Amorphous silicon–oxycarbide (SiCO) can retain large mole fractions of carbon when it is made from controlled pyrolysis of silicon-based polymers. The crystallization resistance of these ceramics, which is quite remarkable, varies with the carbon content. In high-carbon materials, crystallization is usually accompanied by weight loss (resulting from the carbothermal reduction of silica), whereas phase separation can lead to crystallization without significant weight loss in the low-carbon materials. A simple graphical method is developed to estimate the composition of the crystallized material, and the concomitant weight loss. The results are presented as maps for a quick estimate of crystallization and weight loss for any composition. Experiments with a medium–high carbon SiCO are used to quantify the degree of crystallization and the associated weight loss at 1300°C and at 1350°C; these results show that, in the case of medium high carbon content, crystallization begins with phase separation but becomes quickly dominated by weight loss.
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