Density and boron coordination in heat-treated alkali-borosilicate glass

Abstract

The room temperature density of an alkali-borosilicate glass was determined as a function of heat treatment at 415 to 700 °C. It was found to decrease with temperature as expected on the basis of structural relaxation, but at temperatures above about 550 °C it was observed to increase. The increase is explained by the coarsening of the microphases in such glass. NMR studies revealed that the fraction of 4-coordinated borons in the glass decreases on rapid cooling, but that the N4 values are insensitive to subsequent annealing of the glass. Addition of alumina to such glass dramatically reduces the fraction of 4-coordinated borons, presumably due to competition for the Na+ ions by Al3+ ions to form AIO4 tetrahedra.