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Role of redistribution reactions in the polymer route to silicon-carbon-oxygen ceramics

Redistribution reactions are used in the synthesis of several preceramic polymers. Moreover, redistributions play a major role in the pyrolysis of these precursors. Examples of thermal redistributions involving the exchange of Si-O/ Si-X bonds (X = O, H, C,…) in polysiloxanes, precursors to silicon-oxygen-carbon ceramics, are given. Redistributions account for the escape of volatile organosilicon compounds, decreasing the yield and modifying the composition of the final ceramic. Moreover, they deeply modify the environment of Si atoms in the residue. At ∼900°C, Si-O/ Si-C redistributions have reached equilibrium, leading to a random, entropically controlled distribution of SiO x C 4-x sites. At higher temperatures, redistribution equilibria are displaced by the crystallization of SiC. Thus, the structure of the silicon oxycarbide phase (environment of Si atoms) is dependent on the O/Si ratio of the glass and the pyrolysis temperature, but is not directly dependent on the structure of the precursor.