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Characterization and comparison of new bulk superconductors

Applicant Dr.-Ing. Andreas Tschöpe, since 6/2021
Subject Area Synthesis and Properties of Functional Materials
Term from 2017 to 2022
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 391899738
 
Final Report Year 2022

Final Report Abstract

The project had originally three main aims. Firstly, features of two kinds of materials, MgB2 and YBCO were to be compared to each other with respect to applications. Secondly, superconducting foams, a new kind of bulk superconductors, should be studied and characterized. Thirdly, the measurement of the magnetization of the bulk, in situ, in superconducting electrical machines was planned. In the project runtime, a fourth topic was investigated: We have explored the possibilities of the simplest iron-based HTSc, FeSe, which offers a combination of the advatages of MgB2 with the properties of the true HTSc materials, even though the Tc of this material is only about 8-9 K. The first part was done in collaboration with P. Badica (Magurele, Romania), enabling the magnetic characterization and trapped field measurements of MgB2 samples prepared by spark-plasma sintering (SPS). Trapped-field (TF) magnets were constructed using up to 6 MgB2 pellets, and a maximum TF field of 6.78 T (center, 12 K) was recorded. Furthermore, we attempted to perform 3D X-ray measurements on MgB2 and YBCO pellets, resulting in a 3D mapping of the residual pores of the samples. Thus, worthy information about the internal microstructure of the samples could be obtained. The YBCO system was further investigated by high-field cantilever magnetometry (HFML Nijmegen), enabling the recording of magnetic data up to 33 T applied magnetic field. These measurements enabled a determination of the irreversibility fields and the current density down to 40 K, filling an important void in the literature. For the second part, the YBCO foam samples, a variety of characterization measurements of the magnetic properties and the respective microstructure was performed in this project. All the data collected enable a modelling of the superconducting foams. Although the obtained TF values of the foam material are quite low at 77 K, it is clear that the foams can act as TF magnets, and lowering the temperature by means of a cryocooler will improve the field trapping capability considerably. The first results of this modelling already provided valuable information about the limits of such samples, suggesting that a more regular structure would be beneficial. Such structures could be e.g., realized by means of 3D printing. Based on the experimental practice with the foam samples, we could come up with possible applications of TF-magnets in space – the so-called FPDI interfaces of small satellites (cube sats). We have developed a new scheme for this situation, and deduced a novel design of a compact TF-magnet energized by pulsed fields, dubbed PFM. For the use in satellites, the win by reducing the sample weight of the superconductor will bring a larger payload as well as a much more flexible use of these systems, as the TF magnets can be energized in space. The fabrication/characterization of FeSe bulk sample provided insight to a quite complicated material system having several competing phases. Also in this case, the SPS-technique enabled the fabrication of highly dense samples. A remaining task is now the design of additional flux pinning sites in these samples to improve the achievable critical current densities by minimizing the FeSe grain size. This will be a task for future work. Overall, the present project enabled a deeper understanding of all the investigated materials, resulted in 24 publications in refereed journals, and created several new topics and directions for both experiments and modelling in future work.

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