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°C under atmosphere of Ar. The black glasses obtained correspond respectively to i) silicon oxycarbide network with excess of C, ii) stoichiometric SiC_xO_2(1-x) where x = 0.3, and iii) silicon oxycarbide network with deficiency of C, i.e. with excess of Si. At this stage of the treatment, the samples are made up of a single and amorphous phase. A phase separation occurs after further pyrolysis in the high temperature range 1200–1500°C, leading to the formation of nanocrystalline β-SiC and amorphous SiO₂. We used a cubic silica structure factor to model the component due to amorphous silica. This enabled us to apply the Rietveld method to all patterns and to obtain a satisfactory fit of the experimental data. From these refinements, 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 chemical analysis and ²⁹Si Magic Angle Spinning NMR found in the litterature. Concerning the crystalline component β-SiC, its average crystallite size and microstrain were also evaluated. The evolution of the phase separation was then reported versus the pyrolysis temperature and seems to suggest a nucleation-and-growth mechanism.