Selective spectral optical properties and structure of aluminum phosphate for daytime passive radiative cooling application.

Abstract Daytime passive radiative cooling can achieve a cooling surface by reflecting the sunlight and radiating the heat to the outer space without consuming any energy. Aluminum phosphate (AlPO 4) is a novel inorganic daytime passive radiative cooling material which has high solar reflectivity and high mid-infrared emissivity. The influence of heat treatment temperatures on crystallization behavior and optical properties are investigated by X-ray diffraction and spectrographs. The results demonstrate that increased heat treatment temperature will result in increased reflectivity but decreased mid-infrared emissivity. The effect of polyhedral structural differences between tridymite-type AlPO 4 and cristobalite-type AlPO 4 on mid-infrared and selective emissivity are discussed and analyzed. Furthermore, the tridymite-type AlPO 4 fabricated at 700 °C possesses high solar reflectivity of 97% and selective emission with an emissivity of 0.90 across 8–13 µm. When made into coating, the optimum cooling effect could reach a temperature drop of ~4.2 °C lower than the ambient temperature and ~4.8 °C compared with commercial heat insulation coating under direct sunlight. This proves that AlPO 4 is a promising energy-saving material for daytime passive radiative cooling application. Thus, it can greatly lower the indoor temperature and improve comfortability of people's living in buildings. Graphical abstract Aluminum phosphate (AlPO 4) possesses high solar reflectance and high infrared emissivity, is a novel daytime passive radiative cooling material. Tridymite-type AlPO 4 possesses high solar reflectance of 97% and high emissivity of 0.90 across 8–13 µm. When made into coating, the cooling effect drops of 4.2 °C below the ambient temperature under direct sunlight, which can greatly improve people's living comfortability. fx1 Highlights • Aluminum phosphate (AlPO 4) is a novel daytime passive radiative cooling material. • Tridymite-type AlPO 4 possesses high solar reflectance of 97% and high emissivity of 0.90 across 8–13 µm. • For the AlPO 4 coating, under direct sunlight, the coating temperature drops of ~4.2 °C below the ambient temperature and drops of ~4.8 °C compared with commercial heat insulation coating, possessing an excellent radiative cooling effect. [ABSTRACT FROM AUTHOR]