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Title: A molecular modelling study of electron trapping in polyethylene
Author: Meunier, Marc
ISNI:       0000 0001 3395 7157
Awarding Body: University of Wales, Bangor
Current Institution: Bangor University
Date of Award: 2000
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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, and the residence time of electrons is such traps is of the order of a few picoseconds.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Polymers