The Phase Separation in SiO_2-P_2O_5-Na_2O Glasses

The phase separation that occurred in SiO2-P2O5-Na2O glasses has been investigated by the measurements of density, weight loss caused by the acid treatment and by the observation of electron micrographs for their glass specimens.The region of composition in this system, SiO2-P2O5-Na2O, where the formation of glass was easily made is shown in Fig. 1. The relative change in density of the specimens No. 6, 7, 9, and 11 kept at fixed temperatures ranging from 450°to 580°C was determined by the sink float method. The results obtained are shown in Fig. 2. From the curves in the temperatures ranging 490°to 560°C, it is found that at first the density increases, but as the heat-treatment is continued it begins to decrease and reaches an equilibrium value after about 100 hours. This behaviour shows that there are two processes at least. Then we consider the following behaviours, namely, (i) the increase in density is based on the formation of compact structure in quenched glass and (ii) the decrease in one is based on the phase separation in the glass.The weight losses of the specimen-No. 11 from the acid treatments at 50°C with 3N HNO3, 3N H2SO4, and 3N HCl, are respectively shown in Fig. 4. The curves in Fig. 4 show that the weight loss of the specimen from the acid treatment increases as the duration of its heat-treatment at 500°C is prolonged.The electron-microscopic pictures of specimen-No. 9 heat-treated at 500°C for 20, 40 hours and untreated, are respectively shown in Fig. 5. The phase separation in the glass can be observed as the unevenness on the surface of glass etched with 6N HCl, as shown in Fig. 5.