Moisture influence on the electrical properties of cross-linked polyethylene/silica nanocomposites.

摘要

During the last twenty years, nanodielectrics have emerged as an important dielectric material system to provide advanced dielectric properties for power equipment applications, among which, cross-linked polyethylene (XLPE)/silica nanocomposites are regarded as a promising candidate for power cables in the future. Despite the various improvements achieved in XLPE/silica nanocomposites compared to XLPE base resin, the influence of moisture has not been fully explored. For cable insulation materials, water treeing is of particular interest. Therefore, this thesis focuses on the influence of moisture on the electrical properties of XLPE/silica nanocomposites to reveal the facts and mechanisms related to the moisture diffusion and aging phenomena in XLPE/silica nanocomposites in a humid environment.;First, moisture diffusion was monitored in different humid environments. XLPE/silica nanocomposites were found to have an increased moisture uptake compared to XLPE base polymer due to the inclusion of silica particles. Water shells, which have a higher water weight percent than that in XLPE matrix, are believed to form around silica particles/aggregates.;Second, electrical characterization techniques such as dielectric spectroscopy, pulseelectro- acoustic analysis, 60 Hz AC breakdown as well as water treeing were utilized to investigate the influence of moisture on the electrical properties of XLPE/silica nanocomposites. Water shells and the change of the inter-particle/cluster distances due to loading levels and dispersion state are believed to be the two major factors that govern the dielectric behavior in wet XLPE/silica nanocomposites. At a high loading level of 12.5 wt%, percolation of water shells drastically changed the dielectric performance of the composites including increased permittivity, conduction and reduced dielectric strength. However, 5 wt% nanocomposites, even with elevated moisture content, perform comparably to XLPE. At the same time, water treeing was restricted in XLPE/silica nanocomposites regardless of the surface functionalization of the fillers. The impeding of chain movement and the growth direction alternation are attributed to some of the possible reasons to restrict water transport. At the same time, the reduction of interparticle distance due to increased particle weight percent in a nanocomposite might also correlate to the water treeing depression in ways that fatigue effect might be reduced.;Third, a simplified structure model was built for the aggregated nanocomposites to estimate the thickness needed to initiate the percolation of water shells. For 12.5 wt% nanocomposites (aggregated and applied specifically to the nanocomposites studied in this thesis) with a water shell thickness of approximately 50 nm, percolation could be initiated.;At the same time, nanocomposites with near-ideal dispersion achieved by Nanoinfusion techniques were also investigated and compared with the XLPE/silica nanocomposites prepared by conventional mixing techniques. These samples shows that a loading as low as 1 wt% can achieve improvements similar to aggregated XLPE/silica nanocomposites at much higher loadings. This data can be found in the Appendix.