Diffusion path

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A diffusion path is the path through which atoms in a material diffuse. A distinction is made in solid bodies between volume diffusion (lattice diffusion), grain boundary diffusion and boundary or surface diffusion. The distinction is necessary insofar as each path has its own speed and amount of material transport (mass flow).

With volume diffusion, the diffusion takes place directly within the crystallites . Due to the strong binding forces between the atoms in the lattice and the few vacancies, the activation energy of the diffusion process is very high here. Accordingly, according to Fick's law , the mass flow is small in relation to a constant reference diffusion cross section .

In grain boundary diffusion, the atoms migrate across the grain boundary into a neighboring grain. Since grain boundaries are places with a particularly large number of lattice defects , the binding energy between the lattice atoms is lower and thus also the activation energy required for diffusion. In relation to the cross-section , the mass flow is greater than with volume diffusion.

During surface diffusion, some atoms leave the structure, others fill in the voids that are created. This is particularly easy because there are plenty of voids available on the surface at all times. In addition, the difference in concentration between the atoms in the solid and outside it is very large, which strongly favors diffusion. The activation energy is very low in this case and the mass flow is very high in relation to the cross section .

In a material, however, volume diffusion is extremely important, since it has a much larger diffusion cross-section than surface diffusion and thus the mass flow is greatest in volume diffusion.