Effect of Chemical Impurities on Space Charge Density at the Interface Between Polyethylene and Copper

Abstract

The estimation of charge density distribution in polyethylene (PE) especially at the interface region with conducting material such as copper (Cu) is essential at high field conduction. The magnitudes of space charge at that interface depend on the applied electric field and the electrode material. In this work, the charge density is computed for a slab model of Cu (111)/PE (001) in its pure condition and then with chemical impurities present in the PE chains. The density-functional theory (DFT) is used as a computational quantum mechanics to evaluate the charge density in the interface region. An electric field is applied between vacuum regions attached to the model to study the effect of presence of chemical impurities on the distribution of charge density along the interface. The chemical impurities used in this study are carbonyl, vinyl and conjugated double bond. It is found that the peak value of negative charge density near the Cu atoms at the interface region decreases as the impurity-created barrier height decreases. It is also found that changes in the charge density near the interface region occur only within 6 $\text{A}^{\mathrm {o}}$ inside PE chains; the charge density becomes constant thereafter, i.e., in the PE bulk. This paper reveals that the position of chemical impurities in PE chain influences the maximum change in charge density relative to its bulk values. The maximum change in charge density is found to be related to the trap depth in PE band gap, which is created by the presence of chemical impurities. It is concluded that the charge density distribution is affected by chemical impurities in PE and, in turn, affects the charge accumulation process.

Keywords

• Charge density
• chemical impurities
• polyethylene
• copper
• barrier height
• trap depth