Electret Film Research Articles

Multiplying the Stable Electrostatic Field of Electret Based on the Heterocharge-Synergy and Superposition Effect.

Owing to magic charge storage behavior, an electret can exhibit an external electrostatic field, which is widely used in numerous domains such as electronics, energy, healthcare, and environment. However, the theory of the charge storage mechanism still needs further development to enhance the performance and stability of the electret. Herein, a stable charge storage model known as the heterocharge-synergy model (HSM) in electrets is proposed and verified, and the electrostatic field superposition effect of electrets is also proved. Based on the HSM and superposition effect, the stable electrostatic field intensity (average of ≈22.49kV cm-1 and maximum of ≈29.58kV cm-1 , which is close to the minimum air breakdown field intensity of ≈30kV cm-1 ) of the composite electret film is multiplied by simple layer-by-layer stacking. Utilizing the multilayer composite electret films and designing a two-sided electrostatic induction structure, a two-sided bipolar single-electrode non-contact nanogenerator is constructed with transferred charge density up to ≈132.61µC m-2 , which is twice as large as that of the non-contact nanogenerators with one-sided electrostatic induction structure. Clearing and utilizing the charge behaviors of the electret can boost the performance and enhance the stability of electret-based devices in various domains.

Self-powered multifunctional body motion detectors based on highly compressible and stretchable ferroelectrets with an air-filled parallel-tunnel structure

The new kind of ferroelectret electroactive polymers for flexible film sensors represent an attractive opportunity for self-powered wearable devices, which could offer sustainable and convenient health care services for people without the problem of energy supply. However, due to the mechanical heterogeneity, obtaining ferroelectrets with both a high longitudinal and transverse piezoelectric response is still a great challenge, which limits its multifunctional sensing application, especially in body motion tracking. Herein, we develop an air-filled parallel tunnel ferroelectret-based self-powered body motion detectors. Ascribed to the high mechanical compression and tensile properties of the specially designed parallel tunnel void structure and excellent charge storage stability of the fluorinated polyethylene propylene (FEP) electret film, these laminated films exhibit many fascinating merits, including ultrahigh longitudinal piezoelectric response (piezoelectric coefficient of 6200 pC/N, pressure sensitivity of 1950 pC/kPa), wide pressure detection range (0.03–62 kPa), remarkable transverse piezoelectric response (tensile sensitivity of 1600 pC/mm), long-term durability (1,08,0000 cycles), and superior friction force detection capability. This is the first work, to our knowledge, that ever use one kind of piezoelectric mechanism in a single ferroelectret-based sensor to achieve longitudinal stress, transverse stress, and tangential friction force sensing. Furthermore, on the basis of such sensors and machine learning algorithms, we designed an intelligent gesture recognition system for future human-machine interaction applications. Looking forward, our material design methodologies offer an approach to the self-powered flexible multifunctional sensor with great prosperity in household healthcare, smart mobile medical electronics, and humanoid robots.

Performance optimization of electret air filter

With the rapid development of the society, the severe haze weather caused by particulate matters (PMs) occurs frequently. Electret air filters have been playing an important role in purifying the air and alleviating the pollution. In this work, electret air filters were fabricated based on the polypropylene (PP) materials to investigate their filtering performance to the PMs. First, the electret performance of the PP-based films was mainly focused on. The charge storage stability of the electret films was found to be associated with the material recipe and the electret technique. Introducing 1% of tourmaline into the isotactic PP can realize the best charge storage stability, while adding the graphene can weaken the charge stability. And the electret techniques including the high-temperature pre-aging and polarization at elevated temperature were adopted to optimize the charge storage performance. Then, based on the optimized electret materials and electret technique, the purification performance of the electret filter elements dependent on the polarization condition was evaluated. The present work can provide an optimization strategy to improve the performance of electret materials and electret air filters.

MEMS Switching Voltage Regulator Using a Normally-On Electret Relay

We report on a DC/DC step-down switching regulator using a microelectromechanical systems (MEMS) switch for the first time. The output voltage developed in a storage capacitor is used as a feedback control voltage for the electrostatic actuator that drives a micromechanical contact switch inserted between the rectifier and the capacitor. The switch is configured to be normally-on owing to the electrostatic potential remaining in the electret film on the electrodes. When the output voltage exceeds and the feedback control voltage cancels the built-in potential of the electret, the differential voltage acting on the actuator becomes small and the contact is opened, thereby decoupling the rectifier from the storage capacitor. When the output voltage becomes low, on the other hand, the net differential voltage becomes large and the contact is closed again, thereby reconnecting the rectifier to the capacitor and recovering the voltage. Fast repetition of this sequence maintains the DC voltage of the storage capacitor at a constant level. For demonstration, an electret film that is pre-charted to 240 V is used to regulate the output voltage in a range between 12 V and 14 V. Different from the switching regulators using transistors, the MEMS regulator does not require any bias current for operation, being suitable as a low-power voltage regulator for environmental energy harvesters. [2021-0220]

Versatile, rapid and robust nano-positioning of single-photon emitters by AFM-nanoxerography

Atomic force microscopy (AFM) nanoxerography was successfully used to direct the assembly of colloidal nanodiamonds (NDs) containing nitrogen-vacancy (NV) centres on electrostatically patterned surfaces. This study reveals that the number of deposited NDs can be controlled by tuning the surface potentials of positively charged dots on a negatively charged background written by AFM in a thin PMMA electret film, yielding assemblies down to a unique single-photon emitter with very good selectivity. The mechanisms of the ND directed assembly are attested by numerical simulations. This robust deterministic nano-positioning of quantum emitters thus offers great opportunities for ultimate applications in nanophotonics for quantum technologies.

Stability of Charge Distributions in Electret Films on the nm-Scale.

Electret materials find use in various applications, such as microphones or filter media. In recent years, electrets have been used also increasingly on the micrometer scale, for example, in MEMS or for nano-xerography. However, for these applications, it becomes more important to prepare defined charge structures with sub-micrometer features. On the macroscopic level, the technique of isothermal potential decay at elevated temperatures has been developed to study aging effects and charge retention capabilities in electret materials. Here, we extend this technique to the nm-level by means of AFM-based methods, such as contact charging by AFM and the Kelvin probe force microscopy. Defined charge distributions in polyetherimide (PEI) ULTEM 1000 thin-film electrets have been studied for the first time with a high lateral resolution on the nanometer scale. We found a linear correlation between externally applied contact charging potential on the AFM-tip and the resulting relative surface potential on the PEI film. Charge decay at elevated temperatures is independent from the length scale. The same time dependence as for macroscopic, homogenously charged films could be established. We observe a potential decay only at an elevated temperature of 120 °C and no significant lateral charge transport. Thus, we propose a thermally enhanced charge carrier release from surface traps and a subsequent charge migration to the back electrode as the dominant mechanism. This finding is in-line with the observation that potential decay can be reduced also on the nm-level by pre-annealing the film slightly below the glass transition temperature. In contrast to many polymeric or inorganic electrets, no lateral charge migration is observed. Therefore, the charge patterns are preserved for PEI ULTEM 1000 thin-film electrets, which makes it a good candidate as electret for applications in MEMS or similar applications.

Imaging of an Electret Film Fabricated on a Micro-Machined Energy Harvester by a Kelvin Probe Force Microscope

An imaging method for a highly charged material using a Kelvin probe force microscope (KFM) was developed. Energy harvesters generate electric energy from the surrounding energy sources for a stand-alone power supply for Internet-of-Things devices. An electret film is a key component in an electrostatic induction-type energy harvester. However, the evaluation method of the electret film remains a problem for microscale measurements. KFM can provide a microscopic surface potential image corresponding to charge distribution. KFM is generally used to measure low potential distributions below a few volts because the electrostatic force between the cantilever that holds the tip and the sample is negligible. A 50- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> -long tip probe was fabricated, which has a longer tip than any commercially available probes. The surface potential difference of 50 V with a step distribution was measured using the developed KFM with a long tip probe. A potassium ion electret film formed on the sidewall of a comb electrode in an energy harvester fabricated by microelectromechanical systems’ technology was measured using the developed KFM. The measured charging potential distribution was from −30.1 to −28.9 V in a measured area, which was close to the mean charging potential measured using a conventional method of −29.2 V. The developed KFM can contribute to the evaluation of highly charged micro-sized samples.

Piezoelectric-magnetic behavior of ferroelectrets coated with magnetic layer

Electrically charged porous electret films, reported as ferroelectrets or piezoelectrets, have drawn the attention of researchers due to their high piezoelectric activity, low density, and flexibility. This paper introduces a ferroelectret design for the development of piezoelectric-magnetic responses. Samples are prepared with an additional magnetic layer placed above open tubular channels made from fused electret films, and the piezoelectric direct effect of electrically charged samples is monitored. The results of the measurements show approximately 900 pC N−1 piezoelectric coefficients d33. The piezoelectric-magnetic effect is an electrical response from a mechanical deformation caused by an external magnetic force. The magnetic field pulls the magnetic layer, mechanically deforming the electrically charged tubular structure, which creates a charge compensation to be measured as a charge density variation. A setup especially assembled for measuring the piezoelectric-magnetic response is described together with piezoelectric-magnetic values ranging from 94 up to 275 pC T−1.

A Reversible Change in the Nonlinear Optical Properties of Polymer Electret Films Based on Methacrylic Copolymers

A Reversible Change in the Nonlinear Optical Properties of Polymer Electret Films Based on Methacrylic Copolymers

Ultralight ethyl cellulose-based electret fiber membrane for low-resistance and high-efficient capture of PM2.5

Atmospheric particulate matter (PM) pollution is a serious threat to human health, so the preparation of electret films with high efficiency and low resistance capturing particulate has attracted wide attention. Herein, a strategy for the fabrication of all-polymer hybrid electret fiber membranes by electrospinning is reported based on the ethyl cellulose (EC) and polystyrene (PS). The preparation conditions of electret fiber membrane were optimized by focusing on the effect of the mass ratio of EC/PS and the spinning time on the structure, surface potential and purification performance of fiber membrane. The result showed that the fiber membrane has a low fiber diameter of 572 ± 113 nm and porosity of 82.4%, and a better potential stability when the EC/PS mass ratio was 2.5/1 and the spinning time was 10 min. As a result of the enhanced effect of electrostatic attraction on particle capture, the EC/PS composite electret membrane could make the purification efficiency of PM2.5 reach to100% within 11 min. Moreover, the EC/PS composite electret membrane has the properties of ultra-light base weight of 1.08 g·m −2 , low air resistance of 28 Pa and long service life. Therefore, the all-polymer EC/PS electret nanofiber membranes have broad application prospects in the field of air purification. • All-polymer electret EC/PS fibers without nanotoxicity are prepared. • Biomass-based electret based on ethyl cellulose as the main raw material increases the environmental friendliness of the material. • the fabricated electret fiber has an ultralight base weight. • the resultant electret fibers with high efficiency and low resistance can be compared with high efficiency air purification membrane.

Study and Analysis of a Multilayer Multipair Electret-Based Thin-Film Mechanical Antenna

Very low-frequency (VLF) and ultralow-frequency (ULF) electromagnetic waves have the advantage of high penetration and low propagation loss in wireless communication systems and are mainly used for underwater and underground communications, as well as for earthquake and lightning forecasting. At present, VLF and ULF antennas are mostly bulky and require hundreds of antennas and more to be set up, which is costly and inefficient. In this paper, we propose to generate VLF and ULF signals by rotating a multilayer multipair electret thin-film electret driven by an excitation device, which improves the problem of low radiation efficiency of VLF and ULF signals and the large size of conventional low-frequency transmitting antennas. Based on a multilayer, multipair electret film mechanical antenna, a magnetic field propagation model is developed, and the relationship between the magnetic flux density mode and the number of layers of electret films, as well as the relationship between the antenna emission frequency and the motor rotation frequency and the number of pairs of electret films, is analyzed. The selection of a suitable model for practical situations based on conditions such as antenna size and propagation distance is illustrated. The research work is of great importance for guiding the design of mechanical antennas and optimizing antenna structures.

A low voltage-powered soft electromechanical stimulation patch for haptics feedback in human-machine interfaces

One grand challenge in haptic human-machine interface devices is to electromechanically stimulate sensations on the human skin wirelessly by thin and soft patches under a low driving voltage. Here, we propose a soft haptics-feedback system using highly charged, polymeric electret films with an annulus-shape bump structure to induce mechanical sensations on the fingertip of volunteers under an applied voltage range of 5–20 V. As an application demonstration, a 3 × 3 actuators array is used for transmitting patterned haptic information, such as letters of ‘T’, ‘H’, ‘U’ letters and numbers of ‘0’, ‘1’, ‘2’. Moreover, together with flexible lithium batteries and a flexible circuit board, an untethered stimulation patch is constructed for operations of 1 h. The analytical model, design principle, and performance characterizations can be applicable for the integration of other wearable electronics toward practical applications in the fields of AR (augmented reality), VR (virtual reality) and robotics.

Hybrid vibration energy harvesting based on piezoelectric polyline beams with electret coupling

Aiming at harvesting vibration energy more efficiently, a hybrid energy harvester is proposed, which integrates the piezoelectric and electrostatic effects. It is composed of two piezoelectric polyline beams and an electret film. The two piezoelectric polyline beams stand closely, of which one is covered by an electret film, while the other is covered by a counter-electrode. Under base excitations, the electret film interacts with the electrode, by which the two polyline beams can excite each other and form a broad frequency band of large vibration, thereby generating a large output. The dynamical equations are derived and the simulations are carried out. For validation, the experiments are conducted. The experimental results prove that the hybrid harvester is more efficient in harvesting stochastic vibration energy. For PSD = 0.0041 g2/Hz, the two piezoelectric polyline beams can generate the power outputs of 47.9 and 23.6 µW, respectively.

Theoretical analysis and experimental validation of frequency-moldable electrostatic energy harvesters biased with a high elastic electret film

It is an effective strategy to improve the power output of energy harvesters by adjusting resonance frequency of the devices to adapt to complex frequency distribution of vibrations in environments. This paper presents mechanical analysis and experimental validation of a kind of light-weighted, flexible electrostatic vibrational energy harvesters with a customizable resonance frequency range. Such energy harvester consists of a stretchable electret beam, whose high elasticity was achieved by introducing a corrugated structure, an arc-shaped counter electrode, and a light seismic mass. The resonance behavior of the device can be simulated by the corrugated beam with flexible support at both ends. Mechanical analytical modeling allows one to predict resonance frequency of energy harvesters, making the devices potentially interesting for customizable resonance harvesting and broad bandwidth energy harvesting, and thus to satisfy actual application scenarios. The experimental results are in agreement with the theoretical prediction. For a device with an initial size of 15 × 10 × 9 mm3 and a seismic of 0.06 g, by modulating the length of the fluorinated polyethylene propylene electret beam from 15 to 35 mm, a customizable resonance frequency ranging 14–60 Hz, and the normalized output power up to several 100 μW was achieved, demonstrating great superiority.

Evaluating the impact of acoustic impedance matching on the airborne noise rejection and sensitivity of an electrostatic transducer

To capture sound from solid mediums, such as the human body or musical instruments, transducers designed for airborne sound pickup are typically used. Acoustic impedance mismatches between the medium and transducer decrease the energy captured and increase noise corruption. Here, we demonstrate the improved sensitivity and airborne noise rejection of an electrostatic transducer with an acoustic impedance matched to the medium of interest. The transducer produces an electrical response when mechanical vibrations compress the distance between an elastomer with patterned microstructures and a charged electret film. Using a statistical model generated through an I-optimal design of experiments, the elastomer is fabricated with a specific polymer and concentration of nanoparticles to possess a targeted acoustic impedance. Transducers containing elastomers with impedances in the range of 1 to 2.2 MRayls are assembled and characterized on simulators emulating the human body and a wooden instrument. The noise rejection is quantified using the coherence between the captured transducer signal and the simulated ambient noise. Sensitivities are similarly characterized by comparing the transducer signal and input simulator signal. With an impedance closely matched to the medium of interest, the transducer demonstrates a 2V/N sensitivity and 35 dB SNR improvement compared to an airborne transducer.

Tuneable resonance frequency vibrational energy harvester with electret‐embedded variable capacitor

An electret-based electrostatic energy harvester featuring tuneable resonance frequency, small size, light weight, and high output power was designed and its performance predicted by the finite element method and verified by experiment. The device consists of a resilient fluorinated polyethylene propylene (FEP) electret film that is metallised on one side with a small seismic mass attached to its centre and an arc-shaped counter electrode. In principle, such an energy harvester is mechanically a mass-spring system and electrically a self-bias voltage variable capacitor and converts vibrational energy into electrical energy by electromechanical coupling. For an energy harvester sample with dimensions of 30 × 10 × 9 mm for which the last dimension denotes the initial depth of the centre of the harvester, the resonance frequency can be tuned from 17 to 70 Hz by stretching the length of the FEP film loaded with a given seismic mass of 0.06 g. For a seismic mass of 0.1 g, the harvester generated a power up to 797 μW to a matching resistor at its resonance frequency of 17 Hz at an acceleration of 1×g, where g is the gravity of the earth. Such energy harvesters are promising candidates for use in self-powered electronic devices and wireless sensor network nodes.

Electrets: A Remedy for Partial Discharge Caused by Power Electronics Switching

In this article, a novel approach that mitigates partial discharge (PD) by neutralizing local electric fields with the incorporation of electrets is presented. The surface charge density requirement of electret films that enable electric field neutralization is derived and numerical analysis is performed to validate them. The fabrication process of electrets is introduced, and we experimentally demonstrate the electret films reducing PD caused by the high <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dv/dt</i> of power electronics switching voltage stimuli for the first time. The PD measurements of an uncharged polyvinylidene fluoride (PVDF) film and a charged PVDF (electret) film are compared as a function of the rise time of square voltage stimuli. In addition, we compare the PD inception voltage of the two films while maintaining <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dv/dt</i> at a constant value to regulate the impact of capacitive current pulses falsely being detected as PD. The results show great promise in the electret-based approach and suggest that further development of the technology may be able to completely solve PD that has been both chronic and emerging issues in the power industry.

Unexpected bipolar space-charge polarization across transcrystalline interfaces in polypropylene electret films

A double-layer transcrystalline polypropylene (PP) film with a flat central interface layer between its two transcrystalline layers is obtained by recrystallization from the melt between two polytetrafluoroethylene (PTFE) surfaces on both sides of the PP film. Its electret properties are studied and compared with those of a single-layer transcrystalline PP film re-crystallized in contact with only one PTFE surface. Within experimental uncertainty, the two types of transcrystalline films exhibit the same thermal properties and crystallinities. After thermal poling, however, two hetero-charge layers of opposite polarity are found on the internal interfaces of the double-layer transcrystalline films and may together be considered as micrometer-sized dipoles. The unexpected phenomenon does not occur in single-layer transcrystalline samples without a central interface layer, suggesting that the interfaces between the transcrystalline layers and the micrometer-thick central interface layer may be the origin of deeper traps rather than the crystalline structures in the transcrystallites or the spherulites. The origin of the interfacial charges was also studied by means of an injection-blocking charging method, which revealed that intrinsic charge carriers introduced during recrystallization are most likely responsible for the interfacial charges. It is fascinating that a material as familiar as PP can exhibit such intriguing properties with a special bipolar space-charge polarization across the central interface layer after quasi-epitaxial surface moulding into a double-layer transcrystalline form. In addition to applications in electret (micro-)devices for electro-mechanical transduction, the highly ordered structures may also be employed as a new paradigm for studying charge storage and transport in polymer electrets and in dielectrics for DC electrical insulation.

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.

Flexible electrostatic transducer with tuned acoustic impedance for improved sensing of body-and water-borne sounds

Acoustic transducers that are designed for airborne sound suffer from impedance mismatches when used in different media. These mismatches result in lowered energy density at the transducer and increased corruption by external noises. Coupling layers can mitigate mismatches, but restrict real-world applicability through limited bandwidth, thickness- dependent sensitivity, and bonding issues. We introduce a self-powered, flexible, electrostatic transducer with a highly tunable acoustic impedance and wide frequency bandwidth, without matching layers. The transducer can be precisely tuned to impedance values in the range of 1 to 2.5 MRayls, which includes skin, fresh and saltwater, and most plastics. Metal-coated microstructures on the surface of an elastomer create small gaps with the charged polymer electret film that are compressed by mechanical vibrations in the elastomer and induce a change in the voltage output. The transducer exhibits a sensitivity greater than 2V/N and an SNR improvement of 35 dB in highly noisy environments. Major factors that affect the output of the transducer are analyzed, including the dimensions of the microstructures, elastomer thickness, and properties of the electret polymer. The acoustic properties, such as sensitivity and improved rejection of environmental noise, are experimentally evaluated through a body phantom in simulated noise environments in a semi-anechoic chamber.