Cool it: Nano-scale discovery could help prevent overheating in electronics | CU Boulder Today

A team of physicists from CU Boulder has solved the mystery of an obscure phenomenon in the nanosphere: why some ultra-small heat sources cool faster if you collect them closer together. Findings recently appeared in the journal Proceedings of the National Academy of Sciences (PNAS) may one day help the technology industry develop faster electronic devices that overheat less.

“Often heat is a difficult issue when developing electronics. You create a device and then find that it heats up faster than you would like, ”said study co-author Joshua Knoblach, a researcher at JILA, a joint research institute of CU Boulder and the National Institute of Standards and Technology (NIST). “Our goal is to understand fundamental physics so we can develop future devices to effectively control heat flow.”

The laser heats the ultra-thin silicon bars. (Credit: Stephen Burroughs / JILA)

The study began with an unexplained observation. In 2015, researchers led by physicists Margaret Murnain and Henry Captain of JILA experimented with metal rods that were many times thinner than the width of a silicon-based human hair. When they heated these bars with a laser, something strange happened.

“They behaved very unintuitively,” Knoblach said. “These nanoscale heat sources usually do not dissipate heat efficiently. But if you put them close together, they cool down much faster.

Now researchers know why this is happening.

In a new study, they used computer simulations to track the passage of heat from their nanoscale bands. They found that when they placed the heat sources close together, the vibrations of the energy they produced began to reflect off each other, dissipating the heat and cooling the bars.

The group’s results highlight a major challenge in developing the next generation of tiny devices such as microprocessors or quantum computer chips: when you shrink to a very small scale, heat doesn’t always behave the way you think.

Atom after atom

The researchers added that heat transfer in the devices matters. Even minor defects in electronics design, such as computer chips, can cause a rise in temperature, increasing device wear. As technology companies tend to produce smaller and smaller electronics, they will need to pay more attention than ever before to phonons – the vibrations of atoms that transfer heat to solids.

“The heat flow involves very complex processes, making it difficult to control,” Knoblach said. “But if we can understand how phonons behave on a small scale, then we will be able to adapt their transport, which will allow us to build more efficient devices.”

To do this, Murnan and Captain and their team of experimental physicists joined forces with a team of theorists led by Mahmoud Hussein, a professor at the Department of Aerospace Engineering Ann and H.J. Smida. His group specializes in modeling or simulating the motion of phonons.

“On an atomic scale, the very nature of heat transfer is manifested in a new light,” said Hussein, who also has a polite appointment at the Department of Physics.

The researchers, in fact, resumed their experiment a few years ago, but this time entirely on a computer. They modeled a series of silicon bars laid side by side, like slats in a railroad track, and heated them. Hossein Honorvar, the first author of the new work, developed models accurate to the atomic scale.

The simulation was so detailed that the team could track the behavior of each atom in the model – only millions – from start to finish.

“We’ve really pushed the boundaries of the Summit supercomputer’s memory at CU Boulder,” Knoblach said.

Directing heat

The technique bore fruit. Researchers have found, for example, that if they placed their silicon bars far enough apart, the heat would usually emanate from these materials in a predictable way. Energy escaped from the bars and into the material beneath them, dissipating in all directions.

However, when the bars came closer, something else happened. When the heat from these sources dissipated, it actually caused this energy to flow more intensely in a uniform direction from the sources – like a crowd of people in a stadium colliding with each other and eventually jumping out of the exit. The team called this phenomenon “directional thermal channeling”.

“This phenomenon increases heat transfer down into the substrate and away from heat sources,” Knoblach said.

Researchers suspect that one day engineers will be able to take advantage of this unusual behavior to better understand how heat enters small electronics, directing this energy in the desired direction rather than letting it run wild.

At the moment, researchers see in the latest study what scientists from different disciplines can do if they work together.

“This project was such an exciting collaboration of science and technology, where the advanced methods of computational analysis developed by Mahmoud’s group were crucial to understanding the behavior of new materials previously discovered by our group using new ultraviolet quantum light sources,” said Murnain, also a professor. . physics.

This study was supported by the Science and Technology Center of the National Science Foundation STROBE on real-time functional visualization.

Brendan McBennett, a JILA graduate student, also co-authored the new work. Other co-authors included former JILA researchers Travis Fraser, Begonia Abad and Jorge Hernandez-Charpac.

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