Anomalous Avrami Index Recorded During the Non-Isothermal Crystallization of Borotitanosilicate Glass Powders

We investigated the crystallization kinetics of 24.5CaO-24.5SrO-3.7B2O3-6TiO2-41.3SiO2 and 25CaO-25SrO-3.8B2O3-8TiO2-42.2SiO2 (mol%) borotitanosilicate glass powders (<= 43 mu m), herein referred as T6 and T8, respectively, using differential scanning calorimetry (DSC -beta= 2.5 degrees C/min - 40 degrees C/min) and scanning electron microscopy (SEM). The kinetic parameters were calculated through model-free and model-fitting approaches. Despite minor variations in the corresponding activation energy, it was found that the local Avrami exponents changed significantly, and multiple-step reactions govern the crystallization process (with n >= 2 for 0.05 <= alpha <= 0.95). These unexpected n values were attributed to the complexity of the crystallization process, where crystals grow individually and three-dimensionally in the early stages, thereafter tending to grow in a single direction (1D) after meeting each other.