Translucent and persistent luminescent SrAl2O4:Eu2+Dy3+ ceramics
Introduction
Over the past decade, materials that exhibit persistent luminescence have attracted attention due to their great potential for use in electronic displays, high-energy detectors, digital radiography, optical memories and storing images [1], [2]. In many cases, ceramics are more appropriate than single crystals or powders due to their ease of handling and better mechanical robustness. Moreover, ceramics are more economically viable than single crystals in that they are cheaper and easier to fabricate, they can be produced in different shapes and sizes, and they are easier to implement in large scale production facilities.
Silicates and aluminates doped with rare earths, specifically europium and dysprosium, have been shown to exhibit phosphorescent properties and stability that are superior to those of zinc sulfide, thus increasing interest in the study of these phosphorescent hosts [3], [4]. In the aluminate family, the strontium aluminate phases (SrAl2O4, SrAl4O7, Sr3Al2O6 and SrAl12O19) doped with rare earth have been the most suitable candidates for obtaining attractive luminescent properties. The SrAl2O4:Eu2+,Dy3+ (SAED) phase particularly stands out, where persistent luminescence with emission visible to the naked eye has been reported up to 30 h [5], [6]. The SAED phase glows in the dark after previous excitation by solar light or even common lamps, providing its most important application as passive night-time signals. Several synthesis methods have been used to produce SAED [7], [8], [9], [10] and usually an annealing in a reducing atmosphere is applied in order to reduce the europium to the divalent state. This process is necessary to achieve the persistent luminescence property with characteristic green emission from the Eu2+ ion.
Additionally, it has been demonstrated that the transparency of ceramics in the visible spectral region can increase the intensity of the persistent luminescence emission by exciting the entire volume of the host and allowing most of the emitted light to reach the surface [11], [12]. Even though the non-cubic crystal structure of SAED will ultimately limit its transparency, high translucency is possible if high density is achieved during the sintering process. SAED melts congruently at 1960 °C and, given this high temperature, the laser-sintering technique is an attractive alternative fabrication method for making translucent ceramics. Laser sintering allows for rapid processing and has potential for high heating and cooling rates (about 2000 °C) without crucibles, thus reducing the risk of contamination [13]. It additionally enables the sintering of materials that exhibit high melting point such as Y2O3 [14]. The laser-sintering technique has been successfully utilized to produce Bi4Ge3O12 ceramics with good transparency and scintillation properties [15], [16]. Moreover, in previous papers by the Authors, dense oxide ceramics have been obtained with different physical properties using this technique [14], [17], [18] than more conventional ones suggesting kinetic effects can be realized. Reported in this work is the use of laser-sintering as an alternative technique to produce SrAl2O4:Eu2+Dy3+ translucent ceramics with persistent luminescence properties.
Section snippets
Experimental procedure
Strontium aluminate powders doped with europium and dysprosium (SrAl2O4: 0.02Eu, 0.01Dy-SAED) were synthesized using a modified polymeric precursor method as described in [14], [19]. The precursor materials utilized were SrCl2·6H2O (99%, Aldrich), AlCl3·6H2O (99%, Aldrich), Eu(NO3)3 (99.5%, Aldrich) and Dy(NO3)3 5H2O (99.5%, Aldrich). Strontium and aluminum citrates were separately prepared using SrCl2·6H2O (99%, Aldrich) and AlCl3·6H2O (99%, Aldrich) mixed with citric acid (CA), previously
Results and discussion
Fig. 2 shows the DTA/TG curves of the SAED precursor solution after drying at 100 °C/24 h. The thermal decomposition was divided in 3 regions. In the first region, from room temperature to ~430 °C, two small exothermic peaks were observed in DTA, along with a weight loss of 49% mainly due to dehydration and decomposition reactions. In the second region, between 430 °C and 550 °C, an intense exothermic peak in DTA was observed with a weight loss of 35% due to the combustion and crystallization
Conclusions
In conclusion, persistent luminescent SrAl2O4:Eu2+Dy3+ translucent ceramics were successfully fabricated by the first time, presenting transmittance up to 40% between 600 and 800 nm. It was observed a best stabilization of the monoclinic phase attributed to the high sintering temperature and the very high cooling rate achieved during the laser-sintering process. The laser-sintered ceramics exhibited an intense green emission band centered at 514 nm characteristic of 5d→4f transition from Eu2+.
Acknowledgments
The authors wish to acknowledge Brazilian Synchrotron Light Laboratory (LNLS), proposal XAFS01-8760, for the XANES facilities, the financial agencies CNPq, CAPES, FINEP, FAPITEC/SE. Fellowship CAPES-Process no. 99999.007954/2014-00.
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