Flexoelectric response of ferroelectric ceramics with reduced surface layer effect

The flexoelectric response of Na_0.5Bi_0.5TiO₃-based and (Ba,Sr)TiO₃ ferroelectric ceramics with sputtered or fired-on Ag, Au, and Ag/Pd electrodes was investigated. We found that for all the ceramics, the flexoelectric response of the same material is strongly dependent on the methods used to fabricate the electrodes. The samples with fired-on electrodes have a much smaller flexoelectric response than those with sputtered electrodes, and for a certain composition, the flexoelectric response can be reduced by more than 99%. The reduction of flexoelectric response is attributed to the suppression of the contribution from the spontaneously polarized surface layers on ferroelectric ceramics to the flexoelectric response. Three reasons were proposed for the suppression of the surface effect, namely, the diffusion of some substances in electrode pastes to the surface regions of ferroelectric ceramics, stress relief, and the generation of impurity phases caused by the evaporation of volatile element during the firing of electrode materials at a high temperature. We also found that the flexoelectric response can approach the theoretically predicated response after the surface effect is reduced by fabricating fired-on electrodes on ferroelectric ceramics. Our results indicate that the large flexoelectric response of ferroelectric ceramics is mainly from the polarized surface layers and the surface effect is the primary mechanism leading to the deviation of measured flexoelectric response from the intrinsic response in ferroelectric ceramics, which has been an unresolved issue in the flexoelectric research field.