Solubility of Si3N4 in liquid SiO2

The nitrogen solubility in the SiO(2)-rich liquid in the metastable binary SiO(2)-Si(3)N(4) 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 Si(3)N(4) and to adjust the computed phase diagram. The solubility of Si(3)N(4) in fused SiO(2) 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 Si(3)N(4) without prohibitive amounts of network cross-linking. Finally, indications emerged regarding the diffuse nature of the Si(3)N(4)-SiO(2) interface that leads to amorphous regions of higher N content.