Low temperature preparation of MgB 2 tapes using mechanically alloyed powder

297

294

This review focuses on the superconducting properties of MgB2 that are relevant for power applications. The reversible mixed state parameters are the most important, since they define the limiting conditions for loss-free currents: the transition temperature, the upper critical field and the depairing current. They also determine the flux pinning energy, the pinning force and the elastic properties of the flux line lattice and, therefore, strongly influence the critical current densities. The magnetic properties of magnesium diboride are anisotropic and influenced by the two different energy gaps of the σ- and π-bands. Whereas the transition temperature could not be enhanced significantly during the past five years, the upper critical field was considerably increased by impurity scattering or doping. Flux pinning is very weak in MgB2 single crystals and was only improved by irradiation techniques so far. In polycrystalline samples, grain boundary pinning seems to play the dominant role. High critical currents close to the theoretical limit were found in c-axis oriented thin films. The anisotropy of the upper critical field strongly reduces the critical currents in untextured MgB2 at high magnetic fields, where the supercurrents become highly percolative, since not all grains are superconducting anymore. The performance of polycrystalline wires and tapes was significantly improved during the past few years by increasing the upper critical field and by reducing its anisotropy. Pinning seems to be nearly optimized in many forms of this material, but the connectivity between the grains might be further improved.

Section: Influence Of Disordermentioning

confidence: 99%

Magnetic properties and critical currents of MgB₂

Eisterer

2007

297

294

“…There are several technological processes which can be applied for the manufacture of composite MgB 2 wires [1][2][3][4][5]. The commonly used powder-in-tube (PIT) technique has several approaches differing in the initial precursor powder used: (i) in situ (Mg + B) [1], (ii) ex situ (MgB 2 ) [2], (iii) mechanical alloying (Mg + B + MgB 2 ) [3] and (iv) MgB 4 + Mg [4]. Giunchi et al have introduced the socalled 'magnesium liquid infiltration' process using B powder surrounding the central metallic Mg rod [5].…”

Section: Introductionmentioning

confidence: 99%

Critical currents,I_c-anisotropy and stress tolerance of MgB₂wires made by internal magnesium diffusion

Kòváč¹,

Hušek²,

Melíšek³

et al. 2014

Single-and four-core MgB 2 wires have been manufactured by internal magnesium diffusion (IMD) into boron. Initial hydrostatic extrusion was applied for single-core Cu sheathed wire and mostly rolling deformation for assembled four-core wires with Inconel or GlidCop sheath. The highest critical current density (J c ) was measured for the wire with boron densified by cold isostatic pressing by 1.9 GPa. Rolled tapes have exhibited critical current anisotropy increasing with the tape's aspect ratio. Low critical current degradation of IMD wires by twisting has been observed, which can be utilized for effective reduction of AC losses. High engineering current densities of IMD wires are promising for the generation of magnetic fields between 1 and 3 T at cryogen-free conditions, which can be utilized for future MRI systems or wind power generators.

“…The resulting nanocrystalline powder consists of MgB 2 , Mg, B and MgO. This precursor powder is highly reactive and can be further processed to tapes by the powder-in-tube technique (PIT) at relatively low temperatures (500-650 • C) [9,10].…”

Section: Introductionmentioning

confidence: 99%

Improvement of current density by texture andI_canisotropy in thin filament MgB₂/Fe tapes

Kòváč¹,

Hušek²,

Melíšek³

et al. 2006

Thin filament MgB 2 /Fe tapes have been made by the powder-in-tube technique using a precursor powder prepared by mechanical alloying and deformed by two-axial rolling. Increasing critical current density (J c ) in a parallel external magnetic field has been measured for a reduced filament thickness up to 18.5 µm; this could be effective for future practical fine-filamentary MgB 2 conductors. This increase in J c is attributed to an apparent improvement in texture introduced by rolling. Further increase in J c with reduction of filament size is limited due to chemical reaction at the filament/Fe interface during the final heat treatment at 600 • C for 3 h. On the other hand, improved texture leads to a strong critical current anisotropy, which increases with the reduced filament thickness and temperature. High I c anisotropy can lead to an apparent reduction of the total magnet current due to the effect of the radial field component, as in the case of high temperature superconducting coils.