Sep. 20, 2000 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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