Abstract
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 (Tg) is not significantly affected by density, the choice of potential can vary Tg by up to 40%. Our results suggest that amorphous carbon should be interpreted as a glass rather than a solid.
Highlights
Amorphous carbons are often described as one of the allotropes of carbon, along with graphite, diamond and fullerenes
We show that Tg values can be determined and that they provide a robust alternative to the uncertainty associated with melting behavior in the phase diagram of carbon
In a three-dimensional system D is related to the MSD by the expression MSD(t) = 6 Dt, and can be extracted via linear regression
Summary
Amorphous carbons are often described as one of the allotropes of carbon, along with graphite, diamond and fullerenes. For a comprehensive review of the field, summarizing 60 years of research and development, see the review by Vetter [1]. The defining structural characteristics of amorphous carbon are lack of medium- and long-range order, a homogeneous spatial distribution, and a mixture of sp and sp hybridization which varies linearly with density. At the highest density of 3.0–3.1 g/cc the sp fraction is ∼85%, falling to around 10–15% at 1.9 g/cc [2]. This density the carbon network develops pores via inhomogeneity and extended order via graphenization
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