Realizing a 10 °C Cooling Effect in a Flexible Thermoelectric Cooler Using a Vortex Generator

High-Performance Planar Thin-Film Thermoelectric Cooler Based on Sputtered Nanocrystalline Bi2Te3/Bi0.5Sb1.5Te3 Thin Films for On-Chip Cooling

The development of high-performance thin-film thermoelectric coolers (TFTECs) that are compatible with standard integrated circuit processes and can reduce power consumption is critical to achieving large-scale applications. In this work, we fabricate a planar TFTEC based on nanocrystalline p-type Bi0.5Sb1.5Te3 and n-type Bi2Te3 thin films using magnetron sputtering, standard lithography, and postannealing processes. The power factors of the Bi0.5Sb1.5Te3 and Bi2Te3 thin films reach 3.63 and 4.28 mW/mK2, respectively, and the ZT values reach 0.82 and 0.93, which are comparable to those of bulk TE materials. The radial configuration of the device allows the cold-side thermal resistance to be increased and the hot-side thermal resistance to be decreased, thereby facilitating a substantial cooling temperature difference. Furthermore, the large in-plane contact area helps to reduce device resistance and power consumption. At a heating stage temperature of 360 K and a power consumption of 4.76 mW, the net cooling temperature difference of the TFTEC reaches 4 °C. The maximum temperature difference between the hot end and the cold end is 7.26 °C, while the cold end temperature remains below the ambient temperature. The high-performance planar TFTECs demonstrated in this work exhibit both a high net cooling performance and competitive fabrication cost, rendering them ideal for on-chip hotspot cooling.

Substrate-Free Inorganic-Based Films for Thermoelectric Applications

The development of highly integrated electronic components and the Internet of Things demands efficient thermal management and uninterrupted energy harvesting, which provides exciting opportunities for thermoelectric (TE) technology since it allows direct conversion between electricity and thermal energy. The improved output performance of TE devices has traditionally been driven by advancements in inorganic materials. Recently, there has been growing interest in studying substrate-free inorganic-based TE thin films because they provide improved adherence to curved surfaces and offer a more compact size compared to the corresponding rigid form of these materials. This review begins by summarizing various methods for fabricating freestanding inorganic-based TE films, including leveraging the intrinsic plasticity of certain materials, exfoliating layered-structure materials, using sacrificial substrates, and creating composites with flexible components such as polymers and carbon-based materials. A key challenge in achieving high device performance is determining how to maintain the favorable TE properties of inorganic materials. This can be addressed through strategies such as high inorganic content loading, multicomponent engineering, and interfacial structure design. The review also discusses the applications of substrate-free inorganic-based TE devices in both power generation and solid-state cooling. Finally, it outlines current challenges and proposes potential research directions to further advance the field.

Wearable Fluidic Fabric with Excellent Heat Transfer Performance for Sports Recovery

Rapid temperature contrast hydrotherapy by water immersion has been utilized by athletes for effective sports recovery. However, its application at some training or competition venues is limited by high water consumption, bucky size, personal hygiene, and inconvenience. Here, a novel portable system equipped with highly effective, lightweight, and hygienic wearable fluidic fabric device is reported, that replaces direct water immersion. The measured heat transfer coefficient between the skin and the fabric is 98.5 W m-2 K-1, which is 92% of that in direct water immersion at 10 °C and significantly higher than that by previously reported cooling garments. The core layer, a flexible heat transfer panel (FHTP), can switch between cold and hot modes (5-40 °C) over an area of 0.3 m2. The contact condition between skin and the deformable FHTP has been considered in a new verified heat transfer model. Optimization of the parameters has resulted in excellent heat transfer performance. This fluidic fabric also holds potential in diverse applications, such as enhancing thermal safety and comfort in extreme environments (e.g., personal thermal management systems and fire-protection suits), supporting cryotherapy and thermotherapy in rehabilitation and healthcare, and simulation of total tactile sensations in virtual reality.

Wearable Device with High Thermoelectric Performance and Long-Lasting Usability Based on Gel-Thermocells for Body Heat Harvesting

Utilizing the thermogalvanic effect, flexible thermoelectric materials present a compelling avenue for converting heat into electricity, especially in the context of wearable electronics. However, prolonged usage is hampered by the limitation imposed on the thermoelectric device's operational time due to the evaporation of moisture. Deep eutectic solvents (DESs) offer a promising solution for low-moisture gel fabrication. In this study, a bacterial cellulose (BC)/polyacrylic acid (PAA)/guanidinium chloride (GdmCl) gel is synthesized by incorporating BC into the DES. High-performance n-type and p-type thermocells (TECs) are developed by introducing Fe(ClO4)2/3 and K3/4Fe(CN)6, respectively. BC enhances the mechanical properties through the construction of an interpenetrating network structure. The coordination of carboxyl groups on PAA with Fe3+ and the crystallization induced by Gdm+ with [Fe(CN)6]4- remarkably improve the thermoelectric performance, achieving a Seebeck coefficient (S) of 2.4 mV K-1 and ion conductivity (σ) of 1.4 S m-1 for the n-type TEC, and ‒2.8 mV K-1 and 1.9 S m-1 for the p-type TEC. A flexible wearable thermoelectric device is fabricated with a S of 82 mV K-1 and it maintains a stable output over one month. This research broadens the application scope of DESs in the thermoelectric field and offers promising strategies for long-lasting wearable energy solutions.

Advancing flexible thermoelectrics for integrated electronics

With the increasing demand for energy and the climate challenges caused by the consumption of traditional fuels, there is an urgent need to accelerate the adoption of green and sustainable energy conversion and storage technologies. The integration of flexible thermoelectrics with other various energy conversion technologies plays a crucial role, enabling the conversion of multiple forms of energy such as temperature differentials, solar energy, mechanical force, and humidity into electricity. The development of these technologies lays the foundation for sustainable power solutions and promotes research progress in energy conversion. Given the complexity and rapid development of this field, this review provides a detailed overview of the progress of multifunctional integrated energy conversion and storage technologies based on thermoelectric conversion. The focus is on improving material performance, optimizing the design of integrated device structures, and achieving device flexibility to expand their application scenarios, particularly the integration and multi-functionalization of wearable energy conversion technologies. Additionally, we discuss the current development bottlenecks and future directions to facilitate the continuous advancement of this field.

Stretchable and Skin-Conformal Thermoelectric Generator with Highly Flexible and Plastically Bendable Silver Selenide Films

Among inorganic thermoelectric materials, flexible thermoelectric materials have attracted considerable attention. In this study, highly flexible and plastically bendable silver selenide films with excellent thermoelectric performance at room temperature are presented. The flexibility of the freestanding silver selenide films was significantly improved through a simple annealing treatment. The highly flexible silver selenide films with a thickness of 26.0 μm displayed outstanding n-type thermoelectric performance, achieving an in-plane zT value of 0.38 at room temperature. Because silver selenide films are plastically bendable with a bending radius of less than 1 mm, they can be shaped into various forms. To achieve stretchability and skin-conformality in the thermoelectric generator, S-shaped silver selenide strips were used as an n-type thermoelectric element. Effective harvesting of electricity from heat of the human body was successfully demonstrated.

Flexible Ag2Se Thermoelectric Films Enable the Multifunctional Thermal Perception in Electronic Skins

Skin is critical for shaping our interactions with the environment. The electronic skin (E-skin) has emerged as a promising interface for medical devices to replicate the functions of damaged skin. However, exploration of thermal perception, which is crucial for physiological sensing, has been limited. In this work, a multifunctional E-skin based on flexible thermoelectric Ag2Se films is proposed, which utilizes the Seebeck effect to replicate the sensory functions of natural skin. The E-skin can enable capabilities including temperature perception, tactile perception, contactless perception, and material recognition by analyzing the thermal conduction behaviors of various materials. To further validate the capabilities of constructed E-skins, a wearable device with multiple sensory channels was fabricated and tested for gesture recognition. This work highlights the potential for using flexible thermoelectric materials in advanced biomedical applications including health monitoring and smart prosthetics.

Hydraulic-driven adaptable morphing active-cooling elastomer with bioinspired bicontinuous phases

The active-cooling elastomer concept, originating from vascular thermoregulation for soft biological tissue, is expected to develop an effective heat dissipation method for human skin, flexible electronics, and soft robots due to the desired interface mechanical compliance. However, its low thermal conduction and poor adaptation limit its cooling effects. Inspired by the bone structure, this work reports a simple yet versatile method of fabricating arbitrary-geometry liquid metal skeleton-based elastomer with bicontinuous Gyroid-shaped phases, exhibiting high thermal conductivity (up to 27.1 W/mK) and stretchability (strain limit >600%). Enlightened by the vasodilation principle for blood flow regulation, we also establish a hydraulic-driven conformal morphing strategy for better thermoregulation by modulating the hydraulic pressure of channels to adapt the complicated shape with large surface roughness (even a concave body). The liquid metal active-cooling elastomer, integrated with the flexible thermoelectric device, is demonstrated with various applications in the soft gripper, thermal-energy harvesting, and head thermoregulation.

Flexible Active Peltier Coolers Based on Interconnected Magnetic Nanowire Networks

Thermoelectric energy conversion based on flexible materials has great potential for applications in the fields of low-power heat harvesting and solid-state cooling. Here, we show that three-dimensional networks of interconnected ferromagnetic metal nanowires embedded in a polymer film are effective flexible materials as active Peltier coolers. Thermocouples based on Co-Fe nanowires exhibit much higher power factors and thermal conductivities near room temperature than other existing flexible thermoelectric systems, with a power factor for Co-Fe nanowire-based thermocouples of about 4.7 mW/K2m at room temperature. The effective thermal conductance of our device can be strongly and rapidly increased by active Peltier-induced heat flow, especially for small temperature differences. Our investigation represents a significant advance in the fabrication of lightweight flexible thermoelectric devices, and it offers great potential for the dynamic thermal management of hot spots on complex surfaces.

Substrate-Free Thermoelectric 25 μm-Thick Ag2Se Films with High Flexibility and In-Plane zT of 0.5 at Room Temperature

Thermoelectric inorganic films are flexible when sufficiently thin. By removing the substrate, that is, making them free-standing, the flexibility of thermoelectric films can be enhanced to the utmost extent. However, studies on the flexibility of free-standing thermoelectric inorganic films have not yet been reported. Herein, the high thermoelectric performance and flexibility of free-standing thermoelectric Ag₂Se films are reported. Free-standing Ag₂Se films with a thickness of 25.0 ± 3.9 μm exhibited an in-plane zT of 0.514 ± 0.060 at room temperature. These films exhibited superior flexibility compared to Ag₂Se films constrained on a substrate. The flexibility of the Ag₂Se films was systematically investigated in terms of bending strain, bending radius, thickness, and elastic modulus. Using free-standing Ag₂Se films, a substrate-free, flexible thermoelectric generator was fabricated. The energy-harvesting capacity of the thermoelectric generator was also demonstrated.

Personal Cooling Garments: A Review

Thermal comfort is of critical importance to people during hot weather or harsh working conditions to reduce heat stress. Therefore, personal cooling garments (PCGs) is a promising technology that provides a sustainable solution to provide direct thermal regulation on the human body, while at the same time, effectively reduces energy consumption on whole-building cooling. This paper summarizes the current status of PCGs, and depending on the requirement of electric power supply, we divide the PCGs into two categories with systematic instruction on the cooling materials, working principles, and state-of-the-art research progress. Additionally, the application fields of different cooling strategies are presented. Current problems hindering the improvement of PCGs, and further development recommendations are highlighted, in the hope of fostering and widening the prospect of PCGs.