Effect of pre-pyrolysis heat treatment on the preparation of silicon carbide from a polycarbosilane precursor

Polycarbosilane was synthesised directly from chloromethyltrichlorosilane and specimens were heat treated at 200, 300 and 400 degrees C in nitrogen. The as-synthesised polymer and heat treated specimens were characterised using infra-red and Fourier transform infra-red spectroscopy, solid-state nuclear magnetic resonance and differential scanning calorimetry and were also subjected to thermogravimetric analysis in nitrogen. All types of specimens were pyrolysed in nitrogen at 1500 degrees C. The ceramic residue obtained from thermogravimetric and pyrolysis experiments was characterised using X-ray diffraction and scanning electron microscopy. Heating the polymer before pyrolysis improved significantly the yield of the ceramic but the gain beyond 200 degrees C was minimal. It was shown that the heat treatment initially caused the as-synthesised polymer to lose volatile oligomers and subsequently to cross-link. Pyrolysis at 900 degrees C and subsequent crystallisation by further heating to 1500 degrees C of the cross-linked polymer produced homogeneous and fine grained silicon carbide. (C) 1998 Elsevier Science Limited and Techna S.r.l. All rights reserved.