[Davis et al., Nature Nanotechnology, (2009), DOI:  10.1038/nnano.2009.302] 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 Pasquali, one of the researchers 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.

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, 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."