Field-dependent molecular ionization potentials of relevance for insulating oils

Molecular properties are important to consider when designing dielectric liquids because breakdown properties are governed by molecular processes such as ionization. The molecular ionization potential varies with applied field strength, as the positive and negative charge centers in the molecule are separated when subject to an applied electric field, aiding ionization. In this work, we apply an analytical expression for the field-dependent molecular ionization potential based on a Taylor expansion of the molecular charge distribution to six representative molecules: cyclohexane, pyrene, N,N-dimethylaniline, trans-azobenzene, butylated hydroxytoluene and trans-5methyl-3-heptene. Thus, the field dependent molecular ionization potential may be calculated from zero-field properties, namely the molecular energy, dipole moment and polarizability. Vertical and adiabatic ionization potentials are calculated. This model replicates results found with other quantum chemical calculations up to 30 MV/cm, with notable exceptions for trans-azobenzene and N,Ndimethylaniline. The model works reasonably well in the field range 0 to 50 MV/cm, which is considered relevant for fields experienced during dielectric breakdown in transformer insulation liquids. The classical Frenkel term is sufficient to calculate the field-dependent ionization potential at this field range, as additional contributions are generally below ± 0.3 eV compared to the Frenkel term, which is 3.39 eV for all molecules at 20 MV/cm. The difference between the vertical and adiabatic ionization potential is between 0.57 and 0.11 eV for the aforementioned field range, compared to gas-phase ionization potentials of around 7-10 eV.