Kinetics of silicon nitride formation by gas-phase routes in the Si–C–O–N system

Abstract

With the purpose of elucidating gas-phase reaction routes to Si₃N₄ within a C–SiO₂ powder mixture, pieces of carbon and silica have been heat-treated at different temperatures and pressures in the range 1550–1710 °C, 0.5–6.0 MPa. The experiments were performed in gas autoclaves and with a controlled gas flow of N₂ or N₂–CO mixture through the sample container.

Whether or not Si₃N₄ would form from the gas phase at specific T, p and CO content can be approximately predicted from thermodynamic calculations, based on a simple model including N₂(g), SiO(g) and oxygen. The same model was found to be useful in predicting Si₃N₄ formation and growth in SiC powder heated in an N₂–CO gas mixture. The nitridation rate in a fine-grained SiC powder, determined as a function of T and p, was found to increase with increasing p(N₂) and p(SiO) but to decrease with increased oxygen activity. A tentative kinetic expression including these concentration variables has been formulated.