Study On Space Charge Characteristics And DC Electrical Properties Of SiO₂/LDPE Micro-nanocomposites

The accumulation of space charges in polymer dielectrics causes a high field to be formed locally,and reduces the breakdown strength of the insulating materials,which has become an important factor restricting the development of HVDC transmission.Studies have shown that doping an appropriate amount of inorganic nanoparticles in the polymer can inhibit space charge and improve some of its dielectric properties.The degree of improvement is closely related to the dispersion of nanoparticles,and the improvement mechanism is still unclear.Studies have also shown that micro-particles can improve the dispersion of nanoparticles to a certain extent.Micro-nano-particles may have a certain synergistic effect after being doped at the same time.By regulating the carrier transport mechanism,materials with better performance can be obtained.In this study,low-density polyethylene（LDPE）is used as the polymer matrix,and the surface hydrophobically modified micron and nano SiO₂ are used as inorganic fillers.Nano,micro and micro-nano SiO₂/LDPE composites are prepared by melt blending.The influence of the order of adding micron and nano SiO₂ on the performance of the micro-nano composite during the preparation process is firstly discussed.The results show that the micron SiO₂ is added for mixing for a certain period of time,and then the micro-nano composite prepared by adding nano SiO₂.The material performance is the best.Based on this preparation process,the influence of the concentration of micron and nano SiO ₂on the performance of the composites are discussed.Atomic force microscopy（AFM）,polarized light microscopy（PLM）,differential scanning calorimetry（DSC）and X-ray diffractometry（XRD）were used to characterize the crystal morphology and crystal structure size of LDPE and micro-and nano-SiO₂/LDPE composites.The results show that the size of spherulites decreases with the addition of micro-and nano-SiO₂ particles.Heterogeneous nucleation of nano-SiO₂ resulted in the formation of small size and compact spherulite structure in the nanocomposites.The spherulite crystals are perfect and the crystallinity is improved;the heterogeneous nucleation of micron SiO₂ is weak,which hinders the growth of spherulites.As a result,the size of the spherulites in the micro-composites is reduced,and the number of spherulites with imperfect crystallization are increased,and the change of crystallinity is not obvious.The crystallinity of the composites decrease as the concentration of inorganic particles increases.The space charge characteristics of LDPE and micro-and nano-SiO₂/LDPE composites were studied by electro-acoustic pulse method（PEA）experiments.The results show that the nanocomposites accumulate a small amount of homopolar charges near the two poles,while the microcomposites accumulate homopolar charges near the anode and heteropolar charges near the cathode.Nano-SiO₂ has a strong inhibitory effect on space charge,When the content of nano-SiO₂ is 0.5wt%,the inhibition effect of the nanocomposite is the best.When the content of nano-SiO₂ exceeds 0.5wt%,the inhibition effect decreases with the increase of the concentration.Micron SiO₂ has no obvious ability to inhibit space charge,but the short-circuit charge decay rate of the micron composite is high.The space charge characteristics of micro-nano composites are somewhere in between.Experiments have been conducted on the thermally induced current characteristics of LDPE and composites.The results show that th e current peak temperature of the nanocomposite moves to the high temperature direction,the current peak temperature of microcomposite shifts slightly to the low temperature direction,and the current peak of the micro-nano composite is close to that of the nanocomposite.It shows that nano-SiO₂ introduces deep traps inside LDPE,while micron SiO₂ causes more shallow traps inside LDPE.Based on the trap theory,a charge layer shielding model at the electrode interface and a carrier transport model between different traps are established.Experiments have studied the electrical conductivity,dielectric properties and DC breakdown characteristics of LDPE and composites.The results show that the conductance current of composites are lower than that of LDPE,the conductance current of nanocomposites are the lowest,followed by micro-nano composites.With the increase of SiO₂ content,the conductance current of the composite increases.In the J-E curves,LDPE,micro-composites and micro-nano composites with high content of micron SiO₂ have three slope change areas in electrical conductivity.Micron SiO₂ has little effect on the threshold electric field E₁,but it significantly increases the threshold electric field E₂.While nanocomposites and the micro-nano composite with lower content of micron SiO₂ do not appear the third slope change area,and nano-SiO₂ makes the threshold electric field E₁significantly increased.The DC breakdown field strength of nanocomposites are significantly higher than that of LDPE.But as the content of nano-SiO₂ increases,the breakdown field strength gradually decreases.When the content of nano-SiO₂ is 0.5wt%,the breakdown field strength is the highest,which is about 29.2%higher than that of LDPE.The DC breakdown field strength of the micro-composite is lower than that of LDPE and decreases significantly with the increase of the micron SiO₂ content.When the content of micron SiO₂ is low,the breakdown field strength is close to that of LDPE.The breakdown field strength of the micro-nano composite increases with the overall content of SiO₂.However,when the content of micron SiO₂ is low,the breakdown field strength decreases slowly with the increase of nano-SiO₂.When the content of micron SiO₂ is high,with the increase of nano-SiO₂,the breakdown field strength decreases more.Based on the experimental results and the aggregate structure of the composite,and combined with the multi-core model and the dielectric double-layer model and the polymer trap theory,a process model of trap formation,carrier trapping,detrapping and recombination including LDPE and composites is established,as well as the process model of bombardment of LDPE molecular chains by hot electrons.The LDPE model and the nano-SiO₂ model and the interface model between LDPE and nano-SiO₂ modified by methacryloxypropyltrimethoxysilane were established using Materials studio software.The model was optimized and calculated by molecular dynamics,and the trap and breakdown characteristics of the two systems were discussed based on density functional theory calculations.The results show that van der Waals force is the main interaction between nano-SiO₂ and LDPE matrix.The traps in nano-SiO₂/LDPE are deeper,while the traps in LDPE are shallower.The breakdown field strength of SiO₂/LDPE is about 18.2%higher than that of LDPE.