Weiwang Wang

Influence of hydrostatic pressure on dielectric properties of polyethylene/aluminum oxide nanocomposites

The effect of hydrostatic pressure on the dielectric characteristics of polyethylene/aluminum oxide nanocomposites is reported in the present work. Nanocomposite specimens with a thickness of 0.2 mm were placed in a hydrostatic press at pressures ranging from 50 to 200 MPa. Then, ac and dc breakdown and dielectric spectroscopy measurements were carried out on the original and treated specimens. In comparison to the dielectric properties of the original specimens, the results of the treated specimens demonstrate some interesting dielectric behaviors. For example, the breakdown strengths of the treated specimens are enhanced, and the permittivity and dielectric loss are reduced after the hydrostatic pressure treatments. It is found that the movements of molecular chains and polar groups are restricted especially in the interfacial region, leading to the reduction in permittivity and dielectric loss of specimens after hydrostatic pressure treatment. The improvement in breakdown properties is due to the reduction in free volume or a more uniform size distribution of free volume in the nanocomposites after hydrostatic pressure treatments.

Understanding the conduction and breakdown properties of polyethylene nanodielectrics: effect of deep traps

Due to the variation of charge transport characteristics by introduction of nanostructured filler, the conduction and dielectric breakdown properties of nanodielectrics are poorly understood. This work studies on the effect of deep trap on the dc conduction and breakdown properties of nanodielectrics. X-ray diffraction technique is conducted to study the crystallization behavior of LDPE/Al2O3 nanocomposites. Thermally stimulated current (TSC) is applied to measure the trap parameters of specimens. Breakdown strength and volume resistivity are also measured. The results indicate that small amount of nanoalumina enhances the crystallinity, volume resistivity and breakdown strength, and decreases the crystallite size. The TSC results show that the deep trap level and density both increase at low nanoparticle loading samples (<;1 wt%). It is concluded that the independent interfacial region brought by small amount of nanoparticles generates a new potential barrier φ2. It interacts with the original trap sites in LDPE matrix, resulting in the increase of deep traps in nanocomposites. Nanoparticle may act as nucleating agents to modify the morphology and change the deep traps, leading to the reduction of electrical conduction and the improvement of breakdown properties. Both deep trap level and density are benefit to enhance the volume resistivity and breakdown strength. The reduction in mobility of charge carriers, the enhanced height of barrier, the decrease of low density region and the formation of homocharges caused by deep traps are of importance to the reduced conduction and high breakdown performance.

Numerical analysis of space charge accumulation and conduction properties in LDPE nanodielectrics

LDPE nanodielectrics show good space charge suppression performances, reducing the electric field distortions and improving the electric strengths. The decrease of space charge accumulation of LDPE nanodielectrics with increasing the nanoparticle loadings can be explained by the reduction of charge injection, the enhancement of conduction, and so on. However, the phenomena that the conductivities of LDPE nanodielectrics decrease firstly and then may increase with increasing the nanoparticle loadings has not been fully understood. A bipolar charge transport model consisting of charge injection, charge migration, and charge trapping, detrapping, recombination dynamics is used to investigate the space charge accumulation and conduction properties of LDPE nanodielectrics. Based on simulation results and existing experimental results, we discuss the influencing factors for space charge accumulation and conduction properties of LDPE nanodielectrics. It is found that the heightening of injection barrier plays a more important role in the suppression of space charges and the reduction of conductivities of LDPE nanodielectrics. Whereas, the variation of trap density and trap energy will regulate the nanoparticle loading dependent conduction properties.

Linking traps to dielectric breakdown through charge dynamics for polymer nanocomposites

Polymer nanocomposites can change the density and/or energy of traps, suppress the accumulation of space charges, and enhance dielectric breakdown strength. It is of interest to reveal the influencing mechanism of trap properties on the dielectric breakdown of polymer nanocomposites. Results of thermally stimulated depolarization current and surface potential decay were reviewed, showing that incorporating a small amount of nanoparticles into a polymer can increase the density and/or energy of deep traps. Then, the relation between traps and dc breakdown field of several polymer nanocomposites were analyzed. It was found that the increase in the density and energy of deep traps contributes to the improved dielectric breakdown performance. The modifications of traps by nanoparticles and surface treatments affect the charge dynamics in the bulk of polymer nanocomposites. Then, the accumulation of space charges, the distortion of electric field, and the energy gain of free carriers are regulated to improve the performance of dielectric breakdown.

Correlation between trap parameters and breakdown strength of polyethylene/alumina nanocomposites

This paper focuses on the effect of trap parameters on dc breakdown properties of LDPE/Al2O3 nanocomposites. The thermally stimulated discharge current (TSDC) measurement was conducted to characterize the trap parameters of specimens, including trap level and trap density. The experimental results indicated that incorporating of nanoAl2O3 into LDPE changes the trap level and density, which presents that the deep trap level initially increases and then decreases, and the shallow trap level increases with the increase of nanoparticle loading. At 5 wt%, the deep and shallow trap levels are almost the same value. Moreover, the trap density initially decreases and then increases with increasing nanoparticle loadings. The dc breakdown strength initially increases and then decreases with increasing loadings, which corresponding to the variation of the deep trap depth. The enhancement of dc breakdown strength is attributed to the increase in deep trap levels and the increase in trap density. It should be noted that the dc breakdown properties would be improved steadily in a vicinity of 5 wt% nanocomposites with the same deep and shallow trap level.

Modelling of dielectric breakdown through charge dynamics for polymer nanocomposites

A dielectric breakdown model consisted of bipolar charge transport and free volume breakdown criterion is used to investigate the dc breakdown properties of polymer nanocomposites. In the model, we consider charge injection from electrodes, carrier migration with a constant mobility, charge trapping and detrapping associated with deep traps, charge recombination, and energy gained for charge carriers from the local distorted electric field. Since incorporating nanoparticles into a polymer can change its density and/or energy of deep traps, we calculate the dielectric breakdown properties of low-density polyethylene nanocomposites characterized by various densities and energies of deep traps. The simulated results show that the breakdown strength increases with an increase in the density and energy of deep traps, which is consistent with the experimental results. It is shown that the accumulation of homocharges, the distortion of electric field, and the energy gain of free carriers are regulated to improve the performance of breakdown.

Electrical breakdown of polymer nanocomposites modulated by space charges

A method is proposed to calculate the density of deep traps formed in interaction zones based on mesoscopic structure and double electric layer of polymer nanocomposites. Then a space charge modulated breakdown model is utilized to investigate electrical breakdown property and its relation with deep traps in interaction zones. It is found that deep traps formed around independent interaction zones suppress the accumulation of space charges and the distortion of electric field, leading to the improvement of breakdown strength.

Enhanced flashover strength in polyethylene nanodielectrics by secondary electron emission modification

This work studies the correlation between secondary electron emission (SEE) characteristics and impulse surface flashover in polyethylene nanodielectrics both theoretically and experimentally, and illustrates the enhancement of flashover voltage in low-density polyethylene (LDPE) through incorporating Al2O3 nanoparticles. SEE characteristics play key roles in surface charging and gas desorption during surface flashover. This work demonstrates that the presence of Al2O3 nanoparticles decreases the SEE coefficient of LDPE and enhances the impact energy at the equilibrium state of surface charging. These changes can be explained by the increase of surface roughness and of surface ionization energy, and the strong interaction between nanoparticles and the polymer dielectric matrix. The surface charge and flashover voltage are calculated according to the secondary electron emission avalanche (SEEA) model, which reveals that the positive surface charges are reduced near the cathode triple point, while the prese...

Space charge analysis and trap evaluation in silicone rubber by density functional study

This paper presents the fundamental characteristics of the electronic structures of silicone rubber (uncured SR) and cured (crosslinked) SR molecules. Localized states and trapping sites are evaluated by the molecular orbitals (MOs) and the density of state (DOS). The introduction of vinyl groups slightly changes the energy level distribution and introduces the electron-localized states (shallow trapping sites). However, it can be observed clearly that a hole trapping site (0.5 eV) is created by the benzene group. Moreover, some shallower trapping sites are also formed by the vinyl and the benzene groups alternately (0.30.4 eV). The cured SR molecular chains show a reduced energy gap compared with the uncured one. The crosslinked structures introduce the hole and the electron traps. Interestingly, a deep hole trapping site (1.2 eV) was found in the cured SR structure. These localized states or trapping characteristics are ascribed to the interaction of chains, the chemical groups and the crosslinks in the cured SR structure. Space charge of a virgin SR sample can be responsible for the trapping site characteristics, especially for the hole deep trap.

Influence of electron beam irradiation on DC surface flashover of polyimide in vacuum

A satellite operates in a harsh environment including high vacuum and radiations. The vacuum-dielectric interface is the most vulnerable area to flashover discharge. Moreover, the presence of electron radiation will further decrease the flashover voltage. However, the influence of electron radiation on surface flashover remains unclear. We have performed a study on vacuum DC surface flashover during electron beam irradiation as a function of electron kinetic energy and incident direction on polyimide representative of spacecraft dielectrics. The electron radiation was accompanied by electron deposition and bombardment effects on the polyimide surface. The surface flashover voltage was experimentally found to significantly depend on the kinetic energy and incident angle of the electron beam. By calculating the trajectories of incident electron beam, we further found that the surface flashover voltages are closely connected to the irradiated area. During low energy electron beam irradiation, kinetic electrons are repelled away from the dielectric surface by the applied electric field, and deposited electrons will be dominant in the flashover process, promoting the surface flashover voltage. However, when the electron beam energy is high, kinetic electrons can overcome the applied electric field and strike the dielectric surface to generate secondary electrons, which will replace the field-emission electrons to serve as seed electrons, and initiate the flashover at a much lower applied voltage. These observations and analyses are expected to benefit the research into mitigating spacecraft discharge, and promote the knowledge of surface flashover.