Yulia-Trenikhina-thin-films-2016

Extended material characterization of
Nb3Sn-coated Nb cavity cutouts and
Nb coating on Cu
Yulia Trenikhina
Collaborators: S. Posen, M. Sardela, J.-M. Zuo, A.
Romanenko, D. Hall, M. Liepe, S. Calatroni
Thin Films Workshop 2016, JLab

Part I
8/26/2016Yulia Trenikhina | Thin Film Workshop 20162
Material characterization of
Nb3Sn-coated Nb cavity

Nb3Sn coating for SRF cavities: why do we need material
studies?
Nb3Sn is widely accepted as # 1 alternative to Nb
New Fermilab program aims to understand and overcome limitation mechanisms
in Nb3Sn and to scale up to multicell cavities.
We need feedback for coating parameters optimization
T
2
[K
2
]
0
H[mT]
0 82
112
142
162
1820
50
100
150
200
250
0
Hquench
, Nb3
Sn
0
Hsh
, Nb3
Sn
0
Hc1
, Nb3
Sn
Campisi
Hays
S. Posen, N. Valles, and M. Liepe, Phys.
Rev. Lett., 115, 047001 (2015).
• High Q0 at high temperature: simpler cryoplant with
3-4x higher efficiency at 4.2 K vs 2 K
• Predicted superheating field 2x as high as niobium:
potential to increase reach of accelerators
Nb coating chamber
Hot zone
Existing vacuum furnace
Slide is courtesy of S. Posen

What material do we need?
Godeke et. al. Supercond. Sci. Technol.
19 (2006)
1. Stoichiometric A15 Nb3Sn: ~18-26 at.% of Sn
2. Nb3Sn with 24-26 at.% of Sn to get Tc~18K

Nb3Sn coated Nb cavity (ERL1-5) and representative cutouts
Q of 109 @ low fields; significant Q-slope
starting from 5 MV/m
Slide is courtesy of S. Posen
Temperature map was taken at 9 MV/m @ 4.2K
C2
H4

Nb3Sn-coated cavity (ERL1-5) with good and bad half-cell
Good half-cell Bad half-cell

Structure in cavity cutouts: XRD
2theta, degrees
Ln(counts),arb.units
Both cutouts show
A15 structure
acold=5.2889 A
ahot=5.2901 A
Strain~0.7%
200
211
220
310

Cold vs. Hot cutout: SEM/EDS at 20 kV
Cold
Hot
Hot cutout: what are the “patchy” regions?
Nb~75 at.% Sn~25 at.%

Hot cutout: varying EDS probing depth
20kV: probing deep
15kV: probing less deep
10kV: probing shallow
Nb3Sn coating
thickness variation
in hot cutout

Cold vs. Hot: TEM on cross-sectional samples
Cold Hot
Nb3Sn A15 structure is in both
cutouts. Thinner coating is in
hot cutout.
Nb3Sn[011]
Pt
Nb3Sn
Nb
Pt
Nb3Sn
Nb
Nb3Sn[001]
200nm
2000nm ~10 times
difference in
thickness

Cold vs. Hot: STEM/EDS local chemical characterization
Hot
Sn-deficiency
regions are within
individual grains in
cold cutout.
Bottom part of the
coating is Sn-
deficient in hot
cutout? Work in
progress…
Sn map Nb map
Cold Sn map Nb map

Sn-deficiency regions in cold cutouts: work in progress
Difference between normal and Sn-deficiency region inside
an individual Nb3Sn grain is 7-8 at.% (same in 4 samples)
Sn map
Previous studies suggesting Sn-deficiency regions: F. Hellman, J.
Talvacchio, H. Geballe (‘80s), C. Baker, T. Prolier (recently).
Nb-L
Sn-L
Nb-K

What are Sn-deficiency regions in cold cutouts?
Examples of additional
diffraction spots
“Normal” diffraction pattern
without additional reflections
Normal
surface
bulk
Individual Nb3Sn grain with Sn-deficiency spot
What kind of local structural inhomogeneity
is in Sn-deficiency regions?
Work in progress…
Sn map
surface
bulk

What are Sn-deficiency regions in cold cutout?
Welch et. al. J. Phys. Chem.
Solids 45, 11/12, p. 1225
(1984); Besson et. al. J. Phys.
Rev. B 75, 054105 (2007).
One possible scenario (from theoretical work):
Stoichiometric Nb3Sn with atomic disorder: Sn
vacancies are metastable: Sn-vacancy is getting
occupied by Nb atom which creates “split” Nb
vacancy: VSn = NbSn+VNb), amount of NbSn to VNb
relates as 4:1.
(“Split” vacancies in Nb: partial vacancies are
separated by 1D stacking fault)
What is local structure of Sn-deficiency regions?
How to avoid Sn-deficiency regions during the deposition?

421 332
Slow scans of individual peaks
Peak asymmetry:
presence of slightly
different lattice
constant
What are Sn-deficiency regions in cold cutout?
High resolution XRD

Possible relation to SC properties: work in progress
We might have regions with
lower lattice constants and
much lower Tc in cold cutout.
Currently in works: do we have
regions with lower lattice
constants in hot cutout as well?
What are Sn-deficient regions
with lower lattice constant?
Godeke et. al. Supercond. Sci. Technol.
19 (2006)

Conclusions on Nb3Sn coating
• Origin of poor performance of one Nb3Sn coated half-cell was identified.
Not sufficient and varying thickness of Nb3Sn coating in “bad” half-cell can
result either from Nb substrate or geometry of the deposition process.
Both potential causes of thin, irregular Nb3Sn coating are being
investigated.
• Potentially dangerous for the global SC performance, Sn-deficiency
regions were found in cold (and hot?) cutouts. Current research effort is
to understand their role in cavity performance, as well as their structure.
• Future research is aimed to understand the details of nucleation and
growth of Nb3Sn. Since those details are critical for finding optimal
deposition parameters.

Part II
Material characterization of
magnetron-sputtered Nb film on
Cu substrate

Nb film on Cu sustrate
DC magnetron-sputtered Nb film from the group of S. Calatroni
Name Date Kr
pressure
U I P T coating Thickness Yield Comment
mbar V A W min um nm/min
6dc 10/2010 1.2E-03 333 0.63 210 66 0.9 13.2 dcMS
Thanks to A. Sublet for the sample prep details

Overview of Nb film on Cu substrate
Thickness of Nb coating is ~900nm, composed of columnar long
and narrow grains (width is under 100 nm, length is ~900 nm.)
TEM diffraction contrast
Pt ion dep
Nb
Pt e-dep
Cu

Oxide layer of is covering the coating.
The surface of the coating is pretty
rough.
Overview of Nb film on Cu substrate: closer look
TEM diffraction contrast STEM Z-contrast
Nb/Cu interface looks “clean” with no
gaps.

Questions to address
• Heavy elements contamination in
Nb (such as Cu)
• Light elements contamination in
Nb (such as C, O, N), possible
oxidation along GB in Nb
• Structure of surface oxides

Study of possible foreign contaminants in Nb film
No segregation of heavy elements contaminants.
However no ability to detect C, O, N here.
Nb map
STEM/EDS experiment

Work in progress…
No significant
oxides segregation
along Nb grain
boundaries
Check for oxidation in Nb along the grain boundaries
STEM and EELS across the GB
Need to reduce data
Work in progress…
STEM and EDS (optimized for light elements) around GB

Surface oxidation of Nb film
Need to reduce data
Work in progress…
STEM and EELS across surface oxide
Surface oxide
thickness 7-10 nm
Work in progress…but surface oxide “looks” like typical oxide in bulk Nb

Average concentration changes in the first 40 nm
XPS/ sputtering with Ar to
see how average
concentration is changing
in the near-surface of Nb
film
Nb 3p
Nb 3s
Nb 3d
O 1sc
C 1s
C KLL?
O 2s

Carbon and Oxygen concentration changes
After sputtering 2 peaks can
indicate C in hydrocarbons and
C in carbide [Ref. V. Aleshin,
(1981)].
C 1s peak O 1s peak Nb 3d peak
XPS spectra taken from as received sample and subsequently after
every sputtering cycle. XPS probing depth in Nb is ~7 nm.

Comparison of Nb film with typical EP Nb bulk sample
Work in progress…do we have Nb carbides in Nb film?
Nb film: C 1s peak Nb bulk EP: C 1s peak

Carbon and Oxygen concentration changes
XPS probing depth
in Nb is ~7 nm!!!
Removed thickness, nm
Typical bulk EP Nb: after
removal of ~20 nm,
C~15 at%.
Concentration vs. removed thickness from XPS
C concentration in Nb film is comparable to typical EP sample,
but is it the same C chemistry?

Structural characterization of Nb film: XRD
Preliminary: no
Nb carbides were
detected.
Either there is no Nb
carbides in the film or
their size is pretty small

Preliminary conclusions on Nb film on Cu
So far two potential issues for SC performance were found:
• Rough surface of Nb film with sharp transitions from grain to grain,
• Possible small-sized Nb carbides inclusions.
No oxides segregation along Nb grain boundaries.
Surface oxides (7-10 nm) look similar to the bulk Nb sample/cutout.
Future work can be aimed to explore possible Nb carbides presence and
noble gas contamination.

Yulia-Trenikhina-thin-films-2016

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