• E Yanmaz

      Articles written in Bulletin of Materials Science

    • Pinning enhancement in MgB2 superconducting thin films by magnetic nanoparticles of Fe2O3

      E Taylan Koparan A Surdu K Kizilkaya A Sidorenko E Yanmaz

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      MgB2 thin films were fabricated on 𝑟-plane Al2O3 (1$\bar{1}$02) substrates. First, deposition of boron was performed by rf magnetron sputtering on Al2O3 substrates and followed by a post-deposition annealing at 850 °C in magnesium vapour. In order to investigate the effect of Fe2O3 nanoparticles on the structural and magnetic properties of films, MgB2 films were coated with different concentrations of Fe2O3 nanoparticles by spin coating process. The magnetic field dependence of the critical current density 𝐽c was calculated from the M–H loops and magnetic field dependence of the pinning force density, 𝑓p(𝑏), was investigated for the films containing different concentrations of Fe2O3 nanoparticles. The critical current densities, 𝐽c, in 3Tmagnetic field at 5 K were found to be around 2.7 × 104 A/cm2, 4.3 × 104 A/cm2, 1.3 × 105 A/cm2 and 5.2 × 104 A/cm2 for films with concentrations of 0, 25, 50 and 100% Fe2O3, respectively. It was found that the films coated with Fe2O3 nanoparticles have significantly enhanced the critical current density. It can be noted that especially the films coated by Fe2O3 became stronger in the magnetic field and at higher temperatures. It was believed that coated films indicated the presence of artificial pinning centres created by Fe2O3 nanoparticles. The results of AFM indicate that surface roughness of the films significantly decreased with increase in concentration of coating material.

    • Nanoindentation study on Gd-deposited YBaCuO superconductor

      F Yilmaz O Uzun U Kolemen M F Kilicaslan N Basman S Ergen K Ozturk E Yanmaz

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      Nanoindentation technique was used to characterize the mechanical properties of Gd-deposited bulk YBaCuO superconductors fabricated by solid-state reaction method. In order to determine the hardness and reduced modulus of the samples, load-displacement data were analysed by using the Oliver–Pharr method. The hardness values exhibited significant peak load-dependence especially at lower peak loads, while the reduced modulus values were found to be nearly constant at studied loading range. In order to find true hardness of the samples, the peak load-dependency of hardness was analysed by using Meyer’s law,minimum resistance model, elastic/plastic deformation model, energy balance model, Nix–Gao model and Mukhopadhyay approach. Of the aforementioned models, energy balance model and Mukhopadhyay approach were found to be the most effective models to explain the load-dependency of hardness.

    • Effect of Er doping on the superconducting properties of porous MgB2

      O Erdem E Yanmaz

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      MgB2 bulk sample with porous structure was produced by using the in-situ solid-state reaction method under argon (Ar) atmosphere of 10 bar. Elemental Er in powder form was doped into MgB2 with different compositions (Mg1−𝑥Er𝑥)B2, where 𝑥 = 0.00, 0.03 and 0.05, in order to investigate the effect of rare-earth (RE) element Er on the structural and electromagnetic properties of porous MgB2. The Er-doped samples result in small grain size structure compared to the undoped one. The lattice constants 𝑎 and 𝑐 of the doped samples, determined from X-ray diffraction (XRD) analysis, increase with the increasing Er content, and consequently the superconducting transition temperature ($T^{\text{onset}}_{c}$) of MgB2, determined from resistivity measurements, is slightly suppressed. Also, the upper critical field ($B_{c2}$), the irreversibility field ($B_{\text{irr}}$) and the critical current density ($J_{c}$) values are significantly enhanced in the doped samples. For the best sample (𝑥 = 0.03), at 15 K under a magnetic field of 4 T, the $J_{c}$ value reaches 2.4×104 A cm-2, which is higher than that of the porous sample by an order of 103, and the $B_{\text{irr}}$ value at 20 K reaches 9.7 T. These results imply that the RE element Er fills the pores, enhances the density and the grain connectivity. Hence, the superconducting properties of the porous MgB2 sample improve by Er doping.

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