Abstract: 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(PO₄)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 Johnson-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.

Key words: porous material, phospnate glass ceramics, thermodynamic