Experimental thermodynamics of high temperature transformations in single-walled carbon nanotube bundles.

The thermodynamic quantities associated to the transformation of carbon in single-walled carbon nanotube (SWCNT) bundles to carbon in graphite were determined from 750 to 1015 K by a CaF2 solid electrolyte galvanic cell: (-) Mo | Cr3C2,CrF2,C'' | CaF2s.c. | Cr3C2,CrF2,C' | Mo (+). The trend with temperature of the electromotive force of the cell was found to be greatly dependent on temperature and fully reversible with it. The standard enthalpy DeltaH(T)(o) and entropy DeltaS(T)(o) changes are 7.1, 6.0, and 60.2 kJ mol(-1) and 8.6, 14.7, and 72.8 J K(-1) mol(-1) at 778, 883, and 975 K, respectively. This most likely correlates with the different arrangements and shapes of tubes that deviate from the ideal triangular closed packed structure of the SWCNT bundles. The constraints to the thermal expansion of bundles in the electrode containing them generated high internal pressures that were responsible for deformations of tube shape and lattice. Stable bundle states were formed that interconvert as a function of temperature. Comparative analyses by low angle XRD, microRaman, and HR-TEM of SWCNT bundles before and after experiments support this scenario. The cohesion energy and associated entropy changes are also reported for such states. The formation enthalpy of unbundled SWCNTs was calculated equal to 9.5 +/- 0.4 kJ mol(-1).