The mechanical properties of Y-Ba-Cu-O and Gd-Ba-Cu-O/Ag bulk superconductor magnets
Single-grain RE-Ba-Cu-O bulk high temperature superconductors [or (RE)BCO, where RE = rare earth element or yttrium] have demonstrated significant potential for practical applications due to their ability to trap magnetic fields in excess of 17 T, which is an order of magnitude greater than what can be achieved with conventional iron-based permanent magnets. One of the major obstacles to the use of (RE)BCO trapped field magnets is their poor mechanical properties, as bulk samples typically contain a large number of defects, such as pores and micro-cracks. Furthermore, significant electromagnetic stresses develop in bulk superconductors during magnetisation as a result of the Lorentz force, leading frequently to sample failure above around 10 T. Therefore, it is clear that the mechanical properties of bulk (RE)BCO need to be studied comprehensively and improved upon to realise the full potential of this technologically important material. This study first investigated the mechanical strength of YBCO single grains at room temperature by utilising three-point bend and Brazilian tests. This was followed by measurement of the mechanical deformation of GdBCO/Ag single grains in situ, i.e. during high-field magnetisation, to determine the strains and stresses experienced by the samples as a trapped field was established inside them at 64 K. Two techniques for improving the mechanical reliability of (RE)BCO bulk superconductors were subsequently developed. Firstly, samples of YBCO were melt-processed with artificial holes to reduce the defect population and to improve the intrinsic strength of the resultant single grains. As a result, the YBCO sample with artificial holes was able to survive significantly higher magnetisation fields and achieved a surface trapped field of 8.8 T at 30 K without any external reinforcement, which was not possible with the standard YBCO sample. Secondly, a composite structure was proposed, which involved reinforcing GdBCO/Ag single grains with stainless-steel sheets and shrink-fit stainless-steel rings. This preparation technique is also expected to improve the thermal stability of the overall structure. The first composite stack achieved 16.8 T and 17.6 T at 26 K and 22.5 K, respectively, in sequential magnetisation cycles, demonstrating the effectiveness of this reinforcement approach.