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. Na nosized Si/C/N powders are attractive as starting materials to produce SiC and/or Si3N4 fine grained ceramics. Such preceramic powders can b e obtained by combining the ultrasonic injection of a liquid precursor with the emission of an industrial high power CO2 laser. The physical properties of these nanometric powders are strongly correlated to the ir atomic structure. We have investigated Si/C/N powders with a C/N ra tio variable from 0.3 to 1.3 by X-ray absorption spectroscopy at the s ilicon K edge for the as-prepared components, and followed their evolu tion during the annealing at temperatures ranging from 1000 degrees C to 1600 degrees C under N-2 atmosphere. By combining XANES and EXAFS a nalysis results we propose a model for the local structure of the amor phous 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 tr eatment, the structural evolution is strongly dependent on the C/N rat io. For intermediate C/N values, a delay in the crystallization temper ature occurs, leading then to the formation of very small crystallites distributed into a still amorphous matrix. The observed increase of t he crystallization temperature is attributed to a favorable compositio n corresponding, on average to a Si-C2N2 environment for Si atoms. XAN ES calculations using FEFF6 code for alpha and beta Si3N4 are finally presented. (C) 1997 Elsevier Science B.V.