"Wonder Cloth," Feature Article, April 2006

FEATURE FOCUS: Microsystems

wonder cloth

A film made from carbon nanotubes may finally live up to the nanotech hype. But first, it has to get out of the lab.

by Jeffrey Winters, Associate Editor

Stronger than steel, able to glow incandescently, able to support many thousand times its own weight, the ribbons created in Ray Baughman's lab sound less like industrial material than like the stuff of Superman's cape.

Developed at the University of Texas at Dallas, a filmy, very strong material may well be the first breakthrough in bringing carbon nanotubes to the mass market, Baughman says.

Could this be the first breakthrough use for carbon nanotubes, which so far have been longer on promise than on application? It is still too early to say. But the work in Baughman's lab and elsewhere suggests that once the production cost of nanotubes comes down, they will be put to work almost immediately in any number of products.

These scanning electron micrographs show nanotube ribbons being made at the University of Texas at Dallas. As individual tubes are drawn from a dense thicket of nanotubes (top and middle), they pull away adjoining tubes to create a long, horizontal array. The resulting ribbons are then layered atop one another (bottom) to make a superstrong sheet.

Ever since they were first discovered in 1991, carbon nanotubes have been the subject of intense speculation about what they may eventually be able to do. Materials scientists have known that carbon-based molecules had the potential to possess extreme properties. Diamonds, after all, are pure carbon. But common forms of carbon, such as graphite, are soft rather than strong. The key, it turns out, is at the molecular level: The difference between garden-variety graphite and nanotubes is much the same as that between sheet metal and steel tubing.

Thanks to their tubular structure, carbon nanotubes have a measured strength about 50 times that of carbon steel. And due to differences in the way the familiar chicken-wire patterns of atoms line up across the surface, nanotubes may either act as metals (potentially carrying greater current densities than copper) or semiconductors. Some of the first experimental applications using nanotubes have used these electronic properties to create nanoscale transistors.

To a certain extent, one of the biggest stumbling blocks to using carbon nanotubes more widely has been their wild potential. With nanotubes being touted as the components of nanocomputers or of molecular assemblers—or even the cables of space elevators—more mundane applications have seemed a bit, well, boring.

But that's boring only by comparison. Using carbon nanotubes as the basis for materials applications—stronger ceramics, say, or lighter car bodies—could have enormous impact on day-to-day life, similar to the way plastics changed the world in the middle part of the 20th century.

FLEXIBLE FIBERS

A step toward that direction was taken back in 2000 when researcher Phillippe Poulin of the Paul Pascal Research Center in Pessac, France, developed a technique for extruding a fiber from a soup stocked with carbon nanotubes. As the liquid flowed, the nanotubes aligned themselves along the direction of the flow. When dried, the resulting fibers were flexible, but the researchers couldn't find a way to make them very strong.

Baughman's work has built upon Poulin's. In 2003, Baughman's team reported in the journal Nature a success in spinning threads made of nanotubes. The threads were a mixture of carbon nanotubes and polyvinyl alcohol that were extruded and spun into long fibers—as thin as a human hair and hundreds of feet long. But the Texas team was able to get the nanotubes to live up to their reputation. Tests on the material found that the threads were 17 times stronger than Kevlar and about four times tougher than spider silk.

In addition to the obvious applications that rely on high strength—cables and protective clothing—the
superthreads showed an ability to conduct and hold electrical charges, meaning that it could be possible to incorporate electronic devices directly into fabrics containing these fibers.

Even so, there were some stumbling blocks to spinning miles of superthread. The process was relatively slow. And to make a sheet one yard on a side could require more than a mile of thread. Clearly, a faster manufacturing method was needed.

After examining several possible techniques, the Dallas team discovered a new process that had less in common with making yarn than it did with removing a Band-Aid from a hairy arm. Baughman's group started by growing a carpet of multiwalled carbon nanotubes on a metallic substrate. (A multiwalled nanotube is made up of several tubes nested one inside another like Russian dolls. Such tubes are considerably cheaper and easier to make than single-walled tubes, but they aren't as strong.) Nano- tubes, each about 300 micrometers long and around 10 nanometers in diameter, grown on such a substrate can pack as closely as one trillion per square inch.

"The aspect ratio of the nanotubes we're using is such that if the diameter were one inch, the tubes would be a half-mile tall," Baughman said. "And the tubes aren't perfectly straight. They bundle in places with some neighbors."

Turning such a carpet into a sheet might not seem simple, but in an intuitive leap, Baughman's group, in collaboration with Ken Atkinson, a textile expert at the Commonwealth Scientific and Industrial Research Organization in Belmont, Australia, found a way to make a film in a single motion. A thin aerogel ribbon unspools across the top of the carpet. Where the nanotubes and the ribbon meet, tubes catch on the aerogel and are pulled from the substrate. Thanks to the close packing of the tubes and to an attractive force operating at the molecular level, a nanotube breaking away from its base will pull away other tubes. Quickly, most of the nanotubes find themselves realigned: Instead of a forest of tubes, they are arrayed longitudinally along the surface of the ribbon.

The process is actually quite rapid. Baughman said his group has been able to produce as much as 20 feet of ribbon per minute.

TARTAN TAPE

Of course, one layer of nanotubes doesn't account for much material—or strength. So Baughman's team layered several ribbons atop one another in a crosswise pattern. The aerogel substrate was then dissolved, leaving only a web of nanotubes held together by atomic forces. Even so, the membrane had a great deal of strength across the plane. Tests done in the lab showed that a sheet could support some 50,000 times its own weight. Baughman said that, pound for pound, it's stronger than a steel sheet of the same dimensions.

It isn't just the mechanical properties that make the
nanotube fabric a supersheet. The pure carbon membrane turned out to be a very flexible conductor of electricity. A swatch of material placed between two electrodes glowed incandescently when a current was applied.

Such properties open up a number of possibilities. The extraordinary strength and lightness of the nanotube sheets suggest they could first find use in high-end applications such as military aircraft or motor sports—although Baughman denied the rumor that Formula One racing teams were rushing to place the fabric into car bodies.

Other potential uses depend more on the material's electrical properties. The sheets are so thin that they're all but transparent when sandwiched between two sheets of plexiglass, suggesting they could be incorporated in automobile windshields as antennas or heating elements. The sheets also could find a home in photovoltaic cells or in artificial muscles.

But Baughman cautions that the limiting factor is still cost. As long as carbon nanotubes run in the neighborhood of $1,000 per pound, it will be difficult to find uses that are cost effective. Newer production techniques on the horizon, however, could bring down the costs under $50 per pound.

That still sounds like a lot—and for an industrial material, it is. But Baughman's supersheets don't require a lot of carbon. "You could cover an acre with a sheet that weighs only four ounces," Baughman said.

You could make a lot of superhero capes with an acre of fabric.