Interaction of Si<sub>3</sub>N<sub>4</sub> with Carbonyl Iron under Nitrogen or Argon Atmosphere

Abstract

The products and kinetics of the reaction between Si3N4 and iron were studied at 1173 to 1573K under both nitrogen and argon. The following results were obtained:(1) An Fe-Si solid solution was produced at low temperatures, which with an increase in temperature changed to iron silicides in the following order of occurrence: Fe3Si, Fe5Si3 and FeSi.(2) As the reaction proceeded, the lattice constant of the bcc crystal structure successively decreased throughout α-Fe, the Fe-Si solid solution and Fe3Si.(3) The incubation period was long at low temperatures, but shortened markedly with increasing temperature.(4) The initial stage followed a linear rate of reaction. Below 1373K, the activation energies were 626kJ/mol under a nitrogen atmosphere and 477kJ/mol under argon atmosphere. The reaction rate is thought to be controlled by a chemical process. At 1373K and above, an activation energy of 106kJ/mol was obtained under both atmospheres. The rate is thought to be determined by the combination of gas-phase diffusion through interparticle pores and chemical reaction.(5) On producing iron silicides, the reaction kinetics could be described by a parabolic rate law. The activation energy was estimated to be 217kJ/mol under an argon atmosphere. The reaction rate is thought to be determined by solid-state diffusion through the reaction layer. However, the parabolic rate law was unapplicable to the rate data produced under a nitrogen atmosphere, because of the approach to the reaction equilibrium.