BY KATHERINE BOURZAC
Researchers achieve a goal they’ve been after since the 1980s—the advance could make cars and airplanes lighter, and renewable energy more practical.
For the first time, researchers have made carbon-nanotube electrical cables that can carry as much current as copper wires. These nanotube cables could help carry more renewable power farther in the electrical grid, provide lightweight wiring for more-fuel-efficient vehicles and planes, and make connections in low-power computer chips. Researchers at Rice University have now demonstrated carbon-nanotube cables in a practical system and are designing a manufacturing line for commercial production.
Making lightweight, efficient carbon nanotube wiring as conductive as copper has been a goal of nanotechnologists since the 1980s. Individual carbon nanotubes—hollow nanoscale tubes of pure carbon—are mechanically strong and an order of magnitude more conductive than copper. But unless carbon nanotubes are put together just so, larger structures made from them don’t have the superlative properties of the individual tubes.
Years of tinkering in the lab to find the right assembly techniques and ingredients have enabled researchers led by Rice materials science professors Pulickel Ajayan and Enrique Barrera to finally make carbon nanotube cables as good as copper cables. The group’s nano cables boast a combination of properties that’s so far unprecedented. They’re mechanically strong, yet flexible enough to be knotted or woven together into long lengths of wire. They carry about 100,000 amps of current per square centimeter of material, about the same amount as copper wires, but weigh one-sixth as much. They outperform copper on a metric called current density, which means they should be able to carry more electricity over longer distances without losing energy to heat—a problem with today’s electrical grid, and with computer chips. And because they’re made of carbon, not metal, they don’t corrode.
Carbon nanotubes vary in their conductivity, length, and number of layers. The Rice group found that what worked best were relatively long, double-walled nanotubes provided by collaborators from Tsinghua University in Beijing. Electrons move through individual nanotubes very quickly, but current slows down when the electrons must jump from nanotube to nanotube. The longer the nanotubes, the fewer such jumps the electrons have to make in a given length of wire.
The process of making nano cables begins with a lump of double-walled nanotubes that have been treated to remove impurities. The researchers add sulfuric acid to the nanotubes so they can spread them into a thin film. They then grasp the edge of the film with tweezers to start making a fiber, and pull with a steady force to yield a long cable—similar to how wool yarn is made by pulling and twisting fleece. They rinse the acid from the cable and expose it to iodine vapor at high temperatures. The iodine penetrates into the nanotubes within the cable and increases the cable’s conductivity without compromising its mechanical properties. And the Rice group has shown that conductivity isn’t affected when the cables are knotted together to make greater lengths.
To demonstrate that cables made in this way can transmit a standard line voltage, they used one to connect a fluorescent lightbulb to a wall socket and left the light on for days. This work is described online in the journal Nature Scientific Reports.
“It is a testament to how mature these materials are becoming that they are able to measure conductivities that now exceed common metals,” says Michael Strano, a professor of chemical engineering at MIT who was not involved with the work. Surpassing metals, he says, “represents a milestone.”
“This is very exciting, especially considering the enormous importance of decreasing the weight of [electrical] cables in airplanes and cars” to improve fuel efficiency, says Ray Baughman, director of the NanoTech Institute and professor of chemistry at the University of Texas at Dallas. Baughman was not involved with the work.
Aerospace giant Boeing is among the companies supporting the Rice group. Other collaborators and supporters include Chevron, the U.S. Department of Energy, and NanoRidge Materials of Houston.
“The goal is to make an engineered product,” says Rice’s Barrera. “We believe what we’ve been able to do is scalable to continuous production methods.” The group has mapped out how this would be done on a manufacturing line and is currently exploring commercialization with various companies, though they have not disclosed any deals.
Though the cables are now good enough to begin thinking seriously about commercial applications, Ajayan wants to make them even better. Ajayan notes that, so far, they’ve only tested the double-walled cables’ ability to carry alternating current. Electricity is transmitted over long distances in the form of alternating current. A separate goal, Ajayan says, is to make the cables even more conductive than copper. One way to do this is to make workable cables from single-walled carbon nanotubes, which are inherently more conductive, but have been difficult to spin into fibers.