Plasmas Afterglows with N2 for Surface Treatments synthesis 2024
Various ionized gases are plasmas. They are present in the universe (stars), in the ionosphere and in the magnetosphere. They are manufactured on earth for numerous applications such as reactors for controlled fusion, arcs for mineral fusion and for welding, gas laser discharges, arcs for lighting, discharges for chemical synthesis of various products (enrichment of olefins), discharges for surface treatments (deposition of new materials, nitriding). Plasmas are characterized by high temperatures for thermonuclear fusion (108 K), in electrical arcs (104K) and also in various electrical discharges where only the electrons are at high temperatures (104 to 105K), the neutral molecules being near room gas temperature. Such high temperatures specify the plasma medium as the fourth state of matter, after the solid, liquid and gas states. The presently studied reactive plasmas are characterized by hot electrons (104-105K) and a cold neutral gas (300-1000 K). The electron energy is transferred to the neutral gas not only for the gas heating but mainly to excite and to ionize the atoms or molecules on states of high potential energy. The excited ions, atoms or molecules, produced by electron collisions, characterize the reactive plasmas which are of special interest in the process of surface treatments. This book is focused on the production of plasma active species containing N2 which are molecules in electronic, vibrational and rotational states and dissociated atoms or radicals. The book chapters are organized by considering first the plasmas conditions (chap. 1-6) starting from low to high gas pressures and from fundamental (chap. 1-5) to applied results (chap. 6). Then it is joint fundamental results of mixed plasmas and afterglows (chap. 7-11). The larger part of the book concerns the afterglows (chap. 12-27) with the same previous classification of low to high gas pressures and of fundamental (chap. 12-20) to applied results (chap. 21-27). A recent review on low temperature plasma science and technology is reported in the following paper: I.Adamovich, S.Agarwal, E.Ahedo, L.L.Alves, S.Baalrud, N.Babaeva, A.Bogaerts, A.Bourdon, P.J.Bruggeman, C.Canal, E.H.Choi, S.Coulombe, Z.Donkó, D.B.Graves, S.Hamaguchi, D.Hegemann, M.Hori, H-H.Kim, G.M.W.Kroesen, M.J.Kushner, A. Laricchiuta, X.Li, T.E.Magin, S.Mededovic, V.Miller, A.B.Murphy, G.S.Oehrlein, N. Puac,R.M.Sankaran,S.Samukawa, M.Shiratani, M.Šimek, N.Tarasenko, K.Terashima, E.Thomas, J.Trieschmann, S Tsikata, M.Turner, I.J.van der Walt, M.C.M van de Sanden, T.von Woedtke.The 2022 Plasma Roadmap: low temperature plasma science and technology. J. Phys. D: Appl.Phys. 55 (2022) 373001. With the following contents: 1. New plasma excitation and generation approaches 2. Extreme plasma regimes 3. Plasma–liquid interactions 4. Dusty plasmas 5. Atomic layer etching (ALE) and deposition 6. Nanostructures and nanomaterials 7. Additive manufacturing (AM) and coatings 8. Plasma engineering of soft materials and biomaterials 9. Medical plasma applications 10. Contribution of plasma to combat COVID-19 11. Plasma agriculture and innovative food cycles 12. Plasma pollution control 13. Electrification of chemical conversions 14. Plasma propulsion (PP) 15. Plasma assisted combustion and hypersonics 16. Thermal plasma applications 17. Plasma diagnostics 18. Theory, modeling and simulations 19. Diagnostics and modeling of plasma–surface interactions 20. Atomic, molecular and transport data 21. Data-driven plasma science and technology.
Discussion of Plasma Electron Collisions and Optical Spectroscopy
This chapter describes the reactions produced by electron collisions. The nomenclature of the diatomic spectrum is summarized. The He and Ar energy levels and the potential curves of N2, H2 and O2 are reproduced. After examples of N2 and Ar emission spectra, the absorption resonant spectroscopy and the Raman Laser spectroscopy are applied to absolute density measurements of Ar metastable and of N2 (X,v) vibrationally molecules. It is indicated how the actinometry method brings relative density measurements of N, O and H atoms. Emission spectroscopy in gas-pulsed Ar-N2-O2 microwave plasma is briefly reported. Finally, it is indicated how to obtain the electron, vibrational and rotational temperatures of N2 plasma by emission spectroscopy.
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Interaction of Active Species in R (He, Ne, Ar)-Molecular Gas Plasmas
In high pressure, He glows discharges and the 3-body reactions of He (23 S) metastable atoms and He+ ions with He and N2 are in competition to excite the \(N_2^+\) (B,0) radiative states. With [He+] < 1010 cm-3 and [He (23 S)] ~ 1012 cm 3 as found in a DC glow discharge, it is estimated that the 3-body reaction He(23 S) + He + N2 was the dominant process. By using the actinometry method with Ar and Ne impurity in N2 - H2DC glow discharge at 5 Torr, it was found in the negative glow of the DC discharge a small increase of all N2 vibrational states and cathode temperatures as a few H2 was introduced into N2. Finally, it discusses the methods to obtain the plasma gas temperatures from the rotational structures of N2, CN, OH, H2 and O2 radiative states.
Microwave Cavities Generated Active Species in Plasmas
Microwave plasma sources are described from low Ar gas pressure (< 1 Torr) to high gas pressures (> 10 Torr up to the atmospheric gas pressure) in N2 and Ar. At low gas pressure, the plasma diameter is over 1 cm, typically 2.6 cm and at high gas pressure, it needs discharge tubes < 1cm, down to 0.1 cm. The microwave plasmas have been studied from low to atmospheric gas pressure in rare gases (mainly Ar) and gas mixtures containing N2. At low gas pressure, it is found the surface wave plasmas, produced by patented launchers Surfatron and Surfaguide, which were developed by Michel Moisan in the Plasmas Lab. of the Montreal Univ. in 1975 [1] . At high gas pressure from a few Torr up to the atmospheric gas pressure, the SW plasmas Surfatron at low power and Surfaguide at high power are still operating. It is also given the results obtained by a resonant cavity in N2 - xO2 with x = 0-20%. It is presently reported the detection of plasma active species obtained by optical spectroscopy. The main used frequency was 2450 MHz. The plasma power could vary from a few Watts (surfatron) up to a few 103 Watts (Surfaguide, resonant cavity). The plasma can be produced in a static gas flow, mainly at low gas pressure and up to several liters per minute (slpm) at high gas pressures. The optical spectroscopy was set up first to obtain the spatial distribution of radiative species, mainly in Ar gas at low gas pressure in the perspective to characterize the SW plasmas. Then the radial distribution of Ar metastable density was obtained by resonant optical absorption. At medium (a few Torr) up to the atmospheric gas pressure, flowing HF plasmas (resonant cavity) and SW plasmas were produced in gas mixtures with N2 to produce N -atoms and other active species such as N2 metastable molecules and N2+ions which are studied inside the plasma and in afterglow conditions. It discusses the kinetics reactions relating the N2(X,v),N2(A,v'),N2(B,v'),N2(C,v'),N2+(X,v') and N2+(B,v') interfering in the radiative emissions of N2(B,v'),N2(C,v') and N2+(B,v') states. At atmospheric gas pressure, the plasmas are mainly in Ar dominant gas, easier to ionize. In these conditions, the plasmas are characterized by the electron density and the gas temperature which will be reported for specific microwave plasmas sources. Depending on plasma parameters such as powers, flow rates and exit tube diameters, the plasmas were more and less near the Local Thermodynamic Equilibrium (LTE) as demonstrated in Montreal Univ., Lisbon IST and Cordoba Univ. The conclusion was that the N2 excited states are mainly produced from the N2 ground state by electron collisions in a resonant microwave cavity. It was estimated that the LTE was nearly reached in a 100 Torr, 300-500 W N2 plasma in a quartz tube of i.d.10 mm. with equality of vibrational and rotational temperatures.
Time-Resolved Studies of Plasmas and Afterglows for Material Surface Treatments
Time-varying discharges and afterglows have been studied in Ar and N2 gases for several types of discharge structures (DC, RF, DBD, magnetron) at different gas pressures from a few mTorr to the atmospheric gas pressure. Pulsed DC plasmas were performed in N2 and N2 - H2 for steel cathode nitriding. Pulsed RF plasmas and afterglows have been studied in Ar and Ar-N2 with an experimental setup. Both Glow Dielectric Barrier Discharge and Townsend Dielectric Barrier Discharge discharges are homogeneous or diffuse DBDs, free of streamer and microdischarge, and convenient to active polymer It has discussed the effects of reactions between metastable atoms (ArM) and from molecules (N2, A) in long (10-3 s range) and short (10-6 s range) time afterglows in the excitations of Ar high radiative levels, N2(C, v = 1) level and NO*, N2O* bands. Strong emissions of Ti+ and O-atoms when O2 was added to Ar in a short time (10-6 s range), low Ar pressure (10-3Torr) pulsed DC magnetron plasmas which were associated with hard TiO2 coatings.
Decomposition Method of N and C-atoms in R (Ar)-N\(_2\)-CH\(_4\) Flowing Afterglows in High Power RF Jet
The primary purpose of the study is to investigate the decomposition mechanisms of nitrogen (N) and carbon (C) atoms in a plasma afterglow environment consisting of argon (Ar), nitrogen (N2), and methane (CH4). One of the major problems in depollution processes using the plasma technique is the analysis and monitoring of the effluent gases. Radiative species in the Ar RF plasma torch with the addition of H2, CH4 and CF4 polluting gases have been analyzed by emission spectroscopy. An efficient etching by F atoms of reactor quartz tube is detected from Si atom emission when a few 10-3 CF4 was introduced into the Ar plasma. The Si emission disappeared with H2 introduction into the Ar - CF4 gas mixture which has been correlated with HF formation. From C atom emission, it is deduced that CF4 as CH4 polluting gases are largely dissociated into the Ar plasma torch. The C-atom density was evaluated from the N-atom density measured by NO titration by considering the part of N+N recombination which populated the N2 (B,v’) radiative states. It is found that the RF 64 MHz plasma jet at 3 kW brought N and C -atoms densities in an afterglow at 3 10-2s near the atmospheric gas pressure which are respectively ~1016 cm-3 and 1015 cm-3 are 101 and 102 higher than in 2450 MHz afterglows at about 100 W and 10 Torr. The gas temperature determined from the CN and N2 rotational systems is 3200± 200 K in the RF torch and 700± 100 K in the afterglow at 3 10-2 s.
Optical Spectroscopy of Surfaces Plasmas Treatments
Surface plasmas treatments have been studied in Ar - gas mixtures (H2, N2) as film depositions in fields like microelectronics with amorphous a-SiH, iron nitriding and hard coatings with TiN deposition. At the University of Mons, magnetron discharges amplified by an RF coil were studied to determine the efficiency of this process regarding the ionization of the sputtered metal atoms. Several plasmas setups are considered: sputtering plasmas for a-SiH and TiN deposition. To control the process, diagnostics by optical spectroscopy have been developed, in particular the resonant absorption method with adapted optical sources. Densities of species such as rare gas (He, Ar) metastable atoms, Al and Ti sputtered target atoms have been determined. The control of surface layers was performed by emission and absorption spectroscopy, by measurements of rare gases (He, Ar) and metal atoms (Al, Ti, Ti+) metastable densities. The densities of Ti and Ti+ ground and metastable states have been specifically obtained. The behavior of these active species by varying the plasma parameters has been related to the quality of the deposited films.
Exploring Plasma Sources for Optimized Production of Metastable Atoms and Reactive Species
Flowing plasmas and afterglows sustained by DC glow, hollow cathode arc (HCA), and microwave (surfatron) discharges have been studied to obtain a maximum density of R (He, Ne, Ar) metastable atoms and N, O, and H atoms by setting the experimental parameters such as the gas pressure and mixture, the discharge tube diameter, gas flow rates, discharge power, etc. The absolute densities of these active species have been measured by resonant absorption spectroscopy, NO titration with emission spectroscopy, and a catalytic probe. The maximum densities of He (23S), Ne(3P2), and Ar(3P2) metastable atoms obtained in these discharges at reasonable powers or power densities are a few 1011-1012 cm-3 in the range of pressures between 0.1 and 1 Torr. The densities of N, O, and H atoms may be as large as 1015 – 1016 cm-3 in the pressure range between 0.1 and 10 Torr providing the surface of materials facing plasma exhibit a low coefficient for heterogeneous surface recombination. A dissociation fraction of oxygen atoms is often around 10% at reasonable discharge power, more in a mixture with a noble gas. The N-atom density is limited because of the high binding energy of nitrogen molecules and thus a rather poor production rate. The H-atom density tends to be limited by significant surface losses unless the experimental systems are made from quartz glass.
Evaluating Electron and Vibrational Distributions in N\(_2\) Flowing Discharges and Post-discharges at Reduced Gas Pressure
Relaxation of the electron and vibrational distribution functions have been calculated in function of residence time in N2 electrical discharges and post-discharges. Calculations results are compared to the measurements of N2 radiative states intensity and N2(X,v) vibrational density by CARS in an N2 DC positive column where the following plasma parameters: electron density, gas temperature and electric field have been measured. In the discharge, the vibrational temperature gets bigger with the residence time for t < 10 -2 s. In the post-discharge, the vibrational distribution is evolving in such a manner that the high levels are overpopulated as the low vibrational level populations are dropping.
Behaviour of N\(_2\) Active Species in Microwave Plasma and Early Afterglows at Low Pressure
The excited states in N2 plasma and flowing afterglows at reduced pressure are studied for the plasmas and for the afterglows. The N2 flowing microwave discharges have been investigated by emission spectroscopy in the following conditions: N2 at p = 5 Torr, flow rate of 0.3 slpm, t = 1.5×10-3 s and microwave power of P = 150 W. Two experimental setups were analyzed: one with the plasma (produced inside a 5 mm ext. dia. tube) directly connected to a larger post-discharge tube (with a diameter of 15 mm) and another with the introduction of an Ar-NOext mixture after the plasma in the small tube. Under these conditions, the production of N2+ ions in the plasma and in the early afterglow is done predominantly by electron collisions. After the end of the plasma up to the pink afterglow, N2(A) and N2(X,v>11) were the excited species at the origin of the N2(C) and N2+(B) emissions. A self-consistent theoretical model was introduced to explain the production of the N2 active species by particle kinetics. The intensity ratio I\(N_2^+(B)\)/I\(N_2(C)\) increased 15 times from the plasma to the pink afterglow. In this setup, the gas temperature was about 1000 K in the plasma and 700 K in the pink afterglow. In a second setup, the plasma end was at 12 cm of the post-discharge tube. The gas temperature was lower: 750 K at 2 cm and 400 K at the plasma end (3.5 cm). The effect of adding an Ar-NOext gas mixture (used for N-atom titration in the late afterglow inside the 15 mm diameter tube) at the discharge end was studied along the 5 mm diameter tube. It was concluded first that the destruction of N-atoms by NOext decreased the intensity of the N(2P) and N2(B) radiative states. Second, the electrons at the plasma end could excite the N2(C) and N2+(B) radiative states. It was revealed that the destruction of N-atoms caused by the addition of NOext also produced a decrease in the intensities of the N(4P) and N2(B) radiative states. The plasma electrons were still influential in the excitation of the N2(C) and N2+(B) radiative states after the plasma end.
Radial Distributions of Active Species in RF-Jet Plasmas and Afterglows
The radial distributions of plasma and afterglow radiative species have been analyzed with optical fibers connected to an optical spectrometer. With an optical fiber of 1 mm dia. giving a detected light resolution of about 1 cm at a 10 cm distance, the N2 long time afterglow (LA-10-2 s) was studied in a large volume reactor (5 liters). At a pressure of 68 Torr, flow rate 0.5 - 1 slpm, the LA at 10-2 s was a jet of dia. 2 cm with densities of N-atoms and N2(X, v > 13) metastable molecules of 1015 and 0.5 1015 cm-3, respectively. By putting a horizontal slit of 50\(mu\)m in front of the dia. 1 mm optical fiber, a light beam of 1 mm dia. was detected across an afterglow tube of 2 cm dia. In an N2 RF plasma at 8 Torr, 1slpm, 100 W, Bessel-typed profiles were recorded for all the radiative bands in the pink afterglow and flat profiles of the N2 580 nm band in the late afterglow. The N2 580 nm band is produced in the late afterglow by N-atoms rebounding on the tube wall, explaining its flat profile. In the pink afterglow, the N2580 nm band is mainly produced by collisions of the N2(X, v > 13) and N2(A) states where N2(A) is destroyed on the tube wall, which explains its Bessel profile. With a small H2 content into N2, the N2(X, v > 13) and N2(A) states in the pink afterglow are destroyed, lowering the N2 580 nm emission that was only produced by the N atoms, yielding a flat profile. A 1 mm radial profile was also obtained with a collimator set on the 1 mm optical fiber and moved along the radial axis of a surface wave (SW) plasma. With a SW plasma, the electric field (E) reaches a maximum value on the tube wall giving high energy to electrons. As a consequence, the Ar metastable atoms produced by high energy electrons and destroyed by low energy electrons (insensitive to E) will show radial profiles of which the shapes were analyzed to determine the metastable excitation (CM) and de-excitation (CM) rates. In Ar SW plasmas at 600MHz, 0.1 Torr, 2 W, it was determined that CM = (1 - 5)10-11 cm3 s-1 and CM = (1 - 3)10-7 cm3 s-1 as for a DC positive column at same electron density: ne = 31010 cm-3. 3D spatial distributions of the intensity of the optical thin ArI 549.6 nm line were measured in DC and SW plasma columns at 150 mTorr inside tubes of 2 cm dia. and 50 cm long. Bessel-type radial distributions were found in the Ar DC column and more rounded profiles in the Ar SW plasmas at 600 - 900 MHz, with hollow shapes at the plasma end.
Remote Plasmas Chemical Vapour Deposition and Reactive Sputtering in Magnetron Plasmas
Boron trichloride (BCl3) is frequently encountered in plasma surface engineering. Cerium and its compounds have attracted scientific interest as catalytic converters for the reduction of CO emissions in exhaust gases. Moreover, Cerium has been proven to reveal anti-corrosion properties. Two optical spectroscopy methods have been applied to the determination of B-atoms and Ce atom's absolute densities. A microwave discharge (2.45 GHz) of a surfaguide type is produced in a quartz tube (inner diameter 5 mm) either in Ar - BCl3 gas mixture (process 1) or in pure argon (process 2). In process 2, an Ar - BCl3 gas mixture was introduced downstream of the pure argon plasma. The first method on B-atoms was applied to a remote CVD Ar - BCl3 microwave plasma. At the Ar - l. 5%BCl3 plasma end, the B-atom density nearly reach 1015 cm-3, which corresponds to a BCl3 dissociation degree of about 30%. It was observed the reactions of B-atoms on O2 desorbing from the tube wall after B-atoms collisions. The second optical spectroscopy is the resonant absorption applied to Ce - atoms produced in a DC magnetron sputtering of a Ce cathode. In pure Ar plasmas, the Ce density was 1.5 108 cm-3. By introducing O2 in the Ar plasma, it was observed a hysteresis effect of Ce density as for the plasma voltage.
Density of N\(_2\) Active Species in N\(_2\) Dielectric Barrier Discharge Afterglows by Optical Spectroscopy at Atmospheric Gas Pressure
Dielectric Barrier Discharges (DBD) have been used to develop Cold Atmospheric Plasmas and more specifically nitrogen afterglows. The DBD afterglow has been investigated by emission spectroscopy from the VUV (200-300 nm) up to the near IR (1 \(\mu\)m) ranges. A kinetics scheme allowed to determine numerous N2 active species densities. The obtained densities decreased slowly in the afterglow time of (1- 10)10-3s with mean values of 1015 cm-3 for N- atoms, 1012 cm-3 for N2(A) molecules, 1011 cm-3 for O- atoms (coming from air in impurity) and 1010 cm-3 for N2(B,0) radiative molecules. The other excited species (N2(B,11), N2 (X,v>4), NO(X, A, B) and O1S.N2 were found at a lower density between 106 and 108 cm-3.
Examining Microwave Afterglows in N\(_2\) Gas Mixtures Using Spectroscopy and LIF: Application to Surface Cleaning
Variations of N, H, O and C-atoms density have been determined along the reduced pressure flowing afterglows of microwave Ar-N2-H2 discharges that have been experimented with in Plasmas Labs of Nancy, Montreal, Orsay and Toulouse. In the present chapter, previous experiments in N2-O2, N2 - H2 and N2 - CH4 fulfilled by the LSGS(Nancy), Plasmas Lab (Montreal), LPGP (Orsay), Laplace (Toulouse) and SNPE (Bordeaux) are reported. The active species density is obtained by NO titration for N and O-atoms and by line intensity ratios for the H and C-atoms. A small maximum of N-atom density in the range (0.5-3) 1015cm-3 was observed when a few H2 or CH4 was added to N2 and the C/N ratio was about 10-3 in the afterglow of studied microwave setups. The results obtained by LIF measurements in Orsay and Toulouse Labs are reported for N and H -atoms. By comparing the TALIF and the results of H-atoms density obtained from OES in N2-1%H2 afterglows, it has been chosen the appropriate rate coefficient of the N+H+N2 reaction. The surface cleaning by N and O-atoms has been experimented.
Characterization of Active Species in Microwave Discharge Afterglows of N\(_2\)-H\(_2\), Ar-N\(_2\)-H\(_2\), and Ar-N\(_2\)-O\(_2\) Gas Mixtures
This study investigates the densities of active species, including N and O atoms, N\(_2\)(A), N\(_2\)(X, v > 13) metastable molecules, and N\(_2^+\) ions, in the afterglows of N\(_2\)-H\(_2\), Ar-N\(_2\)-H\(_2\), and Ar-N\(_2\)-O\(_2\) microwave discharges. Optical emission spectroscopy and NO titration methods were used for precise calibration. The results reveal significant variations in active species densities across different gas mixtures, providing insights into optimizing plasma conditions. The -atom density is in the range of 1015cm-3 in N2 -H2, Ar - N2 and Ar - N2 - H2. It is lower by a factor of 4 in Ar - N2 - O2 gas mixtures. The -atom density coming from O2 impurity in the N2 - xH2 discharges decrease with x from 7 1013 cm-3 for x=0 to 3 1013cm-3 for x 2.5%. A strong O2 dissociation was found in the Ar-x(N2-5%O2) gas mixtures, decreasing from a full dissociation [0] = 2[O2] at x 20% to a half dissociation ([O]=[O2]) at x 50%.
Characterization of N and O-atoms, of N\(_2\)(A) and N\(_2\)(X,v>13) Metastable Molecules and N\(_2\)+ ion Densities in Ar-N\(_2\) Afterglows Plasmas from Microwave Discharges
The findings underscore the potential of N2(A) and N2+ species to enable sustainable and efficient surface treatments with applications ranging from medical devices to advanced material coatings. Afterglow jets of Ar-N2 flowing microwave discharges are characterized by optical emission spectroscopy. The N and O atoms, the N2(A) and N2(X,V>13) metastable molecules and N2+ ion densities are determined after calibration by NO titration for N and O-atoms and measurements of NO and N2 band intensities. For an Ar-xN2 gas mixture with x increasing from 2 to 100% at 4 Torr, 100 Watt and an afterglow time of 310-3 s at the 5-liter reactor inlet, it is found densities in the ranges of (2-6)1014cm-3 for N - atoms, one order of magnitude lower for N2(X,v>13) and for O - atoms (coming from air impurity), of 1010 - 1011 cm-3 for N2(A) and of 108 - 109cm-3 for N2+.
Measuring the Density of N\(_2\) Active Species in N\(_2\) –xCH\(_4\) Afterglows with x= (0-7.5)10\(^{-4}\) by Mass Spectrometry and Optical Spectroscopy at Low Gas Pressure
The density of N-atoms in N2 pink and late microwave flowing afterglows has been investigated by mass spectrometry and NO titration. By optical spectroscopy, it has been obtained after NO titration the density of N2(B,11), N2(C,0) and N2+(B,0)) radiative states and of N2(A), N2+ metastable and ionic molecules. By introducing a few CH4 into N2, the density of C-atoms added to N-atoms is measured in the pink afterglow. The N2 pink afterglow was observed when the N2(B,v’=16-18) levels appeared after the autodissociative N2(B,v’=13-15) states, contrary to the late afterglow where the N2(B,v’=16-18) levels could not be detected.
Effect of O\(_2\) into N\(_2\) Afterglows in RF and Corona Discharges
The afterglows in N2-O2 gas mixtures have been previously described: N - atoms titration by NO[A-1], oxygen in impurity in N2 at reduced gas pressure and at atmospheric gas pressure. The and O-atoms density versus the O2 percent from 1 to 4% into N2 in HF flowing afterglow (at 7 Torr, 0.5 slpm, 150w) was strongly decreasing for N-atoms from 15 to 21014cm-3 and kept a nearly constant value for O-atoms density: 7-9 1014cm-3, a value which is higher than the N-atom density in the range 2 - 10% O2 into N2. In RF afterglows, it was studied the effect of O-atoms and O2 molecules when O2 was introduced into N2 before or after the plasma (8 Torr, 1 slpm, 100 W ). It is concluded that a quenching of N2 active species by O-atoms more effective than with O2 in the early and late afterglows. At N2 the atmospheric gas pressure of Corona Plasmas with O2 impurity, it has been measured the density of the N2O1S excimer equal to about 1012cm-3, largely less than the N and N2 (A) densities: (5-7) 1014cm-3 .
Determination of N-atom Density in High Ar- N\(_2\) Gas Pressure Microwave Afterglow
This chapter is devoted to the production of N-atoms and other N2 active species in Ar2 - N2 flowing afterglows of microwave discharges at high gas pressure from 100 to 760 Torr (atmospheric gas pressure). The determination of N -atoms density in high-pressure Ar-N2 microwave afterglow is revisited. First, it is observed that pure pink afterglows are mainly obtained at high pressures up to atmospheric gas pressure, which is an aN+N factor of N+N recombination near 0. Second, there is an uncertainty in the range k=10-11-10-12cm3 s-1 on the k-rate coefficient of the reaction N2,v>13+N2(A) \(\rightarrow\) N2(B,v')+N2 which dominates the pink afterglow An enhancement of the N2(B,v'=8)/N2(B,11) density ratio is observed up to the atmospheric gas pressure. It was interpreted by a reduction of the quenching of the N2(B,v'=8) by Ar. Values of the N+N recombination rates were determined when aN+N>0.1. It was found a large uncertainty depending on the N2(B,v'=11) quenching rates found in the literature. It is estimated a decrease in the N+N recombination rates by 0.6 in the Ar-2% N2 in comparison to pure N2 afterglow. The determination of N-atoms density in high-pressure Ar- xN2 microwave afterglow is revisited. First, it is confirmed that pure pink afterglows are mainly observed at high pressures up to atmospheric gas pressure when x <10%, that is aN+N < 0.1.
Quantitative Evaluation of the Densities of Ar-N\(_2\) and N\(_2\) Flowing Afterglows at Atmospheric Gas Pressure
Recombination of N- atoms in N2 flowing afterglows is well characterized by optical emission of the N2, positive yellow band at 580 nm which is considered as a signature of N-atoms in the afterglow at reduced and high gas pressures. Production of N2(B,v') states are analyzed from the N2, positive band intensity in Ar-N2 and N2 afterglows of microwave plasmas and in N2 Corona discharges. In Ar-N2 afterglows of microwave discharges, the N2(B,v') distribution depends on the N2 fraction in Ar. In pure N2 afterglows, it is found that the N2(B,v'=11) state is enhanced by the N+N atom recombination at pressure up to the atmospheric gas pressure. The other N2(B,v'>0) states are also produced in N2 and Ar-N2 Corona and microwave afterglows by the N+N atom recombination, via the N2(A3\(\Sigma^+u,w^3\Delta u\))attractive curves. It appears that the relaxation of vibrational levels of N2(A) and N2(w) states at the benefit of the N2(B,v'=0) state is enhanced in Corona as in Dielectric Barrier Discharge (DBD) afterglows.
N and C-atoms in R (Ar)-N\(_2\)-CH\(_4\) Flowing Afterglows at High Pressure
The plasma and afterglows of N2 and Ar-N2 HF discharges are presently studied to precise the N-atom production in afterglow conditions at high gas pressure (up to the atmospheric gas pressure). The production of N-atom density has been analyzed in Ar-N2 high-pressure flowing afterglows after HF plasmas at a frequency from 13.6 Mhz to 2450 MHz. The plasma length increased when the HF frequency decreased from 2450 to 13.6 MHz and by lowering x in the Ar-xN2 gas mixture. It was found that the N+N recombination on the N2(B,v’) states was reduced by 2.5 for v’=11, by 10 for v’=8 and by 30 for v’=0 by going from pure N2 to Ar-2%N2. The production of N and C-atoms in a double Ar-N2 and Ar-CH4 HF flowing afterglows was in the ranges of 1015cm-3 and 1012cm-3, respectively. In Ar-N2-xCH4 with x=10-3 – 10-2 there was a drop of N and C-atoms at x=10-2 which should result in N+CH3 reactions. The CH3 density was about 4 1011 cm-3 in an Ar-17%N2-1%CH4 gas mixture at 14 Torr.
Effect of HF Wave Frequency on N-atom Production in N\(_2\) HF Plasmas and Its Application in CN Thin Films Deposition
Microwave plasmas are produced in many chapters by launchers named Surfatron at 2450 MHz to form so-called Surface Wave (SW) plasmas. Another plasma RF (13.56 MHz) structure was compared to Surfatron launchers with application to TiO2 nitriding. Variations of N atom density have been measured by emission spectroscopy, calibrated using NO titration, in the N2 late and pink flowing afterglows as a function of the wave frequency f used to generate Surface Wave (SW) plasma. It is found that [N] increases with the absorbed power PA but saturates above a threshold power which decreases with f. At 2450 MHz and a constant pressure of 4 Torr, [N] reaches 1.5 x 1015 cm -3 in the saturation regime while a saturated value of only 7 x10 14 cm -3 is attained at 13.56 MHz. The density variations of [N] as a function of f and PA correlate in the increasing region with that of the vibrational temperature as a result of electron collisions and in the saturated region with the gas temperature when it exceeds 500-600 K. Finally, [N] increases with pressure from 2 to 8 Torr (at constant PA of 100 W) and decreases with the tube diameter (6 to 15 mm), at constant pressure of 4 Torr. These results show that the highest N-atom density is obtained at 2450 MHz. A source of N-atoms produced by such N2 afterglows was combined with laser ablation of graphite for CNX thin film deposition. The XPS and IR analysis of the films have shown that the presence of nitrogen atoms in the post-discharge enhances the incorporation of N into the CNx layers. The N incorporation increases as pressure is increased through the formation of hydrogenated carbon nitride groups while at low pressure of about 10-3Torr, unhydrogenated sp2 and sp3 CN bonds are produced in amorphous forms. The study concluded that the highest [N] value was obtained at 2450 MHz and that the power efficiency was also the highest at this same frequency.
The Iron Nitriding in N\(_2\) and N\(_2\)-H\(_2\) Afterglows
The iron nitriding which allows a hard coating process has been studied in microwave plasma afterglows in Ar-N2, Ar- N2-H2 and Ar-N2-CH4 gas mixtures by optical spectroscopy. The thickness of \(\epsilon\) and \(\gamma\)' layers have been measured by micrography. It is shown that the introduction of H2 and CH4 into N2 removed the native oxide layers and allowed the growth of \(\epsilon\) - layers. To avoid film damage by the plasma high-energy electrons, remote microwave plasmas have given satisfactory results. An iron nitriding with \(\epsilon\) - layers thickness of 10 \(\mu\)m has been obtained in an industrial scale reactor of 90 liters with Ar-N2-H2 gas mixtures heated at 600oC for 2 hours.
A Detailed Comparison between RF and Microwave Afterglows in N\(_2\) Gas Mixtures for Surface Nitriding of TiO\(_2\) Films
It is presently compared to the afterglows after microwave plasmas in Toulouse Univ. and after RF plasmas in Ajou Univ. The synthesis of results after a microwave plasma is detailed in where the densities of N, H and C- atoms are reported in N2 - H2 and N2 - CH4. The applications of the HF and RF afterglows are for TiO2 surface nitriding. We report a detailed comparison between RF and microwave (HF) plasmas of N2 as well as N2 - H2 and N2 - CH4 in the corresponding afterglows by comparing densities of active species at nearly the same discharge conditions of tube diameter (5-6 mm), gas pressure (6-8 Torr), flow rate (0.6-1.0 slpm) and applied power (50-150 W). The analysis reveals an interesting difference between the two cases; the length of the RF plasma (25 cm) is measured to be much longer than that of HF (6 cm). In RF, with a N2 flow rate Q = 1 slpm, a pressure p = 8 Torr and an incident power of 100 Watt, a pink afterglow extends on 15 cm in the 21 mm i.d. tube (from z = 32 cm to z = 47 cm), followed by a late afterglow (after z = 48 cm). In HF, a pink afterglow is observed in the bent part of the 18 mm tube with an N2 flow rate of 0.5 slpm, a pressure p = 5 Torr and an incident power of 100 Watt. The late afterglow extends up to the 5 liter reactor with a characteristic yellow color. This yellow color is the signature of N-atom recombination on the N2(B,11) radiative state as described in the following. This ensures a much longer residence time (10-2s) of the active species in the N2 RF plasma [compared to that (10-3s) of HF], providing a condition for an efficient vibrational excitation of N2(X, v), making the RF plasma more vibrationally excited than the HF one. As a result of high VV plasma excitation in RF, the densities of the vibrationally excited N2(X, v > 13) molecules are higher in the RF afterglow than in the HF afterglow. Destruction of N2(X, v) on the quartz tube wall is estimated to be very similar between the two systems as can be inferred from the \(\gamma\)v destruction probability of N2(X, v > 13) on the tube wall: (2- 3) 10-3 for both cases, obtained from a comparison between the density of N2(X, v > 13) along the afterglows. Interestingly enough, densities of N-atoms and N2(A) metastable molecules in the afterglow regions, however, are measured to be very similar to each other. The measured lower density of N2+ ions than expected in the HF afterglow is rationalized from a high oxygen impurity in the HF setup since N2+ ions are very sensitive to oxygen impurity. In the N2 - H2 studied gas mixtures, the N2(X, v > 13) molecules were more destroyed in RF than in HF and inversely, the NH radicals and H atoms are more populated. The H-atom density was corrected from the N+H+N2 rate coefficient as determined in Chapter 13: k=10-33cm6s-1. The TiO2 surface nitriding in RF was compared to HF at room gas temperature (RT). First, it produced the most rich nitriding layers with a ratio N/Ti of 0.24 in the RF N2 late afterglow which was attributed to a low O-impurity in RF (a few ppm), compared to several 102 ppm in HF. Second, in the N2 - H2 studied gas mixtures, the high H-atoms density appeared to be at the detriment of the nitriding layer with a N/Ti ratio reduced to a few percent in RF as for the HF case. In both RF and HF afterglows, the NH inclusion came with that of N. In progress, a new HF reactor in Toulouse with reduced O-atom impurity (a few ppm) to increase the N/Ti ratio as for the RF Ajou reactor.
N, H and C-atoms Concentrations in Flowing Afterglows of Microwave R/N\(_2\)-H\(_2\) and R/N\(_2\)-CH\(_4\) Discharges with R=N\(_2\), He, Ar and Their Applications to TiO\(_2\) Surface Nitriding
Flowing afterglow plasmas have found their interest in soft surface treatments of sensitive materials in life science as the sterilization of medical instruments. Variations of N,H and C-atoms density has been determined along the reduced pressure flowing afterglows of microwave R/N2-H2 and R/N2-CH4 discharges with R=N2, He and Ar. Density of H and C-atoms and other nitrogen-active species such as N2(A), N2(X,v>13),N2+,NH,N(2D), CN were obtained from that of N-atoms calibrated by NO titration, using the method of band intensity ratios in several conditions (between early and late afterglows). It has been obtained the density of O-atoms and NO molecules coming from air impurities. It has been deduced the wall destruction probability of H,O and C-atoms on the quartz afterglow tube: \(\gamma\)HR,N2=10-3,\(\gamma\)oR,N2=(0.4-1) 10-3 and \(\gamma_C^{N2}\)=(0.7)10-3. The effects of H and C-atoms on N-atoms inclusion inside TiO2 surfaces are reported.
Determining the Effect of Afterglows in N2 Gas Mixtures for Sterilization
Chapter 25 compiles and summarizes the main findings of our earlier works investigating the correlations between the density of active species of N2 and N2-O2 afterglows and the characteristics of different treated surfaces for the sterilization of bacteria. The sterilizing properties of cold plasmas have been demonstrated in the last decades and have raised a wide interest. The employ of N2 microwave afterglows provide a mild platform for the sterilization of thermosensitive and delicate objects, and have allowed the transfer of this technology to the industry by the development of an industrial reactor. It is the aim of this work to compile and summarize the most relevant findings regarding this topic and provide the reader of the most suitable conditions for sterilization of contaminated material, and the outreach of this technique. In particular, the sterilization of E -coli bacteria by N2 microwave flowing afterglows was obtained at 5 Torr, 1slpm, 100w in a treatment time of 40 minutes by heating the bacteria holder at 60oC, and following this promising result, an industrial reactor (Plasmalyse) of 100 liters built by the Satelec company has allowed the sterilization of B-Stearo spores in 30 minutes at 80oC with N2 at 4 Torr, 1 slpm, 300 W. Testing a more complex configuration, the transmission of N -atoms through hollow tubes, interesting the sterilization of endoscopes, was obtained in polyamide tubes of int.dia. 1.5 mm and 50 cm length, by pulsing the plasma gas: pulse 1 s - period 2 s, pressure 0.9 - 2.3 Torr, Q = 1 slpm and 200 W: TN= 3.5%. When increasing the int.dia.to 3 mm and the length to 80 cm, the transmission of N -atoms was TN = 8.5% in the continuous discharge (4 Torr in the 5 liter reactor, 1 slpm and 150 W). It is deduced a destruction probability of the N-atoms on the tube wall: \(\gamma\)N = 1.610-3. With stainless tubes of int.dia. 1.5 mm and length 6.5 cm, it was obtained TN = 1.3% and \(\gamma\)N = 1.610-2.
Plasmalyse - A Novel Plasma Sterilization System for Dental and Medical Applications
Reactors for medical instrument sterilization were studied by starting from 5-liters to 17-liters volumes for dental applications. The first challenge was to find low-cost elements to obtain a prize in the same order as a commercial autoclave. For that, a microwave source from a commercial microwave oven and the small vacuum pump of the autoclave were tested to build up a new plasma sterilizator, named Plasmalyse. Sterilization of 6 Log (10-6 spores survivors) was obtained after a treatment of 7 minutes at 100°C and 120 minutes at 30°C in comparison to 40 minutes at 105°C with the commercial autoclave. After that, Plasmalyse was transformed for the Hospital application, with a 135 liters reactor, connected to 4 microwave sources and with an efficient vacuum pump, to obtain a 6 Log spores inactivation at 60°C in a treatment time of 20 minutes. Moreover, the N-atom transmission and E-coli inactivation through hollow tubes were studied.
Treatment of Textiles via Plasma Afterglow
The use of textiles for technical applications in plasma processes can be of significant interest. Plasma treatment is an environmentally friendly process for sustainable surface modification of textiles. In this work, three different uses which are complementary and allow to explain basic surface phenomena related to the action of cold plasmas in the afterglow on the textile fibers are compiled. In particular, this chapter reproduces the results obtained on chosen fabrics treated in N2 and O2 afterglows where the N and O-atoms are the dominant active species employed to develop process indicators and membranes for sterilisation processes. In this sense, wool fabrics are employed to verify the transmission of N atoms with regard to the modification of materials contained within sterilisation pouches. The hydrophilicity of the wool was found to change from hydrophobic to hydrophilic after 2 minutes of N2 afterglow treatment at 4 Torr, 1 slpm, 100 watts as described in [1]. By comparing plasma and afterglow treatment, it has been remarked that, in N2 afterglows, only the active atoms as N-atoms (and O-atoms in impurity) treated the wool. In the development of sterilisation indicators, cotton yarns knitted with copper wire, and containing a thermocromic dye, revealed a clear colour change from white to magenta after being treated in the same afterglow conditions for 40 minutes. In the last part of this work, wool fabrics have been used to investigate the effects of N and O atoms from N2 and O2 afterglows on conferring them with shrink-resistant properties.