The fiber is unique owing to the strength of its knots. Fibers are typically most likely to snap under tension at the location of the knot, but this new fiber has what the researchers call “100 percent knot efficiency,” meaning the fiber is as likely to fail anywhere along its length as at the knot.
“To see this is very strange,” Rice chemist James Tour said. “The knot is as strong as any other part of the fiber. That never happens in a carbon fiber or polymer fibers.”
The material could be used for raising the strength of products made of carbon fiber, like composites for robust yet light aircraft parts or fabric for bulletproof vests.
It’s all due to the distinctive properties of graphene oxide flakes. Created in a process patented by Rice a few years ago, the flakes are chemically extracted from graphite. They have an average diameter of 22 microns, a quarter the width of an average human hair. Yet they’re huge compared to the petroleum-based pitch used in current carbon fiber.
Ten Thousand Times Larger
“The pitch particles are two nanometers in size, which makes our flakes about ten thousand times larger,” said lead author and Rice graduate student Changsheng Xiang.
As with pitch, the weak van der Waals force binds together the graphene flakes. But unlike pitch, the atom-thick flakes have a massive surface area and cling to each other like the scales on a fish do, when the flakes are pulled into a fiber.
The wet-spinning process used to fabricate the fiber is close to one recently used to make highly conductive fibers out of nanotubes, except in this case Xiang just used water as the solvent rather than a super acid.
Flexibility at the knot is good by reason of the fiber’s bending modulus, Xiang said. “Because graphene oxide has very low bending modulus, it thinks there’s no knot there,” he said.
Stretching the Limits of Carbon Fiber
Industrial carbon fibers, used in ultra-light materials for products as varied as baseball bats to bicycles to stealth fighter jets, have not been improved on much in decades. This because the chemistry involved is already approaching its limits. But the new carbon fibers spun at room temperature at Rice show impressive tensile strength and modulus and have the potential to be even stronger when annealed at higher temperatures.
Changsheng Xiang, Colin C. Young, Xuan Wang, Zheng Yan, Chi-Chau Hwang, Gabriel Cerioti, Jian Lin,Junichiro Kono, Matteo Pasquali,James M. Tour Advanced Materials, DOI: 10.1002/adma.201301065
Photos credit: Tour Group/Rice University