Invisibility to electromagnetic fields has become an exciting theoretical possibility. However, the experimental realization of electromagnetic cloaks has only been achieved starting from simplified approaches (for instance, based on ray approximation, canceling only some terms of the scattering fields, or hiding a bulge in a plane instead of an object in free space). Here, we demonstrate, directly from Maxwell equations, that a specially designed cylindrical superconductor-ferromagnetic bilayer can exactly cloak uniform static magnetic fields, and we experimentally confirmed this effect in an actual setup.

https://science.sciencemag.org/content/sci/335/6075/1466.full.pdf

Experimental Realization of a Magnetic Cloak

The magnetic and transport properties of MgB2 films present performance goals yet to be attained for powder-processed bulk samples and conductors. Carbon-doped films have exhibited upper critical fields, ?0Hc2, as high as 60?T and a possible upper limit of more than twice this value has been predicted. Very high critical current densities, Jc, have also been measured for films, e.g.?25?MA?cm?2 in the self-field and 7?kA?cm?2 in 15?T. Such performance limits are still out of reach for even the best MgB2 magnet wire. In discussing the present status and prospects for improving the performance of powder-based wire we focus attention on (1)?the intrinsic (intragrain) superconducting properties of MgB2?Hc2 and flux pinning, and (2)?the factors that control the efficiency with which current is transported from grain to grain in the conductor, an extrinsic (intergrain) property. With regard to item (1), the role of dopants in Hc2 enhancement is discussed and examples presented. On the other hand their roles in increasing Jc, both via Hc2 enhancement as well as via direct fluxoid/pinning center interaction, are discussed and a comprehensive survey of Hc2 dopants and flux pinning additives is presented. Dopant selection, chemistry, methods of introduction (inclusion), and homogeneity of distribution (via the rounding of the superconducting electronic specific heat transition) are considered. Current transport through the powder-processed wire (an extrinsic property) is partially blocked by the inherent granularity of the material itself and the chemical or other properties of the intergrain surfaces. Overall porosity, including reduced density and intergranular blocking, is quantified in terms of the measured temperature dependence of the normal-state resistivity compared to that of a clean single crystal. Several experimental results are presented in terms of per cent effective cross-sectional area for current transport. These and other such results indicate that in many cases less than 15% of the conductor's cross-sectional area is able to carry transport current. It is pointed out that densification in association with the elimination of grain-boundary blocking phases would yield fivefold-to-tenfold increases in Jc in relevant regimes, enabling the performance of MgB2 in selected applications to compete with that of Nb3Sn.

https://iopscience.iop.org/article/10.1088/0953-2048/21/10/103001/pdf

Prospects for improving the intrinsic and extrinsic properties of magnesium diboride superconducting strands

The authors wish to express their sincere thanks to Dr Nicky Athanassopoulou, Ms Andi Jones and Dr Rob Phaal of Institute for Manufacturing Education and Consultancy Service Ltd for their excellent work in organising and facilitating the Road-Mapping exercise upon which this current work is based. The authors would also like to express their appreciation of the contribution made by the other participants at the workshop: Hari Babu Nadendla (Brunel University), Pavol Diko (Slovak Academy of Sciences), Tomas Hlasek (CAN Superconductors), John Hull (The Boeing Company), Lars Kuhn (evico), Mathias Noe (KIT), Jan Plechacek (CAN Superconductors), Yunhua Shi (University of Cambridge) and Frank Werfel (ATZ). The authors acknowledge financial support for the Road-Mapping exercise on which this work is based from the University of Cambridge EPSRC Impact Acceleration Account (EP/K503757/1).

https://iopscience.iop.org/article/10.1088/1361-6668/aad7ce/pdf

Bulk superconductors: a roadmap to applications

The harnessing of heat emanating from powder-based exothermic reactions to produce advanced materials has been around for many decades, and is manifested in the process of combustion synthesis (CS) A plethora of work has been published on the topic covering fundamental aspects of the process for a large number of material systems Over time, CS has been combined with other processes and effects to potentially improve on conventionally produced CS products and alleviate some of the inherent disadvantages of CS This article discusses processing aspects of CS, and provides a review of CS-related hybrid processes with the intent to exemplify the diversity of CS processing Approaches such as reactant microstructural design, reactive bulk deformation/compaction processes, reactive casting, laser-assisted CS, activation techniques (field/current, mechanical, microwave), and unconventional heat treatments are discussed together with other methods

The diversity of combustion synthesis processing: a review

We succeeded in the fabrication of high-Jc (critical current density) MgB2 /Fe wires applying the internal Mg diffusion (IMD) process with a pure Mg core and SiC addition. A pure Mg rod with a diameter of 2 mm was placed at the center of a Fe tube with an outer diameter of 6 mm and an inner diameter of 4 mm, and the space between the Mg and the Fe tube was filled with B powder or a powder mixture of B–(5 mol%) SiC. The composite was cold worked into a wire with a diameter of 1.2 mm and finally heat treated at temperatures above the melting point of Mg (~650 °C). During the heat treatment, liquid Mg infiltrated into the B layer and reacted with B to form MgB2. X-ray diffraction analysis indicated that the major phase in the reacted layer is MgB2. Scanning electron microscopy (SEM) analysis shows that the density of the MgB2 layer is higher than that of the usual powder-in-tube (PIT) processed wires. The wires with additional 5 mol% SiC heat treated at 670 °C showed Jc values higher than 105 A cm−2 in 8 T and 41 000 A cm−2 in 10 T at 4.2 K. These values are much higher than those of the usual PIT processed wires, even compared to those with additional SiC. The higher density of the MgB2 layer obtained by the diffusion reaction is the major cause of these excellent Jc values.

https://iopscience.iop.org/article/10.1088/0953-2048/21/3/032001/pdf

Fabrication of high-performance MgB2 wires by an internal Mg diffusion process

The reactive liquid Mg infiltration process to produce large superconducting bulk MgB2 manufacts

The shielding of a DC magnetic field has been demonstrated to be complete at 4.2 K, at least up to 2 T, inside the 18 mm bore of a thick MgB2 bulk cylinder, 70 mm tall. No hysteresis has been detected along the cycle of the field application and removal, and no flux jumps have been detected either. The MgB2 cylinder was manufactured by the Reactive Mg Liquid Infiltration process, an 'in situ' technology able to give very dense MgB2 manufacts. According to the previously measured Jc(B, T) characteristics of a MgB2 bulk superconductor of the same origin and quality, a critical current density of about 30 kA cm − 2 can be assumed at the fields of 2–3 T and at the temperature of 4.2 K of the reported shielding experiment. Accordingly, we estimate that the thickness of the MgB2 cylinder filled by the shielding currents is about 5.75 mm, lower than the present cylinder thickness of 8.77 mm.

https://iopscience.iop.org/article/10.1088/0953-2048/23/12/125003/pdf

Magnetic shielding capability of MgB2 cylinders

The possibility to manufacture superconducting MgB2 wires, applying the reactive liquid Mg infiltration (RLI) process, was demonstrated several years ago, obtaining wires characterized by hollow cores. The short samples of these wires gave transport critical currents in line with the best powder in tube (PIT) wires produced by "in situ" technique. Nevertheless the extension of the RLI technology to long superconducting wires initially was prevented owing to difficulties in controlling the liquid magnesium dynamics during the reaction. Now we succeed in controlling the reaction and in the manufacturing wires or tubular long manufacts. For tubular manufacts having large diameter, of several millimetres, useful for current leads or relatively short resistive current limiters, the liquid Mg is also supplied continuously during the reaction, to refill completely the core of the tube. For thin wires, with diameter of the order of a millimetre or less, useful for magnets and racetrack winding and other filamentary manufacts, the movement of liquid Mg is counteracted by the liquid metal surface tension. Concerning the superconducting characteristics of the wires, we have explored some key variables effective on the quality of the crystalline MgB2: they include the choice of the original Boron powders and the addition of doping elements.

Advancements in the Reactive Liquid ${\rm Mg}$ Infiltration Technique to Produce Long Superconducting ${\rm MgB}_{2}$ Tubular Wires

In many superconducting applications the feasibility of a persistent mode operation is a prerequisite of acceptance of the superconducting solution. Regarding the High Temperature Superconducting oxides, this persistency is obtained only for the bulks and, when realized for large samples as for the BSCCO-2212 material, allows the transport of very limited superconducting currents, with densities of the order of some kA/cm2. Instead we were able to maintain in a MgB, ring a persistent current corresponding to several tens of kA/cm2. Designing large superconducting systems, based either on wires or on the bulk materials, as the magnets for MRI or for magnetic separation it will be important to realize persistent superconducting joints between the parts, allowing to sustain the high currents needed for these power applications. In the framework of the MgB2 material development and using the liquid Mg infiltration technique, we have explored the possibility to join wires with bulks and bulks between themselves. The persistency of the joints has been verified by the measurement of the levitation forces of the superconducting system in presence of a permanent magnet and by direct transport current measurements. How to apply the joining technique to the magnet manufacturing is presented.

Superconducting Joints Between ${\hbox {MgB}}_{2}$ Wires and Bulks

Preliminary results of measurements performed on two bulk MgB2 cylinders intended for use as toroidal magnetic shields with a novel cryogenic current comparator are reported up to 37 K, showing an attenuation factor greater than 106 at 27 K, lowering by a factor of 10 at 34 K.

https://iopscience.iop.org/article/10.1088/0953-2048/20/7/L01/pdf

E. Bassani

Papers

The reactive liquid Mg infiltration process to produce large superconducting bulk MgB2 manufacts

Magnetic shielding capability of MgB2 cylinders

Advancements in the Reactive Liquid ${\rm Mg}$ Infiltration Technique to Produce Long Superconducting ${\rm MgB}_{2}$ Tubular Wires

Superconducting Joints Between ${\hbox {MgB}}_{2}$ Wires and Bulks

An MgB2 superconducting shield for a cryogenic current comparator working up to 34 K