Specimens of ZM5 magnesium alloy were dipped into the mixed powder of Al and Zn at （390±5） ℃ for 8 h in argon gas protective environment and subjected to surface diffusion alloying processing （SDAP）. The ero...Specimens of ZM5 magnesium alloy were dipped into the mixed powder of Al and Zn at （390±5） ℃ for 8 h in argon gas protective environment and subjected to surface diffusion alloying processing （SDAP）. The erosion wear behaviors of ZM5 magnesium alloy before and after SDAP were investigated in two different erosion wear environments： oil and quartz environment; tap water and quartz environment. The surfaces of erosion wear specimens exhibited cutting scratch grooves in the oil and quartz environment. Corrosive attack was weak and cutting wear mechanism was responsible for the mass loss. When the erosion wear medium was changed to tap water and quartz environment, corrosion pits and cracks were obviously observed after erosion wear test. The corrosion from tap water and the scour from quartz intensified mass losses. Compared with the untreated specimens, the application of SDAP improved the erosion wear resistance in the same environment.展开更多
Based on the weak formulation for combined surface diffusion and evaporation-condensation,a governing equation of the finite element is derived for simulating the evolution of intergranular microcracks in copper inter...Based on the weak formulation for combined surface diffusion and evaporation-condensation,a governing equation of the finite element is derived for simulating the evolution of intergranular microcracks in copper interconnects induced simultaneously by stressmigration,electromigration and thermomigration.Unlike previously published works,the effect of thermomigration is considered.The results show that thermomigration can contribute to the microcrack splitting and accelerate the drifting process along the direction of the electric field.The evolution of the intergranular microcracks depends on the mechanical stress field,the temperature gradient field,the electric field,the initial aspect ratio and the linewidth.And there exists a critical electric fieldχ_c,a critical stress field■,a critical aspect ratioβ_c and a critical linewidth■.When■or■,the intergranular microcrack will split into two or three small intergranular microcracks.Otherwise,the microcrack will evolve into a stable shape as it migrates along the interconnect line.The critical stress field,the critical electric field and the critical aspect ratio decrease with a decrease in the linewidth,and the critical linewidth increases with an increase in the electric field and the aspect ratio.The increase of the stress field,the electric field or the aspect ratio and the decrease of the linewidth are not only beneficial for the intergranular microcrack to split but also accelerate the microcrack splitting process.展开更多
Based on the bulk free energy density and the degenerate mobility constructed by the quartic double-well potential function,a phase field model is established to simulate the evolution of intragranular microvoids due ...Based on the bulk free energy density and the degenerate mobility constructed by the quartic double-well potential function,a phase field model is established to simulate the evolution of intragranular microvoids due to surface diffusion in a stress field.The corresponding phase field governing equations are derived.The evolution of elliptical microvoids with different stressesΛ,aspect ratiosβand linewidths hˉis calculated using the mesh adaptation finite element method and the reliability of the procedure is verified.The results show that there exist critical values of the stressΛc,the aspect ratioβc and the linewidth hˉc of intragranular microvoids under equivalent biaxial tensile stress.When Λ≥Λ_(c),β≥β_(c) or h≤h_(c),the elliptical microvoids are instable with an extending crack tip.WhenΛ<Λ_(c),β<β_(c) or hˉ>h_(c),the elliptical microvoids gradually cylindricalize and remain a stable shape.The instability time decreases with increasing the stress or the aspect ratio,while increases with increasing the linewidth.In addition,for the interconnects containing two elliptical voids not far apart,the stress will promote the merging of the voids.展开更多
Depositions of Si, Ge and C atoms onto a preliminary Si （001） substrate at different temperatures are investigated by using the molecular dynamics method. The mechanism of atomic self-assembling occurring locally on...Depositions of Si, Ge and C atoms onto a preliminary Si （001） substrate at different temperatures are investigated by using the molecular dynamics method. The mechanism of atomic self-assembling occurring locally on the flat terraces between steps is suggested. Diffusion and arrangement patterns of adatoms at different temperatures are observed. At 900 K, the deposited atoms are more likely to form dimers in the perpendicular [110] direction due to the more favourable movement along the perpendicular [110] direction. C adatoms are more likely to break or reconstruct the dimers on the substrate surface and have larger diffusion distances than Ge and Si adatoms. Exchange between C adatoms and substrate atoms are obvious and the epitaxial thickness is small. Total potential energies of adatoms and substrate atoms involved in the simulation cell are computed. When a newly arrived adatom reaches the stable position, the potential energy of the system will decrease and the curves turns into a ladder-like shape. It is found that C adatoms can lead to more reduction of the system energy and the potential energy of the system will increase as temperature increases.展开更多
Molecular dynamics simulation employing the embedded atom method potential is utilized to investigate nanoscale surface diffusion mechanisms of binary heterogeneous adatoms clusters at 300 K, 500 K, and 700 K. Surface...Molecular dynamics simulation employing the embedded atom method potential is utilized to investigate nanoscale surface diffusion mechanisms of binary heterogeneous adatoms clusters at 300 K, 500 K, and 700 K. Surface diffusion of heterogeneous adatoms clusters can be vital for the binary island growth on the surface and can be useful for the formation of alloy-based thin film surface through atomic exchange process. The results of the diffusion process show that at 300 K, the diffusion of small adatoms clusters shows hopping, sliding, and shear motion; whereas for large adatoms clusters（hexamer and above）, the diffusion is negligible. At 500 K, small adatoms clusters, i.e., dimer, show almost all possible diffusion mechanisms including the atomic exchange process; however no such exchange is observed for adatoms clusters greater than dimer. At 700 K, the exchange mechanism dominates for all types of clusters, where Zr adatoms show maximum tendency and Ag adatoms show minimum or no tendency toward the exchange process. Separation and recombination of one or more adatoms are also observed at 500 K and 700 K. The Ag adatoms also occupy pop-up positions over the adatoms clusters for short intervals. At 700 K, the vacancies are also generated in the vicinity of the adatoms cluster,vacancy formation, filling, and shifting can be observed from the results.展开更多
Consider the motion of immersed hypersurfaces driven by surface diffusion flow and give anlower bound on the life span of a smooth immersed solution, which depends only on how muchthe curvature of the initial surface ...Consider the motion of immersed hypersurfaces driven by surface diffusion flow and give anlower bound on the life span of a smooth immersed solution, which depends only on how muchthe curvature of the initial surface is concentrated in space.展开更多
The diffusion dynamics of small two-dimensional atomic clusters Cux （1≤x≤8） on Cu（lll） surface were studied using the molecular dynamics simulations and a modified analytic embedded-atom method in the temperatur...The diffusion dynamics of small two-dimensional atomic clusters Cux （1≤x≤8） on Cu（lll） surface were studied using the molecular dynamics simulations and a modified analytic embedded-atom method in the temperature range from 200 K to 800 K. The cluster size and temperature dependence of the diffusion coefficients and migration energies are presented. Our simulations show that the diffusion migration energy of the CuT cluster is the highest and the prefactor for the CuT cluster is almost three orders of magnitude larger than that for single atom diffusion. This conclusion is consistent with the experimental results for similar metals. In addition, the dependence of cluster diffusion on film growth is also discussed.展开更多
A detailed first-principles study of the diffusion behaviour of point defects in the O-terminated （0001） surface in wnrtzite ZnO was performed. The 0 vacancy and interstitial are found to diffuse much more easily in...A detailed first-principles study of the diffusion behaviour of point defects in the O-terminated （0001） surface in wnrtzite ZnO was performed. The 0 vacancy and interstitial are found to diffuse much more easily in surface than in bulk. The Zn vacancy has a similar migration barrier for both bulk and surface, but has much smaller barrier for the diffuse-in process. The Zn interstitial is difficult to diffuse in the surface directly, but it can diffuse into the bulk relatively easily. Specific values of corresponding migration barriers are obtained.展开更多
The surface diffusion of an axi-symmetric solid,a whisker,subject to applied uniaxial stress,is studied numerically based on a new boundary integral formulation for periodic stress configurations.An efficient semi-imp...The surface diffusion of an axi-symmetric solid,a whisker,subject to applied uniaxial stress,is studied numerically based on a new boundary integral formulation for periodic stress configurations.An efficient semi-implicit time-stepping scheme is developed to treat the serve stiffness due to high-order derivatives.When the initial perturbation is small the effect of the stress on the motion of the whisker is found to agree with the linear stability analysis.Numerical simulations of a fully nonlinear case are also presented,and a potential break-up of the whisker is observed.展开更多
During heteroepitaxial overlayer growth multiple crystal domains nucleated on a substrate surface compete with each other in such a manner that a domain covered by neighboring ones stops growing.The number density of ...During heteroepitaxial overlayer growth multiple crystal domains nucleated on a substrate surface compete with each other in such a manner that a domain covered by neighboring ones stops growing.The number density of active domains ρ decreases as the height h increases.A simple scaling argument leads to a scaling law of ρ~ h^（-γ） with a coarsening exponent γ=d/z,where d is the dimension of the substrate surface and z the dynamic exponent of a growth front.This scaling relation is confirmed by performing kinetic Monte Carlo simulations of the ballistic deposition model on a two-dimensional（d=2） surface,even when an isolated deposited particle diffuses on a crystal surface.展开更多
Anisotropic evolution of the step edges on the compressive-strained In0.2Ga0.8As/GaAs（001） surface has been investigated by scanning tunneling microscopy （STM）. The experiments suggest that step edges are indeed s...Anisotropic evolution of the step edges on the compressive-strained In0.2Ga0.8As/GaAs（001） surface has been investigated by scanning tunneling microscopy （STM）. The experiments suggest that step edges are indeed sinuous and protrude somewhere a little way along the [110] direction, which is different from the classical waviness predicted by the theoretical model. We consider that the monatomic step edges undergo a morphological instability induced by the anisotropic diffusion of adatoms on the terrace during annealing, and we improve a kinetic model of step edge based on the classical Burton Cabrer-Frank （BCF） model in order to determine the normal velocity of step enlargement. The results show that the normal velocity is proportional to the arc length of the peninsula, which is consistent with the first result of our kinetic model. Additionally, a significant phenomenon is an excess elongation along the [110] direction at the top of the peninsula with a higher aspect ratio, which is attributed to the restriction of diffusion lengths.展开更多
A stable and homogeneous well-aligned air microplasma device for application at atmospheric pressure is designed and its electrical and optical characteristics are investigated. Current-voltage measurements and intens...A stable and homogeneous well-aligned air microplasma device for application at atmospheric pressure is designed and its electrical and optical characteristics are investigated. Current-voltage measurements and intensified charge coupled device （ICCD） images show that the well-aligned air microplasma device is able to generate a large-area and homogeneous discharge at the applied voltages ranging from 12 kV to 14 kV, with a repetition frequency of 5 kHz, which is attributed to the diffusion effect of plasma on dielectric surface. Moreover, this well-aligned microplasma device may result in the uniform and large-area surface modification of heat-sensitive PET polymers without damage, such as optimization in hydrophobicity and biocompatibility. In the biomedical field, the utility of this well-aligned microplasma device is further testified. It proves to be very efficient for the large-area and uniform inactivation of E. coli cells with a density of 103/cm2 on LB agar plate culture medium, and inactivation efficiency can reach up to 99% for 2-min treatment.展开更多
The adsorption dynamics for phenol in aqueous solution of the adsorbent based on polystyrene was studied. In order to distinguish with the Boyd quasi-homogeneous model of the inner structure of ion-exchanger, the part...The adsorption dynamics for phenol in aqueous solution of the adsorbent based on polystyrene was studied. In order to distinguish with the Boyd quasi-homogeneous model of the inner structure of ion-exchanger, the particle diffusion model including surface diffusion model and pore diffusion model was suggested which is suitable to the macroporous adsorbent. The diffusiondetermination step of the adsorption process was established and the effective diffusion coefficient was also determined. The influence of surface diffusion and pore diffusion on the particle diffusion rate was investigated qualitatively. All of these were very important to improve the structure of the macroporous adsorbent in order to improve the mass-transfer rate.展开更多
Atomic surface mobility of glasses plays an important role in understanding glass dynamics and determining many fundamental processes on the surface.However,the diffusion dynamics at the free surface in marginal glass...Atomic surface mobility of glasses plays an important role in understanding glass dynamics and determining many fundamental processes on the surface.However,the diffusion dynamics at the free surface in marginal glasses remains unknown due to limited glass formation ability.In this study,we systematically investigate surface diffusion and relaxation behavior in four marginal glass-forming Fe-based metallic glasses with great application potential.Surface diffusion rates in marginal glassforming Fe-based metallic glasses are significantly faster than those of stable metallic glasses.For the first time,an abnormal β_(t) relaxation mode with thermal activation character is identified betweenαandβrelaxation.Strikingly,the activation energy of surface diffusion matches that ofβt relaxation.A mechanism involving cooperative cluster motion associated withβt relaxation is proposed to explain the ultrafast surface diffusion.These results establish a direct correlation between surface diffusion and bulk relaxation,providing a basis for tailoring surface properties in metallic glasses.展开更多
Membranes with ultramicropores have shown great promise in gas separations.However,achieving high separation selectivity for gas with similar kinetic diameters,such as hydrogen(H_(2))and carbon dioxide(CO_(2)),remains...Membranes with ultramicropores have shown great promise in gas separations.However,achieving high separation selectivity for gas with similar kinetic diameters,such as hydrogen(H_(2))and carbon dioxide(CO_(2)),remains a challenge due to the difficulty in precisely controlling ultramicropores distribution.Herein,we developed a polybenzoxazine polymer-derived carbon molecular sieve(CMS)membrane with uniformly concentrated ultramicropores of 3.5Åto separate H_(2) and CO_(2).This membrane demonstrated a high selectivity of 67.6,with a H_(2) permeance of 118.7 GPU,outperforming most reported samples.The isosteric heat of adsorption(Q_(st))for CO_(2) in these ultramicropores reached 43.1 kJ·mol^(-1),approximately twice that of physical adsorption,indicating that CO_(2) were effectively confined within the concentrated ultramicropores.Permeation tests revealed that the activation energy for CO_(2) permeation in the ultramicropores was 11.3 kJ·mol^(-1),one order of magnitude higher than that for H_(2)(1.3 kJ·mol^(-1)).This significant difference in activation energy magnifies the difference of diffusion rate between H_(2) and CO_(2).The distinct behavior between the free surface diffusion of H_(2) and the activated surface diffusion of CO_(2) is the key to achieving a high H_(2)/CO_(2) separation performance.This discovery presents a promising approach for separating H_(2) and CO_(2) using CMS membranes.展开更多
The inner surface modification process by plasma-based low-energy ion implantation（PBLEII）with an electron cyclotron resonance（ECR）microwave plasma source located at the central axis of a cylindrical tube is model...The inner surface modification process by plasma-based low-energy ion implantation（PBLEII）with an electron cyclotron resonance（ECR）microwave plasma source located at the central axis of a cylindrical tube is modeled to optimize the low-energy ion implantation parameters for industrial applications.In this paper,a magnetized plasma diffusion fluid model has been established to describe the plasma nonuniformity caused by plasma diffusion under an axial magnetic field during the pulse-off time of low pulsed negative bias.Using this plasma density distribution as the initial condition,a sheath collisional fluid model is built up to describe the sheath evolution and ion implantation during the pulse-on time.The plasma nonuniformity at the end of the pulse-off time is more apparent along the radial direction compared with that in the axial direction due to the geometry of the linear plasma source in the center and the difference between perpendicular and parallel plasma diffusion coefficients with respect to the magnetic field.The normalized nitrogen plasma densities on the inner and outer surfaces of the tube are observed to be about 0.39 and 0.24,respectively,of which the value is 1 at the central plasma source.After a 5μs pulse-on time,in the area less than 2 cm from the end of the tube,the nitrogen ion implantation energy decreases from 1.5 keV to 1.3 keV and the ion implantation angle increases from several degrees to more than 40°;both variations reduce the nitrogen ion implantation depth.However,the nitrogen ion implantation dose peaks of about 2×10^（10）-7×10^（10）ions/cm^2 in this area are 2-4 times higher than that of 1.18×10^（10）ions/cm^2 and 1.63×10^（10）ions/cm^2 on the inner and outer surfaces of the tube.The sufficient ion implantation dose ensures an acceptable modification effect near the end of the tube under the low energy and large angle conditions for nitrogen ion implantation,because the modification effect is mainly determined by the ion implantation dose,just as the mass transfer process in PBLEII is dominated by low-energy ion implantation and thermal diffusion.Therefore,a comparatively uniform surface modification by the low-energy nitrogen ion implantation is achieved along the cylindrical tube on both the inner and outer surfaces.展开更多
Mechanical degradation, especially fractures in active particles in an electrode, is a major reason why the capacity of lithiumion batteries fades. This paper proposes a model that couples Li-ion diffusion, stress evo...Mechanical degradation, especially fractures in active particles in an electrode, is a major reason why the capacity of lithiumion batteries fades. This paper proposes a model that couples Li-ion diffusion, stress evolution, and damage mechanics to simulate the growth of central cracks in cathode particles（Li Mn_2 O_4） by an extended finite element method by considering the influence of multiple factors. The simulation shows that particles are likely to crack at a high discharge rate, when the particle radius is large, or when the initial central crack is longer. It also shows that the maximum principal tensile stress decreases and cracking becomes more difficult when the influence of crack surface diffusion is considered. The fracturing process occurs according to the following stages： no crack growth, stable crack growth, and unstable crack growth. Changing the charge/discharge strategy before unstable crack growth sets in is beneficial to prevent further capacity fading during electrochemical cycling.展开更多
Gas sorption and non-Darcy flow are two important issues for shale gas reservoirs. The sorption consists of dissolution and adsorption. Dissolved gas and adsorbed gas are different. The former is dissolved in the shal...Gas sorption and non-Darcy flow are two important issues for shale gas reservoirs. The sorption consists of dissolution and adsorption. Dissolved gas and adsorbed gas are different. The former is dissolved in the shale matrix, while the latter is concentrated near the solid walls of pores. In this paper, the Langmuir equation is used to describe adsorption and Henry’s law is used to describe dissolution. The K coefficient in Henry’s law of 0.052 mmol/(MPa g TOC) is obtained by matching experimental data. The amount of dissolved gas increases linearly when pressure increases. Using only the Langmuir equation without considering dissolution can lead to a significant underestimation of the amount of sorbed gas in shales. For non-Darcy gas flow, the apparent permeability model for free gas is established by combining slip flow and Knudsen flow. For adsorbed gas, the surface diffusion effect is also considered in this model. The surface diffu- sion coefficient is suggested to be of the same scale as the gas self-diffusion coefficient, and the corresponding effective permeability is derived. When 1/ increases,k/ kincreases, but the relationship is not linear as the Klinkenberg effect suggests. The effect of adsorption on the gas flow is significant in nanopores (r≤2 nm). Adsorption increases apparent permeability in shales at low pressures and decreases it at high pressures.展开更多
A modified two-sphere model of sintering neck has been proposed, wherein three diffusion mechanisms including surface diffusion, grain-boundary diffusion and coupled surface and grain-boundary diffusion are assumed. S...A modified two-sphere model of sintering neck has been proposed, wherein three diffusion mechanisms including surface diffusion, grain-boundary diffusion and coupled surface and grain-boundary diffusion are assumed. Sintering neck is appropriately simulated using the modified model. The dynamic change of sintering neck is presented using the simulation. The variational shape of sintering neck in surface diffusion mechanism is continuous, whereas in grain-boundary diffusion mechanism, besides the variational shape of sintering neck being continuous, the center distance between the particles is also assumed to contract. However, the variational shape of sintering neck in coupling diffusion mechanism is integrated using the two diffusion mechanisms mentioned above.展开更多
We perform molecular dynamics simulations of Lennard-Jones particles in a canonical ensemble to study the diffusion of nanodroplets on smooth solid surfaces. Using the droplet-surface interaction to realize a hydrophi...We perform molecular dynamics simulations of Lennard-Jones particles in a canonical ensemble to study the diffusion of nanodroplets on smooth solid surfaces. Using the droplet-surface interaction to realize a hydrophilic or hydrophobic surface and calculating the mean square displacement of the center-of-mass of the nanodroplets, the random motion of nanodroplets could be characterized by short- time subdiffusion, intermediate-time superdiffusion, and long-time normal diffusion. The short-time subdiffusive exponent increases and almost reaches unity （normal diffusion） with decreasing droplet size or enhancing hydrophobicity. The diffusion coefficient of the droplet on hydrophobic surfaces is larger than that on hydrophilie surfaces.展开更多
基金Project (2011JY009) supported by Education Department of Shaanxi Province, China
文摘Specimens of ZM5 magnesium alloy were dipped into the mixed powder of Al and Zn at （390±5） ℃ for 8 h in argon gas protective environment and subjected to surface diffusion alloying processing （SDAP）. The erosion wear behaviors of ZM5 magnesium alloy before and after SDAP were investigated in two different erosion wear environments： oil and quartz environment; tap water and quartz environment. The surfaces of erosion wear specimens exhibited cutting scratch grooves in the oil and quartz environment. Corrosive attack was weak and cutting wear mechanism was responsible for the mass loss. When the erosion wear medium was changed to tap water and quartz environment, corrosion pits and cracks were obviously observed after erosion wear test. The corrosion from tap water and the scour from quartz intensified mass losses. Compared with the untreated specimens, the application of SDAP improved the erosion wear resistance in the same environment.
基金supported by the Natural Science Foundation of Jiangsu Province of China (No.BK20141407)the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions
文摘Based on the weak formulation for combined surface diffusion and evaporation-condensation,a governing equation of the finite element is derived for simulating the evolution of intergranular microcracks in copper interconnects induced simultaneously by stressmigration,electromigration and thermomigration.Unlike previously published works,the effect of thermomigration is considered.The results show that thermomigration can contribute to the microcrack splitting and accelerate the drifting process along the direction of the electric field.The evolution of the intergranular microcracks depends on the mechanical stress field,the temperature gradient field,the electric field,the initial aspect ratio and the linewidth.And there exists a critical electric fieldχ_c,a critical stress field■,a critical aspect ratioβ_c and a critical linewidth■.When■or■,the intergranular microcrack will split into two or three small intergranular microcracks.Otherwise,the microcrack will evolve into a stable shape as it migrates along the interconnect line.The critical stress field,the critical electric field and the critical aspect ratio decrease with a decrease in the linewidth,and the critical linewidth increases with an increase in the electric field and the aspect ratio.The increase of the stress field,the electric field or the aspect ratio and the decrease of the linewidth are not only beneficial for the intergranular microcrack to split but also accelerate the microcrack splitting process.
基金supported by the Natural Science Foundation of Jiangsu Province of China (No. BK20141407)the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions。
文摘Based on the bulk free energy density and the degenerate mobility constructed by the quartic double-well potential function,a phase field model is established to simulate the evolution of intragranular microvoids due to surface diffusion in a stress field.The corresponding phase field governing equations are derived.The evolution of elliptical microvoids with different stressesΛ,aspect ratiosβand linewidths hˉis calculated using the mesh adaptation finite element method and the reliability of the procedure is verified.The results show that there exist critical values of the stressΛc,the aspect ratioβc and the linewidth hˉc of intragranular microvoids under equivalent biaxial tensile stress.When Λ≥Λ_(c),β≥β_(c) or h≤h_(c),the elliptical microvoids are instable with an extending crack tip.WhenΛ<Λ_(c),β<β_(c) or hˉ>h_(c),the elliptical microvoids gradually cylindricalize and remain a stable shape.The instability time decreases with increasing the stress or the aspect ratio,while increases with increasing the linewidth.In addition,for the interconnects containing two elliptical voids not far apart,the stress will promote the merging of the voids.
基金Project supported by the National High Technology Research and Development Program of China (Grant No 2009AA03Z405)the National Natural Science Foundation of China (Grant No 60644004)
文摘Depositions of Si, Ge and C atoms onto a preliminary Si （001） substrate at different temperatures are investigated by using the molecular dynamics method. The mechanism of atomic self-assembling occurring locally on the flat terraces between steps is suggested. Diffusion and arrangement patterns of adatoms at different temperatures are observed. At 900 K, the deposited atoms are more likely to form dimers in the perpendicular [110] direction due to the more favourable movement along the perpendicular [110] direction. C adatoms are more likely to break or reconstruct the dimers on the substrate surface and have larger diffusion distances than Ge and Si adatoms. Exchange between C adatoms and substrate atoms are obvious and the epitaxial thickness is small. Total potential energies of adatoms and substrate atoms involved in the simulation cell are computed. When a newly arrived adatom reaches the stable position, the potential energy of the system will decrease and the curves turns into a ladder-like shape. It is found that C adatoms can lead to more reduction of the system energy and the potential energy of the system will increase as temperature increases.
文摘Molecular dynamics simulation employing the embedded atom method potential is utilized to investigate nanoscale surface diffusion mechanisms of binary heterogeneous adatoms clusters at 300 K, 500 K, and 700 K. Surface diffusion of heterogeneous adatoms clusters can be vital for the binary island growth on the surface and can be useful for the formation of alloy-based thin film surface through atomic exchange process. The results of the diffusion process show that at 300 K, the diffusion of small adatoms clusters shows hopping, sliding, and shear motion; whereas for large adatoms clusters（hexamer and above）, the diffusion is negligible. At 500 K, small adatoms clusters, i.e., dimer, show almost all possible diffusion mechanisms including the atomic exchange process; however no such exchange is observed for adatoms clusters greater than dimer. At 700 K, the exchange mechanism dominates for all types of clusters, where Zr adatoms show maximum tendency and Ag adatoms show minimum or no tendency toward the exchange process. Separation and recombination of one or more adatoms are also observed at 500 K and 700 K. The Ag adatoms also occupy pop-up positions over the adatoms clusters for short intervals. At 700 K, the vacancies are also generated in the vicinity of the adatoms cluster,vacancy formation, filling, and shifting can be observed from the results.
基金Project supported by the National Natural Science Foundation of China(No.10071067)and the Jiangsu Provincial Natural Science Foundation of China.
文摘Consider the motion of immersed hypersurfaces driven by surface diffusion flow and give anlower bound on the life span of a smooth immersed solution, which depends only on how muchthe curvature of the initial surface is concentrated in space.
基金ACKNOWLEDGMENT This work was supported Natural Science Foundation by the Hunan Provincial of China （No.06JJ2100）.
文摘The diffusion dynamics of small two-dimensional atomic clusters Cux （1≤x≤8） on Cu（lll） surface were studied using the molecular dynamics simulations and a modified analytic embedded-atom method in the temperature range from 200 K to 800 K. The cluster size and temperature dependence of the diffusion coefficients and migration energies are presented. Our simulations show that the diffusion migration energy of the CuT cluster is the highest and the prefactor for the CuT cluster is almost three orders of magnitude larger than that for single atom diffusion. This conclusion is consistent with the experimental results for similar metals. In addition, the dependence of cluster diffusion on film growth is also discussed.
基金Project supported by "973 Project" of Ministry of Science and Technology of China (Grant No. 2006CB605102)
文摘A detailed first-principles study of the diffusion behaviour of point defects in the O-terminated （0001） surface in wnrtzite ZnO was performed. The 0 vacancy and interstitial are found to diffuse much more easily in surface than in bulk. The Zn vacancy has a similar migration barrier for both bulk and surface, but has much smaller barrier for the diffuse-in process. The Zn interstitial is difficult to diffuse in the surface directly, but it can diffuse into the bulk relatively easily. Specific values of corresponding migration barriers are obtained.
基金supported by NSF DMS-0511411(X.L.)NSF DMS-0511169(Q.N.).
文摘The surface diffusion of an axi-symmetric solid,a whisker,subject to applied uniaxial stress,is studied numerically based on a new boundary integral formulation for periodic stress configurations.An efficient semi-implicit time-stepping scheme is developed to treat the serve stiffness due to high-order derivatives.When the initial perturbation is small the effect of the stress on the motion of the whisker is found to agree with the linear stability analysis.Numerical simulations of a fully nonlinear case are also presented,and a potential break-up of the whisker is observed.
文摘During heteroepitaxial overlayer growth multiple crystal domains nucleated on a substrate surface compete with each other in such a manner that a domain covered by neighboring ones stops growing.The number density of active domains ρ decreases as the height h increases.A simple scaling argument leads to a scaling law of ρ~ h^（-γ） with a coarsening exponent γ=d/z,where d is the dimension of the substrate surface and z the dynamic exponent of a growth front.This scaling relation is confirmed by performing kinetic Monte Carlo simulations of the ballistic deposition model on a two-dimensional（d=2） surface,even when an isolated deposited particle diffuses on a crystal surface.
基金Project supported by the National Natural Science Foundation of China （Grant No. 60866001）, the Special Project for Senior Researcher of Guizhou Organization Department （Grnat No. TZJF 2006.10）, Doctor Foundation of Guizhou University, the Innovation Fund of Guizhou University （Grant No. 2011008）, the Science and Technological Project for Scholar Abroad, Guizhou Province （Grant No. [2007]03）, and the Guizhou Science and Technology Foundation （Grant No. J[200712176）.
文摘Anisotropic evolution of the step edges on the compressive-strained In0.2Ga0.8As/GaAs（001） surface has been investigated by scanning tunneling microscopy （STM）. The experiments suggest that step edges are indeed sinuous and protrude somewhere a little way along the [110] direction, which is different from the classical waviness predicted by the theoretical model. We consider that the monatomic step edges undergo a morphological instability induced by the anisotropic diffusion of adatoms on the terrace during annealing, and we improve a kinetic model of step edge based on the classical Burton Cabrer-Frank （BCF） model in order to determine the normal velocity of step enlargement. The results show that the normal velocity is proportional to the arc length of the peninsula, which is consistent with the first result of our kinetic model. Additionally, a significant phenomenon is an excess elongation along the [110] direction at the top of the peninsula with a higher aspect ratio, which is attributed to the restriction of diffusion lengths.
基金supported by the Natural Science Foundation of Fujian Province,China(Grant No.2014J01025)the National Natural Science Foundation of China(Grant No.11275261)+1 种基金the Natural Science Foundation of Guangdong Province,China(Grant No.2015A030313005)the Fund from the Fujian Provincia Key Laboratory for Plasma and Magnetic Resonance,China
文摘A stable and homogeneous well-aligned air microplasma device for application at atmospheric pressure is designed and its electrical and optical characteristics are investigated. Current-voltage measurements and intensified charge coupled device （ICCD） images show that the well-aligned air microplasma device is able to generate a large-area and homogeneous discharge at the applied voltages ranging from 12 kV to 14 kV, with a repetition frequency of 5 kHz, which is attributed to the diffusion effect of plasma on dielectric surface. Moreover, this well-aligned microplasma device may result in the uniform and large-area surface modification of heat-sensitive PET polymers without damage, such as optimization in hydrophobicity and biocompatibility. In the biomedical field, the utility of this well-aligned microplasma device is further testified. It proves to be very efficient for the large-area and uniform inactivation of E. coli cells with a density of 103/cm2 on LB agar plate culture medium, and inactivation efficiency can reach up to 99% for 2-min treatment.
文摘The adsorption dynamics for phenol in aqueous solution of the adsorbent based on polystyrene was studied. In order to distinguish with the Boyd quasi-homogeneous model of the inner structure of ion-exchanger, the particle diffusion model including surface diffusion model and pore diffusion model was suggested which is suitable to the macroporous adsorbent. The diffusiondetermination step of the adsorption process was established and the effective diffusion coefficient was also determined. The influence of surface diffusion and pore diffusion on the particle diffusion rate was investigated qualitatively. All of these were very important to improve the structure of the macroporous adsorbent in order to improve the mass-transfer rate.
基金supported by the National Key R&D Program of China(Grant No.2024YFB3813700)the National Natural Science Foundation of China(Grant Nos.52471187,52201194,52222105,92163108,and 52231006)+2 种基金3315 Innovation Youth Talent in Ningbo City(Grant No.2021A123G)the Zhejiang Provincial Natural Science Foundation of China(Grant No.LZY23E010002)the Natural Science Foundation of Ningbo City(Grant No.2023J342).
文摘Atomic surface mobility of glasses plays an important role in understanding glass dynamics and determining many fundamental processes on the surface.However,the diffusion dynamics at the free surface in marginal glasses remains unknown due to limited glass formation ability.In this study,we systematically investigate surface diffusion and relaxation behavior in four marginal glass-forming Fe-based metallic glasses with great application potential.Surface diffusion rates in marginal glassforming Fe-based metallic glasses are significantly faster than those of stable metallic glasses.For the first time,an abnormal β_(t) relaxation mode with thermal activation character is identified betweenαandβrelaxation.Strikingly,the activation energy of surface diffusion matches that ofβt relaxation.A mechanism involving cooperative cluster motion associated withβt relaxation is proposed to explain the ultrafast surface diffusion.These results establish a direct correlation between surface diffusion and bulk relaxation,providing a basis for tailoring surface properties in metallic glasses.
基金supported by the National Key Research and Development Program of China(No.2021YFA1500300)the National Natural Science Foundation of China(No.22275027).
文摘Membranes with ultramicropores have shown great promise in gas separations.However,achieving high separation selectivity for gas with similar kinetic diameters,such as hydrogen(H_(2))and carbon dioxide(CO_(2)),remains a challenge due to the difficulty in precisely controlling ultramicropores distribution.Herein,we developed a polybenzoxazine polymer-derived carbon molecular sieve(CMS)membrane with uniformly concentrated ultramicropores of 3.5Åto separate H_(2) and CO_(2).This membrane demonstrated a high selectivity of 67.6,with a H_(2) permeance of 118.7 GPU,outperforming most reported samples.The isosteric heat of adsorption(Q_(st))for CO_(2) in these ultramicropores reached 43.1 kJ·mol^(-1),approximately twice that of physical adsorption,indicating that CO_(2) were effectively confined within the concentrated ultramicropores.Permeation tests revealed that the activation energy for CO_(2) permeation in the ultramicropores was 11.3 kJ·mol^(-1),one order of magnitude higher than that for H_(2)(1.3 kJ·mol^(-1)).This significant difference in activation energy magnifies the difference of diffusion rate between H_(2) and CO_(2).The distinct behavior between the free surface diffusion of H_(2) and the activated surface diffusion of CO_(2) is the key to achieving a high H_(2)/CO_(2) separation performance.This discovery presents a promising approach for separating H_(2) and CO_(2) using CMS membranes.
基金supported by National Natural Science Foundation of China(Nos.50725519,51271048,51321004)
文摘The inner surface modification process by plasma-based low-energy ion implantation（PBLEII）with an electron cyclotron resonance（ECR）microwave plasma source located at the central axis of a cylindrical tube is modeled to optimize the low-energy ion implantation parameters for industrial applications.In this paper,a magnetized plasma diffusion fluid model has been established to describe the plasma nonuniformity caused by plasma diffusion under an axial magnetic field during the pulse-off time of low pulsed negative bias.Using this plasma density distribution as the initial condition,a sheath collisional fluid model is built up to describe the sheath evolution and ion implantation during the pulse-on time.The plasma nonuniformity at the end of the pulse-off time is more apparent along the radial direction compared with that in the axial direction due to the geometry of the linear plasma source in the center and the difference between perpendicular and parallel plasma diffusion coefficients with respect to the magnetic field.The normalized nitrogen plasma densities on the inner and outer surfaces of the tube are observed to be about 0.39 and 0.24,respectively,of which the value is 1 at the central plasma source.After a 5μs pulse-on time,in the area less than 2 cm from the end of the tube,the nitrogen ion implantation energy decreases from 1.5 keV to 1.3 keV and the ion implantation angle increases from several degrees to more than 40°;both variations reduce the nitrogen ion implantation depth.However,the nitrogen ion implantation dose peaks of about 2×10^（10）-7×10^（10）ions/cm^2 in this area are 2-4 times higher than that of 1.18×10^（10）ions/cm^2 and 1.63×10^（10）ions/cm^2 on the inner and outer surfaces of the tube.The sufficient ion implantation dose ensures an acceptable modification effect near the end of the tube under the low energy and large angle conditions for nitrogen ion implantation,because the modification effect is mainly determined by the ion implantation dose,just as the mass transfer process in PBLEII is dominated by low-energy ion implantation and thermal diffusion.Therefore,a comparatively uniform surface modification by the low-energy nitrogen ion implantation is achieved along the cylindrical tube on both the inner and outer surfaces.
基金support of the National Natural Science Foundation of China (11472165 and 11332005)
文摘Mechanical degradation, especially fractures in active particles in an electrode, is a major reason why the capacity of lithiumion batteries fades. This paper proposes a model that couples Li-ion diffusion, stress evolution, and damage mechanics to simulate the growth of central cracks in cathode particles（Li Mn_2 O_4） by an extended finite element method by considering the influence of multiple factors. The simulation shows that particles are likely to crack at a high discharge rate, when the particle radius is large, or when the initial central crack is longer. It also shows that the maximum principal tensile stress decreases and cracking becomes more difficult when the influence of crack surface diffusion is considered. The fracturing process occurs according to the following stages： no crack growth, stable crack growth, and unstable crack growth. Changing the charge/discharge strategy before unstable crack growth sets in is beneficial to prevent further capacity fading during electrochemical cycling.
基金supported by the Department of Energy under Award Number DE-FE0024311
文摘Gas sorption and non-Darcy flow are two important issues for shale gas reservoirs. The sorption consists of dissolution and adsorption. Dissolved gas and adsorbed gas are different. The former is dissolved in the shale matrix, while the latter is concentrated near the solid walls of pores. In this paper, the Langmuir equation is used to describe adsorption and Henry’s law is used to describe dissolution. The K coefficient in Henry’s law of 0.052 mmol/(MPa g TOC) is obtained by matching experimental data. The amount of dissolved gas increases linearly when pressure increases. Using only the Langmuir equation without considering dissolution can lead to a significant underestimation of the amount of sorbed gas in shales. For non-Darcy gas flow, the apparent permeability model for free gas is established by combining slip flow and Knudsen flow. For adsorbed gas, the surface diffusion effect is also considered in this model. The surface diffu- sion coefficient is suggested to be of the same scale as the gas self-diffusion coefficient, and the corresponding effective permeability is derived. When 1/ increases,k/ kincreases, but the relationship is not linear as the Klinkenberg effect suggests. The effect of adsorption on the gas flow is significant in nanopores (r≤2 nm). Adsorption increases apparent permeability in shales at low pressures and decreases it at high pressures.
文摘A modified two-sphere model of sintering neck has been proposed, wherein three diffusion mechanisms including surface diffusion, grain-boundary diffusion and coupled surface and grain-boundary diffusion are assumed. Sintering neck is appropriately simulated using the modified model. The dynamic change of sintering neck is presented using the simulation. The variational shape of sintering neck in surface diffusion mechanism is continuous, whereas in grain-boundary diffusion mechanism, besides the variational shape of sintering neck being continuous, the center distance between the particles is also assumed to contract. However, the variational shape of sintering neck in coupling diffusion mechanism is integrated using the two diffusion mechanisms mentioned above.
基金This work was supported by the National Natural Science Foundation of China under Grant Nos. 11675008 and 21434001.
文摘We perform molecular dynamics simulations of Lennard-Jones particles in a canonical ensemble to study the diffusion of nanodroplets on smooth solid surfaces. Using the droplet-surface interaction to realize a hydrophilic or hydrophobic surface and calculating the mean square displacement of the center-of-mass of the nanodroplets, the random motion of nanodroplets could be characterized by short- time subdiffusion, intermediate-time superdiffusion, and long-time normal diffusion. The short-time subdiffusive exponent increases and almost reaches unity （normal diffusion） with decreasing droplet size or enhancing hydrophobicity. The diffusion coefficient of the droplet on hydrophobic surfaces is larger than that on hydrophilie surfaces.
