Ferroelectret properties of cellular polymer foams

This joint project of the Johannes Kepler University Linz, the Universities of Erlangen-Nürnberg, Potsdam, and Darmstadt (the latter three as foreign partners) is concerned with the development of ferroelectrets based on a new group of fluoropolymer films with cellular structure and improved thermal and temporal charge stability for electromechanical transducer applications. Ferroelectrets are cellular space-charge electrets with an internal bipolar electret charge, that combine features of both ferroelectrics and space-charge electrets. Ferroelectrets mimic properties typical for polar ferroelectrics e.g. hysteresis in dielectric displacement as well as in mechanical strain versus electric field. Ferroelectrets are therefore also interesting for fundamental research as novel class of "ferroic" materials. The field is still in its infancy, and many fundamental problems were not addressed yet. In the Linz subproject we intend to investigate the nonlinear dielectric, elastic and electromechanical properties of cellular polymers, which are not yet understood in ferroelectretic materials. These properties will be investigated on model systems, beginning with cellular polypropylene and continued with the cellular fluoropolymers developed in Erlangen-Nümberg when available. In comparison to cellular polypropylene, cellular fluoropolymers are expected to have superior material properties, especially long-term and thermally stable piezoelectricity. The nonlinear dielectric response of cellular polymers will be correlated to their piezoelectric and electrostrictive response. This work is not only of pure academic interest, it also provides important parameters for the practical applications developed in Darmstadt, for example distorsion factors in acoustics. Linear and nonlinear elastic properties of cellular polymers will be determined and compared with the corresponding electromechanical properties. Cellular polymers are expected to be strongly nonlinear e astic systems; w h ere the degree of nonlinearity depends-on the relative density and the geometrical structure of the voids within the polymer. Knowledge of the elastic properties is essential for the selection of the most promising fluoropolymer electrets, before they are charged under optimised conditions at the University of Potsdam. Furthermore, knowledge of the elastic nonlinearities may help to provide ways for linearization of the elastic response by suitable pre-stress treatments. This part of the project is also important in transduction applications, where the nonlinearities should be taken into account by suitable electronics or data analysis procedures. The work in the subproject will be closely linked to the work of the applicants in Germany, by helping in the development of the cellular morphology, based on the softness of the polymer structures, by selecting cellular polymers for optimized charging and by providing application relevant properties, like elastic nonlinearities, limiting distortion factors etc.

This joint project of the Johannes Kepler University Linz, the Universities of Erlangen-Nürnberg, Potsdam, and Darmstadt (the latter three as foreign partners) is concerned with the development of ferroelectrets based on a new group of fluoropolymer films with cellular structure and improved thermal and temporal charge stability for electromechanical transducer applications. Ferroelectrets are cellular space-charge electrets with an internal bipolar electret charge, that combine features of both ferroelectrics and space-charge electrets. Ferroelectrets mimic properties typical for polar ferroelectrics e.g. hysteresis in dielectric displacement as well as in mechanical strain versus electric field. Ferroelectrets are therefore also interesting for fundamental research as novel class of "ferroic" materials. The field is still in its infancy, and many fundamental problems were not addressed yet. In the Linz subproject we intend to investigate the nonlinear dielectric, elastic and electromechanical properties of cellular polymers, which are not yet understood in ferroelectretic materials. These properties will be investigated on model systems, beginning with cellular polypropylene and continued with the cellular fluoropolymers developed in Erlangen-Nümberg when available. In comparison to cellular polypropylene, cellular fluoropolymers are expected to have superior material properties, especially long-term and thermally stable piezoelectricity. The nonlinear dielectric response of cellular polymers will be correlated to their piezoelectric and electrostrictive response. This work is not only of pure academic interest, it also provides important parameters for the practical applications developed in Darmstadt, for example distorsion factors in acoustics. Linear and nonlinear elastic properties of cellular polymers will be determined and compared with the corresponding electromechanical properties. Cellular polymers are expected to be strongly nonlinear e astic systems; w h ere the degree of nonlinearity depends-on the relative density and the geometrical structure of the voids within the polymer. Knowledge of the elastic properties is essential for the selection of the most promising fluoropolymer electrets, before they are charged under optimised conditions at the University of Potsdam. Furthermore, knowledge of the elastic nonlinearities may help to provide ways for linearization of the elastic response by suitable pre-stress treatments. This part of the project is also important in transduction applications, where the nonlinearities should be taken into account by suitable electronics or data analysis procedures. The work in the subproject will be closely linked to the work of the applicants in Germany, by helping in the development of the cellular morphology, based on the softness of the polymer structures, by selecting cellular polymers for optimized charging and by providing application relevant properties, like elastic nonlinearities, limiting distortion factors etc.