Managed by UT-Battelle for the Department of Energy PPPL: 2 nd ISLAFD, April 28, 2011 Predrag...

Managed by UT-Battellefor the Department of Energy PPPL: 2nd ISLAFD, April 28, 2011

Predrag Krstić Physics Division, CFADCOak Ridge National Laboratory and Collaborators

Support from: US DOE (OFES and OBES) and ORNL LDRD program

Dynamics of deuterium retention and sputtering of Li-C-O surfaces

OFES Review, ORNL, March 02, 2010Managed by UT-Battellefor the Department of Energy PPPL, 2nd ISLAFD, April 28, 20112

Close collaborators:

Experiment:

Henryk Witek,Taiwan U.

Theory:

Chase Taylor Purdue)

jpg jpg

Paul Kent (CNMS, ORNL)

JP Allain(Purdue)

Alain Allouche (CNRS, Fr)

Jae(ORISE)

Jonny(UTK)

Chris (MTSU)

Eric(Purdue)

Students

K. Morokuma,Kyoto U.

J. Jakowski,(NICS)


All energy from D-T fusion reactions passes through first wall

Vac.

Supercon–ducting magnet

Shield Blanket

Turbine generator

Plasma

a

Plasma heating(rf, microwave, . . .)

Schematic magnetic fusion reactor

3

• Flux of (particles + heat + 14 MeV neutrons) ~10 MW/m2

A FUSION REACTOR IMPLIES MANY INTERFACES BETWEEN THE PLASMA AND MATERIALS

Unlike nuclear fission where energy is volume-distributed

Key role of PMI in fusion research well recognized FESAC 2007, RENEw Workshops, OFES priorities,….

Why lithium in the diveror?First wall?…subject of this Symposium…


PMI has many fundamental processes & synergies

elastic reflection

implantation

re-emission & sputtering & chemistry

trapping/detrappingretention

Plasma Material

diffusion, permeation

Give rise to synergistic effects

Damage Effects:Vacancies, bubbles, blisters, dislocations, voids, neutrons?

Drivers:Multi -T, -n, -species, plasma irradiation,neutronssheath acceleration

ErosionAblationMelting (metals)

Re-depositionCo-deposition

When an ion or neutral arrives at a surface it undergoes a series of elastic and inelastic collisions with the atoms of the solid.


What does flux of 1025 particles/m2s mean (ITER)for a typical atomistic (MD) simulation?

At a box of surface of 3 nm lateral dim?a few thousands atoms (carbon)

The flux is 0.01 particle/nm2ns1)1 particle at the interface surface of the cell each 100 ps.

But for deuterium with impact energy lessthen 100 eV: Penetration is less than 2 nm,typical sputtering process takes up to 50 psEach impact independent, uncorrelated!

In effect interaction of an impact particle with nanosize macromoleculeFunctionalizes it!News is that each particle will change the surface for the subsequent impact!

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Guiding principle:

If Edison had a needle to find in a haystack, he would proceed at once with the diligence of the bee to examine straw after straw until he found the object of his search… I was a sorry witness of such doings, knowing that a little theory and calculation would have saved him 90% of his labor.

–Nikola Tesla, New York Times, October 19, 1931

The traditional trial-and-error approach to PMI for future fusion devices by successively refitting the walls of toroidal plasma devices with different materials and component designs is becoming prohibitively slow and costly

6

How to build an effective science of PMI?


Len

gth

Time

ps ns μs ms s hours years

nm

μm

mm

m

ClassicalMolecular dynamics

Kinetic Monte Carlo

Discrete dislocation dynamics

Rate equations,constitutive equations

QMD

Most relevant region for ITER

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How to position theory and modeling in the PMI science?


Much was learned from carbon surfaces:• PMI extremely difficult interfacial problem (Material mixing create

SURFACE entity; its scale depends on impact energy: for sub-100eV => nm-ns scales

• PMI science can be built from bottom-up recognizing its multiscale character and building form shortest time/spatial scales (fs/Angstrom) up

• Theory&modeling of PMI has to be validated by experiment (and v.v.)

• Irradiation create dynamical surface, changing interface, cumulative bombardment is the key for remarkable agreement with experiment

• Surface responds to synergy in plasma irradiation (angles, energies, particles), NOT following linear superposition principle; NEED plasma irradiation modeling and experiments; dedicated plasma devices a must

• Possible inclusion polarization effects in potentials (Li-C-H, Be-C-H,…) and extension to qunatum0classical approaches an essential step forward ???? : ooops – here is the problem!!!!

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How to handle polarizable materials (Li,…)?

Big problem: Long range interactions between atomsReadily has been “avoided” before: “head in sand” practice


Lithium dynamics: Problem to study theoretically because Li polarizing features when interacting with other elements

Electronegativity is chemical property of an element defining its tendency to attract electrons: Li has it exceptionally low in comparison to H , C, O, Mo, W.

Consequence: Bonding between Li and other atoms covalent and polar;Long-range nonbonding:Coulomb :1/R Lennard-Jones :1/R6, 1/R12

Electronegativity and size of atoms related!


Chemistry and sputtering/reflection dynamics in lithiated carbon material, bombarded by slow deuterium atoms is studied.

The objectives of this research are two-fold:

1)To develop the realistic methods for computational simulation of the Li-C-H, validated by experiments.

2)To explain the specifics of the chemistry of deuterium bonding in lithiated carbon. Experiments from Purdue indicate that bonded C-Li-O? sites are preferable for H bonding: Lithium will always bind with oxygen (when present) and carbon, and incoming deuterium will then interact preferably with existing Li-O and Li-C structures.

Goals:

OFES Review, ORNL, March 02, 2010Managed by UT-Battellefor the Department of Energy

Quaternary system Li-C-O-H

•Li-C and Li-H are of very different electronegativities and therefore these could be considered as ionic solids. •The implication is that, as a result of partial charge transferfrom one element to the other, the dominant long-distance binding force between particles is the Coulombic attraction between opposite charges.

Some carbide structures well studied, like C2H2,

tot bound noboundE E E nobound coul vdWE E E

i jcoul

i j i ij

q qE

r

12 6[( ) 2( ) ]ij ij

vdW iji j i ij ij

E Dr r

Total energy in LiXY mixture

Currently our biggest efforts in th!! (6 members of the team on it):

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Charging changes at each simulation step: Quantum-Classical Molecular dynamics a must

Employed the Self-Consistent-Charge Density-Functional Tight-Binding (SCC-DFTB) method (developed by Bremen Center for Computational Materials Science, Germany) It is an approximation to the DFT method in which only valence orbitals are considered, and difficult Integrals parameterized in advance. In comparison to other TB methods: Improved self-consistent interaction of electronic charges

This enables computational efficiency about 1,000 time faster than ab initio quantum methods (and about 1,000 time slower than Classical Molecular Dynamics)

Parameterization for Li-C-H-O provided by K. Morokuma group (Kyoto-ORNL-Emory)Inclusion of Mo and He in parameterization: H. Witek (Taiwan U.)

Electrons: Quantum mechanically at each step, resulting in charges and forcesNuclei: Classical motion

[1] M. Elstner, D. Porezag, G. Jungnickel, et al, Phys. Rev. B 58, 7260 (1998)[2] G. Zheng, M. Lundberg, J. Jakowski, at al, Int. J. Quantum Chem. 109, 1841 (2007).

PPPL, 2nd ISLAFD, April 28, 2011PPPL, 2nd ISLAFD, April 28, 2011


Simulation of deuterium impact to lithiated and oxidated carbon surface

•Cell of a few hunreds of atoms of lithiated and oxidated amorphous carbon (~30% of Li, < 10% of O), at 300K•By random seed of Li and O in amorphous carbon and energy minimization, followed by thermalization•bombarded by 5 eV D atoms and 2.5 eV H, •Perpendicularly to the shell interface•5004 random trajectories

5004 processors of Cray XT5 (Jaguar, Kraken),Time step 0.2-1fs, 24 hours: 200-400 fs.One run over 120,000 CPU hours (TeraGrid project)

PPPL, 2nd ISLAFD, April 28, 2011

The cell swelled during the structure optimization

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15PPPL, 2nd ISLAFD, April 28, 2011

Only C+H

C-Li-O

Slabs studied: Periodicity in x-y


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Depth (A)-5 0 5 10

sp3 d

ensi

ty (

1/A

3 )

0.000

0.005

0.010

0.015

0.0207.5 eV/D

5 eV/D

15 eV/D

20 eV/D

30 eV/D

surf

ace

impact


Penetration of D and H into C-Li-O easier than into C

Only C

Only C

C-Li-O



Final distribution of charges for retained D (H) atoms reveals the dominant chemistry

No isotope effect for chemistry (also found in D on C-H)


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Large percentage of impact D (40%) prefer closeness of Li to settle down Not much attention to O (already bonded to Li)


XPS: Controlled laboratory experiments

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The formation of new peaks or a reproducible peak shift is an indication of a chemical change.

With lithiu

m

Without lithium

C.N. Taylor, B. Heim, J.P. Allain, Journal of Applied Physics 109, 053306 (2011)

The formation of new peaks or a reproducible peak shift is an indication of a chemical change.

C.N. Taylor, B. Heim, J.P. Allain, Journal of Applied Physics 109, 053306 (2011)



Quantum-mechanical, PWDFT “static” calculations of Alain Allouche of CNRS (Marseille) finger -point in the same direction:

•graphene bilayer with Li and H on the surface

When a lithium atom is co-adsorbed on surface bonding energy of H grows up to values ranging from -2.2 to -2.5 eV, with decreasing the Li-H distance. (compared with -1.9 eV for pure graphite)

The bonding E enhancement is also observed when Li is sandwiched 1 layer below the surface layer, but disappears when Li is 2 layers below the surface.

•graphene 4-layers, Li inside the slab, H on surface



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Es (eV)0.0 0.5 1.0 1.5 2.0 2.5 3.0

Cou

nts

0

50

100

150

200

150 e-Es(eV)/0.6)

E=20 eV/amu


Of C-Li-O

Of C-H

Ejected (reflected) D (H) well thermalized to few (5) thousands K like in sputtering of CH’s of C ?????



Sputtering yield significantly enhancedLithium largest

Significant isotopic effect


Conclusions

•A possibility to get a negatively charged hydrogen in vicinity of Li and of C and O only leads to two (3) peaks in D charge : negative and positive. Consistent with the Purdue experiments. Polarity of Li is a key for enhanced D retention.

•Quantal method seems to be a more efficient and possible only correct choice for self-consistent treatment of polarized materials

•Enhanced chemical sputtering, isotope dependent

NEXT (2011): •Li-C-H-O –Mo-He•H-Cumulative interactions

•W-Be-C-H-He

Chemistry independent on the impact mass!!!

Is it dependent on the impact energy? Answer: NO!



Chemistry independent on the impact energy since it happens when the projectile thermalises

Purdue experiment confirms! C. Taylor, JP Allain)

Managed by UT-Battelle for the Department of Energy PPPL: 2 nd ISLAFD, April 28, 2011 Predrag...

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