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Suche nach „[J.] [Sun]“ hat 27 Publikationen gefunden
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    NachhaltigAngewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

    J. Sun, Thomas Stirner, David Scholz

    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

    Abstract anzeigen

    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

    J. Sun, Thomas Stirner, David Scholz

    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

    Abstract anzeigen

    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.

    NachhaltigAngewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

    S. Liu, J. Sun, C. Sang, Thomas Stirner, S. Dai, 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

    Abstract anzeigen

    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.

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

    J. Sun, Thomas Stirner, D. Wang, Y. Fan, Y. Zou

    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

    DOI: 10.1063/1.4831775

    Abstract anzeigen

    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

    J. Sun, Thomas Stirner, D. Wang, Y. Fan, Y. Zou

    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

    DOI: 10.1063/1.4826215

    Abstract anzeigen

    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.

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

    L. Li, J. Bai, J. Sun, Thomas Stirner, 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

    DOI: 10.1063/1.3695121

    Abstract anzeigen

    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.

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

    S. Liu, J. Sun, 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

    Abstract anzeigen

    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.

    Angewandte Naturwissenschaften und WirtschaftsingenieurwesenExtern

    Beitrag (Sammelband oder Tagungsband)

    J. Sun, Thomas Stirner

    Computer simulation of alpha-chromia and its surface

    Chromium, New York

    2011

    ISBN: 978-1611226560

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Vortrag

    J. Sun, Thomas Stirner, 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

    Abstract anzeigen

    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.

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

    S. Li, J. Sun, Thomas Stirner, et al.

    Molecular dynamics simulation of energy exchanges during hydrogen collision with graphite sheets

    Journal of Applied Physics, vol. 107

    2010

    DOI: 10.1063/1.3428447

    Abstract anzeigen

    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.

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    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

    Abstract anzeigen

    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.

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

    J. Sun, Thomas Stirner

    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

    Abstract anzeigen

    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.

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

    J. Sun, Thomas Stirner

    Non-equilibrium simulation of optically trapped colloidal particles at liquid interfaces

    Computational Materials Science, vol. 43, no. 4, pp. 769-801

    2008

    Abstract anzeigen

    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.

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

    A. Matthews, J. Sun, Thomas Stirner

    Molecular dynamics simulation of the (0001) α-Al2O3 and α-Cr2O3 surfaces

    Surface Science, vol. 601, no. 5, pp. 1358-1364

    2007

    Abstract anzeigen

    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.

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

    A. Matthews, J. Sun, 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

    Abstract anzeigen

    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.

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

    A. Matthews, J. Sun, 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

    Abstract anzeigen

    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.

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

    A. Matthews, J. Sun, Thomas Stirner

    Structure and surface energy of low-index surfaces of stoichiometric α-Al2O3 and α-Cr2O3

    Surface & Coatings Technology, vol. 201, no. 7, pp. 4205-4208

    2006

    Abstract anzeigen

    There is considerable interest in the low-temperature growth of the stable α-phase of alumina, α-Al2O3, due to its superior chemical and mechanical properties compared to the other forms of alumina. Conventional methods for achieving this phase (such as chemical vapour deposition) utilise high temperatures which limits the range of substrate materials that can be beneficially coated. Recently, it has been reported that α-Al2O3 can be grown on α-Cr2O3 templates at a much lower substrate temperature (around 400 °C) by the RF magnetron sputtering deposition technique. Previous studies showed that both α-Al2O3 and α-Cr2O3 (0001) free surfaces relax considerably in comparison with the corresponding bulk structures. From the experimental point of view, a knowledge of the surface structure and stability of both materials will be of great help in determining the facet of α-Cr2O3 which best facilitates the growth of α-Al2O3. Here we present the results of first-principles Hartree–Fock calculations on the surface energy of the low-index planes of both α-alumina and α-chromia. The suitability of possible α-Cr2O3 facets as templates for the growth of α-Al2O3 is discusssed.

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Vortrag

    A. Matthews, J. Sun, Thomas Stirner

    Structure and surface energy of low-index surfaces of stoichiometric α-Al2O3 and α-Cr2O3

    Posterpräsentation

    International Conference on Metallurgical Coatings and Thin Films (ICMCTF 2006), San Diego, CA, USA

    2006

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Vortrag

    A. Matthews, J. Sun, Thomas Stirner

    Calculation of native defect energies in α-Al2O3 and α-Cr2O3 using a modified Matsui potential

    Posterpräsentation

    International Conference on Metallurgical Coatings and Thin Films (ICMCTF 2006), San Diego, CA, USA

    2006