Heterocharge Distribution Research Articles

Deep trapped bipolar heterocharges enable electret nanofibrous membranes for high-efficiency PM0.3 filtration

Recently frequent global pandemic outbreak triggers the demand of high-efficiency aerosol filtration materials. Most of traditional electret air filters can hardly achieve high efficiency (>99.99 %) due to limited charge density and inappropriate charge distribution. The influence mechanism of nanofibers bipolar heterocharge configuration on electrostatic capture of airborne particles is still confusing. Herein, a novel strategy to create high-efficiency air filters based on electret nanofibrous membranes with deep trapped heterocharge is reported. Space charges and dipole charges with opposite polarity are formed in electrospun membranes composed of different polymers under in-situ charge injection and electric poling during electrospinning. Nanoscale spatial surface potential mapping on individual nanofiber characterized by Kelvin probe force microscopy demonstrates bipolar heterocharge distribution. Numerical simulation shows that fiber heterocharge distribution endows the electret membrane with greater electrostatic field intensity gradient, resulting in stronger electrostatic force between nanofibers and airborne particles. With bipolar heterocharge configuration and bimodal fiber diameter distribution, the nanofibrous membranes exhibit high-efficiency low-resistance filtration property against 0.3 μm NaCl particles (99.994 %, 97.4 Pa) at 5.3 cm s−1. Furthermore, benefiting from charges with deep trap energy of 1.12 eV, the membranes still maintain 99.972 % filtration efficiency after 40 h exposure in 95 % relatively humidity. The prepared membranes brings new insight in the design of high-efficiency and long-term stability air filtration materials for public health precaution. This study not only reveals influence mechanism of nanofibers heterocharge distribution on electrostatic attraction, but also provides useful guidance for fiber charge configuration design to improve aerosol filtration performance of electret filters.

Empirical Conductivity Equation for the Simulation of the Stationary Space Charge Distribution in Polymeric HVDC Cable Insulations

Many processes are involved in the accumulation of space charges within the insulation materials of high voltage direct current (HVDC) cables, e.g., the local electric field, a conductivity gradient inside the insulation, and the injection of charges at both electrodes. An accurate description of the time dependent charge distribution needs to include these effects. Furthermore, using an explicit Euler method for the time integration of a suitably formulated transient model, low time steps are used to resolve fast charge dynamics and to satisfy the Courant–Friedrichs–Lewy (CFL) stability condition. The long lifetime of power cables makes the use of a final stationary charge distribution necessary to assess the reliability of the cable insulations. For an accurate description of the stationary space charge and electric field distribution, an empirical conductivity equation is developed. The bulk conductivity, found in literature, is extended with two sigmoid functions to represent a conductivity gradient near the electrodes. With this extended conductivity equation, accumulated bulk space charges and hetero charges are simulated. New introduced constants to specify the sigmoid functions are determined by space charge measurements, taken from the literature. The measurements indicate accumulated hetero charges in about one quarter of the insulation thickness in the vicinity of both electrodes. The simulation results conform well to published measurements and show an improvement to previously published models, i.e., the developed model shows a good approximation to simulate the stationary bulk and hetero charge distribution.

Modulation of DC breakdown strength by direct fluorination in polyethylene

Breakdown performance of polymer under high voltage direct current (DC) and alternating current (AC) fields is receiving more and more attentions with the ever-increasing demand of raising voltage class and developing long-distance transmission. The existing researches indicate that space charge should play an important role in influencing breakdown performance. However, two key issues, that the different effects of space charge on DC breakdown strength (FDC) and AC breakdown strength (FAC), and that the reason for FDC being always higher than FAC, remain not completely clear. In this letter, we study the mechanism of DC and AC breakdown by modulating space charge distribution in low density polyethylene by using direct fluorination process. The results show that FDC closely relates to the space charge modulation under DC field but FAC almost has nothing to do with it. Therefore, direct fluorination process can be used to modulate space charge distribution under DC field and further FDC. DC breakdown models with both homocharge and heterocharge distributions are proposed. They ascribe DC breakdown to field distortion induced by space charge and large defects caused by energetic processes, which happen in middle bulk under homocharge distribution and in marginal bulk under heterocharge distribution. FAC being still lower than FDC with heterocharge distribution proves the important influence of the energetic processes happening at polymer/electrode interface and in marginal bulk, besides the field enhancement caused by heterocharge. The modulation mechanism is discussed combined with trap characteristics of carriers.

Measurement of space charge distribution in epoxy resin

Internal space charge behavior of insulating materials has recently attracted attention of many researchers, and a large number of experimental studies were carried out by using the materials for dc cables, such as XLPE, LDPE, and HDPE. Epoxy resins are used for insulation under strong electric fields in power apparatus and in electronic devices, and we investigated the behavior of internal space charge using the pulsed electroacoustic method. Two types of epoxy resins were studied: insulation-grade and craft-grade. When dc electric fields were applied to the craft resins treated by immersing them into room-temperature water for 8 and 24 h, positive and negative charges accumulated near the anode and the cathode, respectively, and the charge distribution changed with the immersion time. On the other hand, no charge was observed in the insulation-grade epoxy resin. Next, we treated the samples by immersing them into 100 °C water for 8 h. When the sample was treated for 8 h, hetero charge distribution, which means positive charges near the cathode and negative charges near the anode, was observed. This result is consistent with a previous paper reporting that by chemical analysis, secondary decomposition had occurred. These results show that water and temperature influence the internal space charge behavior of epoxy resins. © 1999 Scripta Technica, Electr Eng Jpn, 129(3): 9–16, 1999

エポキシ樹脂中の空間電荷分布の観測

Internal space charge behavior of insulating materials has recently attracted attention of many researchers, and a large number of experimental studies were carried out by using the materials for dc cables, such as XLPE, LDPE, and HDPE. Epoxy resins are used for insulation under strong electric fields in power apparatus and in electronic devices, and we investigated the behavior of internal space charge using the pulsed electroacoustic method. Two types of epoxy resins were studied: insulation-grade and craft-grade. When dc electric fields were applied to the craft resins treated by immersing them into room-temperature water for 8 and 24 h, positive and negative charges accumulated near the anode and the cathode, respectively, and the charge distribution changed with the immersion time. On the other hand, no charge was observed in the insulation-grade epoxy resin. Next, we treated the samples by immersing them into 100 °C water for 8 h. When the sample was treated for 8 h, hetero charge distribution, which means positive charges near the cathode and negative charges near the anode, was observed. This result is consistent with a previous paper reporting that by chemical analysis, secondary decomposition had occurred. These results show that water and temperature influence the internal space charge behavior of epoxy resins. © 1999 Scripta Technica, Electr Eng Jpn, 129(3): 9–16, 1999

Space charge behavior in an antistatic polymer including polymer solid electrolyte

AbstractA polymer dielectric material tends to be easily charged and may cause an electrostatic discharge, due to the serious problems that result when consequent electromagnetic noise damages the electronic circuits in various kinds of apparatus. The antistatic polymers that have been developed to prevent electrostatic discharge contain polymer solid electrolytes so that space charge originally exists inside them. The mechanism of the antistatic discharge effect of the polymer has been investigated by measuring the space charge distribution using the high‐resolution pulsed‐electroacoustic method. The experimental results suggest that the internal space charge of the polymer accumulates at the surface and forms heterocharge distribution at the interface between the electrodes and the specimen. Thus, internal space charge compensates the applied electric field, resulting in preventing the increase of the surface potential. In addition, internal space charge behavior relates to the dispersion condition of the polymer solid electrolyte that depends on the quantity of the electrolytes and the specimen procedures.

Observation of charge behavior in multiply low-density polyethylene

Effects of interfaces such as metal/polymer interfaces and polymer/polymer interfaces on the space-charge distribution in multiply low-density polyethylene have been investigated using a pulsed electroacoustic method. It has been found that the heterocharge was dominant in an artificial interface existing in a polyethylene sample. The time dependence of the heterocharge distribution in the interface on applied voltages and polarity was studied. From these charge distributions, the modified electrical field was calculated based on Poisson’s equation. The calculated result indicated that the actual field was stronger than the applied uniform field at the artificial interface. A new data display method for a three- or two-dimensional plot is employed to display all measurement data on one plot in which the space charge becomes visible so that the results can be easily and conveniently understood.

Experimental observation of charge transport and injection in XLPE at polarity reversal

This paper presents experimental results of space-charge distribution in cross-linked polyethylene (XLPE) by a pulsed electroacoustic method. The influence of polarity reversal on space-charge distribution in degassed XLPE and as-received XLPE is studied to understand the transport and injection of charges under DC voltage. Homocharge distribution is dominant in degassed XLPE whereas heterocharge distribution is dominant in as-received XLPE. After the applied voltage is reversed, for degassed XLPE, previously injected homocharges are neutralized by newly injected ones with opposite polarity; while for as-received XLPE, previously accumulated heterocharges are replaced by newly accumulated ones with opposite polarity. A new data display method of a three-dimensional colour plot is proposed to display all measurement data on one plot by which the time dependence of space-charge distribution can be understood easily and conveniently.

Effect of surface charge on impulse flashover of insulators in SF/sub 6/

The effect of surface charge on the impulse flashover of short spacers in SF/sub 6/ was investigated. A split-electrode arrangement allowed scanning of the entire space surface by a capacitive probe. With a heterocharge distribution which facilitates both inception and propagation of the discharge, the flashover level is reduced by as much as 50% for sinusoidally distributed charge of peak density 40 mu C/m/sup 2/. With monopolar charge, only positive charge near the cathode has any significant effect on breakdown, the reduction in impulse strength being approximately 35% at 40 mu C/m/sup 2/. Tests with metal insets show that although the shielding effect is reduced in the presence of heterocharge the impulse strength of spacers with insets is still superior to that of plain spacers for the same charge density. The possible effects of surface charge on flashover of longer spacers is discussed in terms of a transition from a streamer to a leader type of breakdown mechanism. >