Innovative nanostructured epoxy composites for enhanced high voltage insulation systems

In order to cope with the growing demand in electricity, operating voltages and power ratings have seen an increase in the past years. This means that electro-thermal stresses on the existing electrical insulation systems have increased concomitantly. However, polymeric materials used for high voltage insulation are prone to degradation due to electrical discharges and commonly boast rather low thermal conductivities, which is why there is an impelling need for a new generation of insulating materials with improved dielectric and thermal performances. During the last decades, attention was drawn towards a novel class of dielectric materials: polymer nanocomposites or nanodielectrics. These dielectrics feature nanometric filler particles instead of micrometric particles, which can lead to significantly enhanced performances - such as improved dielectric breakdown strengths - already at very low contents, thus indicating their great potential for application in HV insulation systems. Nevertheless, such nanodielectrics only unfold their full potential when a good dispersion and distribution of those filler particles within the polymer matrix are achieved. Albeit, clusters of nanoparticles with submicrometric or micrometric dimensions are often found, which is due to the incompatibility of inorganic particles with the organic polymer. Such agglomerations will subsequently cancel the beneficial effect seen for well dispersed nanoparticles. In order to enhance the interaction between inorganic filler particles and organic matrix, and hence, improve the dispersion of such particles in polymers, the functionalization of nanofillers has become rather common. Still, little is known about the long-term stability of such functionalizers under electro-thermal stresses, which poses a drawback to their broad industrial use in high voltage engineering. The objective of the presented thesis was to develop innovative, nanostructured epoxy composites that reveal enhanced dielectric and thermal performances, and to evaluate their applicability for high voltage insulation systems. In order to achieve an original contribution to the field of nanodielectrics, a novel approach was explored by using functional nanometric additives, so-called polyhedral oligomeric silsesquioxanes (POSS) instead of applying surface functionalization for the filler particles used. Even more, multifunctional nanostructured composites were to be designed, involving functional POSS additives along with thermally conductive filler, to further enhance the thermal conductivity of the resulting composites. To achieve our objectives, in a first step, epoxy composites with hexagonal boron nitride (h-BN) and cubic boron nitride (c-BN) were developed and analyzed, with filler contents well below the percolation threshold, to find the most promising type of BN filler and its respective size, to boost thermal conductivity in epoxy composites. In this context, it was shown that incorporation of low weight fractions (≤ 5 wt%), of submicrometric and micrometric h-BN particles in epoxy resin resulted in noticeable improvements in corona resistance and thermal conductivity of the resulting composites. Addition of 5 wt% c-BN in submicrometric particle sizes, however, was found to achieve the most significant improvement of the thermal conductivity compared to the h-BN composites. At the same time, all the BN composites have seen a slight reduction in their dielectric breakdown strength of up to 18 % compared to the base epoxy, which is a common phenomenon observed for the filler sizes used. With the respective breakdown strengths being above 130 kV/mm for the h-BN, and above 170 kV/mm for the c-BN composites, thus still significantly higher than common electric stresses in high voltage insulation systems, the improvements found in the composites’ erosion resistance or thermal conductivity should be granted a higher emphasis. In a second step, two different types of functional POSS fillers were used to fabricate composites. POSS is a hybrid inorganic/organic material, which has a silica-like core surrounded by functional groups. These functional groups were of reactive nature in the case of the two POSS additives used, and thus, could covalently bond with the epoxy. The first POSS additive was a Triglycidylisobutyl-POSS (TG-POSS) which had 3 functional groups that were compatible with our epoxy system. It was found that the formation of covalent bonds between POSS and the epoxy matrix significantly improved the filler/matrix interaction, and hence, led to significant improvements in dielectric breakdown (BD) strengths and corona resistances for the TG-POSS composites. This was further supported by the superior performances of the lower content composites with 1 and 2.5 wt% of TG-POSS, where no agglomerations were found, and hence, where the dispersion of POSS in the epoxy can be considered to be at a molecular level. The presence of agglomerations for higher TGPOSS contents (with 5 or 10 wt% POSS) and the concomitant deterioration of the dielectric performances for these composites prevented to exploit higher POSS loadings, to further enhance the resistance to corona discharges for instance. Therefore, a highly functionalized Glycidyl-POSS (G-POSS) was chosen to continue with the study. This time around, it was shown that composites with low G-POSS content have excelled in high BD strengths and notably increased resistances against corona discharges, as well as enhanced thermal conductivities and low dielectric losses. Overall, the addition of 2.5 wt% of both types of POSS in epoxy was found to be an optimal value in terms of dielectric strength and losses. Further increase of the Glycidyl-POSS loading would then contribute towards an even higher resistance to corona discharges, whereas in terms of thermal conductivity for both POSS types, the composites with low contents, of 2.5 wt% POSS and below, have seen the most significant enhancements. In the third step of this study, multiphase composites were produced, which contained both, 1 wt% of POSS and 5 wt% of c-BN particles, in order to investigate the interaction between the hybrid inorganic/organic POSS and the inorganic c-BN. The obtained multiphase samples were compared in terms of their dielectric and thermal properties with the respective singlephase composites, where only 1 wt% POSS or 5 wt% c-BN were incorporated in epoxy. This part of the study revealed that although no complex chemical surface treatment was applied for the c-BN particles used in our study, yet a homogeneous dispersion of the inorganic c-BN articles was seen in the multiphase composites. This effect of POSS, which was shown to act as a dispersant of the inorganic c-BN filler should be regarded as a major point of interest in nanodielectrics or nanocomposites in general, as the dispersion of nanometric inorganic filler particles within polymers is still a very current problematic. And thus, the approach of formulating epoxy composites combining reactive POSS and other filler particles to improve their dispersion within the epoxy matrix, could significantly contribute to the advancement of the implementation of epoxy-based nanocomposites on an industrial level. In terms of thermal conductivity or breakdown strengths no improvement compared to the 5 wt% c-BN composite was seen, when both POSS and c-BN were incorporated together in a multiphase sample. In conclusion, it can be stated that the G-POSS composites feature the overall best performance of a dielectric material for high voltage insulation, with lower dielectric losses, higher BD strength, as well as increased thermal conductivity, compared to the base epoxy. The last part of this work presents a simulation-based analysis of the heat transfer phenomenon observed for the POSS composites. Therefore, 3D FEM simulations of the conductive heat transfer in selected epoxy composites were conducted in COMSOL Multiphysics, given that our results have shown that increasing POSS contents have led to a decrease in terms of thermal conductivity, which is in contradiction with the behavior suggested by theoretical mixing laws. The FEM computational approach finally led to the proposal of a novel model, which can explain the heat transport phenomenon in the presented POSS composites. The Interfacial Restructuration Model (IFRM) points out that the reactive nature of POSS, with its functionalized groups, must have an impact on the morphology of the epoxy/POSS network, in a way that enhances phonon transport through the bulk composite, thus explaining the particular results of thermal conductivities in the POSS composites in question.