New materials with piezoelectric properties

The discovery of a piezoelectric response in low density polypropylene (LDPP) suggests that other material, properly treated, might also exhibit both piezoelectric and pyroelectric properties. The search for new materials is inspired by the relatively low temperature (∼70degC) depolarization of LDPP and the desire to choose independently the mechanical and electrical properties of a transducer material. Composites of two different materials have been suggested to create a material with desirable mechanical properties and high temperature charge storage capabilities. These combination materials are designed primarily to improve thermal and mechanical stability and to produce a Young´s modulus suitable for a given application. It is well known that a small class of polymers (teflon FEP and PTFE) and SiO₂ have excellent charge storage properties. We investigate the piezoelectric and charge storage properties of multilayer teflon FEP and PTFE with various interface materials of a desired shape, size and compliance. Voided teflon FEP, such as is used for insulation in high end audio cables, is also under investigation. Corona charging has been reported in the literature as the desired method to breakdown the air voids in LDPP and other materials. We use a method of charging which allows even higher fields across the sample by applying a DC voltage directly to the sample. Destructive breakdown is avoided by limiting the current through the sample. Biopolymers also offer a variety of possibilities of forming a composite in a manner that clearly separates the electroactive and bulk mechanical properties.