The Thermodynamics of the Transformation of Graphite to Multiwalled Carbon Nanotubes

Abstract

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|Cr(3)C(2), CrF2, MWCNTs|CaF2 s.c.|Cr(3)C(2), CrF2, graphite|Mo with CaF2 as solid electrolyte, Delta(r)H(T) degrees= 8.25 +/- 0.09 kJ mol(-1) and Delta(r)S(T) degrees= 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 Mn(7)C(3) formation starting from graphite or MWCNTs. Thermodynamic values at 298 K were calculated to be: Delta(r)H(298) degrees = 9.0 +/- 0.8 kJ mol(-1) as averaged value from both techniques and Delta(r)S(298) degrees approximately Delta(r)S(T) degrees. 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) degrees approximately Delta(r)H(298) degrees. The latter agrees satisfactorily with the theoretical calculations for the graphite-MWCNTs transformation. On thermodynamic basis, the transformation becomes spontaneous above 704 +/- 13 K.

Kinetic study of carbon nanotubes synthesis by fluidized bed chemical vapor deposition

“…At 5508C, TEM observations revealed the presence of large diameter (100-200 nm) filaments and of numerous encapsulated particles; at 700 and 7508C, such filaments were also present but with less encapsulated particles. The preferential formation of large diameter CNTs at low temperature has already been reported, 3,21 and it can be due to the presence of a-Fe at these temperatures.…”

“…Thermo gravimetric analysis (TGA) was performed to evaluate the amount of carbon deposited in each experiment and also the percentage of other forms of carbon. Those analyses were conducted under air in a Setaram-92-12 apparatus with a 108C min 21 ramp between 20 and 10008C followed by an isotherm at this temperature for 30 min. The distribution of grain diameters before and after MWCNT synthesis was measured with a Malvern-2600C laser granulometer equipped with a Sirocco-2000 sampler in dry mode.…”

On the Deposition Equilibrium of Carbon Nanotubes or Graphite in the Reforming Processes of Lower Hydrocarbon Fuels

“…Such an analysis of the thermodynamic equilibrium was carried out numerically using the specialized, commercial HSC software [23]. Its basic databank was supplemented using information from the literature on the chemical potential function of temperature for pure carbonic nanotubes, both multi-walled [26] and single-walled in bundles [27]. Four pairs of C-H-O mixtures consisting of NG or LPG fuel, with either water steam or carbon dioxide, were analyzed to reflect the equilibrium conditions of reforming the fuels.…”

“…On the other hand, the estimates of the chemical potential of filamentous carbon in the literature, which are based on the reactant partial pressures at the coking threshold, differ greatly [16]. Therefore, only experimental electrochemical data for the purified filamentous forms of solid carbon [26,27] are accepted in this present study. The chemical potentials were estimated from the measured electromotive force of high-temperature solid-state galvanic cells.…”

“…However, the HSC standard database only contains data for graphite, diamond, and amorphous carbon. The database was therefore supplemented with re-formatted literature data [26,27] for the standard chemical potential of multi-walled carbon nanotubes (MWCNT) and single-walled nanotubes in bundles (SWCNT). The format of the additional data was described in Reference [25].…”