Publikationen
Suche nach „[J.] [Sun]“ hat 30 Publikationen gefunden
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Zeitschriftenartikel
A. Hamid, J. Sun, H. Zhang, Thomas Stirner
Molecular dynamics simulation analysis of helium cluster growth conditions under tungsten surfaces
Computational Materials Science, vol. 186, no. January
2021
DOI: 10.1016/j.commatsci.2020.109994
Molecular dynamics simulations have been performed to study the effects of helium fluxes on helium cluster size underneath tungsten surfaces under bombardment of helium atoms with incident energy 30 100 eV at temperature 300 2100 K. The simulation results show that the helium cluster size depends on the magnitude of the helium flux: at a higher flux, the helium clusters on average form in smaller size in tungsten but with larger number; while the clusters form further away from the surface at a lower flux. The coalescence of He atoms and helium bubbles depends on the tungsten temperature: at elevated temperatures around 2000 K, the incident He atoms in tungsten slow down more rapidly than at 1000 K but the number of vacancies per He cluster is smaller. The incident energy has a strong effect on the retention of helium atoms: The helium retention rate increases with the incident energy, and the helium retention depends weakly on temperature in the low energy range of interest. It is also found that the surface orientation plays an important role not only in determining the depth distribution but also in determining the helium retention and cluster size: at the surface {1 1 0}, the retention rate of helium atoms is the lowest, and at the surface {1 1 1}, the clusters grow easily in the lateral direction. The present simulation results suggest that the {0 0 1} surface is favorable for fuzz growth. The results obtained in the present work provide insight to the reasons why the fuzz only grows within a certain parameter range at the atomic level.
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Zeitschriftenartikel
H. Zhang, J. Sun, Y. Wang, Thomas Stirner, A. Hamid, C. Sang
Study of lattice thermal conductivity of tungsten containing bubbles by molecular dynamics simulation
Fusion Engineering and Design, vol. 161, no. December
2020
DOI: 10.1016/j.fusengdes.2020.112004
Exposed to high fluxes of helium/hydrogen isotope particles and heat, tungsten divertor plates will suffer damage thus degrading its performance such as its thermal conductivity. This paper presents a study on the effect of bubbles on the lattice thermal conductivity of tungsten at the atomic level using molecular dynamics simulations. The present study finds that empty bubbles in tungsten lead to a decrease in the lattice thermal conductivity of tungsten. Furthermore, He/D filled bubbles aggravate this decrease. The physical origin of this behavior is discussed. It is also found that the decrease in lattice thermal conductivity depends strongly on both the impurity density in the bubbles and the bubble size.
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Zeitschriftenartikel
Thomas Stirner, David Scholz, J. Sun
Convergence of surface energy calculations for various methods: (001), (012), (100) hematite and the applicability of the standard approach
Journal of Physics: Condensed Matter, vol. 32, no. 18
2020
Three different methods for the calculation of the surface energy, namely the standard approach, the Boettger relation and the linear-fit method, are applied to the (0 0 1), (0 1 2) and (1 0 0) hematite surfaces. The standard approach was previously shown to suffer from a divergence problem, and the Boettger relation was shown to exhibit quantum size effects. While the linear-fit method, in general, leads to a good convergence behavior of the surface energy, the questions arise whether the relative order of the calculated surface energies depends on the chosen calculation method, and whether there is any merit at all in employing the standard approach. The present work investigates these questions with hematite as a benchmark material system. The simulations show that, for the surface facets and slab thicknesses studied here, the relative order of the surface energies is unaffected by the chosen calculation method. A regime is found where the three methods are in reasonably good agreement with respect to the obtained surface energies. Finally, a procedure is put forward to extract meaningful surface energy values from the standard approach.
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Zeitschriftenartikel
Thomas Stirner, David Scholz, J. Sun
Ab initio simulation of structure and surface energy of low-index surfaces of stoichiometric alpha-Fe2O3
Surface Science, vol. 671, no. May, pp. 11-16
2018
DOI: 10.1016/j.susc.2018.01.010
The structure and surface energy of a series of low-index surfaces of stoichiometric α-Fe2O3 (hematite) are investigated using the periodic Hartree–Fock approach with an a posteriori correction of the correlation energy. The simulations show that, amongst the modeled facets, (012) and (0001) are the most stable surfaces of hematite, which is consistent with the fact that the latter are the dominant growth faces exposed on natural α-Fe2O3. The Fe-terminated (0001) surface is shown to exhibit a large relaxation of the surface atoms. It is argued that this arises mainly due to the fact that the surface cations are located opposite empty cation sites in the filled-filled-unfilled cation sequence along the c-axis. In contrast, the (012) plane cuts the crystal through a plane of empty cation sites, thus giving rise to relatively small relaxations and surface energies. The small relaxations and concomitant exposure of five-coordinate cation sites may be important for the catalytic activity of hematite. The simulations also show that the relative stability of the investigated surfaces changes after a full lattice relaxation with the (0001) and (116) facets relaxing disproportionately large. Wherever possible, the simulations are compared with previous simulation data and experimental results. A Wulff–Gibbs construction is also presented.
NachhaltigMaschinenbau und Mechatronik
Zeitschriftenartikel
Thomas Stirner, David Scholz, J. Sun
Hartree-Fock simulation of the (0 0 0 1) surface of hematite with a posteriori calculation of the correlation energy
Computational Materials Science, vol. 137, no. September, pp. 340-345
2017
DOI: 10.1016/j.commatsci.2017.06.011
The results of Hartree-Fock simulations of the (0 0 0 1) surface of hematite including an a posteriori calculation of the correlation energy are presented. Structural as well as electronic and magnetic properties of the surface atoms are considered. Infrared and Raman bands of the hematite slab are also presented. The calculated surface relaxation is shown to be in good agreement with experimental data originating from a recent LEED analysis. The iron-oxygen bond at the single-iron-terminated surface is shown to be less ionic than in the bulk. Also, the magnetic dipole moment of the surface iron atom is smaller than in the bulk. Finally, the surface energy of the (0 0 0 1) facet is presented for hematite, and for comparison also for α-alumina and α-chromia for various hybrid functionals. Here it is shown that the effects of electron correlation play an important role for hematite and chromia due to their localized 3d electrons.
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Zeitschriftenartikel
S. Liu, S. Dai, C. Sang, J. Sun, Thomas Stirner, D. Wang
Molecular dynamics simulation of the formation, growth and bursting of bubbles in tungsten exposed to high fluxes of low energy deuterium
Journal of Nuclear Materials - PLASMA-SURFACE INTERACTIONS 21 — Proceedings of the 21st International Conference on Plasma-Surface Interactions in Controlled Fusion Devices Kanazawa, Japan, May 26-30, 2014, vol. 463, no. August, pp. 363-366
2015
DOI: 10.1016/j.jnucmat.2014.12.060
Molecular dynamics simulations are carried out to investigate the formation, growth and bursting of bubbles in tungsten exposed to the irradiation of an extremely high deuterium flux. It is found that the bubbles form in the region near the location of the implanted ion distribution peaks, and that the effect of the substrate temperature on the bubble formation depth is negligible; it is also found that the percentage of deuterium that is found in D2 molecules increases as the bubble grows, and that the evolution of the bubble’s internal pressure is strongly associated with the properties of its surrounding structure. The simulations display the development of a dome-shaped structure at the tungsten surface during the bubble growth. The merging of two deuterium bubbles is also observed. The present simulations also show that the bubble bursts by generating a partially opened lid, which has already been observed in previous independent experiments.
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Zeitschriftenartikel
J. Sun, Y. Fan, Y. Zou, Thomas Stirner, D. Wang
Investigation of the effects of a thin dielectric layer on low-pressure hydrogen capacitive discharges driven by combined radio frequency and pulse power sources
Physics of Plasmas, vol. 20, no. 11
2013
Negative hydrogen ion sources, for instance for fusion devices, currently attract considerable attention. To generate the precursors—highly rovibrationally excited hydrogen molecules—for negative hydrogen ions effectively by electron excitation, a thin dielectric layer is introduced to cover the surface of the electrically grounded electrode of two parallel metal plates in a low-pressure hydrogen capacitive discharge driven by combined rf and pulse power sources. To understand the characteristics of such discharges, particle-in-cell simulations are conducted to study the effects that the single dielectric layer would bring onto the discharges. The simulation results show that the dielectric layer leads to a much higher plasma density and a much larger production rate of highly vibrationally excited hydrogen molecules compared to discharges without the dielectric layer on the electrode. Further investigation indicates that the nonlinear oscillation of the electrons induced by the nanosecond-pulse continues until it is finally damped down and does not show any dependence on the pulse plateau-time, which is in stark contrast to the case without the dielectric layer present. The physical reason for this phenomenon is explored and explained.
Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Zeitschriftenartikel
Y. Fan, Y. Zou, J. Sun, Thomas Stirner, D. Wang
Study of the effects of a transverse magnetic field on radio frequency argon discharges by two-dimensional particle-in-cell-Monte-Carlo collision simulations
Physics of Plasmas, vol. 20, no. 10
2013
The influence of an applied magnetic field on plasma-related devices has a wide range of applications. Its effects on a plasma have been studied for years; however, there are still many issues that are not understoodwell. This paper reports a detailed kinetic study with thetwo-dimension-in-space and three-dimension-in-velocity particle-in-cellplus Monte Carlo collision method on the role of E×B drift in acapacitive argon discharge, similar to the experiment of You et al.[Thin Solid Films 519, 6981 (2011)]. The parameters chosen in the present study for the external magnetic field are in a range common tomany applications. Two basic configurations of the magnetic field areanalyzed in detail: the magnetic field direction parallel to theelectrode with or without a gradient. With an extensive parametricstudy, we give detailed influences of the drift on the collectivebehaviors of the plasma along a two-dimensional domain, which cannot berepresented by a 1 spatial and 3 velocity dimensions model. By analyzingthe results of the simulations, the occurring collisionless heating mechanism is explained well.
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Zeitschriftenartikel
J. Sun, L. Li, Thomas Stirner, J. Bai, D. Wang
Particle simulation of the nonlinear oscillation of electrons induced by a nanosecond pulse in rf capacitive hydrogen discharges
Physics of Plasmas, vol. 19, no. 3
2012
A particle-in-cell simulation was employed to investigate the nature and physical cause of the nonlinear oscillation of electrons induced by a nanosecond pulse in rf capacitive hydrogen discharges. It was found that the applied nanosecond pulse converted the plasma quickly from the bi-Maxwellian equilibrium formed in the rf capacitive discharge into another temporal bi-Maxwellian equilibrium. When the applied electric field collapses within a few nanoseconds, the electric field arising from the space charge serves as a restoring force to generate a swift oscillation of the electrons. The energy stored in the plasma is converted gradually into the chemical energy during the electron periodic movement. It is also found that the rise-, plateau-, and fall-times of the applied pulse affect the evolution of the electron energy distribution. The collective electron oscillation has a repetition frequency approximately equal to the electron plasma frequency, independent of pulse rise-, plateau-, and fall-times. This oscillation of electrons induced by a nanosecond pulse can be used to generate highly excited vibrational states of hydrogen molecules, which are a necessary precursor for negative hydrogen ions.
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Zeitschriftenartikel
J. Sun, S. Liu, Thomas Stirner, et al.
Dynamic Monte Carlo simulation of film-substrate interface mixing in the deposition of Co on Cu (001)
Surface Science, vol. 605, no. 13-14, pp. 1298-1303
2011
Dynamic Monte Carlo simulations are performed to investigate the interface mixing of Co atoms deposited on a Cu (001) substrate. A tight-binding potential was used to determine the input parameters (jump probabilities and energy barriers) for the Dynamic Monte Carlo model. The results show that more Co adatoms penetrate into the substrate as the temperature rises and/or as the deposition rate decreases, and that the intermixing between the layers becomes concomitantly more pronounced. Cu atoms migrating into the Co layer via exchange processes during the growth of consecutive Co layers are proposed to be responsible for the intermixing. Furthermore, an initial Co clustering followed by a layer-by-layer growth mode was observed in the simulations, with the surface concentration of Cu atoms depending on the fraction of migrating Cu atoms and decaying into the Co film following a power law. The fraction of Cu atoms migrating into the Co layer can be adjusted by varying the deposition rate and the substrate temperature.
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Beitrag (Sammelband oder Tagungsband)
Computer simulation of alpha-chromia and its surface
Chromium, New York
2011
ISBN: 978-1611226560
Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Vortrag
Thomas Stirner, J. Sun, et al.
Ab initio Hartree-Fock simulation of r-plane sapphire
Posterpräsentation
18th International Vacuum Congress (IVC-18), Peking, China
2010
Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen
Zeitschriftenartikel
S. Liu, J. Sun, Thomas Stirner, et al.
A general model for chemical erosion of carbon materials due to low-energy H+ impact
Journal of Applied Physics, vol. 108
2010
Modeling the chemical erosion of carbon materials due to low-energy H+ impact is of paramount importance for the prediction of the behavior of carbon-based plasma-facing components in nuclear fusion devices. In this paper a simple general model describing both energy and temperature dependence of carbon-based chemical erosion is presented. Enlightened by Hopf’s model {Hopf et al. , [J. Appl. Phys.94, 2373 (Year: 2003)}, the chemical erosion is separated into the contributions from three mechanisms: thermal chemical erosion, energetic chemical sputtering, and ion-enhanced chemical erosion. Using input from the Monte Carlo code TRIDYN, this model is able to reproduce experimental data well.
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Zeitschriftenartikel
J. Sun, S. Li, Thomas Stirner, et al.
Molecular dynamics simulation of energy exchanges during hydrogen collision with graphite sheets
Journal of Applied Physics, vol. 107
2010
Experiments show that the energy of particles incident on divertor plates in fusion devices seldom exceeds 100 eV. Trim code and its variants are not suitable to predict the sputtering yield of carbon-based divertor plates for this energy range and, therefore, a dynamic model, taking into account the C–H bond formation and breaking, and the structure of carbon, is needed. In this paper, the molecular dynamics method is employed to investigate collision processes between incident hydrogen atoms and a graphene sheet. The simulation results demonstrate that the collision processes cannot be adequately described by a simple binary approximation. The energy transfer from the projectile to the graphite sheet exhibits a very complicated behavior when the kinetic energy of the incident hydrogen atom is below 30 eV, strongly depending on the impact position. When its kinetic energy is lower than 0.35 eV, the incident hydrogen is always reflected back from the single, perfect graphite sheet; when its kinetic energy is higher than 0.35 eV, then whether the incident particle penetrates the graphite sheet, is reflected back or is adsorbed depends on the impact position. In certain areas of the graphite sheet, either adsorption or reflection of an incident hydrogen atom can occur in two different energy ranges.
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Zeitschriftenartikel
J. Sun, C. Sang, Thomas Stirner
Characteristics of plasma immersion ion implantation with a nanosecond rise-time pulse: particle-in-cell simulations
Journal of Physics D: Applied Physics, vol. 43
2010
DOI: 10.1088/0022-3727/43/27/275201
Processes of plasma immersion ion implantation are analyszed numerically using a one-dimension-in-space and three-dimension-in-velocity particle-in-cell plus Monte Carlo collision (1D3V PIC–MCC) model. The behaviour of ions and electrons between the processed target and the source plasma is simulated after a nanosecond rise-time voltage pulse is applied to the target. The simulation results show that electron–neutral ionization collisions play a significant role in determining the magnitudes of the ion and electron densities when the pulse rise time is very short, and that the plasma density can be enhanced many times. The physical mechanism for this phenomenon is explained in terms of the formation of a reverse electric field inside the plasma chamber.
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Zeitschriftenartikel
Ab initio supercell calculations of the (0001) alpha-Cr203 surface with a partially or totally Al-substituted external layer
Thin Solid Films, vol. 517, pp. 5512-5515
2009
Ab initio supercell calculations employing the periodic Hartree–Fock formalism are presented of the (0001) α-Cr2O3 surface with a partially or totally Al-substituted external layer. In the simulations a fraction of the Cr atoms at the surface of the chromia slab are replaced by Al atoms, and the Al surface coverage is varied between zero (pure chromia) and 100% (Al-terminated chromia). The surface Al atoms are found to relax inwards considerably, with the magnitude of the relaxation decreasing with increasing Al surface coverage. The calculations also reveal that the surface energy of the slab decreases with increasing Al coverage. Finally, the electronic properties at the surface of the Al-substituted (0001) α-Cr2O3 slabs are investigated. Here the calculations show that the substitution of Cr by Al gives rise to an increase in the covalency of the AlO bonds compared to slabs of pure alumina. In contrast, the influence of the surface Al atoms on the electrostatic potential in the (0001) plane of metal ions is relatively small. These findings support the utilisation of α-chromia substrates for the templated growth of α-alumina, which is consistent with recent experiments.
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Zeitschriftenartikel
Non-equilibrium simulation of optically trapped colloidal particles at liquid interfaces
Computational Materials Science, vol. 43, no. 4, pp. 769-801
2008
A theoretical study of the non-equilibrium behaviour of colloidal particle monolayers under the influence of the displacement of a single, optically trapped colloidal particle is presented. Two different types of monolayer (i.e., one relatively rigid and the other relatively soft) confined to a liquid–liquid interface are investigated using the Stokesian dynamics simulation technique. An effective dipole–dipole interaction is assumed to act between the colloidal particles and viscous drag forces acting on the particles are taken into account. The calculations reveal periodic oscillations of the net force on the trapped particle for the soft monolayer and highly non-linear, non-monotonic variations for the rigid monolayer. The highly non-linear variations of the net force for the rigid monolayer are concomitant with rapid, cooperative particle rearrangements and large oscillations in the global orientational order parameter of the monolayer. These findings, combined with the results of optical-tweezer experiments, should be useful for the rheology of liquid interfaces and the investigation of Pickering emulsions.
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Zeitschriftenartikel
J. Sun, A. Matthews, Thomas Stirner
Molecular dynamics simulation of the (0001) α-Al2O3 and α-Cr2O3 surfaces
Surface Science, vol. 601, no. 5, pp. 1358-1364
2007
A simple, rigid pair-potential model is applied to investigate the dynamics of the (0 0 0 1) α-Al2O3 and α-Cr2O3 surfaces using the molecular dynamics technique. The simulations employ a two-stage equilibration process: in the first stage the simulation-cell size is determined via the constant-stress ensemble, and in the second stage the equilibration of the size-corrected simulation cell is continued in the canonical ensemble. The thermal expansion coefficients of bulk alumina and chromia are evaluated as a function of temperature. Furthermore, the surface relaxation and mean-square displacement of the atoms versus depth into the slab are calculated, and their behaviour in the surface region analysed in detail. The calculations show that even moderate temperatures (∼400 °C) give rise to displacements of the atoms at the surface which are similar to the lattice mismatch between α-alumina and chromia. This will help in the initial nucleation stage during thin film growth, and thus facilitate the deposition of α-Al2O3 on (0 0 0 1) α-Cr2O3 templates.
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Zeitschriftenartikel
J. Sun, A. Matthews, Thomas Stirner
Structure and electronic properties calculation of ultrathin α-Al2O3 films on (0001) α-Cr2O3 templates
Surface Science, vol. 601, no. 21, pp. 5050-5056
2007
Ab initio total energy Hartree–Fock calculations of ultrathin films of α-Al2O3 on (0 0 0 1) α-Cr2O3 templates are presented. The surface relaxation, the in-plane reconstruction and the surface and strain energies of the slabs are studied as a function of alumina film thickness. The surface Al layer is found to relax inwards considerably, with the magnitude of the inwards relaxation depending on the thickness of the ultrathin alumina film in a non-linear manner. The calculations also reveal that ultrathin films of alumina lower the surface energy of (0 0 0 1) α-chromia substrates. This indicates that the (0 0 0 1) α-chromia surface provides favourable conditions for the templated growth of α-alumina. However, increasing the alumina film thickness is found to give rise to a significant increase in strain energy. Finally, the electronic properties at the surface of the (0 0 0 1) α-Al2O3/α-Cr2O3 slabs are investigated. Here it is found that the alumina coating gives rise to an increase in the covalency of the bonds at the surface of the slabs. In contrast, the influence of an alumina layer on the electrostatic potential at the surface of the chromia slab is relatively minor, which should also be beneficial for the templated growth of α-alumina on (0 0 0 1) α-chromia substrates.
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Zeitschriftenartikel
J. Sun, A. Matthews, Thomas Stirner
Calculation of native defect energies in α-Al2O3 and α-Cr2O3 using a modified Matsui potential
Surface & Coatings Technology, vol. 201, no. 7, pp. 4201-4204
2006
Alumina and chromia are very important materials in the surface coatings industry, e.g. for corrosion protection and as catalyst supports. The type of defects and the associated formation energy in these materials are of direct relevance to the surface stability and reaction kinetics. In the present work, a modified Matsui potential is applied to calculate the native point defect energies in α-Al2O3 and α-Cr2O3 based on the Mott–Littleton theory. Particular attention is paid to the convergence of the defect energies with the number of atoms surrounding the defect. The results show that the relative values of the defect formation energies are such that the Schottky energy is smaller than either of the Frenkel energies, which is in agreement with experimental data and recent results of first-principles calculations. The implications of these findings for diffusion mechanisms and the associated reaction kinetics are discussed briefly.
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