Microelectronics Journal 35 (2004) 379–382 www.elsevier.com/locate/mejo
SrAl2O4: Eu2þ, Dy3þ phosphors derived from a new sol –gel route Yiqing Lu, Yongxiang Li*, Yuhong Xiong, Dong Wang, Qingrui Yin The State Key Lab of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, China
Abstract The SrAl2O4: Eu2þ, Dy3þ phosphor powders were prepared by a new sol– gel method using aluminum isopropoxide and strontium acetate as precursors. The sol – gel process and the structure of the phosphor powders were investigated by means of DSC-TG and XRD. It was found that the single-phase SrAl2O4 was formed at 900 8C, which is 300 8C lower than that required for the conventional solid-state reaction. The particle morphology, photoluminescence and afterglow properties of the phosphors were studied in this article. q 2003 Elsevier Ltd. All rights reserved. Keywords: Strontium aluminate; Sol–gel process; Rare earth; Phosphor
1. Introduction
2. Experimental
In recent years, there has been a constantly growing market for long-persistent luminescence materials. The rare-earth metal ions doped strontium aluminate phosphors, because of their high quantum efficiency, anomalous long phosphorescence and good stability, have been studied in depth and used widely [1,2]. In particular, SrAl2O4: Eu2þ, Dy3þ has been considered as a useful bluish-green phosphor in the application of luminous clocks and watches as well as potential outdoor night time displays [3 –5]. The solid-state reaction process has been used intensively for phosphor synthesis, but this process often results in poor homogeneity and requires high calcinating temperature. The sol – gel technique, due to its advantages of easier composition control, better homogeneity and low processing temperature, is a new route to synthesize fine powders. It has attracted more and more attention [6 – 8]. In order to prepare SrAl2O4: Eu2þ, Dy3þ phosphors with higher brightness and longer afterglow persistent time, a new sol – gel route was used for the synthesis. Using aluminum isopropoxide and strontium acetate as precursors, we have successfully prepared SrAl2O4: Eu2þ, Dy3þ phosphors at lower temperature and a fine particle size in nanoscale.
2.1. Synthesis of SrAl2O4: Eu2þ, Dy3þ phosphors
* Corresponding author. Tel.: þ 86-21-52411066; fax: þ 86-2152413122. E-mail addresses:
[email protected],
[email protected] (Y. Li). 0026-2692/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0026-2692(03)00250-7
The starting materials used were Eu2O3 (99.99%), Dy2O3 (. 99.9%), Sr(CH3CO2)2·1/2H2O (99%) and Al(i-OC3H7)3 (99%). Aluminum isopropoxide was dissolved in ethylene glycol monoethyl ether at 80 8C. The oxides of Eu, Dy and strontium acetate were dissolved in concentrated nitric acid and deionized water, respectively, then two solutions were mixed while the molar stoichiometry was 0.96:0.04:0.04 for Sr/Eu/Dy. Glycerol was used as additive to the mixed aqueous solution to avoid the undesirable precipitation of Al(OH)3 due to hydrolysis of aluminum isopropoxide, then the solution of aluminum isopropoxide was added dropwise while stirring vigorously. The precursor solution was stirred at room temperature for an addition 1 h and subsequently heated at 60 8C for 12 h. After hydrolysis and condensation, the prepared white gels were dried at 180 8C for 24 h, calcinated in a furnace with air atmosphere at temperatures between 900 and 1250 8C for 4 h, respectively. Finally, the powders were heated at 1200 8C for 2 h in a reducing atmosphere to assure complete conversion of Eu3þ to Eu2þ. The sol – gel process in this work is summarized in a flow chart shown in Fig. 1. 2.2. Characterization of SrAl2O4: Eu2þ, Dy3þ phosphors A small portion of prepared gels were oven-dried at 100 8C for 48 h and investigated by differential scanning
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Fig. 1. Schematic flow chart diagram for the synthesis of SrAl2O4: Eu2þ, Dy3þ phosphors via a sol –gel route.
calorimetry (DSC) and thermogravimetry (TG) using a NETZSCH STA 449C simultaneous thermal analyzer. The sample was heated from room temperature to 1200 8C at a constant heating rate of 10 8C/min. The crystallization process and phase purity of the powders were checked by X-ray Diffraction (XRD) analysis with a Rigaku D/MAX 2550V X-ray diffractometer using Cu Ka radiation. The C content in the final products was quantitatively determined by nonaqueous titration. The phosphor particle morphology was observed with a JEOL JSM-6700F Field Emission Scanning Electron Microscope (FESEM). The emission, excitation spectra and afterglow properties of the phosphors were measured at room temperature using a Perkin– Elmer LS-55 luminescence spectrometer equipped with a xenon discharge lamp as an excitation source.
the organic compounds, the exothermic reactions and accompanied weight loss are observed at 160– 450 8C. The last stage is pyrolysis of the residual organics and crystallization of SrAl2O4. The endothermic peak at 974 8C and accompanied small weight loss can presumably be attributed to the decomposition of SrCO3, as indicated by the following XRD analysis results. The X-ray powder diffraction patterns are shown in Fig. 3. The sample of Fig. 3(a) was made in order to determine the intermediate products formed in the sintering process. SrCO3 as an intermediate product, could be found from this figure. When the temperature raised to 900 8C,
3. Results and discussion 3.1. Formation of SrAl2O4: Eu2þ, Dy3þ The DSC and TG curves of the oven-dried bulk gels are shown in Fig. 2. There are three main stages of weight loss. The initial weight loss results from desorption of the adsorbed moisture and the evaporation of organic solvents in the gels, the endothermic reactions and weight loss are observed up to 160 8C. Next stage is decomposition of
Fig. 2. DSC– TG curves of SrAl2O4: Eu2þ, Dy3þ bulk gels dried at 100 8C.
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the single-phase crystal of monoclinic SrO·Al2O3 was formed. The C content is found to be only 0.10% in the phosphors calcinated at 900 8C. This calcinating temperature is 300 8C less than the conventional solidstate reaction [2]. The results show the most essential advantage of the sol – gel method, e.g. the precursor solutions are homogeneously mixed at the molecular level, leading to high reactivity of starting materials and the reduction of sintering temperature. The observed diffraction peaks become sharper and stronger with increasing calcinating temperature, which indicates that the crystallinity of SrAl2O 4 powders increases. According to the width and intensity of the peaks, the crystallization of the SrAl2O4 powders is nearly completed at 1100 8C. Fig. 3. X-ray powder diffraction patterns of SrAl2O4: Eu2þ, Dy3þ phosphors calcinated at different conditions: (a) 800 8C, 0 h; (b) 900 8C, 4 h; (c) 1100 8C, 4 h; (d) 1250 8C, 4 h.
3.2. Phosphor morphology The SEM photographs of the phosphor are shown in Fig. 4. Irregular spherical grains with size of several tenths of a micron to several microns are observed. This morphology is same with the results observed by Chen et al. [7], and the grain size is slightly smaller than the samples prepared via a solid-state route. 3.3. Luminescent properties of SrAl2O4: Eu2þ, Dy3þ phosphor The excitation and emission spectra of SrAl2O4: Eu2þ, Dy3þ phosphor calcinated at 1100 8C are shown in Fig. 5. The excitation spectra of the SrAl2O4: Eu2þ, Dy3þ phosphor show a broad band from 275 to 400 nm, and its emission is a symmetrical band at 511 nm which is attributed to the typical 4f65d1 – 4f7 transition of Eu2þ. The emission wavelength is in a good agreement with the results of Ravichandran et al. [9] who synthesized the phosphor by microwave processing, but a slightly shorter than the 521 nm
Fig. 4. FESEM images of the SrAl2O4: Eu2þ, Dy3þ phosphor calcinated at 1250 8C.
Fig. 5. The emission and excitation spectra of the SrAl2O4: Eu2þ, Dy3þ phosphor calcinated at 1100 8C.
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the conventional solid-state reaction. The particle size of the phosphors is observed from several tenths of a micron to several microns. The broad excitation band of the phosphors is from 275 to 400 nm, and its symmetrical emission band is peaked at 511 nm, which is ascribed to the typical 4f 65d1 – 4f7 transition of Eu2þ.
Acknowledgements This work was supported by the Knowledge Innovation Foundation of Shanghai Institute of Ceramics (210038, 210041) and the National Natural Science Foundation of China (NSFC No.20151003). Fig. 6. Afterglow decay curve (lex ¼ 340 nm) of SrAl2O4: Eu2þ, Dy3þ phosphor calcinated at 1100 8C.
observed by Palilla et al. [1] who prepared the phosphor by high-temperature ceramic sintering. The afterglow decay curve of the same sample is shown in Fig. 6. The decay rate is very fast in the first 1 ms. the afterglow intensity gradually becomes constant after 20 ms. The phosphorescence characteristic of Eu2þ and Dy3þ co-activated strontium aluminate may be explained by the mechanism of photoconductivity due to holes and the trapping and thermal release of holes by Dy3þ ions in the system [4]. The influence of the different preparation conditions on the photoluminescence and afterglow properties still needs further study.
4. Conclusions Using a new sol –gel method, the SrAl2O4: Eu2þ, Dy3þ phosphor powders were successfully synthesized. The XRD results show that the single-phase SrAl2O4 could be formed at 900 8C. This temperature is 300 8C lower than
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