It is the intention of this study to understand the behavior of photoinduced domain switching by measuring Acoustic Emission(AE) signal and photovoltaic current in $(Pb,La)TiO_3$ ferroelectric ceramics.
In chapter Ⅲ, AE related to domain switching in 1mol% $MnO_2$-doped $(Pb,La)TiO_3$ ferroelectric ceramics during poling was investigated in a preliminary effort to apply AE technique to study of the behavior of photoinduced domain switching. In order to obtain the true AE signals induced by domain switching, it was necessary to eliminate AE signals from the sample's self-vibration which was caused by an electrical coupling between the power supply and the piezoelectricity of the sample. The AE signals in the sample where 90° domains were dominant were distributed in high energy range of 45-70dB. In contrast, those in the sample where 180° domains predominated were distributed in low energy region around 40dB. Both the amount of AE event and 90° domain switching determined by XRD technique increased with poling field and temperature. The amount of AE signals with high energy of 45-70dB had almost a linear relationship with the amount of 90° domain switching.
In chapter Ⅳ, the mechanism of photoinduced domain switching was investigated in $(Pb,La)TiO_3$ ferroelectic ceramics during illumination in the absence of external electric field. The photovoltaic current increased with illumination time. Nonsteady-state photovoltaic current behavior was attributed to the cycle of a series of physical events associated with photoexcited electrons. Some photoexcited electrons were trapped at the grain boundaries and established a space charge field across each grain, while the others still remaining in the conduction state contributed to a photovoltaic current. The space charge field induced domain switching in the direction of remanent polarization so that the level of remanent polarization could increase, eventually which resulted in the increase of photovoltaic current. A series of physical events mentioned above would be repeated until the stored strain prevented the domains from being switched. The detected AE signals revealed that photoinduced domain switching occurred in $(Pb,La)TiO_3$ ferroelectic ceramics during illumination.
In chapter Ⅴ, the variations of the activity of photoinduced domain switching with experimental parameters, such as c/a ratio, poling field, grain size, and light intensity, were investigated by evaluating total AE event count and total photovoltaic current increment in $(Pb,La)TiO_3$ ferroelectric ceramics during illumination. The activity of photoinduced domain switching had a maximum at the intermediate c/a raio(c/a=1.021) and poling field(E=1.5kV/mm) respectively and it increased with grain size and light intensity. The influence of the experimental parameters on the activity of photoinduced domain switching could be explained in terms of the mechanical strain energy, ΔGaM, which acted as the energy barrier for domain wall motion and the energy, ΔGSC, associated with the space charge field which stimulated domain wall motion. On the basis of the analysis of AE energy distribution, the behavior of 90° and 180° domain switching during illumination was investigated. It was possible to distinguish between the AE signals resulting from 90° and 180° domain switching. It revealed that the high energy AE in the range of 45-80dB resulted from 90° domain switching and the low energy AE around 40dB were related to 180° domain switching, respectively. Consequently, it could be confirmed that a space charge field established during illumination allowed both 90° and 180° domains to be switched. It was also confirmed that the energy barrier for 90° domain switching increased with c/a ratio. The quantitative evaluation of AE event count made it possible to predict the change of photovoltaic current as well as the amount of photoinduced domain switching in ferroelectric ceramics during illumination.
