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Graphite rods ready to be wrapped in wood to make pencils. Scientists at the Massachusetts Institute of Technology have shown that heat behaves as a sound when it runs through graphite. But what if the kettle is almost cooled to room temperature almost instantly, losing its heat in a wave that passes through the material near the speed of the sound? Researchers at the Massachusetts Institute of Technology have watched this rare, non-intuitive phenomenon, known as the "second sound," in graphite, the material of lead. They described their results in an article published earlier this week in
You've probably never heard of the term "second sound," although the phenomenon has been known for decades. "This is limited to a handful of materials that are really very cold," said co-author Keith Nelson, limiting its potential utility. There may be a paragraph or two on the topic in your middle textbook in solid state, but the field "was a sort of reversal".
The results of this new study may need to change. Graphite is a very common material and the effect is observed at relatively low (low temperature physical standards) temperatures of about -240 degrees F. The theoretical team models show that it is possible to achieve a graphene effect in something closer. in the future to room temperature, thus opening up a number of potential practical applications. For example, microelectronics simply continues to decrease, which makes the management of heat a difficult task. If the room temperature graphene can quickly emit heat as waves, it can allow even greater miniaturization.
"Heat [normally] does not travel just like a bullet in the right direction."
So what exactly is the "second sound"? Technically, this is an exotic way of transferring heat. Typically, "heat does not travel just like a bullet in the right direction," says Nelson. Rather, it is transported in the air by molecules that move around, constantly colliding with each other and spreading in all directions as they spread out. "phonons." Sound waves usually have long wavelengths that can travel long distances, but the solids coolants have very short wavelengths, on a nanometer scale. The lattice structure of these solids serves as a diffraction grating, so you get the same kind of back scattering and the gradual spread of heat emanating from the source as well as the heat transfer to the air.
"Usually, if you place heat somewhere, it will cool down and spread around, but where you put it [the source] is always the warmest place," Nelson said. "This is because all these acoustic waves that bring the heat around are also constantly scattering back to the origin so it remains warm, which is what
does not happen when there is a second sound."
Instead, reverse diffusion is suppressed and you get an unusual effect where the heat source actually cools down faster than the surrounding area nearby ̵
1; actually almost instantly. This is because the phonons retain momentum and take away the heat
as a wave. "This is the opposite of our experience and our intuition," says Nelson. "I mean, the waves do this all the time, but the heat does not have to move like a wave."
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The background, spread through a crystal lattice, with greatly exaggerated displacements of the atom. Huberman, while studying the transport of phonons into two-dimensional graphene – essentially flat-carbon carbon with only a thick atom. They invented a theoretical model that shows that in a certain temperature range the interaction between the phonon in graphene would keep inertia producing the second sound effect. Their model also predicts the effects of three-dimensional graphite. Ryan Duncan, who works with Nelson, and Duncan have released everything to test these predictions with a technique called a transient thermal grid. First, he postponed the heat in the graphite sample using two crossed laser beams, creating an interference picture – variable light (combs) and dark lines. Heat is absorbed in bright areas, while dark bands remain cool. To make a measurement, he bounced another laser beam of this interfering pox.
Normally, this model gradually decreased when the heat dissipated, and the combs gradually cooled to the same temperature as the pan. But Duncan found that the originally warmed regions were so cool that they had become much cooler than the trough, basically turning the wavy pattern, "I had to sit down"
This is a signaling sign of a second sound. "When I saw this, I had to sit for five minutes," said Duncan, assuming something so wildly opposed to our daily experience could not be real. The heat simply does not move from the cooler regions to warmer. "But I did the experiment at night to see if it happened again, and it turned out very reproducible."
a much more difficult task, as they can not use the same technique they used for graphite. "If you look at the surface of the graphite on the side, you'll see that the warmed areas are a little raised," Nelson said. "There is a surface wave because [thermally] has expanded, but unheated areas have not expanded and this acts as a diffraction grating for our light probe."
However, the Graffen would not have been raised. such an interference model, given its two-dimensional nature. "If you have only one layer of atoms, then there's nothing to expand, because with thermal expansion, that's the distance between atoms, which increases," says Nelson.
Science 2019. 10.1126 /science.aav3548 (For DOIs).