The next generation of carbon fiber composites could be ten times stronger and stiffer than present materials, thanks to a neat trick to reinforce bundles of carbon nanotubes reported at the American Physical Society (APS) meeting in Dallas today. The work is funded by the US military, which is interested in developing stronger lightweight armour and bullet-proof vests.

Carbon nanotubes are some of the strongest and stiffest materials known, and a growing area of research involves spinning them into flexible yarns – bundles of fibers that can be treated to produce superconducting cables, or electrodes for batteries or fuel cells. Textiles, including armour, are another important potential application of nanotube yarns. To date a major problem has been scaling up the strength and stiffness from individual nanotubes to a meter-sized yarn, as bundles of nanotubes tend to slide past each other. The strength limit of a carbon nanotube yarn at the moment is about 2 gigapascals, less than the kevlar that is often used for commercial body armor, and far below the strength of an individual nanotube.

Tobin Filleter of Northwestern University in Evanston, Illinois, and his colleagues attempted to tackle this issue. They exposed bundles of fibers to a dose of high energy electron radiation that created covalent bonds between individual nanotubes. The resulting bundles had strengths of up to 17 Gigapascals (GPA, about 10 times that for unexposed bundles) and a stiffness of 650 GPA (about 13 times that of unexposed). “We envision this could be the next generation carbon fiber composite,” Filleter says.

Ray Baughman of the University of Texas, Dallas, who demonstrated superconducting cables and other applications of treated carbon nanotube yarn in a paper in Science in January – including demonstrating that a nanotube yarn woven into a textile could survive a high-temperature wash cycle — says he’s really impressed by Filleter’s group’s results. “I’ve never seen anyone get up to 17 GPA for bundles,” he says, “I thought it was very fascinating.” Baughman cautions a major problem will be scaling the electron irradiation technique up, as so far the group has only irradiated bundles that are tens of nanometers in size. Meter-sized yarns will be hard to penetrate and the treatment may not work on its interior, leaving spots of weakness in the middle. Filleter agrees that this will be a challenge, but says that he’s hoping to demonstrate the irradiation technique could be used at an earlier stage in the processing, before the bundles are woven into larger yarns. The results are unpublished but have been submitted to Advanced Materials.

Image: a millimetre-sized nanotube yarn / M. Naraghi et al ACS Nano 4, 6463 (2010)