High Temperature Stability of Onion-Like Carbon vs Highly Oriented Pyrolytic Graphite

摘要

AbstractudThe thermodynamic stability of onion-like carbon (OLC) nanostructures with respect to highly oriented pyrolytic graphiteud(HOPG) was determined in the interval 765–1030 K by the electromotive force (emf) measurements of solid electrolyteudgalvanic cell: (Low) Pt|Cr3C2,CrF2,OLC|CaF2s.c.|Cr3C2,CrF2,HOPG|Pt (High). The free energy change of transformationudHOPG = OLC was found positive below 920.6 K crossing the zero value at this temperature. Its trend with temperature wasudwell described by a 3rd degree polynomial. The unexpected too high values ofudLDrHTudLTud udPud~DcPðTÞ jointly to the HR-TEM,udSTEM and EELS evidences that showed OLC completely embedded in rigid cages made of a Cr3C2/CrF2 matrix, suggestedudthat carbon in the electrodes experienced different internal pressures. This was confirmed by the evaluation under constantudvolume ofuddPuddTudby theudaudkudratio for OLC (0.5 MPa K21) and HOPG (8 Pa K21) where a and k are the isobaric thermal expansionudand isothermal compressibility coefficients, respectively. The temperature dependency of the pressure was derived andudutilized to calculate the enthalpy and entropy changes as function of temperature and pressure. The highest value of theudinternal pressure experienced by OLC was calculated to be about 7 GPa at the highest temperature. At 920.6 K, DrH andudDrS values are 95.8 kJ mol21 and 104.1 JK21 mol21, respectively. The surface contributions to the energetic of the systemudwere evaluated and they were found negligible compared with the bulk terms. As a consequence of the high internaludpressure, the values of the enthalpy and entropy changes were mainly attributed to the formation of carbon defects in OLCudconsidered as multishell fullerenes. The change of the carbon defect fraction is reported as a function of temperature.