How to make strong, conductive fibers hundreds of meters long.
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.
Nanotubes made in this reactor contain traces of iron that must be removed before the tubes can be turned into fibers. Graduate student Colin Young fills a glass chamber with nanotubes that have been treated with oxygen in a furnace to oxidize the iron, making it soluble. Inside a fume hood, he fastens the chamber over a flask of hydrochloric acid. He turns on a heating block under the acid to boil it. As it condenses and drips down onto the nanotubes, the acid dissolves the iron; the tubes are left untouched.
After their acid shower, graduate student Natnael Behabtu loads the nanotubes and chlorosulfonic acid into a stainless-steel tube fitted with pistons that rub the nanotubes uniformly in a single direction to encourage them to line up. The resulting viscous solution is 8 percent liquid-crystal nanotubes by weight.
He then detaches half of the chamber, and one of the pistons with it, and replaces it with a part that’s been fitted with a spinning needle. The piston pushes the liquid through a glass filter (which prevents clogging), into the needle, and out into a waiting bath of diethyl ether. The acid is soluble in the ether, but the nanotubes aren’t, so the result is a pure nanotube fiber, 50 to 100 micrometers in diameter and many meters long.
To measure the fibers’ tensile strength, Young uses glue to tack a short length of fiber onto a cardboard frame. He clamps this into the metal vises of a stress tester, cuts the frame, and pulls the fiber from either end until it breaks. The fibers can currently withstand about 350 megapascals of pressure before failing–slightly less than a human hair, which is considered fairly strong for its diameter.
The fibers’ strength depends on the friction generated where nanotube surfaces interact. Longer nanotubes generate more friction and, thus, stronger fibers. The Rice nanotubes–which Pasquali is using for the sake of convenience–are relatively short. But he’s exploring partnerships with fiber-spinning companies and carbon-nanotube manufacturers who can provide additional spinning expertise and longer nanotubes. Pasquali hopes to ultimately increase the fibers’ tensile strength more than tenfold.
There is still one major obstacle to realizing Smalley’s dream of using nanotubes to remake the electrical grid. Pasquali’s fibers have an electrical resistance of 120 microöhms per centimeter, about eight times greater than that of copper wires. The reason is that every method for growing nanotubes results in a mix of conducting and semiconducting versions. For nanotube fibers to carry enough current to displace copper, they’d need to be made up entirely of conducting nanotubes. The Rice group plans to make fibers from conducting nanotubes separated from the nonconducting tubes to determine whether such conductivities are possible. But today’s sorting process makes the nanotubes too expensive for use in electrical transmission.
Pasquali remains optimistic, however, that this second challenge will be overcome, just as he solved the problem of spinning nanotubes into long fibers. And he’s sure that when it is, strong, lightweight nanotube wires can at last replace the heavy and inefficient steel-reinforced aluminum cables used in today’s power grid, just as Smalley imagined.