Source: University Of Pennsylvania (http://www.upenn.edu/)
Date: Posted 9/20/2000
In Another Coup For Carbon Nanotubes, Penn Scientists Find The Tiny Cylinders
Of Pure Carbon May Top All Other Known Materials In Heat Conduction
PHILADELPHIA -- New research from the University of Pennsylvania indicates
that carbon nanotubes, filaments of pure carbon less than one ten-thousandth
the width of a human hair, may be the best heat-conducting material man has
ever known. The findings suggest that these exotic strands, already heralded
for their unparalleled strength and unique ability to adopt the electrical
properties of either semiconductors or perfect metals, may someday also find
applications as miniature heat conduits in a host of devices and materials.
A Penn team led by materials scientist John E. Fischer, Ph.D., and physicist
Alan T. Johnson, Ph.D., offers these first details on carbon nanotubes'
thermal properties in a paper appearing in the Sept. 8 issue of the journal
Science.
For some time, scientists have been intrigued by carbon nanotubes, pure
carbon cylinders with walls just one atom thick. First created a decade ago
by zapping graphite with lasers, the structures have become one of the
marvels of the nanotechnology world -- 100 times as strong as steel and
capable of far greater electrical conductivity than other carbon-based
materials. Researchers have envisioned the miniature strands bulking up
brittle plastics and conducting current in ever-smaller electrical circuits,
among dozens of other possibilities.
Carbon nanotubes' newfound ability to conduct heat suggests applications far
beyond those that call on their strength and electrical conductivity, said
Dr. Johnson, an assistant professor of physics at Penn. As computing power
has skyrocketed, the infinitesimal heat generated by each circuit on a
microchip has proved a headache for computer designers and manufacturers, who
have few ways to dissipate the considerable heat that results from millions
of circuits operating in tandem. Next-generation computer designs might
circumvent this problem with judiciously placed carbon nanotubes to direct
heat away from sensitive circuitry.
Similarly, carbon nanotubes used as heat sinks in electric motors could allow
for the introduction of plastic parts that might otherwise melt under the
motors' intense heat. The tiny structures could also be embedded in materials
regularly called upon to withstand extreme heat, such as those that form the
exterior panels of airplanes and rockets.
Heat energy in nanotubes is carried by sound waves; in materials that are
optimal conductors of heat, these waves move very rapidly in an essentially
one-dimensional direction. Drs. Fischer and Johnson found that sound waves
bearing thermal energy travel straight down individual carbon nanotubes at
roughly 10,000 meters per second, behavior consistent with superior thermal
conductivity. But they also unexpectedly determined that even when carbon
nanotubes are bundled together -- like individual filaments welded together
into the giant cables that support suspension bridges -- the bonds between
the individual nanotubes remain so weak that heat essentially doesn't
transcend them.
"Scientists had predicted that two-dimensional or three-dimensional arrays of
carbon nanotubes would permit the sound waves carrying heat to scatter in all
directions, greatly reducing thermal conductivity," said Dr. Fischer, a
professor of materials science and engineering in Penn's Laboratory for
Research on the Structure of Matter. "Our experiments showed that even within
bundles of nanotubes, sound waves remain remarkably one-dimensional."
"The sound waves don't fan out and dissipate because the bonds between
nanotubes in a bundle are so weak," Dr. Johnson said. "In terms of bonding
strength, you can think of nanotubes in a bundle almost like dried spaghetti
sliding freely back and forth when you shake its box."
Ironically, the same weak linkages that make carbon nanotubes superior for
heat conductance could deflate scientists' earlier expectation that bun-dles
of them would provide unrivaled mechanical strength. While the individual
nanotubes are extremely strong, the weak bonding Drs. Fischer and Johnson
observed between nanotubes would need to be overcome to translate this
strength to a thicker structure.
Drs. Fischer and Johnson were joined in the research by James Hone, a former
Penn postdoctoral researcher now at the California Institute of Technology;
Bertram Batlogg of Lucent Technologies; and Zdenek Benes, a Penn graduate
student. The work was sponsored by the National Science Foundation and the
U.S. Department of Energy.
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-- Note: This story has been adapted from a news release issued by University Of Pennsylvania for journalists and other members of the public. If you wish to quote from any part of this story, please credit University Of Pennsylvania as the original source. You may also wish to include the following link in any citation:http://www.sciencedaily.com/releases/2000/09/000913213024.htm
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