Study of the phase separation in amorphous silicon oxycarbide glasses under heat treatment

The structural evolution of three silicon oxycarbide glasses was studied by X-ray diffraction (XRD) as a function of the pyrolysis temperature. Three compositions were prepared by the sol-gel method and pyrolysed at 1000 degr ees C under atmosphere of Ar. The black glasses obtained correspond respect ively to i) silicon oxycarbide network with excess of C, ii) stoichiometric SiCxO2(1-x) where x = 0.3, and iii) silicon oxycarbide network with defici ency of C, i.e. with excess of Si. At this stage of the treatment, the samp les are made up of a single and amorphous phase. A phase separation occurs after further pyrolysis in the high temperature range 1200-1500 degrees C, leading to the formation of nanocrystalline beta-SIC and amorphous SiO2. We used a cubic silica structure factor to model the component due to amorpho us silica. This enabled us to apply the Rietveld method to all patterns and to obtain a satisfactory fit of the experimental data. From these refineme nts, the amount of each phase (crystalline or amorphous) can be determined, based on the assumption that the electron density of the model agrees with the actual amorphous phase. A comparison is also made with results from ch emical analysis and Si-29 Magic Angle Spinning NMR found in the litterature . Concerning the crystalline component beta-SiC, its average crystallite si ze and microstrain were also evaluated. The evolution of the phase separati on was then reported versus the pyrolysis temperature and seems to suggest a nucleation-and-growth mechanism. (C)1999 Acta Metallurgica Inc.