Carlos M. Ferreira

Plasma kinetics in atmospheric gases

The book Plasma Kinetics in Atmospheric Gases is a worthwhile contribution to the basic phenomena in nitrogen, oxygen and other atmospheric gases. It contains basic introductory chapters on relaxation in translational, rotational (short) and vibrational (extensive) distribution and on the physics of electron excitation and electron distribution functions. In the latter, electron beam excitation (i.e. high electron energies) are included. In the following chapters, much detail follows on spectroscopic constants of various molecules, molecular fragments and ions, on transport coefficients such as diffusion coefficients, relaxation times and rates. The transfer between translational energies and rotational and vibrational energies are treated in this context. Many (electron) excitation rates, electronic lifetimes and rates for chemical reactions for molecules, fragments and ions are given. The background part of the book is completed with an introductory chapter on wall reactions and accommodation coefficients. In this way an enormous wealth of data for nitrogen-, oxygen- and hydrogen-containing molecules can be found in this book. The book continues with the discussion of discharge physics in pure nitrogen and oxygen, and in mixtures. Some examples are given of applications, of which cleaning of atmospheric gases is, in my view, one of the most important. The many processes which play a role in the ionosphere are another important example of the application of the basic material for nitrogen, oxygen and other molecules. The kinetics which play a role in re-entry problems, and which have a limiting effect on the heat flux, are another example of the application of the plasma chemistry of nitrogen and oxygen gases. In this context fits also a short treatment of acoustic and shock waves, with which the book closes. The structure of the book is that of separate chapters which are in a logical order and follow naturally from the preceding ones. The chapters are separate in the sense that there are not many cross-references between them and all contain their own reference lists. Most of the reference lists (but not all) are extensive and contain recent references. A particular value of the book is the coverage of Russian references. However, I missed in several chapters some references as, for example, a recent book by Ricard on a related subject and other publications of, for example, French and Japanese groups which work on N2 and O2 discharges. This means also that some rates are given without mention of the discussion on them. Nevertheless, in my view this is a very useful book and I will use it extensively. The physics and chemistry of nitrogen- and oxygen-containing plasmas present a great challenge, and much is still unknown and not fully understood, in particular also in combination with surface processes. In this research the book will prove to be a valuable contribution. Professor D C Schram Department of Physics, Eindhoven University of Technology, 5600 MB Eindhoven

Kinetic model of a low-pressure N/sub 2/-O/sub 2/ flowing glow discharge

A self-consistent kinetic model is developed to study dc flowing glow discharges in N/sub 2//O/sub 2/ mixtures. This model includes the calculation of electron energy distribution functions and electron rate coefficients coupled with detailed vibrational kinetics of N/sub 2/ molecules, chemical kinetics taking into account a large set of neutral, excited and charged species, interaction of N and O atoms at the discharge tube wall, and the thermal balance of the discharge. The results of this model agree reasonably well with the measurements of the electronic density, the gas temperature, the reduced electric field, the vibrational temperature of N/sub 2/ and the concentration of O, N atoms, NO molecules, N/sub 2/(C), N/sub 2//sup +/(B), and NO(/spl gamma/) excited states. The comparison was performed in a N/sub 2/-O/sub 2/ discharge at pressure p=2 Torr, for discharge currents I=15, 30, and 80 mA, a flow rate Q=100 sccm, and O/sub 2/ percentages ranging from 0 up to 100%,. >

Surface kinetics of N and O atoms in discharges

The probability, , of losses of N and O atoms on Pyrex walls was determined from a fit of calculated to measured concentrations [O] and [NO] in a low-pressure glow discharge in , for percentage concentrations . The kinetic model used for calculations includes a detailed description of the processes occurring in the discharge bulk and of the surface reactions of O and N atoms. It was found that and are functions of the ratio and the wall temperature, . The values of were found to increase from about to about as increases from 1% to 90% (corresponding to ). The probability was found to be independent of and to depend only on in the range . For , however, depends on , its magnitude increasing by a factor of 2 - 5 as . The kinetic model developed here for surface reactions provides closed expressions for and in terms of the rate constants and the activation energies for these reactions. It is shown that the behaviour of and is well explained by the model under the following conditions: (i) the main surface processes for the low wall temperatures involved are reversible adsorption followed by surface diffusion of the adatoms to active sites, where they may either be irreversibly adsorbed or recombine; and (ii) there exist two independent systems of active sites, with different reaction probabilities.

A collisional-radiative model for microwave discharges in helium at low and intermediate pressures

A stationary collisional-radiative model for helium microwave discharges in cylindrical geometry is developed by coupling the rate balance equations for the n<or=6 excited states of helium to the continuity and transport equations for the electrons, He+ atomic ions and He2+ molecular ions, and to the homogeneous Boltzmann equation. The latter is solved using the DC effective field approximation but taking into account stepwise inelastic and superelastic processes from the 23S, 21S and 23P states, as well as electron-electron collisions. A coherent set of electron cross sections is deduced in order to solve the Boltzmann equation. Special attention is paid to the atomic collisions considered (by taking into account /-change reactions and associative ionization reactions), and to the effects of radiation imprisonment. This, together with the inclusion of the kinetics of the molecular ions, allows the range of validity of the model to extend up to atmospheric pressure. The theoretical populations for the excited states, characteristics for the steady-state reduced maintenance electric field and mean absorbed power per electron at unit gas density agree very well with experimental data from surface wave discharges.

Electron kinetics in weakly ionized helium under DC and HF applied electric fields

The electron kinetics in weakly ionized helium under the action of DC and HF fields of angular frequency omega is investigated by solving the homogeneous electron Boltzmann equation using the classical two-term expansion approximation. The analysis is based on a consistent set of electron cross sections which is derived by adjusting experimental cross section data in such a way that calculated and measured electron swarm parameters are in agreement. In the case of HF fields the analysis is based on the DC effective field approximation which is valid for omega > tau e-1, where tau e is the characteristic time for electron energy relaxation by collisions with the atoms. The influence of omega on the electron energy distribution function, transport parameters, rate coefficients and fractional power transfer is investigated and a detailed comparison of the DC and HF situations is made. It is shown that for reduced effective fields in the range 10-16-10-15 V cm2, as typically found in low-pressure discharges, the mean electron kinetic properties are nearly the same in the whole range of omega > tau e-1 as for the DC case.

Non-equilibrium kinetics in nitrogen discharges: a comparative analysis of two theoretical approaches

A comparison of two different theoretical approaches reported in the literature to model kinetic processes in N2 discharges is presented and discussed. One of these approaches is directly concerned with steady-state conditions and the other analysis with the evolution of the properties of the medium from a cold gas situation up to nearly equilibrium conditions as time evolves. Both approaches are based on solutions to the coupled system of the electron Boltzman equation and the kinetic equations for the N2(X,v) vibrational levels, but they differ by the approximations used to model atomic recombination processes and by the choice of collisional data. The conclusions drawn from both models are qualitatively in agreement as to the important role of the coupling between the electron and the vibrational kinetics. However, due to the different data and recombination models used different results are obtained for equilibrium conditions, namely for that concerning the rate of dissociation, the degree of vibrational excitation, and the electron rate coefficients. In particular, the present analysis shows that a deeper knowledge of atomic recombination processes is necessary in order to improve the reliability of both models.

A travelling wave sustained hydrogen discharge: modelling and experiment

A model has been developed for a surface wave sustained hydrogen discharge which, in a self-consistent way, accounts for the main plasma balances governing the discharge production, including bulk and surface processes. The approach used self-consistently describes the axial discharge structure, i.e. the axial distribution of charged particle concentrations, population densities of excited species and neutrals, taking into account inhomogeneous gas heating along the plasma column as well as plasma-wall interactions. A spatially resolved experimental investigation into the distribution of electron density, atomic line intensity and gas temperature confirms the main trends of the model predictions.

Kinetic model of a DC oxygen glow discharge

A simple kinetic model predicting the concentration of oxygen atoms, metastable singlet molecules O2(a1Δ) and negative ions O — in the positive column of a DC glow discharge is developed. The calculated O and O2(a1Δ) concentrations are compared to previously reported measurements for pressuresp=0.2–2 Torr and discharge currentsI=10–80 mA. The electron density calculated from the continuity equationj=nee vd agrees well with experiment. The rate coefficients for electron impact processes used in the balance equations of O, O2(a1Δ), and O− were taken from the literature as a function of the reduced electric fieldE/N forE/N=40–80 Td. A reasonable agreement is obtained between the model and the experiment with a set of 10 reactions for the production and destruction of the above-mentioned species

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.

Self-consistent kinetic model of a surface-wave-sustained discharge in nitrogen

A self-consistent kinetic model based on a set of coupled equations consisting of the local electron Boltzmann equation and the rate balance equations for the most important excited species (vibrationally and electronically excited molecular states) and charged particles in a nitrogen discharge has been developed. The system under analysis is a plasma column produced by a travelling, azimuthally symmetric surface wave. Electron collisions of first and second kind with nitrogen molecules and electron - electron collisions are accounted for in the Boltzmann equation. Therefore, this equation is coupled to the set of equations for electronic and vibrational populations through both inelastic and superelastic collisions. The field strength necessary for the discharge steady-state operation is obtained from the balance between the total rate of ionization (including associative, direct and step-wise ionization) and the total rate of electronic losses (due to diffusion to the wall and bulk recombination). The model determines, as a function of the discharge operating parameters (pressure, tube radius, wave frequency, degree of ionization), the electron energy distribution, the populations of the vibrational levels of the electronic ground state and the most important electronic states as well as the concentrations of and ions, consistently with the discharge maintaining electric field. Theoretical results for the electron energy distribution function and some of its moments are compared with experimental ones obtained in a low-pressure surface-wave-sustained discharge at a wave frequency of 500 MHz.