Abstract: The phase diagram of diamond deposition in atmosphere oxyacetylene flames was calculated according to our non-equilibrium thermodynamic coupling model, and the ranges of substrate temperature and gas flow ratio for diamond growth were obtained theoretically. The suitable range of gas flow ratio (R) is dependent on substrate temperature (T). Since substrate temperatures between 1000K and 1250K are usually used for diamond growth in oxyacetylene flames, the suitable range of flow ratio of oxygen to acctylene will be lying between 0.8 and 1.1. Diamond can not be synthesized for any R value when substrate temperature is too low. The suitable range of substrate temperature for diamond growth is also dependent on gas flow ratio of oxygen to acetylene. The temperature range is widest for R value close to unity, and the range will be narrowed rapidly when R value deviates from unity. The diamond growth conditions obtained in the paper are consistent with many reported perimental
results, and favour experimental designs for diamond deposition in oxyacetylene flames.

Key words: diamond, acetylene, flame, phase diagram