Dielectric Resonance Spectra Research Articles

Characterization of PbWO4 thin films formed by the pulsed laser welding technique

Herein thin films of PbWO4 were formed from bilayers of Pb/WO3 by the pulse laser welding technique (PLW) within couple of seconds in an argon atmosphere. Thin films of Pb (100 nm)/WO3 (500 nm) were deposited by the thermal evaporation technique under a vacuum pressure of 10−5 mbar. The films were then exposed to a pulse laser beam of fixed amplitude and variable pulse width (0.1–10 ms). The produced films were characterized by means of X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy and optical spectrophotometry techniques. An almost pure phase (∼90%) of tetragonal PbWO4 was achieved after the pulse width exceeds 5.0 ms. PbWO4 thin films produced by the PLW technique exhibited a direct and indirect energy band gaps of 2.27 eV and 3.52 eV, respectively. Energy band tails of widths of 1.35 eV were found dominant in these films. In addition fitting of the dielectric resonance spectra for Pb/WO3 and PbWO4 thin films indicated that the formation of PbWO4 was accompanied with a decrease in the high frequency dielectric constant, a decrease in the free hole concentration and an increase in the electronic friction in the films. The fast method of obtaining PbWO4 thin films and the formation of the direct band gap being 2.27 eV together with the nonlinear dielectric and optical properties make the PbWO4 films obtained by the PLW technique suitable for optoelectronic technology.

Energy harvesting from vibration with cross-linked polypropylene piezoelectrets

Piezoelectret films are prepared by modification of the microstructure of polypropylene foam sheets cross-linked by electronic irradiation (IXPP), followed by proper corona charging. Young’s modulus, relative permittivity, and electromechanical coupling coefficient of the fabricated films, determined by dielectric resonance spectra, are about 0.7 MPa, 1.6, and 0.08, respectively. Dynamic piezoelectric d33 coefficients up to 650 pC/N at 200 Hz are achieved. The figure of merit (FOM, d33 ⋅ g33) for a more typical d33 value of 400 pC/N is about 11.2 GPa−1. Vibration-based energy harvesting with one-layer and two-layer stacks of these films is investigated at various frequencies and load resistances. At an optimum load resistance of 9 MΩ and a resonance frequency of 800 Hz, a maximum output power of 120 μW, referred to the acceleration g due to gravity, is obtained for an energy harvester consisting of a one-layer IXPP film with an area of 3.14 cm2 and a seismic mass of 33.7 g. The output power can be further improved by using two-layer stacks of IXPP films in electric series. IXPP energy harvesters could be used to energize low-power electronic devices, such as wireless sensors and LED lights.

Continuum modeling of charging process and piezoelectricity of ferroelectrets

Ferroelectrets in the form of electrically charged micro-porous foams exhibit a very large longitudinal piezoelectric coefficient d33. The structure has hence received wide application interests as sensors particularly in acoustic devices. During charging process, electrical breakdown (Paschen breakdown) takes place in the air pores of the foam and introduces free charge pairs. These charges are separated by electrostatic forces and relocated at the interfaces between the polymer and the electrically broken-down medium, where they are trapped quasistatically. The development of this trapped charge density along the interfaces is key for enabling the piezoelectricity of ferroelectrets. In this article, an internal variable based continuum model is proposed to calculate the charge density development at the interfaces, whereas a Maxwell stress based electromechanical model is used for the bulk behavior, i.e., of the polymer and of the medium where the Paschen breakdown takes place. In the modeling, the electrostatic forces between the separated charge pairs are included, as well as the influence of deformation of the solid layers. The material models are implemented in a nonlinear finite element scheme, which allows a detailed analysis of different geometries. A ferroelectret unit with porous expanded polytetrafluoroethylene (ePTFE) surrounded by fluorinated ethylene propylene is studied first. The simulated hysteresis curves of charge density at the surfaces and the calculated longitudinal piezoelectric constant are in good agreement with experimental results. Simulations show a strong dependency of the interface charge development and thus the remnant charges on the thicknesses of the layers and the permittivity of the materials. According to the calculated relation between d33 and the Young's modulus of ePTFE, the value of the Young's modulus of ePTFE is identified to be around 0.75 MPa, which lies well in the predicted range of 0.45 to 0.80 MPa, determined from the dielectric resonance spectra in the work of Zhang et al. [X. Q. Zhang et al., J. Appl. Phys. 108, 064113 (2010)]. To show the potential of the models, it is also applied to simulation of ferroelectrets with a lens shape. The results indicate that the electrical breakdown happens in a sequential manner, and the local piezoelectric coefficient varies with position. Thereby, the middle point on the surface exhibits the maximum d33. The simulation results obtained by the proposed models will provide insight for device optimization.

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.

Piezoelectric performance of fluor polymer sandwiches with different void structures

Film sandwiches, consisting of two outer layers of fluoroethylenepropylene and one middle layer of patterned porous polytetrafluoroethylene, were prepared by patterning and fusion bonding. Contact charging was conducted to render the films piezoelectric. The critical voltage to trigger air breakdown in the inner voids in the fabricated films was investigated. The piezoelectric d33 coefficients were measured employing the quasistatic method and dielectric resonance spectrum. The results show that the critical voltage for air breakdown in the inner voids is associated with the void microstructure of the films. For the films with patterning factors of 0%, 25% and 44%, the critical values are 300, 230 and 230 kV/cm, respectively. With an increase in the patterning factor, both the piezoelectric d33 coefficients determined from the dielectric resonance spectra and those determined from quasistatic measurements increase, which might be due to a decrease in Young’s modulus for the films. The nonlinearity of d33 becomes increasingly obvious as the patterning factor increases.

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.

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, 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 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">33</sub> coefficient of 50 pC/N at 120°C after one day annealing at the same temperature.

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.

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.