Stabilization of beta-SrAl 2 O 4 with Eu Prepared by Spray Pyrolysis
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Stabilization of beta-SrAl2O4 with Eu Prepared by Spray Pyrolysis W.S. Shi,1) H. Yamada,2) K. Nishikubo,1) H. Kusaba,2) C.N. Xu1,2)∗ 1) National Institute of Advanced Industrial Science and Technology (AIST), Kyushu, Shuku 807-1, Tosu, Saga 841-0052, Japan 2) PRESTO, Japan Science and Technology Corporation (JST), 4 Honcho, Kawaguchi 332-0012, Japan ABSTRACT The effect of doping europium on the structure of strontium aluminate (SrAl2O4:Eu) synthesized by spray pyrolysis was studied. Results revealed that all of the as-sprayed samples were a mixture structure of monoclinic and hexagonal phases. However, the structural behaviors of the SrAl2O4:Eu post-calcinated at 1300°C in reducing ambient for 2hrs strongly depended on the europium concentrations. As increase of the europium concentrations, a characteristic of hexagonal phase could be observed from the post-annealed samples. Up to 12 percent of strontium were replaced by the europium, the structure of the post-annealed sample achieved a pure hexagonal phase. The mechanism of the phase dependence of the SrAl2O4:Eu on europium concentration was discussed. INTRODUCTION The structure of the strontium aluminate (SrAl2O4) has attracted extensive interests since of its potential luminescent application.1-5 It was found that the SrAl2O4 has a phase transformation at around 650°C.4 Below this temperature, the SrAl2O4 is a monoclinic phase (MP). The MP structure would transform to an un-distorted hexagonal phase (HP) at temperature above 650°C. The SrAl2O4 in MP structure is a well-known host for high-efficiency phosphor doped with rare earth europium (Eu) as an activator.6 Some research groups have investigated the role of the Eu on the structure and light emission behaviors of the SrAl2O4:Eu phosphors prepared by solid reaction process.7 Although the concentration quenching of the Eu in the SrAl2O4:Eu was observed, the doping of the Eu was not found to influence the structure of the SrAl2O4:Eu, even Sr were replaced by the Eu up to 12.4%.8 It was revealed that the SrAl2O4:Eu still kept the normal MP structure at room temperature as the increase of the Eu.8 However, It was also reported that the HP SrAl2O4:Eu has a higher light emission efficient than that of the MP one.9 Unfortunately, it has not been possible to achieve a stable SrAl2O4:Eu with a pure HP structure at room temperature until now.4 In order to produce the HP SrAl2O4:Eu at room temperature, a quenching technique was usually employed.4,9 However, this method has a disadvantage of ∗
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instability and incomplete transforming from the MP to the HP structure.4 In the present study, a complete HP SrAl2O4:Eu with high purity and stability was produced by a spray pyrolysis method. Novel structure behavior of the SrAl2O4:Eu synthesized by this method were found to strongly dependent on the Eu concentration. As a result, the pure and stable HP SrAl2O4:Eu is obtained at room temperature by this method without use of any flux and quenching process. In this paper,
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