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Strongest Carbon Nanotubes Revealed Serendipitously

UNPLANNED DISCOVERY
STRONGER FIBER
IDEAL TETRAHEDRAL SYMMETRY


team of researchers led by Vincent Crespi, the Downsborough Associate Professor of Physics at Penn State University, used computer simulations to discover that carbon fibers with mechanical strength comparable to that of diamond can be made. In a paper published in the September 17 issue of Physical Review Letters, Crespi, graduate student Dragan Stojkovic, and recent Ph.D. graduate Peihong Zhang reported results of simulations showing that the incredibly strong and stiff carbon tubes would have a diameter of about 0.4 nanometer. The new fiber would be roughly 10 times stiffer than a comparable amount of steel.

Figure 1: Very First Protostellar Object

In theory, chemists could synthesize the so-called nanotubes, which actually are nearly solid fibers, from simple organic 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.”
Crespi’s team simulated the electronic states and total energies of various carbon molecules while using supercomputers at SDSC, the University of Michigan, and the University of Texas. These resources, provided by the National Science Foundation under NPACI, enable the computationally intensive approach to chemistry research at colleges and universities.

UNPLANNED DISCOVERY

Crespi’s team actually made its nanotube discovery serendipitously while was studying other features of carbon compounds, including their ability to absorb hydrogen. While studying those compounds, Crespi and his students realized that carbon atoms with three weakly bonded groups plus a fourth group that is relatively tightly bound would constitute an ideal molecular substrate for making nanotubes. With this realization, they immediately adjusted their simulations to verify their idea. “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. “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 manufactured as carbon-containing polymers. It is used in the fabrication of items ranging from golf clubs and tennis rackets to bicycle frames and racing yachts.

STRONGER FIBER

Carbon fiber is easy to produce in large quantities, and 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 routinely applied to the sheets to connect the fibers to one another for additional strength.

Crespi’s team has calculated that its nanofiber would be stronger and 10,000 times thinner than commercial carbon fiber. Researchers may be able to produce the new fibers using chemical vapor deposition, a standardized industrial technology in which simple ingredients self assemble. .

IDEAL TETRAHEDRAL SYMMETRY

Several groups of researchers have previously synthesized a variety of carbon nanotubes in their laboratories. The smallest of those has a circumference of about 10 carbon atoms; however, these tubes are unstable and must be grown within larger-diameter carbon tubes or in tiny cylindrical holes in special crystals known as zeolites.

The Penn State team’s key simulation discovery was that a tetrahedral carbon atom with three weakly bonded groups and a tightly bonded one should be ideal for an extremely small-diameter nanotube. Simulations show that when each of these carbon atoms loses its weakly bonded groups and becomes interconnected to other carbon atoms, the resulting matrix is expected to have carbon-carbon bonding angles of roughly 109.5 degrees. This angle is energetically stable for carbon atoms with tetrahedral symmetry.

The Crespi group’s nanotube, when made, would be stiff, small-diameter, and chemically stable. In addition, the tube would have a circumference of only six carbon atoms, or about 0.4 nanometers—the smallest diameter theoretically possible.

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


PRINCIPAL INVESTIGATOR
Vincent Crespi
Penn State University

PARTICIPANTS
Peihong Zhang
Corning, Inc.

Dragan Stojkovic
Penn State University


www.phys/psu.edu/~crespi