Molecular modeling of electron trapping in polymer insulators

Abstract

The presence of space charge in the polymeric insulation of high-voltage cables is correlated with electric breakdown. There is a vast literature concerned with the experimental characterization of space charge and with phenomenological models of space charge formation and discharge. However, a direct link between molecular properties, space charge formation and eventual breakdown has still to be established. In this paper, we suggest a new scheme that constitutes a first step in linking microscopic defects to the formation of space charge. Although our goal is to understand the role of defects at the molecular level in electron trapping and the formation of space charge in polyethylene, we start by considering a “model” material; the wax tridecane (n-C13H28). It is clear that both physical (e.g., conformational defects) and chemical defects (e.g., broken bonds) may be present in insulating materials and may both trap electrons. In the present paper, we focus on the role of physical defects. Our analysis suggests 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. The electron affinity and its variation with wax molecule conformation have been calculated using density functional theory (DFT, as implemented in the code DMol). By performing molecular-dynamics simulations of amorphous waxes, we are able to determine likely conformational defects, and by using ab initio methods estimate the trapping energies. Conformational defects in these waxy materials are predicted to produce shallow traps with energies below 0.3 eV. These results are used to estimate the energy, number, and residence times of electrons in conformational traps in polyethylene.