ON EFFECTS OF VARIATION OF POLARIZING TEMPERATURE AND ADDITIVE CONTENT ON SPACE CHARGE FORMATION AND ELECTRIC FIELD DISTRIBUTION IN PLAIN LDPE AND LDPE DOPED WITH BATIO_3 ADDITIVE : EVIDENCE FROM MEASUREMENTS USING THE THERMAL STEP METHOD

摘要

A thermal step method was used to investigate the effects of polarizing temperature and additive content on space charge formation and electric field distribution in relatively thick samples (～2mm) of plain low density polyethlene (LDPE) and LDPE doped with 5 wt % barium titanate (BaTiO_3). Space charge was formed using de field of about 26 kV / mm at two different polarizing temperatures:25℃ and 50℃. Results indicate that for plain LDPE the remanent space charge density and electric field increase with increasing the polarizing temperature from 25℃ to 50℃. The addition of 5 wt % BaTiO_3 to plain LDPE appears to have remarkably reduced the amounts of the remanent space charge and electric field and changed their distribution patterns when compared with the corresponding values for the plain material.The maximum values of the remanent electric field reached in plain LDPE and doped LDPE are about 85% and 15% of the external applied field respectively. Moreover, the distribution patterns of the remanent space charge and electric field for the doped material appear to be more sensitive to variations of polarizing temperature than those for the plain material. Whereas in plain LDPE, the observed distribution patterns of the remanent space charge exhibit a homocharge at the cathode and a heterocharge at the anode for both polarizing temperatures, for the doped material, the rise of the polarizing temperature from 25℃ to 50℃ appears to have a considerable effect on the distribution pattern of the remanent space charge in this case: at 25 ℃ , the remanent space charge distribution exhibits a heterocharge at the anode and homocharge at the cathode while at 50℃ the remanent space charge distribution pattern is reversed showing a homocharge at the anode and a heterocharge at the cathode.