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The Puzzle Over How Graphene Fails

Graphene may be the world’s strongest material. But put it under enough strain and it simply evaporates into thin air, says a new study of the way graphene breaks.

One important property of any material is its ideal strength: the force per unit area that the stuff can withstand in the absence of any instabilities in its structure. This may sound like an easy thing to measure but it is anything but. Almost all materials are riddled with instabilities such as grain boundaries and dislocations and it is these that give up the ghost, long before the material itself fails.

That’s no big deal. Physicists can well characterise the strength of ordinary materials–dislocations, grain boundaries, warts ‘n’ all–and their understanding of how these materials fail is good.

But that leaves an interesting puzzle. How do materials fail in the absence of these instabilities? What is the mechanism involved? That’s been hard to get to grips with, not least because it’s tough to make materials that are pure enough to determine their ideal strength.

All that has changed in recent years with the explosion of interest in graphene, the chicken-wire form of carbon that naturally forms into large flat sheets. These sheets are more or less perfect and a couple of years ago, a group at Columbia University in New York measured graphene’s ideal strength.

They placed a sheet of the stuff over a tiny circular hole drilled into a slab of silicon dioxide. They then pressed the tip of an atomic force microscope against the sheet until it broke, rather like piercing the seal on a jar of instant coffee. That makes it the world’s strongest material by far.

The strength of graphene turns out to be huge: it has a breaking strain of 0.25. To put that in context, if a sheet of clingfilm had the strength of graphene it could support the weight of an average sized car.

But what happens when graphene breaks? Today, Chris Marianetti and Hannah Yevick, who are also at Columbia, tackle this mystery using a computer model of the way carbon atoms bond together.

In the coffee jar experiment, graphene experiences a strain that is equal in all directions. Marianetti and Yevick say that their model predicts that in these conditions, graphene breaks by undergoing a phase change: it turns into benzene rings.

That’s a fascinating result. Put graphene under enough strain and, essentially, it evaporates into thin air.

But there’s a problem. Their model predicts a breaking strain of just 0.15 and that’s significantly less than was actually measured.

What accounts for the difference? Marianetti and Yevick say there could be a number of factors. For example, they say the tip of the atomic force microscope may have reacted with the graphene influencing the measurement. They also say their calculations assume that graphene is at zero temperature but the measurements were actually carried out at room temperature.

None of these points are fleshed out by Marianetti and Yevick and that’s worrying. The difference between the breaking strain their model predicts and the actual measured value is so great and that, in the absence of a convincing explanation for the difference, another possibility needs to be considered: that their model has failed to capture some important additional mechanism that they’ve overlooked. In other words, it may just be plain wrong.

That would be a shame: mechanical failure by evaporation is just too cool to be wrong!

Ref: arxiv.org/abs/1004.1849: Failure Mechanisms Of Graphene Under Tension

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