Optical? I imagine the flame will include emission bands from CO and so forth. Is that what you mean?
Of course. Not even to mention the fact that combustion of pure carbon is rarely complete - there would be smoke as well.
I meant visually without instrument can you see it burning ? Which I doubt , because the product are invisible . So here comes the question , how can you see a diamond burning with out instrument ?
Why not? You can see coke burning. The CO that is produced as an intermediate gives rise to those little blue flames that are so pretty. I would expect diamond to do exactly the same.
Indeed it does. I remember reading an amusing story about this. Back in the early days when they were investigating the properties of diamonds they initially tried to melt them in air. This seemed to work, although the diamond seemed to sublime rather than melt, but the carbon was never recoverable. Eventually someone got around to looking closely or doing the experiment in a neutral atmosphere (or maybe it was a vacuum) and discovered that the diamond wasn't melting or subliming, but rather, it was burning with a very hot, very blue flame that was all but invisible and small.
Thanks .That is what I was expecting, sublimation , , I believe impurity would give a color , the reaction should be direct to CO and CO2
Wait, what? That's not what I said. I don't recall if it sublimes or melts at STP. They thought, once upon a time that it might be subliming when in fact it had ignited instead.
It is ok I believed from the beginning that it should be like sublimation and it should be a surface burning only, beside I would expect if additional heat is not provided the burning should extinguish it self if is in a non ventilated system.
I posted a link to the phase diagram earlier in this thread. From this it seems the liquid phase only exists at pressures > ~ 0.01GPa, i.e. 10MPa i.e. 100bar. So indeed at ambient pressure it would sublime rather than melt. But as you say, heated in the presence of air it will burn, probably long before it has a high enough vapour pressure to sublime to any significant extent.
Don't forget though, Diamond is only metastable at STP. Heat it enough and it will gain enough energy to revert to Graphite. It seems that in air at least the ignition temperature is comparable to the conversion temperature. Yeah, I could remember I Had seen one earlier in the thread but I was posting from a mobile device so going back to look is a huge PITA. Sounds familiar, although, it seems to convert to graphite at around the same temperature it ignites.
By inferior you appear to mean that the thermal conductivity of diamond is lower than that of graphite. This is incorrect. Perhaps you meant it was a much inferior insulator, with which I would totally agree. As an aside, we can certainly slice through polycrystaline diamond compacts with one of those snazzy electrical arc gizmos whose name quite escapes me. But you wouldn't call that machining.
Yes indeed thanks for the correction re thermal conductivity. But certainly it would be a job to make bricks that fit together exactly, and the holes for fuel rods, in a diamond moderator.
YES! the main component Diamond is carbon, is non-metallic element, after high temperature, it will formed into carbon dioxide.
At the risk of being pedantic it will only do this if oxygen is present. So, why single out a particular instance of its reactivity?
Just a little food for thought...in order for something to combust (burn), it must be in the presence of oxygen. I would expect that any work done with diamond crystallization and phase change experimentation is going to be much more convenient in a controlled evacuated environment (very high or low pressures), and oxygen would not be present.
