ToF-SIMS analysis of the defect chemistry in donor doped PZT

Abstract

Pb(Zr,Ti)O3 (PZT) based electroceramics have become one of the most important materials for piezoelectric applications, such as electromechanical transducers, sensors and actuators. To improve the performance of PZT based applications, a detailed understanding of field-driven degradation phenomena and their defect chemical background is essential. Oxygen vacancies are proposed to play an important role in fatigue and resistance degradation of PZT or other electroceramics. However, quantitative information on the defect chemistry and oxygen transport properties of PZT under field load is still rudimentary. Among others, there is a lack of knowledge on how different electric fields, the duration of field load or electrode materials and geometries affect the oxygen vacancy distribution and the corresponding oxygen diffusivity. To obtain further information on oxygen vacancies in Nd3+ donor doped PZT tracer oxygen diffusion experiments were conducted under applied DC load and subsequently investigated by time-of-flight secondary ion mass spectroscopy (ToF-SIMS). To enable a detailed quantification of the oxygen vacancy diffusivity a novel ToF-SIMS operation mode was developed to accurately determine the bulk and the grain boundary diffusion coefficients of the tracer oxygen. It is shown that initially DC load causes higher diffusivity of oxygen tracer in the bulk and the grain boundaries of PZT. Also a certain asymmetry of the profiles with higher diffusivity close to the cathode is found which indicates that stoichiometry polarization is the cause of these changes. It is particularly interesting that field load immediately causes fast oxygen diffusion along grain boundaries, indicating that an accumulation of oxygen vacancies provides fast diffusion paths under applied electric fields. Evidence is provided that this may be caused by a space charge layer along grain boundaries. After long-term field load the previously high ionic conduction within the grain boundaries vanishes completely. This effect is attributed to irreversible changes of the chemistry of the PZT grain boundaries. Additionally, the effect of Na, Li and La codoping of PZT on the oxygen tracer diffusivity is investigated.