Preparation and cooling performance analysis of double-layer radiative cooling hybrid coatings with TiO2/SiO2/Si3N4 micron particles

Passive daytime radiative cooling is achieved by radiating heat into outer space through electromagnetic waves without energy consumption. A scalable double-layer coating with a mixture of TiO2, SiO2, and Si3N4 micron particles for radiative cooling is proposed in this study. The finite-difference time-domain algorithm is used to analyze the influence of particle size and coating thickness on radiative cooling performance. The results of the simulation show that the particle size of 3 mu m can give the best cooling performance, and the coating thickness should be above 25 mu m for SiO2 coating. Meanwhile, the mixture of SiO2 and Si3N4 significantly improves the overall emissivity. Through sample preparation and characterization, the mixture coating with a 1:1 ratio addition on an Al substrate exhibits high reflectivity with a value of 87.6% in the solar spectrum, and an average emissivity of 92% in the infrared region (2.5 mu m-15 mu m), which can be attributed to the synergy among the optical properties of the material. Both coatings can theoretically be cooled by about 8 degrees C during the day and about 21 degrees C at nighttime with h(c) = 4 W center dot m(-2)center dot K-1. Furthermore, even considering the significant conduction and convection exchanges, the cooling effect persists. Outdoor experimental results show that the temperature of the double-layer radiative cooling coating is always lower than the ambient temperature under direct sunlight during the day, and can be cooled by about 5 degrees C on average, while lower than the temperature of the aluminum film by almost 12 degrees C.