Molecular modeling of ionization processes relevant for electrically insulating liquids

Abstract

Insulating liquids are often used as a dielectric barrier between two electrodes in high-voltage devices and may suffer a breakdown in high electric fields. Breakdown happens when a conductive plasma channel, a streamer, is created in high field regions which propagates through the barrier and bridges the gap between two electrodes. This phenomenon is influenced by the field-dependent molecular properties of the insulating liquid. Among the different properties, the molecular ionization potential (IP) and excitation energies are investigated for molecules relevant for insulating liquids. It is demonstrated how density functional theory (DFT) can be used to propose suitable molecules for the design of new insulating liquids. A model based on constrained DFT is developed for the calculation of field-dependent IP and time-dependent DFT is used to study a few lowest excitation energies in the field. For a dielectric liquid in the electric field, energy is added continuously to the liquid. The liquid releases energy by emitting heat or light in the UV/VIS region. Thus, the excitation energies of molecules in liquids may be important in the streamer experiments. The IP decreases strongly with increasing the field, while the excitation energies are weakly dependent on the field. There is a threshold field for different types of molecules that above it a two-state system consisting of the electronic ground state and the ionized state is obtained. The local electric field is an important parameter in modeling the streamer behavior and is different from the external electric field. A forcefield model is developed to calculate the response of the local field to the external field (local field factor). The local field factors are calculated for liquid benzene by combining the force-field model with the molecular dynamics simulations. The local field factor increases significantly at the absorption frequency for liquid benzene. The force-field model can also be used to calculate different dielectric properties of liquids.