Browsing by Author "Kaynar S.C."

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    Removal of uranium(VI) from aqueous solutions using nanoporous ZnO prepared with microwave-assisted combustion synthesis
    (Kluwer Academic Publishers, 2014) Kaynar U.H.; Ayvacıklı M.; Kaynar S.C.; Hiçsönmez U.
    The adsorption of the uranyl ions from aqueous solutions on the nanoporous ZnO powders has been investigated under different experimental conditions. The adsorption of uranyl on nanoporous ZnO powders were examined as a function of the contact times, pH of the solution, concentration of uranium(VI) and temperature. The ability of this material to remove U(VI) from aqueous solution was followed by a series of Langmuir and Freunlinch adsorption isotherms. The adsorption percent and distribution coefficient for nanoporous ZnO powders were 98.65 % ± 1.05 and 7,304 mL g -1 , respectively. The optimum conditions were found as at pH 5.0, contact time 1 h, at 1/5 Zn 2+ /urea ratio, 50 ppm U(VI) concentration and 303 K. The monomolecular adsorption capacity of nanoporous ZnO powders for U(VI) was found to be 1,111 mg g -1 at 303 K. Using the thermodynamic equilibrium constants obtained at different temperatures, various thermodynamic parameters, such as ΔG, ΔH and ΔS, have been calculated. Thermodynamic parameters (ΔH = 28.1 kJ mol -1 , ΔS = 160.30 J mol -1 K -1 , ΔG = -48.54 kJ mol -1 ) showed the endothermic and spontaneous of the process. The results suggested that nanoporous ZnO powders was suitable as sorbent material for recovery and adsorption of U(VI) ions from aqueous solutions. © 2014 Akadémiai Kiadó, Budapest, Hungary.
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    Influence of laser excitation power on temperature-dependent luminescence behaviour of Ce- and Tb-incorporated BaMgAl10O17 phosphors
    (Elsevier Ltd, 2020) Kaynar Ü.H.; Kaynar S.C.; Ayvacikli M.; Karabulut Y.; Souadi G.O.; Can N.
    BaMgAl10O17 (BAM) is a highly suitable host lattice for various rare earth ions with excellent luminescence properties in different spectral regions, including a strong photoluminescence (PL) emission from the visible spectral region. A new Ce- and Tb-incorporated BaMgAl10O17 phase was synthesized successfully using a wet combustion method and it was studied as a function of the temperature and laser excitation power. We further characterize the obtained phosphors with X-ray diffraction at room temperature. Different fuel/oxidant (f/o) ratios were introduced to investigate the optimum synthesis conditions for the BAM phosphors and optimum ratio was found out to be 8. The photoluminescence (PL) spectra were collected under the excitation light generated by a Nd:YLF pulse laser at 349 nm as the temperature was increased from 10 K to 300 K. A strong green emission of Tb3+ was observed in the green region of the spectrum due to the 5D4→7FJ transition. We also observed a wide emission band from the Ce3+ ion in the wavelength range of 350–650 nm. The luminescence intensities of all phosphors exhibited different patterns with an increase in the temperature. We also evaluated how the PL spectrum of the rare earth-activated BAM host matrix shifts under various laser excitation powers. The PL intensity of Ce-activated BAM significantly shifted (~30 A) to the blue region of the spectrum with an increase in the laser excitation power, however we did observed no shift forTb3+ activated BAM. The present findings suggest that Tb-incorporated BaMgAl10O17 can be effective as a green phosphor candidate material with a wide range of applications. © 2019 Elsevier Ltd
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    Lattice distortion effects induced by Li+ co-doping on ZnO:Tb3+ phosphors: Photoluminescence and unusual hypersensitive ⁵D₄ → ⁷F₀ transition
    (Elsevier Ltd, 2024) Altowyan A.S.; Coban M.B.; Kaynar U.H.; Hakami J.; Çin E.A.; Kaynar S.C.; Ayvacikli M.; Can N.
    A series of Tb3+, Li+ co-doped ZnO phosphors were prepared using a precipitation method. X-ray diffraction (XRD) analysis indicated the successful incorporation of Tb3+ into the ZnO lattice. The influence of Tb3+ doping content and Li+ charge compensator on the photoluminescence (PL) properties of ZnO:Tb3+ was investigated. Under UV excitation, emissions corresponding to electron transitions 5D4→7FJ (J = 0,1,2,3,4,5,6) were observed from Tb3+ ions, including an unusual emission transition at 673 nm, which significantly enriches our understanding of Tb3+ luminescence. The critical concentration quenching of Tb3+ in ZnO:Tb3+ occurs at 7 mol%, as explained by the Van Uitert equation, which attributes this phenomenon to dipole-dipole interactions. Surprisingly, incorporating Li+ for charge balancing led to a reduction in the luminescence intensity of ZnO:7 mol%Tb3+, x%Li+ phosphors (x = 0.01 and 0.07) at 544 nm. This reduction highlights an increased degree of lattice distortion due to Li⁺ inclusion. Furthermore, CIE chromaticity analysis showed that the optimal doping concentration of 0.07 Tb³⁺ shifted the color coordinates towards vivid green, with a color temperature of approximately 6241 K, indicating of neutral white light. © 2024 Elsevier Ltd and Techna Group S.r.l.
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    Photoluminescence properties and structural analysis of Tb³⁺-doped K₃Gd(BO₂)₆: A first study on negative thermal quenching
    (Elsevier Ltd, 2025) Souadi G.; Madkhli A.Y.; Kaynar U.H.; Gok C.; Aydin H.; Coban M.B.; Kaynar S.C.; Ayvacikli M.; Can N.
    In this study, Tb³⁺-doped K₃Gd(BO₂)₆ phosphors were synthesized using the microwave-assisted sol-gel method to explore their photoluminescence (PL) properties and thermal stability. XRD and Rietveld refinement confirmed the incorporation of Tb³⁺ ions, without secondary phases. PL analysis revealed a strong green emission near 542 nm, attributed to the ⁵D₄ → ⁷F₅ transition of Tb³⁺ ions. An optimal Tb³⁺ concentration of 3 wt% was identified, beyond which concentration quenching significantly reduced luminescence intensity. Radiative energy transfer, occurring via reabsorption, was observed at lower concentrations, facilitating efficient energy migration. Conversely, at higher concentrations, non-radiative processes such as cross-relaxation dominated. Remarkably, negative thermal quenching (NTQ) was observed up to 470 K, with an activation energy of 0.96 eV. Additionally, Na⁺ co- doping introduced lattice distortions that enhanced energy transfer between Tb³⁺ ions and improved luminescence efficiency. The chromaticity diagram highlighted a shift towards the yellow-green region with increasing the Tb³⁺ concentration, demonstrating tunable emission properties for solid-state lighting applications. © 2024 Elsevier B.V.