Double-Layer Coating Containing Boron Nitride Powder for Efficient Daytime Radiative Cooling

Daytime radiative cooling outdoors is a passive cooling method that emits thermal radiation toward the sky while reflecting sunlight. Many different daytime radiative coolers have been developed, and some have been commercialized. Coatings offer advantages in ease of application and versatility across different surfaces. Typical daytime radiative cooling coatings are mixtures of powders or particles in polymer hosts. As these paintings reflect sunlight diffusively, the coating thicknesses are submillimeters or thicker. Thick coatings result in high thermal resistance, which is undesirable for cooling objects below the coating. To address this problem, boron nitride (BN) powder was used as a material with high thermal conductivity to reduce thermal resistance. However, the high refractive index of BN in the mid-infrared regions prevents the mid-infrared emissivity of BN-containing coatings from achieving values above 0.9 if the concentration is high. In the current work, we demonstrate a high average emissivity reaching 0.93 and solar reflectance of 0.99 by adding a layer containing silica powder, where the silica layer is instrumental in enhancing the emissivity without deteriorating the solar reflectance. The double-layer coatings exhibit subambient outdoor temperatures in Japan. Introducing a silica layer for thermal emission on top of a BN-containing base layer presents a straightforward method to enhance the daytime radiative cooling performance of a BN-containing layer.