1

EFFECTS  OF CARBON REDEPOSITION ON TUNGSTEN UNDER HIGH-FLUX, LOW ENERGY Ar ION IRRADITAION

AT ELEVATED TEMPERATURE

Lithuanian Energy Institute, LithuaniaVytautas Magnus University, Lithuania

Poitiers University, France

Prof. habil. dr. L. Pranevičius

2006-11-15

2

Outline of the presentationOutline of the presentation

1. Introduction,

2. Sources of carbon redeposition,

3. Simulation of dynamic mixing,

4. Experimental results,

5. Discussions,

6. Conclusions.

1

3

IntroductionIntroduction

Issue: MATERIAL TRANSPORT AND EROSION /DEPOSITION FOR FUSION PROGRAMME

The rate of erosion of the divertor targets and building up of deposited films may ultimately limit the choice of divertor materials and the

operational space for ITER

4

IntroductionIntroduction

1

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0600

800

1000

1200

1400

1600

1800

Time, s

Co

pp

er

Su

rfa

ce

Te

mp

era

ture

, K W

Be

Tm

Cu

on 5 mm Cu Substrate

5 mm W or Be Coating or

60 MJ/m2

300 ms

20 mm Carbon Tiles

VDE

C

Li

Cu Substrate

W/Be/C Coatings or Tiles

Interface

LIST OF PROCESSES

Sketch of divertor

5

IntroductionIntroduction

The present work is an attempt to explain:– the mixing mechanism of C contaminant on W

substrate under high-flux, low-energy ion irradiation;

– the experimentally observable anomalous deep C transport into W under prolonged irradiation at elevated temperature.

The aim:– to deepen the understanding about the behavior

of C contaminant on W .

6

MD simulations for WC targetMD simulations for WC target

Helsinki University, 2005

T=300 K

20 eV H+ 200 eV H+20 eV H+ WC

7

Lithuanian energy institute Materials Research and Testing Laboratory

The goal: to form dense and hard W coatings

The method: plasma activated deposition of W

Plasma activated deposition Magnetron sputter deposition

Samples

Magnetrons

Kick-Off Meeting ASSOCIATION EURATOM 15 November, 2006, Kaunas, Lithuanian energy institute

8

Collaboration in Lithuania

• E-beam deposition of hard coatings Kaunas University of technology

• SIMS carbon profiling Vilnius university

Kick-Off Meeting ASSOCIATION EURATOM 15 November, 2006, Kaunas, Lithuanian energy institute

9

Sources of C redepositionSources of C redeposition

The flux of ejected i atoms:

- wici, where

The flux of redeposited i atoms

- where is the probability for i atom to be back-scattered, and is i atom probability to stick to j atom

)( 1 scw iiiijij

)(/ 10

sCIYw ii

iij

1. Wall collision back-scattering

2 . Working gas collision scattering

3 . Ballistic relocations

4. Redeposition scheme

wici

iiiij cwiii cw

10

ModelModel

j

jiijj

ijj

jjiiii cccwccwtd

cd 11211

)1()1()()(

)( Kia

Kis

Kias

Ki cVcVcVV

dt

dc

ji

jijaj

jjs ckVcwV,

)1()1(where

Surface vacancy

Relocation

Adatom

1

a

K

K + 1

2

3

~ ~ ~ ~ fluxes

Relaxation

The system of rate equations on the surface and for the K

monolayer including sputtering and readsorption processes

11

ModelModel

VI International Conference ION 2006 , Kazimierz DolnyKazimierz Dolny,, Poland, 26-29 June 2006Poland, 26-29 June 2006

dx

cV

x

cD

t

c ix

ief

i

2

2

)()2/1( 20 asef VVhD asx VVhV 0After introduction notations and

It is seen that rate equations can be rewritten as

Three possible cases:

(1) Va > Vs – readsorption prevails (film growth rigime)

(2) Vs > Va – sputtering prevails (surface erosion regime)

(3) Va = Vs - readsorption and sputtering rates are equal (dynamic balance regime)

12

ModelModel

)/exp()(. efxiisti DxVbaxc

as

as

x

ef

VV

VVh

V

Dx

20

0

1

1

1 1 0

2

2 5 0

3

3 1 0 0

4

4 2 0 0

0 .5

0 .6

0 .7

0 .8

0 .9

1 .0

5 1 0 1 5 2 0 2 5 3 0 3 5

Con

cent

ratio

n, c

1()

K

M onolayer num ber

T im e

Surface erosion prevails (Va < Vs)

The steady state solutions

The characteristic thickness of an altered layer

ConclusionConclusion: the steady state mixed layer is formed under simultaneous redeposition and sputtering (Va < Vs)Calculated distribution profiles

13

ModelModel

)2(20

)1(1

20

)1(1

11

ij

sjjiij

jiii ch

DVckc

h

Dkcw

dt

dc

)()( )1()(

20

)1()()1(

20

)(

K

iK

i

K

aK

iK

i

K

s

Ki cc

h

DVcc

h

DV

dt

dc

for K=1

for K1

20

20

1 // hDVhDcwV sjjs 20

20

1 // hDVhDckV ajija

)/2()2/1( 20

20 hDVVhD asef

as VVh

Dxx

1

000

as VVhD 0/The role of diffusion becomes important if

The system of rate equations on the surface and for the K monolayer including

sputtering, redeposition and diffusion processes

14

Experimental proceduresExperimental procedures

The first stage :2 µm-thick W film deposition:- XRD characterization;

- SEM and AFM surface view analysis.

The second stage: erosion by 300 eV Ar+ ion irradiation during C redeposition:

- - SIMS carbon distribution profiles;

- SEM and AFM surface topography analysis.

15

Experimental techniqueExperimental technique

Experimental parameters:

Source power – 200 W,

Ar gas pressure – 10 Pa,

Ar gas flow rate – 1.1 cm3min–1,

Substrate temperature – 300 K

The scheme of experimental device

16

ExperimentalExperimental

Plasma parameters:Electron concentration – 81010 cm-3,

Electron temperature – 3.1 eV,

Sheath bias – 11 V,

Ion flux – 5.51015 cm–2s–1.

Ar plasma

W film

+ + + + + +

GRAPHITE

17

W film characterizationW film characterization

SEM cross-sectional view

2 µ

m

Without bias voltage

Bias voltage – 100 V

Diffraction angle, 2

Diffraction angle, 2

18

Carbon distribution profiles in tungstenCarbon distribution profiles in tungsten

SIMS carbon distribution profiles in W film

0.5 1.0 1.5

0.8

0 20

1

0.5 Pa

5.0 Pa

0.2 Pa

2

3

40 60

0.6

0.4

0.2

0.0

1.0

1.2C

once

ntra

tion,

arb

. u.

Time, s

Depth, m

As-deposited

19

SEM surface views of W film after SEM surface views of W film after irradiation during redepositionirradiation during redeposition

DNQ-117-2-2

DNQ-116-1-1

Adsorption prevails (Va>>Vs) Adsorption prevails (Va>Vs)

Adsorption prevails (VaVs) Sputtering prevails (Vs>Va)

2,5 m

1 m 1 m

5 m

100 m

20

SEM surface views of W film after irradiation SEM surface views of W film after irradiation during redeposition when sputtering prevailsduring redeposition when sputtering prevails

0,5 m 0,5 m

2 m 1 m

21

W surface roughness after irradiation during redeposition

After irradiation during carbon redeposition

Roughness: Ra=2.9 nm Ra=13.5 nm Ra=38.3

Not-irradiated

5 µm

Va > Vs

29 µm

Vs > Va

29 µm

VI International Conference ION 2006 , Kazimierz DolnyKazimierz Dolny,, Poland, 26-29 June 2006Poland, 26-29 June 2006

22

W surface roughness (mechanism)

d t

dtw t w t t

kk k k

1 1

h

01

Ta ik iny s

23456789

1 0

Mon

oslu

oksn

io n

umer

is

Target

Number of monolayer

3

3

-1 5 5 1 0 1 50

0 .2

0 .4

0 .6

0 .8

1 .0

Užp

ildym

o da

lis

M onosluoksn io num eris0-5-1 0

2

21

1

Num

ber

of

mon

olay

er

Coverage

1 – 1 s2 – 5 s3 –10 s

0

0

1

2

2 0

2 0

Rel

jefo

auk

štis

L a ikas

2 5

5

5

1 0

1 0 1 5

1 5 Surface roughness

Time

1. W=2, =12. W=1, =2

23

AFM surface topography sputtering prevails AFM surface topography sputtering prevails redepositionredeposition (V(Vaa>V>Vss))

28 µ

m15 µm

5.1 µ

m

1.5

µm

24

AFM surface topography to the C transport AFM surface topography to the C transport into the W film mechanisminto the W film mechanism

VI International Conference ION 2006 , Kazimierz DolnyKazimierz Dolny,, Poland, 26-29 June 2006Poland, 26-29 June 2006

28 µm 1.5 µm

25

Boundary region

26

XRD patterns of W film on the graphite XRD patterns of W film on the graphite substratesubstrate

W2C

VaVs

Diffraction angle, 2 Diffraction angle, 2

27

XRD patterns of W film on the graphite XRD patterns of W film on the graphite substratesubstrate

Diffraction angle, 2

28

Mechanical erosion by pin-on disc techniqueMechanical erosion by pin-on disc technique

As-deposited W film W film after C redeposition under irradiation

20

0

- 20

-400 400

400

800

800

1200

1200

0

x , m

z,

m

y, m

2

0

-2

-4 0 40 80 120

0 40

80

1

2020

0

- 20

-400 400

400

800

800

1200

1200

0

x , m

z,

m

y, m

0 40 80 120

2

0

-2

-4

0 40

80

1

20

29

DiscussionsDiscussions

The main deduced results:

– the dynamic mixing results in the formation of an layer (modeling);

– the efficient C transport from the surface into W film takes place during the weight decrease regime when W surface is only partially covered by C atoms (experiment);

– the C transport efficiency sharply decreases when continuous amorphous C film is formed on the W surface (experiment).

30

DiscussionsDiscussions

The deduced results may be explained if to assume:

– during high-flux, low-energy ion irradiation the surface chemical potential of W increases and difference of potentials between activated surface and grain boundaries acts as the driving force for C adatoms transporting them into the bulk of W film;

– as continuous amorphous C layer is formed on the W surface the transport of C adatoms from the surface is blocked;

31

ConclusionsConclusions

VI International Conference ION 2006 , Kazimierz DolnyKazimierz Dolny,, Poland, 26-29 June 2006Poland, 26-29 June 2006

The redeposition and surface relocation effects

forms: (i) steady state mixed layer on the surface in the

regime of surface erosion, (ii) formation of continuous

film in the regime when redeposition prevails, and (iii)

mixed layer with thickness increasing in time as

where

tDef )()2/1( 20 asef VVhD

32

ConclusionsConclusions

- The surface roughness increases when

sputtering yield of surface contaminants is low in

comparison with matrix material;

- The efficient carbon transport from the surface into the W film was observed in the regime when sputtering prevails redeposition.

33

The model application to the published The model application to the published experimental resultsexperimental results

Calculated (grey lines) and experimental depth profiles of carbon for target temperatures from 653 K to 1050 K. Beam fluence is 3×1024 m-2.

Y. Ueda, Y. Tanabe, etc., J. Nucl. Mater, 2004,

W by 1.0 ke V of 0.1 % C+ and H3+ beam, flux - 31020 m-2∙s-1,

fluence – 1022 -1024 m-2, T=653 -1050 K

34

The model application to the published The model application to the published experimental resultsexperimental results

Calculated and experimental depth profiles of Ti in natural U

P=10E-2 Pa

Irradiation time -5 min

Ion energy – 2.7 keV

Flux – 1.3×1020 m-2s-1

βU = 0.83, βTi = 0.89

V.I. Safonov, I. G. Marchenko, etc., surf. Coat. Technol., 2003, V by 2.7 keV Ti+, flux - 31020 m-2∙s-1,

time – 5 min, RT