Thermal Decomposition in the System Si‐Y‐AI‐O‐N

The limited transparency of high N (13.3 at.%) glasses in the system Si‐Y‐AI‐O‐N was found to be due to Si precipitates that resulted from thermal decomposition during melting. Thermodynamic analysis supported experimental findings that decomposition in N 2 at 200 kPa resulted from oxide‐nitride reactions and not from simple dissociation of Si 3 N 4 . The Si content of the glass was reduced much more by substitution of AIN for Si 3 N 4 in the glass batch and melting at low N 2 pressure (200 kPa) than it was by merely melting in N 2 at high pressure (1.6 MPa). The thermodynamic analysis suggested that decomposition could be minimized effectively by melting under a modest partial pressure of SiO(g), but this could not be verified experimentally.