Synthesis and characterization of long afterglow phosphors (SrAl2O4:Ce³+, SrAl2O4:Tb³+, CaAlxOy:Tb³+, Y3Al5O12:Eu³+) using solution combustion method
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Date
2011-11
Authors
Foka, Kewele Emily
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University of the Free State
Abstract
This work consists of several aspects of phosphor materials. Strontium, calcium and yttrium aluminate doped with rare earth (Ce, Tb and Eu) have been synthesized by solution combustion method using urea as a fuel for investigations of the luminescent, structure and morphological properties. The phosphors were characterized by several techniques such as X-ray diffraction (XRD), energy dispersive electroscopy (EDS), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and Photoluminescence (PL), PL data were collected using a Cary Eclipse Photoluminescence Spectrophotometer equipped with a 150 W xenon lamp. Cerium doped strontium aluminum oxide (SrAl2O4:Ce3+) were synthesizes. The effects of different concentration of cerium were investigated. X-ray diffraction results confirmed the formation of the SrAl2O4 monoclinic phase (Powder Diffraction Standards (JCPDS) file No 34-0379). The particle sizes of different peaks were estimated and the average particle size was 47 nm. SEM results showed agglomerated as well as small elongated-egg-like shape on particles when taken to higher magnification. The PL spectra show a broad emission consisting of two bands peaking at 374 and 384 nm, corresponding to the transitions from the lowest 5d excited state to the 2F5/2 and 2F7/2 states. The excitation and emission peak position shifted with varying the cerium concentration. This maybe due to uncontrollable electrospinning conditions like air and wetness, which influence the cristal field that surround Ce3+. SrAl2O4:Tb3+ XRD peaks confirmed the formation of the SrAl2O4 monoclinic phase and some impurities were also observed. The photoluminescence characteristics show the emission peaks at 415, 436 and 459 nm which correspond to the 5D3 to 7FJ (J=5, 4, and 3) level and 489, 543, 585, and 622 nm corresponding to 5D4 to 7FJ (J= 6, 5, 4, 3) under excitation at 229 nm and the terbium concentration was varied. The elements of the phosphor SrAl2O4:Tb3+ were shown by energy dispersive spectroscopy. The decay curves were also observed and the decay constants show a higher value at a concentration of 0.25 mol% and lower value at a concentration of 2 mol%. CaAlxOy:Tb3+ green phosphors were obtained at low temperature (500 oC) by a solution- combustion method. The structural analysis revealed the presence of both monoclinic CaAl4O7 and CaAl2O4. The main parent structure of CaAl2O4 monoclinic was revealed when varying the concentration of terbium. The characteristic luminescence properties were investigated using emission spectra. The emission peaks are from transition of the 5D4 state to the 7FJ (J = 6, 5, 4, 3) state. The optimal intensity was obtained when the concentration of Tb3+ was increased to 2 mol%. FTIR was used to identify all the chemical bands. Absorption bands of the condensed matter AlO4 located in the range of 700 cm-1-900 cm-1 and condensed matter AlO6 at 500 cm-1-680 cm-1 are attributed to AlO4 liberation at 600 cm-1-900 cm-1. The decay curves of the phosphor were investigated and showed a higher intensity and longer afterglow time at higher concentration of terbium 2 mol%.Y3Al5O12 known as Yttrium aluminum garnet (YAG) phosphor doped with different concentration of Eu was synthesized by the solution combustion method. The crystalline structure, morphology and luminescent properties of the phosphors were studied. The SEM revealed the agglomerated morphology containing small spherical particles around the pores. FTIR spectra reveal all bonds that exist in the phosphor. The emission spectra revealed three major emission peaks at 592, 615, and 628 nm, corresponding to the 5D0→7F1 (592 nm), 5D0→7F2 (615 nm) and 5D0→7F3 (628 nm) transitions respectively. The luminescence intensity increased with an increase in Eu concentration at 0.7 mol% and then decreases with an increasing of concentration further.
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Keywords
Phosphors, Luminescence, Rare earths, Combustion, Long afterglow, Solution combustion process, Dissertation (M.Sc. (Physics))--University of the Free State (Qwaqwa Campus), 2011