Space shuttles and fighter planes that are lighter, more fuel efficient and better at shielding from space radiation are now a step closer following a discovery by scientists at Binghamton University in New York. The team has found boron nitride nanotubes (BNNTs) form stronger interfaces with epoxy and other polymers than comparable common carbon nanotubes (CNTs).
CNTs have attracted much attention because they are extremely strong, and when mixed with epoxy polymers can reinforce the materials. However, researchers have now found that BNNTs in polymethyl metacrylate (PMMAs) form stronger interfaces than CNTs, with BNNT-epoxy interfaces being even stronger.
Nanotubes like BNNTs are measured in billionths of a meter. Thousands of them bound together are thinner than a strand of human hair. Publishing their findings in Applied Physics Letters, researchers tested the strength by extracting single BNNTs from layers of polymer, applying force until it was ripped free. They then repeated the process to extract the nanotubes from epoxy.
They found the epoxy BNNTs binding strengths were higher than the CNTs – 35% higher than the PMMAs and 20% higher than epoxy. "They are both light and strong," study leader Changhong Ke said. "They have similar mechanical properties, but different electrical properties. Those differences help to add strength to the BNNT interfaces with the polymers."
BNNTs appear to bind more strongly to the polymers because of the way electrons are arranged in the molecules. In CNTs, carbon atoms have equal charges in their nucleus, so atoms share electrons equally. But in BNNTs, the nitrogen atom has more protons, so it monopolises more of the electrons in the bond – the unequal charge distribution leads to a stronger attraction.
As well as being strong, the BNNTs are more stable at higher temperatures and can better absorb neutron radiation. This would be highly beneficial for space travel. Ke said: "We think that this could be the first step in a process that changes the way we design and make materials that affect the future of travel on this planet and exploration of other worlds beyond our own. Those materials may be way off still, but someone needed to take the first step, and we have."
The main drawback at the moment, he added, is the cost. BNNTs are currently far more expensive than CNTs – but Ke said he is optimistic the price will come down eventually. "I think boron nitride nanotubes are the future for making polymer composites for the aerospace industry," he said.