Diagnostic Studies of Microwave Plasmas Containing Hydrocarbons Using Tunable Diode Lasers
109 Seiten, Erscheinungsjahr: 2001
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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
(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
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
(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
(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.