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Pressure Doesn't Get Nanotubes Down
19 January 1999 8:30 pm
Scientists knew that nanotubes combine the strength of a weightlifter with the flexibility of a contortionist. But these tiny carbon hoses may also be near-perfect springs. In a recent issue of Physical Review Letters, researchers report that nanotubes can withstand enormous pressures and then shrug it off as if nothing had happened.
Nanotubes look like tiny, rolled-up fences. And because they're stronger than steel, researchers hope to use them for everything from superstrong cables to puny electrical wires. Under crushing forces, researchers thought the tubes would pack into a tight formation, then resist further compression. That wasn't the case. Physicists at Moscow State University in Russia took a thimbleful of nanotubes and squeezed them with a hydraulic press to 29 kilobars--about the pressure you would feel while swimming 18,000 kilometers underwater. They found that the sample compressed rather easily--it required only a third of the predicted pressure to squish them down into a certain space. When the pressure was off, the nanotubes sprang back to their original volume, indicating that few had broken.
"It was a real head scratcher," says a member of the team, materials scientist John Fischer of the University of Pennsylvania, Philadelphia. After a transatlantic e-mail discussion, the group realized that the tubes get squished flat, like a garden hose underfoot, before springing back to their original shape. Calculations showed that this could indeed provide the extra compressibility.
The tubes' ability to be flattened three to five times more than theory had predicted and then bounce back is "remarkable," says Gerry Lavin, a chemical engineer at DuPont in Wilmington, Delaware. But it makes sense, he says, given their ability to bend individually. What to do with the perfect spring? "I don't know," he says. "Maybe you can make a shock absorber for an automobile that's the size of a paper clip." Such a part could reduce a car's weight and increase its efficiency. It also could be a boon for car thieves: Nanotubes now cost about $1400 a gram.