Dependence of filler size on space charge formation in LDPE/MgO under high electric field at high temperature

Space charge behavior has been investigated in LDPE/MgO composite films including the different size of MgO particles with a diameter of several tens nm (nano) or a few mum (micro) when a dc high stress is applied to the sample at high temperature. It is believed that a positive packet-like charge is injected from the anode, and after that a negative packet charge is injected from the cathode. In the samples with relatively low content of MgO, the apparent velocity of positive packet-like charge in LDPE/MgO (nano) is faster than that in LDPE/MgO (micro) at a field of 150 kV/mm. Such generation of charges causes the enhancement of local electric field in the sample. However, almost no packet-like charge was observed in the sample with relatively high content of MgO irrespective of the filler size. It is considered that the addition of MgO filler to LDPE plays a role to reduce the enhancement of the electric field in the sample by suppression of both the positive and the negative charge injection into the sample.