July 20, 2008
The pencil is mightier than the sword
Researchers have been exploring the different properties of graphene, found in flakes produced when you write with a pencil. It turns out that the lowly tool of scribes and schoolchildren produces one of the most amazing materials known to man.
Graphene crystals are atom-thick sheets of carbon atoms connected together in hexagons, like chicken wire.
Graphene flakes are produced every time we put pencil to paper – the graphite in pencils is simply a 3D structure comprising multiple stacked layers of graphene. And yet graphene was only isolated for the first time in 2004.
In the graphene "gold rush" since then, scientists have scrambled to uncover the material's properties and discover potential applications. The large surface-to-volume ratio and high conductivity already suggest uses in ultra-small electronics.
Now, researchers have discovered that graphene has remarkable mechanical properties too. Changgu Lee and Xiaoding Wei at Columbia University, New York, took flakes of graphene 10 to 20 micrometers in diameter and laid them across a silicon wafer patterned with holes just 1 to 1.5 micrometers in diameter, like a microscopic muffin tray.
The graphene above the tiny holes was unsupported, and Lee and Wei poked at these with the diamond tip of an atomic force microscope to see how readily the graphene deformed and ruptured.
They found 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.
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."
I'm curious about that last bit, "science-fiction applications"; this stuff truly does seem to lend itself to the kind of things we've only seen in the works of Heinlein, Asimov and Clark, as well as Bob Kane.
From gossamer-thin, bullet-proof fabric to light-transmissive, electronically-active camouflage, graphene sounds like a discovery worth getting excited about.
Posted by Mike Lief at July 20, 2008 07:18 AM
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