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Abstract
This thesis investigates the electrical fatigue behaviour of Pb(Zr1-xTix)O3 (PZT), (1-x)Bi1/2Na1/2TiO3 – xBaTiO3 (BNT-BT) and (1-x-y)Bi1/2Na1/2TiO3 – xBaTiO3 – yK0.5Na0.5NbO3 (BNT-BT-KNN) piezoceramics.
Fatigue has always been a major problem for the commercial applications of piezoceramics; and electrical fatigue is shown by several forms of damage – including deterioration of polarization, strain, piezoelectric constant and dielectric constant during electrical cycling. Electrical fatigue also leads to mechanical deterioration in the microstructure, such as microcracking in the material.
PZT is one of the most commonly-used piezoceramics, in which domain pinning and the microcracking mechanism have been suggested as the major causes of fatigue; however, their contributions to fatigue have not been fully quantitatively compared.
Firstly, the electrical fatigue-induced damage in the near electrode regions of PZT is considered, and re-fatigue methods used to determine, quantitatively, the contribution of the cracking and domain pinning mechanisms. The study shows that, under bipolar loading, a large amount of fracture and cracking occurs near the electrodes of the piezoceramics; and the cracking mechanism contributes significantly more than the domain pinning mechanism to electrical fatigue. The cracking in the near-electrode surface regions can be found in both PZT and BNT-BT and the deterioration of piezoelectric properties can be partially restored by removing such regions. Furthermore, by annealing PZT in a reducing gas, the presence of oxygen vacancies is shown to lead to a strong suppression of polarization, or even dielectric breakdown, of the piezoceramic.
Because of the large amount of toxic lead contained in PZT, there has been, in recent years, a large research effort to develop alternative lead-free piezoceramics to replace it. BNT-BT and BNT-BT-KNN piezoceramics are attracting research interest because of their comparable piezoelectric properties [Zhang and Kounga et al. 2007; Rödel et al. 2009]; and this thesis leads investigation into the electrical fatigue behaviour of BNT-BT and BNT-BT-KNN.
Even though the electrical fatigue behaviour of BNT-BT and BNT-BT-KNN is significantly different from that of PZT, the same mechanisms — e.g., domain pinning, microcracking and charge accumulation — can be used to explain the electrical fatigue of BNT-BT or BNT-BT-KNN. However, their electrical fatigue behaviour is further complicated by a proposed pseudo-cubic to tetragonal/rhombohedral or relaxor-to-ferroelectric phase transition. In addition, a strong relaxation effect after fatigue is observed in BNT-BT. Because of the reported large strain on BNT-BT-KNN, its bipolar/unipolar polarization and strain are investigated in addition to fatigue behaviour: it is suggested that the relaxor ferroelectric BNT-BT-KNN is a promising candidate for applications under unipolar loading. Furthermore, the origin of pinching in the P-E hysteresis of BNT-BT-KNN is investigated, showing that BNT-BT-KNN does not have an antiferroelectric characteristic as suggested by earlier studies: this material is in fact a relaxor piezoceramic.