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N2 gaseous mixtures
Accepted Manuscript
Technical Characteristics of a DC Plasma Jet with Ar/N2 and O2 /N2 Gaseous Mixtures A. Barkhordari , A. Ganjovi PII: DOI: Reference:
S0577-9073(18)30105-9 https://doi.org/10.1016/j.cjph.2018.10.017 CJPH 675
To appear in:
Chinese Journal of Physics
Received date: Revised date: Accepted date:
19 January 2018 20 October 2018 20 October 2018
Please cite this article as: A. Barkhordari , A. Ganjovi , Technical Characteristics of a DC Plasma Jet with Ar/N2 and O2 /N2 Gaseous Mixtures, Chinese Journal of Physics (2018), doi: https://doi.org/10.1016/j.cjph.2018.10.017
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ACCEPTED MANUSCRIPT Highlights A DC plasma jet is studied using the optical emission spectroscopy technique.
Its electron density, dissociation rate and all the temperatures are calculated.
At the higher Ar and O2 amounts in both mixtures, all the temperatures are higher.
The electron density is higher at the higher Ar amounts in Ar/N2 gas mixtures.
All the plasma parameters are increased at the larger electrical currents.
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Technical Characteristics of a DC Plasma Jet with Ar/N2 and O2/N2 Gaseous Mixtures A. Barkhordari and A. Ganjovi Laser Research Department, Photonics Research Institute, Institute of Science and High Technology and Environmental Sciences, Graduate University of Advanced Technology, Kerman, Iran Email: [email protected]
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Keyword: DC Plasma Jet, Ar/N2 and O2/N2 Gaseous Mixtures, Physical Properties.
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Abstract: In this paper, the optical emission spectroscopy technique is used to examine the physical properties and technical characteristics of the plasma discharge in a fabricated DC plasma jet. The background gas is nitrogen molecular gas which is mixed with argon and oxygen gases at various percentages. The emission spectra are analyzed and, the plasma density, rotational, vibrational and excitation temperatures are obtained for the formed plasma discharges with Ar/N2 and O2/N2 mixtures. Moreover, the NO and OH lines from , excitation transitions are observed. It is shown that, at the higher argon and oxygen percentages in both the Ar/N2 and O2/N2 gaseous mixture, the emission intensities of argon ions and oxygen atoms are increased. In addition, at the higher Ar and O2 contributions in both the gaseous mixtures, while the rotational, vibrational and excitation temperatures are higher, they are decreased at the O2/N2 gaseous mixture. Moreover, the plasma electron density reduces for O2/N2 and increases for Ar/N2 gaseous mixture. On the other hand, for both the mixtures, the plasma density, rotational, vibrational and excitation temperatures are increased at the higher DC power source currents. Furthermore, it is seen that the dissociation rate increases at the higher DC currents and Ar and O2 percentages in both the mixtures. However, dissociation rate in the formed plasma jet with Ar/N2 gaseous mixture is higher.
1. Introduction
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Owing to the applications of nitrogen plasma discharges in various technological fields, there is a growing interest in studying and characterizing of the nitrogen plasma sources [1-5]. So far, a wide variety of plasma excitation schemes in a broad frequency range, i.e. through DC, low frequency pulsed DC and RF up to microwaves are spanned. On the other hand, the DC plasma jets are the well-known DC powered discharges having the capability of generating the intense plasmas with sufficient densities. Such systems are able to create the remote plasmas with a very homogeneous density distribution. Henriques et al. reported the performed experiments by a plasma jet with different mixtures of Ar/N2 which is produced by a travelling surface wave with the background pressure of about 66–266Pa [6]. Hong et al. developed an AC Atmospheric Pressure Micro-Hollow (APMJ) jet operating at 20 kHz with N2 gas [7]. Generation of long plasma jets of up to 6.5cm operating at 6.3L/min and gas velocity of 535m/s was reported. Moreover, they fabricated an AC APMJ device operating at 20kHz with nitrogen molecular gas [8]. Furthermore, an atmospheric pressure N2 plasma jet was generated from micro-discharges in a porous solid dielectric material [9]. A plasma jet plume with a length of 42mm was produced by feeding nitrogen gas through a porous alumina installed between an outer electrode and a hollow inner electrode and, an AC high voltage which was on the electrodes. In addition, in a recent work, they modified their previous plasma jet device and incorporated the porous alumina at the exit nozzle. Besides, Giuliani et al. used the same experimental configuration with the flow rate of about 10L/min (power of 5–13kV) of air [10]. Additionally, Kim et al. manufactured a similar plasma jet working with the applied frequency of 20 kHz with N2 as feed gas [11]. Blajan et al. studied the emission spectrum of a developed micro-plasma operating at atmospheric pressure non-thermal plasma [12]. The input power was supplied by a high voltage pulsed power source and, nitrogen and argon were used as operating gases. They observed Ar I, OH, N2 Second Positive System, First Negative Systems, and N2 First Positive System peaks in the Ar/N2 gaseous mixture. Lifetime emission signals for N2 Second Positive System for the micro-plasma discharge with 5%N2 in Ar/N2 mixture was around 1µs. Moreover, the rotational and vibrational temperatures were calculated. Qayyum et al. reported the Optical Emission Spectroscopic (OES) features of a formed plasma discharge with the Ar/N2 gaseous mixture with a DC power in a parallel plate configuration of electrodes [13]. The input power was varied between 175–225W and, the filling pressure was kept between 7–9mbar. Their findings shows that the molecular nitrogen emission lines intensity and plasma temperature will increase at the higher argon percentages in the mixture. Bousquet et al. used the OES method to examine the physical characteristics of Ar/N2 plasma discharge in a reactive magnetron sputtering [14]. It was observed that the electron temperature reduces at the higher Ar contributions in the mixture. Using the same OES measurement technique, Kilianova et al. determined the temperature of different species in a discharge plasma medium which was created by helical cavity coupled RF with the Ar/N2 gaseous mixture [15]. The argon percentage in Ar/N2 mixture was varied from 0% to 90%. It was observed that, at the lower argon percentages in the mixtures, the electron temperature decreases. Moreover, a direct relation between the plasma emission intensity and electron temperature was reported. Ohata studied the spectroscopic characteristics and capability of inductively coupled plasma (ICP) with Ar/N2 gas mixture using an axially viewing OES technique [16]. The emission intensity and excitation temperature was measured. Additionally, it was observed that the electron temperature increases at the higher nitrogen gas flow rates. The supersonic jet luminescence spectroscopy of nitrogen clusters (containing nearly 100 molecules per cluster) and Ar/N2 clusters (250 and 400 particles per cluster) was analyzed by Doronin et al. [17]. For the Ar/N2 clusters, it was observed that some transitions to various vibrational levels of the ground state from 2
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single N2 molecules in an argon gaseous environment. Moreover, it was found that the atomic nitrogen ions emission lie between 140 to 220nm [17]. Mohamed et al. [18] studied a micro-plasma jet in the atmospheric pressure molecular gases (nitrogen, oxygen and air). The plasma was generated by blowing the gases through DC Micro-Hollow Cathode Discharges (MHCDs). The applied voltage was sustained between 400-600V and, the maximum current was kept at about 25mA. It was shown that the gas temperature of the microplasma inside the micro-hollow cathode varies between ∼2000K and ∼1000K depending on the current and gas flow rate. Moreover, their finding shows that the temperature in the micro-plasma jet can be accurately controlled and, it is optimized through these parameters with respect to technological applications. Moreover, Schmiedt et al. [19] studied the DC glow discharge at the atmospheric pressures in both the pure oxygen gas and oxygen–nitrogen gaseous mixture (99%O2, 1%N2). The discharge current range was set between 10mA to 40mA. Furthermore, Haraki et al. [20] studied the oxygen radical formation in a DC low pressure plasma discharge system by O2/N2 gaseous mixture. In this paper, using the OES method, the production of active species at different Ar/N 2 and O2/N2 gaseous mixtures in a DC plasma jet is studied. From the emission spectrum of at the wavelength of 391.44nm and using the Gardet model, the rotational temperature is computed. The excitation and vibrational temperatures are evaluated from the spectral emission line intensities of Ar I and O atoms and three vibrational bands i. e., , using the Boltzmann plot method, respectively. Moreover, the plasma electron density and dissociation rates for both the mixtures are compared.
2. Experimental method
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A schematic diagram for the developed experimental setup is shown in figure 1. As seen, a tungsten needle (powered) electrode with 1.6mm diameter is placed in a quartz tube with 6 and 8mm inner and outer diameters, respectively. A stainless steel with a small hole (2mm diameter) at the bottom end (grounded) is placed at the tip of the tube. The spatial distance between the powered electrodes is kept at 1cm. A three dimensional schematic representation of the developed DC plasma jet is shown in figure 2. An electrical discharge is formed between the electrodes by applying high-voltage DC power (0-7kV, 0-50mA voltage and current variations, respectively) through a blocking resistor, R1(1MΩ). By introducing the mixed gas (Ar + N2 and O2 + N2) into the tube, the plasma and gas are spewed out from the nozzle, producing a plasma torch at the atmospheric pressure.
Figure 1. The schematic representation of experimental setup
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Generally, the spectroscopic methods are used to determinate the most important parameters in the plasma discharge media such as plasma temperatures or reactive species concentration. On the other hand, the molecular gas plasma discharges are characterized by the rotational, vibration and excitation temperatures. It must be noted that, the rotational temperature corresponds to the gas temperature. Moreover, the vibrational temperature is important, since the vibrational transitions are the main energy reservoirs. Moreover, the excitation temperature corresponds to energy of electrons, which are the main sources for the molecular and atomic stimulations. In addition, it affects the plasma reactivity via generating the active species via the inelastic collisions. The electron density measurement is important for quantifying the importance of electron dissociation and thermal dissociation of the molecular gases as a function of plasma discharge operating conditions. However, the plasma electron frequency is much larger than the electron-neutral collision frequency. It must be noted that, the degree of dissociation is very important, since it shows that how much of molecules are decomposed to the reactive nitrogen species, such as N, particles. In this work, the manufactured spectrometer by OceanOptics is used. To calibrate this spectrometer, a HG-1 Mercury-Argon lamp is generally used. Its fiber probe was placed at 5mm away from the nozzle and, it is perpendicular to the DC plasma jet at a distance of 10mm. The OES analysis is carried out using a USB2000 OceanOptic spectrometer and SpectraSuite software. Moreover, it stores a full spectrum at every millisecond with a wavelength range sensitivity of 200–1100nm. The various mixtures of nitrogen with argon and oxygen are taken as the main carrier gas. The argon and oxygen are mixed at different percentages with the molecular nitrogen using a gas mixer.
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Figure 2. The 3D schematics of atmospheric pressure DC plasma jet, (1) Tungsten rod (powered electrode); that is sharpened at 300, (2) Gas inlet, (3) PMMA dielectric, (4) Quartz tube, (5) Stainless steel; grounded electrode.
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In this paper, the excitation, vibrational and rotational temperatures along with the electron density and dissociation degree of N2 molecules in the plume of the developed DC plasma jet are estimated by the OES technique. Here, the emission intensities of different emission lines originating from the transition between the levels of atomic and molecular gaseous are measured. The spectra obtained from the plume of the DC plasma jet are analyzed by the SpectraSuite software. The obtained results from the OES technique are used to calculate the plasma characteristics of DC plasma jet such as electron excitation, vibrational and rotational temperatures as well as electron density. In order to measure discharge parameters of the DC plasma jet, the intensity of molecular emission lines of the plasma discharge can be used to estimate the population of the nitrogen molecular electronic levels. It must be noted that, even when one of the four temperatures are not the same, the system has to be considered in non-equilibrium or in Partial Local Thermodynamic Equilibrium (PLTE). Tendero et al. noted that, when the plasma electron density and temperatures lie between and , respectively, the plasma discharge is in PLTE [21]. Moreover, Calzada et al. introduced a criterion to determine the plasma equilibrium conditions [22, 23]. The main hypothesis behind their theory was to consider the atoms as hydrogenic ions with core z ( for neutral atoms; for single charged ions, etc.). However, the results can be applied to the more complex systems such as He, Ar, etc. Moreover, a Maxwellian distribution function was considered for the electron energy. For a hydrogenic ion, each level will be determined by an effective principal quantum number as follows: Where is the Rydberg constant or hydrogen ionization energy and, index of the equilibrium separation for each level p is defined as follows: ⁄
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Where and are the real and Saha-Boltzmann equilibrium populations, respectively, and, Saha-Boltzamann population equals to: ⁄
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is the statistical weight. The
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Where is the ground state population of the plasma discharge species with charge core and, is its statistical weight. Moreover, is equal to the electron density which is obtained from the experimental data for the Stark broadening of the emission line (486.132nm). If for all levels, the ground level is included in the calculations and, the plasma discharge will be in the complete LTE. On the other hand, if such a parameter only equals to the unit for a group of levels, then the plasma discharge will be found in PLTE. Moreover, when , the levels will be overpopulated with respect to the Boltzmann equilibrium population (plasma is in the ionization stage). Finally, when , the levels will be underpopulated with respect to the SahaBoltzmann equilibrium population (plasma is in the recombination stage). Hence, with the obtained electron density from the experiment, , and the plasma discharge is in PLTE [22, 23]. The taken photographs from the developed DC plasma jet in the operational conditions with both Ar/N2 and O2/N2 gaseous mixtures in our laboratory are shown in figure 3. The operational parameters of DC plasma jet are fixed at V=2.5kV and I=40mA. It appears uniform to the human eye and, its length can even reach more than 30mm. The output gas from the gas mixture is injected to the DC plasma jet. It is worthy to mention that, for Ar/N2 mixture, Ar and N2 flow rates are about 15, 1-10 SLM (Standard Liters per Minute), respectively. On the other hand, for O2/N2 mixtures, owing to the difference in the oxygen and argon mass numbers, the flow rates of O2 and N2 are taken at about 10, 1-8 SLM, respectively. Here, the flow rates are controlled by a velocity flow controller.
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Figure 3. Photographs of the developed DC plasma jet in the operational conditions at (a, e) 80%, (b, f) 85% and (c, g) 90% percentages of argon and oxygen in the Ar/N2 and O2/N2 mixtures, respectively, with V=2.5kV and I=40mA.
It must be mentioned that, in this work, the operational features of the developed DC plasma jet are studied in two different working conditions: (1) The Applied DC voltage is fixed at 2.5kV and, the current is varied between 10-40mA and Ar/N2=4 and O2/N2=4, (2) The Ar/N2 and O2/N2 ratio are varied from 19 to 0.3 and 5.5 to 0.5, respectively, while the voltage and current are fixed at 2.5kV, 40mA.
3. Results and Discussion
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In the glow plasma discharges, electron impact process excites some of the plasma species to the higher states decaying and emitting photon at the characteristic wavelengths, which can be detected and analyzed by recording the emission spectrum. Thus, the different reactive species that are produced by the DC plasma jet with Ar/N2 and O2/N2 gaseous mixtures must be identified. Hence, to obtain its operational parameters, the OES technique is used to record the emission spectrum, as seen in figure 4. Four typical emission spectra are recorded at the DC plasma jet plume. These results are related to N2 which is mixed with 70%Ar and 90%Ar (see figure 4(a, c)) and with 70%O2 and 90%O2 (see figure 4(c, d)). The experiments are performed in an atmospheric pressure and DC power at 40mA and 2.5kV, and in the spectral range 200–950 nm. Moreover, the main features of these spectra lie in the UV region. There were NO emission lines from the electronic transitions. The formed NO radical was created from O2 and N2 that can be dissociated at the higher temperatures (>1600 K) or by the electron impact [40]. The NO γ band originated in the following collisions of the N2 metastable state [24, 25]: Excitation: ( ) Radiative transition: Additionally, the highly reactive radicals, i.e. OH emission band at the wavelength of 309nm are detected. It must be noted that, the OH radical formation is generally caused by water vapors molecules in the ambient air [26]. Where, these two distinct populations of excited state (OH) could be probably generated via the direct dissociation electron excitation of water vapor molecules:
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Where is formed by argon metastables and subsequent dissociative recombination in Ar/N2 and O2/N2 mixtures, respectively [27]. Moreover, the excited argon neutrals can dissociate H2O leading to the generation of OH radicals in Ar/N2 [28]:
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Furthermore, as is observable in figure 4, the emission spectra from the wavelength of 300 to wavelength of 450nm are dominated by the presence of Second Positive System N2 , the First Positive System and the First Negative System . These transitions are resulted from the many excitation processes such as electron impact excitation from the molecular ground state , the first metastable state , pooling reaction and transfer of energy between collisional partners [29, 30]. It is worthy to mention that, these long-lived metastable species, with energies below 6eV, are important as reservoirs of energy promoting plasma chemical reactions leading to the condensed products in the flue gas cleaning applications [26].
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Figure 4. Typical emission spectra of the developed DC plasma jet (corrected for the spectrometer’s spectral response) at V=2.5kV, I=45mA and, (a) 70%Ar, (b) 90%Ar in Ar/N2 mixture, (c) 70%O2, (d) 90%O2 in O2/N2 mixture
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( associative excitation, ( ) pooling reaction,
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In the nitrogen plasma discharges, the population of the radiative state N2 processes such as electron impact excitation from the molecular ground state ( ) ( ) first metastable state ,
the energy transfer between the collisional partners and Penning excitation [31-33]:
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Moreover, in the Ar/N2 mixture plasma discharges, the population of the N2 excited state may result from the transfer of the internal energy from a metastable state of argon atoms to the ground state of the nitrogen molecules [33]. For the metastable states of argon, i.e. , the subscript ‘m’ stands for metastables that are having the energies 11.55 and 11.72eV which are higher than the threshold excitation energy (11.1 eV) of nitrogen molecule. Therefore, adding Ar to N2 plasma discharges results in the significant increasing of the emission intensities. Thus, the concentration of the active species can be expected by Penning effect as follows [35]: ( ) from The subsequent radiative decaying processes would cause photon emission from (0–0) band of the Second Positive System at the wavelength of 336.1nm: ( ) As a result, the emission intensity of (0–0) band of Second Positive System is proportional to the population density of the N 2 state [36, 37]. 6
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In the Ar/N2 gas mixture, the
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In both mixtures, the excited state can be populated either by the direct electron impact excitation from the ground state of the ( ) molecule via [36, 37]: ( ) from or based on stepwise electron impact ionization of the N 2 molecule and, then, the subsequent electron impact excitation of the molecular ion [36, 37]: ( ) ( ) from The ( ) state can be further excited to radiative state by electron impact as follows: ( ) from at The subsequent radiative decays of excited state have the capability of emitting the characteristic photons of (0–0) band of First Negative System at wavelength of 391.4nm [36,37]: ( ) The emission intensity of the (0–0) band of the First Negative System is proportional to the population of the state [36, 37]. On the other hand, in the performed experiments with O2/N2 gaseous mixture, there are emission band in the 550–600nm wavelength range and atomic oxygen at the wavelength of 844.6nm . In addition, the strong emissions by O2 are found in the plume of developed plasma jet with the O2/N2 gas mixture as the carrier gas. The most intense emission of oxygen results from the excited neutral atoms and molecules, originating from the dissociative excitation as follow [31]:
excited state can be populated by either one-step process [38]:
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or via the two-step process,
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. For the low temperature plasma discharges (with the species energy below 4eV), there are very few electrons with the energies above 35eV. However, these energies are almost required for the simultaneous excitation and ionization process. Hence, there is a large contribution from the excitation of the ground state ions by two-step process. Indeed, the emission intensity of radiative state provides enough information on the ground state ion density [39]. As shown in figure 4(a, b), in the argon plasma discharge, high intensity peaks corresponding to Ar I peaks are related to the 4s–4p transition. These peaks are measured at the wavelengths of 696.5nm, 706.7nm, 727.3nm and 738.3nm; the transition 4p-6s was measured at 703.2 nm; the transition 4s-4p at 750.4 nm; and the transition 4s-4p at the wavelengths of 763.5 nm and 772.4 nm [40]. The mechanism to excite the 2p1 level of Ar is the one-step electron impact excitation from the ground state. When N2 gas is added, the peaks corresponding to the N2 First Positive System are measured at the wavelengths of 632.3nm, 670.5nm, 676.4nm, 678.6nm, 705.9nm, 716.5nm and 762.7nm [41]. Generally, in the plasma discharge media, the electrical behaviors are determined by the temporal variations of voltage and current using a high voltage probe (Tekteronix, P6015A) and a current probe (Tektronix, TCP202). A DC power supply with selectable polarity was connected in series to a ballast resistor and DC plasma jet. In this work, the power source was able to supply up to 1mA of current. Displacement current is created when a high voltage is applied to the separated electrodes. The discharge current refers to the motion of the charged particles. Therefore, when the plasma discharge is maintained, both displacement and discharge currents will exist. Turning off the plasma discharge, the discharge current will be eliminated. Hence, to determine the displacement and discharge currents, the current should be measured in both situations, i.e. when the plasma discharge is on and, when the plasma discharge is off. When, the plasma discharge is on, the current density can be written as follows: . On the other hand, when the plasma discharge is off, then , and hence, . It must be noted that, the discharge current was used to calculate the discharge voltage using the following formula: (4) Where Vd is the discharge voltage, Vps is the voltage of the power source, I and R b are the current and the used resistor ballast (1MΩ). Furthermore, a stainless steel wire with the diameter of 1.6mm was used as anode and, a stainless steel cylinder was used as cathode. The ballast resistor was connected close to the discharge media and, the circuit was kept small. Thus, the stray capacitance in the circuit is reduced. Hence, this circuit has the capability of operating in a stable mode. The cylindrical electrode is always connected to the ground. The pin electrode was connected to positive terminal of the power supply. Figure 5 shows the V-I characteristics of a positive corona in the developed DC plasma jet for an electrode distance of 10mm and %90 of argon and oxygen in the Ar/N2 and O2/N2 mixtures, respectively. As seen, at the similar discharge currents, the discharge voltage in O2/N2 mixture is higher than that in the Ar/N2 mixture. This is owing to the fact that, compared with argon atoms, the oxygen molecules require more energy to get ionized. It must be noted that, since the electric field in the proximity of the negative biased electrode (cathode cylinder) in a positive corona is not significant, there is no important avalanche increment. Moreover, the electron production depends on the secondary photoemission processes near the sharp tips [42].
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Figure 5. V –I characteristics for the positive corona discharges in the developed DC plasma jet at the similar ratios of Ar/N2 (black) and O2/N2 (blue).
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In this work, the peaks of excited hydrogen atoms and profiles and spectral emission lines broadening are used. In addition, the plume of developed DC plasma jet is generated in ambient air (N2, O2 and water vapor). Hence, the various emitting species could be detected from the emission spectra of the developed DC plasma jet with both the Ar/N2 and O2/N2 gaseous mixtures, such as Ar and N excited atoms, also , O, NO and OH molecular emission bands. On the other hand, determination of the kinetic temperature using the OES technique is based on temperature measurements which are determined from the energy levels of rotationally excited states. Generally, it is assumed that the kinetic temperature and rotational temperature of the ground state are in equilibrium. Moreover, the dipole-allowed transition lifetime of the excited levels are used to determine the rotational distribution. It must be noted that, this lifetime is much shorter than the collision time [43]. Normally, the plasma state is analyzed by the light radiated in the visible region, where the spectral measurements are relatively easy. The rotationally excited molecules frequently execute energy transfer by collision with the other gaseous molecules. As a result, the rotational temperature is nearly equal to the gas temperature. Thus, the gas temperature, Tg, can be determined from the rotational temperature, [44]. In this spectroscopic observation, the spectra of 2nd positive system bands for nitrogen molecule are observable. The rotational temperature can be measured by the rotational spectrum of at wavelength of 391.44nm ( transition), which is used at the high resolutions of the spectrometer. The R branch between the wavelengths of 389nm and 391nm can be exclusively determined. The R lines intensities, , are the function of the rotational temperature, [45]:
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Where is a constant and are the quantum numbers for the excited state ; h, c, and are Planck constant, speed of light, ⁄ Boltzmann and rotational constants, respectively. Here, with the constants is used. The slope of the curves ⁄ to gives the rotational temperature. This has been done with the R lines detecting in the wavelength range of 389–391nm [45]. In this work, the Gardet models are used to obtain the emission spectrum of at the wavelength of 391.44nm. These models are applicable for the low resolutions. In these models, three steps are performed: (1) determination of wavelength positions, , (2) intensity factors calculation, and (3) the so-calculated spectrum convolution with a linear combination of Gaussian and Lorentzian functions which must be used to describe rightly the apparatus function [46]. The number of molecules that are excited into a given vibrational state is defined by the vibrational quantum number v. Moreover, it is proportional to the vibrational energy (E v) by the Boltzmann law to exp(-Ev/kBT). The vibrational band intensity is written as follows [45]:
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(6) Where v′, v′′ is the vibrational quantum number of upper and lower states, respectively. A(v′v′′) is transition probability (can be found in tables and, it corresponds to the electron vibration state functions overlap) and, ν is wavenumber (mainly wavenumber of the band head). The expression for vibrational band intensity shows that the plot of Ln(I '''/4A(''')) versus must be linear. The vibrational temperature can be calculated from the slope of this straight line [45]. Figure 6 shows a typical Boltzmann plot of the relative intensity distributions. Considering the scattered data points and fitting errors, is estimated about 8560 K at 40mA, 2.5kV and 80%Ar in Ar/N2 mixture. The vibrational temperature ( ) of the DC plasma jet is determined by the Boltzmann plot method in the both gas mixtures. Three vibrational bands , , and are chosen to obtain the . The wavelengths of 349.9nm, 353.6nm, 357.6nm, 370.9nm, 375.4nm, 380.4nm, 394.2nm, 399.7nm, 405.8nm are considered for each vibrational transition. The other parameters have taken from ref. [47]. 8
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Figure 6. Typical Boltzmann plot of
vibrational distribution with the vibrational temperature of Tvib=8560K.
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It must be noted that, only for the thermodynamic equilibrium systems, the excitation temperature, Texc, is mostly equal to the electronic temperature, Te [48]. Moreover, in the non-equilibrium plasma discharges, [49, 50]. When one of them is not equal to the others, the system has to be considered in non-equilibrium or in PLTE conditions. In this work, the developed DC plasma jet, with both the Ar/N2 and O2/N2 gaseous mixtures, is in PLTE conditions. However, the Maxwell–Boltzmann equilibrium is established. Furthermore, the atomic emission line intensities of Ar I and O are determined in the wavelength range of 696-912nm of the spectral region. There are strong emission lines from the excited argon and oxygen species in both the Ar/N2 and O2/N2 gaseous mixtures at spectrum. They have reliable transition probabilities that are published in the literature. Assuming that the upper levels of the selected atomic transitions are in the PLTE conditions, the conventional Boltzmann plot technique can be used to obtain the excitation temperature in the plume of the developed DC plasma jet. The following relation defines the relative transition probabilities of two different emission lines [51, 52]: )
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Where I, Aul, gu, λul, Eu and k denotes the total intensity, transition probability, degeneracy of the upper level, wavelength, excitation energy and the Boltzmann constant, respectively. However, the model assumes that the electron collisions between excited atoms are dominant in their populating and depopulating processes. Similar to the vibrational temperature case, a Boltzmann plot based on the ⁄ equation (6) can be used to drive the excitation temperatures from the slope fittings. In this case, the is plotted versus the upper level electronic energy (Eu) for the selected lines of Ar I emission. The atomic parameters of Ar I emission lines are taken from the National Institute of Standards and Technology (NIST) database [59]. It is worthy to mention that the ionization cross sections and ionization potentials for the molecular nitrogen ( and ~15.7 eV) are approximately similar to the argon atom and, they are smaller than the oxygen molecules [53]. Hence, these parameters are not sufficient to explain the observed increase in electron temperature in the Ar/N 2 gaseous mixture. This might be due the difference in the electron energy distribution function (EEDF) and its consequent effects on the higher secondary electron yield at the target in the presence of Ar in the plasma discharge. Moreover, the cross section is a suitable parameter to describe the electron temperature in the O2/N2 gas mixture. On the other hand, the lifetime of absorbing species is longer than that of the emitting species ( ). Therefore, compared with ( ) molecules, metastable species can diffuse farther away from the plasma discharge media, such that their temperature is lower. For Ar/N2 gaseous mixtures, the corresponding values for the First Positive System are similar. Consequently, the argon amount will not significantly affect the gas temperature. The issue for the Second Positive System is quite different. Indeed, an efficient energy transfer between the nitrogen ground state and Ar metastable atoms could involve strong overpopulation of rotational emission lines. Hence, the rotational temperatures are much larger than those obtained from the First Positive System. Figure 7 presents the rotational, vibrational and excitation temperature of the DC plasma jet as a function of the discharge input power, argon and oxygen percentages in both the Ar/N2 and O2/N2 gas mixtures. As seen in figure 7(a), in the Ar/N2 mixture, at the higher DC electrical currents, all the temperatures are increased. This is owing to the fact that, at the higher input powers, owing to the higher electric fields in the discharge medium, the number of high energy electrons will increase in the tail of EEDF. Moreover, the Penning ionization involving argon metastable is higher. Figure 7(b) shows that the electron temperatures are increased at the higher Ar contributions in the mixture. This might be due to the lower electron collision cross section of argon atoms compared with the nitrogen molecules. Thus, the plasma electrons have enough time to get accelerated by the electric field and its consequent effects on the electrons energy. These energetic electrons have the capability of exciting and ionizing nitrogen molecules or generating argon metastable states. This metastable species, in turn, will collide with N2 molecules and excite or ionize them. Hence, each Penning ionization and excitation event will result in the argon neutral ground state atom together with the excitation and ionization of the target species. As seen in figure 7(c), all the temperatures are increased at the higher DC currents. At the higher DC power source currents, the input power to the discharge medium of plasma jet increases. This reflects in the higher electric fields in the discharge 9
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medium and, the plasma electrons will get the higher accelerations. Moreover, the higher intensity of electric fields will result in the higher kinetic energies (temperatures) of different species within the discharge medium of the DC plasma jet. In addition, as seen in figure 7(d), the rotational, vibrational and electronic temperatures are higher at the larger percentages of O 2 in the O2/N2 gaseous mixture. It must be noted that, the increasing of O2 contribution in the mixture will effectively enhance the energy transfer from electrons to molecular O2 via the excitation of rotational, vibrational and electronic levels of oxygen.
Figure 7. Evolution of the excitation, vibration and rotational temperatures as a function of (a, c) electrical current at 2.5kV and 80% Ar and O2 at considered mixtures, (b, d) Ar and O2 percentages at 40mA and 2.5kV.
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The Stark broadening analysis of the spectral profile of the Hβ (486.1nm) emission line which is emitted by the plasma discharge medium is the most used technique to calculate the electron density, ne [49, 55, 56]. If the selection rules for the hydrogen atomic transitions are taken into the account, then the allowed transitions (multipletes) can be related to the Hβ emission line [57]. These transitions lie between 2p–5s, 2s–5p and 2p–5d energy levels [58, 59]. The hydrogen Balmer series (Hβ) is usually used to obtain the plasma electron density without assuming the fine structure of the emission line and ion dynamics [60]. Several broadening mechanisms affect the emission line-shape in plasmas, natural broadening, Doppler broadening, pressure broadening, and Stark broadening [55]. While the natural broadening is usually not important, the Stark broadening ∆λS can be directly related to electron density in plasma discharge. Thus, it can be used for determination of plasma density, ne. On the other hand, the Full-Width at Half-Maximum (FWHM) is a parameter which is widely used to determine the broadened emission line profile. Then, the FWHM broadening values of Stark broadening that may affect the H β emission line can be obtained. The FWHM of Stark broadening is related to the electron density as follows [61]: ⁄ (8) Hence, the Stark broadening can be used to compute the electron density. Where the plasma electron density is in cm −3 unit [62]. Stark broadening at the wavelength of 486.1nm was considered for the Hβ. The Stark broadening is related to Lorentz and van der Waals broadenings using the following relation [62]: (9) Finally, the pressure broadening results from perturbation of the energy levels of the emitting atoms due to the presence of surrounding neutral species. Pressure broadening leads to a Lorentzian profile and, it is subdivided into resonance broadening (when emitters and perturbers are of the same type and either the upper or the lower state of the detected line is a resonance level), and van der Waals (vdw) broadening (when emitters are perturbed by neutrals of a foreign gas). In this work, the density of H atoms was extremely low. Hence, only the van der Waals broadening is considered. The FWHM of the vdw profile is written as follows [62, 63]: 10
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(10) Where p is the pressure in atmosphere and, T is the gas temperature in Kelvin which is equal to the rotational temperature. To find the Lorentz broadening, one could fit the experimental spectrum with Voigt profile which is the superposition of Gauss and Lorentz broadenings. Fitting process is performed using the Origin software and, the Lorentzian FWHM was measured. Thus, knowing Lorentz and van der Waals broadenings, Stark broadening is calculated. Hence, the electron density can be determined. Figure 8 shows the Lorentz broadening which is calculated via fitting of the experimental spectrum to Voigt profile ( ). Inserting the gas pressure of 1atm, temperature of 351K and %90Ar in Ar/N2 gas mixture, the van der Waals broadening is calculated as . Hence, the Stark broadening would be 0.125nm corresponding to the electron density .
Figure 8. Comparison of experimental data and Voigt profile with ∆λL=0.185nm and ∆λG=0.179nm.
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The variations of electron density as a function of DC power source current, Ar and O2 contributions in both the Ar/N2 and O2/N2 gaseous mixtures are shown in figure 9. As seen in figure 9(a), the electron density increases with the input DC current. However, this is owing to the fact that, at the higher DC electrical currents, the input power to the discharge medium of the DC plasma jet is higher. This reflects in the higher intensity of electric fields and, hence, the ionization rate of neutral species is increased. As seen, the electron density for the formed plasma discharge in Ar/N2 mixture is higher than that of the O2/N2 gaseous mixture. This might be due to the higher needful energy for ionization of the oxygen molecules compared with the argon atoms. Thus, the electron number density increases at the higher Ar percentages in the mixture, as depicted in figure 9(b). Consequently, the electron number density increases at the higher molecular nitrogen dissociation rates. Moreover, at the higher Ar contributions, the probability of Ar metastable generation increases. Hence, the production of will be enhanced. In Ar/N2 plasma discharge, the electron impact is the dominant source channel for dissociation process. Moreover, the higher Ar percentages results in the higher number of energetic particles in the tail of the electron energy distribution function, which in turn leads to the higher dissociation rates of N2 molecules [16]. Furthermore, this phenomenon reflects in the higher molecular dissociations. In addition, Figure 9(b) shows the electron density versus O2 percentage in the O2/N2 mixture. As seen, the higher O2 contributions in the mixture reflect in the lower plasma electron densities. It must be noted that, the effects of nitrogen in the mixture can be attributed to an extensive set of vibrational energy levels. Moreover, the nitrogen molecules can absorb a significant amount of energy from the plasma electrons. On the other hand, at the higher percentages of oxygen in the mixture, the collisions occurrence between the metastable nitrogen molecules and oxygen molecules will dissociate the oxygen molecules and, the density of oxygen radicals will increase considerably. Hence, the electron density will decrease due to oxygen electronegativity and, the production of the negative oxygen species will be higher.
Figure 9. Evolution of electron density as a function of (a) DC electrical current at 2.5kV, 80% Ar and O2 in both gaseous mixtures, (b) Ar and O2 percentages at 40mA, 2.5kV. 11
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It is worthy to mention that the dissociation degree is one of the significant parameters of molecular gas plasma which is used for different applications. A simple way to increase dissociation of molecular gas is to introduce another gas in the plasma. On the other hand, the actinometry is a well-known OES technique which has been used for the qualitative and sometimes quantitative determination of atomic densities in the plasma discharges. Here, argon in Ar/N2 mixture and oxygen in O2/N2 mixture play the role of actinometer. Moreover, the [N]/[N2] ratio is considered as the dissociation degree of nitrogen molecule. In the actinometry method, this ratio can be expressed as follows [64]: (11) (12)
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Where IN is the atomic nitrogen emission line intensity at the wavelength of 672.3nm and, IAr is the argon emission line intensity at the wavelength of 750.4nm and IO2 is the oxygen emission line intensity at wavelength of 777nm. Moreover, {[Ar]/[N2]} and {[O2]/[N2]} are the initial ratios between Ar and O2 to N2 percentages when the plasma discharge is off. In addition, , and are the spectral responses of the detection system which are related to the emissions of Ar line, N line and O2 line, respectively. Finally, C1 and are constants. They depend on the excitation coefficient rates, quenching rates, electronic temperature and spectroscopic data [11, 64]. They are not involved in the estimation of the [N]/[N 2] ratio, since all the needful data for their evaluation could not be found. Particularly, those concerning the excitation coefficient rates for the transition of atomic nitrogen. Therefore, the dissociation degree of N2 is determined qualitatively. In this work, the actinometry method is considered as an approximate method, since the argon and oxygen fractions in the mixtures are higher than 5%. Figure 10 shows the evolution of the N2 dissociation degree which is determined using the actinometry method, as a function of (a) DC electrical current, (b) Ar and O2 percentages in both the mixtures. As seen, for both mixtures, similar behaviors with various argon and oxygen percentages in the mixtures are shown. At atmospheric pressures, the dissociation degree increases at the higher argon and oxygen percentages. Due to the energy deposition into the discharge media, the dissociation degree of nitrogen is higher. However, the dissociation degree in Ar/N2 mixture is higher than that in the O2/N2 mixture, as shown in figure 10(a). Furthermore, while N2 dissociation is increased at the higher Ar contributions in the Ar/N2 mixture, it is reduced with O2 contribution for the O2/N2 mixture, as seen in figure 10(b). It must be noted that, owing to argon presence in the Ar/N2 mixture, the quenching reactions are more effective than that in the O2/N2 mixture.
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Figure 10. Evolution of N2 dissociation degree (actinometry method) as a function of (a) DC electrical current at 2.5kV, 80% Ar and O2 contributions , (b) argon and O2 percentages in Ar/N2 and O2/N2 gaseous mixture plasmas, respectively at 40mA and 2.5kV.
4. Conclusion
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In this work, using the OES technique, the physical and technical features of a developed DC plasma jet operating with Ar/N2 and O2/N2 gaseous mixtures are studied. The effects of Ar and O2 gases on the different features of plasma discharge and N2 dissociation process are compared. This experimental study was performed through the calculation of plasma density, rotational, vibrational and electronic temperatures for the formed plasma discharges with both the Ar/N2 and O2/N2 gaseous mixtures. The NO emission lines from the electronic transition were detected at both mixtures. The emission spectra due to the presence of N2 Second Positive System were seen to be dominant. It was shown that, at the higher argon and oxygen percentages in both mixture, the emission intensities from the argon ions and oxygen atoms are increased. In addition, while the rotational temperature with the DC current remains almost constant, both the vibrational and excitation temperatures will increase. Moreover, at the higher Ar and O2 contributions in each mixture, the vibrational and excitation temperatures are increased. It was observed that, while the plasma electrons density increases at the higher DC currents in the both mixtures, it is higher at the higher Ar percentages and lower O2 contributions in Ar/N2 and O2/N2 mixture, respectively. Furthermore, the dissociation fraction of nitrogen molecules increases at the higher DC currents, Ar and O2 percentages in the two mixtures. Finally, it was observed that, effect of atomic argon on the dissociation of molecular nitrogen is higher than the molecular oxygen. Thus, to obtain higher number of active species from the molecular nitrogen in the DC plasma jets, the Ar/N2 gaseous mixture is more suitable than O2/N2 mixture. 12
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Technical Characteristics of a DC Plasma Jet with Ar/N2 and O2 /N2 Gaseous Mixtures A. Barkhordari , A. Ganjovi PII: DOI: Reference:
S0577-9073(18)30105-9 https://doi.org/10.1016/j.cjph.2018.10.017 CJPH 675
To appear in:
Chinese Journal of Physics
Received date: Revised date: Accepted date:
19 January 2018 20 October 2018 20 October 2018
Please cite this article as: A. Barkhordari , A. Ganjovi , Technical Characteristics of a DC Plasma Jet with Ar/N2 and O2 /N2 Gaseous Mixtures, Chinese Journal of Physics (2018), doi: https://doi.org/10.1016/j.cjph.2018.10.017
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ACCEPTED MANUSCRIPT Highlights A DC plasma jet is studied using the optical emission spectroscopy technique.
Its electron density, dissociation rate and all the temperatures are calculated.
At the higher Ar and O2 amounts in both mixtures, all the temperatures are higher.
The electron density is higher at the higher Ar amounts in Ar/N2 gas mixtures.
All the plasma parameters are increased at the larger electrical currents.
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Technical Characteristics of a DC Plasma Jet with Ar/N2 and O2/N2 Gaseous Mixtures A. Barkhordari and A. Ganjovi Laser Research Department, Photonics Research Institute, Institute of Science and High Technology and Environmental Sciences, Graduate University of Advanced Technology, Kerman, Iran Email: [email protected]
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Keyword: DC Plasma Jet, Ar/N2 and O2/N2 Gaseous Mixtures, Physical Properties.
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Abstract: In this paper, the optical emission spectroscopy technique is used to examine the physical properties and technical characteristics of the plasma discharge in a fabricated DC plasma jet. The background gas is nitrogen molecular gas which is mixed with argon and oxygen gases at various percentages. The emission spectra are analyzed and, the plasma density, rotational, vibrational and excitation temperatures are obtained for the formed plasma discharges with Ar/N2 and O2/N2 mixtures. Moreover, the NO and OH lines from , excitation transitions are observed. It is shown that, at the higher argon and oxygen percentages in both the Ar/N2 and O2/N2 gaseous mixture, the emission intensities of argon ions and oxygen atoms are increased. In addition, at the higher Ar and O2 contributions in both the gaseous mixtures, while the rotational, vibrational and excitation temperatures are higher, they are decreased at the O2/N2 gaseous mixture. Moreover, the plasma electron density reduces for O2/N2 and increases for Ar/N2 gaseous mixture. On the other hand, for both the mixtures, the plasma density, rotational, vibrational and excitation temperatures are increased at the higher DC power source currents. Furthermore, it is seen that the dissociation rate increases at the higher DC currents and Ar and O2 percentages in both the mixtures. However, dissociation rate in the formed plasma jet with Ar/N2 gaseous mixture is higher.
1. Introduction
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Owing to the applications of nitrogen plasma discharges in various technological fields, there is a growing interest in studying and characterizing of the nitrogen plasma sources [1-5]. So far, a wide variety of plasma excitation schemes in a broad frequency range, i.e. through DC, low frequency pulsed DC and RF up to microwaves are spanned. On the other hand, the DC plasma jets are the well-known DC powered discharges having the capability of generating the intense plasmas with sufficient densities. Such systems are able to create the remote plasmas with a very homogeneous density distribution. Henriques et al. reported the performed experiments by a plasma jet with different mixtures of Ar/N2 which is produced by a travelling surface wave with the background pressure of about 66–266Pa [6]. Hong et al. developed an AC Atmospheric Pressure Micro-Hollow (APMJ) jet operating at 20 kHz with N2 gas [7]. Generation of long plasma jets of up to 6.5cm operating at 6.3L/min and gas velocity of 535m/s was reported. Moreover, they fabricated an AC APMJ device operating at 20kHz with nitrogen molecular gas [8]. Furthermore, an atmospheric pressure N2 plasma jet was generated from micro-discharges in a porous solid dielectric material [9]. A plasma jet plume with a length of 42mm was produced by feeding nitrogen gas through a porous alumina installed between an outer electrode and a hollow inner electrode and, an AC high voltage which was on the electrodes. In addition, in a recent work, they modified their previous plasma jet device and incorporated the porous alumina at the exit nozzle. Besides, Giuliani et al. used the same experimental configuration with the flow rate of about 10L/min (power of 5–13kV) of air [10]. Additionally, Kim et al. manufactured a similar plasma jet working with the applied frequency of 20 kHz with N2 as feed gas [11]. Blajan et al. studied the emission spectrum of a developed micro-plasma operating at atmospheric pressure non-thermal plasma [12]. The input power was supplied by a high voltage pulsed power source and, nitrogen and argon were used as operating gases. They observed Ar I, OH, N2 Second Positive System, First Negative Systems, and N2 First Positive System peaks in the Ar/N2 gaseous mixture. Lifetime emission signals for N2 Second Positive System for the micro-plasma discharge with 5%N2 in Ar/N2 mixture was around 1µs. Moreover, the rotational and vibrational temperatures were calculated. Qayyum et al. reported the Optical Emission Spectroscopic (OES) features of a formed plasma discharge with the Ar/N2 gaseous mixture with a DC power in a parallel plate configuration of electrodes [13]. The input power was varied between 175–225W and, the filling pressure was kept between 7–9mbar. Their findings shows that the molecular nitrogen emission lines intensity and plasma temperature will increase at the higher argon percentages in the mixture. Bousquet et al. used the OES method to examine the physical characteristics of Ar/N2 plasma discharge in a reactive magnetron sputtering [14]. It was observed that the electron temperature reduces at the higher Ar contributions in the mixture. Using the same OES measurement technique, Kilianova et al. determined the temperature of different species in a discharge plasma medium which was created by helical cavity coupled RF with the Ar/N2 gaseous mixture [15]. The argon percentage in Ar/N2 mixture was varied from 0% to 90%. It was observed that, at the lower argon percentages in the mixtures, the electron temperature decreases. Moreover, a direct relation between the plasma emission intensity and electron temperature was reported. Ohata studied the spectroscopic characteristics and capability of inductively coupled plasma (ICP) with Ar/N2 gas mixture using an axially viewing OES technique [16]. The emission intensity and excitation temperature was measured. Additionally, it was observed that the electron temperature increases at the higher nitrogen gas flow rates. The supersonic jet luminescence spectroscopy of nitrogen clusters (containing nearly 100 molecules per cluster) and Ar/N2 clusters (250 and 400 particles per cluster) was analyzed by Doronin et al. [17]. For the Ar/N2 clusters, it was observed that some transitions to various vibrational levels of the ground state from 2
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single N2 molecules in an argon gaseous environment. Moreover, it was found that the atomic nitrogen ions emission lie between 140 to 220nm [17]. Mohamed et al. [18] studied a micro-plasma jet in the atmospheric pressure molecular gases (nitrogen, oxygen and air). The plasma was generated by blowing the gases through DC Micro-Hollow Cathode Discharges (MHCDs). The applied voltage was sustained between 400-600V and, the maximum current was kept at about 25mA. It was shown that the gas temperature of the microplasma inside the micro-hollow cathode varies between ∼2000K and ∼1000K depending on the current and gas flow rate. Moreover, their finding shows that the temperature in the micro-plasma jet can be accurately controlled and, it is optimized through these parameters with respect to technological applications. Moreover, Schmiedt et al. [19] studied the DC glow discharge at the atmospheric pressures in both the pure oxygen gas and oxygen–nitrogen gaseous mixture (99%O2, 1%N2). The discharge current range was set between 10mA to 40mA. Furthermore, Haraki et al. [20] studied the oxygen radical formation in a DC low pressure plasma discharge system by O2/N2 gaseous mixture. In this paper, using the OES method, the production of active species at different Ar/N 2 and O2/N2 gaseous mixtures in a DC plasma jet is studied. From the emission spectrum of at the wavelength of 391.44nm and using the Gardet model, the rotational temperature is computed. The excitation and vibrational temperatures are evaluated from the spectral emission line intensities of Ar I and O atoms and three vibrational bands i. e., , using the Boltzmann plot method, respectively. Moreover, the plasma electron density and dissociation rates for both the mixtures are compared.
2. Experimental method
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A schematic diagram for the developed experimental setup is shown in figure 1. As seen, a tungsten needle (powered) electrode with 1.6mm diameter is placed in a quartz tube with 6 and 8mm inner and outer diameters, respectively. A stainless steel with a small hole (2mm diameter) at the bottom end (grounded) is placed at the tip of the tube. The spatial distance between the powered electrodes is kept at 1cm. A three dimensional schematic representation of the developed DC plasma jet is shown in figure 2. An electrical discharge is formed between the electrodes by applying high-voltage DC power (0-7kV, 0-50mA voltage and current variations, respectively) through a blocking resistor, R1(1MΩ). By introducing the mixed gas (Ar + N2 and O2 + N2) into the tube, the plasma and gas are spewed out from the nozzle, producing a plasma torch at the atmospheric pressure.
Figure 1. The schematic representation of experimental setup
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Generally, the spectroscopic methods are used to determinate the most important parameters in the plasma discharge media such as plasma temperatures or reactive species concentration. On the other hand, the molecular gas plasma discharges are characterized by the rotational, vibration and excitation temperatures. It must be noted that, the rotational temperature corresponds to the gas temperature. Moreover, the vibrational temperature is important, since the vibrational transitions are the main energy reservoirs. Moreover, the excitation temperature corresponds to energy of electrons, which are the main sources for the molecular and atomic stimulations. In addition, it affects the plasma reactivity via generating the active species via the inelastic collisions. The electron density measurement is important for quantifying the importance of electron dissociation and thermal dissociation of the molecular gases as a function of plasma discharge operating conditions. However, the plasma electron frequency is much larger than the electron-neutral collision frequency. It must be noted that, the degree of dissociation is very important, since it shows that how much of molecules are decomposed to the reactive nitrogen species, such as N, particles. In this work, the manufactured spectrometer by OceanOptics is used. To calibrate this spectrometer, a HG-1 Mercury-Argon lamp is generally used. Its fiber probe was placed at 5mm away from the nozzle and, it is perpendicular to the DC plasma jet at a distance of 10mm. The OES analysis is carried out using a USB2000 OceanOptic spectrometer and SpectraSuite software. Moreover, it stores a full spectrum at every millisecond with a wavelength range sensitivity of 200–1100nm. The various mixtures of nitrogen with argon and oxygen are taken as the main carrier gas. The argon and oxygen are mixed at different percentages with the molecular nitrogen using a gas mixer.
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Figure 2. The 3D schematics of atmospheric pressure DC plasma jet, (1) Tungsten rod (powered electrode); that is sharpened at 300, (2) Gas inlet, (3) PMMA dielectric, (4) Quartz tube, (5) Stainless steel; grounded electrode.
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In this paper, the excitation, vibrational and rotational temperatures along with the electron density and dissociation degree of N2 molecules in the plume of the developed DC plasma jet are estimated by the OES technique. Here, the emission intensities of different emission lines originating from the transition between the levels of atomic and molecular gaseous are measured. The spectra obtained from the plume of the DC plasma jet are analyzed by the SpectraSuite software. The obtained results from the OES technique are used to calculate the plasma characteristics of DC plasma jet such as electron excitation, vibrational and rotational temperatures as well as electron density. In order to measure discharge parameters of the DC plasma jet, the intensity of molecular emission lines of the plasma discharge can be used to estimate the population of the nitrogen molecular electronic levels. It must be noted that, even when one of the four temperatures are not the same, the system has to be considered in non-equilibrium or in Partial Local Thermodynamic Equilibrium (PLTE). Tendero et al. noted that, when the plasma electron density and temperatures lie between and , respectively, the plasma discharge is in PLTE [21]. Moreover, Calzada et al. introduced a criterion to determine the plasma equilibrium conditions [22, 23]. The main hypothesis behind their theory was to consider the atoms as hydrogenic ions with core z ( for neutral atoms; for single charged ions, etc.). However, the results can be applied to the more complex systems such as He, Ar, etc. Moreover, a Maxwellian distribution function was considered for the electron energy. For a hydrogenic ion, each level will be determined by an effective principal quantum number as follows: Where is the Rydberg constant or hydrogen ionization energy and, index of the equilibrium separation for each level p is defined as follows: ⁄
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Where and are the real and Saha-Boltzmann equilibrium populations, respectively, and, Saha-Boltzamann population equals to: ⁄
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Where is the ground state population of the plasma discharge species with charge core and, is its statistical weight. Moreover, is equal to the electron density which is obtained from the experimental data for the Stark broadening of the emission line (486.132nm). If for all levels, the ground level is included in the calculations and, the plasma discharge will be in the complete LTE. On the other hand, if such a parameter only equals to the unit for a group of levels, then the plasma discharge will be found in PLTE. Moreover, when , the levels will be overpopulated with respect to the Boltzmann equilibrium population (plasma is in the ionization stage). Finally, when , the levels will be underpopulated with respect to the SahaBoltzmann equilibrium population (plasma is in the recombination stage). Hence, with the obtained electron density from the experiment, , and the plasma discharge is in PLTE [22, 23]. The taken photographs from the developed DC plasma jet in the operational conditions with both Ar/N2 and O2/N2 gaseous mixtures in our laboratory are shown in figure 3. The operational parameters of DC plasma jet are fixed at V=2.5kV and I=40mA. It appears uniform to the human eye and, its length can even reach more than 30mm. The output gas from the gas mixture is injected to the DC plasma jet. It is worthy to mention that, for Ar/N2 mixture, Ar and N2 flow rates are about 15, 1-10 SLM (Standard Liters per Minute), respectively. On the other hand, for O2/N2 mixtures, owing to the difference in the oxygen and argon mass numbers, the flow rates of O2 and N2 are taken at about 10, 1-8 SLM, respectively. Here, the flow rates are controlled by a velocity flow controller.
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Figure 3. Photographs of the developed DC plasma jet in the operational conditions at (a, e) 80%, (b, f) 85% and (c, g) 90% percentages of argon and oxygen in the Ar/N2 and O2/N2 mixtures, respectively, with V=2.5kV and I=40mA.
It must be mentioned that, in this work, the operational features of the developed DC plasma jet are studied in two different working conditions: (1) The Applied DC voltage is fixed at 2.5kV and, the current is varied between 10-40mA and Ar/N2=4 and O2/N2=4, (2) The Ar/N2 and O2/N2 ratio are varied from 19 to 0.3 and 5.5 to 0.5, respectively, while the voltage and current are fixed at 2.5kV, 40mA.
3. Results and Discussion
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In the glow plasma discharges, electron impact process excites some of the plasma species to the higher states decaying and emitting photon at the characteristic wavelengths, which can be detected and analyzed by recording the emission spectrum. Thus, the different reactive species that are produced by the DC plasma jet with Ar/N2 and O2/N2 gaseous mixtures must be identified. Hence, to obtain its operational parameters, the OES technique is used to record the emission spectrum, as seen in figure 4. Four typical emission spectra are recorded at the DC plasma jet plume. These results are related to N2 which is mixed with 70%Ar and 90%Ar (see figure 4(a, c)) and with 70%O2 and 90%O2 (see figure 4(c, d)). The experiments are performed in an atmospheric pressure and DC power at 40mA and 2.5kV, and in the spectral range 200–950 nm. Moreover, the main features of these spectra lie in the UV region. There were NO emission lines from the electronic transitions. The formed NO radical was created from O2 and N2 that can be dissociated at the higher temperatures (>1600 K) or by the electron impact [40]. The NO γ band originated in the following collisions of the N2 metastable state [24, 25]: Excitation: ( ) Radiative transition: Additionally, the highly reactive radicals, i.e. OH emission band at the wavelength of 309nm are detected. It must be noted that, the OH radical formation is generally caused by water vapors molecules in the ambient air [26]. Where, these two distinct populations of excited state (OH) could be probably generated via the direct dissociation electron excitation of water vapor molecules:
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Where is formed by argon metastables and subsequent dissociative recombination in Ar/N2 and O2/N2 mixtures, respectively [27]. Moreover, the excited argon neutrals can dissociate H2O leading to the generation of OH radicals in Ar/N2 [28]:
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Furthermore, as is observable in figure 4, the emission spectra from the wavelength of 300 to wavelength of 450nm are dominated by the presence of Second Positive System N2 , the First Positive System and the First Negative System . These transitions are resulted from the many excitation processes such as electron impact excitation from the molecular ground state , the first metastable state , pooling reaction and transfer of energy between collisional partners [29, 30]. It is worthy to mention that, these long-lived metastable species, with energies below 6eV, are important as reservoirs of energy promoting plasma chemical reactions leading to the condensed products in the flue gas cleaning applications [26].
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Figure 4. Typical emission spectra of the developed DC plasma jet (corrected for the spectrometer’s spectral response) at V=2.5kV, I=45mA and, (a) 70%Ar, (b) 90%Ar in Ar/N2 mixture, (c) 70%O2, (d) 90%O2 in O2/N2 mixture
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( associative excitation, ( ) pooling reaction,
is caused by many excitation and quenching ,
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In the nitrogen plasma discharges, the population of the radiative state N2 processes such as electron impact excitation from the molecular ground state ( ) ( ) first metastable state ,
the energy transfer between the collisional partners and Penning excitation [31-33]:
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Moreover, in the Ar/N2 mixture plasma discharges, the population of the N2 excited state may result from the transfer of the internal energy from a metastable state of argon atoms to the ground state of the nitrogen molecules [33]. For the metastable states of argon, i.e. , the subscript ‘m’ stands for metastables that are having the energies 11.55 and 11.72eV which are higher than the threshold excitation energy (11.1 eV) of nitrogen molecule. Therefore, adding Ar to N2 plasma discharges results in the significant increasing of the emission intensities. Thus, the concentration of the active species can be expected by Penning effect as follows [35]: ( ) from The subsequent radiative decaying processes would cause photon emission from (0–0) band of the Second Positive System at the wavelength of 336.1nm: ( ) As a result, the emission intensity of (0–0) band of Second Positive System is proportional to the population density of the N 2 state [36, 37]. 6
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In the Ar/N2 gas mixture, the
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In both mixtures, the excited state can be populated either by the direct electron impact excitation from the ground state of the ( ) molecule via [36, 37]: ( ) from or based on stepwise electron impact ionization of the N 2 molecule and, then, the subsequent electron impact excitation of the molecular ion [36, 37]: ( ) ( ) from The ( ) state can be further excited to radiative state by electron impact as follows: ( ) from at The subsequent radiative decays of excited state have the capability of emitting the characteristic photons of (0–0) band of First Negative System at wavelength of 391.4nm [36,37]: ( ) The emission intensity of the (0–0) band of the First Negative System is proportional to the population of the state [36, 37]. On the other hand, in the performed experiments with O2/N2 gaseous mixture, there are emission band in the 550–600nm wavelength range and atomic oxygen at the wavelength of 844.6nm . In addition, the strong emissions by O2 are found in the plume of developed plasma jet with the O2/N2 gas mixture as the carrier gas. The most intense emission of oxygen results from the excited neutral atoms and molecules, originating from the dissociative excitation as follow [31]:
excited state can be populated by either one-step process [38]:
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or via the two-step process,
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. For the low temperature plasma discharges (with the species energy below 4eV), there are very few electrons with the energies above 35eV. However, these energies are almost required for the simultaneous excitation and ionization process. Hence, there is a large contribution from the excitation of the ground state ions by two-step process. Indeed, the emission intensity of radiative state provides enough information on the ground state ion density [39]. As shown in figure 4(a, b), in the argon plasma discharge, high intensity peaks corresponding to Ar I peaks are related to the 4s–4p transition. These peaks are measured at the wavelengths of 696.5nm, 706.7nm, 727.3nm and 738.3nm; the transition 4p-6s was measured at 703.2 nm; the transition 4s-4p at 750.4 nm; and the transition 4s-4p at the wavelengths of 763.5 nm and 772.4 nm [40]. The mechanism to excite the 2p1 level of Ar is the one-step electron impact excitation from the ground state. When N2 gas is added, the peaks corresponding to the N2 First Positive System are measured at the wavelengths of 632.3nm, 670.5nm, 676.4nm, 678.6nm, 705.9nm, 716.5nm and 762.7nm [41]. Generally, in the plasma discharge media, the electrical behaviors are determined by the temporal variations of voltage and current using a high voltage probe (Tekteronix, P6015A) and a current probe (Tektronix, TCP202). A DC power supply with selectable polarity was connected in series to a ballast resistor and DC plasma jet. In this work, the power source was able to supply up to 1mA of current. Displacement current is created when a high voltage is applied to the separated electrodes. The discharge current refers to the motion of the charged particles. Therefore, when the plasma discharge is maintained, both displacement and discharge currents will exist. Turning off the plasma discharge, the discharge current will be eliminated. Hence, to determine the displacement and discharge currents, the current should be measured in both situations, i.e. when the plasma discharge is on and, when the plasma discharge is off. When, the plasma discharge is on, the current density can be written as follows: . On the other hand, when the plasma discharge is off, then , and hence, . It must be noted that, the discharge current was used to calculate the discharge voltage using the following formula: (4) Where Vd is the discharge voltage, Vps is the voltage of the power source, I and R b are the current and the used resistor ballast (1MΩ). Furthermore, a stainless steel wire with the diameter of 1.6mm was used as anode and, a stainless steel cylinder was used as cathode. The ballast resistor was connected close to the discharge media and, the circuit was kept small. Thus, the stray capacitance in the circuit is reduced. Hence, this circuit has the capability of operating in a stable mode. The cylindrical electrode is always connected to the ground. The pin electrode was connected to positive terminal of the power supply. Figure 5 shows the V-I characteristics of a positive corona in the developed DC plasma jet for an electrode distance of 10mm and %90 of argon and oxygen in the Ar/N2 and O2/N2 mixtures, respectively. As seen, at the similar discharge currents, the discharge voltage in O2/N2 mixture is higher than that in the Ar/N2 mixture. This is owing to the fact that, compared with argon atoms, the oxygen molecules require more energy to get ionized. It must be noted that, since the electric field in the proximity of the negative biased electrode (cathode cylinder) in a positive corona is not significant, there is no important avalanche increment. Moreover, the electron production depends on the secondary photoemission processes near the sharp tips [42].
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Figure 5. V –I characteristics for the positive corona discharges in the developed DC plasma jet at the similar ratios of Ar/N2 (black) and O2/N2 (blue).
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In this work, the peaks of excited hydrogen atoms and profiles and spectral emission lines broadening are used. In addition, the plume of developed DC plasma jet is generated in ambient air (N2, O2 and water vapor). Hence, the various emitting species could be detected from the emission spectra of the developed DC plasma jet with both the Ar/N2 and O2/N2 gaseous mixtures, such as Ar and N excited atoms, also , O, NO and OH molecular emission bands. On the other hand, determination of the kinetic temperature using the OES technique is based on temperature measurements which are determined from the energy levels of rotationally excited states. Generally, it is assumed that the kinetic temperature and rotational temperature of the ground state are in equilibrium. Moreover, the dipole-allowed transition lifetime of the excited levels are used to determine the rotational distribution. It must be noted that, this lifetime is much shorter than the collision time [43]. Normally, the plasma state is analyzed by the light radiated in the visible region, where the spectral measurements are relatively easy. The rotationally excited molecules frequently execute energy transfer by collision with the other gaseous molecules. As a result, the rotational temperature is nearly equal to the gas temperature. Thus, the gas temperature, Tg, can be determined from the rotational temperature, [44]. In this spectroscopic observation, the spectra of 2nd positive system bands for nitrogen molecule are observable. The rotational temperature can be measured by the rotational spectrum of at wavelength of 391.44nm ( transition), which is used at the high resolutions of the spectrometer. The R branch between the wavelengths of 389nm and 391nm can be exclusively determined. The R lines intensities, , are the function of the rotational temperature, [45]:
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Where is a constant and are the quantum numbers for the excited state ; h, c, and are Planck constant, speed of light, ⁄ Boltzmann and rotational constants, respectively. Here, with the constants is used. The slope of the curves ⁄ to gives the rotational temperature. This has been done with the R lines detecting in the wavelength range of 389–391nm [45]. In this work, the Gardet models are used to obtain the emission spectrum of at the wavelength of 391.44nm. These models are applicable for the low resolutions. In these models, three steps are performed: (1) determination of wavelength positions, , (2) intensity factors calculation, and (3) the so-calculated spectrum convolution with a linear combination of Gaussian and Lorentzian functions which must be used to describe rightly the apparatus function [46]. The number of molecules that are excited into a given vibrational state is defined by the vibrational quantum number v. Moreover, it is proportional to the vibrational energy (E v) by the Boltzmann law to exp(-Ev/kBT). The vibrational band intensity is written as follows [45]:
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(6) Where v′, v′′ is the vibrational quantum number of upper and lower states, respectively. A(v′v′′) is transition probability (can be found in tables and, it corresponds to the electron vibration state functions overlap) and, ν is wavenumber (mainly wavenumber of the band head). The expression for vibrational band intensity shows that the plot of Ln(I '''/4A(''')) versus must be linear. The vibrational temperature can be calculated from the slope of this straight line [45]. Figure 6 shows a typical Boltzmann plot of the relative intensity distributions. Considering the scattered data points and fitting errors, is estimated about 8560 K at 40mA, 2.5kV and 80%Ar in Ar/N2 mixture. The vibrational temperature ( ) of the DC plasma jet is determined by the Boltzmann plot method in the both gas mixtures. Three vibrational bands , , and are chosen to obtain the . The wavelengths of 349.9nm, 353.6nm, 357.6nm, 370.9nm, 375.4nm, 380.4nm, 394.2nm, 399.7nm, 405.8nm are considered for each vibrational transition. The other parameters have taken from ref. [47]. 8
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Figure 6. Typical Boltzmann plot of
vibrational distribution with the vibrational temperature of Tvib=8560K.
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It must be noted that, only for the thermodynamic equilibrium systems, the excitation temperature, Texc, is mostly equal to the electronic temperature, Te [48]. Moreover, in the non-equilibrium plasma discharges, [49, 50]. When one of them is not equal to the others, the system has to be considered in non-equilibrium or in PLTE conditions. In this work, the developed DC plasma jet, with both the Ar/N2 and O2/N2 gaseous mixtures, is in PLTE conditions. However, the Maxwell–Boltzmann equilibrium is established. Furthermore, the atomic emission line intensities of Ar I and O are determined in the wavelength range of 696-912nm of the spectral region. There are strong emission lines from the excited argon and oxygen species in both the Ar/N2 and O2/N2 gaseous mixtures at spectrum. They have reliable transition probabilities that are published in the literature. Assuming that the upper levels of the selected atomic transitions are in the PLTE conditions, the conventional Boltzmann plot technique can be used to obtain the excitation temperature in the plume of the developed DC plasma jet. The following relation defines the relative transition probabilities of two different emission lines [51, 52]: )
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Where I, Aul, gu, λul, Eu and k denotes the total intensity, transition probability, degeneracy of the upper level, wavelength, excitation energy and the Boltzmann constant, respectively. However, the model assumes that the electron collisions between excited atoms are dominant in their populating and depopulating processes. Similar to the vibrational temperature case, a Boltzmann plot based on the ⁄ equation (6) can be used to drive the excitation temperatures from the slope fittings. In this case, the is plotted versus the upper level electronic energy (Eu) for the selected lines of Ar I emission. The atomic parameters of Ar I emission lines are taken from the National Institute of Standards and Technology (NIST) database [59]. It is worthy to mention that the ionization cross sections and ionization potentials for the molecular nitrogen ( and ~15.7 eV) are approximately similar to the argon atom and, they are smaller than the oxygen molecules [53]. Hence, these parameters are not sufficient to explain the observed increase in electron temperature in the Ar/N 2 gaseous mixture. This might be due the difference in the electron energy distribution function (EEDF) and its consequent effects on the higher secondary electron yield at the target in the presence of Ar in the plasma discharge. Moreover, the cross section is a suitable parameter to describe the electron temperature in the O2/N2 gas mixture. On the other hand, the lifetime of absorbing species is longer than that of the emitting species ( ). Therefore, compared with ( ) molecules, metastable species can diffuse farther away from the plasma discharge media, such that their temperature is lower. For Ar/N2 gaseous mixtures, the corresponding values for the First Positive System are similar. Consequently, the argon amount will not significantly affect the gas temperature. The issue for the Second Positive System is quite different. Indeed, an efficient energy transfer between the nitrogen ground state and Ar metastable atoms could involve strong overpopulation of rotational emission lines. Hence, the rotational temperatures are much larger than those obtained from the First Positive System. Figure 7 presents the rotational, vibrational and excitation temperature of the DC plasma jet as a function of the discharge input power, argon and oxygen percentages in both the Ar/N2 and O2/N2 gas mixtures. As seen in figure 7(a), in the Ar/N2 mixture, at the higher DC electrical currents, all the temperatures are increased. This is owing to the fact that, at the higher input powers, owing to the higher electric fields in the discharge medium, the number of high energy electrons will increase in the tail of EEDF. Moreover, the Penning ionization involving argon metastable is higher. Figure 7(b) shows that the electron temperatures are increased at the higher Ar contributions in the mixture. This might be due to the lower electron collision cross section of argon atoms compared with the nitrogen molecules. Thus, the plasma electrons have enough time to get accelerated by the electric field and its consequent effects on the electrons energy. These energetic electrons have the capability of exciting and ionizing nitrogen molecules or generating argon metastable states. This metastable species, in turn, will collide with N2 molecules and excite or ionize them. Hence, each Penning ionization and excitation event will result in the argon neutral ground state atom together with the excitation and ionization of the target species. As seen in figure 7(c), all the temperatures are increased at the higher DC currents. At the higher DC power source currents, the input power to the discharge medium of plasma jet increases. This reflects in the higher electric fields in the discharge 9
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medium and, the plasma electrons will get the higher accelerations. Moreover, the higher intensity of electric fields will result in the higher kinetic energies (temperatures) of different species within the discharge medium of the DC plasma jet. In addition, as seen in figure 7(d), the rotational, vibrational and electronic temperatures are higher at the larger percentages of O 2 in the O2/N2 gaseous mixture. It must be noted that, the increasing of O2 contribution in the mixture will effectively enhance the energy transfer from electrons to molecular O2 via the excitation of rotational, vibrational and electronic levels of oxygen.
Figure 7. Evolution of the excitation, vibration and rotational temperatures as a function of (a, c) electrical current at 2.5kV and 80% Ar and O2 at considered mixtures, (b, d) Ar and O2 percentages at 40mA and 2.5kV.
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The Stark broadening analysis of the spectral profile of the Hβ (486.1nm) emission line which is emitted by the plasma discharge medium is the most used technique to calculate the electron density, ne [49, 55, 56]. If the selection rules for the hydrogen atomic transitions are taken into the account, then the allowed transitions (multipletes) can be related to the Hβ emission line [57]. These transitions lie between 2p–5s, 2s–5p and 2p–5d energy levels [58, 59]. The hydrogen Balmer series (Hβ) is usually used to obtain the plasma electron density without assuming the fine structure of the emission line and ion dynamics [60]. Several broadening mechanisms affect the emission line-shape in plasmas, natural broadening, Doppler broadening, pressure broadening, and Stark broadening [55]. While the natural broadening is usually not important, the Stark broadening ∆λS can be directly related to electron density in plasma discharge. Thus, it can be used for determination of plasma density, ne. On the other hand, the Full-Width at Half-Maximum (FWHM) is a parameter which is widely used to determine the broadened emission line profile. Then, the FWHM broadening values of Stark broadening that may affect the H β emission line can be obtained. The FWHM of Stark broadening is related to the electron density as follows [61]: ⁄ (8) Hence, the Stark broadening can be used to compute the electron density. Where the plasma electron density is in cm −3 unit [62]. Stark broadening at the wavelength of 486.1nm was considered for the Hβ. The Stark broadening is related to Lorentz and van der Waals broadenings using the following relation [62]: (9) Finally, the pressure broadening results from perturbation of the energy levels of the emitting atoms due to the presence of surrounding neutral species. Pressure broadening leads to a Lorentzian profile and, it is subdivided into resonance broadening (when emitters and perturbers are of the same type and either the upper or the lower state of the detected line is a resonance level), and van der Waals (vdw) broadening (when emitters are perturbed by neutrals of a foreign gas). In this work, the density of H atoms was extremely low. Hence, only the van der Waals broadening is considered. The FWHM of the vdw profile is written as follows [62, 63]: 10
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(10) Where p is the pressure in atmosphere and, T is the gas temperature in Kelvin which is equal to the rotational temperature. To find the Lorentz broadening, one could fit the experimental spectrum with Voigt profile which is the superposition of Gauss and Lorentz broadenings. Fitting process is performed using the Origin software and, the Lorentzian FWHM was measured. Thus, knowing Lorentz and van der Waals broadenings, Stark broadening is calculated. Hence, the electron density can be determined. Figure 8 shows the Lorentz broadening which is calculated via fitting of the experimental spectrum to Voigt profile ( ). Inserting the gas pressure of 1atm, temperature of 351K and %90Ar in Ar/N2 gas mixture, the van der Waals broadening is calculated as . Hence, the Stark broadening would be 0.125nm corresponding to the electron density .
Figure 8. Comparison of experimental data and Voigt profile with ∆λL=0.185nm and ∆λG=0.179nm.
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The variations of electron density as a function of DC power source current, Ar and O2 contributions in both the Ar/N2 and O2/N2 gaseous mixtures are shown in figure 9. As seen in figure 9(a), the electron density increases with the input DC current. However, this is owing to the fact that, at the higher DC electrical currents, the input power to the discharge medium of the DC plasma jet is higher. This reflects in the higher intensity of electric fields and, hence, the ionization rate of neutral species is increased. As seen, the electron density for the formed plasma discharge in Ar/N2 mixture is higher than that of the O2/N2 gaseous mixture. This might be due to the higher needful energy for ionization of the oxygen molecules compared with the argon atoms. Thus, the electron number density increases at the higher Ar percentages in the mixture, as depicted in figure 9(b). Consequently, the electron number density increases at the higher molecular nitrogen dissociation rates. Moreover, at the higher Ar contributions, the probability of Ar metastable generation increases. Hence, the production of will be enhanced. In Ar/N2 plasma discharge, the electron impact is the dominant source channel for dissociation process. Moreover, the higher Ar percentages results in the higher number of energetic particles in the tail of the electron energy distribution function, which in turn leads to the higher dissociation rates of N2 molecules [16]. Furthermore, this phenomenon reflects in the higher molecular dissociations. In addition, Figure 9(b) shows the electron density versus O2 percentage in the O2/N2 mixture. As seen, the higher O2 contributions in the mixture reflect in the lower plasma electron densities. It must be noted that, the effects of nitrogen in the mixture can be attributed to an extensive set of vibrational energy levels. Moreover, the nitrogen molecules can absorb a significant amount of energy from the plasma electrons. On the other hand, at the higher percentages of oxygen in the mixture, the collisions occurrence between the metastable nitrogen molecules and oxygen molecules will dissociate the oxygen molecules and, the density of oxygen radicals will increase considerably. Hence, the electron density will decrease due to oxygen electronegativity and, the production of the negative oxygen species will be higher.
Figure 9. Evolution of electron density as a function of (a) DC electrical current at 2.5kV, 80% Ar and O2 in both gaseous mixtures, (b) Ar and O2 percentages at 40mA, 2.5kV. 11
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It is worthy to mention that the dissociation degree is one of the significant parameters of molecular gas plasma which is used for different applications. A simple way to increase dissociation of molecular gas is to introduce another gas in the plasma. On the other hand, the actinometry is a well-known OES technique which has been used for the qualitative and sometimes quantitative determination of atomic densities in the plasma discharges. Here, argon in Ar/N2 mixture and oxygen in O2/N2 mixture play the role of actinometer. Moreover, the [N]/[N2] ratio is considered as the dissociation degree of nitrogen molecule. In the actinometry method, this ratio can be expressed as follows [64]: (11) (12)
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Where IN is the atomic nitrogen emission line intensity at the wavelength of 672.3nm and, IAr is the argon emission line intensity at the wavelength of 750.4nm and IO2 is the oxygen emission line intensity at wavelength of 777nm. Moreover, {[Ar]/[N2]} and {[O2]/[N2]} are the initial ratios between Ar and O2 to N2 percentages when the plasma discharge is off. In addition, , and are the spectral responses of the detection system which are related to the emissions of Ar line, N line and O2 line, respectively. Finally, C1 and are constants. They depend on the excitation coefficient rates, quenching rates, electronic temperature and spectroscopic data [11, 64]. They are not involved in the estimation of the [N]/[N 2] ratio, since all the needful data for their evaluation could not be found. Particularly, those concerning the excitation coefficient rates for the transition of atomic nitrogen. Therefore, the dissociation degree of N2 is determined qualitatively. In this work, the actinometry method is considered as an approximate method, since the argon and oxygen fractions in the mixtures are higher than 5%. Figure 10 shows the evolution of the N2 dissociation degree which is determined using the actinometry method, as a function of (a) DC electrical current, (b) Ar and O2 percentages in both the mixtures. As seen, for both mixtures, similar behaviors with various argon and oxygen percentages in the mixtures are shown. At atmospheric pressures, the dissociation degree increases at the higher argon and oxygen percentages. Due to the energy deposition into the discharge media, the dissociation degree of nitrogen is higher. However, the dissociation degree in Ar/N2 mixture is higher than that in the O2/N2 mixture, as shown in figure 10(a). Furthermore, while N2 dissociation is increased at the higher Ar contributions in the Ar/N2 mixture, it is reduced with O2 contribution for the O2/N2 mixture, as seen in figure 10(b). It must be noted that, owing to argon presence in the Ar/N2 mixture, the quenching reactions are more effective than that in the O2/N2 mixture.
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Figure 10. Evolution of N2 dissociation degree (actinometry method) as a function of (a) DC electrical current at 2.5kV, 80% Ar and O2 contributions , (b) argon and O2 percentages in Ar/N2 and O2/N2 gaseous mixture plasmas, respectively at 40mA and 2.5kV.
4. Conclusion
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In this work, using the OES technique, the physical and technical features of a developed DC plasma jet operating with Ar/N2 and O2/N2 gaseous mixtures are studied. The effects of Ar and O2 gases on the different features of plasma discharge and N2 dissociation process are compared. This experimental study was performed through the calculation of plasma density, rotational, vibrational and electronic temperatures for the formed plasma discharges with both the Ar/N2 and O2/N2 gaseous mixtures. The NO emission lines from the electronic transition were detected at both mixtures. The emission spectra due to the presence of N2 Second Positive System were seen to be dominant. It was shown that, at the higher argon and oxygen percentages in both mixture, the emission intensities from the argon ions and oxygen atoms are increased. In addition, while the rotational temperature with the DC current remains almost constant, both the vibrational and excitation temperatures will increase. Moreover, at the higher Ar and O2 contributions in each mixture, the vibrational and excitation temperatures are increased. It was observed that, while the plasma electrons density increases at the higher DC currents in the both mixtures, it is higher at the higher Ar percentages and lower O2 contributions in Ar/N2 and O2/N2 mixture, respectively. Furthermore, the dissociation fraction of nitrogen molecules increases at the higher DC currents, Ar and O2 percentages in the two mixtures. Finally, it was observed that, effect of atomic argon on the dissociation of molecular nitrogen is higher than the molecular oxygen. Thus, to obtain higher number of active species from the molecular nitrogen in the DC plasma jets, the Ar/N2 gaseous mixture is more suitable than O2/N2 mixture. 12
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