Decarbonization mechanisms of polycarbosilane during pyrolysis in hydrogen for preparation of silicon carbide fibers

Polycarbosilane (PCS) fibers are cured by electron beam irradiation in helium. Then, the cured fibers are pyrolyzed under hydrogen. The mechanisms of carbon removal during pyrolysis are investigated using chemical elemental analysis, FTIR, Raman, and AES analysis. The development of microstructure and phase is examined by SEM, TEM, and XRD. The results show that the thermal cleavage of relatively weak Si–H and Si–CH₃ bonds takes place first during pyrolysis in hydrogen, generating free radicals. The free radicals then react with C–H bonds or with each other to form Si–CH₂–Si groups, releasing hydrogen and methane. As temperature increases, the Si–CH₂–CH₂–Si groups in PCS begin to dissociate and react with hydrogen to form methane, resulting in the further removal of carbon and giving silicon-rich silicon carbide fibers (i.e. C/Si <1).