Comparison study of the luminescent properties of the white-light long afterglow phosphors: CaxMgSi2O5+x:Dy3+(x=1, 2, 3)
Introduction
In recent years, as a new generation of long afterglow phosphors, rare-earth-doped alkaline earth silicates and aluminates have been researched extensively due to a growing market for their application in traffic signs, emergency exit routes, decoration, and textile printing, among others. Compared with the conventional sulphide materials used earlier, silicates and aluminates long afterglow phosphors yield much better characteristics, such as a longer duration of time of the phosphorescence, brighter luminosity, improved chemical stability, along with relatively easier preparation and lower cost [1], [2], [3].
Colours of the newly developed long-lasting phosphors cover from blue to red. However, no phosphor with white long afterglow has been developed into a commercial application so far, while white light is the most suitable for illuminating light sources. In traditional phosphor industry, the white colour phosphor can be obtained through the combination of the available phosphors with the three primary colours, red, green, and blue. Unfortunately, this method will not work for long afterglow phosphor in practice since we can hardly ensure the very consistent afterglow decay process for every component phosphors thus resulting in the change of colour with time.
Recently, a novel method of achieving white-light long afterglow phosphor was conceived and reported by two groups from China. Lei et al. firstly reported the white-light emitting long afterglow phosphors CdSiO3:Dy3+ [4], [5] and SrSiO3:Dy3+ [6]. Then Liu et al. studied the white persistent luminescence of CaAl2O4:Dy3+ [7], Sr2MgSi2O7:Dy3+ [8], and Y2O2S:Tb3+ [9]. In all these materials, the white-light long afterglow emission is achieved via the combination of different colour emissions from an identical luminescence centre, Dy3+ or Tb3+. For instance, the two main groups of emission lines of Dy3+ are in the wavelength range 470–500 nm (blue) and 570–600 nm (yellow), corresponding to the 4F9/2→6H15/2 and the 4F9/2→6H15/2 transitions, respectively. The two-colour light emissions can produce white light through an appropriate combination in Dy3+-doped materials. Because the afterglow decay process is largely controlled by the properties of traps in the host, the different colour emissions from the identical centre will have the same decay process, thus keeping the afterglow colour unchanged.
The properties of the white long afterglow phosphors thus far reported do not satisfy the demand of practical application. Therefore, search of better white long afterglow phosphors with a new combination of host and activator is still in progress. In this article, a series of Dy3+-doped calcium magnesium silicates, CaxMgSi2O5+x:Dy3+ (x=1, 2, 3), were prepared and investigated, for the calcium magnesium silicates are a kind of important host materials for long afterglow phosphors [10], [11]. The luminescence and the afterglow properties of the samples were measured and compared. Some factors influencing these properties are addressed, in addition to the discussion of the origin of the traps responsible for the long afterglow.
Section snippets
Experimental
The powder polycrystalline samples, CaMgSi2O6:Dy3+, Ca2MgSi2O7:Dy3+, Ca3MgSi2O8:Dy3+, with different doping concentrations were all prepared by the solid-state reaction method. CaCO3(4N), MgO(4N), SiO2(4N), and Dy2O3(4N) were used as raw materials. The nominal Dy3+ molar concentration relative to the host compound was chosen to be varied from 0.2% to 8%. The mixture of raw materials was ground to fine particle before sintering for 5 h in air at 1250 °C for CaMgSi2O6:Dy3+ preparation and 1350 °C
Results and discussion
The phase and structure of the prepared samples were checked using XRD spectra. For comparison, the XRD patterns shown in Fig. 1 are those only for the samples of different hosts with a Dy3+ doping concentration of 4 mol%. It is obvious from Fig. 1 that the single-phased Ca2MgSi2O7:Dy3+ and Ca3MgSi2O8:Dy3+ were obtained by solid-state reaction under 1350 °C. The structure of Ca2MgSi2O7:Dy3+ and Ca3MgSi2O8:Dy3+ can be indexed to JCPDS number of 83-1815 and 73-0382, respectively, corresponding to
Conclusion
In conclusion, a series of silicate compounds, CaxMgSi2O5+x:Dy3+ (x=1, 2, 3), with white long afterglow after irradiation with the mercury lamp have been prepared. The white afterglow comes from the combination of the yellow and the blue emissions corresponding to the transitions between 4f levels of the doping Dy3+ ions. The white afterglow can last for one hour for most of the samples prepared, suggesting promising candidates for the development of the white long afterglow phosphors.
Acknowledgements
This work is supported by the National Natural Science Foundation of China (Grant no. 10404028) and the Natural Science Foundation of Anhui Province of China (Grant no. 070416232).
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