Preparation and characterization of yttrium alumino-borate powders for the development of a new generation of phosphors for lighting

Orateur : Soutenance de thèse de Vinicius FERRAZ GUIMARAES

This work specifies the synthesis and the characterization of amorphous powders belonging to the Y2O3 - Al2O3 - B2O3 system. The main objective of this work was to develop amorphous powders near the YAl3(BO3)4 composition without any rare earth as doping for the development of a new family of phosphors for solid state lighting systems excited by near ultra-violet light. The sol-gel and polymeric precursor methods were applied in order to produce these powders. After these syntheses we optimized several parameters such as thermal treatments : two different routes were tested : direct calcination and pyrolytic decomposition followed by calcination. The temperature, annealing time, heating rate and the effects of impurities on the photoluminescence (PL) were studied. A comparison between these two synthesis methods was done. The powder samples were characterized by thermal analysis technique, X-ray diffraction, photoluminescence and infra-red spectroscopies, transmission and scanning electron microscopy, elemental analysis and electron probe microanalyses, nuclear magnetic resonance, cathodoluminescence, electronic paramagnetic resonance. From the thermal analyzes, it was observed that the powder prepared by polymeric precursor method shows a glass transition temperature (Tg) around 740 °C and crystallization temperatures (Tx) at 815, 850 and 900°C. Amorphous powder showing high photoluminescence emission (between 400 and 750 nm) and quantum yields higher than 90% at 365 nm excitation, without any phase segregation were obtained when the samples are annealed at temperatures at around Tg, above this temperatures the powder start to crystallize decreasing their PL properties. Other compositions were studied by increasing the relative amounts of Y2O3, Al2O3 and B2O3, by removing the yttrium, addition of SiO2. Finally, the first measurements of color coordinates and the preliminary tests on the thermal and photo stability have been done.

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