Study On Space Charge Characteristics And Dielectric Properties Of Polyethylene/Nano-montmorillonite Composites

Space charge which plays an important role in electrical aging andbreakdown of engineering dielectrics is an indispensable factor in thedevelopment of power equipment subjected high voltage. Worldwide attentionhas been attracted in the problem of the space charge in the polymer insulation ofhigh voltage direct current cable. Methods, which could inhibit the generationand accumulation of space charge, have been researched in order to reduce thedeterioration caused by space charge effects in polymer. It is effective to improveits original performance and inhibit the space charge by adding nano particlesinto polymer insulation.In this thesis polyethylene/organic montmorillonite (PE/O-MMT)nanocomposites and the cross-linked polyethylene/organic montmorillonite(XLPE/O-MMT) nanocomposites are prepared by means of melting intercalationprocess and cross-linked reaction after melting intercalation process in whichorganic montmorillonite (O-MMT and S-O-MMT) treated by two different meansindependently as the inorganic filler, low density polyethylene polyethylene(LDPE) as the matrix polymer, maleic anhydride grafted polyethylene (PEMA)as the compatibilizer, and bis (1-(tert-butylperoxy)-1-methylethyl)-benzene(BIPB) as the crosslinking agen. The structure of nano montmorillonite andmorphologies of PE/O-MMT nano composites are analyzed and characterized byx-ray diffraction (XRD), Fourier transform infrared spectrum (FTIR), scanningelectric microscopy (SEM) and atomic force microscope (AFM).The results showthat lamination distances of the montmorillonite expand from1.25nm to2.39nmby the organic treatment. Asymmetric absorption peaks of-CH3-and-CH2-ofinfrared spectra certifies that intercalating agent and coupling agent reacts with cation and hydroxyl group in the montmorillonite. The homogeneous dispersestate of O-MMT with exfoliated lamination in LDPE is presented under thesynergy effect of compatilizer.The space charge properties of PE/O-MMT and XLPE/O-MMTnanocomposites have also been investigated by pulsed electro-acoustic method(PEA). The effects of coupling-agent, compatilizer and nano-MMT content onthe AC breakdown strength, electrical treeing, insulation conductivity anddielectric loss have been comparatively investigated. Both PE/O-MMT andXLPE/O-MMT samples exhibit some advantage inhibiting space charge. Thereare homo-charges around cathode in original LDPE while PE/O-MMT presentsonly a modicum of hetero-charges with relatively low charge release rate atshort-circuit. Compared with PE/C/O-MMT and PE/O-MMT, the hetero-chargesalmost disappear in PE/S-O-MMT with lower charge release rate.The O-MMT introduction reduces permittivity ε with dielectric dissipationfactor tanδ slightly increased, in which the permittivity of nano-composite with3wt%S-O-MMT filler has been most remarkably decreased to comparativelylowest value. The electric conductivities attenuate to some extent while electricbreakdown fields considerably increase, for all the composite samples with O-MMT filler. Especially for the prepared nanocomposites with added compatilizerPEMA, the electric breakdown strength rises to1.35~1.70times as much as thatof LDPE in the experiment temperature range60~90°C. Compared with originalLDPE sample, both propagation length and rate of electrical treeing evidentlyreduce in the composite samples with O-MMT filler under the same experimentalconditions. The major electric tree shapes for LDPE sample appear to be sparse"branches" with non-conductive paths, whereas the PE/O-MMT sample tends toexhibit representative "clusters" with conductive paths.According to the experimental research results of space chargecharacteristics, dielectric properties and electric breakdown performance, thecombination center model of charge traps modulated by the combination status ofinterfaces between polyethylene and inorganic nano-montmorillonite isphysically established by means of trap theory. Combined with multiple-coresmodel suggested by Tanaka and lamination characteristic of inorganic nano-montmorillonite, and the point of views about hot electrons energy dissipated by the laminations of nano-montmorillonite in electronic-phonon coupling mode andresult in diffuse reflectance of electronic transport are initiatively suggestedbased on the nonradiative energy transfer theory.