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- ItemEnhanced reliability of drift-diffusion approximation for electrons in fluid models for nonthermal plasmas(New York, NY : American Inst. of Physics, 2013) Becker, M.M.; Loffhagen, D.Common fluid models used for the description of electron transport in nonthermal discharge plasmas are subject to substantial restrictions if the electron energy transport significantly influences the discharge behaviour. A drift-diffusion approach is presented which is based on a multiterm approximation of the electron velocity distribution function and overcomes some of these restrictions. It is validated using a benchmark model and applied for the analysis of argon discharge plasmas at low and atmospheric pressure. The results are compared to those of common drift-diffusion models as well as to experimental data. It is pointed out that fluid models are able to describe nonlocal phenomena caused by electron energy transport, if the energy transport is consistently described. Numerical difficulties that frequently occur when the conventional drift-diffusion model is consistently applied are avoided by the proposed method.
- ItemDiffusion and interface effects during preparation of all-solid microstructured fibers(Basel : MDPI AG, 2014) Kobelke, J.; Bierlich, J.; Wondraczek, K.; Aichele, C.; Pan, Z.; Unger, S.; Schuster, K.; Bartelt, H.All-solid microstructured optical fibers (MOF) allow the realization of very flexible optical waveguide designs. They are prepared by stacking of doped silica rods or canes in complex arrangements. Typical dopants in silica matrices are germanium and phosphorus to increase the refractive index (RI), or boron and fluorine to decrease the RI. However, the direct interface contact of stacking elements often causes interrelated chemical reactions or evaporation during thermal processing. The obtained fiber structures after the final drawing step thus tend to deviate from the targeted structure risking degrading their favored optical functionality. Dopant profiles and design parameters(e.g., the RI homogeneity of the cladding) are controlled by the combination of diffusion and equilibrium conditions of evaporation reactions. We show simulation results of diffusion and thermal dissociation in germanium and fluorine doped silica rod arrangements according to the monitored geometrical disturbances in stretched canes or drawn fibers. The paper indicates geometrical limits of dopant structures in sub-μm-level depending on the dopant concentration and the thermal conditions during the drawing process. The presented results thus enable an optimized planning of the preform parameters avoiding unwanted alterations in dopant concentration profiles or in design parameters encountered during the drawing process.
- ItemMapping the dissociative ionization dynamics of molecular nitrogen with attosecond resolution(Bristol : IOP Publ., 2015) Klinker, M.; Trabattoni, A.; González-Vázquez, J.; Liu, C.; Sansone, G.; Linguerri, R.; Hochlaf, M..; Klei, J.; Vrakking, M.J.J.; Martin, F.; Nisoli, M.; Calegari, F.We wish to understand the processes underlying the ionization dynamics of N2 as experimentally induced and studied by recording the kinetic energy release (KER) in a XUV-pump/IR-probe setup. To this end a theoretical model was developed describing the ionization process using Dyson Orbitals and, subsequently, the dissociation process using a large set of diabatic potential energy surfaces (PES) on which to propagate. From said set of PES, a small subset is extracted allowing for the identification of one and two photon processes chiefly responsible for the experimentally observed features.
- ItemB12X11(H2)−: exploring the limits of isotopologue selectivity of hydrogen adsorption(London : RSC Publishing, 2021) Wulf, Toshiki; Warneke, Jonas; Heine, ThomasWe study the isotopologue-selective binding of dihydrogen at the undercoordinated boron site of B12X11− (X = H, F, Cl, Br, I, CN) using ab initio quantum chemistry. With a Gibbs free energy of H2 attachment reaching up to 80 kJ mol−1 (ΔG at 300 K for X = CN), these sites are even more attractive than most undercoordinated metal centers studied so far. We thus believe that they can serve as an edge case close to the upper limit of isotopologue-selective H2 adsorption sites. Differences of the zero-point energy of attachment average 5.0 kJ mol−1 between D2 and H2 and 2.7 kJ mol−1 between HD and H2, resulting in hypothetical isotopologue selectivities as high as 2.0 and 1.5, respectively, even at 300 K. Interestingly, even though attachment energies vary substantially according to the chemical nature of X, isotopologue selectivities remain very similar. We find that the H–H activation is so strong that it likely results in the instantaneous heterolytic dissociation of H2 in all cases (except, possibly, for X = H), highlighting the extremely electrophilic nature of B12X11− despite its negative charge. Unfortunately, this high reactivity also makes B12X11− unsuitable for practical application in the field of dihydrogen isotopologue separation. Thus, this example stresses the two-edged nature of strong H2 affinity, yielding a higher isotopologue selectivity on the one hand but risking dissociation on the other, and helps define a window of adsorption energies into which a material for selective adsorption near room temperature should ideally fall.