Zhidong Jia

Effect of electric field distribution uniformity on electrospinning

Recent experiments demonstrated that the essential electrospinning mechanism is a rapidly whipping dielectric liquid jet in an electric field. The effects of three kinds of electric field distributions including a very nonuniform electric field, a slightly nonuniform electric field, and a uniform electric field were studied to analyze the influence of the electric field distribution on the electrospinning process and the fiber morphology. The average electric field strength Eav and the electric field nonuniformity coefficient f were used to describe the electric field. The results show that the length range of the straight jet path for stable electrospinning process was a function of the electric field distribution. The length of the jet straight part increased with the decreasing f. The initial angle of the looping envelope and the helical pitch decreased with decreasing f. The results also show that higher voltages can be used at the nozzle for a stable electrospinning when f is small. A higher voltage ...

Experimental investigation of the governing parameters in the electrospinning of polyethylene oxide solution

Electrospinning is an old but yet immature process which is now used to form nanoscale polymer fibers. This paper describes the use of SEM and measurements of jet currents to investigate the effects of key parameters on fiber formation during electrospinning of a polyethylene oxide solution (PEO). The ambient humidity and voltage polarity are key factors influencing fiber formation, but this process has not been carefully analyzed. The influence of the ambient humidity can be overcome by choosing a solvent with a larger difference between the saturated vapor pressure and the actual vapor pressure in air or by lowering the ambient humidity when water is used as the solvent. The process works with both negative and positive voltages, but the polarity influences the stability and the fiber diameters. Variations of the current were found to correspond with variations of the fiber diameters. Short target distances were found to prevent fiber formation while long distances resulted in a larger distribution of fiber diameters. The experiments also showed that ultra fine fibers were found between the ends of broken fibers due to a strong electrical force on fibers with little solvent

A shield ring enhanced equilateral hexagon distributed multi-needle electrospinning spinneret

The multi-needle electrospinning system is a convenient way to produce fibers with special structures such as core-shell morphologies at a high production rate. In this paper, a specially designed multi-needle electrospinning system is presented. The spinnerets were built-up with an equilateral hexagon array. Each set of 3 needles of the spinnerets were distributed as an equilateral triangle. A coaxial shield ring was used to create an approximate uniform electric field near the tips of the needles and to restrict the collection area. The simulation results also show that the outside needles can help to create a more uniform electric field near the inside tips of the needles and restrict the path of the inside jets, which works almost the same as the additional shield ring. Based on the simulation results, several multi-needle systems were tested. A 7 cm diameter shield ring was used in a 7 needle system, a 9 cm diameter shield ring was used in a 19 needle system and a 10.5 cm diameter shield ring was used in a 37 needle system. Polyethylene Oxide (PEO) aqueous solution was used as the test solution in experiments. The electrospinning results demonstrated that the use of multi-needle spinnerets is robust and that uniform nanofibers can be produced. The more needles used, the smaller the mean fiber diameter for larger mean electric field strengths. These distributions of needles show the scale up possibility of special structure electrospun nanofiber manufacturing.

Study on hydrophobicity transfer of RTV coatings based on a modification of absorption and cohesion theory

Room temperature vulcanized (RTV) silicone rubber coatings are used increasingly on outdoor high voltage (HV) insulators subjected to heavy pollution. An important property of the RTV coatings is the ability to restore the surface hydrophobicity after a pollutant layer has built up on the surface, which can suppress the development of leakage currents, dry band arcing and flashover. The present study analyzes the hydrophobicity and the hydrophobicity transfer of RTV coatings and the factors that affected the hydrophobicity. A modification of absorption and cohesion theory was used to analyze the hydrophobicity transfer mechanisms and the influences of temperature, polydimethylsiloxane (PDMS) molecular weight, degree of cross-linking, extent of soluble and non-soluble components, and the pollutant properties. The hydrophobicity transfer of RTV coatings was also investigated with various conditions of contaminations and various factors, which affect the hydrophobicity transfer, including the composition of RTV coatings and the outside conditions. Results show that the hydrophobicity transfer of RTV coatings is not only dependent on compositions but also on factors of circumstance. The analysis also explains the obvious difference between the hydrophobicity transfer of RTV coatings to kieselguhr and kaolin based on a particle and surface analysis using the Brunauer, Emmett and Teller (BET) method, which explains the relationship between a BET surface area and the hydrophobicity transfer of the RTV coatings

Reducing Ice Accumulation on Insulators by Applying Semiconducting RTV Silicone Coating

Ice accumulation on high voltage insulators can cause accidents by flashover across the ice-covered insulators. The ice accumulation can be reduced by changing the insulator surface properties by application of a semiconducting RTV silicone coating. This method is based on the hydrophobicity and semiconductivity properties of the coating. The mechanism for reducing ice accretion is analyzed theoretically. Experimental results are presented to validate the effectiveness of the semiconducting RTV coating at reducing ice on insulator strings. The results show that the coating on the insulators delays ice formation and reduces the amount of ice.

Bacterial Decontamination of Water by Bipolar Pulsed Discharge in a Gas–Liquid–Solid Three-Phase Discharge Reactor

Inactivation of Escherichia coli by bipolar pulsed discharge in an air-liquid-solid three-phase discharge reactor has been investigated. The effect of several operating parameters on the inactivation efficiency has also been studied. Experimental results showed that bipolar pulsed discharge in the three-phase discharge plasma reactor had good performance in the inactivation of E. coli. Power intensity and gas flow rate had a positive effect on the inactivation, while the initial solution conductivity had little effect. An alkaline solution condition was favorable for the inactivation of E. coli in the reactor. In addition, possible mechanisms of the inactivation were also discussed

Effect of Hydrophobicity on Electric Field Distribution and Discharges Along Various Wetted Hydrophobic Surfaces

According to hydrophobicity and hydrophobicity transfer studies on silicone rubber surfaces, although the silicone rubber can transfer its hydrophobicity to various contaminants deposited on its surface, the hydrophobic states of water droplets on clean silicone rubber surfaces and on various contaminated silicone rubber surfaces were found to be different based on evaluation by contact angle measurement. Furthermore, surface discharge and aging of materials decreases the hydrophobicity of the surface. Focused on the difference of hydrophobicity and the effect of water droplets on the hydrophobic contaminated surfaces, some contrastive computations and tests of water droplets on various surfaces were carried out in this study. The results showed that the water droplets existing on the surface of silicone rubber materials might change the electric field and voltage distribution along the surface, and even reduced the initial voltage of the corona discharge, which was verified with a comparison test on a high voltage side of polymer insulator monitored by a photomultiplier tube (PMT) system. Since the leakage current provides much useful information to diagnose the state of outdoor insulation, this study investigates the time and frequency distributions of surface discharges on ceramic insulators, new room temperature vulcanized (RTV) silicone rubber coated insulators and aging RTV insulators with various hydrophobic states with the short time Fourier transform and Gabor transform methods. Then a correlation was found between the distribution of the spectrum of surface discharges and hydrophobicity via the joint time and frequency analysis of discharges on various hydrophobic surfaces.

Anti-icing performance of RTV coatings on porcelain insulators by controlling the leakage current

Considerable work has been done on preventing ice formation on insulators and the modification of the surface characteristics by increasing the contact angle and decreasing the adherent force have been tried with some degree of success. Heating by electric current has however proved to be an effective and practical method for de-icing transmission lines, but difficult to apply to insulators. A room temperature vulcanized (RTV) silicone rubber coating containing sufficient carbon black to render it partially conducting but not enough carbon to lose its surface hydrophobicity, has been investigated to determine if the heat generated would suffice to inhibit ice growth on the insulators. The heat exchange progress was analyzed and the leakage current through it to prevent ice forming was estimated and coatings developed accordingly. The anti-icing performances of these RTV silicone rubber coatings with different leakage current magnitudes were compared in a climate chamber. The results showed that the surface heating effect, together with the hydrophobicity, could significantly reduce the formation of ice on insulators.

Mechanism of contaminant accumulation and flashover of insulator in heavily polluted coastal area

This paper attempts to address the issue of serious insulator string partial discharge activity and flashover resulted from severe contaminant accumulation of insulators on a 220 kV transmission line in coastal areas during winter when the rain is sparse and heavy contamination accumulates over time. Samples of silicone composite insulators, glass insulator with RTV coating and ceramic station insulator are tested in a series of experiments including ESDD and NSDD measurement, hydrophobicity classification, the measurement of static contact angle, TGA and FTIR analysis. Taking the terrain of the transmission line into consideration, it probes into the mechanism of contaminant accumulation and flashover of insulators in heavily polluted coastal areas. A conclusion has been made that the high salt deposit on the surface of the insulators resulted from peculiar terrain and the dampening effect on hydrophobicity transfer characteristic of silicone composite material lower the flashover voltage of silicone composite insulators and ceramic or glass insulators with RTV coatings leading to severe partial discharge activity and flashovers during dry winter. Finally it sums up the lessons in regards to this problem and proposes appropriate solutions.

PREPARATION AND PROPERTIES OF POLY (VINYL ALCOHOL) NANOFIBERS BY ELECTROSPINNING

In the present contribution, ultrafine fibers were spun from poly(vinyl alcohol) (PVA)/water solution using a homemade electrospinning set-up. The effects of the concentration of polymer solution, spinning voltage, collection distance between the tip to target and the flow rate of the solution on the morphological appearance and average diameter of the as-spun PVA fibers were investigated. A more comprehensive and systematic understanding of process parameters of the electrospinning was obtained. In this experiment fibers with diameter ranging from 100 nm to 1000 nm were obtained. The results showed that the solution concentration significantly affected the morphology and diameters of the as-spun PVA fibers. Lower concentration tended to facilitate the formation of fibers with beads. With increasing solution concentration, the morphology was changed from beaded fiber to smooth and uniform fiber and the fiber diameter was increased. Spinning voltage also had an important influence on the diameters of fibers, while the collection distance slightly affected the diameters of fibers. Thinner fibers were received on the target with lower voltage applied on the nozzle. The flow rate affected the diameters and the dispersion of fibers. The effect on crystallinities by electrospinning was also discussed. The morphology of fibers electrospun from PVA was observed and analyzed by scanning electron microscopy (SEM).