The ability of large grain, REBa2Cu3O7- [(RE)BCO; RE = rare earth] bulk superconductors to trap magnetic field is determined by their critical current. With high trapped fields, however, bulk samples are subject to a relatively large Lorentz force, and their performance is limited primarily by their tensile strength. Consequently, sample reinforcement is the key to performance improvement in these technologically important materials. In this work, we report a trapped field of 17.6 T, the largest reported to date, in a stack of two, silver-doped GdBCO superconducting bulk samples, each of diameter 25 mm, fabricated by top-seeded melt growth (TSMG) and reinforced with shrink-fit stainless steel. This sample preparation technique has the advantage of being relatively straightforward and inexpensive to implement and offers the prospect of easy access to portable, high magnetic fields without any requirement for a sustaining current source.
The ability to generate a permanent, stable magnetic field unsupported by an electromotive force is fundamental to a variety of engineering applications. Bulk high temperature superconducting (HTS) materials can trap magnetic fields of magnitude over ten times higher than the maximum field produced by conventional magnets, which is limited practically to rather less than 2 T. In this paper, two large c-axis oriented, single-grain YBCO and GdBCO bulk superconductors are magnetised by the pulsed field magnetisation (PFM) technique at temperatures of 40 and 65 K and the characteristics of the resulting trapped field profile are investigated with a view of magnetising such samples as trapped field magnets (TFMs) in-situ inside a trapped flux-type superconducting electric machine. A comparison is made between the temperatures at which the pulsed magnetic field is applied and the results have strong implications for the optimum operating temperature for TFMs in trapped fluxtype superconducting electric machines. The effects of inhomogeneities, which occur during the growth process of single-grain bulk superconductors, on the trapped field and maximum temperature rise in the sample are modelled numerically using a 3D finite-element model based on the H-formulation and implemented in Comsol Multiphysics 4.3a. The results agree qualitatively with the observed experimental results, in that inhomogeneities act to distort the trapped field profile and reduce the magnitude of the trapped field due to localised heating within the sample and preferential movement and pinning of flux lines around the growth section regions (GSRs) and growth sector boundaries (GSBs), respectively. The modelling framework will allow further investigation of various inhomogeneities that arise during the processing of (RE)BCO bulk superconductors, including inhomogeneous J c distributions and the presence of current-limiting grain boundaries and cracks, and it can be used to assist optimisation of processing and PFM techniques for practical bulk superconductor applications.
Progress in superconducting bulk materials has been somewhat overshadowed by the considerable effort required to produce practical long-length conductors. There has, however, been steady progress in both the materials science of bulk superconducting materials and the technologies required to use them effectively in engineering applications. In particular, magnetised bulk superconductors are capable of acting as quasi-permanent magnets with the potential of providing magnetic fields of several tesla or greater from a small volume of material, they can act as magnetic shields and they can provide self-stabilised levitation. This roadmap, based on a workshop which involved the participation of a wide range of academic and industrial participants (see doi: 10.17863/CAM.586 for details of the workshop methodology), aims to explore some of the key potential domains of application of bulk superconductors. Detailed technological roadmaps are presented for four key applications that were identified as providing both good market opportunity and feasibility. These are: portable systems for bulk superconductivity; portable, high-field magnet systems for medical devices; ultra-light superconducting rotating machines for next-generation transport & power applications; and magnetic shielding applications for electric machines, equipment and other high-field devices.
Strong evidence for high intergranular critical current densities and large bulk magnetic flux pinning in superconducting polycrystalline MgB 2 has been observed. The presence of strongly-coupled grain boundaries in this material has been confirmed by a dramatic collapse of the magnetic hysteresis loop when a bulk specimen is ground into a fine powder and re-measured under similar conditions. Further evidence for strong intergrain links in polycrystalline MgB 2 is provided by the continuous variation of the remanent magnetic moment up to the full penetration field of a bulk sample. The absence of weak-link nature in this material has profound implications for its potential in a wide range of engineering applications.
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