24-Dec-2015 - State University of New York

Move aside carbon: Boron nitride-reinforced materials are even stronger

Carbon nanotubes are legendary in their strength -- 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, promising 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 -- 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 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 fails, Ke noted.

Plucking Nanotubes

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. 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 researchers found that 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 two forms of polymer: epoxy and poly(methyl methacrylate), or PMMA, which is the same material used for 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 -- 35 percent higher for the PMMA interface and approximately 20 percent higher for the epoxy interface.

The Advantages of Boron Nitride Nanotubes

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. 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 also have additional advantages over carbon nanotubes, Ke said. They are 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, which means they 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 per gram, compared to the $10-20 per gram for carbon nanotubes, Ke said. 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.

Facts, background information, dossiers
More about State University of New York
  • News

    New biobatteries use bacterial interactions to generate power for weeks

    Researchers at Binghamton University, State University of New York have developed a “plug-and-play” biobattery that lasts for weeks at a time and can be stacked to improve output voltage and current. As our tech needs grow and the Internet of Things increasingly connects our devices and sen ... more

    Silver cycle: New evidence for natural synthesis of silver nanoparticles

    Nanoparticles of silver are being found increasingly in the environment—and in environmental science laboratories. Because they have a variety of useful properties, especially as antibacterial and antifungal agents, silver nanoparticles increasingly are being used in a wide variety of indus ... more

    Discovery of ionic elemental crystal against chemical intuition

    An ETH Zurich researcher has developed a computational method for predicting the structure of materials. He used it to solve the structure of a newly synthesized form of pure boron that displays some unusual physical properties and brings a surprise: it is partially ionic. The new structure ... more

More about American Institute of Physics
  • News

    Solar cell keeps working long after sun sets

    About 750 million people in the world do not have access to electricity at night. Solar cells provide power during the day, but saving energy for later use requires substantial battery storage. In Applied Physics Letters, by AIP Publishing, researchers from Stanford University constructed a ... more

    Turning plastic grocery bags into sustainable fuel

    More than 300 million tons of plastic waste are produced annually, which causes serious environmental issues because of plastic's life cycle and the difficulty of eliminating it. Consequently, most plastic waste ends up in either a landfill or the ocean. A significant number of plastics bre ... more

    Turning hazelnut shells into potential renewable energy source

    Biomass is attracting growing interest from researchers as a source of renewable, sustainable, and clean energy. It can be converted into bio-oil by thermochemical methods, such as gasification, liquefaction, and pyrolysis, and used to produce fuels, chemicals, and biomaterials. In Journal ... more