Heteropolar Charging of Ferroelectrets for Flexible Keyboards and Tactile Sensors

Ferroelectrets are flexible and conformable polymer transducer foils with a dominant longitudinal piezoelectric effect. Ferroelectrets also have a negligible transverse piezoelectric effect, which makes them ideal for the fabrication of roll- and bendable keyboards. Cellular polypropylene ferroelectrets with heteropolar charge patterns are useful for a simple and effective key encoding. The preparation of charge patterns is, however, a technical challenge. We have investigated and produced such structures in cellular polypropylene ferroelectrets by corona poling, in combination with shadow masks and unstructured and structured counter electrodes. Furthermore, we have also produced lateral charge distributions by contact poling with specially designed high voltage electrodes. We discuss the two different poling concepts and the related problems, their strengths, weaknesses and experimental pitfalls. We found in particular contact poling to be a reliable and well reproducible poling method for an effective preparation of heteropolar charge patterns of ferroelectrets.