Magnetic properties and critical currents of MgB₂

Abstract

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.

Microscopic role of carbon on MgB2 wire for critical current density comparable to NbTi

“…17 Furthermore, it has recently been argued that the upper critical field is related to the resistivity and the transition temperature. 12 The diffusivities listed in the previous paragraph are decreased by doping, or in other words, the impurity scattering rate is increased (see also Supplementary Table SI). For the almost isotropic p bands originating from the p z orbitals of the boron atoms, the reduction is also quite isotropic.…”

“…In order to clarify this, the field dependence of the critical current was analyzed by the percolation model, 12,32 where the loss of connectivity caused by voids and the upper critical field anisotropy are considered together. The model has four fitting parameters: the effective pinning force maximum F m * , the upper critical field B c2 , the anisotropy parameter g and the percolation threshold p c .…”

“…[11][12][13] Increased interband scattering between the p and s bands could also be a cause for the lower transition temperature. 14 Even without any variation in the interband scattering, the intraband scattering by itself can increase the upper critical field far beyond that of Nb 3 Sn, according to a recent two-band dirty-limit theory.…”

“…15 It is also argued from resistivity measurements or from lattice strain estimated from X-ray diffraction that the impurity scattering rate is closely related to the upper critical field enhancement by carbon doping. 12,16,17 The microscopic origin of the increased scattering rate due to doping is not yet clear. Microscopic imperfections, such as porosity, grain boundaries, stacking faults, dislocations, secondary phases and so on, also can act as pinning sites.…”

Influence of disorder on the in-field Jc of MgB2 wires using highly active pyrene

“…A higher sintering temperature is usually necessary for most of the other C sources, leading to crystalline growth and worse pinning. 13 The use of low temperature processing is also crucial as it allows selection of various sensitive sheath materials and the reduction of production costs.…”