Electret Charge Research Articles

All-polymer hybrid electret fibers for high-efficiency and low-resistance filter media

A one-step and controllable strategy to prepare all-polymer hybrid electret fibers is reported based on the coupling of polystyrene and polyvinylidene fluoride in electric response. The complementary dielectric properties between PS and PVDF generate dual-system electret charges within PS/PVDF fibers, thereby improving the electret effect. The bi-component all-polymer electret fibers show enhanced electret property and structural continuity, contributing to a N95 protective respirator with high filtration efficiency (99.752%), low air resistance (72 Pa) and long service life. The fabrication of all-polymer electret fibers solves the challenge of nanoparticle toxicity for existing polymer/nanoparticle electret fibers.

A Method to Determine the Electret Charge Potential of MEMS Vibrational Energy Harvester Using Pure-White Noise

A high-speed measurement method has been developed to electrically determine the potential voltage of the permanent charge or electret formed in a MEMS (microelectromechanical systems) vibrational energy harvester. While the conventional admittance method requires long time to observe the waveforms of the output power in time domain that diminishes when the built-in electret potential is compensated by an external voltage, this work proposes a new method to uniquely and promptly determine the electret potential by using the FFT (fast Fourier transform) analysis on the vibrational energy harvester excited by the white-noise voltage; the resonant peak in the frequency domain diminishes when the electret potential is compensated by the applied external bias voltage. White noise has been used in conventional method to measure the frequency response of devices of linear characteristics. However, due to the randomness of the input signal, random noise is inherently introduced to the measurement result, which urges us to repeat the measurement and to perform time-averaging to smooth out the noise. Therefore we used a newly developed white noise so-called “pure” white noise, which has a totally flat power spectrum, as it is synthesized from waveforms of a constant magnitude and random phases. We compute the inverse Fourier transform of such pure-white noises to instantly obtain the power spectrum without using mathematical averaging, and thus the throughput of measurement is enhanced ten-times faster.

Improvement of PVDF nanofiltration membrane potential, separation and anti-fouling performance by electret treatment

Electret treatment was a simple method to enhance the charge-electrode properties of polyvinylidene fluoride (PVDF) materials due to the increase of space charge and polarization charge of PVDF materials. The polarization charge was due to the electric dipole orientation change in loose nanofiltration PVDF membrane, which increased the electric dipole moment and improved the polarity of surface potential. Importantly, electret charges were less affected by ambient humidity. Therefore, the electret treatment could improve the surface negative potential of loose nanofiltration PVDF membrane, so as to improve its anti-fouling performance under certain conditions. Based on the above theoretical analysis, the influence and mechanism of the electret treatment on the surface potential, morphology, structure, hydrophilicity and anti-pollution performance of PVDF membrane were studied in this manuscript. When the electret time was 7.5 min and the electret voltage was 30 kV, the surface negative potential was the highest. The content of β phase crystals was 39.1%, which was 12.18% higher than that of untreated membrane. In addition, the surface morphology of PVDF membrane did not change significantly, but the water contact angle decreased slightly, and the pore size increased by 0.36–0.75 nm. Importantly, the flux and the rejection of dye and BSA increased to some extent, and the maximum rejection rate and water flux were increased by 10.34% and 20.25%, respectively. Through the cyclic filtration test and analysis, the anti-fouling performance of membrane was increased due to electrostatic repulsion.

Largely enhanced electrostatic generator based on a bipolar electret charged by patterned contact micro-discharge and optimized substrates

With the rapid development of wearable electronic devices and internet of things, harvesting mechanical energy from the surroundings to generate electric power has attracted intensive interests. Here, we demonstrate a rotary electrostatic generator based on a patterned positive and negative charged electret and optimal substrates of the electrode and electret. The bipolar charged electret is prepared using the patterned contact micro-discharge method with a pair of patterned electrodes. The generator obtains a double effective charge density by bipolar charging, which increases the output power by a factor of 4 compared to that of the conventional one based on unipolar charging. Moreover, the foam electrode substrate with the effective relative permittivity of 1.15 reduces the inherent capacitance of the generator, increasing the maximum average output power by a factor of 1.33 than that with a normal printed circuit board substrate. Totally, the maximum average output power of this generator is 5.3 times as high as that of the conventional one. In addition, we found that the electret substrate had a great influence on the output performance of the generator too, and there is an optimal electret substrate with the relative permittivity of 4.5. An electret substrate with low permittivity reduces the induced charge density on working electrodes of the generator, which contradicts the theory and requires further research.

Multi-block electret-based mechanical antenna model for low frequency communication

In an LF/VLF transmission system, the performance of the antenna is of great importance to the entire system. Currently, the electret-based mechanical LF/VLF antenna uses mechanical movement to accelerate electret charges to produce LF/VLF electromagnetic waves, and the frequency of these electromagnetic waves is limited by the rotation speed of the actuating motor. Based on research that addressed the relationship between antenna structure and electromagnetic wave frequency, this paper — in order to increase the frequency of electromagnetic waves — alters the charge distribution mode of the mechanical antenna while keeping the motor’s rotational speed constant to realize an increase of transmission signal frequency. The effectiveness of this method was verified by model simulation.

Electrification mechanism of corona charged organic electrets

This paper developed a comprehensive model which can fully explain the surface potential behavior of corona charged organic dielectric electrets. Compared to previous studies, both sides of the corona charged films were measured while they were grounded or free-standing. All films showed surface potential with the same magnitude, but opposite polarity on each side while they were measured with the other side grounded. This indicates that in contrast to the previous incomplete model, believing only the corona side containing injected charges, both sides should contain injected charges with the same magnitude but opposite polarity. This dipolar charge distribution is relatively stable and leads to a constant and controllable potential difference ΔVf between each side of the free-standing measured film. Interestingly, free-standing surface potential varied dramatically for films charged with the same parameters. This large variation is caused by the minor number of free charges, which is only ~0.1% of the corona induced dipolar charges. Therefore, in this model, dipolar charges control the surface potential difference between each side and the minor charges control the absolute value on a certain side. By manipulating them together, the surface potential on both sides can be precisely controlled.

Triboelectric charging of polytetrafluoroethylene antithrombotic catheters.

This study proposes that a polytetrafluoroethylene (PTFE) electret tube charged by frictional electricity can prevent the solidification of the indwelling catheter in blood vessels. Coagulation in intravascular indwelling catheters may discontinue the treatment because of thrombus-derived bacteria-adhesion infections or poor blood removal. Current commercially available intravascular catheters lack complete antithrombotic measures, even with heparin or urokinase antithrombotic coatings. Herein, we tested the effectiveness of an antithrombotic treatment that prevents coagulation using a static electric charge on the interior of the PTFE tube via the triboelectric effect by rubbing the tube's inner wall with a round glass rod. The anticoagulation properties were evaluated by enclosing a sample of blood in an electret tube and observing the coagulase adhering to the inner wall using a microscope. To confirm the effectiveness of this treatment, the charge-distribution on the inner surface of the electret tube was measured, surface irregularities were observed, and the elements on the surface were analyzed. The surface potential inside the electret tube was - 366.4V, which proved effective for an antithrombotic treatment, as it discouraged coagulation, and the triboelectric charging process caused neither surface element denaturation nor significant surface irregularities. The nearly uniform negative surface charge on the inside of the tube was responsible for the antithrombotic effect because no surface irregularities or change in the surface element denaturation was observed. Triboelectrically charged PTFE electret tubes are highly useful for intravascular indwelling catheters.

Properties of Polytetrafluoroethylene Films Modified with Titanium and Phosphorus Oxide Structures

Properties of polytetrafluoroethylene films modified with mono- and two-component titanium-phosphorus oxide systems by alternating treatment of the polymeric substrate in a prescribed order with vapors of reagents from the series TiCl4, PCl3, and H2O were examined. It was found that the energy characteristics of the sample surface with two-component oxide systems P–Ti and Ti–P in comparison with polytetrafluoroethylene modified with only phosphorus or titanium oxide functional groups due to the increase in the polar component of the surface energy. Specific features of the effect of stabilization of the electret charge in modified polytetrafluoroethylene films. It was shown that the best electret properties are possessed by titanium-containing polytetrafluoroethylene films, including those with two-component titanium-phosphorus oxide additives. According to the results of a differential-thermal analysis, modified samples exhibit an increased thermo-oxidative stability manifested in that the onset temperature of the polymer decomposition increases by 40–50° and the loss of mass decreases by 17–23% as compared with an unmodified sample at 650°C.

Organic thin film memcapacitors

Organic thin film memcapacitors are designed and implemented by an organic multilayered structure, consisting of a semiconducting carrier accumulation layer and a polymer electret charge trapping layer. The capacitive states of the device can continuously and reversibly change upon programming/erasing at high biases, whereas they show nonvolatility upon reading at low biases or power off. The memcapacitive characteristics are ascribed to the modulation of the carrier accumulation threshold resulting from the cumulative and reversible charge trapping in the polymer electret layer. The organic thin film memcapacitors may provide more opportunities for developing organic-based synaptic electronic devices.

Template-based fluoroethylenepropylene ferroelectrets with enhanced thermal stability of piezoelectricity

In ferroelectrets, the piezoelectricity stems from the charges of both polarities trapped on the inner surfaces of the cavities in the material, so that its thermal stability is defined by the stability of the respective charges. In the present work, a template-based lamination technique has been employed to fabricate tubular-channel ferroelectrets from fluoroethylenepropylene (FEP) films. It has been shown that the piezoelectricity in FEP ferroelectrets decays at relatively low temperatures due to the inherently lower thermal stability of the positive charge. In order to improve charge trapping, we have treated both FEP films and inner surfaces of the ferroelectret cavities with titanium-tetrachloride vapor, using the atomic-layer-deposition technique. Using surface-potential-decay measurements on FEP films, we have found that the charge-decay curves shift by more than 100 °C to the higher temperatures as a result of the surface treatment. Direct measurements of piezoelectric d33 coefficients as a function of temperature have shown that the piezoelectric stability is likewise improved with the d33-decay curves shifted by 60 °C to the right. The improvement of electret/ferroelectret properties can be attributed to the formation of the deeper traps on the chemically modified FEP surface. SEM micrographs and EDS analysis reveal island-like structures with titanium- and oxygen-containing species that can be responsible for the deeper trapping of the electret charges.

An electrostatic energy harvester with sandwiched structure of two electret layers

In this paper, we proposed a fully packaged sandwich electrostatic energy harvester, which consists of two opposite charged electret layers to improve the output performance of the packaged device. The new device is based on the out-of-the-plane gap closing scheme, and the main components are similar to our previous research. This sandwich device mainly contains two variable capacitors, and the volume of the device is about 1.3 × 1.8 × 0.15 cm3. Because of the complementation between the two opposite charged variable capacitors, the output performance of the overall sandwich device is better than the single electret layer devices. We have achieved a maximum power output of 20.78 μW at the acceleration of 19.96 m/s2, which is almost 1.4 times of the packaged single capacitance structure, without changing the overall volume of the packaged device. Also, this sandwich device shows good performance at random vibration.

Evaluations of dielectric property and drug release profile of 5-FU patches based on plasma charged electrets

In the present study, the electret 5-fluorouracil patch was developed, the effective surface potential, piezoelectric coefficient d33, open-circuit thermally stimulated discharge (TSD) current spectra and shear adhesion of the patch were measured. The drug release profile of the patch was determined by using high performance liquid chromatography method. A stable potential difference which was positively dependent on the surface potential of the electret was generated on two sides of the patch. The measurements of d33 coefficient, TSD current spectra and adhesion performance showed that the electrostatic field of the electret could cause polarization and cohesive strength decreasing of the matrix molecules, change the distribution and interaction of the drug molecules in patch, therefore to increase the release of drug from the transdermal patch.

Manipulation of electron transport in graphene by nanopatterned electrostatic potential on an electret

The electron transport characteristics of graphene can be finely tuned using local electrostatic fields. Here, we use a scanning probe technique to construct a statically charged electret gate that enables in-situ fabrication of graphene devices with precisely designed potential landscapes, including p-type and n-type unipolar graphene transistors and p-n junctions. Electron dynamic simulation suggests that electron beam collimation and focusing in graphene can be achieved via periodic charge lines and concentric charge circles. This approach to spatially manipulating carrier density distribution may offer an efficient way to investigate the novel electronic properties of graphene and other low-dimensional materials.

Electret-material enhanced triboelectric energy harvesting from air flow for self-powered wireless temperature sensor network

Energy from wind flow is very common in ambient environment which can be harvested by triboelectric generator effectively. Herein, electret based triboelectric generator (E-TriG) with both electrostatic and triboelectric effects are investigated with enhanced performance comparing with the traditional triboelectric generator based on only contact electrification. Electret materials like PTFE, CYTOP, TOPAS, and COC are prepared with different methods and charged under positive or negative conditions to optimize the material property. It is proved that the performance of the triboelectric generator can be improved by negatively charged electrets, while with positively charged electrets, the power output is weakened. As a demonstration, the E-TriG has been successfully applied for wireless temperature sensing. At a wind flow rate of 18 m/s, a storage capacitor can be fully charged by three E-TriGs devices within 15 s, and afterwards, wireless temperature signal could be read and delivered to the internet every 5 s. An average power of 400 μW is therefore harvested. With enhanced performance from the corona charged electret, the triboelectric generator shows promising application for the future wireless sensor networks.

Manipulation of magnetic ionic liquid droplets and their application in energy harvesting

Controlling discrete fluid droplets using magnetic fields on a hydrophobic surface offers new opportunities in microfluidics. Manipulation of two pure magnetic ionic liquids (MILs) [Emim][FeCl4] and [Bmim][FeCl4] was investigated on a hydrophobic surface under the influence of an external magnetic field. The effects of increasing and decreasing magnetic field strength on the shape of the MIL droplets were first studied at three different temperatures. Both MILs showed good robustness to temperature variations. Moreover, the hysteresis phenomena of height and contact line diameter were observed for both MIL droplets with different volumes. Next, the movement of the MIL droplets under the actuation of a horizontal moving magnet was investigated. It was found that the maximum attainable speed increased linearly with the droplet volume, and it was up to 22 mm s−1 for a 10 µl [Emim][FeCl4] droplet when the surface magnetic field strength was 400 mT. Finally, a electrostatic energy harvester using conductive MIL droplets rolling across a charged electret film was studied. An average output power of 78 nW was obtained with a 20 µl droplet. The manipulation of individual real pure MIL droplets without adding or dispersing magnetic particles was persistent and neither evaporation nor phase separation took place.

Influence of Polarization Charges on the Photoluminescence Properties of Nanocomposites Based on Polyvinylidene Fluoride and Titanium Dioxide Nanoparticles

The purpose of this study is to investigate the electret properties and charge states of the nanocomposites based on the polyvinylidenefluoride and titanium dioxide nanoparticles. It has been found that, large amount of charges are accumulated in the interface as a result of migration polarization and these charges create rather large internal local field in which the polarization of titanium-dioxide nanoparticles occurs. Calculated, the amount of the charges accumulated in the interfacial layer of nanocomposites, the intensity of the field created by them and the value of charge activation energy of PVDF/TiO2 nanocomposites after polarization from thermally stimulated depolarization (TSD) curves. As well as the thickness of the interphase layer of PVDF/TiO2 nanocomposites was calculated and it has been revealed that, as titanium dioxide content increases in the polymeric matrix, the thickness of the interphase layer increases and reaches at its highest value at 5% volume content of TiO2 nanoparticles, after this it commences to decrease.

Accumulation of Free Electret Charges in Small-Sized Electrically Active Systems

The paper focuses on the study of structural features and electret properties of the fine-sized mineral kaolin. Special attention is paid to the mechanism of interphase interaction at the interface of electrically active heterogeneous media that leads to the emergence of internal self-electric field capable of ensuring the current circulation in such systems. The authors consider the possibility of regulating the processes of local change in the polar water structure under the impact of the self-electric field.

Fabrication and performance of pressure-sensing device consisting of electret film and organic semiconductor

We propose a new concept of a pressure-sensitive device that consists of an organic electret film and an organic semiconductor. This device exhibits high sensitivity and selectivity against various types of pressure. The sensing mechanism of this device originates from a modulation of the electric conductivity of the organic semiconductor film induced by the interaction between the semiconductor film and the charged electret film placed face to face. It is expected that a complicated sensor array will be fabricated by using a roll-to-roll manufacturing system, because this device can be prepared by an all-printing and simple lamination process without high-level positional adjustment for printing processes. This also shows that this device with a simple structure is suitable for application to a highly flexible device array sheet for an Internet of Things (IoT) or wearable sensing system.

Peculiarities of the accumulation and transport of electret charges in fine-sized disordered structures due to internal voltage

Electret properties and structural peculiarities have been studied for fine-dispersed heterogeneous systems using the example of mechanoactivated natural-coal grains immersed into a polar aqueous matrix. The observed accumulation of currents with no external voltage applied indicates the formation of a potential difference of an internal electric field in test objects. It has been shown that an aqueous-phase layer with rigid fixation of water molecules is formed at the interfaces. The layer is capable of acquiring functions of a potential barrier that hampers transitions of both free water molecules and water molecules oriented by the system’s internal electric field.

Reduction of the risk of thrombosis and restenosis with negatively charged electret covered stents

Rationale : Endovascular implantation may lead to mechanical injury of the vessel walls, their inflammation and subsequent formation of a thrombus in the stented portion of the vessel, as well as to endothelial growth and restenosis. Intimal injury changes the potential of the damaged area from negative to positive. Aim : To demonstrate the efficacy of corrective negative charge on the stent surface after its endovascular implantation in the reduction of the risk of vessel thrombosis and restenosis. Materials and methods : With a terminal element technique we created a model for distribution of electrostatic potentials in a healthy and in a partially injured vessel, as well as in a vessel with a negatively charged electret stent. Trials of experimental and serial stents in animals were performed with stent placement into the pig neck arteries with subsequent (at day 21) histological examination of the areas of stent placement. Morphological abnormalities and vessel wall reactions after endovascular carotid stent implantation, such as clot formation and endothelial reaction were assessed. Results : Negatively charged electret cover of the stent produces corrective electrostatic field to prevent thrombus formation and vascular intima reaction with subsequent restenosis. The use of the stent with negatively charged electret cover ensures a significant reduction of the risk for positive potential inside the injured vessel, thereby dramatically reducing the risk of vessel thrombosis and restenosis. After placement of stents with negatively charged electret cover to animals, there was very mild proliferation of endothelial cells of the vessel wall neointima, compared to that after placement of the reference stents; no thrombus formation was observed. If the reference uncovered stents were placed, there was a dramatic narrowing of the arterial lumen due to proliferation of endothelial neointimal cells, as well as full thrombotic closure of the vessel. Conclusion : The suggested technology allows for improvement of endovascular stent placement, for reduction of the risk of thrombosis and restenosis after endovascular interventions aimed at revascularization of arterial stenosis.