Dynamics and control of non-classical electrostriction in doped ceria

Electrostrictive and piezoelectric materials, which change their shape in response to external fields, have critical applications in many different contexts, working as a muscle. Nowadays, electromechanical active materials are used in everyday technology as components of cameras or sound transducer as well as advanced systems such as microelectronics, energy harvesting or ultrasound imaging.

The most diffused electrostrictive materials are Pb-based compounds such as Pb(Mn1/3Nb2/3)O3 (PMN). Not only they contain lead (Pb), which is highly toxic, but their use has been restricted by the European restriction of hazardous substances directive (RoHS) in 2006. For these reasons, many efforts have been made to find an environmentally friendly alternative as a substitute to current materials.

Recently, bio-compatible defective oxides have been showed to possess high electrostrictive behavior. In particular cerium oxides, i.e. ceria, display a giant electromechanical effect with magnitude comparable or superior to the best State-of-the-Art materials. The electromechanical mechanism in this kind of compounds depends on the oxygen vacancies configuration within the crystal lattice and it differs from classical electrostriction in intensity and dynamics.

For this reason, doped ceria is considered a promising candidate for a new generation of “smart” materials. This project focuses on the mechanism and operation of electrostrictive doped ceria. Thin films are synthesized by PLD technique using several substrates and electrodes. The reliability and mechanical integrity of actuator thin films are enhanced by integration in full ceramic structures and by planar electrodes devices.

In order to do this, a sub-nanometer displacement characterization tool is designed and assembled. Then, the electrostrictive effect is mapped depending on the crystal geometry in highly coherent thin films. This not only allows to develop a new interpretation of non-classical electrostriction in doped ceria in terms of atomic displacement but also identifies the optimal distortion condition of the material, resulting in a ten times higher electrostrictive effect.

Finally, ultra-thin films coupling with the substrate is studied by x-ray absorption spectroscopy (XAS), highlighting a strong interfacial electrostriction effect controlled by the inclusion of crystal defects in strained structures. In general, during this research, both the operative stability and performances of electrostrictive ceria devices has been dramatically improved.

Moreover, the underlying mechanism behind the effect has been addressed and explained with a rigorous model, supported by experimental data. Finally, means to control electromechanical response in thin films through crystal orientation and strain manipulation have been showed.