We present a self-consistent formulation to study low-pressure traveling wave (azimuthally symmetric surface transverse magnetic mode) driven discharges in nitrogen. The theoretical model is based on a self-consistent treatment of the electron and heavy particle kinetics, wave electrodynamics, gas thermal balance, and plasma–wall interactions. The solution provides the axial variation (as a result of nonlinear wave power dissipation along the wave path) of all discharge quantities and properties of interest, such as the electron energy distribution function and its moments, population densities of all relevant excited and charged species [N2(X 1Σg+,ν),N2(A 3Σu+,a′ 1Σu−,B 3Πg,C 3Πu,a 1Πg,w 1Δu), N2+, N4+, e], gas temperature, degree of dissociation [N(4S)]/N, mean absorbed power per electron, and wave attenuation. A detailed analysis of the energy exchange channels among the degrees of freedom of the heavy particles is presented. Particular attention is paid to the axial variation of the gas and wall temperatures, which affect in a complex way the discharge operation. For the high electron densities and reduced electric fields achieved at 2.45 GHz, it is shown that the contribution of exothermic reactions involving excited molecules in metastable states to the total gas heating can be significant. The role of the triplet N2(A 3Σu+) metastable state as an energy “reservoir” that pumps translational modes of gas particles is pointed out. A strong correlation between the degree of dissociation, the concentration of metastable N2(A 3Σu+), N(2D,2P) particles, and surface kinetics is shown to exist. Spatially resolved measurements of the gas and wall temperatures, electron density, and wave propagation characteristics provide a validation of the model’s predictions.
A theoretical and experimental study on atmospheric pressure microwave plasma-based assembly of free standing graphene sheets is presented. The synthesis method is based on introducing a carbon-containing precursor (C 2 H 5 OH) through a microwave (2.45 GHz) argon plasma environment, where decomposition of ethanol molecules takes place and carbon atoms and molecules are created and then converted into solid carbon nuclei in the 'colder' nucleation zones. A theoretical model previously developed has been further updated and refined to map the particle and thermal fluxes in the plasma reactor. Considering the nucleation process as a delicate interplay between thermodynamic and kinetic factors, the model is based on a set of non-linear differential equations describing plasma thermodynamics and chemical kinetics. The model predictions were validated by experimental results. Optical emission spectroscopy was applied to detect the plasma emission related to carbon species from the 'hot' plasma zone. Raman spectroscopy, scanning electron microscopy (SEM), and x-ray photoelectron spectroscopy (XPS) techniques have been applied to analyze the synthesized nanostructures. The microstructural features of the solid carbon nuclei collected from the colder zones of plasma reactor vary according to their location. A part of the solid carbon was deposited on the discharge tube wall. The solid assembled from the main stream, which was gradually withdrawn from the hot plasma region in the outlet plasma stream directed to a filter, was composed by 'flowing' graphene sheets. The influence of additional hydrogen, Ar flow rate and microwave power on the concentration of obtained stable species and carbon−dicarbon was evaluated. The ratio of sp 3 /sp 2 carbons in graphene sheets is presented. A correlation between changes in C 2 and C number densities and sp 3 /sp 2 ratio was found.
Self-standing graphene sheets were synthesized using microwave plasmas driven by surface waves at 2.45 GHz stimulating frequency and atmospheric pressure. The method is based on injecting ethanol molecules through a microwave argon plasma environment, where decomposition of ethanol molecules takes place. The evolution of the ethanol decomposition was studied in situ by plasma emission spectroscopy. Free gas-phase carbon atoms created in the plasma diffuse into colder zones, both in radial and axial directions, and aggregate into solid carbon nuclei. The main part of the solid carbon is gradually withdrawn from the hot region of the plasma in the outlet plasma stream where nanostructures assemble and grow. Externally forced heating in the assembly zone of the plasma reactor has been applied to engineer the structural qualities of the assembled nanostructures. The synthesized graphene sheets have been analysed by Raman spectroscopy, scanning electron microscopy, high-resolution transmission electron microscopy and x-ray photoelectron spectroscopy. The presence of sp 3 carbons is reduced by increasing the gas temperature in the assembly zone of the plasma reactor. As a general trend, the number of mono-layers decreases when the wall temperature increases from 60 to 100 • C. The synthesized graphene sheets are stable and highly ordered.
Direct assembling of N-graphene, i.e. nitrogen doped graphene, in a controllable manner was achieved using microwave plasmas at atmospheric pressure conditions. The synthesis is accomplished via a single step using ethanol and ammonia as carbon and nitrogen precursors. Tailoring of the high-energy density plasma environment results in a selective synthesis of N-graphene (~0.4% doping level) in a narrow range of externally controlled operational conditions, i.e. precursor and background gas fluxes, plasma reactor design and microwave power. Applying infrared (IR) and ultraviolet (UV) irradiation to the flow of free-standing sheets in the post-plasma zone carries out changes in the percentage of sp2, the N doping type and the oxygen functionalities. X-ray photoelectron spectroscopy (XPS) revealed the relative extension of the graphene sheets π-system and the type of nitrogen chemical functions present in the lattice structure. Scanning Electron microscopy (SEM), Transmission Electron microscopy (TEM) and Raman spectroscopy were applied to determine morphological and structural characteristics of the sheets. Optical emission and FT-IR spectroscopy were applied for characterization of the high-energy density plasma environment and outlet gas stream. Electrochemical measurements were also performed to elucidate the electrochemical behavior of NG for supercapacitor applications.
Discharges in N2–Ar mixtures are experimentally investigated by means of optical emission and absorption spectroscopy, probe diagnostic techniques, and radiophysic methods. The experimental results provide insight into the mechanisms of wave-to-plasma power transfer, N2 dissociation, creation of N2+ ions, and excitation of metastable states [N2(A 3Σu+),Ar(3P2)]. These results are analyzed in the framework of the theoretical predictions of a model developed in a companion article.
A microwave N 2 -H 2 discharge driven by a travelling surface wave is investigated as a source of ground state N( 4 S) and H(1s) atoms. Experimental investigations have been carried out in a plasma source operating at 2.45 GHz at low-pressure conditions (p = 0.5-2 Torr). By means of optical emission spectroscopy and probe diagnostic techniques, the population densities of ground state atoms have been detected. The dissociation kinetics is discussed in the framework of a theoretical model based on a self-consistent treatment of the main discharge balances, wave electrodynamics and plasma-wall interactions. Electron-ion surface recombination processes involving HN + 2 and N + 2 ions are the most important sources of N( 4 S) gas phase atoms for the conditions considered. The relative number of N( 4 S) atoms in respect to the total neutral density remains approximately constant for percentages of H 2 between 10% and 50% at nearly constant electron density. The competitive interplay of two important source channels of H(1s) atoms, namely electron impact dissociation of H 2 and H 2 dissociation via the quenching of nitrogen N 2 (a 1 − u ) and N 2 (A 3 + u ) metastables, determines a smooth decrease of hydrogen dissociation when the amount of hydrogen increases up to 50% in the mixture.
A model for a surface wave sustained nitrogen discharge accounting in a self-consistent way for electron and heavy particles kinetics and discharge electrodynamics has been developed. The system under analysis is a plasma column produced by a traveling, azimuthally symmetric (m=0 mode) surface wave. The model is based on a set of coupled equations consisting of the electron Boltzmann equation and the rate balance equations for the most important excited species—vibrationally, N2(X 1Σg+, ν), and electronically excited states, N2(A 3Σu+, a′ Σu−, B 3Πg, C 3Πu, a 1Πg)—and charged particles (e, N2+, N4−) in the discharge. Electron collisions with nitrogen molecules of the first and the second kind and electron–electron collisions are accounted for in the Boltzmann equation. The field strength necessary for steady-state operation of the discharge is obtained from the balance between the total rates of ionization (including direct, stepwise, and associative ionization) and of electronic losses (due to diffusion to the wall and bulk recombination). The transfer of wave power to the discharge occurs through collisional processes, thus the set of equations is closed by an ordinary differential equation (stemming from basic electrodynamical relations) which associates the axial gradient of the electron density to the wave attenuation. As a result, a self-consistent interdependence between wave propagation and discharge characteristics is obtained over the whole plasma column. The axial profile of the gas temperature and the initial value of the electron density at the position of the wave launcher are used as input parameters. The model determines the axial structure of the discharge—axial variations of the electron energy distribution function and its moments, the vibrational distribution function of the electronic ground state, and the densities of the most important electronically excited states and positive ions—consistently with the electric field and the surface wave dispersion characteristics. A spatially resolved experimental investigation of the electron energy distribution function, the gas and the vibrational temperatures, and the population densities of some electronically excited states along with wave propagation characteristics measurements provides a verification of the model. Strong correlation between different plasma balances, governing the discharge production, and discharge electrodynamics—the basis of surface-wave discharge physics—has been demonstrated both theoretically and experimentally.
In this paper, we present an investigation of the timerelaxation of the electron energy distribution function (EEDF) in the nitrogen afterglow of an 2 = 433 MHz flowing discharge at = 3 3 torr, in a tube with inner radius = 1 9 cm. We solve the time-dependent Boltzmann equation, including the term for creation of new electrons in associative/Penning reactions, coupled to a system of rate balance equations for the heavy-particles. The EEDFs are also obtained experimentally, from second derivatives of digitized probe characteristics measured using a triple probe technique, and compared with the calculations. It is shown that an equilibrium between the vibrational distribution function of ground-state molecules N 2 (1 6 +) and low-energy electrons is rapidly established, in times 10 7 s. In these early instants of the postdischarge, a dip is formed in the EEDF around 4 eV. The EEDF finally reaches a quasi-stationary state for 10 6 s, although the electron density still continues to decrease beyond this instant. Collisions of highly excited N 2 (1 6 + 35) molecules with N(4) atoms are in the origin of a maximum in the electron density occurring downstream from the discharge at 2 10 2 s. These reactions create locally the metastable states N 2 (3 6 +) and N 2 (1 6), which in turn ionize the gas in associative/Penning processes. Slow electrons remain for very long times in the postdischarge and can be involved in electron stepwise processes with energy thresholds smaller than 2-3 eV.
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