Material science plays a critical role in space exploration. So many of the challenges facing both crewed and non-crewed missions come down to factors like weight, thermal and radiation tolerance, and overall material stability. The results of a new study from Young-Kyeong Kim of the Korea Institute of Science and Technology and their colleagues should therefore be exciting for those material scientists who focus on radiation protection. After decades of trying, the authors were able to create a fully complete “sheet” of Boron Nitride Nanotubes (BNNTs).
BNNTs have been on the radar of a number of different applications in space exploration, but one of the biggest one is radiation shielding. Boron - one of their primary components - is widely used in the control rods of nuclear reactors due to its excellent neutron absorbing properties. Studies had shown that BNNTs are particularly effective at limiting a particular type of dangerous neutron known as a “second neutron”, which are created when high-energy particles hit other types of shielding and shatter the atoms of whatever material that shield is made out of. In the past, designers have used aluminum, but it had the disadvantage of being relatively heavy, which is always a disadvantage in space applications.
Given their much lighter weights for the same amount of radiation shielding, BNNTs have long been sought as a potential solution to this problem. However, they have suffered from manufacturability problems since their first experimental synthesization in 1995. They were typically made using a technique called vacuum filtration, and resulted in a material known as “bucky paper”. This process, while effective at creating dense clumps of BNNTs, could not effectively spread them over the whole surface required to create an effective radiation shield.
NASA Video about the hazards of radiation on human spaceflight. Credit - NASA Videos YouTube ChannelEnter the new research from Dr. Kim and their colleagues. Part of the problem is that BNNTs hate water, and tend to clump together when exposed to it rather than spread themselves easily. Typically this problem is solved using a surfactant - an additive chemical that coats the nanotubes to protect them from each other when they would otherwise clump up. The problem with using standard surfactants is that the extra surfactant itself would start clumping together, forming pockets in the material called micelles. These micelles actually push the BNNTs, eliminating the dispersion effect the surfactants surrounding them are trying to enforce.
However, the new research uses a different type of surfactant. Dodecylbenzenesulfonic acid (DBSA) might sound intimidating to anyone who hasn’t taken organic chemistry in a while, but it is a common ingredient in hand soap. The way it interacts with the BNNTs is different than traditional surfactants, creating a bilayer of molecules that both protected the BNNTs from the water, but also didn’t form the micelles that caused them to clump back together. This created a material state known as a Lyotropic Liquid Crystal, where the nanotube aligned all in the same direction.
Alignment is key when depositing them onto a surface, and that is exactly what the researchers did using a coating technique called Doctor Blade. That technique uses the shear force of the created crystal rubbing against a substrate to force the nanotubes to be deposited in a uniform structure, which has all the benefits of a BNNT sheet without any of the holes or gaps associated with traditional manufacturing techniques.
TEDx talk about the potential of BNNTs. Credit - TEDx Talks YouTube ChannelTo prove their newly formed material actually had better radiation properties, the authors ran a simulation that compared the amount of radiation passing through their BNNT film and a similar amount of aluminum. What they found was striking - to get an equivalent amount of radiation protection from aluminum would require 8x the weight. In space exploration terms, that means it would be ⅛ the price to send the same amount of radiation shielding to orbit.
Pretty impressive stuff, though to be fair it has yet to be practically tested on an actual spacecraft. The material itself has to undergo all kinds of stresses and strains on its way to space. BNNTs should have the physical properties to withstand it, but until its proven it remains an assumption. But, after 30 years in the lab, the promise of this radiation-resistent material is finally coming to fruition, and it's now poised to change the future of radiation protection.
Learn More:
Korean National Research Council of Science & Technology - Blocking space radiation threats with nanotubes! 'Boron nitride nanotube space radiation shield' developed
Y.K. Kim et al - High-Density Boron Nitride Nanotube Composites via Surfactant-Stabilized Lyotropic Liquid Crystals for Enhanced Space Radiation Shielding
UT - A New Technique to Make Lighter Radiation Shielding For Spacecraft: Rust.
UT - We Know How Much Radiation Astronauts Will Receive, But We Don't Know How to Prevent it