Synthesis and characterization of strontium (Sr), barium (Ba) and calcium (Ca) aluminate phosophors doped with rare earth ions
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Date
2009-08
Authors
Mothudi, Bakang Moses
Journal Title
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Publisher
University of the Free State
Abstract
The luminescent and structural properties of the alkaline earth aluminate phosphors prepared by solid state reaction, combustion and sol-gel methods are discussed. The Solid state and Sol-gel methods required much longer time (3-9 hours) for preparation of the phosphors. The annealing processes in both methods are performed at very high temperatures (1000 - 1300 0C). Furthermore in order to reduce Eu3+ to Eu2+ toxic gases such as N2 and H2 were introduced during the annealing process. The combustion method is more efficient because the phosphors of high efficiency were obtained at low temperatures (500 – 600 0C) in a very short period of time (5 min). The Eu2+ was obtained by adding a small amount of urea to the mixture during synthesis. SrAl2O4:Eu2+,Dy3+ phosphors prepared by a solid state reaction method at different annealing temperatures (1000 - 1200 0C), in a reducing atmosphere of N2 and 25%H2, were irradiated with an excitation wavelength of 365 nm. The optimum PL intensity was shown by a sample prepared at 1100 0C. The broad emission spectra symmetric at 497 nm can be attributed to the 4f65d1- 4f7 transition of the Eu2+. Eu2+ and Dy3+ co– doped calcium aluminate, barium aluminate and strontium aluminate phosphors were synthesized at an initiating combustion temperature of 500 0C using urea as an organic fuel. The crystallinity of the phosphors was investigated by using X- ray diffraction (XRD) and the morphology was determined by a scanning electron microscope (SEM). The low temperature monoclinic structure for both CaAl2O4 and SrAl2O4 and the hexagonal structure of BaAl2O4 were observed. Photoluminescent (PL) and phosphorescent properties were investigated by using a 325 nm He-Cd Laser and a Cary Eclipse fluorescence spectrophotometer, respectively. The broad band emission spectra with maxima at 449 nm for CaAl2O4:Eu2+,Dy3+, 450 nm (with a shoulder at 500 nm) for BaAl2O4:Eu2+,Dy3+ and 528nm for SrAl2O4:Eu2+,Dy3+ were observed. Ca0.97Al2O4:Eu2+ 0.01,Dy3+ 0.02 phosphors prepared at different initiating combustion temperatures were characterized by XRD), SEM and PL systems (He-Cd Laser and Cary Eclipse Fluorescence spectrophotometer). The PL emission spectra symmetric at 450 nm, in all the phosphors, confirms that only one emitting center, is present (Eu2+). The optimum PL intensity and Phosphorescence was observed from the sample prepared at an initiating combustion temperature of 600 0C. Ba0.97Al2O4:Eu2+ 0.01,Dy3+ 0.02 powder phosphors were prepared at different initiating temperatures ranging from 500 - 800 0C by the combustion method using urea as a comburent. The most crystalline hexagonal structure of BaAl2O4 was observed from samples prepared at the initiating combustion temperature of 500 - 600 0C. Blue-green persistent/long afterglow emission was observed from all the samples. Red photoluminescence was observed from a nanocrystalline SrAl2O4:Eu3+ powder phosphor prepared by a sol-gel process. The preparation process was carried out using Sr(CH3CO2)2. 2 1 H2O, Al (i-OC3H7)3 and Eu2O3 as starting materials. The white foamy gel of SrAl2O4:Eu3+ was dried at 60 – 200oC and calcined at 1000 0C. Nanocrystals of SrAl2O4 exhibited an agglomeration of nano-rodlike particles with an edge thickness of ~27 nm on average. Based on the X-ray diffraction and photoluminescence data, SrAl2O4 was found to crystallize as metastable monoclinic rather than hexagonal phase. The crystalline structure and photoluminescent properties of SrAl2O4:Eu3+ were reported.
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Keywords
Thesis (Ph.D. (Physics))--University of the Free State, 2009, Phosphors, Photoluminescence, Aluminates, Alkaline earth aluminates, Rare earth ions, Long-afterglow, Phosphorescence, Solid state reaction method, Sol- gel method, Combustion method