IOP PUBLISHING PLASMA SOURCES SCIENCE AND TECHNOLOGY Plasma Sources Sci. Technol. 18 (2009) 045006 (8pp) doi:10.1088/0963-0252/18/4/045006 Atmospheric pressure glow discharge plasma in air at frequency 50 Hz A A Garamoon and D M El-zeer Center of Plasma Technology, Al-Azhar University, Nasr City, Cairo, Egypt Received 11 December 2008, in final form 7 June 2009 Published 31 July 2009 Online at stacks.iop.org/PSST/18/045006 Abstract Homogeneous atmospheric pressure discharge using a dielectric barrier discharge in air at 50 Hz was investigated. Porous alumina ceramic sheets were used as a dielectric barrier. In this paper we compare the filamentary discharge initiated by using glass sheets as a dielectric and the glow discharge obtained by the porous alumina sheets as a dielectric. Generally, the discharge in a dielectric barrier arrangement consists of a large number of filaments. Using porous alumina sheets Al2 O3 , an atmospheric pressure glow discharge (APGD) plasma has been obtained. This APGD discharge is characterized by a homogeneous profile of long lifetime (∼5 ms) and the current is one pulse per half cycle of the power supply. The electrical measurements and the emission spectrum confirm that the discharge has been formed in the glow mode. (Some figures in this article are in colour only in the electronic version) species, and so atmospheric discharges exist as streamer, nonuniform microdischarges. In order to minimize this quenching effect, modifying the reactor geometry, [7–12], using fast gas flow were reported [6]. This paper investigates the possibilities of obtaining atmospheric pressure glow discharge (APGD) in air at 50 Hz using alumina ceramic of special configuration and construction as a dielectric. 1. Introduction The generation of uniform discharges in open air is one of the most remarkable achievements in plasma field research. Uniform and stable discharges without additional flowing gases and vacuum chamber have many advantages. This process can be applied to diverse plasma applications in industry, environment and medicine, such as surface modification, deposition, cleaning, removing the pollutant gases and sterilization [1–4]. So far there have been some reports about uniform glow (or glow-like) discharge using a dielectric barrier discharge (DBD) structure in atmospheric pressure. Generally, to generate glow discharge in air, sufficient numbers of seed electrons or metastables are needed. But in the presence of electronegative gases, such as oxygen and water vapor, atmospheric discharges exist as streamer, nonuniform microdischarges. Therefore, most of the atmospheric pressure glow discharges are generated in other gases, such as He, Ar and N2 [5]. In these discharges, metastable species of each gas prevent electron avalanche that induces streamer discharge. These neutral metastable species with long lifetimes move around unrelated to the applied electric field and transfer their energies by collision with other atoms or molecules. Afterwards, every region of plasma zone becomes uniform and stable [6]. But in the case of air discharge, oxygen molecules (represent 21% of air constituents) quench nitrogen metastable 0963-0252/09/045006+08$30.00 2. Experimental setup The experimental setup (figure 1) consists of two copper plane-parallel electrodes of 3 cm diameter. To compare the initiation of the filamentary and the glow discharge, the two electrodes were covered by two different dielectric barriers; in one case they were covered with two porous alumina Al2 O3 sheets of 3.5 mm thickness and 4 cm diameter and in a separate experiment they were covered with two Pyrex glass plates of 1.1 mm thickness and 4 cm diameter. The distance between the two dielectric plates was 1.1 mm. A high voltage transformer (1–10 kV), which generates sinusoidal voltage with a frequency of 50 Hz, was used as an electric power supply to derive the discharge system. The discharge was operated in open air under atmospheric pressure. The applied potential (Va ) across the electrodes and the current (I ) passing through the system were recorded using a digital oscilloscope 1 © 2009 IOP Publishing Ltd Printed in the UK Plasma Sources Sci. Technol. 18 (2009) 045006 A A Garamoon and D M El-zeer R2 V Oscilloscope R3 I Two dielectrics R1 V(K volt) 6 I(mA) 1 4 0.5 2 0 -2 0 0 10 20 30 40 -0.5 -4 -1 -6 -8 Figure 1. Schematic diagram of the discharge cell. 1.5 Time (m sec) The current (mA) Applied voltage (KV) R4 8 -1.5 Figure 3. Voltage–current waveforms of air discharge by using glass sheets on the two electrodes, d = 1.1 mm at Va = 5 kV and at frequency = 50 Hz. Va Vm Cd Vg Cg (a) Pyrex glass as a dielectric barrier. Figure 3 represents the voltage and current oscillograms for air discharge under atmospheric pressure as a function of time. It can be seen that by using Pyrex glass as a dielectric a streamer discharge was formed which is characterized by discrete current spikes. These spikes are related to the formation of microdischarges (filaments) of tens of nanoseconds duration in the gap space [15]. These microdischarges are randomly distributed in the discharge area. The peak of each spike is related to the number of instantaneous microfilaments that were formed at this instant. So a high current spike indicates that a large number of microdischarges are initiated almost simultaneously [16]. Rg I disp Idisch I R1 (b) Porous alumina sheets as a dielectric. Figures 4(a)–(d) represent the voltage and current oscillograms for air discharge under atmospheric pressure and at applied voltages of 3 kV, 3.5 kV, 4 kV and 5 kV, respectively. It is well known that in the case of DBD discharge, the microdischarge results from a streamer development and the current variation is very rapid and the discharge duration is lower than 100 ns [17]. However, in this case a homogeneous discharge can be seen to be formed in which the current variation is slower and of longer duration (∼5 ms), as shown in figure 4. This discharge is characterized by a diffuse APGD discharge, which spreads in the discharge gap between the two electrodes. Also it can be noted that the current pulse and the applied voltage are nearly in phase. As the discharge is initiated by applying a voltage that is sufficient for gas breakdown, the charges start accumulating on the dielectric sheets and induce the memory voltage Vm on the dielectric plate. The accumulated charges on the dielectric induce an electric field that opposes the applied electric field, so the actual voltage applied to gas Vg decreases, and the discharge is extinguished at a much lower value of Vg . In the next half cycle, the discharge is again initiated and the voltage due to charge accumulation (i.e. Vm ) helps to increase Vg more than the breakdown threshold [18]. The decisive criterion for a uniform discharge in DBD is the presence of charge carriers at low electric field, i.e. a ‘memory effect’ producing primary electrons below the breakdown voltage. Without memory effects, the electron multiplication is dominated by the formation of electron avalanches by direct electron collisions which in turn leads to the formation of filamentary discharge [19]. Figure 2. Equivalent electric circuit of the DBD discharge. (HAMEG HM407—40 MHz). The current was measured by the voltage drop across the resistance R1 (=100
) connected in series with the discharge system to the ground as in figure 1. The voltage across the two electrodes was also measured using the potential divider of the resistance system R2 , R3 , where R2 /R3 = 500. An optical spectrometer system was used to measure the intensity of the emitted spectra. It is composed of a McPherson scanning monochromator (model 270) with a grating of 1200 grooves mm−1 and the photomultiplier (type 9558QB) has a resolution time of less than 1 ns. The system works within the wavelength range 300–900 nm. A digital camera was used to record the images of the discharge. 3. Results and discussions 3.1. Electrical characteristics The equivalent electric circuit in the dielectric-controlled APGD is shown in figure 2. The applied voltage Va can be expressed as the sum of gas voltage Vg and memory voltage Vm , which is applied to the dielectric sheets [13, 14], i.e. Vg (t) = Va (t) − Vm (t). (1) 3.1.1. Voltage and current wave forms. The voltage and the current waveforms of the discharge were recorded on the oscilloscope for two types of dielectrics. 2 V (KV) 1 1 0 -1 0 20 40 60 -1 -3 -2 5 Time (m sec) V (KV) I (mA) (c) 40 60 -1 -3 -2 -5 -3 Time (m sec) 0 -2 0 10 20 30 40 -4 506 0 -1 -2 -3 Time (m sec) V (KV) I (mA) (d) 3 2 3 1 1 0 -1 0 20 40 -3 60 -1 -2 -5 The current (mA) 20 1 0 5 0 -1 0 2 3 1 1 2 -6 The current (mA) 3 3 4 -3 2 I (mA) (b) The current (mA) 2 3 6 3 Applied voltage (KV) (a) -5 Applied voltage (KV) V (KV) I (mA) Applied voltage (KV) 5 A A Garamoon and D M El-zeer The current (mA) Applied voltage (KV) Plasma Sources Sci. Technol. 18 (2009) 045006 -3 Time (m sec) Figure 4. Voltage–current waveforms of air discharge by using porous alumina sheets on the two electrodes where d = 1.1 mm and at Va = 3 kV, 3.5 kV, 4 kV and 5 kV, respectively. Obtaining the APGD discharge by using porous alumina sheets has been found due to the special configuration of the alumina sheets, which is characterized by the existence of microholes that are filled with air. Due to the presence of the microholes, the voltage Vm that is applied to the dielectric can be divided into two voltages i.e. the voltage applied to the bulk alumina sheets, Vd , and the voltage applied to the microholes, Vh , where Vm = Vd + Vh . (2) (a) Since the microholes contain air molecules at a certain pressure, when Vh reaches the gas breakdown voltage, a discharge is generated in the microholes and current flows inside these holes. This current is very important in sustaining APGD discharge current. The electron current flows in the microholes and reaches near the alumina surface, hence increasing the probability of electron emission from the alumina surface and in turn increasing the primary seed electrons. Also since the alumina sheet has a large number of microholes between the grains on its surface, these microholes trap most of the electrons that reach the dielectric surface. These trapped electrons contribute to the increase in the seed electrons in the discharge of the next half cycle. In order to understand the physical mechanisms that lead to the formation and the generation of the present APGD discharge by using porous alumina sheets, its surface morphology has been investigated using scanning electron microscopy (SEM). Figure 5 shows the SEM photograph of the surface view (a) and side view of the alumina sheet (b). The surface morphology of the alumina sheet shows the intensive existence of irregular microholes of dimensions up to 100 µm. Also, it can be noticed that there is a network of microchannel connects between the microholes. This network exists not only on the surface but also inside the bulk of the sheet, which connects the microholes between the (b) Figure 5. The SEM photographs of the alumina sheet: (a) the surface view and (b) the side view. two surfaces of the sheet. The discharges generated in the microholes and microchannels can pass through this network of microchannels up to the surfaces of the alumina sheet and therefore provides the seed electrons to help in generating the glow discharge in the gas between the two electrodes [20]. Figure 5(b) shows the existence of sharp edges on the surface of the alumina sheet. These sharp edges are, to a certain 3 Plasma Sources Sci. Technol. 18 (2009) 045006 A A Garamoon and D M El-zeer 0.004 0.0035 I(A) 0.003 0.0025 0.002 0.0015 0.001 0.0005 0 0 500 1000 1500 2000 2500 Va (V) Figure 6. The I –V characteristic curve of the alumina sheet. extent, responsible for the formation of the small filaments that are superimposed on the glow component of the discharge current shown in figure 4. Also from the surface roughness seen in figure 5(b) it can be stated that the formation of neighbor microfilaments on the sharp edges of the alumina surface causes an interference between these neighbor adjacent microfilaments and the closer the microfilaments are the more pronounced the interaction [21]. So decreasing the distance between the neighbor microdischarges allows the interference between these small neighbor filaments and consequently leads to the formation of the homogeneous discharge that fills the discharge gap region. Figure 7. The light emitted by using one porous alumina sheet without the discharge gap. 1.6 60 The current 1.4 Temperature 50 The current (mA) 3.1.2. The impedance of the porous alumina sheet. One porous alumina sheet of thickness 7 mm has been used and two aluminum sheets have been pasted firmly on its two sides. Then a voltage has been applied between the two aluminum sheets only, without any discharge gap. By increasing the applied voltage gradually and recording the voltage and current waveforms, the impedance of the porous alumina sheet has been estimated. The I –V curve of the alumina sheet is shown in figure 6. It can be observed that two lines of different slopes have been obtained and in turn it can be stated that the impedance of the porous alumina sheet decreases from 1.2 M
to about 400 k
, i.e. it has been decreased to a third of its original value, as the applied voltage increases to a certain value. This decrease in the impedance of the alumina sheet is due to the generation of internal discharges inside the microholes of the porous alumina. Also from the data in figure 6 it has been estimated that the discharge voltage in the microholes was in the range 750–1000 V. This value of the discharge voltage (750–1000 V) is comparable to the predetermined minimum breakdown voltage in air using an aluminum oxide cathode which was found to be 416 V [22], taking into consideration that the authors are not certain of the value of (p.d.) in the present case, as the dimensions of microholes and microchannels varies from 1 to 100 µm. The results in figure 6 confirm the fact that the dielectric alumina barrier behaves like a semiconducting material at a certain applied potential due to the generation of discharge inside the microholes. The existence of such a discharge in the microholes has been confirmed by observing the light emission from one porous alumina sheet surface without the discharge gap. As shown in the two images in figure 7 an aluminum 40 1 0.8 30 0.6 20 0.4 The temperature (ºC) 1.2 10 0.2 0 0 0 10 20 3 0 The elapsed time (minutes) Figure 8. The current peak value and the temperature as a function of the discharge running time. sheet has been pasted on one side of the alumina sheet and a metallic grid has been pasted on the other side and a voltage has been applied on the aluminum sheet and metallic grid without the discharge gap. It has been found that a glow has covered the grid which confirms the presence of internal discharges inside the sheet, which provides the seed electrons coming outside the porous alumina sheet that excite the surrounding atmospheric air molecules on the surface that are responsible for the shown glow. Also, the current and temperature of the alumina sheet have been measured by fixing a thermocouple on the alumina sheet and a voltage Va = 2 kV has been applied between the two sides without a gap and the current and temperature were recorded as a function of the elapsed time. Figure 8 shows the current peak value and the temperature as a function of the discharge running time. It can be seen that the current decreases with the increase in the discharge running time and then saturates after passing for about 15 min. The results in figure 8 show that as time elapses, the discharge current in the microholes increases and also the 4 Plasma Sources Sci. Technol. 18 (2009) 045006 A A Garamoon and D M El-zeer temperature increases due to the flow of the discharge current in the microholes. After that, the current drastically decreases which can be attributed to the following: (a) The increase in the pressure inside the microholes due to (i) the increase in the gas density n because of the dissociation of the N2 and O2 in the discharge inside the microholes as time elapses and (ii) this increase in the pressure inside the microholes also due to a lesser extent of the increase in the temperature of the alumina sheet. (b) The continuous build-up of electronegative gas molecules created by the discharge [23]. Due to the drastic decrease in the current, the temperature also decreases and saturates due to the current saturation after a certain time depending on the maximum current that follows initially in the discharge. Figure 9. The total current density, J , the discharge current density, Jdisch , and the displacement current density, Jdisp , as a function of time during one cycle where Va = 4 kV. 3.1.3. Calculation of the electrical parameter of the discharge. The voltage–current waveforms of the discharge can be used to obtain global information about the plasma parameter, i.e. the voltage applied to the gas, Vg , the discharge current, Idisc , the displacement current, Idisp ; also the density of electrons, ne , can be calculated from the electron discharge current Jdisch . According to figure 2, the voltage applied to the gas, Vg , can be calculated from equation (1), i.e. Vg (t) = Va (t) − Vm (t), where, Vm (t) = 1/Cd
t I (t) dt + Vm (t0 ) (3) Figure 10. The applied voltage, Va , the gas voltage, Vg , and the discharge current density, Jdisch , as a function of time during one cycle of the discharge for APGD in air, where Va = 4 kV. t0 and the discharge current can be calculated from   Cg dVa (t) − Cg , Idisch (t) = I (t) 1 + Cd dt (4) The discharge pulse can be subdivided into four regions which are a, b, c and d. In region a of the positive half cycle, the gas voltage, Vg , increases as the applied voltage, Va , increases and the dielectric voltage, Vm , is nearly constant and is negative with respect to the applied voltage because of the memory effect from the previous negative half cycle. When Vg reaches the breakdown voltage of the gas, the discharge current increases and the space charges accumulate on the dielectric surface, which causes an increase in the memory voltage, Vm , toward positive values. So Vg becomes constant with time while the current still increases (region b). During the increase in the discharge current the charges still accumulate on the dielectric surface which causes a further increase in the memory voltage, Vm . So the gas voltage Vg slightly decreases while the discharge current still increases (region c). In region d, the applied voltage Va reaches its maximum value and starts to decrease, and in turn the discharge current starts to decrease. Vm is kept constant due to the charging of the dielectric capacitor of capacitance Cd . To better analyse these observations, it is interesting to compare the variation of the gas voltage, Vg , as a function of the discharge current density, Jdisch , which is a typical voltage– current curve, i.e. characteristic curve of a discharge between two electrodes. where I (t) is the instantaneous discharge current and Cd is the capacitance of the dielectric sheet (Cd was evaluated by using a capacitor of known capacitance instead of the resistance R1 , then the V –Q Lissajous diagram was obtained from which the capacitance of the alumina barrier Cd and the gap capacitance C were determined [16, 24]). Vm (t0 ) is the voltage generated due to space charge accumulation on the dielectric during the previous discharge. Its value was adjusted such that the average gas voltage over a cycle is zero [18]. Figure 9 shows the current density that was measured by the oscilloscope, J , the discharge current density, Jdisch , and the displacement current density, Jdisp , as a function of time during one cycle, where Va = 4 kV. From the same figure it can be seen that the amplitude of the current density is about 0.117 mA cm−2 and is increased for the case of Va = 5 kV to about 0.43 mA cm−2 ; see figure 4. It can be seen that the discharge current is shifted from the total current by the amount of the displacement current. Figure 10 represents a typical variation of the applied voltage, Va , the gas voltage, Vg , the dielectric (memory) voltage, Vm , and the discharge current, Idisch , as a function of time during one cycle of the APGD discharge in air at Va = 4 kV. 5 Plasma Sources Sci. Technol. 18 (2009) 045006 A A Garamoon and D M El-zeer This variation of the gas voltage, Vg , as a function of the discharge current density, Jdisch , in air homogeneous APGD discharge in regions a, b and c is plotted in figure 11. It can be observed from figure 11 that when the breakdown occurs, the gas voltage, Vg , stabilizes and then slightly decreases while the current is still increasing. According to the results of this figure, during each pulse, the APGD in air goes from the non-self-sustained discharge to the Townsend and then the subnormal glow discharge mode starts. The subnormal glow regime is not stable, because the slope of the V –I characteristics is negative, it leads to a decrease in Vg . This decrease in Vg can be due to the increase in the accumulation of the charges on the dielectric and the space charge formation in the subnormal discharge region. Consequently this decrease in Vg does not let the discharge jump to stabilize in the normal glow discharge regime. The electron density can be calculated from the electron conduction current by neglecting the ion current as an approximation, by using the following equation [25]: gas voltage Vg at an applied voltage of 4 kV, the electric field E applied to the gas can be calculated to be nearly constant at ∼3.2 × 104 V cm−1 . For this electric field, the electron mobility is about 5.13 × 102 cm2 V−1 s−1 , which has been estimated from the Boltzmann solver [26] (BOLSIG KINEMA software [27]). Substituting the values of E, µe , Jdisch and e in equation (5), the electron density has been obtained to be of about 0.044 × 109 cm−3 at an applied voltage of 4 kV. 3.2. Spectroscopic measurements 3.2.1. Identification of species in the emission spectra. The typical emission spectra of the present discharge at a discharge current of 0.7 mA is shown in figure 12. The spectra cover the wavelengths from 200 to 800 nm. The remarkable emissions were identified by referring to the basic atomic spectral lines of N, O and H as well as molecular bands of N2 , O2 , OH and CO. The atomic database was constructed from the data, which are provided by the NIST database [28], while the molecular part was created according to Pearse and Gaydon (identification of Molecular Spectra) [29]. The recognized spectra here are the N2 2+ system (SPS) (C 3 u –B 3 g : (1–0) 313.6, (0–0) 337.1, (0–1) 357.7, (0–2) 380.5 nm, etc); also the N2 1+ system (FPS) (B 3 g –A 3 u− : 595.9 nm); N+2 1− (B 2 u+ −X 2 g+ : 419.9 nm); CO at 715.46 and 761 nm, NO at 631.6 nm and the atomic nitrogen N I line at 674.17 nm. The photographic images in figures 13(a) and (b) show the light distribution between the two electrodes (barriers), which have been recorded by using a high resolution digital camera. The camera cannot differentiate between the instantaneous cathode and the instantaneous anode because of its low speed. The existence of a uniform luminous area can be observed at the center of the gap, which has been identified as the negative glow region. Also, there are two regions near both the surfaces of the two barriers of luminosity much lower than the negative glow region, which have been identified as the cathode fall regions. The positive column does not appear in this case because of the small dimension of the discharge gap between the electrodes, d = 1.1 mm. (5) Jdisch = −ne eµe E, where µe is the electron mobility and E is the electric field in the discharge region. From figure 10, which shows the 0.12 2 Jdisch(mA/cm ) 0.10 0.08 0.06 0.04 0.02 0.00 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Vg (kv) 3.2.2. Light emission waveform. The waveform of a single wavelength (at 337.13 nm) has been recorded for the two types of used dielectrics, i.e. Pyrex glass and porous alumina. Figure 11. Variation of gas voltage as a function of discharge current density during the current increase in the air homogeneous APGD plasma. Figure 12. Optical emission spectra of APGD discharge in air in the range 300–800 nm. 6 Plasma Sources Sci. Technol. 18 (2009) 045006 A A Garamoon and D M El-zeer in the two half cycles. This may be due to the slight change in the diameter of the porous holes of the two alumina plates that leads to a difference in the number of the seed electrons in one cycle rather than the other one. The same waveform has been obtained at almost all wavelengths that has been recorded in this work as shown in figure 14(b). (a) 4 mm 4. Conclusion (b) In this work APGD plasma has been realized and investigated at 50 Hz (i.e. without the need for high frequency generators), using porous alumina barriers. It has been found that a discharge takes place inside the microholes and on the surface of the porous alumina. This internal discharge is considered by the authors as an assisting discharge that provides sufficient seed electrons for the initiation and growth of the glow discharge in the gas between the two alumina sheets. I –V waveforms and the electrical characteristics and the light emission spectra confirm that the present discharge is a uniform glow discharge at atmospheric pressure and at a frequency of 50 Hz. 3.7 mm Figure 13. Photographic images of the uniform glow discharge. 1.3 (a) 1.1 0.5 0.9 0 -0.5 0 10 20 30 40 0.7 0.5 -1 0.3 -1.5 Intensity (a. u.) The current (mA) 1 Acknowledgments The authors would like to thank Dr Salah Abd-Elnaeem (Physics Department, Faculty of Science, Al-Azhar University, Nasr City, Cairo, Egypt) for assisting in preparing the alumna sheets and Dr Ahmed Samir Aly (Center of Plasma Technology, Al-Azhar University, Nasr City, Cairo, Egypt) for assisting in the discussion and the experimental work. 0.1 -2 -0.1 5 4 3 2 1 0 -1 0 -2 -3 -4 -5 2.9 (b) 2.4 1.9 1.4 10 20 30 40 0.9 Intensity (a. u.) The current (mA) Time (m sec) References [1] Schulz-von der Gathen V 2005 Atmospheric pressure glow discharges for surface treatment: selected examples 27th ICPIG (Eindhoven, the Netherlands, July 2005) [2] Scott S J, Figgures C C and Dixon D G 2004 Dielectric barrier discharge processing of aerospace materials Plasma Sources Sci. 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