Kinetics of Diamond Formation from Temperature Extrapolatable Kinetics Model built using Molecular Dynamics Simulations

In the atmosphere of icy giant planets, methane is pyrolyzed under extreme conditions and is thought to convert into diamond. However, the exact conditions of diamond formation are poorly understood: depending on the experimental setup, static compression or shockwave experiments, diamond have been observed in conditions that vary by tens of gigapascals and thousands of Kelvins. Rationalizing these differences is complicated by the lack of understanding of the mechanisms of diamond formation from liquid hydrocarbons.

In this work, we present a detailed kinetics model of this mechanism built using molecular dynamics simulations. To correctly describe the reactions between small molecules, but also the growth of long carbon particles, microscopic reactions were defined in terms of atoms and their local environment. This description also allowed us to build a kinetics model that can be accurately extrapolated in time and temperature, so predictions are made on a wide range of temperature and are not limited by the computational cost of molecular dynamics simulations.

We found that additional demixing or dihydrogen absorption mechanisms are necessary to explain the observation of diamond in static compression experiments, and shockwave experiments may be too short to form diamond.