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Supercomputer simulations reveal strongest carbon nanotubes

Jason Ford
A team of researchers Penn State University have used computer simulations to discover carbon fibres with mechanical strength comparable to that of diamond.

A team of researchers led by Vincent Crespi, the Downsborough Associate Professor of Physics at Penn State University, have used computer simulations to discover carbon fibres with mechanical strength comparable to that of diamond.

Crespi's colleagues, graduate student Dragan Stojkovic, and Ph.D. graduate Peihong Zhang report that they discovered incredibly strong and stiff carbon tubes about 0.4 nanometers in diameter, which could theoretically be made from simple starting materials.

'This new fibre hasn't been synthesised yet,' said Crespi, 'but several physicists and chemists are interested in making them, and they may prove very useful in nanotechnology applications.'

Using supercomputers at the San Diego Supercomputer Centre (SDSC), the University of Michigan, and the University of Texas, Crespi's team simulated the electronic states and total energies of various carbon molecules.

The nanotube discovery by Crespi's team was made serendipitously while its members were studying unrelated features of carbon compounds. 'This is one of those sideways inspirations that comes when you're looking at one thing and you suddenly realise it has a different application,' said Crespi. He immediately adjusted the focus of his simulations. 'Actually, I was motivated to make this strong nanotube the moment I realised it could be done.'

Commercially available carbon fibre is 6 to 10 micrometers thick, and made of carbon-containing polymers. While this type of carbon fibre is weaker than carbon nanotubes, it is easy to produce in large quantities.

Manufacturers weave it into sheets, bars and tubes - often in several overlapping layers to increase their strength. Binders such as epoxy resins are often applied to the sheets to connect the fibres to one another for additional strength.

Carbon nanotubes are 10,000 times thinner than commercial carbon fibre. Researchers make them using chemical vapour deposition, a standardised industrial technology in which simple ingredients self assemble.

Crespi said vapour deposition also would most likely be used to make the much stronger version of nanotube that his group discovered.

Not all nanotubes have the same properties. The smallest diameter nanotubes created to date have a circumference of about 10 carbon atoms. These tubes are not stable and must be grown within larger-diameter carbon tubes or in tiny cylindrical holes in special crystals known as zeolites.

The Penn State team recently made a key discovery that a particular type of tetrahedral carbon atom - one with three weakly bonded groups and a relatively tightly bonded group - had special properties.

When connected to one another, these molecules have carbon-carbon bonding angles of about 109.5 degrees, which also is the ideal bonding angle of carbon atoms with tetrahedral symmetry.

In addition, the stiff, small diameter, and chemically stable carbon nanotube discovered by the researchers has a circumference of only six carbon atoms, or about 0.4 nanometers - the smallest diameter theoretically possible.

'Based on our calculations, these new nanotubes are about 40 percent stronger than the other nanotubes formed using the same number of atoms,' said Crespi. 'In fact, the nanotubes we simulated may well be the stiffest one-dimensional systems possible.'


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