ФИЗИКА ПЛАЗМЫ, 2013, том 39, № 12, с. 1141-1152
НИЗКОТЕМПЕРАТУРНАЯ ПЛАЗМА
УДК 533.9
ELECTRICAL AND CHEMICAL PROPERTIES OF XeCl*(308 nm) EXCIPLEX LAMP CREATED BY A DIELECTRIC BARRIER DISCHARGE
© 2013 S. Baadj, Z. Harrache, A. Belasri
Laboratoire de Physique des Plasmas, Matériaux Conducteurs et leurs Application (LPPMCA), Université des Sciences et de la Technologie d'Oran, USTO-MB, Algérie e-mail: zharrache@yahoo.com Поступила в редакцию 06.03.2013 г. Окончательный вариант получен 06.06.2013 г.
The aim of this work is to highlight, through numerical modeling, the chemical and the electrical characteristics of xenon chloride mixture in XeCl* (308 nm) excimer lamp created by a dielectric barrier discharge. A temporal model, based on the Xe/Cl2 mixture chemistry, the circuit and the Boltzmann equations, is constructed. The effects of operating voltage, Cl2 percentage in the Xe/Cl2 gas mixture, dielectric capacitance as well as gas pressure on the 308 nm photon generation, under typical experimental operating conditions, have been investigated and discussed. The importance of charged and excited species, including the major electronic and ionic processes, is also demonstrated. The present calculations show clearly that the model predicts the optimal operating conditions and describes the electrical and chemical properties of the XeCl* ex-ciplex lamp.
DOI: 10.7868/S0367292113120019
1. INTRODUCTION
Non-equilibrium plasma created by dielectric barrier discharge (DBD) is one of the widely used as ultraviolet and vacuum ultraviolet (UV/VUV) sources in excimer lamps [1—8]. These lamps are used especially in biological sterilization, lithography, material deposition in microelectronics, plasma display panels, destruction of pollutants and lighting [9—17]. Recent theoretical and experimental investigations showed that pulse excited DBD is much efficient as UV sources, with typical gas pressure changing between 50 and 700 Torr. The optimum content of chlorine in Xe/Cl2 gas mixtures is about 0.5—5% and the applied voltage is approximately several tens of kV [18, 19]. The presence of a dielectric between the electrodes prevents the transition to an arc [20, 21]. The materials widely used as dielectric barrier are glass, quartz, alumina, and some special ceramics or polymers. Moreover, in order to optimize the discharge luminance efficiency, many experimental and theoretical simulation studies of the plasma kinetics in pulse-excited DBDs have been realized in the last decade [22—36]. It has been shown that the efficiency is rather high and can reach 60% by employing a short pulsed voltage excitation [25, 26, 37—41]. As illustrated in [18], the optimization of the intensities of radiation depends not only on the gas composition but also on the gas pressure, the applied voltage and the number of pulses. In other works, as shown in [19], the optimum pressure when UV-radiation output is in its maximum depends on discharge geometry and voltage waveform.
The aim of this work is to investigate the electrical and chemical characteristics of XeCl*(308 nm) excil-amp excited by a pulsed dielectric barrier discharge in Xe/Cl2 gas mixture. Here, we present a computer modeling of homogeneous dielectric barrier discharge. The main mechanisms of XeCl* molecules formation and the UV production dependence on the total gas pressure, the Cl2 percentage, the applied voltage and the dielectric capacitance are reported and analyzed.
This work is organized as follows. Section 2 describes the physical discharge model and the chemical kinetics scheme. In section 3, the electrical and chemical proprieties of the lamp discharge are presented and discussed, followed by a parametric study. Finally, the conclusion is drawn in section 4.
2. DESCRIPTION OF THE DISCHARGE MODEL
The model of homogeneous discharge (so-called zero-dimensional (0D) model) was based on three modules: the electrical module based on the equation of the discharge circuit, the particle module based on the Boltzmann equation and the chemical kinetic module based on a system of kinetic equations for the considered discharge species [36, 42].
The scheme of the DBD presented in this paper is shown in Fig. 1.
The applied voltage through the discharge is given
by:
Vapp (t) = Vdis (t) + Vdie (t), (1)
1 r - [T
1
Cd1 [ Vapp ^ Cd1
Fig. 1. Scheme of the dielectric-barrier discharge.
where: Vdie (t) is the voltage across the dielectric and given by the formula:
Vde =
VL J1 (t d.
(2)
The discharge current I (t) can be obtained by the following equation:
V„
I (t ) =
dis
R (t )'
(3)
Cl*, XeCl*(B), XeCl*(C), Xe2Cl*, (147, 153, 172, 256, 308, 330, 490 nm). The considered reactions include electron-neutral elastic collisions, direct electron impact ionization, excitation, stepwise ionization, recombination, heavy particle collisions and radiation processes. The reaction rate coefficients of electron—atom or molecule collisions, depending on the reduced electric field E/N, are tabulated by solving the homogenous electron Boltz-mann equation, using the BOLSIG solver [44]. It is worth noting that the rate coefficients of electron collisions with chlorine species are taken from [45] and [46]. They are calculated as function of the electron temperature which is calculated as: 2s
T =
x p
3K,
-, where s is the mean electron energy cal-
The chemical kinetics module constructs differential equations for the evolution of the species density. The system of equations describing the dielectric and the plasma kinetics are solved as follows: for a given voltage at a time t, the plasma kinetics equations coupled with the dielectric equation are solved with the classical GEAR method [43] between the instants t and t + dt.
The full set of reactions used for the present model of Xenon/Chlorine mixture is reported in table 1. It takes into account 22 species regrouped in 74 reactions. These species are electrons, Xe+, Xe+, Cl-, Cl+, Cl, Xe*s, Xemet, Xe**, Xe^1 S+), Xe*(32„+), Xe2*(0+),
culated as a function of the reduced electric field [44] and KB is the Boltzmann constant.
3. RESULTS AND DISCUSSION
We discuss in this section some results obtained from the homogeneous discharge model in the case of a Xe/Cl2 gas mixture for different discharge parameters. The discharge lamp parameters and operating conditions are indicated in table 2.
3.1. Electrical Characteristics
In order to show the profile of some electrical characteristics of the discharge during the pulse, we have plotted in Fig. 2, for a gas mixture of Xe/Cl2(1%), an initial density of preionization of 109 cm-3, an applied voltage of 8 kV and a total gas pressure of 200 Torr, the temporal variations of the voltage across the capacity Vdie, the discharge voltage Vdis, the discharge current I, and the power density. It seems that at any time during
Voltage, kV y Current density, A/cm2 Power density, kW/cm
app
2.0 16
14
1.5 12
10
1.0 8
6
0.5 4
2
0
0.25 0.50 0.75 1.00 1.25 1.50 Time,
0.25 0.50 0.75 Time,
1.00
Fig. 2. Time evolutions of (a) the current density, the applied voltage Vapp, the voltage across the gap Vdis, the voltage across the dielectric Vdie, and (b) the deposited power density.
ELECTRICAL AND CHEMICAL PROPERTIES Table 1. List of reactions and rate coefficients used in the computer simulation of XeCl excilamp
for Xe/Cl2 gas mixture
Process
Rate
Reference
1. Ionization processes (cm3/s)
e + Xe*
e + Xe:
e + Cl2 ■ e + Cl e + Cl
e + Cl ■
e + Cl-
2
2
— 2e + Xe+
— 2e + Xe+
► 2e + Cl2+
► 2e + Cl+ + Cl
- Cl+ + Cl- + e
Cl+ + 2e
- 2e + Cl
e + Xe2Cl* ■
Xe2 + Cl + 2e
2. Electron attachment and detachment e + Cl
e + Cl e + Cl
e + Xe
Cl + Cl- Cl* + Cl
- 2Cl
- Xe** + Xe 3/
3. Charge transfer (cm3/s)
f(E/N) f(E/N)
f(E/N
3.88 x 10-9 x exp(-15.5/rg) 8.55 x 10-10 x exp(-12.65/Te )
3.17 x 10-8 x Te5i x exp(-13.29/Te ) 7 x 10-11
4.0 x 10-5 x Гe x exp(-7.5/T) (cm3/s) f(E/N)
2.0 x 10-8
9.0 x 10-8 x Г
,-0.5
2.0 x 10-7 x те
,-0.5
[44] [44]
[44]
[45] [45]
[45]
[46]
[47]
[44]
[46]
[48]
[47]
Cl+ + Xe — Xe+ + Cl 1.0 x 10-12 [49]
Cl+ + Xe* — Xe+ + Cl 1.0 x 10-12 [46]
Ci+ + Xe** —- Xe+ + Cl 1.0 x 10-12 [46]
Xe+ + Cl2 — Xe + Cl+ 6.0 x 10-11 [46]
Cl+ + Cl2 — Cl+ + Cl 1.0 x 10-9 [46]
4. Penning ionization (cm3/s)
Cl* + Xe —- Xe+ + Cl + e 1.0 x 10-10 [46]
Cl* + Xe* — Xe+ + Cl + e 1.0 x 10-10 [46]
Cl* + Xe** — Xe+ + Cl + e 1.0 x 10-10 [46]
Xe* + Xe* — Xe+ + Xe + e 2.0 x 10-10 [46]
Xe** + Xe** — Xe+ + Xe + e 2.0 x 10-10 [46]
Xe* + Xe* —»- Xe+ + Xe + Xe + e 3.5 x 10-10 [50]
Cl- + Cl — 2Cl + e 5.0 x 10-10 [49]
5. Ion-ion recombination (cm3/s)
Cl+ + Cl- —- Cl2* 2.0 x 10-6 [49]
Cl- + Cl+ — Cl* + 2Cl 2.0 x 10-6 [46]
Xe+ + Cl- — XeCl* Calculated by Flannery formulas [51]
Xe+ + Cl- — XeCl* + Xe Calculated by Flannery formulas [51]
6. Electron-neutral collisions (cm3/s)
e + Xe — Xe* + e f(E/N) [44]
e + Xe — X* * + e f(E/N) [44]
e + Cl2 — 2Cl + e 1.04 x 10-7 x T°e29 x exp(-8.84/Te) [48]
e + Cl2 — e + Cl2* 1.14 x 10-14 x exp(—5.34/Te) [48]
XeCl*(B) + e —- XeCl*(C) + e 5.0 x 10-6 [46]
XeCl*(B) + e —- Xe + Cl + e 1.2 x 10-7 [52]
XeCl*(C) + e — Xe + Cl + e 1.2 x 10-7 [52]
e + Xe2Cl* — e + Xe + Xe + Cl 2.0 x 10-7 [46]
e + Xe2Cl* — XeCl* + Xe + e 3.0 x 10-8 [47]
Table 1. (Contd.)
Process
Rate
Reference
7. Neutral-neutral collisions (cm3/s for two-body reactions and cm6/s for three-body reactions)
2Cl + Cl2 — 2Cl2 5.4 x 0- 32 [53]
2Cl + Xe — Cl2 + Xe 5.4 x 0- 32 [53]
Cl2* + Xe — XeCl*(B) + Cl 1.55 x 0- 10 [49]
Xe* + Xe Xe* + Xe 2.6 x 0- 14 [54]
Xe* + Xe — Xe* + Xe 1.5 x 0- 15 [55]
Xe* + Xe + Xe — Xe2* ( Z+ ) + Xe 6.0 x 0- 33 [46]
Xe* + Xe + Xe — Xe2* (3Z„+ ) + Xe 1.9 x 0- 32 [46]
Xe* + Xe + Xe — Xe2* (O+) + Xe 1.55 x 0- 35 [23]
Xe* + Xe + Cl2 — Xe* + Cl2 5.0 x 0- 32 [56]
Xe* + Cl2 — XeCl* + Cl 7.0 x 0- 10 [46]
Xe** + Cl2 — XeCl* + Cl 7.0 x 0- 10 [46]
Xe** + Xe — Xe* + Xe 2.0 x 0- 11 [23]
Xe** + Xe + Xe — Xe* + Xe 5.0 x 0-
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