Graphene-Based Adaptive Thermal Camouflage

In nature, adaptive coloration has been effectively utilized for concealment and signaling. Various biological mechanisms have evolved to tune the reflectivity for visible and ultraviolet light. These examples inspire many artificial systems for mimicking adaptive coloration to match the visual appearance to their surroundings. Thermal camouflage, however, has been an outstanding challenge which requires an ability to control the emitted thermal radiation from the surface. Here we report a new class of active thermal surfaces capable of efficient real-time electrical-control of thermal emission over the full infrared (IR) spectrum without changing the temperature of the surface. Our approach relies on electro-modulation of IR absorptivity and emissivity of multilayer graphene via reversible intercalation of nonvolatile ionic liquids. The demonstrated devices are light (30 g/m²), thin (<50 μm), and ultraflexible, which can conformably coat their environment. In addition, by combining active thermal surfaces with a feedback mechanism, we demonstrate realization of an adaptive thermal camouflage system which can reconfigure its thermal appearance and blend itself with the varying thermal background in a few seconds. Furthermore, we show that these devices can disguise hot objects as cold and cold ones as hot in a thermal imaging system. We anticipate that, the electrical control of thermal radiation would impact on a variety of new technologies ranging from adaptive IR optics to heat management for outer space applications.

Interlaced, Nanostructured Interface with Graphene Buffer Layer Reduces Thermal Boundary Resistance in Nano/Microelectronic Systems

Improving heat transfer in hybrid nano/microelectronic systems is a challenge, mainly due to the high thermal boundary resistance (TBR) across the interface. Herein, we focus on gallium nitride (GaN)/diamond interface-as a model system with various high power, high temperature, and optoelectronic applications-and perform extensive reverse nonequilibrium molecular dynamics simulations, decoding the interplay between the pillar length, size, shape, hierarchy, density, arrangement, system size, and the interfacial heat transfer mechanisms to substantially reduce TBR in GaN-on-diamond devices. We found that changing the conventional planar interface to nanoengineered, interlaced architecture with optimal geometry results in >80% reduction in TBR. Moreover, introduction of conformal graphene buffer layer further reduces the TBR by ∼33%. Our findings demonstrate that the enhanced generation of intermediate frequency phonons activates the dominant group velocities, resulting in reduced TBR. This work has important implications on experimental studies, opening up a new space for engineering hybrid nano/microelectronics.

Capillary-Flow-Optimized Heat Localization Induced by an Air-Enclosed Three-Dimensional Hierarchical Network for Elevated Solar Evaporation

Solar evaporation is a cost-effective way for obtaining clean water using renewable energy. However, many solar evaporation devices still show unsatisfactory performance and suffer from inefficient utilization of absorbed solar energy. Herein, numerical simulations of solar evaporation demonstrate that the heat management is a key factor governing the solar evaporation efficiency. This prediction is confirmed through using a bilayered solar steam generation architecture [hollow glass microsphere-carbon black (HS-CB)] both in laboratory- and pilot-scale studies. The HS-CB consists of a CB film as a solar-thermal conversion layer and three-dimensional hierarchical polyvinylidene fluoride skeleton cross-linking HSs as a heat localization and water-transporting layer. A balance between thermal insulation and capillary-driven water transport can be reached by tuning the porosity of the thermal-insulating layer, thus inducing optimized heat localization. The proposed structure evaporates water with an efficiency of 82.1% under 1 sun irradiance (1 kW m^-2) in the laboratory and can even stably produce 4.63 L m^-2 d^-1 (average efficiency of 37%) of purified water from highly concentrated industrial waste water in the pilot study, demonstrating its promising potential for applications in seawater desalination and brackish water purification.

Recyclable and Flexible Dual-Mode Electronics with Light and Heat Management

Realizing multiple functions and sustainable manufacturing within the same electronic device would be highly attractive from a design and fabrication perspective. Here we demonstrate a recyclable dual-mode thin-film device that can perform both light emission and heat management simultaneously. The device is composed of a dissolvable emitting layer sandwiched between two undissolvable conducting films. The vertical multilayered device enables a highly flexible and foldable multicolor electroluminescent emission ranging from yellow or blue to white, and the coplanar monolayered conductor achieves tunable Joule heat temperature setting. By utilizing selective dissolution and artificial reconstruction of each layered component, the parent device shows full recyclability and reconstructability without severe performance degradation after several recycles. The proof-of concept device provides an ideal strategy to construct a multifunctional film system with recyclability and makes a significant contribution to scientific and technological advancement in low-cost sustainable electronics and optoelectronics.

Study on the Heat Reduction Effect of Biomimetic Unidirectional Transporting Channels Inspired by Nepenthes alata

Heat control has been a momentous problem in engineering areas which include manufacturing, aeronautics, microchips and so forth for a considerable amount of time. The control of material for thermal deformation, effective cooling are the key components of the aero crafts and compactly laid out microchips are urgently needed for improvement. In a micro-scale, researchers are mainly focused on the mechanism, design, improvement and heat transfer of straight channels other than developing other types of channels. A previous study on the carnivorous plant, Nepenthes alata, indicates that the water can be transported continuously and directionally on the surface of the rim of the pitcher because of its multi-scale structures. Meanwhile, the transporting speed is much higher than what was thought previously. Inspired by this unique phenomenon, the heat management ability of this biological micro channel is investigated in this research. Firstly, based on existing studies, the features of the biological channels are extracted. Then, the unidirectional channels are designed and fabricated by elliptical vibration cutting accordingly. The experimental platform for thermal control was established consequently. Both bio-inspired and straight triangular channels of the same depth and width were set for comparison. Through the comparative experiments, it is concluded preliminarily that the critical point of heat transfer performance of the two channels exists, and the biomimetic structure can improve and strengthen the cooling effects at a large flow rate because of the unique geometric structure. The temperature reduction of the bio-inspired channels can be increased by up to 84 percent compared with straight channels in a single experiment when heated up to 150 centigrade.

Thermal transport in monolayer zinc-sulfide: effects of length, temperature and vacancy defects

Of late, atomically thin two-dimensional zinc-sulfide (2D-ZnS) shows great potential for advanced nanodevices and as a substitute to graphene and transition metal di-chalcogenides owing to its exceptional optical and electronic properties. However, the functional performance of nanodevices significantly depends on the effective heat management of the system. In this paper, we explored the thermal transport properties of 2D-ZnS through molecular dynamics simulations. The impact of length, temperature, and vacancy defects on the thermal properties of 2D-ZnS are systematically investigated. We found that the thermal conductivity (TC) rises monotonically with increasing sheet length, and the bulk TC of ∼30.67 W mK^-1is explored for an infinite length ZnS. Beyond room temperature (300 K), the TC differs from the usual 1/Trule and displays an abnormal, slowly declining behavior. The point vacancy (PV) shows the largest decrease in TC compared to the bi vacancy (BV) defects. We calculated phonon modes for various lengths, temperatures, and vacancies to elucidate the TC variation. Conversely, quantum corrections are used to avoid phonon modes' icing effects on the TC at low temperatures. The obtained phonon density of states (PDOS) shows a softening and shrinking nature with increasing temperature, which is responsible for the anomaly in the TC at high temperatures. Owing to the increase of vacancy concentration, the PDOS peaks exhibit a decrease for both types of defects. Moreover, the variation of the specific heat capacity and entropy with BV and PV signify our findings of 2D-ZnS TC at diverse concentrations along with the different forms of vacancies. The results elucidated in this study will be a guide for efficient heat management of ZnS-based optoelectronic and nano-electronic devices.

Three-Dimensional Spiral Evaporator with Side Channels for Efficient Solar-Driven Water Purification

Solar evaporators have the advantages of not consuming fossil fuels, being environmentally friendly, and nonpolluting, offering a promising sustainable method to obtain fresh water and alleviate the worldwide freshwater shortage crisis. In this work, we report that high-performance solar evaporators can be facilely fabricated by processing a cost-effective polypyrrole (PPy)-coated nonwoven fabric (PCNF) into a three-dimensional (3D) spiral structure and introducing side channels for vapor escape. The coated PPy layer ensures excellent photothermal properties and the chemical stability of the evaporator. Meanwhile, the as-created spiral structure of the evaporator can significantly increase the effective evaporation area and harvest energy from the environment, greatly stimulating the evaporation. The side opening channels can effectively facilitate the escape of vapor generated inside the 3D spiral structure, avoid the internal vapor accumulation, and ultimately promote the evaporation of the inner surface, leading to a boost of the evaporation performance. Combining these features, the resulting evaporator exhibits an ultrahigh evaporation rate of 3.26 kg m-2 h-1 and an energy efficiency of 138% under 1-sun irradiation. More importantly, we show that this evaporator can also be used to collect fresh water from soil and sand, demonstrating its great applicability for obtaining potable water in arid areas.

Ferroelectric Polarization Modulated Thermal Conductivity in Barium Titanate Ceramic

Thermal conductivity k dominates in a heat transfer medium, and a field modulated k would facilitate delicate control in thermal management technology, yet it is hardly realized in a single solid material unless with changing temperature. Herein, in BaTiO3 ceramic, a modulated k was discovered by adjusting ferroelectric polarization P, which was a conventional strategy in ferroelectric functional materials. Four different states (P1, P2, P3, P4) were obtained by controlling poling time and field strength, showing that k leaped from 2.704 ± 0.054 to 3.201 ± 0.070 W (m K)-1 with increased P. Moreover, the strong correlation between P and k was also verified by the thermal depolarization measurement from room temperature to Curie temperature. The underlying origin of P modulated k was attributed to the internal bias field, which is born in the oriented ferroelectric domains, tightening special phonon modes in BaTiO3 ceramics. Raman spectrum, P-E loops, first-order reversible curve, XRD analysis, and PFM measurement were then employed to clarify how ferroelectric polarization structurally influences phonon transport and subsequent thermal conductivity. This work will pave a brand-new research route for conventional ferroelectric ceramic, also potentiating the idea of the electric field-controlled k component and active solid heat-transport device in the future.

Passive Daytime Cooling Foils for Everyone: A Scalable Lamination Process Based on Upcycling Aluminum-Coated Chips Bags

The increasing energy demand for space cooling and environmental pollution caused by post-consumer plastic waste are two of the most challenging issues today. Passive daytime cooling, which dissipates heat to outer space without external energy input, has emerged recently as a sustainable technique for space cooling. In this work, a plastic waste-based passive daytime cooling foil is reported to alleviate both issues simultaneously. The mirror-like aluminum-plastic laminate (APL) waste exhibits a satisfactory solar reflectance of 85.7%. Combining the APL waste with a laminated pouch foil reveals a remarkably simple but effective plastic waste-based cooling foil with a high emissivity of 0.87 in the atmospheric window, resulting in a compelling daytime cooling performance. The sustainable aluminum-plastic laminate waste-based cooling foil is flexible, easily scalable, low-cost, and fabricated with a common laminator. This makes the fabrication of passive cooling materials possible even for nonexperts, which will help to provide advanced sun shelters and comfortable temperatures to a wider community.

Advanced Temperature Design for Dynamic Performance Enhancement of PEMFCs Under High Current Density (HCD)

The dynamic performance of proton exchange membrane fuel cells (PEMFCs) under high current density (HCD) rapid loading is crucial for commercialization. This study introduces an advanced temperature difference (TD) design featuring an in-plane temperature gradient. By reconstructing cooling channels, optimal temperature distribution across the upstream, midstream, and downstream regions achieves balanced water-gas-heat conditions, enhancing the dynamic response of PEMFCs under HCD loading. Various TD designs are investigated across a broad humidity range, innovatively focusing on key moments involving load initiation, transient voltage minimum (TVM), and steady-state voltage (SSV). Comprehensive evaluations encompassing voltage response and energy consumption assess TD enhancements, while electrochemical impedance spectroscopy (EIS) and local current density monitoring further elucidate underlying mechanisms. Results show the positive temperature difference (PTD) design enhances hydration upstream and mitigates flooding downstream under low-humidity conditions. Conversely, the negative temperature difference (NTD) design tends to dehydration upstream and flooding downstream. At RH = 35%, the PTD design increases TVM by 18.2%, decreases voltage undershoot (VU) by 12.5%, raises SSV by 5.67%, and enhances electricity output by 7%. As humidity increases, the positive effect of the PTD design gradually weakens, though it still benefits the current density distribution uniformity.

Understanding and perceiving heat stress risk control: Critical insights from agriculture workers

Software-driven wearable technologies are emerging as a control for heat-related illnesses. Such devices collect biometric data and estimate risk noninvasively. However, little is known about workplace implementation strategies and stakeholder acceptance of the devices. As part of a mixed-methods pilot study to evaluate the feasibility of wearable technologies, the authors invited six agricultural workers with no device experience to participate in a semi-structured focus group, after wearing two devices (e.g., MākuSafe, Des Moines, IA, United States; SlateSafety, V2, Atlanta, GA, United States) for a standard work week. The focus group was separated into two parts: the first assessed the overall understanding of heat stress, and the second captured workers' perceptions of the wearable technologies. For each topic, the authors extracted relevant themes that describe farm workers' general understanding of heat hazards and worker interaction with wearable technology used in heat-related risk. These themes provide relevant answers to the questions outlined in the semi-structured questionnaire that can guide future research into the use of these devices in occupational settings. Wearable technologies continue to be used to control heat-related illnesses. Therefore, it is critically important to gather key strategies for employer implementation and user-interface considerations.

Modulation of the Structure-function Relationship of the "nano-greenhouse effect" towards Optimized Supra-photothermal Catalysis

Photothermal catalytic CO2 hydrogenation holds great promise for relieving the global environment and energy crises. The "nano-greenhouse effect" has been recognized as a crucial strategy for improving the heat management capabilities of a photothermal catalyst by ameliorating the convective and radiative heat losses. Yet it remains unclear to what degree the respective heat transfer and mass transport efficiencies depend on the specific structures. Herein, the structure-function relationship of the "nano-greenhouse effect" was investigated and optimized in a prototypical Ni@SiO2 core-shell catalyst towards photothermal CO2 catalysis. Experimental and theoretical results indicate that modulation of the thickness and porosity of the SiO2 nanoshell leads to variations in both heat preservation and mass transport properties. This work deepens the understandings on the contributing factor of the "nano-greenhouse effect" towards enhanced photothermal conversion. It also provides insights on the design principles of an ideal photothermal catalyst in balancing heat management and mass transport processes.