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Predrag Krstić

Dynamics of deuterium retention and sputtering of Li-C-O surfaces. Predrag Krstić. Physics Division, CFADC Oak Ridge National Laboratory and Collaborators. Support from: US DOE (OFES and OBES) and

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Predrag Krstić

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  1. Dynamics of deuterium retention and sputtering of Li-C-O surfaces Predrag Krstić Physics Division, CFADC Oak Ridge National Laboratory and Collaborators Support from: US DOE (OFES and OBES) and ORNL LDRD program

  2. Close collaborators: Theory: Henryk Witek, Taiwan U. K. Morokuma, Kyoto U. Paul Kent (CNMS, ORNL) Alain Allouche (CNRS, Fr) J. Jakowski, (NICS) jpg jpg Students Experiment: Eric (Purdue) Chris (MTSU) JP Allain (Purdue) Chase Taylor Purdue) Jae (ORISE) Jonny (UTK) 2

  3. All energy from D-T fusion reactions passes through first wall Plasma heating (rf, microwave, . . .) • Flux of (particles + heat + 14 MeV neutrons) ~10 MW/m2 Why lithium in the diveror? First wall? …subject of this Symposium… Schematic magnetic fusion reactor Unlike nuclear fission where energy is volume-distributed A FUSION REACTOR IMPLIES MANY INTERFACES BETWEEN THE PLASMA AND MATERIALS Key role of PMI in fusion research well recognized FESAC 2007, RENEw Workshops, OFES priorities,…. 3

  4. elastic reflection trapping/detrapping retention diffusion, permeation implantation Material Plasma Re-deposition Co-deposition re-emission & sputtering & chemistry Erosion Ablation Melting (metals) PMI has many fundamental processes & synergies When an ion or neutral arrives at a surface it undergoes a series of elastic and inelastic collisions with the atoms of the solid. Damage Effects: Vacancies, bubbles, blisters, dislocations, voids, neutrons? Drivers: Multi -T, -n, -species, plasma irradiation, neutrons sheath acceleration Give rise to synergistic effects 4

  5. 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/nm2ns • 1 particle at the interface surface of • the cell each 100 ps. • But for deuterium with impact energy less • then 100 eV: Penetration is less than 2 nm, • typical sputtering process takes up to • 50 ps • Each impact independent, uncorrelated! In effect interaction of an impact particle with nanosize macromolecule Functionalizes it! News is that each particle will change the surface for the subsequent impact!

  6. 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 How to build an effective science of PMI? 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

  7. How to position theory and modeling in the PMI science? Rate equations, constitutive equations m mm Discrete dislocation dynamics Length μm Most relevant region for ITER Kinetic Monte Carlo ClassicalMolecular dynamics nm QMD ps ns μs ms s hours years Time

  8. 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!!!!

  9. How to handle polarizable materials (Li,…)? Big problem: Long range interactions between atoms Readily has been “avoided” before: “head in sand” practice

  10. 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!

  11. Goals: • Chemistry and sputtering/reflection dynamics in lithiated carbon material, bombarded by slow deuterium atoms is studied. • The objectives of this research are two-fold: • To develop the realistic methods for computational simulation of the Li-C-H, validated by experiments. • 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.

  12. Quaternary system Li-C-O-H Currently our biggest efforts in th!! (6 members of the team on it): • 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 transfer • from 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, Total energy in LiXY mixture

  13. Charging changes at each simulation step: Quantum-Classical Molecular dynamics a must Electrons: Quantum mechanically at each step, resulting in charges and forces Nuclei: Classical motion 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.) [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, 2011

  14. 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 The cell swelled during the structure optimization 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

  15. Slabs studied: Periodicity in x-y C-Li-O Only C+H PPPL, 2nd ISLAFD, April 28, 2011

  16. Penetration of D and H into C-Li-O easier than into C Only C C-Li-O Only C PPPL, 2nd ISLAFD, April 28, 2011

  17. 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) PPPL, 2nd ISLAFD, April 28, 2011

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

  19. XPS: Controlled laboratory experiments With lithium Without lithium The formation of new peaks or a reproducible peak shift is an indication of a chemical change. 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) C.N. Taylor, B. Heim, J.P. Allain, Journal of Applied Physics 109, 053306 (2011) PPPL, 2nd ISLAFD, April 28, 2011

  20. 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) • graphene 4-layers, Li inside the slab, H on surface 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. PPPL, 2nd ISLAFD, April 28, 2011

  21. Ejected (reflected) D (H) well thermalized to few (5) thousands K like in sputtering of CH’s of C ????? Of C-H Of C-Li-O PPPL, 2nd ISLAFD, April 28, 2011

  22. Sputtering yield significantly enhanced Lithium largest Significant isotopic effect PPPL, 2nd ISLAFD, April 28, 2011

  23. 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! PPPL, 2nd ISLAFD, April 28, 2011

  24. Chemistry independent on the impact energy since it happens when the projectile thermalises Purdue experiment confirms! C. Taylor, JP Allain)

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