Effect Of Mg-Ca Substitution On The Spontaneous Crystallization Behaviour Of Phosphosilicate Glasses

The spontaneous crystallization of SiO2-Al2O3-P2O5-Na2O-MgO/CaO glass melts during cooling has been investigated using x-ray diffraction, differential scanning calorimetry and scanning electron microscopy. The focus of the present study is on the influence of the substitution of MgO for CaO on the spontaneous crystallization behaviour and the morphologies of the glasses. Such compositional substitution leads to the formation of different crystalline phases as well as different degrees of crystallization. An increase in R=MgO/(MgO+CaO) up to 0.5 enhances the degree of crystallization, and alters the morphologies of the orthorhombic NaCaPO4 crystals formed. This leads to an increase of opalescencne in these glasses. As R further increases to 0.75, the crystalline phase becomes hexagonal NaCaPO4 rather than orthorhombic NaCaPO4. When R=1.0, Na2MgSiO4 is obtained. Furthermore, the samples become transparent at R=0.75 and 1.0.