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

Abstract

Increasing dissipation-free supercurrent has been the primary issue for practical application of superconducting wires. For magnesium diboride, MgB 2 , carbon is known to be the most effective dopant to enhance high-field properties. However, the critical role of carbon remains elusive, and also low-field critical current density has not been improved. Here, we have undertaken malic acid doping of MgB 2 and find that the microscopic origin for the enhancement of high-field properties is due to boron vacancies and associated stacking faults, as observed by high-resolution transmission electron microscopy and electron energy loss spectroscopy. The carbon from the malic acid almost uniformly encapsulates boron, preventing boron agglomeration and reducing porosity, as observed by three-dimensional X-ray tomography. The critical current density either exceeds or matches that of niobium titanium at 4.2 K. Our findings provide atomic-level insights, which could pave the way to further enhancement of the critical current density of MgB 2 up to the theoretical limit.

Structural Characteristic and Superconducting Performance of MgB2 Fabricated by Mg Diffusion Process

“…This is because the C replacement effect accumulates in the B layer, and dislocations and its associated stacking faults may form when the accumulated strain exceeds the critical value. The existence of lattice defects in C-doped MgB 2 samples has been proven by experimental results (Kazakov et al, 2005;Avdeev et al, 2003;Zhu et al, 2007;Kim et al, 2012). Specifically, the considerable increase in the atomic displacement parameter at the Mg and B sites in C-doped single crystals reflects the enhancement of lattice disorder (Kazakov et al, 2005).…”

Significant enhancement of critical current density by effective carbon-doping in MgB2thin films

“…The flow rates were 100 sccm for the H 2 carrier gas and 50 sccm for the B 2 H 6 /H 2 mixture. CH 4 gas of different flow rates was added to the carrier gas Significant enhancement of critical current density by effective carbon-doping in MgB 2 thin films to dope the film with different carbon concentrations. All the carbon-doped MgB 2 films were fabricated at a substrate temperature of 650 o C. CH 4 was decomposed by the hot filament at 900 o C. A pure MgB 2 film was also prepared under the same conditions for comparison.…”

“…Therefore the improvement of J c under magnetic field is indispensable for the development of MgB 2 material for magnet applications. The substitution of carbon atoms into the boron sites of MgB 2 are known to be the most effective way to improve pinning properties in MgB 2 and hence J c performance under high magnetic fields [4]. The current-carrying performance has been improved by carbon in all the form of MgB 2 superconductors, such as polycrystalline bulk, single crystals, wires, tapes, filaments, fibers and thin films.…”

Magnesium diboride(MgB2) wires for applications

“…In particular, critical current density, J c , has not been increased and was even lower than that of NbTi. Recently, enhancement of the J c and the upper critical field, B c2 , has been reported for MgB 2 wires doped with malic acid additive [1]. The chemical solution method is an economical route for industrial fabrication.…”