Study of thermal properties and crystallization kinetics of Bi-doped 2CaO-Al 2 O 3 -SiO 2 glasses

Bi doped 2CaO-Al 2 O 3 -SiO 2 (gehlenite) glasses were prepared by conventional melting. The concentration of Bi 3+ was 0, 0.25, 0.50 and 1.50 mol. %. The prepared samples were X-ray amorphous except of the undoped sample and the sample with the highest content of Bi 3+ , for which XRD records revealed traces of crystalline gehlenite. Differential thermal analysis (DTA) was used to study the thermal properties of prepared glasses, performed in the atmosphere of nitrogen at 5 different heating rates. The DTA records of all studied samples contained only one exothermic effect. Based on the results of XRD analysis, the exothermic effect was attributed to the crystallization of gehlenite. The XRD patterns of all samples measured after DTA analysis contained gehlenite as the main crystalline phase (01–074-164 COD). The DTA records measured at a heating rate of 10 °C.min −1 revealed a significant decrease in the temperatures of maxima of the exothermic peaks (from 982 °C to 943 °C) with the increasing Bi doping. The model approach using the Johnson–Mehl–Avrami-Kolgomorov (JMAK) equations with six different values (1.5, 2, 2.5, 3, 3.5 and 4) of Avrami parameters ( m ) was used to study crystallization kinetics of gehlenite glasses. The model that best describes the experimental data was found, using the AIC (Akaike criterium) and wAIC (Akaike weight) criteria: A (frequency factor) and E app (apparent activation energy) were determined. Based on kinetic parameters values, it is concluded that the nucleation rate is constant. Irrespective of the Bi content, the movement of the growth zone is controlled by diffusion and 3D growth is characteristic for the gehlenite crystals growth.