On the existence of voids within precursor-derived amorphous Si-C-N ceramics

Several model structures have been generated by molecular dynamics simulations for two precursor-derived amorphous Si-C- N ceramics with stoichiometry Si37C32N31 and Si40C24N36, respectively. These can be considered as possible atomistic model structures for the real amorphous ceramics. Although these models vary in their local atomic arrangements, all have been found to possess the same main structural properties being characteristic for the amorphous state of the precursor-derived ceramics. This is not only the already found phase separation into amorphous Si3N4 and layered amorphous graphitic carbon on the nanoscale with interconnecting Si-C bonds between these two phases, but also the existence of voids within the amorphous structure at the interfaces of the different phases. These voids have only a small extension - like the carbon phase - and their existence is in accordance with the density inhomogeneities found in diffraction experiments. These inhomogeneities can hence be attributed not only to a phase separation but also to the existence of the voids. Because of these voids, the density of the residual structure is definitely larger than the macroscopic one measured in experiment. In the case of Si37C32N31, it is estimated to be 3.0 +/- 0.1 g/cm(3), for Si40C24N36 it is 2.8 +/- 0.1 g/cm(3).