Design and development of high-performance lead-free piezoelectric ceramics

Piezoelectric ceramics have been widely used in the commercial applications of actuators, sensors, etc. Nowadays, Pb(Zr,Ti)O3-based (PZT) ceramics, which contain ≥ 60 wt.% of lead, dominate the market due to their excellent electromechanical properties. The large electric field induced polarization and strain developed in PZT piezoceramics are intimately related to the morphotropic phase boundary (MPB). This has been attributed to the coexistence of tetragonal and rhombohedral perovskite phases. However, toxic lead severely threatens the environment and human health, as a significant amount of lead is released into the air during the fabrication and disposal. Thus, there is an urgent demand worldwide to develop new kinds of green materials, lead-free ceramics, to replace the PZT lead-based compositions with comparable electrical properties. For the past decade, the study has been focused on the BaTiO3-based (BTO), (K0.5Na0.5)NbO3-based (KNN), and (Bi1/2Na1/2)TiO3-based (BNT) solid solutions. Despite the extensive efforts worldwide, the performances of lead-free materials have yet to reach the level of PZT ceramics. In this thesis research, new chemical modifications have been explored and introduced to the lead-free compositions. It is elucidated that the dielectric, ferroelectric and piezoelectric properties are associated with the phase presence and microstructure. Therefore, understanding the structure behavior is the key to enhance the electrical properties efficiently. Usually, the bottleneck for industry application of lead-free materials is the narrow working temperature range, the weak piezoelectric response and the low electric field induced strain. It is well known that the best way to overcome these shortcomings is to form a solid solution by combining two different compounds. Another common way is to decrease the grain size to a submicron level. The results from this dissertation demonstrate that the (Bi1/2A1/2)TiO3 type doping compounds (A = Ag, Li, Na