| Summary: | 博士 === 國立交通大學 === 應用化學系 === 89 === The effect of impurity doping, photoluminescent, afterglow, and thermoluminescent properties and for long phosphorescent SrAl2O4:Eu2+, Dy3+ (SAED) and some analogous phosphors derived from a sol-gel synthetic route have been investigated. The improvement on phosphorescence intensity and the lengthening of afterglow persistent time has been observed in the SAED phases with the addition of boron or silicon. In order to investigate the photoluminescence, afterglow, defects and the depth energy of the traps, we have measured the X-ray diffraction (XRD) profiles, SEM and DTA/TGA, photoluminescence (PL), afterglow (AG) and thermoluminescence (TL) spectra to characterize the microstructure and luminescent properties that are relevant to the nature of defects present in long afterglow SAED phases.
The SAED phases derived from sol-gel route exhibit smaller grain size and poorer crystallinity, as compared to those synthesized by solid-state method. The wavelength of afterglow (lAG) for SAED derived from sol-gel processes was found to be shorter than lAG for those prepared via solid-state route, which was attributed to difference in host crystal field strength for Eu2+. The effect of host compositions on the PL and AG spectra of SAED phases has also been investigated for samples prepared from starting host precursors with different Al/Sr compositions. We found that Sr3Al2O6 dominated in strontium aluminate with the host precursors with Al/Sr < 1; SrAl2O4 was observed in those with 2 < Al/Sr < 3; however, more than 95% of SrAl12O19 was discovered in those with 9 < Al/Sr ≦12. The amount of SrAl2O4 present in the host with various Al/Sr ratios was found to be critical in the determining the afterglow intensity and the afterglow persistent time, as indicated by the AG curves for SAED phases. The coexistence of SrAl12O19 in the SAED host was found to affect the afterglow duration, whereas that of the Sr3Al2O6 phase was found to be independent of the afterglow persistence.
The effect of boron and silicon doping in the SAED phosphors was found to not only increase the crystal defects but also enhance the afterglow intensity. The boron-doped SAED (BSAED) sample was observed to exhibit stronger phosphorescence intensity and longer afterglow duration. This observation can be attributed to the non-homogeneous distribution of glassy strontium borates that promotes the reduction of Eu2+ and increases the trap depth energy, as indicated by surface microstructure analysis of BSAED. The twin peaks observed in the afterglow curves for BSAED phases could probably be attributed to two different traps with different depth energies, as compared to one singlet emission observed in the PL spectra.
The hole trapping effect due to Dy3+ codoping in SrAl2O4:Eu2+,Dy3+ phase can be effective only when the samples were reduced under a reducing H2/N2 atmosphere at 1,300℃, as indicated by the TL curve analysis. Otherwise, shallow traps will form as that found in SrAl2O4:Eu2+. In addition, the codoping of Nd3+ coactivator in the SrAl2O4:Eu2+,Nd3+ phase indicated that the Nd3+-doping only increases the afterglow intensity, but doesn’t increase the trap depth, which is similar to the shallow traps present in the SrAl2O4:Eu2+ phase.
Based on the experimental Hoogenstraaten’s plots, the calculated trap depth energy was found to be 0.57—0.76 eV, 0.43 eV, and 0.18 eV for SrAl2O4:Eu2+0.05,Dy3+0.05,B0.3, SrAl2O4: Eu2+0.05, Dy3+0.05, and SrAl2O4:Eu2+0.05, respectively, as compared to 0.59 — 0.72 eV for BG-300M.manufactured by Nemoto Co. These results indicate the similarity of nature of the trap levels for all strontium aluminate phosphors described in this research.
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