Effect of off-stoichiometry, grain size and chemical substitution on microstructure, global structure, electromechanical and energy storage properties of lead free Na0.5Bi0.5TiO3 (NBT) based piezoceramics
Ferroelectric and piezoelectric materials, especially the polycrystalline ceramics, are promising candidates for application in pressure sensors, actuators, transducers, ultrasonic motors, energy storage capacitors, ferroelectric memories, electrooptic devices and PTC thermistors. Lead zirconate titanate PbZr1-xTixO3 (PZT) based ferroelectric and piezoelectric ceramics have dominated the market owing to their superior electromechanical properties. However, the toxicity of lead has compelled researchers to search for a suitable lead-free alternative which show similar if not superior properties in comparison to lead based ones. Sodium bismuth titanate Na0.5Bi0.5TiO3 (NBT)-based piezoceramics are among the key contenders to replace lead based piezoceramics. Numerous studies in the past have focused attention on the chemical substitution effects in improving the electromechanical response of NBT-based piezoelectrics. Some studies have shown that off-stoichiometry also considerably influences the electromechanical response of NBT-based piezoceramics. However, structure-property correlations in off-stoichiometric NBT-based systems have not received detailed attention. In this thesis we have investigated this aspect extensively. The focus has been on NBT and its solid solution with BaTiO3 (BT); more specifically the compositions close to the morphotropic phase boundary, i.e., 94NBT-0.06BT. In addition to the off-stoichiometric studies, the work has been extended to understand the role of grain size on the dielectric, ferroelectric and piezoelectric properties and the depolarization temperature of NBT and NBT-BT. The ceramics were synthesized via the conventional ceramic synthesis route and characterized by a host of techniques like scanning electron microscopy (SEM), x-ray diffraction, Raman spectroscopy, impedance spectroscopy, ferroelectric, dielectric, piezoelectric and depolarization measurements. While the better resolution of the laboratory x-ray powder diffraction data was used to understand the nature of lattice distortions (spontaneous strain) on the global scale, wherever required, neutron powder diffraction study was also undertaken. The large elastic scattering cross-section of oxygen for thermal neutrons made it possible to capture the subtle structural features such as the subtle in-phase octahedral tilt (not possible to capture with XRD data). This provided new structural insights to explain the property trends as a function of off-stoichiometry and grain size. For example, a correlation between grain size and volume fraction of the structural disorder caused by the in-phase tilt, and its effect on the piezoelectric and depolarization temperature has been unambiguously established in this work. In the end, it is shown that chemical modification of NBT-BT with BiFeO3 (BF) and K0.5Na0.5NbO3 (KNN) yield compositions which are interesting for energy storage applications.