V. Griseri

Space charge modeling in electron-beam irradiated polyethylene: Fitting model and experiments

A numerical model for describing charge accumulation in electron-beam irradiated low density polyethylene has been put forward recently. It encompasses the generation of positive and negative charges due to impinging electrons and their transport in the insulation. However, the model was not optimized to fit all the data available regarding space charge dynamics obtained using up-to-date pulsed electro-acoustic techniques. In the present approach, model outputs are compared with experimental space charge distribution obtained during irradiation and post-irradiation, the irradiated samples being in short circuit conditions or with the irradiated surface at a floating potential. A unique set of parameters have been used for all the simulations, and it encompasses the transport parameters already optimized for charge transport in polyethylene under an external electric field. The model evolution in itself consists in describing the recombination between positive and negative charges according to the Langevin formula, which is physically more accurate than the previous description and has the advantage of reducing the number of adjustable parameters of the model. This also provides a better description of the experimental behavior underlining the importance of recombination processes in irradiated materials.

Simulation of Charge Build Up and Transport in Electron Beam Irradiated Organic Dielectrics: Comparison to Space Charge Measurements

Recently, the pulsed electro acoustic method - PEA - has been adapted to perform space charge measurements in organic dielectrics under electron beam irradiation and during relaxation, considering short-circuit and open-circuit conditions for the insulation. On the other hand, a model has been developed in order to understand the behavior of space charge in spatial conditions. The aim of this paper is to present the numerical model, and to compare simulated results with experimental ones, obtained with the short-circuit PEA and the open PEA, for a polytetrafluoroethylene (PTFE).

New design of the Pulsed Electro-Acoustic upper electrode for measurements under irradiation

In the last past ten years several Pulsed Electro- Acoustic (PEA) set-ups have been designed to perform space charge distribution measurements on irradiated films. These dielectrics are commonly used in spatial applications. Recently, a new Open Ring Electrode (ORE) has been developed to perform measurements during irradiation. In this paper, we will present the first measurements at atmospheric pressure on previously irradiated polymers. The aim is to validate the measurement configuration before introducing it in the irradiation chamber. To do so we will compare the experimental results with the ones obtained by the classic PEA set-up.

Time-resolved space charge measurements during electronic irradiation

Since many years, the behavior of dielectric materials is studied for the energy transportation or space applications. In our case, the Pulsed Electro Acoustic (PEA) method is selected to determine the space charges distribution in polymers submitted to an electron beam. In this work, the PEA measurements in non-contact mode during electronic irradiation are made to evaluate the charge injection dynamic. The measurements are realized with different irradiation conditions as different electronic fluxes and kinetic energies. Then a long relaxation step is performed to monitor the evolution of the injected space charges trapped in the bulk. The results concerning the relaxation phase are discussed in particular to try to explain the appearance of positive charge carriers at the sample surface.

Modelling Charge Generation and Transport in Low Density Polyethylene Irradiated by an Electron-Beam

One way to bypass charge generation due to injection in an insulator sandwiched between parallel electrodes and submitted to an applied voltage is to implant charges in the material with the help of an electron beam. The electrons position and quantity is theoretically known as long as the beam energy and beam current are known. Low density polyethylene (LDPE) has been characterized with in-situ space charge measurements by pulsed electroacoustic method during irradiation, and with exsitu measurements while a DC voltage is applied. A fluid charge transport model has been developed using a commercial software, to reproduce the space charge behaviour during and after irradiation. Simulated results during irradiation are first compared to in-situ space charge measurements, in order to validate the model parameters related to e-beam irradiation. Simulations are then performed on post-irradiated samples, polarized under different electric fields. Space charge measurements and current measurements are available for comparison. Simulated results are in relatively good agreement with experimental ones as long as the model parameters are adapted to irradiated low density polyethylene, compared to a best set of parameters adapted uniquely for non-irradiated polyethylene.

Assessment of the high field behaviour of polyimide films: Experimental methods and impact of electrode material

The assessment of the dielectric behavior of thin Polyimide films submitted to high DC stress is investigated considering different characterization methods. Materials of interest are polyimide films of 11 to 16 μm in thickness deposited by spin coating on a silicon wafer. Comparison is made with structures with a thin SiO2 layer onto the substrate. The laser intensity modulation method appears suited to probe space charges in the films investigated therein. These measurements show that the presence of a SiO2 layer strongly limits the electrons injection.

Polarization of electron-beam irradiated LDPE films to understand space charge generation and transport

The implantation of electrons into insulating materials and the monitoring of the electrical response afterwards can constitute one of the ways of characterizing and modelling dielectrics. However, energetic electrons have on their own secondary effects, e.g. ionization processes that modify the response. To investigate these effects, Low Density Polyethylene (LDPE) films of different thicknesses were irradiated for 10 minutes by a 80 keV electron-beam with a current flux of 1 nA/cm2 under vacuum. Space charge measurements using the Pulsed Electro-Acoustic method were then performed in air under polarization and volt-off. The influence of the electric field polarity, amplitude, and the location of the irradiation zone as regards the electrodes are presented and discussed. Upon DC stressing, the presence of a huge amount of positive charges, due to the creation of electron/hole pairs during irradiation, is put forward.

The combined effect of gamma and e-beam irradiation on the electrical properties of polyethylene Terephthalate

The combined effects of two kinds of irradiation, being gamma and electron-beam, on the PET electrical conductivity are investigated. Gamma ray produces homogeneous irradiation in the material bulk whereas in e-beam with moderate energy (20keV), energy and primary electrons are deposited near the surface. The study is focused on the comparison between the gamma irradiated films and those successively irradiated using gamma and electron sources. 25 μm-thick films were first irradiated in air at room temperature by means of a 60Co gamma source at different absorbed doses (0.1 to 2MGy); thereafter exposed during 5 min to a 20 keV electron-beam with 1 nA/cm2 flux. Transient charging current measurements were performed in the DC field range 40-160 MV/m, at ambient temperature (25°C). The results show that the current increases substantially with the gamma dose. For all irradiated samples, the current-voltage characteristics exhibit a power law Vn, with n greater than two (3 - 5), which could be related to space charge limited conduction process. The variation of the current with the dose is linear for the gamma irradiated samples and exponential on the combined gamma and electron irradiated ones. It was concluded that the electron irradiation tends to attenuate the previous irradiation effects at low gamma doses, whereas it accentuates it at high doses. The change in current-time characteristics and the variation of the exponent n with dose are also discussed.

Space charge buildup and dissipation in charged polyimide films exposed to visible light

Polyimide is photoconductive under visible light, whose blue component is strongly absorbed. A consequence of this absorption is to induce a heterogeneous conductivity within the film. We present space charge and surface potential measurements which provide evidence for a progressive motion of the charge distribution towards the back electrode, in good accordance with a simple model, assuming stability of the deposited charge, fast retrapping of the photoinduced carriers, as well as a conductivity proportional to light absorption. These results may suggest new ideas for characterization of the trapping structure and distribution of the space charge using different light wavelengths.

Continuous Pulsed Electro Acoustic measurements during electronic irradiation

The Pulsed Electro-Acoustic (PEA) set-up which is used in this paper has been designed to perform space charge distribution measurements during electronic irradiation. Dielectrics materials are commonly used in spatial applications and need to be characterized in-situ. The only way to perform measurements during charge injection was to design an electrode with a ring shape. The first results on irradiated PolyEthylene Naphthalate (PEN) using this electrode will be presented and discussed.