Do ferroelectrets always behave like ferroelectrics? [comparison of similarities and differences]

Ferroelectrets are nonpolar porous electrets which are polarized due to internally charged surfaces. Such systems show many similarities to ferroelectrics: they are piezo-and pyroelectric, their polarization can be reversed by means of an external field and hysteresic effects can be observed. Ferroelectrets however are different to conventional ferroelectrics in many respects. We report on the very different charging and switching mechanisms in ferroelectrets, both in a simple model-system and in a cellular polypropylene foam. These mechanisms are crucial and form the basis for the striking phenomenological similarities to ferroelectrics. We also report on specific peculiarities and pitfalls related to present ferroelectrets and their characterization. High electric field strength and anelastic non-linearities can be the cause for false ferroelectric-like hysteresis effects which do not reflect any polarization reversal.