Defect-mediated diffusion

An argument against the defect mediated diffusion model is the same one used earlier that is, there are not enough defects as determined by ESR (Brodsky and Title, 1969, 1976) or Deep Level Transient Spectroscopy measurements (Johnson, 1983) to account for the motion of all of the bonded hydrogen in a-Si H. This objection is removed if the floating bonds are 104-106 times more mobile than the hydrogen atoms. However, such highly mobile defects would rapidly self-annihilate via the process. 

There are two overriding considerations to keep in mind when discussing diffusion in solids the structure of the matrix across which diffusion occurs and the defects present. In a normal crystalline solid, diffusion is mediated by the defects present, and the speed of diffusion will vary significantly if the predominant defect type changes. This is because diffusion involves the movement of a species from a stable position, through some sort of less stable position or bottleneck, to another stable position. Any disorder in the solid due to defects will make this process easier. 

In Fig. 9.1, Dd for nondissociated dislocations is practically equal to DB, which indicates that the diffusion processes in nondissociated dislocation cores and large-angle grain boundaries are probably quite similar. Evidence for this conclusion also comes from the observation that dislocations can support a net diffusional transport of atoms due to self-diffusion [15]. As with grain boundaries, this supports a defect-mediated mechanism. 

The activation volume for diffusion, as measured by the pressure dependence of the diffusivity, is zero to within experimental accuracy [13, 14]. This is unexpected for defect-mediated diffusion, as in such cases, the activation volume for diffusion should consist of the sum of the volume of formation of the defect and the activation volume for the defect migration, and this is usually measurable. 

By use of the proper experimental conditions and Ltting the four models described above, it may be possible to arrive at a reasonable mechanistic interpretation of the experimental data. As an example, the crystal growth kinetics of theophylline monohydrate was studied by Rodriguez-Hornedo and Wu (1991). Their conclusion was that the crystal growth of theophylline monohydrate is controlled by a surface reaction mechanism rather than by solute diffusion in the bulk. Further, they found that the data was described by the screw-dislocation model and by the parabolic law, and they concluded that a defect-mediated growth mechanism occurred rather than a surface nucleation mechanism. 

We begin our discussion with the consideration of diffusion. In earlier chapters, we noted that diffusion is mediated by the presence of point defects. However, until now, we have not given any quantitative analysis of the lubricating effect of other defects in the context of diffusion. We have already seen in fig. 9.1 that bulk diffusion can be negligible in comparison with the diffusion that takes place along surfaces, grain boundaries and dislocations. As a particular realization of this class of problem, we consider the case of surface diffusion with special reference to some of the counterintuitive diffusion mechanisms that take place at surfaces. 

Theoretical studies of defect-mediated turbulence in two-dimensional systems have generally followed two approaches. One starts with local oscillators that couple through a diffusion process [36] another starts from a stationary patterned state [49] (see also the discussion of [10]). Both approaches lead to a turbulent state. However, our data do not distinguish in an obvious way between these two approaches. We do not observe any hint of global oscillations in the neighborhood of the transition from hexagons or stripes to turbulence, as would be expected in the first hypothesis. Nor do we observe any indication of a spatially biperiodic pattern, as would be expected in the second hypothesis. More experiments and analyses are needed to characterize and understand the mechanism of chemical turbulence. 

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