MIT Technology Review reports on 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.
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 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.
Translating the unique characteristics of individual single-walled carbon nanotubes into macroscopic materials such as fibres and sheets has been hindered by ineffective assembly. Fluid-phase assembly is particularly attractive, but the ability to dissolve nanotubes in solvents has eluded researchers for over a decade. Here, we show that single-walled nanotubes form true thermodynamic solutions in superacids, and report the full phase diagram, allowing the rational design of fluid-phase assembly processes. Single-walled nanotubes dissolve spontaneously in chlorosulphonic acid at weight concentrations of up to 0.5wt%, 1,000 times higher than previously reported in other acids. At higher concentrations, they form liquid-crystal phases that can be readily processed into fibres and sheets of controlled morphology. These results lay the foundation for bottom-up assembly of nanotubes and nanorods into functional materials.
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.”
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