Nanotubes Take the Pressure

Nanotubes--tiny carbon straws just billionths of a meter in diameter--are thought to be some of the toughest stuff ever made. But their small size makes these mighty tubes hard to test. Now physicists say that simply by studying the shape of the tubes as they bend, they can tell how they hold up under large compressive forces. They conclude, in the current Physical Review Letters, that nanotubes can take a hundred times the stress of any other known fiber without buckling.

Press on a drinking straw from both ends, and at some point it will buckle. Lacking any way to grip and crush a single nanotube, physicist Daniel Wagner and colleagues at the Weizmann Institute of Science in Rehovot, Israel, tested them en masse. The team threw a dash of nanotubes into a liquid polymer resin and let it harden. As the plastic hardened, it shrunk, putting tremendous pressure on the embedded tubes. Near the surface, some of the tubes happened to be fixed at both ends but free in the middle. These tubes buckled, forming loops and kinks that stuck out like the fibers on a rug.

The arc and shape of the buckles depends on the compressive strength of the tube, Wagner says. Using transmission electron microscopy, the team imaged individual tubes and sized up the kinks. From that they were able to calculate that the tubes could withstand a compressive stress of roughly a hundred billion Pascals (equivalent to the pressure you would feel balancing a battleship on your head) without buckling. That's about what theory predicted, Wagner says, "but the numbers are so gigantic you're almost afraid to publish them."

"What this shows is that the tubes are indeed far stronger than anything else," says Jerzy Bernholc, a physicist at North Carolina State University in Raleigh. It also indicates that the tubes are sticking strongly to the plastic, otherwise the plastic wouldn't be able to bend them, he points out. "What you have is a highly reinforced polymer." If someone can figure out how to make tons of tubes cheaply, Bernholc says, combining them with polymers could make superstrong materials for products such as bullet-proof vests.