Dielectric materials can be used in radiative environment that is why it is of major importance to determine their properties modifications with time. In this paper we focus our attention on PolyTetraFluoroEthylene (PTFE) films submitted to gamma radiations. We investigate the FTIR spectra and show that after exposure to Co 60 the crystallinity of the material is increasing. This result is confirmed by Differential Scanning Calorimetry (DSC) analysis. Some space charge distribution profiles obtained by the Pulse Electro Acoustic (PEA) method show that the irradiation favours the drift of injected charges toward the bulk. Obviously the dose of radiation must be carefully chosen in order to be able to perform electrical measurements. Indeed, we observed that if the radiation dose is too high the sample becomes fragile and most of the characterisation tools cannot be used.

The aim of the present work is to study the dynamics of the space charge in crosslinked polyethylene (XLPE) induced by electron beam irradiation. For this purpose three energies have been chosen so as to implant charges in a controlled way, in the volume, at different depths not too close to the electrodes. Pulsed Electro Acoustic (PEA) method was used to measure space charge in each sample, at the end of the irradiation process, in ambient conditions. The space charge measurements show the existence of two electron implantation sites after irradiation, one in the bulk at a depth depending on the irradiation energy and the other near the irradiated surface and a fixed position (25 µm). The application of the electric field shows that the charge implanted in the volume tends to be evacuated during the polarization. The charge accumulated persists during the relaxation and is stable after 1 hour. The current measurements reveal the impact of the irradiation in XLPE sample: the steady current measured on the irradiated sample is substantially higher compared to the one recorded on the non-irradiated specimens. Also a change in conduction mechanism is observed. The current recorded on XLPE irradiated weakly diverges from linearity with the electric field in the field range up to 40 kV/mm.

The aim of the present work is to study the dynamics of the space charge in cross-linked polyethylene (XLPE) irradiated by gamma rays. For this purpose, film samples peeled from cables have been irradiated with gamma rays at total dose up to 100 kGy, in air at room temperature. To investigate the impact of the irradiation on the electrical behavior, space charge measurements were achieved in ambient conditions after the irradiation using Pulsed Electro Acoustic (PEA) method. The space charge measurements on the irradiated XLPE under an electric field of 40 kV/mm show the development of twoheterocharge regions adjacent to the electrodes. This is in mark contrast with the behavior of un-irradiated XLPE with space charge pattern dominated by charge injection. The charge accumulated during the polarization step persists during the relaxation and is stable after 1 hour.

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Isotactic polypropylene (iPP) films of 25 μm thickness were subjected to gamma-ray irradiation for different doses (30, 60, 90 and 100 kGy) at ambient atmosphere and both molecular modifications and surface properties have been investigated by Fourier transform infrared spectroscopy (FTIR) and atomic force microscopy (AFM), respectively. The occurrence of the oxidation process is identified and quantified by the FTIR. In addition, it is found that the carbonyl index increases with the absorbed dose which confirms that the irradiation process induces the diffusion of the oxygen into the iPP matrix. The AFM image (for virgin and higher dose irradiated samples) showed a dramatic change in the surface topography. The gamma radiation induces the formation of the nanostructured iPP surface. The roughness of the polymer surface, measured by AFM, increases after gamma irradiation. These results revealed that the oxidative degradation may be correlated with the morphological properties.

The electrical properties of Crosslinked PolyEthylene -XLPE-material have been studied for many years as it is used for cable applications. In this work, XLPE peelings, previously submitted to thermal ageing are investigated under electronic irradiation. FTIR spectra did not reveal specific thermal ageing signatures. The space charge distribution has been analyzed by Pulsed Electro-Acoustic method during and after application of a DC field. The space charge profiles appear to be very dependent on the electron beam energy that determines the electron penetration depth and irradiated volume. Most of the space charge features appear linked to charge redistribution in the bulk following irradiation.

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