Calculating Dielectric Constant at the Interface Between Polyethylene and Copper Using Microscopic Polarization Theory

Abstract

The relative dielectric constant is the main electric property of polyethylene (PE) as a dielectric material. Based on experimental findings, the dielectric constant of bulk PE has been known to be constant. However, when interfaced with copper –as it is the case in most power applications- the dielectric constant may differ from that value at interface region in atomic scale, which in turn affects the charge injection process. This is further complicated by the inevitable presence of chemical impurities at the interface, where the dielectric constant becomes position-dependent and depends on the type of impurity. In this work, the dielectric constant is calculated for the cases of a pure slab of PE during the interface between PE and copper, and finally with chemical impurities present in the interface region. In those cases, the microscopic polarization theory is used in calculating the position-dependent relative dielectric constant. This theory makes use of the charge density distribution under an applied electric field, which is computed using the density functional theory (DFT). It is found that the calculated dielectric constant varies at the interface region within a range of a few Angstroms, beyond which it becomes constant well within the bulk polyethylene. The dielectric constant variation at the interface region is then used to study the impact on the injection process using the Schottky injection mechanism. The injection current density is computed from Schottky injection law as a function of applied field at different dielectric constant values. It is found that the injection current increased with decreasing the dielectric constant at the interface region. The paper concludes that the relative dielectric constant varies at the interface due to the morphological deformation between the polyethylene chains and the copper atoms. This variation is further enhanced in the presence of chemical impurities. It is further concluded that the injection current density does not only depend on the barrier height, but also on the varying relative dielectric constant.