A thermodynamic model has been developed to rationalize the enhanced stability of amorphous Si-C-N ceramics derived from polymer precursors. Typically, the complex morphology of such materials is separated into regions of amorphous carbon and amorphous matrix of compositions SiCxNy on the line joining SiC and Si3N4 in the Si-C-N phase diagram. This amorphous matrix has been modelled as a solution phase of atomic constituents of the stoichiometry corresponding to short-range ordered SiCiN4-i atomic configurations. To estimate the parameters of the Gibbs energy of such an a-SICN phase the available thermodynamic assessments of silicon carbide and silicon nitride have been used and some assumptions concerning the bond energies of the phase constituents have been made. The Gibbs energy derived for the a-SICN phase has been compared at temperatures of 1300K T 2000K with the Gibbs energy computed for equivalent compositions of mechanical mixtures from either crystalline or amorphous SiC and Si3N4 phases. This comparison proves an enhanced resistance against crystallization for the a-SICN phase of certain SiCxNy compositions, being in agreement with experimental evidence.
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