Laminated polytetrafluoroethylene (PTFE) films, which are made of compact and porous PTFE layers, are prepared through the process of sintering. The corona charging technique is utilized to make the laminated PTFE films piezoelectric, thus transforming them into piezoelectrets. The crystallinity, Young's modulus in compression mode, stress-strain behavior, charge dynamics, and thermal stability of the fabricated films are investigated by particular techniques, such as differential scanning calorimetry (DSC), dielectric resonance spectra, dynamic mechanical analysis (DMA), thermally stimulated discharge (TSD) spectra, and isothermal annealing, respectively. The results reveal that the crystallinity of the fabricated PTFE films with three and five-layer systems are 79.5 and 59.8 %, respectively. The compressive and tensile moduli at room temperature are 7.4 and 167 MPa for the three-layer system samples. The resulting temperature peak increases by 20°C as the heating rate increases from 2 to 4°C in TSD measurement. Two charge drift mechanisms exist in the films when the samples are thermally stimulated and discharged. With the increase of corona charging voltage from -10 to -25 kV, more and more detrapped charges from the deeper traps in the laminated PTFE films are released, corresponding to the current peaks identified in the temperature range from 130 to 140°C, which prefer to drift through the solid PTFE layers. However, charges also escaped from the relevant shallow traps, corresponding to the current peaks identified in the temperature that range from 80 to 95°C. The charge drift along the surface of the PTFE fibers is always a dominant mechanism, showing resistance of the corona charging voltage under the experimental-study conditions. The sample shows a stable piezoelectric d 33 coefficient of 50 pC/N at 120°C after one day annealing at the same temperature.

The laminated fluoroethylenepropylene (FEP) and porous polytetrafluoroethylene (PTFE) films with regular void structure are prepared by using a rigid template with a periodic-structured surface. The porous PTFE film is firstly patterned with the rigid template surface by applying force on the stack of porous PTFE film and the template, then followed by the fusion bonding process to bond the FEP and patterned porous PTFE together. The corona charging technique is used to make the laminated film piezoelectric, i.e., to become piezoelectrets. The Young’s modulus of the laminated FEP/PTFE films is determined by dielectric resonance spectra. The thermal stability of the piezoelectric d33 coefficients are characterized by measuring the decay of d33 at elevated temperatures. The charge dynamics in such FEP/PTFE piezoelectrets is investigated by analying the thermally stimulated discharge current spectra in short circuit. The results show that laminated FEP/PTFE films with very regular void structure can be made by using rigid template and fusion bonding process. The Young's modulus of such films is about 0.53 MPa. The maximum quasi-static piezoelectric d33 coefficient up to 500 pC/N is achieved. The laminated FEP/PTFE films show improved thermal stability. For example, the remnant d33value is around 22% of the initial value for the sample annealed at 150 ℃ for 5000 min. For the samples after annealing treatment, the main drift path of the detrapped charges is through the solid dielectric layer.

Polytetrafluoroethylene (PTFE) films with void structure are prepared by a fusion bonding process. Such void PTFE films are piezoelectric after proper corona charging. The maximum quasi-static piezoelectric d 33 coefficients of 220 pC/N are achieved. The applied pressure dependence of piezoelectric d 33 coefficients for the void films is associated with the structure of the films. The piezoelectric response of the films is dependent on the duration of the applied force, which is apparently due to the creep of the material.

A hard template process for preparing piezoelectret films with ordered void structure was described. By using the fabrication process, fluorocarbon polymer piezoelectret film was prepared. The scanning electron microscope (SEM) images showed that the fluorocarbon polymer film piezoelectret has very regular void structure as expected. The piezoelectricity of the fluorocarbon polymer film was investigated by measuring the quasi-static piezoelectric d33 coefficient using positive piezoelectric effect. The thermal stability of d33 was studied by taking the measurement of d33 decay at elevated isothermal temperatures. The results showed that a piezoelectric d33 coefficient of 300 pC/N was achieved. Compared with the fluorocarbon polymer film with disordered void structure, the film with ordered void structure has much improved thermal stability of d33, which can be further improved by pre-ageing treatment. The piezoelectric d33 coefficient is dependent on the applied pressure considerably in the range of 2—35 kPa, which is probably associated with the enhancement of Young’s modulus of the film with the increasing pressure.

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 d(33)-coefficients of such films on the grid voltage during the corona charging is investigated. The thermal stability of d(33)-coefficients for the films is characterized by the isothermal method. The Young's Modulus and dynamic d(33)-coefficient are obtained by analyzing the dielectric resonance spectra of the films. The results show that the Young's modulus is around 2.4 MPa and the quasi-static piezoelectric d(33)-coefficient is about 300 pC/N for the laminated PTFE/FEP films. The d(33) value retains 40% of the initial value when the sample was annealed at 90 degrees C for 20 h. For the samples pre-aged at the temperature of 120 degrees C for 5 h, the remained d(33) value is improved to 75% of the initial value in the same conditions. The d(33) value determined by dielectric resonance spectra is smaller than the quasi-static d(33) value, which is properly due to the enhanced Young's modulus with the increase of frequency.

Piezoelectret film made by cross-linked polypropylene foam sheet was prepared by hot-stretching process.The microstructure of stretched films was observed by SEM technique.The piezoelectric d33-coefficients of the samples,with various degrees of elongation,were determined by a quasi-static method.The applied pressure dependence of piezoelectric d33-coefficients was investigated.The influence of surface structure on piezoelectric activity was discussed also.The results show that the piezoelectricity begins at the degree of elongation 70%.The piezoelectric d33-coefficients are enhanced with the increases of elongation degree.The d33 value of 35 pC/N is obtained for the sample stretched to elongation degree of 150%.All the samples show good linearity in the range of applied pressure up to 30 kPa.Improved piezoelectric activity is obtained for the stretched XPP films when the rough surfaces were exposed to corona.

Preparation of piezoelectret films with regular void structure by using a rigid template method is described in this paper. Laminated piezoelectret films with regular void structure are prepared by using porous polytetrafluoroethylene (PTFE) and fluoroethylenepropylene (FEP). The Young's modulus of the laminated regular structure films is determined by dielectric resonance spectra. The quasi-static piezoelectric coefficient d33 is measured by direct piezoelectric effect. The thermal stability of the piezoelectric d33 coefficients is characterized by measuring the decay of d33 at elevated temperatures. The charge dynamics in such regular structure piezoelectrets is investigated by analyzing the thermally stimulated discharge current spectrum in short circuit. The results indicate that the Young's modulus of five-layer structure piezoelectret films is about 0.48 MPa. The quasi-static piezoelectric coefficient d33 is not only up to 500 pC/N, but also shows good thermal stability. The de-trap charges mainly migrate along the surface of holes and ultimately recombinate with opposite charges which deposited on the same hole's relative hole wall.

Preparation of piezoelectret films with ordered void structure by using a template method is described. The structure of films consists of two layers of non-porous fluoroethypropylene (FEP) and one layer of non-porous polytetrafluorothylene (PTFE). The piezoelectricity of such fluorocarbon polymer films is investigated by measuring the quasi-static piezoelectric d33 coefficients using positive piezoelectric effect. And the thermal stability of the films is studied by taking the measurement of d33 decay at elevated temperatures. The results show that the quasi-static piezoelectric d33 coefficients around 300 pC/N are achieved and its Young's modulus is about 0.28 MPa. Compared with the fluorocarbon polymer films with disordered void structure, the films with ordered structure shows a much improved thermal stability of d33. Pre-aging treatment can further improve the thermal stability of d33.

Piezoelectrets, based on the bipolarity space-charge electret, become a new class of artificial piezoelectric functional materials. Their piezoelectric performances are relative to the charge storage capability of the material directly. In this paper, the storage stability and dynamic properties of the charges in the cross-linked polypropylene (XPP) electret films, treated by a hot-pressing process, are investigated by using the measurements of isothermal decay of the surface potential and open-circuit thermally stimulated discharge (TSD) current spectra. The results show that the charge storage stability of the positively charged samples is better than that of the negatively charged samples. When the pre-ageing temperatures are less than 90degC, the surface potentials of positively charged samples still keep above 90% of the initial value after annealing for 5120 min; while the values for the negatively charged samples are below 86%. The results also indicate that most of detrapped charges during TSD are retrapped in deeper traps. Therefore charge transport is controlled by a fast retrapping effect.

Polytetrafluoroethylene (PTFE) films with a void structure are prepared by the sintering process. Such void PTFE films are piezoelectric after proper corona charging. The quasi-static piezoelectric d 33 coefficients up to 250 pC/N are obtained. Pre-ageing treatment is an effective method to further improve the thermal stability. For the samples with pre-ageing treatment, the the d 33 coefficients are very stable when exposed to 120degC. The values of d 33 are slightly applied pressure dependent in the range of 50 kPa.

Piezoelectrets made by cross-linked polypropylene (XPP) foam sheet are prepared by hot-stretching process. The microstructure of stretched films is observed by SEM technique. The piezoelectric d 33 -coefficients of the samples, with various degrees of elongation, are determined by a quasi-static method. The applied pressure dependence of piezoelectric d 33 -coefficients is investigated. The influence of surface structure on piezoelectric activity is discussed also. The results show that the piezoelectricity starts at the degree of elongation 70%. The piezoelectric d 33 -coefficients are enhanced with the increase of elongation degree. The d 33 value of 35pC/N is obtained for the sample stretched to elongation degree of 150%. All the samples show good linearity in the range of applied pressure up to 30 kPa. Improved piezoelectric activity was obtained for the stretched XPP films when the rough surfaces were exposed to corona.

Irradiation cross-linked polypropylene (IXPP) foams show high piezoelectric activity after proper hot-pressing treatment and corona charging. Quasi-static piezoelectric d33 coefficients around 400 pC/N were measured by means of the direct piezoelectric effect. Dynamic values of the inverse piezoelectric d33 coefficients, determined from the dielectric resonance spectra at 220 kHz, is about 68% of the quasi-static d33 values. The difference between the quasi-static and the dynamic values of d33 is probably due to the enhancement of Young’s modulus of IXPP with increasing frequency. The piezoelectric d33 coefficients are slightly dependent on the applied pressure in the range up to 50 kPa. The d33-values decrease by 70% when the samples are exposed to 90 °C for 1 day; and a pre-aging treatment improves the thermal stability of the d33 coefficients.

The piezoelectrets made of porous polytetrafluoroethylene （PTFE） films and nonporous fluoroethylenepropylene （FEP） films are prepared by using a hot-pressing method. The quasi-static piezoelectric d33-coefficients of such piezoelectret films are measured. The thermal stability of d33-coefficients and dynamic characteristics of charges in the piezoelectret films are investigated. The dynamic d33-coefficients are obtained by analyzing the dielectric resonance spectra of the piezoelectret films. The results show that the piezoelectric d33-coefficients up to 300 pC/N are achieved for the laminated PTFE/FEP films. The d33 value retains 40% of the initial value when the sample was annealed at the temperature of 90℃ for 20 h. Pre-ageing is an effective way to further improve the thermal stability of d33-coefficients. The dominant drift path of the detrapped charges in the shallow traps is most likely along the surface of the PTFE fibers, while charge drift through the solid layer of FEP is possibly prevailing for the charges in the deeper traps. The d33 value determined from the dielectric resonance spectrum is smaller than the quasi-static d33 value, which is mainly due to the enhanced Young's modulus with the increase of frequency.

So far, linear polypropylene (PP) foam is the most extensively investigated ferroelectret. In the present studies, cross-linked PP (XPP) foam sheets are modified by a hot-press process, and then corona charged to be piezoelectric. Thereafter their piezoelectric activities are investigated.

Piezoelectrets made by cross-linked polypropylene foam sheet are prepared. The microstructure of the films is observed by SEM technique. The piezoelectric coefficient d33 is determined by a quasi-static method. The thermal stability of d33 is investigated by measurements of isothermal decay and thermally stimulated discharge current spectra. The results show that the grid voltage during corona charging is one of the critical parameters for obtaining piezoelectricity. The films are piezoelectric when the grid voltages are beyond 4 kV. Compared with linear polypropylene, cross-linked polypropylene films show improved linearity and thermal stability.

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 is most likely along the surface of the PTFE fibers, while charge drift through the solid layer of FEP is possibly prevailing at temperatures of around 210°C.