Effects of addition of H2 gas in air DB discharge on its optical and electrical characteristics have been studied. Optical emission spectroscopy is used to investigate the effect of hydrogen admixing on the emission intensity of the nitrogen second positive systems (300 - 420 nm) and the relative population density of states. An obvious decaying of the emission intensity of the nitrogen second positive bands with the introduction of H2 has been observed. It has been concluded that quenching of the nitrogen excited state is the responsible reason of this decaying. Mechanisms of excitation and ionization processes of nitrogen molecules in this mixture have been studied. Processes which are responsible for the decaying of the population density of have been reported. Addition of H2 to air improves the electrical characteristics of the DB discharge. An abrupt increasing in the electron density, reached about thirty fold at H2 flow rate of 3 L/min, as a result of increasing the ionization processes has been reported. The breakdown voltage of the discharge decreased from 1.87 kV to about 1.25 kV by the addition of H2 at flow rate of 3 L/min.
It is known that the working conditions in the plasma may be affected, considerably, by the addition of small amounts of some molecular gases beside the working gas [
The description of gas excitation in various plasmachemical systems can be developed via the analysis of the population dynamics of reference levels for which rate constants of population by electron impact and following depopulation are well known. In the case of air DB discharge plasma, one such reference level is the state of; its population determines radiation intensity of the 2+ nitrogen system transition in discharges with N2. The short radiative lifetime and relatively high excitation rate of this level make this transition suitable for diagnostics of the stationary plasma [
In the present paper the effect of addition of H2 gas to air DB discharge on the optical and electrical characteristics is studied. These effects including the emission intensity of the nitrogen second positive systems (300 - 420 nm) and the relative population density of states. Mechanisms of excitation and ionization processes of nitrogen molecules in this mixture are also studied.
A DBD (dielectric barrier discharge) system, consists of two copper plane-parallel electrodes immersed in porous dielectric plates made of commercial gypsum (CaSO4∙2H2O) material, has been used to produce an atmospheric pressure air discharge. The diameter of the dielectric plates was about 4 cm and its thickness was 2 mm. The distance between the two dielectric plates was kept constant at 1.1 mm. Hydrogen gas has been injected between the two electrodes at flow rates of (1 to 3) L/min. The discharge open reactor was started up using a high voltage transformer (1 to 10 kV), generates sinusoidal voltage with frequency of 50 Hz. The applied potential (Va), and the discharge current (I) were recorded using a digital oscilloscope (HAMEG HM407 - 40 MHz). The current was measured using a voltage drop across the resistance R1 (= 100 Ω) (see
An optical emission spectroscopy (OES) technique consists of a McPherson scanning monochromator [model 270] with a grating of 1200 grooves mm−1 and resolution of less than 2 Å has been used to study the nitrogen spectra in a wavelength range of 300 - 420 nm. The monochromator was then connected to photomultiplier tube (PMT) type 9558 QB, which has a resolution time of less than 1 nanosecond, working at voltage of 1200 volts.
Atmospheric pressure DB discharge can be operated in three different modes namely; the filamentary mode (streamers mode), the glow discharge mode (APGD mode) and the quasi-glow discharge mode.
Typical emission spectra of the air DB discharge are shown in Figures 3(a)-(d) in the range of wavelength of (300 - 420 nm).
related to the de-excitation transitions of molecular nitrogen from the excited electronic state to the low-laying excited state according to: (Equation (1)), [6,7].
and states are lying at 7.4 and 11.0 eV respectively, above the ground electronic state. According to a simple analytical calculation for the electron mean energy in the atmospheric pressure, non-thermal plasma is estimated to be about (2 - 5) eV. Therefore the electronic states of the background molecules can be excited by the high-energy electrons in the tail of the Boltzmann distribution according to Equation (2), [
The de-excitation of will followed by the emission of nitrogen First negative systems according to Equation (5), [
Effect of addition of H2 on the intensity of emitted spectra is shown in Figures 3(b)-(d). It is noticed that:
1) There is an obvious decreasing in the intensity of the nitrogen second positive system by the addition of H2
gas, (see Section 3.2.2).
2) New species such as OH, NO and NH are observed in Figures 3(c) and (d). This can be related to the fact that increasing of H2 flow rate in air discharge gives rise to increase the dissociation processes of the nitrogen, hydrogen and oxygen molecules to form other species such as NO, OH and NH. NO radical is formed as a result of the dissociation of O2 and N2 by electron impact [11,12] according to Equations (6) and (7) and then re-combination of oxygen and nitrogen occurs (Equations (8) and (9)) [
while the species NH and OH are formed according to the following equations;
then
3) No hydrogen lines are observed in Figures 3(b)-(d) e.g. (at 656.2 nm), (at 486.1 nm) or (at 434 nm). The disappearing of hydrogen lines is related to the exhausting of their excitation energy in the Penning ionization processes rather than the radiated decaying processes (see Section 3.3).
The pronounced decay of the nitrogen second positive system is attributed to:
1) Reducing the mean electron energy by the addition of hydrogen molecules to air discharge;
According to the fact that the dissociation energy of the hydrogen molecule (4.3 eV) is very low compared with that of the nitrogen (9.8 eV) or oxygen (5.11 eV) molecules, a considerable amount of electron energy is dissipated in the dissociation process of hydrogen molecules, Equation (10). As a result, the mean electron energy is reduced with the addition of H2 to air discharge. Meanwhile, the production processes of either by direct electron impact, Equations (13) and (14), or by the pooling reaction, Equation (15), is reduced.
2) Reducing the formation of nitrogen excited molecules;
The dissociation processes of N2 molecules increase by the increasing of H2 to form another species such as NH and NO, etc. This in turns decreases the density of nitrogen molecules in the ground state that collide with the electrons to form the nitrogen excited molecules , (Equation (2)), that are responsible for the emission of nitrogen second positive system according to Equation (1).
3) Quenching of the formed excited states before undergo the spontaneous emission to the lowlaying excited state;
This quenching is due to the collision of the nitrogen excited molecules with N2 or with other quenchers such as molecular hydrogen or oxygen respectively according to the collisional deactivation processes, Equation (16), [
where is the quenching rate constant of by N2 and is the quenching rate constant of by M molecule, where M is O2, H2, H* or H2O.
The quenching mechanism of the nitrogen is confirmed by using the Stern-Volmer Equation (20), [16,17] i.e.:
where Io and Ip are the intensities of the special bands system in the absence and the presence of the quencher (which is H2 in the present study), respectively, [Q] is the concentration of the quencher, is the lifetime of the nitrogen excited state, and kQ is the quenching rate constant. In the present work, the hydrogen concentration can be represented by its flow rate in air. Therefore, by plotting the ratio vs the hydrogen flow rate, the quenching efficiency of the different bands of the nitrogen second positive system is estimated.
The quenching rate of the band 337.1 nm (0 - 0) is the highest one (
In order to study the effect of addition of H2 to air
discharge on the population density of the excited nitrogen states, Equation (21), which relates the intensity of the band of the system to the population density of the excited state has been used [
where D is an instrumental constant; is the quantum energy involved in the transition, is the frequency corresponding to the given band; is the electronic transition moment; is the FranckCondon factor for the transition, both latter quantities are theoretically constant.
with and without addition of hydrogen to air discharge.
The enhancement of the discharge current by increasing the amount of H2 is related to the following reasons;
1) More ionization processes are expected to take place in the present of H2 such as the reactions in Equations (19), (22)-(25) i.e.:
(H* here is the hydrogen metastable state which has life time of 0.12 sec [
Also using Equation (7) then
2) Addition of H2 to air discharge enhances the humidity of the porous dielectric that coated the electrodes.
This humidity resulted from the formation of OH and in turns promotes the formation of H2O molecules according to Equations (26)-(28) [
Consequently, the humidity increases the current inside the micro-holes of the porous dielectric and hence increases the seed electrons that sustain the discharge in the glow mode [
3) Addition of H2 to air discharge at atmospheric pressure increases the probability of Penning ionization and energy transfer processes. Addition of H2 enhances also the dissociation processes of the molecular N2 and O2 (Equations (6) and (7)) to form large number of species, of different ionization and excitation energies. For example the product species NO, NH and O2 have ionization energies; of 9.25, 13.1 and 12.07 eV [
The electron density has been calculated using the electron conduction current density [22,23] i.e.:
where J is the discharge current density, μe is the electron mobility and E is the electric field of the discharge region.
Using the data in
The total optical emission intensity of the discharge as a function of time has been measured using the PMT at different H2 flow rates (= 0, 1, 2 and 3 L/min) in air discharge and at the same discharge current (~0.4 mA), see
The total emitted light decreases with the increasing of the H2 flow rate. Addition of hydrogen to air DB discharge plasma enhances the ionization processes, such as Penning ionization, which occurs mainly by the energy transferring between the excited species. Since the present work has been carried out at atmospheric pressure the excited species will exhaust their energies through such ionization processes rather than in radiative decayed processes.
Admixing of hydrogen gas, with different flow rates, to
air DB discharge has an important effect on the optical and electrical characteristics of the discharge. An obvious decaying of the nitrogen second positive bands emission intensity was observed. This decaying of the nitrogen second positive bands resulted from the quenching of the nitrogen excited state. On the other hand, admixing the hydrogen to air discharge enhances the discharge current and in turns the electron density increases as a result of the increasing of the ionization processes e.g. the Penning ionization processes.