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Diagnostic Studies of Microwave Plasmas Containing Hydrocarbons Using Tunable Diode Lasers

Lars Mechold

ISBN 978-3-89722-687-6
109 pages, year of publication: 2001
price: 40.50 €
The observation of the methyl radical and ten related stable species by TDLAS not only shows the versatility of the technique, but has enabled progress to be made in probing the species present and their concentrations under a wide range of oxidizing conditions. For the first time the absolute concentrations of a large group of molecules was measured in H2-O2-Ar microwave plasmas containing small percentages of hydrocarbon precursors either methane or methanol. Their degree of dissociation increased when using oxygen-containing source gas mixtures. The concentration of the methyl radical was found to be in the range from 1010 to 1012 cm-3. The methyl radical and the stable products CH4, C2H2, C2H4, C2H6 showed exponential decays with increasing oxygen amounts. For the first time methane was found as a plasma product in methanol-containing plasmas, as well as formic acid. Formaldehyde was found for the first time in such microwave plasmas containing methane or methanol. Further stable products are carbon monoxide and carbon dioxide. Under conditions with a lot oxygen the concentration of carbon dioxide exceeded that of carbon monoxide by more than one order of magnitude. The water concentration was measured to be nearly independent of the precursor having a maximum at about the 1:1 ratio of hydrogen to oxygen. In order to study the nature of the plasma chemistry all the measurements were done under both, flowing (fc) and static (sc) conditions. The most significant differences of plasma chemical reactions between methane and methanol as hydrocarbon precursors are: (i) the greater dissociation of methanol leading to higher concentrations of carbon dioxide and formaldehyde, (ii) the formation of methane concentrations up to 1014 cm-3 in methanol plasmas in the absence of oxygen, (iii) the gradual increase in formic acid concentration with oxygen in the methanol plasmas, (iv) the absence of any detectable methanol or formic acid down to the detection limit of 1010 cm-3 in methane plasmas. Mass balances were determined for both methane-containing and methanol-containing plasmas. It has been found that only at no or low oxygen amounts the hydrocarbons contribute to it, in fact at most in H2-Ar-CH4 plasmas. With increasing oxygen content the mass balance is dominated by carbon monoxide, carbon dioxide and water. Using moderate and high oxygen contents the sum of CO and CO2 takes 50 % of the consumed carbon in the methane-containing discharge and about 80 % in the methanol-containing discharge. Kinetic modelling has been carried out for H2-O2-Ar-CH4 plasmas. An elaborate set of chemical reactions for the neutral species of the complex plasma was derived. The model calculations were performed with the set of chemical reactions available from the literature and these calculated dissociation rate coefficients. For the first time the comparison of calculated and measured species concentrations of the methyl radical and ten further stable molecules was possible. Based on the comparison between measured and calculated concentrations the modelling was able: (i) to achieve very good agreement for the H2-Ar-CH4 mixture and satisfactory agreement for the O2-Ar-CH4 mixture. (ii) to determine a reaction scheme of H2-Ar-CH4 plasmas containing the most relevant neutral reactions by analysing the balanced production and consumption processes. (iii) to derive for the first time a scheme for the main reactions in O2-H2-Ar-CH4 plasmas. (iv) to predict the concentrations of the further important radicals like e.g. OH, HO2, HCO and CH2 beside the molecules detected in the plasma so far. Because of the promising results the modelling should be improved further especially for the plasma conditions containing oxygen. Measurements of formaldehyde in accordance with the methyl radical were done in Ar-CH3OH microwave plasmas. Both species were identified as intermediates. Results were used to get an idea for the initiating dissociation processes in methanol-containing plasmas. This approach and the experience from the methane-containing plasmas supported the establishment of a reaction scheme deduced from the experimental results containing the most important steps linking the plasma chemistry. This scheme represents a good basis for future studies concerning methanol-containing plasmas.

Keywords:
  • Microwave Plasma
  • Plasma Chemistry
  • TDCAS
  • Hydrocarbons
  • Radical Diagnostics

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