Thermal behaviour of yttrium aluminate glasses studied by DSC, high-temperature X-ray diffraction, SEM and SEM-EDS

DSC, SEM-EDS, XRD and high-temperature XRD analysis was used to study thermal and crystallization behaviour of yttrium aluminate glasses prepared in the form of microspheres. The glasses YA-E (eutectic composition from the pseudo-binary system Al2O3-Y3Al5O12) and YA-G (a composition identical to the stoichiometric Y3Al5O12 (YAG) phase) were prepared by combination of the Pechini method with flame synthesis. The resulting microspheres were largely amorphous, but contained traces of yttrium-aluminium garnet as the main crystalline phase embedded in the yttrium aluminate glass matrix. Crystallization of the YAG phase was observed as the dominant exothermic process on DSC curves. From the DSC records, the basic thermal characteristics of the matrix glass, i.e. T (g) (glass transition temperature), T (x) (onset of crystallization peak temperature), T (f) (temperature of the inflection point of the crystallization peak) and T (p) (maximum of crystallization peak temperature), were determined. HT XRD experiments in the temperature interval 750-1200 A degrees C and isothermal HT XRD experiments at 932, 998 and 1200 A degrees C with 6-h holding time were also performed. Crystallization experiments at lower temperatures 932 A degrees C (YA-E) and 915 A degrees C (YA-G) were conducted to study phase development in a low-temperature region. Crystallization experiments at higher temperatures (1000, 1300 and 1500 A degrees C) with maximum holding time of 6 h were performed to study crystallization of alpha-Al2O3 in the eutectic system. The SEM and SEM-EDS examination of polished cross sections of crystallized microspheres revealed slow volume crystallization of the YAG phase in the AY-E glass. Eventually, polycrystalline microspheres with fine-grained microstructure were prepared after 6-h treatment at 1500 A degrees C.