Transcript Document

Hydrogen Retention in C-W mixed
layers of tungsten surface: the first
principle calculations
Yuexia Wang
Applied Ion Beam Physics Laboratory, Institute of Modern Physics, Fudan
University, 200433 Shanghai, China
Background
• The retention of hydrogen (H) isotope in plasma-facing
materials (PFMs) is an important issue for next step fusion
device. (the recycling of hydrogen and affect plasma
performance)
• W, W alloys, Graphite (the latter which provides feedstock of
C to implant into W; C-W mixed layers forms in W surface
(WC)).
• PFMs are subjected to high heat flux and high particle flux.
Physical and chemical sputtering causes C-W mixed layers in
the surface of PFMs
• Our understanding is still very limited regarding H retention.
(difficult to precisely answer which phase components the CW mixed layers contain.)
• Some experiments have observed that WC layers prevent the
implanted H from escaping from W, which thus lead to blister
formation of H in W:
Nucl. Fusion 44 (2004) 62;
J. Nucl. Mater. 313-316 (2003) 204;
Fus. Eng. Des. 66-68 (2003) 247;
J. Nucl. Mater. 337-339 (2005)1010;
J. Nucl. Mater. 329-333 (2004) 780.
• Other experiments showed that H retention was considerably
small.
Fus. Eng. Des. 81 (2006) 295.
• What is the phenomenon of H retention in the presence of W-C
mixed layers?
Method
• DFT Vienna ab initio simulation package (VASP)
code.
• The projector augmented wave potential (PAW)
method , with the exchange correlation functional
of Perdew and Wang (PW91).
• The plane wave cutoff energy:350 eV.
• bcc W super-cell containing 54 atoms
• 5×5×5 K-points.
The solution energies H and C atom:
EsH = ENW+H – ENW – EH
EsC = ENW+C – ENW – μC
The formation energy of vacancy in bcc W is:
EfV = E (N-1) W – (N–1) ×ENW/N
The binding energy for Hn-1C binding one H atom:
EbH = ENW+C+nH + EW – (ENW+C+ (n-1) H + ENW+H)
Results
The solution behaviour of C in W
Solution energy
Tetrahedral
Octahedral
H
-2.46 eV
-2.08 eV
C
2.27 eV
0.78 eV
In Vacancy
Surface (001)
-1.177 eV
-3.328 eV
2.5
2.0
Binding energy(eV)
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
0
1
2
3
4
5
The number of C atoms
6
7
Bader charges of C atoms in Cn-Vm complexes (|e|), overlap
population of C-C and C-W bonds, and the bond length and
angle of C-C bonds
C1-V1
C2-V1
C3-V1
C4-V1
Barder charges
(|e|)
Overlap
population(|e|)
-1.183(C1)
-0.843 (C1)
-0.939 (C2)
-0.654(C1)
-0.936(C2)
-0.934 (C3)
-0.753(C1)
-0.754(C2)
-0.752(C3)
-0.835 (C4)
0.57 (C-W)
0.48 (C-W)
1.05 (C-C)
Bond Length of Angle of CC bond
C-C (Å)
1.477
0.44 (C-W)
0.95 (C-C)
1.487
108°62′
0.37 (C-W)
0.70 (C-C)
1.574
90°18′
Difficult to precipitate graphite-like species composed of more C atoms since one vacancy
will be saturated by at most 4 C atoms. Surface! Adsorption energy: -3.328 eV
H retention in Cn-V1 complexes
Minimal energy configurations
Binding energy(eV)
1.0
0.8
0.6
0.4
0.2
0.0
0
1
2
3
The number of C atmos
4
5
Eb = 0.862 eV
Eb = -1.859 eV
Charge density distribution
PDOS
Reason:
Charge-depleted region
introduced by vacancies
Note: the distance of C-H: 1.11Å; the bond length in methane: 1.084Å
H trapped by C atom
Binding energy of H with C atom as a function of distance between H and C
Binding energy in
hollow site: 0.42 eV
Reason:
The charge of H:-0.26 |e|
The charge of C:-0.68 |e|
H retention in C1-V1 complex
Binding energy as a function of the number of H atoms
trapped by a vacancy with and without C in the OIS position.
Comparison
Case II
Case III
Binding energy, atomic Bader charges of H and W,
bond populations and bond lengths of H-W.
Deformation
energy(eV)
Binding
energy(eV)
Bader charge (|e|)
Case I
0.019
1.205
-0.321(H)
Case II
0.025
1.060
-0.306(H)
CaseIII
0.270
0.862
0.207(H)
-0.007(W1)
0.125(W2)
0.125(W3)
0.020(W1)
0.105(W2)
0.094(W3)
Overlap
population
(|e|)
0.59(H-W1)
0.18(H-W2)
0.18(H-W3)
0.49(H-W1)
0.25(H-W2)
0.25(H-W3)
1.06(H-C)
Bond length
(Å)
1.903(H-W1)
2.077(H-W2)
2.077(H-W3)
1.923(H-W1)
1.956(H-W2)
1.956(H-W3)
1.112(H-C)
In Case I: more electron transfer from W to H, compared with Case II. High Eb
Atomic Bader charges of C and W, bond
populations and bond lengths of C-W.
Case II
Case III
Bader charge
(|e|)
-1.206(C)
0.103(W1’)
0.283(W2’)
0.289(W3’)
0.304(W4’)
0.308(W5’)
-1.209(C)
0.116(W1’)
0.212(W2’)
0.212(W3’)
0.212(W4’)
0.212(W5’)
Bond length
(Å)
2.111 (C-W1’)
2.141(C-W2’)
2.140(C-W3’)
2.154(C-W4’)
2.155(C-W5’)
2.131(C-W1’)
2.227(C-W2’)
2.227(C-W3’)
2.227(C-W4’)
2.227(C-W5’)
The bond lengths of C-W atoms are elongated, causing the larger deformation
energy, 0.27 eV, and subsequently, the lower binding energy of H, 0.862 eV.
Reason: enhance H retention in WC
Nuclear Fusion 52 (2012) 123003
Discussions
• Some experiments showed that hydrogen is trapped to carbon by
C-H bonding in C-W mixed layers. (High C concentration; C is
the primary component; sp1, sp2, sp3 unsaturated bonds.)
• WC formed under high temperature (600 K, ITER condition.).
The participation of W suppresses the ability of C capturing H.
• Experiments observed that for the sample with higher tungsten
concentration, the amounts of CD4 desorption are considerably
small, which again evidences that H is trapped by W in C-W
mixed layers
• Graphite-like species prefer to be precipitated on the surface of W.
H is not energetically favorable to form H-C bond in W,
hydrocarbon gases prefer to form on the surface of W, while not in
the inner.
Conclusion
• H retention to large extent depends on the phase components
in C-W mixed layers.
• Explanations: In C-W layers with the concentration of W
higher than that of C, the phase components may be mixed
under the co-deposition condition by carbon precipitators,
tungsten carbides and tungsten dissolved by small amount of C
atoms.( two latter phase components: C suppresses the H
isotope retention; the carbon precipitators: desorption
experiments can observe hydrocarbon precursors, such as HD,
CD4 as well as C2D4.
• Explanation: WC layers formed in the surface of W prevents
implanted H atoms from escaping due to increasing diffusion
energy barrier of H. Implanted H atoms will diffuse into the
bulk, which increases the possibility of trapping via intrinsic
defects. The results may explain the enhancement mechanism
of blister formation by carbon impurity implantation.
• Prediction: H tends to bind to W atoms in the presence of C
atoms and C will precipitate on the surface of W. Hence it is
reasonable to predict that the hydrocarbon precursors are much
possibly formed on the surface of W.
• Prediction: It was found that in the matrix W, binding energy
of H is decreased with increasing C concentration, which
corresponds to the desorption temperature of hydrogen being
decreased.
Acknowledgments
• Student：N. Liu，X. D. Ou
• This work was supported by the Natural Science Foundation
of China under Grant Nos. 11175047
• The Special Project for ITER of China under Grant No
2010GB104002.
• The support of the China Scholarship Council.
Thanks for your attention!