Michael Wegener

Understanding the role of the gas in the voids during corona charging of cellular electret films - a way to enhance their piezoelectricity

The influence of the corona-charging process on the piezoelectric transducer coefficient d33 of a cellular electret film has been investigated. An increased corona voltage can be considered as a way to enhance the charge density and thus also the resulting piezoelectric effect. Higher corona-charging voltages are possible with increased ambient pressure or in suitable dielectric gases. The effect of the gas inside the voids has also been studied. Enhanced transducer coefficients were obtained by corona charging in N2 or N2O gas atmospheres at 100-450 or 100-140 kPa pressures, respectively. The highest transducer coefficients of about 790 pCN-1 were obtained when N2 gas was filled into the voids of a cellular polymer film by means of consecutive vacuum and high-pressure treatments at 295 or 313 K.

Controlled inflation of voids in cellular polymer ferroelectrets : optimizing electromechanical transducer properties

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.

Barrier discharges in cellular polypropylene ferroelectrets : How do they influence the electromechanical properties?

Ferroelectrets (i.e., charged cellular polymers) are rendered piezoelectric by means of barrier discharges inside the air-filled voids. The light emission from barrier discharges in cellular polypropylene ferroelectrets was quantitatively studied. Light emission typically occurs above a threshold voltage of 3 kV and then significantly increases with the applied voltage. Time-resolved images reveal discharge processes in individual voids. In addition, a second “back discharge” emission is observed when the voltage is reduced to zero. The buildup of the “effective polarization” in cellular PP ferroelectrets was studied by an acoustic method and dielectric resonance spectroscopy. A polarization-voltage (P-V) hysteresis loop was obtained by analyzing the data with an existing model for the piezoelectric d33 coefficient of ferroelectrets, from which a threshold charging voltage of 3 kV and the back barrier discharges were confirmed and a zero-field “effective polarization” of 0.5 mC/m2 was determined. However,...

Two-step inflation of cellular polypropylene films: void-thickness increase and enhanced electromechanical properties

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 separate 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.

Novel heat durable electromechanical film: processing for electromechanical and electret applications

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 110degC. 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

Thermally stable dynamic piezoelectricity in sandwich films of porous and nonporous amorphous fluoropolymer

Porous amorphous fluoropolymer films of very low stiffness were produced by a solvent evaporation technique. Corona-charged sandwich films consisting of a porous and a nonporous layer exhibit piezoelectric thickness-extension resonances in their dielectric spectrum, through which the temperature dependence of their dynamic stiffness, coupling factor, and piezoelectric coefficient could be determined. Their strong piezoelectricity with coefficients of up to 600 pC/N at temperatures of at least 120 °C could make these polymer electret films interesting candidates for sensor and actuator applications in elevated temperature environments.

Thermal and Temporal Stability of Ferroelectret Films Made from Cellular Polypropylene/Air Composites

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°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°C, however, leads to a decrease in piezoelectricity.

Corona-induced partial discharges, internal charge separation and electromechanical transducer properties in cellular polymer films

Recently, porous or cellular polymer films have received a lot of interest as materials for sensor and actuator applications. Films of cellular polypropylene have shown superior electromechanical thickness response when compared to conventional non-porous piezoelectric polymer materials. The electromechanical effect in cellular films originates from electrical discharges across internal gas-filled cells during preparation. The resulting charge separation is different from the charge-separation and dipole-orientation processes in non-cellular piezoelectric films. The aim of this work is to study the relationship between the partial discharges inside the cellular PP and its piezoelectric response. The focus is on the physical effects in the film at the threshold voltage that leads to a piezoelectric response. We studied the effects of charging voltage and charging time on partial discharges, surface potential and piezoelectric coefficient for a few cellular-film grades with different thicknesses. Also, the visible electroluminescence from numerous spots across the film surface during corona charging has been studied.

Piezoelectric PZT / PVDF-copolymer 0-3 composites: aspects on film preparation and electrical poling

Composite films of lead zirconate titanate (PZT) and different (non-polar and polar) polyvinylidene fluoride (PVDF) copolymers are prepared as 30 to 150 μm thick freestanding, relatively flexible films. For low ceramic-volume fractions the ceramic fillers are homogeneous distributed within the polymer matrix as indicated by scanning electron microscopy studies. Ceramic-volume fractions higher than approximately 0.5 lead to porous composite films which became brittle. The brittle films are difficult to polarize and not suitable as piezoelectric transducers. The permittivities of non-porous composite films follow the Bruggeman model for dielectric mixtures. Different procedures are presented and verified in order to polarize the ferroelectric PZT particles and the ferroelectric polymer matrix. In detail, the overall polarization is discussed by taking into account the polarities of the applied poling voltage and of the measured piezoelectric signals. In summary, for composites with high ceramic-volume fractions piezoelectric coefficients of up to 8.6 pC/N and 22.1 pC/N (for PZT / P(VDFTrFE) composites) and up to 11.3 pC/N and 24.8 pC/N (for PZT / P(VDF-HFP) composites) are reached after short-term, room-temperature and long-term, high-temperature poling, respectively.

Penetration of sulfur hexafluoride into cellular polypropylene films and its effect on the electric charging and electromechanical response of ferroelectrets

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.