Synthesis of Cerium-Activated Yttrium Aluminate Based Fine Phosphors by an Aerosol Route

Abstract

Polycrystalline fine powders of yttrium aluminate doped with Ce3+ were synthesised by spray pyrolysis of a polymeric precursor, which was obtained by dissolving the corresponding nitrates in a solution of ethylenediaminetetraacetic acid (EDTA) in ethylene glycol (EG). Aerosol decomposition was performed at 550 °C followed by an additional thermal treatment (900–1100 °C). The yield of either a single yttrium aluminium perovskite (YAP) phase or a single yttrium aluminium garnet (YAG) phase was investigated as a function of the predefined yttrium/aluminium ratio, the cerium doping concentration, the processing temperature, and the thermal-treatment regime, which included the variation of the heating and cooling rates (dT/dt), the residence time (τ), and the atmosphere. Changes in the composition and structure of the precursor during thermal decomposition were investigated by thermogravimetric and differential thermal analysis (TGA/DTA) and FTIR spectroscopy. The particle morphology and structure were analysed by a combination of scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM/EDS) and by high-resolution transmission electron microscopy (HR-TEM). The structural refinement was based on the phase identification performed by X-ray powder diffraction (XRPD). The emission spectra were recorded within the range 325–800 nm by applying excitation wavelengths of 297 (YAP) and 450 nm (YAG). The employed synthesis conditions assured the formation of spherical, non-agglomerated particles with well-developed surfaces and diameters between 200 and 800 nm. For a predefined Y/Al ratio of 1:1, lower processing temperatures combined with longer heat treatments under stationary conditions resulted in a multiphase system, composed of YAP, YAG, and monoclinic yttrium aluminate (YAM) phases. However, a short heat treatment with a high heating rate (200 °C/min) at higher temperatures results in the formation of a kinetically favoured pure YAP hexagonal phase. On the other hand, for a predefined Y/Al ratio of 3:5, the generation of a thermodynamically favoured pure YAG phase has been confirmed, regardless of the applied heat-treatment conditions. Although incomplete, Ce3+ introduction into the host matrix has been detected by XRPD and luminescence measurements.