Effect of a TiO2 Nucleating Agent on the Nucleation and Crystallization Behavior of MgO-B2O3-SiO2 Glass

The nucleation and crystallization of MgO-B2O3-SiO2 (MBS) glasses with various TiO2 additions (015 wt%) were studied by means of a nonisothermal, thermal analysis technique, X-ray diffraction, and electron microscopy. The effect of the TiO2 on the nucleation rate was determined from the dependence of the inverse temperature at the differential scanning calorimetry (DSC) peak (1/Tp; Tp?crystallization peak temperature) on the nucleation temperature (Tn). A significant increase in the nucleation rate was observed for the MBS glasses with a TiO2 content greater than 3 wt%. The TiO2 was also found to influence the crystallite growth mechanism. In the pure MBS glass the crystallization of the Mg2B2O5 was 3-dimensional (3D) bulk with a diffusion-controlled crystal growth rate. The value of both the Avrami parameter (n) and the morphology index (m) was 1.5 and the crystallization activation energy (E) was in the range 410-440 kJ/mol. The change of the nucleation rate (a) from zero (a = 0) to a constant nucleation rate (a = 1) was observed at 3 wt% TiO2. The change from a diffusion- to an interface-controlled growth rate occurred at 5 wt% TiO2, where n and m became 4 and 3, respectively. According to the analysis of the DSC data using the modified Kissinger method, a small TiO2 concentrations (1 wt%) lowered E, whereas E increased with a further increase in the TiO2 concentration, exceeding at 5 wt% TiO2 the E of pure MBS glass.