The thermodynamics of the transformation of graphite to multiwalled carbon nanotubes

The thermodynamic quantities associated to the transformation from graphite to multiwalled carbon nanotubes ( MWCNTs) were determined by electromotive force ( emf) and differential scanning calorimetry ( DSC) measurements. From the emf versus T data of galvanic cell Mo vertical bar Cr3C2, CrF2, MWCNTs vertical bar CaF2 s.c.vertical bar Cr3C2, CrF2, graphite vertical bar Mo with CaF2 as solid electrolyte, Delta(r) H-T(o) = 8.25 +/- 0.09 kJ mol(-1) and Delta S-r(T)o = 11.72 +/- 0.09 JK(-1) mol(-1) were found at average temperature T = 874 K. The transformation enthalpy was also measured by DSC of the Mn7C3 formation starting from graphite or MWCNTs. Thermodynamic values at 298 K were calculated to be: Delta(r) H-298(o) = 9.0 +/- 0.8 kJ mol(-1) as averaged value from both techniques and Delta(r) S-298(o) approximate to Delta(r) S-T(o). At absolute zero, the residual entropy of MWCNTs was estimated 11.63 +/- 0.09 JK(-1) mol(-1), and transformation enthalpy Delta(r) H-0(o) approximate to Delta(r) H-298(o). The latter agrees satisfactorily with the theoretical calculations for the graphite- MWCNTs transformation. On thermodynamic basis, the transformation becomes spontaneous above 704 +/- 13 K.