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Reforming of methane (CH4) is a process to produce syngas (CO/H2) and other value-added chemicals including oxygenates such as methanol (CH3OH). Atmospheric pressure plasmas have the potential to be more energy efficient than traditional reforming methods as value-added chemicals can be synthesized directly in the plasma without requiring an additional step. In this paper, we discuss the results from a computational investigation of the formation of oxygenates by CH4 oxidation in the presence of Ar, including CH3OH and CH2O, in a nanosecond pulsed dielectric barrier discharge. The plasma is formed in a microfluidic channel whose small dimensions are ideal for plasma formation at atmospheric pressure. The production and consumption mechanisms of dominant radicals and long-lived species are discussed in detail for the base case conditions of Ar/CH4/O2 = 50/25/25. CH3OH is produced primarily by CH3O reacting with CH3O and CH3O2 reacting with OH, while CH2O formation relies on reactions involving CH3O and CH3. The most abundant oxygenate formed is CO (produced by H abstraction from CHO). However, the greenhouse gas CO2 is also formed as a by-product. The effects of gas mixture are examined to maximize the CH3OH and CH2O densities while decreasing the CO2 density. Increasing the Ar percentage from 0% to 95% decreased the CH3OH and CH2O densities. At low Ar percentages, this is due to an increase in consumption of CH3OH and CH2O, while at high Ar percentages (>40% Ar), the production of CH3OH and CH2O is decreased. However, both CO and CO2 reached peak densities at 70%–90% Ar. Changing the CH4/O2 ratio while keeping 50% Ar in the discharge led to increased CH3OH and CH2O production, reaching peak densities at 35%–40% CH4. The CO and CO2 densities decreased beyond 20% CH4, indicating that a CH4 rich discharge is ideal for forming the desired oxygenates.
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E–H transitions in Ar/O2 and Ar/Cl2 inductively coupled plasmas: Antenna geometry and operating conditions
Electronegative inductively coupled plasmas (ICPs) are used for conductor etching in the microelectronics industry for semiconductor fabrication. Pulsing of the antenna power and bias voltages provides additional control for optimizing plasma–surface interactions. However, pulsed ICPs are susceptible to capacitive-to-inductive mode transitions at the onset of the power pulse due to there being low electron densities at the end of the prior afterglow. The capacitive (E) to inductive (H) mode transition is sensitive to the spatial configuration of the plasma at the end of the prior afterglow, circuit (matchbox) settings, operating conditions, and reactor configurations, including antenna geometry. In this paper, we discuss results from a computational investigation of E–H transitions in pulsed ICPs sustained in Ar/Cl2 and Ar/O2 gas mixtures while varying operating conditions, including gas mixture, pulse repetition frequency, duty cycle of the power pulse, and antenna geometry. Pulsed ICPs sustained in Ar/Cl2 mixtures are prone to significant E–H transitions due to thermal dissociative attachment reactions with Cl2 during the afterglow which reduce pre-pulse electron densities. These abrupt E–H transitions launch electrostatic waves from the formation of a sheath at the boundaries of the plasma and under the antenna in particular. The smoother E–H transitions observed for Ar/O2 mixture results from the higher electron density at the start of the power pulse due to the lack of thermal electron attaching reactions to O2. Ion energy and angular distributions (IEADs) incident onto the wafer and the dielectric window under the antenna are discussed. The shape of the antenna influences the severity of the E–H transition and the IEADs, with antennas having larger surface areas facing the plasma producing larger capacitive coupling. Validation of the model is performed by comparison of computed electron densities with experimental measurements.
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- Award ID(s):
- 2009219
- PAR ID:
- 10433328
- Date Published:
- Journal Name:
- Journal of Applied Physics
- Volume:
- 133
- Issue:
- 17
- ISSN:
- 0021-8979
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1063/5.0146168
