PhD Thesis Defence: Mr. GUDETA JAFO MULETA (13/10/26)
Thesis title:
Structural aspects underlying the electromechanical and ferroelectric properties in Na0.5Bi0.5TiO3-based and Pb(Zr, Ti)O3-based piezoceramics
Faculty advisor(s):
Prof. Rajeev Ranjan
When?
13th October, 2026 (Tuesday), 10:30 AM (India Standard Time)
Where
KPA Auditorium, Materials Engineering
Abstract
For over 50 years, due to the excellent electromechanical conversion, Pb(Zr, Ti)O3 (PZT) has been widely used in piezoelectric sensor, actuator, and transducer applications. The superior properties of PZT are attributed to the presence of a morphotropic phase boundary (MPB), where the tetragonal (P4mm) and rhombohedral (R3m) phases coexist. However, the toxicity of Lead has raised health and environmental concerns, encouraging the development of Lead-free alternatives, such as BaTiO3 (BT), BiFeO3 (BF), K0.5Na0.5NbO3 (KNN), K0.5Bi0.5TiO3 (KBT), and Na0.5Bi0.5TiO3 (NBT). Among lead-free systems, NBT-based piezoceramics are promising candidates for sensors, actuators, and energy storage applications. Pure NBT is a nonergodic relaxor (NER) with rhombohedral (R3c) symmetry. Its solid solution with tetragonal (P4mm) BT or KBT exhibits two distinct regions: (i) rhombohedral (R3c) + monoclinic (Cc) mixed state at pre-MPB and (ii) coexistence of rhombohedral and tetragonal phases at the MPB composition. Compared with PZT, NBT-based MPB exhibits modest electromechanical response with a sharp dip in the depolarization temperature; however, the modified NBT-based system demonstrates excellent electrostrain and energy storage performance. The underlying structural mechanisms associated with the relatively low electromechanical responses and the excellent electrostrain and energy storage properties in the NBT-based system are still under investigation. In this thesis, we sought to understand the structural aspects of piezoelectricity, electric-field-induced strain, and energy-storage properties in modified NBT-BT and NBT-KBT-based systems by employing ferroelectric, dielectric, and X-ray diffraction measurements as a function of temperature and electric field. We also extended the structural-property correlation studies to the Lead-based system to elucidate the underlying structural mechanism behind the unusual combination of a large piezoelectric response and a high Curie temperature in PZT-based piezoceramics.