E. E. Hellstrom

Florida State University, Tallahassee, Florida, United States

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Publications (198)555.11 Total impact

  • 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). · 0.70 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. · 2.76 Impact Factor
  • 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.
    04/2014; 27(5).
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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). · 3.79 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; · 35.75 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). · 1.28 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). · 2.76 Impact Factor
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    ABSTRACT: Conductor insulation is one of the key components needed to make Ag-alloy clad Bi2Sr2CaCu2O8+x (Bi-2212/Ag) superconducting round wire (RW) successful for high field magnet applications, as dielectric standoff and high winding current densities (Jw) directly depend on it. In this study, a TiO2–polymer insulation coating developed by nGimat LLC was applied to test samples and a high field test coil. The insulation was investigated by differential thermal analysis (DTA), thermo-gravimetric analysis (TGA), scanning electron microscopy (SEM), dielectric property measurement, and transport critical current (Ic) property measurement. About 29% of the insulation by weight is polymer. When the Bi-2212/Ag wire is fully heat treated, this decomposes with slow heating to 400 ° C in pure O2. After the full reaction, we found that the TiO2 did not degrade the critical current properties, adhered well to the conductor, and provided a breakdown voltage of more than 100 V, which allowed the test coil to survive quenching in 31.2 T background field, while providing a 2.6 T field increment. For Bi-2212/Ag RW with a typical diameter of 1.0–1.5 mm, this ~15 μm thick insulation allows a very high coil packing factor of ~0.74, whereas earlier alumino-silicate braid insulation only allows packing factors of 0.38–0.48.
    Superconductor Science and Technology 05/2013; 26(7):075009. · 2.76 Impact Factor
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    ABSTRACT: We report here that magnetic fields of almost 34 T, far above the upper 24 T limit of Nb3Sn, can be generated using a multifilament round wire conductor made of the high temperature cuprate superconductor Bi2Sr2CaCu2O8-x (Bi-2212). A remarkable attribute of this Bi-2212 conductor is that it does not exhibit macroscopic texture and contains many high angle grain boundaries but nevertheless attains very high superconducting critical current densities Jc of 2500 A/mm2 at 20 T and 4.2 K. This Bi-2212 conductor does not possess the extreme texture that high Jc coated conductors of REBa2Cu3O7-x (REBCO) require, avoiding also its high aspect ratio, large superconducting anisotropy and the inherent sensitivity to defects of a single filament conductor. Bi-2212 wires can be wound or cabled into almost any type of superconducting magnet and will be especially valuable for very high field NMR magnets beyond the present 1 GHz proton resonance limit of Nb3Sn technology. This demonstration that grain boundary limits to high Jc can be practically overcome suggests the huge value of a renewed focus on grain boundary properties in non-ideal geometries, especially with the goal of translating the lessons of this Bi-2212 conductor into fabrication of multifilament round wire REBCO or Fe-based superconductors.
    05/2013;
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    ABSTRACT: Significant progress has been achieved in fabricating high-quality bulk and thin-film iron-based superconductors. In particular, artificial layered pnictide superlattices offer the possibility of tailoring the superconducting properties and understanding the mechanism of the superconductivity itself. For high-field applications, large critical current densities (Jc) and irreversibility fields (Hirr) are indispensable along all crystal directions. On the other hand, the development of superconducting devices such as tunnel junctions requires multilayered heterostructures. Here we show that artificially engineered undoped Ba-122/Co-doped Ba-122 compositionally modulated superlattices produce ab-aligned nanoparticle arrays. These layer and self-assemble along c-axis-aligned defects, and combine to produce very large Jc and Hirr enhancements over a wide angular range. We also demonstrate a structurally modulated SrTiO3(STO)/Co-doped Ba-122 superlattice with sharp interfaces. Success in superlattice fabrication involving pnictides will aid the progress of heterostructured systems exhibiting new interfacial phenomena and device applications.
    Nature Material 05/2013; 12:392-396. · 35.75 Impact Factor
  • F Lu, F Kametani, E E Hellstrom
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    ABSTRACT: In this study, we used a systematic route to optimize the fluorine-free MOD process to achieve a high critical current density (Jc) in BaZrO3 (BZO)-doped YBCO films on RABiTS substrates. The BZO content is given by 1 YBCO+x BZO films, where x is moles of BZO per 1 mole of YBCO. We found x = 0.10 to be the optimal BZO content and ~795–805 °C to be the optimal growth temperature window with 60–90 min processing time. TEM studies show the BZO nanoparticles are ~20 nm in size and spaced ~50–100 nm apart. The in-field Jc and the peak pinning force (Fp) of the film grown at the optimal conditions were greatly increased at 77 K relative to pure YBCO films, achieving ~6.7 GN m−3 at 77 K, H || c in a ~800 nm thick x = 0.10 film. The angular dependence of in-field Jc measurements also shows greatly reduced angular anisotropy at 1 and 4 T at 77 K due to isotropic pinning by BZO nanoparticles.
    Superconductor Science and Technology 03/2013; 26(4):045016. · 2.76 Impact Factor
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    ABSTRACT: We have grown epitaxial, optimally-doped superconducting Ba(Fe0.92Co0.08)2As2 films on SrTiO3, (La, Sr)(Al, Ta)O3 and LaAlO3 substrates, which have a range of lattice mismatch, and studied the strain effect on the structural and transport properties of the films. We found that the superconducting transition temperature increased as the c lattice constant decreased and a lattice constant increased. The thickness dependence of the superconducting transition temperature was studied, which was related to the strain and strain relaxation. A zero-resistance Tc of 21.7 K was obtained in the 120 nm-thick Ba(Fe0.92Co0.08)2As2 film on SrTiO3 substrate.
    03/2013;
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    ABSTRACT: We study the magnetotransport properties of three iron-based high temperature superconductors polycrystalline samples, Ba(Fe0.95Ni0.05)2As2 (Tc= 20.4 K), Ba(Fe0.94Ni0.06)2As2 (Tc= 18.5 K), and Ba(Fe0.91Co0.09)2As2 (Tc= 25.3 K), in magnetic fields of up to 18 T. The thermally activated magnetic flux behavior has been retrieved by plotting ρ vs. 1/T (ρ and T are resistivity and temperature, respectively) and obtaining the activation energies U0 for flux motion near Tc. We show a 3-D plot of the distribution of U0 as a function of T and magnetic field H. We apply the WHH model by measuring dHc2/dT at Tc to estimate the upper critical field Hc2(T=0); we estimate the coherence length ξ(T=0). We study the broadening of resistive transition as a function of the applied magnetic field and compare it to Tinkham's prediction for high-Tc materials.
    03/2013;
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    ABSTRACT: It is well known that longer Bi-2212 conductors have significantly lower critical current density (Jc) than shorter ones, and recently it has become clear that a major cause of this reduction is internal gas pressure generated during heat treatment, which expands the wire diameter and dedensifies the Bi-2212 filaments. Here we report on the length-dependent expansion of 5 to 240 cm lengths of state-of-the-art, commercial Ag alloy-sheathed Bi-2212 wire after full and some partial heat treatments. Detailed image analysis along the wire length shows that the wire diameter increases with distance from the ends, longer samples often showing evident damage and leaks provoked by the internal gas pressure. Comparison of heat treatments carried out just below the melting point and with the usual melt process makes it clear that melting is crucial to developing high internal pressure. The decay of Jc away from the ends is directly correlated to the local wire diameter increase, which decreases the local Bi-2212 filament mass density and lowers Jc, often by well over 50%. It is clear that control of the internal gas pressure is crucial to attaining the full Jc of these very promising round wires and that the very variable properties of Bi-2212 wires are due to the fact that this internal gas pressure has so far not been well controlled.
    Superconductor Science and Technology 02/2013; 26(5). · 2.76 Impact Factor
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    ABSTRACT: Bi-2212 round wire is made by the powder-in-tube technique. An unavoidable property of powder-in-tube conductors is that there is about 30% void space in the as-drawn wire. We have recently shown that the gas present in the as-drawn Bi-2212 wire agglomerates into large bubbles and that they are presently the most deleterious current limiting mechanism. By densifying short 2212 wires before reaction through cold isostatic pressing (CIPping), the void space was almost removed and the gas bubble density was reduced significantly, resulting in a doubled engineering critical current density (JE) of 810 A/mm2 at 5 T, 4.2 K. Here we report on densifying Bi-2212 wire by swaging, which increased JE (4.2 K, 5 T) from 486 A/mm2 for as-drawn wire to 808 A/mm2 for swaged wire. This result further confirms that enhancing the filament packing density is of great importance for making major JE improvement in this round-wire magnet conductor.
    IEEE Transactions on Applied Superconductivity 01/2013; 23(3). · 1.20 Impact Factor
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    ABSTRACT: We report on the superior vortex pinning of single- and multilayer Ba(Fe1−xCox)2As2 thin films with self-assembled c-axis and artificially introduced ab-plane pins. Ba(Fe1−xCox)2As2 can accept a very high density of pins (15–20 vol %) without Tc suppression. The matching field is greater than 12 T, producing a significant enhancement of the critical current density Jc, an almost isotropic Jc(θ, 20 T) > 105 A/cm2, and global pinning force density Fp of ∼50 GN/m3. This scenario strongly differs from the high-temperature superconducting cuprates where the addition of pins without Tc suppression is limited to 2–4 vol %, leading to small HIrr enhancements and improved Jc only below 3–5 T.
    Physical review. B, Condensed matter 12/2012; 86(21). · 3.77 Impact Factor
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    ABSTRACT: The K- and Co-doped BaFe2As2 (Ba-122) superconducting compounds are potentially useful for applications because they have upper critical fields (Hc2) of well over 50 T, Hc2 anisotropy < 2 and thin-film critical current densities Jc exceeding 1MAcm􀀀2 (refs 1–4) at 4.2 K. However, thin-film bicrystals of Co-doped Ba-122 clearly exhibit weak link behaviour for T001U tilt misorientations of more than about 5�, suggesting that textured substrates would be needed for applications, as in the cuprates5,6. Here we present a contrary and very much more positive result in which untextured polycrystalline (Ba0:6K0:4)Fe2As2 bulks and round wires with high grain boundary density have transport critical current densities well over 0:1MAcm􀀀2 (self-field, 4.2 K), more than 10 times higher than that of any other round untextured ferropnictide wire and 4–5 times higher than the best textured flat wire7. The enhanced grain connectivity is ascribed to their much improved phase purity and to the enhanced vortex stiffness of this low-anisotropy compound ( � 1–2) when compared with YBa2Cu3O7􀀀x ( �5).

Publication Stats

2k Citations
555.11 Total Impact Points

Institutions

  • 2008–2014
    • Florida State University
      • Applied Superconductivity Center (ASC)
      Tallahassee, Florida, 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
    • University of Wisconsin - Stout
      • Department of Physics
      Menominee, WI, United States
  • 1997
    • Tsukuba Research Institute
      Edo, Tōkyō, Japan