Multiscale Investigation of BiFeO3-K0.5Bi0.5TiO3-PbTiO3 Ceramics

Abstract

The BiFeO3-K0.5Bi0.5TiO3-PbTiO3 (BF-KBT-PT) ternary system has been proposed in order to develop novel piezoelectric materials for high temperature applications which are beyond the applicable range of commercial PbZrxTi1-xO3 (PZT) ceramics. Although the macroscopic piezoelectric properties and a ferroelectric/relaxor continuum have been reported in the BF-KBT-PT solid solution, the corresponding driving mechanisms related to the corresponding atomic structure and crystallographic evolution with composition variation in such a complex perovskite system are still not fully identified and understood. The objective of this project is to reveal the interplay between crystallographic structure over different length scales and the macroscopic physical properties. Multiscale investigations of selected compositions in the BF-KBT-PT system were realized by a combination of characterization methods including in-situ X-ray diffraction (XRD), transmission electron microscopy (TEM), digital image correlation (DIC), total scattering, ferroelectric and dielectric measurements. At room temperature, the regions of normal ferroelectric, non-ergodic and ergodic relaxors were identified in the studied composition space as well as their corresponding phase boundaries, based on the results of in-situ XRD, macroscopic ferroelectric and dielectric properties. A morphotropic phase boundary (MPB) region between rhombohedral and tetragonal phases was observed in the normal ferroelectric region, where ultrahigh electric field-induced microscopic strain was observed in the tetragonal {200} grain family, up to 5.5×10-3. An electric field-induced phase transition, from pseudocubic to rhombohedral structure, was observed in the non-ergodic relaxor region. According to the quantification method for the microscopic strain developed in this project, the intrinsic lattice strain is twice that of the extrinsic domain switching contribution in the electric field-induced rhombohedral phase. The displacive behavior of cations in corresponding oxygen polyhedra for the non-ergodic composition was evaluated by a further total scattering investigation, which revealed the origin of ferroelectricity in the non-ergodic phase. The effect of ionic size on the local distortion was evaluated based on the statistical analysis of the atomic model obtained from the total scattering investigation. A focused study on the selected composition with core-shell structure was also carried out in this project, which revealed the chemical heterogeneity and strain anisotropy between the core and shell regions.

Date of Award	1 Aug 2019
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Brian Derby (Supervisor) & David Hall (Supervisor)