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Preparation of nonaggregated Y2 O3 : Eu phosphor particles by spray pyrolysis method Yun Chan Kanga) Department of Chemical Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan

Seung Bin Park Department of Chemical Engineering, Korea Advanced Institute of Science and Technology, 373-1 Kusong-dong, Yusong-gu, Taejon 305-701 Korea

I. Wuled Lenggoro and Kikuo Okuyamab) Department of Chemical Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan (Received 10 July 1998; accepted 27 January 1999)

Y2 O3 : Eu phosphor particles were directly prepared by a spray pyrolysis method. Photoluminescence, morphology, and crystallinity of the as-prepared particles were investigated. The as-prepared particles above 600 ±C had good crystallinity, and the crystallinity increased with increasing reactor temperatures. The particles had spherical morphology and were nonaggregated. The mean size of the particles increased from 0.34 to 1.2 mm when the solution concentration was increased from 0.03 to 1 M. The as-prepared particles had good red emission without annealing at high temperatures when excited with uv light. The main emission peak was 612 nm. The brightness of the as-prepared particles increased with increasing temperatures because of good activation and crystallization at high temperatures.

I. INTRODUCTION

Rare-earth-doped oxide particles have been widely studied for application of displays such as high definition (HD), projection televisions (PTVs), and flat panel displays (FPDs). Phosphor materials must have a narrow size distribution, nonagglomeration, and characteristics and spherical morphology for good luminescent characteristics. The mean size of the particles is very important for high resolution and high efficiency.1,2 Also the small particles can improve on aging by forming a densely packed phosphor layer. It has been predicted that optimum phosphor characteristics will be particle sizes on the order of 1 mm. Spherical morphology is also required for high brightness and high resolution. If the phosphor particles have a spherical shape, scattering of light evolved from the phosphors decreases. Additionally, high packing densities can be obtained by using spherical phosphors. Eu-doped Y2 O3 particles are well-known as a good red phosphor for applications in displays and lamps.1–8 Conventionally, Y2 O3 : Eu particles are prepared by a solid state reaction method using Y2 O3 and Eu2 O3 particles. In the solid state reaction methods, high temperature and long heating time are required for activation of rareearth components inside the matrix of the host materials.

a)

On leave from Korea Advanced Institute of Science and Technology. Address all correspondence to this author. e-mail: [email protected]

b)

J. Mater. Res., Vol. 14, No. 6, Jun 1999

Additionally, the prepared particles have irregular shape and tend to agglomerate. Liquid solution methods are also frequently applied to the preparation of Y2 O3 : Eu phosphor particles. These methods have the advantages of low preparation temperature and fine size. On the other hand, the particles can become agglomerated during the annealing process. Jiang et al.2 and Villalobos et al.6 prepared fine size Y2 O3 : Eu particles with spherical morphology by a coprecipitation method. However, liquid solution methods are confined to some monocomponent oxide host materials. Recently, gas phase reaction methods have been applied to the preparation of rare-earth-doped oxide phosphor materials. Generally, the particles prepared by gas phase reactions have fine size and spherical morphology. Sievers et al.1,7 prepared Y2 O3 : Eu phosphors by CO2 -assisted aerosolization. In this technique, supercritical CO2 is combined with aqueous solutions of water-soluble metal nitrates or acetates in a low-deadvolume tee7 to form emulsions at 10 MPa. Spherical and submicron phosphor particles can be formed by rapid desolvation of the aerosol in a tube furnace by dehydration and pyrolysis. Sievers et al. observed red light from as-prepared particles when excited by uv light. However, they focused on the characteristics of particles after annealing. Bihari et al.8 prepared Y2 O3 : Eu nanocrystalline phosphors by gas-phase condensation using CO2 laser heating of ceramic pellets. On the other hand, these gas phase reactions are currently still  1999 Materials Research Society

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Y. C. Kang et al.: Preparation of nonaggregated Y2 O3 : Eu phosphor particles by spray pyrolysis method

confined to the preparation of phosphors with simple composition. In our previous studies, spray pyrolysis was applied to the preparation of rare-earth-doped oxide phosphor materials.9,10 Spray pyrolysis is a method of producing particles in which a misted stream of a precursor solution is dried, precipitated, and decomposed in a tubular furnace reactor.11–14 The phosphor particles prepared by spray pyrolysis had good characteristics such as fine size, narrow size distribution, and spherical morphology after annealing at high temperatures. Multicomponent phosphor particles prepared by spray pyrolysis had a pure phase at low annealing temperatures because of the microscale reaction inside the droplets of several microns in size. Additionally, the particles have maintained sphericity and nonagglomeration characteristics at high temperatures. In this work, Y2 O3 : Eu phosphor particles were directly prepared by spray pyrolysis. The characteristics of the particles such as photoluminescence, crystallinity, and morphology were investigated. II. EXPERIMENTAL

The apparatus used in this work was an ultrasonic spray generator with a 1.7 MHz resonator. The apparatus used in this work is, in principle, the same as that used previously.14 The particles formed were electrostatically collected in a chamber kept at around 250 ±C to prevent water condensation. The starting materials were nitrate precursors of each component. The overall solution concentration was changed from 0.03 to 1 M for size control of the phosphor particles. The flow rate of nitrogen gas used as carrier gas was fixed at 2 lymin. The preparation temperatures were changed from 500 to 1400 ±C for good crystallization and activation of particles, and then the corresponding residence time of particles inside the hot wall reactor was changed from 1.1 to 2.4 s. The prepared particles were characterized by x-ray diffractometry (XRD, Rigaku-Denki Corp., model RINT 1000), and scanning electron microscopy (SEM, Hitachi Corp., model S-3100H2). Optical properties were measured by a spectrophotometer (Shimadzu Corp., model RF-53009c). The xenon lamp was used for excitation of the phosphor particles in the uv region. III. RESULTS AND DISCUSSION

Figure 1 shows the XRD spectra of prepared and annealed particles at different temperatures. The particles prepared at 500 ±C showed broad peaks of Y2 O3 , and sharp peaks were obtained above 700 ±C. The crystallinity of particles was increased with increasing reactor temperatures. The particles are well crystallized at low temperatures and very short residence times because of the fine size of the particles in the gas phase. 2612

FIG. 1. XRD spectra of prepared and annealed particles.

The mean size of the particles is a very important variable for the brightness and processing of phosphor materials. In the spray pyrolysis, changing solution concentrations can easily control the mean size of the particles at constant droplet size. Figure 2 shows the SEM photographs of Y2 O3 : Eu particles prepared at different solution concentrations. In all cases, the particles were nonaggregated and had a spherical morphology because of the gas phase reaction. The mean size of the particles was increased from 0.34 to 1.2 mm when the solution concentration was increased from 0.03 to 1 M. In our previous studies, the particles prepared by spray pyrolysis had no brightness because of the short residence time of particles inside the hot wall reactor on the order of 0.1 s.9,10 In all previous cases, postannealing at high temperature is required for crystallization and activation because of the high refractory characteristics of the studied multicomponent materials. On the other hand, bright red light was obtained from the as-prepared particles in the system of Y2 O3 : Eu, which has a very simple composition in comparison with the studied multicomponent phosphors. The effects of preparation and annealing temperature on the PL intensity and morphology of the particles were investigated. Figure 3 shows the SEM photographs of as-prepared and annealed particles at different temperatures. The morphology of the particles was not changed even at high preparation temperatures because of the short residence time of particles inside the hot wall reactor. The prepared particles have spherical morphology and are nonaggregated. The particles annealed at 1200 ±C for 1 h also have spherical morphology and are nonaggregated. Figure 4 shows the excitation spectra of Y2 O3 : Eu particles prepared at 900 ±C. The prepared particles absorbed excitation energy in the range from 220 to 280 nm, and the maximum excitation wavelength was near 243 nm. In Fig. 5, the luminous intensities of the



FIG. 2. SEM photographs of Y2 O3 particles prepared by ultrasonic spray pyrolysis. Solution concentration (a) 0.03 M, ( b) 0.3 M, and (c) 1 M.

FIG. 3. SEM photographs of particles prepared at different temperatures: (a) P 1200 ±C, ( b) P 1400 ±C, and (c) A 1200 ±C.

FIG. 4. Excitation spectra of Y2 O3 : Eu particles at different doping concentrations.

FIG. 5. Emission spectra of Y2 O3 : Eu particles at different doping concentrations.

as-prepared particles at different doping concentrations of Eu are shown. The particles were prepared at 900 ±C for the investigation of optimum doping concentrations. The optimum brightness was obtained at a doping concentration of 6 at.%. The main emission peak was 612 nm, resulting in a red emission. For the investigation of doping efficiency of the as-prepared

particles at 900 ±C, the particles prepared at different doping concentrations were annealed at 1000 ±C for 3 h. The emission patterns of the annealed particles were also compared to those of the as-prepared particles. Figure 6 shows the relative luminescence intensities of as-prepared and annealed particles at different doping concentrations. In Fig. 6, the relative intensities


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FIG. 6. Relative PL intensities of Y2 O3 : Eu particles at different doping concentrations.

of as-prepared particles were obtained from Fig. 5. In both cases, the particles have similar emission patterns according to the doping concentrations. The optimum doping concentration in annealed particles was also 6 at.%. This result indicates that the europium component is well activated in the gas phase reaction at low temperatures and short residence times. If the activation of europium does not completely occur in the as-prepared particles, the emission according to the concentrations should show different patterns for the annealed particles. In the system of the Y2 O3 : Eu phosphor, diffusion of the dopant into the matrix of the host material occurs at short reaction times of several seconds because of the good distribution of the dopant on a nanometer scale inside the particles. In Fig. 6, the increasing crystallite size after annealing increased the brightness of the annealed particles. The emission characteristics of the rare-earth-doped phosphor particles are strongly affected by crystallinity and doping efficiency of the activator. In Fig. 7, the

effect of preparation temperature on the brightness of Y2 O3 : Eu particles at optimum doping concentration of Eu (6 at.%) was investigated. The residence time of the particles inside the hot wall reactor was changed from 2.4 to 1.1 s when the reactor temperature was varied from 500 to 1400 ±C. The brightness of the as-prepared particles was strongly affected by preparation temperatures. The red light was obtained from the 700 ±C material when the prepared particles were excited by uv light. In the XRD spectra (Fig. 1), well crystallized Y2 O3 was obtained above 700 ±C. The brightness of the as-prepared particles increased with increasing temperatures because of the high crystallinity of the particles at high temperatures. In Fig. 1 (P900A1200), the particles annealed at 1200 ±C for 1 h had better crystallinity than that of the as-prepared particles at 1200 ±C. On the other hand, the as-prepared particles above 1200 ±C had good PL intensities in comparison with the annealed particles. The PL intensity of the as-prepared particles at 1400 ±C (P 1400 ±C) was increased by a factor of 1.2 in comparison with the annealed particles (A 1200 ±C). These results indicate that the Y2 O3 : Eu particles with bright emission can be directly prepared by spray pyrolysis in short residence times. Based on the above results, the Y2 O3 : Eu phosphor with red emission was directly prepared by a gas phase reaction method. The Y2 O3 : Eu particles had fine size, sphericity, and were nonaggregated. Therefore, a milling process for fine size and deagglomeration was not necessary in this system. A milling process is a conventional process reduces the brightness of the phosphor particles. The use of spherical particles should increase screen brightness and improve resolution because of lower scattering of the evolved light and higher packing densities than irregularly shaped particles obtained by conventional methods. These characteristics of particles prepared by spray pyrolysis can be applied to the investigation of the size effect of particles on the brightness and processing of phosphor materials. IV. CONCLUSIONS

FIG. 7. Emission spectra of Y2 O3 : Eu particles at different preparation temperatures. 2614

Europium-doped Y2 O3 phosphor particles were prepared from mixed nitrate solutions by ultrasonic spray pyrolysis. The Y2 O3 : Eu particles directly prepared by spray pyrolysis had fine size, sphericity, and nonaggregation. Therefore, a milling process for fine size and nonagglomeration was not necessary in this system. The as-prepared Y2 O3 : Eu particles had good brightness because of the pure phase, nonaggregation, fine size, and good dispersion of doping components inside the host materials. The brightness of the as-prepared particles increased with increasing preparation temperatures because of good activation and high crystallinity at high temperatures.



ACKNOWLEDGMENTS

One of us (YCK) thanks the KOSEF (Korea Science and Engineering Foundation) for a postdoctoral fellowship at Hiroshima University where this study was performed. Support from the Ministry of Education, Culture and Science of Japan (Grant No. 10650745) and the Hiroshima Industrial Technology Organization are gratefully acknowledged. REFERENCES 1. R. E. Sievers, P. D. Milewski, C. Y. Xu, and B. A. Watkins, Proceedings of the 3rd International Conference on the Science and Technology of Display Phosphors, Huntington Beach, CA, 1997, p. 303. 2. Y. D. Jiang, Z. L. Wang, F. Zhang, H. P. Paris, and C. J. Summers, Proceedings of the 3rd International Conference on the Science and Technology of Display Phosphors, Huntington Beach, CA, 1997, p. 261. 3. J. Koike, T. Kojima, R. Toyonaga, A. Kagami, T. Hase, and S. Inaho, J. Electrochem. Soc. 126, 1008 (1979).

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