Carbon is a main source of a reaction, eg carbothermal reaction is widely used. This reaction is usually exothermic.
Can the presence of amorphous carbon be effective on results in DSC?
Amorphous substances are not in thermodynamic equilibrium and hence on heating they tend to crystallize at a temperature called crystallization temperature as they try to come to the thermodynamically equilibrium state. At this temperature, they release the extra thermal energy which they were having in amorphous state. An exothermic peak occurs in DSC when heat is measured as a function of temperature in non- isothermal run at a constant heating rate. The exothermic peak may shift towards higher temperatures at higher heating rates. In case of amorphous carbon also exothermic peak should occur in DSC on crystallization
Dear Dr. Kumar
thank you so much for your swift respond.
would you please let me know, endothermic peaks maybe observe for carbon in DSC test. I would be grateful if you could inform me.
Transformation of amorphous carbon into something else is extremely slow. Graphitisation is not expected at temperatures below 2500°C in the absence of catalyst, or at temperatures below 800 - 900°C in the present of catalyst such as Ni or Fe nanoparticles. These conditions are far out of range with respect to what usual DSC devices can do. So don't expect any peak of carbon with a normal DSC. At least what you can see is an oxidation above 400°C, but this means that your crucible is not correctly closed and that you have a leak in the gas circuit. But again no peak will appear but a shift of the baseline instead, as oxidation is continuous and never ends once it has started. In conclusion, don't expect any interference in your DSC signal due to the presence of carbon in the system.
Alain
Dear Prof. Alain
tank you so much for your nice respect.
Actually, I am eager to know, how endotermic reaction obtain for carbon
There are many different forms of disordered carbons, depending on their precursor and on their thermal history. I am not aware of any compilation of data from the literature, but given that the conversion of graphite to diamond has an enthalpy of only 1.9 kJ/mol, you can expect even lower values for the transformation of disordered carbon into a more organised form.
Alain
Dear Prof Alain
Based on your suggestion, it is possible that as-milled carbon powder generate peaks of endothermic at DSC test in order to transformation of disordered carbon into a more organised form. I would be grateful if you could inform me.
Armin
I definitely don't think so. Carbon requires huge temperature for changing its atomic structure in the solid state (as obviously carbon never melts). Carbon is inert in DSC. For example, diamond is known to be metastable with respect to graphite (which is the stable form of carbon) but again, except huge temperature or pressure, or catalysts, are used, nothing happens. Another example: glassy carbon is used as resistor for high-temperature ovens, or for crucibles, and despite the extremely high temperature endured by glassy carbon, it doesn't transform into graphite. Nothing can be seen by DSC, except chemical reactions (intercalation by alkali metals, oxidation, ...).
Alain
Dear Prof. Alain
Thank you so much for your respond. I am agree with you. However, some of researchers showed that reaction of carbon by DSC or DTG.
I really appreciate if you could please help me with this problem as follows.
To overcome this, I would like to your attention to attachment papers.
1- In paper 1, DTG test shows a endothermic reaction (i.e. about 10.4 KJ/g), however, results of DSC shows a exothermic reaction. That means that theses results are antithetical. Is is correct?
2- In paper 2, Reaction carbon with vanadium shows a endothermic reaction.
I am extremely grateful to you for your respond.
Armin
OK I understand.
In paper 1, not a true DSC was used, but a TG equipped with a DSC accessory. I have the same and the accuracy is poor. Anyway, the sample was heated in air, so oxidation occurred. You can see the DSC peak occurring along with weight loss and production of CO2. Nothing surprising. This is definitely not the transformation of disordered carbon into another allotropic form, but just carbon burning.
In paper 2, nothing is said about the method, nor about the composition of the phases ! They just measured enthalpies of fusion of these phases, so nothing related to carbon itself.
So papers 1 and 2 present completely different phenomena.
Alain
Dear Prof Alain
I am very obliged for your nice respond.
But, I have a question and I would like draw your attention to Fig.3 in paper 2.
A endothermic peak observed for reaction between vanadium and carbon resulting to VC. Notwithstanding, reaction of transition metals (Ti, V,...) with carbon is exothermic reaction based on thermodynamic data. So, could you please let me know your suggestion about this subject.
Best regards
Armin
The quality of the figure is very poor, but it seems to me that the exothermic direction is downwards, so the first peak seems to be exothermic, and the baseline moved after this peak, giving the impression of a second endothermic peak.
Alain
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