Quasi-static Piezoelectric D33 Coefficient Research Articles

Ultralight and sensitive ferroelectret films with a human skin-like texture

Biologically inspired by natural skin, polypropylene (PP) ferroelectret films with a human skin-like texture, featuring lightweight, light transmittance, small thickness, stretchability, as well as significant longitudinal and transverse piezoelectric activity, are prepared with a simple procedure, and their piezoelectric properties are characterized. The mechanical structure of the fabricated ferroelectret films is of a double-level cellular structure with distributed tight junctions and dipolar charges deposited on the opposite inner walls of cells. The preliminary results show that the area density, light transmittance, and thickness of the films are 3.5 g/m2, 80%, and 10–20 μm, respectively. A quasi-static piezoelectric d33 coefficient of a few thousand pC/N at 3.4 kPa and a dynamic piezoelectric d31 coefficient of −40 pC/N at a frequency of 80 Hz and a stress of 20 kPa are achieved. The improved piezoelectric performance seems mainly owing to the reduction of the elastic modulus for the double-level structure and the enhancement of charges in deep traps in very thin cell walls. The sound pressure level, generated by an ultrasonic emitter made of the PP ferroelectret film, is about 87 dB at 46 kHz as driven by a voltage of 20 V (peak to peak voltage). An ultra-thin tactile sensor based on the fabricated films can localize positions and detect the track and moving speed of applied force. These results indicate that the cost-efficient films may extend the applications of ferroelectrets in flexible electronics.

Fluorinated Polyethylene Propylene Ferroelectrets with an Air-Filled Concentric Tunnel Structure: Preparation, Characterization, and Application in Energy Harvesting.

Fluorinated polyethylene propylene (FEP) bipolar ferroelectret films with a specifically designed concentric tunnel structure were prepared by means of rigid-template based thermoplastic molding and contact polarization. The properties of the fabricated films, including the piezoelectric response, mechanical property, and thermal stability, were characterized, and two kinds of energy harvesters based on such ferroelectret films, working in 33- and 31-modes respectively, were investigated. The results show that the FEP films exhibit significant longitudinal and radial piezoelectric activities, as well as superior thermal stability. A quasi-static piezoelectric d33 coefficient of up to 5300 pC/N was achieved for the FEP films, and a radial piezoelectric sensitivity of 40,000 pC/N was obtained in a circular film sample with a diameter of 30 mm. Such films were thermally stable at 120 °C after a reduction of 35%. Two types of vibrational energy harvesters working in 33-mode and 31-mode were subsequently designed. The results show that a power output of up to 1 mW was achieved in an energy harvester working in 33-mode at a resonance frequency of 210 Hz, referring to a seismic mass of 33.4 g and an acceleration of 1 g (g is the gravity of the earth). For a device working in 31-mode, a power output of 15 μW was obtained at a relatively low resonance frequency of 26 Hz and a light seismic mass of 1.9 g. Therefore, such concentric tunnel FEP ferroelectric films provide flexible options for designing vibrational energy harvesters working either in 33-mode or 31-mode to adapt to application environments.

Energy harvesting from vibration using flexible floroethylenepropylene piezoelectret films with cross-tunnel structure

Flexible fluoroethylenepropylene (FEP) piezoelectret films with a cross-tunnel structure were prepared using a template-based process and contact charging. Energy harvesting from the fabricated FEP films was investigated at various exciting frequencies and load resistances. The results show that the quasi-static piezoelectric d33 coefficients for the FEP piezoelectret samples are of the order of 1000 pCN-1 at an applied pressure of 3.1 kPa. The output voltage increases with the increase in load resistance, and the output power is dependent on the load resistance and vibration frequency. At a load resistance of 0.9 MΩ and vibration frequency of 650 Hz, the output power of 3.5 μWg-2 was obtained for the sample with an area of 3.14 cm2 and a seismic mass of 44.1 g. Such thin, light and flexible FEP piezoelectret films may be applied in wearable environmental energy harvesting system.

Piezoelectric property of cross-linked polypropylene piezoelectret and its application in vibration energy harvester

Piezoelectrets, also known as ferroelectrets, are space-charge electrets based polymer foams with strong piezoelectric effect. The piezoelectric effect in piezoelectrets originates from the regularly arranged dipolar space charges in the polymer matrix, achieved by properly charging the specific foam structure. The large figure of merit (FOM, d33·g33) in piezoelectrets implies that such kinds of materials are promising candidates in energy harvesters. In this article, the electron-irradiated cross-linked polypropylene (IXPP) foam sheets are rendered piezoelectric (i.e. to become piezoelectrets) by modification of the microstructure using hot-pressing process and polarization using corona charging at room temperature. The electromechanical properties of the fabricated IXPP piezoelectrets are investigated by measurements of quasi- and dynamic piezoelectric d33 coefficients, dielectric resonance spectrum, and isothermal decay at elevated temperatures. The energy harvesting from vibrations by using the IXPP piezoelectret films, at various vibration frequencies, load resistances, and seismic masses, are also studied. Results indicate that the quasi-static piezoelectric d33 coefficients of IXPP films up to 620 pC/N can be achieved. The variation of quasi-static piezoelectric d33 coefficient is dependent on the applied pressures, from 0.1 to 1.3 kPa, while it shows good linearity at larger pressures from 1.3 to 15 kPa. The typical values of Young's modulus in the thickness direction and the figure of merit (FOM) are 0.7 MPa and 11.2 GPa-1, respectively. The d33 coefficients will drop to 54%, 43%, 29% of the initial values after annealing the samples for 24 h at 50, 70, 90℃, respectively. At an exciting frequency of 820 Hz, the normalized output power of 65 μW/g2 is obtained from an IXPP film with an area of 3.14 cm2 and a seismic mass of 25.6 g around the optimum load resistance. Such thin, light and flexible IXPP piezoelectret films may be applied in vibration energy harvesters for powering low-power electronic devices.

Influence of Extending Ratio on Mechanical and Piezoelectric Properties of Irradiation Cross-Linked Polypropylene Films

Stretchable piezoelectret films may be applied in sensor skins, wearable equipment, micro-energy harvesters and so on. Irradiation cross-linked polypropylene (IXPP) foam sheets could be stretchable piezoelectrets after proper modification of microstructure and polarization. In this article, commercial IXPP foam sheets were modified by a process consisting of hot-pressing and extending and rendered piezoelectric by corona charging, and the influence of extending ratio on the stretchability and piezoelectric properties were investigated. The quasi-static piezoelectricd33coefficients up to 427 pC/N are achieved, which well retained to the strain up to 11%. The result shows that in the surface parallel plane, the Young’s module in extending and transverse direction show typical anisotropism, which retains as the secondary extending ratio increases from 0% to 110%, while in the surface normal direction, the Young’s module of the film decreases from 0.87 to 0.34 MPa. The stretchability of the modified film is significantly improved compared to which of the origin film.

Quasi-static and dynamic piezoelectric responses of layered polytetrafluoroethylene ferroelectrets

Layered polytetrafluoroethylene (PTFE) films of large area, composed of a porous PTFE core and two compact PTFE cover layers, which are commercially available, were internally charged by application of a corona discharge. The internal charges, which probably reside on the inner surfaces of the compact layers, make the films piezoelectric. Initial quasi-static piezoelectric d33 coefficients are in the range of 1000–2700 pC N−1. After a 1500 min annealing treatment at temperatures of 90 °C, 120 °C and 150 °C, the d33 coefficients stabilize and typically reach about 75%, 40% and 25% of their initial values, respectively. They drop to 18% after a thermally stimulated discharge measurement extending from 30 to 215 °C at a heating rate of 3 °C min−1. A pressure dependence of the piezoelectric d33 coefficients in such films is observed. This is due to the progressive densification of the core between the compact cover layers of the films. Microphones built with such films exhibit a somewhat decreasing frequency response up to about 2 kHz, an increase of the response due to diffraction effects at higher frequencies, and eventually a peak due to a thickness resonance at about 40 kHz. The microphone sensitivity below the diffraction and resonance ranges is about 1 to 1.5 mV Pa−1. This corresponds to dynamic d33 coefficients of 200 to 300 pC N−1.

Fluoroethylenepropylene ferroelectrets with patterned microstructure and high, thermally stable piezoelectricity

Layered fluoroethylenepropylene (FEP) ferroelectret films were prepared from sheets of FEP films by template-patterning followed by a fusion-bonding process and contact charging. The layered ferroelectret films show consistency and regularity in their void structures and good bonding of the layers. For films composed of two 12.5 μm thick FEP layers and a typical void of 60 μm height, the critical voltage necessary for the built-up of the “macro-dipoles” in the inner voids is approximately 800 V. At room temperature, Young’s modulus in the thickness direction, determined from dielectric resonance spectra of the fabricated films with a typical thickness of 85 μm, is about 0.21 MPa. Initial quasistatic piezoelectric d33 coefficients of samples contact charged at a peak voltage of 1500 V are in the range of 1000–3000 pC/N. From these, ferroelectrets with high quasistatic and dynamic (up to 20 kHz) d33 coefficients of up to 1000 pC/N and 400 pC/N, respectively, which are thermally stable at 120°C, can be obtained by proper annealing treatment. This constitutes a significant improvement compared to previous results.

Polarization and piezoelectricity in polymer films with artificial void structure

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 d33 coefficient was investigated at elevated temperatures and via thermally stimulated discharge (TSD) current measurements in short circuit. For 150 μm thick laminated films, consisting of one 25 μm thick PTFE layer, two 12.5 μm thick FEP layers, and a void of 100 μm 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 d33 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 d33 coefficients. The thermal stability of d33 can be further improved by pre-aging. E.g., the reduction of the d33 value in the sample pre-aged at 150°C for 5 h was less than 5% after annealing for 30 h at a temperature of 90°C.

Performance of Piezoelectrets Made of Non-Porous Polytetrafluoroethylene and Fluoroethylenepropylene Layers

The thermal stability of piezoelectricd33coefficients and charge dynamics in the piezoelectret films with tailored microstructure, made of non-porous polytetrafluoroethylene (PTFE) and fluoroethylenepropylene (FEP) layers, are investigated by the measurements of the isothermal decay of piezoelectricd33coefficients at elevated temperatures and the analysis of thermally stimulated discharge (TSD) current spectra in short circuit, respectively. The results show that the quasi-static piezoelectricd33coefficient up to 300 pC/N is achieved and its Young's modulus is about 0.28 MPa. Thed33values in the present studied films show improved thermal stability compared with the films without regular microstructure. The drift path for the most of detrapped charges in the films is through the solid dielectric layer.

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% for samples where positive charges are deposited in both, the porous PTFE and the compact FEP layers.

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 d <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">33</sub> -coefficients of such films on the grid voltage during the corona charging is investigated. The thermal stability of d <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">33</sub> -coefficients for the films is characterized by the isothermal method. The Young's Modulus and dynamic d <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">33</sub> -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 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">33</sub> -coefficient is about 300 pC/N for the laminated PTFE/FEP films. The d <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">33</sub> 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 d <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">33</sub> value is improved to 75% of the initial value in the same conditions. The d <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">33</sub> value determined by dielectric resonance spectra is smaller than the quasi-static d <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">33</sub> value, which is properly due to the enhanced Young's modulus with the increase of frequency.

Thermal stability and charge dynamics of piezoelectrets with tailored micro-structure

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.

Preparation and piezoelectricity of fluorocarbon polymer piezoelectret films with ordered void structure

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.

Polytetrafluoroethylene piezoelectrets prepared by sintering process

Polytetrafluoroethylene (PTFE) films with a void structure are prepared by a sintering process. Such void PTFE films are piezoelectric after proper corona charging. The quasi-static piezoelectric d33 coefficients up to 250 pC/N are achieved for the samples which were made of compact and biaxial-tension porous PTFE layers. Pre-ageing treatment is an effective method to further improve the thermal stability. For the samples with pre-ageing treatment, the reduction of the d33 coefficients is around 2% per day when exposed to 120 °C.

Quasi-static and dynamic piezoelectric d33 coefficients of irradiation cross-linked polypropylene ferroelectrets

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.

Piezoelectricity of laminated polymer films made of nonporous fluoroethylene and porous polytetrafluoroethylene layers

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.

Piezoelectricity of Cellular Polypropylene Films Expanded by a Dichlormethane Evaporation Process

In this paper, a new method for expanding stretched cellular isotactic polypropylene (i-PP) is presented. Firstly, the cellular i-PP films with small cavities were saturated with dichlormethane (DCM) and then, the DCM was abruptly evaporated at elevated temperature to create large-cavity cellular films. Finally, the expanded film samples were quenched in liquid nitrogen to fix the cell structure. After corona charging, the piezoelectric d33-coefficients of such expanded films were measured by quasistatic and interferometric methods. The results show that the highest values of the quasistatic piezoelectric d33-coefficient of up to 2500 pC/N at low frequencies were observed on samples twice expanded with DCM. A high thermal stability of the piezoelectric activity was also observed. The high d33-values are due to the air escape sideways out of the large voids of the cellular films.

Piezoelectric properties and charge transport for fluorocarbon polymers with cellular structure

A new method for prepering piezoelectric polymer with cellular structure (piezoelectret) is introduced. Their piezoelectric activity and thermal stability are investigated for the fluorocarbon piezoelectret films produced in this method. The quasistatic piezoelectric d33 coefficients up to 2200pC/N are obtained for the fluorocarbon piezoelectret films; d33 coefficients are relatively independent on the applied pressure in the range of 20kPa; comparing to polypropylene piezoelectret film the new fluorocarbon films show not only higher values of d33，but also much better thermal stability; Furthermore，the thermal stability of the fluorocarbon piezoelectret film can be further improved by the process of pre-ageing. The enhanced thermal stability of d33 for such fluorocarbon films is due to the good thermal stability of charges in fluoroethylenepropylene (FEP) and polytetrafluoroethylene (PTFE)，and the dimension stability of such cellular structure. The charge recombination is mainly through the drifting of the detraped charges along the inside surfaces of the cavities when the films are thermally stimulated.

Piezoelectric properties of irradiation-crosslinked polypropylene ferroelectrets

The cellular irradiation-cross-linked polypropylene films treated by a hot-press process are corona charged to be piezoelectric. For the samples charged at room temperature, quasistatic piezoelectric d33 coefficients up to 400pC∕N are obtained. The d33 coefficients are slightly dependent on pressure in the range up to 50kPa. The d33 values decrease to 30% when the samples are exposed to 90°C for 1day; a preaging treatment improves the thermal stability of d33. The dominant drift path of the detrapped charges is from one void surface to the adjacent void surface through the bulk of the solid.

Ferroelectrets with improved thermal stability made from fused fluorocarbon layers

Several layers of polytetrafluoroethylene and fluoroethylenepropylene films are fused such that small interfacial gas voids are formed between the layers. After proper charging and annealing, the fused multilayer films show large and thermally stable quasistatic piezoelectric d33 coefficients of about 1000pC∕N which are subject to a minor reduction of 3% per day if exposed to 90°C. Depending on sample processing, the piezoelectric coefficient is relatively independent of applied pressure in the range up to 20kPa and of applied frequency up to about 100kHz. Thermally-stimulated discharge measurements indicate that the decay of the piezoelectric activity at higher temperatures is at least partially due to charge drift along the surfaces of the voids. A microphone built with the cellular ferroelectrets has a sensitivity of 3mV∕Pa at 1kHz.