Thermal Stability of Gels and Glass-Forming Regions in the B sub 2 O sub 3 --Na sub 2 O--Al sub 2 O sub 3 System

The thermal stability of gels in the B sub 2 O sub 3 --Na sub 2 O--Al sub 2 O sub 3 system was evaluated by two methods: observation of the change of amorphous region after heating up to 200-800 deg C at a heating rate of 2 deg C/min and holding for 2 h; and determination of the crystallization temperature T sub c from the DTA curve of gels with heating rate of 5 deg C/min and calculation of the ratio of T sub c to the liquidus temperature T sub l , T sub c /T sub l . After heating gels up to 800 deg C, the original gel region, which extends over a wide area including Al sub 2 O sub 3 , was reduced to only a small amorphous region near the Al sub 2 O sub 3 rich area. The T sub c /T sub l ratio showed the highest value of > 0.8 in the composition range B sub 2 O sub 3 = 55-90 mol% and Al sub 2 O sub 3 = 0-20 mol%, while it was < 0.6 in the Al sub 2 O sub 3 rich composition area. The compositional dependence of T sub c /T sub l ratio was compared with the variation of the glass-forming region obtained from melt at various cooling rates Q in the range of log Q = --2 to 5. It was recognized that the gel region having a high T sub c /T sub l value corresponds to a stable glass region obtainable under slow cooling. It seems that some close relationship exists between the T sub c /T sub l of a gel and the critical cooling rate Q* for glass formation of a melt having the same composition.