By Josh Perry, Editor
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Researchers at the Massachusetts Institute of Technology (MIT) in Cambridge, Mass. discovered a new mode of heat transport, dubbed second sound, in graphite, where heat moves in waves like sound typically moves through air.

Researchers find evidence that heat moves through graphite similar to the way sound moves through air. (Christine Daniloff/MIT)

According to an article from MIT, researchers saw this wave-like movement in graphite at temperatures of 120 kelvin (-240°F).

“Points that were originally warm are left instantly cold, as the heat moves across the material at close to the speed of sound,” the article explained. “The behavior resembles the wavelike way in which sound travels through air, so scientists have dubbed this exotic mode of heat transport ‘second sound.’”  

This is the highest temperature in which this phenomenon has been observed and the first time it has been observed in graphite rather than the more exotic (and hard to control) materials that it was originally observed in. Researchers believe that graphite and graphene could be used to cool microelectronics in ways that haven’t been observed before.

“Normally, heat travels through crystals in a diffusive manner, carried by ‘phonons,’ or packets of acoustic vibrational energy,” the article explained. “The microscopic structure of any crystalline solid is a lattice of atoms that vibrate as heat moves through the material. These lattice vibrations, the phonons, ultimately carry heat away, diffusing it from its source, though that source remains the warmest region, much like a kettle gradually cooling on a stove.”

In typical heat transfer, phonons are scattered in every direction, including back to the source. This back-scattering is suppressed in materials that exhibit second sound. Phonons move as a wave rather than individually and that means the heat source is cooled almost instantly.

A theory about second sound being present in graphene was tested using a 10 mm² sample of graphite. Researchers used transient thermal grating, where laser beams are crossed so that interference causes a ripple effect on the surface of the sample.

“The regions of the sample underlying the ripple’s crests were heated, while those that corresponded to the ripple’s troughs remained unheated,” the article said. “The distance between crests was about 10 microns.”

A third laser beam was diffracted through the ripple and was measured by a photodetector. Its signal was proportional to the height of the ripple, which was dependent on the difference between the hotter bumps and cooler troughs. This was used to measure the flow of heat over time.

“Rather than seeing the crests gradually decay to the same level as the troughs as they cooled, the crests actually became cooler than the troughs, so that the ripple pattern was inverted — meaning that for some of the time, heat actually flowed from cooler regions into warmer regions,” the article continued.

The research was recently published in Science. The abstract stated:

“Wavelike thermal transport in solids, referred to as second sound, is an exotic phenomenon previously limited to a handful of materials at low temperatures. The rare occurrence of this effect restricted its scientific and practical significance.

“We directly observed second sound in graphite at temperatures above 100 K using time-resolved optical measurements of thermal transport on the micrometer-length scale. Our experimental results are in qualitative agreement with ab initio calculations that predict wavelike phonon hydrodynamics.

“We believe these results potentially indicate an important role of second sound in microscale transient heat transport in two-dimensional and layered materials in a wide temperature range.”