Yazar "Yanmaz, E." seçeneğine göre listele

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    Effect of laser irradiation on activation energy, irreversibility field and upper critical magnetic field of bulk MgB2 superconductor
    (Springer, 2016) Erdem, O.; Yanmaz, E.
    In this study, the role of laser irradiation on the upper critical field (H-c2), the irreversibility field (H-irr) and the activation energy (U-0) of bulk MgB2 superconductor was first time investigated with the aid of magnetoresistivity measurements conducted at different applied magnetic fields in a range of 0-15 kOe as a function of temperature from 25 to 40 K. For this aim, a disk shaped MgB2 superconductor was produced and cut into two pieces. One of the pieces was irradiated under high vacuum conditions by using an Nd:YVO4 laser. The obtained results showed an increase in the critical temperature (T-c) by about 1 K after irradiation. Also, the values of H-c2(0) were respectively found to be around 143 and 151 kOe while the values of H-irr(0) were about 122 and 130 kOe for the reference and the irradiated samples. Using the thermally activated flux flow model the dependence of U-0 on magnetic field was determined. The maximum U-0 values of 0.43 and 0.83 eV in zero magnetic field were respectively obtained for the reference and the irradiated samples. Also, the magnetic field dependence of the critical current density J(c)(B) values at different temperatures were slightly increased after the irradiation. These results point out that the superconducting properties of bulk MgB2 can be improved via laser irradiation.
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    Effect of laser irradiation on superconducting properties of bulk MgB2 sintered at different temperatures
    (Springer, 2017) Erdem, O.; Yanmaz, E.
    The surfaces of MgB2 bulk samples produced by a pellet/closed tube method at two different sintering temperatures of 650 and 850 A degrees C, after hot pressing at 200 A degrees C, were irradiated with the same irradiation dose by using an Nd:YVO4 laser in order to study the possible potentiality of laser irradiation to improve pinning performances and critical current density of MgB2 superconductor. The measurements showed that the magnetic field dependence of the critical current density values of irradiated sample sintered at 650 A degrees C slightly increased with a narrowing in superconducting transition region as compared to the reference sample sintered at same temperature. However, irradiated sample sintered at 850 A degrees C showed a decrease in pinning performance and similar critical temperature values as compared to the corresponding reference sample. From these results it can be said that the same laser irradiation dose affects superconducting properties of bulk MgB2 in different ways depending on sintering temperature of the superconductor.
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    Enhanced Pinning Properties of Laser-Irradiated Bulk MgB2 Superconductors
    (Springer, 2017) Erdem, O.; Yanmaz, E.
    In order to investigate the effect of laser irradiation on the superconducting properties of MgB2, a pellet-shaped sample was produced and cut into two pieces. The dot-shaped molten patterns, with the distance of 100 A mu m between them, were formed on the surface of one of the pieces by using an Nd:YVO4 laser. The grains of the irradiated sample were significantly grown due to the laser-induced sintering. The modified surface appeared to have dense and compacted grains as a result of increasing interconnectivity among the superconducting grains. The critical current densities at 5 K in a field of 3 T were found to be 5.38 x 10(4) and 8.96 x 10(4) A cm(-2) for the reference and the irradiated samples, respectively. The increment of critical current density for the irradiated sample is presumably connected with the changing of pinning mechanism after the laser irradiation. For the irradiated sample which showed a higher value of critical current density at 5 K under high fields, Delta kappa pinning was obtained as a dominant pinning mechanism, while the normal point pinning was dominant for the reference one. These results point out that the superconducting properties of bulk MgB2 can be changed and improved via laser irradiation in a short time.
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    Improving magnetic properties of MgB2 bulk superconductors by synthetic engine oil treatment
    (Elsevier Science Bv, 2016) Koparan, E. Taylan; Savaskan, B.; Yanmaz, E.
    The present study focuses on the effects of standby time of the MgB2 samples immersed in synthetic engine oil on the critical current density (J(c) (H)), magnetic field dependence of the pinning force density f(p)(b) and T-c performances of MgB2 bulk superconductors. Synthetic engine oil was used as a product which is cheap and a rich carbon source. Manufactured MgB 2 pellet samples were immersed at different standby time of 30 min, 120 min, 300 min and 1440 min in synthetic engine oil after the first heating process. Finally, MgB2 samples immersed in synthetic engine oil were sintered at 1000 degrees C and kept for 15 min in Ar atmosphere. The critical current density of all of MgB2 samples immersed at different standby time in engine oil in whole field range was better than that of the pure MgB2 sample because of the number of the pinning centers. The MgB2 sample immersed at 300 min standby time in synthetic engine oil has the best performance compared to other samples. The J(c) value for the pure sample is 2.0 x 10(3) A/cm(2), whereas for the MgB 2 sample immersed at 300 min standby time in engine oil the Jc is enhanced to 4.8 x 10(3) A/cm(2) at 5 K and 3 T. The superconducting transition temperature (Tc) did not change with the increasing standby time of the samples in synthetic engine oil at all. The best diamagnetic property was obtained from the sample which kept in synthetic engine oil for 300 min. Synthetic engine oil treatment results in remarkable improvement of the critical current density and pinning force performances of MgB2 superconductors. It was found that all MgB2 samples have a different pinning property at different measuring temperatures. Using synthetic engine oil as a product which is cheap and a rich carbon source in MgB2 bulk superconductors makes MgB 2 samples immersed in synthetic engine oil a good candidate for industrial applications. (C) 2016 Elsevier B.V. All rights reserved.
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    The size effect on the magnetic levitation force of MgB2 bulk superconductors
    (Elsevier Sci Ltd, 2016) Savaskan, B.; Koparan, E. T.; Guner, S. B.; Celik, S.; Yanmaz, E.
    In this study, the size effect on the magnetic levitation performance of disk-shaped MgB2 bulk superconductors and permanent magnets was investigated. MgB2 samples with varying diameters of 13 mm, 15 mm and 18 mm, each of which were 2 g in mass, were prepared by two-step solid state reaction method. Vertical levitation force measurements under both zero-field-cooled (ZFC) and field-cooled (FC) regimes were carried out at different temperatures of 20, 24 and 28 K. It was determined that the levitation force of the MgB2 strongly depends on both the diameters of the sample and the permanent magnet. In ZFC regime, the maximum levitation force value for the permanent magnet and the sample 18 mm in diameters reached to the 8.41 N at 20 K. In addition, in FC regime, attractive and repulsive force increased with increasing diameters of the sample and the permanent magnet. In that, the sample with 18 mm in diameter showed the highest attractive force value -3.46 N at 20 K and FC regime. The results obtained in this study are very useful in magnetic levitation devices as there is no detailed study on the size of superconductors and permanent magnets. (C) 2016 Elsevier Ltd. All rights reserved.
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    Switching and Decoupling Effects in a Single-Phase Transformer Using Extra DC Current
    (IEEE-Inst Electrical Electronics Engineers Inc, 2016) Gecer, S.; Ertekin, E.; Yanmaz, E.; Kosa, J.; Safran, S.; Ozguzel, R.; Kilicarslan, E. S.
    A single-phase current-limiting transformer has been developed based on flux transfer. The transformer consists of two independent iron cores, one primary coil, two separate secondary coils, and superconducting windings/loops. Two iron cores are in a mechanical contact and clamped. Superconducting windings/loops act as coupling/decoupling mechanism between two iron cores. The transformer has been made operational via a switchgear of the applied dc current for a potential use in the current limitation when the transition between the superconducting and normal (quench) states can be triggered. DC and ac currents can be applied simultaneously in fault and no-fault conditions to mimic the realistic cases of use. With decoupling ac energy transfer, we propose that conventional switching off may be replaced with the approach described.
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    Test of 6-kVA Three-Phase Flux Transfer-Type Current-Limiting Transformer
    (Springer, 2017) Ertekin, E.; Gecer, S.; Yanmaz, E.; Safran, S.; Kosa, J.; Kilicarslan, E. S.; Kilic, A.
    A 6-kVA three-phase model of the flux transfer-type current-limiting transformer was developed and tested. In this device, the winding loops of YBCO superconducting tapes couple magnetically two independent iron cores: the primary-side iron core and the secondary-side iron core. The former and the latter are equipped with copper primary and secondary windings, respectively. Because the magnetic fluxes linked to the superconducting winding loops must be kept constant, the magnetic flux is transferred by the superconducting YBCO loops between the two iron cores in order to couple magnetically the primary and secondary coils. While the YBCO loops are superconducting, 100 % of the magnetic flux is transferred and the device shows the similar function as usual transformers. Once the YBCO loops become normal by a fault current in any of the windings, the power transfer between two iron cores is limited and the current in the secondary winding is limited naturally on a result of decoupling the iron cores.