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

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 Eu2+ during the re-firing process. It is well known fact from the literature that the reduction of Eu3+ to Eu2+ in appropriate host materials needs an annealing process in a reducing atmosphere such as H2, H2/N2 mixture or CO. Up to now, the reduction phenomena of Eu3+ → Eu2+ in air have been found in phosphates (Ba3(PO4)2:Eu), sulphates (BaSO4:Eu), borates (SrB4O7:Eu, SrB6O10:Eu and BaB8O13:Eu) and aluminates (Sr4Al14O25: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 energy levels of the co-doped Dy3+ ions, which were thought to act as long-lived hole traps resulting in long afterglow.