Electron trapping in polymer insulators: a new approach using molecular modelling

• Published in: Materials for Advanced Metallization (MAM), European Workshop Vol. 54; no. 287; pp. 66 - 72
• Main Authors: Meunier, M, Quirke, N, Binesti, D
• Format: Journal Article
• Language: English
• Published: 29.06.1998
• ISSN: 1266-0167

It is well known that the presence of space charge in the polymeric insulation of high voltage buried cables is correlated with electric breakdown. Many studies have been carried out on the experimental characterisation of space charge and on phenomenological models of space charge formation and discharge. In this paper we suggest a new scheme based on a molecular modelling approach that employs both quantum calculations and classical molecular dynamics. Although our aim is to understand space charge in polyethylene and the role of defects at the molecular level in electron trapping in polyethylene, we start by considering "model" materials; the wax tridecane (C sub 13 H sub 28 ). It is clear that both physical and chemical defects (i.e. conformational/free volume and additives/broken bonds, respectively) may be present in insulating materials and may both trap electrons. This paper will focus on the role of physical defects. Our analysis suggests that by writing down the defect energy in terms of the molecular electron affinity, a relationship is established between the electron trap and the molecular properties of the waxes. 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 observe the most probable conformational defects, and by using the ab-initio results we can estimate the trapping energies. Conformational defects in these waxy materials are predicted to produce shallow traps with energies below approx0.2 eV.