E. E. Hellstrom

Florida State University, Tallahassee, Florida, United States

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Publications (217)623.72 Total impact

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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.
    IEEE Transactions on Applied Superconductivity 06/2015; 25(3):1-4. DOI:10.1109/TASC.2014.2364779 · 1.32 Impact Factor
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  • Eric Hellstrom
    Superconductor Science and Technology 03/2015; 28(4). DOI:10.1088/0953-2048/28/4/040501 · 2.80 Impact Factor
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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.
    Superconductor Science and Technology 03/2015; 28(3). DOI:10.1088/0953-2048/28/3/032001 · 2.80 Impact Factor
  • Physical Review B 03/2015; 91(10). DOI:10.1103/PhysRevB.91.104525 · 3.66 Impact Factor
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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.
    Superconductor Science and Technology 02/2015; 28(3). DOI:10.1088/0953-2048/28/3/035010 · 2.80 Impact Factor
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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.
    Scientific Reports 02/2015; 5:8285. DOI:10.1038/srep08285 · 5.58 Impact Factor
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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.
    Scientific Reports 12/2014; 4. DOI:10.1038/srep07305 · 5.58 Impact Factor
  • Journal of Superconductivity and Novel Magnetism 12/2014; 27(10):2231. · 0.93 Impact Factor
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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.
    Journal of Low Temperature Physics 12/2014; 178(5-6). DOI:10.1007/s10909-014-1254-x · 1.04 Impact Factor
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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.
    Applied Physics Letters 10/2014; 105(16). DOI:10.1063/1.4898191 · 3.52 Impact Factor
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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.
    Superconductor Science and Technology 10/2014; 27(11):115010. DOI:10.1088/0953-2048/27/11/115010 · 2.80 Impact Factor
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    ABSTRACT: Flux pinning and thermally assisted flux flow are studied in a (BaKFeAs=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 ( shift of the susceptibility curves is modeled by the Anderson-Kim Arrhenius law to determine flux activation energy as a function of ac field and dc magnetic flux density . ranges from 8,822 K (761 meV) at = 0 T to 1,100 K (95 meV) at 18 T for 80 A/m (1 Oe). The energies drop very quickly in a non-linear manner as increases from 0 to 1 T, and more gradually, in a linear-like manner, as 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 = 2 T. As the dc magnetic flux density increases further, this breakdown becomes significant for = 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.
    Journal of Low Temperature Physics 10/2014; 178(3-4). DOI:10.1007/s10909-014-1237-y · 1.04 Impact Factor
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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)(2)As-2(T-c = 25.3 K) sample in magnetic fields up to 18 T. In addition to the upper critical field mu H-0(c2) and the coherence length xi(0), the flux flow activation energy U T H has also been determined. The resistive transition width is proportional to mu H-0, 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 mu H-0(c2), and the strength of the weak pinning decreases to 0 with increasing magnetic field from 0 to 18 T.
    Journal of Superconductivity and Novel Magnetism 10/2014; 27(10):2231-2239. DOI:10.1007/s10948-014-2587-z · 0.93 Impact Factor
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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{\deg}) 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{\deg}) 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.
  • 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(Fe0.95 Ni0.05)2As2 (T c = 20.4 K) and Ba(Fe0.94 Ni0.06)2As2 (T c = 18.5 K) superconductors by transport measurements in magnetic fields up to 18 T. In addition, the upper critical field μ 0H c2(0) and the coherence length ξ(0) are determined. The data is analyzed in the context of the widely accepted Anderson-Kim model and Fischer model. The onset TAFF temperature and the crossover temperature T x from TAFF to flux flow are determined. The flux pinning activation energy U is modeled as U(T,H) = U 0(H) f(t) where f(t) is some temperature function and the modified Anderson-Kim model is used to extract U 0, which is graphed as a function of magnetic field μ 0H near T c. The resistive regime is observed, which is caused by fluctuations. Fisher’s model is applied to determine the glass melting transition temperature; it occurs in the upper TAFF state and not in the expected zero-resistivity vortex solid regime. Furthermore, the resistive transition width is proportional to μ 0H, in contrast to Tinkham’s prediction. The H-T phase diagram is drawn.
    Journal of Superconductivity and Novel Magnetism 09/2014; 27(9). DOI:10.1007/s10948-014-2550-z · 0.93 Impact Factor
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    Applied Superconductivity Conference (ASC 2014), 10-15 Aug. 2014, Charlotte, North Carolina, USA, Charlotte, NC, USA; 08/2014
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    ABSTRACT: The ability of large grain, REBa$_{2}$Cu$_{3}$O$_{7-\delta}$ [(RE)BCO; RE = rare earth] bulk superconductors to trap magnetic field is determined by their critical current. With high trapped fields, however, bulk samples are subject to a relatively large Lorentz force, and their performance is limited primarily by their tensile strength. Consequently, sample reinforcement is the key to performance improvement in these technologically important materials. In this work, we report a trapped field of 17.6 T, the largest reported to date, in a stack of two, silver-doped GdBCO superconducting bulk samples, each of diameter 25 mm, fabricated by top-seeded melt growth (TSMG) and reinforced with shrink-fit stainless steel. This sample preparation technique has the advantage of being relatively straightforward and inexpensive to implement and offers the prospect of easy access to portable, high magnetic fields without any requirement for a sustaining current source.
    Superconductor Science and Technology 06/2014; 27(8):082001. DOI:10.1088/0953-2048/27/8/082001 · 2.80 Impact Factor
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    L B S Da Silva, E E Hellstrom, D Rodrigues Jr
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    ABSTRACT: MgB2 has been catching the attention due to the possibility to apply the material in magnets and electronic devices, operating with cryocoolers. In this work, MgB2 bulks were developed and analyzed with addition of ZrB2, another diboride with the same C32 hexagonal structure as MgB2, and simultaneous addition of different carbon sources (SiC, graphite, and carbon nanotubes). The objective of these additions is to modify the Mg planes with the diborides and to dope the material with carbon, improving the upper critical fields. Besides the doping of the material, this method creates crystalline defects in the superconducting matrix, which can act as pinning centers. As a result we could improve the critical current density of the material and estimate the behavior of dopants on the superconducting properties.
    Journal of Physics Conference Series 05/2014; 507(1):012043. DOI:10.1088/1742-6596/507/1/012043
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    ABSTRACT: The recent discovery that gas bubbles formed in the melt state are a major current-limiting mechanism in Bi2Sr2CaCu2Ox (Bi2212) round wires has prompted explicit examination of the bubble density in split melt processed samples which, under optimized 1 bar processing conditions, can exhibit significant (30-50%) enhancement of critical current density, Jc. By examining quenched and furnace-cooled samples from different points in the split melt processing (SMP), we found that the bubble size correlates well to the Jc. Compared with standard processed samples, the bubble size is smaller in SMP samples which are cooled directly to room temperature by an intermediate cooling from the first melt before being reheated to the second melt. Bubble size and density observations suggest that Jc can only be increased when bubble growth in the second melt is prevented by very tight control of the reheat temperature. Smaller bubble size is favorable for Jc because filament connectivity is determined by the effectiveness of bubble bridging by Bi2212 grain growth on cooling from the second melt. Because SMP appears to allow higher Jc by shrinking bubble size rather than by diminishing the bubble volume fraction, we conclude that SMP is unlikely to offer benefits to newer processes like over-pressure processing which raise Jc much more significantly by full Bi2212 densification and bubble elimination.
    Superconductor Science and Technology 04/2014; 27(5). DOI:10.1088/0953-2048/27/5/055004 · 2.80 Impact Factor

Publication Stats

4k Citations
623.72 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
  • 1988–2010
    • University of Wisconsin, Madison
      • Department of Materials Science and Engineering
      Madison, MS, United States
  • 2005
    • Los Alamos National Laboratory
      Los Alamos, California, United States
  • 2003
    • University of Michigan
      • Department of Materials Science and Engineering
      Ann Arbor, Michigan, United States
    • Jawaharlal Nehru University
      • School of Physical Sciences
      New Delhi, NCT, India
  • 2000
    • Iowa State University
      Ames, Iowa, United States