91 / 2018-08-24 14:27:39

The Influence of Polarization on Charge Transportation in LDPE

polarization,dielectrics,charge transportation,LDPE

It is known that there will be a charge transportation process when an electric field is applied on the dielectrics and simultaneously there will be also a polarization. These electrical phenomena in solid dielectrics are always complicated. Some models were proposed to study the charge transportation process, especially in polymer dielectrics. The Bipolar Charge Transportation model (BCT model) is most widely used to simulate the charge transportation properties in polymers in recent decades. The model features the bipolar charge injection from electrodes, recombination of holes and electrons, charge trapping and de-trapping in dielectrics. Many researches have been carried out to describe surface potential decay, space charge accumulation, electroluminescence and some other electrical phenomena with BCT model. However, the impact of polarization on charge transportation is rarely taken into consideration.
In this paper, we aim to study the effect of polarization on charge transportation of low density polyethylene (LDPE) based on the BCT model. A relative permittivity which depends on temperature is introduced in the model to describe the polarization process. Two conditions that a uniform temperature distribution and a temperature gradient in the plate structure are considered. The charge transportation parameters of electrons and holes are symmetric and a constant charge mobility is adopted in the model to simplify the calculation procedure. The thickness of the sample is 200μm and the circumstance temperature is set as 30℃ (i.e., 303K). In the first case of uniform temperature distribution, the temperature is ambient temperature 30℃ (i.e., 303K) and an electric field of 40kV/mm is applied. Two polarization mechanisms with different relaxation time of ~280s and ~28000s which depends on relaxation activation energy and temperature are implemented in our model. The polarization with relaxation time of ~280s needs much shorter time to complete polarization than that with relaxation time of ~28000s and so we call them fast polarization and slow polarization in this paper, respectively. In the second case, the parameters in the model is the same as first case except that there is a temperature difference of 20℃ (i.e., 293K) between cathode and anode. And only a slow polarization with a relaxation time depending on temperature is taken into consideration in the second case. According to the proposed models, the influences of slow and fast polarization on the charge transportation under different temperature distributions are discussed in details.
The results show that the polarization has a great influence on the electric field distribution and charge transportation process. The slow polarization can bring more obvious effect than fast polarization in charge transportation. And the ratio of the difference between the electric field with polarization and the one without polarization to the latter field is defined as a filed distortion ratio. The field distortion ratio is much more apparent in slow polarization than that in fast polarization. When there is a temperature gradient, the maximum field distortion ratio is almost twice higher than that under uniform temperature. The maximum field distortion happens in the vicinity of electrodes, which will have an effect on charge injection and will influence charge transportation process further more.