Nanoscale near-surface investigation of cutouts from niobium cavities is the most direct way to understand the material features affecting the SRF performance. We will present temperature dependent TEM studies on cutouts from Q-disease free cavities prepared by EP, EP+120C baking, and N doping. We have found that several niobium hydride phases form upon cooldown to <100K in the penetration depth for EP and EP/120C cutouts with the difference in the density/size caused by 120C. No hydrides were found in the first few tens of nanometers of the N doped cutouts.These findings support the proposed involved of niobium nanohydrides in the high field Q slope, and the effect of N doping.

1. ILLINOIS INSTITUTE
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TEM studies of cavity cutouts from EP niobium
SRF cavities prepared by different treatments.
Yulia Trenikhina
SRF Workshop
10/07/2014

2. ILLINOIS INSTITUTE
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Outline
Cutouts from Nb EP 120°C baked/not baked cavities (HFQS):
•TEM diffraction: room and cryogenic T
•Direct observation of Nb nanohydrides for the 1st time
•High Resolution TEM: no oxidation along grain
boundaries
Nitrogen doping for high Q0 (MFQS):
•Treatment characterization: Nb nitrides on the surface,
nitrogen doping deeper.
•TEM diffraction at room and cryogenic T: Nb hydrides
precipitation is the cause?
Are Nb nanohydrides responsible for HFQS and MFQS?

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Effect of 120°C on Q0
16
14
12
HFQS elimination 10
in FG EP
cavities after 120°C bake
8
6
50 100 150 200 250 300 2
16
14
EP + 120C baking, Bpeak = 119 3X0-4
Electropolished, Bpeak = 119 mT
Angle (deg)
Sensor number
310-1011
1010
100
Nb 310-10
Nb 3X0-10
10 FG
FGB
50 100 150 9
Q0
Bpeak (mT)
(a)
(b)
(c)
(d)

4. Origin of Hot and Cold cavity cutout
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Hot spot: from EP cavity
Cold spot: from EP+120°C
baked cavity
Cutout (d=11mm, t=3mm)
“useful near-surface area”
~10 μm
~3 μm
Cu grid
SEM of FIB sample

5. Room T Comparison of Hot and Cold spot
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[113] [011]
[-111]
“useful near-surface area”
[001]
NED: Hot (not baked) and Cold
(baked) spot at room T
Electron diffraction: only Nb at room T
H in solid solution (α-phase)
~10 μm
~3 μm
Cu grid
SEM of FIB sample
[100]

6. Cryogenic T investigations of cavity cutouts
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Diffraction mapping with low intensity beam
Hot (not baked) spot at 94K 120°C baked stop at 94K
ε β
ε+β
ε β
ε β β
ε ε ε+β ε+β
ε ε ε
ε
ε
ε
ε
ε
ε ε
ε
ε+β
ε+β
ε+β β ε
_ _ _ _
Nb +ε,β
Nb+β(NbH)
Nb+ε(Nb4H3)
Nb hydrides precipitation
Nb
Nb
NO Nb hydrides precipitation

7. Cryogenic T investigations of cavity cutouts
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Diffraction with brighter beam, better S/N
NED: Nb hydrides precipitation in all cutouts,
amount and/or size of NbHx is different
44%-68%
probed
spots
26%-29%
probed
spots
Hot (not baked) at 94K
120°C baked stop at 94K

8. The 10th Workshop on RF Superconductivity, 2001, Tsukuba, Japan
MATERIAL SCIENCE OF Nb RF ACCELERATOR CAVITIES:
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grain 1
Grain boundary investigation
HRTEM
WHERE DO WE STAND 2001?
grain 2
SEM image of GB
HRTEM: No visible oxide layer along GB
No evidence!
NbO (x 1) x %
C H-OH x y
H O-OH 2 1nm
J. Halbritter, SRF 2001
J. Halbritter
Forschungszentrum Karlsruhe, Institut für Materialforschung I
Postfach 3640, 76021 Karlsruhe , Germany
Abstract
The rf losses, especially actual level and increase with
rf fields, limit most stringently the application of
superconducting rf cavities. This is due to the needed
cooling power to be supplied locally to the high field re-gion
causing rf breakdown. The rf losses are due to two
sources based on different physics: dielectric rf losses
proportional to REE!2 and shielding current losses pro-portional
to RHH||2. Material science wise intrinsic losses
RBCS are separate from extrinsic, rf residual losses Rres.
The separation of Rres(T,f,H) from the BCS losses
RBCS(T,f,H) yields the quasi-exponential increases of the
electric surface resistance with the electric field E! per-pendicular
to the surface "RE(E!) # exp (-c/E!) and the
power law increases of the magnetic surface impedances
with the magnetic field H|| parallel to the surface "RH(H||)
# (H||)2n (n = 1, 2. .). By Nb/Nb2O5-y interfaces of external
and internal surfaces RH
res(T,f) and RE
res(f,E!) can be
explained quantitatively by localized states nL of Nb2O5-y
in close exchange with extended states nm of Nb. Espe-cially,
the Q-drop # 1/RE(E!) and its reduction by EP-and
BCP-smoothening and by UHV anneal at T$100°C
are well accounted for by interface tunnel exchange. The
UHV anneal not only reduces surface scattering and RE
but also enforces the RBCS(T, 1.3 GHz, H < 10 mT)-drop
and reduces RBCS(T, $ GHz, $ 10 mT) by more than a
Secondly, high pressure (80 bar) water rinsing (HPR) [4]
is able to reduce the dust on Nb surfaces sufficiently.
Thirdly, we are left with intrinsic Nb corrosion yielding
after electropolishing (EP) or buffered chemical polishing
(BCP), followed in both cases by HPR, some inhomoge-neities,
as sketched in Fig. 1.
1nm
Nb
NbO (x 0.02) x $
Nb O2 5-y
Fig. 1: Nb surface with crack corrosion by oxidation by Nb2O5
volume expansion (factor 3). Nb2O5-y-NbOx weak links/segregates
(y, x < 1) extend up to depths between 0.01 – 1/ 1-10 μm for
good – bad Nb quality and weak - strong oxidation [8].
Embedded in the adsorbate layer of H2O/CxHyOH (& 2 nm)
being chemisorbed by hydrogen bonds to NbOx(OH)y,
adsorbate covered dust is found. This dust yields enhanced

9. ILLINOIS INSTITUTE
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Possible effect of 120°C bake
EP cavity EP cavity after 120°C bake
Nb2O5
NbOx
Nb
H
Nb2O5
NbOx
Nb
H-vacancy
complex
~40 nm
Mild vacuum 120°C bake
Introduction of H-Vac complexes
Less/no NbHx precipitation
A. Romanenko, C.J. Edwardson, P.G. Coleman, and P.J. Simpson Appl. Phys. Lett. 10, 232601 (2013)
B. Visentin, M.F. Bathe, V. Moineau, and P. Desgardin, Phys. Rev. ST Accel. Beams 13, 052002 (2010)

10. Nitrogen doping: a breakthrough in Q0
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Nitrogen doping => up to 4 times higher Q!
Standard state-of-the
art preparation
A. Grassellino et al, 2013 Supercond. Sci.
Technol. 26 102001 (Rapid Communication)
1.3 GHz
This was the highest Q
possible up to last year

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N-doping-production-ready
LCLS-II spec
• Technology
immediately
adopted for SLAC
• 100+ single cell tests
with high Qs
• 10s of 9-cell tests
with the
“production”
protocol for LCLS-II
–We have 8 nine-cell
cavities lined up for the
first cryomodule at
FNAL
T=2K

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N-doping treatment
I. Reacting bulk niobium cavities with N2 gas (N2 p.p ~
2x10-2 Torr) at 800°C in UHV furnace for ~20 min
followed by 30 min with no N;
II. Material removal via electropolishing (EP) followed by
high-pressure water rinsing (HPR).

13. Investigation of N treatment: step I
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Nb samples processed parallel with cavities
XRD: hexagonal
NbN0.5
120x103
115
95 counts, arb.units
110
105
100
treatments at 800Cº
fit
peak A
peak B
peak C
XPS N2 1s:
~20 at.% of N 150x103
406 404 402 400 398 396 394 392 390
binding energy, eV
100
50
0
counts, arb.units
SEM of Nb surface after step I
treatment at 800Cº
fit
doublet A
doublet B
doublet C XPS Nb 3d:
214 212 210 208 206 204 202 200 198
binding energy, eV
mixture of
NbNx, NbNxOy and
Nb2O5
XRD, XPS, SEM: we have NbNx (β-NbN) after the 1st step

14. Investigation of N treatment: step I
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~2 μm
surface after step I TEM sample
TEM low mag Nb [113] TEM low mag
NbN0.5
NbN0.5+ NbNx
Pt protective layer Pt protective layer
TEM, NED: NbN0.5+NbNx within at least first 2 μm.
Poor SRF performance after step I: Q~ 107

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Investigation of N treatment: step I
grain
boundary
TEM image TEM image
SEM image
NbNx extend ~2μm along GB

16. Investigation of N treatment: step II
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TEM image
Cutout from N treated cavity
Nb
TEM image
Pt protective layer
Pt protective layer
XRD, XPS, TEM: NO Nb nitrides after step II

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N depth profiles by SIMS
Nitrides Interstitial N in Nb
Doped
Non-doped
Depth (um)
Set of N-doped
samples using
different
temperatures and
duration –
comparison with
the non-doped

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SIMS on cutouts
40 ppm of N

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Possible effect of N doping
SEND at 94K: NO Nb hydrides formation in cutouts
from N-treated cavities, similar to baked cutout.
N traps H at interstitials close to
tetrahedral. No/less NbHx precipitation
• Pfieffer et. al. (J. Phys. F: Metal Phys., V.6(2), 1976);
• Rush et. al. (Europhys. Lett., 48(2), 187-193, 1999);
• Magerl (Phys. Rev. B, V.27(2), 1983)
• Baker et. al. (Acta Metallurgica, V.21, 1973);
• ...

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Conclusions
First direct cryogenic T observation of Nb nanohydrides in cutouts
from EP baked/not baked cavities
• Size/distribution of NbHx define Q0
GB don’t appear to have an oxide layer
Understanding of N doping of Nb on microscopic material level
• Possible scenario: Nitrogen traps hydrogen
Our data are inline with proposed model

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Acknowledgments
UIUC MRL: Dr. J.Kwon, Prof. J.-M. Zuo, Dr. J. Mabon
Fermilab: Dr. Anna Grassellino
Thank you for your attention!