Influence of modifiers and glass-forming ions on the crystallization of glasses of the NaCaPO4–SiO2 system

Abstract

Silicate–phosphate glasses of the XYPO4–SiO2 and XYPO4–SiO2–AlPO4 (where X = Na+ and/or K+ and Y = Ca2+ and/or Mg2+) systems have been the subject of the presented investigations. Bioactive glasses from these systems are the base for obtaining glass-crystalline biomaterials through a direct crystallization. However, growth of crystalline phases very adversely affects the bioactivity of the glasses. Uncontrolled growth of crystalline phases can be reduced by means of a glass phase separation phenomenon in the silicate–phosphate glasses because boundaries of inclusion-matrix phase may be a barrier limiting the growth of crystalline phases. Microscopic and EDX investigations which have been carried out have shown that glass phase separation occurs in glasses belonging to XYPO4–SiO2 and XYPO4–SiO2–AlPO4 systems. Introduction of aluminum ions into the glass structure leads to a rapid homogenization of its texture. Based on DSC examinations it has been found out that crystallization of the glasses of XYPO4–SiO2 systems is a multistep process. The presence of several (the number depends on the type of modifiers and glass-forming ions) clearly separated exothermic peaks in DSC curves of investigated glasses makes it possible to crystallize only the inclusions with the matrix remaining amorphous or vice versa. It has been shown that, crystallization of glasses of XYPO4–SiO2–AlPO4 system is single-stage process, which is the consequence of the homogenizing effect of aluminum ions on their texture.