Une approche du vieillissement électrique des isolants polymères par mesure d'électroluminescence et de cathodoluminescence

摘要

Electroluminescence (EL) of insulating polymers is a subject of great interest because it is associated with electrical ageing and could provide the signature of excited species under electric field. Electrical ageing and breakdown in insulating polymers is of fundamental interest to the researchers, the design engineers, the manufacturers and the customers of electrical apparatus. In this respect, Partial Discharge (PD) is a harmful process leading to ageing and failure of insulating polymers. However, with the development of the materials and apparatus, PDs can be weakened or avoided in some situations, e.g. extra high voltage cables, capacitors, etc. Therefore, there is urgent demand for understanding electrical degradation mechanisms under high electric field, which can be triggered by energetic charge carriers. In this work, Electroluminescence, EL, and cathodoluminescence, CL, excited under electron beam, along with other luminescence-family techniques are carried out for probing polyolefins and other insulating polymers. In order to uncover the excitons formation in Polypropylene (PP) and Polyethylene (PE) thin films, the field dependence of EL and current under DC stress and field dependence of EL and phase-resolved EL under AC stress, are investigated. The EL spectra of both PP and PE have the same main peak at approximately 570 nm, pointing towards similar chemical structures and defects in both polyolefins, and same route to degradation. This main peak can be complemented by an emission at approximately 750 nm dominating at low field. Electrode effect on the EL of Polyethylene Naphthalte (PEN) was investigated to understand the origin of the red emission at 750 nm. Through field dependence of EL and phase-resolved EL of Au or ITO electrodes, we proved the red component is due to the nature of electrode, more precisely Surface Plasmons and/or interface states. Further thorough study was carried out on cathodoluminescence of insulating polymers. Thin films of PP, PE, along with Polyethylene Naphthalate (PEN) and Polyether Ether Ketone (PEEK) were irradiated under electron beam up to 5 keV to be excited. We could reconstruct EL and CL spectra of both PE and PP using four elementary components: i.e. Fluorescence, Chemiluminescence, Recombination-induced Luminescence, and main component of the EL spectrum at 570 nm reported above and constituting an ageing marker. For the first time the nature of both EL and CL in polyolefins is uncovered, containing four basic components with different relative contributions. Identification of these spectral components is helpful to interpret the nature of light emission from polyolefins and other insulating polymers and to bridge the gap between space charge distribution and electrical ageing or breakdown. Through researches on EL and CL in several insulating polymers, i.e. polyolefins and a polyester, excitons formation and relaxation processes under electric stress and kinetic electrons are evidenced. More importantly, the spectral components analyses and reconstruction uncovers the nature of luminescence and its correlation to electrical ageing. In the future, luminescence measurement can be developed to be a standard method to probe and analyze insulating polymers.