Numerical simulation on molecular displacement and DC breakdown of LDPE

It is generally known that the dc breakdown strength of low density polyethylene (LDPE) decreases with as the thickness and temperature of the sample increase. The breakdown strength is influenced by the charge transport and electric field distortion, and is also related to the molecular chain displacement and fracture. This paper investigates mutual relations among the charge transport, molecular chain displacement, and thickness dependent dc breakdown of LDPE. A model that combines the dynamics of charge transport and molecular displacement (CTMD) is used to calculate the space charge accumulation, molecular chain displacement, and dc breakdown properties of LDPE with various thicknesses at various constant voltage ramping rates. It is assumed that breakdown occurs when the molecular chain displacement reaches a critical value. The simulation results show that the breakdown field as a function of sample thickness satisfies an inverse power law with a power index of about 0.43 for various voltage ramping rates. This is consistent with experimental results. The CTMD model considers both the distortion of electric field and the displacement kinetics of molecular chains, resulting in a power index closer to the experiment than that calculated only from the electric field distortion. Adopted a Williams-Landel-Ferry type molecular chain mobility in the CTMD model, the simulation results are consistent with the results calculated by applying experimental results on polyisobutylene and polymethyl methacrylate to the free volume breakdown theory. It is also found that the CTMD model with temperature-dependent molecular chain mobility controlled by piecewise Arrhenius equations can explain well the temperature dependent breakdown experimental results of LDPE.