A new method for assembling carbon nanotubes has been used to create fibers hundreds of meters long. Individual carbon nanotubes are strong, lightweight, and electrically conductive, and could be valuable as, among other things, electrical transmission wires. But aligning masses of the nanotubes into well-ordered materials such as fibers has proven challenging at a scale suitable for manufacturing. By processing carbon nanotubes in a solution called a superacid, researchers at Rice University have made long fibers that might be used as lightweight, efficient wires for the electrical grid or as the basis of structural materials and conductive textiles.
Others have made carbon-nanotube fibers by pulling the tubes from solid hair-like arrays or by spinning them like wool as they emerge from a chemical reactor. The problem with starting from a solid, says Rice chemical engineering professor Matteo Pasquali, is that “the alignment is not spectacular, and these methods are difficult to scale up.” The better aligned and ordered the individual nanotubes in a larger structure, the better the collective structure’s electrical and mechanical properties. Using the Rice methods, well-aligned nanotube fibers can be made on a large scale, shot out from a nozzle similar to a showerhead.
The late Nobel laureate Richard Smalley started the Rice project in 2001. Smalley knew solution-processing would be a good way to assemble nanotube fibers and films because of nanotubes’ shape. Carbon nanotubes are much longer than they are wide, so when they’re in a flowing solution, they line up like logs floating down a river. But carbon nanotubes aren’t soluble in conventional solvents. The Rice group laid the foundations for liquid processing of the nanotubes five years ago, when they discovered that sulfuric acid brings the nanotubes into solution by coating their surfaces with positively charged ions.
For the past five years, the Rice group has used microscopy to study nanotube solutions made in several different acids. “There was no quick experiment,” Pasquali says. “We had to be very deliberate. We now understand how the solution processing works, the knobs to control the nanotubes, and how to predict what they’ll do.” The best solvent for processing the tubes, according to work published this month in the journal Nature Nanotechnology, is chlorosulphonic acid. Nanotubes spontaneously dissolve in this acid at concentrations 1,000 times greater than they do in any other solvent.
The Rice group has used acid processing methods to assemble carbon nanotubes into fibers 50 micrometers thick and hundreds of meters long. “There are no limitations on the fiber length,” says Pasquali. The Rice group demonstrated its assembly method with high-quality, single-walled carbon nanotubes.
So far, the group has made fibers that are highly conductive but not as strong as other carbon materials. Pasquali says the strength of the fibers could probably be improved tenfold by using longer carbon nanotubes. “We’re now working on a project for making electrical transmission lines,” says Pasquali. “Metallic nanotubes conduct electricity better than copper, they’re lighter, and they fail less often.”
One important hurdle for large-scale manufacturing of carbon nanotubes remains: Today, there aren’t any good methods for making the nanotubes themselves in large, pure batches. In order to make nanotube transmission lines, for example, the Rice group would need to start with a large batch of nanotubes containing all metallic nanotubes and no semiconducting ones. Last month, chemists at the Honda Research Institute published a paper in Science describing a method for making large amounts of metallic nanotubes that Pasquali says is promising. “For transmission lines you need to make tons, and there are no methods now to do that,” he says. “We are one miracle away.”