Amorphous silica has high potential as a precursor for synthetic applications due to its active and large specific surface area. This work aims to identify the comparative features of the self-propagating high-temperature synthesis (SHS) of silicon carbide obtained by magnesiocarbothermic reduction of amorphous and crystalline silica. It was demonstrated that the synthesis pathway and microstructure depend on the structure of silica or, in other words, on the rate of rupture of Si–O–Si bonds. Moreover, the introduction of small amounts of polytetrafluoroethylene (PTFE) significantly intensifies the combustion reaction and governs the formation of specific microstructural entities, e.g., sheets and nanoflakes. Particularly, when using amorphous silica as a precursor, it is fully reduced, resulting in a higher amount of high-temperature α-SiC compared to samples reduced from crystalline silica. This is expressed by the dominant presence of sheet-like structures in the product, the amount of which significantly increases with the introduction of PTFE into the system. In contrast, in the case of crystalline silica, the combustion is characterized by a comparatively lower temperature and lower yield. Complete reduction of the crystalline silica was possible only if PTFE was added. The fine-grained microstructure of the obtained β-SiC was preserved regardless of the amount of PTFE. According to the results of scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM/EDS) examinations, in both cases, a homogeneous distribution of the constituent elements (Si, C) was observed. The precursor morphology, combustion parameters, and PTFE were found to play a crucial role in tailoring the microstructural characteristics and degree of conversion of the combustion products.
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