Superconducting MgB2 Wires Research Articles

Study on the Sintering and Mg Composition Effect on MgB<sub>2</sub> Superconducting Bulks and Wires Synthesized by Powder-in-Sealed-Tube Method

Numerous research efforts aimed at the MgB2(Magnesium diboride) as a superconducting material due to its higher critical temperature than Nb-based superconductors such as NbTi, Nb3Sn. Nowadays MgB2is becoming more popular as the candidate to be applied on medical devices and large-scale applications because of its full coherence lengths, improved critical current density and fields, and simple crystal structure. In this study, we fabricated the 4 mm MgB2superconducting wires by mixing stoichiometric mole ratio of Mg: B with 1.0:2.0 and 1.1:2.0 through the Powder-In-Sealed-Tube (PIST) method to optimize high critical temperature (TC) than the conventional MgB2bulk and wire. Furthermore, we decreased the diameter of 4 mm to 1.8 mm wire and analyze the effect of critical temperature. The specimens were sintered at a different temperature to investigate the sintering effect of MgB2superconducting wire. The resistivity versus temperature relationship, surface morphology, and crystal phase was characterized using Cryogenic system, SEM (Scanning Electron Microscopy), and XRD (X-ray Diffractometer), respectively. We optimized the high Tc,onset for the bulk and 4 mm wire compared to other studies that are 42.1K and 40.3K respectively at 800°C sintered temperature. Finally, the results suggest that the stoichiometric ratio of MgB2exhibited excellent feasibility to prepare conventional MgB2superconducting wire.

Superconducting MgB2 Wire Drawing Considering Anisotropic Hardening Behavior and Hydrostatic Effect

Superconducting MgB2 Wire Drawing Considering Anisotropic Hardening Behavior and Hydrostatic Effect

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.

Effect of Cu Cladding on the Mechanical Properties of Iron Sheath Material in the Drawing of Superconducting MgB2 Wires

In the present study, a novel choice of sheath materials for drawing long superconducting MgB2 wire by using the powder-in-tube technique (PIT) is reported. This would eliminate the need for an intermediate strain-relieving annealing process and would reduce the time and cost of fabrication. Our strategy involved the use of a composite sheath instead of a sheath made of a single material. The relatively inert Fe constituted the inner sheath around the MgB2 powder while the Cu—which is capable of efficient heat dissipation—was used as the outer sheath. Important mechanical properties of the wire such as elastic modulus, ultimate tensile strength, yield strength, hardness, and microstructure were carefully studied at different stages of the drawing process using tensile and microhardness tests. To clearly delineate the effect of Cu cladding on the ductile behavior of the iron sheath, another MgB2 wire with only an Fe sheath was prepared; its mechanical properties were measured and compared with those of the composite Cu–Fe-sheathed MgB2 wire. After a few drawing steps, the composite Cu–Fe-sheathed wire showed a lower elastic modulus and tensile strength than those of its Fe sheath counterpart. While both types of wires showed an increase in hardness as the drawing process progressed, the composite-sheath wire consistently showed a lower hardness than that of its counterpart, implying its lower susceptibility to fracture; it can therefore be safely drawn to small diameters without the need for intermediate annealing during the wire drawing process.

Thermal conductivities and thermal runaways of superconducting MgB2 wires stabilized by an Al + Al2O3 sheath

This paper presents a study of the thermal transport properties of extra-light MgB2/Ti/Al + Al2O3 composite wires. The longitudinal thermal conductivity has been investigated as a function of temperature and magnetic field on wires differing only by an outer Al + Al2O3 sheath. The correlation between the thermal transport properties, the microstructure of the Al + Al2O3 sheath and the Al/Ti interface reactions provide important information for these thermally stabilized extra-light superconductors. The thermal runaways observed by the current–voltage characteristics of MgB2/Ti/Al + Al2O3 wires correlate well with the conductor’s temperature affected by the thermal conductivity and the resistance of the outer sheath.

Experiment and Simulation for Normal Zone Propagation of Multifilament MgB2 Superconducting Wire Cooled by Liquid Hydrogen

Developing applications of liquid-hydrogen (LH 2 )-cooled superconducting devices is a challenging issue. Since the boiling point of LH 2 is 20.4 K, MgB 2 , whose critical temperature is 39 K, can be cooled with a sufficient temperature margin. Furthermore, MgB 2 wire is expected to be used for superconducting equipment due to the low production and material cost. Therefore, in order to design MgB 2 superconducting equipment, the knowledge of normal zone propagation phenomena is important for the thermal stability and the quench protection. In this study, normal zone propagation and minimum quench energy (MQE) with a multi-filamentary MgB 2 superconducting wire produced by Hitachi, Ltd. were observed under immersed in LH 2 . In the experiment, heat pulse, which initiates a normal zone, was injected to the center area of 200 cm long MgB 2 wire. Then, the MQE and the normal zone propagation velocity (NZPV) were measured under specific conditions. NZPV was in the order of several cm/s and MQE was in the order of a few J at 30 K under LH 2 cooling. In order to clarify temperature distribution along the wire during the normal zone propagating phenomena, the simulation model of MgB 2 wire cooled by LH 2 was created and analyzed using a finite element method simulation software.

Exploring Fully Superconducting Air-Core Machine Topology for Off-Shore Wind Turbine Applications

This paper presents an electromagnetic (EM) analysis and feasibility design study of a 10 MW air-core fully superconducting (SC) generator for off-shore wind turbine applications. This machine topology uses MgB2 SC wires in both armature and field windings to reduce the generator cost. To reduce the weight of the machine, stator iron core in the traditional SC machine design is replaced with shield coils to contain the flux inside the machine. This air-core design allows the air-gap flux density to be increased without saturation to achieve an increased power density in machine design. Further, eliminating the iron core also eliminates the core losses. This is an attractive feature for off-shore wind turbine applications where machine weight can be reduced with increased efficiency. Preliminary EM studies are performed on a 10 MW, 10 r/min fully SC air-core machine design and its associated ac losses are evaluated. Design specification of the machine is provided with the armature, field, and shield winding parameter details. Finite element (FE) analysis is used to evaluate the flux density and torque profile of the machine. To reduce the ac losses in the armature coils, magnetic flux density at the armature winding is set to be 2 T. The preliminary results show this fully SC air-core generators show better performance in ac losses compared to the similar fully SC MW wind turbine designs.

Preparation of MgB2 superconducting wires by the rapid heating and quenching method

In order to reduce the long-term annealing process and prevent excessive grain growth, we attempted to fabricate MgB2 superconducting wires by the rapid heating and quenching (RHQ) method. In this paper, we have successfully prepared MgB2 superconductive wires with self-made RHQ equipment, and the effect of various Joule heating currents on the properties of MgB2 was investigated. The XRD results showed that the sample had relatively strongest diffraction peaks and least heterogenous peaks when the loading heating current was 350 A, so the superconducting transition temperature (Tc) could reach 38.5 K. Because the MgB2 phase particles were small and uniformly distributed, there was a good critical current density (Jc). That is, in the self-field, the Jc value of the 350 A sample reached up to 1.39 × 105 A/cm2, and the downward trend was the flattest. When the magnetic field was 7 T, the critical current density was still as high as 300 A/cm2.

Effect of SiC and Sintering Temperature Variations on the Characteristic of Fe-Sheathed MgB<sub>2</sub> Superconductor Wires

MgB2superconductor with relatively high critical temperature (Tc=40 K) has been developed for possibilities utilization in various practical applications such as Magnetic Resonance Imaging. In order to enhance the process, the material was prepared by Powder-In-Tube (PIT) method, while the superconducting properties was improved by incorporating 0 wt.%, 10 wt.% and 20 wt.% SiC nanoparticles into MgB2structure. This study aimed at analyzing the effect of sintering temperature on the microstructure, resistivity and phase of Fe-sheathed MgB2superconducting wires. Three different compositions of MgB2powders were inserted into Fe tube with inner ø of 4 mm and outer ø of 6 mm. This tube was then rolled and drawn into a ø 2.5 mm wire. Wire samples with three different compositions of SiC were heat treated at 600°C and 800°C respectively. All samples were characterized to analyze the morphology, resistivity and crystal structure. X-ray diffraction (XRD) analysis showed that some Mg may react with SiC to form MgSi and with oxygen to form MgO. Scanning electron microscope (SEM) images revealed that with no addition of SiC, MgB2was formed and dispersed uniformly in wire, but with 10 wt.% and 20 wt.% SiC nanoparticles, the whisker morphology was observed resulting in degradation of the superconducting properties.

Lightweight MgB2 wires with a high temperature aluminum sheath made of variable purity Al powder and Al2O3 content

The properties of high temperature aluminum (HITEMAL, hereinafter HIT) composites made of powders with different particle sizes ranging from 0.8–3 μm and purities of 99.9%–99.996% were tested. The thermal, electrical, and mechanical properties of these materials were examined and compared with pure Al. Three different HIT materials were used for the outer sheath of single-core MgB2 wires with a Ti barrier manufactured by internal magnesium diffusion into a boron process. The critical currents, thermal stability, and strain tolerances of the MgB2/Ti/HIT wires were measured at liquid He temperature. It was found that the Al powder purity and particle size used for HIT affect the performance of MgB2 wires. A sheath made of high purity Al powder allows better thermal stability of MgB2 wire, but the strain tolerance is lower in comparison to the wire with HIT made of less pure powder and higher Al2O3 content. The presented wire configurations are promising for lightweight, thermally stabile, and strain tolerant superconducting MgB2 wires.

Lightweight Al-stabilized MgB2 conductor made by the IMD process

Light single-core MgB2 wire stabilized by a unique Al–Al2O3 metal matrix composite (MMC) was fabricated by the internal magnesium diffusion (IMD) process. Mg/B/Ta/Al–Al2O3 composite was cold-deformed into a wire with a cross section of ∼1 mm, and finally heat-treated at temperatures of 635 °C–645 °C for times of 8–60 min. Critical temperatures and critical currents were evaluated by transport and magnetic measurements. Thermal stability and AC loss were characterized by the quench power from I–V curves and by the hysteresis loss measured by a calibration-free method. The results obtained clearly confirmed that the Al–Al2O3 MMC mechanically supports the Mg/B/Ta wire core well during cold-working operations and subsequent heat treatment to form MgB2, while at the same time it retains its advantageous mechanical properties, and it can be utilized as a material for the outer sheath of MgB2 superconductors. Critical temperatures and critical current densities of MgB2/Ta/Al–Al2O3 are comparable with the values for other superconductors with MgB2 core fabricated by the IMD process using different sheath materials. The composition presented here shows promising potential for future lightweight and thermally stable superconducting MgB2 wires with high engineering current density.

Formation of High-Field Pinning Centers in Superconducting MgB2 Wires by Using High Hot Isostatic Pressure Process

This paper demonstrates the effects of hot isostatic pressure (HIP) on the structure and transport critical parameters of in situ MgB2 wires without a barrier. Our results show that only HIP and nano-boron allow the formation of more high-field pinning centers, which lead to the increase in critical current density (Jc) at high applied magnetic fields. Nano-boron and annealing at a low pressure increase the Jc in the low magnetic field. This indicates that nano-particles create more high-field pinning centers. In addition, the results show that nano-boron improves the connection between the grains. Scanning electron microscope results show that HIP increases the reaction rate between Mg and B, density, and homogeneity of the MgB2 material. Additionally, HIP allows to create a structure with small grains and voids and eliminates the significance of the number of voids. High isostatic pressure allows to obtain high Jc of 10 A/mm2 (at 4.2 K) in 10 T and increases irreversible magnetic field (Birr) and upper critical field (Bc2). Measurements show that these wires have high critical temperature of 37 K.

Very Large Engineering Critical Current Density Properties in a (6+1)-Structure MgB2 Superconducting Cable at 20 K

We report the fabrication process and critical current-carrying properties of a 20-m-length (6+1)-structure MgB 2 superconducting cable with a Nb-Ti central reinforcement wire. The results suggest that the twisting process hardly degrades the critical current properties of MgB 2 superconducting wire, when the cables are prepared by the twist and react process. Comparing the common single-filament wire (CW) with Ic of 118.4 A at 4.2 K and 4 T, the Ic of the single-filament wire extracted from the cable (EW) has almost with no change, about 116.7 A at the same conditions. Moreover, for the (6+1)-structure MgB 2 superconducting cable, which is 2.40 mm in diameter and 45 mm in twist pitch, its critical current-carrying capability is up to about 8 kA and its engineering current density Jce reaches 2270 A/mm 2 at 20 K and self-field. This paper strongly implies that MgB 2 superconducting cable prepared by the in situ solid-station method is very promising for large-scale engineering applications.

Neutron Microtomography of MgB2 Superconducting Multifilament Wire

Neutron imaging of sub-10-micrometres spatial resolution has been recently achieved in 2D mode within the framework of the Neutron Microscope project at the Paul Scherrer Institut. Here we report on the development of the PSI Neutron Microscope instrument and the results of the first microtomographic imaging experiment of multifilament superconducting MgB2 wire. The sample of MgB2 superconducting 37 multifilaments embedded in copper-nickel matrix was investigated –in microtomographic mode– with the scientific interest regarding the distribution of boron within the individual superconducting filaments (about 40μm in diameter). The resulting tomographic dataset revealed the distribution of boron within the entire 0.8mm thick multifilamental wire with the isotropic voxel size of 2.6 micrometres.

A defect detection method for MgB2 superconducting and iron-based Ba(Fe,Co)2As2 wires

In this article, a method allowing for the detection of damage in iron-based superconducting bulks and MgB2 wires is presented. MgB2 wires were made of in situ material with ex situ MgB2 barrier. The iron superconductor studied was Ba(Fe,Co)2As2. This material was surrounded by Nb barrier and placed inside an iron tube. All samples were annealed in the isostatic pressure of 1 GPa. Transport measurements were made using a four-contact probe. The transition of Nb and ex situ MgB2 barrier from superconducting into resistive state (first transition) was observed at a temperature range from 5 K to 10 K and can be attributed to damage in either the Nb or ex situ MgB2 barrier in these samples. For samples with a damaged barrier, it was not possible to determine the critical current density of the wires. The analysis indicates that annealing at 1 GPa leads to the Ba(Fe,Co)2As2 material with critical temperatures of 27 K and 21.5 K at upper critical flux density (Bc2) of 14 T.

Quench Protection using CLIQ of a MgB<sub>2</sub> 0.5 T Persistent Mode Magnet

A protection system using the coupling loss induced quench (CLIQ) method has been simulated for an eight coil 0.5 T MRI main magnet designed with MgB 2 superconducting wire. The temperature rise was modeled in MATLAB using the Douglas-Gunn method to solve the three-dimensional heat equation and Heun's method was used to solve the circuit equations associated with the CLIQ electrical circuit. The wire's twist pitch and the voltage and capacitance of the CLIQ unit can be varied to study their impact on quench protection performance. Simulations of CLIQ protection were performed on the entire 0.5 T eight coil magnet. The peak temperature varied with the wire's twist pitch. The optimal twist pitch was around 3.5 and 5.5 cm assuming the lowest and highest transverse resistivity, respectively. For the entire main magnet using CLIQ protection, the peak temperature reaches 188 K with a 10 mF capacitor at 1.3 kV.

Cryogenic Tension Response of Monel Sheath MgB2 Superconducting Wires

We report the effects of cryogenic temperature on tensile responses for the heat-treated Monel sheath MgB2 super-conducting wires. A variable temperature cryostat system was used to provide a cooling environment from room temperature (RT) to the liquid nitrogen temperature (LNT), and a small cryogenic-type extensometer was used to measure strains of the MgB2 wires under tension loading. Under a range of cryogenic temperatures, we recorded the corresponding stress-strain relations during the wire stretching and obtained the tensile and yield strengths, Young's modulus and toughness energy of the Monel sheath MgB2 wires. It was shown that their ultimate tensile strengths always increased linearly with decreasing temperature, but an obvious nonlinear feature on the yield strength and toughness energy appeared, whereas the Young's modulus was almost independent on the cryogenic temperature in our tested range. In addition, for the purpose of comparison and practical application, the properties at 4.2 K were extrapolated for the Monel sheath MgB2 composite wires based on measured data of tensile behaviors from RT to LNT, which were compared with the experimental data in the other literature.

The electric field jump—detection of damaged Nb barrier in MgB2 wires annealed under pressure

In this paper we present the results of critical current (Ic) measurements of MgB2 wires made with two different set-ups of the four-point probe method: current sweep type—constant magnetic field and increasing current, and field sweep type—constant current and rapidly increasing magnetic field. Results from magnet field sweep type measurements can be interpreted by a new physical concept—a jump of the electric field in low magnetic fields. This physical concept can be correlated with damages in the Nb-barrier existing in the MgB2 wire and be employed as a detection scheme. The damage in Nb barrier reduces critical current density (Jc) and complicates the study on critical temperature (Tc), upper critical field (Bc2), irreversible magnetic field (Birr), pinning force (Fp), and pinning centers in superconducting MgB2 wires. Our proposed method to detect damages in Nb barrier would benefit efforts in development and applications of MgB2 wires.

Experimental research on electric field jump in low magnetic fields: Detection of damage in new ex-situ MgB2 barriers in MgB2 wires

We explored the incorporation of field sweep (constant current and rapidly increasing magnetic field) into the four-probe method as a new technique to detect defects in barrier layers in superconducting MgB2 wires. This method allows us to observe jumps in the electric field in low magnetic fields. The scanning electron microscopy results indicate that such a jump originates from cracks in Nb barriers and ex-situ MgB2 barriers. Our research indicates that the field sweep allows us to detect damage to barriers that are made of superconducting materials. This method can be the basis for an industrial method for detecting damages in MgB2 wires. These defects reduce the critical current of MgB2 wire. Detection and removal of these defects will allow us to produce MgB2 wires with ex-situ MgB2 and Nb barriers that will have improved critical current density. Manufacturing of MgB2 wires with new ex-situ MgB2 barriers is a new technological concept. This type of barrier is cheaper and easier to manufacture, leading to cheaper MgB2 wires. Moreover, we show that critical current can be measured by two methods: current sweep (constant magnetic field and quickly increasing current) and field sweep.

Winding Configuration and Performance Investigations of a Tubular Superconducting Flux-Switching Linear Generator

Primary permanent-magnet linear generators have an advantage of a simple secondary structure, which is suitable for the application of wave energy conversion. Based on vernier hybrid machines (VHMs), which are widely used for direct-drive wave energy converters, this paper proposes a tubular superconducting flux-switching linear generator (TSFSLG), which can effectively improve the performance of this kind of generators. Magnesium-diboride (MgB 2 )-type direct current superconducting windings are used in the generator to increase the magnetic energy and overcome the disadvantages of easily irreversible demagnetization of the VHMs and high-voltage regulation of a permanent-magnet flux-switching linear generator (PMFSLG). The alternating current superconducting windings are implemented to improve the efficiency. Based on the MgB 2 superconducting wire produced and measured in the laboratory, the superconducting windings structure used for the generator is designed. The coil-EMF vector is optimized, and the losses of superconducting windings are analyzed. In addition, by using the finite-element method, the no-load performances of the generator are analyzed and compared with the ones of a VHM and a PMFSLG. Finally, the on-load performance of the TSFSLG is obtained by finite-element analysis and compared with the VHM and the PMFSLG. The results validate that the proposed generator is more suitable for wave energy conversion than its counterparts.