Effect of various mechanical deformation processes on critical current density and microstructure in MgB<sub>2</sub>tapes and wires

Zhou, Sihai; Pan, Alexey V.; Liu, Huan; Horvat, J.; Dou, Shi Xue

doi:10.1088/0953-2048/15/11/303

Cited by 10 publications

(11 citation statements)

References 18 publications

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“…The ironsheathed wires were prepared by first filling the powder into iron tubes, which were drawn to desired diameter, sealed into another iron tube, and then sintered in flowing high purity argon. The exact preparation procedure for these samples was described elsewhere 9,10 . The iron sheath was removed before the magnetic measurements, to avoid its contribution to the signal and magnetic interaction between the iron and MgB 2 11 .…”

Section: Methodsmentioning

confidence: 99%

Superconducting screening on different length scales in high-quality bulk MgB2 superconductor

Horvat

Soltanian

Pan

et al. 2004

Journal of Applied Physics

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Field dependence of irreversible magnetic moment ∆m was obtained from magnetic hysteresis loops for a number of bulk MgB 2 superconductors. The field dependence of ∆m exhibits inflections at two characteristic fields, H t and H i (H t < H i ). These two fields increase linearly with the logarithm of the sample size, each extrapolating to zero at a different characteristic sample size. Magneto-optical, optical and scanning electron microscopic examinations show that these extrapolated sample sizes match the sizes of the main microscopic features of the samples. The inflection at H t and the sample size dependence of ∆m and H t are associated with voids scattered through the samples, which are observed for all bulk MgB 2 samples. The voids encircle cells of MgB 2 material of tens of micrometers in size. The cells are connected by narrow bridges. The superconducting currents screening the whole of the sample have to cross these bridges and they are concentrated in them. This promotes additional superconducting screening around the cells, where the current density would otherwise be smaller than the critical current density J c . The field dependence of the currents pertinent to each of the screenings follows the stretched exponential law. However, these two screening currents decrease with H with a different rate, and H t is a field of the transition from the dominance of one of the currents in ∆m to the other. The screening that gives predominant contribution to ∆m for H < H t is confined to inside the cells, circulating on ~10 µm-scale. For H t < H < H i , the dominant screening currents circulate the whole of the sample, percolating through the bridges. If J c is calculated from the critical state model assuming only the screening around the whole of the sample, erroneous value of J c and its functional dependence on the field are obtained. This also leads to an artifact of the sample size-dependent J c . However, because these two stretched exponential contributions decrease with H with a different rate, they can be separated for most of the samples. The magnetically obtained J c for H t < H < H i is defined by the screening currents percolating around the whole of the sample, and it is in good agreement with the transport J c . There is also a third tier of superconducting screening on a micrometer scale, inside the cells. It is associated with clusters of superconducting crystals that make up the cells. However, its contribution to ∆m is negligible for H < H i , which are the fields of technological interest.

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Section: Methodsmentioning

confidence: 99%

Superconducting screening on different length scales in high-quality bulk MgB2 superconductor

Horvat

Soltanian

Pan

et al. 2004

Journal of Applied Physics

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“…In this work [3], only hard sheath materials were used, which are beneficial for the ex situ preparation due to chemical compatibility and the possibility of achieving high core densities inside the sheath. Nevertheless, the poor ability of MgB 2 particles to form a bulk monolithic structure remains an important issue for further J c enhancement, as was shown, for example, by the two-stage wire fabrication process discussed in [4,5]. This process involved an initial in situ wire preparation from an unreacted Mg+2B powder mixture, as the first stage, and a quasi ex situ second stage consisting of an additional mechanical deformation of the wires, which crashed the core that had been formed in order to increase its density, followed by another heat treatment.…”

Section: Introductionmentioning

confidence: 99%

Properties of superconducting MgB₂wires:in situversusex situreaction technique

Pan

Zhou

Liu

et al. 2003

Supercond. Sci. Technol.

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We have fabricated a series of iron-sheathed superconducting wires prepared by the powder-in-tube technique from (MgB 2 ) 1−x :(Mg+2B) x initial powder mixtures taken with different proportions, so that x varies from 0 to 1. It turned out that ex situ prepared wire (x = 0) has considerable disadvantages compared to all the other wires in which in situ assisted (0 < x < 1) or pure in situ (x = 1) preparation was used due to weaker inter-grain connectivity. As a result, higher critical current densities J c were measured over the entire range of applied magnetic fields B a for all the samples with x > 0. Pinning of vortices in MgB 2 wires is shown to be due to grain boundaries. J c (B a ) behaviour is governed by an interplay between the transparency of grain boundaries and the amount of 'pinning' grain boundaries. Differences between thermo-magnetic flux-jump instabilities in the samples and a possible threat to practical applications are also discussed.

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“…On the other hand, the core density of in situ MgB 2 wires after the formation of the superconducting phase is rather low, and its microstructure is extremely porous [14,15]. Deformation of in situ MgB 2 wires after phase formation is unavoidable due to mass density considerations because densification of the packed MgB 2 powders is incomplete when the wire fabrication process is completed [15,16]. Even in multifilament wires with smaller filament size, MgB 2 filaments are not fully dense since Mg + 2B has a lower theoretical density than MgB 2 .…”

Section: Introductionmentioning

confidence: 99%

“…Even in multifilament wires with smaller filament size, MgB 2 filaments are not fully dense since Mg + 2B has a lower theoretical density than MgB 2 . Porosity occurs in the superconducting core after heat treatment; an extra densification is necessary to get rid of this porous structure, and with no doubt, there is a cost in terms of critical current density [16].…”

Section: Introductionmentioning

confidence: 99%

“…The application of various mechanical deformations and subsequent heat treatment is a common approach [15][16][17][18], but processing of in situ MgB 2 wires and tapes has not yet been completely resolved [19]. In this study, a series of experiments has been planned in order to increase the superconducting core density of the mono-core in situ Fe/MgB 2 wire samples.…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Microstructure and Transport Properties of Compaction-Modified In Situ Fe/MgB2 Wires

Şenol

Karaboğa

2016

J Supercond Nov Magn

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This study has shown the effect of excessive mechanical deformation by pressing and consecutive heat treatment, respectively, on the microstructure and transport properties of mono-core sintered in situ Fe/MgB 2 wires. These wires were pressed into short tapes under 1 GPa uniaxial pressure. Transport properties like critical temperature, transition width and engineering critical current density of the same samples were determined before and after pressing and after annealing. Phase formation and grain size of the samples were studied with XRD. The SEM analysis indicated that the mechanical deformation reduced initial voids to a large extent but caused cracks, increased the number of grain boundaries and introduced weak links. The subsequent heat treatment on the pressed samples improved the microstructure and the grain connectivity. Possibility of a final densification of the reacted in situ wires was discussed in terms of transport properties.

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Effect of various mechanical deformation processes on critical current density and microstructure in MgB₂tapes and wires

Cited by 10 publications

References 18 publications

Superconducting screening on different length scales in high-quality bulk MgB2 superconductor

Superconducting screening on different length scales in high-quality bulk MgB2 superconductor

Properties of superconducting MgB₂wires:in situversusex situreaction technique

Microstructure and Transport Properties of Compaction-Modified In Situ Fe/MgB2 Wires

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Effect of various mechanical deformation processes on critical current density and microstructure in MgB2tapes and wires

Cited by 10 publications

References 18 publications

Superconducting screening on different length scales in high-quality bulk MgB2 superconductor

Superconducting screening on different length scales in high-quality bulk MgB2 superconductor

Properties of superconducting MgB2wires:in situversusex situreaction technique

Microstructure and Transport Properties of Compaction-Modified In Situ Fe/MgB2 Wires

Contact Info

Product

Resources

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Effect of various mechanical deformation processes on critical current density and microstructure in MgB₂tapes and wires

Properties of superconducting MgB₂wires:in situversusex situreaction technique