A molecular modelling study of electron trapping in polythylene

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 characterisation 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 thesis, a new scheme that constitutes a first step in linking microscopic defects to
the formation of space charge is suggested. Although the goal is to understand the role
of defects at the molecular level in electron trapping and the formation of space charge
in polyethylene, at first a "model" material is considered: 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 thesis, the focus is on the role of physical defects. The 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. A
methodology to accurately compute the electron affinity of a wide range of atoms and
molecules has been developed. The electron affinity and its variation with wax molecule
conformation have been calculated using Density Functional Theory. By performing
molecular dynamics simulations of amorphous waxes, likely conformational defects can
be identified, and by using ab-initio methods, the trapping energies can be estimated.
Conformational defects in these waxy materials are predicted to produce shallow traps
with energies below 0.3 eV, their density is estimated to be 3.1 1020 traps.cm-3, and the
residence time of electrons is such traps is of the order of a few picoseconds.

Date of Award	Jul 2000
Original language	English
Awarding Institution	Bangor University
Supervisor	Nicholas Quirke (Supervisor)