Scientists create material that can both move and block heat
Unusual material could improve the reliability of electronics and other devices

Date:
September 30, 2021
Source:
University of Chicago
Summary:
Scientists have invented a new way to funnel heat around at the
microscopic level: a thermal insulator made using an innovative
technique. They stack ultra-thin layers of crystalline sheets
on top of each other, but rotate each layer slightly, creating a
material with atoms that are aligned in one direction but not in
the other. The result is a material that is extremely good at both
containing heat and moving it, albeit in different directions --
an unusual ability at the microscale, and one that could have very
useful applications in electronics and other technology.



FULL STORY ========================================================================== Moving heat around where you want it to go -- adding it to houses and hairdryers, removing it from car engines and refrigerators -- is one of
the great challenges of engineering.


==========================================================================
All activity generates heat, because energy escapes from everything we
do. But too much can wear out batteries and electronic components --
like parts in an aging laptop that runs too hot to actually sit on your
lap. If you can't get rid of heat, you've got a problem.

Scientists at the University of Chicago have invented a new way to funnel
heat around at the microscopic level: a thermal insulator made using an innovative technique. They stack ultra-thin layers of crystalline sheets
on top of each other, but rotate each layer slightly, creating a material
with atoms that are aligned in one direction but not in the other.

"Think of a partly-finished Rubik's cube, with layers all rotated
in random directions," said Shi En Kim, a graduate student with the
Pritzker School of Molecular Engineering who is the first author of the
study. "What that means is that within each layer of the crystal, we still
have an ordered lattice of atoms, but if you move to the neighboring
layer, you have no idea where the next atoms will be relative to the
previous layer -- the atoms are completely messy along this direction."
The result is a material that is extremely good at both containing heat
and moving it, albeit in different directions -- an unusual ability
at the microscale, and one that could have very useful applications in electronics and other technology.

"The combination of excellent heat conductivity in one direction and
excellent insulation in the other direction does not exist at all in
nature," said study lead author Jiwoong Park, professor of chemistry
and molecular engineering at the University of Chicago. "We hope this
could open up an entirely new direction for making novel materials."
'Just amazingly low'


========================================================================== Scientists are constantly on the search for materials with unusual
properties, because they can unlock completely new capabilities for
devices such as electronics, sensors, medical technology or solar
cells. For example, MRI machines were made possible by the discovery of
a strange material that can perfectly conduct electricity.

Park's group had been investigating ways to make extremely thin layers
of materials, which are just a few atoms thick. Normally, the materials
used for devices are made up of extremely regular, repeating lattices
of atoms, which makes it very easy for electricity (and heat) to move
through the material. But the scientists wondered what would happen if
they instead rotated each successive layer slightly as they stacked them.

They measured the results and found that a microscopic wall made of
this material was extremely good at preventing heat from moving between compartments. "The thermal conductivity is just amazingly low -- as
low as air, which is still one of the best insulators we know," said
Park. "That in itself is surprising, because it's very unusual to find
that property in a material that is a dense solid -- those tend to be
good heat conductors." But the point that was really exciting for the scientists was when they measured the material's ability to transport
heat along the wall, and found it could do so very easily.

Those two properties in combination could be very useful. For example,
making computer chips smaller and smaller results in more and more
power running through a small space, creating an environment with a high
"power density" -- a dangerous hotspot, said Kim.



========================================================================== "You're basically baking your electronic devices at power levels as if
you are putting them in a microwave oven," she said. "One of the biggest challenges in electronics is to take care of heat at that scale, because
some components of electronics are very unstable at high temperatures.

"But if we can use a material that can both conduct heat and insulate heat
at the same time in different directions, we can siphon heat away from
the heat source -- such as the battery -- while avoiding the more fragile
parts of the device." That capability could open doors to experiment
with materials that have been too heat-sensitive for engineers to use
in electronics. In addition, creating an extreme thermal gradient --
where something is very hot on one side and cool on the other -- is
difficult to do, particularly at such small scales, but could have many applications in technology.

"If you think of what the windowpane did for us -- being able to keep
the outside and inside temperatures separate -- you can get a sense of
how useful this could be," Park said.

The scientists only tested their layering technique in one material,
called molybdenum disulfide, but think this mechanism should be general
across many others. "I hope this opens up a whole new direction for
making exotic thermal conductors," Kim said.

The research used the University of Chicago Materials Research Science
and Engineering Center and the Pritzker Nanofabrication Facility.

Other coauthors were UChicago graduate students Fauzia Mujid and Preeti
Poddar; postdoctoral fellows Chibeom Park (now at Samsung Electronics Semiconductor Research Center), Joonki Suh (now at UNIST) and Yu Zhong;
as well as David Cahill and Akash Rai with the University of Illinois
at Urbana-Champaign, Paul Erhart, Fredrik Eriksson, and Erik Fransson
with the Chalmers University of Technology in Sweden, and David Muller
and Ariana Ray with Cornell University.

========================================================================== Story Source: Materials provided by University_of_Chicago. Original
written by Louise Lerner.

Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Shi En Kim, Fauzia Mujid, Akash Rai, Fredrik Eriksson, Joonki
Suh, Preeti
Poddar, Ariana Ray, Chibeom Park, Erik Fransson, Yu Zhong, David A.

Muller, Paul Erhart, David G. Cahill, Jiwoong Park. Extremely
anisotropic van der Waals thermal conductors. Nature, 2021; 597
(7878): 660 DOI: 10.1038/s41586-021-03867-8 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/09/210930125013.htm

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