Ferroelectret

In solid-state physics, pyroelectricity, piezoelectricity and ferroelectricity is defined from the free-energy of a material in a rigorous but restrictive way as an intrinsic property of the noncentrosymmetric crystal classes that exhibit polarization. Ferroelectric materials are always pyro- and piezoelectric, additionally they exhibit hysteresis phenomena in the polarization versus the applied electric field. For materials with a metastable polarization, like poled ceramics, polymers, and glasses a simpler and broader definition of pyro- and piezoelectricity is used: Any material that undergoes a change in displacement D in response to a temperature change T (pyroelectricity) or mechanical stress X (direct piezoelectricity) or a change in entropy S (electrocaloric effect) or strain x (converse piezoelectricity) in response to an electric field E is termed pyro- or piezoelectric. This formal description of pyro- and piezoelectricity with the independent variables displacement D, electric field E, entropy  Q , temperature q, strain S, and stress T is only appropriate when the thermal expansion coefficient and compressibility are negligibly small. In polymers, dimensional changes of the sample significantly influence the pyro- and piezoelectric response. For such materials it is better to use the charge Q on the electrodes, the applied voltage for defining pyroelectricity and the electrocaloric effect, and the total force F and sample thickness s for piezoelectricity. This general transducer definition of pyro- and piezoelectricity is also applicable to internally charged cellular space charge electrets, also called electromechanical film or EMFi. The trapped charges in these cellular polymers generate pyro- and piezoelectricity since the elastic properties of the electret material are macroscopically non-uniform, the respective materials may be called “pyro”- and “piezoelectrets”. Some of the cellular space charge polymers also exhibit hysteresis and switching, very similar to phenomena observed in ferromagnetic materials. Therefore, this subclass of cellular polymers enlarges the family of ferroic materials and are hence called “ferroelectrets”. Cellular space charge electrets lead to new paradigms of pyro- and piezoelectricity as well as of ferroic material behavior. Soft cellular space charge polymer electrets with longitudinal piezoelectric responses exceeding those of conventional poled and ferroelectric polymers by orders of magnitude are state of the art.

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For more details see: S. Bauer, R. Gerhard-Multhaupt, and G. M. Sessler, Ferroelectrets: Soft Electroactive Foams for Transducers, Physics Today Vol. 57, pages 37-43, February 2004. M. Wegener and S. Bauer, Microstorms in Cellular Polymers: A route to soft piezoelectric transducer materials with engineered macroscopic dipoles, ChemPhysChem, Vol. 6, pages 1014-1025, 2005.