Effect of gas composition on Si2N20 formation in the Si -C-N-O system

Formation of silicon oxynitride was observed during silicon nitride whisker synthesis from silica, carbon and nitrogen. The silicon oxynitride formation was limited to a bottom area of the charged powders and found both in whisker and powder. The possible reason for this localized Si2N20 formation is analysed, based on the effect of gas phase composition on the phase stability among 13-SiaN4, 13-SIC and Si2N20. The phase stability is closely related to the Psio/PN= ratio in the gas phase. To suppress Si2N20 formation, the Pslo/PN2 ratio must be lower than that of the phase boundary in the SiaN4/Si2N20 equilibrium, whereas it must be higher than that of the phase boundary in the SiC/Si2N20 equilibrium. The formation of silicon oxynitride during silicon nitride whisker formation was caused most likely by fluctuation in the Psio/PN2 ratio in the gas phase surrounding the lower area.