Investigation On The Trap Characteristics And Electrical Properties Of Polyethylene Based Nanocomposite

Nanocomposite is an effective way to enhance the electrical properties of insulation materials used in the electrical equipments. The investigation of polymer based inorganic nanocomposite is of great practical significance and theoretical value. Traps have prominent effects on the electrical conduction, ageing and breakdown of polymers. The investigations on the trap characteristics of the polymer and its based nanocomposites and on the role of traps in determining their electrical properties are helpful to reveal the mechanism of electrical properties of polymer based nanocomposites. In this thesis, the low density polyethylene (LDPE) is chosen as the matrix. The main effort is devoted to the investigations on the effects of ZnO, ZnxMg1-xO(x=0.5) and ZnO+MMT (montmorillonite) nanoparticle fillers and their surface treatment on the trap characteristics and electrical properties of LDPE. By taking the trap characteristics as the entry point, the mechanisms corresponding to the electrical properties of nanodielectrics are studied. The main work is shown as follows:(1) LDPE/ZnO, LDPE/ZnxMg1-xO and LDPE/ZnO+MMT nanocomposites are prepared by melt bending method and the physical and chemical structures have been investigated by scanning electron microscope (SEM), infrared spectroscopy (IR), X-ray diffraction (XRD) and ultraviolet-visible absorption spectroscopy (UV-vis). The dispersion of nanoparticles in the LDPE matrix, the compatibility of nanoparticles with the matrix, the physical and chemical action between the nanoparticles and the matrix at the interface, the LDPE crystallinity and the crystallize, the UV-vis characteristics of the nanocomposites and the coupling agent treatment on these properties are investigated. Results show that the diameter of the nanoparticles in the nanocomposites is in the range50-200nm. Coupling agents are chemically bonded to the nanoparticles and have decreased the aggregate size of the nanoparticles. The mixed fillers of ZnO and MMT have increased the layer space of MMT and promoted their dispersion. ZnO and MMT have formed pilotaxitic structures. The ZnO and ZnxMg1-xO fillers have increased the crystallinity of LDPE. The LDPE/ZnO and LDPE/ZnxMg1-xO nanocomposites exhibit strong absorption for the ultraviolet light in the range250-400nm. (2) Considering the drawback of the traditional thermally stimulated current (TSC) method in the quantitative investigation of trap characteristics of polymers, the modified analysis methods of isothermal discharge current (MIDC) and thermally stimulated current suitable for the study of the trap characteristics in polymers with continuous energy levels are proposed. The effectiveness of the methods has been verified by experiments and analysis. The trap characteristics of the nanocomposites are obtained by the proposed methods. Research indicates that the trap level density in LDPE is increased about2-6times by nanofillers and the trap level depths are basically unchanged. The interface traps are cavities essentially and come from the space configuration defects of polymer chains. The change of the trap level density is closely related to the property and shapes of the nanoparticles, nanofiller loadings and treatment by coupling agent.(3) The micro mechanism of the trap level density changes in LDPE by nanofillers are investigate and the theoretical mechanism model of trapping effect at the interface for the property improvement of the nanocomposite is proposed and confirmed. The trap density increase in the nanocomposite is derived from two aspects. Firstly, the nanofillers have promoted the heterogeneous nucleation process of LDPE, leading to the formation of uniformly distributed small spherulites and increase of the interface areas between the crystallites and amorphous domains. These interfaces can generate large amounts of cavity traps. Secondly, the interface areas between the nanoparticles and the LDPE matrix are different in molecular mobility, morphology structures and free volume, in which cavity traps can also be generated. The two kinds of trapping effects at the interface are the main cause for the trap level density increase in the nanocomposite.(4) The effect of different nanofillers on the electrical properties, especially charge transportation characteristics of LDPE is investigated and the physical mechanisms of the decrease in electrical conduction and increase in electrical breakdown strength and electrical treeing life in the nanocomposites are given based on the trapping mechanism at the interface. Results show that the carrier mobility of the nanocomposites is about1/4～1/20times that of LDPE and the volume resistivity is about2to20times that of LDPE. The high field conductivity of the nanocomposites is about1/8to1/2that of LDPE at least. The accumulated heterocharge and homocharge are about1/8to1/3that of LDPE at least. Coupling agent treatment is helpful to space charge inhibition. The electrical treeing-resistant life is30times that of LDPE at most. The nanocomposites filled with ZnO and MMT mixtures have the best treeing resistant ability and then are those filled with ZnxMg1-xO and ZnO fillers. The electrical breakdown strength is enhanced by10%to20%than LDPE at most. It has been concluded that the trapping effect at the interface by nanofillers and the properties of nanofillers are the key factors leading to the electrical property improvement of nanocomposites.(5) In order to go deep into the space charge behavior and its mechanism, the theoretical model of trapping mechanism for space charge inhibition in LDPE/ZnO nanocomposite is proposed via the experimental and theoretical analysis of the modulating action of interface trap on the charge injection process from electrodes and transportation process. Research concludes that the trapping effect at the interface caused by nanofillers has effectively decreased the accumulation depth of the injected charge and is helpful to promote trapping of injected charge at the interface around the electrodes, leading to the reduction of interface electric field. This can weaken the impurity ionization and inhibit the hetero space charge accumulation around the electrodes. On the other hand, the reduction of interface electric field decreases the charge injection from electrodes. The small amount of injected charge can be easily conducted out of the bulk. Thus it can suppress the homo space charge accumulation in the material. This is meaningful for the development of electrical insulation materials for high voltage and extra high voltage cables.