Browsing by Author "Koc N.S."

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The Effect of Ar Ambient Pressure and Annealing Duration on the Microstructure, Superconducting Properties and Activation Energies of MgB2 Superconductors
(2017-05-01) Ozturk O.; Asikuzun E.; Kaya S.; Koc N.S.; Erdem M.
MgB2 samples are produced by ex situ reaction method under vacuum, and various (0, 10, 20 Bar) Ar pressure for 0.5 and 1 h. The effect of ambient pressure and annealing duration on the microstructure and electrical properties of bulk samples are investigated. XRD, SEM, and magnetoresistance measurements are made. The a and c lattice parameters and the grain size values are the highest for the samples produced under vacuum and their values decrease with increasing Ar pressure. Moreover, these values increase when the annealing duration increases from 0.5 to 1 h. The increasing pressure reduces the bond lengths between the atoms thus the grain sizes decrease. Smaller grain size promotes the connection between grains which results in an increase of the critical current density (Jc). SEM micrographs reveal that the produced samples have granular structure which is a characteristic feature of MgB2. The decrease of grain sizes and thus enhancement in grain connectivity with increasing pressure is also confirmed by SEM images. Magneto resistivity measurements show that Tc values of the samples produced under vacuum are the highest. Tc values decrease with increasing ambient pressure and applied magnetic field. The activation energies (U0) of the samples are calculated using Arrhenius plots due to thermally activated flux flow theory. Existence and increase of Ar pressure causes increase of activation energies. The samples produced with 0.5-h annealing have higher activation energies than the ones produced with 1-h annealing.
The structural and dynamic nanohardness properties of yttrium substituted layered (La, Ca)3Mn2O7 manganites
(2023-01-01) Koc N.S.; Terzioglu R.; Ozturk O.; Terzioglu C.
This study devotes the structural and dynamic nanohardness properties of yttrium (Y) substituted within La1.4Ca1.6Mn2O7 manganites. XRD, SEM, EDS and dynamic nanohardness measurements are carried out to determine the structural properties such as grain sizes, impurity phases, crystallite sizes, average strain, and surface morphologies. The obtained results were compared with other rare-earth (RE) substituted double-layered lanthanum manganites published in the literature that have the same A-site radius. The lattice parameters are not affected substantially by the Y addition while the average grain size is significantly different from the other RE-substituted samples. The mechanical properties obtained from the dynamic micro-hardness tests such as the contact depth, area of the indent imprint, stiffness, creep, apparent hardness, reduced elastic modulus and the elastic recovery ratio (ERR) are calculated for the Y-substituted sample and compared with previously published studies. Different models are applied to determine the measured and true micro-hardness values. The microstructural and mechanical properties of Y-substituted double-layered manganites are discussed in detail.