Ferroelectric-like properties of cellular ´ferroelectrets´

Space-charge electrets are dielectric materials exhibiting a quasi-permanent electrical charge. They are highly demanded for advanced electret devices, like microphones, dosimeters, micro-relay switches, and for the electrostatic anchoring of nanoparticles in nanoxerography. They are also interesting for fundamental research in order to better understand charge trapping mechanisms in polymers. Nonpolar cellular polymers with an internal bipolar electret charge are a sub-class of space-charge electrets. They are considered for transducer applications, since they mimic piezoelectricity by their unusual electromechanical properties. Recently, we and others have shown that these materials also mimic other useful properties: It came as quite a surprise that completely nonpolar materials exhibit features typical of ferroelectrics, e.g. hysteresis in dielectric displacement and mechanical strain versus electric field. For this reason, we have introduced the term "ferroelectret" for this material class, since they combine features of both ferroelectrics and space charge electrets. Ferroelectrets may therefore also become interesting for fundamental research as a novel class of ferroic materials. The field is still in its infancy, and many fundamental problems have not been addressed yet. In this project, we intend to investigate and to model the ferroelectretic properties of ferroelectrets, and assess the results in comparison to the behavior of ferroelectric materials. In ferroelectrics, polarization results from a non-centrosymmetric arrangement of charged particles in the unit cell of the material. Polarization switching is achieved by a reversal of ferroelectric domains. In ferroelecrets, charged macrodipoles are formed by the bipolar electret charge within the voids of the cellular material. "Polarization" switching is achieved by "destroying" and "recreating" the macrodipole by means of dielectric barrier microdischarges at atmospheric pressure. We intend to investigate the discharges within voids with respect to their duration and spatial size, in order to establish an analogy between "domains" in ferroelectric and ferroelectret materials. In addition, we intend to investigate other practical aspects of highly localized dielectric barrier microdischarges in materials science, e.g. surface modification, etching and thin film deposition of materials. An example is the spatially structured surface modification of polymers for biomedical applications. Ferroelectric materials display a variety of effects useful in practical applications, such as pyroelectricity, piezoelectricity, and second-order nonlinear optical effects, like the linear electro-optical Pockels effect and second-harmonic generation of light. No attempt to study optical nonlinearities in ferroelectrets has been reported yet. In view of the analogies between ferroelectric materials and ferroelectrets we intend to investigate second- order optical nonlinearities in ferroelectret materials. For this purpose, a simple model system of a ferroelectret will be employed, a stack of two highly transparent polymer space charge electrets, separated by a well-defined air-gap. Such a model system is not only of pure academic interest, it may also pave the way for micro-machined ferroelectret transducer systems.

Space-charge electrets are dielectric materials exhibiting a quasi-permanent electrical charge. They are highly demanded for advanced electret devices, like microphones, dosimeters, micro-relay switches, and for the electrostatic anchoring of nanoparticles in nanoxerography. They are also interesting for fundamental research in order to better understand charge trapping mechanisms in polymers. Nonpolar cellular polymers with an internal bipolar electret charge are a sub-class of space-charge electrets. They are considered for transducer applications, since they mimic piezoelectricity by their unusual electromechanical properties. Recently, we and others have shown that these materials also mimic other useful properties: It came as quite a surprise that completely nonpolar materials exhibit features typical of ferroelectrics, e.g. hysteresis in dielectric displacement and mechanical strain versus electric field. For this reason, we have introduced the term "ferroelectret" for this material class, since they combine features of both ferroelectrics and space charge electrets. Ferroelectrets may therefore also become interesting for fundamental research as a novel class of ferroic materials. The field is still in its infancy, and many fundamental problems have not been addressed yet. In this project, we intend to investigate and to model the ferroelectretic properties of ferroelectrets, and assess the results in comparison to the behavior of ferroelectric materials. In ferroelectrics, polarization results from a non-centrosymmetric arrangement of charged particles in the unit cell of the material. Polarization switching is achieved by a reversal of ferroelectric domains. In ferroelecrets, charged macrodipoles are formed by the bipolar electret charge within the voids of the cellular material. "Polarization" switching is achieved by "destroying" and "recreating" the macrodipole by means of dielectric barrier microdischarges at atmospheric pressure. We intend to investigate the discharges within voids with respect to their duration and spatial size, in order to establish an analogy between "domains" in ferroelectric and ferroelectret materials. In addition, we intend to investigate other practical aspects of highly localized dielectric barrier microdischarges in materials science, e.g. surface modification, etching and thin film deposition of materials. An example is the spatially structured surface modification of polymers for biomedical applications. Ferroelectric materials display a variety of effects useful in practical applications, such as pyroelectricity, piezoelectricity, and second-order nonlinear optical effects, like the linear electro-optical Pockels effect and second-harmonic generation of light. No attempt to study optical nonlinearities in ferroelectrets has been reported yet. In view of the analogies between ferroelectric materials and ferroelectrets we intend to investigate second-order optical nonlinearities in ferroelectret materials. For this purpose, a simple model system of a ferroelectret will be employed, a stack of two highly transparent polymer space charge electrets, separated by a well-defined air-gap. Such a model system is not only of pure academic interest, it may also pave the way for micro-machined ferroelectret transducer systems.