High stability of atmospheric pressure plasmas containing carbon tetrafluoride and sulfur hexafluoride

Abstract

The properties of electronegative discharges operating at atmospheric pressure have been investigated. The plasma source consisted of two parallel metal electrodes into which helium was fed with 0.5-8.0 Torr carbon tetrafluoride or sulfur hexafluoride. It was found that the ionization mechanism changed from the α-to the γ-mode at a critical RF power level. In pure helium, this resulted in an abrupt drop in the voltage with increasing current, whereas for the fluorine-containing plasmas, a smooth, continuous transition was observed along the I-V curve. The electron densities in the plasmas fed with 4.0 Torr CF 4 and SF 6 were calculated to be 7.0 ± 2.0 × 10 11 cm −3 , or about 30 times lower than that estimated for the pure helium case. Free electrons were consumed by electron attachment to the gas molecules, which boosted the density of negative ions to ∼10 13 cm −3. Compared with pure He, the lower electron density in the fluorine-containing plasmas removed the ionization instabilities encountered in the γ-mode and made it possible to sustain these discharges over substantially larger areas.

Characterization of an Atmospheric Pressure Radio-Frequency Capacitive Plasma Jet

“…Current and drift velocity of the electrons are in phase. In many cases n e is than calculated by using where µ e is the electron mobility and E the time‐dependent electric field strength 3,4,10,11,13. According to Raizer and converted into SI units µ e in He is constant for reduced electrical field strengths E / p in the range of 0.45–7.5 V · m −1 · Pa −1 and is given by and µ e in Ar is constant for reduced electrical field strengths E / p in the range of 0.75–9.75 V · m −1 · Pa −1 and is given by where µ e p is given in m 2 · Pa · V −1 · s −1 18.…”

“…The discharge was identified as the alpha( α )‐mode of an RF discharge 3–5. Up to now a variety of process gas mixtures have been investigated, but they were always restricted to helium or argon with only a small admixture of another gas 3–11. Whereas in the case of dielectric barrier discharges (DBD) a systematic investigation of helium/argon process gas mixtures was very recently performed,12 it is still lacking for APPJs.…”

Recent Advances in the Research on Non‐Equilibrium Atmospheric Pressure Plasma Jets

“…The uniform discharge in the APPJ was identified as the α ‐mode of an RF‐discharge 33–35. Since the introduction of the APPJ, quite a few investigations have been reported 34–70. In the following sections, the results of these studies will be summarized.…”

“…The most frequently used design consists of two solid parallel planar electrodes (Figure 2b). 33–35, 41, 42, 56, 57, 61 The gas flows along the electrodes through the rectangular cross section between them. Since the electrodes may be rather large, this design is sometimes referred as large area APPJ 35.…”

“…Meanwhile, quite a few investigations have been performed concerning the observation of the discharge patterns, and the electrical characterization by current, voltage and phase angle measurements 33–35, 55–57, 67, 70. It was found that basically two types of RF discharges were observed in APPJs, the α ‐ and the γ ‐mode,55 similar to the RF discharges at medium pressure 74.…”