CONVERSION MECHANISMS OF A POLYCARBOSILANE PRECURSOR INTO AN SIC-BASED CERAMIC MATERIAL

The pyrolysis of a PCS precursor has been studied up to 1600-degrees-C through the analysis of the gas phase and the characterization of the solid residue by thermogravimetric analysis, extended X-ray absorption fine structure, electron spectrocopy for chemical analysis, transmission electron microscopy, X-ray diffraction, Raman and Auger electron spectroscopy microanalyses, as well as electrical conductivity measurements. The pyrolysis mechanism involves three main steps: (1) an organometallic mineral transition (550 < T(p) < 800-degrees-C) leading to an amorphous hydrogenated solid built on tetrahedral SiC, SiO2 and silicon oxycarbide entities, (2) a nucleation of SiC (1000 < T(p) < 1200-degrees-C) resulting in SiC nuclei (less than 3 nm in size) surrounded with aromatic carbon layers, and (3) a SiC grain-size coarsening (T(p) > 1400-degrees-C) consuming the residual amorphous phases and giving rise simultaneously to a probable evolution of SiO and CO. The formation of free carbon results in a sharp insulator-quasimetal transition with a percolation effect.