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 (B0.39N0.39C0.06O0.16) 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.