I just read that diamond melts at 3550C at 10 million times normal atmospheric pressure, is this true? and if it is another thought occurs since the strength of diamond comes from its atomic arrangement does this mean that if you did melt it down then let it cool again the result would no longer be diamond but a weak/er carbon structure?
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See the phase diagram at the reference. Its triple point is around 10 MPa. One atmosphere is about 100 kPa, so the triple point is at only about 100 atmospheres.
You are correct. When diamond melts, its carbon is no longer crystalline but amorphous. It's a liquid with no crystal structure.
You are correct. When diamond melts, its carbon is no longer crystalline but amorphous. It's a liquid with no crystal structure.
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No, I think it would stay stable as diamond. I think you'd need to heat it well over 4000K to force a phase transition to liquid, or heat it over 1500K in a vacuum to force a phase transition to graphite.
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Diamond is just one of the solid phases of carbon. Here is the phase diagram:
http://www.mathewpeet.org/images/carbon_…
Note that atmospheric pressure is about 1 bar, which isn't shown on this diagram (which uses a log scale for pressure). Most of the diagram is probably predicted through thermodynamic calculations rather than experimentally measured.
If you heat up a diamond under atmospheric pressure, it will eventually convert to graphite, which is the stable phase at low pressure, then it will sublime to a gas around 3600 C.
The only way you can have diamond go directly to the liquid phase without converting to graphite first is to increase the pressure. You'd need to get to about 2*10^5 bar (200,000 times atmospheric pressure) and it would melt at around 4250 C. Increasing the pressure actually increases the melting point. Above about 2 million times atmospheric pressure it won't melt even at 10,000 C. Carbon will never form a stable liquid below ~4100 C.
As long as you maintain the pressure, it would still be diamond when it cooled back down. If you decrease the pressure too, you'll probably end up with mostly graphite.
http://www.mathewpeet.org/images/carbon_…
Note that atmospheric pressure is about 1 bar, which isn't shown on this diagram (which uses a log scale for pressure). Most of the diagram is probably predicted through thermodynamic calculations rather than experimentally measured.
If you heat up a diamond under atmospheric pressure, it will eventually convert to graphite, which is the stable phase at low pressure, then it will sublime to a gas around 3600 C.
The only way you can have diamond go directly to the liquid phase without converting to graphite first is to increase the pressure. You'd need to get to about 2*10^5 bar (200,000 times atmospheric pressure) and it would melt at around 4250 C. Increasing the pressure actually increases the melting point. Above about 2 million times atmospheric pressure it won't melt even at 10,000 C. Carbon will never form a stable liquid below ~4100 C.
As long as you maintain the pressure, it would still be diamond when it cooled back down. If you decrease the pressure too, you'll probably end up with mostly graphite.
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I don't think anyone has actually melted one, but if you did, when it resolidified it would no longer be diamond, just some other form of carbon.
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impossible to reach that pressure so no-one knows