Pyroelectricity in heterogeneous, charged polymer electrets

Research project P 14358 Pyroelectricity in heterogeneous, charged polymer electrets Siegfried BAUER 09.10.2000 Piezo- and pyroelectricity is the basis for a large number of applications in sensors and actuators. The most widely investigated piezo- and pyroelectric materials are ferroelectric ceramics or crystals, such as lead zirconate titanate (PZT) or lithium tantalate (LiTaO3 ). Low density piezoelectric materials with an acoustic impedance matched to water or thermally insulating pyroelectric materials for hybrid integrated thermal imaging systems are required for many mass applications. The development of new piezo- and pyroelectric materials is thus a matter of urgency. The basis of piezo- and pyroelectric materials is very broad and not limited to ferroelectrics: piezoand pyroelectricity is observed in polar glasses and in heterogeneous charged electrets. Most recently, charged microporous polypropylene electret foils as well as charged stacks of "stiff" (microporous polytetrafluoroethylene) and ,stiff` (perfluorinated benzocyclobutene) electret foils were identified to show very large piezoelectric coefficients d33 up to 600pC/N. Little is known about the charge trapping, the internal electric field distribution and the microscopic origin of the piezo- and pyroelectric response of these new class of electroactive materials. The aim of this project is the detennination of the distribution of trapped charges and internal electric fields, the measurement of the piezo- and pyroelectric coefficients, the determination of their thermal and temporal stability and the optimization of the poling process of charged heterogeneous electrets by employing the thermal-pulse technique. While acoustic techniques fail for the determination of internal electric field distributions due to the large acoustic attenuation, the thermal pulse technique is ideally suited for this purpose, since heat diffusion is also effective in porous media. The proposal is a direct continuation of the FWF project P12898NAW, based on the experimental techniques developed in the project. It is a natural extension of the project, since it underlines the importance of temperature dependent thermal-pulse measurements with and without voltage biasing for dielectric materials characterization. The results of the continuation project may help in the optimization of the poling process, and thus in the preparation of improved soft piezoelectrics, with a large scope for applications as sensors and actuators. From a fundamental point of view, new insights are also expected in our understanding of the microscopic origin of the piezo- and pyroelectric effect in this new class of electroactive polymers.