Surface discharge characteristics and initiation mechanism of PEEK in nitrogen under semi-square voltage

The surface discharge behaviors of polyetheretherketone (PEEK) in nitrogen under both positive and negative semi-square voltage were investigated using the current pulse detection method by non-inductive resistor and optical signal detection method by photomultiplier and digital camera. A 50Hz positive or negative semi-square voltage was applied to a rod-plane electrode system, and two types of surface discharge in a voltage cycle were found: one forward discharge and one back discharge. The characteristics and initiation mechanism of the two types of surface discharge were explored. The experimental results showed that surface discharges between cycles were independent. Based on the independence, the time-lag characteristics of surface discharge were obtained. An improvement on Laue-Zuber time-lag theory was made to fit the time-lag characteristics of forward discharge and back discharge. The mean time-lag decreased with the increase of voltage for both forward discharge and back discharge, and the relation between mean time-lag and applied voltage could be modeled by inverse power law. Comparative analysis on the characteristics of back discharge and forward discharge under opposite voltage was performed, and the characteristics of back discharge and forward discharge under opposite voltage were homogenous. In addition, the initiation mechanism of the two types of discharge was discussed. The initiation of forward discharge is believed to be mainly determined by the time-lag and the electric field caused by the applied voltage, while the initiation of back discharge is believed to be mainly determined by the time-lag and the electric field induced by the residual surface charge left by the forward discharge in the same cycle. Verification tests under positive semi-square voltage with different fall time were conducted to validate the initiation mechanism.