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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.

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.

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.

The activities of the young STU MTF research team focused on the material properties of superconductors were reviewed for the first time. We performed structural analyses on multiple types of superconducting tapes in order to get deeper insight into the correlation between their structure and electromagnetic properties, both experimentally and by modelling. We also addressed the joining of tapes by lead-free solders.

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.

This paper presents a topical review of the current state of the art in modelling the magnetization of bulk superconductors, including both (RE)BCO (where RE = rare earth or Y) and MgB2 materials. Such modelling is a powerful tool to understand the physical mechanisms of their magnetization, to assist in interpretation of experimental results, and to predict the performance of practical bulk superconductor-based devices, which is particularly important as many superconducting applications head towards the commercialisation stage of their development in the coming years. In addition to the analytical and numerical techniques currently used by researchers for modelling such materials, the commonly used practical techniques to magnetize bulk superconductors are summarised with a particular focus on pulsed field magnetization (PFM), which is promising as a compact, mobile and relatively inexpensive magnetizing technique. A number of numerical models developed to analyse the issues related to PFM and optimise the technique are described in detail, including understanding the dynamics of the magnetic flux penetration and the influence of material inhomogeneities, thermal properties, pulse duration, magnitude and shape, and the shape of the magnetization coil(s). The effect of externally applied magnetic fields in different configurations on the attenuation of the trapped field is also discussed. A number of novel and hybrid bulk superconductor structures are described, including improved thermal conductivity structures and ferromagnet-superconductor structures, which have been designed to overcome some of the issues related to bulk superconductors and their magnetization and enhance the intrinsic properties of bulk superconductors acting as trapped field magnets (TFMs). Finally, the use of hollow bulk cylinders/tubes for shielding is analysed.

National audience; 1. Contexte scientifique et pertinenceL’utilisation de supraconducteurs de dernière génération est d’un intérêt primordial pour le développement des applications supraconductrices dans le domaine du génie électrique. Le tableau 1 résume les avantages de l'utilisation des dispositifs supraconducteurs à haute température (HTS pour High Temperature Superconductors) pour la conversion de l'énergie, le transport et la distribution. Il est remarquable de constater que les avantages sont de natures différentes, allant de l'amélioration de l'efficacité énergétique aux nouvelles technologies de réseau et au respect environnemental [1].Ne présentant aucune perte en courant DC, les supraconducteurs engendrent néanmoins des pertes par hystérésis et par courants induits lorsqu’ils sont soumis à un champ magnétique variable ou lorsqu’ils sont parcourus par un courant variable. Dès 100 Hz, les pertes générées dans ces matériaux refroidis à basse température entraînent une augmentation de la puissance nécessaire au refroidissement. A titre d’exemple, 1 W de pertes à froid peut représenter plus de 100 W à température ambiante. Cela conduit donc à une augmentation considérable de la taille et du poids du système de refroidissement. La connaissance des pertes et leur maîtrise est un des verrous scientifiques et technologique pour les futures applications de la supraconductivité dans le Génie Electrique.2. Enjeux et caractère exploratoireBien que le mécanisme générant les pertes AC dans un matériau supraconducteur soit bien connu, e.g. [2], dans de nombreux cas, il n’est pas simple de comprendre le comportement des pertes AC observé dans des conducteurs ou des dispositifs. Même au niveau d’un simple conducteur sous forme de ruban, les caractéristiques de pertes AC peuvent être très différentes de ceux d'un supraconducteur pur, si des matériaux magnétiques sont présents par exemple [3].Les mesures de pertes sont habituellement effectuées électriquement. Ces mesures ne sont toutefois pas sans difficultés. En effet, la composante de la tension qui produit des pertes est entièrement masquée dans la partie inductive. Le positionnement des prises de potentiel a une influence capitale sur la mesure car ils captent une partie du flux magnétique produit par l'échantillon et aussi une partie induite provenant des pièces environnantes. Une des difficultés expérimentales et de s’affranchir de tout métal conducteur à proximité du supraconducteur, y compris le cryostat, afin de ne pas interférer su les pertes mesurées qui sont très faibles. Il faut donc un cryostat spécifique en fibre de verre.Dans le cadre de ce projet, nous souhaitons étudier les pertes dans des fils supraconducteurs de dernière génération, i.e. BiSCCO, YBCO et MgB2, à différentes températures et pour différents champs magnétiques appliqués. Les aspects modélisation et expérimentaux seront traités en parallèle pour confronter la véracité des résultats obtenus. En effet, la modélisation de tels dispositifs est tout aussi délicate que les mesures à effectuer.3. Programme de travailL’approche choisie se décompose en deux temps. Dans un 1er temps, avec l’expérience acquise des 2 laboratoires en supraconductivité, il s’agira de définir les limites de l’étude telles que les dimensions des fils à tester, la fréquence d’alimentation, les températures envisagées, afin de spécifier clairement le besoin et d’acheter le matériel nécessaire. Comme évoqué précédemment, le problème est complexe car il est fortement contraint par des choix antagonistes au niveau électrique, magnétique, thermique et mécanique. Des pré-dimensionnements réalisés avec l’aide de simulations numériques sont nécessaires. Dans un 2ème temps, ce sera une phase de mesures de pertes sur différentes échantillons. Ces mesures pourraient être réalisées par un stagiaire de Master. La modélisation finale des expériences de mesures de pertes sera également effectuée et confrontée aux résultats obtenus en pratique.4. Partenariats et complémentaritésLes deux laboratoires impliqués dans le projet proposé ont tous deux une forte expérience dans le dimensionnement et l’étude de systèmes supraconducteurs mais des compétences différentes.L’intérêt d’Arnaud Badel, de Guillaume Escamez et de Pascal Tixador se porte plus sur la modélisation numérique des pertes dans les supraconducteurs. L’équipe commune Institut Néel/G2Elab possède également de fortes compétences en cryogénie, et une entrée au LNCMI où des caractérisations de matériaux peuvent êtres effectuées en champ magnétique continu jusqu’à 20 T.Kévin Berger,Bruno Douine et Jean Lévêque ont des compétences dans les modèles analytiques de pertes des supraconducteurs. Ils étudient et développent actuellement des bancs de mesures de pertes à la température de l’azote liquide. Ils ont également mis au point plusieurs outils et méthodes permettant de caractériser les supraconducteurs, ce qui est indispensable pour la modélisation.[1] M. Noe. Superconducting power applications and their potential to increase efficiency. Symposium on Superconducting Devices for Wind Energy Systems, Barcelona, 25 February 2011.[2] S. Elschner, B. Douine, E. Demencik, F. Grilli, W. Goldacker, A. Kudymow, M. Vojenciak, V. Zermeno, M. Stemmle, M. Noe. Experimental setup of a superconducting power transmission cable for AC-loss investigations under controlled current distribution. European Conference on Applied Superconductivity, EUCAS 2013. Genova, Italy, 15-19 September 2013.[3] F. Grilli, S.P, Ashworth & L. Civale. Interaction of magnetic field and magnetic history in high-temperature superconductors. Journal of Applied Physics , vol. 102, pp. 073909, 2007.