EFFECT OF HEAT‐TREATMENT ON THE DENSITY AND CONSTITUTION OF HIGH‐SILICA GLASSES OF THE BOROSILICATE TYPE*

Abstract

AbstractSome evidence leading to the supposition that certain borosilicate glasses containing a high percentage of silica are made up of two‐component glasses is presented. One of these components, which consists mainly of silica and has a low expansivity, forms a rigid network throughout the body of the glass while the temperature is still high enough to maintain the other component in a liquid state. At lower temperatures, the latter component also forms a rigid network that is intricately entwined with that of the high‐temperature component. The composition of the network that forms at relatively low temperatures is presumably not unlike that of a borosilicate glass containing a moderate percentage of silica. Consequently, the expansivity of this low‐temperature component is definitely higher than that of the other. Because of the difference between the expansivities, the low‐temperature component, as soon as it behaves as a rigid network on cooling, compresses the high‐temperature network and is correspondingly distended. By cooling very rapidly, these elastic strains are developed at temperatures within and even above the upper part of the annealing range of the low‐temperature component. When such strains are developed at high temperatures, they are relaxed by heat‐treatments at lower annealing temperatures, and this relaxation causes an expansion of the undercooled glass while at a constant temperature. However, in the early stages of the treatment, the expansion caused by the relaxation of the strains may be overshadowed by a contraction that results from the shrinkage of the low‐temperature component as it approaches equilibrium from its undercooled condition. These concepts concerning the constitution of borosilicate glasses that contain a high percentage of silica are used to explain the very unusual density changes that are observed when these glasses are subjected to heat‐treatments at different annealing temperatures.