Short term DC breakdown and complex permittivity of Al2O3- and MgO-epoxy nanocomposites

Electrical insulation based on epoxy resin is commonly used in high voltage applications. Examples are transformers, terminations, plugs or connectors. But, it is also used in high voltage applications where energy transportation or distribution is not the main purpose, e.g. in medical and industrial X-ray systems, or radar. This paper addresses the changes in the structure due to the introduction of surface functionalized nanoscale particles, namely magnesium oxide and aluminum oxide. Short term DC breakdown tests were performed alongside dielectric spectroscopy. The BD strength was measured for negative DC ramp voltages. Dielectric properties have been acquired by broadband dielectric spectroscopy. The base polymer is a commercially available bisphenol A epoxy with anhydrite hardener. As filler material magnesium oxide powder was used with an average particle size of 22 nm and alumina filler with 50 nm average diameter. Both particle types were modified with a silane coupling agent, in order to achieve a uniform dispersion of particles in the host material. Neat epoxy samples were used as a reference. Both composites showed remarkable DC breakdown strength for low fillgrades. A possible explanation for this unique behavior is given and the differences between samples filled with aluminum oxide and magnesium oxide are discussed.