Critical current densities and microstructures in Rod-in-Tube and Tube Type Nb3Sn strands - Present status and prospects for improvement

TL;DR

This paper compares the microstructures and critical current densities of Rod-in-Tube and Tube Type Nb3Sn strands, identifying microstructural factors affecting performance and proposing architectural improvements for better superconducting properties.

Contribution

It provides a detailed microstructural analysis linking A15 phase area fraction to critical current density differences and suggests a simple architectural change to enhance strand performance.

Findings

Tube Type strands have smaller magnetizations and better low-field stability.

Lower A15 area fraction causes reduced Jc in Tube Type strands.

Proposed architectural change aims to increase the A15 area fraction.

Abstract

In this work, the transport and magnetization properties of distributed-barrier Rod-in-Tube (RIT) strands and Tube Type strands are studied. While Tube Type strands had smaller magnetizations and thus better stabilities in the low field region, their 12 T non-Cu Jcs were somewhat smaller than those of the RIT strands. Microstructures were investigated in order to find out the reasons for the difference in non-Cu Jc values. Their grain size and stoichiometry were found to be comparable, leading to similar layer Jcs. Accordingly it was determined that the lower A15 area fraction rather than the quality of A15 layer was the cause of the discrepancy in non-Cu Jc. Subsequently, the area utilizations of subelements were investigated. While for a RIT strand the fine grain (FG) A15 area occupies ~60% of a subelement, for a Tube Type strand it is no more than 40%. Further analysis indicates that the low FG area fraction in a Tube Type strand is attributed to its much larger unreacted Nb area fraction. Finally, a simple change in strand architecture is proposed to reduce the unreacted Nb area fraction.