Superheated steam-induced surface-accelerated amorphous-to-crystalline transition in an aluminosilicate inorganic polymer

Abstract

This study demonstrates that the physical properties of amorphous materials can be effectively controlled by superheated steam and that inorganic glass, as an inorganic polymer, serves as a valuable model system for investigating thermal behavior that is not easily accessible with organic polymers. To elucidate the full sequence of behavior, from glass transition to crystallization, under superheated steam, powder compacts and plate-shaped CaO–Al₂O₃–SiO₂ (CAS) glass specimens, with a primary composition of 28.6CaO–12.6Al₂O₃–58.8SiO₂ in mol% and well-characterized crystallization behavior, were calcined at 800–1050 °C under superheated steam. Thermal analysis of the powder specimens revealed that the onset temperature of the glass transition decreased by 60 °C, and the first and second crystallization steps were lowered by 25 and 15 °C, respectively. X-ray diffraction analysis indicated that reflections from surface crystalline phases such as anorthite and wollastonite appeared at lower temperatures. Additionally, photographs and scanning electron microscopy revealed an increase in the thickness of the surface crystalline layer, indicating enhanced surface crystallization under superheated steam. These results demonstrated that superheated steam promotes surface-accelerated amorphous-to-crystalline transitions of amorphous materials, as demonstrated using the CAS glass as an inorganic polymer.