Quasi-static and dynamic piezoelectric responses of layered polytetrafluoroethylene ferroelectrets

Abstract

Layered polytetrafluoroethylene (PTFE) films of large area, composed of a porous PTFE core and two compact PTFE cover layers, which are commercially available, were internally charged by application of a corona discharge. The internal charges, which probably reside on the inner surfaces of the compact layers, make the films piezoelectric. Initial quasi-static piezoelectric d33 coefficients are in the range of 1000–2700 pC N−1. After a 1500 min annealing treatment at temperatures of 90 °C, 120 °C and 150 °C, the d33 coefficients stabilize and typically reach about 75%, 40% and 25% of their initial values, respectively. They drop to 18% after a thermally stimulated discharge measurement extending from 30 to 215 °C at a heating rate of 3 °C min−1. A pressure dependence of the piezoelectric d33 coefficients in such films is observed. This is due to the progressive densification of the core between the compact cover layers of the films. Microphones built with such films exhibit a somewhat decreasing frequency response up to about 2 kHz, an increase of the response due to diffraction effects at higher frequencies, and eventually a peak due to a thickness resonance at about 40 kHz. The microphone sensitivity below the diffraction and resonance ranges is about 1 to 1.5 mV Pa−1. This corresponds to dynamic d33 coefficients of 200 to 300 pC N−1.