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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; 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.

International audience; Various MgB2 thin films and single crystals were found in the literature to exhibit a sharp, narrow peak at low fields in the volume pinning force, Fp(H)-diagrams. The origin of this peak is associated with a steep drop of the current density when applying external magnetic fields and is ascribed to sample purity. We show here that bulk MgB2 prepared by spark-plasma sintering also shows the sharp, narrow peak in Fp. The peak is also seen in the volume pinning force scaling, Fp/Fp,max vs h = H/H irr. Furthermore, polycrystalline bulk MgB2 samples prepared close to the optimum reaction temperature reveal this peak effect as well, but other samples of the series show a regular scaling behavior. The combination of magnetization data with data from electric transport measurements on the same samples demonstrates the origin of this peak effect. On increasing preparation temperature, the pinning force scaling changes from grain boundary pinning to point pinning and the grain connectivity gets worse. Hence, the sharp, low-field peak in Fp vanishes. Therefore, the occurrence of the peak effect in Fp gives important information on the grain coupling in the MgB2 samples.

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.

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.