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Superconducting YBa2Cu3Oy (YBCO) foams were prepared using commercial open-cell, polyurethane foams as starting material to form ceramic Y2BaCuO5 foams which are then converted into superconducting YBCO by using the infiltration growth process. For modelling the superconducting and mechanical properties of the foam samples, a Kelvin-type cell may be employed as a first approach as reported in the literature for pure polyurethane foams. The results of a first modelling attempt in this direction are presented concerning an estimation of the possible trapped fields (TFs) and are compared to experimental results at 77 K. This simple modelling revealed already useful information concerning the best suited foam structure to realize large TF values, but it also became obvious that for various other parameters like magnetostriction, mechanical strength, percolative current flow and the details of the TF distribution, a refined model of a superconducting foam sample incorporating the real sample structure must be considered. Thus, a proper description of the specific microstructure of the superconducting YBCO foams is required. To obtain a set of reliable data, YBCO foam samples were investigated using optical microscopy, scanning electron microscopy and electron backscatter diffraction (EBSD). A variety of parameters including the size and shape of the cells and windows, the length and shape of the foam struts or ligaments and the respective intersection angles were determined to better describe the real foam structure. The investigation of the foam microstructures revealed not only the differences to the original polymer foams used as base material, but also provided further insights to the infiltration growth process via the large amount of internal surface in a foam sample.

We compare the temperature and field dependence of the critical current densities of high- $T_c$ superconductor materials intended for various bulk applications such as trapped-field magnets. This comprises bulk samples of YBa $_2$ Cu $_3$ O $_x$ (YBCO), MgB $_2$ , and iron-based materials, including also various versions of the YBCO compound such as melt-textured ones, infiltration-growth processed ones, and YBCO foams. Critical current densities and flux pinning forces were obtained from magnetization loops measured using Quantum Design SQUID and physical property measurement system (PPMS) systems with applied magnetic fields of up to $\pm$ 9 T. The obtained data are compared to each other with respect of the optimal cooling temperature possible using modern cryocoolers. Furthermore, we plot the temperature dependencies of the critical current densities versus the normalized temperature $t=T/T_c$ . This enables a direct judgement of the performance of the material in the trapped-field applications.

High-Temperature Superconductors (HTS) considerably accelerate the development of superconducting machines for electrical engineering applications such as fully electrical aircraft. This present contribution is an overview of different superconducting materials that can be used as magnetic screens for the inductor of high specific power electrical machines. The impact of the material properties, such as the critical temperature (Tc) and the critical current density (Jc), on the machine performances is evaluated. In addition, the relevance to flux modulation machines of different HTS bulk synthesis methods are addressed.

International audience; With the objective to concurrence the well-known melt texturing YBCO samples in term of irreversible field and critical current, infiltration growth YBCO bulks (IG-processed) show a recent increase of interest due to their advantage of better controlling the shape of the final product and better mechanical stability. Two types of IG-processed YBCO samples were developed: standard bulk samples and superconducting foams which show the advantage of a huge cooling capacity and a low weight due to his low density, particularly appreciated for space applications. Intended for use as permanent superconducting magnets (trapped field (TF) magnets), precedents works were done to evaluate the pinning properties of such materials by pinning force scaling (Dew-Hughes and Kramer) but only under relatively low magnetic field (up to 9 T). Considering a very high irreversible field (Hirr), even at 77 K, a lot of assumptions and extrapolations were needed. Small pieces of both designs (standard bulk and foam) were cut and mechanically polished (1 x 1 x 0.1 mm3) according to the crystallographic orientation (H // c). To experimentally determine the pinning properties for magnetic field above 9 T, we were able to make measurements at high magnetic field (above 30 T) for different temperatures between 85 K and 40 K with the help of EMFL facilities using Cantilever Torque Magnetometry at the High Magnetic Field Laboratory of Nijmegen.

International audience; For possible applications as trapped field (TF) magnets, it is essential to fabricate large, polycrystalline bulk samples from the FeSe compound, the simplest high-Tc superconductor (HTSc) possible. FeSe is relatively cheap to prepare, and does not contain any rare-earth material. The grain boundaries in this compound are not acting as weak links as it is the case for the YBCO compound. Although the transition temperature, Tc, is just below 10 K, the upper critical fields are comparable with other HTSc. Preparing the FeSe samples using solid-state sintering yields samples exhibiting strong magnetic hysteresis loops (MHLs), and the superconducting contribution is only visible after subtracting MHLs from above Tc. Due to the complicated phase diagram [1], the samples are a mixture of several phases, α-FeSe, β-FeSe, δ-FeSe (Fe7Se8) and metallic α-Fe [2]. The amount of the latter two phases depends directly on the Se loss during the sintering process. The δ-FeSe is antiferromagnetic, and α-Fe is ferromagnetic [3]. In the present contribution, we show MHLs of a variety of samples measured up to ±7 T and determine the magnetic characteristics, together with the amount of superconductivity determined from M(T) measurements. We performed a thorough analysis of the microstructures using polarization microscopy, Kerr effect, MFM, SEM, EBSD and TEM in order to establish a relation between microstructure and sample properties. To prepare good superconducting samples, the presence of the (anti)ferromagnetic phases must be reduced by carefully adjusting the Se content using Ti foils as getter materials. Measuring magnetoresistance of these samples [4] implies that the samples are always cooled in the own local field, and thus, the analysis of the resistance data calculating the fluctuation-induced conductivity above Tc [5] is strongly affected by this local magnetic field. We demonstrate the importance of preparing phase-pure FeSe samples, which are essential for the various applications envisaged. This work is part of the SUPERFOAM international project funded by ANR and DFG under the references ANR-17-CE05-0030 and DFG-ANR Ko2323-10, respectively. References [1] H. Okamoto, J. Phase Equilibria 12, 383 (1991). [2] P. Diko et al., Physica C 476, 29 (2012). [3] A. J. Williams, T. M. McQueen, R. J. Cava, Solid State Commun.149, 1507 (2009). [4] T- Karwoth et al., J. Phys. Conf. Ser. 1054, 012018 (2018). [5] A. Almessiere et al., Mat. Chem. Phys. 243, 122665 (2020).

International audience; The development of the sintering process for the synthetization of superconducting Iron-selenide (FeSe) polycrystal enables a quick and cheap fabrication of superconducting bulks for trapped field applications without the use of Rare Earths or toxic elements. A relevant number of papers were published in the literature in order to determine the best way to produce a pure β-phase, responsible of the superconductivity without any other non-desirable phases (α-FeSe, unreacted Fe, FeSe – also called δ-FeSe). Using the collected information, we produced a pure β-phase by classic sintering process at temperatures above 685°Cfollowed by annealing below 450°C for optimizing the sample and improvement of the critical currents. Using the Spark Plasma Sintering method enables an improved homogeneity and a better grain coupling, which leads to a higher quality for FeSe superconducting bulks and an increased sample density. The consequence is another increase of the critical currents which makes the samples competitive for trapped bulks applications.

Poster; International audience; Pieces of bulk, superconducting FeSe samples prepared by solid-state sintering were irradiated with 2.5 MeV electrons (T = 23.5 K) at SIRIUS facility using two different fluences, 2×10^19 electrons/cm2 and 4×10^19 electrons/cm2. In contrast to previous work performed on FeSe single crystals, the bulk FeSe samples are polycrystalline without specific texture and exhibit a density of ~4 g/cm3 (= 78% of theoret. density) due to pores. Furthermore, the samples contain magnetic impurities which lead to a ferromagnetic signal of the sample. Such bulk FeSe samples are intended as base material for superconducting trapped field magnets operating at 5 K, with the benefit of a higher upper critical field, Hc2, and reduced flux jump probability as compared to MgB2. The electron irradiation introduced point defects to the FeSe grains. As result, the superconducting transition temperature, Tc, is slightly reduced, depending on the fluence, but the critical currents are increased by about 20-30%. We discuss the changes of Tc, of the magnetic background and the superconducting properties of the samples before and after the electron irradiation.

Poster; International audience; We suggest the use of superconducting trapped field magnets (TF) for Flux-Pinning Docking Interfaces (FPDI) in satellite systems. Previous work introducing the FPDI concept to cube-sat satellites was using bulk high-Tc superconductors and permanent magnets of Nd-Fe-B. This arrangement was proven to work both experimentally and theoretically. However, such FPDIs have limited magnetic field strength, consist of heavy-weight material, and can only be used with a single purpose. Replacing the magnetic material in the FPDI by a TF-magnet would enable the interface to operate for both purposes, i.e., generating a (stronger) magnetic field and trapping it. We show the requirements for such a system and discuss the possible gains when using a TF-FPDI in satellites. A further possibility to reduce the system weight would be the use of superconducting foams as superconducting material.

International audience; The FeSe compound is the simplest high-temperature superconductor (HTSc) possible, and relatively cheap, not containing any rare-earth material. Although the transition temperature, Tc, is just below 10 K, the upper critical fields are comparable with other HTSc. Preparing FeSe using solid-state sintering yields samples exhibiting strong ferromagnetic hysteresis loops (MHLs), and the superconducting contribution is only visible after subtracting MHLs from above Tc. Due to the complicated phase diagram, the samples are a mixture of several phases, the superconducting beta-FeSe, and the non-superconducting delta-FeSe and gamma-FeSe. Furthermore, antiferromagnetic Fe7Se8 and ferromagnetic alpha-Fe may be contained, depending directly on the Se loss during the sintering process. Here, we show MHLs measured up to ±7 T and determine the magnetic characteristics, together with the amount of superconductivity determined from M(T ) measurements. We also performed a thorough analysis of the microstructures in order to establish a relation between microstructure and the resulting sample properties.

International audience; The current flow and the flux pinning properties on struts of superconducting YBa2Cu3Ox (YBCO) foams are analyzed in detail in the temperature range 60 K < T < Tc. For this purpose, magnetization loops were measured taken from various positions of a 5 × 2 × 2 cm3 large foam sample prepared at RWTH Aachen. From these data, the critical current densities, jc, and the flux pinning forces, Fp = jc × B, were calculated and pinning force scaling diagrams Fp/Fp,max vs. h = Ha/Hirr were established. The scaling in the temperature range 60 K < T < 90 K was found to be well developed for all samples with peak positions, h0, above 0.4, which is an indication of δTc-pinning. From microstructure analysis of the foam struts employing the electron backscatter diffraction technique (EBSD) [1] it is known that the struts are fully converted into the 123-phase with embedded 211 particles being relatively small. These small 211 particles and their specific distribution within the samples play an important role to explain the observations. The variation of Fp depending on the position of the struts in the original sample directly reveals the changes of the texture as found employing EBSD.[1] M.R. Koblischka, A. Koblischka-Veneva, E. S. Reddy, G. Schmitz, J. Adv. Ceram. 3, 317 (2014).

International audience; Modelling the mechanical and superconducting properties of superconducting, open-cell foam samples requires a proper description of their specific microstructure. For this purpose, foam samples are investigated using optical microscopy, SEM and x-ray tomography, enabling to identify the parameters important for modelling.

International audience; The Infiltration-Growth prepared YBa2Cu3O7-y (IG-YBCO) bulks recently received an increase of interest due to a possibility to control the shape and the density as compared to the well-known melt-textured YBCO bulks, IG method also gives the possibility to adapt the synthetization process like preparing a foam [1]–[3] for some particular uses like space applications, when a low-density sample is preferred. To optimize the performance of IG-YBCO an ultrasonic treatment at 300 W was done on the Y-211 powder and for different durations (15, 30, 45 and 60 min) in order to reduce the average grain size. Then, different YBCO bulk single grains were grown using the pre-treated powders with the infiltration-growth technique [4]. Intended for use as trapped field (TF) magnets, previous works were done on samples to evaluate and compare the superconducting properties such as the evaluation of the field dependence critical currents (Jc) or the pinning properties and magnetization loops using an MPMS-SQUID for up to 7 T. But the dependence of the magnetic field sweep rate on the flux creep into the sample and the evaluation of the dynamic relaxation rate is still needed in order to characterize and compare previous high field measurements where a relatively high sweep rate was needed (33 mT/s). Small pieces were cut and mechanically polished (2 x 2 x 0.4 mm3) according to the crystallographic orientation (H // c-axis). Magnetic measurements at various field sweep rates (from 1.25 to 40 mT/s) were done on the samples at various constant temperatures and up to 7 T using MPMS-SQUID in order to determine the impact of the flux creep on the sample properties at high fields such as Jc and irreversible fields (Hirr) as well as the associated dynamic relaxation rate.[1] E. S. Reddy et al., Supercond. Sci. Technol., vol. 15, no. 8, pp. L21–L24, Aug. 2002, doi: 10.1088/0953-2048/15/8/101.[2] J. G. Noudem, et al., Phys. C Supercond., vol. 390, no. 4, pp. 286–290, Jul. 2003, doi: 10.1016/S0921-4534(03)00755-X.[3] M. R. Koblischka et al., IEEE Trans. Appl. Supercond., vol. 29, no. 5, pp. 1–5, Aug. 2019, doi: 10.1109/TASC.2019.2894712.[4] S. Pavan Kumar Naik et al., Appl. Phys. Express, vol. 12, no. 6, p. 063002, Jun. 2019, doi: 10.7567/1882-0786/ab1c72.

Poster presentation; International audience; Solid Nitrogen (SN2) can provide a uniform and stable cryogenic environment for High Temperature Superconducting (HTS) systems such as bulk samples during their magnetization and/or characterization. In this paper, a SN2 cooling system consisting of a cryocooler Sumitomo CH-110 and an exchanger in a Liquid Nitrogen (LN2) bath is studied. In order to design this cooling system, an analytical model based on a nodal method coupled to a formulation of the thermal capacity was realized. The model considers the thermal parameters variation as well as the phase change of the Nitrogen. In order to compare our results, we performed a 3D simulation on COMSOL Multiphysics.

International audience; In this contribution, we compare the temperature and field dependence of the critical current densities of high-Tc superconductor materials intended for various bulk applications like trapped-field magnets. This comprises bulk samples of YBa2Cu3Ox (YBCO), MgB2 and FeSe, including also various versions of the YBCO compound like melt-textured ones, infiltration-growth processed ones, and YBCO foams. Critical current densities and flux pinning forces were obtained from magnetization loops measured using Quantum Design SQUID and PPMS systems with applied magnetic fields of up to 9 T. The obtained data are compared to each other with respect of the optimal cooling temperature possible using modern cryocoolers. Furthermore, we plot the temperature dependencies of the critical current densities versus a normalized temperature t = T/Tc. This enables a direct judgement of the performance of the material in the trapped field applications.

Présentation poster; National audience; Avec le développement des technologies supraconductrices ainsi que le besoin de décarboner notre énergie, de plus en plus d’acteurs industriels commencent à investir dans la supraconductivité. Ainsi, le besoin de connaitre plus en détail les propriétés de matériaux supraconducteurs de formes et de tailles variées (câbles, rubans, massifs, pastilles ou encore mousses supraconductrices) nécessite la mise en place de plateformes de mesures dédiées. C’est dans cette optique qu’au GREEN nous mettons en place des équipements pour préparer des échantillons (synthèse et/ou recuit, usinage, mise en condition) mais également pour déterminer leurs propriétés supraconductrices critiques (Tc, Jc, Hc, Hirr) par des méthodes magnétiques et/ou électriques. Parmi les équipements disponibles, un aimant de 9 T avec un trou de 150 mm sera utilisé pour des mesures de champs piégés sur des massifs supraconducteurs de taille centimétrique, de magnétométrie par cantilever et de transport de courant sous champs et à des températures variant de 4.2 à 300 K. Une alimentation permettant de générer des champs et des courants impulsionnels de plusieurs kA vient compléter cette installation.Enfin un cryostat a été développé pour la mise en place de nouvelles expériences d’ensembles plus complexes en utilisant l’azote gelé de 25 K à 63 K comme refroidissement.