SRF2017 - List of Authors (Lee, P.J.)

Paper	Title	Page
TUXBA05	Hydrogen Distribution and Hydride Precipitation in SRF Nb Revealed by Metallographic Techniques
	S. Balachandran, P.J. Lee NHMFL, Tallahassee, Florida, USA S. Chetri, D.C. Larbalestier ASC, Tallahassee, Florida, USA P. Dhakal JLab, Newport News, Virginia, USA
	The current issues that are of importance for SRF Nb cavities include: flux trapping depending on temperature gradient at superconducting transition, variation in flux trapping depending on precursor Nb material, and quality or process control of SRF Nb sheets that would lead to consistent performance. Our work relates to exploring microstructure correlations relevant to the above topics using coupon SRF Nb samples. Of particular importance are: a) hydride precipitation which could occur during cavity cooling below cryogenic temperatures, b) variation in surface superconducting properties and N doping, c) extent of surface damage in as received Nb sheets, and d) distribution of dislocation and substructures which lead to lattice strains in the material. The main results of our studies suggest the role of GB's in hydride formation, direct evidence of N doping preventing hydrides, and occurrence of strained lattice features depending on the surface treatment of SRF Nb. We also look at how this type of study can supplement and strengthen the effort to include quantitative microstructure based features in our understanding, and modelling of SRF superconductivity in Nb.
	Slides TUXBA05 [12.981 MB]
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THPB002	Role of Nitrogen on Hydride Nucleation in Pure Niobium by First Principles Calculations	741
	P. Garg, I. Adlakha, K.N. Solanki Arizona State University, Tempe, USA S. Balachandran, P.J. Lee NHMFL, Tallahassee, Florida, USA T.R. Bieler Michigan State University, East Lansing, Michigan, USA
	It is known that formation and growth of Nb hydride degrades superconducting radio frequency (SRF) properties of Nb cavities and the treatments that reduce H concentration improve quality factor. Recently it is has also been shown that addition of N through doping or infusion improves the quality factor. Thus, we probe role of N addition in Nb on hydride precipitation and stability through first principles calculations & compared with coupon samples. In presence of N, energetic preference for H to occupy interstitial sites in the vicinity of N is reduced. Furthermore, presence of N forces H to occupy interstitial octahedral site instead of a tetrahedral site. The thermodynamic stability of hydride is decreased in the presence of N in Nb.The quantum insights using charge transfer and density of states show a strong tendency of N to accumulate charge, thereby decreasing the bond strength of neighboring Nb and H atoms. These atomic scale results explain the lesser tendency of surface hydride formation in SRF Nb cavities in presence of N. These results are consistent with metallographic examination of N-treated Nb coupons, which show suppressed hydride formation near N-treated surface.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-THPB002
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THPB026	Investigation of the Effect of Strategically Selected Grain Boundaries on Superconducting Properties of SRF Cavity Niobium	787
	M. Wang, T.R. Bieler Michigan State University, East Lansing, Michigan, USA S. Balachandran, P.J. Lee NHMFL, Tallahassee, Florida, USA S. Chetri, A. Polyanskii ASC, Tallahassee, Florida, USA C. Compton FRIB, East Lansing, Michigan, USA
	Funding: Research supported by DOE/OHEP contracts DE-SC0009962, DE-SC0009960, NSF-DMR-1157490, and the State of Florida. High purity Nb is commonly used for fabricating SRF cavities due to its high critical temperature and its formability. However, microstructural defects such as dislocations and grain boundaries in niobium can serve as favorable sites for pinning centers of magnetic flux that can degrade SRF cavity performance. In this study, two bi-crystal niobium samples extracted from strategically selected grain boundaries were investigated for the effect of grain misorientation on magnetic flux behavior. Laue X-ray and EBSD-OIM crystallographic analyses were used to characterize grain orientations and orientation gradients. Cryogenic Magneto-Optical Imaging (MOI) was used to directly observe magnetic flux penetration at about 5-8 K. Flux penetration was observed along one of the grain boundaries, as well as along a low angle boundary that was not detected prior to MOI imaging. Hydride scars on the sample surface after MOI were examined using atomic force microscopy (AFM) analysis. The relationships between dislocation content, cryo-cooling, flux penetration and grain boundaries are examined.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-THPB026
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Paper

Title

Page

TUXBA05

Hydrogen Distribution and Hydride Precipitation in SRF Nb Revealed by Metallographic Techniques

S. Balachandran, P.J. Lee
NHMFL, Tallahassee, Florida, USA
S. Chetri, D.C. Larbalestier
ASC, Tallahassee, Florida, USA
P. Dhakal
JLab, Newport News, Virginia, USA

The current issues that are of importance for SRF Nb cavities include: flux trapping depending on temperature gradient at superconducting transition, variation in flux trapping depending on precursor Nb material, and quality or process control of SRF Nb sheets that would lead to consistent performance. Our work relates to exploring microstructure correlations relevant to the above topics using coupon SRF Nb samples. Of particular importance are: a) hydride precipitation which could occur during cavity cooling below cryogenic temperatures, b) variation in surface superconducting properties and N doping, c) extent of surface damage in as received Nb sheets, and d) distribution of dislocation and substructures which lead to lattice strains in the material. The main results of our studies suggest the role of GB's in hydride formation, direct evidence of N doping preventing hydrides, and occurrence of strained lattice features depending on the surface treatment of SRF Nb. We also look at how this type of study can supplement and strengthen the effort to include quantitative microstructure based features in our understanding, and modelling of SRF superconductivity in Nb.

Slides TUXBA05 [12.981 MB]

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reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

THPB002

Role of Nitrogen on Hydride Nucleation in Pure Niobium by First Principles Calculations

741

P. Garg, I. Adlakha, K.N. Solanki
Arizona State University, Tempe, USA
S. Balachandran, P.J. Lee
NHMFL, Tallahassee, Florida, USA
T.R. Bieler
Michigan State University, East Lansing, Michigan, USA

It is known that formation and growth of Nb hydride degrades superconducting radio frequency (SRF) properties of Nb cavities and the treatments that reduce H concentration improve quality factor. Recently it is has also been shown that addition of N through doping or infusion improves the quality factor. Thus, we probe role of N addition in Nb on hydride precipitation and stability through first principles calculations & compared with coupon samples. In presence of N, energetic preference for H to occupy interstitial sites in the vicinity of N is reduced. Furthermore, presence of N forces H to occupy interstitial octahedral site instead of a tetrahedral site. The thermodynamic stability of hydride is decreased in the presence of N in Nb.The quantum insights using charge transfer and density of states show a strong tendency of N to accumulate charge, thereby decreasing the bond strength of neighboring Nb and H atoms. These atomic scale results explain the lesser tendency of surface hydride formation in SRF Nb cavities in presence of N. These results are consistent with metallographic examination of N-treated Nb coupons, which show suppressed hydride formation near N-treated surface.

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-THPB002

Export •

reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

THPB026

Investigation of the Effect of Strategically Selected Grain Boundaries on Superconducting Properties of SRF Cavity Niobium

787

M. Wang, T.R. Bieler
Michigan State University, East Lansing, Michigan, USA
S. Balachandran, P.J. Lee
NHMFL, Tallahassee, Florida, USA
S. Chetri, A. Polyanskii
ASC, Tallahassee, Florida, USA
C. Compton
FRIB, East Lansing, Michigan, USA

Funding: Research supported by DOE/OHEP contracts DE-SC0009962, DE-SC0009960, NSF-DMR-1157490, and the State of Florida.
High purity Nb is commonly used for fabricating SRF cavities due to its high critical temperature and its formability. However, microstructural defects such as dislocations and grain boundaries in niobium can serve as favorable sites for pinning centers of magnetic flux that can degrade SRF cavity performance. In this study, two bi-crystal niobium samples extracted from strategically selected grain boundaries were investigated for the effect of grain misorientation on magnetic flux behavior. Laue X-ray and EBSD-OIM crystallographic analyses were used to characterize grain orientations and orientation gradients. Cryogenic Magneto-Optical Imaging (MOI) was used to directly observe magnetic flux penetration at about 5-8 K. Flux penetration was observed along one of the grain boundaries, as well as along a low angle boundary that was not detected prior to MOI imaging. Hydride scars on the sample surface after MOI were examined using atomic force microscopy (AFM) analysis. The relationships between dislocation content, cryo-cooling, flux penetration and grain boundaries are examined.

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-THPB026

Export •

reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)