Fe Sheath Research Articles

Structural and superconducting properties of PIT processed sintered MgB 2/Fe wires

In this paper, we report the structural and superconducting properties of MgB 2/Fe wires that are produced by different sintering conditions. Good quality MgB 2/Fe wires are fabricated by the powder-in-tube (PIT) method using commercially available MgB 2 powder at ambient pressure. In order to check the annealing effect, the different pieces of the as-rolled wires are sintered at 1323 K for 0.30–1.20 ks. XRD data confirms that they are hexagonal MgB 2 structure. All the sintered samples show higher T c values in the range of 38.2–38.4 K with high J c than the as-rolled sample. No significant change in T c is seen among the annealed samples. On the other hand, the annealed samples show significant change in the J c values as well as in micro-structural features due to variable sintering time. On annealing at 1323 K for 0.60 ks, we obtain the best quality sample with a J c of 372 A/mm 2 at 33.1 K in self-field with a maximum T c of 38.4 K. Among the annealed samples, 1.20 ks sample shows lower J c. Longer annealing time reduced J c, indicating a possible interfacial reaction between the Fe sheath and the MgB 2 core.

Critical current densities of superconducting MgB 2 tapes prepared on the base of mechanically alloyed precursors

Cu- and Fe-cladded monofilamentary tapes and 19-filamentary tapes with Fe matrix have been prepared by the powder-in-tube (PIT) method using mechanically alloyed precursor powders consisting of the constituents Mg, B, and MgB 2. Despite of the low T c values of about 31 K for monofilamentary tapes with Fe sheath maximum critical current densities ( J c) of 29 and 6 kA/cm 2 at 4.2 K in external magnetic fields of 7.5 and 10 T, respectively, were achieved. The maximum J c value of 18.5 kA/cm 2 at 20 K in a field of 2 T was obtained for these conductors. The heat treatment of the monofilamentary tapes under uniaxial pressure of 0.5 GPa led to enhanced J c values compared to those of tapes annealed under normal pressure. Up to 27 and 11 kA/cm 2 were achieved at 4.2 K in external fields of 8 and 10 T, respectively, for samples annealed under pressure. For the 19-filamentary tape J c values up to 35 and 9 kA/cm 2 in fields of 7.5 and 10 T, respectively, were measured at 4.2 K. Investigation of the microstructure by optical microscopy, SEM/WDX and XRD have shown that the high J c values may be mainly due to the remarkably small grain size of the MgB 2 phase and defects particularly precipitates of magnesium oxide. J c of the Cu-cladded tapes is one order of magnitude below that of Fe-cladded ones. The main reason for this discrepancy is the higher porosity of the superconducting cores with Cu sheath compared to that with Fe sheath. At higher sintering temperatures strong interactions between the Cu sheath and constituents of the precursor contribute to the suppression of the superconducting properties of Cu-sheathed conductors.

Critical current densities of superconducting MgB2 tapes prepared on the base of mechanically alloyed precursors

Cu- and Fe-cladded monofilamentary tapes and 19-filamentary tapes with Fe matrix have been prepared by the powder-in-tube (PIT) method using mechanically alloyed precursor powders consisting of the constituents Mg, B, and MgB 2 . Despite of the low T c values of about 31 K for monofilamentary tapes with Fe sheath maximum critical current densities ( J c ) of 29 and 6 kA/cm 2 at 4.2 K in external magnetic fields of 7.5 and 10 T, respectively, were achieved. The maximum J c value of 18.5 kA/cm 2 at 20 K in a field of 2 T was obtained for these conductors. The heat treatment of the monofilamentary tapes under uniaxial pressure of 0.5 GPa led to enhanced J c values compared to those of tapes annealed under normal pressure. Up to 27 and 11 kA/cm 2 were achieved at 4.2 K in external fields of 8 and 10 T, respectively, for samples annealed under pressure. For the 19-filamentary tape J c values up to 35 and 9 kA/cm 2 in fields of 7.5 and 10 T, respectively, were measured at 4.2 K. Investigation of the microstructure by optical microscopy, SEM/WDX and XRD have shown that the high J c values may be mainly due to the remarkably small grain size of the MgB 2 phase and defects particularly precipitates of magnesium oxide. J c of the Cu-cladded tapes is one order of magnitude below that of Fe-cladded ones. The main reason for this discrepancy is the higher porosity of the superconducting cores with Cu sheath compared to that with Fe sheath. At higher sintering temperatures strong interactions between the Cu sheath and constituents of the precursor contribute to the suppression of the superconducting properties of Cu-sheathed conductors.

Iron-sheath influence on the superconductivity ofMgB2 core in wires and tapes

The magnetic systems of superconducting wires and tapes, consisting of a magnesium diboride(MgB2) superconducting core surrounded by a ferromagnetic iron (Fe) sheath, have beeninvestigated. The interaction of the superconductor with the soft magnetic environment hasbeen experimentally shown to lead to significant enhancement of dissipation-freesuper-current densities. The maximum densities of the super-currents can exceed thecritical current densities obtained in the same ‘bare’ wire (with its iron sheath removed) bymore than one order of magnitude under the same experimental conditions. This currentdensity enhancement is referred to as the overcritical state, which has been observed over awide range of magnetic fields applied transversely to the wire. No overcritical currents areobserved for the longitudinal field orientation. The irreversibility field is shownto be considerably suppressed by the influence of the iron sheath for both fieldorientations. Different geometries of the wire cross section have been investigated.

Interfacial reactions and oxygen distribution inMgB2 wires in Fe, stainless steel and Nb sheaths

Superconducting MgB2 wires have been made by powder-in-tube (PIT) processing of commercialMgB2 powder in Fe, stainless steel and composite metallic sheaths using annealing temperatures from 850 to1100 °C. Technologicallyinteresting Jc values around 106 A cm−2 in the self-field at 4.2 K have been achieved. However, as the annealing temperatureis increased, substantial chemical reactions have been observed at theMgB2/sheath interface. We report on a detailed study using electron probe microanalysis of the reactionproducts in the interfacial region and the distribution of oxygen in the core. Iron andchromium borides are formed at the core/sheath interface in Fe and stainless steel sheathsrespectively, with consequent depletion of B from the superconducting core. A similar effectis caused by the formation of a solid solution of B in Nb sheaths. Milling the startingpowder before wire manufacture results in a very significant increase in the oxygen contentof the core, which probably explains the relatively poor performance of these wires.

The effect of used deformation, metal sheath and heat treatment on the I–V curve of ex situ MgB2 composite

Current–voltage characteristics of MgB 2 composite wires made of ex situ process using Cu, Fe and stainless steel (SS) sheaths have been measured at 4.2 K in the self-field and external magnetic field between 2 and 4 T. It was found that the used mode of deformation (drawing, rotary swaging and two-axial rolling) influences the slope of I–V curve due to affected grain connectivity. Final heat treatment at 950 °C/0.5 h improves grain connectivity apparently, which results in I–V characteristic with very high n -exponents. The n -exponents of multi-core wires measured in external field are apparently lower due to a current sharing among the non-uniform filaments surrounded by high resistance matrix.

The Ic(B) behavior of MgB2 composite with ferromagnetic sheath

Single core MgB 2 composites have been made by the powder-in-tube method using commercial MgB 2 powder (Alfa Aesar) and Fe or Cu sheath. Transport currents of as-deformed tapes were measured at 4.2 K and external magnetic fields B =0–6 T. A different I c ( B ) curves were measured for Fe and Cu sheathed tapes. This behavior is attributed to the Fe sheath magnetization by self-field and external field. Field distribution in the conductor cross section was analysed with the finite element method. The results were in agreement with the measurements. The presence of ferromagnetic layer in MgB 2 composite influence also the pinning curves and Kramer’s plots derived from measured data. This phenomenon has to be taken into account when the basic superconducting characteristics of the composite (wires/tapes) with magnetic sheaths are described and discussed.

The effect of Fe-magnetization on Ic(B) and Ic(α) characteristics of iron-sheathed MgB2 composite wires

Single- and four-core MgB2 composites have been made by the powder-in-tube (PIT) method using commercial MgB2 powder in an Fe sheath. Transport currents of as-deformed wires were measured at 4.2 K and external magnetic fields B = 0–4 T. Different Ic(B) curves were observed for single- and four-core wires depending on the value of the self-field current. The behaviour Ic(B) and Ic(α) is influenced by Fe sheath magnetization by self-field and external field. Field distribution and Ic(B) curves were calculated using the finite element method (FEM) and they are used for an explanation of the experimental characteristics.

High critical current density in MgB2/Fe wires

High-quality monofilamentary MgB2wires with Fe sheath were fabricated by the powder-in-tube (PIT) method at ambient pressure. The phase compositions, microstructure features and flux pinning properties are investigated by using x-ray diffractometer, SEM, TEM and a standard four-probe technique. It is found that MgB2 has no reaction with Fe. The results indicate that MgB2 wires are dense and only MgB2 and few MgO phases exist in the core area. MgB2/Fe wires exhibit very high transport critical current densities of 1.43 × 105 A cm−2 (4.2 K, 4 T), 2.8 × 104 A cm−2 (4.2 K, 6 T), 3.72 × 104 A cm−2 (15 K, 4 T) and 2.34 × 104 A cm−2 (25 K, 3 T). The maximum flux pinning force shows a peak at 4 T at 4.2 K, suggesting strong pinning at high fields. The reasons for the large Jc in the sample was also discussed.

Transport critical current density of Fe sheath MgB 2 tapes sintered at different temperatures

Fe sheath MgB 2 tapes sintered at 650, 750, 850 and 950 °C, respectively, for 2 h in a high purity argon gas were prepared using the powder-in-tube method. Transport critical current densities of tapes were measured in the fields up to 10 T from 4 to 30 K. Both tapes sintered at 850 and 950 °C completely lost the capacity of carrying superconducting current over the temperature range from 4 to 30 K. Compared to a tape sintered at 650 °C, an improvement in the in-field critical current density J c and irreversibility field μ 0 H *( T) was observed in a tape sintered at 750 °C. At 20 K, the J c value was ∼1.32×10 5 A/cm 2 in self-field and the μ 0 H * value 4.2 T for the tape sintered at 750 °C, whereas the J c in self-field and μ 0 H * values were ∼5.9×10 4 A/cm 2 and 2.8 T for that sintered at 650 °C.

Continuous- and batch-processed MgB 2/Fe strands––transport and magnetic properties

The tube filling/forming powder-in-tube (PIT) technique was employed at Hyper Tech Research for the continuous processing of MgB 2/Fe composite strand. Demonstration lengths of up to 70 m of 1.2 mm diameter precursor strand––an Fe-clad Mg + B elemental powder mixture––have so far been produced. Short samples of this strand, heat treated for 3–30 min/800–950 °C, have yielded transport critical current densities ( J c) of 2–3×10 4 A/cm 2 at 4 K/4 T, 4×10 4 A/cm 2 at 4 K/2 T, and at 4 K in self field an extrapolated J c of at least 2×10 5 A/cm 2. Batch-processed PIT MgB 2/Fe strands and tapes were made at the University of Wollongong’s Institute for Superconducting and Electronic Materials. These were the focus of magnetization- and magnetic-shielding studies. Depending on the thickness of the Fe sheath, the superconducting core could be completely shielded at low fields (e.g. below 2 kOe). Taking this into account, the magnetic J c is extracted, and found to be comparable to the extrapolated transport value.

Properties of as-deformed and post-annealed MgB2/Fe(Fe-alloy) composite wires

Composite MgB2 wires have been made by the powder-in-tube method using commercial MgB2 powder (Alfa Aesar) in Fe and Fe-alloy (stainless steel (SS)) sheaths. Different deforming processes have been used. Two-axial rolling (TAR) produces the highest powder density leading to the best Jc (0 T, 4.2 K) of 55000 A cm−2 in as-deformed wire. Therefore, four-core wire has also been made by TAR. The short samples were subjected to annealing at temperatures from 850 to 1100 °C for 30 min in argon, which leads to apparently improved Jc values, 25000 A cm−2 at 4 T and around 106 A cm−2 in the self-field (at 4.2 K). An interdiffusion layer between MgB2 and the Fe or SS sheath was observed and analysed. The SS sheath reacts more intensely than Fe due to the presence of Ni and Cr elements. Transport currents were measured at temperatures 4.2–25 K and an external magnetic field B = 0–8 T. The engineering current density level of 104 A cm−2 is used to estimate the magnetic field possibly generated by coils wound from MgB2 composite wires.

Superconducting properties of MgB 2 tapes and wires

An overview of the current state of development of MgB 2 wires and tapes prepared by several techniques is presented, based on the known literature data and our own results. We focus on the powder-in-tube processing method, using pre-reacted MgB 2 powders surrounded by an Fe sheath (ex situ processing route). The study of the effect of the initial MgB 2 particle size shows that after reducing the initial particles by ball milling to sizes of the order of 3 μm, the degree of texturing and the upper critical field, B c2, remain unchanged, while both the critical current density, J c, and the irreversibility field, B irr, show a considerable increase, followed by a decrease after longer milling times. After various recrystallization times and temperatures we show that a critical amount of impurities introduced during the particle reduction process is responsible for the observed maximum of J c and B irr. Our analysis indicates that this is a grain boundary effect, thus pointing the way for further improvement of the transport properties. A preferential orientation along the c-axis has been observed in Fe/MgB 2 tapes by X-ray diffraction. An anisotropy ratio of 1.3 for both B c2 and B irr was found. The Fe/MgB 2 tapes exhibit a very high n factor, which opens up the possibility for a persistent mode operation at 4.2 K and moderate fields. Transport J c values above 10 4 A/cm 2 are obtained in monofilamentary Fe/MgB 2 tapes at 4.2 K and 6.5 T and at 25 K and 2.25 T. Multifilamentary tapes were found to exhibit lower J c values due to the presently used deformation process yielding a lower density with respect to monofilaments. In all measured tapes, quenching was observed at the lowest applied fields. Improvement of the thermal stability of MgB 2 tapes will be one of the major challenges in future developments.

Transport currents of two-axially rolled and post-annealed MgB2/Fe wires at 4.2 K

Single- and four-core MgB2 wires have been made by the powder-in-tube (PIT) method using a commercial MgB2 powder (Alfa Aesar) and two-axial rolling deformation in an Fe sheath. The deformed wires were subjected to annealing at temperatures from 950 °C up to 1100 °C for 30 minutes in argon. The interface layer thickness of 10–25.4 μm caused by inter-diffusion and reaction between the MgB2 core and the Fe sheath has been observed in all annealed wires. Transport currents of as-deformed and post-annealed wires were measured at the temperature 4.2 K and external magnetic field B = 0–5.5 T. The presence of a ferromagnetic sheath in MgB2 composite wire influences the Ic(B) characteristics leading to Ic hysteresis and Ic anisotropy, which is more pronounced for those that have not been annealed (weakly linked grains) in comparison to well-connected MgB2 grains after recrystallization. A comparison of the overall current densities versus the external magnetic field of NbTi, Nb3Sn and Bi-2223 superconductors with four-core MgB2 wire is also presented.

Ic anisotropy and Ic hysteresis in MgB2/Fe/Cu tape

Single core MgB2 tape has been made by the powder-in-tube (PIT) method using commercial MgB2 powder (Alfa Aesar) and two-axial rolling deformation in an Fe/Cu sheath. Transport currents were measured at temperatures 4.2 K–15 K in parallel and perpendicular to the external magnetic field B = 0–5.5 T and at a constant field B = 1 T of variable orientation to the tape surface (0–180°). The transport current anisotropy of the tape with different orientations of the tape surface with respect to the external magnetic field was measured, showing higher currents in perpendicular field orientation. The Ic anisotropy of MgB2/Fe/Cu tape was observed only for low external fields (0–2.5 T). This behaviour is explained by the Fe sheath magnetization and confirmed by the isotropic behaviour of MgB2/Cu tape prepared by the similar PIT process and by hysteretic Ic(B) curves measured for increased and decreased external field in MgB2/Fe/Cu. The presence of the ferromagnetic barrier in MgB2 composite wire influences the Ic(B) characteristics and may lead to increased hysteretic losses for possible future ac applications.

Influence of Ag, Cu and Fe sheaths on MgB2 superconducting tapes

Copper, iron and silver MgB2 sheathed tapes have been manufactured under different conditions. It has been found that copper-sheathed tapes can show a higher critical current density than iron-sheathed tapes if heat-treated at temperatures below 850 °C. The influence of different overall mechanical deformation rates has been studied for tapes sheathed by all three types of metals. By increasing the deformation rate the critical current density was improved by about an order of magnitude in the case of the copper-sheathed tapes, while the critical current density of the iron-sheathed tapes remained constant.

High-transport critical current density above 30 K in pure Fe-clad MgB 2 tape

Fe-clad MgB 2 long tapes have been fabricated using a powder-in-tube technique. An Mg+2B mixture was used as the central conductor core and reacted in situ to form MgB 2. The tapes were sintered in pure Ar at 800°C for 1 h at ambient pressure. SEM shows a highly dense core with a large grain size of 100 μm. The Fe-clad tape shows a sharp transition with transition width of ΔT c of 0.2 K and a T c0 of 37.5 K. We have achieved the highest transport critical current reported so far at 1.7×10 4 A/cm 2 for both 29.5 K in 1 T and 33 K in zero applied field. Resistivity temperature dependence and transport critical current were also measured in magnetic fields applied perpendicular and parallel to the tape plane. Not only is the use of an Fe sheath necessary for the successful processing of in situ reacted powder-in-tube MgB 2, it confers on the finished wire the additional benefit of magnetic screening.