Spring 2002 -- Vol. 19, No. 1

Supercomputer Simulations Reveal Strongest Carbon Nanotubes

A team of researchers led by Vincent Crespi, the Downsborough Associate Professor of Physics at Penn State University, has used computer simulations to discover carbon fibers with mechanical strength comparable to that of diamond. In a paper published in the 17 September 2001 issue of Physical Review Letters, Crespi, graduate student Dragan Stojkovic, and recent Ph.D. graduate Peihong Zhang report that they discovered incredibly strong and stiff carbon tubes about 0.4 nanometers in diameter. The so-called nanotubes could theoretically be made from simple starting materials.

"This new fiber hasn’t been synthesized 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 Center (SDSC), the University of Michigan, and the University of Texas, Crespi’s team simulated the electronic states and total energies of various carbon molecules. This computationally intensive approach to chemistry research at colleges and universities has been made possible with supercomputers provided by the National Science Foundation under its National Partnership for Advanced Computational Infrastructure (NPACI). SDSC, which is on the campus of the University of California, San Diego, is the leading-edge site for NPACI.

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 realize 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 realized it could be done."

Commercially available "carbon fiber" is 6 to 10 micrometers thick, or one-fifth the thickness of a human hair, and made of carbon-containing polymers. It is used to make items ranging from golf clubs and tennis rackets to bicycle frames and racing yachts. While this type of carbon fiber is weaker than carbon nanotubes, it is easy to produce in large quantities. Manufacturers weave it into sheets, bars, tubes, and other shapes – often in several overlapping layers to increase their strength. Binders such as epoxy resins are often applied to the sheets to connect the fibers to one another for additional strength.

Carbon nanotubes are 10,000 times thinner than commercial carbon fiber. Researchers make them using chemical vapor deposition, a standardized industrial technology in which simple ingredients self assemble. Crespi said vapor 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."

This research was funded by the National Science Foundation and the Army Research Office.
Ron Graham, San Diego Supercomputer Center

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