Nitridation of phosphate glass melts with ammonia

Abstract

A sodium metaphosphate glass melt was nitrided with ammonia in a newly designed tilted tube furnace with a rotating crucible. An optimum nitridation temperature of 780 °C led to 7.2 wt% of nitrogen in a clear glass in only 10 h. Crystallization of this glass was greatly reduced by incorporation of nitrogen. From the present results a reaction mechanism and structural model is developed for this incorporation. This model provides a better understanding of the nitridation process and may lead to successful nitridation of compositions with a P/Na ratio > 1. Based on thermodynamic considerations, the use of a graphite crucible is thought to play a role in the development of brown glasses at temperatures greater than 780 °C via reduction to phosphorus.