Structural Rearrangements in Silicate Glasses During Annealing

JSGT 1956 V40 T338-T352

The equilibrium temperature coefficients of refractivity, or optical sensitivity t prolonged annealing, for 23 silicate glasses were plotted as functions of chemical compositions expressed as fractions of non-silica on a basis varying from completely molecular for highest silica content to completely ionic for lowest silica. Evidence is found that sensitivities to prolonged annealing are roughly proportional to the fraction of the non-silica particles and more or less independent of their size and chemical nature except in so far as such properties, like bond character, may influence the number of ions (or discrete particles such as ion groups) that are loosely bonded and not an integral part of the silicon-oxygen framework. The data are in accord with a view that during the annealing of silicate glasses the tetrahedral framework is steadily undergoing a sluggish and limited degree of collapse or folding by orientation of the oxygen-to-silicon bonds that form angles of about 180°  in fused silica but only 144°  in quartz. The rate of collapse is assumed to be governed by thermal agitation of the non-framework particles that are located in the interstices of the tenacious tetrahedral framework. It is concluded that various degrees of progressive structural rearrangement induced by heat treatment are, potentially, equally homogeneous. Consequently, lower annealing temperatures and longer annealing periods do not necessarily produce more homogeneous optical glass.

Leroy W. Tilton