Mechanism and kinetics of the carbothermal nitridation synthesis of alpha-silicon nitride

The carbothermal nitridation synthesis of alpha-Si3N4 is studied using electron microscopy techniques (FEG/SEM and TEM) and chemical composition analysis to characterize the reaction at various degrees of conversion, The reaction follows a nucleation-growth mechanism, Without ''seed'' alpha-Si3N4 in the precursor, the reaction rate is controlled by the formation of nuclei which are associated with a Si-O-C intermediate phase. In the presence of ''seed,'' the limiting step is growth of alpha-Si3N4 onto the ''seed'' nuclei. Growth appears to follow a gas-phase route and is characterized by an irregular porous layer which grows onto the ''seed.'' The porous structure is the result of reaction around carbon particles which are consumed during the process, The presence of admired ''seed'' Si3N4 in the precursor formulation increases the reaction rate since the nucleation step is eliminated. An activation energy of E = 457 +/- 55 kJ/mol for the overall reaction closely approximates that previously reported for the formation of SiO. This result, along with the finding that residual crystalline SiO2 is present at all stages of the reaction, indicates that the overall reaction rate is controlled by the reduction of SiO2. Since reaction at the carbon and SiO2 contact points is fast, the rate-limiting step is most likely the gas-phase carbon reduction of SiO2 with CO.