Effect of BaZrO₃ amounts on the domain structure and electrical properties of lead-free piezoelectric KNN-based films

K_0.5Na_0.5NbO₃ (KNN)-based bulk ceramics show superior piezoelectric coefficient by constructing the morphotropic phase boundary (MPB) between rhombohedral and tetragonal phases. However, it is a challenge for KNN films to obtain the superior electrical properties with the same MPB composition, because of the serious volatilization of alkali ions and substrate-induced stress. Therefore, the MPB composition in films could be different from bulk ceramics. The lead-free piezoelectric 0.92KNN-0.07BaZrO₃ (BZ)-0.01Bi_0.5Na_0.5TiO₃ (BNT) and 0.91KNN-0.08BZ-0.01BNT films were prepared, referring to the MPB composition of bulk ceramics. For comparison, the 0.95KNN-0.04BZ-0.01BNT and 0.88KNN-0.11BZ-0.01BNT films far away from the MPB composition of bulk ceramics were prepared together. Their crystallographic phase, domain morphologies and ferroelectric switching show a dependence of BZ amount. With increase of the BZ content, the (1 0 0)-preferential orientation and out-of-plane self-polarization of films enhanced. Because the 0.08 BZ film shows the higher self-polarization and lower KNN content, compared with 0.04 BZ film, it shows the stronger piezoelectric response but not a higher dielectric constant.