Crystallization kinetics of gehlenite glass microspheres

The glass of gehlenite composition was prepared by fame synthesis in the form of microspheres. The powder precursor was synthesised by standard solid-state reaction method using SiO2, Al2O3 and CaCO3. The prepared glasses were characterized from the point of view of surface morphology, phase composition and thermal properties by optical microscopy, scanning electron microscopy (SEM), X-ray difraction (XRD) and diferential scanning calorimetry (DSC), respectively. The prepared samples contained only completely re-melted spherical particles. SEM did not reveal any features indicating the presence of crystalline phases. However, traces of crystalline gehlenite were detected by XRD. The high-temperature XRD measurements (HT XRD) were carried out to identify the phase evolution during glass crystallization. In the studied temperature range, gehlenite phase was identifed as the main crystalline phase. Non-isothermal DSC analysis of prepared glass microspheres was carried out from room temperature up to 1200 °C at fve diferent heating rates: 2, 4, 6, 8 and 10 °C/ min to determine the thermal properties of microspheres. In order to study the crystallization kinetics, the DSC curves were transformed into dependence of fractional extent of crystallization (α) on temperature. The Johnson–Mehl–Avrami–Kolmogorov model was found to be suitable for description of crystallization kinetics. Frequency factor A=5.56×1029±1.73 ×1029 min−1, apparent activation energy Eapp=722±3 kJ mol−1 and the Avrami coefcient m=2 were determined. In the studied system, the linear temperature dependence of nucleation rate, difusion controlled crystal growth interface and a 2D crystal growth were confrmed