A silicon-29 MAS-NMR study of α-silicon nitride and amorphous silicon oxynitride fibres

Si-29-MAS-NMR with the support of elemental and XRD analyses has been used to assess the structure, the chemical and the phase composition of two types (C and A) of Si-N-(O) fibres prepared according to a gas phase process. The type C fibre consists of alpha-Si3N4 and traces of beta-Si3N4. It however contains 1-2 at% of oxygen which has been suggested to be distributed within the alpha-Si3N4 structure as a silicon oxynitride. The Si-29-NMR peak at - 56.4 ppm may originate from the original Si-N3O tetrahedral site involved in this structure. The type A fibre consists of a single amorphous silicon oxynitride phase of composition Si-N4(1-x/3)O-2x. with x = 0.14-0.16 (12-13 at% of oxygen). Silicon atoms in this meterial are in a tetrahedral environment of statistically distributed O and N atoms (Si-N4-nOn, with n = 0, 1, ..., 4). These different environments give rise-fo characteristic Si-29-NMR bands, with distinct chemical shifts. Because of the amorphous structure, these bands overlap in the case of the Type A fibre, yielding a single broad NMR band. Their respective intensities and therefore, the shape of the band, are related to the oxygen concentration of the fibre. (C) 1998 Elsevier Science Limited. All rights reserved.