We found that the transport Jc of the ex-situ PIT processed (Ba,K)Fe2As2 (Ba-122) wire with single Ag sheath can be significantly enhanced by repeating the combined process of rolling and heat treatment. The transport Jc (4.2 K and 10 T) of 4.4 x 103 A/cm2 (Ic =15.7 A) was obtained for a thin tape of 0.3 mm in thickness, which is the highest value reported so far for the PIT processed 122 wires and tapes with a Ag sheath and processed by a conventional route. The measurement by a hybrid magnet showed that Jc-H curve maintains very small field dependence up to the strong magnetic field of 28 T as expected from the previously reported high Hc2 value. The core of the thin tape shows dense grain structure with less cracks and voids and indicates the development of c-axis alignment, although it is still incomplete. The researches to elucidate the origin of Jc enhancement and to have further improvement of transport Jc are now ongoing. The process is simple using a Ag single sheath and, therefore, more realistic technique for long length wire production.
We fabricated in situ powder-in-tube processed MgB2/Fe tapes using aromatic hydrocarbon of benzene, naphthalene, and thiophene as additives, and investigated the superconducting properties. We found that these aromatic hydrocarbons were very effective for increasing the Jc values. The Jc values of 20mol% benzene-added tapes reached 130A/mm2 at 4.2K and 10T. This value was almost comparable to that of 10mol% SiC-added tapes and about four times higher than that of tapes with no additions. Microstructure analyses suggest that this Jc enhancement is due to both the substitution of carbon for boro…
Magneto-optical current reconstruction has been used for detailed analysis of the local critical current density (Jc) variation in monocore Bi-2223 tapes. We find, even in high quality tapes with bulk transport Jc ~ 40 kA/cm^2 (77K, 0T), that there exist local regions which possess current densities of more than 200 kA/cm^2. Overpressure processing at 148 bar significantly improved Jc to 48 kA/cm^2 by improving the connectivity. For the overpressure-processed sample we find that the current distribution is more uniform and that the maximum local current density at 77 K is increased almost to 3…
MgB2 tapes and wires were fabricated by the Powder-In-Tube method. Stainless steel and Cu-Ni tubes were used as sheath materials, and no heat treatment was applied. The tapes made of stainless steel showed transport critical current density Jc of about 10,000A/cm2 at 4.2K and 5T. A high Jc of about 300,000A/cm2 was obtained by extrapolating the Jc-B curves to zero field. Multifilamentary(7-core) MgB2 wire was successfully fabricated using Cu-Ni tubes. For both tapes and wires the grain connectivity of MgB2 was as good as a high-pressure sintered bulk sample. However, the Jc of the Cu-Ni sheath…
We report that high transport critical current density exceeding 104 A/cm2 at 4.2 K and 10 T can be obtained with good reproducibility for Ag sheathed (Ba,K)Fe2As2 (Ba 122) tapes by modifying the ex-situ powder-in-tube (PIT) method. The process involves cycles of the cold deformation and subsequent heat treatment. The intermediate cycles of flat rolling and heat treatment produce uniform fine-grained structure, resulting in an enhancement of transport Jc in applied magnetic fields. However, the rolled tapes contain cracks running traverse to the tape axis, which still limit the current pass al…
The development of the PIT fabrication process of iron pnictide superconducting wires and tapes has been carried out in order to enhance their transport properties. Silver was found to be the best sheath material, since no reaction layer was observed between the silver sheath and the superconducting core. The grain connectivity of iron pnictide wires and tapes has been markedly improved by employing Ag or Pb as dopants. At present, critical current densities in excess of 3750 A/cm^2 (Ic = 37.5 A) at 4.2 K have been achieved on Ag-sheathed SrKFeAs wires prepared with the above techniques, which…
The effect of nano-C doping on the microstructure and superconducting properties of Fe-sheathed MgB2 tapes prepared through the in-situ powder-in-tube method was studied. Heat treatment was performed at a low temperature of 650C for 1 h. Scanning electron microscopy investigation revealed that the smaller grain size of MgB2 in the samples with the C-doping. Further, the a-axis lattice parameter and transition temperature decreased monotonically with increasing doping level, which is due to the C substitution for B. High critical current density Jc values in magnetic fields were achieved in the…
We found that transport critical current density (Jc) of the Ba1-xKxFe2As2/Ag tapes is significantly enhanced by the combination process of cold flat rolling and uniaxial pressing. At 4.2 K, the Jc reaches a high value of 9.1x104 A/cm2 in 4 T, which is almost the practical level of 105 A/cm2. The Jc-H curve shows extremely small magnetic field dependence and keeps a high value of 6.9x104 A/cm2 in 10 T. These are the highest values reported so far for iron based superconducting wires. We also show our successful attempts of the fabrication of multi-filamentary tapes. A high Jc value of 5.3x104 …
Improving transport current has been the primary issue for practical application of superconducting wires and tapes. However, the porous nature of powder-in-tube (PIT) processed iron-based superconducting tapes is thought to be one of the important reasons for the low Jc values. In this work, the superconducting core density of ex-situ Sr0.6K0.4Fe2As2 tapes was significantly improved by employing a simple hot pressing as an alternative route for final sintering. At 4.2 K, the transport Jc values showed excellent values of 5.1x10^4 A/cm^2 at 10 T and 4.3x10^4 A/cm^2 at 14 T, respectively, which…
Conventional powder-in-tube (PIT) method has been the most effective technique for fabricating iron-based superconducting wires and tapes. Tremendous advances have been made during the last few years, especially for 122 family pnictide tapes. Here we review some of the most recent and significant developments in making high-performance iron-based tapes by the ex-situ PIT process, paying particular attention to several fabrication techniques to realize high-field Jc performance in terms of increase of core density and improvement of texture. At 4.2 K, the practical level transport Jc up to 0.12…
Among the recently discovered Fe-based superconducting compounds, the (K,Ba)Fe2As2 phase is attracting large interest within the scientific community interested in conductor developments. In fact, after some years of development, critical current densities Jc of about 105 A/cm2 at fields up to more than 10 T have been obtained in powder in tube (PIT) processed wires and tapes. Here we explore the crucial points in the wire/tape fabrication by means of the ex-situ PIT method. We focus on scaling up processes which are crucial for the industrial fabrication. We analyzed the effects on the micros…
We have fabricated prototype Bi,Pb(2223)/Ag superconduting multifilamentary wires with a square cross-section. The AC losses of these wires were measured, compared with those of tapes and also compared with the theory. Wires show largely reduced AC magnetic field losses at 47 Hz, compared to those of tapes in a perpendicular magnetic field. The effects of the twist pitch and of the use of the high resisitive barriers were also investigated. The results show that the use of a twist pitch of about 10 mm is sufficient for decoupling the filaments, and that the use of very short twist pitches does…
Nanocarbon-doped Fe-sheathed MgB2 tapes with different doping levels were prepared by the in situ powder-in-tube method. Compared to the undoped tapes, Jc for all the C-doped samples was enhanced by more than an order of magnitude in magnetic fields above 9 T. At 4.2 K, the transport Jc for the 5 at% doped tapes reached 1.85x104 A/cm2 at 10 T and 2.8x103 A/cm2 at 14 T, respectively. Moreover, the critical temperature for the doped tapes decreased slightly. Transmission electron microscopy showed a number of intra-granular dislocations and the dispersed nanoparticles embedded within MgB2 grains…
The effect of the quality of starting powders on the microstructure and superconducting properties of in-situ processed Fe-sheathed MgB2 tapes has been investigated. Three different types of commercial atomized spherical magnesium powder and two different purities of amorphous boron powder were employed. When using the 10-micrometre magnesium as precursor powders, the Mg reacted with boron more uniformly and quickly, thus the uniformity of the fabricated MgB2 was improved and the grain size of the MgB2 was decreased, hence significant critical current density (Jc) enhancements were achieved fo…
Iron-based superconductors, with Tc values up to 55 K, are of great interest for applications, due to their lower anisotropies and ultrahigh upper critical fields. In the past 4 years, great progress has been made in the fabrication of iron-based superconducting wires and tapes using the powder-in-tube (PIT) processing method, including main three types of 122, 11, and 1111 iron-based parent compounds. In this article, an overview of the current state of development of iron-based superconducting wires and tapes is presented. We focus on the fabrication techniques used for 122 pnictide wires an…
We report on the superconducting performance of the ex-situ SiC doped MgB2 monofilamentary tapes. Polycrystalline powders of MgB2 doped with 5 and 10 wt% SiC were synthesized by conventional solid-state reaction route and characterized for their superconducting performance. It is found that superconducting parameters i.e. upper critical field (Hc2), irreversibility field (Hirr) and critical current density (Jc) are all improved significantly with SiC addition. Also it was found that relatively lower synthesis temperature (700 C) resulted in further improved superconducting parameters. As synth…
Multifilamentary MgB2/Fe wires and tapes with high transport critical current densities have been fabricated using a straightforward powder-in-tube (PIT) process. After annealing, we measured transport jc values up to 1.1 * 105 A/cm2 at 4.2 K and in a field of 2 T in a MgB2/Fe square wire with 7 filaments fabricated by two-axial rolling, and up to 5 * 104 A/cm2 at 4.2 K in 1 T in a MgB2/Fe tape with 7 filaments. For higher currents these multifilamentary wires and tapes quenched due to insufficient thermal stability of filaments. Both the processing routes and deformation methods were found to…
From the application point of view, large critical current densities Jc (H) for superconducting wires are required, preferably for magnetic fields higher than 5 T. Here we show that strong c-axis textured Sr1-xKxFe2As2 tapes with nearly isotropic transport Jc were fabricated by an ex-situ powder-in-tube (PIT) process. At 4.2 K, the Jc values show extremely weak magnetic field dependence and reach high values of 1.7x10^4 A/cm^2 at 10 T and 1.4x10^4 A/cm^2 at 14 T, respectively, these values are by far the highest ever reported for iron based wires and approach the Jc level desired for practical…
Mono- and multifilamentary MgB2/Fe tapes and wires with high transport critical current densities have been prepared using the powder-in-tube (PIT) process. The fabrication details are described. The effect of powder grain sizes and recrystallization temperature on jc has been investigated. At 25K and 1 T, jc values close to 105 A/cm2 were measured, while jc of 106 A/cm2 were extrapolated for 4.2K/0T in our monofilamentary tape. MgB2/Fe tapes exhibit high exponential n factors for the resistive transition: n ~ 80 and 40 were found at 5 T and 7 T, respectively. The highest transport jc values o…
We investigate theoretically the dependence of magnetization loss of a helically wound superconducting tape on the round core radius $R$ and the helical conductor pitch in a ramped magnetic field. Using the thin-sheet approximation, we identify the two-dimensional equation that describes Faraday's law of induction on a helical tape surface in the steady state. Based on the obtained basic equation, we simulate numerically the current streamlines and the power loss $P$ per unit tape length on a helical tape. For $R \gtrsim w_0$ (where $w_0$ is the tape width), the simulated value of $P$ saturate…
We demonstrate a large enhancement of critical current density (Jc) up to 1.0x10^7 A/cm2 at 5 K under self-field in (Ba,K)Fe2As2 single crystals by irradiating 320 MeV Au ions. With the very promising potential of this material in mind, we have fabricated a (Ba,K)Fe2As2 superconducting wire through a powder-in-tube method combined with the hot isostatic pressing technique, whose effectiveness has been proven in industrial Bi2223 tapes. The Jc in the wire at 4.2 K has reached 37 kA/cm2 under self-field and 3.0 kA/cm2 at 90 kOe. Magneto-optical imaging of the wire confirmed the large intergranul…
Magnetization loss on a twisted superconducting (SC) tape in a ramped magnetic field is theoretically investigated through the use of a power law for the electric field--current density characteristics and a sheet current approximation. First, the Maxwell equation in a helicoidal coordinate system is derived to model a twisted SC tape, taking account of the response to the perpendicular field component in the steady state. We show that a loosely twisted tape can be viewed as the sum of a portion of tilted flat tapes of infinite length by examining the perpendicular field distribution on a twis…
Minimization of ac losses is essential for economic operation of high-temperature superconductor (HTS) ac power cables. A favorable configuration for the phase conductor of such cables has two counter-wound layers of HTS tape-shaped wires lying next to each other and helically wound around a flexible cylindrical former. However, if magnetic materials such as magnetic substrates of the tapes lie between the two layers, or if the winding pitch angles are not opposite and essentially equal in magnitude to each other, current distributes unequally between the two layers. Then, if at some point in …
In this work, the feasibility condition of Powder-In-Tube (PIT) processed wires of Fe(Se,Te) superconductor has been investigated. We faced several technical issues that are extensively described and discussed. In particular, we tested different metals and alloys as external sheaths (Cu, Ag, Nb, Ta, Ni, Fe, cupronickel, brass) concluding that the only sheath that does not affect substantially the Fe(Se,Te) phase is Fe. On the other hand, Fe sheath introduces excess iron in the Fe(Se,Te) phase, which affects the superconducting properties; we investigated the effects of the thermal treatments a…
An advanced internal Mg infiltration method (AIMI) in this paper has been shown to be effective in producing superconducting wires containing dense MgB2 layers with high critical current densities. In this study, the in-field critical current densities of a series of AIMI-fabricated MgB2 strands were investigated in terms of C doping levels, heat treatment (HT) time and filament numbers. The highest layer Jc for our monofilamentary AIMI strands is 1.5 x 10^5 A/cm2 at 10 T, 4.2 K, when the C concentration was 3 mol% and the strand was heat-treated at 675 °C for 4 hours. Transport critical curre…
We succeeded in the fabrication of high-Jc MgB2/Fe wires applying the internal Mg diffusion (IMD) process with pure Mg core and SiC addition. A pure Mg rod with 2 mm diameter was placed at the center of a Fe tube, and the space between Mg and Fe tube was filled with B powder or the powder mixture of B-(5mol%)SiC. The composite was cold worked into 1.2mm diameter wire and finally heat treated at temperatures above the melting point of Mg(~650oC). During the heat treatment liquid Mg infiltrated into B layer and reacted with B to form MgB2. X-ray diffraction analysis indicated that the major phas…
We present the fabrication and test results of Hot-Isostatic-Pressed (HIPed) Powder-in-Tube (PIT) MgB$_2$ coils. The coils properties were measured by transport and magnetization at different applied fields ($H$) and temperatures ($T$). The engineering critical current ($J_e$) value is the largest reported in PIT MgB$_2$ wires or tapes. At 25 K our champion 6-layer coil was able to generate a field of 1 T at self-field ($I_c >$ 220 A, $J_e \sim 2.8 \times 10^4$ A/cm$^2$). At 4 K this coil generated 1.6 T under an applied field of 1.25 T ($I_c \sim350$ A, $J_e \sim 4.5 \times 10^4$ A/cm$^2$). T…
Low AC loss high transport current HTS cables (>1 kA) are required for application in transformers, generators and are considered for future generations of fusion reactors coils. 2G coated conductors are suitable candidates for high field application at quite high operation temperatures of 50-77 K, which is crucial precondition for economical cooling costs. As a feasibility study we present the first ROEBEL bar cable of approx. 35 cm length made from industrial DyBCO coated conductor (THEVA GmbH, Germany). Meander shaped ROEBEL strands of 4 mm width with a twist pitch of 180 mm were cut from 1…
Promising results reported in our previous works led us to think that production of B powder plays a crucial role in MgB2 synthesis. A new method for boron preparation has been developed in our laboratory. This particular process is based on magnesiothermic reaction (Moissan process) with the addition of an initial step that gives boron powder with nano-metric grain size. In this paper we report our efforts regarding optimization of PIT method for these nanometric powders and the resolution of problems previously highlighted such as the difficulty in powder packaging and the high friction phen…
