Conversion of polycarbosilane (PCS) to SiC-based ceramic Part II Pyrolysis and characterisation

Abstract

The conversion to ceramic of a commercial polycarbosilane (PCS) under various pyrolysis conditions has been investigated. The products of pyrolysis have been characterised by solid state ²⁹Si and ¹³C NMR spectroscopy and X-ray diffraction (XRD). Some of the phases identified in the present study were found to differ from those reported previously, particularly in the earlier literature. Oxidation-cured PCS, when pyrolyzed up to 1400 °C in argon, generally produced silicon oxycarbide (SiO _x C _y ) as the second major phase with β-SiC as the major phase, and smaller amounts of free carbon. With increasing temperature above 1200 °C, the silicon oxycarbide phase decomposed to give β-SiC. Silica (SiO₂) was also found to evolve from this silicon oxycarbide phase. Loss of some of the silica, probably by reaction with carbon, was found at 1400 °C, possibly yielding SiO, CO and SiC. At 1500 °C, crystalline α-cristobalite was found as a minor phase with β-SiC as the major phase and a lower amount of free carbon. Pyrolysis in vacuum leads to production and crystallization of β-SiC at a lower temperature than required if pyrolyzed in argon flow. After pyrolysis at 1600 ° in vacuum, the cured PCS converted to almost stoichiometric β-SiC.