In Super-Chilled Graphite, Heat Can Flow From Cold To Warm

The graphite in pencils has very unusual thermal properties at high temperatures, and it turns out even stranger ones at very low ones, which may have important applications. Miriam Doerr Martin Frommherz/Shutterstock

Against all logic and experience, heat can, under rare conditions, flow from colder areas to warmer ones. When the phenomenon was first discovered, it was thought restricted to ultra-pure materials close to absolute zero. Now it has been observed in an everyday material at easily replicated temperatures.

Heat is the random movement of particles, and in crystals is normally transferred in packets known as phonons. When part of a crystal becomes hot, the phonons disperse through the lattice until the temperature evens out. If another part of the crystal momentarily becomes hotter, energy would backscatter to the source.

In a phenomenon known as “second sound” however, the heat travels through material like sound waves – which under the right circumstances can be focused to be louder elsewhere than at the source. The heat acts as if it has momentum, and follows the Bose-Einstein equations drawn up to describe bizarre behavior normally only seen in the subatomic world where quantum mechanics reigns, or in super-cold fluids.

Second sound has been observed in exotic, pure materials at temperatures below 20 Kelvin (-253ºC, -423ºF). The observation of second sound behavior in graphite at 120 K (-153ºC -243ºF) shifts it from a curiosity to something with potentially valuable applications. Although still far colder than natural temperatures anywhere on Earth, this temperature is above liquid nitrogen freezing point, making it reachable in a high school laboratory. Graphite, famously used in "lead" pencils, is also easily available, although the experiment used the purest commercially available grade.

After modeling of phonon interactions in graphite suggested second sound should occur up to 120 K, Professor Keith Nelson of MIT decided to test it experimentally. In Science, he reports observing heat flowing from cool areas to warm ones in a 10 mm2 (0.016 square inch) block of graphite heated with lasers.

The team observed this behavior in great detail at 85 K, then again at 125 K, only seeing it disappear at 150 K. Intriguingly, signs of second sound were also lost when temperatures were lowered below 50 K, and while the experimental results matched the modeling well from 80-120 K, results diverged from expectations below 80 K. They expect the second sound “window” will vary depending on the graphite's isotopic purity and defects.

Nelson anticipates major applications if, as he anticipates, he can replicate the results at even higher temperatures in graphene, sheets of graphite a single atom thick.

"There's a huge push to make things smaller and denser for devices like our computers and electronics, and thermal management becomes more difficult at these scales," Nelson said in a statement. If Nelson's hunch holds up, graphene could be the perfect material to remove heat from overheating microscopic computers.

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