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 composi tion analysis to characterize the reaction at various degrees of conve rsion, The reaction follows a nucleation-growth mechanism, Without ''s eed'' alpha-Si3N4 in the precursor, the reaction rate is controlled by the formation of nuclei which are associated with a Si-O-C intermedia te phase. In the presence of ''seed,'' the limiting step is growth of alpha-Si3N4 onto the ''seed'' nuclei. Growth appears to follow a gas-p hase route and is characterized by an irregular porous layer which gro ws onto the ''seed.'' The porous structure is the result of reaction a round carbon particles which are consumed during the process, The pres ence of admired ''seed'' Si3N4 in the precursor formulation increases the reaction rate since the nucleation step is eliminated. An activati on energy of E = 457 +/- 55 kJ/mol for the overall reaction closely ap proximates that previously reported for the formation of SiO. This res ult, along with the finding that residual crystalline SiO2 is present at all stages of the reaction, indicates that the overall reaction rat e is controlled by the reduction of SiO2. Since reaction at the carbon and SiO2 contact points is fast, the rate-limiting step is most likel y the gas-phase carbon reduction of SiO2 with CO.