In a Rice University lab, a black fiber the diameter of a human hair spools into a beaker of ether. Made up of pure nanotubes, the strand is the culmination of nearly a decade of experimentation. Chemical engineer Matteo Pasquali and his colleagues have spun nanotubes into fibers several hundred meters long, proving that commercially useful manufacturing techniques can be developed to produce macroscale materials from these cylindrical molecules of pure carbon.
Making carbon nanotubes into fibers was a particular dream of the late Rice professor Richard Smalley, who shared the 1996 Nobel Prize in chemistry for his discovery of the spherical carbon molecules called buckyballs. Individual nanotubes have remarkable properties: they’re lightweight, they’re strong, and they can be electrically conductive. But assembling them into large structures with these properties has been difficult.
In 2001, Smalley began trying to use liquid processing to spin carbon nanotubes into fibers that retained the tubes’ electrical and mechanical properties over kilometer lengths–an idea that, he admitted, was “really lunatic extreme” (see “Wires of Wonder,” March 2001). Such fibers would be stronger than steel and more conductive than copper. Smalley imagined them woven into cables that could efficiently carry electricity from remote wind and solar farms to populated areas–without losing energy to heat. Pasquali, who was part of the project from the beginning and took over after Smalley’s death in 2005, acknowledges that he started out as a skeptic. “I thought that it was complete lunacy, because carbon nanotubes are not soluble in fluid–and I’m a fluid guy,” he says.
Other researchers have made macroscale fibers from dry nanotubes, pulling them from vertical arrays or spinning them like wool as they emerge from a reactor. But the individual nanotubes in these fibers don’t line up, and proper alignment is critical: tangled masses of the molecules don’t carry electricity well, and they’re not strong. Pasquali knew that nanotubes brought into solution would line up like logs floating down a river, resulting in well-ordered fibers.
The group had a breakthrough in 2004, when they reasoned that the methods used to manufacture Kevlar fibers, a component of bulletproof vests, might also work with nanotubes. Like nanotubes, the Kevlar polymer is long, thin, and difficult to dissolve in solution; the fibers are made by mixing the polymer with sulfuric acid and then shooting the solution through needles grouped like the holes in a showerhead.
The Rice researchers managed to dissolve only small amounts of nanotubes using sulfuric acid. But when they used chlorosulfonic acid–a so-called superacid–they could get high concentrations of nanotubes into solution. The tubes form a liquid crystal, in which they are already aligned–a tremendous advantage in making them into fibers.
Spinning a line
Pasquali’s group starts its spinning process with single-walled nanotubes made in a nearby lab using a process originally developed by Smalley. In a high-pressure reactor where temperatures reach 1,000 °C, carbon monoxide alights on droplets of pure iron catalyst and decomposes. The carbon atoms build up into hollow cylinders about a nanometer in diameter and a few hundred nanometers long. These nanotubes emerge from the reactor in fluffy black drifts; they’re kept in five-gallon buckets stacked to the ceiling, each holding just 200 grams.