Large Grain CBMM Nb Ingot Slices:

An Ideal Test Bed for Exploring Microstructure-Electromagnetic Property Relationships Relevant to SRFZu-Hawn Sung *, Peter J. Lee, Anatolii Polyanskii ,

Shreyas Balachandran, Santosh Chetri, David C. Larbalestier – Applied Superconductivity Center/NHMFL* Now at US Steel

Mingmin Wang, Christopher Compton and Thomas R. Bieler – Michigan State University

Ingot Niobium Summary Workshop – December 4, 2015

Outline

Premature flux penetration along the grain boundary (MO)

Angular effect on GB flux penetration (MO)

The effects of surface chemical treatments on GB flux penetration (MO)

GB flux penetration by DC transport V-I characterization

Microstructural features at the BCP’ed GB region (TEM)

Preferential flux penetration in single crystal after very low strains (MO)

Magneto-Optical Imaging

Allows direct Imaging of Bz in plane above superconductor

Double Faraday effect occurs in reflective mode using Bi-doped YFe indicator film with in-plane magnetization

Field Cooled (FC): shows bulk property of SC

Zero Field Cooled (ZFC): shows surface property of SC

A. Polyanskii

Tri-crystalTri-crystal

Tri-crystal

Tri-crystal

Bi-crystalGB (#1)

Normal to Surface

Bi-crystalGB (#1)

Normal to Surface

Tri-crystalTri-crystal

Thickness of sheet is 1.88 mm

GB #2

GB #1RF field in-plane

JLab slice1: Multiple samples from GBs

GB1: Premature flux penetration along the grain boundary

Our earliest magneto optical studies on large grain Nb samples showed that there was preferential flux penetration at grain boundaries.

Misorientation angle between grains ≈17.8o Orientation Imaging Microscopy (OIM): from Abraimov

After BCP

MO

H

Magneto Optical Sequences: ZFC

JLAB #1 ZFC T=5.6 K JLAB #1 ZFC T=7 K

MSC

H

Magneto Optical Sequences: ZFC

JLAB #1 ZFC T=8.3 K JLAB #1 ZFC T=8 K

MSC

H

Zig-zag domain walls nucleate above the Meissner state due to in-plane components Hx , which are equal to zero above the zig-zag walls where only vertical components Hz exist.

Non-perpendicular GB: No penetration

In this more complex GB triple point the GBs twist through the specimen and are not aligned with the ⊥magnetic field – no premature flux penetration along GB is observed.

MO

H

Angular effect on GB flux penetration

GB trace on top face of sample

No GB trace on front face

GB trace on bottom face of sample

This face has been imaged by MO, when sample was turned by 900

H

MO indicator

2.78mm

2.17

mm1.89mm

H = 80 mTZFC

Flux penetration again when Hext GB∥

Isolated a small sample from the above bi-crystal then, rotated by ~90° to align the GB plane parallel to the surface normal

And compared the BCP’ed with the smooth-polished.

H = 80 mT H = 112 mT

H = 0 mT

ZFC T = 6.2 K FC T = 6.2 K

Before polishing

H = 80 mT H = 100 mT

1 mm

H = 0 mT

FC T = 6.4 K

ZFC T = 6.4 K

GB

After polishing

The bulk pinning of magnetic flux is symmetric and only the flux penetration is asymmetric.There is no topological effects on the preferential flux penetration

H

BCP vs. EP: Similar flux penetration

H = 58 mT

FC T = 6 K

H = 0 mT

ZFC T = 6 K

H = 72 mT H = 0 mT

BCP’ed

EP’ed ZFC T = 6.5 K

FC T = 6.5 K

Reduced thickness, and then compared with the EP’ed bi-crystal cut from same GB

1 mm

GB is a weak link only when Hext is aligned parallel with the GB planeThe GB groove may not the cause of the preferential flux penetration

HBoth: GB∥Hext

~20-30 mT

200mT

As-received Mechanically ground

Surface image of I-shape sample after BCP treatment

1 mm

100μm

Expand the gap between Hc1 (170 mT) and Hc2 (200 mT) at 4.2 K → Make vortex penetration at lower Hext

1. Cut samples into I-shape with wire-EDM2. Mechanically grind down the bottom of

the sample surface to ~150-250μm, so the top surface remain as-received condition

3. Ultra fine polish with vibratory polisher (Vibromet ® Buehler)

4. Finalize all surfaces with either BCP or EP

- Make surfaces representative of real cavity surface

5. Further reduce the bridges of some I-shape single- & bi- crystals with extra BCP

6. Artificially groove with FIB and mechanically smear away the grooved produced by the chemical treatments

The procedures

DC transport V-I characterization with 1T Electromagnet

Higher-Hc SCNb

Preferential flux flow on the deep GB groove

A deep (3-5 μm) and highly inclined groove No groove (~0.5–2.0μm roughness)

• The V-J characteristics show that the grain boundary is a channel of preferential flux flow (FF) by weakly pinned vortices.

• Flux flow evidence from H = 0.08 T to 0.28 T

• However, the slightly non-ohmic V-I response suggests that flux flow is not just confined to a single vortex row flowing along the grain boundary

0.05T0.08T

0.10T

BCP'ed Single Crystal BCP'ed Bi-Crystal

Angular dependence of preferential GB flux flow

A deep (3-5μm) and highly inclined groove

Preferential flux flow Hext = 0.08 T to 0.28 T when the

GB plane // Hext

The # is the angle between the GB plane and Hext

0.08T0.10T

0.05T

Hext

Does BCP vs. EP influence V-J?

Linear coordinates

Flux flow?

• Very different responses. • No distinct flux flow evidence at the electropolished GB, similar to BCP’ed Single crystal• However, traces of flux flow along the electropolished GB are visible

Linear coordinates

BCP’ed EP’ed

GB flux flow

Do topological features play an important role in preferential flux flow on V-I characteristics?

Artificially grooved single crystal (using FIB)

Very flat surface by ultra-fine polishing

V-J response of 26° Bi-crystalV-J response of Single crystal

0.08T0.10T

0.05T0.13T

0.18T

0.20T

Flux flow

Preferential flux flow at the grain boundary may be not triggered by surface topological features when GB plane is parallel to Hext

by FIB(Focused Ion beam)

Angular dependence of Hext on GB

BCP’ed – groove effect Flattened – No surface effect

H = 0.08T

• Angular dependency of flux flow at GB becomes more pronounced in non-grooved sample compared to BCP’ed, GB-grooved one.

• When GB is angularly aligned to external filed, the GB may split vortices treading at the GB into two or more parts or enlarge the length of vortex channel. Thus GB enhances Jc

The number of degree indicates the angle of between a plane of GB and external magnetic field (GB vs Hext)

11012

1

Darker contrast due to high misorientation angle across GB

Grain Boundary Dislocations Possible dislocations Pile-up at G.BPrepared by 30min BCP after

mechanical thinning ~30-50 μm

Precipitation contrast at GBSeveral strain and dislocation contrast

Uniform transmission contrast indicates no step at GB

Prepared by 50min BCP after mechanical thinning ~80-100 μm

Bright Field (BF) TEM images at the GB

BF TEM image of the cross-sectional view of the GB

19

GB

Au-Pd

Oxide

GB

Au-PdOxide

Λ ~ 40nm

GBGB

OxideOxide

Au-PdAu-Pd

Inclusions

Native oxide : Nb2O5 5-10 nm

Interface : sub oxides + interstitial oxygen : some monolayers.

interstitials : what concentration, what depth profile ?

Grain boundaries

Chemical residue ~

40nm

• Shallow oxide indentation at the GB• Thickness of Nb oxide; ~ 5-7nm

Λ ~ 40nm

A

B

Halbritter’s widely accepted model

GB microchemistry produced after BCP

Successful GB TEM foils allow us to perform µ chemical investigation

Thin Nb2O5 film Reference –

Gatan Atlas (HV = 200 kV BF)

Energy loss spectrum position

Spectrometer entrance aperture position (diaphragm : ~100nm)

Nb – M3

Nb – M4,5

Nb – M2

O – K

Example with oxygenExample w/o oxygen

Illustration of sampling area for

EELS

Illustration of sampling area for

EELS

Location of peaks in example analyses with and without oxygen

Multiple EELS Analyses on Plan-view image

Oxygen-K

Possible O-K knee?

• Oxygen-K peak is detectable in about 80% of in grain regions (50 20 µm away from GB)

• Oxygen peak (K shell) not clearly visible in 100 nm diameter grain boundary analysis regions

Fourier deconvolution & background subtraction

Courtesy of R.F. Egerton

Within the integration window (∆) ≈ 75eVD. Bach, et al. Micro. Micronal. 12, (2006)

Preferential flux penetration in single crystal after very low strains

• Single crystal from a large

grain high purity SRF-grade

Nb ingot slice• Slip system evaluation with

Laue crystallographic analysis• 5% uniaxial tensile test, then

sliced into small pieces (2.5

by 2.5 mm, 1.5 mm thickness)

by EDM in order to properly

align the dislocation slip

system parallel to the

direction of the external

magnetic fields.

Optical image after wire-EDM cutting of a tensile-tested single crystal

screw dislocations out of surface

screwand edgedislocations in slip plane

b

b

and

1.5m

m

d

MOI: Flux penetration along LAGBs on 5% uniaxial deformed single crystals: sample b

View || Hext

After Mechanical Polishing + 30 min BCP

Zero Field Cooled (ZFC)

T = 7 K

Rem, T = 7 K, H = 0 mT (after H= 60 mT)

ZFC, T = 7 K, H = 68 mT Remn, T = 7 K, H = 0 mT (after H = 68 mT)

Edge dislocation line direction and slip plane trace

Slip plane

Hext

Hext

Flux penetration

Low angle grain boundary (GB) trace revealed by BCP

Surface optical image

b

Flux penetration

b

MOI: Flux penetration along LAGBs on 5% uniaxial deformed single crystals: sample d

After Mechanical Polishing + 30 m BCP

Surface optical image

Grain boundary (GB) trace revealed by BCP

View || Hext

Screw dislocation

Slip plane

ZFC, T = 8.2 K Remn, T = 8.2 K, H = 0 mT (after H = 52 mT)

ZFC, T = 8.2 K, H = 60 mT

Remn, T = 8.2 K, H = 0 mT (after H = 60 mT)

Flux penetration

Flux penetration

Hext

Hext

d

+ 800 °C/2 h anneal: Minor flux penetration

ZFC, T = 7.2 K, H = 35.6 mT FC, T = 7.2 K, H = 0 mT (after FC in H = 68 mT)

LAGBSs

Surface optical image

Flux penetration

Roof pattern for strong

bulk SC current

ZFC, T = 7.2 K FC, T = 7.2 K H=0 mT (after FC in H = 120 mT)

LAGBSs

Flux penetration

Roof pattern for

strong bulk SC current

b

d

Preferential NbH1-x segregation along LAGBs

At room tempAfter cryogenic treatment in MO imaging

Clean zones: no dislocations & no NbHx segregations

Pits or craters of NbHx segregationsFE-SEM image

Surface optical image

Trace of NbHx segregations along the LAGB

~0.5-0.6° misorientatio

n

Grain orientation across the LAGBIPF Local misorientation map Misorientation angle profile

NbHx segregation (< 100 K) highly favorable during MO imaging below Tc of Nb ~ 9.2 K

5% Strain: Summary

5% uniaxial mechanical deformation produced low angle grain boundaries (LAGBs) on SRF-grade Nb single crystal. This LAGB is dislocation-substructure which consists of consecutive geometrical necessary boundaries (GNBs)

Magneto-optical imaging shows that LAGBs preferentially admit magnetic flux before the surface defects of the single Nb crystal.

Comparison of surface topological features before and after MO imaging below Tc of Nb ~ 9.2 K shows that niobium hydride (NbH1-x) preferentially segregates along LAGBs.

800 °C/2 h annealing likely re-distributes NbH1-x over the bulk of Nb.

This study shows the first evidence of the effect of LAGBs on SRF Nb superconducting properties, which is initiated by only 5% mechanical deformation.

Summary

High quality large grain Nb ingots have allowed us to sample individual grains (single

crystals) and grain boundaries (bi-crystals and tri-crystals) for detailed analysis of their

properties. Multiple samples can be obtained from the same grain boundary.

GBs in Nb are shown to be an intrinsic local defect (not just the source of topological

changes) that can significantly suppress both local and bulk superconductivity.

However, this localized GB weakness is only observed if the direction of the external magnetic field lies close to the GB plane. This sensitivity to orientation may help explain why not all GBs are deleterious to cavity performance.

MO imaging analysis on minimally deformed single crystal, with small strain levels

indicate that not only do traditional GB’s but low angle grain boundaries (LAGBs)

generated during deformation composed of dislocation substructures can preferentially

cause external magnetic flux penetration.

We show a high density of niobium hydrides segregated at LAGBs, which could cause premature breakdown.

Transport V-I measurements show preferential AJ flux flow along the GBs of SRF Nb.

We these techniques established we can explore the impact of new processing such as

doping on the GB properties.

Acknowledgements

This work was supported by the US DOE under awards DE-SC0009960, DE-

FG02-09ER41638, DE-FG02-05ER41392, DE-FG02-07ER41451, with

additional support from the State of Florida.

The TEM/STEM and EBSD-OIM facility at FSU is funded and supported by the

Florida State University Research Foundation, and the National High Magnetic

Field Laboratory, which is supported in part by the National Science Foundation

Cooperative Agreement DMR-1157490.

The ingot niobium international collaborations were initiated under the

Cooperative Research And Development Agreement JSA 2004S002 between

CBMM and Jefferson Lab under U.S. DOE Contract No. DE-AC05-06OR23177.

The authors would like to thank G. Myneni and colleagues at Thomas Jefferson

National Accelerator Laboratory for providing the large grain high purity Nb slice.

Special thanks to P. Bauer and C. Antoine for their support and valuable

discussions in the early stages of this work.