MgB2 Wires Research Articles

Fundamental Study of MgB2 Superconducting Coil for Storage Devices

The authors carried out a basic study on a power storage coil made of MgB2 superconducting wires which was expected to require less cooling and lower manufacturing costs. First, for manufacturing the storage coil, the basic characteristics of MgB2 superconducting wire were evaluated. The characteristics at 20 K under 1.5 T were a critical current of 170 A for the in-situ MgB2 wire and 200 A for the ex-situ MgB2 wire. Based on these results, MgB2 conductors and prototype storage coils were produced and their performance was evaluated. The prototype was successfully energized to 600 A under 1.5 T at less than 25 K, which are the basic requirements for several 10 kJ class coils, and there was no significant deterioration through conductor and coil processing.

Enhanced critical current density at high magnetic fields in MgB2 wire processed by in-situ spark plasma sintering

Sparking plasma sintering (SPS) technique has the characteristics of rapid heating and cooling rate, high mass density, and grain activation sintering. Therefore, it may be used to improve the grain connection, refine the grains, and enhance the superconducting current-carrying capacity of MgB2. In this paper, the MgB2 wire prepared by the in-situ method is rapidly treated by the SPS method, and the effects of different SPS sintering conditions on the phase formation, microstructure characteristics and superconductivity of the MgB2 material are studied, and compared with the MgB2 prepared by conventional sintering and the ex-situ SPS method. It was found that the SPS-treated samples were in a rapidly changing non-equilibrium state during the entire sintering process, so there are existing both flake particles and nanoparticles in the samples, which significantly improves the connection of the grains and refine the grains, resulting in significantly improved superconducting performance for MgB2. The critical current density Jc of SPS-treated sample at 850 ℃ reached 1354 A/cm2 @ (10 K, 5 T) and 1615 A/cm2 @ (20 K, 3 T), which were 3.7 times and 2.6 times that of conventional sintering sample. Our research shows that SPS is an effective method to improve the superconductivity of in-situ MgB2 wire.

Homogeneity of SiC distribution in IMD MgB2 wires

In this work, MgB2 wires with different SiC doping levels were processed using the internal Mg diffusion method and the evolving MgB2 reaction layers were thoroughly analyzed with an electron probe micro-analyzer. It was also shown that high performance wire can be obtained by optimizing SiC contents and heat treatment conditions. The SiC dopant is uniformly distributed in the boron matrix at the macroscale. The highest layer J c value of 5 × 104A cm−2 at 4.2 K and 10 T is achieved in the sample with 7 at% SiC doping and sintered at 640 °C for 15 h. This high layer J c is considered to be due to the doping effect as well as uniform dispersion of SiC particles. The SiC distribution homogeneity can be explained by matrix-to-additive particle size ratio.

Effect of Heat Treatments under High Isostatic Pressure on the Transport Critical Current Density at 4.2 K and 20 K in Doped and Undoped MgB2 Wires.

Annealing undoped MgB2 wires under high isostatic pressure (HIP) increases transport critical current density (Jtc) by 10% at 4.2 K in range magnetic fields from 4 T to 12 T and significantly increases Jtc by 25% in range magnetic fields from 2 T to 4 T and does not increase Jtc above 4 T at 20 K. Further research shows that a large amount of 10% SiC admixture and thermal treatment under a high isostatic pressure of 1 GPa significantly increases the Jtc by 40% at 4.2 K in magnetic fields above 6 T and reduces Jtc by one order at 20 K in MgB2 wires. Additionally, our research showed that heat treatment under high isostatic pressure is more evident in wires with smaller diameters, as it greatly increases the density of MgB2 material and the number of connections between grains compared to MgB2 wires with larger diameters, but only during the Mg solid-state reaction. In addition, our study indicates that smaller wire diameters and high isostatic pressure do not lead to a higher density of MgB2 material and more connections between grains during the liquid-state Mg reaction.

Superconducting MgB2 Wire Drawing Considering Anisotropic Hardening Behavior and Hydrostatic Effect

Superconducting MgB2 Wire Drawing Considering Anisotropic Hardening Behavior and Hydrostatic Effect

Longitudinal uniformity of MgB2 wires made by an internal magnesium diffusion process

Single- and multi-core MgB2 wires have been fabricated by an internal Mg diffusion (IMD) process. Wires uniformity after drawing and/or rolling deformation has been studied by optical microscopy and x-ray micro-tomography. Several short samples taken lengthwise between both ends of the wire were annealed and their critical current measured. It was found that I c of the single-core barrier-free MgB2/Fe wire with high fill factor (32.5% of MgB2) vary by 100% towards the averaged I c. Multi-core wire having ∼15.0% of MgB2 and Cu outer sheath shows better uniformity with 25% deviation of I c over the 10 m length, which is still quite low and needs some improvements. It is expected, that improved in-length uniformity can be reached only for filamentary IMD wires with optimized deforming process and mechanically stronger outer sheath.

Superconducting joints using multifilament MgB2 wires for MRI application

We present superconducting joints using 36 filaments magnesium diboride (MgB2) wires in continuation joint orientation and their characterisation results for the first time. One of the joints demonstrated the critical temperature (offset) of 34.4 K and the critical current (Ic) of 38.5 A at 10 K in self-field. Unconventional Ic behaviour of the joint was observed at 20 K in self-field when the Ic was measured in a high to a low field due to the effect of field trapping in the MgB2 bulk within the joint. Elemental mapping at the interface between the MgB2 bulk and the filament revealed oxidation, which was attributed to being responsible for reducing the Ic of the joint. This work will notably contribute towards MgB2 superconducting joint technology development for magnetic resonance imaging (MRI) application.

Influence of Amorphous Boron Grain Size, High Isostatic Pressure, Annealing Temperature, and Filling Density of Unreacted Material on Structure, Critical Parameters, N-Value, and Engineering Critical Current Density in Mgb2 Wires.

Our results show that a lower density of unreacted Mg + B material during an Mg solid-state synthesis reaction leads to a significant reduction in the quantity of the superconducting phase and lowers the homogeneity of the superconducting material. It also significantly reduces the irreversible magnetic field (Birr), critical temperature (Tc), upper magnetic field (Bc2), engineered critical current density (Jec), and n-value, despite high isostatic pressure (HIP) treatment and the use of nanoboron in the sample. Our measurements show that samples with large boron grains with an 8% higher density of unreacted Mg + B material allow better critical parameters to be achieved. Studies have shown that the density of unreacted material has little effect on Birr, Tc, Bc2, Jec, and the n-value for an Mg liquid-state synthesis reaction. The results show that the critical parameters during an Mg liquid-state synthesis reaction depend mainly on grain size. Nanoboron grains allow for the highest Birr, Tc, Bc2, Jec, and n-values. Scanning electron microscopy (SEM) images taken from the longitudinal sections of the wires show that the samples annealed under low isostatic pressure have a highly heterogeneous structure. High isostatic pressure heat treatment greatly improves the homogeneity of MgB2.

Feasibility study of novel rapid ramp-down procedure in MgB2 MRI magnet using persistent current switch with high off-resistivity

Dry magnets using high temperature superconductors and MgB2 are attractive options to depart from dependence on liquid helium in MRI scanners. In the dry magnets, however, lack of thermal mass of cryogen makes a controlled quench difficult and extends time for restarting the magnets after the quench. In this study, a novel rapid ramp-down procedure, which can be substituted for the controlled quench in emergency rundown, is proposed, and its feasibility is proven for a 1.5 T whole-body MgB2 MRI magnet. In this procedure, a power supply receives current from a persistent current switch (PCS), the PCS is turned off by heating, the power supply is interrupted by a breaker, and the stored energy in the magnet is mostly consumed at an external resistor. Owing to the large energy margin of MgB2 wires, the AC loss during the ramp-down does not bring a quench of the MgB2 coils. A niobium–titanium sheathed MgB2 wire 0.60 mm in diameter is made, and a PCS with high off-resistivity is designed using this wire. The shunt current during the ramp-down does not bring the burnout of the PCS when the wire length is sufficiently long, typically hundreds of metres. Because heat generation inside the cryostat during the ramp-down is a few per cent of the stored energy in the magnet, the magnet is not heated excessively. As a result, the proposed ramp-down procedure should shorten the downtime of MRI scanners.

Low‐purity Cu and Al sheathed multi‐core MgB2 wires made by IMD process

Up to now, the highest current densities of MgB2 superconducting wires have been obtained by the internal magnesium diffusion (IMD) method. MgB2 superconductors are especially suitable for DC or AC windings generating low or medium magnetic fields, where thermal stability and low AC losses are important issues. Mechanical, thermal and electrical parameters of the wires are strongly influenced by metallic materials used for the outer sheath. While highly electrical conductive sheath material is beneficial for thermally stable behaviour, it has a great effect on the generation of high eddy current losses. To minimize the contribution of eddy current losses, multi-core MgB2 wires with low purity Cu and Al sheaths were prepared by the IMD process and characterized in detail. Results of low temperature measurements are analysed and discussed in the present work.

In-Situ Manufacturing of SiC-Doped MgB2 Used for Superconducting Wire

Magnesium diboride (MgB2) is a highly potential superconducting material, in substitution of Nb3Sn, which has a critical temperature of ~ 39 K. This synthesis and manufacturing of MgB2 wire were conducted by in-situ powder in tube (PIT). The method doped with silicon carbide (SiC) was aimed to study the effect of phase formation on carbon substitution and morphological characteristics with the motivation to improve superconductivity properties. Magnesium, boron, and SiC powders were synthesized and functionally processed with stainless Steel 304 tube. Heat treatment was conducted at 750℃, 800℃, and 850℃ for 2 hours followed by furnace cooling. Characterization was carried out by x-ray diffractometer (XRD), scanning electron microscopy (SEM), and cryogenic magnet testing. The results showed that 1% SiC optimally increased the zero critical temperature of MgB2 ~ 37.18 K along with the sintering at 750℃ for 2 hours.

Superior engineering critical current density obtained via hot isostatic pressing of MgB2 wires manufactured using nano-amorphous isotopic boron

The effects of heat treatment temperature, time and isostatic pressure on the grain connectivity and superconducting properties of Mg11B2 wires prepared using nano-amorphous isotopic boron powder (11B) have been investigated. The article presents detailed studies of the Mg11B2 material structure of the wires using scanning electron microscope (SEM). Heat treatment at low and high temperature for different annealing time and isostatic pressure does not create an internal macro-defect that disrupts connectivity in the Mg11B2 superconducting wires. Our study shows that dense longitudinal superconducting layers are formed at high annealing temperature of 800 °C for 60 min. Formation of dense longitudinal superconducting layers under high isostatic pressure (HIP) allow improved grain connectivity and high density of pinning centers and as a result, high engineering critical current density, Jec, at 4.2 K has been achieved. Also, high upper critical field (Bc2) and high irreversible magnetic field (Birr) were obtained using HIP process while superconducting transition temperature (Tc) was not significantly affected. Better grain connectivity obtained using high annealing temperature and long annealing time under low isotopic pressure leads to Jec enhancement at high magnetic fields at 20 K. Our results show that the annealing temperature of 800 °C for 60 min under isostatic pressure of 1 GPa yields the required Jec of 25 A/mm2 and 50 A/mm2 for ITER applications in magnetic fields of 8 T and 7 T, respectively. The promising results obtained in this work could pave the way for designing low activation superconducting Mg11B2 wires to be considered for next generation fusion magnets.

Effect of grain size selection in ex-situ made MgB2 wires

Commercial MgB2 powder manufactured by Alfa Aesar was mechanically sieved in order to obtain powder samples with different particle size. As-sieved powders were used for single-core wires with FeNiCo sheath made by ex-situ process and heat treated at the same conditions, 950 °C/30 min. Resistive transitions of extracted MgB2 cores allowed to obtain the critical temperature and normal state resistivity for different particle size and estimate the connectivity by the Rowell approach. In field critical currents were also affected by the particle size of sieved MgB2 powder. The highest current densities were measured for the wire with particles 15-25 µm, which is explained by the dominant effect of grain-boundary pinning.

BaZrO3 dopant interactions during MgB2 wire formation by modified internal magnesium diffusion process

Undoped and BaZrO3 nanopowder-doped MgB2/Nb/Cu single filament wires were prepared by modified internal magnesium diffusion (IMD) process using 0, 2 and 5 wt% BaZrO3 addition in amorphous B powder. Superconducting MgB2 phase was formed during heat treatment at temperatures from 650 to 670 °C. The wire microstructure was analysed using XRD, SEM, TEM and EDS to study BaZrO3 dopant interaction with Mg and B precursors during the heat treatment and effect of dopant on MgB2 superconductor. The BaZrO3 nanopowder decomposed and its products reacted with Mg and B producing relatively large BaB6 grains and nanograins of ZrB2 and MgO phases which were not detected by conventional XRD measurements due to their low grain size. Measured MgB2 lattice parameters varied with the dopant addition and heat treatment temperature. Different possible origins of MgB2 lattice deformation were discussed, and it was shown that Zr substitution in MgB2 lattice could not cause this deformation.

Increase in the infield critical current density of MgB2 thin films by high-temperature post-annealing

We propose a novel fabrication technique based on the formation of a Nb protective layer on a MgB2 thin film and high-temperature post-annealing to increase the critical current density (J c) of MgB2 films under an external magnetic field. Analyses of the crystal structure and the composition of the processed MgB2 films confirmed the suppression of the evaporation and oxidation of Mg by high-temperature annealing above 550 °C. The MgB2 film annealed at 650 °C exhibited a J c of 1.62 MA cm−2 under 5 T, which is the highest reported value for MgB2 films, wires, and bulk samples to date.

Critical current density defined at low electric field criterion and energy margin of superconducting MgB2 wires in wide temperature and magnetic field range

High-temperature superconductors (HTS) and MgB2 may potentially improve the usability of superconducting magnets dramatically owing to their large energy margin. When HTS and MgB2 wires are used for magnets operated in the persistent current mode, such as in magnetic resonance imaging (MRI) scanners, the electric field generated in the wires must be lower than 10−10 V m−1. In this paper, critical current density, J c, defined at an electric field criterion of 10−10 V m−1 is evaluated from a magnetisation decay measurement for state-of-the-art monofilamentary MgB2 wires. By using the obtained J c, a critical line of our multifilamentary MgB2 wire is shown on the temperature-magnetic field plane. Here, the critical line is defined as a line on which an electric field of 10−10 V m−1 is generated at a coil current density of 150 A mm−2. The area inside the critical line is demonstrated to be large enough to fulfil the requirement of 1.5 T MRI scanners operated at 10K–15K. In addition, the iso energy-margin lines are shown on the temperature-magnetic field plane and compared with those of NbTi wires. The MgB2 wire has an order of magnitude greater energy margin than the NbTi wires in most of the area inside the critical line. This suggests that the MgB2 wire is highly unlikely to be quenched due to mechanical disturbances.

Enhanced higher temperature irreversibility field and critical current density in MgB2 wires with Dy2O3 additions

Bulk samples of magnesium diboride (MgB2) doped with 0.5 wt% of the rare earth oxides (REOs) Nd2O3 and Dy2O3 (named B-ND and B-DY) prepared by standard powder processing, and wires of MgB2 doped with 0.5 wt% Dy2O3 (named W-DY) prepared by a commercial powder-in-tube processing were studied. Investigations included x-ray diffractometry, scanning- and transmission electron microscopy, magnetic measurement of superconducting transition temperature (T c), magnetic and resistive measurements of upper critical field (B c2) and irreversibility field (B irr), as well as magnetic and transport measurements of critical current densities versus applied field (J cm(B) and J c(B), respectively). It was found that although the products of REO doping did not substitute into the MgB2 lattice, REO-based inclusions resided within grains and at grain boundaries. Curves of bulk pinning force density (F p) versus reduced field (b = B/B irr) showed that flux pinning was by predominantly by grain boundaries, not point defects. At all temperatures the F p(b) of W-DY experienced enhancement by inclusion-induced grain boundary refinement but at higher temperatures F p(b) was still further increased by a Dy2O3 additive-induced increase in B irr of about 1 T at all temperatures up to 20 K (and beyond). It is noted that Dy2O3 increases B irr and that it does so, not just at 4 K, but in the higher temperature regime. This important property, shared by a number of REOs and other oxides promises to extend the applications range of MgB2 conductors.

Nano-sized amorphous/semi-crystalline boron ratio and external Mg effects on transport and structural properties in MgB2 IMD wires

In the present study, Mg diffusion process has been investigated by transport measurements and surface morphology analysis in round MgB2 wires made with internal Mg diffusion (IMD) method. These wires were produced using semi-amorphous boron, nano-sized amorphous boron or mixtures of them via cold drawing process. In addition to the conventional IMD wire production technique, a thin layer of magnesium was initially coated on the interior of the iron sheath and diffused into the boron powder during sintering. It has been found that Mg coating is effective on improving the engineering current density of IMD wires having relatively thick MgB2 cores. Surprisingly, Mg coating is also effective in non-stoichiometric hybrid IMD wires containing highly excessive amount of Mg.

Improved superconducting properties of multifilament internal Mg diffusion processed MgB2 wires by rapid thermal processing

6+1-filament MgB2 long wires with high critical current densities were fabricated using an internal Mg diffusion (IMD) process. The effect of high temperature treatment by conventional furnace annealing and rapid thermal processing on the superconducting properties and microstructure of IMD wire was investigated. The rapid thermal processing can improve the superconducting properties of multifilament internal Mg diffusion processed MgB2 wires obviously. A best layer Jc and Je as high as 2.8 × 105 A/cm2 and 3.1 × 104 A/cm2 at 4.2 K and 6 T were obtained by rapid thermal processing in the continuous heat treatment furnace. The Je is about 1.0 × 104 A/cm2 and 5.2 × 103 A/cm2 at 8 and 10 T, respectively. The engineering Je and MgB2 fill factor of MgB2 wires were improved obviously. Our obtained results show that the IMD wires with excellent superconductivity and mechanical property are suitable for practical applications.

Method for the production of pure and C-doped nanoboron powders tailored for superconductive applications

The present paper describes the improvement of the performances of boron powder obtained applying the freeze-drying process (FDP) for the nanostructuration and doping of B2O3, which is here used as boron precursor. After the nanostructuration process, B2O3 is reduced to elemental nanoboron (nB) through magnesiothermic reaction with Mg. For this work, the usefulness of the process was tested focusing on the carbon-doping (C-doping), using Cblack, inulin and haemoglobin as C sources. The choice of these molecules, their concentration, size and shape, aims at producing improvements in the final compound of boron: in this case the superconductive magnesium diboride, which has been prepared and characterized both as powder and wire. The characteristics of B2O3, B and MgB2 powder, as well as MgB2 wire were tested and compared with that obtained using the best commercial precursors: H. C. Starck micrometric boron and Pavezyum nanometric boron. Both the FDP and the magnesiothermic reaction were carried out with simplicity and a great variety of doping sources, i.e. elements or compounds, which can be organic or inorganic and soluble or insoluble. The FDP allows to produce nB suitable for numerous applications. This process is also very competitive in terms of scalability and production costs if compared to the via gas technique adopted by nanoboron producers currently available on the world market.