MgB2 Filament Research Articles

Measurement of Mechanical Behavior of 11B-Enriched MgB2 Wire Using a Pulsed Neutron Source

MgB2 represents a hexagonal superconductive material renowned for its straightforward composition, which has facilitated the development of cost-effective practical wires. Its capacity to function at temperatures as low as liquid hydrogen (LH2) has made it a prominent candidate as wire material for the coils of next-generation fusion reactors. Much like other superconducting wires, a prevalent issue arises when these wires are employed in coils, wherein electromagnetic forces induce tensile stress and strain within the wire. This, in turn, diminishes the critical current, which is the maximum current capable of flowing within the generated magnetic field and strain. The techniques and methods for accurately measuring the actual strain on the filaments are of paramount importance. While strain measurements have been conducted with synchrotron radiation and neutrons for other practical wires in the past, no such measurements have been undertaken for MgB2. Presumably, this lack of measurement is attributed to its relatively greater thickness, making it less suitable for synchrotron radiation measurements. Additionally, the high absorption cross-section of the included boron-10 poses challenges in obtaining elastic scattering data for neutron measurements. In response, we fabricated a wire enriched with boron-11, an isotope with a smaller neutron absorption cross-section. We then embarked on the endeavor to measure its strain under tensile loading using pulsed neutrons. Consequently, we succeeded in obtaining changes in the lattice constant under tensile loading through Rietveld analysis. This marks the inaugural instance of strain measurement on an MgB2 filament, signifying a significant milestone in superconductivity research.

Mechanical analysis of an MgB2 1.5 T MRI main magnet protected using Coupling Loss Induced Quench

Mechanical analysis of the stress and strains developed in the coils were calculated for a ten coil 1.5 T MRI magnet design with magnesium diboride (MgB2) wire protected with Coupling Loss Induced Quench (CLIQ). The temperature distribution inside the coils was first simulated in MATLAB to solve the governing heat and circuit equations. Simulations were performed on the magnet, in which each coil was divided into two subsections, with two CLIQ units while the capacitor ranged from 5 to 20 mF and the initial charging voltage ranged from 2.6 kV to 1.3 kV in order to keep the total stored energy in the CLIQ system constant. The wire’s filamentary twist pitch remained constant at 5 cm for all simulations. The exported temperature distribution was expanded to form a representative unit cell (RUC) representing the wire composite and then imported into ANSYS to calculate the 1st principle strain in the MgB2 filament and shear stress across the epoxy for the coils. A peak temperature of 191 K occurred inside the coil with the initial quench when the CLIQ unit had a 20 mF capacitor charged to 1.3 kV. According to the mechanical simulations, the largest resulting peak strain in the wire was 0.034%, and peak shear stress was 44 MPa.

Point contact spectroscopy of superconductors via nanometer scale point contacts formed by resistive switching

Point contact spectroscopy is commonly used to investigate electronic properties of superconductors. Here we show that nanometer scale point contacts, which enable to study the superconductor properties locally, can be created by means of the resistive switching phenomenon. Our experiments were performed on sandwiched MgB2/Al/TiO2/Pt structures, where multiple bipolar resistive switching cycles were conducted. The differential conductance as a function of voltage was measured at temperatures below the critical temperature of the MgB2 superconductor. In the low-resistance state the MgB2 and Pt electrodes are connected by an ultrathin metallic filament which creates at the MgB2 electrode the Sharvin point contact with diameter below 10 nm. In this case the differential conductance data demonstrate the Andreev reflections due to the carrier transport between the superconducting MgB2 electrode and filament. From these data the two-gap superconductivity of MgB2 is clearly visible which also confirms the fit by the Blonder-Tinkham-Klapwijk model. If the bottom electrode is made of a superconductor with known gap, our approach allows us to estimate from the Andreev reflection spectrum the resistance of both the filament and point contact. We can then determine from the Sharvin formula the cross-section size of the point contact and thus also the filament cross-section size. In the high resistance state when the filament is ruptured, the differential conductance data demonstrate the spectrum typical for tunneling between two normal metals, with a zero-bias anomaly due to the Altshuler-Aronov effect. This suggests that the filament is not ruptured at the superconducting MgB2 electrode but elsewhere.

Investigation of multifilament MgB2 superconducting joint technique for development of MRI magnets

This study presents the investigation of superconducting joints fabricated using multifilament magnesium diboride (MgB2) wires for the development of persistent-current mode magnetic resonance imaging (MRI) magnets. The critical current of the jointed samples decreased with increasing cutting angle because the smaller cutting angle allowed greater exposure of the MgB2 filament, thereby increasing the contact area for the wire-bulk-wire connection. In addition, an appropriate pressing pressure (300 MPa) was necessary to establish the multifilament MgB2 joint without significant degradation of superconducting properties. The resistance of the optimal MgB2 joint, measured using the field-decay technique, was <1.5 × 10-14 Ω. Therefore, the proposed joint technique can be employed for developing multifilament MgB2 MRI magnets operating in the persistent-current mode.

The effect of boron powder on the microstructure of MgB2 filaments prepared by the modified internal magnesium diffusion technique

We have analyzed the microstructure of MgB2 prepared by the modified internal magnesium diffusion technique resulting in filaments without a central hole. Three different amorphous boron powders with a nominal purity of 98.5%–99.0% and a different particle size were used. It was shown that the real purity of the powders was different from the nominal purity, and the microstructure as well as the critical current densities of the MgB2 were affected by using boron. Larger B particles with a size of 400–500 nm led to the presence of large B-rich grains inside the MgB2 matrix. For one wire series, boron powder containing residual chlorine from the boron preparation process was used. It was found that just a small quantity of chlorine in the B powder caused significant MgB2 filament inhomogeneity, an apparent decrease in the critical current density and high reactivity with air. We estimated the ideal packing density for achieving a large volume of dense MgB2.

High-performance dense MgB2 superconducting wire fabricated from mechanically milled powder

Owing to the relatively high critical temperature and the low manufacturing cost, MgB2 superconducting wires are promising for liquid helium-free superconducting applications. Today, commercially available MgB2 wires are manufactured by either an in situ or ex situ powder-in-tube process, the in situ process being more effective to obtain high critical current density. In in situ-processed wires, however, the critical current density is seriously suppressed by the high porosity of MgB2 filaments. To resolve this problem, we propose an innovative method of using precursor powder prepared by mechanical milling of magnesium, boron, and coronene powders. This precursor powder has a metal–matrix–composite structure, in which boron particles are dispersed in a magnesium matrix. The plastic deformation of the precursor powder through wire processing leads to compact packing, and a dense MgB2 filament is generated after heat treatment. As a result, the limitation of critical current density that occurs for the typical in situ process is overcome, and the practical critical current density of 103 A mm−2 is obtained at 10 K and 6.1 T, at 15 K and 4.8 T, and at 20 K and 3.3 T.

Three-Dimensional Analysis of MgB2Wire by use of X-ray Micro-Tomography

For the further development of MgB2 wires, investigation of 3-D structures for MgB2 filaments and sheath materials is important. However, typical cross-sectional observation such as optical microscopy and scanning electron microscopy are essentially 2-D, therefore, not enough to understand the 3-D filament structures. In this paper, we have adopted X-ray microtomography and carried out a 3-D analysis for MgB2 wire. X-ray microtomography (Skyscan 1172, Bruker) allows us to visualize the internal filament structure of a centimeter-long MgB2 wire with a high spatial resolution of 1.1 $\mu\text{m}$ , with a nondestructive manner. We investigated the boundary between the sheath material and the MgB2 filament of an internal Mg-diffusion-processed MgB2 wire. Furthermore, by improving the signal-to-noise ratio for the X-ray microtomography, we have also identified the boundary between the MgB2 filament and the hole (air) in spite of the low X-ray absorption coefficient for boron and magnesium. From the 3-D analysis, we can reveal the shape and area of MgB2 filament quantitatively and also their statistical distribution.

High density and connectivity of a MgB2 filament made using the internal magnesium diffusion technique

In order to allow precise and detailed physical studies of an MgB2 filament made by the internal magnesium diffusion process (IMD), a modified approach (MIMD) using a Mg tube filled with boron powder deformed into wire was introduced. The MIMD process allows easy extraction of the MgB2 filament after the final heat treatment and performance of four-probe resistive measurements and density estimation, which is not possible for standard IMD wires. The Rowell approach has been applied for the grain connectivity from R(T) data of extracted MgB2 for the first time. The filament’s density has been estimated from the precise volume measured by x-ray micro-tomography and mass. The high connectivity and density of the MgB2 filament made by the diffusion process are discussed and compared with those of filaments made by other processes.

Effects of impurities addition in MgB2/Nb tapes on flux jumps instability and critical current density

In order to understand the effect of impurities inclusions on the transport properties of MgB2 tapes, by means of magnetization measurements we have analysed non-commercial monofilamentary tapes in which titanium (Ti), tungsten (W) and silicon carbide (SiC) normal particles were included. All the samples under analysis have the same cross section and their MgB2 filament is embedded in a superconducting niobium matrix. For all the tapes a magnetic field (Ha) up to 65 kOe has been applied parallel to the filament, and we have compared their magnetization loops M(Ha) measured in the temperature range from 2 K up to 30 K. The results show that the process of adding normal particles in the material, in order to increase the transport properties, produces at low field opposite effects to those at higher field. Moreover, all the particles included produce the common result of an enhancement in the thermomagnetic instability, and consequently a decrease of the transport properties, at low temperature and magnetic fields.

Quantitative study of the inhomogeneous distribution of phases in Fe-sheathed exsitu MgB2 tapes

Quantitative phase analyses have been performed on Fe-sheathed exsitu MgB2 tapes after the deformation process and after annealing at 700, 850 and920 °C in vacuum. The techniques used were x-ray diffraction, differential thermalanalysis and scanning electron microscopy. All superconducting cores were foundto contain certain amounts of MgO, the largest amounts being observed nearthe sheath, and after annealing at the highest temperature. Small amounts ofMgB4 were detected in the inner part of the core, presumably formed by the decomposition ofMgB2. Atthe MgB2/Fe interface, a reaction layer was observed, which was identified as the borideFe2B. In addition,tapes annealed at 700 °C revealed the presence of the metastable phaseFe23B6. The interactionof the MgB2 filament with the Fe sheath and the presence of trapped oxygen limit the possibilities of thecurrent tape fabrication techniques.

Anisotropic grain morphology, crystallographic texture and their implications for flux pinning mechanisms in MgB2 pellets, filaments and thin films

Grain morphology and crystallographic texture were investigated by electron microscopy in four different polycrystalline forms of superconducting MgB2. The materials included a hot-pressed sintered MgB2 pellet, a pellet reacted in situ from Mg and B, an in situ reacted MgB2 filament and a pulsed-laser-deposited thin film grown on a single crystalline [111] oriented SrTiO3 substrate. Thick plate-shaped grains with an aspect ratio of ∼3 and large faces parallel to (0001) planes dominated the microstructure in all four types of sample. The intermediate-sized plate-shaped grains (0.1 μm × 0.3 μm on average) in the electromagnetically most homogeneous parts of the hot-pressed pellets were strongly facetted, but not textured. Large (3–5 μm) plate-shaped grains were seen in the pellet reacted directly from stoichiometric Mg and B. A tendency for parallel alignment of the [0001] axes of the considerably larger grains (∼0.25 μm × 1 μm) in the filament was observed near its W core, but degradation of this texture away from the core was apparent. The very small grains (∼10 nm) of the thin film possessed a well-defined fibre texture with [0001] parallel to the film normal and no preferred orientation in the plane of the film. Electrical resistivity of the finest grain samples was some 103 times higher than the largest grain sample and their critical current density about one order of magnitude higher. We conclude that, in contrast to the cuprate-based high-Tc superconductors, grain boundaries do not limit the critical current density of polycrystalline MgB2 and indeed act as flux-pinning centres, which enhance the critical current density.