Design and characterisation of ex situ bulk MgB₂ superconductors containing a nanoscale dispersion of artificial pinning centres

Abstract

MgB2 pellets containing a nanoscale dispersion of artificial pinning centres have been successfully manufactured through a powder metallurgy route based on the oxide dispersion strengthened (ODS) concept more usually used for steels and superalloys. Commercial MgB2 powder and Y2O3 nano-powder were mechanically alloyed in a high energy planetary ball mill and consolidated using the field assisted sintering technique. The composite powders were ball milled for different times up to 12 h and characterised by means of particle size analysis, x-ray diffraction (XRD) and scanning transmission electron microscopy (STEM). The microstructure and superconducting properties were characterised by density, XRD, STEM and magnetic property measurements. The powder microstructure comprised Y2O3 particles dissolved into the MgB2 matrix. After consolidation there was a near-uniform dispersion of precipitated YB4 and MgO particles. A bulk 0.5 wt% Y2O3-MgB2 composite showed the best superconducting performance with a significant improvement in J c at high field compared with unmodified MgB2, and only a small reduction in T c . The results suggest that the ODS concept is promising to improve the superconducting properties of MgB2.

Flux pinning and microstructure of a bulk MgB2 doped with diverse additives

“…The sample doped with Dy 2 O 3 was the second best, followed by the sample with MgB 4 . All these results are within the relatively narrow range of 1-6 × 10 5 A cm −2 , where high-quality self-field J c data are published in the literature [14,16,17,24,25,34]. The combined electromagnetic and microstructure analyses indicate an interesting combination of grain-boundary pinning with core pinning induced by normal secondary phase precipitates segregated at the MgB 2 grain boundaries.…”

Effect of the sintering temperature on the microstructure and superconducting properties of MgB2 bulks manufactured by the field assisted sintering technique

“…All the consolidated samples showed a higher MgO fraction which suggests that new MgO particles were formed during sintering. This was also observed by Matthews et al [17] in composite MgB2 bulks manufactured by FAST.…”

Dense MgB2 Ceramics by Ultrahigh Pressure Field-Assisted Sintering

“…Figure 4 shows the temperature and pressure dependence of MgB 2 resistivity. The superconducting properties of MgB 2 are highly dependent on the density of the ceramic processed as well as on the grain boundaries acting as pinning sites [19,20] for large magnetic moments.…”

“…Figure 4 shows the temperature and pressure dependence of MgB2 resistivity. The superconducting properties of MgB2 are highly dependent on the density of the ceramic processed as well as on the grain boundaries acting as pinning sites [19,20] for large magnetic moments. One would expect that, provided the materials were the same, dense samples would appear to have lower resistivity than porous samples, as the resistivity was calculated from the resistance and sample dimensions reported by those who made samples at lower pressures, and the effective crosssectional area was smaller than the measured one (because the material does not fill the whole space).…”

“…Figure 4a shows the benefit of identical temperature for 3 and 5 GPa where a threshold of resistivity appeared from 950 • C. The benefit of pressure in Figure 4b at 1.7 GPa probably derived from the high-density ceramics that were reached in this range of pressure. Resistivity values decreased systematically with increasing processing temperature, suggesting the MgB 2 lattice contains fewer defects and likely results in the superconductor having no parasitic phases [20,21]. Up to now, the majority of dense samples have been made by high-temperature and high-pressure (HT-HP) processes.…”