Surface Potential Decay Rate Research Articles

Beneficial role of manganese dioxide for reducing surface resistivity to increase surface flashover voltage of vacuum insulating ceramics

Electric vacuum insulated devices generate surface flashover and breakdown under instantaneous high voltage. Flashover voltage can be increased through coating without changing the structure of the ceramic device. Herein, multiphase coatings with different manganese dioxide contents are prepared on α-Al2O3 insulating ceramic surface by screen printing and solid-state sintering methods to yield an average film thickness of about 12.38 μm with good bonding strength. X-ray diffraction shows that the crystal phases of the coating are (Al0.9Cr0.1)2O3 and MnAl2O4. Scanning electron micrographs and atomic force microscopy reveal that greater surface height difference and deeper surface depth are more conducive to increasing the surface roughness and the formation of "pores", and decreasing the surface potential decay rate. When MnO2 content is 0.0069 mol, the deep trap center energy level on the material surface reaches its maximum, increasing from 1.506 eV to 1.550 eV, and the surface resistivity of ceramics decreases from 1016 Ω to 1013 Ω. The formation of deep trap centers on the surface makes it difficult for surface charges to detach, which reduces the charge mobility and surface potential decay rate, and leads to decrease in surface resistivity and increase in surface flashover voltage.

Plasma dielectric barrier discharge fluorination modified epoxy resin and its ageing behavior

The modification effect of plasma on the material will be weakened with the storage time, that is, it shows a certain timeliness, which limits further development of plasma modification technology. To explore ageing behavior of plasma dielectric barrier discharge (DBD) fluorinated epoxy resin, the surface fluorination of epoxy resin was realized by plasma dielectric barrier discharge. Scanning electron microscopy (SEM), surface profilometer, X-ray photoelectron spectroscopy (XPS), contact angle tester, high resistance meter, flashover voltage and surface potential testing system were used to characterize the physical morphology, chemical composition and electrical properties of epoxy resin surface before modification as well as being placed in 25 ℃ aging box for 0−30 d after modification. The experimental results show that fluoride grafting on the surface of epoxy resin is realized by DBD fluorination modification, which reduces the surface energy, surface resistivity and trap level of epoxy resin, thus speeds up the surface potential decay rate and increases the flashover voltage along the surface. After storage of 30 d, the fluorine content decreased, the surface energy increased, the attenuation rate of surface potential slowed down slightly, and the flashover voltage also decreased, but it is still higher than that of the untreated sample.

Effects of ambient humidity and thermal aging on properties of Nomex insulation in mining dry‐type transformer

Temperature and humidity are the main factors that cause the decomposition of Nomex paper widely used as turn-to-turn insulation material of mining dry-type transformers. In this study, for understanding the effect of ambient humidity and thermal aging on the properties of Nomex paper, specimens with the initial moisture content of 7% were aged in the oven for 168, 504, 672 h at 180°C, respectively. After thermal aging, corona discharge experiments of unaged and aged specimens with different aging degrees were carried out at different relative humidity (RH) levels from 10 to 70%. The characteristic parameters of corona discharge, such as the average discharge amplitude and the number of discharges, were measured. Then, the surface conductivity and the surface potential decay rate were measured. The morphology and attenuated total internal reflectance Fourier transform infrared spectroscopy were also analysed for explaining the difference of properties between unaged and aged specimens. The experiment results indicate that the average discharge amplitude decreases with the increase of RH or the deepening of the aging degree of specimens, but the number of discharges shows the reversal tendency. The changes in the physical structure and chemical composition, which is induced by the hydrolysis reaction of Nomex paper, accelerate the surface potential decay and increase the surface conductivity.

Filler concentration effect on breakdown strength and trap level of epoxy resin–Al2O3 nanocomposites

In this work, epoxy resin samples incorporated with nano-Al2O3 of various concentrations 0, 1, 3 and 5 wt% were prepared. Surface treatment of nano-Al2O3 was performed by the saline coupling agent of γ-aminopropyltriethoxysilane (KH550) to restrict the aggregation. The thermal gravimetric analysis (TGA) of the epoxy resin/Al2O3 nanocomposites was observed at different filler concentrations. TGA analysis indicated that incorporation of nano-Al2O3 increases the decomposition temperature as compared to pure epoxy resin. Moreover, 3 wt% filler concentration showed increased decomposition temperature because nanoparticles form close connections between molecules due to which thermal stability improved at higher temperatures. Atomic force microscopy was employed for the surface morphology of epoxy resin/Al2O3 nanocomposites which has indicated that surface roughness of epoxy resin/Al2O3 nanocomposites increased slightly with the filler concentration. The distribution of trap energy and trap density obtained by isothermal surface potential decay measurements revealed that 1 wt% filler concentration results in decrease in surface potential decay rate. In addition, significant increase in deep trap energy level and trap density was obvious at 1 wt% filler concentration. AC and DC breakdown strength of the samples was measured to evaluate the effect of filler concentrations on the electrical characteristics of epoxy resin. The results showed that breakdown strength of the epoxy resin/Al2O3 nanocomposites increases with the nano-Al2O3 concentration. Moreover, increase in trap energy results in an increase in the breakdown strength of the nanoparticles.

Understanding the surface condition of gamma irradiated epoxy alumina nanocomposites adopting wavelets and fractal technique

The influence of alumina nanofiller and gamma irradiation on the surface potential variation of epoxy-alumina nanocomposites was investigated. The surface potential decay rate of nanocomposites has increased and the trap depth decreased with alumina nanoparticles addition to the matrix as well as upon exposure to gamma irradiation, Surface roughness was estimated using the wavelets and fractal technique. Daubechies wavelet of order 4 (db4) wavelet was chosen as the most suitable mother wavelet for surface roughness measurement. Multi resolution signal decomposition (MRSD) analysis of surface profile has revealed that with increasing wt% of alumina nanofiller in the nanocomposites, reduction in surface roughness of nanocomposites was observed. Upon gamma irradiation, the surface roughness factor at each level of MRSD has increased marginally. Fractal dimension and lacunarity were calculated for unaged and gamma ray irradiated samples and it exhibits inverse correlation.

Investigation of surface strain by digital image correlation and charge trap characteristics of epoxy alumina nanocomposites

Epoxy alumina nanocomposites were fabricated through shear mixing of alumina nanoparticles in to epoxy resin under optimum process conditions for its potential application as insulator in transformer. The fundamental insight in to the influence of ageing of nanocomposite insulator due to its continuous exposure to UV irradiation and water in terms of their charge trap characteristics was explored. The surface potential decay and strain variation of nanocomposite samples were measured by using Electrostatic voltmeter and Digital Image Correlation technique (DIC), respectively. Diffusion of water into epoxy alumina nanocomposite is less compared with pure epoxy resin. It has been observed that surface potential decay rate and trap depth have shown increasing and decreasing trend, respectively, with the addition of alumina nanoparticles in to epoxy resin. The UV and water aged nanocomposites exhibited a decreasing trend of surface potential decay rate and the governing reasons could be due to surface damage causing increase in deep trap formation. The tensile strength and stiffness of the samples have increased with the addition of alumina nanoparticles in to the epoxy resin. It could be due to the better bonding of alumina particles with epoxy resin. The surface strain induced in to the sample during tensile loading and the surface potential decay rate of nanocomposite has shown a direct correlation, as observed in the present work.

Investigation on space charge and charge trap characteristics of gamma‐irradiated epoxy micro–nano composites

Epoxy nano–micro composite specimen prepared with micro silica and ion trapping nanoparticle, by shear mixing process, was exposed to gamma radiation and its performance for space charge and charge trap characteristics were analysed. The threshold for space charge accumulation of epoxy nanocomposites reduces and rate of space charge accumulation increases with an increase in dosage of gamma irradiation. The average growth of space charge density during poling and charge decay rate during depoling are relatively higher for gamma-irradiated specimens than the virgin specimen. The initial surface potential has a marginal reduction with increase in the dosage of gamma radiation, but the surface potential decay rate has increased significantly. Trap distribution characteristics indicate more number of shallow traps and increase in charge mobility after irradiation. The relative permittivity and loss tangent of the specimens have high impact due to gamma irradiation. The activation energy calculated from DC conductivity by Arrhenius law reduces with increment in radiation dose. Laser-induced breakdown spectroscopy reflected no change in elemental composition with gamma-irradiated specimen. The variation in plasma temperature and ion line to atomic line intensity ratio with dosage of gamma radiation have direct correlation to the Vickers hardness number of the specimens.

Enhancement of insulating properties of polyethylene blends by delocalization type voltage stabilizers

This paper reports on the effects of four voltage stabilizers, m-aminophenylboric acid, 2-methoxy-5-pyridineboric acid, m-aminobenzoic acid and 4-dimethylaminobenzoic acid, on the insulation properties of polyethylene blends. 1 wt% voltage stabilizers were added into low density polyethylene (LDPE) blend containing 10 wt% high density polyethylene (HDPE) by a solution method. Physicochemical and electrical properties of the blends were analyzed by infrared spectroscopy, thermo-gravimetric analysis, differential scanning calorimetry, electric tree initiation tests, space charge and surface potential decay measurements. The results showed that the addition of the voltage stabilizers had a little effect on the thermal stability and melting properties of the LDPE/HDPE blend. When the voltage stabilizers were included into the blends, the tree initiation voltage (TIV) increased, in particular the blend with m-aminobenzoic acid showed the highest increase of 41% compared to the reference LDPE/HDPE. It was found that space charge accumulations of the blends were apparently suppressed by adding voltage stabilizers. Moreover, the blend with the m-aminobenzoic acid had the least space charge accumulation. The surface potential decay rate of the blends was significantly increased after adding voltage stabilizers. Quantum chemical calculations illustrated that the HOMO-LUMO energy gap was closely related to the TIV of the blends, whereas the m-aminobenzoic acid has the lowest energy gap level and the best effect of restraining the electrical tree initiation. The trap level calculation showed that the trap depth in the blends decreased with the addition of the voltage stabilizers, which was beneficial to suppress the accumulation of space charge in the materials.

Moisture effect on surface fluorinated epoxy resin for high-voltage DC applications

Charge accumulation under high-voltage DC is a major concern in the transmission system as its presence distorts the local electric field. By performing chemical treatment on the polymeric insulation via fluorination process, the charge dynamics of the treated material can be modified. Surface fluorination treatment has been proven to be an effective tool to enhance the dielectric properties. The treatment slightly improves the surface conductivity value and enhances the surface potential decay rate, which helps to prevent the accumulation of surface charge and consequently improve the breakdown strength of epoxy samples. However, the authors suspected the introduction of a fluorinated layer on top of the epoxy sample may have the capacity to absorb moisture from the environment. The increase in the surface conductivity by the fluorination treatment may not come from the fluorine layer itself, but rather from the absorbed moisture in the surface layer. The loss of moisture from the surface may lead to a dip in dielectric performance of the treated materials. Hence, a study on the moisture effect on the surface fluorinated epoxy resin was designed. The fluorinated samples were characterised by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDX) to analyse the morphology as well as the concentration of elements in the treated materials. The samples then were dried in vacuum oven at 105°C and/or inside nitrogen gas chamber at room temperature for 24 hours to forcefully dry out the absorbed surface moisture. The measurements of surface DC conductivity and surface potential decay were taken before and after the drying process. It has been found that the fluorination treatment on epoxy resins did give rise to high moisture content on the surface which plays an important contribution towards the dielectric enhancement properties.

粒径の異なる高分子粉体層の交流コロナ帯電挙動

The behavior of charges in polymer powder layers charged by an alternating current (AC) corona discharge were examined by surface potential measurements and the separation of powder layer by applying an electrical field. A commercial epoxy-polyester paint powder was used as the sample polymer powder in this study. The relationship between the surface potential of the powder layers of various particle sizes and charge distributions within the powder layers was studied. The surface potential of the powder layer consisting of small-sized particles was higher than that of the powder layer consisting of large-sized particles. This result was attributable to the difference in charge distribution within the powder layers which was caused by the total surface area of the powder layer. The charge distribution within the powder layer and the surface potential decay rate depended on the AC corona discharge conditions for the large particle-sized powder layer. The surface potential decay rate changed with the fraction of the large particle-sized particles in the powder layer.

Electrical and structural studies of plasma-polymerized fluorocarbon films

Plasma-polymerized fluorocarbon films up to 8 mu m in thickness were prepared by high-frequency glow discharge deposition, the purpose being to investigate the materials' charge storage (electret) properties. Film compositions were characterized by infrared and by X-ray photoelectron spectroscopies. Under 'mild' plasma conditions, materials with high fluorine concentration (F/C<1.9) could be obtained, while films with lower F/C were found to be partially oxidized. Besides surface potential decay rate of corona charged specimens, other electrical measurements performed include those of complex permittivity and of thermally stimulated depolarization. The latter revealed relaxations (designated gamma and beta ) at -80 degrees C and +50 degrees C, attributed to local motions of chain segments and to rotation of carbonyl dipoles, respectively. While fluorine-rich films have low dielectric loss and relatively good charge stability (characteristic charge decay time tau >1700 h), a systematic decrease in tau and increase in tan delta could be observed for specimens with decreasing F/C ratio.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>