Electro-chemo-mechanical Contribution to Mechanical Actuation in Nanocrystalline Gd-doped Ceria Membranes

Gadolinium-doped ceria (CGO), one of the most extensively studied ion-conductors, exhibits electrostrictive strain that is comparable in magnitude to the field-induced strain of commercial, lead-based electrostrictors. Given the ecological preference for ceria over Pb, we fabricated electrically and mechanically robust nanocrystalline CGO self-supported MEMS structures in the form of millimeter-sized membranes, bridges, and cantilevers, using only Si-compatible processes and materials.

Electro-mechanical measurements, combined with electrical and mechanical characterization, show that in addition to a 2^nd order electrostriction response, Al/Ti/CGO/Ti/Al membranes display 1^st harmonic electro-chemo-mechanical (ECM) actuation, i.e., mechanical deformation induced by the change in composition due to an electro-chemical oxidation/reduction reaction at the Ti/CGO interface. Unlike the electrostriction response that has a Debye-type relaxation time of ~16 sec, the ECM response (associated with a “Warburg” like constant phase element) is limited by the oxygen diffusion rate and, therefore, is proportional to the square-root of time (relaxation was not observed even after 5min). Michelson-Morley interferometry measurements revealed that the amplitude of the 1st harmonic ECM response is an exponentially increasing function of temperature with activation energy very similar to that for oxygen vacancy diffusion.

The ECM effect is known to be seriously detrimental in batteries, sensors and fuel-cells because it can produce large stresses and strains, which may lead to catastrophic device failure. However, prior to this work, the room temperature ECM effect in oxygen ion conductors was not investigated because at room temperature, the oxygen diffusion coefficient is too low (between 10^-16 and 10^-17 cm² s^-1) to produce a significant response. Our data suggest that with proper choice of materials, ECM response can be a viable mechanism for mechanical actuation at ambient, and also at slightly elevated temperatures.