Crystalline phase development in cordierite glass-ceramics
Cordierite glass-ceramics are characterised by their excellent physical properties viz. high mechanical strength and hardness, good thermal shock resistance and chemical durability, and therefore are widely used in materials technology. The microstructure and phase development of these materials during the initial stages of crystallisation has been investigated using X-ray diffraction, Differential Thermal Analysis and Transmission Electron Microscopy. Annealing temperatures were chosen both above and below the glass transition temperature, Tg, which was determined as 720°C by Differential Scanning Calorimetry. Infra-red, u. v-visible and electron paramagnetic resonance spectroscopy were employed as additional techniques to investigate structural changes in the glass during phase transformations. It has been shown that annealing above Tg promotes glass-in-glass phase separation on a scale of ~ 100 Å by spinodal decomposition which may well be enhanced by an incipient ordering during the annealing process. The presence of phase separation is seen to have a profound influence on the subsequent nucleation of a magnesium dititanate phase and the growth of the metastable ß-quartz solid solutions, magnesianpetalite and μ-cordierite which are capable of co-existing over the temperature range 800-100°C. The effects of annealing and phase separation are manifested as variations in both phase composition and growth morphology for a given crystallisation temperature and holding time.