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, J(c). To minimize such grain boundary obstacles, HTS conductors such as REBa2Cu3O(7-x) and (Bi, Pb)2Sr2Ca2Cu3O(10-x) are both made as tapes with a high aspect ratio and a large superconducting anisotropy. Here we report that Bi2Sr2CaCu2O(8-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 J(c) 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.
High Field Magnets With HTS Conductors Weijers, H W; Trociewitz, U P; Markiewicz, W D ...
IEEE transactions on applied superconductivity,
06/2010, Letnik:
20, Številka:
3
Journal Article, Conference Proceeding
Recenzirano
Development of high-field magnets using high temperature superconductors (HTS) is a core activity at the NHMFL. Magnet technology based on both YBCO-coated tape conductors and Bi-2212 round wires is ...being pursued. Two specific projects are underway. The first is a user magnet with a 17 T YBCO coil set which, inside an LTS outsert, will generate a combined field of 32 T. The second is a 7 T Bi2212 demonstration coil set to be operated in a large bore resistive magnet to generate a combined magnetic field of 25 T. Owing to the substantial technological differences of the two conductor types, each project faces different conductor and magnet technology challenges. Two small coils have been tested in a 38-mm cold bore cryostat inserted in a 31 T resistive magnet: a Bi2212 round-wire layer-wound insert coil that generated 1.1 T for a total of 32.1 T and a YBCO double-pancake insert that generated 2.8 T for a total central field of 33.8 T. Four larger layer-wound coils have been manufactured and tested in a 20 T, 186-mm cold bore resistive magnet: a sizeable Bi-2212 coil and three thin large-diameter YBCO coils. The test results are discussed. The current densities and stress levels that these coils tolerate underpin our conviction that >30 T all-superconducting magnets are viable.
The discovery of superconductivity at 39 K in magnesium diboride, MgB2, raises many issues, a critical one being whether this material resembles a high-temperature copper oxide superconductor or a ...low-temperature metallic superconductor in terms of its behaviour in strong magnetic fields. Although the copper oxides exhibit very high transition temperatures, their in-field performance is compromized by their large anisotropy, the result of which is to restrict high bulk current densities to a region much less than the full magnetic-field-temperature (H-T) space over which superconductivity is found. Moreover, the weak coupling across grain boundaries makes transport current densities in untextured polycrystalline samples low and strongly sensitive to magnetic field. Here we report that, despite the multiphase, untextured, microscale, subdivided nature of our MgB2 samples, supercurrents flow throughout the material without exhibiting strong sensitivity to weak magnetic fields. Our combined magnetization, magneto-optical, microscopy and X-ray investigations show that the supercurrent density is mostly determined by flux pinning, rather than by the grain boundary connectivity. Our results therefore suggest that this new superconductor class is not compromized by weak-link problems, a conclusion of significance for practical applications if higher temperature analogues of this compound can be discovered.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The discovery of superconductivity at 39 K in magnesium diboride offers the possibility of a new class of low-cost, high-performance superconducting materials for magnets and electronic applications. ...This compound has twice the transition temperature of Nb3Sn and four times that of Nb-Ti alloy, and the vital prerequisite of strongly linked current flow has already been demonstrated. One possible drawback, however, is that the magnetic field at which superconductivity is destroyed is modest. Furthermore, the field which limits the range of practical applications-the irreversibility field H*(T)-is approximately 7 T at liquid helium temperature (4.2 K), significantly lower than about 10 T for Nb-Ti (ref. 6) and approximately 20 T for Nb3Sn (ref. 7). Here we show that MgB2 thin films that are alloyed with oxygen can exhibit a much steeper temperature dependence of H*(T) than is observed in bulk materials, yielding an H* value at 4.2 K greater than 14 T. In addition, very high critical current densities at 4.2 K are achieved: 1 MA cm-2 at 1 T and 105 A cm-2 at 10 T. These results demonstrate that MgB2 has potential for high-field superconducting applications.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Recent study of the current-limiting mechanisms in Bi-2212 round wires has suggested that agglomeration of the residual Bi-2212 powder porosity into bubbles of filament-diameter size occurs on ...melting the Bi-2212 filaments. These pores introduce a major obstacle to current flow, which greatly reduces the critical current density (J sub(c)). Here we present an in situ non-destructive tomographic and diffraction study of the changes occurring during the heat treatment of wires and starting powder, as well as a room temperature study of ex situ processed wires. The in situ through-process study shows that the agglomeration of residual porosity is more complex than previously seen. Filament changes start with coalescence of the quasi-uniform and finely divided powder porosity into lens-shaped defects at about 850 degree C when the Bi-2201 impurity phase decomposes before the Bi-2212 starts to melt. These lens-shaped voids grow to bubbles of a filament diameter on melting of the Bi-2212 and continue to lengthen and then to agglomerate across multiple filaments while the filaments are in the liquid state. The experiment makes clear why melt processing is vital to developing high J sub(c) and also shows how rearrangement of the residual filament porosity on melting imposes a strong longitudinal inhomogeneity in each filament. Reducing the bubble density is clearly an important path to reaching much higher J sub(c) values in Bi-2212 round wires. Synchrotron micro-tomography is an exceptionally powerful technique for studying the spatial extent of the porosity on a scale of about 2 mu m and larger.
We have studied the influence of the oxygen partial pressure pO 2 up to 5.5 bar on the phase changes that occur during the melt processing of a state-of-the-art Bi-2212 multifilamentary wire. Phase ...changes have been monitored in situ by high energy synchrotron X-ray diffraction. We found that the stability of the Bi-2212 phase is reduced with increasing pO 2 . For pO 2 > 1 bar, a significant amount of the Bi-2212 phase decomposes upon heating in the range of 400 °C-650 °C. The extent of decomposition strongly increases with increasing pO 2 , and at pO 2 = 5.5 bar Bi-2212 completely decomposes in the solid state. Textured Bi-2212 can be formed during solidification when pO 2 is reduced to 0.45 bar when the precursor is molten. Since the formation of current-limiting second phases is very sensitive to pO 2 when it exceeds 1 bar, we recommend to reduce the oxygen partial pressure below the commonly used pO 2 = 1 bar, in order to increase the pO 2 margins and to make the overpressure process more robust.
We report a significant enhancement of the upper critical field Hc2 of different MgB2 samples alloyed with nonmagnetic impurities. By studying films and bulk polycrystals with different resistivities ...rho, we show a clear trend of an increase in Hc2 as rho increases. One particular high resistivity film had a zero-temperature Hc2(0) well above the Hc2 values of competing non-cuprate superconductors such as Nb3Sn and Nb-Ti. Our high-field transport measurements give record values T and T for high resistivity films and T for untextured bulk polycrystals. The highest Hc2 film also exhibits a significant upward curvature of Hc2(T) and a temperature dependence of the anisotropy parameter opposite to that of single crystals: gamma(T) decreases as the temperature decreases, from to . This remarkable Hc2 enhancement and its anomalous temperature dependence are a consequence of the two-gap superconductivity in MgB2, which offers special opportunities for further Hc2 increases by tuning of the impurity scattering by selective alloying on Mg and B sites. Our experimental results can be explained by a theory of two-gap superconductivity in the dirty limit. The very high values of Hc2(T) observed suggest that MgB2 can be made into a versatile, competitive high-field superconductor.
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.
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.
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