Shape memory metals have been useful in many applications as actuators that can control the movement of various devices. These materials can change shape simply by being heated or otherwise triggered. The discovery of a brand-new class of ceramic shape memory material as opposed to metal ones could expand the range of possible uses, particularly for high-temperature settings like actuators inside jet engines or deep boreholes.
The new findings will be published in the journal Nature in a paper written by professors Gregory Olson and Christopher Schuh from the MIT Department of Materials Science and Engineering, along with former doctoral candidate Edward Pang, PhD ’21.
According to Schuh, materials with shape memory can alternate between two different shapes. He says that they can easily change shape in a way that creates force when exposed to heat, mechanical stress, or electric or magnetic fields.
According to him, “They are interesting materials because they’re kind of like a solid-state piston”—a machine that can push against something. However, whereas a piston is made up of numerous parts, a “solid-state material that does all of that is called a shape memory material. It does not need a system. There aren’t many parts required. It is merely a substance that spontaneously changes shape. It is functional. As a result, it’s intriguing as “smart material.” He claims
Shape-memory metals have been used as simple actuators in many different devices for a long time, but their use is restricted by the highest service temperatures that can be reached for the metals, typically a few hundred degrees Celsius. Ceramics are notorious for their brittleness but can withstand much higher temperatures, sometimes up to thousands of degrees. The MIT team has now found a way to get around this problem and make a ceramic material that can move without getting damaged. This means that it can be used over and over again as a shape memory material without breaking down.
According to Schuh, all shape memory material currently in use are made of metal. “There is a lot of damage that can be done when you alter a material’s shape at the atomic level. Atoms must rearrange and modify their structure. Furthermore, since atoms are constantly shifting and moving, it’s relatively simple to place them incorrectly, leading to flaws and material damage that eventually cause the materials to wear out and disintegrate. ”
He added that “The result is materials that can deform a few times before eventually degrading and crumbling. The field has really focused on metals because they can tolerate internal damage and are somewhat more damage resistant due to their high degree of ductility. ”
In contrast, ceramics don’t take damage well at all and typically don’t bend; instead, they fracture. Zirconia is one material with a well-known shape-memory property, but it easily accumulates damage during a shape-memory cycle, a characteristic known as high hysteresis. “With this work, we aimed to create a new ceramic that would specifically address that hysteresis. We wanted to create a ceramic whose [shape] transformation is still incredibly large because we wanted to put a lot of effort into it. But on the atomic level, it is softer inside. ”
Pang, who oversaw the project, “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 address the issue of producing such a material, according to Schuh.
A new type of zirconia was produced as a result. According to Schuh, “it’s basically zirconia.” The zirconia that people are already familiar with and use, such as cubic zirconia in jewelry, is exactly how it looks, smells, and tastes. But some atoms from various other elements have been added to its structure in a way that changes some of its characteristics. These elements “dissolve into the lattice, they sculpt it, they change that transformation, they make it more gentle at the atomic scale.”
According to Schuh, the hysteresis has changed so drastically that it now resembles that of metals. We’re talking about a factor of 10 change, so that was a huge, huge change. Due to the material’s 10% deformation capacity, a rod made of it could lengthen by about 10% when activated, which is enough to do a lot of work.
One typical use of shape memory material is in relief valves, which automatically open to release pressure and prevent explosions if a tank of something reaches a certain critical temperature. This capability could now be expanded to temperatures that current materials couldn’t handle thanks to the new ceramic material.
Pang cites a possible application as actuators that control airflow inside jet engines. The area is hot overall, but there are several controlled airflow channels, so those flows could be used to activate a shape memory ceramic by directing cooler or hotter air onto the device as necessary.
Because they break down after a few cycles, Schuh describes the shape-memory ceramics that are currently made as “sort of a laboratory curiosity.” “This is a good step toward making something that can work reliably and over and over again in service.”
The group plans to do more research on the substance, such as figuring out how to make it in larger quantities and more complicated shapes and testing how well it holds up to being changed over and over.
According to Schuh, the project’s potential for numerous applications is what drew him to it in the first place. “The idea that you can replace a complex package of components with a single material that has the functionality built in at the atomic level is appealing to me because it reduces large, complex things to small, simple things. There are things we do with complex mechanical systems that have many parts and assemblies. In some ways, it is analogous to switching from vacuum tubes to transistors. ”
The areas in which this material will find its first applications are difficult to predict, but Schuh notes that, for instance, “a hydraulic piston is very difficult to reduce in size. Making that on a small scale is difficult. ” But now, “I’ve always believed there are numerous applications for microscale motions, and the notion that you have a solid-state version of that at very small scales intrigues me. Smart materials like these could be useful for microrobots in cramped spaces, lab-on-a-chip valves, and many other tiny things that require actuation. ”
The work was funded by the U.S. Army Research Office, the U.S. National Science Foundation, and MIT’s Institute for Soldier Nanotechnologies.
