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Laminated polymer-film systems with well-defined void structures were prepared from fluoroethylenepropylene (FEP) and polytetrafluoroethylene (PTFE) layers. First the PTFE films were patterned and then fusion-bonded with the FEP films. The laminates were subjected to either corona or contact charging in order to obtain the desired piezoelectricity. The build-up of the "macro-dipoles" in the laminated films was studied by recording the electric hysteresis loops. The resulting electro-mechanical properties were investigated by means of dielectric resonance spectroscopy (DRS) and direct measurements of the stress-strain relationship. Moreover, the thermal stability of the piezoelectric d (33) coefficient was investigated at elevated temperatures and via thermally stimulated discharge (TSD) current measurements in short circuit. For 150 mu m thick laminated films, consisting of one 25 mu m thick PTFE layer, two 12.5 mu m thick FEP layers, and a void of 100 mu m height, the critical voltage necessary for the build-up of the "macro-dipoles" in the inner voids was approximately 1400 V, which agrees with the value calculated from the Paschen Law. A quasi-static piezoelectric d (33) coefficient up to 300 pC/N was observed after corona charging. The mechanical properties of the film systems are highly anisotropic. At room temperature, the Young's moduli of the laminated film system are around 0.37 MPa in the thickness direction and 274 MPa in the lateral direction, respectively. Using these values, the theoretical shape anisotropy ratio of the void was calculated, which agrees well with experimental observation. Compared with films that do not exhibit structural regularity, the laminates showed improved thermal stability of the d (33) coefficients. The thermal stability of d (33) can be further improved by pre-aging. E.g., the reduction of the d (33) value in the sample pre-aged at 150A degrees C for 5 h was less than 5% after annealing for 30 h at a temperature of 90A degrees C.
Piezoelectrets are novel transducer materials which can be widely applied in sensors and actuators. Here, three techniques for determining piezoelectric d(33) coefficients of piezoelectrets are reviewed and compared. Two types of piezoelectrets, polyethylene-naphthalate (PEN) polymer-foam piezoelectrets and fluorinated ethylene-propylene (FEP) copolymer-layer piezoelectrets, have been prepared and measured by means of dynamic, resonance, and acoustical methods. The dynamic measurements show that the d(33) coefficient of PEN-foam samples clearly decreases with increasing stress, but 80% of the initial d(33) can be retained after 1800 cycles of a continuous dynamic measurement in a mechanical fatigue test. The resonance measurements demonstrate that both PEN-foam and FEP-layer samples exhibit clear electro-mechanical resonances. PEN-foam samples show elastic moduli in the range from 1 to 12 MPa and d(33) values up to 500 pC N-1, while FEP-layer samples show homogeneous elastic moduli of about 0.3 MPa and d(33) values of about 280 pC N-1. The acoustical measurements reveal that both PEN-foam and FEP-layer samples exhibit stable frequency responses in the range from 5.7 to 20 kHz. In addition, d(33) coefficients obtained with different experimental methods are in good agreement with each other, which confirms the reliability of all three techniques.
Dielectric elastomers (DE) are soft polymer materials exhibiting large deformations under electrostatic stress. When a prestretched elastomer is stuck to a flat plastic frame, a complex structure that can be used as an actuator (DEA) is formed due to self-organization and energy minimization. Here, such a DEA was equipped with a ferroelectret film. Ferroelectrets are internally charged polymer foams or void-containing polymer-film systems combining large piezoelectricity with mechanical flexibility and elastic compliance. In their dielectric spectra, ferroelectrets show piezoelectric resonances that can be used to analyze their electromechanical properties. The antiresonance frequencies ( ) of ferroelectret films not only are directly related to their geometric parameters, but also are sensitive to the boundary conditions during measurement. In this paper, a fluoroethylenepropylene (FEP) ferroelectret film with tubular void channels was glued to a plastic frame prior to the formation of self-organized minimum-energy DEA structure. The dielectric resonance spectrum (DRS) of the ferroelectret film was measured in-situ during the actuation of the DEA under applied voltage. It is found that the antiresonance frequency is a monotropic function of the bending angle of the actuator. Therefore, the actuation of DEAs can be used to modulate the of ferroelectrets, while the can also be taken for in-situ diagnosis and for precise control of the actuation of the DEA. Combination of DEAs and ferroelectrets brings a number of possibilities for application.
Fluoropolymer piezoelectrets with tubular channels resonance behavior controlled by channel geometry
(2012)
Ferro- or piezoelectrets are dielectric materials with two elastically very different macroscopic phases and electrically charged interfaces between them. One of the newer piezoelectret variants is a system of two fluoroethylenepropylene (FEP) films that are first laminated around a polytetrafluoroethylene (PTFE) template. Then, by removing the PTFE template, a two-layer FEP structure with open tubular channels is obtained. After electrical charging, the channels form easily deformable macroscopic electric dipoles whose changes under mechanical or electrical stress lead to significant direct or inverse piezoelectricity, respectively. Here, different PTFE templates are employed to generate channel geometries that vary in height or width. It is shown that the control of the channel geometry allows a direct adjustment of the resonance frequencies in the tubular-channel piezoelectrets. By combining several different channel widths in a single ferroelectret, it is possible to obtain multiple resonance peaks that may lead to a rather flat frequency-response region of the transducer material. A phenomenological relation between the resonance frequency and the geometrical parameters of a tubular channel is also presented. This relation may help to design piezoelectrets with a specific frequency response.
A template-based lamination technique for the manufacture of ferroelectrets from uniform electret films was recently reported. In the present work, this technique is used to prepare similar ferroelectret structures from low-density polyethylene (LDPE) films and from fluoro-ethylene-propylene (FEP) copolymer films. A comparative analysis of the pressure-, temperature-, and frequency-dependent piezoelectric properties has been performed on the two ferroelectret systems. It is observed that the FEP ferroelectrets exhibit better piezoelectric responses and are thermally more stable. The difference between the piezoelectric d(33) coefficients of the two ferroelectret systems is partially explained here by their different elastic moduli. The anti-resonance peaks of both structures have been investigated by means of dielectric resonance spectroscopy and electroacoustic sound-pressure measurements. A difference of more than 10 kHz is observed between the anti-resonance frequencies of the two ferroelectret systems.
In recent communications from these laboratories, we observed that amine-rich thin organic layers are very efficient surfaces for the adhesion of mammalian cells. We prepare such deposits by plasma polymerization at low pressure, atmospheric pressure, or by vacuum-ultraviolet photo-polymerization. More recently, we have also investigated a commercially available material, Parylene diX AM. In this article we first briefly introduce literature relating to electrostatic interactions between cells, proteins, and charged surfaces. We then present certain selected cell-response results that pertain to applications in orthopedic and cardiovascular medicine: we discuss the influence of surface properties on the observed behaviors of two particular cell lines, human U937 monocytes, and Chinese hamster ovary cells. Particular emphasis is placed on possible electrostatic attractive forces due to positively charged R-NH3+ groups and negatively charged proteins and cells, respectively. Experiments carried out with electrets, polymers with high positive or negative surface potentials are added for comparison.
Temporal evolution of the re-breakdown voltage in small gaps from nanoseconds to milliseconds
(2013)
A detailed understanding of electric breakdown in dielectrics is of scientific and technological interest. In gaseous dielectrics, a so-called re-breakdown is sometimes observed after extinction of the previous discharge. Although time-dependent re-breakdown voltage is essentially known, its behavior immediately after the previous discharge is not precisely understood. We present an electronic circuit for accurate measurements of the time-dependent re-breakdown voltage in small gaps from tens of nanoseconds to several milliseconds after the previous spark. Results from such experiments are compared with earlier findings, and relevant physical mechanisms such as heating of the gas, decay of the plasma, and ionization of excited atoms and molecules are discussed. It is confirmed that the thermal model is not valid at times below several microseconds.
Direct hysteresis measurements on ferroelectret films by means of a modified Sawyer-Tower circuit
(2013)
Ferro- and piezo-electrets are non-polar polymer foams or film systems with internally charged cavities. Since their invention more than two decades ago, ferroelectrets have become a welcome addition to the range of piezo-, pyro-, and ferro-electric materials available for device applications. A polarization-versus-electric-field hysteresis is an essential feature of a ferroelectric material and may also be used for determining some of its main properties. Here, a modified Sawyer-Tower circuit and a combination of unipolar and bipolar voltage waveforms are employed to record hysteresis curves on cellular-foam polypropylene ferroelectret films and on tubular-channel fluoroethylenepropylene copolymer ferroelectret film systems. Internal dielectric barrier discharges (DBDs) are required for depositing the internal charges in ferroelectrets. The true amount of charge transferred during the internal DBDs is obtained from voltage measurements on a standard capacitor connected in series with the sample, but with a much larger capacitance than the sample. Another standard capacitor with a much smaller capacitance-which is, however, still considerably larger than the sample capacitance-is also connected in series as a high-voltage divider protecting the electrometer against destructive breakdown. It is shown how the DBDs inside the polymer cavities lead to phenomenological hysteresis curves that cannot be distinguished from the hysteresis loops found on other ferroic materials. The physical mechanisms behind the hysteresis behavior are described and discussed.
Electrically charged cellular polymer films can exhibit very high piezoelectric activity and are therefore more and more often employed in advanced electromechanical and electro-acoustical transducers. In this paper, we report an optimized sequence of steps for preparing such ferroelectrets from commercial nonvoided ploy(ethylene terephthalate) (PETP) films by means of foaming with CO2 biaxial mechanical stretching, controlled void inflation, and bipolar electric charging. The nonvoid PETP films foamed with supercritical CO2 at a suitably high pressure and subsequently annealed for stabilization. The cellular foam structure was further optimized by means of well controlled biaxial stretching in a commercial stretcher and sometimes subsequent inflamation in a pressure chamber. Bipolar electric charging of the internal voids was achieved through the application of high electric fields in an SF0 atmosphere. The new optimized PETP ferroelectric exhibit quite large piezoelectric coefficients up to almost 500 pCN(-1), for which unusually low elastic stiffness of only around 0.3 MPa are essential. The PETP foam ferroelectrics posses unclamped thickenss-extension resonance frequences between approximately 120 and 250 kHz, and are thus highly suitable for several established as well as novel ultrasonic-transductant applications.
When exposed to sufficiently high electric fields, polymer-foam electret materials with closed cells exhibit ferroelectric-like behavior and may therefore be called ferroelectrets. In cellular ferroelectrets, the influence of the cell size and shape distributions on the application-relevant properties is not yet understood. Therefore, controlled inflation experiments were carried out on cellular polypropylene films, and the resulting elastical and electromechanical parameters were determined. The elastic modulus in the thickness direction shows a minimum with a corresponding maximum in the electromechanical transducer coefficient. The resonance frequency shifts as a function of the elastic modulus and the relative density of the inflated cellular films. Therefore, the transducer properties of cellular ferroelectrets can be optimized by means of controlled inflation. (C) 2004 American Institute of Physics