  We present calculations on the deformation of two- and three-layer electret
systems. The electrical field is coupled with the stress-strain equations by
means of the Maxwell stress tensor. In the simulations, two-phase systems are
considered, and intrinsic relative dielectric permittivity and Young's modulus
of the phases are altered. The numerically calculated electro-mechanical
activity is compared to an analytical expression. Simulations are performed on
two- and three-layer systems. Various parameters in the model are
systematically varied and their influence on the resulting piezoelectricity is
estimated. In three-layer systems with bipolar charge, the piezoelectric
coefficients exhibit a strong dependence on the elastic moduli of the phases.
However, with mono-polar charge, there is no significant piezoelectric effect.
A two-dimensional simulation illustrated that higher piezoelectricity
coefficients can be obtained for non-uniform surface charges and low Poisson's
ratio of phases. Irregular structures considered exhibit low piezoelectric
activity compared to two-layer structures.
