Abstract
OH radical profiles were measured in the near-deposition-surface region in low-pressure, stagnation-flow diamond-forming flames using laser-induced fluorescence (LIF). The objective of these measurements was to explore the chemistry of the diamond chemical-vapor deposition (CVD) process by resolving the OH profile near the deposition substrate. Detailed OH LIF measurements were performed in stagnation-flow diamond-forming C 2H 2/O 2 flames with equivalence ratios of 1.80, 2.10, and 2.32. The flame with an equivalence ratio of 1.80 exhibited no diamond film growth, while the flame with an equivalence ratio of 2.32 exhibited significant diamond film growth. The LIF measurements were performed in the linear LIF regime and the laser frequency was scanned across the spectral profile of the P 1(3) line in the (1,0) band of the A 2 Σ +– X 2 Π electronic transition at each spatial location. In earlier H 2 CARS measurements, the temperature gradients near the deposition substrate were resolved and good agreement between the measured temperature profiles and the numerical modeling results was demonstrated. The measured LIF profiles were corrected for electronic quenching and the variation of the Boltzmann fraction with temperature. Major species profiles were calculated using the results of the numerical flame code. The laser beam spatial profile was accounted for by convolving the theoretical OH profiles from the numerical model with the measured laser beam profile. The experimental OH profiles for all three cases are compared with theoretical results calculated using a stagnation-flame model that includes the diamond-formation surface chemistry. The measured OH concentrations near the deposition surface are substantially lower than theoretical results from a flame model that does not include the surface chemistry, but somewhat higher than for the model that does include the diamond-formation surface chemistry, especially for the flames with equivalence ratios of 1.80 and 2.10. The measured OH profile for the flame with an equivalence ratio of 2.32 is in good agreement with the profile calculated using a theoretical model with diamond-formation surface chemistry.
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