Isothermal and non-isothermal crystallization of a fresnoite glass

Several analytical models have been widely applied to determine the activation energy of crystallization, E, and the Avrami index, n. In this work, the parameters E and n for the crystallization of fresnoite (2BaO·TiO2·2SiO2) glass were calculated using different approaches (Ozawa, Kissinger, and Matusita & Sakka). Monolithic glass samples (≈ 36 mg) were heated in a DSC furnace from room temperature to 650 °C at 40 °C/min, and held at that temperature for 5 min. To induce the formation of new nuclei mostly in the nucleation zone, these samples were heat-treated in the temperature range of 650 °C to 750 °C at heating rates of 10 °C/min and 20 °C/min. They were then reheated at different heating rates (ϕc = 6, 8, 10, 15 and 20 °C/min) until they became completely crystallized. The experiments were carried out in a DSC furnace under an argon atmosphere (20 ml/min), using Pt crucibles. Additionally, to confirm the Avrami coefficient and the predominant crystallization mechanism, the evolution of the crystallized fraction over time was determined independently by powder X-ray diffraction of the fresnoite glass. In this case, the samples were heat-treated in a tubular furnace at 720 °C for different lengths of time. The Avrami index obtained by the non-isothermal DSC and XRD experiments are in excellent agreement and demonstrate that fresnoite crystallizes predominantly in the volume and occurs by simultaneous crystal nucleation and growth. The activation energy of crystallization predicted by Kissinger and Matusita & Sakkaʹs models was EC = (688 ± 22) kJ/mol, and ECK = (940 ± 70) kJ/mol, respectively.