Diffusion at grain boundaries

Those that affect diffusivity at grain boundaries by modification of boundary defects. 

FeZOCrBAI-O.ITYjOs 1100 Oxygen diffusion at grain boundaries... 

Figure C2.11.6. The classic two-particle sintering model illustrating material transport and neck growtli at tire particle contacts resulting in coarsening (left) and densification (right) during sintering. Surface diffusion (a), evaporation-condensation (b), and volume diffusion (c) contribute to coarsening, while volume diffusion (d), grain boundary diffusion (e), solution-precipitation (f), and dislocation motion (g) contribute to densification.

In polycrystalline materials, ion transport within the grain boundary must also be considered. For oxides with close-packed oxygens, the O-ion almost always diffuses much faster in the boundary region than in the bulk. In general, second phases at grain boundaries are less close packed and provide a pathway for more rapid diffusion of ionic species. Thus the simplified picture of bulk ionic conduction is made more complex by these additional effects. 

Said this, we can let the reader to recall Fig. 1.15, where we depicted amorphous-like phase regions at grain boundaries as the pathways open for preferential diffusion of hydrogen atoms. Apparently, an alloy can benefit from some fraction of amorphous phase to improve kinetics of hydrogen absorption, but complete amorphization of crystalline lattice lowers capacity for storing hydrogen [156]. Mechanochemical activation is therefore a complex process where kinetic and thermodynamic effects must be firstly well understood, and then optimized. 

There are many situations, particularly at low temperatures, where short-circuit diffusion along grain boundaries and free surfaces is the dominant mode of diffusional transport and therefore controls important kinetic phenomena in materials ... 

Without zinc, silicon diffusion to the surface is slow under MCS conditions. Furthermore, when only tin is used as a promoter, no Cu is observed at the surface. Table 3 shows the elemental concentration under various conditions. Under MCS reaction conditions, when zinc was present silicon was not depleted from the subsurface, and when zinc was absent the subsurface was depleted in silicon. Zinc causes the rate of silicon diffusion and copper dispersion to increase. Zinc accumulates at grain boundaries and lowers the free energy of CU3SL Tin and zinc appear to work synergistically but tin does not enhance silicon diffusion on its own. Tin does appear to lower the surface energy of silicon/copper. 

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