Monday, November 19, 2007

Superstrong Carbon-Nanotube Fibers

A new process could make nanotube fibers that are strong enough to stop bullets.

By Mason Inman

Nano order: Researchers have improved techniques for spinning fibers of carbon nanotubes: they make the nanotubes align in the fiber, creating fibers as strong as, or stronger than, materials such as Kevlar that are used in bullet-proof vests. Also, the nanotube fibers, unlike regular ropes, can be knotted without hurting their strength much. Credit: Juan Vilatela

Multimedia •  See how carbon nanotubes are spun into ultrastrong fiber.

Carbon nanotubes spun to form long yarnlike fibers could outperform even the strongest bullet-proof materials on the market, but turning nanotubes into such materials has proved to be a challenge. Now researchers say that they have improved the method of making the fibers: they can pull them from a hot furnace faster, make the nanotubes line up better, and vastly improve their strength. While the carbon-nanotube fibers can still be made only in small batches--and only in short lengths, experts say--the fibers show great promise for ultrastrong, resilient materials, with possible applications from body armor to oil drilling.

Carbon nanotubes are pipelike carbon molecules with walls just one atom thick. They are extremely strong, electrically conductive--and hard to make reliably. Many research groups have been toiling to create longer carbon nanotubes and build them into longer strands that could be used for tough fabrics, and even efficient power lines. (See "10 Emerging Technologies.")

Alan Windle, a professor of materials science at the University of Cambridge, in England, made and tested the new nanotube fibers along with researchers at the Natick Soldier Research Development Center, in Massachusetts. Windle and his colleagues tugged on the nanotube fibers, finding that the weaker ones snapped at stresses around one gigapascal, making them comparable to steel, gram for gram.

The better-performing carbon-nanotube fibers broke at around six gigapascals, beating the strengths that manufacturers report for materials used in bullet-proof vests, such as Kevlar. These nanotube fibers matched the highest reported strengths for a couple of the strongest commercially available fibers, Zylon and Dyneema, also used in bullet-proof vests. A lone, extremely strong nanotube fiber was off the charts, reaching nine gigapascals of stress--far beyond any other reported material--before breaking. Earlier work with carbon nanotubes has produced fibers that withstand at most three gigapascals.

"We're pleased with the results, but I wouldn't say we're surprised," Windle says. "It's known that the properties of individual nanotubes are still five times better." He adds, "This makes me optimistic. There's still a huge amount of room for improvement."

To make the fibers, the researchers used a method pioneered by Windle's group in 2004 in which a furnace vaporizes carbon and blows out a stream of nanotubes. When these carbon nanotubes are captured in midair and spun around a spool, they form a fiber composed of billions of the molecules aligned along the length of the nanotube.

By tweaking the temperature of the furnace and adjusting how quickly they wind up the fiber, the researchers optimized the process, making fibers 0.3 times stronger than those that other groups have made. The researchers report that the improvement is largely because with faster winding, the nanotubes align better and pack together more tightly. They also added a step to make the fibers more dense. The team ran the fibers through acetone gas, which condensed on the fibers, forming a liquid. "There's a surface tension effect that pulls the nanotubes together," which boosts their strength, Windle says.