Shielding Effectiveness of Boron Nitride Nanotube Composites for Space Radiation

In space environment, materials are exposed to micrometeoroids and space radiation such as charged particle radiation, solar particle events (SPE), galactic cosmic radiation (GCR), secondary neutron, and gamma rays. Material selection is an extremely important consideration for space missions in parallel with other radiation shielding strategies. Boron nitride nanotubes (BNNT) demonstrate excellent structural reinforcement to polymers with enhanced radiation shielding properties, leading to extensive research for applications in extreme space environments.
In this study, the radiation shielding effectiveness of BNNT and BNNT polymer composite materials are investigated using three methods. First, a set of BNNT polymer composite films such as BNNT polydimethylsiloxane, BNNT epoxy, BNNT polyethylene, and BNNT styrene-butadiene rubber composites are exposed to neutron radiation to study shielding effectiveness of the composites as a function of BNNT concentration. Results demonstrate that BNNTs significantly improve neutron radiation shielding properties of the composite materials while offering mechanical and thermal reinforcement. Second, BNNT polymer composite samples such as BNNT epoxy composites are exposed to Low Earth Orbit (LEO) environment for six months as part of the NASA Materials International Space Station Experiment (MISSE) project to analyze the effect of low flux grazing atomic oxygen (AO), ultra-violet (UV) radiation, space radiation, thermal cycling, and the vacuum environment. The exposed MISSE samples were retrieved and analyzed with microscopy (optical and electron), spectroscopy (IR and Raman), profilometer, and contact angle goniometer to compare with the unexposed reference samples. The analyses suggest that the exposed surface underwent significant chemical degradation from the increase in absorption of reactive species and increase in hydrophilicity of the surface. In addition, the surface roughness of the exposed region is higher than unexposed regions, possibly due to surface erosion caused by AO. Third, BNNT polymer composite materials are modeled using the On-Line Tool for the Assessment of Radiation in Space (OLTARIS) in LEO, Lunar surface GCR, and Lunar surface SPE environments for dose equivalent or differential flux at different areal densities. Materials with maximum hydrogen content yield the highest radiation shielding effectiveness against GCR and SPE environments. BNNT polymer composites demonstrate minimum tradeoff between radiation shielding properties and structural enhancements. In this study, a wide range of BNNT polymer composites with different polymer matrices, each with varying BNNT concentrations have been evaluated for radiation shielding properties under various space environments, demonstrating potential as multifunctional materials for aerospace structures.