OUTGASSING OF HYDROGEN M. Leisch

Institute of Solid State Physics 

OUTGASSING OF HYDROGEN 

Manfred Leisch 


Graz University of Technology, Austria 

Professor Manfred Leisch Horst Cerjak, 19.12.2005 63rd IUVSTA Workshop Avila September 17, 2010 

1


Contributions by 

TU 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 Steiermark 

Project No 119 

WS&M Software by Nanotec (E) 


2


Background 

Accelerators 

Mainspring for 

achieving extremly 

low pressures (XHV) 

(Images copyright CERN) 


3


Materials 

Austenitic stainless steel (Type AISI 304 and 316) is one of the 

most 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 …. 


4


Hydrogen as impurity 

Solubility of Hydrogen in stainless steel 

The H content in standard austenitic 

stainless steel is about 1 ppm in weight 

(~ 56 at ppm). 

Amount equates ~ 0.1 mbar·l / cm 3 , 

at typical outgassing rates of 

10 -11 mbar.l/cm 2 .s from 2 mm wall 

source for more than 50 years 


5


Reduction of outgassing 

Is essential for obtaining ultimate pressures 

1. Decreasing the H Concentration: 

-- Bakeout and high temperature bakeout (vacuum firing) 

2. Hindering H Desorption from the Surface: 

– Treatments to produce barrier layer on the surface: air bakeout 

– Passive coatings on the surface 

– Active coatings (NEG) 

[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) 


6


Decreasing the concentration 

Vacuum firing 


7


Mechanism for H 2 outgassing 

Diffusion in the bulk– Recombinative desorption from surface 

Polany – Wigner Equation: 

- dN/dt = ν (Θ) 2 exp (-E des / kT) 


8


Diffusion limited outgassing (DLM) 

described by Fick‘s equation 

Kinetic models 

Recombination limited outgassing 

(RLM) 

Flat bulk concentration in pure RLM 

with recombination coefficient 


9


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. 


10



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 


11



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 (2006) [CAS Platjo d‘Aro, Spain] 

The outgassing of H after vacuum firing can be reasonable described by a diffusion 

model only if the pressure of H during the treatment is taken into account 

J. Setina (2006) 

[53rd AVS Symposium, San Francisco 2006 ] 

- Outgassing measurements (250° C, 300 h) and 

model calculations solved with FEM support RLM 


12


Surface Analysis 

Recombination of atomic Hydrogen to the Hydrogen molecule is 

the essential step. 

Recombination process strongly relates on surface morphology 

• surface structure of the material? 

• surface composition of the material? 

Application of Surface characterization tools like 

TDS, AES, XPS, AFM, STM and Atom Probe 


13


Thermal Desorption Spectroscopy 

Useful technique to provide insight on thermodynamics and kinetics of gas – 

surface reactions (surface states, sticking coefficient, activation energy) 

[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 


14



15


Surface Imaging 

Characterization of surface morphology 

Surface roughness 

Grain boundary structure 

Surface reconstruction 

Surface defects 

Atomic level information by STM 


16


Experimental setup 

Preparation 

Chamber 

• 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 


18


Samples for AFM and STM study 

304L stainless steel 

C Mn P S Si Cr Ni Mo 



100 nm 

100 nm 

304L 3hours@300°C 

304L 15min@1000°C 

Surface morphology after low temperature bakeout and after vacuum firing 


20


3hours@300°C 


15min@1000°C 

200 

600 

150 

500 

Z[nm] 

100 

Z[nm] 

400 

300 

50 

200 

100 

0 

0 

0.5 

1 

1.5 

2 

2.5 

3 

3.5 

0 

0 

2 

4 

6 

8 

10 

X[µm] 

X[µm] 


21



Line profile show Δh up to 150 nm 

140 

120 

100 

Z[nm] 

80 

60 

40 

20 

1 µm 

0 

0 

1 

2 

3 

4 

5 

6 

7 

8 

X[µm] 

AFM micrograph 316L (B) 20min@1000°C 

(10x10µm², derivated image) 


22




AFM micrograph 316L (U) 1h@1100°C 

(8.6x8,6µm², derivated image) 


23




70 

60 

50 

Z[nm] 

40 

30 

20 

10 

0 

0 

0.2 

0.4 

0.6 

0.8 

1 

X[µm] 

STM micrograph 316L (U) 1h@1100°C 

(1x1 µm², 3D image, top view) 


24


Precipitates 

b 

c 

200 nm 200 nm 

100 nm 

100 nm 

304L (1000x1000nm², U=-0.5V, I=0.1nA) 20min@1000°C 

Nanoprecipitate in reconstructed surface 


25


Facets and atomic steps 


AFM micrograph 316L (U) 48h@450°C 

(2.2x2.2 µm², derivated image) 


26



304L 15min@1000°C 

(1000x1000nm², U=-0.5V, I=0.1nA) 

(1000x1000nm², U=-0.5V, I=0.1nA) 


27



25 

20 

Z[nm] 

15 

10 

5 

1 µm 

0 

0 

0.5 

1 

1.5 

2 

X[µm] 

AFM micrograph 316L (B) 20min@1000°C 



28



100 

80 

Z[nm] 

60 

40 

20 

0.5 µm 

0 

0 

200 

400 

600 

800 

1000 

X[nm] 

AFM micrograph 316L (B) 

20min@1000°C 



29



6 

5 

4 

Z[nm] 

3 

2 

1 

0 

0 

50 

100 

150 

200 

250 

X[nm] 

2 

1.5 

Z[nm] 

1 

0.5 

STM 316L 20min@1000°C 

0 

0 

50 

100 

150 

(1000x1000nm², U=-0.5V, I=0.1nA) 

X[nm] 


30


304L 20min@1000°C 


(500x500nm², U=-0.5V, I=0.1nA) 

(300x300nm², U=-0.5V, I=0.1nA) 


31


Role of stepped surface 

(111) terraces 

(110) faceted 

The bunched steps and facets offer preferred surface sites for Hydrogen recombination 


32


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. 

Site coord. nr. ∆n total ∆n sp ∆n d 

bulk 12 0 0 0 

d 

b 

a 

c 

surface 9 -0.11 -0.29 0.18 

atom a 7 -0.18 -0.49 0.31 

step 

atom b 7 -0.18 -0.50 0.32 

atom c 11 -0.04 -0.08 0.04 

atom d 10 -0.08 -0.19 0.11 

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

and d electrons 

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


33


304L 20min@1000°C 

STM images show vacancies 

(300x300nm², U=-0.1V, I=0.1nA) 

(10x10nm², U=-0.1V, I=0.1nA) 


34


Role of vacancies 

Recently performed theoretical studies and simulations 

on the interaction of hydrogen with lattice 

imperfections 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 with 

lower energetic levels 

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


35


Role of vacancy 

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


36


STM close ups 

304L (10x10nm², U=-0.1V, I=0.1nA) 

Comparison: Vanadium 


37


Comparison TDS on V (100) surface 

ΔE v 

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

Surface defects are channels for H diffusion and sites for recombination 


38


Effect of pinhole 

Pinhole gas throughput is strongly enhanced by lateral diffusion 


39



40


From: Paolo Chiggiato, CAS 2006 Platjo d‘Aro, Spain 

Surface composition 


41


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 


42


Analysis of surface composition 

AES gives sign for slight enrichment of Ni and 

depletion of Cr but due to information depth of this 

technique composition of topmost layer almost 

uncertain. 

Atom probe provides true atomic layer composition. 

Quantitative analysis simply by counting of the field 

evaporated ions. 


43


Atom probe depth profiling analysis 

Samples 

Samples 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 


44


Field ion imaging and atom probe analysis 

Field ion image of a vacuum fired 304 stainless steel 

(1 . 10 -5 mbar Ne, U=10kV, T=40K) 


45


Atom probe principle 

Pulsed field desorption of individual ions, mass from time-of-flight, lateral position 

from screen, 3D reconstruction of probed volume from data set 


46


3D atom probe result before vacuum firing 

Cr and Ni distribution in space 

(Fe matrix not displayed) 

Probed volume ca. 9x9x5 nm 3 

Combined features of Cr and Ni 

• Cr 

• Ni 


47


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). 

100 

% 

90 

80 

70 

60 

50 

40 

30 

20 

10 

0 

0 2 4 6 8 10 12 14 16 18 

atomic layer 


Cr 

Ni 

Fe 

48


3D atom probe result after vacuum firing 

% 

100 

90 

80 

70 

60 

50 

40 

30 

20 

10 

0 

0 2 4 6 8 10 12 

atomic layer 

Cr 

Ni 

Fe 

Depth profile of a 304 stainless steel sample after vacuum firing 

(20s@ 900°C). 


49


Atom probe results 

• After vacuum firing an enrichment of Nickel was found within the 

first atomic layer. 

• Chromium depletion on the first atomic layer after vacuum firing. 

• Thermodynamic model calculation of first atomic layer 

composition gives equalitative explanation for behavior. 

• Consequences of Ni enrichment on surface? 


50


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 


51


•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 


52


Hindering the desorption 

• Airbake 

• Passive Layers 

• Active Layers 

• Internal gettering – new materials 


53


Air bakeout 

Such processing is reported to decrease hydrogen diffusion 

rate by 10 3 times (D.G. Bills, JVST 6(1969)166) 

Air bakeout 38h@450° C and in-situ bakeout 168h@150°C 

(Bernardini et al.) results in: 

Question arises is benefit of treatment due to hydrogen 

depletion or not? 

TDS on air baked samples show H peak shift 

Hydrogen trapped by oxide layer 

Extraction measurements gives reason for H depletion 


54


Passive barriers 

Deposited thin films (Al2O3, BN, TiN, ZrO etc.) should entirely block 

Hydrogen outgassing 

Experimental results have shown that only partial reduction of the flux is 

attained. 

Attributed to defects on the coating (pinholes) 

Pinhole gas throughput is strongly enhanced by lateral diffusion 


55


Active barriers 

Active barriers absorb H exothermically (negative 

solution enthalpy). They should absorb the H atoms 

coming from the substrate. 

Application of non evaporable getter (NEG) layers e.g. 

sputter-deposited Ti-Zr-V alloys. 

Potentiality of active barriere is obtained only after 

surface activation (activation e.g. 24h@180°C). 


56


Other materials 

Titanium Chambers: 


57


Materials on ITER 


58


Complex system 


59


Challenge for the future 


60


The End 

© ITER.org 

Thank you for your 

attention 


61

OUTGASSING OF HYDROGEN M. Leisch

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