructured PbTe for power generation

Thermoelectric power generation from the prototype π-shaped module composed of p-type colusite (Cu26Cr2Ge6S32) and n-type nanostructured PbTe (Pb0.98Ga0.02Te-3% GeTe) was demonstrated in this study. The thermoelectric figure of merit zT of Cu26Cr2Ge6S32 and Pb0.98Ga0.02Te-3% GeTe was ∼0.8 and ∼1.2 at 665 K, respectively. In PbTe, transmission electron microscopic images and energy-dispersive x-ray elemental maps reveal the insertion of nanoscale precipitates induced by the GeTe alloying. Contact layers based on Au and Co-Fe were used for p- and n-type thermoelectric legs, respectively, which allow the low electrical specific contact resistances of ≤10 × 10−10 Ω m2 at room temperature. Maximum thermoelectric conversion efficiency ηmax of ∼5.5% was obtained for the Cu26Cr2Ge6S32 and Pb0.98Ga0.02Te-3% GeTe-based two-pair module when the hot-side Th and cold-side Tc temperatures were maintained at 673 and 283 K, respectively. A three-dimensional finite-element simulation predicts the ηmax of ∼7.1% for the module at Th and Tc of 673 and 283 K, respectively.

Performance investigation of a two-stage thermoelectric cooler with inhomogeneous materials

A novel model of two-stage thermoelectric cooler with inhomogeneous thermal conductivity in steady-state operating condition is established. The modification of the constant properties model allows controlling the distribution of Joule heat. Considering internal irreversibilities of the thermoelectric cooler, expressions for the cooling capacity, COP, and exergy efficiency are derived. By utilizing numerical methods, the temperature profile along the thermoelectric legs is presented. The optimal operating regions are explored. The COP vs. cooling capacity describing optimal operating regions in inhomogeneity materials are plotted. Meanwhile, the influence of the main parameters such as the variation of thermal conductivity distribution, cold-end temperature and the number of thermoelectric modules on the cooling performance is discussed in detail. Results indicate that the cooling capacity, COP and exergy efficiency are improved compared to those of homo?geneous two-stage thermoelectric coolers when an appropriate inhomogeneous property parameter is applied. The work can provide guidance on design of actual two-stage thermoelectric coolers with inhomogeneous materials.

Performance Investigation of Frustum-Shaped Thermoelectric Legs Used in Milliwatt Radioisotope Power Supply Based on Three-Dimensional Multiphysics Model

Performance Investigation of Frustum-Shaped Thermoelectric Legs Used in Milliwatt Radioisotope Power Supply Based on Three-Dimensional Multiphysics Model

Performance Investigation of Frustum-Shaped Thermoelectric Legs Used in Milliwatt Radioisotope Power Supply Based on Three-Dimensional Multiphysics Model

Performance Investigation of Frustum-Shaped Thermoelectric Legs Used in Milliwatt Radioisotope Power Supply Based on Three-Dimensional Multiphysics Model

Design of cascade thermoelectric generation systems with improved thermal reliability

This paper presents a comprehensive analysis of novel, unileg cascade thermoelectric systems. Unileg systems offer the flexibility of selecting a single (p- or n-) thermoelectric material in a thermoelectric system compared to two (n- and p-) materials. Finite element analysis simulations were performed to design and analyze two and three stages of cascade systems. Simulation results show that unileg cascade systems perform significantly (∼75%) better than their unicouple counterparts in both two and three stage configurations due to the elimination of the poorly performing thermoelectric leg. In addition to the enhanced thermoelectric power generation, thermal stress is lowered in unileg systems since the mismatch of the thermal expansion coefficient originating between different TE legs material can be minimized. Overall, the presented unileg systems show their promise for the future thermoelectric energy generation or cooling applications for electronics.

Stretchable thermoelectric generator for wearable power source and temperature detection applications

Wearable thermoelectric generators are attracting tremendous interest due to their capability of converting the human body heat into electricity for powering electronics. In order to reduce the parasitic thermal resistance and enhance the efficiency of the energy harvesting process, the compliance of the device is required by introducing stretchable structures or materials. Thus, in this work, we present a novel stretchable thermoelectric generator (STEG) with 50 pairs of bulk thermoelectric legs connected by the kirigami-inspired flexible electrodes. The device was fabricated by a sacrificial layer assisted soldering fabrication method and its generation performances were characterized under natural and forced convection, as well as constant temperature difference conditions, respectively. The results indicated that the generated power density of the STEG under convection was enhanced by improving the cold-side air velocity, and the power density of the device under constant temperature difference conditions could be promoted with order of magnitudes to 2.7 mW/cm2 at ΔT = 50 K. Besides, the STEG features reliable during the cycling of 30 % tensile strain load and bending with a radius of 5 mm, respectively. Finally, various practical applications of the STEG were investigated including the wearable power supply and temperature detector for human proximity/contact testing and high-temperature alert. Our results demonstrate that the proposed STEG configuration displays high output performance that could be utilized for wearable electronics applications.

Realizing High Thermoelectric Performance of Bi-Sb-Te-Based Printed Films through Grain Interface Modification by an In Situ-Grown β-Cu2-δSe Phase

It has been a substantial challenge to develop a printed thermoelectric (TE) material with a figure-of-merit ZT > 1. In this work, high ZT p-type Bi0.5Sb1.5Te3-based printable TE materials have been advanced by interface modification of the TE grains with a nonstoichiometric β-Cu2-δSe-based inorganic binder (IB) through a facile printing-sintering process. As a result, a very high TE power factor of ∼17.5 μW cm-1 K-2 for a p-type printed material is attained in the optimized compounds at room temperature (RT). In addition, a high ZT of ∼1.2 at RT and of ∼1.55 at 360 K is realized using thermal conductivity (κ) of a pellet made of the prepared printable material containing 10 wt % of IB. Using the same material for p-type TE legs and silver paste for n-type TE legs, a printed TE generator (print-TEG) of four thermocouples has been fabricated for demonstration. An open-circuit voltage (VOC) of 14 mV and a maximum power output (Pmax) of 1.7 μW are achieved for ΔT = 40 K for the print-TEG.

House electrical generation using thermoelectric cinder block: Case study on Lebanese hollow block

The work presented in this article describes thermoelectric effect in cinder blocks in order to generate electricity in houses through heat waste harvesting. The study consists of developing a numerical model formed by a heat transfer equation coupled to thermoelectric equations to study the performance of thermoelectric generators(TEG) incorporated inside Lebanese hollow blocks through two simulations using finite difference and finite element schemes. Results showed a voltage of 5.85 mV produced from a single 8.6 × 0.4 × 0.4 cm3 thermoelectric leg made of Bismuth Antimony Telluride for a temperature difference ΔT = 30 K. Then, a design of three TEGs with 123 thermoelectric legs incorporated inside a hollow block was tested and validated numerically using both methods, the main results obtained for ΔT = 30 K, showed a voltage ΔV = 0.72 V, a current I = 0.06 A and a figure of merit ZT = 0.55. The design was then optimized for economic purposes; an optimum model was chosen with a final cost of 8880 $. Different suggestions were also presented to enhance the thermoelectric performance of the model.

Termoelektrik Soğutucunun Modelleme Çalışması ve Performans İncelemesi

Thermoelectric refrigerators are widely used in electronics, medical, and food industry application areas. A refrigeration effect can also be achieved without using any moving parts by merely passing a small current through a closed circuit made up of two dissimilar materials. This effect is called the Peltier effect, and a refrigerator that works on this principle is called a thermoelectric refrigerator. They consist of several thermoelectric legs sandwiched between two thermally conductive plates, one cold and one hot. Thermoelectric refrigerators presently cannot compete with the vapor-compression refrigeration system because of their low- coefficient of performance (COP). However, some applications have been preferred because of their small size, simplicity, quietness, and reliability. In this study, a thermoelectric cooler having a maximum cooling power of 50 W, having a dimension of 40mmx40mmx3.6 mm, is modeled in multi-physics software. Also, the performance of a thermoelectric refrigerator is investigated. It is computed the temperature difference between ceramics plates versus electric current and COP for a temperature difference between ceramics plates. The simulation results are compared with experimental data. The data obtained from the analyses have been compared with the experimental results and found to agree with each other. For the surface temperatures of 25 oC and 50 oC, the maximum coefficients of performance have been computed to be 1.091 and 1.445, respectively. In general, as the temperature of hot surfaces has increased for the same temperature differences, the COP of the thermoelectric cooler has increased.

Fabrication and Cooling Performance Optimization of Stretchable Thermoelectric Cooling Device

Stretchable electronic devices are known as elastic electronics or circuits. They are typically built by deposition or embedding of devices and circuits onto stretchable substrates, which can sustain large strains without failure. The research on this emerging technology has focused on the development of components based on insulators, conductors, or semiconductors to overcome inherent difficulties of generating mechanically stable layers and interconnections. In addition to the mechanical challenges, the heat-dissipation problem, remains yet to be solved, since stretchable electronics are generally fabricated using materials with poor thermal conductivities. To simultaneously address stretchability and heat dissipation, novel cooling platforms are required. As one promising pathway, this paper presents the fabrication and performance optimization of a thermoelectric device that is stretchable and effectively dissipates heat. The device is easy to fabricate and has a relatively simple structure, consisting of a silicone elastomer (Ecoflex), liquid metal, and thermoelectric legs. It is mechanically stable over 1000 stretching-and-releasing cycles with 30% strain. A thermal analysis reveals that the Ecoflex substrates impose significant thermal resistances to the entire device, degrading its cooling performance. To address this issue, alumina powder is added into the Ecoflex substrates. For the case of Ecoflex/alumina composite substrates with 60 wt % alumina, the maximum temperature drop is enhanced by 80 and 50% at 0 and 30% strains, respectively, under no heat load conditions. The devices with alumina addition also show a highly improved cooling per unit electrical power input, suggesting their potential merit in cost and energy saving. The cooling capability of the stretchable thermoelectric devices is further confirmed under both conductive and convective heat load conditions.

Effects of interfacial properties on conversion efficiency of Bi2Te3-based segmented thermoelectric devices

The conversion efficiency η of a thermoelectric (TE) device can be effectively improved by constructing segmented TE legs, but the specific interfaces between the heterogeneous materials inevitably degrade the performance. Focusing on the Bi2Te3-based two-segmented module, we systematically investigated the influences of the Peltier effect, interfacial electrical resistance Re, and interfacial thermal resistance Rt on the conversion efficiency η. It is found that the Peltier heat can increase the conversion efficiency if the Seebeck coefficient increases along the direction of an electric current. An applicable Re should be kept on the order of magnitudes of 10−5 Ω cm2 for segmented TE devices, since the increased Re significantly decreases η. With a determined Re, η depends on the leg height L rather than the cross-sectional area A. In contrast, η is hardly affected by the variation in the interfacial thermal resistance Rt, while both the input heat flux Qin and output power P decrease with the increasing Rt.

Garment-integrated thermoelectric generator arrays for wearable body heat harvesting

Wearable thermoelectric generator arrays have the potential to use waste body heat to power on-body sensors and create, for example, self-powered health monitoring systems. In this work, we demonstrate that a surface coating of a conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT-Cl), created on one face of a wool felt using a chemical vapor deposition method was able to manifest a Seebeck voltage when subjected to a temperature gradient. The wool felt devices can produce voltage outputs of up to 120 mV when measured on a human body. Herein, we present a strategy to create arrays of polymer-coated fabric thermopiles and to integrate such arrays into familiar garments that could become a part of a consumer’s daily wardrobe. Using wool felt as the substrate fabric onto which the conducting polymer coating is created allowed for a higher mass loading of the polymer on the fabric surface and shorter thermoelectric legs, as compared to our previous iteration. Six or eight of these PEDOT-Cl coated wool felt swatches were sewed onto a backing/support fabric and interconnected with silver threads to create a coupled array, which was then patched onto the collar of a commercial three-quarter zip jacket. The observed power output from a six-leg array while worn by a healthy person at room temperature (ΔT = 15 °C) was 2 µW, which is the highest value currently reported for a polymer thermoelectric device measured at room temperature.

Evaluation of the double-tuned functionally graded thermoelectric material approach for the fabrication of n-type leg based on Pb0.75Sn0.25Te

Thermoelectric (TE) technologies realize the generation of electrical energy from the waste heat. The one bottleneck, which significantly restricts the wide use of these technologies, relates to the low energy conversion efficiency of the commercial devices. In this work, the double-tuned functionally graded thermoelectric material (DT-FGTM) approach was proposed to achieve the high-performance TE leg through the increase in the average TE figure of merit (ZT)ave. The essence of this idea is connected with the precise control of the bandgap Eg and chemical potential μc over the entire temperature range. Considering Pb0.75Sn0.25Te solid solution, as an example, and using the three band Kane model, we evaluated the best conditions for the highest thermoelectric performance in this material. Within the offered herein DT-FGTM approach, we fabricated the thermoelectric n-type Pb0.75Sn0.25Te1−xIx leg and measured its output energy characteristics. The efficiency of energy conversion for the prepared DT-FGTM leg reaches a very high value of ∼12.0% at temperature difference ΔT = 540 K. Furthermore, the thermal treatment of the fabricated leg should not injure the carrier concentration distribution through the leg, as the hot end of the leg is heavily doped, and the chemical diffusion between segments would be only beneficial. Our demonstration shows that the DT-FGTM approach has significant practical interest and can be utilized for the other TE materials.

Glass‐ceramic composites as insulation material for thermoelectric oxide multilayer generators

AbstractThermoelectric generators can be used as energy harvesters for sensor applications. Adapting the ceramic multilayer technology, their production can be highly automated. In such multilayer thermoelectric generators, the electrical insulation material, which separates the thermoelectric legs, is crucial for the performance of the device. The insulation material should be adapted to the thermoelectric regarding its averaged coefficient of thermal expansion α and its sintering temperature while maintaining a high resistivity.In this study, starting from theoretical calculations, a glass‐ceramic composite material adapted for multilayer generators from calcium manganate and calcium cobaltite is developed. The material is optimized towards an α of 11 × 10−6 K−1 (20–500°C), a sintering temperature of 900°C, and a high resistivity up to 800°C. Calculated and measured α are in good agreement. The chosen glass‐ceramic composite with 45 vol.% quartz has a resistivity of 1 × 107 Ωcm and an open porosity of &lt;3%. Sintered multilayer samples from tape‐cast thermoelectric oxides and screen‐printed insulation show only small reaction layers. It can be concluded that glass‐ceramic composites are a well‐suited material class for insulation layers as their physical properties can be tuned by varying glass composition or dispersion phases.