Time-temperature-transformation diagrams for borosilicate glasses and preparation of chemically durable porous glasses

To clarify the effects of glass composition and heat-treatment conditions on phase separation and crystallization, time-temperature-transformation (TTT) diagrams for the system (65 - x)SiO₂·25B₂O₃·(10 - y)Na₂O·yCaO·xZrO₂, which is used for fabricating alkali-durable porous glasses, were determined. The microstructures and properties, such as resistance to alkaline attack as a function of ZrO₂ content, of the porous glasses fabricated on the basis of the developed TTT diagrams were examined. CaO and ZrO₂ additions were shown to affect the locations of the noses in the TTT curves for phase separation and crystallization. The substituting of CaO for Na₂O was effective for retaining ZrO₂ in the skeleton of the porous glasses. The alkali resistance of ZrO₂-containing porous glasses was 8-10 times superior to that of the porous glass without ZrO₂. The results demonstrate that TTT diagrams can be used as guides for heat-treatment scheduling in processing or predicting of glass-forming ability and the onset of phase separation and crystallization in glass-forming systems.