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The trapped field in Y–Ba–Cu–O (YBCO) bulk single grain superconductors correlates directly with the critical current density J c, which, in turn, is dependent on the microstructure of the bulk material. It has been shown recently and indirectly that porosity influences J c of these technologically important materials, in addition to the presence of well-researched Y2BaCuO5 (Y-211) particles in the bulk sample. In this work, we report the direct impact of porosity on the critical current density of a single grain YBCO bulk superconductor using 3D x-ray computer tomography scans and superconducting quantum interference device measurements. It is concluded that porosity has a considerably more substantial impact on the measured critical current density than Y-211 on the micrometre scale with, predictably, a decrease in porosity leading to an increase in J c. J c is directly proportional to the trapped field, so any method that can reduce porosity, therefore, improves J c, and, subsequently, the trapped field of these technologically important bulk superconductors.

It is of great significance to enhance the mechanical properties in single grain YBCO bulk superconductors, as mechanical failure limits the suitability of these materials for high-field applications. The influence of the microstructure on the mechanical properties of YBCO bulk materials was studied in 11 partially oxygenated YBCO, 11 oxygenated YBCO, and 10 oxygenated YBCO(Ag) melt-processed bulk single grains in a statistically significant way using Brazilian testing. Surprisingly, the oxygenation cracks evolving during the tetragonal to orthorhombic phase transition do not influence the average tensile strength of YBCO bulk single grains, with compaction cracks and pores as the main microstructural flaws responsible for mechanical failure. The mechanical properties of bulk YBCO single grains even improve following oxygenation, as indicated by an increase in Weibull modulus. An addition of 10 wt% Ag₂O to YBCO increases the average tensile strength and the Weibull modulus further, making the material mechanically more robust. However, an overlap in the measured tensile strengths of oxygenated YBCO and oxygenated YBCO(Ag) bulk materials reveals that not all silver-containing samples have better mechanical properties compared to oxygenated YBCO bulk samples.

It is necessary to fabricate (RE)BCO bulk high temperature superconductors in the form of individual single grains in order to maximise the length scale over which current flows, and hence the trapped magnetic field. However, inherent difficulties in the grain growth process place limitations on the diameter and height of the single grain that may be achieved by existing melt processes. A practical approach to increase the height of the sample and the trapped field at its surface is by assembling individual single grains in a stack formation with their ab planes aligned parallel, primarily to avoid the expensive process of fabricating large, individual monoliths. The trapped fields observed at the top and bottom surfaces of a single grain sample are frequently different since both the superconducting and physical properties of single grain (RE)BCO samples are generally non-uniform. This leads to challenges in determining how to spatially arrange stacks of single grain samples to generate the largest and most uniform trapped field overall. In this study, we report the optimisation of two-stack configurations involving a total of 8 individual GdBCO/Ag single grains. The samples were arranged in four pairs and configured with different surfaces in contact in the assembly of the stack. The primary superconducting properties for trapped field and total flux distributions were measured at 77 K and compared for each stack arrangement. The initial results indicate that surfaces with inferior flux trapping properties (measured in terms of the overall trapped field value) of a two-sample stack should be positioned at the middle of the assembly to achieve the best overall trapped field and higher total flux at the external, and therefore, usable surface of the stack sample. A numerical modelling method that incorporates different Jc-B characteristics for the top and bottom layers of a single grain to take account of the variability in physical properties and spatial non-uniformity confirmed the optimised experimental arrangement of the stacked bulk samples. Furthermore, the optimisation of single grains of ring geometry to achieve a longer and wider uniform magnetic field zone inside the bore was also performed.

The intrinsic mechanical properties of single‐grain RE‐Ba‐Cu‐O bulk high‐temperature superconductors can be improved by employing a thin‐wall geometry. This is where the samples are melt‐processed with a predefined network of artificial holes to decrease the effective wall thickness. In this study, the tensile strengths of thin‐wall YBCO disks were determined using the Brazilian test at room temperature. Compared with conventional single grain YBCO disks, the thin‐wall YBCO disks displayed an average tensile strength that is 93% higher when the holes were filled with Stycast epoxy resin. This implies a thin‐wall sample should, in theory, be able to sustain a trapped field that is 39% higher without exceeding the mechanical limit of the sample. High‐field 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 thin‐wall 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 field‐cooling the thin‐wall sample with 11 T at 30 K, a surface field of 8.8 T was trapped successfully without requiring any external ring reinforcement.

The magnitude of the maximum trapped magnetic field in a bulk, single-grain superconductor is a key performance figure of merit. This is determined, generally, by the magnitude of the critical current density, J c , and the length scale over which it flows. As with all type-II superconductors, J c is related closely to the microstructure of the superconducting material and, in the case of RE–Ba–Cu–O [(RE)BCO, where RE is a rare-earth element or yttrium] single grains, RE2BaCuO5 (RE-211) inclusions in the superconducting REBa2Cu3O7−δ (RE-123) phase matrix are key microstructural features that act effectively as flux pinning centres. Although the distribution of RE-211 in single-grain bulk superconductors has been studied extensively, the variation of J c within a given sample has been much investigated much less thoroughly. A detailed experimental understanding of the variation of J c in these technologically important materials, therefore, is required given the growing popularity and significance of numerical techniques for modelling the behaviour of type-II bulk superconductors. Here we report a systematic investigation of the correlation between Gd-211 particle density and sample porosity, which are microstructural features, and T c and J c in a Gd–Ba–Cu–O bulk, single grain fabricated using a buffer layer and a supply of additional liquid phase. This was performed by cutting the sample into numerous sub-specimens of approximate dimensions 1.8 × 2.8 × 1.5 mm3. We observe that J c decreases with distance from the seed, although more strongly with distance along the c-axis than along the a–b plane. In contrast to what might be expected given the assumed contribution of RE-211 inclusions to flux pinning, we find no evidence of a clear correlation between the local RE-211 precipitate density and local critical current on a length scale of mm. We observe that the porosity of the sample is a more dominant factor in determining the distribution of J c within a single grain.

The superconducting properties, melting temperatures and crystal growth rates of single grain, RE–Ba–Cu–O [(RE)BCO] bulk superconductors (where RE = a rare earth element or yttrium) decrease with the RE-element sequence of Nd, Sm, Eu, Gd, Dy and Y. The mechanical properties of these technologically important materials, on the other hand, however, improve in the same sequence. Consequently, one promising approach for optimising the balance between mechanical and superconducting properties of bulk (RE)BCO superconductors, or for adjusting growth rate, is the use of combinations of different rare earth elements. In this study, we explore combinations of Gd and Y in the formation of (Gd–Y)–Ba–Cu–O single grains. We describe the optimisation of the growth process for this multi-RE element system and use optical and scanning electron microscopy to study the microstructure of both non-superconducting (Gd–Y)2BaCuO5 [(Y–Gd)-211] phase inclusions and the (Y–Gd)Ba2Cu3O7-δ [(Y–Gd)-123] phase matrix itself. We demonstrate that (Gd–Y)–Ba–Cu–O single grains can be fabricated reliably and that they exhibit reasonably good superconducting properties. We observe that there is an increase in RE-211 particle size in this mixed rare earth system, which, ultimately, limits sample performance, and conclude that this may be a general disadvantage of this approach to the synthesis of single grains for high field engineering applications.

Bulk (RE)BCO, where RE is a rare-earth element or yttrium, superconductors fabricated in the form of rings are potentially useful for a variety of solenoidal-type applications, such as small, high field nuclear magnetic resonance and electromagnetic undulators. It is anticipated that the practical exploitation of these technologically important materials will involve pulse field magnetization (PFM) and, consequently, it is important to understand the behavior of ring-shaped samples subjected to the PFM process. Macroscopic flux jumps were observed in PFM experiments on ring-shaped bulk samples when the peak applied field reaches a threshold magnitude, similar to behavior reported previously in cylindrical samples. Magnetic flux jumps inward when the thermal instability is triggered, however it subsequently flows outwards from the sample, resulting in a relatively low trapped field. This behavior is attributed to a variety of effects, including the inhomogeneity of the material, which may lead to the formation of localized hot spots during the PFM process. In order to further elucidate this phenomena, the properties of a structure consisting of a bulk superconducting ring with a cylindrical superconductor core were studied. We observe that, although a flux jump occurs consistently in the ring, a critical state is established at the boundary of the ring-shaped sample and the core. We provide a detailed account of these experimental observations and provide an explanation in terms of the current understanding of the PFM process.

The widespread use of melt processed, single grain (RE)–Ba–Cu–O 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 Ba–Cu–O (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.