Ferroelectrets: polymer-foam space-charge electrets with ferroelectric-like behaviour

Cellular space-charge polymer electrets can be internally charged by silent or partial discharges ("microstorms") within the voids of the polymer foam. The resulting material, which carries positive and negative charges on the internal void surfaces is called a ferroelectret. Ferroelectrets behave like typical ferroelectrics, hence they provide a novel class of ferroic materials. The soft foams are strongly piezoelectric, and can be used in a wide range of applications as transducers for interconverting mechanical and electrical signals. A review on the preparation of cellular polymers by physical foaming (extrusion, biaxial stretching and controlled inflation by pressure treatments), on the charging by "microstorms", on the piezo- and pyroelectric properties and on analogies to ferroelectric materials is given. The huge potential of internally charged cellular space-charge electrets is illustrated with a brief tour d\´horizon through selected applications.