Flexible Al2O3/polymethyl methacrylate composite nanofibers for high-performance sun shading and radiative cooling

Radiative cooling utilizes high solar reflectivity and high infrared emissivity to achieve temperature cooling down, which has attracted extensive research interest as a zero-energy consumption manner. However, the design of photonic structures and metamaterials for radiative cooling do not exploit commercialization prospects due to high production costs and structural complexity. Here we propose an economical and scalable electrospinning method for fabricating high-performance sun-shading and radiative cooling composite flexible film. The composite film is a non-woven fabric of nanofibers with polymethyl methacrylate (PMMA) and aluminum oxide (Al2O3) nanoparticles. The diameter of the nanofibers distributes in the range of 300–1200 nm and Al2O3 nanoparticles are wrapped in the nanofibers. The nanofibers strongly scatter sunlight to enable high solar reflectivity and back-reflection layer is no more necessary. Molecular vibrations of PMMA and phonon polarization resonance of Al2O3 nanoparticles synergistically result in high infrared emissivity. At a thickness of 0.30 mm and with an Al2O3 content of 8%, this material exhibits exceptional solar reflectance levels of approximately 95% between wavelengths of 1.1–2.5 µm. Additionally, it demonstrates an extremely low rate of solar absorption of approximately 5.5% between wavelengths of 0.3–1.1 µm, distinguishing itself as a highly effective solar selective surface even without the inclusion of a back-reflective layer. Furthermore, the material exhibits ultrahigh infrared emissivity levels of approximately 0.98 within atmospheric windows ranging from 8 to 13 µm. These remarkable characteristics position this material as a highly promising candidate for use in solar thermal energy conversion systems. As a result, the device shows a cooling performance that reaches a temperature decrease of up to 4.3 °C under 1000 W/m2 peak solar intensity and a decrease of 5.9 °C during nighttime. The composite film has enormous potential for use in vehicle shading and personal thermal management. Moreover, its mechanical properties allow for the flexibility needed to meet the demands of everyday use. Our work provides a straightforward and scalable method to manufacture products with exceptional sun shading and radiative cooling capabilities.