Many important technologies—from battery electrodes and superconducting wires to the catalysts in fuel cells—rely on materials containing powdered particles, which can be tricky to manage. Now, in a feat that could simplify the production of many such technologies and might point the way toward some radical new ones, researchers at the University of Texas have demonstrated a way to spin yarn out of nanotubes infused with useful powdered materials.
The researchers have used the method to make strips of yarn that function as a battery electrode, others with superconducting properties, and self-cleaning yarns.
“Powders are very important functional materials because they have very high surface area,” says Ray Baughman, who directs the MacDiarmid NanoTech Institute at the University of Texas at Dallas. “The problem is that powders without form are difficult to use.”
Lithium-ion battery electrodes, for example, take advantage of the high surface area of powders to achieve greater storage density. But typically either powders must be held together by binders that add weight and solidity, or they must be sintered together into solid structures, the processes for which are complicated.
Baughman says the technology developed by his group should make it easier to work with a wide range of powdered materials. “You can take almost any powder and make a sewable, knittable, knotable, braidable yarn,” he says.
The researchers start by growing a forest of vertically aligned carbon nanotubes in a chemical reactor. Then they drag a roller over the nanotubes, which separate from the surface and get tangled up in a long, stretchy ribbon—a so-called nanotube web. These webs, Baughman’s team has discovered, can act as a host for nanoparticles and powders. The researchers spray the surface of the web with the powder and then twist it into a yarn. The powder is confined inside the spirals of the nanotube web. “When you wash it, almost all the powder is retained,” he says. The resulting yarns can be 95 to 99 percent powder by weight.
Baughman’s group used a mixture of powdered boron and magnesium to make superconducting yarns by a simple process. The conventional process for making superconducting wires involves packing the powders in copper tubes and heating and drawing them tens of times to stretch them into wires. But the superconducting yarns are heated just once to anneal the powders and form a superconducting thread.
The powders retain the properties that make them so useful, says Matteo Pasquali, professor of chemical and biomolecular engineering at Rice University, who was not involved with the work. Baughman’s method is essentially “turning particles into fibers,” he says. Chemicals can readily move in and out of the sparse nanotubes and interact with the surface of the particles trapped inside.