PhD Thesis Colloquium: Mr. Gudeta Jafo Muleta (27/04/26)
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?
27th April, 2026 (Monday), 03:00 PM (India Standard Time)
Where
KPA Auditorium, Department of 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 them, NBT-based systems exhibit large electrostrain, which is promising for actuator 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) an MPB with coexistence of rhombohedral and tetragonal phases. Among NBT-based systems, NBT-KBT and NBT-BT modified with other lead-free systems exhibit large electrostrain at the NER/ER (ergodic relaxor) phase boundary. The significance of this phase boundary in electrostrain enhancement remains unclear. In this thesis, this aspect has been explored in SrTi0.875Nb0.1O3 (STN), K0.5Na0.5NbO3 (KNN), and NaNbO3 (NN) modified NBT-KBT systems. An electric-field-dependent in-situ X-ray diffraction method was employed to understand the nature and structural evolution of the NER and ER states. Although the structural crossover at pre-MPB in the NBT-BT system is well established, compositional modification of its (R3c + Cc) mixed phase and subsequent changes in piezoelectric and ferroelectric properties have not been studied. This issue is addressed in this thesis by combining temperature-dependent X-ray diffraction with ferroelectric and dielectric property measurements on the NaTaO3 (NT)-modified 0.98NBT-0.02BT system. In one of the systems, NBT-KBT-NN, discussed in this thesis, the Eu3+ photoluminescence (PL) method has been adopted to study local structural evolution that XRD has not detected. However, there is an open question as to whether this technique can be utilized to probe structural changes in modified PZT systems. In the last part of this thesis, we used a combination of Eu3+ PL, Raman spectroscopy, and XRD to investigate the underlying mechanisms behind a peculiar, simultaneous increase in d33 and Curie point, properties observed in rare-earth-doped PZT.