Date post: | 22-Jul-2015 |
Category: |
Science |
Upload: | thinfilmsworkshop |
View: | 193 times |
Download: | 4 times |
ILLINOIS INSTITUTE OF TECHNOLOGY
TEM studies of cavity cutouts from EP niobium SRF cavities prepared by different treatments.
Yulia Trenikhina
SRF Workshop10/07/2014
ILLINOIS INSTITUTE OF TECHNOLOGY
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?
ILLINOIS INSTITUTE OF TECHNOLOGY
50 100 150 200 250 300 350
2
4
6
8
10
12
14
16EP + 120C baking, Bpeak = 119 mT
Angle (deg)
Sens
or n
umbe
r
1016254063100159252400
!T (mK)
3X0-10
50 100 150 200 250 300 350
2
4
6
8
10
12
14
16Electropolished, Bpeak = 119 mT
Angle (deg)Se
nsor
num
ber
1016254063100159252400
!T (mK)
310-10
50 100 150
1
10
100
Nb 310-10 Nb 3X0-10
!T (m
K)
B (mT)
50 100 150109
1010
1011
FG FGB
Q0
Bpeak (mT)
(a)
(b)
(c)
(d)
HFQS elimination in FG EP cavities after 120°C bake
Effect of 120°C on Q0
ILLINOIS INSTITUTE OF TECHNOLOGY
Cutout (d=11mm, t=3mm)
Origin of Hot and Cold cavity cutout
Hot spot: from EP cavity Cold spot: from EP+120°C
baked cavity
~10 µm~
3 µm
“useful near-surface area”
Cu grid
SEM of FIB sample
ILLINOIS INSTITUTE OF TECHNOLOGY
Room T Comparison of Hot and Cold spot
[113] [011]
[-111]
[001]
NED: Hot (not baked) and Cold (baked) spot at room T
Electron diffraction: only Nb at room TH in solid solution (α-phase)
~10 µm
~3
µm
“useful near-surface area”
Cu grid
SEM of FIB sample[100]
ILLINOIS INSTITUTE OF TECHNOLOGY
βε
ε+β
βε
ε β β
ε ε ε+β ε+β
ε ε ε
ε
ε
ε
ε
ε ε
ε
ε
ε+β
ε+β
εε+β β_ _ _ _
Hot (not baked) spot at 94K 120°C baked stop at 94K
Nb+ε(Nb4H3)
Nb+β(NbH)
Nb +ε,β
Nb hydrides precipitation
Cryogenic T investigations of cavity cutouts
Nb
NO Nb hydrides precipitation
Nb
Diffraction mapping with low intensity beam
ILLINOIS INSTITUTE OF TECHNOLOGY
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
Cryogenic T investigations of cavity cutouts
Diffraction with brighter beam, better S/N
120°C baked stop at 94K
ILLINOIS INSTITUTE OF TECHNOLOGY
grain 1
grain 2
SEM image of GB
Grain boundary investigation
HRTEM: No visible oxide layer along GB
HRTEM
No evidence!
J. Halbritter, SRF 2001
MATERIAL SCIENCE OF Nb RF ACCELERATOR CAVITIES:WHERE DOWE STAND 2001?
J. HalbritterForschungszentrum Karlsruhe, Institut für Materialforschung I
Postfach 3640, 76021 Karlsruhe , Germany
AbstractThe rf losses, especially actual level and increase with
rf fields, limit most stringently the application ofsuperconducting rf cavities. This is due to the neededcooling power to be supplied locally to the high field re-gion causing rf breakdown. The rf losses are due to twosources based on different physics: dielectric rf lossesproportional to REE!2 and shielding current losses pro-portional to RHH||2. Material science wise intrinsic lossesRBCS are separate from extrinsic, rf residual losses Rres.The separation of Rres(T,f,H) from the BCS lossesRBCS(T,f,H) yields the quasi-exponential increases of theelectric surface resistance with the electric field E! per-pendicular to the surface "RE(E!) # exp (-c/E!) and thepower law increases of the magnetic surface impedanceswith the magnetic field H|| parallel to the surface "RH(H||)# (H||)2n (n = 1, 2. .). By Nb/Nb2O5-y interfaces of externaland internal surfaces RHres(T,f) and REres(f,E!) can beexplained quantitatively by localized states nL of Nb2O5-yin 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°Care well accounted for by interface tunnel exchange. TheUHV anneal not only reduces surface scattering and REbut also enforces the RBCS(T, 1.3 GHz, H < 10 mT)-dropand reduces RBCS(T, $ GHz, $ 10 mT) by more than afactor 2. The interrelations RBCS-drop, RE and RBCS withthe material science of Nb2O5-y/Nb interfaces, e.g., by EPor UHV anneal, will be elucidated.
1 INTRODUCTIONThe technology to produce Nb rf accelerator cavities
with gradients above Eacc $ 10 MeV/m and Qo (2K; 1GHz) $ 1010 is now in hand and delivered by industry, asthe result of more than 30 years intensive research and de-velopments started first at Stanford [1] and Karlsruhe [2].But still progress is needed to fulfill the ever growingdesire of, e.g., the high energy physics community [3].Highlights in the recent progress are discussed below.Progress usually starts with a name, like Q-decease or Q-drop, followed by quantification and then the decease canbe overcome. The progress to be reported in Nb cavitiyperformance is based on basic research carried through bygroups at CEBAF, DESY, INFM, KEK and SACLAY,discussed in Sects. 4 and 5.Where stands the materials science of superconducting
Nb rf cavities and surfaces at the moment? Firstly, Nbfree of inclusions, like big Ta- or NbOx-lumps, with a dcresistance ratio RRR > 200 is now available [3].
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 yieldingafter electropolishing (EP) or buffered chemical polishing(BCP), followed in both cases by HPR, some inhomoge-neities, as sketched in Fig. 1.
1nm
NbC H -OHx y
H O-OH2 1nm
NbO (x 0.02)x $
Nb O2 5-y
NbO (x 1)x %
Fig. 1: Nb surface with crack corrosion by oxidation by Nb2O5volume expansion (factor 3). Nb2O5-y-NbOx weak links/segregates(y, x < 1) extend up to depths between 0.01 – 1/ 1-10 µm forgood – 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 enhancedfield emission (EFE [7]) summarized in Sect. 3.1.
Crucial are the scales: Nb is coated by less than 0.5 nmNbOx(x %1) and by 1 – 3 nm Nb2O5-y covered with hydro-gen bonded H2O/CxHy (OH)z of similar thickness.Optimal superconducting Nb properties have to hold, atleast, in a penetration depth 'H(T<Tc/2) $ 40 nm. Anotherscale is the dimension of Cooper pairs (F $ 60 nm, wheremetallic defects in Nb have to be much smaller in size tosustain overall good Nb properties. Similarly on Nb2O5,dust and protrusions have to be reduced in amount andsize to get good electric field properties. This neededhomogeneity in and on Nb is achieved now yielding Eacc &30 MeV/m The scales of 1 - 10 nm for Nb cleanliness andhomogeneity have to be compared to accelerator lengthsof 0.1 -1 km, showing a reproducibility over 10 order inmagnitude being now achieved. This, by itself, is a bigachievement. But as we see from semiconductor industry,there is still more to come. As material scientist, I will notdwell on this but I want to outline old and new Nb resultsand understandings, where, in my view, progress, hasbeen made in the last years and will be made in the nextyears.
The 10th Workshop on RF Superconductivity, 2001, Tsukuba, Japan
292
ILLINOIS INSTITUTE OF TECHNOLOGY
Possible effect of 120°C bake
NbOx
Nb2O5
Nb
H
NbOx
Nb2O5
Nb
H-vacancycomplex
EP cavity EP cavity after 120˚C bake
Mild vacuum 120˚C bakeIntroduction 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)
~40 nm
ILLINOIS INSTITUTE OF TECHNOLOGY
Standard state-of-the art preparation
Nitrogen doping => up to 4 times higher Q!
A. Grassellino et al, 2013 Supercond. Sci. Technol. 26 102001 (Rapid Communication)
1.3 GHz
Nitrogen doping: a breakthrough in Q0
This was the highest Q possible up to last year
ILLINOIS INSTITUTE OF TECHNOLOGY
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
N-doping-production-ready
ILLINOIS INSTITUTE OF TECHNOLOGY
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).
ILLINOIS INSTITUTE OF TECHNOLOGY
XRD: hexagonal NbN0.5
120x103
115
110
105
100
95
cou
nts,
arb
.uni
ts
406 404 402 400 398 396 394 392 390binding energy, eV
treatments at 800Cº fit peak A peak B peak C
XPS N2 1s: ~20 at.% of N 150x103
100
50
0
cou
nts,
arb
.uni
ts
214 212 210 208 206 204 202 200 198binding energy, eV
treatment at 800Cº fit doublet A doublet B doublet C XPS Nb 3d:
mixture of NbNx, NbNxOy and Nb2O5
XRD, XPS, SEM: we have NbNx (β-NbN) after the 1st step
Nb samples processed parallel with cavitiesSEM of Nb surface after step I
Investigation of N treatment: step I
ILLINOIS INSTITUTE OF TECHNOLOGY
~2 µ
m
TEM low mag Nb [113] TEM low mag
NbN0.5
NbN0.5+ NbNx
TEM, NED: NbN0.5+NbNx within at least first 2 µm.Poor SRF performance after step I: Q~ 107
Pt protective layer Pt protective layer
Investigation of N treatment: step Isurface after step I TEM sample
ILLINOIS INSTITUTE OF TECHNOLOGY
Investigation of N treatment: step I
grainboundary
SEM image
TEM image TEM image
NbNx extend ~2µm along GB
ILLINOIS INSTITUTE OF TECHNOLOGY
TEM image
XRD, XPS, TEM: NO Nb nitrides after step II
Nb
Investigation of N treatment: step II
TEM image
Pt protective layer
Pt protective layer
Cutout from N treated cavity
ILLINOIS INSTITUTE OF TECHNOLOGY
Non-doped
Doped
Depth (um)
Interstitial N in NbNitrides
N depth profiles by SIMS
Set of N-doped samples using different temperatures and duration – comparison with the non-doped
ILLINOIS INSTITUTE OF TECHNOLOGY
SIMS on cutouts
40 ppm of N
ILLINOIS INSTITUTE OF TECHNOLOGY
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);
• ...
Possible effect of N doping
SEND at 94K: NO Nb hydrides formation in cutouts from N-treated cavities, similar to baked cutout.
ILLINOIS INSTITUTE OF TECHNOLOGY
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
Conclusions
ILLINOIS INSTITUTE OF TECHNOLOGY
Acknowledgments
UIUC MRL: Dr. J.Kwon, Prof. J.-M. Zuo, Dr. J. MabonFermilab: Dr. Anna Grassellino
Thank you for your attention!