This diagram illustrates how the thin-film material bends from its normal flat state (center) as oxygen is taken up by its structure (right) or released (left). This behavior allows the film’s shape to be controlled remotely by altering the applied electric charge. Image courtesy of the researchers.Carrying out maintenance tasks inside a nuclear plant puts severe strains on equipment, due to extreme temperatures that are hard for components to endure without degrading. Now, researchers at Massachusetts Institute of Technology (MIT) and elsewhere have come up with a radically new method for making actuators that can be used in such extremely hot environments.
The method relies on oxide materials similar to those used in many of today's rechargeable batteries, in that ions move in and out of the material during charging and discharging cycles. Whether the ions are lithium ions, in the case of lithium ion batteries, or oxygen ions, in the case of the oxide materials, their reversible motion causes the material to expand and contract.
Such expansion and contraction can be a major issue affecting the usable lifetime of a battery or fuel cell, as the repeated changes in volume can cause cracks to form, potentially leading to short-circuits or degraded performance. But for high-temperature actuators, these volume changes are a desired result rather than an unwelcome side effect.
The findings are described in a paper in Nature Materials by Jessica Swallow, an MIT graduate student, Krystyn Van Vliet, professor of materials science and engineering, Harry Tuller, professor of materials science and engineering, and five others.
"The most interesting thing about these materials is that they function at temperatures above 500°C," Swallow explains. That suggests that their predictable bending motions could be harnessed, for example, for maintenance robotics inside a nuclear reactor, or actuators inside jet engines or spacecraft engines.
By coupling these oxide materials with other materials whose dimensions remain constant, it would be possible to make actuators that bend when the oxide expands or contracts. This action is similar to the way bimetallic strips work in thermostats, where heating causes one metal to expand more than another that is bonded to it, leading the bonded strip to bend. For these tests, the researchers used a material called praseodymium-doped cerium oxide (PCO).
Conventional materials that move in response to electric charge, such as piezoelectric devices, don't work nearly as well at such high temperatures, so these new materials could open up a new field of high-temperature sensors and actuators. Such devices could be used, for example, to open and close valves in hot environments, the researchers say.
According to Van Vliet, this finding was made possible by a high-resolution, probe-based mechanical measurement system for high-temperature conditions that she and her co-workers have developed over the years. The system provides "precision measurements of material motion that here relate directly to oxygen levels," she says, allowing the researchers to measure exactly how the oxygen is cycling in and out of the metal oxide.
To make these measurements, the scientists begin by depositing a thin layer of metal oxide on a substrate and then use the detection system, which can measure small displacements on a scale of nanometers. "These materials are special," she says, "because they 'breathe' oxygen in and out, and change volume, and that causes the substrate to bend."
While they demonstrated this process using PCO, the researchers say the findings could apply broadly to a variety of oxide materials, and even to other kinds of ions in addition to oxygen, moving in and out of the oxide layer.
This story is adapted from material from MIT, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.