Non-uniform electric field in poling of structurally graded monolithic piezoactuator

In this paper the effect of a non-uniform electric poling field and its optimisation for structurally graded piezoelectric actuators are investigated. The two compared actuator structures were both based on commercial PZT 5H bulk discs with thicknesses of 375 µm (Ø 25 mm), where one was machined into a graded structure with a step-like decrease of the thickness towards the centre and one intact. The hysteresis loops of the pre-stressed concave shaped actuators were measured under 0.5–5.0 kV/mm electric fields at 25–100°C temperatures, and the remanent polarization and coercive electric field were determined. The graded structured actuator obtained ～10% higher coercive field compared to the non-graded actuator, when measured at 25°C and 5.0 kV/mm. On the other hand the remanent polarisation values of the graded actuator were slightly lower than non graded bulk values. However the maximum decrease was only 9.6% under 5.0 kV/mm. The results show that strain and stress gradients in the structure are generated when exposed to an electric field. Furthermore, as a consequence of the restricted dimension changes, an inherent bending of the monolithic ceramic structure was obtained which can be utilised, for example, in miniaturised micro-machined actuators or in larger pre-stressed benders.