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/cm³, for Si₄₀C₂₄N₃₆ it is 2.8±0.1 g/cm³.

Keywords:

• Precursor ceramics
• Amorphous
• Atomic structure
• Phase separation
• Graphitic carbon
• Voids
• Pores
• Modeling
• Molecular dynamics
• DFTB

Acknowledgment

The author is indebted to the Deutsche Forschungsgemeinschaft for financial support within the DFG Schwerpunktprogramm “Neue Precursorkeramik aus kondensierten molekularen Vorstufen” for part of the work and acknowledges stimulating and helpful discussions with Drs. Michael Hoffmann and Peter König. The provision of the experimental structure factors and pair correlation functions of the precursor‐derived ceramic Si₃₇C₃₂N₃₁ by Dr. Hans‐Peter Lamparter is highly appreciated.