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Scientists Watch As Heat Moves Through ‘Pencil Lead’ at the Speed of Sound



Scientists have seen something magical happen inside graphite, the material your pencil is made of: the heat moved in waves at the speed of sound.

This is rather rad for a couple of reasons: the heat should not move like a wave – it usually spreads and bounces on molecules that shake in every direction; If heat can travel as a wave, it can move in a mass direction away from its source, a sort of zapping energy all at once from an object. One day, this heat transfer behavior in graphite could be used to cool the microelectronics in an instant. That is, if they can make it work at a reasonable temperature (they worked in bone cooling temperatures of minus 240 degrees Fahrenheit, or minus 1

51 degrees Celsius).

"If you do it at room temperature in some materials, then there would be some prospects for some applications," study researcher Keith Nelson, an MIT chemist, told Live Science, adding that this is the highest temperature in which anyone has seen this behavior. [The 18 Biggest Unsolved Mysteries in Physics]

The researchers describe the "normal" movement of heat using a heated kettle – After turning off the burner, that thermal energy makes a turn on the air molecules, which the other collide and pass the heat in the process. These molecules bounce in every direction; some of these molecules pour into the kettle. Over time, the water in the kettle and the surrounding environment reach the balance at the same temperature.

In solids, the molecules do not move because the atoms are locked in place. "The thing that can move is sound waves," said Nelson, who spoke with Live Science along with co-author Gang Chen, a mechanical engineer at MIT.

Rather, the heat jumps on phonons, or small packets of sound vibrations; the phonons can bounce and disperse, transporting heat like the air molecules make from the kettle. [What’s That Noise? 11 Strange and Mysterious Sounds on Earth]

This is not what happened in this new experiment.

Chen's previous theoretical work predicted that heat could travel as a wave when moving through graphite or graphene. To test this, the MIT researchers crossed two laser beams on the surface of their graphite, creating what is called an interference pattern in which there were parallel lines of light and no light. This created the same pattern of heated and unheated regions on the graphite surface. Then, they aimed another laser beam at the setup to see what happened once they hit the graphite.

"Normally, the heat gradually spread from the heated regions to the unheated regions, until the temperature pattern was washed away," Nelson said. "Instead, the heat flowed from the heated to the unheated regions, and continued to flow even after the temperature was equalized everywhere, so the unheated regions were actually warmer than the originally heated regions." The heated regions, meanwhile, have become even fresher than the unheated regions. And it all happened extremely fast, at about the same speed as the sound normally travels in graphite. [8 Ways You Can See Einstein’s Theory of Relativity in Real Life]

"The heat flowed much faster because it moved in a wave-like fashion without dispersion," Nelson told Live Science.

How did they get this strange behavior, which scientists call "second sound" occurs in graphite?

"From a fundamental perspective, this is not just ordinary behavior." The second sound was measured only in a handful of materials, at any temperature. "Anything we observe that is very out of the ordinary challenges us to understand and explain it," Nelson said.

Here's what they think is happening: graphite, or a 3D material, has a layered structure in which the thin layers of carbon barely know that the other is there, and therefore behave somehow like graphene, which is a 2D material. Because of what Nelson calls this "low dimensionality", phonons that carry heat in a graphite layer are less likely to bounce and disperse on other layers. Furthermore, the phonons that can form in graphite have wave lengths that are mostly too large to reflect back after crashing into the atoms in the lattice, a phenomenon known as backscatter. These little sound packs spread out a little, but travel mainly in one direction, which means that on average they could travel much faster.

Their research was published today (March 14) in the journal Science.

Editor's Note: This article has been updated to clarify some of the methods in the experiment and the fact that the heat traveled at about the same rate as the sound would have traveled through graphite, not the Air, as previously stated.

Originally published in Live Science .


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