Diffusion at extended defects

A second fundamental theme that will be taken up in this chapter in which there is an interplay between the various types of defects introduced earlier is that of mass transport assisted deformation. Our discussion will build on the analysis of diffusion at extended defects. We will begin with an examination of the phenomenology of creep. 

Diffusion at Extended Defects 11.2.1 Background on Short-Circuit Diffusion In chap. 7, we discussed the fundamental role of point defects in diffusion. We also hinted at the serious amendment to diffusion rates that is mediated by the presence of extended defects such as dislocations and grain boundaries (see fig. 9.1). As was noted above, diffusion rates along defects can often be so much larger than those in... 

Our present task is to build on the foundations laid in chap. 7, but now with special reference to the diffusive processes that take place at extended defects. The basic argument will be that by virtue of the more open atomic-level environments near extended defects, the activation energy both for point defect formation and migration will often be reduced relative to bulk values. We will build our case around a fundamental case study through the consideration of diffusion at surfaces. The surface diffusion example will illustrate not only how diffusive processes are amended at extended defects, but will also illustrate the shortcomings of the transition state formalism when the detailed atomic-level mechanisms are not known a priori. 

A key aspect of the deposition and diffusion of metal atoms and clusters on oxide surfaces is the role played by point and extended defects. There is little doubt that the nucleation and growth of clusters and small metal particles occur at defect sites [26-33]. A recent example is that of atomically resolved STM images of Au atoms on Ti02 [34] (Fig. 2.2). 

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