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- ItemApplication of the transferred matrix method to a unified evaluation of the cathodic electron emission(New York, NY : American Inst. of Physics, 2018) Baeva, M.The work is concerned with the Transfer Matrix Method for solving the steady-state Schrödinger equation applied for a unified evaluation of the emission current density from non-refractory cathodes. The method is applicable to arbitrary shapes of the potential barrier and its transmission probability is obtained without any analytical approximations. The Fermi-Dirac distribution for the free electrons in the metal is considered as a supply function. The results, obtained for a work function of the cathode material of 4.5 eV over a wide range of values of the surface temperature and the electric field strength, clearly show a growing deviation from those obtained by the classical Jeffreys-Wentzel-Kramers-Brillouin approximation with the increase of the electric field strength. Preliminary results are obtained to demonstrate the applicability of the Transfer Matrix method to the evaluation of the ion-assisted electron emission. A significant local enhancement of the emission current density is obtained as a result of the presence of an ion at a fixed position near the metal surface. The effect becomes very strongly pronounced at an appropriate value of the electric field strength, for which a resonant ion contribution appears.
- ItemMicrowave plasma discharges for biomass pretreatment: Degradation of a sodium carboxymethyl cellulose model(New York, NY : American Inst. of Physics, 2020) Honnorat, B.; BrĂ¼ser, V.; Kolb, J.F.Biogas production is an important component of an environmentally benign renewable energy strategy. However, the cost-effectiveness of biogas production from biomass is limited by the presence of polymeric structures, which are recalcitrant to digestion by bacteria. Therefore, pretreatments must often be applied prior to anaerobic fermentation to increase yields of biogas. Many physico-chemical pretreatments have a high energy demand and are generally costly. An alternative could be the ignition of a plasma directly in the biomass substrate. The reactive species that are generated by plasma-liquid interactions, such as hydroxyl radicals and hydrogen peroxides, could contribute significantly to the disintegration of cell walls and the breakage of poorly digestible polymers. With respect to economic, processing, and other potential benefits, a microwave instigated and sustained plasma was investigated. A microwave circuit transmitted 2-kW pulses into a recirculated sodium carboxymethyl cellulose solution, which mimicked the rheological properties of biomass. Each microwave pulse had a duration of 12.5 ms and caused the ignition of a discharge after a vapor bubble had formed. Microwaves were absorbed in the process with an efficiency of ∼97%. Slow-motion imaging showed the development of the discharge. The plasma discharges provoked a decrease in the viscosity, probably caused by the shortening of polymer chains of the cellulose derivative. The decrease in viscosity by itself could reduce processing costs and promotes bacterial activity in actual biomass. The results demonstrate the potential of microwave in-liquid plasma discharges for the pretreatment of biomass. © 2020 Author(s).
- ItemComprehensive characterization of osseous tissues from impedance measurements by effective medium approximation(New York, NY : American Inst. of Physics, 2021) Wei, Wenzuo; Shi, Fukun; Zhuang, Jie; Kolb, Juergen F.A unified mixing (UM) model was developed to derive microstructural information of trabecular bone, i.e., bone volume fraction (BV/TV), from electrical impedance spectroscopy. A distinct advantage of the UM-model over traditional methods, such as equivalent circuit models and multivariate analysis, is that the influence of both the environment (hydroxyapatite) and different inclusions (water, fat, and air) can be taken into account simultaneously. In addition, interactions between the different components such as interfacial polarization can be addressed by a dedicated fitting parameter v. Accordingly, values of BV/TV for different bone samples, e.g., including or not including water, were determined in the higher frequency range of 1-5 MHz. Results showed good agreement with experimental data obtained by micro-computer tomography. In particular, predictions for dielectric parameters that were derived for 3 and 4 MHz were found most promising for the assessment and distinction of osteopathic conditions and differences. This was shown by a clear differentiation of osseous tissues, e.g., the greater trochanter, femoral head, and femoral neck.
- ItemOn the relationship between SiF4plasma species and sample properties in ultra low-k etching processes(New York, NY : American Inst. of Physics, 2020) Haase, Micha; Melzer, Marcel; Lang, Norbert; Ecke, Ramona; Zimmermann, Sven; van Helden, Jean-Pierre H.; Schulz, Stefan E.The temporal behavior of the molecular etching product SiF4 in fluorocarbon-based plasmas used for the dry etching of ultra low-k (ULK) materials has been brought into connection with the polymer deposition on the surface during plasma treatment within the scope of this work. For this purpose, time-resolved measurements of the density of SiF4 have been performed by quantum cascade laser absorption spectroscopy. A quantification of the non-linear time dependence was achieved by its characterization via a time constant of the decreasing SiF4 density over the process time. The time constant predicts how fast the stationary SiF4 density is reached. The higher the time constant is, the thicker the polymer film on top of the treated ultra low-k surface. A correlation between the time constant and the ULK damage was also found. ULK damage and polymer deposition were proven by Variable Angle Spectroscopic Ellipsometry and X-ray Photoelectron Spectroscopy. In summary, the observed decay of the etching product concentration over process time is caused by the suppressed desorption of the SiF4 molecules due to a more dominant adsorption of polymers. © 2020 Author(s).
- 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.