Solubility of Si3N4 in liquid SiO2

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.