Influence of H 2/NH 3 mixtures on the composition of SiCNYO and SiCNAl(O) nanopowders

Abstract

We performed pyrolysis of SiCNAlH and SiCNYOH nanopowder precursors under a reactive atmosphere (Ar/NH 3/H 2) with various compositions of ammonia (NH 3) and dihydrogen (H 2) to diminish C content, which is deleterious for thermal stability and sintering of the powders. This paper continues a previous work on the fabrication of an Si 3N 4/SiC composite without free C by studying the effect of H 2 on the C/N atomic ratio of the powder. We studied the influence of the nature of the gaseous mixture (Ar/NH 3/H 2) on the powder composition. Elemental analysis showed that the introduction of H 2 in the pyrolysis atmosphere limited the decomposition of NH 3 and allowed for control of the C/N ratio. This behaviour can be explained by the structural evolution observed by 29Si NMR spectrometry but also by Fourier transform infrared and Raman spectroscopy. An Si 3N 4/SiC composite, with traces of free C, was obtained after post-pyrolysis heat treatment of the powders synthesized with 10 wt.% of H 2 and 25 wt.% NH 3.