Engineers make first pure nanotube fibers

Engineers make first pure nanotube fibers

BY JADE BOYD
Rice News Staff

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.

The research was published online this week by the journal Macromolecules. 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. So far, no large-scale objects have been made of pure nanotubes, due to a lack of processing methods that are viable on an
industrial scale.

Rice’s research team, led by Matteo Pasquali, assistant professor in 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. This new processing route uses no polymeric additives nor detergents, which were used in previous processing methods and are known to be an obstacle to commercial scalability and final product purity.

“As the concentration increases, the nanotubes first align themselves into spaghetti-like strands, and eventually they form tightly packed liquid crystals that can be processed into pure fibers,” said Pasquali.

Nanotubes are hollow cylinders of pure carbon that are just one atom thick. In addition to being very strong, nanotubes can also be either metals or semiconductors, which means they could be used to manufacture materials that are both “smart” and ultrastrong. NASA, for example, is researching how nanotubes could be used in aircraft and spacecraft.

Chemically, 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 soap and polymer and convert the nanotubes back into their pure form.

“To produce large objects out of nanotubes, chemical processes must use a liquid that can disperse large concentrations of pristine tubes,” Pasquali said. “Based on our findings, 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 paper is titled “Phase Behavior and Rheology of SWNTs in Superacids.” Co-authors of the article include Pasquali; Smalley; Robert Hauge, distinguished faculty fellow; Wade Adams, director, Center for Nanoscale Science and Technology; W.E. Billups, professor of chemistry; research scientists Carter Kittrell, S. Ramesh and Rajesh Saini; graduate students Virginia Davis, Lars Ericson, Nicholas Parra-Vasquez, Hua Fan and Yuhuang Wang; and undergraduate students Valentin Prieto and Jason Longoria.

This research was 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/>.