Built-in bias in Gd-doped ceria films and its implication for electromechanical actuation devices

Electromechanical response of 1.5 μm thick 2 mm diameter self-supported films (membranes) of 20 mol% Gd-doped ceria with Ti electrodes was measured at two temperatures (25 and 75 °C) as a function of direct (U _DC) and alternating (U _AC, 20 Hz) voltages. The films assumed a soup-panbowl shape and application of the external voltage resulted in a uniform vertical shift of the flat area. In the absence of superimposed U _DC, U _AC induces electromechanical response at the 1st and 2nd harmonics at both temperatures. The amplitude of the 2nd harmonic is proportional to U _AC ² at these temperatures and it is independent of U _DC, which identifies it as due to the electrostriction effect. Direct measurement of the built-in bias via minimization of the 1st harmonic response, as well as electrical impedance and I–V measurements indicate that, although asymmetry of the contacts may contribute to the appearance of the 1st harmonic, it is insufficient to explain it. Based on the fact that the force-displacement curve of the membrane measured with AFM is hysteretic, we hypothesize that the 1st harmonic response is related to the non-linear mechanical deformation of the buckled film.