With the continuous growth of societal demands for high-performance electronics and communication technologies, along with the decreasing size of electronic components, the power density of electronic components has been continuously rising. This places higher demands on thermal management strategies for electronic components. Passive thermal management technologies have attracted increasing interest due to their zero energy consumption, higher compactness, and lower maintenance costs. Solid-liquid phase change materials (PCMs) are the most commonly used materials in existing passive thermal management strategies for electronic components. Despite extensive efforts by scholars at home and abroad to improve the performance of PCMs, low phase change enthalpy (usually below 200 J/gPCM) remains a fundamental limitation. The liquid-vapor phase change of water (i.e., desorption cooling) offers a new avenue for passive thermal management due to its extremely high phase change enthalpy (approximately 2400 J/g), inspired by the natural process of mammals regulating their body temperature through sweating.Currently, this technology primarily relies on hydrogels and MOFs materials. Specifically, hygroscopic adsorbents (hydrogels or MOFs) provide a highly porous structure that can capture moisture from the atmosphere. The water stored in the adsorbent can evaporate during peak periods of electronic device operation, extracting a significant amount of heat to prevent device overheating (i.e., desorption cooling). During non-peak periods, hygroscopic adsorbents can spontaneously absorb moisture from the surrounding atmosphere to restore their cooling capacity (i.e., self-regeneration). However, hydrogels (～10-12-10-11) and MOFs (～10-13-10-11) have relatively low mass diffusion coefficients, resulting in extremely slow regeneration rates. Generally, passive thermal management strategies based on hydrogels need to rely on actively replenishing water to maintain high cooling capacity, while the high cost of MOFs (over 10,000 USD/kg) limits their large-scale application. In contrast, hygroscopic inorganic salts, such as lithium chloride and lithium bromide, are expected to solve these issues due to their extremely high hygroscopic capacity and low cost.
Recently, Associate Professor Wu Wei from City University of Hong Kong collaborated with Professor Yang Ronggui from Huazhong University of Science and Technology to propose a passive thermal management technology based on the evaporation process of moisture in hygroscopic salt solutions to suppress the temperature rise of electronic components.In this study, the moisture desorption process in low-cost hygroscopic salt solutions was utilized to extract the heat generated during the operation of electronic components to prevent overheating; importantly, this passive technology can automatically restore its cooling capacity during non-working periods (or non-peak periods). Experimental results demonstrated that this technology can provide approximately 400 minutes of effective cooling capacity (ΔTmax = 11.5 °C), with tested heat fluxes reaching up to 75 kW/m2. Applying this technology to actual computing devices can enhance device performance by 32.65%. Compared to existing passive thermal management technologies, this technology exhibits record-high cost-effectiveness. Notably, this technology has high scalability and can be applied to other scenarios requiring intermittent thermal regulation, such as batteries, LEDs, photovoltaics, and construction (the team is conducting related research). The research results were published under the title “Membrane-encapsulated, Moisture-desorptive Passive Cooling for High-performance, Ultralow-cost, and Long-duration Electronics Thermal Management” in Cell sister journal Device. PhD student Sui Zengguang from City University of Hong Kong is the first author of this paper, and Associate Professor Wu Wei from City University of Hong Kong and Professor Yang Ronggui from Huazhong University of Science and Technology are the corresponding authors.This technology is inspired by the natural phenomenon of mammals sweating to regulate their body temperature, and its working principle and manufacturing process are illustrated in Figure 1. During the desorption cooling process, moisture evaporates from the salt solution to carry away the heat generated by electronic components, similar to how mammals sweat to lower their body temperature. During the absorption regeneration process, the cooler concentrated salt solution spontaneously absorbs moisture from the surrounding atmosphere to autonomously restore its cooling capacity, akin to how mammals drink water to replenish their body fluids. The porous membrane prevents solution leakage and equipment corrosion while allowing water vapor to pass through, similar to mammalian skin. The device primarily consists of an aluminum heat sink, anti-corrosion coating, salt solution, porous membrane, and support plate, featuring a simple structure and low cost.Figure 1 Working principle of HSMHS: (A) Schematic diagram of desorption and absorption processes; (B) Schematic diagram of HSMHS manufacturing process; (C) HSMHS prototype; (D) High-magnification SEM image of PTFE membrane with a pore size of 0.45 μm; (E) Schematic diagram of HSMHS heat dissipation process.In preliminary experiments, researchers first removed the porous membrane to observe changes in the solution. A PI heating film was used to simulate the chip, with multiple thermocouples used to record temperature changes, and a high-precision balance to monitor mass changes, with the entire device placed in a constant temperature and humidity chamber for testing. Experimental data is shown in Figure 2, where researchers measured the impact of different solution thicknesses and concentrations on cooling performance.Figure 2 Desorption/absorption experiments under typical environmental conditions (RH 60% and 25°C): (A) Schematic diagram of HSHS experiments in a constant temperature and humidity chamber; (B) Temperature and mass changes of HSHS with different solution layer thicknesses; (C) Infrared images of HSHS under different solution layer thicknesses; (D) Temperature changes under different heat flux conditions; (E) Comparison of maximum transient cooling power and temperature; (F) Thermal resistance caused by salt solution during desorption.At the same time, researchers also measured the cycling performance of this device. As shown in Figure 3, this work discusses in detail the impact of regeneration time on the cooling performance of the device. Experimental results prove that this device can provide stable and long-lasting thermal management for devices requiring intermittent thermal regulation.Figure 3 Cycling experiments of HSHS under typical environmental conditions (relative humidity 60% and 25°C): (A) Relationship between desorption time and regeneration time; (B) Cycling stability testing; (C) Mass changes during desorption and absorption processes within each cycle; (D) Impact of regeneration time on cooling performance.Based on the above experimental research, this work tested the influence of the membrane on the cooling performance of the device, as shown in Figure 4. This work discusses the impact of the membrane on the effective cooling time of the device. At the same time, researchers defined a cost-effectiveness index to compare existing passive thermal management technologies. The results indicate that the thermal management technology proposed in this work has record-high cost-effectiveness.
Figure 4 Testing of HSMHS cooling performance: (A) Impact of the membrane on the mass change of the testing device; (B) Impact of the membrane on the temperature change of the testing device; (C) Cost-effectiveness comparison.Finally, this work also tested the thermal management capability of the device under high heat flux conditions. Actual test results indicate that the applicable heat flux of this device can reach up to 75 kW/m2. To test the performance of this technology in practical scenarios, the device was assembled onto the ODROID-XU4 single-board computer. Results indicated that this device significantly reduced the chip temperature (11°C). Thanks to the excellent thermal management capability of this device, the performance of the ODROID-XU4 can be improved by 32.65%.Figure 5 Practical application of the proposed thermal management strategy: (A) Impact of different solution thicknesses on regeneration time; (B) Cycling experiments of HSMNHS; (C) Temperature comparison of HSMNHS and FHS under high heat flux conditions; (D) Physical images of ODROID-XU4 with HSMNHS and original heat sink; (E) Temperature changes under two thermal management strategies; (F) Transient input power changes under two thermal management strategies.【Conclusion】This work proposes a novel passive thermal management technology utilizing low-cost hygroscopic salt solutions. This technology achieves efficient thermal management for electronic components by introducing a porous PTFE membrane and salt solution, without the risk of solution leakage and corrosion. Thanks to the high hygroscopic characteristics of hygroscopic salts, this device exhibits excellent self-regeneration capability. Compared to existing passive thermal management strategies, the proposed thermal management strategy has record-high cost-effectiveness and shows immense commercial potential. Additionally, researchers have developed an experimentally validated CFD heat transfer and mass transfer model to guide further optimization of this technology. The thermal management strategy proposed in this work is both economical and highly scalable. This technology provides a new thermal management approach for devices requiring intermittent thermal regulation, applicable to electronic components, batteries, photovoltaic panels, and construction, with virtually no technical barriers.
