The ceramic-based material could be used for highly efficient actuators for aircraft or other uses, with minimal moving parts
From: Massachusetts Institute of Technology
October 5, 2022 -- Engineers
have created shape-memory materials made of ceramic rather than of traditional
metal. The development opens a new range of applications, especially for
actuators in high-temperature settings.
Shape-memory metals,
which can revert from one shape to a different one simply by being warmed or
otherwise triggered, have been useful in a variety of applications, as
actuators that can control the movement of various devices. Now, the discovery
of a new category of shape-memory materials made of ceramic rather than of
metal could open up a new range of applications, especially for
high-temperature settings, such as actuators inside a jet engine or a deep
borehole.
The new findings will
be reported in the journal Nature, in a paper by former doctoral
student Edward Pang PhD '21 and professors Gregory Olson and Christopher Schuh,
all in MIT's Department of Materials Science and Engineering.
Shape-memory materials,
Schuh explains, have two distinct shapes, and can switch back and forth between
them. They can be easily triggered by temperature, mechanical stress, or
electric or magnetic fields, to change shape in a way that exerts force, he
says.
"They are
interesting materials because they're sort of like a solid-state piston,"
he says -- in other words, a device that can push against something. But while
a piston is an assembly of many parts, a "shape-memory material is a
solid-state material that does all of that. It doesn't need a system. It
doesn't need many parts. It's just a material, and it changes its shape
spontaneously. It can do work. So, it's interesting as a 'smart
material,'" he says.
Shape-memory metals
have long been used as simple actuators in a variety of devices but are limited
by the achievable service temperatures of the metals used, usually a few
hundred degrees Celsius at most. Ceramics can withstand much higher
temperatures, sometimes up to thousands of degrees, but are known for their
brittleness. Now, the MIT team has found a way to overcome that and produce a
ceramic material that can actuate without accumulating damage, thus making it
possible for it to function reliably as a shape-memory material through many
cycles of use.
"The shape-memory
materials that are out there in the world, they're all metal," says Schuh.
"When you change a material'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."
He adds that "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."
Ceramics, by contrast,
don't tolerate damage well at all, and normally don't bend but fracture.
Zirconia is one that is known to have a shape-memory property, but it
accumulates damage very easily during a shape memory cycle -- a property
measured as high hysteresis. "What we wanted to do with this work was
design a new ceramic and specifically target that hysteresis. We wanted to
design a ceramic where the [shape] transformation is somehow still gigantic: We
want to do a lot of work. But internally, at the atomic scale, it's more
gentle."
Schuh explains that
Pang, who led the work, "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" in order to solve the problem
of creating such a material.
The result was a new
variation of zirconia. "Basically, it's zirconia," Schuh says.
"It looks and smells and tastes just like zirconia that people already
know and use, including like cubic zirconia in jewelry." But some atoms of
different elements have been introduced into its structure in a way that alters
some of its properties. These 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 says. "That
was a huge, huge change -- we're talking about a factor of 10." And the
deformation that the material can achieve amounts to about 10 percent, meaning
that a rod of this material could get 10 percent longer when triggered --
enough to do significant work.
One common application
of shape-memory materials is relief valves, where if a tank of something
exceeds a certain critical temperature, the valve is triggered by that heat,
automatically opening to relieve pressure and prevent explosion. The new
ceramic material could now extend that capability to far higher-temperature
situations than present materials could handle.
For example, actuators
that direct airflow inside a jet engine might be a useful application, Pang
says. While the overall environment there is hot, there are various channels of
airflow being controlled, so those flows could be used to trigger a
shape-memory ceramic by directing cooler or hotter air on the device as needed.
Today, shape-memory
ceramics that exist "are sort of a laboratory curiosity," because
they fall apart after a few cycles, Schuh says. "This is a step in the
direction of making something that can reproducibly and reliably operate many,
many times in service."
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 many cycles of
transformation.
What attracted him to
this project in the first place, Schuh says, is its potential for broad
applications. "There are things we do with complex mechanical systems that
have lots of parts and assemblies, and the idea that you can replace a
complicated package of things with a single material that has the functionality
built in at the atomic scale -- to me, that's attractive because it makes
large, complicated things into small, simple things. In some ways it's like
replacing vacuum tubes with transistors."
While it's hard to
predict the areas where this material will find its first practical uses, Schuh
says that, for example, "it's very hard to scale down a hydraulic piston.
It's hard to make that on the micro scale." But now, "the idea that
you have a solid-state version of that at very small scales -- I've always felt
there are a lot of applications for microscale motions. Microrobots in small
places, lab-on-a-chip valves, lots of small things that need actuation could
benefit from smart materials like this."
The work was supported
by the U.S. Army Research Office, in part through MIT's Institute for Soldier
Nanotechnologies, and by the U.S. National Science Foundation.
https://www.sciencedaily.com/releases/2022/10/221005111905.htm
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