Temperature profile measurements in stagnation-flow, diamond-forming flames using hydrogen cars spectroscopy

Abstract

Gas-phase temperature profiles were measured near the deposition substrate in atmospheric-pressure, stagnation-flow, diamond-forming flames. In these flames, an acetylene-oxygen-hydrogen mixture accelerates through a nozzle and impinges on a water-cooled molybdenum substrate, stabilizing a flat flame approximately 1 mm below the substrate. A thin, polycrystalline diamond film is deposited on the substrate under appropriate conditions of flame stoichiometry and substrate temperature. Coherent anti-Stokes Raman scattering (CARS) spectroscopy of H 2 was used to determine the temperature at various points layer (the region between the premixed flame reaction zone and the substrate) is approximately 0.6–0.8 mm thick along the stagnation streamline, depending on the velocity of the incoming jet. The CARS measurements also show peak flame temperatures ∼200–300 K above the adiabatic equilibrium temperature—an effect of the short residence times in the flame. Measured temperature profiles are in good agreement with a theoretical calculation of the profile performed by Meeks and coworkers at Sandia National Laboratories, except that the measured distance between the substrate and the reaction zone is much less than predicted. H 2 CARS was also compared to N 2 CARS in a calibration burner and found to be in good agreement, with the H 2 CARS temperatures an average of 3.6% higher than the N 2 CARS results. The estimated H 2 CARS temperature accuracy is ±4% in the diamond-forming flames, and the estimated spatial resolution is ±50 μ m perpendicular to the deposition surface.