Electronic and phononic characteristics of high-performance radiative cooling pigments h-BN: A comparative study to BaSO4

A thin layer, lightweight, and ultra-white hexagonal boron nitride (h-BN) nanoporous paint has been developed recently. However, the underlying atomic and nanostructural physics of the paint’s radiative cooling performance remains quite elusive. In this work, a multiscale, multiphysics computational framework is employed to gain atomic level insights of the high radiative cooling performance. By leveraging first-principles calculations to study the electronic transitions and phonon dynamics, the refractive index and extinction coefficient are predicted across solar and mid-infrared (mid-IR) spectra, which are then used to calculate the optical properties of a single nanoparticle either by Mie Theory or computationally solving Maxwell’s Equations. Subsequently, the photon Monte Carlo simulation is used to predict the photon transport in nanoplatelet-matrix nanocomposites, by including the anisotropic optical properties of nanoplatelets for the first time. The predicted solar reflectance and sky window emissivity of the nanocomposites agree well with the experiments. By comparing with BaSO₄-based paint, we attribute the high solar reflectance of h-BN paint at a lower thickness to its higher refractive index and nanoplatelet morphology, and attribute the relatively lower sky window emissivity to its lower extinction coefficient in mid-IR. Surprisingly, aligning the nanoplatelets horizontally does not significantly improve the solar reflectance at $150\mu m$ coating thickness due to diminishing returns. Finally, we compile many radiative cooling pigments and order the following few in decreasing refractive index: h-BN, BaSO₄, CaCO₃, SiO₂. Our work advances the understanding of atomic-scale features in designing radiative cooling materials.