An atomistic description of the high-field degradation of dielectric polyethylene

Abstract

A microscopic mechanism governing the initiating step in the high-field aging of crystalline polyethylene is proposed, based on density functional calculations and ab initio molecular dynamics simulations. It is assumed that electrons, holes, and excitons are present in the system. While the additional individual electrons or holes are not expected to lead to significant degradation, the presence of triplet excitons are concluded to be rather damaging. The electron and hole states of the exciton localize on a distorted region of polyethylene, significantly weakening nearby C-H bonds and facilitating C-H bond scission. The barrier to cleavage of the weakened C-H bonds is estimated and is comparable to the thermal energy, suggesting that this mechanism may be responsible for the degradation of polyethylene when placed under electrical stress, e.g., in high-voltage cables.

Unraveling the luminescence signatures of chemical defects in polyethylene

“…[12][13][14] In the case of PE, such work is in a state of infancy. [15][16][17][18][19] Past DFT work on PE is dominated by the use of (semi)local exchangecorrelation functionals to treat the quantum mechanical part of the electron-electron interaction within a singleparticle framework. Defect states are identified as the one-electron Kohn-Sham eigenenergies.…”

On the nature of high field charge transport in reinforced silicone dielectrics: Experiment and simulation

“…This may be interpreted in analogy to the progressive degradation in gate dielectrics that continuous trap creation and enhanced trap-assisted tunneling cause an increase in the leakage current close to breakdown 45 . In the case of a polymer, the injected charges can induce exciton formation, catalyze bond cleavage, generate free radicals and encourage oxidations as they diffuse throughout the material 46 , which is likely to create more deep traps along its path so that direct tunneling between deep traps becomes more favorable and thus may explain the increase in the apparent charge mobility. The experiment also suggests that the injected charges can still remain chemically active and are able to catalyze material degradation even after the voltage is removed.…”

Molecular modeling and electron transport in polyethylene

“…In contrast, very recent DFT calculations [54] have found a direct channel for the recombination of the exciton via the breaking of a C-H bond. The discrepancy with the results in [53] might be due to the exchange-correlation functional employed or the different methods chosen to improve the deficiencies of standard DFT.…”

“…For example there has been very little work on the fate of the energy given up by trapped electrons in polyethylene (but see [58] for trapped excitons) which may alter the environment leading to local damage and ageing. The very recent DFT results [54] suggesting PE degradation by exciton recombination provide an interesting scenario that needs to be confirmed. In this regard, though molecular modeling studies provide a great insight in the understanding of polymeric dielectrics, these studies need to be assisted by experiments on very pure polymeric materials (including polyethylene) in which the effects of chemical traps from additive chemicals and/or radiation damage on electrical properties can be separated from those of physical traps related to the various polymer morphologies present in real materials.…”

“…The discrepancy with the results in [53] might be due to the exchange-correlation functional employed or the different methods chosen to improve the deficiencies of standard DFT. However, reference [54] presents a convincing scenario in which the recombination of an electron-hole pair under electrical stress could lead to degradation of PE.…”