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

Abstract

29Si–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 α-Si 3N 4 and traces of β-Si 3N 4. It however contains 1–2 at% of oxygen which has been suggested to be distributed within the α-Si 3N 4 structure as a silicon oxynitride. The 29Si–NMR peak at −56·4 ppm may originate from the original Si–N 3O tetrahedral site involved in this structure. The type A fibre consists of a single amorphous silicon oxynitride phase of composition Si–N 4 (1− x)/3 O 2 x . with x=0·14–0·16 (12–13 at% of oxygen). Silicon atoms in this material are in a tetrahedral environment of statistically distributed O and N atoms (Si–N 4– n O n with n=0, 1, …, 4). These different environments give rise to characteristic 29Si–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.