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Gd2O3:Eu phosphor particles prepared from spray solution containing boric acid flux and polymeric precursor by spray pyrolysis
Optical Materials 28 (2006) 530–535 www.elsevier.com/locate/optmat
Gd2O3:Eu phosphor particles prepared from spray solution containing boric acid flux and polymeric precursor by spray pyrolysis Dae Soo Jung, Seung Kwon Hong, Hyo Jin Lee, Yun Chan Kang
*
Department of Chemical Engineering, Konkuk University, 1 Hwayang-dong, Gwangjin-gu, Seoul 143-701, South Korea Received 5 October 2004; accepted 24 March 2005 Available online 1 June 2005
Abstract Micron size Gd2O3:Eu phosphor particles with spherical shape and dense morphology were prepared from spray solution containing boric acid, citric acid and polyethylene glycol by spray pyrolysis. Boric acid used as flux material was effective in improving the photoluminescence intensity of micron size Gd2O3:Eu phosphor particles prepared by spray pyrolysis. Boric acid did not affect on the morphology of the phosphor particles prepared from spray solution containing citric acid and polyethylene glycol after posttreatment at high temperature. The optimum content of boric acid in the preparation of Gd2O3:Eu phosphor particles from spray solution containing polymeric precursors by spray pyrolysis was 1 wt.%. The optimum post-treatment temperature of Gd2O3:Eu phosphor particles showing the maximum photoluminescence intensity was 1050 °C. The Gd2O3:Eu phosphor particles post-treated at 1050 °C had complete spherical shape, non-aggregation characteristics and high crystallinity. The photoluminescence intensity of the Gd2O3:Eu phosphor particles prepared from spray solution containing polymeric precursors and boric acid flux was 149% of the phosphor particles prepared from spray solution without polymeric precursor and boric acid flux. Ó 2005 Elsevier B.V. All rights reserved. PACS: 81.20.Rg; 78.55.Hx; 78.40.Ha; 81.05.Hd; 81.40.Tv Keywords: Phosphor; Spray pyrolysis; Flux material; Functional ceramic
1. Introduction Phosphors are required to have good luminescence properties like high luminescence efficiency, suitable emitting colors, and proper decay time. In addition to the luminescence properties, they should have good screening properties to produce phosphor layers with good uniformity, high packing density and proper adhesion strength to the substrate for practical application. The screening properties depend on many factors such
*
Corresponding author. Tel.: +82 2 2049 6010; fax: +82 2 458 3504. E-mail address: [email protected] (Y.C. Kang).
0925-3467/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2005.03.014
as chemical stability, particle size, particle size distribution, surface properties and particle shape [1]. The characteristics of phosphor particles are strongly affected by synthetic process [2–4]. Spray pyrolysis is one of the promising processes for the preparation of improved phosphor particles. In this method, a misted stream of precursor solution is dried, precipitated, and decomposed in a tubular furnace reactor continuously [5–9]. Phosphor particles synthesized by spray pyrolysis have relative uniformity in size and composition, spherical shape, fine size, and non-aggregation characteristics because of the micro-scale reaction within a droplet and the lack of milling process. However, despite these advantages, phosphor particles prepared by spray
D.S. Jung et al. / Optical Materials 28 (2006) 530–535
pyrolysis suffer from problems such as hollowness and high porosity, which cause the reduction in brightness and long-term stability. In large-scale spray pyrolysis for mass production, the prepared particles tend to have further hollow and/or porous structure because the severe operation conditions such as high solution concentration, short residence time within reactor, and high temperature variation induced by large-size tubular reactor. Thus, a new technique is required to synthesize spherical phosphor powders with solid structure by a large-scale spray pyrolysis. In the spray pyrolysis, polymeric precursors and flux material were separately used to the preparation of oxide phosphor particles. Polymeric precursors were introduced to control the spherical morphology with micron size phosphor particles in the spray pyrolysis [9]. The esterification reaction between carboxyl group in citric acid and hydroxyl group in ethylene glycol or polyethylene glycol within droplet forms highly viscous gel consisting of a three-dimensional network of polymer. The viscous gel promoted the volume precipitation and resulted in the formation of the particles with a spherical shape, filled morphology, and non-aggregation characteristics. In conventional solid state reaction and liquid solution methods, flux is used to improve wetting of reactants and to form well-crystallized particles. The flux also plays an important role in the control of mean size, size distribution, and shape of phosphor particles [10–14]. Fluxes are usually compounds of alkali- or alkaline earth metals having low melting points. In the spray pyrolysis, flux materials were used to improve the brightness of spherical shape phosphor particles with micron size [15]. In the previous work, polymeric precursors and flux material were simultaneously used to prepare the redemitting phosphor particles in the spray pyrolysis [16]. The as-prepared Gd2O3:Eu phosphor particles prepared from spray solution containing polymeric precursors and lithium carbonate flux material by spray pyrolysis had spherical shape and filled morphology. The asprepared particles had micron size because one particle was obtained from one droplet in gas phase reaction. However, the morphology of the as-prepared Gd2O3:Eu phosphor particles prepared from spray solution containing polymeric precursors and lithium carbonate flux changed after post-treatment at high temperature. The spherical shape of the as-prepared particles disappeared and nano-sized Gd2O3:Eu phosphor particles were obtained after post-treatment. In this work, Gd2O3:Eu phosphor particles with micron size, spherical shape and high brightness were prepared from spray solution containing polymeric precursor and flux material by spray pyrolysis. Citric acid and polyethylene glycol as polymeric precursors and boric acid as flux material were simultaneously used to prepare the red-emitting Gd2O3:Eu phosphor particles in the spray pyrolysis.
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2. Experimental Spherical shape Gd2O3:Eu phosphor particles were prepared by ultrasonic spray pyrolysis from spray solution with polymeric precursors and flux materials. Spray solution were prepared by dissolving gadolinium nitrate, europium nitrate, citric acid, polyethylene glycol and flux material. Citric acid and polyethylene glycol (M.W. 400) were used as polymeric precursors. Various types of flux materials including boric acid, alkaline earth metal carbonate and chloride were studied as flux materials for producing the high brightness Gd2O3:Eu phosphor particles. The total concentration of metal components was 0.5 M. The concentration of citric acid and polyethylene glycol used as polymeric precursors was 0.2 M, respectively. The content of flux material was varied from 0 wt.% to 5 wt.% of Gd2O3:Eu phosphor particles. The spray solution was atomized into micron size droplets by ultrasonic spray generator with six resonators. The flow rate of air used as a carrier gas was 45 L/min and the residence time of the particles inside the reactor was 0.6 s. The as-prepared particles were obtained at 900 °C by ultrasonic spray pyrolysis. The asprepared particles with spherical shape and micron size were post-treated at various temperatures from 700 to 1200 °C for 3 h in muffle furnace. The crystal structures of the particles were studied by X-ray diffraction (XRD, RIGAKU, D/MAX-RB) with Cu-K radiation (k = 1.5418). The morphologies of particles were investigated using scanning electron microscopy (SEM). Photoluminescence measurement was performed with spectrofluorophotometer (SHIMADZU, RF-5301PC) using a Xe lamp excitation source.
3. Results and discussion Various types of flux materials including boric acid, lithium carbonate, sodium carbonate, potassium carbonate and lithium chloride were tried to the preparation of Gd2O3:Eu phosphor particles with spherical shape and dense morphology. In the large-scale ultrasonic spray pyrolysis process, polymeric precursors such as citric acid and ethylene glycol (or polyethylene glycol) were necessary to obtain the red-emitting Gd2O3:Eu phosphor particles with spherical shape and dense morphology. However, the combination of polymeric precursor and flux material as the additives into spray solution caused the change of the morphology of the Gd2O3:Eu phosphor particles. In the present work, applying the flux materials with the exception of boric acid in spray solution including polymeric precursor destroyed the spherical shape of the as-prepared particles obtained by spray pyrolysis after post-treatment at high temperature for improving the crystallinity and photoluminescence intensity of the Gd2O3:Eu phosphor
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particles. Therefore, boric acid was selected as the flux material in the preparation of Gd2O3:Eu phosphor particles with micron size, spherical shape and dense morphology. The effect of the boric acid flux and polymeric precursors on the morphology of Gd2O3:Eu phosphor particles was shown in Fig. 1. The content of boric acid used as flux material was varied from 1 wt.% to 4 wt.% of Gd2O3:Eu phosphor particles. The as-prepared particles obtained by spray pyrolysis were post-treated at 1050 °C, in which the Gd2O3:Eu phosphor particles exhibited the maximum photoluminescence intensity.
The Gd2O3:Eu phosphor particles prepared from spray solution without polymeric precursors and boric acid flux had hollow and porous morphology. In the spray pyrolysis, the severe preparation conditions such as short residence time and high heating rate of the droplets caused the Gd2O3:Eu phosphor particles with hollow and porous structure. On the other hand, the phosphor particles prepared from spray solution containing citric acid and polyethylene glycol had complete spherical shape and dense morphology even after posttreatment at 1050 °C regardless of addition of boric acid flux into spray solution. Citric acid and polyethylene
Fig. 1. SEM photographs of Gd3O3:Eu phosphor particles prepared from different spray solutions.
D.S. Jung et al. / Optical Materials 28 (2006) 530–535
glycol used as polymeric precursors produced the Gd2O3:Eu phosphor particles with spherical shape and dense morphology. The content of the boric acid flux below 2 wt.% of Gd2O3:Eu phosphor did not change the morphology of the phosphor particles. The Gd2O3:Eu phosphor particles (Fig. 1c) prepared from spray solution containing citric acid, polyethylene glycol and 1 wt.% boric acid flux had complete spherical shape, micron size, and non-aggregation characteristics. The Gd2O3:Eu phosphor particles (Fig. 1e) with the content of 4 wt.% boric acid had spherical shape and slightly aggregated morphology after post-treatment. However, Gd2O3:Eu phosphor particles prepared from spray solution containing polymeric precursor and flux materials such as alkaline earth metal carbonates and chlorides by spray pyrolysis had non-spherical shape and fine size after post-treatment above 1000 °C. Fig. 2 shows the photoluminescence spectra of the Gd2O3:Eu phosphor particles as shown in Fig. 1. The Gd2O3:Eu phosphor particles prepared from spray solution containing citric acid and polyethylene glycol had higher photoluminescence intensity than that of the phosphor particles prepared from spray solution containing no polymeric precursor. The dense morphology of the Gd2O3:Eu phosphor particles prepared from spray solution containing citric acid and polyethylene glycol improved the photoluminescence intensity of the phosphor particles. The effect of the flux material on the photoluminescence intensity of the phosphor particles was also appeared in the system of Gd2O3:Eu phosphor particles prepared from spray solution containing polymeric precursors. The phosphor particles prepared from spray solution containing boric acid flux had higher photoluminescence intensity than that of the phosphor particles prepared from spray solution containing no boric acid flux. The photoluminescence intensity of the Gd2O3:Eu phosphor particles prepared from
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spray solution containing polymeric precursors and boric acid flux was 149% of the phosphor particles prepared from spray solution without polymeric precursor and flux material. Fig. 3 shows the effect of the additional amount of boric acid flux on the photoluminescence intensity of the Gd2O3:Eu phosphor particles prepared from spray solution containing citric acid and polyethylene glycol. The phosphor particles prepared from spray solution with different additional amount of the boric acid were post-treated at 1050 °C for 3 h. The phosphor particles with low content of boric acid below 2 wt.% of Gd2O3:Eu phosphor had higher photoluminescence intensities than that of the phosphor particles prepared from solution containing no boric acid flux. The phosphor particles with high content of boric acid above 4 wt.% of Gd2O3:Eu phosphor had low photoluminescence intensities, in which boric acid produced impurity phase by reaction with gadolinium component. The optimum content of boric acid in the preparation of Gd2O3:Eu phosphor particles from spray solution containing polymeric precursors by spray pyrolysis was 1 wt.%. The effect of the post-treatment temperature on the photoluminescence intensities of Gd2O3:Eu phosphor particles prepared from spray solution containing polymeric precursor and boric acid flux was shown in Fig. 4. The Gd2O3:Eu phosphor particles with the content of 1 wt.% boric acid were post-treated at different temperature for 3 h. In the spray pyrolysis, the optimum post-treatment temperature of Gd2O3:Eu phosphor particles showing the maximum photoluminescence intensity was 1050 °C. The phosphor particles post-treated at 1000 °C had comparable photoluminescence intensity with that of the commercial Y2O3:Eu phosphor particles prepared by solid state reaction method applying flux
140
1wt.% H3BO3
140 0.2M CA/PEg + 1wt.% H3BO3
120
2wt.% H3BO3
120
0wt.% H3BO3
100
Intensity
0.2M CA/PEG
100
Intensity
No additive
80
80
4wt.% H3BO3
60
60 40 40 20 20 0 560
0 560
580
600
620
640
580
600
620
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Wavelength (nm)
Wavelength (nm)
Fig. 2. Photoluminescence spectra of Gd3O3:Eu phosphor particles prepared from different spray solutions.
Fig. 3. Photoluminescence spectra of Gd3O3:Eu phosphor particles prepared from spray solutions containing different content of boric acid.
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D.S. Jung et al. / Optical Materials 28 (2006) 530–535 140
1050°C 1100°C
1200 °C
120 1200°C 1000°C
100
Commertial
80
Intensity
Intensity
1100 °C 800°C
60
1050 °C
40
1000 °C
20
800 °C 0 560
580
600 Wavelength (nm)
620
640
Fig. 4. Photoluminescence spectra of Gd2O3:Eu phosphor particles at different post-treatment temperature.
material. The photoluminescence intensity of the Gd2O3:Eu phosphor particles showing the maximum brightness was 120% of that of the commercial Y2O3:Eu phosphor particles. The phosphor particles prepared from spray solution containing polymeric precursor and boric acid flux by spray pyrolysis had high photoluminescence intensity at lower post-treatment tempera-
20
30
40
50
60
2θ
Fig. 5. XRD spectra of Gd3O3:Eu phosphor particles at different posttreatment temperature.
ture than that of the solid state reaction method. The chelating effect of polymeric precursor and flux effect of boric acid improved the photoluminescence intensities of the Gd2O3:Eu phosphor particles prepared by spray pyrolysis method at low post-treatment temperature. Fig. 5 shows the XRD spectra of the Gd2O3:Eu phosphor particles prepared from spray solution con-
Fig. 6. SEM photographs of Gd3O3:Eu phosphor particles at different post-treatment temperature.
D.S. Jung et al. / Optical Materials 28 (2006) 530–535
taining polymeric precursor and boric acid flux at different post-treatment temperature. The phosphor particles post-treated at 800 °C had low photoluminescence intensity because of poor crystallinity as shown in Fig. 5. On the other hand, the Gd2O3:Eu phosphor particles post-treated at 1050 °C had high crystallinity and high photoluminescence intensity in Figs. 4 and 5. The effect of the post-treatment temperature on the morphology of the Gd2O3:Eu phosphor particles prepared from spray solution containing polymeric precursor and 1 wt.% boric acid flux was shown in Fig. 6. The phosphor particles post-treated at temperature below 1050 °C had complete spherical shape and non-aggregation characteristics. Aggregation between particles occurred at post-treatment temperature above 1100 °C and hard-aggregated phosphor particles were obtained at 1200 °C. Therefore, the optimum post-treatment temperature for producing the Gd2O3:Eu phosphor particles with spherical shape, micron size, non-aggregation characteristics and high photoluminescence intensity was 1050 °C.
4. Conclusion In the spray pyrolysis, flux material was applied to improve the photoluminescence intensity of the Gd2O3:Eu phosphor particles with spherical shape and micron size obtained from spray solution containing citric acid and polyethylene glycol. Various types of flux materials including boric acid, lithium carbonate, sodium carbonate, potassium carbonate and lithium chloride were tried to the preparation of Gd2O3:Eu phosphor particles. Applying the flux materials with the exception of boric acid into spray solution including polymeric precursor destroyed the spherical shape of the as-prepared particles obtained by spray pyrolysis after post-treatment at high temperature. The Gd2O3:Eu phosphor particles prepared from spray solution containing citric acid, polyethylene glycol and boric acid had good morphology and high photoluminescence intensity under ultraviolet. The Gd2O3:Eu phosphor particles prepared from spray solution containing citric
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acid and polyethylene glycol had complete spherical shape, dense and non-aggregation characteristics. Addition of boric acid flux into spray solution improved the photoluminescence intensity of the spherical shape Gd2O3:Eu phosphor particles without destroying the morphology of the phosphor particles.
Acknowledgement This work was supported by the Korea Science and Engineering Foundation (KOSEF, R08-2004-00010160-0).
References [1] M. Tamatani, N. Matsuda, M. Okumura, A.K. Albessard, Y. Inoue, S. Yokota, K. Kawasaki, in: Proc. 3rd Int. Display Workshop (IDWÕ96), 1996, p. 29. [2] D . Ravichandran, R. Roy, W.B. White, S. Erdei, J. Mater. Res. 12 (3) (1997) 819. [3] S. Erdei, R. Roy, G. Harshe, H. Juwhari, D. Agrawal, F.W. Ainger, W.B. White, Mater. Res. Bull. 30 (6) (1995) 745. [4] M. Kottaaisamy, D. Jeyakumar, R. Jagannathan, M.M. Rao, Mater. Res. Bull. 31 (8) (1996) 1013. [5] M. Nyman, J. Caruso, M.J. Hampden-Smith, T.T. Kodas, J. Am. Ceram. Soc. 80 (5) (1997) 1231. [6] Y.C. Kang, S.B. Park, I.W. Lenggoro, K. Okuyama, J. Phys. Chem. Solids 60 (1999) 379. [7] M.J. Hampden-Smith, T.T. Kodas, J. Caruso, US Patent 6,180,029 B1, 2001. [8] K. Vanheusden, C.H. Seager, W.L. Warren, D.R. Tallant, J. Caruso, M.J. Hampden-Smith, T.T. Kodas, J. Lumin. 75 (1) (1997) 11. [9] E.J. Kim, Y.C. Kang, H.D. Park, S.G. Yoo, Mater. Res. Bull. 38 (2003) 515. [10] K. Ohno, T. Abe, J. Electrochem. Soc. 133 (3) (1986) 638. [11] K. Ohno, T. Abe, J. Electrochem. Soc. 134 (8) (1987) 2072. [12] S. Oshio, T. Matsuoka, S. Tanaka, H. Kobayashi, J. Electrochem. Soc. 145 (11) (1998) 3898. [13] S. Ekambaram, K.C. Patil, J. Alloys Compd. 248 (1997) 7. [14] C.R. Ronda, B.M.J. Smets, J. Electrochem. Soc. 136 (2) (1989) 570. [15] Y.C. Kang, H.S. Roh, S.B. Park, J. Am. Ceram. Soc. 84 (2) (2001) 447. [16] Y.C. Kang, H.S. Roh, E.J. Kim, H.D. Park, J. Electrochem. Soc. 150 (2003) H93.
Gd2O3:Eu phosphor particles prepared from spray solution containing boric acid flux and polymeric precursor by spray pyrolysis Dae Soo Jung, Seung Kwon Hong, Hyo Jin Lee, Yun Chan Kang
*
Department of Chemical Engineering, Konkuk University, 1 Hwayang-dong, Gwangjin-gu, Seoul 143-701, South Korea Received 5 October 2004; accepted 24 March 2005 Available online 1 June 2005
Abstract Micron size Gd2O3:Eu phosphor particles with spherical shape and dense morphology were prepared from spray solution containing boric acid, citric acid and polyethylene glycol by spray pyrolysis. Boric acid used as flux material was effective in improving the photoluminescence intensity of micron size Gd2O3:Eu phosphor particles prepared by spray pyrolysis. Boric acid did not affect on the morphology of the phosphor particles prepared from spray solution containing citric acid and polyethylene glycol after posttreatment at high temperature. The optimum content of boric acid in the preparation of Gd2O3:Eu phosphor particles from spray solution containing polymeric precursors by spray pyrolysis was 1 wt.%. The optimum post-treatment temperature of Gd2O3:Eu phosphor particles showing the maximum photoluminescence intensity was 1050 °C. The Gd2O3:Eu phosphor particles post-treated at 1050 °C had complete spherical shape, non-aggregation characteristics and high crystallinity. The photoluminescence intensity of the Gd2O3:Eu phosphor particles prepared from spray solution containing polymeric precursors and boric acid flux was 149% of the phosphor particles prepared from spray solution without polymeric precursor and boric acid flux. Ó 2005 Elsevier B.V. All rights reserved. PACS: 81.20.Rg; 78.55.Hx; 78.40.Ha; 81.05.Hd; 81.40.Tv Keywords: Phosphor; Spray pyrolysis; Flux material; Functional ceramic
1. Introduction Phosphors are required to have good luminescence properties like high luminescence efficiency, suitable emitting colors, and proper decay time. In addition to the luminescence properties, they should have good screening properties to produce phosphor layers with good uniformity, high packing density and proper adhesion strength to the substrate for practical application. The screening properties depend on many factors such
*
Corresponding author. Tel.: +82 2 2049 6010; fax: +82 2 458 3504. E-mail address: [email protected] (Y.C. Kang).
0925-3467/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2005.03.014
as chemical stability, particle size, particle size distribution, surface properties and particle shape [1]. The characteristics of phosphor particles are strongly affected by synthetic process [2–4]. Spray pyrolysis is one of the promising processes for the preparation of improved phosphor particles. In this method, a misted stream of precursor solution is dried, precipitated, and decomposed in a tubular furnace reactor continuously [5–9]. Phosphor particles synthesized by spray pyrolysis have relative uniformity in size and composition, spherical shape, fine size, and non-aggregation characteristics because of the micro-scale reaction within a droplet and the lack of milling process. However, despite these advantages, phosphor particles prepared by spray
D.S. Jung et al. / Optical Materials 28 (2006) 530–535
pyrolysis suffer from problems such as hollowness and high porosity, which cause the reduction in brightness and long-term stability. In large-scale spray pyrolysis for mass production, the prepared particles tend to have further hollow and/or porous structure because the severe operation conditions such as high solution concentration, short residence time within reactor, and high temperature variation induced by large-size tubular reactor. Thus, a new technique is required to synthesize spherical phosphor powders with solid structure by a large-scale spray pyrolysis. In the spray pyrolysis, polymeric precursors and flux material were separately used to the preparation of oxide phosphor particles. Polymeric precursors were introduced to control the spherical morphology with micron size phosphor particles in the spray pyrolysis [9]. The esterification reaction between carboxyl group in citric acid and hydroxyl group in ethylene glycol or polyethylene glycol within droplet forms highly viscous gel consisting of a three-dimensional network of polymer. The viscous gel promoted the volume precipitation and resulted in the formation of the particles with a spherical shape, filled morphology, and non-aggregation characteristics. In conventional solid state reaction and liquid solution methods, flux is used to improve wetting of reactants and to form well-crystallized particles. The flux also plays an important role in the control of mean size, size distribution, and shape of phosphor particles [10–14]. Fluxes are usually compounds of alkali- or alkaline earth metals having low melting points. In the spray pyrolysis, flux materials were used to improve the brightness of spherical shape phosphor particles with micron size [15]. In the previous work, polymeric precursors and flux material were simultaneously used to prepare the redemitting phosphor particles in the spray pyrolysis [16]. The as-prepared Gd2O3:Eu phosphor particles prepared from spray solution containing polymeric precursors and lithium carbonate flux material by spray pyrolysis had spherical shape and filled morphology. The asprepared particles had micron size because one particle was obtained from one droplet in gas phase reaction. However, the morphology of the as-prepared Gd2O3:Eu phosphor particles prepared from spray solution containing polymeric precursors and lithium carbonate flux changed after post-treatment at high temperature. The spherical shape of the as-prepared particles disappeared and nano-sized Gd2O3:Eu phosphor particles were obtained after post-treatment. In this work, Gd2O3:Eu phosphor particles with micron size, spherical shape and high brightness were prepared from spray solution containing polymeric precursor and flux material by spray pyrolysis. Citric acid and polyethylene glycol as polymeric precursors and boric acid as flux material were simultaneously used to prepare the red-emitting Gd2O3:Eu phosphor particles in the spray pyrolysis.
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2. Experimental Spherical shape Gd2O3:Eu phosphor particles were prepared by ultrasonic spray pyrolysis from spray solution with polymeric precursors and flux materials. Spray solution were prepared by dissolving gadolinium nitrate, europium nitrate, citric acid, polyethylene glycol and flux material. Citric acid and polyethylene glycol (M.W. 400) were used as polymeric precursors. Various types of flux materials including boric acid, alkaline earth metal carbonate and chloride were studied as flux materials for producing the high brightness Gd2O3:Eu phosphor particles. The total concentration of metal components was 0.5 M. The concentration of citric acid and polyethylene glycol used as polymeric precursors was 0.2 M, respectively. The content of flux material was varied from 0 wt.% to 5 wt.% of Gd2O3:Eu phosphor particles. The spray solution was atomized into micron size droplets by ultrasonic spray generator with six resonators. The flow rate of air used as a carrier gas was 45 L/min and the residence time of the particles inside the reactor was 0.6 s. The as-prepared particles were obtained at 900 °C by ultrasonic spray pyrolysis. The asprepared particles with spherical shape and micron size were post-treated at various temperatures from 700 to 1200 °C for 3 h in muffle furnace. The crystal structures of the particles were studied by X-ray diffraction (XRD, RIGAKU, D/MAX-RB) with Cu-K radiation (k = 1.5418). The morphologies of particles were investigated using scanning electron microscopy (SEM). Photoluminescence measurement was performed with spectrofluorophotometer (SHIMADZU, RF-5301PC) using a Xe lamp excitation source.
3. Results and discussion Various types of flux materials including boric acid, lithium carbonate, sodium carbonate, potassium carbonate and lithium chloride were tried to the preparation of Gd2O3:Eu phosphor particles with spherical shape and dense morphology. In the large-scale ultrasonic spray pyrolysis process, polymeric precursors such as citric acid and ethylene glycol (or polyethylene glycol) were necessary to obtain the red-emitting Gd2O3:Eu phosphor particles with spherical shape and dense morphology. However, the combination of polymeric precursor and flux material as the additives into spray solution caused the change of the morphology of the Gd2O3:Eu phosphor particles. In the present work, applying the flux materials with the exception of boric acid in spray solution including polymeric precursor destroyed the spherical shape of the as-prepared particles obtained by spray pyrolysis after post-treatment at high temperature for improving the crystallinity and photoluminescence intensity of the Gd2O3:Eu phosphor
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D.S. Jung et al. / Optical Materials 28 (2006) 530–535
particles. Therefore, boric acid was selected as the flux material in the preparation of Gd2O3:Eu phosphor particles with micron size, spherical shape and dense morphology. The effect of the boric acid flux and polymeric precursors on the morphology of Gd2O3:Eu phosphor particles was shown in Fig. 1. The content of boric acid used as flux material was varied from 1 wt.% to 4 wt.% of Gd2O3:Eu phosphor particles. The as-prepared particles obtained by spray pyrolysis were post-treated at 1050 °C, in which the Gd2O3:Eu phosphor particles exhibited the maximum photoluminescence intensity.
The Gd2O3:Eu phosphor particles prepared from spray solution without polymeric precursors and boric acid flux had hollow and porous morphology. In the spray pyrolysis, the severe preparation conditions such as short residence time and high heating rate of the droplets caused the Gd2O3:Eu phosphor particles with hollow and porous structure. On the other hand, the phosphor particles prepared from spray solution containing citric acid and polyethylene glycol had complete spherical shape and dense morphology even after posttreatment at 1050 °C regardless of addition of boric acid flux into spray solution. Citric acid and polyethylene
Fig. 1. SEM photographs of Gd3O3:Eu phosphor particles prepared from different spray solutions.
D.S. Jung et al. / Optical Materials 28 (2006) 530–535
glycol used as polymeric precursors produced the Gd2O3:Eu phosphor particles with spherical shape and dense morphology. The content of the boric acid flux below 2 wt.% of Gd2O3:Eu phosphor did not change the morphology of the phosphor particles. The Gd2O3:Eu phosphor particles (Fig. 1c) prepared from spray solution containing citric acid, polyethylene glycol and 1 wt.% boric acid flux had complete spherical shape, micron size, and non-aggregation characteristics. The Gd2O3:Eu phosphor particles (Fig. 1e) with the content of 4 wt.% boric acid had spherical shape and slightly aggregated morphology after post-treatment. However, Gd2O3:Eu phosphor particles prepared from spray solution containing polymeric precursor and flux materials such as alkaline earth metal carbonates and chlorides by spray pyrolysis had non-spherical shape and fine size after post-treatment above 1000 °C. Fig. 2 shows the photoluminescence spectra of the Gd2O3:Eu phosphor particles as shown in Fig. 1. The Gd2O3:Eu phosphor particles prepared from spray solution containing citric acid and polyethylene glycol had higher photoluminescence intensity than that of the phosphor particles prepared from spray solution containing no polymeric precursor. The dense morphology of the Gd2O3:Eu phosphor particles prepared from spray solution containing citric acid and polyethylene glycol improved the photoluminescence intensity of the phosphor particles. The effect of the flux material on the photoluminescence intensity of the phosphor particles was also appeared in the system of Gd2O3:Eu phosphor particles prepared from spray solution containing polymeric precursors. The phosphor particles prepared from spray solution containing boric acid flux had higher photoluminescence intensity than that of the phosphor particles prepared from spray solution containing no boric acid flux. The photoluminescence intensity of the Gd2O3:Eu phosphor particles prepared from
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spray solution containing polymeric precursors and boric acid flux was 149% of the phosphor particles prepared from spray solution without polymeric precursor and flux material. Fig. 3 shows the effect of the additional amount of boric acid flux on the photoluminescence intensity of the Gd2O3:Eu phosphor particles prepared from spray solution containing citric acid and polyethylene glycol. The phosphor particles prepared from spray solution with different additional amount of the boric acid were post-treated at 1050 °C for 3 h. The phosphor particles with low content of boric acid below 2 wt.% of Gd2O3:Eu phosphor had higher photoluminescence intensities than that of the phosphor particles prepared from solution containing no boric acid flux. The phosphor particles with high content of boric acid above 4 wt.% of Gd2O3:Eu phosphor had low photoluminescence intensities, in which boric acid produced impurity phase by reaction with gadolinium component. The optimum content of boric acid in the preparation of Gd2O3:Eu phosphor particles from spray solution containing polymeric precursors by spray pyrolysis was 1 wt.%. The effect of the post-treatment temperature on the photoluminescence intensities of Gd2O3:Eu phosphor particles prepared from spray solution containing polymeric precursor and boric acid flux was shown in Fig. 4. The Gd2O3:Eu phosphor particles with the content of 1 wt.% boric acid were post-treated at different temperature for 3 h. In the spray pyrolysis, the optimum post-treatment temperature of Gd2O3:Eu phosphor particles showing the maximum photoluminescence intensity was 1050 °C. The phosphor particles post-treated at 1000 °C had comparable photoluminescence intensity with that of the commercial Y2O3:Eu phosphor particles prepared by solid state reaction method applying flux
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1wt.% H3BO3
140 0.2M CA/PEg + 1wt.% H3BO3
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2wt.% H3BO3
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0wt.% H3BO3
100
Intensity
0.2M CA/PEG
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Intensity
No additive
80
80
4wt.% H3BO3
60
60 40 40 20 20 0 560
0 560
580
600
620
640
580
600
620
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Wavelength (nm)
Wavelength (nm)
Fig. 2. Photoluminescence spectra of Gd3O3:Eu phosphor particles prepared from different spray solutions.
Fig. 3. Photoluminescence spectra of Gd3O3:Eu phosphor particles prepared from spray solutions containing different content of boric acid.
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D.S. Jung et al. / Optical Materials 28 (2006) 530–535 140
1050°C 1100°C
1200 °C
120 1200°C 1000°C
100
Commertial
80
Intensity
Intensity
1100 °C 800°C
60
1050 °C
40
1000 °C
20
800 °C 0 560
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600 Wavelength (nm)
620
640
Fig. 4. Photoluminescence spectra of Gd2O3:Eu phosphor particles at different post-treatment temperature.
material. The photoluminescence intensity of the Gd2O3:Eu phosphor particles showing the maximum brightness was 120% of that of the commercial Y2O3:Eu phosphor particles. The phosphor particles prepared from spray solution containing polymeric precursor and boric acid flux by spray pyrolysis had high photoluminescence intensity at lower post-treatment tempera-
20
30
40
50
60
2θ
Fig. 5. XRD spectra of Gd3O3:Eu phosphor particles at different posttreatment temperature.
ture than that of the solid state reaction method. The chelating effect of polymeric precursor and flux effect of boric acid improved the photoluminescence intensities of the Gd2O3:Eu phosphor particles prepared by spray pyrolysis method at low post-treatment temperature. Fig. 5 shows the XRD spectra of the Gd2O3:Eu phosphor particles prepared from spray solution con-
Fig. 6. SEM photographs of Gd3O3:Eu phosphor particles at different post-treatment temperature.
D.S. Jung et al. / Optical Materials 28 (2006) 530–535
taining polymeric precursor and boric acid flux at different post-treatment temperature. The phosphor particles post-treated at 800 °C had low photoluminescence intensity because of poor crystallinity as shown in Fig. 5. On the other hand, the Gd2O3:Eu phosphor particles post-treated at 1050 °C had high crystallinity and high photoluminescence intensity in Figs. 4 and 5. The effect of the post-treatment temperature on the morphology of the Gd2O3:Eu phosphor particles prepared from spray solution containing polymeric precursor and 1 wt.% boric acid flux was shown in Fig. 6. The phosphor particles post-treated at temperature below 1050 °C had complete spherical shape and non-aggregation characteristics. Aggregation between particles occurred at post-treatment temperature above 1100 °C and hard-aggregated phosphor particles were obtained at 1200 °C. Therefore, the optimum post-treatment temperature for producing the Gd2O3:Eu phosphor particles with spherical shape, micron size, non-aggregation characteristics and high photoluminescence intensity was 1050 °C.
4. Conclusion In the spray pyrolysis, flux material was applied to improve the photoluminescence intensity of the Gd2O3:Eu phosphor particles with spherical shape and micron size obtained from spray solution containing citric acid and polyethylene glycol. Various types of flux materials including boric acid, lithium carbonate, sodium carbonate, potassium carbonate and lithium chloride were tried to the preparation of Gd2O3:Eu phosphor particles. Applying the flux materials with the exception of boric acid into spray solution including polymeric precursor destroyed the spherical shape of the as-prepared particles obtained by spray pyrolysis after post-treatment at high temperature. The Gd2O3:Eu phosphor particles prepared from spray solution containing citric acid, polyethylene glycol and boric acid had good morphology and high photoluminescence intensity under ultraviolet. The Gd2O3:Eu phosphor particles prepared from spray solution containing citric
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acid and polyethylene glycol had complete spherical shape, dense and non-aggregation characteristics. Addition of boric acid flux into spray solution improved the photoluminescence intensity of the spherical shape Gd2O3:Eu phosphor particles without destroying the morphology of the phosphor particles.
Acknowledgement This work was supported by the Korea Science and Engineering Foundation (KOSEF, R08-2004-00010160-0).
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