Understanding mechanics and stress effects in Rainbow and Thunder stress-biased actuators

Rainbow and Thunder actuators constitute a family of "stress-biased" devices that display enhanced strain and load-bearing capabilities in comparison to traditional flextensional devices. For both of these actuators, doming occurs during the cooling phase of the fabrication process to relieve thermal expansion mismatch between the metallic and piezoelectric layers. Accompanying dome formation is the development of a tensile stress within the surface region of the piezoelectric layer that can approach 400 MPa. This tensile stress affects the ferroelectric domain configuration and improves the 90 degrees domain wall movement within the surface region of the piezoelectric under an applied electric field. It has been reported that this effect is responsible for the enhanced electromechanical performance of these devices. The results of the present study, however, suggest that in addition to stress, other mechanical and mass loading effects may also play a role in the enhanced performance of these devices. Equivalent circuit and finite element modeling studies of these stress-biased actuators are reported, and in particular, the effects of specimen geometry on internal stress in the piezoelectric layer are discussed. Finite element analysis shows that in the surface region of the piezoelectric, the highest tensile stresses are, in fact, predicted for those devices that display the greatest displacement performance; i.e., devices that have a piezoelectric layer that is approximately twice as thick as the "metallic" layer. However, equivalent circuit studies show that the highest predicted strains should also be observed for samples with similar geometries yet this approach does not include stress effects. This implies that not only stress, but also mass loading and other mechanical effects must also be considered in predicting optimum design geometries.