7 research outputs found
Recommended from our members
Characterisation of the mechanical failure and fracture mechanisms of single grain YâBaâCuâO bulk superconductors
Abstract: The widespread use of melt processed, single grain (RE)âBaâCuâO bulk superconductors [(RE)BCO], where RE = Y, Gd or Sm, is limited predominantly by the poor mechanical properties of these inherently brittle, ceramic-like materials. The high density of flaws, such as cracks and voids, within the single grain microstructure leads directly to a low fracture toughness. As a result, the Lorentz forces, generated when these materials carry current in the presence of a large magnetic field, create stresses sufficiently large to cause brittle failure. The addition of BaâCuâO (liquid phase) and Ag to the precursor composition prior to melt-growth has been demonstrated to be effective in improving the mechanical properties of these technologically important materials. In this work, we characterise the mechanical failure of single grain YBCO bulk superconductors in terms of a Weibull statistical distribution. In addition, differences in fracture mechanisms have been studied to provide a better understanding of how the provision of additional liquid phase and silver produces YBCO single grains with better resulting mechanical properties and how these can be improved further
Recommended from our members
Improved trapped field performance of single grain YâBaâCuâO bulk superconductors containing artificial holes
Abstract: The intrinsic mechanical properties of singleâgrain REâBaâCuâO bulk highâtemperature superconductors can be improved by employing a thinâwall geometry. This is where the samples are meltâprocessed with a predefined network of artificial holes to decrease the effective wall thickness. In this study, the tensile strengths of thinâwall YBCO disks were determined using the Brazilian test at room temperature. Compared with conventional single grain YBCO disks, the thinâwall YBCO disks displayed an average tensile strength that is 93% higher when the holes were filled with Stycast epoxy resin. This implies a thinâwall sample should, in theory, be able to sustain a trapped field that is 39% higher without exceeding the mechanical limit of the sample. Highâfield magnetization experiments were performed by applying magnetization fields of up to 11.5 T, specifically to break the samples in order to verify the effect of increased mechanical strength (and improved cooling) on the ability of bulk (RE)BCO to trap field successfully. The standard YBCO sample failed when it was magnetized with a field of 10 T at 35 K, suffering permanent damage. As a result, the standard sample could only trap a maximum surface field of 7.6 T without failure. On the other hand, the thinâwall YBCO sample survived all magnetization cycles, including a maximum magnetization field of 11.5 T at 35 K, demonstrating a greater intrinsic ability to withstand significantly higher electromagnetic stresses. By subsequently fieldâcooling the thinâwall sample with 11 T at 30 K, a surface field of 8.8 T was trapped successfully without requiring any external ring reinforcement