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.

A new class of 'powder-in-tube' Mg-B superconducting conductors has been prepared using two different methods: an in situ technique where an Mg + 2B mixture was used as a central conductor core and reacted in situ to form MgB 2 , and an ex situ technique where fully reacted MgB 2 powder was used to fill the metal tube. Conductors were prepared using silver, copper and bimetallic silver/stainless steel tubes. Wires manufactured by the in situ technique, diffusing Mg to B particles experienced ∼25.5% decrease in density from the initial value after cold deformation, due to the phase transformation from Mg + 2(β − B) → MgB 2 all with hexagonal structure.A comparative study of the intergranular current and grain connectivity in wires was conducted by AC susceptibility measurements and direct four point transport measurements. Using a SQUID magnetometer, magnetization versus magnetic field (M-H ) curves of the round wires before and after sintering and reactive diffusion were measured at 5 K and in magnetic fields up to 5 T to define the J cmag . The direct current measurements were performed in self field at 4.2 K. A comparison between zero-field-cooled (ZFC) and field-cooled (FC) susceptibility measurements for sintered Ag/MgB 2 , and reacted Cu/Mg + 2B conductors revealed systematic differences in the flux pinning in the wires which is in very good agreement with direct high transport current measurements.

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.

Investigating and predicting the magnetization of bulk superconducting materials and developing practical magnetizing techniques is crucial to using them as trapped field magnets (TFMs) in engineering applications. The pulsed field magnetization (PFM) technique is considered to be a compact, mobile and relative inexpensive way to magnetize bulk samples, requiring shorter magnetization times (on the order of milliseconds) and a smaller and less complicated magnetization fixture; however, the trapped field produced by PFM is generally much smaller than that of slower zero field cooling (ZFC) or field cooling (FC) techniques, particularly at lower operating temperatures. In this paper, the PFM of two, standard Ag-containing Gd-Ba-Cu-O samples is carried out using two types of magnetizing coils: 1) a solenoid coil, and 2) a split coil, both of which make use of an iron yoke to enhance the trapped magnetic field. It is shown that a significantly higher trapped field can be achieved using a split coil with an iron yoke, and in order to explain these how this arrangement works in detail, numerical simulations using a 2D axisymmetric finite element method based on the H-formulation are carried to qualitatively reproduce and analyse the magnetization process from both electromagnetic and thermal points of view. It is observed that after the pulse peak significantly less flux exits the bulk when the iron core is present, resulting in a higher peak trapped field, as well as more overall trapped flux, after the magnetization process is complete. The results have important implications for practical applications of bulk superconductors as such a split coil arrangement with an iron yoke could be incorporated into the design of a portable, high magnetic field source/magnet to enhance the available magnetic field or in an axial gap-type bulk superconducting electric machine, where iron can be incorporated into the stator windings to 1) improve the trapped field from the magnetization process, and 2) increase the effective air-gap magnetic field.

Single-crystal superconductors of the general formula (LRE)-Ba-Cu-O (light rare earth, LRE = Nd, Sm, Eu and Gd) have considerable potential for engineering applications because of their ability to trap magnetic fields significantly higher than those achievable with permanent magnets. But the lack of a process by which these materials can be fabricated reliably and economically in the form of large single grains has severely hindered their development. We report a practical processing method for the fabrication in air of single-crystal (RE)BCO. The technique is economical and offers considerable freedom in terms of the processing parameters and reproducibility in growth of oriented single grains. The process is based primarily on the development of a new type of generic seed crystal that can effectively promote the epitaxial nucleation of any (RE)BCO system, and secondly on suppressing the formation of RE-Ba solid solution in a controlled manner within large grains processed in air.