Papers by Keyword: Surface Diffusion

Abstract: This paper presents a level set framework for the modelling of doping effect during surfacediffusion phenomena in a granular packing. The molecular flux of the doped compound is related tothe chemical potentials of all the diffusion species. The evolution of the grain compact is simulatedin three dimensions, based on the resulting kinetic law relating the surface diffusion velocity to thethermodynamic driving force. An anisotropic adaptive mesh, based on the level set function propertiesis used to refine the mesh in the surroundings of the grain surface. The simulations have been perfomedby using parallel computing strategy.

Abstract: Within the general context of solid-state sintering process, this work presents a numericalmodelling approach, at the grain scale, of ceramic grain packing consolidation. Typically, the sinteringprocess triggers several matter diffusion routes that are thermally activated: surface, grain boundaryand volume diffusions. Including this physics into a high-performance computing framework wouldpermit to investigate and to track the changes occurring into a granular packing during sintering. Inperforming this kind of simulations, one will face several challenges: the strong topological changesappear during sintering simulation at the grains scale, the evolution of the structure is mainly drivenby the surface tension phenomena through the Laplace's law, and the mechanical properties of thegrains could, possibly, be different. The proposed numerical simulations are carried out within anEulerian Finite Element framework and the Level-Set method is used to cope with changes in themicrostructure. The results obtained with this numerical strategy are compared with success to theusual geometrical models.

Abstract: The creep deformation in pure aluminum was investigated using helicoid spring samples at room temperature, 298 K, and σ < 1.19 MPa. It was found that the stress exponent is n = 0, which means the creep behavior in this region is independent on applied stress but some physical properties of materials. The creep behavior was suggested to be controlled by surface diffusion based on the strongly effect of surface area on creep behavior only in this creep region (n = 0). The threshold creep rate, , called intrinsic deformation limit, decided by surface diffusion was suggested. This discovery provided a new perspective to understand the extremely slow deformation in the nature.

Abstract: The diffusion of particles adsorbed on a patchwise surface with two non-equivalentsites is investigated in the framework of the lattice-gas model. The coverage dependencies of thecenter-of-mass and Fickian diffusion coefficients are calculated for some representative valuesof the lateral interaction. We propose the analytical expressions for the diffusion coefficients.We compare the theoretical dependencies with the numerical data obtained by the kineticMonte Carlo simulations. The good coincidence of the data obtained by the two quite differentmethods corroborates strongly the approach developed to describe the particle migration onsuch complex, heterogeneous lattices.

Abstract: Erosion wear tests under different erosion slurry mediums were carried out to investigate erosion properties of the treated samples of ZM5 magnesium alloy by surface diffusion alloying processing (SDAP).The result indicated that the micro-hardness (HV172) near the surface of alloyed specimen was about twice more than that of its substrate(HV82) after treated at 390°C for 8h.The erosion wear resistance β (0.68×104 mm2/g) of the treated specimen was better than that of untreated specimen (0.48×104mm2/g) under the conditions of erosion slurry of oil and quartz. The difference of erosion wear resistance between them was primary due to cutting wear mechanism. While erosion mediums was changed to the slurry of water and quartz, the wear resistance β of the treated specimen increase up to 0.24×104 (mm2/g), and that of the untreated specimen was only 0.09×104 (mm2/g),the difference between them resulted from corrosion wear mechanism predominating, which worsen the erosion wear resistance β.

Abstract: Recently) the point contact reactions between silicon nanowires (covered by natural oxide) and nanowires or nanodots of metals (nickel, cobalt, platinum) were discovered and studied. These reactions have at least three remarkable characteristics: (1) the reaction product phase is quite different from thin film or bulk reactions (for example, in Ni-Si reaction the appearing phase is Ni1Si1 or Ni1Si2, depending on the orientation of Si, instead of common Ni2Si phase); (2) Phase is formed not in the contact zone but, instead, near the wire tip or between two point contacts; (3) Subsequent phase growth of silicide inside silicon nanowire is a “stop-and-go” process consisting of waiting time before formation of critical island of each new atomic layer and then the fast filling of this new layer (mechanism suggested in 30-s for ideal crystals) Models of surface diffusion along and penetration through silicon oxide are presented. Nucleation models need more place and will be discussed in subsequent parts

Abstract: Nowadays, the most promising methods for high purity hydrogen production are membranes separation such as polymer, metal, ceramic and composites. It is well known that Pd and Pd-alloys membranes have excellent properties for hydrogen separation. However, it has hydrogen embrittlement and high cost for practical applications. Therefore, most scientists have studied new materials instead of Pd and Pd-alloys. On the other hand, TiN powders are great in resistance to acids and chemically stable under high operating temperature. In order to get specimens for hydrogen permeation, the TiN powders synthesized were consolidated together with pure Co powders by hot press sintering. During the consolidation of powders at HPS, heating rate was 10K/min and the pressure was 10MPa. It was characterized by XRD, SEM, and BET. Also, we estimated the hydrogen permeability by Sievert's type hydrogen permeation membrane equipment.

Abstract: Thin graded W/Cu coatings plus subsequent heat treatment at 800°C are used to improve the interfacial adhesion between W and Cu. Specimen with ~500 nm thick graded W/Cu coatings were characterized and analyzed after thermal treatment at 550°C, 650°C and 800°C. At 800°C a significant change in the nanostructure is observed. The better adhesion is caused by W/W grain boundary diffusion processes and Oswald ripening of the nanometre-sized grains leading to their interconnection between each other and the W substrate. The phase and texture analysis of graded W/Cu indicates grain growth and diffusion processes of pure W and Cu. The stress analysis shows that the changes in the nanostructure of the W/Cu coatings correlates to the stress relieve of W at temperature starting from 650°C. After cooling of the coating to RT the residue intrinsic tensile stress is caused by thermal mismatches of the substrate and the 100% W layer of the coating.

Abstract: A general model of transport of gases in an artificial epidermal layer (membrane) was established. The model was developed based on Dusty Gas Model (DGM), solution diffusion and surface diffusion. As a result, solutions of the model for different transport conditions were derived. In this investigation, parameters of oxygen and carbon dioxide gases through an artificial “epidermal” membrane of varying porosity were used to calculate semi-empirical solutions of the general model. In other words, the solutions of the general model were analytically obtained for different transport conditions, using experimentally obtained parameters of oxygen and carbon dioxide gases through the artificial “epidermal” membrane of varying porosity. The obtained solutions of the general model were for the oxygen and carbon dioxide gases through the artificial “epidermal” membrane of the varying porosity.

Abstract: With the aim to study the regularities of surface diffusion in coadsorbed layers, we investigated diffusion of lithium on the (112) surfaces of Mo and W precovered with submonolayers of dysprosium and strontium, which have substantially lower mobilities than lithium. Experiments were carried out using scanning contact-potential microscopy, and Li diffusion parameters were extracted from diffusional evolution of coverage profiles. Dy and Sr preadsorbed in amounts of ∼10–1 of a monolayer were found to reduce the diffusion rate of Li by orders of magnitude. The strong impact of coadsorbates with low mobility on Li diffusion can be caused by important role of collective mechanisms in surface diffusion, which entails pronounced pinning effects, as well as by the possibility of formation of surface alloys and surface vitrification.