Chemistry of a series of aluminum-modified polysilazanes: Synthesis, pyrolysis behaviour and microstructural evolution

A series of polyaluminosilazanes was synthesized by reaction of dimethylethylamine alane complex with polysilazanes. Starting from different dichlorosilanes, polysilazanes were obtained by means of ammonolysis. The latter reacted with the alane complex and led to solid polyalumino(carbo)silazanes that can be described as “custom-made” polymers since all the steps were monitored and controlled (from the choice of the molecular precursor to the polymerization and the functionalization with aluminum) in order to study the effect of the introduction of Al on the different reactive sites of the Si- backbone in the polysilazanes. Detailed information on the polymer structures were obtained by FTIR and multinuclear solid state NMR spectroscopies. The transition from the polymeric state to the ceramic inorganic state was investigated by means of solid-state NMR spectroscopy and finally the structural evolution of the final ceramics by high angle XRD, Raman spectroscopy and TEM imaging. It is noted that the polymer-derived SiAlCN ceramics are X-ray amorphous even at high temperature under nitrogen with some local crystallization. Upon further heating these materials tend to further crystallize into thermodynamically stable phases at a given chemical composition, such as SiC, Si3N4, free C, or AlN. Finally, the present study shows that the amorphous-to-crystalline transition is closely related to the nature and structure of the preceramic polymer.