Characterization of W and C surfaces exposed to low energy plasmas and ion beams

R.D. Kolasinski, D.A. Buchenauer, R.A. Causey, J. WhaleySandia National Laboratories, Livermore, CA

M. Shimada, J.P. Sharpe, R.J. PawelkoIdaho National Laboratory, Idaho Falls, ID

K.R. Umstadter, R. DoernerUniversity of California, San Diego, CA

Characterization of W and C surfaces exposed to low energy plasmas and ion beams

Plasma Facing Components Meeting / Boston, MA / July 8-10, 2009

Sandia National Laboratories / University of California, San Diego / Idaho National Laboratory

overview

• Part 1: – Characterization of surface

morphology and retention in tungsten materials exposed to high fluxes of D ions.

• Part 2: – The impact of specific

surface area on the retention of D in carbon fiber composites.

• Part 3:– Low energy ion scattering

measurements of W surfaces.

• Part 4:– Testing of tritium permeation

barriers.

150

100

50

0

y (m

icro

ns)

200150100500x (microns)

2.5

2.0

1.5

1.0

0.5

0.0

-0.5

-1.0

height (micro

ns)


part 1: surface morphology studies

Motivation: W is known to blister when exposed to low energy H plasmas.• Tritium can precipitate in blister cavities and other voids beneath the

surface, affecting transport into the material.• Melting of blister caps is possible during transient high heat loads,

providing a contamination mechanism for the core plasma.

Experiment: Tungsten samples were exposed to a high flux, low energy deuterium plasma in the Tritium Plasma Experiment (TPE) over a range of temperatures (147 °C ≤ Tsurf. ≤ 704 °C)

Surface characterization: • Vertical scanning interferometery• SEM• AES (depth profiling)• XRD

R.D. Kolasinski, M. Shimada, D.A. Buchenauer, R.A. Causey, T. Otsuka, W.M. Clift, J.M. Shea, T.R. Allen, P. Calderoni, J.P. Sharpe, Physica Scripta (submitted, PFMC-12 Proceedings.)


surface profilometery results

(a)147 ºC

W08a

1.2

0.8

0.4

0.0

(b)173 ºC

W08b

1.0

0.8

0.6

0.4

0.2

0.0

-0.2

(d)449 ºC

W08c

543210

-1-2

(c)385 ºC

W08d

4

3

2

1

0

-1

(f)704 ºC

W08f

3

2

1

0

height(mm)

height(mm)

height(mm)

height(mm)

height(mm)

561 ºC (e)

W08e

8

6

4

2

0

-2

Exposure conditions: E=70 eV, Φ=1.1×1022 m-2s-1, F=8.7×1025 m-2

Sample material: Warm rolled PLANSEE W, annealed for 1 hr at 1000 °C, 1 mm thick

Note: Each panel above represents a 500 μm × 500 μm area.


SEM images

• SEM results provide insight into the size of blisters relative to the grain boundaries.

• Blisters found in this study (PLANSEE W) are much larger than those observed under similar exposure conditions with re-crystallized W.

• Small fissures visible in blister caps located at grain boundaries, suggesting release of D is possible.

5 μm 10 μm 20 μm

(a) (b) (c)


Auger depth profiling

• AES depth profiles obtained using 500 eV Ar+ ions to sputter the target material; etch rate is 10 Å/min.

• For 385 °C≤Tsurface≤561 °C, a carbide layer was formed up to depths of 30-40 nm deep into the material (corresponds to more highly blistered cases.)

• Samples exposed outside of this temperature range showed no carbide formation.

• XRD confirms carbide layer exists prior to AES depth profiling (i.e. carbide not created by Ar beam.)

• Due to T contamination concerns, it was not possible to analyze sample until after TDS.

• Further work in this area is needed to determine root cause of carbide formation.

100

80

60

40

20

0

ato

mic

con

c.(%

)

3020100

profile time (min.)

AES depth profilesTsurf=561 °C

Flux=1.1×1022 m-2s-1

Fluence=8.7×1025

m-2

WC (carbide)C (graphite)O

100

80

60

40

20

0

ato

mic

con

c.(%

)

302520151050

profile time (min)

WC (carbide/graphite)O

AES depth profilesTsurf=147 °C

Flux=1.1×1022

m-2

s-1

Fluence=8.7×1025

m-2


part 2: CFC retentionMotivation: Internal porosity of CFC materials can affect retention of hydrogen

isotopes in carbon fiber composites. Our objective is to determine the effect of specific surface area on retention directly.

Experiment: • Performed specific surface area

measurements for CFC N11, and Hitco and K-Karb composites.

• Samples exposed to a low energy (130 eV), high flux (2×1022 m-2s-1) plasma in PISCES-A (UCSD) at a sample temperature of 200 °C.

(a)

(b)

(c)

100 μm(d)

20 μm(e)

Test Matrix: SEM images showing the fiber structure of the plasma exposed surfaces of the CFC tiles: (a) Hitco CC139C (b) Hitco CC389L (c) K-Karb (d) N11, showing side view of fiber bundle (e) N11, showing end view of fiber bundle. The 100 μm scale applies to images (a)-(d), and the 20 μm scale applies to image (e).

R.D. Kolasinski, K. Umstadter, J.P. Sharpe, R.A. Causey, R. Pawelko, J. Whaley, D. Buchenauer, M. Shimada, Fusion Eng. & Design, (2009).


BET surface area measurementsThe Brunauer-Emmett-Teller (BET) gas adsorption technique was used to determine the specific surface area of the samples.

Experimental Analysis:–Measurements obtained with a Micromeritics ASAP 2000 instrument with Kr gas serving as the adsorptive.–Adsorption isotherms for Hitco and K-Karb composites show a marked increase in the volume of gas adsorbed (Va) at a pressure ratio P/Po≈0.3. (A non-porous material would be characterized by a linear increase in Va until P/Po≈0.9.)–Composite N11 shows a much more abrupt change in Va at P/Po≈0.3.

BET isotherms indicate each material contains a population of mesopores (>2 nm in diameter) in addition to a population of macropores (>50 nm in diameter).

1.0

0.9

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0.4

0.3

Va

(nor

mal

ized

)

Hitco CC139C Hitco CC389L K-Karb

(a)

0.14

0.12

0.10

0.08

Va (

cm3 /g

)

0.50.40.30.20.10.0

P/Po

6

5

4

3

2

1

BE

T T

erm: P

/Va (P

-Po )

CFC N11 (Va) CFC N11 (BET)

(b)

BET Results: BET surface areas for the materials described in this study ranged from 0.14-0.55 m2/g (CFC N11 had a BET surface area of 0.37 m2/g.)


CFC retention results• Results:

– A strong correlation is noted between retention and BET surface area.

– Results compared with exposure of N11 composite from a recent study by Roth [1] under nearly identical conditions in PISCES-A.

– Measurements show a factor of 5 reduction in retention may be achieved using a material with lower BET surface area.

– Further studies needed with the NB31 composite.

1020

2

4

6

810

21

2

4

6

810

22

2

4

tota

l ret

ain

ed (

D/m

2)

1021

1022

1023

1024

1025

1026

incident fluence (D/m2)

PISCES Hitco CC139C K-Karb Hitco CC389L N11 (Roth et al.)

Ion beam (Roth et al.)

NB31 200 °C NB31 20 °C

CFC type BET surface

area

(m2/g)

Fluence

(m-2)

Retention

(m-2)

Retained Fraction

K-Karb 0.14 5×1025 5.6×1020 1.1×10-5

1×1025 3.0×1020 3.0×10-5

5×1025 1.0×1021 2.0×10-5

Hitco CC139C

0.15

1×1026 2.5×1021 2.5×10-5

CFC N11 0.37 -- -- --

Hitco CC389L

0.55 5×1025 2.6×1021 5.2×10-5

[1] J. Roth et al., J. Nucl. Mater., 363-365, 822-826 (2007)


part 3: low energy ion scattering measurements

• Low energy ion scattering (LEIS) measurements of W surfaces– Low energy ion scattering is one of the

only techniques capable of detecting H on surfaces.

– Can provide detailed information regarding H recoil processes from surfaces, adsorbate binding sites, and binding energies.

Local atomic structure derived from shadowing effects.

• Incident ions are deflected around target atoms.

• A shadow cone represents a region behind the scattering center where incident ions can not reach.

• Beam angle of incidence α can be related to an intersection distance on the target surface d.

• Scattering intensity enhanced when shadow cone envelope intersects neighboring atoms.

-2.0

-1.5

-1.0

-0.5

0.0

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1.0

z di

stan

ce (

Å)

43210-1

r distance (Å)

scatteringcenter

nearestneighbor

target surface

shadow coneenvelope

d

α

incident beam

-2.0

-1.5

-1.0

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1.0

z di

stan

ce (

Å)

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r distance (Å)

scatteringcenter

nearestneighbor

target surface

shadow coneenvelope

d

α

incident beam


H recoil maps for W(100)signal

intensity(counts/nC)

signalintensity

(counts/nC)

(a)

(b)

(a) Dosed with molecular hydrogen

(b) Residual hydrogen

Which H binding site is preferred?

3.0

2.5

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1.5

1.0

0.5

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log(

coun

ts/n

C)

1.00.80.60.40.20.0

relative energy (E/Eo)

ARIES ion energy spectrum

1 keV Ne+ W(100)

=78° / =0° / =45°

O(s) O(r)

W(s)

H(r)

W(r)


shadowing mechanismssignal

intensity(counts/nC)

signalintensity

(counts/nC)

(a)

(b)

Structure associated with bridge site more prominent when the sample is dosed with H2(g).

Pattern sensitive to height of H above surface.

hbridge=1.25 Å

h4-fold=0.4 Å


MARLOWE H(r) mapsMARLOWE simulations reproduce the experimental recoil maps if binding in both four-fold hollow and bridge sites are assumed.

hollow site: h=0.4 Å

bridge site: h=1.25 Å

The differences between the recoil patterns from the two sites is striking.

0.20

0.15

0.10

0.05

0.00

R-f

acto

r

1.51.41.31.21.11.0

ARIES/MARLOWEbridge site R-factorcomparison

(a)

1.51.41.31.21.11.0

adsorbate height (Å)

MARLOWE/MARLOWEbridge site R-factorcomparison

(b)

0.60.40.2

0.025

0.020

0.015

0.010

0.005

0.000

R-factor

(c)

ARIES/MARLOWEfour-fold hollow R-factorcomparison

Tritium Permeation Barriers are Needed forITER and DEMO

Gas driven permeation of various materials is being studiedCurrent system: T ≤ 500 ºC and deuterium gas (< 1 atm)Example: ZrO2 on Zircaloy-4 used in current fission reactors

Newly designed system under constructionfor studying SiC (SBIR with Ultramet)

T ≤ 1000 ºCImproved sensitivity (mass discrimination)Oxidation control


Concluding remarks / future work• Part 1: Surface morphology and retention of D in W

– Blister formation on W showed a strong temperature dependence, with largest defects approximately 10 μm high.

– More analysis needed to determine the cause of carbide formation.

• Part 2: The impact of specific surface area on CFC retention– Test matrix included four different CFC materials, including the N11 composite used in

Tore Supra.– BET measurements indicate surface connected porosity consisting of populations of

mesopores (>2 nm dia.) and macropores (>50 nm dia.)– Strong dependence of retention on BET surface area.– For better comparison with NB31 data of Roth, we need to consider lower flux / lower

fluence measurements.

• Part 3: Low energy ion scattering measurements of the W surface– Determined height and binding site of H adsorbed on W(100).– Further work will include monitoring surface concentration with temperature to

determine the binding energy.


Acknowledgements

Work by Sandia is supported by the U. S. DOE under contract DE-AC04-Work by Sandia is supported by the U. S. DOE under contract DE-AC04-94AL85000. Sandia is a multi-program laboratory operated by Sandia 94AL85000. Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Corporation, a Lockheed Martin Company, for the United States Department of Energy.Department of Energy.

We express our appreciation to:

Ed Waters (INL)

Tammy Trowbridge (INL)

Norm Bartelt (SNL/CA)

Kevin McCarty (SNL/CA)

Thank you for your attention!

Characterization of W and C surfaces exposed to low energy plasmas and ion beams

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