J. Henriques

Wave driven N2–Ar discharge. I. Self-consistent theoretical model

A theoretical model for a low pressure surface wave driven N2–Ar discharge accounting in a self-consistent way for electron and heavy particle kinetics, gas thermal balance, and wave electrodynamics is developed. The inhomogeneous wave power transfer along the discharge and the concentrations of active species as a function of the spatial position and mixture composition are investigated. N2+ are the dominant ions over a wide range of Ar percentages in the mixture due to the contributions of charge transfer processes between Ar+ and N2 and an effective associative ionization from N2(A 3Σu+). Dissociative recombination of N2+ with electrons provides an increase in the dissociation degree of nitrogen molecules at high Ar fractional concentration.

Wave driven N2–Ar discharge. II. Experiment and comparison with theory

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.

Microwave air plasma source at atmospheric pressure: Experiment and theory

An experimental and theoretical investigation of the axial structure of a surface wave (2.45 GHz) driven atmospheric plasma source in air with a small admixture (1%) of water vapor has been performed. Measurements of the gas temperature and of the intensities of the O(777.4 nm), O(844.6 nm), and O(630 nm) atomic lines and the NO(γ) molecular band versus input power and axial position were carried out. Amplitude and phase sensitive measurements have also been performed to derive the surface wave dispersion characteristics. The experimental results are analyzed in terms of a one-dimensional theoretical model based on a self-consistent treatment of particle kinetics, gas dynamics, and wave electrodynamics. The predicted gas temperature and emission line intensities variations with power and axial position are shown to compare well with experiment. “Hot” excited O atoms (with kinetic energy ∼2 eV) have been detected.

Effect of gas heating on the spatial structure of a traveling wave sustained Ar discharge

In this work we report a theoretical and experimental study of the influence of gas heating on the spatial structure of a microwave Ar discharge sustained by a traveling surface wave. The theoretical analysis is based on a discharge model which couples in a self-consistent way electron and heavy particle kinetics, discharge electrodynamics, and gas thermal balance. The set of coupled equations used includes the electron Boltzmann equation, the rate balance equations for the most important excited species and charged particles, the gas thermal balance equation, and the equations describing wave propagation and power dissipation. The principal collisional and radiative processes which determine the populations in the Ar(3p54s) and Ar(3p54p) levels are accounted for. The field strength necessary for steady-state discharge operation is obtained from the balance between total rates of ionization (including direct and step-wise ionization and energy pooling reactions) and of electron loss due to the diffusion t...

Microwave plasmas applied for the synthesis of free standing graphene sheets

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 sp3 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.

Microwave N2–Ar plasma torch. II. Experiment and comparison with theory

Spatially resolved emission spectroscopy techniques have been used to determine the gas temperature, the electron, and N2+ ion densities and the relative emission intensities of radiative species in a microwave (2.45 GHz) plasma torch driven by a surface wave. The experimental results have been analyzed in terms of a two-dimensional theoretical model based on a self-consistent treatment of particles kinetics, gas dynamics, and wave electrodynamics. The measured spatial variations in the various quantities agree well with the model predictions. The radially averaged gas temperature is around 3000 K and varies only slowly along the discharge zone of the source but it drops sharply down to about 400 K in the postdischarge. The experimental wave dispersion characteristics nearly follow the theoretical ones, thus confirming that this plasma source is driven by a surface wave.

Spatial structure of a slot-antenna excited microwave N2–Ar plasma source

The spatial structure of a large-scale, slot-antenna excited (2.45GHz) surface wave plasma source operating in N2–Ar mixtures is investigated. A self-consistent theoretical model is developed in the local approximation to investigate the entire spatial structure of the system, including the discharge zone sustained by the field of the TM140 surface mode and the remote plasma zone. Maxwell’s equations and the rate balance equations for the most important excited species—vibrationally and electronically excited states, ions, and N(S4) atoms—and the electron Boltzmann are consistently solved. The pumping of the higher νth levels of N2(XΣg+1,ν) molecules is shown to be very effective and to strongly influence the remote plasma kinetics. Collisions of N2(XΣg+1,ν) molecules with N(S4) atoms are responsible for the increase in the number densities of electrons and electronically excited states N2(AΣu+3,BΠg3,CΠu3,a′Σu−1) in the “far” remote plasma zone.

Microwave plasma based single step method for free standing graphene synthesis at atmospheric conditions

Microwave atmospheric pressure plasmas driven by surface waves were used to synthesize graphene sheets from vaporized ethanol molecules carried through argon plasma. In the plasma, ethanol decomposes creating carbon atoms that form nanostructures in the outlet plasma stream, where external cooling/heating was applied. It was found that the outlet gas stream temperature plays an important role in the nucleation processes and the structural quality of the produced nanostructures. The synthesis of few layers (from one to five) graphene has been confirmed by high-resolution transmission electron microscopy. Raman spectral studies were conducted to determine the ratio of the 2D to G peaks (>2). Disorder D-peak to G-peak intensity ratio decreases when outlet gas stream temperature decreases.

Microwave N2–Ar plasma torch. I. Modeling

The spatial structure of a microwave plasma torch driven by an azimuthally symmetric surface wave operating in a N2–Ar mixture at atmospheric pressure is investigated. A two-dimensional (2D) self-consistent theoretical model is developed to investigate the entire spatial structure of the source, including the discharge zone, sustained by the field of the surface TM00 mode, and the postdischarge plasma. Maxwell’s equations, the rate balance equations for the most important excited species—vibrationally and electronically excited states, ions and nitrogen atoms N(S4)—and the Boltzmann equation for electrons are consistently solved. Model calculations of the 2D spatial distributions of species of interest such as charged particles (electrons and positive ions), N2(Χ Σ1g+,v) vibrationally excited molecules, N2(A Σ3u+) metastable molecules, and N(S4) ground state atoms are presented and discussed.

Modelling of large-scale microwave plasma sources

Theoretical models have been developed for large-scale microwave discharges driven by surface waves which account in a self-consistent way for the main plasma balances, including bulk and surface elementary processes, as well as wave electrodynamics. The systems under consideration are long tubular and large-volume, slot-antenna excited plasma sources. As an example of application, discharges in N2–Ar mixtures, which are characterized by a complex kinetics, are analysed in some detail. The approach used describes self-consistently the spatial structure of the plasma sources, i.e. the spatial distribution of population densities of excited species, charged particles and ground-state molecules and atoms taking into account the main energy exchange pathways as well as plasma–wall interactions. The model predictions are shown to compare well with experiment in the case of both sources. It is demonstrated that the self-consistent models developed provide deep physical insight into the discharge workings, being also instrumental as a tool for the optimization of these plasma sources in view of specific applications. (Some figures in this article are in colour only in the electronic version)