Browsing by Author "Koz, C."

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    Homogeneity range of ternary 11-type chalcogenides Fe1 + yTe1−xSex
    (Springer, 2017) Koz, C.; Rößler, S.; Wirth, S.; Schwarz, U.
    The 11-type Fe-chalcogenides belong to the family of Fe-based superconductors. In these compounds, the interstitial Fe is known to strongly influence the magnetic and superconducting properties. Here, we present the chemical homogeneity range of ternary compounds Fe1 + yTe1−xSex based on powder x-ray diffraction, energy dispersive x-ray analysis, and magnetization measurements. Our investigations show that the maximum amount of excess Fe in homogeneous Fe1 + yTe1−xSex decreases with increase in Se substitution for Te. Using our synthesis procedure, single-phase Fe1 + yTe1−xSex, with 0.5 ≤ x < 1 could not be formed for any amount of excess Fe. Further, the superconducting volume fraction in the material is found to be strongly suppressed by excess Fe.
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    Nanohybrid structured RuO2/Mn2O3/CNF as a catalyst for Na-O2 batteries
    (Institute of Physics Publishing, 2018) Tovini, M. F.; Patil, B.; Koz, C.; Uyar, Tamer; Yılmaz, E.
    A 3D RuO2/Mn2O3/carbon nanofiber (CNF) composite has been prepared in this study by a facile two step microwave synthesis, as a bi-functional electrocatalyst towards oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). RuO2 nanoparticles with the mean size of 1.57 nm are uniformly distributed on Mn2O3 nano-rods grown on electrospun CNFs. The electrocatalytic activity of the composites are investigated towards ORR/OER under alkaline condition. The ternary RuO2/Mn2O3/CNF composite showed superior ORR activity in terms of onset potential (0.95 V versus RHE) and Tafel slope (121 mV dec-1) compared to its RuO2/CNF and Mn2O3/CNF counterparts. In the case of OER, the RuO2/Mn2O3/CNF exhibited 0.34 V over-potential value measured at 10 mA cm-2 and 52 mV dec-1 Tafel slope which are lower than those of the other synthesized samples and as compared to state of the art RuO2 and IrO x type materials. RuO2/Mn2O3/CNF also exhibited higher specific capacity (9352 mAh ) than CNF (1395 mAh ), Mn2O3/CNF (3108 mAh ) and RuO2/CNF (4859 mAh g carbon -1) as the cathode material in Na-O2 battery, which indicates the validity of the results in non-aqueous medium. Taking the benefit of RuO2 and Mn2O3 synergistic effect, the decomposition of inevitable side products at the end of charge occurs at 3.838 V versus Na/Na+ by using RuO2/Mn2O3/CNF, which is 388 mV more cathodic compared with CNF.
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    Spectroscopic characterization of tungstated zirconia prepared by equilibrium adsorption from hydrogen peroxide solutions of tungsten(VI) precursors
    (2007) Kantcheva, M.; Koz, C.
    Two series of WO x /ZrO2 samples are prepared by equilibrium adsorption from H2O2 solutions at pH 1.8 containing two different precursor anions, [W2O3(O 2)4(H2O)2]2- and [H 2W12O40]6-. The starting material is amorphous zirconium oxyhydroxide. The maximum W densities obtained are larger than that reported in the literature for systems synthesized by the same method using aqueous non-peroxide solutions. In the case of the metatungstate precursor, this increase is attributed to the generation of additional anchoring sites by interaction between the amorphous support and H2O 2. The high uptake achieved when the peroxo complex is used as a precursor is a result of both the ZrO x (OH)4-2x -H 2O2 interaction and low nuclearity of the adsorbing anion. The materials are characterized by XRD, DR-UV-vis, Micro-Raman and FT-IR spectroscopy. The surface acidities of samples with identical W loading prepared by equilibrium adsorption from the [H2W12O 40]6--H2O2 system and by impregnation with aqueous solution of ammonium metatungstate are investigated by FT-IR spectroscopy of CO adsorbed at 80 K. © Springer Science+Business Media, LLC 2007.
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    X-ray Raman spectroscopy of lithium-ion battery electrolyte solutions in a flow cell
    (Wiley-Blackwell, 2018) Ketenoglu, D.; Spiekermann, G.; Harder, M.; Oz, E.; Koz, C.; Yagci, M. C.; Yilmaz, E.; Yin, Z.; Sahle, C. J.; Detlefs, B.; Yavaş, H.
    The effects of varying LiPF6 salt concentration and the presence of lithium bis(oxalate)borate additive on the electronic structure of commonly used lithium-ion battery electrolyte solvents (ethylene carbonate-dimethyl carbonate and propylene carbonate) have been investigated. X-ray Raman scattering spectroscopy (a non-resonant inelastic X-ray scattering method) was utilized together with a closed-circle flow cell. Carbon and oxygen K-edges provide characteristic information on the electronic structure of the electrolyte solutions, which are sensitive to local chemistry. Higher Li+ ion concentration in the solvent manifests itself as a blue-shift of both the π∗ feature in the carbon edge and the carbonyl π∗ feature in the oxygen edge. While these oxygen K-edge results agree with previous soft X-ray absorption studies on LiBF4 salt concentration in propylene carbonate, carbon K-edge spectra reveal a shift in energy, which can be explained with differing ionic conductivities of the electrolyte solutions.Electronic structures of commonly used lithium-ion battery electrolyte solutions measured by non-resonant inelastic X-ray scattering method are presented.