Diagnosis of Methane Plasma Generated in an Atmospheric Pressure DBD Micro-Jet by Optical Emission Spectroscopy

Abstract

Diagnosis of methane plasma, generated in an atmospheric pressure dielectric barrier discharge (DBD) microplasma jet with a quartz tube as dielectric material by a 25 kHz sinusoidal ac power source, is conducted by optical emission spectroscopy (OES). The reactive radicals in methane plasma such as CH, C2, and H𝛼 are detected insitu by OES. The possible dissociation mechanism of methane in diluted Ar plasma is deduced from spectra. In addition, the density of CH radical, which is considered as one of the precursors in diamond-like (DLC) film formation, affected by the parameters of input voltage and the feed gas flow rate, is emphasized. With the Boltzmann plots, four Ar atomic spectral lines (located at 675.28 nm, 687.13 nm, 738.40 nm and 794.82 nm, respectively) are chosen to calculate the electron temperature, and the dependence of electron temperature on discharge parameters is also investigated.

Conversion of Pyrolysis Fuel Oils by a Dielectric Barrier Discharge Reactor in the Presence of Methane and Ethane

“…The lifetime of CH 3 is long, i.e., several milliseconds. The absence of CH 3 radical peaks could be due to the lack of suitable conditions for emitting light in the presented wavelengths 24. The absence of the CH 2 line in the spectra is most likely because the CH 2 radical has an extremely short lifetime to be probed.…”

“…It was documented that the active species for radical attachment in plasma are CH, CH 3 , CH 2 , and H radicals as the following reactions 24: CH 4 + e* → CH 3 + H + e CH 3 + e* → CH 2 + H + e CH 2 + e* → CH + H + e CH 4 + e* → CH 2 + H 2 + e CH 3 + e* → CH + H 2 + e …”

“…The emission intensity of the radicals was significantly higher in argon‐added plasma than in pure methane plasma or pure ethane plasma, which very likely is caused by Penning discharge in CH 4 /C 2 H 6 +Ar plasma. The CH 4 /C 2 H 6 gases were effectively dissociated by electron and argon metastable atom impacts in the Penning discharge model, creating sufficient CH χ radicals 24. Also, in many studies about current hydrocracking or plasma cracking, it was stated that cracking is enhanced by raising the temperature and H 2 concentration 26–29.…”

“…The CH radical intensity increased due to a larger population of excited particles at higher applied voltage. It is known that at lower voltages the contents of electron energy as well as the impact cross section are relatively small 24. As a result, the collision probability between the electron and CH is low, which leads to an infrequent CH radical population generated in the plasma.…”

Mechanism of high growth rate for diamond-like carbon films synthesized by helicon wave plasma chemical vapor deposition

“…As is well known, the disappearance of the CH 3 radical line is because it does not emit light in the spectrograph range [25]. In addition, the absence of the CH 2 radical line can be expressed via CH 2 →CH+H [26]. The C 2 species are formed from the recombination reaction of CH 4 and other carbon-containing radicals (CH x species) spread all over the chamber [27].…”