High temperature superconductors (HTSs) are enablers of extensive electrification for aircraft propulsion. Indeed, if used in electrical machines, HTS materials can drastically improve their performance in terms of the power-to-weight ratio. Among the different topologies of superconducting electrical machines, a flux modulation machine based on HTS bulks is of interest for its compactness and light weight. Such a machine is proposed in the FROST (Flux-barrier Rotating Superconducting Topology) project led by Airbus to develop new technologies as part of their decarbonization goals driven by international policies. The rotor of the machine will house large ring-segment-shaped HTS bulks in order to increase the output power. However, the properties of those bulks are scarcely known and have barely been investigated in the literature. In this context, the present work aims to fill out partially this scarcity within the framework of FROST. Thus, a thorough characterisation of the performances and homogeneity of 11 large REBaCuO bulks was carried out. Ten of the bulks are to be utilized in the machine prototype, originally keeping the eleventh bulk as a spare. A first set of characterisation was conducted on the eleven bulks. For this set, the trapped field mapping and the critical current were estimated. Then, a series of in-depth characterisations on the eleventh bulk followed. It included critical current measurement, X-ray diffraction, and scanning electron microscopy on different millimetre-size samples cut out from the bulk at various locations. The X-ray diffraction and scanning electron microscopy showed weakly oxygenated regions inside the bulk explaining the local drop or loss in superconducting properties. The objective was to determine the causes of the inhomogeneities found in the trapped field measured on all the bulks, sacrificing one of them, here the spare one. To help obtain a clearer picture, a numerical model was then elaborated to reproduce the field map of the eleventh bulk using the experimental data obtained from the characterisation of its various small samples. It is concluded that further characterisations, including the statistics on various bulks, are still needed to understand the underlying reasons for inhomogeneity in the trapped field. Nonetheless, all the bulks presented enough current density to be usable in the construction of the proposed machine.

Flux-Pinning Docking Interfaces (FPDI) in satellite systems were developed using bulk superconductors and permanent magnets in previous works. However, such FPDIs have limited magnetic field strength, consist of heavy-weight material, and can only be used with a single purpose, i.e., as chasing or docking satellite. Replacing the magnetic material in the FPDI by a trapped field (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. To reduce the system weight, the use of superconducting foams as superconducting material is discussed in detail. Furthermore, the use of superconducting foams, the size of which can be easily upscaled, may also comprise the function of the damping material, so even more weight could be saved for the payload.

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.

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 β-FeSe, and the non-superconducting δ-FeSe and γ-FeSe. Furthermore, antiferromagnetic Fe7Se8 and ferromagnetic α-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.

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.

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.

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/Hirr. 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.

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 YBa2Cu3Ox (YBCO), MgB2, 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 +/- 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.

Bulk, polycrystalline samples of FeSe are interesting candidates for trapped field applications as the material can be prepared by simple solid-state sintering, is free of toxic elements and exhibits magnetic properties like the copper-based high-Tc superconductors, i.e., high upper critical fields Hc2, and relatively small anisotropy. Polycrystalline, sintered FeSe material shows further promising features such as strong grain coupling. The FeSe material consists of well-coupled, but randomly oriented, platelet-like round grains with a diameter between 1 and 6 µm.However, the critical currents of the FeSe system are still low, but comparable to sintered MgB2. The goal of the present work is to explore the possibilities of the FeSe system in application-type samples (i.e., polycrystalline material, not single crystals) using electron irradiation. 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. The electron irradiation introduced point defects to the FeSe grains. The changes of Tc due to irradiation and the critical currents were measured using SQUID magnetometry. As result, the superconducting transition temperature, Tc, is slightly reduced, depending on the fluence, but the critical currents are increased by about 20-30%, which demonstrates that one can introduce additional disorder to FeSe to improve the flux pinning properties also in the polycrystalline material.

The current flow and the flux pinning properties on struts of superconducting YBa 2 Cu 3 O x foams are analyzed in detail in the temperature range 60 K <; T <; T c . For this purpose, magnetization loops were measured on foam struts taken from various positions of a 5 x 2 x 2 cm 3 large foam sample prepared at RWTH Aachen. From these data, the critical current densities, j c , and the flux pinning forces, F p = j c x B, were calculated and pinning force scaling diagrams F p /F p,max versus h = H a /H irr 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, h 0 , above 0.4, which is an indication of δT c -pinning. The shape of the pinning functions is found to be completely different from all other high-T c materials and varies only slightly with position. This specific dome shape cannot be described by an addition of several pinning functions, and the parameters p and q do not fit to the description of Dew-Hughes. Therefore, we employ Kramer plots to obtain more information on the flux pinning mechanism.

Synthèse, caractérisation et comparaison de nouveaux supraconducteurs massifs Des matériaux supraconducteurs sont d'ores et déjà utilisés pour diverses applications technologiques comme les IRM, les accélérateurs de particules, les TOKAMAKs ou les câbles supraconducteurs. Avec ceci, de nouveaux projets sont en cours de développement comme des moteurs électriques supraconducteurs ou des écrans magnétiques ce qui peut nécessiter l'utilisation de massifs supraconducteurs comme aimant permanent ou écran magnétique. Dans cette optique, cette thèse est centrée sur la synthèse, la caractérisation et la comparaison de différents supraconducteurs massifs tels que les supraconducteurs à base de Fer ou d'oxydes de cuivre. Les synthèses par four ou par frittage flash ont été utilisés pour synthétiser des supraconducteurs en Fer-Sélénium pour optimiser leurs propriétés et la pureté de la phase supraconductrice. La caractérisation des propriétés supraconductrices et cristallographiques a été réalisés sur les matériaux synthétisés mais également sur d'autres matériaux comme les supraconducteurs REBaCuO (Terra rare, Baryum et oxyde de Cuivre). Différentes méthodes de caractérisation sont présentées dans ce manuscrit comme la magnétométrie à l'aide d'un MPMS-SQUID (Magnetic Property Measurement System, Superconducting Quantum Interference Device), de la cartographie de champs piégés, de la diffraction de rayon X ou bien de la microscopie optique ou à balayage électronique. J'ai également eu l'occasion de faire de la magnétométrie par cantilever sous de fort champs magnétiques allant jusqu'à 33 T sur des supraconducteurs YBaCuO fabriqué par infiltration. Une comparaison des propriétés mesurées avec d'autres matériaux supraconducteurs plus développés a été fait pour déterminer la maturité de cette technologie et sa compétitivité sur les applications potentielles en génie électrique.