Boron nitride nanotubes (BNNTs) that are long enough to be twisted into yarns have been made for the first time, scientists claim. BNNTs are of interest because, like carbon nanotubes (CNTs), they are strong yet light and have potentially useful thermal, electronic and quantum properties that could see them put to use in aerospace, body armour or even space suits.
The researchers from the NASA Langley Research Center in Hampton, US, developed the pressurised vapour-condenser (PVC) method to grow BNNTs 100 times longer than those grown using closely related methods (Nanotechnology, doi: 10.1088/0957-4484/20/50/505604). They then twisted the resulting fluffy clumps of nanotubes into a yarn 30mm long and 1mm thick.
‘Other labs can make really good nanotubes that are short or really crummy ones that are long,’ says author Michael Smith. ‘We’ve developed a technique that makes really good ones that are really long.’
In the PVC method, boron is heated to 4000ºC using a laser and the vapour is blown through nitrogen at ambient temperatures. A cool wire is moved across the chamber, providing a nucleation site for droplets of boron, which stretch into BNNT fibrils as the two gases meet.
BNNTs represent technology that has yet to find applications, but Smith says they are close. One of the first is likely to be toughened ceramics for high temperature aerospace components, such as jet engine parts, that are of interest to NASA. ‘Ceramics tend to be brittle and fracture easily,’ Smith explains. ‘That’s why jet engines are still made out of metal. With BNNTs as a filler, ceramics will be made that can take shock and “give” a bit.’
Smith expects BNNTs will be cheaper than currently available high grade CNTs. Furthermore, early toxicology data suggest that BNNTs are safer than CNTs.
‘This work is an important step toward increasing the production rate of ultimately pure and long multi-walled boron nitride nanotubes,’ says Dmitri Goldberg, at the National Institute for Materials Science in Ibaraki, Japan. BNNTs have been underestimated to date, he adds, due to low yields and manufacturing difficulties or the high costs of production.
But Ying Chen, a nanotechnology professor at the Australian National University in Canberra, says ‘the authors have not commented accurately’ on other methods, which can also lead to high quality BNNTs. In addition, he is not optimistic about scale-up. ‘The process uses extremely high temperature and high pressure, which will be very difficult for scalingup to increase the production from current gram to kilogram quantities,’ he says. The production costs will be much higher compared to other techniques, such as chemical vapour deposition and ball milling, he adds. ‘Only NASA can afford to use their products.’