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 ($T_c$) and the critical current density ($J_c$), on the machine performances is evaluated. In addition, the relevance to flux modulation machines of different HTS bulk synthesis methods are addressed.

The FeSe compound is the simplest high-temperature superconductor (HTSc) possible, and relatively cheap, not containing any rare-earth material. Although the transition temperature, T_c, 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 T_c. 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.

We compare the temperature and field dependence of the critical current densities of high- $T_c$ superconductor ma-terials intended for various bulk applications like trapped-field magnets. This comprises bulk samples of $\rm YBa_{2}Cu_{3}O_x$ (YBCO), $\rm MgB_2$ and iron-based materials, 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 the normalized temperature $t = T/T_c$ . This enables a direct judgement of the performance of the material in the trapped field applications.

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 $\leq T \leq T_c$ . For this purpose, magnetization loops were measured on foam struts taken from various positions of a 5 × 2 × 2 cm $^3$ large foam sample prepared at RWTH Aachen. From these data, the critical current densities, $j_c$ , and the flux pinning forces, $F_p = j_c \times B$ , were calculated and pinning force scaling diagrams $F_p/F_{p,{\max}}$ versus $h = H_a/H_{\rm irr}$ 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 $\delta T_c$ -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.

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, F_p(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 F_p. The peak is also seen in the volume pinning force scaling, F_p/F_p,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 F_p vanishes. Therefore, the occurrence of the peak effect in F_p gives important information on the grain coupling in the MgB₂ samples.

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.