Tunable Band Gap in Hydrogenated Quasi-Free-Standing Graphene

Eur. Phys. J. Appl. Phys. 24, 67–74 (2003) DOI: 10.1051/epjap:2003051

THE EUROPEAN PHYSICAL JOURNAL APPLIED PHYSICS

Electrical study of DC positive corona discharge in dry and humid air containing carbon dioxide S. Lachaud and J.F. Loiseaua ´ Laboratoire d’Electronique des Gaz et des Plasmas, Université de Pau et des Pays de l’Adour, 64000 Pau, France Received: 17 January 2003 / Revised in final form: 25 April 2003 / Accepted: 2 June 2003 c EDP Sciences Published online: 16 July 2003 – Abstract. As most part of the industrial effluents contain carbon dioxide and water vapour, it is interesting to study from an electric point of view ‘basic’ gas mixtures including air and various amounts of these gases, in order to precise how the corona discharge inception takes place in such mixtures. In a DC pointto-plane reactor, distinct parameters are varied, such as discharge current, partial pressure of CO2 , and relative humidity. Gap voltage and streamer frequency are experimentally measured as functions of the discharge current. Current waveforms are recorded in various gas mixtures and electrical conditions. DC and impulsional current components, as well as streamer frequency are also plotted as functions of the CO2 partial pressure for dry and water saturated mixtures. For dry or wet mixtures, 5% CO2 in volume appears to be the proportion allowing the establishment of a stable corona discharge with the lowest energy cost. PACS. 51.50.+v Electrical properties (ionization, breakdown, electron and ion mobility, etc.)

1 Introduction Gaseous pollutants are always presents in industrial activities, and effluents like H2 S, NOx , SOx or Volatile Organic Compounds (VOC) are one of the most frequent cause of breathing diseases. Many devices have been developed for the last 40 years, such as mechanical filtration, chemical action or washing of gaseous effluents. At the present time, other processes are developed with the aim to be more efficient than classical methods and cold plasmas is one of these new methods [1]. Cold plasmas are mainly generated by an electrical discharge between two electrodes with various particular geometries. For industrial treatments, which concern gaseous effluents with relatively high flow rates, depollution must be performed at atmospheric pressure, and the most used devices are pulsed corona discharges, and DBD (Dielectric Barrier Discharge, which needs AC power supply) [2–4]. The geometry of electrodes can be wireplane (electrostatic precipitators), multipoint-plane, wirecylinder or concentric cylinders (flue gas depollution) [4]. Ionizing fronts, which in corona discharges are referred to as streamers [5–9], are essential in any cold plasma depollution process: during their propagation in the interelectrode gap, ions and excited species are created, as well as radicals like O, N, O3 , OH or HO2 . These active species intervene in reactions with pollutants which are destroyed or transformed into innocuous molecules [4]. a

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Most of the chemical kinetics takes place in the time intervals between streamer propagation and also, in the case of pulsed corona, between two subsequent impulses. For the fundamental (electrical or chemical) study of a corona discharge reactor, these configurations may be replaced by point-to-plane geometry, which is less efficient but more easy to manage in a laboratory environment. As far as we are concerned with fundamental processes, the experimental study must take place in a closed vessel with controlled atmosphere and surrounded by the electrical and optical devices for discharge and plasma diagnostics. Even if a pulsed supply is much more efficient for depollution than a DC one, at small time scale it may be considered as a ‘chopped DC corona’, and during the pulses the main features of the discharge are essentially the same. For this reason, as the electrical problems (stray capacitances and self-inductions) are much simpler in DC devices, the present study has been carried on with a DC point-toplane reactor. The electrical results may be extrapolated to pulsed coronas provided the frequency is significantly lower than 10 kHz (so that many streamers can occur during the duration of one ‘voltage slot’). Of course, the extrapolation would not be so simple for chemical kinetics of depollution. In order to improve gas depollution processes using cold plasmas, a precise characterisation of the discharge regime (voltage-current curves, waveforms and frequency of current impulses) is needed for the ‘basic’ gas mixtures which generally include air, carbone dioxide and water vapour. For corona discharge, the electrical behaviour

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The European Physical Journal Applied Physics

(value of gap voltage, current threshold for the beginning of streamer regime, current waveforms) is modified with small amounts of electronegative gases [9,10], and also when polar molecules are present, leading to the formation of clusters [4, Chap. 8] and [11]. Mass spectrometry studies [12] have proved that complex mechanisms take place, involving ozone in dry air and clusters in humid air. The present work is a part of an experimental study [13], here from electrical point of view, of corona discharge at atmospheric pressure in various gas mixtures which constitute the basis of industrial effluents. A next paper will involve the chemical processes taking place in these gas mixtures. The experimental device is a DC reactor with short gap point-to-plane configuration: the experimental set-up is described in Section 2. The electrical measurements performed for various gas mixtures are displayed in Section 3, and followed by a physical discussion (Sect. 4). Concluding remarks are given in Section 5.

2 Experimental set-up The experimental device (Fig. 1) consists in a stainless steel cylindrical reactor (capacity: 24 litres) filled with controlled atmosphere. The anode, connected to a positive DC high-voltage supply (maximal voltage 30 kV) and fixed on a stainless steel rod, is a rhodium point with 100 µm tip curvature radius. The cathode is an earthed stainless steel plane axed on the anode’s direction, with a 100 mm diameter) and the interelectrode gap is d = 10 mm. This reactor is equipped with quartz windows for optical purposes. Work pressure is 760 torr and the gas mixtures used are bottled dry air (N2 : 80%, O2 : 20%), hereafter denoted as ‘synthetic air’, with various ratios of CO2 and/or H2 O (vapour): these are the basic mixtures of industrial effluents (power plants and transports). The relevant parameters are CO2 partial pressure (PCO2 ) and Relative Hygrometry (RH). All the experiments are performed without gas flow (it was noted in previous works [10,14,15], that electrical measurements in a gas flow give less precise and reproducible results), and a primary pumping device is used before any new gas filling. Of course, industrial devices use corona discharges in flue gas. So, as a rule, a gas flow should have been used and considered as a parameter; without it, the gas mixture proportions change slowly with time and the discharge regime may depend on the concentration of by-products such as ozone, metastables or chemically active radicals. However, the electric wind generated by the filamentary discharge in the reactor [16,17] drains off the by-products from the discharge volume (which is very small when compared to the reactor volume) and if the electrical measurements are made within a time lag sufficiently short to avoid an appreciable amount coming back through vortex circulation [17], they may be neglected as chemical components of the mixture. This assumption yields in fact a good approximation of a flue gas discharge.

Fig. 1. Experimental set-up.

A loading resistor R = 100 MΩ (ballast) is inserted between the high-voltage supply and the anode in order to limit the discharge current. The gap voltage is measured by a numerical voltmeter across a high voltage probe and the discharge current by a low resistive ammeter (precision better than 1 µA). The current waveform is recorded by a numerical oscilloscope (Tektronix) at the plane through a 50 Ω resistor which may replace the ammeter.

3 Electrical measurements 3.1 General features The experimental results have been separated into three cases (respectively displayed in Sects. 3.2, 3.3 and 3.4), according to the gas mixture considered: – synthetic air (80% N2 ; 20% O2 ) with various values of PCO2 ; – synthetic air (80% N2 ; 20% O2 ) with various values of RH; – mixtures of synthetic water saturated air (RH = 100%) with various CO2 ratios. A comparison of ‘dry’ and ‘wet’ mixtures of air and carbon dioxide is also performed in Section 3.4, and a summary is given in Section 3.5. The electrical study of the corona discharge has been performed with two distinct approaches: a static one through the so-called V -I curves, plotting the gap voltage V versus the average discharge current I (Figs. 2, 6 and 8), and a time resolved one through the current waveforms recorded with an oscilloscope with various time scales (Figs. 3, 4, 5, 7 and 9).

S. Lachaud and J.F. Loiseau: Corona discharge in dry and humid air containing carbon dioxide

In the case of a DC positive point electrode, at low applied voltages, a luminous phenomenon appears in the anode vicinity (anodic glow), the interelectrode gap remaining dark: this regime is often called ‘dark discharge’ and corresponds to the classical Townsend regime in homogeneous fields [10,14,15] It is characterized by a DC current upon which high frequency (1−10 MHz) and low amplitude oscillations may be superimposed at low pressure [14] and sometimes, as will be observed, at atmospheric pressure. When the applied voltage is increased, the so-called prebreakdown streamers precede either a direct transition to arcing or a transition to the proper corona regime where the gap is partially or totally illuminated by the successive streamers. The corresponding V -I curves always display a positive dV /dI slope [9,10,14,15], and a ‘rather smooth’ slope breaking occurs at the eventual transition to the streamer regime. The streamer regime displays a DC current component upon which are superimposed current impulses related to the ionizing fronts (streamers). The corresponding waveforms and their repetition rate may give further informations [18,19]. Throughout the following study, the word ‘frequency’ will be used to denote the inverse of the average time lag between the streamer impulses. This is more a statistical quantity than a real frequency; it is obtained by using oscillograms such as those presented in Figures 4d and 5d which take into account the superimposition of 512 successive waveforms and display a ‘jitter’ on the impulse following the triggered one. Then, the frequency-current characteristic curves (F -I curves) can be plotted (Fig. 10). When the jitter becomes too considerable, the phenomenon cannot be considered as periodic or quasi-periodic (there are only repetitive streamers with random time spacing) and no frequency can be given (interruption in the F -I curve). This is often the case in the 10−25 µA range for the discharge current, especially for great amounts of CO2 (PCO2 > 100 torr). Above I = 25 µA, all the CO2 and/or H2 O ratios give a streamer regime with a relatively stable repetition rate which may be interpreted as a frequency in the meaning above defined, and plotted versus PCO2 (Fig. 11). Two other quantities can be plotted versus PCO2 : the amplitude of the dc component of the discharge current (Fig. 12), and also the streamer ‘amplitude’ (Fig. 13), which is in fact the maximum amplitude of the corresponding current impulses (detected by an adjustment of the trigger level of the oscilloscope). 3.2 Synthetic air + CO2 The first mixture studied is dry synthetic air (80% N2 ; 20% O2 ) with a variable amount of CO2 added. Results show that in dry air without CO2 , no streamer regime can be observed. Only dark discharge exists on the whole V -I curve (Fig. 2), up to arcing. The signal recorded during the dark discharge displays only a DC current without impulses. The amplitude of this DC current increases when the running point follows the V -I curve (Fig. 3).

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P : pressure in Torr d : gap between anode tip and cathode ρ : anode curvature radius

Fig. 2. V -I curves in synthetic air + CO2 .

Fig. 3. Current signals in pure synthetic air. (a): I = 15 µA; (b): I = 43 µA. Current scale: 20 µA/maj. div.; time scale: 100 ns/maj. div.

If a small quantity of CO2 is injected in the mixture, the streamer regime appears with the lowest available currents (Fig. 4). The amplitude of the DC component decreases until PCO2 reaches 40 torr (Fig. 12). For a fixed value of I, the gap voltage V decreases with PCO2 in the 0−40 torr pressure range, then it increases for higher pressures (Fig. 2). With PCO2 ≈ 300 torr (40% CO2 ), the V -I curve is quite similar to the case of pure synthetic air up to I = 30 µA, but the current waveform is very different (streamer regime instead of dark discharge). The streamer frequency is relatively ‘well defined’ (in the statistical meaning defined in Sect. 3.1), and this stability increases with the discharge current (compare Figs. 4d and 5d). With pure CO2 , at low current a few streamers occur randomly, then recurrent sparks looking like a transient arcing appear at I ≈ 20 µA, but a further increase of applied voltage results in a stable streamer regime, similar to the one obtained with lower CO2 amounts (but the gap voltage is about 2 kV higher). In order to obtain for every discharge current a well defined frequency (cf. Sect. 3.1), the F -I curve has been

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(2,56.1016 cm-3) (2,99.1017 cm-3) (8,53.1017 cm-3)

s

Fig. 4. Current signals in synthetic air+CO2 . PCO2 = 38 torr; I = 5 µA. (a, b): impulse waveform; (c, d): repetition rate [d: 512 signals]. Current scale: (a, c, d): 20 µA/maj. div. (b): 1 mA/maj. div.; time scale: 100 ns/maj. div.


Fig. 6. V -I curves in synthetic air with various water vapour concentrations (correspondence between RH and concentration is for T = 300 K).

(below 100 torr) at the transition between dark discharge and the ‘classical’ glow discharge [10,14,15]. 3.3 Synthetic air + H2 O Experiments have been performed with water vapour injected in synthetic air, the following protocol being used in order to adjust the Relative Hygrometry (RH) of mixture is the following: – – – –

Fig. 5. Current signals in synthetic air+CO2 . PCO2 = 38 torr; I = 30 µA. (a, b): impulse waveform; (c, d): repetition rate [d: 512 signals]. Current scale: (a, c, d): 20 µA/maj. div. (b): 2 mA/maj. div.; time scale: 100 ns/maj. div.

plotted for a small (PCO2 = 38 torr) CO2 amount (Fig. 10). Provided I > 10 µA, the streamer frequency increases with the discharge current. When CO2 is present in the gas mixture, a system of more or less chaotic oscillations appears before the streamer impulse (Fig. 5a). This phenomenon may be called ‘α signal’ by reference (however there is no certainty that it is really the same phenomenon) to the oscillations observed in nitrogen and synthetic air at lower pressure

closing of pump after reactor under vacuum; injection of liquid water in reactor; evaporation of water in reactor; injection of dry mixture in reactor.

The effect on electrical discharge is perceptible even with a small amount of water vapour. However, the scattering of curves is lower than in the case of dry mixture air + CO2 (Fig. 6). A slope breaking, relatively smooth, can be observed on the corresponding V -I curves. Contrarily to the case of CO2 (where the gap voltage variation with PCO2 was not monotonic), the gap voltage is always increasing with increasing RH for low currents, up to the slope breaking. Impulses of current (streamers) are always present. However, contrarily to the case of dry air + CO2 , the ionizing fronts occur randomly, and a real streamer regime, with high repetition rate but no ‘well defined’ frequency appears only for the highest current values, just before arcing (plateau on the V -I curves of Fig. 6). Contrarily to the waveforms observed in dry air + CO2 mixtures, the α signals do appear with the lowest measured currents. With increasing discharge currents, they organize themselves when current increases until appearance of a well-defined frequency around 10 MHz (Fig. 7c).


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s (8,53.1017 cm-3)

Fig. 7. Current signals in synthetic water saturated air. (a): I = 5 µA (DC component + α signals). (b): I = 20 µA (DC component + α signals). (c, d): I = 30 µA (DC component+α signals + streamer impulse). Current scale: 20 µA/maj. div.; time scale: 100 ns/maj. div.

However, their amplitude fluctuates and presents a growing tendency. They seem to occur with any RH value, even if no streamer regime can be established (at low currents for instance). The streamers seem to appear when a sequence of α signals reaches a threshold amplitude, the last ones of the sequence being less and less sinusoidal (Fig. 7d). 3.4 Synthetic air + CO2 , saturated with H2 O In this last case, the V -I curves are not very different from those of dry mixtures air + CO2 (compare Fig. 8 to Fig. 2). Without CO2 , the streamer regime (characterized by a luminous column in the interelectrode gap) exists between 25 µA and 40 µA. When CO2 is added, this regime is present for every curve of Figure 8. As in the ‘dry’ case, the cases PCO2 = 0 and PCO2 = 304 torr (40% CO2 in volume) are very close to each other, up to the slope breaking. The potential drop on the V -I curve (recurrent sparks), which happened only with pure (or almost pure) ‘dry’ CO2 , appears in the ‘wet’ case with PCO2 = 570 torr (75% CO2 in volume). With water saturated pure CO2 , in order to protect the experimental device, experiment was rapidly interrupted as the sparks seemed to lead directly to arcing at I = 20 µA. Considering now the DC component (Fig. 12), it grows monotonically and, except for the lowest values of CO2 , stays in the same order of magnitude (15 to 20 µA) for either ‘dry’ or ‘wet’ mixtures of synthetic air + CO2 . The variation of the peak current value in the streamer regime (Fig. 13) is also similar, slowly increasing up to PCO2 ≈ 80 torr and then decreasing.


Fig. 8. V -I curves in synthetic air + CO2 , water saturated. (RH = 100%, T = 300 K).

Fig. 9. Current signals in synthetic air + CO2 , water saturated. PCO2 = 38 torr; RH = 100%; I = 30 µA (DC component + α signals + streamer impulse). Current scale: (a): 20 µA/maj. div. (b): 4 mA/maj. div.; time scale: 100 ns/maj. div.

The α signals are also present in the wet mixtures and their amplitude increase just before giving birth to a streamer impulse (compare Fig. 5a to Fig. 9a), and the F -I curve for streamers is similar to the case of a dry mixture (Fig. 10). It must be noted also that for greater amounts of CO2 (for instance PCO2 = 304 torr), as in the dry case, a current value between 10 µA and 25 µA gives a greater statistical dispersion of the time interval between two successive streamers (cf. Sect. 3.1) so that no frequency can be clearly defined in this current range.

3.5 Summary All these results show that a small quantity of water or carbon dioxide added to air is sufficient to trigger off the

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(8,53.1017 cm-3)

FF-I curve RH = 0 % FF-I curve RH = 100 % 0

Fig. 10. V -I and F -I curves in dry and wet mixtures. PCO2 = 38 torr; RH = 0% and 100%.

Fig. 12. DC current component vs. PCO2 (dry and wet mixtures, I = 30 µA).

(Figs. 6 and 8), on which the streamer regime initiates with a slope breaking; – for a fixed current corresponding to a well established streamer regime (here, I = 30 µA), the partial pressure PCO2 = 38 torr (5% CO2 in volume) gives the lower gap voltage (Figs. 2 and 8), so that this mixture allows the establishment of a stable streamer regime with the lowest energy cost. This is an interesting information for industrial applications.

4 Physical discussion According to classical models [5–8], the establishment of a streamer regime demands two distinct requirements: Fig. 11. Streamer frequency vs. PCO2 (dry and wet mixtures, I = 30 µA).

streamer regime. The evolution of streamer regime depends on the amount of CO2 or H2 O injected: – adding a small quantity of CO2 allows the gap voltage to decrease, but beyond 40 torr of CO2 partial pressure, the gap voltage increases again (Figs. 2 and 8); – without CO2 , in the stable streamer regime (on the plateau after the slope breaking), the more RH is high, the more gap voltage is low (Fig. 6). The following results can be stated: – dry (RH = 0%) and wet (RH = 100%) mixtures of air and carbon dioxide behave similarly from electrical point of view; – the intensity range in which the streamers do appear is variable with CO2 proportion: as a general rule, the streamer regime extension in current increases with RH: this can be observed on the V -I curves

(i) at least one avalanche must reach a critical charge (∼108 , Meek’s criterion) to create a local distorsion of the electric field and initiate the first streamer. In positive point-to-plane configuration, this phenomenon takes place close to the point tip (in the cathode direction); (ii) electrons must be present ‘before’ the streamer’s head, in order to create new avalanches, the streamer’s head playing the role of a spatial extension of the anode. In any gas, the first condition can be fulfilled provided the gap voltage is high enough: a sharp point makes it easier, the high electric field facilitating electron multiplication by ionization in its vicinity. But in pure gases as N2 or CO2 , the second requirement cannot be realized only by the electrons coming from the cosmical background, except if a high overvoltage creates a high field in a region larger than the point vicinity where the first critical avalanches are initiated (this was investigated by numerical simulation [20–22]). Even in this case, experimental studies generally show that the streamer regime is a transient one, leading immediately or very quickly to arc.


Fig. 13. Peak current value in the streamer regime vs. PCO2 (dry and wet mixtures, I = 30 µA).

It is generally admitted that the second requirement is fulfilled by photoionization which mainly acts through the following mechanisms: – step-ionization is probably efficient for noble gases such as argon [23,24] and may be present also in other gases and favourized by the existence of metastables [25]; – an experimental study on laser-induced streamers [26] has shown that if multiphoton ionization is possible, it is concealed by other processes at high pressure; – ionization of a precise species can be attributed to the photons resulting from de-excitation of high energy levels of other molecules [8] or dissociation products [27] (in fact, streamers do occur in pure oxygen [28]). Such processes are likely to happen more easily in gas mixtures. Raizer claims that the conditions for photoionization enhancement of avalanches are realized in the case of N2 /O2 ‘although there is hardly any experimental confirmation of this’ [8, p. 337]; according to this author, ‘such events are infrequent’. However, the probability of their occurrence may be increased in a gas mixture with several species and various dissociation opportunities. As a matter of fact, it is difficult to observe a stable streamer regime in very dry air [13] and the above study show that pure CO2 is unable to initiate a streamer regime at low voltage (although a few streamer do occur randomly) and that, when the gap voltage is high enough, the streamer regime is initiated by a transient arcing (see Sect. 3.2 and Fig. 2). But a very small amount of water vapour enables the inception of a streamer regime. Electronegativity is sometimes invoked to explain why a stable streamer regime cannot be established in short gaps. Electronegative species like O2 reduce (by attachment) the electronic population, and as the effective ionization rate is lower, a longer path is needed for the avalanches to get the critical size so that, according to Raizer, dry air in short gaps goes to breakdown through

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avalanche multiplication rather than streamers [8, p. 340]. This might be confirmed by the case of pure synthetic air treated above (Sect. 3.2): the random oscillations on the DC current in Figure 3 could be due to avalanches which never reach the critical size. However, Raizer’s argument is only valid for homogeneous fields or, in inhomogeneous ones, for the low field region where attachment takes place. In the point anode vicinity or close to the streamer’s head (if a propagating front has already been initiated), the field value is too high for attachment processes: on the contrary, electrons are possibly released by detachment (as the initial density of negative ions is negligible, this concerns the negative ions previously created by attachment of secondary electrons coming from the cathode). Essentially, the negative ions work as ‘electron carriers’ until they release them in the high field region close to the anode or a streamer’s head. In addition, the transported electrons may result from secondary effects in the cathode region [7, Chap. 8]. As a consequence, these processes seem to increase the electron density before the streamer’s head and then to make easier the front propagation. So, the classical explanations are incomplete, or somewhat fuzzy, about the influence of both photoionisation and electronegativity on streamer inception, and there must be other good reasons to explain why O2 , which is an electronegative molecule, is of no help to initiate a streamer regime in dry air, whereas adding CO2 (which is electronegative too) favourizes the streamer inception. . . and also why a very small amount of water vapour has the same effect. For mixtures containing water vapour, the possible formation, around the ions, of clusters due to the polarity of H2 O molecules, may play an important role [4,11]. Clusters have a very low mobility, which varies with the number of water molecules sticked to the ions. In the same way as the bare negative ions, they carry the negative charge and release it in the high field regions. The statistical dispersion of their mobilities could explain, at least partly, the corresponding dispersion of the time intervals between successive streamers: in fact, the observed ‘jitter’ (Figs. 4d and 5d) is increased in the case of ‘wet’ mixtures, sometimes in such proportions that the frequency cannot be well defined. As noted at the end of Section 3.2, the α signals present a waveform similar to the so-called ‘α oscillations’ observed in nitrogen or air discharges at lower pressure [14,15]. These oscillations, which modulate the DC component of current in the dark discharge regime, were interpreted as precursors of the transition to glow discharge and associated to the oscillations of an electrostatic double layer. With increasing applied voltage, the amplitude of these oscillations become larger and their waveform less and less sinusoidal until a destruction gives birth to a large ‘γ impulse’ identified to a ‘pseudo-streamer’ of glow regime. The double layer appears because the gap is filled by two distinct sources of electrons [14]: the ionization region close to the point, and the secondary effects taking place on, or just above the cathode surface.

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Of course, a model which was established for pressures below 50 torr cannot be transposed to high pressure without careful examination of specific conditions (mean free path, cross sections, radial diffusion, etc.), but the similarity in waveform patterns is striking, and in both cases, in such a short gap, the secondary effects on the cathode plane cannot be neglected as an electron source for discharge sustainment. This track, which is coherent with a continuity, already investigated, of phenomena from low pressure (glow discharge) to high pressures (corona) [10,29], must be followed in the future.

5 Conclusion This experimental study of positive point-to-plane discharge in gas mixtures containing air, carbon dioxide and/or water in various proportions allows to state the following points: (i) (ii) (iii)

(iv)

(v)

dry synthetic air or pure CO2 cannot give birth to a stable streamer (corona regime); there are no major differences between dry and wet mixtures of air and CO2 ; when water vapour is present, the time intervals between two successive streamers, as well as the amplitude of the associated current impulses are more statistically scattered; for dry as for wet mixtures, 5% CO2 in volume appears to be the proportion allowing the establishment of a stable corona discharge with the lowest energy cost; a few observations, particularly concerning the current waveform at small time scale (∼100 ns), may imply a similarity with discharge mechanisms occuring at lower pressure.

The authors are grateful to Pr. B. Held and Dr. N. Soulem (University of Pau, France) and to Pr. N. Spyrou (University of Patras, Greece) for very interesting discussions about corona discharge and its physical mechanisms. They are also indebted to Dr. R. Peyrous for having initiated in Pau, more than twenty years ago, experiments on point-to-plane discharges in humid air.

References 1. M. Popescu, J.M. Blanchard, J. Carré, Analyse et traitement physico-chimique des rejets atmosph´ eriques industriels (Paris: Lavoisier/Tec & Doc, 1998) 2. G. Larroche, M. Orfeuil, Les Plasmas dans l’Industrie (Avon (France): Electra/Dopee & Paris: Lavoisier, 1991) 3. B.M. Penetrante, S.E. Schultheis, Non-Thermal Plasma Techniques for Pollution Control, NATO ASI Series G (Berlin: Springer-Verlag, 1993), Vol. 34 4. E.M. Van Veldhuizen, Electrical discharges for environmental purposes – Fundamentals and applications (New-York: Nova Science Publishers, Inc., 2000)

5. H. Raether, Electron avalanches and breakdown in gases (London: Butterworth, 1964) 6. L.B. Loeb, Electrical coronas (Berkeley: University of California Press, 1965) 7. E. Nasser, Fundamentals of gaseous ionization and plasma electronic (New-York: Wiley-Interscience, 1971) 8. Yu.P. Raizer, Gas Discharge Physics (Berlin: SpringerVerlag, 1991) 9. B. Held, Proc.11th Int. Conf. on Gas Discharges and their Applications, Tokyo (Japan), 1995, Vol. II, pp. 514–526 ´ 10. A.E. Ercilbengoa, Etude expérimentale de régimes de décharge continue positive dans l’azote et l’air pour différentes pressions, Doctorate Thesis, Université de Pau et des Pays de l’Adour (France), 1999 11. R.M. Lapeyre, R. Peyrous, Environ. Technol. Lett. 2, 29 (1981) 12. B. Held, R. Peyrous, Czech. J. Phys. 49, 301 (1999) 13. S. Lachaud, Décharge pointe-pan dans les mélanges gazeux correspondant aux effluents industriels : ´ etude électrique et physico-chimique, application a ` la destruction du dioxyde d’azote, Doctorate Thesis, Université de Pau et des Pays de l’Adour (France), 2002 14. A.E. Ercilbengoa, J.F. Loiseau, N. Spyrou, J. Phys. D: Appl. Phys. 33, 2425 (2000) 15. A.E. Ercilbengoa, N. Spyrou, J.F. Loiseau, J. Phys. D: Appl. Phys. 34, 584 (2001) 16. J. Baricos, J. Dupuy, R. Peyrous, G. Schreiber, J. Phys. D: Appl. Phys. 11, L187 (1978) 17. J.F. Loiseau, J. Batina, F. Noël, R. Peyrous, J. Phys. D: Appl. Phys. 35, 1020 (2002) 18. R.S. Sigmond, Corona Discharges in Electrical Breakdown in Gases, edited by J.M. Meek, J.D. Craggs (New-York: John Wiley and Sons, 1978), pp. 319–384 19. M. Goldmanand, A. Goldman, Corona Discharges in Gaseous Electronics, edited by M.N. Hirsch, H.J. Oskam (New-York: Academic Press, 1978), Vol. II, pp. 219–290 20. J.F. Loiseau, F. Grangé, N. Spyrou, N. Soulem, B. Held, Proc.11th Int. Conf. on Gas Discharges and their Applications, Tokyo (Japan), 1995, Vol. II, pp. 492–495 21. F. Grangé, N. Soulem, J.F. Loiseau, N. Spyrou, J. Phys. D: Appl. Phys. 28, 1619 (1995) 22. F. Grangé, J.F. Loiseau, N. Spyrou, Proc. 5th Int. Sympos. on High Pressure Low Temperature Plasma Chemistry, HAKONE V, Milovy (Czech Republic), 1996, pp. 205–209 23. N.L. Aleksandrov, E.M. Bazelyan, A.Yu. Gorunov, I.V. Kochetov, J. Phys. D: Appl. Phys. 32, 2636 (1999) 24. N.L. Aleksandrov, E.M. Bazelyan, G.A. Novitskii, J. Phys. D: Appl. Phys. 34, 1374 (2001) 25. F.M. van Veldhuizen, W.R. Rutgers, J. Phys. D: Appl. Phys. 35, 2169 (2002) 26. N. Soulem, B. Held, J. Chapelle, J. Phys. D: Appl. Phys. 29, 1952 (1996) 27. G.W. Penney, G.T. Hummert, J. Appl. Phys. 41, 572 (1970) 28. F. Nicolas, J.F. Loiseau, A.E. Ercilbengoa, R. Peyrous, J. Phys. D: Appl. Phys. 31, 3108 (1998) 29. N. Soulem, Application de l’interaction laser-gaz a ` l’étude des régimes de fonctionnement d’une décharge électrique, Doctorate Thesis, Université de Pau et des Pays de l’Adour (France), 1996

Tunable Band Gap in Hydrogenated Quasi-Free-Standing Graphene

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