Piezoelectric ceramic components are mainly based on lead. However, considering the hazardous effects of lead materials on human health and the environment, there is need to align with stringent measures and regulations on environmental pollution by developing alternative lead-free piezoelectric ceramics. Among the available lead-free piezoelectric materials, Na0.5Bi0.5TiO3-TiO3 (NBT)-based materials, particularly NBT-BT, have been extensively investigated for high power applications owing to their remarkable piezoelectric properties. However, the piezoelectricity of the NBT-BT materials significantly reduces at depolarization temperature (Td), thus limiting their use at high temperatures. Despite the several approaches developed to address the issue, most of them increase the depolarization temperature at the risk of decreasing the piezoelectric properties. Recently, quenching has been identified as a promising method for increasing Td without compromising the piezoelectric properties. Nonetheless, to effectively achieve strong Td enhancement, it is important to understand the factors influencing Td in quenched NBT-based ceramics.
To this note, a team of researchers at the Xi’an University of Technology: Professor Pengrong Ren, Dr. Jiale Wang, Dr. Yike Wang and Professor Gaoyang Zhao in collaboration with Professor Lalitha K. V. from the Technical University of Darmstadt explored the origin of enhanced depolarization temperature in quenched NBT-based piezoelectric ceramics. In their approach, NBT-6BT was heat-treated by quenching in air and liquid oxygen. This was followed by subsequent annealing at oxygen and nitrogen atmospheres. Furthermore, a combination of X-ray diffraction and field-emission scanning electron microscopy techniques were used to characterize the crystalline structure and microstructure of the ceramics. Their research work is currently published in the Journal of the European Ceramic Society.
The research team reported an increase in the depolarization temperature of the quenched NBT-6BT ceramics. This was attributed to the residual stress that significantly contributed to the stabilization of the ferromagnetic phase after the quenching process. Specifically, the residual stress induced a pseudocubic to rhombohedral phase transformation responsible for the stabilization of the rhombohedral ferroelectric phase. A comparison of the oxygen vacancy concentration confirmed that oxygen is not the primary reason for the enhancement of Td in quenched NBT-6B. For instance, annealing in the oxygen atmosphere compensated for the increased vacancy concentration for the quenched samples. In contrast, annealing in the nitrogen atmosphere increased the vacancy concentration due to a low oxygen pressure. However, for both quenched samples followed by oxygen and nitrogen annealing, Td recovers back to the value of unquenched samples. Thus, residual stress was identified as one of the primary factors influencing the electric properties of a quenched piezoelectric ceramics.
In summary, the study investigated the underlying mechanism of increasing depolarization temperature in quenched NBT-BT piezoelectric ceramics. The experimental results correlated the enhancement of the thermal depolarization temperature to the formation of the residual stress, which was identified as the main factor influencing the electrical properties of quenched piezoelectric ceramics. In a statement to Advances in Engineering, Professor Pengrong Ren said the study provided useful insights on the alternative mechanism for the enhancement temperature stability of NBT ceramics and would pave the way for improving the depolarization temperature and piezoelectric properties of other ferroelectric materials.
Ren, P., Wang, J., Wang, Y., K. V., L., & Zhao, G. (2020). Origin of enhanced depolarization temperature in quenched Na0.5Bi0.5TiO3-BaTiO3 ceramics. Journal of the European Ceramic Society, 40(8), 2964-2969.