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Diamond growth in low pressure flames

Citation

Glumac, Nick (1994) Diamond growth in low pressure flames. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-08152005-103353

Abstract

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The results of an experimental and computational study of diamond chemical vapor deposition in low pressure flames are presented. In acetylene/oxygen low pressure flat flames in a stagnation-point flow arrangement over a constant temperature substrate, experimental conditions were identified in which diamond particles and films were grown on the substrate surface. The effects of substrate temperature, burner to substrate distance, and equivalence ratio on the nature of the deposit were examined.

At low specific flow rates, only diamond particles on silicon and molybdenum substrates were grown. Only continuous non-diamond films were observed at these conditions. Diamond particle growth rates of up to 1 [...]/hour (based on particle radius) were observed. The nucleation density in these flames was found to be low compared with other common diamond CVD methods. However, nucleation density was successfully increased by deposition of a small amount of non-diamond carbon on the surface early in growth experiments.

At higher specific flow rates, diamond films were deposited. Linear film growth rates of up to 1.0 [...]/hour were observed. Film growth rates were observed to be temperature dependent. The diamond films exhibited excellent spatial uniformity of deposit quality and thickness. For the highest growth rate case, the deposition process required 40 standard liters of acetylene per milligram of diamond deposit. This deposition efficiency is comparable to those of atmospheric pressure torch reactors.

A flame model is used to analyze the experimental results. The model accounts for complex gas phase chemistry, some surface chemistry (including deposition and etching of diamond and recombination of some radical species), and all transport processes. The results of the model were tested by diagnostic measurements using laser-induced fluorescence of OH and mass spectrometry. The flames tested were similar to those used to deposit diamond. The results of the diagnostic measurements indicate that the flame model predicts gas phase temperatures to within 175 K and that the predictions of the surface mole fractions of some important species are within experimental uncertainty. The predicted relative spatial OH concentration profile matches experiment very well, and the observed dependence of surface species mole fractions on equivalence ratio is well reproduced by the model for all measured species.

The model indicated that, at low specific flow rates, the acetylene/oxygen low pressure flat flame growth environment has a very low [H]/[CH3] ratio at the substrate and a large amount of surface [O2] as compared to other deposition methods. The results suggest that etching of non-diamond carbon by O2 and possibly OH are essential reactions in the diamond growth process under these conditions. At the higher specific flow rates, the model predicts substantial increases in [H] and [H]/[CH3] and a significant reduction in surface [O2] under film growth conditions. For these conditions, etching reactions by OH and O2 are likely less important.

Diamond growth in low pressure hydrogen/oxygen flames with post-flame injection of methane is also investigated. Deposition by this method offers the potential of an extremely high level of control over the chemical growth environment. Several experimental conditions for growth of diamond particles were identified. Modeling and diagnostic studies were also performed for a single injection geometry. The results suggest that mixing of the injectant with the post-flame gases can be well approximated by a simplified flow model with limited chemistry. Further analysis suggests that the technique could be useful commercially if mixing at the substrate could be improved.

Item Type:Thesis (Dissertation (Ph.D.))
Degree Grantor:California Institute of Technology
Major Option:Mechanical Engineering
Thesis Availability:Restricted to Caltech community only
Thesis Committee:
  • Goodwin, David G. (chair)
Defense Date:25 April 1994
Record Number:CaltechETD:etd-08152005-103353
Persistent URL:http://resolver.caltech.edu/CaltechETD:etd-08152005-103353
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:3124
Collection:CaltechTHESIS
Deposited By: Imported from ETD-db
Deposited On:16 Aug 2005
Last Modified:26 Dec 2012 02:57

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