Scientists have devised a way to manufacture hexagonal boron nitride nanotubes on an industrial scale. This ‘white graphene’ is strong, heat-resistant and electrically insulating and can, for example, be used as a material for supersonic aircraft.
‘Hexagonal boron nitride’ is a material that is somewhat similar to graphene. Both materials consist of a single layer of atoms arranged in a honeycomb structure. With graphene these are carbon atoms, with boron nitride it is a regular alternation between boron and nitrogen atoms. Both materials are strong and very resistant to heat – boron nitride slightly better than graphene.
The main difference is that graphene is an electrical conductor and boron nitride is an insulator. Because graphene as a solid is black and hexagonal boron nitride is white, the latter material is also called ‘white graphene’.
Nanotubes of this white graphene, that is, tiny cylinders a few or tens of nanometers in size, are even stronger and more heat-resistant than the flat version. The hollow fibers are therefore a perfect ingredient for composite materials to be used in space shields or supersonic aircraft. They are also suitable as insulation material in electronic devices or as part of membranes for water filters.
Crystallize on a platform
Until now, it has been quite difficult to make robust nanotubes from hexagonal boron nitride, but engineers from MIT and the University of Tokyo have now succeeded. They produced multi-centimeter platforms containing hundreds of billions of aligned hollow boron nitride nanotubes.
This provides the first insight into the possibility of producing the boron nitride fibers on an industrial scale, they wrote on October 31 in the scientific journal. ACS Nano.
The engineers’ trick was to make a kind of carbon scaffolding on which the tubes could rise like a kind of nanoforest.
Researchers at the University of Tokyo described an earlier version of this process two years ago. They first created micrometer-scale carbon tubes in the usual way through a process called ‘chemical vapor deposition’ – that’s a factor of a thousand times larger than the nanoscale.
These tubes were then coated with boron and nitrogen gas and then placed in a very hot oven (think temperatures of seven hundred to one thousand degrees). There, the nanostructures of hexagonal boron nitride crystallized out on the carbon tubes from the coating.
In the new method, published last week, the carbon tubes are no longer needed as an intermediate step. The boron nitride nanotubes now grow directly on the carbon scaffold.
The engineers did this by adjusting the temperature, pressure and composition of the vapor that had to settle. “The first few times it was a complete mess,” said Brian Wardle, lead author of the research paper, in an MIT press release. “All the tubes clumped together into a ball.” In the end, however, we managed to find the right combination.
Because the boron nitride nanotubes can (still) withstand heat better than graphene, the carbon scaffold could then simply be removed by setting the oven to a thousand degrees Celsius and thus burning it.
To demonstrate the technique’s flexibility, the team also made some larger (centimeter-scale) carbon structures—such as woven mats or sheets with more randomly oriented fibers—on which they applied a coating of boron nitride nanotubes and then fired the carbon structure itself. This left them with a copy of the carbon structure, but made of boron nitride.
The next step is to turn it into usable material, say the researchers. The future users seem to see this positively: the research was partially sponsored by Airbus, ANSYS, Boeing, Embraer, Lockheed Martin, Saab AB and Teijin Carbon America.
Opening image: Benjah-bmm27, Wikimedia Commons, Public Domain
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