Atmospheric-pressure diffuse dielectric barrier discharges in Ar/O₂gas mixture using 200 kHz/13.56 MHz dual frequency excitation

Abstract

IntroductionDevelopment of a large linear or volumetric source of diffuse non-thermal plasma at atmospheric pressure remains to be an essential challenge both from a scientific and a technological point of view. Such a source is required in various applications including plasma-assisted gas phase chemical conversion, surface modification and synthesis of high quality functional thin films [1][2][3][4]. The emerging atmospheric-pressure plasma enhanced chemical vapour deposition (AP-PECVD) technology can potentially achieve a considerable cost efficiency in comparison to the common low-pressure plasma methods, by the possibility of in-line production and by avoiding expensive large-footprint vacuum equipment [5]. One of the common ways to create a large area non-thermal plasma at atmospheric pressure is the dielectric barrier discharge (DBD) AbstractAtmospheric-pressure diffuse dielectric barrier discharges (DBDs) were obtained in Ar/O 2 gas mixture using dual-frequency (DF) excitation at 200 kHz low frequency (LF) and 13.56 MHz radio frequency (RF). The excitation dynamics and the plasma generation mechanism were studied by means of electrical characterization and phase resolved optical emission spectroscopy (PROES). The DF excitation results in a time-varying electric field which is determined by the total LF and RF gas voltage and the spatial ion distribution which only responds to the LF component. By tuning the amplitude ratio of the superimposed LF and RF signals, the effect of each frequency component on the DF discharge mechanism was analysed. The LF excitation results in a transient plasma with the formation of an electrode sheath and therefore a pronounced excitation near the substrate. The RF oscillation allows the electron trapping in the gas gap and helps to improve the plasma uniformity by contributing to the pre-ionization and by controlling the discharge development. The possibility of temporally modifying the electric field and thus the plasma generation mechanism in the DF discharge exhibits potential applications in plasma-assisted surface processing and plasma-assisted gas phase chemical conversion.

Numerical simulation of atmospheric-pressure 200 kHz/13.56 MHz dual-frequency dielectric barrier discharges

“…9(d) and (i)) is found in both LF and DF discharges. The asymmetrical structure of the discharge emissions between two opposite polarities of the voltage is attributed to the particular electrode configuration (a top curved electrode and a bottom flat electrode) in this study [44,45] . The ionization rate ( Figure. 9(c) and (h)) is a few times lower than the excitation rate ( Figure.…”

“…The detailed description of the experimental setup for the atmospheric-pressure DF discharge has been given elsewhere [44,45] . Here only the main parameters of the system are briefly introduced.…”

“…For the DF discharges, due to the extra RF electric field, the DF equivalent electrical circuit should be modified with an extra variable resistor RRF(t) and an equivalent capacitor ζRF(t) [45] , see Figure. 8(b). During the "plasmaoff" phase (AB and DE in Figure.…”

“…Furthermore, by tuning the amplitude ratio of the superimposed LF and RF signals, the DF excitation also exhibits capability of modifying the electron energy distribution function (EEDF), which can be applied to the other fields e.g. plasma-assisted gas phase chemical conversion [45] .…”

Electrical Diagnostics of Dielectric Barrier Discharges

“…The answer is that while i plasma (t) always remains greater than zero during 'plasma off' phases, it is negligibly small compared to the large plasma currents occurring during 'plasma on' phases. The tendency for Q-V diagrams to become more elliptical, especially with increasing power can, however, be observed as small increases to the 'plasma off' gradient [24,44,45]. This apparent increase in the C cell capacitance can be attributed to the formation of plasma sheaths between the weak residual plasma in the centre of the gas gap and the surrounding surfaces, which can be treated as a small (and likely timedependent) additional capacitance parallel to C gap in the equivalent circuit.…”