Phase separation and crystallization in the system SiO2–Al2O3–P2O5–B2O3–Na2O glasses

The phase separation and crystallization behavior in the system (80 − X)SiO2 · X(Al2O3 + P2O5) · 5B2O3 · 15Na2O (mol%) glasses was investigated. Glasses with X = 20 and 30 phase separated into two phases, one of which is rich in Al2O3–P2O5–SiO2 and forms a continuous phase. Glasses containing a larger amount of Al2O3–P2O5 (X = 40 and 50) readily crystallize and precipitates tridymite type AlPO4 crystals. It is estimated that the phase separation occurs forming continuous Al2O3–P2O5–SiO2 phase at first, and then tridymite type AlPO4 crystals precipitate and grow in this phase. Highly transparent glass–ceramics comparable to glass can be successfully obtained by controlling heat treatment precisely. The crystal size and percent crystallinity of these transparent glass–ceramics are 20–30 nm and about 50%, respectively.