Thin layer lightweight and ultrawhite hexagonal boron nitride nanoporous paints for daytime radiative cooling

Thin and lightweight radiative cooling paints are needed for many weight-sensitive applications. However, it is difficult to achieve high solar reflectance with thin layers. This work develops ultrawhite hBN-acrylic paints that achieve solar reflectance of 97.9% and sky window emissivity of 0.83 with only 150 mm thickness and 0.029 g/cm(2) weight, representing significant reductions from previous radiative cooling paints. The high refractive index and nanoplatelet morphology of hBN enable a unique combination of Mie scattering-like high scattering coefficient and Rayleigh scattering-like strong backscattering, and a porosity of 44.3% offers high refractive index contrast between hBN and air; all contribute to achieve high solar reflectance with a thin coating. Field tests show full daytime cooling under direct sunlight, reaching 5-6 degrees C below ambient temperature on average during daylight hours. Our hBN-acrylic paint demonstrates comparable cooling performance with recent best technologies, and the thinness and light weight reduce barriers toward many practical applications.