Combustion synthesis of diamond

The effects of flame structure and chemistry in low pressure combustion-enhanced diamond synthesis were studied numerically and experimentally. A numerical model which combines both aspects of combustion and chemical vapor deposition (CVD) processes was developed to help understand low pressure combustion diamond CVD. The model solves the two-dimensional conservation equations in a stagnation-point geometry, and accounts for gas-phase and surface reaction kinetics and transport processes.; Example simulations were performed for low pressure {dollar}rm Csb2Hsb2{dollar}-{dollar}rm Osb2{dollar} flames diluted with argon. It was found that the level of {dollar}rm CHsb3{dollar} in the postflame region is established as a balance between hydrocarbon oxidation and cyclization. Optimum diamond growth conditions were predicted, and these results provided a starting point for experimental investigation.; A low pressure combustion facility was constructed, and flame operating conditions for the deposition of high quality diamond films were identified in premixed {dollar}rm Csb2Hsb2{dollar}-{dollar}rm Osb2{dollar} flames at growth rates as high as 4-5 {dollar}rmmu m/h,{dollar} representing the highest growth rate ever reported for {dollar}rm Csb2Hsb2{dollar}-{dollar}rm Osb2{dollar} flames. Recognizing that the low pressure {dollar}rm Csb2Hsb2{dollar}-{dollar}rm Osb2{dollar} flame approach is still an expensive diamond synthesis technology mainly due to the high cost of acetylene, the use of alternative fuels were explored, and diamond films were also successfully deposited in low pressure {dollar}rm Csb2Hsb4{dollar}-{dollar}rm Osb2{dollar} and {dollar}rm CHsb4{dollar}-{dollar}rm Osb2{dollar} flames.; The flame CVD model was tested by comparing the predicted diamond growth rates with those measured, and for its ability to predict the flame temperature profiles. It was found that the model can predict the observed growth rates within a factor of 2-3. Limitations in the surface mechanism were identified, and a simple four-step oxidation mechanism was proposed to improve the capability of the model. The results show better agreement between the model predictions and experimental results. Flame temperature measurements were performed using a thermocouple probe and optical emission spectroscopy. The radiation-corrected thermocouple temperatures were found to be in good agreement with the predicted temperatures. The emission measurements for rotational temperatures of CH also showed good agreement with the model predictions. Temperature overshoots observed in {dollar}rm Csb2Hsb2{dollar}-{dollar}rm Osb2{dollar} and {dollar}rm Csb2Hsb4{dollar}-{dollar}rm Osb2{dollar} flames were explained in part by a superequilibrium level of unburned {dollar}rm Csb2Hsb2{dollar} in the postflame region.