Realization of densified microstructure and large piezoelectricity in KNN ceramics via the addition of oxide additives

Lead-free potassium sodium niobate [(K, Na)NbO3, KNN]-based ceramics have aroused great interest due to their excellent piezoelectricity and high Curie temperature. However, the deteriorative microstructure of KNN-based ceramics restricts their further developments. In this work, various kinds of oxide additives (Al2O3, Co2O3, Nb2O5, SnO2 and HfO2) are introduced into the KNN-based ceramics to increase the grain size and wipe the microcracks to improve their microstructures. Herein, large piezoelectric coefficient values (d33 ≈ 450–500 pC/N) and high Curie temperature values (TC ≈ 255–263 °C) together with high electromechanical coupling coefficient values (kp ≈ 0.50–0.56) are obtained in constructed R-O-T multiphase coexisted KNN-based ceramics. Moreover, the KNN ceramic with the addition of Al2O3 exhibits an optimal unipolar strain value (S = 0.124%) and a high inverse piezoelectric constant (d33* = 494 pm/V). We believe that our findings provide a simple, effective, and applicable approach to optimize the electrical performances of KNN-based system materials.