Search results for "Surface diffusion"
showing 10 items of 30 documents
AES and CEMS analysis of the formation of layers on Si steel under thermal treatment in a flux of H2/water vapour
1991
The near surface diffusion and reaction processes in iron-silicon steel (3.1 wt.%Si) during 10 min decarburization in water vapour/hydrogen have been studied. The decarburization temperature has been varied between 506 and 714°C for the fixed partial pressure ratio pH2O/pH2=0.017. An outer layer of SiO2 forms on the surface with its thickness increasing with temperature. From 600°C upwards, the decarburization process is hindered and a cementite layer is formed below the SiO2 layer. The formation of fayalite at the surface has been studied at a fixed decarburization temperature with pH2O/pH2 ranging from 0.017 to 0.49. The scale thickness reduces abruptly just before the ratio pH2O/pH2 nece…
Modeling of PAH elimination by adiabatic steam stripping and intraparticle desorption
2003
The decontamination process of solids loaded with PAH in a pilot plant is modeled. This process is separated into two main stages: fast steam stripping at the entrance of the separation tube and the subsequent slow desorption of PAH from the interior of the soil particles. The stripping process also occurs in two stages: fast diffusion controlled saturation of the partial pressures followed by the heat transfer controlled evaporation. The phase diagram of PAH/water is constructed using both the microscopic model and empirical relationships for systems with wide miscibility gaps. The numerical basis for the calculations describing desorption from the soil particles is discussed. The model fo…
The coupling of the electrostatic potential with the transport and adsorption mechanisms in ion-exchange chromatography systems: Theory and experimen…
2005
The coupling of the constitutive expression for the electrostatic potential as specified through Poisson's equation together with the constitutive equations for the mechanisms of convection, diffusion, electrophoretic migration, and adsorption provides the necessary set of constitutive expressions to be employed in the material balance equations of ion-exchange chromatography systems to construct macroscopic continuum models that could be used to design and simulate the dynamic behavior of systems involving a single charged adsorbate or multiple charged adsorbates. A physically relevant and consistent macroscopic continuum model that can predict, as has been observed experimentally by UV co…
Hybrid chalcogenide nanoparticles: 2D-WS2 nanocrystals inside nested WS2 fullerenes.
2013
The MOCVD assisted formation of nested WS2 inorganic fullerenes (IF-WS2) was performed by enhancing surface diffusion with iodine, and fullerene growth was monitored by taking TEM snapshots of intermediate products. The internal structure of the core–shell nanoparticles was studied using scanning electron microscopy (SEM) after cross-cutting with a focused ion beam (FIB). Lamellar reaction intermediates were found occluded in the fullerene particles. In contrast to carbon fullerenes, layered metal chalcogenides prefer the formation of planar, plate-like structures where the dangling bonds at the edges are stabilized by excess S atoms. The effects of the reaction and annealing temperatures o…
The role of surface diffusion in the growth mechanism of III-nitride nanowires and nanotubes.
2021
Abstract The spontaneous growth of GaN nanowires (NWs) in absence of catalyst is controlled by the Ga flux impinging both directly on the top and on the side walls and diffusing to the top. The presence of diffusion barriers on the top surface and at the frontier between the top and the sidewalls, however, causes an inhomogeneous distribution of Ga adatoms at the NW top surface resulting in a GaN accumulation in its periphery. The increased nucleation rate in the periphery promotes the spontaneous formation of superlattices in InGaN and AlGaN NWs. In the case of AlN NWs, the presence of Mg can enhance the otherwise short Al diffusion length along the sidewalls inducing the formation of AlN …
Enhanced Thermal Stability of Gold and Silver Nanorods by Thin Surface Layers
2007
Using in situ transmission electron microscopy, we find that a carbon shell governs the morphological transitions of gold and silver nanorods upon heating. Encapsulated Ag nanorods show a surprising nonuniform sublimation behavior starting from one side and leaving behind the shell. Uncovered gold nanorods transform their shape to spheres well below the bulk melting temperature through surface diffusion, which is prevented by a thin carbon shell.
Comment on “Surface diffusion near the points corresponding to continuous phase transitions” [J. Chem. Phys. 109, 3197 (1998)]
1999
It is well known that unlike static equilibrium properties, kinetic quantities in Monte Carlo simulations are very sensitive to the details of the algorithm used for the microscopic transition rates. This is particularly true near the critical region where fluctuations are pronounced. We demonstrate that when diffusion of oxygen adatoms near the order–disorder transition of a lattice-gas model of the O/W(110) model system is studied, the transition rates must be chosen carefully. In particular, we show that the choice by Uebing and Zhdanov [J. Chem. Phys. 109, 3197 (1998)] is inappropriate for the study of critical effects in diffusion.
Reply to "comment on 'Monte Carlo simulations for a Lotka-type model with reactant surface diffusion and interactions' ".
2002
As is well known, a wide class of physical problems, including the kinetics of heterogeneous catalytic reactions, is traditionally described in terms of the master equations ~ME!. The definition of ME allows us not only to perform Monte Carlo ~MC! simulations, but also to develop at the same time appropriate analytical methods @mean field~MF!, cluster approximations, etc. #@ 1#. ME is formally defined when all possible states of a system and the transition rates between these states are specified. This is enough to define only the transition rates K(i! j ) for such elementary processes as particle adsorption, desorption, diffusion, reaction, etc., from the initial state i to the final state…
Diffusion-Driven Formation of MoS2 Nanotube Bundles Containing MoS2 Nanopods
2011
MoS2 nanotube bundles, along with embedded nested fullerenes, were formed in a gas-phase reaction of molybdenum carbonyl and H2S gas with the assistance of I2. The amorphous Mo-S-I precursor particles obtained by quenching a modified metal organic chemical vapor deposition (MOCVD) reaction in a large temperature gradient were annealed at elevated temperature in an inert atmosphere. Under the influence of the iodine, the amorphous precursor formed a surface film with an enhanced mobility of the molybdenum and sulfur components. Point defects within the MoS2 layers, combined with the enhanced surface diffusion, lead to a scrolling of the inherently unstable MoS2 lamellae. The role of the reac…
Mechanical properties and accommodation processes on metallic interfaces
1998
Bimetallic joints Al/Pb, Al/Sn, Pb/Sn, Pb/Pb, etc. with clean interfaces, obtained by a special cold welding method, are used as a model of phase boundaries for investigation of accommodation processes and strength properties of interfaces. To reduce the volume diffusion-induced relaxation processes, investigations were carried out at relatively low temperatures 0.1-0.5 T m . The role of surface diffusion and the effect of phase boundary energy on the healing of micropore ensembles on interfaces was investigated. The accommodation processes on the phase boundaries are considered as a result of mechanoactivation of both the interaction and structure formation of such non-equilibrium systems.…