Title: Improvement of small to large grain A15 ratio in Nb ₃ Sn PIT wires by inverted multistage heat treatments

Abstract

The next generation of superconducting accelerator magnets for the Large Hadron Collider at CERN will require large amounts of Nb ₃Sn superconducting wires and the Powder-In-Tube (PIT) process, which utilizes a NbSn ₂-rich powder core within tubes of Nb(7.5wt%Ta) contained in a stabilizing Cu matrix, is a potential candidate. But, the critical current density, J _c , is limited by the formation of a large grain (LG) A15 layer which does not contribute to transport current, but occupies 25-30% of the total A15 area. Thus it is important to understand how this layer forms, and if it can be minimized in favor of the beneficial small grain (SG) A15 morphology which carries the supercurrent. The ratio of SG/LG A15 is our metric here, where an increase signals improvement in the wires A15 morphology distribution. We have made a critical new observation that the initiation of the LG A15 formation can be controlled at a wide range of temperatures relative to the formation of the small grain (SG) A15. The LG A15 can be uniquely identified as a decomposition product of the Nb6Sn5(Cu x ), surrounded by a layer of rejected Cu, thus the LG A15 is not only of lowmore » pin density, but is not continuous grain to grain. We have found that in single stage reactions limited to 630 °C - 690 °C, the maximum SG A15 layer thickness prior to LG A15 formation is very sensitive to temperature, with a maximum around 670 °C. This result led to the design of four novel heat treatments which all included a short, high temperature stage early in the reaction, followed by a slow cooling to a more typical reaction temperature of 630 °C. We also found that this heat treatment (HT) modification increased the SG A15 layer thickness while simultaneously suppressing LG A15 morphology, with no additional consumption of the diffusion barrier. In the best heat treatment the SG/LG A15 ratio improved by 30%. Unfortunately, J _c values suffered slightly, however further exploration of this high temperature reaction region is required to understand the limits to A15 formation in Nb3Sn PIT conductors.« less

Authors:

Segal, Christopher  ^[1];

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Tarantini, Chiara  ^[1];

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Lee, Peter J.  ^[1];

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• Search OSTI.GOV for ORCID "0000-0002-8849-8995"
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Larbalestier, David C.  ^[1]

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• Search OSTI.GOV for ORCID "0000-0001-7098-7208"
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+ Show Author Affiliations

1. Florida State Univ., Tallahassee, FL (United States). Applied Superconductivity Center

Publication Date:

2017-12-30

Research Org.:

Florida State Univ., Tallahassee, FL (United States)

Sponsoring Org.:

USDOE Office of Science (SC), High Energy Physics (HEP); National Science Foundation (NSF)

OSTI Identifier:

1415990

Grant/Contract Number:  

SC0012083; DMR-1157490

Resource Type:

Journal Article: Accepted Manuscript

Journal Name:

IOP Conference Series. Materials Science and Engineering

Additional Journal Information:

Journal Volume: 279; Conference: International Cryogenic Materials Conference (ICMC) 201, Madison, WI (United States), 9-13 Jul 2017; Journal ID: ISSN 1757-8981

Publisher:

IOP Publishing

Country of Publication:

United States

Language:

English

Subject:

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; 36 MATERIALS SCIENCE; 43 PARTICLE ACCELERATORS; Nb3Sn; superconductors; powder-in-tube technique; PIT; flux-pinning; critical current density; microstructure