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Pure binderless nanopolycrystalline diamond can be produced from graphite when the graphite is treated with more than 12 GPa pressure and at least 2100°C temperature .

If we treated graphite or amorphous carbon with a somewhat lower pressure of 6-7 GPa but with a very high temperature of 3000°C - 3500°C , would these conditions be sufficient to convert graphite to diamond without use of a catalyst ?

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Chemical vapor deposition is used to produce diamonds at low pressure https://en.wikipedia.org/wiki/Synthetic_diamond#Chemical_vapor_deposition

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  • $\begingroup$ thanks, but I talking about HPHT treatment, CVD is very very slow process. $\endgroup$ – abduoman Jun 27 '15 at 23:37
  • $\begingroup$ @abduoman - "HPHT" stands for "high pressure". How can you talk about that process and ask for "low pressure" at the same time. At any rate - please clarify in your question that this is the process you want to know about. I think this answer addresses the question you posed. But if it's not what you wanted, you might want to modify the question. $\endgroup$ – Floris Jun 28 '15 at 0:50
  • $\begingroup$ @abduoman: 50-150 micron per hour (pnas.org/content/99/20/12523.full ) does not look "very very slow" to me. $\endgroup$ – akhmeteli Jun 28 '15 at 0:56
  • $\begingroup$ @Floris: I talking about relative low pressure , not absolutely low. $\endgroup$ – abduoman Jun 28 '15 at 1:57
  • $\begingroup$ @akhmeteli: 150 micron/hr when compared to a piece of diamond has 10 mm diameter & 10 mm length synthesized using HPHT treatment for no more than 30 min. I think HPHT win. $\endgroup$ – abduoman Jun 28 '15 at 2:07
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According to the phase diagram of diamond (see for example http://files.umwblogs.org/blogs.dir/6093/files/2011/10/carbon_phase_diagram2.jpg) there is a region where diamond is stable and graphite is metastable, at pressures and temperatures in the range you are asking about:

enter image description here

Given the slope on the diagram, you would actually expect that this reaction is more thermodynamically favorable at lower temperatures... but the rate of the reaction is probably quite slow.

CVD is indeed the more common method for slowly growing diamond at lower temperatures. Getting these very high temperatures and pressures at the same time places enormous demands on the surrounding materials systems - which is why natural diamond tends to be formed "under a large and heavy pile of dirt" (deep inside the earth).

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  • $\begingroup$ that is mean lower temperature is more favourable than higher temperature for synthesis of diamond at this range of pressure , right ? or it mean the diamond produced will be not stable ?! sorry, but I can't compeletly understand. $\endgroup$ – abduoman Jun 28 '15 at 2:15
  • $\begingroup$ It says that at lower temperature, diamond is more stable than graphite at lower pressure. This does not tell you about the rate at which one turns into the other - that is a very difficult calculation (because of the metastable state). $\endgroup$ – Floris Jun 28 '15 at 2:20

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