IN-SITU MASS SAMPLING DURING SUPERSONIC ARCJET SYNTHESIS OF DIAMOND

A study of in-situ mass spectrometry during diamond film synthesis in supersonic, low density arcjet flows is described. With the arcjet operating entirely on hydrogen, hydrocarbon reactants are introduced downstream of the expansion nozzle where the flow is highly rarefied. Diamond is grown by impinging the flow on a water-cooled molybdenum substrate. Diamond of high quality is shown to be synthesized from injected acetylene, under conditions where acetylene is expected to be the dominant hydrocarbon species transported to the molybdenum substrate. In-situ through-the-substrate mass sampling analysis of the gas transported to the growth surface provides a basis for estimating the acetylene ''sticking coefficient'' and a comparison of the relative diamond growth efficiency for C1 species and acetylene in this particular arcjet reactor. A study of diamond growth with injected acetylene has permitted us to speculate on a simplified acetylene-based surface mechanism. The extracted rate coefficients for this simplified mechanism are presented and used to estimate surface thermochemistry for global addition-desorption reactions.