High Trapped Fields in C-doped MgB2 Bulk Superconductors Fabricated by Infiltration and Growth Process

A G Bhagurkar,D A Cardwell,T A Aljohani,A R Dennis,L Wang,A Yamamoto,J H Durrell,M Xia,N H Babu

doi:10.1038/s41598-018-31416-3

A G Bhagurkar, D A Cardwell + Show 7 more

Open Access

https://doi.org/10.1038/s41598-018-31416-3

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Abstract

The grain boundaries in superconducting MgB2 are known to form effective magnetic flux pinning sites and, consequently, bulk MgB2 containing a fine-grain microstructure fabricated from nanoscale Mg and B precursor powders exhibits good magnetic field-trapping performance below 20 K. We report here that the trapped field of MgB2 bulk superconductors fabricated by an infiltration and growth process to yield a dense, pore-free microstructure, can be enhanced significantly by carbon-doping, which increases intra-band scattering within the superconducting grains. A maximum trapped field of 4.15 T has been measured at 7.5 K at the centre of a five-sample stack of Mg(B1−xiCxi)2 bulk superconductors processed by infiltration and growth, which not only represents a ~40% increase in trapped field observed compared to undoped bulk MgB2, but also is the highest trapped field reported to date in MgB2 samples processed under ambient pressure. The trapped field is observed to decay at a rate of <2%/day at 10 K, which suggests that bulk MgB2 superconductors fabricated using the infiltration and growth technique can be used potentially to generate stable, high magnetic fields for a variety of engineering applications.

Highlights

A bulk superconductor can act effectively as a quasi-permanent magnet when magnetized by an applied magnetic field below its superconducting transition temperature, Tc
In order to realize the advantages offered by C-doping on a larger scale, Mg channel free MgB2 bulk superconductors were fabricated by combining C-doping and a Modified Precursor Infiltration and Growth (MPIG) technique, as described in ref.[40]
The MPIG samples, 5% B4C (MPIG) and 10% B4C (MPIG), exhibited a second transition at 34.8 K and 33.8 K respectively, below the onset Tc (37.8 K). Such high Tc onset of 37.8 K in MPIG samples suggests that C-doping has not taken place in the pre-reacted MgB2

Summary

Introduction

A bulk superconductor can act effectively as a quasi-permanent magnet when magnetized by an applied magnetic field below its superconducting transition temperature, Tc. The key advantages of this technique were demonstrated subsequently by Giunchi et al (RLI) for the fabrication of state-of-the art bulk MgB2 artefacts[12,13] Such a relatively simple, ambient pressure process results in the formation of hard, dense structures but can be applied to fabricate complex geometries that are not achieved using conventional sintering techniques. Jc in self-field as high as 106 A/cm[2] has been measured in bulk samples, the performance of MgB2 tends to drop-off rapidly with applied magnetic field[16] It is, essential that the in-field performance of bulk MgB2 is enhanced if Nb based low temperature superconducting materials used widely in existing practical applications and which typically require expensive liquid helium (LHe) as a coolant during operation, are to be replaced. Trapped field measurements were performed on the doped bulk samples and the results discussed

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