School of Engineering and Materials Science
Research Student Awards
PhD Thesis: Domain wall dynamics in ferroelectric ceramics under mechanical stress
Author: HU, Dunzhong
Supervisor(s): Mike Reece
Non-180° domain walls are twin-like and will have an associated twinning dislocation. The motion of a twinning dislocation is opposed by short-range interactions and long-range internal stresses. The relaxation, creep and Rayleigh analysis results show a very small strain rate sensitivity for PZT (Pb[Zr,Ti]O3). This means the effective stress which produces domain wall switching is small and almost strain-rate insensitive at room temperature. So most of the resistance to domain wall movement comes from the build-up of internal mechanical stresses and electric fields. The different behaviour between poled 5H, 5A, 4D and depoled 5A in their stress-strain and stress-charge displacement behaviour and degradation is explained by their internal stress states. It has been shown that both the direct piezoelectric response and the compliance have a Rayleigh type dependence on the stress amplitude. But the observation of a logarithmic dependence of the piezoelectric and elastic coefficient on frequency suggests that the intrinsic Rayleigh parameters at room temperature are not really “intrinsic”. Knowing the reversible elastic/piezoelectric (Sinit/dinit) contribution from the Rayleigh analysis, the true depolarisation curve can be constructed. The depolarisation includes recoverable domain wall movement and irrecoverable domain wall switching. The turning point of linear relationship between intrinsic lattice and applied stress indicates the beginning of domain wall switching from recoverable domain wall movement with increasing stress. Rayleigh analysis is a promising tool for describing the influence of microstructural and other parameters on the domain-wall displacement in ferroelectric devices. The “deageing” effect in hard 4D due to the Vo - Fe complex defects is more significant than for soft 5H and 5A. It is apparent in the P-E loops, but also in the degradation behaviour and activation volume.