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.
Nano loom: Carbon-nanotube webs are pulled and twisted to make yarns. The four webs here are 5.5 centimeters wide.
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.
Pasquali heads a project at Rice aimed at making carbon-nanotube fibers that are very dense and therefore very strong and conductive. These pure nanotube fibers could eventually be used as low-loss electrical transmission cables or in super-strong structural materials. "Once you have a fiber, you can weave it, put it into a polymer [such as fiberglass], or make a fabric," says Pasquali, who notes that textile processing is relatively low-cost.
Baughman's team made a battery-electrode fabric using lithium-iron-phosphate powders. The fabric is almost 99 percent active material, so it could be used to make lightweight batteries.
And of course, once something can be made into a fabric, it can also be worn.
Yi Cui, associate professor of materials science and engineering at Stanford University, is also developing textile-based energy-storage devices. He believes that wearable power supplies could one day power everyday gadgets. The important qualities are cost, weight, and the ability to charge and discharge rapidly, and textile electrodes seem to be ideal in these respects. But they are unlikely to scale well for applications that require a high total energy-storage capacity, like batteries for electric cars.
The University of Texas researchers plan to take the project in several directions. In addition to testing different powders, they are experimenting with different ways of depositing them.
Someday, the yarns could be used to produce large quantities of material for structural manufacturing. "Right now it's more sensible to talk about batteries, not airplane wings, because of the tonnage [of materials] required," says Baughman. His group is working with a few companies to further develop the yarns, including chemical manufacturer Lintech and carbon-nanotube textile maker Nanocomp.
By Katherine Bourzac
From Technology Review
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