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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