Sr₄Al₁₄O₂₅:(Eu²⁺,Dy³⁺) phosphors were prepared by high temperature solid state reaction. SrCO₃, Al₂O₃, Eu₂O₃, and Dy₂O₃ were used as the starting materials of Sr, Al, Eu, and Dy, respectively. H₃BO₃ was used as the flux agent to reduce the reaction temperature. SrCO₃, Al₂O₃, and Dy₂O₃ were mixed according to the atomic ratio of Sr:Al:Dy=4:14:0.28, and Eu₂O₃ was added in a range of 1%-4%. The mixed starting materials were then ball milled for 2 h, and then loaded into a corundum crucible. Solid state reaction was carried out in a muff furnace at 1250-1400℃ with carbon powder as the reducing agent for 2 h.

The crystalline phases of the persistent luminescent samples were characterized by a X-ray diffractometer (XRD, Dandong Tongda, TD-3500, CuKα₁, 30 kV, 50 mA). The luminescence spectrum was recorded by a photoluminescence (PL) spectrometer (Shimadu, RF5301). In order to record the decay curve in a very long period (>24 h), the PL spectrometer was left on with the Xenon lamp off after illumination for 10 min.

H₃BO₃was added as flux agent to reduce the solid state reaction temperature of REDAs. However, it is also found that boron has effects on the luminescence properties of the REDAs^{[ 13]}. In this section, a set of five samples were prepared at 1400℃ with different amounts of H₃BO₃addition. The amounts of H₃BO₃addition for the five samples are 0, 5at%, 10at%, 15at%, and 20at%, respectively. Figure 1 shows the XRD patterns of the five samples. The addition of H₃BO₃has great effect on the crystalline structure of the solid reaction products. For the sample without H₃BO₃, no peak corresponding to Sr₄Al₁₄O₂₅ phase is observed, and the product is mainly SrAl₂O₄ (2 θ=28.387º for (211), 2 θ=29.276º for (220), and 2 θ=29.923º for (211), JSPDS 34-379). As the amount of H₃BO₃ increases, peak intensities corresponding to Sr₄Al₁₄O₂₅ increases (2 θ=27.824º for (021), 2 θ=30.201º for (620) and (611), and 2 θ=31.408º for (421), JSPDS74-1810), and peak intensities corresponding to SrAl₂O₄ phase diminishes. Peak at 2 θ=31.9º is also from SrAl₂O₄, and peak at 2 θ=32.1º is from Sr₄Al₁₄O₂₅, as listed in JSPDS 34-379 and JSPDS74-1810, but no specific crystalline faces were denoted to them. For samples with H₃BO₃ amount more than 10at%, peak intensities corresponding to Sr₄Al₁₄O₂₅ phase decreases, which is in agreement with the results found by Haranath, et al for CaAl₂O₄:Eu²⁺, Nd³⁺ phosphor^{[ 11]}.

Fig. 1

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	Fig. 1 XRD patterns of the samples prepared with different amounts of H3BO3

Figure 2 is the photoluminescence spectra of the samples excited at 365 nm. Only green emission (510 nm) was observed without H₃BO₃ addition.

Fig. 2

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	Fig. 2 Emission spectra of the samples prepared with different amounts of H3BO3

Figure 3 shows the comparison of the dependence of the photoluminescence intensity of the green-blue emission (490 nm) and the XRD intensity of peak of the Sr₄Al₁₄O₂₅ phase (2 θ=31.4º) on amount of H₃BO₃. It is clear that close relation exists between the blue-green emission intensity and the amounts of Sr₄Al₁₄O₂₅ phase in the reaction products. The luminescence intensity increases as the amounts of H₃BO₃ increase, but it turns down when the amount of H₃BO₃ is more than 10at%, i.e., the optimal amount of H₃BO₃ is about 10at%. Similar results were found by Haranath, et alfor CaAl₂O₄:Eu²⁺, Nd³⁺ phosphors^{[ 15]}. According to the above results, the role that H₃BO₃ mainly plays as flux agent that promotes the formation the Sr₄Al₁₄O₂₅ phase, rather than the substitution of Al in Sr₄Al₁₄O₂₅. Our results is different from that of B₂O₃ in SrAl₂O₄:Eu, Dy which B substitute Al as discovered by Nag and Kutty^{[ 16]}.

Fig. 3

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	Fig. 3 Comparison between PL intensity and XRD intensity of Sr4Al14O25 phase for samples with different amounts of H3BO3

In this section, a set of samples were prepared at various solid reaction temperatures with same amount of H₃BO₃ addition (10at%,) and Eu₂O₃ doping (Eu/Sr atomic ratio=3%). Solid reaction temperatures for these samples were 1250℃, 1300℃, 1350℃ and 1400℃, respectively.

Figure 4 shows the photoluminescence spectra of these samples. As shown in Fig. 4, the photoluminescence intensity of the samples increases as the solid reaction temperature increases. It means that higher reaction tempera- ture is needed to get higher photoluminescence intensity. This is because that, for SrO-Al₂O₃-B₂O₃ system, the glass-forming temperature is about 1500℃^{[ 17]}, and as the temperature close to the glass-forming temperature, the product will be denser, which will enhance the luminescence.

Fig. 4

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	Fig. 4 Emission luminescence spectra of the samples prepared at different temperatures

In this section, the effects of Eu contents on the PL intensity were studied. The amount of H₃BO₃ was fixed at 10at%, and the solid reaction temperature was fixed at 1400℃. Eu/Sr ratios of each sample are 1%, 2%, 3% and 4%, respectively.

Figure 5 shows the photoluminescence spectra of these samples. The PL intensity does not increase proportionally as the content of Eu increases. This phenomenon shows that the persistent luminescence process is not Eu²⁺ concentration limited at the Eu doping levels of 1%-4%, but trapsEu²⁺ transition controlled, i.e., the rate and the number of electrons transfer from the trap levels to Eu²⁺ level are the dominate factors, which limit the intensity and decay time of afterglow luminescence^{[ 12, 13, 17]}. The decrease of the PL intensity with Eu content more than 3% is caused by the quenching phenomenon^{[ 18, 19]}.

Fig. 5

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	Fig. 5 Emission luminescence spectra of the samples doped with different amounts of Eu

According to the above results, a set of optimal parameters was selected, i.e., with Eu/Sr=3%, the H₃BO₃amount of 10at%, and solid reaction temperature of 1400℃, respectively. The afterglow persistent luminescence can be observed for more than 24 h by naked eyes. Figure 6 shows the afterglow luminescence of this sample in log-log scale. In order to record the decay curve in a very long period, the PL spectrometer is left on with the Xenon lamp off after the first irradiation of 10 min at wavelength of 430 nm and an outlet slit width of 5 nm. Data fitting shows that the decay constant is 249.8 min, or 4.16 h, for the afterglow time longer than 16 h.

Fig. 6

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	Fig. 6 Persistent luminescence intensity decay curves vs time

Persistent luminescent of Sr₄Al₁₄O₂₅:(Eu²⁺,Dy³⁺) green-blue phosphors with afterglow time longer than 24 h were prepared. The following conclusions were obtained.

1) The flux agent H₃BO₃affects the crystalline structure of the solid reaction products, which promotes the formation of Sr₄Al₁₄O₂₅ phase.

2) Quantitative relationship exists between the persistent luminescence of the green-blue emission and the Sr₄Al₁₄O₂₅ phase, which indicates that H₃BO₃ is mainly as flux agent rather than the substitution of Al in Sr₄Al₁₄O₂₅.

3) The dependence of persistent luminescence intensity on Eu content shows that the process is controlled by the electron transitions from deep traps to Eu²⁺ levels for Eu content in the range of 1%-4%. It is suggested that reaction temperature above 1400℃ is favorable for more intense luminescence.