Synthesis and luminescence properties of phase-pure ultrafine Y2.9Tb0.1Al5−xGaxO12 phosphors

Journal of Alloys and Compounds 474 (2009) 441–444

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Synthesis and luminescence properties of phase-pure ultrafine Y2.9 Tb0.1 Al5−x Gax O12 phosphors Quan-Ming Li a , Fa-Gui Qiu b,∗ , Jun-Ji Zhang c , Xi-Peng Pu c , Xue-Jian Liu c , Wan-Xi Zhang a a

Department of Materials Science and Engineering, Jilin University, No. 5333, Xi’an Road, Changchun City 130062, People’s Republic of China Department of Quartermaster Engineering, Jilin University, No. 5333, Xi’an Road, Changchun City 130062, People’s Republic of China c Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, People’s Republic of China b

a r t i c l e

i n f o

Article history: Received 22 April 2008 Received in revised form 13 June 2008 Accepted 24 June 2008 Available online 21 August 2008 Keywords: Optical material Sol–gel chemistry Luminescence X-ray diffraction

a b s t r a c t Ultrafine Y2.9 Tb0.1 Al5−x Gax O12 phosphor powders with different Ga concentration were prepared by a nitrate–citrate sol–gel combustion process. Mono-phase cubic phosphors were obtained at 900 ◦ C by directly crystallizing from amorphous materials determined by X-ray diffraction (XRD) technique and no intermediate phase was observed. The resultant phosphor powders heat-treated at 1000 ◦ C are uniform with good dispersity and the particle size is about 80 nm. The photoluminescence (PL) spectra of Tb3+ substituted for Y3+ in Yttrium aluminum garnet (Y3 Al5 O12 , YAG) with different Ga concentrations were measured on samples calcined at 1000 ◦ C. The luminescence intensity was effectively improved because of the replacement of Al with Ga in YAG. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Yttrium aluminum garnet material has been widely studied in the application of fluorescence and solid-state lasers [1–3]. YAG is a durable material that can withstand a high-energy electron beam and has been considered as an ideal candidate for phosphor in large-size projection cathode-ray tubes (CRTs) systems, which requires both higher electron accelerating energy and current density to increase the brightness [4,5]. YAG:Tb phosphor has luminescence properties fairly insensitive to temperature variation and shows little tendency to saturate at high current excitation. Furthermore, YAG:Tb is a characteristic narrow-band phosphor suitable for contrast-enhanced display application in high ambient illumination conditions [6]. Recently, much effort has been made to improve the brightness and resolution of projection. Y3−x Tbx Al5−y Gay O12 phosphor has been prepared using solid-state reaction technique, formed by partial substitution of Al in Y3−x Tbx Al5 O12 with Ga, which exhibits considerably better saturation characteristics at higher density electron beam excitation than other Tb3+ activated garnet, therefore, providing superior performance under severe cathode-ray tubes operating conditions.

Traditionally, YAG phosphors doped with activators are mainly synthesized by solid-state reaction techniques [4,5]. To achieve desired phase purity and required particle size, the process of solidstate reaction usually needs lengthy high temperature treatment (>1600 ◦ C) and extensive ball milling, which generally introduces additional impurities and defects. Furthermore, high temperature processing does not yield sufficient fine particles required to achieve enhanced screen resolution in phosphor applications. Recently, a nitrate–citrate sol–gel combustion process was used to synthesize YAG phosphors [7,8]. By this process, Mono-phase cubic YAG phosphors were prepared at temperature as low as 900 ◦ C and the particle size of the phosphor can be easily controlled between nano-size to several micrometers by modulating the heat-treating temperature. The photoluminescence (PL) properties of YAG:Tb phosphors with different Tb concentration were also discussed. In the present work, Mono-phase cubic Y2.9 Tb0.1 Al5−x Gax O12 (x = 0–5) phosphors with different Ga concentration were synthesized using nitrate–citrate sol–gel combustion process. The crystalline evolution and the particle size of the product were investigated and the influence of Ga concentration on the photoluminescence properties of the phosphors was reported. 2. Experimental

∗ Corresponding author. Tel.: +86 431 87835754; fax: +86 431 87836356. E-mail address: [email protected] (F.-G. Qiu). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.06.102

The samples were prepared by the nitrate–citrate sol–gel combustion process. Al(NO3 )3 ·9H2 O (analytical grade), Y(NO3 )3 ·6H2 O (99.99% purity), TbF3 (99.99% purity), C6 H8 O7 ·H2 O (hydrated citric acid, analytical grade), and metal gallium (99.99% purity) were used as starting materials. High-purity TbF3 and metal gallium

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were dissolved in HNO3 and then dissolved in deionized water with a stoichiometric amount of yttrium nitrate, aluminum nitrate and appropriate dosage of citric acid to yield a composition with the general formula Y2.9 Tb0.1 Al5−x Gax O12 with x = 0.0, 0.5, 1.0, 2.0, 3.0, and 5.0. The ratio of nitrate to citrate used in the present work was 1:1. After the mixed solution was heated at 60 ◦ C and continuously stirred using a magnetic agitator for several hours, the solution turned to yellowish sol. Then, heated at 80 ◦ C and stirred constantly, the sol transformed into transparent sticky gel. The gel was rapidly heated to 180 ◦ C and an auto combustion process took place companying with the evolution of brown fume. Finally, a yellowish product, fluffy precursor, was yielded. The precursor was then heat-treated at varying temperatures from 800 to 1000 ◦ C for 2 h in a muffle furnace in air atmosphere. The crystalline development of the product was identified by X-ray diffraction analysis (XRD, Model D/MAX-2550V) using nickel filtered Cu K␣ radiation in the range of 2 = 10–70◦ . The particle size and morphology of the heat-treated powders were examined using the transmission electronic microscope (TEM, Model 200CX, JEOL, Tokyo, Japan). Excitation and luminescence spectra were measured using a Hitachi F-4500 fluorescence spectrophotometer at room temperature.

3. Results and discussion

Fig. 2. X-ray diffraction patterns of Y2.9 Tb0.1 Al5−x Gax O12 .

XRD patterns of the precursor and powders with the formula composition Y2.9 Tb0.1 Al5 O12 calcined at various temperatures are shown in Fig. 1. No obvious diffraction peaks are observed, so it can be concluded that the precursor is amorphous and remains amorphous below calcined temperature of 800 ◦ C. At 800 ◦ C, the characteristic peaks of YAG phase appear with rather weak intensity, which is an indication of microcrystalline YAG. Above 900 ◦ C, continued refinement of peak shapes are observed, indicating crystallite growth of the YAG grains as the temperature increases. The phase evolution shown by XRD indicates that YAG is the only phase detected during heat treatment. It can be concluded that YAG appears to crystallize directly from the amorphous precursor without the formation of any intermediate phase, such as YAP (YAlO3 ) and YAM (Y4 Al2 O9 ) that usually appear as intermediate phases in other wet-chemical powder preparation methods [9–11]. Fig. 2 shows the diffraction patterns of the samples with the formula Y2.9 Tb0.1 Al5−x Gax O12 (x = 0–5) calcined at 1000 ◦ C for 2 h in air. It can be seen that no any detrimental phases (YAP, YAM, YGaM, and YGaP) are detected in the samples. Namely, the raw materials, yttrium, aluminum, and gallium powders are also fully converted to monophase cubic YAG, which is essential to obtain a high-luminescence efficiency [5]. The samples for luminescence spectra measurement are calcined at 1000 ◦ C for 2 h in air. Before the spectra are measured, the micrograph of the powders is observed by TEM as shown in Fig. 3. It can be seen that the particle size of the produced phosphors is about 80 nm. The powders are uniform with good dispersity. For phosphor applications, one of the requirements is the particle size

distribution. In some applications, uniform and ultrafine phosphors are desirable to achieve high resolution, and high chemical purity is essential to optimize chromaticity and brightness. Thus, the sol–gel combustion process employed in the present study has successfully synthesized ultrafine phosphors with desired phase purity. Because of its fine particle and good dispersity, the phosphors can be used directly without milling.

Fig. 1. XRD patterns of the precursor and Y2.9 Tb0.1 Al5 O12 powders calcined at various temperature.

Fig. 4. Excitation spectrum of Tb3+ 545 nm emission of Y2.9 Tb0.1 Al4 Ga1 O12 calcined at 1000 ◦ C.

Fig. 3. TEM micrograph of the Y2.9 Tb0.1 Al5 O12 powders calcined at 1000 ◦ C.

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Fig. 5. Photoluminescence spectra of Y2.9 Tb0.1 Al5−x Gax O12 calcined at 1000 ◦ C.

Fig. 4 shows the excitation spectra of the Tb3+ 545-nm emission in the phosphors calcined at 1000 ◦ C. The excitation spectra of the samples for different Ga content are the same, so only the spectrum with formula composition Y2.9 Tb0.1 Al4 Ga1 O12 is shown in the figure. From 240 to 400 nm, there are two bands: a strong excitation at about 274 nm and a weak excitation bands located at about 322 nm, respectively, all corresponding to the Tb3+ 4f-5d absorption. The emission spectra of Y2.9 Tb0.1 Al5−x Gax O12 (x = 0.0, 0.5, 1.0, 2.0, 3.0, 5.0) excited by Ultraviolet (UV,  = 274 nm) are shown in Fig. 5. As well known, the fluorescence of Tb3+ in solid originates mainly from the transitions of 5 D3 –7 Fj and 5 D4 –7 Fj [12]. From Fig. 5 it can be seen that the fluorescence spectra of the phosphor cal-

cined at 1000 ◦ C are very weak, which results from the high Tb3+ concentration (3.3%) used in the present work. As shown in Fig. 5, replacing a portion of Al with Ga has no influence on the shape of the luminescence spectra. However, the intensity of luminescence spectra varies with different Al and Ga ratio for a constant Tb3+ concentration. Fig. 6 shows the integrated intensity of the emission of Tb3+ in the range of 350–650 nm as a function of Al to Ga ratio. As shown in Fig. 6, the highest luminescence intensity among Y2.9 Tb0.1 Al5−x Gax O12 appears in the Y2.9 Tb0.1 Al4 Ga1 O12 (Al:Ga = 4:1), which is about 150% that of Y2.9 Tb0.1 Al5 O12 . This may be due to the change in crystal field on the Tb3+ caused by the larger ionic radius of Ga replacing Al. 4. Conclusions Phase-pure Y2.9 Tb0.1 Al5−x Gax O12 phosphors can be synthesized at temperature as low as 900 ◦ C by nitrate–citrate sol–gel combustion process. Mono-phase cubic Y2.9 Tb0.1 Al5−x Gax O12 is formed by directly crystallizing from amorphous materials and no detrimental intermediate phases (YAM, YAP, YGaM, YGaP) are observed. Uniform, ultrafine Y2.9 Tb0.1 Al5−x Gax O12 phosphor powders are prepared in the present work. The particle size of the Y2.9 Tb0.1 Al5−x Gax O12 powders calcined at 1000 ◦ C is about 80 nm. The replacement of Al with Ga in YAG-based phosphors effectively improved the luminescence intensity. The highest intensity of Y2.9 Tb0.1 Al5−x Gax O12 intensity appears at the composition of Al:Ga = 4:1 and the intensity of Y2.9 Tb0.1 Al4 Ga1 O12 is about 150% that of Y2.9 Tb0.1 Al5 O12 . References

Fig. 6. Dependence of the intensity for Al and Ga ratio.

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