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PdD defence : Mallys BANDA

par Agnès Gaunie Picart - publié le

Measurement and modelling of dielectric materials behaviour under electron-beam irradiation

*Friday 1st december 09:30 AM , Salle des Colloques du Bâtiment 3R2 ( Université Paul Sabatier Site)

Jury :

Gilles DAMAMME, Directeur de Recherche, Commissariat à l’Energie Atomique, Rapporteur
Omar JBARA, Professeur, Université Reims-Champagne Ardennes, Rapporteur
Thierry PAILLAT, Professeur, Université de Poitiers, Examinateur
David MALEC, Professeur, Université Toulouse III, Invité
Gilbert TEYSSÈDRE, Directeur de Recherche, CNRS, Directeur de thèse
Séverine Le ROY, Chargée de Recherche, CNRS, Co-directrice de thèse
Virginie GRISERI, Maître de Conférence, Université Toulouse III, Co-directrice de thèse

Abstract
In their common uses as electrical insulators, organic solidmaterials are constitutive of electric power transmission cables,power control and conversion circuits as well as (micro)
electronic components or embedded systems (thermal coating of satellites, batteries of accumulators, etc.). Under various constraints of use (electric field, radiation, temperature, humidity ...) they can accumulate charges in their bulk which could affect the reliability of the systems in which they are employed. One of the commonly used means to study the electrical behavior of these charges is to measure the
spatiotemporal distribution of charges by subjecting the dielectrics to a continuous potential difference between two electrodes. However, this method does not always allow clearly distinguishing the contribution of charges due to generation on the one hand and the one due to transport phenomena on the other hand. This study proposes an alternative approach, consisting in generating charges (electrons) within the electrical insulation using an electron-beam under vacuum. The charges
are hence deposited at a known position and in a controlled
quantity. Other physical processes related to the implantation of electrons must then be taken into account in order to predict and model the behavior of these irradiated materials. Lowdensity polyethylene (LDPE) films, prepared by thermal
molding, were irradiated by a 80 keV electron-beam with a current flux of 1 nA/cm2. Space charge measurements using the Pulsed Electro-Acoustic (PEA) method, performed first in-situ and then ex-situ under DC electrical polarization, confirm an
effective localization of charges within the material. The results under electrical polarization after irradiation show an important amount of positive charges in the irradiated zone of the dielectric. The electrical characterizations of irradiated
LDPE films show a completely different behavior compared to the same non-irradiated material, suggesting a modification of the chemical structure of the material. Physico-chemical measurements (infrared spectroscopy, Photoluminescence and Differential Scanning Calorimetry-DSC) on these irradiated
PEbd films do not show a significant degradation of the chemical structure of the dielectric which would explain the observed electrical behavior under post-irradiation
polarization. Additional measurements show the reversible behavior of the irradiated then polarized PEbd, which would be only related to the presence of the charges generated by the beam. The experimental data of this study have simultaneously
fed a numerical model of charge transport, developed to take into account the irradiation constraints. This model allows reproducing the in-situ results of charge implantation by the electron beam as well as the majority of the electrical processes
observed on irradiated and polarized LDPE. It confirms the impact of the electron-beam deposited charge on the behavior under polarization and allows concluding on the origin of the positive charges observed after irradiation, which would be due
to injection at the electrodes as well as to the creation of electron-hole pairs by the electron-beam during irradiation.