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  • Küçük Resim
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    Ac Susceptibility measurements and mechanical performance of Bulk MgB2
    (Springer, 2015) Öztürk, Özgür; Aşıkuzun, Elif; Kaya, Şeydanur; Erdem, Murat; Safran, Serap; Kılıç,Ahmet; Terzioğlu, Cabir
    The effects of Ar ambient pressure (vacuum and 0, 10, and 20 B) and annealing times (0.5 and 1 h) on microstructural, superconducting, and mechanical properties of bulk superconducting MgB2 are investigated. The samples are produced using the solid-state reaction method. X-ray diffraction (XRD) and scanning electron microscopy (SEM) measurements are performed for determination of the crystal structure and surface morphology of MgB2 samples, respectively. The superconducting properties are studied by AC magnetic susceptibility measurements. Microhardness analyses are made using the Vickers microhardness test for determination of mechanical properties of all samples. Increasing the Ar pressure decreases the lattice parameters and hence the average grain size. Increasing the annealing time results in larger lattice parameters and larger grain formation. The susceptibility measurements revealed a two-step transition which is reminiscent of granular superconductors. The intra-grain transition temperature is determined to be 38.4 K for all samples. The inter-grain transition temperature of 37.2 K is obtained for samples produced under Ar ambient pressure. The samples produced under Ar ambient pressure have better superconducting properties than the ones produced in vacuum. Increasing the annealing time under vacuum further decreases the superconducting properties probably due to Mg loss.
  • Küçük Resim
    Öğe
    The effect of Ar ambient pressure and annealing duration on the microstructure, superconducting properties and activation energies of MgB2 superconductors
    (Springer, 2017) Öztürk, Özgür; Aşıkuzun, Elif; Kaya, Şeydanur; Koç, Nevin Soylu; Erdem, Murat
    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 (J(c)). 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 T-c values of the samples produced under vacuum are the highest. Tc values decrease with increasing ambient pressure and applied magnetic field. The activation energies (U-0) 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.