Structural Evolution of Silicon Carbide Phase from the Polycarbosilane Cured with Iodine: NMR Study

The structural evolution of silicon carbide phase from polycarbosilane fibers cured with iodine in air was investigated using nuclear magnetic resonance (NMR) together with in situ gas analysis up to 1400 °C by thermogravimetry coupled with mass spectroscopy (TG-MS). The investigation with solid-state ¹H, ¹³C, and ²⁹Si NMR analyses showed the influence of the oxygen affinity of Si atoms on the chemical structural changes of the SiOCH system during pyrolysis (up to 800 °C). In particular, the mechanism of phase segregation (SiOC → β-SiC + SiO₂ + C) in amorphous SiOC structure at 800–1250 °C was determined. Carbon in the Si–O–C networks is replaced by silicon, forming the Si-O-Si network, while the cleaved carbon atoms, which have unpaired electrons, combine, forming C=C bonds. This mechanism accounts for the structural rearrangement from O₂SiC₂ to O₃SiC to SiO₄ (from the silicon-centered standpoint, i.e., SiO₂ phase), the growth of β-SiC crystallites, and the carbon clustering.