Scientists with the University of Chicago have
discovered a way to create a material that can be made like a plastic, but
conducts electricity more like a metal.
The research, published
Oct. 26 in Nature, shows how to make a kind of material in which the molecular
fragments are jumbled and disordered, but can still conduct electricity extremely
well.
This goes against all
of the rules we know about for conductivity—to a scientist, it's kind of like
seeing a car driving on water and still going 70 mph. But the finding could
also be extraordinarily useful; if you want to invent something revolutionary,
the process often first starts with discovering a completely new material.
"In principle,
this opens up the design of a whole new class of materials that conduct
electricity, are easy to shape, and are very robust in everyday
conditions," said John Anderson, an associate professor of chemistry at
the University of Chicago and the senior author on the study.
"Essentially, it suggests new possibilities for an extremely important
technological group of materials," said Jiaze Xie (Ph.D. '22, now at
Princeton), the first author on the paper.
'There isn't a solid theory to explain this'
Conductive materials
are absolutely essential if you're making any kind of electronic device,
whether it be an iPhone, a solar panel, or a television. By far the oldest and
largest group of conductors is the metals: copper, gold, aluminum. Then, about
50 years ago, scientists were able to create conductors made out of organic
materials, using a chemical treatment known as "doping," which
sprinkles in different atoms or electrons through the material.
This is advantageous
because these materials are more flexible and easier to process than
traditional metals, but the trouble is they aren't very stable; they can lose
their conductivity if exposed to moisture or if the temperature gets too high.
But fundamentally, both
of these organic and traditional metallic conductors share a common
characteristic. They are made up of straight, closely packed rows of atoms or
molecules. This means that electrons can easily flow through the material, much
like cars on a highway. In fact, scientists thought a material had to have
these straight, orderly rows in order to conduct electricity efficiently.
Then Xie began
experimenting with some materials discovered years ago, but largely ignored. He
strung nickel atoms like pearls into a string of of molecular beads made of
carbon and sulfur, and began testing.
To the scientists'
astonishment, the material easily and strongly conducted electricity. What's
more, it was very stable. "We heated it, chilled it, exposed it to air and
humidity, and even dripped acid and base on it, and nothing happened,"
said Xie. That is enormously helpful for a device that has to function in the
real world.
But to the scientists,
the most striking thing was that the molecular structure of the material was
disordered. "From a fundamental picture, that should not be able to be a
metal," said Anderson. "There isn't a solid theory to explain
this."
Xie, Anderson, and
their lab worked with other scientists around the university to try to
understand how the material can conduct electricity. After tests, simulations,
and theoretical work, they think that the material forms layers, like sheets in
a lasagna. Even if the sheets rotate sideways, no longer forming a neat lasagna
stack, electrons can still move horizontally or vertically—as long as the
pieces touch.
The end result is
unprecedented for a conductive material. "It's almost like conductive
Play-Doh—you can smush it into place and it conducts electricity," Anderson
said.
The scientists are
excited because the discovery suggests a fundamentally new design principle for
electronics technology. Conductors are so important that virtually any new
development opens up new lines for technology, they explained.
One of the material's
attractive characteristics is new options for processing. For example, metals
usually have to be melted in order to be made into the right shape for a chip
or device, which limits what you can make with them, since other components of
the device have to be able to withstand the heat needed to process these
materials.
The new material has no
such restriction because it can be made at room temperatures. It can also be
used where the need for a device or pieces of the device to withstand heat,
acid or alkalinity, or humidity has previously limited engineers' options to develop
new technology.
The team is also
exploring the different forms and functions the material might make. "We
think we can make it 2D or 3D, make it porous, or even introduce other
functions by adding different linkers or nodes," said Xie.
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