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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.

(RE)BCO, rare-earth based high temperature superconductors fabricated in the form of large, single grain bulk samples can trap comparatively large magnetic fields in relatively small sample volumes, unlike conventional permanent magnetic materials. Fabrication of (RE)BCO single grains has been achieved largely following the development of processing techniques based on melt growth (MG). In the present study, the recently developed alternative fabrication technique of infiltration and growth (IG) is discussed and its significance highlighted in the context of obtaining (RE)BCO bulk superconductors with dense microstructures. The necessity of employing a buffer layer in the IG methodology is elucidated. The path followed in solving the complex problem of controlling the amount of RE2BaCuO5 (RE-211) present in the microstructure of the end product to achieve enhanced and optimized flux pinning is described. A brief overview of the recently developed 2-step, buffer-assisted top-seeded infiltration and growth (BA-TSIG) fabrication technique, which enables successful fabrication of (RE)BCO bulks, is presented. Finally, two novel experiments based on the TSIG technique – fabrication of a bar-shaped YBCO sample (with size: 72 mm x 24 mm x 15 mm) and multi-seeding of YBCO (with two NdBCO seeds in 0°-0° configuration, with aligned a-b planes) – are described and further potential options for the fabrication of complex-shaped (RE)BCO bulk components for specific practical applications are outlined.

In this work, a series of melt processed Y Ba Cu O (YBCO) single-grains have been fabricated by the top-seeded melt growth (TSMG) technique. The melt processing is accepted widely as an effective way to grow bulk, single grain YBCO superconductors; however, this process is extremely complex and every step can affect the final properties of prepared bulk. Therefore, the impact of precursor powder preparation and growth conditions was studied for the first time. Cost-effective in-house made powder and commercially available precursor powders were employed and samples were grown employing top seeded melt growth, following the isothermal and the under-cooling growth techniques. The bulk microstructure including Y2BaCuO5 (Y-211) particle size and distribution, superconducting properties (Tc, Jc) and field trapping potential were investigated. The cost-effective high performance batch processing methodology was optimized. The fabricated YBCO bulks (diameter of 28 mm) exhibited average trapped field of 0.85 T at 77 K. Furthermore, other possibilities to achieve advancement in processing (RE)BCO bulks, are proposed.

© 2016 American Chemical Society.Single grain, rare earth-barium-copper oxide [(RE)BCO] bulk superconductors, fabricated either individually or assembled in large or complicated geometries, have a significant potential for a variety of potential engineering applications. Unfortunately, (RE)BCO single grains have intrinsically very low growth rates, which limits the sample size that may be achieved in a practical, top seeded melt growth process. As a result, a melt process based on the use of two or more seeds (so-called multiseeding) to control the nucleation and subsequent growth of bulk (RE)BCO superconductors has been developed to fabricate larger samples and to reduce the time taken for the melt process. However, the formation of regions that contain non-superconducting phases at grain boundaries has emerged as an unavoidable consequence of this process. This leads to the multiseeded sample behaving as if it is composed of multiple, singly seeded regions. In this work we have examined the factors that lead to the accumulation of non-superconducting phases at grain boundaries in multiseeded (RE)BCO bulk samples. We have studied the microstructure and superconducting properties of a number of samples fabricated by the multiseeded process to explore how the severity of this problem can be reduced significantly, if not eliminated completely. We conclude that, by employing the techniques described, multiseeding is a practical approach to the processing of large high performance superconducting bulk samples for engineering applications.

Large, single-grain bulk high-temperature superconducting materials can trap high magnetic fields in comparison with conventional permanent magnets, making them ideal candidates to develop more compact and efficient devices, such as actuators, magnetic levitation systems, flywheel energy storage systems and electric machines. However, macrosegregation of Y-211 inclusions in melt-processed Y-Ba-Cu-O (YBCO) limits the macroscopic critical current density of such bulk superconductors, and hence, the potential trapped field. A new fabrication technique using graded precursor powders has recently been developed by our research group, which results in a more uniform distribution of Y-211 particles, in order to further improve the superconducting properties and trapped field capability of such materials. Large, single-grain bulk high-temperature superconducting materials can trap high magnetic fields in comparison with conventional permanent magnets, making them ideal candidates to develop more compact and efficient devices, such as actuators, magnetic levitation systems, flywheel energy storage systems and electric machines. However, macrosegregation of Y-211 inclusions in melt-processed Y-Ba-Cu-O (YBCO) limits the macroscopic critical current density of such bulk superconductors, and hence, the potential trapped field. A new fabrication technique using graded precursor powders has recently been developed by our research group, which results in a more uniform distribution of Y-211 particles, in order to further improve the superconducting properties and trapped field capability of such materials.

We review recent progress in the successful and reliable fabrication of large grain (RE)BCO and (RE)BCO-Ag bulk high temperature superconductors by three primary processing methodologies: top-seeded melt growth (TSMG), top-seeded infiltration growth (TSIG) and a combined processing route comprising both TSMG and TSIG. Several significant and useful modifications have been made to these processing routes over the recent years in order to grow (RE)BCO single grains reliably and to enhance their superconducting properties. With respect to the reliable growth of (RE)BCO single grains, we have further developed the so-called buffer technique for reliable seeding, which includes identifying and providing an appropriate liquid-rich phase at the bottom of pellet. This modification has proved to be critical to the success of the newly-developed fabrication techniques presented here. In addition, we have addressed existing and significant challenges in the context of improving the superconducting properties of the bulk samples by controlling the microstructures of single grain materials (porosity and the RE-211 distribution, in particular) and by suppressing the extent of RE/Ba substitution during grain growth. Finally, we provide a short summary of the potential pathways towards realising practical applications of these technologically important materials in the near future.

Large, single-grain bulk high-temperature superconductors (HTS) can trap magnetic fields over 17 T below 30 K and up to 3 T at 77 K, and have significant potential to replace permanent magnets, the fields from which are limited to significantly less than 2 T. Therefore, bulk HTS samples are ideal candidates to develop more compact and efficient devices, such as actuators, magnetic levitation systems, flywheel energy storage systems and electric machines. In electric machines, in particular, the higher flux density improves the power density of the machine, resulting in smaller, lighter devices. However, in a real electric machine environment, bulk HTS samples can be exposed to AC magnetic field fluctuations that can affect the distribution of the supercurrent in the material and attenuate the trapped field, leading to a reduction in the magnetic loading of the machine, and in some cases, full demagnetisation. In this paper, the variation of trapped field with the frequency and magnitude of an external time-varying magnetic field is analysed numerically, and the mechanisms of the attenuation of the trapped field in HTS bulks are investigated using a two-dimensional (2D) axisymmetric finite-element model based on the H-formulation, considering both the electromagnetic and thermal behaviour of the bulk sample.

The multi-seeding process has the potential to enlarge the sample size of (RE)BCO (where RE = rare earth or Y) single-grain, bulk superconductors with improved fabrication speed, and in previous studies on multi-seeding, a significant improvement was made in the alignment of the seeds in such samples using a novel bridge-seeding technique. In this paper, we report the experimental measurements of the pulsed field magnetization (PFM) of a 0°-0° bridge-seeded Y-Ba-Cu-O (YBCO) sample. The PFM is carried out using a solenoid coil, as well as a split coil arrangement with an iron yoke, at temperature of 65, 40 and 20 K, and the resultant trapped fields and magnetic flux dynamics for these two PFM techniques are compared. It is shown that such bridge-seeded bulk YBCO can be fabricated that performs as a bulk magnet with trapped fields comparable to or better than standard, single-seeded high-$J_\text{c}$ samples, with the potential of enlarging the sample size. Furthermore, the split coil arrangement with an iron yoke is useful to enhance the trapped field and has a positive effect on the maximum temperature rise in the sample, which increases at lower temperatures and seriously impacts the achievable trapped field from PFM.

The study reports phase and microstructural evolution in MgB2 bulk superconductors fabricated by an infiltration and growth (IG) process. Three distinct stages, (1) intermediate boride formation, (2) bulk liquid Mg infiltration, and (3) MgB2 layer formation, were identified in IG process after detailed examination of series of samples prepared with varied heating conditions. The intermediate phase Mg2B25, isomorphous to β-boron, was detected prior to MgB2 phase formation in stage (1). Due to volume expansion involved in stage 1, cracks formed in the β-boron particles and propagated radially inwards during stage 3. The growing MgB2 particles sintered simultaneously with formation of grain boundaries during the process, as evidenced by the measured hardness and critical current density in these samples. From our observations, we estimate the total time needed for complete transformation to MgB2., Authors acknowledge financial support from KACST-Cambridge Joint Centre of Excellence in Advanced Materials and Manufacturing (CAMM) based at the University of Cambridge, UK. Partial financial support from Engineering and Physical Sciences Research Council, UK (Grant: EP/K031422/1) is gratefully acknowledged.

We have investigated, both experimentally and numerically, the trapped field characteristics of a standard Y–Ba–Cu–O bulk of 30 mm in diameter and 14 mm in thickness magnetized by pulsed field magnetization (PFM) using a split coil, in which three kinds of iron yoke are inserted in the bore of the coil: soft iron with a flat surface, soft iron with a taper, and permendur (50Fe + 50Co alloy) with a flat surface. The highest trapped field, $B_\text{Tmax}$, of 2.93 T was achieved at 40 K in the case of the permendur yoke, which was slightly higher than that obtained for the flat soft iron or the tapered soft iron yokes, and was much higher than 2.20 T in the case without the yoke. The insertion effect of the yoke on the trapped field characteristics was also investigated using numerical simulations. The results suggest that the saturation magnetic flux density, $B_\text{sat}$, of the yoke acts to reduce the flux flow due to its hysteretic magnetization curve and the higher electrical conductivity, $\sigma$, of the yoke material also acts to suppress the flux increase rate. A flux jump (or flux leap) can be reproduced in the ascending stage of PFM using numerical simulation, using an assumption of relatively high $J_c$($T, B$) characteristics. The insertion effect of the yoke with high $B_\text{sat}$ and $\sigma$ is discussed to enhance the final trapped field from both electromagnetic and thermal points of view.

A trapped magnetic field of greater than 3 T has been achieved in a single grain GdBa$_2$Cu$_3$O$_{7-}$$_\delta$ (GdBaCuO) bulk superconductor of diameter 30 mm by employing pulsed field magnetization. The magnet system is portable and operates at temperatures between 50 K and 60 K. Flux jump behaviour was observed consistently during magnetization when the applied pulsed field, $B_a$, exceeded a critical value (e.g., 3.78 T at 60 K). A sharp d$B_a$/d$t$ is essential to this phenomenon. This flux jump behaviour enables the magnetic flux to penetrate fully to the centre of the bulk superconductor, resulting in full magnetization of the sample without requiring an applied field as large as that predicted by the Bean model. We show that this flux jump behaviour can occur over a wide range of fields and temperatures, and that it can be exploited in a practical quasi-permanent magnet system.

REBa 2 Cu 3 O 7-δ bulk superconductors are able to trap large magnetic fields and as a result they have potential to work as strong magnets. In this study, highly textured yttrium, gadolinium, and samarium based cuprate bulk superconductors were prepared using conventional top-seeded melt-textured growth, and magnetized by employing both field-cooling magnetization (FCM) and pulsed-field magnetization (PFM) techniques at 77 K. The same peak value of a trapped field was achieved using FCM and PFM, which means all the bulk samples were fully magnetized by using the PFM method. In addition, the magnitude of the pulsed field required to fully magnetize the bulk samples varied with rare-earth element in the (RE)BCO formulation.

A major limitation to the widespread application of Y–Ba–Cu–O (YBCO) bulk superconductors is the relative complexity and low yield of the top seeded melt growth (TSMG) process, by which these materials are commonly fabricated. It has been demonstrated in previous work on the recycling of samples in which the primary growth had failed, that the provision of an additional liquid-rich phase to replenish liquid lost during the failed growth process leads to the reliable growth of relatively high quality recycled samples. In this paper we describe the adaptation of the liquid phase enrichment technique to the primary TSMG fabrication process. We further describe the observed differences between the microstructure and superconducting properties of samples grown with additional liquid-rich phase and control samples grown using a conventional TSMG process. We observe that the introduction of the additional liquid-rich phase leads to the formation of a higher concentration of Y species at the growth front, which leads, in turn, to a more uniform composition at the growth front. Importantly, the increased uniformity at the growth front leads directly to an increased homogeneity in the distribution of the Y-211 inclusions in the superconducting Y-123 phase matrix and to a more uniform Y-123 phase itself. Overall, the provision of an additional liquid-rich phase improves significantly both the reliability of grain growth through the sample thickness and the magnitude and homogeneity of the superconducting properties of these samples compared to those fabricated by a conventional TSMG process.

Superconducting single-grain YBCO bulk samples with the ability to trap high magnetic fields can be grown using the top-seeded melt-growth (TSMG) process. Multiseeding techniques have the potential to enable larger diameter bulks to be grown, but the performance of these materials is not yet comparable to the single-seeded bulks. Here we carry out detailed three-dimensional microstructural characterisation on a multiseeded sample grown with the seeds aligned in the 0◦ -0◦ geometry using high resolution microanalysis techniques. Chemical and structural variations have been correlated with the trapped field distribution in three separate slices of the sample. The top slice of the sample shows four peaks in trapped field, indicating that the current flows in four separate loops rather than in one large loop within the sample. This has been explained by the build-up in insulating Y-211 particles where the growth fronts from the two seeds meet, forming a barrier to current flow, as well as the low Y-211 content (and hence low Jc) of the large c-axis growth sector.

SmBa$_2$Cu$_3$O$_{7−δ}$ (Sm-123) is a light-rare-earth barium-cuprate (LRE-BCO) high-temperature superconductor (HTS) with significant potential for high field industrial applications. We report the fabrication of large, single grain bulk [Sm–Ba–Cu–O (SmBCO)] superconductors containing 1 wt% CeO$_2$ and 0.1 wt% Pt using a top-seeded melt growth process. The performance of the SmBCO bulk superconductors containing the different dopants was evaluated based on an analysis of their superconducting properties, including critical transition temperature, $T_c$ and critical current density, $J_c$ , and on sample microstructure. We find that both CeO$_2$ and Pt dopants refine the size of Sm$_2$BaCuO$_5$ (Sm-211) particles trapped in the Sm-123 superconducting phase matrix, which act as effective flux pinning centres, although the addition of CeO$_2$ results in broadly improved superconducting performance of the fully processed bulk single grain. However, 1 wt% CeO$_2$ is significantly cheaper than 0.1 wt% Pt, which has clear economic benefits for use in medium to large scale production processes for these technologically important materials. Finally, the use of CeO$_2$ results generally in the formation of finer Sm-211 particles and to the generation of fewer macro-cracks and Sm-211 free regions in the sample microstructure.

Rare earth–barium–copper oxide bulk superconductors fabricated in large or complicated geometries are required for a variety of engineering applications. Initiating crystal growth from multiple seeds reduces the time taken to melt-process individual samples and can reduce the problem of poor crystal texture away from the seed. Grain boundaries between regions of independent crystal growth can reduce significantly the flow of current due to crystallographic misalignment and the agglomeration of impurity phases. Enhanced supercurrent flow at such boundaries has been achieved by minimising the depth of the boundary between $\small \textit{A}$ growth sectors generated during the melt growth process by reducing second phase agglomerations and by a new technique for initiating crystal growth that minimises the misalignment between different growth regions. The trapped magnetic fields measured for the resulting samples exhibit a single trapped field peak indicating they are equivalent to conventional single grains.

Large, single-grain high-temperature superconducting (HTS) bulks have significant potential to replace permanent magnets in various engineering applications. However, based on our previous research, the trapped field in a bulk superconductor can be attenuated or even erased when a bulk is subjected to a time-varying, external magnetic field. Therefore, it is important to develop a method to protect bulks from demagnetization by (a) improving the thermal conduction of the bulk and/or (b) reducing AC losses. Improvement in the thermal conduction of bulks involves modification of the material fabrication process, which may have a detrimental effect on its superconducting properties. Employing shielding materials around a bulk helps to decrease the AC losses, but also provides a durable way to maintain the original material properties. In this paper, two shielding cases are proposed and evaluated numerically: ring-shaped shielding with a copper coil, and surface shielding with a ferromagnetic material. Based on the numerical modelling results, the ring-shaped coil works well for externally applied AC fields of larger magnitude and higher frequency. However, the ferromagnetic material was preferable for surface shielding for relatively lower fields. Finally, an optimal shield design is presented.

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.

Bulk (RE)–Ba–Cu–O ((RE)BCO, where RE stands for rare-earth), single grain superconductors can trap magnetic fields of several tesla at low temperatures and therefore can function potentially as high field magnets. Although top seeded melt growth (TSMG) is an established process for fabricating relatively high quality single grains of (RE)BCO for high field applications, this technique suffers from inherent problems such as sample shrinkage, a large intrinsic porosity and the presence of (RE)$_2$BaCuO$_5$ (RE-211)-free regions in the single grain microstructure. Seeded infiltration and growth (SIG), therefore, has emerged as a practical alternative to TSMG that overcomes many of these problems. Until now, however, the superconducting properties of bulk materials processed by SIG have been inferior to those fabricated using the TSMG technique. In this study, we identify that the inferior properties of SIG processed bulk superconductors are related to the presence of a relatively large Y-211 content (~41.8%) in the single grain microstructure. Controlling the RE-211 content in SIG bulk samples is particularly challenging because it is difficult to regulate the entry of the liquid phase into the solid RE-211 preform during the infiltration process. In an attempt to solve this issue, we have investigated the effect of careful control of both the infiltration temperature and the quantity of liquid phase powder present in the sample preforms prior to processing. We conclude that careful control of the infiltration temperature is the most promising of these two process variables. Using this knowledge, we have fabricated successfully a YBCO bulk single grain using the SIG process of diameter 25 mm that exhibits a trapped field of 0.69 T at 77 K, which is the largest value reported to date for a sample fabricated by the SIG technique.

The characterisation of the critical current density of bulk high temperature superconductors is typically performed using magnetometry, which involves numerous assumptions including, significantly, that Jc within the sample is uniform. Unfortunately, magnetometry is particularly challenging to apply where a local measurement of Jc across a feature, such as a grain boundary, is desired. Although transport measurements appear to be an attractive alternative to magnetization, it is extremely challenging to reduce the cross-sectional area of a bulk sample sufficiently to achieve a sufficiently low critical current that can be generated by a practical current source. In the work described here, we present a technique that enables transport measurements to be performed on sections of bulk superconductors. Metallographic techniques and resin reinforcement were used to create an I-shaped sample of bulk superconductor from a section of Gd-Ba-Cu-O containing 15 wt % Ag2O. The resulting superconducting track had a cross-sectional area of 0.44 mm2. The sample was found to support a critical current of 110 A using a field criterion in the narrowed track region of 1 μV cm-1. We conclude, therefore, that it is possible to measure critical current densities in excess of 2.5 x 108 A m-2 in sections of a bulk superconductor.

Large, single-grain (RE)BCO (where RE = rare earth or Y) bulk superconductors with complicated geometries are required for a variety of potential applications, such as rotating machines, magnetic bearings and magnetic separation. As a consequence, the top multi-seeded melt growth process has been studied over many years in an attempt to deliver large, single grains for practical applications. Among these techniques, the so-called bridge-seeding produces the best alignment of two seeds during melt processing of multi-seed samples. In this paper, the trapped field performance and magnetic flux dynamics of two bridge-seeded, multi-seed samples magnetized by pulsed field magnetization (PFM) are analysed: one with a 45⁰-45⁰ and another with a 0⁰-0⁰ bridge seed. Based on an analysis of the flux penetration across the seeds and in-between the seeds of the 45⁰-45⁰ multi-seed sample, an estimated Jc distribution over the ab-plane was determined, which provides the basis for further analysis via numerical simulation. A 3D finite-element model, developed to qualitatively reproduce and interpret the experimental results, was employed to investigate the influence of the length of the bridge seed for such multi-seed samples. The simulation results agree well with the observed experimental results, in that the multi-seed sample’s particular inhomogeneous Jc distribution acts to distort the trapped field profile from a traditional conical Bean’s profile, which is determined by the length and direction of the bridge seed on the bulk surface.

In this paper single domain, bulk SmBCO samples have been fabricated successfully in air in a top-seeded melt growth (TSMG) process using a conventional chamber furnace, in addition to samples doped with ZrO2. In order to improve the reliability of seeding, Sm-123+ Sm-211 mixed-powder buffers were used to increase the success of the SmBCO single grain growth process. SmBCO single grains of diameter as large as 20 mm and 10 mm in thickness with and without ZrO2 were fabricated successfully using Sm-123 + Sm2BaCuO5 (Sm-211) mixed-powder buffer layers. The geometric configurations of the buffers were also optimized as part of this study to further increase the success of the single grain growth process. Superconducting properties of Jc and Tc of the specimens under the buffer layer have also been investigated.

Bulk, high temperature superconductors have significant potential for use as powerful permanent magnets in a variety of practical applications due to their ability to trap record magnetic fields. In this paper, soft ferromagnetic sections are combined with a bulk, large grain Y-Ba-Cu-O (YBCO) high temperature superconductor to form superconductor/ferromagnet (SC/FM) hybrid structures. We study how the ferromagnetic sections influence the shape of the profile of the trapped magnetic induction at the surface of each structure and report the surface magnetic flux density measured by Hall probe mapping. These configurations have been modelled using a 2D axisymmetric finite element method based on the H-formulation and the results show excellent qualitative and quantitative agreement with the experimental measurements. The model has also been used to study the magnetic flux distribution and predict the behaviour for other constitutive laws and geometries. The results show that the ferromagnetic material acts as a magnetic shield, but the flux density and its gradient are enhanced on the face opposite to the ferromagnet. The thickness and saturation magnetization of the ferromagnetic material are important and a characteristic ferromagnet thickness d* is derived: below d*, saturation of the ferromagnet occurs, and above d*, a weak thickness-dependence is observed. The influence of the ferromagnet is observed even if its saturation magnetization is lower than the trapped flux density of the superconductor. Conversely, thin ferromagnetic discs can be driven to full saturation even though the outer magnetic field is much smaller than their saturation magnetization.

In this paper, the authors investigate numerically the in-field behaviour of high-temperature superconducting (HTS) coils and a method to potentially improve their performance using ferromagnetic material as a flux diverter. The ability to accurately predict the electromagnetic behaviour of superconductors in complex geometries and electromagnetic environments is crucial to the design of commercially-viable superconductor-based electrical devices, such as power transmission cables, superconducting fault current limiters, transformers, and motors and generators. The analysis is carried out using a two-dimensional (2D) axisymmetric model of a circular pancake coil based on the H-formulation and implemented in Comsol Multiphysics 4.3a. We explore the use of flux diverters to improve an HTS coil’s performance with respect to its DC (maximum allowable/critical current) and AC (AC loss) characteristics, for various background magnetic fields. It is found that while flux diverters can improve the AC properties of coils, they can be detrimental to the DC properties in this particular configuration.

The existence of a zone containing a low concentration of Y2BaCuO5 (Y-211) phase particles in the vicinity of the seed in single grain, bulk Y–Ba–Cu–O (YBCO) superconductors fabricated by the top seeded melt growth (TSMG) technique has been reported, suggesting the presence of low intrinsic magnetic flux pinning, and hence reduced trapped field, in this region of the sample. The possibility of introducing additional pinning centers in the vicinity of the seed in single grain YBCO, therefore, may be effective in improving the applied magnetic properties of these technologically important materials. Single grain, bulk YBCO superconductors fabricated from precursor pellets containing a graded CeO2 composition have been prepared corresponding to a secondary phase content of 2 wt % CeO2 beneath the seed and 1 wt % CeO2 at the outer, “U” shape cross-section, of the sample. The resulting graded single grain YBCO superconductors exhibit enhanced trapped field compared to standard YBCO superconductors fabricated by TSMG from precursor pellets containing purely 1 wt % CeO2. The influence of CeO2 content on the microstructure, growth process, and superconducting properties of bulk, single grain YBCO superconductors fabricated by TSMG has been investigated and used to explain the increased trapped field ability of the graded composition.

The engineering of fine Y2Ba1Cu1O5 (Y-211) inclusions of average particle size 1−2 μm within the continuous, superconducting YBa2Cu3O7−δ (Y-123) phase matrix of single-grain, bulk high temperature Y–Ba–Cu–O (YBCO) superconductors is fundamental to achieving high critical current density in these materials. However, bulk samples fabricated using fine Y-211 precursor powders generally suffer from incomplete growth of the Y-123 phase compared to samples fabricated with coarser Y-211 particles of average particle size >5 μm in the precursor powder. In this study, the effects of Y-211 particle size on processing parameters during growth of large single YBCO grains are reported. Chemical analysis using EDX of cross-sections of single YBCO grains indicates that the loss of liquid phase during melt processing is greater when finer Y-211 precursor powder is employed, which accounts for the observed growth limitations and low sample yield. Specific modifications to the single grain growth process have been made based on the results of this investigation to reduce liquid loss and to enable the use of fine Y-211 precursor powder without compromising complete single grain growth or yield.

Single-grain, (RE)–Ba–Cu–O [(RE)BCO)] bulk high-temperature superconductors have significant potential for application as trapped field magnets in a range of engineering devices. However, it is not trivial to fabricate single grains of (RE)BCO due to the complexity of the growth process, especially when the sample diameter exceeds 25 mm. In particular, difficulties associated with the seed crystal can lead to poor grain growth or to complete growth failure. We have employed an optimized buffer technique, which was determined by optimizing targeted critical parameters of the buffer pellet, including the choice of the buffer pellet composition and its aspect ratio, for the reliable fabrication of large, single grains of (RE)BCO. Potential candidates for the buffer pellet composition have been identified to yield successful grain growth and good superconducting properties. The optimum aspect ratio of the buffer pellet was also determined as part of this study. The optimized buffer pellet capped with the seed crystal has been demonstrated to work effectively as an efficient seed crystal and to aid significantly the growth of the Y-123 phase. We show that this optimized buffer technique ameliorates problems associated with both interfacial stress (commonly occurring at the seed/sample interface) and problems of grain contamination. We have fabricated a 40.8 mm diameter single-grain bulk superconductor and more than 25 single-grain YBCO samples with diameters in the range 25–35 mm by a significantly improved top-seeded melt growth process.

Single grain (RE)-Ba-Cu-O (RE: Sm, Gd, Y) high temperature superconductors are able to generate high magnetic fields. However, the relatively high cost of the raw materials and the low yield of the manufacturing process have impeded the development of practical applications of these materials to date. This article describes a simple, reliable and economical method of recycling failed bulk (RE)-Ba-Cu-O (RE: Sm, Gd, Y) samples. Sixty-four failed bulk samples, with diameters up to 31 mm, were recycled with a yield of 90%. The key innovation in this recycling process involves reintroducing the liquid phase into the melt process, which is normally lost during the primary peritectic processing of these materials. This enables the direct re-growth of failed samples from solid form without the need for re-grinding into powder. We also demonstrate that the superconducting performance and microstructure of the recycled samples is similar to that of the primary grown samples.

—High-temperature superconducting (HTS) coils play an important role in a number of large-scale engineering applications, such as electric machines employing HTS coated conductors. Non-uniformities and anisotropy in the properties of the coated conductor along its length and width can have a large impact on the performance of the tape, which directly influences the performance of an HTS electric machine. In this paper, the specific influences of non-uniformity and anisotropy on the dc properties of coils, such as the maximum allowable dc current, and the ac properties, such as ac loss, are analyzed using a numerical model based on the H formulation. It is found that non-uniformity along the conductor width has a large effect on the ac properties (i.e., ac loss) of a coil, but a relatively small effect on the dc properties (i.e., critical current). Conversely, non-uniformity along the length has a small effect on the ac coil properties, but has a large effect on the dc properties. Index Terms—AC loss, critical current density (superconductivity), high-temperature superconductors, numerical analysis, superconducting coils, transport ac loss.

Silver (Ag) is an established additive for improving the mechanical properties of single grain, (RE)BCO bulk superconductors (where RE = Sm, Gd and Y). The presence of Ag in the (RE)BCO bulk composition, however, typically reduces the melting temperature of the single crystal seed in the top seeded melt growth (TSMG) process, which complicates significantly the controlled nucleation and subsequent epitaxial growth of a single grain, which is essential for high field engineering applications. The reduced reliability of the seeding process in the presence of Ag is particularly acute for the SmBCO system, since the melting temperature of SmBCO is very close to that of the generic NdBCO(MgO) seed. SmBCO has the highest superconducting transition temperature, Tc, and exhibits the most pronounced "peak" effect at higher magnetic field of all materials in the family of (RE)BCO bulk superconductors and, therefore, has the greatest potential for use in practical applications (compared to GdBCO and YBCO, in particular). Development of an effective seeding process, therefore, is one of the major challenges of the TSMG process for the growth of large, high quantity single grain superconductors. In this paper, we report a novel technique that involves introducing a buffer layer between the seed crystal and the precursor pellet, primarily to inhibit the diffusion of Ag from the green body to the seed during melt processing in order to prevent the melting of the seed. The success rate of the seeding process using this technique is 100% for relatively small batch samples. The superconducting properties, Tc, Jc and trapped fields, of the single grains fabricated using the buffers are reported and the micro-structures in the vicinity of the buffer of single grains fabricated by the modified technique are analysed., 10 pages, 10 figures

YBaCuO and GdBaCuO + 15 wt% Ag large, single-grain, bulk superconductors have been fabricated via the top-seeded, melt-growth (TSMG) process using a generic NdBCO seed. The mechanical behavior of both materials has been investigated by means of three-point bending (TPB) and transversal tensile tests at 77 and 300 K. The strength, fracture toughness and hardness of the samples were studied for two directions of applied load to obtain comprehensive information about the effect of microstructural anisotropy on the macroscopic and microscopic mechanical properties of these technologically important materials. Splitting (Brazilian) tests were carried out on as-melt-processed cylindrical samples following a standard oxygenation process and with the load applied parallel to the growth-facet lines characteristic of the TSMG process. In addition, the elastic modulus of each material was measured by three different techniques and related to the microstructure of each sample using optical microscopy. The results show that both the mechanical properties and the elastic modulus of both YBCO and GdBCP/Ag are improved at 77 K. However, the GdBCO/Ag samples are less anisotropic and exhibit better mechanical behavior due to the presence of silver particles in the bulk, superconducting matrix. The splitting tensile strength was determined at 77 K and both materials were found to exhibit similar behavior, independently of their differences in microstructure.

The ability of large grain, REBa$_{2}$Cu$_{3}$O$_{7-\delta}$ [(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., Comment: Updated submission to reflect licence change to CC-BY. This is the "author accepted manuscript" and is identical in content to the published version

Bulk high temperature superconductor magnets (HTSM) have a higher flux-generating capability compared to conventional permanent magnets (PMs). These materials potentially can be used in high temperature superconducting (HTS) linear synchronous motors (LSMs) as superconducting secondary magnets, what will result in a reduced volume and weight as well as in higher force density and efficiency of these devices when compared to conventional PMs. The focus of this paper is on the effect of size of the secondary HTSM on the static performance (thrust force and normal force) of a LSM. In order to obtain high-field HTSM as the secondary, single grain bulk GdBCO-Ag superconductors of diameter 20 mm, 30 mm and 40 mm, which have higher Jc and trapped fields than YBCO superconductors, were used in this device for the first time following application by the same optimized magnetization condition. It was found that both thrust and normal forces increase and saturate with the increasing size of the HTSM secondary at the small size range, and then potentially distort when the physical size of the HTSM secondary approaches the pole pitch of the linear three-phase primary windings of the LSM. Furthermore, more experiments of a larger-sized multi-seeded HTSM secondary, confirmed that the relationship between the HTSM secondary size and the pole pitch of the primary is an important factor for achieving higher thrust and normal forces. It is suggested that the multi-pole HTSM secondary will be more beneficial to future HTS LSM designs since the single-pole HTSM secondary size should be equal to or smaller than the stator pole pitch in the paper.

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 magnetized by the pulsed field magnetization (PFM) technique at temperatures of 40 and 65 K and the characteristics of the resulting trapped field profile are investigated with a view of magnetizing 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 flux-type 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 localized 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 Jc distributions and the presence of current-limiting grain boundaries and cracks, and it can be used to assist optimization of processing and PFM techniques for practical bulk superconductor applications.