A team of physicists at 抖阴旅行射 Boulder has solved the mystery behind a perplexing phenomenon in the nano realm: why some ultra-small heat sources cool down faster if you pack them closer together. The findings, , could one day help the tech industry design speedier electronic devices that overheat less.
鈥淥ften听heat is a challenging consideration in designing electronics. You build a device then discover that it鈥檚 heating up faster than desired,鈥� said study co-author Joshua Knobloch, postdoctoral research associate at , a joint research institute between 抖阴旅行射 Boulder and the National Institute of Standards and Technology (NIST). 鈥淥ur goal is to understand the fundamental physics involved so we can engineer future devices to efficiently manage the flow of heat.鈥�
A laser heats up听ultra-thin bars of silicon. (Credit: Steven Burrows/JILA)
The research听began with an unexplained observation.听In 2015, researchers led by physicists Margaret Murnane and Henry Kapteyn at JILA that were many times thinner than the width of a human hair on a silicon base. When they heated those bars up with a laser, something strange occurred.
鈥淭hey behaved very counterintuitively,鈥� Knobloch said. 鈥淭hese nano-scale heat sources do not usually dissipate heat efficiently. But if you pack them close together, they cool down much more quickly.鈥�
Now, the researchers know why this happens.听
In the new study, they used computer-based simulations to track the passage of heat from their nano-sized bars. They discovered that when they placed the heat sources close together, the vibrations of energy they produced began to bounce off each other, scattering heat away and cooling the bars down.听
The group鈥檚 results highlight a major challenge in designing the next generation of tiny devices, such as microprocessors or quantum computer chips: When you shrink down to very small scales, heat does not always behave the way you think it should.
Atom by atom
The transmission of heat in devices matters, the researchers added. Even minute defects in the design of electronics like computer chips can allow temperature to build up, adding wear and tear to a device. As tech companies strive to produce smaller and smaller electronics, they鈥檒l need to pay more attention than ever before to phonons鈥攙ibrations of atoms that carry heat in solids.
鈥淗eat flow involves very complex processes, making it hard to control,鈥� Knobloch said. 鈥淏ut if we can understand how phonons behave on the small scale, then we can tailor their transport, allowing us to build more efficient devices.鈥�
To do just that, Murnane and Kapteyn and their team of experimental physicists joined forces with a group of theorists led by Mahmoud听Hussein, professor in the Ann and H.J. Smead Department of Aerospace Engineering Sciences. His group听specializes in simulating, or modeling, the motion of phonons.
鈥淎t the atomic scale, the very nature of heat transfer emerges in a new light,鈥� said Hussein who also has a听courtesy appointment in the Department of Physics.
The researchers听essentially听recreated their experiment from several years before, but this time, entirely on a computer. They modeled a series of silicon bars, laid side by side like the slats in a train track and heated them up.听Hossein Honarvar, the first author on the new paper, developed the models with atomic-scale precision.
The simulations were so detailed that the team could follow the behavior of each and every atom in the model鈥攎illions of them in all鈥攆rom start to finish.听
鈥淲e were really pushing the limits of memory of the Summit Supercomputer at 抖阴旅行射 Boulder,鈥� Knobloch said.
Directing heat
The technique paid off. The researchers found, for example, that when they spaced their silicon bars far enough apart, heat tended to escape away from those materials in a predictable way. The energy leaked from the bars and into the material below them, dissipating in every direction.
When the bars got closer together, however, something else happened. As the heat from those sources scattered, it effectively forced that energy to flow more intensely in a uniform direction away from the sources鈥攍ike a crowd of people in a stadium jostling against each other and eventually leaping out of the exit. The team denoted this phenomenon 鈥渄irectional thermal channeling.鈥澨�
鈥淭his phenomenon increases the transport of heat down into the substrate and away from the heat sources,鈥� Knobloch said.
The researchers suspect that engineers could one day tap into this unusual behavior to gain a better handle on how heat flows in small electronics鈥攄irecting that energy along a desired path, instead of letting it run wild.
For now, the researchers see the latest study as what scientists from different disciplines can do when they work together.听
鈥淭his project was such an exciting collaboration between science and engineering鈥攚here advanced computational analysis methods developed by Mahmoud鈥檚 group were critical for understanding new materials behavior uncovered earlier by our group using new extreme ultraviolet quantum light sources,鈥� said Murnane, also a professor of physics.
This research was supported by the .
Brendan McBennett, a graduate student at JILA, was also a coauthor of the new paper. Other coauthors included former JILA researchers Travis Frazer, Bego帽a Abad and Jorge Hernandez-Charpak.
