Impossible scenario: the scientists observed the movement of heat on the speed of sound

Ryan Duncan froze. Only that he had a new experiment for the study of conventional graphite — the same one their pencil — but the results seemed impossible physically: the heat that normally dissipates slowly passed through the graphite at the speed of sound. It’s like putting a pot of water on a hot stove and instead of counting the long minutes until the water starts to boil, watch as it immediately begins to boil.

The speed with which spreads the heat?

No wonder Duncan, a graduate student at mit, could not believe his eyes. To make sure he was not mistaken, had four times to recheck everything in the setup, re-run the experiment and make a good break. “I tried to sleep, knowing that I won’t be able to determine whether the experiment is successful or not, a few hours, but the disconnect was quite difficult,” he recalls. When the next morning the alarm clock rang Duncan, he PJ’s and ran to the computer and looked at the new dimensions. The results were the same: the heat was moving incredibly fast.

The results of their work, Duncan has published in the journal Science. This phenomenon, known as “second sound” leads the physicists into raptures — partly because it can pave the way for advanced microelectronics, and partly because it is a very strange phenomenon.

To understand just imagine how heat moves through the air. It is molecules that constantly collide among themselves and dissipate heat in all directions: forward, sideways and even backwards. This fundamental inefficiency makes the relatively slow conduction of heat (radiant heat, by comparison, moves at the speed of light in the form of infrared radiation). The same slowness is saved for the heat that moves through a rigid body. Here the phonons (packets of acoustic vibrational energy) transfer heat as molecules in the air, allowing it to dissipate in all directions and slowly disintegrating. “It’s a bit like if you put a drop of food coloring in the water and allow it to spread,” says Kate Nelson, counselor of Duncan in MIT. “It isn’t going straight as an arrow, from point of contact”. But it is a consequence Duncan got from the experiment. In the second sound backscattering of phonons strongly choked, with the result that the heat went off ahead. The movement of the wave. “If you are in the pool and run from the wave, it will take you. But for the heat this abnormal behavior.”

Second sound was first discovered in liquid helium 75 years ago, and subsequently in three more solids. “All the signs pointed to the fact that it will be limited to a small number of materials and appear at very low temperatures”. Scientists thought had stalled. It was not clear what else could be the second sound in addition to scientific statements, so many years this area was without news.

However, significant improvements in numerical modeling have helped to revive this area about five years ago, and scientists have recognized that this phenomenon may be more common. Gang Chen, an engineer from the Massachusetts Institute of technology, for example, could predict that the second sound can occur in graphite at rather mild temperatures. This prediction filled Duncan, who checked it and, eventually, met with mixed results.

First, Duncan took to heat a sample of graphite using two crossed laser beam to generate interference patterns — alternating bright and dark areas that correspond to the crests and troughs of oncoming light waves. Initially, the combs were heated graphite, and the depression remained cool. But as soon as Duncan was supposed to turn off the lasers, the picture had to start slowly to change, and heat flow from the hot crests to cold troughs. The experiment would reach its end when the entire sample had attained uniform temperature. At least, this is what happens. But when the lasers ceased to glow, the graphite had other plans: the warmth continued to flow until hot combs do not become colder than the depression. As if cooking surface has turned to ice in that moment, when you turned it off, not cool gradually to ambient temperature. “It’s weird — the heat is not supposed to do that.”

And certainly not supposed to do that at such high temperatures. Also the Duncan’s experiment allowed to establish the limit of high temperature where does the second sound: about 120 Kelvins — more than 10 times higher than in the previous measurements.

What practical applications these results would be found in the future? First, the manipulation of temperature, not cryogenic cooling, more practical. Second, graphite is a quite common material. These two properties will help engineers to overcome the acute problem of thermal management in microelectronics. Just imagine what heat will be dissipated at the speed of sound, allowing the materials and devices to cool much faster.

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