ZHU Hai ¹, BAI Mingmin ¹, LI Weixin ², ZHANG Yiyang ¹, XIE Jun ¹
(1. School of Materials Science and Engineering, Jingdezhen Ceramic University, Jingdezhen 333403, Jiangxi, China; 2. College of Humanities, Jingdezhen University, Jingdezhen 333032, Jiangxi, China)

Extended Abstract: [Background and purpose] With the advancement of society and the increase in population, the energy crisis is becoming increasingly severe. Cooling and refrigeration account for about 20% of the total electricity consumption in buildings worldwide and 10% of the global electricity consumption. Therefore, reducing the energy consumption of active cooling and exploring new cooling technologies have become important research topics. Radiative cooling is a passive cooling method that involves radiating heat in the form of infrared waves through the atmospheric window (primarily between 8 and 13 μm) to outer space. This study was aimed to prepare PVDF-HFP (polyvinylidene fluoride-hexafluoropropylene) porous network structured film for radiative cooling. The goal is to achieve high solar reflectance and high infrared emissivity through spectral selectivity, thereby enhancing radiative cooling performance.[Methods] The main materials used are PVDF-HFP powder, acetone and deionized water. The PVDF-HFP powder was mixed with acetone and water in a specific ratio. The mixture was subjected to ultrasonic oscillation, heated and stirred to form a precursor solution. The solution was then coated onto an aluminum plate and dried at room temperature to form PVDF-HFP film.The effects of the content of acetone, reaction temperature and reaction time on microstructure and radiative performance of the PVDF-HFP film were systematically studied. Phase composition and microstructure of the film were characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Solar spectral reflectance and infrared emissivity of the film were tested. Outdoor radiative cooling tests were conducted to measure the temperature reduction effect of the film under outdoor conditions.[Results] The PVDF-HFP films were successfully fabricated using the phase inversion method, and the effects of various preparation parameters on the film's microstructure and radiative cooling performance were systematically investigated. The optimal preparation conditions were determined to be a mass ratio of PVDF-HFP powder to acetone to water of 1:11:1, a reaction temperature of 65 ℃, and a reaction time of 2 hours. Under these conditions, the films exhibited a highly porous network structure, which significantly enhanced the scattering and reflection of solar light.The solar reflectance of the films was measured to be 80% across the solar spectrum, and the infrared emissivity within the atmospheric window (8 μm to 13 μm) was found to be 97%. These properties are crucial for effective radiative cooling, as they enable the film to reflect a large portion of incoming solar radiation and efficiently emit heat in the form of infrared radiation through the atmospheric window to outer space.Outdoor radiative cooling tests conducted in Jingdezhen, a temperate climate region, demonstrated the practical cooling capabilities of the PVDF-HFP films. The temperature of an aluminum sheet coated with the PVDF-HFP film under direct sunlight was up to 14 ℃ lower than the ambient temperature. This significant temperature reduction highlights the film's potential as an efficient, environmentally friendly, and energy-saving cooling solution, particularly for building cooling in hot regions.The high reflectance and emissivity of the PVDF-HFP films can be attributed to their porous network structure, which effectively scatters solar light and enhances infrared emission. The absence of impurities or new substances during the preparation process ensures the film's purity and consistent performance. The results confirm that the PVDF-HFP films are a promising material for passive radiative cooling applications, offering a viable alternative to traditional cooling methods.[Conclusions] The PVDF-HFP film with high reflectance and high emissivity was successfully prepared, and no impurities or new substances were formed during the melting process. Under the optimal preparation conditions (PVDF-HFP: acetone: water mass ratio of 1:11:1, reaction temperature of 65 ℃, and reaction time of 2 hours), the film's reflectance to sunlight is 80%, and the infrared emissivity within the atmospheric window is 97%. The actual radiative cooling tests demonstrate that the PVDF-HFP film has excellent radiative cooling capabilities and can significantly reduce surface temperatures, providing an efficient, environmentally friendly, and energy-saving cooling method, especially suitable for building cooling in hot regions.
Key words: PVDF-HFP film; reflecting sunlight; porous network structure; radiation cooling