Zhongfu Xia

High surface-charge stability of porous polytetrafluoroethylene electret films at room and elevated temperatures

Porous polytetrafluoroethylene films were positively or negatively corona-charged at room or elevated temperatures and their charge-storage behaviour was investigated by means of isothermal surface-potential and thermally stimulated discharge-current measurements. In addition, electron micrographs of the sample morphology were taken and the influence of high humidities on the surface-charge decay was investigated. For comparison, nominally non-porous polytetrafluoroethylene films were studied in the same manner. It was found that porosity may lead to significantly enhanced surface-charge stability for both polarities if the relative humidity is not too high. Further investigations are under way in order to better understand this behaviour and to employ it for electret applications.

Porous PTFE space-charge electrets for piezoelectric applications

Porous polytetrafluoroethylene (PTFE) films were positively or negatively corona charged at room or elevated temperatures. Their charge storage behavior was investigated by means of isothermal surface potential measurements in direct comparison to nominally nonporous samples of the same polymer. It was found that porosity may lead to significantly enhanced surface-charge stability for both polarities. Direct piezoelectricity was studied on quadruple, double, and single layer samples by means of quasi-static measurements. For the determination of indirect piezoelectricity, frequency-dependent acoustical-transducer experiments were carried out. Both applications-relevant measurements yielded piezoelectric d/sub 33/ coefficients of up to approximately 600 pC/N or 600 pm/V. These values are more than one order of magnitude higher than in conventional piezoelectric polymers such as polyvinylidenefluoride (PVDF) and almost comparable to the highest known values of inorganic piezoelectrics. Consequently, the novel piezoelectric porous-fluoropolymer spacecharge electrets exhibit an outstanding potential for various device applications that are very briefly discussed.

Piezoelectrets from laminated sandwiches of porous polytetrafluoroethylene films and nonporous fluoroethylenepropylene films

In this paper, the piezoelectric properties of laminated films made of polytetrafluoroethylene (PTFE) and tetrafluoroethylene-hexafluoropropylene (FEP) copolymer by an improved process and charged by a corona method are investigated by measurements of the pressure dependence of the piezoelectric d33 coefficents, the isothermal decay of d33 at various temperatures, and thermally stimulated discharge current spectra. The results show that the structure of the laminated films is mechanically stable. The quasistatic piezoelectric d33 coefficents can reach 400pC∕N and they are relatively independent of the static pressure in the range up to 16kPa. The decay of the d33 coefficients is primarily due to charge detrapping. Compared to polypropylene ferroelectrets, the thermal stability of the piezoelectric activity in such laminated films at 90°C is improved by a factor of 2 with respect to the percentage of the d33 values remaining. The dominant drift path of the detrapped charges at temperatures of about 130°C i...

Fabrication of fluoropolymer piezoelectrets by using rigid template: Structure and thermal stability

Fluorocarbon films with regular void structure, made of compact fluoroethylenepropylene (FEP), or skived polytetrafluoroethylene (PTFE), and patterned porous PTFE layers, are successfully fabricated by using a rigid template and fusion bonding process. A corona charging technique is used to make the films piezoelectric, i.e., to be piezoelectrets. The results show that the typical Young’s moduli of the films are in the range of 0.45–0.80 MPa. A maximum quasistatic piezoelectric d33 coefficient up to 500 pC/N is achieved. Compared to the laminated FEP/porous PTFE piezoelectrets without regular void structure, the presently fabricated films show significantly improved thermal stability. Furthermore, when the films are designed, fabricated, and corona charged such that positive charges are deposited in the porous PTFE layers, the thermal stability of d33 coefficients can be further improved. For example, after annealing at 90 °C for 4500 min, these samples have a remaining d33 value of 86% compared with 77% ...

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.

Charge storage and its dynamics in porous polytetrafluoroethylene (PTFE) film electrets

The outstanding space charge storage stability of porous polytetrafluoroethylene (PTFE) film electrets is studied by isothermal surface potential decay measurements and open-circuit thermally stimulated discharge (TSD) experiments after corona charging at room and elevated temperatures, or corona charging at RT and then aging at different temperatures. Charge storage properties of porous PTFE, nonporous PTFE (Teflon/spl reg/ PTFE) and nonporous FEP (Teflon/spl reg/ FEP) electrets are compared. The results show that porous PTFE has the best charge storage stability of organic materials for both negative and positive charges, especially at high temperatures. The structure of porous PTFE, investigated by a scanning electron microscope (SEM), is important for understanding the electret properties of this material. Charge dynamics, including the influence of environmental humidity and temperature on charge stability and shift of mean charge depth, and the kinetics of detrapped charges for the porous PTFE film electrets were also investigated by means of isothermal surface potential decay measurements and analysis of the TSD current spectra in combination with the heat pulse technique. It is found that from about RT to 200/spl deg/C slow retrapping plays a dominant role; from about 200/spl deg/C to 300/spl deg/C fast retrapping controls the transport.

Piezoelectricity of cellular and porous polymer electrets

A number of experiments were conducted to further investigate the piezoelectric properties of cellular and porous electrets. The samples studied were films of cellular polypropylene (PP) and of porous polytetrafluoroethylene (PTFE), with thicknesses between 50 and 100 /spl mu/m. In some cases, multilayers consisting of one of these polymers plus an additional air or solid layer were also investigated. Results show that single layers of cellular PP, metalized on both sides, have piezoelectric d/sub 33/-constants of up to 350 pC/N while such samples of porous PTFE have much lower constants. Multilayers consisting of one cellular or porous layer plus an air layer exhibit significantly larger constants, exceeding 20,000 pC/N, due to the softness of the air. While the d/sub 33/-constant of the multilayer systems decreases with load due to the compression of the softer layer, cellular PP shows fairly constant d/sub 33/ values for static pressures of up to 10 kPa thus indicating a rather large linear regime of the nonlinear stress-strain relationship. In this static pressure range a linear frequency response of the cellular foils was found if the foils were unstretched or under constant stress during the measurement. Stretching of the cellular PP foils results in a significant increase of the d/sub 33/-constants and thus represents an optimization method for the piezoelectric activity.

Influence of fluorination on piezoelectric properties of cellular polypropylene ferroelectrets

Direct fluorination using fluorine gas, as one of the most effective approaches to the chemical modification of polymers, was used to improve the thermal stability of piezoelectricity of the polypropylene (PP) ferroelectrets. High fluorination degree was obtained as indicated by attenuated total reflection infrared spectroscopy. The results of the isothermal decay of the piezoelectric d33-coefficient at 70 ◦ C indicated the improved thermal stability of piezoelectricity and the enhanced piezoelectric activity of the fluorinated PP ferroelectrets. The improved thermal stability of piezoelectricity is attributed to the improvement in thermal stability of the charges on the internal void surfaces as indicated by the thermally stimulated discharge measurements, while the enhanced piezoelectric activity is ascribed not only to the improved thermal stability of the charges but also to the reduction in Young’s modulus of the PP ferroelectrets due to fluorination as revealed by the dielectric resonance analyses.

Piezoelectric properties and charge dynamics in poly(vinylidene fluoride-hexafluoropropylene) copolymer films with different content of HEP

In this paper, the electret properties of three types of castings prepared P(VDF-HFP) (poly(vinylidene fluoride-hexafluoropropylene)) copolymer films whose contents of HFP are 4.2, 4.7 and 5.9% are discussed. Experimental results point out that the copolymer film is a polar material, i.e. there exists both oriented dipoles and deposited space charge in the film after charging. Estimating polarizations at different charging fields based on analysis of charging current curves indicate a comparatively high polarization formed under the charging field above 200 MVm-1 for the copolymer film. The mechanically thermal stretching process not only increases the density of the oriented dipoles and deposited space charge but also improves the thermal stability of deposited space charge. However, the increase of HFP content in the copolymer has a negative impact on the density of oriented dipoles and deposited space charge. The measurements by means of laser doppler vibrometer show that the copolymer in which the HFP content is 4.2% and 4.7%, respectively has a notable converse piezoelectric coefficient as high as 140 pmV-1 and 130 pmV -1 . Dynamics of interaction of space charge with oriented dipoles for the copolymer with HFP content of 4.7% are investigated by means of isothermal surface potential decay at different temperatures. At the same time, pyroelectric effect of the copolymer is also discussed

Piezoelectricity and dynamic characteristics of laminated fluorocarbon films

The piezoelectrets made of porous polytetrafluoroethylene (PTFE) and nonporous fluoroethylenepropylene (FEP) layers are prepared by using a hot-pressing method. The dependence of the quasi-static piezoelectric d33-coefficients of such films on the grid voltage during the corona charging is investigated. The thermal stability of d33-coefficients for the films is characterized by the isothermal method. The Youngs Modulus and dynamic d33-coefficient are obtained by analyzing the dielectric resonance spectra of the films. The results show that the Youngs modulus is around 2.4 MPa and the quasi-static piezoelectric d33-coefficient is about 300 pC/N for the laminated PTFE/FEP films. The d33 value retains 40% of the initial value when the sample was annealed at 90°C for 20 h. For the samples pre-aged at the temperature of 120°C for 5 h, the remained d33 value is improved to 75% of the initial value in the same conditions. The d33 value determined by dielectric resonance spectra is smaller than the quasi-static d33 value, which is properly due to the enhanced Youngs modulus with the increase of frequency.