Plasma Chemistry and Plasma Processing最新文献

Abstract

The effects of neutral gas heating along the direction of the gas flow inside the discharge channel of a parallel plate micro atmospheric pressure plasma jet, the COST-jet, on the spatio-temporal dynamics of energetic electrons are investigated by experiments and simulations. The plasma source is driven by a single frequency sinusoidal voltage waveform at 13.56 MHz in helium with an admixture (0.05–0.2%) of nitrogen. Optical emission spectroscopy measurements are applied to determine the spatio-temporally resolved electron impact excitation dynamics from the ground state into the He I (3s) (^3) S (_1) state and the rotational temperature of nitrogen molecules at different positions along the direction of the gas flow inside the 30 mm long discharge channel. The gas temperature, which is assumed to be equal to the N (_2) rotational temperature, is found to increase along the discharge channel. This effect is attenuated as the nitrogen concentration is increased in the gas mixture, leading to an eventually constant temperature profile. The experimental data also reveal a plasma operating mode transition along the discharge channel from the (Omega) - to the Penning-mode and show good agreement with the results of 1d3v kinetic simulations, which spatially resolve the inter-electrode space and use the gas temperature as an input value. The simulations demonstrate that the increase of the gas temperature leads to the observed mode transition. The results suggest the possibility of using the nitrogen admixture and the feed gas temperature as additional control parameters, (i) to tailor the plasma operating mode along the direction of the gas flow so that the production of specific radicals is optimized; and (ii) to control the final gas temperature of the effluent. The latter could be particularly interesting for biological applications, where the upper gas temperature limit is dictated by the rather low thermal damage threshold of the treated material.

Experimental investigations of n-butane oxidation under atmospheric-pressure plasma conditions and in He-dilution have provided detailed information on the power-dependence of the conversion of (text {C}_{4}text {H}_{10}) into CO and (text {CO}_{2}) at 450 K surface temperature. The rf-plasma discharge has been equipped with a (text {MnO}_{2})-catalyst, and a significant impact on the reaction chain due to the presence of the catalyst surface could be observed. We report on ongoing data-based model development. Recently, a reaction kinetic model has been published, which agrees well with the experimental data (Stewig et al. in Plasma Sources Sci Technol 32:105006, 2023). However, that model could not clearly identify the main mechanisms in the interaction of plasma and catalyst. We show that various models can be found that explain the data similarly well. Detailed sensitivity analysis shows that only a maximum of three parameters can be identified in all the models considered for the currently limited data. Despite this limitation, we intend to continue the data analysis using more general models and introduce possible surface effects. Such unified models simultaneously describe the experimental data from both measurements with and without catalyst using a single set of physical parameters. To evaluate the hypotheses, we present numerical results for certain ranges of experimental parameters, which, in a subsequent experimental verification, allows to exclude or confirm one or another model.

Understanding the individual effects of species in plasma activated water (PAW) is indeed necessary for its selective application as an antimicrobial agent. The current study describes a setup and a methodology to generate species-specific high strength PAW at neutral pH. Three types of PAW i.e., ROS-rich PAW (Reactive Oxygen Species-rich Plasma Activated Water), RNS-rich PAbW (Reactive Nitrogen Species- Rich Plasma Activated buffer Water) and hs-PAbW (High strength Plasma Activated buffer Water) were generated in the developed set-up, the concentration of species was measured, shelf-life studies under ambient condition were carried out and each water type was studied for its antimicrobial activity. Results show that, after 60 min of activation, hs-PAbW had ROS and RNS concentrations of 215 mg/l and 650 mg/l respectively. The ROS-rich PAW had ROS and RNS concentration of 218 mg/l and 0 mg/l respectively. The RNS-rich PAbW had ROS and RNS concentration of 16 mg/l and 858 mg/l respectively after 60 min of activation. Antimicrobial studies showed that individual species i.e., high concentrations of ROS alone and high concentrations of RNS alone, were ineffective in achieving microbial degradation, whereas hs-PAbW containing high concentrations of both ROS and RNS showed significant antimicrobial effect. This study marks an important step in generating high concentrations of species-specific PAW, which is crucial to understand the mechanistic pathways of bacterial death in PAW systems.

Abstract

We study the bactericidal efficacy of surface dielectric barrier discharge low-temperature plasma treatments, powered by nanosecond high voltage pulses. We achieve (sim ) 4-log reduction in Escherichia coli population, after 10 min treatments, at a distance of 1.5 cm from the plasma surface. To investigate the reactive oxygen and nitrogen species (RONS) responsible for the bactericidal effect, we employ in-situ fourier transform infrared (FTIR) spectroscopy to measure a selection of relevant species, such as O (_3) , NO (_2) , N (_2) O and N (_2) O (_5) . The measurements are taken under various relative humidity conditions to replicate the bacteria treatment environment. While RONS originating from nitrogen chemistry are detected, nitric oxide (NO), a pivotal molecule in nitrate production, is absent due to the sensitivity limitations of FTIR detection. To overcome this limitation, we employ laser induced fluorescence utilizing a picosecond-pulsed laser to measure the kinetics of NO produced by the plasma. Our results show that the NO concentration is smaller than 1 ppm and primarily localized near the plasma surface, with concentrations increasing proportionally with relative humidity. Notably, at a distance of 1.5 cm from the plasma surface, at which the E. coli is treated, the concentration of NO falls below 50 ppb. Although NO is pivotal in generating secondary reactive species within the plasma, our results suggest that it does not directly contribute to the bacteria inactivation process. Instead, other molecules, such as O (_3) , NO (_2) , and N (_2) O, which are found in higher concentrations, may be responsible for the bactericidal properties observed in indirect plasma treatments.

New collision integrals and transport cross sections for O((^{3})P)–O((^{3})P) interaction are reported in the 300–30000 K range. Those values are based on a new set of potential energy curves (PECs) calculated with the multireference configuration interaction method. The results of the classical and semiclassical WKB (Wentzel–Kramers–Brillouin) methods are compared, excellent performance of the classical approach is shown (discrepancy much lower than 1% even at room temperature). In particular, the classical and WKB methods agree very well for the repulsive potentials effectively reducing overall uncertainty.

Abstract

The use of fused silica material is crucial in various scientific applications; however, its chemical inertness and brittle nature pose challenges to machining and fabrication processes. The present study introduced a dynamic plasma flow system for medium-pressure plasma processing of fused silica substrate to address this issue. The results indicate that the new plasma flow system can significantly enhance the material removal rate compared to existing systems, with a 300% increase in material removal rate. Importantly, this process enables a sustained linear material removal rate, essential for long process durations. Despite the higher material removal rate, there is no deterioration in surface finish observed, and in fact, an improvement in surface integrity is noted after plasma processing. Confocal Raman microscopy characterization further confirms this improvement, revealing reduced stress-induced defect peaks compared to a confined plasma system.

The glow-to-arc transition of a convection-stabilized atmospheric pressure air discharge is numerically investigated. Two separate models are considered: a one-dimensional axisymmetric time-dependent fluid model of the positive column, describing the thermal-instability, and a sheath model of a cold cathode describing the field-emission instability, which must then be properly matched together. The fluid model considers the most important chemical reactions in air plasma, including thermal ionization in atomic collisions. The radial electric field in the plasma is obtained from the Poisson equation. The voltage–current characteristic of the discharge is simulated for a time-varying current up to 300 mA. It is found that at some critical value slightly above 200 mA, the contraction of the positive column arises from a vibrational–translational energy relaxation. The subsequent increases in the discharge current density in the positive column drive in turn a field-emission instability in the cathode, which is accompanied by a large voltage drop. Simulation results are validated against available experimental data.

In this paper, we present a plasma rapid heater (PRH) designed to uniformly heat methane with electric arc thermal plasma and reduce loss rate. Our model uses a simplified and detailed mechanism to analyze the rapid and intense mixing process between a plasma mainstream and circumferential cold jets of methane in a hydrogen environment. The research focuses on three areas: the plasma mainstream section, the circumferential jet mixing chamber, and the reaction chamber. We investigate the characteristics of the mainstream and explore the impact of the jet momentum ratio on the mixing process and the losses from methane heating. We explain these phenomena using Damkohler numbers to demonstrate the relationship from a time-scale perspective. The findings indicate that an increased momentum ratio improves mixing, reduces temperature and material non-uniformity, and minimizes losses from pyrolysis during methane heating. Additionally, we provide a formula for calculating the penetration depth of the jet. The examination of Damkohler numbers also suggests that the momentum ratio primarily reduces methane losses by extending the reaction time scale. This work offers guidance on extending the usage of plasma heaters and integrating them into other industry processes in the future.

The epoxidation reaction of propylene (C₃H₆) for propylene oxide (PO) production was, for the first time of its kind, investigated in a low-temperature parallel-plate dielectric barrier discharge system with two frosted glass plates and a separate C₃H₆ feed under ambient condition. For the mixed feed experiments, the maximum PO selectivity was found at a feed molar air-to-C₃H₆ ratio of 0.5:1, an applied voltage of 7 kV, an input of 550 Hz, and a feed flow rate of the mixed gas of 100 cm³/min (corresponding to a residence time (RT) of 12.1 s). At the optimum operational conditions, a separate C₃H₆ feed position fraction of 1 (complete separation of C₃H₆) with total reactant feed flow rate of 100 cm³/min (corresponding to an O₂ residence time (RT) of 24.1 s), the greatest propylene oxide selectivity of 16.40% was achevied. The relatively high PO selectivity with very low selectivities for all other products and with the absence of both CO and CO₂ obseved in this studied resulted from less active of C₃H₆ due to the separate feed of C₃H₆. The C₃Hseparate feed is fundamentally responsible for the reduction in all the undesired reactions including cracking, dehydrogenation, coupling, and oxidations of C₃H₆.

Graphical Abstract

Abstract

The use of a dielectric barrier discharge plasma reactor with liquid-phase discharge was investigated in the production of methyl esters by transesterification reaction at room temperature, atmospheric pressure and without any previous reactant stirring step. The effects of applied voltage, reaction time and catalyst content were analyzed on ester production. The transesterification reactions of vegetable oil and monoesters were compared to understand the effect of dielectric barrier discharge plasma on miscible and poorly miscible liquid phases. The power transmitted to the plasma was evaluated for different conditions in order to evaluate the energy expenditure of the plasma reactor in the transesterification reaction and the factors that influence it. Results of the transesterification of vegetable oil were analyzed through sustainability indicators regarding mass productivity, energy consumption and cost of production. The transesterification of monoester showed higher conversion rates when compared to the transesterification of vegetable oil due to the fact that reactants are miscible. The analysis of the power transmitted to the plasma showed that conditions with lower energy expenditure are obtained with more polar solutions and with lower applied voltages to the plasma. Conditions with low plasma applied voltage showed significantly better results in the energy indicators. The results with sustainability indicators suggest that an optimized route using a plasma-assisted catalytic reactor is competitive when compared with conventional reactors for biodiesel production.