A study of time stability of piezoelectricity in porous polypropylene electrets

Time stability of piezoelectricity in porous polypropylene (PP) electrets was studied. Piezoelectric e₃₃ and d₃₃ constants and the electromechanical coupling factor k_t were evaluated by dielectric resonance method. The amount of storage charge and its temperature stability were measured by short circuit TSD measurement. The experimental results showed e₃₃, d₃₃ and k_t decreased after a day passed from charging and became stable after that. The TSD spectra showed three peaks at 65°C, 90°C and 120°C. The decrease of piezoelectricity was in correspond with a decrease of TSD peak at 65°C. Temperature spectra of Young´s modulus and permittivity showed ß-relaxation around 10°C and ¿-relaxation around 100°C. The peak temperatures of TSD were covered with the temperature range of ¿-relaxation in a crystal region of PP. Airborne ultrasonic transducer using the porous PP electrets showed the maximum SPL of 95 dB at 150 kHz.