Researchers tested the force required to pluck a boron nitride nanotube (BNNT) 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 an actual image on the right. Credit: Changhong Ke/State University of New York at Binghamton
Carbon nanotubes are legendary in their strength – at least 30X 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, promising lightweight and strong materials for airplanes, spaceships, cars and even sports equipment. Now a different nanotube – made from boron nitride – could offer even more strength per unit of weight.
Boron nitride, like carbon, can form single-atom-thick sheets that are 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,” said A/prof Changhong Ke. 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 fails. Ke’s team devised a novel way to test the strength of the nanotube-polymer link. They sandwiched boron nitride nanotubes between 2 thin layers of polymer, with some of the nanotubes left sticking out. They selected only the tubes that were sticking straight out of the polymer, and then welded the nanotube to the tip of a tiny cantilever beam. The team applied a force on the beam and tugged increasingly harder on the nanotube until it was ripped free of the polymer.
The force required to pluck out a nanotube at first increased with the nanotube length, but then plateaued. The 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 2 forms of polymer: epoxy and poly(methyl methacrylate), or PMMA, which is the same material used for Plexiglas. The epoxy-boron nitride nanotube interface was stronger than the PMMA-nanotube interface. Both polymer-boron nitride nanotube binding strengths were higher than those reported for carbon nanotubes – 35% higher for PMMA interface and ~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. In carbon nanotubes, all carbon atoms have equal charges in their nucleus, so the atoms share electrons equally. In boron nitride, the N has more protons than the boron atom, so it hogs more of the electrons in the bond. The unequal charge distribution leads to a stronger attraction between the boron nitride and the polymer molecules, as verified by molecular dynamics simulations performed by Ke’s colleagues in Dr. Xianqiao Wang’s group at the University of Georgia.
Boron nitride nanotubes are also more stable at high temperatures and they can better absorb neutron radiation, both advantageous properties in the extreme environment of outer space. In addition, boron nitride nanotubes are piezoelectric, ie can generate an electric charge when stretched. This property means the material offers energy harvesting as well as sensing and actuation capabilities. The main drawback to boron nitride nanotubes is the cost. Currently they sell for about $1,000/g vs $10-20/g for carbon nanotubes. He is optimistic that the price will come down, though, 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. https://publishing.aip.org/publishing/journal-highlights/move-aside-carbon-boron-nitride-reinforced-materials-are-even-stronger?TRACK=Gallery
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