HYDROGEN OUTGASSING OF STAINLESS STEEL OUR ...Manfred Leisch 1st Vacuum Symposium UK February 11, 2010 Samples for AFM and STM study 304L stainless steel

Institute of Solid State Physics

Professor Horst Cerjak, 19.12.20051

Manfred Leisch 1st Vacuum Symposium UK February 11, 2010

Manfred LeischInstitute of Solid State Physics

Graz University of Technology, Austria

HYDROGEN OUTGASSING OF STAINLESS STEEL

OUR PRESENT KNOWLEDGE




Graz




Background

Accelerators

Mainspring forachieving extremlylow pressures (XHV)

(Images copyright CERN)




The Material

Austenitic stainless steel (Type AISI 304 and 316) is one of themost important construction materials in UHV and XHV

– corrosion resistant and chemically inert – nonmagnetic– standard machining and welding procedures

– relatively cheap– negligible vapor pressure at room temperature– negligible permeation of atmospheric gasses (fcc lattice)

but ….




Hydrogen as impuritySolubility of Hydrogen in stainless steel

The H content in standard austeniticstainless steel is about 1 ppm in weight(~ 56 at ppm).

Amount equates ~ 0.1 mbar·l / cm3 ,at typical outgassing rates of10-11 mbar.l/cm2.s from 2 mm wall source for more than 50 years




Is essential for obtaining ultimate pressures

Actions performed:

– Surface treatments to reduce surface roughness (electropolishing, surface machining…)

– Surface treatments to create oxide or other films to act as a barrier for diffusion of H from the bulk.

– High temperature bakeout (vacuum firing) to reduce amount of dissolved H.

[1] P. A. Redhead: Extreme high vacuum, CERN Report No 99-05, 213 (1999)[2] R. Dobrozemsky: Our present understanding of outgassing, EVC-9, Paris (2005)

Reduction of outgassing




Mechanism for H2 outgassing

Polany – Wigner Equation:

- dN/dt = ν (Θ)2 exp (-Edes / kT)

Diffusion in the bulk– Recombinative desorption from surface




Outgassing modelsDiffusion limited outgassing (DLM)described by Fick‘s equation

Recombination limited outgassing(RLM)Flat bulk concentration in pure RLM

with recombination coefficient




Documented work

Calder and Lewin (1969):[R. Calder and G. Lewin, Brit. J. Appl. Phys. 18, 1459 (1969)]

– poor correlation between experiment and calculation (diffusion limited outgassing expected)

Moore (1995):[B. C. Moore, J. Vac. Sci. Technol. A 13(3), 545 (1995)]

– Calculations of H concentration profile support recombination limited outgassing

L. Westerberg (1997):[L. Westerberg et al., Vacuum 48, 771 (1997)]- Assignment of difference in the outgassing rate to different transport properties

due to bulk states .




Documented work

Jousten (1998):[K. Jousten, Vacuum 49, 359 (1998)]

– There is a significant influence of the surface on outgassing

Fremery (1999):[J. K. Fremery, Vacuum 53, 197 (1999)]

– At low H concentration in the bulk outgassing is limited by surface recombination

B. Zajec, V. Nemanic (2001):[B. Zajec, V. Nemanic, Vacuum 61, 447 (2001)]

- neither DLM nor RLM fits, most H strongly bound in traps, precipitates, surface states , just a fraction in the interstitial state




J.-P. Bacher et al. JVST A21(2003)167

J.P. Bacher et al. (2003):[J.-P. Bacher et al. JVST A21(2003)167]

- TDS study (up to 1200°C) on different SS types and treatments (vac firing, air bake, vac bake) gives reason for oxide-layer traps, lattice defects due to precipitates, recrystallization

Paolo Chiggiato, CAS 2006 Platjo d‘Aro, Spain

The outgassing of H aftervacuum firing can be reasonabledescribed by a diffusion modelonly if the pressure of H duringthe treatment is taken intoaccount

Documented work




Documented work

J. Setina (2006)[53rd AVS Symposium, San Francisco

2006 ]

- Outgassing measurements(250°C, 300 h) and modelcalculations solved with FEM support RLM

© janez.setina @imt.si

1.0E-12

1.0E-11

1.0E-10

1.0E-09

1.0E-08

1.0E-07

1.0E-06

Out

gass

ing

rate

mba

rl/s/

cm^2

0 100 200 300 400 Time / hours

Diffusion model Recombination model

T=250 C




Surface characterization

Recombination process strongly relates on surface morphology

• surface structure after vacuum firing?• surface composition after vacuum firing?

Goal :

Surface characterization after vacuum firing on atomic level

by AFM, STM and Atom Probe




Experimental setup

PreparationChamber• QMA• sample

heating stage• Sputtergun• Auger CMA

STM • OMICRON

STM 1

Atom Probe• FIM• TOF

Main Chamber• Tip heating

stage• Tip sputter

gun• Vacuum

lock for transfer




Samples for AFM and STM study304L stainless steel

-8-1218-20<0.75<0.03<0.045<2<0.03

MoNiCrSiSPMnC

In situ thermal treatment:– Low temperature bakeout

– Vacuum firing at 1000°C

(e-beam bombardment, temperature measurement by micro pyrometer and thermocouple)

2-310-1416-18<1<0.03<0.045<2<0.03

MoNiCrSiSPMnC

316L stainless steel




AFM images after vacuum firing

100 nm200nm

Surface morphology after low temperature bakeout and after vacuum firing

100 nm

3hours@300°C 15min@1000°C




AFM images after vacuum firing

1086420

600

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X[µm]

Z[n

m]

15min@1000°C3hours@300°C

3.532.521.510.50

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Z[n

m]




STM images after vacuum firing304L 15min@1000°C

(1000x1000nm², U=-0.5V, I=0.1nA) (1000x1000nm², U=-0.5V, I=0.1nA)














STM after vacuum firing

100 nm200nm

Nanoprecipitate in reconstructed surface

100 nm

200 nm 200 nm

b c

20min@1000°C304L (1000x1000nm², U=-0.5V, I=0.1nA)




316L after vacuum firing

The different alloy compostion (addition of Mo) leads to a noticable different reconstruction of the surface




316L(Upp): vacuum bake 48h@450°C

AFM micrograph

(2.2x2.2 µm², derivated image)

Line profile show ∆h up to 25 nm




316L: vacuum fired 20min@1000°C

AFM micrograph

(10x10µm², derivated image)

876543210

140

120

100

80

60

40

20

0

X[µm]

Z[n

m]

1 µm





316L(Upp): vacuum fired 1h@1100°C

AFM micrograph

(8.6x8,6µm², derivated image)






AFM micrograph


21.510.50

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m]

1 µm





AFM micrograph


10008006004002000

100

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m]

0.5 µm




316L(Upp): vacuum fired 1h@1100°C

STM micrograph

(1x1 µm², 3D image, top view)


10.80.60.40.20

70

60

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0

X[µm]Z

[nm

]





250200150100500

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(1000x1000nm², U=-0.5V, I=0.1nA)

STM





(50x50nm², U=-0.5V, I=0.1nA)

2520151050

2

1.5

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]

STM




Results from AFM and STM

• After vacuum firing the surface shows significant reconstructiondepending on alloy.

• Formation of (111) terraces with monoatomic steps and stacking faults and point defects (vacancies).

• Width of (111) terraces increases with annealing time, step bunching and formation of facets, corresponding to (110) and (100) planes.

• Grain boundaries became predominate as consequence of recrystallization (pronounced on 304L).

• From the look on the surface: still a high number of active sites like steps and vacancies remain.

An outgassing rate of 10-12 mbar.l /s.cm2 = 2.45x107 molecules /s.cm2

which corresponds to 10-8 of a monolayer at room temperature, RL according PW about 10-4 ML)




Surface compositionFrom: Paolo Chiggiato, CAS 2006 Platjo d‘Aro, Spain




Surface inspection by Auger

304L after bakeout 3h@300°C after vacuum firing 5min @1000°C

Enrichment: P, S, Fe, Ni Depletion: C, N, O, Cr




Analysis of surface composition

AES gives sign for slight enrichment of Ni and depletion of Cr but due to information depth of thistechnique composition of topmost layer almostuncertain.

Atom probe provides true atomic layer composition. Quantitative analysis simply by counting of the fieldevaporated ions.




Atom probe depth profiling analysis

SamplesSamples were cut by low speed diamond saw from stainless steel samples (needles 0.3mm x 0.3mm), electropolished to fine tips (tip radius >10nm) and mounted on Mo heating loop.

Thermal treatment:– vacuum firing in situ

by resistive heating– Temperature control

by micro pyrometer




Field ion imaging and atom probe analysis

Field ion image of a vacuum fired 304 stainless steel (1.10-5mbar Ne, U=10kV, T=40K)




Atom probe principle

Pulsed field desorption of individual ions, mass from time-of-flight, lateral positionfrom screen, 3D reconstruction of probed volume from data set




Cr and Ni distribution in space (Fe matrix not displayed)Probed volume ca. 9x9x5 nm3

Combined features of Cr and Ni

• Cr• Ni

3D atom probe result before vacuum firing




Depth profile of a 302 stainless steel sample without thermal treatment Measured bulk composition close to nominal composition.. (200 ions correspond to one atomic layer).

0 2 4 6 8 10 12 14 16 180

10

20

30

40

50

60

70

80

90

100

%

atomic layer

Cr Ni Fe

3D atom probe result before vacuum firing




Depth profile of a 304 stainless steel sample after vacuum firing (20s@ 900°C).

0 2 4 6 8 10 120

10

20

30

40

50

60

70

80

90

100

%

atomic layer

Cr Ni Fe

3D atom probe result after vacuum firing




Depth profile of a 316L stainless steel sample after 20s vacuum firing @ 900°C

3D atom probe result after vacuum firing




Atom probe results

• After vacuum firing an enrichment of Nickel was found within thefirst atomic layer.

• The total amount of Chromium within the first 10 atomic layers decreases during vacuum firing.

• Thermodynamic model calculation of first atomic layer composition gives explanation for behavior qualitatively.

• Consequences of Ni enrichment on surface?




Sticking coefficients from TDS

H.F. Berger et. al. Surface Sci 251/252(1991)882 A. Winkler et. al. I Rev Phys Chem 11(1992)101

Nearly 10x higher sticking coefficients on Ni! Recombination of H is promoted




Role of stepped surface

The bunched steps and facets offer preferred surface sites for recombination promotedby segregated Ni

(111) terraces

(110) faceted




Electronic structure on stepped surface

Tersoff (1981):Calculations for flat and stepped Ni (111) surfaces show that sites of highest coordination have a less completely filled d band and tend to be the most active sites on a surface.

0.04-0.08-0.0411atom c

0.11-0.19-0.0810atom d

0.32-0.50-0.187atom b

0.31-0.49-0.187atom a

step

0.18-0.29-0.119surface

00012bulk

∆nd∆nsp∆ntotalcoord. nr.Site

ab cd

[1] J. Tersoff and L. M. Falicov, Phys. Rev. B 24 (2), 754 (1981)

∆n… change in total electron occupation (with respect to the bulk) for s, pand d electrons




Role of vacancies

Recently performed theoretical studies and simulationson the interaction of hydrogen with latticeimperfections by Alfredo Juan provide a new insight.

Energy calculations using ASED method (Atom Superposition and Electron Delocalization) result in lower energy levels in tetrahedral sites in Fe vacancies*.

Surface and subsurface defects are forming traps withlower energetic levels

* D. Rey Saravia, A. Juan, G. Brizuela, S. Simonetti, J Hydrogen Energy 34(2009)8302




Role of vacancy

Alfredo Juan, 17th Conf. on Materials, Portoroz, 2009




STM close ups

304L (10x10nm², U=-0.1V, I=0.1nA) Comparison: Vanadium




Comparison TDS on V (100) surface

G. Krenn et. al. Surface Sci 445(2000)343

Surface defects are channels for H desorption

∆Ev




Conclusion

The AFM and STM clearly show that the surface reconstructs during vacuum firing to in order to minimize the surface free energy.

1 µm1 µm 0.3 µm




•Atom Probe show segregation of Ni accompanied by slight Cr depletion.•The steps and facets provide still a considerable number of active sites which can promote the recombination of hydrogen.•Vacancies give reason for subsurface states which act as traps for H.•This surface and subsurface states may control the outgassing behaviour.•From the look on the surface: In all probability diffusion to the surface may be the limiting process.

0.1 µm




Contributions byTU Graz:A. Stupnik (STM, AP)F. Lackner (AP)P. Frank, H. Plank, E. List (AFM)A. Winkler (TDS)K.D. Rendulic (TDS)R. Schennach (SurfChem)

R. Dobrozemsky (TU Wien)J. Setina (IMT Ljubljana)E. Hedlund (Uppsala Univ.)L. Westerberg (Uppsala Univ.)A. Juan (Bahia Blanca, Argentina)

Zukunftsfonds des Landes SteiermarkProject No 119

WS&M Software by Nanotec (E)




The End

Thank you for yourattention

© Andréas M. Winter




OMICRON STM 1

• UHV (or air)

• room temperature

• piezo tripodOMICRON TS1 scannermax. scan size ~(1.2x1.2) µm²piezo sensitivity: 5nm/V

piezo tripod

tip carrier

sample

eddy current dampingmechanism

spring suspension

vibration insulation:

sampleslider




Fully-predictive preparationof STM probe tip

• A control over tip geometry is essential

• Tips are field evaporated to desired end form and imaged by using Field Ion Microscopy (FIM).

• Tip with one single atom on top can be obtained. Shape can be documented in the FIM image.

Field ion micrograph of a W tip with a single atom at the apex position.(He, T=80K, R~10nm)




Vacuum firing




CAS




CAS (P.G.)

Documents

HYDROGEN OUTGASSING OF STAINLESS STEEL OUR ...Manfred Leisch 1st Vacuum Symposium UK February 11, 2010 Samples for AFM and STM study 304L stainless steel