Thermal Stability of Gels and Glass-Forming Regions in the B2O3-Na2O-Al2O3 System

The thermal stability of gels in the B2O3-Na2O-Al2O3 system was evaluated by two methods; (1) observation of the change of amorphous region after heating up to 200-800°C at a heating rate of 1°C/min and keeping for 2h, and (2) determination of the crystallization temperature Tc from the DTA curve of gels with heating rate of 5°C/min and calculation of the ratio of Tc to the liquidus temperature Tl, Tc/Tl. After heating gels up to 800°C, the original gel region, which extends over a wide area including Al2O3, was reduced to only a small amorphous region near the Al2O3 rich area. The Tc/Tl ratio showed the highest value of more than 0.8 in the composition range B2O3=55-90mol% and Al2O3=0-20mol%, while it was less than 0.6 value in the Al2O3 rich composition area. The compositional dependence of Tc/Tl ratio was compared with the variation of the glass-forming region obtained from melt at various cooling rates Q in the range of logQ=-2 to 5. It was recognized that the gel region having a high Tc/Tl value corresponds to a stable glass region obtainable under slow cooling. It seems that some close relationship exists between the Tc/Tl of a gel and the critical cooling rate Q* for glass formation of a melt having the same composition.