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The synthesis of well-crystallized diamond has been achieved by a wide variety of CVD techniques including plasma assisted methods (DC, RF and microwave), hot wire methods and by oxyacetylene combustion. These can be differentiated from other carbon coatings (graphite, vitreous carbons, and DLC) by Raman spectroscopy, X-ray diffraction and scanning electron microscopy. The high linear growth rates reported for the small area flame deposition of diamond and the recent demonstration of optically transparent continuous diamond films using 'torch' growth methods has led to speculation that by enlarging the area of uniform combustion it might be possible to deposit tens of square centimeters of transparent diamond at reasonable rates using relatively inexpensive equipment. We have previously reported that samples prepared by oxyacetylene combustion at low pressure were highly non-uniform and that higher oxygen/acetylene ratios were required than those found successful at atmospheric pressure using common brazing or welding torches. This paper reports low pressure flat flame combustion growth at still higher 02/C2H2 gas ratios and a new Raman feature at —1280 cm1 which was observed in some experiments in addition to the expected diamond peak at 1332 cm. Using data from previously published work on a similar flame composition, we speculate that the locations in the flame yielding diamond (as opposed to non-diamond forms of solid carbon) correspond to regions of the highest atomic hydrogen mole fraction (partial pressure) which in turn is closely related to the highest partial pressure of numerous reactive hydrocarbon radicals including the methyl radical. The increased oxygen/acetylene ratio needed for diamond deposition in these experiments may well be related to the much lower than adiabatic flame temperature expected and observed in previous work on oxyacetylene combustion at low pressure using a water cooled flat flame burner.
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