Advantages of multi-seeded (RE)–Ba–Cu–O superconductors for magnetic levitation applications

Yunhua Shi,Anthony R Dennis,Kaiyuan Huang,David A Cardwell,Difan Zhou,John H Durrell

doi:10.1088/1361-6668/aad4f1

Abstract

Large, single grain (RE)Ba2Cu3O7 (where RE is a rare-earth element or yttrium) high temperature superconductors are technologically important materials due to their ability to trap large magnetic fields and to provide stable magnetic levitation for a number of potential high field applications. The fabrication of samples in the large singe grain form is a challenge, however, due to the characteristic slow growth rate of these materials and the need to produce samples that are electrically well-connected in order to generate trapped magnetic fields that are significantly greater than those produced by conventional permanent magnets (PM). In this work, we investigate whether large, single grain samples are optimum for the generation of high levitation forces for engineering applications. Three large bar-shaped Y–Ba–Cu–O samples of dimensions 60 × 20 × 12 mm3 were prepared for this investigation, including one single-seeded, one multi-seeded and one consisting of three square samples packed together closely in an array. The processing of these samples is described and their trapped field and levitation performance at 77 K measured using different PM arrays. We find that the multi-seeded samples and those assembled from smaller, individual bulk superconductors are able to achieve a higher levitation force than an equivalent single seed sample arrangement, at least in some geometries. This result is significant in that it suggests clearly that it is not always necessary to fabricate bulk (RE)BCO superconductors in the form of very large single grains for levitation applications, although the specific configuration of the system does need to be considered on an application-by-application basis.

Highlights

This pattern can be seen for Sample #2 and Sample #1. This explains the levitation force results in figure 4, where the external field applied to each sample changes. It can be seen from the measured levitation forces shown in figures 6(a), (d) and (g) that, when the applied field is generated by a permanent magnets (PM) stack of length 6 mm, the total trapped magnetic flux decreases in the following order: single-seeded; close packed array; multi-seeded
The fabrication of a large, YBCO single grain of dimensions 60 × 20 × 12 mm3 enabled the comparison of levitation forces at 77 K generated by three samples of the same overall geometry; single-seeded, multi-seeded and a close packed array of bulk single grains
The measured levitation forces of these samples show that the maximum levitation force, in descending order, is: multi-seeded; close packed; and singleseeded

Summary

Introduction

It is interesting to note, that, at a distance of 2.0 mm away from the top surface, the multi-seeded Sample #2 exhibits higher trapped fields Bz along the vertical axis at the centres of each single grain compared the close packed single grains, Sample #3 shows higher maximum peaks between the positions of the seeds. The order of the levitation forces of the samples, in descending order, is: Sample #2 (multi-seeded); Sample #3 (close packed array); Sample #1 (single-seeded), which is the reverse of the order observed for the peak trapped field measurements at the immediate top surfaces of the samples.

Results

Conclusion