Research

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

Ryan Duncan froze. He had just conducted a new experiment on the study of ordinary graphite — the very one, their pencil core — but the results seemed physically impossible: the heat, which usually dissipates slowly, passed through the graphite at the speed of sound. It is like putting a pot of water on a hot stove and instead of counting long minutes until the water boils, watch how it boils immediately.

How fast does the heat spread?

No wonder Duncan, a graduate studentMassachusetts Institute of Technology, could not believe my eyes. To make sure that he was not mistaken, I had to double-check everything that was in the installation, run the experiment again and have a good break. “I tried to sleep, knowing that I couldn’t determine if the experiment was successful or not, for a few more hours, but it was quite difficult to disconnect,” he recalls. When the next morning, Duncan’s alarm clock rang, he ran straight to his computer in his pajamas and looked at the new measurements. The results were all the same: the heat moved incredibly fast.

The results of his work Duncan published inScience magazine. This phenomenon, known as the “second sound,” delights physicists - partly because it can pave the way for advanced microelectronics, and partly because it is a very strange phenomenon.

To understand, just imagine how warmmoves through the air. It is carried by molecules that constantly collide with each other and dissipate heat in all directions: forward, sideways and even backward. This fundamental inefficiency makes the conductivity of heat relatively slow (radiant heat, by comparison, moves at the speed of light in the form of infrared radiation). The same slowness is maintained for heat that moves through a solid. Here, phonons (acoustic vibration energy packets) transfer heat like molecules in air, allowing it to dissipate in all directions and slowly decay. “It’s a bit like putting a drop of food coloring in the water and letting it spread,” says Kate Nelson, Duncan’s advisor to MIT. "It does not move in a straight line, like an arrow, from the point of impact." But this is precisely the result Duncan obtained from the experiment. In the second sound, the reverse phonon scattering strongly suppressed, with the result that the heat shot forward. Wave motion. “If you are in the pool and start a wave from yourself, it will leave you. But for heat, this is abnormal behavior. ”

The second sound was first detected in liquid helium.75 years ago and later on in three solids. "All signs indicated that it would be limited to a small amount of materials and manifest itself at very low temperatures." Scientists thought they were deadlocked. It was not clear what could be the second sound other than scientific approval, so for many years this area was without news.

However, significant improvements in the numericalThe modeling helped revive this area about five years ago, and scientists recognized that this phenomenon may be more common. Gang Chen, an engineer at the Massachusetts Institute of Technology, for example, was able to predict that a second sound could appear in graphite at fairly mild temperatures. This prediction charged Duncan, who tested it and eventually ran into conflicting results.

First, Duncan removed heat to the sample.graphite, using two crossed laser beams to create an interference pattern — alternating bright and dark areas that correspond to the crests and troughs of opposing light waves. Initially, the combs heated the graphite, and the hollows remained cool. But as soon as Duncan had to turn off the lasers, the picture had to begin to change slowly, and the heat flow from the hot ridges to the cold depressions. The experiment would reach its end when the entire sample reached a uniform temperature. At least that's what usually happens. But when the lasers stopped glowing, graphite had other plans: the heat continued to flow until the hot ridges became colder than the hollows. It was as if the cooking surface had become icy at the moment when you turned it off, and not gradually cooled down to ambient temperature. "It is strange - the heat should not do that."

And certainly it should not do so at such hightemperatures. Also, the Duncan experiment allowed us to establish a high temperature limit at which the second sound manifests itself: about 120 Kelvin - more than 10 times higher than during previous measurements.

What practical applications are such results?would you find in the future? First, temperature manipulations, rather than cryogenic cooling, are more practical. Secondly, graphite is quite common material. These two properties will help engineers overcome the acute problem of heat management in microelectronics. Just imagine that heat will dissipate at the speed of sound, allowing materials and devices to cool much faster.

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