Bipolar Electret Research Articles

Fully Degradable, Highly Sensitive Pressure Sensor Based on Bipolar Electret for Biomechanical Signal Monitoring

Fully Degradable, Highly Sensitive Pressure Sensor Based on Bipolar Electret for Biomechanical Signal Monitoring

Multiphase Bipolar Electret Rotary Generator for Energy Harvesting and Rotation Monitoring

Inspired by an electromagnetic generator containing multi-pole radial ring magnets, we designed a bipolar electret rotatory power generator (e-RPG) with a multi-phase core-shell structure to boost output performance and achieve high-precision self-sensing capabilities. A selectively localized rotary corona discharging method is invented to uniformly inject both positive and negative charges into a single electret’s curved surface. The experimental results show that the output power of the bipolar-charged e-RPG is increased to 384% compared with those of positive or negative charging alone, which is consistent with the results obtained from the theoretical model. By introducing the core-shell-structured multi-phase electrode design, both the rotation direction and speed can be precisely detected. The frequency sensitivity and linearity <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{R}^{2}$ </tex-math></inline-formula> (coefficient of determination) are up to 3.0 rpm/Hz and 99.9%, respectively. The proposed e-RPG has potential applications in the Internet of Things, self-powered sensing and self-sensing of rotational motions.

The influence of orthophosphoric-acid surface modification on charge-storage enhancement in polypropylene electrets

Bipolar electrets from polypropylene (PP) are essential, e.g., in electret air filters and in cellular-foam ferroelectrets. Therefore, the mechanism of surface-charge stability enhancement on PP electrets via orthophosphoric-acid surface treatment is investigated in detail. It is shown that the significant charge-stability enhancement can be mainly attributed to deeper surface traps originating from deposited chemicals and topographic features on the modified surfaces. Thermally stimulated discharge of chemically treated and non-treated PP films with different surface-charge densities is used to test the limits of the newly formed deep traps in terms of the capacity for hosting surface charges. When the initial surface-charge density is very high, more charges are forced into shallower original traps on the surface or in the bulk of the treated PP samples, reducing the effect of the deeper surface traps brought by the surface modification. The well-known crossover phenomenon (of the surface-charge decay curves) has been observed between modified PP electrets charged to ±2 kV and to ±3 kV. Acoustically probed charge distributions in the thickness direction of PP electrets at different stages of thermal discharging indicate that the deep surface trapping sites may have preference for negative charges, resulting in the observed asymmetric charge stability of the modified PP films.

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.

A bipolar electret-based micro-vibration-driven energy harvester with multi-air-gap structure

Micro mechanical energy harvester as a kind of micro power source for micro electro mechanical systems (MEMS) has been paid considerable attention. Electret-based vibration energy harvester (E-MVEH) has been a research highlight due to its high sensitivity, simplicity and compatibility to the fabrication process of MEMS. In this manuscript, a novel E-MVEH with multiple-air-gap structure based on bipolar electrets is comparatively investigated. The E-MVEH with single-air-gap structure consists of an electret membrane (EM) with an electrode on one surface and a counter-electrode supported by means of micro-springs. The E-MVEH with double air gaps is made of a bipolar electret membrane put in the middle of two counter-electrodes that are supported by means of micro-springs. It is essentially equivalent to two variable capacitors in series. The E-MVEH with four air gaps consists of two double-air-gaps E-MVEH in parallel connection. When an external vibration causes a relative displacement between the electret membrane (EM) and the counter-electrode, the capacitance varies, leading to re-organization of the charges on the counter-electrode. Thus, an alternating current is generated and the vibration energy is converted into electric energy. This alternating current can be changed into direct current through a full bridge rectifier. The bipolar EM used in this study is a sandwich FEP/THV/FEP perfluoropolymer EM reported previously by our group. It can display persistent and strong electrostatic field with both polarities on the top and bottom surface, respectively. The experimental results of the output characteristic for the proposed E-MVEH show that there exists a resonant frequency in the vibration frequency range of 30−180 Hz, at which the open-circuit voltage and short-circuit current and hence the output power reach their respective maximum. The resonant frequency moves towards the low frequency with increasing vibration intensity, whereas the effect of air-gap number on the resonant frequency is rather small. On the basis of experimental results and theoretical analysis, the maximum output power is positively correlated to the vibration driving force, the surface potential and the area of the electret membrane, and the air-gap number. When the surface potential of the electret membrane is 1200±50 V, the maximum output power is only 0.19 μW for single-air-gap structure and reaches 27.02 μW for the four-air-gap structures under a vibration driving force of 0.5 N and a load resistance of 55 MΩ. For the four-air-gap structures, the maximum output power increases to 59.12 μW when the surface potential of the electret membrane increases to 2150 V. The multi-air-gap energy harvester fabricated with bipolar electret membrane can produce larger variable capacitance; therefore, the output power is significantly enhanced. This novel E-MVEH provides an up-to-date strategy for the design of self-powered sensor systems.

A micro-oscillation-driven energy harvester based on a flexible bipolar electret membrane with high output power

This paper reports the successful assembly of a micro-oscillation-driven energy harvester with bipolar electret membrane and dual air gap.

Piezoelectricity of porous polytetrafluoroethylene single- and multiple-film electrets containing high charge densities of both polarities

Single-film bipolar electrets of porous polytetrafluoroethylene are generated by means of a two-step corona-charging process at elevated temperatures. Quasi-static direct piezoelectric coefficients of up to 0.15 nC/N have been observed on these films. In addition, multiple-layer stacks of porous and non-porous polytetrafluoroethylene films with monopolar charge were also investigated. While the piezoelectric responses of the stacks were not as high as those of the single films, the multiple-film arrangements may have other advantages such as better electrical shielding or tunable mechanical properties and adjustable acoustical impedance. Our new results are discussed in the context of the emerging field of porous polymer electrets with many potential device applications.

Theoretical and experimental study of the electromechanical electret slit-effect transducer

Theoretical and experimental investigations of the electromechanical electret slit-effect transducer are presented. After deriving the basic equations of the bidirectional (or reciprocal) transducer from known relations for electret fields, the force and the electric response are calculated for a very general case. From these fundamental equations, the responses of slit-effect devices with symmetric bipolar electrets, with monopolar electrets, and with one-sided metallised electrets are deduced. Experimental results confirm the theoretically predicted linearity and yield an electromechanical reciprocity constant of the order of 1 mu NV-1 for a single-element slit-effect transducer. Furthermore, force and dynamic voltage-response measurements of a 20-element transducer are reported. Finally, technological problems and possible applications of slit-effect devices are discussed.

Slot-effect electrometers with monopolar electrets

Slot-effect electrometers with monopolar electrets as active elements are, in several aspects, superior to similar electrometers with bipolar electrets. In particular, the sensitivity of electrometers with monopolar electrets does not depend on the electret thickness and on the electret–electrode gap. Therefore, monopolar foil electrets can be used in such meters, and the electrometers can be constructed with less precision than when bipolar electrets are used. Two prototype electrometers are described: one with a disk-shaped electret, the other with a cylindrical electret. The sensitivity of the former is 1000 V or 10−7 C per full-scale deflection, that of the latter is 100 V or 10−9 C per full-scale deflection.