Tiny Tubes With the Strength of Steel

Nanoscopic size and unusual electrical properties have made carbon nanotubes one of the hottest new materials to come out of physics labs in recent years. They're also one of the strongest, according a study published in the 12 June issue of Physical Review Letters; in fact, researchers say they're so sturdy that they make promising ingredients for new, ultratough lightweight materials.

Carbon nanotubes, discovered in 1991 in Japan as by-products of the manufacture of buckyballs, are chicken-wire-like lattices of carbon atoms rolled up in tubes. The thinnest ones are single-walled structures just over a nanometer in diameter. Many consider them to be elongated buckyballs, hence their nickname "buckytubes."

To determine the tubes' strength, researchers prepared samples of "buckytube paper," dense pads of nanotubes, which they tore apart. From the ragged edges dangled bundles of single-walled tubes. Then they used an electron beam from a scanning electron microscope to "nanoweld" a bundle (or "rope") of 100 to 200 single-walled nanotubes to the tip of an atomic force microscope (AFM), a tiny cantilever equipped with a very sharp tip. Next, the team carefully stretched the miniature rope by exerting a force on the tip, enabling them to measure the force at which it broke. After dividing that force by the number of nanotubes in the rope, the team concluded that each nanotube was about 10 times stronger than a same-sized steel wire.

Current AFM technology doesn't allow the researchers to grab just one nanotube and pull it, but that's the ultimate goal, says physicist Rodney Ruoff of Washington University in St. Louis, who led the team. And he expects those measurements to show that single tubes are the strongest material ever made.

The current study will stimulate research into the use of nanotubes in new materials to replace composite materials containing carbon fibers, says Pierre Petit of the University of Strasbourg in France. Ruoff is convinced that nanotube-based materials will eventually be used in airplanes and spacecraft--and perhaps even the deployable mirrors of future space instruments, such as the Next Generation Space Telescope.