Cellular Electrets Research Articles

Influence of soft x-ray and ultraviolet irradiations on sensitivity of sensors made with piezoelectret films

Abstract Piezo/ferroelectrets are a kind of cellular electret materials exhibiting piezoelectric effect. Owing to their large piezoelectric activity, flexibility, lightweight, low cost and large area, a list of promising applications have been suggested, including wearable sensors, actuators, micro energy harvesters, and etc. In terms of actual application, one of the most important performance indexes is the stability of piezoelectric response in complex application environments. This article mainly focuses on the influence of soft x-ray and ultraviolet irradiations on piezoelectric sensitivity of two kinds of sensors, made with irradiation cross-linked polypropylene (IXPP) and air-filled parallel-tunnel fluorinated ethylene propylene (FEP) piezoelectret films. A versatile encapsulation process with silica gel for sensor fabrication was introduced to improve the stability against radiations. The results show that soft x-ray irradiation can accelerate the depolarization of the piezoelectrets, while the influence of ultraviolet irradiation on piezoelectrets is not prominent. Encapsulation of the sensors with silica gel may significantly improve their stability under the radiation of soft x-ray. As indicated, the piezoelectric sensitivities of the encapsulated samples, made with IXPP and parallel-tunnel FEP piezoelectret films, are 50.2% and 6.9% of the initial values after 150 min irradiation of the soft x-ray with an energy of 9.5 keV, respectively.

History and recent progress in ferroelectrets produced in Brazil

More than 30 years ago, a group of researchers in Tampere–Finland developed a thin foamed polymeric material for capacitive sensors. Such soft-voided films exhibited electrical charging characteristics, forming a powerful combination, which resulted in a smart-material with ferroelectric properties. The discovery of the electro-thermo mechanical film (ETMF) has sparked the curiosity of the electret community, leading to the development of several studies. At that time, ETMF became known as cellular electrets and, later, as ferroelectrets or piezoelectrets regarding their electromechanical properties. This paper provides a timeline review of the research on ferroelectrets produced in Brazil, between the years 1990 and 2020, towards demonstrating how the interest in the electret electrical charging mechanism has resulted in the use of ferroelectrets with well-controlled cavities for ultrasound applications.

Understanding the Role of Soft X-ray in Charging Solid-Film and Cellular Electrets

Solid-film electrets and cellular electrets are defined as promising insulating dielectric materials containing permanent electrostatic and polarizations. High-performance charging methods are critical for electret transducers. Unlike dielectric barrier discharge (DBD) charging, the soft X-ray charging method, with its strong penetration ability, has been widely used in electrets after packaging and has even been embedded in high-aspect-ratio structures (HARSs). However, the related charging model and the charging effect of the soft X-ray irradiation remain unclear. In this study, the charge carrier migration theory and the one-dimensional electrostatic model were employed to build the soft X-ray charging models. The influence of soft X-ray irradiation under deferent poling voltages was investigated theoretically and experimentally. The conducted space charge measurement based on a pulsed electro-acoustic (PEA) system with a soft X-ray generator revealed that soft X-ray charging can offer higher surface charge densities and piezoelectricity to cellular electrets under the critical poling voltage lower than twice the breakdown voltage.

Influence of shape effect on dynamic surface charge transport mechanism of cellular electret after corona discharge

Surface charge accumulation and transport on cellular polypropylene play an important role in nanogenerators, which could have a potential impact on energy harvesting and wearable devices for zero carbon energy systems and the internet of things. Different shapes have different charge accumulation and decay characteristics of the polymer. Therefore, we studied the influence of the sample’s shape on the surface charge decay by experiment and modeling. The surface potential of square and circular cellular polypropylene was measured by a two-dimensional surface potential measurement system with electrostatic capacitive probe. The experimental result shows that the surface potential distribution of the square sample dissipates non-uniformly from the bell shape to a one-sided collapsed shape, while that of the circular sample dissipates uniformly from the bell shape to the crater-like shape. Moreover, the simulated results of the initial surface potential distributions of the square and circular cellular polypropylene are consistent with the experimental results. The investigation demonstrates that the charge transport process is correlated with the shape of the sample, which provides significant reference for designing electret material used for highly efficient nanogenerators.

High performance laminated FEP/PTFE piezoelectret films for air-borne sound transducers

Piezoelectret (also known as ferroelectret) is a kind of cellular electret material with strong piezoelectric effect. Such a material exhibits flexibility, low density and small acoustic impedance. Therefore, piezoelectret is an ideal material for air-borne flexible sound transducers. Aiming at high-sensitivity and thermal-stability sound transducers, in this work, laminated fluorinated polyethylene propylene (FEP) and polytetrafluoroethylene (PTFE) piezoelectret film with a regular cellular microstructure is prepared by a procedure involving template-based cellular structure formation and polarization. The results show that the characteristic acoustic impedance of such a laminated FEP/PTFE film is 0.02 MRayl. The quasi-static piezoelectric charge coefficient <i>d</i><sub>33</sub> up to 800 pC/N is achieved in a small applied pressure range. The maximum value of sensitivity of the microphones based on laminated FEP/PTFE piezoelectrets film can reach to 6.4 mV/Pa at 1 kHz. Besides, the frequency response curve of the device is flat in the whole audio range. For an ultrasonic transmitter with a diameter of 20 mm, driven by a voltage of 600 V (<i>V</i><sub>p</sub>), the sound pressure level (SPL) generated by it increases from 80 to 90 dB (Ref. 20 µPA) as frequency increases from 40 to 80 kHz. The thermal stability of the sensitivity for the transducers made of such a laminated FEP/PTFE piezoelectret film is much superior to that of polypropylene (PP) piezoelectret based device. The sensitivity of the present device remains 26% of the initial value after being annealed at 125 ℃ for 211 h. The improvement of thermal stability is attributed to the excellent space charge storage stability of FEP and PTFE.

Theoretical modeling of longitudinal piezoelectric characteristic for cellular polymers

Efficient energy harvesting is a difficult challenge that consists in the development of systems allowing charging autonomous and low-power devices. In addition to traditional piezoelectric polymers, mono-crystals, and ceramics, cellular electrets offer consistent solutions by converting wasted vibration energy from the environment to usable electrical energy. This paper presents an electromechanical model to study the energy harvesting capability of cellular polymers. The theoretical study models the response of these materials to investigate the effect of different parameters on the piezoelectric coefficient d33, particularly. The model considers the percentage of porosity, surface charge density in each polymer–gas surface, the properties of the polymer matrix and the gas encapsulated in the pores, and the Young’s modulus of the porous film. For poly(ethylene-co-vinyl acetate), the results showed that the piezoelectric performance of the film declines with the increase of the film thickness. However, the variation of the d33 as a function of the percentage of porosity is exponential and can achieve 4.24 pC/N for a porosity of 80%. Compared to a previously published experiment, the theoretical results have proven a good agreement with only 3.3% error.

Piezo-electret vibration sensors designed for acoustic-electric guitars

A piezo-electret vibration sensor designed for acoustic-electric guitars is evaluated. A piezo-electret, which is a cellular polypropylene electret film, is laminated with a 150-μm-thick polyethylene terephthalate (PET) film to achieve sufficient durability for general use. Additionally, a 10-μm-thick silver electrode is printed on the top surface as a shield electrode. This electrode adds a mass of only 63 mg to the sensor. The sensor is attached to the soundboard of a guitar body. An impact force is applied to the top of the saddle of the guitar body. The output voltage of the sensor owing to the vibration of the soundboard is measured by a voltage follower and an FFT analyzer. Conventional sensors that use lead zirconate titanate (PZT) ceramics are also tested. The results show that only the piezo-electret sensor detects the vibration of the soundboard over a wide frequency range up to 10 kHz. The time response of the sensor also corresponds to the vibration of the soundboard. However, the output voltage of the sensor is 1/10 to 1/100 of that of the PZT sensors. The effective d 33 value of the sensor is 1.58 pC/N at 1 kHz. The value is only 1/280 of the apparent d 33 value of the piezo-electret, owing to an increase in the effective elastic modulus in the thickness direction caused by lamination with the PET film.

Surface charge decay patterns of insulating polymers based on potential-mapping method

The accumulation of surface charge on insulators can lead to electric field distortion. Therefore, measurement of the surface potential decay tendency has proven to be a practical technique for evaluation of the charge dissipation characteristics of dielectrics after corona polarization. The traditional method for evaluation of the surface charge dissipation behavior is based on using a stable suspended electrostatic probe. However, this method was found to be inadequate for analysis of the surface charge transport mechanism because the electrostatic probe was focused only on one point on the sample surface. Therefore, a scanning and mapping technique has been applied in the surface potential measurement that uses a movable electrostatic probe. This technique enables measurement of the complete surface charge distribution profile of the cellular electrets and provides a potential-mapping graph that reflects the exact surface charge distribution and decay tendency of the functional dielectric after discharge. This work describes the phenomenon of the surface charge diffusion pattern based on the mapping of the dynamic surface potential profile from a scanning and mapping graph. This method could be developed into an efficient tool for evaluation of surface charge dissipation on insulating polymers used in high-voltage apparatuses and energy-storage devices.

HIGH PRECISION SIMULATION ON SURFACE POTENTIAL OF CELLULAR ELECTRETS BY CHARGE SIMULATION METHOD

Surface potential is one of the key properties of charge retain ability and energy storage performance of cellular electrets. Based on charge simulation method (CSM), a high precision simulation algorithm of surface potential of functional dielectrics is proposed in this paper, which can be applied to the simulations of cellular polypropylene after high-voltage corona discharge. In order to validate the modeling, the surface potential of insulating polymer after corona discharge is measured by an electrostatic measurement system and compared with simulation results based on CSM and finite element method (FEM). The result shows that the numerical simulation based on CSM is well consistent with the experimental data, which indicates that the modeling based on CSM can be used in the calculation and analysis of charge storage ability of capacitor and electrostatic generator based on functional electrets. Moreover, as simulation based on CSM has higher precision than FEM, investigation of surface charge storage ability and prediction of electrostatic hazards of insulating dielectrics could be realized by using CSM.

Epidermis-Inspired Ultrathin 3D Cellular Sensor Array for Self-Powered Biomedical Monitoring

Sensing devices with wearability would open the door to many advanced applications including soft robotics, artificial intelligence, and healthcare monitoring. Here, inspired by the configuration of the human epidermis, we present a flexible three-dimensional (3D) cellular sensor array (CSA) via a one-step thermally induced phase separation method. The CSA was framed by the 3D cellular electret with caged piezoelectric nanoparticles, which was ultrathin (80 μm), lightweight, and highlyrobust. For biomedical sensing, the 3D-CSA holds a decent pressure sensitivity up to 0.19 V kPa-1 with a response time of less than 16 ms. Owing to its rigid structural symmetry, the 3D-CSA could be identically operated from itsboth sides.It was demonstrated to successfully measure the human heartbeat, detect the eyeball motion for sleeping monitoring, and tactile imaging. Mimicking the functionalities of the human skin with a self-powered operation feature, the 3D-CSA was expected to represent a substantial advancement in wearable electronics for healthcare.

Electrostrictive energy conversion property of cellular electrets after corona discharge

In this paper, the authors present the electrostrictive energy conversion ability of cellular electrets after the high-voltage corona polarization. Moreover, the electrostrictive effect of such foamed polymer before and after corona polarization has also been compared and discussed. The enhancement of electrostrictive effect of cellular electrets after corona polarization was observed. In particular, the impact on the electrostrictive effect of the macroscopic electric dipoles inside of cellular polymer which are generated by high-voltage corona poling procedure has been investigated. The present research has promoted the development of the application of electret in the field of energy conversion, actuator, transducers, etc.

Surface charge self-recovering electret film for wearable energy conversion in a harsh environment

A laminated cellular electret film with surface charge self-recovery property for reliable wearable devices was developed.

Increase of Piezoelectric Constant and Thermal Durability of Polypropylene Electret by Introducing SiO2 and Kaolin Filler and Creating a Cellular Structure

This article presents a method for preparing and testing the piezoelectric properties and stability of cellular electret based on polypropylene (PP). Introducing 5% mineral filler as a mixture of crystalline silica, colloidal silica, and kaolin to isotactic polypropylene followed by a film stretching process resulted in the formation of a composite cellular structure. To manufacture electrets, the films were polarized at a constant electric field in the range from 100 V/μm to 125 V/μm, in a climatic chamber heated up to 80°C. The durability of the electrets was determined using thermostimulated discharge currents and approximate calculations of depolarization process activation energy. For electrets made of cellular films, the depolarization temperature Tm at which the density of the discharge current assumes the highest value was ∼108°C and the activation energy was 6.25 eV. The response of the polarized composite film to mechanical stress expressed as the piezoelectric constant d33 was about 3 times higher than for a-PP film of the prevailing atactic phase and poly(vinylidene fluoride) film without a cellular structure. In the range of stress of 1 kPa to 120 kPa it was 135 pC/N for lower stresses and 60 pC/N for higher stresses.

Piezoelectricity and ferroelectricity of cellular polypropylene electrets films characterized by piezoresponse force microscopy

Cellular electrets polymer is a new ferroelectret material exhibiting large piezoelectricity and has attracted considerable attentions in researches and industries. Property characterization is very important for this material and current investigations are mostly on macroscopic properties. In this work, we conduct nanoscale piezoelectric and ferroelectric characterizations of cellular polypropylene (PP) films using piezoresponse force microscopy (PFM). First, both the single-frequency PFM and dual-frequency resonance-tracking PFM testings were conducted on the cellular PP film. The localized piezoelectric constant d33 is estimated to be 7–11pC/N by correcting the resonance magnification with quality factor and it is about one order lower than the macroscopic value. Next, using the switching spectroscopy PFM (SS-PFM), we studied polarization switching behavior of the cellular PP films. Results show that it exhibits the typical ferroelectric-like phase hysteresis loops and butterfly-shaped amplitude loops, which is similar to that of a poly(vinylidene fluoride) (PVDF) ferroelectric polymer film. However, both the phase and amplitude loops of the PP film are intensively asymmetric, which is thought to be caused by the nonzero remnant polarization after poling. Then, the D-E hysteresis loops of both the cellular PP film and PVDF film were measured by using the same wave form as that used in the SS-PFM, and the results show significant differences. Finally, we suggest that the ferroelectric-like behavior of cellular electrets films should be distinguished from that of typical ferroelectrics, both macroscopically and microscopically.

Enhancement of electrostrictive polymer efficiency for energy harvesting with cellular polypropylene electrets

The purpose of this paper is to propose new means for harvesting energy using electrostrictive polymers. The recent development of electrostrictive polymers has generated new opportunities for high-strain actuators. At the current time, the investigation of using electrostrictive polymer for energy harvesting, or mechanical-to-electrical energy conversion, is beginning to show its potential for this application. The objective of this work was to study the effect of cellular polypropylene electrets after high-voltage corona poling on an electrostrictive polyurethane composite filled with 1vol.% carbon black at a low applied voltage in order to increase the efficiency of the electromechanical conversion with electrostrictive polymers. Theoretical analysis supported by experimental investigations showed that an energy harvesting with this structure rendered it possible to obtain harvested power up to 13.93nW using a low electric field of 0.4V/μm and a transverse strain of 3% at a mechanical frequency of 15Hz. This represents an efficiency of 78.14% at low frequency. This percentage is very significant compared to other structures. Finally, it was found that the use of polypropylene electrets with electrostrictive polymers was the best way to decrease the power of polarization in order to obtain a good efficiency of the electromechanical conversion for energy harvesting.

Trench-filled cellular parylene electret for piezoelectric transducer

Cellular ferroelectrets with charged voids are promising for piezoelectric transducers, but the long-term stability or reproducibility due to the conventional “stochastic” fabrication and the corona charging methods is of concern. We microfabricated a high-density cellular piezoelectret with high-aspect-ratio polymer structures based on the trench-filled parylene technology. Vertical walls of the parylene structures are charged using soft x-ray to realize uniform artificial dipoles, of which moments could vary along with parylene structural deformation driven by the inertia of a seismic mass. Charge and voltage sensitivities of 9600 pC/N and 960 V/N with respect to the in-plane resonant oscillation have been achieved at 149 Hz, respectively.

Enhanced electromechanical performance of cellular polypropylene electrets charged at a high temperature

In order to enhance the electromechanical performance of polypropylene (PP) electrets, cellular PP electrets were heated during corona poling at a high temperature in order to accelerate its chemical structure change and increase the quantity of formed dipole within the electrets. We investigated the behaviour of surface potential decay after corona poling, and the results showed that corona poling at a high temperature can improve the charge retention of commercially used cellular PP. The results of thermally stimulated depolarization current indicated the different localized states of the electric charges in cellular PP electrets after corona poling. Then, enhanced electromechanical ability was measured by a deformation measurement system and large strain (about 5%) under a moderate applied electric field intensity (30 MV m−1) was obtained.

Trap capture properties of space charge and void breakdown charge in a cellular polypropylene electret film

The following aspects have been indicated by systematic studies on the cellular polypropylene (PP) electret film. Trap capture properties of space charges changed with their injected amount or surface potential of the sample. The space charges were captured largely in the deep traps of the sample surface and the second-deep traps of the near-surface region at small amount of charge injection. They would fill these traps in the surface layer (surface and near-surface region) and the shallow traps in bulk at the same time in the case of greater amount of charge injection. The charge detrapping temperatures of the three kinds of trap centers are about 160℃，138℃ and 92℃，respectively. The charges formed due to void breakdown (called the void breakdown charges) were captured not only in the surface layers of voids which have similar traps as the surface layer of the sample，but also there were numerous void breakdown charges which penetrated the void surface layer，entered into the bulk，and were captured by the shallow bulk traps.

Nanomechanical and electrical characterization of a new cellular electret sensor–actuator

Electrically charged cellular polymers are known to display pseudo-piezoelectric effects thatendow them with interesting mechano-electrical characteristics. When a film of such apolymer is compressed, charge is generated across its thickness, and conversely,applying an oscillatory or static potential elicits mechanical motions. This dualsensor–actuator behaviour can be embedded in one material and presents distinctadvantages of functional integration. A novel electroactive foam is presented here thatembeds such a sensor–actuator function. The foam has a sensitivity constant(d33)of 330 pC N−1. Interestingly, the resonant behaviour of the cellular film can be altered by variation in theDC offset across the material. Such adaptive capacity could be of great advantage fortuning polymer-based mechanical devices to be either efficient sound radiators andmechanical actuators, or sensitive and coherent sensors. Possible applications inmicrofluidics are also discussed.

Influence of macroscopic dipoles on the charge storage and charge dynamics of polypropylene ferroelectret films

In this work, influence of the formation of macroscopic dipoles with space charges enclosed in a void (electric domain) and their polarization enhancement on the charging current characteristic and the conductivity of cellular polypropylene (PP) film were studied by the corona charging with a grid in combination with the reverse-polarity corona charging compensation method. By measuring the isothermal surface potential decay and analyses of open-circuit and short-circuit thermally stimulated discharging current spectra, the charge stability and charge dynamics of PP ferroelectrets in relation to the formation and density variety of macroscopy dipoles were also discussed. The experimental results indicate that the decrease of charge stability of cellular PP film with high polarization should be attributed to the increase of the conductivity induced by the self-field of macroscopic dipoles. For cellular electret film with weak polarization, macroscopic dipoles formed by space charges should only be trapped at two sharp ends of lens-like void and, therefore, detrapped charges will transport along the side surfaces at both ends of the void. When the sample has high polarization, the trapped charges are distributed in the whole surface layer of the void. Most of the detrapped charges excited by the electret field drift through the solid dielectric layer between adjacent voids, which leads to their decay.