Charge and polarization profiles in polymer electrets

Laser-induced pressure pulses (LIPP's) of < 500 ps duration have been used to measure the ultrasonic attenuation in fluoroethylenepropylene (FEP), polyethyleneterephthalate (PETP) and polyimide (PI) as well as the charge, polarization, and piezoelectricity profiles in the thickness direction of polyvinylidenefluoride (PVDF) and FEP electrets with a resolution of 1 to 2 μm. Measurements on PVDF, obtained during and after poling, reveal a uniform polarization profile for samples poled at room temperature and a non-uniform polarization, concentrated near the positive electrode, for samples poled at elevated temperatures. In FEP, charges injected by electron beams are stored close to the CSDA range while negative carriers deposited by corona discharges or liquid-contact electrodes are trapped at, or close to, the surface.