Molecular Dynamics modelling of mixed layer formation K. Nordlund, C. Björkas, N. Juslin, K....

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Molecular Dynamics modelling of mixed layer formation K. Nordlund , C. Björkas, N. Juslin, K. Vörtler Accelerator Laboratory, University of Helsinki P. Erhart Lawrence Livermore National Laboratory, Livermore, USA K. Henriksson Kungliga Tekniska Högskolan, Stockholm, Sweden

Transcript of Molecular Dynamics modelling of mixed layer formation K. Nordlund, C. Björkas, N. Juslin, K....

Page 1: Molecular Dynamics modelling of mixed layer formation K. Nordlund, C. Björkas, N. Juslin, K. Vörtler Accelerator Laboratory, University of Helsinki P.

Molecular Dynamics modelling of mixed layer formation

K. Nordlund, C. Björkas, N. Juslin, K. Vörtler

Accelerator Laboratory, University of Helsinki

P. Erhart

Lawrence Livermore National Laboratory, Livermore, USA

K. Henriksson

Kungliga Tekniska Högskolan, Stockholm, Sweden

Page 2: Molecular Dynamics modelling of mixed layer formation K. Nordlund, C. Björkas, N. Juslin, K. Vörtler Accelerator Laboratory, University of Helsinki P.

Carolina Björkas and Kai Nordlund 20072

Outline

Motivation for MD simulation of mixed materials What is needed to model mixed materials by

molecular dynamics? Current status of BeWCH potential development

Simulations of mixed material formation and erosion WC formation from melt

WC structural formation and erosion by D ions

Outlook/workplan WC cobombardment

BeC

BeW

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Motivation

Central phenomena for mixed materials in ITER are erosion,

sputtering and redeposition All of these originate in atomic level processes

To attempt to understand these theoretically it is natural to use

atomic-level methods

In this context 4 main levels of methods can be considered

relevant: Binary collision approximation (BCA):

- Ideal for purely ballistic effects, e.g. linear cascade sputtering

Quantum chemistry (in practice mainly DFT methods):

- Individual chemical reactions for a few tens of atoms

Molecular dynamics (MD):

- Long time scale processes for thousands of atoms

Kinetic Monte Carlo (KMC)

- Diffusion of impurities in surfaces and bulk

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What is needed for MD

We do all of these levels of simulations, but in the

remainder of this talk I will concentrate on MD MD simulations always require the existence of an

interatomic potential for the system to be modelled A potential must describe the relevant interactions at

least qualitatively correctly For fusion reactor issues the potentials must in addition

be fully reactive in the sense that they should be able to

describe all coordination states and chemical bond

breaking => molecular mechanics models are out

Until recently, reactive MD potentials existed only for

elemental and binary systems Clearly not enough for the ITER BeWCH materials mix…

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Examples of past MD plasma-wall work

“Swift chemical sputtering”

mechanism which explains

the low-temperature

chemical erosion of C in

divertors Now much additional work

by LLNL, ORNL, FOM on

this; good agreement on

main points

H and He interactions with

W surfaces: big difference

between H and He

clustering explained

[Salonen, EPL 52 (2000) 50; PRB 63 (2000) 195415 ]

[Henriksson et al, Nucl. Instr. Meth. B 244 (2005) 377]

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Status of availability of divertor-relevant potentials: overview

H He C W Be

H

He

C

W

Be

2002, done by others

2006, CarWMod

[Juslin et al, J. Appl. Phys. 98, 123520 (2005)]

BeTunCMod aim 2008

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What is needed for a new potential

We use a 3-stage fitting procedure, which has been

proven to work well in a wide range of complex systems e.g. Si, C, SiC, WCH, Pt, Zn, ZnO, GaAs, GaN, Fe, ...

Our fitting stages are, for each interaction pair:

1. Obtain data to be fit: obtained from

experiments and DFT calculations,

relevant to problems of interest

2. Fit a Tersoff-Brenner-like potential

into the data

3. Test important non-fitted properties, iterate

back to stage 2 as long as necessary…

Expected effort needed for success/interaction pair: 2-12

person-months depending on complexity

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Status of availability of divertor-relevant potentials: status of today

H He C W Be

H

He

C

W

Be

2002

2006, CarWMod

1. DFT

2. Pot. fit

Almost all DFT data for fitting calculated, some more

for Be-H and Be-He coming from Alain Allouche Be-Be has good fit, Be-C fit still to be improved

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WC formation from melt

One way to form WC mixed layers is by cooling from a

melt, ‘quenching’ Not necessarily same structure as that formed during

irradiation

But good as ‘ground state’

comparison point for

irradiated layers

- E.g. in Si same comparison

shows similar densities between

irradiated and quenched cells

We obtained reasonable WC amorphous layers WxC1-x, x = 0.5 – 0.9

considering e.g. density compared to crystals and melt

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D irradiation of WC

D irradiation of initially crystalline showed a clear

amorphization effect in the cells What is surprising is that the amorphization correlates

with D implantation depth and not deposited energy Not only ballistic collisions, but also chemistry needed

20 eV 50 eV

100 eV 200 eV

Original 10 eV

1000 eV 2000 eV

Structure of WC after 2000 D impacts

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D irradiation of WC

We observed (as expected) preferential sputtering of C →

increasing W surface concentration

Results for change of W surface concentration compared to

experiment:

Experiment at 300 eV D on WC:

63 ± 10 at.% /(1018 ions/cm2)

Simulation at 300 eV D on WC:

crystalline:

57 ± 7 at.% /(1018 ions/cm2)

amorphous:

110 ± 10 at.% /(1018 ions/cm2)

Excellent agreement considering

there are no adjusted parameters!

[Träskelin et al, Phys. Rev. B 75 (2007) 174113]

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Outlook

Potential construction: Complete Be-C-H system potential, test it

Make Be-W system potential, test total Be-W-C-H

potential

Make He-Be potential

WC mixed layer erosion/formation: Response of WC to mixed D + 10% He/Ne/Ar/C/W

bombardment

- Especially C/W interesting: deposition vs. erosion

- Details still to be determined

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Outlook

Formation and erosion of BeC and BeW mixed layers: Construction of quenched a-BeC and a-BeW layers and

simulation of their erosion by D

- Relatively fast

Simulation of formation of mixed layers by Be/C/W

deposition on Be or C or W

- Very slow simulations

- Comparable to results of Doerner/Linsmeier/Krug

presented earlier during this meeting

- Time scale a challenge, but high T might work

Details still to be determined

- Will be fixed based on discussion with EFDA, you,

and technical simulation limitations

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Conclusions

It is now possible to simulate on an atomic level mixed

layer formation and erosion in the ternary WCH system Plus any noble gas (He/Ne/Ar/…) with a pair potential

Results until now show no obvious major shortcomings =>

results likely to be qualitatively reliable

Potentials now under development for the quaternary

BeWCH system We are hopeful this will do at least as well as the WCH one

But this is one of the first fully reactive potentials ever made

for a quaternary system for any application area. Complexity

is high and I am sure there will be shortcomings

somewhere.

- But some potential is still better than no potential…

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Thank you for your attention!Thank you for your attention!