Sol-gel synthesis and the effect of boron addition on the phosphorescent properties of SrAl 2 O 4 :Eu 2+ ,Dy 3+ phosphor
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The effects of boron addition on the microstructure and afterglow properties of the long-phosphorescent SrAl2O4:Eu2+,Dy3+ (SAED), synthesized via a novel sol-gel route, were systematically investigated. Significant improvement on luminescence intensity and the lengthening of afterglow persistent time in boron-added SAED (BSAED) phases were observed, as compared to those without boron addition and commercial phosphors. Typical bluish-green emissions attributed to the doublet phosphorescence with wavelengths peaking at 412 and 501 nm for BSAED phase and 398 and 486 nm for the pristine SAED phase were observed. Afterglow with wavelengths peaking at 403 and 485 nm was observed for BSAED phase, whereas that with wavelengths peaking at 486.5 nm was found for the pristine SAED phase, as indicated by time-dependent afterglow decay profiles. Results from scanning electron microscopic morphological studies were used to investigate the modification of microstructure of the BSAED phases. I. INTRODUCTION
Eu2+-activated alkaline earth aluminates are known as phosphorescent phosphors for their high quantum efficiency in the visible region as indicated in investigations on the MAl2O4:Eu2+ and MAl12O19:Eu2+ (M ⳱ Ca, Sr, Ba) phases described by Blasse et al.1 Palilla et al. investigated the phosphorescent properties of SrAl2O4:Eu2+ green phosphor with emission wavelength (em) of 520 nm and attributed the phosphorescence to the 4f 65d → 4f 7 transition of Eu2+.2 Later, the Eu2+- and Dy3+-coactivated SrAl2O4 (SAED) phase with tridymite-type structure was reported by Murayama et al.3 to exhibit long persistent and bright bluish-green phosphorescence and greater chemical stability than traditional sulfide-based ZnS:Cu,Co phosphors. Thus SrAl2O4:Eu2+,Dy3+ has been considered as a useful bluish-green phosphor for use in dials for luminous watches and clocks as well as a potential outdoor nighttime display. However, basing their conclusions on the investigations of the phosphorescence, thermoluminescence, and photoconductivity characteristics of polycrystalline SrAl2O4:Eu2+,Dy3+, Matsuzawa et al. suggested a mechanism indicating that the phosphorescence is caused by the presence of holes and to the trapping and thermal release of holes by Dy3+ ions in the system.4
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J. Mater. Res., Vol. 16, No. 3, Mar 2001 Downloaded: 25 Jun 2014
Furthermore, Katsumata et al.5 studied the host composition effect on the phosphorescent properties of SAED crystals by varying the mole ratio of Al/Sr (from 2.05 to 2.22) and Dy/Eu (from 0 to 3.55). They concluded that em attributed to Eu2+ ions did not vary with the host and/or activator compositions.5 On the contrary, Sakai et al. investigated the effect of composition on the phosphorescence of BaAl2O4:Eu2+,Dy3+ crystals and concluded that the intensity of afterglow phosphorescence, the trap depth, and the trap density derived from the thermoluminescence measurements vary with the host compositio
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