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Sputtering of lithiated and oxidated carbon surfaces by low-energy deuterium irradiation
Journal of Nuclear Materials 492 (2017) 56e61
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Sputtering of lithiated and oxidated carbon surfaces by low-energy deuterium irradiation rrez, P.S. Krsti F.J. Domínguez-Gutie c* Institute for Advanced Computational Science, Stony Brook University, Stony Brook, NY 11749, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 January 2017 Received in revised form 20 April 2017 Accepted 12 May 2017 Available online 13 May 2017
We study sputtering of lithiated and oxidized amorphous carbon surfaces by deuterium impact in energy range 5e30 eV. Using classical molecular dynamics, we obtain the sputtering yield, the mass and energy spectra, as well as the angular distribution of ejected atoms and molecules of the surfaces saturated by accumulated deuterium impacts. Our results are compared with existing experimental and theoretical data for amorphous a-C:D surfaces, showing that presence of lithium reduces erosion of carbon, while oxygen further enhances this effect. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Plasma-material interactions (PMI) have an important role in the operation of a fusion reactor because high particle flux from the fusion plasma can damage the material of the plasma facing components (PFC's) and limit its lifetime, the particles sputtered from the PFC's can pollute the plasma, and the plasma particles retained in the material can significantly influence the fusion fuel recycling. In the National Spherical Torus Experiment lithium deposition on graphite has been an important advance for plasma control, reaching low-recycling regimes with enhanced plasma performance [1e7]. Chemical sputtering measurements of plain and lithiated ATJ graphite have been performed at impact energies of 1e2 keV showing that lithium deposition on ATJ graphite causes suppression of methane from the initial set of experiments reported in ref [8]. The authors report 69% of suppression of sputtered methane due to lithium coating on ATJ graphite at 453 K target temperature. It is interesting to note that deposited lithium on carbon does not form a layer, but it rather intercalates inside the carbon material. Sputtering yields on plane ATJ-graphite, pure lithium, and lithiated ATJ-graphite were measured at 500 eV/ion, 45 incidence, and sample temperatures of 25 C and 200 C. These experiments show that ejection of hydrocarbons is notably lower than that observed from a plane graphite [9]. In the NSTX, lithium sputtering
* Corresponding author. E-mail address: [email protected] (P.S. Krsti c). http://dx.doi.org/10.1016/j.jnucmat.2017.05.014 0022-3115/© 2017 Elsevier B.V. All rights reserved.
yields from lithium-coated graphite plasma facing components increases as function of the temperature of the target [10,11]. In this work we carry out a molecular dynamics study to analyze the effect chemical sputtering of lithiated, deuterated and oxidized carbon surfaces at 300 K by deuterium atoms in the impact energy range of 5e30 eV, which has not been studied before. It is known that the interaction of the divertor plasma with carbon-based materials is mainly chemical erosion and that the formation of ejected compounds depends on the impact energy, temperature of the surface and level of deuterated saturation. Our previous results based on quantum-classical molecular dynamics (QCMD) simulations and as well as on X-ray photoelectron spectroscopy (XPS) measurements in NSTX-U [12e15] have demonstrated that increased oxygen content in the top carbon surface layers is the main player for deuterium uptake in lithiated carbon surfaces. Thus, while the oxygen concentration in lithiated carbon surfaces is below 10%, it can reach over 30% upon sufficient irradiation by deuterium [13e15]. Lithium in this case plays a role of “catalyst”, keeping the oxygen from degassing from the surface [13]. Quantum-classical molecular dynamics is required in modeling of lithiated, and possible oxidized carbon surfaces because of a big difference in electronegativities of Li and C, O and D atoms. The difference causes polarization of Li (positive) and oxygen and deuterium (mainly negative) [12,13]. This causes long range Coulomb type interactions between the surface constituents, which depend on instantaneous coordinates of the atoms. QCMD can treat the change of the electron cloud during the dynamics [12,13], but this requires an excessive computational time and resources. The
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quantum component of the QCMD solves the multielectron eigenvalue problem at each time step of the system evolution, which could be computationally formidable problem even if density-functional theory (DFT) is used. The use of approximations to DFT, like is Self-Consistent-Charge Tight Binding DFT (SCC-DFTB) [16] can make the QCMD problem computationally feasible for study of deuterium retention or chemical sputtering at low energies (5 eV) [12,13]. However, study of chemical sputtering in a range of energies 5e30 eV is computationally too intensive, not only because the calculation has to be repeated for various energies, but also because higher energies require larger computational cells, i.e. larger number of atoms. This is the reason that we have chosen in this work the combination of classical molecular dynamics (CMD) and electronegativity equalization method (EEM). The latter is a semi-empirical method that applies a set of pre-calibrated, material and coordinate dependent parameters [17,18] to estimate change of charges of each atom as their coordinates change during the collision cascade of an impact atom in the surface. EEM is implemented [17] in the Large Scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) [19] with Reactive Force Field method and potentials (ReaxFF) of van Duim et al. [20e22]. The EEM slows down computation by an order of magnitude, but this is still orders of magnitudes faster than QCMD, even when approximation to DFT is used for the quantum-mechanical component [12,13]. On the other hand, ReaxFF is a bond-order potential, with proven capability to treat correctly chemical processes in the material of interest [20]. This approach was applied successfully in our previous work [23] to study deuterium retention in boronized and lithiated carbon surfaces. The paper is organized as follows: In section 2, we provide a brief description of the computational approach used to prepare the target surface. In section 3 we report our results for the sputtering yield, energy and angular spectra of various ejected particles, as function of the impact energy. Section 4 contains our concluding remarks. 2. Surface preparation The amorphous target surfaces are prepared for each studied impact energy. Thus, for a virgin LiCO we use a computational method depicted in Ref. [13]. We choose computational cell of about 400 atoms with a predefined random initial atomic distribution of 20% O, 20% of Li, and 60% of C. This cell is then energy optimized in a succession of heating and annealing processes, and finally thermalized to 300 K, using Langevin thermostat with time constant of 100 fs. We consider periodic boundary conditions in x and y directions, with the D impacts in z-direction. The lateral dimensions of the cell is 2.0 nm in z direction and about 1.75 nm in x and y directions. The depth of the cell is chosen to be large enough to prevent D atoms in the considered energy range to reach the cell bottom and have artificial reflections. To prepare a deuterated surface (LiCO:D), saturated with D for each considered collision energy we apply a cumulative bombardment with D impact each 50 ps. We note that this time separation of successive impacts is sufficient for the thermostat-independent cascade evolution (20 ps), followed by thermalization at 300 K (20 ps) to remove the excessive energy deposited by D impact, then a relaxation of the system is done for 10 ps. In Fig. 1 a), we show the percentage of D accumulated in the LiCO:D surface at 5 and 30 eV which is calculated as
Dacc ¼ ND =ðND þ NC þ NLi þ NO Þ; where ND, NC, NLi, and NO are instantaneous numbers of deuterium, carbon, lithium, and oxygen atoms, respectively. The cumulative
57
Fig. 1. a) Percentage of implanted D as function of time to prepare the LiCO:D surface at 5, 20 and 30 eV. b) Distribution of implantation volume density, calculated in bins of Dz ¼ 1 Å height along the depth in the LiCO:D target cell for various impact energies.
bombardment ends when the variation Dacc is lower than 0.5% [24, 26]. In Fig. 1 b), we show the implantation volume density distributions along the depth of the LiCO:D target, for various impact energies. In Table 1 we present the obtained average percentages of Dacc, for C, Li, and O upon reaching saturation at different energies. These percentages are calculated with respect to the whole cell, but comparison with Fig. 1b) shows that the implanted D distributions are accumulated around the implantation density maxima. The integrals of the distributions in Fig. 1b) for one impact energy are equal to the total number of implanted D atoms at that energy in Table 1. We have also prepared a deuterated and saturated LiC:D surfaces for different impact energies, starting from a LiC mixture with an atomic concentration of 20% of Li and 80% of C. For this surface, the obtained percentages of D saturation, carbon, and lithium are shown in Table 2. Comparison of sputtering of the LiCO:D and LiC:D surfaces enabled us to analyze the effect of oxygen for sputtering of the lithiated surfaces.
3. Results With prepared LiC:D and LiCO:D surfaces, for each energy of the deuterium impacts, instantaneous sputtering data are calculated by 15,000 independent impacts at the same surface with randomly chosen impact locations. This was done by the Li-RED institutional supercomputer in a successive series of simulations (30 s per simulation). Each D impact was emitted about 0.7 nm from the surface and directed perpendicularly to the surface interface. All ejected particles are counted once they passed a parallel plane located at 2.5 nm above the surface. For each calculated data x, we
Table 1 Percentage and total number of atoms for Dacc, C, Li, and O upon reaching saturation for the LiCO:D surface at different impact energies. Energy
5 eV
Element
%
# atoms
%
10 eV # atoms
%
20 eV # atoms
%
30 eV # atoms
Dacc C Li O
10.2 54.3 17.5 18.0
43 229 74 76
11.5 53.6 17.2 17.6
49 228 73 75
13.0 52.8 17.0 17.2
56 227 73 74
14.2 52.1 16.5 17.2
61 224 71 74
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Table 2 Percentage and the total number of atoms for D, Li, and C in the LiC:D target cell. Percentage are calculated respect to the whole cell at each impact energy. Energy
5 eV
Element
%
# Atoms
%
10 eV # Atoms
%
20 eV # Atoms
%
30 eV # Atoms
Dacc C Li
9.5 72.5 18.0
40 306 76
10.6 71.8 17.6
45 306 75
11.9 70.9 17.2
51 305 74
13.5 69.6 16.9
59 304 74
report standard error defined as
s ss ¼ pffiffiffiffi N
where s is the standard deviation of our sample for N ¼ 15,0000 cases. Using our relatively large statistical sample the standard error in all results reported in this work is not bigger than 25%. In this section we report the data on ejection yield per impact D, mass spectra of ejected particles, average translational kinetic energy spectra of sputtered atoms and (center-of-mass) molecules, and the angular distributions of the sputtered particles. 3.1. Ejection yield per impact D The sputtering yields for various particles is defined as NS/NP, where NS is a number of ejected particles and NP is the total number of incident deuterium atoms (15000) at a considered impact energy. In Fig. 2, we present the calculated total sputtering yields per D for carbon, oxygen, and lithium, summing up all ejected atoms and molecules of a particular kind. We consider both LiC:D (Fig. 2a) and LiCO:D (Fig. 2b) surfaces in order to estimate role of oxygen in the sputtering. The results are also compared with the published data for amorphous hydro-carbons (a-C:D) [25]. Total sputtering yield for LiC:D surface is quite high, reaching 10%, contributed
Fig. 2. a) Total sputtering yields (including molecules) for C and Li of LiC:D, and b) for C, Li, and O from the LiCO:D surface. Presence of oxygen in the LiC mixture reduces the sputtering of carbon and lithium. We compare our results with total sputtering yield of a-C:D surface [25]. In b) we also compare our data with the normalized data obtained by SCC-DFTB at 5 eV impact energy [13]. Gray band in a) represents the experimental result for lithium sputtering yield [5].
almost equally by Li (about 4.5%) and carbon (about 5.5%) for energies above 20 eV. Lithium dominates sputtering at impact energy 10 eV due to the low bond energy of Li to C (1.6 eV) in comparison to the C-C bond (3.6 eV) [27]. We compare our data for the lithium sputtering with the experimental data for Li sputtering from graphite substrate of heated LLD in NSTX at about 370 K (Fig. 23 at [5]). Because of uncertain impact energies of deuterium in the divertor we present the measured range of the Li sputtering yield by a shaded gray band in Fig. 2a). The agreement of the band with our data is good in the whole range of considered impact energies. When oxygen is present in the surface at concentration close to 20% (see Table 1), as is a case in LiCO:D surface, the total sputtering yield is reduced by about factor 3 (Fig. 2b). The ejection of carbons is dominant in the sputtering process, reaching 2% at 30 eV, still about factor 2 smaller than in the LiC:D case. Lithium sputtering yield reaches about 1.2% at 30 eV, reflecting its stronger bonding to O (3.5 eV) [27] than to C [27]. Ejection of oxygen has the lowest sputtering yield (stays below 0.7%). It is important to stress that presence of Li suppresses sputtering of carbon from LiC:D by about factor 2 and by about factor 5 from LiCO:D in comparison to a-C:D surface. The suppression of chemical erosion for the LiCO:D with respect to the a-C:D is in good agreement with plasma experiment of Yagi et al. [11], where hydrogen irradiation on the lithiated ATJ graphite (initial surface atom composition of ~10% of oxygen, ~20% of lithium, and ~70% of carbon) reduces the sputtering yield by a factor of 9 when the target temperature is below 450 K [11]. The results for total sputtering and sputtering of carbon form LICO:D surface, obtained by SCC-DFTB in Ref. [13], do not compare well with our data. The reason could be in quite different number of atoms in the surfaces of the two calculations. Still, approximate, mainly qualitative MD methods, based on REAXFF and SCC-DFTB, should not be expected to agree in absolute values of the results. These should rather give correct trends and ratios of the processes. Indeed, when we normalized carbon sputtering (Fig. 2b) from SCCDFTB to our result in Fig. 2b), the total sputtering data in the two calculations almost coincided. In Fig. 3, we show the sputtering yields of carbon and lithium ejected in atomic form as functions of the impact energy. We compare our results with the sputtering yield of ejected carbon atoms from the a-C:D surface [25] noticing a suppression of sputtering of atomic carbon by a factor of 3 when lithium is present in the carbon surface (Fig. 3a) and by factor 9 for LiCO:D surface (Fig. 3b). Lithium atoms sputtering is also reduced with presence of oxygen by about factor 2 in the whole studied energy range. Ejection of oxygen atoms has the lowest contribution, being below 0.3%. In Fig. 4, we show the chemical sputtering yields for different ejected hydrocarbons of the LiC:D and LiCO:D surfaces, as function
Fig. 3. a) Sputtering yield of ejected C and Li atoms of the LiC:D surface. b) Oxygen, carbon, and lithium atoms sputtering yield as function of the impact energy of the LiCO:D. We compare our with those for carbon atoms from the a-C:D surface [25].
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the calculated sputtering yields for various lithium compounds ejected from the LiC:D and LiCO:D surfaces as functions of deuterium impact energy. We include the total sum of ejected atomic lithium and lithium compounds for each of the two surfaces. Sputtering yield of LiD molecules decreases with energy for the LiC:D surface but seems to reach a maximum at 20 eV for the LiCO:D surface, being reduced by about factor 3 with presence of oxygen. When oxygen is present in the surface the dominant ejected product containing Li is LiO molecule. Ejection of LiC molecules is not observed at energies below 30 eV due to the strong bond of the lithium and oxygen (~3.5 eV) inside the surface in comparison to more than a factor 2 weaker LiC bond (1.6 eV) [27]. Fig. 5 b) shows the calculated sputtering yields for various oxygen compounds ejected from the LiCO:D surface. We note that ejected molecules such as oxygen with deuterium (OD) and oxygen with carbon (CO) are the main products at the lowest impact energies, however when the energy increases a variety of oxygen compounds is present such as CO, OLi, and OD. Fig. 4. The total, CD, CD2, and CD3þCD4 sputtering yields per D as function of the impact energy for a) LiC:D surface and for b) LiCO:D surface. We compare our calculated results with the theoretical [25] and experimental data [27, 28] for a-C:D. Ejection of CD4 molecules is not observed for the LiCO:D (Fig. 4b) in good agreement with the results of Yagi et al. [11].
of the D impact energy. We have compared our results with the published theoretical and experimental results for a-C:D [25, 27]. In Fig. 4 a) Presence of lithium reduces the ejection of CD3þCD4 molecules, but the ejection of CD and CD2 molecules is increased by about a factor of 2. In Fig. 4 b), we show sputtering yields for the CxDy molecules of the LiCO:D surface. Comparison with the theoretical [25] and experimental [27, 28] results for the a-C:D surface is included in the same figure. The presence of oxygen in lithiated carbon surface somewhat reduces C2D2 and the total ejection of hydrocarbons with respect to the ATJ graphite [25, 27]. The suppression of ejected CD3þCD4 in our simulations is in good agreement with the analysis reported by P. Raman et al. [5] for a lithiated ATJ graphite and impacts of 1 keV D ions, and reported by Yagi et al. [11] for sputtering of ATJ graphite at 50e200 eV. Fig. 4 shows only ejected CD molecules. In Fig. 5 a) we present
Fig. 5. a) Sputtering yield per D for ejected lithium atoms and LiX molecules, from the LiC:D and LiCO:D surfaces as function of the impact energy. X ¼ D, C, or O; LiþLiX means total yield of lithium atoms and all lithium compounds ejected. b) OþOX is the total yield of oxygen atoms and all ejected combinations for oxygen molecules.
3.2. Mass spectra of ejected particles The decomposition by mass of sputtered particles can reveal additional features of the sputtering of lithiated and oxygenated carbon surfaces as may be reported in measurements of sputtering yield by mass spectrometers. In Fig. 6 a), we show the mass spectra of sputtered particles from the LiC:D surface. We notice that lithium atoms at 7 amu are dominant in the whole impact energy range. At the lowest impact energy (5 eV) the generation of Li-D molecules at ~ 9 amu is dominating over the CDx molecules by a factor of 3. However, for projectiles with impact energy above 10 eV the production of Li-D is lower than ejected hydrocarbons by a factor of 2. We note that CD3 at ~18 amu and CD4 at ~20 amu have the same sputtering yields per D, approaching1%. In Fig. 6 b), we show the mass spectra at various impact energies for the LiCO:D surface. At the lowest energy (5 eV) the ejection of lithium at 7 amu is dominant like in LiCO:D. Atomic lithium (~7 amu), carbon (~12 amu), and oxygen (~16 amu) atoms ejected from the surface are the main sputtering products, reaching about the same yield per D at 30 eV. The presence of more complex species such as C2D2 at 28 amu, C-O at 28 amu, and Li-O at 23 amu, is observed in the whole energy range. The sputtering yields for C2D2 and LiO molecules increase as function of the impact energy. We notice the evident suppression of ejected CD4 and CD3 molecules from this surface in comparison with the yields from the LiC:D surface.
Fig. 6. Mass spectra for various ejected atoms and molecules of a) LiC:D, and b) LiCO:D for various impact energies.
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3.3. Translational energy spectra of sputtered particles In order to closer characterize sputtering of ejected atoms and molecules, we have calculated their kinetic energy as
KE ¼ M=2 v2 where M is the total mass of a particle, v is velocity of the center of mass of the particle when its trajectory reaches 2.5 nm above the surface, with z-component of velocity oriented from the surface. Kinetic energies of all ejected atoms or compounds are averaged as
Eave ¼
X
, KEN
N;
N
where N is the total number of ejected atoms or compounds of a particular type, In Fig. 7 a), we show the average translation energy of the particles ejected from the LiC:D surface, as function of the mass (in amu) for various impact energies. Standard errors in the graph are smaller than the symbol size. The solid gray line represents the average of the translation kinetic energies over impact energies which is calculated as
KEðaveÞ ¼ ðXð5evÞ þ Xð10eVÞ þ Xð20eVÞ þ Xð30eVÞÞ=4; where X(E) is average kinetic energy of an ejected particle at an impact energy E. Sputtered atoms of lithium and carbon have the lowest kinetic energy, below 1 eV, while more complex CDx and LiD molecules have kinetic energy between 1 eV and 2.5 eV. Similarly, in Fig. 7 b), we present the average kinetic energy of sputtered particles of the LiCO:D surface. Carbon and lithium atoms again have the lowest kinetic energy below 1 eV, while particles with highest kinetic energy (~3 eV) are oxygen compounds and oxygen atoms.
dynamics of these particles only depends on the polar angle q of a particle motion direction to the surface normal. The angular distribution of ejected atoms/molecules is calculated as [23, 24]
dN Nðq; DqÞ ¼C ; dV 2p sin qDq
(1)
where N (q, Dq) is the number of a particle center-of-mass ejected in the interval (q - Dq/2, q þ Dq/2) and C is a normalization constant such that the maximum value of the angular distribution is at the unit circle [24]. The ejection is defined when the center of mass crosses plane 2.5 nm above the surface. We present the angular distributions by polar graphs in Fig. 8 for carbon atoms, as well as for CD and CD2 molecules, while Fig. 9 shows the distributions of Li atoms and Li-D molecule. In both figures the sputtered particles were ejected form LiCO:D surface. The span of the angular distributions of the c.m. momenta for various impact energies in Figs. 8 and 9 are shown by shaded gray areas. The averages over impact energy of the data in bins of 5 and in bin steps of 5 are shown by hollow symbols. Finally, we fitted these average data to the least squares curves in form (cosq)a, where a is the fitting parameter. The quality of the fits is excellent, with cross-correlation parameter larger than 0.99 in all five cases. The obtained values of a are 2.82, 3.06 and 2.79 for C, CD and CD2 distributions, respectively. The values of a for the Li and Li-D in Fig. 9 are 5.99 and 5.36, respectively. As expected, all shown distributions are much sharper than cosq. Similar overcos distribution were obtained in Ref. [26] for chemical sputtering of hydrocarbons and deuterium atoms and molecules. 4. Concluding remarks
The angular distribution of ejected particles is independent of the azimuthal angle in the plane of the interfacial surface and the
Classical molecular dynamics simulations with reactive force field potentials were performed to study the sputtering process of lithiated, oxidized and deuterated carbon surfaces by deuterium irradiation in 5e30 eV impact energy range. The capability of ReaxFF to treat atomic charges dynamically is a key feature for lithium material mixtures, due to polarization features of material induced by low electronegativity of lithium. We have calculated the sputtering yields for various particles ejected from LiC:D and LiCO:D surfaces. In absence of oxygen, we find that the total carbon
Fig. 7. Average kinetic energy as function of the mass of ejected atoms and molecules from the a) LiC:D surface, and b) LiCO:D. The solid gray lines represent the average of the translation kinetic energies of ejected particles over the impact energies of D.
Fig. 8. Solid angle distribution of ejected carbon atoms and CDx molecules from the LiCO:D surface. Shaded areas are spans of sputtered distributions at various impact energies of D. Hollow symbols are the values of the sputtering yields averaged over impact energy, while the solid curved lines are curves of the form (cosq)a, fitted through the symbols. Details of the fits are explained in the text.
3.4. Angular distributions
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Acknowledgments Research supported by the National Council on Science and Technology, Mexico (CONACyT) through the postdoctoral fellowship # 267898 (FJDG) and by DOE-OFES grant DE-SC0013752 through RF of SUNY (PSK). Simulation were obtained using Li-red cluster of the IACS-SBU.
References Fig. 9. Angular distributions of sputtered lithium atoms and LiD molecules from the LiCO:D surface. Details are the same as in Fig. 8.
erosion (of which 40% is atomic carbon) is suppressed in comparison to a-C:D by about factor 2. However, Li (of which 60% is in atomic form) contributes almost equally to carbon to the total sputtering at 30 eV, while its contribution at lower energies dominates due to the weak bond of Li to C. In presence of oxygen, the carbon erosion (40% in atomic form) is reduced by factor 4 in comparison to a-C:D, while Li sputtering (of which 75% is now in atomic form) is reduced by factor 4 in comparison to LiC:D. At the other hand, sputtering of hydrocarbon molecules (dominance of CD3 and CD4) of LiC:D is increased by factor 2 in comparison to aC:D, but reduced by factor 2 in presence of oxygen. Oxygen sputtering stays low, below 0.7%, over the whole studied energy range. The analysis of kinetic energy spectra of ejected particles shows that sputtered molecules are more energetic than atoms, which is expected due to the larger masses of molecules. Angular distributions of ejected lithium and carbon atoms, as well as of CDx and LiD molecules follow sharp (cosq)a function of polar angle, where a is between 3.6 and 6.
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Sputtering of lithiated and oxidated carbon surfaces by low-energy deuterium irradiation rrez, P.S. Krsti F.J. Domínguez-Gutie c* Institute for Advanced Computational Science, Stony Brook University, Stony Brook, NY 11749, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 January 2017 Received in revised form 20 April 2017 Accepted 12 May 2017 Available online 13 May 2017
We study sputtering of lithiated and oxidized amorphous carbon surfaces by deuterium impact in energy range 5e30 eV. Using classical molecular dynamics, we obtain the sputtering yield, the mass and energy spectra, as well as the angular distribution of ejected atoms and molecules of the surfaces saturated by accumulated deuterium impacts. Our results are compared with existing experimental and theoretical data for amorphous a-C:D surfaces, showing that presence of lithium reduces erosion of carbon, while oxygen further enhances this effect. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Plasma-material interactions (PMI) have an important role in the operation of a fusion reactor because high particle flux from the fusion plasma can damage the material of the plasma facing components (PFC's) and limit its lifetime, the particles sputtered from the PFC's can pollute the plasma, and the plasma particles retained in the material can significantly influence the fusion fuel recycling. In the National Spherical Torus Experiment lithium deposition on graphite has been an important advance for plasma control, reaching low-recycling regimes with enhanced plasma performance [1e7]. Chemical sputtering measurements of plain and lithiated ATJ graphite have been performed at impact energies of 1e2 keV showing that lithium deposition on ATJ graphite causes suppression of methane from the initial set of experiments reported in ref [8]. The authors report 69% of suppression of sputtered methane due to lithium coating on ATJ graphite at 453 K target temperature. It is interesting to note that deposited lithium on carbon does not form a layer, but it rather intercalates inside the carbon material. Sputtering yields on plane ATJ-graphite, pure lithium, and lithiated ATJ-graphite were measured at 500 eV/ion, 45 incidence, and sample temperatures of 25 C and 200 C. These experiments show that ejection of hydrocarbons is notably lower than that observed from a plane graphite [9]. In the NSTX, lithium sputtering
* Corresponding author. E-mail address: [email protected] (P.S. Krsti c). http://dx.doi.org/10.1016/j.jnucmat.2017.05.014 0022-3115/© 2017 Elsevier B.V. All rights reserved.
yields from lithium-coated graphite plasma facing components increases as function of the temperature of the target [10,11]. In this work we carry out a molecular dynamics study to analyze the effect chemical sputtering of lithiated, deuterated and oxidized carbon surfaces at 300 K by deuterium atoms in the impact energy range of 5e30 eV, which has not been studied before. It is known that the interaction of the divertor plasma with carbon-based materials is mainly chemical erosion and that the formation of ejected compounds depends on the impact energy, temperature of the surface and level of deuterated saturation. Our previous results based on quantum-classical molecular dynamics (QCMD) simulations and as well as on X-ray photoelectron spectroscopy (XPS) measurements in NSTX-U [12e15] have demonstrated that increased oxygen content in the top carbon surface layers is the main player for deuterium uptake in lithiated carbon surfaces. Thus, while the oxygen concentration in lithiated carbon surfaces is below 10%, it can reach over 30% upon sufficient irradiation by deuterium [13e15]. Lithium in this case plays a role of “catalyst”, keeping the oxygen from degassing from the surface [13]. Quantum-classical molecular dynamics is required in modeling of lithiated, and possible oxidized carbon surfaces because of a big difference in electronegativities of Li and C, O and D atoms. The difference causes polarization of Li (positive) and oxygen and deuterium (mainly negative) [12,13]. This causes long range Coulomb type interactions between the surface constituents, which depend on instantaneous coordinates of the atoms. QCMD can treat the change of the electron cloud during the dynamics [12,13], but this requires an excessive computational time and resources. The
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quantum component of the QCMD solves the multielectron eigenvalue problem at each time step of the system evolution, which could be computationally formidable problem even if density-functional theory (DFT) is used. The use of approximations to DFT, like is Self-Consistent-Charge Tight Binding DFT (SCC-DFTB) [16] can make the QCMD problem computationally feasible for study of deuterium retention or chemical sputtering at low energies (5 eV) [12,13]. However, study of chemical sputtering in a range of energies 5e30 eV is computationally too intensive, not only because the calculation has to be repeated for various energies, but also because higher energies require larger computational cells, i.e. larger number of atoms. This is the reason that we have chosen in this work the combination of classical molecular dynamics (CMD) and electronegativity equalization method (EEM). The latter is a semi-empirical method that applies a set of pre-calibrated, material and coordinate dependent parameters [17,18] to estimate change of charges of each atom as their coordinates change during the collision cascade of an impact atom in the surface. EEM is implemented [17] in the Large Scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) [19] with Reactive Force Field method and potentials (ReaxFF) of van Duim et al. [20e22]. The EEM slows down computation by an order of magnitude, but this is still orders of magnitudes faster than QCMD, even when approximation to DFT is used for the quantum-mechanical component [12,13]. On the other hand, ReaxFF is a bond-order potential, with proven capability to treat correctly chemical processes in the material of interest [20]. This approach was applied successfully in our previous work [23] to study deuterium retention in boronized and lithiated carbon surfaces. The paper is organized as follows: In section 2, we provide a brief description of the computational approach used to prepare the target surface. In section 3 we report our results for the sputtering yield, energy and angular spectra of various ejected particles, as function of the impact energy. Section 4 contains our concluding remarks. 2. Surface preparation The amorphous target surfaces are prepared for each studied impact energy. Thus, for a virgin LiCO we use a computational method depicted in Ref. [13]. We choose computational cell of about 400 atoms with a predefined random initial atomic distribution of 20% O, 20% of Li, and 60% of C. This cell is then energy optimized in a succession of heating and annealing processes, and finally thermalized to 300 K, using Langevin thermostat with time constant of 100 fs. We consider periodic boundary conditions in x and y directions, with the D impacts in z-direction. The lateral dimensions of the cell is 2.0 nm in z direction and about 1.75 nm in x and y directions. The depth of the cell is chosen to be large enough to prevent D atoms in the considered energy range to reach the cell bottom and have artificial reflections. To prepare a deuterated surface (LiCO:D), saturated with D for each considered collision energy we apply a cumulative bombardment with D impact each 50 ps. We note that this time separation of successive impacts is sufficient for the thermostat-independent cascade evolution (20 ps), followed by thermalization at 300 K (20 ps) to remove the excessive energy deposited by D impact, then a relaxation of the system is done for 10 ps. In Fig. 1 a), we show the percentage of D accumulated in the LiCO:D surface at 5 and 30 eV which is calculated as
Dacc ¼ ND =ðND þ NC þ NLi þ NO Þ; where ND, NC, NLi, and NO are instantaneous numbers of deuterium, carbon, lithium, and oxygen atoms, respectively. The cumulative
57
Fig. 1. a) Percentage of implanted D as function of time to prepare the LiCO:D surface at 5, 20 and 30 eV. b) Distribution of implantation volume density, calculated in bins of Dz ¼ 1 Å height along the depth in the LiCO:D target cell for various impact energies.
bombardment ends when the variation Dacc is lower than 0.5% [24, 26]. In Fig. 1 b), we show the implantation volume density distributions along the depth of the LiCO:D target, for various impact energies. In Table 1 we present the obtained average percentages of Dacc, for C, Li, and O upon reaching saturation at different energies. These percentages are calculated with respect to the whole cell, but comparison with Fig. 1b) shows that the implanted D distributions are accumulated around the implantation density maxima. The integrals of the distributions in Fig. 1b) for one impact energy are equal to the total number of implanted D atoms at that energy in Table 1. We have also prepared a deuterated and saturated LiC:D surfaces for different impact energies, starting from a LiC mixture with an atomic concentration of 20% of Li and 80% of C. For this surface, the obtained percentages of D saturation, carbon, and lithium are shown in Table 2. Comparison of sputtering of the LiCO:D and LiC:D surfaces enabled us to analyze the effect of oxygen for sputtering of the lithiated surfaces.
3. Results With prepared LiC:D and LiCO:D surfaces, for each energy of the deuterium impacts, instantaneous sputtering data are calculated by 15,000 independent impacts at the same surface with randomly chosen impact locations. This was done by the Li-RED institutional supercomputer in a successive series of simulations (30 s per simulation). Each D impact was emitted about 0.7 nm from the surface and directed perpendicularly to the surface interface. All ejected particles are counted once they passed a parallel plane located at 2.5 nm above the surface. For each calculated data x, we
Table 1 Percentage and total number of atoms for Dacc, C, Li, and O upon reaching saturation for the LiCO:D surface at different impact energies. Energy
5 eV
Element
%
# atoms
%
10 eV # atoms
%
20 eV # atoms
%
30 eV # atoms
Dacc C Li O
10.2 54.3 17.5 18.0
43 229 74 76
11.5 53.6 17.2 17.6
49 228 73 75
13.0 52.8 17.0 17.2
56 227 73 74
14.2 52.1 16.5 17.2
61 224 71 74
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Table 2 Percentage and the total number of atoms for D, Li, and C in the LiC:D target cell. Percentage are calculated respect to the whole cell at each impact energy. Energy
5 eV
Element
%
# Atoms
%
10 eV # Atoms
%
20 eV # Atoms
%
30 eV # Atoms
Dacc C Li
9.5 72.5 18.0
40 306 76
10.6 71.8 17.6
45 306 75
11.9 70.9 17.2
51 305 74
13.5 69.6 16.9
59 304 74
report standard error defined as
s ss ¼ pffiffiffiffi N
where s is the standard deviation of our sample for N ¼ 15,0000 cases. Using our relatively large statistical sample the standard error in all results reported in this work is not bigger than 25%. In this section we report the data on ejection yield per impact D, mass spectra of ejected particles, average translational kinetic energy spectra of sputtered atoms and (center-of-mass) molecules, and the angular distributions of the sputtered particles. 3.1. Ejection yield per impact D The sputtering yields for various particles is defined as NS/NP, where NS is a number of ejected particles and NP is the total number of incident deuterium atoms (15000) at a considered impact energy. In Fig. 2, we present the calculated total sputtering yields per D for carbon, oxygen, and lithium, summing up all ejected atoms and molecules of a particular kind. We consider both LiC:D (Fig. 2a) and LiCO:D (Fig. 2b) surfaces in order to estimate role of oxygen in the sputtering. The results are also compared with the published data for amorphous hydro-carbons (a-C:D) [25]. Total sputtering yield for LiC:D surface is quite high, reaching 10%, contributed
Fig. 2. a) Total sputtering yields (including molecules) for C and Li of LiC:D, and b) for C, Li, and O from the LiCO:D surface. Presence of oxygen in the LiC mixture reduces the sputtering of carbon and lithium. We compare our results with total sputtering yield of a-C:D surface [25]. In b) we also compare our data with the normalized data obtained by SCC-DFTB at 5 eV impact energy [13]. Gray band in a) represents the experimental result for lithium sputtering yield [5].
almost equally by Li (about 4.5%) and carbon (about 5.5%) for energies above 20 eV. Lithium dominates sputtering at impact energy 10 eV due to the low bond energy of Li to C (1.6 eV) in comparison to the C-C bond (3.6 eV) [27]. We compare our data for the lithium sputtering with the experimental data for Li sputtering from graphite substrate of heated LLD in NSTX at about 370 K (Fig. 23 at [5]). Because of uncertain impact energies of deuterium in the divertor we present the measured range of the Li sputtering yield by a shaded gray band in Fig. 2a). The agreement of the band with our data is good in the whole range of considered impact energies. When oxygen is present in the surface at concentration close to 20% (see Table 1), as is a case in LiCO:D surface, the total sputtering yield is reduced by about factor 3 (Fig. 2b). The ejection of carbons is dominant in the sputtering process, reaching 2% at 30 eV, still about factor 2 smaller than in the LiC:D case. Lithium sputtering yield reaches about 1.2% at 30 eV, reflecting its stronger bonding to O (3.5 eV) [27] than to C [27]. Ejection of oxygen has the lowest sputtering yield (stays below 0.7%). It is important to stress that presence of Li suppresses sputtering of carbon from LiC:D by about factor 2 and by about factor 5 from LiCO:D in comparison to a-C:D surface. The suppression of chemical erosion for the LiCO:D with respect to the a-C:D is in good agreement with plasma experiment of Yagi et al. [11], where hydrogen irradiation on the lithiated ATJ graphite (initial surface atom composition of ~10% of oxygen, ~20% of lithium, and ~70% of carbon) reduces the sputtering yield by a factor of 9 when the target temperature is below 450 K [11]. The results for total sputtering and sputtering of carbon form LICO:D surface, obtained by SCC-DFTB in Ref. [13], do not compare well with our data. The reason could be in quite different number of atoms in the surfaces of the two calculations. Still, approximate, mainly qualitative MD methods, based on REAXFF and SCC-DFTB, should not be expected to agree in absolute values of the results. These should rather give correct trends and ratios of the processes. Indeed, when we normalized carbon sputtering (Fig. 2b) from SCCDFTB to our result in Fig. 2b), the total sputtering data in the two calculations almost coincided. In Fig. 3, we show the sputtering yields of carbon and lithium ejected in atomic form as functions of the impact energy. We compare our results with the sputtering yield of ejected carbon atoms from the a-C:D surface [25] noticing a suppression of sputtering of atomic carbon by a factor of 3 when lithium is present in the carbon surface (Fig. 3a) and by factor 9 for LiCO:D surface (Fig. 3b). Lithium atoms sputtering is also reduced with presence of oxygen by about factor 2 in the whole studied energy range. Ejection of oxygen atoms has the lowest contribution, being below 0.3%. In Fig. 4, we show the chemical sputtering yields for different ejected hydrocarbons of the LiC:D and LiCO:D surfaces, as function
Fig. 3. a) Sputtering yield of ejected C and Li atoms of the LiC:D surface. b) Oxygen, carbon, and lithium atoms sputtering yield as function of the impact energy of the LiCO:D. We compare our with those for carbon atoms from the a-C:D surface [25].
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the calculated sputtering yields for various lithium compounds ejected from the LiC:D and LiCO:D surfaces as functions of deuterium impact energy. We include the total sum of ejected atomic lithium and lithium compounds for each of the two surfaces. Sputtering yield of LiD molecules decreases with energy for the LiC:D surface but seems to reach a maximum at 20 eV for the LiCO:D surface, being reduced by about factor 3 with presence of oxygen. When oxygen is present in the surface the dominant ejected product containing Li is LiO molecule. Ejection of LiC molecules is not observed at energies below 30 eV due to the strong bond of the lithium and oxygen (~3.5 eV) inside the surface in comparison to more than a factor 2 weaker LiC bond (1.6 eV) [27]. Fig. 5 b) shows the calculated sputtering yields for various oxygen compounds ejected from the LiCO:D surface. We note that ejected molecules such as oxygen with deuterium (OD) and oxygen with carbon (CO) are the main products at the lowest impact energies, however when the energy increases a variety of oxygen compounds is present such as CO, OLi, and OD. Fig. 4. The total, CD, CD2, and CD3þCD4 sputtering yields per D as function of the impact energy for a) LiC:D surface and for b) LiCO:D surface. We compare our calculated results with the theoretical [25] and experimental data [27, 28] for a-C:D. Ejection of CD4 molecules is not observed for the LiCO:D (Fig. 4b) in good agreement with the results of Yagi et al. [11].
of the D impact energy. We have compared our results with the published theoretical and experimental results for a-C:D [25, 27]. In Fig. 4 a) Presence of lithium reduces the ejection of CD3þCD4 molecules, but the ejection of CD and CD2 molecules is increased by about a factor of 2. In Fig. 4 b), we show sputtering yields for the CxDy molecules of the LiCO:D surface. Comparison with the theoretical [25] and experimental [27, 28] results for the a-C:D surface is included in the same figure. The presence of oxygen in lithiated carbon surface somewhat reduces C2D2 and the total ejection of hydrocarbons with respect to the ATJ graphite [25, 27]. The suppression of ejected CD3þCD4 in our simulations is in good agreement with the analysis reported by P. Raman et al. [5] for a lithiated ATJ graphite and impacts of 1 keV D ions, and reported by Yagi et al. [11] for sputtering of ATJ graphite at 50e200 eV. Fig. 4 shows only ejected CD molecules. In Fig. 5 a) we present
Fig. 5. a) Sputtering yield per D for ejected lithium atoms and LiX molecules, from the LiC:D and LiCO:D surfaces as function of the impact energy. X ¼ D, C, or O; LiþLiX means total yield of lithium atoms and all lithium compounds ejected. b) OþOX is the total yield of oxygen atoms and all ejected combinations for oxygen molecules.
3.2. Mass spectra of ejected particles The decomposition by mass of sputtered particles can reveal additional features of the sputtering of lithiated and oxygenated carbon surfaces as may be reported in measurements of sputtering yield by mass spectrometers. In Fig. 6 a), we show the mass spectra of sputtered particles from the LiC:D surface. We notice that lithium atoms at 7 amu are dominant in the whole impact energy range. At the lowest impact energy (5 eV) the generation of Li-D molecules at ~ 9 amu is dominating over the CDx molecules by a factor of 3. However, for projectiles with impact energy above 10 eV the production of Li-D is lower than ejected hydrocarbons by a factor of 2. We note that CD3 at ~18 amu and CD4 at ~20 amu have the same sputtering yields per D, approaching1%. In Fig. 6 b), we show the mass spectra at various impact energies for the LiCO:D surface. At the lowest energy (5 eV) the ejection of lithium at 7 amu is dominant like in LiCO:D. Atomic lithium (~7 amu), carbon (~12 amu), and oxygen (~16 amu) atoms ejected from the surface are the main sputtering products, reaching about the same yield per D at 30 eV. The presence of more complex species such as C2D2 at 28 amu, C-O at 28 amu, and Li-O at 23 amu, is observed in the whole energy range. The sputtering yields for C2D2 and LiO molecules increase as function of the impact energy. We notice the evident suppression of ejected CD4 and CD3 molecules from this surface in comparison with the yields from the LiC:D surface.
Fig. 6. Mass spectra for various ejected atoms and molecules of a) LiC:D, and b) LiCO:D for various impact energies.
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3.3. Translational energy spectra of sputtered particles In order to closer characterize sputtering of ejected atoms and molecules, we have calculated their kinetic energy as
KE ¼ M=2 v2 where M is the total mass of a particle, v is velocity of the center of mass of the particle when its trajectory reaches 2.5 nm above the surface, with z-component of velocity oriented from the surface. Kinetic energies of all ejected atoms or compounds are averaged as
Eave ¼
X
, KEN
N;
N
where N is the total number of ejected atoms or compounds of a particular type, In Fig. 7 a), we show the average translation energy of the particles ejected from the LiC:D surface, as function of the mass (in amu) for various impact energies. Standard errors in the graph are smaller than the symbol size. The solid gray line represents the average of the translation kinetic energies over impact energies which is calculated as
KEðaveÞ ¼ ðXð5evÞ þ Xð10eVÞ þ Xð20eVÞ þ Xð30eVÞÞ=4; where X(E) is average kinetic energy of an ejected particle at an impact energy E. Sputtered atoms of lithium and carbon have the lowest kinetic energy, below 1 eV, while more complex CDx and LiD molecules have kinetic energy between 1 eV and 2.5 eV. Similarly, in Fig. 7 b), we present the average kinetic energy of sputtered particles of the LiCO:D surface. Carbon and lithium atoms again have the lowest kinetic energy below 1 eV, while particles with highest kinetic energy (~3 eV) are oxygen compounds and oxygen atoms.
dynamics of these particles only depends on the polar angle q of a particle motion direction to the surface normal. The angular distribution of ejected atoms/molecules is calculated as [23, 24]
dN Nðq; DqÞ ¼C ; dV 2p sin qDq
(1)
where N (q, Dq) is the number of a particle center-of-mass ejected in the interval (q - Dq/2, q þ Dq/2) and C is a normalization constant such that the maximum value of the angular distribution is at the unit circle [24]. The ejection is defined when the center of mass crosses plane 2.5 nm above the surface. We present the angular distributions by polar graphs in Fig. 8 for carbon atoms, as well as for CD and CD2 molecules, while Fig. 9 shows the distributions of Li atoms and Li-D molecule. In both figures the sputtered particles were ejected form LiCO:D surface. The span of the angular distributions of the c.m. momenta for various impact energies in Figs. 8 and 9 are shown by shaded gray areas. The averages over impact energy of the data in bins of 5 and in bin steps of 5 are shown by hollow symbols. Finally, we fitted these average data to the least squares curves in form (cosq)a, where a is the fitting parameter. The quality of the fits is excellent, with cross-correlation parameter larger than 0.99 in all five cases. The obtained values of a are 2.82, 3.06 and 2.79 for C, CD and CD2 distributions, respectively. The values of a for the Li and Li-D in Fig. 9 are 5.99 and 5.36, respectively. As expected, all shown distributions are much sharper than cosq. Similar overcos distribution were obtained in Ref. [26] for chemical sputtering of hydrocarbons and deuterium atoms and molecules. 4. Concluding remarks
The angular distribution of ejected particles is independent of the azimuthal angle in the plane of the interfacial surface and the
Classical molecular dynamics simulations with reactive force field potentials were performed to study the sputtering process of lithiated, oxidized and deuterated carbon surfaces by deuterium irradiation in 5e30 eV impact energy range. The capability of ReaxFF to treat atomic charges dynamically is a key feature for lithium material mixtures, due to polarization features of material induced by low electronegativity of lithium. We have calculated the sputtering yields for various particles ejected from LiC:D and LiCO:D surfaces. In absence of oxygen, we find that the total carbon
Fig. 7. Average kinetic energy as function of the mass of ejected atoms and molecules from the a) LiC:D surface, and b) LiCO:D. The solid gray lines represent the average of the translation kinetic energies of ejected particles over the impact energies of D.
Fig. 8. Solid angle distribution of ejected carbon atoms and CDx molecules from the LiCO:D surface. Shaded areas are spans of sputtered distributions at various impact energies of D. Hollow symbols are the values of the sputtering yields averaged over impact energy, while the solid curved lines are curves of the form (cosq)a, fitted through the symbols. Details of the fits are explained in the text.
3.4. Angular distributions
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Acknowledgments Research supported by the National Council on Science and Technology, Mexico (CONACyT) through the postdoctoral fellowship # 267898 (FJDG) and by DOE-OFES grant DE-SC0013752 through RF of SUNY (PSK). Simulation were obtained using Li-red cluster of the IACS-SBU.
References Fig. 9. Angular distributions of sputtered lithium atoms and LiD molecules from the LiCO:D surface. Details are the same as in Fig. 8.
erosion (of which 40% is atomic carbon) is suppressed in comparison to a-C:D by about factor 2. However, Li (of which 60% is in atomic form) contributes almost equally to carbon to the total sputtering at 30 eV, while its contribution at lower energies dominates due to the weak bond of Li to C. In presence of oxygen, the carbon erosion (40% in atomic form) is reduced by factor 4 in comparison to a-C:D, while Li sputtering (of which 75% is now in atomic form) is reduced by factor 4 in comparison to LiC:D. At the other hand, sputtering of hydrocarbon molecules (dominance of CD3 and CD4) of LiC:D is increased by factor 2 in comparison to aC:D, but reduced by factor 2 in presence of oxygen. Oxygen sputtering stays low, below 0.7%, over the whole studied energy range. The analysis of kinetic energy spectra of ejected particles shows that sputtered molecules are more energetic than atoms, which is expected due to the larger masses of molecules. Angular distributions of ejected lithium and carbon atoms, as well as of CDx and LiD molecules follow sharp (cosq)a function of polar angle, where a is between 3.6 and 6.
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