Graphite, of course. As it is thernodinamically stable phase.

Data for graphite can be found in any handbook. Thermodynamic characteristics of carbon fiber and MWCNTs I did not seen across in the scientific literature, as these are methastable phases and their characteristics depend both on the method of production and on the macrostructure of the samples (there are not and cannot be standard). This means that all the characteristics of the samples that you receive apply only to your samples, moreover, these will changed from one batch to another. No one will check you (I do not know is it good or bad). On the other hand, the differences in the characteristics of these forms should not be too different (within measurement accuracy) from both graphite and soot.

Dear Dimitris Baltzis,

Graphite is considered as the most thermodynamically stable material.

Research publications preferably with indicative thermodynamic values.

01. Article Synthesis, Properties, and Applications of Low-Dimensional C...

02. https://brainly.in/question/3437447

03. Article Carbon allotropes: A suggested classification scheme based o...

04. https://www.sciencedirect.com/science/article/pii/S0955221918304217

05. https://www.science.gov/topicpages/c/carbon+allotropes+graphite

I hope the information will help you.

With Best Wishes,

Samir G. Pandya

It always depends, of course, on PVT conditions. But it is easy to intuitively demonstrate that graphite is more stable than CNT and carbon fibers (or even graphene):

Carbon fibers have some amorphous carbon and more edges than graphite, all of which will increase the energy of the structure thus decreasing its stability.

Graphene (I'll go to CNT in a sec... just need this first) lacks the layer stacking of graphite. The graphene stacking in graphite provides van der Waals interlayer bonding which decreases the energy. Graphite is thus more stable than graphene.

Finally, to make a CNT we need to roll a graphene layer, thus bending all the sp2 bonds and increasing the energy through torsion. CNT is thus less stable than graphene (which is less stable than graphite).

By the way, wikipedia has a great article:

https://en.wikipedia.org/wiki/Allotropes_of_carbon

and this reference is excellent:

Article Homo Citans and Carbon Allotropes: For an Ethics of Citation