Mechanisms of electro-mechanical coupling in polycrystalline piezoelectric ceramic materials

Download files
Access & Terms of Use
open access
Copyright: Hossain, Mohammad
Altmetric
Abstract
In this thesis a sample cell has been developed which is capable of measuring the structural variations of piezoelectric ceramic materials using low-energy X-ray scattering techniques in reflection geometry during the application of an electric field, while simultaneously collecting macroscopic strain data using a linear displacement sensor. The results show that the macroscopic strain measured using the cell can be directly correlated with the microscopic response of the material obtained from diffraction data. The electro-mechanical coupling mechanisms in polycrystalline ferroelectric materials including a soft PZT and a lead-free 0.9375(Bi1/2Na1/2)TiO3-0.0625BaTiO3 (BNT-6.25BT) have been contrasted using surface sensitive in situ low-energy (12.4 keV) and bulk sensitive in situ high-energy (73 keV) synchrotron XRD during the application of electric fields. The results allow a direct comparison of the microscopic responses between the bulk grains constrained in three dimensions and the surface grains which have one dimension of mechanical freedom. It is shown that for both PZT and BNT-6.25BT the intrinsic lattice strains and extrinsic non-180° domain switching strains are larger at the surface than those in the bulk of the samples. The property difference is believed to result from the fact that surface grains are not constrained in three dimensions and consequently domain reorientation and lattice expansion in surface grains along the field direction occur more freely. The magnitude of the property difference between the surface and bulk is higher for PZT than for BNT-6.25BT due to the magnitude of anisotropy in the strain mechanism. The comparison of the results from different methods reveals that the grain-to-grain interactions have a significant influence on the electric-field-induced electro-mechanical responses in bulk polycrystalline ferroelectrics. The structure-property relationships in a series of BNT-BT solid solutions with the BT content ranging from 5 mol% to 8 mol% in 0.25 mol% steps were also studied using in situ high-energy synchrotron XRD. This fine compositional deference helps to make a comprehensive picture of field-induced phases particularly in the “pseudo-cubic” region of the phase diagram. Unipolar stress cycling with a maximum stress of approximately 600 MPa and bipolar electric-field cycling with a maximum field of 5 kV/mm were applied in two separate experiments. In the as-processed state, BNT-5BT exhibited rhombohedral crystallographic symmetry, while the rest of BNT-xBT compositions (5.25 ≤ x ≤ 8) exhibited the pseudo-cubic symmetry. During the application of stress and electric field in two separate experiments, lower BT content samples (x = 5.25 and 5.5) tended to transform to rhombohedral symmetry, while the compositions with higher BT contents (7 ≤ x ≤ 8) tended to transform to tetragonal symmetry. Compositions between these (5.5 < x <7) tended to transform to mixed tetragonal-rhombohedral phase symmetry. The results show that the stress and electric-field-induced phase transformation mechanisms are highly analogous. Based on the results of the surface and bulk response, and the compositional dependent studies, it is suggested that the best piezoelectric performance in a polycrystalline BNT-xBT material occurs when the magnitude of anisotropy of the response mechanism is minimum. In this case, compositions which transform to a mixed phase structure are likely to have superior piezoelectric performance, as all grains in the system strain with a similar magnitude along the applied field direction. This study of field-induced strain generation mechanisms in polycrystalline piezoelectric ceramics has identified an important structural aspect which may be utilised to improve the piezoelectric properties in future developed systems.
Persistent link to this record
Link to Publisher Version
Link to Open Access Version
Additional Link
Author(s)
Hossain, Mohammad
Supervisor(s)
Daniels, John
Creator(s)
Editor(s)
Translator(s)
Curator(s)
Designer(s)
Arranger(s)
Composer(s)
Recordist(s)
Conference Proceedings Editor(s)
Other Contributor(s)
Corporate/Industry Contributor(s)
Publication Year
2016
Resource Type
Thesis
Degree Type
PhD Doctorate
UNSW Faculty
Files
download public version.pdf 4.73 MB Adobe Portable Document Format
Related dataset(s)