Characteristic Study On Atmospheric-pressure Radio-frequency Capacitively Coupled Ar/N₂Plasma Discharge

Atmospheric-pressure plasma can effectively reduce equipment investment, maintenance and production costs, and can realize continuous production by elimination of the need for an expensive vacuum system. Therefore, there is a widespread concern for its applications in recent years. In this paper, a uniform and stable atmospheric-pressure radio-frequency Ar/N2glow discharge was produced in a parallel bare metal plate reactor by means of radio-frequency capacitively coupled discharge through pre-excitation technique at the inlet of the working gas.First, three different discharge modes of Ar/N2glow discharge were revealed from the discharge forms:normal a-mode glow discharge mode, abnormal a-mode glow discharge and y-mode/arc discharge, respectively. The variation of the critical voltage and current values versus the nitrogen-to-argon ratios was given while the Ar/N2radio-frequency plasma translated into abnormal a mode and y mode/arc discharge. The variation of current-voltage characteristic (CVC) curves for the Ar/N2plasma versus nitrogen percentages was analyzed, also. Since the electrical characteristic of a gas discharge is an important basis for diagnosis of plasma, the impedance, phase angle, resistance and reactance of the Ar/N2glow discharge plasma versus the nitrogen-to-argon ratios was measured, respectively. The experiment results showed that the critical voltage values increased monotonously with increasing addition of nitrogen, while the critical current values changed slightly; it was found that each of the current-voltage curves consists of two segments which corresponds to a normal a-mode discharge and an abnormal a-mode discharge, each varying almost linearly at different slopes with respect to two segments. Second, When optical emission spectroscopy (OES) is used as an important means of diagnosis of plasma, the vibrational distribution function (VDF) of the nitrogen molecules (C3Πu, u’=0-3) state and the normalized distribution characteristic (NDC) curves was analyzed using the second positive band of N2spectrum. The experiment results showed that the VDF is closed to Boltzmann distribution, and the vibrational temperature varied slightly, which can be estimated to be about2216K with respect to the uncertainty of5%. Additionally, the relative variation of the N-atom density with the nitrogen-to-argon ratios was obtained actinometrically. It was founded that the N-atom density increased coincidently with addition of nitrogen. Last, the hydrophilic modification of slide glass treated by Ar/N2plasma was studied experimentally, and a qualitative explanation was given.Furthermore, the matching issue of radio-frequency capacitively coupled microplasma discharge has been investigated with respect to the influence of transmission line. It is by changing the frequency of signal source and the length of coaxial lines between the matching box and the plasma reactor that a high resonance voltage was generated and supplied for an atmospheric-pressure capacitively coupled dielectric-barrier plasma source to strike and maintain plasma discharge at atmospheric pressure. First, in order to obtain the variation tendency of capacitances of coaxial lines versus their length, the impedance characteristics of coaxial lines was measured by using RF impedance analyzer; Second, the variation tendency of circuit resonance frequency versus the length of coaxial lines was determined by changing the frequency of the signal generator; Finally, the variation tendency of the resonance voltage in the terminal plasma reactor versus the length of coaxial lines was obtained. Also, the experiment results were analyzed by means of circuitous philosophy with introduction of an equivalent circuit, which gives a satisfactory agreement. The experiment results showed that coaxial transmission lines exhibited capacitances in the working frequency range and their capacitances increased linearly with the length of the transmission line; the circuit resonance frequency and the terminal reactor voltage decreased with the length of coaxial transmission line.