Development of analytical bond-order potentials for the Be-C-W-H system C. Björkas , N. Juslin, K. Vörtler, H. Timkó, K. Nordlund Department of Physics, University of Helsinki K. Henriksson Department of Chemistry, University of Helsinki P. Erhart Lawrence Livermore National Laboratory, Livermore, USA Joint TFE-SEWG - Material Migration and Material Mixing meeting
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Development of analytical bond- order potentials for the Be-C-W-H system C. Björkas, N. Juslin, K. Vörtler, H. Timkó, K. Nordlund Department of Physics,
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Development of analytical bond-order potentials for the Be-C-W-H system
C. Björkas, N. Juslin, K. Vörtler, H. Timkó, K. Nordlund
Department of Physics, University of Helsinki
K. Henriksson
Department of Chemistry, University of Helsinki
P. Erhart
Lawrence Livermore National Laboratory, Livermore, USA
Joint TFE-SEWG - Material Migration and Material Mixing meeting
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Motivation
We all want to understand what is going on in a fusion
reactor Erosion, redeposition, formation of mixed materials, ...
Ideally we would be able to test every possible
situation that could occur But, experiments are timely and difficult
The interesting phenomena take place at the atom-
level and at very small time scales Hence, they are hardly accessible to experiments
Therefore Molecular Dynamics (MD) can be used
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MD
Approximations: Atoms treated as objects
with no internal structure No electronic structure
calculations done
- May include electronic
stopping and electron-
phonon coupling as
friction
E.g. 41·106 atoms, 72 x 72 x 72 nm, 35 ps, 1024
processors = 36 h
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MD algorithm
Give atoms initial positions r0
Calculate forces F = V(r)and a = F/m
Move atoms: r = r + v t +1/2 a t2 + correction terms
Advance time: t = t + t
Repeat until done
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Potentials
Only with a reliable interatomic potential can we model
things correctly A potential must at least be able to reproduce:
Ground state properties and non-equilibrium processes
such as different phases, melting point, defect structures
and energetics, ... Otherwise we don't know what will happen:
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Potentials
To model plasma-wall interactions, we need potentials
for the Be-C-W-H system W-W, W-H, C-C and H-C made earlier
[Brenner PRB 42 (1990) 9458 and Juslin et al. JAP 98 (2005) 123520]
Now we develop Be-C, Be-Be, Be-H and Be-W
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Potentials
Many different forms of potentials exist Simple pair potentials, EAM, MEAM, BOP
We chose the BOP formalism It is based on Linus Pauling's bond order concept It can describe:
The angular dependency of covalent bonds Breaking of bonds
It has successfully been applied to many systems e.g. Si, C, SiC, W, Pt, Zn, ZnO, GaAs, GaN, Fe, ...
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BOP: Formalism
Bond Order ≈ the strength of the bond between two
atoms depends on the surroundings of the bond
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BOP: Formalism
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BOP: Construction
There are all in all 11 parameters that must be
specified Constructing a potential means finding suitable values
for these Done by fitting to different experimental or DFT values of
both ground state and hypothetical phases Not a trivial task!
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Beryllium
Be has hcp as ground phase bcc and fcc may also exist
Potential vs. experiments
c
Z = 12
BOP exp.
-3.32 eV -3.32 eVa 2.3 Å 2.29 Åc/a 1.57 1.57B 120 Gpa 116.8 Gpa
1560 K
Ecoh
Tmelt 1550±50 K
αV 38.2·10-6 1/K 29.0·10-6 1/K
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Beryllium
Be Pauling plot
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Be-H
The Be-H potential was fitted
to molecules and H defects in
Be Almost the right ground state
interstitial H in Be Most of Be-H
n molecules ok
D and T are also modellable
with this potential
Interstitials BOP DFT I DFT IIBT (eV) 1.22 0.8 1.58O (eV) 1.46 unstable 1.79Ground state (T) (eV) 1.04 0.8 1.58
BT to O 0.43 0.38migration barrier (eV)
Molecules BOP DFT
Be-H
-1.3 -1.3
1.34 1.34
-1.65 -2.13
1.35 1.33
-1.31 -1.35
1.41 1.47
EC
(eV)
rb (Å)
Be-H2 linear
EC
(eV)
rb (Å)
Be-H3 D3h
EC
(eV)
rb (Å)
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Be-C
Be-C poorly known experimentally Only one phase observed, the
ionic antifluorite Be2C
Potential vs. experiments
rb
a BOP exp.
-5.34 eV -5.4 – -5.85 eVa 4.57 Å 4.34 ÅB 227 Gpa > 233 Gpa
3150±50 K 2670 K
Ecoh
Tmelt
αV 4.5·10-6 1/K 5.8·10-6 1/K
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Be-C
We ensured that there are no false minima Cooled a random melt (Be:C = 2:1) to zero K The atoms crystallized into the antifluorite structure The correct Be
2C really is the ground structure of the
potential
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Be-W
Complex phase diagram: Be2W, Be
12W, Be
22W seen
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Be2W
Initial test of the Be2W phase: What becomes of the
ideal hexagonal Laves structure?
DFT: Ecoh
= -7.03 eV/at BOP: Ecoh
= -6.72
eV/at
a = 4.46 Å a = 4.70 Å
c/a = 1.64 c/a = 1.60
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Self sputtering
20 – 100 eV Be ion irradiation flux ~2·1028 m-2s-1 , @ room T
Sput. threshold 20 – 50 eV Yield agrees with extrapolated exp.
Be does not amorphize
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C irradiation of Be
At 1500 K
Layers of Be2C are formed close
to the initial Be surface
Sputtering threshold 20 - 50 eV
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Conclusions
Potentials for pure Be, Be-H, and Be-C ready
and tested Simulations with them already in process
Be-W potential under development
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