Giant electrostrictors for electromechanical devices – MEGAEM

The ability to generate acoustic waves with an electric field enables the design of electromechanical applications such as sonars, hydrophones, ultrasonic probes, frequency filters, as well as micro-electro-mechanical systems (MEMS).

These electromechanical applications are mostly based on the piezoelectric effect, where the deformation is proportional to the applied electric field. More than 99% of these applications are based on PZT, a solid solution between lead titanate and lead zirconate, which has been dominating the market very wildly for several decades now. The current environmental and European regulations currently do not apply, by exception, to this lead content, that remains nevertheless an issue. Lead-free piezoelectric alternatives, despite a large effort from the research community, exhibit so far performances at best similar to those of PZT. They therefore do not provide a strong enough cost/benefit ratio to induce a change of the industrial technology.

Electrostriction is another electromechanical property but the induced deformation is now proportional to the square of the electric displacement induced by an electric field. For linear dielectrics, this deformation is therefore proportional to the electric field squared. Until recently, the electrostrictive properties were not strong enough to compete with piezoelectrics; what is more the best electrostrictors contained lead. In 2012, the discovery of a "giant" electrostrictive effect, a million time larger than expected, enables electrostrictive applications with higher performances than piezoelectrics, with the additional advantage that this material (gadolinium-doped ceria) does not contain lead. In 2016 and 2018, two other materials, namely bismuth oxide (delta-Bi2O3) and LAMOX (La2Mo2O9) have been discovered, with similar performances. These discoveries are surprising as these materials have been studied so far for their ionic conduction properties for solid oxide fuel cells (SOFC).

This research field is not currently very active (about twenty articles over seven years), probably because the piezoelectric community is not much interested in these oxygen-vacancy containing materials and the SOFC community is barely interested in electromechanical properties. This means that a consortium composed of specialists of these materials and of the electromechanical measurements should provide a strong momentum to the field. In addition to these experimental expertise, a specialist of ab initio calculations shall contribute to the consortium in order to pinpoint, together with the experiments, the mechanism at the heart of this effect. This will enable to identify the structural parameters to optimise in order to exacerbate some more this response. Our industrial partner will help us ensure that this research is compatible with industrial applications.

Our project therefore aims to validate the potential of the "giant" electrostriction of the already-discovered materials for applications based on bulk electrostrictors, especially sonars. For this, we shall determine which are the optimal compositions and their properties as a function of temperature and frequency. Combining structural characterisations under electric field and simulations, we shall determine the strategies to optimise even more these performances for the target devices. It is indeed unlikely that the materials discovered first are the best ones. We therefore hope to provide a momentum able to trigger a larger research effort on these theme, resulting in electromechanical applications with superior performances and environmentally friendly.