High magnetic shielding properties of an MgB₂ cup obtained by machining a spark-plasma-sintered bulk cylinder

Abstract

Superconductors are key materials for shielding quasi-static magnetic fields. In this work, we investigated the shielding properties of an MgB 2 cup-shaped shield with small aspect-ratio of height/outer radius. Shape and aspect-ratio were chosen in order to address practical requirements of both high shielding factors (SFs) and space-saving solutions. To obtain large critical current densities (J c ), which are crucial for achieving high magnetic-mitigation performance, a highpurity starting MgB 2 powder was selected. Then, processing of the starting MgB 2 powder into high density bulks was performed by spark plasma sintering. The as-obtained material is fully machinable and was shaped into a cup-shield. Assessment of the material by scaling of the pinning force showed a non-trivial pinning behaviour. The MgB 2 powder selection was decisive in enlarging the range of external fields where efficient shielding occurs. The shield's properties were measured in both axial-and transverse-field configurations using Hall probes. Despite a height/outer radius aspect ratio of 2.2, shielding factors higher than 10 4 at T=20 K up to a threshold field of 1.8 T were measured in axial-field geometry at a distance of 1 mm from the closed extremity of the cup, while SFs>10 2 occurred in the inner half of the cup. As expected, this threshold field decreased with increased temperature, but SFs still exceeding the above mentioned values were found up to 0.35 T at 35 K. The shield's shape limits the SF values achievable in transverse-field configuration. Nevertheless, the in-field J c of the sample supported SFs over 40 at T=20 K up to a field of 0.8 T, 1 mm away from the cup closure.

On the pinning force in high density MgB2 samples

“…The details of raw materials, additives, preparation conditions, as well as the structure, microstructure and physical properties of the samples are presented in the references attached to each sample. The samples selected for analysis are: ( i ) pure MgB 2 16 ; ( ii ) (MgB 2 ) 0.99 (Te 0.25 (HoO 1.5 ) 0.75 ) 0.01 17 ; ( iii ) (MgB 2 ) 0.99 (B 4 C) 0.01 18 ; ( iv ) weakly oriented MgB 2 (orientation degree ~ 21%) 19 ; ( v ) highly oriented MgB 2 (orientation degree ~ 40.5%) 20 . The partial c -axis orientation was induced in the green compacts of the samples ( iv ) and ( v ) by field assisted slip casting (FASC) under a high magnetic field of 12 T. The subsequent SPS procedure enhanced the orientation.…”

Modelling and Performance Analysis of MgB2 and Hybrid Magnetic Shields

“…Equation ( 1) ensures the collinearity between the current density and the electric field, expected in superconductors with isotropic properties, as the polycrystalline MgB 2 bulks that we characterized are assumed to be [49]. This equation slightly differs from that reported in Ref.…”

“…To this aim, we considered the critical current density values achieved at T = 30 K from magnetic flux density measurements carried out on the axis of tube-and cup-shaped shields while the applied field was cycling [48,54]. These shields were obtained by carving fully machinable MgB 2 cylinders, fabricated by spark plasma sintering of BN-added MgB 2 powders [48,49]. We chose the working temperature of 30 K because it guarantees a negligible flux-jump occurrence, thus allowing the J c calculation [49], joined with still noteworthy shielding properties (by way of example, the cup shielding performances are still comparable with those found at 20 K on a YBa 2 Cu 3 O 7 cup with similar aspect ratio [8]).…”

“…A cap addition with a superconducting joint proved to significantly enhance the shielding ability of a short tube [11,48]. However, also in this case, the efficiency of the device results remarkably reduced when a field tilted with respect to the shield axis is applied [49,52]. Therefore, an analogous study on the effect of the superimposition of an FM shell was carried out in the case of cup-shaped shields.…”

“…The material choice was driven by the MgB 2 specific features, such as the low-cost, the use of non-toxic precursors (e.g., not containing rare earth elements), the low weight density, and the long coherence length. In particular, the last characteristic implies that boundaries among well connected grains do not prevent the flow of high current densities, thus opening to the employment of large polycrystalline manufacts-fabricated by in situ or ex situ sintering processes [45][46][47][48][49] or by infiltration processes [9,50,51]-for large scale applications.…”