Two-axial Rolling Research Articles

Fabrication and AC transport losses for Ag-sheathed Bi-2223 tapes using rectangular deformation process

The AC transport losses in self-field at 77 K have been investigated for the Ag-sheathed Bi-2223 multifilamentary tapes prepared by rectangular deformation process. The rectangular wires with 20 (5 × 4 arrangement) filaments and different aspect ratio of their cross-section were fabricated by two-axial rolling machine, and subsequently they were converted to the tape-form conductors by the one-axial flat rolling and heat treatments. The initial configurations of rectangular wires before applying the flat rolling affect the filament shape near the tape edge in the final tapes. Furthermore, transport loss behaviors for investigated tapes also depend on the initial configurations of rectangular wires, although the main contribution to the losses comes from the hysteresis loss of the superconductor. This may be caused by the difference in filament shape and lateral J c distributions in the cross-section of each tape samples.

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.

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We investigated the applicability of a Cu sheath to MgB2 wires that were prepared utilizing the in-situ synthesis PIT (powder-in-tube) method and compared it with that of stainless steel. Since the critical current density of MgB2 increases with TiH2 doping, we prepared TiH2-doped Cu sheath MgB2 wires 40 m in length and φ1.0 mm or 0.5x1.0 mm2 in cross-sectional dimension utilizing rotary swaging, drawing, and two-axial rolling at room temperature. We then annealed the samples at 600∼850°C for 1∼5hr in an Ar gas atmosphere. The critical current of the TiH2 (6%)-doped MgB2/Cu short sample annealed at 650°C reached 139A(JC = 132kA/cm2) at 4.2 K and ambient field. We also investigated the effect of repeated rolling and annealing on the critical current density of MgB2/SUS wires and confirmed that the enhancement of JC is caused by the densification of MgB2 cores. Finally, we concluded that Cu-sheathed MgB2 wires produced utilizing the in-situ PIT method are promising for practical use.

Effects of the high-pressure treatment of ex situ MgB2 superconductors

Single-core MgB2/Cu and MgB2/Fe/Cu wires and tapes have been made by the powder-in-tube method with the use of commercial MgB2 powder (Alfa Aesar) involving hydrostatic extrusion and two-axial rolling processes. High-pressure post-treatments such as cold isostatic pressing, double hydrostatic extrusion and uniaxial pressing were applied. The grain connectivity, Ic(B) curves, pinning in MgB2 grains, upper critical field, deformation homogeneity, compact density and preferable alignment crystallites for the [001] (c-axis) direction were all improved and enhanced by application of various high-pressure treatments for the MgB2 compound and composites. Hydrostatically extruded MgB2 wires have been shown to be better than rolled wires and are very promising for possible application in windings.

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.

Structural inhomogeneity of superconducting ex situ MgB2/Cu wires made by the powder-in-tube technique

Single core MgB2 wires have been made by the powder-in-tube (PIT) method using commercial MgB2 powder (Alfa Aesar). Using the two-axial rolling process, composites have been made in a Cu sheath. Energy dispersive x-ray (EDX) mapping and scanning electron microscopy backscattered electron observations prove that there is high element segregation (phase inhomogeneity) for as-rolled and as-sintered wires, although the wire after sintering is more inhomogeneous than the as-rolled (non-sintered) wire. X-ray diffraction diffractograms confirm the presence of isotropy and the absence of texturization in the ‘macroscopic’ phase within the MgB2 cores, despite their thermomechanical history. The observed decrease in the value of Jc in the wire after sintering was attributed to macroscopic cracking and the worsening of the intergrain connectivity. EDX line scans have also shown high anisotropy and high inhomogeneity in the distribution of elements within the MgB2 cores. Inhomogeneity in the final product (wires) has resulted from the high macroscopic inhomogeneity observed in the starting MgB2 powder. The Cu sheath has been shown to be neutral (non-poisoning) for MgB2 cores made ex situ. However, due to its softness, it cannot properly constrain the MgB2 core or give enough support required for adequate grain connectivity.

Fabrication of square and round Ag/Bi(2223) wires and their ac loss behaviour

Square and round wires have been fabricated by using a combination of two-axial rolling and drawing in order to reduce the effect of field anisotropy and to confirm the decrease of ac losses predicted by calculation. Critical current measurements, as a function of magnetic field angle, confirmed that the symmetrical configuration of filaments inside the wire leads to an almost angle-insensitive behaviour of Jc(B). The Bi(2223) grains in our wires have a higher misalignment angle (around 8°) with respect to flat tapes, due to the particular wire configuration and the use of two-axial rolling. Therefore, the Jc values in our square wire are somewhat lower, 11 kA cm−2 at 77 K and zero field. AC loss measurements showed that the combination of wire geometry and twisting leads to strongly reduced ac losses in square Ag/Bi(2223) wires when compared to flat tapes: the value of 0.53 mW A−1 m−1 was measured, regardless of the field direction.

Structure, grain connectivity and pinning of as-deformed commercial MgB2 powder in Cu and Fe/Cu sheaths

Single-core MgB2 wires and tapes have been made by the powder-in-tube (PIT) method using commercial MgB2 powder (Alfa Aesar). Composites have been made using the two-axial rolling process in Cu and/or Fe/Cu sheaths. Alternative deformations by wire drawing, rotary swaging and cold isostatic pressing have been applied to PIT wires and tapes. Current–voltage characteristics and transport current densities in the self-field and in the external field were measured. It was found that the grain connectivity of ex situ MgB2 is affected by the applied sheath and the mode of deformation. Two-axial rolling has generated the highest powder density resulting in the best grain connectivity. The highest transport current densities of 8700 A cm−2 and 55 830 A cm−2 were measured for Cu and Fe/Cu sheathed square wires, respectively. Cold isostatic pressing at 1.5 GPa has increased current density and n-exponent, which suggests an improvement in grain connectivity. It was found that the external pressure improves the inter-grain connectivity but decreases the pinning in MgB2 cores.

Fabrication and transport critical currents of multifilamentary MgB2/Fe wires and tapes

Multifilamentary MgB2/Fe wires and tapes with high transport critical current densities have been fabricated using a straightforward powder-in-tube process. After annealing, we measured transport jc values up to 1.1 × 105 A cm−2 at 4.2 K and in a field of 2 T in a MgB2/Fe square wire with seven filaments fabricated by two-axial rolling, and up to 5 × 104 A cm−2 at 4.2 K in 1 T in a MgB2/Fe tape with seven filaments. For higher currents these multifilamentary wires and tapes quenched due to the insufficient thermal stability of the filaments. Both the processing routes and deformation methods were found to be important factors for fabricating multifilamentary MgB2 wires and tapes with high transport jc values.

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.

Square and round Bi(2 2 2 3) wire configurations and their AC losses

In order to reduce the effect of field anisotropy, but also in view of future cabling facilities, a series of Bi(2 2 2 3) wires with various square and round configurations were prepared using a combination of two-axial rolling and drawing. As previously reported, two-axis rolling of a wire configuration, with filaments being perpendicular or inclined to each other, cannot lead to an optimized rolling texture for both directions, e.g. parallel and perpendicular to the tape surface: it gives an average. As a result, even using the same precursor powders, the values of J c in wire configurations are inherently lower than those obtained on standard powder-in-tube tapes. SEM images show that particular care has to be taken in order to avoid a distorted alignment of the filaments in certain local areas. The influence of local stresses on the filaments and their influence on the final texture were studied systematically. The critical current measurements as a function of magnetic field showed that wires with the filament configuration with a rotation symmetry exhibit the most angle-insensitive behavior of J c( B). Due to the wire configuration, the expected relatively low J c values, 11 kA/cm 2 at 77 K and zero field, were measured for square wires. AC loss measurements were performed in wires with various configurations and with twist pitches down to 2 mm. The results show that both wire geometry and twisting have a positive effect on the AC losses.

BSCCO/Ag tapes made by a tape-in-rectangular tube process

Multi-core tape composites with various filament configurations havebeen made by a tape-in-rectangular tube (TIRT) process. This TIRT process isbased on the deformation of multifilamentary composites assembled from arectangular Ag (Ag-alloy) tube filled by pre-deformed single-core BSCCO/Agtapes. Two-axial rolling induces texture in the filaments in two rolling planes,with the filament planes oriented parallel or perpendicular to the tape width,thus reducing the high Ic anisotropy which can degrade the total current incoils working at high temperatures (77 K). The homogeneity of the filaments wasstudied by a magneto-optical technique. Roll-sintered samples were measured at77 K in an external magnetic field up to 1 T and at constant fields (0.1 T and0.5 T) at various orientations (between parallel and perpendicular) to the tapesurface. The results demonstrate the possibility of using these tapes forselected parts of a coil winding to reduce the apparent effect of the radialfield component on the reduction of the total coil current.

Filament aspect ratio and transport currents of Bi(2223)/Ag at 77 K

Monocore BSCCO/Ag tapes of various aspect ratio ( b/ a) have been made by two-axial rolling. After the final treatment (sinter or roll-sinter), the critical currents in self and external magnetic field as well as at various field orientation have been measured. The higher the aspect ratio b/ a, the higher is the value of the self-field transport current and the better is the J c( B) characteristics. It is a consequence of the improved pinning and diminishing influence of weak-links as well. For small b/ a<3, mainly the ratio of two perpendicular 2223 grains orientation influences the I c anisotropy, while for b/ a<3 the major influence on I c anisotropy comes from the different improvement of pinning in ab and c directions.

Evaluation of core density during the two-axial rolling of BSCCO/Ag composite

Two-axial rolling is a promising deforming technique for BSCCO/Ag composite because of several benefits. In comparison to flat rolling, it allows us to increase filament density, to improve the homogeneity and to make texture in two perpendicular planes. Up to now, this deformation has not yet been well understood. Therefore, the density evolution at the transverse cross-sections of two-axially rolled BSCCO cores of single and multicore Ag-sheathed wires was studied by microhardness profiles (HV0.01). The obtained results allow the better understanding of two-axial rolling of BSCCO/Ag composite and also show what deforming parameters should be used to obtain homogeneous and dense ceramic filaments by the powder-in-tube method.

Calculation of the critical currents of Bi(2223)/Ag tapes and coils with reduced anisotropy in Ic( B) characteristic — effect of different proportional representations of the filaments oriented parallel and perpendicularly to the tape surface

A mathematical model was developed to allow for the prediction of critical currents in Bi(2223)/Ag-sheathed tapes that consist of two groups of filaments, having crystalline texture ( c-axis) predominantly in two directions orthogonal to each other and to the length of the conductor (filaments oriented parallel and perpendicular to the tape surface). Using the theory presented, it is possible to estimate the upper limit of critical currents in the tapes prepared with a two-axial rolling technique. Magnetic field and angular dependencies of critical currents were obtained at 77 K measuring tapes with various proportional representations of filaments oriented in both preferred directions, i.e., the parallel and perpendicular directions. The results of calculation indicate that the anisotropy in the I c( B) characteristic should be reduced considerably using the two-axial rolling. Unfortunately, the effect of anisotropy reduction is accompanied by simultaneous decrease in the current-carrying capacity of the tapes. The data obtained for short tape samples were utilised to estimate the critical currents of the individual turns of a small cylindrical magnet, assembled of eight pancake coils. It is shown that only a slight increase in the critical current of the magnet can be expected if the winding should be made of the tape with reduced anisotropy in the I c( B) characteristic. The factors that limit the increase of the magnet critical current are discussed in detail.

Enhancing the engineering J/sub c/ of Bi-2223 multifilamentary tapes by two-axial rolling and periodic pressing

Bi-2223 multifilamentary tapes with a filling factor from 20 to 51% have been produced by a modified Powder-In-Tube method, where a two-axial rolling method is used to replace the standard drawing process. Because the two-axial rolling process does not apply tensile force during wire fabrication and every filament receives a very similar reduction during the deformation processing of the wire into a tape, the size homogeneity of filaments can be significantly improved for tapes with high filling factor. Combining this with a periodic pressing process, the engineering critical current density has been increased from 4 to 8 kA/cm/sup 2/ (77 K, 0 T) for tapes with a filling factor of 35%. The effect of deformation and heat-treatment conditions on the microstructure and transport properties has been found to be quite different in tapes with high filling factor.

Upper limit of the critical currents and magnetic fields of cylindrical coils made of Bi(2223)Ag tapes with reduced -anisotropy

The results of model calculations of the critical currents of silver sheathed Bi(2223) multifilamentary tapes with reduced -anisotropy, which can be achieved by assembling the filaments into several preferred orientations with respect to the external transverse magnetic field, are presented. Such tapes can be prepared by a two-axial rolling technique. It is shown that the properties of tape with higher disordered orientation of the filaments will approach those of isotropic tape. The characteristics of this new class of tapes have been constructed with respect to the angular dependence of the mutual orientation of the magnetic field and the tape surface. A comparison of the evaluated critical current values with those of an original tape with the same cross-section was performed. Further, these data were used for calculation of the critical current of a small model magnet consisting of cylindrical pancake coils. Unexpected results were obtained, showing that practically no appreciable change in the absolute value of the resulting critical current of the magnet can be achieved when the magnet winding is made of the tape with reduced anisotropy. It is shown that the increase in the critical currents of some turns is accompanied by the simultaneous decrease of the critical currents of other turns. However, a slight increase in the magnet critical current at 77 K could be achieved by combining both tape types in the winding, i.e. the tape showing considerable anisotropy and the tape with reduced anisotropy. The influence of the tape widening on the resulting engineering current density is discussed.

Application of two-axial rolling for multicore Bi(2223)/Ag tapes

Multicore Ag-sheathed Bi(2223) tapes have been prepared by OPIT technique using the two-axial rolling deformation by active `Turk's head' roller. In comparison to wire drawing, the presented technique allows one to avoid the tensile stress during composite deformation and to increase the BSCCO-core density. A better homogeneity of filaments has been obtained by two-axial rolling. The transport critical current and its angular dependencies have been measured for single and 36 filament rectangular wires and tapes. Total tape widening influences the transport current density considerably but anisotropy at 0.1 T and 77 K is affected only slightly. Reduced anisotropy has been measured in a single core wire which has texturing of grains in two perpendicular rolling planes.