Investigation of phase evolotion during crystallization of long afterglow strontium aluminate phosphor (Sr4Al14O25): The B2O3 effect

Official URL: https://risc01.sabanciuniv.edu/record=b2553608 _(Table of contents)

Strontium aluminate ceramics, when doped with rare earth elements, have an impressive capability for temporary storage and slow release of light, making them attractive for zero-energy consumption lighting. While persistent luminescence has been well known in Eu2+ and Dy3+ co-doped Sr4Al14O25 (S4A7ED) as the most efficient luminescing compound in strontium aluminate family, the addition of B2O3, initially merely as a sintering flux, increases the brightness and dramatically extends the afterglow persistence from several minutes to > 14 hours. Previous works, centered on the influence of boron on electronic structure, suggested that the extended afterglow might be attributed to clustering of ionic defects induced by presence of B2O3. However, the results were solely based on indirect experimental evidences obtained by global characterizations. The objective of this dissertation is to understand the pivotal role played by B2O3 through investigating its effect on microstructural evolution that supports long afterglow. To set the stage, we have systematically analyzed the impact of B2O3 content on microstructural evolution of stoichiometric Sr4Al14O25 compounds processed by a modified Pechini method under thermal and kinetic control. Spatially resolved analyses based on advanced electron microscopy techniques were used to elucidate the nature of boron incorporation by investigation of local coordination of B, as well as the influence of B2O3 on diffusion kinetics and dopant distribution in the microstructure, giving rise to persistent luminescence. Our key results demonstrate that, incorporation of B2O3 facilitates the evolution of the long afterglow Sr4Al14O25 phosphors through reducing the migration energy of the Sr2+ diffusing species in between the aluminum polyhedra. In the absence of B2O3, the mechanism of phase evolution from amorphous state is controlled by a diffusion-limited solid-state reaction. However, evolution of Sr4Al14O25 phase from the thermodynamically meta-stable phases is facilitated by a liquid phase assisted diffusion due to precipitation of a borate glassy phase in between the grains. At very high concentration of B2O3, our results suggest that, the crystallization mechanism completely changes to precipitation from liquid phase. Our study reveals that B plays another role in the evolution of the microstructure that supports persistent afterglow—optimizing the Eu2+ and Dy3+ concentration below the solubility limit of the Sr4Al14O25 phase—in addition to fostering their energy-transfer interactions. The experimental evidences obtained in this work, based on spatially resolved assessment of the B2O3 effect in the microstructure of Sr4Al14O25: Eu, Dy compound, contributes significantly to the knowledge in the field and provides new insights for rational design of afterglow behavior in strontium aluminate pigments