Thermodynamic properties of graphite-like nanostructures prepared by thermobaric treatment of fullerite C60

Abstract

Temperature dependences of the heat capacities of disordered graphite like nanostructures prepared by the thermobaric treatment of fullerite C 60 (p = 2 and 8 GPa, T = 1373 K) were measured in the temperature ranges from 7 to 360 K in an adiabatic vacuum calorimeter and from 330 to 650 K in a differential scanning calorimeter. At Т < 50 K, the dependences obtained were analyzed using the Debye theory of the heat capacity of solids and its multifractal version. The fractal dimensions D were determined and some conclusions on the heterodynamic charac ter of the structures studied were made. The thermodynamic functions С р°( Т), Н°(Т) -Н°(0), S°(T) -S°(0), and G°(T) -H°(0) were calculated in the temperature range from Т → 0 to 610 (650) K. The thermodynamic properties of the graphite like nanostructures studied and some carbon allotropes were compared. The standard entropies of formation Δ f S° of the graphite nanostructures studied and diamond were calculated along with the standard entropies of the reactions of their synthesis from the face centered cubic phase of fullerite C 60 and their inter conversions at Т = 298.15 K.There is considerable literature on the studies of fullerenes, their derivatives, and materials based on them. 1-8 The thermobaric treatment of fullerite C 60 followed by abrupt pressure drop from several GPa to р = 0.1 MPa and lowering the temperature to 298.15 K pro duces new carbon materials. 4-12 Under these conditions, the systems are thermodynamically metastable but kineti cally stable. In some cases, the thermobaric treatment of the face centered cubic (fcc) phase of fullerite C 60 causes breakdown of the molecular cage and formation of di sordered nanostructures 4,8,11-13 containing the sp 2 and sp 3 hybridized carbon atoms. These structures are similar in physicochemical properties to graphite and diamond. The fraction of the sp 3 hybridized C atoms in the fi nal product significantly increases under pressures of 8-10 GPa. 14 Interest in carbon materials is due to their unique properties, e.g., superhardness, 4,8,15 superconductivity, 16,17 high plasticity, 4,15 and controlled anisotropy of elastic properties. 12 There is a demand for studies of the thermo dynamic properties of carbon materials and optimization of technology of their production.The purpose of this work is to carry out a calorimetric study of the temperature dependences of the heat capacity of the graphite like nanostructures (GNS) produced by the thermobaric treatment of fullerite C 60 in the tempera ture interval from 7 to 610-650 K; to analyze these de pendences at T < 50 K in the framework of the Debye theory of the heat capacity of solids and its multifractal version; to calculate the standard thermodynamic func tions in the temperature range from Т → 0 to 610-650 K; to compare the standard thermodynamic properties of the GNS and some carbon allotropes; and to establish how their physicochemical properties depend on the structure. ExperimentalDoubly sublimed, finely crystalline powdered fullerite C 60 (grain size ~10...

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