We present a characterization of chemical conditions that could cause electron emission from a polyethylene surface, triggering a partial discharge in an isolated vacuum.
In the electrical industry, polyethylene is commonly the most used material for forming the polymer insulation layer in electrical cables. Unfortunately, in AC current, this polymer is known to suffer degradation, usually associated with the treeing process.
Treeing begins within a gaseous defect embedded in the polymer matrix, within which the dielectric stiffness of the material decreases significantly, and an electron is emitted from the surface of the polymer towards the vacuum. This creates the conditions for triggering a series of partial discharges which degrades the material from the inside, creating a tree of cavities in continuous, and self-sustaining, expansion.
The mechanism by which the emission that causes the discharge occurs is, most likely, the Schottky effect. It is therefore very important to define the chemical conditions that most favor surface electronic emission in the initial phase.
In the present study, we made a series of density functional theory calculations to characterize the electronic structure of different chemically defective polyethylene systems. Our aim was to find the right combination of defects that can significantly reduce the extraction work, and give a Schottky emission current potentially in line with the experimental reference. The extraction of each system was the key parameter to follow to define the Schottky emission properties. According to the numerous tests we conducted, we establish that it is very unlikely to observe Schottky emission from neutral polyethylene, i.e. in the absence of a certain quantity of residual electronic charge on the surface, which needs to be localized through the presence of additional electronic states, created by chemical defects.
In particular, we found that a surface of negatively charged oxidized polyethylene presents an extraction consistent with the experiments