Hi everybody!

I was looking for apparently banal data: the vapour pressure of carbon as a function of temperature, and possibly also the erosion rate in vacuum, around 2700..3300K.

But data isn't that abundant, and worse, highly inconsistent. Some sources tell "sublimation at 3825°C" (I guess the critical point, well over 1 bar) others "sublimation at 2200K", some say "usable at 3000°C in vacuum".

My guess is that industrial carbon or graphite are inconsistent materials anyway. I would be most interested in carbon fiber reinforced carbon.

Also: does anybody know how opaque carbon vapour is? I mean: to visible light, at 1mbar or less, and thicknesses of 0.1m typically. Probably very bright, but in this case, I want it to absorb light.

By the way, how many atoms does gaseous carbon have? The CPropep software for instance shows C3 as the main species, but I'm wary of this less-than-perfect software, especially under unusual conditions.

Thanks!

It seems that graphite simply is an inconsistent material. Such high temperatures don't make measurements easy, neither.

One good reason is that pure crystallized graphite looks very uncommon. The Handbook of Chemistry and Physics gives a density of 2360km/m3, as opposed to most graphite suppliers who indicate 1740kg/m3 or a bit more, depending on the densifying process...

Even the Handbook of Ch&Ph is highly inconsistent. mmHg instead of bar at one place, and the mistake(s) could be much bigger.

It's hard to believe, but one of the most common elements in our live is widely unknown. Even for the triple point, sources hesitate between 1bar and 100bar! Nanotubes and fullerenes seem to be better known.

As of C3: The Hdbk only knows C1 as a gas, some Nasa papers know it but as a minority species, and Cpropep considers it much more abundant than C1.

So if somebody know data that is somewhat credible... I still look for
- The vapour pressure of carbon fiber reinforced carbon around 2700..3300K
- The opacity of carbon vapour, similar temperatures, around 1mbar.

The unary phase diagram for carbon is known. The thermodynamic data for all the solid and vapor species are also available (see JANAF tables). So, you can plot the vapor pressure of carbon (graphite/vapor equilibrium) as a function of temperature using the Clapeyron relation.

Thank you Martensite!
Seen your reply, even though I hadn't reacted.

I couldn't find the Janaf tables for free on the Web (quite expensive in fact), but they are integrated in some free software. I have Guipep for instance, but it still doesn't work properly.

The other programme I use, CPropepShell, does not rely on the Janaf tables, so comparing the results should bring some healthy uncertainty...

JANAF tables are available in most libraries in the reference section, so you can photocopy the page. Alternatively, as you know you may try some thermo databases/software such as HSC Chemistry, FactSage, or ThermoCalc. I think the NIST Chemistry WebBook has an online database that is available for free. If the thermo data is given in the form of functions for the specific heats (Cp's), then you do not need the tabulated JANAF tables. Since you are looking at a solid/vapor reaction, the transformational enthalpies and entropies are the ones that are more important, since the difference in the Cp's can often be ignored.

I remember that Kubaschewski's book (80's edition or earlier) on Metallurgical Thermochemistry had experimental vapor pressure curves for many elements, so perhaps you can locate that in a library. The chapter in the thermodynamics book by DeHoff on unary phase diagrams may be helpful since it has computed vapor pressure curves for many elements.