Opalescence and clearing techniques were used to determine the metastable immiscibility surface for sodium borosilicate solutions. These results indicate that a three-liquid region, which may or may not be metastable to two-liquid regions, underlies the immiscibility surface.
Crystallization rates were measured in vacuum, dry nitrogen, and water-saturated nitrogen atmospheres from 1300' to 1540'C. In all cases the observed rates were linear. Three reactions appeared to contribute to crystallization : the intrinsic crystallization, the impurity effect of H20 vapor, and furnace contamination. Enhancement of crystalkation by both water vapor and furnace contamination is attributed to the breaking of silicon-oxygen bonds of the glass structure. Competitive adsorption mechanisms were proposed to characterize the adsorption of water and impurity species. The activation energy for apparent intrinsic crystallization was 134 kcd/ mole; the activation energy for crystallization in H20 vapor was 77 kcd/mole.
It has recently been observed that cristobalite can heterogeneously nucleate and grow internally in vitreous silica. When the glass-crystal composite is cooled to room temperature, p-cristobalite is metastably retained as a result of the large tensile stresses developed during cooling.Although these crystals are not obvious, they can be readily observed with a polarizing microscope or polariscope. As long as the cristobalite does not transform when cooled to room temperature, repeated increments of crystal growth can be measured on the same growing crystal. Crystal growth was measured on electrically melted quartz glass at 1350" to 162OOC. The observed growth rates were linear with time and were the lowest that have been measured in vitreous silica. The kinetic data agreed with the general form of the crystal growth equation p = (AIAT)/q. Since the growth is internal and free from surface contamination, the measured rates are considered to be very near the intrinsic rates for the material.
Using the phase diagram and a simple solution model, a subliquidus miscibility gap was estimated for the B2O3‐SiO2 system. The predicted coexistence boundary, showing a consolute temperature of 520°C, was flat and symmetrical and extended across the complete binary. Gradient furnace heat treatments of selected compositions in this system resulted in phase separation which corresponded closely to the coexistence boundary initially predicted. Calculations and preliminary experimental results indicate that temperatures and compositions exist wherein metastable three‐liquid immiscibility occurs in R20–B203‐SiO2systems.
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