Search

Bulk, single grain RE–Ba–Cu–O (where RE = rare earth or yttrium) [(RE)BCO] high temperature superconductors could potentially be used to generate stable magnetic fields for magnetic resonance imaging (MRI) and nuclear magnetic resonance (NMR). In these applications, however, the homogeneity of the magnetic field is of critical importance. As a result, the spatial distribution of critical current density, J c, within the bulk single grain and the effects of the magnetisation process, which are primary drivers of the uniformity of the achievable trapped magnetic field, are fundamental to assessing the performance of these technologically important materials. This paper reports the systematic measurement of the distribution of J c–B at 77 K over a vertical cross-section of a single grain along a facet line and through the seed crystal [(110)-F] at 20 positions within a 20 mm diameter Gd–Ba–Cu–O sample in an attempt to understand and assess the distribution of J c along this microstructural feature. A comparison of the data within the whole vertical plane across the seed measured along the a or b direction within the [(100)-a] plane shows that J c–B at 77 K at the facet line is more than 10% higher for applied fields between 0.2 T and 2.5 T. The effect of the J c–B relationship of the facet line on the overall trapped field measured in an individual bulk sample was investigated by measuring the magnitudes of trapped fields and their contour maps for sections cut from four single grain samples of GdBCO–Ag at different sizes and shapes parallel to the ab-plane from the top to the bottom of the bulk sample. Based on the results reported here, we demonstrate a method to achieve more uniform trapped fields through an optimal arrangement of an assembly of sections of individual GdBCO single grains.

Funder: Engineering and Physical Sciences Research Council; doi: http://dx.doi.org/10.13039/501100000266, Funder: King Abdulaziz City for Science and Technology; doi: http://dx.doi.org/10.13039/501100004919, Bulk (RE)-Ba-Cu-O [(RE)BCO] cuprate HTS have been developed steadily towards a wide range of sustainable engineering and technological applications since their discovery in 1986 based primarily on their unique potential to trap very large magnetic fields (>5 T) at temperatures that are accessible potentially by thermo-electric cooling techniques. Trapped fields of ∼10 T at the surface of individual (RE)BCO bulk single grains and in excess of 17 T in a reinforced two-sample stack are now being achieved reliably. This paper reviews the current state of the art of the processing of large, single grain (RE)BCO bulk superconductors required to trap fields of this magnitude, and specifically via two advanced fabrication approaches; the traditional TSMG process and the more recently developed TSIG technique. The focus of the review is on optimising the critical processing parameters to achieve high-quality, high performance single grain (RE)BCO bulk superconductors specifically for high-field applications. The review also summarises recent advances in processing, such as the integration of the so-called buffer technique into the TSMG and TSIG processing methodologies to achieve improved reliability in single grain growth with a success rate exceeding 90%, the development of a Mg-doped NdBCO generic seed crystal for the successful growth of all rare-earth and light-rare earth based bulk superconductors [(RE)BCO and (LRE)BCO] and the introduction of nano-size stable, non-superconducting phase(s) to the bulk microstructure to improve the intrinsic flux pinning strength of the material, and hence trapped magnetic field. Details of the two-step buffer-aided TSIG technique developed recently that yields dense, near-net shaped, high performance (RE)BCO bulk superconductors with improved superconducting and mechanical properties are also presented. Suitable sample-seed configurations for effective multi-seeding are discussed, which enables the production of high aspect ratio, bar-shaped (RE)BCO quasi-single grains that exhibit improved levitation forces required in Maglev-based applications, for example, are discussed. The electrical, mechanical, microstructural and magnetic properties (including those achieved from a pulsed-field magnetisation approach) of the different (RE)BCO systems are presented and the relevant correlation in properties and performance highlighted, accordingly. Finally, a brief summary of existing applications and prospects for near-future exploitation of these remarkable, technologically important materials, and particularly in the medical and pharma-industries, is provided.

The production of large (RE)Ba–Cu–O single grains ((RE)BCO), where RE = Y, Gd or Sm, of complex geometries is presently limited by the intrinsic slowness of the grain growth process. Additionally, the shapes achievable using established melt processing are constrained by the small number of post-processing techniques available. These materials are brittle and hard, which makes machining a difficult task and largely eliminates the possibility of producing highly intricate shapes. An alternative to this slow and inflexible growth process would be to join many small single grains to form one large composite grain, connected by high-performance superconducting joints. A reliable joining technique would also overcome the need for the careful and time-consuming post-growth machining processes. In this work we report on the use of single grain YBCO–Ag as an interface medium to achieve superconducting joints between (RE)BCO bulks. This joining technique is relatively quick and does not require tight process parameter control as there is no need to re-grow the interface joining material. We report on six joints produced from samples cut and joined in a variety of orientations. In addition, a joint was produced using bulk YBCO from two independent single grains. The trapped field properties of the resulting joined sample were measured and the microstructure at the joint was examined. We show that this simple but effective joining technique makes it possible to produce multiple composite grains with comparable superconducting properties to those of a single grain of the same size.

A robust and reliable in-situ magnetization method is essential for exploiting the outstanding magnetic flux trapping ability of bulk superconductors in practical applications. We report a 4.8 T peak trapped magnetic field, B T, achieved at 30 K in a 36 mm diameter GdBa2Cu3O7-δ –Ag bulk superconductor using pulsed field magnetization (PFM). To realize this, we have developed a reliable two-step multi-pulse PFM process based on understanding and exploiting the avalanche-like flux jump phenomenon observed in these materials. The magnitude of the applied pulsed magnetic field (B a) necessary to trap 4.8 T was merely 5.29 T, corresponding to a remarkable magnetization efficiency (B T/B a) of 90%.

Bulk high temperature superconductors (HTS) based on the rare-earth barium cuprates [(RE)BCO] have the potential to be applied in a variety of engineering and technological applications such as trapped field magnets, rotating electrical machines, magnetic bearings and flywheel energy storage systems. The key materials figure of merit for most practical applications of bulk superconductors is simply the product of the maximum current density that can be supported, which correlates directly with the maximum achievable trapped magnetic field, and the physical length scale over which the current flows. Unfortunately, however, bulk (RE)BCO superconductors exhibit relatively poor mechanical properties due to their inherent ceramic nature. Consequently, the performance of these materials as trapped field magnets is limited significantly by their tensile strength, rather than critical current and size, given that the relatively large Lorentz forces produced in the generation of large magnetic fields can lead to catastrophic mechanical failure. In the present work, we describe a simple, but effective and reliable reinforcement methodology to enhance the mechanical properties of (RE)BCO bulk superconductors by incorporating hybrid SiC fibres consisting of a tungsten core with SiC cladding within the bulk microstructure. An improvement in tensile strength by up to 40% has been achieved via this process and, significantly, without compromising the superconducting performance of the bulk material.

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.

Bulk, single grain (RE)Ba2Cu3O7- [(RE)BCO, where RE is a rare earth element or yttrium] high temperature superconductors (HTS) exhibit significant potential for use in a variety of engineering applications due to their ability to trap large magnetic fields, which can be up to 10 times greater than those generated by conventional, iron-based magnets. Limitations on the maximum size to which single grains can be grown, however, are a major obstacle to the further development of these materials. Indeed, multiple samples are often required to achieve the required superconducting properties in particular applications. The geometry of bulk (RE)BCO single grains samples plays an important role in determining the superconducting properties of a given technical arrangement. In order to gain a better understanding of the full applications potential of bulk single grain superconductors, three, relatively long, cylindrical YBCO single grains of different diameters were fabricated and their trapped field and total trapped flux measured at 77 K as a function of sample height. The effects of size and aspect ratio of YBCO single grains on these key applied properties have been investigated experimentally and the results compared qualitatively with the predictions of an established theoretical model. Conclusions based on the trapped field measurements on a variety of single grain samples are presented in this study and the possibilities of using assemblies of smaller samples for engineering devices, in particular, are discussed.

The two-step Top Seeded Infiltration and Growth (TSIG) melt process has emerged as a successful and reliable technique for the fabrication of single grain (RE)Ba2Cu3O7- (where RE is a rare-earth element or yttrium) bulk high temperature superconductors with engineered microstructures that exhibit improved superconducting properties. In this study, the performance of these materials in large applied magnetic fields has been investigated by field cooling single grain samples in a magnetic field of 18 T. YBa2Cu3O7- samples processed without added Ag by the TSIG technique, in the two-sample stack configuration, trapped a magnetic field of 14.3 T at 28 K after field cooling from 100 K and subsequent removal of the applied field. This result is particularly significant in that, previously, only single grain (RE)Ba2Cu3O7- bulk superconductors containing Ag have been reported to be able to tolerate the large stresses on the samples inherent in the magnetisation process at large fields. The samples prepared in the present study were pre-stressed using a reinforcing stainless-steel ring, although, otherwise, they did not contain any additives, dopants or resin impregnation. The ability of samples processed by TSIG to withstand large tensile forces without Ag-addition is attributed to the reduced incidence of intrinsic cracks/pores in the single grain microstructure.

Bulk large-grain superconductors can be used as high-field permanent magnets. Although the properties of such individual trapped field magnets are well documented, much less is known concerning their behaviour when two are brought together. In this work, the interaction between two cylindrical bulk YBa2Cu3O7 (YBCO) superconductors is described. Two sets of experiments were carried out. The first set of experiments involved the simultaneous magnetization of two bulk superconductors placed a short distance apart. The applied magnetic field was aligned parallel to the c-axis of one bulk, while the other had its c-axis directed at different angles. For a centre-to-centre distance equal to twice the sample height, the presence of the second sample is found not to alter the current distribution inside the first. Consequently, the contribution of both samples simply sums, thus increasing the magnetic flux density between them. In the second set of experiments, the translational approach of the superconductors with parallel c-axes was investigated. The following configurations were considered: (i) face to face approach (with anti-parallel trapped field orientation) and (ii) sideways approach (with parallel trapped field orientation). An irreversible decrease of the trapped field was measured when the samples are separated again. Repeated approach cycles showed that the irreversible loss of trapped field is most significant for the first approach.

Large, single grain (RE)Ba2Cu3O7 (where RE is a rare-earth element or yttrium) high temperature superconductors (HTS) are technologically important materials due to their ability to trap large magnetic fields and to provide stable magnetic levitation for a number of potential high field applications. The fabrication of samples in the form of large singe grain is a challenge, however, due to the characteristic slow growth rate of these materials and the need to produce samples that are electrically well-connected in order to generate trapped magnetic fields that are significantly greater than those produced by conventional permanent magnets (PM). In this work, we investigate whether large, single grain samples are optimum for the generation of high levitation forces for engineering applications. Three large bar-shaped Y-Ba-Cu-O (YBCO) samples of dimensions 60 x 20 x 12 mm3 were prepared for this investigation, including one single-seeded, one multi-seeded and one consisting of three square samples packed together closely in an array. The processing of these samples is described and their trapped field and levitation performance at 77 K measured using different permanent magnet arrays. We find that the multi-seeded samples and those assembled from smaller, individual bulks are able to achieve a higher levitation force than an equivalent single seed sample arrangement, at least in some geometries. This result is significant in that it suggests clearly that it is not always necessary to fabricate bulk (RE)BCO superconductors in the form of very large single grains for levitation applications, although the specific configuration of the system does need to be considered on a case-by-case basis.

Trapped fields of over 20 T are, in principle, achievable in bulk, single-grain high temperature cuprate superconductors. The principle barriers to realizing such performance are, firstly, the large tensile stresses that develop during the magnetization of such trapped-field magnets as a result of the Lorentz force, which lead to brittle fracture of these ceramic-like materials at high fields and, secondly, catastrophic thermal instabilities as a result of flux movement during magnetization. Moreover, for a batch of samples nominally fabricated identically, the statistical nature of the failure mechanism means the best performance (i.e. trapped fields of over 17 T) cannot be attained reliably. The magnetization process, particularly to higher fields, also often damages the samples such that they cannot repeatedly trap high fields following subsequent magnetization. In this study, we report the sequential trapping of magnetic fields of ~ 17 T, achieving 16.8 T at 26 K initially and 17.6 T at 22.5 K subsequently, in a stack of two Ag-doped GdBa2Cu3O7-{\delta} bulk superconductor composites of diameter 24 mm reinforced with (1) stainless-steel laminations, and (2) shrink-fit stainless steel rings. A trapped field of 17.6 T is, in fact, comparable with the highest trapped fields reported to date for bulk superconducting magnets of any mechanical and chemical composition, and this was achieved using the first composite stack to be fabricated by this technique., Comment: Accepted revised version for Superconductor Science and Technology

The flux-pinning landscape in type-II superconductors determines the response of the flux line lattice to changing magnetic fields. Typically, the flux vortex behaviour is hysteretic and well described within the framework of the Bean critical-state model and its extensions. However, if the changing magnetic field does not move the flux vortices from their pinning sites, their response remains linear and reversible. The vortex displacement, then, is characterised by the Campbell penetration depth, which itself is related directly to the effective size of the pinning potential. Here, we present measurements of the Campbell penetration depth (and the effective size of the pinning potential) as a function of magnetic field in a single-grain bulk GdBa2Cu3O 7 − δ superconductor using a pick-up coil method. Hence, the hysteretic losses, which take into account the reversible vortex movement, are established.

Magnetisation is one of the main barriers to practical use of bulk superconductors as high field magnets. Recently several authors have reported a flux jump effect that allows penetration of magnetic flux into a bulk superconductor during pulsed field magnetisation (PFM) at lower fields than that would be predicted on the basis of the Bean model. We have systematically investigated macroscopic flux jumps in single grain GdBa2Cu3O7-δ-Ag (GdBCO-Ag) bulk superconductors with diameters of up to 30 mm when subjected to pulsed magnetic fields. Flux jumps were observed at temperatures between 30 K and 77 K and in applied magnetic fields of up to 7 T. The applied pulsed field required to trigger the instability or flux jump field, Bj, was determined experimentally and found to increase with decreasing temperature. An extended instability criterion based on a 2D axisymmetric model was used to predict Bj at various temperatures and the results are in good agreement with experiments. A significant temperature rise has been measured experimentally during the magnetisation process which indicates that local heat generation due to the sharp rise of the applied field in the PFM process is the primary cause of the flux jumps. The experimental results suggest further that the critical current density reduces to almost zero in the warm part of the sample during the short period of non-equilibrium. A peak trapped field of 4.1 T at the surface and 5.3 T between a stack of two GdBCO-Ag bulk superconductors was achieved at 30 K by means of an optimized two- step pulse sequence with the assistance of the flux jumps, which is extremely promising for potential applications of these technologically important materials.

The Insertion Device group of the Paul Scherrer Institute has started an R&D program on a high temperature superconducting undulator to reduce the period length and increase the undulator's magnetic field well beyond the present capability. Simulation results for a 10 mm period and 4 mm magnetic gap staggered array of GdBCO bulks predict peak magnetic field above 2 T. Building on the existing working principle of undulator design and simulated performance, the first experimental results of a 5 period 6.0 mm gap short undulator measured in the new test facility available at the University of Cambridge will be presented together with details of the experimental setup and sample preparation.