At relatively balmy temperatures, heat behaves like sound when moving through graphite, study reports — ScienceDaily

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The next time you boil the kettle, consider the following: After turning off the burner, do not keep the heat and slowly heat the surrounding kitchen and stove. The kettle will quickly cool to room temperature and its heat will heat away from boiling. The heat wave.

We know that in our daily environment, heat does not. But now researchers at the Massachusetts Institute of Technology have observed this seemingly unbelievable heat transfer model, known as the "second sound," in a fairly common material: graphite – pencil lead stuff.

At 120 Kelvin or -240 degrees Fahrenheit, they saw obvious signs that heat traveled through the graphite in a wavy motion. The initial warm spot is kept cold immediately as the heat moves over the material at a speed close to the speed of sound. This behavior is similar to how waves travel in the air, so scientists call this unusual heat transfer a "second sound."

The new result represents the highest temperature at which the scientist observes the second sound. More importantly, graphite is a commercially available material that stands in stark contrast to cleaner, uncontrollable materials that exhibit a second sound at 20 K (-420 F) – these temperatures are for any It's too cold for practical applications.

The findings published in Science show that graphite, perhaps its high performance relative to graphene, can effectively remove heat from microelectronic devices in ways not previously recognized.

"For devices like our computers and electronics, it is much smaller and more intensive, and thermal management becomes more difficult on these scales, which is a huge boost," Massachusetts Institute of Technology College Haslam and Dewey Chemistry professor Keith Nelson said. “There is good reason to believe that the second sound in graphene may be more pronounced, even at room temperature. If it turns out that graphene can effectively remove heat as a wave, it will be wonderful.”

The results came from a long-term interdisciplinary collaboration between Nelson's research team and Carl Chen, professor of mechanical engineering and power engineering, Carl Chen Soderberg. The co-authors of the Massachusetts Institute of Technology are lead authors Sam Huberman and Ryan Duncan, Ke Chen, Bai Song, Vazrik Chiloyan, Zhiwei Ding and Alexei Maznev.

"In the Fast Lane"

Typically, heat travels through the crystal in a diffuse manner, carried by a "phonon" or acoustic vibration energy pack. The microstructure of any crystalline solid is an atomic lattice that vibrates as heat passes through the material. These lattice vibrations, phonons, eventually take away heat and diffuse from its source, although the source is still the warmest area, like a kettle that gradually cools on the stove.

The kettle is still the warmest place, because heat is carried away by molecules in the air, and these molecules are constantly scattered in all directions, including back to the kettle. This "backscattering" also occurs on the phonons, and even if the heat spreads, the original heated area of ​​the solid remains the warmest point.

However, in materials exhibiting a second sound, such backscattering is severely suppressed. Instead of phonons, the phonons preserve momentum and collectively wash away, and the heat stored in the phonons is transmitted as waves. Therefore, the point of initial heating cools almost immediately, approaching the speed of sound.

Previous theoretical studies by the Chen group have shown that phonons in graphene may interact primarily in a momentum-conserving manner over a range of temperatures, suggesting that graphene may exhibit a second sound. Last year, Humberman, a member of Chen's lab, wondered if the general materials such as graphite were suitable.

Based on tools previously developed in Chen's graphene research group, he developed a complex model for numerically simulating the transmission of phonons in graphite samples. For each phonon, he tracks every possible scatter event based on the direction and energy of each phonon. He simulated at a temperature range of 50 K to room temperature and found that in the temperature range of 80 to 120 K, heat may flow in a manner similar to the second sound.

Hubermann worked with Duncan of the Nielsen Group to develop another project. When he shared his predictions with Duncan, the experimentalists decided to test Huber's calculations.

"This is an amazing cooperation," Chen said. “Ryan basically gave up all of these experiments in a short period of time.”

"We are really on the fast lane," Duncan added.

Cancellation of the norm

Duncan's experiment revolves around a small, commercially available graphite sample of 10 square millimeters.

He uses a technique called a transient thermal grating that passes through two laser beams, causing the interference of light to create a "ripple" pattern on the surface of a small piece of graphite sample. The sample area below the corrugated top is heated while the area corresponding to the corrugated groove is still unheated. The distance between the peaks is approximately 10 microns.

Duncan then illuminates a third laser beam on the sample, the light of which is diffracted by the ripple, the signal of which is measured by a photodetector. This signal is proportional to the height of the corrugated pattern, which depends on the peak being hotter than the trough. In this way, Duncan can track how heat flows through the sample over time.

If the heat in the sample flows normally, Duncan will see that the surface ripple slowly weakens as the heat moves from the peak to the trough, clearing the ripple pattern. Instead, he observed "different behavior" at 120 K.

Instead of seeing the peak gradually decay to the same level as the trough as it cools, the peak actually becomes colder than the trough, so the ripple pattern is reversed – which means that at some point the heat is actually flowing From cooler areas to warmer areas.

"This completely contradicts our daily experience and the heat transfer of almost any material at any temperature," Duncan said. "It looks really like the second. When I saw this, I had to sit for five minutes, I said to myself, this couldn't be true." But I was in the night. I experimented to see if it happened again, and it turned out to be very repeatable."

According to Huberman's prediction, the two-dimensional graphene of graphite may exhibit the characteristics of the second sound at an even higher temperature near or above room temperature. If it is the case they plan to test, graphene may be a practical choice for cooling more dense microelectronic devices.

"This is one of the few professional highlights I'm looking forward to, and the results really subvert the way you usually think of it," Nelson said. “Because it depends on where it starts, there may be some interesting applications in the future, and this fact becomes even more exciting. From a basic point of view, there is no doubt that it is really unusual and exciting. "

This research was funded in part by the Office of Naval Research, the Department of Energy, and the National Science Foundation.

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