NMR and X-ray diffraction studies of amorphous and crystallized pyrolysis residues from pre-ceramic polymers

Abstract

²⁹Si MAS NMR and X-ray diffraction studies are presented of black and white pyrolysis residues obtained by initial 1100°C pyrolyses in N₂ and NH₃ atmospheres followed by 1550°C pyrolyses in Ar, N₂ or vacuum atmospheres of a polycarbosilane and four polysilazane precursors to SiC and Si₃N₄ ceramics. Amorphous white pyrolysis residues crystallized under the various conditions to give not only Si₃N₄ but also Si₂N₂O, SiC, SiO₂ and Si, while black amorphous pyrolysis residues crystallized to form only Si₃N₄ or SiC. In general, the crystalline ceramic products observed depended on a variety of factors, i.e. moisture sensitivity of polymer, the initial 1100°C pyrolysis gas (N₂/NH₃), the dryness of the 1100°C-NH₃ pyrolysis gas and the 1550°C pyrolysis atmosphere (N₂, Ar, vacuum). An additional factor of interest affecting product distribution was the choice of crucible (alumina/graphite) employed in the 1550°C pyrolysis. The combined studies suggest that the white amorphous pyrolysis residues are complex silicon oxycarbonitrides (Si_xN_yO_zC_a), while the amorphous black residues are silicon carbonitrides (Si_xN_yC_z).