Synthesis, structure and properties of lead-free piezoelectric ceramics
Tan, Ivan Chee Kiang
Date of Issue2013
School of Materials Science and Engineering
Since the discovery of PbZrO3-PbTiO3 (PZT) ceramic in 1950s, lead-based piezoelectric materials have been widely used in sensors, actuators, transducers and many other electronic devices. The increasing application of these devices generates increasingly aggravated health and environmental concerns because these conventional lead-based ceramics contain more than 60% of lead by weight. During recent years, intensive efforts have been made to develop lead-free alternatives. Candidates of the lead-free ceramic systems 0.94(K0.5Na0.5)NbO3-0.06LiNbO3 (0.94KNN-0.06LN) and 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 (0.5BZT-0.5BCT) exhibit promising properties with application potential among the lead-free piezoelectric ceramics. Hence, they are chosen for the current investigation in this project. However, lead-free KNN based ceramics usually suffer from volatile loss of the alkali ions during the calcination process, which is more serious before the formation of the perovskite structure. This leads to stoichiometry deviation and consequent degradation of properties. Therefore, special attention should be paid to suppress alkali volatility by promoting synthesis at lower temperatures. Another problem with solid state synthesis involves the long hours of ball milling, which is time consuming and is a potential source for contamination. Two approaches are investigated in this project to address alkali volatility. Firstly, ultrasonic irradiation during synthesis is systematically studied. Results show that ultrasonic irradiation facilitates the carbonate decomposition and promotes the formation of perovskite phase at a substantially lower temperature. Secondly, effect of polyvinylpyrrolidone (PVP) as a chemical additive to suppress the loss of alkali ions is investigated. The results of the addition reveals that chemical interaction exists between PVP and the starting alkali carbonates in their uncalcined state which subsequently reduces the volatilization of potassium and sodium during calcination. The in-situ heat generated by decomposition of PVP might also promote decomposition of the carbonates and promote the perovskite phase formation. In addition, two additional lead-free compositions are explored. One is Ba0.85Ca0.15Zr0.1Ti0.9O3 (BCZT) doped with LiF and synthesized with ultrasonic irradiation assistance. In this ceramic, the LiF dopant lowers the polymorphic phase transition (PPT) temperature and enhances the electrical properties at room temperature. Second is another novel KNN-LN based ceramic with BCZT as the dopant which was prepared with ultrasonic irradiation. A new compositional morphotropic phase boundary (MPB) was discovered with improved piezoelectric properties.