Synthesis of Si₃N₄ from Na₂SiF₆ as a solid precursor: Microstructural evolution

The effect of processing parameters on the weight loss of the silicon solid precursor (Na₂SiF₆) and the deposition characteristics and morphology of Si₃N₄ formed onto SiC_p/Si porous substrates by CVD has been investigated. The results show that the weight loss of Na₂SiF₆ is most significantly affected by the processing temperature, followed by the processing time and the type of nitrogen precursor. Formation of Si₃N₄ is mostly influenced by the substrate temperature, followed by the type of nitrogen precursor and processing time. An increase in processing time and temperature from 60 to 120 min and from 900 to 1300°C, respectively, favors dissociation of Na ₂SiF₆ and formation of Si₃N₄. Moreover, N₂ enhances Na₂SiF₆ dissociation and hampers Si₃N₄ formation, while the N₂-NH ₃ mixture hinders the solid precursor dissociation and favors Si ₃N₄ formation. With regard to microstructure evolution, it is found that in N₂ the amount of Si₃N₄ increases with temperature and the morphology changes from wool-like and light fibers to thicker and compact fibers. When N₂-NH₃ is used and the processing temperature is increased, the morphology of Si ₃N₄ is modified from deposits with wool-like and compact appearance to whiskers and spheres and finally to thick and compact fibers.