High performance laminated FEP/PTFE piezoelectret films for air-borne sound transducers

Abstract

Piezoelectret (also known as ferroelectret) is a kind of cellular electret material with strong piezoelectric effect. Such a material exhibits flexibility, low density and small acoustic impedance. Therefore, piezoelectret is an ideal material for air-borne flexible sound transducers. Aiming at high-sensitivity and thermal-stability sound transducers, in this work, laminated fluorinated polyethylene propylene (FEP) and polytetrafluoroethylene (PTFE) piezoelectret film with a regular cellular microstructure is prepared by a procedure involving template-based cellular structure formation and polarization. The results show that the characteristic acoustic impedance of such a laminated FEP/PTFE film is 0.02 MRayl. The quasi-static piezoelectric charge coefficient <i>d</i>₃₃ up to 800 pC/N is achieved in a small applied pressure range. The maximum value of sensitivity of the microphones based on laminated FEP/PTFE piezoelectrets film can reach to 6.4 mV/Pa at 1 kHz. Besides, the frequency response curve of the device is flat in the whole audio range. For an ultrasonic transmitter with a diameter of 20 mm, driven by a voltage of 600 V (<i>V</i>_p), the sound pressure level (SPL) generated by it increases from 80 to 90 dB (Ref. 20 µPA) as frequency increases from 40 to 80 kHz. The thermal stability of the sensitivity for the transducers made of such a laminated FEP/PTFE piezoelectret film is much superior to that of polypropylene (PP) piezoelectret based device. The sensitivity of the present device remains 26% of the initial value after being annealed at 125 ℃ for 211 h. The improvement of thermal stability is attributed to the excellent space charge storage stability of FEP and PTFE.