E. E. Hellstrom

Florida State University, Tallahassee, Florida, United States

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Publications (236)610.56 Total impact

  • Source
    M. Nikolo · X. Shi · E.S. Choi · J. Jiang · J.D. Weiss · E.E. Hellstrom
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    ABSTRACT: Thermally activated flux de-pinning and flux activation de-pinning energies are studied in a (Ba0.56K0.44)Fe2As2 (Tc=38.5 K) bulk superconductor in DC magnetic fields up to 18 T. Ac susceptibility was measured as a function of temperature, DC and AC magnetic fields, and frequency. Ac susceptibility curves shift to higher temperatures as the frequency is increased from 75 to 1997 Hz in all fields. We model this data by Arrhenius law to determine flux activation energies as a function of AC and DC magnetic fields. The activation energy ranges from 8822 K at μ0 Hdc = 0 T to 1100 K at 18 T for Hac =80 A/m. The energies drop quickly in a non-linear manner as DC field rises above 0 T and around 1 T, which we describe as pinning transition field, the drop levels and continues more slowly in a linear like manner as DC field approaches to 18 T. Furthermore, the activation energy drops quickly as AC field increases from 80 A/m to 800 A/m at 0 DC field. As the DC field rises above 0, the activation energy has significantly weaker dependence on the AC field amplitude. Extensive map of the de-pinning, or irreversibility, lines shows broad dependence on the magnitude of the small AC field, frequency, in addition to the DC field.
    Preview · Article · Dec 2015
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    M. Nikolo · X. Shi · J. Jiang · J.D. Weiss · E.E. Hellstrom
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    ABSTRACT: Thermally assisted flux flow (TAFF) is studied in bulk Ba(Fe0.95Ni0.05)2As2 (Tc = 20.4 K), Ba(Fe0.94Ni0.06)2As2(Tc = 18.5 K) superconductors by transport measurements in magnetic fields up to 18 T.The onset TAFF temperature and the crossover temperature Tx from TAFF to flux flow are determined. The flux pinning activation energy U is modeled as U(T,H) = U0(H) f(t) where f(t) is temperature function and the modified Anderson-Kim model is used to extract U0, which is graphed as a function of magnetic field μ0H near Tc. Maximum pinning energies at 0 T are about 1600 K for Ni6 doped sample and at about 4800 K for Ni5 doped sample. Sub-TAFF resistive regime caused by fluctuations is observed. The upper critical fields Hc2 are measured in a range of temperatures below Tc to find Hc2 (T=0) by applying the WHH model.
    Preview · Article · Dec 2015
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    M. Nikolo · X. Shi · E.S. Choi · J. Jiang · J.D. Weiss · E.E. Hellstrom
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    ABSTRACT: Thermally assisted flux flow (TAFF) and flux pinning energiesare studied in a Ba(Fe0.91Co0.09)2As2 (Tc = 25.3 K) sample via resistivity and AC susceptibility measurements in magnetic fields up to 18T. The flux pinning energy U(T,H) is determined from the Arrhenius law. The pinning maxima well determined by resistivity measurements ranged from 1724 K at 0 T to 585 K at 18 T with a sharp drop off so that U(T=Tc) varied with the applied field H as . The pinning activation energies determined from the AC susceptibility data but were by a factor of three higher, which is explained here. Both inter- and intra-granular pinning energies are determined in low fields. The onset of TAFF temperature and the crossover temperature Tx from TAFF to flux flow are determined, showing the limitations of the Anderson-Kim model.
    Preview · Article · Dec 2015
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    ABSTRACT: The grain boundaries (GBs) of high-temperature superconductors (HTSs) intrinsically limit the maximum achievable inter-grain current density (Jc), when the misalignment between the crystallographic axes of adjacent grains exceeds a certain value. A prominent effect resulting from large-angle GBs is a hysteresis of Jc between the increasing and decreasing field branches. Here, we investigate this feature for K- and Co-doped Ba-122 polycrystalline bulks with systematically varied grain size and find that the widely accepted explanation for this effect -- the return field of the grains -- fails. We use large-area scanning Hall-probe microscopy to distinguish Jc from the intra-granular current density (JG) in order to clarify their interactions. Measurements on Ba-122 bulks reveal that a large Jc results from a small JG as well as small grains. An extended version of the model proposed by Svistunov and D’yachenko is successfully applied to quantitatively evaluate this behavior. The excellent agreement between the model and experiments suggests that the GBs limit the macroscopic current in all of the samples and that the inter-grain coupling is governed by Josephson tunneling. The predictions of the model are promising in view of realizing high-field HTS magnets. Our main result is that the field dependence of the Jc of an untextured wire can be significantly reduced by reducing the grain size, which results in much higher currents at high magnetic fields. This result is not limited to the investigated iron-based materials and is therefore of interest in the context of other HTS materials.
    No preview · Article · Dec 2015 · Superconductor Science and Technology
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    ABSTRACT: A trapped field of over 1 T at 5 K and 0.5 T at 20 K has been measured between a stack of magnetized cylinders of bulk polycrystalline Ba0.6K0.4Fe2As2 superconductors 10 mm in diameter and 18 mm in combined thickness. The trapped field showed a low magnetic creep rate (~3% after 24 hours at 5 K), while magneto-optical imaging revealed a trapped field distribution corresponding to uniform macroscopic current loops circulating through the sample. The superconductors were manufactured by hot isostatic pressing of pre-reacted powders using the scalable powder-in-tube technique. A high Vickers hardness of ~3.5 GPa and a reasonable fracture toughness of ~2.35 MPa m0.5 were measured. Given the untextured polycrystalline nature of the cylinders and their large irreversibility field (> 90 T), it is expected that larger bulks could trap fields in excess of 10 T.
    No preview · Article · Nov 2015 · Superconductor Science and Technology
  • Lucas B. S. Da Silva · Eric E. Hellstrom · Durval Rodrigues
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    ABSTRACT: High-energy ball milling is generally used in granular materials to increase grain connectivity and densification. In this study, different high-energy ball milling times were investigated to optimize the MgB2 superconducting properties in samples with added ZrB2. The objectives of this addition were to modify the Mg planes and to create crystalline defects in the superconducting matrix that can act as pinning centers. We observed the behavior of the dopant on the superconducting properties, and found the optimal ball milling time of 300 min improved .ic in high magnetic field and Hirr from 7.2 to 10.0 T at 4.2 K.
    No preview · Article · Jun 2015 · IEEE Transactions on Applied Superconductivity
  • Source

    Full-text · Dataset · Apr 2015
  • Source
    Eric Hellstrom

    Preview · Article · Mar 2015 · Superconductor Science and Technology
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    ABSTRACT: The critical current (Ic) of dense Bi2Sr2CaCu2O8+x (Bi-2212) round wires with Ag-alloy matrices was measured as a function of axial applied strain (), to determine whether the behavior is improved in comparison to wires with a large (30-50%) void fraction in the Bi-2212 filaments. Wires were reacted at approximately under a 1% O2 in Ar gas mixture at 100 bar pressure to densify the Bi-2212 fraction during the partial melt reaction. After measurement of the Ic at 4.2 K and 5 T at Florida State University, wire sections were sent to Lawrence Berkeley National Laboratory where was measured at 4.2 K at 5 and 15 T using a U-shaped bending spring. We found that has a 0.3% wide linear reversible strain range. An unexpected result is that the width of this reversible range seems comparable to that found for porous samples reacted at only 1 bar, apparently negating an earlier plausible supposition that fuller density samples would be more resilient.
    Full-text · Article · Mar 2015 · Superconductor Science and Technology
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    ABSTRACT: The nanoscale structural, chemical, and electronic properties of artificial engineered superlattice thin films consisting of superconducting Co-doped BaFe2As2(Ba-122) and nonsuperconducting SrTiO3(STO) layers are determined by using atomically resolved scanning transmission electron microscopy and electron energy loss spectroscopy. The bonding of Ba-122/STO occurring between As (Ba) and SrO(TiO2) terminated layers has been identified. The thin STO (3 unit cell) insertion layers are a mixture of cations (Ba, Sr, Fe, and Ti) and rich in oxygen vacancies and the Ba-122 layers (10 unit cell) are free of vertical second phases. Our results explain why these superlattices show anisotropic transport response to an external magnetic field, i.e., strong ab-axis pinning (enhancing critical current density) and no c-axis pinning, which is opposite to single layer Ba-122 thin films. These findings reveal physical and chemical properties of superconducting/nonsuperconducting heterostructures and provide important insights into engineering of superconducting devices.
    No preview · Article · Mar 2015 · Physical Review B
  • Source
    H Kandel · J Lu · J Jiang · P Chen · M Matras · N Craig · U P Trociewitz · E E Hellstrom · D C Larbalestier
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    ABSTRACT: We have developed TiO2 coating on Ag-alloy sheathed Bi2Sr2CaCu2O8−x (Bi-2212) round-wire conductor for electrical insulation in Bi-2212 magnets. The green coating has a base layer comprised of TiO2, polyvinyl butyral (PVB) and a small amount of polysilicate and a top layer made of polyacrylic. The coating was applied on the conductor using a continuous reel-to-reel dip coating process and showed very good adherence and flexibility that is suitable for magnet coil winding. The thickness of the coating is a function of slurry viscosity, wire withdrawal speed and wire radius. Small test coils were built with the coated Bi-2212 round-wires and were heat treated at 100 atm pressure. During the heat treatment, the PVB and polyacrylic were removed from the green coating and the polysilicate decomposed to SiO2 that served as a sintering aid for TiO2. After the heat treatment, the coating remained strongly adhered to the conductor and did not have a detrimental effect on the critical current (I c) values. The breakdown voltage was about 150 V across a 7 μm thick heat treated coating on Bi-22112 round-wire conductor, corresponding to a dc dielectric strength of about 21 MV m−1.
    Full-text · Article · Feb 2015 · Superconductor Science and Technology
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    F Kametani · J Jiang · M Matras · D Abraimov · E E Hellstrom · D C Larbalestier
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    ABSTRACT: Why Bi2Sr2CaCu2Ox (Bi2212) allows high critical current density Jc in round wires rather than only in the anisotropic tape form demanded by all other high temperature superconductors is important for future magnet applications. Here we compare the local texture of state-of-the-art Bi2212 and Bi2223 ((Bi,Pb)2Sr2Ca2Cu3O10), finding that round wire Bi2212 generates a dominant a-axis growth texture that also enforces a local biaxial texture (FWHM <15°) while simultaneously allowing the c-axes of its polycrystals to rotate azimuthally along and about the filament axis so as to generate macroscopically isotropic behavior. By contrast Bi2223 shows only a uniaxial (FWHM <15°) c-axis texture perpendicular to the tape plane without any in-plane texture. Consistent with these observations, a marked, field-increasing, field-decreasing Jc(H) hysteresis characteristic of weak-linked systems appears in Bi2223 but is absent in Bi2212 round wire. Growth-induced texture on cooling from the melt step of the Bi2212 Jc optimization process appears to be the key step in generating this highly desirable microstructure.
    Full-text · Article · Feb 2015 · Scientific Reports
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    ABSTRACT: The preparation and production of bulk High – Temperature Superconductors (HTS) since the discovery is analysed and reported in their development and progress. For the three material groups REBCO (RE: Y, Sm, Gd, Nd, …), BiSCCO (Bi2122, Bi2223), and MgB2 the processing routes are described and evaluated. While BiSCCO compounds play an important role in the conductor development the REBCO melt texture processes provide large crystalline bulks for magnetic application. Top-seed melt growth (TSMG) fabrication methods dominate the bulk production by using single or multiple seeds to prevent unwanted parasitic multi- crystalline nucleation. Large single grain bulk fabrication of 50–100 mm requires exact precursor phase preparation of RE123 and RE211 and a temperature/ oxygen-controlled furnace process. Multiple seeding of large plane bulks, rings and cylinders improves the magnetic properties and ensures an optimum geometry for application. Doping strategy of REBCO bulks is reviewed and the mechanical limitations of REBCO bulks under high trapped field values above 5 Tesla explained. Bulk HTS are characterized by measurements of the critical current density and the maximum magnetic trapped flux density. Physical properties of bulk HTS are given and the magnetic performance of the different REBCO materials (RE= Y, Sm, Nd, Gd) compared in view of applicability.
    No preview · Article · Jan 2015
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    ABSTRACT: Ba(Fe1-xCox)2As2 is the most tunable of the Fe-based superconductors (FBS) in terms of acceptance of high densities of self-assembled and artificially introduced pinning centres which are effective in significantly increasing the critical current density, Jc. Moreover, FBS are very sensitive to strain, which induces an important enhancement in critical temperature, Tc, of the material. In this paper we demonstrate that strain induced by the substrate can further improve Jc of both single and multilayer films by more than that expected simply due to the increase in Tc. The multilayer deposition of Ba(Fe1-xCox)2As2 on CaF2 increases the pinning force density Fp by more than 60% compared to a single layer film, reaching a maximum of 84 GN/m^3 at 22.5T and 4.2 K, the highest value ever reported in any 122 phase.
    Full-text · Article · Dec 2014 · Scientific Reports
  • M. Nikolo · X. Shi · E. S. Choi · J. Jiang · J. D. Weiss · E. E. Hellstrom
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    ABSTRACT: Flux pinning dynamics are studied in a Ba(Fe \(_{0.91}\) Co \(_{0.09})_{2}\) As \(_{2}\) ( \(T_\mathrm{{c}}=25.3\) K) bulk samples via ac susceptibility measurements. Ac susceptibility curves shift to higher temperatures as the frequency of small ac fields is increased from 75 to 1997 Hz in all magnetic fields ranging from 0 to 18 T. The temperature profile of the ac susceptibility curves shows narrower ac loss distribution in temperature for higher frequencies and gradually narrowing frequency shift as the temperature sweeps the full range from 2 K to the upper critical field temperature. The frequency ( \(f)\) shift of the susceptibility curves is modeled by the Anderson–Kim Arrhenius law \(f = f_{0} \mathrm {exp}(- {E}_\mathrm{{a}} /kT)\) to determine flux activation energy \(E_\mathrm{{a}}/k\) as a function of magnetic field. Extensive mapping of the irreversibility lines shows broad dependence on the magnitude and the frequency of the ac field, in addition to the dc magnetic field. The irreversibility lines were just below the upper critical field \(H_\mathrm{{c2}}\) lines at 0 T in the \(H-T\) plane, but they moved significantly below the \(H_\mathrm{{c2}}\) line at higher magnetic fields, placing constraints on the use of these materials at higher magnetic fields such as 10 T and above.
    No preview · Article · Dec 2014 · Journal of Low Temperature Physics
  • M. Nikolo · X. Shi · E. S. Choi · J. Jiang · J. Weiss · E. Hellstrom

    No preview · Article · Dec 2014 · Journal of Superconductivity and Novel Magnetism
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    ABSTRACT: Some polycrystalline forms of the K- and Co-doped BaFe2As2 and SrFe2As2 superconductors now have a critical current density (J(c)) within a factor of similar to 5 of that required for real applications, even though it is known that some grain boundaries (GBs) block current, thus, raising the question of whether this blocking is intrinsic or extrinsically limited by artefacts amenable to improvement by better processing. Herein, we utilize atom-probe tomography (APT) to study the grain and GB composition in high J(c) K- and Co-doped BaFe2As2 polycrystals. We find that all GBs studied show significant compositional variations on the scale of a few coherence lengths (xi), as well as strong segregation of oxygen impurities, which we believe are largely introduced in the starting materials. Importantly, these findings demonstrate that APT enables quantitative analysis of the highest J(c) K-doped BaFe2As2 samples, where analytical transmission electron microscopy (TEM) fails because of the great reactivity of thin TEM samples. The observations of major chemical perturbations at GBs make us cautiously optimistic that there is a large extrinsic component to the GB current blocking, which will be ameliorated by better processing, for which APT will likely be a crucial instrument. (C) 2014 AIP Publishing LLC.
    No preview · Article · Oct 2014 · Applied Physics Letters
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    ABSTRACT: A comprehensive microstructural study was conducted on optimally-doped epitaxial Ba(Fe1−xCox)2As2 thin films grown by pulsed laser deposition on various substrates of a wide range of lattice constants: SrTiO3, LaAlO3, (La,Sr)(Al,Ta)O3, MgO, CaF2, and BaF2. We found that epitaxial strain directly affects the superconductivity in the film, with the transition temperature decreasing linearly with increasing in-plane lattice constant of the film. However, the strain is not determined by the lattice mismatch between the film and substrate. Instead, the mosaic spread of the grain orientation in the film and the thermal expansion coefficient of the substrate were found to correlate well with the in-plane lattice constant of the film. The result confirms the importance of structural distortions to the superconductivity in the Ba(Fe1−xCox)2As2 films.
    No preview · Article · Oct 2014 · Superconductor Science and Technology
  • Source
    M. Nikolo · X. Shi · J. Jiang · E.S. Choi · J. D. Weiss · E. E. Hellstrom
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    ABSTRACT: Flux pinning and thermally assisted flux flow are studied in a (Ba\(_{0.6}\)K\(_{0.4})\)Fe\(_{2}\)As\(_{2}(T_\mathrm{c}\)=38.3 K) bulk samples in magnetic fields up to 18 T via ac susceptibility measurements. Ac susceptibility curves shift to higher temperatures as the frequency is increased from 75 to 1,997 Hz in all fields. The frequency (\(f)\) shift of the susceptibility curves is modeled by the Anderson-Kim Arrhenius law \(f = f_{0}\mathrm{exp}(-E_\mathrm{a}{ /kT})\) to determine flux activation energy \(E_\mathrm{a}/k\) as a function of ac field \(H_\mathrm{ac}\) and dc magnetic flux density \(\mu \) \(_\mathrm{0} H_\mathrm{dc}\). \(E_\mathrm{a}/k\) ranges from 8,822 K (761 meV) at \(\mu \) \(_{0} H_{dc}\) = 0 T to 1,100 K (95 meV) at 18 T for \(H_\mathrm{ac}=\)80 A/m (1 Oe). The energies drop very quickly in a non-linear manner as \(\mu \) \(_{0} H_\mathrm{dc}\) increases from 0 to 1 T, and more gradually, in a linear-like manner, as \(\mu \) \(_{0} H_\mathrm{dc}\) increases further to 18 T, suggesting some kind of vortex transition. For ac fields of 400 A/m (5 Oe) and higher, the Arrhenius model starts breaking down, at around \(\mu \) \(_{0} H_{ \mathrm dc}\) = 2 T. As the dc magnetic flux density increases further, this breakdown becomes significant for \(\mu _{0} H_\mathrm{dc}\) = 15 and 18 T at ac fields of 400 A/m and higher. Extensive mapping of the de-pinning, or irreversibility, lines shows broad dependence on the magnitude of the ac field, frequency, in addition to the dc magnetic flux density.
    Full-text · Article · Oct 2014 · Journal of Low Temperature Physics
  • M. Nikolo · X. Shi · E. S. Choi · J. Jiang · J. D. Weiss · E. E. Hellstrom
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    ABSTRACT: Thermally assisted flux flow (TAFF) based on magneto-resistivity and ac susceptibility measurements is studied in a Ba(Fe0.91Co0.09)2As2(T c=25.3 K) sample in magnetic fields up to 18 T. In addition to the upper critical field μ 0H c2 and the coherence length ξ(0), the flux flow activation energy U(T,H) has also been determined. The resistive transition width is proportional to μ 0H, in contrast to Tinkham’s theoretical prediction. By applying Fisher’s model, the glass melting transition temperature T g, which occurs in the upper TAFF state and not in the zero resistivity vortex solid regime, is calculated. The onset of TAFF temperature and the crossover temperature T x from TAFF to flux flow are determined. By contrasting the ac susceptibility data with the resistivity data, considerable flux penetration appears even in the zero resistivity state, in addition to ac losses. The H-T phase diagram is drawn and shows weak pinning regime as the field approaches μ 0H c2, and the strength of the weak pinning decreases to 0 with increasing magnetic field from 0 to 18 T.
    No preview · Article · Oct 2014 · Journal of Superconductivity and Novel Magnetism

Publication Stats

5k Citations
610.56 Total Impact Points

Institutions

  • 2008-2015
    • Florida State University
      • Applied Superconductivity Center (ASC)
      Tallahassee, Florida, United States
  • 2010-2013
    • National High Magnetic Field Laboratory
      Tallahassee, Florida, United States
  • 1987-2007
    • University of Wisconsin, Madison
      • • Department of Materials Science and Engineering
      • • Department of Physics
      Madison, MS, United States
  • 2003
    • Jawaharlal Nehru University
      • School of Physical Sciences
      New Dilli, NCT, India
    • University of Michigan
      • Department of Materials Science and Engineering
      Ann Arbor, Michigan, United States