MIT scientists: Heat can act like sound wave when moving through pencil lead

Enlarge / Graphite rods ready to be encased in wood to make pencils. MIT scientists have shown that heat behaves like sound when moving through graphite. (credit: Jean-Christophe Verhaegen/AFP/Getty Images)

A boiling tea kettle diffuses its heat to gradually warm surrounding air, yet it will still be the warmest region even as it, too, slowly cools. But what if the kettle cooled down to room temperature almost instantly, losing its heat in a wave traveling through the material close to the speed of sound? MIT researchers have observed this rare, counterintuitive phenomenon-known as "second sound"-in graphite, the stuff of pencil lead. They described their results in a paper published earlier this week in Science.

Chances are you've never heard of the concept of "second sound," even though the phenomenon has been known for decades. "It's been confined to only a handful of materials that are really very low temperature," said co-author Keith Nelson, severely limiting its potential usefulness. There might be a paragraph or two on the topic in your average solid-state textbook, but the field "has been kind of a backwater."

With the results of this new research, that may be about to change. Graphite is a very common material, and the effect was observed at a relatively balmy (by low-temperature physics standards) temperature of around -240 degrees F. The team's theoretical models indicate it might be possible to produce the effect in graphene at something closer to room temperature in the future, thereby opening up any number of potential practical applications. For instance, microelectronics just keep getting smaller, making heat management a daunting challenge. If room-temperature graphene could rapidly carry off heat as waves, it might allow even more miniaturization.

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