Ferroelectrets: adjustment of electric, elastic and piezoelectric properties by means of foam-structure modification

In ferroelectrets, the piezoelectricity is based on bipolar charging of voids in the foam structure, which then represent (macroscopic) dipoles. It was found that inflation by gas-diffusion expansion is an useful processing step in order to optimize the cellular structure by adjusting the void heights (dipole sizes) and the elastic and piezoelectric properties. Here, we describe the inflation of cellular PP films over a range from the initial film thickness with fiat lens-like voids to six-fold film thicknesses with spherical voids. We analyze the resulting void sizes and void-size distributions as well as electrical, elastic and piezoelectric properties of the differently inflated films. Furthermore, the penetration dynamics of different gases such as nitrogen and sulfur hexafluoride into the voided structure are investigated.