MIT scientists: heat can act as a sound wave when going through a pencil lead

A boiling tea kettle gradually diffuses its heat to warm the ambient air while remaining the hottest region, even if it cools slowly. But what if the kettle comes back almost instantly to room temperature, losing its heat in a wave crossing the material at a speed close to that of the sound? MIT researchers have observed this rare and counter-intuitive phenomenon – called "second sound" – in graphite, the pencil lead. They described their findings in an article published earlier this week in Science.

It is likely that you have never heard of the concept of "second sound," even though the phenomenon has been known for decades. "It has been limited to a handful of materials that are actually at very low temperatures," said co-author Keith Nelson, severely limiting its potential usefulness. There may be a paragraph or two on the subject in your average solid state manual, but the field "has been a kind of backwater."

With the results of this new research, this could change. Graphite is a very common material, and the effect has been observed at a relatively mild temperature (according to the standards of low temperature physics) of about -240 ° F. Theoretical models of l & # 39; 39; team indicate that it might be possible to produce the effect in graphene more closely. ambient temperature in the future, opening up many potential practical applications. For example, microelectronics continues to shrink, making heat management a challenge. If graphene at room temperature could quickly dissipate heat in the form of waves, it could allow even greater miniaturization.

"Heat [normally] does not move like a ball in a straight direction. "

So, what exactly are we talking about when we talk about "second sound"? Technically, it is an exotic mode of heat transport. Normally, "the heat does not move like a ball in a straight direction," Nelson said. Instead, it is transported in the air by molecules that move, constantly bumping into each other and dispersing in all directions as they spread to the outside.

Sound, or acoustic waves, can carry thermal energy through a solid via vibratory energy packets called "phonons". The sound waves are usually long wavelengths, capable of traveling long distances, but the heat – carrying phonons in a solid have very short wavelengths, at the nanoscale. The lattice structure of such solids serves as a diffraction grating. This gives you the same kind of backscattering and gradual diffusion of heat radiating from the source to the outside as you would with heat transport in the air.

"Normally, if you put heat somewhere, it will cool and spread, but where you put it [the source] It's always the hottest place, "Nelson said. This is because all these acoustic wavelets that carry the heat are also constantly dispersed to the origin, so that it stays warm. That's what is not pass when there is a second sound ".

Instead, the backscatter is removed and you get an unusual effect that the heat source cools faster than the nearby surrounding area, almost instantly, in fact. Indeed, the phonons keep their momentum and evacuate the heat in mass like a wave. "It goes against our experience and our intuition," Nelson said. "I mean, the waves do it all the time, but the heat should not move like a wave."

The experiment draws on the previous theoretical work of co-authors Gang Chen and Sam Huberman, while studying the transport of phonons in two-dimensional graphene, essentially flat carbon sheets of only one atom. They developed a theoretical model indicating that in a certain temperature range, the interaction between the phonons in graphene would retain the momentum, thus producing the second sound effect. Their model also predicts the effect in a three-dimensional graphite.

Huberman spoke about the results to Ryan Duncan, who works with Nelson, and Duncan left everything to test these predictions with a technique called transient thermal network. First, he deposited heat into the graphite sample with the help of two crossed laser beams, creating a pattern of interference alternating lines of light (crests) and dark (hollow). The heat has been absorbed in the light regions, while the dark bands remain cold. To perform a measurement, he returned another laser beam of this interference pattern.

Normally, this pattern slowly decreases as the heat dissipates, the crests gradually cooling up to reach the same temperature as the hollows. But Duncan discovered that the initially heated areas cooled so quickly that they became much colder than the hollows, essentially reversing the ripple pattern.

"I had to sit down"

This is a revealing signature of the second sound. "When I saw that, I had to sit down for five minutes," Duncan said, finding that something so fiercely opposed to our daily experience could not be real. . The heat simply does not flow from the coldest to the hottest regions. "But I did experience overnight to see if this was happening again and it was very reproducible."

The next step is to try to measure the second sound in a graphene sample, where it could be even more pronounced – a much more daunting challenge as they can not use the same technique as used for graphite. "If you look at the surface of the graphite aside, you would see that the heated areas are a little elevated," Nelson said. "There is a ripple pattern on the surface because they have [thermally] extended. But the unheated regions did not expand and this acts as a diffraction grating for our probe. "

Graphene, however, would not produce such a raised interference pattern, given its two-dimensional nature. "If you only have a layer of atoms, there's nothing to dilate, because in thermal expansion, it's the distance between atoms that increases," he said. Nelson.

DO I: Science, 2019. 10.1126 / science.aav3548 (About DOIs).