Modulation of DC breakdown strength by direct fluorination in polyethylene

Breakdown performance of polymer under high voltage direct current (DC) and alternating current (AC) fields is receiving more and more attentions with the ever-increasing demand of raising voltage class and developing long-distance transmission. The existing researches indicate that space charge should play an important role in influencing breakdown performance. However, two key issues, that the different effects of space charge on DC breakdown strength (F) and AC breakdown strength (F), and that the reason for F being always higher than F, remain not completely clear. In this letter, we study the mechanism of DC and AC breakdown by modulating space charge distribution in low density polyethylene by using direct fluorination process. The results show that F closely relates to the space charge modulation under DC field but F almost has nothing to do with it. Therefore, direct fluorination process can be used to modulate space charge distribution under DC field and further F. DC breakdown models with both homocharge and heterocharge distributions are proposed. They ascribe DC breakdown to field distortion induced by space charge and large defects caused by energetic processes, which happen in middle bulk under homocharge distribution and in marginal bulk under heterocharge distribution. F being still lower than F with heterocharge distribution proves the important influence of the energetic processes happening at polymer/electrode interface and in marginal bulk, besides the field enhancement caused by heterocharge. The modulation mechanism is discussed combined with trap characteristics of carriers.