Sweating the small stuff Home

Free-Electron Laser Improves
Production of Carbon Nanotubes

By James Schultz

Scientists and technologists of all stripes are working intensively to explore the possibilities of an extremely strong and versatile cylinder so tiny that millions -- which in bunches look like an ebony snowflake -- could fit easily on the tip of a pin. The objects in question are known as carbon nanotubes, first discovered in 1991 as the elongated form of an all-carbon molecule.

Sometimes called CNTs, nanotubes take up an extremely small space but can connect together materials with different properties, even as their own properties can be adjusted depending on formulation. The tubes' "aspect ratio" is enormous: that is, they are very long but not wide, and like an ultra-strong rope, can be extended without sacrificing strength. CNTs have potential applications in molecular and quantum computing and as components for microelectromechanical sensors, or MEMS. The tubes could also function as a "lab on a chip," with attached microelectronics and components that could detect toxins and nerve agents in vanishingly small concentrations.

Nanotubes could also lead to an entirely new generation of materials: as strong or stronger than steel, but very lightweight. CNTs are amazingly damage-tolerant, generally displaying nearly total "elastic recovery," even under high-deformation conditions. If bent, buckled or creased the tubes are usually able to reassume their original shape once external stressors are removed.

"Nanotubes take up a very small amount of space but can connect a lot of material together," says Brian Holloway, an assistant professor in the College of William & Mary's Department of Applied Science. "You can imagine replacing metal components with nanotubes that could weigh maybe a tenth as much. One of the big reasons NASA is interested is obviously because of the cost of getting to space."

A research team led by Holloway is also intrigued by the tubes' potential. Holloway's group has used Jefferson Lab's Free-Electron Laser (FEL) to explore the fundamental science of how and why nanotubes form, paying close attention to the atomic and molecular details. Already, in experiments, the William & Mary/NASA Langley collaboration has produced tubes as good as if not better than those at other laboratories or in industry.

The next step will be to increase quantity while holding costs down, which Holloway believes will be possible using the Lab's upgrade of the FEL to 10 kilowatts.

"Right now we're interested in making more nanotubes," Holloway says. "The FEL offers a way to efficiently and cost-effectively make large amounts of high-quality tubes. Nanotubes come in a variety of flavors; the thought is we could eventually control what we call 'tube chiralities,' [properties like] structure, length and diameter."

The CNT collaboration makes the tubes by striking a metal-impregnated carbon target with FEL light. The laser vaporizes layers of a graphite annulus, essentially a thick ring mounted on a spinning quartz rod. Atoms discharge from the annulus surface, creating a plume, a kind of nanotube "spray." Under the right conditions trillions upon trillions of nanotubes can be so formed within an hour.

Conventional means of nanotube production involves a tabletop laser. In this more traditional manufacturing approach, perhaps 10 milligrams -- about one-tenth of an aspirin-bottle full -- of the tubes can be produced per hour at costs up to $200 per gram. Conversely, with a one-kilowatt FEL, up to two grams per hour, or about 100 times more nanotubes can be made, at a cost of $100 per gram. A 10-kilowatt FEL could radically alter that equation. To that end, Holloway is seeking funding from NASA and the Office of Naval Research for a three-year project whose goal would be to optimize nanotube production with the upgraded FEL in order to manufacture large quantities quickly and cheaply.

According to Gwyn Williams, FEL Basic Research Program manager, researchers are anticipating learning much more about the details of the photochemical processes involved in nanotube production once the new FEL comes back on line in 2003. Demand for the tubes is intense and growing. Whoever finds a way to make them reliably and affordably could reap the rewards, financially and otherwise, as commercial interests beat a figurative path to researchers' doors.

"A lot of people can make nanotubes. Very few can make grams or kilograms of nanotubes on time scales less than weeks," Holloway points out. "Factors other than price can drive demand. Right now there's no one who could sell you one kilogram of nanotubes per month all of the same quality, at any price."

Contact: Linda Ware
DOE/Thomas Jefferson National Accelerator Facility