High performance fluorinated polyethylene propylene ferroelectrets with an air-filled parallel-tunnel structure

Fluorinated polyethylene propylene (FEP) ferroelectrets with an air-filled parallel-tunnel structure and regularly distributed bipolar space charges can stimulate integration of ferroelectrets in transducers due to their unique piezoelectric and mechanical properties. Such FEP ferroelectret films exhibit not only large longitudinal piezoelectric activity but also a strong transverse piezoelectric effect which allows for application in flexible transducers either working in 33 mode or in 31 mode. In this article, the transducer relevant properties of FEP ferroelectret films have been characterized. The experimental results show that the directly measurable, quasi-static piezoelectric d33- and g31-coefficients of the FEP films reach up to 4700 pC N-1 and 4.3 Vm N-1, respectively, and dynamic values in the frequency range from 10 to 100 Hz are 2600-1500 pC N-1 and 1.2-0.78 Vm N-1, respectively. No significant reduction of d33-coefficient is observed at strains up to 2.1%, showing a high elasticity of the film. Both isothermal decay of d33-coefficient and short circuit thermally stimulated discharge (TSD) current spectra show that the ferroelectric FEP films exhibit excellent thermal stability. After 14-day of storage in an atmosphere with a relative humidity of 99%, 75% of the initial piezoelectric d33-coefficient was still retained. Also a mechanical test with 1.6 million cycles does not harm the piezoelectric coefficients indicating that air-filled parallel-tunnel FEP ferroelectret films are promising candidates for a variety of flexible transducer applications.