Influence of porosity on properties in fluorocarbon piezoelectrets

The fluorocarbon polymer films with regular void structure, made from fluoroethylenepropylene (FEP) and porous polytetrafluoroethylene (PTFE) layers were prepared by using the template patterning and fusion bonding process. After proper corona or contact charging, the films were piezoelectric. The microstructure, mechanical property, piezoelectricity and viscoelasticity of the fabricated films were studied by means of scanning electron microscope (SEM) technique, measurements of dielectric resonance spectra, and analysis of quasi-static d₃₃ coefficient in time domain. The results show that, there ate threshold values of polarization about -4000 V, -3000 V and -2000 V in the films with porosities of 0,25% and 44%, respectively, and above the threshold voltage, the piezoelectric coefficient d₃₃ of the films increases significantly with the enhancement of the polarization voltage. For the films with porosities of 0,25% and 44%, the measured dielectric anti-resonance frequencies are around 104 kHz, 119 kHz and 132 kHz, the calculated Young´s modulus are about 0.9 MPa, 0.58 MPa and 0.53 MPa and the densities of surface charge are 0.009 μC/cm², 0.029 μC/cm² and 0.046 μC/cm², respectively. The piezoelectric response in time domain reflects the viscoelastic behaviors in the films. In addition, the films with large porosity show an improved piezoelectricity, which is perhaps due to the relative high density of charge and low Young´s modulus in them.