Fluoroethylenepropylene ferroelectrets with patterned microstructure and high, thermally stable piezoelectricity

Layered fluoroethylenepropylene (FEP) ferroelectret films were prepared from sheets of FEP films by template-patterning followed by a fusion-bonding process and contact charging. The layered ferroelectret films show consistency and regularity in their void structures and good bonding of the layers. For films composed of two 12.5 mu m thick FEP layers and a typical void of 60 mu m height, the critical voltage necessary for the built-up of the macro-dipoles in the inner voids is approximately 800 V. At room temperature, Young's modulus in the thickness direction, determined from dielectric resonance spectra of the fabricated films with a typical thickness of 85 mu m, is about 0.21 MPa. Initial quasistatic piezoelectric (33) coefficients of samples contact charged at a peak voltage of 1500 V are in the range of 1000-3000 pC/N. From these, ferroelectrets with high quasistatic and dynamic (up to 20 kHz) (33) coefficients of up to 1000 pC/N and 400 pC/N, respectively, which are thermally stable at 120A degrees C, can be obtained by proper annealing treatment. This constitutes a significant improvement compared to previous results.