Carbothermal reduction and nitridation reaction of SiO_x and preoxidized SiO_x: Formation of α-Si₃N₄ fibers

The chemical composition of amorphous SiO_x has been analyzed by oxidation studies and is found to be SiO_1.7. SiO_1.7 appears to be a monophasic amorphous material on the basis of ²⁹Si nuclear magnetic resonance, high resolution electron microscopy, and comparative behavior of a physical mixture of Si and SiO₂. Carbothermal reduction and nitridation reactions have been carried out on amorphous SiO_1.7 and on amorphous SiO₂ obtained from oxidation of SiO_1.7. At 1623 K reactions of SiO_1.7 lead exclusively to the formation of Si₂N₂O, while those of SiO₂ lead exclusively to the formation of Si₃N₄. Formation of copious fibers of α-Si₃N₄ was observed in the latter reaction. It is suggested that the partial pressure of SiO in equilibrium with reduced SiO_1.7 and SiO₂ during the reaction is the crucial factor that determines the chemistry of the products. The differences in the structures of SiO₂ and SiO_1.7 have been considered to be the origin of the differences in the SiO partial pressures of the reduction products formed prior to nitridation. The effect of the ratios, C:SiO_1.7 and C:SiO₂, in the reaction mixture as well as the effect of the temperature on the course of the reactions have also been investigated.