Composition Gradient Actuators

Piezoelectric functionally graded materials (FGMs) are attractive alternatives to homogenous-single phase materials for actuator applications because of their reduced internal mechanical stresses and lower production costs. Furthermore, such FGM structures have increased band width if used as an ultrasonic transducer. One of the most effective ways to prepare piezoelectric and dielectric gradients based creating a gradient in chemical composition by powder processing prior to sintering. The sharp chemical interfaces between the green layers disappear because of diffusion during sintering. The chemical gradient is then transformed into a gradient in the piezoelectric properties by a poling process after sintering. Several models have been developed for the description of poling of layered systems, which is a formidable challenge. The ferroelectric properties, such as spontaneous polarization and coercive field strength, also depend on the local chemical composition. This causes an inhomogeneous electric field distribution, which is usually not stable in time because of conductive currents and space charges.

In this chapter, different types of composition gradients for bending actuators are described. Combinations of hard and soft piezoelectric ceramics and electrostrictive and electroconductive materials are presented. The theoretical results are compared with experimental data for lead-free systems based on barium titanate.