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Author: Subject: What is molten Carbon? Diamond or graphite?
radioboy
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[*] posted on 19-4-2016 at 08:56
What is molten Carbon? Diamond or graphite?


If we apply conditions neccassary for melting C (which are temperature and pressure), what allotrope will we get? I think only diamond, because graphite and other allotropes don't exist under pressure neccassary for making diamond. Or simply said "graphite can't be melted". But what is then glassy carbon? So, am I right saying that "diamond is molten carbon" or "molten carbon can only be diamond"?
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Boffis
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[*] posted on 19-4-2016 at 09:48


Niether, they differ in crystal structure only and molten carbon has no crystal structure. Diamond is definitely not molten carbon!

I am not entirely sure about "glassy" carbon but given the mode of preparation I suspect that it is probably amorphous like most glassy materials.

I suspect that on cooling molten carbon you would get graphite. It is a common misconception that diamonds are stable at high temperature. Diamond is the low temperature - high pressure polymorph of carbon. At STP graphite is the stable form theoretically but diamond exist indefinitely as a metastable quenched phase under these condition. As the temperature increases the pressure required to form diamond also increases. At the melting point of carbon at the diamond-graphite junction (roughly 4100 C) you need a pressure of about 14 GPa to form diamond I doubt that the vapour pressure of carbon at this point is anywhere near this value and hence at a pressure just sufficent to stop the carbon subliming the solid phase that forms on freezing will be graphite. Check out the phase diagrams, there are several available on google.
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Chemist_Cup_Noodles
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[*] posted on 19-4-2016 at 09:51


First off your wording is rather different. And do you really need to classify it as either, not just liquid carbon? You really might want to clarify this, maybe edit it?

I'm going to attempt to go from what points I believe you are trying to make. You mention "conditions", and in order to successfully melt carbon (which is immensely difficult) it requires very low pressure and extremely high heat. Even then you usually can only get it to a triple point, I don't believe there is such thing as just liquified carbon. Here is the theoretical phase diagram for carbon also:


But see just how extreme the temperatures are in order to get the liquid? And at that point it's not a question of allotropes anymore. Someone else correct me if I'm wrong, but the allotropes only have to do with how the carbon molecule in question is bonded. Whether it's a network covalent (diamond) or planar (graphite), mainly. But the nature of these bonds tend to make diamonds a single macromolecule. So there's no intERmolecular forces to overcome in between a solid and liquid phase, only intRAmolecular forces, or bond energies. Now once you overcome this with very extreme heat, at this point I'd just think you would have a lot of elemental carbon, throwing allotropes out of the window.




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Oscilllator
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[*] posted on 20-4-2016 at 00:28


I think you're correct Chemist_Cup_Noodles. The whole point of a liquid is that it has no crystal structure, so the answer is of course that it is neither diamond nor graphite.
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[*] posted on 20-4-2016 at 04:08


I think it is an open question on what exactly the structure of liquid carbon is. Is it individual atoms or is there some kind of molecular structure? I can envisage short carbon chains or tens of atoms in a graphite-like structure or maybe buckyball-type structures.
I don't know and I am not really sure how one would go about testing this. But if there is some kind of molecular thing happening in liquid carbon then it may be that freshly-melted diamond is different from freshly-melted graphite -- at least until an equilibrium is reached.

I guess this is all pretty academic. I think the temperatures and pressures needed preclude any immediately-obvious practical applications.

[Edited on 20-4-2016 by j_sum1]




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radioboy
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[*] posted on 20-4-2016 at 06:46


No, I mean what we get when we melt and resolidify (cool) Carbon. Of course, under minimum pressure needed for it to liquify (critical pressure?). Because I have never seen liquid carbon (it would probably glow strongly of course), or at least one piece of graphite (like metal or glass, not powdered glued). Or is it glassy carbon?
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[*] posted on 20-4-2016 at 07:00


Well, it will depend on your pressure. See the phase diagram posted by CC-Nodles.

Above 10GPa you will most likely form diamond.
Below 10GPa you will most likely form graphite.

Bear in mind that
(a) this particular phase diagram is largely theoretical
(b) kinetic factors will come into play here. There is a zone (shaded) where both forms are almost equally stable and so thermodynamic stability won't really be a factor. The kinetics of crystallisation is likely more significant and will probably determine the product.




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