Study On The Properties And Preparation Of Polyethylene Aluminum Nanocomposite Dielectrics

The rapid development of economy around the world, especially in our country brings out the rapid increase of the demand of electric power, which leads the development of electric transmission and distribution systems to be directed forward the trends of high voltage (HV) or extra high voltage (EHV). And also, the quantitive and qualitive demands for power cables become larger and larger with the rapid urbanization process and with the operation condition growing severer and severer. Cable terminations are considered to occupy an important part of the electric transmission and distribution systems and also to be one of the weakest parts of electric transmission and distribution cable networks. At cable terminations, especially for medium and high voltage cables, the electric field has both radial and tangential components because of their complex configurations, which may be intensive enough to cause the surface discharge or flashover and finally cause the failure of them. Besides cable terminations, the stress control in bushings and the coils of motors and generators have attracted much attention of the investigators. The use of high-dielectric-constant materials has been strongly recommended in order to equalize the inhomogeneous distribution of electric field in the electric power apparatuses.Polyethylene is known to be one of the most widely used polymeric insulating materials. The high dielectric strength, very low electrical conductivity, low dielectric loss at high frequencies and remarkable mechanical properties allow it to be used as an outstanding insulation in wire and cable applications, particularly in high frequency cables. However, its low dielectric constant makes it impossible for PE to be used for electrical stress control in cable terminations. One effective method for expanding its application to cable terminations is to increase its dielectric constant.1111The common approach to increase the dielectric constant of a polymer, e.g. PE, is known to adopt metal particles, conductive fibers and carbon tubes. Polymer/conductor composites are classical percolation systems, in which the permittivity of a composite showing the percolation behavior is inversely proportional to the difference between the real filling volume fraction of fillers and the critical filling volume fraction (the threshold value of percolation). Hence, if high values of permittivity are needed for composite materials, the filling volume fraction of fillers should be similar to the critical value but not higher than it; if an appropriate value of the filling volume fraction is selected, then very high permittivity value of the composite material can be realized. However, the dielectric properties of the composites having the percolation characteristic are known to be quite sensitive to the constitution of the material; a little change in the constitution can produce significant changes in the performances of composites. A very low value of percolation threshold gives challenges to production of a composite and reproducibility of performance indices of the material.1111In our work, we have chosen aluminum (Al) nanoparticles as fillers. Al is a self-passivation metal, where the self-passivated oxide layer forms a thin insulating boundary around the surface of its metallic core, allowing the polyethylene/Al composites as a percolation system to have a high dielectric constant. In addition, Aluminum not only is low in density, but also has not any catalysis and degradation effects on polyethylene in contrast to copper, which leads polyethylene/Al nanocomposites to be a kind of the promising materials for application in cable terminations.In our work, we mainly focused on the following five scientific topics: (1) the filler concentration and frequency dependences of dielectric behaviors of PE/Al nanocomposites; (2) the mechanism/model for the higher dielectric constant of the PE/Al nanocomposites; (3) the influence of the chemistry characteristics of Al nanoparticle surface on the microstructure, electrical properties and rheological behaviors of PE/Al nanocomposites; (4) the correlation between rheological, electrical, and microstructure characteristics of PE/Al nanocomposites; (5) the factors influencing the dielectric strength of PE/Al nanocomposites and some methods used to improve the dielectric strength of the nanocompsites.(1) The filler concentration and frequency dependences of dielectric behaviors of PE/Al nanocomposites: The dielectric properties (dielectric permittivity and loss tangent) of PE/Al nanocomposites are dependent not only on the Al nanofiller concentration, but also on the measuring frequency. The significant differences of the dielectric behaviors are found in the extra-low frequency range of 0.1Hz to 10Hz. In the case of quite low loading levels, agglomerates can be hardly found and any dielectric dispersion can not be observed in the extra-low frequency range, which may be attributed to that the formation of the interfaces between Al nanoparticles and PE matrix can be still neglected because of the small number and extra-small diameter of the nanoparticles. The higher the nanofiller loading level, the larger the total effective area of the interfaces between the polymer matrix and the fillers, and the thinner the insulating spacers separating Al nanoparticles, which results in the significant increment of the interfacial space-charge polarization. The values of dielectric characteristics becomes lower again as the filler loading exceeds 24wt%, which may be ascribed to voiding from imperfect filler packing and solvent evaporation, the decrease of the effective area of the interface between polymer matrix and the nanofillers, the increase of the average thickness of the equivalent oxide shell of any Al nanofiller cluster, and the decrease of the volume fraction of the polymer matrix due to the increase of that of the nanofiller cluster. It is found that the dielectric permittivity of the PE/Al nanocomposites does not show any percolative characteristics which have been reported in classical polymer/conductor composites.(2) The mechanism/model for the higher dielectric constant of the PE/Al nanocomposites: PE/Al nanocomposites were modeled with a general equivalent electric circuit consisting of CPE, CAl, RAl, Co, Ro, Ci and Ri (CPE represents the average capacitance of PE domain between two electrodes; CAl, RAl are the equivalent average capacitances and ac resistances between the Al core and the internal interface of the oxide layer, respectively; Co, Ro are the equivalent average capacitance and ac resistance of the oxide layer, respectively; Ci, Ri are the equivalent average capacitance and ac resistance between nanoparticles or between nanoparticles and the electrodes, respectively). It has been found that the equivalent electric circuit can be used to clearly explain the filler concentration and frequency dependences of dielectric behaviors of PE/Al nanocomposites(3) The influence of the chemistry characteristics of Al nanoparticle surface on the microstructure, electrical properties and rheological behaviors of PE/Al nanocomposites: The surface modification of the nanoparticle has been shown to significantly improve the particle dispersion in the PE matrix. The percolation theory has been used to investigate the effect of the surface modification of Al nanofillers on the electrical conductivity of the nanocomposites. The value of percolation threshold for the PE nanocomposites containing octyl-trimethoxysilane-coated Al nanoparticles is higher than for those loaded with un-treated nanoparticles, while both of these values are lower than the predicted ones. The dc critical exponent values are lower for PE nanocomposites filled with un-treated Al nanoparticles than with octyl-trimethoxysilane-coated ones, while the values for these two cases are much higher than the predicted ones. It can be understood that the low values of percolation threshold of the nanocomposites may be related to the small size of the Al nanoparticles and their agglomeration inside the composites, and the much higher critical exponents of the nanocomposites could be ascribed to the nature of the inter-particle contact. The differences of threshold and critical exponent between the nanocomposites can be closely related to the fact that the surface modification can improve the dispersion of the nanoparticles in the polymer matrix. Our results also indicate that the surface modification makes it possible to easily control the values of dielectric permittivity in the comparatively wide range and also to provide an excellent approach able to considerably reduce the dielectric loss of the nanocomposites.(4) The correlation between rheological, electrical, and microstructure characteristics in PE/Al nanocomposites: A strong correlation between the time and concentration dependences of dc conductivity and the rheological properties has been observed in the different nanocomposite systems. The rheological threshold of the composites is smaller than the percolation threshold of electrical conductivity for both of the nanocomposite systems. The difference in percolation threshold is understood in terms of the smaller particle-particle distance required for electrical conduction as compared to that required to impede polymer mobility.(5) The factors influencing the dielectric strength of PE/Al nanocomposites and some methods used to improve the dielectric strength of the nanocompsites: Combined with the observation results on the microstructure of the nanocomposites, the main factor to determine the dielectric strength of the PE/Al nanocomposites is the particle dispersion properties and the externally-introduced charge carriers may only be the secondary factor. It has been also found that only the Al nanofiller surface-treated with the silane coupling agent makes it possible for the PE/Al nanocomposites to still keep the relatively higher breakdown strength even in the higher Al loading levels above 14vol%, which may be ascribed to the existence of very thin barriers of polymeric material between metallic clusters. The dielectric strength results of the macro- and nanocomposites hint at that the surface modification of the metal nanoparticle is surely necessary for preparing the useful polymer/metal composites with high dielectric permittivity and low dielectric loss but without any significant reduction of their dielectric breakdown strength.The innovations of this dissertation are listed as follows:1. It is known that the polymer/metal composites with high metal loading levels show the typical conductive behaviors. In this paper, a self-passivated Al has been chosen as fillers, and PE/Al nanocomposites were prepared for the development of a high permittivity material. It has been found that the composites with relatively high Al concentration have high dielectric constant while they still keep good electrical properties even in so high filler loadings. On the basis of this finding, we have expanded the applications of polymer/metal composites in this work.2. It has been found that the nanocomposites at high filler concentrations still keep good mechanical properties and breakdown strength, which is very important for practical applications.