The Influence of Molecular Weights on Dispersion and Thermoelectric Performance of Alkoxy Side-Chain Polythiophene/Carbon Nanotube Composite Materials.

In the development of wearable electronic devices, the composite modification of conductive polymers and single-walled carbon nanotubes (SWCNTs) has become a burgeoning research area. This study presents the synthesis of a novel polythiophene derivative, poly(3-alkoxythiophene) (P3(TEG)T), with alkoxy side chains. Different molecular weight variants of P3(TEG)T (P1-P4) were prepared and combined with SWCNTs to form composite materials. Density functional theory (DFT) calculations revealed a reduced bandgap for P3(TEG)T. Raman spectroscopy demonstrated π-π interactions between P3(TEG)T and SWCNTs, facilitating the dispersion of single-walled carbon nanotubes and the formation of a continuous conductive network. Among the composite films, P4/SWCNTs-0.9 exhibited the highest thermoelectric performance, with a power factor (PF) value of 449.50 µW m-1 K-2. The fabricated flexible thermoelectric device achieved an output power of 3976.92 nW at 50 K, with a tensile strength of 59.34 MPa for P4/SWCNTs. Our findings highlight the strong interfacial interactions between P3(TEG)T and SWCNTs in the composite material, providing an effective charge transfer pathway. Furthermore, an improvement in the tensile performance was observed with an increase in the molecular weight of the polymer used in the composite, offering a viable platform for the development of high-performance flexible organic thermoelectric materials.

Grain Manipulation by Annealing Treatment Realizes High-Performance N-Type Bi2Te2.4Se0.6 Thermoelectric Material and Device.

Bi2Te3-based materials play a crucial role in solid cooling and power generation, but the rapidly deteriorated ZT with rising temperatures above 450 K severely limits further applications. Here, this paper reports a novel preparation method of annealing treatment for molten ingot, which can enhance the thermoelectric performance of n-type Bi2Te2.4Se0.6 in a wide temperature range. Instead of conventional halides, copper is adopted to regulate the carrier concentration and grain size to optimal levels. During the process of annealing at 573 K for 4 h, the number of twins significantly increases and the grains of Cu-doped samples become larger and more oriented. These optimizations lead to higher carrier mobility with similar carrier concentration compared with the sample without heat treatment. The synergistic effects of Cu doping and annealing treatment realize a high average ZT of 0.89 within 300-600 K in n-type Cu0.02Bi2Te2.4Se0.6. Combined with p-type (Bi,Sb)2Te3, the fabricated thermoelectric device exhibits a high conversion efficiency of 6.9% at a temperature difference of 300 K. This study suggests that annealing treatment is a simple and effective scheme to promote the applications of n-type Bi2(Te,Se)3 in a wide temperature range.

Enhancing Thermoelectric and Cooling Performance of Bi0.5Sb1.5Te3 through Ferroelectric Polarization in Flexible Ag/PZT/PVDF/Bi0.5Sb1.5Te3 Film.

Bi2Te3-based thin films are gaining recognition for their remarkable room temperature thermoelectric performance. Beyond the conventional "process-composition-performance" paradigm, it is highly desirable to explore new methods to enhance their performance further. Here, we designed a sandwich-structured Ag/PZT/PVDF/Bi0.5Sb1.5Te3(BST) thin film device and effectively regulated the performance of the BST film by controlling the polarization state of the PZT/PVDF layers. Results indicate that polarization induces interlayer charge redistribution and charge transfer between PZT/PVDF and BST, thereby achieving the continuous modulation of the electrical transport characteristics of BST films. Finally, following polarization at a saturation voltage of 3 kV, the power factor of the BST film increased by 13% compared to the unpolarized condition, reaching 20.8 µW cm-1 K-2. Furthermore, a device with 7 pairs of P-N legs was fabricated, achieving a cooling temperature difference of 11.0 K and a net cooling temperature difference of 2.4 K at a current of 10 mA after the saturation polarization of the PZT/PVDF layer. This work reveals the critical effect of introducing ferroelectric layer polarization to achieve excellent thermoelectric performance of the BST film.

Localized Surface Doping Induced Ultralow Contact Resistance between Metal and (Bi,Sb)2Te3 Thermoelectric Films.

Micro thermoelectric devices are expected to further improve the cooling density for the temperature control of electronic devices; nevertheless, the high contact resistivity between metals and semiconductors critically limits their applications, especially in chip cooling with extremely high heat flux. Herein, based on the calculated results, a low specific contact resistivity of ∼10-7 Ω cm2 at the interface is required to guarantee a desirable cooling power density of micro devices. Thus, we developed a generally applicable interfacial modulation strategy via localized surface doping of thermoelectric films, and the feasibility of such a doping approach for both n/p-type (Bi,Sb)2Te3 films was demonstrated, which can effectively increase the surface-majority carrier concentration explained by the charge transfer mechanism. With a proper doping level, ultralow specific contact resistivities at the interfaces are obtained for n-type (6.71 × 10-8 Ω cm2) and p-type (3.70 × 10-7 Ω cm2) (Bi,Sb)2Te3 layers, respectively, which is mainly attributed to the carrier tunneling enhancement with a narrowed interfacial contact barrier width. This work provides an effective scheme to further reduce the internal resistance of micro thermoelectric coolers, which can also be extended as a kind of universal interfacial modification technique for micro semiconductor devices.

Long-Lasting Heat Dissipation of Flexible Heat Sinks for Wearable Thermoelectric Devices.

Flexible wearable thermoelectric (TE) devices hold great promise for a wide range of applications in human thermal management and self-powered systems. Currently, the main challenge faced by flexible TE devices is the inadequate dissipation of heat, which hinders the maintenance of significant temperature differences over prolonged periods. Most existing heat sinks, being rigid in nature, compromise the overall flexibility of the device. Therefore, the challenge lies in maintaining device flexibility while ensuring effective heat dissipation. In this study, we developed a flexible phase-change material (FPCM) heat sink to address this issue and enhance the heat dissipation capabilities of TE devices (FPCM-TED). When used as a thermoelectric cooler (TEC), the FPCM heat sink efficiently absorbs heat from the hot end, enabling long-lasting and high-performance cooling of the TEC. This capability effectively reduces body temperature by up to 11.21 °C and can be sustained for at least 300 s. Additionally, when employed as a thermoelectric generator (TEG), the FPCM absorbs heat at the cold end, thereby increasing the temperature difference between the hot and cold ends and enhancing the output performance of the device. By integrating FPCM-TED into a fabric wristband, we successfully developed a self-powered wireless pedometer sensing system. This breakthrough lays a solid foundation for the application of wearable, smart clothing.

High-Performance Thermoelectric Fibers from Metal-Backboned Polymers for Body-Temperature Wearable Power Devices.

Metal-backboned polymers (MBPs), with a unique backbone consisting of bonded metal atoms, are promising for optic, electric, magnetic, and thermoelectric fields. However, the application of MBP remains relatively understudied. Here, we develop a shear-induced orientation method to construct a flexible nickel-backboned polymer/carbon nanotube (NBP/CNT) thermoelectric composite fiber. It demonstrated a power factor of 719.48 µW ⋅m-1 K-2, which is ca. 3.5 times as high as the bare CNT fiber. Remarkably, with the regulation of carrier mobility and carrier concentration of NBP, the composite fiber further showed simultaneous increases in electrical conductivity and Seebeck coefficient in comparison to the bare CNT fiber. The NBP/CNT fiber can be integrated into fabrics to harvest thermal energy of human body to generate an output voltage of 3.09 mV at a temperature difference of 8 K. This research opens a new avenue for the development of MBPs in power supply.

Passive Self-Sustained Thermoelectric Devices Powering the 24 h Wireless Transmission via Radiation-Cooling and Selective Photothermal Conversion.

The rapid development of the Internet of Things has triggered a huge demand for self-sustained technology that can provide a continuous electricity supply for low-power electronics. Here, a self-sustained power supply solution is demonstrated that can produce a 24 h continuous and unipolar electricity output based on thermoelectric devices by harvesting the environmental temperature difference, which is ingeniously established utilizing radiation cooling and selective photothermal conversion. The developed prototype system can stably maintain a large temperature difference of about 1.8 K for a full day despite the real-time changes in environmental temperature and solar radiation, thereby driving continuous electricity output using the built-in thermoelectric device. Specifically, the large output voltage of >102 mV and the power density of >4.4 mW m-2 could be achieved for a full day, which are outstanding among the 24 h self-sustained thermoelectric devices and far higher than the start-up values of the wireless temperature sensor and also the light-emitting diode, enabling the 24 h remote data transmission and lighting, respectively. This work highlights the application prospects of self-sustained thermoelectric devices for low-power electronics.

The Failure Mechanism of Micro Thermoelectric Devices under the Action of the Temperature Field.

The micro thermoelectric device (m-TED) boasts features such as adjustable volume, straightforward structure, and precise, rapid temperature control, positioning it as the only current solution for managing the temperature of microelectronic systems. It is extensively utilized in 5G optical modules, laser lidars, and infrared detection. Nevertheless, as the size of the m-TED diminishes, the growing proportion of interface damages the device's operational reliability, constraining the advancement of the m-TED. In this study, we used commercially available bismuth telluride materials to construct the m-TED. The device's reliability was tested under various temperatures: -40, 85, 125, and 150 °C. By deconstructing and analyzing the devices that failed during the tests, we discovered that the primary cause of device failure was the degradation of the solder layer. Moreover, we demonstrated that encapsulating the device with polydimethylsiloxane (PDMS) could effectively delay the deterioration of its performance. This study sparks new insights into the service reliability of m-TEDs and paves the way for further optimizing device interface design and enhancing the device manufacturing process.

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.

Programmable and Surface-Conformable Origami Design for Thermoelectric Devices.

Thermoelectric devices (TEDs) show great potential for waste heat energy recycling and sensing. However, existing TEDs cannot be self-adapted to the complex quadratic surface, leading to significant heat loss and restricting their working scenario. Here, surface-conformable origami-TEDs (o-TEGs) are developed through programmable crease-designed origami substrates and the screen-printing TE legs. Compared with "π" structured TEDs, the origami design (with heat conductive materials) changed the heat-transferring direction of the laminated TE legs, resulting in an enhancement in enlarging ΔT/THot and Vout by 5.02 and 3.51 times. Four o-TEDs with different creases designs are fabricated to verify the heat recycling ability on plane and central quadratic surfaces. Demonstrating a high Vout density (up to 0.98 -2 at ΔT of 50 K) and good surface conformability, o-TEDs are further used in thermal touch panels attached to multiple surfaces, allowing information to be wirelessly transferred on a remote display via finger-writing.

Gibbs Adsorption and Zener Pinning Enable Mechanically Robust High-Performance Bi2 Te3 -Based Thermoelectric Devices.

Bi2 Te3 -based alloys have great market demand in miniaturized thermoelectric (TE) devices for solid-state refrigeration and power generation. However, their poor mechanical properties increase the fabrication cost and decrease the service durability. Here, this work reports on strengthened mechanical robustness in Bi2 Te3 -based alloys due to thermodynamic Gibbs adsorption and kinetic Zener pinning at grain boundaries enabled by MgB2 decomposition. These effects result in much-refined grain size and twofold enhancement of the compressive strength and Vickers hardness in (Bi0.5 Sb1.5 Te3 )0.97 (MgB2 )0.03 compared with that of traditional powder-metallurgy-derived Bi0.5 Sb1.5 Te3 . High mechanical properties enable excellent cutting machinability in the MgB2 -added samples, showing no missing corners or cracks. Moreover, adding MgB2 facilitates the simultaneous optimization of electron and phonon transport for enhancing the TE figure of merit (ZT). By further optimizing the Bi/Sb ratio, the sample (Bi0.4 Sb1.6 Te3 )0.97 (MgB2 )0.03 shows a maximum ZT of ≈1.3 at 350 K and an average ZT of 1.1 within 300-473 K. As a consequence, robust TE devices with an energy conversion efficiency of 4.2% at a temperature difference of 215 K are fabricated. This work paves a new way for enhancing the machinability and durability of TE materials, which is especially promising for miniature devices.

Enhanced Contact Performance and Thermal Tolerance of Ni/Bi2Te3 Joints for Bi2Te3-Based Thermoelectric Devices.

Ni metal has been widely used as a barrier layer in Bi2Te3-based thermoelectric devices, which establishes stable joints to link Bi2Te3-based legs and electrodes. However, the Ni/Bi2Te3 joints become very fragile when the devices were exposed to high temperature, causing severe performance deterioration and even device failure. Herein, stable Ni/Bi2Te3 joints have been established by arc spraying of the Ni barrier layer on the Bi2Te3-based alloys. The interface microstructure and contact performance including the bonding strength and contact resistivity of the arc-sprayed Ni/Bi2Te3 joints are investigated. The results indicate that, as compared with traditional Ni/Bi2Te3 joints, the arc-sprayed Ni/Bi2Te3 joints have comparably low contact resistivity while possessing a 50% higher bonding strength. Aging the joints as an exposure to high-temperature circumstances, the arc-sprayed Ni/Bi2Te3 joints exhibit much better tolerance to the thermal shock with stable bonding strength and contact resistivity. The enhanced interfacial contact performance and thermal tolerance should be attributed to the thick Ni barrier layer and interface reaction layer with good Ohmic contact. This work provides an effective strategy to establish stable joints for the Bi2Te3-based thermoelectric devices with improved thermal stability.

Highly Stable Metal─Na0.02Pb0.98Te Contacts for Medium Temperature Thermoelectric Devices.

In the medium temperature (600-850 K) range, Na0.02Pb0.98Te is a highly efficient p-type thermoelectric compound. Device fabrication utilizing this compound for power generation demands highly stable low-contact resistance contacts with metal electrodes. This work investigates the microstructural, electrical, mechanical, and thermochemical stability of Na0.02Pb0.98Te-metal (Ni, Fe, and Co) contacts made by a one-step vacuum hot pressing process. Direct contact mostly resulted in either an interface with poor mechanical integrity, as in Co and Fe, or poisoning of the TE compound, as in the case of Ni, which results in high specific contact resistance (rc). In Ni and Co, adding a SnTe interlayer lowers the rc and strengthens the contact. It does not, however, effectively stop Ni from diffusing into Na0.02Pb0.98Te. The bonding is poor in the Fe/SnTe/Na0.02Pb0.98Te contacts due to the absence of any reaction at the Fe/SnTe interface. A composite buffer layer Co + 75 vol % SnTe with SnTe improves the mechanical stability of the Co contact with moderately lesser rc than pure SnTe alone. However, a similar approach with Fe does not yield stable contact. The Co/Co + 75 vol % SnTe/SnTe/Na0.02Pb0.98Te contact exhibits rc less than 50 µΩ cm2 and has good microstructural and mechanical stability after annealing at 723 K for 170 h.

A low-cost device for cryoanesthesia of neonatal rodents.

Studying the development of neural circuits in rodent models requires surgical access to the neonatal brain. Since commercially available stereotaxic and anesthetic equipment is designed for use in adults, reliable targeting of brain structures in such young animals can be challenging. Hypothermic cooling (cryoanesthesia) has been used as a preferred anesthesia approach in neonates. This commonly involves submerging neonates in ice, an approach that is poorly controllable. We have developed an affordable, simple to construct device - CryoPup - that allows for fast and robust cryoanesthesia of rodent pups. CryoPup consists of a microcontroller controlling a Peltier element and a heat exchanger. It is capable of both cooling and heating, thereby also functioning as a heating pad during recovery. Importantly, it has been designed for size compatibility with common stereotaxic frames. We validate CryoPup in neonatal mice, demonstrating that it allows for rapid, reliable and safe cryoanesthesia and subsequent recovery. This open-source device will facilitate future studies into the development of neural circuits in the postnatal brain.

High-Performance Skutterudite/Half-Heusler Cascaded Thermoelectric Module Using the Transient Liquid Phase Sintering Joining Technique.

Thermoelectric (TE) materials have made rapid advancement in the past decade, paving the pathway toward the design of solid-state waste heat recovery systems. The next requirement in the design process is realization of full-scale multistage TE devices in the medium to high temperature range for enhanced power generation. Here, we report the design and manufacturing of full-scale skutterudite (SKD)/half-Heusler (hH) cascaded TE devices with 49-couple TE legs for each stage. The automated pick-and-place tool is employed for module fabrication providing overall high manufacturing process efficiency and repeatability. Optimized Ti/Ni/Au coating layers are developed for metallization as the diffusion barrier and electrode contact layers. The Cu-Sn transient liquid phase sintering technique is utilized for SKD and hH stages, which provides a high strength bonding and very low contact resistance. A remarkably high output power of 38.3 W with a device power density of 2.8 W·cm-2 at a temperature gradient of 513 °C is achieved. These results provide an avenue for widespread utilization of TE technology in waste heat recovery applications.

Enhancement of thermoelectric power factor via electron energy filtering in Cu doped MoS2 on carbon fabric for wearable thermoelectric generator applications.

The design and construction of state-of-the-art wearable thermoelectric materials are important for the development of self-powered wearable thermoelectric generators (WTEGs). Molybdenum disulfide (MoS2) has been reported as a noteworthy thermoelectric (TE) material because of its large intrinsic bandgap and high carrier mobility. In this work, Cu-doped two-dimensional layered MoS2 nanosheets were grown on carbon fabric (CF) via a hydrothermal method. The electrical conductivity, Seebeck coefficient, and power factor for the Cu-doped MoS2 were found to increase with increasing temperature. The maximum Seebeck coefficient was obtained for a MoS2 sample doped with 4 at% of Cu (CM4) was ∼10 µV/K at 303 K and ∼13 µV/K at 373 K. The enhancement in the Seebeck coefficient was attributed to an energy-filtering effect caused by the interfacial barrier between MoS2 and Cu. In addition, a thermoelectric device was designed with four pairs of TE materials, where CM4 (4 at%) was used as a p-type material and Cu wire was used as an n-type material. These p- and n-type materials were connected electrically in series and thermally in parallel to generate a voltage of 190.7 µV at a temperature gradient of 8 K.

Bonding Performance of Ag-Cu Brazing Solders and Half-Heusler Alloys for High-Performance Thermoelectric Generators.

Due to the uncertainty of the brazing solder composition and its unknown effect on the long-term stability of the interface, the brazing interface connection process for half-Heusler (hH) thermoelectric (TE) devices is still partially concealed and incomplete. In this work, we selected different types of Ag-Cu-based brazing solders with different Ag and Cu contents to assemble hH TE devices, observed the microstructure of the interface contact, and analyzed its formation mechanism. It is found that when the Cu element in the brazing solder is high, it tends to form an intermetallic compound (IMC) layer at the interface, which threatens the life of the device. On the contrary, when the content of the Ag element is high, the formation of the IMC layer will be avoided. Then, the long-term stability of the interface brazed by Ag72Cu28 with high Ag content was verified: the interface connection showed good contact resistivity stability and mechanical reliability; the fabricated uni-couple TE module achieved a maximum output power of 0.28 W and a maximum conversion efficiency of 7.34% at a temperature difference of 538 K. This work summarizes the selection principle of Cu-Ag-based brazing solder when assembling hH TE modules and verifies the long-term stability of the brazed connection interface. The experiment results can provide a reference for the actual fabrication of hH TE devices.

Cement-Based Thermoelectric Device for Protection of Carbon Steel in Alkaline Chloride Solution.

The thermoelectric cement-based materials can convert heat into electricity; this makes them promising candidates for impressed current cathodic protection of carbon steel. However, attempts to use the thermoelectric cement-based materials for energy conversion usually results in low conversion efficiency, because of the low electrical conductivity and Seebeck coefficient. Herein, we deposited polyaniline on the surface of MnO2 and fabricated a cement-based thermoelectric device with added PANI/MnO2 composite for the protection of carbon steel in alkaline chloride solution. The nanorod structure (70~80 nm in diameter) and evenly dispersed conductive PANI provide the PANI/MnO2 composite with good electrical conductivity (1.9 ± 0.03 S/cm) and Seebeck coefficient (-7.71 × 103 ± 50 µV/K) and, thereby, increase the Seebeck coefficient of cement-based materials to -2.02 × 103 ± 40 µV/K and the electrical conductivity of cement-based materials to 0.015 ± 0.0003 S/cm. Based on this, the corrosion of the carbon steel was delayed after cathodic protection, which was demonstrated by the electrochemical experiment results, such as the increased resistance of the carbon steel surface from 5.16 × 102 Ω·cm2 to 5.14 × 104 Ω·cm2, increased charge transfer resistance from 11.4 kΩ·cm2 to 1.98 × 106 kΩ·cm2, and the decreased corrosion current density from 1.67 µA/cm2 to 0.32 µA/cm2, underlining the role of anti-corrosion of the PANI/MnO2 composite in the cathodic protection system.

Variable Range Hopping Model Based on Gaussian Disordered Organic Semiconductor for Seebeck Effect in Thermoelectric Device.

We investigate the carrier concentration dependent Seebeck coefficient in Gaussian disordered organic semiconductors (GD-OSs) for thermoelectric device applications. Based on the variable-range hopping (VRH) theory, a general model predicting the Seebeck effect is developed to reveal the thermoelectric properties in GD-OSs. The proposed model could interpret the experimental data on carrier concentration- and temperature-dependence of the Seebeck coefficient, including various kinds of conducting polymer film and small molecule based field-effect transistors (FETs). Compared with the conventional Mott's VRH and mobility edge model, our model has a much better description of the relationship between the Seebeck coefficient and conductivity. The model could deepen our insight into charge transport in organic semiconductors and provide instructions for the optimization of thermoelectric device performance in a disordered system.

Fabrication and Excellent Performances of Bismuth Telluride-Based Thermoelectric Devices.

The barrier layer between thermoelectric (TE) legs and electrodes has crucial impact on the electrothermal conversion efficiency of the TE device; however, the interfacial reaction of the Ni metal barrier layer with TE legs in traditional Bi2Te3-based devices is harmful to the device performance. Herein, a high-quality barrier layer of a Ni-based alloy has been fabricated on both n-type and p-type Bi2Te3-based TE legs by the electroplating method. The in situ XRD results indicate that the as-prepared Bi2Te3-based TE legs with a Ni-based alloy barrier layer remain stable even at 300 °C. The high-resolution high-angle annular dark field scanning transmission electron microscopy images reveal that the Ni-based alloy barrier layer has more excellent stability than that of the Ni metal barrier layer. The Bi2Te3-based TE devices with excellent structural and performance stabilities were assembled with the as-grown high-performance n-type and p-type Bi2Te3-based leg with a Ni-based alloy barrier layer, which have lower internal resistance and higher cooling and power generation performances. A maximum cooling temperature difference over 65 K and a maximum cooling capacity of 55 W were obtained for the high-performance Bi2Te3-based TE devices. This work provides a new strategy for high-temperature applications of commercial Bi2Te3-based TE devices.