Little tiny hairs are way cool


Fisher (above) and Johnson with a soot generator. The soot is the source of the graphite used to make nanotubes.

Photo by Candace diCarlo


A piece of lint may hold the key to faster computers, cooler motors and more heat-resistant aircraft. But first, scientists have to get the lint in line.

Needless to say, this is no ordinary lint. The stuff we’re talking about here contains hundreds of thousands of nanotubes — cylinders of pure carbon about 1/10,000th the width of a human hair.

Scientists experimenting with these tiny tubes have already discovered their incredible strength and their superior electrical conductivity. Now, Penn scientists have found that they’re excellent heat conductors, too.

Alan T. Johnson, assistant professor of physics, said that this most recent discovery is in part the result of an oversight.

Soon after the first nanotubes were made by zapping graphite with lasers, he said, “People realized right away that they were interesting for their electronic properties, and because of their strength — the carbon-carbon bond is really strong; diamonds confirm that. But thermal properties in general just slipped by the wayside, even though diamonds are also excellent thermal conductors.”

That began to change when a Michigan State University theoretical physicist, David Tomanek, did calculations that predicted that a single nanotube would be 10 times more efficient at conducting heat than any other material. At about the same time, Professor of Materials Science and Engineering John E. Fischer, Penn postdoctoral fellow James Hone (now at Caltech) and Johnson were performing physical experiments on bundles of nanotubes to determine their heat-conducting characteristics.

The Penn team’s results backed up Tomanek’s predictions, but also revealed something even better: when the nanotubes were packed into bundles, heat continued to flow along each tube rather than from tube to tube as usually happens when linear heat conductors are grouped together. This means nanotubes are more efficient at channeling heat.

This discovery has significant implications. Indeed, said Fischer, “Our theoretical friends, who have these amazing imaginations, have looked at our work and imagined a [sound wave] laser, which would do for heat what the laser did for light.”

But that’s a long way off. Before that can happen, scientists have to refine the structure of nanotubes first. Today’s nanotubes do not live up to their full potential. The ones the Penn team studied were actually five times worse at conducting heat than diamonds were. But this was still much better than calculations had predicted, and Johnson said that the results still supported the higher performance claim.

Fischer attributed the discrepancy to defects in this brand-new material. “We know diamonds are superb thermal conductors because they have very few defects,” he said. “These nanotubes have only been around for five years, while diamonds have been around since people began making engagement rings.”

In the meantime, in order to use nanotubes’ superior heat conductivity in real-world applications such as computer chips or electric motors, one of two things has to happen, said Fischer. “You either have to get nanotubes to the low-defect state or work on small-length scale applications where the defects are not an issue.”

This should keep materials scientists around the world busy for a while.


Originally published on October 12, 2000