Modelling and simulation of amorphous silicon oxycarbide

Abstract

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 sp³-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.