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How nanotubes break

Researchers from Rice University and the University of Minnesota have devised a computer model to predict nanotube breaks. This model "plots the likelihood or probability that a nanotube will break -- and how it's likely to break -- based on only four key variables" such as symmetry or temperature.
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Written by Roland Piquepaille, Inactive on

The media have widely spread the idea that carbon nanotubes are about 100 times stronger than steel and several times lighter. But they rarely mentioned that these nanotubes often break without warning, making their production difficult. This is why researchers from Rice University and the University of Minnesota have devised a computer model to predict nanotube breaks. This model "plots the likelihood or probability that a nanotube will break -- and how it's likely to break -- based on only four key variables" such as symmetry or temperature.

Here is the introduction of the Rice University news release about this computer model.

In theory, carbon nanotubes are 100 times stronger than steel, but in practice, scientists have struggled make nanotubes that live up to those predictions, in part, because there are still many unanswered questions about how nanotubes break and under what conditions.

This is why researchers have developed a predictive tool for nanotube breaks.

"Nanotubes break in one of two ways: the bonds either snap in a brittle fashion or they stretch and deform," said Boris Yakobson, professor of mechanical engineering and materials science and of chemistry. "We found that the underlying mechanisms that cause both types of breaks are each present at the same time."
"Even in a particular test, either type of break can occur, but we were able to map out a pattern -- based on statistical probabilities -- of what was likely to occur in a range of conditions for the whole catalog of nanotube species."

On the figure below, you can see how a perfect nanotube can break. For more information, about this process, see the reference at the bottom of this post (Credit: Proceedings of the National Academy of Sciences).

How nanotubes break

And here are more details about this phenomenon.

"The breaking mechanism for a particular nanotube depends to a great extent on its intrinsic twist called chirality," said co-author Traian Dumitrica, a former Rice postdoctoral researcher who is now assistant professor of mechanical engineering at the University of Minnesota. "Yet, temperature still influences the outcome. We were able to summarize the intricate dependence on parameters in a map, which stands as a striking example for the predictive power of simulations in materials science research."

[Note: According to Wikipedia, "in chemistry, a molecule is chiral if is not superimposable on its mirror image regardless of how it is contorted. Your hands are also chiral - mirror images of one another and non-superimposable - and chiral molecules are often described as being 'left handed' or 'right-handed.']

This research work has been published by the Proceedings of the National Academy of Sciences under the title "Symmetry-, time-, and temperature-dependent strength of carbon nanotubes" (March 31, 2006). Here is a link to the abstract.

Although the strength of carbon nanotubes has been of great interest, their ideal value has remained elusive both experimentally and theoretically. Here, we present a comprehensive analysis of underlying atomic mechanisms and evaluate the yield strain for arbitrary nanotubes at realistic conditions.
For this purpose, we combine detailed quantum mechanical computations of failure nucleation and transition-state barriers with the probabilistic approach of the rate theory. The numerical results are then summarized in a concise set of equations for the breaking strain. We reveal a competition between two alternative routes of brittle bond breaking and plastic relaxation, determine the domains of their dominance, and map the nanotube strength as a function of chiral symmetry, tensile test time, and temperature.

And for more details, you also can read the full paper (PDF format, 5 pages, 1.05 MB) from which the above illustration has been extracted.

Sources: Rice University news release, March 27, 2006; and PLoS Biology web site

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