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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.

measuring up

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.

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Credit: Tommy Lavergne
Video by Katherine Bourzac, edited by Brittany Sauser

Tagged: Computing, Materials

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