This paper examines the role that COS plays in the inhibition of carbon deposition onto a Si-free 20Cr-25Ni-Nb-stabilized austenitic stainless steel. We have shown that the addition of just 240 vppb of COS to a standard gas deposit mixture of high carbon activity (1000 vppm C 2 H 4 , 1% CO, balance CO 2 ) yields a dramatic fall in the amount of filamentous carbon deposited. The inhibitive effect persists when treatment is continued after COS has been subsequently removed from the treatment gas. Clearly, in these experiments the phenomenon is a result of substrate surface modification by the COS rather than changes in gas phase kinetics. Deposition onto surfaces where filamentous carbon has already been laid down is similarly inhibited.
A technique for sampling ions and neutral molecules from a radio-frequency discharge into a conventional single-focusing mass spectrometer has been developed. Using this technique the variation in positive-ion concentration in discharges in carbon dioxide, carbon monoxide and oxygen, and the degree of decomposition of the sampled neutral gas has been investigated. The observed results are discussed in terms of possible ionization mechanisms and charge-transfer reactions.
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