Rersearch On Preparation And Properties Of High Performance Insulating Polyethylene Nanocomposites And Semi-Conductive Shield Polyethylene Nanocomposites

High voltage direct current (HVDC) cables are attracting more and more interesting for its low loss, long life, high reliability and so on. With the rapid development of electrical engineering and electronic technology, a demand for flexible conducting polymer composites is attracting a broad attention. In some applications, functional polymer-based composites with high dielectric permittivity and high electrical conductivity have a potential ability to meet some special needs. For instance, polyethylene (PE) is one of the excellent insulators in the field of high voltage direct current (HVDC) transmission.A functional shield layer often consisting of PE-based composite with high dielectric permittivity and electrical conductivity is important in the HVDC cable field because this type of shield layer can prevent the formation of space charges. Because of the space charges produced in the shield layer, they may also be released due to the high conductivity of the shield layer. In addition, the induced opposite electrical field can also force the high electric field at irregular surface of conductors in the cables to decrease. Therefore, the suitable inner shield layer materials with high dielectric permittivities and electrical conductivities are key factors to sustain the safe running of HVDC cables.The present paper takes LDPE as an excellent insulation. A use nanotechnology and the appropriate method of preparation were proposed to synthesize LDPE/A12O3, LDPE/A1N and LDPE/MWCNT nanocomposites. The dielectric properties and the performance enhancement were studied under the special electric field. The influence of space charge formation and distribution on the electric field and the migration and accumulation in the dielectrics were also studied. The insulating structures and the novel shield layer were developed by controlling the dielectric properties and microstructures of materials. The main conclusions are as follows.LDPE/A12O3 and LDPE/A1N nanocomposites were prepared by using melting method and subsequently molded via hot pressing methods with different cooling processes. The influences of nano-Al2O3 and nano-AIN concentration on crystallization, the dielectric properties, breakdown strength and space charge distribution of the composites were explored. The LDPE/nano-Al2O3 composites were also studied by Scanning electron microscopy (SEM), Differential Scanning Calorimeter (DSC) and X-ray diffraction (XRD). Results show that the cooling process influences thermal stability of the composites and the well-dispersed Nano-materials in the matrix have taken great effect on the crystalline and electrical properties of the composites. With the increase of Al2O3 and A1N fillers, the dielectric properties of the composites are improved, breakdown strength decreased and the accumulation of space charge changed significantly.There is increasing evidence that degradation of polymer insulation under high electric stress is associated with space charge formation. In fact, space charge accumulation in DC regime is the main reason that still limits the use of polymers as insulation for high voltage DC power cables. The space charge dynamics in LDPE/A12O3 and LDPE/A1N nanocomposites have been investigated by using the Pulsed Electroacoustic Method (PEA) technique. Bipolar charge injection has taken place in pure LDPE film. The amount of space charge injected increases with the duration of applied voltage. The space charge characteristics in nanocomposites containing the nano-Al2O3 or nano-AIN particles are very different from the pure LDPE film. Heterocharges dominate the distribution indicating ionization occurrence. The addition of the nano-Al2O3 or nano-A1N particles into LDPE matrix may also hinder charge injection process. Test results showed that that the 0.5 wt% nano-Al2O3 added to LDPE greatly decreases space charge accumulation and increases space charge mobility. While, nano-AIN blends can't improve space chare accumulation and mobility.LDPE/MWCNT nanocomposites were prepared by using melting method and subsequently molded via hot pressing methods with different cooling processes. The influences of MWCNT concentration on crystallization, the dielectric properties, and conductivity of the composites were explored. The LDPE/MWCNT composites were also studied by Scanning electron microscopy (SEM), Differential Scanning Calorimeter (DSC). Enhanced dielectric-permittivity and conductivity are observed in the LDPE/MWCNT nanocomposites. Conductivity and dielectric permittivity of the LDPE/MWCNT nanocomposites first increase gently with the increase of MWCNT concentration, and they increase significantly as the MWCNT volume fraction was near 0.081. The results are explained well by employing the percolation theory. A relative high percolation threshold (fc=0.081) is discovered because the MWCNT is broken easily during the melting mixture process due to the strong mechanical agitation. The LDPE/MWCNT nanocomposite at fMWCNT=0.08 displays the high dielectric permittivity and the high conductivity, which would satisfy the need as a potential shield layer materials in the HVDC field.