Engineers Make First Pure Nanotube Fibers

Researchers at Rice University have discovered how to create continuous fibers out of pristine single-walled carbon nanotubes.

The process, which is similar to the one used to make Kevlar on an industrial scale, offers the first real hope of making threads, cables, and sheets of pure carbon nanotubes.

Matteo Pasquali, assistant professor of chemical engineering, displays a strand of carbon nanotube fiber.

Scientists estimate nanotubes are about 100 times stronger than steel at one-sixth the weight. By comparison, Kevlar—the fiber used in bulletproof body armor—is about five times stronger than an equal weight of steel. Hollow cylinders of pure carbon that are just one atom thick, nanotubes also can be semiconductors, which means they could be used to manufacture materials that are “smart” as well as ultrastrong. NASA, for example, is researching how nanotubes could be used in aircraft and spacecraft. Due to a lack of processing methods that are viable on an industrial scale, however, no large objects have been made of pure nanotubes.

Part of the problem is that carbon nanotubes are difficult to work with. They are strongly attracted to one another and tend to stick together in hairball-like clumps. Scientists have developed ways to untangle and sort nanotubes, but storing them after processing is difficult. To date, the medium of choice has been detergent and water solutions that contain less than 1 percent of dispersed nanotubes by volume and are processed using polymer solutions. Such concentrations are too low to support industrial processes aimed at making large nanotube fibers. Moreover, scientists haven’t found a way to remove all the detergent and polymer and convert the nanotubes back into their pure form.

Rice’s research team, led by Matteo Pasquali, assistant professor of chemical engineering, and Richard Smalley, University Professor, the Gene and Norman Hackerman Professor of Chemistry, and professor of physics, believes it has overcome the major hurdle to industrial production of large-scaled objects made of single-walled nanotubes—finding a way to store large amounts of nanotubes in liquid form. By dissolving nanotubes in strong sulfuric acid, a team of chemists and chemical engineers was able to achieve solutions containing up to 10 percent by weight of pure carbon nanotubes—more than 10 times the highest concentrations previously achieved.

“As the concentration increases, the nanotubes first align themselves into spaghetti-like strands,” said Pasquali. “Eventually, they form tightly packed liquid crystals that can be processed into pure fibers. We believe superacids can be used to make macroscale fibers and sheets of nanotubes using methods that are quite similar to those in widespread use by the chemical industry.”

The research was published online in December by the journal Macromolecules, in a paper titled “Phase Behavior and Rheology of SWNTs in Superacids,” and is funded by the Office of Naval Research, NASA, the Robert A. Welch Foundation, and the Nanoscale Science and Engineering Initiative of the National Science Foundation. The paper is available at http://pubs.acs.org/journals/mamobx/.

—Jade Boyd