Smalley: There's Buckytubes in Our Future
Diamonds may be a girl's best friend, but to chemists like Nobel laureate Richard Smalley, ordinary graphite is the real stuff of dreams.Smalley, who delivered the annual Fritz London chemical physics lecture at Duke April 10, has good reason to be fond of graphite. In 1985, he and his associates at Rice University figured out how to "fold" sheets of the soft gray material into a completely new form of carbon - a soccer-ball-shaped molecule containing 60 carbon atoms, dubbed buckminsterfullerene because of its resemblance to architect Buckminster Fuller's geodesic domes. Smalley shared the 1996 Nobel Prize in chemistry for his discovery, which launched an entire sub-field of research devoted to buckminsterfullerene and its chemical cousins.
Some of the most intriguing discoveries in this field involve carbon nanotubes - long, hollow "buckytubes" a few nanometers in diameter, fashioned from the same repeating hexagon/pentagon pattern of carbon atoms as buckminsterfullerene. These tubes can act as a metal, a semiconductor or a semi-metal, depending on how they are put together. In each case, buckytubes match or beat the properties of their less exotic counterparts: better than silicon as a semiconductor, comparable to copper or gold as a metal, better at heat conduction than pure diamond.
The carbon-ring structure of buckytubes also makes them incredibly strong. Although the tubes are no wider than the double helix found in DNA molecules, a carbon nanotube fiber would be 600 times as strong as a steel fiber of the same weight.
"It would be the strongest fiber you could ever make out of anything - the all-time, hands-down winner," Smalley said.
Even more impressive than a buckytube's outright strength is its response to bending, said Smalley. If you bend a tube far enough, it will buckle under the weight, just like a soda straw. Unlike a soda straw, though, a tube will snap back into place after it is released, leaving no sign that it was ever bent.
"These things take a licking and keep on ticking," Smalley said. "You can twist them and bend them and tie them in a knot, and still the bottom of the tube won't bond with the top."
The key to buckytubes' stability lies in the characteristics of the graphene form of carbon. Graphene is made up of thin sheets of carbon rings, and each sheet resembles microscopic chicken wire. In a graphite crystal, the sheets are stacked on top of each other and held together by weak intermolecular forces, called London dispersion forces in honor of their discoverer, Fritz London, a Duke professor of chemistry and physics from 1939 until his death in 1954. In most materials, London dispersion forces are too weak to carry much weight, but for carbon, their importance is "stunning," Smalley said.
"Every atom in a graphene sheet is completely naked to the outside world on the top and bottom, but it will not bond with other atoms," he said. "Most elements, when they pack together as bulk materials, try to grab onto as many other atoms as they can possibly get in there.
"But here this crazy atom is able to fulfill all its bonding desires with one dimension tied behind its back. And the particular carbon bond [in buckytubes] is very directionally demanding. Once you admit it into this position, it's going to stay that way until you do something really ferocious to it."
A buckytube's innate strength, stability and conductivity should make it an ideal building material for future nanomachinery, Smalley said. Within the next five or 10 years, Smalley predicts that nanotubes will find their way into flat panel displays, making bulky television sets a thing of the past.
On a more fanciful note, buckytube fibers may prove to be the key ingredient in a futuristic elevator to outer space. The cables for a space elevator would be a few thousand miles long, and would have to withstand 63 gigapascals of pressure - almost twice the pressure exerted by Earth's entire atmosphere - to avoid snapping under their own weight.
By some estimates, buckytubes can endure 100 gigapascals of pressure - more than enough to haul astronauts, materials or even whole satellites into low-Earth orbit without expending vast amounts of shuttle fuel.
Before any of these ideas become reality, chemists must learn how to make nanotubes in far greater quantities than have been cooked up in laboratories so far. Smalley's research group hopes to make 10 kilograms of buckytubes by January 2002 - a small amount by most standards, but still almost five times the current supply.
One of Smalley's former postdoctoral students, Jie Liu, is now an assistant professor at Duke; he, too, is working on a process for making carbon nanotubes in bulk. Research published in last week's issue of Science shows that pure buckytubes can be grown like crystals under special conditions, raising scientists' hopes that someday buckytubes will be both as remarkable as diamonds and as plentiful as graphite.
"Eventually, you'll be able to get any kind of tube you like from the store," Smalley said. "From there, the sky's the limit. I think you're going to be hearing London lectures for Lord knows how many decades to come [on] this incredible element called carbon, and what it's doing - particularly in tubes."
Written by Margaret Harris.