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

Abstract

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.

All‐Organic Flexible Ferroelectret Nanogenerator with Fabric‐Based Electrodes for Self‐Powered Body Area Networks

“…Ferroelectrets with a parallel tunnel or hierarchical elastomer structure fabricated from FEP or fibrous PTFE—sometimes around a template—were extensively studied recently, [ 25,32–34 ] and can be used in sensitive pressure sensors because of their extremely high piezoelectric d 33 coefficient. [ 35 ] However, the extreme softness of this ferroelectret caused by large artificial air gaps made it unsuitable for applications in human energy harvesting, for example, during walking. Nevertheless, FEP/ePTFE/FEP‐sandwich ferroelectrets were investigated in detail and may be applied in energy‐harvesting systems.…”

Highly Efficient Piezoelectrets through Ultra-Soft Elastomeric Spacers

“…It is, however, also known from the literature that a positive charge in FEP can be stabilized by poling FEP films [46] and also fluoropolymer-based ferroelectrets [20,24,31,34] at elevated temperatures. The physics behind this stabilization is explained by the existence of a large amount of shallow hole traps and a relatively small amount of deep hole traps [46].…”

“…The still observed decay for the high-temperature poled samples is not clearly understood, but it may be related to charging of the residual air gaps near the FEP/TPU interfaces, the physics of which has yet to be explored in detail in upcoming studies. It is, however, also known from the literature that a positive charge in FEP can be stabilized by poling FEP films [46] and also fluoropolymer-based ferroelectrets [20,24,31,34] at elevated temperatures. The physics behind this stabilization is explained by the existence of a large amount of shallow hole traps and a relatively small amount of deep hole traps [46].…”

“…Depending on the pore structure, ferroelectrets can be divided into three groups: (a) ferroelectrets with a closed pore structure and lens-like air voids [9,[12][13][14][15][16], (b) multilayer configurations with open-porous foams embedded between two thin, solid polymer films [17][18][19][20][21][22][23], and (c) polymer arrays with artificially introduced air voids [24][25][26][27][28][29][30][31]. All of the above structures are originally non-polar and must be polarized in a sufficiently large external electric field to exhibit piezoelectricity.…”

“…In this work, a different strategy was used to increase the overall piezoelectric response. Ultrasoft thermoplastic polyurethane (TPU) [40,41] with a very low Young's modulus was used as spacer material in order to significantly reduce the apparent stiffness of the device, while dielectric films of polyfluoroethylene propylene (FEP) were chosen as dielectric films to keep the polarization characteristics of the air tunnels similar to previous publications [18][19][20][21][22][23][24][25][26][27][28]30,31,33,34,37]. The question of how geometrical, electrical, and mechanical parameters influence the d 33 piezocoefficient was the main objective of the present publication.…”