This review focuses on the superconducting properties of MgB2 that are relevant for power applications. The reversible mixed state parameters are the most important, since they define the limiting conditions for loss-free currents: the transition temperature, the upper critical field and the depairing current. They also determine the flux pinning energy, the pinning force and the elastic properties of the flux line lattice and, therefore, strongly influence the critical current densities. The magnetic properties of magnesium diboride are anisotropic and influenced by the two different energy gaps of the σ- and π-bands. Whereas the transition temperature could not be enhanced significantly during the past five years, the upper critical field was considerably increased by impurity scattering or doping. Flux pinning is very weak in MgB2 single crystals and was only improved by irradiation techniques so far. In polycrystalline samples, grain boundary pinning seems to play the dominant role. High critical currents close to the theoretical limit were found in c-axis oriented thin films. The anisotropy of the upper critical field strongly reduces the critical currents in untextured MgB2 at high magnetic fields, where the supercurrents become highly percolative, since not all grains are superconducting anymore. The performance of polycrystalline wires and tapes was significantly improved during the past few years by increasing the upper critical field and by reducing its anisotropy. Pinning seems to be nearly optimized in many forms of this material, but the connectivity between the grains might be further improved.

https://iopscience.iop.org/article/10.1088/0953-2048/20/12/R01/pdf

Magnetic properties and critical currents of MgB2

Experimental investigation for enhanced ferrofluid heat transfer under magnetic field effect

Heat transfer augmentation using a magnetic fluid under the influence of a line dipole

Detection of neutrons, at high total efficiency, with greater resolution in kinetic energy, time and/or real-space position, is fundamental to the advance of subfields within nuclear medicine, high-energy physics, non-proliferation of special nuclear materials, astrophysics, structural biology and chemistry, magnetism and nuclear energy. Clever indirect-conversion geometries, interaction/transport calculations and modern processing methods for silicon and gallium arsenide allow for the realization of moderate- to high-efficiency neutron detectors as a result of low defect concentrations, tuned reaction product ranges, enhanced effective omnidirectional cross sections and reduced electron–hole pair recombination from more physically abrupt and electronically engineered interfaces. Conversely, semiconductors with high neutron cross sections and unique transduction mechanisms capable of achieving very high total efficiency are gaining greater recognition despite the relative immaturity of their growth, lithographic processing and electronic structure understanding. This review focuses on advances and challenges in charged-particle-based device geometries, materials and associated mechanisms for direct and indirect transduction of thermal to fast neutrons within the context of application. Calorimetry- and radioluminescence-based intermediate processes in the solid state are not included.

https://iopscience.iop.org/article/10.1088/0953-8984/22/44/443201/pdf

The physics of solid-state neutron detector materials and geometries

Hybrid physical–chemical vapor deposition (HPCVD) has been the most effective technique for depositing MgB2 thin films. It generates high magnesium vapor pressures and provides a clean environment for the growth of high purity MgB2 films. The epitaxial pure MgB2 films grown by HPCVD show higher-than-bulk Tc due to tensile strain in the films. The HPCVD films are the cleanest MgB2 materials reported, allowing basic research, such as on magnetoresistance, that reveals the two-band nature of MgB2. The carbon-alloyed HPCVD films demonstrate record-high Hc2 values promising for high magnetic field applications. The HPCVD films and multilayers have enabled the fabrication of high quality MgB2 Josephson junctions.

MgB2 thin films by hybrid physical–chemical vapor deposition

As-grown superconducting MgB2 thin films were prepared using a multiple-target sputtering system The targets were pure B and Mg metal The MgB2 thin films were deposited at substrate temperatures (Ts) ranging from 207°C to 268°C on Al2O3 (1102) substrates We found that the critical temperatures (Tc) of the films were increased with increase of the Ts The film grown at a Ts of 252°C showed the highest Tc,onset value of 278 K and had a sharp transition (ΔTc=05 K) in the resistivity

https://iopscience.iop.org/article/10.1143/JJAP.41.L127/pdf

As-grown deposition of superconducting MgB2 thin films by multiple-target sputtering system

We developed a novel nondestructive and contactless method for measurements of critical current density (J/sub c/) in high-temperature superconductor (HTS) thin films by using a permanent magnet (Sm/sub 2/Co/sub 17/). This measurement technique is based on a repulsive force (F/sub r/) and attractive force (F/sub a/) between the magnet and HTS film caused by the shielding currents and pinning forces. The F/sub r/ and F/sub a/ were measured by using a high-resolution load sensor changing the distance (L) between the magnet and HTS film. An effective maximum repulsive force (F/sub m,eff/) could be determined from an extrapolated value of the F/sub r/ vs. L curve for L=0 mm. We investigated the relationship of the J/sub c/ to the F/sub m,eff/ in many HTS (YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// and ErBa/sub 2/Cu/sub 3/O/sub 7-/spl delta//) samples. The standard J/sub c/ of HTS films was measured by using an inductive measurement system produced from THEVA GmbH. We found that the J/sub c/ is almost proportional to the two-thirds power of the F/sub m,eff/ normalized by the film thickness. This result indicates that the simple permanent magnet technique can be easily estimate the J/sub c/ of the HTS films. This system has good reproducibility for the J/sub c/ measurements and is useful to provide a rapid, routine characterization of the superconductivity in the HTS materials.

A simple measurement technique for critical current density by using a permanent magnet

Natural convection of a magnetic fluid in concentric horizontal annuli under nonuniform magnetic fields

We present the results of non-destructive and contactless measurement of the critical current density ( J c ) of high-temperature superconductor (HTS) thin films using a permanent magnet method. A cylindrical permanent magnet (Sm 2 Co 17 ) with a height of 5.0 mm and diameter d of 5.0 mm was used. To investigate the margin of the sample setting for the vertical direction, load vs. distance curves for a minimum distance L min which varied from 0.5 to 2.0 mm were measured. Here, L is the distance from the permanent magnet to a surface of the HTS film. The effective maximum repulsive force F m,eff was almost constant below L min  = 1.2 mm. This result indicates that the measurement system has a wide margin with respect to sample setting in the vertical direction. We investigated the dependence of F m,eff on the sample area from diameter l  = 20–4 mm and found that the relationship of the critical current density J c  = 1.7 × 10 −7 ( F m,eff / t ) can be applied for l / d  > 2 and the required sample size of the HTS films is approximately l  = 2 d for this system. Here, t is the thickness of the HTS thin film. These results indicate that the permanent magnet method can be used to easily estimate the J c of HTS films at 77 K. Additionally, this system has good reproducibility for J c measurements. It is thus promising as a replacement for conventional inductive measurements and for measurements for J c mapping of HTS films, tapes, and wires using a small magnet.

Rapid and simple measurement of critical current density in HTS thin films using a permanent magnet method

It is difficult to detect neutrons compared to charged particles. A novel metallic superconductor MgB 2 has a high T c of 40 K. It is well known that the 10 B has a huge cross-section for the nuclear reaction of 10 B( n , α ) 7 Li. It reaches 3800 barns for thermal neutrons. The Li particle has 0.84 MeV while α particle has 1.47 MeV. We propose to use the MgB 2 as a superconducting neutron detector. We design a prototype of a MgB 2 neutron detector from an MgB 2 film where the electron beam lithography is employed to fabricate a meandering MgB 2 stripe line of the 1-μm width. The hot spot caused by the nuclear reaction is responsible for detecting neutrons.

Atsushi Saito

Papers

As-grown deposition of superconducting MgB2 thin films by multiple-target sputtering system

A simple measurement technique for critical current density by using a permanent magnet

Natural convection of a magnetic fluid in concentric horizontal annuli under nonuniform magnetic fields

Rapid and simple measurement of critical current density in HTS thin films using a permanent magnet method

Design of neutron detector by using a novel superconductor MgB2