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LESTINSKY ET AL.: AC MICRODISCHARGES IN POROUS CERAMICSFigure 4. Discharge power as a function of the applied voltage (gas mixtures).Figure 5. Current pulse amplitude as a function of the pore size (20% O 2 in N 2 ).We recorded oscilloscopic waveforms of the applied voltage and the current pulses. The microdischargesoccurred both in negative and positive polarity of the applied voltage. During one half-period severalbreakdowns were observed and they were accompanied by a voltage drop almost to zero and a sharp currentpulses. The current pulses had a short duration of less than 100 ns and relatively high amplitude reaching severaltenths of amps. The amplitude of the current pulses increased with the applied voltage and the pore size. At theconstant power, the maximum amplitude of the current pulses was observed with 50 and 80 µm pore sizeceramics (Figure 5).III. Optical propertiesWe also recorded a photographs to visualize the macroscopic changes of microdischarges, depending onthe gas composition and the pore size. This observations showed that the spatial and temporal distribution of thechannels was not steady but randomly changed. The emission intensity increased with the discharge powerresulting from the increase of the number of discharge channels and the discharge current.We found that the increase of the oxygen amount in the gas mixture resulted in a redistribution of themicrodischarge channels inside the ceramics. In nitrogen, the light emission was relatively homogenouslydistributed over the whole surface of the ceramics. With the increase of oxygen a gradual migration of thechannels toward the rim of the ceramics occurred. Similar effect we observed also in CO 2 (Figure 6). Theincrease of oxygen caused also a reduction of the total number of discharge channels, which resulted in a136
LESTINSKY ET AL.: AC MICRODISCHARGES IN POROUS CERAMICSdecrease of the discharge current/power at a given voltage. Comparing all ceramics, at the given power and gasmixture, the most homogenous distribution of the channels inside the ceramics was observed for the pore size of80 µm (Figure 7).N 2 O 2 Air CO 2Figure 6. The effect of gas mixture (pore size 80 µm, P ~ 2-4 W).10 µm 30 µm 80 µm 120 µmFigure 7. The effect of pore size (P = 6.4 W, 20% O 2 in N 2 ).Figure 8. Emission spectra of microdischarges in UV-VIS region (pore size 80 µm, U = 16.5 kV).Figure 8 presents the emission spectra of microdischarges in UV-VIS-NIR region taken at the constantpower in mixtures with various amounts of oxygen. In nitrogen containing mixtures, the 2 nd positive system ofN 2 (C 3 Π u – B 3 Π g ), the 1 st positive system of N 2 (B 3 Π g – A 3 Σ u + ) and atomic N and O lines were observed.Based on the spectral bands of the N 2 2 nd positive system we were able to determine rotational (T r ) andvibrational (T v ) temperatures by fitting the experimental spectra with the simulated ones using Specair softwarefor spectral simulation [6]. Owing to fast collisional relaxation at atmospheric pressure, the gas temperature T gequals with T r . Vibrational temperature T v > T r indicates the non-equilibrium in the plasma. The temperaturesmeasured for the ceramics with 80 µm pore size were the following:N 2 T g ~ T r = (300 ± 50) K T v = (2000 ± 300) K5% O 2 in N 2 T g ~ T r = (350 ± 50) K T v = (2500 ± 300) K20% O 2 in N 2 T g ~ T r = (400 ± 50) K T v = (2800 ± 300) KThe temperatures increased with the increasing amount of O 2 . This is because O 2 is an electronegative gasand so decreases the number of electrons by attachment under formation of negative ions. Higher energy is thusrequired to sustain the discharge at the same parameters than in nitrogen, thus resulting in higher temperatures.137
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