Scalable, High Thermal Conductive Electrospun/Sprayed Radiative Cooler for Efficient Thermal Management

Passive radiative cooling enables an object to maintain a temperature lower than the ambient air temperature without external energy consumption. To achieve efficient radiative cooling performance, the thermal emitter should exhibit solar spectrum reflection (0.28-2.5 μm) while emitting heat as radiation within the atmospheric window (8-13 μm) [1]. In practical applications with significant cooling requirements, such as data centers, vehicles, and communication base stations, the objects that need cooling often operate at temperatures higher than the surrounding ambient temperature due to substantial internal heat generation [2]. However, polymer-based photonic structures, commonly employed for their material properties and porosity to provide insulation, exhibit low thermal conductivity. Consequently, when these coolers are applied to high-temperature objects, they encounter heat resistance resulting from their low thermal conductivity, leading to the accumulation of cooling heat and a decline in cooling performance. To overcome these challenges, researchers have been investigating radiative coolers that possess enhanced thermal conductivity. One approach involves incorporating nanofillers, such as boron nitride, into the radiative cooler to improve both the overall thermal conductivity and solar reflectivity [3]. Nevertheless, achieving adequate thickness is crucial to ensure high reflectivity, which simultaneously increases thermal resistance. Consequently, a significant amount of high thermal conductivity filler material is required to offset this effect. On the other hand, reducing the thickness enhances thermal conductivity but diminishes the reflectivity characteristics. Therefore, optimizing the cooling device's properties, including its thickness, reflectivity, and thermal conductivity, is of utmost importance in achieving effective cooling performance.<br/>In this study, we fabricated an electrospun/sprayed radiative cooler (ESRC) with high thermal conductivity as a thermal management method for high-temperature objects under outdoor conditions. The ESRC was fabricated using poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) and boron nitride nanosheets (BNNS) through electrospinning and electrospraying techniques. The nanofiber membranes were manufactured with a thickness of 700 nm, and the BNNS diameter was set at 500 nm to induce multiple scattering within the solar spectrum range. Electrospraying was utilized to minimize BNNS aggregation and ensure particle dispersion for effective light scattering and thermal diffusion. Furthermore, a hot press process was employed to stack the layers, creating a heat transfer path between BNNS layers, and reducing overall thickness. As a result, the reflectance of the entire solar spectrum was measured to be over 94%. Moreover, the ESRC demonstrated high thermal radiation within the atmospheric window, with measured emittance exceeding 95%. The total thickness of the fabricated ESRC was only 120 μm, resulting in exceptional flexibility. The thermal conductivity of the ESRC was determined using the hot disk method, revealing a through-plane thermal conductivity of 1.5 K/mK with 8.7 wt% loading of BNNSs. Compared to pure PVDF-HFP, the thermal conductivity was increased by a factor of 13.<br/>[1] Byun, Sang-Hyuk, et al. "Self-Cooling Gallium-Based Transformative Electronics with a Radiative Cooler for Reliable Stiffness Tuning in Outdoor Use." <i>Advanced Science</i> 9.24 (2022): 2202549.<br/>[2] Li, Pengli, et al. "Thermo-optically designed scalable photonic films with high thermal conductivity for subambient and above-ambient radiative cooling." <i>Advanced Functional Materials</i> 32.5 (2022): 2109542.<br/>[3] Yu, Xi, et al. "Thermoconductive, moisture-permeable, and superhydrophobic nanofibrous membranes with interpenetrated boron nitride network for personal cooling fabrics." <i>ACS applied materials & interfaces</i> 12.28 (2020): 32078-32089.