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Posted: Jun 29th, 2007
Nanotechnology in space
(Nanowerk Spotlight) Nanotechnology will play an important role in future space missions. Nanosensors, dramatically improved high-performance materials, or highly efficient propulsion systems are but a few examples. In previous Nanowerk Spotlights we reported about nanotechnology propulsion technology, such as nano field emission thrusters, and the use of carbon nanotubes to harden electronic components in space. This last aspect, radiation shielding, is also where nanotechnology could make a major contribution to human space flight. NASA says that the risks of exposure to space radiation are the most significant factor limiting humans’ ability to participate in long-duration space missions. A lot of research therefore focuses on developing countermeasures to protect astronauts from those risks. To meet the needs for radiation protection as well as other requirements such as low weight and structural stability, spacecraft designers are looking for materials that help them develop multifunctional spacecraft hulls. Advanced nanomaterials such as the newly developed, isotopically enriched boron nanotubes could pave the path to future spacecraft with nanosensor-integrated hulls that provide effective radiation shielding as well as energy storage.
Space radiation is qualitatively different from the radiation humans encounter on Earth. Once astronauts leave the Earth's protective magnetic field and atmosphere, they become exposed to ionizing radiation in the form of charged atomic particles traveling at close to the speed of light. Highly charged, high-energy particles known as HZE particles pose the greatest risk to humans in space. A long-term exposure to this radiation can lead to DNA damage and cancer. One of the shielding materials under study is boron 10. Scientists have known about the ability of boron 10 to capture neutrons since the 1930s and use it as a radiation shield in geiger counters as well as a shielding layer in nuclear reactors.
Boron is a non-metallic chemical element that is used in numerous applications, from use in its elemental form as a dopant in the semiconductor industry to boron compounds that play important roles as light structural materials, nontoxic insecticides and preservatives, and reagents for chemical synthesis.
"Boron nanotubes have many of the excellent properties of the well-known carbon nanotubes (CNTs)because they share the same structure" Dr. Ying Chen explains to Nanowerk. "Compared to CNTs, boron nanotubes have some better properties such as high chemical stability, high resistance to oxidation at high temperatures and are a stable wide band-gap semiconductor. Because of these properties, they can be used for applications at high temperatures or in corrosive environments such as batteries, fuel cells, super capacitors, high-speed machines as solid lubricant."
Chen, a Senior Fellow at Electronic Materials Engineering at The Australian National University, together with his department's colleagues, demonstrates the synthesis of isotopically enriched boron nanotubes (10BN nanotubes – isotopically enriched means that the boron nitride has a higher concentration of the isotope boron 10; normally, boron nitride is 80% or more composed of boron 11) for the first time with a high yield and in large quantities using a ball-milling/annealing process. Their recent paper on this process ("Isotopically Enriched 10BN Nanotubes") reveals the special role of high-energy ball-milling in reducing the nitriding temperature, leading to the growth of thin, cylindrical tubes.
Chen says that the specific application of isotopic 10BN nanotubes includes radiation shielding, multifunctional materials for energy storage, sensing and shell of future space ship, environmental protection, nuclear industry, neutron related medical applications and cancer diagnostics and treatment.
"The large-quantity production of pure boron nanotubes is a major problem for future practical applications such as radiation shielding because it requires a large quantity of this material" says Chen. "We have developed the ball milling method that can solve this problem. Using this method, we can produce such quantities that we even started to sell boron nanotubes. In our recent study we show that boron nanotubes with different isotopics, structures, and sizes can be produced."
Chen and colleagues have been refining the ball milling process for preparing boron nanotubes for several years now. It involves grinding down a powder of boron into nanoparticles in a ball mill in which steel balls tumble against each other for hundreds of hours. The role of the high-energy ball-milling is to reduce the reaction temperature by creating a chemically reactive structure and to introduce metal catalysts into 10B. The fine boron material is then heated in an atmosphere of nitrogen. This process gives the researchers not only a a mass production method but also full control over nanotube size and structure, which is very important for tuning nanotube mechanical and physical properties.
"I have communicated with researchers at NASA about the possible application of our boron nanotubes in space missions" says Chen. "Several years ago they asked me to prepare boron nanotube samples for tests on the space station. We also have discussed the possible use of 10BN nanotubes. Currently we are conducting radiation tests on the nanotubes at the Australian Nuclear Science and Technology Organization."
The 10BN nanotubes might find use in various applications – not only in space – where there is a need for strong, light weight, cost effective radiation shielding.
"For example" says Chen, "there is a lot of talk about developing fusion energy to feed an energy- hungry world. One of the major challenges in developing fusion energy on a commercial basis is coming up with materials that can provide shielding from the high neutron fluxes produced by the fusion process. Boron nanotubes might just fit the bill."