Multimetastability effect on the intermediate stage of phase separation in BaO-${\mathrm{SiO}}_2$ glass

Controlling the phase separation phenomenon can enhance the properties of glass materials, such as transparency and strength. However, the initial and intermediate stages of phase separation of amorphous glass are yet to be understood completely. In this study, we performed an in situ observation on glass through scanning transmission electron microscopy, which possesses a high spatial resolution and chemical sensitivity. We visualized the phase-separated structure in the initial and intermediate stages of phase separation and observed a local and rapid change in the phase-separated structures and the formation of regions with advanced and delayed degrees of phase separation. The results were compared with the phase-field simulation and it was concluded that the characteristic change of the phase-separated structures is attributable to the multimetastability of the amorphous phase.

Figure 1

(a) Thickness map and (b) composition map of the sample before heating. (c)Thickness map and (d) composition map of the sample after heating. The sample thickness increases from left to right in the image, while the composition becomes nonuniform after heating.

Figure 2

Temporal change in the compositional distribution. These images show the distribution after heating for (a) 0 s, (b) 2 130 s, (c) 6 040 s, (d) 8 740 s, (e) 11 440 s, and (f) 23 150 s. The blue and red regions correspond to the ${\mathrm{SiO}}_2$ phases (low BaO composition) and BaO phases (high BaO composition), respectively. The volume fraction of the ${\mathrm{SiO}}_2$ phase is low (minor) while that of the BaO phase is high (major). Temporal changes in the (g) average BaO composition of the BaO phases, (h) average BaO composition of the ${\mathrm{SiO}}_2$ phases, (i) the average area of the ${\mathrm{SiO}}_2$ phases, and (j) the number of ${\mathrm{SiO}}_2$ phases. The white, light gray, and dark gray regions in these graphs indicate the nucleation stage (initial stage), growth stage (intermediate stage), and coarsening stage (final stage), respectively.

Figure 3

Compositional difference between the two consecutive images. (a) 8730–7830 s, (b) 9370–8730 s, (c) 10 550–9370 s, (d) 11 440–10 550, (e) 12 330–11 440 s, and (f) 13 250–12 330 s. The red and blue colors indicate the increase and decrease in the BaO composition, respectively. The depth of color corresponds to the amount of the increase/decrease of BaO composition. Arrows in the images from (b) to (f) indicate the regions where rapid change in the composition is observed.

Figure 4

Temporal change in (a) the average composition of the BaO phase measured from the region in the black rectangle [Fig. 2] and (b) the area of the ${\mathrm{SiO}}_2$ phase indicated by the white arrow [Fig. 2]. Green arrows indicate the rapid increase in the composition and area during the region movement (10 550–12 330 s) where the phase separation rapidly proceeds. Both the composition and area slightly change after the movement of the region.

Figure 5

(a) The Gibbs energy curve under the regular solution approximation. The vertical dash-dot line indicates the average composition. (b) The initial compositional distribution with a boundary between different degrees of phase separation progress (right top: delayed, left bottom: progressed). (b)–(g) Temporal changes in the compositional distribution. (h) Temporal change of the average composition of the matrix phase measured from the black rectangle in (b). Temporal change of the areas of phases (i) P1, (j) P2, and (k) P3.

Figure 6

(a) The Gibbs energy curve under the polynomial expression. There are five basins in the Gibbs free energy curve. Basins 2, 3, and 4 are the metastable states, and basins 1 and 5 are the most stable states. The vertical dash-dot line indicates the average composition. (b) The initial compositional distribution with a boundary between the different degrees of progress of phase separation (right top: delayed, left bottom: progressed). (b)–(h) Temporal change in the compositional distribution. (i) Temporal change in the average composition of the matrix phase measured from the black rectangle region in (c). Temporal changes in the areas of phases (j) P4, (k) P5, and (l) P6.