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.
In theory, chemists could
synthesize the so-called nanotubes, which actually are nearly solid
fibers, from simple organic starting materials. This new fiber
hasnt been synthesized yet, said Crespi, but several
physicists and chemists are interested in making them, and they
may prove very useful in nanotechnology applications.
Very First Protostellar Object
Crespis 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.
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 youre 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.
Carbon fiber is easy
to produce in large quantities, and manufacturers weave it into
sheets, bars, tubes, and other shapesoften 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.
Crespis 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. .
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 teams 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 groups 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 nanometersthe 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.
Penn State University
Penn State University