We report on a combined experimental and modelling approach towards the design and fabrication of efficient bulk shields for low-frequency magnetic fields. To this aim, MgB2 is a promising material when its growing technique allows the fabrication of suitably shaped products and a realistic numerical modelling can be exploited to guide the shield design. Here, we report the shielding properties of an MgB2 tube grown by a novel technique that produces fully machinable bulks, which can match specific shape requirements. Despite a height/radius aspect ratio of only 1.75, shielding factors higher than 175 and 55 were measured at temperature T = 20 K and in axially-applied magnetic fields μ0Happl = 0.1 and 1.0 T, respectively, by means of cryogenic Hall probes placed on the tube’s axis. The magnetic behaviour of the superconductor was then modelled as follows: first we used a two-step procedure to reconstruct the macroscopic critical current density dependence on magnetic field, Jc(B), at different temperatures from the local magnetic induction cycles measured by the Hall probes. Next, using these Jc(B) characteristics, by means of finite-element calculations we reproduced the experimental cycles remarkably well at all the investigated temperatures and positions along the tube’s axis. Finally, this validated model was exploited to study the influence both of the tube’s wall thickness and of a cap addition on the shield performance. In the latter case, assuming the working temperature of 25 K, shielding factors of 105 and 104 are predicted in axial applied fields μ0Happl = 0.1 and 1.0 T, respectively.
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.
High density (above 93%) superconducting bulks of MgB 2 with addition of hexagonal BN (h-BN) and cubic BN (c-BN) with compositions ((MgB 2 ) + (BN) x , x = 0.01, 0.03, 0.05) were obtained by ex situ spark plasma sintering. All the investigated samples have the critical temperature T c = 38.8 K. The variation of the critical current density J c with the external magnetic field H for h-BN added sample is almost overlapping the J c (H) dependence for the pristine MgB 2 sample. On the other hand, J c for the samples added with c-BN is larger at high magnetic fields, while the decrease of J c at low H is very small. At T = 20 K, a J c of 10 2 A cm −2 is determined for the sample with x c-BN = 0.005 at H = 58 kOe, and for the sample with x c-BN = 0.01 at 54 kOe. Magnetic relaxation measurements indicate a significant flux pinning enhancement in MgB 2 samples added with c-BN. It is proposed that the disorder at the interface caused by the convenient lattice matching relationship between the lateral plane of the MgB 2 crystal prism and the face of the c-BN crystal cube is responsible for the observed vortex pinning increase.
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