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Preparation of dense BaMgAl
10O
17:Eu
2+particles and their surface treatment
Dae-Won Leea, Jin-Hyo Boob andHa-Kyun Junga
aAdvanced Materials Division, Korea Research Institute of Chemical Technology, P.O. Box 107, Yuseong, Daejeon 305-600, Korea
bDepartment of Chemistry, Sungkyunkwan University, Suwon 440-746, Korea Phone: +82-42-860-7313, E-mail: krader@krict.re.kr
Abstract
Dense BaMgAl10O17:Eu2 + phosphor particles with a spherical shape have been synthesized through spray pyrolysis method using basic aluminum nitrate precursor as a spray solution. This BaMgAl10O17:Eu2 + particles prepared by the spray pyrolysis have shown the stronger emission intensity compared to the commercially-available BaMgAl10O17:Eu2 +. However, thermal stability of the BAM:Eu b lue phosphor is very poor due to changing from Eu2+ to Eu3 + at the thermal process, so brightness of the phosphor decreases. To improve the thermal stability of the dense BAM:Eu phosphor, the spherical BAM:Eu particles were coated with pure BaMgAl10O17 layer using the hydrolysis reaction in a solution system. The synthesized powders were characterized by XRD, SEM and PL. On the other hand, the emission properties of the BAM:Eu phosphors coated with BaMgAl10O17 layer before and after thermal treatment at 500℃ for 30 min were estimated under VUV excitation. The brightness of the coated phosphor was higher than that of the uncoated phosphor. Also, the coating thickness of BAM layer in the BAM:Eu particles was optimized.
1. Introduction
BaMgAl10O17:Eu2 + phosphor particles have been studied for applications in fluorescent lamps and plasma display panels (PDP). In PDP, Eu2+-activated barium magnesium aluminate (BAM) phosphor has been conventionally adopted as a blue emitting component due to its availability and high quantum efficiency [1]. It is known that the spray pyrolysis using an ultrasonic resonator is an excellent technique for preparation of spherical BAM:Eu phosphor particles [2]. However, in a common spray pyrolysis experiment, the BAM:Eu particles prepared from conventional water-soluble salts are a hollow usually.
To overcome this problem, the precursor solution containing basic aluminum nitrate was introduced in
the spray pyrolysis system under the served preparation condition. The concentration of aluminum in the spray solution was optimized on the basis of the morphology, the dependency of the relative density and the relative emission intensity of the fabricated BAM:Eu phosphor particles. On the other hand, brightness of BAM:Eu phosphor particles decreases by thermal treatment and long-lasting VUV irradiation[3, 4]. It is possible to minimize the degradation either by heating in a reducing ambience during the manufacturing step of panels or by coating the phosphor particles[5]. In this work, to improve the thermal stability of BAM:Eu phosphor, the BAM:Eu powders prepared by spray pyrolysis were coated with non-activated BAM layer by liquid phase synthesis technique. The emission properties of the BAM:Eu phosphors coated with BAM layer before and after thermal treatment at 500℃ for 30 min were estimated under VUV excitation.
2. Experimental
The BAM phosphor particles were prepared by spray pyrolysis from a mixture solution of metal salts and basic aluminum nitrate. A given amount of aluminum nitrate nonahydrate was dissolved in distilled water to form the solution with concentration of 0.75 M and then heated up to 85 ℃ during 30 minute. Aluminum metal powders were added into the solution by bits over two minutes. The temperature of the mixture solution was raised to between 95℃ and 100℃, and maintained at this temperature for 3 h at least, or until all the Al metal powders are dissolved completely. In next process, the spray solution for spray pyrolysis was prepared by dissolving barium, magnesium, and europium nitrate salts in the as- prepared basic aluminum nitrate solution. The precursor particles obtained from spray pyrolysis equipment were calcined at 900 for 5 h in air and ℃ then fired at 1400℃ for 4 h under 5% H2 gas mixed
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95% N2 gas flowing, and finally ground. On the other hand, the coatings of BAM on the BAM:Eu particles were performed using Al(NO3)3 as a start material based on hydrolysis reaction of alkoxide. The ratio of BAM to the phosphor was changed from 0.05wt% to 10wt%. The required amount of phosphor particles were firstly dispersed in aluminum solution and then heated to 84℃ and mainta ined at this temperature for at least 4 h. In next step, BAM and the aluminum hydroxide particles synthesized were suspended into a mixture solution of Ba2 + and Mg2 +, then ammonium oxalate precipitant was added into the suspension to co-precipitate the precursor. The particles were filtered and dried and then fired at 1400℃ for 1 h under 5% H2 gas mixed 95% N2 gas flowing, and finally ground. The crystallinity and morphology of particles were investigated using X-ray diffractometer (XRD) and scanning electron microscope (SEM), respectively. The PL characteristics of the prepared particles were measured under vacuum ultraviolet (147 nm) by D2 lamp.
3. Results and Discussion
As the particles in the spray pyrolysis equipment had too short residence time , the post-heat treatments in air ambience and 5% H2/N2 gas flowing are need.
Figure 1. XRD patterns of the commercial BAM:Eu phosphor and the BAM:Eu phosphor particles prepared by spray-pyrolysis.
Figure 1 shows the XRD patterns of commercially- available BAM and spherical BAM:Eu phosphor particles prepared by spray-pyrolysis. The crystalline phase of both BAM:Eu phosphor particles are exactly accordance with each other. Therefore, both the samples have the single phase of ß-alumina structure.
In order to effectively prepare spherical and dense BAM particles, the preparation method of the basic aluminum nitrate solution has been modified, as described in the experimental section. The mean size of particles produced by spray pyrolysis can be controlled by changing the concentration of this spray solution.
Figure 2. Scanning electron micrographs of BAM phosphor particles prepared by spray from Al concentration of (a) 0.75 M (b) 1.0 M (c) 1.25 M (d) 1.5 M, respectively.
Figure 2 is SEM photographs of BAM:Eu phosphors synthesized from the spray solutions using the 0.75, 1.0, 1.25, and 1.5 M of Al concentrations in aqueous basic aluminum nitrate solution. As the Al concentration of the spray solution increases from 0.75 to 1.5 M, the mean particle size of the phosphors decreases from 2.7 to 1.2㎛. Although a majority of as-prepared particles have a spherical shape, a few of non-spherical particles are observed in case of lower Al concentration than 1 M. From the comparison of the photographs, therefore, it could be concluded that the post-heat treatment at 1400℃ changes the shape of the particles prepared from the nitrate solution, whereas the spherical shape of BAM:Eu particles prepared from the basic aluminum nitrate solution is maintained without variation of the shape. In view of the results, the formation of spherical and filled- structured particles from high concentration of solution is essential for production of phosphor particles using spray pyrolysis.
Figure 3 shows the relative density of phosphor on the Al concentration changes. As the concentration of Al ingredient in the spray solution increases, the relative density of the phosphor increases up to 1.5 M and this concentration shows about 89% of relative
1 0 2 0 3 0 4 0 5 0 60 70
Intensity (a.u)
2 Θ
( a)Commercially available BAM
(b)BaMgAl1 0O17:Eu by spray pyrolysis
1 0 2 0 3 0 4 0 5 0 60 70
Intensity (a.u)
2 Θ
( a)Commercially available BAM
(b)BaMgAl1 0O17:Eu by spray pyrolysis
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0.50 0.75 1.00 1.25 1.50
65 70 75 80 85 90
Al concentration (M)
Relative density (%)
density for the theoretical one .
Figure 3. The relative density of phosphor on the Al concentration changes in the spray solution.
Figure 4 shows TEM images for the BAM coated BAM:Euphosphors with BAM contents of (a)0.1, (b)1 and (c)10 wt%, respectively. As the BAM contents from 0.1 to 10 wt%, the thickness of the coated layer for the phosphor increases from 2 ㎚ to 10 ㎚. And it is observed that the surface of BAM:Eu particles is uniformlywell-coated with a thin layer of BAM.
(a) (b)
(c)
Figure 4. TEM image of the-coated BAM particle s .
Figure 5 expresses relative emission intensities of the coated BAM:Eu particles. A majority of VUV
emission spectra showed that the emission intensity was reduced after the baking and only after baking at 500℃ for 30 min. The degree of thermal degradation for 0.2wt% BAM-coated BAM:Eu was 4.3%, while that of commercial BAM:Eu particles was around 25%. Therefore, it is concluded that the thermal degradation of BAM:Eu phosphor can be improved by forming uniform BAM layer on the surface of BAM:Eu particles.
350 400 450 500 550
0.0 0.2 0.4 0.6 0.8 1.0 1.2
(A)
j
h f g
d, e, i b
c
a
Normalized Intensity
Wavelength (nm)
a : commercial b : synthesis c : 0.05Wt% coatting d : 0.075Wt% coatting e : 0.1Wt% coatting f : 0.15Wt% coatting g : 0.2Wt% coatting h : 0.3Wt% coatting i : 1Wt% coatting j : 10Wt% coatting
350 400 450 500 550
0.0 0.2 0.4 0.6 0.8 1.0 1.2
(B)
j i
h f
g d e
c b a
Normalized Intensity
Wavelength (nm)
a : commercial baking b : synthesis baking c : 0.05Wt% baking d : 0.075Wt% baking e : 0.1Wt% baking f : 0.15Wt% baking g : 0.2Wt% baking h : 0.3Wt% baking i : 1Wt% baking j : 10Wt% baking
Figure 5. Emission spectra of (A) BAM phosphors coated with various BAM contents and (B) the coated BAM phosphors fired at 500℃ for 30 min.
Figure 6 shows relative XPS spectra of the coated BAM:Eu and commercial BAM:Eu after baking at 500℃ for 30 min. XPS analysis measured in the near surface . In order to compare content of Eu2+ ions after baking process, the areas of Eu2 + and Eu3 + in XPS spectra were proportion to the concentration of Eu, respectively. The concentration of Eu2 + with the coated BAM:Eu was 26.4%, while that of commercial
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BAM:Eu particles was 17.4%. As the results, the coated BAM:Eu particles was protected the oxidation of Eu2+ during the baking process.
Figure 6. Eu3d XPS spectra of phosphors after baking at 500℃ for 30 min.
4. Conclusions
Dense blue-emitting BAM:Euphosphor particles for plasma display panels were fabricated by spray pyrolysis from basic aluminum nitrate solution. The BAM:Eu phosphor particles had a completely spherical shape and filled morphology even after post- treatment at 1400℃. The optimum Al concentration of the spray solution was 1.5 M. The decrease in the emission intensity of 0.2wt % BAM-coated particles was 4.3%, while that of comercial BAM:Eu around 25%.
5. Acknowledgments
This work was supported from Information Display R & D Center, one of the 21th Century frontier R & D Program funded by the Ministry of Commerce, Industry and Energy of Korea.
6. References
[1] C.-H. Kim, I.-E. Kwo n, C.-H. Park, Y.-J. Hwang, H.-S.
Bae, B.-Y. Yu, C.-H. Pyun and G.-Y. Hong, J. Alloys Comp., 311, 33 (2000).
[2] Y.C. Kang, H.S. Roh and S.B. Park, Adv. Mater., 12, 451 (2000).
[3] K. Yokota, S.-X. Zhang, K. Kimura and A. Sakamoto,
J. Lumin., 92, 223 (2001).
[4] S. Zhang, T. Kono, A. Ito, T. Yasaka and H. Uchiike, J. Lumin., 106, 39 (2004).
[5] C. Guo, B. Chu, M. Wu and Q. Su, J. Lumin., 105, 121 (2003).
1145 1140 1135 1130 1125 1120 1115
b a
b: Coated BAM baking a: Commercial BAM baking
1124.9 1124.8 1135.2
1135.23
Intensity (a.u.)
Binding Energy (eV)
