An internal variable based model for the electric breakdown and piezoelectricity of ferroelectrets
Baixiang Xu (TU Darmst), Dietmar Gross (Division of Solid Mechancis, TU Darmstadt)
Prager Medal Symposium in honor of George Weng: Micromechanics, Composites and Multifunctional Materials
Tue 10:45 - 12:15
MacMillan 117
Ferroelectrets are known as electrically charged micro-porous foam and have very large longitudinal piezoelectric coefficients. Wide applications as sensors have been received particularly in acoustical devices. During a charging process electric breakdown (Paschen breakdown) may take place in the air pores of the foam and introduce free charge pairs, which can be separated by electrostatic forces and relocated at the interfaces between the polymer and the electrically broke-down medium. The development of this free charge density along the interfaces is the key for the piezoelectricity of ferroelectrets. In this work, an internal variable based continuum model is proposed to recapitulate the charge density development at the interfaces during the electric breakdown, whereas a Maxwell stress based electromechanical model is used for the bulk behavior. In the models, the electrostatic forces between the separated free charge pairs are included, as well as the inuence of deformation. The material models are implemented in a nonlinear nite element scheme which allows a detailed analysis of different setups and structures. A ferroelectret unit with porous expanded polytetrauoroethylene (ePTFE) surrounded by uorinated ethylene propylene (FEP) is rst studied. The simulated hysteresis curves of charge density at the interfaces, as well as the calculated longitudinal piezoelectric constant, are compared with experimental results. The model is also applied to calculate the ferroelectret unit of a lens shape. It shows that the electric breakdown happens in a sequential sense and that the local piezoelectric coefficient varies with the position.