Fluoroethylenepropylene Films Research Articles

Fluoroethylenepropylene ferroelectret films with cross-tunnel structure for piezoelectric transducers and micro energy harvesters

Layered fluoroethylenepropylene (FEP) ferroelectret films with cross-tunnel structure were fabricated from sheets of FEP films by template-patterning followed by a fusion-bonding process and contact charging. The typical piezoelectric d33 coefficients, measured by a quasi-static method of samples not annealed, are in the range of 1000–3700 pC/N. The resonance behavior of the samples is analyzed by dielectric spectroscopy which also yields Young's modulus. Microphones built with such films exhibit a somewhat decreasing frequency response up to 1 kHz, an increase of the responses due to diffraction effects at higher frequencies, and eventually a peak probably due to a thickness resonance at about 40 kHz. Annealing at 125 °C indicates that the sensitivity stabilizes at about 40% of the original value. From this data, stable dynamic d33 coefficients of up to 300 pC/N can be calculated. A micro energy harvesting generator utilizing these films and based on the excitation of thickness vibrations is also described. With an active area of 4.3 cm2 and a seismic mass of 69.5 g, power up to 0.5 μW referred to an acceleration of 1 g can be generated at a frequency of 120 Hz.

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 μm thick FEP layers and a typical void of 60 μm 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 μm, is about 0.21 MPa. Initial quasistatic piezoelectric d33 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) d33 coefficients of up to 1000 pC/N and 400 pC/N, respectively, which are thermally stable at 120°C, can be obtained by proper annealing treatment. This constitutes a significant improvement compared to previous results.

Template-based fluoroethylenepropylene piezoelectrets with tubular channels for transducer applications

We describe the concept, the fabrication, and the most relevant properties of a piezoelectric-polymer system: Two fluoroethylenepropylene (FEP) films with good electret properties are laminated around a specifically designed and prepared polytetrafluoroethylene (PTFE) template at 300 °C. After removing the PTFE template, a two-layer FEP film with open tubular channels is obtained. For electric charging, the two-layer FEP system is subjected to a high electric field. The resulting dielectric barrier discharges inside the tubular channels yield a ferroelectret with high piezoelectricity. d33 coefficients of up to 160 pC/N have already been achieved on the ferroelectret films. After charging at suitable elevated temperatures, the piezoelectricity is stable at temperatures of at least 130 °C. Advantages of the transducer films include ease of fabrication at laboratory or industrial scales, a wide range of possible geometrical and processing parameters, straightforward control of the uniformity of the polymer system, flexibility, and versatility of the soft ferroelectrets, and a large potential for device applications e.g., in the areas of biomedicine, communications, production engineering, sensor systems, environmental monitoring, etc.

Ferroelectrets with improved thermal stability made from fused fluorocarbon layers

Several layers of polytetrafluoroethylene and fluoroethylenepropylene films are fused such that small interfacial gas voids are formed between the layers. After proper charging and annealing, the fused multilayer films show large and thermally stable quasistatic piezoelectric d33 coefficients of about 1000pC∕N which are subject to a minor reduction of 3% per day if exposed to 90°C. Depending on sample processing, the piezoelectric coefficient is relatively independent of applied pressure in the range up to 20kPa and of applied frequency up to about 100kHz. Thermally-stimulated discharge measurements indicate that the decay of the piezoelectric activity at higher temperatures is at least partially due to charge drift along the surfaces of the voids. A microphone built with the cellular ferroelectrets has a sensitivity of 3mV∕Pa at 1kHz.

Charge distribution in Teflon FEP (fluoroethylenepropylene) negatively corona-charged to high potentials

The charge distribution in negatively corona-charged Teflon FEP (fluoroethylenepropylene) films was investigated with the laser-induced pressure-pulse method. The 12.7- and 25.4-μm samples were charged to potentials up to −2500 and −4500 V, respectively, and thereafter stored at room temperature. Measurements of the charge distribution were taken at time intervals ranging from 1 min to several days after charging. The results indicate that the corona charge is initially trapped at the surface of the samples. A negative bulk charge which is nonuniformly distributed over the entire volume with a density of ∼1013 e/cm3 is found for samples kept at the highest voltages for periods of the order of hours. Within the resolution of the method, a spatial broadening of the surface-charge layer is not observed. These findings suggest that in highly surface-charged samples shallow-trapped holes in the bulk of the material, which compensate deeply trapped electrons, drift to the charged surface and thus cause the observed net-negative charge in the bulk.