CONVERSION MECHANISM OF POLYBOROSILAZANE INTO SILICON NITRIDE-BASED CERAMICS

The pyrolysis of polyborosilazane in anhydrous ammonia has been studie d up to 1200 degrees C through the analysis of the gas phase and the c haracterization of the solid residue by elemental analyses, thermograv imetric analysis, Fourier transform-infrared analysis, X-ray photoelec tron spectroscopy, X-ray diffraction and Si-29 cross-polarization/magi c angle spinning-nuclear magnetic resonance. The pyrolysis mechanism i nvolves four main steps: (1) below 400 degrees C, an evaporation of re sidual solvent and reaction with ammonia leading to an increase of nit rogen content; (2) from 400-600 degrees C, reaction with ammonia leadi ng to an increase of nitrogen content and formation of preceramic poly mer-ceramics intermediate solid with a three-dimensional network along with evolution of gaseous species; (3) from 600-800 degrees C, comple tion of loss of C-H functionalities and progress of formation of an am orphous ceramic, accompanied by evolution of CH4 and H-2; (4) from 800 -1200 degrees C, completion of formation of an amorphous hydrogenated Si-B-O-N solid composed of SiN4-xOx(x=0,1,2,3) and BN3-yOy (y=0, 1), a long with evolution of H-2