Gongxun Cao

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% ...

Piezoelectricity of cross-linked polypropylene films treated by hot-stretching

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 d33-coefficients of the samples, with various degrees of elongation, are determined by a quasi-static method. The applied pressure dependence of piezoelectric d33-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 d33-coefficients are enhanced with the increase of elongation degree. The d33 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 30kPa. Improved piezoelectric activity was obtained for the stretched XPP films when the rough surfaces were exposed to corona.

Mechanical and electrical properties of laminated polytetrafluoroethylene films

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, Youngs 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 d33 coefficient of 50 pC/N at 120°C after one day annealing at the same temperature.

Preparation and Piezoelectricity of Piezoelectrets Made of Polytetrafluoroethylene

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.

Charge storage and transport in oriented and non-oriented polytetrafluoroethylene films

The charge storage and transport in oriented and non-oriented polytetrafluoroethylene (PTFE) films, charged by the corona method at room temperature, were investigated by isothermal decay and thermally stimulated discharge (TSD) techniques. The crystallinity of the PTFE films was determined by differential scanning calorimetry (DSC). The results obtained from isothermal decay of the surface potential and the spectra of the TSD charge and current indicate that the thermal stability of both positive and negative charges in oriented PTFE films is significantly better than that of non-oriented PTFE films. This is probably due to the higher crystallinity in oriented PTFE films. The transport of the de-trapped positive charges in oriented PTFE films, which occurred due to thermal excitation, can be explained by a model that takes thermal ion emission into consideration.

Charge storage capability of cross-linked polypropylene electret films

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 90°C, the surface potentials of positively charged samples still keep above 90% of the initial value after annealing for 5120min; 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.

Polarization and Properties of Laminated Fluoropolymer Films

Laminated fluoropolymer films with regular void structure, fabricated by using a process consisting of the patterning and fusion bonding steps, are polarized to be piezoelectric. The influence of the applied voltage on the piezoelectric d33 coefficient is investigated. The measurements of ferroelectric-like polarization-voltage hysteresis loops are taken to further understand the capability of polarization in the laminated films. The compressive Young’s moduli of the films are determined from the dielectric resonance spectra. The results show that the laminated fluoropolymer films are piezoelectric after proper charging. The maximum d33 coefficients of the five-layer laminated piezoelectrets are achieved at the applied voltage of 5 kV. The remnant charge density of 0.3 mC/m2 is obtained from the polarization-voltage hysteresis loop at a bias voltage of 4 kV. The measured anti-resonance frequency and calculated compressive Young’s modulus for the five-layer laminated films are 112 kHz and 0.48 MPa, respectively.

Influence of porosity on properties in fluorocarbon piezoelectrets

The ifluorocarbon polymer films with regular void structure, made from fluoroethylenepropylene (FEP) and porous polytetrafluoroethylene (PTFE) layers were prepared by using the template patterning and fusion bonding process. After proper corona or contact charging, the films were piezoelectric. The microstructure, mechanical property, piezoelectricity and viscoelasticity of the fabricated films were studied by means of scanning electron microscope (SEM) technique, measurements of dielectric resonance spectra, and analysis of quasi-static d33 coefficient in time domain. The results show that, there ate threshold values of polarization about −4000 V, −3000 V and −2000 V in the films with porosities of 0,25% and 44%, respectively, and above the threshold voltage, the piezoelectric coefficient d33 of the films increases significantly with the enhancement of the polarization voltage. For the films with porosities of 0,25% and 44%, the measured dielectric anti-resonance frequencies are around 104 kHz, 119 kHz and 132 kHz, the calculated Youngs modulus are about 0.9 MPa, 0.58 MPa and 0.53 MPa and the densities of surface charge are 0.009 µC/cm2, 0.029 µC/cm2 and 0.046 µC/cm2, respectively. The piezoelectric response in time domain reflects the viscoelastic behaviors in the films. In addition, the films with large porosity show an improved piezoelectricity, which is perhaps due to the relative high density of charge and low Youngs modulus in. them.

Fabrication and piezoelectricity of fluoropolymer films with patterned structure

Laminated fluoroethylenepropylene (FEP) and porous polytetrafluoroethylene (PTFE) films with patterned void structure were successfully fabricated. An improved model taking into account of both the mechanical structure and charge distribution was used to describe the behavior of the fabricated films. The Youngs modulus of the three-layer FEP/PTFE film is ~0.5 MPa. Maximum quasi-static piezoelectric d33 coefficient up to 500 pC/N is achieved. Compared to the PP piezoelectrets, which loose their piezoelectric activity completely at the annealing temperature of 150 °C within 60 min, the laminated FEP/PTFE films show significantly improved thermal stability. For example, the d33-coefficient retains ~22% of the initial value for samples annealed at 150 °C for 4500 min.

Performances of regular structure piezoelectrets made by template

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 Youngs modulus of the laminated regular structure films is determined by dielectric resonance spectra. The quasi-static piezoelectric coefficient d 33 is measured by direct piezoelectric effect. The thermal stability of the piezoelectric d 33 coefficients is characterized by measuring the decay of d 33 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 Youngs modulus of five-layer structure piezoelectret films is about 0.48 MPa. The quasi-static piezoelectric coefficient d 33 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 holes relative hole wall.