Effects of high-temperature annealing on structural and mechanical properties of amorphous carbon materials investigated by molecular dynamics simulations

We investigated effects of thermal annealing on structural and mechanical properties of amorphous model carbon materials with molecular dynamics (MD) simulations using a reactive force field. Annealed amorphous carbon materials were prepared by formation of amorphous carbons from disconnected carbon atoms at 4 different initial densities of 1.5, 2.0, 2,5, and 3.0 g/cc and subsequent high-temperature annealing simulations. Carbon atoms with three bonds as in graphite dominated at later stages of annealing at all densities while the fraction of carbon atoms with four bonds increased with the increase of the densities. Carbon ring structures with varying sizes were found in annealed carbon materials. Hexagon rings were observed most frequently, and pentagon-pentagon or heptagon-heptagon neighboring ring pairs were rarely observed. Internal ring structures consistent with planar structures were found more frequently at smaller densities and with smaller ring sizes. Distributions of inter-ring distances along directions parallel and normal to the ring plane were calculated. Peaks in distribution of inter-ring distances normal to the ring plane matched closely those found in graphite. We characterized elastic mechanical properties of annealed carbon materials at different densities by estimating Young's moduli from tensile deformation simulations. We also investigated effects of annealing on structural properties of amorphous carbon with density functional tight binding (DFTB) MD simulation. The configuration of an annealed carbon material prepared with DFTB MD simulation displayed a multi-vacancy defect structure and more planar arrangements of carbon rings than the configuration prepared with MD simulation with a reactive force field.