IMF2017

Lead-free piezoelectric ceramics with the composition of 0.915K_0.5Na_0.5NbO₃-0.075BaZrO₃ -0.01Bi_0.5Na_0.5TiO₃ (KNN-BZ-BNT) show excellent piezoelectric properties with a good thermal stability, as the formation of vertical morphotropic phase boundary. In this work, the films of KNN-BZ-BNT were grown on Nb-doped SrTiO₃ substrates with different orientations to investigate the crystal phase and further optimize the performances of films. The thin films show highly preferential orientations in accordance with the orientations of single crystalline substrates. Crystalline phase of the samples was determined to be rhombohedral, which has the spontaneous polarization along [111] direction. Thus the (100)-oriented film demonstrates the most superior piezoelectric properties. All the films exhibit a weak contrast and remarkable homogeneity in the local static out-of-plane piezo-response phase images, suggesting a strong self-polarization. Detailed TEM investigation indicates that the self-polarization in KNN-BZ-BNT films is attributed to the formation internal built-in electric field from the bottom electrode. The La_0.7Sr_0.3MnO₃ layer deposited on SrTiO₃ as the bottom electode is more favorible to formation the self-polariztion effect. The layered-redistribution of Ba and Zr ions in the films is one of the possible reasons. The lattice parameters in the Zr-rich and Ba-rich areas also show the difference, which this phenomenon might lead to formation of a super structure and be a reason for improvement of electrical properties of KNN films by Ba and Zr co-doping.