Diffusion and clustering on the titanium (0001) surface

In this paper, we present a study of diffusion and clustering of atoms on the titanium (0001) surface using molecular dynamics and molecular statics methods. The formation energy of each defect cluster is calculated using a combination of annealing and quenching techniques. Diffusion mechanisms of clusters are elucidated by the analysis of atomic trajectories, and are confirmed by calculating energy states along the diffusion path and by fitting the diffusion coefficients to one or more Arrhenius functions. Our simulations show that the clusters diffuse by a sequence of atomic jumps; once an atom moves forward by half an atomic diameter, the others follow it. Such a correlated process involves an activation energy that is comparable to that of an adatom. Atoms in each cluster, especially those in clusters consisting of three and seven atoms, are tightly bound. However, due to high mobility, the clusters are less likely to be the critical nuclei in three-dimensional growth of thin films on the (0001) surface. Even the cluster consisting of seven atoms swaps a linear distance of 60 nm during the growth of one monolayer of thin film under typical deposition conditions and at room temperature, making it ineffective as a critical nucleus above room temperature. This revised version was published online in July 2006 with corrections to the Cover Date.     

碳纳米管内气体吸附与扩散的分子模拟研究

气体在纳米管中的输运具有非连续性的特点,这与宏观条件下稀薄气体的输运类似,同时考虑到纳米管的比表面积非常大,那么吸附气体对输运的贡献就不可忽略。本文基于平衡分子动力学和巨正则蒙特卡洛方法,模拟了不同气体分子在不同直径碳纳米管内的扩散和吸附行为。运用通量相关函数计算了气体的输运扩散系数,并结合稀薄气体动力学推导出了气体的表面扩散系数。结果表明,气体在碳纳米管中的输运扩散系数比克努森公式计算出的结果高数倍甚至十数倍,碳纳米管内吸附分子的表面扩散是气体快速输运的原因,表面扩散系数随表面覆盖度的增大而增大。     

分子动力学模拟Gd原子在Cu低密勒表面的扩散过程

为了分析Gd吸附原子在Cu（001）、Cu（110）和Cu（111）表面的扩散机制，本文用分子动力学对该扩散过程进行模拟。模拟结果表明在Cu（001）和Cu（111）表面，Gd原子通过跳跃机制扩散；在Cu（110）表面，在[110]方向Gd原子通过跳跃机制扩散，而且多步跳跃频率很高，而在[001]方向则通过交换机制扩散。通过对扩散频率的拟合，发现在各种扩散机制都符合Arrhenius公式，从而确定了在Cu（001）和Cu（111）表面扩散势垒分别为0．19eV和0．013eV，在Cu（110）表面，沿[110]方向跳跃扩散势垒和沿[001]方向交换扩散势垒分别为0．097eV和0．33eV。     

The effects of different precipitant agents on the formation of alumina-magnesia composite powders as the magnesium aluminate spinel precursor

Al₂O₃/MgO composite powders were synthesized via a partially wet chemical method. The effects of precipitant agent on the morphology, size and chemical composition of the resultant powders were investigated. The structures of rod-like with polygonal prism surface, platelet-like and uniform spherical Mg-compound particles were successfully prepared by using ammonium hydrogen carbonate, sodium hydroxide and ammonia water as precipitant agents, respectively. Analysis results proved that using ammonium hydrogen carbonate as precipitant agent produced nesquehonite (MgCO₃·3H₂O) Mg-compounds but in the case of other two precipitant agents, Mg-compound particles with magnesium hydroxide (Mg(OH)₂) chemical composition were obtained. The morphological features of MgO particles in Al₂O₃/MgO composite powders were the same as individual Mg-compound particles. Furthermore, conversion of the Al₂O₃/Mg-compound precursor synthesized with ammonia water to pure magnesium aluminate spinel particles was studied. The precursor converted to pure magnesium aluminate spinel phase with 220?nm particle size at 1200?°C.     

Surface self-diffusion of adatom on Pt cluster with truncated octahedron structure

Surface diffusion of single Pt adatom on Pt cluster with truncated octahedron structure is investigated through a combination of molecular dynamics and nudged elastic band method. Using an embedded atom method to describe the atomic interactions, the minimum energy paths are determined and the energy barriers for adatom diffusion across and along step are evaluated. The diffusion of adatom crossing step edge between {111} and {100} facets has a surprisingly low barrier of 0.03 eV, which is 0.12 eV lower than the barrier for adatom diffusion from {111} to neighboring {111} facet. Owing to the small barrier of adatom diffusion across the step edge between {111} and {100} facets, the diffusion of adatom along the step edge cannot occur. The molecular dynamics simulations at low temperatures also support these results. Our results show that mass transport will prefer step with {100} microfacet and the Pt clusters can have only {111} facets in epitaxial growth.     

Controllable degradation of biomedical magnesium by chromium and oxygen dual ion implantation

The surface stability of biodegradable magnesium is crucial to tissue growth on implants in the initial healing stage. Inspired by the design principle of biomedical stainless steels, we implant chromium as a passive element into pure magnesium to alter the surface biodegradation behavior. However, because Cr exists in the metallic state in the implanted layer to induce the galvanic effect, excessively fast degradation is observed. Ensuing implantation of oxygen produces a thicker surface oxidized layer composed of chromium oxide, which successfully retards the surface degradation of pure magnesium. The dual implantation process offers a promising means to improve the initial surface stability of Mg in the physiological environment.     

Insights into the structures,energies and electronic properties of nesquehonite surfaces by first-principles calculations

By using the first-principles calculations, the structure, energies and electronic properties of four commonly exposed surfaces for the nesquehonite crystal were investigated. The needle-like nesquehonite whisker is well developed with smooth side faces and irregular hexagonal end faces. Surface energy results indicate that the (1 0 1) surface is the most stable surface and corresponds to the side face. The density of dangling bond has a positive relationship with surface energy and the (1 0 1) surface has the least dangling bonds. In terms of relaxed surface energy, the order of relaxed surfaces is (1 0 1) < (2 0 0)-H < (3 0 1) < (2 0 0)-M < (0 0 4). During surface relaxation, the changes in the length of Mg-O bonds and hydrogen bonds contribute to generating a more stable surface with a lower surface energy. The PDOS (partial density of states) of these surfaces are mainly dominated by Mg and O atoms. A small peak value is found in the PDOS of (1 0 1) and (3 0 1) surfaces, which have less exposed Mg-O bonds. Electron transfer causes changes in the length of Mg-O bonds. A more active surface will obtain a larger value of transferred electrons during surface relaxation.     

扩渗时间对纯镁表面扩渗合金层形成行为的影响

研究了扩渗温度为390℃条件下，扩散时间对纯镁扩渗合金层的显微特征影响。结果表明：随着扩渗时间从4h-8h-12h，表面合金层逐渐由Al6Mg10Zn发展为Al5Mg11Zn4，随着扩渗时间的延长，Al5Mg11Zn4较Al6Mg10Zn更为稳定；依据所发现的反应层组成和结构特征，提出了Zn作为主控元素的概念。     

Laser-assisted maskless microdeposition of silver nano-particles on a magnesium substrate

Laser-assisted maskless microdeposition and silver nano-particles are used to create micro-scale patterns with nano-scale topography on a magnesium substrate for potential biomedical applications. By controlling the processing parameters, various patterns and properties are achieved. The electron microscopy analysis of a sample prepared by focused ion beam microscope provides evidence for excellent bounding and effective surface alloying across the interface.     

Molecular dynamics description of silver adatom diffusion on Ag(1 0 0) and Ag(1 1 1) surfaces

The self-diffusion processes of single adatoms on Ag(1 0 0) and Ag(1 1 1) surfaces have been studied using molecular-dynamics simulations and a many-body potential derived in the framework of the second-moment approximation to the tight-binding model. Our results for the (1 0 0) surface indicate that, although the migration energy for hopping is lower than that of the exchange mechanism, the exchange diffusion is higher than hopping diffusion for temperatures above 600 K. The migration energy for the hopping mechanism is in very good agreement with the experiment and the results of ab initio calculations. We also find that for the Ag(1 1 1) face the dominant mechanism is the hopping, which exhibit Arrhenius behaviour with two distinct temperature ranges, corresponding to two different migration energies. The diffusion in the high temperature region is mainly due to correlated jumps requiring an activation energy which is in excellent agreement with the experimental data. In addition the temperature dependence of the mean-square-displacements and the relaxations of both surface atoms and adatoms are presented and compared with previous studies.     

Effect of silicon on damping capacities of pure magnesium and magnesium alloys

A new idea of using a stiffer metallic compound Mg₂Si to enhance the damping capacities of pure magnesium and its alloys is successfully attempted. The present paper focuses on the relation between damping capacities and the addition amount of Si. The results show that damping capacities increase with increasing amount of Si. Particular emphasis is placed on the increasing dislocation density around matrix-particulate interface and the refinement of grain size promoted by big growth restriction factor of Si.     

Cluster diffusion and surface morphological transitions on Pt (111) via reptation and concerted motion

Embedded-atom molecular dynamics simulations were used to follow the diffusion dynamics of compact Pt clusters with up to 19 atoms on Pt (111) surfaces. The results reveal a novel cluster diffusion mechanism, involving successive shear translations of adjacent subcluster regions, which give rise to reptation, a snake-like gliding motion. We show that for compact clusters with 4 to 6 atoms, this mechanism competes energetically with that of island diffusion through concerted motion. However, as the cluster size increases from >7 to 20 atoms, reptation becomes the energetically favored diffusion mechanism. The concerted shear motion of subcluster regions, leading to reptation, is also shown to play a significant role in dendritic-to-compact morphological transitions of Pt islands.     

Influence of carboxyl group formation on ammonia adsorption of NiO-templated nanoporous carbon surfaces

The scope of this work was to control the surface functional groups of nanoporous carbons (NPs) by oxidizing agents (nitric acid and hydrogen peroxide) treatments and to investigate the relation between carboxyl group and ammonia removal efficiency. The NPs were directly prepared from a cation exchange resin by the carbonization of a mixture with Ni acetate at 900 °C. N₂/-196 °C adsorption, Boehm's titrations, and X-ray photoelectron spectroscopy (XPS) analyzes were employed to confirm the physicochemical properties of NPs. The ammonia removal efficiency was confirmed by temperature programmed desorption (TPD) technique. In the result, the oxygen content of NPs increased after various treatments and the highest content of carboxyl group formation appeared at a 2:3 volume ratio of HNO₃/H₂O₂. It was also found that the oxidation treatment led to an increase in ammonia removal efficiency of NPs, mainly due to an increase of acid oxygen functional groups (such as carboxyl) on NPs surfaces.     

The properties of gas tungsten arc deposited SiCP and Al surface coating on magnesium alloy AZ31

SiC particles and aluminum powders were used to deposit on the surface of magnesium alloy AZ31 by pulse square-wave alternating current gas tungsten arc (GTA) processing. This method is an effective technique in producing a high performance surface modified composite layer. The microstructure, microhardness, wear resistance and corrosion behavior of the GTA surface modified composite layer were evaluated. It was proved that no reaction products were formed at the SiC-matrix interface and no melting or dissolution of the SiC particle occurred during GTA surface modification. The microhardness of GTA surface modified composite layer was between 100 and 150 HV according to the variation of the GTA processing parameters. The microhardness, wear resistance and corrosion behavior of the GTA surface modified layer were superior to that of the as-received AZ31. The optimum processing parameters of the GTA surface modification of magnesium alloy AZ31 with SiC + Al for the formation of a homogeneous crack/defect-free and grain refinement microstructure were established.     

AZ91D镁合金阳极氧化与热扩散渗铝复合膜层的研究

将阳极氧化与热扩散相结合对AZ91D镁合金表面进行低温渗铝处理,利用扫描电镜、光学显微镜和X射线衍射仪对所得膜层的形貌、相结构及耐腐蚀性能进行了研究,并对复合膜层改善镁合金基体耐腐蚀性能的过程进行了分析.研究表明,该复合膜层是由MgO和金属间化合物Mg17Al12以及被γ相(Mg17Al12)包裹住的α-Mg晶粒共同组成;AZ91D镁合金表面的阳极氧化和热扩散渗铝复合膜层能显著提高镁合金基体材料的耐腐蚀性能.     

Fabrication of hydrophobic/super-hydrophobic nanofilms on magnesium alloys by polymer plating

Hydrophobic/super-hydrophobic nanofilms with improved corrosion resistance were fabricated on the surfaces of Mg-Mn-Ce magnesium alloy by a surface modification technique, named as polymer plating, which has been developed to modify superficial characteristics of magnesium alloys with polymeric nanofilms through synthesized organic compounds of triazine dithiol containing functional groups. The nanofilms were prepared by the electrochemical and polymerization reactions during polymer plating analyzed from characteristics of Fourier transform infrared spectrophotometer, X-ray photoelectron spectroscopy and scanning electron microscopy. The fabricated nanofilms changed the surface wettability of blank magnesium alloy from hydrophilic to hydrophobic with contact angle 119.0° of distilled water with lower surface free energy of 20.59 mJ/m² and even super-hydrophobic with contact angle 158.3° with lowest surface free energy of 4.68 mJ/m² by different functional nanofilms on their surfaces. Alteration of wettability from hydrophilic to hydrophobic and super-hydrophobic resulted from their low surface free energy and surface morphology with micro- and nano-rough structures. The corrosion behaviors from potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) show that the super-hydrophobic nanofilm has higher corrosion resistance and stability in 0.1 mol/L NaCl solution and lower corrosion current density (I_corr) with R_ct increasing two orders of magnitude of 16,500 Ω·cm² compared to that obtained for blank of 485 Ω·cm².     

The Influence of the Interaction Potential on the Diffusion of Neutral Particles in Oxyfluoride Glasses

A systematic study of the effects of the interaction potential upon the diffusion coefficients of neutral particles (P0s) and ions in the 60GeO₂–40PbF₂ glass has been performed using molecular dynamics simulations. The P0–P0 and P0–ions pair potentials were described by a Lennard–Jones potential with two parameters: the well depth (ε) and the size (σ). These parameters were varied in the ranges: (σ) = 0.1-1.0 eV and σ=1.0−4.0 ?. The diffusion coefficients were calculated for several combinations at several temperatures (300–1200 K). The estimated glass transition temperature (T _g) presented a slight dependence upon the potential parameters, but it is near the crystallization temperature of a glass of similar composition. Whereas, the diffusion coefficients of the P0s presented a strong dependence with the potential parameters, where increasing ε and σ causes a decrease of their diffusion coefficients. These results suggest that the atoms should be responsible for forming any surface film, since neutral particles larger than atom, such as, nanoparticles, should have negligible diffusion.     

Pore formation during hybrid laser-tungsten inert gas arc welding of magnesium alloy AZ31B—mechanism and remedy

One of the major concerns during high speed welding of magnesium alloys is the presence of porosity in the weld metal that can deteriorate mechanical properties. This study seeks to analyze the presence method and quantity of pore during hybrid laser-tungsten inert gas arc (TIG) welding of magnesium alloy AZ31B by radiography, optical microscopy and electron probe microanalysis (EMPA). At the same time, it identifies both the mechanism of pore formation and a remedy for this problem. The experimental results indicate that lacking of shielding gas for laser beam is the dominant cause of macroporosity formation during the hybrid of laser-TIG welding of magnesium Alloys AZ31B plate, and hydrogen is not main cause to form large pores. A favorable weld without porosity can be obtained by appending lateral shielding gas for laser beam.     

Gradient microstructure and enhanced mechanical performance of magnesium alloy by severe impact loading

Subjecting a workpiece to a surface treatment with severe impact loading is a novel severe plastic deformation procedure to fabricate gradient microstructures through the thickness and longitudinal direction.Mechanical performance is a function of twin density and the newly-formed grain size gradients.{1012}tensile twins created from processing without excessive grain refinement lead to strength enhancement with retained ductility.Creation of residual strain by a single impact results in a significant reduction in time and cost of the process.This paper investigates the effect of applying severe impact loading on mechanical and microstructural properties of magnesium for various impact velocities.     

Effect of impurities and electrolyte thickness on degradation of pure magnesium: A finite element study

The aim of this work is to study the degradation of magnesium due to the presence of impurities, by finite element method (FEM), when different thickness of physiological medium bathes the surface. The electrochemical experimental data obtained from polarization curves are used to model mathematically the corrosion process by solving the Laplace equation and the proper boundary conditions by means of FEM. The results show that when Mg is covered by a thin film of electrolyte, galvanic corrosion is focused only on the areas located really close to the cathodic sites, and far from the impurities, the Mg matrix remains near to its corrosion potential with a natural corrosion process. However, if the Mg matrix is completely covered by a thick layer of electrolyte the potentials obtained in the Mg surface far from the impurity are higher than its corrosion potential, so the Mg suffers more severe galvanic corrosion. On the other hand, when a higher number of impurities is considered, the Mg matrix is anodically polarized and it suffers severe galvanic corrosion, independently of h. The thickness of the electrolyte h must be considered as an important variable that affects the in vivo degradation.