Key-features in processing and microstructure for achieving giant electrostriction in gadolinium doped ceria thin films
Gadolinium doped ceria is a well-known oxygen ion conduction material for solid fuel cell electrolytes. With its centrosymmetric average lattice and relatively low bulk dielectric constant it does not look interesting for electromechanical applications. However, a giant electrostriction (ES) effect was recently found in Ce0.8Gd0.2O2-X thin films. It was explained by the dynamic response of oxygen vacancies to an external electric field. In this work, the giant ES response has been reproduced in sputter deposited thin films. The proper transverse bulk ES coefficient has been derived from the measured clamped value. For this purpose, the Young's modulus was measured by nanoindentation. The highest ES coefficient was found as 9.0 x 10(-19) (m/V)(2) for the strain coefficient, and 2.3 x 10(-7) N/V-2 for the effective stress coefficient. Specific growth conditions must be chosen in order to obtain a microstructure exhibiting the giant ES effect. There is evidence for a higher oxygen deficiency than needed to compensate the gadolinium dopants (Gd'). It was observed that the nature of the bottom electrode impacts on the size of the effect. The highest response was obtained at films grown on Al bottom electrodes. To learn more on the mechanisms of the giant ES effect, a bipolar cycling was performed to test the delay time for the ionic reorientation in changing the sign of polarization, as observed in the stress loop. The maximal response was observed below 100 Hz in this bipolar mode, showing that the time for 180 degree reorientation amounts to several milliseconds. (C) 2016 Acta MateriaIia Inc. Published by Elsevier Ltd. All rights reserved.