Download files
Abstract
Due to the toxicity of lead, there is a demand for lead-free alternatives to PZT that not only have comparable piezoelectric performance but also reliability during long-term application. Due to a high piezoelectric coefficient, BCTZ piezoelectric ceramics have been considered as a promising lead-free alternate piezoelectric material at room temperature range. However, the BCTZ system has not been fully understood and only limited compositions in the system have been synthesised and investigated in the literature.
In this thesis seven specifically selected compositions in the BCTZ system are investigated including six new compositions that have not been previously reported. All six compositions have phase coexistence at room temperature and exhibit high piezoelectric performance. It is found that the phase coexisting temperature can be predicted for each composition, which enables the selection of high performance compositions for different working temperature.
For investigation of ageing behaviour, samples were left at poled state up to 36 days and showed a shift of the hysteresis loop along the field direction and development of asymmetry in strain and permittivity hysteresis loop over time. The origin of this ageing behaviour is proposed as the local defect dipoles and the migration of the charged defects to the grain boundaries.
The stability of the piezoelectric performance during both unipolar and bipolar cycling is investigated. For unipolar cycling, samples were cycled at 10Hz up to 5x106 cycles. Development of a bias field, offset polarization, and asymmetry in strain and dielectric hysteresis loops are observed during bipolar measurements. These changes are mainly attributed to the migration of charged carriers driven by the unscreened depolarization field at grain boundaries.
For bipolar cycling, two compositions with different Ca and Zr doping were cycled at 10 Hz up to approximately 107 cycles. Both investigated compositions exhibited high bipolar fatigue resistance compared to other ceramics reported in the literatures. The high fatigue resistance is originated from the lack of mechanical damage and a weak domain wall pinning effect due to their location in the phase transition region. It was also found that the pore morphology affected the bipolar fatigue behaviour.