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The equipment used for testing the new materials. (Credit: MIT)

Extreme temperatures can severely strain a mechanical component because its material may have trouble enduring the heat without degrading. To address the problem, researchers at MIT developed a new material that expands and contracts as it lets oxygen in and out. The result is a new way to make actuators that could be used in extremely hot environments.

The system relies on oxide materials similar to those used in many of today’s rechargeable batteries, in which ions move in and out of the material during charging and discharging cycles. Whether the ions are lithium ions or oxygen ions, 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 most interesting thing about these materials is that they function at temperatures above 500 °C,” Jessica Swallow, an MIT graduate student, 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 is 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 compound dubbed PCO, for praseodymium-doped cerium oxide.

Conventional materials used to create motion by applying electricity, such as piezoelectric devices, don’t work nearly as well at such high temperatures, so the new system could open up a new area of high-temperature sensors and actuators. Such devices could be used, for example, to open and close valves in these hot environments, the researchers say.

Krystyn Van Vliet, Professor of Materials Science and Engineering, says the research was facilitated 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 relate directly to oxygen levels,” she says, enabling researchers to measure exactly how the oxygen is cycling in and out of the metal oxide.

To make these measurements, scientists begin by depositing a thin layer of metal oxide on a substrate, then use the detection system, which can measure small displacements on a scale of nanometers, or billionths of a meter. “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 the process using one particular oxide compound, 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.

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