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Öğe Ac loss and ramp up related heating effects in superconducting mgb(2) coils(Ieee-Inst Electrical Electronics Engineers Inc, 2020) Yetiş, Hakan; Akdoğan, Mustafa; Karaboğa, Fırat; Avcı, Doğan; Belenli, İbrahimSuperconducting coils modeled by using single core MgB2 wires having copper or iron or both in the cross section as sheath materials have been examined numerically by using the magnetic field formulation and heat transfer modules of COMSOL Multiphysics software. Heating effects due to the core ac losses and skin effects in MgB2 coils are revealed for alternating currents in a liquid helium environment. Temperature changes, magnetic flux density profiles, core ac losses for each coil are obtained in one full cycle of applied alternating current. The effect of copper layer thickness on core ac loss behavior of MgB2/Cu coil and transport losses in MgB2/Cu and MgB2/Fe coils as a function of current ramp rate are investigated for an applied direct current up to 300 A.Öğe AC susceptibility analysis on MgB2 bulk and Ti-sheathed wire superconductors(Elsevier Sci Ltd, 2014) Çiçek, Özlem; Yetiş, Hakan; Gencer, AliWe report a comparative study for bulk MgB2 superconductors and monofilamentary Ti/MgB2 wires as functions of the boron powder purity, average particle size of magnesium, and applied pressure. The structural and magnetic characterizations of the bulk samples were performed by means of XRD and AC susceptibility measurements, respectively. We found that the applied pressure did not cause any significant change on the onset transition temperature T-c,T-onset and transition width. This is also confirmed by the calculation of the lattice parameters. On the other hand, T-c,T-onset values were measured as 37.3 K, 38.0 K, and 38.6 K for the samples prepared from the precursor boron powders with purities of 95.2%, >= 95%, and >98%, respectively. However Mg-particle size (Mg1: AlfaAesar Mg powder, avg. par. size: -325 mesh, purity: 99.8%; Mg2: SigmaAldrich Mg powder, avg. par. size: 20-230 mesh, purity: 98%) did not affect the T-c,T-onset but increasing the Mg particle size caused a significant broadening at the superconducting transition width for all precursor boron powder purity levels. Magnetization measurements showed that the critical current density of Ti/MgB2 wire prepared from AlfaAesar Mg powder (avg. par. size: -325 mesh) and Aldrich B powder (amorphous, purity >= 95%) is greater than that of prepared by SigmaAldrich Mg powder (avg. par. size: 20-230 mesh) and Pavezyum B powder (amorphous, avg. par. size: 0,3 mu m (max), purity > 98%) as 2.6 x 10(5) A/cm(2) and 1.4 x 10(5) A/cm(2) at T = 10 K and mu H-0 = 0.5 T, respectively. In addition, SigmaAldrich Mg and Pavezyum B wire sample has a lower pinning force, F-p, value at each temperature. (C) 2014 Elsevier Ltd. All rights reserved.Öğe Analysis of observed voltage oscillations in silver doped high temperature superconductor ybco(Polish Acad Sciences Inst Physics, 2018) Altınkök, Atılgan; Olutaş, Murat; Yetiş, Hakan; Kılıç, Atilla; Kılıç, KıvılcımThe effect of square wave current was investigated by the voltage time (V-t curves) measurements at various external magnetic fields and periods (P) in silver doped YBCO. The general sinusoidal behavior of V-t curves was mainly interpreted as a dynamic competition between driving and pinning forces. It is observed that as the period of square wave current is increased, the amplitude of oscillation is also increased. The observed oscillations in the voltage was fitted by an common sinusoidal equation V(t) similar to sin(omega t + phi). It is also found that there is different phase angle phi values for each cycle. Fast Fourier transform measurements is applied to oscillation period (P-I) of the square wave current. The results give us that a physical mechanism is related with charge density waves. Intrinsically, in Ag doped YBCO, the pinned flux line system evokes the general behavior of charge density waves. Due to properties of the converting the square wave current to sinusoidal voltage oscillations, Ag doped YBCO sample can behave as double-integrator for the defined period, amplitude of square wave current and magnetic field values in time.Öğe Antidot effects on micromagnetic behavior of Py ferromagnetic samples(Elsevier, 2016) Yetiş, Hakan; Denizli, HalukThe coercivity and magnetic hysteresis behavior of permalloy (Py) samples have been studied in the presence of square arrays of the circular antidots. The open source OOMMF micromagnetic software is used to numerically solve the Landau-Lifshitz-Gilbert (LLG) equation. In calculations, Py samples are designed in such a way that they include a different number of antidot in an array which possess the same total surface area. In this way, the total Py region stayed unchanged despite the growing number of antidots in a fixed sample size. We found significant increase in the coercive field for the sample with the smallest antidot spacing. The results are discussed within the framework of superdomain (SD) and superdomain wall (SDW) formation. (C) 2016 Elsevier B.V. All rights reserved.Öğe Antidot shape dependence of switching mechanism in permalloy samples(Elsevier, 2017) Yetiş, Hakan; Denizli, HalukWe study antidot shape dependence of the switching magnetization for various permalloy samples with square and triangular arrays of nanometer scale antidots. The remnant magnetization, squareness ratio, and coercive fields of the samples are extracted from the hysteresis loops which are obtained by solving the Landau-Lifshitz-Gilbert (LLG) equation numerically. We find several different magnetic spin configurations which reveal the existence of superdomain wall structures. Our results are discussed in terms of the local shape anisotropy, array geometry, and symmetry properties in order to explain the formation of inhomogeneous domain structures. (C) 2016 Elsevier B.V. All rights reserved.Öğe Coherent voltage oscillations in superconducting polycrystalline Y(1)Ba(2)Cu(3)O(7-x)(Iop Publishing Ltd, 2006) Altınkök, Atılgan; Yetiş, Hakan; Olutaş, Murat; Kılıç, Kıvılcım; Kılıç, Atilla; Çetin, OktayWe have investigated the voltage response of superconducting polycrystalline bulk Y(1)Ba(2)Cu(3)O(7-x) (YBCO) material to a bidirectional square wave current with long periods and dc current by means of the evolution of the voltage-time (V-t) curves near the critical temperature. In a well-defined range of amplitudes and periods of driving current, and temperatures, it was observed that a non-linear response to bidirectional square wave current rides on a time independent background voltage value and manifests itself as regular sinusoidal-like voltage oscillations. It was found that the non-linear response disappears when the bidirectional current was switched to dc current. The spectral content of the voltage oscillations analyzed by the Fast Fourier Transform of the corresponding V-t curves revealed that the fundamental harmonics is comparable to the frequency of bidirectional square wave current. The coherent voltage oscillations were discussed mainly in terms of the dynamic competition between pinning and depinning together with the disorder in the coupling strength between the superconducting grains (i.e Josephson coupling effects). The density fluctuations and semi-elastic coupling of the flux lines with the pinning centers were also considered as possible physical mechanisms in the interpretation of the experimental results.Öğe Current dependent reorganization in superconducting Y1Ba2Cu3O7-delta(Springer, 2005) Kılıç, Atilla; Kılıç, Kıvılcım; Yetiş, Hakan; Çetin, OktayWe present strong non-linear dynamic responses developing due to magnitude and type of driving current in bulk polycrystalline superconducting Y1Ba2Cu3O7-delta sample at zero magnetic field. Several novel types of dynamic changes induced by the transport current were observed via the time evolution of the voltage ( V - t curves). The physical observations appearing in V - t curves were interpreted mainly with the reorganization of driving current in a multiply connected network of weak-link structure. It was found that such a dynamic process could cause an enhancement or suppression in superconducting order parameter due to the magnitude of the driving current and coupling strength of weak link structure together with the chemical and anisotropic states of the sample. It was shown that the general behavior of decays evolving in V - t curves is consistent with an exponential relation which is analogous to the glassy state relaxation.Öğe Current-induced voltage oscillations in superconducting Y1Ba2Cu3O7-delta(Springer, 2005) Kılıç, Kıvılcım; Kılıç, Atilla; Altınkök, Atılgan; Yetiş, Hakan; Çetin, OktayWe investigated influence of bidirectional square wave current with long periods and dc current on the evolution of the voltage-time (V - t) curves in superconducting polycrystalline bulk Y1Ba2Cu3O7-delta (YBCO) material at the temperatures near the critical temperature. In a well-defined range of amplitudes and periods of driving current, and temperatures, novel type of non-linear dynamic responses was observed by means of the V-t curves. It was seen that such a non-linear response to bidirectional square wave current sometimes reflects itself as regular sinusoidal-like voltage oscillations. The sinusoidal-like and non-sinusoidal oscillations were discussed mainly in terms of the dynamic competition between pinning and depinning and significant relaxation effects which appear in this competing process. The density fluctuations associated with the current induced self-magnetic flux (SMF) lines and semi-elastic coupling of SMF lines with the pinning centers were also considered as possible physical mechanisms in the interpretation of the experimental results.Öğe Design, fabrication, and testing of MgB2/Fe racetrack coils(Ieee-Inst Electrical Electronics Engineers Inc, 2018) Karaboğa, Fırat; Yetiş, Hakan; Akdoğan, Mustafa; Gajda, Daniel; Belenli, İbrahimWe fabricated four superconducting racetrack coils wound by bare in situ MgB2/Fe mono and multifilamentary wires produced in our laboratory by using the wind-and-react method. Transport measurements in self-field were performed in a liquid helium dewar. The magnetic field flux density B = 25 mT for I = 92 A was measured to verify how the current flowed inside the coil for one of the coils by means of a cryogenic axial Hall sensor placed into the central bore region of the coil. The coil with 36 turns wound by mono MgB2/Fe wire of l = 24 m has a highest current-carrying capacity of more than 150 A at T = 4.2 K and self-field among the coils. The coils fabricated by multifilamentary wires have relatively low engineering critical current values of I-ce = 67 A for 18 + 1 and I-ce = 57 A for 4 + 5 superconducting/copper filament wires at T = 4.2 K in self-field. Our results form essential contribution toward determination of the possible use of low-cost MgB2/Fe wires in low-field superconductor coil applications.Öğe Effect of columnar defects on magnetization of mesoscopic superconductors(Springer/Plenum Publishers, 2012) Yetiş, HakanIn this work we investigated magnetization and vortex configurations in mesoscopic superconducting samples in the presence of square columnar defects (CDs). We solved numerically the nonlinear TDGL equations for different samples to study magnetization as a function of the applied magnetic field. In calculations, we focused mainly on four samples with different numbers of CDs which have the same total surface area. In this way, the total superconducting area remained the same with increasing the number of CDs for a fixed sample size. We found that the superconducting regions still exist inside the sample at high applied magnetic fields with increasing the number of CDs but irreversible effects became increasingly prominent, when the field is returned to zero. The results are discussed in frame of surface and pinning effects in mesoscopic systems.Öğe Effect of different-sized h-BN nano-particles on some properties of MgB2 superconductors(Springer, 2016) Karaboğa, Fırat; Yetiş, Hakan; Öz, Muhammed; Belenli, İbrahimThe present study reports the effect of hexagonal boron nitride (h-BN) doping on the superconducting properties of bulk MgB2 samples. Structural, electrical and magnetic properties of 3 wt% h-BN doped MgB2 samples are studied in terms of three different sized h-BN nanoparticles. The results show that h-BN doping has no significant change on critical transition temperature (+/- 0.5 K deviation), but decreases the magnetic J (c) with increasing the h-BN particle size. Among the doped samples, a highest J (c) (T = 10 K) value of 1.8 x 10(4) A/cm(2) is achieved for the sample doped with the smallest sized h-BN at zero external magnetic field but it is still lower than the 2.5 x 10(4) A/cm(2) obtained for the un-doped sample. In the scope of this study, it is investigated whether or not the nano-sized h-BN particle is a possible candidate to improve the pinning properties of the MgB2 sample and we suggest that particle size is very important parameter in h-BN doping into MgB2.Öğe Effect of the initial filling density on the critical current of in-situ Fe/MgB2 wires(Elsevier Science Sa, 2015) Akdoğan, Mustafa; Yetiş, Hakan; Gajda, Daniel; Karaboğa, Fırat; Ülgen, Asaf Tolga; Demirtürk, Emine; Belenli, İbrahimWe report a comparative study on the fabrication of in-situ monofilamentary superconducting Fe sheathed MgB2 wires as a function of the initial tube filling density. The wire samples are fabricated by means of a standard Powder-in-Tube (PIT) method with two different initial powder filling densities. Transport measurements are applied under high magnetic fields, of up to 10 T, obtained in a Bitter magnet. We find that increasing the initial filling density improved the critical current of the Fe/MgB2 wires, and a high I-c (4.2 K) = 140 A at B = 5 T is achieved. (C) 2015 Elsevier B.V. All rights reserved.Öğe Effects of mg-coating and boron type on electromechanical properties of IMD MgB2/Fe wires(Springer, 2021) Avcı, Doğan; Yetiş, Hakan; Karaboğa, Fırat; Akdoğan, Mustafa; Belenli, İbrahimIn this study, mechanical and electrical characterization measurements were performed on non-bent and gradually bent IMD MgB2/Fe superconducting round wires having 0.81 mm outer diameter. Four straight IMD wires were successively bent to 150 mm, 125 mm, and 100 mm bending diameters and characterized after each bending step. IMD wires were produced using two types of boron powders (PVZ Nano Boron, purity > 98.5%, particle sizes < 250 nm, and PVZ boron, purity of 95-97%, particle sizes < 1 mu m) by filling them into Mg-coated and uncoated iron tubes. The coated Mg was considered as an extra source of Mg that diffuses into the Mg + 2B core without causing any porosity during the reaction. We have investigated the effect of Mg coating on the transport and electromechanical properties of the wires and found that the Mg coating process improves the J(ce) before and after bending. Also, our results have revealed that crack formation in core regions of mono filament bent IMD wires depends on the type of boron, since the use of nano-sized boron significantly increases the hardness of the MgB2 core.Öğe Fabrication and characterization of C15H12O2 doped MgB2 bulk superconductors(Elsevier Sci Ltd, 2014) Ağıl, Hasan; Yetiş, Hakan; Akdoğan, Mustafa; Altuğ, Cevher; Aktürk, SelçukThe synthesis and characterization of bulk MgB2 superconductors doped with various amounts of organic compound C15H12O2 are reported. We prepared MgB2 + C15H12O2 samples by means of wet-mixing process in ethyl toluene. A high J(c) value 3.6 x 10(5) A/cm(2) in self field was achieved at 20 K for 5 wt% C15H12O2 doped MgB2 sample. The thermal and microstructural analysis of the doped samples revealed the existence of significant differences in their granular structures. The results were interpreted in terms of the additive effects of C15H12O2. Our findings suggested that C15H12O2 promises to be an appropriate material to improve J(c) in a wide magnetic field range at low dopant levels. (C) 2014 Elsevier Ltd. All rights reserved.Öğe Flux dynamics and magnetovoltage measurements in a macroscopic cylindrical hole drilled in BSCCO(Elsevier, 2007) Yetiş, Hakan; Altınkök, Atılgan; Olutaş, Murat; Kılıç, Atilla; Kılıç, KıvılcımSlow transport relaxation measurements (V-t curves) and magnetovoltage measurements (V-H curves) were carried out in a polycrystalline sample of Bi1.7Pb0.3Sr2Ca2Cu30, (BSCCO) with a macroscopic cylindrically drilled hole (CH). The time evolution of quenched state in V-t curves was interpreted in terms of enhancement of the superconducting order parameter and the relaxation of moving entity. Upon cycling of the external magnetic field with different sweep rates, unusual counter clockwise hysteresis effects and asymmetry in V-H curves are observed in BSCCO sample with CH, which can also be correlated to the trapping of the macroscopic flux bundles in CH. (C) 2007 Elsevier B.V. All rights reserved.Öğe Flux dynamics and time effects in a carved out superconducting polycrystalline Bi-sr-ca-cu-o sample(Iop Publishing Ltd, 2006) Olutaş, Murat; Yetiş, Hakan; Altınkök, Atılgan; Sözeri, Hüseyin; Kılıç, Kıvılcım; Kılıç, Atilla; Çetin, OktaySystematic slow transport relaxation (V-t curves) and magnetovoltage measurements (V-H curves) have been carried out in a carved out superconducting polycrystalline Bi-Sr-Ca-Cu-O sample as a function of current (I), temperature (T), and external field (H). The V-t curves reveal the details of the time evolution of the penetrated state within the granular structure of the sample and also give a direct evidence of the relaxation of the flux trapped inside the drilled hole on the time scale of the experiment. On the other hand, V-H curves exhibit several unusual interesting properties upon cycling of the external magnetic field in forward and reverse directions, and, in addition to irreversibilities, strong reversible effects are observed, which is associated with the trapping of the macroscopic flux bundles in the drilled hole. It is also observed that the field sweep rate influences dramatically the reversible and irreversible behavior of V-H curves. The experimental results were mainly interpreted in terms of current and field induced organization of the vortices.Öğe Flux trapping in a macroscopic cylinderical hole drilled in Bi-Sr-Ca-Cu-O(Springer, 2006) Kılıç, Atilla; Kılıç, Kıvılcım; Yetiş, Hakan; Olutaş, Murat; Altınkök, Atılgan; Sözeri, Hüseyin; Çetin, OktayThe flux dynamics in a polycrystalline sample of Bi1.7Pb0.3Sr2Ca2Cu3Ox with a macroscopic cylindrical hole (CH) drilled was investigated by slow transport relaxation (V-t curves) and magnetovoltage measurements (V-H curves). It was monitored that there are several discontinuities in the time evolution of quenched state in V-t curves, which was attributed to the leaving of quantized flux lines trapped through CH together with surface superconducting effects. We observed that asymmetric V-H curves demonstrate unusual remarkable counter clockwise hysteresis effects upon cycling of field. This interesting result was correlated mainly to the flux trapping inside the CH that acts as a macroscopic attractive pinning center for flux lines. Further, the hysteresis effects in V-H curves for a fixed transport current provide a direct evidence that the number of flux lines, measured dissipation and relative decrease/increase in irreversibilities could be determined by sweeping rate of external magnetic field (dH/dt) which leads also to peculiar time effects.Öğe Formation of high-field pinning centers in superconducting MgB2 wires by using high hot isostatic pressure process(Springer, 2017) Gajda, Daniel; Morawski, Andrzej; Zaleski, Andrzej Janusz; Akdoğan, Mustafa; Yetiş, Hakan; Karaboğa, Fırat; Belenli, İbrahimThis paper demonstrates the effects of hot isostatic pressure (HIP) on the structure and transport critical parameters of in situ MgB2 wires without a barrier. Our results show that only HIP and nano-boron allow the formation of more high-field pinning centers, which lead to the increase in critical current density (J (c)) at high applied magnetic fields. Nano-boron and annealing at a low pressure increase the J (c) in the low magnetic field. This indicates that nano-particles create more high-field pinning centers. In addition, the results show that nano-boron improves the connection between the grains. Scanning electron microscope results show that HIP increases the reaction rate between Mg and B, density, and homogeneity of the MgB2 material. Additionally, HIP allows to create a structure with small grains and voids and eliminates the significance of the number of voids. High isostatic pressure allows to obtain high J (c) of 10 A/mm(2) (at 4.2 K) in 10 T and increases irreversible magnetic field (B (irr)) and upper critical field (B (c2)). Measurements show that these wires have high critical temperature of 37 K.Öğe How do different locations, floors and aspects influence indoor radon concentrations? An empirical study using neural networks for a university campus in Northwestern Turkey(Sage Publications Ltd, 2013) Atik, Şeyma Yılmaz; Yetiş, Hakan; Denizli, Haluk; Evrendilek, FatihIndoor radon (Rn-222) concentrations were measured at a 10-min interval during October 2011 and January 2012. The monitoring followed a randomised and repeated pattern of experimental design, and was carried out at six faculty buildings of the Abant Izzet Baysal University, on five floor levels and two aspect directions (south vs. north) using an AlphaGUARD P30 Radon Monitor. The University campus area located in northwestern Turkey is near the North Anatolian Fault, a major active right lateral-moving strike-slip fault which runs along the transform boundary between the Eurasian Plate and the Anatolian Plate. Best artificial neural networks (ANNs) emulating indoor Rn-222 levels were selected as a function of air temperature (T-a), air pressure (P-a), relative humidity (RH), T-a by RH interaction, local time, location, floor and aspect. Elevated levels of indoor Rn-222 concentrations were measured at the south-facing offices and on the first floor levels of the building. Lower concentrations were found on the upper floor levels. Out of 27 ANNs, GFF-1-B-L and MLP-2-B-L performed best and could be contributing to the 35.6% and 87.2% of variations in spatio-temporal dynamics of indoor Rn-222 levels as a function of location or floor level and aspect, respectively, in addition to T-a, P-a, RH, T-a by RH interaction and time.Öğe Hysteresis effects and flux pinning in a macroscopic cylindrical hole drilled in superconducting Bi-Sr-Ca-Cu-O(Elsevier, 2012) Kılıç, Atilla; Yetiş, Hakan; Kılıç, Kıvılcım; Altınkök, Atılgan; Olutaş, Murat; Sözeri, HüseyinWe report experimental results on the flux dynamics in a polycrystalline superconducting Bi1.7Pb0.3Sr2Ca2Cu3Ox (BSCCO) sample with a macroscopic cylindrical hole (CH) drilled through it. Current-voltage (I - V) curve measurements were carried out as a function of current sweep rates (dI/dt) of the transport current at zero magnetic field (H = 0) and under an external magnetic field (H not equal 0) at a temperature just below T-c on the BSCCO sample before and after drilling the CH, in order to investigate the effects of the CH on the transport properties. The hysteresis effects in the I - V curves of the sample without the CH disappear for all given dI/dt values in the presence of an external magnetic field, and the V curves exhibit a nearly linear behavior, which can be taken as an indication of flux flow effects. However, for the same magnetic field, temperature, and dI/dt values, the hysteresis effects in the I - V curves of the sample with the CH are still observed. This interesting observation suggests that the CH acts as an attractive macroscopic pinning center for flux lines and inhibits the flux motion. In the I - V curves of the sample with the CH, a downward curvature is observed in the low current region when I-max has a relatively high value, as the transport current is increased with high values of dI/dt. The hysteresis effects in the I - V curves are considered as a kind of superheated state of coherent motion of the flux lines which try to keep their integrity. In this procedure, at low values of I-max, the voltage response of the sample with a CH increases beginning from zero, which indicates the presence of an obstruction to the penetration of transport current due to both the surface effects and the CH. How the current sweep rate controls the degree of current-induced organization of the flux lines in the sample is also discussed.
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