Composition Gradient Actuators

Abstract

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. IntroductionFunctionally graded materials (FGMs) have a one or more dimensional gradient of at least one material property. Usually, several material properties change their values continuously in the direction of the gradient. Thus, a gradient in piezoelectric coefficients is often combined with a gradient in dielectric or elastic coefficients. Piezoelectric FGMs are suitable for applications where materials with different properties need to be mechanically connected. Thereby, residual mechanical stresses at interfaces between the different materials can be reduced. Piezoelectric FGMs with a gradient of the electromechanical properties can be used in micromechanical systems (MEMS), or as actuators for connecting piezoelectric active parts with substrates or other passive parts. In this way, reliability and lifetime of electromechanical devices can be improved. Furthermore, FGMs can be used to improve the bandwidth in ultrasonic applications (Sakamura et al. 2000).Monolithic bending actuators are a special application of piezoelectric FGMs. The bending effect strongly depends on the difference of the lateral strain at the top and the bottom surface of the actuator. Usually, two layers with different piezoelectric properties or with an electrode in between the layers are bonded together. It was shown that piezoelectric FGMs can reduce the mechanical stresses at the interface (Hauke et al. 2000). A good performance of the bending actuator depends on the quality of the piezoelectric gradient.Usually, the piezoelectric gradient in monolithic samples is obtained by creating a chemical composition gr...