Air-coupled ultrasonic transducers based on laminated fluorinated ethylene propylene and porous polytetrafluoroethylene ferroelectrets

Laminated fluorocarbon films with an artificially controlled periodic microstructure consisting of solid fluorinated ethylene propylene (FEP) and porous polytetrafluoroethylene (PTFE) layers were successfully fabricated by using a patterning and fusion bonding method. They were made piezoelectric and turned into ferroelectrets by contact charging. The acoustic impedance of the fabricated ferroelectrets determined from dielectric spectra is about 0.012 MRayl. This provides a good match to air. Air-coupled transducers based on such films exhibit a broad bandwidth. For a transducer with a diameter of 20 mm, the transmitting sensitivity is 1.6 mPa/V at 60 cm distance and 50 kHz, while the receiving sensitivity is about 1.8 mV/Pa in a broad frequency range from 25 to 35 kHz. These laminated films, designed in such a way that only positive charges are deposited into the porous PTFE layers, show a significantly improved thermal stability. This arises from the good thermal stability of positive charges in porous PTFE, which is demonstrated by the open circuit thermally-stimulated discharge current spectra.