There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

https://cyberleninka.org/article/n/818074.pdf

Recent advances in iron-based superconductors toward applications

Numerical modeling of superconductors is widely recognized as a powerful tool for interpreting experimental results, understanding physical mechanisms, and predicting the performance of high-temperature-superconductor (HTS) tapes, wires, and devices. This is particularly true for ac loss calculation since a sufficiently low ac loss value is imperative to make these materials attractive for commercialization. In recent years, a large variety of numerical models, which are based on different techniques and implementations, has been proposed by researchers around the world, with the purpose of being able to estimate ac losses in HTSs quickly and accurately. This paper presents a literature review of the methods for computing ac losses in HTS tapes, wires, and devices. Technical superconductors have a relatively complex geometry (filaments, which might be twisted or transposed, or layers) and consist of different materials. As a result, different loss contributions exist. In this paper, we describe the ways of computing such loss contributions, which include hysteresis losses, eddy-current losses, coupling losses, and losses in ferromagnetic materials. We also provide an estimation of the losses occurring in a variety of power applications.

Computation of Losses in HTS Under the Action of Varying Magnetic Fields and Currents

Iron with a large magnetic moment was widely believed to be harmful to the emergence of superconductivity because of the competition between the static ordering of electron spins and the dynamic formation of electron pairs (Cooper pairs). Thus, the discovery of a high critical temperature (Tc) iron-based superconductor (IBSC) in 2008 was accepted with surprise in the condensed matter community and rekindled extensive study globally. IBSCs have since grown to become a new class of high-Tc superconductors next to the high-Tc cuprates discovered in 1986. The rapid research progress in the science and technology of IBSCs over the past decade has resulted in the accumulation of a vast amount of knowledge on IBSC materials, mechanisms, properties, and applications with the publication of more than several tens of thousands of papers. This article reviews recent progress in the technical applications (bulk magnets, thin films, and wires) of IBSCs in addition to their fundamental material characteristics. Highlights of their applications include high-field bulk magnets workable at 15-25 K, thin films with high critical current density (Jc) > 1 MA/cm2 at ~10 T and 4 K, and an average Jc of 1.3*104 A/cm2 at 10 T and 4 K achieved for a 100-m-class-length wire. These achievements are based on the intrinsically advantageous properties of IBSCs such as the higher crystallographic symmetry of the superconducting phase, higher critical magnetic field, and larger critical grain boundary angle to maintain high Jc. These properties also make IBSCs promising for applications using high magnetic fields.

Achievement of practical level critical current densities in Ba 1−x K x Fe 2 As 2 /Ag tapes by conventional cold mechanical deformation

/pdf/achievement-of-practical-level-critical-current-densities-in-4swxdr82q5.pdf

Achievement of practical level critical current densities in Ba1−xKxFe2As2/Ag tapes by conventional cold mechanical deformation

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.

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

Multi-core MgB2 wire reinforced by stainless steel (SS) has been made by an in situ approach and subjected to additional deformations and mechanical stressing. A critical current density Jc of 104 A cm − 2 at 4.2 K was measured in an external field of 9.45–10 T. The filament density of drawn wire was increased by cold deformation without axial tension, which resulted in an increased Jc by approximately 17%. SS reinforced wire shows a high resistance to mechanical stresses. Only 10% Ic degradation was measured for wire twisted with Lt = 7.14 mm. The presented SS reinforced wire has the highest irreversible strain eirr = 0.9% so far published.

Stainless steel reinforced multi-core MgB2 wire subjected to variable deformations, heat treatments and mechanical stressing

Multi-core MgB2 wires with an NbTi barrier protecting the stabilized copper and reinforced with stainless steel have been made. A critical current density of 104 A cm−2 at 4.2 K was measured in the external field of 10 T. Though the filament size is reduced to 26 µm, the Jc values are not much decreased. Circular 1+6 cable containing 49 filaments 35 µm in size was also demonstrated. The NbTi barrier is still superconducting after annealing at 650–850 °C (up to 9 T), which allows easy current transfer and also contributes slightly to the wire Jc (maximally at 7 T). Inter-diffusion between the NbTi barrier and Cu stabilizer was analyzed by energy dispersive x-ray spectrometry. This is influenced by the annealing temperature and by the pressure stress induced by the stainless steel sheath. Ti diffusion worsens the conductivity of Cu stabilization and also changes the mechanical properties of composite wire. Lowered wire resistances to the axial tension were measured for annealing at 850 °C.

https://iopscience.iop.org/article/10.1088/0953-2048/23/2/025014/pdf

Cu stabilized MgB2 composite wire with an NbTi barrier

Single- and four-core MgB2 wires have been manufactured by internal magnesium diffusion (IMD) into boron. Initial hydrostatic extrusion was applied for single-core Cu sheathed wire and mostly rolling deformation for assembled four-core wires with Inconel or GlidCop sheath. The highest critical current density (Jc) was measured for the wire with boron densified by cold isostatic pressing by 1.9 GPa. Rolled tapes have exhibited critical current anisotropy increasing with the tape’s aspect ratio. Low critical current degradation of IMD wires by twisting has been observed, which can be utilized for effective reduction of AC losses. High engineering current densities of IMD wires are promising for the generation of magnetic fields between 1 and 3 T at cryogen-free conditions, which can be utilized for future MRI systems or wind power generators.

https://iopscience.iop.org/article/10.1088/0953-2048/27/6/065003/pdf

Critical currents, I c -anisotropy and stress tolerance of MgB 2 wires made by internal magnesium diffusion

Transport current densities of MgB2 wire, cable and continually transposed conductor

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