Virginie Griseri

Decay of space charge in a glassy epoxy resin following voltage removal

The PEA technique is used to measure the distribution of space charge in an epoxy resin after polarisation for one week at an applied field of 7.14 kV/mm over a range of temperatures. The decay of the space charge is followed for times up to 114 h after removal of the voltage and analysed in terms of a number of alternative decay mechanisms. It is shown that the rate-determining stage of the decay mechanism is that of a thermally activated process that has been associated with charge de-trapping. At times greater than 102 s the de-trapping process behaves as though the space charge field does not exist and the retention time of the space charge depends only upon the depth of the deepest occupied traps and the temperature

Measurement of internal charge distribution in dielectrics using the pulsed electro-acoustic method in non contact mode

We introduce a pulsed electro acoustic (PEA) device enabling space charge measurement inside dielectric films without the need of sample to excitation unit contact. The new device -called open PEA because one face of the film is at a floating potential- has been used to investigate the internal charge of electron beam irradiated dielectrics. Details are given on the system and its calibration for performing quantitative charge measurements. Charge distributions in electron beam irradiated polytetrafluoroethylene (PTFE) films are compared using the classical and the open PEA. Due to the short-circuit condition in the classical PEA, the implanted charges measured using the two systems are drastically different. Surface potential and current measurements performed during irradiation have been used to validate the measurement in the open configuration which is well suited to investigate the behaviour of dielectrics in space environment.

Photoluminescence, recombination induced luminescence and electroluminescence in epoxy resin

The dielectric breakdown of epoxies is preceded by light emission, or so-called electroluminescence, from the solid state material. Very little is known about the luminescence properties of epoxies. The aim of this paper is to derive information that can be used as a basis with which to understand the nature of the excited states and their involvement in the electrical degradation processes. Three different kinds of stimulation were used to excite the material luminescence. Photoluminescence was performed on the base resin, the hardener and the cured resin. Luminescence excited by a silent discharge has been analysed to identify which of the luminescent centres are optically active upon the recombination of electrical charges and could therefore act as charge traps. Finally, the electroluminescence spectrum has been acquired and compared with the previous ones. Although the identification of the origin of these emissions is far from being complete, it has been found that the photoluminescence from the cured resin is due to in-chain chromophores, which act as trapping centres. The excited states involved in photoluminescence also seem to be involved in electroluminescence, but other components are detected as well, which could be due to the degradation of the resin molecule under the effect of the electric stress.

Electroluminescence excitation mechanisms in an epoxy resin under divergent and uniform field

Electroluminescence (EL) excitation mechanisms have been investigated in epoxy resin under divergent and uniform field situations. Metallic wires embedded in the resin were used to produce field divergence whereas film samples were metallised to obtain a uniform field. EL under divergent field was stimulated by a pulse voltage. Light was emitted on the positive and negative fronts of the pulses when the field exceeded 20 kV/mm at the wire surface, with equal intensity and without polarity dependence. There was evidence of space charge accumulation around the wires in multiple-pulse experiments. Charge injection and extraction occurring at both fronts of the pulse provide the condition for EL excitation. Further excitation of the EL during the plateau of the voltage pulse is prevented by the opposite field of the trapped charge. Field computation with and without space charge supports the proposed interpretation. A dc voltage was used for the uniform field experiments. A continuous level of EL is found at 175 kV/mm. Charging/discharging current measurements and space charge profile analyses using the pulsed electroacoustic (PEA) technique were performed at different fields up to the EL level. Dipolar orientation generates a long lasting transient current that prevents the conduction level being reached within the experimental protocol (one hour poling time). The continuous EL emission is nevertheless associated with a regime where the conduction becomes dominant over the orientational polarization. Polarization and space charge contribute to the PEA charge profiles. Homocharge injection at anode and cathode is seen at 20 kV/mm and a penetration of positive space charge in the bulk is detected above 100 kV/mm, suggesting an excitation of the continuous EL by bipolar charge recombination.

Space charge behaviour in electron irradiated polymers

The materials under investigation were introduced into an irradiation chamber specially equipped with electron beam facilities able to reproduce a spatial environment. In situ surface potential and bulk space charge distribution measurements were performed to investigate various materials (polyfluoroethylene FEP, epoxy resin, polymetacrylate of methyl PMMA). A new space charge detection set-up based on the pulse electro acoustic (PEA) method specially adapted for space environment monitoring was developed for this purpose.

Improvement of PEA signal analysis using simulations for complex geometry samples

Complex geometry structures, such as produced by imperfections, cause the electric field to reach a very high local value even at low applied voltages. These are often the initiators of breakdown. Though direct measurement methods for space charge and electric field can be used only for planar or coaxial geometry, the authors propose to apply the pulsed-electro-acoustic (PEA) method to such complex geometry structures by using a simulation technique. The physical background of the simulation is presented. Comparisons between simulated and experimental signals are shown to be in good agreement meaning that simulations can be used as a tool for signal analysis.

Space charge observation of a filler free epoxy resin

Space charge behaviour in epoxy resin and its effect on electrical ageing have been investigated for several years. For example, copper ion migration in an epoxy resin of a printed circuit board was observed by using the pulsed electroacoustic (PEA) space charge measurement system. In this work, a type of filler-free resins (CY1301) was used as a specimen in order to investigate the temperature dependence of space charge behaviour in an epoxy resin of which the glass transition temperature was just below 40/spl deg/C. Space charge profiles were measured by the PEA method at 20/spl deg/C and 40/spl deg/C either under the electric field of 15 kV/mm or after the electrodes were short circuited. At 20/spl deg/C, injected charges appeared near the electrodes. At 40/spl deg/C, on the other hand, only surface charge on the electrode remained even though the electrodes were short-circuited. Although further investigation is required, it is clear that the space charge phenomena is strongly influenced by the temperature, and it could be related to the glass transition temperature of the resin.

Preliminary measurements on dielectric materials by the pulsed electro-acoustic method using a ring electrode

In the last past thirteen years many Pulsed Electro Acoustic (PEA) set-ups have been developed to study space charge behavior in irradiated material mainly used for space applications. In this paper we will present our first measurements with a ring electrode. In these preliminary studies measurements are realized in the air. A dielectric film is polarized under a positive or a negative voltage then PEA measurements are realized using the open PEA set up and the new Open Ring Electrode (ORE). After the record of a reference signal and a deconvolution process, the space charge profile is obtained. We will compare and discuss the results recorded from both configurations and will explain which improvement still need to be brought.

Pulse electro-acoustic measurements with contact and contact-less on electron irradiated polymers

Several types of polymeric materials are used in satellites wings and body. When these dielectrics are submitted to an electronic irradiation, sudden discharge might occur after a period of accumulation. To get a better knowledge on the charging and discharging phenomena in dielectrics, experimental studies are performed on earth. We focused our studies on the effect of the electronic irradiation, which is the main cause of charging, on polymeric films. Between two periods of irradiation and during the relaxation classic surface potential measurements are performed. The space charge distribution in the bulk is followed thanks to the pulse electro acoustic (PEA) method adapted to work under the appropriated configuration. We completed our studies by a series of simulations.

Three-dimensional space charge measurements of printed circuit boards

Investigated the ion migration phenomena of printed circuit boards, and found that the pulsed electroacoustic space charge measurement system can monitor the copper ion migration into the epoxy resin from the electrode surface. The migration, however, can proceed in any direction in the specimen simultaneously; therefore, it should be measured in three dimensions. We have measured typical test pieces of polyimide printed circuit boards and metal-based epoxy resin insulated printed circuit boards by a newly developed three-dimensional pulsed electroacoustic method. The experimental results show that the space charge density varies in the lateral direction when ion migration occurs in the specimen.