Optimally Designed Multimaterial Microparticle–Polymer Composite Paints for Passive Daytime Radiative Cooling

Passive daytime radiative cooling (PDRC) devices have enabled subambient cooling of terrestrial objects without any energy input, offering great potential to future clean energy technology. Among various PDRC structures, random dielectric particles in a polymer matrix or paint-like coatings have displayed powerful radiative cooling performances with excellent scalability and easy fabrication. While modeling and analyzing such a system is nontrivial to enhance the cooling effect and engineer the structures to be utilized in various applications, it is essential to understand its complex physical relations and determine the optimal design conditions. In this work, we have thoroughly analyzed the optical properties and radiative cooling performances of PDRC paints composed of two-material particles (SiO2 and Al2O3) using 2D FDTD simulation and investigated the optimal design conditions. Specifically, we have studied the effects of design parameters, such as particle size, size distribution, binder volume ratio, and coating thickness. Subsequently, we have conducted an outdoor cooling measurement of the fabricated PDRC paints to demonstrate their radiative cooling potential and to analyze and understand their performance based on our numerical investigations. The fabricated PDRC paints exhibited high solar reflectance (0.958) and strong long-wave infrared emission (0.937) in the atmospheric transparency window, achieving a maximum temperature drop of 9.1 °C. This comprehensive study provides a detailed characterization of the structure and material parameters of the multimaterial PDRC paint system.