Piezoelectric performance of polytetrafluoroethylene ferroelectrets

Commercially available polytetrafluoroethylene (PTFE) films, composed of a porous PTFE core and two compact PTFE cover layers, were made piezoelectric (i.e. ferroelectrets) by corona charging. Most of the internal charges probably reside on the inner surfaces of the compact PTFE layers. Typical quasi-static piezoelectric d₃₃ coefficients are in the range of 1000-2000 pC/N at an applied pressure of 3.2 kPa. After a 1500 min-annealing treatment at temperatures of 90, 120, and 150 °C, the quasi-static piezoelectric d₃₃ coefficients stabilize and reach 77, 41, and 23% of the initial values, respectively. The quasi-static d₃₃ coefficients drop to 20% after a thermally stimulated discharge measurement extending from 30 to 215 oC at a heating rate of 3 oC/min. A pressure dependence of the piezoelectric d₃₃ coefficients in such films is observed. This could be due to the rough surface and very soft porous PTFE core of the films. Microphones built with such films exhibit a relatively flat frequency response up to about 2 kHz, show the expected diffraction and resonance peaks at higher frequencies, and have sensitivities of about 1mV/Pa in a broad range from 100 Hz to 50 kHz.