Tech Briefs Magazine - February 2023 - 24

Materials & Coatings
somehow still gigantic: We want to do a
lot of work. But internally, at the atomic
scale, it's more gentle, " he said.
Schuh explained that the team " took
all of the modern tools of science, everything
you can name - computational
thermodynamics, phase transformation
physics, crystallographic calculations,
machine learning - and he put all these
tools together in a totally new way " to
solve this problem.
Diagrams show the two different ways that the atomic structure of the shape-memory material,
zirconia ceramic, can be configured. An external trigger such as a temperature change can shift the
configuration from one shape to the other, changing its dimensions and allowing it to exert pressure
or do other work. The background is an electron microscope image of the material, with the
two colors indicating the two different configurations. (Image: Edward Pang)
terial's shape down at the atomic level,
there's a whole lot of damage that can
be created. Atoms have to reshuffle and
change their structure. And as atoms
are moving and reshuffling, it's sort of
easy to get them in the wrong spots and
create defects and damage the material,
which leads them to fatigue and eventually
fall apart.
" You end up with materials that can
deform a few times, but then eventually
they degrade and they can fall apart.
And because metals are so ductile,
they're a little more damage-resistant,
and so the field has really focused on
metals because when a metal is damaged
on the inside, it can tolerate it, "
added Schuh.
The team aimed to design a new ceramic
and specifically target that hysteresis.
" We wanted to design a ceramic
where the [shape] transformation is
The result was a new variation of zirconia,
but some atoms of different elements
have been introduced into its
structure in a way that alters some of its
properties. The elements " dissolve into
the lattice, and they sculpt it, and they
change that transformation, they make it
more gentle at the atomic scale. "
The hysteresis changed so dramatically
that it now resembles that of metals,
Schuh said. And the deformation that
the material can achieve amounts to
about 10 percent.
Actuators that direct airflow inside
a jet engine might be a useful application,
the team noted. While that overall
environment is hot, there are various
channels of airflow being controlled, so
those flows could be used to trigger a
shape-memory ceramic.
The team plans to continue exploring
the material, finding ways to produce
it in bigger batches and more complex
shapes, and testing its ability to withstand
more cycles of transformation.
For more information, contact Abby
Abazorius at abbya@mit.edu; 617-2532709.
New
Method to Determine How 2D Materials Expand
A technique that accurately measures how atom-thin materials expand when heated could
help engineers develop faster, more powerful electronic devices.
Massachusetts Institute of Technology, Cambridge, MA
T
wo-dimensional materials, which consist
of just a single layer of atoms, can
be packed together more densely than
conventional materials, so they could be
used to make transistors, solar cells, LEDs,
and other devices that run faster and perform
better. One issue holding back these
next-generation electronics is the heat they
generate when in use. Conventional electronics
typically reach about 80 °C, but the
materials in 2D devices are packed so
densely in such a small area that the devices
can become twice as hot. This temperature
increase can damage the device.
24
This problem is compounded by the
fact that scientists don't have a good understanding
of how 2D materials expand
when temperatures rise. Because the materials
are so thin and optically transparent,
their thermal expansion coefficient
(TEC) - the tendency for the material
to expand when temperatures increase
- is nearly impossible to measure using
standard approaches.
" When people measure the thermal
expansion coefficient for some bulk material,
they use a scientific ruler or a microscope
because with a bulk material,
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you have the sensitivity to measure them.
The challenge with a 2D material is that
we cannot really see them, so we need to
turn to another type of ruler to measure
the TEC, " said Yang Zhong, a graduate
student in mechanical engineering.
Zhong is co-lead author of a research
paper that demonstrates just such a
" ruler. " Rather than directly measuring
how the material expands, they use laser
light to track vibrations of the atoms
that comprise the material. Taking measurements
of one 2D material on three
different surfaces, or substrates, allows
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