Abstract
In order to study the crystallinity of different density polyethylenes, this paper conducts an experimental study on the transformation of the conductance mechanism under a high electric field. In this experiment, X-ray diffraction (XRD), differentials scanning calorimetry (DSC), direct current (DC) breakdown of low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), medium-density polyethylene (MDPE) and high-density polyethylene (HDPE), as well as the conductivity characteristics under an electric field of 5–200 kV/mm are tested. In addition, the electric field–current density curves of the four kinds of polyethylene are fitted to analyze their conductance transition in non-ohmic regions under different high field strengths, through applying the mathematical formula of a variety of conductance mechanisms. The experimental results are as follows: as the density of polyethylene increases, the crystallinity increases continuously. Moreover, the continuous increase of crystallinity causes the electric conduction flow under the same field strength to decrease significantly. The field strength corresponding to the two turning points in the conductance characteristic curve increases simultaneously, and the breakdown field strength increases accordingly; through analysis, it is found that in the high field, as the electric field increases, the conductance mechanism develops from the ohmic conductance of the low field strength region to the bulk effect of the high field strength region (Poole–Frenkel effect). Then, it develops into the electrode effect to the high field strength (Schottky effect), although the threshold field strength of this conductance mechanism transition increases with the increase of crystallinity.
Highlights
Polyethylene is a partially crystalline solid, whose properties are highly dependent on the relative content of the crystalline phase and amorphous phase, i.e., crystallinity
Based on the existing research on polyethylene and polyethylene nano-polyethylene [19,20], this paper studies the effects of different crystallinity on the direct current (DC) breakdown strength as well as the conductance property of different density polyethylene
According to the E-I curve of the four different density polyethylenes in Figure 6, we know that the electric conductivity values of low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), medium-density polyethylene (MDPE), and high-density polyethylene (HDPE), respectively, decrease in the same field strength, so the conductivity is in the same field strength
Summary
Polyethylene is a partially crystalline solid, whose properties are highly dependent on the relative content of the crystalline phase and amorphous phase, i.e., crystallinity. It is widely applied in the insulating material of power cables due to its symmetrical molecular structure without polar groups, which gives it excellent electrical and mechanical properties [1,2]. According to different polymerization methods, polyethylene can be classified into linear low-density polyethylene (LLDPE), low-density polyethylene (LDPE), medium-density polyethylene (MDPE), and high-density polyethylene (HDPE). LLDPE possesses a regular short-chain structure, and, its crystallinity and density are similar to those of LDPE, Materials 2019, 12, 2657; doi:10.3390/ma12172657 www.mdpi.com/journal/materials. The macromolecules of LDPE have many branches, which cannot be closely and regularly arranged with each other. Medium-high density polyethylene is a linear macromolecule with low branching degree and regular structure [3]
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