Gas Phase Chemistry in a DC Arcject Diamond CVD Reactor

Reaction products contained in the exhaust gas of a dc plasma jet reactor system were detected

using mass spectrometry. The major reaction products formed from a feed gas mixture of 96%

H2, 3% CH4 , and 1% 02 injected into an argon plasma jet were CH4 , C2H2 , C2H6 , CO, and

H20. The products formed without 02 in the reactor feed were CH4 , C2H2 , and C2H6. Addition

of 02 to the reactor gas feed increased the diamond deposition rate by -30% with no

degradation in quality. A chemical kinetics model for flames was adapted to this reactor and

accurately predicted the major reaction products formed for a reactor feed without 02,

indicating the gas phase chemistry may be described by a reaction set developed for hydrocarbon

combustion and the chemistry is thermally driven. When compared to experimental results, the

model predicts: (1) a maximum temperature in the gas of 3000 K, (2) only 1.2% of the H2 in

the reactor feed is dissociated, and (3) CH3 is the primary diamond growth precursor.