Thermodynamically Predicted C-H-O Phase Diagram

A C-H-O diagram for carbon deposition from gas phase was thermodynamically calculated from free energies of formation of the limited chemical species, CH4, C2H2, CO and CO2, and C-H-O gas composition for diamond deposition was discussed. The feasible area of diamond deposition is in the vicinity of carbon solubility limit in the diagram. Solubility limit of carbon to hydrogen is much lower than that of carbon to oxygen, because the formation free energies of C2H2 and CH4 in C-H system are much higher than those of CO and CO2 in C-O system. The necessity of low C/H ratio in gas composition for diamond deposition comes from the low solubility limit of carbon to hydrogen. CO becomes extremely stable at higher temperatures. Therefore carbon is soluble up to C/O=1, which appears as the solubility line, called the 'CO line', in the C-H-O diagram. The vicinity of this solubility line shows a good agreement with C-H-O gas composition for diamond deposition.