Electronic structure of defected polyethylene for Schottky emission

Highlights: • Schottky emission (SE) from a neutral polyethylene (PE) surface is unlikely. • SE is also related to the nature of chemical defects. • Chemical defects should act as a trap even when the system is charged. • An oxidized PE surface with an excess electron is consistent with experimental data. Polyethylene is one of the most used solid state insulators in electrical power industry. It is particularly used to electrically insulate high-voltage cables. Under the stresses associated with AC power supplies, this material undergoes ageing, which is often associated with treeing. It is thought that this phenomenon starts from gaseous defects embedded in the insulator bulk, leading to the formation of a cluster of cavities. Treeing is able to dig the matrix until complete breakdown of the insulating components. Cavities are generated by a sequence of partial discharges. Each discharge is triggered by an electron emission from the surface at the interface with gas. The Schottky effect is believed to be the most likely mechanism able to cause this electron emission. Our DFT modelling has suggested that electron emission is highly unlikely to occur if the surface is neutral. DOS analysis has revealed that the Schottky effect is also related to chemical defects. The latter must exhibit electronic states slightly under the conduction band. Furthermore, these sites must be able to act as a trap for negative charge excess. A polyethylene system with an excess electron, combined with specific oxidative groups, has proved to be consistent with experimental data.