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

Lead-free potassium sodium niobate [(K, Na)NbO 3 , 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 (Al 2 O 3 , Co 2 O 3 , Nb 2 O 5 , SnO 2 and HfO 2 ) 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 ( d 33 ≈ 450–500 pC/N) and high Curie temperature values ( T C ≈ 255–263 °C) together with high electromechanical coupling coefficient values ( k p ≈ 0.50–0.56) are obtained in constructed R-O-T multiphase coexisted KNN-based ceramics. Moreover, the KNN ceramic with the addition of Al 2 O 3 exhibits an optimal unipolar strain value ( S = 0.124%) and a high inverse piezoelectric constant ( d 33 * = 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.