Direct Growth of Transparent Boron Nitride Neutron Shielding Layer for Space Window

Cubic boron nitride (c-BN) and hexagonal boron nitride (h-BN) are known for their transparency and high ¹⁰B density, which provides a large thermal-neutron cross-section, yet their potential for space neutron shielding has not been explored. The fabrication of transparent c-BN films remains challenging, and the chemical vapor deposition growth of h-BN beyond 70 nm, or with precise thickness control and high uniformity, has not been reported except by our group. Here, we present a space window design integrating an h-BN-based neutron shielding layer with advanced ceramic bulletproof layers and a γ-ray shielding layer. By incorporating C and O into h-BN, sp²-sp³ hybridized BN (HBN) reduces the refractive index mismatch with the SiO₂ substrate, achieving 90.9% transmission at 550 nm at 11.9 µm thickness and enabling stable, transparent growth up to 79.2 µm with minimized thermal expansion mismatch. The optically optimized HBN (B_0.39N_0.39C_0.06O_0.16) shows reduced boron content, but the enriched formation of 63.4% c-BN, with its higher boron density, compensates for this loss. The resultant density is 3.01 g cm^-3, evaluated from neutron-shielding probability, and HBN achieves the same neutron-shielding efficiency as h-BN at 3% reduced thickness.