| 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 (Bi 1/2Na1/2)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)TiO 3 type doping compounds (A = Ag, Li, Na, K, Rb, Cs) in BTO ceramic stabilize the tetragonal phase and increase the Curie temperature (TC). Meanwhile, in BNT-based systems, La modification transforms the structure of (Bi1/2A1/2)TiO3-BaTiO3 (BNT-BT) from rhombohedral R3c phase with micron-sized complex domains into a coexistence of rhombohedral R3c and tetragonal P4bm phases with nanodomains. Large electric field-induced strains (S) and temperature-stable dielectric permittivity at high temperatures are also revealed. Since chemical modified (Bi1/2A 1/2)TiO3-(Bi1/2K1/2)TiO3 (BNK-BKT) piezoceramics develop the largest electrostrain among lead-free solid solutions, the electrical properties and microstructure evolution under the electric field in Nb and Sr co-doped BNT-BKT ceramics have been systematically studied. Giant electrostrains, which we believe are the largest value reported in lead-free polycrystalline ceramics, and their correspondingly high normalized strain d33* (Smax/Emax ) were observed. The origin of this abnormally large electrostrain is attributed to the reversible phase transition between the ferroelectric phase with aligned lamellar domains and the ergodic relaxor phase with randomly orientated nanometer sized domains under the electric field, according to in-situ transmission electron microscopy examinations. In addition, the introduction of donor dopant Ta to simplify the composition, instead of Nb and Sr, into the BNT-BKT system results in enhanced piezoelectricity with large strain comparable even to that of lead-free single crystals.;The phase presence and microstructure of modified ceramics, revealed by X-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) examinations, are affected by chemical modifications and further correlate to the electrical properties. Thus, understanding the phase transition and domain configuration helps to develop lead-free candidates with piezoelectric properties comparable to lead-containing materials. Furthermore, in-situ TEM technique is introduced to study the real time domain evolution under electric field. |
