Atomic-scale analyses of Nb3Sn on Nb prepared by vapor diffusion for superconducting radiofrequency cavity applications: a correlative study

Jaeyel Lee,Sam Posen,Zugang Mao,Daniel L Hall,Matthias Liepe,David N Seidman,Kai He,Yulia Trenikhina

doi:10.1088/1361-6668/aaf268

Abstract

We report on atomic-scale analyses of the microstructure of an Nb3Sn coating on Nb, prepared by a vapor diffusion process for superconducting radiofrequency (SRF) cavity applications using transmission electron microscopy, electron backscatter diffraction and first-principles calculations. Epitaxial growth of Nb3Sn on a Nb substrate is found and four types of orientation relationships (ORs) at the Nb3Sn/Nb interface are identified by electron diffraction or high-resolution scanning transmission electron microscopy (HR-STEM) analyses. Thin Nb3Sn grains are observed in regions with a low Sn flux and they have a specific OR: Nb3Sn //Nb and Nb3Sn //Nb The Nb3Sn/Nb interface of thin grains has a large lattice mismatch, 12.3%, between Nb and Nb3Sn (002) and a high density of misfit dislocations as observed by HR-STEM. Based on our microstructural analyses of the thin grains, we conclude that the thin regions are probably a result of a slow interfacial migration with this particular OR. The Sn-deficient regions are seen to form initially at the Nb3Sn/Nb interface and remain in the grains due to the slow diffusion of Sn in bulk Nb3Sn. The formation of Sn-deficient regions and the effects of interfacial energies on the formation of Sn-deficient regions at different interfaces are estimated by first-principles calculations. The finding of ORs at the Nb3Sn/Nb interface provides important information about the formation of imperfections in Nb3Sn coatings, such as large thin-regions and Sn-deficient regions, which are critical to the performance of Nb3Sn SRF cavities for accelerators.

Highlights

Nb3Sn is an A15-type superconductor, which has been actively studied and used in superconducting wire applications [1]
5Northwestern University Center for Atomic Probe Tomography (NUCAPT), Evanston, IL, 60208, USA Abstract We report on atomic-scale analyses of the microstructure of an Nb3Sn coating on Nb, prepared by a vapor diffusion process for superconducting radiofrequency (SRF) cavity applications using transmission electron microscopy (TEM), electron backscatter diffraction (EBSD) and first-principles calculations
A number of studies have employed Nb3Sn coatings on Nb for superconducting radiofrequency (SRF) cavity applications and these studies were motivated by the high critical temperature (Tc) and quality factor (Q0) of this superconductor at a given temperature, compared to Nb [2,3,4]; Q0 is defined by the surface resistance (Rs) and the geometric factor (G) of a cavity as G/Rs

Summary

Introduction

Nb3Sn is an A15-type superconductor, which has been actively studied and used in superconducting wire applications [1]. A number of studies have employed Nb3Sn coatings on Nb for superconducting radiofrequency (SRF) cavity applications and these studies were motivated by the high critical temperature (Tc) and quality factor (Q0) of this superconductor at a given temperature, compared to Nb [2,3,4]; Q0 is defined by the surface resistance (Rs) and the geometric factor (G) of a cavity as G/Rs. Nb3Sn has lower surface resistance and a higher Tc than the Nb [5, 6]. Studies at Cornell [7, 8] reported a high Q-factor of approximately 1010 at 4.2 K, with a maximum accelerating electric field gradient up to 17 MV/m for ~2 μm thick Nb3Sn coatings on Nb, prepared by a vapor diffusion process. Active research in vapor diffusion Nb3Sn films is on-going at Fermilab, Cornell University and Jefferson Laboratory [9]

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Discussion

Conclusion