钙钛磷酸盐微晶玻璃的过析晶作用及其热力学分析

5Na₂O-20TiO₂-32P₂O₅-43CaO系统玻璃在690℃核化和730℃晶体生长的热处理后,可以得到CaTi_4(PO₄)₆和Ca₃(PO₄)₂两种晶相,经酸滤去Ca₃(PO₄)₂相后,可获得主晶相为CaTi4(PO4)6的多孔微晶玻璃.本文介绍了一种扩大孔径的方法,在该组成玻璃微晶化后及酸滤析之前,在更高的温度880℃下作过析晶处理,此两析晶相能自发地进行固相反应,生成TiO₂和Ca₂P₂O₇,后者容易被酸滤析,使得更多的钙和磷自微晶玻璃体中被酸溶出,从而增大了多孔微晶玻璃的孔径.本文还应用热力学第一近似方程,对过析晶过程中的固相反应的自由焓变化作了估算,其数值为-1394kJ/mol,说明在过析晶温度下,固相反应能自发地进行,试样在过析晶前后的XRD物相分析的结果验证了此固相反应机理.本文还利用Johnson-Avirami-Mehl公式对析晶的动力学参数作了近似估算.其析晶活化能为584.7kJ/mol,晶化指数为3.2,说明试样的析晶是以整体析晶的方式进行.此计算的结果也符合试样形貌的SEM观察,即过析晶过程使得整体微晶玻璃的孔径增大.

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(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.