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 d 33 -and g 31 -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 d 33 -coefficient is observed at strains up to 2.1%, showing a high elasticity of the film. Both isothermal decay of d 33 -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 d 33 -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.
Polymers with electrically charged internal air cavities (ferroelectrets) reveal a pronounced piezoelectric response and are regarded as soft electroactive multi-functional materials. This work presents preliminary results on the preparation and piezoelectric effect of ferroelectrets based on the polylactic acid (PLA) polymer. A distinctive feature of the manufactured films is that they are biodegradable. After a microstructure modification of carbon dioxide (CO2) foamed PLA sheets by hot-pressing treatment and corona polarization, these cellular films reveal large piezoelectric d33 and d31 responses in both quasi-static and dynamic modes. For freshly charged films, the maximum quasi-static piezoelectric coefficients are about 600 pC/N (d33) and 44 pC/N (d31) for a relatively thick film of 360 μm and a nominal porosity of about 60%. During the first 20 days after polarization, the piezoelectric activity decreases by half compared to the primary value, but then remains almost unchanged for a long time. Due to an already established inherent biocompatibility of PLA polymers, these eco-friendly ferroelectrets can be potentially used in various biological applications such as biosensors and microenergy harvesters embedded in tissue and artificial muscles.
Energy harvesting from vibrations provides power to low-energy-consuming electronics for standalone and wearable devices as well as for wireless and remote sensing. In this contribution, compact tubular ferroelectret energy harvesters utilizing a single-tube design are presented. Such single-tube harvesters can be fabricated from commercially available fluorinated ethylene propylene (FEP) tubes with wall thicknesses of 25 and 50 μm, respectively, by mechanical deformation at elevated temperature. It is demonstrated that the generated power is highly dependent on parameters such as wall thickness, load resistance, and seismic mass. Utilizing a seismic mass of 80 g at resonance frequencies around 80 Hz and an input acceleration of 1 Â g (9.81 m s À2 rms), output powers up to 300 μW can be reached for a transducer with 25 μm thick walls.
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