Atomic-network conditions for bulk-modulus enhancement in amorphous SiOC through carbon-clustering suppression: Molecular dynamics comparison of C-C repulsion and nitrogen co-doping

We performed molecular dynamics simulations to examine whether suppressing carbon clustering can enhance the bulk modulus of amorphous SiOC and to identify the atomic-network conditions responsible for this enhancement. Two types of models were compared under O-rich and O-poor stoichiometries: a-SiOC with an artificial short-range C-C repulsion constraint (SiOC-CCRep) and equal-C/N-co-doped a-SiOCN. Under O-poor conditions, the bulk modulus of SiOC-CCRep reaches about 165 GPa at the carbon-rich end, substantially above the value of about 135 GPa for conventional melt–quench a-SiOC. This enhancement is associated with an increased total bond density and the incorporation of carbon mainly into Si-rich fourfold coordination environments. Under O-rich conditions, carbon remains in oxygen-containing low-coordination environments, and no comparable enhancement occurs. For the equal-C/N a-SiOCN compositions examined, C-C bonding is reduced at higher carbon fractions, but the total bond density and bulk modulus remain close to those of conventional a-SiOC because Si-rich fourfold carbon environments do not become dominant. These results show that suppressing C-C bonding enhances the bulk modulus only when it increases the total bond density by promoting carbon incorporation into Si-rich fourfold coordination environments.