Oxide film after sufficient mechanical expansion can produce fractures, observable as dark cracks within the film material.A new study has developed a type of low-voltage electromechanical oxide actuator made from a material that expands and contracts as it lets oxygen in and out, and which can operate in very hot environments. The materials, which bend at the same time as they “breathe”, could assist in maintenance work carried out in nuclear installations or as actuators operating inside the engines of jets spacecraft where equipment components often become degraded.
A team led by researchers at MIT, whose work featured in Nature Materials [Swallow et al. Nat. Mater. (2017) DOI: 10.1038/nmat4898], presented a system that depends on oxide materials akin to that in many rechargeable batteries, where ions move in and out of the material during the charging and discharging cycles. These tend to be lithium ions for lithium ion batteries or oxygen ions for oxide materials, but it is the reversible motion that results in the expansion and contraction of the material. Although such continual motion can be a problem for batteries since repeated volume changes can result in cracks that produce short-circuits or limit performance, they are actually a good thing for high-temperature actuators.
A high-resolution, probe-based mechanical measurement system provided a precise measure of material motion to help assess how the oxygen is moving in and out of a thin layer of metal oxide deposited on a substrate. The detection system measured tiny displacements in the order of nanometers – despite the film only expanding a few nanometers, placing it on a flexible structure amplifies the displacement effect in a predictable and significant way.
“These results demonstrate a novel type of high temperature actuator that can achieve high strains for relatively low applied electrical voltage, using a mechanism that is quite different from the more standard piezoelectric type of actuator”Krystyn Van Vliet
On combining their oxide material – a compound called praseodymium-doped cerium oxide – with other materials with constant dimensions, the actuators were found to bend when the oxide expands or contracts. The new materials were also found to function at temperatures greater than 5000C, giving hope that such predictable motion could be harnessed, including for valves that open and close in extremely hot environments.
The measurement, learning and design of the materials and devices offered a better understanding of the material in its operating environment, and demonstrated that the similar materials and structures could provide new high temperature actuating devices. As Krystyn Van Vliet points out, “These results demonstrate a novel type of high temperature actuator that can achieve high strains for relatively low applied electrical voltage, using a mechanism that is quite different from the more standard piezoelectric type of actuator”.
While the coupling process was shown to work with only one oxide compound, the findings could be extended to other electrochemomechanically coupled oxide films at extreme temperatures, and perhaps also to other kinds of ions in addition to oxygen.