July 18, 2008

Graphene is the strongest material ever

A new and highly reliable approach for making graphene membranes of a macroscopic size (currently up to 100 μm in diameter) provided a lot fo graphene samples for strength tests Such measurements had never been taken before because they must be performed on perfect samples of graphene, with no tears or missing atoms.

Graphene transistors could take the heat of being very high performance computer processors. Graphene would make durable, mechanically operated electrical switch for communications devices including cell phones and advanced radar.

James Hone, mechanical-engineering professors at Columbia University, compares his material strength test to stretching a piece of plastic wrap over the top of a coffee cup, and measuring the force that it takes to puncture it with a pencil. If he could get a large enough piece of the material to lay over the top of a coffee cup, he says, graphene would be strong enough to support the weight of a car balanced atop the pencil.

New Scientist magazine indicates that the graphene could be pushed downwards by 100 nanometres with a force of up to 2.9 micronewtons before rupturing. The researchers estimate that graphene has a breaking strength of 55 newtons per metre.

"As a way of visualising the force needed to break the membranes, imagine trying to puncture a sheet of graphene that is as thick as ordinary plastic food wrap – typically 100 micrometers thick," says James Hone, head of the laboratory at Columbia in which Lee studies. "It would require a force of over 20,000 newtons, equivalent to the weight of a 2000 kilogram car."

That strength puts graphene literally "off the chart" of the strongest materials measured, Hone says. "These measurements constitute a benchmark of strength that a macroscopic system will never achieve, but can hope to approach," he says.

In separate work, Tim Booth and Peter Blake at the University of Manchester, UK, are well on the way to bringing atomically perfect graphene out of the nanoscopic and into to the macroscopic world. Their team has patented a new method to produce free-standing graphene flakes up to 100 micrometers in diameter.

Booth and Blake have realised that acrylic glass (PMMA) has the same optical properties as silicon and can also highlight graphene flakes. It however easily dissolves away in acetone, a less aggressive chemical that doesn't alter graphene. Using their technique, Booth and Blake can easily isolate large crystals.

We are limited only by the size of graphene flakes available," says Booth. "There is no reason that the method will not scale up to much larger flakes."

Using these flakes, Booth and Blake have also found that graphene is extraordinarily stiff. A crystal supported on just one side extends nearly 10 micrometers without any support – equivalent to an unsupported sheet of paper 100 metres in length. It had previously been assumed that graphene would curl up if left unsupported.

Graphene could be added to polymers to form super-strength composites, Booth says. "However, it is likely the most interesting applications will result from a unique combination of graphene's properties: transparency, electronic structure, stiffness, thermal conductivity," he says. "That could help achieve science-fiction applications."