Xu, Bai-Xiang and von Seggern, Heinz and Zhukov, Sergey and Gross, Dietmar (2014):
An internal-variable-based interface model for the charging process of ferroelectrets.
In: European Journal of Mechanics - A/Solids, 48, pp. 97-111. Elsevier Science Publishing, [Article]

Abstract

Electrically charged micro-porous foams, referred to as ferroelectrets, exhibit a very large longitudinal piezoelectric coefficient. In the present configuration the microporous material is sandwiched between two solid materials which can block the motion of charges. During the charging process of the ferroelectret electrical breakdown (Paschen breakdown) takes place in the air pores of the foam and introduce free charge pairs. These pairs are separated by the electric field induced by the applied voltage. Depending on the polarity they are relocated at the interfaces between the polymer and the electrically broken-down medium, where they are trapped quasistatically. Charging of the interfaces is key for the observed piezoelectricity of ferroelectrets. In the present article, an internal-variable-based interface model is proposed to simulate the charging and discharging of ferroelectrets. The model includes also the electrostatic force between the interface charges. For the bulk behavior, an electromechanical model based on the Maxwell stress is used. In particular, a 2D nonlinear finite element implementation of the models is elaborated, which involves a novel embedded interface element. Simulations of a sandwiched ferroelectret show that the interface model can reproduce the hysteresis behavior of the interface charge density, as it is demonstrated by the comparison with the corresponding experimental results. The model is further used to numerically study the influences of geometric, elastic and electrical parameters on the hysteresis and the piezoelectric coefficients. The models are also used to simulate a lens-shaped ferroelectret unit.

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