Transcript presentation of prof. L.Pranevičius

EFFECTS OF CARBON REDEPOSITION ON
TUNGSTEN UNDER HIGH-FLUX, LOW
ENERGY Ar ION IRRADITAION AT
ELEVATED TEMPERATURE
Lithuanian Energy Institute, Lithuania
Vytautas Magnus University, Lithuania
Poitiers University, France
Prof. habil. dr. L. Pranevičius
2006-11-15
1
1
Outline of the presentation
1.
2.
3.
4.
5.
6.
Introduction,
Sources of carbon redeposition,
Simulation of dynamic mixing,
Experimental results,
Discussions,
Conclusions.
2
Introduction
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
3
1
Introduction
LIST OF PROCESSES
Copper Surface Temperature, K
1800
5 mm W or Be Coating or
20 mm Carbon Til es
W
1600
on 5 mm Cu Substr ate
TmCu
1400
VDE
60 MJ/ m
300 ms
1200
2
1000
Be
C
800
600
0.0
Li
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Tim e , s
W/ Be/ C Coatings or Ti les
Inter face
Cu Substrate
Sketch of divertor
4
Introduction
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 .
5
Helsinki University, 2005
MD simulations for WC target
T=300 K
20 eV H+
WC
20 eV H+
200 eV H+
6
Lithuanian energy institute
Materials Research and Testing Laboratory
The goal: to form dense and hard W coatings
The method: plasma activated deposition of W
Samples
Magnetrons
Plasma activated deposition
Kick-Off Meeting ASSOCIATION EURATOM
Magnetron sputter deposition
15 November, 2006, Kaunas, Lithuanian energy institute 7
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 8
Sources of C redeposition
1. Wall collision backscattering
2 . Working gas
collision scattering
The flux of ejected i atoms:
- wici,
3 . Ballistic relocations
wici
i wi ci
iji wi ci
1
where wi  Yi I0 / C (s )
The flux of redeposited i atoms
4. Redeposition scheme
ij-  iji wi ci (s 1 ) where  i is the probability for
i atom to be back-scattered, and  ij is i atom probability to
stick to j atom
9
Model
The system of rate equations on the surface and for the K
monolayer including sputtering and readsorption processes
Surface
vacancy
1
Relocation
Adatom
a
1 2
1 1
d ci
  wi ci  ci  w j c j    ij c j  ci   ji
dt
j
j
j
dci( K )
 (Vs  Va )ci( K )  Vs ci( K 1)  Va ci( K 1)
dt
1
2
3
~
~
Relaxation
fluxes
~~
K
K+1
where Vs   w j c (j1)
j
Va   kij c (j1)
i, j
10
Model
After introduction notations Def  (1/ 2)h02 (Vs  Va ) and Vx  h0 Vs  Va 
ci
 2 ci
ci
 Def
 Vx
It is seen that rate equations can be rewritten as
2
t
dx
x
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)
VI International Conference ION 2006 , Kazimierz Dolny, Poland, 26-29 June 2006
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Model
Surface erosion prevails (Va < Vs)
1.0
The steady state solutions
0.9
(K)
Concentration, c1
ci.st ( x)  ai  bi exp(Vx x / Def )
Time
0.8
1
2
10
50
3
4
100
The characteristic thickness
of an altered layer
200
0.7
0.6
2
1
3
x0 
4
Def
Vx

h0 Vs  Va
2 Vs  Va
0.5
1
5
10
15
20
25
30
35
Monolayer number
Calculated distribution profiles
Conclusion: the steady state mixed layer
is formed under simultaneous redeposition
and sputtering (Va < Vs)
12
Model
The system of rate equations on the surface and for the K monolayer including
sputtering, redeposition and diffusion processes
(1) 
(1) 


dc11
D
D




1
1
   wi ci   k ji  2  ci   k ji c j  Vs  2  ci( 2)
dt
h0 
h0 
j
j


for K=1
dciK  
D( K )  ( K 1) ( K ) 
D( K 1)  ( K ) ( K 1)
 (ci  ci
 Vs  2  (ci
 ci )  Va 
) for K1
2

dt
h0 
h0 


Va   kij cj1  D / h02  Va  D / h02
Vs   w j cj1  D / h02  Vs  D / h02
Def  (1/ 2)h02 (Vs  Va  2D / h02 )
D 1
h0 Vs  Va
The role of diffusion becomes important if D / h0  Vs  Va 
x0  x0 
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Experimental 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.
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Experimental technique
Experimental parameters:
Source power – 200 W,
Ar gas pressure – 10 Pa,
Ar gas flow rate – 1.1 cm3min–1,
Substrate temperature – 300 K
The scheme of experimental device
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Experimental
Ar plasma
+
+
+
+
+
+
W film
GRAPHITE
Plasma parameters:
Electron concentration – 81010 cm-3,
Electron temperature – 3.1 eV,
Sheath bias – 11 V,
Ion flux – 5.51015 cm–2s–1.
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W film characterization
2 µm
SEM cross-sectional view
Without bias voltage
Bias voltage – 100 V
Diffraction angle, 2
Diffraction angle, 2
17
Carbon distribution profiles in tungsten
Depth, m
0.5
1.0
1.5
1.2
Concentration, arb. u.
1.0
0.2 Pa
5.0 Pa
0.8
0.5 Pa
0.6
0.4
3
0.2
2
1
0.0
0
20
40
60
Time, s
SIMS carbon distribution profiles in W film
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SEM surface views of W film after
irradiation during redeposition
100 m
DNQ-117-2-2
5 m
Adsorption prevails (Va>>Vs)
2,5 m
Adsorption prevails (Va>Vs)
DNQ-116-1-1
1 m
Adsorption prevails (VaVs)
1 m
Sputtering prevails (Vs>Va)
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SEM surface views of W film after irradiation
during redeposition when sputtering prevails
0,5 m
0,5 m
2 m
1 m
20
W surface roughness after irradiation during
redeposition
Not-irradiated
After irradiation during carbon redeposition
Va > Vs
Roughness: Ra=2.9 nm
Ra=13.5 nm
Vs > Va
Ra=38.3
VI International Conference ION 2006 , Kazimierz Dolny, Poland, 26-29 June 2006
21
h
0
1
2
3
4
5
6
7
8
9
10

Number
of monolayer
Monosluoksnio
numeris
W surface roughness (mechanism)
d  k   t 
Target
Taikinys
dt
 w  k  1  t   w     k   t     k 1  t 
Surface
Reljeforoughness
aukštis
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Coverage
Užpildymo
dalis
1.0
1– 1s
2– 5s
0.8
0.6
1 2
3
3 –10 s
0.4
0.2
1
3
1
20
15
2
10
1. W=2, =1
2. W=1, =2
5
2
0
-15
-10
-5
0
5
10
Monosluoksnio
numeris
Number
of monolayer
15
0
0
5
10
15
20
Time
Laikas
22
AFM surface topography sputtering prevails
redeposition (Va>Vs)
23
AFM surface topography to the C transport
into the W film mechanism
VI International Conference ION 2006 , Kazimierz Dolny, Poland, 26-29 June 2006
24
Boundary region
25
XRD patterns of W film on the graphite
substrate
VaVs
W2C
Diffraction angle, 2
Diffraction angle, 2
26
XRD patterns of W film on the graphite
substrate
Diffraction angle, 2
27
Mechanical erosion by pin-on disc technique
1200
20
2
0
400
40
800
80
x, m
1200
120
As-deposited W film
800
m
00
y,

400
- 20
-2
z, m
m
y,

z, m
800
00
-40
-4
00
1200
20
2
400
- 20
-2
-40
-4
00
0
400
40
800
80
1200
120
x, m
W film after C redeposition
under irradiation
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Discussions
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).
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Discussions
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;
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Conclusions
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
Def t where D  (1/ 2)h 2 (V  V )
ef
0
s
a
VI International Conference ION 2006 , Kazimierz Dolny, Poland, 26-29 June 2006
31
Conclusions
- 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.
32
The model application to the published
experimental results
Y. Ueda, Y. Tanabe, etc., J. Nucl. Mater, 2004,
W by 1.0 ke V of 0.1 % C+ and H3+ beam, flux - 31020 m-2∙s-1,
fluence – 1022 -1024 m-2, T=653 -1050 K
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.
33
The model application to the published
experimental results
V.I. Safonov, I. G. Marchenko, etc., surf. Coat. Technol., 2003,
V by 2.7 keV Ti+, flux - 31020 m-2∙s-1,
time – 5 min, RT
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
34