Boron nitride beats carbon in nanocomposite strength test
Researchers tested the force required to pluck a boron nitride nanotube from a polymer by welding a cantilever to the nanotube and pulling. The experimental set-up is shown in a schematic on the left and in an actual image on the right. Image: Changhong Ke/State University of New York at Binghamton.
Carbon nanotubes are legendary for their strength ¨C at least 30 times stronger than bullet-stopping Kevlar by some estimates. When mixed with lightweight polymers such as plastics and epoxy resins, the tiny tubes reinforce the material, like the rebar in a block of concrete, producing lightweight and strong materials for airplanes, spaceships, cars and even sports equipment.
While such carbon nanotube-polymer nanocomposites have attracted enormous interest from the materials research community, a group of scientists now has evidence that a different nanotube ¨C made from boron nitride ¨C could offer even more strength per unit of weight. They publish their results in Applied Physics Letters.
Boron nitride, like carbon, can form single-atom-thick sheets, which can then be rolled into cylinders to create nanotubes. By themselves, boron nitride nanotubes are almost as strong as carbon nanotubes, but their real advantage in a composite material comes from the way they stick strongly to the polymer.
"The weakest link in these nanocomposites is the interface between the polymer and the nanotubes," explained Changhong Ke, an associate professor in the mechanical engineering department at the State University of New York at Binghamton. If you break a composite, the nanotubes left sticking out have clean surfaces, as opposed to having chunks of polymer still stuck to them. The clean break indicates that the connection between the tubes and the polymer has failed, Ke noted.
Ke and his colleagues devised a novel way to test the strength of the nanotube-polymer link. They sandwiched boron nitride nanotubes between two thin layers of polymer, with some of the nanotubes left sticking out, and then welded these exposed nanotubes to the tip of a tiny cantilever beam. Next, by applying a force to the cantilever beam, they tugged increasingly hard on each exposed nanotube until it was ripped free of the polymer.
The researchers found that the force required to pluck out a nanotube at first increased with the nanotube length, but then plateaued. This behavior is a sign that the connection between the nanotube and the polymer is failing through a crack that forms and then spreads, Ke said.
The researchers tested two forms of polymer: epoxy and poly(methyl methacrylate), or PMMA, which is the same material used in Plexiglas. They found that the epoxy-boron nitride nanotube interface was stronger than the PMMA-nanotube interface. They also found that both polymer-boron nitride nanotube binding strengths were higher than those reported for carbon nanotubes ¨C 35% higher for the PMMA interface and approximately 20% higher for the epoxy interface.
Boron nitride nanotubes likely bind more strongly to polymers because of the way the electrons are arranged in the molecules, Ke explained. In carbon nanotubes, all carbon atoms have equal charges in their nucleus, so the atoms share electrons equally. In boron nitride, the nitrogen atom has more protons than the boron atom, so it hogs more of the electrons in the bond. This unequal charge distribution leads to a stronger attraction between the boron nitride and the polymer molecules, as verified by molecular dynamics simulations performed by a group led by Xianqiao Wang at the University of Georgia.
Boron nitride nanotubes also have additional advantages over carbon nanotubes, Ke said. They are more stable at high temperatures and they can better absorb neutron radiation, both of which are advantageous properties in the extreme environment of outer space. In addition, boron nitride nanotubes are piezoelectric, able to generate an electric charge when stretched. This property means the material offers energy harvesting capabilities, in addition to sensing and actuation capabilities.
The main drawback to boron nitride nanotubes is the cost. Currently they sell for about $1000 per gram, compared to $10¨C20 per gram for carbon nanotubes, but Ke is optimistic that the price will come down, noting that carbon nanotubes were similarly expensive when they were first developed. "I think boron nitride nanotubes are the future for making polymer composites for the aerospace industry," he said.