The space charge measurement in MgO/LDPE nanocomposite up to the breakdown under DC ramp voltage

To understand basic electric properties of nano-sized magnesium oxide (MgO) / low-density polyethylene (LDPE) nanocomposite under DC voltage application, DC breakdown strength and the space charge up to the breakdown under DC ramp voltage were investigated. DC breakdown strength of 1 phr sample contained 1 phr MgO nanofiller (hereafter, called 1phr sample) increased compared with that of 0 phr sample. In the case of 0 phr sample, the homo charge, containing a huge negative charge and a little positive charge, was finally observed near the electrodes shortly before the breakdown. In the case of 1 phr sample, the positive charge increased with increasing average field until the average field reached certain value. After then, the positive charge decreased with increasing average field until the breakdown occurred. The field enhancement ratio (=maximum field / average field) of 0 phr sample increased with increasing the average field and was saturated. After showing the peak, the field enhancement ratio of 1 phr sample turned to decrease with increasing the average field. From these results, it is thought that the suppression effect of conduction current determined by the space charge leads to higher DC breakdown strength of 1 phr sample, rather than the suppression effect of electronic avalanche with the addition of the MgO nano-filler.