Abstracts

In order to meet the increasing demand for electricity and electrical powers efficient transmission, power ratings and operating voltages have seen a continuous rise over the years, which inevitably brings greater electro-thermal stresses to power cable insulation. Polyethylene (PE), an insulating material widely used in underground power cables, is susceptible to electrical degradation and exhibits a low thermal conductivity. Consequently, there is a compelling need to improve its dielectric and thermal performances.During the last decades, considerable attention has been given to a new class of dielectric materialnanodielectrics, i.e. dielectric composites containing nanometric fillers. They are frequently reported to have superior dielectric properties as compared to neat polymers and microcomposites, and thus have great potential to serve as the insulating materials for highvoltage (HV) power cables. Nonetheless, such property enhancements can only be achieved when the nano-filler has a good size dispersion and spatial distribution within the host dielectric. However, due to nano-fillers strong tendency to agglomerate, and their generally poor compatibility with polymers, their dispersion is often compromised with aggregates of micrometric sizes. Furthermore, hydrophilic nano-fillers attract water to the filler-matrix interface, not only impairing the crucial role of the interphase, but also causing property and material degradation. In order to facilitate a homogeneous nanoscaled filler dispersion and prevent water absorption, inorganic nano-fillers are commonly treated with dispersants and coupling agents. However, this adds extra work to material fabrication. Moreover, little is known about the long-term stability of these surface modifications under electro-thermal stresses. Furthermore, a thorough removal of hydrophilic groups may not be straightforward, and so is the complete prevention of filler aggregation and water absorption.In this context, the objective of this PhD research is to develop PE-based nanodielectrics with enhanced dielectric and thermal performances, as insulating materials for HV underground power cables.In order to avoid the aforementioned problems concerning surface modifications, polyhedral oligomeric silsesquioxanes (POSS), which are by nature nanoscaled molecules bearing builtin functionalities, were used. POSS selection, POSS loading and fabrication method all play an important role in developing PE/POSS nanodielectrics with enhanced performances. In this project, three types of POSS with different alkyl substituents were studiedsolid octamethyl-POSS (OmPOSS, OM), solid octaisobutyl-POSS (OibPOSS, OIB) and viscous-liquid isooctyl-POSS (IoPOSS, IO); 1 wt% and 5 wt% POSS loadings were investigated; and three fabrication methods were attemptedball milling (BM), xylene solution blending (XSB), and extrusion E). The obtained composites were examined concerning their dielectric and thermal properties. Additional characterizations such as scanning electron microscopy (SEM)