Phase separation, crystallization and leaching of microporous glass ceramics in the CaO–TiO2–P2O5 system

Abstract

Microporous glass ceramics belonging to the CaO–TiO 2 –P 2 O 5 system were prepared with the assumption of a 2:1 mole ratio for β-Ca 3 (PO 4 ) 2 :CaTi 4 (PO 4 ) 6 , the anticipated crystalline phases in the end product. The glasses formulated according to the above composition were melted and cast onto a steel mold and were crystallized to glass ceramics containing the above phases. Dilatometric/differential thermal analysis (DTA) techniques were utilized to determine the appropriate phase separation–nucleation and crystallization temperatures. The crystalline products and resulting microstructures in various stages of process were determined and observed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). By leaching the resulting glass ceramics in HCl, β-Ca 3 (PO 4 ) 2 was dissolved out leaving a porous skeleton of CaTi 4 (PO 4 ) 6 . It was found that the volume porosity, specific surface area and mean pore diameter of microporous glass ceramics can be managed through the proper selection of heat treatment conditions. In the optimized conditions for fabricating glass ceramics of minimum mean pore size the values of 41 ± 4%, 26 ± 3 m 2 /g and 14.3 ± 2 nm were obtained for porosity, surface area and pore diameter respectively.