Polymer-ceramic conversion of a highly branched liquid polycarbosilane for SiC-based ceramics

Liquid polycarbosilane (LPCS) with a highly branched structure was characterized by fourier-transform infrared spectrometry (FT-IR) and H-1, C-13, Si-29 nuclear magnetic resonance spectrometry (NMR). The LPCS was then cured and pyrolysized up to 1,600 degrees C under flowing argon. The structural evolution process was studied by thermogravimetric analysis and differential scanning calorimetry (TG-DSC), FT-IR, and X-ray diffraction (XRD). Hydrosilylation, dehydrocoupling, and polymerization cross-linking reactions between Si-H and C=C groups occurred at low temperatures, which mainly accounted for the high ceramic yield (70%) up to 1,400 degrees C. The organic groups gradually decomposed and the structure rearranged at high temperatures. The FT-IR analysis revealed that Si-CH2-Si chains, the backbone of original polymer, can be retained up to 1,200 degrees C. At temperatures higher than 1,200 degrees C, the Si-CH2-Si chains broke down and crystalline SiC began to form. The final crystalline products were beta-SiC and a small amount of carbon.