A laser heats ultra-thin silicon bars. Credit: Steven Burrows/JILA
A team of CU Boulder physicists has discovered the secret to a mysterious phenomenon in the nano realm. It is how ultra-small heat sources cool faster if they are packed closer together. These findings were published in the journal Proceedings of the National Academy of Sciences. They could be used to help the tech industry create faster electronic devices that heat less.
Heat is often a difficult consideration when designing electronics. Joshua Knobloch (postdoctoral researcher at JILA), a joint research center between CU Boulder and NIST, said that you can build a device and then find it heating up too fast. "Our goal is understanding the fundamentals of heat flow management so that we can engineer future devices."
The research started with an unexplained observation. In 2015, JILA researchers Margaret Murnane (physicist) and Henry Kapteyn (physicist) experimented with metal bars that were thinner than a human hair. Something strange happened when they heated the bars with a laser.
Knobloch stated that they behaved "very counterintuitively". These nano-scale heat sources don't usually dissipate heat well. They cool much faster if they are packed together.
Researchers now know why this happens.
They used computer-based simulations in the new study to track heat's passage from the nano-sized bars. The researchers discovered that heat energy produced by heat sources placed close together effected the flow of heat, cooling down the bars and scattering it off.
The results of the group highlight a significant challenge when designing the next generation microprocessors and quantum computer chips. Heat behaves differently at very small scales.
Atom by atom
Researchers also noted that heat transmission in devices is important. Even small defects in electronics, such as computer chips, can cause temperature buildup and add wear to devices. Tech companies will need to pay greater attention to the phononsvibrations caused by atoms carrying heat in solids as they strive to make smaller electronics.
Knobloch stated that heat flow is a complex process, which makes it difficult to control. "But, if we understand the behavior of phonons on a small scale, we can tailor their transport to allow us to create more efficient devices."
Murnane, Kapteyn, their experimental physicists, and a team of them joined forces with Mahmoud Hussein (professor in the Ann and H.J. Smead Department of Aerospace Engineering Sciences. His research group is specialized in modeling the motion of phonons.
Hussein also holds a courtesy appointment at the Department of Physics.
Researchers essentially recreated the experiment they had done years ago, but entirely on a computer. They created a series silicon bars that were laid side-by-side like the rails on a train track. Then they heated them up.
Knobloch stated that simulations were so precise that they allowed the team to follow the behaviorof every single atom in the modelmillions at a time from start to finish.
He said, "We were pushing the limits of memory on the Summit Supercomputer at CU Boulder."
Heat direction
This worked. For example, the researchers discovered that heat tends to escape from silicon materials in predictable ways if they space their silicon bars sufficiently apart. The heat escaped from the bars into the material below, dissipating all directions.
Something else happened when the bars were closer together. The heat from these sources was scattered and it caused that energy to flow more strongly away from the sources. It was like a crowd jostling for each other in a stadium, eventually jumping out of the exit. This phenomenon was called "directional thermal channeling" by the team.
Knobloch stated that this phenomenon "increases the transport of heat down to the substrate and away the heat sources."
Researchers believe engineers might be able to tap into this unique behavior and gain insight into how heat flows in small electronic devices. They could direct that energy in a desired direction, rather than letting it flow wild.
The researchers view the new study as a demonstration of what scientists can achieve when they collaborate across disciplines.
Murnane, a professor of Physics, said that "this project was such a thrilling collaboration between science and engineering where advanced computational analysis methods developed Mahmoud’s group were crucial for understanding new materials behaviour uncovered earlier in our group using new ultraviolet quantum light sources."
Hossein Honarvar (CU Boulder postdoctoral researcher in aerospace engineering sciences, JILA) and Brendan McBennett (a graduate student at JILA), are other coauthors of the new research. The study also included Travis Frazer, Begoa Abbad, and Jorge Hernandez-Charpak, former JILA researchers.
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More information: Directional Thermal Channeling: A phenomenon that is triggered by tight packaging of heat sources, Proceedings of the National Academy of Sciences (2021). Information from Proceedings of the National Academy of Sciences Directional Thermal Channeling: A phenomenon that is triggered by tight packaging of heat sources (2021). DOI: 10.1073/pnas.2109056118
