Uma condutividade evanescente, dependente do tempo, tem sido detectada nos filmes de PVDF da BembergFolien. Após muitos ciclos de polarização e despolarização, a corrente de absorção torna-se fortemente reduzida, para o mesmo nível das correntes de despolarização. Esta condutividade é facilmente observada, aparecendo na corrente de polarização pela aplicação de altos campos. É observada recuperação em amostras deixadas em atmosfera úmida. Nós interpretamos estas observações como sendo uma limpeza, pelo campo elétrico, dos íons gerados por uma acentuação da dissociação de moléculas de água absorvida no volume, pela ação deste campo. / A time dependent evanescent conductivity has been detected in BembergFoliem PVDF films. After many polarization-depolarization cycles, the absorption current became strongly reduced, to the same level as the depolarization one. It is easily seen at higher fields by a bump appearing in the polarization current Recovering is observed in samples left in humid atmosphere. We have interpreted these observations as an electric field ion cleaning of an electric field enhanced dissociation of bulk absorbed water molecules.

Neste trabalho obtivemos curvas de corrente induzida por raios-x no teflon FEP que apresentou as seguintes características: a) Inicialmente a corrente subiu, atingindo um máximo em torno dos 10s; b) Decaiu lentamente durante aproximadamente meia-hora, e; c) Atingiu um estado estacionário daí por diante. Ao se desligar a radiação, registrou-se a componente atrasada desta corrente. Usamos amostras de 25&#956m de espessura e área irradiada foi de 12,5cm2 ; o campo aplicado da ordem de 104V/cm e taxa de exposição da ordem de 102 R/S. Verificamos que depois de completada de uma medida da corrente induzida num amostra, esta não voltava a se repetir se realizada depois algumas horas; a corrente então não apresentava um máximo, indo diretamente ao valor estacionário; porém se recuperava com o tempo, repetindo a primeira medida depois de algumas semanas. Para mostrar que esta subida e descida da corrente induzida, não era devido a um efeito de campo, realizamos uma medida onde aplicamos o campo intermitente por curtos períodos de tempo, enquanto a amostra era irradiada. O resultado se mostrou igual aos realizados com tensão aplicada permanentemente. Estudamos ainda a dependência da condutividade induzida com a taxa de exposição e com o campo. Finalmente construímos um modelo teórico para o material que permitiu a obtenção de parâmetros do mesmo concordantes com o esperado. Acrescentamos a este trabalho uma curva que mostra o efeito da variação da temperatura sobre uma medida longa da absorção dielétrica. / In this work we measured X-ray indeuced currents in teflon FEP wich show the following features: a) At the beginning the current increases and reaches a maximum at about 10s; b) It decays slowly during 30 minutes, when a steady state is reached slowly during 30 minutes, when a steady state is reached the delayed conductvity was also measured. The sample were 25&#956m thick and the irradiated área was 12,5cm2; the applied field was of the order of 104 V/cm and the dose rate of the order of 102 R/S. It was observed that a new measurement of the induced conductivity does not duplicate the first one, but after a few hours it come backs to the original one. In order to show that the increase and the deacrease of the current is not caused by na electric field effect we realized a measurement where we polled the sample intermitently while it was irradiated. The current thus obtained had about the same values of the first measuments, when the voltage was applied all the time during the measurement. We also measured the absorption current f a teflon sample wich shows after some days the effect of its variation due the variation of the ambiental temperature.

Computational quantum mechanics in the frame work of density functional theory (DFT) was used to investigate the effect of chemical impurities on high field conduction in polyethylene (PE). The impurity states in the band gap caused by common chemical impurities were characterized in terms of their “depth”, i.e. energy relative to their relevant band edge (valence band or conduction band), and in terms of the extent to which their wavefunctions were localized to a single polymer chain or extended across chains. It was found that impurity states can affect high field phenomena by providing “traps” for carriers, the depths of which were computed from first principle in agreement with estimates in literature. Since the square of the wavefunction is proportional to the spatial electron probability density, transfer of charge between chains requires wavefunctions which are extended across chains. Impurity states which are extended between chains can facilitate the inherently limited interchain charge transfer in PE, as the DFT study of iodine doped PE revealed. The introduction of iodine into PE increases conductivity by several orders of magnitude, increases hole mobility to a much greater extent than electron mobility, and decreases the activation energy of conduction from about 1 eV to about 0.8 eV. These characteristics were explained in terms of the impurity states introduced by iodine and wavefunctions of those states. Understanding the effect of iodine on conduction in PE provided a basis for understanding the effect of common chemical impurities on conduction therein. In particular, carbonyl and vinyl impurities create states which should promote hole mobility in a manner very similar to that caused by iodine. It was demonstrated that in the context of high field conduction in PE, besides the traditional focus on the depth of impurity states, it is important to study the spatial features of the states wavefunctions which are neither discussed nor accounted for in present models.

Computational quantum mechanics in the frame work of density functional theory (DFT) was used to investigate the effect of chemical impurities on high field conduction in polyethylene (PE). The impurity states in the band gap caused by common chemical impurities were characterized in terms of their “depth”, i.e. energy relative to their relevant band edge (valence band or conduction band), and in terms of the extent to which their wavefunctions were localized to a single polymer chain or extended across chains. It was found that impurity states can affect high field phenomena by providing “traps” for carriers, the depths of which were computed from first principle in agreement with estimates in literature. Since the square of the wavefunction is proportional to the spatial electron probability density, transfer of charge between chains requires wavefunctions which are extended across chains. Impurity states which are extended between chains can facilitate the inherently limited interchain charge transfer in PE, as the DFT study of iodine doped PE revealed. The introduction of iodine into PE increases conductivity by several orders of magnitude, increases hole mobility to a much greater extent than electron mobility, and decreases the activation energy of conduction from about 1 eV to about 0.8 eV. These characteristics were explained in terms of the impurity states introduced by iodine and wavefunctions of those states. Understanding the effect of iodine on conduction in PE provided a basis for understanding the effect of common chemical impurities on conduction therein. In particular, carbonyl and vinyl impurities create states which should promote hole mobility in a manner very similar to that caused by iodine. It was demonstrated that in the context of high field conduction in PE, besides the traditional focus on the depth of impurity states, it is important to study the spatial features of the states wavefunctions which are neither discussed nor accounted for in present models.

Neste trabalho obtivemos curvas de corrente induzida por raios-x no teflon FEP que apresentou as seguintes características: a) Inicialmente a corrente subiu, atingindo um máximo em torno dos 10s; b) Decaiu lentamente durante aproximadamente meia-hora, e; c) Atingiu um estado estacionário daí por diante. Ao se desligar a radiação, registrou-se a componente atrasada desta corrente. Usamos amostras de 25&#956m de espessura e área irradiada foi de 12,5cm2 ; o campo aplicado da ordem de 104V/cm e taxa de exposição da ordem de 102 R/S. Verificamos que depois de completada de uma medida da corrente induzida num amostra, esta não voltava a se repetir se realizada depois algumas horas; a corrente então não apresentava um máximo, indo diretamente ao valor estacionário; porém se recuperava com o tempo, repetindo a primeira medida depois de algumas semanas. Para mostrar que esta subida e descida da corrente induzida, não era devido a um efeito de campo, realizamos uma medida onde aplicamos o campo intermitente por curtos períodos de tempo, enquanto a amostra era irradiada. O resultado se mostrou igual aos realizados com tensão aplicada permanentemente. Estudamos ainda a dependência da condutividade induzida com a taxa de exposição e com o campo. Finalmente construímos um modelo teórico para o material que permitiu a obtenção de parâmetros do mesmo concordantes com o esperado. Acrescentamos a este trabalho uma curva que mostra o efeito da variação da temperatura sobre uma medida longa da absorção dielétrica. / In this work we measured X-ray indeuced currents in teflon FEP wich show the following features: a) At the beginning the current increases and reaches a maximum at about 10s; b) It decays slowly during 30 minutes, when a steady state is reached slowly during 30 minutes, when a steady state is reached the delayed conductvity was also measured. The sample were 25&#956m thick and the irradiated área was 12,5cm2; the applied field was of the order of 104 V/cm and the dose rate of the order of 102 R/S. It was observed that a new measurement of the induced conductivity does not duplicate the first one, but after a few hours it come backs to the original one. In order to show that the increase and the deacrease of the current is not caused by na electric field effect we realized a measurement where we polled the sample intermitently while it was irradiated. The current thus obtained had about the same values of the first measuments, when the voltage was applied all the time during the measurement. We also measured the absorption current f a teflon sample wich shows after some days the effect of its variation due the variation of the ambiental temperature.

Uma condutividade evanescente, dependente do tempo, tem sido detectada nos filmes de PVDF da BembergFolien. Após muitos ciclos de polarização e despolarização, a corrente de absorção torna-se fortemente reduzida, para o mesmo nível das correntes de despolarização. Esta condutividade é facilmente observada, aparecendo na corrente de polarização pela aplicação de altos campos. É observada recuperação em amostras deixadas em atmosfera úmida. Nós interpretamos estas observações como sendo uma limpeza, pelo campo elétrico, dos íons gerados por uma acentuação da dissociação de moléculas de água absorvida no volume, pela ação deste campo. / A time dependent evanescent conductivity has been detected in BembergFoliem PVDF films. After many polarization-depolarization cycles, the absorption current became strongly reduced, to the same level as the depolarization one. It is easily seen at higher fields by a bump appearing in the polarization current Recovering is observed in samples left in humid atmosphere. We have interpreted these observations as an electric field ion cleaning of an electric field enhanced dissociation of bulk absorbed water molecules.

L'électroluminescence (EL) de isolants polymères est étudiée car elle peut permettre d'approcher les phénomènes de vieillissement électrique en fournissant la signature optique d'espèces excitées sous champ électrique. Le vieillissement et la rupture diélectrique dans les isolants polymères est d'un intérêt fondamental pour les chercheurs, concepteurs et fabricants de dispositif du génie électrique. À cet égard, les décharges partielles (DPs) sont un des principaux processus conduisant au vieillissement et à la défaillance des isolants. Cependant, avec le développement des matériaux et procédés, les DPs sont évitées dans certaines situations, par exemple, les câbles haute tension, les condensateurs, etc. Par conséquent, le besoin reste prégnant pour la compréhension des mécanismes de dégradation électrique sous forte contrainte électrique, qui peut être initiée par des porteurs énergétiques. Dans ce travail, l'EL, la cathodoluminescence (CL) excitée sous faisceau d'électrons, ainsi que d'autres techniques de luminescence ont été appliquées à la caractérisation de polyoléfines et d'autres polymères isolants. Afin de comprendre la formation d'excitons dans des films minces de Polypropylène (PP) et Polyéthylène (PE), la dépendance en champ de l'EL et du courant sous contrainte continue, et de l'EL et de sa résolution selon la phase sous contrainte AC, sont étudiées. Les spectres d'EL du PP et du PE ont le même pic principal à environ 570 nm, ce qui implique des structures et des défauts chimiques similaires pour les deux matériaux, et le même processus de dégradation. Le pic principal peut être complété par une émission à environ 750 nm dominante à faible champ. L'impact de la nature des électrodes a été étudiée sur du PEN pour comprendre l'origine de l'émission dans le rouge. A travers la dépendance en champ de l'EL et sa résolution selon la phase avec des métallisations or et ITO, on montre que l'émission dans le rouge est liée à la nature des électrodes et correspond à l'excitation de plasmons de surface ou d'états d'interface. Une étude plus approfondie est effectuée sur la cathodoluminescence d'isolants polymères. Des couches minces de PP, PE, ainsi que de Polyethylene Naphthalate (PEN) et de Polyether Ether Ketone (PEEK) ont été irradiés par faisceau d'électrons jusqu'à 5 keV. Nous avons pu reconstruire les spectres de CL et d'EL du PE et du PP à partir de quatre composants élémentaires: fluorescence, chimiluminescence, luminescence induite par recombinaison, et composante principale du spectre d'EL à 570nm décrite plus haut et considérée comme signature du vieillissement. Pour la première fois, la nature de l'EL et de la CL de polyoléfines est décomposée en quatre composantes de base avec des contributions relatives différentes. L'identification de ces composantes spectrales est utile pour interpréter la luminescence de polyoléfines et autres isolants polymères, et établir les liens entre distribution de charge d'espace et vieillissement diélectrique. A travers ces recherches sur l'EL et la CL dans plusieurs isolants polymères, i.e. polyoléfines ou polyesters, la formation d'excitons et les processus de relaxation d'énergie sous contrainte électrique et électrons énergétiques sont mis en évidence. Surtout, l'analyse en composantes spectrales et la reconstruction des spectres donne accès aux mécanismes d'excitation de la luminescence et à une corrélation avec le vieillissement électrique. A l'avenir, les mesures de luminescence peuvent devenir une méthode standard pour sonder et analyser les isolants polymères. / 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.