M Kulich

MgB2 composite wires with Fe, Nb and Ta sheaths

Single-core MgB2 composite wires have been made by the powder-in-tube method using commercial Mg, B and MgB2 powders (Alfa Aesar) in Fe, Nb and Ta tubes and both in situ and ex situ processes. Prepared wires were subjected to annealing at temperatures ranging from 600 °C up to 950 °C for 30 min in argon atmosphere. Resistive (R(T)) and transport current (Ic(μ0H)) measurements have shown how the sheath material that was used influences the critical temperature and critical current density. Inter-diffusion and reaction has been observed only for the iron sheath. Niobium has appeared as the best sheath material for MgB2 wires made by the in situ process but as the worst sheath material for the ex situ method. The reason is mainly due to the large transversal cracks generated in the fully Nb sheathed ex situ wires during the deformation, which are not healed by the subsequent heat treatment.

Progress in electrical and mechanical properties of rectangular MgB2 wires

Critical current densities and mechanical resistance of MgB2 wires made by the rectangular wire-in-tube technique (RWIT) have been studied. Wires prepared from different precursor powders and variable sheath materials are compared. The best electrical performance (104 A cm−2 at 11.3 T) was measured for the wire with mechanically alloyed powder doped by SiC. While the critical current densities, Jc, at 4.2 K are considerably influenced by the powder used, the differences at 20 K are much smaller. Flattened wires show different levels of critical current anisotropy influenced by the precursor powder used. Stress–strain characteristics and critical current degradation are strongly affected by the applied metallic materials and also by the filaments strength. The highest irreversible strain eirr = 0.55% was measured for Ti/Cu/Monel sheathed wire with filaments from mechanically alloyed powder.

Anisotropy of the critical current in MgB 2 tapes made of high energy milled precursor powder

For applications of MgB2 wires or tapes, high critical currents in high magnetic fields are essential. By using tapes in superconducting coils the anisotropic behaviour of the critical current, i.e. the dependence on the direction of the external field in relation to the tape surface, has to be taken into account. The anisotropy of MgB2 tapes with mechanically alloyed (MA) precursor powder and different sheath materials which can be much higher than the intrinsic anisotropy is discussed. Furthermore tapes with pure and C doped MA-MgB2 precursor are compared. Tapes with a hard Fe-sheath and undoped precursor show a high extrinsic anisotropy of the critical current density which can be reduced considerably by carbon doping. A texture of the MgB 2 phase in the tape filaments introduced by flat rolling was observed by synchrotron x-ray diffraction. Using this texture information the observed macroscopic Ic-anisotropy of the tapes can be explained by calculations based on the percolation model. (Some figures in this article are in colour only in the electronic version)

Fast creation of dense MgB2 phase in wires made by IMD process

Single-core MgB2 wires have been made by internal magnesium diffusion (IMD) into boron process. Heat treatments were performed at variable periods (4–90 min) for two adjusted temperatures 635 °C and 650 °C, with an overshoot of ≈20 °C after 5 min. Critical currents of differently treated MgB2/Ti/GlidCop wires have been measured and related with the MgB2 layers. The fast creation MgB2 compound has been observed after 8 min annealing at both temperatures and showing the critical current maxima. The less uniform MgB2 containing cracks was created for longer annealing periods (≥10 min), which resulted in lowered critical currents and worsened thermal stability. The fast creation of dense MgB2 allows it to utilize a continual and short heat treatment process for high current density wires made by IMD.

Properties of MgB2 wires made by internal magnesium diffusion into different boron powders

Different boron powders were used for MgB2 wires manufactured by internal magnesium diffusion. The structure of the MgB2 core, critical temperature and critical currents of Cu/Ti sheathed wires differing only in boron powder were analyzed and compared. It was found that the particle size and purity of boron powders influence the creation of the MgB2 phase and, consequently, also considerably influence its superconducting properties. The highest critical current density in the low external field was measured for wire with a boron purity of 98.5% produced by Pavezyum. It was used also for stabilized multi-core MgB2 wire with high engineering current densities in low magnetic fields at 20 K, which may be attractive for some low field applications, e.g. high-power wind generators.

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 filaments 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.

Electromechanical properties of iron and silver sheathed Sr0.6K0.4Fe2As2 tapes

This work describes the electromechanical behaviour of iron- and silver-sheathed ex situ Sr0.6Ka0.4Fe2As2 (Sr-122) tape superconductors. Free-standing Sr-122 tapes were stressed by axial tension at 4.2 K under external fields at 5–6 T, and critical currents have been simultaneously measured. An attempt to reinforce Sr-122/Ag mechanically was done by simple soldering of two stainless steel strips of 40 μm to both sides of the tape. The obtained results are compared with the electromechanical property of MgB2/Fe. The mechanical limits of Sr-122/Fe and Sr-122/Ag regarding possible high field applications are also discussed.

Effect of cold isostatic pressing on the transport current of filamentary MgB2 wire made by the IMD process

This work describes the effect of cold isostatic pressing applied to as-drawn filamentary wires in a GlidCop and/or Cu sheath made by the internal magnesium diffusion process. Critical currents of as-drawn and isostatically pressed wires at high pressures up to 2.0 GPa followed by heat treatment at 640 °C for 40 min were measured. The obtained results show an improvement in boron powder density resulting in an increase of the critical current of MgB2 layers. The engineering current density increases by 4–13 times after the high-pressure treatment, and is influenced by the density of the boron powder and by the mechanical strength of the outer sheath.

Comparison on Effects of

Several MgB₂ monofilamentary samples are prepared to evaluate the effects that the doping compounds B₄C, Al₂O₃, and SiC have on critical current density (<i>J</i>_c) and the <i>J</i>_c dependency of magnetic flux density (<i>B</i>) of the sample. All of the samples in this study have Nb sheaths and are manufactured using in situ precursor powders and the Powder-in-Tube fabrication method. The amounts of added impurities are 10 wt.% for B₄C and Al₂O₃, and 3, 10, and 20 wt.% for SiC. A reference sample without any doping is also prepared. All of the samples are heat treated between 650 and 800°C for 30 minutes. The transport <i>J</i>_c(<i>B</i>) characteristics are measured at 4.2 K and the magnetic <i>J</i>_c(<i>B</i>) behavior is obtained in varied temperatures up to 30 K. The changes in connectivity, or effective cross-section, are also evaluated by performing resistivity measurements on the samples after removal of the sheath. Furthermore, the pinning forces of the samples are evaluated from the magnetic measurements.

{\rm B}_{4}{\rm C}

Actually, MgB2 is the lightest superconducting compound. Its connection with lightweight metals like Ti (as barrier) and Al (as outer sheath) would result in a superconducting wire with the minimal mass. However, pure Al is mechanically soft metal to be used in drawn or rolled composite wires, especially if applied for the outer sheath, where it cannot provide the required densification of the boron powder inside. This study reports on a lightweight MgB2 wire sheathed with aluminum stabilized by nano-sized γ-Al2O3 particles (named HITEMAL) and protected against the reaction with magnesium by Ti diffusion barrier. Electrical and mechanical properties of single-core MgB2/Ti/HITEMAL wire made by internal magnesium diffusion (IMD) into boron were studied at low temperatures. It was found that the ultra-lightweight MgB2 wire exhibited high critical current densities and also tolerances to mechanical stress. This predetermines the potential use of such lightweight superconducting wires for aviation and space applications, and for powerful offshore wind generators, where reducing the mass of the system is required.