| Doping an appropriate number of inorganic nanoparticles into polymers can significantly improve some of its electrical properties,and giving them excellent mechanical and thermal properties.However,the electrical properties of semi-crystalline polymers strongly depend on the aggregated structure of the polymer itself,such as crystallinity,crystalline size,grain boundaries and internal defects.The addition of nanoparticles in semi-crystalline polymers will inevitably affect the molecular chain mobility and aggregation structure.Therefore,it is necessary to explore the effects of different inorganic particles on the aggregation structure of polymers and reveal the microscopic mechanism of electrical properties of inorganic particles/polymer micro-nanocomposites.In this paper,SiO2/LDPE nanocomposite,MMT/LDPE composite and MMT/SiO2/LDPE multi-element composites were prepared by melting blending method using low-density polyethylene?LDPE?as the matrix,nano-SiO2modified by surface hydrophobic and organic montmorillonite?MMT?treated by ultrasound as inorganic particles.The crystallization kinetics,crystal morphology and crystal structure parameters of LDPE,SiO2/LDPE nanocomposite,MMT/LDPE composite and MMT/SiO2/LDPE multi-element composites were characterized by Differential scanning calorimeter?DSC?,X-ray diffraction?XRD?,polarizing microscope?PLM?and atomic force microscopy?AFM?.The results show that nano-SiO2 particles and MMT particles hardly changed the grain size and lamella thickness of LDPE crystal structure.The nano-SiO2particles in SiO2/LDPE nanocomposite form a large number of spherulites with smaller size through heterogeneous nucleation.The laminated structure of MMT in MMT/LDPE composite limits the development of spherulites,and most MMT slices are dispersed in amorphous regions.The electrical tree growth characteristics of LDPE,SiO2/LDPE nanocomposite,MMT/LDPE composite and MMT/SiO2/LDPE multi-element composites under power-frequency AC 7kV voltage were studied experimentally.Based on the chemical etching method,the samples containing electrical tree branches were sliced and etched.The correlation between the crystalline structure and the growth path of electrical tree branches was studied by PLM.The results show that most of the electrical trees in semi-crystalline LDPE grow along the spherulite boundary and form branches,showing a small fractal dimension of dendritic fractal characteristics.In SiO2/LDPE nanocomposite,due to the heterogeneous nucleation of nano-SiO2 particles,a large number of spherulites with compact structure and small spherulite size are formed,which makes the development path of electrical trees more tortuous.Therefore,the development of electrical trees in SiO2/LDPE nanocomposites has the characteristics of slow growth rate,long growth time and more inflection points of electrical trees.The MMT slices distributed in amorphous area in MMT/LDPE composite play a leading role in inhibiting the growth of electrical trees.The development of electrical trees in MMT/LDPE composites has the characteristics of slow growth rate,short length and dense morphology of electrical trees.In MMT/SiO2/LDPE multi-element composites,due to the heterogeneous nucleation of nano-SiO2 particles and the blocking effect of MMT slices in the amorphous region on the growth path of electric trees,MMT/SiO2/LDPE multi-element composites exhibit better electrical tree resistance,especially when the ratio of MMT particles to nano-SiO2 particles was 1:2,the electrical trees resistance of MMT/SiO2/LDPE multi-element composites was improved most obviously.A combined measurement system of electrical tree initiation and partial discharge was established.The effects of organic montmorillonite?O-MMT?and ultrasonic dispersion montmorillonite?US-O-MMT?on the morphological changes,growth characteristics and partial discharge characteristics of electrical tree during its growth were comparative studied.The results show that the ultrasonic treatment of organic montmorillonite can improve the exfoliation degree of montmorillonite slices,and further improve the electrical trees resistance of MMT/LDPE composite.The time-domain analysis of partial discharge signals in the electric tree growth process of LDPE,O-MMT/LDPE and US-O-MMT/LDPE samples shows that the maximum discharge quantity of LDPE at the early growth stage,growth stage and lag stage were 50pC,90pC and110pC,respectively.MMT/LDPE samples were 40pC,90pC and 80pC,while US-O-MMT/LDPE samples were 30pC,60pC and 40pC.The spectrum analysis of discharge signals shows that the frequency of discharge signals of LDPE,O-MMT/LDPE and US-O-MMT/LDPE samples in the initial stage of tree growth were distributed at 5-12MHz,5-17MHz and 5-10MHz,respectively,while the frequency components expand slightly to 5-15MHz,5-20MHz and 5-15MHz respectively when the tree grows to the lag stage.A time-frequency analysis method based on partial discharge signal is proposed to estimate the growth stage and development form of electrical trees.The trap characteristics,broadband polarization characteristics,volt-ampere characteristics and power frequency AC breakdown strength of LDPE,SiO2/LDPE nanocomposite,MMT/LDPE composite and MMT/SiO2/LDPE multi-element composites were studied experimentally.The experimental results show that the peak value,peak width and peak area of thermally stimulated currents of SiO2/LDPE nanocomposite and MMT/LDPE composite increase significantly compared with LDPE to certain extent.In addition,another thermally stimulated current peak appears near 90?C in SiO2/LDPE nanocomposite,which means that deep traps are introduced into SiO2/LDPE nanocomposite.Both SiO2/LDPE,MMT/LDPE and MMT/SiO2/LDPE composite show relaxation polarization and loss characteristics to some extent,and the DC threshold field strength and AC breakdown field strength of SiO2/LDPE nanocomposite and MMT/SiO2/LDPE multi-element composites are higher than those of LDPE and MMT/LDPE composite.Based on the principle of correlation between aggregate structure and dielectric properties of polymers and their composites,it may be considered that a large number of microcrystalline structures formed by heterogeneous nucleation of nano-SiO2 particles in SiO2/LDPE nanocomposite and MMT/SiO2/LDPE multi-element composites introduce more crystal/amorphous interfaces and charge traps at interface.The interfacial charge layer of nano-SiO2 particles/LDPE matrix not only affects the low-frequency polarization behavior of composite dielectrics,but also influences the migration of carriers along electrical field,which is the micro-mechanism of the relaxation polarization characteristics,the increase of DC threshold electric field and the increase of AC electrical breakdown strength.However,the trap characteristics,carrier migration and broadband polarization behavior of MMT/LDPE composite are resulted from the prevent effect of MMT lamellae on the molecular chain motion. |
PDF Full Text Request