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Carbon nanotubes 'heal' themselves
Carbon nanotubes are stronger and lighter than steel but unlike other materials, the miniscule cylinders of carbon, no wider than a strand of DNA, remain remarkably robust even when assaulted with heat or radiation. A new study by Rice scientists offers the first explanation: tiny blemishes crawl over the skin of the damaged tubes, sewing up larger holes as they go.
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"The shape and direction of this imperfection does not change, and it never gets any larger," said lead researcher Boris Yakobson, professor of mechanical engineering and materials science and of chemistry. "We were amazed by it, but upon further study we found a good explanation. The atomic irregularity acts as a kind of safety valve, allowing the nanotube to release excess energy.”
The research appeared in Feb. 16 issue of Physical Review Letters.
Carbon nanotubes are hollow cylinders of pure carbon that measure about a billionth of a meter, or one nanometer, across. They are longer than they are wide, similar in shape to a garden hose, and are 100 times stronger than steel at one-sixth the weight.
The carbon atoms in nanotubes are joined in six-sided hexagons, so when scientists sketch their configuration they resemble rolled up coils of chicken wire. Yakobson's “smart repair machine” is a deformity in this pattern. The blemish consists of a pentagon joined to a heptagon, and contains 10 atoms.
Yakobson, who specializes in using computers to decipher the atomic peculiarities of materials, discovered several years ago that mechanically stressed nanotubes are predisposed to developing these five-to-seven defects due to the complex interplay of thermodynamic forces at work in the nanotube.
In the latest study, Yakobson, research associate Feng Ding and students examined the effects of other types of stress, including exposure to heat and radiation. The tests confirmed the predisposition and revealed the unexpected healing powers.
“The five-to-seven blemishes move across the surface of the nanotube like a steamship, giving off puffs of carbon gas,” said Ding. “In their wake, the skin of the tube appears pristine, in its characteristic hexagonal arrangement.”
The healing, however, comes with a price.
“In their role as a safety valve, the five-to-seven steamers give off energy and mass, which is released as pairs of gaseous carbon atoms,” Yakobson said. “Since they never change shape or stop moving, they ever so slowly eat away the surface of the nanotube, one pair of atoms at a time.”
Yakobson said the five-to-seven blemishes turn when they reach the end of the nanotube and return in the opposite direction. Only another five-to-seven blemish can stop them. If two blemishes run together, they neutralize each other and disappear.
Research co-authors are graduate students Kun Jiao and Mingqi Wu. The research was supported by the Office of Naval Research, the National Science Foundation and the Robert A. Welch Foundation.
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