2. Materials Two compositions were investigated APS: within the immiscibility gap NoAPS: outside the immiscibility gap APS: 67SiO 2.11TiO 2.22BaO NoAPS:49SiO 2.17TiO 2.34BaO The raw material was cast after melting at 1560°C for 2h. The glass was then annealed for 12h at 600°C The crystallization treatment was carried at 1000°C (from 1 min to 1h) 5. DRX analysis on bulk samples Both samples crystallize to fresnoite (Ba 2 TiSi 2 O 8 ) NoAPS exhibits oriented crystallization along [001] and [002] APS shows no preferred growth direction typical of a bulk crystallization process 4. Microstructures: APS versus NoAPS Influence of Amorphous Phase Separation on the crystallization mechanisms in the BaO-TiO 2 -SiO 2 system Emilie Boulay and Stéphane Godet Service Matières et Matériaux, Faculté des Sciences Appliquées, Université Libre de Bruxelles (ULB), avenue Franklin Roosevelt 50, CP 194/3, 1050 Bruxelles 1. Summary The control of microstructures is a major challenge to enhance the properties of base glasses. More specifically, the controlled crystallization of glasses through a Prior Amorphous Phase Separation (APS) may be an elegant way to ensure a bulk process. Glasses in the BaO-TiO 2 -SiO 2 system undergo APS for specific composition ranges. The interfaces created can have a significant effect on the crystallization behavior since it allows bulk nucleation of fresnoite to be obtained, instead of surface crystallization. This phenomenon can enhance optical properties such as blue photoluminescence, particularly interesting for plasma screen applications. The aim of this work is to compare the crystallization behavior with and without prior APS. The microstructures were investigated using Scanning and Transmission Electron Microscopy (SEM & TEM). Differential Scanning Calorimetry (DSC) and X-Ray Diffraction (DRX) were also used to characterize the crystallization process. The prior APS is shown to have a major influence for the glass compositions investigated Consequently, a modeling effort is presently being carried out in order to determine the immiscibility region, in order to compare compositions closer to the immiscibility boundary. 6. Need of a thermodynamical assessment of the immiscibility gap In order to minimize the influence of the composition of the glasses investigated, compositions closer to the immiscibility boundary should be studied The BaO-TiO 2 -SiO 2 system has not been fully covered nor critically assessed in databases of thermodynamical software The boundaries were defined in literature based on the immiscibility regions of BaO-SiO 2 and TiO 2 - SiO 2. An assessment is required to localize more precisely the immiscibility area versus temperature Thermo-Calc ® software was chosen to assess the system, using the Ionic Two Sublattice model Minimization of the Gibbs Free Energy of the system and optimization of the interaction parameters through fitting experimental data Results from binaries, together with additional experimental measurements will allow the ternary thermodynamical properties and immiscibility area to be evaluated. This assessment is still under investigation 7. Conclusions and perspectives In the BaO-TiO 2 -SiO 2 system, APS influences the crystallization mechanisms and the corresponding microstructures in the same way. A prior APS ensures a bulk crystallization process. To minimize the effect that the difference in composition may have, compositions closer to the immiscibility boundary must be compared. The assessment of the immiscibility gap is under investigation. Further work will consist in characterizing the start of the crystallization in order to determine the influence of the interfaces between the amorphous droplets and the matrix that crystallizes. Eventually, the interplay between the microstructural control and the corresponding optical properties will be analyzed. We acknowledge the financial support of FRIA. Michel Bogaerts and Jinichiro Nakano are also greatly acknowledged for the fruitful discussions. 3. DSC analysis Two crystal phases (fresnoite and cristobalite) + an endotherm around 1250 °C APS: TEM observations – Amorphous rich-SiO 2 droplets within an homogeneous crystallized matrix APS NoAPS NoAPS: SEM observations - Strong surface crystallization Droplets Matrix 50 µm 100nm Binaries extrapolation+experimental data Experimental data NoAPSAPS [0 0 1] [2 1 0] [0 0 2] [0 0 1] [2 1 0] [0 0 2] Start: 25°C Rate: 40°C/min End 1300°C Start: 25°C Rate: 40°C/min End 1300°C Immiscibility area 30 µm +(ternary terms) 2 µm NoAPS APS