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Abstract:
Pound for pound, carbon nanotubes are stronger and lighter than steel, but
unlike other materials, the miniscule carbon cylinders remain remarkably
robust even when chunks of their bodies are blasted away with heat or
radiation. A Rice University study in the Feb. 16 issue of Physical Review
Letters offers the first explanation: tiny blemishes crawl over the skin of
the damaged nanotubes, sewing up larger holes as they go.

Rice science news - Nanotube, heal thyself

Houston, TX | Posted on February 15th, 2007

"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, in much the way that a valve allows steam to escape
from a kettle."

The research appears Feb. 16 issue of in 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 much longer than
they are wide, akin in shape to 100-foot garden hose, and they're 100 times
stronger than steel at one-sixth the weight.

The carbon atoms in nanotubes are joined together in six-sided hexagons, so
when scientists sketch out the arrangement on paper, nanotubes look
something like a rolled up tube of chicken wire. Yakobson's "smart repair
machine" is a deformity, a blemish in this pattern. The blemish consists of
a five-sided pentagon joined to a seven-sided heptagon and contains a total
of ten atoms. Yakobson, who specializes in using computers to decipher the
atomic pecularities of materials, discovered several years ago that
mechanically stressed nanotubes - like those being pulled very hard from
both ends - are predisposed to develop these 5/7-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 of nanotubes to
develop the 5/7 blemishes, and they revealed the blemishes' unexpected
healing powers.

"The 5/7-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."

Yakobson said the blemishes consume all larger defects, and chug along
indefinitely, rearranging atoms and healing the skin of the damaged
nanotubes. This explains how nanotubes retain their strength, even when
severely damaged. But the healing comes with a price.

"In their role as a safety valve, the 57-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 5/7-blemishes turn when they reach the end of the nanotube
and return in the opposite direction. In fact, there's only one thing that
can stop them: another 5/7 blemish. If two of the blemishes run headlong
into one other, they cancel each other out and disappear.

Research co-authors include 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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About Rice University
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research universities. It is distinguished by its: size—2,850 undergraduates
and 1,950 graduate students; selectivity—10 applicants for each place in the
freshman class; resources—an undergraduate student-to-faculty ratio of
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