The discovery of a new category of shape-memory materials — ceramic as opposed to metal — could open a new range of applications, especially for high-temperature settings, such as actuators inside a jet engine.
Shape-memory materials 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.
“They are interesting materials because they’re sort of like a solid-state piston,” said Professor Christopher Schuh, Professor in MIT’s Department of Materials Science and Engineering. Put another way, they’re a device that can push against something.
However, 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 added.
Shape-memory metals have long been used as simple actuators in a variety of devices, but they’re limited by the achievable service temperatures of the metals used — usually a few hundred degrees C, tops.
Ceramics can withstand much higher temperatures, sometimes up to thousands of degrees, than shape-memory metals but are brittle. A team at MIT has found a way to 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,” said 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.
“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 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.
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.