Enhanced Thermal Emissivity through Boron Nitride Nanofiber Scatterers for Passive Radiative Cooling

Spontaneous cooling offers a sustainable, electricity-free strategy to mitigate global warming and reduce urban heat buildup. Building on our previous experimental work with nanoporous films and glass bubble–enhanced Passive Radiative Cooling (PRC) paints - demonstrating solar reflectance above 94% and broadband emissivity exceeding 95% - this study advances PRC design through full-wave electromagnetic modeling. Using the Finite Element Method, we optimized multilayer disordered optical coatings incorporating randomly distributed dielectric boron nitride nanofibers as light scatterers. Two architectures were investigated: a five-layer stacked (5LS) structure with graded particle sizes (100 - 600 nm), and a single-layer dense-to-coarse (DtC) configuration featuring a vertical gradient in scatterer size. Polarization-resolved simulations and diffraction order decomposition were employed to generate accurate, unpolarized reflectance spectra across the full solar range (280 - 2500 nm). Both designs exhibited total and solar-weighted reflectance values exceeding 97%, demonstrating their effectiveness for Passive Daytime Radiative Cooling (PDRC). Coupled with high emissivity in the sky transparency window, these optical properties were integrated into a tailored thermal model simulating realistic conditions. The optimized coatings achieved an ideal temperature drop of up to 7°C below ambient, approaching the performance of ideal PDRC materials.