Identification of the A15 Contours in Multifilamentary Nb${3}$Sn Wires by Means of a Magnetic Method

Mattia Ortino,Michael Eisterer

doi:10.1109/tasc.2021.3063996

Abstract

A model for evaluating thecontours of the A15 layer in Nb 3 Sn wires is proposed. These borders, crucial for the correct assessment of the layer current density J c and the volume layer pinning force F p (B) are in fact non trivial to assess in some cases from scanning electron microscopy (SEM) polished surface images, eventually leading to non-negligible discrepancies between independent measurements reporting the same m-H or I c values. A self-reliant method from magnetometry based (Bean) models is proposed, allowing the outer (r o ) and inner (r i ) contours of the A15 phase to be assessed. The magnetic moment of short samples of a prototype powder in tube (PIT) wire with artificial pinning centres (APC) was measured between 6.5 T (H 0 ) and 6.95 T at 4.2 K in a superconducting quantum interference device (SQUID) magnetometer, allowing the calculation of r o from the initial slope of dm/dH. Then, the derived function for the remanent magnetic moment m rem is fit to the experimental data in order to derive r i , under the condition that the saturation moment exactly matches the experimental value.

Highlights

The evaluation of the layer Jc, the critical current flowing through the effective superconducting cross-section of a wire/tape, has always been crucial for the determination of the pinning characteristics of technical superconductors
We proposed a magnetic method for assessing the cross section of the effectively current-carrying elements of a filamentarystructured technical superconductor, when scanning electron microscopy (SEM) evaluations are not sufficiently reliable
This solution is suited for N b3Sn multi-filamentary wires, where the contours of the finegrained A15 region are not clear and applicable to all types of wires whose filaments or sub-elements can be modelled as tubes

Summary

INTRODUCTION

The evaluation of the layer Jc, the critical current flowing through the effective superconducting cross-section of a wire/tape, has always been crucial for the determination of the pinning characteristics of technical superconductors. In the specific case of multi-filamentary Nb3Sn wires, especially at their prototype stage, the right identification of the effective superconducting phase (”A15 phase”) borders turns out to be sometimes ambiguous, possibly leading to non-negligible discrepancies between independent measurements. This is usuallyr due to: Morphological differences among the filaments through the cross-section, especially in non-optimized wires (subr elements bigger than 50 μm); Longitudinal inhomogeneities:. We propose a method to cross-check the ro and Aeff , from magnetometry based models

EXPERIMENTAL

CRITICAL CURRENT DENSITY EVALUATION

EVALUATION OF SUB ELEMENT GEOMETRY

CONCLUSION