E. E. Hellstrom

Florida State University, Tallahassee, Florida, United States

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Publications (205)596.39 Total impact

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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). · 2.80 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 01/2015; 28(3). · 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. 01/2015; 5:8285.
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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. · 5.08 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; · 1.04 Impact Factor
  • Journal of Superconductivity and Novel Magnetism 12/2014; 27(10):2231. · 0.93 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. · 2.80 Impact Factor
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    Journal of Low Temperature Physics 10/2014; · 1.04 Impact Factor
  • Journal of Superconductivity and Novel Magnetism 10/2014; 27(10):2231-2239. · 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.
    09/2014;
  • 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). · 0.93 Impact Factor
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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. · 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.
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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). · 2.80 Impact Factor
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    ABSTRACT: It has been recently reported (S. Lee et al., Nature Materials 12, 392, 2013) that superlattices where layers of the 8% Co-doped BaFe2As2 superconducting pnictide are intercalated with non superconducting ultrathin layers of either SrTiO3 or of oxygen-rich BaFe2As2, can be used to control flux pinning, thereby increasing critical fields and currents, without significantly affecting the critical temperature of the pristine superconducting material. However, little is known about the electron properties of these systems. Here we investigate the electrodynamics of these superconducting pnictide superlattices in the normal and superconducting state by using infrared reflectivity, from THz to visible range. We find that multi-gap structure of these superlattices is preserved, whereas some significant changes are observed in their electronic structure with respect to those of the original pnictide. Our results suggest that possible attempts to further increase the flux pinning may lead to a breakdown of the pnictide superconducting properties.
    Applied Physics Letters 04/2014; 104(22). · 3.52 Impact Factor
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    ABSTRACT: Magnets are the principal market for superconductors, but making attractive conductors out of the high-temperature cuprate superconductors (HTSs) has proved difficult because of the presence of high-angle grain boundaries that are generally believed to lower the critical current density, Jc. To minimize such grain boundary obstacles, HTS conductors such as REBa2Cu3O7-x and (Bi, Pb)2Sr2Ca2Cu3O10-x are both made as tapes with a high aspect ratio and a large superconducting anisotropy. Here we report that Bi2Sr2CaCu2O8-x (Bi-2212) can be made in the much more desirable isotropic, round-wire, multifilament form that can be wound or cabled into arbitrary geometries and will be especially valuable for high-field NMR magnets beyond the present 1 GHz proton resonance limit of Nb3Sn technology. An appealing attribute of this Bi-2212 conductor is that, being without macroscopic texture, it contains many high-angle grain boundaries but nevertheless attains a very high Jc of 2,500 A mm(-2) at 20 T and 4.2 K. The large potential of the conductor has been demonstrated by building a small coil that generated almost 2.6 T in a 31 T background field. This demonstration that grain boundary limits to high Jc can be practically overcome underlines the value of a renewed focus on grain boundary properties in non-ideal geometries.
    Nature Material 03/2014; · 36.43 Impact Factor
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    ABSTRACT: Here we report an optical investigation in the terahertz region of a 40 nm ultrathin BaFe$_{1.84}$Co$_{0.16}$As$_2$ superconducting film with superconducting transition temperature T$_c$ = 17.5 K. A detailed analysis of the combined reflectance and transmittance measurements showed that the optical properties of the superconducting system can be described in terms of a two-band, two-gap model. The zero temperature value of the large gap $\Delta_B$, which seems to follow a BCS-like behavior, results to be $\Delta_B$(0) = 17 cm$^{-1}$. For the small gap, for which $\Delta_A$(0) = 8 cm$^{-1}$, the temperature dependence cannot be clearly established. These gap values and those reported in the literature for the BaFe$_{2-x}$Co$_{x}$As$_2$ system by using infrared spectroscopy, when put together as a function of T$_c$, show a tendency to cluster along two main curves, providing a unified perspective of the measured optical gaps. Below a temperature around 20 K, the gap-sizes as a function of T$_c$ seem to have a BCS-like linear behavior, but with different slopes. Above this temperature, both gaps show different supra-linear behaviors.
    Physics of Condensed Matter 06/2013; 86(6). · 1.46 Impact Factor
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    ABSTRACT: We report on an infrared study on the undoped compound BaFe2As2 as a function of pressure (up to about 10 GPa) at three temperatures (300, 160, and 110 K). The evolution with pressure and temperature of the optical conductivity shows that by increasing pressure, the mid-infrared absorptions associated with magnetic order are lowered while the Drude term increases, indicating the evolution towards a conventional metallic state. We evaluate the spectral weight dependence on pressure comparing it to that previously found upon doping. All the optical results indicate that lattice modifications cannot be recognized as the only parameter determining the low-energy electrodynamics in these compounds.
    Physical Review B 06/2013; 85(17). · 3.66 Impact Factor
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    ABSTRACT: BaFe2As2 (Ba-122) and (Ba0.6K0.4)Fe2As2 (K-doped Ba-122) powders were successfully synthesized from the elements using a reaction method, which incorporates a mechanochemical reaction using high-impact ball milling. Mechanically-activated, self-sustaining reactions (MSR) were observed while milling the elements together to form these compounds. After the MSR, the Ba-122 phase had formed, the powder had an average grain size < 1 {\mu}m, and the material was effectively mixed. X-ray diffraction confirmed Ba-122 was the primary phase present after milling. Heat treatment of the K-doped MSR powder at high temperature and pressure yielded dense samples with high phase purity but only granular current flow could be visualized by magneto optical imaging. In contrast, a short, low temperature, heat treatment at ambient pressure resulted in global current flow throughout the bulk sample even though the density was lower and impurity phases were more prevalent. An optimized heat treatment involving a two-step, low temperature, heat treatment of the MSR powder produced bulk material with very high critical current density above 0.1 MAcm-2 (4.2 K, 0 T).
    Superconductor Science and Technology 05/2013; 26(7). · 2.80 Impact Factor

Publication Stats

4k Citations
596.39 Total Impact Points

Institutions

  • 2008–2015
    • Florida State University
      • Applied Superconductivity Center (ASC)
      Tallahassee, Florida, United States
  • 2014
    • Saint Louis University
      Сент-Луис, Michigan, United States
  • 2009–2013
    • National High Magnetic Field Laboratory
      Tallahassee, Florida, United States
    • University of São Paulo
      • Departamento de Engenharia de Materiais (LOM) (Lorena)
      Ribeirão Preto, Estado de Sao Paulo, Brazil
  • 1988–2013
    • University of Wisconsin, Madison
      • Department of Materials Science and Engineering
      Madison, MS, United States
  • 2011
    • University of North Carolina at Asheville
      • Department of Chemistry
      Asheville, NC, United States
  • 2010
    • Northwestern University
      • Department of Physics and Astronomy
      Evanston, IL, United States
  • 2005
    • Los Alamos National Laboratory
      Los Alamos, California, United States
  • 2003
    • Jawaharlal Nehru University
      • School of Physical Sciences
      New Delhi, NCT, India
    • University of Michigan
      • Department of Materials Science and Engineering
      Ann Arbor, Michigan, United States