SYNTHESIS OF UNIFORM DIAMOND FILMS BY FLAT FLAME COMBUSTION OF ACETYLENE HYDROGEN OXYGEN MIXTURES

It is shown that a uniform film of polycrystalline diamond of good quality can be deposited near the stagnation point of a watercooled substrate from a flat flame of mixtures of acetylene, oxygen, and hydrogen stabilized in the stagnation boundary layer. An experimental study of the effects of reactant composition and flow rates on the stability of the flame is reported. A mantle of hydrogen around the reactant gases, from the surrounding nozzle, was used to prevent formation of a conical flame of the reactant gases stabilized on the burner rim. Variables were tested over the following ranges: mantle flow rates 2-4 L/min; carbon/oxygen ratios in the reactant gas 0.9-1.3; hydrogen/oxygen ratios in the reactant gas 0-0.6; and reactant gas velocity 35-58 m/s. The stable operating range increases with increased flow of the mantle gas up to about 3.5 L/min; further increases have no effect. With the condition that the acetylene/oxygen ratio be maintained so as to produce abundant C2 and CH radicals (C/O greater-than-or-equal-to 0.9), the addition of hydrogen to the reactant gases lowers the burning velocity and extends the region of flat flame stability. Increased flow velocity also extends the stable operating range. The quality of the diamond film was estimated by means of Raman spectra and electron micrographs.