Heat moves over ‘Pencil Lead’ at the Speed of Sound

Scientists have seen something magical happening inside graphite, the stuff that your pencil lead is made of: Heat moved in waves at the speed of sound.

Heat moves over 'Pencil Lead' at the Speed of Sound

Heat isn’t supposed to move like a wave — it usually diffuses and bounces off of jiggling molecules in every direction; If heat can travel as a wave, it can move in one direction enmasse away from its source, sort of zapping energy all at once from an object.

Some day, this heat-transfer behavior in graphite could be used to cool down microelectronics in a snap. That is, if they can get it to work a reasonable temperature.

In solids, molecules don’t move because the atoms are locked into position. “The thing that can move is sound waves,” said Nelson co-author Gang Chen, a mechanical engineer at MIT.

Graphite, or a 3D material, has a layered structure in which the thin carbon layers hardly know the other is there, and so they sort of behave like graphene, which is a 2D material.

Because of what Nelson calls this “low dimensionality,” the phonons carrying the heat in one layer of the graphite are much less likely to bounce about and scatter off other layers.

Also,the phonons that can form in graphite have wavelengths that are mostly too big to reflect backwards after crashing into atoms in the lattice, a phenomenon known as backscatter.

These little sound packets do scatter a bit, but travel mostly in one direction, meaning that on average, they could travel a large distance much faster.

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