Effects of Si4+ and B3+ doping on the photoluminescence of BaMgAl10O17:Eu2+ phosphor under UV and VUV excitation

Journal of Alloys and Compounds 478 (2009) 801–804

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Effects of Si4+ and B3+ doping on the photoluminescence of BaMgAl10 O17 :Eu2+ phosphor under UV and VUV excitation Zhan Hui Zhang a,b,∗ , Yu Hua Wang b , Xiao Xia Li c a b c

School of Materials Science and Engineering, Wuhan Institute of Technology, Wuhan 430073, China Department of Materials Science, Lanzhou University, Lanzhou 730000, China School of Electronics, Jiangxi University of Finance & Economics, Nanchang 330013, China

a r t i c l e

i n f o

Article history: Received 3 September 2008 Received in revised form 5 December 2008 Accepted 13 December 2008 Available online 24 December 2008 Keywords: Phosphor Optical properties Luminescence

a b s t r a c t Si4+ and B3+ were doped into the spinel block of blue-emitting BaMgAl10 O17 :Eu2+ phosphor. Their photoluminescence, such as emission characteristic and thermal stability, were investigated under ultraviolet (UV) and vacuum ultraviolet (VUV) excitation. The results showed that proper quantities of Si4+ or B3+ doping could obviously improve the emission intensity. However, the thermal stability of Si4+ or B3+ doped phosphors was worse than that of undoped samples. Based on lattice defect theory, the mechanisms of these interesting phenomena were discussed and explained, which revealed that the thermal degradation of BaMgAl10 O17 :Eu2+ had great correlation with the lattice environment. © 2008 Elsevier B.V. All rights reserved.

1. Introduction BaMgAl10 O17 :Eu2+ (BAM) has been regarded as the preferred blue-emitting phosphor for fluorescent lamps, plasma display panels (PDPs) and Hg-free lamps applications because of its high luminance efficiency and good color purity under ultraviolet (UV) and vacuum ultraviolet (VUV) excitation [1]. However, BaMgAl10 O17 :Eu2+ undergoes degradation on the luminance efficiency and color quality during the manufacturing process of lamps or PDPs in which the phosphors suffer a temperature of about 500 ◦ C in air [2]. This degradation is one of the most significant shortcomings in the applications of BaMgAl10 O17 :Eu2+ because of its bad influences on the longevity and quality of lamps or displays. It is well known that BaMgAl10 O17 :Eu2+ crystallizes in the space group of P63 /mmc with the ␤-alumina type crystal structure which consists of spinel blocks (MgAl10 O16 ) separated by mirror planes (BaO or EuO). The oxidation of Eu2+ within mirror planes has been well accepted as the mechanism of thermal degradation, thus, several methods were proposed to improve the thermal stability based on reducing the oxidation degree, such as compositional variation [3], coating [4] and baking of fabrication process in a not-oxidizing atmosphere [4], but the degradation was not overcome completely. In our previous researches, it has been confirmed that the migration of Eu2+ from mirror planes to spinel blocks is another essential

∗ Corresponding author. Tel.: +86 27 87195661; fax: +86 27 87195661. E-mail address: [email protected] (Z.H. Zhang). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.12.031

mechanism of thermal degradation besides the oxidation of Eu2+ to Eu3+ [5,6]. Therefore, the thermal stability would be changed by the variation of the lattice environments, which could prevent or accelerate the migration of Eu2+ . However, the correlation of the thermal degradation with the lattice environment has not been justified by any experimental evidences. In this work, different concentrations of Si4+ or B3+ were introduced into BaMgAl10 O17 :Eu2+ to change its lattice environment. In terms of their ionic radii, the doping ions would occupy the Al3+ sites within spinel blocks. The dependence of the photoluminescence besides the thermal stability on the lattice environment was systematically investigated under UV and VUV excitation. 2. Experimental BaMgAl10 O17 :Eu2+ doped with 0–6% additional Si4+ or B3+ , i.e. BaMg (Al1−x Six )10 O17+5x :Eu2+ and BaMg(Al1−y By )10 O17 :Eu2+ (0 ≤ (x, y) ≤ 0.06), were prepared by a co-precipitation process at 1300 ◦ C. Ba(NO3 )2 (AR), Mg(NO3 )2 ·6H2 O (AR), Al(NO3 )3 ·9H2 O (AR), tetraethyl orthosilicate (TEOS) (AR), H3 BO3 (AR) and Eu2 O3 (99.99%) were used as starting materials. The content of Eu2+ was 10% in mole ratio relative to Ba2+ in all samples [7]. Stoichiometric amounts of each starting materials except H3 BO3 were dissolved in nitric acid and diluted with distilled water/ethanol (2:1 in volume ratio). (NH4 )2 CO3 in large excess was added into above mixed solution under vigorous stirring at 60 ◦ C until the pH ≈ 8. The obtained slurry was filtered, washed and then dried at 110 ◦ C in air. For B3+ -doped samples, H3 BO3 was added into the dried precursor and well blended. The precursor was calcined in a reducing atmosphere (5% H2 in N2 ) at 1300 ◦ C for 4 h. The post-annealing process on the as-prepared phosphors was carried out at 500 ◦ C for 30 min in air. The as-prepared samples were determined by powder X-ray diffraction (XRD) (Rigaku D/max–2400 X-ray diffractometer with Ni-filtered Cu K˛ radiation). The emission spectra were measured at room temperature by FLS-920T fluorescence spectrophotometer with a VM-504-type vacuum monochromator. A deuterium lamp was used as the lighting source. The unit cell parameters were calculated by the

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Fig. 1. The XRD patterns of BaMgAl10 O17 :Eu2+ without doping and doped with 6% of Si4+ or B3+ , respectively.

Fig. 3. The relative emission intensity as a function of the doping contents under 147 nm excitation (em = 452 nm, before annealing).

Before annealing, the effects of the doping contents on the relative emission intensities of BaMgAl10 O17 :Eu2+ are presented in Fig. 2 (ex = 254 nm) and Fig. 3 (ex = 147 nm), respectively. The emission intensity of BaMgAl10 O17 :Eu2+ without doping was assigned to be 100. It can be detected from Figs. 2 and 3 that the emission intensity of BaMgAl10 O17 :Eu2+ could be enhanced by proper quantities of Si4+ or B3+ doping. It is clarify that the Si–O group efficiently absorbs UV and VUV lights and transfers the energy to the luminescent centers [8–11]. Moreover, it is well known that B–O group can absorb

VUV lights [12–14] and H3 BO3 as the B source also acts as flux [15] which could improve the luminescence of phosphors. Therefore, it is easy to explicit that the doping of Si4+ or B3+ could enhance the emission intensity of BaMgAl10 O17 :Eu2+ . We also noticed that the emission intensity was damaged when large concentration of Si4+ or B3+ was doped. This damage could be attributed to the lattice distortion which resulting from the differences on the ionic radii between Al3+ and Si4+ or B3+ . The emission spectrum of BaMgAl10 O17 :Eu2+ under 254 or 147 nm excitation exhibited an asymmetrical broad band with the maximum at about 452 nm. This asymmetry is due to the multiple crystalline sites for Eu2+ to occupy in BaMgAl10 O17 host [16]. According to Refs. [17,18], there are at least three different Eu2+ sites named with BR, a-BR and mO sites, which have maximal emission at about 2.79 (445 nm), 2.61 (476 nm), and 2.43 eV (511 nm), respectively. Therefore, the change of Eu2+ distribution in different sites will undoubtedly result in different features of emission spectra, such as peak position and full-width at half-maximum (FWHM), and then lead to different chromaticity coordinates. Figs. 4 and 5 represent the effects of the doping contents on FWHM of the emission spectra under 254 and 147 nm excitation, respectively. Figs. 6 and 7 show the effects of the doping contents on y value of the chromaticity coordinates under 254 and 147 nm excitation, respectively. In this study, we did not observe any changes on peak positions (452 nm) and x value of chromaticity coordinates (0.147) under 254 or 147 nm excitation. From Figs. 4–7, it can be

Fig. 2. The relative emission intensity as a function of the doping contents under 254 nm excitation (em = 452 nm, before annealing).

Fig. 4. FWHM of the emission spectra as a function of the doping contents under 254 nm excitation (before annealing).

peak top method and the least square fitting after using silicon powder as internal standard.

3. Results and discussion 3.1. XRD All XRD patterns of the as-prepared samples were well in agreement with the standard JCPDS 26-0163. Fig. 1 shows the full scale XRD patterns of BaMgAl10 O17 :Eu2+ , BaMg(Al1−x Six )10 O17+5x :Eu2+ and BaMg(Al1−y By )10 O17 :Eu2+ in which x or y = 0.06. Each pattern in Fig. 1 presents a single phase nature of BaMgAl10 O17 and it means that the doping of Si4+ or B3+ did not change the phase structure of BaMgAl10 O17 . 3.2. Luminescent properties before annealing

Z.H. Zhang et al. / Journal of Alloys and Compounds 478 (2009) 801–804

Fig. 5. FWHM of the emission spectra as a function of the doping contents under 147 nm excitation (before annealing).

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Fig. 8. The relative emission intensity as a function of the doping contents under 254 nm excitation (em = 452 nm, after annealing).

cent enhancement of Si4+ or B3+ doped samples at 452 nm was just because the above damage was cancelled out by the absorption of Si–O groups and B-O groups in UV or VUV regions, or the flux effect of H3 BO3 . 3.3. Luminescent properties after annealing

Fig. 6. y Value of the chromaticity coordinates as a function of the doping contents under 254 nm excitation (before annealing).

noted that the samples doped with Si4+ or B3+ had wider FWHM and larger y values of chromaticity coordinates than the sample without doping. It indicated that the distribution of Eu2+ was changed by the ions doping. The wider FWHM and larger y values of chromaticity coordinates meant that the luminescent centers which emitting at longer wavelength were increased in quantity by the doping. This change of distribution would undoubtedly damage the emission at shorter wavelength (about 452 nm). Well the observed lumines-

Fig. 7. y Value of the chromaticity coordinates as a function of the doping contents under 147 nm excitation (before annealing).

After annealing at 500 ◦ C for 30 min, Figs. 8 and 9 give the effects of the doping contents on the relative emission intensity under 254 and 147 nm excitation, respectively. It was obvious that the relative emission intensity had a decreasing tendency along with the increasing of the doping content of Si4+ or B3+ . This was a completely inconsistent phenomenon against the observed enhancement on the emission intensity of Si4+ or B3+ doped samples before annealing. The reason of this inconsistency could be directly correlated with the thermal degradation mechanism of BaMgAl10 O17 :Eu2+ , i.e. the migration of Eu2+ from mirror planes to spinel blocks. As far as Si4+ doping was concerned, the substitution of Si4+ for Al3+ is a substitution of higher valence cations for lower valence cations within the spinel block of BaMgAl10 O17 :Eu2+ . Considering charge compensating mechanism and lattice defect theory, the following reaction formula could occur, Al O



2 3 3SiO2 −→ 3Si•Al + VAl + 6OO

(1)

Fig. 9. The relative emission intensity as a function of the doping contents under 147 nm excitation (em = 452 nm, after annealing).

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The defects of VAl made Eu2+ easy to accommodate within the spinel block and would undoubtedly accelerate the migration of Eu2+ from mirror plane to spinel block in the annealing process. Therefore, the samples of Si4+ doped BaMgAl10 O17 :Eu2+ had worse thermal stability than the sample of BaMgAl10 O17 :Eu2+ without doping. For the case of B3+ doping, the substitution of B3+ for Al3+ is an equivalent substitution and do not need charge compensation. However, the ionic radius of B3+ (0.027 nm) is just one half of the radius of Al3+ (0.054 nm), therefore, serious lattice distortion and large lattice interspaces would be formed in this substitution and results in the bad thermal stability of the samples of B3+ doped BaMgAl10 O17 :Eu2+ . A comparison between the effects of Si4+ and B3+ doping on the luminescent intensity of BaMgAl10 O17 :Eu2+ after annealing could be detected from Figs. 8 and 9. Whenever under 254 or 147 nm excitation, the thermal degradation degree on luminescence of Si4+ doped samples was much worse than that of B3+ -doped samples with same doping concentration. This was because that the former mainly came from lattice defect and the latter just resulted from the lattice distortion. Therefore, an important conclusion could be obtained that the lattice environment was deeply influenced by different doping ions and the thermal degradation of BaMgAl10 O17 :Eu2+ had great correlation with the lattice environment.

or VUV regions, and flux effect of H3 BO3 which was the B source. The thermal stability of Si4+ doped phosphor was worse than that of undoped samples and this could be explained by the generation  of VAl which accelerating the migration of Eu2+ from mirror plane to spinel block. The B3+ doped phosphor also had worse thermal stability than the undoped one and this was due to the serious lattice distortion resulted from the large difference in ionic radius. The correlation of the thermal degradation with the lattice environment was clarified by direct experimental evidences.

4. Conclusions

[10] [11] [12]

The photoluminescence and thermal stability of BaMgAl10 O17 :Eu2+ doped with Si4+ or B3+ were investigated and compared with those of BaMgAl10 O17 :Eu2+ without doping. The experimental results revealed that proper quantities of Si4+ or B3+ doping observably enhanced the emission intensity of BaMgAl10 O17 :Eu2+ both under UV and VUV excitation. These were because of the absorption of Si–O groups and B–O groups in UV

Acknowledgment This work was supported by the Research Foundation of Education Bureau of Hubei Province, China (grant no. Q20081509). References [1] [2] [3] [4] [5] [6] [7] [8] [9]

[13] [14] [15] [16] [17] [18]

T. Jüstel, J.C. Krupa, D.U. Wiechert, J. Lumin. 93 (2001) 179. T. Jüstel, H. Nikol, Adv. Mater. 12 (2000) 527. K. Yokota, S.X. Zhang, K. Kimura, A. Sakamoto, J. Lumin. 92 (2001) 223. G. Bizarri, B. Moine, J. Lumin. 113 (2005) 199. Y.H. Wang, Z.H. Zhang, Electrochem. Solid-State Lett. 8 (2005) H97. Z.H. Zhang, Y.H. Wang, X.X. Li, Y.K. Du, W.J. Liu, J. Lumin. 122–123 (2007) 1003. Z.H. Zhang, Y.H. Wang, Mater. Sci. Forum 475–479 (2005) 1701. S. Mikoshiba, S. Shirai, S. Shinada, M. Fukushima, J. Appl. Phys. 50 (1979) 1088. K.C. Mishra, K.H. Johnson, B.G. DeBoer, J.K. Berkowitz, J. Olsen, E.A. Dale, J. Lumin. 47 (1991) 197. Y. Wang, Y. Hao, L. Yuwen, J. Alloys Compd. 425 (2006) 339. Y. Hao, Y. Wang, J. Lumin. 122–123 (2007) 1006. Y. Wang, X. Guo, T. Endo, Y. Murakami, M. Ushirozawa, J. Solid State Chem. 177 (2004) 2242. H.C. Yang, C.Y. Li, H. He, Y. Tao, J.H. Xu, Q. Su, J. Lumin. 118 (2006) 61. X.X. Li, Y.H. Wang, Y. Hao, L.L. Wang, J. Electrochem. Soc. 153 (2006) G807. S.S. Lee, H.J. Kim, S.H. Byeon, J.C. Park, D.K. Kim, Ind. Eng. Chem. Res. 44 (2005) 4300. S. Zhang, M. Kokubu, H. Fujii, H. Uchiike, J. SID 10/1 (2002) 25. K.C. Mishra, M. Raukas, A. Ellens, K.H. Johnson, J. Lumin. 96 (2002) 95. T.H. Kwon, M.S. Kang, J.P. Kim, G.J. Kim, J. SID 10/3 (2002) 241.