ON THE IGNITION VOLTAGE AND STRUCTURE OF COPLANAR BARRIER DISCHARGES

Transcription

1 ON THE IGNITION VOLTAGE AND STRUCTURE OF COPLANAR BARRIER DISCHARGES L. Hulka and G. J. Pietsch Electrical Engineering and Gas Discharge Technology, RWTH Aachen, Schinkelstr. 2, Aachen, GERMANY Abstract. Coplanar discharges (CDs) appear in arrangements where pair(s) of electrodes are embedded in a solid dielectric. The ignition voltage of CDs is much higher than that in common dielectric barrier discharge arrangements with a gas gap comparable with the electrode distance of CD arrangements. From calculations of the initial field strength distributions the ignition voltage can be found for different electrode distances. In order to analyze the discharge structure an intensified CCD camera has been used. From highly resolved photographs follows that CDs consists of more or less parallel discharge channels on the surface similar to those in common arrangements with gas gaps. 1. INTRODUCTION Dielectric barrier discharges (DBDs) are characterized by discharge gaps and the presence of one or more insulating layers in between the gaps. Typical operation conditions are pressures between 0.1 and 10 bar and frequencies in the range of 50 Hz to 100 khz. There exists a broad range of applications for DBDs like ozone generation, pollution control, surface modification and light production in e. g. CO 2 lasers, UV and VUV lamps as well as in ac plasma display panels [1]. Three basic configurations to produce DBDs can be distinguished [2], the volume discharge (VD), the surface discharge (SD) and the coplanar discharge (CD) arrangement. The VD arrangement is most common. It consists in general of a gas gap between parallel electrode plates with one or both electrodes covered by a dielectric layer. The ignition voltage is mainly determined by the size of the gap, the kind of gas and its pressure. SD arrangements consist of one or more long high voltage electrodes on the surface of a dielectric and an extended counter electrode on its reverse side. Pure SDs appear on the surface of the dielectric. The discharge ignites at Paschen minimum conditions. Its area on the surface is proportional to the amplitude of the applied voltage [3]. FIGURE 1. Principle of a coplanar discharge arrangement. The CD arrangement on the other hand consists of pair(s) of extended electrodes that are embedded in a dielectric close its surface. Most often the electrode separation is in the order of 100 µm. The discharge occurs on the surface of the dielectric in between the location of the electrodes. A principal arrangement is shown in Fig. 1. The advantages of VDs (like constant electrode gap) and SDs (like direct contact with the surface) are combined in this type of discharge. In order to be able to optimize

2 the properties of CDs for special applications it is necessary to understand their behavior. For this purpose the ignition conditions as well as the structure of CDs has been investigated. 2. EXPERIMENTAL SETUP In order to investigate the discharge behavior in CD arrangements, a simple model device has been developed with the possibility to change easily the electrode distance and the thickness of the dielectric layer above the electrodes. In order to facilitate optical investigations, the electrode distance was chosen in the range of a few millimeters. The model device consists of a Teflon plate with milled deep grooves. Thin electrodes are placed in these grooves and they are poured with epoxy resin in order to improve the dielectric strength of the device (Fig. 2). FIGURE 2. Sketch of the model coplanar discharge arrangement (D - electrode separation, d electrode width, - thickness of the Teflon layer above the electrodes). 3. IGNITION VOLTAGE The ignition voltage of DBDs depends on the arrangement. It differs considerably. While it is comparatively low in SD arrangements (ignition at the Paschen minimum), it is rather high in CD arrangements, even much higher than in VD arrangements with a comparable electrode distance. The reason is found in the different field strength distributions. While the field strength distribution in VD devices is rather homogeneous in the gas gap, the situation is quite different in CD devices (Fig. 3). Here the field strength maximum is in between the electrodes within the dielectric bloc. For the breakdown in the gas region the tangential component of the field strength in parallel to the surface of the dielectric is of importance. FIGURE 3. Equipotential lines of a coplanar arrangement with round electrodes and added field strength lines in the upper left part of the figure. In Fig. 4 the absolute value of the electric field strength in the gas region above the dielectric surface is given for different electrode distances (round electrodes). The shape changes with growing electrode

3 distance. At 2 mm distance a single maximum appears, at the higher values two of them at the location of the electrode edges. From the figure follows an electric field strength of 3.2 kv/cm in the gas region between the location of the electrodes with a distance of 2 mm. In a VD arrangement with a 2 mm gas gap the electric field strength is higher, 5 kv/cm, with the consequence of a remarkable lower ignition voltage. For larger electrode distances the breakdown voltage is mainly determined by the value of the field strength minimum, which occurs in the gas space in between the location of the electrodes. 3,5 3,0 2,5 D= 2mm D= 5mm D=10mm d = 2mm E [kv/cm] 2,0 1,5 1,0 FIGURE 4. 0,5 left electrode 0 kv right electrode 1 kv 0, [mm] 6 Absolute value of the electric field strength in the gas region above the dielectric surface for different electrode distances D (round electrode with a diameter of d = 2 mm). The electric field strength in the gas region depends not only on the electrode distance, but also on the thickness of the dielectric, its permittivity and the shape of the electrodes. In Fig. 5 the electric field strength distributions in the gas region above the electrodes are given for three different electrode shapes. The sharp edges of rectangular shaped electrodes cause a higher field strength than round electrodes even with a diameter of 1 mm. 2,5 2,0 E [kv/cm] 1,5 1,0 round d=1mm round d=2mm rectangular d=2mm 0,5 left electrode 0 kv right electrode 1 kv 0, [mm] 6 FIGURE 5. Absolute value of the electric field strength in the gas region above the dielectric surface for different shapes of the electrode (electrode distance 5 mm). With the knowledge of the minimum field strength in the gas space at the location between the electrodes, the ignition voltage can be estimated. Dividing the critical field strength value (about

4 30 kv/cm for air at atmospheric pressure) by this minimum value the ignition voltage for the given arrangement results. In Fig. 6 the ignition voltages resulting from such estimations are depicted together with measured values for different electrode distances. At small electrode distances a certain influence of the electrode shape on the ignition voltage is given. The larger the electrode distance the less is this influence. The measured values are in good agreement with those found by estimation. 40 ignition voltage [kv] estimation rectangular estimation round measurement round electrode distance D [mm] FIGURE 6. Estimated and measured values for the ignition voltage for different electrode distances D in air at atmospheric pressure. 4. OPTICAL INVESTIGATIONS In Fig. 7 a photograph of a discharge channel in a coplanar arrangement just at ignition voltage with short exposure time is shown. The position of the two round electrodes with a diameter of 2 mm inside the Teflon plate is marked by dashed lines. On the left side is the cathode and on the right the anode. This photo was taken with an intensified CCD camera in ambient air and an electrode distance of 2 mm. FIGURE 7. Photograph of a discharge channel on the dielectric surface from the top (CD arrangement, cathode left, 2 mm electrode diameter and distance, dielectric Teflon, ambient conditions, exposure time 200 ns). The discharge channel in between the position of the electrodes looks quite similar to those, which are taken from VD arrangements. There exist a dark zone in front of the cathode and the channel looks a bit like cone shaped. The discharge propagates even on the area above the electrodes. The structure in these regions differ. Above the cathode weak channels can be distinguished, above the anode is a homogeneous discharge distribution. These patterns corresponds to those of the corresponding

5 footprints of microdischarges in VD arrangements [2, 4]. In Fig. 8 photographs of CDs are shown of an arrangement with 2 mm electrode distance, however, with electrode widths of 8 mm. The left one is taken during voltage rise, the right one nearby voltage amplitude, both with an exposure time of 5 µs (period duration 50 µs). A number of nearly parallel channels appears with almost equal distances from one another. The patterns of the channels during voltage rise corresponds to those at ignition voltage (Fig. 7), while at enlarged voltage the character changes. The channels become longer and stronger and seem to discharge larger areas above the electrodes farther from the gap midst. With SDs and high voltage values a similar phenomenon has been observed [5]. Within the exposure time some additional rather faint channels are detected. They result from discharging a limited area in between strong channels due to the rise of voltage. The far distant areas are already discharged by the strong channels so that a limited area is left. FIGURE 8. Photographs of discharge channels on the dielectric surface from the top, left during voltage rise and right at voltage maximum (exposure time 5 µs, AC of 20 khz, voltage amplitude 30 kv, ambient air, electrode distance 2 mm, width 8 mm, cathode at the top). 5. CONCLUSIONS Investigating the initial field strength distribution of CD arrangements it has been found that the character depends on electrode distance. At larger distances two field strength maxima appear. The value of the minimum between them allows to estimate ignition voltages, which are in good agreement with measured values. The shape of the electrodes has only a limited influence on ignition voltage. In comparison to VDs the ignition voltage of CDs is remarkable higher. From photographs taken with an intensified CCD camera follows that the CDs consist of more or less parallel discharge channels, which discharge even the area above the full width of the electrodes at high voltage amplitudes. Their structure is similar to those of microdischarges in VD arrangements. REFERENCES [1] Kogelschatz U., Eliasson B., and Egli W., Dielectric-Barrier Discharges - Principle and Applications, J. Physique IV, C (1997) [2] Gibalov V. I., and Pietsch G. J., The Development of Dielectric Barrier Discharges in Gas Gaps and on Surfaces, J. Phys. D: Appl. Phys. 33, (2000) [3] Haacke M., and Pietsch G. J., Some Features of Dielectric Barrier Discharges, In: 13 th Int. Conf. on Gas Discharges and Their Applications, Glasgow, UK, 2000, pp [4] Heuser C., and Pietsch G., Prebreakdown phenomena between glass-glass and glass-metal electrodes, In: 6 th Int. Conf. on Gas Discharges and Their Applications, Edinburgh, IEE Conf. Publ. 189, 1980, pp [5] Saveliev A. B., and Pietsch G. J., On the Structure of Dielectric Barrier Surface Discharges, In: Proc., 8 th Int. Symp. on High Pressure, Low Temperature Plasma Chemistry (Hakone VIII), Pühajärve, Estonia, 2002