Crystallization phenomena in iron-rich glasses

In this study, results of the crystallization of iron-rich glasses are summarized. Thermogravimetry (TG)-differential thermal analysis (DTA) were utilized to explain the phase formations and the surface oxidation of FeO to yield Fe2O3. The crystal phases fraction was evaluated utilizing X-ray diffraction analysis (XRD). Low angle XRD technique was used to investigate the distribution of the crystal phases on the surface and in the bulk as a function of the heat treatment. Transmission electron microscopy (TEM) was employed to detect the evolution of the crystalline structure and to determine the variation of the residual glass composition. The crystallization kinetics were investigated in isothermal conditions by measuring the variation of the density. The activation energy of crystal growth was calculated using isothermal and non-isothermal methodologies. The values of 377 and 298 kJ/mol were obtained for the temperature ranges 620-660 degreesC and 720-780 degreesC, respectively. Similar values, 368 and 321 kJ/mol, were estimated for the energy of viscous flow in the same temperature ranges. The results indicate that magnetite and pyroxene are the main crystal phases and that the kinetics of pyroxene formation can be explained as growth on a fixed number of magnetite nuclei. In powder samples, heat-treated in air, the crystallization is inhibited by the surface oxidation of Fe2+ to yield Fe3+ and a layer of haematite is formed on the surface. (C) 2001 Elsevier Science B.V. All rights reserved.