How to cool it: Nano-scale discovery could help prevent overheating in electronics
By Daniel Strain -- University of
Colorado Boulder
September 20, 2021 --A team of
physicists at CU 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, which will publish this week in the
journal Proceedings of the National Academy of Sciences (PNAS), could one day
help the tech industry design speedier electronic devices that overheat less.
“Often heat is a challenging
consideration in designing electronics. You build a device then discover that
it’s heating up faster than desired,” said study co-author Joshua Knobloch,
postdoctoral research associate at JILA, a joint research institute between CU
Boulder and the National Institute of Standards and Technology (NIST). “Our
goal is to understand the fundamental physics involved so we can engineer
future devices to efficiently manage the flow of heat.”
The research began with an unexplained
observation. In 2015, researchers led by physicists Margaret Murnane and
Henry Kapteyn at JILA were experimenting with bars of metal 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.
“They behaved very counterintuitively,”
Knobloch said. “These nano-scale heat sources do not usually dissipate heat
efficiently. But if you pack them close together, they cool down much more
quickly.”
Now, the researchers know why this
happens.
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.
The group’s 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’ll need to pay more attention than ever before to
phonons—vibrations of atoms that carry heat in solids.
“Heat flow involves very complex processes,
making it hard to control,” Knobloch said. “But if we can understand how
phonons behave on the small scale, then we can tailor their transport, allowing
us to build more efficient devices.”
To do just that, Murnane and Kapteyn and
their team of experimental physicists joined forces with a group of theorists
led by Mahmoud Hussein, professor in the Ann and H.J. Smead Department of
Aerospace Engineering Sciences. His group specializes in simulating, or modeling,
the motion of phonons.
“At the atomic scale, the very nature of
heat transfer emerges in a new light,” said Hussein who also has
a courtesy appointment in the Department of Physics.
The researchers essentially 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.
The simulations were so detailed,
Knobloch said, that the team could follow the behavior of each and every atom
in the model—millions of them in all—from start to finish.
“We were really pushing the limits of
memory of the Summit Supercomputer at CU Boulder,” he 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—like a crowd of people in a stadium jostling against each
other and eventually leaping out of the exit. The team denoted this phenomenon
“directional thermal channeling.”
“This phenomenon increases the transport
of heat down into the substrate and away from the heat sources,” 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—directing 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.
“This project was such an exciting
collaboration between science and engineering—where advanced computational
analysis methods developed by Mahmoud’s group were critical for understanding
new materials behavior uncovered earlier by our group using new extreme
ultraviolet quantum light sources,” said Murnane, also a professor of physics.
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