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Numerical investigation of Ar–NH3mixture in homogenous DBDs

更新时间:2016-07-05

1.Introduction

In recent years,the creation of non-thermal equilibrium plasma by dielectric barriers discharges(DBDs)near atmospheric pressure has received an intensive attention by scientific community [1].The DBDscan found many applications such as ozone generation[2],biological sterilization[3],controlling of pollutants[4],material deposition in microelectronics[5,6]and photovoltaic industries[7,8].For photovoltaic application,the homogenous discharge generation in the DBD is required than filamentary discharge.In the literature,the homogenous DBDs can be easily produced and operated in atmospheric pressure[9–13].Due to the specific advantages of homogenous DBDs application in surface treatment and thin film deposition,many published works studied this kind of discharges using homogenous models for He[14],Ne[15]rare gases and N2[16,17].In[18]the authors proposed a new technique for stabilizing the homogenous glow discharge at atmospheric pressure by a 50Hz source in any gas,this technique allows to distinguish between atmospheric pressure glow discharge(APGD)and the silent electrical filamentary discharge.In the case of APGD,the discharge behavior was studied numerically and experimentally by many authors[19–21].Massines et al[22]proved that to obtain a homogenous and stable APGD generated by a dielectric barrier with specific operating conditions,a temporal and spatial analysis of most important physical quantities are needed.The numerical calculations are in agreement with experimental investigations.A detailed analysis of electrical and kinetic properties of homogenous DBD for excimer lamps has been investigated by using a computer modeling[2,23–26]to predict the optimal operating conditions and improve the performances of the lamp.We note here that the model used in the present work is similar to[24,25].Other interest works on homogenous DBDs at atmospheric pressure were carried out in order to give a better understanding of different physical mechanisms that allowed such homogenous DBDs[23–30].In particular,the DBD working with Ar–NH3mixture at atmospheric pressure provides Efficiency in deposit silicon nitride material on silicon solar cells[31–34].However,this Efficiency is dependent on the discharge homogeneity and power deposition in the DBD.The results obtained show that the homogeneity of the discharge should be achieved only at high excitation frequencies.The subject of this work is to simulate a homogenous dielectric barrier discharge DBD at atmospheric pressure for SiNxphotovoltaic cells deposition.The gas mixture considered here is Ar/NH3/SiH4.The homogenous model is used here to describe the kinetic scheme of Ar–NH3gas mixture and all possible energy transfers into the discharge are considered.

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2.Description of discharge model

A zero-dimensional model is used in this work to describe a homogenous DBD in Ar–NH3.This model combines the circuit module,Boltzmann equation module,and plasma kinetic module.The kinetics of the plasma is represented by differential equations describing the evolution in time of the densities of the different species existing in the plasma.The discharge plasma is assumed to be homogeneous.The particle continuity equations are solved to calculate the time dependant population densities of all neutral and charged species in the plasma.Electrons are described by the continuity equations:

where S(t)is the source term.This takes into account the electron creation and loss.The electron continuity equation is simultaneously resolved with the kinetic equations[35].We take into account in the present work the following species:e,and photons hv(120nm,135nm).Therepresents the three first vibrational levels of NH3.The transport coefficients are precalculated and tabulated by solving the Boltzmann equations of electrons in the Ar–NH3gas mixture,using the Bolsig+[36].The solution of the system of kinetic equations is based on a set of reactions indicated in table 1.

3.Results and discussions

The reactor has symmetric parallel plane discharge geometry with two electrodes covered by dielectrics.The operating conditions of the reactor are resumed in table 2.These parameters and the geometry of the discharge are selected according to[28].

The two currents have the same shape and the same order of magnitude.An offset of 6 μs between the two current peaks is observed.This is due to our zero-dimensional approach;we did not consider the secondary emission caused by the impact of ions and photons.And consequently the breakdown of our gas occurs rapidly.

The shift between the peak current and the peak voltage demonstrates that the discharge is capacitive the phase shift was calculated,it is 0.5π.From the second alternation of voltage,there are two current pulses by alternation.A residual pulse appears at 3μs of the first.This residual current pulse is similar to a residual peak which appears in the power deposition in figure 3.

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Temporal evolution of the ion density is plotted in figure 6.The density of the molecular ions ofis obtained by electron impact of excited molecules ofand the impact of two molecules of thus by transfer of charge of Ar+.These ions disappear by transfer of charge to NH3to give more The temporal evolution of the charged particles densities are presented in figure 7.The ionization energy of the NH3molecule is 10.2 eV which is slightly lower than the excitation energy of the Argon metastable which is 11.55 eV.So when NH3is added to pure Argon Penning ionization will be very efficient between the metastable generated in the antecedent discharge and the NH3molecules.These methods can provide a good number of seed electrons sufficient to sustain the previous discharge.With 200 ppm of NH3in Argon and the conditions of our discharge at the end of the first voltage alternation,the density of the,ions reaches 1.6 × 1010cm−3.And during the remainder of the discharge this density varies from 1.8 × 1010to 2.6 × 1010cm−3.Due to penning ionization the +NH3ions are the dominant ions in the discharge.All Penning-type reactions involve the metastable levels of Argon and ammonia.

In figure 1,we plotted the time variation of the current of the discharge of our simulation and the discharge current measured experimentally by Bazinette[29].

After the first current pulse the charges accumulate on the dielectric surfaces this accumulation of charges creates a residual voltage across the plasma Vpl,which causes an electron displacement.In DBDs,the first and second current pulses are comparable in amplitude with similar energy deposition.The current and the voltages are periodic and consequently the power is also.The maxima of the power coincide with the extreme of the current.It has a maximum of 4.6 W cm−3.In figure 4 illustrates the time variation of the electronic density.The electron density becomes maximum when the applied voltage is zero[41].

Figure 5 reports the temporal evolution of the ion Ar+density.The density of the Ar+atomic ions that are created essentially by electron impact with Argon atoms with a high energy of the order of 15.76eV or by collision of two metastables Argon atoms is low not exceeding 5.5 × 104cm−3but these ions disappear givingmolecular ions according to this reaction and ions of ArH+by the reactions ArH+ + NH2and Ar+ + H2 → ArH+ + H.

The shape of the electron density is the same as the total current in the discharge.It can be seen that the electronic density reaches 6 × 109cm−3at the third current pulse.This large number of electrons can initiate a second discharge at low electric field;this helps us to obtain a uniform discharge instead a filamentary discharge.The time shift between the first two current pulses is 3μs,this time is so short that the charged particles created during the first current pulse accumulate in the discharge space and do not have enough time to dissipate.Before the second breakdown the density of electrons is in the range 5.8 × 109–1.8 × 109cm−3.

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Figure 2 illustrate time evolution of the applied voltage Va,voltage across the plasma Vpl,the voltage across the dielectric VDand the current in the gap of the discharge.The behavior of the different voltages and the current are similar on the two positive and negative alternations.The gas becomes conductive when the gas voltage is equal to the breakdown voltage which is equal under these calculation conditions to 1.88μs for the first pulse of the current.After the breakdown of the gas,the passage of the current on the dielectric surfaces cause an increase in the voltage across the dielectric Vd,the increase in Vdis faster than the applied voltage,which causes a voltage drop in the plasma Vp.These provoke the extinction of the discharge before passing through an arc regime.

Table 1.Kinetic scheme and rate coefficient used in the present simulation of Ar–NH3mixture.

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Figure 1.Temporal variation of the discharge current of our simulation and the discharge current measured experimentally by Bazinette[29].

Figure 2.Temporal evolution of the applied voltage Va,voltage across the plasma Vpl,the voltage across the dielectric VDand the current in the gap of the discharge.

Table 2.Reactor parameters and operating conditions.

Parameters Values Gas Ar–NH3mixture(200 ppm NH3)Gas pressure P = 760 Torr Gap length d = 0.1 cm Electrode area S = 10 cm2 Gas temperature T = 300 K Initial electron density N0 = 109cm−3 Dielectric capacitance Cdiel = 0.18 nF External resistance R = 3.6 kΩ Frequency F = 50 kHz Peak value of applied voltage V = 900 volts Applied voltage form Va(t) = V sin(ωt)

Figure 3.Temporal evolution of the power deposited in the plasma,current of the discharge and the applied voltage.

Figure 4.Temporal evolution of the electronic density.

Figure 5.Temporal evolution of the ion Ar+density.

Figure 6.Temporal evolution of the ion density.

Figure 7.Temporal evolution of the charged particles.

Figure 8.Temporal evolution of the excited particles.

During the collision,the internal energy of the metastable Argon is used to ionize or dissociate the ammonia.The most of the volume production of electrons is caused by the two Penning ionization reactions from the analysis of the source terms.ArH+ions are obtained by charge transfer of the atomic and molecular ions of Argon.

The value of the ArH+ion density increases from 1 × 106cm−3to 7 × 106cm−3.Figure 8 shows the time evolution of excited particles.The increase in metastable densities reflects the increase in the production of metastables by electron excitation of Argon.

完成地面控制点与像片控制点的像平面坐标获取后,在光束法区域网平差内引入GPS摄站坐标(作为带权观测值)。出于消除系统漂移误差和坐标变化误差的考虑,区域两端加设架构航线1条,四角设置平高地面控制点1个。加密结果如表1所示。

4.Conclusion

A 0D model for a mixture of Ar/NH3dielectric barrier discharge at atmospheric pressure has been investigated.We have tried to compare our discharge current with the discharge current measured experimentally by Bazinette[28].We noticed that the secondary emission due to the impact of ions and photons are very important to describe the electrical properties of a discharge,these parameters are not taken into account in our model.However this model allowed us to describe the formation of homogeneous plasma in a dielectric barrier discharge at atmospheric pressure.A kinetic pattern of discharge has been defined,including electron excitation and direct ionization of argon,Penning ionization,molecular conversion of excited levels of argon to dimer,quenching by collision with argon,argon of the excited levels of argon and the UV emission of the dimer.The discharge was studied more precisely in the case of an Ar–NH3mixture(200ppm)and a gap of 1mm.The existence of a positive charge of space created by ions,major ions despite the low concentration of NH3in the mixture has been demonstrated.It is clear that the addition of NH3and the Penning reactions and the metastable states of argon contribute to obtaining a homogeneous discharge.

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Barkahoum LAROUCI,Soumia BENDELLA,Ahmed BELASRI
《Plasma Science and Technology》2018年第3期文献

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