Substituting nitrogen for oxygen in glass structures to form oxynitride glasses has been shown to improve a variety of mechanical and other properties of these glasses. For example, oxynitride glasses exhibit better chemical durability, higher glass transition temperature and lower coefficients of thermal expansion then oxide glasses. Furthermore, oxynitride glasses retain their electrical insulating qualities at temperatures at which ordinary glasses become conductive. Because these glasses are stable at high temperatures and are leach resistant, they may be useful for encapsulating nuclear waste for long-term storage. These glasses are also useful in other high temperature applications, for example the joining of structural ceramics such as silicon nitride.
Oxynitride glasses are more difficult to prepare than conventional glasses for several reasons. For example, temperatures up to 1750.degree. C. to 1800.degree. C. are required for melting and homogenization of the melt. A low oxygen partial pressure is required during melting and homogenization to avoid oxidizing the glass-forming components. The number of nitride compounds which can serve as sources of nitrogen for the glasses is severely limited. Most oxynitride glasses are prepared with Si.sub.3 N.sub.4 as the nitrogen source, although AlN, Mg.sub.3 N.sub.2, YN Si.sub.2 ON.sub.2 or Li.sub.3 N can also be used in some glasses.
As hereinbefore stated, most oxynitride glasses are quite refractory and require high temperatures, ranging from 1500.degree. to 1800.degree.C., for melting and homogenization. The upper temperature limit is determined by the thermal decomposition of Si.sub.3 N.sub.4 or other nitrogen source used, while the lower temperature limit is set by the melting temperature of the particular glass-forming batch composition. In one method of preparing oxynitride glasses, Na.sub.2 CO.sub.3 CaCO.sub.3 and SiO.sub.2 are melted together to form a base glass. This glass is ground and mixed with varying amount of Si.sub.3 N.sub.4 ranging up to 4.55 mol %. The mixture is placed in a molybdenum crucible and melted in a furnace, under a flowing atmosphere of high-purity N.sub.2 at 1600.degree. C. for 1.5 to 2.5 hours. The amount of retained nitrogen in glasses prepared by this method is up to about 3.23 mol %. Glasses prepared by this or similar methods are predominately a grey mass with overtones of blue and black. The material is usually translucent in one millimeter or thicker sections. In the Journal of Non-Crystalline Solids 71 (1985) p.287-294 another method is described for preparing oxynitride glasses in which glass-forming batch compositions, containing Si.sub.3 N.sub.4 as the nitrogen source, are first dry mixed and then cold isostatically pressed at 60,000 psi to reduce powder volume. The pre-compacted batch of material is placed into a boron nitride crucible and heated in a hot isostatic press (HIP) under high-purity nitrogen gas pressurized to 30,000 psi to a temperature of about 1900.degree. C. The HIP is held at temperature and pressure for about 60 minutes. Glasses prepared by this method can contain up to about 15 at. % nitrogen.
While the HIP process for producing oxynitride glasses is effective and produces glasses of consistent quality, the primary disadvantage of HIP glass production is the expense due to the high pressures and high temperatures required. Any large volume commercial use of oxynitride glass will require production methods which are less expensive than methods currently available.