Extreme temperatures are hard for mechanical components to endure without degrading. To address the problem, researchers at MIT worked with several other universities to develop a new way to make actuators that could be used in exceptionally hot environments.

Figure 1. 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 enables the film’s shape to be controlled remotely by changing its electric charge. (Image courtesy of the researchers)
Figure 2. MIT graduate student Jessica Swallow pictured with the equipment used for testing the new materials. (Image courtesy of the researchers)

The system relies on oxide materials like those used in many rechargeable batteries, in which ions move in and out of the material during the 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.

These materials function at temperatures above 500 °C. 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 works 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 (Figure 1). For these tests, the researchers used a compound dubbed PCO (praseodymium-doped cerium oxide).

Conventional materials used to create motion by applying electricity, such as piezoelectric devices, don’t work well at such high temperatures, so the technology could enable a new class 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.

MIT Professor Krystyn Van Vliet explains that the finding was made possible because of a high-resolution, probe-based mechanical measurement system for high-temperature conditions that she and her co-workers have developed over the years (Figure 2). The system provides “precision measurements of material motion that here 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.”

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