A numerical model for describing bipolar charge transport and storage in polyethylene has been developed recently. The present paper proposes a comparison of the model outputs with experimental data in three different direct current (DC) voltage application protocols (step field increase and polarization/depolarization schemes). Three kinds of measurement have been realized for the three different protocols: space charge distribution using the pulsed electro-acoustic method, external current and electroluminescence. Simulation under AC stress has also been attempted on the basis of the model parameters that were derived from the DC case. Model limitations and possible improvements are discussed.
We present a conduction model aimed at describing bipolar transport and space charge phenomena in low density polyethylene under dc stress. In the first part we recall the basic requirements for the description of charge transport and charge storage in disordered media with emphasis on the case of polyethylene. A quick review of available conduction models is presented and our approach is compared with these models. Then, the bases of the model are described and related assumptions are discussed. Finally, results on external current, trapped and free space charge distributions, field distribution and recombination rate are presented and discussed, considering a constant dc voltage, a step-increase of the voltage, and a polarization–depolarization protocol for the applied voltage. It is shown that the model is able to describe the general features reported for external current, electroluminescence and charge distribution in polyethylene.
We introduce and develop two bipolar transport models which are based on appreciably different physical assumptions regarding the distribution function in the energy levels of trap states. In the first model, conduction is described by an effective mobility of the carriers and the accumulation of stored space charge is taken into account through a single trapping level. In the second model the hypothesis of an exponential distribution function of trap depth is made, with conduction taking place via a hopping process from site to site. The results of simulations of the two models are compared with experimental data for the external current and the space-time evolution of the electrical space charge distribution. The two descriptions are evaluated in a critical way, and the prospects for these models to adequately describe real systems are given.
The dielectric relaxation mode associated with the liquid-glass transition of a fluorinated co-polymer, P(VDF-TRFE) 75/25 mol.%, has been resolved experimentally by the technique of fractional polarizations, and a discrete distribution of relaxation times has been obtained. This distribution shows a compensation pattern characteristic of the glass transition/relaxation of polymers. Attention has been paid to the effect of the experimental set-up on the obtained activation parameters by using polarization windows Delta Tp of 0, 5 and 10 degrees C. It was shown that the activation energy increased as the polarization window was reduced. The latter also had a slight influence on the compensation parameters. This behaviour has been explained in terms of an increase in the width of the distribution of relaxation times associated with elementary peaks as Delta Tp increased. Thus, the Bucci-Fieschi-Guidi (BFG) analysis based on monokinetic relaxations is no longer valid as this distribution broadens. Overall, the use of Delta Tp=5 degrees C as polarization window constituted the best compromise between validity of the BFG analysis and the experimental resolution of the depolarization current. The errors in the compensation temperature have been estimated and a test has been applied in order to reject the hypothesis of a compensation pattern due to a propagation of experimental errors. Thus, the compensation phenomena described true kinetic effects, independently from the experimental set-up. It was shown that data pertaining to different methods of analysis could also describe a compensation law, which was actually statistical in nature.
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