Temperature Dependent Space Charge Formation up to the Breakdown under DC Ramp Voltage in MgO/LDPE Nanocomposite

To understand basic electric properties of nano-sized magnesium oxide (MgO) / low-density polyethylene (LDPE) nanocomposite materials, the temperature dependence of breakdown strength and the space charge up to the breakdown has been investigated under DC ramp field. At temperatures at which these investigations were carried out, DC breakdown strength of the sample with the MgO nano-filler (hereafter, called 0 phr sample) exhibited higher DC breakdown strength compared with the sample with the MgO nano-filler (hereafter, called 1 phr sample). DC breakdown strength of 0 phr and 1 phr samples decreased with increasing temperature. In the case of 0 phr sample at 303 K, the negative charge was observed in the bulk and was found to be saturated before the breakdown. The hetero charge was observed at 333 K shortly before the breakdown. In the case of 1 phr sample at 303 K, the positive charge increased with increasing average field until the average field reached a certain value and then decreased with increasing average field until the breakdown occurred. The field increment (maximum field - average field), brought by the space charge, of 0 phr sample at 303 and 333 K increased with increasing average field and found to be saturated before the breakdown. As different from 0 phr sample, the field increment of 1 phr sample turned to decrease after showing a peak under the high field.