Thermodynamic Analysis on the Post-crystallization for Calcium Titanium Phosphate Glass Ceramics

Porous glass ceramics with CaTi4(PO4)6 as the main crystalline phase could be produced by heat-treating the 5Na2O-20TiO2-32P2O5-43CaO glass system at 690℃ and 735℃ separatively and then by acid-leaching the crystallization product Ca3(PO4)2. In this paper, a method of enlarging the size of pores was introduced. A high temperature heat-treatment (880℃), that called post-crystallization, could be inserted into the processes between crystallization and acid-leaching. In the post-crystallization, there was a solid reaction where the CaTi4(PO4)6 and 5Ca3(P04)2 reacted and then products TiO2 and Ca2P2O7 produced. The later phase could be easily leached from the glass-ceramic. The pores became larger, since more Ca and P were leached. In the present study, the change in free enthalpy of the solid reaction was estimated as -333Kcal/mol by applying the approximate first thermodynamic equation. The phase analysis, XRD before and after post-crystallization, confirmed that this solid reaction could happen spontaneously. The dynamic parameters for crystallization activation energy and reaction index were calculated as 584.7kJ/mol and 3.2 respectively by applying Jormson-Arirami-Mehl equation while implied that the sample was crystallized in bulk. This calculation was also confirmed by the observation under SEM, that the pore size enlarged in the whole area of the sample after post-crystallization.