Molecular modeling of electron traps in polymer insulators: Chemical defectsand impurities

摘要

The presence of space charge in the polymeric insulation of high-voltage cables is thought to be correlated with electric breakdown. However, a direct link between molecular properties, space charge formation and eventual breakdown has ~till to be established. It is clear that both physical (e.g., conformational disorder) and chemical defects (e.g., broken bonds and impurities) are present in insulating materials and that both may trap electrons. We have shown that by defining the defect energy in terms of the molecular electron affinity, a relationship is established between the electron trap and the molecular properties of the material. In a recent paper [M.. Meunier and N. Quirke, I. Chem. Phys. 113,369 (2000)] we proposed methods that have made it possible to provide estimates of the energy, number and residence times of electrons ift conformational traps in polyethylene. Typical physical trap energies are of the order of 0.15 eV and all are less than 0.3 eV. In the present paper we focus on the role of chemical defects, where we expect much deeper traps but at very low concentrations. Following the methodology used in our previous paper we have used density-functional theory to calculate trap energies for a set of chemical impurities and additives commonly found in polyethylene used for high-voltage cable insulation. In an extension of our approach we have estimated the effect of neighboring molecul~ on the trap energies of suchdefec;ts. The resulting trap energy-trap density distribution reveals some very deep (> leV) traps presumably implicated in the formation of long-lived space charge in polymeric insulators and consequently to changes in the dielectri~ properties of the material.