Space Charge Mechanism of Polyethylene and Polytetrafluoroethylene by Electrode/Dielectrics Interface Study using Quantum Chemical Method

This work studies the space charge mechanisms of PE and PTFE by involving the combination of electronic structures and charge transfer processes at the interfaces of electrode/dielectrics. Experimentally, significant positive charge packets were observed in LDPE when subjected to a high electric stress. However, slight charge accumulations (mainly homo-charges) were observed inside polytetrafluoroethylene (PTFE) under the same condition. The electronic structures of PE and PTFE were calculated by density functional theory (DFT). After that, we obtained the electronic parameters, such as energy level, Fermi level, band gap and electron affinity. A negative electron affinity is determined in PE, indicating an interchain electron charge transport at the bottom of the conduction band. Nevertheless, a positive electron affinity in PTFE exhibits an intrachain electron charge transport. Moreover, injection and charge transfer at electrode/dielectrics interfaces were studied based on the calculated parameters. A low hole injection barrier dominates the positive charge injection and accumulation in LDPE, while a high electron barrier hinders the electron charge injection. In contrast, the hole injection barrier of PTFE is higher than that of LDPE. We discussed these different processes in this paper.