Evidence For Glass Behavior In Amorphous Carbon

Amorphous carbons are disordered carbons with densities of circa 1.9-3.1 g/cc and a mixture of sp2 and sp3 hybridization. Using molecular dynamics simulations, we simulate diffusion in amorphous carbons at different densities and temperatures to investigate the transition between amorphous carbon and the liquid state. Arrhenius plots of the self-diffusion coefficient clearly demonstrate that there is a glass transition rather than a melting point. We consider five common carbon potentials (Tersoff, REBO-II, AIREBO, ReaxFF and EDIP) and all exhibit a glass transition. Although the glass-transition temperature (T-g) is not significantly affected by density, the choice of potential can varyTgby up to 40%. Our results suggest that amorphous carbon should be interpreted as a glass rather than a solid.