Charge Transport Investigation In Low-Density Polyethylene/Silica Nanocomposite Based On Experiment And Numerical Simulation

Polymer has been widely used as insulating materials in electrical and electronicsengineering. But some problems may occur during operation, especially the space chargeaccumulation under DC electric field. Space charges formed by trapped charge carriers mayincrease the local electric field and thus lead to a breakdown of the insulator. When thenanofillers are homogeneously dispersed in the polymer matrix, even if they are small inconcentration, polymer’s properties, such as chemical structure, mechanical, thermal andelectrical characteristics, can be effectively improved. To elucidate the underlying phenomena,many researchers are participating in the study of polymer nanocomposite and large amountof data are currently available in the literature. Some models have been suggested to interpretthe effect of nanofiller in polymer matrix. However, charge injection, transportation, andtrapping in nanocomposites are still complicated and require sufficient experiments andfurther theoretical analysis.In this paper, the space charge distribution, polarization relaxation current, depolarizationrelaxation current and quasi-steady high field conduction were investigated in low-densitypolyethylene (LDPE) and LDPE/silica nanocomposites filled with various concentrations ofhydrophobic nanosilica. The concentration of hydrophobic nanosilica was from0to5.0wt%and the average particle size was7nm and16nm, respectively. The variations of trap density,trap depth and apparent mobility in LDPE/silica nanocomposite with the introduction ofnanosilica were evaluation by relevant theories based on experiment results. The nanosilicaeffect on charge transport in LDPE/silica nanocomposite was discussed. Furthermore, thepreliminary numerical simulation of space charge behavior was study based on theexperiment research.Compared with the depolarization property of space charge and the property of conduction current, it is clearly that threshold field for remarkable charge injection andremarkable space charge accumulation is consistent in pure LDPE and nanocomposite. Theeffect of nanofiller is not distinct for space charge accumulation and conduction at lowelectric field. At high electric field, the heterocharge accumulation in pure LDPE clearlyappears in the vicinity of both electrodes. After the introduction of nanofiller, the space chargeaccumulation varies with the increase of nanofiller concentration from0~5.0wt%.(1) At lownanofiller concentration, the density of heterocharge near both electrodes reduces innanocomposite, while the conduction is distinctly larger, compared with that of pure LDPE.(2)As the nanofiller concentration further increases, homocharge is formed in nanocompositenear both electrodes. Moreover, with the increase of nanofiller concentration, the amount ofhomocharge space charge accumulated firstly increases and then reduces, while theconduction sequentially reduces.The depolarization current of pure LDPE acquired from0.1to3600s is associated withdeep trap depth from0.71to0.98eV. Three trap peaks are obviously observed in trapdistribution evaluated from isothermal depolarization current theory (IDC). It indicates thatdepolarization relaxation current is originated from detrapping of charge in these three typesof deep traps. The trap depth evaluated from depolarization property of space charge from2to3600s is in the range from0.9-1.06eV, included in the range estimated based on IDC theory.The density of deep trap significantly reduces after introduction of nanofiller, evaluated by theIDC theory and the ε＂proportional to trap density. It means that the introduction ofnanofiller could suppress the formulation of deep trap. Moreover, the density of deep trap issmaller in nanocomposite filled with7nm nanofiller, compared with that filled with16nmnanofiller. It indicates that the suppression of deep trap is more effective for nanofiller atsmaller particle size. The threshold Et-cproportional to total trap density H increases with theincrease of filler concentration. However, it is significantly lower in the nanocomposite at lowfiller concentration and apparent larger at high filler concentration, compared with that ofpure LDPE.The space charge behavior in pure LDPE and LDPE/silica nanocomposite is simulatedbased on the experiment research of space charge and conduction. The charge transport modeland numerical scheme are investigated in the paper.(1) To represent the effect of impurity ionexisting in polymer on space charge behavior, the impurity ion transport model is proposed based on the bipolar charge transport model.(2) The method of reasonable variation regionfor charge migration is proposed based on the Courant-Friedrichs-Lewy (CFL) relation. Thesimulation result indicates that it could solve the oscillation and negative density in schemeduring simulation and reduce the numerical diffusion. Furthermore, the numerical simulationsoftware is developed for the dynamic simulation of space charge behavior based on bipolarcharge transport model and impurity ion transport model. The charge injection, transport,trapping and recombination could be monitored during polarization and depolarizationprocesses, respectively.Compared with the result of simulation and experiment, it is clear that there are somelimitations on the bipolar charge transport model and impurity ion transport model, due to thelimitation on charge source. The bipolar charge transport model could only simulate thehomocharge accumulation, while the impurity ion transport model could only simulate theheterocharge accumulation. They are difficult to simulate the polarity variation of spacecharge accumulation in nanocomposite with the increase of nanofiller. In the paper, theimpurity ion transport model is introduced into the bipolar charge transport model. Thesimulation result indicated that the combination of these two models could not only treat theheterocharge accumulation in pure LDPE, but also simulate the threshold field characteristicfor apparent space charge accumulation. Furthermore, it could simulate the trend fromheterocharge accumulation to homocharge accumulation in nanocomposite with the increaseof filler concentration.