Molecular modeling of ionization processes relevant for electrically insulating liquids
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- Institutt for kjemi 
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.
Består avPaper 1: Davari, Nazanin; Åstrand, Per-Olof; Ingebrigtsen, Stian; Unge, Mikael. Excitation energies and ionization potentials at high electric fields for molecules relevant for electrically insulating liquids. Journal of Applied Physics 2013 ;Volum 113. s http://dx.doi.org/10.1063/1.4800118 (C) 2013 American Institute of Physics
Paper 2: Davari, Nazanin; Åstrand, Per-Olof; Van Voorhis, Troy. Field-dependent ionisation potential by constrained density functional theory. Molecular Physics 2013 ;Volum 111.(9-11) s. 1456-1461. Is not included due to copyright available at http://dx.doi.org/10.1080/00268976.2013.800243
Paper 3: Davari, Nazanin; Åstrand, Per-Olof; Unge, Mikael; Lundgaard, Lars Esben; Linhjell, Dag. Field-dependent molecular ionization and excitation energies: Implications for electrically insulating liquids. AIP Advances 2014 ;Volum 4.(3) http://dx.doi.org/10.1063/1.4869311 All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported license
Paper 4: Davari, Nazanin; Åstrand, Per-Olof; Unge, Mikael. Density-functional calculations of field-dependent ionization potentials and excitation energies of aromatic molecules. Chemical Physics 2015 ;Volum 447. s. 22-29 http://dx.doi.org/10.1016/j.chemphys.2014.11.023 This article is reprinted with kind permission from Elsevier, sciencedirect.com
Paper 5: Davari, Nazanin; Haghdani, Shokouh; Åstrand, Per-Olof; Schatz, George C.. Local electric field factors by a combined charge-transfer and point-dipole interaction model. RSC Advances 2015 ;Volum 5.(40) s. 31594-31605 http://dx.doi.org/10.1039/c5ra04183j This journal is © The Royal Society of Chemistry 2015
Paper 6: N. Davari, C. D. Daub, P.-O. Åstrand, and M. Unge, “Local electric field factors and dielectric properties of liquid benzene. this manuscript has been accepted and published in Journal of Physical Chemistry B http://dx.doi.org/10.1021/acs.jpcb.5b07043 Reproduced with permission from Journal of Physical Chemistry B (C) 2015 American Chemical Society.