Current Flow and Flux Pinning Properties of YBCO Foam Struts

Abstract

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.

Superconducting YBCO Foams as Trapped Field Magnets

“…On side (4), the maximum trapped field of 400 G is recorded. This is a surprising result, as we would have expected the maximum trapped field either on side (1) or on side (3), as position-dependent measurements of the properties of foam struts had shown increasing j c towards the bottom of the foam sample [30].…”

Superconducting YBCO Foams as Trapped Field Magnets

“…On Side (4), the maximum trapped field of 400 G is recorded. This is a surprising result, as we would have expected the maximum trapped field either on Side (1) or on Side (3), as position-dependent measurements of the properties of foam struts had shown increasing jc towards the bottom of the foam sample [30]. Remarkably, there are many of the small, sharp peaks especially on Side (3), the bottom side of the foam sample.…”

Microstructural Parameters for Modelling of Superconducting Foams

“…From the bottom side, which was closest to the liquid source, the shape is found to change from the parabolic type (which contains more material on the ligands) to a more cusp-like type at the top face of the foam sample, which indicates the amount of liquid which was present during the processing. Previous work on flux pinning and critical currents in the foam sample [ 52 , 53 ] also revealed a clear dependence on the position within the original foam, and the TF measurements on all sides of a foam sample demonstrated that the top side was the weakest one [ 13 ]. Thus, this finding is another important issue for improving the properties of future YBCO foam samples.…”