Role of Viscosity Playing in the Formation of B₂O₃-Na₂O Binary Glasses by Quenching Method

Abstract

The glass-forming region in the B₂O₃-Na₂O system was tried to interpret on the basis of an assumption concerning the glass-forming ability of a substance that higher viscosity melts will make a glass when quenched at an appropriately rapid rate, while lower viscosity melts will not. Ten grams of B₂O₃-Na₂O melts were water-quenched at a rate of 7°C/sec, and two glass-forming regions were determined. The upper glass region was 100∼61 mole per cent B₂O₃ and the lower glass region was 32∼28 mole per cent B₂O₃. Viscosity was measured by a counter-balanced viscometer at elevated temperatures above the liquidus temperature, and the liquidus viscosity log η_l was obtained for the compositions of 100∼25 mole per cent B₂O₃. The liquidus viscosity curve plotted as a function of B₂O₃ content for the B₂O₃-Na₂O system exhibited two maxima and two minima in viscosity. It was found that the glass regions, 100∼61 and 32∼28 mole per cent B₂O₃, corresponded quite closely with the regions where the liquidus viscosity was higher than log η_l (poise)=0∼0.1. The single-bond strength B_M-O and the Rawson's parameter B_M-O/T_l for glass formation were calculated and were compared with the glass-forming regions of this system. These parameters were found unsatisfactory to explain the glass-forming ability of the lower glass region. But the liquidus viscosity described rather quantitatively the existence of the separate glass regions in the B₂O₃-Na₂O system.