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A simulation model for second mode positive streamers in dielectric liquids is presented. Initiation and propagation is modeled by an electron-avalanche mechanism and the Townsend-Meek criterion. The electric breakdown is simulated in a point-plane gap, using cyclohexane as a model liquid. Electrons move in a Laplacian electric field arising from the electrodes and streamer structure, and turn into electron avalanches in high-field regions. The Townsend-Meek criterion determines when an avalanche is regarded as a part of the streamer structure. The results show that an avalanche-driven breakdown is possible, however, the inception voltage is relatively high. Parameter variations are included to investigate how the parameter values affect the model. visible light [3], re-illuminations, from one or more of its branches. Above the breakdown voltage, streamers may change between the 2nd, the 3rd, and the 4th mode during propagation. There are usually more reilluminations in the 3rd mode than the 2nd mode. The inception of the 4th mode is associated with a drastic increase in speed and fewer, more luminous, branches [2].There are numerous mechanisms that can be involved in the streamer phenomena, the challenge is identifying their importance during initiation and propagation. Applying a potential to a needle can cause charge injection, giving a space-charge limited current [16] causing Joule heating [16], which in turn can cause bubble nucleation [17]. A breakdown in the gas bubble can then propagate the needle potential, and the process may repeat. This is one way to explain 1st mode propagation. Electric fields can also cause electrohydrodynamic flow, which could cause streamer formation through cavitation [18]. Electrostatic cracking has also been proposed as a cavitation mechanism [19]. A main topic of discussion is whether a lowering of the liquid density is needed before charge generation can occur. Electron avalanches are important in gas discharge, but their importance in liquid breakdown is still disputed. In water, strong scattering could prevent electrons from forming avalanches in the liquid phase [20]. Therefore, discharges in micro-bubbles can be important for charge generation [10,14,20]. The same mechanism was also proposed for non-polar liquids [19], however, the relative permittivity is about 80 in water and about 2 in a typical oil, and this difference can prove important since the field enhancement within a bubble in oil is much lower than in water. Contrary to water, there are indications of electron avalanches in non-polar liquids [16,21,22], furthermore, while the initiation and the propagation length of 2nd mode streamers are dependent on the pressure, their propagation velocity is not pressure dependent [16,23]. This implies that the mechanism responsible for propagation occurs in the liquid phase and that the gaseous channel follows as a consequence. In very high electric fields, field-ionization can occur [24,25], and this mechanism has been proposed for the fast 3rd and 4th propagation mode...

The electric-field dependence of the molecular ionization potential and excitation energies is investigated by density-functional theory calculations. It is demonstrated that the ionization potential has a strong field dependence and decreases with increasing field. The excitation energies depend weakly on the field and the number of available excited states decreases with increasing field since the ionization potential has a stronger field dependence. Above a specific field, different for each molecule, a two-state model is obtained consisting of the electronic ground state and the ionized state. Implications for streamer propagation and electrically insulating materials are discussed.

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In this letter, we experimentally demonstrate that the lightning impulse breakdown characteristics of natural ester liquids can be significantly enhanced by dispersing molecular additives possessing lower values of both the ionization potential and 1st excitation energies as compared to the base liquid. One such additive contributed to an increase in the breakdown and acceleration voltage of the base liquid by 32% and 90%, respectively. Apart from the expected influence of the low ionization potential of the additives, results also indicate a positive effect of lower value of 1st excitation energy.

Contribution of the metal ∕ Si O 2 interface potential to photoinduced switching in molecular single-electron tunneling junctions J. Appl. Phys. 97, 073513 (2005); 10.1063/1.1862319Energy level alignment at Alq/metal interfaces Appl.

In order to increase our fundamental knowledge about high-voltage cable insulation materials, realistic polyethylene (PE) structures, generated with a novel molecular modeling strategy, have been analyzed using first principle electronic structure simulations. The PE structures were constructed by first generating atomistic PE configurations with an off-lattice Monte Carlo method and then equilibrating the structures at the desired temperature and pressure using molecular dynamics simulations. Semicrystalline, fully crystalline and fully amorphous PE, in some cases including crosslinks and short-chain branches, were analyzed. The modeled PE had a structure in agreement with established experimental data. Linear-scaling density functional theory (LS-DFT) was used to examine the electronic structure (e.g., spatial distribution of molecular orbitals, bandgaps and mobility edges) on all the materials, whereas conventional DFT was used to validate the LS-DFT results on small systems. When hybrid functionals were used, the simulated bandgaps were close to the experimental values. The localization of valence and conduction band states was demonstrated. The localized states in the conduction band were primarily found in the free volume (result of gauche conformations) present in the amorphous regions. For branched and crosslinked structures, the localized electronic states closest to the valence band edge were positioned at branches and crosslinks, respectively. At 0 K, the activation energy for transport was lower for holes than for electrons. However, at room temperature, the effective activation energy was very low (∼0.1 eV) for both holes and electrons, which indicates that the mobility will be relatively high even below the mobility edges and suggests that charge carriers can be hot carriers above the mobility edges in the presence of a high electrical field.

The electronic structure of tetracyanoethylene ͑TCNE͒ has been studied both in its pristine state and upon stepwise rubidium intercalation, by UV and x-ray photoelectron spectroscopy as well as with theoretical calculations. The intercalated system may serve as a model for TCNE-based organometallic magnets, of which the electronic structure remains largely unknown. Rubidium is found to n-dope the TCNE molecules forming Rb ϩ TCNE Ϫ with almost complete charge transfer. Calculations show a spin splitting of the former highest occupied molecular orbital level upon Rb doping. We see no evidence for the formation of doubly charged TCNE molecules. A gap opens up at the Fermi energy for Rb ϩ TCNE Ϫ due to on-site Coulomb interactions. We estimate the on-site Coulomb interaction of amorphous TCNE doped with Rb to be ϳ2 eV.