Synthesis and Characterization of MAl 2 O 4 (M= Ba, Ca, Sr) Phosphor by Self-propagating High Temperature Synthesis

Transcription

1 Available online at Energy Procedia 9 (2011 ) th Eco-Energy and Materials Science and Engineering Symposium Synthesis and Characterization of MAl 2 O 4 (M= Ba, Ca, Sr) Phosphor by Self-propagating High Temperature Synthesis Taschaporn Sathaporn a,c and Sutham Niyomwas b,c* a Department of Mining and Materials Engineering, Faculty of Engineering, Prince of Songkla University, Thailand b Department of Mechanical Engineering, Faculty of Engineering, Prince of Songkla University, Thailand c NANOTEC Center of Excellence at Prince of Songkla University, Hat Yai 90112, Thailand Abstract The preparation of Eu 2+ doped barium aluminate (BaAl 2 O 4 :Eu 2+ ), calcium aluminate (CaAl 2 O 4 :Eu 2+ ), strontium aluminate (SrAl 2 O 4 :Eu 2+ ) with high brightness and long afterglow by self-propagating high temperature synthesis (SHS) method were described in this study. The reactions were carried out in a SHS reactor under static argon gas at a pressure of 0.5 MPa. The morphologies and the phase structures of the products have been characterized by X-ray diffraction (XRD) and scanning electron microscope technique (SEM). The emission spectra of the products have been measured by an Ocean potics spectrometer at room temperature. Broad band UV excited luminescence was observed for BaAl 2 O 4 :Eu 2+ and SrAl 2 O 4 :Eu 2+ in the green region peak at max = 501 nm, 517 nm, respectively and CaAl 2 O 4 :Eu 2+ in the blue region at 437 nm Published by Elsevier Ltd. Open access under CC BY-NC-ND license. Selection and/or peer-review under responsibility of CEO of Sustainable Energy System, Rajamangala University of Technology Thanyaburi (RMUTT). Keywords: Barium aluminate, calcium aluminate, strontium aliminate, phosphorescent, self-propagating high temperature synthesis 1. Introduction Persistent luminescence, or afterglow, is the phenomenon observed when a phosphor exposed to UVradiation or even to light shows visible luminescence in the dark for a long time, preferably for tens of hours.[1] Phosphor materials in visible region with long persistence are widely used in different displays in signing and devices [2],[3]. Several aluminate compositions are investigated and used as photoluminescence, catholuminescence and plasma display panel. For a long time many kinds of sulfide, such as CaS: Eu 2+ CaS:Eu 2+, SrS:Eu 2+, CaS:Ce 3+, SrS:Ce 3+, CaS:Eu 2+,Ce 3+ and SrS:Eu 2+,Ce 3+ show * Corresponding author. Tel.: ; fax: address: sutham.n@psu.ac.th Published by Elsevier Ltd. Open access under CC BY-NC-ND license. Selection and/or peer-review under responsibility of CEO of Sustainable Energy System, Rajamangala University of Technology Thanyaburi (RMUTT). doi: /j.egypro

2 Taschaporn Sathaporn and Sutham Niyomwas / Energy Procedia 9 ( 2011 ) chemically unstable and degraded light resistance [4]-[5]. Recently, interests has been focused on Eu 2+ - doped alkaline earth aluminate MAl 2 O 4 : Eu 2+ (M=Ba, Ca, Sr) which showed good luminescent performances, such as high initial luminescent intensity, long lasting time, suitable emitting color and chemical stability [6]. So far phosphorescence materials have been manufactured by conventional process such as solid-state reaction, sol-gel, mechanical alloying and flame spray pyrolysis. But it should be noted that the following problems concerning the process of the solid-state reaction are unavoidable: (i) extremely high temperatures and a lengthy period of time are needed to prepare the products [7]; (ii) the grain size of powders is in the range of several tens of micrometers and the distribution of components is asymmetrical; (iii) it is difficult to crush the hard phosphor blocks into small particles, which will decrease the luminescence intensity;(iv) the samples should be prepared in a reductive atmosphere. In the present study we have prepared MAl 2 O 4 :Eu 2+ (M=Ba, Ca, Sr) phosphor with keeping Eu 2+ fixed at 0.47 mol% by self-propagating high-temperature synthesis (SHS) method with the development of the synthesis technologies. There are many merits of the combustion synthesis. 2. Experimental Procedure 2.1. Preparation of MAl 2 O 4 :Eu 2+ Specimens were prepared from barium peroxide (BaO 2 : Fluka, 93.0%), calcium peroxide (CaO 2 : Aldrich, 75%), strontium peroxide (SrO 2 : Aldrich, 98%) aluminium oxide (Al 2 O 3 : Fluka, 99%), aluminium (Al: Himedia Laboratories, 99%) and Europium oxide (Eu 2 O 3 : Aldrich, 99.5%) powders. An experimental procedure was shown in Fig. 1. Powders were mixed and milled with ball mill for 30 min then pressed into cylindrical compacts under 40 kn. The reaction was carried out in a SHS reactor under static argon gas at the pressure of 0.5 MPa, as shown in Fig. 2. The morphology and crystalline structure of the product were characterized by powder X-ray diffraction and scanning electron microscopy analysis. The photoluminescence properties of phosphor were measured using ocean optics USB4000 spectrometer at room temperature under UV light BaO 2, CaO 2, SrO 2 Al 2 O 3 Al Eu 2 O 3 Mixing Press SHS BaAl 2 O 4 :Eu 2+ CaAl 2 O 4 :Eu 2+ SrAl 2 O 4 :Eu 2+ XRD, SEM, Ocean optics USB 4000 Spectrometer Fig. 1. Experimental procedure.

3 412 Taschaporn Sathaporn and Sutham Niyomwas / Energy Procedia 9 ( 2011 ) Fig. 2. A schematic of the experimental setup Characterization The prepared compositions were characterized for their phases, crystallinity and morphology by powder X-ray diffraction (PHILIPS with Cu Kα radiation) and scanning electron microscopy (JEOL, JSM-5800 LV) analysis. The photoluminescence properties of phosphor were measured using Ocean optics USB4000 spectrometer at room temperature under UV light 3. Result and Discussion 3.1. Thermodynamics analysis The equilibrium composition of the BaO 2 -Al-Al 2 O 3 -Eu 2 O 3, CaO 2 -Al-Al 2 O 3 -Eu 2 O 3 and SrO 2 -Al- Al 2 O 3 -Eu 2 O 3 systems at different temperatures were calculated using a computer program [8] based on Gibbs energy minimization method [9] and the results were shown in Figure 3. The overall chemical reactions can be expressed as: 1.5 BaO 2 (s) + Al (s) + Al 2 O 3 (s) Eu 2 O 3 (s ) = 1.5 BaAl 2 O 4 (s ) Eu 2 O 3 (s) (1) 1.5 CaO 2 (s) + Al (s) + Al 2 O 3 (s) Eu 2 O 3 (s) = 1.5 CaAl 2 O 4 (s) Eu 2 O 3 (s) (2) 1.5 SrO 2 (s) + Al (s) + Al 2 O 3 (s) Eu 2 O 3 (s) = 1.5 SrAl 2 O 4 (s) Eu 2 O 3 (s) (3) It has been accepted that the reaction can be a self-sustained combustion when the adiabatic temperature of the reaction is higher than 1800 C. [10] The calculated adiabatic temperature of the reaction are C, C and C thus using SHS is feasible for BaO 2 -Al-Al 2 O 3 -Eu 2 O 3, CaO 2 -Al-Al 2 O 3 -Eu 2 O 3 and SrO 2 -Al-Al 2 O 3 -Eu 2 O 3 system, respectively. It can be seen from Fig. 3 that it is thermodynamically feasible to synthesize BaAl 2 O 4 -Eu 2 O 3, CaO 2 -Al-Al 2 O 3 -Eu 2 O 3 and SrO 2 -Al-Al 2 O 3 - Eu 2 O 3 by igniting the reactant of reaction (1), (2) and (3). Due to a highly exothermic reaction at room temperature (ΔH = , and kj) for reaction (1), (2) and (3), respectively and thermodynamic instability at room temperature, the reactant phases of MO 2, Al and Al 2 O 3 were not shown in the calculated stable phases in Figure 3. After ignition the reaction (1), (2) and (3) took place to form MAl 2 O 4 -Eu 2 O 3 phases. The presence of other phase or some compound can be attributed to the fact that the combustion wave is not uniform and portion of the reactant might not react completely during

4 Taschaporn Sathaporn and Sutham Niyomwas / Energy Procedia 9 ( 2011 ) combustion process.[11] When the reaction front moved further away the products cooled down and rearranged phases in such a way that was shown in Figure 3. Fig. 3. Equilibrium composition of (a) BaO 2-Al-Al 2O 3-Eu 2O 3; (b) CaO 2-Al-Al 2O 3-Eu 2O 3; (c) SrO 2-Al-Al 2O 3-Eu 2O 3 systems in Ar gas atmosphere.

5 414 Taschaporn Sathaporn and Sutham Niyomwas / Energy Procedia 9 ( 2011 ) Product charaterization Figure 4(a) and (b) indicates the phase composition of BaAl 2 O 4 and SrAl 2 O 4 phosphors doped with Eu 2+ ions. The results proved that the phosphor samples prepared in this work are almost single BaAl 2 O 4 and SrAl 2 O 4 phase, no other product or starting material was observed and the little amount of doped rare earth ions have almost no effect on the BaAl 2 O 4 and SrAl 2 O 4 phase composition. As can be seen in Fig. 4(c) the phase diffraction peaks of CaAl 2 O 4 are predominant in the XRD patterns of the product. Besides the predominant peaks of CaAl 2 O 4, weak diffraction peaks of CaAl 4 O 7 were observed at the same time. Fig. 4. XRD patterns of as-synthesis product (a) BaAl 2O 4:Eu + 2 ; (b) SrAl 2O 4:Eu + 2 ; (c) CaAl 2O 4:Eu + 2.

6 Taschaporn Sathaporn and Sutham Niyomwas / Energy Procedia 9 ( 2011 ) The microstructure of the MAl 2 O 4 : Eu 2+ (M=Ba, Ca, Sr) phosphor from SHS reaction are studied on SEM micrograph as shown in Fig. 5. The morphology of BaAl 2 O 4 :Eu 2+ reveals an agglomerated structure, with grains size about μm in diameter (Fig. 5a). It is believed that the agglomeration of fine BaAl 2 O 4 :Eu 2+ particles occurred because of the liquid phase reaction followed by recrystallization result from high temperature of reaction front in SHS process. According to the phosphor prepared by solid-state reaction, which normally has an average diameter size about μm [12], product from SHS process has the same grain size range. From Fig. 5(b) and 5(c), we can see that after the SHS process the CaAl 2 O 4 :Eu 2+ and SrAl 2 O 4 :Eu 2+ particles are melted with hard agglomeration accompanied with the formation of necked crystallites. Figure 6(a) shows the image of as-synthesis products show pale yellow and purple color under visible light. The afterglow image of the products is also shown in Fig. 6(b) emitted the blue-green, purple-blue and yellow-green light of BaAl 2 O 4 :Eu 2+, CaAl 2 O 4 :Eu 2+ and SrAl 2 O 4 :Eu 2+ under UV light, respectively. Figure 7 shows the phosphorescence spectrum of as-synthesis product at room temperature, solid line represent emission spectra of BaAl 2 O 4 : Eu 2+ at the main peak 501 nm, dash line represent emission of CaAl 2 O 4 : Eu 2+ at the main peak 437 nm and dotted line represent emission spectra of SrAl 2 O 4 : Eu 2+ at the main peak 517 nm, correspondingly, the phosphorescence change from blue-green to blue-purple and then to yellow-green. This phenomenon is derived from the changing matrix crystal structure [10]. a b c Fig. 5. SEM image of (a) BaAl 2O 4:Eu 2+ ; (b) CaAl 2O 4:Eu + 2 ; (c) SrAl 2O 4:Eu 2+.

7 416 Taschaporn Sathaporn and Sutham Niyomwas / Energy Procedia 9 ( 2011 ) Fig. 6. The images of (a) as-synthesis product under visible light; (b) as-synthesis product under UV light. Fig. 7. Phosphorescence spectrum from SHS method of BaAl 2O 4:Eu 2+, CaAl 2O 4:Eu 2+ and SrAl 2O 4:Eu 2+.

8 Taschaporn Sathaporn and Sutham Niyomwas / Energy Procedia 9 ( 2011 ) Conclusions Phosphor of barium aluminate (BaAl 2 O 4 :Eu 2+ ), calcium aluminate (CaAl 2 O 4 :Eu 2+ ) and strontium aluminate (SrAl 2 O 4 :Eu 2+ ) with keeping Eu 2+ fixed at 0.47 mol% were prepared successfully by selfpropagating high-temperature synthesis (SHS) method. BaAl 2 O 4 : Eu 2+ and SrAl 2 O 4 : Eu 2+, CaAl 2 O 4 : Eu 2+ which show long after-glow phosphorescence with green and blue region phosphorescence. Moreover, the preparation conditions and the optical properties will be studied in more detail to obtain more information on optimize the persistent luminescence performance of MAl 2 O 4 :Eu 2+ (M=Ba, Ca, Sr ). Acknowledgements The authors are pleased to acknowledge the financial support from the National Nanotechnology Center (NANOTEC), NSTDA, Ministry of Science and Technology, Thailand, through its Program of Center of Excellence Network (NANOTEC Center of Excellence at Prince of Songkla University), and the Faculty of Engineering, Prince of Songkla University, Thailand and The Ocean Optics USB 4000 Spectrometer was obtained from Asst. Prof. Dr. Chayut Nantadusit (Department of Mechanical Engineering, Faculty of Engineering, Prince of Songkla University) References [1] Shi, C.S. and Qi, Z.M New development of long afterglow phosphorescent materials. journal of Inorganic Materials.19(5): [2] Jeong, I.K.; Park, H.L. and Mho, S Photoluminescence of ZnGa 2O 4 mixed with InGaZnO 4. Solid State Communications. 108(11): [3] Jia, D.; Wang, X.J.; Van der Kolk, E. and Yen, W.M Site dependent thermoluminescence of long persistent phosphorescence of BaAl 2O 4:Ce 3+. Optics Communications.204(1-6): [4] Xiong, Z.; Chen, Y.; Chen, Z.; and Song, C Modification of Luminescent Properties of Red Sulphide Phosphors for White LED Lighting. Journal of Rare Earths. 24(1): [5] Guo, C.; Huang, D. And Su, Q Methods to improve the fluorescence intensity of CaS:Eu 2+ red-emitting phosphor for white LED. Materials Science and Engineering B. 130(1-3): [6] Qiu, Z.; Zhou, Y.; Lu, M.; Zhang, A. and Ma, Q Combustion synthesis of long-persistent luminescent MAl 2O 4:Eu 2+, R 3+ (M = Sr, Ba, Ca, R = Dy, Nd and La) nanoparticle and luminescence mechanism research. Acta Materialia. 55(8): [7] Saitoh, H.; Kawahara, K.; Ohshio, S.; Nakamura, A. and Nambu, N Synthesis of blue phosphor by decomposition of metal complex powder. J. Ceram Soc. Jpn.110(): [8] Outokumpu HSC Chemistry for Windows, HSC Finland::Outokumpu Research Oy [9] Gokcen, N.A. and Reddy, R.G Thermodynamics, Plenum Press, New York, NY, U.S.A., pp [10] Moore, J. and Feng, H Combustion synthesis of advanced materials: Part Ireaction parameters. Prog.Mater. Sci. 39(4-5): [11] D. Haiyen, L. Gengshen and S. Jaiyue Journal Rare Earths.25: [12] Kim, J.S.; Park, Y.H.; Choi, J.C.; Park, H.L.; Kim, G. and Joong, H.Y Color tunability of nanophosphors by changing cations for solid-state lighting. Solid State Commun 137(4):