The materials research in our laboratory is focussed on dielectrics storing charges for long times (electrets). The properties of these materials are important for many applications such as electro-acoustical and electro-mechanical transducers, sensors and actuators, solar cells, dosimeters, gasfilters etc.
Polymer electrets possess excellent charge-storage properties or piezoelectric properties and are available as flexible thin films. Among them are Teflon FEP and PTFE, Mylar/Hostaphan, Polypropylene, Kapton/Polyimide and ferroelectric polymers such as Polyvinylidenfluoride (PVDF) and its copolymers with Trifluorethylen (TrFE).
A new class of polymer electrets are permanently charged cellular materials with high piezoelectric d33 and d31 – coefficient, the so-called ferroelectrets . The charging is such that the lens-like air cavities are all charged with the same polarity on their „upper“ surface and with the opposite polarity on their „lower“ surface. This causes the strong piezoelectric effect. Of present interest are the investigation of the charging of these materials and the study of the electric, elastic, and piezoelectric properties as well as the improvement of their thermal stability.
Electrets based on silicon are very important for applications realized in silicon technology. Charge storage and conduction phenomena are measured for silicon dioxide, silicon nitride, silicon-oxynitride and multilayers of these materials.
Some of the specific research activities in this field are charge storage and decay, static and time-dependent analysis of charge profiles, charger dynamics at various temperatures and piezo- and pyroelectric properties. Charge and polarization profiles are measured by the Laser-Induced Pressure Pulse method (LIPP method) and the Thermal-Pulse method. Charge storage is analyzed by isothermal and thermally stimulated discharge (TSD) techniques.
