Packet-like space charge dynamics in polyethylene with various crystallinity

As the phenomenon of packet-like space charge behavior in polyethylene was firstly indentified near thirty years ago, much effort has been done to clarify its physical mechanism. The packet-like space charge density and migrating velocity are of the two main parameters to characterize this phenomenon. Both the parameters could be serious influenced by the micro-structure of the material, since the micro-structure could significantly influence charge trap density and trap level, which therefore influence the space charge migration. In this study, linear low density polyethylene (LLDPE) samples with various crystalline rate and gradient crystalline were prepared by hot pressing process with controlled cooling rate. The evolution of space charge packet in the various LLDPE samples sandwiched by semiconducting electrodes was monitored by pressure wave propagation (PWP) method under DC 40 MV/m stress at 40°C. The influence of crystallinity and micro-morphology on charge injecting rate, mobility and decaying procedure in LLDPE are discussed. The results show that the heating treating procedure could significantly influence the dynamics of packet-like space charge and the charge decay process. The slow cooling rate could lead to deeper trap level and therefore slower packet charge.