The effects of re-firing process under oxidizing atmosphere and temperatures on the properties of strontium aluminate phosphors

Graphical abstract: The comparative emission spectra of standard and re-fired Phosphor A under oxidizing atmosphere at various temperatures. The colour of Phosphor A re-fired at higher temperatures above 900 °C shifted from yellowish-green to bluish-green in the dark. But, the bluish-green emission could only be seen when it was exposed to UV and disappeared as soon as the light source was removed. Moreover, the emission intensities decreased as the re-firing temperatures increased. This could be attributed to the oxidation of Eu{sup 2+} during the re-firing process. It is well known fact from the literature that the reduction of Eu{sup 3+} to Eu{sup 2+} in appropriate host materials needs an annealing process in a reducing atmosphere such as H{sub 2}, H{sub 2}/N{sub 2} mixture or CO. Up to now, the reduction phenomena of Eu{sup 3+} → Eu{sup 2+} in air have been found in phosphates (Ba{sub 3}(PO{sub 4}){sub 2}:Eu), sulphates (BaSO{sub 4}:Eu), borates (SrB{sub 4}O{sub 7}:Eu, SrB{sub 6}O{sub 10}:Eu and BaB{sub 8}O{sub 13}:Eu) and aluminates (Sr{sub 4}Al{sub 14}O{sub 25}:Eu). Interestingly, an apparent blue shift in the phosphorescence spectrum was observed in the samples re-fired at 1000 °C and above, indicating a minimal effect on the oxidation state or the electronic energymore » levels of the co-doped Dy{sup 3+} ions, which were thought to act as long-lived hole traps resulting in long afterglow. - Highlights: • This study examines the effects re-firing at oxidizing atmosphere of photoluminescence of three different commercial SrAl{sub 2}O{sub 4}:Eu{sup 2+},Dy{sup 3+}-phosphors. • All the commercial SrAl{sub 2}O{sub 4}:Eu{sup 2+},Dy{sup 3+}-phosphors completely lost their phosphorescence after being re-fired at 1300 °C. • Oxidizing environment and re-firing temperature naturally affecting the valance of Eu{sup 2+} may cause the basic lattice structure to be modified and also limit their applications at higher temperatures, such as third firing vetrosa decor or glaze applications in ceramic industry. • It was thought that this kind of study may be promising to provide many outcome studies, may act to encourage additional efforts along these lines. - Abstract: Inorganic phosphorescent pigments have a very wide application, consisting of ceramic decoration and glazes. Therefore, in the present study 3 different commercially available strontium aluminate phosphors (coded Phosphor A, B and C) emitting yellowish-green light were investigated to determine their behaviour when re-fired under oxidizing atmosphere. The phosphors were re-fired at 10 different temperatures and consequent effects were studied. All the samples were characterized by X-ray fluorescence (XRF), X-ray diffraction (XRD), laser particle size, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX) and fluorescence spectrometer analysis. It was found that firing them under oxidizing atmosphere strongly affected their phosphorescence abilities especially over 800–900 °C, because doped europium ions (Eu{sup 2+}) in their structures are oxidized to Eu{sup 3+}. It was also concluded that the phosphor powders clumped as the temperature rose, leading to their particle size distributions to change.« less