Contrasting strain mechanisms in lead-free piezoelectric ceramics

Abstract

Piezoelectric ceramics find a wide range of applications in advanced technological fields. Most of the currently used piezoelectric ceramics contain lead (Pb). Environmental concerns and limitations in high-temperature performances of market leading Pb(Zr,Ti)O3 has spurred the field of lead-free electroceramics research. Compositions based on Bi1/2Na1/2TiO3, Na1/2K1/2NbO3, BiFeO3 and BaTiO3 have long been considered as candidates to replace lead containing piezoelectric ceramics. Although lead-free compositions based on these systems exhibit electromechanical properties for potential device application, further enhancement of properties and reliability is required. To achieve this, extensive knowledge of structure- property relationships at multiple length scales, especially during the actuation condition is essential. However, systematic studies of the multi- length-scale structural contributions (lattice deformation, domain wall motion, phase transformations) to the field-induced strain in lead-free electroceramics are lacking. To understand the microscopic origin of strain in lead-free electroceramics several compositions have been studied using in situ high-energy x-ray diffraction. Their microscopic strain response has been elucidated under electric field. Strain contributions have been analysed for each system and have been correlated with their macroscopic properties. The first material was a ceramic/ceramic composite of particulate phase 0.93(Bi1/2Na1/2)TiO3-0.07BaTiO3 and matrix phase 0.92(Bi1/2Na1/2)TiO3-0.06BaTiO3-0.02(Na1/2K1/2NbO3). Under applied electric field, this material system showed that the local response of the particulate phase can be used to tailor bulk material properties. A microscopic strain mechanism in ceramic/ceramic composite system was proposed on the basis of these diffraction studies. A microscopic strain mechanism in ceramic/ceramic composite system was proposed on the basis of these diffraction studies. Domain switching in a core-shell BaTiO3-KNbO3 piezoelectric ceramics has also been investigated. The core-shell BT-KN has been shown to exhibit remarkably large reversible domain switching during the application of electric fields. Electromechancial coupling behavior in bulk BiFeO3 has been quantified from diffraction data. The strain mechanism has been contrasted with thin-film BiFeO3. Interestingly, despite the different mechanism, rhombohedral bulk BiFeO3 ceramic can exhibit a similar strain/field ratio to thin-film BiFeO3. This comparative study of strain responses will enable the research field to focus on identified important structural aspects that are essential to improve piezoelectric properties in future lead-free systems.