Nanometric Si/C/N powders: Description of the short range atomic structure by X-ray absorption spectroscopy

The growing interest in ceramics with nanophase structure is explained by their anticipated improved thermomechanical properties and example of superplastic nanocomposite ceramics has already been evidenced. Nanosized Si/C/N powders are attractive as starting materials to produce SiC and/or Si 3N 4 fine grained ceramics. Such preceramic powders can be obtained by combining the ultrasonic injection of a liquid precursor with the emission of an industrial high power CO 2 laser. The physical properties of these nanometric powders are strongly correlated to their atomic structure. We have investigated Si/C/N powders with a C N ratio variable from 0.3 to 1.3 by X-ray absorption spectroscopy at the silicon K edge for the as-prepared components, and followed their evolution during the annealing at temperatures ranging from 1000°C to 1600°C under N 2 atmosphere. By combining XANES and EXAFS analysis results we propose a model for the local structure of the amorphous as-prepared powders, and for the structural evolution during the annealing. These models are based on the hypothesis of the existence of a “solid solution” of Si, C, N atoms at the sites of a distorted network. They have been tested by the FEFF code. During the thermal treatment, the structural evolution is strongly dependent on the C N ratio. For intermediate C N values, a delay in the crystallization temperature occurs, leading then to the formation of very small crystallites distributed into a still amorphous matrix. The observed increase of the crystallization temperature is attributed to a favorable composition corresponding, on average to a SiC 2N 2 environment for Si atoms. XANES calculations using FEFF6 code for α and β Si 3N 4 are finally presented.