Abstract

The nitrogen solubility in the SiO2-rich liquid in the metastable binary SiO2-Si3N4 system has been determined by analytical TEM to be 1%-4% of N/(O + N) at 1973-2223 K. Analysis of the near edge structure of the electron energy loss peak indicates that nitrogen is Incorporated into the silicate network rather than being present as molecular N-2. A regular solution model with a positive enthalpy of mixing for the liquid was used to match the data for the metastable solubility of N in the presence of crystalline Si3N4 and to adjust the computed phase diagram. The solubility of Si3N4 in fused SiO2 is far less than reported in liquid silicates also containing Al, Mg, and/or Y. Apparently, these cations act as modifiers that break anion bridges in the silicate network and, thereby, allow further incorporation of Si3N4 without prohibitive amounts of network cross-linking. Finally, indications emerged regarding the diffuse nature of the Si3N4-SiO2 interface that leads to amorphous regions of higher N content.