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 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 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, 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. The shape of the pinning functions is found to be completely different from all other high-Tc 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. Index Terms-Foam, YBCO, critical currents, flux pinning, scaling. I. INTRODUCTION S UPERCONDUCTING foams are an interesting class of high-T c materials for various applications due to several advantages as compared to conventional bulk samples [1]-[5]. These advantages include facile oxygenation and effective cooling processes, scalability, light weight of the samples, easy shaping and reduced material costs [6], [7]. The superconducting foams may be useful for a variety of applications, starting from fault-current limiters to trapped field magnets and elements in electric motors and generators, where the reduced weight may play an important role. However, the high-T c superconducting foam samples prepared up to now pose many new questions concerning the current flow in such a 3D material and the corresponding flux pinning properties, which are essential to be answered for future applications of these materials. One important issue is the detailed understanding of Manuscript received xx.xx.xxxx. This work is a part of the SUPERFOAM international project funded by ANR and DFG under the references ANR-17-CE05-0030 and DFG-ANR Ko2323-10.
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