Piezoelectricity Induced by Charge Injection in Thin Polymer Films

The vibrations produced in the electrodes surrounding a polypropylene foil in a three‐layer capacitive microphone under an a‐c field are analyzed. These vibrations can be due either to the electrostatic pressure acting on the electrodes (direct excitation) or to the vibrations of the central dielectric foil itself transmitted to the electrodes (transmitted excitation). Experimental evidence is given showing the separation between these two types of excitations. In the case of transmitted excitation, electrostriction of the central dielectric layer was to be ruled out as an explanation of the observed effects. They were interpreted in terms of piezoelectricity and were proved to be due, in this case, not to a high field effect, but to charge injection, even in the absence of macroscopic space charge.