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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.
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.
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
In cellular, electromechanically active polymer films, the so-called ferroelectrets, the cell size and shape distributions can be varied through a controlled inflation process. Up to now, high-pressure treatments were usually performed at elevated temperatures. There are, however, significant experimental limitations and complications if the pressure and temperature treatments are performed at the same time. Here, we demonstrate the controlled inflation of cellular polypropylene films by means of sepal-ate pressure and temperature treatments. Separate procedures are Much easier to implement. Excellent electromechanical properties were achieved with Such a two-step inflation process. The technique has significant potential for inflating large-area transducer films for electromechanical and electroacoustical applications
Elastic properties and electromechanical coupling factor of inflated polypropylene ferroelectrets
(2006)
Polyvinylidene fluoride was dissolved together with solid sodium hydroxide as catalyst in a dimethylsulfoxide/ acetone mixture and moderately dehydrofluorinated. The dehydrofluorination leads to a partial degradation of the fluorohydrocarbons, and in particular to main-chain scission and to formation of carbon double or triple bonds. This enhances the absorption at UV-vis frequencies. The degradation process also generates a large amount of excess charges in the polymer, which influence the electrical polarization behavior of the dehydrofluorinated polymer. Uniaxial stretching of moderately dehydrofluorinated polyvinylidene fluoride leads to films in a polar phase. Dipole polarization in the degraded and stretched films is demonstrated by means of switching experiments
Piezo-, pyro- and ferroelectricity in poly (vinylidene fluoride-hexafluoropropylene) copolymer films
(2004)
Thin films of poly(vinylidene fluoride-hexafluoropropylene) P(VDF-HFP) show significant electroactive properties, such as piezoelectricity, pyroelectricity and electrostriction. Suitable polar P(VDF-HFP) copolymer films can be prepared by melt-pressing or solution-casting. Dipolar orientation causes the macroscopic polarization and thus also the symmetry breaking necessary for electroactive properties. We discuss the polarization build-up in thin, stretched and non-stretched, films of P(VDF-HFP) copolymer with a HFP content of 15%. Poling currents measured in-situ during electric poling are analyzed and the polarization is calculated. Suitable electric poling leads to hysteresis phenomena of the polarization as a function of the electric field as well as to significant polarization during switching experiments. Our results indicate dipolar orientation also in non-stretched P(VDF-HFP) films
In this paper, a measuring technique is presented for the detection of radial oscillations of tube walls excited by changes in internal air pressure. On organ pipes, the oscillations were investigated by means of piezoelectric polymer films slightly tensioned around the pipe bodies. Employing sensors with patterned electrodes, the well-known elliptical oscillation of the cross section as well as an additional monopole breathing of the organ-pipe body were detected. For the monopole breathing, a close relationship between the pressure distribution of the air-column resonances inside the pipe and the circumference variations along the pipe was observed
Piezoelectric cellular polypropylene films, so-called ferroelectrets, are assembled in a stack with two active transducer layers. The stack is characterized with respect to its linear and quadratic response in a frequency range from 1 kHz to 80 kHz. A relatively smooth frequency response in the sound-pressure level is found for the individual layers as well as for both layers driven in phase. The piezoelectric response of the two-layer stack is twice the response of an individual layer over a rather broad frequency range. Furthermore, the influence of the preparation conditions on the resonance frequency and the effect of the quadratic distortion on the radiated sound are investigated both for the individual transducer films in the stack and for the stack system as a whole
Voided space-charge electrets : piezoelectric transducer materials for electro-acoustic applications
(2004)
In contrast to molecular-dipole polymers, such as PVDF, ferroelectrets are a new class of flexible spatially heterogeneous piezoelectric polymers with dosed or open voids that act as deformable macro-dipoles after charging.
With a spectrum of manufacturing processes being developed to engineer the heterogeneous structures, ferroelectrets are made with attractive piezoelectric properties well-suited for applications, such as pressure sensors, acoustic transducers, etc.
However, the sources of the macro-dipole charges have usually been the same, microscopic dielectric barrier discharges within the voids, induced when the ferroelectrets are poled under a large electric field typically via a so-called corona poling, resulting in the separation and trapping of opposite charges into the interior walls of the voids.
Such a process is inherently self-limiting, as the reverse internal field from the macro-dipoles eventually extinguishes the microdischarges, resulting in limited density of ions and not too high overall piezoelectric performance. Here, a new method to form ferroelectrets with gigantic electroactivity is proposed and demonstrated with the aid of an external ion booster.
A laminate consisting of expanded polytetrafluoroethylene (ePTFE) and fluorinated-ethylene-propylene (FEP) was prefilled with bipolar ions produced externally by an ionizer and sequentially poled to force the separation of positive and negative ions into the open fibrous structure, rendering an impressive piezoelectric d(33)( )coefficient of 1600 pC/N-an improvement by a factor of 4 in comparison with the d(33) of a similar sandwich poled with nonenhanced corona poling.
The (pre)filling dearly increases the ion density in the open voids significantly. The charges stored in the open-cell structure stays at a high level for at least 4 months. In addition, an all-organic nanogenerator was made from an ePTFE-based ferroelectret, with conducting poly(3,4-ethylene dioxythiophene): poly(styrenesulfonate) (PEDOT: PSS) coated fabric electrodes.
When poled with this ion-boosting process, it yielded an output power twice that of a similar sample poled in a conventional corona-only process. The doubling in output power is mainly brought about by the significantly higher charge density achieved with the aid of external booster.
Furthermore, aside from the bipolar ions, extra monopolar ions can during the corona poling be blown into the open pores by using for instance a negative ionic hair dryer to produce a unipolar ePTFE-based ferroelectret with its d(33) coefficient enhanced by a factor of 3. Ion-boosting poling thus unleashes a new route to produce bipolar or unipolar open-cell ferroelectrets with highly enhanced piezoelectric response.
Due to their electrically polarized air-filled internal pores, optimized ferroelectrets exhibit a remarkable piezoelectric response, making them suitable for energy harvesting. Expanded polytetrafluoroethylene (ePTFE) ferroelectret films are laminated with two fluorinated-ethylene-propylene (FEP) copolymer films and internally polarized by corona discharge. Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)-coated spandex fabric is employed for the electrodes to assemble an all-organic ferroelectret nanogenerator (FENG). The outer electret-plus-electrode double layers form active device layers with deformable electric dipoles that strongly contribute to the overall piezoelectric response in the proposed concept of wearable nanogenerators. Thus, the FENG with spandex electrodes generates a short-circuit current which is twice as high as that with aluminum electrodes. The stacking sequence spandex/FEP/ePTFE/FEP/ePTFE/FEP/spandex with an average pore size of 3 mu m in the ePTFE films yields the best overall performance, which is also demonstrated by the displacement-versus-electric-field loop results. The all-organic FENGs are stable up to 90 degrees C and still perform well 9 months after being polarized. An optimized FENG makes three light emitting diodes (LEDs) blink twice with the energy generated during a single footstep. The new all-organic FENG can thus continuously power wearable electronic devices and is easily integrated, for example, with clothing, other textiles, or shoe insoles.
Bipolar electrets from polypropylene (PP) are essential, e.g., in electret air filters and in cellular-foam ferroelectrets. Therefore, the mechanism of surface-charge stability enhancement on PP electrets via orthophosphoric-acid surface treatment is investigated in detail. It is shown that the significant charge-stability enhancement can be mainly attributed to deeper surface traps originating from deposited chemicals and topographic features on the modified surfaces. Thermally stimulated discharge of chemically treated and non-treated PP films with different surface-charge densities is used to test the limits of the newly formed deep traps in terms of the capacity for hosting surface charges. When the initial surface-charge density is very high, more charges are forced into shallower original traps on the surface or in the bulk of the treated PP samples, reducing the effect of the deeper surface traps brought by the surface modification. The well-known crossover phenomenon (of the surface-charge decay curves) has been observed between modified PP electrets charged to +/- 2kV and to +/- 3kV. Acoustically probed charge distributions in the thickness direction of PP electrets at different stages of thermal discharging indicate that the deep surface trapping sites may have preference for negative charges, resulting in the observed asymmetric charge stability of the modified PP films.
A double-layer transcrystalline polypropylene (PP) film with a flat central interface layer between its two transcrystalline layers is obtained by recrystallization from the melt between two polytetrafluoroethylene (PTFE) surfaces on both sides of the PP film. Its electret properties are studied and compared with those of a single-layer transcrystalline PP film re-crystallized in contact with only one PTFE surface. Within experimental uncertainty, the two types of transcrystalline films exhibit the same thermal properties and crystallinities. After thermal poling, however, two hetero-charge layers of opposite polarity are found on the internal interfaces of the double-layer transcrystalline films and may together be considered as micrometer-sized dipoles. The unexpected phenomenon does not occur in single-layer transcrystalline samples without a central interface layer, suggesting that the interfaces between the transcrystalline layers and the micrometer-thick central interface layer may be the origin of deeper traps rather than the crystalline structures in the transcrystallites or the spherulites. The origin of the interfacial charges was also studied by means of an injection-blocking charging method, which revealed that intrinsic charge carriers introduced during recrystallization are most likely responsible for the interfacial charges. It is fascinating that a material as familiar as PP can exhibit such intriguing properties with a special bipolar space-charge polarization across the central interface layer after quasi-epitaxial surface moulding into a double-layer transcrystalline form. In addition to applications in electret (micro-)devices for electro-mechanical transduction, the highly ordered structures may also be employed as a new paradigm for studying charge storage and transport in polymer electrets and in dielectrics for DC electrical insulation.
The low surface-charge stability of polypropylene (PP) frequently limits its application as an electret material. In this paper, we demonstrate how the treatment of PP-film surfaces with orthophosphoric acid (H3PO4) enhances their charge stability. To discriminate between the effects of chemical modification and thermal treatment, as-received and annealed PP films are used as reference samples. The electret properties of treated and non-treated PP films are characterized with thermally stimulated discharge (TSD) and isothermal surface-potential decay (ISPD) experiments, from which considerable improvement in thermal and temporal charge stability is observed for samples modified with H3PO4. The half-value temperature (T-1/2) observed on TSD curves of chemically treated PP increases to 131 and 145 degrees C for positive and negative charges, respectively. The enhancement might be attributed to the phosphoric compounds detected on the H3PO4-modified surfaces via attenuated-total-reflection infrared spectroscopy. Deeper surface traps formed at the "foreign" phosphorus-containing structures are able to capture the charges over longer time periods and at higher temperatures, thus leading to significant improvements in the temporal and thermal surface-charge stabilities of PP electrets. Published by AIP Publishing.
Previous work has shown that surface modification with orthophosphoric acid can significantly enhance the charge stability on polypropylene (PP) surface by generating deeper traps. In the present study, thermally stimulated potential-decay measurements revealed that the chemical treatment may also significantly increase the number of available trapping sites on the surface. Thus, as a consequence, the so-called "cross-over" phenomenon, which is observed on as-received and thermally treated PP electrets, may be overcome in a certain range of initial charge densities. Furthermore, the discharge behavior of chemically modified samples indicates that charges can be injected from the treated surface into the bulk, and/or charges of opposite polarity can be pulled from the rear electrode into the bulk at elevated temperatures and at the high electric fields that are caused by the deposited charges. In the bulk, a lack of deep traps causes rapid charge decay already in the temperature range around 95 degrees C.
Spherulite-related space-charge electret properties of polypropylene (PP) have been widely discussed in the past decades. In the present paper, a less-common crystalline structure in PP-transcrystalline PP-is studied regarding its electret behavior in comparison with the typical spherulitic morphology. Polarized light microscopy and differential scanning calorimetry were employed to characterize the crystallite types and crystallinities of transcrystalline and spherulitic PP. Their electret functionality is investigated by means of thermally stimulated discharge experiments, where the cross-over phenomenon is observed on transcrystalline PP films, whereas surface-potential saturation and undercharging on the surface occur on the spherulitic samples. Besides, an asymmetrical behavior of positive and negative surface-charge stabilities is found on PP with spherulites, the negatively charged spherulitic surfaces show a better charge stability. It is shown that PP electrets are very sensitive to changes in the microscopic crystalline structures and their interfaces as well as in the molecular conformations controlled through adjustments of the respective processing steps. In addition, surface and bulk nanocomposites of PP or low-density polyethylene with inorganic particles are included in the comparison. In view of recent developments in the areas of PP-based electret-fiber filters and cellular-foam ferroelectrets, the observed changes in the charge-storage properties may have particular relevance, as the required film, fiber, or foam processing might significantly modify crystalline morphologies and nano-scale interfaces in PP electrets. Limitations in the charge-storage capabilities of interface structures may also be of interest in the context of high-voltage electrical-insulation materials where reduced space-charge accumulation and slightly increased charge transport can be advantageous.
Tailoring the secondary surface morphology of electro-spun nanofibers has been highly desired, as such delicate structures equip nanofibers with distinct functions. Here, we report a simple strategy to directly reconstruct the surface of polyvinyl alcohol/polyvinylpyrrolidone (PVA/PVP) nanofibers by water evaporation. The roughness and diameter of the nanofibers depend on the temperature during vacuum drying. Surface changes of the nanofibers from smooth to rough were observed at 55 degrees C, with a significant drop in nanofiber diameter. We attribute the formation of the secondary surface morphology to the intermolecular forces in the water vapor, including capillary and the compression forces, on the basis of the results from the Fourier-transform infrared (FTIR) and X-ray photoelectron (XPS) spectroscopy. The strategy is universally effective for various electro-spun polymer nanofibers, thus opening up avenues toward more detailed and sophisticated structure design and implementation for nanofibers.
Thin films of ferroelectric beta-phase poly(vinylidene fluoride) (PVDF) were spin-coated from a solution that contained small amounts of the ionic liquid (IL) 1-ethyl-3-methylimidazolium nitrate. A remanent polarization of 60 mC/m(2) and a quasi-static pyroelectric coefficient of 19 mu C/m(2)K at 30 degrees C were observed in the films. It is suggested that the IL promotes the formation of the beta phase through dipolar interactions between PVDF chain-molecules and the IL. The dipolar interactions are identified as Coulomb attraction between hydrogen atoms in PVDF chains and anions in IL. The strong crystallinity increase is probably caused by the same dipolar interaction as well.
beta-phase poly(vinylidene fluoride-hexafluoropropylene) (P(VDF-HFP)) copolymer films were prepared by uniaxially stretching solution-cast or melt-quenched samples. Different preparation routes lead to different amounts of the crystalline alpha and beta phases in the films, as detected by means of Fourier-transform infrared spectroscopy and X-ray diffractometry. The beta phase is significantly enhanced in melt-quenched and stretched films in comparison to solution-cast and stretched films. This is particularly true for copolymer samples with higher HFP content. The beta- phase enhancement is also observed in ferroelectric-hysteresis experiments where a rather high polarization of 58 mC/ m(2) was found on melt-quenched and stretched samples after poling at electric fields of 140 MV/m. After poling at 160 MV/m, one of these samples exhibited a piezoelectric d(33) coefficient as high as 21 pC/N. An electric-field-induced partial transition from the alpha to the beta phase was also observed on the melt-quenched and stretched samples. This effect leads to a further increase in the applications-relevant dipole polarization. Uniaxially stretched ferroelectric- polymer films are highly anisotropic. Dielectric resonance spectroscopy reveals a strong increase of the transverse piezoelectric d(32) coefficient and a strong decrease of the transverse elastic modulus c(32) upon heating from 20 to 50 degrees C.
We present calculations on the deformation of two- and three-layer electret systems. The electrical field is coupled with the stress-strain equations by means of the Maxwell stress tensor. In the simulations, two-phase systems are considered, and intrinsic relative dielectric permittivity and Young's modulus of the phases are altered. The numerically calculated electro-mechanical activity is compared to an analytical expression. Simulations are performed on two- and three-layer systems. Various parameters in the model are systematically varied and their influence on the resulting piezoelectricity is estimated. In three-layer systems with bipolar charge, the piezoelectric coefficients exhibit a strong dependence on the elastic moduli of the phases. However, with mono-polar charge, there is no significant piezoelectric effect. A two-dimensional simulation illustrated that higher piezoelectric coefficients can be obtained for non-uniform surface charges and low Poisson's ratio of phases. Irregular structures considered exhibit low piezoelectric activity compared to two-layer structures. (C) 2004 Elsevier B.V. All rights reserved
In this paper, a recently developed numerical method to analyze dielectric-spectroscopy data is applied to alpha-phase polyvinylidene fluoride (PVDF). The numerical procedure is non-parametric and does not contain any of the extensively used empirical formulas mentioned in the literature. The method basically recovers the unknown distribution of relaxation times of the generalized dielectric function representation by simultaneous application of the Monte Carlo integration method and of the constrained least-squares optimization. The relaxation map constructed after the numerical analysis is compared to a-phase PVDF data presented in the literature and results of the parametric analysis with a well- known empirical formula. (c) 2005 Elsevier B.V. All rights reserved
The conductivity of alpha-polyvinylidene fluoride is obtained from dielectric measurements performed in the frequency domain at several temperatures. At temperatures above the glass-transition, the conductivity can be interpreted as an ionic conductivity, which confirms earlier results reported in the literature. Our investigation shows that the observed ionic conductivity is closely related to the amorphous phase of the polymer. (C) 2005 American Institute of Physics
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.
In our fast-changing world, human-machine interfaces (HMIs) are of ever-increasing importance. Among the most ubiquitous examples are touchscreens that most people are familiar with from their smartphones. The quality of such an HMI can be improved by adding haptic feedback-an imitation of using mechanical buttons-to the touchscreen. Thin-film actuators on the basis of electro-mechanically active polymers (EAPs), with the electroactive material sandwiched between two compliant electrodes, offer a promising technology for haptic surfaces. In thin-film technology, the thickness and the number of stacked layers of the electroactive dielectric are key parameters for tuning a system. Therefore, we have experimentally investigated the influence of the thickness of a single EAP layer on the electrical and the electro-mechanical performance of the transducer. In order to achieve high electro-mechanical actuator outputs, we have employed relaxor-ferroelectric ter-fluoropolymers that can be screen-printed. By means of a model-based approach, we have also directly compared single- and multi-layer actuators, thus providing guidelines for optimized transducer configurations with respect to the system requirements of haptic applications for which the operation frequency is of particular importance.
A new variant of the Laser-Induced Pressure-Pulse (LIPP) method for repeatable, time-resolved space-charge profile measurements is proposed and demonstrated. Automated deposition of a fresh laser-target film before each illumination leads to good repeatability of the LIPP and thus allows for the detection of time-resolved changes in the space-charge distribution over many hours. We describe and discuss the experimental setup and its features, compare the repeatability of the LIPP measurements on the same sample without and with re-preparation of the test cell, and present the time-resolved evolution of the space-charge profile in a two-layer arrangement of a silicone-grease and a silicone-elastomer film as an example. Finally, the temperature dependence of the space-charge evolution during polarization under high voltage and during depolarization in short circuit is shown. Possible uses and future developments of the new LIPP approach are also discussed.
We report a process for preparing polymer ferroelectrets by means of screen printing-a technology that is widely used for the two-dimensional patterning of printed layers. In order to produce polymer-film systems with cavities that are suitable for bipolar electric charging, a screen-printing paste is deposited through a screen with a pre-designed pattern onto the surface of a polymer electret film. Another such polymer film is placed on top of the printed pattern, and well-defined cavities are formed in-between. During heating and curing, the polymer films are tightly bonded to the patterned paste layer so that a stable three-layer system is obtained. In the present work, polycarbonate (PC) films have been employed as electret layers. Screen printing, curing and charging led to PC ferroelectret systems with a piezoelectric d (33) coefficient of about 28 pC/N that is stable up to 100 C-a similar to. Due to the rather soft patterned layer, d (33) strongly decreases already for static pressures of tens of kPa. The results demonstrate the suitability of screen printing for the preparation of ferroelectret systems.
New ferroelectrets were developed on the basis of foams from cyclo-olefin polymers and copolymers. The results obtained on the cyclo-olefin polymer foam demonstrate a significant improvement of the service temperature for ferroelectret transducer materials. Suitable compounding and preparation led to cyclo-olefin ferroelectrets with an electromechanical activity of around 15 pC/N, which is thermally stable at least up to 110 degrees C. The properties in sensor and actuator applications are strongly dependent on the processing parameters related to film-making, sensor and actuator preparation, gas content and electric charging. The processing window for the film stretching was very narrow compared to the earlier developed polypropylene ferroelectrets. The film porosity, softness and thus the electromechanical activity are adjusted by gas-diffusion expansion. The activity of the electromechanically operating sensors and actuators was increased by stacking several layers of cellular cyclo-olefin film. For applications such as flat loudspeakers, the foamed films are tuned by tensioning them on a support frame. Correct tensioning was essential also for reducing the distortion levels.
Tetrafluoroethylene-hexafluoropropylene copolymer (FEP) films were treated with titanium-tetrachloride vapor in a molecular-layer deposition process. As a result of the surface treatment, significant improvements of the thermal and temporal charge stability were observed. Charge-decay measurements revealed enhancements of the half-value temperatures and the relaxation times of positively charged FEP electrets by at least 120 A degrees C and two orders of magnitude, respectively. Beyond previous publications on fluoropolymer electrets with surface modification, we here report enhanced charge stabilities of the FEP films charged in negative as well as in positive corona discharges. Even though the improvement for negatively charged FEP films is moderate (half-value temperature about 20 A degrees C higher), our experiments show that the asymmetry in positive and negative charge stability that is typical for FEP electrets can be overcome by means of chemical surface treatments. The results are discussed in the context of the formation of modified surface layers with enhanced charge-trapping properties.
Tetrafluoroethylene-hexafluoropropylene copolymer (FEP) films were treated with titanium-tetrachloride vapor by means of molecular-layer deposition. The treatment leads to considerable improvements of the electret-charge stability on positively charged films. A slight improvement is also observed for negatively charged films. In line with our previous findings, we attribute the improvement in electret properties to the formation of deeper traps on the chemically modified polymer surfaces. Here, we investigate the influence of the charge density on electret stability of FEP films with modified surfaces. Trap-energy spectra obtained from thermally-stimulated-discharge measurements indicate that the charge stability on modified FEP films depends on how the surface traps are populated and on the availability of additional deeper traps.
Enhanced electret charge stability on Polyethylene Films treated with Titanium-Tetrachloride vapor
(2012)
Low-density polyethylene (LDPE) films have been treated with titanium-tetrachloride vapor by means of the molecular-layer-deposition method. It is shown that such a treatment leads to a considerable improvement of the electret properties for both positively and negatively charged films. The temperature stability of the electret homo-charge has been increased by approximately 60 degrees C. At the same time, the temporal stability of charge is also considerably improved. Modified low-density polyethylene films show no "cross-over phenomenon" when charged to higher voltages. Thus, it is now possible to produce electrets from polyethylene films with high initial charge densities, but without a strongly reduced charge stability. The influence of a chemical treatment with titanium-tetrachloride vapor on charge injection from aluminum electrodes into polyethylene films was also investigated. It is found that the interface between an aluminum electrode and a modified LDPE surface layer has different injection properties for positive and negative charges. Electrons can be injected across the modified interface, whereas injection of holes is either very limited or non-existent.
The surface of polytetrafluoroethylene films was treated with titanium-tetrachloride vapor. The treatment was carried out in a flow reactor by means of molecular-layer deposition, a method from the arsenal of chemical nanotechnology. X-ray photoelectron spectroscopy reveals that such a treatment results in considerable changes in the chemical composition at and near the surface of the fluoropolymer film. Both, defluorination and oxidation of the surface were observed. At the same time, samples treated with titanium tetrachloride show a significant enhancement in the thermal stability of the positive homocharge. The thermally stimulated surface-potential-decay curves were observed to shift to higher temperatures by more than 100 degrees C.
Ferroelectrets have been fabricated from low-density polyethylene (LDPE) films by means of a template-based lamination. The temperature dependence of the piezoelectric d(33) coefficient has been investigated. It was found that low-density polyethylene ferroelectrets have rather low thermal stability with the piezoelectric coefficient decaying almost to zero already at 100 degrees C. This behavior is attributed to the poor electret properties of the polyethylene films used for the fabrication of the ferroelectrets. In order to improve the charge trapping and the thermal stability of electret charge and piezoelectricity, LDPE ferroelectrets were treated with orthophosphoric acid. The treatment resulted in considerable improvements of the charge stability in LDPE films and in ferroelectret systems made from them. For example, the charge and piezoelectric-coefficient decay curves shifted to higher temperatures by 60 K and 40 K, respectively. It is shown that the decay of the piezoelectric coefficient in LDPE ferroelectrets is governed by the relaxation of less stable positive charges. The treatment also leads to noticeable changes in the chemical composition of the LDPE surface. Infrared spectroscopy reveals absorption bands attributed to phosphorus-containing structures, while scanning electron microscopy shows new island-like structures, 50-200 nm in diameter, on the modified surface.
The zeta potential of the motile spores of the green alga (seaweed) Ulva linza was quantified by video microscopy in combination with optical tweezers and determined to be -19.3ñ1.1 mV. The electrostatic component involved in the settlement and adhesion of spores was studied using electret surfaces consisting of PTFE and bearing different net charges. As the surface chemistry remains the same for differently charged surfaces, the experimental results isolate the influence of surface charge and thus electrostatic interactions. Ulva spores were demonstrated to have a reduced tendency to settle on negatively charged surfaces and when they did settle the adhesion strength of settled spores was lower than with neutral or positively charged surfaces. These observations can be ascribed to electrostatic interactions.
Poly(vinylidenefluoride-trifluoroethylene)-based (P(VDF-TrFE)-based) terpolymers represent a new class of electroactive polymer materials that are relaxor-ferroelectric (RF) polymers and that offer unique and attractive property combinations in comparison with conventional ferroelectric polymers. The RF state is achieved by introducing a fluorine-containing termonomer as a "defect" into the ferroelectric P(VDF-TrFE) copolymer, which reduces the interaction between the VDF/TrFE dipoles. The resulting terpolymer exhibits a low Curie transition temperature and small remanent and coercive fields yielding a slim hysteresis loop that is typical for RF materials. Though the macroscopic behavior is similar to RF ceramics, the mechanisms of relaxor ferroelectricity in semi-crystalline polymers are different and not fully understood yet. Structure-property relationships play an important role in RF terpolymers, as they govern the final RF properties. Hence, a review of important characteristics, previous studies and relevant developments of P(VDF-TrFE)-based terfluoropolymers with either chlorofluoroethylene (CFE) or chlorotrifluoroethylene (CTFE) as the termonomer is deemed useful. The role of the termonomer and of its composition, as well as the effects of the processing conditions on the semi-crystalline structure which in turn affects the final RF properties are discussed in detail. In addition, the presence of noteworthy transition(s) in the mid-temperature range and the influence of preparation conditions on those transitions are reviewed. A better understanding of the fundamental aspects affecting the semi-crystalline structures will help to elucidate the nature of RF activity in VDF-based terpolymers and also help to further improve their applications-relevant electroactive properties.
Non-linear dielectric spectroscopy (NLDS) is employed as an effective tool to study relaxation processes and phase transitions of a poly(vinylidenefluoride-trifluoroethylene-chlorofluoroethylene) (P(VDF-TrFE-CFE)) relaxor-ferroelectric (R-F) terpolymer in detail. Measurements of the non-linear dielectric permittivity epsilon 2 ' reveal peaks at 30 and 80 degrees C that cannot be identified in conventional dielectric spectroscopy. By combining the results from NLDS experiments with those from other techniques such as thermally stimulated depolarization and dielectric-hysteresis studies, it is possible to explain the processes behind the additional peaks. The former peak, which is associated with the mid-temperature transition, is found in all other vinylidene fluoride-based polymers and may help to understand the non-zero epsilon 2 ' values that are detected on the paraelectric phase of the terpolymer. The latter peak can also be observed during cooling of P(VDF-TrFE) copolymer samples at 100 degrees C and is due to conduction and space-charge polarization as a result of the accumulation of real charges at the electrode-sample interface.
The effects of thermal processing on the micro- and nanostructural features and thus also on the relaxor-ferroelectric properties of a P(VDF-TrFE-CFE) terpolymer were investigated in detail by means of dielectric experiments, such as dielectric relaxation spectroscopy (DRS), dielectric hysteresis loops, and thermally stimulated depolarization currents (TSDCs). The results were correlated with those obtained from differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and Fourier-transform infrared spectroscopy (FTIR). The results from DRS and DSC show that annealing reduces the Curie transition temperature of the terpolymer, whereas the results from WAXD scans and FTIR spectra help to understand the shift in the Curie transition temperatures as a result of reducing the ferroelectric phase fraction, which by default exists even in terpolymers with relatively high CFE contents. In addition, the TSDC traces reveal that annealing has a similar effect on the midtemperature transition by altering the fraction of constrained amorphous phase at the interphase between the crystalline and the amorphous regions. Changes in the transition temperatures are in turn related to the behavior of the hysteresis curves on differently heat-treated samples. During heating, evolution of the hysteresis curves from ferroelectric to relaxor-ferroelectric, first exhibiting single hysteresis loops and then double hysteresis loops near the Curie transition of the sample, is observed. When comparing the dielectric-hysteresis loops obtained at various temperatures, we find that annealed terpolymer films show higher electric-displacement values and lower coercive fields than the nonannealed sample, irrespective of the measurement temperature, and also exhibit ideal relaxor- ferroelectric behavior at ambient temperatures, which makes them excellent candidates for applications at or near room temperature. By tailoring the annealing conditions, it has been shown that the application temperature could be increased by fine tuning the induced micro- and nanostructures.
Polyvinylidene fluoride (PVDF)-based copolymers with tetrafluoroethylene (P(VDF-TFE)), trifluoroethylene (P(VDF-TrFE)) or hexafluoropropylene (P(VDF-HFP)) are of strong interest due to the underlying fundamental mechanisms and the potential ferro-, pyro- and piezo-electrical applications. Their flexibility and their adaptability to various shapes are advantageous in comparison to inorganic ferroelectrics. Here, we study the influence of stretching temperature on the crystalline phases and the dielectric properties in P(VDF-TFE) films by means of Dielectric Relaxation Spectroscopy (DRS), Fourier-Transform InfraRed spectroscopy (FTIR), Wide-Angle X-ray Diffraction (WAXD), Differential Scanning Calorimetry (DSC) and Dynamic Mechanical Analysis (DMA). Especially, the effect of stretching and the influence of the temperature of stretching on the mid-temperature (T-mid) transition are studied in detail. The results show that stretching has a similar effect as that on PVDF, and we observe an increase in the fraction of ferroelectric beta-phase with a simultaneous increment in both melting point (T-m) and crystallinity (chi(c)) of the copolymer. While an increase in the stretching temperature does not have a profound impact on the amount of ferroelectric phase, the stability of the ferroelectric phase seems to improve - as seen in the reduction of the Full Width at Half Maximum (FWHM) of the WAXD peaks in both parallel and perpendicular directions to the molecular chain axis. The observation is also supported by the reduction of dissipation losses with an increase in stretching temperature - as seen in DRS measurements. Finally, both stretching itself and the temperature of stretching affect the various molecular processes taking place in the temperature range of the T-mid transition.
Dielectric Relaxation Spectroscopy (DRS) and Thermally Stimulated Depolarization Current (TSDC) measurements were employed to study dielectric-relaxation processes, structural transitions and electric-polarization phenomena in poly(vinylidenefluoride-trifluoroethylene-chlorofluoroethylene) (P(VDF-TrFE-CFE)) terpolymer films. Results from DRS confirm the existence of two separate dispersion regions related to a para-to-ferroelectric phase transition and to the glass transition. The dipolar TSDC peak correlates with the loss peak of the ? relaxation that represents the glass transition. The electric polarization calculated from the dipolar TSDC peak (glass transition) shows a non-linear electric-field dependence and saturates at high electric poling fields. As the observed behaviour is essentially the same as that of the electric polarization obtained from direct polarization-versus-electric-field hysteresis measurements, TSDC experiments are also suitable for studying the polarization in relaxor-ferroelectric polymers. A saturation polarization of 44 mC m(?2) was found for an electric field of 190 MV m(?1).
Dielectric relaxation processes and structural transitions in Poly(vinylidenefluoride-trifluoroethylene-chlorofluoroethylene) (P(VDF-TrFE-CFE)) terpolymer films with two different monomer compositions were investigated in comparison with Poly(vinylidenefluoride-trifluoroethylene) (P(VDF-TrFE)) copolymer films as reference material. Differential Scanning Calorimetry was employed to assess annealing effects on phase transitions and crystalline structure, while relaxation processes were investigated by means of Dielectric Relaxation Spectroscopy, the results of which indicate the existence of two separate dispersion regions, denoted as processes A and B, respectively. Process A appears at a certain temperature independent of frequency, but is strongly influenced by the crystallisation temperature and the CFE content, while peak B shows typical features of a relaxation process and is less influenced by crystallisation temperature and CFE content. Furthermore, peak B is related to the glass transition which is more pronounced in the terpolymer than in P(VDF-TrFE). A closer analysis indicates that the addition of CFE and thermal annealing gradually shift the ferro-to-paraelectric transition in P(VDF-TrFE) to lower temperatures, while the phase transition is transformed more and more into a relaxation.
The existence of an intermediate transition between the glass and the Curie/melting temperatures in Poly(vinylidene fluoride) (PVDF) and some of its co- and ter-polymers has been reported by several authors. In spite (or because?) of various different explanations in the literature, the origins of the transition are still not clear. Here, we try to understand the extra transition in more detail and study it with thermal and dielectric methods on PVDF, on its co-polymers with trifluoroethylene (P(VDF-TrFE)) and tetrafluoroethylene (P(VDF-TFE)), and on its ter-polymer with trifluoroethylene and chlorofluoroethylene (P(VDF-TrFE-CFE). Based on interpretations from the literature and our experimental studies, we propose the new hypothesis that the intermediate transition should have several interrelated origins. Especially since the relevant range is not far above room temperature, better understanding and control of their properties may also have practical implications for the use of the respective polymer materials in devices.
The existence of an intermediate transition between the glass and the Curie/melting temperatures in Poly(vinylidene fluoride) (PVDF) and some of its co- and ter-polymers has been reported by several authors. In spite (or because?) of various different explanations in the literature, the origins of the transition are still not clear. Here, we try to understand the extra transition in more detail and study it with thermal and dielectric methods on PVDF, on its co-polymers with trifluoroethylene (P(VDF-TrFE)) and tetrafluoroethylene (P(VDF-TFE)), and on its ter-polymer with trifluoroethylene and chlorofluoroethylene (P(VDF-TrFE-CFE). Based on interpretations from the literature and our experimental studies, we propose the new hypothesis that the intermediate transition should have several interrelated origins. Especially since the relevant range is not far above room temperature, better understanding and control of their properties may also have practical implications for the use of the respective polymer materials in devices.
The influence of chemical composition and crystallisation conditions on the ferroelectric and paraelectric phases and the resulting morphology in Poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) (P(VDF-TrFE-CFE)) terpolymer films with 55.4/37.2/7.3 mol% or with 62.2/29.4/8.4 mol% of VDF/TrFE/CFE was studied. Poly(vinylidene fluoride trifluoroethylene) (P(VDF-TrFE)) with 75/25 mol% VDF/TrFE was employed as reference material. Fourier-Transform Infrared Spectroscopy (FTIR) was used to determine the fractions of the relevant terpolymer phases, and X-Ray Diffraction (XRD) was employed to assess the crystalline morphology. The FTIR results show an increase of the fraction of paraelectric phases after annealing. On the other hand, XRD results indicate a more stable paraelectric phase in the terpolymer with higher CFE content.
Cellular polypropylene (PP) ferroelectrets combine a large piezoelectricity with mechanical flexibility and elastic compliance. Their charging process represents a series of dielectric barrier discharges (DBDs) that generate a cold plasma with numerous active species and thus modify the inner polymer surfaces of the foam cells. Both the threshold for the onset of DBDs and the piezoelectricity of ferroelectrets are sensitive to repeated DBDs in the voids. It is found that the threshold voltage is approximately halved and the charging efficiency is clearly improved after only 10(3) DBD cycles. However, plasma modification of the inner surfaces from repeated DBDs deteriorates the chargeability of the voids, leading to a significant reduction of the piezoelectricity in ferroelectrets. After a significant waiting period, the chargeability of previously fatigued voids shows a partial recovery. The plasma modification is, however, detrimental to the stability of the deposited charges and thus also of the macroscopic dipoles and of the piezoelectricity. Fatigue from only 10(3) DBD cycles already results in significantly less stable piezoelectricity in cellular PP ferroelectrets. The fatigue rate as a function of the number of voltage cycles follows a stretched exponential. Fatigue from repeated DBDs can be avoided if most of the gas molecules inside the voids are removed via a suitable evacuation process.
Polarization and Hysteresis in Tubular-Channel Fluoroethylenepropylene-Copolymer Ferroelectrets
(2014)
Polarization-vs.-applied-voltage hysteresis curves are recorded on tubular-channel fluoroethylene-propylene (FEP) copolymer ferroelectrets by means of a modified Sawyer-Tower circuit. Dielectric barrier discharges (DBDs) inside the cavities are triggered when the applied voltage is sufficiently high. During the DBDs, the cavities become man-made macroscopic dipoles which build up an effective polarization in the ferroelectret. Therefore, a phenomenological hysteresis curve is observed. From the hysteresis loop, the remanent polarization and the coercive field can be determined. Furthermore, the polarization can be related to the respective piezoelectric coefficient of the ferroelectret. The proposed method is easy to implement and is useful for characterization, further development and optimization of ferro- or piezoelectrets.
The influence of the temperature in the gas-filled cavities on the charging process of ferroelectret film systems has been studied in hysteresis measurements. The threshold voltage and the effective polarization of the ferroelectrets were determined as functions of the charging temperature TP. With increasing TP, the threshold voltage for triggering dielectric barrier discharges in ferroelectrets decreases. Thus, increasing the temperature facilitates the charging of ferroelectrets. However, a lower threshold voltage reduces the attainable remanent polarization because back discharges occur at lower charge levels, as soon as the charging voltage is turned off. The results are discussed in view of Paschen's law for electrical breakdown, taking into account the respective gas temperature and a simplified model for ferroelectrets. Our results indicate that the thermal poling scheme widely used for conventional ferroelectrics is also useful for electrically charging ferroelectrets.
Ferroelectrets (sometimes also called piezoelectrets) are relatively new members of the family of piezo-, pyro-, and ferroelectric materials.1–5 As their name indicates, ferroelectrets are space-charge electrets that show ferroic behavior. They are non-uniform electret materials or materials systems with electrically charged internal cavities. As space-charge electrets, ferroelectrets usually do not contain any molecular dipoles. However, the cavities inside the material can be turned into macroscopic dipoles through a series of micro-plasma discharges at high electric fields, so-called dielectric barrier discharges (DBDs).6–8 The gas inside the cavities is ionized when the internal electric field exceeds the threshold for electrical breakdown, generating charges of both polarities.9 The positive and negative charges travel in opposite directions, and are eventually trapped at the internal top and bottom surfaces of the cavities, respectively. After charging, the cavities may be regarded as macroscopic dipoles that can be switched by reversing the applied voltage.
An electric-polarization-vs.-electric-field (P(E)) hysteresis is considered as an essential criterion for ferroelectricity. P(E)-hysteresis curves are usually characterized by the spontaneous polarization, the coercive field, and the remanent polarization. Recently, we have demonstrated P(E)-hysteresis loops on two different types of ferroelectrets, namely, cellular polypropylene ferroelectrets and tubular-channel fluoroethylene-polypropylene copolymer ferroelectrets.10,11 The P(E)-hysteresis loops not only prove the ferroic behavior of ferroelectrets, but also allow us to determine such parameters as the coercive field and the remanent polarization.
It is widely accepted that Paschen breakdown is the underlying mechanism for the inception of DBDs in ferroelectrets.12–14 On this basis, the charging behavior and the resulting piezoelectricity of ferroelectrets in different gases at various pressures have been studied.15–17 Paschen's law describes the conditions for electrical breakdown in a gas at a constant temperature (usually room temperature), and it needs to be modified for gas breakdown at other temperatures. The temperature stability of the piezoelectricity in ferroelectrets after charging at elevated temperatures was investigated by several researchers.18–21 Recently, a preliminary report about the effects of the charging temperature on the hysteresis loops in ferroelectrets has been presented.22
In this letter, the influence of the gas temperature on the charging of ferroelectret systems is investigated in more detail by means of quasi-ferroelectric hysteresis-loop measurements. Teflon™ fluoroethylenepropylene (FEP) copolymer samples with tubular channels were prepared via thermal lamination as described previously.23 To this end, two FEP films with a thickness of 50 μm each were laminated at 300 ° C
around a 100 μm thick polytetrafluoroethylene (PTFE) template (total area 35 mm × 45 mm) that contains parallel rectangular openings (area 1.5 mm × 40 mm each). After lamination, the template was removed, which results in an FEP film system with open tubular channels. The samples were metallized on both surfaces with aluminum electrodes of 20 mm diameter.
P(E)-hysteresis loops were obtained with a modified Sawyer–Tower (ST) circuit.10,11 A high-voltage (HV) capacitor C1 (3 nF) and a large standard capacitor Cm (1 μF) were connected in series with the sample. A bipolar sinusoidal voltage with a frequency of 10 mHz was applied from an HV power supply (FUG HCB 7-6500) controlled by an arbitrary-waveform generator (HP 33120a). The voltage Vout
on Cm is measured by means of an electrometer (HP 3458a), and the charge flowing through the circuit is determined as Q(t)=CmVout(t)
. The experiments were carried out at isothermal conditions in a Novocontrol® Quatro cryosystem.
With the modified ST circuit, Q–V loops have been measured on a tubular-channel FEP ferroelectret system at different temperatures. The sample capacitance of about 34.5 pF is determined by a linear fit of the initial part of the Q–V curve recorded at 20 °C
, where the voltage has been raised up from zero on a fresh sample. The hysteresis loops are obtained from the Q–V curves by subtracting the contribution that results from charging of the sample capacitance.10 Figure 1 shows the hysteresis loops of the sample at −100, 0, and +100 ° C, respectively. According to previous theoretical and experimental studies,24,25 the length of each of the horizontal sides of the parallelogram-like hysteresis loops is given by 2Vth where Vth is the threshold voltage. As the charging temperature decreases, the hysteresis loop becomes wider and less high, i.e., the threshold voltage increases, while the polarization at maximum voltage decreases.
Spectroscopic study of dielectric barrier discharges in cellular polypropylene ferroelectrets
(2007)
The transient light emission from the dielectric barrier discharges (DBDs) in cellular polypropylene ferroelectrets subjected to high electric poling fields was spectroscopically measured. The spectrum shows strong emission from the second positive system of molecular nitrogen, N-2(C (3)Pi(u))-> N-2(B (3)Pi(g)), and the first negative system of N-2(+), N-2(+)(B (2)Sigma(+)(u))-> N-2(+)(X (2)Sigma(+)(g)), consistent with a DBD in air. When a dc voltage is applied stepwise to the ferroelectret film, light emission starts above a threshold, coinciding with the threshold voltage in obtaining piezoelectricity. From selected vibronic band strength ratios, the electric field in the discharge was determined and found to agree with Townsend breakdown.
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.
Polymer foams are in industrial use for several decades. More recently, non-polar polymer foams were found to be piezoelectric (so-called piezoelectrets) after internal electrical charging of the cavities. So far, few studies have been carried out on the electrical-insulation properties of polymer foams. Here, we compare the piezoelectric and the DC-voltage electrical-insulation properties of cellular polypropylene (PP) foams. Their cavity microstructure can be adjusted via inflation in high-pressure nitrogen gas in combination with a subsequent thermal treatment. While inflation is effective for improving the piezoelectricity, it is detrimental for the electrical-insulation properties. The original cellular PP foam shows a breakdown strength of approximately 230 MV/m, within the same range as that of solid PP. The breakdown strength decreases with increasing degree of inflation, and the dependence on the foam thickness follows an inverse power law with an exponent of 1.2. Nevertheless, up to a thickness of 140 mu m (3.5 times the original thickness), the breakdown strength of cellular-foam PP films is at least 7 times that of an air gap with the same thickness. In addition, the influence of high temperatures and high humidities on the piezoelectricity and the breakdown strength of cellular PP was studied. It was found that the piezoelectric d(33) coefficient decays rapidly already at 70 degrees C, while the breakdown strength slightly increases during storage at 70 or 90 degrees C. Under a relative humidity of 95%, the breakdown strength increases with storage time, while the piezoelectric d(33) coefficient slightly decreases.
Polymer foams and void-containing polymer-film systems with internally charged voids combine large piezoelectricity with mechanical flexibility and elastic compliance. This new class of soft materials (often called ferro-or piezoelectrets) has attracted considerable attention from science and industry. It has been found that the voids can be internally charged by means of dielectric barrier discharges (DBDs) under high electric fields. The charged voids can be considered as man-made macroscopic dipoles. Depending on the ferroelectret structure and the pressure of the internal gas, the voids may be highly compressible. Consequently, very large dipole-moment changes can be induced by mechanical or electrical stresses, leading to large piezoelectricity. DBD charging of the voids is a critical process for rendering polymer foams piezoelectric. Thus a comprehensive exploration of DBD charging is essential for the understanding and the optimization of piezoelectricity in ferroelectrets. Recent studies show that DBDs in the voids are triggered when the internal electric field reaches a threshold value according to Townsend's model of Paschen breakdown. During the DBDs, charges of opposite polarity are generated and trapped at the top and bottom internal surfaces of the gas-filled voids, respectively. The deposited charges induce an electric field opposite to the externally applied one and thus extinguish the DBDs. Back discharges may eventually be triggered when the external voltage is reduced or turned off. In order to optimize the efficiency of DBD charging, the geometry (in particular the height) of the voids, the type of gas and its pressure inside the voids are essential factors to be considered and to be optimized. In addition, the influence of the plasma treatment on the internal void surfaces during the DBDs should be taken into consideration.
Here, piezoelectric transducers consisting of a P(VDF-TrFE) layer with either silver or PEDOT:PSS screen-printed electrodes are studied. The influence of electrodes on the dielectric and electroacoustic properties are studied in dielectric-spectroscopy and ferroelectric-hysteresis measurements. Only when both the bottom and the top electrodes are made of silver, the typical dielectric relaxation of the P(VDF-TrFE) layer is clearly observed. When one or two of the electrodes are of PEDOT:PSS, a Debye-like relaxation is present. Compared with silver electrodes, PEDOT:PSS electrodes allow for moderate self-healing. Consequently, samples with bottom and top PEDOT:PSS electrodes can be poled to saturation, while samples with silver electrodes can hardly be poled to saturation due to destructive electric breakdown. Acoustic transducer measurements show that silver electrodes facilitate higher and broader frequency operation, while PEDOT:PSS electrodes bring slightly lower total harmonic distortion. Overall, the acoustic performance shows no significant deviations between differently electroded samples so that silver electrodes do not offer any advantages for the transducers studied here due to their much higher tendency for destructive electric breakdown.
Cellular polypropylene (PP) films were treated with sulfur hexafluoride (SF6) gas in order to study the SF6 penetration behaviour and optimize the electric charging conditions. There were differences in the penetration of SF6 for different cellular PP materials, depending on the microscopic properties, which manifest themselves in the voided structure as well as in the mechanical stiffnesses of the cellular films. The penetration of SF6 after long-term pressure treatment is confirmed in strongly inflated cellular PP films with a low mechanical stiffness of about 1 MPa. No SF6 penetration occurs for slightly inflated cellular PP films with smaller void sizes and higher mechanical stiffnesses of around 5.8 MPa. The observed thickness variations, the higher charging fields during corona charging because of SF6 penetration and the SF6 environment, as well as the resulting electromechanical properties are discussed
In this paper, two non-destructive thermal methods are used in order to determine, with a high degree of accuracy, three-dimensional polarization distributions in thin films (12 mu m) of poly(vinylidenefluoride- trifluoroethylene) (PVDF-TrFE). The techniques are the frequency-domain Focused Laser Intensity Modulation Method (FLIMM) and time-domain Thermal-Pulse Tomography (TPT). Samples were first metalized with grid-shaped electrode and poled. 3D polarization mapping yielded profiles which reproduce the electrode-grid shape. The polarization is not uniform across the sample thickness. Significant polarization values are found only at depths beyond 0.5 mu m from the sample surface. Both methods provide similar results, TPT method being faster, whereas the FLIMM technique has a better lateral resolution.
In the present study, the charge distribution and the charge transport across the thickness of 2- and 3-dimensional polymer nanodielectrics was investigated. Chemically surface-treated polypropylene (PP) films and low-density polyethylene nanocomposite films with 3 wt % of magnesium oxide (LDPE/MgO) served as examples of 2-D and 3-D nanodielectrics, respectively. Surface charges were deposited onto the non-metallized surfaces of the one-side metallized polymer films and found to broaden and to thus enter the bulk of the films upon thermal stimulation at suitable elevated temperatures. The resulting space-charge profiles in the thickness direction were probed by means of Piezoelectrically-generated Pressure Steps (PPSs). It was observed that the chemical surface treatment of PP which led to the formation of nano-structures or the use of bulk nanoparticles from LDPE/MgO nanocomposites enhance charge trapping on or in the respective polymer films and also reduce charge transport inside the respective samples.
Published results on LDPE/MgO nanocomposites (3wt%) show that they promise to be good electrical-insulation materials. In this work, the nanocomposites are examined as a potential (ferro-)electret material as well. Isothermal surface-potential decay measurements show that charged LDPE/MgO films still exhibit significant surface potentials after heating for 4 hours at 80 degrees C, which suggests good capabilities of LDPE/MgO nanocomposites to hold electric charges of both polarities. Open-tubular-channel ferroelectrets prepared from LDPE/MgO nanocomposite films show significant piezoelectricity with d(33) coefficients of about 20 pC/N after charging and are stable up to temperatures of at least 80 degrees C. Thus LDPE/MgO nanocomposites may become available as a new ferroelectret material. To increase their d(33) coefficients, it is desirable to optimize the charging conditions and the ferroelectret structure.
Charging properties and time-temperature stability of innovative polymeric cellular ferroelectrets
(2007)
After appropriate mechanical and electrical treatments, some cellular polymers become able to retain space charge for a long time, i.e. they acquire electret behavior. The electrical treatment consists of charging under high levels of DC electric field. The mechanical treatment, based on the application of stretching forces to cellular polymer slabs that were before expanded under pressurized gas, affects the cavity size and shape, and therefore also the effectiveness of the charging process itself. An investigation of charging mechanisms, as well as of mechanical treatment, is therefore fundamental for optimizing the ferro- and piezo-electret properties. The aim of this paper is to discuss the effect of the physical dimension of the cavities on the charging behavior of cellular ferroelectrets and to focus on the time-temperature stability for two families of polymeric cellular ferroelectrets based on polypropylene (PP) and on a cyclo-olefin copolymer (COC). Emphasis will be given to the stretching process and in particular to the expansion rate applied during the manufacturing process (which affects the radial dimension and the height of the cavities, respectively). Space-charge and partial-discharge measurements as a function of time and temperature are the main tools to infer the influence of the cavity size on charging and stability characteristics.
Ferroelectrets are thin films of polymer foams, exhibiting piezoelectric properties after electrical charging. Ferroelectret foams usually consist of a cellular polymer structure filled with air. Polymer-air composites are elastically soft due to their high air content as well as due to the size and shape of the polymer walls. Their elastically soft composite structure is one essential key for the working principle of ferroelectrets, besides the permanent trapping of electric charges inside the polymer voids. The elastic properties allow large deformations of the electrically charged voids. However, the composite structure can also possibly limit the stability and consequently the range of applications because of, e. g., penetration of gas and liquids accompanied by discharge phenomena or because of a mechanical pre-load which may be required during the application. Here, we discuss various stability aspects related to the piezoelectric properties of polypropylene ferroelectrets. Near and below room temperature, the piezoelectric effect and the stability of the trapped charges are practically independent from humidity during long-time storage in a humid atmosphere or water, or from operating conditions, such as continuous mechanical excitation. Thermal treatment of cellular polypropylene above -10 degrees C leads to a softening of the voided structure which is apparent from the decreasing values of the elastic modulus. This decrease results in an increase of the piezoelectric activity. Heating above 60 degrees C, however, leads to a decrease in piezoelectricity
High-resolution, large-area three-dimensional mapping of polarization profiles in electret polymers was carried out by means of a fast thermal pulse technique with a focused laser beam. A lateral resolution of 38 mu m and a near- surface depth resolution of less than 0.5 mu m was achieved. At larger depths, fast thermal diffusion in the metal electrode rather than the laser spot size becomes the limiting factor for the lateral resolution. (c) 2005 American Institute of Physics
Space-charge depth profiles in various electret polymers have been measured in both the time and the frequency domain using thermal pulses and waves, respectively. A comparison of the two techniques on corona-charged polytetrafluoroethylene showed that the thermal-pulse method yielded similar results as the thermal-wave technique, but approximately 20-50 times faster. The article discusses sensitivity limitations as well as possible applications, including the real-time monitoring of space-charge decay under UV irradiation. (C) 2005 American Institute of Physics
Dielectric spectra of a partially fluorinated chromophore / amorphous Teflon AF guest-host system
(1999)
Zerstörungsfreie Tomographie von Raumladungs- und Polarisationsverteilungen mittles Wärmepulsen
(2007)
Non-destructive, three-dimensional imaging of space-charge and polarization distributions in electret materials has been implemented by means of laser-induced thermal pulses. In pyroelectric films of poled poly(vinylidene fluoride), images of up to 45 x 45 pixels with a depth resolution of less than 0.5 mu m and a lateral resolution of 40 mu m were recorded, the latter being limited by fast thermal diffusion in the absorbing metallic front electrode. Initial applications include the analysis of polarization distributions in corona-poled piezoelectric sensor cables and the detection of patterned space-charge distributions in polytetrafluoroethylene films.
The stability of space charge in electrets such as polytetrafluoroethylene (PTFE), polyethylene terephthalate (PETP) and polypropylene (PP) under ultraviolet irradiation has been investigated using photostimulated discharge spectroscopy. While only weak discharge currents were observed in PTFE coated with semitransparent gold electrodes, up to 15 pA/cm(2) were found in PETP around the UV absorption edge near 310 nm. Space charge profiles obtained with the piezoelectrically generated pressure step method indicate that near-surface charges were almost completely removed. In PP foam, recent findings of a UV-reduced d(33) coefficient were confirmed for exposure times of up to 3.5 h, and a discharge peak at 200 urn could be assigned to the charges stored on the surfaces of the voids. The unique morphology and the (quasi-) piezoelectric properties of cellular PP make it a role model for the future investigation of charge storage in electrets
A recently reported novel class of elastomers was tested with respect to its dielectric properties. The new elastomer material is based on a commercially available poly(dimethylsiloxane) composition, which has been modified by embedding glycerol droplets into its matrix. The approach has two major advantages that make the material useful in a dielectric actuator. First, the glycerol droplets efficiently enhance the dielectric constant, which can reach astonishingly high values in the composite. Second, the liquid filler also acts as a softener that effectively decreases the elastic modulus of the composite. In combination with very low cost and easy preparation, the two property enhancements lead to an extremely attractive dielectric elastomer material. Experimental permittivity data are compared to various theoretical models that predict relative permittivity changes as a function of filler loading, and the applicability of the models is discussed. (c) 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 44153.
Nowadays, structural health monitoring of critical infrastructures is considered as of primal importance especially for managing transport infrastructure however most current SHM methodologies are based on point-sensors that show various limitations relating to their spatial positioning capabilities, cost of development and measurement range. This publication describes the progress in the SENSKIN EC co-funded research project that is developing a dielectric-elastomer sensor, formed from a large highly extensible capacitance sensing membrane and is supported by an advanced micro-electronic circuitry, for monitoring transport infrastructure bridges. The sensor under development provides spatial measurements of strain in excess of 10%, while the sensing system is being designed to be easy to install, require low power in operation concepts, require simple signal processing, and have the ability to self-monitor and report. An appropriate wireless sensor network is also being designed and developed supported by local gateways for the required data collection and exploitation. SENSKIN also develops a Decision-Support-System (DSS) for proactive condition-based structural interventions under normal operating conditions and reactive emergency intervention following an extreme event. The latter is supported by a life-cycle-costing (LCC) and life-cycle-assessment (LCA) module responsible for the total internal and external costs for the identified bridge rehabilitation, analysis of options, yielding figures for the assessment of the economic implications of the bridge rehabilitation work and the environmental impacts of the bridge rehabilitation options and of the associated secondary effects respectively. The overall monitoring system will be evaluated and benchmarked on actual bridges of Egnatia Highway (Greece) and Bosporus Bridge (Turkey).
Structural health monitoring activities are of primal importance for managing transport infrastructure, however most SHM methodologies are based on point-based sensors that have limitations in terms of their spatial positioning requirements, cost of development and measurement range. This paper describes the progress on the SENSKIN EC project whose objective is to develop a dielectric-elastomer and micro-electronics-based sensor, formed from a large highly extensible capacitance sensing membrane supported by advanced microelectronic circuitry, for monitoring transport infrastructure bridges. Such a sensor could provide spatial measurements of strain in excess of 10%. The actual sensor along with the data acquisition module, the communication module and power electronics are all integrated into a compact unit, the SENSKIN device, which is energy-efficient, requires simple signal processing and it is easy to install over various surface types. In terms of communication, SENSKIN devices interact with each other to form the SENSKIN system; a fully distributed and autonomous wireless sensor network that is able to self-monitor. SENSKIN system utilizes Delay-/Disruption-Tolerant Networking technologies to ensure that the strain measurements will be received by the base station even under extreme conditions where normal communications are disrupted. This paper describes the architecture of the SENSKIN system and the development and testing of the first SENSKIN prototype sensor, the data acquisition system, and the communication system.
Three poly(tetrafluoroethylene-hexafluoropropylene-vinylidenefluoride) (TFE-HFP-VDF or THV) terpolymers (Dyneon (R)) with different monomer ratios are investigated to demonstrate the concept of "modified" PTFE for space-charge electrets. HFP and VDF monomers distort the highly ordered PTFE molecules, which effectively enhances processability and adversely affects space-charge storage. Particularly, VDF component renders the material polar and probably also more conductive, partially undermining the space-charge-storage capabilities of PTFE. Nevertheless, the terpolymer THV815 with a TFE/HFP/VDF wt% ratio of 76.1/10.9/13 combines easy processability and relatively good space-charge stability. Our results shed light on novel concepts for space-charge electret materials with enhanced processing properties and reasonable charge-storage capabilities.
Cork is a natural cellular and electrically insulating material which may have the capacity to store electric charges on or in its cell walls. Since natural cork has many voids, it is difficult to obtain uniform samples with the required dimensions. Therefore, a more uniform material, namely commercial cork agglomerate, usually used for floor and wall coverings, is employed in the present study. Since we know from our previous work that the electrical properties of cork are drastically affected by absorbed and adsorbed water, samples were protected by means of different polymer coatings (applied by spin-coating or soaking). Corona charging and isothermal charging and discharging currents were used to study the electrical trapping and detrapping capabilities of the samples. A comparison of the results leads to the conclusion that the most promising method for storing electric charges in this cellular material consists of drying and coating or soaking with a hydrophobic, electrically insulating polymer such as polytetraflouroethylene (Teflon (R)). (c) 2007 Elsevier B.V. All rights reserved.