Ingot Niobium
Summary
Workshop –
December 4, 2015
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Zero Field Cooled (ZFC): shows surface property of SC
Field Cooled (FC): shows bulk property of SC
A. Polyanskii
Tri-crystal
Tri-crystal
GB #1
RF field
in-plane
GB #2
Tricrystal
Bi-crystal
GB (#1)
Normal to Surface
Thickness of sheet is 1.88 mm
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After
BCP
MO
H
Misorientation angle between grains ≈17.8o Orientation
Imaging Microscopy (OIM): from Abraimov
H
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SC
M
H
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SC
M
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.
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MO
H
GB trace on top
face of sample
ZFC
H = 80 mT
No GB trace on front
face
This face has been imaged
by MO, when sample was
turned by 900
H
MO indicator
GB trace on bottom
face of sample
1.89mm
2.78mm
Before
polishing
ZFC T = 6.4
K
FC
T=
6.4 K
GB
H = 80 mT
H = 100 mT
1 mm
After polishing
Isolated a small
sample from the
above bi-crystal then,
rotated by ~90° to
align the GB plane
parallel to the surface
normal
ZFC T = 6.2 K
H = 80 mT
H=0
mT
FC
T=
6.2 K
H = 112
mT
H = 0 mT
And compared the
BCP’ed with the
smooth-polished.
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’ed
Both:
GB∥Hext
Reduced
thickness, and
then
compared with
the EP’ed bicrystal cut
from same GB
ZFC
6K
T=
FC
T=6K
H
H = 58 mT
H=0
mT
1 mm
EP’ed
ZFC
K
T = 6.5
H = 72 mT
FC
T = 6.5 K
H = 0 mT
GB is a weak link only when Hext is aligned parallel with the GB plane
The GB groove may not the cause of the preferential flux penetration
DC transport V-I characterization with 1T Electromagnet
~20-30 mT
Expand the gap between Hc1 (170 mT) and Hc2
(200 mT) at 4.2 K → Make vortex penetration at lower
Hext
The procedures
200mT
Higher-Hc SC
Nb
As-received
Mechanically ground
1. Cut samples into I-shape with wire-EDM
2. 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
1 mm
100μm
Surface image of I-shape sample after BCP treatment
5. Further reduce the bridges of some Ishape single- & bi- crystals with extra
BCP
6. Artificially groove with FIB and
mechanically smear away the grooved
produced by the chemical treatments
No groove (~0.5–2.0μm roughness)
A deep (3-5 μm) and highly inclined groove
0.10T
0.08T 0.05T
BCP'ed Single Crystal
BCP'ed Bi-Crystal
• Flux flow evidence from H = 0.08 T to 0.28 T
• The V-J characteristics show that the grain boundary is a channel of preferential flux
flow (FF) by weakly pinned vortices.
• 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
A deep (3-5μm) and highly inclined groove
The # is the angle between the GB plane and Hext
0.10T
0.08T
0.05T
Hext
Preferential flux flow Hext
= 0.08 T to 0.28 T when
the GB plane // Hext
BCP’ed
EP’ed
Linear coordinates
Linear coordinates
GB flux
flow
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
Artificially grooved single
crystal (using FIB)
Very flat surface by ultra-fine
polishing
V-J response of Single crystal
V-J response of 26° Bi-crystal
by FIB
(Focused Ion
beam)
Flux flow
0.20T
0.18T
0.10T0.08T
0.13T
0.05T
Preferential flux flow at the grain boundary may be not triggered by surface
topological features when GB plane is parallel to Hext
The number of degree indicates the angle of
between a plane of GB and external magnetic field (GB vs Hext)
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
Uniform transmission contrast
indicates no step at GB
Precipitation contrast at GB
Several strain and dislocation
contrast
Prepared by 30min BCP after
mechanical thinning ~30-50 μm
Darker contrast due to high
misorientation angle across GB
Grain Boundary
Dislocations
Possible dislocations Pile-up at
G.B
Prepared by 50min BCP after
mechanical thinning ~80-100 μm
A
Λ ~ 40nm
GB
GB
Au-Pd
Au-Pd
Oxide
• Shallow oxide indentation at
the GB
• Thickness of Nb oxide; ~ 57nm
Inclusions
Chemica
l residue
Oxide
~
40nm
B
GB
GB
Λ ~ 40nm
Native oxide : Nb2O5
5-10 nm
Interface : sub
oxides + interstitial
oxygen : some
monolayers.
Au-Pd
Au-Pd
Oxide
Oxide
19
interstitials : what
concentration,
what depth profile
?
Grain boundaries
Halbritter’s widely accepted
model
Successful GB TEM foils allow us to perform µchemical investigation
Nb – M4,5
Illustration of
sampling area for
EELS
Example with oxygen
Example w/o oxygen
Nb – M3
Thin Nb2O5 film Reference –
Gatan Atlas (HV = 200 kV BF)
Nb – M2
O–K
Energy loss spectrum
position
Spectrometer entrance aperture
position (diaphragm : ~100nm)
Location of peaks in example analyses with
and without oxygen
• Oxygen-K peak is detectable in
about 80% of in grain regions (5020 µm away from GB)
• Oxygen peak (K shell) not clearly
visible in 100 nm diameter grain
boundary analysis regions
Oxygen-K
Fourier deconvolution & background subtraction
Possible OK knee?
Within the integration window (∆) ≈ 75eV
D. Bach, et al. Micro. Micronal. 12, (2006) Courtesy of R.F. Egerton
• Si
Optical image after wire-EDM cutting of a
tensile-tested single crystal
b
d
b
screwand edge
dislocations in
slip plane
screwdislocations
out of surface
and
After Mechanical Polishing +
30 min BCP
Surface optical image
Flux
penetration
Hext
b
Low angle grain
boundary (GB)
trace revealed by
BCP
Zero Field Cooled (ZFC)
T=7K
Flux
penetration
Edge
dislocation line
direction and
slip plane trace
b
Rem, T = 7 K, H = 0 mT (after
H= 60 mT)
Hext
Slip plane
View || Hext
ZFC, T = 7 K, H = 68 mT
Remn, T = 7 K, H = 0 mT (after H
= 68 mT)
After Mechanical
Polishing + 30 m BCP
Flux
penetration
Surface optical image
Hext
d
ZFC, T = 8.2 K
Remn, T = 8.2 K, H = 0 mT
(after H = 52 mT)
Grain boundary (GB)
trace revealed by BCP
Flux
penetration
Slip
plane
View ||
Hext
Screw
dislocation
Hext
ZFC, T = 8.2 K, H =
60 mT
Remn, T = 8.2 K, H = 0 mT
(after H = 60 mT)
Flux
penetration
Roof
pattern for
strong
bulk SC
current
b
LAGBSs
FC, T = 7.2 K H=0 mT (after
FC in H = 120 mT)
ZFC, T = 7.2 K
Surface optical image
Flux penetration
Roof pattern
for strong bulk
SC current
d
LAGBSs
ZFC, T = 7.2 K, H = 35.6 mT
FC, T = 7.2 K, H = 0 mT
(after FC in H = 68 mT)
NbHx
segregation
(< 100 K)
highly
favorable
during MO
imaging
below Tc of
Nb ~ 9.2 K
At room temp
After cryogenic
treatment in MO imaging
Clean zones: no
dislocations & no
NbHx segregations
FE-SEM image
Pits or craters of
NbHx
segregations
Trace of NbHx
segregations
along the LAGB
Surface
optical
image
Grain orientation across the LAGB
IPF
Local misorientation map
Misorientation angle profile
~0.5-0.6°
misorientatio
n
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