Epoxy Based Nanodielectrics for High Voltage DC Applications: Synthesis, Dielectric Properties and Space Charge Dynamics

Main goal of the research described in this PhD thesis was to determine the influences of filler size, material and distribution on the DC breakdown strength, permittivity and space charge behaviour of nanocomposites. This should lay the groundwork for tailored insulation materials for HVDC applications. Examples for this are medical and industrial X-ray imaging, radar and cable terminations. In the course of this project a manufacturing process was devised, which enabled the fabrication of epoxy based nanocomposites with a good dispersion of different types of nanoparticles. Models from literature, which explain the behaviour nanodielectrics exhibit, are discussed: electric double-layer model, intensity model, multi-core model and the interphase volume model. Based on these theories, a new model was devised for explaining the behaviour of epoxy based nanocomposites: the polymer chain alignment model. The underlying idea of this model is that the restructuring of the base polymer on the molecular scale, due to the presence of surface modified nanoparticles, plays a fundamental part in the properties of the bulk material. Each modified particle will act as centre for crosslinking of the polymer, leading to a rigid layer of polymer chains around each particle. These rigid layers have a much lower permittivity than both host and filler material, thus their presence can easily be identified by dielectric spectroscopy, since the relative permittivity of the bulk material decreases. In literature it is shown, that the strong bonding of particles and host material due to the surface modification gives rise to improved resistance to partial discharges and electrical treeing. More energy is needed to break these bonds than it would be the case in unmodified polymers. The particles themselves can also act as recombination centres for electrons and holes, which travel between or along polymer chains. This has an effect on the space charge dynamics. Agglomerations of nanoparticles can nullify these effects however: it is explained how agglomerations can act as charge traps, lead to field enhancements and cause interfacial polarization. Claims from theory are tested with three measurement methods: short term DC breakdown tests, dielectric spectroscopy and space charge measurement. It is shown that nanocomposites exhibit improved DC breakdown strength for very low fillgrades of 0.5 to 2 % by weight. Compared to the unmodified base material improvements of up to 80 % could be measured. Dielectric spectroscopy reveals that the relative permittivity in nanocomposites is lower than of the host and filler materials, with a minimum at a fillgrade of approximately 2 % by weight. For higher fillgrades the permittivity of the composite increases depending on the ratio between the permittivity values of filler and host material. Above 2 wt.% the permittivity of the filler material starts to overshadow the low permittivity of the rigid layers around the particles. Results from space charge measurement with the pulsed electro-acoustic method show that the quality of particle dispersion has an impact on the charge intake. Based on these measurements it is concluded that particle agglomerations act as charge traps, while the amount of charges in nanocomposites with good particle dispersion is lower than in the unmodified epoxy. This confirms that the particles indeed act as recombination centres, actively mitigating charge buildup inside the material. These results show why nanocomposites are very interesting for HVDC equipment. Space charges are a limiting factor for DC applications. Their reduction improves the reliability of the insulation system. The increased DC breakdown strength enables more compact high voltage equipment, respectively the utilization at higher field strengths. The work presented here is a stepping stone on the way to industrial applications of nanostructured insulation material and fundament for further investigations on topics like nanofluids.