Field-enhanced diffusion

Changes in the composition of a specimen over the analyzed depth can be caused by beam heating or by beam charging of the specimen. Beam heating can lead to selective vaporization of some elements or diffrisional redistribution of the elements. If the surflice charges up to some potential, then electric-field enhanced diffusion can selectively redistribute certain elements. Beam-heating effects usually... 

Recently Fu et al. [22] considered the state of reduction of the bulk oxide and its inflnence on the SMSI state. They concluded that, although complicated, the bulk must be reduced (or n-type doped) to enable the encapsulation process to occur and their study proposed electric field enhanced diffusion of Ti interstitials as a driver for the formation of the SMSI state. Furthermore, Pd islands on reduced SrTiOjCOOl) single crystals also show hexagonal structures on the flat facets of the islands when vacuum annealed [23]. Here, the reduced SrTiOj(OOl) surfaces are predominantly TiO terminated, so it appears that reduced Ti oxides are a requirement to induce the SMSI state. 

R. Ramirez, R. Gonzalez, J. Colera, and Y. Chen, Electric-field-enhanced diffusion of deuterons and protons in a-A1203 crystals, Phys. Rev. B 55, 237-242 (1997). 

Spurious effects can also occur due to space charge, as when insulating materials are treated by charged-particle beams, and may include electric-field-enhanced diffusion, sample degradation by surface flashover, and peak shifting and broadening during XPS analysis [34, 35]. 

It should be kept in mind that all transport processes in electrolytes and electrodes have to be described in general by irreversible thermodynamics. The equations given above hold only in the case that asymmetric Onsager coefficients are negligible and the fluxes of different species are independent of each other. This should not be confused with chemical diffusion processes in which the interaction is caused by the formation of internal electric fields. Enhancements of the diffusion of ions in electrode materials by a factor of up to 70000 were observed in the case of LiiSb [15]. 

The velocity, viscosity, density, and channel-height values are all similar to UF, but the diffusivity of large particles (MF) is orders-of-magnitude lower than the diffusivity of macromolecules (UF). It is thus quite surprising to find the fluxes of cross-flow MF processes to be similar to, and often higher than, UF fluxes. Two primary theories for the enhanced diffusion of particles in a shear field, the inertial-lift theory and the shear-induced theory, are explained by Davis [in Ho and Sirkar (eds.), op. cit., pp. 480-505], and Belfort, Davis, and Zydney [/. Membrane. Sci., 96, 1-58 (1994)]. While not clear-cut, shear-induced diffusion is quite large compared to Brownian diffusion except for those cases with very small particles or very low cross-flow velocity. The enhancement of mass transfer in turbulent-flow microfiltration, a major effect, remains completely empirical. 

While the growth of thermal oxides is dominated by high-temperature diffusion of oxygen in the oxide matrix, anodic oxide growth is dominated by field-enhanced hydroxyl diffusion at RT. These different growth mechanisms result in pronounced differences in the morphological, chemical and electrical properties of the oxide. 

What happens when the dimensions are furthermore reduced Initially, an enhanced diffusive mass transport would be expected. That is true, until the critical dimension is comparable to the thickness of the electrical double layer or the molecular size (a few nanometers) [7,8]. In this case, diffusive mass transport occurs mainly across the electrical double layer where the characteristics (electrical field, ion solvent interaction, viscosity, density, etc.) are different from those of the bulk solution. An important change is that the assumption of electroneutrality and lack of electromigration mass transport is not appropriate, regardless of the electrolyte concentration [9]. Therefore, there are subtle differences between the microelectrodic and nanoelectrodic behaviour. 

Another illustration of the power of molecular dynamics simulation can be drawn from the sphere of enzyme catalysis. Many enzyme-catalyzed reactions proceed at a rate that depends on the diffusion-limited association of the substrate with the active site. Sharp et al. [28] have carried out Brownian dynamics simulations of the association of superoxide anions with superoxide dismutase (SOD). The active center in SOD is a positively charged copper atom. The distribution of charge over the enzyme is not uniform, and so an electric field is produced. Using their model, Sharp et al. [28] have shown that the electric field enhances the association of the substrate with the enzyme by a factor of 30 or more. Their calculations also predict correctly the response of the association rate to changes in ionic strength and amino... 

On the very small scale of individual lines, vias, pole pieces, etc., current-distribution behavior is dominated by three characteristics. Firstly, ohmic potential drop becomes nearly insignificant because the distances are so short. Secondly, since small features can be smaller than or at least comparable in size to diffusion boundary layers, geometric concentration-field effects, such as radially and spherically enhanced diffusion, can play an important role. Thirdly, since the lateral dimensions of the feature are often comparable to the final thickness of the deposit, the geometry of the problem (which is responsible for mass-transfer nonuniformity) constantly evolves during deposition. Hence it may be necessary to include cumulative shape-change effects in studies on the feature scale. 

Recovery of CO2 in the oil and gas production is of major importance to promote enhanced oil recovery (FOR) from depleted fields High pressure CO2 is then pumped back into the reservoir at the periphery of the field and diffuses through the formation to drive residual oil toward the wells. The recycled gas generally needs to have a purity of at least 95 vol% CO2. 

It has been shown recently [75] that the diffusion of boron and aluminium in SiC occurs by a mechanism very similar to that of gold in silicon, i.e. it breaks the equilibrium of intrinsic point defects in the system. Thus, if aluminium concentration is high and the diffusion temperature is not too high, the enhanced diffusion will take place along dislocations. In the vicinity of an imperfection the p-n junction profile will be distorted, and a needle-shaped projection will appear (FIGURE 17(c)). This suggests a very strong electric field concentration and microplasma formation in a reverse-biased p-n junction. 

A. H. Marshak and R. Shrivastava, Calculation of the electric field enhancement for a degenerate diffusion process, Solid State Electron. 25 151-153. 

Diffusion in ionic materials occurs primarily by the movement of charged species. Therefore, the application of an electric field can provide a very powerful driving force for mass transport. There have been numerous studies on the effects of electric fields on transport phenomena. Several studies have been performed on the evaporation of alkali halides in the presence of an external field. These investigations showed that the application of an electric field enhanced the evaporation of the crystal species. Similar studies have been performed on oxide ionic conductors, including ZrOi and p-aluminas. However, only a few experiments have been performed on classical insulating oxides such as a-A Os and MgO (perhaps because they are insulators). 

Aside from cavitation, the enhanced mass transfer rates in acoustic fields can be attributed to plug flow of capillary liquid as well as to enhanced dispersion of the liquid and vapor moisture due to alternating compression and expansion cycles, which result in reduced viscosity of the liquid-vapor mixture. In fact, a substantial increase in the amount of liquid diffusing through porous solids has been noted in the presence of ultrasound (Fairbanks and Chen, 1969 Woodford and Morrison, 1969 Kuznetsov and Subbotina, 1965). The enhanced diffusion appears to be of the directional type since mass transfer was hindered when ultrasound irradiation was opposed to the direction of diffusive flow (Kuznetsov and Subbotina, 1965). 

The main virtue of SFC compared with HPLC is that it provides enhanced diffusivity. Enhanced diffusivity can also be obtained, however, in other ways. First, CO2 in its liquid state provides higher diffusivity than do conventional liquids application of this mobile phase is often called subcritical fluid chromatography. Another possibility to improve diffusivity is to use mixtures of liquid CO2 and conventional liquids as the mobile phase. This approach has been taken by Olesik and co-workers (Cui and Olesik, 1991 Lee and Olesik, 1995 Lee, Olesik and Fields, 1995) who showed that the mixing of methanol with up to 50% liquid CO2 resulted in increased diffusivity and decreased viscosity and the polarity was largely maintained. A third approach to HPLC with enhanced diffusivity is the application of enhanced separation temperatures as demonstrated by Trones, Iveland and Greibrokk (1995). 

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