Solubility of Si3N4 in Liquid SiO2

Abstract

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.