Jpn. J. Appl. Phys. Vol. 38 (1999) pp. 2013–2016 Part 1, No. 4A, April 1999 c °1999 Publication Board, Japanese Journal of Applied Physics
Photoluminescence Properties of Ce1−x Tb x MgAl11 O19 Phosphor Particles Prepared by Spray Pyrolysis Yun Chan K ANG ∗1 , Seung Bin PARK1 , I. Wuled L ENGGORO and Kikuo O KUYAMA ∗2 Department of Chemical Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan 1 Department of Chemical Engineering, Korea Advanced Institute of Science and Technology, 373-1, Kusong-dong, Yusong-gu, Taejon, 305-701, Korea (Received May 11, 1998; accepted for publication September 21, 1998)
Spray pyrolysis method was applied to the preparation of submicron Ce1−x Tbx MgAl11 O19 phosphor particles. The characteristics such as photoluminescence, crystallinity, and morphology were investigated under various preparation conditions. While the as-prepared particles by spray pyrolysis at temperature of 900◦ C had amorphous phase, the particles calcined above 1400◦ C had phase-pure magnetoplumbite structure, and its crystallinity increased with increasing the calcining temperature. The as-prepared particles had spherical morphology, submicron size, and narrow size distribution. The mean size of the particles increased from 0.5 to 1.4 µm when the spray solution concentration increased from 0.03 to 1.5 M. After calcination, the particles showed 545 nm green emission characteristic of Tb3+ ions with optimum excitation at 280 nm. The maximum luminous intensity was obtained at x = 0.4. The optimum calcining temperature for good brightness was 1500◦ C. Particles directly prepared by spray pyrolysis at 1500◦ C show good green emission characteristics and maintain its sphericity and non-aggregation characteristics. KEYWORDS: phosphor, spray pyrolysis, plasma display panel, magnetoplumbite structure, photoluminescence, fine particles
1. Introduction In recent years much effort has been made to improve the characteristics of phosphor materials. In the new developing displays such as plasma display panels (PDPs), field emission displays (FEDs), electroluminescence (EL) panels, more defined phosphor powders than those of current phosphors are required. In these applications, fine grain size (about 1 µm), phase purity and spherical morphology are also required for good brightness of phosphors and its post-processing.1, 2) The mean size of phosphor powders prepared through conventional solid state reactions is above 5 µm and thus milling process is necessary to obtain finer grain size. Milling process reduces the brightness of phosphor particles due to the damages of surface structure. Surface characteristics of phosphor powders are important since the phenomena such as excitation and emission occur near the surface. Ce1−x Tbx MgAl11 O19 , which is magnetoplumbite structure, is a promising green phosphor in PDPs.3, 4) The compound CeTbMgAl11 O19 luminesces effectively in the UV region when excited by 254 nm.5, 6) By the addition of Tb, excitation energy is transferred from Ce3+ to Tb3+ , resulting in mainly green luminescence. The typical synthesis method of magnetoplumbites involves repeated heating, grinding and firing of the powders. However magnetoplumbites are highly refractory, so it is difficult to obtain phase-pure powders by direct firing. Therefore, some new methods are attempted to the production these phosphor materials. Ravichandran et al. applied a combustion method to the preparation of Ce1−x Tbx MgAl11 O19 phosphors.3) They prepared phase-pure magnetoplumbite at low operation temperatures but the prepared particles had irregular morphology. Spray pyrolysis is a gas phase reaction method to prepare particles.7–10) In this method, a misted stream of precursor solution is dried, precipitated, and decomposed in a tubular furnace reactor. Particles produced by spray pyrolysis are rel∗1 On
leave from Korea Advanced Institute of Science and Technology. ∗2 E-mail address:
[email protected]
atively uniform in size and composition because of the microscale reaction within a droplet and the lack of milling process. In our previous works, spray pyrolysis has been applied to the preparation of multicomponent phosphor materials with a small amount of doping.11, 12) In the spray pyrolysis, we could prepare multicomponent phosphor particles with pure phase at lower temperatures in comparison with conventional solid-state reaction methods. The prepared particles had submicron, spherical morphology, and non-agglomeration. Additionally, milling process was not necessary to obtain the fine grain size. In this work, we investigated the characteristics of Ce1−x Tbx MgAl11 O19 phosphors prepared by spray pyrolysis. 2. Experimental The apparatus used in this work was an ultrasonic spray generator with 1.7 MHz resonator. The apparatus used in this work is in principle the same as that used previously.7) 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 components. The overall solution concentration was changed from 0.03 to 1.5 M to control the mean size of phosphor particles. Nominal compositions Ce1−x Tbx MgAl11 O19 with Tb (0.1 ≤ x ≤ 0.7) were directly prepared at the furnace temperatures of 900 and 1500◦ C. The flow rate of nitrogen gas used as carrier gas was maintained at 2 L/min, and the residence time of particles inside the hot wall reactor was 1.6 s. The prepared particles were characterized with X-ray diffractometry (XRD, Rigaku-Denki Corp., model RINT 1000), scanning electron microscopy (SEM, Hitachi Corp., model S3100H2). Optical properties were measured by spectrophotometer (Shimadzu Corp., model RF-53009c). Xenon lamp was used for excitation of phosphor particles in the UV region. 3. Results and Discussion The particles prepared by spray pyrolysis at the temperature of 900◦ C had amorphous phase because of its short residence time as 1.6 s. Figure 1 shows the XRD spectra of
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Fig. 1. XRD spectra of particles prepared at 900◦ C and calcined at different temperatures.
Ce0.5 Tb0.5 MgAl11 O19 particles calcined at different temperatures. The particles calcined above 1200◦ C had main peaks of magnetoplumbite structure but the particles calcined below 1300◦ C had small impurity. Phase-pure magnetoplumbite structure (JCPDS # 36–73) was obtained at the calcining temperature above 1400◦ C. Figure 2 shows the SEM photographs of particles prepared at different solution concentrations. The prepared particles have spherical morphology in all solution concentrations. The mean size measured from the SEM photographs increased from 0.5 to 1.4 µm when the overall solution concentration changed from 0.03 to 1.5 M. It is well known that using spray pyrolysis, changing the solution concentration can control the mean size of particles. Therefore an additional process like milling causing surface defects is not necessary for preparing finer particles. These as-prepared (900◦ C) particles were calcined above 1100◦ C for its crystallization and activation. In the case of the particles which were directly prepared at temperature of 900◦ C, the activation of dopant did not occur because of the short residence time of particles inside the hot wall reactor. Figure 3 shows the SEM photographs of Ce0.5 Tb0.5 MgAl11 O19 particles calcined at high temperatures for 5 h [Figs. 3(a)–3(c)] and the particles directly prepared at 1500◦ C without calcination [Fig. 3(d)]. For the particles prepared at 900◦ C, their sphericity was maintained in the calcining below 1200◦ C but tended to disappear above 1300◦ C. The particles calcined below 1400◦ C still showed micron size and non-agglomeration, but the hard agglomeration of particles occured after calcining above 1500◦ C. On the other hand, the particles directly prepared at 1500◦ C [Fig. 3(d)] are spherical and still separately formated, thus no milling process is necessary. As desribed later, the particles directly prepared at 1500◦ C could show the green emission characteristics. These results emphasize that by using spray pyrolysis, spherical and non-aggregated Ce1−x Tbx MgAl11 O19 phosphor particles could be prepared in a few seconds, without the calcination at several long hours. Photoluminescence characteristics of crystalline Ce1−x Tbx MgAl11 O19 particles prepared by spray pyrolysis were investigated. Figure 4 shows the excitation spectra of Ce1−x Tbx MgAl11 O19 particles at different doping concentrations of Tb. The particles absorbed excitation energy from 220 to 330 nm, and had the optimum value at 280 nm. In
Fig. 2. SEM photographs of particles prepared at different concentrations; 0.03 (a), 0.2 (b), 1.5 M (c). Preparation temperature 900◦ C.
Fig. 3. SEM photographs of particles prepared at 900◦ C and calcined at different temperatures; 1200 (a), 1400 (b), 1500◦ C (c), and directly prepared at 1500◦ C without calcination (d).
Fig. 5, luminous intensities as a function of Tb concentration at 280 nm were shown. In Figs. 4 and 5, the prepared particles were calcined at 1500◦ C for 5 h. The emission spectra had four sharp peaks at 486, 545, 585, and 624 nm, and had the maximum brightness at 545 nm, resulting in a dominant green emission. These excitation and emission characteristics of Ce1−x Tbx MgAl11 O19 particles are well coincided with the results reported in literature.3–5) The brightness of Ce1−x Tbx MgAl11 O19 particles was strongly affected by doping concentrations of Tb. Luminescence intensity had maximum value at x = 0.4, and concentration quenching occurred at high doping concentrations (x ≤ 0.5). This result was well coincided with that of previous report, in which quenching of the Ce3+ emission in Ce1−x Tbx MgAl11 O19 particles was
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Fig. 4. Excitation spectra of Ce1−x Tbx MgAl11 O19 particles prepared by spray pyrolysis. Prepared temperature 900◦ C and calcined at 1500◦ C.
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Fig. 6. Emission spectra of Ce1−x Tbx MgAl11 O19 particles prepared at 900◦ C and calcined at different temperatures (symbols ‘S’) and directly prepared at 1500◦ C without calcination (symbol P1500).
It was proved that the particles prepared in the present study are in the optimum order of general phosphor particles, e.g. 1 µm order, and more spherical than those reported in the literature. Recently, Ravichandran et al.3) showed the Ce1−x Tbx MgAl11 O19 particles with platy morphology and having the size of 10–20 µm. Therefore, it should be noted that by using the spray pyrolysis method, no extreme grinding is necessary on the preparation of the Ce1−x Tbx MgAl11 O19 phosphors particles. 4. Conclusion
Fig. 5. Emission spectra of Ce1−x Tbx MgAl11 O19 particles prepared at different doping concentrations. Prepared temperature 900◦ C and calcined at 1500◦ C.
completely occurred at x = 0.4, and the phosphor had maximum Tb3+ emissions in the near of x = 0.4.6) In the case of low concentrations of Tb, excitation energy transfer from Ce3+ to Tb3+ did not completely occur. The effect of calcination temperature on the luminous intensities of particles at constant doping concentration (x = 0.5) was shown in Fig. 6. The samples calcined above 1200◦ C showed green light, and luminous intensities increased with increasing calcination temperatures until 1500◦ C. These optical characteristics of particles coincide well with those of XRD. The optimum calcination temperature for high brightness and somewhat good morphology of Ce1−x Tbx MgAl11 O19 phosphor was 1400◦ C. The PL intensity of particles calcined at 1400◦ C had about two times in comparison with that of particles calcined at 1300◦ C. At high calcining temperatures above 1600◦ C, vaporization of particles was severely occurred, and luminous intensities decreased. The PL characteristic of particles directly prepared at temperature of 1500◦ C was also showed in Fig. 6. Even the PL of the particles shows lower intensity compared with those calcined, it can be considered that the spray pyrolysis with the reaction temperature of 1500◦ C could prepare directly the useful Ce1−x Tbx MgAl11 O19 phosphor particles.
Spray pyrolysis using ultrasonic nebulizer is applied to the preparation of Ce1−x Tbx MgAl11 O19 phosphors. This material is one of the promising green phosphors for plasma display panels (PDPs). Without calcination, the particles prepared by spray pyrolysis have good characteristics such as fine size, narrow size distribution, and spherical morphology. In the spray pyrolysis, changing the solution concentration of liquid precursors can control the mean size of particles from submicron to several micron. Multicomponent particles prepared by spray pyrolysis have pure phase at low calcined temperatures because of the microscale reaction inside droplets with several micron sizes. Additionally, the phospor particles are maintained their sphericity and non-agglomeration characteristics after directly prepared at the temperature of 1500◦ C and also after the calcination below the temperature of 1400◦ C. In the system of Ce1−x Tbx MgAl11 O19 phosphors, the particles prepared by spray pyrolysis have good brightness because of pure phase and high crystallinity. Acknowledgements Support from the Ministry Education, Science, Sports and Culture of Japan (Grant No. 10650745) and the Hiroshima Industrial Technology Organization is gratefully acknowledged. One of the authors (YCK) thanks the KOSEF (Korea Science and Engineering Foundation) for his postdoctoral fellowship.
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