Crystallization behavior and microstructure of ZnO-Al2O3-SiO2-B2O3-based glass-ceramics using MgO and CaO modifiers

The role of glass-structure modifiers was examined by analyzing the crystallization behavior and microstructure of a ZnO-Al2O3-SiO2-B2O3 glass system with MgO and CaO substitutions. Differential thermal analysis indicated that the crystallization temperature (Tp) increased with MgO substitution but decreased with CaO substitution. Xray diffraction analysis demonstrated that increasing the amounts of MgO and CaO led to the formation of a gahnite crystalline phase after treatment at 1200 C-degrees. Nano-sized crystals were observed when the specimen was heat-treated at 1000 C-degrees, and dendrite crystals were observed on the surface when treated at 1200( degrees)C. Nonisothermal analysis revealed that the substitution with MgO and CaO resulted in surface crystallization behaviors with high activation energies (271.72 and 301.75 k & sdot;J & sdot;mol(-1)) and low Avrami constants (2.05 and 1.67). Micro-Vickers hardness showed maximum values of 7.65 and 6.86 GPa when MgO and CaO were substituted, respectively.