Modelling and simulation of amorphous silicon oxycarbide

Structures and properties of stoichiometric amorphous silicon oxycarbide glasses of various compositions and various densities are studied on network models consisting of 112 - 196 atoms using density functional theory. We find a perfect random network structure of the glassy phase for low carbon concentrations. In this regime, many properties of the network scale with the amount of incorporated carbon, according to a rule of mixture between amorphous silica and silicon carbide. Beyond a critical limit, however, at about 12.5 at% C or 25 wt% SiC, the perfect network structure is disrupted and structural defects develop. The critical limit coincides with the onset of Si - C bond percolation throughout the structure and is accompanied by a discontinuous behavior of the bulk modulus. We thus propose a threshold value for the incorporation of tetrahedral sp(3)-C into a chemically perfectly ordered a-SiCO network. Further investigations show the development of voids and pores in low-density structures of a-SiCO. For a given composition, we calculate a linear dependence of the elastic properties with the density of the material. The free internal energy of the a-SiCO phase turns out to be independent over a wide range of densities. Car Parrinello molecular simulations at elevated temperatures show that the model structures constitute locally stable configurations of a-SiCO. The simulations of the dynamic evolution provide a qualitative insight into the mechanisms during a reorganization of the network structure that will happen at much longer time scales on annealing.