Modelling polymer-derived ceramics

We investigate structure, energy, and elastic properties of some polymer-derived nitride ceramics, amorphous silicon nitride, a-Si3N4, and its ternary derivatives a-Si3B3N7, a-SiNO, and a-SiCN. Model structures consist of 104-448 atoms. They are first generated by an empirical network algorithm and then further computed using density functional methods including extensive ab initio molecular dynamics simulations. Optimized structures we obtain exhibit perfect chemical order consistent with the perception of an inorganic network derived from polymer precursors. We find a random network structure for a-SiNO, but phase segregation of BN and C sub-structures in a-Si3B3N7 and a-SiCN, respectively. The phase partitioning is driven by energy and benefits from a low density of the material, since we find the segregated phase predominantly at internal surfaces of voids and pores. Energies calculated for a-SiCN phases support a non-solubility of carbon in a-Si3N4 or stoichiometric a-SiCN. The bulk modulus of a-Si3B3N7 and a-SiCN is comparable to that of a-Si3N4 but at 5-10% lower density of the material.