Hexagonal boron nitride and alumina dual-layer coating for space solar thermal shielding

Reducing the temperature of a solar probe or an orbital spacecraft has been successful in the spectral selectivity regime by maximizing the sunlight reflection and irradiating heat to the cold universe. Despite the extensive work on the adjustment of the absorbance/emissivity ratio, few efforts have been devoted to the modulation of heat conduction. In fact, the backlit-side of the spacecraft facing the cold universe is ideal for radiative cooling, as long as the sunlight generated heat is transferred effectively to the backlit surface with minimum heating of the interior structures. Here, by using a hexagonal‑boron nitride (h-BN) and Al2O3 dual-layer coating, we demonstrate a strategy for space solar thermal shielding that combines radiative cooling and anisotropic thermal conduction. The in-plane alignment of the top-layer h-BN flakes to the coating surface leads to a highly anisotropic thermal diffusion behavior, enabling rapid in-plane heat transfer and effective out-of-plane thermal insulation. The spectral selectivity optimization allows the dual-layer coating to reflect nearly 89% of solar irradiation while having an emissivity >0.86 across the thermal infrared wavelength. Moreover, the coating exhibits remarkable spectral stability under proton irradiation, showing its great potential in aerospace applications.