Hakan Yetis

The paper presents the results of the influence of SiC dopant, annealing temperature, and annealing time on the morphology of MgB2 material in superconducting wires. The results of measurements of critical temperature (Tc), irreversible magnetic field (Birr), resistance in the normal state (Rn), and transport critical current density (Jct) at the temperature range from 15 K to 30 K are presented. The MgB2 material is characterized by the presence of two specific regions. The first region with high density, excess Mg, and rectangular MgB2 grains is located outside the voids surrounding them. The second region occurs inside the ceramic core, away from voids, and its chemical composition corresponds to a stoichiometric Mg to B ratio (1:2), and it is characterized by the presence of spherical grains and lower material density. A higher amount of SiC admixtures (6 at.%) causes an increase in the first region surface area. This kind of structure observation in MgB2 superconducting wires has never been reported previously. The transport measurements showed that higher SiC dopant leads to lower Jct at higher temperatures and high magnetic fields. The studies showed that the point-dominant mechanism and the first region allow for obtaining high Jct at 30 K.

In this study, a new method called three-hole internal magnesium diffusion (TH-IMD) was applied to the fabrication of superconducting monofilament MgB₂/Fe wires. In the TH-IMD method, three thin magnesium rods were placed within the boron powder in a triangular arrangement instead of a single magnesium rod in the middle of the wire, as in the standard IMD method. The aim of the TH-IMD method is to ensure the complete diffusion of magnesium into the surrounding boron during the heat treatment of IMD wires and to obtain uniform MgB₂ structure throughout the core region. To determine optimal heat treatment conditions and wire diameters, transport measurements (R–T and I–V) were conducted under magnetic fields up to 11 T. Microstructural analysis was performed using optical microscopy, while phase composition was determined via XRD.

In this study, we have successfully produced a Fe-sheathed 6+1 multifilament wire using Cu/Nb/MgB2 monocore wires. The mono filament wire was prepared using Mg+2B powder mixture by powder-in-tube method without any intermediate heat treatment. The powder mixture of the amorphous nano boron (PVZ Nano Boron, purity of 98.5%, particle sizes < 250 nm) and high purity Mg powder (PVZ Mg, purity of 99%, particle size 74µm) were used. The multifilament wire was produced using groove rolling and cold drawing machines. The geometrical form of the filaments was examined using wire pieces taken from the wire at different steps throughout the production process. Finally, the multifilament wires produced in two different diameters of 1.02 mm and 0.82 mm were investigated in terms of filament uniformity, crack formation, surface roughness, and electrical transport properties. The structural examination was done on Nb filaments after the Fe and Cu sheaths were etched using HCl and HNO3 solution. The I – V measurements of the multifilament wires heat treated at 650 °C for 15, 30, 45, 60, and 240 minutes, and 700 °C for 60 minutes were carried out for the applied current up to 1 A at 25 K under various external magnetic field.

Currently, MgB₂ wires made by the powder-in-tube (PIT) method are most often used in the construction and design of superconducting devices. In this work, we investigated the impact of heat treatment under both low and high isostatic pressures on the formation of a layered structure in PIT MgB₂ wires manufactured using the Mg coating method. The microstructure, chemical composition, and density of the obtained superconductive wires were investigated using scanning electron microscopy (SEM) with an energy-dispersive X-ray spectroscopy (EDS) analyzer and optical microscopy with Kameram CMOS software (version 2.11.5.6). Transport measurements of critical parameters were made by using the Physical Property Measurement System (PPMS) for 100 mA and 19 Hz in a perpendicular magnetic field. We observed that the Mg coating method can significantly reduce the reactions of B with the Fe sheath. Moreover, the shape, uniformity, and continuity of the layered structure (cracks, gaps) depend on the homogeneity of the B layer before the synthesis reaction. Additionally, the formation of a layered structure depends on the annealing temperature (for Mg in the liquid or solid-state), isostatic pressure, type of boron, and density of layer B before the synthesis reaction.

A novel type of superconducting joining technique has been introduced to join unreacted internal Mg diffusion (IMD) single-core MgB₂ wires. Our method is based on fabricating a small diameter joint mould obtained by deforming an Nb/Cu composite tube with a longitudinal semi-cylindrical Mg and B core into a thick round wire. The small diameter of the joint provided advantages such as rapid cooling, low resistance, and the unique core design inside the joint ensured a uniform MgB₂ phase formation. Scanning electron microscope analysis revealed that the IMD MgB₂ wires had excellent contact with the superconducting MgB₂ bulk material inside the joint. The joint resistance, calculated from the decay of the trapped magnetic field over time, is a quite low value of 6.44 × 10⁻¹⁶ Ω at 20 K. The transport critical current (I _c) of the joint is 62 A at 20 K under a self-magnetic field, and the n-value of the joint is 66 at 20 K under 1.5 T. The results showed that the I _c of our joint can be determined precisely, regardless of whether the magnetic field is applied from low to high or from high to low value during I–V measurements.

We report transport, electromechanical, and structural properties of single core MgB₂/Fe wire produced using a new fabrication method, called designed internal Mg diffusion (IMD) process, which relies on the use of non-stoichiometric Mg + B pellets with excess Mg in place of a central Mg rod used in the standard IMD method. Structural analysis revealed the successful formation of a porous MgB₂ structure in the center and a dense circular MgB₂ layer surrounding this structure in the designed-IMD wire. Fast transport I–V measurements showed that the designed IMD method increased engineering critical current density (J _e) up to twice that of the IMD wires in self-field. The central porous MgB₂ structure shared the applied current and indirectly behaved as an internal stabilizer against quench damage at high applied currents.

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 (B_irr), critical temperature (T_c), upper magnetic field (B_c2), engineered critical current density (J_ec), 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 B_irr, T_c, B_c2, J_ec, 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 B_irr, T_c, B_c2, J_ec, 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.

Superconducting coils modeled by using single core MgB2 wires having copper or iron or both in the cross-section as sheath materials have been examined numerically by using the magnetic field formulation and heat transfer modules of Comsol Multiphysics software. Heating effects due to the core AC losses and skin effects in MgB2 coils are revealed for alternating currents in a liquid helium environment. Temperature changes, magnetic flux density profiles, core AC losses for each coil are obtained in one full cycle of applied alternating current. The effect of copper layer thickness on core AC loss behavior of MgB2/Cu coil and transport losses in MgB2/Cu and MgB2/Fe coils as a function of current ramp rate are investigated for an applied direct current up to 300 A.