FIG 4 - uploaded by Júlio Paulo Henriques
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Discharges in N 2 – Ar mixtures are experimentally investigated by means of optical emission and absorption spectroscopy, probe diagnostic techniques, and radiophysic methods. The experimental results provide insight into the mechanisms of wave-to-plasma power transfer, N 2 dissociation, creation of N 2 ions, and excitation of metastable states N 2...
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When gas discharge and plasma devices shrink to the microscale, the electrode distance in the device approaches the mean free path of electrons and they experience few collisions. As microscale gas discharge and plasma devices become more prevalent, the behavior of discharges at these collisionless and near-collisionless conditions need to be under...
Citations
... However, due to high bond energy, it is quite difficult to break the molecular nitrogen to achieve a higher dissociation fraction (DF) [14][15][16]. Although several works have been published on pure nitrogen plasma, produced via various kinds of discharge methods, such as microwave [17][18][19][20][21], radio frequency (RF) [8,9,22] and helicon discharge [7], for all the cases, the reported DF of nitrogen molecules stays at a minimal level in the low power regime (<600 W). Apart from this, it is observed that when nitrogen is mixed with other gases like hydrogen, argon etc, the DF gets a boost and for argon mixing, the stability of the overall plasma remains intact even at higher argon proportions [15,16,[23][24][25]. ...
The behaviour of nitrogen plasma mixed with varying proportions of argon (10%-80%) has been investigated under different RF discharge conditions. It is observed that at a relatively low RF power of 200 W (E-mode) the dissociation fraction (DF) of nitrogen increases with the growing concentration of argon whereas the opposite happens for a higher RF power of 1000 W (H-mode) when the DF rapidly falls from a high value as argon percentage starts moving upwards. This rising trend of DF closely follows the argon metastable fraction (MF) in the E-mode and for H-mode it does not follow until the argon percentage crosses 20% mark. The electron density, temperature and electron energy probability function (EEPF) has been obtained using a RF compensated Langmuir probe and to evaluate the vibrational and rotational temperature, DF, MF etc. a separate optical emission spectroscopy technique is incorporated. At 5×10-3 mbar of working pressure and 10% argon content the EEPF profile reveals that the plasma changes from non-Maxwellian to Maxwellian as the RF power jumps from 200 W to 1000 W, and for a fixed RF power the high energy tail tends to move upwards with the gradual increment of argon. These observations have been re-verified theoretically by considering electron-electron collision frequency and electron bounce frequency as a function of electron temperature. Overall, all the major experimental phenomena in this study have been explained in terms of EEPF profile, electron-electron collision effect, electron and gas temperature, electron density and argon metastable population.
... 1-There is the case where SW power is reflected at the end of the column (from a vacuum metallic cap, for example), generating a standing (sine form) wave pattern of electron density as a function of axial position. The most pronounced electron density variation is located close to the end of the plasma column [28]. 2-The length of the space-wave radiation region in the vicinity of the field applicator increases as frequency is lowered. ...
Surface-wave (SW) sustained plasmas belong to the category of gaseous discharges set up by the electric field component of an electromagnetic (EM) wave. Tubular plasmas of this kind can be achieved over an unrivalled wide range of operating conditions, namely gas nature and pressure, EM field frequency, (dielectric) discharge tube characteristics. The modeling of SW produced plasmas has rest on assuming an intrinsic connection between the wave and the generated plasma column, the SW traveling along the plasma column as its propagating medium and the plasma column influencing in return the SW properties. In contrast, the current paper advocates that there is no action of the plasma column on the characteristics of the surface wave that sustains it. This argument stems from the, amply-documented, experimental fact that the axial distribution of electron density along SWDs is linear with a constant slope till the very end of the plasma column. This feature thus rejects any possible influence of axial inhomogeneity along the plasma column on the SW properties. It is further confirmed experimentally that the slope of the axial gradient of electron density is entirely imposed by the operating conditions, even at the minimum observable length of plasma column. A paradigm shift is thus proposed, promoting the idea that the features of the plasma column are fully determined by the SW power flow running along the air-dielectric tube interface, excluding the plasma column as a propagating medium. The plasma column develops from the electrons generated by the wave electric-field radial component present at the discharge tube inner wall. The previous modeling endeavors cannot come to the conclusion that the plasma column is passive toward the SW and explain why the axial distribution of electron density is, in all cases examined, perfectly linear with a constant slope.
... These hysteresis loops were used to measure coercivity (H c ) and remanence magnetization (M r ) whose values are reported in Table 3. Observations show that increase in temperature decreases the magnetic behavior of samples under investigation. Reports indicate similar results with varying compositions synthesized by various methods [40,41]. ...
Mixed Li(0.5−x/2)Fe(2.5−x/2)CrxO4 (X = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) nano-ferrite system was synthesized by using Citrate gel auto-combustion method. Structural parameters such as lattice constant, hopping length, and X-ray density were reported and discussed with composition. The surface morphology was presented and explained with TEM analysis and SAED patterns. XRD patterns show the prepared samples are single-phase cubic spinel structure and the average particle size lies between 17 to 27 nm. TEM analysis shows prepared sample are the crystallite formation in nano-size. DC electrical properties presented variation with temperature and composition were discussed and studied well behind the Curie temperature. Magnetic measurements are carryout with VSM (Vibrating Sample Magnetometer), Observations have shown that magnetic properties are strongly influenced due to the substitution of Cr³⁺ in Lithium ferrites. Two samples Li0.5Cr0.6Fe1.9O4 and Li0.5Cr1.0Fe1.5O4 were subjected to an applied field of 100 Oe between 5 and 375 K temperature for ZFC and FC (Zero Field Cooled and Field Cooled) measurements of magnetization which indicated blocking temperature at around 350 K for both samples beyond which they exhibited super-paramagnetic behavior.
... Henriques et al. reported the performed experiments by a plasma jet with different mixtures of Ar/N 2 which is produced by a travelling surface wave with the background pressure of about 66-266 Pa [6]. Hong et al. developed an AC Atmospheric Pressure Micro-Hollow (APMJ) jet operating at 20 kHz with N 2 gas [7]. ...
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 NOγ(A²Σ⁺→X²Πr), (A²Σ,v=0→X²Π,v′=0) 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 N2 dissociation rate increases at the higher DC currents and Ar and O2 percentages in both the mixtures. However, N2 dissociation rate in the formed plasma jet with Ar/N2 gaseous mixture is higher.
... . Although the rate coefficients for reactions R3.1 and R3.2 are different in the previous research, 55,56 k R3:1 ¼ 4:45 Â 10 À10 cm 3 s À1 and k R3:2 ¼ 2:81 Â 10 À10 cm 3 s À1 are employed in this paper as they have been validated by experiments in Ref.57. Since the excited-state Ar atoms may play an important role in the Penning dissociation pro- ...
A two-dimensional self-consistent fluid model and the experimental diagnostic are employed to investigate the dependencies of species concentrations on the gas proportion in the capacitive N2/Ar discharges operated at 60 MHz, 50 Pa, and 140 W. The results indicate that the N2/Ar proportion has a considerable impact on the species densities. As the N2 fraction increases, the electron density, as well as the Ar⁺ and Arm densities, decreases remarkably. On the contrary, the N2+ density is demonstrated to increase monotonically with the N2 fraction. Moreover, the N density is observed to increase significantly with the N2 fraction at the N2 fractions below 40%, beyond which it decreases slightly. The electrons are primarily generated via the electron impact ionization of the feed gases. The electron impact ionization of Ar essentially determines the Ar⁺ density. For the N2+ production, the charge transition process between the Ar⁺ ions and the feed gas N2 dominates at low N2 fraction, while the electron impact ionization of N2 plays the more important role at high N2 fraction. At any gas mixtures, more than 60% Arm atoms are generated through the radiative decay process from Ar(4p). The dissociation of the feed gas N2 by the excited Ar atoms and by the electrons is responsible for the N formation at low N2 fraction and high N2 fraction, respectively. To validate the simulation results, the floating double probe and the optical emission spectroscopy are employed to measure the total positive ion density and the emission intensity originating from Ar(4p) transitions, respectively. The results from the simulation show a qualitative agreement with that from the experiment, which indicates the reliable model.
... The principles of this method and its use for plasma diagnostics have been thoroughly described elsewhere. [48][49][50] This method has been successfully applied for different gas mixtures containing molecules like N 2 , [51] O 2 , [52] H 2 , [53] and CO 2 . [44] It involves addition of a small, known, amount of a tracer specie-an actinometer-to the gaseous medium of interest. ...
The growing interest to plasma-based greenhouse gas decomposition requires the knowledge of the different kinetic mechanisms inherent in CO2 discharges and post-discharges. This automatically involves extensive plasma diagnostics research work. Among different types of plasma diagnostics, the ones based on optical spectroscopy are of particular relevance due to their non-intrusiveness. The recent progress in optical diagnostics of plasma-assisted CO2 decomposition process is discussed in this work. A microwave surfaguide discharge operating at 2.45 GHz in several CO2-containing mixtures was investigated. Optical emission spectroscopy is used to characterize the discharge area of the reactor in terms of fundamental plasma parameters. At the same time, two-photon absorption laser-induced fluorescence is applied for investigation of oxygen and carbon monoxide concentrations in the post-discharge.
... Several methods have been proposed for the measurement of N atomic density and dissociation fraction of N 2 . Henriques et al. 5 determined the N atomic density by actinometric method in Ar-N 2 surface wave plasma in the pressure range of 0.2-2 Torr. An increase in the dissociation of N 2 was reported, particularly at the higher concentration of argon. ...
... 7,[15][16][17][18][19] Trace rare gas optical emission actinometry permits quantitative determination of atomic densities in a discharge. 5,20 This technique is based on the following assumptions: ...
A Magnetic Pole Enhanced inductively coupled RF He-N2/ Ar plasma is characterized using a
Langmuir probe and optical emission spectroscopy (OES) techniques. The effect of helium mixing
on electron density ðneÞ and temperature ðTeÞ, electron energy probability functions (EEPFs), [N]
atomic density, and N2 dissociation is investigated. A Langmuir probe and a zero slope method
based on trace rare gas-optical emission spectroscopy (TRG-OES) are employed to measure the electron
temperature. It is noted that the electron temperature shows an increasing trend for both methods.
However, the temperature measured by a zero slope method TeðZ SÞ approaches the
temperature measured by a Langmuir probe; TeðL PÞ at 56% and above helium concentration in the
discharge. “Advance actinometry” is employed to monitor the variation in [N] atomic density with
helium concentration and gas pressure. It is noted that [N] atomic density increases at 56% and above
helium in the discharge, which is consistent with the trend of electron temperature and EEPFs. A
drastic enhancement in N2 dissociation fraction D1 determined by “advance actinometry” is noted at
56% and above helium concentration in the mixture due to modifications in different population and
depopulation mechanisms. However, it is also noted that the dissociation fraction D2 determined by
intensity ratio method increases linearly with helium addition.
... Several methods have been proposed for the measurement of N atomic density and dissociation fraction of N 2 . Henriques et al. 5 determined the N atomic density by actinometric method in Ar-N 2 surface wave plasma in the pressure range of 0.2-2 Torr. An increase in the dissociation of N 2 was reported, particularly at the higher concentration of argon. ...
... 7,[15][16][17][18][19] Trace rare gas optical emission actinometry permits quantitative determination of atomic densities in a discharge. 5,20 This technique is based on the following assumptions: ...
A Magnetic Pole Enhanced inductively coupled RF He-N2/ Ar plasma is characterized using a
Langmuir probe and optical emission spectroscopy (OES) techniques. The effect of helium mixing
on electron density ðneÞ and temperature ðTeÞ, electron energy probability functions (EEPFs), [N]
atomic density, and N2 dissociation is investigated. A Langmuir probe and a zero slope method
based on trace rare gas-optical emission spectroscopy (TRG-OES) are employed to measure the electron
temperature. It is noted that the electron temperature shows an increasing trend for both methods.
However, the temperature measured by a zero slope method TeðZ � SÞ approaches the
temperature measured by a Langmuir probe; TeðL � PÞ at 56% and above helium concentration in the
discharge. “Advance actinometry” is employed to monitor the variation in [N] atomic density with
helium concentration and gas pressure. It is noted that [N] atomic density increases at 56% and above
helium in the discharge, which is consistent with the trend of electron temperature and EEPFs. A
drastic enhancement in N2 dissociation fraction D1 determined by “advance actinometry” is noted at
56% and above helium concentration in the mixture due to modifications in different population and
depopulation mechanisms. However, it is also noted that the dissociation fraction D2 determined by
intensity ratio method increases linearly with helium addition.
... This fact may be attributed as follows: at many occasions, the electron energy distribution function does not remain Maxwellian, and the atomic/ionic densities are not in Boltzmann equilibrium; that is, excitation and de-excitation are not controlled by collisions with electrons. Under these circumstances, the local thermo-dynamical equilibrium model is not valid, and the use of the simple Boltzmann plot method only provides the excitation temperature instead of electron temperature (32,33). In this method, the spectral intensities of several spectral lines having different threshold excitation energies are employed to determine the excitation temperature of electrons. ...
The N2-H2 plasma gas mixture, generated in a 50 Hz pulsed dc discharge system with active screen cage, is characterized by optical emission spectroscopy (OES), as a function of gas pressure, the fractions of hydrogen and current density. The N2 dissociation degree and N atomic density was measured with actinometery where argon gas is used as actinometer. It was shown that the increase in hydrogen fraction enhances the dissociation of N2, until the maximum of 40%. The excitation temperature is determined from Ar-I emission line intensities by using the simple Boltzmann plot method. The dissociation fraction and excitation temperature is found to increase with hydrogen mixing in nitrogen plasma.
... Various magnetic parameters (coercivity H c and remanence magnetization M r ) for both the samples were measured from the hysteresis loops and are tabulated in Table 2. From the table, it is observed that by increasing the temperature, magnetic behavior of investigated samples decreases. Similar results were observed by other researchers with various compositions prepared by different methods [28,29]. ...
An attempt has been made to prepare nanocrystalline nickel substituted lithium ferrites, having chemical composition Li0.5-0.5xNi x Fe2.5-0.5xO4 (where x=0.0-1.0 with a step increment of 0.2) by a citrate-gel auto combustion method. Structural characterization of the synthesized samples was carried out by X-ray diffraction analysis (XRD) and field emission scanning electron microscopy (FESEM). Magnetic measurements were carried out using vibrating sample magnetometer (VSM) and Mossbauer spectroscopy. From these measurements it is observed that Ni2+ substitution in lithium ferrites has strong influence on magnetic properties. Hysteresis loops indicate that the saturation magnetization values were decreased from 56emu/gm to 28emu/gm by increasing the Ni2+ content in the Li-Ni ferrite samples. This signifies the fact that the lesser magnetic Ni2+ ions are substituted for the magnetic Fe3+ ions in the octahedral sub-lattice of the ferrites. Field Cooled (FC) and Zero Field Cooled (ZFC) magnetization measurements under an applied field of 100Oe and 1000Oe in the temperature range of 5-375K were performed on two samples Li0.1Ni0.8Fe2.1O4 and NiFe2O4. The magnetization as a function of an applied field 10T was carried out at two different temperatures 5K and 310K. These measurements showed the blocking temperature of the two samples at around 350K above which the materials show super-paramagnetic behavior where the coercivity and remanence magnetization are almost zero.
