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Characterization of Srβ-alumina prepared by sol–gel and spray pyrolysis methods
Materials Chemistry and Physics 85 (2004) 286–293
Characterization of Sr-alumina prepared by sol–gel and spray pyrolysis methods G. Paruthimal Kalaignan a,∗ , Dae Jong Seo b , Seung Bin Park b b
a Department of Industrial Chemistry, Alagappa University, Karaikudi 630003, Tamilnadu, India Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, 373-1 Kusong-dong, Yusong-gu, Taejon 305-701, South Korea
Received 20 October 2003; received in revised form 10 December 2003; accepted 8 January 2004
Abstract Eu2+ doped -alumina, Sr 1−x MgAl10 O17 Eux 2+ (x = 0.01–0.07) were successfully prepared by sol–gel and spray pyrolysis techniques with the same precursor materials. Sr-alumina doped with Eu2+ (SrMgAl10 O17 :Eu2+ ) prepared from sol–gel method showed three photoluminescence (PL) peaks at 390, 418 and 459 nm after excitation wavelength at 254 nm and one PL peak at 461 nm when excitation was at 365 nm. The same powder was prepared from spray pyrolysis technique showed the six PL peaks at 323, 397, 415, 443, 480 and 508 nm after excitation at 254 nm. Also two PL peaks at 440 and 480 nm were observed after the excitation at 365 nm. These PL peaks were dependent on the excitation wavelength. The effect of different annealing temperatures of sol–gel powders, preparation conditions of spray pyrolysis powders and reduction atmospheres of both sol–gel and spray pyrolysis powders of various compositions of Eu2+ doped Sr-alumina were also studied. Both the powders were characterized by scanning electron microscopy, X-ray diffraction and PL techniques and comparison between the two preparation methods. Sol–gel prepared powder had eight times higher PL intensity and brightness than the spray pyrolysis prepared powder. The suggested good composition of Sr-alumina is Sr0.93 MgAl10 O17 :Eu0.07 for both sol–gel and spay pyrolysis methods. © 2004 Elsevier B.V. All rights reserved. Keywords: Sr-alumina; Sol–gel; Spray pyrolysis; SEM; Photoluminescence
1. Introduction Physical and optical properties and crystal structure configuration of -aluminas have been investigated [1–3]. -Alumina has a layer structure with the stacking sequence of conduction plane and spinel block. It is possible to incorporate a large variety of divalent, trivalent and rare earth ions into the conduction plane and the spinel block. The emissions are associated with the position of Eu2+ ions as well as the arrangement of cations and anions in the conductance plane and the spinel block of -alumina structure. There are two methods employed for the preparation of -alumina, i.e., ion exchange method starting from Na-alumina [4] and direct high temperature solid phase synthesis using stabilizers [5–7]. Matsui et al. reported the phosphorescence behavior of Eu2+ doped Sr-alumina prepared by solid phase reaction [8]. But SrAl2 O4 :Eu, dry materials has been prepared by various methods like solid ∗ Corresponding author. Tel.: +91-4565-235307; fax: +91-4565-225202. E-mail address: [email protected] (G.P. Kalaignan).
0254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphs.2004.01.008
state reaction [9], floating zone technique [10,11], pedestal growth method [12], gel method [13] and hydrothermal reaction [14]. In this paper, we report the preparation of Sr-alumina powders doped with Eu2+ by sol–gel and spray pyrolysis methods, characterization by X-ray diffraction (XRD), scanning electron microscopy (SEM) and photoluminescence (PL) techniques and comparison between the two preparation methods. From sol–gel method high purity, good homogenous and fine particles can be achieved. The sol–gel method has been greatly applied in preparing fine ceramic powders. The sol–gel method require lower processing temperatures and shorter heating times than conventional processing. They show high reproducibility, good control of stoichiometry and the size and shape of the particles obtained. Powders obtained by spray pyrolysis method was uniform spherical shape with fine micron size. In the present study, we have selected strontium acetate, magnesium acetate, europium acetate and aluminum sec-butoxide (Aldrich) as the starting precursor materials to prepare Sr-alumina. These starting materials are low cost and easy to handle.
G.P. Kalaignan et al. / Materials Chemistry and Physics 85 (2004) 286–293
2. Experimental Fig. 1 shows the preparation process for Sr-alumina (SrMgAl10 O17 :Eu) by sol–gel technique. Since the starting materials are very sensitive to moisture, the handling of chemicals and procedures were carried out in a dry nitrogen atmosphere. All the precursors were separately dissolved in dry ethanol and then mixed in a two neck round bottom flask with the addition of ethylene glycol as stabilizing agent. The ratio of ethanol and ethylene glycol was 1:0.25. The contents were refluxed at 25 ◦ C in N2 atmosphere for 7 h, yielding a homogenous solution. Then dried in an air oven at 110 ◦ C for 24 h and ground with a mortar and pestle. The powders were calcined at different temperatures ranging from 1300 to 1600 ◦ C at a heating rate of 2 ◦ C min−1 for 5 h. The crystallinity of the powders was controlled by changing the calcination temperature. The calcined powders were reduced at 1050 ◦ C for 5 h in a 2% H2 /N2 and 5% H2 /N2 gas mixtures to reduce Eu3+ to Eu2+ ions. In spray pyrolysis method, the same sol–gel process precursors were used. But all the precursors were dissolved in H2 O and HNO3 (2:1) mixture. All the mixtures were vigorously stirred by magnetic stirrer for 3 h. This clear solution was loaded in the round flask connected with the nebulizer (1.67 MHz) and the furnace with the length of 80 cm. The droplets produced by the nebulizer were carried by air of 1 l min−1 into the furnace maintaining at temperature between 950 and 1100 ◦ C. The prepared particles were collected with thimble filter (28 mm × 10 mm, Toyo Roshi Kaisha Ltd.). The powders were calcined at different temperatures ranging from 1100
Fig. 1. Schematic flow chart of the synthesis of the SrMgAl10 O17 :Eu2+ powder by sol–gel technique.
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to 1600 ◦ C at a heating rate of 2 ◦ C min−1 for 5 h. But in the first stage heating temperature was 800 ◦ C at a heating rate of 2 ◦ C min−1 for 1 h to avoid the agglomeration of the powders. The calcined powders were reduced at 1050 ◦ C for 5 h in a 2% H2 /N2 and 5% H2 /N2 mixtures to reduce Eu3+ to Eu2+ ions. Based on the above two experimental methods various compositions of Eu were doped in the powder SrMgAl10 O17 . The Eu doped powder compositions were identified and characterized by PL spectrum. The crystal structure was investigated by X-ray diffraction using a Rigaku D/MAX-III diffractometer with Cu K␣ radiation. Surface morphology and the grain structures of the powders were analyzed by SEM (Philips, XL 30S/FEG, Netherlands). Luminescent properties of the powders were analyzed by photoluminescence spectrometer (Perkin-Elmer Instruments, LS50B).
3. Results and discussion Fig. 2a–f show the SEM photographs of SrMgAl10 O17 :Eu phosphor particles prepared from sol–gel method. Most of the as prepared particles were less than 1 m size (Fig. 2a). Good crystallinity as manifested by shape, were obtained for particles calcined at 1600 ◦ C at a heating rate of 10 ◦ C min−1 for 5 h (Fig. 2d). In the case of two step calcination at 800 ◦ C, 1 h and 1600 ◦ C, 5 h at a heating rate of 2 ◦ C min−1 , the grain size of the particles were increased to microns (Fig. 2e). It suggests that the heating rate of calcination give a large influence on the shape and size of the particles. After reduction, irregular shaped particles with different grain sizes were present (Fig. 2f). Fig. 3a–e show the SEM photographs of SrMgAl10 O17 :Eu phosphor particles prepared from spray pyrolysis method. All the as prepared particles were spherical in shape (Fig. 3a and b). Particles calcined at 800 ◦ C, 1 h and 1100 ◦ C, 5 h at a heating rate of 2 ◦ C min−1 showed no change in spherical shape of the particles (Fig. 3d). Spherical shape has been changed to irregular shape after two step calcination at 800 ◦ C, 1 h and 1600 ◦ C, 5 h at a heating rate of 2 ◦ C min−1 (Fig. 3c and e). After reduction, there was no change in size and shape of the phosphor particles. Compared with spray pyrolysis, as prepared sol–gel particles were smaller in size. But in two step calcined sol–gel particles were bigger in size than the spray pyrolysis (Figs. 2e, 3c and e). Figs. 4 and 5 show the XRD patterns for SrMgAl10 O17 :Eu phosphor powders prepared from sol–gel and spray pyrolysis methods, respectively. These XRD patterns showed the exact composition of the powders prepared (SrMgAl10 O17 ), which is consistent with the results reported previously [8] for the same powder prepared by solid state reaction method. The lattice parameters of the hexagonal SrMgAl10 O17 powder prepared by sol–gel method are a = 5.61, c = 22.45 and spray pyrolysis method are a = 5.62, c = 22.46. These
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Fig. 2. The SEM of SrMgAl10 O17 :Eu phosphor powders prepared by sol–gel method: (a) as prepared, (b) calcination at 1300 ◦ C, (c) calcination at 1500 ◦ C, (d) calcination at 1600 ◦ C, (e) two step calcination at 800 and 1600 ◦ C, (f) after reduction at 1050 ◦ C.
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Fig. 3. The SEM of SrMgAl10 O17 :Eu phosphor powders prepared by spray pyrolysis method: (a) as prepared at 950 ◦ C (sample-1), (b) as prepared at 1100 ◦ C (sample-2), (c) after calcination of (sample-1) at 800 and 1600 ◦ C, (d) after calcination of (sample-2) at 800 and 1100 ◦ C, (e) after calcination of (sample-2) at 800 and 1600 ◦ C.
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Fig. 4. XRD patterns for SrMgAl10 O17 :Eu powders prepared from (a) solid phase reaction, (b) sol–gel method.
lattice parameters are consistent with the JCPDS no. 26-0879 for the same phosphor powder. Fig. 6 shows the PL spectrum excited at a wavelength of 254 nm for sol–gel prepared SrMgAl10 O17 :Eu powder reduced with N2 –2% H2 . It showed three PL peaks at 390, 418 and 459 nm after excitation wavelength at 254 nm. When the
Fig. 5. XRD patterns for SrMgAl10 O17 :Eu powders prepared from (a) solid phase reaction, (b) spray pyrolysis method.
composition of Eu increases from 0.01 to 0.07, the strong and well defined PL peak also shifted from 453 to 459 nm. But higher intensity and brightness were obtained for 0.07 and 0.03 Eu doped in Sr-alumina. The excitation spectrum of -alumina has many peaks which are clearly separated between the high excitation band centered at 254 nm and low band at 365 nm. The PL spectrum was found to be greatly
Fig. 6. PL spectrum of sol–gel prepared SrMgAl10 O17 :Eu powder reduced with N2 –2% H2 and excited at a wavelength of 254 nm.
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Fig. 7. PL spectrum of sol–gel prepared SrMgAl10 O17 :Eu powder reduced with N2 –2% H2 and excited at a wavelength of 365 nm.
dependent on the excitation wavelength. Fig. 7 shows the PL spectrum excited at a wavelength of 365 nm for sol–gel prepared SrMgAl10 O17 :Eu powder reduced with N2 –2% H2 . The well defined, broad PL peak was observed between 454 and 461 nm at 365 nm excitation wavelength with doped Eu composition varies from 0.01 to 0.07. Higher intensity was obtained for 0.07 and 0.03 Eu doped in Sr-alumina. Tamatani [2] suggested that -aluminas contained more than one type of Eu2+ center based on the results of the Eu2+ concentration dependence and the difference between the PL spectra excited at 254 and 365 nm. Matsui et al. [8] pro-
posed that three types of luminescence centers of Eu2+ exist in SrMgAl10 O17 , of which two sites are responsible for the green emissions observed during phosphorescence and the other produced the blue emission in fluorescence. Luminescent properties are greatly dependent on the grain size, shape, preparation method and conditions. When the particle size reaches less than 1 m, the luminescent materials will show some attractive properties. Fig. 8 represents the PL spectrum of sol–gel prepared Sr0.93 MgAl10 O17 :Eu0.07 powder with calcination temperature 1600 ◦ C at a heating rate of 10 ◦ C min−1 and two step
Fig. 8. PL spectrum of sol–gel prepared Sr0.93 MgAl10 O17 :Eu0.07 powder with different calcination temperature, rate of heating and various N2 –H2 reduction gas mixture at 254 nm.
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Fig. 9. PL spectrum of spray pyrolysis prepared at 950 and 1100 ◦ C Sr0.93 MgAl10 O17 :Eu0.07 powder at 254 and 365 nm.
calcination temperature 800 and 1600 ◦ C at a heating rate of 2 ◦ C min−1 and reduced with N2 –2% H2 or N2 –5% H2 gas mixtures at 254 nm. It showed that one low intensity PL peak in the range of 390–396 nm and another high intensity, well defined PL peak range from 458 to 459 nm. Based on the above results, single step calcination temperature 1600 ◦ C at a heating rate of 10 ◦ C min−1 and reduced with N2 –2% H2 gas mixture condition was suitable for sol–gel prepared Sr-alumina powder. Fig. 9 shows the PL spectrum of Sr0.93 MgAl10 O17 :Eu0.07 powder prepared by spray pyrolysis at 950 and 1100 ◦ C at 254 and 365 nm excitation. It showed six PL peaks of low intensity with broad spectrum at 323, 397, 415, 443, 480 and 508 nm after excitation wavelength at 254 nm. The photoluminescence properties of the Sr-alumina:Eu pow-
der depend on the ionization state of Eu, and the spectrum in Eu3+ state has several weak peaks [15]. This may be due to the incomplete reduction of Eu3+ to Eu2+ ions. In the case of excitation wavelength at 365 nm, two PL peaks of very low intensity with broad spectrum at 440 and 480 nm were obtained. Spray pyrolysis powder prepared at 1100 ◦ C was having higher intensity than the 950 ◦ C. Fig. 10 represents the PL spectrum of spray pyrolysis prepared Sr0.93 MgAl10 O17 :Eu0.07 powder at N2 –2% H2 and N2 –5% H2 reduction gas mixtures at 254 nm. It showed six PL peaks of low intensity with broad spectrum at 398, 415, 448, 480, 508 and 528 nm. Based on the above results, N2 –2% H2 reduction gas mixture was suitable for the spray pyrolysis prepared Sr-alumina powder. Sol–gel prepared Sr0.93 MgAl10 O17 :Eu0.07 powder was having eight times higher PL intensity and brightness than the spray pyrolysis prepared powder.
4. Conclusions
Fig. 10. PL spectrum of spray pyrolysis Sr0.93 MgAl10 O17 :Eu0.07 powder prepared at 1100 ◦ C with various N2 –H2 reduction gas mixture at 254 nm.
SrMgAl10 O17 :Eu powder was prepared by sol–gel and spray pyrolysis methods with the same precursor solutions. As prepared sol–gel powder was less than 1 m size with irregular shape. But the spray pyrolysis powder was less than 1 m size to slightly higher microns with spherical shape. The good condition based on the crystallinity and fineness for the sol–gel powder preparation was 1600 ◦ C calcination temperature for 5 h at a heating rate of 10 ◦ C min−1 and reduced at 1050 ◦ C for 5 h in N2 –2% H2 gas mixture. Whereas the best condition for the spray pyrolysis powder prepared at 1100 ◦ C was calcined at 800 ◦ C, 1 h and 1600 ◦ C, 5 h at a heating rate of 2 ◦ C min−1 and reduced at 1050 ◦ C for 5 h in N2 –2% H2 gas mixture. PL spectrum of sol–gel prepared
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powder showed two low intensity peaks and one broad, well defined, high intensity peak at 254 nm excitation and one broad well defined peak at 365 nm. PL spectrum of spray pyrolysis prepared powder showed six low intensity peaks and two very low intensity peaks at the excitation wavelength of 254 and 365 nm, respectively. Sol–gel prepared powder had eight times higher PL intensity and brightness than the spray pyrolysis prepared powder. The suggested good composition of Sr-alumina is Sr0.93 MgAl10 O17 :Eu0.07 for both sol–gel and spay pyrolysis methods. Sol–gel method is best for Sr-alumina preparation compared to spray pyrolysis. Acknowledgements The authors wish to thank the Brain Korea 21 Programme of the Ministry of Science and Technology of South Korea in 2002 for financial support. References [1] A.L.N. Stevels, A.D.M. Schrama-de pauw, J. Electrochem. Soc. 123 (5) (1976) 691.
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[2] M. Tamatani, . Jpn. J. Appl. Phys. 13 (1974) 950. [3] P. Bicchi, M. Meucci, M. Tonelli, M. Montagna, J. Electrochem. Soc. 138 (1991) 3509. [4] G.W. Schafer, W. Weppner, Solid State Ionics 53–56 (1991) 559. [5] G.W. Schafer, A.V. Zyl, W. Weppner, Solid State Ionics 40–41 (1991) 154. [6] S. Yamaguchi, K. Kimura, M. Tange, Y. Iguchi, A. Imari, Solid State Ionics 26 (1988) 183. [7] S. Yamaguchi, A. Imari, Y. Iguchi, Solid State Ionics 40–41 (1990) 87. [8] H. Matsui, C.N. Xu, T. Watanabe, M. Akiyama, X.G. Zheng, J. Electrochem. Soc. 147 (12) (2000) 4692. [9] T. Matsuzawa, Y. Aoki, N. Takeuchi, Y. Murayama, J. Electrochem. Soc. 143 (8) (1996) 2670. [10] T. Katsumata, T. Nabae, K. Sasajima, T. Matsuzawa, J. Crystal Growth 183 (1998) 361. [11] T. Katsumata, K. Sasajima, T. Nabae, S. Komuro, T. Morkikawa, J. Am. Ceram. Soc. 81 (2) (1998) 413. [12] W. Jia, H. Yuan, L. Lu, H. Liu, W.M. Yen, J. Lumin. 76–77 (1998) 424. [13] T. Zilong, Z. Feng, Z. Zhongtai, H. Chuanyong, L. Yuanhua, J. Eur. Ceram. Soc. 20 (2000) 2129. [14] T.R.N. Kutty, R. Jagannathan, R.P. Rao, Mater. Res. Bull. 25 (1990) 1355. [15] K. Kato, I. Tsutai, T. Kamimura, F. Kaneko, K. Shinbo, M. Ohta, T. Kawakami, J. Lumin. 82 (1999) 213.
Characterization of Sr-alumina prepared by sol–gel and spray pyrolysis methods G. Paruthimal Kalaignan a,∗ , Dae Jong Seo b , Seung Bin Park b b
a Department of Industrial Chemistry, Alagappa University, Karaikudi 630003, Tamilnadu, India Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, 373-1 Kusong-dong, Yusong-gu, Taejon 305-701, South Korea
Received 20 October 2003; received in revised form 10 December 2003; accepted 8 January 2004
Abstract Eu2+ doped -alumina, Sr 1−x MgAl10 O17 Eux 2+ (x = 0.01–0.07) were successfully prepared by sol–gel and spray pyrolysis techniques with the same precursor materials. Sr-alumina doped with Eu2+ (SrMgAl10 O17 :Eu2+ ) prepared from sol–gel method showed three photoluminescence (PL) peaks at 390, 418 and 459 nm after excitation wavelength at 254 nm and one PL peak at 461 nm when excitation was at 365 nm. The same powder was prepared from spray pyrolysis technique showed the six PL peaks at 323, 397, 415, 443, 480 and 508 nm after excitation at 254 nm. Also two PL peaks at 440 and 480 nm were observed after the excitation at 365 nm. These PL peaks were dependent on the excitation wavelength. The effect of different annealing temperatures of sol–gel powders, preparation conditions of spray pyrolysis powders and reduction atmospheres of both sol–gel and spray pyrolysis powders of various compositions of Eu2+ doped Sr-alumina were also studied. Both the powders were characterized by scanning electron microscopy, X-ray diffraction and PL techniques and comparison between the two preparation methods. Sol–gel prepared powder had eight times higher PL intensity and brightness than the spray pyrolysis prepared powder. The suggested good composition of Sr-alumina is Sr0.93 MgAl10 O17 :Eu0.07 for both sol–gel and spay pyrolysis methods. © 2004 Elsevier B.V. All rights reserved. Keywords: Sr-alumina; Sol–gel; Spray pyrolysis; SEM; Photoluminescence
1. Introduction Physical and optical properties and crystal structure configuration of -aluminas have been investigated [1–3]. -Alumina has a layer structure with the stacking sequence of conduction plane and spinel block. It is possible to incorporate a large variety of divalent, trivalent and rare earth ions into the conduction plane and the spinel block. The emissions are associated with the position of Eu2+ ions as well as the arrangement of cations and anions in the conductance plane and the spinel block of -alumina structure. There are two methods employed for the preparation of -alumina, i.e., ion exchange method starting from Na-alumina [4] and direct high temperature solid phase synthesis using stabilizers [5–7]. Matsui et al. reported the phosphorescence behavior of Eu2+ doped Sr-alumina prepared by solid phase reaction [8]. But SrAl2 O4 :Eu, dry materials has been prepared by various methods like solid ∗ Corresponding author. Tel.: +91-4565-235307; fax: +91-4565-225202. E-mail address: [email protected] (G.P. Kalaignan).
0254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphs.2004.01.008
state reaction [9], floating zone technique [10,11], pedestal growth method [12], gel method [13] and hydrothermal reaction [14]. In this paper, we report the preparation of Sr-alumina powders doped with Eu2+ by sol–gel and spray pyrolysis methods, characterization by X-ray diffraction (XRD), scanning electron microscopy (SEM) and photoluminescence (PL) techniques and comparison between the two preparation methods. From sol–gel method high purity, good homogenous and fine particles can be achieved. The sol–gel method has been greatly applied in preparing fine ceramic powders. The sol–gel method require lower processing temperatures and shorter heating times than conventional processing. They show high reproducibility, good control of stoichiometry and the size and shape of the particles obtained. Powders obtained by spray pyrolysis method was uniform spherical shape with fine micron size. In the present study, we have selected strontium acetate, magnesium acetate, europium acetate and aluminum sec-butoxide (Aldrich) as the starting precursor materials to prepare Sr-alumina. These starting materials are low cost and easy to handle.
G.P. Kalaignan et al. / Materials Chemistry and Physics 85 (2004) 286–293
2. Experimental Fig. 1 shows the preparation process for Sr-alumina (SrMgAl10 O17 :Eu) by sol–gel technique. Since the starting materials are very sensitive to moisture, the handling of chemicals and procedures were carried out in a dry nitrogen atmosphere. All the precursors were separately dissolved in dry ethanol and then mixed in a two neck round bottom flask with the addition of ethylene glycol as stabilizing agent. The ratio of ethanol and ethylene glycol was 1:0.25. The contents were refluxed at 25 ◦ C in N2 atmosphere for 7 h, yielding a homogenous solution. Then dried in an air oven at 110 ◦ C for 24 h and ground with a mortar and pestle. The powders were calcined at different temperatures ranging from 1300 to 1600 ◦ C at a heating rate of 2 ◦ C min−1 for 5 h. The crystallinity of the powders was controlled by changing the calcination temperature. The calcined powders were reduced at 1050 ◦ C for 5 h in a 2% H2 /N2 and 5% H2 /N2 gas mixtures to reduce Eu3+ to Eu2+ ions. In spray pyrolysis method, the same sol–gel process precursors were used. But all the precursors were dissolved in H2 O and HNO3 (2:1) mixture. All the mixtures were vigorously stirred by magnetic stirrer for 3 h. This clear solution was loaded in the round flask connected with the nebulizer (1.67 MHz) and the furnace with the length of 80 cm. The droplets produced by the nebulizer were carried by air of 1 l min−1 into the furnace maintaining at temperature between 950 and 1100 ◦ C. The prepared particles were collected with thimble filter (28 mm × 10 mm, Toyo Roshi Kaisha Ltd.). The powders were calcined at different temperatures ranging from 1100
Fig. 1. Schematic flow chart of the synthesis of the SrMgAl10 O17 :Eu2+ powder by sol–gel technique.
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to 1600 ◦ C at a heating rate of 2 ◦ C min−1 for 5 h. But in the first stage heating temperature was 800 ◦ C at a heating rate of 2 ◦ C min−1 for 1 h to avoid the agglomeration of the powders. The calcined powders were reduced at 1050 ◦ C for 5 h in a 2% H2 /N2 and 5% H2 /N2 mixtures to reduce Eu3+ to Eu2+ ions. Based on the above two experimental methods various compositions of Eu were doped in the powder SrMgAl10 O17 . The Eu doped powder compositions were identified and characterized by PL spectrum. The crystal structure was investigated by X-ray diffraction using a Rigaku D/MAX-III diffractometer with Cu K␣ radiation. Surface morphology and the grain structures of the powders were analyzed by SEM (Philips, XL 30S/FEG, Netherlands). Luminescent properties of the powders were analyzed by photoluminescence spectrometer (Perkin-Elmer Instruments, LS50B).
3. Results and discussion Fig. 2a–f show the SEM photographs of SrMgAl10 O17 :Eu phosphor particles prepared from sol–gel method. Most of the as prepared particles were less than 1 m size (Fig. 2a). Good crystallinity as manifested by shape, were obtained for particles calcined at 1600 ◦ C at a heating rate of 10 ◦ C min−1 for 5 h (Fig. 2d). In the case of two step calcination at 800 ◦ C, 1 h and 1600 ◦ C, 5 h at a heating rate of 2 ◦ C min−1 , the grain size of the particles were increased to microns (Fig. 2e). It suggests that the heating rate of calcination give a large influence on the shape and size of the particles. After reduction, irregular shaped particles with different grain sizes were present (Fig. 2f). Fig. 3a–e show the SEM photographs of SrMgAl10 O17 :Eu phosphor particles prepared from spray pyrolysis method. All the as prepared particles were spherical in shape (Fig. 3a and b). Particles calcined at 800 ◦ C, 1 h and 1100 ◦ C, 5 h at a heating rate of 2 ◦ C min−1 showed no change in spherical shape of the particles (Fig. 3d). Spherical shape has been changed to irregular shape after two step calcination at 800 ◦ C, 1 h and 1600 ◦ C, 5 h at a heating rate of 2 ◦ C min−1 (Fig. 3c and e). After reduction, there was no change in size and shape of the phosphor particles. Compared with spray pyrolysis, as prepared sol–gel particles were smaller in size. But in two step calcined sol–gel particles were bigger in size than the spray pyrolysis (Figs. 2e, 3c and e). Figs. 4 and 5 show the XRD patterns for SrMgAl10 O17 :Eu phosphor powders prepared from sol–gel and spray pyrolysis methods, respectively. These XRD patterns showed the exact composition of the powders prepared (SrMgAl10 O17 ), which is consistent with the results reported previously [8] for the same powder prepared by solid state reaction method. The lattice parameters of the hexagonal SrMgAl10 O17 powder prepared by sol–gel method are a = 5.61, c = 22.45 and spray pyrolysis method are a = 5.62, c = 22.46. These
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Fig. 2. The SEM of SrMgAl10 O17 :Eu phosphor powders prepared by sol–gel method: (a) as prepared, (b) calcination at 1300 ◦ C, (c) calcination at 1500 ◦ C, (d) calcination at 1600 ◦ C, (e) two step calcination at 800 and 1600 ◦ C, (f) after reduction at 1050 ◦ C.
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Fig. 3. The SEM of SrMgAl10 O17 :Eu phosphor powders prepared by spray pyrolysis method: (a) as prepared at 950 ◦ C (sample-1), (b) as prepared at 1100 ◦ C (sample-2), (c) after calcination of (sample-1) at 800 and 1600 ◦ C, (d) after calcination of (sample-2) at 800 and 1100 ◦ C, (e) after calcination of (sample-2) at 800 and 1600 ◦ C.
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Fig. 4. XRD patterns for SrMgAl10 O17 :Eu powders prepared from (a) solid phase reaction, (b) sol–gel method.
lattice parameters are consistent with the JCPDS no. 26-0879 for the same phosphor powder. Fig. 6 shows the PL spectrum excited at a wavelength of 254 nm for sol–gel prepared SrMgAl10 O17 :Eu powder reduced with N2 –2% H2 . It showed three PL peaks at 390, 418 and 459 nm after excitation wavelength at 254 nm. When the
Fig. 5. XRD patterns for SrMgAl10 O17 :Eu powders prepared from (a) solid phase reaction, (b) spray pyrolysis method.
composition of Eu increases from 0.01 to 0.07, the strong and well defined PL peak also shifted from 453 to 459 nm. But higher intensity and brightness were obtained for 0.07 and 0.03 Eu doped in Sr-alumina. The excitation spectrum of -alumina has many peaks which are clearly separated between the high excitation band centered at 254 nm and low band at 365 nm. The PL spectrum was found to be greatly
Fig. 6. PL spectrum of sol–gel prepared SrMgAl10 O17 :Eu powder reduced with N2 –2% H2 and excited at a wavelength of 254 nm.
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Fig. 7. PL spectrum of sol–gel prepared SrMgAl10 O17 :Eu powder reduced with N2 –2% H2 and excited at a wavelength of 365 nm.
dependent on the excitation wavelength. Fig. 7 shows the PL spectrum excited at a wavelength of 365 nm for sol–gel prepared SrMgAl10 O17 :Eu powder reduced with N2 –2% H2 . The well defined, broad PL peak was observed between 454 and 461 nm at 365 nm excitation wavelength with doped Eu composition varies from 0.01 to 0.07. Higher intensity was obtained for 0.07 and 0.03 Eu doped in Sr-alumina. Tamatani [2] suggested that -aluminas contained more than one type of Eu2+ center based on the results of the Eu2+ concentration dependence and the difference between the PL spectra excited at 254 and 365 nm. Matsui et al. [8] pro-
posed that three types of luminescence centers of Eu2+ exist in SrMgAl10 O17 , of which two sites are responsible for the green emissions observed during phosphorescence and the other produced the blue emission in fluorescence. Luminescent properties are greatly dependent on the grain size, shape, preparation method and conditions. When the particle size reaches less than 1 m, the luminescent materials will show some attractive properties. Fig. 8 represents the PL spectrum of sol–gel prepared Sr0.93 MgAl10 O17 :Eu0.07 powder with calcination temperature 1600 ◦ C at a heating rate of 10 ◦ C min−1 and two step
Fig. 8. PL spectrum of sol–gel prepared Sr0.93 MgAl10 O17 :Eu0.07 powder with different calcination temperature, rate of heating and various N2 –H2 reduction gas mixture at 254 nm.
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Fig. 9. PL spectrum of spray pyrolysis prepared at 950 and 1100 ◦ C Sr0.93 MgAl10 O17 :Eu0.07 powder at 254 and 365 nm.
calcination temperature 800 and 1600 ◦ C at a heating rate of 2 ◦ C min−1 and reduced with N2 –2% H2 or N2 –5% H2 gas mixtures at 254 nm. It showed that one low intensity PL peak in the range of 390–396 nm and another high intensity, well defined PL peak range from 458 to 459 nm. Based on the above results, single step calcination temperature 1600 ◦ C at a heating rate of 10 ◦ C min−1 and reduced with N2 –2% H2 gas mixture condition was suitable for sol–gel prepared Sr-alumina powder. Fig. 9 shows the PL spectrum of Sr0.93 MgAl10 O17 :Eu0.07 powder prepared by spray pyrolysis at 950 and 1100 ◦ C at 254 and 365 nm excitation. It showed six PL peaks of low intensity with broad spectrum at 323, 397, 415, 443, 480 and 508 nm after excitation wavelength at 254 nm. The photoluminescence properties of the Sr-alumina:Eu pow-
der depend on the ionization state of Eu, and the spectrum in Eu3+ state has several weak peaks [15]. This may be due to the incomplete reduction of Eu3+ to Eu2+ ions. In the case of excitation wavelength at 365 nm, two PL peaks of very low intensity with broad spectrum at 440 and 480 nm were obtained. Spray pyrolysis powder prepared at 1100 ◦ C was having higher intensity than the 950 ◦ C. Fig. 10 represents the PL spectrum of spray pyrolysis prepared Sr0.93 MgAl10 O17 :Eu0.07 powder at N2 –2% H2 and N2 –5% H2 reduction gas mixtures at 254 nm. It showed six PL peaks of low intensity with broad spectrum at 398, 415, 448, 480, 508 and 528 nm. Based on the above results, N2 –2% H2 reduction gas mixture was suitable for the spray pyrolysis prepared Sr-alumina powder. Sol–gel prepared Sr0.93 MgAl10 O17 :Eu0.07 powder was having eight times higher PL intensity and brightness than the spray pyrolysis prepared powder.
4. Conclusions
Fig. 10. PL spectrum of spray pyrolysis Sr0.93 MgAl10 O17 :Eu0.07 powder prepared at 1100 ◦ C with various N2 –H2 reduction gas mixture at 254 nm.
SrMgAl10 O17 :Eu powder was prepared by sol–gel and spray pyrolysis methods with the same precursor solutions. As prepared sol–gel powder was less than 1 m size with irregular shape. But the spray pyrolysis powder was less than 1 m size to slightly higher microns with spherical shape. The good condition based on the crystallinity and fineness for the sol–gel powder preparation was 1600 ◦ C calcination temperature for 5 h at a heating rate of 10 ◦ C min−1 and reduced at 1050 ◦ C for 5 h in N2 –2% H2 gas mixture. Whereas the best condition for the spray pyrolysis powder prepared at 1100 ◦ C was calcined at 800 ◦ C, 1 h and 1600 ◦ C, 5 h at a heating rate of 2 ◦ C min−1 and reduced at 1050 ◦ C for 5 h in N2 –2% H2 gas mixture. PL spectrum of sol–gel prepared
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powder showed two low intensity peaks and one broad, well defined, high intensity peak at 254 nm excitation and one broad well defined peak at 365 nm. PL spectrum of spray pyrolysis prepared powder showed six low intensity peaks and two very low intensity peaks at the excitation wavelength of 254 and 365 nm, respectively. Sol–gel prepared powder had eight times higher PL intensity and brightness than the spray pyrolysis prepared powder. The suggested good composition of Sr-alumina is Sr0.93 MgAl10 O17 :Eu0.07 for both sol–gel and spay pyrolysis methods. Sol–gel method is best for Sr-alumina preparation compared to spray pyrolysis. Acknowledgements The authors wish to thank the Brain Korea 21 Programme of the Ministry of Science and Technology of South Korea in 2002 for financial support. References [1] A.L.N. Stevels, A.D.M. Schrama-de pauw, J. Electrochem. Soc. 123 (5) (1976) 691.
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[2] M. Tamatani, . Jpn. J. Appl. Phys. 13 (1974) 950. [3] P. Bicchi, M. Meucci, M. Tonelli, M. Montagna, J. Electrochem. Soc. 138 (1991) 3509. [4] G.W. Schafer, W. Weppner, Solid State Ionics 53–56 (1991) 559. [5] G.W. Schafer, A.V. Zyl, W. Weppner, Solid State Ionics 40–41 (1991) 154. [6] S. Yamaguchi, K. Kimura, M. Tange, Y. Iguchi, A. Imari, Solid State Ionics 26 (1988) 183. [7] S. Yamaguchi, A. Imari, Y. Iguchi, Solid State Ionics 40–41 (1990) 87. [8] H. Matsui, C.N. Xu, T. Watanabe, M. Akiyama, X.G. Zheng, J. Electrochem. Soc. 147 (12) (2000) 4692. [9] T. Matsuzawa, Y. Aoki, N. Takeuchi, Y. Murayama, J. Electrochem. Soc. 143 (8) (1996) 2670. [10] T. Katsumata, T. Nabae, K. Sasajima, T. Matsuzawa, J. Crystal Growth 183 (1998) 361. [11] T. Katsumata, K. Sasajima, T. Nabae, S. Komuro, T. Morkikawa, J. Am. Ceram. Soc. 81 (2) (1998) 413. [12] W. Jia, H. Yuan, L. Lu, H. Liu, W.M. Yen, J. Lumin. 76–77 (1998) 424. [13] T. Zilong, Z. Feng, Z. Zhongtai, H. Chuanyong, L. Yuanhua, J. Eur. Ceram. Soc. 20 (2000) 2129. [14] T.R.N. Kutty, R. Jagannathan, R.P. Rao, Mater. Res. Bull. 25 (1990) 1355. [15] K. Kato, I. Tsutai, T. Kamimura, F. Kaneko, K. Shinbo, M. Ohta, T. Kawakami, J. Lumin. 82 (1999) 213.