Diffusion Kirkendal effect

The reaction may occur by a number of mechanisms, provided electroneutrality is maintained. An extreme assumption could be, for example, that the only mobile cation is AP. Electro-neutrality involves a parallel diffusion of three 0 ions, with every two AP ions. An important consequence of such a diffusion mechanism is that the marker would be displaced both alumina ions would be transported towards the spinel-MgO boundary while the spinel-Al203 boundary progresses into the alumina side. The displacement of the marker is known as the Kirkendal effect. 

Kirkendal Effect When we previously discussed the transient interdiffiision of two semi-infinite bodies (material A and material B, respectively), we explicitly specified that the diffusion of A in B and that of B in A were identical and could therefore be described by a single diffusion coefficient. In many solids, however, this is not true. For example, the diffusivity of zinc in copper is much larger than the diffiisivity of copper in zinc. If a block of brass (a copper-zinc alloy) and a block of pure copper are bonded together at high temperatures, the zinc atoms will diffuse out of the brass and into the copper at a much faster rate than the copper atoms diffuse into the brass block. The net result is that the effective interface between the brass and copper blocks moves toward the brass, as illustrated schematically in Figure 4.17. This phenomenon is known as the Kirkendal effect and it occurs in many solid-state systems. 

FIGURE 4.17 Kirkendal effect iUustTated for a brass/copper diffusion couple. Since the diffusion of zinc into copper is mnch faster than the diffusion of copper into brass, a net flow of mass occurs from the brass side to the copper side. As a result, the location of the interface between the brass and the copper moves toward the brass side. 

As A and B diffuse across the eutectic interface into the P and a phases, respectively, the interface will move (just as we discussed previously in the context of the Kirkendal effect). This will occur even if the difftisivities of A and B in a and P are equal, because the local concentration gradients on the two sides of the interface will likely be different. (Remember, the flux depends on the diffusivity and the concentration gradient.)... 

Alivisatos and coworkers have synthesized hollow nanocrystals by a process analogous to the Kirkendal effect observed in the bulk [712]. In bulk matter, pores are formed in alloying or oxidation reactions due to large differences in the solid-state diffusion rates of the constituents. By reacting Co nanocrystals... 

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